Molecular	O
Dissection	O
of	O
Xyloglucan	B-chemical
Recognition	O
in	O
a	O
Prominent	O
Human	B-species
Gut	O
Symbiont	O

Polysaccharide	B-gene
utilization	I-gene
loci	I-gene
(	O
PUL	B-gene
)	O
within	O
the	O
genomes	O
of	O
resident	O
human	B-species
gut	O
Bacteroidetes	B-taxonomy_domain
are	O
central	O
to	O
the	O
metabolism	O
of	O
the	O
otherwise	O
indigestible	O
complex	O
carbohydrates	B-chemical
known	O
as	O
“	O
dietary	O
fiber	O
.”	O
However	O
,	O
functional	O
characterization	O
of	O
PUL	B-gene
lags	O
significantly	O
behind	O
sequencing	O
efforts	O
,	O
which	O
limits	O
physiological	O
understanding	O
of	O
the	O
human	B-species
-	O
bacterial	B-taxonomy_domain
symbiosis	O
.	O

In	O
particular	O
,	O
the	O
molecular	O
basis	O
of	O
complex	B-chemical
polysaccharide	I-chemical
recognition	O
,	O
an	O
essential	O
prerequisite	O
to	O
hydrolysis	O
by	O
cell	O
surface	O
glycosidases	B-protein_type
and	O
subsequent	O
metabolism	O
,	O
is	O
generally	O
poorly	O
understood	O
.	O

Here	O
,	O
we	O
present	O
the	O
biochemical	B-experimental_method
,	I-experimental_method
structural	I-experimental_method
,	I-experimental_method
and	I-experimental_method
reverse	I-experimental_method
genetic	I-experimental_method
characterization	I-experimental_method
of	O
two	O
unique	O
cell	B-protein_type
surface	I-protein_type
glycan	I-protein_type
-	I-protein_type
binding	I-protein_type
proteins	I-protein_type
(	O
SGBPs	B-protein_type
)	O
encoded	O
by	O
a	O
xyloglucan	B-gene
utilization	I-gene
locus	I-gene
(	O
XyGUL	B-gene
)	O
from	O
Bacteroides	B-species
ovatus	I-species
,	O
which	O
are	O
integral	O
to	O
growth	O
on	O
this	O
key	O
dietary	O
vegetable	B-taxonomy_domain
polysaccharide	B-chemical
.	O

Biochemical	B-experimental_method
analysis	I-experimental_method
reveals	O
that	O
these	O
outer	B-protein_type
membrane	I-protein_type
-	I-protein_type
anchored	I-protein_type
proteins	I-protein_type
are	O
in	O
fact	O
exquisitely	O
specific	O
for	O
the	O
highly	O
branched	O
xyloglucan	B-chemical
(	O
XyG	B-chemical
)	O
polysaccharide	B-chemical
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
SGBP	B-protein
-	I-protein
A	I-protein
,	O
a	O
SusD	B-protein
homolog	O
,	O
with	O
a	O
bound	B-protein_state
XyG	B-chemical
tetradecasaccharide	B-chemical
reveals	O
an	O
extended	O
carbohydrate	B-site
-	I-site
binding	I-site
platform	I-site
that	O
primarily	O
relies	O
on	O
recognition	O
of	O
the	O
β	B-chemical
-	I-chemical
glucan	I-chemical
backbone	O
.	O

The	O
unique	O
,	O
tetra	B-structure_element
-	I-structure_element
modular	I-structure_element
structure	B-evidence
of	O
SGBP	B-protein
-	I-protein
B	I-protein
is	O
comprised	O
of	O
tandem	B-structure_element
Ig	I-structure_element
-	I-structure_element
like	I-structure_element
folds	I-structure_element
,	O
with	O
XyG	B-chemical
binding	O
mediated	O
at	O
the	O
distal	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
.	O

Despite	O
displaying	O
similar	O
affinities	B-evidence
for	O
XyG	B-chemical
,	O
reverse	B-experimental_method
-	I-experimental_method
genetic	I-experimental_method
analysis	I-experimental_method
reveals	O
that	O
SGBP	B-protein
-	I-protein
B	I-protein
is	O
only	O
required	O
for	O
the	O
efficient	O
capture	O
of	O
smaller	O
oligosaccharides	B-chemical
,	O
whereas	O
the	O
presence	O
of	O
SGBP	B-protein
-	I-protein
A	I-protein
is	O
more	O
critical	O
than	O
its	O
carbohydrate	B-chemical
-	O
binding	O
ability	O
for	O
growth	O
on	O
XyG	B-chemical
.	O
Together	O
,	O
these	O
data	O
demonstrate	O
that	O
SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
play	O
complementary	O
,	O
specialized	O
roles	O
in	O
carbohydrate	B-chemical
capture	O
by	O
B	B-species
.	I-species
ovatus	I-species
and	O
elaborate	O
a	O
model	O
of	O
how	O
vegetable	B-taxonomy_domain
xyloglucans	B-chemical
are	O
accessed	O
by	O
the	O
Bacteroidetes	B-taxonomy_domain
.	O

Our	O
combined	O
analysis	O
illuminates	O
new	O
fundamental	O
aspects	O
of	O
complex	B-chemical
polysaccharide	I-chemical
recognition	O
,	O
cleavage	O
,	O
and	O
import	O
at	O
the	O
Bacteroidetes	B-taxonomy_domain
cell	O
surface	O
that	O
may	O
facilitate	O
the	O
development	O
of	O
prebiotics	O
to	O
target	O
this	O
phylum	O
of	O
gut	O
bacteria	B-taxonomy_domain
.	O

This	O
microbial	B-taxonomy_domain
community	O
is	O
largely	O
bacterial	B-taxonomy_domain
,	O
with	O
the	O
Bacteroidetes	B-taxonomy_domain
,	O
Firmicutes	B-taxonomy_domain
,	O
and	O
Actinobacteria	B-taxonomy_domain
comprising	O
the	O
dominant	O
phyla	O
.	O

However	O
,	O
there	O
is	O
a	O
paucity	O
of	O
data	O
regarding	O
how	O
the	O
vast	O
array	O
of	O
complex	B-chemical
carbohydrate	I-chemical
structures	O
are	O
selectively	O
recognized	O
and	O
imported	O
by	O
members	O
of	O
the	O
microbiota	B-taxonomy_domain
,	O
a	O
critical	O
process	O
that	O
enables	O
these	O
organisms	O
to	O
thrive	O
in	O
the	O
competitive	O
gut	O
environment	O
.	O

The	O
human	B-species
gut	O
bacteria	B-taxonomy_domain
Bacteroidetes	B-taxonomy_domain
share	O
a	O
profound	O
capacity	O
for	O
dietary	O
glycan	B-chemical
degradation	O
,	O
with	O
many	O
species	O
containing	O
>	O
250	O
predicted	O
carbohydrate	O
-	O
active	O
enzymes	O
(	O
CAZymes	O
),	O
compared	O
to	O
50	O
to	O
100	O
within	O
many	O
Firmicutes	B-taxonomy_domain
and	O
only	O
17	O
in	O
the	O
human	B-species
genome	O
devoted	O
toward	O
carbohydrate	O
utilization	O
.	O

A	O
remarkable	O
feature	O
of	O
the	O
Bacteroidetes	B-taxonomy_domain
is	O
the	O
packaging	O
of	O
genes	O
for	O
carbohydrate	O
catabolism	O
into	O
discrete	O
polysaccharide	B-gene
utilization	I-gene
loci	I-gene
(	O
PUL	B-gene
),	O
which	O
are	O
transcriptionally	O
regulated	O
by	O
specific	O
substrate	O
signatures	O
.	O

The	O
Sus	B-complex_assembly
includes	O
a	O
lipid	B-protein_state
-	I-protein_state
anchored	I-protein_state
,	O
outer	O
membrane	O
endo	B-protein_type
-	I-protein_type
amylase	I-protein_type
,	O
SusG	B-protein
;	O
a	O
TonB	B-protein_type
-	I-protein_type
dependent	I-protein_type
transporter	I-protein_type
(	O
TBDT	B-protein_type
),	O
SusC	B-protein
,	O
which	O
imports	O
oligosaccharides	B-chemical
with	O
the	O
help	O
of	O
an	O
associated	O
starch	B-protein_type
-	I-protein_type
binding	I-protein_type
protein	I-protein_type
,	O
SusD	B-protein
;	O
two	O
additional	O
carbohydrate	B-protein_type
-	I-protein_type
binding	I-protein_type
lipoproteins	I-protein_type
,	O
SusE	B-protein
and	O
SusF	B-protein
;	O
and	O
two	O
periplasmic	O
exo	B-protein_type
-	I-protein_type
glucosidases	I-protein_type
,	O
SusA	B-protein
and	O
SusB	B-protein
,	O
which	O
generate	O
glucose	B-chemical
for	O
transport	O
into	O
the	O
cytoplasm	O
.	O

The	O
importance	O
of	O
PUL	B-gene
as	O
a	O
successful	O
evolutionary	O
strategy	O
is	O
underscored	O
by	O
the	O
observation	O
that	O
Bacteroidetes	B-taxonomy_domain
such	O
as	O
B	B-species
.	I-species
thetaiotaomicron	I-species
and	O
Bacteroides	B-species
ovatus	I-species
devote	O
~	O
18	O
%	O
of	O
their	O
genomes	O
to	O
these	O
systems	O
.	O

Moving	O
beyond	O
seminal	O
genomic	O
and	O
transcriptomic	O
analyses	O
,	O
the	O
current	O
state	O
-	O
of	O
-	O
the	O
-	O
art	O
PUL	B-gene
characterization	O
involves	O
combined	O
reverse	B-experimental_method
-	I-experimental_method
genetic	I-experimental_method
,	I-experimental_method
biochemical	I-experimental_method
,	I-experimental_method
and	I-experimental_method
structural	I-experimental_method
studies	I-experimental_method
to	O
illuminate	O
the	O
molecular	O
details	O
of	O
PUL	B-gene
function	O
.	O

Cleavage	O
sites	O
for	O
BoXyGUL	B-gene
glycosidases	B-protein_type
(	O
GHs	B-protein_type
)	O
are	O
indicated	O
for	O
solanaceous	B-taxonomy_domain
xyloglucan	B-chemical
.	O
(	O
B	O
)	O
BtSus	B-gene
and	O
BoXyGUL	B-gene
.	O
(	O
C	O
)	O
Localization	O
of	O
BoXyGUL	B-gene
-	O
encoded	O
proteins	O
in	O
cellular	O
membranes	O
and	O
concerted	O
modes	O
of	O
action	O
in	O
the	O
degradation	O
of	O
xyloglucans	B-chemical
to	O
monosaccharides	O
.	O

XyG	B-chemical
variants	O
(	O
Fig	O
.	O
1A	O
)	O
constitute	O
up	O
to	O
25	O
%	O
of	O
the	O
dry	O
weight	O
of	O
common	O
vegetables	B-taxonomy_domain
.	O

Analogous	O
to	O
the	O
Sus	B-gene
locus	I-gene
,	O
the	O
xyloglucan	B-gene
utilization	I-gene
locus	I-gene
(	O
XyGUL	B-gene
)	O
encodes	O
a	O
cohort	O
of	O
carbohydrate	B-protein_type
-	I-protein_type
binding	I-protein_type
,	I-protein_type
-	I-protein_type
hydrolyzing	I-protein_type
,	I-protein_type
and	I-protein_type
-	I-protein_type
importing	I-protein_type
proteins	I-protein_type
(	O
Fig	O
.	O
1B	O
and	O
C	O
).	O

The	O
number	O
of	O
glycoside	B-protein_type
hydrolases	I-protein_type
(	O
GHs	B-protein_type
)	O
encoded	O
by	O
the	O
XyGUL	B-gene
is	O
,	O
however	O
,	O
more	O
expansive	O
than	O
that	O
by	O
the	O
Sus	B-gene
locus	I-gene
(	O
Fig	O
.	O
1B	O
),	O
which	O
reflects	O
the	O
greater	O
complexity	O
of	O
glycosidic	O
linkages	O
found	O
in	O
XyG	B-chemical
vis	O
-	O
à	O
-	O
vis	O
starch	B-chemical
.	O

In	O
the	O
archetypal	O
starch	B-complex_assembly
utilization	I-complex_assembly
system	I-complex_assembly
of	O
B	B-species
.	I-species
thetaiotaomicron	I-species
,	O
starch	O
binding	O
to	O
the	O
cell	O
surface	O
is	O
mediated	O
at	O
eight	O
distinct	O
starch	B-site
-	I-site
binding	I-site
sites	I-site
distributed	O
among	O
four	O
surface	B-protein_type
glycan	I-protein_type
-	I-protein_type
binding	I-protein_type
proteins	I-protein_type
(	O
SGBPs	B-protein_type
):	O
two	O
within	O
the	O
amylase	B-protein_type
SusG	B-protein
,	O
one	O
within	O
SusD	B-protein
,	O
two	O
within	O
SusE	B-protein
,	O
and	O
three	O
within	O
SusF	B-protein
.	O
The	O
functional	O
redundancy	O
of	O
many	O
of	O
these	O
sites	O
is	O
high	O
:	O
whereas	O
SusD	B-protein
is	O
essential	O
for	O
growth	O
on	O
starch	B-chemical
,	O
combined	O
mutations	O
of	O
the	O
SusE	B-protein
,	O
SusF	B-protein
,	O
and	O
SusG	B-protein
binding	B-site
sites	I-site
are	O
required	O
to	O
impair	O
growth	O
on	O
the	O
polysaccharide	B-chemical
.	O

Bacteroidetes	B-taxonomy_domain
PUL	B-gene
ubiquitously	O
encode	O
homologs	O
of	O
SusC	B-protein
and	O
SusD	B-protein
,	O
as	O
well	O
as	O
proteins	O
whose	O
genes	O
are	O
immediately	O
downstream	O
of	O
susD	B-gene
,	O
akin	O
to	O
susE	B-gene
/	I-gene
F	I-gene
,	O
and	O
these	O
are	O
typically	O
annotated	O
as	O
“	O
putative	B-protein_state
lipoproteins	B-protein_type
”.	O

The	O
genes	O
coding	O
for	O
these	O
proteins	O
,	O
sometimes	O
referred	O
to	O
as	O
“	O
susE	B-gene
/	I-gene
F	I-gene
positioned	O
,”	O
display	O
products	O
with	O
a	O
wide	O
variation	O
in	O
amino	O
acid	O
sequence	O
and	O
which	O
have	O
little	O
or	O
no	O
homology	O
to	O
other	O
PUL	B-gene
-	O
encoded	O
proteins	O
or	O
known	O
carbohydrate	B-protein_type
-	I-protein_type
binding	I-protein_type
proteins	I-protein_type
.	O

We	O
describe	O
here	O
the	O
detailed	O
functional	B-experimental_method
and	I-experimental_method
structural	I-experimental_method
characterization	I-experimental_method
of	O
the	O
noncatalytic	B-protein_state
SGBPs	B-protein_type
encoded	O
by	O
Bacova_02651	B-gene
and	O
Bacova_02650	B-gene
of	O
the	O
XyGUL	B-gene
,	O
here	O
referred	O
to	O
as	O
SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
,	O
to	O
elucidate	O
their	O
molecular	O
roles	O
in	O
carbohydrate	O
acquisition	O
in	O
vivo	O
.	O

Combined	O
biochemical	B-experimental_method
,	I-experimental_method
structural	I-experimental_method
,	I-experimental_method
and	I-experimental_method
reverse	I-experimental_method
-	I-experimental_method
genetic	I-experimental_method
approaches	I-experimental_method
clearly	O
illuminate	O
the	O
distinct	O
,	O
yet	O
complementary	O
,	O
functions	O
that	O
these	O
two	O
proteins	O
play	O
in	O
XyG	B-chemical
recognition	O
as	O
it	O
impacts	O
the	O
physiology	O
of	O
B	B-species
.	I-species
ovatus	I-species
.	O

SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
are	O
cell	B-protein_type
-	I-protein_type
surface	I-protein_type
-	I-protein_type
localized	I-protein_type
,	I-protein_type
xyloglucan	I-protein_type
-	I-protein_type
specific	I-protein_type
binding	I-protein_type
proteins	I-protein_type
.	O

SGBP	B-protein
-	I-protein
A	I-protein
,	O
encoded	O
by	O
the	O
XyGUL	B-gene
locus	O
tag	O
Bacova_02651	B-gene
(	O
Fig	O
.	O
1B	O
),	O
shares	O
26	O
%	O
amino	O
acid	O
sequence	O
identity	O
(	O
40	O
%	O
similarity	O
)	O
with	O
its	O
homolog	O
,	O
B	B-species
.	I-species
thetaiotaomicron	I-species
SusD	B-protein
,	O
and	O
similar	O
homology	O
with	O
the	O
SusD	B-protein_type
-	I-protein_type
like	I-protein_type
proteins	I-protein_type
encoded	O
within	O
syntenic	O
XyGUL	B-gene
identified	O
in	O
our	O
earlier	O
work	O
.	O

In	O
contrast	O
,	O
SGBP	B-protein
-	I-protein
B	I-protein
,	O
encoded	O
by	O
locus	O
tag	O
Bacova_02650	B-gene
,	O
displays	O
little	O
sequence	O
similarity	O
to	O
the	O
products	O
of	O
similarly	O
positioned	O
genes	O
in	O
syntenic	O
XyGUL	B-gene
nor	O
to	O
any	O
other	O
gene	O
product	O
among	O
the	O
diversity	O
of	O
Bacteroidetes	B-taxonomy_domain
PUL	B-gene
.	O

Whereas	O
sequence	O
similarity	O
among	O
SusC	B-protein
/	O
SusD	B-protein
homolog	O
pairs	O
often	O
serves	O
as	O
a	O
hallmark	O
for	O
PUL	B-gene
identification	O
,	O
the	O
sequence	O
similarities	O
of	O
downstream	O
genes	O
encoding	O
SGBPs	B-protein_type
are	O
generally	O
too	O
low	O
to	O
allow	O
reliable	O
bioinformatic	O
classification	O
of	O
their	O
products	O
into	O
protein	O
families	O
,	O
let	O
alone	O
prediction	O
of	O
function	O
.	O

Hence	O
,	O
there	O
is	O
a	O
critical	O
need	O
for	O
the	O
elucidation	O
of	O
detailed	O
structure	O
-	O
function	O
relationships	O
among	O
PUL	B-gene
SGBPs	B-protein_type
,	O
in	O
light	O
of	O
the	O
manifold	O
glycan	B-chemical
structures	O
in	O
nature	O
.	O

Immunofluorescence	B-experimental_method
of	O
formaldehyde	O
-	O
fixed	O
,	O
nonpermeabilized	O
cells	O
grown	O
in	O
minimal	O
medium	O
with	O
XyG	B-chemical
as	O
the	O
sole	O
carbon	O
source	O
to	O
induce	O
XyGUL	B-gene
expression	O
,	O
reveals	O
that	O
both	O
SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
are	O
presented	O
on	O
the	O
cell	O
surface	O
by	O
N	O
-	O
terminal	O
lipidation	B-ptm
,	O
as	O
predicted	O
by	O
signal	O
peptide	O
analysis	O
with	O
SignalP	O
(	O
Fig	O
.	O
2	O
).	O

SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
visualized	O
by	O
immunofluorescence	B-experimental_method
.	O

Formalin	O
-	O
fixed	O
,	O
nonpermeabilized	O
B	B-species
.	I-species
ovatus	I-species
cells	O
were	O
grown	O
in	O
minimal	O
medium	O
plus	O
XyG	B-chemical
,	O
probed	O
with	O
custom	O
rabbit	O
antibodies	O
to	O
SGBP	B-protein
-	I-protein
A	I-protein
or	O
SGBP	B-protein
-	I-protein
B	I-protein
,	O
and	O
then	O
stained	O
with	O
Alexa	O
Fluor	O
488	O
goat	O
anti	O
-	O
rabbit	O
IgG	O
.	O
(	O
A	O
)	O
Overlay	B-experimental_method
of	O
bright	B-evidence
-	I-evidence
field	I-evidence
and	I-evidence
FITC	I-evidence
images	I-evidence
of	O
B	B-species
.	I-species
ovatus	I-species
cells	O
labeled	O
with	O
anti	O
-	O
SGBP	O
-	O
A	O
.	O
(	O
B	O
)	O
Overlay	B-experimental_method
of	O
bright	B-evidence
-	I-evidence
field	I-evidence
and	I-evidence
FITC	I-evidence
images	I-evidence
of	O
B	B-species
.	I-species
ovatus	I-species
cells	O
labeled	O
with	O
anti	O
-	O
SGBP	O
-	O
B	O
.	O
(	O
C	O
)	O
Bright	B-evidence
-	I-evidence
field	I-evidence
image	I-evidence
of	O
ΔSGBP	B-mutant
-	I-mutant
B	I-mutant
cells	O
labeled	O
with	O
anti	O
-	O
SGBP	O
-	O
B	O
antibodies	O
.	O

(	O
D	O
)	O
FITC	B-evidence
images	I-evidence
of	O
ΔSGBP	B-mutant
-	I-mutant
B	I-mutant
cells	O
labeled	O
with	O
anti	O
-	O
SGBP	O
-	O
B	O
antibodies	O
.	O

Cells	O
lacking	B-protein_state
SGBP	B-protein
-	I-protein
A	I-protein
(	O
ΔSGBP	B-mutant
-	I-mutant
A	I-mutant
)	O
do	O
not	O
grow	O
on	O
XyG	B-chemical
and	O
therefore	O
could	O
not	O
be	O
tested	O
in	O
parallel	O
.	O

Additional	O
affinity	B-experimental_method
PAGE	I-experimental_method
analysis	O
(	O
Fig	O
.	O
3	O
)	O
demonstrates	O
that	O
SGBP	B-protein
-	I-protein
A	I-protein
also	O
has	O
moderate	O
affinity	O
for	O
the	O
artificial	O
soluble	O
cellulose	O
derivative	O
hydroxyethyl	B-chemical
cellulose	I-chemical
[	O
HEC	B-chemical
;	O
a	O
β	B-chemical
(	I-chemical
1	I-chemical
→	I-chemical
4	I-chemical
)-	I-chemical
glucan	I-chemical
]	O
and	O
limited	O
affinity	O
for	O
mixed	B-chemical
-	I-chemical
linkage	I-chemical
β	I-chemical
(	I-chemical
1	I-chemical
→	I-chemical
3	I-chemical
)/	I-chemical
β	I-chemical
(	I-chemical
1	I-chemical
→	I-chemical
4	I-chemical
)-	I-chemical
glucan	I-chemical
(	O
MLG	B-chemical
)	O
and	O
glucomannan	B-chemical
(	O
GM	B-chemical
;	O
mixed	O
glucosyl	B-chemical
and	O
mannosyl	B-chemical
backbone	O
),	O
which	O
together	O
indicate	O
general	O
binding	O
to	O
polysaccharide	B-chemical
backbone	O
residues	O
and	O
major	O
contributions	O
from	O
side	O
-	O
chain	O
recognition	O
.	O

In	O
contrast	O
,	O
SGBP	B-protein
-	I-protein
B	I-protein
bound	O
to	O
HEC	B-chemical
more	O
weakly	O
than	O
SGBP	B-protein
-	I-protein
A	I-protein
and	O
did	O
not	O
bind	O
to	O
MLG	B-chemical
or	O
GM	B-chemical
.	O

Neither	O
SGBP	B-protein_type
recognized	O
galactomannan	B-chemical
(	O
GGM	B-chemical
),	O
starch	B-chemical
,	O
carboxymethylcellulose	B-chemical
,	O
or	O
mucin	B-chemical
(	O
see	O
Fig	O
.	O
S1	O
in	O
the	O
supplemental	O
material	O
).	O

Notably	O
,	O
the	O
absence	O
of	O
carbohydrate	B-site
-	I-site
binding	I-site
modules	I-site
in	O
the	O
GHs	B-protein_type
encoded	O
by	O
the	O
XyGUL	B-gene
implies	O
that	O
noncatalytic	O
recognition	O
of	O
xyloglucan	B-chemical
is	O
mediated	O
entirely	O
by	O
SGBP	B-protein
-	I-protein
A	I-protein
and	O
-	B-protein
B	I-protein
.	O

SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
preferentially	O
bind	O
xyloglucan	B-chemical
.	O

Affinity	B-experimental_method
electrophoresis	I-experimental_method
(	O
10	O
%	O
acrylamide	O
)	O
of	O
SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
with	O
BSA	B-protein
as	O
a	O
control	O
protein	O
.	O

All	O
samples	O
were	O
loaded	O
on	O
the	O
same	O
gel	O
next	O
to	O
the	O
BSA	B-protein
controls	O
;	O
thin	O
black	O
lines	O
indicate	O
where	O
intervening	O
lanes	O
were	O
removed	O
from	O
the	O
final	O
image	O
for	O
both	O
space	O
and	O
clarity	O
.	O

The	O
percentage	O
of	O
polysaccharide	B-chemical
incorporated	O
into	O
each	O
native	O
gel	O
is	O
displayed	O
.	O

The	O
vanguard	O
endo	B-protein_type
-	I-protein_type
xyloglucanase	I-protein_type
of	O
the	O
XyGUL	B-gene
,	O
BoGH5	B-protein
,	O
preferentially	O
cleaves	O
the	O
polysaccharide	B-chemical
at	O
unbranched	O
glucosyl	B-chemical
residues	O
to	O
generate	O
xylogluco	B-chemical
-	I-chemical
oligosaccharides	I-chemical
(	O
XyGOs	B-chemical
)	O
comprising	O
a	O
Glc4	B-structure_element
backbone	I-structure_element
with	O
variable	B-structure_element
side	I-structure_element
-	I-structure_element
chain	I-structure_element
galactosylation	I-structure_element
(	O
XyGO1	B-chemical
)	O
(	O
Fig	O
.	O
1A	O
;	O
n	O
=	O
1	O
)	O
as	O
the	O
limit	O
of	O
digestion	O
products	O
in	O
vitro	O
;	O
controlled	B-experimental_method
digestion	I-experimental_method
and	I-experimental_method
fractionation	I-experimental_method
by	O
size	B-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
allow	O
the	O
production	O
of	O
higher	O
-	O
order	O
oligosaccharides	B-chemical
(	O
e	O
.	O
g	O
.,	O
XyGO2	B-chemical
)	O
(	O
Fig	O
.	O
1A	O
;	O
n	O
=	O
2	O
).	O

ITC	B-experimental_method
demonstrates	O
that	O
SGBP	B-protein
-	I-protein
A	I-protein
binds	O
to	O
XyG	B-chemical
polysaccharide	B-chemical
and	O
XyGO2	B-chemical
(	O
based	O
on	O
a	O
Glc8	B-structure_element
backbone	I-structure_element
)	O
with	O
essentially	O
equal	O
affinities	B-evidence
,	O
while	O
no	O
binding	O
of	O
XyGO1	B-chemical
(	O
Glc4	B-structure_element
backbone	I-structure_element
)	O
was	O
detectable	O
(	O
Table	O
1	O
;	O
see	O
Fig	O
.	O
S2	O
and	O
S3	O
in	O
the	O
supplemental	O
material	O
).	O

Together	O
,	O
these	O
data	O
clearly	O
suggest	O
that	O
polysaccharide	B-chemical
binding	O
of	O
both	O
SGBPs	B-protein_type
is	O
fulfilled	O
by	O
a	O
dimer	B-oligomeric_state
of	O
the	O
minimal	B-structure_element
repeat	I-structure_element
,	O
corresponding	O
to	O
XyGO2	B-chemical
(	O
cf	O
.	O

The	O
observation	O
by	O
affinity	B-experimental_method
PAGE	I-experimental_method
that	O
these	O
proteins	O
specifically	O
recognize	O
XyG	B-chemical
is	O
further	O
substantiated	O
by	O
their	O
lack	O
of	O
binding	O
for	O
the	O
undecorated	O
oligosaccharide	B-chemical
cellotetraose	B-chemical
(	O
Table	O
1	O
;	O
see	O
Fig	O
.	O
S3	O
).	O

Furthermore	O
,	O
SGBP	B-protein
-	I-protein
A	I-protein
binds	O
cellohexaose	B-chemical
with	O
~	O
770	O
-	O
fold	O
weaker	O
affinity	B-evidence
than	O
XyG	B-chemical
,	O
while	O
SGBP	B-protein
-	I-protein
B	I-protein
displays	O
no	O
detectable	O
binding	O
to	O
this	O
linear	O
hexasaccharide	B-chemical
.	O

To	O
provide	O
molecular	O
-	O
level	O
insight	O
into	O
how	O
the	O
XyGUL	B-gene
SGBPs	B-protein_type
equip	O
B	B-species
.	I-species
ovatus	I-species
to	O
specifically	O
harvest	O
XyG	B-chemical
from	O
the	O
gut	O
environment	O
,	O
we	O
performed	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
analysis	O
of	O
both	O
SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGPB	B-protein
-	I-protein
B	I-protein
in	O
oligosaccharide	B-complex_assembly
-	I-complex_assembly
complex	I-complex_assembly
forms	I-complex_assembly
.	O

Summary	O
of	O
thermodynamic	O
parameters	O
for	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
obtained	O
by	O
isothermal	B-experimental_method
titration	I-experimental_method
calorimetry	I-experimental_method
at	O
25	O
°	O
Ca	O

Carbohydrate	O
Ka	O
(	O
M	O
−	O
1	O
)	O
ΔG	B-evidence
(	O
kcal	O
⋅	O
mol	O
−	O
1	O
)	O
ΔH	O
(	O
kcal	O
⋅	O
mol	O
−	O
1	O
)	O
TΔS	O
(	O
kcal	O
⋅	O
mol	O
−	O
1	O
)	O
SGBP	O
-	O
A	O
SGBP	O
-	O
B	O
SGBP	O
-	O
A	O
SGBP	O
-	O
B	O
SGBP	O
-	O
A	O
SGBP	O
-	O
B	O
SGBP	O
-	O
A	O
SGBP	O
-	O
B	O
XyGb	O
(	O
4	O
.	O
4	O
±	O
0	O
.	O
1	O
)	O
×	O
105	O
(	O
5	O
.	O
7	O
±	O
0	O
.	O
2	O
)	O
×	O
104	O
−	O
7	O
.	O
7	O
−	O
6	O
.	O
5	O
−	O
14	O
±	O
3	O
−	O
14	O
±	O
2	O
−	O
6	O
.	O
5	O
−	O
7	O
.	O
6	O
XyGO2c	O
3	O
.	O
0	O
×	O
105	O
2	O
.	O
0	O
×	O
104	O
−	O
7	O
.	O
5	O
−	O
5	O
.	O
9	O
−	O
17	O
.	O
2	O
−	O
17	O
.	O
6	O
−	O
9	O
.	O
7	O
−	O
11	O
.	O
7	O
XyGO1	O
NBd	O
(	O
2	O
.	O
4	O
±	O
0	O
.	O
1	O
)	O
×	O
103	O
NB	O
−	O
4	O
.	O
6	O
NB	O
−	O
4	O
.	O
4	O
±	O
0	O
.	O
2	O
NB	O
0	O
.	O
2	O
Cellohexaose	O
568	O
.	O
0	O
±	O
291	O
.	O
0	O
NB	O
−	O
3	O
.	O
8	O
NB	O
−	O
16	O
±	O
8	O
NB	O
−	O
12	O
.	O
7	O
NB	O
Cellotetraose	O
NB	O
NB	O
NB	O
NB	O
NB	O
NB	O
NB	O
NB	O

SGBP	B-protein
-	I-protein
A	I-protein
is	O
a	O
SusD	B-protein
homolog	O
with	O
an	O
extensive	O
glycan	B-site
-	I-site
binding	I-site
platform	I-site
.	O

Specifically	O
,	O
SGBP	B-protein
-	I-protein
A	I-protein
overlays	B-experimental_method
B	B-species
.	I-species
thetaiotaomicron	I-species
SusD	B-protein
(	O
BtSusD	B-protein
)	O
with	O
a	O
root	B-evidence
mean	I-evidence
square	I-evidence
deviation	I-evidence
(	O
RMSD	B-evidence
)	O
value	O
of	O
2	O
.	O
2	O
Å	O
for	O
363	O
Cα	O
pairs	O
,	O
which	O
is	O
notable	O
given	O
the	O
26	O
%	O
amino	O
acid	O
identity	O
(	O
40	O
%	O
similarity	O
)	O
between	O
these	O
homologs	O
(	O
Fig	O
.	O
4C	O
).	O

The	O
SGBP	B-complex_assembly
-	I-complex_assembly
A	I-complex_assembly
:	I-complex_assembly
XyGO2	I-complex_assembly
complex	O
superimposes	B-experimental_method
closely	O
with	O
the	O
apo	B-protein_state
structure	B-evidence
(	O
RMSD	B-evidence
of	O
0	O
.	O
6	O
Å	O
)	O
and	O
demonstrates	O
that	O
no	O
major	O
conformational	O
change	O
occurs	O
upon	O
substrate	O
binding	O
;	O
small	O
deviations	O
in	O
the	O
orientation	O
of	O
several	O
surface	O
loops	O
are	O
likely	O
the	O
result	O
of	O
differential	O
crystal	O
packing	O
.	O

It	O
is	O
particularly	O
notable	O
that	O
although	O
the	O
location	O
of	O
the	O
ligand	B-site
-	I-site
binding	I-site
site	I-site
is	O
conserved	B-protein_state
between	O
SGBP	B-protein
-	I-protein
A	I-protein
and	O
SusD	B-protein
,	O
that	O
of	O
SGBP	B-protein
-	I-protein
A	I-protein
displays	O
an	O
~	O
29	O
-	O
Å	O
-	O
long	O
aromatic	B-site
platform	I-site
to	O
accommodate	O
the	O
extended	O
,	O
linear	O
XyG	B-chemical
chain	O
(	O
see	O
reference	O
for	O
a	O
review	O
of	O
XyG	B-chemical
secondary	O
structure	O
),	O
versus	O
the	O
shorter	O
,	O
~	O
18	O
-	O
Å	O
-	O
long	O
,	O
site	B-site
within	O
SusD	B-protein
that	O
complements	O
the	O
helical	O
conformation	O
of	O
amylose	B-chemical
(	O
Fig	O
.	O
4C	O
and	O
D	O
).	O

Molecular	O
structure	B-evidence
of	O
SGBP	B-protein
-	I-protein
A	I-protein
(	O
Bacova_02651	B-gene
).	O
(	O
A	O
)	O
Overlay	B-experimental_method
of	O
SGBP	B-protein
-	I-protein
A	I-protein
from	O
the	O
apo	B-protein_state
(	O
rainbow	O
)	O
and	O
XyGO2	B-chemical
(	O
gray	O
)	O
structures	B-evidence
.	O

An	O
omit	B-evidence
map	I-evidence
(	O
2σ	O
)	O
for	O
XyGO2	B-chemical
(	O
orange	O
and	O
red	O
sticks	O
)	O
is	O
displayed	O
.	O

(	O
B	O
)	O
Close	O
-	O
up	O
view	O
of	O
the	O
omit	B-evidence
map	I-evidence
as	O
in	O
panel	O
A	O
,	O
rotated	O
90	O
°	O
clockwise	O
.	O

(	O
C	O
)	O
Overlay	B-experimental_method
of	O
the	O
Cα	O
backbones	O
of	O
SGBP	B-protein
-	I-protein
A	I-protein
(	O
black	O
)	O
with	O
XyGO2	B-chemical
(	O
orange	O
and	O
red	O
spheres	O
)	O
and	O
BtSusD	B-protein
(	O
blue	O
)	O
with	O
maltoheptaose	B-chemical
(	O
pink	O
and	O
red	O
spheres	O
),	O
highlighting	O
the	O
conservation	O
of	O
the	O
glycan	B-site
-	I-site
binding	I-site
site	I-site
location	O
.	O

(	O
D	O
)	O
Close	O
-	O
up	O
of	O
the	O
SGBP	B-protein
-	I-protein
A	I-protein
(	O
black	O
and	O
orange	O
)	O
and	O
SusD	B-protein
(	O
blue	O
and	O
pink	O
)	O
glycan	B-site
-	I-site
binding	I-site
sites	I-site
.	O

The	O
backbone	O
glucose	B-chemical
residues	O
are	O
numbered	O
from	O
the	O
nonreducing	O
end	O
;	O
xylose	B-chemical
residues	O
are	O
labeled	O
X1	B-residue_name_number
and	O
X2	B-residue_name_number
.	O

Indeed	O
,	O
the	O
electron	B-evidence
density	I-evidence
for	O
the	O
ligand	O
suggests	O
some	O
disorder	O
,	O
which	O
may	O
arise	O
from	O
multiple	O
oligosaccharide	B-chemical
orientations	O
along	O
the	O
binding	B-site
site	I-site
.	O

Three	O
aromatic	O
residues	O
—	O
W82	B-residue_name_number
,	O
W283	B-residue_name_number
,	O
W306	B-residue_name_number
—	O
comprise	O
the	O
flat	B-site
platform	I-site
that	O
stacks	B-bond_interaction
along	O
the	O
naturally	O
twisted	O
β	B-chemical
-	I-chemical
glucan	I-chemical
backbone	O
(	O
Fig	O
.	O
4E	O
).	O

Contrasting	O
with	O
the	O
clear	O
importance	O
of	O
these	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
,	O
there	O
are	O
remarkably	O
few	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonding	I-bond_interaction
interactions	I-bond_interaction
with	O
the	O
ligand	B-chemical
,	O
which	O
are	O
provided	O
by	O
R65	B-residue_name_number
,	O
N83	B-residue_name_number
,	O
and	O
S308	B-residue_name_number
,	O
which	O
are	O
proximal	O
to	O
Glc5	B-residue_name_number
and	O
Glc3	B-residue_name_number
.	O

Most	O
surprising	O
in	O
light	O
of	O
the	O
saccharide	B-evidence
-	I-evidence
binding	I-evidence
data	I-evidence
,	O
however	O
,	O
was	O
a	O
lack	O
of	O
extensive	O
recognition	O
of	O
the	O
XyG	B-chemical
side	O
chains	O
;	O
only	O
Y84	B-residue_name_number
appeared	O
to	O
provide	O
a	O
hydrophobic	B-site
interface	I-site
for	O
a	O
xylosyl	B-chemical
residue	O
(	O
Xyl1	B-residue_name_number
).	O

Summary	O
of	O
thermodynamic	O
parameters	O
for	O
site	O
-	O
directed	O
mutants	O
of	O
SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
obtained	O
by	O
ITC	B-experimental_method
with	O
XyG	B-chemical
at	O
25	O
°	O
Ca	O

Weak	O
binding	O
represents	O
a	O
Ka	B-evidence
of	O
<	O
500	O
M	O
−	O
1	O
.	O

Ka	B-evidence
fold	O
change	O
=	O
Ka	B-evidence
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
protein	O
/	O
Ka	B-evidence
of	O
mutant	O
protein	O
for	O
xyloglucan	B-chemical
binding	O
.	O

SGBP	B-protein
-	I-protein
B	I-protein
has	O
a	O
multimodular	O
structure	O
with	O
a	O
single	O
,	O
C	O
-	O
terminal	O
glycan	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SGBP	B-protein
-	I-protein
B	I-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
XyGO2	B-chemical
(	O
2	O
.	O
37	O
Å	O
,	O
Rwork	B-evidence
=	O
19	O
.	O
9	O
%,	O
Rfree	B-evidence
=	O
23	O
.	O
9	O
%,	O
residues	O
34	B-residue_range
to	I-residue_range
489	I-residue_range
)	O
(	O
Table	O
2	O
)	O
revealed	O
an	O
extended	O
structure	B-evidence
composed	O
of	O
three	O
tandem	B-structure_element
immunoglobulin	I-structure_element
(	I-structure_element
Ig	I-structure_element
)-	I-structure_element
like	I-structure_element
domains	I-structure_element
(	O
domains	O
A	B-structure_element
,	O
B	B-structure_element
,	O
and	O
C	B-structure_element
)	O
followed	O
at	O
the	O
C	O
terminus	O
by	O
a	O
novel	O
xyloglucan	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
(	O
domain	O
D	B-structure_element
)	O
(	O
Fig	O
.	O
5A	O
).	O

These	B-structure_element
domains	I-structure_element
also	O
display	O
similarity	O
to	O
the	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sandwich	I-structure_element
domains	I-structure_element
of	O
many	O
GH13	B-protein_type
enzymes	I-protein_type
,	O
including	O
the	O
cyclodextrin	B-protein_type
glucanotransferase	I-protein_type
of	O
Geobacillus	B-species
stearothermophilus	I-species
(	O
Fig	O
.	O
5B	O
).	O

Such	B-structure_element
domains	I-structure_element
are	O
not	O
typically	O
involved	O
in	O
carbohydrate	B-chemical
binding	O
.	O

Indeed	O
,	O
visual	B-experimental_method
inspection	I-experimental_method
of	O
the	O
SGBP	B-protein
-	I-protein
B	I-protein
structure	B-evidence
,	O
as	O
well	O
as	O
individual	O
production	O
of	O
the	O
A	B-structure_element
and	O
B	B-structure_element
domains	O
and	O
affinity	B-experimental_method
PAGE	I-experimental_method
analysis	O
(	O
see	O
Fig	O
.	O
S5	O
in	O
the	O
supplemental	O
material	O
),	O
indicates	O
that	O
these	O
domains	O
do	O
not	O
contribute	O
to	O
XyG	B-chemical
capture	O
.	O

On	O
the	O
other	O
hand	O
,	O
production	B-experimental_method
of	O
the	O
fused	B-mutant
domains	I-mutant
C	I-mutant
and	I-mutant
D	I-mutant
in	O
tandem	O
(	O
SGBP	B-protein
-	I-protein
B	I-protein
residues	O
230	B-residue_range
to	I-residue_range
489	I-residue_range
)	O
retains	O
complete	O
binding	O
of	O
xyloglucan	B-chemical
in	O
vitro	O
,	O
with	O
the	O
observed	O
slight	O
increase	O
in	O
affinity	O
likely	O
arising	O
from	O
a	O
reduced	O
potential	O
for	O
steric	O
hindrance	O
of	O
the	O
smaller	O
protein	O
construct	O
during	O
polysaccharide	B-chemical
interactions	O
(	O
Table	O
3	O
).	O

While	O
neither	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
protein	O
nor	O
domain	O
D	B-structure_element
displays	O
structural	O
homology	O
to	O
known	O
XyG	B-protein_type
-	I-protein_type
binding	I-protein_type
proteins	I-protein_type
,	O
the	O
topology	O
of	O
SGBP	B-protein
-	I-protein
B	I-protein
resembles	O
the	O
xylan	B-protein_type
-	I-protein_type
binding	I-protein_type
protein	I-protein_type
Bacova_04391	B-protein
(	O
PDB	O
3ORJ	O
)	O
encoded	O
within	O
a	O
xylan	B-chemical
-	O
targeting	O
PUL	B-gene
of	O
B	B-species
.	I-species
ovatus	I-species
(	O
Fig	O
.	O
5C	O
).	O

The	O
structure	B-experimental_method
-	I-experimental_method
based	I-experimental_method
alignment	I-experimental_method
of	O
these	O
proteins	O
reveals	O
17	O
%	O
sequence	O
identity	O
,	O
with	O
a	O
core	O
RMSD	B-evidence
of	O
3	O
.	O
6	O
Å	O
for	O
253	O
aligned	O
residues	O
.	O

Multimodular	O
structure	O
of	O
SGBP	B-protein
-	I-protein
B	I-protein
(	O
Bacova_02650	B-gene
).	O
(	O
A	O
)	O
Full	B-protein_state
-	I-protein_state
length	I-protein_state
structure	B-evidence
of	O
SGBP	B-protein
-	I-protein
B	I-protein
,	O
color	O
coded	O
by	O
domain	O
as	O
indicated	O
.	O

An	O
omit	B-evidence
map	I-evidence
(	O
2σ	O
)	O
for	O
XyGO2	B-chemical
is	O
displayed	O
to	O
highlight	O
the	O
location	O
of	O
the	O
glycan	B-site
-	I-site
binding	I-site
site	I-site
.	O

(	O
B	O
)	O
Overlay	O
of	O
SGBP	B-protein
-	I-protein
B	I-protein
domains	O
A	B-structure_element
,	O
B	B-structure_element
,	O
and	O
C	B-structure_element
(	O
colored	O
as	O
in	O
panel	O
A	O
),	O
with	O
a	O
C	O
-	O
terminal	O
Ig	B-structure_element
-	I-structure_element
like	I-structure_element
domain	I-structure_element
of	O
the	O
G	B-species
.	I-species
stearothermophilus	I-species
cyclodextrin	B-protein_type
glucanotransferase	I-protein_type
(	O
PDB	O
1CYG	O
[	O
residues	O
375	B-residue_range
to	I-residue_range
493	I-residue_range
])	O
in	O
green	O
.	O
(	O
C	O
)	O
Cα	O
overlay	B-experimental_method
of	O
SGBP	B-protein
-	I-protein
B	I-protein
(	O
gray	O
)	O
and	O
Bacova_04391	B-protein
(	O
PDB	O
3ORJ	O
)	O
(	O
pink	O
).	O

(	O
D	O
)	O
Close	O
-	O
up	O
omit	B-evidence
map	I-evidence
for	O
the	O
XyGO2	B-chemical
ligand	O
,	O
contoured	O
at	O
2σ	O
.	O
(	O
E	O
)	O
Stereo	O
view	O
of	O
the	O
xyloglucan	B-site
-	I-site
binding	I-site
site	I-site
of	O
SGBP	B-protein
-	I-protein
B	I-protein
,	O
displaying	O
all	O
residues	O
within	O
4	O
Å	O
of	O
the	O
ligand	O
.	O

The	O
backbone	O
glucose	B-chemical
residues	O
are	O
numbered	O
from	O
the	O
nonreducing	O
end	O
,	O
xylose	B-chemical
residues	O
are	O
shown	O
as	O
X1	B-residue_name_number
,	O
X2	B-residue_name_number
,	O
and	O
X3	B-residue_name_number
,	O
potential	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonding	I-bond_interaction
interactions	I-bond_interaction
are	O
shown	O
as	O
dashed	O
lines	O
,	O
and	O
the	O
distance	O
is	O
shown	O
in	O
angstroms	O
.	O

Domains	O
A	B-structure_element
,	O
B	B-structure_element
,	O
and	O
C	B-structure_element
do	O
not	O
pack	O
against	O
each	O
other	O
.	O

Moreover	O
,	O
the	O
five	B-structure_element
-	I-structure_element
residue	I-structure_element
linkers	I-structure_element
between	O
these	O
first	O
three	O
domains	O
all	O
feature	O
a	O
proline	B-residue_name
as	O
the	O
middle	B-structure_element
residue	I-structure_element
,	O
suggesting	O
significant	O
conformational	O
rigidity	O
(	O
Fig	O
.	O
5A	O
).	O

Any	O
mobility	O
of	O
SGBP	B-protein
-	I-protein
B	I-protein
on	O
the	O
surface	O
of	O
the	O
cell	O
(	O
beyond	O
lateral	O
diffusion	O
within	O
the	O
membrane	O
)	O
is	O
likely	O
imparted	O
by	O
the	O
eight	B-structure_element
-	I-structure_element
residue	I-structure_element
linker	I-structure_element
that	O
spans	O
the	O
predicted	O
lipidated	B-protein_state
Cys	B-residue_name
(	O
C28	B-residue_name_number
)	O
and	O
the	O
first	B-structure_element
β	I-structure_element
-	I-structure_element
strand	I-structure_element
of	O
domain	O
A	B-structure_element
.	O
Other	O
outer	B-protein_type
membrane	I-protein_type
proteins	I-protein_type
from	O
various	O
Sus	B-complex_assembly
-	I-complex_assembly
like	I-complex_assembly
systems	I-complex_assembly
possess	O
a	O
similar	O
10	B-structure_element
-	I-structure_element
to	I-structure_element
20	I-structure_element
-	I-structure_element
amino	I-structure_element
-	I-structure_element
acid	I-structure_element
flexible	I-structure_element
linker	I-structure_element
between	O
the	O
lipidated	B-protein_state
Cys	B-residue_name
that	O
tethers	O
the	O
protein	O
to	O
the	O
outside	O
the	O
cell	O
and	O
the	O
first	O
secondary	O
structure	O
element	O
.	O

Analogously	O
,	O
the	O
outer	B-protein_state
membrane	I-protein_state
-	I-protein_state
anchored	I-protein_state
endo	B-protein_type
-	I-protein_type
xyloglucanase	I-protein_type
BoGH5	B-protein
of	O
the	O
XyGUL	B-gene
contains	O
a	O
100	B-structure_element
-	I-structure_element
amino	I-structure_element
-	I-structure_element
acid	I-structure_element
,	I-structure_element
all	I-structure_element
-	I-structure_element
β	I-structure_element
-	I-structure_element
strand	I-structure_element
,	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
module	I-structure_element
and	O
flexible	B-structure_element
linker	I-structure_element
that	O
imparts	O
conformational	O
flexibility	O
and	O
distances	O
the	O
catalytic	B-structure_element
module	I-structure_element
from	O
the	O
cell	O
surface	O
.	O

XyG	B-chemical
binds	B-protein_state
to	I-protein_state
domain	O
D	B-structure_element
of	O
SGBP	B-protein
-	I-protein
B	I-protein
at	O
the	O
concave	B-site
interface	I-site
of	O
the	O
top	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
,	O
with	O
binding	O
mediated	O
by	O
loops	B-structure_element
connecting	O
the	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
.	O

Six	O
glucosyl	B-chemical
residues	O
,	O
comprising	O
the	O
main	O
chain	O
,	O
and	O
three	O
branching	O
xylosyl	B-chemical
residues	O
of	O
XyGO2	B-chemical
can	O
be	O
modeled	O
in	O
the	O
density	B-evidence
(	O
Fig	O
.	O
5D	O
;	O
cf	O
.	O

The	O
aromatic	B-site
platform	I-site
created	O
by	O
W330	B-residue_name_number
,	O
W364	B-residue_name_number
,	O
and	O
Y363	B-residue_name_number
spans	O
four	O
glucosyl	B-chemical
residues	O
,	O
compared	O
to	O
the	O
longer	B-protein_state
platform	B-site
of	O
SGBP	B-protein
-	I-protein
A	I-protein
,	O
which	O
supports	O
six	O
glucosyl	B-chemical
residues	O
(	O
Fig	O
.	O
5E	O
).	O

The	O
Y363A	B-mutant
site	B-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutant	I-experimental_method
of	O
SGBP	B-protein
-	I-protein
B	I-protein
displays	O
a	O
20	O
-	O
fold	O
decrease	O
in	O
the	O
Ka	B-evidence
for	O
XyG	B-chemical
,	O
while	O
the	O
W364A	B-mutant
mutant	B-protein_state
lacks	B-protein_state
XyG	I-protein_state
binding	I-protein_state
(	O
Table	O
3	O
;	O
see	O
Fig	O
.	O
S6	O
in	O
the	O
supplemental	O
material	O
).	O

There	O
are	O
no	O
additional	O
contacts	O
between	O
the	O
protein	O
and	O
the	O
β	B-chemical
-	I-chemical
glucan	I-chemical
backbone	O
and	O
surprisingly	O
few	O
interactions	O
with	O
the	O
side	O
-	O
chain	O
xylosyl	B-chemical
residues	O
,	O
despite	O
that	O
fact	O
that	O
ITC	B-experimental_method
data	O
demonstrate	O
that	O
SGBP	B-protein
-	I-protein
B	I-protein
does	O
not	O
measurably	O
bind	O
the	O
cellohexaose	B-chemical
(	O
Table	O
1	O
).	O

F414	B-residue_name_number
stacks	B-bond_interaction
with	O
the	O
xylosyl	B-chemical
residue	O
of	O
Glc3	B-residue_name_number
,	O
while	O
Q407	B-residue_name_number
is	O
positioned	O
for	O
hydrogen	B-bond_interaction
bonding	I-bond_interaction
with	O
the	O
O4	O
of	O
xylosyl	B-chemical
residue	O
Xyl1	B-residue_name_number
.	O

Surprisingly	O
,	O
an	O
F414A	B-mutant
mutant	B-protein_state
of	O
SGBP	B-protein
-	I-protein
B	I-protein
displays	O
only	O
a	O
mild	O
3	O
-	O
fold	O
decrease	O
in	O
the	O
Ka	B-evidence
value	O
for	O
XyG	B-chemical
,	O
again	O
suggesting	O
that	O
glycan	B-chemical
recognition	O
is	O
primarily	O
mediated	O
via	O
contact	O
with	O
the	O
β	O
-	O
glucan	O
backbone	O
(	O
Table	O
3	O
;	O
see	O
Fig	O
.	O
S6	O
).	O

Additional	O
residues	B-structure_element
surrounding	O
the	O
binding	B-site
site	I-site
,	O
including	O
Y369	B-residue_name_number
and	O
E412	B-residue_name_number
,	O
may	O
contribute	O
to	O
the	O
recognition	O
of	O
more	O
highly	O
decorated	O
XyG	B-chemical
,	O
but	O
precisely	O
how	O
this	O
is	O
mediated	O
is	O
presently	O
unclear	O
.	O

The	O
CD	B-structure_element
domains	I-structure_element
of	O
the	O
truncated	B-protein_state
and	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
proteins	O
superimpose	B-experimental_method
with	O
a	O
0	O
.	O
4	O
-	O
Å	O
RMSD	B-evidence
of	O
the	O
Cα	O
backbone	O
,	O
with	O
no	O
differences	O
in	O
the	O
position	O
of	O
any	O
of	O
the	O
glycan	B-site
-	I-site
binding	I-site
residues	I-site
(	O
see	O
Fig	O
.	O
S7A	O
in	O
the	O
supplemental	O
material	O
).	O

While	O
density	B-evidence
is	O
observed	O
for	O
XyGO2	B-chemical
,	O
the	O
ligand	O
could	O
not	O
be	O
unambiguously	O
modeled	O
into	O
this	O
density	B-evidence
to	O
achieve	O
a	O
reasonable	O
fit	O
between	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
data	I-evidence
and	O
the	O
known	O
stereochemistry	O
of	O
the	O
sugar	O
(	O
see	O
Fig	O
.	O
S7B	O
and	O
C	O
).	O

SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
have	O
distinct	O
,	O
coordinated	O
functions	O
in	O
vivo	O
.	O

To	O
disentangle	O
the	O
functions	O
of	O
SGBP	B-protein
-	I-protein
A	I-protein
and	O
SGBP	B-protein
-	I-protein
B	I-protein
in	O
XyG	B-chemical
recognition	O
and	O
uptake	O
,	O
we	O
created	O
individual	O
in	B-experimental_method
-	I-experimental_method
frame	I-experimental_method
deletion	I-experimental_method
and	I-experimental_method
complementation	I-experimental_method
mutant	I-experimental_method
strains	O
of	O
B	B-species
.	I-species
ovatus	I-species
.	O

Growth	O
on	O
glucose	B-chemical
displayed	O
the	O
shortest	O
lag	B-evidence
time	I-evidence
for	O
each	O
strain	O
,	O
and	O
so	O
lag	B-evidence
times	I-evidence
were	O
normalized	O
for	O
each	O
carbohydrate	B-chemical
by	O
subtracting	O
the	O
lag	B-evidence
time	I-evidence
of	O
that	O
strain	O
in	O
glucose	B-chemical
(	O
Fig	O
.	O
6	O
;	O
see	O
Fig	O
.	O
S8	O
in	O
the	O
supplemental	O
material	O
).	O

A	O
strain	O
in	O
which	O
the	O
entire	O
XyGUL	B-gene
is	O
deleted	B-experimental_method
displays	O
a	O
lag	B-evidence
of	O
24	O
.	O
5	O
h	O
during	O
growth	O
on	O
glucose	B-chemical
compared	O
to	O
the	O
isogenic	O
parental	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
WT	B-protein_state
)	O
Δtdk	B-mutant
strain	O
,	O
for	O
which	O
exponential	O
growth	O
lags	B-evidence
for	O
19	O
.	O
8	O
h	O
(	O
see	O
Fig	O
.	O
S8D	O
).	O

It	O
is	O
unknown	O
whether	O
this	O
is	O
because	O
cultures	O
were	O
not	O
normalized	O
by	O
the	O
starting	O
optical	O
density	O
(	O
OD	O
)	O
or	O
viable	O
cells	O
or	O
reflects	O
a	O
minor	O
defect	O
for	O
glucose	B-chemical
utilization	O
.	O

The	O
former	O
seems	O
more	O
likely	O
as	O
the	O
growth	O
rates	O
are	O
nearly	O
identical	O
for	O
these	O
strains	O
on	O
glucose	B-chemical
and	O
xylose	B-chemical
.	O

The	O
ΔXyGUL	B-mutant
and	O
WT	B-protein_state
Δtdk	B-mutant
strains	O
display	O
normalized	O
lag	B-evidence
times	I-evidence
on	O
xylose	B-chemical
within	O
experimental	O
error	O
,	O
and	O
curiously	O
some	O
of	O
the	O
mutant	O
and	O
complemented	O
strains	O
display	O
a	O
nominally	O
shorter	O
lag	B-evidence
time	I-evidence
on	O
xylose	B-chemical
than	O
the	O
WT	B-protein_state
Δtdk	B-mutant
strain	O
.	O

The	O
reason	O
for	O
this	O
observation	O
on	O
XyGO2	B-chemical
is	O
unclear	O
,	O
as	O
the	O
ΔSGBP	B-mutant
-	I-mutant
B	I-mutant
mutant	B-protein_state
does	O
not	O
have	O
a	O
significantly	O
different	O
growth	O
rate	O
from	O
the	O
WT	B-protein_state
on	O
XyGO2	B-chemical
.	O

Growth	O
of	O
select	O
XyGUL	B-gene
mutants	O
on	O
xyloglucan	B-chemical
and	O
oligosaccharides	B-chemical
.	O

B	B-species
.	I-species
ovatus	I-species
mutants	O
were	O
created	O
in	O
a	O
thymidine	B-mutant
kinase	I-mutant
deletion	I-mutant
(	O
Δtdk	B-mutant
)	O
mutant	O
as	O
described	O
previously	O
.	O

SGBP	B-mutant
-	I-mutant
A	I-mutant
*	I-mutant
denotes	O
the	O
Bacova_02651	B-gene
(	O
W82A	B-mutant
W283A	B-mutant
W306A	B-mutant
)	O
allele	O
,	O
and	O
the	O
GH9	B-protein
gene	O
is	O
Bacova_02649	B-gene
.	O

Solid	O
bars	O
indicate	O
conditions	O
that	O
are	O
not	O
statistically	O
significant	O
from	O
the	O
WT	B-protein_state
Δtdk	B-mutant
cultures	O
grown	O
on	O
the	O
indicated	O
carbohydrate	B-chemical
,	O
while	O
open	O
bars	O
indicate	O
a	O
P	O
value	O
of	O
<	O
0	O
.	O
005	O
compared	O
to	O
the	O
WT	B-protein_state
Δtdk	B-mutant
strain	O
.	O

Conditions	O
denoted	O
by	O
the	O
same	O
letter	O
(	O
b	O
,	O
c	O
,	O
or	O
d	O
)	O
are	O
not	O
statistically	O
significant	O
from	O
each	O
other	O
but	O
are	O
significantly	O
different	O
from	O
the	O
condition	O
labeled	O
“	O
a	O
.”	O
Complementation	O
of	O
ΔSGBP	B-mutant
-	I-mutant
A	I-mutant
and	O
ΔSBGP	B-mutant
-	I-mutant
B	I-mutant
was	O
performed	O
by	O
allelic	O
exchange	O
of	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
genes	O
back	O
into	O
the	O
genome	O
for	O
expression	O
via	O
the	O
native	O
promoter	O
:	O
these	O
growth	O
curves	O
,	O
quantified	O
rates	O
and	O
lag	B-evidence
times	I-evidence
are	O
displayed	O
in	O
Fig	O
.	O
S8	O
in	O
the	O
supplemental	O
material	O
.	O

Fig	O
.	O
1B	O
)	O
was	O
completely	O
incapable	O
of	O
growth	O
on	O
XyG	B-chemical
,	O
XyGO1	B-chemical
,	O
and	O
XyGO2	B-chemical
,	O
indicating	O
that	O
SGBP	B-protein
-	I-protein
A	I-protein
is	O
essential	O
for	O
XyG	B-chemical
utilization	O
(	O
Fig	O
.	O
6	O
).	O

This	O
result	O
mirrors	O
our	O
previous	O
data	O
for	O
the	O
canonical	O
Sus	B-complex_assembly
of	O
B	B-species
.	I-species
thetaiotaomicron	I-species
,	O
which	O
revealed	O
that	O
a	O
homologous	O
ΔsusD	B-mutant
mutant	B-protein_state
is	O
unable	O
to	O
grow	O
on	O
starch	B-chemical
or	O
malto	B-chemical
-	I-chemical
oligosaccharides	I-chemical
,	O
despite	O
normal	O
cell	O
surface	O
expression	O
of	O
all	O
other	O
PUL	B-gene
-	O
encoded	O
proteins	O
.	O

More	O
recently	O
,	O
we	O
demonstrated	O
that	O
this	O
phenotype	O
is	O
due	O
to	O
the	O
loss	O
of	O
the	O
physical	O
presence	O
of	O
SusD	B-protein
;	O
complementation	B-experimental_method
of	O
ΔsusD	B-mutant
with	O
SusD	B-mutant
*,	I-mutant
a	O
triple	B-protein_state
site	I-protein_state
-	I-protein_state
directed	I-protein_state
mutant	I-protein_state
(	O
W96A	B-mutant
W320A	B-mutant
Y296A	B-mutant
)	O
that	O
ablates	B-protein_state
glycan	I-protein_state
binding	I-protein_state
,	O
restores	O
B	B-species
.	I-species
thetaiotaomicron	I-species
growth	O
on	O
malto	B-chemical
-	I-chemical
oligosaccharides	I-chemical
and	O
starch	B-chemical
when	O
sus	B-gene
transcription	O
is	O
induced	O
by	O
maltose	B-chemical
addition	O
.	O

Similarly	O
,	O
the	O
function	O
of	O
SGBP	B-protein
-	I-protein
A	I-protein
extends	O
beyond	O
glycan	B-chemical
binding	O
.	O

Complementation	B-experimental_method
of	O
ΔSGBP	B-mutant
-	I-mutant
A	I-mutant
with	O
the	O
SGBP	B-mutant
-	I-mutant
A	I-mutant
*	I-mutant
(	O
W82A	B-mutant
W283A	B-mutant
W306A	B-mutant
)	O
variant	O
,	O
which	O
does	O
not	B-protein_state
bind	I-protein_state
XyG	B-chemical
,	O
supports	O
growth	O
on	O
XyG	B-chemical
and	O
XyGOs	B-chemical
(	O
Fig	O
.	O
6	O
;	O
ΔSGBP	B-mutant
-	I-mutant
A	I-mutant
::	O
SGBP	B-mutant
-	I-mutant
A	I-mutant
*),	I-mutant
with	O
growth	O
rates	O
that	O
are	O
~	O
70	O
%	O
that	O
of	O
the	O
WT	B-protein_state
.	O

In	O
previous	O
studies	O
,	O
we	O
observed	O
that	O
carbohydrate	B-chemical
binding	O
by	O
SusD	B-protein
enhanced	O
the	O
sensitivity	O
of	O
the	O
cells	O
to	O
limiting	O
concentrations	O
of	O
malto	O
-	O
oligosaccharides	O
by	O
several	O
orders	O
of	O
magnitude	O
,	O
such	O
that	O
the	O
addition	O
of	O
0	O
.	O
5	O
g	O
/	O
liter	O
maltose	B-chemical
was	O
required	O
to	O
restore	O
growth	O
of	O
the	O
ΔsusD	B-mutant
::	O
SusD	B-mutant
*	I-mutant
strain	O
on	O
starch	B-chemical
,	O
which	O
nonetheless	O
occurred	O
following	O
an	O
extended	O
lag	B-evidence
phase	I-evidence
.	O

In	O
contrast	O
,	O
the	O
ΔSGBP	B-mutant
-	I-mutant
A	I-mutant
::	O
SGBP	B-mutant
-	I-mutant
A	I-mutant
*	I-mutant
strain	O
does	O
not	O
display	O
an	O
extended	O
lag	B-evidence
time	I-evidence
on	O
any	O
of	O
the	O
xyloglucan	B-chemical
substrates	O
compared	O
to	O
the	O
WT	B-protein_state
(	O
Fig	O
.	O
6	O
).	O

However	O
,	O
the	O
modest	O
rate	O
defect	O
displayed	O
by	O
the	O
SGBP	B-protein
-	I-protein
A	I-protein
::	O
SGBP	B-mutant
-	I-mutant
A	I-mutant
*	I-mutant
strain	O
suggests	O
that	O
recognition	O
of	O
XyG	B-chemical
and	O
product	O
import	O
is	O
somewhat	O
less	O
efficient	O
in	O
these	O
cells	O
.	O

10	O
-	O
fold	O
more	O
weakly	O
than	O
XyGO2	B-chemical
and	O
XyG	B-chemical
(	O
Fig	O
.	O
6	O
;	O
Table	O
1	O
).	O

As	O
such	O
,	O
the	O
data	O
suggest	O
that	O
SGBP	B-protein
-	I-protein
A	I-protein
can	O
compensate	O
for	O
the	O
loss	O
of	O
function	O
of	O
SGBP	B-protein
-	I-protein
B	I-protein
on	O
longer	O
oligo	B-chemical
-	I-chemical
and	I-chemical
polysaccharides	I-chemical
,	O
while	O
SGBP	B-protein
-	I-protein
B	I-protein
may	O
adapt	O
the	O
cell	O
to	O
recognize	O
smaller	O
oligosaccharides	B-chemical
efficiently	O
.	O

Indeed	O
,	O
a	O
double	B-protein_state
mutant	I-protein_state
,	O
consisting	O
of	O
a	O
crippled	B-protein_state
SGBP	B-protein
-	I-protein
A	I-protein
and	O
a	O
deletion	B-experimental_method
of	I-experimental_method
SGBP	B-protein
-	I-protein
B	I-protein
(	O
ΔSGBP	B-mutant
-	I-mutant
A	I-mutant
::	O
SGBP	B-mutant
-	I-mutant
A	I-mutant
*/	I-mutant
ΔSGBP	B-mutant
-	I-mutant
B	I-mutant
),	O
exhibits	O
an	O
extended	O
lag	B-evidence
time	I-evidence
on	O
both	O
XyG	B-chemical
and	O
XyGO2	B-chemical
,	O
as	O
well	O
as	O
XyGO1	B-chemical
.	O

This	O
additional	O
role	O
of	O
SGBP	B-protein
-	I-protein
B	I-protein
is	O
especially	O
notable	O
in	O
the	O
context	O
of	O
studies	O
on	O
BtSusE	B-protein
and	O
BtSusF	B-protein
(	O
positioned	O
similarly	O
in	O
the	O
archetypal	O
Sus	B-gene
locus	I-gene
)	O
(	O
Fig	O
.	O
1B	O
),	O
for	O
which	O
growth	O
defects	O
on	O
starch	B-chemical
or	O
malto	B-chemical
-	I-chemical
oligosaccharides	I-chemical
have	O
never	O
been	O
observed	O
.	O

However	O
,	O
combined	B-experimental_method
deletion	I-experimental_method
of	I-experimental_method
the	I-experimental_method
genes	I-experimental_method
encoding	I-experimental_method
GH9	B-protein
(	O
encoded	O
by	O
Bacova_02649	B-gene
)	O
and	O
SGBP	B-protein
-	I-protein
B	I-protein
does	O
not	O
exacerbate	O
the	O
growth	O
defect	O
on	O
XyGO1	B-chemical
(	O
Fig	O
.	O
6	O
;	O
ΔSGBP	B-mutant
-	I-mutant
B	I-mutant
/	O
ΔGH9	B-mutant
).	O

The	O
necessity	O
of	O
SGBP	B-protein
-	I-protein
B	I-protein
is	O
elevated	O
in	O
the	O
SGBP	B-mutant
-	I-mutant
A	I-mutant
*	I-mutant
strain	O
,	O
as	O
the	O
ΔSGBP	B-mutant
-	I-mutant
A	I-mutant
::	O
SGBP	B-mutant
-	I-mutant
A	I-mutant
*/	I-mutant
ΔSGBP	B-mutant
-	I-mutant
B	I-mutant
mutant	B-protein_state
displays	O
an	O
extended	O
lag	B-evidence
during	O
growth	O
on	O
XyG	B-chemical
and	O
xylogluco	B-chemical
-	I-chemical
oligosaccharides	I-chemical
,	O
while	O
growth	O
rate	O
differences	O
are	O
more	O
subtle	O
.	O

The	O
precise	O
reason	O
for	O
this	O
lag	B-evidence
is	O
unclear	O
,	O
but	O
recapitulating	O
our	O
findings	O
on	O
the	O
role	O
of	O
SusD	B-protein
in	O
malto	B-chemical
-	I-chemical
oligosaccharide	I-chemical
sensing	O
in	O
B	B-species
.	I-species
thetaiotaomicron	I-species
,	O
this	O
extended	O
lag	B-evidence
may	O
be	O
due	O
to	O
inefficient	O
import	O
and	O
thus	O
sensing	O
of	O
xyloglucan	B-chemical
in	O
the	O
environment	O
in	O
the	O
absence	O
of	O
glycan	B-chemical
binding	O
by	O
essential	O
SGBPs	B-protein_type
.	O

Our	O
previous	O
work	O
demonstrates	O
that	O
B	B-species
.	I-species
ovatus	I-species
cells	O
grown	O
in	O
minimal	O
medium	O
plus	O
glucose	B-chemical
express	O
low	O
levels	O
of	O
the	O
XyGUL	B-gene
transcript	O
.	O

Thus	O
,	O
in	O
our	O
experiments	O
,	O
we	O
presume	O
that	O
each	O
strain	O
,	O
initially	O
grown	O
in	O
glucose	B-chemical
,	O
expresses	O
low	O
levels	O
of	O
the	O
XyGUL	B-gene
transcript	O
and	O
thus	O
low	O
levels	O
of	O
the	O
XyGUL	B-gene
-	O
encoded	O
surface	O
proteins	O
,	O
including	O
the	O
vanguard	O
GH5	B-protein
.	O

Presumably	O
without	O
glycan	B-chemical
binding	O
by	O
the	O
SGBPs	B-protein_type
,	O
the	O
GH5	B-protein
protein	O
cannot	O
efficiently	O
process	O
xyloglucan	B-chemical
,	O
and	O
/	O
or	O
the	O
lack	O
of	O
SGBP	B-protein_type
function	O
prevents	O
efficient	O
capture	O
and	O
import	O
of	O
the	O
processed	O
oligosaccharides	B-chemical
.	O

In	O
the	O
BtSus	B-gene
,	O
SusD	B-protein
and	O
the	O
TBDT	B-protein_type
SusC	B-protein
interact	O
,	O
and	O
we	O
speculate	O
that	O
this	O
interaction	O
is	O
necessary	O
for	O
glycan	B-chemical
uptake	O
,	O
as	O
suggested	O
by	O
the	O
fact	O
that	O
a	O
ΔsusD	B-mutant
mutant	B-protein_state
cannot	O
grow	O
on	O
starch	B-chemical
,	O
but	O
a	O
ΔsusD	B-mutant
::	O
SusD	B-mutant
*	I-mutant
strain	O
regains	O
this	O
ability	O
if	O
a	O
transcriptional	B-protein_type
activator	I-protein_type
of	O
the	O
sus	B-gene
operon	I-gene
is	O
supplied	O
.	O

However	O
,	O
unlike	O
the	O
Sus	B-complex_assembly
,	O
in	O
which	O
elimination	B-experimental_method
of	I-experimental_method
SusE	B-protein
and	O
SusF	B-protein
does	O
not	O
affect	O
growth	O
on	O
starch	B-chemical
,	O
SGBP	B-protein
-	I-protein
B	I-protein
appears	O
to	O
have	O
a	O
dedicated	O
role	O
in	O
growth	O
on	O
small	O
xylogluco	B-chemical
-	I-chemical
oligosaccharides	I-chemical
.	O

The	O
ability	O
of	O
gut	O
-	O
adapted	O
microorganisms	B-taxonomy_domain
to	O
thrive	O
in	O
the	O
gastrointestinal	O
tract	O
is	O
critically	O
dependent	O
upon	O
their	O
ability	O
to	O
efficiently	O
recognize	O
,	O
cleave	O
,	O
and	O
import	O
glycans	B-chemical
.	O

The	O
human	B-species
gut	O
,	O
in	O
particular	O
,	O
is	O
a	O
densely	O
packed	O
ecosystem	O
with	O
hundreds	O
of	O
species	O
,	O
in	O
which	O
there	O
is	O
potential	O
for	O
both	O
competition	O
and	O
synergy	O
in	O
the	O
utilization	O
of	O
different	O
substrates	O
.	O

Recent	O
work	O
has	O
elucidated	O
that	O
Bacteroidetes	B-taxonomy_domain
cross	O
-	O
feed	O
during	O
growth	O
on	O
many	O
glycans	B-chemical
;	O
the	O
glycoside	B-protein_type
hydrolases	I-protein_type
expressed	O
by	O
one	O
species	O
liberate	O
oligosaccharides	B-chemical
for	O
consumption	O
by	O
other	O
members	O
of	O
the	O
community	O
.	O

Here	O
,	O
we	O
demonstrate	O
that	O
the	O
surface	B-protein_type
glycan	I-protein_type
binding	I-protein_type
proteins	I-protein_type
encoded	O
within	O
the	O
BoXyGUL	B-gene
play	O
unique	O
and	O
essential	O
roles	O
in	O
the	O
acquisition	O
of	O
the	O
ubiquitous	O
and	O
abundant	O
vegetable	B-taxonomy_domain
polysaccharide	B-chemical
xyloglucan	B-chemical
.	O

Yet	O
,	O
a	O
number	O
of	O
questions	O
remain	O
regarding	O
the	O
molecular	O
interplay	O
of	O
SGBPs	B-protein_type
with	O
their	O
cotranscribed	O
cohort	O
of	O
glycoside	B-protein_type
hydrolases	I-protein_type
and	O
TonB	B-protein_type
-	I-protein_type
dependent	I-protein_type
transporters	I-protein_type
.	O

A	O
direct	O
interaction	O
between	O
the	O
BtSusC	B-protein
TBDT	B-protein_type
and	O
the	O
SusD	B-protein
SGBP	B-protein_type
has	O
been	O
previously	O
demonstrated	O
,	O
as	O
has	O
an	O
interaction	O
between	O
the	O
homologous	O
components	O
encoded	O
by	O
an	O
N	O
-	O
glycan	B-chemical
-	O
scavenging	O
PUL	B-gene
of	O
Capnocytophaga	B-species
canimorsus	I-species
.	O

It	O
is	O
yet	O
presently	O
unclear	O
whether	O
this	O
interaction	O
is	O
static	O
or	O
dynamic	O
and	O
to	O
what	O
extent	O
the	O
association	O
of	O
cognate	O
TBDT	B-protein_type
/	O
SGBPs	B-protein_type
is	O
dependent	O
upon	O
the	O
structure	O
of	O
the	O
carbohydrate	B-chemical
to	O
be	O
imported	O
.	O

On	O
the	O
other	O
hand	O
,	O
there	O
is	O
clear	O
evidence	O
for	O
independent	O
TBDTs	B-protein_type
in	O
Bacteroidetes	B-taxonomy_domain
that	O
do	O
not	O
require	O
SGBP	B-protein_type
association	O
for	O
activity	O
.	O

For	O
example	O
,	O
it	O
was	O
recently	O
demonstrated	O
that	O
expression	O
of	O
nanO	B-gene
,	O
which	O
encodes	O
a	O
SusC	B-protein_type
-	I-protein_type
like	I-protein_type
TBDT	I-protein_type
as	O
part	O
of	O
a	O
sialic	O
-	O
acid	O
-	O
targeting	O
PUL	B-gene
from	O
B	B-species
.	I-species
fragilis	I-species
,	O
restored	O
growth	O
on	O
this	O
monosaccharide	B-chemical
in	O
a	O
mutant	O
strain	O
of	O
E	B-species
.	I-species
coli	I-species
.	O

In	O
this	O
instance	O
,	O
coexpression	O
of	O
the	O
susD	B-gene
-	O
like	O
gene	O
nanU	B-gene
was	O
not	O
required	O
,	O
nor	O
did	O
the	O
expression	O
of	O
the	O
nanU	B-gene
gene	O
enhance	O
growth	O
kinetics	O
.	O

Thus	O
,	O
the	O
strict	O
dependence	O
on	O
a	O
SusD	B-protein_type
-	I-protein_type
like	I-protein_type
SGBP	I-protein_type
for	O
glycan	B-chemical
uptake	O
in	O
the	O
Bacteroidetes	B-taxonomy_domain
may	O
be	O
variable	O
and	O
substrate	O
dependent	O
.	O

Such	O
is	O
the	O
case	O
for	O
XyGUL	B-gene
from	O
related	O
Bacteroides	B-taxonomy_domain
species	O
,	O
which	O
may	O
encode	O
either	O
one	O
or	O
two	O
of	O
these	O
predicted	O
SGBPs	B-protein_type
,	O
and	O
these	O
proteins	O
vary	O
considerably	O
in	O
length	O
.	O

The	O
extremely	O
low	O
similarity	O
of	O
these	O
SGBPs	B-protein_type
is	O
striking	O
in	O
light	O
of	O
the	O
moderate	O
sequence	O
conservation	O
observed	O
among	O
homologous	O
GHs	B-protein_type
in	O
syntenic	O
PUL	B-gene
.	O

This	O
,	O
together	O
with	O
the	O
observation	O
that	O
these	O
SGBPs	B-protein_type
,	O
as	O
exemplified	O
by	O
BtSusE	B-protein
and	O
BtSusF	B-protein
and	O
the	O
XyGUL	B-gene
SGBP	B-protein
-	I-protein
B	I-protein
of	O
the	O
present	O
study	O
,	O
are	O
expendable	O
for	O
polysaccharide	B-chemical
growth	O
,	O
implies	O
a	O
high	O
degree	O
of	O
evolutionary	O
flexibility	O
to	O
enhance	O
glycan	B-chemical
capture	O
at	O
the	O
cell	O
surface	O
.	O

However	O
,	O
the	O
natural	O
diversity	O
of	O
these	O
proteins	O
represents	O
a	O
rich	O
source	O
for	O
the	O
discovery	O
of	O
unique	O
carbohydrate	B-structure_element
-	I-structure_element
binding	I-structure_element
motifs	I-structure_element
to	O
both	O
inform	O
gut	O
microbiology	O
and	O
generate	O
new	O
,	O
specific	O
carbohydrate	B-chemical
analytical	O
reagents	O
.	O

In	O
conclusion	O
,	O
the	O
present	O
study	O
further	O
illuminates	O
the	O
essential	O
role	O
that	O
surface	B-protein_type
-	I-protein_type
glycan	I-protein_type
binding	I-protein_type
proteins	I-protein_type
play	O
in	O
facilitating	O
the	O
catabolism	O
of	O
complex	O
dietary	O
carbohydrates	B-chemical
by	O
Bacteroidetes	B-taxonomy_domain
.	O

The	O
ability	O
of	O
our	O
resident	O
gut	O
bacteria	B-taxonomy_domain
to	O
recognize	O
polysaccharides	B-chemical
is	O
the	O
first	O
committed	O
step	O
of	O
glycan	B-chemical
consumption	O
by	O
these	O
organisms	O
,	O
a	O
critical	O
process	O
that	O
influences	O
the	O
community	O
structure	O
and	O
thus	O
the	O
metabolic	O
output	O
(	O
i	O
.	O
e	O
.,	O
short	O
-	O
chain	O
fatty	O
acid	O
and	O
metabolite	O
profile	O
)	O
of	O
these	O
organisms	O
.	O

Inhibiting	O
complex	O
IL	B-protein
-	I-protein
17A	I-protein
and	O
IL	B-protein
-	I-protein
17RA	I-protein
interactions	O
with	O
a	O
linear	O
peptide	B-chemical

IL	B-protein
-	I-protein
17A	I-protein
is	O
a	O
pro	O
-	O
inflammatory	O
cytokine	B-protein_type
that	O
has	O
been	O
implicated	O
in	O
autoimmune	O
and	O
inflammatory	O
diseases	O
.	O

HAP	B-chemical
binds	O
specifically	O
to	O
IL	B-protein
-	I-protein
17A	I-protein
and	O
inhibits	O
the	O
interaction	O
of	O
the	O
cytokine	B-protein_type
with	O
its	O
receptor	B-protein_type
,	O
IL	B-protein
-	I-protein
17RA	I-protein
.	O

Crystal	B-experimental_method
structure	I-experimental_method
studies	I-experimental_method
revealed	O
that	O
two	O
HAP	B-chemical
molecules	O
bind	O
to	O
one	O
IL	B-protein
-	I-protein
17A	I-protein
dimer	B-oligomeric_state
symmetrically	O
.	O

The	O
N	O
-	O
terminal	O
portions	O
of	O
HAP	B-chemical
form	O
a	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
that	O
inserts	O
between	O
two	O
IL	B-protein
-	I-protein
17A	I-protein
monomers	B-oligomeric_state
while	O
the	O
C	O
-	O
terminal	O
section	O
forms	O
an	O
α	B-structure_element
helix	I-structure_element
that	O
directly	O
blocks	O
IL	B-protein
-	I-protein
17RA	I-protein
from	O
binding	O
to	O
the	O
same	O
region	O
of	O
IL	B-protein
-	I-protein
17A	I-protein
.	O

IL	B-protein
-	I-protein
17A	I-protein
signals	O
through	O
a	O
specific	O
cell	O
surface	O
receptor	B-protein_type
complex	O
which	O
consists	O
of	O
IL	B-protein
-	I-protein
17RA	I-protein
and	O
IL	B-protein
-	I-protein
17RC	I-protein
.	O

IL	B-protein
-	I-protein
17A	I-protein
’	O
s	O
downstream	O
signaling	O
leads	O
to	O
increased	O
production	O
of	O
inflammatory	O
cytokines	B-protein_type
such	O
as	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
,	O
IL	B-protein_type
-	I-protein_type
8	I-protein_type
,	O
CCL	B-protein_type
-	I-protein_type
20	I-protein_type
and	O
CXCL1	B-protein_type
by	O
various	O
mechanisms	O
including	O
stimulation	O
of	O
transcription	O
and	O
stabilization	O
of	O
mRNA	B-chemical
.	O

Although	O
various	O
cell	O
types	O
have	O
been	O
reported	O
to	O
express	O
IL	B-protein
-	I-protein
17RA	I-protein
,	O
the	O
highest	O
responses	O
to	O
IL	B-protein
-	I-protein
17A	I-protein
come	O
from	O
epithelial	O
cells	O
,	O
endothelial	O
cells	O
,	O
keratinocytes	O
and	O
fibroblasts	O
.	O

IL	B-protein
-	I-protein
17A	I-protein
and	O
its	O
signaling	O
is	O
important	O
in	O
host	O
defense	O
against	O
certain	O
fungal	O
and	O
bacterial	O
infections	O
as	O
demonstrated	O
by	O
patients	O
with	O
autoantibodies	O
against	O
IL	B-protein
-	I-protein
17A	I-protein
and	O
IL	B-protein
-	I-protein
17F	I-protein
,	O
or	O
with	O
inborn	O
errors	O
of	O
IL	B-protein_type
-	I-protein_type
17	I-protein_type
immunity	O
.	O

In	O
addition	O
to	O
its	O
physiological	O
role	O
,	O
IL	B-protein
-	I-protein
17A	I-protein
is	O
a	O
key	O
pathogenic	O
factor	O
in	O
inflammatory	O
and	O
autoimmune	O
diseases	O
.	O

In	O
phase	O
II	O
and	O
III	O
clinical	O
trials	O
,	O
neutralizing	O
monoclonal	O
antibodies	B-protein_type
against	O
IL	B-protein
-	I-protein
17A	I-protein
(	O
secukinumab	B-chemical
and	O
ixekizumab	B-chemical
)	O
or	O
its	O
receptor	B-protein_type
IL	B-protein
-	I-protein
17RA	I-protein
(	O
brodalumab	B-chemical
)	O
are	O
highly	O
efficacious	O
in	O
treating	O
moderate	O
to	O
severe	O
plaque	O
psoriasis	O
and	O
psoriatic	O
arthritis	O
.	O

Secukinumab	B-chemical
has	O
been	O
approved	O
recently	O
as	O
a	O
new	O
psoriasis	O
drug	O
by	O
the	O
US	O
Food	O
and	O
Drug	O
Administration	O
(	O
Cosentyx	B-chemical
™).	I-chemical

In	O
addition	O
to	O
psoriasis	O
and	O
psoriatic	O
arthritis	O
,	O
IL	B-protein
-	I-protein
17A	I-protein
blockade	O
has	O
also	O
shown	O
preclinical	O
and	O
clinical	O
efficacies	O
in	O
ankylosing	O
spondylitis	O
and	O
rheumatoid	O
arthritis	O
.	O

Among	O
IL	B-protein_type
-	I-protein_type
17	I-protein_type
cytokines	I-protein_type
,	O
IL	B-protein
-	I-protein
17A	I-protein
and	O
IL	B-protein
-	I-protein
17F	I-protein
share	O
the	O
highest	O
homology	O
.	O

Structures	B-evidence
are	O
known	O
for	O
apo	B-protein_state
IL	B-protein
-	I-protein
17F	I-protein
and	O
its	O
complex	B-protein_state
with	I-protein_state
IL	B-protein
-	I-protein
17RA	I-protein
,	O
for	O
apo	B-protein_state
IL	B-protein
-	I-protein
17A	I-protein
,	O
its	O
complex	B-protein_state
with	I-protein_state
an	O
antibody	B-protein_type
Fab	B-structure_element
,	O
and	O
its	O
complex	B-protein_state
with	I-protein_state
IL	B-protein
-	I-protein
17RA	I-protein
.	O

Developing	O
small	O
molecules	O
targeting	O
protein	O
-	O
protein	O
interactions	O
is	O
difficult	O
with	O
particular	O
challenges	O
associated	O
with	O
the	O
large	O
,	O
shallow	O
IL	B-site
-	I-site
17A	I-site
/	I-site
IL	I-site
-	I-site
17RA	I-site
interfaces	I-site
.	O

Our	O
efforts	O
resulted	O
in	O
discovery	O
of	O
a	O
high	B-chemical
affinity	I-chemical
IL	I-chemical
-	I-chemical
17A	I-chemical
peptide	I-chemical
antagonist	I-chemical
(	O
HAP	B-chemical
),	O
which	O
we	O
attempted	O
to	O
increase	O
the	O
functional	O
production	O
and	O
pharmacokinetics	O
after	O
fusing	B-experimental_method
HAP	B-chemical
to	O
antibodies	B-protein_type
for	O
evaluation	O
as	O
a	O
bispecific	O
therapeutic	O
in	O
animal	O
studies	O
.	O

Unfortunately	O
,	O
this	O
past	O
work	O
revealed	O
stability	O
issues	O
of	O
the	O
uncapped	B-protein_state
HAP	B-chemical
in	O
cell	O
culture	O
Here	O
,	O
we	O
provide	O
the	O
details	O
of	O
the	O
discovery	O
and	O
optimization	O
that	O
led	O
to	O
HAP	B-chemical
and	O
report	O
the	O
complex	B-evidence
structure	I-evidence
of	O
IL	B-protein
-	I-protein
17A	I-protein
with	O
HAP	B-chemical
,	O
which	O
provides	O
structure	O
based	O
rationalization	O
of	O
peptide	B-experimental_method
optimization	I-experimental_method
and	O
structure	B-experimental_method
activity	I-experimental_method
relationship	I-experimental_method
(	O
SAR	B-experimental_method
).	O

Single	O
clones	O
were	O
isolated	O
and	O
sub	O
-	O
cultured	O
in	O
growth	O
medium	O
,	O
and	O
culture	O
supernatants	O
were	O
used	O
in	O
an	O
enzyme	B-experimental_method
-	I-experimental_method
linked	I-experimental_method
immunosorbent	I-experimental_method
assay	I-experimental_method
(	O
ELISA	B-experimental_method
)	O
to	O
identify	O
specific	O
IL	B-protein
-	I-protein
17A	I-protein
-	O
binding	O
clones	O
.	O

Approximately	O
10	O
%	O
of	O
the	O
clones	O
that	O
specifically	O
bound	O
to	O
IL	B-protein
-	I-protein
17A	I-protein
also	O
prevented	O
the	O
cytokine	B-protein_type
from	O
binding	O
to	O
IL	B-protein
-	I-protein
17RA	I-protein
.	O

A	O
15	O
-	O
mer	O
linear	O
peptide	B-chemical
1	I-chemical
was	O
shown	O
to	O
block	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
IL	I-complex_assembly
-	I-complex_assembly
17RA	I-complex_assembly
binding	O
with	O
an	O
IC50	B-evidence
of	O
80	O
nM	O
in	O
the	O
competition	B-experimental_method
ELISA	I-experimental_method
assay	I-experimental_method
(	O
Table	O
1	O
).	O

This	O
peptide	O
was	O
then	O
tested	O
in	O
a	O
cell	B-experimental_method
-	I-experimental_method
based	I-experimental_method
functional	I-experimental_method
assay	I-experimental_method
wherein	O
production	O
of	O
GRO	B-protein
-	I-protein
α	I-protein
in	O
BJ	O
human	B-species
fibroblast	O
cells	O
was	O
measured	O
as	O
a	O
function	O
of	O
IL	B-protein
-	I-protein
17A	I-protein
stimulation	O
using	O
1	O
ng	O
/	O
ml	O
IL	B-protein
-	I-protein
17A	I-protein
.	O

Peptide	B-chemical
1	I-chemical
was	O
found	O
to	O
be	O
active	O
in	O
this	O
functional	B-experimental_method
assay	I-experimental_method
with	O
an	O
IC50	B-evidence
of	O
370	O
nM	O
.	O

Optimization	O
of	O
IL	B-protein
-	I-protein
17A	I-protein
peptide	O
inhibitors	O

A	O
SAR	B-experimental_method
campaign	O
was	O
undertaken	O
to	O
improve	O
the	O
potency	O
of	O
peptide	B-chemical
1	I-chemical
.	O

When	O
alanine	B-residue_name
was	O
already	O
present	O
(	O
positions	O
7	B-residue_number
and	O
15	B-residue_number
),	O
substitution	B-experimental_method
was	O
made	O
with	O
lysine	B-residue_name
(	O
Table	O
1	O
,	O
peptides	B-chemical
3	I-chemical
–	I-chemical
17	I-chemical
).	O

Positions	O
1	B-residue_number
,	O
2	B-residue_number
,	O
4	B-residue_number
,	O
5	B-residue_number
,	O
7	B-residue_number
,	O
14	B-residue_number
and	O
15	B-residue_number
were	O
shown	O
to	O
be	O
amenable	O
to	O
substitution	O
without	O
significant	O
loss	O
(	O
less	O
than	O
3	O
-	O
fold	O
)	O
of	O
binding	B-evidence
affinity	I-evidence
as	O
measured	O
by	O
the	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
IL	I-complex_assembly
-	I-complex_assembly
17RA	I-complex_assembly
competition	B-experimental_method
ELISA	I-experimental_method
.	O

In	O
order	O
to	O
rapidly	O
evaluate	O
the	O
effects	O
of	O
substitution	B-experimental_method
of	O
natural	O
amino	O
acids	O
at	O
tolerant	O
positions	O
identified	O
by	O
the	O
alanine	B-experimental_method
scan	I-experimental_method
,	O
the	O
lead	O
sequence	O
was	O
subjected	O
to	O
site	B-experimental_method
-	I-experimental_method
specific	I-experimental_method
saturation	I-experimental_method
mutagenesis	I-experimental_method
using	O
MBP	B-experimental_method
.	O

Each	O
of	O
the	O
seven	O
positions	O
identified	O
by	O
the	O
alanine	B-experimental_method
scan	I-experimental_method
was	O
individually	O
modified	O
while	O
keeping	O
the	O
rest	O
of	O
the	O
sequence	O
constant	O
.	O

Peptides	O
with	O
beneficial	O
point	B-experimental_method
mutations	I-experimental_method
at	O
positions	O
2	B-residue_number
,	O
5	B-residue_number
,	O
and	O
14	B-residue_number
were	O
synthesized	B-experimental_method
and	O
evaluated	O
in	O
the	O
competition	B-experimental_method
ELISA	I-experimental_method
(	O
Table	O
1	O
).	O

Two	O
of	O
the	O
changes	O
,	O
V2H	B-mutant
(	O
18	B-chemical
)	O
or	O
V2T	B-mutant
(	O
21	B-chemical
)	O
displayed	O
improved	O
binding	O
in	O
the	O
competition	B-experimental_method
ELISA	I-experimental_method
.	O

Introduction	B-experimental_method
of	O
a	O
methionine	B-residue_name
(	O
27	B-chemical
)	O
or	O
a	O
carboxamide	B-chemical
(	O
28	B-chemical
and	O
29	B-chemical
)	O
at	O
position	O
14	B-residue_number
was	O
shown	O
to	O
improve	O
the	O
binding	B-evidence
affinity	I-evidence
of	O
the	O
lead	O
peptide	O
.	O

In	O
general	O
,	O
there	O
was	O
good	O
agreement	O
between	O
the	O
respective	O
binding	B-evidence
affinities	I-evidence
of	O
the	O
synthesized	O
peptides	O
and	O
their	O
MBP	B-experimental_method
fusion	I-experimental_method
counterparts	O
,	O
except	O
for	O
substitution	B-experimental_method
of	O
valine	B-residue_name
at	O
position	O
2	B-residue_number
to	O
a	O
tryptophan	B-residue_name
(	O
22	B-chemical
),	O
which	O
resulted	O
in	O
a	O
fivefold	O
loss	O
of	O
affinity	B-evidence
,	O
for	O
the	O
free	O
peptide	O
when	O
compared	O
with	O
the	O
MBP	B-experimental_method
fusion	I-experimental_method
.	O

Combining	O
the	O
key	O
amino	O
-	O
acid	O
residues	O
identified	O
by	O
SAR	B-experimental_method
into	O
a	O
single	O
peptide	O
sequence	O
resulted	O
in	O
peptide	B-chemical
30	I-chemical
,	O
named	O
high	B-chemical
affinity	I-chemical
peptide	I-chemical
(	O
HAP	B-chemical
),	O
that	O
was	O
found	O
to	O
inhibit	O
IL	B-protein
-	I-protein
17A	I-protein
signaling	O
in	O
a	O
BJ	O
human	B-species
fibroblast	O
cell	O
assay	O
with	O
an	O
IC50	B-evidence
of	O
17	O
nM	O
,	O
a	O
more	O
than	O
20	O
-	O
fold	O
improvement	O
over	O
the	O
phage	B-experimental_method
peptide	B-chemical
1	I-chemical
(	O
Table	O
2	O
and	O
Supplementary	O
Figure	O
S2	O
).	O

We	O
also	O
examined	O
the	O
effect	O
of	O
removing	O
the	O
acetyl	O
group	O
at	O
the	O
N	O
-	O
terminus	O
of	O
HAP	B-chemical
(	O
which	O
is	O
present	O
in	O
all	O
the	O
peptides	O
made	O
,	O
see	O
Supplementary	O
Material	O
).	O

The	O
un	B-protein_state
-	I-protein_state
capped	I-protein_state
peptide	B-chemical
(	I-chemical
31	I-chemical
)	I-chemical
had	O
an	O
IC50	B-evidence
of	O
420	O
nM	O
in	O
the	O
cell	B-experimental_method
-	I-experimental_method
based	I-experimental_method
assay	I-experimental_method
.	O

The	O
loss	O
of	O
cellular	O
activity	O
of	O
31	B-chemical
was	O
most	O
likely	O
due	O
to	O
the	O
degradation	O
of	O
the	O
N	O
-	O
terminus	O
of	O
31	B-chemical
,	O
since	O
peptide	O
31	B-chemical
was	O
shown	O
to	O
be	O
able	O
to	O
bind	O
to	O
IL	B-protein
-	I-protein
17A	I-protein
with	O
similar	O
affinity	O
as	O
HAP	B-chemical
itself	O
.	O

Furthermore	O
,	O
our	O
previous	O
work	O
had	O
reported	O
that	O
in	O
antibody	B-experimental_method
fusions	I-experimental_method
the	O
uncapped	B-protein_state
peptide	B-chemical
was	O
degraded	O
under	O
cell	O
assay	O
conditions	O
with	O
removal	B-experimental_method
of	I-experimental_method
the	O
first	B-residue_range
1	I-residue_range
-	I-residue_range
3	I-residue_range
residues	I-residue_range
to	O
inactive	O
products	O
with	O
the	O
same	O
N	O
-	O
terminal	O
sequences	O
as	O
peptides	B-chemical
32	I-chemical
–	I-chemical
34	I-chemical
.	O

C	O
-	O
terminal	O
truncations	B-experimental_method
showed	O
a	O
more	O
gradual	O
reduction	O
in	O
activity	O
(	O
35	B-chemical
–	I-chemical
37	I-chemical
;	O
Table	O
2	O
).	O

After	O
deletion	B-experimental_method
of	I-experimental_method
three	B-residue_range
amino	I-residue_range
acids	I-residue_range
from	O
the	O
C	O
-	O
terminal	O
end	O
(	O
37	B-chemical
),	O
the	O
peptide	O
is	O
no	O
longer	O
active	O
.	O

We	O
reasoned	O
that	O
since	O
the	O
IL	B-protein
-	I-protein
17A	I-protein
protein	O
is	O
almost	O
exclusively	O
present	O
in	O
a	O
dimeric	B-oligomeric_state
form	O
,	O
dimerizing	B-oligomeric_state
the	O
IL	B-protein
-	I-protein
17A	I-protein
binding	O
peptides	O
could	O
result	O
in	O
an	O
improvement	O
in	O
binding	B-evidence
affinity	I-evidence
and	O
inhibitory	O
activity	O
.	O

Homodimers	B-oligomeric_state
of	O
HAP	B-chemical
were	O
made	O
through	O
attachment	O
of	O
polyethylene	B-chemical
glycol	I-chemical
(	O
PEG	B-chemical
)	O
spacers	O
of	O
different	O
lengths	O
at	O
amino	O
acids	O
4	B-residue_number
,	O
7	B-residue_number
and	O
14	B-residue_number
,	O
as	O
these	O
positions	O
were	O
identified	O
in	O
the	O
alanine	B-experimental_method
scan	I-experimental_method
analysis	I-experimental_method
as	O
not	O
contributing	O
significantly	O
to	O
the	O
activity	O
,	O
and	O
at	O
each	O
N	O
-	O
terminus	O
(	O
Supplementary	O
Table	O
S2	O
).	O

Due	O
to	O
the	O
high	O
reactivity	O
of	O
the	O
pentafluoroester	B-chemical
(	O
PFP	B-chemical
)	O
group	O
used	O
as	O
the	O
activating	O
group	O
in	O
the	O
PEG	B-chemical
,	O
the	O
histidine	B-residue_name
at	O
position	O
2	B-residue_number
and	O
the	O
lysine	B-residue_name
at	O
position	O
15	B-residue_number
were	O
replaced	O
with	O
threonine	B-residue_name
and	O
dimethyllysine	B-residue_name
respectively	O
to	O
prevent	O
formation	O
of	O
side	O
products	O
,	O
which	O
resulted	O
in	O
peptide	B-chemical
38	I-chemical
that	O
was	O
comparable	O
in	O
activity	O
with	O
HAP	B-chemical
.	O

This	O
exercise	O
revealed	O
that	O
several	O
dimeric	B-oligomeric_state
peptides	B-chemical
with	O
the	O
longer	O
PEG21	B-chemical
spacer	O
were	O
significantly	O
more	O
potent	O
than	O
the	O
monomer	B-oligomeric_state
peptide	O
in	O
the	O
cell	B-experimental_method
-	I-experimental_method
based	I-experimental_method
assay	I-experimental_method
(	O
Supplementary	O
Table	O
S2	O
).	O

The	O
species	O
cross	O
-	O
reactivity	O
of	O
the	O
dimeric	B-oligomeric_state
peptide	B-chemical
45	I-chemical
and	O
HAP	B-chemical
were	O
assessed	O
in	O
a	O
murine	B-experimental_method
functional	I-experimental_method
cell	I-experimental_method
assay	I-experimental_method
using	O
15	O
ng	O
/	O
ml	O
murine	B-taxonomy_domain
IL	B-protein
-	I-protein
17A	I-protein
.	O

Peptide	B-chemical
45	I-chemical
blocked	O
the	O
receptor	B-protein_type
binding	O
of	O
murine	B-taxonomy_domain
IL	B-protein
-	I-protein
17A	I-protein
although	O
with	O
potency	O
two	O
orders	O
of	O
magnitude	O
weaker	O
than	O
that	O
observed	O
against	O
human	B-species
IL	B-protein
-	I-protein
17A	I-protein
(	O
IC50	B-evidence
=	O
41	O
nM	O
vs	O
IC50	B-evidence
=	O
0	O
.	O
1	O
nM	O
,	O
respectively	O
).	O

The	O
monomer	B-oligomeric_state
HAP	B-chemical
was	O
much	O
weaker	O
(	O
IC50	B-evidence
>	O
1	O
μM	O
)	O
in	O
inhibiting	O
murine	B-taxonomy_domain
IL	B-protein
-	I-protein
17A	I-protein
signaling	O
(	O
Supplementary	O
Figure	O
S4	O
).	O

Although	O
the	O
dimeric	B-oligomeric_state
peptide	B-chemical
45	I-chemical
is	O
much	O
more	O
potent	O
than	O
HAP	B-chemical
in	O
the	O
cell	B-experimental_method
-	I-experimental_method
based	I-experimental_method
assay	I-experimental_method
,	O
in	O
subsequent	O
studies	O
we	O
decided	O
to	O
focus	O
our	O
efforts	O
solely	O
on	O
characterizations	O
of	O
the	O
monomeric	B-oligomeric_state
peptide	O
HAP	B-chemical
in	O
hopes	O
to	O
identify	O
smaller	O
peptide	O
inhibitors	O
containing	O
the	O
best	O
minimal	O
functional	O
group	O
.	O

HAP	B-chemical
binds	O
to	O
immobilized	O
human	B-species
IL	B-protein
-	I-protein
17A	I-protein
homodimer	B-oligomeric_state
tightly	O
(	O
Table	O
3	O
).	O

It	O
has	O
slightly	O
weaker	O
affinity	B-evidence
for	O
human	B-species
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
F	I-complex_assembly
heterodimer	B-oligomeric_state
and	O
>	O
10	O
fold	O
weaker	O
affinity	B-evidence
for	O
mouse	B-taxonomy_domain
IL	B-protein
-	I-protein
17A	I-protein
(	O
Table	O
3	O
).	O

HAP	B-chemical
does	O
not	O
show	O
significant	O
binding	O
to	O
immobilized	O
human	B-species
IL	B-protein
-	I-protein
17F	I-protein
homodimer	B-oligomeric_state
or	O
IL	B-protein
-	I-protein
17RA	I-protein
at	O
concentrations	O
up	O
to	O
100	O
nM	O
.	O

Additionally	O
,	O
we	O
investigated	O
the	O
antagonism	O
of	O
the	O
human	B-species
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
IL	I-complex_assembly
-	I-complex_assembly
17RA	I-complex_assembly
interaction	O
by	O
HAP	B-chemical
using	O
orthogonal	O
methods	O
including	O
SPR	B-experimental_method
and	O
Förster	B-experimental_method
resonance	I-experimental_method
energy	I-experimental_method
transfer	I-experimental_method
(	I-experimental_method
FRET	I-experimental_method
)	I-experimental_method
competition	I-experimental_method
assays	I-experimental_method
(	O
Fig	O
.	O
1B	O
,	O
C	O
).	O

In	O
both	O
assays	O
,	O
incubation	O
of	O
IL	B-protein
-	I-protein
17A	I-protein
with	O
HAP	B-chemical
effectively	O
blocks	O
the	O
binding	O
of	O
IL	B-protein
-	I-protein
17A	I-protein
to	O
immobilized	B-protein_state
IL	B-protein
-	I-protein
17RA	I-protein
with	O
similar	O
sub	O
-	O
nM	O
IC50	B-evidence
(	O
Table	O
3	O
).	O

While	O
either	O
IL	B-protein
-	I-protein
17A	I-protein
or	O
TNF	B-protein
-	I-protein
α	I-protein
alone	O
can	O
stimulate	O
the	O
release	O
of	O
multiple	O
inflammatory	O
cytokines	B-protein_type
,	O
when	O
acting	O
together	O
they	O
can	O
synergistically	O
enhance	O
each	O
other	O
’	O
s	O
effects	O
(	O
Supplementary	O
Figure	O
S5	O
).	O

These	O
integrative	O
responses	O
to	O
IL	B-protein
-	I-protein
17A	I-protein
and	O
TNF	B-protein
-	I-protein
α	I-protein
in	O
human	B-species
keratinocytes	O
have	O
been	O
reported	O
to	O
account	O
for	O
key	O
inflammatory	O
pathogenic	O
circuits	O
in	O
psoriasis	O
.	O

Thus	O
,	O
we	O
chose	O
to	O
study	O
HAP	B-chemical
’	O
s	O
efficacy	O
in	O
blocking	O
the	O
production	O
of	O
IL	B-protein_type
-	I-protein_type
8	I-protein_type
,	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
and	O
CCL	B-protein_type
-	I-protein_type
20	I-protein_type
by	O
primary	O
human	B-species
keratinocytes	O
stimulated	O
by	O
IL	B-protein
-	I-protein
17A	I-protein
in	O
the	O
presence	O
of	O
TNF	B-protein
-	I-protein
α	I-protein
,	O
an	O
assay	O
which	O
may	O
be	O
more	O
disease	O
-	O
relevant	O
.	O

HAP	B-chemical
inhibits	O
the	O
production	O
of	O
all	O
three	O
cytokines	B-protein_type
in	O
a	O
dose	O
-	O
dependent	O
fashion	O
(	O
Fig	O
.	O
1D	O
).	O

Significantly	O
,	O
the	O
baseline	O
levels	O
of	O
IL	B-protein_type
-	I-protein_type
8	I-protein_type
,	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
and	O
CCL	B-protein_type
-	I-protein_type
20	I-protein_type
stimulated	O
by	O
TNF	B-protein
-	I-protein
α	I-protein
alone	O
are	O
not	O
inhibited	O
by	O
HAP	B-chemical
,	O
further	O
indicating	O
the	O
selectivity	O
of	O
HAP	B-chemical
(	O
Fig	O
.	O
1D	O
).	O

Such	O
pharmacological	O
selectivity	O
may	O
be	O
important	O
to	O
suppress	O
inflammatory	O
pathogenic	O
circuits	O
in	O
psoriasis	O
,	O
while	O
sparing	O
the	O
anti	O
-	O
infectious	O
immune	O
responses	O
produced	O
by	O
TNF	B-protein
-	I-protein
α	I-protein
.	O

As	O
a	O
reference	O
,	O
a	O
commercial	O
anti	O
-	O
IL	B-protein
-	I-protein
17A	I-protein
antibody	B-protein_type
(	O
R	O
&	O
D	O
Systems	O
)	O
inhibits	O
the	O
production	O
of	O
IL	B-protein_type
-	I-protein_type
8	I-protein_type
with	O
an	O
IC50	B-evidence
of	O
13	O
(±	O
6	O
)	O
nM	O
(	O
N	O
=	O
3	O
).	O

Indeed	O
,	O
the	O
IC50	B-evidence
was	O
14	O
(±	O
9	O
)	O
nM	O
(	O
N	O
=	O
12	O
)	O
for	O
HAP	B-chemical
inhibition	O
of	O
IL	B-protein_type
-	I-protein_type
8	I-protein_type
production	O
when	O
only	O
5	O
ng	O
/	O
ml	O
IL	B-protein
-	I-protein
17A	I-protein
was	O
used	O
in	O
this	O
assay	O
.	O

Similar	O
to	O
keratinocytes	B-experimental_method
assay	I-experimental_method
results	O
,	O
while	O
HAP	B-chemical
inhibits	O
IL	B-protein
-	I-protein
17A	I-protein
stimulated	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
production	O
by	O
BJ	O
human	B-species
fibroblast	O
potently	O
(	O
IC50	B-evidence
of	O
17	O
nM	O
),	O
it	O
does	O
not	O
inhibit	O
TNF	B-protein
-	I-protein
α	I-protein
stimulated	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
production	O
at	O
concentrations	O
up	O
to	O
10	O
μM	O
(	O
Supplementary	O
Figure	O
S2	O
).	O

Extensive	O
crystallization	B-experimental_method
trials	I-experimental_method
,	O
either	O
by	O
co	B-experimental_method
-	I-experimental_method
crystallization	I-experimental_method
or	O
by	O
soaking	B-experimental_method
HAP	B-chemical
into	O
preformed	O
apo	B-protein_state
IL	B-protein
-	I-protein
17A	I-protein
crystals	B-evidence
,	O
failed	O
to	O
lead	O
to	O
an	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
HAP	I-complex_assembly
complex	O
crystals	B-evidence
.	O

We	O
theorized	O
that	O
HAP	B-chemical
binding	O
induced	O
large	O
conformational	O
changes	O
in	O
IL	B-protein
-	I-protein
17A	I-protein
that	O
led	O
to	O
the	O
difficulty	O
of	O
getting	O
an	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
HAP	I-complex_assembly
binary	O
complex	O
crystal	B-evidence
.	O

We	O
hypothesized	O
that	O
HAP	B-chemical
may	O
target	O
the	O
N	O
-	O
terminal	O
of	O
IL	B-protein
-	I-protein
17A	I-protein
which	O
is	O
known	O
to	O
be	O
more	O
flexible	O
than	O
its	O
C	O
-	O
terminal	O
and	O
conformational	O
changes	O
needed	O
for	O
HAP	B-chemical
binding	O
may	O
be	O
more	O
likely	O
there	O
.	O

We	O
designed	O
an	O
antibody	B-protein_type
Fab	B-structure_element
known	O
to	O
target	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
half	I-structure_element
of	O
IL	B-protein
-	I-protein
17A	I-protein
based	O
on	O
a	O
published	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
Fab	I-complex_assembly
complex	O
crystal	B-evidence
structure	I-evidence
,	O
and	O
produced	O
it	O
in	O
HEK293	O
cells	O
.	O

In	O
an	O
SPR	B-experimental_method
assay	I-experimental_method
HAP	B-chemical
and	O
this	O
Fab	B-structure_element
were	O
able	O
to	O
co	O
-	O
bind	O
IL	B-protein
-	I-protein
17A	I-protein
without	O
large	O
changes	O
in	O
their	O
binding	B-evidence
affinities	I-evidence
and	O
kinetics	B-evidence
,	O
confirming	O
our	O
hypothesis	O
(	O
Supplementary	O
Figure	O
S6	O
).	O

These	O
were	O
,	O
respectively	O
,	O
a	O
presumably	O
more	O
homogeneous	O
form	O
of	O
IL	B-protein
-	I-protein
17A	I-protein
that	O
lacked	B-protein_state
the	O
disordered	B-protein_state
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
peptide	I-structure_element
and	O
a	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
form	O
of	O
the	O
cytokine	B-protein_type
with	O
a	O
full	O
complement	O
of	O
disulfide	B-ptm
bonds	I-ptm
.	O

Both	O
complexes	O
crystallized	B-experimental_method
in	O
the	O
space	O
group	O
of	O
P321	O
,	O
with	O
half	O
the	O
complex	O
(	O
1	O
Fab	B-structure_element
/	O
1	O
IL	B-protein
-	I-protein
17A	I-protein
monomer	B-oligomeric_state
/	O
1	O
HAP	B-chemical
)	O
in	O
the	O
asymmetric	O
unit	O
.	O

The	O
intact	B-protein_state
complex	O
can	O
be	O
generated	O
by	O
applying	O
crystallographic	O
2	O
-	O
fold	O
symmetry	O
.	O

Electron	B-evidence
densities	I-evidence
for	O
HAP	B-chemical
residues	O
Ile1	B-residue_range
-	I-residue_range
Asn14	I-residue_range
were	O
readily	O
interpretable	O
with	O
the	O
exception	O
of	O
Lys15	B-residue_name_number
,	O
which	O
is	O
disordered	B-protein_state
.	O

When	O
considering	O
the	O
protein	O
,	O
the	O
complex	B-evidence
structure	I-evidence
containing	O
the	O
full	B-protein_state
length	I-protein_state
IL	B-protein
-	I-protein
17A	I-protein
is	O
identical	O
to	O
that	O
of	O
the	O
truncated	B-protein_state
IL	B-protein
-	I-protein
17A	I-protein
,	O
with	O
the	O
exception	O
of	O
Cys106	B-residue_name_number
(	O
Ser106	B-residue_name_number
in	O
the	O
truncated	B-protein_state
IL	B-protein
-	I-protein
17A	I-protein
),	O
which	O
is	O
disordered	B-protein_state
.	O

Cys106	B-residue_name_number
is	O
covalently	O
linked	O
to	O
Cys10	B-residue_name_number
that	O
resides	O
in	O
the	O
disordered	B-protein_state
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
peptide	I-structure_element
in	O
the	O
full	B-protein_state
length	I-protein_state
IL	B-protein
-	I-protein
17A	I-protein
.	O

Overall	O
structure	B-evidence
of	O
Fab	B-complex_assembly
/	I-complex_assembly
IL	I-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
HAP	I-complex_assembly
complex	O

In	O
a	O
similar	O
manner	O
to	O
the	O
published	O
structure	B-evidence
of	O
Fab	B-complex_assembly
/	I-complex_assembly
IL	I-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
complex	O
,	O
two	O
Fab	B-structure_element
molecules	O
bind	O
symmetrically	O
to	O
the	O
C	O
-	O
terminal	O
of	O
the	O
cytokine	B-protein_type
dimer	B-oligomeric_state
,	O
interacting	O
with	O
epitopes	O
from	O
both	O
monomers	B-oligomeric_state
(	O
Fig	O
.	O
2A	O
).	O

Based	O
on	O
disclosed	O
epitopes	O
of	O
Secukinumab	B-chemical
and	O
Ixekizumab	B-chemical
,	O
HAP	B-chemical
binds	O
to	O
IL	B-protein
-	I-protein
17A	I-protein
at	O
an	O
area	O
that	O
is	O
also	O
different	O
from	O
those	O
of	O
those	O
two	O
antibodies	B-protein_type
.	O

The	O
N	O
-	O
terminal	O
5	B-residue_range
residues	I-residue_range
of	O
HAP	B-chemical
,	O
1IHVTI	B-chemical
,	O
form	O
an	O
amphipathic	B-protein_state
β	B-structure_element
-	I-structure_element
strand	I-structure_element
that	O
inserts	O
between	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
4	I-structure_element
of	O
one	O
IL	B-protein
-	I-protein
17A	I-protein
monomer	B-oligomeric_state
and	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
0	I-structure_element
(	O
the	O
first	O
ordered	O
peptide	O
of	O
IL	B-protein
-	I-protein
17A	I-protein
)	O
of	O
the	O
second	O
monomer	B-oligomeric_state
.	O

This	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
is	O
parallel	O
to	O
both	O
strands	B-structure_element
0	I-structure_element
and	I-structure_element
4	I-structure_element
(	O
Fig	O
.	O
3B	O
).	O

Strands	B-structure_element
0	I-structure_element
of	O
two	O
IL	B-protein
-	I-protein
17A	I-protein
monomer	B-oligomeric_state
are	O
antiparallel	O
,	O
as	O
appeared	O
in	O
other	O
IL	B-protein
-	I-protein
17A	I-protein
structures	B-evidence
.	O

As	O
a	O
comparison	O
,	O
an	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
IL	I-complex_assembly
-	I-complex_assembly
17RA	I-complex_assembly
complex	B-evidence
structure	I-evidence
(	O
PDB	O
code	O
4HSA	O
)	O
is	O
also	O
shown	O
with	O
IL	B-protein
-	I-protein
17A	I-protein
in	O
the	O
same	O
orientation	O
(	O
Fig	O
.	O
2C	O
).	O

IL	B-protein
-	I-protein
17RA	I-protein
binds	O
IL	B-protein
-	I-protein
17A	I-protein
at	O
three	O
regions	O
on	O
the	O
IL	B-protein
-	I-protein
17A	I-protein
homodimer	B-oligomeric_state
.	O

HAP	B-chemical
binds	O
IL	B-protein
-	I-protein
17A	I-protein
at	O
region	B-structure_element
I	I-structure_element
.	O
Region	B-structure_element
I	I-structure_element
is	O
formed	O
by	O
residues	O
at	O
the	O
ends	O
of	O
β	B-structure_element
strands	I-structure_element
0	I-structure_element
and	I-structure_element
4	I-structure_element
,	O
and	O
from	O
loops	B-structure_element
1	I-structure_element
–	I-structure_element
2	I-structure_element
and	O
3	B-structure_element
–	I-structure_element
4	I-structure_element
of	O
IL	B-protein
-	I-protein
17A	I-protein
(	O
Fig	O
.	O
2	O
).	O

The	O
most	O
significant	O
interactions	O
between	O
the	O
α	B-structure_element
helix	I-structure_element
of	O
HAP	B-chemical
and	O
IL	B-protein
-	I-protein
17A	I-protein
involve	O
Trp12	B-residue_name_number
of	O
HAP	B-chemical
,	O
which	O
binds	O
in	O
a	O
hydrophobic	B-site
pocket	I-site
in	O
IL	B-protein
-	I-protein
17A	I-protein
formed	O
by	O
the	O
side	O
chains	O
of	O
Phe110	B-residue_name_number
,	O
Tyr62	B-residue_name_number
,	O
Pro59	B-residue_name_number
and	O
the	O
hydrophobic	O
portion	O
of	O
the	O
Arg101	B-residue_name_number
side	O
chain	O
(	O
Fig	O
.	O
3A	O
).	O

The	O
Trp12	B-residue_name_number
side	O
chain	O
of	O
HAP	B-chemical
donates	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
to	O
the	O
main	O
chain	O
oxygen	O
of	O
Pro69	B-residue_name_number
of	O
IL	B-protein
-	I-protein
17A	I-protein
.	O

The	O
positively	O
charged	O
Arg101	B-residue_name_number
side	O
chain	O
of	O
the	O
IL	B-protein
-	I-protein
17A	I-protein
engages	O
in	O
a	O
charge	B-bond_interaction
-	I-bond_interaction
helix	I-bond_interaction
dipole	I-bond_interaction
interaction	I-bond_interaction
with	O
the	O
main	O
chain	O
oxygen	O
of	O
Trp12	B-residue_name_number
.	O

Additionally	O
,	O
Leu9	B-residue_name_number
and	O
Ile13	B-residue_name_number
of	O
the	O
HAP	B-chemical
have	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
with	O
IL	B-protein
-	I-protein
17A	I-protein
,	O
and	O
the	O
Asp8	B-residue_name_number
side	O
chain	O
has	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
and	O
ion	B-bond_interaction
pair	I-bond_interaction
interactions	I-bond_interaction
with	O
Tyr62	B-residue_name_number
and	O
Lys114	B-residue_name_number
of	O
IL	B-protein
-	I-protein
17A	I-protein
,	O
respectively	O
.	O

In	O
region	B-structure_element
I	I-structure_element
,	O
an	O
IL	B-protein
-	I-protein
17RA	I-protein
peptide	O
interacts	O
with	O
IL	B-protein
-	I-protein
17A	I-protein
in	O
a	O
very	O
similar	O
fashion	O
to	O
the	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
of	O
HAP	B-chemical
.	O

The	O
IL	B-protein
-	I-protein
17RA	I-protein
peptide	O
has	O
sequences	O
of	O
27LDDSWI	B-chemical
,	O
and	O
part	O
of	O
the	O
peptide	O
is	O
also	O
α	B-structure_element
-	I-structure_element
helical	I-structure_element
(	O
Fig	O
.	O
3B	O
).	O

Leu7	B-residue_name_number
,	O
Trp31	B-residue_name_number
and	O
Ile32	B-residue_name_number
of	O
IL	B-protein
-	I-protein
17RA	I-protein
interact	O
very	O
similarly	O
with	O
the	O
same	O
residues	O
of	O
IL	B-protein
-	I-protein
17A	I-protein
as	O
Leu9	B-residue_name_number
,	O
Trp12	B-residue_name_number
and	O
Ile13	B-residue_name_number
of	O
HAP	B-chemical
(	O
Fig	O
.	O
3B	O
).	O

In	O
this	O
sense	O
,	O
the	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
of	O
HAP	B-chemical
with	O
a	O
sequence	O
of	O
9LWDWI	B-chemical
is	O
a	O
good	O
mimetic	O
of	O
the	O
27LDDSWI	B-chemical
peptide	O
of	O
IL	B-protein
-	I-protein
17RA	I-protein
.	O

The	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
of	O
HAP	B-chemical
has	O
no	O
equivalent	O
in	O
IL	B-protein
-	I-protein
17RA	I-protein
.	O

The	O
amphipathic	B-protein_state
β	B-structure_element
-	I-structure_element
strand	I-structure_element
of	O
HAP	B-chemical
orients	O
the	O
hydrophilic	O
side	O
chains	O
of	O
His2	B-residue_name_number
and	O
Thr4	B-residue_name_number
outwards	O
,	O
and	O
the	O
hydrophobic	O
side	O
chains	O
of	O
Ile1	B-residue_name_number
,	O
Val3	B-residue_name_number
and	O
Ile5	B-residue_name_number
inward	O
(	O
Fig	O
.	O
3A	O
).	O

β	B-structure_element
-	I-structure_element
strand	I-structure_element
0	I-structure_element
in	O
IL	B-protein
-	I-protein
17A	I-protein
is	O
also	O
amphipathic	B-protein_state
with	O
the	O
sequence	O
of	O
21TVMVNLNI	B-chemical
.	O

In	O
all	O
IL	B-protein
-	I-protein
17A	I-protein
structures	B-evidence
obtained	O
to	O
date	O
,	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
0	I-structure_element
orients	O
the	O
hydrophilic	O
side	O
chains	O
of	O
Thr21	B-residue_name_number
,	O
Asn25	B-residue_name_number
and	O
Asn27	B-residue_name_number
outward	O
,	O
and	O
the	O
hydrophobic	O
side	O
chains	O
of	O
Val22	B-residue_name_number
,	O
Val24	B-residue_name_number
,	O
Leu26	B-residue_name_number
and	O
Ile28	B-residue_name_number
inward	O
.	O

The	O
binding	B-site
pocket	I-site
occupied	O
by	O
either	O
Trp12	B-residue_name_number
of	O
HAP	B-chemical
or	O
Trp31	B-residue_name_number
of	O
IL	B-protein
-	I-protein
17RA	I-protein
is	O
not	O
formed	O
in	O
the	O
apo	B-protein_state
IL	B-protein
-	I-protein
17A	I-protein
structure	B-evidence
(	O
Fig	O
.	O
3C	O
).	O

Particularly	O
for	O
HAP	B-chemical
,	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
0	I-structure_element
have	O
to	O
shift	O
out	O
of	O
the	O
hydrophobic	B-site
cleft	I-site
formed	O
by	O
the	O
main	B-structure_element
body	I-structure_element
of	O
the	O
IL	B-protein
-	I-protein
17A	I-protein
by	O
as	O
much	O
as	O
10	O
Å	O
between	O
Cα	O
atoms	O
(	O
Fig	O
.	O
3C	O
).	O

Disruptions	O
of	O
the	O
apo	B-protein_state
IL	B-protein
-	I-protein
17A	I-protein
structure	B-evidence
by	O
HAP	B-chemical
binding	O
are	O
apparently	O
compensated	O
for	O
by	O
formation	O
of	O
the	O
new	O
interactions	O
that	O
involve	O
almost	O
the	O
entire	O
HAP	B-chemical
molecule	O
(	O
Fig	O
.	O
3B	O
).	O

The	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
HAP	I-complex_assembly
complex	B-evidence
structure	I-evidence
obtained	O
is	O
very	O
consistent	O
with	O
the	O
observed	O
SAR	B-experimental_method
of	O
our	O
identified	O
peptide	O
inhibitors	O
,	O
explaining	O
well	O
how	O
the	O
evolution	O
of	O
the	O
initial	O
phage	B-experimental_method
peptide	B-chemical
1	I-chemical
to	O
HAP	B-chemical
and	O
45	B-chemical
improved	O
its	O
potency	O
(	O
Supplementary	O
Figure	O
S7	O
).	O

The	O
important	O
interactions	O
involving	O
Trp12	B-residue_name_number
of	O
HAP	B-chemical
explain	O
the	O
>	O
90	O
times	O
drop	O
in	O
potency	O
of	O
the	O
W12A	B-mutant
variant	O
(	O
6	O
vs	O
1	O
,	O
Table	O
1	O
).	O

The	O
amphipathic	B-protein_state
nature	O
of	O
the	O
HAP	B-chemical
β	B-structure_element
-	I-structure_element
strand	I-structure_element
explains	O
the	O
preference	O
of	O
the	O
hydrophilic	O
residues	O
at	O
the	O
2	B-residue_number
and	O
4	B-residue_number
positions	O
of	O
peptides	O
(	O
14	B-chemical
,	O
18	B-chemical
,	O
19	B-chemical
,	O
21	B-chemical
and	O
23	B-chemical
vs	O
1	B-chemical
and	O
22	B-chemical
,	O
Table	O
1	O
).	O

All	O
N	O
-	O
terminal	O
residues	O
of	O
HAP	B-chemical
are	O
part	O
of	O
the	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
with	O
β	B-structure_element
-	I-structure_element
stands	I-structure_element
0	I-structure_element
and	I-structure_element
4	I-structure_element
of	O
IL	B-protein
-	I-protein
17A	I-protein
,	O
which	O
explains	O
why	O
removal	B-experimental_method
of	I-experimental_method
the	O
first	B-residue_range
1	I-residue_range
–	I-residue_range
3	I-residue_range
residues	I-residue_range
completely	O
abolishes	O
the	O
ability	O
of	O
HAP	B-chemical
to	O
block	O
IL	B-protein
-	I-protein
17A	I-protein
cell	O
signaling	O
(	O
31	B-chemical
,	O
32	B-chemical
and	O
33	B-chemical
,	O
Table	O
2	O
).	O

Each	O
peptide	O
monomer	B-oligomeric_state
in	O
45	B-chemical
may	O
not	O
necessarily	O
be	O
more	O
potent	O
than	O
HAP	B-chemical
,	O
but	O
two	O
monomer	B-oligomeric_state
peptides	O
within	O
the	O
same	O
molecule	O
that	O
can	O
simultaneously	O
bind	O
to	O
IL	B-protein
-	I-protein
17A	I-protein
can	O
greatly	O
improve	O
its	O
potency	O
due	O
to	O
avidity	O
effects	O
.	O

HAP	B-chemical
targets	O
region	B-structure_element
I	I-structure_element
of	O
IL	B-protein
-	I-protein
17A	I-protein
,	O
an	O
area	O
that	O
has	O
the	O
least	O
sequence	O
conservation	O
in	O
IL	B-protein_type
-	I-protein_type
17	I-protein_type
cytokines	I-protein_type
.	O

This	O
lack	O
of	O
sequence	O
conservation	O
in	O
the	O
HAP	B-site
binding	I-site
site	I-site
explains	O
the	O
observed	O
specificity	O
of	O
HAP	B-chemical
binding	O
to	O
human	B-species
IL	B-protein
-	I-protein
17A	I-protein
.	O

This	O
Phe	B-structure_element
-	I-structure_element
Phe	I-structure_element
motif	I-structure_element
is	O
missing	B-protein_state
in	O
IL	B-protein
-	I-protein
17A	I-protein
.	O

Sequence	B-experimental_method
alignments	I-experimental_method
between	O
human	B-species
and	O
mouse	B-taxonomy_domain
IL	B-protein
-	I-protein
17A	I-protein
indicated	O
that	O
among	O
IL	B-protein
-	I-protein
17A	I-protein
residues	O
that	O
interacting	O
with	O
HAP	B-chemical
,	O
majority	O
differences	O
occur	O
in	O
strand	B-structure_element
0	I-structure_element
of	O
IL	B-protein
-	I-protein
17A	I-protein
which	O
interacts	O
with	O
the	O
N	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
of	O
HAP	B-chemical
.	O

In	O
human	B-species
IL	B-protein
-	I-protein
17A	I-protein
the	O
sequences	O
are	O
21TVMVNLNI	B-chemical
,	O
and	O
in	O
mouse	B-taxonomy_domain
they	O
are	O
21NVKVNLKV	B-chemical
.	O

Using	O
a	O
combination	O
of	O
phage	B-experimental_method
display	I-experimental_method
and	O
SAR	B-experimental_method
we	O
have	O
discovered	O
novel	O
peptides	O
that	O
are	O
IL	B-protein
-	I-protein
17A	I-protein
antagonists	O
.	O

One	O
of	O
those	O
peptides	O
,	O
HAP	B-chemical
,	O
also	O
shows	O
activity	O
in	O
inhibiting	O
the	O
production	O
of	O
multiple	O
inflammatory	O
cytokines	B-protein_type
by	O
primary	O
human	B-species
keratinocytes	O
stimulated	O
by	O
IL	B-protein
-	I-protein
17A	I-protein
and	O
TNF	B-protein
-	I-protein
α	I-protein
,	O
a	O
disease	O
relevant	O
-	O
model	O
.	O

With	O
two	O
HAP	B-chemical
molecules	O
covering	O
both	O
faces	O
of	O
the	O
IL	B-protein
-	I-protein
17A	I-protein
dimer	B-oligomeric_state
,	O
HAP	B-chemical
can	O
block	O
IL	B-protein
-	I-protein
17RA	I-protein
approaching	O
from	O
either	O
face	O
.	O

To	O
form	O
the	O
1	O
:	O
2	O
complex	O
observed	O
in	O
crystal	B-evidence
structure	I-evidence
,	O
it	O
is	O
important	O
that	O
there	O
is	O
no	O
strong	O
negative	O
cooperativity	O
in	O
the	O
binding	O
of	O
two	O
HAP	B-chemical
molecules	O
.	O

In	O
fact	O
,	O
in	O
native	B-experimental_method
electrospray	I-experimental_method
ionization	I-experimental_method
mass	I-experimental_method
spectrometry	I-experimental_method
analysis	O
only	O
1	O
:	O
2	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
HAP	I-complex_assembly
complex	O
was	O
observed	O
even	O
when	O
IL	B-protein
-	I-protein
17A	I-protein
was	O
in	O
excess	O
(	O
Supplementary	O
Figure	O
S8	O
),	O
indicating	O
a	O
positive	O
binding	O
cooperativity	O
that	O
favors	O
inhibition	O
of	O
IL	B-protein
-	I-protein
17RA	I-protein
binding	O
by	O
HAP	B-chemical
.	O

Due	O
to	O
the	O
discontinuous	O
nature	O
of	O
the	O
IL	B-site
-	I-site
17A	I-site
/	I-site
IL	I-site
-	I-site
17RA	I-site
binding	I-site
interface	I-site
,	O
it	O
is	O
classified	O
as	O
having	O
tertiary	O
structural	O
epitopes	O
on	O
both	O
binding	O
partners	O
,	O
and	O
is	O
therefore	O
hard	O
to	O
target	O
using	O
small	O
molecules	O
.	O

Our	O
studies	O
of	O
HAP	B-chemical
demonstrated	O
an	O
uncommon	O
mode	O
of	O
action	O
for	O
a	O
peptide	O
in	O
inhibiting	O
such	O
a	O
difficult	O
protein	O
-	O
protein	O
interaction	O
target	O
,	O
and	O
suggest	O
further	O
possible	O
improvements	O
in	O
its	O
binding	O
potency	O
.	O

Homo	O
-	O
dimerization	O
of	O
HAP	B-chemical
(	O
45	B-chemical
)	O
achieved	O
sub	O
-	O
nanomolar	O
potency	O
against	O
human	B-species
IL	B-protein
-	I-protein
17A	I-protein
in	O
cell	O
assay	O
.	O

In	O
the	O
crystal	B-evidence
structure	I-evidence
,	O
the	O
distance	O
between	O
the	O
carbonyl	O
of	O
Asn14	B-residue_name_number
of	O
one	O
HAP	B-chemical
molecule	O
and	O
the	O
N	O
-	O
terminus	O
of	O
the	O
second	O
is	O
only	O
15	O
.	O
7	O
Å	O
,	O
suggesting	O
the	O
potential	O
for	O
more	O
potent	O
dimeric	B-oligomeric_state
peptides	B-chemical
to	O
be	O
designed	O
by	O
using	O
linkers	O
of	O
different	O
lengths	O
at	O
different	O
positions	O
.	O

Another	O
direction	O
of	O
improving	O
HAP	B-chemical
is	O
by	O
reducing	O
its	O
size	O
.	O

In	O
summary	O
,	O
these	O
peptide	O
-	O
based	O
anti	O
-	O
IL	B-protein
-	I-protein
17A	I-protein
modalities	O
could	O
be	O
further	O
developed	O
as	O
alternative	O
therapeutic	O
options	O
to	O
the	O
reported	O
monoclonal	O
antibodies	B-protein_type
.	O

We	O
are	O
also	O
very	O
interested	O
in	O
finding	O
non	O
-	O
peptidic	O
small	O
molecule	O
IL	B-protein
-	I-protein
17A	I-protein
antagonists	O
,	O
and	O
HAP	B-chemical
can	O
be	O
used	O
as	O
an	O
excellent	O
tool	O
peptide	O
.	O

The	O
strategy	O
utilized	O
in	O
generating	O
the	O
complex	O
structures	B-evidence
of	O
HAP	B-chemical
may	O
also	O
be	O
useful	O
for	O
enabling	O
structure	O
based	O
design	O
of	O
some	O
known	O
small	O
molecule	O
IL	O
-	O
17A	O
antagonists	O
.	O

Binding	O
of	O
HAP	B-chemical
to	O
IL	B-protein
-	I-protein
17A	I-protein
and	O
inhibition	O
of	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
IL	I-complex_assembly
-	I-complex_assembly
17RA	I-complex_assembly
are	O
measured	O
by	O
SPR	B-experimental_method
,	O
FRET	B-experimental_method
and	O
cell	B-experimental_method
-	I-experimental_method
based	I-experimental_method
assays	I-experimental_method
.	O

(	O
A	O
)	O
Typical	O
SPR	B-experimental_method
sensorgrams	B-evidence
(	O
black	O
)	O
of	O
HAP	B-chemical
at	O
indicated	O
concentrations	O
binding	O
to	O
biotinylated	B-protein_state
human	B-species
IL	B-protein
-	I-protein
17A	I-protein
immobilized	O
on	O
a	O
streptavidin	O
chip	O
surface	O
,	O
fitted	O
with	O
single	B-evidence
site	I-evidence
binding	I-evidence
model	I-evidence
curves	I-evidence
(	O
red	O
).	O

Kinetic	O
parameters	O
(	O
ka	B-evidence
,	O
kd	B-evidence
)	O
were	O
obtained	O
by	O
a	O
global	O
fit	O
using	O
three	O
concentrations	O
in	O
triplicate	O
.	O

Data	O
are	O
mean	O
and	O
error	O
bars	O
of	O
+/−	O
standard	O
deviation	O
of	O
three	O
measurements	O
.	O
(	O
C	O
)	O
Inhibition	O
of	O
IL	B-protein
-	I-protein
17A	I-protein
and	O
IL	B-protein
-	I-protein
17RA	I-protein
binding	O
by	O
HAP	B-chemical
measured	O
by	O
FRET	B-experimental_method
assay	I-experimental_method
.	O

Data	O
are	O
mean	O
and	O
error	O
bars	O
of	O
+/−	O
standard	O
deviation	O
from	O
299	O
experiments	O
,	O
each	O
performed	O
in	O
duplicate	O
.	O
(	O
D	O
)	O
Example	O
of	O
HAP	B-chemical
selective	O
inhibition	O
of	O
the	O
production	O
of	O
IL	B-protein_type
-	I-protein_type
8	I-protein_type
(	O
triangles	O
),	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
(	O
squares	O
)	O
and	O
CCL	B-protein_type
-	I-protein_type
20	I-protein_type
(	O
circles	O
)	O
by	O
primary	O
human	B-species
keratinocyte	O
cells	O
synergistically	O
stimulated	O
by	O
100	O
ng	O
/	O
ml	O
IL	B-protein
-	I-protein
17A	I-protein
and	O
10	O
ng	O
/	O
ml	O
TNF	B-protein
-	I-protein
α	I-protein
.	O

HAP	B-chemical
does	O
not	O
inhibit	O
the	O
baseline	O
production	O
of	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
,	O
IL	B-protein_type
-	I-protein_type
8	I-protein_type
and	O
CCL	B-protein_type
-	I-protein_type
20	I-protein_type
stimulated	O
by	O
10	O
ng	O
/	O
ml	O
TNF	B-protein
-	I-protein
α	I-protein
alone	O
(	O
gray	O
lines	O
and	O
symbols	O
).	O

Two	O
HAP	B-chemical
molecules	O
are	O
colored	O
blue	O
and	O
red	O
,	O
and	O
IL	B-protein
-	I-protein
17A	I-protein
monomers	B-oligomeric_state
are	O
colored	O
ice	O
blue	O
and	O
pink	O
,	O
respectively	O
.	O

(	O
A	O
)	O
Overview	O
of	O
the	O
distinct	O
binding	B-site
sites	I-site
of	O
Fab	B-structure_element
and	O
HAP	B-chemical
to	O
IL	B-protein
-	I-protein
17A	I-protein
.	O

(	O
B	O
)	O
Close	O
-	O
in	O
view	O
of	O
the	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
HAP	I-complex_assembly
structure	B-evidence
.	O

IL	B-protein
-	I-protein
17A	I-protein
β	B-structure_element
-	I-structure_element
strands	I-structure_element
are	O
labelled	O
.	O

Each	O
of	O
the	O
two	O
bound	B-protein_state
HAP	B-chemical
interacts	O
with	O
both	O
monomers	B-oligomeric_state
of	O
the	O
IL	B-protein
-	I-protein
17A	I-protein
dimer	B-oligomeric_state
.	O

Three	O
distinct	O
areas	O
IL	B-site
-	I-site
17A	I-site
/	I-site
IL	I-site
-	I-site
17RA	I-site
interface	I-site
are	O
labeled	O
.	O

Mechanism	O
of	O
the	O
inhibition	O
of	O
the	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
IL	I-complex_assembly
-	I-complex_assembly
17RA	I-complex_assembly
interaction	O
by	O
HAP	B-chemical
.	O

IL	B-protein
-	I-protein
17A	I-protein
dimer	B-oligomeric_state
is	O
in	O
surface	O
presentation	O
(	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
0	I-structure_element
shown	O
as	O
ribbons	O
for	O
clarity	O
).	O

HAP	B-chemical
residues	O
as	O
well	O
as	O
key	O
IL	B-protein
-	I-protein
17A	I-protein
residues	O
are	O
labeled	O
.	O

For	O
clarity	O
,	O
a	O
few	O
HAP	B-chemical
residues	O
are	O
also	O
shown	O
in	O
stick	O
model	O
with	O
carbon	O
atoms	O
colored	O
green	O
,	O
oxygen	O
in	O
red	O
and	O
nitrogen	O
in	O
blue	O
.	O

(	O
B	O
)	O
I	B-protein
-	I-protein
17RA	I-protein
(	O
ribbon	O
in	O
gold	O
)	O
peptide	O
Leu27	B-residue_range
-	I-residue_range
Ile32	I-residue_range
binds	O
to	O
the	O
same	O
area	O
as	O
the	O
HAP	B-chemical
α	B-structure_element
-	I-structure_element
helix	I-structure_element
.	O

Trp31	B-residue_name_number
of	O
IL	B-protein
-	I-protein
17RA	I-protein
binds	O
to	O
the	O
same	O
pocket	B-site
in	O
IL	B-protein
-	I-protein
17A	I-protein
as	O
Trp12	B-residue_name_number
of	O
HAP	B-chemical
.	O
(	O
C	O
)	O
As	O
illustrated	O
by	O
overlay	B-experimental_method
a	O
single	O
HAP	B-chemical
molecule	O
and	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
0	I-structure_element
(	O
grey	O
)	O
of	O
the	O
IL	B-complex_assembly
-	I-complex_assembly
17A	I-complex_assembly
/	I-complex_assembly
HAP	I-complex_assembly
complex	O
in	O
the	O
apo	B-protein_state
IL	B-protein
-	I-protein
17A	I-protein
structure	B-evidence
,	O
conformational	O
changes	O
in	O
region	B-structure_element
I	I-structure_element
of	O
IL	B-protein
-	I-protein
17A	I-protein
are	O
needed	O
for	O
binding	O
of	O
both	O
the	O
β	B-structure_element
-	I-structure_element
stand	I-structure_element
and	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
of	O
the	O
HAP	B-chemical
.	O

Molecular	O
Basis	O
of	O
Ligand	O
-	O
Dependent	O
Regulation	O
of	O
NadR	B-protein
,	O
the	O
Transcriptional	B-protein_type
Repressor	I-protein_type
of	O
Meningococcal	B-taxonomy_domain
Virulence	O
Factor	O
NadA	B-protein

Neisseria	B-protein
adhesin	I-protein
A	I-protein
(	O
NadA	B-protein
)	O
is	O
present	O
on	O
the	O
meningococcal	B-taxonomy_domain
surface	O
and	O
contributes	O
to	O
adhesion	O
to	O
and	O
invasion	O
of	O
human	B-species
cells	O
.	O

NadA	B-protein
is	O
also	O
one	O
of	O
three	O
recombinant	O
antigens	O
in	O
the	O
recently	O
-	O
approved	O
Bexsero	O
vaccine	O
,	O
which	O
protects	O
against	O
serogroup	B-taxonomy_domain
B	I-taxonomy_domain
meningococcus	I-taxonomy_domain
.	O

In	O
the	O
presence	B-protein_state
of	I-protein_state
4	B-chemical
-	I-chemical
hydroxyphenylacetate	I-chemical
(	O
4	B-chemical
-	I-chemical
HPA	I-chemical
),	O
a	O
catabolite	O
present	O
in	O
human	B-species
saliva	O
both	O
under	O
physiological	O
conditions	O
and	O
during	O
bacterial	B-taxonomy_domain
infection	O
,	O
the	O
binding	O
of	O
NadR	B-protein
to	O
the	O
nadA	B-gene
promoter	O
is	O
attenuated	O
and	O
nadA	B-gene
expression	O
is	O
induced	O
.	O

To	O
gain	O
insights	O
into	O
the	O
regulation	O
of	O
NadR	B-protein
mediated	O
by	O
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
we	O
combined	O
structural	B-experimental_method
,	I-experimental_method
biochemical	I-experimental_method
,	I-experimental_method
and	I-experimental_method
mutagenesis	I-experimental_method
studies	I-experimental_method
.	O

In	O
particular	O
,	O
two	O
new	O
crystal	B-evidence
structures	I-evidence
of	O
ligand	B-protein_state
-	I-protein_state
free	I-protein_state
and	O
ligand	B-protein_state
-	I-protein_state
bound	I-protein_state
NadR	B-protein
revealed	O
(	O
i	O
)	O
the	O
molecular	O
basis	O
of	O
‘	O
conformational	O
selection	O
’	O
by	O
which	O
a	O
single	O
molecule	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
binds	O
and	O
stabilizes	O
dimeric	B-oligomeric_state
NadR	B-protein
in	O
a	O
conformation	O
unsuitable	O
for	O
DNA	O
-	O
binding	O
,	O
(	O
ii	O
)	O
molecular	O
explanations	O
for	O
the	O
binding	O
specificities	O
of	O
different	O
hydroxyphenylacetate	B-chemical
ligands	O
,	O
including	O
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
which	O
is	O
produced	O
during	O
inflammation	O
,	O
(	O
iii	O
)	O
the	O
presence	O
of	O
a	O
leucine	B-residue_name
residue	O
essential	O
for	O
dimerization	O
and	O
conserved	B-protein_state
in	O
many	O
MarR	B-protein_type
family	O
proteins	O
,	O
and	O
(	O
iv	O
)	O
four	O
residues	O
(	O
His7	B-residue_name_number
,	O
Ser9	B-residue_name_number
,	O
Asn11	B-residue_name_number
and	O
Phe25	B-residue_name_number
),	O
which	O
are	O
involved	O
in	O
binding	O
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
and	O
were	O
confirmed	O
in	O
vitro	O
to	O
have	O
key	O
roles	O
in	O
the	O
regulatory	O
mechanism	O
in	O
bacteria	B-taxonomy_domain
.	O

Overall	O
,	O
this	O
study	O
deepens	O
our	O
molecular	O
understanding	O
of	O
the	O
sophisticated	O
regulatory	O
mechanisms	O
of	O
the	O
expression	O
of	O
nadA	B-gene
and	O
other	O
genes	O
governed	O
by	O
NadR	B-protein
,	O
dependent	O
on	O
interactions	O
with	O
niche	O
-	O
specific	O
signal	O
molecules	O
that	O
may	O
play	O
important	O
roles	O
during	O
meningococcal	B-taxonomy_domain
pathogenesis	O
.	O

The	O
Bexsero	O
vaccine	O
protects	O
against	O
MenB	B-species
and	O
has	O
recently	O
been	O
approved	O
in	O
>	O
35	O
countries	O
worldwide	O
.	O

Neisseria	B-protein
adhesin	I-protein
A	I-protein
(	O
NadA	B-protein
)	O
present	O
on	O
the	O
meningococcal	B-taxonomy_domain
surface	O
can	O
mediate	O
binding	O
to	O
human	B-species
cells	O
and	O
is	O
one	O
of	O
the	O
three	O
MenB	B-species
vaccine	O
protein	O
antigens	O
.	O

A	O
deep	O
understanding	O
of	O
nadA	B-gene
expression	O
is	O
therefore	O
important	O
,	O
otherwise	O
the	O
contribution	O
of	O
NadA	B-protein
to	O
vaccine	O
-	O
induced	O
protection	O
against	O
meningococcal	B-taxonomy_domain
meningitis	O
may	O
be	O
underestimated	O
.	O

These	O
findings	O
shed	O
light	O
on	O
the	O
regulation	O
of	O
NadR	B-protein
,	O
a	O
key	O
MarR	B-protein_type
-	O
family	O
virulence	O
factor	O
of	O
this	O
important	O
human	B-species
pathogen	O
.	O

The	O
‘	O
Reverse	B-experimental_method
Vaccinology	I-experimental_method
’	O
approach	O
was	O
pioneered	O
to	O
identify	O
antigens	O
for	O
a	O
protein	O
-	O
based	O
vaccine	O
against	O
serogroup	B-species
B	I-species
Neisseria	I-species
meningitidis	I-species
(	O
MenB	B-species
),	O
a	O
human	B-species
pathogen	O
causing	O
potentially	O
-	O
fatal	O
sepsis	O
and	O
invasive	O
meningococcal	B-taxonomy_domain
disease	O
.	O

Indeed	O
,	O
Reverse	B-experimental_method
Vaccinology	I-experimental_method
identified	O
Neisseria	B-protein
adhesin	I-protein
A	I-protein
(	O
NadA	B-protein
),	O
a	O
surface	O
-	O
exposed	O
protein	O
involved	O
in	O
epithelial	O
cell	O
invasion	O
and	O
found	O
in	O
~	O
30	O
%	O
of	O
clinical	O
isolates	O
.	O

Recently	O
,	O
we	O
reported	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
NadA	B-protein
,	O
providing	O
insights	O
into	O
its	O
biological	O
and	O
immunological	O
functions	O
.	O

Recombinant	O
NadA	B-protein
elicits	O
a	O
strong	O
bactericidal	O
immune	O
response	O
and	O
is	O
therefore	O
included	O
in	O
the	O
Bexsero	O
vaccine	O
that	O
protects	O
against	O
MenB	B-species
and	O
which	O
was	O
recently	O
approved	O
in	O
over	O
35	O
countries	O
worldwide	O
.	O

Previous	O
studies	O
revealed	O
that	O
nadA	B-gene
expression	O
levels	O
are	O
mainly	O
regulated	O
by	O
the	O
Neisseria	B-protein
adhesin	I-protein
A	I-protein
Regulator	I-protein
(	O
NadR	B-protein
).	O

Studies	O
of	O
NadR	B-protein
also	O
have	O
broader	O
implications	O
,	O
since	O
a	O
genome	O
-	O
wide	O
analysis	O
of	O
MenB	B-species
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
nadR	B-gene
knock	B-protein_state
-	I-protein_state
out	I-protein_state
strains	O
revealed	O
that	O
NadR	B-protein
influences	O
the	O
regulation	O
of	O
>	O
30	O
genes	O
,	O
including	O
maf	O
genes	O
,	O
from	O
the	O
multiple	O
adhesin	B-protein_type
family	O
.	O

These	O
genes	O
encode	O
a	O
wide	O
variety	O
of	O
proteins	O
connected	O
to	O
many	O
biological	O
processes	O
contributing	O
to	O
bacterial	B-taxonomy_domain
survival	O
,	O
adaptation	O
in	O
the	O
host	O
niche	O
,	O
colonization	O
and	O
invasion	O
.	O

NadR	B-protein
belongs	O
to	O
the	O
MarR	B-protein_type
(	O
Multiple	B-protein_type
Antibiotic	I-protein_type
Resistance	I-protein_type
Regulator	I-protein_type
)	O
family	O
,	O
a	O
group	O
of	O
ligand	B-protein_type
-	I-protein_type
responsive	I-protein_type
transcriptional	I-protein_type
regulators	I-protein_type
ubiquitous	O
in	O
bacteria	B-taxonomy_domain
and	O
archaea	B-taxonomy_domain
.	O

MarR	B-protein_type
family	O
proteins	O
can	O
promote	O
bacterial	B-taxonomy_domain
survival	O
in	O
the	O
presence	O
of	O
antibiotics	O
,	O
toxic	O
chemicals	O
,	O
organic	O
solvents	O
or	O
reactive	O
oxygen	O
species	O
and	O
can	O
regulate	O
virulence	O
factor	O
expression	O
.	O

MarR	B-protein_type
homologues	O
can	O
act	O
either	O
as	O
transcriptional	O
repressors	O
or	O
as	O
activators	O
.	O

Although	O
>	O
50	O
MarR	B-protein_type
family	O
structures	B-evidence
are	O
known	O
,	O
a	O
molecular	O
understanding	O
of	O
their	O
ligand	O
-	O
dependent	O
regulatory	O
mechanisms	O
is	O
still	O
limited	O
,	O
often	O
hampered	O
by	O
lack	O
of	O
identification	O
of	O
their	O
ligands	O
and	O
/	O
or	O
DNA	O
targets	O
.	O

A	O
potentially	O
interesting	O
exception	O
comes	O
from	O
the	O
ligand	B-protein_state
-	I-protein_state
free	I-protein_state
and	O
salicylate	B-protein_state
-	I-protein_state
bound	I-protein_state
forms	O
of	O
the	O
Methanobacterium	B-species
thermoautotrophicum	I-species
protein	O
MTH313	B-protein
which	O
revealed	O
that	O
two	O
salicylate	B-chemical
molecules	O
bind	O
to	O
one	O
MTH313	B-protein
dimer	B-oligomeric_state
and	O
induce	O
large	O
conformational	O
changes	O
,	O
apparently	O
sufficient	O
to	O
prevent	O
DNA	O
binding	O
.	O

However	O
,	O
it	O
is	O
unknown	O
whether	O
salicylate	B-chemical
is	O
a	O
relevant	O
in	O
vivo	O
ligand	O
of	O
either	O
of	O
these	O
two	O
proteins	O
,	O
which	O
share	O
~	O
20	O
%	O
sequence	O
identity	O
with	O
NadR	B-protein
,	O
rendering	O
unclear	O
the	O
interpretation	O
of	O
these	O
findings	O
in	O
relation	O
to	O
the	O
regulatory	O
mechanisms	O
of	O
NadR	B-protein
or	O
other	O
MarR	B-protein_type
family	O
proteins	O
.	O

NadR	B-protein
binds	O
nadA	B-gene
on	O
three	O
different	O
operators	O
(	O
OpI	O
,	O
OpII	O
and	O
OpIII	O
).	O

4	B-chemical
-	I-chemical
HPA	I-chemical
is	O
a	O
small	O
molecule	O
derived	O
from	O
mammalian	B-taxonomy_domain
aromatic	O
amino	O
acid	O
catabolism	O
and	O
is	O
released	O
in	O
human	B-species
saliva	O
,	O
where	O
it	O
has	O
been	O
detected	O
at	O
micromolar	O
concentration	O
.	O

In	O
the	O
presence	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
NadR	B-protein
is	O
unable	O
to	O
bind	O
the	O
nadA	B-gene
promoter	O
and	O
nadA	B-gene
gene	O
expression	O
is	O
induced	O
.	O

In	O
vivo	O
,	O
the	O
presence	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
in	O
the	O
host	O
niche	O
of	O
N	B-species
.	I-species
meningitidis	I-species
serves	O
as	O
an	O
inducer	O
of	O
NadA	B-protein
production	O
,	O
thereby	O
promoting	O
bacterial	B-taxonomy_domain
adhesion	O
to	O
host	O
cells	O
.	O

Further	O
,	O
we	O
recently	O
reported	O
that	O
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
,	O
produced	O
during	O
inflammation	O
,	O
is	O
another	O
inducer	O
of	O
nadA	B-gene
expression	O
.	O

Extending	O
our	O
previous	O
studies	O
based	O
on	O
hydrogen	B-experimental_method
-	I-experimental_method
deuterium	I-experimental_method
exchange	I-experimental_method
mass	I-experimental_method
spectrometry	I-experimental_method
(	O
HDX	B-experimental_method
-	I-experimental_method
MS	I-experimental_method
),	O
here	O
we	O
sought	O
to	O
reveal	O
the	O
molecular	O
mechanisms	O
and	O
effects	O
of	O
NadR	B-protein
/	O
HPA	B-chemical
interactions	O
via	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
,	O
NMR	B-experimental_method
spectroscopy	I-experimental_method
and	O
complementary	O
biochemical	B-experimental_method
and	I-experimental_method
in	I-experimental_method
vivo	I-experimental_method
mutagenesis	I-experimental_method
studies	I-experimental_method
.	O

Standard	O
chromatographic	O
techniques	O
were	O
used	O
to	O
obtain	O
a	O
highly	O
purified	O
sample	O
of	O
NadR	B-protein
(	O
see	O
Materials	O
and	O
Methods	O
).	O

These	O
data	O
showed	O
that	O
NadR	B-protein
was	O
dimeric	B-oligomeric_state
in	O
solution	O
,	O
since	O
the	O
theoretical	O
molecular	O
mass	O
of	O
the	O
NadR	B-protein
dimer	B-oligomeric_state
is	O
33	O
.	O
73	O
kDa	O
;	O
and	O
,	O
there	O
was	O
no	O
change	O
in	O
oligomeric	O
state	O
on	O
addition	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
.	O

The	O
thermal	O
stability	O
of	O
NadR	B-protein
was	O
examined	O
using	O
differential	B-experimental_method
scanning	I-experimental_method
calorimetry	I-experimental_method
(	O
DSC	B-experimental_method
).	O

The	O
Tm	B-evidence
of	O
NadR	B-protein
was	O
67	O
.	O
4	O
±	O
0	O
.	O
1	O
°	O
C	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
ligand	I-protein_state
,	O
and	O
was	O
unaffected	O
by	O
salicylate	B-chemical
.	O

However	O
,	O
an	O
increased	O
thermal	O
stability	O
was	O
induced	O
by	O
4	B-chemical
-	I-chemical
HPA	I-chemical
and	O
,	O
to	O
a	O
lesser	O
extent	O
,	O
by	O
3	B-chemical
-	I-chemical
HPA	I-chemical
.	O

Interestingly	O
,	O
NadR	B-protein
displayed	O
the	O
greatest	O
Tm	B-evidence
increase	O
upon	O
addition	O
of	O
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
(	O
Table	O
1	O
and	O
Fig	O
1B	O
).	O

Stability	O
of	O
NadR	B-protein
is	O
increased	O
by	O
small	O
molecule	O
ligands	O
.	O

Melting	B-evidence
-	I-evidence
point	I-evidence
(	O
Tm	B-evidence
)	O
and	O
its	O
ligand	O
-	O
induced	O
increase	O
(	O
ΔTm	B-evidence
)	O
derived	O
from	O
DSC	B-experimental_method
thermostability	B-experimental_method
experiments	I-experimental_method
.	O

Dissociation	B-evidence
constants	I-evidence
(	O
KD	B-evidence
)	O
of	O
the	O
NadR	B-protein
/	O
ligand	O
interactions	O
from	O
SPR	B-experimental_method
steady	I-experimental_method
-	I-experimental_method
state	I-experimental_method
binding	I-experimental_method
experiments	I-experimental_method
.	O

Ligand	O
Tm	B-evidence
(°	O
C	O
)	O
ΔTm	B-evidence
(°	O
C	O
)	O
KD	B-evidence
(	O
mM	O
)	O
No	O
ligand	O
67	O
.	O
4	O
±	O
0	O
.	O
1	O
n	O
.	O
a	O
.	O
n	O
.	O
a	O
.	O

NadR	B-protein
displays	O
distinct	O
binding	B-evidence
affinities	I-evidence
for	O
hydroxyphenylacetate	B-chemical
ligands	O

The	O
SPR	B-experimental_method
sensorgrams	B-evidence
revealed	O
very	O
fast	O
association	O
and	O
dissociation	O
events	O
,	O
typical	O
of	O
small	O
molecule	O
ligands	O
,	O
thus	O
prohibiting	O
a	O
detailed	O
study	O
of	O
binding	O
kinetics	O
.	O

3	B-chemical
-	I-chemical
HPA	I-chemical
showed	O
a	O
weaker	O
interaction	O
,	O
with	O
a	O
KD	B-evidence
of	O
2	O
.	O
7	O
mM	O
,	O
while	O
salicylate	B-chemical
showed	O
only	O
a	O
very	O
weak	O
response	O
that	O
did	O
not	O
reach	O
saturation	O
,	O
indicating	O
a	O
non	O
-	O
specific	O
interaction	O
with	O
NadR	B-protein
.	O
A	O
ranking	O
of	O
these	O
KD	B-evidence
values	O
showed	O
that	O
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
was	O
the	O
tightest	O
binder	O
,	O
and	O
thus	O
matched	O
the	O
ranking	O
of	O
ligand	O
-	O
induced	O
Tm	B-evidence
increases	O
observed	O
in	O
the	O
DSC	B-experimental_method
experiments	O
.	O

Although	O
these	O
KD	B-evidence
values	O
indicate	O
rather	O
weak	O
interactions	O
,	O
they	O
are	O
similar	O
to	O
the	O
values	O
reported	O
previously	O
for	O
the	O
MarR	B-protein_type
/	O
salicylate	B-chemical
interaction	O
(	O
KD	O
~	O
1	O
mM	O
)	O
and	O
the	O
MTH313	B-protein
/	O
salicylate	B-chemical
interaction	O
(	O
KD	O
2	O
–	O
3	O
mM	O
),	O
and	O
approximately	O
20	O
-	O
fold	O
tighter	O
than	O
the	O
ST1710	B-protein
/	O
salicylate	B-chemical
interaction	O
(	O
KD	O
~	O
20	O
mM	O
).	O

Crystal	B-evidence
structures	I-evidence
of	O
holo	B-protein_state
-	O
NadR	B-protein
and	O
apo	B-protein_state
-	O
NadR	B-protein

First	O
,	O
we	O
crystallized	B-experimental_method
NadR	B-protein
(	O
a	O
selenomethionine	B-experimental_method
-	I-experimental_method
labelled	I-experimental_method
derivative	I-experimental_method
)	O
in	O
the	O
presence	O
of	O
a	O
200	O
-	O
fold	O
molar	O
excess	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
.	O

The	O
structure	B-evidence
of	O
the	O
NadR	B-complex_assembly
/	I-complex_assembly
4	I-complex_assembly
-	I-complex_assembly
HPA	I-complex_assembly
complex	O
was	O
determined	O
at	O
2	O
.	O
3	O
Å	O
resolution	O
using	O
a	O
combination	O
of	O
the	O
single	B-experimental_method
-	I-experimental_method
wavelength	I-experimental_method
anomalous	I-experimental_method
dispersion	I-experimental_method
(	O
SAD	B-experimental_method
)	O
and	O
molecular	B-experimental_method
replacement	I-experimental_method
(	O
MR	B-experimental_method
)	O
methods	O
,	O
and	O
was	O
refined	O
to	O
R	B-evidence
work	I-evidence
/	I-evidence
R	I-evidence
free	I-evidence
values	O
of	O
20	O
.	O
9	O
/	O
26	O
.	O
0	O
%	O
(	O
Table	O
2	O
).	O

Despite	O
numerous	O
attempts	O
,	O
we	O
were	O
unable	O
to	O
obtain	O
high	O
-	O
quality	O
crystals	B-evidence
of	O
NadR	B-protein
complexed	B-protein_state
with	I-protein_state
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
,	O
3	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
,	O
3	B-chemical
-	I-chemical
HPA	I-chemical
or	O
DNA	O
targets	O
.	O

However	O
,	O
it	O
was	O
eventually	O
possible	O
to	O
crystallize	B-experimental_method
apo	B-protein_state
-	O
NadR	B-protein
,	O
and	O
the	O
structure	B-evidence
was	O
determined	O
at	O
2	O
.	O
7	O
Å	O
resolution	O
by	O
MR	B-experimental_method
methods	O
using	O
the	O
NadR	B-complex_assembly
/	I-complex_assembly
4	I-complex_assembly
-	I-complex_assembly
HPA	I-complex_assembly
complex	O
as	O
the	O
search	O
model	O
.	O

The	O
apo	B-protein_state
-	O
NadR	B-protein
structure	B-evidence
was	O
refined	O
to	O
R	B-evidence
work	I-evidence
/	I-evidence
R	I-evidence
free	I-evidence
values	O
of	O
19	O
.	O
1	O
/	O
26	O
.	O
8	O
%	O
(	O
Table	O
2	O
).	O

The	O
asymmetric	O
unit	O
of	O
the	O
NadR	B-complex_assembly
/	I-complex_assembly
4	I-complex_assembly
-	I-complex_assembly
HPA	I-complex_assembly
crystals	B-evidence
(	O
holo	B-protein_state
-	O
NadR	B-protein
)	O
contained	O
one	O
NadR	B-protein
homodimer	B-oligomeric_state
,	O
while	O
the	O
apo	B-protein_state
-	O
NadR	B-protein
crystals	B-evidence
contained	O
two	O
homodimers	B-oligomeric_state
.	O

Moreover	O
,	O
our	O
SE	B-experimental_method
-	I-experimental_method
HPLC	I-experimental_method
/	I-experimental_method
MALLS	I-experimental_method
analyses	O
(	O
see	O
above	O
)	O
revealed	O
that	O
in	O
solution	O
NadR	B-protein
is	O
dimeric	B-oligomeric_state
,	O
and	O
previous	O
studies	O
using	O
native	B-experimental_method
mass	I-experimental_method
spectrometry	I-experimental_method
(	O
MS	B-experimental_method
)	O
revealed	O
dimers	B-oligomeric_state
,	O
not	O
tetramers	B-oligomeric_state
.	O

The	O
NadR	B-protein
homodimer	B-oligomeric_state
bound	B-protein_state
to	I-protein_state
4	B-chemical
-	I-chemical
HPA	I-chemical
has	O
a	O
dimerization	B-site
interface	I-site
mostly	O
involving	O
the	O
top	O
of	O
its	O
‘	O
triangular	B-protein_state
’	O
form	O
,	O
while	O
the	O
two	O
DNA	B-structure_element
-	I-structure_element
binding	I-structure_element
domains	I-structure_element
are	O
located	O
at	O
the	O
base	O
(	O
Fig	O
2A	O
).	O

High	O
-	O
quality	O
electron	B-evidence
density	I-evidence
maps	I-evidence
allowed	O
clear	O
identification	O
of	O
the	O
bound	B-protein_state
ligand	O
,	O
4	B-chemical
-	I-chemical
HPA	I-chemical
(	O
Fig	O
2B	O
).	O

The	O
overall	O
structure	B-evidence
of	O
NadR	B-protein
shows	O
dimensions	O
of	O
~	O
50	O
×	O
65	O
×	O
50	O
Å	O
and	O
a	O
large	O
homodimer	B-site
interface	I-site
that	O
buries	O
a	O
total	O
surface	O
area	O
of	O
~	O
4800	O
Å2	O
.	O

Each	O
NadR	B-protein
monomer	B-oligomeric_state
consists	O
of	O
six	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
and	O
two	O
short	B-structure_element
β	I-structure_element
-	I-structure_element
strands	I-structure_element
,	O
with	O
helices	B-structure_element
α1	B-structure_element
,	O
α5	B-structure_element
,	O
and	O
α6	B-structure_element
forming	O
the	O
dimer	B-site
interface	I-site
.	O

Together	O
,	O
these	O
structural	O
elements	O
constitute	O
the	O
winged	B-structure_element
helix	I-structure_element
-	I-structure_element
turn	I-structure_element
-	I-structure_element
helix	I-structure_element
(	O
wHTH	B-structure_element
)	O
DNA	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
and	O
,	O
together	O
with	O
the	O
dimeric	B-oligomeric_state
organization	O
,	O
are	O
the	O
hallmarks	O
of	O
MarR	B-protein_type
family	O
structures	B-evidence
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
NadR	B-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
4	B-chemical
-	I-chemical
HPA	I-chemical
.	O

(	O
A	O
)	O
The	O
holo	B-protein_state
-	O
NadR	B-protein
homodimer	B-oligomeric_state
is	O
depicted	O
in	O
green	O
and	O
blue	O
for	O
chains	B-structure_element
A	I-structure_element
and	I-structure_element
B	I-structure_element
respectively	O
,	O
while	O
yellow	O
sticks	O
depict	O
the	O
4	B-chemical
-	I-chemical
HPA	I-chemical
ligand	O
(	O
labelled	O
).	O

For	O
simplicity	O
,	O
secondary	O
structure	O
elements	O
are	O
labelled	O
for	O
chain	B-structure_element
B	I-structure_element
only	O
.	O

Red	O
dashes	O
show	O
hypothetical	O
positions	O
of	O
chain	B-structure_element
B	I-structure_element
residues	O
88	B-residue_range
–	I-residue_range
90	I-residue_range
that	O
were	O
not	O
modeled	O
due	O
to	O
lack	O
of	O
electron	B-evidence
density	I-evidence
.	O

(	O
B	O
)	O
A	O
zoom	O
into	O
the	O
pocket	B-site
occupied	O
by	O
4	B-chemical
-	I-chemical
HPA	I-chemical
shows	O
that	O
the	O
ligand	O
contacts	O
both	O
chains	B-structure_element
A	I-structure_element
and	I-structure_element
B	I-structure_element
;	O
blue	O
mesh	O
shows	O
electron	B-evidence
density	I-evidence
around	O
4	B-chemical
-	I-chemical
HPA	I-chemical
calculated	O
from	O
a	O
composite	B-evidence
omit	I-evidence
map	I-evidence
(	O
omitting	O
4	B-chemical
-	I-chemical
HPA	I-chemical
),	O
using	O
phenix	B-experimental_method
.	O

A	O
single	O
conserved	B-protein_state
leucine	B-residue_name
residue	O
(	O
L130	B-residue_name_number
)	O
is	O
crucial	O
for	O
dimerization	O

The	O
NadR	B-protein
dimer	B-site
interface	I-site
is	O
formed	O
by	O
at	O
least	O
32	O
residues	O
,	O
which	O
establish	O
numerous	O
inter	O
-	O
chain	O
salt	B-bond_interaction
bridges	I-bond_interaction
or	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
,	O
and	O
many	O
hydrophobic	B-bond_interaction
packing	I-bond_interaction
interactions	I-bond_interaction
(	O
Fig	O
3A	O
and	O
3B	O
).	O

To	O
determine	O
which	O
residues	O
were	O
most	O
important	O
for	O
dimerization	O
,	O
we	O
studied	O
the	O
interface	B-site
in	O
silico	O
and	O
identified	O
several	O
residues	O
as	O
potential	O
mediators	O
of	O
key	O
stabilizing	O
interactions	O
.	O

Each	O
mutant	B-protein_state
NadR	B-protein
protein	O
was	O
purified	O
,	O
and	O
then	O
its	O
oligomeric	O
state	O
was	O
examined	O
by	O
analytical	B-experimental_method
SE	I-experimental_method
-	I-experimental_method
HPLC	I-experimental_method
.	O

Almost	O
all	O
the	O
mutants	O
showed	O
the	O
same	O
elution	O
profile	O
as	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
WT	B-protein_state
)	O
NadR	B-protein
protein	O
.	O

Only	O
the	O
L130K	B-mutant
mutation	O
induced	O
a	O
notable	O
change	O
in	O
the	O
oligomeric	O
state	O
of	O
NadR	B-protein
(	O
Fig	O
3C	O
).	O

Further	O
,	O
in	O
SE	B-experimental_method
-	I-experimental_method
MALLS	I-experimental_method
analyses	O
,	O
the	O
L130K	B-mutant
mutant	B-protein_state
displayed	O
two	O
distinct	O
species	O
in	O
solution	O
,	O
approximately	O
80	O
%	O
being	O
monomeric	B-oligomeric_state
(	O
a	O
19	O
kDa	O
species	O
),	O
and	O
only	O
20	O
%	O
retaining	O
the	O
typical	O
native	O
dimeric	B-oligomeric_state
state	O
(	O
a	O
35	O
kDa	O
species	O
)	O
(	O
Fig	O
3D	O
),	O
demonstrating	O
that	O
Leu130	B-residue_name_number
is	O
crucial	O
for	O
stable	O
dimerization	O
.	O

In	O
contrast	O
,	O
most	O
of	O
the	O
other	O
residues	O
identified	O
in	O
the	O
NadR	B-protein
dimer	B-site
interface	I-site
were	O
poorly	B-protein_state
conserved	I-protein_state
in	O
the	O
MarR	B-protein_type
family	O
.	O

Analysis	O
of	O
the	O
NadR	B-protein
dimer	B-site
interface	I-site
.	O

(	O
A	O
)	O
Both	O
orientations	O
show	O
chain	B-structure_element
A	I-structure_element
,	O
green	O
backbone	O
ribbon	O
,	O
colored	O
red	O
to	O
highlight	O
all	O
locations	O
involved	O
in	O
dimerization	O
;	O
namely	O
,	O
inter	O
-	O
chain	O
salt	B-bond_interaction
bridges	I-bond_interaction
or	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
involving	O
Q4	B-residue_name_number
,	O
S5	B-residue_name_number
,	O
K6	B-residue_name_number
,	O
H7	B-residue_name_number
,	O
S9	B-residue_name_number
,	O
I10	B-residue_name_number
,	O
N11	B-residue_name_number
,	O
I15	B-residue_name_number
,	O
Q16	B-residue_name_number
,	O
R18	B-residue_name_number
,	O
D36	B-residue_name_number
,	O
R43	B-residue_name_number
,	O
A46	B-residue_name_number
,	O
Q59	B-residue_name_number
,	O
C61	B-residue_name_number
,	O
Y104	B-residue_name_number
,	O
D112	B-residue_name_number
,	O
R114	B-residue_name_number
,	O
Y115	B-residue_name_number
,	O
D116	B-residue_name_number
,	O
E119	B-residue_name_number
,	O
K126	B-residue_name_number
,	O
E136	B-residue_name_number
,	O
E141	B-residue_name_number
,	O
N145	B-residue_name_number
,	O
and	O
the	O
hydrophobic	B-bond_interaction
packing	I-bond_interaction
interactions	I-bond_interaction
involving	O
I10	B-residue_name_number
,	O
I12	B-residue_name_number
,	O
L14	B-residue_name_number
,	O
I15	B-residue_name_number
,	O
R18	B-residue_name_number
,	O
Y115	B-residue_name_number
,	O
I118	B-residue_name_number
,	O
L130	B-residue_name_number
,	O
L133	B-residue_name_number
,	O
L134	B-residue_name_number
and	O
L137	B-residue_name_number
.	O

Chain	B-structure_element
B	I-structure_element
,	O
grey	O
surface	O
,	O
is	O
marked	O
blue	O
to	O
highlight	O
residues	O
probed	O
by	O
site	B-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
(	O
E136	B-residue_name_number
only	O
makes	O
a	O
salt	B-bond_interaction
bridge	I-bond_interaction
with	O
K126	B-residue_name_number
,	O
therefore	O
it	O
was	O
sufficient	O
to	O
make	O
the	O
K126A	B-mutant
mutation	O
to	O
assess	O
the	O
importance	O
of	O
this	O
ionic	B-bond_interaction
interaction	I-bond_interaction
;	O
the	O
H7	B-residue_name_number
position	O
is	O
labelled	O
for	O
monomer	B-oligomeric_state
A	B-structure_element
,	O
since	O
electron	B-evidence
density	I-evidence
was	O
lacking	O
for	O
monomer	B-oligomeric_state
B	B-structure_element
).	O
(	O
B	O
)	O
A	O
zoom	O
into	O
the	O
environment	O
of	O
helix	B-structure_element
α6	B-structure_element
to	O
show	O
how	O
residue	O
L130	B-residue_name_number
chain	B-structure_element
B	I-structure_element
(	O
blue	O
side	O
chain	O
)	O
is	O
a	O
focus	O
of	O
hydrophobic	B-bond_interaction
packing	I-bond_interaction
interactions	I-bond_interaction
with	O
L130	B-residue_name_number
,	O
L133	B-residue_name_number
,	O
L134	B-residue_name_number
and	O
L137	B-residue_name_number
of	O
chain	B-structure_element
A	I-structure_element
(	O
red	O
side	O
chains	O
).	O

(	O
C	O
)	O
SE	B-experimental_method
-	I-experimental_method
HPLC	I-experimental_method
analyses	O
of	O
all	O
mutant	B-protein_state
forms	O
of	O
NadR	B-protein
are	O
compared	O
with	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
WT	B-protein_state
)	O
protein	O
.	O

The	O
WT	B-protein_state
and	O
most	O
of	O
the	O
mutants	O
show	O
a	O
single	O
elution	O
peak	O
with	O
an	O
absorbance	O
maximum	O
at	O
17	O
.	O
5	O
min	O
.	O

The	O
holo	B-protein_state
-	O
NadR	B-protein
structure	B-evidence
presents	O
only	O
one	O
occupied	O
ligand	B-site
-	I-site
binding	I-site
pocket	I-site

The	O
tunnel	B-site
was	O
lined	O
with	O
rather	O
hydrophobic	O
amino	O
acids	O
,	O
and	O
did	O
not	O
contain	O
water	B-chemical
molecules	O
.	O

Unexpectedly	O
,	O
only	O
one	O
monomer	B-oligomeric_state
of	O
the	O
holo	B-protein_state
-	O
NadR	B-protein
homodimer	B-oligomeric_state
contained	O
4	B-chemical
-	I-chemical
HPA	I-chemical
in	O
the	O
binding	B-site
pocket	I-site
,	O
whereas	O
the	O
corresponding	O
pocket	B-site
of	O
the	O
other	O
monomer	B-oligomeric_state
was	O
unoccupied	O
by	O
ligand	O
,	O
despite	O
the	O
large	O
excess	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
used	O
in	O
the	O
crystallization	O
conditions	O
.	O

Inspection	O
of	O
the	O
protein	B-site
-	I-site
ligand	I-site
interaction	I-site
network	I-site
revealed	O
no	O
bonds	O
from	O
NadR	B-protein
backbone	O
groups	O
to	O
the	O
ligand	O
,	O
but	O
several	O
key	O
side	O
chain	O
mediated	O
hydrogen	B-bond_interaction
(	I-bond_interaction
H	I-bond_interaction
)-	I-bond_interaction
bonds	I-bond_interaction
and	O
ionic	B-bond_interaction
interactions	I-bond_interaction
,	O
most	O
notably	O
between	O
the	O
carboxylate	O
group	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
and	O
Ser9	B-residue_name_number
of	O
chain	B-structure_element
A	I-structure_element
(	O
SerA9	B-residue_name_number
),	O
and	O
chain	B-structure_element
B	I-structure_element
residues	O
TrpB39	B-residue_name_number
,	O
ArgB43	B-residue_name_number
and	O
TyrB115	B-residue_name_number
(	O
Fig	O
4A	O
).	O

Atomic	O
details	O
of	O
NadR	B-protein
/	O
HPA	B-chemical
interactions	O
.	O

A	O
)	O
A	O
stereo	O
-	O
view	O
zoom	O
into	O
the	O
binding	B-site
pocket	I-site
showing	O
side	O
chain	O
sticks	O
for	O
all	O
interactions	O
between	O
NadR	B-protein
and	O
4	B-chemical
-	I-chemical
HPA	I-chemical
.	O

4	B-chemical
-	I-chemical
HPA	I-chemical
is	O
shown	O
in	O
yellow	O
sticks	O
,	O
with	O
oxygen	O
atoms	O
in	O
red	O
.	O

A	O
water	B-chemical
molecule	O
is	O
shown	O
by	O
the	O
red	O
sphere	O
.	O

H	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
up	O
to	O
3	O
.	O
6Å	O
are	O
shown	O
as	O
dashed	O
lines	O
.	O

The	O
entire	O
set	O
of	O
residues	O
making	O
H	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
or	O
non	B-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
contacts	I-bond_interaction
with	O
4	B-chemical
-	I-chemical
HPA	I-chemical
is	O
as	O
follows	O
:	O
SerA9	B-residue_name_number
,	O
AsnA11	B-residue_name_number
,	O
LeuB21	B-residue_name_number
,	O
MetB22	B-residue_name_number
,	O
PheB25	B-residue_name_number
,	O
LeuB29	B-residue_name_number
,	O
AspB36	B-residue_name_number
,	O
TrpB39	B-residue_name_number
,	O
ArgB43	B-residue_name_number
,	O
ValB111	B-residue_name_number
and	O
TyrB115	B-residue_name_number
(	O
automated	O
analysis	O
performed	O
using	O
PDBsum	B-experimental_method
and	O
verified	O
manually	O
).	O

Side	O
chains	O
mediating	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
are	O
shown	O
in	O
orange	O
.	O
(	O
B	O
)	O
A	O
model	O
was	O
prepared	O
to	O
visualize	O
putative	O
interactions	O
of	O
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
(	O
pink	O
)	O
with	O
NadR	B-protein
,	O
revealing	O
the	O
potential	O
for	O
additional	O
contacts	O
(	O
dashed	O
lines	O
)	O
of	O
the	O
chloro	O
moiety	O
(	O
green	O
stick	O
)	O
with	O
LeuB29	B-residue_name_number
and	O
AspB36	B-residue_name_number
.	O

Notably	O
,	O
the	O
phenyl	O
ring	O
of	O
PheB25	B-residue_name_number
was	O
positioned	O
parallel	O
to	O
the	O
phenyl	O
ring	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
potentially	O
forming	O
π	B-bond_interaction
-	I-bond_interaction
π	I-bond_interaction
parallel	I-bond_interaction
-	I-bond_interaction
displaced	I-bond_interaction
stacking	I-bond_interaction
interactions	I-bond_interaction
.	O

Consequently	O
,	O
residues	O
in	O
the	O
4	B-site
-	I-site
HPA	I-site
binding	I-site
pocket	I-site
are	O
mostly	O
contributed	O
by	O
NadR	B-protein
chain	B-structure_element
B	I-structure_element
,	O
and	O
effectively	O
created	O
a	O
polar	O
‘	O
floor	O
’	O
and	O
a	O
hydrophobic	O
‘	O
ceiling	O
’,	O
which	O
house	O
the	O
ligand	O
.	O

Collectively	O
,	O
this	O
mixed	O
network	O
of	O
polar	B-bond_interaction
and	I-bond_interaction
hydrophobic	I-bond_interaction
interactions	I-bond_interaction
endows	O
NadR	B-protein
with	O
a	O
strong	O
recognition	O
pattern	O
for	O
HPAs	B-chemical
,	O
with	O
additional	O
medium	O
-	O
range	O
interactions	O
potentially	O
established	O
with	O
the	O
hydroxyl	O
group	O
at	O
the	O
4	O
-	O
position	O
.	O

Structure	O
-	O
activity	O
relationships	O
:	O
molecular	O
basis	O
of	O
enhanced	O
stabilization	O
by	O
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical

We	O
modelled	B-experimental_method
the	O
binding	O
of	O
other	O
HPAs	B-chemical
by	O
in	B-experimental_method
silico	I-experimental_method
superposition	I-experimental_method
onto	O
4	B-chemical
-	I-chemical
HPA	I-chemical
in	O
the	O
holo	B-protein_state
-	O
NadR	B-protein
structure	B-evidence
,	O
and	O
thereby	O
obtained	O
molecular	O
explanations	O
for	O
the	O
binding	O
specificities	O
of	O
diverse	O
ligands	O
.	O

For	O
example	O
,	O
similar	O
to	O
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
the	O
binding	O
of	O
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
could	O
involve	O
multiple	O
bonds	O
towards	O
the	O
carboxylate	O
group	O
of	O
the	O
ligand	O
and	O
some	O
to	O
the	O
4	O
-	O
hydroxyl	O
group	O
.	O

Additionally	O
,	O
the	O
side	O
chains	O
of	O
LeuB29	B-residue_name_number
and	O
AspB36	B-residue_name_number
would	O
be	O
only	O
2	O
.	O
6	O
–	O
3	O
.	O
5	O
Å	O
from	O
the	O
chlorine	O
atom	O
,	O
thus	O
providing	O
van	B-bond_interaction
der	I-bond_interaction
Waals	I-bond_interaction
’	I-bond_interaction
interactions	I-bond_interaction
or	O
H	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
to	O
generate	O
the	O
additional	O
binding	B-evidence
affinity	I-evidence
observed	O
for	O
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
(	O
Fig	O
4B	O
).	O

Finally	O
,	O
salicylate	B-chemical
is	O
presumably	O
unable	O
to	O
specifically	O
bind	O
NadR	B-protein
due	O
to	O
the	O
2	O
-	O
hydroxyl	O
substitution	O
and	O
the	O
shorter	O
aliphatic	O
chain	O
connecting	O
its	O
carboxylate	O
group	O
(	O
Fig	O
1A	O
):	O
the	O
compound	O
simply	O
seems	O
too	O
small	O
to	O
simultaneously	O
establish	O
the	O
network	O
of	O
beneficial	O
bonds	O
observed	O
in	O
the	O
NadR	B-protein
/	O
HPA	B-chemical
interactions	O
.	O

Analysis	O
of	O
the	O
pockets	B-site
reveals	O
the	O
molecular	O
basis	O
for	O
asymmetric	O
binding	O
and	O
stoichiometry	O

However	O
,	O
studies	O
based	O
on	O
tryptophan	B-experimental_method
fluorescence	I-experimental_method
were	O
confounded	O
by	O
the	O
fluorescence	O
of	O
the	O
HPA	B-chemical
ligands	O
,	O
and	O
isothermal	B-experimental_method
titration	I-experimental_method
calorimetry	I-experimental_method
(	O
ITC	B-experimental_method
)	O
was	O
unfeasible	O
due	O
to	O
the	O
need	O
for	O
very	O
high	O
concentrations	O
of	O
NadR	B-protein
in	O
the	O
ITC	B-experimental_method
chamber	O
(	O
due	O
to	O
the	O
relatively	O
low	O
affinity	O
),	O
which	O
exceeded	O
the	O
solubility	O
limits	O
of	O
the	O
protein	O
.	O

However	O
,	O
it	O
was	O
possible	O
to	O
calculate	O
the	O
binding	B-evidence
stoichiometry	I-evidence
of	O
the	O
NadR	B-complex_assembly
-	I-complex_assembly
HPA	I-complex_assembly
interactions	O
using	O
an	O
SPR	B-experimental_method
-	O
based	O
approach	O
.	O

This	O
approach	O
relies	O
on	O
the	O
assumption	O
that	O
the	O
captured	O
protein	O
(‘	O
the	O
ligand	O
’,	O
according	O
to	O
SPR	B-experimental_method
conventions	O
)	O
is	O
100	O
%	O
active	O
and	O
freely	O
-	O
accessible	O
to	O
potential	O
interactors	O
(‘	O
the	O
analytes	O
’).	O

Overall	O
,	O
the	O
superposition	B-experimental_method
revealed	O
a	O
high	O
degree	O
of	O
structural	O
similarity	O
(	O
Cα	O
root	B-evidence
mean	I-evidence
square	I-evidence
deviation	I-evidence
(	O
rmsd	B-evidence
)	O
of	O
1	O
.	O
5Å	O
),	O
though	O
on	O
closer	O
inspection	O
a	O
rotational	O
difference	O
of	O
~	O
9	O
degrees	O
along	O
the	O
long	O
axis	O
of	O
helix	B-structure_element
α6	B-structure_element
was	O
observed	O
,	O
suggesting	O
that	O
4	B-chemical
-	I-chemical
HPA	I-chemical
induced	O
a	O
slight	O
conformational	O
change	O
(	O
Fig	O
5A	O
).	O

Most	O
notably	O
,	O
atomic	O
clashes	O
between	O
the	O
ligand	O
and	O
the	O
side	O
chains	O
of	O
MetA22	B-residue_name_number
,	O
PheA25	B-residue_name_number
and	O
ArgA43	B-residue_name_number
would	O
occur	O
if	O
4	B-chemical
-	I-chemical
HPA	I-chemical
were	O
present	O
in	O
the	O
monomer	B-oligomeric_state
A	B-structure_element
pocket	B-site
(	O
Fig	O
5B	O
).	O

Subsequently	O
,	O
analyses	O
of	O
the	O
pockets	B-site
in	O
apo	B-protein_state
-	O
NadR	B-protein
revealed	O
that	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
ligand	I-protein_state
the	O
long	O
Arg43	B-residue_name_number
side	O
chain	O
was	O
always	O
in	O
the	O
open	O
‘	O
outward	B-protein_state
’	O
position	O
compatible	O
with	O
binding	O
to	O
the	O
4	B-chemical
-	I-chemical
HPA	I-chemical
carboxylate	O
group	O
.	O

Structural	O
differences	O
of	O
NadR	B-protein
in	O
ligand	B-protein_state
-	I-protein_state
bound	I-protein_state
or	O
free	B-protein_state
forms	O
.	O

(	O
A	O
)	O
Aligned	B-experimental_method
monomers	B-oligomeric_state
of	O
holo	B-protein_state
-	O
NadR	B-protein
(	O
chain	B-structure_element
A	I-structure_element
:	O
green	O
;	O
chain	B-structure_element
B	I-structure_element
:	O
blue	O
),	O
reveal	O
major	O
overall	O
differences	O
by	O
the	O
shift	O
of	O
helix	B-structure_element
α6	B-structure_element
.	O
(	O
B	O
)	O
Comparison	B-experimental_method
of	O
the	O
two	O
binding	B-site
pockets	I-site
in	O
holo	B-protein_state
-	O
NadR	B-protein
shows	O
that	O
in	O
the	O
ligand	B-protein_state
-	I-protein_state
free	I-protein_state
monomer	B-oligomeric_state
A	B-structure_element
(	O
green	O
)	O
residues	O
Met22	B-residue_name_number
,	O
Phe25	B-residue_name_number
and	O
Arg43	B-residue_name_number
adopt	O
‘	O
inward	B-protein_state
’	O
positions	O
(	O
highlighted	O
by	O
arrows	O
)	O
compared	O
to	O
the	O
ligand	B-protein_state
-	I-protein_state
occupied	I-protein_state
pocket	B-site
(	O
blue	O
residues	O
);	O
these	O
‘	O
inward	B-protein_state
’	O
conformations	O
appear	O
unfavorable	O
for	O
binding	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
due	O
to	O
clashes	O
with	O
the	O
4	O
-	O
hydroxyl	O
group	O
,	O
the	O
phenyl	O
ring	O
and	O
the	O
carboxylate	O
group	O
,	O
respectively	O
.	O

In	O
these	O
crystals	B-evidence
,	O
the	O
ArgA43	B-residue_name_number
side	O
chain	O
showed	O
two	O
alternate	O
conformations	O
,	O
modelled	O
with	O
50	O
%	O
occupancy	O
in	O
each	O
state	O
,	O
as	O
indicated	O
by	O
the	O
two	O
‘	O
mirrored	O
’	O
arrows	O
.	O

Finally	O
,	O
we	O
applied	O
15N	B-experimental_method
heteronuclear	I-experimental_method
solution	I-experimental_method
NMR	I-experimental_method
spectroscopy	I-experimental_method
to	O
examine	O
the	O
interaction	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
with	O
apo	B-protein_state
NadR	B-protein
.	O
We	O
collected	O
NMR	B-experimental_method
spectra	B-evidence
on	O
NadR	B-protein
in	B-protein_state
the	I-protein_state
presence	I-protein_state
and	O
absence	B-protein_state
of	I-protein_state
4	B-chemical
-	I-chemical
HPA	I-chemical
(	O
see	O
Materials	O
and	O
Methods	O
).	O

The	O
1H	B-experimental_method
-	I-experimental_method
15N	I-experimental_method
TROSY	I-experimental_method
-	I-experimental_method
HSQC	I-experimental_method
spectrum	B-evidence
of	O
apo	B-protein_state
-	O
NadR	B-protein
,	O
acquired	O
at	O
25	O
°	O
C	O
,	O
displayed	O
approximately	O
140	O
distinct	O
peaks	O
(	O
Fig	O
6A	O
),	O
most	O
of	O
which	O
correspond	O
to	O
backbone	O
amide	O
N	O
-	O
H	O
groups	O
.	O

Upon	O
the	O
addition	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
over	O
45	O
peaks	O
showed	O
chemical	O
shift	O
perturbations	O
,	O
i	O
.	O
e	O
.	O
changed	O
position	O
in	O
the	O
spectrum	O
or	O
disappeared	O
,	O
while	O
the	O
remaining	O
peaks	O
remained	O
unchanged	O
.	O

This	O
observation	O
showed	O
that	O
4	B-chemical
-	I-chemical
HPA	I-chemical
was	O
able	O
to	O
bind	O
NadR	B-protein
and	O
induce	O
notable	O
changes	O
in	O
specific	O
regions	O
of	O
the	O
protein	O
.	O

NMR	B-experimental_method
spectra	B-evidence
of	O
NadR	B-protein
in	B-protein_state
the	I-protein_state
presence	I-protein_state
and	O
absence	B-protein_state
of	I-protein_state
4	B-chemical
-	I-chemical
HPA	I-chemical
.	O

(	O
A	O
)	O
Superposition	B-experimental_method
of	O
two	O
1H	B-experimental_method
-	I-experimental_method
15N	I-experimental_method
TROSY	I-experimental_method
-	I-experimental_method
HSQC	I-experimental_method
spectra	B-evidence
recorded	O
at	O
25	O
°	O
C	O
on	O
apo	B-protein_state
-	O
NadR	B-protein
(	O
cyan	O
)	O
and	O
on	O
NadR	B-protein
in	O
the	O
presence	B-protein_state
of	I-protein_state
4	B-chemical
-	I-chemical
HPA	I-chemical
(	O
red	O
).	O

The	O
spectra	B-evidence
acquired	O
at	O
10	O
°	O
C	O
are	O
excluded	O
from	O
panel	O
A	O
for	O
simplicity	O
.	O

However	O
,	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
the	O
1H	B-experimental_method
-	I-experimental_method
15N	I-experimental_method
TROSY	I-experimental_method
-	I-experimental_method
HSQC	I-experimental_method
spectrum	B-evidence
of	O
NadR	B-protein
displayed	O
approximately	O
140	O
peaks	O
,	O
as	O
for	O
apo	B-protein_state
-	O
NadR	B-protein
,	O
i	O
.	O
e	O
.	O
two	O
distinct	O
stable	O
conformations	O
(	O
that	O
might	O
have	O
potentially	O
revealed	O
the	O
molecular	O
asymmetry	O
observed	O
crystallographically	B-experimental_method
)	O
were	O
not	O
notable	O
.	O

These	O
doubled	O
peaks	O
may	O
therefore	O
reveal	O
that	O
the	O
cooler	O
temperature	O
partially	O
trapped	O
the	O
existence	O
in	O
solution	O
of	O
two	O
distinct	O
states	O
,	O
in	O
presence	B-protein_state
or	O
absence	B-protein_state
of	I-protein_state
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
with	O
minor	O
conformational	O
differences	O
occurring	O
at	O
least	O
in	O
proximity	O
to	O
the	O
binding	B-site
pocket	I-site
.	O

Apo	B-protein_state
-	O
NadR	B-protein
structures	B-evidence
reveal	O
intrinsic	O
conformational	O
flexibility	O

The	O
apo	B-protein_state
-	O
NadR	B-protein
crystal	B-evidence
structure	I-evidence
contained	O
two	O
homodimers	B-oligomeric_state
in	O
the	O
asymmetric	O
unit	O
(	O
chains	B-structure_element
A	I-structure_element
+	I-structure_element
B	I-structure_element
and	O
chains	B-structure_element
C	I-structure_element
+	I-structure_element
D	I-structure_element
).	O

Upon	O
overall	O
structural	B-experimental_method
superposition	I-experimental_method
,	O
these	O
dimers	B-oligomeric_state
revealed	O
a	O
few	O
minor	O
differences	O
in	O
the	O
α6	B-structure_element
helix	I-structure_element
(	O
a	O
major	O
component	O
of	O
the	O
dimer	B-site
interface	I-site
)	O
and	O
the	O
helices	B-structure_element
α4	B-structure_element
-	I-structure_element
α5	I-structure_element
(	O
the	O
DNA	B-site
binding	I-site
region	I-site
),	O
and	O
an	O
rmsd	B-evidence
of	O
1	O
.	O
55Å	O
(	O
Fig	O
7A	O
).	O

The	O
slightly	O
larger	O
rmsd	B-evidence
between	O
the	O
two	O
apo	B-protein_state
-	O
homodimers	B-oligomeric_state
,	O
rather	O
than	O
between	O
apo	B-protein_state
-	O
and	O
holo	B-protein_state
-	O
homodimers	B-oligomeric_state
,	O
further	O
indicate	O
that	O
apo	B-protein_state
-	O
NadR	B-protein
possesses	O
a	O
notable	O
degree	O
of	O
intrinsic	O
conformational	O
flexibility	O
.	O

Overall	O
apo	B-protein_state
-	O
and	O
holo	B-protein_state
-	O
NadR	B-protein
structures	B-evidence
are	O
similar	O
.	O

(	O
A	O
)	O
Pairwise	B-experimental_method
alignment	I-experimental_method
of	O
the	O
two	O
distinct	O
apo	B-protein_state
-	O
NadR	B-protein
homodimers	B-oligomeric_state
(	O
AB	B-structure_element
and	O
CD	B-structure_element
)	O
present	O
in	O
the	O
apo	B-protein_state
-	O
NadR	B-protein
crystals	B-evidence
.	O
(	O
B	O
)	O
Alignment	B-experimental_method
of	O
the	O
holo	B-protein_state
-	O
NadR	B-protein
homodimer	B-oligomeric_state
(	O
green	O
and	O
blue	O
chains	O
)	O
onto	O
the	O
apo	B-protein_state
-	O
NadR	B-protein
homodimers	B-oligomeric_state
.	O

Here	O
,	O
larger	O
differences	O
are	O
observed	O
in	O
the	O
α6	B-structure_element
helices	I-structure_element
(	O
top	O
).	O

4	B-chemical
-	I-chemical
HPA	I-chemical
stabilizes	O
concerted	O
conformational	O
changes	O
in	O
NadR	B-protein
that	O
prevent	O
DNA	O
-	O
binding	O

To	O
further	O
investigate	O
the	O
conformational	O
rearrangements	O
of	O
NadR	B-protein
,	O
we	O
performed	O
local	B-experimental_method
structural	I-experimental_method
alignments	I-experimental_method
using	O
only	O
a	O
subset	O
of	O
residues	O
in	O
the	O
DNA	B-structure_element
-	I-structure_element
binding	I-structure_element
helix	I-structure_element
(	O
α4	B-structure_element
).	O

By	O
selecting	B-experimental_method
and	O
aligning	B-experimental_method
residues	O
Arg64	B-residue_range
-	I-residue_range
Ala77	I-residue_range
of	O
one	O
α4	B-structure_element
helix	I-structure_element
per	O
dimer	B-oligomeric_state
,	O
superposition	B-experimental_method
of	O
the	O
holo	B-protein_state
-	O
homodimer	B-oligomeric_state
onto	O
the	O
two	O
apo	B-protein_state
-	O
homodimers	B-oligomeric_state
revealed	O
differences	O
in	O
the	O
monomer	B-oligomeric_state
conformations	O
of	O
each	O
structure	B-evidence
.	O

While	O
one	O
monomer	B-oligomeric_state
from	O
each	O
structure	B-evidence
was	O
closely	O
superimposable	O
(	O
Fig	O
8A	O
,	O
left	O
side	O
),	O
the	O
second	O
monomer	B-oligomeric_state
displayed	O
quite	O
large	O
differences	O
(	O
Fig	O
8A	O
,	O
right	O
side	O
).	O

Most	O
notably	O
,	O
the	O
position	O
of	O
the	O
DNA	B-chemical
-	O
binding	O
helix	B-structure_element
α4	B-structure_element
shifted	O
by	O
as	O
much	O
as	O
6	O
Å	O
(	O
Fig	O
8B	O
).	O

Accordingly	O
,	O
helix	B-structure_element
α4	B-structure_element
was	O
also	O
found	O
to	O
be	O
one	O
of	O
the	O
most	O
dynamic	O
regions	O
in	O
previous	O
HDX	B-experimental_method
-	I-experimental_method
MS	I-experimental_method
analyses	O
of	O
apo	B-protein_state
-	O
NadR	B-protein
in	O
solution	O
.	O

The	O
α4	B-structure_element
helices	I-structure_element
aligned	O
closely	O
,	O
Cα	O
rmsd	B-evidence
0	O
.	O
2Å	O
for	O
14	O
residues	O
.	O

(	O
B	O
)	O
The	O
relative	O
positions	O
of	O
the	O
α4	B-structure_element
helices	I-structure_element
of	O
the	O
4	B-protein_state
-	I-protein_state
HPA	I-protein_state
-	I-protein_state
bound	I-protein_state
holo	B-protein_state
homodimer	B-oligomeric_state
chain	B-structure_element
B	I-structure_element
(	O
blue	O
),	O
and	O
of	O
apo	B-protein_state
homodimers	B-oligomeric_state
AB	B-structure_element
and	O
CD	B-structure_element
(	O
showing	O
chains	B-structure_element
B	I-structure_element
and	I-structure_element
D	I-structure_element
)	O
in	O
pale	O
blue	O
.	O

Dashes	O
indicate	O
the	O
Ala77	B-residue_name_number
Cα	O
atoms	O
,	O
in	O
the	O
most	O
highly	O
shifted	O
region	O
of	O
the	O
‘	O
non	O
-	O
fixed	O
’	O
α4	B-structure_element
helix	I-structure_element
.	O

(	O
C	O
)	O
The	O
double	O
-	O
stranded	O
DNA	B-chemical
molecule	O
(	O
grey	O
cartoon	O
)	O
from	O
the	O
OhrR	B-complex_assembly
-	I-complex_assembly
ohrA	I-complex_assembly
complex	O
is	O
shown	O
after	O
superposition	B-experimental_method
with	O
NadR	B-protein
,	O
to	O
highlight	O
the	O
expected	O
positions	O
of	O
the	O
NadR	B-protein
α4	B-structure_element
helices	I-structure_element
in	O
the	O
DNA	B-chemical
major	O
grooves	O
.	O

For	O
clarity	O
,	O
only	O
the	O
α4	B-structure_element
helices	I-structure_element
are	O
shown	O
in	O
panels	O
(	O
B	O
)	O
and	O
(	O
C	O
).	O
(	O
D	O
)	O
Upon	O
comparison	O
with	O
the	O
experimentally	O
-	O
determined	O
OhrR	B-complex_assembly
:	I-complex_assembly
ohrA	I-complex_assembly
structure	B-evidence
(	O
grey	O
),	O
the	O
α4	B-structure_element
helix	I-structure_element
of	O
holo	B-protein_state
-	O
NadR	B-protein
(	O
blue	O
)	O
is	O
shifted	O
~	O
8Å	O
out	O
of	O
the	O
major	O
groove	O
.	O

In	O
summary	O
,	O
compared	O
to	O
ligand	B-protein_state
-	I-protein_state
stabilized	I-protein_state
holo	B-protein_state
-	O
NadR	B-protein
,	O
apo	B-protein_state
-	O
NadR	B-protein
displayed	O
an	O
intrinsic	O
flexibility	O
focused	O
in	O
the	O
DNA	B-site
-	I-site
binding	I-site
region	I-site
.	O

This	O
was	O
also	O
evident	O
in	O
the	O
greater	O
disorder	O
(	O
i	O
.	O
e	O
.	O
less	O
well	O
-	O
defined	O
electron	B-evidence
density	I-evidence
)	O
in	O
the	O
β1	B-structure_element
-	I-structure_element
β2	I-structure_element
loops	I-structure_element
of	O
the	O
apo	B-protein_state
dimers	B-oligomeric_state
(	O
density	B-evidence
for	O
16	O
residues	O
per	O
dimer	B-oligomeric_state
was	O
missing	O
)	O
compared	O
to	O
the	O
holo	B-protein_state
dimer	B-oligomeric_state
(	O
density	B-evidence
for	O
only	O
3	O
residues	O
was	O
missing	O
).	O

In	O
holo	B-protein_state
-	O
NadR	B-protein
,	O
the	O
distance	O
separating	O
the	O
two	O
DNA	O
-	O
binding	O
α4	B-structure_element
helices	I-structure_element
was	O
32	O
Å	O
,	O
while	O
in	O
apo	B-protein_state
-	O
NadR	B-protein
it	O
was	O
29	O
Å	O
for	O
homodimer	B-oligomeric_state
AB	B-structure_element
,	O
and	O
34	O
Å	O
for	O
homodimer	B-oligomeric_state
CD	B-structure_element
(	O
Fig	O
8C	O
).	O

Pairwise	B-experimental_method
superpositions	I-experimental_method
showed	O
that	O
the	O
NadR	B-protein
apo	B-protein_state
-	O
homodimer	B-oligomeric_state
AB	B-structure_element
was	O
the	O
most	O
similar	O
to	O
OhrR	B-protein
(	O
rmsd	B-evidence
2	O
.	O
6	O
Å	O
),	O
while	O
the	O
holo	B-protein_state
-	O
homodimer	B-oligomeric_state
was	O
the	O
most	O
divergent	O
(	O
rmsd	B-evidence
3	O
.	O
3	O
Å	O
)	O
(	O
Fig	O
8C	O
).	O

Assuming	O
the	O
same	O
DNA	B-chemical
-	O
binding	O
mechanism	O
is	O
used	O
by	O
OhrR	B-protein
and	O
NadR	B-protein
,	O
the	O
apo	B-protein_state
-	O
homodimer	B-oligomeric_state
AB	B-structure_element
seems	O
ideally	O
pre	O
-	O
configured	O
for	O
DNA	B-chemical
binding	O
,	O
while	O
4	B-chemical
-	I-chemical
HPA	I-chemical
appeared	O
to	O
stabilize	O
holo	B-protein_state
-	O
NadR	B-protein
in	O
a	O
conformation	O
poorly	O
suited	O
for	O
DNA	B-chemical
binding	O
.	O

Specifically	O
,	O
in	O
addition	O
to	O
the	O
different	O
inter	B-evidence
-	I-evidence
helical	I-evidence
translational	I-evidence
distances	I-evidence
,	O
the	O
α4	B-structure_element
helices	I-structure_element
in	O
the	O
holo	B-protein_state
-	O
NadR	B-protein
homodimer	B-oligomeric_state
were	O
also	O
reoriented	O
,	O
resulting	O
in	O
movement	O
of	O
α4	B-structure_element
out	O
of	O
the	O
major	O
groove	O
,	O
by	O
up	O
to	O
8Å	O
,	O
and	O
presumably	O
preventing	O
efficient	O
DNA	B-chemical
binding	O
in	O
the	O
presence	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
(	O
Fig	O
8D	O
).	O

NadR	B-protein
residues	O
His7	B-residue_name_number
,	O
Ser9	B-residue_name_number
,	O
Asn11	B-residue_name_number
and	O
Phe25	B-residue_name_number
are	O
essential	O
for	O
regulation	O
of	O
NadA	B-protein
expression	O
in	O
vivo	O

While	O
previous	O
studies	O
had	O
correctly	O
suggested	O
the	O
involvement	O
of	O
several	O
NadR	B-protein
residues	O
in	O
ligand	O
binding	O
,	O
the	O
crystal	B-evidence
structures	I-evidence
presented	O
here	O
revealed	O
additional	O
residues	O
with	O
previously	O
unknown	O
roles	O
in	O
dimerization	O
and	O
/	O
or	O
binding	O
to	O
4	B-chemical
-	I-chemical
HPA	I-chemical
.	O

To	O
explore	O
the	O
functional	O
involvement	O
of	O
these	O
residues	O
,	O
we	O
characterized	O
the	O
behavior	O
of	O
four	O
new	O
NadR	B-protein
mutants	O
(	O
H7A	B-mutant
,	O
S9A	B-mutant
,	O
N11A	B-mutant
and	O
F25A	B-mutant
)	O
in	O
an	O
in	O
vivo	O
assay	O
using	O
the	O
previously	O
described	O
MC58	B-mutant
-	I-mutant
Δ1843	I-mutant
nadR	B-gene
-	O
null	O
mutant	B-protein_state
strain	O
,	O
which	O
was	O
complemented	O
either	O
by	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
nadR	B-gene
or	O
by	O
the	O
nadR	B-gene
mutants	B-protein_state
.	O

NadA	B-protein
protein	O
abundance	O
levels	O
were	O
assessed	O
by	O
Western	B-experimental_method
blotting	I-experimental_method
to	O
evaluate	O
the	O
ability	O
of	O
the	O
NadR	B-protein
mutants	B-protein_state
to	O
repress	O
the	O
nadA	B-gene
promoter	O
,	O
in	O
the	O
presence	O
or	O
absence	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
.	O

The	O
nadR	B-gene
H7A	B-mutant
,	O
S9A	B-mutant
and	O
F25A	B-mutant
complemented	O
strains	O
showed	O
hyper	O
-	O
repression	O
of	O
nadA	B-gene
expression	O
in	O
vivo	O
,	O
i	O
.	O
e	O
.	O
these	O
mutants	O
repressed	O
nadA	B-gene
more	O
efficiently	O
than	O
the	O
NadR	B-protein
WT	B-protein_state
protein	O
,	O
either	O
in	O
the	O
presence	O
or	O
absence	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
while	O
complementation	O
with	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
nadR	B-gene
resulted	O
in	O
high	O
production	O
of	O
NadA	B-protein
only	O
in	O
the	O
presence	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
(	O
Fig	O
9	O
).	O

Interestingly	O
,	O
and	O
on	O
the	O
contrary	O
,	O
the	O
nadR	B-gene
N11A	B-mutant
complemented	O
strain	O
showed	O
hypo	O
-	O
repression	O
(	O
i	O
.	O
e	O
.	O
exhibited	O
high	O
expression	O
of	O
nadA	B-gene
both	O
in	O
absence	O
and	O
presence	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
).	O

Structure	B-experimental_method
-	I-experimental_method
based	I-experimental_method
point	I-experimental_method
mutations	I-experimental_method
shed	O
light	O
on	O
ligand	O
-	O
induced	O
regulation	O
of	O
NadR	B-protein
.	O

Complementation	O
of	O
ΔNadR	B-mutant
with	O
WT	B-protein_state
NadR	B-protein
enables	O
induction	O
of	O
nadA	B-gene
expression	O
by	O
4	B-chemical
-	I-chemical
HPA	I-chemical
.	O

A	O
detailed	O
understanding	O
of	O
the	O
in	O
vitro	O
repression	O
of	O
nadA	B-gene
expression	O
by	O
the	O
transcriptional	B-protein_type
regulator	I-protein_type
NadR	B-protein
is	O
important	O
,	O
both	O
because	O
it	O
is	O
a	O
relevant	O
disease	O
-	O
related	O
model	O
of	O
how	O
small	O
-	O
molecule	O
ligands	O
can	O
regulate	O
MarR	B-protein_type
family	O
proteins	O
and	O
thereby	O
impact	O
bacterial	B-taxonomy_domain
virulence	O
,	O
and	O
because	O
nadA	B-gene
expression	O
levels	O
are	O
linked	O
to	O
the	O
prediction	O
of	O
vaccine	O
coverage	O
.	O

The	O
repressive	O
activity	O
of	O
NadR	B-protein
can	O
be	O
relieved	O
by	O
hydroxyphenylacetate	B-chemical
(	O
HPA	B-chemical
)	O
ligands	O
,	O
and	O
HDX	B-experimental_method
-	I-experimental_method
MS	I-experimental_method
studies	O
previously	O
indicated	O
that	O
4	B-chemical
-	I-chemical
HPA	I-chemical
stabilizes	O
dimeric	B-oligomeric_state
NadR	B-protein
in	O
a	O
configuration	O
incompatible	O
with	O
DNA	O
binding	O
.	O

Despite	O
these	O
and	O
other	O
studies	O
,	O
the	O
molecular	O
mechanisms	O
by	O
which	O
ligands	O
regulate	O
MarR	B-protein_type
family	O
proteins	O
are	O
relatively	O
poorly	O
understood	O
and	O
likely	O
differ	O
depending	O
on	O
the	O
specific	O
ligand	O
.	O

Firstly	O
,	O
we	O
confirmed	O
that	O
NadR	B-protein
is	O
dimeric	B-oligomeric_state
in	O
solution	O
and	O
demonstrated	O
that	O
it	O
retains	O
its	O
dimeric	B-oligomeric_state
state	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
indicating	O
that	O
induction	O
of	O
a	O
monomeric	B-oligomeric_state
status	O
is	O
not	O
the	O
manner	O
by	O
which	O
4	B-chemical
-	I-chemical
HPA	I-chemical
regulates	O
NadR	B-protein
.	O
These	O
observations	O
were	O
in	O
agreement	O
with	O
(	O
i	O
)	O
a	O
previous	O
study	O
of	O
NadR	B-protein
performed	O
using	O
SEC	B-experimental_method
and	O
mass	B-experimental_method
spectrometry	I-experimental_method
,	O
and	O
(	O
ii	O
)	O
crystallographic	B-experimental_method
studies	I-experimental_method
showing	O
that	O
several	O
MarR	B-protein_type
homologues	O
are	O
dimeric	B-oligomeric_state
.	O

We	O
also	O
used	O
structure	B-experimental_method
-	I-experimental_method
guided	I-experimental_method
site	I-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
to	O
identify	O
an	O
important	O
conserved	B-protein_state
residue	O
,	O
Leu130	B-residue_name_number
,	O
which	O
stabilizes	O
the	O
NadR	B-protein
dimer	B-site
interface	I-site
,	O
knowledge	O
of	O
which	O
may	O
also	O
inform	O
future	O
studies	O
to	O
explore	O
the	O
regulatory	O
mechanisms	O
of	O
other	O
MarR	B-protein_type
family	O
proteins	O
.	O

Secondly	O
,	O
we	O
assessed	B-experimental_method
the	I-experimental_method
thermal	I-experimental_method
stability	I-experimental_method
and	O
unfolding	O
of	O
NadR	B-protein
in	B-protein_state
the	I-protein_state
presence	I-protein_state
or	O
absence	B-protein_state
of	I-protein_state
ligands	O
.	O

All	O
DSC	B-experimental_method
profiles	B-evidence
showed	O
a	O
single	O
peak	O
,	O
suggesting	O
that	O
a	O
single	O
unfolding	O
event	O
simultaneously	O
disrupted	O
the	O
dimer	B-oligomeric_state
and	O
the	O
monomer	B-oligomeric_state
.	O

HPA	O
ligands	O
specifically	O
increased	O
the	O
stability	O
of	O
NadR	B-protein
.	O
The	O
largest	O
effects	O
were	O
induced	O
by	O
the	O
naturally	O
-	O
occurring	O
compounds	O
4	B-chemical
-	I-chemical
HPA	I-chemical
and	O
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
,	O
which	O
,	O
in	O
SPR	B-experimental_method
assays	I-experimental_method
,	O
were	O
found	O
to	O
bind	O
NadR	B-protein
with	O
KD	B-evidence
values	O
of	O
1	O
.	O
5	O
mM	O
and	O
1	O
.	O
1	O
mM	O
,	O
respectively	O
.	O

Although	O
these	O
NadR	B-protein
/	O
HPA	B-chemical
interactions	O
appeared	O
rather	O
weak	O
,	O
their	O
distinct	O
affinities	O
and	O
specificities	O
matched	O
their	O
in	O
vitro	O
effects	O
and	O
their	O
biological	O
relevance	O
appears	O
similar	O
to	O
previous	O
proposals	O
that	O
certain	O
small	O
molecules	O
,	O
including	O
some	O
antibiotics	O
,	O
in	O
the	O
millimolar	O
concentration	O
range	O
may	O
be	O
broad	O
inhibitors	O
of	O
MarR	B-protein_type
family	O
proteins	O
.	O

Indeed	O
,	O
4	B-chemical
-	I-chemical
HPA	I-chemical
is	O
found	O
in	O
human	B-species
saliva	O
and	O
3Cl	B-chemical
,	I-chemical
4	I-chemical
-	I-chemical
HPA	I-chemical
is	O
produced	O
during	O
inflammatory	O
processes	O
,	O
suggesting	O
that	O
these	O
natural	O
ligands	O
are	O
encountered	O
by	O
N	B-species
.	I-species
meningitidis	I-species
in	O
the	O
mucosa	O
of	O
the	O
oropharynx	O
during	O
infections	O
.	O

It	O
is	O
also	O
possible	O
that	O
NadR	B-protein
responds	O
to	O
currently	O
unidentified	O
HPA	B-chemical
analogues	O
.	O

Indeed	O
,	O
in	O
the	O
NadR	B-complex_assembly
/	I-complex_assembly
4	I-complex_assembly
-	I-complex_assembly
HPA	I-complex_assembly
complex	O
there	O
was	O
a	O
water	B-chemical
molecule	O
close	O
to	O
the	O
carboxylate	O
group	O
and	O
also	O
a	O
small	O
unfilled	O
tunnel	B-site
~	O
5Å	O
long	O
,	O
both	O
factors	O
suggesting	O
that	O
alternative	O
larger	O
ligands	O
could	O
occupy	O
the	O
pocket	O
.	O

The	O
ability	O
to	O
respond	O
to	O
various	O
ligands	O
might	O
enable	O
NadR	B-protein
in	O
vivo	O
to	O
orchestrate	O
multiple	O
response	O
mechanisms	O
and	O
modulate	O
expression	O
of	O
genes	O
other	O
than	O
nadA	B-gene
.	O
Ultimately	O
,	O
confirmation	O
of	O
the	O
relevance	O
of	O
each	O
ligand	O
will	O
require	O
a	O
deeper	O
understanding	O
of	O
the	O
available	O
concentration	O
in	O
vivo	O
in	O
the	O
host	O
niche	O
during	O
bacterial	B-taxonomy_domain
colonization	O
and	O
inflammation	O
.	O

Here	O
,	O
we	O
determined	O
the	O
first	O
crystal	B-evidence
structures	I-evidence
of	O
apo	B-protein_state
-	O
NadR	B-protein
and	O
holo	B-protein_state
-	O
NadR	B-protein
.	O
These	O
experimentally	O
-	O
determined	O
structures	B-evidence
enabled	O
a	O
new	O
detailed	O
characterization	O
of	O
the	O
ligand	B-site
-	I-site
binding	I-site
pocket	I-site
.	O

In	O
holo	B-protein_state
-	O
NadR	B-protein
,	O
4	B-chemical
-	I-chemical
HPA	I-chemical
interacted	O
directly	O
with	O
at	O
least	O
11	O
polar	O
and	O
hydrophobic	O
residues	O
.	O

Several	O
,	O
but	O
not	O
all	O
,	O
of	O
these	O
interactions	O
were	O
predicted	O
previously	O
by	O
homology	B-experimental_method
modelling	I-experimental_method
combined	O
with	O
ligand	B-experimental_method
docking	I-experimental_method
in	O
silico	O
.	O

Subsequently	O
,	O
we	O
established	O
the	O
functional	O
importance	O
of	O
His7	B-residue_name_number
,	O
Ser9	B-residue_name_number
,	O
Asn11	B-residue_name_number
and	O
Phe25	B-residue_name_number
in	O
the	O
in	O
vitro	O
response	O
of	O
meningococcus	B-taxonomy_domain
to	O
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
via	O
site	B-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
.	O

More	O
unexpectedly	O
,	O
the	O
crystal	B-evidence
structure	I-evidence
revealed	O
that	O
only	O
one	O
molecule	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
was	O
bound	B-protein_state
per	O
NadR	B-protein
dimer	B-oligomeric_state
.	O

We	O
also	O
used	O
heteronuclear	B-experimental_method
NMR	I-experimental_method
spectroscopy	I-experimental_method
to	O
detect	O
substantial	O
conformational	O
changes	O
of	O
NadR	B-protein
occurring	O
in	O
solution	O
upon	O
addition	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
.	O

Moreover	O
,	O
NMR	B-experimental_method
spectra	B-evidence
at	O
10	O
°	O
C	O
suggested	O
the	O
existence	O
of	O
two	O
distinct	O
conformations	O
of	O
NadR	B-protein
in	O
the	O
vicinity	O
of	O
the	O
ligand	B-site
-	I-site
binding	I-site
pocket	I-site
.	O

More	O
powerfully	O
,	O
our	O
unique	O
crystallographic	B-evidence
observation	I-evidence
of	O
this	O
‘	O
occupied	B-protein_state
vs	O
unoccupied	B-protein_state
site	O
’	O
asymmetry	O
in	O
the	O
NadR	B-complex_assembly
/	I-complex_assembly
4	I-complex_assembly
-	I-complex_assembly
HPA	I-complex_assembly
interaction	O
is	O
,	O
to	O
our	O
knowledge	O
,	O
the	O
first	O
example	O
reported	O
for	O
a	O
MarR	B-protein_type
family	O
protein	O
.	O

Such	O
a	O
mechanism	O
indicates	O
negative	O
cooperativity	O
,	O
which	O
may	O
enhance	O
the	O
ligand	O
-	O
responsiveness	O
of	O
NadR	B-protein
.	O

Comparisons	O
of	O
the	O
NadR	B-complex_assembly
/	I-complex_assembly
4	I-complex_assembly
-	I-complex_assembly
HPA	I-complex_assembly
complex	O
with	O
available	O
MarR	B-protein_type
family	O
/	O
salicylate	B-chemical
complexes	O
revealed	O
that	O
4	B-chemical
-	I-chemical
HPA	I-chemical
has	O
a	O
previously	O
unobserved	O
binding	O
mode	O
.	O

Briefly	O
,	O
in	O
the	O
M	B-species
.	I-species
thermoautotrophicum	I-species
MTH313	B-protein
dimer	B-oligomeric_state
,	O
one	O
molecule	O
of	O
salicylate	B-chemical
binds	O
in	O
the	O
pocket	B-site
of	O
each	O
monomer	B-oligomeric_state
,	O
though	O
with	O
two	O
rather	O
different	O
positions	O
and	O
orientations	O
,	O
only	O
one	O
of	O
which	O
(	O
site	B-site
-	I-site
1	I-site
)	O
is	O
thought	O
to	O
be	O
biologically	O
relevant	O
(	O
Fig	O
10A	O
).	O

In	O
NadR	B-protein
,	O
the	O
single	O
molecule	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
binds	O
in	O
a	O
position	O
distinctly	O
different	O
from	O
the	O
salicylate	B-site
binding	I-site
site	I-site
:	O
translated	O
by	O
>	O
10	O
Å	O
and	O
with	O
a	O
180	O
°	O
inverted	O
orientation	O
(	O
Fig	O
10C	O
).	O

(	O
A	O
)	O
A	O
structural	B-experimental_method
alignment	I-experimental_method
of	O
MTH313	B-protein
chains	B-structure_element
A	I-structure_element
and	I-structure_element
B	I-structure_element
shows	O
that	O
salicylate	B-chemical
is	O
bound	B-protein_state
in	O
distinct	O
locations	O
in	O
each	O
monomer	B-oligomeric_state
;	O
site	B-site
-	I-site
1	I-site
(	O
thought	O
to	O
be	O
the	O
biologically	O
relevant	O
site	O
)	O
and	O
site	B-site
-	I-site
2	I-site
differ	O
by	O
~	O
7Å	O
(	O
indicated	O
by	O
black	O
dotted	O
line	O
)	O
and	O
also	O
by	O
ligand	O
orientation	O
.	O

(	O
C	O
)	O
Addition	O
of	O
holo	B-protein_state
-	O
NadR	B-protein
(	O
chain	B-structure_element
B	I-structure_element
,	O
blue	O
)	O
to	O
the	O
alignment	B-experimental_method
reveals	O
that	O
bound	B-protein_state
4	B-chemical
-	I-chemical
HPA	I-chemical
differs	O
in	O
position	O
by	O
>	O
10	O
Å	O
compared	O
to	O
salicylate	B-chemical
,	O
and	O
adopts	O
a	O
novel	O
orientation	O
.	O

Interestingly	O
,	O
a	O
crystal	B-evidence
structure	I-evidence
was	O
previously	O
reported	O
for	O
a	O
functionally	O
-	O
uncharacterized	O
meningococcal	B-taxonomy_domain
homologue	O
of	O
NadR	B-protein
,	O
termed	O
NMB1585	B-protein
,	O
which	O
shares	O
16	O
%	O
sequence	O
identity	O
with	O
NadR	B-protein
.	O
The	O
two	O
structures	B-evidence
can	O
be	O
closely	O
aligned	O
(	O
rmsd	B-evidence
2	O
.	O
3	O
Å	O
),	O
but	O
NMB1585	B-protein
appears	O
unsuited	O
for	O
binding	O
HPAs	B-chemical
,	O
since	O
its	O
corresponding	O
‘	B-site
pocket	I-site
’	O
region	O
is	O
occupied	O
by	O
several	O
bulky	O
hydrophobic	O
side	O
chains	O
.	O

It	O
can	O
be	O
speculated	O
that	O
MarR	B-protein_type
family	O
members	O
have	O
evolved	O
separately	O
to	O
engage	O
distinct	O
signaling	O
molecules	O
,	O
thus	O
enabling	O
bacteria	B-taxonomy_domain
to	O
use	O
the	O
overall	O
conserved	O
MarR	B-protein_type
scaffold	O
to	O
adapt	O
and	O
respond	O
to	O
diverse	O
changing	O
environmental	O
stimuli	O
experienced	O
in	O
their	O
natural	O
niches	O
.	O

The	O
apo	B-protein_state
-	O
NadR	B-protein
crystal	B-evidence
structures	I-evidence
revealed	O
two	O
dimers	B-oligomeric_state
with	O
slightly	O
different	O
conformations	O
,	O
most	O
divergent	O
in	O
the	O
DNA	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
.	O

It	O
is	O
not	O
unusual	O
for	O
a	O
crystal	B-evidence
structure	I-evidence
to	O
reveal	O
multiple	O
copies	O
of	O
the	O
same	O
protein	O
in	O
very	O
slightly	O
different	O
conformations	O
,	O
which	O
are	O
likely	O
representative	O
of	O
the	O
lowest	O
-	O
energy	O
conformations	O
sampled	O
by	O
the	O
dynamic	O
ensemble	O
of	O
molecular	O
states	O
occurring	O
in	O
solution	O
,	O
and	O
which	O
likely	O
have	O
only	O
small	O
energetic	O
differences	O
,	O
as	O
described	O
previously	O
for	O
MexR	B-protein
(	O
a	O
MarR	B-protein_type
protein	O
)	O
or	O
more	O
recently	O
for	O
the	O
solute	B-protein_type
-	I-protein_type
binding	I-protein_type
protein	I-protein_type
FhuD2	B-protein
.	O

Further	O
,	O
the	O
holo	B-protein_state
-	O
NadR	B-protein
structure	B-evidence
was	O
overall	O
more	O
different	O
from	O
the	O
two	O
apo	B-protein_state
-	O
NadR	B-protein
structures	B-evidence
(	O
rmsd	B-evidence
values	O
~	O
1	O
.	O
3Å	O
),	O
suggesting	O
that	O
the	O
ligand	O
selected	O
and	O
stabilized	O
yet	O
another	O
conformation	O
of	O
NadR	B-protein
.	O
These	O
observations	O
suggest	O
that	O
4	B-chemical
-	I-chemical
HPA	I-chemical
,	O
and	O
potentially	O
other	O
similar	O
ligands	O
,	O
can	O
shift	O
the	O
molecular	O
equilibrium	O
,	O
changing	O
the	O
energy	O
barriers	O
that	O
separate	O
active	B-protein_state
and	O
inactive	B-protein_state
states	O
,	O
and	O
stabilizing	O
the	O
specific	O
conformation	O
of	O
NadR	B-protein
poorly	O
suited	O
to	O
bind	O
DNA	B-chemical
.	O

Comparisons	O
of	O
the	O
apo	B-protein_state
-	O
and	O
holo	B-protein_state
-	O
NadR	B-protein
structures	B-evidence
revealed	O
that	O
the	O
largest	O
differences	O
occurred	O
in	O
the	O
DNA	B-chemical
-	O
binding	O
helix	B-structure_element
α4	B-structure_element
.	O

The	O
shift	O
of	O
helix	B-structure_element
α4	B-structure_element
in	O
holo	B-protein_state
-	O
NadR	B-protein
was	O
also	O
accompanied	O
by	O
rearrangements	O
at	O
the	O
dimer	B-site
interface	I-site
,	O
involving	O
helices	B-structure_element
α1	B-structure_element
,	O
α5	B-structure_element
,	O
and	O
α6	B-structure_element
,	O
and	O
this	O
holo	B-protein_state
-	O
form	O
appeared	O
poorly	O
suited	O
for	O
DNA	B-chemical
-	O
binding	O
when	O
compared	O
with	O
the	O
known	O
OhrR	B-complex_assembly
:	I-complex_assembly
ohrA	I-complex_assembly
complex	O
.	O

One	O
of	O
the	O
two	O
conformations	O
of	O
apo	B-protein_state
-	O
NadR	B-protein
appeared	O
ideally	O
suited	O
for	O
DNA	B-chemical
-	O
binding	O
.	O

Overall	O
,	O
these	O
analyses	O
suggest	O
that	O
the	O
apo	B-protein_state
-	O
NadR	B-protein
dimer	B-oligomeric_state
has	O
a	O
pre	O
-	O
existing	O
equilibrium	O
that	O
samples	O
a	O
variety	O
of	O
conformations	O
,	O
some	O
of	O
which	O
are	O
compatible	O
with	O
DNA	B-chemical
binding	O
.	O

Subsequently	O
,	O
upon	O
ligand	O
binding	O
,	O
holo	B-protein_state
-	O
NadR	B-protein
adopts	O
a	O
structure	O
less	O
suited	O
for	O
DNA	B-chemical
-	O
binding	O
and	O
this	O
conformation	O
is	O
selected	O
and	O
stabilized	O
by	O
a	O
network	O
of	O
protein	O
-	O
ligand	O
interactions	O
and	O
concomitant	O
rearrangements	O
at	O
the	O
NadR	B-protein
holo	B-protein_state
dimer	B-site
interface	I-site
.	O

In	O
an	O
alternative	O
and	O
less	O
extensive	O
manner	O
,	O
the	O
binding	O
of	O
two	O
salicylate	B-chemical
molecules	O
to	O
the	O
M	B-species
.	I-species
thermoautotrophicum	I-species
protein	O
MTH313	B-protein
appeared	O
to	O
induce	O
large	O
changes	O
in	O
the	O
wHTH	B-structure_element
domain	I-structure_element
,	O
which	O
was	O
associated	O
with	O
reduced	O
DNA	O
-	O
binding	O
activity	O
.	O

Here	O
we	O
have	O
presented	O
two	O
new	O
crystal	B-evidence
structures	I-evidence
of	O
the	O
transcription	B-protein_type
factor	I-protein_type
,	O
NadR	B-protein
,	O
which	O
regulates	O
expression	O
of	O
the	O
meningococcal	B-taxonomy_domain
surface	O
protein	O
,	O
virulence	O
factor	O
and	O
vaccine	O
antigen	O
NadA	B-protein
.	O
Detailed	O
structural	B-experimental_method
analyses	I-experimental_method
provided	O
a	O
molecular	O
explanation	O
for	O
the	O
ligand	O
-	O
responsive	O
regulation	O
by	O
NadR	B-protein
on	O
the	O
majority	O
of	O
the	O
promoters	O
of	O
meningococcal	B-taxonomy_domain
genes	O
regulated	O
by	O
NadR	B-protein
,	O
including	O
nadA	B-gene
.	O
Intriguingly	O
,	O
NadR	B-protein
exhibits	O
a	O
reversed	O
regulatory	O
mechanism	O
on	O
a	O
second	O
class	O
of	O
promoters	O
,	O
including	O
mafA	B-gene
of	O
the	O
multiple	O
adhesin	O
family	O
–	O
i	O
.	O
e	O
.	O
NadR	B-protein
represses	O
these	O
genes	O
in	O
the	O
presence	O
but	O
not	O
absence	O
of	O
4	B-chemical
-	I-chemical
HPA	I-chemical
.	O

Ultimately	O
,	O
knowledge	O
of	O
the	O
ligand	O
-	O
dependent	O
activity	O
of	O
NadR	B-protein
will	O
continue	O
to	O
deepen	O
our	O
understanding	O
of	O
nadA	B-gene
expression	O
levels	O
,	O
which	O
influence	O
meningococcal	B-taxonomy_domain
pathogenesis	O
.	O

The	O
structure	O
of	O
NMB1585	B-protein
,	O
a	O
MarR	O
-	O
family	O
regulator	O
from	O
Neisseria	O
meningitidis	O

The	O
nuclear	B-protein_type
hormone	I-protein_type
receptor	I-protein_type
RORγ	B-protein
regulates	O
transcriptional	O
genes	O
involved	O
in	O
the	O
production	O
of	O
the	O
pro	O
-	O
inflammatory	O
interleukin	B-protein_type
IL	B-protein_type
-	I-protein_type
17	I-protein_type
which	O
has	O
been	O
linked	O
to	O
autoimmune	O
diseases	O
such	O
as	O
rheumatoid	O
arthritis	O
,	O
multiple	O
sclerosis	O
and	O
inflammatory	O
bowel	O
disease	O
.	O

This	O
transcriptional	O
activity	O
of	O
RORγ	B-protein
is	O
modulated	O
through	O
a	O
protein	O
-	O
protein	O
interaction	O
involving	O
the	O
activation	B-structure_element
function	I-structure_element
2	I-structure_element
(	I-structure_element
AF2	I-structure_element
)	I-structure_element
helix	I-structure_element
on	O
the	O
ligand	B-structure_element
binding	I-structure_element
domain	I-structure_element
of	O
RORγ	B-protein
and	O
a	O
conserved	B-protein_state
LXXLL	B-structure_element
helix	I-structure_element
motif	I-structure_element
on	O
coactivator	O
proteins	O
.	O

We	O
identified	O
a	O
novel	O
series	O
of	O
synthetic	O
benzoxazinone	B-chemical
ligands	O
having	O
an	O
agonist	B-protein_state
(	O
BIO592	B-chemical
)	O
and	O
inverse	B-protein_state
agonist	I-protein_state
(	O
BIO399	B-chemical
)	O
mode	O
of	O
action	O
in	O
a	O
FRET	B-experimental_method
based	I-experimental_method
assay	I-experimental_method
.	O

We	O
show	O
that	O
the	O
AF2	B-structure_element
helix	I-structure_element
of	O
RORγ	B-protein
is	O
proteolytically	B-protein_state
sensitive	I-protein_state
when	O
inverse	B-protein_state
agonist	I-protein_state
BIO399	B-chemical
binds	O
.	O

Using	O
x	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
we	O
show	O
how	O
small	O
modifications	O
on	O
the	O
benzoxazinone	B-chemical
agonist	B-protein_state
BIO592	B-chemical
trigger	O
inverse	O
agonism	O
of	O
RORγ	B-protein
.	O

The	O
proteolytic	O
sensitivity	O
of	O
the	O
AF2	B-structure_element
helix	I-structure_element
of	O
RORγ	B-protein
demonstrates	O
that	O
it	O
destabilizes	O
upon	O
BIO399	B-chemical
inverse	B-protein_state
agonist	I-protein_state
binding	O
perturbing	O
the	O
coactivator	B-site
protein	I-site
binding	I-site
site	I-site
.	O

Even	O
though	O
a	O
high	O
degree	O
of	O
sequence	O
similarity	O
exists	O
between	O
the	O
RORs	B-protein_type
,	O
their	O
functional	O
roles	O
in	O
regulation	O
for	O
physiological	O
processes	O
involved	O
in	O
development	O
and	O
immunity	O
are	O
distinct	O
.	O

During	O
development	O
,	O
RORγ	B-protein
regulates	O
the	O
transcriptional	O
genes	O
involved	O
in	O
the	O
functioning	O
of	O
multiple	O
pro	O
-	O
inflammatory	O
lymphocyte	O
lineages	O
including	O
T	O
helper	O
cells	O
(	O
TH17cells	O
)	O
which	O
are	O
necessary	O
for	O
IL	B-protein_type
-	I-protein_type
17	I-protein_type
production	O
.	O

IL	B-protein_type
-	I-protein_type
17	I-protein_type
is	O
a	O
pro	O
-	O
inflammatory	O
interleukin	B-protein_type
linked	O
to	O
autoimmune	O
diseases	O
such	O
as	O
rheumatoid	O
arthritis	O
,	O
multiple	O
sclerosis	O
and	O
inflammatory	O
bowel	O
disease	O
;	O
making	O
its	O
transcriptional	O
regulation	O
through	O
RORγ	B-protein
an	O
attractive	O
therapeutic	O
target	O
.	O

RORγ	B-protein
consists	O
of	O
an	O
N	O
-	O
terminal	O
DNA	B-structure_element
binding	I-structure_element
domain	I-structure_element
(	O
DBD	B-structure_element
)	O
connected	O
to	O
a	O
C	O
-	O
terminal	O
ligand	B-structure_element
binding	I-structure_element
domain	I-structure_element
(	O
LBD	B-structure_element
)	O
via	O
a	O
flexible	O
hinge	B-structure_element
region	I-structure_element
.	O

The	O
DBD	B-structure_element
is	O
composed	O
of	O
two	O
zinc	B-structure_element
fingers	I-structure_element
that	O
allow	O
it	O
to	O
interact	O
with	O
specifically	O
encoded	O
regions	O
on	O
the	O
DNA	O
called	O
the	O
nuclear	B-structure_element
receptor	I-structure_element
response	I-structure_element
elements	I-structure_element
.	O

The	O
LBD	B-structure_element
consists	O
of	O
a	O
coactivator	B-site
protein	I-site
binding	I-site
pocket	I-site
and	O
a	O
hydrophobic	B-site
ligand	I-site
binding	I-site
site	I-site
(	O
LBS	B-site
)	O
which	O
are	O
responsible	O
for	O
regulating	O
transcription	O
.	O

The	O
coactivator	B-site
binding	I-site
pocket	I-site
of	O
RORγ	B-protein
recognizes	O
a	O
conserved	B-protein_state
helix	B-structure_element
motif	I-structure_element
LXXLL	I-structure_element
(	O
where	O
X	O
can	O
be	O
any	O
amino	O
acid	O
)	O
on	O
transcriptional	O
coactivator	O
complexes	O
and	O
recruits	O
it	O
to	O
activate	O
transcription	O
.	O

In	O
RORγ	B-protein
,	O
the	O
conformation	O
of	O
the	O
AF2	B-structure_element
helix	I-structure_element
required	O
to	O
form	O
the	O
coactivator	B-site
binding	I-site
pocket	I-site
is	O
mediated	O
by	O
a	O
salt	B-bond_interaction
bridge	I-bond_interaction
between	O
His479	B-residue_name_number
and	O
Tyr502	B-residue_name_number
in	O
addition	O
to	O
π	B-bond_interaction
-	I-bond_interaction
π	I-bond_interaction
interactions	I-bond_interaction
between	O
Tyr502	B-residue_name_number
and	O
Phe506	B-residue_name_number
.	O

The	O
conformation	O
of	O
the	O
AF2	B-structure_element
helix	I-structure_element
can	O
be	O
modulated	O
through	O
targeted	O
ligands	O
which	O
bind	O
the	O
LBS	B-site
and	O
increase	O
the	O
binding	O
of	O
the	O
coactivator	O
protein	O
(	O
agonists	O
)	O
or	O
disrupt	O
binding	O
(	O
inverse	O
agonists	O
)	O
thereby	O
enhancing	O
or	O
inhibiting	O
transcription	O
.	O

Since	O
RORγ	B-protein
has	O
been	O
demonstrated	O
to	O
play	O
an	O
important	O
role	O
in	O
pro	O
-	O
inflammatory	O
gene	O
expression	O
patterns	O
implicated	O
in	O
several	O
major	O
autoimmune	O
diseases	O
,	O
our	O
aim	O
was	O
to	O
develop	O
RORγ	B-protein
inverse	O
agonists	O
that	O
would	O
help	O
down	O
regulate	O
pro	O
-	O
inflammatory	O
gene	O
transcription	O
.	O

Finally	O
,	O
comparing	O
binding	B-evidence
modes	I-evidence
of	O
our	O
benzoxazinone	B-chemical
RORγ	B-protein
crystal	B-evidence
structures	I-evidence
to	O
other	O
ROR	B-protein_type
structures	B-evidence
,	O
we	O
hypothesize	O
a	O
new	O
mode	O
of	O
action	O
for	O
achieving	O
inverse	O
agonism	O
and	O
selectivity	O
.	O

Interestingly	O
,	O
the	O
structural	O
difference	O
between	O
the	O
agonist	B-protein_state
BIO592	B-chemical
and	O
inverse	B-protein_state
agonist	I-protein_state
BIO399	B-chemical
was	O
minor	O
;	O
with	O
the	O
2	B-chemical
,	I-chemical
3	I-chemical
-	I-chemical
dihydrobenzo	I-chemical
[	I-chemical
1	I-chemical
,	I-chemical
4	I-chemical
]	I-chemical
oxazepin	I-chemical
-	I-chemical
4	I-chemical
-	I-chemical
one	I-chemical
ring	O
system	O
of	O
BIO399	B-chemical
being	O
3	O
atoms	O
larger	O
than	O
the	O
benzo	B-chemical
[	I-chemical
1	I-chemical
,	I-chemical
4	I-chemical
]	I-chemical
oxazine	I-chemical
-	I-chemical
3	I-chemical
-	I-chemical
one	I-chemical
ring	O
system	O
of	O
BIO592	B-chemical
.	O

In	O
order	O
to	O
understand	O
how	O
small	O
changes	O
in	O
the	O
core	O
ring	O
system	O
leads	O
to	O
inverse	O
agonism	O
,	O
we	O
wanted	O
to	O
structurally	O
determine	O
the	O
binding	O
mode	O
of	O
both	O
BIO592	B-chemical
and	O
BIO399	B-chemical
in	O
the	O
LBS	B-site
of	O
RORγ	B-protein
using	O
x	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
.	O

Structure	B-evidence
of	O
the	O
RORγ518	B-complex_assembly
-	I-complex_assembly
BIO592	I-complex_assembly
-	I-complex_assembly
EBI96	I-complex_assembly
ternary	O
complex	O
is	O
in	O
a	O
transcriptionally	O
active	B-protein_state
conformation	O

RORγ518	B-protein
bound	B-protein_state
to	I-protein_state
agonist	B-protein_state
BIO592	B-chemical
was	O
crystallized	B-experimental_method
with	O
a	O
truncated	B-protein_state
form	O
of	O
the	O
coactivator	O
peptide	O
EBI96	B-chemical
to	O
a	O
resolution	O
of	O
2	O
.	O
6	O
Å	O
(	O
Fig	O
.	O
2a	O
).	O

The	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
between	O
His479	B-residue_name_number
and	O
Tyr502	B-residue_name_number
has	O
been	O
reported	O
to	O
be	O
critical	O
for	O
RORγ	B-protein
agonist	B-protein_state
activity	O
.	O

Disrupting	O
this	O
interaction	O
through	O
mutagenesis	B-experimental_method
reduced	O
transcriptional	O
activity	O
of	O
RORγ	B-protein
.	O

This	O
reduced	O
transcriptional	O
activity	O
has	O
been	O
attributed	O
to	O
the	O
inability	O
of	O
the	O
AF2	B-structure_element
helix	I-structure_element
to	O
complete	O
the	O
formation	O
of	O
the	O
coactivator	B-site
binding	I-site
pocket	I-site
necessary	O
for	O
coactivator	O
proteins	O
to	O
bind	O
.	O

This	O
interaction	O
is	O
further	O
stabilized	O
through	O
a	O
conserved	B-protein_state
charged	B-structure_element
clamp	I-structure_element
wherein	O
the	O
backbone	O
amide	O
of	O
Tyr7	B-residue_name_number
and	O
carbonyl	O
of	O
Leu11	B-residue_name_number
of	O
EBI96	B-chemical
form	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
Glu504	B-residue_name_number
(	O
helix12	B-structure_element
)	O
and	O
Lys336	B-residue_name_number
(	O
helix3	B-structure_element
)	O
of	O
RORγ	B-protein
.	O

Formation	O
of	O
this	O
charged	B-structure_element
clamp	I-structure_element
is	O
essential	O
for	O
RORγ	B-protein
’	O
s	O
function	O
for	O
playing	O
a	O
role	O
in	O
transcriptional	O
activation	O
and	O
this	O
has	O
been	O
corroborated	O
through	O
mutagenic	B-experimental_method
studies	I-experimental_method
in	O
this	O
region	O
.	O

BIO592	B-chemical
binds	O
in	O
a	O
collapsed	B-protein_state
conformation	O
stabilizing	O
the	O
agonist	B-protein_state
conformation	O
of	O
RORγ	B-protein

a	O
Collapsed	O
binding	O
mode	O
of	O
agonist	B-protein_state
BIO592	B-chemical
in	O
the	O
hydrophobic	O
LBS	B-site
of	O
RORγ	B-protein
.	O

The	O
sulfonyl	B-chemical
group	O
faces	O
the	O
entrance	O
of	O
the	O
pocket	B-site
,	O
while	O
the	O
CF3	O
makes	O
a	O
hydrophobic	B-bond_interaction
contact	I-bond_interaction
with	O
Ala327	B-residue_name_number
.	O

RORγ	B-protein
AF2	B-structure_element
helix	I-structure_element
is	O
sensitive	O
to	O
proteolysis	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
Inverse	B-protein_state
Agonist	I-protein_state
BIO399	B-chemical

Next	O
,	O
we	O
attempted	O
co	B-experimental_method
-	I-experimental_method
crystallization	I-experimental_method
with	O
the	O
inverse	B-protein_state
agonist	I-protein_state
BIO399	B-chemical
.	O

However	O
,	O
extensive	O
crystallization	B-experimental_method
efforts	O
with	O
BIO399	B-chemical
and	O
RORγ518	B-protein
or	O
other	O
AF2	B-structure_element
intact	B-protein_state
constructs	O
did	O
not	O
produce	O
crystals	B-evidence
.	O

We	O
hypothesized	O
that	O
the	O
RORγ518	B-protein
coactivator	O
peptide	O
interaction	O
in	O
the	O
FRET	B-experimental_method
assay	I-experimental_method
was	O
disrupted	O
upon	O
BIO399	B-chemical
binding	O
and	O
that	O
a	O
conformational	O
rearrangement	O
of	O
the	O
AF2	B-structure_element
helix	I-structure_element
could	O
have	O
occurred	O
,	O
hindering	O
crystallization	B-experimental_method
.	O

The	O
unfolding	O
of	O
the	O
AF2	B-structure_element
helix	I-structure_element
has	O
been	O
observed	O
for	O
other	O
nuclear	B-protein_type
hormone	I-protein_type
receptors	I-protein_type
when	O
bound	B-protein_state
to	I-protein_state
an	O
inverse	B-protein_state
agonist	I-protein_state
or	O
antagonist	O
.	O

We	O
used	O
partial	B-experimental_method
proteolysis	I-experimental_method
in	O
combination	O
with	O
mass	B-experimental_method
spectrometry	I-experimental_method
to	O
determine	O
if	O
BIO399	B-chemical
was	O
causing	O
the	O
AF2	B-structure_element
helix	I-structure_element
to	O
unfold	O
.	O

Results	O
of	O
the	O
Actinase	B-experimental_method
E	I-experimental_method
proteolysis	I-experimental_method
experiments	O
on	O
RORγ518	B-protein
,	O
the	O
ternary	O
complex	O
of	O
RORγ518	B-protein
with	O
agonist	B-protein_state
BIO592	B-chemical
and	O
coactivator	O
EBI96	B-chemical
,	O
or	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
inverse	B-protein_state
agonist	I-protein_state
BIO399	B-chemical
supported	O
our	O
hypothesis	O
.	O

Analysis	O
of	O
the	O
fragmentation	B-evidence
pattern	I-evidence
showed	O
minimal	O
proteolytic	O
removal	O
of	O
the	O
AF2	B-structure_element
helix	I-structure_element
by	O
Actinase	B-protein
E	I-protein
on	O
RORγ518	B-protein
alone	O
(	O
ending	O
at	O
504	B-residue_range
to	I-residue_range
506	I-residue_range
)	O
and	O
the	O
ternary	B-protein_state
complex	I-protein_state
remained	O
primarily	O
intact	O
(	O
ending	O
at	O
515	B-residue_number
/	O
518	B-residue_number
)	O
(	O
Additional	O
file	O
4	O
).	O

We	O
attributed	O
the	O
inability	O
to	O
form	O
crystals	B-evidence
to	O
the	O
unfolding	O
of	O
the	O
AF2	B-structure_element
helix	I-structure_element
induced	O
by	O
BIO399	B-chemical
.	O

AF2	B-protein_state
truncated	I-protein_state
RORγ	B-complex_assembly
BIO399	I-complex_assembly
complex	O
is	O
more	O
amenable	O
to	O
crystallization	B-experimental_method

a	O
The	O
binary	O
structure	B-evidence
of	O
AF2	B-protein_state
-	I-protein_state
truncated	I-protein_state
RORγ	B-protein
and	O
BIO399	B-chemical
.	O

b	O
The	O
superposition	B-experimental_method
of	O
inverse	B-protein_state
agonist	I-protein_state
BIO399	B-chemical
(	O
Cyan	O
)	O
and	O
agonist	B-protein_state
BIO592	B-chemical
(	O
Green	O
).	O

c	O
Movement	O
of	O
Met358	B-residue_name_number
and	O
His479	B-residue_name_number
in	O
the	O
BIO399	B-chemical
(	O
Cyan	O
)	O
and	O
BIO592	B-chemical
(	O
Green	O
)	O
structures	B-evidence

The	O
Actinase	B-protein
E	I-protein
treated	O
RORγ518	B-complex_assembly
BIO399	I-complex_assembly
ternary	O
complex	O
(	O
aeRORγ493	B-complex_assembly
/	I-complex_assembly
4	I-complex_assembly
)	O
co	B-experimental_method
-	I-experimental_method
crystallized	I-experimental_method
readily	O
in	O
several	O
PEG	O
based	O
conditions	O
.	O

The	O
structure	B-evidence
of	O
aeRORγ493	B-complex_assembly
/	I-complex_assembly
4	I-complex_assembly
BIO399	I-complex_assembly
complex	O
was	O
solved	B-experimental_method
to	O
2	O
.	O
3	O
Å	O
and	O
adopted	O
a	O
similar	O
core	O
fold	O
to	O
the	O
BIO592	B-chemical
agonist	B-protein_state
crystal	B-evidence
structure	I-evidence
(	O
Fig	O
.	O
5a	O
,	O
Additional	O
file	O
3	O
).	O

Inverse	B-protein_state
agonist	I-protein_state
BIO399	B-chemical
uses	O
Met358	B-residue_name_number
as	O
a	O
trigger	O
for	O
inverse	O
agonism	O

The	O
majority	O
of	O
the	O
side	O
chains	O
within	O
4	O
Å	O
of	O
BIO399	B-chemical
and	O
BIO592	B-chemical
adopt	O
similar	O
rotomer	O
conformations	O
with	O
the	O
exceptions	O
of	O
Met358	B-residue_name_number
and	O
His479	B-residue_name_number
(	O
Fig	O
.	O
5c	O
).	O

The	O
difference	B-evidence
density	I-evidence
map	I-evidence
showed	O
clear	O
positive	B-evidence
density	I-evidence
for	O
Met358	B-residue_name_number
in	O
an	O
alternate	O
rotomer	O
conformation	O
compared	O
to	O
the	O
one	O
observed	O
in	O
the	O
molecular	B-experimental_method
replacement	I-experimental_method
model	I-experimental_method
or	O
the	O
other	O
agonist	B-protein_state
containing	O
models	O
(	O
Additional	O
file	O
6	O
).	O

We	O
tried	O
to	O
refine	O
Met358	B-residue_name_number
in	O
the	O
same	O
conformation	O
as	O
the	O
molecular	B-experimental_method
replacement	I-experimental_method
model	I-experimental_method
or	O
the	O
other	O
agonist	B-protein_state
containing	O
models	O
,	O
but	O
the	O
results	O
clearly	O
indicated	O
that	O
this	O
was	O
not	O
possible	O
,	O
thus	O
confirming	O
the	O
new	O
rotamer	O
conformation	O
for	O
the	O
Met358	B-residue_name_number
sidechain	O
in	O
the	O
inverse	B-protein_state
agonist	I-protein_state
bound	I-protein_state
structure	B-evidence
.	O

The	O
change	O
in	O
rotomer	O
conformation	O
of	O
Met358	B-residue_name_number
between	O
the	O
agonist	B-protein_state
and	O
inverse	B-protein_state
agonist	I-protein_state
structures	B-evidence
is	O
attributed	O
to	O
the	O
gem	O
-	O
dimethyl	O
group	O
on	O
the	O
larger	O
7	O
membered	O
benzoxazinone	B-chemical
ring	O
system	O
of	O
BIO399	B-chemical
.	O

The	O
comparison	B-experimental_method
of	O
the	O
two	O
structures	B-evidence
shows	O
that	O
the	O
agonist	B-protein_state
conformation	O
observed	O
in	O
the	O
BIO592	B-chemical
structure	B-evidence
would	O
be	O
perturbed	O
by	O
BIO399	B-chemical
pushing	O
Met358	B-residue_name_number
into	O
Phe506	B-residue_name_number
of	O
the	O
AF2	B-structure_element
helix	I-structure_element
indicating	O
that	O
Met358	B-residue_name_number
is	O
a	O
trigger	O
for	O
inducing	O
inverse	O
agonism	O
in	O
RORγ	B-protein
(	O
Fig	O
.	O
5c	O
).	O

b	O
Overlay	B-experimental_method
of	O
M358	B-residue_name_number
in	O
RORγ	B-protein
structure	B-evidence
BIO596	B-chemical
(	O
Green	O
),	O
BIO399	B-chemical
(	O
Cyan	O
),	O
Digoxin	B-chemical
(	O
Yellow	O
),	O
Compound	O
2	O
(	O
Grey	O
),	O
Compound	O
48	O
(	O
Salmon	O
)	O
and	O
Compound	O
4j	O
(	O
Orange	O
)	O

The	O
co	B-evidence
-	I-evidence
crystal	I-evidence
structure	I-evidence
of	O
RORγ	B-protein
with	O
T0901317	B-chemical
(	O
PDB	O
code	O
:	O
4NB6	O
),	O
an	O
inverse	B-protein_state
agonist	I-protein_state
of	O
RORγ	B-protein
(	O
IC50	B-evidence
of	O
54nM	O
in	O
an	O
SRC1	B-experimental_method
displacement	I-experimental_method
FRET	I-experimental_method
assay	I-experimental_method
and	O
an	O
IC50	B-evidence
of	O
59nM	O
in	O
our	O
FRET	B-experimental_method
assay	I-experimental_method
(	O
Additional	O
file	O
7	O
))	O
shows	O
that	O
it	O
adopts	O
a	O
collapsed	B-protein_state
conformation	O
similar	O
to	O
the	O
structure	B-evidence
of	O
BIO399	B-chemical
described	O
here	O
.	O

The	O
two	O
compounds	O
superimpose	B-experimental_method
with	O
an	O
RMSD	B-evidence
of	O
0	O
.	O
81	O
Å	O
(	O
Fig	O
.	O
6a	O
).	O

The	O
CF3	O
group	O
on	O
the	O
hexafluoropropanol	B-chemical
group	O
of	O
T0901317	B-chemical
was	O
reported	O
to	O
fit	O
the	O
electron	B-evidence
density	I-evidence
in	O
two	O
conformations	O
one	O
of	O
which	O
pushes	O
Met358	B-residue_name_number
into	O
the	O
vicinity	O
of	O
Phe506	B-residue_name_number
in	O
the	O
RORγ	B-protein
BIO592	B-chemical
agonist	B-protein_state
structure	B-evidence
.	O

Co	B-evidence
-	I-evidence
crystal	I-evidence
structures	I-evidence
of	O
RORγ	B-protein
have	O
been	O
generated	O
with	O
several	O
potent	O
inverse	O
agonists	O
adopting	O
a	O
linear	B-protein_state
conformation	O
distinct	O
from	O
the	O
collapsed	B-protein_state
conformations	O
seen	O
for	O
BIO399	B-chemical
and	O
T090131718	B-chemical
.	O

BIO399	B-chemical
neither	O
orients	O
the	O
sidechain	O
of	O
Trp317	B-residue_name_number
toward	O
Tyr502	B-residue_name_number
nor	O
forms	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
His479	B-residue_name_number
suggesting	O
its	O
mode	O
of	O
action	O
is	O
distinct	O
from	O
linear	O
inverse	O
agonists	O
(	O
Additional	O
file	O
8	O
).	O

In	O
the	O
linear	O
inverse	B-protein_state
agonist	I-protein_state
crystal	B-evidence
structures	I-evidence
the	O
side	O
chain	O
of	O
Met358	B-residue_name_number
resides	O
in	O
a	O
similar	O
position	O
as	O
the	O
rotomer	O
observed	O
in	O
RORγ	B-protein
agonist	B-protein_state
structures	B-evidence
with	O
BIO592	B-chemical
described	O
here	O
or	O
as	O
observed	O
in	O
the	O
hydroxycholesterol	B-chemical
derivatives	O
and	O
therefore	O
would	O
not	O
trigger	O
inverse	O
agonism	O
with	O
these	O
ligands	O
(	O
Fig	O
.	O
6b	O
).	O

BIO399	B-chemical
shows	O
selectivity	O
for	O
RORγ	B-protein
over	O
RORα	B-protein
and	O
RORβ	B-protein
in	O
a	O
GAL4	B-experimental_method
Cellular	I-experimental_method
Reporter	I-experimental_method
Assay	I-experimental_method

a	O
Overlay	B-experimental_method
of	O
RORα	B-protein
(	O
yellow	O
),	O
β	B-protein
(	O
pink	O
)	O
and	O
γ	B-protein
(	O
cyan	O
)	O
showing	O
side	O
chain	O
differences	O
at	O
Met358	B-residue_name_number
inverse	O
agonism	O
trigger	O
position	O
and	O
(	O
b	O
)	O
around	O
the	O
benzoxazinone	B-chemical
ring	O
system	O
of	O
BIO399	B-chemical

In	O
order	O
to	O
assess	O
the	O
in	O
vivo	O
selectivity	O
profile	O
of	O
BIO399	B-chemical
a	O
cellular	B-experimental_method
reporter	I-experimental_method
assay	I-experimental_method
was	O
implemented	O
where	O
the	O
ligand	B-structure_element
binding	I-structure_element
domains	I-structure_element
of	O
ROR	B-protein_type
α	B-protein
,	O
β	B-protein
and	O
γ	B-protein
were	O
fused	B-experimental_method
to	I-experimental_method
the	O
DNA	B-structure_element
binding	I-structure_element
domain	I-structure_element
of	O
the	O
transcriptional	B-protein_type
factor	I-protein_type
GAL4	B-protein
.	O

The	O
ROR	B-protein_type
-	O
GAL4	B-protein
fusion	O
proteins	O
were	O
expressed	O
in	O
cells	O
with	O
the	O
luciferase	O
reporter	O
gene	O
under	O
the	O
control	O
of	O
a	O
GAL4	B-protein
promoter	O
.	O

BIO399	B-chemical
inhibited	O
the	O
luciferase	O
activity	O
when	O
added	O
to	O
the	O
cells	O
expressing	O
the	O
RORγ	B-protein
-	O
GAL4	B-protein
fusion	O
with	O
an	O
in	O
vivo	O
IC50	B-evidence
of	O
42	O
.	O
5nM	O
while	O
showing	O
>	O
235	O
and	O
28	O
fold	O
selectivity	O
over	O
cells	O
expressing	O
GAL4	B-protein
fused	O
to	O
the	O
LBD	B-structure_element
of	O
ROR	B-protein_type
α	B-protein
or	O
β	B-protein
,	O
respectively	O
(	O
Table	O
1	O
).	O

The	O
LBS	B-site
of	O
RORs	B-protein_type
share	O
a	O
high	O
degree	O
of	O
similarity	O
.	O

This	O
selectivity	O
profile	O
for	O
BIO399	B-chemical
is	O
attributed	O
to	O
the	O
shorter	O
leucine	B-residue_name
side	O
chain	O
in	O
RORα	B-protein
and	O
β	B-protein
which	O
would	O
not	O
reach	O
the	O
phenylalanine	B-residue_name
on	O
the	O
AF2	B-structure_element
helix	I-structure_element
further	O
underscoring	O
the	O
role	O
of	O
Met358	B-residue_name_number
as	O
a	O
trigger	O
for	O
RORγ	B-protein
specific	O
inverse	O
agonism	O
(	O
Fig	O
.	O
7a	O
).	O

We	O
hypothesize	O
that	O
the	O
two	O
phenylalanine	B-residue_name
residues	O
in	O
the	O
LBS	B-site
of	O
RORα	B-protein
occlude	O
the	O
dihydrobenzoxazepinone	B-chemical
ring	O
system	O
of	O
BIO399	B-chemical
from	O
binding	O
it	O
and	O
responsible	O
for	O
the	O
increase	O
in	O
selectivity	O
for	O
RORα	B-protein
over	O
β	B-protein
.	O

We	O
have	O
identified	O
a	O
novel	O
series	O
of	O
synthetic	O
benzoxazinone	B-chemical
ligands	O
which	O
modulate	O
the	O
transcriptional	O
activity	O
of	O
RORγ	B-protein
in	O
a	O
FRET	B-experimental_method
based	I-experimental_method
assay	I-experimental_method
.	O

Using	O
partial	B-experimental_method
proteolysis	I-experimental_method
we	O
show	O
a	O
conformational	O
change	O
which	O
destabilizes	O
the	O
AF2	B-structure_element
helix	I-structure_element
of	O
RORγ	B-protein
when	O
the	O
inverse	B-protein_state
agonist	I-protein_state
BIO399	B-chemical
binds	O
.	O

The	O
two	O
RORγ	B-protein
co	B-evidence
-	I-evidence
crystal	I-evidence
structures	I-evidence
reported	O
here	O
show	O
how	O
a	O
small	O
change	O
to	O
the	O
core	O
ring	O
system	O
can	O
modulate	O
the	O
mode	O
of	O
action	O
from	O
agonist	B-protein_state
(	O
BIO592	B-chemical
)	O
to	O
inverse	O
agonism	O
(	O
BIO399	B-chemical
).	O

Finally	O
,	O
we	O
are	O
reporting	O
a	O
newly	O
identified	O
trigger	O
for	O
achieving	O
RORγ	B-protein
specific	O
inverse	O
agonism	O
in	O
an	O
in	O
vivo	O
setting	O
through	O
Met358	B-residue_name_number
which	O
perturbs	O
the	O
agonist	B-protein_state
conformation	O
of	O
the	O
AF2	B-structure_element
helix	I-structure_element
and	O
prevents	O
coactivator	O
protein	O
binding	O
.	O

Bacterial	B-taxonomy_domain
Microcompartments	B-complex_assembly
(	O
BMCs	B-complex_assembly
)	O
are	O
proteinaceous	O
organelles	O
that	O
encapsulate	O
critical	O
segments	O
of	O
autotrophic	O
and	O
heterotrophic	O
metabolic	O
pathways	O
;	O
they	O
are	O
functionally	O
diverse	O
and	O
are	O
found	O
across	O
23	O
different	O
phyla	O
.	O

The	O
core	O
enzyme	O
phosphotransacylase	B-protein_type
(	O
PTAC	B-protein_type
)	O
recycles	O
Coenzyme	B-chemical
A	I-chemical
and	O
generates	O
an	O
acyl	B-chemical
phosphate	I-chemical
that	O
can	O
serve	O
as	O
an	O
energy	O
source	O
.	O

The	O
PTAC	B-protein_type
predominantly	O
associated	O
with	O
metabolosomes	B-complex_assembly
(	O
PduL	B-protein_type
)	O
has	O
no	O
sequence	O
homology	O
to	O
the	O
PTAC	B-protein_type
ubiquitous	O
among	O
fermentative	B-taxonomy_domain
bacteria	I-taxonomy_domain
(	O
Pta	B-protein_type
).	O

Here	O
,	O
we	O
report	O
two	O
high	O
-	O
resolution	O
PduL	B-protein_type
crystal	B-evidence
structures	I-evidence
with	B-protein_state
bound	I-protein_state
substrates	I-protein_state
.	O

The	O
PduL	B-protein_type
fold	B-structure_element
is	O
unrelated	O
to	O
that	B-structure_element
of	O
Pta	B-protein_type
;	O
it	O
contains	O
a	O
dimetal	B-site
active	I-site
site	I-site
involved	O
in	O
a	O
catalytic	O
mechanism	O
distinct	O
from	O
that	O
of	O
the	O
housekeeping	B-protein_state
PTAC	B-protein_type
.	O

The	O
PduL	B-protein_type
structure	B-evidence
,	O
in	O
the	O
context	O
of	O
the	O
catalytic	O
core	O
,	O
completes	O
our	O
understanding	O
of	O
the	O
structural	O
basis	O
of	O
cofactor	O
recycling	O
in	O
the	O
metabolosome	B-complex_assembly
lumen	O
.	O

This	O
study	O
describes	O
the	O
structure	B-evidence
of	O
a	O
novel	O
phosphotransacylase	B-protein_type
enzyme	O
that	O
facilitates	O
the	O
recycling	O
of	O
the	O
essential	O
cofactor	O
acetyl	B-chemical
-	I-chemical
CoA	I-chemical
within	O
a	O
bacterial	B-taxonomy_domain
organelle	O
and	O
discusses	O
the	O
properties	O
of	O
the	O
enzyme	O
'	O
s	O
active	B-site
site	I-site
and	O
how	O
it	O
is	O
packaged	O
into	O
the	O
organelle	O
.	O

The	O
phosphotransacylase	B-protein_type
(	O
Pta	B-protein_type
)	O
enzyme	O
catalyzes	O
the	O
conversion	O
between	O
acyl	B-chemical
-	I-chemical
CoA	I-chemical
and	O
acyl	B-chemical
-	I-chemical
phosphate	I-chemical
.	O

This	O
reaction	O
directly	O
links	O
an	O
acyl	B-chemical
-	I-chemical
CoA	I-chemical
with	O
ATP	B-chemical
generation	O
via	O
substrate	O
-	O
level	O
phosphorylation	O
,	O
producing	O
short	B-chemical
-	I-chemical
chain	I-chemical
fatty	I-chemical
acids	I-chemical
(	O
e	O
.	O
g	O
.,	O
acetate	B-chemical
),	O
and	O
also	O
provides	O
a	O
path	O
for	O
short	B-chemical
-	I-chemical
chain	I-chemical
fatty	I-chemical
acids	I-chemical
to	O
enter	O
central	O
metabolism	O
.	O

Due	O
to	O
this	O
key	O
function	O
,	O
Pta	O
is	O
conserved	B-protein_state
across	O
the	O
bacterial	B-taxonomy_domain
kingdom	I-taxonomy_domain
.	O

Recently	O
,	O
a	O
new	O
type	O
of	O
phosphotransacylase	B-protein_type
was	O
described	O
that	O
shares	O
no	O
evolutionary	O
relation	O
to	O
Pta	B-protein_type
.	O

Not	O
only	O
does	O
PduL	B-protein_type
facilitate	O
substrate	O
level	O
phosphorylation	O
,	O
but	O
it	O
also	O
is	O
critical	O
for	O
cofactor	O
recycling	O
within	O
,	O
and	O
product	O
efflux	O
from	O
,	O
the	O
organelle	O
.	O

We	O
solved	B-experimental_method
the	O
structure	B-evidence
of	O
this	O
convergent	B-protein_state
phosphotransacylase	B-protein_type
and	O
show	O
that	O
it	O
is	O
completely	O
structurally	O
different	O
from	O
Pta	B-protein_type
,	O
including	O
its	O
active	B-site
site	I-site
architecture	O
.	O

Bacterial	B-taxonomy_domain
Microcompartments	B-complex_assembly
(	O
BMCs	B-complex_assembly
)	O
are	O
organelles	O
that	O
encapsulate	O
enzymes	O
for	O
sequential	O
biochemical	O
reactions	O
within	O
a	O
protein	O
shell	B-structure_element
.	O

The	O
shell	B-structure_element
is	O
typically	O
composed	O
of	O
three	O
types	O
of	O
protein	O
subunits	O
,	O
which	O
form	O
either	O
hexagonal	B-protein_state
(	O
BMC	B-complex_assembly
-	I-complex_assembly
H	I-complex_assembly
and	O
BMC	B-complex_assembly
-	I-complex_assembly
T	I-complex_assembly
)	O
or	O
pentagonal	B-protein_state
(	O
BMC	B-complex_assembly
-	I-complex_assembly
P	I-complex_assembly
)	O
tiles	O
that	O
assemble	O
into	O
a	O
polyhedral	B-protein_state
shell	B-structure_element
.	O

The	O
vitamin	B-complex_assembly
B12	I-complex_assembly
-	I-complex_assembly
dependent	I-complex_assembly
propanediol	I-complex_assembly
-	I-complex_assembly
utilizing	I-complex_assembly
(	I-complex_assembly
PDU	I-complex_assembly
)	I-complex_assembly
BMC	I-complex_assembly
was	O
one	O
of	O
the	O
first	O
functionally	O
characterized	O
catabolic	B-protein_state
BMCs	B-complex_assembly
;	O
subsequently	O
,	O
other	O
types	O
have	O
been	O
implicated	O
in	O
the	O
degradation	O
of	O
ethanolamine	B-chemical
,	O
choline	B-chemical
,	O
fucose	B-chemical
,	O
rhamnose	B-chemical
,	O
and	O
ethanol	B-chemical
,	O
all	O
of	O
which	O
produce	O
different	O
aldehyde	B-chemical
intermediates	O
(	O
Table	O
1	O
).	O

More	O
recently	O
,	O
bioinformatic	B-experimental_method
studies	I-experimental_method
have	O
demonstrated	O
the	O
widespread	O
distribution	O
of	O
BMCs	B-complex_assembly
among	O
diverse	O
bacterial	B-taxonomy_domain
phyla	I-taxonomy_domain
and	O
grouped	O
them	O
into	O
23	O
different	O
functional	O
types	O
.	O

The	O
reactions	O
carried	O
out	O
in	O
the	O
majority	O
of	O
catabolic	B-protein_state
BMCs	B-complex_assembly
(	O
also	O
known	O
as	O
metabolosomes	B-complex_assembly
)	O
fit	O
a	O
generalized	O
biochemical	O
paradigm	O
for	O
the	O
oxidation	O
of	O
aldehydes	B-chemical
(	O
Fig	O
1	O
).	O

This	O
involves	O
a	O
BMC	B-complex_assembly
-	O
encapsulated	O
signature	O
enzyme	O
that	O
generates	O
a	O
toxic	O
and	O
/	O
or	O
volatile	O
aldehyde	B-chemical
that	O
the	O
BMC	B-complex_assembly
shell	B-structure_element
sequesters	O
from	O
the	O
cytosol	O
.	O

These	O
two	O
cofactors	O
are	O
relatively	O
large	O
,	O
and	O
their	O
diffusion	O
across	O
the	O
protein	B-structure_element
shell	I-structure_element
is	O
thought	O
to	O
be	O
restricted	O
,	O
necessitating	O
their	O
regeneration	O
within	O
the	O
BMC	B-complex_assembly
lumen	O
.	O

The	O
final	O
product	O
of	O
the	O
BMC	B-complex_assembly
,	O
an	O
acyl	B-chemical
-	I-chemical
phosphate	I-chemical
,	O
can	O
then	O
be	O
used	O
to	O
generate	O
ATP	B-chemical
via	O
acyl	B-protein_type
kinase	I-protein_type
,	O
or	O
revert	O
back	O
to	O
acyl	B-chemical
-	I-chemical
CoA	I-chemical
by	O
Pta	B-protein_type
for	O
biosynthesis	O
.	O

Collectively	O
,	O
the	O
aldehyde	B-protein_type
and	I-protein_type
alcohol	I-protein_type
dehydrogenases	I-protein_type
,	O
as	O
well	O
as	O
the	O
PTAC	B-protein_type
,	O
constitute	O
the	O
common	O
metabolosome	B-complex_assembly
core	O
.	O

General	O
biochemical	O
model	O
of	O
aldehyde	B-protein_state
-	I-protein_state
degrading	I-protein_state
BMCs	B-complex_assembly
(	O
metabolosomes	B-complex_assembly
)	O
illustrating	O
the	O
common	O
metabolosome	B-complex_assembly
core	O
enzymes	O
and	O
reactions	O
.	O

Characterized	O
and	O
predicted	O
catabolic	B-protein_state
BMC	B-complex_assembly
(	O
metabolosome	B-complex_assembly
)	O
types	O
that	O
represent	O
the	O
aldehyde	B-chemical
-	O
degrading	O
paradigm	O
(	O
for	O
definition	O
of	O
types	O
see	O
Kerfeld	O
and	O
Erbilgin	O
).	O

Name	O
PTAC	B-protein_type
Type	O
Sequestered	O
Aldehyde	B-chemical
PDU	B-complex_assembly
*	O
PduL	B-protein_type
propionaldehyde	B-chemical
EUT1	B-complex_assembly
PTA_PTB	B-protein_type
acetaldehyde	B-chemical
EUT2	B-complex_assembly
PduL	B-protein_type
acetaldehyde	B-chemical
ETU	B-complex_assembly
None	O
acetaldehyde	B-chemical
GRM1	B-complex_assembly
/	I-complex_assembly
CUT	I-complex_assembly
PduL	B-protein_type
acetaldehyde	B-chemical
GRM2	B-complex_assembly
PduL	B-protein_type
acetaldehyde	B-chemical
GRM3	B-complex_assembly
*,	I-complex_assembly
4	I-complex_assembly
PduL	B-protein_type
propionaldehyde	B-chemical
GRM5	B-complex_assembly
/	I-complex_assembly
GRP	I-complex_assembly
PduL	B-protein_type
propionaldehyde	B-chemical
PVM	B-complex_assembly
*	O
PduL	B-protein_type
lactaldehyde	B-chemical
RMM1	B-complex_assembly
,	I-complex_assembly
2	I-complex_assembly
None	O
unknown	O
SPU	B-complex_assembly
PduL	B-protein_type
unknown	O

*	O
PduL	B-protein_type
from	O
these	O
functional	O
types	O
of	O
metabolosomes	B-complex_assembly
were	O
purified	O
in	O
this	O
study	O
.	O

The	O
concerted	O
functioning	O
of	O
a	O
PTAC	B-protein_type
and	O
an	O
acetate	B-protein_type
kinase	I-protein_type
(	O
Ack	B-protein_type
)	O
is	O
crucial	O
for	O
ATP	B-chemical
generation	O
in	O
the	O
fermentation	O
of	O
pyruvate	B-chemical
to	O
acetate	B-chemical
(	O
see	O
Reactions	O
1	O
and	O
2	O
).	O

Both	O
enzymes	O
are	O
,	O
however	O
,	O
not	O
restricted	O
to	O
fermentative	B-taxonomy_domain
organisms	I-taxonomy_domain
.	O

Reaction	O
1	O
:	O
acetyl	B-chemical
-	I-chemical
S	I-chemical
-	I-chemical
CoA	I-chemical
+	O
Pi	B-chemical
←→	O
acetyl	B-chemical
phosphate	I-chemical
+	O
CoA	B-chemical
-	I-chemical
SH	I-chemical
(	O
PTAC	B-protein_type
)	O

Reaction	O
2	O
:	O
acetyl	B-chemical
phosphate	I-chemical
+	O
ADP	B-chemical
←→	O
acetate	B-chemical
+	O
ATP	B-chemical
(	O
Ack	B-protein_type
)	O

Pta	B-protein_type
has	O
been	O
extensively	O
characterized	O
due	O
to	O
its	O
key	O
role	O
in	O
fermentation	O
.	O

More	O
recently	O
,	O
a	O
second	O
type	O
of	O
PTAC	B-protein_type
without	O
any	O
sequence	O
homology	O
to	O
Pta	B-protein_type
was	O
identified	O
.	O

This	O
protein	O
,	O
PduL	B-protein_type
(	O
Pfam	O
domain	O
PF06130	B-structure_element
),	O
was	O
shown	O
to	O
catalyze	O
the	O
conversion	O
of	O
propionyl	B-chemical
-	I-chemical
CoA	I-chemical
to	O
propionyl	B-chemical
-	I-chemical
phosphate	I-chemical
and	O
is	O
associated	O
with	O
a	O
BMC	B-complex_assembly
involved	O
in	O
propanediol	O
utilization	O
,	O
the	O
PDU	B-complex_assembly
BMC	I-complex_assembly
.	O

Both	O
pduL	B-gene
and	O
pta	B-gene
genes	O
can	O
be	O
found	O
in	O
genetic	O
loci	O
of	O
functionally	O
distinct	O
BMCs	B-complex_assembly
,	O
although	O
the	O
PduL	B-protein_type
type	O
is	O
much	O
more	O
prevalent	O
,	O
being	O
found	O
in	O
all	O
but	O
one	O
type	O
of	O
metabolosome	B-gene
locus	I-gene
:	O
EUT1	B-gene
(	O
Table	O
1	O
).	O

Furthermore	O
,	O
in	O
the	O
Integrated	O
Microbial	O
Genomes	O
Database	O
,	O
91	O
%	O
of	O
genomes	O
that	O
encode	O
PF06130	B-structure_element
also	O
encode	O
genes	O
for	O
shell	O
proteins	O
.	O

As	O
a	O
member	O
of	O
the	O
core	O
biochemical	O
machinery	O
of	O
functionally	O
diverse	O
aldehyde	B-protein_state
-	I-protein_state
oxidizing	I-protein_state
metabolosomes	B-complex_assembly
,	O
PduL	B-protein_type
must	O
have	O
a	O
certain	O
level	O
of	O
substrate	O
plasticity	O
(	O
see	O
Table	O
1	O
)	O
that	O
is	O
not	O
required	O
of	O
Pta	B-protein_type
,	O
which	O
has	O
generally	O
been	O
observed	O
to	O
prefer	O
acetyl	B-chemical
-	I-chemical
CoA	I-chemical
.	O
PduL	B-protein_type
from	O
the	O
PDU	B-complex_assembly
BMC	I-complex_assembly
of	O
Salmonella	B-species
enterica	I-species
favors	O
propionyl	B-chemical
-	I-chemical
CoA	I-chemical
over	O
acetyl	B-chemical
-	I-chemical
CoA	I-chemical
,	O
and	O
it	O
is	O
likely	O
that	O
PduL	B-protein_type
orthologs	O
in	O
functionally	O
diverse	O
BMCs	B-complex_assembly
would	O
have	O
substrate	O
preferences	O
for	O
other	O
CoA	B-chemical
derivatives	O
.	O

EPs	B-structure_element
have	O
also	O
been	O
observed	O
to	O
cause	O
proteins	O
to	O
aggregate	O
,	O
and	O
this	O
has	O
recently	O
been	O
suggested	O
to	O
be	O
functionally	O
relevant	O
as	O
an	O
initial	O
step	O
in	O
metabolosome	B-complex_assembly
assembly	O
,	O
in	O
which	O
a	O
multifunctional	O
protein	O
core	O
is	O
formed	O
,	O
around	O
which	O
the	O
shell	B-structure_element
assembles	O
.	O

Of	O
the	O
three	O
common	O
metabolosome	B-complex_assembly
core	O
enzymes	O
,	O
crystal	B-evidence
structures	I-evidence
are	O
available	O
for	O
both	O
the	O
alcohol	B-protein_type
and	I-protein_type
aldehyde	I-protein_type
dehydrogenases	I-protein_type
.	O

In	O
contrast	O
,	O
the	O
structure	B-evidence
of	O
PduL	B-protein_type
,	O
the	O
PTAC	B-protein_type
found	O
in	O
the	O
vast	O
majority	O
of	O
catabolic	B-protein_state
BMCs	B-complex_assembly
,	O
has	O
not	O
been	O
determined	O
.	O

This	O
is	O
a	O
major	O
gap	O
in	O
our	O
understanding	O
of	O
metabolosome	B-complex_assembly
-	O
encapsulated	O
biochemistry	O
and	O
cofactor	O
recycling	O
.	O

Moreover	O
,	O
it	O
will	O
be	O
useful	O
for	O
guiding	O
efforts	O
to	O
engineer	O
novel	O
BMC	B-complex_assembly
cores	O
for	O
biotechnological	O
applications	O
.	O

No	O
available	O
protein	O
structures	O
contain	O
the	O
PF06130	B-structure_element
domain	O
,	O
and	O
homology	B-experimental_method
searches	I-experimental_method
using	O
the	O
primary	O
structure	O
of	O
PduL	B-protein_type
do	O
not	O
return	O
any	O
significant	O
results	O
that	O
would	O
allow	O
prediction	O
of	O
the	O
structure	B-evidence
.	O

Moreover	O
,	O
the	O
evident	O
novelty	O
of	O
PduL	B-protein_type
makes	O
its	O
structure	B-evidence
interesting	O
in	O
the	O
context	O
of	O
convergent	O
evolution	O
of	O
PTAC	B-protein_type
function	O
;	O
to	O
-	O
date	O
,	O
only	O
the	O
Pta	B-protein_type
active	B-site
site	I-site
and	O
catalytic	O
mechanism	O
is	O
known	O
.	O

We	O
propose	O
a	O
catalytic	O
mechanism	O
analogous	O
but	O
yet	O
distinct	O
from	O
the	O
ubiquitous	O
Pta	B-protein_type
enzyme	O
,	O
highlighting	O
the	O
functional	O
convergence	O
of	O
two	O
enzymes	O
with	O
completely	O
different	O
structures	O
and	O
metal	O
requirements	O
.	O

We	O
also	O
investigate	O
the	O
quaternary	O
structures	O
of	O
three	O
different	O
PduL	B-protein_type
homologs	O
and	O
situate	O
our	O
findings	O
in	O
the	O
context	O
of	O
organelle	O
biogenesis	O
in	O
functionally	O
diverse	O
BMCs	B-complex_assembly
.	O

We	O
cloned	B-experimental_method
,	I-experimental_method
expressed	I-experimental_method
,	I-experimental_method
and	I-experimental_method
purified	I-experimental_method
three	O
different	O
PduL	B-protein_type
homologs	O
from	O
functionally	O
distinct	O
BMCs	B-complex_assembly
(	O
Table	O
1	O
):	O
from	O
the	O
well	O
-	O
studied	O
pdu	B-gene
locus	I-gene
in	O
S	B-species
.	I-species
enterica	I-species
Typhimurium	I-species
LT2	I-species
(	O
sPduL	B-protein
),	O
from	O
the	O
recently	O
characterized	O
pvm	B-gene
locus	I-gene
in	O
Planctomyces	B-species
limnophilus	I-species
(	O
pPduL	B-protein
),	O
and	O
from	O
the	O
grm3	B-gene
locus	I-gene
in	O
Rhodopseudomonas	B-species
palustris	I-species
BisB18	I-species
(	O
rPduL	B-protein
).	O

While	O
purifying	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
sPduL	B-protein
,	O
we	O
observed	O
a	O
tendency	O
to	O
aggregation	O
as	O
described	O
previously	O
,	O
with	O
a	O
large	O
fraction	O
of	O
the	O
expressed	O
protein	O
found	O
in	O
the	O
insoluble	O
fraction	O
in	O
a	O
white	O
,	O
cake	O
-	O
like	O
pellet	O
.	O

Similar	O
differences	O
in	O
solubility	O
were	O
observed	O
for	O
pPduL	B-protein
and	O
rPduL	B-protein
when	O
comparing	O
EP	B-protein_state
-	I-protein_state
truncated	I-protein_state
forms	O
to	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
protein	O
,	O
but	O
none	O
were	O
quite	O
as	O
dramatic	O
as	O
for	O
sPduL	B-protein
.	O
We	O
confirmed	O
that	O
all	O
homologs	O
were	O
active	B-protein_state
(	O
S1a	O
and	O
S1b	O
Fig	O
).	O

Among	O
these	O
,	O
we	O
were	O
only	O
able	O
to	O
obtain	O
diffraction	B-evidence
-	I-evidence
quality	I-evidence
crystals	I-evidence
of	O
rPduL	B-protein
after	O
removing	B-experimental_method
the	O
N	O
-	O
terminal	O
putative	O
EP	B-structure_element
(	O
33	B-residue_range
amino	I-residue_range
acids	I-residue_range
,	O
also	O
see	O
Fig	O
2a	O
)	O
(	O
rPduLΔEP	B-mutant
).	O

Truncated	B-protein_state
rPduLΔEP	B-mutant
had	O
comparable	O
enzymatic	O
activity	O
to	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
enzyme	O
(	O
S1a	O
Fig	O
).	O

Structural	O
overview	O
of	O
R	B-species
.	I-species
palustris	I-species
PduL	B-protein_type
from	O
the	O
grm3	B-gene
locus	I-gene
.	O

(	O
a	O
)	O
Primary	O
and	O
secondary	O
structure	O
of	O
rPduL	B-protein
(	O
tubes	O
represent	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
,	O
arrows	O
β	B-structure_element
-	I-structure_element
sheets	I-structure_element
and	O
dashed	O
line	O
residues	O
disordered	O
in	O
the	O
structure	B-evidence
.	O

The	O
first	B-residue_range
33	I-residue_range
amino	I-residue_range
acids	I-residue_range
are	O
present	O
only	O
in	O
the	O
wildtype	O
construct	O
and	O
contains	O
the	O
predicted	O
EP	B-structure_element
alpha	B-structure_element
helix	I-structure_element
,	O
α0	B-structure_element
);	O
the	O
truncated	B-protein_state
rPduLΔEP	B-mutant
that	O
was	O
crystallized	B-experimental_method
begins	O
with	O
M	B-residue_name
-	O
G	B-residue_name
-	O
V	B-residue_name
.	O
Coloring	O
is	O
according	O
to	O
structural	O
domains	O
(	O
domain	B-structure_element
1	I-structure_element
D36	B-residue_range
-	I-residue_range
N46	I-residue_range
/	O
Q155	B-residue_range
-	I-residue_range
C224	I-residue_range
,	O
blue	O
;	O
loop	B-structure_element
insertion	I-structure_element
G61	B-residue_range
-	I-residue_range
E81	I-residue_range
,	O
grey	O
;	O
domain	B-structure_element
2	I-structure_element
R47	B-residue_range
-	I-residue_range
F60	I-residue_range
/	O
E82	B-residue_range
-	I-residue_range
A154	I-residue_range
,	O
red	O
).	O

Metal	B-site
coordination	I-site
residues	I-site
are	O
highlighted	O
in	O
light	O
blue	O
and	O
CoA	B-site
contacting	I-site
residues	I-site
in	O
magenta	O
,	O
residues	O
contacting	O
the	O
CoA	B-chemical
of	O
the	O
other	O
chain	O
are	O
also	O
outlined	O
.	O

(	O
b	O
)	O
Cartoon	O
representation	O
of	O
the	O
structure	B-evidence
colored	O
by	O
domains	O
and	O
including	O
secondary	O
structure	B-evidence
numbering	O
.	O

Coenzyme	B-chemical
A	I-chemical
is	O
shown	O
in	O
magenta	O
sticks	O
and	O
Zinc	B-chemical
(	O
grey	O
)	O
as	O
spheres	O
.	O

We	O
collected	B-experimental_method
a	I-experimental_method
native	I-experimental_method
dataset	I-experimental_method
from	O
rPduLΔEP	B-mutant
crystals	B-evidence
diffracting	O
to	O
a	O
resolution	O
of	O
1	O
.	O
54	O
Å	O
(	O
Table	O
2	O
).	O

Using	O
a	O
mercury	B-experimental_method
-	I-experimental_method
derivative	I-experimental_method
crystal	I-experimental_method
form	O
diffracting	O
to	O
1	O
.	O
99	O
Å	O
(	O
Table	O
2	O
),	O
we	O
obtained	O
high	O
quality	O
electron	B-evidence
density	I-evidence
for	O
model	O
building	O
and	O
used	O
the	O
initial	O
model	O
to	O
refine	O
against	O
the	O
native	O
data	O
to	O
Rwork	B-evidence
/	O
Rfree	B-evidence
values	O
of	O
18	O
.	O
9	O
/	O
22	O
.	O
1	O
%.	O

There	O
are	O
two	O
PduL	B-protein_type
molecules	O
in	O
the	O
asymmetric	O
unit	O
of	O
the	O
P212121	O
unit	O
cell	O
.	O

We	O
were	O
able	O
to	O
fit	O
all	O
of	O
the	O
primary	O
structure	O
of	O
PduLΔEP	B-mutant
into	O
the	O
electron	B-evidence
density	I-evidence
with	O
the	O
exception	O
of	O
three	O
amino	O
acids	O
at	O
the	O
N	O
-	O
terminus	O
and	O
two	O
amino	O
acids	O
at	O
the	O
C	O
-	O
terminus	O
(	O
Fig	O
2a	O
);	O
the	O
model	O
is	O
of	O
excellent	O
quality	O
(	O
Table	O
2	O
).	O

Structurally	O
,	O
PduL	B-protein_type
consists	O
of	O
two	O
domains	B-structure_element
(	O
Fig	O
2	O
,	O
blue	O
/	O
red	O
),	O
each	O
a	O
beta	B-structure_element
-	I-structure_element
barrel	I-structure_element
that	O
is	O
capped	O
on	O
both	O
ends	O
by	O
short	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
.	O

β	B-structure_element
-	I-structure_element
Barrel	I-structure_element
2	I-structure_element
consists	O
mainly	O
of	O
the	O
central	O
segment	O
of	O
primary	O
structure	O
(	O
β2	B-structure_element
,	O
β5	B-structure_element
–	I-structure_element
β9	I-structure_element
;	O
residues	O
47	B-residue_range
–	I-residue_range
60	I-residue_range
and	O
82	B-residue_range
–	I-residue_range
154	I-residue_range
)	O
(	O
Fig	O
2	O
,	O
red	O
),	O
but	O
is	O
interrupted	O
by	O
a	O
short	B-structure_element
two	I-structure_element
-	I-structure_element
strand	I-structure_element
beta	I-structure_element
sheet	I-structure_element
(	O
β3	B-structure_element
-	I-structure_element
β4	I-structure_element
,	O
residues	O
61	B-residue_range
–	I-residue_range
81	I-residue_range
).	O

This	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
is	O
involved	O
in	O
contacts	O
between	O
the	O
two	O
domains	O
and	O
forms	O
a	O
lid	O
over	O
the	O
active	B-site
site	I-site
.	O

Residues	O
in	O
this	O
region	O
(	O
Gln42	B-residue_name_number
,	O
Pro43	B-residue_name_number
,	O
Gly44	B-residue_name_number
),	O
covering	O
the	O
active	B-site
site	I-site
,	O
are	O
strongly	B-protein_state
conserved	I-protein_state
(	O
Fig	O
3	O
).	O

This	O
structural	O
arrangement	O
is	O
completely	O
different	O
from	O
the	O
functionally	O
related	O
Pta	B-protein_type
,	O
which	O
is	O
composed	O
of	O
two	O
domains	B-structure_element
,	O
each	O
consisting	O
of	O
a	O
central	O
flat	O
beta	B-structure_element
sheet	I-structure_element
with	O
alpha	B-structure_element
-	I-structure_element
helices	I-structure_element
on	O
the	O
top	O
and	O
bottom	O
.	O

Residues	O
100	O
%	O
conserved	O
across	O
all	O
PduL	B-protein_type
homologs	O
in	O
our	O
dataset	O
are	O
noted	O
with	O
an	O
asterisk	O
,	O
and	O
residues	O
conserved	O
in	O
over	O
90	O
%	O
of	O
sequences	O
are	O
noted	O
with	O
a	O
colon	O
.	O

There	O
are	O
two	O
PduL	B-protein_type
molecules	O
in	O
the	O
asymmetric	O
unit	O
forming	O
a	O
butterfly	B-protein_state
-	I-protein_state
shaped	I-protein_state
dimer	B-oligomeric_state
(	O
Fig	O
4c	O
).	O

Consistent	O
with	O
this	O
,	O
results	O
from	O
size	B-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
of	O
rPduLΔEP	B-mutant
suggest	O
that	O
it	O
is	O
a	O
dimer	B-oligomeric_state
in	O
solution	O
(	O
Fig	O
5e	O
).	O

The	O
interface	B-site
between	O
the	O
two	O
chains	O
buries	O
882	O
Å2	O
per	O
monomer	B-oligomeric_state
and	O
is	O
mainly	O
formed	O
by	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
2	I-structure_element
and	I-structure_element
4	I-structure_element
and	O
parts	O
of	O
β	B-structure_element
-	I-structure_element
sheets	I-structure_element
12	I-structure_element
and	I-structure_element
14	I-structure_element
,	O
as	O
well	O
as	O
a	O
π	O
–	O
π	O
stacking	O
of	O
the	O
adenine	B-chemical
moiety	O
of	O
CoA	B-chemical
with	O
Phe116	B-residue_name_number
of	O
the	O
adjacent	O
chain	O
(	O
Fig	O
4c	O
).	O

The	O
folds	O
of	O
the	O
two	O
chains	O
in	O
the	O
asymmetric	O
unit	O
are	O
very	O
similar	O
,	O
superimposing	B-experimental_method
with	O
a	O
rmsd	B-evidence
of	O
0	O
.	O
16	O
Å	O
over	O
2	O
,	O
306	O
aligned	O
atom	O
pairs	O
.	O

The	O
peripheral	O
helices	B-structure_element
and	O
the	O
short	B-structure_element
antiparallel	I-structure_element
β3	I-structure_element
–	I-structure_element
4	I-structure_element
sheet	I-structure_element
mediate	O
most	O
of	O
the	O
crystal	O
contacts	O
.	O

Details	O
of	O
active	B-site
site	I-site
,	O
dimeric	B-oligomeric_state
assembly	O
,	O
and	O
sequence	O
conservation	O
of	O
PduL	B-protein_type
.	O

(	O
a	O
,	O
b	O
)	O
Proposed	O
active	B-site
site	I-site
of	O
PduL	B-protein_type
with	O
relevant	O
residues	O
shown	O
as	O
sticks	O
in	O
atom	O
coloring	O
(	O
nitrogen	B-chemical
blue	O
,	O
oxygen	B-chemical
red	O
,	O
sulfur	B-chemical
yellow	O
),	O
zinc	B-chemical
as	O
grey	O
colored	O
spheres	O
and	O
coordinating	O
ordered	O
water	B-chemical
molecules	O
in	O
red	O
.	O

(	O
c	O
)	O
View	O
of	O
the	O
dimer	B-oligomeric_state
in	O
the	O
asymmetric	O
unit	O
from	O
the	O
side	O
,	O
domains	B-structure_element
1	I-structure_element
and	I-structure_element
2	I-structure_element
colored	O
as	O
in	O
Fig	O
2	O
and	O
the	O
two	O
chains	O
differentiated	O
by	O
blue	O
/	O
red	O
versus	O
slate	O
/	O
firebrick	O
.	O

(	O
d	O
)	O
Surface	O
representation	O
of	O
the	O
structure	B-evidence
with	O
indicated	O
conservation	O
(	O
red	O
:	O
high	O
,	O
white	O
:	O
intermediate	O
,	O
yellow	O
:	O
low	O
).	O

All	O
chromatograms	B-evidence
are	O
cropped	O
to	O
show	O
only	O
the	O
linear	O
range	O
of	O
separation	O
based	O
on	O
standard	O
runs	O
,	O
shown	O
in	O
black	O
squares	O
with	O
a	O
dashed	O
linear	O
trend	O
line	O
.	O

Active	B-site
Site	I-site
Properties	O

CoA	B-chemical
and	O
the	O
metal	O
ions	O
bind	O
between	O
the	O
two	O
domains	O
,	O
presumably	O
in	O
the	O
active	B-site
site	I-site
(	O
Figs	O
2b	O
and	O
4a	O
).	O

To	O
identify	O
the	O
bound	O
metals	O
,	O
we	O
performed	O
an	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
fluorescence	I-experimental_method
scan	I-experimental_method
on	O
the	O
crystals	B-evidence
at	O
various	O
wavelengths	O
(	O
corresponding	O
to	O
the	O
K	O
-	O
edges	O
of	O
Mn	B-chemical
,	O
Fe	B-chemical
,	O
Co	B-chemical
,	O
Ni	B-chemical
,	O
Cu	B-chemical
,	O
and	O
Zn	B-chemical
).	O

There	O
was	O
a	O
large	O
signal	O
at	O
the	O
zinc	O
edge	O
,	O
and	O
we	O
tested	O
for	O
the	O
presence	O
of	O
zinc	B-chemical
by	O
collecting	B-experimental_method
full	I-experimental_method
data	I-experimental_method
sets	I-experimental_method
before	I-experimental_method
and	I-experimental_method
after	I-experimental_method
the	I-experimental_method
Zn	I-experimental_method
K	I-experimental_method
-	I-experimental_method
edge	I-experimental_method
(	I-experimental_method
1	I-experimental_method
.	I-experimental_method
2861	I-experimental_method
and	I-experimental_method
1	I-experimental_method
.	I-experimental_method
2822	I-experimental_method
Å	I-experimental_method
,	O
respectively	O
).	O

The	O
large	O
differences	O
between	O
the	O
anomalous	O
signals	O
confirm	O
the	O
presence	O
of	O
zinc	B-chemical
at	O
both	O
metal	O
sites	O
(	O
S3	O
Fig	O
).	O

The	O
first	O
zinc	B-chemical
ion	O
(	O
Zn1	B-chemical
)	O
is	O
in	O
a	O
tetrahedral	O
coordination	O
state	O
with	O
His48	B-residue_name_number
,	O
His50	B-residue_name_number
,	O
Glu109	B-residue_name_number
,	O
and	O
the	O
CoA	B-chemical
sulfur	B-chemical
(	O
Fig	O
4a	O
).	O

The	O
nitrogen	O
atom	O
coordinating	O
the	O
zinc	B-chemical
is	O
the	O
Nε	O
in	O
each	O
histidine	B-residue_name
residue	O
,	O
as	O
is	O
typical	O
for	O
this	O
interaction	O
.	O

The	O
phosphate	B-protein_state
-	I-protein_state
bound	I-protein_state
structure	B-evidence
aligns	B-experimental_method
well	O
with	O
the	O
CoA	B-protein_state
-	I-protein_state
bound	I-protein_state
structure	B-evidence
(	O
0	O
.	O
43	O
Å	O
rmsd	B-evidence
over	O
2	O
,	O
361	O
atoms	O
for	O
the	O
monomer	B-oligomeric_state
,	O
0	O
.	O
83	O
Å	O
over	O
5	O
,	O
259	O
aligned	O
atoms	O
for	O
the	O
dimer	B-oligomeric_state
).	O

Conserved	B-protein_state
Arg103	B-residue_name_number
seems	O
to	O
be	O
involved	O
in	O
maintaining	O
the	O
phosphate	B-chemical
in	O
that	O
position	O
.	O

An	O
additional	O
phosphate	B-chemical
molecule	O
is	O
bound	O
at	O
a	O
crystal	O
contact	O
interface	O
,	O
perhaps	O
accounting	O
for	O
the	O
14	O
Å	O
shorter	O
c	O
-	O
axis	O
in	O
the	O
phosphate	B-protein_state
-	I-protein_state
bound	I-protein_state
crystal	O
form	O
(	O
Table	O
2	O
).	O

Interestingly	O
,	O
some	O
of	O
the	O
residues	O
important	O
for	O
dimerization	O
of	O
rPduL	B-protein
,	O
particularly	O
Phe116	B-residue_name_number
,	O
are	O
poorly	B-protein_state
conserved	I-protein_state
across	O
PduL	B-protein_type
homologs	O
associated	O
with	O
functionally	O
diverse	O
BMCs	B-complex_assembly
(	O
Figs	O
4c	O
and	O
3	O
),	O
suggesting	O
that	O
they	O
may	O
have	O
alternative	O
oligomeric	O
states	O
.	O

We	O
tested	O
this	O
hypothesis	O
by	O
performing	O
size	B-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
on	O
both	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
and	O
truncated	O
variants	O
(	O
lacking	B-protein_state
the	O
EP	B-structure_element
,	O
ΔEP	B-mutant
)	O
of	O
sPduL	B-protein
,	O
rPduL	B-protein
,	O
and	O
pPduL	B-protein
.	O
These	O
three	O
homologs	O
are	O
found	O
in	O
functionally	O
distinct	O
BMCs	B-complex_assembly
(	O
Table	O
1	O
).	O

It	O
has	O
been	O
proposed	O
that	O
the	O
catabolic	B-protein_state
BMCs	B-complex_assembly
may	O
assemble	O
in	O
a	O
core	O
-	O
first	O
manner	O
,	O
with	O
the	O
luminal	O
enzymes	O
(	O
signature	O
enzyme	O
,	O
aldehyde	B-protein_type
,	I-protein_type
and	I-protein_type
alcohol	I-protein_type
dehydrogenases	I-protein_type
and	O
the	O
BMC	B-complex_assembly
PTAC	B-protein_type
)	O
forming	O
an	O
initial	O
bolus	O
,	O
or	O
prometabolosome	O
,	O
around	O
which	O
a	O
shell	B-structure_element
assembles	O
.	O

We	O
found	O
that	O
not	O
only	O
did	O
the	O
different	O
orthologs	O
appear	O
to	O
assemble	O
into	O
different	O
oligomeric	O
states	O
,	O
but	O
that	O
quaternary	O
structure	O
was	O
dependent	O
on	O
whether	O
or	O
not	O
the	O
EP	B-structure_element
was	O
present	O
.	O

Full	B-protein_state
-	I-protein_state
length	I-protein_state
sPduL	B-protein
was	O
unstable	O
in	O
solution	O
—	O
precipitating	O
over	O
time	O
—	O
and	O
eluted	O
throughout	O
the	O
entire	O
volume	O
of	O
a	O
size	O
exclusion	O
column	O
,	O
indicating	O
it	O
was	O
nonspecifically	O
aggregating	O
.	O

However	O
,	O
when	O
the	O
putative	O
EP	B-structure_element
(	O
residues	O
1	B-residue_range
–	I-residue_range
27	I-residue_range
)	O
was	O
removed	B-experimental_method
(	O
sPduL	B-mutant
ΔEP	I-mutant
),	O
the	O
truncated	B-protein_state
protein	O
was	O
stable	O
and	O
eluted	O
as	O
a	O
single	O
peak	O
(	O
Fig	O
5a	O
)	O
consistent	O
with	O
the	O
size	O
of	O
a	O
monomer	B-oligomeric_state
(	O
Fig	O
5d	O
,	O
blue	O
curve	O
).	O

In	O
contrast	O
,	O
both	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
rPduL	B-protein
and	O
pPduL	B-protein
appeared	O
to	O
exist	O
in	O
two	O
distinct	O
oligomeric	O
states	O
(	O
Fig	O
5b	O
and	O
5c	O
respectively	O
,	O
orange	O
curves	O
),	O
one	O
form	O
of	O
the	O
approximate	O
size	O
of	O
a	O
dimer	B-oligomeric_state
and	O
the	O
second	O
,	O
a	O
higher	O
molecular	O
weight	O
oligomer	B-oligomeric_state
(~	O
150	O
kDa	O
).	O

Upon	O
deletion	B-experimental_method
of	O
the	O
putative	O
EP	B-structure_element
(	O
residues	O
1	B-residue_range
–	I-residue_range
47	I-residue_range
for	O
rPduL	B-protein
,	O
and	O
1	B-residue_range
–	I-residue_range
20	I-residue_range
for	O
pPduL	B-protein
),	O
there	O
was	O
a	O
distinct	O
change	O
in	O
the	O
elution	O
profiles	O
(	O
Fig	O
5b	O
and	O
5c	O
respectively	O
,	O
blue	O
curves	O
).	O

In	O
contrast	O
,	O
rPduLΔEP	B-mutant
eluted	O
as	O
one	O
smaller	O
oligomer	O
,	O
possibly	O
a	O
dimer	B-oligomeric_state
.	O

We	O
also	O
analyzed	O
purified	O
rPduL	B-protein
and	O
rPduLΔEP	B-mutant
by	O
size	B-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
coupled	O
with	O
multiangle	B-experimental_method
light	I-experimental_method
scattering	I-experimental_method
(	O
SEC	B-experimental_method
-	I-experimental_method
MALS	I-experimental_method
)	O
for	O
a	O
complementary	O
approach	O
to	O
assessing	O
oligomeric	O
state	O
.	O

SEC	B-experimental_method
-	I-experimental_method
MALS	I-experimental_method
analysis	O
of	O
rPdulΔEP	B-mutant
is	O
consistent	O
with	O
a	O
dimer	B-oligomeric_state
(	O
as	O
observed	O
in	O
the	O
crystal	B-evidence
structure	I-evidence
)	O
with	O
a	O
weighted	B-evidence
average	I-evidence
(	I-evidence
Mw	I-evidence
)	I-evidence
and	I-evidence
number	I-evidence
average	I-evidence
(	I-evidence
Mn	I-evidence
)	I-evidence
of	I-evidence
the	I-evidence
molar	I-evidence
mass	I-evidence
of	O
58	O
.	O
4	O
kDa	O
+/−	O
11	O
.	O
2	O
%	O
and	O
58	O
.	O
8	O
kDa	O
+/−	O
10	O
.	O
9	O
%,	O
respectively	O
(	O
S4a	O
Fig	O
).	O

This	O
corresponds	O
to	O
an	O
oligomeric	O
state	O
of	O
six	B-oligomeric_state
subunits	I-oligomeric_state
(	O
calculated	O
molecular	B-evidence
weight	I-evidence
of	O
144	O
kDa	O
).	O

Collectively	O
,	O
these	O
data	O
strongly	O
suggest	O
that	O
the	O
N	O
-	O
terminal	O
EP	B-structure_element
of	O
PduL	B-protein_type
plays	O
a	O
role	O
in	O
defining	O
the	O
quaternary	O
structure	O
of	O
the	O
protein	O
.	O

The	O
BMC	B-complex_assembly
shell	B-structure_element
not	O
only	O
sequesters	O
specific	O
enzymes	O
but	O
also	O
their	O
cofactors	O
,	O
thereby	O
establishing	O
a	O
private	O
cofactor	O
pool	O
dedicated	O
to	O
the	O
encapsulated	O
reactions	O
.	O

In	O
catabolic	B-protein_state
BMCs	B-complex_assembly
,	O
CoA	B-chemical
and	O
NAD	B-chemical
+	I-chemical
must	O
be	O
continually	O
recycled	O
within	O
the	O
organelle	O
(	O
Fig	O
1	O
).	O

Curiously	O
,	O
while	O
the	O
housekeeping	B-protein_state
Pta	B-protein_type
could	O
provide	O
this	O
function	O
,	O
and	O
indeed	O
does	O
so	O
in	O
the	O
case	O
of	O
one	O
type	O
of	O
ethanolamine	B-complex_assembly
-	I-complex_assembly
utilizing	I-complex_assembly
(	I-complex_assembly
EUT	I-complex_assembly
)	I-complex_assembly
BMC	I-complex_assembly
,	O
the	O
evolutionarily	O
unrelated	O
PduL	B-protein_type
fulfills	O
this	O
function	O
for	O
the	O
majority	O
of	O
metabolosomes	B-complex_assembly
using	O
a	O
novel	O
structure	B-evidence
and	O
active	B-site
site	I-site
for	O
convergent	O
evolution	O
of	O
function	O
.	O

The	O
Tertiary	O
Structure	O
of	O
PduL	B-protein_type
Is	O
Formed	O
by	O
Discontinuous	O
Segments	O
of	O
Primary	O
Structure	O

The	O
structure	B-evidence
of	O
PduL	B-protein_type
consists	O
of	O
two	B-structure_element
β	I-structure_element
-	I-structure_element
barrel	I-structure_element
domains	I-structure_element
capped	O
by	O
short	B-structure_element
alpha	I-structure_element
helical	I-structure_element
segments	I-structure_element
(	O
Fig	O
2b	O
).	O

The	O
two	O
domains	O
are	O
structurally	O
very	O
similar	O
(	O
superimposing	B-experimental_method
with	O
a	O
rmsd	B-evidence
of	O
1	O
.	O
34	O
Å	O
(	O
over	O
123	O
out	O
of	O
320	O
/	O
348	O
aligned	O
backbone	O
atoms	O
,	O
S5a	O
Fig	O
).	O

However	O
,	O
the	O
amino	O
acid	O
sequences	O
of	O
the	O
two	O
domains	O
are	O
only	O
16	O
%	O
identical	O
(	O
mainly	O
the	O
RHxH	B-structure_element
motif	I-structure_element
,	O
β2	B-structure_element
and	O
β10	B-structure_element
),	O
and	O
34	O
%	O
similar	O
.	O

Our	O
structure	B-evidence
reveals	O
that	O
the	O
two	O
assigned	O
PF06130	B-structure_element
domains	O
(	O
Fig	O
3	O
)	O
do	O
not	O
form	O
structurally	O
discrete	O
units	O
;	O
this	O
reduces	O
the	O
apparent	O
sequence	O
conservation	O
at	O
the	O
level	O
of	O
primary	O
structure	O
.	O

One	O
strand	B-structure_element
of	O
the	O
domain	B-structure_element
1	I-structure_element
beta	B-structure_element
barrel	I-structure_element
(	O
shown	O
in	O
blue	O
in	O
Fig	O
2	O
)	O
is	O
contributed	O
by	O
the	O
N	O
-	O
terminus	O
,	O
while	O
the	O
rest	O
of	O
the	O
domain	O
is	O
formed	O
by	O
the	O
residues	O
from	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
half	I-structure_element
of	O
the	O
protein	B-protein_type
.	O

When	O
aligned	B-experimental_method
by	O
structure	B-evidence
,	O
the	O
β1	B-structure_element
strand	I-structure_element
of	O
the	O
first	B-structure_element
domain	I-structure_element
(	O
Fig	O
2a	O
and	O
2b	O
,	O
blue	O
)	O
corresponds	O
to	O
the	O
final	B-structure_element
strand	I-structure_element
of	O
the	O
second	B-structure_element
domain	I-structure_element
(	O
β9	B-structure_element
),	O
effectively	O
making	O
the	O
domains	O
continuous	O
if	O
the	O
first	O
strand	O
was	O
transplanted	O
to	O
the	O
C	O
-	O
terminus	O
.	O

The	O
closest	O
structural	O
homolog	O
of	O
the	O
PduL	B-protein_type
barrel	B-structure_element
domain	I-structure_element
is	O
a	O
subdomain	O
of	O
a	O
multienzyme	O
complex	O
,	O
the	O
alpha	B-structure_element
subunit	I-structure_element
of	O
ethylbenzene	B-protein_type
dehydrogenase	I-protein_type
(	O
S5b	O
Fig	O
,	O
rmsd	B-evidence
of	O
2	O
.	O
26	O
Å	O
over	O
226	O
aligned	O
atoms	O
consisting	O
of	O
one	O
beta	B-structure_element
barrel	I-structure_element
and	O
one	O
capping	B-structure_element
helix	I-structure_element
).	O

In	O
contrast	O
to	O
PduL	B-protein_type
,	O
there	O
is	O
only	O
one	O
barrel	B-structure_element
present	O
in	O
ethylbenzene	B-protein_type
dehydrogenase	I-protein_type
,	O
and	O
there	O
is	O
no	O
comparable	O
active	B-site
site	I-site
arrangement	O
.	O

The	O
PduL	B-protein_type
signature	O
primary	O
structure	O
,	O
two	O
PF06130	B-structure_element
domains	O
,	O
occurs	O
in	O
some	O
multidomain	O
proteins	O
,	O
most	O
of	O
them	O
annotated	O
as	O
Acks	B-protein_type
,	O
suggesting	O
that	O
PduL	B-protein_type
may	O
also	O
replace	O
Pta	B-protein_type
in	O
variants	O
of	O
the	O
phosphotransacetylase	B-protein_type
-	O
Ack	B-protein_type
pathway	O
.	O

These	O
PduL	B-protein_type
homologs	O
lack	B-protein_state
EPs	B-structure_element
,	O
and	O
their	B-protein_type
fusion	O
to	O
Ack	B-protein_type
may	O
have	O
evolved	O
as	O
a	O
way	O
to	O
facilitate	O
substrate	O
channeling	O
between	O
the	O
two	O
enzymes	O
.	O

For	O
BMC	B-complex_assembly
-	O
encapsulated	O
proteins	O
to	O
properly	O
function	O
together	O
,	O
they	O
must	O
be	O
targeted	O
to	O
the	O
lumen	O
and	O
assemble	O
into	O
an	O
organization	O
that	O
facilitates	O
substrate	O
/	O
product	O
channeling	O
among	O
the	O
different	O
catalytic	B-site
sites	I-site
of	O
the	O
signature	O
and	O
core	O
enzymes	O
.	O

The	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
extension	I-structure_element
on	O
PduL	B-protein_type
homologs	O
may	O
serve	O
both	O
of	O
these	O
functions	O
.	O

The	B-structure_element
extension	I-structure_element
shares	O
many	O
features	O
with	O
previously	O
characterized	O
EPs	B-structure_element
:	O
it	O
is	O
present	O
only	O
in	O
homologs	O
associated	O
with	O
BMC	B-gene
loci	I-gene
,	O
and	O
it	O
is	O
predicted	O
to	O
form	O
an	O
amphipathic	B-protein_state
α	B-structure_element
-	I-structure_element
helix	I-structure_element
.	O

Moreover	O
,	O
its	O
removal	B-experimental_method
affects	O
the	O
oligomeric	O
state	O
of	O
the	O
protein	O
.	O

EP	B-structure_element
-	O
mediated	O
oligomerization	O
has	O
been	O
observed	O
for	O
the	O
signature	O
and	O
core	O
BMC	B-complex_assembly
enzymes	O
;	O
for	O
example	O
,	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
propanediol	B-protein_type
dehydratase	I-protein_type
and	O
ethanolamine	B-protein_type
ammonia	I-protein_type
-	I-protein_type
lyase	I-protein_type
(	O
signature	O
enzymes	O
for	O
PDU	B-complex_assembly
and	O
EUT	B-complex_assembly
BMCs	I-complex_assembly
)	O
subunits	O
are	O
also	O
insoluble	O
,	O
but	O
become	O
soluble	O
upon	O
removal	O
of	O
the	O
predicted	O
EP	B-structure_element
.	O

sPduL	B-protein
has	O
also	O
previously	O
been	O
reported	O
to	O
localize	O
to	O
inclusion	O
bodies	O
when	O
overexpressed	B-experimental_method
;	O
we	O
show	O
here	O
that	O
this	O
is	O
dependent	O
on	O
the	O
presence	O
of	O
the	O
EP	B-structure_element
.	O

This	O
propensity	O
of	O
the	O
EP	B-structure_element
to	O
cause	O
proteins	O
to	O
form	O
complexes	O
(	O
Fig	O
5	O
)	O
might	O
not	O
be	O
a	O
coincidence	O
,	O
but	O
could	O
be	O
a	O
necessary	O
step	O
in	O
the	O
assembly	O
of	O
BMCs	B-complex_assembly
.	O

Structured	O
aggregation	O
of	O
the	O
core	O
enzymes	O
has	O
been	O
proposed	O
to	O
be	O
the	O
initial	O
step	O
in	O
metabolosome	B-complex_assembly
assembly	O
and	O
is	O
known	O
to	O
be	O
the	O
first	O
step	O
of	O
β	O
-	O
carboxysome	O
biogenesis	O
,	O
where	O
the	O
core	O
enzyme	O
Ribulose	B-protein_type
Bisphosphate	I-protein_type
Carboxylase	I-protein_type
/	I-protein_type
Oxygenase	I-protein_type
(	O
RuBisCO	B-protein_type
)	O
is	O
aggregated	O
by	O
the	O
CcmM	B-protein_type
protein	O
.	O

Likewise	O
,	O
CsoS2	B-protein_type
,	O
a	O
protein	O
in	O
the	O
α	B-complex_assembly
-	I-complex_assembly
carboxysome	I-complex_assembly
core	O
,	O
also	O
aggregates	O
when	O
purified	O
and	O
is	O
proposed	O
to	O
facilitate	O
the	O
nucleation	O
and	O
encapsulation	O
of	O
RuBisCO	B-protein_type
molecules	O
in	O
the	O
lumen	O
of	O
the	O
organelle	O
.	O

This	O
role	O
for	O
EPs	B-structure_element
in	O
BMC	B-complex_assembly
assembly	O
is	O
in	O
addition	O
to	O
their	O
interaction	O
with	O
shell	O
proteins	O
.	O

Our	O
PduL	B-protein_type
crystals	B-evidence
contained	O
CoA	B-chemical
that	O
was	O
captured	O
from	O
the	O
Escherichia	B-species
coli	I-species
cytosol	O
,	O
indicating	O
that	O
the	O
“	O
ground	O
state	O
”	O
of	O
PduL	B-protein_type
is	O
in	O
the	O
CoA	B-protein_state
-	I-protein_state
bound	I-protein_state
form	O
;	O
this	O
could	O
provide	O
an	O
elegantly	O
simple	O
means	O
of	O
guaranteeing	O
a	O
1	O
:	O
1	O
ratio	O
of	O
CoA	B-complex_assembly
:	I-complex_assembly
PduL	I-complex_assembly
within	O
the	O
metabolosome	B-complex_assembly
lumen	O
.	O

Active	B-site
Site	I-site
Identification	O
and	O
Structural	O
Insights	O
into	O
Catalysis	O

The	O
active	B-site
site	I-site
of	O
PduL	B-protein_type
is	O
formed	O
at	O
the	O
interface	B-site
of	O
the	O
two	O
structural	O
domains	B-structure_element
(	O
Fig	O
2b	O
).	O

As	O
expected	O
,	O
the	O
amino	O
acid	O
sequence	O
conservation	O
is	O
highest	O
in	O
the	O
region	O
around	O
the	O
proposed	O
active	B-site
site	I-site
(	O
Fig	O
4d	O
);	O
highly	B-protein_state
conserved	I-protein_state
residues	O
are	O
also	O
involved	O
in	O
CoA	B-chemical
binding	O
(	O
Figs	O
2a	O
and	O
3	O
,	O
residues	O
Ser45	B-residue_name_number
,	O
Lys70	B-residue_name_number
,	O
Arg97	B-residue_name_number
,	O
Leu99	B-residue_name_number
,	O
His204	B-residue_name_number
,	O
Asn211	B-residue_name_number
).	O

All	O
of	O
the	O
metal	B-site
-	I-site
coordinating	I-site
residues	I-site
(	O
Fig	O
2a	O
)	O
are	O
absolutely	B-protein_state
conserved	I-protein_state
,	O
implicating	O
them	O
in	O
catalysis	O
or	O
the	O
correct	O
spatial	O
orientation	O
of	O
the	O
substrates	O
.	O

Arg103	B-residue_name_number
,	O
which	O
contacts	O
the	O
phosphate	B-chemical
(	O
Fig	O
4b	O
),	O
is	O
present	O
in	O
all	O
PduL	B-protein_type
homologs	O
.	O

The	O
close	O
resemblance	O
between	O
the	O
structures	O
binding	O
CoA	B-chemical
and	O
phosphate	B-chemical
likely	O
indicates	O
that	O
no	O
large	O
changes	O
in	O
protein	O
conformation	O
are	O
involved	O
in	O
catalysis	O
,	O
and	O
that	O
our	O
crystal	B-evidence
structures	I-evidence
are	O
representative	O
of	O
the	O
active	B-protein_state
form	O
.	O

There	O
is	O
a	O
pocket	B-site
nearby	O
the	O
active	B-site
site	I-site
between	O
the	O
well	B-protein_state
-	I-protein_state
conserved	I-protein_state
residues	O
Ser45	B-residue_name_number
and	O
Ala154	B-residue_name_number
,	O
which	O
could	O
accommodate	O
the	O
propionyl	O
group	O
(	O
S6	O
Fig	O
).	O

A	O
homology	B-experimental_method
model	I-experimental_method
of	O
sPduL	B-protein
indicates	O
that	O
the	O
residues	O
making	O
up	O
this	O
pocket	B-site
and	O
the	O
surrounding	O
active	B-site
site	I-site
region	O
are	O
identical	O
to	O
that	O
of	O
rPduL	B-protein
,	O
which	O
is	O
not	O
surprising	O
,	O
because	O
these	O
two	O
homologs	O
presumably	O
have	O
the	O
same	O
propionyl	B-chemical
-	I-chemical
CoA	I-chemical
substrate	O
.	O

The	O
homology	B-experimental_method
model	I-experimental_method
of	O
pPduL	B-protein
also	O
has	O
identical	O
residues	O
making	O
up	O
the	O
pocket	B-site
,	O
but	O
with	O
a	O
key	O
difference	O
in	O
the	O
vicinity	O
of	O
the	O
active	B-site
site	I-site
:	O
Gln77	B-residue_name_number
of	O
rPduL	B-protein
is	O
replaced	O
by	O
a	O
tyrosine	B-residue_name
(	O
Tyr77	B-residue_name_number
)	O
in	O
pPduL	B-protein
.	O
The	O
physiological	O
substrate	O
of	O
pPduL	B-protein
(	O
Table	O
1	O
)	O
is	O
thought	O
to	O
be	O
lactyl	B-chemical
-	I-chemical
CoA	I-chemical
,	O
which	O
contains	O
an	O
additional	O
hydroxyl	O
group	O
relative	O
to	O
propionyl	B-chemical
-	I-chemical
CoA	I-chemical
.	O
The	O
presence	O
of	O
an	O
aromatic	B-protein_state
residue	B-structure_element
at	O
this	O
position	O
may	O
underlie	O
the	O
substrate	O
preference	O
of	O
the	O
PduL	B-protein_type
enzyme	O
from	O
the	O
pvm	B-gene
locus	I-gene
.	O

The	O
catalytic	O
mechanism	O
of	O
Pta	B-protein_type
involves	O
the	O
abstraction	O
of	O
a	O
thiol	O
hydrogen	O
by	O
an	O
aspartate	B-residue_name
residue	O
,	O
resulting	O
in	O
the	O
nucleophilic	O
attack	O
of	O
thiolate	O
upon	O
the	O
carbonyl	O
carbon	O
of	O
acetyl	B-chemical
-	I-chemical
phosphate	I-chemical
,	O
oriented	O
by	O
an	O
arginine	B-residue_name
and	O
stabilized	O
by	O
a	O
serine	B-residue_name
—	O
there	O
are	O
no	O
metals	O
involved	O
.	O

In	O
contrast	O
,	O
in	O
the	O
rPduL	B-protein
structure	B-evidence
,	O
there	O
are	O
no	O
conserved	O
aspartate	B-residue_name
residues	O
in	O
or	O
around	O
the	O
active	B-site
site	I-site
,	O
and	O
the	O
only	O
well	B-protein_state
-	I-protein_state
conserved	I-protein_state
glutamate	B-residue_name
residue	O
in	O
the	O
active	B-site
site	I-site
is	O
involved	O
in	O
coordinating	B-bond_interaction
one	O
of	O
the	O
metal	O
ions	O
.	O

These	O
observations	O
strongly	O
suggest	O
that	O
an	O
acidic	B-protein_state
residue	B-structure_element
is	O
not	O
directly	O
involved	O
in	O
catalysis	O
by	O
PduL	B-protein_type
.	O
Instead	O
,	O
the	O
dimetal	B-site
active	I-site
site	I-site
of	O
PduL	B-protein_type
may	O
create	O
a	O
nucleophile	O
from	O
one	O
of	O
the	O
hydroxyl	O
groups	O
on	O
free	O
phosphate	B-chemical
to	O
attack	O
the	O
carbonyl	O
carbon	O
of	O
the	O
thioester	O
bond	O
of	O
an	O
acyl	B-chemical
-	I-chemical
CoA	I-chemical
.	O
In	O
the	O
reverse	O
direction	O
,	O
the	O
metal	O
ion	O
(	O
s	O
)	O
could	O
stabilize	O
the	O
thiolate	O
anion	O
that	O
would	O
attack	O
the	O
carbonyl	O
carbon	O
of	O
an	O
acyl	B-chemical
-	I-chemical
phosphate	I-chemical
;	O
a	O
similar	O
mechanism	O
has	O
been	O
described	O
for	O
phosphatases	B-protein_type
where	O
hydroxyl	O
groups	O
or	O
hydroxide	O
ions	O
can	O
act	O
as	O
a	O
base	O
when	O
coordinated	O
by	O
a	O
dimetal	B-site
active	I-site
site	I-site
.	O

Our	O
structures	B-evidence
provide	O
the	O
foundation	O
for	O
studies	O
to	O
elucidate	O
the	O
details	O
of	O
the	O
catalytic	O
mechanism	O
of	O
PduL	B-protein_type
.	O
Conserved	B-protein_state
residues	O
in	O
the	O
active	B-site
site	I-site
that	O
may	O
contribute	O
to	O
substrate	O
binding	O
and	O
/	O
or	O
transition	O
state	O
stabilization	O
include	O
Ser127	B-residue_name_number
,	O
Arg103	B-residue_name_number
,	O
Arg194	B-residue_name_number
,	O
Gln107	B-residue_name_number
,	O
Gln74	B-residue_name_number
,	O
and	O
Gln	B-residue_name_number
/	O
Glu77	B-residue_name_number
.	O

In	O
the	O
phosphate	B-protein_state
-	I-protein_state
bound	I-protein_state
crystal	B-evidence
structure	I-evidence
,	O
Ser127	B-residue_name_number
and	O
Arg103	B-residue_name_number
appear	O
to	O
position	O
the	O
phosphate	B-chemical
(	O
Fig	O
4b	O
).	O

Alternatively	O
,	O
Arg103	B-residue_name_number
might	O
act	O
as	O
a	O
base	O
to	O
render	O
the	O
phosphate	B-chemical
more	O
nucleophilic	O
.	O

The	O
functional	O
groups	O
of	O
Gln74	B-residue_name_number
,	O
Gln	B-residue_name_number
/	O
Glu77	B-residue_name_number
,	O
and	O
Arg194	B-residue_name_number
are	O
directed	O
away	O
from	O
the	O
active	B-site
site	I-site
in	O
both	O
CoA	B-protein_state
and	O
phosphate	B-protein_state
-	I-protein_state
bound	I-protein_state
crystal	B-evidence
structures	I-evidence
and	O
do	O
not	O
appear	O
to	O
be	O
involved	O
in	O
hydrogen	B-bond_interaction
bonding	I-bond_interaction
with	O
these	O
substrates	O
,	O
although	O
they	O
could	O
be	O
important	O
for	O
positioning	O
an	O
acyl	B-chemical
-	I-chemical
phosphate	I-chemical
.	O

This	O
hypothesis	O
is	O
strengthened	O
by	O
the	O
fact	O
that	O
the	O
CoA	B-protein_state
-	I-protein_state
bound	I-protein_state
crystals	B-evidence
were	O
obtained	O
without	O
added	O
CoA	B-chemical
,	O
indicating	O
that	O
the	O
protein	O
bound	B-protein_state
CoA	B-chemical
from	O
the	O
E	B-species
.	I-species
coli	I-species
expression	O
strain	O
and	O
retained	O
it	O
throughout	O
purification	O
and	O
crystallization	O
.	O

Functional	O
,	O
but	O
Not	O
Structural	O
,	O
Convergence	O
of	O
PduL	B-protein_type
and	O
Pta	B-protein_type

PduL	B-protein_type
and	O
Pta	B-protein_type
are	O
mechanistically	O
and	O
structurally	O
distinct	O
enzymes	O
that	O
catalyze	O
the	O
same	O
reaction	O
,	O
a	O
prime	O
example	O
of	O
evolutionary	O
convergence	O
upon	O
a	O
function	O
.	O

There	O
are	O
several	O
examples	O
of	O
such	O
functional	O
convergence	O
of	O
enzymes	O
,	O
although	O
typically	O
the	O
enzymes	O
have	O
independently	O
evolved	O
similar	O
,	O
or	O
even	O
identical	O
active	B-site
sites	I-site
;	O
for	O
example	O
,	O
the	O
carbonic	B-protein_type
anhydrase	I-protein_type
family	O
.	O

However	O
,	O
apparently	O
less	O
frequent	O
is	O
functional	O
convergence	O
that	O
is	O
supported	O
by	O
distinctly	O
different	O
active	B-site
sites	I-site
and	O
accordingly	O
catalytic	O
mechanism	O
,	O
as	O
revealed	O
by	O
comparison	O
of	O
the	O
structures	O
of	O
Pta	B-protein_type
and	O
PduL	B-protein_type
.	O
One	O
well	O
-	O
studied	O
example	O
of	O
this	O
is	O
the	O
β	B-protein_type
-	I-protein_type
lactamase	I-protein_type
family	O
of	O
enzymes	O
,	O
in	O
which	O
the	O
active	B-site
site	I-site
of	O
Class	O
A	O
and	O
Class	O
C	O
enzymes	O
involve	O
serine	O
-	O
based	O
catalysis	O
,	O
but	O
Class	O
B	O
enzymes	O
are	O
metalloproteins	B-protein_type
.	O

This	O
is	O
not	O
surprising	O
,	O
as	O
β	B-protein_type
-	I-protein_type
lactamases	I-protein_type
are	O
not	O
so	O
widespread	O
among	O
bacteria	B-taxonomy_domain
and	O
therefore	O
would	O
be	O
expected	O
to	O
have	O
evolved	O
independently	O
several	O
times	O
as	O
a	O
defense	O
mechanism	O
against	O
β	O
-	O
lactam	O
antibiotics	O
.	O

However	O
,	O
nearly	O
all	O
bacteria	B-taxonomy_domain
encode	O
Pta	B-protein_type
,	O
and	O
it	O
is	O
not	O
immediately	O
clear	O
why	O
the	O
Pta	B-protein_type
/	O
PduL	B-protein_type
functional	O
convergence	O
should	O
have	O
evolved	O
:	O
it	O
would	O
seem	O
to	O
be	O
evolutionarily	O
more	O
resourceful	O
for	O
the	O
Pta	B-gene
-	I-gene
encoding	I-gene
gene	I-gene
to	O
be	O
duplicated	O
and	O
repurposed	O
for	O
BMCs	B-complex_assembly
,	O
as	O
is	O
apparently	O
the	O
case	O
in	O
one	O
type	O
of	O
BMC	B-complex_assembly
—	I-complex_assembly
EUT1	I-complex_assembly
(	O
Table	O
1	O
).	O

Further	O
biochemical	O
comparison	O
between	O
the	O
two	O
PTACs	B-protein_type
will	O
likely	O
yield	O
exciting	O
results	O
that	O
could	O
answer	O
this	O
evolutionary	O
question	O
.	O

BMCs	B-complex_assembly
are	O
now	O
known	O
to	O
be	O
widespread	O
among	O
the	O
bacteria	B-taxonomy_domain
and	O
are	O
involved	O
in	O
critical	O
segments	O
of	O
both	O
autotrophic	O
and	O
heterotrophic	O
biochemical	O
pathways	O
that	O
confer	O
to	O
the	O
host	O
organism	O
a	O
competitive	O
(	O
metabolic	O
)	O
advantage	O
in	O
select	O
niches	O
.	O

As	O
one	O
of	O
the	O
three	O
common	O
metabolosome	B-complex_assembly
core	O
enzymes	O
,	O
the	O
structure	B-evidence
of	O
PduL	B-protein_type
provides	O
a	O
key	O
missing	O
piece	O
to	O
our	O
structural	O
picture	O
of	O
the	O
shared	O
core	O
biochemistry	O
(	O
Fig	O
1	O
)	O
of	O
functionally	O
diverse	O
catabolic	B-protein_state
BMCs	B-complex_assembly
.	O

We	O
have	O
observed	O
the	O
oligomeric	O
state	O
differences	O
of	O
PduL	B-protein_type
to	O
correlate	O
with	O
the	O
presence	O
of	O
an	O
EP	B-structure_element
,	O
providing	O
new	O
insight	O
into	O
the	O
function	O
of	O
this	O
sequence	O
extension	O
in	O
BMC	B-complex_assembly
assembly	O
.	O

Moreover	O
,	O
our	O
results	O
suggest	O
a	O
means	O
for	O
Coenzyme	B-chemical
A	I-chemical
incorporation	O
during	O
metabolosome	B-complex_assembly
biogenesis	O
.	O

The	O
fact	O
that	O
PduL	B-protein_type
is	O
confined	O
almost	O
exclusively	O
to	O
metabolosomes	B-complex_assembly
can	O
be	O
used	O
to	O
develop	O
an	O
inhibitor	O
that	O
blocks	O
only	O
PduL	B-protein_type
and	O
not	O
Pta	B-protein_type
as	O
a	O
way	O
to	O
selectively	O
disrupt	O
BMC	B-complex_assembly
-	O
based	O
metabolism	O
,	O
while	O
not	O
affecting	O
most	O
commensal	O
organisms	O
that	O
require	O
PTAC	B-protein_type
activity	O
.	O

Biochemistry	O
and	O
Crystal	B-evidence
Structure	I-evidence
of	O
Ectoine	B-protein_type
Synthase	I-protein_type
:	O
A	O
Metal	B-protein_state
-	I-protein_state
Containing	I-protein_state
Member	O
of	O
the	O
Cupin	B-protein_type
Superfamily	I-protein_type

Ectoine	B-chemical
is	O
a	O
compatible	O
solute	O
and	O
chemical	O
chaperone	O
widely	O
used	O
by	O
members	O
of	O
the	O
Bacteria	B-taxonomy_domain
and	O
a	O
few	O
Archaea	B-taxonomy_domain
to	O
fend	O
-	O
off	O
the	O
detrimental	O
effects	O
of	O
high	O
external	O
osmolarity	O
on	O
cellular	O
physiology	O
and	O
growth	O
.	O

Ectoine	B-protein_type
synthase	I-protein_type
(	O
EctC	B-protein_type
)	O
catalyzes	O
the	O
last	O
step	O
in	O
ectoine	B-chemical
production	O
and	O
mediates	O
the	O
ring	O
closure	O
of	O
the	O
substrate	O
N	B-chemical
-	I-chemical
gamma	I-chemical
-	I-chemical
acetyl	I-chemical
-	I-chemical
L	I-chemical
-	I-chemical
2	I-chemical
,	I-chemical
4	I-chemical
-	I-chemical
diaminobutyric	I-chemical
acid	I-chemical
through	O
a	O
water	B-chemical
elimination	O
reaction	O
.	O

However	O
,	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
ectoine	B-protein_type
synthase	I-protein_type
is	O
not	O
known	O
and	O
a	O
clear	O
understanding	O
of	O
how	O
its	O
fold	O
contributes	O
to	O
enzyme	O
activity	O
is	O
thus	O
lacking	O
.	O

Using	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
from	O
the	O
cold	O
-	O
adapted	O
marine	B-taxonomy_domain
bacterium	I-taxonomy_domain
Sphingopyxis	B-species
alaskensis	I-species
(	O
Sa	B-species
),	O
we	O
report	O
here	O
both	O
a	O
detailed	O
biochemical	O
characterization	O
of	O
the	O
EctC	B-protein
enzyme	O
and	O
the	O
high	O
-	O
resolution	O
crystal	B-evidence
structure	I-evidence
of	O
its	O
apo	B-protein_state
-	O
form	O
.	O

Structural	B-experimental_method
analysis	I-experimental_method
classified	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
as	O
a	O
member	O
of	O
the	O
cupin	B-protein_type
superfamily	I-protein_type
.	O

The	O
interface	B-site
of	O
the	O
dimer	B-oligomeric_state
assembly	O
is	O
shaped	O
through	O
backbone	O
-	O
contacts	O
and	O
weak	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
mediated	O
by	O
two	O
beta	B-structure_element
-	I-structure_element
sheets	I-structure_element
within	O
each	O
monomer	B-oligomeric_state
.	O

We	O
found	O
that	O
EctC	B-protein
not	O
only	O
effectively	O
converts	O
its	O
natural	O
substrate	O
N	B-chemical
-	I-chemical
gamma	I-chemical
-	I-chemical
acetyl	I-chemical
-	I-chemical
L	I-chemical
-	I-chemical
2	I-chemical
,	I-chemical
4	I-chemical
-	I-chemical
diaminobutyric	I-chemical
acid	I-chemical
into	O
ectoine	B-chemical
through	O
a	O
cyclocondensation	O
reaction	O
,	O
but	O
that	O
it	O
can	O
also	O
use	O
the	O
isomer	O
N	B-chemical
-	I-chemical
alpha	I-chemical
-	I-chemical
acetyl	I-chemical
-	I-chemical
L	I-chemical
-	I-chemical
2	I-chemical
,	I-chemical
4	I-chemical
-	I-chemical
diaminobutyric	I-chemical
acid	I-chemical
as	O
its	O
substrate	O
,	O
albeit	O
with	O
substantially	O
reduced	O
catalytic	B-evidence
efficiency	I-evidence
.	O

An	O
assessment	O
of	O
enzyme	O
activity	O
and	O
iron	B-chemical
content	O
of	O
these	O
mutants	O
give	O
important	O
clues	O
for	O
understanding	O
the	O
architecture	O
of	O
the	O
active	B-site
site	I-site
positioned	O
within	O
the	O
core	O
of	O
the	O
EctC	B-protein
cupin	B-structure_element
barrel	I-structure_element
.	O

Ectoine	B-chemical
[(	O
S	B-chemical
)-	I-chemical
2	I-chemical
-	I-chemical
methyl	I-chemical
-	I-chemical
1	I-chemical
,	I-chemical
4	I-chemical
,	I-chemical
5	I-chemical
,	I-chemical
6	I-chemical
-	I-chemical
tetrahydropyrimidine	I-chemical
-	I-chemical
4	I-chemical
-	I-chemical
carboxylic	I-chemical
acid	I-chemical
]	O
and	O
its	O
derivative	O
5	B-chemical
-	I-chemical
hydroxyectoine	I-chemical
[(	O
4S	B-chemical
,	I-chemical
5S	I-chemical
)-	I-chemical
5	I-chemical
-	I-chemical
hydroxy	I-chemical
-	I-chemical
2	I-chemical
-	I-chemical
methyl	I-chemical
-	I-chemical
1	I-chemical
,	I-chemical
4	I-chemical
,	I-chemical
5	I-chemical
,	I-chemical
6	I-chemical
-	I-chemical
tetrahydropyrimidine	I-chemical
-	I-chemical
4	I-chemical
-	I-chemical
carboxylic	I-chemical
acid	I-chemical
]	O
are	O
such	O
compatible	O
solutes	O
.	O

Both	O
marine	B-taxonomy_domain
and	I-taxonomy_domain
terrestrial	I-taxonomy_domain
microorganisms	I-taxonomy_domain
produce	O
them	O
widely	O
in	O
response	O
to	O
osmotic	O
or	O
temperature	O
stress	O
.	O

Synthesis	O
of	O
ectoine	B-chemical
occurs	O
from	O
the	O
intermediate	O
metabolite	O
L	B-chemical
-	I-chemical
aspartate	I-chemical
-	I-chemical
ß	I-chemical
-	I-chemical
semialdehyde	I-chemical
and	O
comprises	O
the	O
sequential	O
activities	O
of	O
three	O
enzymes	O
:	O
L	B-protein_type
-	I-protein_type
2	I-protein_type
,	I-protein_type
4	I-protein_type
-	I-protein_type
diaminobutyrate	I-protein_type
transaminase	I-protein_type
(	O
EctB	B-protein_type
;	O
EC	O
2	O
.	O
6	O
.	O
1	O
.	O
76	O
),	O
2	B-protein_type
,	I-protein_type
4	I-protein_type
-	I-protein_type
diaminobutyrate	I-protein_type
acetyltransferase	I-protein_type
(	O
EctA	B-protein_type
;	O
EC	O
2	O
.	O
3	O
.	O
1	O
.	O
178	O
),	O
and	O
ectoine	B-protein_type
synthase	I-protein_type
(	O
EctC	B-protein_type
;	O
EC	O
4	O
.	O
2	O
.	O
1	O
.	O
108	O
)	O
(	O
Fig	O
1	O
).	O

The	O
ectoine	B-chemical
derivative	O
5	B-chemical
-	I-chemical
hydroxyectoine	I-chemical
,	O
a	O
highly	O
effective	O
stress	O
protectant	O
in	O
its	O
own	O
right	O
,	O
is	O
synthesized	O
by	O
a	O
substantial	O
subgroup	O
of	O
the	O
ectoine	B-chemical
producers	O
.	O

The	O
remarkable	O
function	O
preserving	O
effects	O
of	O
ectoines	B-chemical
for	O
macromolecules	O
and	O
cells	O
,	O
frequently	O
also	O
addressed	O
as	O
chemical	O
chaperones	O
,	O
led	O
to	O
a	O
substantial	O
interest	O
in	O
exploiting	O
these	O
compounds	O
for	O
biotechnological	O
purposes	O
and	O
medical	O
applications	O
.	O

Biosynthetic	O
routes	O
for	O
ectoine	B-chemical
and	O
5	B-chemical
-	I-chemical
hydroxyectoine	I-chemical
.	O

Here	O
we	O
focus	O
on	O
ectoine	B-protein_type
synthase	I-protein_type
(	O
EctC	B-protein
),	O
the	O
key	O
enzyme	O
of	O
the	O
ectoine	B-chemical
biosynthetic	O
route	O
(	O
Fig	O
1	O
).	O

Biochemical	O
characterizations	B-experimental_method
of	O
ectoine	B-protein_type
synthases	I-protein_type
from	O
the	O
extremophiles	B-taxonomy_domain
Halomonas	B-species
elongata	I-species
,	O
Methylomicrobium	B-species
alcaliphilum	I-species
,	O
and	O
Acidiphilium	B-species
cryptum	I-species
,	O
and	O
from	O
the	O
nitrifying	B-taxonomy_domain
archaeon	I-taxonomy_domain
Nitrosopumilus	B-species
maritimus	I-species
have	O
been	O
carried	O
out	O
.	O

Each	O
of	O
these	O
enzymes	O
catalyzes	O
as	O
their	O
main	O
activity	O
the	O
cyclization	O
of	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
acetyl	I-chemical
-	I-chemical
L	I-chemical
-	I-chemical
2	I-chemical
,	I-chemical
4	I-chemical
-	I-chemical
diaminobutyric	I-chemical
acid	I-chemical
(	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
),	O
the	O
reaction	O
product	O
of	O
the	O
2	B-protein_type
,	I-protein_type
4	I-protein_type
-	I-protein_type
diaminobutyrate	I-protein_type
acetyltransferase	I-protein_type
(	O
EctA	B-protein_type
),	O
to	O
ectoine	B-chemical
with	O
the	O
concomitant	O
release	O
of	O
a	O
water	B-chemical
molecule	O
(	O
Fig	O
1	O
).	O

In	O
side	O
reactions	O
,	O
EctC	B-protein
can	O
promote	O
the	O
formation	O
of	O
the	O
synthetic	O
compatible	O
solute	O
5	B-chemical
-	I-chemical
amino	I-chemical
-	I-chemical
3	I-chemical
,	I-chemical
4	I-chemical
-	I-chemical
dihydro	I-chemical
-	I-chemical
2H	I-chemical
-	I-chemical
pyrrole	I-chemical
-	I-chemical
2	I-chemical
-	I-chemical
carboxylate	I-chemical
(	O
ADPC	B-chemical
)	O
through	O
the	O
cyclic	O
condensation	O
of	O
two	O
glutamine	B-chemical
molecules	O
and	O
it	O
also	O
possesses	O
a	O
minor	O
hydrolytic	O
activity	O
for	O
ectoine	B-chemical
and	O
synthetic	O
ectoine	B-chemical
derivatives	O
with	O
either	O
reduced	O
or	O
expanded	O
ring	O
sizes	O
.	O

Although	O
progress	O
has	O
been	O
made	O
with	O
respect	O
to	O
the	O
biochemical	O
characterization	O
of	O
ectoine	B-protein_type
synthase	I-protein_type
,	O
a	O
clear	O
understanding	O
of	O
how	O
its	O
structure	B-evidence
contributes	O
to	O
its	O
enzyme	O
activity	O
and	O
reaction	O
mechanism	O
is	O
still	O
lacking	O
.	O
With	O
this	O
in	O
mind	O
,	O
we	O
have	O
biochemically	B-experimental_method
characterized	I-experimental_method
the	O
ectoine	B-protein_type
synthase	I-protein_type
from	O
the	O
cold	O
-	O
adapted	O
marine	B-taxonomy_domain
bacterium	I-taxonomy_domain
Sphingopyxis	B-species
alaskensis	I-species
(	O
Sa	B-species
).	O

We	O
demonstrate	O
here	O
for	O
the	O
first	O
time	O
that	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
is	O
a	O
metal	B-chemical
-	O
dependent	O
enzyme	O
,	O
with	O
iron	B-chemical
as	O
the	O
most	O
likely	O
physiologically	O
relevant	O
co	O
-	O
factor	O
.	O

Overproduction	B-experimental_method
,	O
purification	B-experimental_method
and	O
oligomeric	O
state	O
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
in	O
solution	O

We	O
focused	O
our	O
biochemical	B-experimental_method
and	I-experimental_method
structural	I-experimental_method
studies	I-experimental_method
on	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
from	O
S	B-species
.	I-species
alaskensis	I-species
[(	O
Sa	B-species
)	O
EctC	B-protein
],	O
a	O
cold	O
-	O
adapted	O
marine	B-taxonomy_domain
ultra	I-taxonomy_domain
-	I-taxonomy_domain
microbacterium	I-taxonomy_domain
,	O
from	O
which	O
we	O
recently	O
also	O
determined	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
ectoine	B-protein_type
hydroxylase	I-protein_type
(	O
EctD	B-protein_type
)	O
in	B-protein_state
complex	I-protein_state
with	I-protein_state
either	O
its	O
substrate	O
or	O
its	O
reaction	O
product	O
.	O

We	O
expressed	O
a	O
codon	O
-	O
optimized	O
version	O
of	O
the	O
S	B-species
.	I-species
alaskensis	I-species
ectC	B-gene
gene	O
in	O
E	B-species
.	I-species
coli	I-species
to	O
produce	O
a	O
recombinant	O
protein	O
with	O
a	O
carboxy	O
-	O
terminally	O
attached	O
Strep	B-experimental_method
-	I-experimental_method
tag	I-experimental_method
II	I-experimental_method
affinity	I-experimental_method
peptide	I-experimental_method
to	O
allow	O
purification	O
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
-	O
Strep	B-experimental_method
-	I-experimental_method
Tag	I-experimental_method
-	I-experimental_method
II	I-experimental_method
protein	O
by	O
affinity	B-experimental_method
chromatography	I-experimental_method
.	O

Conventional	O
size	B-experimental_method
-	I-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
(	O
SEC	B-experimental_method
)	O
has	O
already	O
shown	O
that	O
(	O
Sa	B-species
)	O
EctC	B-protein
preparations	O
produced	O
in	O
this	O
fashion	O
are	O
homogeneous	O
and	O
that	O
the	O
protein	O
forms	O
dimers	B-oligomeric_state
in	O
solution	O
.	O

High	B-experimental_method
performance	I-experimental_method
liquid	I-experimental_method
chromatography	I-experimental_method
coupled	O
with	O
multi	B-experimental_method
-	I-experimental_method
angle	I-experimental_method
light	I-experimental_method
-	I-experimental_method
scattering	I-experimental_method
detection	I-experimental_method
(	O
HPLC	B-experimental_method
-	I-experimental_method
MALS	I-experimental_method
)	O
experiments	O
carried	O
out	O
here	O
confirmed	O
that	O
the	O
purified	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
was	O
mono	O
-	O
disperse	O
and	O
possessed	O
a	O
molecular	O
mass	O
of	O
33	O
.	O
0	O
±	O
2	O
.	O
3	O
kDa	O
(	O
S2b	O
Fig	O
).	O

This	O
value	O
corresponds	O
very	O
well	O
with	O
the	O
theoretically	O
calculated	O
molecular	O
mass	O
of	O
an	O
(	O
Sa	B-species
)	O
EctC	B-protein
dimer	B-oligomeric_state
(	O
molecular	O
mass	O
of	O
the	O
monomer	B-oligomeric_state
,	O
including	O
the	O
Strep	B-experimental_method
-	I-experimental_method
tag	I-experimental_method
II	I-experimental_method
affinity	I-experimental_method
peptide	I-experimental_method
:	O
16	O
.	O
3	O
kDa	O
).	O

Such	O
a	O
quaternary	O
assembly	O
as	O
dimer	B-oligomeric_state
has	O
also	O
been	O
reported	O
for	O
the	O
EctC	B-protein_type
proteins	I-protein_type
from	O
H	B-species
.	I-species
elongata	I-species
and	O
N	B-species
.	I-species
maritimus	I-species
.	O

The	O
EctA	B-protein
-	O
produced	O
substrate	O
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
,	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
acetyl	I-chemical
-	I-chemical
L	I-chemical
-	I-chemical
2	I-chemical
,	I-chemical
4	I-chemical
-	I-chemical
diaminobutyric	I-chemical
acid	I-chemical
(	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
)	O
(	O
Fig	O
1	O
),	O
is	O
commercially	O
not	O
available	O
.	O

We	O
used	O
alkaline	O
hydrolysis	O
of	O
ectoine	B-chemical
and	O
subsequent	O
chromatography	O
on	O
silica	O
gel	O
columns	O
to	O
obtain	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
in	O
chemically	O
highly	O
purified	O
form	O
(	O
S1a	O
Fig	O
).	O

This	O
procedure	O
also	O
yielded	O
the	O
isomer	O
of	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
,	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
acetyl	I-chemical
-	I-chemical
L	I-chemical
-	I-chemical
2	I-chemical
,	I-chemical
4	I-chemical
-	I-chemical
diaminobutyric	I-chemical
acid	I-chemical
(	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical
)	O
(	O
S1b	O
Fig	O
).	O

Using	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
as	O
the	O
substrate	O
,	O
we	O
initially	O
evaluated	O
a	O
set	O
of	O
biochemical	O
parameters	O
of	O
the	O
recombinant	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
.	O

S	B-species
.	I-species
alaskensis	I-species
,	O
from	O
which	O
the	O
studied	O
ectoine	B-protein_type
synthase	I-protein_type
was	O
originally	O
derived	O
,	O
is	O
a	O
microorganism	B-taxonomy_domain
that	O
is	O
well	O
-	O
adapted	O
to	O
a	O
life	O
in	O
permanently	O
cold	O
ocean	O
waters	O
.	O

Consistent	O
with	O
the	O
physicochemical	O
attributes	O
of	O
this	O
habitat	O
,	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
was	O
already	O
enzymatically	B-protein_state
active	I-protein_state
at	O
5	O
°	O
C	O
,	O
had	O
a	O
temperature	O
optimum	O
of	O
15	O
°	O
C	O
and	O
was	O
able	O
to	O
function	O
over	O
a	O
broad	O
range	O
of	O
temperatures	O
(	O
S3a	O
Fig	O
).	O

It	O
possessed	O
an	O
alkaline	B-protein_state
pH	O
optimum	O
of	O
8	O
.	O
5	O
(	O
S3b	O
Fig	O
),	O
a	O
value	O
similar	O
to	O
the	O
ectoine	B-protein_type
synthases	I-protein_type
from	O
the	O
halo	B-protein_state
-	I-protein_state
tolerant	I-protein_state
H	B-species
.	I-species
elongata	I-species
(	O
pH	O
optimum	O
of	O
8	O
.	O
5	O
to	O
9	O
.	O
0	O
),	O
the	O
alkaliphile	B-taxonomy_domain
M	B-species
.	I-species
alcaliphilum	I-species
(	O
pH	O
optimum	O
of	O
9	O
.	O
0	O
),	O
and	O
the	O
acidophile	B-taxonomy_domain
Acidiphilium	B-species
cryptum	I-species
(	O
pH	O
optimum	O
of	O
8	O
.	O
5	O
to	O
9	O
.	O
0	O
),	O
whereas	O
the	O
EctC	B-protein
protein	O
from	O
N	B-species
.	I-species
maritimus	I-species
has	O
a	O
neutral	B-protein_state
pH	I-protein_state
optimum	O
(	O
pH	O
7	O
.	O
0	O
).	O

The	O
salinity	O
of	O
the	O
assay	O
buffer	O
had	O
a	O
significant	O
influence	O
on	O
the	O
maximal	O
enzyme	O
activity	O
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
.	O

An	O
increase	O
in	O
either	O
the	O
NaCl	B-chemical
or	O
the	O
KCl	B-chemical
concentration	O
led	O
to	O
an	O
approximately	O
5	O
-	O
fold	O
enhancement	O
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
activity	O
.	O

The	O
maximum	O
enzyme	O
activity	O
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
occurred	O
around	O
250	O
mM	O
NaCl	B-chemical
or	O
KCl	B-chemical
,	O
respectively	O
.	O

Considerations	O
based	O
on	O
bioinformatics	O
suggests	O
that	O
EctC	B-protein
belongs	O
to	O
the	O
cupin	B-protein_type
superfamily	I-protein_type
.	O

Most	O
of	O
these	O
proteins	O
contain	O
catalytically	O
important	O
transition	O
state	O
metals	O
such	O
as	O
iron	B-chemical
,	O
copper	B-chemical
,	O
zinc	B-chemical
,	O
manganese	B-chemical
,	O
cobalt	B-chemical
,	O
or	O
nickel	B-chemical
.	O

Cupins	B-protein_type
contain	O
two	O
conserved	B-protein_state
motifs	O
:	O
G	B-structure_element
(	I-structure_element
X	I-structure_element
)	I-structure_element
5HXH	I-structure_element
(	I-structure_element
X	I-structure_element
)	I-structure_element
3	I-structure_element
,	I-structure_element
4E	I-structure_element
(	I-structure_element
X	I-structure_element
)	I-structure_element
6G	I-structure_element
and	O
G	B-structure_element
(	I-structure_element
X	I-structure_element
)	I-structure_element
5PXG	I-structure_element
(	I-structure_element
X	I-structure_element
)	I-structure_element
2H	I-structure_element
(	I-structure_element
X	I-structure_element
)	I-structure_element
3N	I-structure_element
(	O
the	O
letters	O
in	O
bold	O
represent	O
those	O
residues	O
that	O
often	O
coordinate	O
the	O
metal	B-chemical
).	O

Inspection	O
of	O
a	O
previous	O
alignment	B-experimental_method
of	I-experimental_method
the	I-experimental_method
amino	I-experimental_method
acid	I-experimental_method
sequences	I-experimental_method
of	O
440	O
EctC	B-protein_type
-	I-protein_type
type	I-protein_type
proteins	I-protein_type
revealed	O
that	O
the	O
canonical	O
metal	B-structure_element
-	I-structure_element
binding	I-structure_element
motif	I-structure_element
(	O
s	O
)	O
of	O
cupin	B-protein_type
-	I-protein_type
type	I-protein_type
proteins	I-protein_type
is	O
not	B-protein_state
conserved	I-protein_state
among	O
members	O
of	O
the	O
extended	O
ectoine	B-protein_type
synthase	I-protein_type
protein	I-protein_type
family	I-protein_type
.	O

An	O
abbreviated	O
alignment	B-experimental_method
of	I-experimental_method
the	I-experimental_method
amino	I-experimental_method
acid	I-experimental_method
sequence	I-experimental_method
of	O
EctC	B-protein_type
-	I-protein_type
type	I-protein_type
proteins	I-protein_type
is	O
shown	O
in	O
Fig	O
2	O
.	O

Abbreviated	O
alignment	B-experimental_method
of	O
EctC	B-protein_type
-	I-protein_type
type	I-protein_type
proteins	I-protein_type
.	O

Strictly	B-protein_state
conserved	I-protein_state
amino	O
acid	O
residues	O
are	O
shown	O
in	O
yellow	O
.	O

Dots	O
shown	O
above	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
sequence	O
indicate	O
residues	O
likely	O
to	O
be	O
involved	O
in	O
iron	B-chemical
-	O
binding	O
(	O
red	O
),	O
ligand	O
-	O
binding	O
(	O
green	O
)	O
and	O
stabilization	O
of	O
the	O
loop	O
-	O
architecture	O
(	O
blue	O
).	O

The	O
conserved	B-protein_state
residue	O
Tyr	B-residue_name_number
-	I-residue_name_number
52	I-residue_name_number
with	O
so	O
-	O
far	O
undefined	O
functions	O
is	O
indicated	O
by	O
a	O
green	O
dot	O
circled	O
in	O
red	O
.	O

Secondary	O
structural	O
elements	O
(	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
and	O
β	B-structure_element
-	I-structure_element
sheets	I-structure_element
)	O
found	O
in	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
crystal	B-evidence
structure	I-evidence
are	O
projected	O
onto	O
the	O
amino	O
acid	O
sequences	O
of	O
EctC	B-protein_type
-	I-protein_type
type	I-protein_type
proteins	I-protein_type
.	O

For	O
this	O
analysis	O
we	O
used	O
recombinant	O
(	O
Sa	B-species
)	O
EctC	B-protein
preparations	O
from	O
three	O
independent	O
protein	O
overproduction	O
and	O
purification	O
experiments	O
.	O

The	O
ICP	B-experimental_method
-	I-experimental_method
MS	I-experimental_method
analyses	O
yielded	O
an	O
iron	B-chemical
content	O
of	O
0	O
.	O
66	O
±	O
0	O
.	O
06	O
mol	O
iron	B-chemical
per	O
mol	O
of	O
protein	O
and	O
the	O
used	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
preparations	O
also	O
contained	O
a	O
minor	O
amount	O
of	O
zinc	B-chemical
(	O
0	O
.	O
08	O
mol	O
zinc	B-chemical
per	O
mol	O
of	O
protein	O
).	O

All	O
other	O
assayed	O
metals	O
(	O
copper	B-chemical
and	O
nickel	B-chemical
)	O
were	O
only	O
present	O
in	O
trace	O
amounts	O
(	O
0	O
.	O
01	O
mol	O
metal	B-chemical
per	O
mol	O
of	O
protein	O
,	O
respectively	O
).	O

The	O
presence	O
of	O
iron	B-chemical
in	O
these	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
preparations	O
was	O
further	O
confirmed	O
by	O
a	O
colorimetric	B-experimental_method
method	I-experimental_method
that	O
is	O
based	O
on	O
an	O
iron	B-chemical
-	O
complexing	O
reagent	O
;	O
this	O
procedure	O
yielded	O
an	O
iron	B-chemical
-	O
content	O
of	O
0	O
.	O
84	O
±	O
0	O
.	O
05	O
mol	O
per	O
mol	O
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
.	O

Hence	O
,	O
both	O
ICP	B-experimental_method
-	I-experimental_method
MS	I-experimental_method
and	O
the	O
colorimetric	B-experimental_method
method	I-experimental_method
clearly	O
established	O
that	O
the	O
recombinantly	O
produced	O
ectoine	B-protein_type
synthase	I-protein_type
from	O
S	B-species
.	I-species
alaskensis	I-species
is	O
an	O
iron	B-chemical
-	O
containing	O
protein	O
.	O

The	O
reason	O
for	O
this	O
difference	O
is	O
not	O
known	O
,	O
but	O
indicates	O
that	O
the	O
well	O
established	O
colorimetric	B-experimental_method
assay	I-experimental_method
probably	O
overestimates	O
the	O
iron	B-chemical
content	O
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
preparations	O
to	O
a	O
certain	O
degree	O
.	O

The	O
iron	B-chemical
detected	O
in	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
preparations	O
could	O
serve	O
a	O
structural	O
role	O
,	O
or	O
most	O
likely	O
,	O
could	O
be	O
critical	O
for	O
enzyme	O
catalysis	O
as	O
is	O
the	O
case	O
for	O
many	O
members	O
of	O
the	O
cupin	B-protein_type
superfamily	I-protein_type
.	O

The	O
addition	O
of	O
very	O
low	O
concentrations	O
of	O
EDTA	B-chemical
(	O
0	O
.	O
05	O
mM	O
)	O
to	O
the	O
EctC	B-protein
enzyme	O
already	O
led	O
to	O
a	O
noticeable	O
inhibition	O
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
activity	O
and	O
the	O
presence	O
of	O
1	O
mM	O
EDTA	B-chemical
completely	O
inhibited	O
the	O
enzyme	O
(	O
Fig	O
3a	O
).	O

(	O
a	O
)	O
Impact	O
of	O
the	O
iron	B-chemical
-	O
chelator	O
EDTA	B-chemical
on	O
the	O
enzyme	O
activity	O
of	O
the	O
purified	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
.	O

Metal	B-experimental_method
depletion	I-experimental_method
and	I-experimental_method
reconstitution	I-experimental_method
experiments	I-experimental_method
with	O
(	O
b	O
)	O
stoichiometric	O
and	O
(	O
c	O
)	O
excess	O
amounts	O
of	O
metals	O
.	O

The	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
was	O
present	O
at	O
a	O
concentration	O
of	O
10	O
μM	O
.	O
The	O
level	O
of	O
enzyme	O
activity	O
given	O
in	O
(	O
b	O
)	O
is	O
benchmarked	O
relative	O
to	O
that	O
of	O
ectoine	B-protein_type
synthase	I-protein_type
enzyme	B-experimental_method
assays	I-experimental_method
in	O
which	O
1	O
mM	O
FeCl2	B-chemical
was	O
added	O
.	O

The	O
addition	O
of	O
FeCl2	B-chemical
to	O
the	O
enzyme	B-experimental_method
assay	I-experimental_method
restored	O
enzyme	O
activity	O
to	O
about	O
38	O
%,	O
whereas	O
the	O
addition	O
of	O
ZnCl2	B-chemical
or	O
CoCl2	B-chemical
rescued	O
(	O
Sa	B-species
)	O
EctC	B-protein
enzyme	O
activity	O
only	O
to	O
5	O
%	O
and	O
3	O
%,	O
respectively	O
.	O

All	O
other	O
tested	O
metals	O
,	O
including	O
Fe3	B-chemical
+,	I-chemical
were	O
unable	O
to	O
restore	O
activity	O
(	O
Fig	O
3b	O
).	O

When	O
the	O
concentration	O
of	O
the	O
various	O
metals	O
in	O
the	O
enzyme	B-experimental_method
assay	I-experimental_method
was	O
increased	O
100	O
-	O
fold	O
,	O
Fe2	B-chemical
+	I-chemical
exhibited	O
again	O
the	O
strongest	O
stimulating	O
effect	O
on	O
enzyme	O
activity	O
,	O
and	O
rescued	O
enzyme	O
activity	O
to	O
a	O
degree	O
similar	O
to	O
that	O
exhibited	O
by	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
preparations	O
that	O
had	O
not	O
been	O
inactivated	O
through	O
EDTA	B-chemical
treatment	O
(	O
Fig	O
3c	O
).	O

However	O
,	O
a	O
large	O
molar	O
excess	O
of	O
other	O
transition	O
-	O
state	O
metals	O
(	O
zinc	B-chemical
,	O
cobalt	B-chemical
,	O
nickel	B-chemical
,	O
copper	B-chemical
,	O
and	O
manganese	B-chemical
)	O
typically	O
found	O
in	O
members	O
of	O
the	O
cupin	B-protein_type
superfamily	I-protein_type
allowed	O
the	O
partial	O
rescue	O
of	O
ectoine	B-protein_type
synthase	I-protein_type
activity	O
as	O
well	O
(	O
Fig	O
3c	O
).	O

This	O
is	O
in	O
line	O
with	O
literature	O
data	O
showing	O
that	O
cupin	B-protein_type
-	I-protein_type
type	I-protein_type
enzymes	I-protein_type
are	O
often	O
promiscuous	O
with	O
respect	O
to	O
the	O
use	O
of	O
the	O
catalytically	O
important	O
metal	B-chemical
.	O

Kinetic	O
parameters	O
of	O
EctC	B-protein
for	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
and	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical

Based	O
on	O
the	O
data	O
presented	O
in	O
S3	O
Fig	O
,	O
we	O
formulated	O
an	O
optimized	O
activity	B-experimental_method
assay	I-experimental_method
for	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
of	O
S	B-species
.	I-species
alaskensis	I-species
and	O
used	O
it	O
to	O
determined	O
the	O
kinetic	O
parameters	O
for	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
enzyme	O
for	O
both	O
its	O
natural	O
substrate	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
and	O
the	O
isomer	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical
.	O

Given	O
the	O
chemical	O
relatedness	O
of	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical
to	O
the	O
natural	O
substrate	O
(	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
)	O
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
(	O
S1a	O
and	O
S1b	O
Fig	O
),	O
we	O
wondered	O
whether	O
(	O
Sa	B-species
)	O
EctC	B-protein
could	O
also	O
use	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical
to	O
produce	O
ectoine	B-chemical
.	O

However	O
,	O
both	O
the	O
affinity	B-evidence
(	O
Km	B-evidence
)	O
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
and	O
its	O
catalytic	B-evidence
efficiency	I-evidence
(	O
kcat	B-evidence
/	I-evidence
Km	I-evidence
)	O
were	O
strongly	O
reduced	O
in	O
comparison	O
with	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
.	O

Both	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
and	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical
are	O
concomitantly	O
formed	O
during	O
the	O
enzymatic	O
hydrolysis	O
of	O
the	O
ectoine	B-chemical
ring	O
during	O
catabolism	O
.	O

Our	O
finding	O
that	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical
is	O
a	O
substrate	O
for	O
ectoine	B-protein_type
synthase	I-protein_type
has	O
bearings	O
for	O
an	O
understanding	O
of	O
the	O
physiology	O
of	O
those	O
microorganisms	B-taxonomy_domain
that	O
can	O
both	O
synthesize	O
and	O
catabolize	O
ectoine	B-chemical
.	O

However	O
,	O
these	O
types	O
of	O
microorganisms	B-taxonomy_domain
should	O
still	O
be	O
able	O
to	O
largely	O
avoid	O
a	O
futile	O
cycle	O
since	O
the	O
affinity	B-evidence
of	O
ectoine	B-protein_type
synthase	I-protein_type
for	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
and	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical
,	O
and	O
its	O
catalytic	B-evidence
efficiency	I-evidence
for	O
the	O
two	O
compounds	O
,	O
differs	O
substantially	O
(	O
S4a	O
and	O
S4b	O
Fig	O
).	O

Crystallization	B-experimental_method
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O

Since	O
no	O
crystal	B-evidence
structure	I-evidence
of	O
ectoine	B-protein_type
synthase	I-protein_type
has	O
been	O
reported	O
,	O
we	O
set	O
out	O
to	O
crystallize	B-experimental_method
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
.	O

Attempts	O
to	O
obtain	O
crystals	B-evidence
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
in	B-protein_state
complex	I-protein_state
either	O
with	O
its	O
substrate	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
or	O
its	O
reaction	O
product	O
ectoine	B-chemical
were	O
not	O
successful	O
.	O

Attempts	O
to	O
solve	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
by	O
molecular	B-experimental_method
replacement	I-experimental_method
has	O
previously	O
failed	O
.	O

However	O
,	O
we	O
were	O
able	O
to	O
obtain	O
crystals	B-evidence
of	O
form	O
B	O
that	O
were	O
derivatized	O
with	O
mercury	B-chemical
and	O
these	O
diffracted	O
up	O
to	O
2	O
.	O
8	O
Å	O
(	O
S1	O
Table	O
).	O

This	O
dataset	O
was	O
used	O
to	O
derive	O
an	O
initial	O
structural	B-evidence
model	I-evidence
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
,	O
which	O
in	O
turn	O
was	O
employed	O
as	O
a	O
template	O
for	O
molecular	B-experimental_method
replacement	I-experimental_method
to	O
phase	O
the	O
native	O
dataset	O
(	O
2	O
.	O
0	O
Å	O
)	O
of	O
crystal	O
form	O
B	O
.	O
After	O
several	O
rounds	O
of	O
manual	O
model	O
building	O
and	O
refinement	O
,	O
four	O
monomers	B-oligomeric_state
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
were	O
identified	O
and	O
the	O
crystal	B-evidence
structure	I-evidence
was	O
refined	O
to	O
a	O
final	O
Rcryst	B-evidence
of	O
21	O
.	O
1	O
%	O
and	O
an	O
Rfree	B-evidence
of	O
24	O
.	O
8	O
%	O
(	O
S1	O
Table	O
).	O

The	O
two	O
EctC	B-protein
structures	B-evidence
that	O
we	O
determined	O
revealed	O
that	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
belongs	O
to	O
the	O
cupin	B-protein_type
superfamily	I-protein_type
with	O
respect	O
to	O
its	O
overall	O
fold	O
(	O
Fig	O
4a	O
–	O
4c	O
).	O

However	O
,	O
they	O
represent	O
two	O
different	O
states	O
of	O
the	O
137	B-residue_range
amino	I-residue_range
acids	I-residue_range
comprising	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
(	O
Fig	O
2	O
).	O

First	O
,	O
the	O
1	O
.	O
2	O
Å	O
structure	B-evidence
reveals	O
the	O
spatial	O
configuration	O
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
ranging	O
from	O
amino	O
acid	O
Met	B-residue_range
-	I-residue_range
1	I-residue_range
to	I-residue_range
Glu	I-residue_range
-	I-residue_range
115	I-residue_range
;	O
hence	O
,	O
it	O
lacks	B-protein_state
22	B-residue_range
amino	I-residue_range
acids	I-residue_range
at	O
the	O
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
of	O
the	O
authentic	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
.	O

In	O
this	O
structure	B-evidence
no	O
metal	B-chemical
co	O
-	O
factor	O
was	O
identified	O
.	O

The	O
second	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
was	O
solved	B-experimental_method
at	O
a	O
resolution	O
of	O
2	O
.	O
0	O
Å	O
and	O
contained	O
four	O
molecules	O
of	O
the	O
protein	O
in	O
the	O
asymmetric	O
unit	O
of	O
which	O
protomer	B-oligomeric_state
A	B-structure_element
comprised	O
amino	O
acid	O
Met	B-residue_range
-	I-residue_range
1	I-residue_range
to	I-residue_range
Gly	I-residue_range
-	I-residue_range
121	I-residue_range
and	O
adopts	O
a	O
closed	B-protein_state
conformation	O
.	O

Hence	O
,	O
it	O
still	O
lacks	B-protein_state
16	B-residue_range
amino	I-residue_range
acid	I-residue_range
residues	O
of	O
the	O
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
of	O
the	O
authentic	O
137	B-residue_range
amino	I-residue_range
acids	I-residue_range
comprising	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
(	O
Fig	O
2	O
).	O

We	O
therefore	O
cannot	O
exclude	O
that	O
this	O
crystal	B-evidence
structure	I-evidence
does	O
not	O
represent	O
the	O
fully	B-protein_state
closed	I-protein_state
state	O
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
;	O
consequently	O
,	O
we	O
tentatively	O
termed	O
it	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
.	O

Overall	O
structure	B-evidence
of	O
the	O
“	O
open	B-protein_state
”	O
and	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
crystal	B-evidence
structures	I-evidence
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
.	O

(	O
a	O
)	O
The	O
overall	O
structure	B-evidence
of	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
resolved	O
at	O
2	O
.	O
0	O
Å	O
is	O
depicted	O
in	O
green	O
in	O
a	O
cartoon	O
(	O
upper	O
panel	O
)	O
and	O
surface	O
(	O
lower	O
panel	O
)	O
representation	O
.	O

(	O
b	O
)	O
The	O
overall	O
structure	B-evidence
of	O
the	O
“	O
open	B-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
was	O
resolved	O
at	O
1	O
.	O
2	O
Å	O
and	O
is	O
depicted	O
in	O
yellow	O
in	O
a	O
cartoon	O
(	O
upper	O
panel	O
)	O
and	O
surface	O
(	O
lower	O
panel	O
)	O
representation	O
.	O

The	O
overall	O
structure	B-evidence
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
is	O
basically	O
the	O
same	O
in	O
both	O
crystals	B-evidence
except	O
for	O
the	O
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
,	O
which	O
covers	O
the	O
entry	O
of	O
one	O
side	O
of	O
the	O
cupin	B-structure_element
barrel	I-structure_element
from	O
the	O
surroundings	O
in	O
monomer	B-oligomeric_state
A	B-structure_element
in	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
structure	B-evidence
.	O

This	O
is	O
reflected	O
by	O
the	O
calculated	O
root	B-evidence
mean	I-evidence
square	I-evidence
deviation	I-evidence
(	O
RMSD	B-evidence
)	O
of	O
the	O
Cα	O
atoms	O
that	O
was	O
about	O
0	O
.	O
56	O
Å	O
(	O
over	O
117	O
residues	O
)	O
when	O
the	O
four	O
“	O
open	B-protein_state
”	O
monomers	B-oligomeric_state
were	O
compared	O
with	O
each	O
other	O
.	O

However	O
,	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
monomer	B-oligomeric_state
has	O
a	O
slightly	O
higher	O
RMSD	B-evidence
of	O
1	O
.	O
4	O
Å	O
(	O
over	O
117	O
residues	O
)	O
when	O
compared	O
with	O
the	O
“	O
open	B-protein_state
”	O
2	O
.	O
0	O
Å	O
structure	B-evidence
.	O

Therefore	O
,	O
we	O
describe	O
in	O
the	O
following	O
the	O
overall	O
structure	B-evidence
for	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
form	O
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
and	O
subsequently	O
highlight	O
the	O
structural	O
differences	O
between	O
the	O
“	O
open	B-protein_state
”	O
and	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
forms	O
in	O
more	O
detail	O
.	O

The	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
form	O
two	O
anti	B-structure_element
-	I-structure_element
parallel	I-structure_element
β	I-structure_element
-	I-structure_element
sheets	I-structure_element
:	O
β2	B-structure_element
β3	B-structure_element
,	O
β4	B-structure_element
,	O
β11	B-structure_element
,	O
β6	B-structure_element
,	O
and	O
β9	B-structure_element
,	O
and	O
a	O
smaller	O
three	B-structure_element
-	I-structure_element
stranded	I-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
(	O
β7	B-structure_element
,	O
β8	B-structure_element
,	O
and	O
β10	B-structure_element
),	O
respectively	O
.	O

These	O
two	O
β	B-structure_element
-	I-structure_element
sheets	I-structure_element
pack	O
against	O
each	O
other	O
,	O
forming	O
a	O
cup	B-structure_element
-	I-structure_element
shaped	I-structure_element
β	I-structure_element
-	I-structure_element
sandwich	I-structure_element
with	O
a	O
topology	O
characteristic	O
for	O
the	O
cupin	B-structure_element
-	I-structure_element
fold	I-structure_element
.	O

In	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
structure	B-evidence
,	O
a	O
longer	O
carboxy	B-structure_element
-	I-structure_element
terminal	I-structure_element
tail	I-structure_element
is	O
visible	O
in	O
the	O
electron	B-evidence
density	I-evidence
,	O
folding	O
into	O
a	O
small	B-structure_element
helix	I-structure_element
(	O
α	B-structure_element
-	I-structure_element
II	I-structure_element
)	O
that	O
closes	O
the	O
active	B-site
site	I-site
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
(	O
Fig	O
4a	O
).	O

Structural	B-experimental_method
comparison	I-experimental_method
analyses	I-experimental_method
using	O
the	O
DALI	B-experimental_method
server	I-experimental_method
revealed	O
that	O
(	O
Sa	B-species
)	O
EctC	B-protein
adopts	O
a	O
fold	O
similar	O
to	O
other	O
members	O
of	O
the	O
cupin	B-protein_type
superfamily	I-protein_type
.	O

The	O
highest	O
structural	O
similarities	O
are	O
observed	O
for	O
the	O
Cupin	B-protein
2	I-protein
conserved	I-protein
barrel	I-protein
domain	I-protein
protein	I-protein
(	O
YP_751781	B-protein
.	I-protein
1	I-protein
)	O
from	O
Shewanella	B-species
frigidimarina	I-species
(	O
PDB	O
accession	O
code	O
:	O
2PFW	O
)	O
with	O
a	O
Z	B-evidence
-	I-evidence
score	I-evidence
of	O
13	O
.	O
1	O
and	O
an	O
RMSD	B-evidence
of	O
2	O
.	O
2	O
Å	O
over	O
104	O
Cα	O
-	O
atoms	O
(	O
structural	O
data	O
for	O
this	O
protein	O
have	O
been	O
deposited	O
in	O
the	O
PDB	O
but	O
no	O
publication	O
connected	O
to	O
this	O
structure	B-evidence
is	O
currently	O
available	O
),	O
a	O
manganese	B-protein
-	I-protein
containing	I-protein
cupin	I-protein
(	O
TM1459	B-protein
)	O
from	O
Thermotoga	B-species
maritima	I-species
(	O
PDB	O
accession	O
code	O
:	O
1VJ2	O
)	O
with	O
a	O
Z	B-evidence
-	I-evidence
score	I-evidence
of	O
12	O
.	O
8	O
and	O
an	O
RMSD	B-evidence
of	O
2	O
.	O
0	O
Å	O
over	O
103	O
Cα	O
-	O
atoms	O
,	O
the	O
cyclase	B-protein_type
RemF	B-protein
from	O
Streptomyces	B-species
resistomycificus	I-species
(	O
PDB	O
accession	O
code	O
:	O
3HT1	O
with	O
a	O
Z	B-evidence
-	I-evidence
score	I-evidence
of	O
11	O
.	O
9	O
and	O
an	O
RMSD	B-evidence
of	O
1	O
.	O
9	O
Å	O
over	O
102	O
Cα	O
-	O
atoms	O
),	O
and	O
an	O
auxin	B-protein
-	I-protein
binding	I-protein
protein	I-protein
1	I-protein
from	O
Zea	B-species
mays	I-species
(	O
PDB	O
accession	O
code	O
:	O
1LR5	O
)	O
with	O
an	O
Z	B-evidence
-	I-evidence
score	I-evidence
of	O
11	O
.	O
8	O
and	O
an	O
RMSD	B-evidence
of	O
2	O
.	O
8	O
Å	O
over	O
104	O
Cα	O
-	O
atoms	O
).	O

Next	O
to	O
RemF	B-protein
and	O
the	O
aldos	B-protein_type
-	I-protein_type
2	I-protein_type
-	I-protein_type
ulose	I-protein_type
dehydratase	I-protein_type
/	O
isomerase	B-protein_type
,	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
is	O
only	O
the	O
third	O
characterized	O
dehydratase	B-protein_type
within	O
the	O
cupin	B-protein_type
superfamily	I-protein_type
.	O

In	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
crystal	B-evidence
structure	I-evidence
,	O
(	O
Sa	B-species
)	O
EctC	B-protein
has	O
crystallized	B-experimental_method
as	O
a	O
dimer	B-oligomeric_state
of	O
dimers	B-oligomeric_state
within	O
the	O
asymmetric	O
unit	O
.	O

This	O
dimer	B-oligomeric_state
(	O
Fig	O
5a	O
and	O
5b	O
)	O
is	O
composed	O
of	O
two	O
monomers	B-oligomeric_state
arranged	O
in	O
a	O
head	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
tail	I-protein_state
orientation	O
and	O
is	O
stabilized	O
via	O
strong	O
interactions	O
mediated	O
by	O
two	O
antiparallel	B-structure_element
β	I-structure_element
-	I-structure_element
strands	I-structure_element
,	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
β1	B-structure_element
(	O
sequence	O
1MIVRN5	B-structure_element
)	O
from	O
monomer	B-oligomeric_state
A	B-structure_element
and	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
β8	B-structure_element
from	O
monomer	B-oligomeric_state
B	B-structure_element
(	O
sequence	O
82GVMYAL87	B-structure_element
)	O
(	O
Fig	O
5c	O
).	O

The	O
strong	O
interactions	O
between	O
these	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
rely	O
primarily	O
on	O
backbone	O
contacts	O
.	O

In	O
addition	O
to	O
these	O
interactions	O
,	O
some	O
weaker	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
are	O
also	O
observed	O
between	O
the	O
two	O
monomers	B-oligomeric_state
in	O
some	O
loops	B-structure_element
connecting	O
the	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
.	O

Both	O
values	O
fall	O
within	O
the	O
range	O
for	O
known	O
functional	O
dimers	B-oligomeric_state
.	O

Crystal	B-evidence
structure	I-evidence
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
.	O

(	O
a	O
)	O
Top	O
-	O
view	O
of	O
the	O
dimer	B-oligomeric_state
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
.	O

The	O
position	O
of	O
the	O
water	B-chemical
molecule	O
,	O
described	O
in	O
detail	O
in	O
the	O
text	O
,	O
is	O
shown	O
in	O
one	O
of	O
the	O
monomers	B-oligomeric_state
as	O
an	O
orange	O
sphere	O
.	O
(	O
b	O
)	O
Side	O
-	O
view	O
of	O
a	O
(	O
Sa	B-species
)	O
EctC	B-protein
dimer	B-oligomeric_state
allowing	O
an	O
assessment	O
of	O
the	O
dimer	B-site
interface	I-site
formed	O
by	O
two	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
of	O
each	O
monomer	B-oligomeric_state
.	O

(	O
c	O
)	O
Close	O
-	O
up	O
representation	O
of	O
the	O
dimer	B-site
interface	I-site
mediated	O
by	O
beta	B-structure_element
-	I-structure_element
strand	I-structure_element
β1	B-structure_element
and	O
β6	B-structure_element
.	O

Indeed	O
,	O
a	O
similar	O
dimer	B-oligomeric_state
configuration	O
to	O
the	O
one	O
described	O
for	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
is	O
observed	O
with	O
the	O
same	O
monomer	B-oligomeric_state
-	O
monomer	B-oligomeric_state
interactions	O
mediated	O
by	O
the	O
two	O
β	B-structure_element
-	I-structure_element
sheets	I-structure_element
.	O

The	O
crystallographic	O
two	O
-	O
fold	O
axis	O
present	O
within	O
the	O
crystal	O
symmetry	O
is	O
located	O
exactly	O
in	O
between	O
the	O
two	O
monomers	B-oligomeric_state
,	O
resulting	O
in	O
a	O
monomer	B-oligomeric_state
within	O
the	O
asymmetric	O
unit	O
.	O

Hence	O
,	O
the	O
same	O
dimer	B-oligomeric_state
observed	O
in	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
structure	B-evidence
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
can	O
also	O
be	O
observed	O
in	O
the	O
“	O
open	B-protein_state
”	O
structure	B-evidence
.	O

Interestingly	O
,	O
the	O
proteins	O
identified	O
by	O
the	O
above	O
-	O
described	O
DALI	B-experimental_method
search	I-experimental_method
not	O
only	O
have	O
folds	O
similar	O
to	O
EctC	B-protein
,	O
but	O
are	O
also	O
functional	O
dimers	B-oligomeric_state
that	O
adopt	O
similar	O
monomer	B-oligomeric_state
-	O
monomer	B-oligomeric_state
interactions	O
within	O
the	O
dimer	B-oligomeric_state
assembly	O
as	O
deduced	O
from	O
the	O
inspection	O
of	O
the	O
corresponding	O
PDB	O
files	O
(	O
2PFW	O
,	O
3HT1	O
,	O
1VJ2	O
,	O
1LR5	O
).	O

Structural	O
rearrangements	O
of	O
the	O
flexible	B-protein_state
(	O
Sa	B-species
)	O
EctC	B-protein
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element

The	O
cupin	O
core	O
represents	O
the	O
structural	O
framework	O
of	O
ectoine	B-protein_type
synthase	I-protein_type
(	O
Figs	O
4	O
and	O
5	O
).	O

The	O
major	O
difference	O
in	O
the	O
two	O
crystal	B-evidence
structures	I-evidence
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
reported	O
here	O
is	O
the	O
orientation	O
of	O
the	O
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
.	O

Some	O
amino	O
acids	O
located	O
in	O
the	O
carboxy	B-structure_element
-	I-structure_element
terminal	I-structure_element
region	I-structure_element
of	O
the	O
137	B-residue_range
amino	I-residue_range
acids	I-residue_range
comprising	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
are	O
highly	B-protein_state
conserved	I-protein_state
(	O
Fig	O
2	O
)	O
within	O
the	O
extended	B-protein_state
EctC	B-protein_type
protein	I-protein_type
family	O
.	O

At	O
the	O
end	O
of	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
β11	B-structure_element
,	O
two	O
consecutive	O
conserved	B-protein_state
proline	B-residue_name
residues	O
(	O
Pro	B-residue_name_number
-	I-residue_name_number
109	I-residue_name_number
and	O
Pro	B-residue_name_number
-	I-residue_name_number
110	I-residue_name_number
)	O
are	O
present	O
that	O
are	O
responsible	O
for	O
a	O
turn	O
in	O
the	O
main	O
chain	O
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
.	O

In	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
,	O
the	O
visible	O
electron	B-evidence
density	I-evidence
of	O
the	O
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
is	O
extended	O
by	O
7	B-residue_range
amino	I-residue_range
acid	I-residue_range
residues	I-residue_range
and	O
ends	O
at	O
position	O
Gly	B-residue_name_number
-	I-residue_name_number
121	I-residue_name_number
.	O

Furthermore	O
,	O
this	O
helix	B-structure_element
is	O
stabilized	O
via	O
interactions	O
with	O
the	O
loop	B-structure_element
region	I-structure_element
between	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
β4	B-structure_element
and	O
β6	B-structure_element
,	O
thereby	O
inducing	O
a	O
structural	O
rearrangement	O
.	O

This	O
induces	O
the	O
formation	O
of	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
β5	B-structure_element
,	O
which	O
is	O
not	O
present	O
when	O
the	O
small	B-structure_element
C	I-structure_element
-	I-structure_element
terminal	I-structure_element
helix	I-structure_element
is	O
absent	B-protein_state
as	O
observed	O
in	O
the	O
“	O
open	B-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
.	O

The	O
position	O
of	O
this	O
His	B-residue_name
residue	O
is	O
slightly	O
shifted	O
in	O
both	O
(	O
Sa	B-species
)	O
EctC	B-protein
structures	B-evidence
,	O
likely	O
the	O
result	O
of	O
the	O
formation	O
of	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
β5	B-structure_element
.	O

The	O
consecutive	O
Pro	B-residue_name_number
-	I-residue_name_number
109	I-residue_name_number
and	O
Pro	B-residue_name_number
-	I-residue_name_number
110	I-residue_name_number
residues	O
found	O
at	O
the	O
end	O
of	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
β11are	B-structure_element
highly	B-protein_state
conserved	I-protein_state
in	O
EctC	B-protein_type
-	I-protein_type
type	I-protein_type
proteins	I-protein_type
(	O
Fig	O
2	O
).	O

They	O
are	O
responsible	O
for	O
redirecting	O
the	O
main	O
chain	O
of	O
the	O
remaining	O
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
(	O
27	B-residue_range
amino	I-residue_range
acid	I-residue_range
residues	I-residue_range
)	O
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
to	O
close	O
the	O
cupin	B-structure_element
fold	I-structure_element
.	O

In	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
structure	B-evidence
this	O
results	O
in	O
a	O
complete	O
closure	O
of	O
the	O
entry	O
of	O
the	O
cupin	B-structure_element
barrel	I-structure_element
(	O
Fig	O
4a	O
to	O
4c	O
).	O

A	O
search	O
for	O
partners	O
interacting	O
with	O
Pro	B-residue_name_number
-	I-residue_name_number
109	I-residue_name_number
revealed	O
that	O
it	O
interacts	O
via	O
its	O
backbone	O
oxygen	O
with	O
the	O
side	O
chain	O
of	O
His	B-residue_name_number
-	I-residue_name_number
55	I-residue_name_number
as	O
visible	O
in	O
both	O
the	O
“	O
open	B-protein_state
”	O
and	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
structures	B-evidence
.	O

The	O
Pro	B-residue_name_number
-	I-residue_name_number
109	I-residue_name_number
/	O
His	B-residue_name_number
-	I-residue_name_number
55	I-residue_name_number
interaction	O
ensures	O
the	O
stable	B-protein_state
orientation	O
of	O
both	O
proline	B-residue_name
residues	O
at	O
the	O
end	O
of	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
β11	B-structure_element
.	O

In	O
addition	O
to	O
the	O
interactions	O
between	O
Pro	B-residue_name_number
-	I-residue_name_number
109	I-residue_name_number
and	O
His	B-residue_name_number
-	I-residue_name_number
55	I-residue_name_number
,	O
the	O
carboxy	B-structure_element
-	I-structure_element
terminal	I-structure_element
region	I-structure_element
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
is	O
held	O
in	O
position	O
via	O
an	O
interaction	O
of	O
Glu	B-residue_name_number
-	I-residue_name_number
115	I-residue_name_number
with	O
His	B-residue_name_number
-	I-residue_name_number
55	I-residue_name_number
,	O
which	O
stabilizes	O
the	O
conformation	O
of	O
the	O
small	B-structure_element
helix	I-structure_element
in	O
the	O
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
further	O
.	O

Architecture	O
of	O
the	O
presumed	O
metal	B-site
-	I-site
binding	I-site
site	I-site
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
and	O
its	O
flexible	B-protein_state
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
.	O

(	O
a	O
)	O
The	O
described	O
water	B-chemical
molecule	O
(	O
depicted	O
as	O
orange	O
sphere	O
)	O
is	O
bound	O
via	O
interactions	O
with	O
the	O
side	O
chains	O
of	O
Glu	B-residue_name_number
-	I-residue_name_number
57	I-residue_name_number
,	O
Tyr	B-residue_name_number
-	I-residue_name_number
85	I-residue_name_number
,	O
and	O
His	B-residue_name_number
-	I-residue_name_number
93	I-residue_name_number
.	O

The	O
position	O
occupied	O
by	O
this	O
water	B-chemical
molecule	O
represents	O
probably	O
the	O
position	O
of	O
the	O
Fe2	B-chemical
+	I-chemical
cofactor	O
in	O
the	O
active	B-site
side	I-site
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
.	O

His	B-residue_name_number
-	I-residue_name_number
55	I-residue_name_number
interacts	O
with	O
the	O
double	B-structure_element
proline	I-structure_element
motif	I-structure_element
(	O
Pro	B-residue_name_number
-	I-residue_name_number
109	I-residue_name_number
and	O
Pro	B-residue_name_number
-	I-residue_name_number
110	I-residue_name_number
).	O

It	O
is	O
further	O
stabilized	O
via	O
an	O
interaction	O
with	O
the	O
side	O
chain	O
of	O
Glu	B-residue_name_number
-	I-residue_name_number
115	I-residue_name_number
which	O
is	O
localized	O
in	O
the	O
flexible	B-protein_state
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
(	O
colored	O
in	O
orange	O
)	O
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
that	O
is	O
visible	O
in	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
.	O

Since	O
(	O
Sa	B-species
)	O
EctC	B-protein
is	O
a	O
metal	B-chemical
containing	O
protein	O
(	O
Fig	O
3	O
),	O
we	O
tried	O
to	O
fit	O
either	O
Fe2	B-chemical
+,	I-chemical
or	O
Zn2	B-chemical
+	I-chemical
ions	O
into	O
this	O
density	B-evidence
and	O
also	O
refined	B-experimental_method
occupancy	I-experimental_method
.	O

Only	O
the	O
refinement	O
of	O
Fe2	B-chemical
+	I-chemical
resulted	O
in	O
a	O
visibly	O
improved	O
electron	B-evidence
density	I-evidence
,	O
however	O
with	O
a	O
low	O
degree	O
of	O
occupancy	O
.	O

This	O
possible	O
iron	B-chemical
molecule	O
is	O
bound	O
via	O
interactions	O
with	O
Glu	B-residue_name_number
-	I-residue_name_number
57	I-residue_name_number
,	O
Tyr	B-residue_name_number
-	I-residue_name_number
85	I-residue_name_number
and	O
His	B-residue_name_number
-	I-residue_name_number
93	I-residue_name_number
(	O
Fig	O
6a	O
and	O
6b	O
).	O

The	O
distance	O
between	O
the	O
side	O
chains	O
of	O
these	O
residues	O
and	O
the	O
(	O
putative	O
)	O
iron	B-chemical
co	O
-	O
factor	O
is	O
3	O
.	O
1	O
Å	O
for	O
Glu	B-residue_name_number
-	I-residue_name_number
57	I-residue_name_number
,	O
2	O
.	O
9	O
Å	O
for	O
Tyr	B-residue_name_number
-	I-residue_name_number
85	I-residue_name_number
,	O
and	O
2	O
.	O
9	O
Å	O
for	O
His	B-residue_name_number
-	I-residue_name_number
93	I-residue_name_number
,	O
respectively	O
.	O

The	O
position	O
of	O
this	O
water	B-chemical
molecule	O
is	O
described	O
in	O
more	O
detail	O
below	O
and	O
is	O
highlighted	O
in	O
Figs	O
5a	O
and	O
5b	O
and	O
6a	O
and	O
6b	O
as	O
a	O
sphere	O
.	O

Interestingly	O
,	O
all	O
three	O
amino	O
acids	O
coordinating	O
this	O
water	B-chemical
molecule	O
are	O
strictly	B-protein_state
conserved	I-protein_state
within	O
an	O
alignment	B-experimental_method
of	O
440	O
members	O
of	O
the	O
EctC	B-protein_type
protein	I-protein_type
family	O
(	O
for	O
an	O
abbreviated	O
alignment	O
of	O
EctC	B-protein_type
-	I-protein_type
type	I-protein_type
proteins	I-protein_type
see	O
Fig	O
2	O
).	O

In	O
the	O
“	O
open	B-protein_state
”	O
structure	B-evidence
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
,	O
electron	B-evidence
density	I-evidence
is	O
visible	O
where	O
the	O
presumptive	O
iron	B-chemical
is	O
positioned	O
in	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
structure	B-evidence
.	O

However	O
,	O
this	O
electron	B-evidence
density	I-evidence
fits	O
perfectly	O
to	O
a	O
water	B-chemical
molecule	O
and	O
not	O
to	O
an	O
iron	B-chemical
,	O
and	O
the	O
water	B-chemical
molecule	O
was	O
clearly	O
visible	O
after	O
the	O
refinement	O
at	O
this	O
high	O
resolution	O
(	O
1	O
.	O
2	O
Å	O
)	O
of	O
the	O
“	O
open	B-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
.	O

In	O
a	O
superimposition	B-experimental_method
of	O
both	O
(	O
Sa	B-species
)	O
EctC	B-protein
crystal	B-evidence
structures	I-evidence
,	O
the	O
spatial	O
arrangements	O
of	O
the	O
side	O
chains	O
of	O
the	O
three	O
amino	O
acids	O
(	O
Glu	B-residue_name_number
-	I-residue_name_number
57	I-residue_name_number
,	O
Tyr	B-residue_name_number
-	I-residue_name_number
85	I-residue_name_number
,	O
and	O
His	B-residue_name_number
-	I-residue_name_number
93	I-residue_name_number
)	O
likely	O
to	O
contact	O
the	O
iron	B-chemical
in	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
structure	B-evidence
match	O
nicely	O
with	O
those	O
of	O
the	O
corresponding	O
residues	O
of	O
the	O
“	O
iron	B-protein_state
-	I-protein_state
free	I-protein_state
”	O
“	O
open	B-protein_state
”	O
structure	B-evidence
(	O
Fig	O
6b	O
).	O

In	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
structure	B-evidence
,	O
the	O
hydroxyl	O
-	O
group	O
of	O
the	O
side	O
-	O
chain	O
of	O
Tyr	B-residue_name_number
-	I-residue_name_number
52	I-residue_name_number
points	O
towards	O
the	O
iron	B-chemical
(	O
Fig	O
6a	O
and	O
6b	O
),	O
but	O
the	O
corresponding	O
distance	O
(	O
3	O
.	O
9	O
Å	O
)	O
makes	O
it	O
highly	O
unlikely	O
that	O
Tyr	B-residue_name_number
-	I-residue_name_number
52	I-residue_name_number
is	O
directly	O
involved	O
in	O
metal	B-chemical
binding	O
.	O

Nevertheless	O
,	O
its	O
substitution	B-experimental_method
by	O
an	O
Ala	B-residue_name
residue	O
causes	O
a	O
strong	O
decrease	O
in	O
iron	B-chemical
-	O
content	O
and	O
enzyme	O
activity	O
of	O
the	O
mutant	B-protein_state
protein	O
(	O
Table	O
1	O
).	O

Since	O
Tyr	B-residue_name_number
-	I-residue_name_number
52	I-residue_name_number
is	O
strictly	B-protein_state
conserved	I-protein_state
in	O
an	O
alignment	B-experimental_method
of	O
440	O
EctC	B-protein_type
-	I-protein_type
type	I-protein_type
proteins	I-protein_type
(	O
Fig	O
2	O
),	O
we	O
speculate	O
that	O
it	O
might	O
be	O
involved	O
in	O
contacting	O
the	O
substrate	O
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
and	O
that	O
the	O
absence	B-protein_state
of	I-protein_state
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
in	O
our	O
(	O
Sa	B-species
)	O
EctC	B-protein
crystal	B-evidence
structures	I-evidence
might	O
endow	O
the	O
side	O
chain	O
of	O
Tyr	B-residue_name_number
-	I-residue_name_number
52	I-residue_name_number
with	O
extra	O
spatial	O
flexibility	O
.	O

To	O
further	O
analyze	O
the	O
putative	O
iron	B-site
binding	I-site
site	I-site
(	O
Fig	O
6a	O
),	O
we	O
performed	O
structure	B-experimental_method
-	I-experimental_method
guided	I-experimental_method
site	I-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
and	O
assessed	O
the	O
resulting	O
(	O
Sa	B-species
)	O
EctC	B-protein
variants	O
for	O
their	O
iron	B-chemical
content	O
and	O
studied	O
their	O
enzyme	O
activity	O
.	O

When	O
those	O
three	O
residues	O
(	O
Glu	B-residue_name_number
-	I-residue_name_number
57	I-residue_name_number
,	O
Tyr	B-residue_name_number
-	I-residue_name_number
85	I-residue_name_number
,	O
His	B-residue_name_number
-	I-residue_name_number
93	I-residue_name_number
)	O
that	O
likely	O
form	O
the	O
mono	B-site
-	I-site
nuclear	I-site
iron	I-site
center	I-site
in	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
crystal	B-evidence
structure	I-evidence
were	O
individually	O
replaced	B-experimental_method
by	O
an	O
Ala	B-residue_name
residue	O
,	O
both	O
the	O
catalytic	O
activity	O
and	O
the	O
iron	B-chemical
content	O
of	O
the	O
mutant	B-protein_state
proteins	O
was	O
strongly	O
reduced	O
(	O
Table	O
1	O
).	O

For	O
some	O
of	O
the	O
presumptive	O
iron	B-site
-	I-site
coordinating	I-site
residues	I-site
,	O
additional	O
site	B-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
experiments	O
were	O
carried	O
out	O
.	O

To	O
verify	O
the	O
importance	O
of	O
the	O
negative	O
charge	O
in	O
the	O
position	O
of	O
Glu	B-residue_name_number
-	I-residue_name_number
57	I-residue_name_number
,	O
we	O
created	O
an	O
Asp	B-residue_name
variant	B-protein_state
.	O

This	O
mutant	B-protein_state
protein	O
rescued	O
the	O
enzyme	O
activity	O
and	O
iron	B-chemical
content	O
of	O
the	O
Ala	B-residue_name
substitution	B-experimental_method
substantially	O
(	O
Table	O
1	O
).	O

Collectively	O
,	O
these	O
data	O
suggest	O
that	O
the	O
hydroxyl	O
group	O
of	O
the	O
Tyr	B-residue_name_number
-	I-residue_name_number
85	I-residue_name_number
side	O
chain	O
is	O
needed	O
for	O
the	O
binding	O
of	O
the	O
iron	B-chemical
(	O
Fig	O
6a	O
).	O

We	O
also	O
replaced	B-experimental_method
the	O
presumptive	O
iron	B-site
-	I-site
binding	I-site
residue	I-site
His	B-residue_name_number
-	I-residue_name_number
93	I-residue_name_number
by	O
an	O
Asn	B-residue_name
residue	O
,	O
yielding	O
a	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
variant	O
that	O
possessed	O
an	O
enzyme	O
activity	O
of	O
23	O
%	O
and	O
iron	B-chemical
content	O
of	O
only	O
14	O
%	O
relative	O
to	O
that	O
of	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
protein	O
(	O
Table	O
1	O
).	O

Collectively	O
,	O
the	O
data	O
addressing	O
the	O
functionality	O
of	O
the	O
putative	O
iron	B-site
-	I-site
coordinating	I-site
residues	I-site
(	O
Glu	B-residue_name_number
-	I-residue_name_number
57	I-residue_name_number
,	O
Tyr	B-residue_name_number
-	I-residue_name_number
85	I-residue_name_number
,	O
His	B-residue_name_number
-	I-residue_name_number
93	I-residue_name_number
)	O
buttress	O
our	O
notion	O
that	O
the	O
Fe2	B-chemical
+	I-chemical
present	O
in	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
is	O
of	O
catalytic	O
importance	O
.	O

A	O
chemically	O
undefined	O
ligand	O
in	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
provides	O
clues	O
for	O
the	O
binding	O
of	O
the	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
substrate	O

Despite	O
considerable	O
efforts	O
,	O
either	O
by	O
trying	O
co	B-experimental_method
-	I-experimental_method
crystallization	I-experimental_method
or	O
soaking	B-experimental_method
experiments	I-experimental_method
,	O
we	O
were	O
not	O
able	O
to	O
obtain	O
a	O
(	O
Sa	B-species
)	O
EctC	B-protein
crystal	B-evidence
structures	I-evidence
that	O
contained	O
either	O
the	O
substrate	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
,	O
or	O
ectoine	B-chemical
,	O
the	O
reaction	O
product	O
of	O
ectoine	B-protein_type
synthase	I-protein_type
(	O
Fig	O
1	O
).	O

However	O
,	O
in	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
where	O
the	O
carboxy	B-structure_element
-	I-structure_element
terminal	I-structure_element
loop	I-structure_element
is	O
largely	O
resolved	O
,	O
a	O
long	O
stretched	O
electron	B-evidence
density	I-evidence
feature	O
was	O
detected	O
in	O
the	O
predicted	O
active	B-site
site	I-site
of	O
the	O
enzyme	O
;	O
it	O
remained	O
visible	O
after	O
crystallographic	B-experimental_method
refinement	I-experimental_method
.	O

We	O
tried	O
to	O
fit	O
all	O
compounds	O
used	O
in	O
the	O
buffers	O
during	O
purification	B-experimental_method
and	O
crystallization	B-experimental_method
into	O
the	O
observed	O
electron	B-evidence
density	I-evidence
,	O
but	O
none	O
matched	O
.	O

This	O
observation	O
indicates	O
that	O
the	O
chemically	O
undefined	O
ligand	O
was	O
either	O
trapped	O
by	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
during	O
its	O
heterologous	O
production	O
in	O
E	B-species
.	I-species
coli	I-species
or	O
during	O
crystallization	B-experimental_method
.	O

Estimating	O
from	O
the	O
dimensions	O
of	O
the	O
electron	B-evidence
density	I-evidence
feature	I-evidence
,	O
we	O
modeled	O
the	O
chemically	O
undefined	O
compound	O
trapped	O
by	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
as	O
a	O
hexane	B-chemical
-	I-chemical
1	I-chemical
,	I-chemical
6	I-chemical
-	I-chemical
diol	I-chemical
molecule	O
(	O
PDB	O
identifier	O
:	O
HEZ	O
)	O
to	O
best	O
fit	O
the	O
observed	O
electron	B-evidence
density	I-evidence
.	O

However	O
,	O
to	O
the	O
best	O
of	O
our	O
knowledge	O
,	O
hexane	B-chemical
-	I-chemical
1	I-chemical
,	I-chemical
6	I-chemical
-	I-chemical
diol	I-chemical
is	O
not	O
part	O
of	O
the	O
E	B-species
.	I-species
coli	I-species
metabolome	O
.	O

We	O
note	O
that	O
both	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
and	O
hexane	B-chemical
-	I-chemical
1	I-chemical
,	I-chemical
6	I-chemical
-	I-chemical
diol	I-chemical
are	O
both	O
C6	O
-	O
compounds	O
and	O
display	O
similar	O
length	O
(	O
Fig	O
7a	O
).	O

A	O
chemically	O
undefined	O
ligand	O
is	O
captured	O
in	O
the	O
active	B-site
site	I-site
of	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
crystal	B-evidence
structure	I-evidence
.	O

(	O
a	O
)	O
The	O
observed	O
electron	B-evidence
density	I-evidence
in	O
the	O
active	B-site
site	I-site
of	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
structure	B-evidence
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
is	O
modeled	O
as	O
a	O
hexane	B-chemical
-	I-chemical
1	I-chemical
,	I-chemical
6	I-chemical
-	I-chemical
diol	I-chemical
molecule	O
and	O
compared	O
with	O
the	O
electron	B-evidence
density	I-evidence
of	O
the	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
substrate	O
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
to	O
emphasize	O
the	O
similarity	O
in	O
size	O
of	O
these	O
compounds	O
.	O

(	O
b	O
)	O
The	O
presumable	O
binding	B-site
site	I-site
of	O
the	O
iron	B-chemical
co	O
-	O
factor	O
and	O
of	O
the	O
modeled	O
hexane	B-chemical
-	I-chemical
1	I-chemical
,	I-chemical
6	I-chemical
-	I-chemical
diol	I-chemical
molecule	O
is	O
depicted	O
.	O

The	O
amino	O
acid	O
side	O
chains	O
involved	O
in	O
iron	B-chemical
-	O
ligand	O
binding	O
are	O
colored	O
in	O
blue	O
and	O
those	O
involved	O
in	O
the	O
binding	O
of	O
the	O
chemically	O
undefined	O
ligand	O
are	O
colored	O
in	O
green	O
using	O
a	O
ball	O
and	O
stick	O
representation	O
.	O

The	O
flexible	B-protein_state
carboxy	B-structure_element
-	I-structure_element
terminal	I-structure_element
loop	I-structure_element
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
is	O
highlighted	O
in	O
orange	O
.	O

We	O
refined	B-experimental_method
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
with	O
the	O
trapped	O
compound	O
,	O
and	O
by	O
doing	O
so	O
,	O
the	O
refinement	O
parameters	O
(	O
especially	O
R	B-evidence
-	I-evidence
and	I-evidence
Rfree	I-evidence
-	I-evidence
factor	I-evidence
)	O
dropped	O
by	O
1	O
.	O
5	O
%.	O

Remarkably	O
,	O
all	O
of	O
these	O
residues	O
are	O
highly	B-protein_state
conserved	I-protein_state
throughout	O
the	O
extended	O
EctC	B-protein_type
protein	I-protein_type
family	O
(	O
Fig	O
2	O
).	O

Structure	B-experimental_method
-	I-experimental_method
guided	I-experimental_method
site	I-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
of	O
the	O
catalytic	B-site
core	I-site
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type

In	O
a	O
previous	O
alignment	B-experimental_method
of	I-experimental_method
the	I-experimental_method
amino	I-experimental_method
acid	I-experimental_method
sequences	I-experimental_method
of	O
440	O
EctC	B-protein_type
-	I-protein_type
type	I-protein_type
proteins	I-protein_type
,	O
13	O
amino	O
acids	O
were	O
identified	O
as	O
strictly	B-protein_state
conserved	I-protein_state
residues	O
.	O

Amino	O
acid	O
residues	O
Gly	B-residue_name_number
-	I-residue_name_number
64	I-residue_name_number
,	O
Pro	B-residue_name_number
-	I-residue_name_number
109	I-residue_name_number
,	O
and	O
Gly	B-residue_name_number
-	I-residue_name_number
113	I-residue_name_number
likely	O
fulfill	O
structural	O
roles	O
since	O
they	O
are	O
positioned	O
either	O
at	O
the	O
end	O
or	O
at	O
the	O
beginning	O
of	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
and	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
.	O

We	O
considered	O
the	O
remaining	O
ten	O
residues	O
as	O
important	O
either	O
for	O
ligand	O
binding	O
,	O
for	O
catalysis	O
,	O
or	O
for	O
the	O
structurally	O
correct	O
orientation	O
of	O
the	O
flexible	B-protein_state
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
.	O

In	O
view	O
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
with	O
the	O
serendipitously	O
trapped	O
compound	O
(	O
Fig	O
7b	O
),	O
we	O
probed	O
the	O
functional	O
importance	O
of	O
the	O
seven	O
residues	O
that	O
contact	O
this	O
ligand	O
by	O
structure	B-experimental_method
-	I-experimental_method
guided	I-experimental_method
site	I-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
(	O
Table	O
1	O
).	O

We	O
benchmarked	O
the	O
activity	O
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
variants	O
in	O
a	O
single	B-experimental_method
time	I-experimental_method
-	I-experimental_method
point	I-experimental_method
enzyme	I-experimental_method
assay	I-experimental_method
under	O
conditions	O
where	O
10	O
μM	O
of	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
converted	O
almost	O
completely	O
the	O
supplied	O
10	O
mM	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
substrate	O
to	O
9	O
.	O
33	O
mM	O
ectoine	B-chemical
within	O
a	O
time	O
frame	O
of	O
20	O
min	O
.	O

In	O
addition	O
,	O
we	O
determined	O
the	O
iron	B-chemical
content	O
of	O
each	O
of	O
the	O
mutant	B-protein_state
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
by	O
a	O
colorimetric	B-experimental_method
assay	I-experimental_method
(	O
Table	O
1	O
).	O

The	O
side	O
chains	O
of	O
the	O
evolutionarily	B-protein_state
conserved	I-protein_state
Trp	B-residue_name_number
-	I-residue_name_number
21	I-residue_name_number
,	O
Ser	B-residue_name_number
-	I-residue_name_number
23	I-residue_name_number
,	O
Thr	B-residue_name_number
-	I-residue_name_number
40	I-residue_name_number
,	O
Cys	B-residue_name_number
-	I-residue_name_number
105	I-residue_name_number
,	O
and	O
Phe	B-residue_name_number
-	I-residue_name_number
107	I-residue_name_number
residues	O
(	O
Fig	O
2	O
)	O
make	O
contacts	O
with	O
the	O
chemically	O
undefined	O
ligand	O
that	O
we	O
observed	O
in	O
the	O
“	O
semi	B-protein_state
-	I-protein_state
closed	I-protein_state
”	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
(	O
Fig	O
7b	O
).	O

Thr	B-residue_name_number
-	I-residue_name_number
40	I-residue_name_number
is	O
positioned	O
on	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
β5	B-structure_element
and	O
its	O
side	O
chain	O
protrudes	O
into	O
the	O
lumen	O
of	O
the	O
cupin	B-structure_element
barrel	I-structure_element
formed	O
by	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
(	O
Fig	O
7b	O
).	O

We	O
also	O
replaced	B-experimental_method
Phe	B-residue_name_number
-	I-residue_name_number
107	I-residue_name_number
with	O
either	O
an	O
Tyr	B-residue_name
or	O
an	O
Trp	B-residue_name
residue	O
:	O
the	O
Phe	B-mutant
-	I-mutant
107	I-mutant
/	I-mutant
Tyr	I-mutant
substitution	B-experimental_method
possessed	O
near	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
enzyme	O
activity	O
(	O
about	O
95	O
%)	O
and	O
the	O
full	O
iron	B-chemical
content	O
,	O
but	O
the	O
Phe	B-mutant
-	I-mutant
107	I-mutant
/	I-mutant
Trp	I-mutant
substitution	B-experimental_method
possessed	O
only	O
12	O
%	O
enzyme	O
activity	O
and	O
72	O
%	O
iron	B-chemical
content	O
compared	O
to	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
protein	O
.	O

The	O
properties	O
of	O
these	O
mutant	B-protein_state
proteins	O
indicate	O
that	O
the	O
aromatic	O
side	O
chain	O
at	O
position	O
107	B-residue_number
of	O
(	O
Sa	B-species
)	O
EctC	B-protein
is	O
of	O
importance	O
but	O
that	O
a	O
substitution	B-experimental_method
with	O
a	O
bulky	O
aromatic	O
side	O
chain	O
is	O
strongly	O
detrimental	O
to	O
enzyme	O
activity	O
and	O
concomitantly	O
moderately	O
impairs	O
iron	B-chemical
binding	O
.	O

Replacement	B-experimental_method
of	O
the	O
only	O
Cys	B-residue_name
residue	O
in	O
(	O
Sa	B-species
)	O
EctC	B-protein
(	O
Cys	B-residue_name_number
-	I-residue_name_number
105	I-residue_name_number
;	O
Fig	O
2	O
)	O
by	O
a	O
Ser	B-residue_name
residue	O
,	O
a	O
configuration	O
that	O
is	O
naturally	O
found	O
in	O
two	O
EctC	B-protein_type
proteins	I-protein_type
among	O
440	O
inspected	O
amino	O
acid	O
sequences	O
,	O
yielded	O
a	O
(	O
Sa	B-species
)	O
EctC	B-protein
variant	B-protein_state
with	O
84	O
%	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
activity	O
and	O
an	O
iron	B-chemical
content	O
similar	O
to	O
that	O
of	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
protein	O
.	O

Since	O
the	O
side	O
-	O
chains	O
of	O
Cys	B-residue_name
residues	O
are	O
chemically	O
reactive	O
and	O
often	O
participate	O
in	O
enzyme	O
catalysis	O
,	O
Cys	B-residue_name_number
-	I-residue_name_number
105	I-residue_name_number
(	O
or	O
Ser	B-residue_name_number
-	I-residue_name_number
105	I-residue_name_number
)	O
might	O
serve	O
such	O
a	O
role	O
for	O
ectoine	B-protein_type
synthase	I-protein_type
.	O

Based	O
on	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
crystal	B-evidence
structures	I-evidence
that	O
we	O
present	O
here	O
,	O
we	O
can	O
currently	O
not	O
firmly	O
understand	O
why	O
the	O
replacement	B-experimental_method
of	O
Tyr	B-residue_name_number
-	I-residue_name_number
52	I-residue_name_number
by	O
Ala	B-residue_name
impairs	O
enzyme	O
function	O
and	O
iron	B-chemical
content	O
so	O
drastically	O
(	O
Table	O
1	O
).	O

This	O
is	O
different	O
for	O
the	O
His	B-mutant
-	I-mutant
55	I-mutant
/	I-mutant
Ala	I-mutant
substitution	O
.	O

The	O
individual	O
substitution	B-experimental_method
of	O
either	O
Glu	B-residue_name_number
-	I-residue_name_number
115	I-residue_name_number
or	O
His	B-residue_name_number
-	I-residue_name_number
55	I-residue_name_number
by	O
an	O
Ala	B-residue_name
residue	O
is	O
predicted	O
to	O
disrupt	O
this	O
interactive	B-site
network	I-site
and	O
therefore	O
should	O
affect	O
enzyme	O
activity	O
.	O

Indeed	O
,	O
the	O
Glu	B-mutant
-	I-mutant
115	I-mutant
/	I-mutant
Ala	I-mutant
and	O
the	O
His	B-mutant
-	I-mutant
55	I-mutant
/	I-mutant
Ala	I-mutant
substitutions	O
possessed	O
only	O
21	O
%	O
and	O
16	O
%	O
activity	O
of	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
protein	O
,	O
respectively	O
(	O
Table	O
1	O
).	O

We	O
also	O
replaced	B-experimental_method
Glu	B-residue_name_number
-	I-residue_name_number
115	I-residue_name_number
with	O
a	O
negatively	O
charged	O
residue	O
(	O
Asp	B-residue_name
);	O
this	O
(	O
Sa	B-species
)	O
EctC	B-protein
variant	O
possessed	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
levels	O
of	O
iron	B-chemical
and	O
still	O
exhibited	O
77	O
%	O
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
enzyme	O
activity	O
.	O

Collectively	O
,	O
these	O
data	O
suggest	O
that	O
the	O
correct	O
positioning	O
of	O
the	O
carboxy	B-structure_element
-	I-structure_element
terminus	I-structure_element
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
is	O
of	O
structural	O
and	O
functional	O
importance	O
for	O
the	O
activity	O
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
.	O

Residues	O
Leu	B-residue_name_number
-	I-residue_name_number
87	I-residue_name_number
and	O
Asp	B-residue_name_number
-	I-residue_name_number
91	I-residue_name_number
are	O
highly	B-protein_state
conserved	I-protein_state
in	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
protein	O
family	O
.	O

The	O
replacement	B-experimental_method
of	O
Leu	B-residue_name_number
-	I-residue_name_number
87	I-residue_name_number
by	O
Ala	B-residue_name
led	O
to	O
a	O
substantial	O
drop	O
in	O
enzyme	O
activity	O
(	O
Table	O
1	O
).	O

Conversely	O
,	O
the	O
replacement	B-experimental_method
of	O
Asp	B-residue_name_number
-	I-residue_name_number
91	I-residue_name_number
by	O
Ala	B-residue_name
and	O
Glu	B-residue_name
,	O
resulted	O
in	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
variants	O
with	O
80	O
%	O
and	O
98	O
%	O
enzyme	O
activity	O
,	O
respectively	O
(	O
Table	O
1	O
).	O

We	O
currently	O
cannot	O
comment	O
on	O
possible	O
functional	O
role	O
Asp	B-residue_name_number
-	I-residue_name_number
91	I-residue_name_number
.	O

However	O
,	O
Leu	B-residue_name_number
-	I-residue_name_number
87	I-residue_name_number
is	O
positioned	O
at	O
the	O
end	O
of	O
one	O
of	O
the	O
β	B-structure_element
-	I-structure_element
sheets	I-structure_element
that	O
form	O
the	O
dimer	B-site
interface	I-site
(	O
Fig	O
5c	O
)	O
and	O
it	O
might	O
therefore	O
possess	O
a	O
structural	O
role	O
.	O

It	O
is	O
also	O
located	O
near	O
Tyr	B-residue_name_number
-	I-residue_name_number
85	I-residue_name_number
,	O
one	O
of	O
the	O
residues	O
that	O
probably	O
coordinate	O
the	O
iron	B-chemical
molecule	O
with	O
in	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
active	B-site
site	I-site
(	O
Fig	O
6a	O
)	O
and	O
therefore	O
might	O
exert	O
indirect	O
effects	O
.	O

We	O
note	O
that	O
His	B-residue_name_number
-	I-residue_name_number
117	I-residue_name_number
is	O
located	O
close	O
to	O
the	O
chemically	O
undefined	O
ligand	O
in	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
(	O
Fig	O
7b	O
)	O
and	O
might	O
thus	O
play	O
a	O
role	O
in	O
contacting	O
the	O
natural	O
substrate	O
of	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
.	O

Both	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
variants	O
exhibited	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
level	O
enzyme	O
activities	O
and	O
possessed	O
a	O
iron	B-chemical
content	O
matching	O
that	O
of	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
Table	O
1	O
).	O

This	O
illustrates	O
that	O
not	O
every	O
amino	O
acid	O
substitution	O
in	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
leads	O
to	O
an	O
indiscriminate	O
impairment	O
of	O
enzyme	O
function	O
and	O
iron	B-chemical
content	O
.	O

The	O
crystallographic	B-evidence
data	I-evidence
presented	O
here	O
firmly	O
identify	O
ectoine	B-protein_type
synthase	I-protein_type
(	O
EctC	B-protein
),	O
an	O
enzyme	O
critical	O
for	O
the	O
production	O
of	O
the	O
microbial	B-taxonomy_domain
cytoprotectant	O
and	O
chemical	O
chaperone	O
ectoine	B-chemical
,	O
as	O
a	O
new	O
member	O
of	O
the	O
cupin	B-protein_type
superfamily	I-protein_type
.	O

The	O
overall	O
fold	O
and	O
bowl	O
shape	O
of	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
protein	O
(	O
Figs	O
4	O
and	O
5	O
)	O
with	O
its	O
11	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
(	O
β1	B-structure_element
-	I-structure_element
β11	I-structure_element
)	O
and	O
two	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
(	O
α	B-structure_element
-	I-structure_element
I	I-structure_element
and	O
α	B-structure_element
-	I-structure_element
II	I-structure_element
)	O
closely	O
adheres	O
to	O
the	O
design	O
principles	O
typically	O
found	O
in	O
crystal	B-evidence
structures	I-evidence
of	O
cupins	B-protein_type
.	O

In	O
addition	O
to	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
,	O
the	O
polyketide	B-protein_type
cyclase	I-protein_type
RemF	B-protein
is	O
the	O
only	O
other	O
currently	O
known	O
cupin	B-protein_type
-	I-protein_type
related	I-protein_type
enzyme	O
that	O
catalyze	O
a	O
cyclocondensation	O
reaction	O
although	O
the	O
substrates	O
of	O
EctC	B-protein
and	O
RemF	B-protein
are	O
rather	O
different	O
.	O

The	O
pro	B-taxonomy_domain
-	I-taxonomy_domain
and	O
eukaryotic	B-taxonomy_domain
members	O
of	O
the	O
cupin	B-protein_type
superfamily	I-protein_type
perform	O
a	O
variety	O
of	O
both	O
enzymatic	O
and	O
non	O
-	O
enzymatic	O
functions	O
that	O
are	O
built	O
upon	O
a	O
common	O
structural	O
scaffold	O
.	O

Most	O
cupins	B-protein_type
contain	O
transition	O
state	O
metals	O
that	O
can	O
promote	O
different	O
types	O
of	O
chemical	O
reactions	O
.	O

We	O
report	O
here	O
for	O
the	O
first	O
time	O
that	O
the	O
ectoine	B-protein_type
synthase	I-protein_type
is	O
a	O
metal	B-chemical
-	O
dependent	O
enzyme	O
.	O

ICP	B-experimental_method
-	I-experimental_method
MS	I-experimental_method
,	O
metal	B-experimental_method
-	I-experimental_method
depletion	I-experimental_method
and	I-experimental_method
reconstitution	I-experimental_method
experiments	I-experimental_method
(	O
Fig	O
3	O
)	O
consistently	O
identify	O
iron	B-chemical
as	O
the	O
biologically	O
most	O
relevant	O
metal	B-chemical
for	O
the	O
EctC	B-protein
-	O
catalyzed	O
cyclocondensation	O
reaction	O
.	O

However	O
,	O
as	O
observed	O
with	O
other	O
cupins	B-protein_type
,	O
EctC	B-protein
is	O
a	O
somewhat	O
promiscuous	O
enzyme	O
as	O
far	O
as	O
the	O
catalytically	O
important	O
metal	B-chemical
is	O
concerned	O
when	O
they	O
are	O
provided	O
in	O
large	O
molar	O
excess	O
(	O
Fig	O
3c	O
).	O

Although	O
some	O
uncertainty	O
remains	O
with	O
respect	O
to	O
the	O
precise	O
identity	O
of	O
amino	O
acid	O
residues	O
that	O
participate	O
in	O
metal	B-chemical
binding	O
by	O
(	O
Sa	B-species
)	O
EctC	B-protein
,	O
our	O
structure	B-experimental_method
-	I-experimental_method
guided	I-experimental_method
site	I-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
experiments	O
targeting	O
the	O
presumptive	O
iron	B-site
-	I-site
binding	I-site
residues	I-site
(	O
Fig	O
6a	O
and	O
6b	O
)	O
demonstrate	O
that	O
none	O
of	O
them	O
can	O
be	O
spared	O
(	O
Table	O
1	O
).	O

The	O
three	O
residues	O
(	O
Glu	B-residue_name_number
-	I-residue_name_number
57	I-residue_name_number
,	O
Tyr	B-residue_name_number
-	I-residue_name_number
85	I-residue_name_number
,	O
His	B-residue_name_number
-	I-residue_name_number
93	I-residue_name_number
)	O
that	O
we	O
deem	O
to	O
form	O
it	O
(	O
Figs	O
6	O
and	O
7b	O
)	O
are	O
strictly	B-protein_state
conserved	I-protein_state
in	O
a	O
large	O
collection	O
of	O
EctC	B-protein_type
-	I-protein_type
type	I-protein_type
proteins	I-protein_type
originating	O
from	O
16	O
bacterial	B-taxonomy_domain
and	O
three	O
archaeal	B-taxonomy_domain
phyla	O
(	O
Fig	O
2	O
).	O

We	O
also	O
show	O
here	O
for	O
the	O
first	O
time	O
that	O
,	O
in	O
addition	O
to	O
its	O
natural	O
substrate	O
N	B-chemical
-	I-chemical
γ	I-chemical
-	I-chemical
ADABA	I-chemical
,	O
EctC	B-protein
also	O
converts	O
the	O
isomer	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical
into	O
ectoine	B-chemical
,	O
albeit	O
with	O
a	O
73	O
-	O
fold	O
reduced	O
catalytic	B-evidence
efficiency	I-evidence
(	O
S3a	O
and	O
S3b	O
Fig	O
).	O

Our	O
finding	O
that	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical
serves	O
as	O
a	O
substrate	O
for	O
ectoine	B-protein_type
synthase	I-protein_type
has	O
physiologically	O
relevant	O
ramifications	O
for	O
those	O
microorganisms	B-taxonomy_domain
that	O
can	O
both	O
synthesize	O
and	O
catabolize	O
ectoine	B-chemical
,	O
since	O
they	O
need	O
to	O
prevent	O
a	O
futile	O
cycle	O
of	O
synthesis	O
and	O
degradation	O
when	O
N	B-chemical
-	I-chemical
α	I-chemical
-	I-chemical
ADABA	I-chemical
is	O
produced	O
as	O
an	O
intermediate	O
in	O
the	O
catabolic	O
route	O
.	O

Although	O
we	O
cannot	O
identify	O
the	O
true	O
chemical	O
nature	O
of	O
the	O
C6	B-chemical
compound	O
that	O
was	O
trapped	O
in	O
the	O
(	O
Sa	B-species
)	O
EctC	B-protein
structure	B-evidence
nor	O
its	O
precise	O
origin	O
,	O
we	O
treated	O
this	O
compound	O
as	O
a	O
proxy	O
for	O
the	O
natural	O
substrate	O
of	O
ectoine	B-protein_type
synthase	I-protein_type
,	O
which	O
is	O
a	O
C6	O
compound	O
as	O
well	O
(	O
Fig	O
7a	O
).	O

Indeed	O
,	O
site	B-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
of	O
those	O
five	O
residues	O
that	O
contact	O
the	O
unknown	O
C6	O
compound	O
(	O
Fig	O
7b	O
)	O
yielded	O
(	O
Sa	B-species
)	O
EctC	B-protein
variants	O
with	O
strongly	O
impaired	O
enzyme	O
function	O
but	O
near	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
levels	O
of	O
iron	B-chemical
(	O
Table	O
1	O
).	O

We	O
therefore	O
surmise	O
that	O
our	O
crystallographic	B-evidence
data	I-evidence
and	O
the	O
site	B-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
study	I-experimental_method
reported	O
here	O
provide	O
a	O
structural	O
and	O
functional	O
view	O
into	O
the	O
architecture	O
of	O
the	O
EctC	B-protein
active	B-site
site	I-site
(	O
Fig	O
7b	O
).	O

The	O
ectoine	B-protein_type
synthase	I-protein_type
from	O
the	O
cold	O
-	O
adapted	O
marine	B-taxonomy_domain
bacterium	I-taxonomy_domain
S	B-species
.	I-species
alaskensis	I-species
can	O
be	O
considered	O
as	O
a	O
psychrophilic	O
enzyme	O
(	O
S3a	O
Fig	O
),	O
types	O
of	O
proteins	O
with	O
a	O
considerable	O
structural	O
flexibility	O
.	O

It	O
is	O
hoped	O
that	O
these	O
can	O
be	O
further	O
employed	O
to	O
obtain	O
EctC	B-protein
crystal	B-evidence
structures	I-evidence
with	O
either	O
the	O
substrate	O
or	O
the	O
reaction	O
product	O
.	O

Together	O
with	O
our	O
finding	O
that	O
ectoine	B-protein_type
synthase	I-protein_type
is	O
metal	B-protein_state
dependent	I-protein_state
,	O
these	O
crystal	B-evidence
structures	I-evidence
should	O
allow	O
a	O
more	O
detailed	O
understanding	O
of	O
the	O
chemistry	O
underlying	O
the	O
EctC	B-protein
-	O
catalyzed	O
cyclocondensation	O
reaction	O
.	O

ADARs	B-protein_type
(	O
adenosine	B-protein_type
deaminases	I-protein_type
acting	I-protein_type
on	I-protein_type
RNA	I-protein_type
)	O
are	O
editing	B-protein_type
enzymes	I-protein_type
that	O
convert	O
adenosine	B-residue_name
(	O
A	B-residue_name
)	O
to	O
inosine	B-residue_name
(	O
I	B-residue_name
)	O
in	O
duplex	B-structure_element
RNA	I-structure_element
,	O
a	O
modification	O
reaction	O
with	O
wide	O
-	O
ranging	O
consequences	O
on	O
RNA	B-chemical
function	O
.	O

Our	O
understanding	O
of	O
the	O
ADAR	B-protein_type
reaction	O
mechanism	O
,	O
origin	O
of	O
editing	B-site
site	I-site
selectivity	O
and	O
effect	O
of	O
mutations	O
is	O
limited	O
by	O
the	O
lack	O
of	O
high	O
-	O
resolution	O
structural	B-evidence
data	I-evidence
for	O
complexes	O
of	O
ADARs	B-protein_type
bound	B-protein_state
to	I-protein_state
substrate	O
RNAs	B-chemical
.	O

Here	O
we	O
describe	O
four	O
crystal	B-evidence
structures	I-evidence
of	O
the	O
deaminase	B-structure_element
domain	I-structure_element
of	O
human	B-species
ADAR2	B-protein
bound	B-protein_state
to	I-protein_state
RNA	B-structure_element
duplexes	I-structure_element
bearing	O
a	O
mimic	O
of	O
the	O
deamination	O
reaction	O
intermediate	O
.	O

These	O
structures	B-evidence
,	O
together	O
with	O
structure	B-experimental_method
-	I-experimental_method
guided	I-experimental_method
mutagenesis	I-experimental_method
and	O
RNA	B-experimental_method
-	I-experimental_method
modification	I-experimental_method
experiments	I-experimental_method
,	O
explain	O
the	O
basis	O
for	O
ADAR	B-protein_type
deaminase	B-structure_element
domain	I-structure_element
’	O
s	O
dsRNA	B-chemical
specificity	O
,	O
its	O
base	O
-	O
flipping	O
mechanism	O
,	O
and	O
nearest	O
neighbor	O
preferences	O
.	O

In	O
addition	O
,	O
an	O
ADAR2	B-protein
-	O
specific	O
RNA	B-structure_element
-	I-structure_element
binding	I-structure_element
loop	I-structure_element
was	O
identified	O
near	O
the	O
enzyme	O
active	B-site
site	I-site
rationalizing	O
differences	O
in	O
selectivity	O
observed	O
between	O
different	O
ADARs	B-protein_type
.	O

RNA	B-chemical
editing	O
reactions	O
alter	O
a	O
transcript	O
’	O
s	O
genomically	O
encoded	O
sequence	O
by	O
inserting	O
,	O
deleting	O
or	O
modifying	O
nucleotides	O
.	O

Deamination	O
of	O
adenosine	B-residue_name
(	O
A	B-residue_name
),	O
the	O
most	O
common	O
form	O
of	O
RNA	B-chemical
editing	O
in	O
humans	B-species
,	O
generates	O
inosine	B-residue_name
(	O
I	B-residue_name
)	O
at	O
the	O
corresponding	O
nucleotide	O
position	O
.	O

Since	O
I	B-residue_name
base	O
pairs	O
with	O
cytidine	B-residue_name
(	O
C	B-residue_name
),	O
it	O
functions	O
like	O
guanosine	B-residue_name
(	O
G	B-residue_name
)	O
in	O
cellular	O
processes	O
such	O
as	O
splicing	O
,	O
translation	O
and	O
reverse	O
transcription	O
.	O

A	O
to	O
I	O
editing	O
has	O
wide	O
-	O
ranging	O
consequences	O
on	O
RNA	B-chemical
function	O
including	O
altering	O
miRNA	B-site
recognition	I-site
sites	I-site
,	O
redirecting	O
splicing	O
and	O
changing	O
the	O
meaning	O
of	O
specific	O
codons	O
.	O

ADAR	B-protein_type
activity	O
is	O
required	O
for	O
nervous	O
system	O
function	O
and	O
altered	O
editing	O
has	O
been	O
linked	O
to	O
neurological	O
disorders	O
such	O
as	O
epilepsy	O
and	O
Prader	O
Willi	O
Syndrome	O
.	O

In	O
addition	O
,	O
mutations	O
in	O
the	O
ADAR1	B-protein
gene	O
are	O
known	O
to	O
cause	O
the	O
autoimmune	O
disease	O
Aicardi	O
-	O
Goutieres	O
Syndrome	O
(	O
AGS	O
)	O
and	O
the	O
skin	O
disorder	O
Dyschromatosis	O
Symmetrica	O
Hereditaria	O
(	O
DSH	O
).	O

Hyper	O
editing	O
has	O
been	O
observed	O
at	O
certain	O
sites	O
in	O
cancer	O
cells	O
,	O
such	O
as	O
in	O
the	O
mRNA	B-chemical
for	O
AZIN1	B-protein
(	O
antizyme	B-protein
inhibitor	I-protein
1	I-protein
).	O

However	O
,	O
hypo	O
editing	O
also	O
occurs	O
in	O
cancer	O
-	O
derived	O
cell	O
lines	O
exemplified	O
by	O
reduced	O
editing	O
observed	O
in	O
the	O
message	O
for	O
glioma	B-protein
-	I-protein
associated	I-protein
oncogene	I-protein
1	I-protein
(	O
Gli1	B-protein
).	O

The	O
ADAR	B-protein_type
proteins	O
have	O
a	O
modular	O
structure	O
with	O
double	B-structure_element
stranded	I-structure_element
RNA	I-structure_element
binding	I-structure_element
domains	I-structure_element
(	O
dsRBDs	B-structure_element
)	O
and	O
a	O
C	O
-	O
terminal	O
deaminase	B-structure_element
domain	I-structure_element
(	O
see	O
Fig	O
.	O
1a	O
for	O
hADAR2	B-protein
domains	O
).	O

ADARs	B-protein_type
efficiently	O
deaminate	O
specific	O
adenosines	B-residue_name
in	O
duplex	B-structure_element
RNA	I-structure_element
while	O
leaving	O
most	O
adenosines	B-residue_name
unmodified	O
.	O

The	O
mechanism	O
of	O
adenosine	B-residue_name
deamination	O
requires	O
ADAR	B-protein_type
to	O
flip	O
the	O
reactive	O
base	O
out	O
of	O
an	O
RNA	B-chemical
double	I-chemical
helix	I-chemical
to	O
access	O
its	O
active	B-site
site	I-site
.	O

How	O
an	O
enzyme	O
could	O
accomplish	O
this	O
task	O
with	O
a	O
duplex	B-structure_element
RNA	I-structure_element
substrate	O
is	O
not	O
known	O
.	O

To	O
address	O
these	O
knowledge	O
gaps	O
,	O
we	O
set	O
out	O
to	O
trap	O
the	O
human	B-species
ADAR2	B-protein
deaminase	B-structure_element
domain	I-structure_element
(	O
aa299	O
–	B-residue_range
701	I-residue_range
,	O
hADAR2d	B-mutant
)	O
bound	B-protein_state
to	I-protein_state
different	O
duplex	B-structure_element
RNAs	I-structure_element
and	O
solve	O
structures	B-evidence
for	O
the	O
resulting	O
complexes	O
using	O
x	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
.	O

Trapping	O
the	O
flipped	B-protein_state
conformation	O

The	O
ADAR	B-protein_type
reaction	O
involves	O
the	O
formation	O
of	O
a	O
hydrated	O
intermediate	O
that	O
loses	O
ammonia	O
to	O
generate	O
the	O
inosine	B-residue_name
-	O
containing	O
product	O
RNA	B-chemical
(	O
for	O
reaction	O
scheme	O
see	O
Fig	O
.	O
1b	O
).	O

The	O
covalent	O
hydrate	O
of	O
the	O
nucleoside	O
analog	O
8	B-chemical
-	I-chemical
azanebularine	I-chemical
(	O
N	B-chemical
)	O
mimics	O
the	O
proposed	O
high	O
-	O
energy	O
intermediate	O
(	O
for	O
reaction	O
scheme	O
see	O
Fig	O
.	O
1b	O
).	O

In	O
addition	O
,	O
for	O
one	O
of	O
these	O
duplexes	O
(	O
Bdf2	B-chemical
),	O
we	O
positioned	O
the	O
8	B-chemical
-	I-chemical
azanebularine	I-chemical
opposite	O
either	O
uridine	B-residue_name
or	O
cytidine	B-residue_name
to	O
mimic	O
an	O
A	B-residue_name
-	O
U	B-residue_name
pair	O
or	O
A	B-residue_name
-	O
C	B-residue_name
mismatch	O
at	O
the	O
editing	B-site
site	I-site
creating	O
a	O
total	O
of	O
three	O
different	O
RNA	B-chemical
substrates	O
for	O
structural	O
studies	O
(	O
Fig	O
.	O
1c	O
).	O

The	O
hADAR2d	B-mutant
protein	O
(	O
without	B-protein_state
RNA	I-protein_state
bound	I-protein_state
)	O
has	O
been	O
previously	O
crystallized	B-experimental_method
and	O
structurally	O
characterized	O
revealing	O
features	O
of	O
the	O
active	B-site
site	I-site
including	O
the	O
presence	O
of	O
zinc	B-chemical
.	O

For	O
crystallization	B-experimental_method
of	O
hADAR2d	B-complex_assembly
-	I-complex_assembly
RNA	I-complex_assembly
complexes	O
,	O
we	O
used	O
both	O
the	O
wild	B-protein_state
type	I-protein_state
(	O
WT	B-protein_state
)	O
deaminase	B-structure_element
domain	I-structure_element
and	O
a	O
mutant	B-protein_state
(	O
E488Q	B-mutant
)	O
that	O
has	O
enhanced	O
catalytic	O
activity	O
.	O

A	O
description	O
of	O
the	O
crystallization	O
conditions	O
,	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
diffraction	I-experimental_method
data	I-experimental_method
collection	I-experimental_method
and	I-experimental_method
solution	I-experimental_method
of	O
the	O
structures	B-evidence
can	O
be	O
found	O
in	O
Online	O
Methods	O
.	O

Four	O
protein	O
-	O
RNA	B-chemical
combinations	O
generated	O
diffracting	O
crystals	B-evidence
that	O
resulted	O
in	O
high	O
-	O
resolution	O
structures	B-evidence
(	O
hADAR2d	B-complex_assembly
WT	I-complex_assembly
–	I-complex_assembly
Bdf2	I-complex_assembly
-	I-complex_assembly
U	I-complex_assembly
,	O
hADAR2d	B-complex_assembly
WT	I-complex_assembly
–	I-complex_assembly
Bdf2	I-complex_assembly
-	I-complex_assembly
C	I-complex_assembly
,	O
hADAR2d	B-complex_assembly
E488Q	I-complex_assembly
–	I-complex_assembly
Bdf2	I-complex_assembly
-	I-complex_assembly
C	I-complex_assembly
,	O
hADAR2d	B-complex_assembly
E488Q	I-complex_assembly
–	I-complex_assembly
Gli1	I-complex_assembly
)	O
(	O
Table	O
1	O
).	O

The	O
large	O
binding	B-site
site	I-site
(	O
1493	O
Å2	O
RNA	O
surface	O
area	O
and	O
1277	O
Å2	O
protein	O
surface	O
area	O
buried	O
)	O
observed	O
for	O
hADAR2d	B-mutant
is	O
consistent	O
with	O
recent	O
footprinting	B-experimental_method
studies	I-experimental_method
.	O

Both	O
strands	O
of	O
the	O
RNA	B-chemical
contact	O
the	O
protein	O
with	O
the	O
majority	O
of	O
these	O
interactions	O
mediated	O
through	O
the	O
phosphodiester	O
-	O
ribose	O
backbone	O
near	O
the	O
editing	B-site
site	I-site
(	O
Fig	O
.	O
2c	O
,	O
Supplementary	O
Fig	O
.	O
2	O
b	O
–	O
d	O
).	O

The	O
structures	B-evidence
show	O
a	O
large	O
deviation	O
from	O
A	B-structure_element
-	I-structure_element
form	I-structure_element
RNA	B-chemical
conformation	O
at	O
the	O
editing	B-site
site	I-site
(	O
Fig	O
.	O
2	O
,	O
Fig	O
.	O
3	O
,	O
Supplementary	O
Video	O
1	O
).	O

This	O
interaction	O
was	O
expected	O
given	O
the	O
proposed	O
role	O
of	O
E396	B-residue_name_number
in	O
mediating	O
proton	O
transfers	O
to	O
and	O
from	O
N1	O
of	O
the	O
substrate	O
adenosine	B-residue_name
.	O

The	O
2	O
’-	O
hydroxyl	O
of	O
8	B-chemical
-	I-chemical
azanebularine	I-chemical
H	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
to	O
the	O
backbone	O
carbonyl	O
of	O
T375	B-residue_name_number
while	O
the	O
T375	B-residue_name_number
side	O
chain	O
contacts	O
its	O
3	O
’-	O
phosphodiester	O
.	O

R455	B-residue_name_number
and	O
K376	B-residue_name_number
help	O
position	O
the	O
flipped	B-protein_state
nucleotide	B-chemical
in	O
the	O
active	B-site
site	I-site
by	O
fastening	O
the	O
phosphate	O
backbone	O
flanking	O
the	O
editing	B-site
site	I-site
.	O

RNA	B-chemical
binding	O
does	O
not	O
alter	O
IHP	B-chemical
binding	O
or	O
the	O
H	B-site
-	I-site
bonding	I-site
network	I-site
linking	O
IHP	B-chemical
to	O
the	O
active	B-site
site	I-site
.	O

The	O
ADAR2	B-protein
base	B-structure_element
-	I-structure_element
flipping	I-structure_element
loop	I-structure_element
,	O
bearing	O
residue	O
488	B-residue_number
,	O
approaches	O
the	O
RNA	B-structure_element
duplex	I-structure_element
from	O
the	O
minor	B-site
groove	I-site
side	O
at	O
the	O
editing	B-site
site	I-site
.	O

For	O
instance	O
,	O
in	O
the	O
complex	B-protein_state
with	I-protein_state
hADAR2d	B-mutant
E488Q	B-mutant
and	O
the	O
Bdf2	B-chemical
-	I-chemical
C	I-chemical
duplex	I-chemical
,	O
the	O
protein	O
recognizes	O
an	O
orphaned	B-protein_state
C	B-residue_name
by	O
donating	O
H	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
from	O
Nε2	O
to	O
cytosine	B-residue_name
N3	O
and	O
from	O
its	O
backbone	O
NH	O
to	O
cytosine	B-residue_name
O2	O
(	O
Fig	O
.	O
3b	O
).	O

Interestingly	O
,	O
the	O
E488Q	B-mutant
mutant	B-protein_state
was	O
discovered	O
in	O
a	O
screen	O
for	O
highly	B-protein_state
active	I-protein_state
ADAR2	B-protein
mutants	B-protein_state
and	O
this	O
residue	O
was	O
suggested	O
to	O
be	O
involved	O
in	O
base	O
flipping	O
given	O
its	O
effect	O
on	O
editing	O
substrates	O
with	O
a	O
fluorescent	O
nucleobase	O
at	O
the	O
editing	B-site
site	I-site
.	O

ADARs	B-protein_type
react	O
preferentially	O
with	O
adenosines	B-residue_name
in	O
A	B-structure_element
•	I-structure_element
C	I-structure_element
mismatches	O
and	O
A	B-structure_element
-	I-structure_element
U	I-structure_element
pairs	I-structure_element
over	O
A	B-structure_element
•	I-structure_element
A	I-structure_element
and	O
A	B-structure_element
•	I-structure_element
G	I-structure_element
mismatches	O
.	O

A	O
purine	B-chemical
at	O
the	O
orphan	B-protein_state
base	B-chemical
position	O
(	O
in	O
its	O
anti	O
conformation	O
)	O
would	O
clash	O
with	O
the	O
488	B-residue_number
residue	O
explaining	O
the	O
preference	O
for	O
pyrimidines	B-chemical
here	O
.	O

The	O
interaction	O
of	O
the	O
488	B-residue_number
residue	O
with	O
the	O
orphaned	B-protein_state
base	B-chemical
is	O
reminiscent	O
of	O
an	O
interaction	O
observed	O
for	O
Hha	B-protein_type
I	I-protein_type
DNA	I-protein_type
methyltransfersase	I-protein_type
(	O
MTase	B-protein_type
),	O
a	O
duplex	B-structure_element
DNA	I-structure_element
modifying	O
enzyme	O
that	O
also	O
uses	O
a	O
base	O
flipping	O
mechanism	O
to	O
access	O
2	B-residue_name
’-	I-residue_name
deoxycytidine	I-residue_name
(	O
dC	B-residue_name
)	O
for	O
methylation	O
.	O

For	O
that	O
enzyme	O
,	O
Q237	B-residue_name_number
H	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
to	O
an	O
orphaned	B-protein_state
dG	B-residue_name
while	O
it	O
fills	O
the	O
void	O
left	O
by	O
the	O
flipped	B-protein_state
out	I-protein_state
dC	B-residue_name
(	O
Supplementary	O
Fig	O
.	O
4b	O
).	O

In	O
addition	O
,	O
two	O
glycine	B-residue_name
residues	O
flank	O
Q237	B-residue_name_number
allowing	O
the	O
loop	B-structure_element
to	O
adopt	O
the	O
conformation	O
necessary	O
for	O
penetration	O
into	O
the	O
helix	B-structure_element
.	O

Such	O
an	O
approach	O
requires	O
deeper	O
penetration	O
of	O
the	O
intercalating	B-site
residue	I-site
to	O
access	O
the	O
H	B-site
-	I-site
bonding	I-site
sites	I-site
on	O
the	O
orphaned	B-protein_state
base	B-chemical
,	O
necessitating	O
an	O
additional	O
conformational	O
change	O
in	O
the	O
RNA	B-structure_element
duplex	I-structure_element
.	O

This	O
change	O
includes	O
shifting	O
of	O
the	O
base	O
pairs	O
immediately	O
5	O
’	O
to	O
the	O
editing	B-site
site	I-site
toward	O
the	O
helical	O
axis	O
and	O
a	O
widening	O
of	O
the	O
major	B-site
groove	I-site
opposite	O
the	O
editing	B-site
site	I-site
(	O
Figs	O
.	O
4a	O
,	O
4b	O
,	O
Supplementary	O
Video	O
1	O
).	O

In	O
the	O
case	O
of	O
the	O
hADAR2d	B-complex_assembly
WT	I-complex_assembly
–	I-complex_assembly
Bdf2	I-complex_assembly
-	I-complex_assembly
U	I-complex_assembly
RNA	B-chemical
,	O
this	O
shift	O
is	O
accompanied	O
by	O
a	O
shearing	O
of	O
the	O
U11	B-residue_name_number
-	O
A13	B-residue_name_number
'	O
base	O
pair	O
with	O
U11	B-residue_name_number
shifted	O
further	O
in	O
the	O
direction	O
of	O
the	O
major	B-site
groove	I-site
creating	O
an	O
unusual	O
U	B-structure_element
-	I-structure_element
A	I-structure_element
""""	I-structure_element
wobble	I-structure_element
""""	I-structure_element
interaction	O
with	O
adenine	B-residue_name
N6	O
and	O
N1	O
within	O
H	B-bond_interaction
-	I-bond_interaction
bonding	I-bond_interaction
distance	O
to	O
uracil	B-residue_name
N3H	O
and	O
O2	O
,	O
respectively	O
(	O
Fig	O
.	O
4c	O
,	O
Supplementary	O
Fig	O
.	O
3b	O
).	O

This	O
type	O
of	O
wobble	O
pair	O
has	O
been	O
observed	O
before	O
and	O
requires	O
either	O
the	O
imino	O
tautomer	O
of	O
adenine	B-residue_name
or	O
the	O
enol	O
tautomer	O
of	O
uracil	B-residue_name
.	O

This	O
kink	B-structure_element
is	O
stabilized	O
by	O
interactions	O
of	O
the	O
side	O
chains	O
of	O
R510	B-residue_name_number
and	O
S495	B-residue_name_number
with	O
phosphodiesters	O
in	O
the	O
RNA	B-chemical
backbone	O
of	O
the	O
unedited	O
strand	O
(	O
Fig	O
.	O
4a	O
).	O

Interestingly	O
,	O
ADAR2	B-protein
’	O
s	O
flipping	B-structure_element
loop	I-structure_element
approach	O
from	O
the	O
minor	B-site
groove	I-site
side	O
is	O
like	O
that	O
seen	O
with	O
certain	O
DNA	B-protein_type
repair	I-protein_type
glycosylases	I-protein_type
(	O
e	O
.	O
g	O
.	O
UDG	B-protein
,	O
HOGG1	B-protein
,	O
and	O
AAG	B-protein
)	O
that	O
also	O
project	O
intercalating	O
residues	O
from	O
loops	B-structure_element
bound	B-protein_state
in	I-protein_state
the	O
minor	B-site
groove	I-site
(	O
Supplementary	O
Fig	O
.	O
5a	O
).	O

However	O
,	O
these	O
enzymes	O
typically	O
bend	O
the	O
DNA	B-chemical
duplex	I-chemical
at	O
the	O
site	O
of	O
modification	O
to	O
allow	O
for	O
penetration	O
of	O
intercalating	O
residues	O
and	O
damage	O
recognition	O
.	O

While	O
hADAR2d	B-mutant
clearly	O
alters	O
the	O
duplex	O
conformation	O
to	O
gain	O
access	O
to	O
the	O
modification	O
site	O
from	O
the	O
minor	B-site
groove	I-site
,	O
it	O
does	O
not	O
bend	O
the	O
RNA	B-structure_element
duplex	I-structure_element
(	O
Figs	O
.	O
2a	O
,	O
2b	O
,	O
4b	O
).	O

Furthermore	O
,	O
ADARs	B-protein_type
do	O
not	O
modify	O
duplex	B-structure_element
DNA	I-structure_element
.	O

For	O
instance	O
,	O
ADAR	B-protein_type
can	O
readily	O
penetrate	O
an	O
A	B-structure_element
-	I-structure_element
form	I-structure_element
helix	I-structure_element
from	O
the	O
minor	B-site
groove	I-site
side	O
and	O
place	O
the	O
helix	O
-	O
penetrating	O
residue	O
in	O
the	O
space	O
occupied	O
by	O
the	O
editing	B-site
site	I-site
base	O
(	O
Supplementary	O
Fig	O
.	O
6	O
).	O

However	O
,	O
this	O
residue	O
cannot	O
penetrate	O
the	O
minor	B-site
groove	I-site
enough	O
to	O
occupy	O
the	O
base	O
position	O
in	O
a	O
B	B-structure_element
-	I-structure_element
form	I-structure_element
helix	I-structure_element
(	O
Supplementary	O
Fig	O
.	O
6	O
).	O

Furthermore	O
,	O
DNA	B-chemical
lacks	O
the	O
2	O
’	O
hydroxyls	O
that	O
are	O
used	O
by	O
ADAR	B-protein_type
for	O
substrate	O
recognition	O
(	O
Fig	O
.	O
2c	O
).	O

Thus	O
,	O
hADAR2d	B-mutant
uses	O
a	O
substrate	O
recognition	O
and	O
base	O
flipping	O
mechanism	O
with	O
similarities	O
to	O
other	O
known	O
nucleic	B-protein_type
acid	I-protein_type
-	I-protein_type
modifying	I-protein_type
enzymes	I-protein_type
but	O
uniquely	O
suited	O
for	O
reaction	O
with	O
adenosine	B-residue_name
in	O
the	O
context	O
of	O
duplex	B-structure_element
RNA	I-structure_element
.	O

Structures	B-evidence
explain	O
nearest	O
neighbor	O
preferences	O

Also	O
,	O
the	O
minor	B-site
groove	I-site
edge	O
of	O
this	O
pair	O
is	O
juxtaposed	O
to	O
the	O
protein	O
backbone	O
at	O
G489	B-residue_name_number
.	O

2AP	B-structure_element
is	O
an	O
adenosine	B-residue_name
analog	O
that	O
forms	O
a	O
base	O
pair	O
with	O
uridine	B-residue_name
of	O
similar	O
stability	O
to	O
a	O
U	B-structure_element
-	I-structure_element
A	I-structure_element
pair	I-structure_element
,	O
but	O
places	O
an	O
amino	O
group	O
in	O
the	O
minor	B-site
groove	I-site
(	O
Fig	O
.	O
5b	O
).	O

Importantly	O
,	O
this	O
substitution	O
also	O
resulted	O
in	O
an	O
80	O
%	O
reduction	O
in	O
rate	O
,	O
illustrating	O
the	O
detrimental	O
effect	O
of	O
the	O
amino	O
group	O
in	O
the	O
minor	B-site
groove	I-site
at	O
this	O
location	O
.	O

In	O
each	O
of	O
the	O
hADAR2d	B-complex_assembly
-	I-complex_assembly
RNA	I-complex_assembly
structures	B-evidence
reported	O
here	O
,	O
the	O
backbone	O
carbonyl	O
oxygen	O
at	O
S486	B-residue_name_number
accepts	O
an	O
H	B-bond_interaction
-	I-bond_interaction
bond	I-bond_interaction
from	O
the	O
2	O
-	O
amino	O
group	O
of	O
the	O
G	B-residue_name
on	O
the	O
3	O
’	O
side	O
of	O
the	O
edited	O
nucleotide	O
(	O
Fig	O
.	O
5d	O
).	O

Guanine	B-residue_name
is	O
the	O
only	O
common	O
nucleobase	O
that	O
presents	O
an	O
H	B-bond_interaction
-	I-bond_interaction
bond	I-bond_interaction
donor	O
in	O
the	O
RNA	B-site
minor	I-site
groove	I-site
suggesting	O
that	O
other	O
nucleotides	O
in	O
this	O
position	O
would	O
reduce	O
editing	O
efficiency	O
.	O

Indeed	O
,	O
mutating	B-experimental_method
this	O
base	O
to	O
A	B-residue_name
,	O
C	B-residue_name
or	O
U	B-residue_name
,	O
while	O
maintaining	O
base	O
pairing	O
at	O
this	O
position	O
,	O
reduced	O
the	O
rate	O
of	O
deamination	O
by	O
hADAR2d	B-mutant
in	O
Gli1	B-protein
mRNA	B-chemical
model	O
substrates	O
(	O
Supplementary	O
Fig	O
.	O
7	O
a	O
–	O
b	O
).	O

To	O
test	O
the	O
importance	O
of	O
the	O
amino	O
group	O
on	O
the	O
3	O
’	O
G	B-residue_name
in	O
the	O
hADAR2d	B-mutant
reaction	O
,	O
we	O
prepared	O
RNA	B-structure_element
duplex	I-structure_element
substrates	O
with	O
purine	O
analogs	O
on	O
the	O
3	O
’	O
side	O
of	O
the	O
edited	B-protein_state
A	B-residue_name
(	O
Fig	O
.	O
5e	O
).	O

We	O
tested	O
a	O
G	B-residue_name
analog	O
that	O
lacks	O
the	O
2	O
-	O
amino	O
group	O
(	O
inosine	B-residue_name
,	O
I	B-residue_name
)	O
and	O
one	O
that	O
blocks	O
access	O
to	O
this	O
amino	O
group	O
(	O
N2	O
-	O
methylguanosine	O
(	O
N2MeG	O
).	O

In	O
addition	O
,	O
we	O
compared	O
a	O
3	O
’	O
A	B-residue_name
to	O
a	O
3	O
’	O
2AP	B-structure_element
since	O
2AP	B-structure_element
could	O
form	O
the	O
H	B-bond_interaction
-	I-bond_interaction
bonding	I-bond_interaction
interaction	I-bond_interaction
observed	O
with	O
S486	B-residue_name_number
.	O

We	O
found	O
the	O
substrate	O
with	O
a	O
3	O
’	O
N2MeG	O
to	O
be	O
unreactive	O
to	O
hADAR2d	B-mutant
-	O
catalyzed	O
deamination	O
confirming	O
the	O
importance	O
of	O
the	O
observed	O
close	O
approach	O
by	O
the	O
protein	O
to	O
the	O
3	O
’	O
G	B-residue_name
2	O
-	O
amino	O
group	O
(	O
Fig	O
.	O
5f	O
).	O

This	O
conclusion	O
is	O
further	O
supported	O
by	O
the	O
observation	O
that	O
deamination	O
in	O
the	O
substrate	O
with	O
a	O
3	O
’	O
2AP	B-structure_element
is	O
faster	O
than	O
in	O
the	O
substrate	O
with	O
a	O
3	O
’	O
A	B-residue_name
(	O
Fig	O
.	O
5f	O
).	O

The	O
structures	B-evidence
reported	O
here	O
identify	O
RNA	B-structure_element
-	I-structure_element
binding	I-structure_element
loops	I-structure_element
of	O
the	O
ADAR	B-protein_type
catalytic	B-structure_element
domain	I-structure_element
and	O
suggest	O
roles	O
for	O
several	O
amino	O
acids	O
not	O
previously	O
known	O
to	O
be	O
important	O
for	O
editing	O
,	O
either	O
substrate	O
binding	O
or	O
catalysis	O
(	O
Fig	O
.	O
6	O
).	O

The	O
side	O
chain	O
for	O
R510	B-residue_name_number
ion	B-bond_interaction
-	I-bond_interaction
pairs	I-bond_interaction
with	O
the	O
3	O
’	O
phosphodiester	O
of	O
the	O
orphaned	B-protein_state
nucleotide	B-chemical
(	O
Figs	O
.	O
3a	O
,	O
3c	O
).	O

This	O
residue	O
is	O
conserved	B-protein_state
in	O
ADAR2s	B-protein_type
and	O
ADAR1s	B-protein_type
,	O
but	O
is	O
glutamine	B-residue_name
in	O
the	O
editing	B-protein_state
-	I-protein_state
inactive	I-protein_state
ADAR3s	B-protein_type
(	O
Supplementary	O
Table	O
1	O
).	O

Mutation	B-experimental_method
of	O
hADAR2d	B-mutant
at	O
this	O
site	O
to	O
either	O
glutamine	B-residue_name
(	O
R510Q	B-mutant
)	O
or	O
to	O
alanine	B-residue_name
(	O
R510A	B-mutant
)	O
reduced	O
the	O
measured	O
deamination	B-evidence
rate	I-evidence
constant	I-evidence
by	O
approximately	O
an	O
order	O
of	O
magnitude	O
(	O
Fig	O
.	O
6c	O
).	O

In	O
addition	O
,	O
the	O
contact	O
point	O
near	O
the	O
5	O
’	O
end	O
of	O
the	O
unedited	O
strand	O
involves	O
G593	B-residue_name_number
,	O
K594	B-residue_name_number
and	O
R348	B-residue_name_number
,	O
residues	O
completely	B-protein_state
conserved	I-protein_state
in	O
the	O
family	O
of	O
ADAR2s	B-protein_type
(	O
Fig	O
.	O
2c	O
,	O
Supplementary	O
Table	O
1	O
).	O

RNA	B-chemical
binding	O
leads	O
to	O
an	O
ordering	O
of	O
the	O
454	B-residue_range
–	I-residue_range
477	I-residue_range
loop	B-structure_element
,	O
which	O
was	O
disordered	B-protein_state
in	O
the	O
RNA	B-protein_state
-	I-protein_state
free	I-protein_state
hADAR2d	B-mutant
structure	B-evidence
(	O
Fig	O
.	O
1d	O
,	O
green	O
)	O
(	O
Supplementary	O
Video	O
2	O
).	O

This	O
loop	B-structure_element
sequence	O
is	O
conserved	B-protein_state
in	O
ADAR2s	B-protein_type
but	O
different	O
in	O
the	O
family	O
of	O
ADAR1s	B-protein_type
(	O
Fig	O
.	O
6d	O
).	O

The	O
substantial	O
difference	O
in	O
sequence	O
between	O
the	O
ADARs	B-protein_type
in	O
this	O
RNA	B-structure_element
-	I-structure_element
binding	I-structure_element
loop	I-structure_element
suggests	O
differences	O
in	O
editing	B-site
site	I-site
selectivity	O
between	O
the	O
two	O
ADARs	B-protein_type
arise	O
,	O
at	O
least	O
in	O
part	O
,	O
from	O
differences	O
in	O
how	O
this	O
loop	B-structure_element
binds	O
RNA	B-chemical
substrates	O
.	O

DNA	B-protein_type
methylases	I-protein_type
,	O
DNA	B-protein_type
repair	I-protein_type
glycosylases	I-protein_type
and	O
RNA	B-protein_type
loop	I-protein_type
modifying	I-protein_type
enzymes	I-protein_type
are	O
known	O
that	O
flip	O
a	O
nucleotide	B-chemical
out	O
of	O
a	O
base	O
pair	O
.	O

However	O
,	O
none	O
of	O
the	O
structurally	O
characterized	O
base	B-protein_type
-	I-protein_type
flipping	I-protein_type
enzymes	I-protein_type
access	O
their	O
reactive	B-site
sites	I-site
from	O
within	O
a	O
normal	B-protein_state
base	I-protein_state
-	I-protein_state
paired	I-protein_state
RNA	B-structure_element
duplex	I-structure_element
.	O

We	O
are	O
aware	O
of	O
one	O
other	O
protein	O
-	O
induced	O
nucleotide	O
flipping	O
from	O
an	O
RNA	B-structure_element
duplex	I-structure_element
region	O
.	O

Bacterial	B-taxonomy_domain
initiation	B-protein
factor	I-protein
1	I-protein
(	O
IF1	B-protein
)	O
binds	O
to	O
the	O
30S	B-complex_assembly
ribosomal	I-complex_assembly
subunit	I-complex_assembly
at	O
helix	B-structure_element
44	I-structure_element
of	O
16S	B-chemical
RNA	I-chemical
with	O
A1492	B-residue_name_number
and	O
A1493	B-residue_name_number
flipped	B-protein_state
out	I-protein_state
of	O
the	O
helix	O
and	O
bound	B-protein_state
into	I-protein_state
protein	B-site
pockets	I-site
(	O
Supplementary	O
Fig	O
.	O
5b	O
).	O

Because	O
the	O
modification	B-site
sites	I-site
are	O
not	O
flanked	O
on	O
both	O
sides	O
by	O
normal	B-protein_state
duplex	B-structure_element
,	O
these	O
enzymes	O
do	O
not	O
experience	O
the	O
same	O
limits	O
in	O
approach	O
to	O
the	O
substrate	O
that	O
ADARs	B-protein_type
do	O
.	O

The	O
fact	O
that	O
ADARs	B-protein_type
must	O
induce	O
flipping	O
from	O
a	O
normal	B-protein_state
duplex	B-structure_element
has	O
implications	O
on	O
its	O
preference	O
for	O
adenosines	B-residue_name
flanked	O
by	O
certain	O
base	O
pairs	O
,	O
a	O
phenomenon	O
that	O
was	O
not	O
well	O
understood	O
prior	O
to	O
this	O
work	O
.	O

In	O
our	O
structures	B-evidence
,	O
the	O
flipped	B-protein_state
out	I-protein_state
8	B-chemical
-	I-chemical
azanebularine	I-chemical
is	O
hydrated	O
,	O
mimicking	O
the	O
tetrahedral	O
intermediate	O
predicted	O
for	O
deamination	O
of	O
adenosine	B-residue_name
(	O
Figs	O
.	O
1b	O
,	O
3a	O
,	O
Supplementary	O
Fig	O
.	O
3	O
a	O
–	O
b	O
).	O

In	O
addition	O
,	O
8	B-chemical
-	I-chemical
azanebularine	I-chemical
was	O
found	O
to	O
adopt	O
a	O
2	O
’-	O
endo	O
sugar	O
pucker	O
with	O
its	O
2	O
’-	O
hydroxyl	O
H	B-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
to	O
the	O
protein	O
backbone	O
carbonyl	O
at	O
T375	B-residue_name_number
.	O

The	O
2	O
’	O
endo	O
conformation	O
appears	O
to	O
facilitate	O
access	O
of	O
the	O
nucleobase	O
to	O
the	O
zinc	B-chemical
-	O
bound	O
water	B-chemical
for	O
nucleophilic	O
attack	O
at	O
C6	O
.	O

For	O
hADAR2	B-protein
,	O
E488	B-residue_name_number
serves	O
this	O
role	O
.	O

In	O
the	O
two	O
structures	B-evidence
with	O
wild	B-protein_state
type	I-protein_state
hADAR2	B-protein
,	O
the	O
E488	B-residue_name_number
residue	O
and	O
orphan	B-protein_state
base	B-chemical
are	O
in	O
nearly	O
identical	O
positions	O
(	O
see	O
Supplementary	O
Fig	O
.	O
4a	O
for	O
overlay	B-experimental_method
).	O

The	O
pKa	B-evidence
of	O
E488	B-residue_name_number
in	O
the	O
ADAR	B-complex_assembly
-	I-complex_assembly
RNA	I-complex_assembly
complex	O
has	O
not	O
been	O
measured	O
,	O
but	O
proximity	O
to	O
H	B-bond_interaction
-	I-bond_interaction
bond	I-bond_interaction
acceptors	O
,	O
such	O
as	O
cytidine	B-residue_name
N3	O
,	O
and	O
insertion	O
between	O
stacked	O
nucleobases	O
,	O
would	O
undoubtedly	O
elevate	O
this	O
value	O
and	O
could	O
lead	O
to	O
a	O
substantial	O
fraction	O
in	O
the	O
protonated	B-protein_state
state	O
at	O
physiologically	O
relevant	O
pH	O
.	O
Since	O
the	O
glutamine	B-residue_name
side	O
chain	O
is	O
fully	B-protein_state
protonated	I-protein_state
under	O
physiologically	O
relevant	O
conditions	O
,	O
a	O
rate	O
enhancement	O
for	O
the	O
E488Q	B-mutant
mutant	B-protein_state
would	O
be	O
expected	O
if	O
the	O
reaction	O
requires	O
E488	B-residue_name_number
protonation	O
.	O

The	O
interactions	O
of	O
hADAR2d	B-mutant
with	O
base	O
pairs	O
adjacent	O
to	O
the	O
editing	B-site
site	I-site
adenosine	B-residue_name
explain	O
the	O
known	O
5	O
’	O
and	O
3	O
’	O
nearest	O
neighbor	O
preferences	O
(	O
Fig	O
.	O
5	O
).	O

While	O
these	O
studies	O
indicate	O
the	O
ADAR2	B-protein
catalytic	B-structure_element
domain	I-structure_element
makes	O
an	O
important	O
contact	O
to	O
the	O
3	O
’	O
nearest	O
neighbor	O
G	B-residue_name
,	O
Stefl	O
et	O
al	O
.	O
suggested	O
the	O
3	O
’	O
G	B-residue_name
preference	O
arises	O
from	O
dsRBD	B-structure_element
binding	O
selectivity	O
for	O
ADAR2	B-protein
.	O

These	O
authors	O
reported	O
a	O
model	O
for	O
ADAR2	B-protein
’	O
s	O
dsRBDs	B-structure_element
bound	B-protein_state
to	I-protein_state
an	O
editing	O
substrate	O
based	O
on	O
NMR	B-experimental_method
data	O
from	O
the	O
isolated	B-protein_state
dsRBDs	B-structure_element
(	O
lacking	B-protein_state
the	O
deaminase	B-structure_element
domain	I-structure_element
)	O
and	O
short	O
RNA	B-chemical
fragments	O
derived	O
from	O
the	O
GluR	B-protein
-	I-protein
B	I-protein
R	B-site
/	I-site
G	I-site
site	I-site
RNA	B-chemical
.	O

They	O
describe	O
an	O
interaction	O
wherein	O
the	O
3	O
’	O
G	B-residue_name
2	O
-	O
amino	O
group	O
H	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
to	O
the	O
backbone	O
carbonyl	O
of	O
S258	B-residue_name_number
found	O
in	O
the	O
β1	B-structure_element
-	I-structure_element
β2	I-structure_element
loop	I-structure_element
of	O
ADAR2	B-protein
’	O
s	O
dsRBDII	B-structure_element
.	O

It	O
is	O
not	O
possible	O
for	O
the	O
S486	B-residue_name_number
-	O
3	O
’	O
G	B-residue_name
interaction	O
we	O
describe	O
here	O
and	O
the	O
S258	B-residue_name_number
-	O
3	O
’	O
G	B-residue_name
interaction	O
reported	O
by	O
Stefl	O
et	O
al	O
.	O
to	O
exist	O
in	O
the	O
same	O
complex	O
since	O
both	O
involve	O
protein	O
loops	O
bound	B-protein_state
in	I-protein_state
the	O
RNA	B-chemical
minor	B-site
groove	I-site
at	O
the	O
same	O
location	O
.	O

Because	O
our	O
structures	B-evidence
have	O
captured	O
the	O
edited	B-protein_state
nucleotide	B-chemical
in	O
the	O
conformation	O
required	O
to	O
access	O
the	O
active	B-site
site	I-site
,	O
the	O
interactions	O
observed	O
here	O
are	O
highly	O
likely	O
to	O
occur	O
during	O
the	O
deamination	O
reaction	O
at	O
the	O
editing	B-site
site	I-site
.	O

It	O
is	O
also	O
possible	O
that	O
ADAR	B-protein_type
dsRBD	B-structure_element
and	O
catalytic	B-structure_element
domain	I-structure_element
binding	O
are	O
sequential	O
,	O
with	O
release	O
of	O
the	O
dsRBD	B-structure_element
from	O
the	O
RNA	B-chemical
taking	O
place	O
prior	O
to	O
catalytic	B-structure_element
domain	I-structure_element
engagement	O
and	O
base	O
flipping	O
.	O

Given	O
the	O
conservation	O
in	O
RNA	B-site
binding	I-site
surface	I-site
and	O
active	B-site
site	I-site
residues	O
,	O
we	O
expect	O
the	O
hADAR1	B-protein
catalytic	B-structure_element
domain	I-structure_element
to	O
bind	O
RNA	B-chemical
with	O
a	O
similar	O
orientation	O
of	O
the	O
helix	O
found	O
in	O
our	O
hADAR2d	B-complex_assembly
-	I-complex_assembly
RNA	I-complex_assembly
structures	B-evidence
.	O

When	O
one	O
maps	O
the	O
locations	O
of	O
the	O
AGS	O
-	O
associated	O
mutations	O
onto	O
the	O
hADAR2d	B-complex_assembly
-	I-complex_assembly
RNA	I-complex_assembly
complex	O
,	O
two	O
mutations	O
involve	O
residues	O
in	O
close	O
proximity	O
to	O
the	O
RNA	B-chemical
(<	O
4	O
Å	O
)	O
(	O
Supplementary	O
Fig	O
.	O
8a	O
).	O

G487	B-residue_name_number
of	O
hADAR2	B-protein
is	O
found	O
on	O
the	O
flipping	B-structure_element
loop	I-structure_element
near	O
the	O
RNA	B-chemical
(	O
Fig	O
.	O
3b	O
).	O

Sequence	O
in	O
this	O
loop	B-structure_element
is	O
highly	B-protein_state
conserved	I-protein_state
among	O
ADARs	B-protein_type
and	O
corresponds	O
to	O
G1007	B-residue_name_number
in	O
hADAR1	B-protein
(	O
Supplementary	O
Table	O
2	O
).	O

Also	O
,	O
K376	B-residue_name_number
forms	O
salt	B-bond_interaction
bridges	I-bond_interaction
with	O
both	O
the	O
5	O
’	O
and	O
3	O
’	O
phosphodiesters	O
of	O
the	O
guanosine	B-residue_name
on	O
the	O
3	O
’	O
side	O
of	O
the	O
editing	B-site
site	I-site
(	O
Fig	O
.	O
2	O
).	O

The	O
corresponding	O
residue	O
in	O
hADAR1	B-protein
(	O
R892	B-residue_name_number
)	O
could	O
form	O
similar	O
contacts	O
and	O
the	O
R892H	B-mutant
mutation	O
would	O
likely	O
alter	O
this	O
interaction	O
.	O

a	O
,	O
Domain	O
map	O
for	O
human	B-species
ADAR2	B-protein
b	O
,	O
ADAR	B-protein_type
reaction	O
showing	O
intermediate	O
and	O
8	B-chemical
-	I-chemical
azanebularine	I-chemical
(	I-chemical
N	I-chemical
)	I-chemical
hydrate	I-chemical
that	O
mimics	O
this	O
structure	B-evidence
c	O
,	O
Duplex	B-structure_element
RNAs	I-structure_element
used	O
for	O
crystallization	B-experimental_method
.	O

Bdf2	B-chemical
duplex	I-chemical
sequence	O
is	O
derived	O
from	O
an	O
editing	B-site
site	I-site
found	O
in	O
S	B-species
.	I-species
cerevisiae	I-species
Bdf2	B-chemical
mRNA	I-chemical
and	O
Gli1	B-protein
duplex	O
has	O
sequence	O
surrounding	O
the	O
human	B-species
Gli1	B-protein
mRNA	B-chemical
editing	B-site
site	I-site
.	O

Structure	B-evidence
of	O
hADAR2d	B-mutant
E488Q	B-mutant
bound	B-protein_state
to	I-protein_state
the	O
Bdf2	B-chemical
-	I-chemical
C	I-chemical
RNA	I-chemical
duplex	I-chemical
at	O
2	O
.	O
75	O
Å	O
resolution	O

a	O
,	O
View	O
of	O
structure	O
perpendicular	O
to	O
the	O
dsRNA	B-chemical
helical	O
axis	O
.	O

Colors	O
correspond	O
to	O
those	O
in	O
Figs	O
.	O
1a	O
and	O
1c	O
;	O
flipped	B-protein_state
out	I-protein_state
base	O
N	O
is	O
highlighted	O
red	O
,	O
zinc	B-chemical
in	O
grey	O
space	O
-	O
filling	O
sphere	O
,	O
Q488	B-residue_name_number
in	O
yellow	O
,	O
previously	O
disordered	B-protein_state
aa454	O
–	B-residue_range
477	I-residue_range
loop	B-structure_element
in	O
green	O
and	O
inositol	B-chemical
hexakisphosphate	I-chemical
(	O
IHP	B-chemical
)	O
in	O
space	O
filling	O
.	O

ADAR	B-protein_type
recognition	O
of	O
the	O
flipped	B-protein_state
out	I-protein_state
and	O
orphaned	B-protein_state
nucleotides	B-chemical

a	O
,	O
Contacts	O
to	O
the	O
editing	B-site
site	I-site
nucleotide	B-chemical
(	O
N	O
)	O
in	O
the	O
active	B-site
site	I-site
.	O

c	O
,	O
Orphan	B-protein_state
nucleotide	B-chemical
recognition	O
in	O
the	O
hADAR2d	B-complex_assembly
WT	I-complex_assembly
–	I-complex_assembly
Bdf2	I-complex_assembly
-	I-complex_assembly
U	I-complex_assembly
complex	O
.	O

a	O
,	O
hADAR2d	B-mutant
shifts	O
the	O
position	O
of	O
U11	B-residue_name_number
-	O
A13	B-residue_name_number
’	O
base	O
pair	O
from	O
ideal	O
A	B-structure_element
-	I-structure_element
form	I-structure_element
RNA	I-structure_element
helix	I-structure_element
(	O
yellow	O
).	O

b	O
,	O
Overlay	B-experimental_method
of	O
Bdf2	B-chemical
duplex	I-chemical
RNA	I-chemical
and	O
idealized	O
A	B-structure_element
form	I-structure_element
duplex	I-structure_element
of	O
same	O
sequence	O
(	O
yellow	O
)	O
illustrating	O
kink	O
in	O
strand	O
and	O
widening	O
of	O
major	B-site
groove	I-site
opposite	O
editing	B-site
site	I-site
induced	O
by	O
hADAR2d	B-mutant
.	O

Interactions	O
with	O
editing	B-site
site	I-site
nearest	O
neighbor	O
nucleotides	B-chemical

c	O
,	O
Comparison	O
of	O
deamination	B-evidence
rate	I-evidence
constants	I-evidence
by	O
hADAR2d	B-mutant
at	O
the	O
editing	B-site
site	I-site
adenosine	B-residue_name
(	O
red	O
)	O
for	O
duplexes	O
bearing	O
different	O
5	O
’	O
nearest	O
neighbors	O
;	O
krel	B-evidence
=	O
kobs	B-evidence
/(	O
kobs	B-evidence
for	O
unmodified	B-protein_state
RNA	B-chemical
).	O

d	O
,	O
hADAR2	B-protein
S486	B-residue_name_number
backbone	O
H	B-bond_interaction
-	I-bond_interaction
bond	I-bond_interaction
with	O
3	O
’	O
G	B-residue_name
2	O
-	O
amino	O
group	O
;	O
e	O
,	O
RNA	B-structure_element
duplex	I-structure_element
substrates	O
prepared	O
with	O
different	O
3	O
’	O
nearest	O
neighbor	O
nucleotides	O
adjacent	O
to	O
editing	B-site
site	I-site
indicated	O
in	O
red	O
(	O
I	B-residue_name
=	O
inosine	B-residue_name
,	O
N2MeG	O
=	O
N2	O
-	O
methylguanosine	O
,	O
2AP	B-structure_element
=	O
2	B-structure_element
-	I-structure_element
aminopurine	I-structure_element
).	O

krel	B-evidence
=	O
kobs	B-evidence
/(	O
kobs	B-evidence
for	O
unmodified	B-protein_state
RNA	B-chemical
).	O

RNA	B-structure_element
-	I-structure_element
binding	I-structure_element
loops	I-structure_element
in	O
the	O
ADAR	B-protein_type
catalytic	B-structure_element
domain	I-structure_element

a	O
,	O
hADAR2	B-protein
residues	O
that	O
contact	O
phosphodiester	O
backbone	O
near	O
5	O
’	O
end	O
of	O
unedited	O
strand	O
.	O

b	O
,	O
Location	O
of	O
mutations	O
introduced	O
at	O
protein	B-site
-	I-site
RNA	I-site
interface	I-site
.	O

c	O
,	O
Comparison	O
of	O
deamination	B-evidence
rate	I-evidence
constants	I-evidence
of	O
the	O
different	O
hADAR2d	B-mutant
mutants	O
(	O
Log	O
scale	O
).	O

krel	B-evidence
=	O
kobs	B-evidence
for	O
mutant	B-protein_state
/	O
kobs	B-evidence
for	O
WT	B-protein_state
.	O

d	O
,	O
Sequence	B-experimental_method
alignment	I-experimental_method
of	O
ADAR2s	B-protein_type
(	O
A2	O
)	O
and	O
ADAR1s	B-protein_type
(	O
A1	O
)	O
from	O
different	O
organisms	O
with	O
different	O
levels	O
of	O
conservation	O
colored	O
(	O
Yellow	O
:	O
conserved	B-protein_state
in	O
all	O
ADAR1s	B-protein_type
and	O
ADAR2s	B-protein_type
,	O
red	O
:	O
conserved	B-protein_state
in	O
ADAR2s	B-protein_type
,	O
blue	O
:	O
conserved	B-protein_state
in	O
ADAR1s	B-protein_type
.	O

e	O
,	O
Interaction	O
of	O
the	O
ADAR	B-structure_element
-	I-structure_element
specific	I-structure_element
RNA	I-structure_element
-	I-structure_element
binding	I-structure_element
loop	I-structure_element
near	O
the	O
5	O
’	O
end	O
of	O
the	O
edited	O
strand	O
.	O

Colors	O
as	O
in	O
d	O
,	O
white	O
:	O
not	B-protein_state
conserved	I-protein_state
,	O
flipped	B-protein_state
out	I-protein_state
base	B-chemical
is	O
shown	O
in	O
pink	O
.	O

Structural	O
basis	O
for	O
the	O
regulation	O
of	O
enzymatic	O
activity	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
by	O
domain	O
-	O
domain	O
interactions	O

Here	O
,	O
we	O
report	O
the	O
structures	B-evidence
of	O
four	O
domains	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
from	O
Mus	B-species
musculus	I-species
—	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
(	O
NTD	B-structure_element
),	O
PilT	B-structure_element
N	I-structure_element
-	I-structure_element
terminus	I-structure_element
like	I-structure_element
(	O
PIN	B-structure_element
)	O
domain	O
,	O
zinc	B-structure_element
finger	I-structure_element
(	O
ZF	B-structure_element
)	O
domain	O
and	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
(	O
CTD	B-structure_element
).	O

The	O
PIN	B-structure_element
domain	O
harbors	O
the	O
RNase	B-protein_type
catalytic	B-site
center	I-site
;	O
however	O
,	O
it	O
is	O
insufficient	O
for	O
enzymatic	O
activity	O
.	O

We	O
found	O
that	O
the	O
NTD	B-structure_element
associates	O
with	O
the	O
PIN	B-structure_element
domain	O
and	O
significantly	O
enhances	O
its	O
RNase	B-protein_type
activity	O
.	O

The	O
PIN	B-structure_element
domain	O
forms	O
a	O
head	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
tail	I-protein_state
oligomer	B-oligomeric_state
and	O
the	O
dimer	B-site
interface	I-site
overlaps	O
with	O
the	O
NTD	B-site
binding	I-site
site	I-site
.	O

These	O
results	O
suggest	O
that	O
Regnase	B-protein
-	I-protein
1	I-protein
RNase	B-protein_type
activity	O
is	O
tightly	O
controlled	O
by	O
both	O
intramolecular	O
(	O
NTD	B-structure_element
-	O
PIN	B-structure_element
)	O
and	O
intermolecular	O
(	O
PIN	B-structure_element
-	O
PIN	B-structure_element
)	O
interactions	O
.	O

The	O
initial	O
sensing	O
of	O
infection	O
is	O
mediated	O
by	O
a	O
set	O
of	O
pattern	B-protein_type
-	I-protein_type
recognition	I-protein_type
receptors	I-protein_type
(	O
PRRs	B-protein_type
)	O
such	O
Toll	B-protein_type
-	I-protein_type
like	I-protein_type
receptors	I-protein_type
(	O
TLRs	B-protein_type
)	O
and	O
the	O
intracellular	O
signaling	O
cascades	O
triggered	O
by	O
TLRs	B-protein_type
evoke	O
transcriptional	O
expression	O
of	O
inflammatory	O
mediators	O
that	O
coordinate	O
the	O
elimination	O
of	O
pathogens	O
and	O
infected	O
cells	O
.	O

Regnase	B-protein
-	I-protein
1	I-protein
(	O
also	O
known	O
as	O
Zc3h12a	B-protein
and	O
MCPIP1	B-protein
)	O
is	O
an	O
RNase	B-protein_type
whose	O
expression	O
level	O
is	O
stimulated	O
by	O
lipopolysaccharides	B-chemical
and	O
prevents	O
autoimmune	O
diseases	O
by	O
directly	O
controlling	O
the	O
stability	O
of	O
mRNAs	B-chemical
of	O
inflammatory	O
genes	O
such	O
as	O
interleukin	O
(	B-protein_type
IL	I-protein_type
)-	I-protein_type
6	I-protein_type
,	O
IL	B-protein_type
-	I-protein_type
1β	I-protein_type
,	O
IL	B-protein_type
-	I-protein_type
2	I-protein_type
,	O
and	O
IL	B-protein_type
-	I-protein_type
12p40	I-protein_type
.	O

Regnase	B-protein
-	I-protein
1	I-protein
accelerates	O
target	O
mRNA	B-chemical
degradation	O
via	O
their	O
3	B-structure_element
′-	I-structure_element
terminal	I-structure_element
untranslated	I-structure_element
region	I-structure_element
(	O
3	B-structure_element
′	I-structure_element
UTR	I-structure_element
),	O
and	O
also	O
degrades	O
its	O
own	O
mRNA	B-chemical
.	O

Several	O
CCCH	B-structure_element
-	I-structure_element
type	I-structure_element
ZF	I-structure_element
motifs	I-structure_element
in	O
RNA	B-protein_type
-	I-protein_type
binding	I-protein_type
proteins	I-protein_type
have	O
been	O
reported	O
to	O
directly	O
bind	O
RNA	B-chemical
.	O

Here	O
,	O
we	O
performed	O
structural	B-experimental_method
and	I-experimental_method
functional	I-experimental_method
analyses	I-experimental_method
of	O
individual	O
domains	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
derived	O
from	O
Mus	B-species
musculus	I-species
in	O
order	O
to	O
understand	O
the	O
catalytic	O
activity	O
in	O
vitro	O
.	O

The	O
NTD	B-structure_element
plays	O
a	O
crucial	O
role	O
in	O
efficient	O
cleavage	O
of	O
target	O
mRNA	B-chemical
,	O
through	O
intramolecular	O
NTD	B-structure_element
-	O
PIN	B-structure_element
interactions	O
.	O

Moreover	O
,	O
Regnase	B-protein
-	I-protein
1	I-protein
functions	O
as	O
a	O
dimer	B-oligomeric_state
through	O
intermolecular	O
PIN	B-structure_element
-	O
PIN	B-structure_element
interactions	O
during	O
cleavage	O
of	O
target	O
mRNA	B-chemical
.	O

Our	O
findings	O
suggest	O
that	O
Regnase	B-protein
-	I-protein
1	I-protein
cleaves	O
its	O
target	O
mRNA	B-chemical
by	O
an	O
NTD	B-protein_state
-	I-protein_state
activated	I-protein_state
functional	B-protein_state
PIN	B-structure_element
dimer	B-oligomeric_state
,	O
while	O
the	O
ZF	B-structure_element
increases	O
RNA	B-chemical
affinity	O
in	O
the	O
vicinity	O
of	O
the	O
PIN	B-structure_element
dimer	B-oligomeric_state
.	O

Domain	O
structures	B-evidence
of	O
Regnase	B-protein
-	I-protein
1	I-protein

We	O
analyzed	O
Rengase	B-protein
-	I-protein
1	I-protein
derived	O
from	O
Mus	B-species
musculus	I-species
and	O
solved	B-experimental_method
the	O
structures	B-evidence
of	O
the	O
four	O
domains	O
;	O
NTD	B-structure_element
,	O
PIN	B-structure_element
,	O
ZF	B-structure_element
,	O
and	O
CTD	B-structure_element
individually	O
by	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
or	O
NMR	B-experimental_method
(	O
Fig	O
.	O
1a	O
–	O
e	O
).	O

X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
was	O
attempted	O
for	O
the	O
fragment	O
containing	O
both	O
the	O
PIN	B-structure_element
and	O
ZF	B-structure_element
domains	O
,	O
however	O
,	O
electron	B-evidence
density	I-evidence
was	O
observed	O
only	O
for	O
the	O
PIN	B-structure_element
domain	O
(	O
Fig	O
.	O
1c	O
),	O
consistent	O
with	O
a	O
previous	O
report	O
on	O
Regnase	B-protein
-	I-protein
1	I-protein
derived	O
from	O
Homo	B-species
sapiens	I-species
.	O

This	O
suggests	O
that	O
the	O
PIN	B-structure_element
and	O
ZF	B-structure_element
domains	O
exist	O
independently	O
without	O
interacting	O
with	O
each	O
other	O
.	O

The	O
domain	O
structures	B-evidence
of	O
NTD	B-structure_element
,	O
ZF	B-structure_element
,	O
and	O
CTD	B-structure_element
were	O
determined	O
by	O
NMR	B-experimental_method
(	O
Fig	O
.	O
1b	O
,	O
d	O
,	O
e	O
).	O

The	O
NTD	B-structure_element
and	O
CTD	B-structure_element
are	O
both	O
composed	O
of	O
three	O
α	B-structure_element
helices	I-structure_element
,	O
and	O
structurally	O
resemble	O
ubiquitin	B-protein
conjugating	I-protein
enzyme	I-protein
E2	I-protein
K	I-protein
(	O
PDB	O
ID	O
:	O
3K9O	O
)	O
and	O
ubiquitin	B-protein
associated	I-protein
protein	I-protein
1	I-protein
(	O
PDB	O
ID	O
:	O
4AE4	O
),	O
respectively	O
,	O
according	O
to	O
the	O
Dali	B-experimental_method
server	I-experimental_method
.	O

Although	O
the	O
PIN	B-structure_element
domain	O
is	O
responsible	O
for	O
the	O
catalytic	O
activity	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
,	O
the	O
roles	O
of	O
the	O
other	O
domains	O
are	O
largely	O
unknown	O
.	O

First	O
,	O
we	O
evaluated	O
a	O
role	O
of	O
the	O
NTD	B-structure_element
and	O
ZF	B-structure_element
domains	O
for	O
mRNA	B-chemical
binding	O
by	O
an	O
in	B-experimental_method
vitro	I-experimental_method
gel	I-experimental_method
shift	I-experimental_method
assay	I-experimental_method
(	O
Fig	O
.	O
1f	O
).	O

Fluorescently	B-protein_state
5	I-protein_state
′-	I-protein_state
labeled	I-protein_state
RNA	B-chemical
corresponding	O
to	O
nucleotides	O
82	O
–	O
106	O
of	O
the	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
mRNA	B-chemical
3	B-structure_element
′	I-structure_element
UTR	I-structure_element
and	O
the	O
catalytically	O
inactive	B-protein_state
mutant	B-protein_state
(	O
D226N	B-mutant
and	O
D244N	B-mutant
)	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
—	O
hereafter	O
referred	O
to	O
as	O
the	O
DDNN	B-mutant
mutant	B-protein_state
—	O
were	O
utilized	O
.	O

Based	O
on	O
the	O
decrease	O
in	O
the	O
free	O
RNA	B-chemical
fluorescence	O
band	O
,	O
we	O
evaluated	O
the	O
contribution	O
of	O
each	O
domain	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
to	O
RNA	B-chemical
binding	O
.	O

While	O
the	O
RNA	B-chemical
binding	O
ability	O
was	O
not	O
significantly	O
changed	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
NTD	B-structure_element
,	O
it	O
increased	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
the	O
ZF	B-structure_element
domain	O
(	O
Fig	O
.	O
1f	O
,	O
g	O
and	O
Supplementary	O
Fig	O
.	O
1	O
).	O

Direct	O
binding	O
of	O
the	O
ZF	B-structure_element
domain	O
and	O
RNA	B-chemical
were	O
confirmed	O
by	O
NMR	B-experimental_method
spectral	B-evidence
changes	I-evidence
.	O

The	O
fitting	O
of	O
the	O
titration	B-evidence
curve	I-evidence
of	O
Y314	B-residue_name_number
resulted	O
in	O
an	O
apparent	O
dissociation	B-evidence
constant	I-evidence
(	O
Kd	B-evidence
)	O
of	O
10	O
±	O
1	O
.	O
1	O
μM	O
(	O
Supplementary	O
Fig	O
.	O
2	O
).	O

These	O
results	O
indicate	O
that	O
not	O
only	O
the	O
PIN	B-structure_element
but	O
also	O
the	O
ZF	B-structure_element
domain	O
contribute	O
to	O
RNA	B-chemical
binding	O
,	O
while	O
the	O
NTD	B-structure_element
is	O
not	O
likely	O
to	O
be	O
involved	O
in	O
direct	O
interaction	O
with	O
RNA	B-chemical
.	O

Contribution	O
of	O
each	O
domain	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
to	O
RNase	B-protein_type
activity	O

The	O
apparent	O
half	O
-	O
life	O
(	O
T1	O
/	O
2	O
)	O
of	O
the	O
RNase	B-protein_type
activity	O
was	O
about	O
20	O
minutes	O
.	O

Regnase	B-protein
-	I-protein
1	I-protein
lacking	B-protein_state
the	O
ZF	B-structure_element
domain	O
generated	O
a	O
smaller	O
but	O
appreciable	O
amount	O
of	O
cleaved	O
product	O
(	O
T1	O
/	O
2	O
~	O
70	O
minutes	O
),	O
while	O
those	O
lacking	B-protein_state
the	O
NTD	B-structure_element
did	O
not	O
generate	O
cleaved	O
products	O
(	O
T1	O
/	O
2	O
>	O
90	O
minutes	O
).	O

It	O
should	O
be	O
noted	O
that	O
NTD	B-mutant
-	I-mutant
PIN	I-mutant
(	I-mutant
DDNN	I-mutant
)-	I-mutant
ZF	I-mutant
,	O
which	O
possesses	O
the	O
NTD	B-structure_element
but	O
lacks	B-protein_state
the	O
catalytic	B-site
residues	I-site
in	O
PIN	B-structure_element
,	O
completely	O
lost	O
all	O
RNase	B-protein_type
activity	O
(	O
Fig	O
.	O
1g	O
,	O
right	O
panel	O
),	O
as	O
expected	O
,	O
confirming	O
that	O
the	O
RNase	B-protein_type
catalytic	B-site
center	I-site
is	O
located	O
in	O
the	O
PIN	B-structure_element
domain	O
.	O

Taken	O
together	O
with	O
the	O
results	O
in	O
the	O
previous	O
section	O
,	O
we	O
conclude	O
that	O
the	O
NTD	B-structure_element
is	O
crucial	O
for	O
the	O
RNase	B-protein_type
activity	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
in	O
vitro	O
,	O
although	O
it	O
does	O
not	O
contribute	O
to	O
the	O
direct	O
mRNA	B-chemical
binding	O
.	O

Dimer	B-oligomeric_state
formation	O
of	O
the	O
PIN	B-structure_element
domains	O

By	O
comparison	B-experimental_method
with	I-experimental_method
the	I-experimental_method
elution	I-experimental_method
volume	I-experimental_method
of	I-experimental_method
standard	I-experimental_method
marker	I-experimental_method
proteins	I-experimental_method
,	O
the	O
PIN	B-structure_element
domain	O
was	O
assumed	O
to	O
be	O
in	O
equilibrium	O
between	O
a	O
monomer	B-oligomeric_state
and	O
a	O
dimer	B-oligomeric_state
in	O
solution	O
at	O
concentrations	O
in	O
the	O
20	O
–	O
200	O
μM	O
range	O
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
PIN	B-structure_element
domain	O
has	O
been	O
determined	O
in	O
three	O
distinct	O
crystal	B-evidence
forms	I-evidence
with	O
a	O
space	O
group	O
of	O
P3121	O
(	O
form	O
I	O
in	O
this	O
study	O
and	O
PDB	O
ID	O
3V33	O
),	O
P3221	O
(	O
form	O
II	O
in	O
this	O
study	O
),	O
and	O
P41	O
(	O
PDB	O
ID	O
3V32	O
and	O
3V34	O
),	O
respectively	O
.	O

We	O
found	O
that	O
the	O
PIN	B-structure_element
domain	O
formed	O
a	O
head	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
tail	I-protein_state
oligomer	B-oligomeric_state
that	O
was	O
commonly	O
observed	O
in	O
all	O
three	O
crystal	B-evidence
forms	I-evidence
in	O
spite	O
of	O
the	O
different	O
crystallization	O
conditions	O
(	O
Supplementary	O
Fig	O
.	O
3	O
).	O

On	O
the	O
other	O
hand	O
,	O
single	B-experimental_method
mutations	I-experimental_method
of	O
side	O
chains	O
involved	O
in	O
the	O
PIN	B-structure_element
–	O
PIN	B-structure_element
oligomeric	O
interaction	O
resulted	O
in	O
monomer	B-oligomeric_state
formation	O
,	O
judging	O
from	O
gel	B-experimental_method
filtration	I-experimental_method
(	O
Fig	O
.	O
2a	O
,	O
b	O
).	O

Wild	B-protein_state
type	I-protein_state
and	O
monomeric	B-oligomeric_state
PIN	B-structure_element
mutants	B-protein_state
(	O
P212A	B-mutant
and	O
D278R	B-mutant
)	O
were	O
also	O
analyzed	O
by	O
NMR	B-experimental_method
.	O

The	O
spectra	B-evidence
indicate	O
that	O
the	O
dimer	B-site
interface	I-site
of	O
the	O
wild	B-protein_state
type	I-protein_state
PIN	B-structure_element
domain	O
were	O
significantly	O
broadened	O
compared	O
to	O
the	O
monomeric	B-oligomeric_state
mutants	B-protein_state
(	O
Supplementary	O
Fig	O
.	O
4	O
).	O

Interestingly	O
,	O
the	O
monomeric	B-oligomeric_state
PIN	B-structure_element
mutants	B-protein_state
P212A	B-mutant
,	O
R214A	B-mutant
,	O
and	O
D278R	B-mutant
had	O
no	O
significant	O
RNase	B-protein_type
activity	O
for	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
mRNA	B-chemical
in	O
vitro	O
(	O
Fig	O
.	O
2c	O
).	O

Domain	O
-	O
domain	O
interaction	O
between	O
the	O
NTD	B-structure_element
and	O
the	O
PIN	B-structure_element
domain	O

While	O
the	O
NTD	B-structure_element
does	O
not	O
contribute	O
to	O
RNA	B-chemical
binding	O
(	O
Fig	O
.	O
1f	O
,	O
g	O
,	O
and	O
Supplementary	O
Fig	O
.	O
1	O
),	O
it	O
increases	O
the	O
RNase	B-protein_type
activity	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
(	O
Fig	O
.	O
1h	O
).	O

In	O
order	O
to	O
gain	O
insight	O
into	O
the	O
molecular	O
mechanism	O
of	O
the	O
NTD	B-structure_element
-	O
mediated	O
enhancement	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
RNase	B-protein_type
activity	O
,	O
we	O
further	O
investigated	O
the	O
domain	O
-	O
domain	O
interaction	O
between	O
the	O
NTD	B-structure_element
and	O
the	O
PIN	B-structure_element
domain	O
using	O
NMR	B-experimental_method
.	O

We	O
used	O
the	O
catalytically	B-protein_state
inactive	I-protein_state
monomeric	B-oligomeric_state
PIN	B-structure_element
mutant	B-protein_state
possessing	O
both	O
the	O
DDNN	B-mutant
and	O
D278R	B-mutant
mutations	O
to	O
avoid	O
dimer	B-oligomeric_state
formation	O
of	O
the	O
PIN	B-structure_element
domain	O
.	O

The	O
NMR	B-experimental_method
signals	O
from	O
the	O
PIN	B-structure_element
domain	O
(	O
residues	O
V177	B-residue_name_number
,	O
F210	B-residue_range
-	I-residue_range
T211	I-residue_range
,	O
R214	B-residue_name_number
,	O
F228	B-residue_range
-	I-residue_range
L232	I-residue_range
,	O
and	O
F234	B-residue_range
-	I-residue_range
S236	I-residue_range
)	O
exhibited	O
significant	O
chemical	O
shift	O
changes	O
upon	O
addition	B-experimental_method
of	I-experimental_method
the	O
NTD	B-structure_element
(	O
Fig	O
.	O
3a	O
).	O

These	O
results	O
clearly	O
indicate	O
a	O
direct	O
interaction	O
between	O
the	O
PIN	B-structure_element
domain	O
and	O
the	O
NTD	B-structure_element
.	O

Based	O
on	O
the	O
titration	B-evidence
curve	I-evidence
for	O
the	O
chemical	B-evidence
shift	I-evidence
changes	I-evidence
of	O
L58	B-residue_name_number
,	O
the	O
apparent	O
Kd	B-evidence
between	O
the	O
isolated	O
NTD	B-structure_element
and	O
PIN	B-structure_element
was	O
estimated	O
to	O
be	O
110	O
±	O
5	O
.	O
8	O
μM	O
.	O
Considering	O
the	O
fact	O
that	O
the	O
NTD	B-structure_element
and	O
PIN	B-structure_element
domains	O
are	O
attached	O
by	O
a	O
linker	B-structure_element
,	O
the	O
actual	O
binding	B-evidence
affinity	I-evidence
is	O
expected	O
much	O
higher	O
in	O
the	O
native	B-protein_state
protein	O
.	O

Mapping	O
the	O
residues	O
with	O
chemical	O
shift	O
changes	O
reveals	O
the	O
putative	O
PIN	B-site
/	I-site
NTD	I-site
interface	I-site
,	O
which	O
includes	O
a	O
helix	B-structure_element
that	O
harbors	O
catalytic	O
residues	O
D225	B-residue_name_number
and	O
D226	B-residue_name_number
on	O
the	O
PIN	B-structure_element
domain	O
(	O
Fig	O
.	O
3a	O
).	O

Interestingly	O
,	O
the	O
putative	O
binding	B-site
site	I-site
for	O
the	O
NTD	B-structure_element
overlaps	O
with	O
the	O
PIN	B-site
-	I-site
PIN	I-site
dimer	I-site
interface	I-site
,	O
implying	O
that	O
NTD	B-structure_element
binding	O
can	O
“	O
terminate	O
”	O
PIN	B-structure_element
-	O
PIN	B-structure_element
oligomerization	O
(	O
Fig	O
.	O
2b	O
).	O

An	O
in	B-experimental_method
silico	I-experimental_method
docking	I-experimental_method
of	O
the	O
NTD	B-structure_element
and	O
PIN	B-structure_element
domains	O
using	O
chemical	B-evidence
shift	I-evidence
restraints	I-evidence
provided	O
a	O
model	O
consistent	O
with	O
the	O
NMR	B-experimental_method
experiments	O
(	O
Fig	O
.	O
3c	O
).	O

Residues	O
critical	O
for	O
Regnase	B-protein
-	I-protein
1	I-protein
RNase	B-protein_type
activity	O

To	O
gain	O
insight	O
into	O
the	O
residues	O
critical	O
for	O
Regnase	B-protein
-	I-protein
1	I-protein
RNase	B-protein_type
activity	O
,	O
each	O
basic	O
or	O
aromatic	O
residue	O
located	O
around	O
the	O
catalytic	B-site
site	I-site
of	O
the	O
PIN	B-structure_element
oligomer	B-oligomeric_state
was	O
mutated	B-experimental_method
to	I-experimental_method
alanine	B-residue_name
,	O
and	O
the	O
oligomerization	O
and	O
RNase	B-protein_type
activity	O
were	O
investigated	O
(	O
Fig	O
.	O
4	O
).	O

From	O
the	O
gel	B-experimental_method
filtration	I-experimental_method
assays	I-experimental_method
,	O
all	O
mutants	B-protein_state
except	O
R214A	B-mutant
formed	O
dimers	B-oligomeric_state
,	O
suggesting	O
that	O
any	O
lack	O
of	O
RNase	B-protein_type
activity	O
in	O
the	O
mutants	B-protein_state
,	O
except	O
R214A	B-mutant
,	O
was	O
directly	O
due	O
to	O
mutational	O
effects	O
of	O
the	O
specific	O
residues	O
and	O
not	O
to	O
abrogation	O
of	O
dimer	B-oligomeric_state
formation	O
.	O

The	O
W182A	B-mutant
,	O
R183A	B-mutant
,	O
and	O
R214A	B-mutant
mutants	B-protein_state
markedly	O
lost	O
cleavage	O
activity	O
for	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
mRNA	B-chemical
as	O
well	O
as	O
for	O
Regnase	B-protein
-	I-protein
1	I-protein
mRNA	B-chemical
.	O

The	O
K184A	B-mutant
,	O
R215A	B-mutant
,	O
and	O
R220A	B-mutant
mutants	B-protein_state
moderately	O
but	O
significantly	O
decreased	O
the	O
cleavage	O
activity	O
for	O
both	O
target	O
mRNAs	B-chemical
.	O

The	O
importance	O
of	O
K219	B-residue_name_number
and	O
R247	B-residue_name_number
was	O
slightly	O
different	O
for	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
and	O
Regnase	B-protein
-	I-protein
1	I-protein
mRNA	B-chemical
;	O
both	O
K219	B-residue_name_number
and	O
R247	B-residue_name_number
were	O
more	O
important	O
in	O
the	O
cleavage	O
of	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
mRNA	B-chemical
than	O
for	O
Regnase	B-protein
-	I-protein
1	I-protein
mRNA	B-chemical
.	O

The	O
other	O
mutated	O
residues	O
—	O
K152	B-residue_name_number
,	O
R158	B-residue_name_number
,	O
R188	B-residue_name_number
,	O
R200	B-residue_name_number
,	O
K204	B-residue_name_number
,	O
K206	B-residue_name_number
,	O
K257	B-residue_name_number
,	O
and	O
R258	B-residue_name_number
—	O
were	O
not	O
critical	O
for	O
RNase	B-protein_type
activity	O
.	O

The	O
importance	O
of	O
residues	O
W182	B-residue_name_number
and	O
R183	B-residue_name_number
can	O
readily	O
be	O
understood	O
in	O
terms	O
of	O
the	O
monomeric	B-oligomeric_state
PIN	B-structure_element
structure	B-evidence
as	O
they	O
are	O
located	O
near	O
to	O
the	O
RNase	B-protein_type
catalytic	B-site
site	I-site
;	O
however	O
,	O
the	O
importance	O
of	O
residue	O
K184	B-residue_name_number
,	O
which	O
points	O
away	O
from	O
the	O
active	B-site
site	I-site
is	O
more	O
easily	O
rationalized	O
in	O
terms	O
of	O
the	O
oligomeric	O
structure	B-evidence
,	O
in	O
which	O
the	O
“	O
secondary	O
”	O
chain	O
’	O
s	O
residue	O
K184	B-residue_name_number
is	O
positioned	O
near	O
the	O
“	O
primary	B-protein_state
”	I-protein_state
chain	O
’	O
s	O
catalytic	B-site
site	I-site
(	O
Fig	O
.	O
4	O
).	O

In	O
contrast	O
,	O
R214	B-residue_name_number
is	O
important	O
for	O
oligomerization	O
of	O
the	O
PIN	B-structure_element
domain	O
and	O
the	O
“	O
secondary	O
”	O
chain	O
’	O
s	O
residue	O
R214	B-residue_name_number
is	O
also	O
positioned	O
near	O
the	O
“	O
primary	B-protein_state
”	O
chain	O
’	O
s	O
active	B-site
site	I-site
within	O
the	O
dimer	B-site
interface	I-site
.	O

It	O
should	O
be	O
noted	O
that	O
the	O
putative	B-site
-	I-site
RNA	I-site
binding	I-site
residues	I-site
K184	B-residue_name_number
and	O
R214	B-residue_name_number
are	O
unique	O
to	O
Regnase	B-protein
-	I-protein
1	I-protein
among	O
PIN	B-structure_element
domains	O
.	O

Molecular	O
mechanism	O
of	O
target	O
mRNA	B-chemical
cleavage	O
by	O
the	O
PIN	B-structure_element
dimer	B-oligomeric_state

One	O
group	O
consisted	O
of	O
catalytically	B-protein_state
active	I-protein_state
PIN	B-structure_element
domains	O
with	O
mutation	B-experimental_method
of	I-experimental_method
basic	O
residues	O
found	O
in	O
the	O
previous	O
section	O
to	O
confer	O
decreased	O
RNase	B-protein_type
activity	O
(	O
Fig	O
.	O
4	O
).	O

These	O
were	O
paired	O
with	O
a	O
DDNN	B-mutant
mutant	B-protein_state
that	O
had	O
no	O
RNase	B-protein_type
activity	O
by	O
itself	O
.	O

When	O
any	O
members	O
of	O
the	O
two	O
groups	O
are	O
mixed	O
,	O
two	O
kinds	O
of	O
heterodimers	B-oligomeric_state
can	O
be	O
formed	O
:	O
one	O
is	O
composed	O
of	O
a	O
DDNN	B-mutant
primary	B-protein_state
PIN	B-structure_element
and	O
a	O
basic	O
residue	O
mutant	B-protein_state
secondary	B-protein_state
PIN	B-structure_element
and	O
is	O
expected	O
to	O
exhibit	O
no	O
RNase	B-protein_type
activity	O
;	O
the	O
other	O
is	O
composed	O
of	O
a	O
basic	O
residue	O
mutant	B-protein_state
primary	B-protein_state
PIN	B-structure_element
and	O
a	O
DDNN	B-mutant
secondary	B-protein_state
PIN	B-structure_element
and	O
is	O
predicted	O
to	O
rescue	O
RNase	B-protein_type
activity	O
(	O
Fig	O
.	O
5a	O
).	O

When	O
we	O
compared	O
the	O
fluorescence	B-evidence
intensity	I-evidence
of	O
uncleaved	B-protein_state
IL	B-protein_type
-	I-protein_type
6	I-protein_type
mRNA	B-chemical
,	O
basic	O
residue	O
mutants	B-protein_state
W182A	B-mutant
,	O
K184A	B-mutant
,	O
R214A	B-mutant
,	O
and	O
R220A	B-mutant
were	O
rescued	O
upon	O
addition	O
of	O
the	O
DDNN	B-mutant
mutant	B-protein_state
(	O
Fig	O
.	O
5b	O
).	O

Rescue	O
of	O
K184A	B-mutant
and	O
R214A	B-mutant
by	O
the	O
DDNN	B-mutant
mutant	B-protein_state
was	O
also	O
confirmed	O
by	O
a	O
significant	O
increase	O
in	O
the	O
cleaved	O
products	O
.	O

This	O
is	O
particularly	O
significant	O
because	O
the	O
side	O
chains	O
of	O
K184	B-residue_name_number
and	O
R214	B-residue_name_number
in	O
the	O
primary	B-protein_state
PIN	B-structure_element
are	O
oriented	O
away	O
from	O
their	O
own	O
catalytic	B-site
center	I-site
,	O
while	O
those	O
in	O
the	O
secondary	B-protein_state
PIN	B-structure_element
face	O
toward	O
the	O
catalytic	B-site
center	I-site
of	O
the	O
primary	B-protein_state
PIN	B-structure_element
.	O

Taken	O
together	O
,	O
the	O
rescue	O
experiments	O
above	O
support	O
the	O
proposed	O
model	O
in	O
which	O
the	O
head	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
tail	I-protein_state
dimer	B-oligomeric_state
is	O
functional	O
in	O
vitro	O
.	O

We	O
determined	O
the	O
individual	O
domain	O
structures	B-evidence
of	O
Regnase	B-protein
-	I-protein
1	I-protein
by	O
NMR	B-experimental_method
and	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
.	O

A	O
Regnase	B-protein
-	I-protein
1	I-protein
construct	O
consisting	O
of	O
PIN	B-structure_element
and	O
ZF	B-structure_element
domains	O
derived	O
from	O
Mus	B-species
musculus	I-species
was	O
crystallized	B-experimental_method
;	O
however	O
,	O
the	O
electron	B-evidence
density	I-evidence
of	O
the	O
ZF	B-structure_element
domain	O
was	O
low	O
,	O
indicating	O
that	O
the	O
ZF	B-structure_element
domain	O
is	O
highly	B-protein_state
mobile	I-protein_state
in	O
the	O
absence	B-protein_state
of	I-protein_state
target	O
mRNA	B-chemical
or	O
possibly	O
other	O
protein	O
-	O
protein	O
interactions	O
.	O

These	O
results	O
indicate	O
that	O
Regnase	B-protein
-	I-protein
1	I-protein
directly	O
binds	O
to	O
RNA	B-chemical
and	O
precipitates	O
under	O
such	O
experimental	O
conditions	O
.	O

The	O
previously	O
reported	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
Regnase	B-protein
-	I-protein
1	I-protein
PIN	B-structure_element
domain	O
derived	O
from	O
Homo	B-species
sapiens	I-species
is	O
nearly	O
identical	O
to	O
the	O
one	O
derived	O
from	O
Mus	B-species
musculus	I-species
in	O
this	O
study	O
,	O
with	O
a	O
backbone	O
RMSD	B-evidence
of	O
0	O
.	O
2	O
Å	O
.	O
The	O
amino	O
acid	O
sequences	O
corresponding	O
to	O
PIN	B-structure_element
(	O
residues	O
134	B-residue_range
–	I-residue_range
295	I-residue_range
)	O
are	O
the	O
two	O
non	O
-	O
identical	O
residues	O
are	O
substituted	O
with	O
similar	O
amino	O
acids	O
.	O

Both	O
the	O
mouse	B-taxonomy_domain
and	O
human	B-species
PIN	B-structure_element
domains	O
form	O
head	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
tail	I-protein_state
oligomers	B-oligomeric_state
in	O
three	O
distinct	O
crystal	B-evidence
forms	I-evidence
.	O

Rao	O
and	O
co	O
-	O
workers	O
previously	O
argued	O
that	O
PIN	B-structure_element
dimerization	O
is	O
likely	O
to	O
be	O
a	O
crystallographic	O
artifact	O
with	O
no	O
physiological	O
significance	O
,	O
since	O
monomers	B-oligomeric_state
were	O
dominant	O
in	O
their	O
analytical	B-experimental_method
ultra	I-experimental_method
-	I-experimental_method
centrifugation	I-experimental_method
experiments	O
.	O

In	O
contrast	O
,	O
our	O
gel	B-experimental_method
filtration	I-experimental_method
data	O
,	O
mutational	B-experimental_method
analyses	I-experimental_method
,	O
and	O
NMR	B-experimental_method
spectra	B-evidence
all	O
indicate	O
that	O
the	O
PIN	B-structure_element
domain	O
forms	O
a	O
head	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
tail	I-protein_state
dimer	B-oligomeric_state
in	O
solution	O
in	O
a	O
manner	O
similar	O
to	O
the	O
crystal	B-evidence
structure	I-evidence
.	O

Single	B-experimental_method
mutations	I-experimental_method
to	O
residues	O
involved	O
in	O
the	O
putative	O
oligomeric	O
interaction	O
of	O
PIN	B-structure_element
monomerized	B-oligomeric_state
as	O
expected	O
and	O
these	O
mutants	B-protein_state
lost	O
their	O
RNase	B-protein_type
activity	O
as	O
well	O
.	O

Moreover	O
,	O
our	O
structure	B-experimental_method
-	I-experimental_method
based	I-experimental_method
mutational	I-experimental_method
analyses	I-experimental_method
showed	O
these	O
two	O
Regnase	B-protein
-	I-protein
1	I-protein
specific	O
basic	O
regions	O
were	O
essential	O
for	O
target	O
mRNA	B-chemical
cleavage	O
in	O
vitro	O
.	O

The	O
cleavage	B-experimental_method
assay	I-experimental_method
also	O
showed	O
that	O
the	O
NTD	B-structure_element
is	O
crucial	O
for	O
efficient	O
mRNA	B-chemical
cleavage	O
.	O

Moreover	O
,	O
we	O
found	O
that	O
the	O
NTD	B-structure_element
associates	O
with	O
the	O
oligomeric	B-site
surface	I-site
of	O
the	O
primary	B-protein_state
PIN	B-structure_element
,	O
docking	O
to	O
a	O
helix	B-structure_element
that	O
harbors	O
its	O
catalytic	B-site
residues	I-site
(	O
Figs	O
2b	O
and	O
3a	O
).	O

Taken	O
together	O
,	O
this	O
suggests	O
that	O
the	O
NTD	B-structure_element
and	O
the	O
PIN	B-structure_element
domain	O
compete	O
for	O
a	O
common	B-site
binding	I-site
site	I-site
.	O

The	O
affinity	B-evidence
of	O
the	O
domain	O
-	O
domain	O
interaction	O
between	O
two	O
PIN	B-structure_element
domains	O
(	O
Kd	B-evidence
=	O
~	O
10	O
−	O
4	O
M	O
)	O
is	O
similar	O
to	O
that	O
of	O
the	O
NTD	B-structure_element
-	O
PIN	B-structure_element
(	O
Kd	B-evidence
=	O
110	O
±	O
5	O
.	O
8	O
μM	O
)	O
interactions	O
;	O
however	O
,	O
the	O
covalent	O
connection	O
corresponding	O
to	O
residues	O
90	B-residue_range
–	I-residue_range
133	I-residue_range
between	O
the	O
NTD	B-structure_element
and	O
the	O
primary	B-protein_state
PIN	B-structure_element
will	O
greatly	O
enhance	O
the	O
intramolecular	O
domain	O
interaction	O
in	O
the	O
case	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
Regnase	B-protein
-	I-protein
1	I-protein
.	O

Based	O
on	O
these	O
structural	B-experimental_method
and	I-experimental_method
functional	I-experimental_method
analyses	I-experimental_method
of	O
Regnase	B-protein
-	I-protein
1	I-protein
domain	O
-	O
domain	O
interactions	O
,	O
we	O
performed	O
docking	B-experimental_method
simulations	I-experimental_method
of	O
the	O
NTD	B-structure_element
,	O
PIN	B-structure_element
dimer	B-oligomeric_state
,	O
and	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
mRNA	B-chemical
.	O

The	O
docking	B-experimental_method
result	O
revealed	O
multiple	O
RNA	B-chemical
binding	O
modes	O
that	O
satisfied	O
the	O
experimental	O
results	O
in	O
vitro	O
(	O
Supplementary	O
Fig	O
.	O
7c	O
,	O
d	O
),	O
however	O
,	O
it	O
should	O
be	O
noted	O
that	O
,	O
in	O
vivo	O
,	O
there	O
would	O
likely	O
be	O
many	O
other	O
RNA	B-protein_type
-	I-protein_type
binding	I-protein_type
proteins	I-protein_type
that	O
would	O
protect	O
loop	B-structure_element
regions	O
from	O
cleavage	O
by	O
Regnase	B-protein
-	I-protein
1	I-protein
.	O

In	O
the	O
absence	B-protein_state
of	I-protein_state
target	O
mRNA	B-chemical
,	O
the	O
PIN	B-structure_element
domain	O
forms	O
head	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
tail	I-protein_state
oligomers	B-oligomeric_state
at	O
high	O
concentration	O
.	O

While	O
further	O
investigations	O
on	O
the	O
domain	O
-	O
domain	O
interactions	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
in	O
vivo	O
are	O
necessary	O
,	O
these	O
intramolecular	O
and	O
intermolecular	O
domain	O
interactions	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
appear	O
to	O
structurally	O
constrain	O
Regnase	B-protein
-	I-protein
1activity	I-protein
,	O
which	O
,	O
in	O
turn	O
,	O
enables	O
tight	O
regulation	O
of	O
immune	O
responses	O
.	O

Structural	B-experimental_method
and	I-experimental_method
functional	I-experimental_method
analyses	I-experimental_method
of	O
Regnase	B-protein
-	I-protein
1	I-protein
.	O

Catalytic	B-protein_state
Asp	B-residue_name
residues	O
were	O
shown	O
in	O
sticks	O
.	O

Three	O
Cys	B-residue_name
residues	O
and	O
one	O
His	B-residue_name
residue	O
responsible	O
for	O
Zn2	O
+-	O
binding	O
were	O
shown	O
in	O
sticks	O
.	O

All	O
the	O
structures	B-evidence
were	O
colored	O
in	O
rainbow	O
from	O
N	O
-	O
terminus	O
(	O
blue	O
)	O
to	O
C	O
-	O
terminus	O
(	O
red	O
).	O

Fluorescence	B-evidence
intensity	I-evidence
of	O
the	O
free	B-protein_state
IL	B-protein_type
-	I-protein_type
6	I-protein_type
in	O
each	O
sample	O
was	O
indicated	O
as	O
the	O
percentage	O
against	O
that	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
Regnase	B-protein
-	I-protein
1	I-protein
.	O

The	O
percentage	O
of	O
the	O
bound	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
was	O
calculated	O
based	O
on	O
the	O
fluorescence	B-evidence
intensities	I-evidence
of	O
the	O
free	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
quantified	O
in	O
(	O
f	O
).	O

Head	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
tail	I-protein_state
oligomer	B-oligomeric_state
formation	O
of	O
the	O
PIN	B-structure_element
domain	O
is	O
crucial	O
for	O
the	O
RNase	B-protein_type
activity	O
of	O
Regnase	B-protein
-	I-protein
1	I-protein
.	O

Two	O
PIN	B-structure_element
molecules	O
in	O
the	O
crystal	B-evidence
were	O
colored	O
white	O
and	O
green	O
,	O
respectively	O
.	O

Catalytic	B-site
residues	I-site
and	O
mutated	O
residues	O
were	O
shown	O
in	O
sticks	O
.	O

Residues	O
important	O
for	O
the	O
oligomeric	O
interaction	O
were	O
colored	O
red	O
,	O
while	O
R215	B-residue_name_number
that	O
was	O
dispensable	O
for	O
the	O
oligomeric	O
interaction	O
was	O
colored	O
blue	O
.	O
(	O
c	O
)	O
RNase	B-protein_type
activity	O
of	O
monomeric	B-oligomeric_state
mutants	B-protein_state
for	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
mRNA	B-chemical
was	O
analyzed	O
.	O

Domain	O
-	O
domain	O
interaction	O
between	O
the	O
NTD	B-structure_element
and	O
the	O
PIN	B-structure_element
domain	O
.	O

(	O
a	O
)	O
NMR	B-experimental_method
analyses	I-experimental_method
of	O
the	O
NTD	B-structure_element
-	O
binding	O
to	O
the	O
PIN	B-structure_element
domain	O
.	O

The	O
residues	O
with	O
significant	O
chemical	O
shift	O
changes	O
were	O
labeled	O
in	O
the	O
overlaid	B-experimental_method
spectra	B-evidence
(	O
left	O
)	O
and	O
colored	O
red	O
on	O
the	O
surface	O
and	O
ribbon	O
structure	O
of	O
the	O
PIN	B-structure_element
domain	O
(	O
right	O
).	O

Pro	B-residue_name
and	O
the	O
residues	O
without	O
analysis	O
were	O
colored	O
black	O
and	O
gray	O
,	O
respectively	O
.	O

(	O
b	O
)	O
NMR	B-experimental_method
analyses	I-experimental_method
of	O
the	O
PIN	B-structure_element
-	O
binding	O
to	O
the	O
NTD	B-structure_element
.	O

The	O
NTD	B-structure_element
and	O
the	O
PIN	B-structure_element
domain	O
are	O
shown	O
in	O
cyan	O
and	O
white	O
,	O
respectively	O
.	O

Catalytic	B-site
residues	I-site
of	O
the	O
PIN	B-structure_element
domain	O
are	O
shown	O
in	O
sticks	O
,	O
and	O
the	O
residues	O
that	O
exhibited	O
significant	B-evidence
chemical	I-evidence
shift	I-evidence
changes	I-evidence
in	O
(	O
a	O
,	O
b	O
)	O
were	O
labeled	O
.	O

(	O
a	O
)	O
In	B-experimental_method
vitro	I-experimental_method
cleavage	I-experimental_method
assay	I-experimental_method
of	O
basic	O
residue	O
mutants	B-protein_state
for	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
mRNA	B-chemical
.	O

(	O
b	O
)	O
In	B-experimental_method
vitro	I-experimental_method
cleavage	I-experimental_method
assay	I-experimental_method
of	O
basic	O
residue	O
mutants	B-protein_state
for	O
Regnase	B-protein
-	I-protein
1	I-protein
mRNA	B-chemical
.	O

The	O
fluorescence	B-evidence
intensity	I-evidence
of	O
the	O
uncleaved	B-protein_state
mRNA	B-chemical
was	O
quantified	O
and	O
the	O
results	O
were	O
mapped	O
on	O
the	O
PIN	B-structure_element
dimer	B-oligomeric_state
structure	B-evidence
.	O

Mutated	O
basic	O
residues	O
were	O
shown	O
in	O
sticks	O
and	O
those	O
with	O
significantly	O
reduced	O
RNase	B-protein_type
activities	O
were	O
colored	O
red	O
or	O
yellow	O
.	O

(	O
a	O
)	O
Cartoon	O
representation	O
of	O
the	O
concept	O
of	O
the	O
experiment	O
.	O
(	O
b	O
)	O
In	B-experimental_method
vitro	I-experimental_method
cleavage	I-experimental_method
assay	I-experimental_method
of	O
Regnase	B-protein
-	I-protein
1	I-protein
for	O
IL	B-protein_type
-	I-protein_type
6	I-protein_type
mRNA	B-chemical
.	O

(	O
c	O
)	O
In	B-experimental_method
vitro	I-experimental_method
cleavage	I-experimental_method
assay	I-experimental_method
of	O
Regnase	B-protein
-	I-protein
1	I-protein
for	O
Regnase	B-protein
-	I-protein
1	I-protein
mRNA	B-chemical
.	O

The	O
fluorescence	B-evidence
intensity	I-evidence
of	O
the	O
uncleaved	B-protein_state
mRNA	B-chemical
was	O
quantified	O
and	O
the	O
results	O
were	O
mapped	O
on	O
the	O
PIN	B-structure_element
dimer	B-oligomeric_state
.	O

The	O
mutations	O
whose	O
RNase	B-protein_type
activities	O
were	O
not	O
increased	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
DDNN	B-mutant
mutant	B-protein_state
were	O
colored	O
in	O
blue	O
on	O
the	O
primary	O
PIN	B-structure_element
.	O

Schematic	O
representation	O
of	O
regulation	O
of	O
the	O
Regnase	B-protein
-	I-protein
1	I-protein
catalytic	O
activity	O
through	O
the	O
domain	O
-	O
domain	O
interactions	O
.	O

RAD51	B-protein
forms	O
oligomers	B-oligomeric_state
by	O
binding	O
to	O
another	O
molecule	O
of	O
RAD51	B-protein
via	O
an	O
‘	O
FxxA	B-structure_element
’	O
motif	O
,	O
and	O
the	O
same	O
recognition	O
sequence	O
is	O
similarly	O
utilised	O
to	O
bind	O
BRCA2	B-protein
.	O

We	O
use	O
mutants	B-protein_state
of	O
a	O
tetrapeptide	B-chemical
sequence	O
to	O
probe	O
the	O
binding	O
interaction	O
,	O
using	O
both	O
isothermal	B-experimental_method
titration	I-experimental_method
calorimetry	I-experimental_method
and	O
X	B-experimental_method
‐	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
.	O

Where	O
possible	O
,	O
comparison	O
between	O
our	O
tetrapeptide	B-experimental_method
mutational	I-experimental_method
study	I-experimental_method
and	O
the	O
previously	O
reported	O
mutations	O
is	O
made	O
,	O
discrepancies	O
are	O
discussed	O
and	O
the	O
importance	O
of	O
secondary	O
structure	O
in	O
interpreting	O
alanine	B-experimental_method
scanning	I-experimental_method
and	O
mutational	O
data	O
of	O
this	O
nature	O
is	O
considered	O
.	O

Eukaryotic	B-taxonomy_domain
RAD51	B-protein
,	O
archeal	B-taxonomy_domain
RadA	B-protein
and	O
prokaryotic	B-taxonomy_domain
RecA	B-protein
are	O
a	O
family	O
of	O
ATP	B-protein_type
‐	I-protein_type
dependent	I-protein_type
recombinases	I-protein_type
involved	O
in	O
homologous	O
recombination	O
(	O
HR	O
)	O
of	O
double	O
‐	O
strand	O
breaks	O
in	O
DNA	O
1	O
.	O

BRCA2	B-protein
especially	O
has	O
garnered	O
much	O
attention	O
in	O
a	O
clinical	O
context	O
,	O
as	O
many	O
mutations	O
have	O
been	O
identified	O
that	O
drive	O
an	O
increased	O
risk	O
of	O
cancer	O
in	O
individuals	O
4	O
,	O
5	O
.	O

Although	O
the	O
inactivation	O
of	O
the	O
BRCA2	B-complex_assembly
:	I-complex_assembly
RAD51	I-complex_assembly
DNA	O
repair	O
pathway	O
can	O
cause	O
genomic	O
instability	O
and	O
eventual	O
tumour	O
development	O
,	O
an	O
inability	O
to	O
repair	O
breaks	O
in	O
DNA	O
may	O
also	O
engender	O
a	O
sensitivity	O
to	O
ionising	O
radiation	O
6	O
,	O
7	O
.	O

For	O
this	O
reason	O
it	O
is	O
hypothesised	O
that	O
in	O
tumour	O
cells	O
with	O
an	O
intact	O
BRCA2	B-complex_assembly
:	I-complex_assembly
RAD51	I-complex_assembly
repair	O
pathway	O
,	O
small	O
molecules	O
which	O
prevent	O
the	O
interaction	O
between	O
RAD51	B-protein
and	O
BRCA2	B-protein
may	O
confer	O
radiosensitivity	O
by	O
disabling	O
the	O
HR	O
pathway	O
8	O
.	O

The	O
interaction	O
between	O
the	O
two	O
proteins	O
is	O
mediated	O
by	O
eight	O
BRC	B-structure_element
repeats	I-structure_element
,	O
which	O
are	O
characterised	O
by	O
a	O
conserved	B-protein_state
‘	B-structure_element
FxxA	I-structure_element
’	I-structure_element
motif	I-structure_element
9	I-structure_element
.	O

RAD51	B-protein
and	O
RadA	B-protein
proteins	O
also	O
contain	O
an	O
‘	O
FxxA	B-structure_element
’	O
sequence	O
(	O
FTTA	B-structure_element
for	O
human	B-species
RAD51	B-protein
)	O
through	O
which	O
it	O
can	O
bind	O
to	O
other	O
RAD51	B-protein
and	O
RadA	B-protein
molecules	O
,	O
and	O
oligomerise	O
to	O
form	O
higher	O
order	O
filament	O
structures	O
on	O
DNA	O
.	O

The	O
common	O
FxxA	B-structure_element
motifs	O
of	O
both	O
the	O
BRC	B-structure_element
repeats	I-structure_element
and	O
RAD51	B-protein
oligomerisation	B-structure_element
sequence	I-structure_element
are	O
recognised	O
by	O
the	O
same	O
FxxA	B-site
‐	I-site
binding	I-site
site	I-site
of	O
RAD51	B-protein
.	O

In	O
general	O
,	O
the	O
dominant	O
contribution	O
of	O
certain	O
residues	O
to	O
the	O
overall	O
binding	B-evidence
energy	I-evidence
of	O
a	O
protein	O
–	O
protein	O
interaction	O
are	O
known	O
as	O
‘	O
hot	B-site
‐	I-site
spot	I-site
’	O
residues	O
.	O

Interestingly	O
,	O
small	O
molecule	O
inhibitors	O
of	O
PPIs	O
are	O
often	O
found	O
to	O
occupy	O
the	O
same	O
pockets	B-site
which	O
are	O
otherwise	O
occupied	O
by	O
hot	B-site
‐	I-site
spot	I-site
residues	O
in	O
the	O
native	B-protein_state
complex	O
.	O

It	O
is	O
therefore	O
of	O
great	O
interest	O
to	O
identify	O
hot	B-site
‐	I-site
spots	I-site
in	O
an	O
effort	O
to	O
guide	O
drug	O
discovery	O
efforts	O
against	O
a	O
PPI	O
.	O

Further	O
,	O
a	O
correlation	O
between	O
residues	O
that	O
are	O
strongly	B-protein_state
conserved	I-protein_state
and	O
hot	B-site
‐	I-site
spot	I-site
residues	O
has	O
been	O
reported	O
10	O
.	O

Purely	O
based	O
on	O
the	O
amino	O
acid	O
consensus	O
sequence	O
reported	O
by	O
Pellegrini	O
et	O
al	O
.,	O
11	O
phenylalanine	B-residue_name
and	O
alanine	B-residue_name
would	O
both	O
be	O
expected	O
to	O
be	O
hot	B-site
‐	I-site
spots	I-site
and	O
to	O
a	O
lesser	O
extent	O
,	O
threonine	B-residue_name
.	O

The	O
importance	O
of	O
residues	O
in	O
the	O
FxxA	B-structure_element
motif	O
has	O
been	O
probed	O
by	O
a	O
variety	O
of	O
techniques	O
,	O
collated	O
in	O
Table	O
1	O
.	O

Briefly	O
,	O
mutating	B-experimental_method
phenylalanine	B-residue_name
to	O
glutamic	B-residue_name
acid	I-residue_name
inactivated	B-protein_state
the	O
BRC4	B-chemical
peptide	O
and	O
prevented	O
RAD51	B-protein
oligomerisation	O
11	O
,	O
12	O
.	O

A	O
phenylalanine	B-protein_state
‐	I-protein_state
truncated	I-protein_state
BRC4	B-chemical
is	O
also	O
found	O
to	O
be	O
inactive	B-protein_state
13	O
,	O
however	O
,	O
introducing	B-experimental_method
a	O
tryptophan	B-residue_name
for	O
phenylalanine	B-residue_name
was	O
found	O
to	O
have	O
no	O
significant	O
effect	O
on	O
BRC4	B-chemical
affinity	B-evidence
12	O
.	O

A	O
glutamine	B-residue_name
replacing	B-experimental_method
the	O
histidine	B-residue_name
in	O
BRC4	B-chemical
maintains	O
BRC4	B-chemical
activity	O
13	O
.	O

Similarly	O
,	O
mutating	B-experimental_method
alanine	B-residue_name
to	O
glutamic	B-residue_name
acid	I-residue_name
in	O
the	O
RAD51	B-protein
oligomerisation	B-structure_element
sequence	I-structure_element
11	O
or	O
to	O
serine	B-residue_name
in	O
BRC4	B-chemical
13	O
leads	O
to	O
loss	O
of	O
interaction	O
in	O
both	O
cases	O
.	O

Mutations	O
identified	O
in	O
the	O
clinic	O
,	O
in	O
the	O
FxxA	B-structure_element
region	O
of	O
the	O
BRC	B-structure_element
repeats	I-structure_element
of	O
BRCA2	B-protein
are	O
collated	O
in	O
Table	O
1	O
14	O
.	O

For	O
completeness	O
,	O
we	O
present	O
them	O
here	O
with	O
this	O
caveat	O
,	O
and	O
to	O
make	O
the	O
comment	O
that	O
these	O
clinical	O
mutations	O
are	O
consistent	O
with	O
abrogating	O
the	O
interaction	O
with	O
RAD51	B-protein
.	O

Summary	O
of	O
FxxA	B-structure_element
‐	O
relevant	O
mutations	O
previously	O
reported	O
and	O
degree	O
of	O
characterisation	O
.	O

Mutation	O
contexta	O
Mutation	O
FxxA	B-structure_element
motif	O
Technique	O
used	O
Effect	O
RAD51	B-protein
(	O
FTTA	B-structure_element
)	O
F86E	B-mutant
ETTA	B-structure_element
Immunoprecipitation	B-experimental_method
11	O
No	O
binding	O
BRC4	B-chemical
(	O
FHTA	B-structure_element
)	O
F1524E	B-mutant
EHTA	B-structure_element
Competitive	B-experimental_method
ELISA	I-experimental_method
12	O
Peptide	O
inactive	B-protein_state
BRC4	B-chemical
(	O
FHTA	B-structure_element
)	O
F1524W	B-mutant
WHTA	B-structure_element
Competitive	B-experimental_method
ELISA	I-experimental_method
12	O
Comparable	O
activity	O
to	O
WT	B-protein_state
BRC4	B-chemical
(	O
FHTA	B-structure_element
)	O
F1524V	B-mutant
VHTA	B-structure_element
BRCA2	B-protein
mutations	O
database	O
14	O
–	O
BRC4	B-chemical
(	O
FHTA	B-structure_element
)	O
ΔF1524	B-mutant
‐	O
HTA	B-structure_element
Dissociation	O
of	O
RAD51	B-complex_assembly
‐	I-complex_assembly
DNA	I-complex_assembly
complex	O
13	O
Peptide	O
inactive	B-protein_state
BRC4	B-chemical
(	O
FHTA	B-structure_element
)	O
H1525Q	B-mutant
FQTA	B-structure_element
Dissociation	O
of	O
RAD51	B-complex_assembly
‐	I-complex_assembly
DNA	I-complex_assembly
complex	O
13	O
Comparable	O
activity	O
BRC7	B-chemical
(	O
FSTA	B-structure_element
)	O
S1979R	B-mutant
FRTA	B-structure_element
BRCA2	B-protein
mutations	O
database	O
14	O
–	O
BRC3	B-chemical
(	O
FQTA	B-structure_element
)	O
T1430A	B-mutant
FQAA	B-structure_element
RAD51	B-complex_assembly
:	I-complex_assembly
DNA	I-complex_assembly
bandshift	B-experimental_method
assay	I-experimental_method
3	O
Peptide	O
inactive	B-protein_state
BRC3	B-chemical
(	O
FQTA	B-structure_element
)	O
T1430A	B-mutant
FQAA	B-structure_element
Electron	B-experimental_method
microscopic	I-experimental_method
visualisation	O
of	O
nucleoprotein	O
filaments	O
3	O
Peptide	O
inactive	B-protein_state
BRC1	B-chemical
(	O
FRTA	B-structure_element
)	O
T1011R	B-mutant
FRRA	B-structure_element
BRCA2	B-protein
mutations	O
database	O
14	O
–	O
BRC2	B-chemical
(	O
FYSA	B-structure_element
)	O
S1221P	B-mutant
FYPA	B-structure_element
BRCA2	B-protein
mutations	O
database	O
14	O
–	O
BRC2	B-chemical
(	O
FYSA	B-structure_element
)	O
S1221Y	B-mutant
FYYA	B-structure_element
BRCA2	B-protein
mutations	O
database	O
14	O
–	O
RAD51	B-protein
(	O
FTTA	B-structure_element
)	O
A89E	B-mutant
FTTE	B-structure_element
Immunoprecipitation	B-experimental_method
11	O
No	O
binding	O
BRC4	B-chemical
(	O
FHTA	B-structure_element
)	O
A1527S	B-mutant
FHTS	B-structure_element
Dissociation	O
of	O
RAD51	B-complex_assembly
‐	I-complex_assembly
DNA	I-complex_assembly
complex	O
13	O
Peptide	O
inactive	B-protein_state

In	O
this	O
work	O
,	O
we	O
report	O
the	O
most	O
detailed	O
study	O
of	O
systematic	B-experimental_method
mutations	I-experimental_method
of	O
peptides	O
to	O
probe	O
the	O
FxxA	B-structure_element
‐	I-structure_element
binding	I-structure_element
motif	I-structure_element
to	O
date	O
.	O

We	O
have	O
chosen	O
to	O
focus	O
on	O
tetrapeptides	B-chemical
,	O
which	O
allows	O
us	O
to	O
examine	O
the	O
effect	O
of	O
mutation	B-experimental_method
on	O
the	O
fundamental	O
unit	O
of	O
binding	O
,	O
FxxA	B-structure_element
,	O
rather	O
than	O
in	O
the	O
context	O
of	O
either	O
the	O
BRC	B-structure_element
repeat	I-structure_element
or	O
self	B-structure_element
‐	I-structure_element
oligomerisation	I-structure_element
sequence	I-structure_element
.	O

The	O
use	O
of	O
ITC	B-experimental_method
is	O
generally	O
perceived	O
as	O
a	O
gold	O
‐	O
standard	O
in	O
protein	O
–	O
ligand	O
characterisation	O
,	O
rather	O
than	O
a	O
competitive	B-experimental_method
assay	I-experimental_method
which	O
may	O
be	O
prone	O
to	O
aggregation	O
artefacts	O
.	O

Conservation	O
of	O
FxxA	B-structure_element
motif	O
(	O
A	O
)	O
BRC4	B-chemical
peptide	O
(	O
green	O
cartoon	O
)	O
bound	B-protein_state
to	I-protein_state
truncated	B-protein_state
human	B-species
RAD51	B-protein
(	O
grey	O
surface	O
)	O
(	O
PDB	O
:	O
1n0w	O
,	O
11	O
).	O

The	O
blue	O
dashed	O
box	O
highlights	O
the	O
FxxA	B-site
interaction	I-site
pocket	I-site
.	O

(	O
B	O
)	O
Two	O
interacting	O
protein	O
molecules	O
of	O
RAD51	B-protein
from	O
Saccharomyces	B-species
cerevisiae	I-species
are	O
shown	O
.	O

The	O
N	O
‐	O
terminal	O
domain	O
of	O
one	O
RAD51	B-protein
protomer	B-oligomeric_state
is	O
highlighted	O
in	O
pink	O
for	O
clarity	O
and	O
the	O
green	O
arrow	O
indicates	O
the	O
location	O
of	O
this	O
protomer	B-oligomeric_state
'	O
s	O
FxxA	B-structure_element
oligomerisation	I-structure_element
sequence	I-structure_element
(	O
PDB	O
:	O
1szp	O
,	O
29	O
).	O
(	O
C	O
)	O
Conservation	O
of	O
FxxA	B-structure_element
motif	O
across	O
the	O
human	B-species
BRC	B-structure_element
repeats	I-structure_element
and	O
(	O
D	O
)	O
across	O
21	O
eukaryotic	B-taxonomy_domain
RAD51s	B-protein_type
and	O
24	O
RadAs	B-protein_type
,	O
with	O
the	O
size	O
of	O
the	O
letters	O
proportional	O
to	O
the	O
degree	O
of	O
conservation	O
.	O

We	O
have	O
mutated	B-experimental_method
and	I-experimental_method
truncated	I-experimental_method
the	O
tetrapeptide	B-chemical
epitope	O
FHTA	B-structure_element
,	O
and	O
examined	O
the	O
effects	O
both	O
structurally	O
and	O
on	O
the	O
binding	B-evidence
affinity	I-evidence
with	O
humanised	B-protein_state
RadA	B-protein
.	O
As	O
a	O
comparative	O
reference	O
,	O
we	O
are	O
using	O
the	O
FHTA	B-structure_element
sequence	O
derived	O
from	O
the	O
most	O
tightly	O
binding	O
BRC	B-structure_element
repeat	I-structure_element
,	O
BRC4	B-chemical
22	O
.	O

The	O
peptides	O
used	O
are	O
N	B-protein_state
‐	I-protein_state
acetylated	I-protein_state
and	O
C	O
‐	O
amide	O
terminated	O
in	O
order	O
to	O
provide	O
the	O
most	O
relevant	O
peptide	O
in	O
the	O
context	O
of	O
a	O
longer	O
peptide	O
chain	O
.	O

Phe1524	B-residue_name_number
of	O
BRC4	B-chemical
binds	O
in	O
a	O
small	O
surface	B-site
pocket	I-site
of	O
human	B-species
RAD51	B-protein
,	O
defined	O
by	O
the	O
hydrophobic	O
side	O
chains	O
of	O
residues	O
Met158	B-residue_name_number
,	O
Ile160	B-residue_name_number
,	O
Ala192	B-residue_name_number
,	O
Leu203	B-residue_name_number
and	O
Met210	B-residue_name_number
.	O

The	O
residue	O
is	O
highly	B-protein_state
conserved	I-protein_state
across	O
BRC	B-structure_element
repeats	I-structure_element
and	O
oligomerisation	B-structure_element
sequences	I-structure_element
.	O

Consistent	O
with	O
this	O
,	O
the	O
truncated	B-protein_state
HTA	B-structure_element
tripeptide	B-chemical
could	O
not	O
be	O
detected	O
to	O
bind	O
to	O
humanised	B-protein_state
,	O
monomeric	B-oligomeric_state
RadA	B-protein
,	O
HumRadA2	B-mutant
(	O
Table	O
2	O
,	O
entry	O
13	O
).	O

As	O
previously	O
discussed	O
,	O
there	O
is	O
some	O
evidence	O
that	O
substituting	B-experimental_method
a	O
tryptophan	B-residue_name
for	O
the	O
phenylalanine	B-residue_name
at	O
this	O
position	O
was	O
tolerated	O
in	O
the	O
context	O
of	O
BRC4	B-chemical
12	O
.	O

Therefore	O
,	O
the	O
WHTA	B-structure_element
peptide	O
was	O
tested	O
and	O
found	O
to	O
not	O
only	O
be	O
tolerated	O
,	O
but	O
to	O
increase	O
the	O
binding	B-evidence
affinity	I-evidence
of	O
the	O
peptide	O
approximately	O
threefold	O
.	O

The	O
second	O
position	O
of	O
the	O
tetrapeptide	B-chemical
was	O
found	O
to	O
be	O
largely	O
invariant	O
to	O
changes	O
in	O
the	O
side	O
chains	O
that	O
were	O
investigated	O
.	O

Replacing	B-experimental_method
the	O
histidine	B-residue_name
with	O
an	O
asparagine	B-residue_name
,	O
chosen	O
to	O
potentially	O
mimic	O
the	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
donor	O
–	O
acceptor	O
nature	O
of	O
histidine	B-residue_name
,	O
resulted	O
in	O
a	O
moderate	O
,	O
twofold	O
decrease	O
in	O
potency	O
(	O
Table	O
2	O
,	O
entry	O
4	O
).	O

Mutating	B-experimental_method
to	O
an	O
alanine	B-residue_name
,	O
recapitulated	O
the	O
potency	O
of	O
FHTA	B-structure_element
,	O
implying	O
that	O
the	O
interactions	O
made	O
by	O
histidine	B-residue_name
do	O
not	O
contribute	O
overall	O
to	O
binding	B-evidence
affinity	I-evidence
(	O
Table	O
2	O
,	O
entry	O
3	O
).	O

Modelling	O
suggests	O
that	O
a	O
proline	B-residue_name
in	O
the	O
second	O
position	O
would	O
be	O
expected	O
to	O
clash	O
sterically	O
with	O
the	O
surface	O
of	O
the	O
protein	O
,	O
and	O
provides	O
a	O
rationale	O
for	O
the	O
lack	O
of	O
binding	O
observed	O
.	O

Threonine	B-residue_name
was	O
mutated	B-experimental_method
to	O
an	O
alanine	B-residue_name
,	O
resulting	O
in	O
only	O
a	O
moderately	O
weaker	O
K	B-evidence
D	I-evidence
(	O
twofold	O
,	O
Table	O
2	O
,	O
entry	O
7	O
).	O

In	O
the	O
context	O
of	O
a	O
tetrapeptide	B-chemical
at	O
least	O
,	O
this	O
result	O
implies	O
a	O
lack	O
of	O
importance	O
of	O
a	O
threonine	B-residue_name
at	O
this	O
position	O
.	O

While	O
the	O
importance	O
of	O
the	O
phenylalanine	B-residue_name
may	O
be	O
possible	O
to	O
predict	O
from	O
examination	O
of	O
the	O
crystal	B-evidence
structure	I-evidence
,	O
the	O
alanine	B-residue_name
appears	O
to	O
be	O
of	O
much	O
less	O
importance	O
in	O
this	O
regard	O
.	O

It	O
is	O
,	O
however	O
,	O
a	O
highly	B-protein_state
conserved	I-protein_state
residue	O
and	O
clearly	O
of	O
interest	O
for	O
systematic	O
mutation	O
.	O

Removing	B-experimental_method
the	O
alanine	B-residue_name
residue	O
entirely	O
produced	O
the	O
truncated	B-protein_state
tripeptide	B-chemical
FHT	B-structure_element
,	O
which	O
did	O
not	O
bind	O
(	O
Table	O
2	O
,	O
entry	O
12	O
).	O

The	O
unnatural	O
amino	O
acid	O
,	O
α	B-chemical
‐	I-chemical
amino	I-chemical
butyric	I-chemical
acid	I-chemical
(	O
U	B-chemical
),	O
was	O
introduced	O
at	O
the	O
fourth	O
position	O
,	O
positioning	O
an	O
ethyl	O
group	O
into	O
the	O
alanine	B-site
pocket	I-site
(	O
Table	O
2	O
,	O
entry	O
9	O
).	O

Structural	B-experimental_method
characterisation	I-experimental_method
of	O
peptide	O
complexes	O

The	O
corresponding	O
PDB	O
codes	O
are	O
indicated	O
in	O
Table	O
2	O
and	O
crystallographic	B-evidence
data	I-evidence
are	O
found	O
in	O
the	O
Supporting	O
Information	O
.	O

WHTA	B-structure_element
peptide	O
shows	O
a	O
relative	O
dislocation	O
when	O
compared	O
to	O
FHTA	B-structure_element
(	O
Fig	O
2A	O
),	O
with	O
the	O
entire	O
ligand	O
backbone	O
of	O
WHTA	B-structure_element
shifted	O
to	O
accommodate	O
the	O
change	O
in	O
the	O
position	O
of	O
the	O
main	O
chain	O
carbon	O
of	O
the	O
first	O
residue	O
,	O
as	O
the	O
larger	O
indole	O
side	O
chain	O
fills	O
the	O
Phe	B-site
pocket	I-site
.	O

This	O
shift	O
is	O
translated	O
all	O
the	O
way	O
to	O
the	O
alanine	B-residue_name
side	O
chain	O
.	O

It	O
is	O
possible	O
that	O
this	O
mutation	B-experimental_method
is	O
beneficial	O
in	O
the	O
tetrapeptide	B-chemical
context	O
and	O
neutral	O
in	O
the	O
full	B-protein_state
‐	I-protein_state
length	I-protein_state
BRC4	B-chemical
context	O
because	O
the	O
smaller	O
peptide	O
is	O
less	O
constrained	O
and	O
allowed	O
to	O
explore	O
more	O
conformations	O
.	O

An	O
attempt	O
to	O
combine	O
both	O
the	O
tryptophan	B-residue_name
and	O
proline	B-residue_name
mutations	B-experimental_method
,	O
however	O
,	O
led	O
to	O
no	O
improvement	O
for	O
WHPA	B-structure_element
peptide	O
compared	O
to	O
FHTA	B-structure_element
.	O

One	O
possible	O
explanation	O
is	O
that	O
the	O
‘	O
shifted	O
’	O
binding	O
mode	O
observed	O
in	O
WHTA	B-structure_element
was	O
not	O
compatible	O
with	O
the	O
conformational	O
restriction	O
that	O
the	O
proline	B-residue_name
of	O
WHPA	B-structure_element
introduced	O
.	O

Comparison	O
of	O
different	O
peptide	O
complexes	O
(	O
A	O
)	O
Overlay	B-experimental_method
with	O
FHTA	B-structure_element
(	O
grey	O
)	O
and	O
WHTA	B-structure_element
(	O
purple	O
)	O
showing	O
a	O
small	O
relative	O
displacement	O
of	O
the	O
peptide	O
backbone	O
.	O
(	O
B	O
)	O
Superposition	B-experimental_method
of	O
FHTA	B-structure_element
(	O
grey	O
)	O
and	O
FHPA	B-structure_element
(	O
yellow	O
),	O
showing	O
conservation	O
of	O
backbone	O
orientation	O
(	O
C	O
)	O
Overlay	B-experimental_method
of	O
FHTU	B-structure_element
(	O
green	O
),	O
FHTA	B-structure_element
(	O
grey	O
)	O
and	O
FHTG	B-structure_element
(	O
cyan	O
).	O

Although	O
ΔH	B-evidence
and	O
ΔS	B-evidence
are	O
tabulated	O
,	O
the	O
K	B-evidence
Ds	I-evidence
measured	O
are	O
relatively	O
weak	O
and	O
necessarily	O
performed	O
under	O
low	O
c	O
‐	O
value	O
conditions	O
.	O

In	O
this	O
experimental	O
regime	O
,	O
nonsigmoidal	O
curves	O
are	O
generated	O
and	O
therefore	O
errors	O
in	O
ΔH	B-evidence
are	O
expected	O
to	O
be	O
much	O
higher	O
than	O
the	O
errors	O
from	O
model	O
fitting	O
given	O
in	O
Table	O
2	O
16	O
.	O

Such	O
effects	O
have	O
been	O
discussed	O
by	O
Klebe	O
24	O
and	O
Chodera	O
and	O
Mobley	O
25	O
and	O
will	O
frustrate	O
attempts	O
to	O
interpret	O
the	O
measured	O
ΔΔH	B-evidence
and	O
ΔΔS	B-evidence
.	O

The	O
conserved	B-protein_state
phenylalanine	B-residue_name
and	O
alanine	B-residue_name
residues	O
of	O
the	O
FHTA	B-structure_element
sequence	O
were	O
both	O
found	O
to	O
be	O
essential	O
for	O
binding	O
by	O
ITC	B-experimental_method
.	O

Conversely	O
the	O
second	O
position	O
histidine	B-residue_name
residue	O
,	O
corresponding	O
to	O
the	O
unconserved	B-protein_state
His1525	B-residue_name_number
in	O
the	O
BRC4	B-chemical
sequence	O
,	O
could	O
be	O
mutated	B-experimental_method
without	O
significant	O
effect	O
on	O
the	O
peptide	B-evidence
affinity	I-evidence
.	O

The	O
more	O
general	O
correlation	O
between	O
hot	B-site
‐	I-site
spot	I-site
residues	O
in	O
protein	O
–	O
protein	O
interactions	O
and	O
the	O
high	B-protein_state
conservation	I-protein_state
of	O
such	O
residues	O
has	O
been	O
previously	O
reported	O
10	O
,	O
26	O
.	O

Interestingly	O
,	O
however	O
,	O
the	O
highly	B-protein_state
conserved	I-protein_state
threonine	B-residue_name
residue	O
could	O
be	O
mutated	B-experimental_method
without	O
affecting	O
the	O
peptide	B-evidence
affinity	I-evidence
.	O

This	O
unexpected	O
result	O
,	O
in	O
the	O
light	O
of	O
its	O
very	O
high	B-protein_state
conservation	I-protein_state
in	O
the	O
BRC	B-structure_element
and	O
oligomerisation	B-structure_element
sequences	I-structure_element
,	O
begs	O
the	O
question	O
of	O
what	O
the	O
role	O
of	O
Thr1526	B-residue_name_number
is	O
and	O
highlights	O
a	O
potential	O
pitfall	O
and	O
need	O
for	O
caution	O
in	O
the	O
experimental	O
design	O
of	O
alanine	B-experimental_method
mutation	I-experimental_method
studies	I-experimental_method
.	O

As	O
the	O
FHTA	B-structure_element
peptide	B-chemical
is	O
potentially	O
a	O
surrogate	O
peptide	O
for	O
both	O
the	O
BRC	B-structure_element
repeat	I-structure_element
peptides	O
and	O
the	O
RAD51	B-protein
self	B-structure_element
‐	I-structure_element
oligomerisation	I-structure_element
peptide	I-structure_element
,	O
it	O
is	O
useful	O
to	O
examine	O
the	O
role	O
of	O
Thr1526	B-residue_name_number
(	O
BRC4	B-chemical
)	O
and	O
the	O
analogous	O
Thr87	B-residue_name_number
(	O
RAD51	B-protein
)	O
in	O
both	O
binding	O
contexts	O
in	O
more	O
detail	O
.	O

Also	O
,	O
Thr1520	B-residue_name_number
is	O
constrained	O
by	O
crystal	O
contacts	O
in	O
this	O
structure	B-evidence
.	O

Lack	B-protein_state
of	I-protein_state
conservation	I-protein_state
of	O
this	O
residue	O
supports	O
the	O
idea	O
that	O
this	O
interaction	O
is	O
not	O
crucial	O
for	O
RAD51	B-complex_assembly
:	I-complex_assembly
BRC	I-complex_assembly
repeat	I-complex_assembly
binding	O
.	O

Residue	O
numbering	O
relates	O
to	O
the	O
S	B-species
.	I-species
cerevisiae	I-species
RAD51	B-protein
protein	O
,	O
the	O
corresponding	O
human	B-species
residues	O
are	O
in	O
parentheses	O
.	O

With	O
respect	O
to	O
understanding	O
the	O
RAD51	B-complex_assembly
:	I-complex_assembly
RAD51	I-complex_assembly
interaction	O
,	O
no	O
human	B-species
crystal	B-evidence
structure	I-evidence
has	O
been	O
published	O
,	O
however	O
,	O
several	O
oligomeric	O
structures	B-evidence
of	O
archaeal	B-taxonomy_domain
RadA	B-protein
as	O
well	O
that	O
of	O
Saccharomyces	B-species
cerevisiae	I-species
RAD51	B-protein
have	O
been	O
reported	O
27	O
,	O
28	O
,	O
29	O
.	O

Figure	O
3B	O
shows	O
a	O
highlight	O
of	O
the	O
FxxA	B-structure_element
portion	O
of	O
oligomerisation	B-structure_element
peptide	I-structure_element
from	O
the	O
S	B-species
.	I-species
cerevisiae	I-species
RAD51	B-protein
structure	B-evidence
,	O
with	O
residues	O
in	O
parentheses	O
corresponding	O
to	O
the	O
human	B-species
RAD51	B-protein
protein	O
.	O

In	O
both	O
structural	O
contexts	O
,	O
the	O
role	O
of	O
the	O
third	O
position	O
threonine	B-residue_name
in	O
FxxA	B-structure_element
seems	O
to	O
be	O
in	O
stabilising	O
secondary	O
structure	O
;	O
a	O
β	B-structure_element
‐	I-structure_element
turn	I-structure_element
in	O
the	O
case	O
of	O
BRC	B-structure_element
binding	O
and	O
an	O
α	B-structure_element
‐	I-structure_element
helix	I-structure_element
in	O
the	O
case	O
of	O
RAD51	B-protein
oligomerisation	O
.	O

In	O
the	O
tetrapeptide	B-chemical
context	O
these	O
secondary	O
interactions	O
are	O
not	O
present	O
and	O
mutation	B-experimental_method
of	O
threonine	B-residue_name
to	O
alanine	B-residue_name
would	O
be	O
expected	O
to	O
have	O
little	O
effect	O
on	O
affinity	B-evidence
.	O

In	O
line	O
with	O
this	O
,	O
although	O
we	O
observe	O
a	O
slight	O
twofold	O
weakening	O
of	O
peptide	B-evidence
affinity	I-evidence
,	O
the	O
effect	O
is	O
far	O
from	O
being	O
as	O
drastic	O
or	O
inactivating	O
as	O
reported	O
in	O
longer	O
peptide	O
backgrounds	O
3	O
.	O
It	O
would	O
be	O
interesting	O
to	O
investigate	O
the	O
importance	O
of	O
this	O
residue	O
in	O
the	O
context	O
of	O
the	O
BRC4	B-chemical
peptide	O
,	O
and	O
the	O
oligomerisation	B-structure_element
peptide	I-structure_element
.	O

Rather	O
than	O
indifference	O
to	O
alanine	B-residue_name
mutation	B-experimental_method
,	O
a	O
significant	O
effect	O
,	O
via	O
lack	O
of	O
secondary	O
structure	O
stabilisation	O
,	O
would	O
be	O
predicted	O
,	O
as	O
indeed	O
has	O
been	O
reported	O
for	O
BRC3	B-chemical
3	O
.	O

Proline	B-residue_name
at	O
the	O
third	O
position	O
similarly	O
improved	O
potency	O
.	O

Activity	O
was	O
lost	O
by	O
mutating	B-experimental_method
the	O
terminal	O
alanine	B-residue_name
to	O
glycine	B-residue_name
,	O
but	O
recovered	O
somewhat	O
with	O
the	O
novel	O
α	B-chemical
‐	I-chemical
amino	I-chemical
butyric	I-chemical
acid	I-chemical
(	O
U	B-chemical
).	O

Threonine	B-residue_name
was	O
found	O
to	O
be	O
relatively	O
unimportant	O
in	O
the	O
tetrapeptides	B-chemical
but	O
has	O
been	O
previously	O
reported	O
to	O
be	O
crucial	O
in	O
the	O
context	O
of	O
BRC3	B-chemical
.	O

This	O
may	O
lead	O
to	O
a	O
more	O
general	O
caution	O
,	O
that	O
hot	B-site
‐	I-site
spot	I-site
data	O
should	O
be	O
interpreted	O
by	O
considering	O
the	O
bound	O
interaction	O
with	O
the	O
protein	O
,	O
as	O
well	O
as	O
the	O
potential	O
role	O
in	O
stabilising	O
the	O
bound	O
peptide	O
secondary	O
structure	O
.	O

In	O
either	O
case	O
,	O
the	O
requirement	O
for	O
structural	O
data	O
in	O
correctly	O
interpreting	O
alanine	B-experimental_method
‐	I-experimental_method
scanning	I-experimental_method
experiments	I-experimental_method
is	O
reinforced	O
.	O

Summary	O
of	O
key	O
observations	O
(	O
A	O
)	O
FxxA	B-structure_element
motif	O
sequence	O
conservation	O
of	O
Rad51	B-protein
oligomerisation	O
sequences	O
and	O
BRC	B-structure_element
repeats	I-structure_element
.	O
(	O
B	O
)	O
Highlight	O
of	O
SAR	O
identified	O
for	O
the	O
tetrapeptide	B-chemical
.	O

Purple	O
carbon	O
is	O
WHTA	B-structure_element
,	O
light	O
blue	O
is	O
FATA	B-structure_element
,	O
yellow	O
is	O
FHPA	B-structure_element
,	O
cyan	O
is	O
FHTG	B-structure_element
and	O
grey	O
carbon	O
is	O
FHTA	B-structure_element
.	O

Note	O
the	O
C	O
‐	O
terminal	O
amide	O
changes	O
position	O
in	O
FHTG	B-structure_element
without	O
the	O
anchoring	O
methyl	O
group	O
.	O

Crystal	B-evidence
Structure	I-evidence
and	O
Activity	B-experimental_method
Studies	I-experimental_method
of	O
the	O
C11	B-protein_type
Cysteine	B-protein_type
Peptidase	I-protein_type
from	O
Parabacteroides	B-species
merdae	I-species
in	O
the	O
Human	B-species
Gut	O
Microbiome	O
*	O

Clan	B-protein_type
CD	I-protein_type
cysteine	I-protein_type
peptidases	I-protein_type
,	O
a	O
structurally	O
related	O
group	O
of	O
peptidases	B-protein_type
that	O
include	O
mammalian	B-taxonomy_domain
caspases	B-protein_type
,	O
exhibit	O
a	O
wide	O
range	O
of	O
important	O
functions	O
,	O
along	O
with	O
a	O
variety	O
of	O
specificities	O
and	O
activation	O
mechanisms	O
.	O

However	O
,	O
for	O
the	O
clostripain	B-protein_type
family	I-protein_type
(	O
denoted	O
C11	B-protein_type
),	O
little	O
is	O
currently	O
known	O
.	O

PmC11	B-protein
is	O
a	O
monomeric	B-oligomeric_state
cysteine	B-protein_type
peptidase	I-protein_type
that	O
comprises	O
an	O
extended	B-structure_element
caspase	I-structure_element
-	I-structure_element
like	I-structure_element
α	I-structure_element
/	I-structure_element
β	I-structure_element
/	I-structure_element
α	I-structure_element
sandwich	I-structure_element
and	O
an	O
unusual	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
.	O

It	O
shares	O
core	O
structural	O
elements	O
with	O
clan	B-protein_type
CD	I-protein_type
cysteine	I-protein_type
peptidases	I-protein_type
but	O
otherwise	O
structurally	O
differs	O
from	O
the	O
other	O
families	O
in	O
the	O
clan	O
.	O

Collectively	O
,	O
these	O
data	O
provide	O
insights	O
into	O
the	O
mechanism	O
and	O
activity	O
of	O
PmC11	B-protein
and	O
a	O
detailed	O
framework	O
for	O
studies	O
on	O
C11	B-protein_type
peptidases	I-protein_type
from	O
other	O
phylogenetic	O
kingdoms	O
.	O

In	O
the	O
MEROPS	O
peptidase	O
database	O
,	O
clan	B-protein_type
CD	I-protein_type
contains	O
groups	O
(	O
or	O
families	O
)	O
of	O
cysteine	B-protein_type
peptidases	I-protein_type
that	O
share	O
some	O
highly	B-protein_state
conserved	I-protein_state
structural	O
elements	O
.	O

Although	O
seven	O
families	O
(	O
C14	O
is	O
additionally	O
split	O
into	O
three	O
subfamilies	O
)	O
have	O
been	O
described	O
for	O
this	O
clan	O
,	O
crystal	B-evidence
structures	I-evidence
have	O
only	O
been	O
determined	O
from	O
four	O
:	O
legumain	B-protein
(	O
C13	B-protein_type
),	O
caspase	B-protein
(	O
C14a	B-protein_type
),	O
paracaspase	B-protein
(	O
C14b	B-protein_type
(	I-protein_type
P	I-protein_type
),	O
metacaspase	B-protein
(	O
C14b	B-protein_type
(	I-protein_type
M	I-protein_type
),	O
gingipain	B-protein
(	O
C25	B-protein_type
),	O
and	O
the	O
cysteine	B-structure_element
peptidase	I-structure_element
domain	I-structure_element
(	O
CPD	B-structure_element
)	O
of	O
various	O
toxins	O
(	O
C80	B-protein_type
).	O

No	O
structural	O
information	O
is	O
available	O
for	O
clostripain	B-protein
(	O
C11	B-protein_type
),	O
separase	B-protein
(	O
C50	B-protein_type
),	O
or	O
PrtH	B-protein
-	I-protein
peptidase	I-protein
(	O
C85	B-protein_type
).	O

Clan	B-protein_type
CD	I-protein_type
enzymes	I-protein_type
have	O
a	O
highly	B-protein_state
conserved	I-protein_state
His	B-site
/	I-site
Cys	I-site
catalytic	I-site
dyad	I-site
and	O
exhibit	O
strict	O
specificity	O
for	O
the	O
P1	B-residue_number
residue	O
of	O
their	O
substrates	O
.	O

Interestingly	O
,	O
little	O
is	O
known	O
about	O
the	O
structure	O
or	O
function	O
of	O
the	O
C11	B-protein_type
proteins	O
,	O
despite	O
their	O
widespread	O
distribution	O
and	O
its	O
archetypal	O
member	O
,	O
clostripain	B-protein
from	O
Clostridium	B-species
histolyticum	I-species
,	O
first	O
reported	O
in	O
the	O
literature	O
in	O
1938	O
.	O

As	O
part	O
of	O
an	O
ongoing	O
project	O
to	O
characterize	O
commensal	O
bacteria	B-taxonomy_domain
in	O
the	O
microbiome	O
that	O
inhabit	O
the	O
human	B-species
gut	O
,	O
the	O
structure	B-evidence
of	O
C11	B-protein_type
peptidase	I-protein_type
,	O
PmC11	B-protein
,	O
from	O
Parabacteroides	B-species
merdae	I-species
was	O
determined	O
using	O
the	O
Joint	O
Center	O
for	O
Structural	O
Genomics	O
(	O
JCSG	O
)	O
4	O
HTP	O
structural	O
biology	O
pipeline	O
.	O

The	O
structure	B-experimental_method
was	I-experimental_method
analyzed	I-experimental_method
,	O
and	O
the	O
enzyme	O
was	O
biochemically	B-experimental_method
characterized	I-experimental_method
to	O
provide	O
the	O
first	O
structure	O
/	O
function	O
correlation	O
for	O
a	O
C11	B-protein_type
peptidase	I-protein_type
.	O

Structure	B-evidence
of	O
PmC11	B-protein

The	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
catalytically	B-protein_state
active	I-protein_state
form	O
of	O
PmC11	B-protein
revealed	O
an	O
extended	B-structure_element
caspase	I-structure_element
-	I-structure_element
like	I-structure_element
α	I-structure_element
/	I-structure_element
β	I-structure_element
/	I-structure_element
α	I-structure_element
sandwich	I-structure_element
architecture	O
comprised	O
of	O
a	O
central	O
nine	B-structure_element
-	I-structure_element
stranded	I-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
,	O
with	O
an	O
unusual	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
(	O
CTD	B-structure_element
),	O
starting	O
at	O
Lys250	B-residue_name_number
.	O

The	O
central	O
nine	B-structure_element
-	I-structure_element
stranded	I-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
(	O
β1	B-structure_element
–	I-structure_element
β9	I-structure_element
)	O
of	O
PmC11	B-protein
consists	O
of	O
six	O
parallel	B-structure_element
and	O
three	O
anti	B-structure_element
-	I-structure_element
parallel	I-structure_element
β	I-structure_element
-	I-structure_element
strands	I-structure_element
with	O
4	O
↑	O
3	O
↓	O
2	O
↑	O
1	O
↑	O
5	O
↑	O
6	O
↑	O
7	O
↓	O
8	O
↓	O
9	O
↑	O
topology	O
(	O
Fig	O
.	O
1A	O
)	O
and	O
the	O
overall	O
structure	B-evidence
includes	O
14	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
with	O
six	O
(	O
α1	B-structure_element
–	I-structure_element
α2	I-structure_element
and	O
α4	B-structure_element
–	I-structure_element
α7	I-structure_element
)	O
closely	O
surrounding	O
the	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
in	O
an	O
approximately	O
parallel	O
orientation	O
.	O

Helices	B-structure_element
α1	B-structure_element
,	O
α7	B-structure_element
,	O
and	O
α6	B-structure_element
are	O
located	O
on	O
one	O
side	O
of	O
the	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
with	O
α2	B-structure_element
,	O
α4	B-structure_element
,	O
and	O
α5	B-structure_element
on	O
the	O
opposite	O
side	O
(	O
Fig	O
.	O
1A	O
).	O

Helix	B-structure_element
α3	B-structure_element
sits	O
at	O
the	O
end	O
of	O
the	O
loop	B-structure_element
following	O
β5	B-structure_element
(	O
L5	B-structure_element
),	O
just	O
preceding	O
the	O
Lys147	B-residue_name_number
cleavage	B-site
site	I-site
,	O
with	O
both	O
L5	B-structure_element
and	O
α3	B-structure_element
pointing	O
away	O
from	O
the	O
central	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
and	O
toward	O
the	O
CTD	B-structure_element
,	O
which	O
starts	O
with	O
α8	B-structure_element
.	O

A	O
,	O
primary	B-experimental_method
sequence	I-experimental_method
alignment	I-experimental_method
of	O
PmC11	B-protein
(	O
Uniprot	O
ID	O
A7A9N3	O
)	O
and	O
clostripain	B-protein
(	O
Uniprot	O
ID	O
P09870	O
)	O
from	O
C	B-species
.	I-species
histolyticum	I-species
with	O
identical	O
residues	O
highlighted	O
in	O
gray	O
shading	O
.	O

Connecting	O
loops	B-structure_element
are	O
colored	O
gray	O
,	O
the	O
main	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
is	O
in	O
orange	O
,	O
with	O
other	O
strands	O
in	O
olive	O
,	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
are	O
in	O
blue	O
,	O
and	O
the	O
nonapeptide	B-structure_element
linker	I-structure_element
of	O
clostripain	B-protein
that	O
is	O
excised	O
upon	O
autocleavage	B-ptm
is	O
underlined	O
in	O
red	O
.	O

Sequences	O
around	O
the	O
catalytic	B-site
site	I-site
of	O
clostripain	B-protein
and	O
PmC11	B-protein
align	O
well	O
.	O

The	O
position	O
of	O
the	O
catalytic	B-site
dyad	I-site
(	O
H	B-residue_name
,	O
C	B-residue_name
)	O
and	O
the	O
processing	B-site
site	I-site
(	O
Lys147	B-residue_name_number
)	O
are	O
highlighted	O
.	O

Helices	O
(	O
1	O
–	O
14	O
)	O
and	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
(	O
1	O
–	O
9	O
and	O
A	O
-	O
F	O
)	O
are	O
numbered	O
from	O
the	O
N	O
terminus	O
.	O

The	O
core	B-structure_element
caspase	I-structure_element
-	I-structure_element
fold	I-structure_element
is	O
highlighted	O
in	O
a	O
box	O
.	O

C	O
,	O
tertiary	O
structure	O
of	O
PmC11	B-protein
.	O

Loops	O
are	O
colored	O
gray	O
,	O
the	O
main	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
is	O
in	O
orange	O
,	O
with	O
other	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
in	O
yellow	O
,	O
and	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
are	O
in	O
blue	O
.	O

The	O
CTD	B-structure_element
of	O
PmC11	B-protein
is	O
composed	O
of	O
a	O
tight	B-structure_element
helical	I-structure_element
bundle	I-structure_element
formed	O
from	O
helices	B-structure_element
α8	B-structure_element
–	I-structure_element
α14	I-structure_element
and	O
includes	O
strands	B-structure_element
βC	B-structure_element
and	O
βF	B-structure_element
,	O
and	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
βD	B-structure_element
–	I-structure_element
βE	I-structure_element
.	O
The	O
CTD	B-structure_element
sits	O
entirely	O
on	O
one	O
side	O
of	O
the	O
enzyme	O
interacting	O
only	O
with	O
α3	B-structure_element
,	O
α5	B-structure_element
,	O
β9	B-structure_element
,	O
and	O
the	O
loops	B-structure_element
surrounding	O
β8	B-structure_element
.	O

This	B-structure_element
helix	I-structure_element
makes	O
a	O
total	O
of	O
eight	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
the	O
CTD	B-structure_element
,	O
including	O
one	O
salt	B-bond_interaction
bridge	I-bond_interaction
(	O
Arg191	B-residue_name_number
-	O
Asp255	B-residue_name_number
)	O
and	O
is	O
surrounded	O
by	O
the	O
CTD	B-structure_element
on	O
one	O
side	O
and	O
the	O
main	B-structure_element
core	I-structure_element
of	O
the	O
enzyme	O
on	O
the	O
other	O
,	O
acting	O
like	O
a	O
linchpin	O
holding	O
both	O
components	O
together	O
(	O
Fig	O
.	O
1C	O
).	O

The	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
is	O
unique	O
to	O
PmC11	B-protein
within	O
clan	B-protein_type
CD	I-protein_type
and	O
structure	B-experimental_method
comparisons	I-experimental_method
for	O
this	B-structure_element
domain	I-structure_element
alone	I-structure_element
does	O
not	O
produce	O
any	O
hits	O
in	O
the	O
PDB	O
(	O
DaliLite	B-experimental_method
,	O
PDBeFold	B-experimental_method
),	O
suggesting	O
a	O
completely	O
novel	O
fold	O
.	O

As	O
the	O
archetypal	O
and	O
arguably	O
most	O
well	O
studied	O
member	O
of	O
clan	B-protein_type
CD	I-protein_type
,	O
the	O
caspases	B-protein_type
were	O
used	O
as	O
the	O
basis	O
to	O
investigate	O
the	O
structure	O
/	O
function	O
relationships	O
in	O
PmC11	B-protein
,	O
with	O
caspase	B-protein
-	I-protein
7	I-protein
as	O
the	O
representative	O
member	O
.	O

Six	O
of	O
the	O
central	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
in	O
PmC11	B-protein
(	O
β1	B-structure_element
–	I-structure_element
β2	I-structure_element
and	O
β5	B-structure_element
–	I-structure_element
β8	I-structure_element
)	O
share	O
the	O
same	O
topology	O
as	O
the	O
six	B-structure_element
-	I-structure_element
stranded	I-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
found	O
in	O
caspases	B-protein_type
,	O
with	O
strands	B-structure_element
β3	B-structure_element
,	O
β4	B-structure_element
,	O
and	O
β9	B-structure_element
located	O
on	O
the	O
outside	O
of	O
this	O
core	B-structure_element
structure	I-structure_element
(	O
Fig	O
.	O
1B	O
,	O
box	O
).	O

A	O
multiple	B-experimental_method
sequence	I-experimental_method
alignment	I-experimental_method
of	O
C11	B-protein_type
proteins	O
revealed	O
that	O
these	O
residues	O
are	O
highly	B-protein_state
conserved	I-protein_state
(	O
data	O
not	O
shown	O
).	O

Biochemical	B-experimental_method
and	I-experimental_method
structural	I-experimental_method
characterization	I-experimental_method
of	O
PmC11	B-protein
.	O

A	O
,	O
ribbon	O
representation	O
of	O
the	O
overall	O
structure	O
of	O
PmC11	B-protein
illustrating	O
the	O
catalytic	B-site
site	I-site
,	O
cleavage	O
site	O
displacement	O
,	O
and	O
potential	O
S1	B-site
binding	I-site
site	I-site
.	O

The	O
two	O
ends	O
of	O
the	O
autolytic	B-site
cleavage	I-site
site	I-site
(	O
Lys147	B-residue_name_number
and	O
Ala148	B-residue_name_number
,	O
green	O
)	O
are	O
displaced	O
by	O
19	O
.	O
5	O
Å	O
(	O
thin	O
black	O
line	O
)	O
from	O
one	O
another	O
and	O
residues	O
in	O
the	O
potential	O
substrate	B-site
binding	I-site
pocket	I-site
are	O
highlighted	O
in	O
blue	O
.	O

B	O
,	O
size	B-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
of	O
PmC11	B-protein
.	O

PmC11	O
migrates	O
as	O
a	O
monomer	B-oligomeric_state
with	O
a	O
molecular	O
mass	O
around	O
41	O
kDa	O
calculated	O
from	O
protein	O
standards	O
of	O
known	O
molecular	O
weights	O
.	O

Elution	O
fractions	O
across	O
the	O
major	O
peak	O
(	O
1	O
–	O
6	O
)	O
were	O
analyzed	O
by	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
on	O
a	O
4	O
–	O
12	O
%	O
gel	O
in	O
MES	O
buffer	O
.	O

C	O
,	O
the	O
active	B-protein_state
form	O
of	O
PmC11	B-protein
and	O
two	O
mutants	O
,	O
PmC11C179A	B-mutant
(	O
C	O
)	O
and	O
PmC11K147A	B-mutant
(	O
K	O
),	O
were	O
examined	O
by	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
(	O
lane	O
1	O
)	O
and	O
Western	B-experimental_method
blot	I-experimental_method
analysis	O
using	O
an	O
anti	O
-	O
His	O
antibody	O
(	O
lane	O
2	O
)	O
show	O
that	O
PmC11	B-protein
autoprocesses	B-ptm
,	O
whereas	O
mutants	O
,	O
PmC11C179A	B-mutant
and	O
PmC11K147A	B-mutant
,	O
do	O
not	O
show	O
autoprocessing	B-ptm
in	O
vitro	O
.	O

D	O
,	O
cysteine	O
peptidase	O
activity	O
of	O
PmC11	B-protein
.	O

Km	O
and	O
Vmax	B-evidence
of	O
PmC11	B-protein
and	O
K147A	B-mutant
mutant	O
were	O
determined	O
by	O
monitoring	O
change	O
in	O
the	O
fluorescence	O
corresponding	O
to	O
AMC	O
release	O
from	O
Bz	B-chemical
-	I-chemical
R	I-chemical
-	I-chemical
AMC	I-chemical
.	O

PmC11C179A	O
(	O
20	O
μg	O
)	O
was	O
incubated	O
overnight	O
at	O
37	O
°	O
C	O
with	O
increasing	O
amounts	O
of	O
processed	O
PmC11	B-protein
and	O
analyzed	O
on	O
a	O
10	O
%	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
gel	O
.	O

F	O
,	O
activity	B-evidence
of	O
PmC11	B-protein
against	O
basic	O
substrates	O
.	O

G	O
,	O
electrostatic	O
surface	O
potential	O
of	O
PmC11	B-protein
shown	O
in	O
a	O
similar	O
orientation	O
,	O
where	O
blue	O
and	O
red	O
denote	O
positively	O
and	O
negatively	O
charged	O
surface	O
potential	O
,	O
respectively	O
,	O
contoured	O
at	O
±	O
5	O
kT	O
/	O
e	O
.	O

Five	O
of	O
the	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
surrounding	O
the	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
of	O
PmC11	B-protein
(	O
α1	B-structure_element
,	O
α2	B-structure_element
,	O
α4	B-structure_element
,	O
α6	B-structure_element
,	O
and	O
α7	B-structure_element
)	O
are	O
found	O
in	O
similar	O
positions	O
to	O
the	O
five	O
structurally	B-protein_state
conserved	I-protein_state
helices	B-structure_element
in	O
caspases	B-protein_type
and	O
other	O
members	O
of	O
clan	B-protein_type
CD	I-protein_type
,	O
apart	O
from	O
family	O
C80	B-protein_type
.	O

Autoprocessing	B-ptm
of	O
PmC11	B-protein

Purification	B-experimental_method
of	O
recombinant	O
PmC11	B-protein
(	O
molecular	O
mass	O
=	O
42	O
.	O
6	O
kDa	O
)	O
revealed	O
partial	O
processing	O
into	O
two	O
cleavage	O
products	O
of	O
26	O
.	O
4	O
and	O
16	O
.	O
2	O
kDa	O
,	O
related	O
to	O
the	O
observed	O
cleavage	B-ptm
at	O
Lys147	B-residue_name_number
in	O
the	O
crystal	B-evidence
structure	I-evidence
(	O
Fig	O
.	O
2A	O
).	O

Incubation	B-experimental_method
of	O
PmC11	B-protein
at	O
37	O
°	O
C	O
for	O
16	O
h	O
,	O
resulted	O
in	O
a	O
fully	B-protein_state
processed	I-protein_state
enzyme	O
that	O
remained	O
as	O
an	O
intact	B-protein_state
monomer	B-oligomeric_state
when	O
applied	O
to	O
a	O
size	O
-	O
exclusion	O
column	O
(	O
Fig	O
.	O
2B	O
).	O

The	O
single	O
cleavage	B-site
site	I-site
of	O
PmC11	B-protein
at	O
Lys147	B-residue_name_number
is	O
found	O
immediately	O
after	O
α3	B-structure_element
,	O
in	O
loop	B-structure_element
L5	B-structure_element
within	O
the	O
central	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
(	O
Figs	O
.	O
1	O
,	O
A	O
and	O
B	O
,	O
and	O
2A	O
).	O

Moreover	O
,	O
the	O
C	O
-	O
terminal	O
side	O
of	O
the	O
cleavage	B-site
site	I-site
resides	O
near	O
the	O
catalytic	B-site
dyad	I-site
with	O
Ala148	B-residue_name_number
being	O
4	O
.	O
5	O
and	O
5	O
.	O
7	O
Å	O
from	O
His133	B-residue_name_number
and	O
Cys179	B-residue_name_number
,	O
respectively	O
.	O

Thus	O
,	O
the	O
cleavage	B-ptm
would	O
be	O
required	O
for	O
full	B-protein_state
activation	I-protein_state
of	O
PmC11	B-protein
.	O

To	O
investigate	O
this	O
possibility	O
,	O
two	O
mutant	O
forms	O
of	O
the	O
enzyme	O
were	O
created	O
:	O
PmC11C179A	B-mutant
(	O
a	O
catalytically	B-protein_state
inactive	I-protein_state
mutant	I-protein_state
)	O
and	O
PmC11K147A	B-mutant
(	O
a	O
cleavage	B-protein_state
-	I-protein_state
site	I-protein_state
mutant	I-protein_state
).	O

Initial	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
and	O
Western	B-experimental_method
blot	I-experimental_method
analysis	O
of	O
both	O
mutants	O
revealed	O
no	O
discernible	O
processing	O
occurred	O
as	O
compared	O
with	O
active	B-protein_state
PmC11	B-protein
(	O
Fig	O
.	O
2C	O
).	O

The	O
PmC11K147A	B-mutant
mutant	B-protein_state
enzyme	O
had	O
a	O
markedly	O
different	O
reaction	B-evidence
rate	I-evidence
(	O
Vmax	B-evidence
)	O
compared	O
with	O
WT	B-protein_state
,	O
where	O
the	O
reaction	B-evidence
velocity	I-evidence
of	O
PmC11	B-protein
was	O
10	O
times	O
greater	O
than	O
that	O
of	O
PmC11K147A	B-mutant
(	O
Fig	O
.	O
2D	O
).	O

Taken	O
together	O
,	O
these	O
data	O
reveal	O
that	O
PmC11	B-protein
requires	O
processing	O
at	O
Lys147	B-residue_name_number
for	O
optimum	O
activity	O
.	O

This	O
suggests	O
that	O
cleavage	B-ptm
of	O
PmC11C179A	B-mutant
was	O
most	O
likely	O
an	O
effect	O
of	O
the	O
increasing	O
concentration	O
of	O
PmC11	B-protein
and	O
intermolecular	O
cleavage	O
.	O

Substrate	O
Specificity	O
of	O
PmC11	B-protein

The	O
autocatalytic	B-ptm
cleavage	I-ptm
of	O
PmC11	B-protein
at	O
Lys147	B-residue_name_number
(	O
sequence	O
KLK	O
∧	O
A	O
)	O
demonstrates	O
that	O
the	O
enzyme	O
accepts	O
substrates	O
with	O
Lys	B-residue_name
in	O
the	O
P1	B-residue_number
position	O
.	O

As	O
expected	O
,	O
PmC11	B-protein
showed	O
no	O
activity	O
against	O
substrates	O
with	O
Pro	B-residue_name
or	O
Asp	B-residue_name
in	O
P1	B-residue_number
but	O
was	O
active	B-protein_state
toward	O
substrates	O
with	O
a	O
basic	O
residue	O
in	O
P1	B-residue_number
such	O
as	O
Bz	B-chemical
-	I-chemical
R	I-chemical
-	I-chemical
AMC	I-chemical
,	O
Z	B-chemical
-	I-chemical
GGR	I-chemical
-	I-chemical
AMC	I-chemical
,	O
and	O
BOC	B-chemical
-	I-chemical
VLK	I-chemical
-	I-chemical
AMC	I-chemical
.	O

The	O
rate	O
of	O
cleavage	O
was	O
∼	O
3	O
-	O
fold	O
greater	O
toward	O
the	O
single	O
Arg	B-residue_name
substrate	O
Bz	B-chemical
-	I-chemical
R	I-chemical
-	I-chemical
AMC	I-chemical
than	O
for	O
the	O
other	O
two	O
(	O
Fig	O
.	O
2F	O
)	O
and	O
,	O
unexpectedly	O
,	O
PmC11	B-protein
showed	O
no	O
activity	O
toward	O
BOC	B-chemical
-	I-chemical
K	I-chemical
-	I-chemical
AMC	I-chemical
.	O

The	O
catalytic	B-site
dyad	I-site
of	O
PmC11	B-protein
sits	O
near	O
the	O
bottom	O
of	O
an	O
open	B-protein_state
pocket	B-site
on	O
the	O
surface	O
of	O
the	O
enzyme	O
at	O
a	O
conserved	B-protein_state
location	I-protein_state
in	O
the	O
clan	O
CD	B-protein_type
family	I-protein_type
.	O

The	O
PmC11	B-protein
structure	B-evidence
reveals	O
that	O
the	O
catalytic	B-site
dyad	I-site
forms	O
part	O
of	O
a	O
large	O
acidic	B-site
pocket	I-site
(	O
Fig	O
.	O
2G	O
),	O
consistent	O
with	O
a	O
binding	B-site
site	I-site
for	O
a	O
basic	O
substrate	O
.	O

This	O
pocket	B-site
is	O
lined	O
with	O
the	O
potential	O
functional	O
side	O
chains	O
of	O
Asn50	B-residue_name_number
,	O
Asp177	B-residue_name_number
,	O
and	O
Thr204	B-residue_name_number
with	O
Gly134	B-residue_name_number
,	O
Asp207	B-residue_name_number
,	O
and	O
Met205	B-residue_name_number
also	O
contributing	O
to	O
the	O
pocket	B-site
(	O
Fig	O
.	O
2A	O
).	O

Interestingly	O
,	O
these	O
residues	O
are	O
in	O
regions	O
that	O
are	O
structurally	B-protein_state
similar	I-protein_state
to	O
those	O
involved	O
in	O
the	O
S1	B-site
binding	I-site
pockets	I-site
of	O
other	O
clan	B-protein_type
CD	I-protein_type
members	I-protein_type
(	O
shown	O
in	O
Ref	O
.).	O

A	O
structure	B-experimental_method
overlay	I-experimental_method
of	O
PmC11	B-protein
with	O
the	O
MALT1	B-protein
-	I-protein
paracacaspase	I-protein
(	O
MALT1	B-protein
-	I-protein
P	I-protein
),	O
in	O
complex	B-protein_state
with	O
Z	B-chemical
-	I-chemical
VRPR	I-chemical
-	I-chemical
FMK	I-chemical
,	O
revealed	O
that	O
the	O
PmC11	B-protein
dyad	B-site
sits	O
in	O
a	O
very	O
similar	O
position	O
to	O
that	O
of	O
active	B-protein_state
MALT1	B-protein
-	I-protein
P	I-protein
and	O
that	O
Asn50	B-residue_name_number
,	O
Asp177	B-residue_name_number
,	O
and	O
Asp207	B-residue_name_number
superimpose	O
well	O
with	O
the	O
principal	O
MALT1	B-protein
-	I-protein
P	I-protein
inhibitor	B-site
binding	I-site
residues	I-site
(	O
Asp365	B-residue_name_number
,	O
Asp462	B-residue_name_number
,	O
and	O
Glu500	B-residue_name_number
,	O
respectively	O
(	O
VRPR	B-chemical
-	I-chemical
FMK	I-chemical
from	O
MALT1	B-protein
-	I-protein
P	I-protein
with	O
the	O
corresponding	O
PmC11	B-protein
residues	O
from	O
the	O
structural	B-experimental_method
overlay	I-experimental_method
is	O
shown	O
in	O
Fig	O
.	O
1D	O
),	O
as	O
described	O
in	O
Ref	O
.).	O

Asp177	B-residue_name_number
is	O
located	O
near	O
the	O
catalytic	B-protein_state
cysteine	B-residue_name
and	O
is	O
conserved	B-protein_state
throughout	I-protein_state
the	O
C11	B-protein_type
family	I-protein_type
,	O
suggesting	O
it	O
is	O
the	O
primary	O
S1	B-site
binding	I-site
site	I-site
residue	I-site
.	O

In	O
the	O
structure	B-evidence
of	O
PmC11	B-protein
,	O
Asp207	B-residue_name_number
resides	O
on	O
a	O
flexible	O
loop	B-structure_element
pointing	O
away	O
from	O
the	O
S1	B-site
binding	I-site
pocket	I-site
(	O
Fig	O
.	O
3C	O
).	O

However	O
,	O
this	O
loop	B-structure_element
has	O
been	O
shown	O
to	O
be	O
important	O
for	O
substrate	O
binding	O
in	O
clan	B-protein_type
CD	I-protein_type
and	O
this	O
residue	O
could	O
easily	O
rotate	O
and	O
be	O
involved	O
in	O
substrate	O
binding	O
in	O
PmC11	B-protein
.	O

Thus	O
,	O
Asn50	B-residue_name_number
,	O
Asp177	B-residue_name_number
,	O
and	O
Asp207	B-residue_name_number
are	O
most	O
likely	O
responsible	O
for	O
the	O
substrate	O
specificity	O
of	O
PmC11	B-protein
.	O

PmC11	B-protein
binds	O
and	O
is	O
inhibited	O
by	O
Z	B-chemical
-	I-chemical
VRPR	I-chemical
-	I-chemical
FMK	I-chemical
and	O
does	O
not	O
require	O
Ca2	B-chemical
+	I-chemical
for	O
activity	O
.	O

A	O
,	O
PmC11	O
activity	O
is	O
inhibited	O
by	O
Z	B-chemical
-	I-chemical
VRPR	I-chemical
-	I-chemical
FMK	I-chemical
.	O

PmC11	O
was	O
incubated	B-experimental_method
with	O
(+)	O
or	O
without	O
(−)	O
Z	B-chemical
-	I-chemical
VRPR	I-chemical
-	I-chemical
FMK	I-chemical
and	O
the	O
samples	O
analyzed	O
on	O
a	O
10	O
%	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
gel	O
.	O

A	O
size	B-evidence
shift	I-evidence
can	O
be	O
observed	O
in	O
the	O
larger	O
processed	O
product	O
of	O
PmC11	B-protein
(	O
26	O
.	O
1	O
kDa	O
).	O

C	O
,	O
PmC11	B-protein
with	O
the	O
Z	B-chemical
-	I-chemical
VRPR	I-chemical
-	I-chemical
FMK	I-chemical
from	O
the	O
MALT1	B-protein
-	I-protein
paracacaspase	I-protein
(	O
MALT1	B-protein
-	I-protein
P	I-protein
)	O
superimposed	B-experimental_method
.	O

A	O
three	B-experimental_method
-	I-experimental_method
dimensional	I-experimental_method
structural	I-experimental_method
overlay	I-experimental_method
of	O
Z	B-chemical
-	I-chemical
VRPR	I-chemical
-	I-chemical
FMK	I-chemical
from	O
the	O
MALT1	B-protein
-	I-protein
P	I-protein
complex	O
onto	O
PmC11	B-protein
.	O

The	O
position	O
and	O
orientation	O
of	O
Z	B-chemical
-	I-chemical
VRPR	I-chemical
-	I-chemical
FMK	I-chemical
was	O
taken	O
from	O
superposition	B-experimental_method
of	O
the	O
PmC11	B-protein
and	O
MALTI_P	B-protein
structures	B-evidence
and	O
indicates	O
the	O
presumed	O
active	B-site
site	I-site
of	O
PmC11	B-protein
.	O

Residues	O
surrounding	O
the	O
inhibitor	O
are	O
labeled	O
and	O
represent	O
potentially	O
important	O
binding	B-site
site	I-site
residues	I-site
,	O
labeled	O
in	O
black	O
and	O
shown	O
in	O
an	O
atomic	O
representation	O
.	O

C	O
,	O
divalent	O
cations	O
do	O
not	O
increase	O
the	O
activity	O
of	O
PmC11	B-protein
.	O

Clostripain	B-protein
from	O
C	B-species
.	I-species
histolyticum	I-species
is	O
the	O
founding	O
member	O
of	O
the	O
C11	B-protein_type
family	I-protein_type
of	O
peptidases	B-protein_type
and	O
contains	O
an	O
additional	O
149	B-residue_range
residues	I-residue_range
compared	O
with	O
PmC11	B-protein
.	O

Unlike	O
PmC11	B-protein
,	O
clostripain	B-protein
has	O
two	O
cleavage	B-site
sites	I-site
(	O
Arg181	B-residue_name_number
and	O
Arg190	B-residue_name_number
),	O
which	O
results	O
in	O
the	O
removal	O
of	O
a	O
nonapeptide	B-structure_element
,	O
and	O
is	O
required	O
for	O
full	B-protein_state
activation	I-protein_state
of	O
the	O
enzyme	O
(	O
highlighted	O
in	O
Fig	O
.	O
1A	O
).	O

As	O
studies	O
on	O
clostripain	B-protein
revealed	O
addition	O
of	O
Ca2	B-chemical
+	I-chemical
ions	O
are	O
required	O
for	O
full	B-protein_state
activation	I-protein_state
,	O
the	O
Ca2	B-chemical
+	I-chemical
dependence	O
of	O
PmC11	B-protein
was	O
examined	O
.	O

Surprisingly	O
,	O
Ca2	B-chemical
+	I-chemical
did	O
not	O
enhance	O
PmC11	B-protein
activity	O
and	O
,	O
furthermore	O
,	O
other	O
divalent	O
cations	O
,	O
Mg2	B-chemical
+,	I-chemical
Mn2	B-chemical
+,	I-chemical
Co2	B-chemical
+,	I-chemical
Fe2	B-chemical
+,	I-chemical
Zn2	B-chemical
+,	I-chemical
and	O
Cu2	B-chemical
+,	I-chemical
were	O
not	O
necessary	O
for	O
PmC11	B-protein
activity	O
(	O
Fig	O
.	O
3D	O
).	O

In	O
support	O
of	O
these	O
findings	O
,	O
EGTA	B-chemical
did	O
not	O
inhibit	O
PmC11	B-protein
suggesting	O
that	O
,	O
unlike	O
clostripain	B-protein
,	O
PmC11	B-protein
does	O
not	O
require	O
Ca2	B-chemical
+	I-chemical
or	O
other	O
divalent	O
cations	O
,	O
for	O
activity	O
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
PmC11	B-protein
now	O
provides	O
three	O
-	O
dimensional	O
information	O
for	O
a	O
member	O
of	O
the	O
clostripain	B-protein
C11	B-protein_type
family	I-protein_type
of	O
cysteine	B-protein_type
peptidases	I-protein_type
.	O

The	O
enzyme	O
exhibits	O
all	O
of	O
the	O
key	O
structural	O
elements	O
of	O
clan	B-protein_type
CD	I-protein_type
members	I-protein_type
,	O
but	O
is	O
unusual	O
in	O
that	O
it	O
has	O
a	O
nine	O
-	O
stranded	O
central	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
with	O
a	O
novel	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
.	O

The	O
structural	O
similarity	O
of	O
PmC11	B-protein
with	O
its	O
nearest	O
structural	O
neighbors	O
in	O
the	O
PDB	O
is	O
decidedly	O
low	O
,	O
overlaying	O
better	O
with	O
six	O
-	O
stranded	O
caspase	B-protein
-	I-protein
7	I-protein
than	O
any	O
of	O
the	O
other	O
larger	O
members	O
of	O
the	O
clan	O
(	O
Table	O
2	O
).	O

The	O
substrate	O
specificity	O
of	O
PmC11	B-protein
is	O
Arg	B-residue_name
/	O
Lys	B-residue_name
and	O
the	O
crystal	B-evidence
structure	I-evidence
revealed	O
an	O
acidic	B-site
pocket	I-site
for	O
specific	O
binding	O
of	O
such	O
basic	O
substrates	O
.	O

In	O
addition	O
,	O
the	O
structure	B-evidence
suggested	O
a	O
mechanism	O
of	O
self	O
-	O
inhibition	O
in	O
both	O
PmC11	B-protein
and	O
clostripain	B-protein
and	O
an	O
activation	O
mechanism	O
that	O
requires	O
autoprocessing	B-ptm
.	O

PmC11	B-protein
differs	O
from	O
clostripain	B-protein
in	O
that	O
is	O
does	O
not	O
appear	O
to	O
require	O
divalent	O
cations	O
for	O
activation	O
.	O

Several	O
other	O
members	O
of	O
clan	B-protein_type
CD	I-protein_type
require	O
processing	B-ptm
for	O
full	B-protein_state
activation	I-protein_state
including	O
legumain	B-protein
,	O
gingipain	B-protein
-	I-protein
R	I-protein
,	O
MARTX	B-protein
-	I-protein
CPD	I-protein
,	O
and	O
the	O
effector	B-protein_type
caspases	I-protein_type
,	O
e	O
.	O
g	O
.	O
caspase	B-protein
-	I-protein
7	I-protein
.	O

To	O
date	O
,	O
the	O
effector	B-protein_type
caspases	I-protein_type
are	O
the	O
only	O
group	O
of	O
enzymes	O
that	O
require	O
cleavage	B-ptm
of	O
a	O
loop	B-structure_element
within	O
the	O
central	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
.	O

All	O
other	O
clan	B-protein_type
CD	I-protein_type
members	I-protein_type
requiring	O
cleavage	B-ptm
for	O
full	B-protein_state
activation	I-protein_state
do	O
so	O
at	O
sites	B-site
external	O
to	O
their	O
central	O
sheets	B-structure_element
.	O

In	O
addition	O
,	O
several	O
members	O
of	O
clan	B-protein_type
CD	I-protein_type
exhibit	O
self	O
-	O
inhibition	O
,	O
whereby	O
regions	B-structure_element
of	O
the	O
enzyme	O
block	O
access	O
to	O
the	O
active	B-site
site	I-site
.	O

Like	O
PmC11	B-protein
,	O
these	O
structures	O
show	O
preformed	O
catalytic	O
machinery	O
and	O
,	O
for	O
a	O
substrate	O
to	O
gain	O
access	O
,	O
movement	O
and	O
/	O
or	O
cleavage	B-ptm
of	O
the	O
blocking	B-structure_element
region	I-structure_element
is	O
required	O
.	O

The	O
structure	B-evidence
of	O
PmC11	B-protein
gives	O
the	O
first	O
insight	O
into	O
this	O
class	O
of	O
relatively	O
unexplored	O
family	O
of	O
proteins	O
and	O
should	O
allow	O
important	O
catalytic	O
and	O
substrate	O
binding	O
residues	O
to	O
be	O
identified	O
in	O
a	O
variety	O
of	O
orthologues	O
.	O

Indeed	O
,	O
insights	O
gained	O
from	O
an	O
analysis	O
of	O
the	O
PmC11	B-protein
structure	B-evidence
revealed	O
the	O
identity	O
of	O
the	O
Trypanosoma	B-species
brucei	I-species
PNT1	B-protein
protein	O
as	O
a	O
C11	B-protein_type
cysteine	I-protein_type
peptidase	I-protein_type
with	O
an	O
essential	O
role	O
in	O
organelle	O
replication	O
.	O

The	O
chemically	O
most	O
complex	O
modification	O
in	O
eukaryotic	B-taxonomy_domain
rRNA	B-chemical
is	O
the	O
conserved	B-protein_state
hypermodified	B-protein_state
nucleotide	B-chemical
N1	B-chemical
-	I-chemical
methyl	I-chemical
-	I-chemical
N3	I-chemical
-	I-chemical
aminocarboxypropyl	I-chemical
-	I-chemical
pseudouridine	I-chemical
(	O
m1acp3Ψ	B-chemical
)	O
located	O
next	O
to	O
the	O
P	B-site
-	I-site
site	I-site
tRNA	B-chemical
on	O
the	O
small	O
subunit	O
18S	B-chemical
rRNA	I-chemical
.	O

While	O
S	B-chemical
-	I-chemical
adenosylmethionine	I-chemical
was	O
identified	O
as	O
the	O
source	O
of	O
the	O
aminocarboxypropyl	B-chemical
(	O
acp	B-chemical
)	O
group	O
more	O
than	O
40	O
years	O
ago	O
the	O
enzyme	O
catalyzing	O
the	O
acp	B-chemical
transfer	O
remained	O
elusive	O
.	O

In	O
functionally	O
impaired	O
Tsr3	B-protein
-	O
mutants	B-protein_state
,	O
a	O
reduced	O
level	O
of	O
acp	B-chemical
modification	O
directly	O
correlates	O
with	O
increased	O
20S	B-chemical
pre	I-chemical
-	I-chemical
rRNA	I-chemical
accumulation	O
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
archaeal	B-taxonomy_domain
Tsr3	B-protein
homologs	O
revealed	O
the	O
same	O
fold	O
as	O
in	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
RNA	I-protein_type
-	I-protein_type
methyltransferases	I-protein_type
but	O
a	O
distinct	O
SAM	B-site
binding	I-site
mode	I-site
.	O

Structurally	O
,	O
Tsr3	B-protein
therefore	O
represents	O
a	O
novel	O
class	O
of	O
acp	B-protein_type
transferase	I-protein_type
enzymes	O
.	O

During	O
eukaryotic	B-taxonomy_domain
ribosome	O
biogenesis	O
several	O
dozens	O
of	O
rRNA	B-chemical
nucleotides	B-chemical
become	O
chemically	O
modified	O
.	O

The	O
most	O
abundant	O
rRNA	B-chemical
modifications	O
are	O
methylations	B-ptm
at	O
the	O
2	O
′-	O
OH	O
ribose	B-chemical
moieties	O
and	O
isomerizations	O
of	O
uridine	B-chemical
residues	O
to	O
pseudouridine	B-chemical
,	O
catalyzed	O
by	O
small	B-complex_assembly
nucleolar	I-complex_assembly
ribonucleoprotein	I-complex_assembly
particles	I-complex_assembly
(	O
snoRNPs	B-complex_assembly
).	O

In	O
addition	O
,	O
18S	B-chemical
and	O
25S	B-chemical
(	O
yeast	B-taxonomy_domain
)/	O
28S	B-chemical
(	O
humans	B-species
)	O
rRNAs	B-chemical
contain	O
several	O
base	O
modifications	O
catalyzed	O
by	O
site	O
-	O
specific	O
and	O
snoRNA	B-chemical
-	O
independent	O
enzymes	O
.	O

In	O
Saccharomyces	B-species
cerevisiae	I-species
18S	B-chemical
rRNA	I-chemical
contains	O
four	O
base	O
methylations	B-ptm
,	O
two	O
acetylations	B-ptm
and	O
a	O
single	O
3	B-chemical
-	I-chemical
amino	I-chemical
-	I-chemical
3	I-chemical
-	I-chemical
carboxypropyl	I-chemical
(	O
acp	B-chemical
)	O
modification	O
,	O
whereas	O
six	O
base	O
methylations	B-ptm
are	O
present	O
in	O
the	O
25S	B-chemical
rRNA	I-chemical
.	O

Ribosomal	B-chemical
RNA	I-chemical
modifications	O
have	O
been	O
suggested	O
to	O
optimize	O
ribosome	O
function	O
,	O
although	O
in	O
most	O
cases	O
this	O
remains	O
to	O
be	O
clearly	O
established	O
.	O

They	O
might	O
contribute	O
to	O
increased	O
RNA	B-chemical
stability	O
by	O
providing	O
additional	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
(	O
pseudouridines	B-chemical
),	O
improved	O
base	B-bond_interaction
stacking	I-bond_interaction
(	O
pseudouridines	B-chemical
and	O
base	B-ptm
methylations	I-ptm
)	O
or	O
an	O
increased	O
resistance	O
against	O
hydrolysis	O
(	O
ribose	B-ptm
methylations	I-ptm
).	O

Most	O
modified	O
rRNA	B-chemical
nucleotides	B-chemical
cluster	O
in	O
the	O
vicinity	O
of	O
the	O
decoding	B-site
or	O
the	O
peptidyl	B-site
transferase	I-site
center	I-site
,	O
suggesting	O
an	O
influence	O
on	O
ribosome	O
functionality	O
and	O
stability	O
.	O

The	O
chemically	O
most	O
complex	O
modification	O
is	O
located	O
in	O
the	O
loop	B-structure_element
capping	I-structure_element
helix	I-structure_element
31	I-structure_element
of	O
18S	B-chemical
rRNA	I-chemical
(	O
Supplementary	O
Figure	O
S1B	O
).	O

There	O
a	O
uridine	B-residue_name
(	O
U1191	B-residue_name_number
in	O
yeast	B-taxonomy_domain
)	O
is	O
modified	O
to	O
1	B-chemical
-	I-chemical
methyl	I-chemical
-	I-chemical
3	I-chemical
-(	I-chemical
3	I-chemical
-	I-chemical
amino	I-chemical
-	I-chemical
3	I-chemical
-	I-chemical
carboxypropyl	I-chemical
)-	I-chemical
pseudouridine	I-chemical
(	O
m1acp3Ψ	B-chemical
,	O
Figure	O
1A	O
).	O

This	O
base	O
modification	O
was	O
first	O
described	O
in	O
1968	O
for	O
hamster	B-taxonomy_domain
cells	O
and	O
is	O
conserved	B-protein_state
in	I-protein_state
eukaryotes	B-taxonomy_domain
.	O

This	O
hypermodified	B-protein_state
nucleotide	B-chemical
,	O
which	O
is	O
located	O
at	O
the	O
P	B-site
-	I-site
site	I-site
tRNA	B-chemical
,	O
is	O
synthesized	O
in	O
three	O
steps	O
beginning	O
with	O
the	O
snR35	B-chemical
H	B-structure_element
/	I-structure_element
ACA	I-structure_element
snoRNP	B-complex_assembly
guided	O
conversion	O
of	O
uridine	B-chemical
into	O
pseudouridine	B-chemical
.	O

In	O
a	O
second	O
step	O
,	O
the	O
essential	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
methyltransferase	I-protein_type
Nep1	B-protein
/	O
Emg1	B-protein
modifies	O
the	O
pseudouridine	B-chemical
to	O
N1	B-chemical
-	I-chemical
methylpseudouridine	I-chemical
.	O

The	O
final	O
acp	B-chemical
modification	O
leading	O
to	O
N1	B-chemical
-	I-chemical
methyl	I-chemical
-	I-chemical
N3	I-chemical
-	I-chemical
aminocarboxypropyl	I-chemical
-	I-chemical
pseudouridine	I-chemical
occurs	O
late	O
during	O
40S	B-complex_assembly
biogenesis	O
in	O
the	O
cytoplasm	O
,	O
while	O
the	O
two	O
former	O
reactions	O
are	O
taking	O
place	O
in	O
the	O
nucleolus	O
and	O
nucleus	O
,	O
and	O
is	O
independent	O
from	O
pseudouridylation	B-ptm
or	O
methylation	O
.	O

Both	O
the	O
methyl	O
and	O
the	O
acp	O
group	O
are	O
derived	O
from	O
S	B-chemical
-	I-chemical
adenosylmethionine	I-chemical
(	O
SAM	B-chemical
),	O
but	O
the	O
enzyme	O
responsible	O
for	O
acp	B-chemical
modification	O
remained	O
elusive	O
for	O
more	O
than	O
40	O
years	O
.	O

The	O
asterisk	O
indicates	O
the	O
C1	O
-	O
atom	O
labeled	O
in	O
the	O
14C	B-experimental_method
-	I-experimental_method
incorporation	I-experimental_method
assay	I-experimental_method
.	O

(	O
B	O
)	O
RP	B-experimental_method
-	I-experimental_method
HPLC	I-experimental_method
elution	B-evidence
profile	I-evidence
of	O
yeast	B-taxonomy_domain
18S	B-chemical
rRNA	I-chemical
nucleosides	B-chemical
.	O

Wild	B-protein_state
type	I-protein_state
(	O
WT	B-protein_state
)	O
and	O
plasmid	O
encoded	O
18S	B-chemical
rRNA	I-chemical
(	O
U1191U	B-mutant
)	O
show	O
the	O
14C	B-chemical
-	I-chemical
acp	I-chemical
signal	O
,	O
whereas	O
the	O
14C	B-chemical
-	I-chemical
acp	I-chemical
signal	O
is	O
missing	O
in	O
the	O
U1191A	B-mutant
mutant	B-protein_state
plasmid	O
encoded	O
18S	B-chemical
rRNA	I-chemical
(	O
U1191A	B-mutant
)	O
and	O
Δtsr3	B-mutant
mutants	O
(	O
Δtsr3	B-mutant
).	O

All	O
samples	O
were	O
loaded	O
on	O
the	O
gel	O
with	O
two	O
different	O
amounts	O
of	O
5	O
and	O
10	O
μl	O
.	O
(	O
D	O
)	O
Primer	B-experimental_method
extension	I-experimental_method
analysis	I-experimental_method
of	O
acp	B-chemical
modification	O
in	O
yeast	B-taxonomy_domain
18S	B-chemical
rRNA	I-chemical
(	O
right	O
gel	O
)	O
including	O
a	O
sequencing	O
ladder	O
(	O
left	O
gel	O
).	O

The	O
primer	O
extension	O
stop	O
at	O
nucleotide	O
1191	B-residue_number
is	O
missing	O
exclusively	O
in	O
Δtsr3	B-mutant
mutants	O
and	O
Δtsr3	B-mutant
Δsnr35	I-mutant
recombinants	O
.	O

(	O
E	O
)	O
Primer	B-experimental_method
extension	I-experimental_method
analysis	I-experimental_method
of	O
human	B-species
18S	B-chemical
rRNA	I-chemical
after	O
siRNA	B-experimental_method
knockdown	I-experimental_method
of	O
HsNEP1	B-protein
/	O
EMG1	B-protein
(	O
541	O
,	O
542	O
and	O
543	O
)	O
and	O
HsTSR3	B-protein
(	O
544	O
and	O
545	O
)	O
(	O
right	O
gel	O
),	O
including	O
a	O
sequencing	O
ladder	O
(	O
left	O
gel	O
).	O

The	O
primer	O
extension	O
arrest	O
is	O
reduced	O
in	O
HTC116	O
cells	O
transfected	O
with	O
siRNAs	B-chemical
544	O
and	O
545	O
.	O

The	O
efficiency	O
of	O
siRNA	B-chemical
mediated	O
HsTSR3	B-protein
repression	O
correlates	O
with	O
the	O
primer	B-evidence
extension	I-evidence
signals	I-evidence
(	O
see	O
Supplementary	O
Figure	O
S2A	O
).	O

Only	O
a	O
few	O
acp	B-chemical
transferring	O
enzymes	O
have	O
been	O
characterized	O
until	O
now	O
.	O

During	O
the	O
biosynthesis	O
of	O
wybutosine	B-chemical
,	O
a	O
tricyclic	O
nucleoside	B-chemical
present	O
in	O
eukaryotic	B-taxonomy_domain
and	O
archaeal	B-taxonomy_domain
phenylalanine	B-chemical
tRNA	B-chemical
,	O
Tyw2	B-protein
(	O
Trm12	B-protein
in	O
yeast	B-taxonomy_domain
)	O
transfers	O
an	O
acp	B-chemical
group	O
from	O
SAM	B-chemical
to	O
an	O
acidic	O
carbon	O
atom	O
.	O

Another	O
acp	B-chemical
modification	O
has	O
been	O
described	O
in	O
the	O
diphtamide	B-chemical
biosynthesis	O
pathway	O
,	O
where	O
an	O
acp	B-chemical
group	O
is	O
transferred	O
from	O
SAM	B-chemical
to	O
the	O
carbon	O
atom	O
of	O
a	O
histidine	B-residue_name
residue	O
of	O
eukaryotic	B-taxonomy_domain
translation	B-protein_type
elongation	I-protein_type
factor	I-protein_type
2	I-protein_type
by	O
use	O
of	O
a	O
radical	O
mechanism	O
.	O

It	O
is	O
highly	B-protein_state
conserved	I-protein_state
among	O
eukaryotes	B-taxonomy_domain
and	O
archaea	B-taxonomy_domain
(	O
Supplementary	O
Figure	O
S1A	O
)	O
and	O
its	O
deletion	O
leads	O
to	O
an	O
accumulation	O
of	O
the	O
20S	B-chemical
pre	I-chemical
-	I-chemical
rRNA	I-chemical
precursor	O
of	O
18S	B-chemical
rRNA	I-chemical
,	O
suggesting	O
an	O
influence	O
on	O
D	B-site
-	I-site
site	I-site
cleavage	O
during	O
the	O
maturation	O
of	O
the	O
small	O
ribosomal	O
subunit	O
.	O

However	O
,	O
its	O
function	O
remained	O
unclear	O
although	O
recently	O
a	O
putative	O
nuclease	O
function	O
during	O
18S	B-chemical
rRNA	I-chemical
maturation	O
was	O
predicted	O
.	O

Here	O
,	O
we	O
identify	O
Tsr3	B-protein
as	O
the	O
long	O
-	O
sought	O
acp	B-protein_type
transferase	I-protein_type
that	O
catalyzes	O
the	O
last	O
step	O
in	O
the	O
biosynthesis	O
of	O
the	O
hypermodified	B-protein_state
nucleotide	B-chemical
m1acp3Ψ	B-chemical
in	O
yeast	B-taxonomy_domain
and	O
human	B-species
cells	O
.	O

Furthermore	O
using	O
catalytically	B-protein_state
defective	I-protein_state
mutants	O
of	O
yeast	B-taxonomy_domain
Tsr3	B-protein
we	O
demonstrated	O
that	O
the	O
acp	B-chemical
modification	O
is	O
required	O
for	O
18S	B-chemical
rRNA	I-chemical
maturation	O
.	O

Surprisingly	O
,	O
the	O
crystal	B-evidence
structures	I-evidence
of	O
archaeal	B-taxonomy_domain
homologs	O
revealed	O
that	O
Tsr3	B-protein
is	O
structurally	O
similar	O
to	O
the	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
RNA	I-protein_type
methyltransferases	I-protein_type
.	O

Interestingly	O
,	O
the	O
two	O
structurally	O
very	O
different	O
enzymes	O
use	O
similar	O
strategies	O
in	O
binding	O
the	O
SAM	B-chemical
-	O
cofactor	O
in	O
order	O
to	O
ensure	O
that	O
in	O
contrast	O
to	O
methyltransferases	B-protein_type
the	O
acp	B-chemical
and	O
not	O
the	O
methyl	O
group	O
of	O
SAM	B-chemical
is	O
transferred	O
to	O
the	O
substrate	O
.	O

Tsr3	B-protein
is	O
the	O
enzyme	O
responsible	O
for	O
18S	B-chemical
rRNA	I-chemical
acp	B-chemical
modification	O
in	O
yeast	B-taxonomy_domain
and	O
humans	B-species

For	O
the	O
Δtsr3	B-mutant
deletion	O
strain	O
the	O
HPLC	B-evidence
elution	I-evidence
profile	I-evidence
of	O
18S	B-chemical
rRNA	I-chemical
nucleosides	B-chemical
(	O
Figure	O
1B	O
)	O
was	O
very	O
similar	O
to	O
that	O
of	O
the	O
pseudouridine	B-protein_type
-	I-protein_type
N1	I-protein_type
methyltransferase	I-protein_type
mutant	B-protein_state
Δnep1	B-mutant
,	O
where	O
a	O
shoulder	O
at	O
∼	O
7	O
.	O
4	O
min	O
elution	O
time	O
was	O
missing	O
in	O
the	O
elution	O
profile	O
.	O

As	O
previously	O
reported	O
this	O
shoulder	O
was	O
identified	O
by	O
ESI	B-experimental_method
-	I-experimental_method
MS	I-experimental_method
as	O
corresponding	O
to	O
m1acp3Ψ	B-chemical
.	O

In	O
order	O
to	O
directly	O
analyze	O
the	O
presence	O
of	O
the	O
acp	B-chemical
modification	O
of	O
nucleotide	B-chemical
1191	B-residue_number
we	O
used	O
an	O
in	B-experimental_method
vivo14C	I-experimental_method
incorporation	I-experimental_method
assay	I-experimental_method
with	O
1	B-chemical
-	I-chemical
14C	I-chemical
-	I-chemical
methionine	I-chemical
.	O

Whereas	O
the	O
acp	B-chemical
labeling	O
of	O
18S	B-chemical
rRNA	I-chemical
was	O
clearly	O
present	O
in	O
the	O
wild	B-protein_state
type	I-protein_state
strain	O
no	O
radioactive	O
labeling	O
could	O
be	O
observed	O
in	O
a	O
Δtsr3	B-mutant
strain	O
(	O
Figure	O
1C	O
).	O

As	O
previously	O
shown	O
,	O
only	O
the	O
acp	B-chemical
but	O
none	O
of	O
the	O
other	O
modifications	O
at	O
U1191	B-residue_name_number
of	O
yeast	B-taxonomy_domain
18S	B-chemical
rRNA	I-chemical
blocks	O
reverse	O
transcriptase	O
activity	O
.	O

Therefore	O
the	O
presence	O
of	O
the	O
acp	B-chemical
modification	O
can	O
be	O
directly	O
assessed	O
by	O
primer	B-experimental_method
extension	I-experimental_method
.	O

Indeed	O
,	O
in	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
yeast	B-taxonomy_domain
a	O
strong	O
primer	B-evidence
extension	I-evidence
stop	I-evidence
signal	I-evidence
occurred	O
at	O
position	O
1192	B-residue_number
.	O

In	O
contrast	O
,	O
in	O
a	O
Δtsr3	B-mutant
mutant	B-protein_state
no	O
primer	O
extension	O
stop	O
signal	O
was	O
present	O
at	O
this	O
position	O
.	O

As	O
expected	O
,	O
in	O
a	O
Δsnr35	B-mutant
deletion	B-experimental_method
preventing	O
pseudouridylation	B-ptm
and	O
N1	B-ptm
-	I-ptm
methylation	I-ptm
(	O
resulting	O
in	O
acp3U	B-chemical
)	O
as	O
well	O
as	O
in	O
a	O
Δnep1	B-mutant
deletion	O
strain	O
where	O
pseudouridine	B-chemical
is	O
not	B-protein_state
methylated	I-protein_state
(	O
resulting	O
in	O
acp3Ψ	B-chemical
)	O
a	O
primer	B-evidence
extension	I-evidence
stop	I-evidence
signal	I-evidence
of	O
similar	O
intensity	O
as	O
in	O
the	O
wild	B-protein_state
type	I-protein_state
was	O
observed	O
.	O

In	O
a	O
Δtsr3	B-mutant
Δsnr35	I-mutant
double	O
deletion	O
strain	O
the	O
18S	B-chemical
rRNA	I-chemical
contains	O
an	O
unmodified	B-protein_state
U	B-chemical
and	O
the	O
primer	O
extension	O
stop	O
signal	O
was	O
missing	O
(	O
Figure	O
1D	O
).	O

Human	B-species
18S	B-chemical
rRNA	I-chemical
has	O
also	O
been	O
shown	O
to	O
contain	O
m1acp3Ψ	B-ptm
in	O
the	O
18S	B-chemical
rRNA	I-chemical
at	O
position	O
1248	B-residue_number
.	O

After	O
siRNA	B-experimental_method
-	I-experimental_method
mediated	I-experimental_method
depletion	I-experimental_method
of	O
Tsr3	B-protein
in	O
human	B-species
colon	O
carcinoma	O
HCT116	O
(+/+)	O
cells	O
the	O
acp	B-evidence
primer	I-evidence
extension	I-evidence
arrest	I-evidence
was	O
reduced	O
in	O
comparison	O
to	O
cells	O
transfected	O
with	O
a	O
non	O
-	O
targeting	O
scramble	O
siRNA	B-chemical
control	O
(	O
Figure	O
1E	O
,	O
compare	O
lanes	O
544	O
and	O
scramble	O
).	O

The	O
efficiency	O
of	O
siRNA	B-chemical
-	O
mediated	O
depletion	O
was	O
established	O
by	O
RT	B-experimental_method
-	I-experimental_method
qPCR	I-experimental_method
and	O
found	O
to	O
be	O
very	O
high	O
with	O
siRNA	B-chemical
544	O
(	O
Supplementary	O
Figure	O
S2A	O
,	O
remaining	O
TSR3	B-protein
mRNA	O
level	O
of	O
2	O
%).	O

Thus	O
,	O
HsTsr3	B-protein
is	O
also	O
responsible	O
for	O
the	O
acp	B-chemical
modification	O
of	O
18S	B-chemical
rRNA	I-chemical
nucleotide	B-chemical
Ψ1248	B-ptm
in	O
helix	B-structure_element
31	I-structure_element
.	O

Phenotypic	O
characterization	O
of	O
Δtsr3	B-mutant
mutants	O

However	O
,	O
the	O
Δtsr3	B-mutant
deletion	O
was	O
synthetic	O
sick	O
with	O
a	O
Δsnr35	B-mutant
deletion	O
preventing	O
pseudouridylation	B-ptm
and	O
Nep1	B-protein
-	O
catalyzed	O
methylation	O
of	O
nucleotide	O
1191	B-residue_number
(	O
Figure	O
2A	O
).	O

Interestingly	O
,	O
no	O
increased	O
growth	O
defect	O
could	O
be	O
observed	O
for	O
Δtsr3	B-mutant
Δnep1	I-mutant
recombinants	O
containing	O
the	O
nep1	B-gene
suppressor	O
mutation	O
Δnop6	B-mutant
as	O
well	O
as	O
for	O
Δtsr3	B-mutant
Δsnr35	I-mutant
Δnep1	I-mutant
recombinants	O
with	O
unmodified	B-protein_state
U1191	B-residue_name_number
(	O
Supplementary	O
Figure	O
S2D	O
and	O
E	O
).	O

Phenotypic	O
characterization	O
of	O
yeast	B-taxonomy_domain
TSR3	B-protein
deletion	O
(	O
Δtrs3	B-mutant
)	O
and	O
human	B-species
TSR3	B-protein
depletion	O
(	O
siRNAs	B-chemical
544	O
and	O
545	O
)	O
and	O
cellular	O
localization	O
of	O
yeast	B-taxonomy_domain
Tsr3	B-protein
.	O
(	O
A	O
)	O
Growth	O
of	O
yeast	B-taxonomy_domain
wild	B-protein_state
type	I-protein_state
,	O
Δtsr3	B-mutant
,	O
Δsnr35	B-mutant
and	O
Δtsr3	B-mutant
Δsnr35	I-mutant
segregants	O
after	O
meiosis	O
and	O
tetrad	O
dissection	O
of	O
Δtsr3	B-mutant
/	O
TSR3	B-protein
Δsnr35	B-mutant
/	O
SNR35	B-protein
heterozygous	O
diploids	O
.	O

The	O
Δtsr3	B-mutant
deletion	O
is	O
synthetic	O
sick	O
with	O
a	O
Δsnr35	B-mutant
deletion	O
preventing	O
U1191	B-residue_name_number
pseudouridylation	O
.	O

(	O
B	O
)	O
In	O
agar	B-experimental_method
diffusion	I-experimental_method
assays	I-experimental_method
the	O
yeast	B-taxonomy_domain
Δtsr3	B-mutant
deletion	B-protein_state
mutant	I-protein_state
shows	O
a	O
hypersensitivity	O
against	O
paromomycin	B-chemical
and	O
hygromycin	B-chemical
B	I-chemical
which	O
is	O
further	O
increased	O
by	O
recombination	O
with	O
Δsnr35	B-mutant
.	O
(	O
C	O
)	O
Northern	B-experimental_method
blot	I-experimental_method
analysis	I-experimental_method
with	O
an	O
ITS1	O
hybridization	O
probe	O
after	O
siRNA	B-experimental_method
depletion	I-experimental_method
of	O
HsTSR3	B-protein
(	O
siRNAs	B-chemical
544	O
and	O
545	O
)	O
and	O
a	O
scrambled	O
siRNA	B-chemical
as	O
control	O
.	O

The	O
accumulation	O
of	O
18SE	B-chemical
and	O
47S	B-chemical
and	O
/	O
or	O
45S	B-chemical
pre	I-chemical
-	I-chemical
RNAs	I-chemical
is	O
enforced	O
upon	O
HsTSR3	B-protein
depletion	O
.	O

Right	O
gel	O
:	O
Ethidium	O
bromide	O
staining	O
showing	O
18S	B-chemical
and	O
28S	B-chemical
rRNAs	I-chemical
.	O

(	O
D	O
)	O
Cytoplasmic	O
localization	O
of	O
yeast	B-taxonomy_domain
Tsr3	B-protein
shown	O
by	O
fluorescence	B-experimental_method
microscopy	I-experimental_method
of	O
GFP	B-mutant
-	I-mutant
fused	I-mutant
Tsr3	I-mutant
.	O

From	O
left	O
to	O
right	O
:	O
differential	B-experimental_method
interference	I-experimental_method
contrast	I-experimental_method
(	O
DIC	B-experimental_method
),	O
green	O
fluorescence	O
of	O
GFP	B-mutant
-	I-mutant
Tsr3	I-mutant
,	O
red	O
fluorescence	O
of	O
Nop56	B-mutant
-	I-mutant
mRFP	I-mutant
as	O
nucleolar	O
marker	O
,	O
and	O
merge	O
of	O
GFP	B-mutant
-	I-mutant
Tsr3	I-mutant
/	O
Nop56	B-mutant
-	I-mutant
mRFP	I-mutant
with	O
DIC	B-experimental_method
.	O
(	O
E	O
)	O
Elution	B-evidence
profile	I-evidence
(	O
A254	O
)	O
after	O
sucrose	B-experimental_method
gradient	I-experimental_method
separation	I-experimental_method
of	O
yeast	B-taxonomy_domain
ribosomal	B-complex_assembly
subunits	I-complex_assembly
and	O
polysomes	B-complex_assembly
(	O
upper	O
part	O
)	O
and	O
western	B-experimental_method
blot	I-experimental_method
analysis	O
of	O
3xHA	B-chemical
tagged	O
Tsr3	B-protein
(	O
Tsr3	B-mutant
-	I-mutant
3xHA	I-mutant
)	O
after	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
separation	O
of	O
polysome	O
profile	O
fractions	O
taken	O
every	O
20	O
s	O
(	O
lower	O
part	O
).	O

The	O
TSR3	B-protein
gene	O
was	O
genetically	O
modified	O
at	O
its	O
native	O
locus	O
,	O
resulting	O
in	O
a	O
C	O
-	O
terminal	O
fusion	B-protein_state
of	O
Tsr3	B-protein
with	O
a	O
3xHA	B-chemical
epitope	O
expressed	O
by	O
the	O
native	O
promotor	O
in	O
yeast	B-taxonomy_domain
strain	O
CEN	O
.	O
BM258	O
-	O
5B	O
.	O

The	O
influence	O
of	O
the	O
acp	B-chemical
modification	O
of	O
nucleotide	B-chemical
1191	B-residue_number
on	O
ribosome	O
function	O
was	O
analyzed	O
by	O
treating	O
Δtsr3	B-mutant
mutants	O
with	O
protein	O
synthesis	O
inhibitors	O
.	O

A	O
minor	O
effect	O
on	O
20S	B-chemical
rRNA	I-chemical
accumulation	O
was	O
also	O
observed	O
for	O
Δsnr35	B-mutant
,	O
but	O
-	O
probably	O
due	O
to	O
different	O
strain	O
backgrounds	O
–	O
to	O
a	O
weaker	O
extent	O
than	O
described	O
earlier	O
.	O

In	O
human	B-species
cells	O
,	O
the	O
depletion	B-experimental_method
of	I-experimental_method
HsTsr3	B-protein
in	O
HCT116	O
(+/+)	O
cells	O
caused	O
an	O
accumulation	O
of	O
the	O
human	B-species
20S	B-chemical
pre	I-chemical
-	I-chemical
rRNA	I-chemical
equivalent	O
18S	B-chemical
-	I-chemical
E	I-chemical
suggesting	O
an	O
evolutionary	O
conserved	O
role	O
of	O
Tsr3	B-protein
in	O
the	O
late	O
steps	O
of	O
18S	B-chemical
rRNA	I-chemical
processing	O
(	O
Figure	O
2C	O
and	O
Supplementary	O
Figure	O
S2B	O
).	O

Surprisingly	O
,	O
early	O
nucleolar	O
processing	O
reactions	O
were	O
also	O
inhibited	O
,	O
and	O
this	O
was	O
observed	O
in	O
both	O
yeast	B-taxonomy_domain
Δtsr3	B-mutant
cells	O
(	O
see	O
accumulation	O
of	O
35S	B-complex_assembly
in	O
Supplementary	O
Figure	O
S2C	O
)	O
and	O
Tsr3	B-protein
depleted	O
human	B-species
cells	O
(	O
see	O
47S	B-complex_assembly
/	O
45S	B-complex_assembly
accumulation	O
in	O
Figure	O
2C	O
and	O
Northern	B-experimental_method
blot	I-experimental_method
quantification	O
in	O
Supplementary	O
Figure	O
S2B	O
).	O

Consistent	O
with	O
its	O
role	O
in	O
late	O
18S	B-chemical
rRNA	I-chemical
processing	O
,	O
TSR3	B-protein
deletion	O
leads	O
to	O
a	O
ribosomal	O
subunit	O
imbalance	O
with	O
a	O
reduced	O
40S	B-complex_assembly
to	O
60S	B-complex_assembly
ratio	O
of	O
0	O
.	O
81	O
(	O
σ	O
=	O
0	O
.	O
024	O
)	O
which	O
was	O
further	O
increased	O
in	O
a	O
Δtsr3	B-mutant
Δsnr35	I-mutant
recombinant	O
to	O
0	O
.	O
73	O
(	O
σ	O
=	O
0	O
.	O
023	O
)	O
(	O
Supplementary	O
Figure	O
S2F	O
).	O

Cellular	O
localization	O
of	O
Tsr3	B-protein
in	O
S	B-species
.	I-species
cerevisiae	I-species

Fluorescence	B-experimental_method
microscopy	I-experimental_method
of	O
GFP	B-protein_state
-	I-protein_state
tagged	I-protein_state
Tsr3	B-protein
localized	O
the	O
fusion	O
protein	O
in	O
the	O
cytoplasm	O
of	O
yeast	B-taxonomy_domain
cells	O
and	O
no	O
co	O
-	O
localization	O
with	O
the	O
nucleolar	O
marker	O
protein	O
Nop56	B-protein
could	O
be	O
observed	O
(	O
Figure	O
2D	O
).	O

This	O
agrees	O
with	O
previous	O
biochemical	O
data	O
suggesting	O
that	O
the	O
acp	B-chemical
modification	O
of	O
18S	B-chemical
rRNA	I-chemical
occurs	O
late	O
during	O
40S	B-complex_assembly
subunit	O
biogenesis	O
in	O
the	O
cytoplasm	O
,	O
and	O
makes	O
an	O
additional	O
nuclear	O
localization	O
as	O
reported	O
in	O
a	O
previous	O
large	O
-	O
scale	O
analysis	O
unlikely	O
.	O

After	O
polysome	B-experimental_method
gradient	I-experimental_method
separation	I-experimental_method
C	O
-	O
terminally	O
epitope	O
-	O
labeled	O
Tsr3	B-mutant
-	I-mutant
3xHA	I-mutant
was	O
exclusively	O
detectable	O
in	O
the	O
low	O
-	O
density	O
fraction	O
(	O
Figure	O
2E	O
).	O

Such	O
distribution	B-evidence
on	I-evidence
a	I-evidence
density	I-evidence
gradient	I-evidence
suggests	O
that	O
Tsr3	B-protein
only	O
interacts	O
transiently	O
with	O
pre	B-complex_assembly
-	I-complex_assembly
40S	I-complex_assembly
subunits	I-complex_assembly
,	O
which	O
presumably	O
explains	O
why	O
it	O
was	O
not	O
characterized	O
in	O
pre	B-experimental_method
-	I-experimental_method
ribosome	I-experimental_method
affinity	I-experimental_method
purifications	I-experimental_method
.	O

Searches	O
for	O
sequence	O
homologs	O
of	O
S	B-species
.	I-species
cerevisiae	I-species
Tsr3	B-protein
(	O
ScTsr3	B-protein
)	O
by	O
us	O
and	O
others	O
revealed	O
that	O
the	O
genomes	O
of	O
many	O
archaea	B-taxonomy_domain
contain	O
genes	O
encoding	O
Tsr3	B-protein_type
-	I-protein_type
like	I-protein_type
proteins	I-protein_type
.	O

To	O
locate	O
the	O
domains	O
most	O
important	O
for	O
Tsr3	B-protein
activity	O
,	O
ScTsr3	B-protein
fragments	O
of	O
different	O
lengths	O
containing	O
the	O
highly	B-protein_state
conserved	I-protein_state
central	O
part	O
were	O
expressed	B-experimental_method
in	O
a	O
Δtsr3	B-mutant
mutant	B-protein_state
(	O
Figure	O
3A	O
)	O
and	O
analyzed	O
by	O
primer	B-experimental_method
extension	I-experimental_method
(	O
Figure	O
3B	O
)	O
and	O
Northern	B-experimental_method
blotting	I-experimental_method
(	O
Figure	O
3C	O
).	O

Domain	O
characterization	O
of	O
yeast	B-taxonomy_domain
Tsr3	B-protein
and	O
correlation	O
of	O
acp	B-chemical
modification	O
with	O
late	O
18S	B-chemical
rRNA	I-chemical
processing	O
steps	O
.	O
(	O
A	O
)	O
Scheme	O
of	O
the	O
TSR3	B-protein
gene	O
with	O
truncation	O
positions	O
in	O
the	O
open	O
reading	O
frame	O
.	O

TSR3	B-protein
fragments	O
of	O
different	O
length	O
were	O
expressed	O
under	O
the	O
native	O
promotor	O
from	O
multicopy	O
plasmids	O
in	O
a	O
Δtsr3	B-mutant
deletion	O
strain	O
.	O

N	O
-	O
terminal	O
deletions	B-experimental_method
of	O
36	B-residue_range
or	O
45	B-residue_range
amino	O
acids	O
and	O
C	O
-	O
terminal	O
deletions	B-experimental_method
of	O
43	B-residue_range
or	O
76	B-residue_range
residues	O
show	O
a	O
primer	B-evidence
extension	I-evidence
stop	I-evidence
comparable	O
to	O
the	O
wild	B-protein_state
type	I-protein_state
.	O

Tsr3	B-protein
fragments	O
37	B-residue_range
–	I-residue_range
223	I-residue_range
or	O
46	B-residue_range
–	I-residue_range
223	I-residue_range
cause	O
a	O
nearly	O
complete	O
loss	O
of	O
the	O
arrest	O
signal	O
.	O

The	O
box	O
highlights	O
the	O
shortest	O
Tsr3	B-protein
fragment	O
(	O
aa	O
46	B-residue_range
–	I-residue_range
270	I-residue_range
)	O
with	O
wild	B-protein_state
type	I-protein_state
activity	O
(	O
strong	O
primer	B-evidence
extension	I-evidence
block	I-evidence
).	O
(	O
C	O
)	O
Northern	B-experimental_method
blot	I-experimental_method
analysis	O
of	O
20S	B-chemical
pre	I-chemical
-	I-chemical
rRNA	I-chemical
accumulation	O
.	O

A	O
weak	O
20S	B-chemical
rRNA	I-chemical
signal	O
,	O
indicating	O
normal	O
processing	O
,	O
is	O
observed	O
for	O
Tsr3	B-protein
fragment	O
46	B-residue_range
–	I-residue_range
270	I-residue_range
(	O
highlighted	O
in	O
a	O
box	O
)	O
showing	O
its	O
functionality	O
.	O

Strong	O
20S	O
rRNA	O
accumulation	O
similar	O
to	O
that	O
of	O
the	O
Δtsr3	B-mutant
deletion	B-experimental_method
is	O
observed	O
for	O
Tsr3	B-protein
fragments	O
37	B-residue_range
–	I-residue_range
223	I-residue_range
or	O
46	B-residue_range
–	I-residue_range
223	I-residue_range
.	O

Thus	O
,	O
the	O
archaeal	B-taxonomy_domain
homologs	O
correspond	O
to	O
the	O
functional	O
core	O
of	O
Tsr3	B-protein
.	O

In	O
order	O
to	O
define	O
the	O
structural	O
basis	O
for	O
Tsr3	B-protein
function	O
,	O
homologs	O
from	O
thermophilic	B-taxonomy_domain
archaea	I-taxonomy_domain
were	O
screened	O
for	O
crystallization	B-experimental_method
.	O

We	O
focused	O
on	O
archaeal	B-taxonomy_domain
species	O
containing	O
a	O
putative	O
Nep1	B-protein
homolog	O
suggesting	O
that	O
these	O
species	O
are	O
in	O
principle	O
capable	O
of	O
synthesizing	O
N1	B-chemical
-	I-chemical
methyl	I-chemical
-	I-chemical
N3	I-chemical
-	I-chemical
acp	I-chemical
-	I-chemical
pseudouridine	I-chemical
.	O

Well	O
diffracting	O
crystals	B-evidence
were	O
obtained	O
for	O
Tsr3	B-protein
homologs	O
from	O
the	O
two	O
crenarchaeal	B-taxonomy_domain
species	O
Vulcanisaeta	B-species
distributa	I-species
(	O
VdTsr3	B-protein
)	O
and	O
Sulfolobus	B-species
solfataricus	I-species
(	O
SsTsr3	B-protein
)	O
which	O
share	O
36	O
%	O
(	O
VdTsr3	B-protein
)	O
and	O
38	O
%	O
(	O
SsTsr3	B-protein
)	O
identity	O
with	O
the	O
ScTsr3	B-protein
core	B-structure_element
region	I-structure_element
(	O
ScTsr3	B-protein
aa	O
46	B-residue_range
–	I-residue_range
223	I-residue_range
).	O

Crystals	B-evidence
of	O
VdTsr3	B-protein
diffracted	O
to	O
a	O
resolution	O
of	O
1	O
.	O
6	O
Å	O
whereas	O
crystals	B-evidence
of	O
SsTsr3	B-protein
diffracted	O
to	O
2	O
.	O
25	O
Å	O
.	O
Serendipitously	O
,	O
VdTsr3	B-protein
was	O
purified	O
and	O
crystallized	B-experimental_method
in	B-protein_state
complex	I-protein_state
with	I-protein_state
endogenous	B-protein_state
(	O
E	B-species
.	I-species
coli	I-species
)	O
SAM	B-chemical
(	O
Supplementary	O
Figure	O
S4	O
)	O
while	O
SsTsr3	B-protein
crystals	B-evidence
contained	O
the	O
protein	O
in	O
the	O
apo	B-protein_state
state	O
.	O

The	O
structure	B-evidence
of	O
SsTsr3	B-protein
was	O
solved	O
by	O
molecular	B-experimental_method
replacement	I-experimental_method
using	O
VdTsr3	B-protein
as	O
a	O
search	O
model	O
(	O
see	O
Supplementary	O
Table	O
S1	O
for	O
data	O
collection	O
and	O
refinement	O
statistics	O
).	O

The	O
structure	B-evidence
of	O
VdTsr3	B-protein
can	O
be	O
divided	O
into	O
two	O
domains	O
(	O
Figure	O
4A	O
).	O

The	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
(	O
aa	O
1	B-residue_range
–	I-residue_range
92	I-residue_range
)	O
has	O
a	O
mixed	O
α	B-structure_element
/	I-structure_element
β	I-structure_element
-	I-structure_element
structure	I-structure_element
centered	O
around	O
a	O
five	B-structure_element
-	I-structure_element
stranded	I-structure_element
all	I-structure_element
-	I-structure_element
parallel	I-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
(	O
Figure	O
4B	O
)	O
with	O
the	O
strand	O
order	O
β5	B-structure_element
↑-	I-structure_element
β3	B-structure_element
↑-	I-structure_element
β4	B-structure_element
↑-	I-structure_element
β1	B-structure_element
↑-	I-structure_element
β2	B-structure_element
↑.	I-structure_element
The	O
loops	B-structure_element
connecting	O
β1	B-structure_element
and	O
β2	B-structure_element
,	O
β3	B-structure_element
and	O
β4	B-structure_element
and	O
β4	B-structure_element
and	O
β5	B-structure_element
include	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
α1	B-structure_element
,	O
α2	B-structure_element
and	O
α3	B-structure_element
,	O
respectively	O
.	O

The	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
(	O
aa	O
93	B-residue_range
–	I-residue_range
184	I-residue_range
)	O
has	O
a	O
globular	B-structure_element
all	I-structure_element
α	I-structure_element
-	I-structure_element
helical	I-structure_element
structure	I-structure_element
comprising	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
α4	B-structure_element
to	I-structure_element
α9	I-structure_element
.	O

Remarkably	O
,	O
the	O
entire	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
(	O
92	B-residue_range
aa	I-residue_range
)	O
of	O
the	O
protein	O
is	O
threaded	O
through	O
the	O
loop	B-structure_element
which	O
connects	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
β3	B-structure_element
and	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
α2	B-structure_element
of	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
.	O

The	O
structure	B-evidence
of	O
SsTsr3	B-protein
in	O
the	O
apo	B-protein_state
state	O
is	O
very	O
similar	O
to	O
that	O
of	O
VdTsr3	B-protein
(	O
Figure	O
4C	O
)	O
with	O
an	O
RMSD	B-evidence
for	O
equivalent	O
Cα	O
atoms	O
of	O
1	O
.	O
1	O
Å	O
.	O
The	O
only	O
significant	O
difference	O
in	O
the	O
global	O
structure	B-evidence
of	O
the	O
two	O
proteins	O
is	O
the	O
presence	O
of	O
an	O
extended	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
α8	B-structure_element
and	O
the	O
absence	B-protein_state
of	I-protein_state
α	B-structure_element
-	I-structure_element
helix	I-structure_element
α9	B-structure_element
in	O
SsTsr3	B-protein
.	O

Tsr3	B-protein
has	O
a	O
fold	O
similar	O
to	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
RNA	I-protein_type
methyltransferases	I-protein_type
.	O
(	O
A	O
)	O
Cartoon	O
representation	O
of	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
structure	I-evidence
of	O
VdTsr3	B-protein
in	O
two	O
orientations	O
.	O

The	O
bound	O
S	B-chemical
-	I-chemical
adenosylmethionine	I-chemical
is	O
shown	O
in	O
a	O
stick	O
representation	O
and	O
colored	O
by	O
atom	O
type	O
.	O

The	O
color	O
coding	O
is	O
the	O
same	O
as	O
in	O
(	O
A	O
).	O
(	O
C	O
)	O
Structural	B-experimental_method
superposition	I-experimental_method
of	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
structures	I-evidence
of	O
VdTsr3	B-protein
in	O
the	O
SAM	B-protein_state
-	I-protein_state
bound	I-protein_state
state	O
(	O
red	O
)	O
and	O
SsTsr3	B-protein
(	O
blue	O
)	O
in	O
the	O
apo	B-protein_state
state	O
.	O

The	O
locations	O
of	O
the	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
α8	B-structure_element
which	O
is	O
longer	O
in	O
SsTsr3	B-protein
and	O
of	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
α9	B-structure_element
which	O
is	O
only	O
present	O
in	O
VdTsr3	B-protein
are	O
indicated	O
.	O
(	O
D	O
)	O
Secondary	O
structure	O
cartoon	O
(	O
left	O
)	O
of	O
S	B-species
.	I-species
pombe	I-species
Trm10	B-protein
(	O
pdb4jwf	O
)—	O
the	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
RNA	I-protein_type
methyltransferase	I-protein_type
structurally	O
most	O
similar	O
to	O
Tsr3	B-protein
and	O
superposition	B-experimental_method
of	O
the	O
VdTsr3	B-protein
and	O
Trm10	B-protein
X	B-evidence
-	I-evidence
ray	I-evidence
structures	I-evidence
(	O
right	O
).	O
(	O
E	O
)	O
Analytical	B-experimental_method
gel	I-experimental_method
filtration	I-experimental_method
profiles	B-evidence
for	O
VdTsr3	B-protein
(	O
red	O
)	O
and	O
SsTsr3	B-protein
(	O
blue	O
)	O
show	O
that	O
both	O
proteins	O
are	O
monomeric	B-oligomeric_state
in	O
solution	O
.	O

Vd	B-species
,	O
Vulcanisaeta	B-species
distributa	I-species
;	O
Ss	B-species
,	O
Sulfolobus	B-species
solfataricus	I-species
.	O

However	O
,	O
no	O
structural	O
similarity	O
to	O
an	O
RLI	B-structure_element
-	I-structure_element
domain	I-structure_element
was	O
detectable	O
.	O

This	O
is	O
in	O
accordance	O
with	O
the	O
functional	O
analysis	O
of	O
alanine	B-experimental_method
replacement	I-experimental_method
mutations	I-experimental_method
of	O
cysteine	B-residue_name
residues	O
in	O
ScTsr3	B-protein
(	O
Supplementary	O
Figure	O
S3	O
).	O

The	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
topology	I-structure_element
and	O
the	O
deep	O
C	O
-	O
terminal	O
trefoil	B-structure_element
knot	I-structure_element
of	O
archaeal	B-taxonomy_domain
Tsr3	B-protein
are	O
the	O
structural	O
hallmarks	O
of	O
the	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
RNA	I-protein_type
-	I-protein_type
methyltransferase	I-protein_type
fold	O
.	O

In	O
comparison	O
to	O
Tsr3	B-protein
the	O
central	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
element	I-structure_element
of	O
Trm10	B-protein
is	O
extended	O
by	O
one	O
additional	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
pairing	O
to	O
β2	B-structure_element
.	O

Furthermore	O
,	O
the	O
trefoil	B-structure_element
knot	I-structure_element
of	O
Trm10	B-protein
is	O
not	O
as	O
deep	O
as	O
that	O
of	O
Tsr3	B-protein
(	O
Figure	O
4D	O
).	O

Interestingly	O
,	O
Nep1	B-protein
—	O
the	O
enzyme	O
preceding	O
Tsr3	B-protein
in	O
the	O
biosynthetic	O
pathway	O
for	O
the	O
synthesis	O
of	O
m1acp3Ψ	B-chemical
—	O
also	O
belongs	O
to	O
the	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
of	I-protein_type
RNA	I-protein_type
methyltransferases	I-protein_type
.	O

However	O
,	O
the	O
structural	O
similarities	O
between	O
Nep1	B-protein
and	O
Tsr3	B-protein
(	O
DALI	B-evidence
Z	I-evidence
-	I-evidence
score	I-evidence
4	O
.	O
4	O
)	O
are	O
less	O
pronounced	O
than	O
between	O
Tsr3	B-protein
and	O
Trm10	B-protein
.	O

Most	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
RNA	I-protein_type
-	I-protein_type
methyltransferases	I-protein_type
are	O
homodimers	B-oligomeric_state
.	O

So	O
far	O
,	O
structural	O
information	O
is	O
only	O
available	O
for	O
one	O
other	O
enzyme	O
that	O
transfers	O
the	O
acp	B-chemical
group	O
from	O
SAM	B-chemical
to	O
an	O
RNA	B-chemical
nucleotide	B-chemical
.	O

Cofactor	O
binding	O
of	O
Tsr3	B-protein

The	O
SAM	B-site
-	I-site
binding	I-site
site	I-site
of	O
Tsr3	B-protein
is	O
located	O
in	O
a	O
deep	O
crevice	O
between	O
the	O
N	B-structure_element
-	I-structure_element
and	I-structure_element
C	I-structure_element
-	I-structure_element
terminal	I-structure_element
domains	I-structure_element
in	O
the	O
vicinity	O
of	O
the	O
trefoil	B-structure_element
knot	I-structure_element
as	O
typical	O
for	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
RNA	I-protein_type
-	I-protein_type
methyltransferases	I-protein_type
(	O
Figure	O
4A	O
).	O

The	O
adenine	B-chemical
base	O
of	O
the	O
cofactor	O
is	O
recognized	O
by	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
between	O
its	O
N1	O
nitrogen	O
and	O
the	O
backbone	O
amide	O
of	O
L93	B-residue_name_number
directly	O
preceding	O
β5	B-structure_element
as	O
well	O
as	O
between	O
its	O
N6	O
-	O
amino	O
group	O
and	O
the	O
backbone	O
carbonyl	O
group	O
of	O
Y108	B-residue_name_number
located	O
in	O
the	O
loop	B-structure_element
connecting	O
β5	B-structure_element
in	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
and	O
α4	B-structure_element
in	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
(	O
Figure	O
5A	O
).	O

Furthermore	O
,	O
the	O
adenine	B-chemical
base	O
of	O
SAM	B-chemical
is	O
involved	O
in	O
hydrophobic	B-bond_interaction
packing	I-bond_interaction
interactions	I-bond_interaction
with	O
the	O
side	O
chains	O
of	O
L45	B-residue_name_number
(	O
β3	B-structure_element
),	O
P47	B-residue_name_number
and	O
W73	B-residue_name_number
(	O
α3	B-structure_element
)	O
in	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
as	O
well	O
as	O
with	O
L93	B-residue_name_number
,	O
L110	B-residue_name_number
(	O
both	O
in	O
the	O
loop	B-structure_element
connecting	O
β5	B-structure_element
and	O
α4	B-structure_element
)	O
and	O
A115	B-residue_name_number
(	O
α5	B-structure_element
)	O
in	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
.	O

The	O
ribose	B-chemical
2	O
′	O
and	O
3	O
′	O
hydroxyl	O
groups	O
of	O
SAM	B-chemical
are	O
hydrogen	B-bond_interaction
bonded	I-bond_interaction
to	O
the	O
backbone	O
carbonyl	O
group	O
of	O
I69	B-residue_name_number
.	O

The	O
acp	B-chemical
side	O
chain	O
of	O
SAM	B-chemical
is	O
fixed	O
in	O
position	O
by	O
hydrogen	B-bond_interaction
bonding	I-bond_interaction
of	O
its	O
carboxylate	O
group	O
to	O
the	O
backbone	O
amide	O
and	O
the	O
side	O
chain	O
hydroxyl	O
group	O
of	O
T19	B-residue_name_number
in	O
α1	B-structure_element
as	O
well	O
as	O
the	O
backbone	O
amide	O
group	O
of	O
T112	B-residue_name_number
in	O
α4	B-structure_element
(	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
).	O

Most	O
importantly	O
,	O
the	O
methyl	O
group	O
of	O
SAM	B-chemical
is	O
buried	O
in	O
a	O
hydrophobic	B-site
pocket	I-site
formed	O
by	O
the	O
sidechains	O
of	O
W73	B-residue_name_number
and	O
A76	B-residue_name_number
both	O
located	O
in	O
α3	B-structure_element
(	O
Figure	O
5A	O
and	O
B	O
).	O

W73	B-residue_name_number
is	O
highly	B-protein_state
conserved	I-protein_state
in	O
all	O
known	O
Tsr3	B-protein_type
proteins	I-protein_type
,	O
whereas	O
A76	B-residue_name_number
can	O
be	O
replaced	O
by	O
other	O
hydrophobic	O
amino	B-chemical
acids	I-chemical
.	O

Consequently	O
,	O
the	O
accessibility	O
of	O
this	O
methyl	O
group	O
for	O
a	O
nucleophilic	O
attack	O
is	O
strongly	O
reduced	O
in	O
comparison	O
with	O
RNA	B-protein_type
-	I-protein_type
methyltransferases	I-protein_type
such	O
as	O
Trm10	B-protein
(	O
Figure	O
5B	O
,	O
C	O
).	O

In	O
contrast	O
,	O
the	O
acp	B-chemical
side	O
chain	O
of	O
SAM	B-chemical
is	O
accessible	O
for	O
reactions	O
in	O
the	O
Tsr3	B-protein_state
-	I-protein_state
bound	I-protein_state
state	O
(	O
Figure	O
5B	O
).	O

(	O
A	O
)	O
Close	O
-	O
up	O
view	O
of	O
the	O
SAM	B-site
-	I-site
binding	I-site
pocket	I-site
of	O
VdTsr3	B-protein
.	O

Nitrogen	O
atoms	O
are	O
dark	O
blue	O
,	O
oxygen	O
atoms	O
red	O
,	O
sulfur	B-chemical
atoms	O
orange	O
,	O
carbon	O
atoms	O
of	O
the	O
protein	O
light	O
blue	O
and	O
carbon	O
atoms	O
of	O
SAM	B-chemical
yellow	O
.	O

Hydrogen	B-bond_interaction
bonds	I-bond_interaction
are	O
indicated	O
by	O
dashed	O
lines	O
.	O

(	O
B	O
)	O
Solvent	O
accessibility	O
of	O
the	O
acp	B-chemical
group	O
of	O
SAM	B-chemical
bound	B-protein_state
to	I-protein_state
VdTsr3	B-protein
.	O

The	O
solvent	O
accessible	O
surface	O
of	O
the	O
protein	O
is	O
shown	O
in	O
semitransparent	O
gray	O
whereas	O
SAM	B-chemical
is	O
show	O
in	O
a	O
stick	O
representation	O
.	O

A	O
red	O
arrow	O
indicates	O
the	O
reactive	O
CH2	O
-	O
moiety	O
of	O
the	O
acp	B-chemical
group	O
.	O
(	O
C	O
)	O
Solvent	O
accessibility	O
of	O
the	O
SAM	B-chemical
methyl	O
group	O
for	O
SAM	B-chemical
bound	B-protein_state
to	I-protein_state
the	O
RNA	B-protein_type
methyltransferase	I-protein_type
Trm10	B-protein
.	O

A	O
red	O
arrow	O
indicates	O
the	O
SAM	B-chemical
methyl	O
group	O
.	O
(	O
D	O
)	O
Binding	O
of	O
SAM	B-chemical
analogs	O
to	O
SsTsr3	B-protein
.	O

Tryptophan	B-evidence
fluorescence	I-evidence
quenching	I-evidence
curves	I-evidence
upon	O
addition	O
of	O
SAM	B-chemical
(	O
blue	O
),	O
5	B-chemical
′-	I-chemical
methyl	I-chemical
-	I-chemical
thioadenosine	I-chemical
(	O
red	O
)	O
and	O
SAH	B-chemical
(	O
black	O
).	O

(	O
E	O
)	O
Binding	O
of	O
14C	B-chemical
-	I-chemical
labeled	I-chemical
SAM	I-chemical
to	O
SsTsr3	B-protein
.	O

Radioactively	O
labeled	O
SAM	B-chemical
is	O
retained	O
on	O
a	O
filter	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
SsTsr3	B-protein
.	O

Addition	O
of	O
unlabeled	O
SAM	B-chemical
competes	O
with	O
the	O
binding	O
of	O
labeled	O
SAM	B-chemical
.	O

This	O
correlates	O
with	O
a	O
20S	B-chemical
pre	I-chemical
-	I-chemical
rRNA	I-chemical
accumulation	O
comparable	O
to	O
the	O
Δtsr3	B-mutant
deletion	O
(	O
right	O
:	O
northern	B-experimental_method
blot	I-experimental_method
).	O

Binding	B-evidence
affinities	I-evidence
for	O
SAM	B-chemical
and	O
its	O
analogs	O
5	B-chemical
′-	I-chemical
methylthioadenosin	I-chemical
and	O
SAH	B-chemical
to	O
SsTsr3	B-protein
were	O
measured	O
using	O
tryptophan	B-experimental_method
fluorescence	I-experimental_method
quenching	I-experimental_method
.	O

SsTsr3	B-protein
bound	B-protein_state
SAM	B-chemical
with	O
a	O
KD	B-evidence
of	O
6	O
.	O
5	O
μM	O
,	O
which	O
is	O
similar	O
to	O
SAM	B-evidence
-	I-evidence
KD	I-evidence
'	I-evidence
s	I-evidence
reported	O
for	O
several	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
methyltransferases	I-protein_type
.	O

5	B-chemical
′-	I-chemical
methylthioadenosin	I-chemical
—	O
the	O
reaction	O
product	O
after	O
the	O
acp	B-chemical
-	O
transfer	O
—	O
binds	O
only	O
∼	O
2	O
.	O
5	O
-	O
fold	O
weaker	O
(	O
KD	O
=	O
16	O
.	O
7	O
μM	O
)	O
compared	O
to	O
SAM	B-chemical
.	O

This	O
suggests	O
that	O
the	O
hydrophobic	B-bond_interaction
interaction	I-bond_interaction
between	O
SAM	B-chemical
'	O
s	O
methyl	O
group	O
and	O
the	O
hydrophobic	B-site
pocket	I-site
of	O
Tsr3	B-protein
is	O
thermodynamically	O
important	O
for	O
the	O
interaction	O
.	O

On	O
the	O
other	O
hand	O
,	O
the	O
loss	O
of	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
between	O
the	O
acp	B-chemical
sidechain	O
carboxylate	O
group	O
and	O
the	O
protein	O
appears	O
to	O
be	O
thermodynamically	O
less	O
important	O
but	O
these	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
might	O
play	O
a	O
crucial	O
role	O
for	O
the	O
proper	O
orientation	O
of	O
the	O
cofactor	O
side	O
chain	O
in	O
the	O
substrate	B-site
binding	I-site
pocket	I-site
.	O

Accordingly	O
,	O
a	O
W66A	B-mutant
-	O
mutation	B-experimental_method
(	O
W73	B-residue_name_number
in	O
VdTsr3	B-protein
)	O
of	O
SsTsr3	B-protein
significantly	O
diminished	O
SAM	B-evidence
-	I-evidence
binding	I-evidence
in	O
a	O
filter	B-experimental_method
binding	I-experimental_method
assay	I-experimental_method
compared	O
to	O
the	O
wild	B-protein_state
type	I-protein_state
(	O
Figure	O
5E	O
).	O

Furthermore	O
,	O
a	O
W	B-experimental_method
to	I-experimental_method
A	I-experimental_method
mutation	I-experimental_method
at	O
the	O
equivalent	O
position	O
W114	B-residue_name_number
in	O
ScTsr3	B-protein
strongly	O
reduced	O
the	O
in	O
vivo	O
acp	B-protein_type
transferase	I-protein_type
activity	O
(	O
Figure	O
5F	O
).	O

The	O
side	O
chain	O
hydroxyl	O
group	O
of	O
T19	B-residue_name_number
seems	O
of	O
minor	O
importance	O
for	O
SAM	B-chemical
binding	O
since	O
mutations	B-experimental_method
of	O
T17	B-residue_name_number
(	O
T19	B-residue_name_number
in	O
VdTsr3	B-protein
)	O
to	O
either	O
A	B-residue_name
or	O
D	B-residue_name
did	O
not	O
significantly	O
influence	O
the	O
SAM	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
SsTsr3	B-protein
(	O
KD	B-evidence
'	O
s	O
=	O
3	O
.	O
9	O
or	O
11	O
.	O
2	O
mM	O
,	O
respectively	O
).	O

Nevertheless	O
,	O
a	O
mutation	B-experimental_method
of	O
the	O
equivalent	O
position	O
S62	B-residue_name_number
of	O
ScTsr3	B-protein
to	O
D	B-residue_name
,	O
but	O
not	O
to	O
A	B-residue_name
,	O
resulted	O
in	O
reduced	O
acp	B-chemical
modification	O
in	O
vivo	O
,	O
as	O
shown	O
by	O
primer	B-experimental_method
extension	I-experimental_method
analysis	I-experimental_method
(	O
Figure	O
5F	O
).	O

The	O
acp	B-chemical
-	O
transfer	O
reaction	O
catalyzed	O
by	O
Tsr3	B-protein
most	O
likely	O
requires	O
the	O
presence	O
of	O
a	O
catalytic	O
base	O
in	O
order	O
to	O
abstract	O
a	O
proton	O
from	O
the	O
N3	O
imino	O
group	O
of	O
the	O
modified	O
pseudouridine	B-chemical
.	O

This	O
residue	O
is	O
conserved	B-protein_state
as	I-protein_state
D	B-residue_name
or	O
E	B-residue_name
both	O
in	O
archaeal	B-taxonomy_domain
and	O
eukaryotic	B-taxonomy_domain
Tsr3	B-protein
homologs	O
.	O

Mutations	B-experimental_method
of	O
the	O
corresponding	O
residue	O
in	O
SsTsr3	B-protein
to	O
A	B-residue_name
(	O
D63	B-residue_name_number
)	O
does	O
not	O
significantly	O
alter	O
the	O
SAM	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
the	O
protein	O
(	O
KD	B-evidence
=	O
11	O
.	O
0	O
μM	O
).	O

RNA	B-chemical
-	O
binding	O
of	O
Tsr3	B-protein

Analysis	B-experimental_method
of	I-experimental_method
the	I-experimental_method
electrostatic	I-experimental_method
surface	I-experimental_method
properties	I-experimental_method
of	O
VdTsr3	B-protein
clearly	O
identified	O
positively	B-site
charged	I-site
surface	I-site
patches	I-site
in	O
the	O
vicinity	O
of	O
the	O
SAM	B-site
-	I-site
binding	I-site
site	I-site
suggesting	O
a	O
putative	O
RNA	B-site
-	I-site
binding	I-site
site	I-site
(	O
Figure	O
6A	O
).	O

Furthermore	O
,	O
a	O
negatively	O
charged	O
MES	B-chemical
-	O
ion	O
is	O
found	O
in	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
VdTsr3	B-protein
complexed	B-protein_state
to	I-protein_state
the	O
side	O
chain	O
of	O
K22	B-residue_name_number
in	O
helix	B-structure_element
α1	B-structure_element
.	O

Its	O
negatively	O
charged	O
sulfate	B-chemical
group	O
might	O
mimic	O
an	O
RNA	B-chemical
backbone	O
phosphate	O
.	O

Helix	B-structure_element
α1	B-structure_element
contains	O
two	O
more	O
positively	O
charged	O
amino	O
acids	O
K17	B-residue_name_number
and	O
R25	B-residue_name_number
as	O
does	O
the	O
loop	B-structure_element
preceding	O
it	O
(	O
R9	B-residue_name_number
).	O

Some	O
of	O
these	O
amino	O
acids	O
are	O
conserved	B-protein_state
between	O
archaeal	B-taxonomy_domain
and	O
eukaryotic	B-taxonomy_domain
Tsr3	B-protein
(	O
Supplementary	O
Figure	O
S1A	O
).	O

RNA	O
-	O
binding	O
of	O
Tsr3	B-protein
.	O

(	O
A	O
)	O
Electrostatic	O
charge	O
distribution	O
on	O
the	O
surface	O
of	O
VdTsr3	B-protein
.	O

SAM	B-chemical
is	O
shown	O
in	O
a	O
stick	O
representation	O
.	O

Also	O
shown	O
in	O
stick	O
representation	O
is	O
a	O
negatively	O
charged	O
MES	B-chemical
ion	O
.	O

Conserved	B-protein_state
basic	O
amino	B-chemical
acids	I-chemical
are	O
labeled	O
.	O
(	O
B	O
)	O
Comparison	O
of	O
the	O
secondary	O
structures	O
of	O
helix	B-structure_element
31	I-structure_element
from	O
the	O
small	O
ribosomal	O
subunit	O
rRNAs	B-chemical
in	O
S	B-species
.	I-species
cerevisiae	I-species
and	O
S	B-species
.	I-species
solfataricus	I-species
with	O
the	O
location	O
of	O
the	O
hypermodified	B-protein_state
nucleotide	B-chemical
indicated	O
in	O
red	O
.	O

(	O
C	O
)	O
Binding	O
of	O
SsTsr3	B-protein
to	O
RNA	B-chemical
.	O

5	O
′-	O
fluoresceine	B-chemical
labeled	O
RNA	B-chemical
oligonucleotides	O
corresponding	O
either	O
to	O
the	O
native	B-protein_state
(	O
20mer	B-oligomeric_state
–	O
see	O
inset	O
)	O
or	O
a	O
stabilized	B-protein_state
(	O
20mer_GC	B-oligomeric_state
-	O
inset	O
)	O
helix	B-structure_element
31	I-structure_element
of	O
the	O
small	O
ribosomal	O
subunit	O
rRNA	B-chemical
from	O
S	B-species
.	I-species
solfataricus	I-species
were	O
titrated	B-experimental_method
with	I-experimental_method
increasing	I-experimental_method
amounts	I-experimental_method
of	O
SsTsr3	B-protein
and	O
the	O
changes	O
in	O
the	O
fluoresceine	B-chemical
fluorescence	B-evidence
anisotropy	I-evidence
were	O
measured	O
and	O
fitted	O
to	O
a	O
binding	B-evidence
curve	I-evidence
(	O
20mer	B-oligomeric_state
–	O
red	O
,	O
20mer_GC	B-oligomeric_state
–	O
blue	O
).	O

Oligo	B-chemical
-	I-chemical
U9	I-chemical
-	I-chemical
RNA	I-chemical
was	O
used	O
for	O
comparison	O
(	O
black	O
).	O

The	O
20mer_GC	B-oligomeric_state
RNA	B-chemical
was	O
also	O
titrated	B-experimental_method
with	O
SsTsr3	B-protein
in	O
the	O
presence	O
of	O
2	O
mM	O
SAM	B-chemical
(	O
purple	O
).	O
(	O
D	O
)	O
Mutants	B-protein_state
of	O
ScTsr3	B-protein
R60	B-residue_name_number
,	O
K65	B-residue_name_number
or	O
R131	B-residue_name_number
(	O
equivalent	O
to	O
K17	B-residue_name_number
,	O
K22	B-residue_name_number
and	O
R91	B-residue_name_number
in	O
VdTsr3	B-protein
)	O
expressed	B-experimental_method
in	O
Δtsr3	B-mutant
yeast	B-taxonomy_domain
cells	O
show	O
a	O
primer	B-evidence
extension	I-evidence
stop	I-evidence
comparable	O
to	O
the	O
wild	B-protein_state
type	I-protein_state
.	O

Combination	B-experimental_method
of	I-experimental_method
the	I-experimental_method
three	I-experimental_method
point	I-experimental_method
mutations	I-experimental_method
(	O
R60A	B-mutant
/	O
K65A	B-mutant
/	O
R131A	B-mutant
)	O
leads	O
to	O
a	O
strongly	O
reduced	O
acp	B-chemical
modification	O
of	O
18S	B-chemical
rRNA	I-chemical
.	O

In	O
order	O
to	O
explore	O
the	O
RNA	O
-	O
ligand	O
specificity	O
of	O
Tsr3	B-protein
we	O
titrated	B-experimental_method
SsTsr3	B-protein
prepared	O
in	O
RNase	B-protein_state
-	I-protein_state
free	I-protein_state
form	O
with	O
5	O
′-	O
fluoresceine	B-chemical
-	O
labeled	O
RNA	B-chemical
and	O
determined	O
the	O
affinity	B-evidence
by	O
fluorescence	B-experimental_method
anisotropy	I-experimental_method
measurements	I-experimental_method
.	O

SsTsr3	B-protein
in	O
the	O
apo	B-protein_state
state	O
bound	B-protein_state
a	O
20mer	B-oligomeric_state
RNA	B-chemical
corresponding	O
to	O
helix	B-structure_element
31	I-structure_element
of	O
S	B-species
.	I-species
solfataricus	I-species
16S	B-chemical
rRNA	I-chemical
(	O
Figure	O
6B	O
)	O
with	O
a	O
KD	B-evidence
of	O
1	O
.	O
9	O
μM	O
and	O
to	O
a	O
version	O
of	O
this	O
hairpin	B-structure_element
stabilized	O
by	O
additional	O
GC	O
base	O
pairs	O
(	O
20mer	B-oligomeric_state
-	I-oligomeric_state
GC	I-oligomeric_state
)	O
with	O
a	O
KD	B-evidence
of	O
0	O
.	O
6	O
μM	O
(	O
Figure	O
6C	O
).	O

A	O
single	O
stranded	O
oligoU	B-chemical
-	I-chemical
RNA	I-chemical
bound	B-protein_state
with	O
a	O
10	O
-	O
fold	O
-	O
reduced	O
affinity	B-evidence
(	O
6	O
.	O
0	O
μM	O
).	O

The	O
presence	O
of	O
saturating	O
amounts	O
of	O
SAM	B-chemical
(	O
2	O
mM	O
)	O
did	O
not	O
have	O
a	O
significant	O
influence	O
on	O
the	O
RNA	B-evidence
-	I-evidence
affinity	I-evidence
of	O
SsTsr3	B-protein
(	O
KD	B-evidence
of	O
1	O
.	O
7	O
μM	O
for	O
the	O
20mer	B-oligomeric_state
-	I-oligomeric_state
GC	I-oligomeric_state
-	O
RNA	B-chemical
)	O
suggesting	O
no	O
cooperativity	O
in	O
substrate	O
binding	O
.	O

U1191	B-residue_name_number
is	O
the	O
only	O
hypermodified	B-protein_state
base	O
in	O
the	O
yeast	B-taxonomy_domain
18S	B-chemical
rRNA	I-chemical
and	O
is	O
strongly	B-protein_state
conserved	I-protein_state
in	O
eukaryotes	B-taxonomy_domain
.	O

Unexpectedly	O
,	O
archaeal	B-taxonomy_domain
Tsr3	B-protein
has	O
a	O
structure	B-evidence
similar	O
to	O
SPOUT	B-protein_type
-	I-protein_type
class	I-protein_type
RNA	I-protein_type
methyltransferases	I-protein_type
,	O
and	O
it	O
is	O
the	O
first	O
example	O
for	O
an	O
enzyme	O
of	O
this	O
class	O
transferring	O
an	O
acp	B-chemical
group	O
,	O
due	O
to	O
a	O
modified	O
SAM	B-site
-	I-site
binding	I-site
pocket	I-site
that	O
exposes	O
the	O
acp	B-chemical
instead	O
of	O
the	O
methyl	O
group	O
of	O
SAM	B-chemical
to	O
its	O
RNA	B-chemical
substrate	O
.	O

Similar	O
to	O
the	O
structurally	O
unrelated	O
Rossmann	B-protein_type
-	I-protein_type
fold	I-protein_type
Tyw2	I-protein_type
acp	I-protein_type
transferase	I-protein_type
,	O
the	O
SAM	B-chemical
methyl	O
group	O
of	O
Tsr3	B-protein
is	O
bound	O
in	O
an	O
inaccessible	O
hydrophobic	B-site
pocket	I-site
whereas	O
the	O
acp	B-chemical
side	O
chain	O
becomes	O
accessible	O
for	O
a	O
nucleophilic	O
attack	O
by	O
the	O
N3	O
of	O
pseudouridine	B-chemical
.	O

Thus	O
,	O
additional	O
examples	O
for	O
acp	B-protein_type
transferase	I-protein_type
enzymes	O
might	O
be	O
found	O
with	O
similarities	O
to	O
other	O
structural	O
classes	O
of	O
methyltransferases	B-protein_type
.	O

In	O
contrast	O
to	O
Nep1	B-protein
,	O
the	O
enzyme	O
preceding	O
Tsr3	B-protein
in	O
the	O
m1acp3Ψ	B-chemical
biosynthesis	O
pathway	O
,	O
Tsr3	B-protein
binds	O
rather	O
weakly	O
and	O
with	O
little	O
specificity	O
to	O
its	O
isolated	O
substrate	O
RNA	B-chemical
.	O

This	O
suggests	O
that	O
Tsr3	B-protein
is	O
not	O
stably	O
incorporated	O
into	O
pre	B-complex_assembly
-	I-complex_assembly
ribosomal	I-complex_assembly
particles	I-complex_assembly
and	O
that	O
its	O
binding	O
to	O
the	O
nascent	O
ribosomal	B-complex_assembly
subunit	I-complex_assembly
possibly	O
requires	O
additional	O
interactions	O
with	O
other	O
pre	O
-	O
ribosomal	O
components	O
.	O

Consistently	O
,	O
in	O
sucrose	B-experimental_method
gradient	I-experimental_method
analysis	I-experimental_method
,	O
Tsr3	B-protein
was	O
found	O
in	O
low	O
-	O
molecular	O
weight	O
fractions	O
rather	O
than	O
with	O
pre	B-complex_assembly
-	I-complex_assembly
ribosome	I-complex_assembly
containing	O
high	O
-	O
molecular	O
weight	O
fractions	O
.	O

In	O
contrast	O
to	O
several	O
enzymes	O
that	O
catalyze	O
base	O
specific	O
modifications	O
in	O
rRNAs	B-chemical
Tsr3	B-protein
is	O
not	O
an	O
essential	O
protein	O
.	O

Typically	O
,	O
other	O
small	B-protein_type
subunit	I-protein_type
rRNA	I-protein_type
methyltransferases	I-protein_type
as	O
Dim1	B-protein
,	O
Bud23	B-protein
and	O
Nep1	B-protein
/	O
Emg1	B-protein
carry	O
dual	O
functions	O
,	O
in	O
ribosome	O
biogenesis	O
and	O
rRNA	B-chemical
modification	O
,	O
and	O
it	O
is	O
their	O
involvement	O
in	O
pre	B-chemical
-	I-chemical
RNA	I-chemical
processing	O
that	O
is	O
essential	O
rather	O
than	O
their	O
RNA	O
-	O
methylating	O
activity	O
(,	O
discussed	O
in	O
7	O
).	O

This	O
demonstrates	O
that	O
,	O
unlike	O
the	O
other	O
small	O
subunit	O
rRNA	B-chemical
base	O
modifications	O
,	O
the	O
acp	B-chemical
modification	O
is	O
required	O
for	O
efficient	O
pre	B-chemical
-	I-chemical
rRNA	I-chemical
processing	O
.	O

Recently	O
,	O
structural	B-experimental_method
,	I-experimental_method
functional	I-experimental_method
,	I-experimental_method
and	I-experimental_method
CRAC	I-experimental_method
(	I-experimental_method
cross	I-experimental_method
-	I-experimental_method
linking	I-experimental_method
and	I-experimental_method
cDNA	I-experimental_method
analysis	I-experimental_method
)	I-experimental_method
experiments	I-experimental_method
of	O
late	O
assembly	O
factors	O
involved	O
in	O
cytoplasmic	O
processing	O
of	O
40S	B-complex_assembly
subunits	I-complex_assembly
,	O
along	O
with	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
studies	O
of	O
the	O
late	B-protein_state
pre	B-complex_assembly
-	I-complex_assembly
40S	I-complex_assembly
subunits	I-complex_assembly
have	O
provided	O
important	O
insights	O
into	O
late	O
pre	B-complex_assembly
-	I-complex_assembly
40S	I-complex_assembly
processing	O
.	O

Apart	O
from	O
most	O
of	O
the	O
ribosomal	O
proteins	O
,	O
cytoplasmic	O
pre	B-complex_assembly
-	I-complex_assembly
40S	I-complex_assembly
particles	I-complex_assembly
contain	O
20S	B-chemical
rRNA	I-chemical
and	O
at	O
least	O
seven	O
non	B-protein_type
-	I-protein_type
ribosomal	I-protein_type
proteins	I-protein_type
including	O
the	O
D	B-protein_type
-	I-protein_type
site	I-protein_type
endonuclease	I-protein_type
Nob1	B-protein
as	O
well	O
as	O
Tsr1	B-protein
,	O
a	O
putative	O
GTPase	B-protein_type
and	O
Rio2	B-protein
which	O
block	O
the	O
mRNA	B-site
channel	I-site
and	O
the	O
initiator	B-site
tRNA	I-site
binding	I-site
site	I-site
,	O
respectively	O
,	O
thus	O
preventing	O
translation	O
initiation	O
.	O

The	O
cleavage	O
step	O
most	O
likely	O
acts	O
as	O
a	O
quality	O
control	O
check	O
that	O
ensures	O
the	O
proper	O
40S	B-complex_assembly
subunit	I-complex_assembly
assembly	O
with	O
only	O
completely	O
processed	O
precursors	O
.	O

Interestingly	O
,	O
differences	O
in	O
the	O
level	O
of	O
acp	B-chemical
modification	O
were	O
demonstrated	O
for	O
different	O
steps	O
of	O
the	O
cytoplasmic	O
pre	B-complex_assembly
-	I-complex_assembly
40S	I-complex_assembly
subunit	I-complex_assembly
maturation	O
after	O
analyzing	O
purified	O
20S	B-chemical
pre	I-chemical
-	I-chemical
rRNAs	I-chemical
using	O
different	O
purification	O
bait	O
proteins	O
.	O

In	O
contrast	O
,	O
late	O
pre	B-complex_assembly
-	I-complex_assembly
40S	I-complex_assembly
subunits	I-complex_assembly
containing	O
Nob1	B-protein
and	O
Rio1	B-protein
or	O
already	O
associated	O
with	O
60S	B-complex_assembly
subunits	I-complex_assembly
in	O
80S	B-complex_assembly
-	I-complex_assembly
like	I-complex_assembly
particles	I-complex_assembly
showed	O
acp	B-chemical
modification	O
levels	O
comparable	O
to	O
mature	B-protein_state
40S	B-complex_assembly
subunits	I-complex_assembly
.	O

Thus	O
,	O
the	O
acp	B-chemical
transfer	O
to	O
m1Ψ1191	B-residue_name_number
occurs	O
during	O
the	O
step	O
at	O
which	O
Rio2	B-protein
leaves	O
the	O
pre	B-complex_assembly
-	I-complex_assembly
40S	I-complex_assembly
particle	I-complex_assembly
.	O

These	O
data	O
and	O
the	O
finding	O
that	O
a	O
missing	O
acp	B-chemical
modification	O
hinders	O
pre	B-chemical
-	I-chemical
20S	I-chemical
rRNA	I-chemical
processing	O
,	O
suggest	O
that	O
the	O
acp	B-chemical
modification	O
together	O
with	O
the	O
release	O
of	O
Rio2	B-protein
promotes	O
the	O
formation	O
of	O
the	O
decoding	B-site
site	I-site
and	O
thus	O
D	B-site
-	I-site
site	I-site
cleavage	O
by	O
Nob1	B-protein
.	O

The	O
interrelation	O
between	O
acp	B-chemical
modification	O
and	O
Rio2	B-protein
release	O
is	O
also	O
supported	O
by	O
CRAC	B-experimental_method
analysis	I-experimental_method
showing	O
that	O
Rio2	B-protein
binds	O
to	O
helix	B-structure_element
31	I-structure_element
next	O
to	O
the	O
Ψ1191	B-residue_name_number
residue	O
that	O
receives	O
the	O
acp	B-chemical
modification	O
.	O

In	O
summary	O
,	O
by	O
identifying	O
Tsr3	B-protein
as	O
the	O
enzyme	O
responsible	O
for	O
introducing	O
the	O
acp	B-chemical
group	O
to	O
the	O
hypermodified	B-protein_state
m1acp3Ψ	B-chemical
nucleotide	B-chemical
at	O
position	O
1191	B-residue_number
(	O
yeast	B-taxonomy_domain
)/	O
1248	B-residue_number
(	O
humans	B-species
)	O
of	O
18S	B-chemical
rRNA	I-chemical
we	O
added	O
one	O
of	O
the	O
last	O
remaining	O
pieces	O
to	O
the	O
puzzle	O
of	O
eukaryotic	B-taxonomy_domain
small	B-chemical
ribosomal	I-chemical
subunit	I-chemical
rRNA	I-chemical
modifications	O
.	O

The	O
current	O
data	O
together	O
with	O
the	O
finding	O
that	O
acp	B-chemical
modification	O
takes	O
place	O
at	O
the	O
very	O
last	O
step	O
in	O
pre	B-complex_assembly
-	I-complex_assembly
40S	I-complex_assembly
subunit	I-complex_assembly
maturation	O
indicate	O
that	O
the	O
acp	B-chemical
modification	O
probably	O
supports	O
the	O
formation	O
of	O
the	O
decoding	B-site
site	I-site
and	O
efficient	O
20S	B-chemical
pre	I-chemical
-	I-chemical
rRNA	I-chemical
D	B-site
-	I-site
site	I-site
cleavage	O
.	O

Furthermore	O
,	O
our	O
structural	B-evidence
data	I-evidence
unravelled	O
how	O
the	O
regioselectivity	O
of	O
SAM	B-chemical
-	O
dependent	O
group	O
transfer	O
reactions	O
can	O
be	O
tuned	O
by	O
distinct	O
small	O
evolutionary	O
adaptions	O
of	O
the	O
ligand	B-site
binding	I-site
pocket	I-site
of	O
SAM	B-protein_type
-	I-protein_type
binding	I-protein_type
enzymes	I-protein_type
.	O

Structural	O
insights	O
into	O
the	O
regulatory	O
mechanism	O
of	O
the	O
Pseudomonas	B-species
aeruginosa	I-species
YfiBNR	B-complex_assembly
system	O

YfiBNR	B-complex_assembly
is	O
a	O
recently	O
identified	O
bis	B-chemical
-(	I-chemical
3	I-chemical
’-	I-chemical
5	I-chemical
’)-	I-chemical
cyclic	I-chemical
dimeric	I-chemical
GMP	I-chemical
(	O
c	B-chemical
-	I-chemical
di	I-chemical
-	I-chemical
GMP	I-chemical
)	O
signaling	O
system	O
in	O
opportunistic	O
pathogens	O
.	O

Here	O
,	O
we	O
report	O
the	O
crystal	B-evidence
structures	I-evidence
of	O
YfiB	B-protein
alone	B-protein_state
and	O
of	O
an	O
active	B-protein_state
mutant	B-protein_state
YfiBL43P	B-mutant
complexed	B-protein_state
with	I-protein_state
YfiR	B-protein
with	O
2	O
:	O
2	O
stoichiometry	O
.	O

In	O
addition	O
,	O
our	O
crystallographic	B-experimental_method
analyses	I-experimental_method
revealed	O
that	O
YfiR	B-protein
binds	O
Vitamin	B-chemical
B6	I-chemical
(	O
VB6	B-chemical
)	O
or	O
L	B-chemical
-	I-chemical
Trp	I-chemical
at	O
a	O
YfiB	B-site
-	I-site
binding	I-site
site	I-site
and	O
that	O
both	O
VB6	B-chemical
and	O
L	B-chemical
-	I-chemical
Trp	I-chemical
are	O
able	O
to	O
reduce	O
YfiBL43P	B-mutant
-	O
induced	O
biofilm	O
formation	O
.	O

An	O
increase	O
in	O
c	B-chemical
-	I-chemical
di	I-chemical
-	I-chemical
GMP	I-chemical
promotes	O
biofilm	O
formation	O
,	O
and	O
a	O
decrease	O
results	O
in	O
biofilm	O
degradation	O
(	O
Boehm	O
et	O
al	O
.,;	O
Duerig	O
et	O
al	O
.,;	O
Hickman	O
et	O
al	O
.,;	O
Jenal	O
,;	O
Romling	O
et	O
al	O
.,).	O

The	O
c	B-chemical
-	I-chemical
di	I-chemical
-	I-chemical
GMP	I-chemical
level	O
is	O
regulated	O
by	O
two	O
reciprocal	O
enzyme	O
systems	O
,	O
namely	O
,	O
diguanylate	B-protein_type
cyclases	I-protein_type
(	O
DGCs	B-protein_type
)	O
that	O
synthesize	O
c	B-chemical
-	I-chemical
di	I-chemical
-	I-chemical
GMP	I-chemical
and	O
phosphodiesterases	B-protein_type
(	O
PDEs	B-protein_type
)	O
that	O
hydrolyze	O
c	B-chemical
-	I-chemical
di	I-chemical
-	I-chemical
GMP	I-chemical
(	O
Kulasakara	O
et	O
al	O
.,;	O
Ross	O
et	O
al	O
.,;	O
Ross	O
et	O
al	O
.,).	O
Many	O
of	O
these	O
enzymes	O
are	O
multiple	O
-	O
domain	O
proteins	O
containing	O
a	O
variable	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
that	O
commonly	O
acts	O
as	O
a	O
signal	O
sensor	O
or	O
transduction	O
module	O
,	O
followed	O
by	O
the	O
relatively	B-protein_state
conserved	I-protein_state
GGDEF	B-structure_element
motif	I-structure_element
in	O
DGCs	B-protein_type
or	O
EAL	B-structure_element
/	I-structure_element
HD	I-structure_element
-	I-structure_element
GYP	I-structure_element
domains	I-structure_element
in	O
PDEs	B-protein_type
(	O
Hengge	O
,;	O
Navarro	O
et	O
al	O
.,;	O
Schirmer	O
and	O
Jenal	O
,).	O

In	O
Pseudomonas	B-species
aeruginosa	I-species
in	O
particular	O
,	O
42	O
genes	O
containing	O
putative	O
DGCs	B-protein_type
and	O
/	O
or	O
PDEs	B-protein_type
were	O
identified	O
(	O
Kulasakara	O
et	O
al	O
.,).	O

However	O
,	O
due	O
to	O
the	O
intricacy	O
of	O
c	B-chemical
-	I-chemical
di	I-chemical
-	I-chemical
GMP	I-chemical
signaling	O
networks	O
and	O
the	O
diversity	O
of	O
experimental	O
cues	O
,	O
the	O
detailed	O
mechanisms	O
by	O
which	O
these	O
signaling	O
pathways	O
specifically	O
sense	O
and	O
integrate	O
different	O
inputs	O
remain	O
largely	O
elusive	O
.	O

Biofilm	O
formation	O
protects	O
pathogenic	O
bacteria	B-taxonomy_domain
from	O
antibiotic	O
treatment	O
,	O
and	O
c	O
-	O
di	O
-	O
GMP	O
-	O
regulated	O
biofilm	O
formation	O
has	O
been	O
extensively	O
studied	O
in	O
P	B-species
.	I-species
aeruginosa	I-species
(	O
Evans	O
,;	O
Kirisits	O
et	O
al	O
.,;	O
Malone	O
,;	O
Reinhardt	O
et	O
al	O
.,).	O

In	O
the	O
lungs	O
of	O
cystic	O
fibrosis	O
(	O
CF	O
)	O
patients	O
,	O
adherent	O
biofilm	O
formation	O
and	O
the	O
appearance	O
of	O
small	O
colony	O
variant	O
(	O
SCV	O
)	O
morphologies	O
of	O
P	B-species
.	I-species
aeruginosa	I-species
correlate	O
with	O
prolonged	O
persistence	O
of	O
infection	O
and	O
poor	O
lung	O
function	O
(	O
Govan	O
and	O
Deretic	O
,;	O
Haussler	O
et	O
al	O
.,;	O
Haussler	O
et	O
al	O
.,;	O
Parsek	O
and	O
Singh	O
,;	O
Smith	O
et	O
al	O
.,).	O

The	O
YfiBNR	B-complex_assembly
system	O
contains	O
three	O
protein	O
members	O
and	O
modulates	O
intracellular	O
c	B-chemical
-	I-chemical
di	I-chemical
-	I-chemical
GMP	I-chemical
levels	O
in	O
response	O
to	O
signals	O
received	O
in	O
the	O
periplasm	O
(	O
Malone	O
et	O
al	O
.,).	O

More	O
recently	O
,	O
this	O
system	O
was	O
also	O
reported	O
in	O
other	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
negative	I-taxonomy_domain
bacteria	I-taxonomy_domain
,	O
such	O
as	O
Escherichia	B-species
coli	I-species
(	O
Hufnagel	O
et	O
al	O
.,;	O
Raterman	O
et	O
al	O
.,;	O
Sanchez	O
-	O
Torres	O
et	O
al	O
.,),	O
Klebsiella	B-species
pneumonia	I-species
(	O
Huertas	O
et	O
al	O
.,)	O
and	O
Yersinia	B-species
pestis	I-species
(	O
Ren	O
et	O
al	O
.,).	O

YfiN	B-protein
is	O
an	O
integral	O
inner	O
-	O
membrane	O
protein	O
with	O
two	O
potential	O
transmembrane	B-structure_element
helices	I-structure_element
,	O
a	O
periplasmic	O
Per	B-structure_element
-	I-structure_element
Arnt	I-structure_element
-	I-structure_element
Sim	I-structure_element
(	O
PAS	B-structure_element
)	O
domain	O
,	O
and	O
cytosolic	O
domains	O
containing	O
a	O
HAMP	B-structure_element
domain	I-structure_element
(	O
mediate	O
input	O
-	O
output	O
signaling	O
in	O
histidine	B-protein_type
kinases	I-protein_type
,	O
adenylyl	B-protein_type
cyclases	I-protein_type
,	O
methyl	B-protein_type
-	I-protein_type
accepting	I-protein_type
chemotaxis	I-protein_type
proteins	I-protein_type
,	O
and	O
phosphatases	B-protein_type
)	O
and	O
a	O
C	O
-	O
terminal	O
GGDEF	B-structure_element
domain	I-structure_element
indicating	O
a	O
DGC	B-protein_type
’	O
s	O
function	O
(	O
Giardina	O
et	O
al	O
.,;	O
Malone	O
et	O
al	O
.,).	O

YfiN	B-protein
is	O
repressed	B-protein_state
by	I-protein_state
specific	O
interaction	O
between	O
its	O
periplasmic	O
PAS	B-structure_element
domain	I-structure_element
and	O
the	O
periplasmic	O
protein	O
YfiR	B-protein
(	O
Malone	O
et	O
al	O
.,).	O

YfiB	B-protein
is	O
an	O
OmpA	B-protein_type
/	I-protein_type
Pal	I-protein_type
-	I-protein_type
like	I-protein_type
outer	O
-	O
membrane	O
lipoprotein	B-protein_type
(	O
Parsons	O
et	O
al	O
.,)	O
that	O
can	O
activate	O
YfiN	B-protein
by	O
sequestering	O
YfiR	B-protein
(	O
Malone	O
et	O
al	O
.,)	O
in	O
an	O
unknown	O
manner	O
.	O

Whether	O
YfiB	B-protein
directly	O
recruits	O
YfiR	B-protein
or	O
recruits	O
YfiR	B-protein
via	O
a	O
third	O
partner	O
is	O
an	O
open	O
question	O
.	O

It	O
has	O
been	O
reported	O
that	O
the	O
activation	O
of	O
YfiN	B-protein
may	O
be	O
induced	O
by	O
redox	O
-	O
driven	O
misfolding	O
of	O
YfiR	B-protein
(	O
Giardina	O
et	O
al	O
.,;	O
Malone	O
et	O
al	O
.,;	O
Malone	O
et	O
al	O
.,).	O

It	O
is	O
also	O
proposed	O
that	O
the	O
sequestration	O
of	O
YfiR	B-protein
by	O
YfiB	B-protein
can	O
be	O
induced	O
by	O
certain	O
YfiB	B-protein
-	O
mediated	O
cell	O
wall	O
stress	O
,	O
and	O
mutagenesis	B-experimental_method
studies	I-experimental_method
revealed	O
a	O
number	O
of	O
activation	B-structure_element
residues	I-structure_element
of	O
YfiB	B-protein
that	O
were	O
located	O
in	O
close	O
proximity	O
to	O
the	O
predicted	B-protein_state
first	B-structure_element
helix	I-structure_element
of	O
the	O
periplasmic	B-structure_element
domain	I-structure_element
(	O
Malone	O
et	O
al	O
.,).	O

In	O
addition	O
,	O
quorum	O
sensing	O
-	O
related	O
dephosphorylation	O
of	O
the	O
PAS	B-structure_element
domain	I-structure_element
of	O
YfiN	B-protein
may	O
also	O
be	O
involved	O
in	O
the	O
regulation	O
(	O
Ueda	O
and	O
Wood	O
,;	O
Xu	O
et	O
al	O
.,).	O

Recently	O
,	O
we	O
solved	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
YfiR	B-protein
in	O
both	O
the	O
non	B-protein_state
-	I-protein_state
oxidized	I-protein_state
and	O
the	O
oxidized	B-protein_state
states	O
,	O
revealing	O
breakage	O
/	O
formation	O
of	O
one	O
disulfide	B-ptm
bond	I-ptm
(	O
Cys71	B-residue_name_number
-	O
Cys110	B-residue_name_number
)	O
and	O
local	O
conformational	O
change	O
around	O
the	O
other	O
one	O
(	O
Cys145	B-residue_name_number
-	O
Cys152	B-residue_name_number
),	O
indicating	O
that	O
Cys145	B-residue_name_number
-	O
Cys152	B-residue_name_number
plays	O
an	O
important	O
role	O
in	O
maintaining	O
the	O
correct	O
folding	O
of	O
YfiR	B-protein
(	O
Yang	O
et	O
al	O
.,).	O

Most	O
recently	O
,	O
Li	O
and	O
coworkers	O
reported	O
the	O
crystal	B-evidence
structures	I-evidence
of	O
YfiB	B-protein
(	O
27	B-residue_range
–	I-residue_range
168	I-residue_range
)	O
alone	B-protein_state
and	O
YfiRC71S	B-mutant
in	B-protein_state
complex	I-protein_state
with	I-protein_state
YfiB	B-protein
(	O
59	B-residue_range
–	I-residue_range
168	I-residue_range
)	O
(	O
Li	O
et	O
al	O
.,).	O

Compared	O
with	O
the	O
reported	O
complex	O
structure	O
,	O
YfiBL43P	B-mutant
in	O
our	O
YfiB	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
complex	O
structure	B-evidence
has	O
additional	O
visible	O
N	O
-	O
terminal	O
residues	O
44	B-residue_range
–	I-residue_range
58	I-residue_range
that	O
are	O
shown	O
to	O
play	O
essential	O
roles	O
in	O
YfiB	B-protein
activation	O
and	O
biofilm	O
formation	O
.	O

Therefore	O
,	O
we	O
are	O
able	O
to	O
visualize	O
the	O
detailed	O
allosteric	O
arrangement	O
of	O
the	O
N	O
-	O
terminal	O
structure	O
of	O
YfiB	B-protein
and	O
its	O
important	O
role	O
in	O
YfiB	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
interaction	O
.	O

In	O
addition	O
,	O
we	O
found	O
that	O
the	O
YfiBL43P	B-mutant
shows	O
a	O
much	O
higher	O
PG	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
than	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
YfiB	B-protein
,	O
most	O
likely	O
due	O
to	O
its	O
more	O
compact	O
PG	B-site
-	I-site
binding	I-site
pocket	I-site
.	O

Overall	O
structure	B-evidence
of	O
YfiB	B-protein
.	O
(	O
A	O
)	O
The	O
overall	O
structure	B-evidence
of	O
the	O
YfiB	B-protein
monomer	B-oligomeric_state
.	O
(	O
B	O
)	O
A	O
topology	O
diagram	O
of	O
the	O
YfiB	B-protein
monomer	B-oligomeric_state
.	O
(	O
C	O
and	O
D	O
)	O
The	O
analytical	B-experimental_method
ultracentrifugation	I-experimental_method
experiment	O
results	O
for	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
YfiB	B-protein
and	O
YfiBL43P	B-mutant

The	O
“	O
back	B-protein_state
to	I-protein_state
back	I-protein_state
”	O
dimer	B-oligomeric_state
.	O

In	O
addition	O
,	O
there	O
is	O
a	O
short	O
helix	B-structure_element
turn	I-structure_element
connecting	O
the	O
β4	B-structure_element
strand	I-structure_element
and	O
α4	B-structure_element
helix	I-structure_element
(	O
Fig	O
.	O
1A	O
and	O
1B	O
).	O

Here	O
,	O
we	O
refer	O
to	O
the	O
two	O
dimeric	B-oligomeric_state
types	O
as	O
“	O
head	B-protein_state
to	I-protein_state
head	I-protein_state
”	O
and	O
“	O
back	B-protein_state
to	I-protein_state
back	I-protein_state
”	O
according	O
to	O
the	O
interacting	O
mode	O
(	O
Fig	O
.	O
2A	O
and	O
2E	O
),	O
with	O
the	O
total	O
buried	O
surface	O
areas	O
being	O
316	O
.	O
8	O
Å2	O
and	O
554	O
.	O
3	O
Å2	O
,	O
respectively	O
.	O

The	O
“	O
head	B-protein_state
to	I-protein_state
head	I-protein_state
”	O
dimer	B-oligomeric_state
exhibits	O
a	O
clamp	B-protein_state
shape	I-protein_state
.	O

The	O
“	O
back	B-protein_state
to	I-protein_state
back	I-protein_state
”	O
dimer	B-oligomeric_state
presents	O
a	O
Y	B-protein_state
shape	I-protein_state
.	O

Overall	O
structure	B-evidence
of	O
the	O
YfiB	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
complex	O
and	O
the	O
conserved	B-site
surface	I-site
in	O
YfiR	B-protein
.	O
(	O
A	O
)	O
The	O
overall	O
structure	B-evidence
of	O
the	O
YfiB	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
complex	O
.	O

The	O
YfiBL43P	B-mutant
molecules	O
are	O
shown	O
in	O
cyan	O
and	O
yellow	O
.	O

To	O
illustrate	O
the	O
differences	O
between	O
apo	B-protein_state
YfiB	B-protein
and	O
YfiR	B-protein_state
-	I-protein_state
bound	I-protein_state
YfiBL43P	B-mutant
,	O
the	O
apo	B-protein_state
YfiB	B-protein
is	O
shown	O
in	O
pink	O
,	O
except	O
residues	O
34	B-residue_range
–	I-residue_range
70	I-residue_range
are	O
shown	O
in	O
red	O
,	O
whereas	O
the	O
YfiR	B-protein_state
-	I-protein_state
bound	I-protein_state
YfiBL43P	B-mutant
is	O
shown	O
in	O
cyan	O
,	O
except	O
residues	O
44	B-residue_range
–	I-residue_range
70	I-residue_range
are	O
shown	O
in	O
blue	O
.	O
(	O
C	O
)	O
Close	O
-	O
up	O
view	O
of	O
the	O
differences	O
between	O
apo	B-protein_state
YfiB	B-protein
and	O
YfiR	B-protein_state
-	I-protein_state
bound	I-protein_state
YfiBL43P	B-mutant
.	O

The	O
residues	O
proposed	O
to	O
contribute	O
to	O
YfiB	B-protein
activation	O
are	O
illustrated	O
in	O
sticks	O
.	O

The	O
key	O
residues	O
in	O
apo	B-protein_state
YfiB	B-protein
are	O
shown	O
in	O
red	O
and	O
those	O
in	O
YfiBL43P	B-mutant
are	O
shown	O
in	O
blue	O
.	O
(	O
D	O
)	O
Close	O
-	O
up	O
views	O
showing	O
interactions	O
in	O
regions	B-structure_element
I	I-structure_element
and	I-structure_element
II	I-structure_element
.	O

YfiBL43P	B-mutant
and	O
YfiR	B-protein
are	O
shown	O
in	O
cyan	O
and	O
green	O
,	O
respectively	O
.	O
(	O
E	O
and	O
F	O
)	O
The	O
conserved	B-site
surface	I-site
in	O
YfiR	B-protein
contributes	O
to	O
the	O
interaction	O
with	O
YfiB	B-protein
.	O
(	O
G	O
)	O
The	O
residues	B-structure_element
of	O
YfiR	B-protein
responsible	O
for	O
interacting	O
with	O
YfiB	B-protein
are	O
shown	O
in	O
green	O
sticks	O
,	O
and	O
the	O
proposed	O
YfiN	B-site
-	I-site
interacting	I-site
residues	I-site
are	O
shown	O
in	O
yellow	O
sticks	O
.	O

The	O
red	O
sticks	O
,	O
which	O
represent	O
the	O
YfiB	B-site
-	I-site
interacting	I-site
residues	I-site
,	O
are	O
also	O
responsible	O
for	O
the	O
proposed	O
interactions	O
with	O
YfiN	B-protein

It	O
has	O
been	O
reported	O
that	O
single	B-experimental_method
mutants	I-experimental_method
of	I-experimental_method
Q39	B-residue_name_number
,	O
L43	B-residue_name_number
,	O
F48	B-residue_name_number
and	O
W55	B-residue_name_number
contribute	O
to	O
YfiB	B-protein
activation	O
leading	O
to	O
the	O
induction	O
of	O
the	O
SCV	O
phenotype	O
in	O
P	B-species
.	I-species
aeruginosa	I-species
PAO1	I-species
(	O
Malone	O
et	O
al	O
.,).	O

Therefore	O
,	O
we	O
constructed	B-experimental_method
two	I-experimental_method
such	I-experimental_method
single	I-experimental_method
mutants	I-experimental_method
of	O
YfiB	B-protein
(	O
YfiBL43P	B-mutant
and	O
YfiBF48S	B-mutant
).	O

As	O
expected	O
,	O
both	O
mutants	O
form	O
a	O
stable	B-protein_state
complex	B-protein_state
with	I-protein_state
YfiR	B-protein
.	O
Finally	O
,	O
we	O
crystalized	B-experimental_method
YfiR	B-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
the	O
YfiBL43P	B-mutant
mutant	B-protein_state
and	O
solved	O
the	O
structure	B-evidence
at	O
1	O
.	O
78	O
Å	O
resolution	O
by	O
molecular	B-experimental_method
replacement	I-experimental_method
using	O
YfiR	B-protein
and	O
YfiB	B-protein
as	O
models	O
.	O

The	O
YfiB	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
complex	O
is	O
a	O
2	O
:	O
2	O
heterotetramer	B-oligomeric_state
(	O
Fig	O
.	O
3A	O
)	O
in	O
which	O
the	O
YfiR	B-protein
dimer	B-oligomeric_state
is	O
clamped	O
by	O
two	O
separated	O
YfiBL43P	B-mutant
molecules	O
with	O
a	O
total	O
buried	O
surface	O
area	O
of	O
3161	O
.	O
2	O
Å2	O
.	O

The	O
YfiR	B-protein
dimer	B-oligomeric_state
in	O
the	O
complex	O
is	O
identical	O
to	O
the	O
non	B-protein_state
-	I-protein_state
oxidized	I-protein_state
YfiR	B-protein
dimer	B-oligomeric_state
alone	B-protein_state
(	O
Yang	O
et	O
al	O
.,),	O
with	O
only	O
Cys145	B-residue_name_number
-	O
Cys152	B-residue_name_number
of	O
the	O
two	O
disulfide	B-ptm
bonds	I-ptm
well	O
formed	O
,	O
suggesting	O
Cys71	B-residue_name_number
-	O
Cys110	B-residue_name_number
disulfide	B-ptm
bond	I-ptm
formation	O
is	O
not	O
essential	O
for	O
forming	O
YfiB	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
complex	O
.	O

The	O
observed	O
changes	O
in	O
conformation	O
of	O
YfiB	B-protein
and	O
the	O
results	O
of	O
mutagenesis	B-experimental_method
suggest	O
a	O
mechanism	O
by	O
which	O
YfiB	B-protein
sequesters	O
YfiR	B-protein
.	O

The	O
YfiB	B-site
-	I-site
YfiR	I-site
interface	I-site
can	O
be	O
divided	O
into	O
two	O
regions	O
(	O
Fig	O
.	O
3A	O
and	O
3D	O
).	O

Region	B-structure_element
I	I-structure_element
is	O
formed	O
by	O
numerous	O
main	O
-	O
chain	O
and	O
side	O
-	O
chain	O
hydrophilic	B-bond_interaction
interactions	I-bond_interaction
between	O
residues	O
E45	B-residue_name_number
,	O
G47	B-residue_name_number
and	O
E53	B-residue_name_number
from	O
the	O
N	O
-	O
terminal	O
extended	O
loop	B-structure_element
of	O
YfiB	B-protein
and	O
residues	O
S57	B-residue_name_number
,	O
R60	B-residue_name_number
,	O
A89	B-residue_name_number
and	O
H177	B-residue_name_number
from	O
YfiR	B-protein
(	O
Fig	O
.	O
3D	O
-	O
I	O
(	O
i	O
)).	O

In	O
region	B-structure_element
II	I-structure_element
,	O
the	O
side	O
chains	O
of	O
R96	B-residue_name_number
,	O
E98	B-residue_name_number
and	O
E157	B-residue_name_number
from	O
YfiB	B-protein
interact	O
with	O
the	O
side	O
chains	O
of	O
E163	B-residue_name_number
,	O
S146	B-residue_name_number
and	O
R171	B-residue_name_number
from	O
YfiR	B-protein
,	O
respectively	O
.	O

Additionally	O
,	O
the	O
main	O
chains	O
of	O
I163	B-residue_name_number
and	O
V165	B-residue_name_number
from	O
YfiB	B-protein
form	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
the	O
main	O
chains	O
of	O
L166	B-residue_name_number
and	O
A164	B-residue_name_number
from	O
YfiR	B-protein
,	O
respectively	O
,	O
and	O
the	O
main	O
chain	O
of	O
P166	B-residue_name_number
from	O
YfiB	B-protein
interacts	O
with	O
the	O
side	O
chain	O
of	O
R185	B-residue_name_number
from	O
YfiR	B-protein
(	O
Fig	O
.	O
3D	O
-	O
II	O
).	O

These	O
two	O
regions	O
contribute	O
a	O
robust	O
hydrogen	B-site
-	I-site
bonding	I-site
network	I-site
to	O
the	O
YfiB	B-site
-	I-site
YfiR	I-site
interface	I-site
,	O
resulting	O
in	O
a	O
tightly	O
bound	O
complex	O
.	O

Based	O
on	O
the	O
observations	O
that	O
two	O
separated	O
YfiBL43P	B-mutant
molecules	O
form	O
a	O
2	O
:	O
2	O
complex	O
structure	B-evidence
with	O
YfiR	B-protein
dimer	B-oligomeric_state
,	O
we	O
performed	O
an	O
analytical	B-experimental_method
ultracentrifugation	I-experimental_method
experiment	O
to	O
check	O
the	O
oligomeric	O
states	O
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
YfiB	B-protein
and	O
YfiBL43P	B-mutant
.	O

The	O
results	O
showed	O
that	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
YfiB	B-protein
exists	O
in	O
both	O
monomeric	B-oligomeric_state
and	O
dimeric	B-oligomeric_state
states	O
in	O
solution	O
,	O
while	O
YfiBL43P	B-mutant
primarily	O
adopts	O
the	O
monomer	B-oligomeric_state
state	O
in	O
solution	O
(	O
Fig	O
.	O
1C	O
–	O
D	O
).	O

This	O
suggests	O
that	O
the	O
N	O
-	O
terminus	O
of	O
YfiB	B-protein
plays	O
an	O
important	O
role	O
in	O
forming	O
the	O
dimeric	B-oligomeric_state
YfiB	B-protein
in	O
solution	O
and	O
that	O
the	O
conformational	O
change	O
of	O
residue	O
L43	B-residue_name_number
is	O
associated	O
with	O
the	O
stretch	O
of	O
the	O
N	O
-	O
terminus	O
and	O
opening	O
of	O
the	O
dimer	B-oligomeric_state
.	O

The	O
PG	B-site
-	I-site
binding	I-site
site	I-site
of	O
YfiB	B-protein

The	O
PG	B-site
-	I-site
binding	I-site
site	I-site
in	O
YfiB	B-protein
.	O
(	O
A	O
)	O
Structural	B-experimental_method
superposition	I-experimental_method
of	O
the	O
PG	B-site
-	I-site
binding	I-site
sites	I-site
of	O
the	O
H	B-species
.	I-species
influenzae	I-species
Pal	B-complex_assembly
/	I-complex_assembly
PG	I-complex_assembly
-	I-complex_assembly
P	I-complex_assembly
complex	O
and	O
YfiR	B-protein_state
-	I-protein_state
bound	I-protein_state
YfiBL43P	B-mutant
complexed	B-protein_state
with	I-protein_state
sulfate	B-chemical
ions	O
.	O

(	O
B	O
)	O
Close	O
-	O
up	O
view	O
showing	O
the	O
key	O
residues	O
of	O
Pal	B-protein_type
interacting	O
with	O
the	O
m	B-chemical
-	I-chemical
Dap5	I-chemical
ε	I-chemical
-	I-chemical
carboxylate	I-chemical
group	O
of	O
PG	B-chemical
-	I-chemical
P	I-chemical
.	O
Pal	B-protein_type
is	O
shown	O
in	O
wheat	O
and	O
PG	B-chemical
-	I-chemical
P	I-chemical
is	O
in	O
magenta	O
.	O

Apo	B-protein_state
YfiB	B-protein
is	O
shown	O
in	O
yellow	O
and	O
YfiR	B-protein_state
-	I-protein_state
bound	I-protein_state
YfiBL43P	B-mutant
in	O
cyan	O
.	O
(	O
E	O
and	O
F	O
)	O
MST	B-experimental_method
data	O
and	O
analysis	O
for	O
binding	B-evidence
affinities	I-evidence
of	O
(	O
E	O
)	O
YfiB	B-protein
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
(	O
F	O
)	O
YfiBL43P	B-mutant
with	O
PG	B-chemical
.	O
(	O
G	O
)	O
The	O
sequence	B-experimental_method
alignment	I-experimental_method
of	O
P	B-species
.	I-species
aeruginosa	I-species
and	O
E	B-species
.	I-species
coli	I-species
sources	O
of	O
YfiB	B-protein
,	O
Pal	B-protein_type
and	O
the	O
periplasmic	B-structure_element
domain	I-structure_element
of	O
OmpA	B-protein_type

Previous	O
homology	B-experimental_method
modeling	I-experimental_method
studies	O
suggested	O
that	O
YfiB	B-protein
contains	O
a	O
Pal	B-site
-	I-site
like	I-site
PG	I-site
-	I-site
binding	I-site
site	I-site
(	O
Parsons	O
et	O
al	O
.,),	O
and	O
the	O
mutation	B-experimental_method
of	I-experimental_method
two	I-experimental_method
residues	I-experimental_method
at	O
this	O
site	O
,	O
D102	B-residue_name_number
and	O
G105	B-residue_name_number
,	O
reduces	O
the	O
ability	O
for	O
biofilm	O
formation	O
and	O
surface	O
attachment	O
(	O
Malone	O
et	O
al	O
.,).	O

In	O
the	O
YfiB	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
complex	O
,	O
one	O
sulfate	B-chemical
ion	O
is	O
found	O
at	O
the	O
bottom	O
of	O
each	O
YfiBL43P	B-mutant
molecule	O
(	O
Fig	O
.	O
3A	O
)	O
and	O
forms	O
a	O
strong	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
D102	B-residue_name_number
of	O
YfiBL43P	B-mutant
(	O
Fig	O
.	O
4A	O
and	O
4C	O
).	O

Structural	B-experimental_method
superposition	I-experimental_method
between	O
YfiBL43P	B-mutant
and	O
Haemophilus	B-species
influenzae	I-species
Pal	B-protein_type
complexed	B-protein_state
with	I-protein_state
biosynthetic	O
peptidoglycan	B-chemical
precursor	I-chemical
(	O
PG	B-chemical
-	I-chemical
P	I-chemical
),	O
UDP	B-chemical
-	I-chemical
N	I-chemical
-	I-chemical
acetylmuramyl	I-chemical
-	I-chemical
L	I-chemical
-	I-chemical
Ala	I-chemical
-	I-chemical
α	I-chemical
-	I-chemical
D	I-chemical
-	I-chemical
Glu	I-chemical
-	I-chemical
m	I-chemical
-	I-chemical
Dap	I-chemical
-	I-chemical
D	I-chemical
-	I-chemical
Ala	I-chemical
-	I-chemical
D	I-chemical
-	I-chemical
Ala	I-chemical
(	O
m	B-chemical
-	I-chemical
Dap	I-chemical
is	O
meso	B-chemical
-	I-chemical
diaminopimelate	I-chemical
)	O
(	O
PDB	O
code	O
:	O
2aiz	O
)	O
(	O
Parsons	O
et	O
al	O
.,),	O
revealed	O
that	O
the	O
sulfate	B-chemical
ion	O
is	O
located	O
at	O
the	O
position	O
of	O
the	O
m	B-chemical
-	I-chemical
Dap5	I-chemical
ϵ	I-chemical
-	I-chemical
carboxylate	I-chemical
group	O
in	O
the	O
Pal	B-complex_assembly
/	I-complex_assembly
PG	I-complex_assembly
-	I-complex_assembly
P	I-complex_assembly
complex	O
(	O
Fig	O
.	O
4A	O
).	O

Similarly	O
,	O
in	O
the	O
YfiR	B-protein_state
-	I-protein_state
bound	I-protein_state
YfiBL43P	B-mutant
structure	B-evidence
,	O
the	O
sulfate	B-chemical
ion	O
interacts	O
with	O
the	O
side	O
-	O
chain	O
atoms	O
of	O
D102	B-residue_name_number
(	O
corresponding	O
to	O
D71	B-residue_name_number
in	O
Pal	B-protein_type
)	O
and	O
R117	B-residue_name_number
(	O
corresponding	O
to	O
R86	B-residue_name_number
in	O
Pal	B-protein_type
)	O
and	O
the	O
main	O
-	O
chain	O
amide	O
of	O
N68	B-residue_name_number
(	O
corresponding	O
to	O
D37	B-residue_name_number
in	O
Pal	B-protein_type
).	O

Moreover	O
,	O
a	O
water	B-chemical
molecule	O
was	O
found	O
to	O
bridge	O
the	O
sulfate	B-chemical
ion	O
and	O
the	O
side	O
chains	O
of	O
N67	B-residue_name_number
and	O
D102	B-residue_name_number
,	O
strengthening	O
the	O
hydrogen	B-site
bond	I-site
network	I-site
(	O
Fig	O
.	O
4C	O
).	O

Compared	O
to	O
YfiBL43P	B-mutant
,	O
the	O
N68	B-residue_name_number
-	O
containing	O
loop	B-structure_element
of	O
the	O
apo	B-protein_state
YfiB	B-protein
flips	O
away	O
about	O
7	O
Å	O
,	O
and	O
D102	B-residue_name_number
and	O
R117	B-residue_name_number
swing	O
slightly	O
outward	O
;	O
thus	O
,	O
the	O
PG	B-site
-	I-site
binding	I-site
pocket	I-site
is	O
enlarged	O
with	O
no	O
sulfate	B-chemical
ion	O
or	O
water	B-chemical
bound	O
(	O
Fig	O
.	O
4D	O
).	O

As	O
the	O
experiment	O
is	O
performed	O
in	B-protein_state
the	I-protein_state
absence	I-protein_state
of	I-protein_state
YfiR	B-protein
,	O
it	O
suggests	O
that	O
an	O
increase	O
in	O
the	O
PG	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
YfiB	B-protein
is	O
not	O
a	O
result	O
of	O
YfiB	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
interaction	O
and	O
is	O
highly	O
coupled	O
to	O
the	O
activation	O
of	O
YfiB	B-protein
characterized	O
by	O
a	O
stretched	B-protein_state
N	I-protein_state
-	I-protein_state
terminal	I-protein_state
conformation	I-protein_state
.	O

The	O
conserved	B-site
surface	I-site
in	O
YfiR	B-protein
is	O
functional	O
for	O
binding	O
YfiB	B-protein
and	O
YfiN	B-protein

Interestingly	O
,	O
the	O
majority	O
of	O
this	O
conserved	B-site
surface	I-site
contributes	O
to	O
the	O
interaction	O
with	O
YfiB	B-protein
(	O
Fig	O
.	O
3E	O
and	O
3F	O
).	O

Malone	O
JG	O
et	O
al	O
.	O
have	O
reported	O
that	O
F151	B-residue_name_number
,	O
E163	B-residue_name_number
,	O
I169	B-residue_name_number
and	O
Q187	B-residue_name_number
,	O
located	O
near	O
the	O
C	O
-	O
terminus	O
of	O
YfiR	B-protein
,	O
comprise	O
a	O
putative	O
YfiN	B-site
binding	I-site
site	I-site
(	O
Malone	O
et	O
al	O
.,).	O

F151	B-residue_name_number
,	O
E163	B-residue_name_number
and	O
I169	B-residue_name_number
form	O
a	O
hydrophobic	B-site
core	I-site
while	O
,	O
Q187	B-residue_name_number
is	O
located	O
at	O
the	O
end	O
of	O
the	O
α6	B-structure_element
helix	I-structure_element
.	O

E163	B-residue_name_number
and	O
I169	B-residue_name_number
are	O
YfiB	B-site
-	I-site
interacting	I-site
residues	I-site
of	O
YfiR	B-protein
,	O
in	O
which	O
E163	B-residue_name_number
forms	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
R96	B-residue_name_number
of	O
YfiB	B-protein
(	O
Fig	O
.	O
3D	O
-	O
II	O
)	O
and	O
I169	B-residue_name_number
is	O
involved	O
in	O
forming	O
the	O
L166	B-residue_name_number
/	O
I169	B-residue_name_number
/	O
V176	B-residue_name_number
/	O
P178	B-residue_name_number
/	O
L181	B-residue_name_number
hydrophobic	B-site
core	I-site
for	O
anchoring	O
F57	B-residue_name_number
of	O
YfiB	B-protein
(	O
Fig	O
.	O
3D	O
-	O
I	O
(	O
ii	O
)).	O

YfiR	B-protein
binds	O
small	O
molecules	O

The	O
electron	B-evidence
densities	I-evidence
of	O
VB6	B-chemical
and	O
Trp	B-chemical
are	O
countered	O
at	O
3	O
.	O
0σ	O
and	O
2	O
.	O
3σ	O
,	O
respectively	O
,	O
in	O
|	B-evidence
Fo	I-evidence
|-|	I-evidence
Fc	I-evidence
|	I-evidence
maps	I-evidence
.	O
(	O
C	O
)	O
Superposition	B-experimental_method
of	O
the	O
hydrophobic	B-site
pocket	I-site
of	O
YfiR	B-protein
with	O
VB6	B-chemical
,	O
L	B-chemical
-	I-chemical
Trp	I-chemical
and	O
F57	B-residue_name_number
of	O
YfiB	B-protein

Intriguingly	O
,	O
a	O
Dali	B-experimental_method
search	I-experimental_method
(	O
Holm	O
and	O
Rosenstrom	O
,)	O
indicated	O
that	O
the	O
closest	O
homologs	O
of	O
YfiR	B-protein
shared	O
the	O
characteristic	O
of	O
being	O
able	O
to	O
bind	O
several	O
structurally	O
similar	O
small	O
molecules	O
,	O
such	O
as	O
L	B-chemical
-	I-chemical
Trp	I-chemical
,	O
L	B-chemical
-	I-chemical
Phe	I-chemical
,	O
B	O
-	O
group	O
vitamins	O
and	O
their	O
analogs	O
,	O
encouraging	O
us	O
to	O
test	O
whether	O
YfiR	B-protein
can	O
recognize	O
these	O
molecules	O
.	O

For	O
this	O
purpose	O
,	O
we	O
co	B-experimental_method
-	I-experimental_method
crystallized	I-experimental_method
YfiR	B-protein
or	O
soaked	B-experimental_method
YfiR	B-protein
crystals	B-evidence
with	O
different	O
small	O
molecules	O
,	O
including	O
L	B-chemical
-	I-chemical
Trp	I-chemical
and	O
B	O
-	O
group	O
vitamins	O
.	O

Fortunately	O
,	O
we	O
found	O
obvious	O
small	B-evidence
-	I-evidence
molecule	I-evidence
density	I-evidence
in	O
the	O
VB6	B-protein_state
-	I-protein_state
bound	I-protein_state
and	O
Trp	B-protein_state
-	I-protein_state
bound	I-protein_state
YfiR	B-protein
crystal	B-evidence
structures	I-evidence
(	O
Fig	O
.	O
5A	O
and	O
5B	O
),	O
and	O
in	O
both	O
structures	B-evidence
,	O
the	O
YfiR	B-protein
dimers	B-oligomeric_state
resemble	O
the	O
oxidized	B-protein_state
YfiR	B-protein
structure	B-evidence
in	O
which	O
both	O
two	O
disulfide	B-ptm
bonds	I-ptm
are	O
well	O
formed	O
(	O
Yang	O
et	O
al	O
.,).	O

Functional	O
analysis	O
of	O
VB6	B-chemical
and	O
L	B-chemical
-	I-chemical
Trp	I-chemical
.	O
(	O
A	O
and	O
B	O
)	O
The	O
effect	B-experimental_method
of	I-experimental_method
increasing	I-experimental_method
concentrations	I-experimental_method
of	O
VB6	B-chemical
or	O
L	B-chemical
-	I-chemical
Trp	I-chemical
on	O
YfiBL43P	B-mutant
-	O
induced	O
attachment	O
(	O
bars	O
).	O

The	O
relative	B-evidence
optical	I-evidence
density	I-evidence
is	O
represented	O
as	O
curves	O
.	O

Wild	B-protein_state
-	I-protein_state
type	I-protein_state
YfiB	B-protein
is	O
used	O
as	O
negative	O
control	O
.	O

(	O
C	O
and	O
D	O
)	O
BIAcore	B-experimental_method
data	O
and	O
analysis	O
for	O
binding	B-evidence
affinities	I-evidence
of	O
(	O
C	O
)	O
VB6	B-chemical
and	O
(	O
D	O
)	O
L	B-chemical
-	I-chemical
Trp	I-chemical
with	O
YfiR	B-protein
.	O
(	O
E	O
–	O
G	O
)	O
ITC	B-experimental_method
data	O
and	O
analysis	O
for	O
titration	B-experimental_method
of	O
(	O
E	O
)	O
YfiB	B-protein
wild	B-protein_state
-	I-protein_state
type	I-protein_state
,	O
(	O
F	O
)	O
YfiBL43P	O
,	O
and	O
(	O
G	O
)	O
YfiBL43P	B-mutant
/	O
F57A	B-mutant
into	O
YfiR	B-protein

Structural	B-experimental_method
analyses	I-experimental_method
revealed	O
that	O
the	O
VB6	B-chemical
and	O
L	B-chemical
-	I-chemical
Trp	I-chemical
molecules	O
are	O
bound	B-protein_state
at	I-protein_state
the	O
periphery	O
of	O
the	O
YfiR	B-protein
dimer	B-oligomeric_state
,	O
but	O
not	O
at	O
the	O
dimer	B-site
interface	I-site
.	O

Interestingly	O
,	O
VB6	B-chemical
and	O
L	B-chemical
-	I-chemical
Trp	I-chemical
were	O
found	O
to	O
occupy	O
the	O
same	O
hydrophobic	B-site
pocket	I-site
,	O
formed	O
by	O
L166	B-residue_name_number
/	O
I169	B-residue_name_number
/	O
V176	B-residue_name_number
/	O
P178	B-residue_name_number
/	O
L181	B-residue_name_number
of	O
YfiR	B-protein
,	O
which	O
is	O
also	O
a	O
binding	B-site
pocket	I-site
for	O
F57	B-residue_name_number
of	O
YfiB	B-protein
,	O
as	O
observed	O
in	O
the	O
YfiB	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
complex	O
(	O
Fig	O
.	O
5C	O
).	O

To	O
evaluate	O
the	O
importance	O
of	O
F57	B-residue_name_number
in	O
YfiBL43P	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
interaction	O
,	O
the	O
binding	B-evidence
affinities	I-evidence
of	O
YfiBL43P	B-mutant
and	O
YfiBL43P	B-mutant
/	O
F57A	B-mutant
for	O
YfiR	B-protein
were	O
measured	O
by	O
isothermal	B-experimental_method
titration	I-experimental_method
calorimetry	I-experimental_method
(	O
ITC	B-experimental_method
).	O

The	O
results	O
showed	O
Kd	B-evidence
values	O
of	O
1	O
.	O
4	O
×	O
10	O
−	O
7	O
mol	O
/	O
L	O
and	O
5	O
.	O
3	O
×	O
10	O
−	O
7	O
mol	O
/	O
L	O
for	O
YfiBL43P	B-mutant
and	O
YfiBL43P	B-mutant
/	O
F57A	B-mutant
,	O
respectively	O
,	O
revealing	O
that	O
the	O
YfiBL43P	B-mutant
/	O
F57A	B-mutant
mutant	B-protein_state
caused	O
a	O
3	O
.	O
8	O
-	O
fold	O
reduction	O
in	O
the	O
binding	B-evidence
affinity	I-evidence
compared	O
with	O
the	O
YfiBL43P	B-mutant
mutant	B-protein_state
(	O
Fig	O
.	O
6F	O
and	O
6G	O
).	O

Growth	B-experimental_method
and	I-experimental_method
surface	I-experimental_method
attachment	I-experimental_method
assays	I-experimental_method
were	O
carried	O
out	O
for	O
the	O
yfiB	B-mutant
-	I-mutant
L43P	I-mutant
strain	O
in	O
the	O
presence	O
of	O
increasing	B-experimental_method
concentrations	I-experimental_method
of	O
VB6	B-chemical
or	O
L	B-chemical
-	I-chemical
Trp	I-chemical
.	O

As	O
shown	O
in	O
Fig	O
.	O
6A	O
and	O
6B	O
,	O
the	O
over	B-experimental_method
-	I-experimental_method
expression	I-experimental_method
of	O
YfiBL43P	B-mutant
induced	O
strong	O
surface	O
attachment	O
and	O
much	O
slower	O
growth	O
of	O
the	O
yfiB	B-mutant
-	I-mutant
L43P	I-mutant
strain	O
,	O
and	O
as	O
expected	O
,	O
a	O
certain	O
amount	O
of	O
VB6	B-chemical
or	O
L	B-chemical
-	I-chemical
Trp	I-chemical
(	O
4	O
–	O
6	O
mmol	O
/	O
L	O
for	O
VB6	B-chemical
and	O
6	O
–	O
10	O
mmol	O
/	O
L	O
for	O
L	B-chemical
-	I-chemical
Trp	I-chemical
)	O
could	O
reduce	O
the	O
surface	O
attachment	O
.	O

In	O
Helicobacter	B-species
pylori	I-species
in	O
particular	O
,	O
VB6	B-chemical
biosynthetic	O
enzymes	O
act	O
as	O
novel	O
virulence	O
factors	O
,	O
and	O
VB6	B-chemical
is	O
required	O
for	O
full	O
motility	O
and	O
virulence	O
(	O
Grubman	O
et	O
al	O
.,).	O

In	O
E	B-species
.	I-species
coli	I-species
,	O
mutants	O
with	O
decreased	O
tryptophan	B-chemical
synthesis	O
show	O
greater	O
biofilm	O
formation	O
,	O
and	O
matured	O
biofilm	O
is	O
degraded	O
by	O
L	B-chemical
-	I-chemical
tryptophan	I-chemical
addition	O
(	O
Shimazaki	O
et	O
al	O
.,).	O

To	O
answer	O
the	O
question	O
whether	O
competition	O
of	O
VB6	B-chemical
or	O
L	B-chemical
-	I-chemical
Trp	I-chemical
for	O
the	O
YfiB	B-protein
F57	B-site
-	I-site
binding	I-site
pocket	I-site
of	O
YfiR	B-protein
plays	O
an	O
essential	O
role	O
in	O
inhibiting	O
biofilm	O
formation	O
,	O
we	O
measured	O
the	O
binding	B-evidence
affinities	I-evidence
of	O
VB6	B-chemical
and	O
L	B-chemical
-	I-chemical
Trp	I-chemical
for	O
YfiR	B-protein
via	O
BIAcore	B-experimental_method
experiments	O
.	O

The	O
results	O
showed	O
relatively	O
weak	O
Kd	B-evidence
values	O
of	O
35	O
.	O
2	O
mmol	O
/	O
L	O
and	O
76	O
.	O
9	O
mmol	O
/	O
L	O
for	O
VB6	B-chemical
and	O
L	B-chemical
-	I-chemical
Trp	I-chemical
,	O
respectively	O
(	O
Fig	O
.	O
6C	O
and	O
6D	O
).	O

Previous	O
studies	O
suggested	O
that	O
in	O
response	O
to	O
cell	O
stress	O
,	O
YfiB	B-protein
in	O
the	O
outer	O
membrane	O
sequesters	O
the	O
periplasmic	O
protein	O
YfiR	B-protein
,	O
releasing	O
its	O
inhibition	O
of	O
YfiN	B-protein
on	O
the	O
inner	O
membrane	O
and	O
thus	O
inducing	O
the	O
diguanylate	O
cyclase	O
activity	O
of	O
YfiN	B-protein
to	O
allow	O
c	B-chemical
-	I-chemical
di	I-chemical
-	I-chemical
GMP	I-chemical
production	O
(	O
Giardina	O
et	O
al	O
.,;	O
Malone	O
et	O
al	O
.,;	O
Malone	O
et	O
al	O
.,).	O

Here	O
,	O
we	O
report	O
the	O
crystal	B-evidence
structures	I-evidence
of	O
YfiB	B-protein
alone	B-protein_state
and	O
an	O
active	B-protein_state
mutant	B-protein_state
YfiBL43P	B-mutant
in	B-protein_state
complex	I-protein_state
with	I-protein_state
YfiR	B-protein
,	O
indicating	O
that	O
YfiR	B-protein
forms	O
a	O
2	O
:	O
2	O
complex	B-protein_state
with	I-protein_state
YfiB	B-protein
via	O
a	O
region	O
composed	O
of	O
conserved	O
residues	O
.	O

In	O
addition	O
to	O
the	O
preceding	B-residue_range
8	I-residue_range
aa	I-residue_range
loop	B-structure_element
(	O
from	O
the	O
lipid	O
acceptor	O
Cys26	B-residue_range
to	I-residue_range
Gly34	I-residue_range
),	O
the	O
full	B-protein_state
length	I-protein_state
of	O
the	O
periplasmic	O
portion	O
of	O
apo	B-protein_state
YfiB	B-protein
can	O
reach	O
approximately	O
60	O
Å	O
.	O
It	O
was	O
reported	O
that	O
the	O
distance	O
between	O
the	O
outer	O
membrane	O
and	O
the	O
cell	O
wall	O
is	O
approximately	O
50	O
Å	O
and	O
that	O
the	O
thickness	O
of	O
the	O
PG	O
layer	O
is	O
approximately	O
70	O
Å	O
(	O
Matias	O
et	O
al	O
.,).	O

Thus	O
,	O
YfiB	B-protein
alone	B-protein_state
represents	O
an	O
inactive	B-protein_state
form	O
that	O
may	O
only	O
partially	O
insert	O
into	O
the	O
PG	O
matrix	O
.	O

In	O
addition	O
to	O
the	O
17	B-residue_range
preceding	I-residue_range
intracellular	I-residue_range
residues	I-residue_range
(	O
from	O
the	O
lipid	O
acceptor	O
Cys26	B-residue_range
to	I-residue_range
Leu43	I-residue_range
),	O
the	O
length	O
of	O
the	O
intracellular	O
portion	O
of	O
active	B-protein_state
YfiB	B-protein
may	O
extend	O
over	O
100	O
Å	O
,	O
assuming	O
a	O
fully	B-protein_state
stretched	I-protein_state
conformation	I-protein_state
.	O

The	O
periplasmic	B-structure_element
domain	I-structure_element
of	O
YfiB	B-protein
and	O
the	O
YfiB	B-complex_assembly
-	I-complex_assembly
YfiR	I-complex_assembly
complex	O
are	O
depicted	O
according	O
to	O
the	O
crystal	B-evidence
structures	I-evidence
.	O

The	O
lipid	O
acceptor	O
Cys26	B-residue_name_number
is	O
indicated	O
as	O
blue	O
ball	O
.	O

The	O
PAS	B-structure_element
domain	I-structure_element
of	O
YfiN	B-protein
is	O
shown	O
as	O
pink	O
oval	O
.	O

In	O
this	O
model	O
,	O
in	O
response	O
to	O
a	O
particular	O
cell	O
stress	O
that	O
is	O
yet	O
to	O
be	O
identified	O
,	O
the	O
dimeric	B-oligomeric_state
YfiB	B-protein
is	O
activated	B-protein_state
from	O
a	O
compact	B-protein_state
,	O
inactive	B-protein_state
conformation	B-protein_state
to	O
a	O
stretched	B-protein_state
conformation	I-protein_state
,	O
which	O
possesses	O
increased	O
PG	B-chemical
binding	O
affinity	O
.	O

The	O
YfiBNR	B-complex_assembly
system	O
provides	O
a	O
good	O
example	O
of	O
a	O
delicate	O
homeostatic	O
system	O
that	O
integrates	O
multiple	O
signals	O
to	O
regulate	O
the	O
c	B-chemical
-	I-chemical
di	I-chemical
-	I-chemical
GMP	I-chemical
level	O
.	O

Homologs	O
of	O
the	O
YfiBNR	B-complex_assembly
system	O
are	O
functionally	B-protein_state
conserved	I-protein_state
in	O
P	B-species
.	I-species
aeruginosa	I-species
(	O
Malone	O
et	O
al	O
.,;	O
Malone	O
et	O
al	O
.,),	O
E	B-species
.	I-species
coli	I-species
(	O
Hufnagel	O
et	O
al	O
.,;	O
Raterman	O
et	O
al	O
.,;	O
Sanchez	O
-	O
Torres	O
et	O
al	O
.,),	O
K	B-species
.	I-species
pneumonia	I-species
(	O
Huertas	O
et	O
al	O
.,)	O
and	O
Y	B-species
.	I-species
pestis	I-species
(	O
Ren	O
et	O
al	O
.,),	O
where	O
they	O
affect	O
c	B-chemical
-	I-chemical
di	I-chemical
-	I-chemical
GMP	I-chemical
production	O
and	O
biofilm	O
formation	O
.	O

Hemi	B-chemical
-	I-chemical
methylated	I-chemical
DNA	I-chemical
opens	O
a	O
closed	B-protein_state
conformation	O
of	O
UHRF1	B-protein
to	O
facilitate	O
its	O
histone	B-protein_type
recognition	O

UHRF1	B-protein
is	O
an	O
important	O
epigenetic	O
regulator	O
for	O
maintenance	O
DNA	O
methylation	B-ptm
.	O

Here	O
we	O
show	O
that	O
UHRF1	B-protein
adopts	O
a	O
closed	B-protein_state
conformation	O
,	O
in	O
which	O
a	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
region	I-structure_element
(	O
Spacer	B-structure_element
)	O
binds	B-protein_state
to	I-protein_state
the	O
tandem	B-structure_element
Tudor	I-structure_element
domain	I-structure_element
(	O
TTD	B-structure_element
)	O
and	O
inhibits	O
H3K9me3	B-protein_type
recognition	O
,	O
whereas	O
the	O
SET	B-structure_element
-	I-structure_element
and	I-structure_element
-	I-structure_element
RING	I-structure_element
-	I-structure_element
associated	I-structure_element
(	O
SRA	B-structure_element
)	O
domain	O
binds	B-protein_state
to	I-protein_state
the	O
plant	B-structure_element
homeodomain	I-structure_element
(	O
PHD	B-structure_element
)	O
and	O
inhibits	O
H3R2	B-site
recognition	O
.	O

Hm	B-chemical
-	I-chemical
DNA	I-chemical
impairs	O
the	O
intramolecular	O
interactions	O
and	O
promotes	O
H3K9me3	B-protein_type
recognition	O
by	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
.	O

The	O
Spacer	B-structure_element
also	O
facilitates	O
UHRF1	B-complex_assembly
–	I-complex_assembly
DNMT1	I-complex_assembly
interaction	O
and	O
enhances	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
the	O
SRA	B-structure_element
.	O

When	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
binds	B-protein_state
to	I-protein_state
H3K9me3	B-protein_type
,	O
SRA	B-structure_element
-	I-structure_element
Spacer	I-structure_element
may	O
exist	O
in	O
a	O
dynamic	O
equilibrium	O
:	O
either	O
recognizes	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
or	O
recruits	O
DNMT1	B-protein
to	O
chromatin	O
.	O

UHRF1	B-protein
is	O
involved	O
in	O
the	O
maintenance	O
of	O
DNA	O
methylation	B-ptm
,	O
but	O
the	O
regulatory	O
mechanism	O
of	O
this	O
epigenetic	O
regulator	O
is	O
unclear	O
.	O

Here	O
,	O
the	O
authors	O
show	O
that	O
it	O
has	O
a	O
closed	B-protein_state
conformation	O
and	O
are	O
able	O
to	O
make	O
conclusions	O
about	O
the	O
mechanism	O
of	O
recognition	O
of	O
epigenetic	O
marks	O
.	O

DNA	O
methylation	B-ptm
is	O
an	O
important	O
epigenetic	O
modification	O
for	O
gene	O
repression	O
,	O
X	O
-	O
chromosome	O
inactivation	O
,	O
genome	O
imprinting	O
and	O
maintenance	O
of	O
genome	O
stability	O
.	O

Mammalian	B-taxonomy_domain
DNA	O
methylation	B-ptm
is	O
established	O
by	O
de	O
novo	O
DNA	B-protein_type
methyltransferases	I-protein_type
DNMT3A	B-protein
/	I-protein
3B	I-protein
,	O
and	O
DNA	O
methylation	B-ptm
patterns	O
are	O
maintained	O
by	O
maintenance	O
DNA	B-protein
methyltransferase	I-protein
1	I-protein
(	O
DNMT1	B-protein
)	O
during	O
DNA	O
replication	O
.	O

Ubiquitin	B-protein
-	I-protein
like	I-protein
,	I-protein
containing	I-protein
PHD	I-protein
and	I-protein
RING	I-protein
fingers	I-protein
domains	I-protein
,	I-protein
1	I-protein
(	O
UHRF1	B-protein
,	O
also	O
known	O
as	O
ICBP90	B-protein
and	O
NP95	B-protein
in	O
mouse	B-taxonomy_domain
)	O
was	O
shown	O
to	O
be	O
essential	O
for	O
maintenance	O
DNA	O
methylation	B-ptm
through	O
recruiting	O
DNMT1	B-protein
to	O
replication	O
forks	O
in	O
S	O
phase	O
of	O
the	O
cell	O
cycle	O
.	O

UHRF1	B-protein
is	O
essential	O
for	O
S	O
phase	O
entry	O
and	O
is	O
involved	O
in	O
heterochromatin	O
formation	O
.	O

UHRF1	B-protein
is	O
a	O
multi	O
-	O
domain	O
containing	O
protein	O
connecting	O
histone	B-protein_type
modification	O
and	O
DNA	B-chemical
methylation	B-ptm
.	O

As	O
shown	O
in	O
Fig	O
.	O
1a	O
,	O
UHRF1	B-protein
is	O
comprised	O
of	O
an	O
N	O
-	O
terminal	O
ubiquitin	B-structure_element
-	I-structure_element
like	I-structure_element
domain	I-structure_element
,	O
followed	O
by	O
a	O
tandem	B-structure_element
Tudor	I-structure_element
domain	I-structure_element
(	O
TTD	B-structure_element
containing	O
TTDN	B-structure_element
and	O
TTDC	B-structure_element
sub	O
-	O
domains	O
),	O
a	O
plant	B-structure_element
homeodomain	I-structure_element
(	O
PHD	B-structure_element
),	O
a	O
SET	B-structure_element
-	I-structure_element
and	I-structure_element
-	I-structure_element
RING	I-structure_element
-	I-structure_element
associated	I-structure_element
(	O
SRA	B-structure_element
)	O
domain	O
,	O
and	O
a	O
C	O
-	O
terminal	O
really	B-structure_element
interesting	I-structure_element
new	I-structure_element
gene	I-structure_element
(	O
RING	B-structure_element
)	O
domain	O
.	O

We	O
and	O
other	O
groups	O
demonstrated	O
that	O
the	O
TTD	B-structure_element
and	O
the	O
PHD	B-structure_element
coordinately	O
recognize	O
histone	B-protein_type
H3K9me3	B-protein_type
,	O
in	O
which	O
residue	O
R2	B-residue_name_number
is	O
recognized	O
by	O
the	O
PHD	B-structure_element
and	O
tri	B-ptm
-	I-ptm
methylation	I-ptm
of	O
residue	O
K9	B-residue_name_number
(	O
K9me3	B-ptm
)	O
is	O
recognized	O
by	O
the	O
TTD	B-structure_element
.	O

The	O
SRA	B-structure_element
preferentially	O
binds	B-protein_state
to	I-protein_state
hemi	B-chemical
-	I-chemical
methylated	I-chemical
DNA	I-chemical
(	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
).	O

Recent	O
studies	O
show	O
that	O
the	O
SRA	B-structure_element
directly	O
binds	B-protein_state
to	I-protein_state
replication	B-structure_element
focus	I-structure_element
targeting	I-structure_element
sequence	I-structure_element
(	O
RFTS	B-structure_element
)	O
of	O
DNMT1	B-protein
(	O
RFTSDNMT1	B-protein
).	O

A	O
spacer	B-structure_element
region	I-structure_element
(	O
Fig	O
.	O
1a	O
,	O
designated	O
Spacer	B-structure_element
hereafter	O
)	O
connecting	O
the	O
SRA	B-structure_element
and	O
the	O
RING	B-structure_element
is	O
rich	O
in	O
basic	O
residues	O
and	O
predicted	O
to	O
be	O
unstructured	B-protein_state
for	O
unknown	O
function	O
.	O

Recent	O
study	O
shows	O
that	O
phosphatidylinostiol	B-chemical
phosphate	I-chemical
PI5P	B-chemical
binds	B-protein_state
to	I-protein_state
the	O
Spacer	B-structure_element
and	O
induces	O
a	O
conformational	O
change	O
of	O
UHRF1	B-protein
to	O
allow	O
the	O
TTD	B-structure_element
to	O
recognize	O
H3K9me3	B-protein_type
(	O
ref	O
.).	O

These	O
studies	O
indicate	O
that	O
UHRF1	B-protein
connects	O
dynamic	O
regulation	O
of	O
DNA	B-chemical
methylation	B-ptm
and	O
H3K9me3	B-protein_type
,	O
which	O
are	O
positively	O
correlated	O
in	O
human	B-species
genome	O
.	O

Upon	O
binding	B-protein_state
to	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
,	O
UHRF1	B-protein
impairs	O
the	O
intramolecular	O
interactions	O
and	O
promotes	O
the	O
H3K9me3	B-protein_type
recognition	O
by	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
,	O
which	O
may	O
further	O
enhance	O
its	O
genomic	O
localization	O
.	O

Our	O
study	O
reveals	O
the	O
mechanism	O
for	O
regulation	O
of	O
H3K9me3	B-protein_type
and	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
recognition	O
by	O
UHRF1	B-protein
.	O

Hm	B-chemical
-	I-chemical
DNA	I-chemical
facilitates	O
histone	B-protein_type
H3K9me3	B-protein_type
recognition	O
by	O
UHRF1	B-protein

We	O
first	O
performed	O
an	O
in	B-experimental_method
vitro	I-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assay	I-experimental_method
using	O
biotinylated	B-protein_state
histone	B-protein_type
H3	B-protein_type
peptides	O
and	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
Supplementary	O
Table	O
1	O
).	O

As	O
shown	O
in	O
Fig	O
.	O
1b	O
,	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
largely	O
enhanced	O
the	O
interaction	O
between	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
and	O
unmethylated	B-protein_state
histone	B-protein_type
H3	B-protein_type
(	O
H3K9me0	B-protein_type
)	O
or	O
H3K9me3	B-protein_type
peptide	O
.	O

Compared	O
with	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
,	O
um	B-chemical
-	I-chemical
DNA	I-chemical
(	O
unmethylated	B-protein_state
DNA	B-chemical
)	O
or	O
fm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
fully	B-protein_state
methylated	I-protein_state
DNA	B-chemical
)	O
showed	O
marginal	O
effect	O
on	O
facilitating	O
the	O
interaction	O
between	O
UHRF1	B-protein
and	O
histone	B-protein_type
peptides	O
,	O
which	O
is	O
consistent	O
with	O
previous	O
studies	O
that	O
UHRF1	B-protein
prefers	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
for	O
chromatin	O
association	O
(	O
Supplementary	O
Fig	O
.	O
1a	O
).	O

In	O
contrast	O
,	O
histone	B-protein_type
peptides	O
showed	O
no	O
enhancement	O
on	O
the	O
interaction	O
between	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
and	O
UHRF1	B-protein
(	O
Fig	O
.	O
1c	O
).	O

Our	O
previous	O
studies	O
show	O
that	O
the	O
PHD	B-structure_element
recognizes	O
H3K9me0	B-protein_type
and	O
the	O
TTD	B-structure_element
and	O
the	O
PHD	B-structure_element
together	O
(	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
)	O
coordinately	O
recognize	O
H3K9me3	B-protein_type
(	O
refs	O
.).	O

We	O
noticed	O
that	O
the	O
isolated	B-protein_state
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
showed	O
much	O
higher	O
(∼	O
31	O
-	O
fold	O
)	O
binding	B-evidence
affinity	I-evidence
to	O
H3K9me3	B-protein_type
peptide	O
than	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
(	O
Fig	O
.	O
1d	O
and	O
Supplementary	O
Table	O
2	O
),	O
and	O
the	O
isolated	O
PHD	B-structure_element
showed	O
much	O
higher	O
(∼	O
34	O
-	O
fold	O
)	O
binding	B-evidence
affinity	I-evidence
to	O
H3K9me0	B-protein_type
peptide	O
than	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
(	O
Fig	O
.	O
1e	O
).	O

Intramolecular	O
interaction	O
within	O
UHRF1	B-protein

Interestingly	O
,	O
the	O
TTD	B-structure_element
directly	O
bound	B-protein_state
to	I-protein_state
SRA	B-structure_element
-	I-structure_element
Spacer	I-structure_element
but	O
not	O
the	O
SRA	B-structure_element
,	O
suggesting	O
that	O
the	O
Spacer	B-structure_element
(	O
residues	O
587	B-residue_range
–	I-residue_range
674	I-residue_range
)	O
is	O
important	O
for	O
the	O
intramolecular	O
interaction	O
(	O
Fig	O
.	O
2a	O
).	O

The	O
GST	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assay	I-experimental_method
also	O
shows	O
that	O
the	O
PHD	B-structure_element
bound	B-protein_state
to	I-protein_state
the	O
SRA	B-structure_element
,	O
which	O
was	O
further	O
confirmed	O
by	O
the	O
ITC	B-experimental_method
measurements	O
(	O
KD	B-evidence
=	O
26	O
.	O
7	O
μM	O
;	O
Fig	O
.	O
2a	O
,	O
d	O
).	O

Taken	O
together	O
,	O
UHRF1	B-protein
seems	O
to	O
adopt	O
a	O
closed	B-protein_state
form	O
through	O
intramolecular	O
interactions	O
(	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
and	O
PHD	B-structure_element
-	I-structure_element
SRA	I-structure_element
),	O
which	O
inhibit	O
histone	B-protein_type
H3	B-protein_type
tail	O
recognition	O
by	O
UHRF1	B-protein
.	O

Overall	O
structure	B-evidence
of	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element

To	O
investigate	O
the	O
intramolecular	O
interaction	O
within	O
UHRF1	B-protein
,	O
we	O
first	O
mapped	O
the	O
minimal	O
regions	O
within	O
the	O
Spacer	B-structure_element
for	O
the	O
interaction	O
with	O
the	O
TTD	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
2a	O
).	O

Internal	O
deletions	B-experimental_method
of	O
the	O
Spacer	B-structure_element
,	O
including	O
SpacerΔ660	B-mutant
–	I-mutant
664	I-mutant
,	O
SpacerΔ665	B-mutant
–	I-mutant
669	I-mutant
,	O
SpacerΔ670	B-mutant
–	I-mutant
674	I-mutant
and	O
Spacer642	B-mutant
–	I-mutant
674	I-mutant
,	O
bound	B-protein_state
to	I-protein_state
the	O
TTD	B-structure_element
with	O
comparable	O
binding	B-evidence
affinities	I-evidence
to	O
that	O
of	O
the	O
Spacer	B-structure_element
,	O
whereas	O
Spacer587	B-mutant
–	I-mutant
641	I-mutant
showed	O
no	O
detectable	O
interaction	O
.	O

We	O
next	O
determined	O
the	O
solution	B-evidence
structure	I-evidence
of	O
the	O
TTD	B-structure_element
(	O
residues	O
134	B-residue_range
–	I-residue_range
285	I-residue_range
)	O
bound	B-protein_state
to	I-protein_state
Spacer627	B-residue_range
–	I-residue_range
674	I-residue_range
by	O
conventional	O
NMR	B-experimental_method
techniques	O
(	O
Supplementary	O
Table	O
3	O
and	O
Supplementary	O
Fig	O
.	O
3a	O
,	O
b	O
).	O

In	O
the	O
complex	B-evidence
structure	I-evidence
,	O
each	O
Tudor	B-structure_element
domain	I-structure_element
adopts	O
a	O
‘	B-structure_element
Royal	I-structure_element
'	I-structure_element
fold	I-structure_element
containing	O
a	O
characteristic	O
five	B-structure_element
-	I-structure_element
stranded	I-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
and	O
the	O
two	O
Tudor	B-structure_element
domains	I-structure_element
tightly	O
pack	O
against	O
each	O
other	O
with	O
a	O
buried	O
area	O
of	O
573	O
Å2	O
(	O
Fig	O
.	O
3a	O
).	O

The	O
TTD	B-structure_element
adopts	O
similar	O
fold	O
to	O
that	O
in	O
TTD	B-complex_assembly
–	I-complex_assembly
PHD	I-complex_assembly
–	I-complex_assembly
H3K9me3	I-complex_assembly
complex	O
structure	B-evidence
(	O
PDB	O
:	O
4GY5	O
)	O
with	O
a	O
root	B-evidence
-	I-evidence
mean	I-evidence
-	I-evidence
square	I-evidence
deviation	I-evidence
of	O
1	O
.	O
09	O
Å	O
for	O
128	O
Cα	O
atoms	O
,	O
indicating	O
that	O
the	O
Spacer	B-structure_element
does	O
not	O
result	O
in	O
obvious	O
conformational	O
change	O
of	O
the	O
TTD	B-structure_element
(	O
Fig	O
.	O
3b	O
).	O

The	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
interaction	O
is	O
mediated	O
by	O
a	O
number	O
of	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
(	O
Fig	O
.	O
3d	O
).	O

The	O
side	O
chain	O
of	O
residue	O
K648	B-residue_name_number
forms	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
the	O
carbonyl	O
oxygen	O
atom	O
of	O
D189	B-residue_name_number
and	O
side	O
chain	O
of	O
D190	B-residue_name_number
of	O
the	O
TTD	B-structure_element
.	O

The	O
side	O
chain	O
of	O
residue	O
R649	B-residue_name_number
packs	B-bond_interaction
against	I-bond_interaction
an	O
acidic	O
surface	O
mainly	O
formed	O
by	O
residues	O
D142	B-residue_name_number
and	O
E153	B-residue_name_number
.	O

Residue	O
S651	B-residue_name_number
forms	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
the	O
main	O
chain	O
of	O
residues	O
G236	B-residue_name_number
and	O
W238	B-residue_name_number
.	O

The	O
interaction	O
is	O
further	O
supported	O
by	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
formed	O
between	O
residues	O
K650	B-residue_name_number
,	O
A652	B-residue_name_number
,	O
G653	B-residue_name_number
and	O
G654	B-residue_name_number
of	O
the	O
Spacer	B-structure_element
and	O
residues	O
N228	B-residue_name_number
,	O
G236	B-residue_name_number
and	O
W238	B-residue_name_number
of	O
the	O
TTD	B-structure_element
,	O
respectively	O
.	O

In	O
support	O
of	O
above	O
structural	B-experimental_method
analyses	I-experimental_method
,	O
mutation	B-experimental_method
D142A	B-mutant
/	O
E153A	B-mutant
of	O
the	O
TTD	B-structure_element
abolished	O
its	O
interaction	O
with	O
the	O
Spacer	B-structure_element
(	O
Fig	O
.	O
3e	O
).	O

As	O
negative	O
control	O
,	O
mutations	B-experimental_method
S639D	B-mutant
and	O
S666D	B-mutant
of	O
the	O
Spacer	B-structure_element
showed	O
little	O
effect	O
on	O
the	O
interaction	O
.	O

Interestingly	O
,	O
phosphorylation	B-ptm
at	O
residue	O
S651	B-residue_name_number
of	O
UHRF1	B-protein
was	O
observed	O
in	O
previous	O
mass	B-experimental_method
-	I-experimental_method
spectrometry	I-experimental_method
analyses	O
.	O

Compared	O
with	O
the	O
unmodified	B-protein_state
peptide	O
of	O
Spacer642	B-mutant
–	I-mutant
664	I-mutant
,	O
a	O
phosphorylation	B-ptm
at	O
S651	B-residue_name_number
markedly	O
decreased	O
the	O
binding	B-evidence
affinity	I-evidence
to	O
the	O
TTD	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
2b	O
),	O
suggesting	O
that	O
the	O
phosphorylation	B-ptm
may	O
regulate	O
the	O
intramolecular	O
interaction	O
within	O
UHRF1	B-protein
.	O

The	O
spacer	B-structure_element
binds	B-protein_state
to	I-protein_state
the	O
TTD	B-structure_element
by	O
competing	O
with	O
the	O
linker	B-structure_element

Previous	O
studies	O
indicate	O
that	O
the	O
TTD	B-structure_element
binds	B-protein_state
to	I-protein_state
a	O
linker	B-structure_element
region	I-structure_element
connecting	O
the	O
TTD	B-structure_element
and	O
PHD	B-structure_element
(	O
residues	O
286	B-residue_range
–	I-residue_range
306	I-residue_range
,	O
designated	O
Linker	B-structure_element
,	O
Fig	O
.	O
1a	O
),	O
and	O
TTD	B-structure_element
–	I-structure_element
Linker	I-structure_element
interaction	O
is	O
essential	O
for	O
H3K9me3	B-protein_type
recognition	O
by	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
.	O

In	O
TTD	B-complex_assembly
–	I-complex_assembly
PHD	I-complex_assembly
–	I-complex_assembly
H3K9me3	I-complex_assembly
structure	B-evidence
,	O
residues	O
R295	B-residue_name_number
,	O
R296	B-residue_name_number
and	O
S298	B-residue_name_number
of	O
the	O
Linker	B-structure_element
adopt	O
almost	O
identical	O
conformation	O
to	O
residues	O
K648	B-residue_name_number
,	O
R649	B-residue_name_number
and	O
S651	B-residue_name_number
of	O
the	O
Spacer	B-structure_element
in	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
structure	B-evidence
,	O
respectively	O
.	O

Similar	O
intramolecular	O
contacts	O
(	O
TTD	B-structure_element
–	I-structure_element
Linker	I-structure_element
and	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
)	O
were	O
observed	O
in	O
the	O
two	O
structures	B-evidence
(	O
Fig	O
.	O
3b	O
,	O
d	O
and	O
Supplementary	O
Fig	O
.	O
4a	O
).	O

To	O
test	O
this	O
hypothesis	O
,	O
we	O
first	O
investigated	O
the	O
potential	O
competition	O
between	O
the	O
Linker	B-structure_element
and	O
the	O
Spacer	B-structure_element
for	O
their	O
interaction	O
with	O
the	O
TTD	B-structure_element
.	O

The	O
competitive	B-experimental_method
ITC	I-experimental_method
experiments	O
show	O
that	O
TTD	B-evidence
–	I-evidence
Spacer	I-evidence
binding	I-evidence
affinity	I-evidence
decreased	O
by	O
a	O
factor	O
of	O
two	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
the	O
Linker	B-structure_element
,	O
whereas	O
TTD	B-structure_element
–	I-structure_element
Linker	I-structure_element
interaction	O
was	O
abolished	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
the	O
Spacer	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
4c	O
).	O

Compared	O
with	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
interaction	O
(	O
KD	B-evidence
=	O
1	O
.	O
48	O
μM	O
),	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
decreased	O
the	O
binding	B-evidence
affinity	I-evidence
to	O
the	O
Spacer	B-structure_element
(	O
KD	B-evidence
=	O
10	O
.	O
68	O
μM	O
),	O
whereas	O
mutation	B-experimental_method
R295D	B-mutant
/	O
R296D	B-mutant
(	O
within	O
the	O
Linker	B-structure_element
and	O
important	O
for	O
TTD	B-structure_element
–	I-structure_element
Linker	I-structure_element
interaction	O
)	O
of	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
showed	O
minor	O
decrease	O
in	O
the	O
binding	B-evidence
affinity	I-evidence
(	O
KD	B-evidence
=	O
2	O
.	O
69	O
μM	O
;	O
Fig	O
.	O
3g	O
),	O
indicating	O
a	O
competition	O
between	O
the	O
Spacer	B-structure_element
and	O
the	O
Linker	B-structure_element
on	O
the	O
same	O
binding	B-site
site	I-site
of	O
the	O
TTD	B-structure_element
.	O

As	O
indicated	O
in	O
Fig	O
.	O
3h	O
,	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
and	O
UHRF1ΔSRA	B-mutant
showed	O
no	O
interaction	O
with	O
GST	B-protein_state
-	I-protein_state
tagged	I-protein_state
TTD	B-structure_element
,	O
Linker	B-structure_element
or	O
Spacer	B-structure_element
,	O
suggesting	O
that	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
interaction	O
in	B-protein_state
-	I-protein_state
cis	I-protein_state
within	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
or	O
UHRF1ΔSRA	B-mutant
prohibits	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
complex	O
formation	O
in	B-protein_state
-	I-protein_state
trans	I-protein_state
.	O

In	O
contrast	O
,	O
UHRF1ΔTTD	B-mutant
bound	B-protein_state
to	I-protein_state
GST	B-experimental_method
-	O
TTD	B-structure_element
,	O
and	O
UHRF1Δ627	B-mutant
–	I-mutant
674	I-mutant
bound	B-protein_state
to	I-protein_state
GST	B-experimental_method
-	O
Spacer	B-structure_element
,	O
indicating	O
that	O
lack	O
of	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
interaction	O
in	B-protein_state
-	I-protein_state
cis	I-protein_state
,	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
complex	O
could	O
form	O
in	B-protein_state
-	I-protein_state
trans	I-protein_state
,	O
supporting	O
that	O
the	O
TTD	B-structure_element
binds	B-protein_state
to	I-protein_state
the	O
Spacer	B-structure_element
in	O
the	O
context	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
.	O

Moreover	O
,	O
GST	B-experimental_method
-	O
Linker	B-structure_element
showed	O
very	O
weak	O
if	O
not	O
undetectable	O
interaction	O
with	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
or	O
deletions	O
of	O
UHRF1	B-protein
,	O
suggesting	O
that	O
TTD	B-structure_element
–	I-structure_element
Linker	I-structure_element
interaction	O
is	O
much	O
weaker	O
than	O
that	O
of	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
.	O

The	O
spacer	B-structure_element
inhibits	O
H3K9me3	B-protein_type
recognition	O
by	O
the	O
isolated	O
TTD	B-structure_element

Our	O
previous	O
study	O
indicates	O
that	O
H3K9me3	B-protein_type
binds	B-protein_state
to	I-protein_state
the	O
TTD	B-structure_element
in	O
different	O
manner	O
in	O
TTD	B-complex_assembly
–	I-complex_assembly
PHD	I-complex_assembly
–	I-complex_assembly
H3K9me3	I-complex_assembly
(	O
ref	O
.)	O
and	O
TTD	B-complex_assembly
-	I-complex_assembly
H3K9me3	I-complex_assembly
(	O
PDB	O
:	O
2L3R	O
)	O
structures	B-evidence
.	O

As	O
shown	O
in	O
Supplementary	O
Fig	O
.	O
4e	O
,	O
the	O
Spacer	B-structure_element
inhibited	O
TTD	B-complex_assembly
–	I-complex_assembly
H3K9me3	I-complex_assembly
interaction	O
,	O
whereas	O
its	O
TTD	B-protein_state
-	I-protein_state
binding	I-protein_state
defective	I-protein_state
mutants	B-protein_state
of	O
the	O
Spacer	B-structure_element
or	O
the	O
SRA	B-structure_element
(	O
a	O
negative	O
control	O
)	O
markedly	O
decreased	O
the	O
inhibition	O
.	O

Compared	O
with	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
,	O
UHRF1Δ627	B-mutant
–	I-mutant
674	I-mutant
enhanced	O
H3K9me3	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
by	O
a	O
factor	O
of	O
four	O
(	O
Supplementary	O
Fig	O
.	O
4f	O
).	O

The	O
restoration	O
of	O
H3K9me3	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
is	O
not	O
dramatic	O
because	O
the	O
PHD	B-structure_element
still	O
binds	B-protein_state
to	I-protein_state
histone	B-protein_type
H3	B-protein_type
in	O
both	O
proteins	O
.	O

To	O
exclude	O
this	O
effect	O
,	O
we	O
performed	O
the	O
assay	O
using	O
UHRF1D334A	B-mutant
,	O
which	O
abolishes	B-protein_state
H3R2	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
the	O
PHD	B-structure_element
.	O

UHRF1D334A	B-mutant
showed	O
undetectable	O
H3K9me3	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
,	O
whereas	O
UHRF1D334A	B-mutant
&	O
Δ627	B-mutant
–	I-mutant
674	I-mutant
dramatically	O
restored	O
its	O
H3K9me3	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
(	O
KD	B-evidence
=	O
8	O
.	O
69	O
μM	O
;	O
Supplementary	O
Fig	O
.	O
4f	O
),	O
indicating	O
that	O
H3K9me3	B-protein_type
recognition	O
by	O
the	O
TTD	B-structure_element
is	O
blocked	O
by	O
the	O
Spacer	B-structure_element
through	O
competitive	O
interaction	O
with	O
the	O
TTD	B-structure_element
.	O

Moreover	O
,	O
the	O
R295D	B-mutant
/	O
R296D	B-mutant
mutant	B-protein_state
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
showed	O
decreased	O
binding	B-evidence
affinity	I-evidence
to	O
H3K9me3	B-protein_type
(	O
eightfold	O
lower	O
than	O
wild	B-protein_state
type	I-protein_state
),	O
suggesting	O
that	O
mutation	B-experimental_method
of	O
R295D	B-mutant
/	O
R296D	B-mutant
favours	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
interaction	O
and	O
therefore	O
promotes	O
UHRF1	B-protein
to	O
exhibit	O
a	O
more	O
stable	O
closed	B-protein_state
conformation	O
(	O
Supplementary	O
Fig	O
.	O
4g	O
).	O

TTD	B-complex_assembly
–	I-complex_assembly
PHD	I-complex_assembly
–	I-complex_assembly
H3K9me3	I-complex_assembly
complex	O
inhibits	O
TTD	B-structure_element
–	I-structure_element
spacer	I-structure_element
interaction	O

Interestingly	O
,	O
pre	B-experimental_method
-	I-experimental_method
incubation	I-experimental_method
of	O
H3K9me3	B-protein_type
peptide	O
completely	O
blocked	O
the	O
interaction	O
between	O
the	O
Spacer	B-structure_element
and	O
the	O
TTD	B-structure_element
alone	B-protein_state
or	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
(	O
Supplementary	O
Fig	O
.	O
4h	O
),	O
whereas	O
the	O
presence	B-protein_state
of	I-protein_state
the	O
Spacer	B-structure_element
partially	O
impaired	O
the	O
interaction	O
between	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
and	O
H3K9me3	B-protein_type
(	O
Fig	O
.	O
2c	O
).	O

The	O
results	O
are	O
also	O
consistent	O
with	O
the	O
previous	O
observation	O
that	O
the	O
interaction	O
between	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
and	O
the	O
Spacer	B-structure_element
is	O
much	O
weaker	O
(	O
KD	B-evidence
=	O
10	O
.	O
68	O
μM	O
,	O
Fig	O
.	O
3g	O
)	O
than	O
that	O
between	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
and	O
H3K9me3	B-protein_type
(	O
KD	B-evidence
=	O
0	O
.	O
15	O
μM	O
,	O
Fig	O
.	O
1d	O
).	O

These	O
results	O
suggest	O
that	O
once	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
binds	B-protein_state
to	I-protein_state
H3K9me3	B-protein_type
,	O
UHRF1	B-protein
will	O
be	O
locked	O
by	O
H3K9me3	B-protein_type
and	O
the	O
Spacer	B-structure_element
is	O
unlikely	O
to	O
fold	O
back	O
for	O
the	O
intramolecular	O
interaction	O
.	O

Hm	B-chemical
-	I-chemical
DNA	I-chemical
disrupts	O
intramolecular	O
interaction	O
within	O
UHRF1	B-protein

To	O
investigate	O
whether	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
could	O
open	B-protein_state
the	O
closed	B-protein_state
conformation	O
of	O
UHRF1	B-protein
,	O
we	O
first	O
measured	O
the	O
intramolecular	O
interaction	O
using	O
UHRF1	B-protein
truncations	B-experimental_method
in	O
the	O
presence	B-protein_state
or	O
absence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
.	O

The	O
GST	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assays	I-experimental_method
show	O
that	O
the	O
PHD	B-structure_element
bound	B-protein_state
to	I-protein_state
the	O
SRA	B-structure_element
and	O
such	O
interaction	O
was	O
impaired	O
by	O
the	O
addition	O
of	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
Fig	O
.	O
4a	O
).	O

H3	B-experimental_method
peptide	I-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assays	I-experimental_method
show	O
that	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
only	O
enhanced	O
the	O
H3K9me0	B-evidence
-	I-evidence
binding	I-evidence
affinities	I-evidence
of	O
UHRF1	B-protein
truncations	B-experimental_method
containing	O
PHD	B-structure_element
-	I-structure_element
SRA	I-structure_element
,	O
such	O
as	O
PHD	B-structure_element
-	I-structure_element
SRA	I-structure_element
,	O
TTD	B-structure_element
-	I-structure_element
PHD	I-structure_element
-	I-structure_element
SRA	I-structure_element
,	O
TTD	B-structure_element
-	I-structure_element
PHD	I-structure_element
-	I-structure_element
SRA	I-structure_element
-	I-structure_element
Spacer	I-structure_element
,	O
UHRF1ΔTTD	B-mutant
and	O
UHRF1ΔSpacer	B-mutant
(	O
Fig	O
.	O
4b	O
).	O

The	O
result	O
indicates	O
that	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
disrupts	O
PHD	B-structure_element
–	I-structure_element
SRA	I-structure_element
interaction	O
and	O
facilitates	O
H3K9me0	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
the	O
PHD	B-structure_element
in	O
a	O
manner	O
independent	O
on	O
the	O
TTD	B-structure_element
or	O
the	O
Spacer	B-structure_element
.	O

Moreover	O
,	O
the	O
TTD	B-structure_element
or	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
bound	B-protein_state
to	I-protein_state
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
and	O
the	O
interaction	O
was	O
impaired	O
by	O
the	O
addition	O
of	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
Fig	O
.	O
4c	O
).	O

The	O
ITC	B-experimental_method
measurements	O
show	O
that	O
the	O
presence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
markedly	O
impaired	O
the	O
interaction	O
between	O
the	O
TTD	B-structure_element
and	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
(	O
Supplementary	O
Fig	O
.	O
5a	O
).	O

However	O
,	O
the	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
interaction	O
was	O
not	O
affected	O
by	O
the	O
presence	B-protein_state
of	I-protein_state
the	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
,	O
indicating	O
that	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
displaces	O
the	O
Spacer	B-structure_element
from	O
the	O
TTD	B-structure_element
in	O
a	O
SRA	B-structure_element
-	O
dependent	O
manner	O
(	O
Supplementary	O
Fig	O
.	O
5b	O
).	O

To	O
investigate	O
whether	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
disrupts	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
interaction	O
in	O
the	O
context	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
,	O
we	O
monitored	O
the	O
conformational	O
changes	O
of	O
UHRF1	B-protein
using	O
its	O
histone	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
as	O
read	O
-	O
out	O
.	O

UHRF1D334A	B-mutant
was	O
used	O
to	O
exclude	O
the	O
effect	O
of	O
H3K9me0	B-protein_type
recognition	O
by	O
the	O
PHD	B-structure_element
.	O

As	O
expected	O
,	O
all	O
D334A	B-mutant
-	O
containing	O
mutants	B-protein_state
showed	O
undetectable	O
interaction	O
with	O
H3K9me0	B-protein_type
(	O
Fig	O
.	O
4d	O
).	O

UHRF1D334A	B-mutant
bound	B-protein_state
to	I-protein_state
H3K9me3	B-protein_type
peptide	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
,	O
but	O
showed	O
no	O
interaction	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
,	O
which	O
is	O
consistent	O
with	O
the	O
ITC	B-experimental_method
experiments	O
(	O
Supplementary	O
Fig	O
.	O
4f	O
).	O

In	O
contrast	O
,	O
UHRF1D334A	B-mutant
&	O
Δ627	B-mutant
–	I-mutant
674	I-mutant
strongly	O
bound	B-protein_state
to	I-protein_state
H3K9me3	B-protein_type
even	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
Fig	O
.	O
4d	O
),	O
indicating	O
that	O
the	O
deletion	B-experimental_method
of	O
the	O
Spacer	B-structure_element
releases	O
otherwise	O
blocked	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
for	O
H3K9me3	B-protein_type
recognition	O
.	O

The	O
results	O
further	O
support	O
the	O
conclusion	O
that	O
the	O
Spacer	B-structure_element
binds	B-protein_state
to	I-protein_state
the	O
TTD	B-structure_element
in	O
the	O
context	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
and	O
the	O
intramolecular	O
interactions	O
are	O
disrupted	O
by	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
.	O

We	O
next	O
performed	O
similar	O
peptide	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assay	I-experimental_method
using	O
two	O
mutants	B-protein_state
(	O
N228C	B-mutant
/	O
G653C	B-mutant
and	O
R235C	B-mutant
/	O
G654C	B-mutant
)	O
generated	O
on	O
UHRF1D334A	B-mutant
.	O

As	O
negative	O
controls	O
,	O
H3K9me3	B-protein_type
recognition	O
by	O
UHRF1D334A	B-mutant
or	O
UHRF1D334A	B-mutant
&	O
Δ627	B-mutant
–	I-mutant
674	I-mutant
is	O
not	O
affected	O
by	O
DTT	B-chemical
.	O

We	O
have	O
previously	O
demonstrated	O
that	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
also	O
disrupts	O
PHD	B-structure_element
–	I-structure_element
SRA	I-structure_element
interaction	O
and	O
facilitates	O
H3K9me0	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
the	O
PHD	B-structure_element
in	O
a	O
manner	O
independent	O
on	O
the	O
TTD	B-structure_element
or	O
the	O
Spacer	B-structure_element
.	O

Taken	O
together	O
,	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
disrupts	O
the	O
intramolecular	O
interactions	O
within	O
UHRF1	B-protein
,	O
and	O
therefore	O
facilitates	O
the	O
coordinate	O
recognition	O
of	O
H3K9me3	B-protein_type
by	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
.	O

The	O
spacer	B-structure_element
enhances	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
the	O
SRA	B-structure_element

To	O
investigate	O
how	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
impairs	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
interaction	O
,	O
we	O
tested	O
whether	O
the	O
Spacer	B-structure_element
is	O
involved	O
in	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
recognition	O
by	O
the	O
SRA	B-structure_element
,	O
which	O
is	O
the	O
only	O
known	O
domain	O
for	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
recognition	O
within	O
UHRF1	B-protein
.	O

In	O
the	O
electrophoretic	B-experimental_method
mobility	I-experimental_method
-	I-experimental_method
shift	I-experimental_method
assay	I-experimental_method
,	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
showed	O
higher	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
than	O
the	O
SRA	B-structure_element
alone	B-protein_state
(	O
Supplementary	O
Fig	O
.	O
6a	O
).	O

ITC	B-experimental_method
measurements	O
show	O
that	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
bound	B-protein_state
to	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
with	O
a	O
much	O
higher	O
binding	B-evidence
affinity	I-evidence
(	O
KD	B-evidence
=	O
1	O
.	O
75	O
μM	O
)	O
than	O
the	O
SRA	B-structure_element
(	O
KD	B-evidence
=	O
25	O
.	O
12	O
μM	O
),	O
whereas	O
the	O
Spacer	B-structure_element
alone	B-protein_state
showed	O
no	O
interaction	O
with	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
Fig	O
.	O
5a	O
).	O

In	O
the	O
fluorescence	B-experimental_method
polarization	I-experimental_method
(	I-experimental_method
FP	I-experimental_method
)	O
measurements	O
,	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
,	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
and	O
UHRF1ΔTTD	B-mutant
showed	O
comparable	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinities	I-evidence
(	O
Fig	O
.	O
5b	O
and	O
Supplementary	O
Table	O
4	O
),	O
suggesting	O
that	O
UHRF1	B-protein
binds	B-protein_state
to	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
no	O
matter	O
UHRF1	B-protein
adopts	O
a	O
closed	B-protein_state
form	O
or	O
not	O
.	O

In	O
contrast	O
,	O
UHRF1ΔSRA	B-mutant
abolished	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
,	O
indicating	O
that	O
the	O
SRA	B-structure_element
is	O
essential	O
for	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
recognition	O
.	O

Compared	O
with	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
,	O
UHRF1Δ627	B-mutant
–	I-mutant
674	I-mutant
decreased	O
the	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
by	O
a	O
factor	O
of	O
14	O
(	O
Fig	O
.	O
5b	O
),	O
further	O
supporting	O
that	O
the	O
Spacer	B-structure_element
plays	O
an	O
important	O
role	O
in	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
recognition	O
in	O
the	O
context	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
.	O

In	O
addition	O
,	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinities	I-evidence
of	O
SRA	B-structure_element
or	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
did	O
not	O
obviously	O
vary	O
upon	O
the	O
change	O
of	O
DNA	O
lengths	O
but	O
did	O
decrease	O
with	O
the	O
increasing	O
salt	O
concentrations	O
(	O
Supplementary	O
Fig	O
.	O
6b	O
,	O
c	O
and	O
Supplementary	O
Table	O
5	O
).	O

We	O
next	O
mapped	O
the	O
minimal	O
region	O
of	O
the	O
Spacer	B-structure_element
for	O
the	O
enhancement	O
of	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
.	O

SRA	B-mutant
–	I-mutant
Spacer	I-mutant
-	I-mutant
661	I-mutant
(	O
residues	O
414	B-residue_range
–	I-residue_range
661	I-residue_range
)	O
still	O
maintained	O
strong	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
comparable	O
to	O
that	O
of	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
(	O
residues	O
414	B-residue_range
–	I-residue_range
674	I-residue_range
),	O
whereas	O
SRA	B-mutant
–	I-mutant
Spacer	I-mutant
-	I-mutant
652	I-mutant
and	O
SRA	B-mutant
–	I-mutant
Spacer	I-mutant
-	I-mutant
642	I-mutant
markedly	O
decreased	O
their	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinities	I-evidence
(	O
Fig	O
.	O
5c	O
),	O
indicating	O
that	O
residues	O
642	B-residue_range
–	I-residue_range
661	I-residue_range
are	O
important	O
for	O
enhancing	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
the	O
SRA	B-structure_element
.	O

This	O
minimal	O
region	O
largely	O
overlaps	O
with	O
the	O
Spacer	B-structure_element
region	O
(	O
643	B-residue_range
–	I-residue_range
655	I-residue_range
)	O
essential	O
for	O
TTD	B-structure_element
interaction	O
.	O

We	O
also	O
determined	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
bound	B-protein_state
to	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
at	O
3	O
.	O
15	O
Å	O
resolution	O
(	O
Supplementary	O
Table	O
6	O
and	O
Supplementary	O
Fig	O
.	O
7a	O
).	O

Intriguingly	O
,	O
no	O
electron	B-evidence
density	I-evidence
was	O
observed	O
for	O
the	O
Spacer	B-structure_element
.	O

A	O
possible	O
explanation	O
is	O
that	O
the	O
Spacer	B-structure_element
facilitates	O
SRA	B-complex_assembly
–	I-complex_assembly
hm	I-complex_assembly
-	I-complex_assembly
DNA	I-complex_assembly
interaction	O
through	O
nonspecific	O
salt	B-bond_interaction
bridge	I-bond_interaction
contacts	O
because	O
DNA	B-chemical
is	O
rich	O
in	O
acidic	O
groups	O
and	O
the	O
Spacer	B-structure_element
is	O
rich	O
in	O
basic	O
residues	O
(	O
Supplementary	O
Fig	O
.	O
7b	O
).	O

The	O
nonspecific	O
interaction	O
is	O
consistent	O
with	O
the	O
previous	O
observation	O
that	O
UHRF1	B-protein
has	O
no	O
DNA	B-chemical
sequence	O
selectivity	O
besides	O
hm	B-chemical
-	I-chemical
CpG	I-chemical
dinucleotide	I-chemical
.	O

The	O
spacer	B-structure_element
is	O
important	O
for	O
PCH	O
localization	O
of	O
UHRF1	B-protein

For	O
the	O
NIH3T3	O
cells	O
expressing	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
UHRF1	B-protein
,	O
most	O
cells	O
(∼	O
74	O
.	O
6	O
%)	O
showed	O
a	O
focal	O
pattern	O
of	O
protein	O
that	O
is	O
co	O
-	O
localized	O
with	O
4	B-chemical
,	I-chemical
6	I-chemical
-	I-chemical
diamidino	I-chemical
-	I-chemical
2	I-chemical
-	I-chemical
phenylindole	I-chemical
(	O
DAPI	B-chemical
)	O
foci	O
(	O
Fig	O
.	O
5d	O
),	O
whereas	O
the	O
rest	O
cells	O
showed	O
a	O
diffuse	O
nuclear	O
staining	O
pattern	O
.	O

The	O
result	O
is	O
consistent	O
with	O
the	O
previous	O
studies	O
that	O
UHRF1	B-protein
is	O
mainly	O
localized	O
to	O
highly	B-protein_state
methylated	I-protein_state
pericentromeric	O
heterochromatin	O
(	O
PCH	O
).	O

In	O
contrast	O
,	O
for	O
the	O
cells	O
expressing	O
UHRF1Δ627	B-mutant
–	I-mutant
674	I-mutant
,	O
a	O
spacer	B-protein_state
deletion	I-protein_state
mutant	I-protein_state
with	O
decreased	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
(	O
Fig	O
.	O
5b	O
),	O
only	O
∼	O
22	O
.	O
1	O
%	O
cells	O
showed	O
co	O
-	O
localization	O
with	O
DAPI	B-chemical
.	O

Previous	O
reports	O
have	O
shown	O
that	O
the	O
H3K9me3	B-protein_type
recognition	O
of	O
UHRF1	B-protein
also	O
plays	O
an	O
important	O
role	O
in	O
its	O
heterochromatin	O
localization	O
.	O

Because	O
manipulation	O
of	O
endogenous	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
in	O
cells	O
is	O
technically	O
challenging	O
,	O
we	O
used	O
UHRF1ΔSRA	B-mutant
(	O
lacks	B-protein_state
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
but	O
maintains	O
closed	B-protein_state
conformation	O
,	O
Figs	O
3h	O
and	O
5b	O
)	O
to	O
test	O
whether	O
closed	B-protein_state
conformation	O
of	O
UHRF1	B-protein
exists	O
in	O
vivo	O
.	O

In	O
NIH3T3	O
cells	O
,	O
UHRF1ΔSRA	B-mutant
largely	O
decreased	O
chromatin	O
association	O
(	O
Fig	O
.	O
5d	O
).	O

Only	O
∼	O
4	O
.	O
8	O
%	O
cells	O
expressing	O
UHRF1ΔSRA	B-mutant
showed	O
an	O
intermediate	O
enrichment	O
,	O
but	O
not	O
characteristic	O
focal	O
pattern	O
,	O
at	O
DAPI	B-chemical
foci	O
,	O
whereas	O
the	O
majority	O
of	O
the	O
cells	O
showed	O
a	O
diffuse	O
nuclear	O
staining	O
pattern	O
.	O

The	O
spacer	B-structure_element
facilitates	O
UHRF1	B-complex_assembly
–	I-complex_assembly
DNMT1	I-complex_assembly
interaction	O

Previous	O
studies	O
show	O
that	O
UHRF1	B-protein
recruits	O
DNMT1	B-protein
to	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
for	O
maintenance	O
DNA	B-chemical
methylation	B-ptm
through	O
the	O
interaction	O
between	O
the	O
SRA	B-structure_element
and	O
RFTSDNMT1	B-protein
(	O
refs	O
).	O

We	O
confirmed	O
the	O
direct	O
interaction	O
between	O
RFTSDNMT1	B-protein
and	O
the	O
SRA	B-structure_element
in	O
a	O
solution	O
with	O
low	O
salt	O
concentration	O
(	O
50	O
mM	O
NaCl	B-chemical
),	O
but	O
observed	O
weak	O
or	O
undetectable	O
interaction	O
in	O
a	O
solution	O
with	O
higher	O
salt	O
concentrations	O
(	O
100	O
or	O
150	O
mM	O
NaCl	B-chemical
)	O
(	O
Supplementary	O
Fig	O
.	O
8a	O
).	O

Compared	O
with	O
the	O
SRA	B-structure_element
,	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
exhibited	O
stronger	O
interaction	O
with	O
RFTSDNMT1	B-protein
.	O

In	O
addition	O
,	O
RFTSDNMT1	B-protein
bound	B-protein_state
to	I-protein_state
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
with	O
a	O
binding	B-evidence
affinity	I-evidence
of	O
7	O
.	O
09	O
μM	O
,	O
but	O
showed	O
no	O
detectable	O
interaction	O
with	O
the	O
SRA	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
8b	O
).	O

Interestingly	O
,	O
the	O
addition	O
of	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
abolished	O
the	O
interaction	O
between	O
RFTSDNMT1	B-protein
and	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
,	O
suggesting	O
that	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
also	O
regulates	O
UHRF1	B-complex_assembly
–	I-complex_assembly
DNMT1	I-complex_assembly
interaction	O
(	O
Supplementary	O
Fig	O
.	O
8c	O
).	O

These	O
results	O
indicate	O
that	O
the	O
Spacer	B-structure_element
facilitates	O
the	O
interaction	O
between	O
RFTSDNMT1	B-protein
and	O
the	O
SRA	B-structure_element
,	O
and	O
the	O
interaction	O
is	O
impaired	O
by	O
the	O
presence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
.	O

We	O
next	O
tested	O
whether	O
the	O
UHRF1	B-complex_assembly
–	I-complex_assembly
DNMT1	I-complex_assembly
interaction	O
is	O
regulated	O
by	O
the	O
conformational	O
change	O
of	O
UHRF1	B-protein
.	O

Because	O
the	O
addition	O
of	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
disrupts	O
the	O
interaction	O
between	O
the	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
and	O
RFTSDNMT1	B-protein
,	O
we	O
used	O
various	O
truncations	B-experimental_method
to	O
mimic	O
open	B-protein_state
and	O
closed	B-protein_state
forms	O
of	O
UHRF1	B-protein
.	O

As	O
the	O
deletion	B-experimental_method
of	I-experimental_method
the	O
TTD	B-structure_element
allows	O
UHRF1	B-protein
to	O
adopt	O
an	O
open	B-protein_state
conformation	O
,	O
the	O
results	O
suggest	O
that	O
RFTSDNMT1	B-protein
binds	B-protein_state
to	I-protein_state
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
when	O
UHRF1	B-protein
adopts	O
an	O
open	B-protein_state
conformation	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
.	O

In	O
support	O
of	O
above	O
observations	O
,	O
the	O
addition	B-experimental_method
of	O
large	O
amount	O
of	O
RFTSDNMT1	B-protein
impaired	O
the	O
interaction	O
between	O
UHRF1	B-protein
and	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
Supplementary	O
Fig	O
.	O
8d	O
),	O
suggesting	O
an	O
existence	O
of	O
dynamic	O
equilibrium	O
between	O
UHRF1	B-complex_assembly
–	I-complex_assembly
hm	I-complex_assembly
-	I-complex_assembly
DNA	I-complex_assembly
and	O
UHRF1	B-complex_assembly
–	I-complex_assembly
DNMT1	I-complex_assembly
complexes	O
.	O

According	O
to	O
the	O
above	O
results	O
,	O
we	O
here	O
proposed	O
a	O
working	O
model	O
for	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
-	O
mediated	O
regulation	O
of	O
UHRF1	B-protein
conformation	O
(	O
Fig	O
.	O
5f	O
).	O

In	O
the	O
absence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
A	O
),	O
UHRF1	B-protein
prefers	O
a	O
closed	B-protein_state
conformation	O
,	O
in	O
which	O
the	O
Spacer	B-structure_element
binds	B-protein_state
to	I-protein_state
the	O
TTD	B-structure_element
by	O
competing	O
with	O
the	O
Linker	B-structure_element
and	O
the	O
SRA	B-structure_element
binds	B-protein_state
to	I-protein_state
the	O
PHD	B-structure_element
.	O

As	O
a	O
result	O
,	O
the	O
recognition	O
of	O
histone	B-protein_type
H3K9me3	B-protein_type
by	O
the	O
TTD	B-structure_element
is	O
blocked	O
by	O
the	O
Spacer	B-structure_element
,	O
and	O
recognition	O
of	O
unmodified	B-protein_state
histone	B-protein_type
H3	B-protein_type
(	O
H3R2	B-site
)	O
by	O
the	O
PHD	B-structure_element
is	O
inhibited	O
by	O
the	O
SRA	B-structure_element
.	O

The	O
interaction	O
between	O
UHRF1	B-protein
and	O
DNMT1	B-protein
is	O
also	O
weak	O
because	O
the	O
Spacer	B-structure_element
is	O
unable	O
to	O
facilitate	O
the	O
intermolecular	O
interaction	O
.	O

In	O
the	O
presence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
B	O
),	O
UHRF1	B-protein
prefers	O
an	O
open	B-protein_state
conformation	O
,	O
in	O
which	O
the	O
SRA	B-structure_element
binds	B-protein_state
to	I-protein_state
the	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
;	O
the	O
Spacer	B-structure_element
dissociates	O
from	O
the	O
TTD	B-structure_element
and	O
facilitates	O
the	O
interaction	O
between	O
the	O
SRA	B-structure_element
and	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
;	O
the	O
Linker	B-structure_element
binds	B-protein_state
to	I-protein_state
the	O
TTD	B-structure_element
and	O
allows	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
to	O
recognize	O
histone	B-protein_type
H3K9me3	B-protein_type
.	O

When	O
UHRF1	B-protein
adopts	O
an	O
open	B-protein_state
conformation	O
and	O
has	O
already	O
bound	B-protein_state
to	I-protein_state
H3K9me3	B-protein_type
(	O
B	O
),	O
the	O
interaction	O
between	O
H3K9me3	B-protein_type
and	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
further	O
prevents	O
the	O
Spacer	B-structure_element
from	O
folding	O
back	O
to	O
interact	O
with	O
the	O
TTD	B-structure_element
,	O
and	O
therefore	O
locks	O
UHRF1	B-protein
in	O
an	O
open	B-protein_state
conformation	O
.	O

The	O
association	O
of	O
UHRF1	B-protein
to	O
the	O
histone	B-protein_type
may	O
facilitate	O
the	O
ubiquitination	B-ptm
of	O
histone	B-protein_type
tail	O
(	O
mediated	O
by	O
RING	B-structure_element
domain	O
)	O
for	O
DNMT1	B-protein
targeting	O
.	O

Moreover	O
,	O
through	O
a	O
mechanism	O
yet	O
to	O
be	O
fully	O
elucidated	O
,	O
DNMT1	B-protein
targets	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
for	O
maintenance	O
DNA	O
methylation	B-ptm
,	O
probably	O
through	O
interaction	O
with	O
the	O
histone	B-protein_type
ubiquitylation	B-ptm
and	O
/	O
or	O
SRA	B-structure_element
-	I-structure_element
Spacer	I-structure_element
.	O

The	O
P	B-evidence
(	I-evidence
r	I-evidence
)	I-evidence
function	I-evidence
obtained	O
from	O
small	B-experimental_method
-	I-experimental_method
angle	I-experimental_method
X	I-experimental_method
-	I-experimental_method
ray	I-experimental_method
scattering	I-experimental_method
(	O
SAXS	B-experimental_method
)	O
measurements	O
of	O
TTD	B-complex_assembly
–	I-complex_assembly
PHD	I-complex_assembly
–	I-complex_assembly
SRA	I-complex_assembly
–	I-complex_assembly
Spacer	I-complex_assembly
–	I-complex_assembly
hm	I-complex_assembly
-	I-complex_assembly
DNA	I-complex_assembly
complex	O
showed	O
a	O
broader	O
distribution	O
than	O
that	O
of	O
the	O
TTD	B-complex_assembly
–	I-complex_assembly
PHD	I-complex_assembly
–	I-complex_assembly
SRA	I-complex_assembly
–	I-complex_assembly
Spacer	I-complex_assembly
alone	O
,	O
supporting	O
the	O
proposed	O
model	O
that	O
UHRF1	B-protein
adopts	O
an	O
open	B-protein_state
conformation	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
Supplementary	O
Fig	O
.	O
8e	O
).	O

We	O
have	O
tried	O
crystallizing	B-experimental_method
more	O
than	O
three	O
sub	O
-	O
constructs	O
with	B-protein_state
and	O
without	B-protein_state
DNA	B-chemical
across	O
over	O
1	O
,	O
200	O
crystallization	O
conditions	O
but	O
failed	O
to	O
determine	O
the	O
structure	B-evidence
of	O
TTD	B-complex_assembly
–	I-complex_assembly
PHD	I-complex_assembly
–	I-complex_assembly
SRA	I-complex_assembly
–	I-complex_assembly
Spacer	I-complex_assembly
in	O
the	O
absence	B-protein_state
or	O
presence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
.	O

Our	O
previous	O
studies	O
show	O
that	O
phosphorylation	B-ptm
at	O
S639	B-residue_name_number
within	O
the	O
Spacer	B-structure_element
disrupts	O
interaction	O
between	O
UHRF1	B-protein
and	O
deubiquitylase	B-protein_type
USP7	B-protein
and	O
decreases	O
UHRF1	B-protein
stability	O
in	O
the	O
M	O
phase	O
of	O
the	O
cell	O
cycle	O
.	O

The	O
Spacer	B-structure_element
was	O
predicted	O
to	O
contain	O
two	O
nuclear	B-structure_element
localization	I-structure_element
signals	I-structure_element
,	O
residues	O
581	B-residue_range
–	I-residue_range
600	I-residue_range
and	O
648	B-residue_range
-	I-residue_range
670	I-residue_range
(	O
ref	O
.).	O

In	O
this	O
report	O
,	O
we	O
found	O
that	O
the	O
Spacer	B-structure_element
(	O
i	O
)	O
binds	B-protein_state
to	I-protein_state
the	O
TTD	B-structure_element
in	O
the	O
closed	B-protein_state
form	O
of	O
UHRF1	B-protein
and	O
inhibits	O
its	O
interaction	O
with	O
H3K9me3	B-protein_type
;	O
(	O
ii	O
)	O
facilitates	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
recognition	O
by	O
the	O
SRA	B-structure_element
and	O
(	O
iii	O
)	O
facilitates	O
the	O
interaction	O
between	O
the	O
SRA	B-structure_element
and	O
RFTSDNMT1	B-protein
.	O

The	O
result	O
suggests	O
that	O
PI5P	B-chemical
may	O
facilitate	O
the	O
conformational	O
change	O
of	O
UHRF1	B-protein
induced	O
by	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
when	O
UHRF1	B-protein
is	O
recruited	O
to	O
chromatin	O
.	O

In	O
addition	O
,	O
mass	B-experimental_method
-	I-experimental_method
spectrometry	I-experimental_method
analyses	O
have	O
identified	O
several	O
phosphorylation	B-site
sites	I-site
(	O
S639	B-residue_name_number
,	O
S651	B-residue_name_number
,	O
S661	B-residue_name_number
)	O
within	O
the	O
Spacer	B-structure_element
,	O
suggesting	O
that	O
post	O
-	O
translational	O
modification	O
may	O
add	O
another	O
layer	O
of	O
regulation	O
of	O
UHRF1	B-protein
(	O
refs	O
).	O

It	O
has	O
been	O
well	O
characterized	O
that	O
the	O
SRA	B-structure_element
of	O
UHRF1	B-protein
preferentially	O
recognizes	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
through	O
a	O
base	O
-	O
flipping	O
mechanism	O
.	O

Our	O
study	O
demonstrates	O
that	O
the	O
Spacer	B-structure_element
markedly	O
enhances	O
the	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
the	O
SRA	B-structure_element
and	O
the	O
deletion	B-experimental_method
of	I-experimental_method
the	O
Spacer	B-structure_element
impairs	O
heterochromatin	O
localization	O
of	O
UHRF1	B-protein
,	O
indicating	O
that	O
the	O
Spacer	B-structure_element
is	O
essential	O
for	O
recognition	O
of	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
in	O
the	O
context	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
.	O

Thus	O
,	O
although	O
UHRF2	B-protein
exhibits	O
the	O
histone	O
-	O
and	O
hm	O
-	O
DNA	O
-	O
binding	O
activities	O
,	O
the	O
difference	O
in	O
the	O
Spacer	B-structure_element
region	O
may	O
contribute	O
to	O
the	O
functional	O
differences	O
between	O
UHRF1	B-protein
and	O
UHRF2	B-protein
.	O

This	O
is	O
also	O
consistent	O
with	O
previous	O
finding	O
that	O
UHRF2	B-protein
is	O
unable	O
to	O
replace	O
UHRF1	B-protein
to	O
maintain	O
the	O
DNA	B-chemical
methylation	B-ptm
.	O

However	O
,	O
little	O
is	O
known	O
about	O
the	O
crosstalk	O
between	O
these	O
two	O
epigenetic	O
marks	O
within	O
UHRF1	B-protein
.	O

We	O
have	O
shown	O
that	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
and	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
strongly	O
bind	O
to	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
0	O
.	O
35	O
and	O
0	O
.	O
49	O
μM	O
,	O
respectively	O
)	O
and	O
the	O
Spacer	B-structure_element
plays	O
an	O
important	O
role	O
in	O
PCH	O
localization	O
(	O
Fig	O
.	O
5d	O
).	O

As	O
a	O
result	O
,	O
when	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
dissociates	O
from	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
and	O
binds	B-protein_state
to	I-protein_state
DNMT1	B-protein
with	O
a	O
currently	O
unknown	O
mechanism	O
,	O
UHRF1	B-protein
may	O
keep	O
the	O
complex	O
associated	O
with	O
chromatin	O
through	O
the	O
interaction	O
between	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
and	O
H3K9me3	B-protein_type
(	O
or	O
PHD	B-structure_element
-	O
H3	B-protein_type
),	O
and	O
make	O
it	O
possible	O
for	O
DNMT1	B-protein
to	O
target	O
proper	O
DNA	B-chemical
substrate	O
for	O
methylation	B-ptm
.	O

This	O
explanation	O
agrees	O
nicely	O
with	O
previous	O
observations	O
and	O
clarifies	O
the	O
importance	O
of	O
coordinate	O
recognition	O
of	O
H3K9me3	B-protein_type
and	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
by	O
UHRF1	B-protein
for	O
maintenance	O
DNA	O
methylation	B-ptm
.	O

UHRF1	B-protein
is	O
essential	O
for	O
maintenance	O
DNA	B-chemical
methylation	B-ptm
through	O
recruiting	O
DNMT1	B-protein
to	O
DNA	B-chemical
replication	O
forks	O
during	O
S	O
phase	O
.	O

DNMT1	B-protein
binds	B-protein_state
to	I-protein_state
ubiquitylated	B-protein_state
histone	B-protein_type
H3	B-protein_type
and	O
ubiquitylation	B-ptm
is	O
required	O
for	O
maintenance	O
of	O
DNA	B-chemical
methylation	B-ptm
in	O
vivo	O
.	O

In	O
this	O
study	O
,	O
we	O
found	O
that	O
both	O
TTD	B-structure_element
and	O
PHD	B-structure_element
are	O
regulated	O
by	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
to	O
recognize	O
histone	B-protein_type
tail	O
.	O

Thus	O
,	O
the	O
closed	B-protein_state
form	O
UHRF1	B-protein
may	O
prevent	O
miss	O
localization	O
of	O
URHF1	B-protein
,	O
whereas	O
only	O
the	O
UHRF1	B-protein
in	O
open	B-protein_state
conformation	O
(	O
induced	O
by	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
)	O
could	O
properly	O
binds	B-protein_state
to	I-protein_state
histone	B-protein_type
tail	O
for	O
ubiquitylation	B-ptm
and	O
subsequent	O
DNA	B-chemical
methylation	B-ptm
.	O

Binding	O
of	O
UHRF1	B-protein
to	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
may	O
serve	O
as	O
a	O
switch	O
for	O
its	O
recruitment	O
of	O
DNMT1	B-protein
.	O

Hm	B-chemical
-	I-chemical
DNA	I-chemical
facilities	O
histione	B-protein_type
tails	O
recognition	O
by	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
.	O

(	O
a	O
)	O
Colour	O
-	O
coded	O
domain	O
structure	O
of	O
human	B-species
UHRF1	B-protein
.	O

Note	O
that	O
the	O
conserved	B-protein_state
motif	O
(	O
green	O
background	O
)	O
of	O
the	O
Linker	B-structure_element
(	O
residues	O
286	B-residue_range
–	I-residue_range
306	I-residue_range
)	O
and	O
the	O
Spacer	B-structure_element
(	O
residues	O
587	B-residue_range
–	I-residue_range
674	I-residue_range
)	O
bind	O
to	O
the	O
TTD	B-structure_element
in	O
a	O
similar	O
manner	O
(	O
Fig	O
.	O
3b	O
).	O
(	O
b	O
)	O
Hm	B-chemical
-	I-chemical
DNA	I-chemical
facilities	O
histone	B-protein_type
H3	B-protein_type
and	O
H3K9me3	B-protein_type
recognition	O
by	O
UHRF1	B-protein
.	O

Purified	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
was	O
incubated	O
with	O
biotinylated	B-protein_state
H3	B-protein_type
(	O
1	B-residue_range
–	I-residue_range
21	I-residue_range
)	O
or	O
H3K9me3	B-protein_type
(	O
1	B-residue_range
–	I-residue_range
21	I-residue_range
)	O
peptides	O
in	O
the	O
presence	O
or	O
absence	B-protein_state
of	I-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
(	O
molar	O
ratio	O
UHRF1	B-protein
/	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
=	O
1	O
:	O
2	O
).	O

Full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
was	O
incubated	B-experimental_method
with	I-experimental_method
biotinylated	B-protein_state
hm	B-chemical
-	I-chemical
DNA	I-chemical
in	O
the	O
presence	O
or	O
absence	B-protein_state
of	I-protein_state
H3	B-protein_type
(	O
1	B-residue_range
–	I-residue_range
17	I-residue_range
)	O
or	O
H3K9me3	B-protein_type
(	O
1	B-residue_range
–	I-residue_range
17	I-residue_range
)	O
peptides	O
and	O
analysed	O
as	O
in	O
b	O
.	O
(	O
d	O
,	O
e	O
)	O
Superimposed	O
ITC	B-experimental_method
enthalpy	B-evidence
plots	I-evidence
for	O
binding	O
of	O
H3K9me3	B-protein_type
peptide	O
(	O
1	B-residue_range
–	I-residue_range
17	I-residue_range
)	O
to	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
and	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
(	O
d	O
),	O
and	O
H3	B-protein_type
peptide	O
(	O
1	B-residue_range
–	I-residue_range
17	I-residue_range
)	O
to	O
the	O
PHD	B-structure_element
and	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
(	O
e	O
).	O

The	O
estimated	O
binding	B-evidence
affinities	I-evidence
(	O
KD	B-evidence
)	O
are	O
listed	O
.	O

(	O
a	O
)	O
GST	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assays	I-experimental_method
for	O
the	O
intramolecular	O
interactions	O
.	O

The	O
isolated	O
domains	O
of	O
UHRF1	B-protein
were	O
incubated	B-experimental_method
with	O
GST	B-protein_state
-	I-protein_state
tagged	I-protein_state
TTD	B-structure_element
or	O
PHD	B-structure_element
immobilized	O
on	O
glutathione	O
resin	O
.	O

The	O
bound	O
proteins	O
were	O
analysed	O
by	O
SDS	B-experimental_method
–	I-experimental_method
PAGE	I-experimental_method
and	O
Coomassie	O
blue	O
staining	O
.	O

(	O
b	O
,	O
d	O
)	O
Superimposed	O
ITC	B-experimental_method
enthalpy	B-evidence
plots	I-evidence
for	O
the	O
intramolecular	O
interactions	O
of	O
isolated	O
UHRF1	B-protein
domains	O
.	O

ND	O
,	O
not	O
detectable	O
.	O
(	O
c	O
)	O
Superimposed	O
ITC	B-experimental_method
enthalpy	B-evidence
plots	I-evidence
for	O
the	O
binding	O
of	O
H3K9me3	B-protein_type
to	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
in	O
the	O
absence	B-protein_state
or	O
presence	B-protein_state
of	I-protein_state
the	O
Spacer	B-structure_element
(	O
molar	O
ratio	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
/	O
Spacer	B-structure_element
=	O
1	O
:	O
2	O
).	O
(	O
e	O
)	O
Superimposed	O
ITC	B-experimental_method
enthalpy	B-evidence
plots	I-evidence
for	O
the	O
binding	O
of	O
H3	B-protein_type
to	O
PHD	B-structure_element
–	I-structure_element
SRA	I-structure_element
or	O
PHD	B-structure_element
in	O
the	O
absence	B-protein_state
or	O
presence	B-protein_state
of	I-protein_state
the	O
SRA	B-structure_element
(	O
molar	O
ratio	O
PHD	B-structure_element
/	O
SRA	B-structure_element
=	O
1	O
:	O
1	O
or	O
1	O
:	O
2	O
).	O

NMR	B-experimental_method
structure	B-evidence
of	O
the	O
TTD	B-structure_element
bound	B-protein_state
to	I-protein_state
the	O
Spacer	B-structure_element
.	O

(	O
a	O
)	O
Ribbon	O
representation	O
of	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
structure	B-evidence
.	O

N	O
-	O
and	O
C	O
-	O
termini	O
of	O
the	O
Spacer	B-structure_element
are	O
indicated	O
.	O

The	O
TTD	B-structure_element
is	O
coloured	O
in	O
green	O
,	O
and	O
the	O
Spacer	B-structure_element
is	O
coloured	O
in	O
yellow	O
.	O

(	O
b	O
)	O
Superimposition	B-experimental_method
of	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
and	O
TTD	B-complex_assembly
–	I-complex_assembly
PHD	I-complex_assembly
–	I-complex_assembly
H3K9me3	I-complex_assembly
(	O
4GY5	O
.	O
PDB	O
)	O
structures	B-evidence
shown	O
in	O
ribbon	O
representations	O
.	O

The	O
TTD	B-structure_element
is	O
coloured	O
in	O
green	O
and	O
the	O
Spacer	B-structure_element
in	O
yellow	O
in	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
structure	B-evidence
.	O

(	O
c	O
)	O
Electrostatic	O
potential	O
surface	O
representation	O
of	O
the	O
TTD	B-structure_element
with	O
the	O
Spacer	B-structure_element
shown	O
in	O
ribbon	O
representation	O
.	O

The	O
critical	O
residues	O
on	O
the	O
Spacer	B-structure_element
for	O
the	O
interaction	O
are	O
shown	O
in	O
stick	O
representation	O
.	O

(	O
d	O
)	O
Close	O
-	O
up	O
view	O
of	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
interaction	O
.	O

Hydrogen	B-bond_interaction
bonds	I-bond_interaction
are	O
indicated	O
as	O
dashed	O
lines	O
.	O

(	O
e	O
–	O
g	O
)	O
Superimposed	O
ITC	B-experimental_method
enthalpy	B-evidence
plots	I-evidence
for	O
the	O
interaction	O
between	O
the	O
Spacer	B-structure_element
and	O
the	O
TTD	B-structure_element
(	O
or	O
TTD	B-structure_element
–	I-structure_element
PHD	I-structure_element
)	O
with	O
the	O
estimated	O
binding	B-evidence
affinity	I-evidence
(	O
KD	B-evidence
)	O
indicated	O
.	O

Wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
mutant	B-protein_state
proteins	O
for	O
the	O
measurements	O
are	O
indicated	O
.	O

(	O
h	O
)	O
GST	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assays	I-experimental_method
for	O
the	O
intramolecular	O
interactions	O
.	O

The	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
or	O
indicated	O
truncations	O
of	O
UHRF1	B-protein
were	O
incubated	O
with	O
GST	B-protein_state
-	I-protein_state
tagged	I-protein_state
TTD	B-structure_element
,	O
Linker	B-structure_element
or	O
Spacer	B-structure_element
.	O

Hm	B-chemical
-	I-chemical
DNA	I-chemical
impairs	O
the	O
intramolecular	O
interaction	O
of	O
UHRF1	B-protein
and	O
facilitates	O
its	O
histone	B-protein_type
recognition	O
.	O

(	O
a	O
)	O
Hm	B-chemical
-	I-chemical
DNA	I-chemical
impairs	O
the	O
intramolecular	O
interaction	O
of	O
PHD	B-structure_element
–	I-structure_element
SRA	I-structure_element
.	O

The	O
SRA	B-structure_element
was	O
incubated	B-experimental_method
with	O
GST	B-protein_state
-	I-protein_state
tagged	I-protein_state
PHD	B-structure_element
in	O
the	O
presence	B-protein_state
of	I-protein_state
increasing	O
concentrations	O
of	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
and	O
immobilized	O
on	O
glutathione	O
resin	O
.	O

(	O
b	O
)	O
Purified	O
fragments	O
of	O
UHRF1	B-protein
were	O
analysed	O
by	O
histone	B-experimental_method
peptide	I-experimental_method
(	O
H3K9me0	B-protein_type
)	O
pull	B-experimental_method
-	I-experimental_method
down	I-experimental_method
assay	I-experimental_method
as	O
described	O
in	O
Fig	O
.	O
1b	O
.	O
(	O
c	O
)	O
Hm	B-chemical
-	I-chemical
DNA	I-chemical
impairs	O
the	O
intramolecular	O
interaction	O
of	O
TTD	B-structure_element
–	I-structure_element
Spacer	I-structure_element
.	O

The	O
assays	O
were	O
performed	O
in	O
the	O
presence	O
(+	O
DTT	B-chemical
)	O
or	O
absence	O
(−	O
DTT	B-chemical
)	O
of	O
15	O
mM	O
DTT	B-chemical
.	O

The	O
Spacer	B-structure_element
facilitates	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
–	O
SRA	B-structure_element
interaction	O
and	O
DNMT1	B-complex_assembly
–	I-complex_assembly
UHRF1	I-complex_assembly
interaction	O
.	O

(	O
a	O
)	O
Superimposed	O
ITC	B-experimental_method
enthalpy	B-evidence
plots	I-evidence
for	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinities	I-evidence
of	O
the	O
SRA	B-structure_element
,	O
the	O
Spacer	B-structure_element
and	O
SRA	B-structure_element
–	I-structure_element
Spacer	I-structure_element
.	O
(	O
b	O
,	O
c	O
)	O
Superimposed	O
fluorescence	B-evidence
polarization	I-evidence
(	I-evidence
FP	I-evidence
)	I-evidence
plots	I-evidence
for	O
hm	B-evidence
-	I-evidence
DNA	I-evidence
-	I-evidence
binding	I-evidence
affinities	I-evidence
of	O
truncations	O
or	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
UHRF1	B-protein
.	O

Scale	O
bar	O
,	O
5	O
μm	O
.	O
(	O
e	O
)	O
GST	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
experiment	I-experimental_method
for	O
the	O
interactions	O
between	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
or	O
truncations	B-experimental_method
of	O
UHRF1	B-protein
and	O
RFTSDNMT1	B-protein
as	O
described	O
in	O
Fig	O
.	O
2a	O
.	O
(	O
f	O
)	O
Working	O
model	O
for	O
hm	B-chemical
-	I-chemical
DNA	I-chemical
-	O
mediated	O
conformational	O
changes	O
of	O
UHRF1	B-protein
,	O
as	O
described	O
in	O
the	O
Discussion	O
.	O

X	B-evidence
-	I-evidence
ray	I-evidence
Crystallographic	I-evidence
Structures	I-evidence
of	O
a	O
Trimer	B-oligomeric_state
,	O
Dodecamer	B-oligomeric_state
,	O
and	O
Annular	B-site
Pore	I-site
Formed	O
by	O
an	O
Aβ17	B-protein
–	B-residue_range
36	I-residue_range
β	B-structure_element
-	I-structure_element
Hairpin	I-structure_element

High	O
-	O
resolution	O
structures	B-evidence
of	O
oligomers	B-oligomeric_state
formed	O
by	O
the	O
β	B-protein
-	I-protein
amyloid	I-protein
peptide	I-protein
Aβ	B-protein
are	O
needed	O
to	O
understand	O
the	O
molecular	O
basis	O
of	O
Alzheimer	O
’	O
s	O
disease	O
and	O
develop	O
therapies	O
.	O

Two	O
covalent	O
constraints	O
act	O
in	O
tandem	O
to	O
stabilize	O
the	O
Aβ17	B-protein
–	B-residue_range
36	I-residue_range
peptide	O
in	O
a	O
hairpin	B-structure_element
conformation	O
:	O
a	O
δ	B-protein_state
-	I-protein_state
linked	I-protein_state
ornithine	B-residue_name
turn	B-structure_element
connecting	O
positions	O
17	B-residue_number
and	O
36	B-residue_number
to	O
create	O
a	O
macrocycle	O
and	O
an	O
intramolecular	O
disulfide	B-ptm
linkage	I-ptm
between	O
positions	O
24	B-residue_number
and	O
29	B-residue_number
.	O

Three	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
monomers	B-oligomeric_state
assemble	O
to	O
form	O
a	O
triangular	B-protein_state
trimer	B-oligomeric_state
,	O
four	O
trimers	B-oligomeric_state
assemble	O
in	O
a	O
tetrahedral	O
arrangement	O
to	O
form	O
a	O
dodecamer	B-oligomeric_state
,	O
and	O
five	O
dodecamers	B-oligomeric_state
pack	O
together	O
to	O
form	O
an	O
annular	B-site
pore	I-site
.	O

High	O
-	O
resolution	O
structures	B-evidence
of	O
oligomers	B-oligomeric_state
formed	O
by	O
the	O
β	B-protein
-	I-protein
amyloid	I-protein
peptide	I-protein
Aβ	B-protein
are	O
desperately	O
needed	O
to	O
understand	O
the	O
molecular	O
basis	O
of	O
Alzheimer	O
’	O
s	O
disease	O
and	O
ultimately	O
develop	O
preventions	O
or	O
treatments	O
.	O

Over	O
the	O
last	O
two	O
decades	O
the	O
role	O
of	O
Aβ	B-protein
oligomers	B-oligomeric_state
in	O
the	O
pathophysiology	O
of	O
Alzheimer	O
’	O
s	O
disease	O
has	O
begun	O
to	O
unfold	O
.	O

Aβ	B-protein
isolated	O
from	O
the	O
brains	O
of	O
young	O
plaque	O
-	O
free	O
Tg2576	O
mice	B-taxonomy_domain
forms	O
a	O
mixture	O
of	O
low	O
molecular	O
weight	O
oligomers	B-oligomeric_state
.	O

A	O
56	O
kDa	O
soluble	O
oligomer	B-oligomeric_state
identified	O
by	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
was	O
found	O
to	O
be	O
especially	O
important	O
within	O
this	O
mixture	O
.	O

This	O
oligomer	B-oligomeric_state
was	O
termed	O
Aβ	B-complex_assembly
*	I-complex_assembly
56	I-complex_assembly
and	O
appears	O
to	O
be	O
a	O
dodecamer	B-oligomeric_state
of	O
Aβ	B-protein
.	O

Smaller	O
oligomers	B-oligomeric_state
with	O
molecular	O
weights	O
consistent	O
with	O
trimers	B-oligomeric_state
,	O
hexamers	B-oligomeric_state
,	O
and	O
nonamers	B-oligomeric_state
were	O
also	O
identified	O
within	O
the	O
mixture	O
of	O
low	O
molecular	O
weight	O
oligomers	B-oligomeric_state
.	O

Recently	O
,	O
Aβ	B-protein
trimers	B-oligomeric_state
and	O
Aβ	B-complex_assembly
*	I-complex_assembly
56	I-complex_assembly
were	O
identified	O
in	O
the	O
brains	O
of	O
cognitively	O
normal	O
humans	B-species
and	O
were	O
found	O
to	O
increase	O
with	O
age	O
.	O

A	O
type	O
of	O
large	O
oligomers	B-oligomeric_state
called	O
annular	B-complex_assembly
protofibrils	I-complex_assembly
(	O
APFs	B-complex_assembly
)	O
have	O
also	O
been	O
observed	O
in	O
the	O
brains	O
of	O
transgenic	O
mice	B-taxonomy_domain
and	O
isolated	O
from	O
the	O
brains	O
of	O
Alzheimer	O
’	O
s	O
patients	O
.	O

APFs	B-complex_assembly
were	O
first	O
discovered	O
in	O
vitro	O
using	O
chemically	B-protein_state
synthesized	I-protein_state
Aβ	B-protein
that	O
aggregated	O
into	O
porelike	B-structure_element
structures	I-structure_element
that	O
could	O
be	O
observed	O
by	O
atomic	B-experimental_method
force	I-experimental_method
microscopy	I-experimental_method
(	O
AFM	B-experimental_method
)	O
and	O
transmission	B-experimental_method
electron	I-experimental_method
microscopy	I-experimental_method
(	O
TEM	B-experimental_method
).	O

Lashuel	O
et	O
al	O
.	O
observed	O
APFs	B-complex_assembly
with	O
an	O
outer	O
diameter	O
that	O
ranged	O
from	O
7	O
–	O
10	O
nm	O
and	O
an	O
inner	O
diameter	O
that	O
ranged	O
from	O
1	O
.	O
5	O
–	O
2	O
nm	O
,	O
consistent	O
with	O
molecular	O
weights	O
of	O
150	O
–	O
250	O
kDa	O
.	O

Kayed	O
et	O
al	O
.	O
observed	O
APFs	B-complex_assembly
with	O
an	O
outer	O
diameter	O
that	O
ranged	O
from	O
8	O
–	O
25	O
nm	O
,	O
which	O
were	O
composed	O
of	O
small	B-protein_state
spherical	I-protein_state
Aβ	B-protein
oligomers	B-oligomeric_state
,	O
3	O
–	O
5	O
nm	O
in	O
diameter	O
.	O

Although	O
the	O
APFs	B-complex_assembly
in	O
these	O
studies	O
differ	O
in	O
size	O
,	O
they	O
share	O
a	O
similar	O
annular	O
morphology	O
and	O
appear	O
to	O
be	O
composed	O
of	O
smaller	O
oligomers	B-oligomeric_state
.	O

APFs	B-complex_assembly
have	O
also	O
been	O
observed	O
in	O
the	O
brains	O
of	O
APP23	O
transgenic	O
mice	B-taxonomy_domain
by	O
immunofluorescence	B-experimental_method
with	O
an	O
anti	O
-	O
APF	B-complex_assembly
antibody	O
and	O
were	O
found	O
to	O
accumulate	O
in	O
neuronal	O
processes	O
and	O
synapses	O
.	O

In	O
a	O
subsequent	O
study	O
,	O
APFs	B-complex_assembly
were	O
isolated	O
from	O
the	O
brains	O
of	O
Alzheimer	O
’	O
s	O
patients	O
by	O
immunoprecipitation	B-experimental_method
with	O
an	O
anti	O
-	O
APF	B-complex_assembly
antibody	O
.	O

These	O
APFs	B-complex_assembly
had	O
an	O
outer	O
diameter	O
that	O
ranged	O
from	O
11	O
–	O
14	O
nm	O
and	O
an	O
inner	O
diameter	O
that	O
ranged	O
from	O
2	O
.	O
5	O
–	O
4	O
nm	O
.	O

Dimers	B-oligomeric_state
of	O
Aβ	B-protein
have	O
also	O
been	O
isolated	O
from	O
the	O
brains	O
of	O
Alzheimer	O
’	O
s	O
patients	O
.−	O
Aβ	B-protein
dimers	B-oligomeric_state
inhibit	O
long	O
-	O
term	O
potentiation	O
in	O
mice	B-taxonomy_domain
and	O
promote	O
hyperphosphorylation	B-ptm
of	O
the	O
microtubule	B-protein
-	I-protein
associated	I-protein
protein	I-protein
tau	I-protein
,	O
leading	O
to	O
neuritic	O
damage	O
.	O

Aβ	B-protein
dimers	B-oligomeric_state
have	O
only	O
been	O
isolated	O
from	O
human	B-species
or	O
transgenic	O
mouse	B-taxonomy_domain
brains	O
that	O
contain	O
the	O
pathognomonic	O
fibrillar	B-protein_state
Aβ	B-protein
plaques	O
associated	O
with	O
Alzheimer	O
’	O
s	O
disease	O
.	O

Furthermore	O
,	O
the	O
endogenous	O
rise	O
of	O
Aβ	B-protein
dimers	B-oligomeric_state
in	O
the	O
brains	O
of	O
Tg2576	O
and	O
J20	O
transgenic	O
mice	B-taxonomy_domain
coincides	O
with	O
the	O
deposition	O
of	O
Aβ	B-protein
plaques	O
.	O

These	O
observations	O
suggest	O
that	O
the	O
Aβ	B-protein
trimers	B-oligomeric_state
,	O
hexamers	B-oligomeric_state
,	O
dodecamers	B-oligomeric_state
,	O
and	O
related	O
assemblies	O
may	O
be	O
associated	O
with	O
presymptomatic	O
neurodegeneration	O
,	O
while	O
Aβ	B-protein
dimers	B-oligomeric_state
are	O
more	O
closely	O
associated	O
with	O
fibril	O
formation	O
and	O
plaque	O
deposition	O
during	O
symptomatic	O
Alzheimer	O
’	O
s	O
disease	O
.−	O

Techniques	O
such	O
as	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
,	O
TEM	B-experimental_method
,	O
and	O
AFM	B-experimental_method
have	O
only	O
provided	O
information	O
about	O
the	O
molecular	O
weights	O
,	O
sizes	O
,	O
morphologies	O
,	O
and	O
stoichiometry	O
of	O
Aβ	B-protein
oligomers	B-oligomeric_state
.	O

High	O
-	O
resolution	O
structural	B-experimental_method
studies	I-experimental_method
of	O
Aβ	B-protein
have	O
primarily	O
focused	O
on	O
Aβ	B-protein
fibrils	B-oligomeric_state
and	O
Aβ	B-protein
monomers	B-oligomeric_state
.	O

Solid	B-experimental_method
-	I-experimental_method
state	I-experimental_method
NMR	I-experimental_method
spectroscopy	I-experimental_method
studies	O
of	O
Aβ	B-protein
fibrils	B-oligomeric_state
revealed	O
that	O
Aβ	B-protein
fibrils	B-oligomeric_state
are	O
generally	O
composed	O
of	O
extended	O
networks	O
of	O
in	B-structure_element
-	I-structure_element
register	I-structure_element
parallel	I-structure_element
β	I-structure_element
-	I-structure_element
sheets	I-structure_element
.−	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallographic	I-experimental_method
studies	I-experimental_method
using	O
fragments	O
of	O
Aβ	B-protein
have	O
provided	O
additional	O
information	O
about	O
how	O
Aβ	B-protein
fibrils	B-oligomeric_state
pack	O
.	O

Solution	B-experimental_method
-	I-experimental_method
phase	I-experimental_method
NMR	I-experimental_method
and	O
solid	B-experimental_method
-	I-experimental_method
state	I-experimental_method
NMR	I-experimental_method
have	O
been	O
used	O
to	O
study	O
the	O
structures	B-evidence
of	O
the	O
Aβ	B-protein
monomers	B-oligomeric_state
within	O
oligomeric	O
assemblies	O
.−	O
A	O
major	O
finding	O
from	O
these	O
studies	O
is	O
that	O
oligomeric	O
assemblies	O
of	O
Aβ	B-protein
are	O
primarily	O
composed	O
of	O
antiparallel	B-structure_element
β	I-structure_element
-	I-structure_element
sheets	I-structure_element
.	O

Many	O
of	O
these	O
studies	O
have	O
reported	O
the	O
monomer	B-oligomeric_state
subunit	B-structure_element
as	O
adopting	O
a	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
conformation	O
,	O
in	O
which	O
the	O
hydrophobic	O
central	B-structure_element
and	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
regions	I-structure_element
form	O
an	O
antiparallel	B-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
.	O

In	O
2008	O
,	O
Hoyer	O
et	O
al	O
.	O
reported	O
the	O
NMR	B-experimental_method
structure	B-evidence
of	O
an	O
Aβ	B-protein
monomer	B-oligomeric_state
bound	B-protein_state
to	I-protein_state
an	O
artificial	B-chemical
binding	I-chemical
protein	I-chemical
called	O
an	O
affibody	B-chemical
(	O
PDB	O
2OTK	O
).	O

Sequestering	O
Aβ	B-protein
within	O
the	O
affibody	B-chemical
prevents	O
its	O
fibrilization	O
and	O
reduces	O
its	O
neurotoxicity	O
,	O
providing	O
evidence	O
that	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
structure	O
may	O
contribute	O
to	O
the	O
ability	O
of	O
Aβ	B-protein
to	O
form	O
neurotoxic	O
oligomers	B-oligomeric_state
.	O

In	O
a	O
related	O
study	O
,	O
Sandberg	O
et	O
al	O
.	O
constrained	O
Aβ	B-protein
in	O
a	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
conformation	O
by	O
mutating	B-experimental_method
residues	O
A21	B-residue_name_number
and	O
A30	B-residue_name_number
to	O
cysteine	B-residue_name
and	O
forming	O
an	O
intramolecular	O
disulfide	B-ptm
bond	I-ptm
.	O

The	O
oligomers	B-oligomeric_state
with	O
a	O
molecular	O
weight	O
of	O
∼	O
100	O
kDa	O
that	O
were	O
isolated	O
by	O
SEC	B-experimental_method
were	O
toxic	O
toward	O
neuronally	O
derived	O
SH	O
-	O
SY5Y	O
cells	O
.	O

This	O
study	O
provides	O
evidence	O
for	O
the	O
role	O
of	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
structure	O
in	O
Aβ	B-protein
oligomerization	O
and	O
neurotoxicity	O
.	O

Inspired	O
by	O
these	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
structures	B-evidence
,	O
our	O
laboratory	O
developed	O
a	O
macrocyclic	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
peptide	O
derived	O
from	O
Aβ17	B-protein
–	B-residue_range
36	I-residue_range
designed	O
to	O
mimic	O
an	O
Aβ	B-protein
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
and	O
reported	O
its	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structure	I-evidence
.	O

This	O
peptide	O
(	O
peptide	B-mutant
1	I-mutant
)	O
consists	O
of	O
two	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
comprising	O
Aβ17	B-protein
–	B-residue_range
23	I-residue_range
and	O
Aβ30	B-protein
–	B-residue_range
36	I-residue_range
covalently	O
linked	O
by	O
two	O
δ	B-protein_state
-	I-protein_state
linked	I-protein_state
ornithine	B-residue_name
(	O
δOrn	B-structure_element
)	O
β	B-structure_element
-	I-structure_element
turn	I-structure_element
mimics	O
.	O

The	O
δOrn	B-structure_element
that	O
connects	O
residues	O
D23	B-residue_name_number
and	O
A30	B-residue_name_number
replaces	O
the	O
Aβ24	B-protein
–	B-residue_range
29	I-residue_range
loop	B-structure_element
.	O

The	O
δOrn	B-structure_element
that	O
connects	O
residues	O
L17	B-residue_name_number
and	O
V36	B-residue_name_number
enforces	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
structure	O
.	O

We	O
incorporated	O
an	O
N	O
-	O
methyl	O
group	O
at	O
position	O
G33	B-residue_name_number
to	O
prevent	O
uncontrolled	O
aggregation	O
and	O
precipitation	O
of	O
the	O
peptide	O
.	O

To	O
improve	O
the	O
solubility	O
of	O
the	O
peptide	O
we	O
replaced	B-experimental_method
M35	B-residue_name_number
with	O
the	O
hydrophilic	O
isostere	O
of	O
methionine	B-residue_name
,	O
ornithine	B-residue_name
(	O
α	B-protein_state
-	I-protein_state
linked	I-protein_state
)	O
(	O
Figure	O
1B	O
).	O

The	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structure	I-evidence
of	O
peptide	B-mutant
1	I-mutant
reveals	O
that	O
it	O
folds	O
to	O
form	O
a	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
that	O
assembles	O
to	O
form	O
trimers	B-oligomeric_state
and	O
that	O
the	O
trimers	B-oligomeric_state
further	O
assemble	O
to	O
form	O
hexamers	B-oligomeric_state
and	O
dodecamers	B-oligomeric_state
.	O

(	O
A	O
)	O
Cartoon	O
illustrating	O
the	O
design	O
of	O
peptides	B-chemical
1	I-chemical
and	I-chemical
2	I-chemical
and	O
their	O
relationship	O
to	O
an	O
Aβ17	B-protein
–	B-residue_range
36	I-residue_range
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
.	O

(	O
B	O
)	O
Chemical	O
structure	O
of	O
peptide	B-mutant
1	I-mutant
illustrating	O
Aβ17	B-protein
–	O
23	O
and	O
Aβ30	B-protein
–	O
36	O
,	O
M35Orn	O
,	O
the	O
N	O
-	O
methyl	O
group	O
,	O
and	O
the	O
δ	B-protein_state
-	I-protein_state
linked	I-protein_state
ornithine	B-residue_name
turns	B-structure_element
.	O
(	O
C	O
)	O
Chemical	O
structure	O
of	O
peptide	B-mutant
2	I-mutant
illustrating	O
Aβ17	B-protein
–	O
36	O
,	O
the	O
N	O
-	O
methyl	O
group	O
,	O
the	O
disulfide	B-ptm
bond	I-ptm
across	O
positions	O
24	B-residue_number
and	O
29	B-residue_number
,	O
and	O
the	O
δ	B-protein_state
-	I-protein_state
linked	I-protein_state
ornithine	B-residue_name
turn	B-structure_element
.	O

Our	O
design	O
of	O
peptide	B-mutant
1	I-mutant
omitted	O
the	O
Aβ24	B-protein
–	B-residue_range
29	I-residue_range
loop	B-structure_element
.	O

To	O
visualize	O
the	O
Aβ24	B-protein
–	B-residue_range
29	I-residue_range
loop	B-structure_element
,	O
we	O
performed	O
replica	B-experimental_method
-	I-experimental_method
exchange	I-experimental_method
molecular	I-experimental_method
dynamics	I-experimental_method
(	O
REMD	B-experimental_method
)	O
simulations	B-experimental_method
on	O
Aβ17	B-protein
–	B-residue_range
36	I-residue_range
using	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
coordinates	I-evidence
of	O
Aβ17	B-protein
–	B-residue_range
23	I-residue_range
and	O
Aβ30	B-protein
–	B-residue_range
36	I-residue_range
from	O
peptide	B-mutant
1	I-mutant
.	O

These	O
studies	O
provided	O
a	O
working	O
model	O
for	O
a	O
trimer	B-oligomeric_state
of	O
Aβ17	B-protein
–	B-residue_range
36	I-residue_range
β	B-structure_element
-	I-structure_element
hairpins	I-structure_element
and	O
demonstrated	O
that	O
the	O
trimer	B-oligomeric_state
should	O
be	O
capable	O
of	O
accommodating	O
the	O
Aβ24	B-protein
–	B-residue_range
29	I-residue_range
loop	B-structure_element
.	O

We	O
designed	O
peptide	B-mutant
2	I-mutant
as	O
a	O
homologue	O
of	O
peptide	B-mutant
1	I-mutant
that	O
embodies	O
these	O
ideas	O
.	O

Peptide	B-mutant
2	I-mutant
contains	O
a	O
methionine	B-residue_name
residue	O
at	O
position	O
35	B-residue_number
and	O
an	O
Aβ24	B-protein
–	B-residue_range
29	I-residue_range
loop	B-structure_element
with	O
residues	O
24	B-residue_number
and	O
29	B-residue_number
(	O
Val	B-residue_name
and	O
Gly	B-residue_name
)	O
mutated	B-experimental_method
to	O
cysteine	B-residue_name
and	O
linked	O
by	O
a	O
disulfide	B-ptm
bond	I-ptm
(	O
Figure	O
1C	O
).	O

Here	O
,	O
we	O
describe	O
the	O
development	O
of	O
peptide	B-mutant
2	I-mutant
and	O
report	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structures	I-evidence
of	O
the	O
trimer	B-oligomeric_state
,	O
dodecamer	B-oligomeric_state
,	O
and	O
annular	B-site
pore	I-site
observed	O
within	O
the	O
crystal	B-evidence
structure	I-evidence
.	O

Development	O
of	O
Peptide	B-mutant
2	I-mutant

We	O
developed	O
peptide	B-mutant
2	I-mutant
from	O
peptide	B-mutant
1	I-mutant
by	O
an	O
iterative	O
process	O
,	O
in	O
which	O
we	O
first	O
attempted	O
to	O
restore	O
the	O
Aβ24	B-protein
–	B-residue_range
29	I-residue_range
loop	B-structure_element
without	O
a	O
disulfide	B-ptm
linkage	I-ptm
.	O

We	O
envisioned	O
peptide	B-mutant
3	I-mutant
as	O
a	O
homologue	O
of	O
peptide	B-mutant
1	I-mutant
with	O
the	O
Aβ24	B-protein
–	B-residue_range
29	I-residue_range
loop	B-structure_element
in	O
place	O
of	O
the	O
δOrn	B-structure_element
that	O
connects	O
D23	B-residue_name_number
and	O
A30	B-residue_name_number
and	O
p	B-chemical
-	I-chemical
iodophenylalanine	I-chemical
(	O
FI	B-chemical
)	O
in	O
place	O
of	O
F19	B-residue_name_number
.	O

After	O
determining	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structure	I-evidence
of	O
the	O
p	B-chemical
-	I-chemical
iodophenylalanine	I-chemical
variant	O
we	O
attempt	O
to	O
determine	O
the	O
structure	B-evidence
of	O
the	O
native	O
phenylalanine	B-residue_name
compound	O
by	O
isomorphous	B-experimental_method
replacement	I-experimental_method
.	O

We	O
postulate	O
that	O
the	O
loss	O
of	O
the	O
δOrn	B-structure_element
constraint	O
leads	O
to	O
conformational	O
heterogeneity	O
that	O
prevents	O
peptide	B-mutant
3	I-mutant
from	O
crystallizing	O
.	O

We	O
designed	O
peptide	B-mutant
4	I-mutant
to	O
embody	O
this	O
idea	O
,	O
mutating	B-experimental_method
Val24	B-residue_name_number
and	O
Gly29	B-residue_name_number
to	O
cysteine	B-residue_name
and	O
forming	O
an	O
interstrand	O
disulfide	B-ptm
linkage	I-ptm
.	O

Residues	O
V24	B-residue_name_number
and	O
G29	B-residue_name_number
form	O
a	O
non	B-bond_interaction
-	I-bond_interaction
hydrogen	I-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
pair	I-bond_interaction
,	O
which	O
can	O
readily	O
accommodate	O
disulfide	B-ptm
linkages	I-ptm
in	O
antiparallel	B-structure_element
β	I-structure_element
-	I-structure_element
sheets	I-structure_element
.	O

Disulfide	B-ptm
bonds	I-ptm
across	O
non	B-bond_interaction
-	I-bond_interaction
hydrogen	I-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
pairs	I-bond_interaction
stabilize	O
β	B-structure_element
-	I-structure_element
hairpins	I-structure_element
,	O
while	O
disulfide	B-ptm
bonds	I-ptm
across	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
pairs	I-bond_interaction
do	O
not	O
.	O

Although	O
the	O
disulfide	B-ptm
bond	I-ptm
between	O
positions	O
24	B-residue_number
and	O
29	B-residue_number
helps	O
stabilize	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
,	O
it	O
does	O
not	O
alter	O
the	O
charge	O
or	O
substantially	O
change	O
the	O
hydrophobicity	O
of	O
the	O
Aβ17	B-protein
–	B-residue_range
36	I-residue_range
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
.	O

We	O
were	O
gratified	O
to	O
find	O
that	O
peptide	B-mutant
4	I-mutant
afforded	O
crystals	B-evidence
suitable	O
for	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
.	O

As	O
the	O
next	O
step	O
in	O
the	O
iterative	O
process	O
,	O
we	O
determined	B-experimental_method
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structure	I-evidence
of	O
this	O
peptide	O
(	O
PDB	O
5HOW	O
).	O

We	O
completed	O
the	O
iterative	O
process	O
—	O
from	O
1	O
to	O
3	O
to	O
4	O
to	O
2	O
—	O
by	O
successfully	O
determining	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structure	I-evidence
of	O
peptide	B-mutant
2	I-mutant
(	O
PDB	O
5HOX	O
and	O
5HOY	O
).	O

The	O
following	O
sections	O
describe	O
the	O
synthesis	O
of	O
peptides	B-mutant
2	I-mutant
–	I-mutant
4	I-mutant
and	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structure	I-evidence
of	O
peptide	B-mutant
2	I-mutant
.	O

Synthesis	O
of	O
Peptides	B-mutant
2	I-mutant
–	I-mutant
4	I-mutant

We	O
synthesized	O
peptides	B-mutant
2	I-mutant
–	I-mutant
4	I-mutant
by	O
similar	O
procedures	O
to	O
those	O
we	O
have	O
developed	O
for	O
other	O
macrocyclic	O
peptides	O
.	O

In	O
synthesizing	O
peptides	B-mutant
2	I-mutant
and	I-mutant
4	I-mutant
we	O
formed	O
the	O
disulfide	B-ptm
linkage	I-ptm
after	O
macrolactamization	O
and	O
deprotection	O
of	O
the	O
acid	O
-	O
labile	O
side	O
chain	O
protecting	O
groups	O
.	O

Crystallization	B-experimental_method
,	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
Crystallographic	I-experimental_method
Data	I-experimental_method
Collection	I-experimental_method
,	O
Data	O
Processing	O
,	O
and	O
Structure	B-experimental_method
Determination	I-experimental_method
of	O
Peptides	B-mutant
2	I-mutant
and	I-mutant
4	I-mutant

We	O
screened	B-experimental_method
crystallization	I-experimental_method
conditions	I-experimental_method
for	O
peptide	B-mutant
4	I-mutant
in	O
a	O
96	O
-	O
well	O
-	O
plate	O
format	O
using	O
three	O
different	O
Hampton	O
Research	O
crystallization	O
kits	O
(	O
Crystal	O
Screen	O
,	O
Index	O
,	O
and	O
PEG	O
/	O
Ion	O
)	O
with	O
three	O
ratios	O
of	O
peptide	O
and	O
mother	O
liquor	O
per	O
condition	O
(	O
864	O
experiments	O
).	O

Peptide	B-mutant
4	I-mutant
afforded	O
crystals	B-evidence
in	O
a	O
single	O
set	O
of	O
conditions	O
containing	O
HEPES	O
buffer	O
and	O
Jeffamine	B-chemical
M	I-chemical
-	I-chemical
600	I-chemical
—	O
the	O
same	O
crystallization	O
conditions	O
that	O
afforded	O
crystals	B-evidence
of	O
peptide	B-mutant
1	I-mutant
.	O

We	O
further	O
optimized	O
these	O
conditions	O
to	O
rapidly	O
(∼	O
72	O
h	O
)	O
yield	O
crystals	B-evidence
suitable	O
for	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
.	O

Crystal	B-evidence
diffraction	I-evidence
data	I-evidence
for	O
peptides	B-mutant
4	I-mutant
and	I-mutant
2	I-mutant
were	O
collected	O
in	O
-	O
house	O
with	O
a	O
Rigaku	O
MicroMax	O
007HF	O
X	O
-	O
ray	O
diffractometer	O
at	O
1	O
.	O
54	O
Å	O
wavelength	O
.	O

Data	O
from	O
peptides	B-mutant
4	I-mutant
and	I-mutant
2	I-mutant
suitable	O
for	O
refinement	O
at	O
2	O
.	O
30	O
Å	O
were	O
obtained	O
from	O
the	O
diffractometer	O
;	O
data	O
from	O
peptide	B-mutant
2	I-mutant
suitable	O
for	O
refinement	O
at	O
1	O
.	O
90	O
Å	O
were	O
obtained	O
from	O
the	O
synchrotron	O
.	O

Phases	B-evidence
for	O
peptide	B-mutant
4	I-mutant
were	O
determined	O
by	O
single	B-experimental_method
-	I-experimental_method
wavelength	I-experimental_method
anomalous	I-experimental_method
diffraction	I-experimental_method
(	O
SAD	B-experimental_method
)	O
phasing	B-experimental_method
by	O
using	O
the	O
coordinates	O
of	O
the	O
iodine	B-evidence
anomalous	I-evidence
signal	I-evidence
from	O
p	B-chemical
-	I-chemical
iodophenylalanine	I-chemical
.	O

Phases	B-evidence
for	O
peptide	B-mutant
2	I-mutant
were	O
determined	O
by	O
isomorphous	B-experimental_method
replacement	I-experimental_method
of	O
peptide	B-mutant
4	I-mutant
.	O

The	O
structures	B-evidence
of	O
peptides	B-mutant
2	I-mutant
and	I-mutant
4	I-mutant
were	O
solved	B-experimental_method
and	O
refined	O
in	O
the	O
P6122	O
space	O
group	O
.	O

The	O
asymmetric	O
unit	O
of	O
each	O
peptide	B-chemical
consists	O
of	O
six	O
monomers	B-oligomeric_state
,	O
arranged	O
as	O
two	O
trimers	B-oligomeric_state
.	O

Peptides	B-mutant
2	I-mutant
and	I-mutant
4	I-mutant
form	O
morphologically	O
identical	O
structures	O
and	O
assemblies	O
in	O
the	O
crystal	B-evidence
lattice	I-evidence
.	O

X	B-evidence
-	I-evidence
ray	I-evidence
Crystallographic	I-evidence
Structure	I-evidence
of	O
Peptide	B-mutant
2	I-mutant
and	O
the	O
Oligomers	B-oligomeric_state
It	O
Forms	O

Eight	O
residues	O
make	O
up	O
each	O
surface	O
of	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
:	O
L17	B-residue_name_number
,	O
F19	B-residue_name_number
,	O
A21	B-residue_name_number
,	O
D23	B-residue_name_number
,	O
A30	B-residue_name_number
,	O
I32	B-residue_name_number
,	O
L34	B-residue_name_number
,	O
and	O
V36	B-residue_name_number
make	O
up	O
one	O
surface	O
;	O
V18	B-residue_name_number
,	O
F20	B-residue_name_number
,	O
E22	B-residue_name_number
,	O
C24	B-residue_name_number
,	O
C29	B-residue_name_number
,	O
I31	B-residue_name_number
,	O
G33	B-residue_name_number
,	O
and	O
M35	B-residue_name_number
make	O
up	O
the	O
other	O
surface	O
.	O

The	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
of	O
the	O
monomers	B-oligomeric_state
in	O
the	O
asymmetric	O
unit	O
are	O
virtually	O
identical	O
,	O
differing	O
primarily	O
in	O
rotamers	O
of	O
F20	B-residue_name_number
,	O
E22	B-residue_name_number
,	O
C24	B-residue_name_number
,	O
C29	B-residue_name_number
,	O
I31	B-residue_name_number
,	O
and	O
M35	B-residue_name_number
(	O
Figure	O
S1	O
).	O

We	O
refined	B-experimental_method
three	O
of	O
the	O
β	B-structure_element
-	I-structure_element
hairpins	I-structure_element
with	O
intact	B-protein_state
disulfide	B-ptm
linkages	I-ptm
and	O
three	O
with	O
thiols	O
to	O
represent	O
cleaved	B-protein_state
disulfide	B-ptm
linkages	I-ptm
in	O
the	O
synchrotron	O
data	O
set	O
(	O
PDB	O
5HOX	O
).	O

X	O
-	O
ray	O
crystallographic	O
structure	O
of	O
peptide	B-mutant
2	I-mutant
(	O
PDB	O
5HOX	O
,	O
synchrotron	O
data	O
set	O
).	O
(	O
A	O
)	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structure	I-evidence
of	O
a	O
representative	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
monomer	B-oligomeric_state
formed	O
by	O
peptide	B-mutant
2	I-mutant
.	O
(	O
B	O
)	O
Overlay	B-experimental_method
of	O
the	O
six	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
monomers	B-oligomeric_state
in	O
the	O
asymmetric	O
unit	O
.	O

The	O
β	B-structure_element
-	I-structure_element
hairpins	I-structure_element
are	O
shown	O
as	O
cartoons	O
to	O
illustrate	O
the	O
differences	O
in	O
the	O
Aβ25	B-protein
–	B-residue_range
28	I-residue_range
loops	B-structure_element
.	O

The	O
Aβ25	B-protein
–	B-residue_range
28	I-residue_range
loops	B-structure_element
of	O
the	O
six	O
monomers	B-oligomeric_state
within	O
the	O
asymmetric	O
unit	O
vary	O
substantially	O
in	O
backbone	O
geometry	O
and	O
side	O
chain	O
rotamers	O
(	O
Figures	O
2B	O
and	O
S1	O
).	O

The	O
electron	B-evidence
density	I-evidence
for	O
the	O
loops	B-structure_element
is	O
weak	O
and	O
diffuse	O
compared	O
to	O
the	O
electron	B-evidence
density	I-evidence
for	O
the	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
.	O

The	O
B	B-evidence
values	I-evidence
for	O
the	O
loops	B-structure_element
are	O
large	O
,	O
indicating	O
that	O
the	O
loops	B-structure_element
are	O
dynamic	O
and	O
not	O
well	O
ordered	O
.	O

Thus	O
,	O
the	O
differences	O
in	O
backbone	O
geometry	O
and	O
side	O
chain	O
rotamers	O
among	O
the	O
loops	B-structure_element
are	O
likely	O
of	O
little	O
significance	O
and	O
should	O
be	O
interpreted	O
with	O
caution	O
.	O

Peptide	B-mutant
2	I-mutant
assembles	O
into	O
oligomers	B-oligomeric_state
similar	O
in	O
morphology	O
to	O
those	O
formed	O
by	O
peptide	B-mutant
1	I-mutant
.	O

Like	O
peptide	B-mutant
1	I-mutant
,	O
peptide	B-mutant
2	I-mutant
forms	O
a	O
triangular	B-protein_state
trimer	B-oligomeric_state
,	O
and	O
four	O
trimers	B-oligomeric_state
assemble	O
to	O
form	O
a	O
dodecamer	B-oligomeric_state
.	O

In	O
the	O
higher	O
-	O
order	O
assembly	O
of	O
the	O
dodecamers	B-oligomeric_state
formed	O
by	O
peptide	B-mutant
2	I-mutant
a	O
new	O
structure	B-evidence
emerges	O
,	O
not	O
seen	O
in	O
peptide	B-mutant
1	I-mutant
,	O
an	O
annular	B-site
pore	I-site
consisting	O
of	O
five	O
dodecamers	B-oligomeric_state
.	O

Trimer	B-oligomeric_state

Peptide	B-mutant
2	I-mutant
forms	O
a	O
trimer	B-oligomeric_state
,	O
much	O
like	O
that	O
which	O
we	O
observed	O
previously	O
for	O
peptide	B-mutant
1	I-mutant
,	O
in	O
which	O
three	O
β	B-structure_element
-	I-structure_element
hairpins	I-structure_element
assemble	O
to	O
form	O
an	O
equilateral	B-structure_element
triangle	I-structure_element
(	O
Figure	O
3A	O
).	O

The	O
two	O
trimers	B-oligomeric_state
are	O
almost	O
identical	O
in	O
structure	O
,	O
differing	O
slightly	O
among	O
side	O
chain	O
rotamers	O
and	O
loop	B-structure_element
conformations	O
.	O

X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structure	I-evidence
of	O
the	O
trimer	B-oligomeric_state
formed	O
by	O
peptide	B-mutant
2	I-mutant
.	O
(	O
A	O
)	O
Triangular	B-protein_state
trimer	B-oligomeric_state
.	O

The	O
three	O
water	B-chemical
molecules	O
in	O
the	O
center	O
hole	O
of	O
the	O
trimer	B-oligomeric_state
are	O
shown	O
as	O
spheres	O
.	O
(	O
B	O
)	O
Detailed	O
view	O
of	O
the	O
intermolecular	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
between	O
the	O
main	O
chains	O
of	O
V18	B-residue_name_number
and	O
E22	B-residue_name_number
and	O
δOrn	B-structure_element
and	O
C24	B-residue_name_number
,	O
at	O
the	O
three	O
corners	O
of	O
the	O
triangular	B-protein_state
trimer	B-oligomeric_state
.	O
(	O
C	O
)	O
The	O
F19	B-residue_name_number
face	O
of	O
the	O
trimer	B-oligomeric_state
,	O
with	O
key	O
side	O
chains	O
shown	O
as	O
spheres	O
.	O
(	O
D	O
)	O
The	O
F20	B-residue_name_number
face	O
of	O
the	O
trimer	B-oligomeric_state
,	O
with	O
key	O
side	O
chains	O
as	O
spheres	O
.	O

At	O
the	O
corners	O
of	O
the	O
trimer	B-oligomeric_state
,	O
the	O
pairs	O
of	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
monomers	B-oligomeric_state
form	O
four	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
:	O
two	O
between	O
the	O
main	O
chains	O
of	O
V18	B-residue_name_number
and	O
E22	B-residue_name_number
and	O
two	O
between	O
δOrn	B-structure_element
and	O
the	O
main	O
chain	O
of	O
C24	B-residue_name_number
(	O
Figure	O
3B	O
).	O

At	O
each	O
corner	O
,	O
the	O
side	O
chains	O
of	O
residues	O
L17	B-residue_name_number
,	O
F19	B-residue_name_number
,	O
and	O
V36	B-residue_name_number
of	O
one	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
pack	O
against	O
the	O
side	O
chains	O
of	O
residues	O
A21	B-residue_name_number
,	O
I32	B-residue_name_number
,	O
L34	B-residue_name_number
,	O
and	O
also	O
D23	B-residue_name_number
of	O
the	O
adjacent	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
to	O
create	O
a	O
hydrophobic	B-site
cluster	I-site
(	O
Figure	O
3C	O
).	O
The	O
three	O
hydrophobic	B-site
clusters	I-site
create	O
a	O
large	O
hydrophobic	B-site
surface	I-site
on	O
one	O
face	O
of	O
the	O
trimer	B-oligomeric_state
.	O

The	O
other	O
face	O
of	O
the	O
trimer	B-oligomeric_state
displays	O
a	O
smaller	O
hydrophobic	B-site
surface	I-site
,	O
which	O
includes	O
the	O
side	O
chains	O
of	O
residues	O
V18	B-residue_name_number
,	O
F20	B-residue_name_number
,	O
and	O
I31	B-residue_name_number
of	O
the	O
three	O
β	B-structure_element
-	I-structure_element
hairpins	I-structure_element
(	O
Figure	O
3D	O
).	O

In	O
subsequent	O
discussion	O
,	O
we	O
designate	O
the	O
former	O
surface	O
the	O
“	O
F19	B-residue_name_number
face	O
”	O
and	O
the	O
latter	O
surface	O
the	O
“	O
F20	B-residue_name_number
face	O
”.	O

Dodecamer	B-oligomeric_state

Four	O
trimers	B-oligomeric_state
assemble	O
to	O
form	O
a	O
dodecamer	B-oligomeric_state
.	O

The	O
four	O
trimers	B-oligomeric_state
arrange	O
in	O
a	O
tetrahedral	B-protein_state
fashion	O
,	O
creating	O
a	O
central	B-site
cavity	I-site
inside	O
the	O
dodecamer	B-oligomeric_state
.	O
Because	O
each	O
trimer	B-oligomeric_state
is	O
triangular	B-protein_state
,	O
the	O
resulting	O
arrangement	O
resembles	O
an	O
octahedron	B-protein_state
.	O

Figure	O
4B	O
illustrates	O
the	O
tetrahedral	B-protein_state
arrangement	O
of	O
the	O
four	O
trimers	B-oligomeric_state
.	O

X	O
-	O
ray	O
crystallographic	O
structure	O
of	O
the	O
dodecamer	B-oligomeric_state
formed	O
by	O
peptide	B-mutant
2	I-mutant
.	O
(	O
A	O
)	O
View	O
of	O
the	O
dodecamer	B-oligomeric_state
that	O
illustrates	O
the	O
octahedral	B-protein_state
shape	O
.	O
(	O
B	O
)	O
View	O
of	O
the	O
dodecamer	B-oligomeric_state
that	O
illustrates	O
the	O
tetrahedral	B-protein_state
arrangement	O
of	O
the	O
four	O
trimers	B-oligomeric_state
that	O
comprise	O
the	O
dodecamer	B-oligomeric_state
.	O
(	O
C	O
)	O
View	O
of	O
two	O
trimer	B-oligomeric_state
subunits	B-structure_element
from	O
inside	O
the	O
cavity	B-site
of	O
the	O
dodecamer	B-oligomeric_state
.	O

The	O
F19	B-residue_name_number
faces	O
of	O
the	O
trimers	B-oligomeric_state
line	O
the	O
interior	O
of	O
the	O
dodecamer	B-oligomeric_state
.	O

At	O
the	O
six	O
vertices	O
,	O
hydrophobic	B-bond_interaction
packing	I-bond_interaction
between	O
the	O
side	O
chains	O
of	O
L17	B-residue_name_number
,	O
L34	B-residue_name_number
,	O
and	O
V36	B-residue_name_number
helps	O
stabilize	O
the	O
dodecamer	B-oligomeric_state
(	O
Figures	O
4C	O
and	O
D	O
).	O

Salt	O
bridges	O
between	O
the	O
side	O
chains	O
of	O
D23	B-residue_name_number
and	O
δOrn	B-structure_element
at	O
the	O
vertices	O
further	O
stabilize	O
the	O
dodecamer	B-oligomeric_state
.	O

Each	O
of	O
the	O
six	O
vertices	O
includes	O
two	O
Aβ25	B-protein
–	B-residue_range
28	I-residue_range
loops	B-structure_element
that	O
extend	O
past	O
the	O
core	B-structure_element
of	O
the	O
dodecamer	B-oligomeric_state
without	O
making	O
any	O
substantial	O
intermolecular	O
contacts	O
.	O

In	O
the	O
crystal	B-evidence
lattice	I-evidence
,	O
each	O
F20	B-residue_name_number
face	O
of	O
one	O
dodecamer	B-oligomeric_state
packs	O
against	O
an	O
F20	B-residue_name_number
face	O
of	O
another	O
dodecamer	B-oligomeric_state
.	O

Although	O
the	O
asymmetric	O
unit	O
comprises	O
half	O
a	O
dodecamer	B-oligomeric_state
,	O
the	O
crystal	B-evidence
lattice	I-evidence
may	O
be	O
thought	O
of	O
as	O
being	O
built	O
of	O
dodecamers	B-oligomeric_state
.	O

The	O
shape	O
and	O
length	O
of	O
the	O
electron	B-evidence
density	I-evidence
is	O
consistent	O
with	O
the	O
structure	B-evidence
of	O
Jeffamine	B-chemical
M	I-chemical
-	I-chemical
600	I-chemical
,	O
which	O
is	O
an	O
essential	O
component	O
of	O
the	O
crystallization	O
conditions	O
.	O

Although	O
Jeffamine	B-chemical
M	I-chemical
-	I-chemical
600	I-chemical
is	O
a	O
heterogeneous	O
mixture	O
with	O
varying	O
chain	O
lengths	O
and	O
stereochemistry	O
,	O
we	O
modeled	O
a	O
single	O
stereoisomer	O
with	O
nine	O
propylene	O
glycol	O
units	O
(	O
n	O
=	O
9	O
)	O
to	O
fit	O
the	O
electron	B-evidence
density	I-evidence
.	O

The	O
Jeffamine	B-chemical
M	I-chemical
-	I-chemical
600	I-chemical
appears	O
to	O
stabilize	O
the	O
dodecamer	B-oligomeric_state
by	O
occupying	O
the	O
central	B-site
cavity	I-site
and	O
making	O
hydrophobic	B-bond_interaction
contacts	I-bond_interaction
with	O
residues	O
lining	O
the	O
cavity	B-site
(	O
Figure	O
S3	O
).	O

In	O
a	O
dodecamer	B-oligomeric_state
formed	O
by	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
Aβ	B-protein
,	O
the	O
hydrophobic	O
C	O
-	O
terminal	O
residues	O
(	O
Aβ37	B-protein
–	B-residue_range
40	I-residue_range
or	O
Aβ37	B-protein
–	B-residue_range
42	I-residue_range
)	O
might	O
play	O
a	O
similar	O
role	O
in	O
filling	O
the	O
dodecamer	B-oligomeric_state
and	O
thus	O
create	O
a	O
packed	O
hydrophobic	B-site
core	I-site
within	O
the	O
central	B-site
cavity	I-site
of	O
the	O
dodecamer	B-oligomeric_state
.	O

Five	O
dodecamers	B-oligomeric_state
assemble	O
to	O
form	O
an	O
annular	O
porelike	B-structure_element
structure	O
(	O
Figure	O
5A	O
).	O

Hydrophobic	B-bond_interaction
packing	I-bond_interaction
between	O
the	O
F20	B-residue_name_number
faces	O
of	O
trimers	B-oligomeric_state
displayed	O
on	O
the	O
outer	O
surface	O
of	O
each	O
dodecamer	B-oligomeric_state
stabilizes	O
the	O
porelike	O
assembly	O
.	O

Hydrophobic	B-bond_interaction
packing	I-bond_interaction
between	O
the	O
side	O
chains	O
of	O
F20	B-residue_name_number
,	O
I31	B-residue_name_number
,	O
and	O
E22	B-residue_name_number
stabilizes	O
these	O
interfaces	B-site
(	O
Figure	O
5D	O
and	O
E	O
).	O

The	O
eclipsed	B-protein_state
interfaces	B-site
occur	O
between	O
dodecamers	B-structure_element
1	I-structure_element
and	I-structure_element
2	I-structure_element
,	O
1	B-structure_element
and	I-structure_element
5	I-structure_element
,	O
and	O
3	B-structure_element
and	I-structure_element
4	I-structure_element
,	O
as	O
shown	O
in	O
Figure	O
5A	O
.	O

The	O
annular	B-site
pore	I-site
is	O
not	O
completely	O
flat	O
,	O
instead	O
,	O
adopting	O
a	O
slightly	O
puckered	O
shape	O
,	O
which	O
accommodates	O
the	O
eclipsed	B-protein_state
and	O
staggered	B-protein_state
interfaces	B-site
.	O

The	O
hydrophilic	O
side	O
chains	O
of	O
S26	B-residue_name_number
,	O
N27	B-residue_name_number
,	O
and	O
K28	B-residue_name_number
decorate	O
the	O
hole	O
.	O

(	O
B	O
)	O
Eclipsed	B-site
interface	I-site
between	O
dodecamers	B-structure_element
1	I-structure_element
and	I-structure_element
2	I-structure_element
(	O
side	O
view	O
).	O

The	O
diameter	O
of	O
the	O
hole	O
in	O
the	O
center	O
of	O
the	O
pore	B-site
is	O
∼	O
2	O
nm	O
.	O

The	O
thickness	O
of	O
the	O
pore	B-site
is	O
∼	O
5	O
nm	O
,	O
which	O
is	O
comparable	O
to	O
that	O
of	O
a	O
lipid	O
bilayer	O
membrane	O
.	O

It	O
is	O
important	O
to	O
note	O
that	O
the	O
annular	B-site
pore	I-site
formed	O
by	O
peptide	B-mutant
2	I-mutant
is	O
not	O
a	O
discrete	O
unit	O
in	O
the	O
crystal	B-evidence
lattice	I-evidence
.	O

The	O
crystal	B-evidence
lattice	I-evidence
shows	O
how	O
the	O
dodecamers	B-oligomeric_state
can	O
further	O
assemble	O
to	O
form	O
larger	O
structures	O
.	O

Each	O
dodecamer	B-oligomeric_state
may	O
be	O
thought	O
of	O
as	O
a	O
tetravalent	O
building	O
block	O
with	O
the	O
potential	O
to	O
assemble	O
on	O
all	O
four	O
faces	O
to	O
form	O
higher	O
-	O
order	O
supramolecular	O
assemblies	O
.	O

The	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallographic	I-experimental_method
study	I-experimental_method
of	O
peptide	B-mutant
2	I-mutant
described	O
here	O
provides	O
high	O
-	O
resolution	O
structures	B-evidence
of	O
oligomers	B-oligomeric_state
formed	O
by	O
an	O
Aβ17	B-protein
–	B-residue_range
36	I-residue_range
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
.	O

The	O
crystallographic	B-evidence
assembly	I-evidence
of	O
peptide	B-mutant
2	I-mutant
into	O
a	O
trimer	B-oligomeric_state
,	O
dodecamer	B-oligomeric_state
,	O
and	O
annular	B-site
pore	I-site
provides	O
a	O
model	O
for	O
the	O
assembly	O
of	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
Aβ	B-protein
peptide	O
to	O
form	O
oligomers	B-oligomeric_state
.	O

The	O
dodecamers	B-oligomeric_state
further	O
assemble	O
to	O
form	O
an	O
annular	B-site
pore	I-site
(	O
Figure	O
6	O
).	O

Model	O
for	O
the	O
hierarchical	O
assembly	O
of	O
an	O
Aβ	B-protein
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
into	O
a	O
trimer	B-oligomeric_state
,	O
dodecamer	B-oligomeric_state
,	O
and	O
annular	B-site
pore	I-site
based	O
on	O
the	O
crystallographic	O
assembly	O
of	O
peptide	B-mutant
2	I-mutant
.	O

Monomeric	B-oligomeric_state
Aβ	B-protein
folds	O
to	O
form	O
a	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
in	O
which	O
the	O
hydrophobic	O
central	B-structure_element
and	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
regions	I-structure_element
form	O
an	O
antiparallel	B-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
.	O

Five	O
dodecamers	B-oligomeric_state
assemble	O
to	O
form	O
an	O
annular	B-site
pore	I-site
.	O

The	O
model	O
put	O
forth	O
in	O
Figure	O
6	O
is	O
consistent	O
with	O
the	O
current	O
understanding	O
of	O
endogenous	O
Aβ	B-protein
oligomerization	O
and	O
explains	O
at	O
atomic	O
resolution	O
many	O
key	O
observations	O
about	O
Aβ	B-protein
oligomers	B-oligomeric_state
.	O

Two	O
general	O
types	O
of	O
endogenous	O
Aβ	B-protein
oligomers	B-oligomeric_state
have	O
been	O
observed	O
:	O
Aβ	B-protein
oligomers	B-oligomeric_state
that	O
occur	O
on	O
a	O
pathway	O
to	O
fibrils	B-oligomeric_state
,	O
or	O
“	O
fibrillar	B-protein_state
oligomers	B-oligomeric_state
”,	O
and	O
Aβ	B-protein
oligomers	B-oligomeric_state
that	O
evade	O
a	O
fibrillar	B-protein_state
fate	O
,	O
or	O
“	O
nonfibrillar	B-protein_state
oligomers	B-oligomeric_state
”.−	O
Fibrillar	B-protein_state
oligomers	B-oligomeric_state
accumulate	O
in	O
Alzheimer	O
’	O
s	O
disease	O
later	O
than	O
nonfibrillar	B-protein_state
oligomers	B-oligomeric_state
and	O
coincide	O
with	O
the	O
deposition	O
of	O
plaques	O
.	O

Nonfibrillar	B-protein_state
oligomers	B-oligomeric_state
accumulate	O
early	O
in	O
Alzheimer	O
’	O
s	O
disease	O
before	O
plaque	O
deposition	O
.	O

Fibrillar	B-protein_state
oligomers	B-oligomeric_state
are	O
recognized	O
by	O
the	O
OC	O
antibody	O
but	O
not	O
the	O
A11	O
antibody	O
,	O
whereas	O
nonfibrillar	B-protein_state
oligomers	B-oligomeric_state
are	O
recognized	O
by	O
the	O
A11	O
antibody	O
but	O
not	O
the	O
OC	O
antibody	O
.	O

These	O
criteria	O
have	O
been	O
used	O
to	O
classify	O
the	O
Aβ	B-protein
oligomers	B-oligomeric_state
that	O
accumulate	O
in	O
vivo	O
.	O

Aβ	B-protein
dimers	B-oligomeric_state
have	O
been	O
classified	O
as	O
fibrillar	B-protein_state
oligomers	B-oligomeric_state
,	O
whereas	O
Aβ	B-protein
trimers	B-oligomeric_state
,	O
Aβ	B-complex_assembly
*	I-complex_assembly
56	I-complex_assembly
,	O
and	O
APFs	B-complex_assembly
have	O
been	O
classified	O
as	O
nonfibrillar	B-protein_state
oligomers	B-oligomeric_state
.	O

Larson	O
and	O
Lesné	O
proposed	O
a	O
model	O
for	O
the	O
endogenous	O
production	O
of	O
nonfibrillar	B-protein_state
oligomers	B-oligomeric_state
that	O
explains	O
these	O
observations	O
.	O

In	O
this	O
model	O
,	O
folded	B-protein_state
Aβ	B-protein
monomer	B-oligomeric_state
assembles	O
into	O
a	O
trimer	B-oligomeric_state
,	O
the	O
trimer	B-oligomeric_state
further	O
assembles	O
into	O
hexamers	B-oligomeric_state
and	O
dodecamers	B-oligomeric_state
,	O
and	O
the	O
dodecamers	B-oligomeric_state
further	O
assemble	O
to	O
form	O
annular	B-complex_assembly
protofibrils	I-complex_assembly
.	O

The	O
hierarchical	O
assembly	O
of	O
peptide	B-mutant
2	I-mutant
is	O
consistent	O
with	O
this	O
model	O
;	O
and	O
the	O
trimer	B-oligomeric_state
,	O
dodecamer	B-oligomeric_state
,	O
and	O
annular	B-site
pore	I-site
formed	O
by	O
peptide	B-mutant
2	I-mutant
may	O
share	O
similarities	O
to	O
the	O
trimers	B-oligomeric_state
,	O
Aβ	B-complex_assembly
*	I-complex_assembly
56	I-complex_assembly
,	O
and	O
APFs	B-complex_assembly
observed	O
in	O
vivo	O
.	O

The	O
crystallographically	B-evidence
observed	I-evidence
annular	B-site
pore	I-site
formed	O
by	O
peptide	B-mutant
2	I-mutant
is	O
morphologically	O
similar	O
to	O
the	O
APFs	B-complex_assembly
formed	O
by	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
Aβ	B-protein
.	O

The	O
annular	B-site
pore	I-site
formed	O
by	O
peptide	B-mutant
2	I-mutant
is	O
comparable	O
in	O
size	O
to	O
the	O
APFs	B-complex_assembly
prepared	O
in	O
vitro	O
or	O
isolated	O
from	O
Alzheimer	O
’	O
s	O
brains	O
(	O
Figure	O
7	O
and	O
Table	O
1	O
).	O

The	O
dodecamers	B-oligomeric_state
that	O
comprise	O
the	O
annular	B-site
pore	I-site
exhibit	O
two	O
modes	O
of	O
assembly	O
—	O
eclipsed	B-protein_state
interactions	O
and	O
staggered	B-protein_state
interactions	O
between	O
the	O
F20	B-residue_name_number
faces	O
of	O
trimers	B-oligomeric_state
within	O
dodecamers	B-oligomeric_state
.	O

These	O
two	O
modes	O
of	O
assembly	O
might	O
reflect	O
a	O
dynamic	O
interaction	O
between	O
dodecamers	B-oligomeric_state
,	O
which	O
could	O
permit	O
assemblies	O
of	O
more	O
dodecamers	B-oligomeric_state
into	O
larger	O
annular	B-site
pores	I-site
.	O

Annular	B-site
Pores	I-site
Formed	O
by	O
Aβ	B-protein
and	O
Peptide	B-mutant
2	I-mutant

annular	B-site
pore	I-site
source	O
outer	O
diameter	O
inner	O
diameter	O
observation	O
method	O
peptide	B-chemical
2	O
∼	O
11	O
–	O
12	O
nm	O
∼	O
2	O
nm	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
synthetic	B-protein_state
Aβ	B-protein
7	O
–	O
10	O
nm	O
1	O
.	O
5	O
–	O
2	O
nm	O
TEM	B-experimental_method
synthetic	O
Aβ	B-protein
16	O
nm	O
not	O
reported	O
AFM	B-experimental_method
synthetic	O
Aβ	B-protein
8	O
–	O
25	O
nm	O
not	O
reported	O
TEM	B-experimental_method
Alzheimer	O
’	O
s	O
brain	O
11	O
–	O
14	O
nm	O
2	O
.	O
5	O
–	O
4	O
nm	O
TEM	B-experimental_method

Dot	B-experimental_method
blot	I-experimental_method
analysis	O
shows	O
that	O
peptide	B-mutant
2	I-mutant
is	O
reactive	O
toward	O
the	O
A11	O
antibody	O
(	O
Figure	O
S5	O
).	O

Further	O
studies	O
are	O
needed	O
to	O
elucidate	O
the	O
species	O
that	O
peptide	B-mutant
2	I-mutant
forms	O
in	O
solution	O
and	O
to	O
study	O
their	O
biological	O
properties	O
.	O

Preliminary	O
attempts	O
to	O
study	O
these	O
species	O
by	O
SEC	B-experimental_method
and	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
have	O
not	O
provided	O
a	O
clear	O
measure	O
of	O
the	O
structures	B-evidence
formed	O
in	O
solution	O
.	O

The	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structure	I-evidence
and	O
A11	O
reactivity	O
of	O
peptide	B-mutant
2	I-mutant
support	O
the	O
model	O
proposed	O
by	O
Larsen	O
and	O
Lesné	O
and	O
suggest	O
that	O
β	B-structure_element
-	I-structure_element
hairpins	I-structure_element
constitute	O
a	O
fundamental	O
building	O
block	O
for	O
nonfibrillar	B-protein_state
oligomers	B-oligomeric_state
.	O

What	O
makes	O
β	B-structure_element
-	I-structure_element
hairpins	I-structure_element
special	O
is	O
that	O
three	O
β	B-structure_element
-	I-structure_element
hairpins	I-structure_element
can	O
nestle	O
together	O
to	O
form	O
trimers	B-oligomeric_state
,	O
stabilized	O
by	O
a	O
network	O
of	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
and	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
.	O

This	O
mode	O
of	O
assembly	O
is	O
not	O
unique	O
to	O
Aβ	B-protein
.	O

The	O
foldon	B-structure_element
domain	I-structure_element
of	O
bacteriophage	B-species
T4	I-species
fibritin	B-protein
is	O
composed	O
of	O
three	O
β	B-structure_element
-	I-structure_element
hairpins	I-structure_element
that	O
assemble	O
into	O
a	O
triangular	B-protein_state
trimer	B-oligomeric_state
similar	O
to	O
the	O
triangular	B-protein_state
trimer	B-oligomeric_state
formed	O
by	O
peptide	B-mutant
2	I-mutant
.	O

Additionally	O
,	O
our	O
research	O
group	O
has	O
observed	O
a	O
similar	O
assembly	O
of	O
a	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
peptide	O
derived	O
from	O
β2	B-protein
-	I-protein
microglobulin	I-protein
.	O

Although	O
we	O
began	O
these	O
studies	O
with	O
a	O
relatively	O
simple	O
hypothesis	O
—	O
that	O
the	O
trimers	B-oligomeric_state
and	O
dodecamers	B-oligomeric_state
formed	O
by	O
peptide	B-mutant
1	I-mutant
could	O
accommodate	O
the	O
Aβ24	B-protein
–	B-residue_range
29	I-residue_range
loop	B-structure_element
—	O
an	O
even	O
more	O
exciting	O
finding	O
has	O
emerged	O
—	O
that	O
the	O
dodecamers	B-oligomeric_state
can	O
assemble	O
to	O
form	O
annular	B-site
pores	I-site
.	O

This	O
finding	O
could	O
not	O
have	O
been	O
anticipated	O
from	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystallographic	I-evidence
structure	I-evidence
of	O
peptide	B-mutant
1	I-mutant
and	O
reveals	O
a	O
new	O
level	O
of	O
hierarchical	O
assembly	O
that	O
recapitulates	O
micrographic	O
observations	O
of	O
annular	B-complex_assembly
protofibrils	I-complex_assembly
.	O

The	O
crystallographically	B-evidence
observed	I-evidence
dodecamer	B-oligomeric_state
,	O
in	O
turn	O
,	O
recapitulates	O
the	O
observation	O
of	O
Aβ	B-complex_assembly
*	I-complex_assembly
56	I-complex_assembly
,	O
which	O
appears	O
to	O
be	O
a	O
dodecamer	B-oligomeric_state
of	O
Aβ	B-protein
.	O

We	O
believe	O
this	O
iterative	O
,	O
“	O
bottom	O
up	O
”	O
approach	O
of	O
identifying	O
the	O
minimal	O
modification	O
required	O
to	O
crystallize	B-experimental_method
Aβ	B-protein
peptides	O
will	O
ultimately	O
allow	O
larger	O
fragments	O
of	O
Aβ	B-protein
to	O
be	O
crystallized	B-experimental_method
,	O
thus	O
providing	O
greater	O
insights	O
into	O
the	O
structures	B-evidence
of	O
Aβ	B-protein
oligomers	B-oligomeric_state
.	O

Some	O
estrogen	B-protein
receptor	I-protein
‐	I-protein
α	I-protein
(	O
ERα	B-protein
)‐	O
targeted	O
breast	O
cancer	O
therapies	O
such	O
as	O
tamoxifen	B-chemical
have	O
tissue	O
‐	O
selective	O
or	O
cell	O
‐	O
specific	O
activities	O
,	O
while	O
others	O
have	O
similar	O
activities	O
in	O
different	O
cell	O
types	O
.	O

Ligands	O
that	O
regulate	O
the	O
dynamics	O
and	O
stability	O
of	O
the	O
coactivator	B-site
‐	I-site
binding	I-site
site	I-site
in	O
the	O
C	O
‐	O
terminal	O
ligand	B-structure_element
‐	I-structure_element
binding	I-structure_element
domain	I-structure_element
,	O
called	O
activation	B-structure_element
function	I-structure_element
‐	I-structure_element
2	I-structure_element
(	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
),	O
showed	O
similar	O
activity	O
profiles	O
in	O
different	O
cell	O
types	O
.	O

Such	O
ligands	O
induced	O
breast	O
cancer	O
cell	O
proliferation	O
in	O
a	O
manner	O
that	O
was	O
predicted	O
by	O
the	O
canonical	O
recruitment	O
of	O
the	O
coactivators	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
and	O
induction	O
of	O
the	O
GREB1	B-protein
proliferative	O
gene	O
.	O

Small	O
‐	O
molecule	O
ligands	O
control	O
receptor	O
activity	O
by	O
modulating	O
recruitment	O
of	O
effector	O
enzymes	O
to	O
distal	O
regions	O
of	O
the	O
receptor	O
,	O
relative	O
to	O
the	O
ligand	B-site
‐	I-site
binding	I-site
site	I-site
.	O

For	O
example	O
,	O
selective	O
estrogen	B-protein_type
receptor	I-protein_type
modulators	I-protein_type
(	O
SERMs	B-protein_type
)	O
such	O
as	O
tamoxifen	B-chemical
(	O
Nolvadex	B-chemical
®;	I-chemical
AstraZeneca	O
)	O
or	O
raloxifene	B-chemical
(	O
Evista	B-chemical
®;	I-chemical
Eli	O
Lilly	O
)	O
(	O
Fig	O
1A	O
)	O
block	O
the	O
ERα	B-protein
‐	O
mediated	O
proliferative	O
effects	O
of	O
the	O
native	O
estrogen	B-chemical
,	O
17β	B-chemical
‐	I-chemical
estradiol	I-chemical
(	O
E2	B-chemical
),	O
on	O
breast	O
cancer	O
cells	O
,	O
but	O
promote	O
beneficial	O
estrogenic	O
effects	O
on	O
bone	O
mineral	O
density	O
and	O
adverse	O
estrogenic	O
effects	O
such	O
as	O
uterine	O
proliferation	O
,	O
fatty	O
liver	O
,	O
or	O
stroke	O
(	O
Frolik	O
et	O
al	O
,	O
1996	O
;	O
Fisher	O
et	O
al	O
,	O
1998	O
;	O
McDonnell	O
et	O
al	O
,	O
2002	O
;	O
Jordan	O
,	O
2003	O
).	O

Allosteric	O
control	O
of	O
ERα	B-protein
activity	O

E2	B-chemical
‐	O
rings	O
are	O
numbered	O
A	O
‐	O
D	O
.	O
The	O
E	O
‐	O
ring	O
is	O
the	O
common	O
site	O
of	O
attachment	O
for	O
BSC	O
found	O
in	O
many	O
SERMS	B-protein_type
.	O

Schematic	O
illustration	O
of	O
the	O
canonical	O
ERα	B-protein
signaling	O
pathway	O
.	O

Branched	O
causality	O
model	O
for	O
ERα	B-protein
‐	O
mediated	O
cell	O
proliferation	O
.	O

ERα	B-protein
contains	O
structurally	B-protein_state
conserved	I-protein_state
globular	B-structure_element
domains	I-structure_element
of	O
the	O
nuclear	B-protein_type
receptor	I-protein_type
superfamily	I-protein_type
,	O
including	O
a	O
DNA	B-structure_element
‐	I-structure_element
binding	I-structure_element
domain	I-structure_element
(	O
DBD	B-structure_element
)	O
that	O
is	O
connected	O
by	O
a	O
flexible	B-protein_state
hinge	B-structure_element
region	I-structure_element
to	O
the	O
ligand	B-structure_element
‐	I-structure_element
binding	I-structure_element
domain	I-structure_element
(	O
LBD	B-structure_element
),	O
as	O
well	O
as	O
unstructured	B-protein_state
AB	B-structure_element
and	O
F	B-structure_element
domains	O
at	O
its	O
amino	O
and	O
carboxyl	O
termini	O
,	O
respectively	O
(	O
Fig	O
1B	O
).	O

The	O
LBD	B-structure_element
contains	O
a	O
ligand	O
‐	O
dependent	O
coactivator	B-site
‐	I-site
binding	I-site
site	I-site
called	O
activation	B-structure_element
function	I-structure_element
‐	I-structure_element
2	I-structure_element
(	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
).	O

However	O
,	O
the	O
agonist	O
activity	O
of	O
SERMs	B-protein_type
derives	O
from	O
activation	B-structure_element
function	I-structure_element
‐	I-structure_element
1	I-structure_element
(	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
)—	O
a	O
coactivator	B-site
recruitment	I-site
site	I-site
located	O
in	O
the	O
AB	B-structure_element
domain	O
(	O
Berry	O
et	O
al	O
,	O
1990	O
;	O
Shang	O
&	O
Brown	O
,	O
2002	O
;	O
Abot	O
et	O
al	O
,	O
2013	O
).	O

AF	B-structure_element
‐	I-structure_element
1	I-structure_element
and	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
bind	O
distinct	O
but	O
overlapping	O
sets	O
of	O
coregulators	O
(	O
Webb	O
et	O
al	O
,	O
1998	O
;	O
Endoh	O
et	O
al	O
,	O
1999	O
;	O
Delage	O
‐	O
Mourroux	O
et	O
al	O
,	O
2000	O
;	O
Yi	O
et	O
al	O
,	O
2015	O
).	O

AF	B-structure_element
‐	I-structure_element
2	I-structure_element
binds	O
the	O
signature	O
LxxLL	B-structure_element
motif	I-structure_element
peptides	O
of	O
coactivators	O
such	O
as	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
(	O
also	O
known	O
as	O
SRC	B-protein
‐	I-protein
1	I-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
).	O

In	O
the	O
canonical	O
model	O
of	O
the	O
ERα	B-protein
signaling	O
pathway	O
(	O
Fig	O
1C	O
),	O
E2	B-protein_state
‐	I-protein_state
bound	I-protein_state
ERα	B-protein
forms	O
a	O
homodimer	B-oligomeric_state
that	O
binds	O
DNA	O
at	O
estrogen	B-site
‐	I-site
response	I-site
elements	I-site
(	O
EREs	B-site
),	O
recruits	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
(	O
Metivier	O
et	O
al	O
,	O
2003	O
;	O
Johnson	O
&	O
O	O
'	O
Malley	O
,	O
2012	O
),	O
and	O
activates	O
the	O
GREB1	B-protein
gene	O
,	O
which	O
is	O
required	O
for	O
proliferation	O
of	O
ERα	B-protein
‐	O
positive	O
breast	O
cancer	O
cells	O
(	O
Ghosh	O
et	O
al	O
,	O
2000	O
;	O
Rae	O
et	O
al	O
,	O
2005	O
;	O
Deschenes	O
et	O
al	O
,	O
2007	O
;	O
Liu	O
et	O
al	O
,	O
2012	O
;	O
Srinivasan	O
et	O
al	O
,	O
2013	O
).	O

However	O
,	O
ERα	B-protein
‐	O
mediated	O
proliferative	O
responses	O
vary	O
in	O
a	O
ligand	O
‐	O
dependent	O
manner	O
(	O
Srinivasan	O
et	O
al	O
,	O
2013	O
);	O
thus	O
,	O
it	O
is	O
not	O
known	O
whether	O
this	O
canonical	O
model	O
is	O
widely	O
applicable	O
across	O
diverse	O
ERα	B-protein
ligands	O
.	O

The	O
simplest	O
response	O
model	O
for	O
ligand	O
‐	O
specific	O
proliferative	O
effects	O
is	O
a	O
linear	O
causality	O
model	O
,	O
where	O
the	O
degree	O
of	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
recruitment	O
determines	O
GREB1	B-protein
expression	O
,	O
which	O
in	O
turn	O
drives	O
ligand	O
‐	O
specific	O
cell	O
proliferation	O
(	O
Fig	O
1D	O
).	O

To	O
test	O
these	O
signaling	O
models	O
,	O
we	O
profiled	O
a	O
diverse	O
library	O
of	O
ERα	B-protein
ligands	O
using	O
systems	O
biology	O
approaches	O
to	O
X	B-experimental_method
‐	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
and	O
chemical	B-experimental_method
biology	I-experimental_method
(	O
Srinivasan	O
et	O
al	O
,	O
2013	O
),	O
including	O
a	O
series	O
of	O
quantitative	O
bioassays	O
for	O
ERα	B-protein
function	O
that	O
were	O
statistically	O
robust	O
and	O
reproducible	O
,	O
based	O
on	O
the	O
Z	B-evidence
’‐	I-evidence
statistic	I-evidence
(	O
Fig	O
EV1A	O
and	O
B	O
;	O
see	O
Materials	O
and	O
Methods	O
).	O

Our	O
findings	O
here	O
indicate	O
that	O
specific	O
structural	O
perturbations	O
can	O
be	O
tied	O
to	O
ligand	O
‐	O
selective	O
domain	O
usage	O
and	O
signaling	O
patterns	O
,	O
thus	O
providing	O
a	O
framework	O
for	O
structure	O
‐	O
based	O
design	O
of	O
improved	O
breast	O
cancer	O
therapeutics	O
,	O
and	O
understanding	O
the	O
different	O
phenotypic	O
effects	O
of	O
environmental	O
estrogens	B-chemical
.	O

High	O
‐	O
throughput	O
screens	O
for	O
ERα	B-protein
ligand	O
profiling	O

Summary	O
of	O
ligand	B-experimental_method
screening	I-experimental_method
assays	I-experimental_method
used	O
to	O
measure	O
ER	O
‐	O
mediated	O
activities	O
.	O

ERE	B-structure_element
,	O
estrogen	B-structure_element
‐	I-structure_element
response	I-structure_element
element	I-structure_element
;	O
Luc	B-experimental_method
,	O
luciferase	B-experimental_method
reporter	I-experimental_method
gene	I-experimental_method
;	O
M2H	B-experimental_method
,	O
mammalian	B-experimental_method
2	I-experimental_method
‐	I-experimental_method
hybrid	I-experimental_method
;	O
UAS	B-structure_element
,	O
upstream	B-structure_element
‐	I-structure_element
activating	I-structure_element
sequence	I-structure_element
.	O

Strength	O
of	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
signaling	O
does	O
not	O
determine	O
cell	O
‐	O
specific	O
signaling	O

These	O
include	O
15	O
indirect	O
modulator	O
series	O
,	O
which	O
lack	B-protein_state
a	O
SERM	B-protein_type
‐	I-protein_type
like	I-protein_type
side	O
chain	O
and	O
modulate	O
coactivator	O
binding	O
indirectly	O
from	O
the	O
ligand	B-site
‐	I-site
binding	I-site
pocket	I-site
(	O
Fig	O
2A	O
–	O
E	O
;	O
Dataset	O
EV1	O
)	O
(	O
Zheng	O
et	O
al	O
,	O
2012	O
)	O
(	O
Zhu	O
et	O
al	O
,	O
2012	O
)	O
(	O
Muthyala	O
et	O
al	O
,	O
2003	O
;	O
Seo	O
et	O
al	O
,	O
2006	O
)	O
(	O
Srinivasan	O
et	O
al	O
,	O
2013	O
)	O
(	O
Wang	O
et	O
al	O
,	O
2012	O
)	O
(	O
Liao	O
et	O
al	O
,	O
2014	O
)	O
(	O
Min	O
et	O
al	O
,	O
2013	O
).	O

Ligand	B-experimental_method
profiling	I-experimental_method
using	O
our	O
quantitative	B-experimental_method
bioassays	I-experimental_method
revealed	O
a	O
wide	O
range	O
of	O
ligand	O
‐	O
induced	O
GREB1	B-protein
expression	O
,	O
reporter	O
gene	O
activities	O
,	O
ERα	B-protein
‐	O
coactivator	O
interactions	O
,	O
and	O
proliferative	O
effects	O
on	O
MCF	O
‐	O
7	O
breast	O
cancer	O
cells	O
(	O
Figs	O
EV1	O
and	O
EV2A	O
–	O
J	O
).	O

This	O
wide	O
variance	O
enabled	O
us	O
to	O
probe	O
specific	O
features	O
of	O
ERα	B-protein
signaling	O
using	O
ligand	B-experimental_method
class	I-experimental_method
analyses	I-experimental_method
,	O
and	O
identify	O
signaling	O
patterns	O
shared	O
by	O
specific	O
ligand	O
series	O
or	O
scaffolds	O
.	O

Classes	O
of	O
compounds	O
in	O
the	O
ERα	B-protein
ligand	O
library	O

Structural	O
details	O
of	O
the	O
ERα	B-protein
LBD	B-structure_element
bound	B-protein_state
to	I-protein_state
the	O
indicated	O
ligands	O
.	O

Unlike	O
E2	B-chemical
(	O
PDB	O
1GWR	O
),	O
TAM	B-chemical
is	O
a	O
direct	O
modulator	O
with	O
a	O
BSC	O
that	O
dislocates	O
h12	B-structure_element
to	O
block	O
the	O
NCOA2	B-site
‐	I-site
binding	I-site
site	I-site
(	O
PDB	O
3ERT	O
).	O

OBHS	B-chemical
is	O
an	O
indirect	O
modulator	O
that	O
dislocates	O
the	O
h11	B-structure_element
C	O
‐	O
terminus	O
to	O
destabilize	O
the	O
h11	B-site
–	I-site
h12	I-site
interface	I-site
(	O
PDB	O
4ZN9	O
).	O

To	O
this	O
end	O
,	O
we	O
compared	O
the	O
average	O
ligand	O
‐	O
induced	O
GREB1	B-protein
mRNA	O
levels	O
in	O
MCF	O
‐	O
7	O
cells	O
and	O
3	B-experimental_method
×	I-experimental_method
ERE	I-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
reporter	O
gene	O
activity	O
in	O
Ishikawa	O
endometrial	O
cancer	O
cells	O
(	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
)	O
or	O
in	O
HepG2	O
cells	O
transfected	O
with	O
wild	B-protein_state
‐	I-protein_state
type	I-protein_state
ERα	B-protein
(	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-protein
‐	O
WT	B-protein_state
)	O
(	O
Figs	O
3A	O
and	O
EV2A	O
–	O
C	O
).	O

Direct	O
modulators	O
showed	O
significant	O
differences	O
in	O
average	O
activity	O
between	O
cell	O
types	O
except	O
OBHS	B-chemical
‐	I-chemical
ASC	I-chemical
analogs	O
,	O
which	O
had	O
similar	O
low	O
agonist	O
activities	O
in	O
the	O
three	O
cell	O
types	O
.	O

While	O
it	O
was	O
known	O
that	O
direct	O
modulators	O
such	O
as	O
tamoxifen	B-chemical
drive	O
cell	O
‐	O
specific	O
signaling	O
,	O
these	O
experiments	O
reveal	O
that	O
indirect	O
modulators	O
also	O
drive	O
cell	O
‐	O
specific	O
signaling	O
,	O
since	O
eight	O
of	O
fourteen	O
classes	O
showed	O
significant	O
differences	O
in	O
average	O
activity	O
(	O
Figs	O
3A	O
and	O
EV2A	O
–	O
C	O
).	O

Ligand	O
‐	O
specific	O
signaling	O
underlies	O
ERα	B-protein
‐	O
mediated	O
cell	O
proliferation	O

(	O
A	O
)	O
Ligand	O
‐	O
specific	O
ERα	B-protein
activities	O
in	O
HepG2	O
,	O
Ishikawa	O
and	O
MCF	O
‐	O
7	O
cells	O
.	O

The	O
ligand	O
‐	O
induced	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-protein
‐	O
WT	B-protein_state
and	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
activities	O
and	O
GREB1	B-protein
mRNA	O
levels	O
are	O
shown	O
by	O
scaffold	O
(	O
mean	O
+	O
SD	O
).	O

(	O
B	O
)	O
Ligand	O
class	B-experimental_method
analysis	I-experimental_method
of	O
the	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-protein
‐	O
WT	B-protein_state
and	O
ERα	B-mutant
‐	I-mutant
ΔAB	I-mutant
activities	O
in	O
HepG2	O
cells	O
.	O

Significant	O
sensitivity	O
to	O
AB	B-structure_element
domain	O
deletion	O
was	O
determined	O
by	O
Student	B-experimental_method
'	I-experimental_method
s	I-experimental_method
t	I-experimental_method
‐	I-experimental_method
test	I-experimental_method
(	O
n	O
=	O
number	O
of	O
ligands	O
per	O
scaffold	O
in	O
Fig	O
2	O
).	O

Correlation	B-experimental_method
and	I-experimental_method
regression	I-experimental_method
analyses	I-experimental_method
in	O
a	O
large	O
test	O
set	O
.	O

In	O
cluster	O
1	O
,	O
the	O
first	O
three	O
comparisons	O
(	O
rows	O
)	O
showed	O
significant	O
positive	O
correlations	O
(	O
F	B-experimental_method
‐	I-experimental_method
test	I-experimental_method
for	O
nonzero	O
slope	O
,	O
P	B-evidence
≤	O
0	O
.	O
05	O
).	O

In	O
cluster	O
2	O
,	O
only	O
one	O
of	O
these	O
comparisons	O
revealed	O
a	O
significant	O
positive	O
correlation	O
,	O
while	O
none	O
was	O
significant	O
in	O
cluster	O
3	O
.	O
+,	O
statistically	O
significant	O
correlations	O
gained	O
by	O
deletion	B-experimental_method
of	O
the	O
AB	B-structure_element
or	O
F	B-structure_element
domains	O
.	O

−,	O
significant	O
correlations	O
lost	O
upon	O
deletion	O
of	O
AB	B-structure_element
or	O
F	B-structure_element
domains	O
.	O

Tamoxifen	B-chemical
depends	O
on	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
for	O
its	O
cell	O
‐	O
specific	O
activity	O
(	O
Sakamoto	O
et	O
al	O
,	O
2002	O
);	O
therefore	O
,	O
we	O
asked	O
whether	O
cell	O
‐	O
specific	O
signaling	O
observed	O
here	O
is	O
due	O
to	O
a	O
similar	O
dependence	O
on	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
for	O
activity	O
(	O
Fig	O
EV1	O
).	O

While	O
E2	B-chemical
showed	O
similar	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-protein
‐	O
WT	B-protein_state
and	O
ERα	B-mutant
‐	I-mutant
ΔAB	I-mutant
activities	O
,	O
tamoxifen	B-chemical
showed	O
complete	O
loss	O
of	O
activity	O
without	B-protein_state
the	O
AB	B-structure_element
domain	O
(	O
Fig	O
EV1B	O
).	O

These	O
“	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
‐	O
sensitive	O
”	O
activities	O
were	O
exhibited	O
by	O
both	O
direct	O
and	O
indirect	O
modulators	O
,	O
and	O
were	O
not	O
limited	O
to	O
scaffolds	O
that	O
showed	O
cell	O
‐	O
specific	O
signaling	O
(	O
Fig	O
3A	O
and	O
B	O
).	O

Thus	O
,	O
the	O
strength	O
of	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
signaling	O
does	O
not	O
determine	O
cell	O
‐	O
specific	O
signaling	O
.	O

Identifying	O
cell	O
‐	O
specific	O
signaling	O
clusters	O
in	O
ERα	B-protein
ligand	O
classes	O

For	O
each	O
ligand	O
class	O
or	O
scaffold	O
,	O
we	O
calculated	O
the	O
Pearson	B-evidence
'	I-evidence
s	I-evidence
correlation	I-evidence
coefficient	I-evidence
,	O
r	B-evidence
,	O
for	O
pairwise	O
comparison	O
of	O
activity	O
profiles	O
in	O
breast	O
(	O
GREB1	B-protein
),	O
liver	O
(	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
),	O
and	O
endometrial	O
cells	O
(	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
).	O

The	O
value	O
of	O
r	B-evidence
ranges	O
from	O
−	O
1	O
to	O
1	O
,	O
and	O
it	O
defines	O
the	O
extent	O
to	O
which	O
the	O
data	O
fit	O
a	O
straight	O
line	O
when	O
compounds	O
show	O
similar	O
agonist	O
/	O
antagonist	O
activity	O
profiles	O
between	O
cell	O
types	O
(	O
Fig	O
EV3A	O
).	O

We	O
also	O
calculated	O
the	O
coefficient	B-evidence
of	I-evidence
determination	I-evidence
,	O
r	B-evidence
2	I-evidence
,	O
which	O
describes	O
the	O
percentage	O
of	O
variance	O
in	O
a	O
dependent	O
variable	O
such	O
as	O
proliferation	O
that	O
can	O
be	O
predicted	O
by	O
an	O
independent	O
variable	O
such	O
as	O
GREB1	B-protein
expression	O
.	O

We	O
present	O
both	O
calculations	O
as	O
r	B-evidence
2	I-evidence
to	O
readily	O
compare	O
signaling	O
specificities	O
using	O
a	O
heat	O
map	O
on	O
which	O
the	O
red	O
–	O
yellow	O
palette	O
indicates	O
significant	O
positive	O
correlations	O
(	O
P	B-evidence
≤	O
0	O
.	O
05	O
,	O
F	B-experimental_method
‐	I-experimental_method
test	I-experimental_method
for	O
nonzero	O
slope	O
),	O
while	O
the	O
blue	O
palette	O
denotes	O
negative	O
correlations	O
(	O
Fig	O
3C	O
–	O
F	O
).	O

The	O
side	O
chain	O
of	O
OBHS	B-chemical
‐	I-chemical
BSC	I-chemical
analogs	O
induces	O
cell	O
‐	O
specific	O
signaling	O

Correlation	O
analysis	O
of	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-mutant
‐	I-mutant
ΔAB	I-mutant
activity	O
versus	O
endogenous	O
ERα	B-protein
activity	O
of	O
OBHS	B-chemical
analogs	O
.	O

In	O
panel	O
(	O
D	O
),	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-protein
‐	O
WT	B-protein_state
activity	O
from	O
panel	O
(	O
B	O
)	O
is	O
shown	O
for	O
comparison	O
.	O

Correlation	O
analysis	O
of	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-mutant
‐	I-mutant
ΔF	I-mutant
activity	O
versus	O
endogenous	O
ERα	B-protein
activities	O
of	O
OBHS	B-chemical
analogs	O
.	O

Correlation	O
analysis	O
of	O
MCF	O
‐	O
7	O
cell	O
proliferation	O
versus	O
NCOA2	B-protein
/	I-protein
3	I-protein
recruitment	O
or	O
GREB1	B-protein
levels	O
observed	O
in	O
response	O
to	O
(	O
G	O
)	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
and	O
(	O
H	O
)	O
OBHS	B-chemical
‐	I-chemical
BSC	I-chemical
analogs	O
.	O

Scaffolds	O
in	O
cluster	O
1	O
exhibited	O
strongly	O
correlated	O
GREB1	B-protein
levels	O
,	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
and	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
activity	O
profiles	O
across	O
the	O
three	O
cell	O
types	O
(	O
Fig	O
3C	O
lanes	O
1	O
–	O
4	O
),	O
suggesting	O
these	O
ligands	O
use	O
similar	O
ERα	B-protein
signaling	O
pathways	O
in	O
the	O
breast	O
,	O
endometrial	O
,	O
and	O
liver	O
cell	O
types	O
.	O

This	O
cluster	O
includes	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
,	O
OBHS	B-chemical
,	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
,	O
and	O
triaryl	B-chemical
‐	I-chemical
ethylene	I-chemical
analogs	O
,	O
all	O
of	O
which	O
are	O
indirect	O
modulators	O
.	O

This	O
cluster	O
includes	O
two	O
classes	O
of	O
direct	O
modulators	O
(	O
cyclofenil	B-chemical
‐	I-chemical
ASC	I-chemical
and	O
WAY	B-chemical
dimer	I-chemical
),	O
and	O
six	O
classes	O
of	O
indirect	O
modulators	O
(	O
2	B-chemical
,	I-chemical
5	I-chemical
‐	I-chemical
DTP	I-chemical
,	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTP	I-chemical
,	O
S	B-chemical
‐	I-chemical
OBHS	I-chemical
‐	I-chemical
2	I-chemical
and	O
S	B-chemical
‐	I-chemical
OBHS	I-chemical
‐	I-chemical
3	I-chemical
,	O
furan	B-chemical
,	O
and	O
WAY	B-chemical
‐	I-chemical
D	I-chemical
).	O

For	O
example	O
,	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTP	I-chemical
,	O
furan	B-chemical
,	O
and	O
S	B-chemical
‐	I-chemical
OBHS	I-chemical
‐	I-chemical
2	I-chemical
drove	O
positively	O
correlated	O
GREB1	B-protein
levels	O
and	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
but	O
not	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-protein
‐	O
WT	B-protein_state
activity	O
(	O
Fig	O
3C	O
lanes	O
5	O
–	O
7	O
).	O

In	O
contrast	O
,	O
WAY	B-chemical
dimer	I-chemical
and	O
WAY	B-chemical
‐	I-chemical
D	I-chemical
analogs	O
drove	O
positively	O
correlated	O
GREB1	B-protein
levels	O
and	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-protein
‐	O
WT	B-protein_state
but	O
not	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
activity	O
(	O
Fig	O
3C	O
lanes	O
8	O
and	O
9	O
).	O

This	O
is	O
demonstrated	O
by	O
directly	O
comparing	O
the	O
signaling	O
specificities	O
of	O
matched	O
OBHS	B-chemical
(	O
indirect	O
modulator	O
,	O
cluster	O
1	O
)	O
and	O
OBHS	B-chemical
‐	I-chemical
BSC	I-chemical
analogs	O
(	O
direct	O
modulator	O
,	O
cluster	O
3	O
),	O
which	O
differ	O
only	O
in	O
the	O
basic	O
side	O
chain	O
(	O
Fig	O
2E	O
).	O

The	O
activities	O
of	O
OBHS	B-chemical
analogs	O
were	O
positively	O
correlated	O
across	O
the	O
three	O
cell	O
types	O
,	O
but	O
the	O
side	O
chain	O
of	O
OBHS	B-chemical
‐	I-chemical
BSC	I-chemical
analogs	O
was	O
sufficient	O
to	O
abolish	O
these	O
correlations	O
(	O
Figs	O
3C	O
lanes	O
1	O
and	O
19	O
,	O
and	O
EV3A	O
–	O
C	O
).	O

This	O
demonstrates	O
that	O
the	O
signaling	O
specificities	O
underlying	O
these	O
positive	O
correlations	O
are	O
not	O
modified	O
by	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
.	O

Despite	O
this	O
nearly	O
complete	O
lack	O
of	O
activity	O
,	O
the	O
pattern	O
of	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-mutant
‐	I-mutant
ΔAB	I-mutant
activity	O
was	O
still	O
highly	O
correlated	O
with	O
the	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
activity	O
and	O
GREB1	B-protein
expression	O
(	O
Fig	O
EV3D	O
and	O
E	O
),	O
demonstrating	O
that	O
very	O
small	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
activities	O
can	O
be	O
amplified	O
by	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
to	O
produce	O
robust	O
signals	O
.	O

Similarly	O
,	O
deletion	B-experimental_method
of	I-experimental_method
the	O
F	B-structure_element
domain	O
did	O
not	O
abolish	O
correlations	O
between	O
the	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
and	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
or	O
GREB1	B-protein
levels	O
induced	O
by	O
OBHS	B-chemical
analogs	O
(	O
Fig	O
EV3F	O
).	O

Deletion	B-experimental_method
of	I-experimental_method
the	O
AB	B-structure_element
or	O
F	B-structure_element
domain	O
altered	O
correlations	O
for	O
six	O
of	O
the	O
eight	O
scaffolds	O
in	O
this	O
cluster	O
(	O
2	B-chemical
,	I-chemical
5	I-chemical
‐	I-chemical
DTP	I-chemical
,	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTP	I-chemical
,	O
S	B-chemical
‐	I-chemical
OBHS	I-chemical
‐	I-chemical
3	I-chemical
,	O
WAY	B-chemical
‐	I-chemical
D	I-chemical
,	O
WAY	B-chemical
dimer	I-chemical
,	O
and	O
cyclofenil	B-chemical
‐	I-chemical
ASC	I-chemical
)	O
(	O
Fig	O
3D	O
lanes	O
5	O
–	O
12	O
).	O

For	O
ligands	O
in	O
cluster	O
3	O
,	O
we	O
could	O
not	O
eliminate	O
a	O
role	O
for	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
in	O
determining	O
signaling	O
specificity	O
,	O
since	O
this	O
cluster	O
lacked	O
positively	O
correlated	O
activity	O
profiles	O
(	O
Fig	O
3C	O
),	O
and	O
deletion	B-experimental_method
of	I-experimental_method
the	O
AB	B-structure_element
or	O
F	B-structure_element
domain	O
rarely	O
induced	O
such	O
correlations	O
(	O
Fig	O
3D	O
),	O
except	O
for	O
A	B-chemical
‐	I-chemical
CD	I-chemical
and	O
OBHS	B-chemical
‐	I-chemical
ASC	I-chemical
analogs	O
,	O
where	O
deletion	B-experimental_method
of	I-experimental_method
the	O
AB	B-structure_element
domain	O
or	O
F	B-structure_element
domain	O
led	O
to	O
positive	O
correlations	O
with	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
activity	O
and	O
/	O
or	O
GREB1	B-protein
levels	O
(	O
Fig	O
3D	O
lanes	O
13	O
and	O
18	O
).	O

Ligand	O
‐	O
specific	O
control	O
of	O
GREB1	B-protein
expression	O

To	O
determine	O
whether	O
ligand	O
classes	O
control	O
expression	O
of	O
native	O
ERα	B-protein
target	O
genes	O
through	O
the	O
canonical	O
linear	O
signaling	O
pathway	O
,	O
we	O
performed	O
pairwise	B-experimental_method
linear	I-experimental_method
regression	I-experimental_method
analyses	I-experimental_method
using	O
ERα	B-complex_assembly
–	I-complex_assembly
NCOA1	I-complex_assembly
/	I-complex_assembly
2	I-complex_assembly
/	I-complex_assembly
3	I-complex_assembly
interactions	O
in	O
M2H	B-experimental_method
assay	I-experimental_method
as	O
independent	O
predictors	O
of	O
GREB1	B-protein
expression	O
(	O
the	O
dependent	O
variable	O
)	O
(	O
Figs	O
EV1	O
and	O
EV2A	O
,	O
F	O
–	O
H	O
).	O

In	O
cluster	O
1	O
,	O
the	O
recruitment	O
of	O
NCOA1	B-protein
and	O
NCOA2	B-protein
was	O
highest	O
for	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
,	O
followed	O
by	O
triaryl	B-chemical
‐	I-chemical
ethylene	I-chemical
,	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
,	O
and	O
OBHS	B-chemical
series	O
,	O
while	O
for	O
NCOA3	B-protein
,	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
compounds	O
induced	O
the	O
most	O
recruitment	O
and	O
OBHS	B-chemical
ligands	O
were	O
inverse	O
agonists	O
(	O
Fig	O
EV2F	O
–	O
H	O
).	O

GREB1	B-protein
levels	O
induced	O
by	O
OBHS	B-chemical
analogs	O
were	O
determined	O
by	O
recruitment	O
of	O
NCOA1	B-protein
but	O
not	O
NCOA2	B-protein
/	I-protein
3	I-protein
(	O
Fig	O
3E	O
lane	O
1	O
),	O
suggesting	O
that	O
there	O
may	O
be	O
alternate	O
or	O
preferential	O
use	O
of	O
these	O
coactivators	O
by	O
different	O
classes	O
.	O

However	O
,	O
in	O
cluster	O
1	O
,	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
recruitment	O
generally	O
predicted	O
GREB1	B-protein
levels	O
(	O
Fig	O
3E	O
lanes	O
1	O
–	O
4	O
),	O
consistent	O
with	O
the	O
canonical	O
signaling	O
model	O
(	O
Fig	O
1D	O
).	O

For	O
clusters	O
2	O
and	O
3	O
,	O
GREB1	B-protein
activity	O
was	O
generally	O
not	O
predicted	O
by	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
recruitment	O
.	O

Direct	O
modulators	O
showed	O
low	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
recruitment	O
(	O
Fig	O
EV2F	O
–	O
H	O
),	O
but	O
only	O
OBHS	B-chemical
‐	I-chemical
ASC	I-chemical
analogs	O
had	O
NCOA2	B-protein
recruitment	O
profiles	O
that	O
predicted	O
a	O
full	O
range	O
of	O
effects	O
on	O
GREB1	B-protein
levels	O
(	O
Figs	O
3E	O
lanes	O
9	O
,	O
11	O
,	O
18	O
–	O
19	O
,	O
and	O
EV2A	O
).	O

The	O
indirect	O
modulators	O
in	O
clusters	O
2	O
and	O
3	O
stimulated	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
recruitment	O
and	O
GREB1	B-protein
expression	O
with	O
substantial	O
variance	O
(	O
Figs	O
3A	O
and	O
EV2F	O
–	O
H	O
).	O

However	O
,	O
ligand	O
‐	O
induced	O
GREB1	B-protein
levels	O
were	O
generally	O
not	O
determined	O
by	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
recruitment	O
(	O
Fig	O
3E	O
lanes	O
5	O
–	O
19	O
),	O
consistent	O
with	O
an	O
alternate	O
causality	O
model	O
(	O
Fig	O
1E	O
).	O

Out	O
of	O
11	O
indirect	O
modulator	O
series	O
in	O
cluster	O
2	O
or	O
3	O
,	O
only	O
the	O
S	B-chemical
‐	I-chemical
OBHS	I-chemical
‐	I-chemical
3	I-chemical
class	O
had	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
recruitment	O
profiles	O
that	O
predicted	O
GREB1	B-protein
levels	O
(	O
Fig	O
3E	O
lane	O
12	O
).	O

To	O
determine	O
mechanisms	O
for	O
ligand	O
‐	O
dependent	O
control	O
of	O
breast	O
cancer	O
cell	O
proliferation	O
,	O
we	O
performed	O
linear	B-experimental_method
regression	I-experimental_method
analyses	I-experimental_method
across	O
the	O
19	O
scaffolds	O
using	O
MCF	O
‐	O
7	O
cell	O
proliferation	O
as	O
the	O
dependent	O
variable	O
,	O
and	O
the	O
other	O
activities	O
as	O
independent	O
variables	O
(	O
Fig	O
3F	O
).	O

In	O
cluster	O
1	O
,	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
and	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
activities	O
,	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
recruitment	O
,	O
and	O
GREB1	B-protein
levels	O
generally	O
predicted	O
the	O
proliferative	O
response	O
(	O
Fig	O
3F	O
lanes	O
2	O
–	O
4	O
).	O

With	O
the	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
compounds	O
,	O
NCOA3	B-protein
and	O
GREB1	B-protein
showed	O
near	O
perfect	O
prediction	O
of	O
proliferation	O
(	O
Fig	O
EV3G	O
),	O
with	O
unexplained	O
variance	O
similar	O
to	O
the	O
noise	O
in	O
the	O
assays	O
.	O

The	O
lack	O
of	O
significant	O
predictors	O
for	O
OBHS	B-chemical
analogs	O
(	O
Fig	O
3F	O
lane	O
1	O
)	O
reflects	O
their	O
small	O
range	O
of	O
proliferative	O
effects	O
on	O
MCF	O
‐	O
7	O
cells	O
(	O
Fig	O
EV2I	O
).	O

3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTP	I-chemical
,	O
cyclofenil	B-chemical
,	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTPD	I-chemical
,	O
and	O
imidazopyridine	B-chemical
analogs	O
had	O
NCOA1	B-protein
/	I-protein
3	I-protein
recruitment	O
profiles	O
that	O
predicted	O
their	O
proliferative	O
effects	O
,	O
without	O
determining	O
GREB1	B-protein
levels	O
(	O
Fig	O
3E	O
and	O
F	O
,	O
lanes	O
5	O
and	O
14	O
–	O
16	O
).	O

Similarly	O
,	O
S	B-chemical
‐	I-chemical
OBHS	I-chemical
‐	I-chemical
3	I-chemical
,	O
cyclofenil	B-chemical
‐	I-chemical
ASC	I-chemical
,	O
and	O
OBHS	B-chemical
‐	I-chemical
ASC	I-chemical
had	O
positively	O
correlated	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
recruitment	O
and	O
GREB1	B-protein
levels	O
,	O
but	O
none	O
of	O
these	O
activities	O
determined	O
their	O
proliferative	O
effects	O
(	O
Fig	O
3E	O
and	O
F	O
lanes	O
11	O
–	O
12	O
and	O
18	O
).	O

For	O
ligands	O
that	O
show	O
cell	O
‐	O
specific	O
signaling	O
,	O
ERα	B-protein
‐	O
mediated	O
recruitment	O
of	O
other	O
coregulators	O
and	O
activation	O
of	O
other	O
target	O
genes	O
likely	O
determine	O
their	O
proliferative	O
effects	O
on	O
MCF	O
‐	O
7	O
cells	O
.	O

We	O
also	O
questioned	O
whether	O
promoter	O
occupancy	O
by	O
coactivators	O
is	O
statistically	O
robust	O
and	O
reproducible	O
for	O
ligand	O
class	O
analysis	O
using	O
a	O
chromatin	B-experimental_method
immunoprecipitation	I-experimental_method
(	I-experimental_method
ChIP	I-experimental_method
)‐	I-experimental_method
based	I-experimental_method
quantitative	I-experimental_method
assay	I-experimental_method
,	I-experimental_method
and	O
whether	O
it	O
has	O
a	O
better	O
predictive	O
power	O
than	O
the	O
M2H	B-experimental_method
assay	I-experimental_method
.	O

ERα	B-protein
and	O
NCOA3	B-protein
cycle	O
on	O
and	O
off	O
the	O
GREB1	B-protein
promoter	O
(	O
Nwachukwu	O
et	O
al	O
,	O
2014	O
).	O

Therefore	O
,	O
we	O
first	O
performed	O
a	O
time	B-experimental_method
‐	I-experimental_method
course	I-experimental_method
study	I-experimental_method
,	O
and	O
found	O
that	O
E2	B-chemical
and	O
the	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
analog	O
,	O
AAPII	B-chemical
‐	I-chemical
151	I-chemical
‐	I-chemical
4	I-chemical
,	O
induced	O
recruitment	O
of	O
NCOA3	B-protein
to	O
the	O
GREB1	B-protein
promoter	O
in	O
a	O
temporal	O
cycle	O
that	O
peaked	O
after	O
45	O
min	O
in	O
MCF	O
‐	O
7	O
cells	O
(	O
Fig	O
4A	O
).	O

At	O
this	O
time	O
point	O
,	O
other	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
analogs	O
also	O
induced	O
recruitment	O
of	O
NCOA3	B-protein
at	O
this	O
site	O
to	O
varying	O
degrees	O
(	O
Fig	O
4B	O
).	O

The	O
Z	B-evidence
’	I-evidence
for	O
this	O
assay	O
was	O
0	O
.	O
6	O
,	O
showing	O
statistical	O
robustness	O
(	O
see	O
Materials	O
and	O
Methods	O
).	O

We	O
prepared	O
biological	O
replicates	O
with	O
different	O
cell	O
passage	O
numbers	O
and	O
separately	O
prepared	O
samples	O
,	O
which	O
showed	O
r	B-evidence
2	I-evidence
of	O
0	O
.	O
81	O
,	O
demonstrating	O
high	O
reproducibility	O
(	O
Fig	O
4C	O
).	O

Kinetic	B-experimental_method
ChIP	I-experimental_method
assay	I-experimental_method
examining	O
recruitment	O
of	O
NCOA3	B-protein
to	O
the	O
GREB1	B-protein
gene	O
in	O
MCF	O
‐	O
7	O
cells	O
stimulated	O
with	O
E2	B-chemical
or	O
the	O
indicated	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
analog	O
.	O

NCOA3	B-protein
occupancy	O
at	O
GREB1	B-protein
was	O
compared	O
by	O
ChIP	B-experimental_method
assay	I-experimental_method
45	O
min	O
after	O
stimulation	O
with	O
vehicle	O
,	O
E2	B-chemical
,	O
or	O
the	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
analogs	O
.	O

In	O
panel	O
(	O
B	O
),	O
the	O
average	O
recruitment	O
of	O
two	O
biological	O
replicates	O
are	O
shown	O
as	O
mean	O
+	O
SEM	O
,	O
and	O
the	O
Z	B-evidence
‐	I-evidence
score	I-evidence
is	O
indicated	O
.	O

Linear	B-experimental_method
regression	I-experimental_method
analyses	I-experimental_method
comparing	O
the	O
ability	O
of	O
NCOA3	B-protein
recruitment	O
,	O
measured	O
by	O
ChIP	B-experimental_method
or	O
M2H	B-experimental_method
,	O
to	O
predict	O
other	O
agonist	O
activities	O
of	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
analogs	O
.	O
*	O
Significant	O
positive	O
correlation	O
(	O
F	B-experimental_method
‐	I-experimental_method
test	I-experimental_method
for	O
nonzero	O
slope	O
,	O
P	B-evidence
‐	I-evidence
value	I-evidence
).	O

Thus	O
,	O
the	O
simplified	O
coactivator	B-experimental_method
‐	I-experimental_method
binding	I-experimental_method
assay	I-experimental_method
showed	O
much	O
greater	O
predictive	O
power	O
than	O
the	O
ChIP	B-experimental_method
assay	I-experimental_method
for	O
ligand	O
‐	O
specific	O
effects	O
on	O
GREB1	B-protein
expression	O
and	O
cell	O
proliferation	O
.	O

One	O
difference	O
between	O
MCF	O
‐	O
7	O
breast	O
cancer	O
cells	O
and	O
Ishikawa	O
endometrial	O
cancer	O
cells	O
is	O
the	O
contribution	O
of	O
ERβ	B-protein
to	O
estrogenic	O
response	O
,	O
as	O
Ishikawa	O
cells	O
may	O
express	O
ERβ	B-protein
(	O
Bhat	O
&	O
Pezzuto	O
,	O
2001	O
).	O

When	O
overexpressed	B-experimental_method
in	O
MCF	O
‐	O
7	O
cells	O
,	O
ERβ	B-protein
alters	O
E2	B-chemical
‐	O
induced	O
expression	O
of	O
only	O
a	O
subset	O
of	O
ERα	B-protein
‐	O
target	O
genes	O
(	O
Wu	O
et	O
al	O
,	O
2011	O
),	O
raising	O
the	O
possibility	O
that	O
ligand	O
‐	O
induced	O
ERβ	B-protein
activity	O
may	O
contribute	O
to	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
activities	O
,	O
and	O
thus	O
underlie	O
the	O
lack	O
of	O
correlation	O
between	O
the	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
and	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-protein
‐	O
WT	B-protein_state
activities	O
or	O
GREB1	B-protein
levels	O
induced	O
by	O
cell	O
‐	O
specific	O
modulators	O
in	O
cluster	O
2	O
and	O
cluster	O
3	O
(	O
Fig	O
3C	O
).	O

To	O
test	O
this	O
idea	O
,	O
we	O
determined	O
the	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERβ	O
activity	O
profiles	O
of	O
the	O
ligands	O
(	O
Fig	O
EV1	O
).	O

All	O
direct	O
modulator	O
and	O
two	O
indirect	O
modulator	O
scaffolds	O
(	O
OBHS	B-chemical
and	O
S	B-chemical
‐	I-chemical
OBHS	I-chemical
‐	I-chemical
3	I-chemical
)	O
lacked	O
ERβ	O
agonist	O
activity	O
.	O

Nevertheless	O
,	O
the	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
activities	O
of	O
both	O
2	B-chemical
,	I-chemical
5	I-chemical
‐	I-chemical
DTP	I-chemical
and	O
cyclofenil	B-chemical
analogs	O
were	O
better	O
predicted	O
by	O
their	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERα	B-protein
‐	O
WT	B-protein_state
than	O
L	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
ERβ	B-protein
activities	O
(	O
Fig	O
EV4A	O
and	O
B	O
).	O

ERβ	B-protein
activity	O
is	O
not	O
an	O
independent	O
predictor	O
of	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
activity	O

ERβ	B-protein
activity	O
in	O
HepG2	O
cells	O
rarely	O
correlates	O
with	O
E	B-experimental_method
‐	I-experimental_method
Luc	I-experimental_method
activity	O
.	O

Data	O
information	O
:	O
The	O
r	O
2	O
and	O
P	B-evidence
values	I-evidence
for	O
the	O
indicated	O
correlations	O
are	O
shown	O
in	O
both	O
panels	O
.	O
*	O
Significant	O
positive	O
correlation	O
(	O
F	B-experimental_method
‐	I-experimental_method
test	I-experimental_method
for	O
nonzero	O
slope	O
,	O
P	B-evidence
‐	I-evidence
value	I-evidence
)	O

To	O
overcome	O
barriers	O
to	O
crystallization	B-experimental_method
of	O
ERα	B-protein
LBD	B-structure_element
complexes	O
,	O
we	O
developed	O
a	O
conformation	B-experimental_method
‐	I-experimental_method
trapping	I-experimental_method
X	I-experimental_method
‐	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
approach	O
using	O
the	O
ERα	B-mutant
‐	I-mutant
Y537S	I-mutant
mutation	O
(	O
Nettles	O
et	O
al	O
,	O
2008	O
;	O
Bruning	O
et	O
al	O
,	O
2010	O
;	O
Srinivasan	O
et	O
al	O
,	O
2013	O
).	O

To	O
further	O
validate	O
this	O
approach	O
,	O
we	O
solved	B-experimental_method
the	O
structure	B-evidence
of	O
the	O
ERα	B-mutant
‐	I-mutant
Y537S	I-mutant
LBD	B-structure_element
in	B-protein_state
complex	I-protein_state
with	I-protein_state
diethylstilbestrol	B-chemical
(	O
DES	B-chemical
),	O
which	O
bound	O
identically	O
in	O
the	O
wild	B-protein_state
‐	I-protein_state
type	I-protein_state
and	O
ERα	B-mutant
‐	I-mutant
Y537S	I-mutant
LBDs	B-structure_element
,	O
demonstrating	O
again	O
that	O
this	O
surface	O
mutation	O
stabilizes	O
h12	B-structure_element
dynamics	O
to	O
facilitate	O
crystallization	O
without	O
changing	O
ligand	O
binding	O
(	O
Appendix	O
Fig	O
S1A	O
and	O
B	O
)	O
(	O
Nettles	O
et	O
al	O
,	O
2008	O
;	O
Bruning	O
et	O
al	O
,	O
2010	O
;	O
Delfosse	O
et	O
al	O
,	O
2012	O
).	O

Using	O
this	O
approach	O
,	O
we	O
solved	B-experimental_method
76	O
ERα	B-protein
LBD	B-structure_element
structures	B-evidence
in	O
the	O
active	B-protein_state
conformation	I-protein_state
and	O
bound	B-protein_state
to	I-protein_state
ligands	I-protein_state
studied	O
here	O
(	O
Appendix	O
Fig	O
S1C	O
).	O

Eleven	O
of	O
these	O
structures	B-evidence
have	O
been	O
published	O
,	O
while	O
65	O
are	O
new	O
,	O
including	O
the	O
DES	B-protein_state
‐	I-protein_state
bound	I-protein_state
ERα	B-mutant
‐	I-mutant
Y537S	I-mutant
LBD	B-structure_element
.	O

We	O
present	O
57	O
of	O
these	O
new	O
structures	B-evidence
here	O
(	O
Dataset	O
EV2	O
),	O
while	O
the	O
remaining	O
eight	O
new	O
structures	B-evidence
bound	B-protein_state
to	I-protein_state
OBHS	B-chemical
‐	I-chemical
N	I-chemical
analogs	O
will	O
be	O
published	O
elsewhere	O
(	O
S	O
.	O
Srinivasan	O
et	O
al	O
,	O
in	O
preparation	O
).	O

The	O
indirect	O
modulator	O
scaffolds	O
in	O
cluster	O
1	O
did	O
not	O
show	O
cell	O
‐	O
specific	O
signaling	O
(	O
Fig	O
3C	O
),	O
but	O
shared	O
common	O
structural	O
perturbations	O
that	O
we	O
designed	O
to	O
modulate	O
h12	B-structure_element
dynamics	O
.	O

Based	O
on	O
our	O
original	O
OBHS	B-chemical
structure	B-evidence
,	O
the	O
OBHS	B-chemical
,	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
,	O
and	O
triaryl	B-chemical
‐	I-chemical
ethylene	I-chemical
compounds	O
were	O
modified	O
with	O
h11	B-structure_element
‐	O
directed	O
pendant	O
groups	O
(	O
Zheng	O
et	O
al	O
,	O
2012	O
;	O
Zhu	O
et	O
al	O
,	O
2012	O
;	O
Liao	O
et	O
al	O
,	O
2014	O
).	O

Superposing	B-experimental_method
the	O
LBDs	B-structure_element
based	O
on	O
the	O
class	O
of	O
bound	O
ligands	O
provides	O
an	O
ensemble	O
view	O
of	O
the	O
structural	O
variance	O
and	O
clarifies	O
what	O
part	O
of	O
the	O
ligand	B-site
‐	I-site
binding	I-site
pocket	I-site
is	O
differentially	O
perturbed	O
or	O
targeted	O
.	O

We	O
observed	O
that	O
the	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
analogs	O
displaced	O
h11	B-structure_element
along	O
a	O
vector	O
away	O
from	O
Leu354	B-residue_name_number
in	O
a	O
region	O
of	O
h3	B-structure_element
that	O
is	O
unaffected	O
by	O
the	O
ligands	O
,	O
and	O
toward	O
the	O
dimer	B-site
interface	I-site
.	O

For	O
the	O
triaryl	B-chemical
‐	I-chemical
ethylene	I-chemical
analogs	O
,	O
the	O
displacement	O
of	O
h11	B-structure_element
was	O
in	O
a	O
perpendicular	O
direction	O
,	O
away	O
from	O
Ile424	B-residue_name_number
in	O
h8	B-structure_element
and	O
toward	O
h12	B-structure_element
.	O

Remarkably	O
,	O
these	O
individual	O
inter	B-evidence
‐	I-evidence
atomic	I-evidence
distances	I-evidence
showed	O
a	O
ligand	O
class	O
‐	O
specific	O
ability	O
to	O
significantly	O
predict	O
proliferative	O
effects	O
(	O
Fig	O
5E	O
and	O
F	O
),	O
demonstrating	O
the	O
feasibility	O
of	O
developing	O
a	O
minimal	O
set	O
of	O
activity	O
predictors	O
from	O
crystal	B-evidence
structures	I-evidence
.	O

Structure	B-experimental_method
‐	I-experimental_method
class	I-experimental_method
analysis	I-experimental_method
of	O
triaryl	B-chemical
‐	I-chemical
ethylene	I-chemical
analogs	O
.	O

Arrows	O
indicate	O
chemical	O
variance	O
in	O
the	O
orientation	O
of	O
the	O
different	O
h11	B-structure_element
‐	O
directed	O
ligand	O
side	O
groups	O
(	O
PDB	O
5DK9	O
,	O
5DKB	O
,	O
5DKE	O
,	O
5DKG	O
,	O
5DKS	O
,	O
5DL4	O
,	O
5DLR	O
,	O
5DMC	O
,	O
5DMF	O
and	O
5DP0	O
).	O

Panel	O
(	O
B	O
)	O
shows	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
a	O
triaryl	B-chemical
‐	I-chemical
ethylene	I-chemical
analog	O
‐	O
bound	O
ERα	B-protein
LBD	B-structure_element
(	O
PDB	O
5DLR	O
).	O

The	O
h11	B-site
–	I-site
h12	I-site
interface	I-site
(	O
circled	O
)	O
includes	O
the	O
C	O
‐	O
terminal	O
part	O
of	O
h11	B-structure_element
.	O

This	O
region	O
was	O
expanded	O
in	O
panel	O
(	O
C	O
),	O
where	O
the	O
10	O
triaryl	B-chemical
‐	I-chemical
ethylene	I-chemical
analog	O
‐	O
bound	O
ERα	B-protein
LBD	B-structure_element
structures	B-evidence
(	O
see	O
Datasets	O
EV1	O
and	O
EV2	O
)	O
were	O
superposed	B-experimental_method
to	O
show	O
variations	O
in	O
the	O
h11	B-structure_element
C	O
‐	O
terminus	O
(	O
PDB	O
5DK9	O
,	O
5DKB	O
,	O
5DKE	O
,	O
5DKG	O
,	O
5DKS	O
,	O
5DL4	O
,	O
5DLR	O
,	O
5DMC	O
,	O
5DMF	O
,	O
and	O
5DP0	O
).	O

Inter	B-evidence
‐	I-evidence
atomic	I-evidence
distances	I-evidence
predict	O
the	O
proliferative	O
effects	O
of	O
specific	O
ligand	O
series	O
.	O

Ile424	B-residue_name_number
–	O
His524	B-residue_name_number
distance	B-evidence
measured	O
in	O
the	O
crystal	B-evidence
structures	I-evidence
correlates	O
with	O
the	O
proliferative	O
effect	O
of	O
triaryl	B-chemical
‐	I-chemical
ethylene	I-chemical
analogs	O
in	O
MCF	O
‐	O
7	O
cells	O
.	O

In	O
contrast	O
,	O
the	O
Leu354	B-residue_name_number
–	O
Leu525	B-residue_name_number
distance	B-evidence
correlates	O
with	O
the	O
proliferative	O
effects	O
of	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
analogs	O
in	O
MCF	O
‐	O
7	O
cells	O
.	O

Structure	B-experimental_method
‐	I-experimental_method
class	I-experimental_method
analysis	I-experimental_method
of	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
analogs	O
.	O

WAY	B-chemical
‐	I-chemical
C	I-chemical
side	O
groups	O
subtly	O
nudge	O
h12	B-structure_element
Leu540	B-residue_name_number
.	O

ERα	B-protein
LBD	B-structure_element
structures	B-evidence
bound	B-protein_state
to	I-protein_state
4	O
distinct	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
analogs	O
were	O
superposed	B-experimental_method
(	O
PDB	O
4	O
IU7	O
,	O
4IV4	O
,	O
4IVW	O
,	O
4IW6	O
)	O
(	O
see	O
Datasets	O
EV1	O
and	O
EV2	O
).	O

Structure	B-experimental_method
‐	I-experimental_method
class	I-experimental_method
analysis	I-experimental_method
of	O
indirect	O
modulators	O

Structure	B-experimental_method
‐	I-experimental_method
class	I-experimental_method
analysis	I-experimental_method
of	O
indirect	O
modulators	O
in	O
cluster	O
1	O
.	O

Crystal	B-evidence
structures	I-evidence
of	O
the	O
ERα	B-protein
LBD	B-structure_element
bound	B-protein_state
to	I-protein_state
OBHS	B-chemical
and	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
analogs	O
were	O
superposed	B-experimental_method
.	O

Arrows	O
indicate	O
chemical	O
variance	O
in	O
the	O
orientation	O
of	O
the	O
different	O
h11	B-structure_element
‐	O
directed	O
ligand	O
side	O
groups	O
.	O

Panel	O
(	O
B	O
)	O
shows	O
the	O
ligand	O
‐	O
induced	O
conformational	O
variation	O
at	O
the	O
C	O
‐	O
terminal	O
region	O
of	O
h11	B-structure_element
(	O
OBHS	B-chemical
:	O
PDB	O
4ZN9	O
,	O
4ZNH	O
,	O
4ZNS	O
,	O
4ZNT	O
,	O
4ZNU	O
,	O
4ZNV	O
,	O
and	O
4ZNW	O
;	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
:	O
PDB	O
4ZUB	O
,	O
4ZUC	O
,	O
4ZWH	O
,	O
4ZWK	O
,	O
5BNU	O
,	O
5BP6	O
,	O
5BPR	O
,	O
and	O
5BQ4	O
).	O

Structure	B-experimental_method
‐	I-experimental_method
class	I-experimental_method
analysis	I-experimental_method
of	O
indirect	O
modulators	O
in	O
clusters	O
2	O
and	O
3	O
.	O

Crystal	B-evidence
structures	I-evidence
of	O
the	O
ERα	B-protein
LBD	B-structure_element
bound	B-protein_state
to	I-protein_state
ligands	O
with	O
cell	O
‐	O
specific	O
activities	O
were	O
superposed	B-experimental_method
.	O

As	O
visualized	O
in	O
four	O
LBD	B-structure_element
structures	B-evidence
(	O
Srinivasan	O
et	O
al	O
,	O
2013	O
),	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
analogs	O
were	O
designed	O
with	O
small	O
substitutions	O
that	O
slightly	O
nudge	O
h12	B-structure_element
Leu540	B-residue_name_number
,	O
without	O
exiting	O
the	O
ligand	B-site
‐	I-site
binding	I-site
pocket	I-site
(	O
Fig	O
5G	O
and	O
H	O
).	O

Therefore	O
,	O
changing	O
h12	B-structure_element
dynamics	O
maintains	O
the	O
canonical	O
signaling	O
pathway	O
defined	O
by	O
E2	B-chemical
(	O
Fig	O
1D	O
)	O
to	O
support	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
‐	O
driven	O
signaling	O
and	O
recruit	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
for	O
GREB1	B-protein
‐	O
stimulated	O
proliferation	O
.	O

Therefore	O
,	O
we	O
examined	O
another	O
50	O
LBD	B-structure_element
structures	B-evidence
containing	O
ligands	O
in	O
clusters	O
2	O
and	O
3	O
.	O

These	O
structures	B-evidence
demonstrated	O
that	O
cell	O
‐	O
specific	O
activity	O
derived	O
from	O
altering	O
the	O
shape	O
of	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
without	O
an	O
extended	O
side	O
chain	O
.	O

Ligands	O
in	O
cluster	O
2	O
and	O
cluster	O
3	O
showed	O
conformational	O
heterogeneity	O
in	O
parts	O
of	O
the	O
scaffold	O
that	O
were	O
directed	O
toward	O
multiple	O
regions	O
of	O
the	O
receptor	O
including	O
h3	B-structure_element
,	O
h8	B-structure_element
,	O
h11	B-structure_element
,	O
h12	B-structure_element
,	O
and	O
/	O
or	O
the	O
β	B-structure_element
‐	I-structure_element
sheets	I-structure_element
(	O
Fig	O
EV5C	O
–	O
G	O
).	O

This	O
difference	O
in	O
ligand	O
positioning	O
altered	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
via	O
a	O
shift	O
in	O
the	O
N	O
‐	O
terminus	O
of	O
h12	B-structure_element
,	O
which	O
directly	O
contacts	O
the	O
coactivator	O
.	O

This	O
effect	O
is	O
evident	O
in	O
a	O
single	O
structure	B-evidence
due	O
to	O
its	O
1	O
Å	O
magnitude	O
(	O
Fig	O
6A	O
and	O
B	O
).	O

The	O
shifts	O
in	O
h12	B-structure_element
residues	O
Asp538	B-residue_name_number
and	O
Leu539	B-residue_name_number
led	O
to	O
rotation	O
of	O
the	O
coactivator	O
peptide	O
(	O
Fig	O
6C	O
).	O

Thus	O
,	O
cell	O
‐	O
specific	O
activity	O
can	O
stem	O
from	O
perturbation	O
of	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
without	O
an	O
extended	O
side	O
chain	O
,	O
which	O
presumably	O
alters	O
the	O
receptor	O
–	O
coregulator	O
interaction	O
profile	O
.	O

S	B-chemical
‐	I-chemical
OBHS	I-chemical
‐	I-chemical
2	I-chemical
/	I-chemical
3	I-chemical
analogs	O
subtly	O
distort	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
.	O

The	O
h3	B-site
–	I-site
h12	I-site
interface	I-site
(	O
circled	O
)	O
at	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
(	O
pink	O
)	O
was	O
expanded	O
in	O
panels	O
(	O
B	O
,	O
C	O
).	O

The	O
S	B-protein_state
‐	I-protein_state
OBHS	I-protein_state
‐	I-protein_state
2	I-protein_state
/	I-protein_state
3	I-protein_state
‐	I-protein_state
bound	I-protein_state
ERα	B-protein
LBDs	B-structure_element
were	O
superposed	B-experimental_method
to	O
show	O
shifts	O
in	O
h3	B-structure_element
(	O
panel	O
B	O
)	O
and	O
the	O
NCOA2	B-protein
peptide	O
docked	O
at	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
(	O
panel	O
C	O
).	O

Crystal	B-evidence
structures	I-evidence
show	O
that	O
2	B-chemical
,	I-chemical
5	I-chemical
‐	I-chemical
DTP	I-chemical
analogs	O
shift	O
h3	B-structure_element
and	O
h11	B-structure_element
further	O
apart	O
compared	O
to	O
an	O
A	O
‐	O
CD	O
‐	O
ring	O
estrogen	B-chemical
(	O
PDB	O
4PPS	O
,	O
5DRM	O
,	O
5DRJ	O
).	O

The	O
2F	O
o	O
‐	O
F	O
c	O
electron	O
density	O
map	O
and	O
F	O
o	O
‐	O
F	O
c	O
difference	O
map	O
of	O
a	O
2	B-protein_state
,	I-protein_state
5	I-protein_state
‐	I-protein_state
DTP	I-protein_state
‐	I-protein_state
bound	I-protein_state
structure	B-evidence
(	O
PDB	O
5DRJ	O
)	O
were	O
contoured	O
at	O
1	O
.	O
0	O
sigma	O
and	O
±	O
3	O
.	O
0	O
sigma	O
,	O
respectively	O
.	O

Average	O
(	O
mean	O
+	O
SEM	O
)	O
α	B-evidence
‐	I-evidence
carbon	I-evidence
distance	I-evidence
measured	O
from	O
h3	B-structure_element
Thr347	B-residue_name_number
to	O
h11	B-structure_element
Leu525	B-residue_name_number
of	O
A	B-protein_state
‐	I-protein_state
CD	I-protein_state
‐,	I-protein_state
2	I-protein_state
,	I-protein_state
5	I-protein_state
‐	I-protein_state
DTP	I-protein_state
‐,	I-protein_state
and	I-protein_state
3	I-protein_state
,	I-protein_state
4	I-protein_state
‐	I-protein_state
DTPD	I-protein_state
‐	I-protein_state
bound	I-protein_state
ERα	B-protein
LBDs	B-structure_element
.	O

*	O
Two	O
‐	O
tailed	O
Student	B-experimental_method
'	I-experimental_method
s	I-experimental_method
t	I-experimental_method
‐	I-experimental_method
test	I-experimental_method
,	O
P	B-evidence
=	O
0	O
.	O
002	O
(	O
PDB	O
A	B-chemical
‐	I-chemical
CD	I-chemical
:	O
5DI7	O
,	O
5DID	O
,	O
5DIE	O
,	O
5DIG	O
,	O
and	O
4PPS	O
;	O
2	B-chemical
,	I-chemical
5	I-chemical
‐	I-chemical
DTP	I-chemical
:	O
4IWC	O
,	O
5DRM	O
,	O
and	O
5DRJ	O
;	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTPD	I-chemical
:	O
5DTV	O
and	O
5DU5	O
).	O

Crystal	B-evidence
structures	I-evidence
show	O
that	O
a	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTPD	I-chemical
analog	O
shifts	O
h3	B-structure_element
(	O
F	B-structure_element
)	O
and	O
the	O
NCOA2	B-protein
(	O
G	O
)	O
peptide	O
compared	O
to	O
an	O
A	B-chemical
‐	I-chemical
CD	I-chemical
‐	O
ring	O
estrogen	B-chemical
(	O
PDB	O
4PPS	O
,	O
5DTV	O
).	O

The	O
2	B-chemical
,	I-chemical
5	I-chemical
‐	I-chemical
DTP	I-chemical
analogs	O
showed	O
perturbation	O
of	O
h11	B-structure_element
,	O
as	O
well	O
as	O
h3	B-structure_element
,	O
which	O
forms	O
part	O
of	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
.	O

These	O
compounds	O
bind	O
the	O
LBD	B-structure_element
in	O
an	O
unusual	O
fashion	O
because	O
they	O
have	O
a	O
phenol	O
‐	O
to	O
‐	O
phenol	O
length	O
of	O
~	O
12	O
Å	O
,	O
which	O
is	O
longer	O
than	O
steroids	O
and	O
other	O
prototypical	O
ERα	B-protein
agonists	O
that	O
are	O
~	O
10	O
Å	O
in	O
length	O
.	O

To	O
quantify	O
this	O
difference	O
,	O
we	O
compared	O
the	O
distance	B-evidence
between	O
α	O
‐	O
carbons	O
at	O
h3	B-structure_element
Thr347	B-residue_name_number
and	O
h11	B-structure_element
Leu525	B-residue_name_number
in	O
the	O
set	O
of	O
structures	B-evidence
containing	O
2	B-chemical
,	I-chemical
5	I-chemical
‐	I-chemical
DTP	I-chemical
analogs	O
(	O
n	O
=	O
3	O
)	O
or	O
A	B-chemical
‐	I-chemical
CD	I-chemical
‐	O
ring	O
analogs	O
(	O
n	O
=	O
5	O
)	O
(	O
Fig	O
6E	O
).	O

We	O
observed	O
a	O
difference	O
of	O
0	O
.	O
4	O
Å	O
that	O
was	O
significant	O
(	O
two	O
‐	O
tailed	O
Student	B-experimental_method
'	I-experimental_method
s	I-experimental_method
t	I-experimental_method
‐	I-experimental_method
test	I-experimental_method
,	O
P	B-evidence
=	O
0	O
.	O
002	O
)	O
due	O
to	O
the	O
very	O
tight	O
clustering	O
of	O
the	O
2	B-chemical
,	I-chemical
5	I-chemical
‐	I-chemical
DTP	I-chemical
‐	O
induced	O
LBD	B-structure_element
conformation	O
.	O

The	O
shifts	O
in	O
h3	B-structure_element
suggest	O
these	O
compounds	O
are	O
positioned	O
to	O
alter	O
coregulator	O
preferences	O
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
ERα	B-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
a	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTP	I-chemical
is	O
unknown	O
;	O
however	O
,	O
we	O
solved	B-experimental_method
two	O
crystal	B-evidence
structures	I-evidence
of	O
ERα	B-protein
bound	B-protein_state
to	I-protein_state
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTPD	I-chemical
analogs	O
and	O
one	O
structure	B-evidence
containing	O
a	O
furan	B-chemical
ligand	O
—	O
all	O
of	O
which	O
have	O
a	O
3	O
,	O
4	O
‐	O
diaryl	O
configuration	O
(	O
Fig	O
2	O
;	O
Datasets	O
EV1	O
and	O
EV2	O
).	O

In	O
these	O
structures	B-evidence
,	O
the	O
A	O
‐	O
ring	O
mimetic	O
of	O
the	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTPD	I-chemical
scaffold	O
bound	O
h3	B-structure_element
Glu353	B-residue_name_number
as	O
expected	O
,	O
but	O
the	O
other	O
phenol	O
wrapped	O
around	O
h3	B-structure_element
to	O
form	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
Thr347	B-residue_name_number
,	O
indicating	O
a	O
change	O
in	O
binding	O
epitopes	O
in	O
the	O
ERα	B-protein
ligand	B-site
‐	I-site
binding	I-site
pocket	I-site
(	O
Fig	O
6F	O
).	O

The	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTPD	I-chemical
analogs	O
also	O
induced	O
a	O
shift	O
in	O
h3	B-structure_element
positioning	O
,	O
which	O
translated	O
again	O
into	O
a	O
shift	O
in	O
the	O
bound	O
coactivator	O
peptide	O
(	O
Fig	O
6F	O
).	O

Therefore	O
,	O
these	O
indirect	O
modulators	O
,	O
including	O
S	B-chemical
‐	I-chemical
OBHS	I-chemical
‐	I-chemical
2	I-chemical
,	O
S	B-chemical
‐	I-chemical
OBHS	I-chemical
‐	I-chemical
3	I-chemical
,	O
2	B-chemical
,	I-chemical
5	I-chemical
‐	I-chemical
DTP	I-chemical
,	O
and	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTPD	I-chemical
analogs	O
—	O
all	O
of	O
which	O
show	O
cell	O
‐	O
specific	O
activity	O
profiles	O
—	O
induced	O
shifts	O
in	O
h3	B-structure_element
and	O
h12	B-structure_element
that	O
were	O
transmitted	O
to	O
the	O
coactivator	O
peptide	O
via	O
an	O
altered	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
.	O

To	O
test	O
whether	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
shows	O
changes	O
in	O
shape	O
in	O
solution	O
,	O
we	O
used	O
the	O
microarray	B-experimental_method
assay	I-experimental_method
for	I-experimental_method
real	I-experimental_method
‐	I-experimental_method
time	I-experimental_method
coregulator	I-experimental_method
–	I-experimental_method
nuclear	I-experimental_method
receptor	I-experimental_method
interaction	I-experimental_method
(	O
MARCoNI	B-experimental_method
)	O
analysis	O
(	O
Aarts	O
et	O
al	O
,	O
2013	O
).	O

Here	O
,	O
the	O
ligand	O
‐	O
dependent	O
interactions	O
of	O
the	O
ERα	B-protein
LBD	B-structure_element
with	O
over	O
150	O
distinct	O
LxxLL	B-structure_element
motif	I-structure_element
peptides	O
were	O
assayed	O
to	O
define	O
structural	O
fingerprints	O
for	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
,	O
in	O
a	O
manner	O
similar	O
to	O
the	O
use	O
of	O
phage	B-experimental_method
display	I-experimental_method
peptides	I-experimental_method
as	O
structural	O
probes	O
(	O
Connor	O
et	O
al	O
,	O
2001	O
).	O

However	O
,	O
there	O
was	O
a	O
unique	O
cluster	O
of	O
peptides	O
that	O
were	O
recruited	O
by	O
E2	B-chemical
but	O
not	O
the	O
2	B-chemical
,	I-chemical
5	I-chemical
‐	I-chemical
DTP	I-chemical
analogs	O
.	O

In	O
contrast	O
,	O
3	B-chemical
,	I-chemical
4	I-chemical
‐	I-chemical
DTP	I-chemical
analogs	O
dismissed	O
most	O
of	O
the	O
peptides	O
from	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
(	O
Fig	O
6H	O
).	O

Indirect	O
modulators	O
in	O
cluster	O
1	O
avoid	O
this	O
by	O
perturbing	O
the	O
h11	B-site
–	I-site
h12	I-site
interface	I-site
,	O
and	O
modulating	O
the	O
dynamics	O
of	O
h12	B-structure_element
without	O
changing	O
the	O
shape	O
of	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
when	O
stabilized	O
.	O

We	O
found	O
a	O
very	O
strong	O
set	O
of	O
predictors	O
,	O
where	O
ligands	O
in	O
cluster	O
1	O
,	O
defined	O
by	O
similar	O
signaling	O
across	O
cell	O
types	O
,	O
showed	O
indirect	O
modulation	O
of	O
h12	B-structure_element
dynamics	O
via	O
the	O
h11	B-site
–	I-site
12	I-site
interface	I-site
or	O
slight	O
contact	O
with	O
h12	B-structure_element
.	O

For	O
ligands	O
in	O
cluster	O
1	O
,	O
deletion	B-experimental_method
of	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
reduced	O
activity	O
to	O
varying	O
degrees	O
,	O
but	O
did	O
not	O
change	O
the	O
underlying	O
signaling	O
patterns	O
established	O
through	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
.	O

In	O
contrast	O
,	O
an	O
extended	O
side	O
chain	O
designed	O
to	O
directly	O
reposition	O
h12	B-structure_element
and	O
completely	O
disrupt	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
results	O
in	O
cell	O
‐	O
specific	O
signaling	O
.	O

Compared	O
to	O
cluster	O
1	O
,	O
the	O
structural	O
rules	O
are	O
less	O
clear	O
in	O
clusters	O
2	O
and	O
3	O
,	O
but	O
a	O
number	O
of	O
indirect	O
modulator	O
classes	O
perturbed	O
the	O
LBD	B-structure_element
conformation	O
at	O
the	O
intersection	O
of	O
h3	B-structure_element
,	O
the	O
h12	B-structure_element
N	O
‐	O
terminus	O
,	O
and	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
.	O

For	O
direct	O
and	O
indirect	O
modulators	O
in	O
cluster	O
2	O
or	O
3	O
,	O
the	O
canonical	O
ERα	B-protein
signaling	O
pathway	O
involving	O
recruitment	O
of	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
and	O
induction	O
of	O
GREB1	B-protein
did	O
not	O
generally	O
predict	O
their	O
proliferative	O
effects	O
,	O
indicating	O
an	O
alternate	O
causal	O
model	O
(	O
Fig	O
1E	O
).	O

These	O
principles	O
outlined	O
above	O
provide	O
a	O
structural	O
basis	O
for	O
how	O
the	O
ligand	B-site
–	I-site
receptor	I-site
interface	I-site
leads	O
to	O
different	O
signaling	O
specificities	O
through	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
and	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
.	O

Completely	O
blocking	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
with	O
an	O
extended	O
side	O
chain	O
or	O
altering	O
the	O
shape	O
of	O
AF	B-structure_element
‐	I-structure_element
2	I-structure_element
changes	O
the	O
preference	O
away	O
from	O
NCOA1	B-protein
/	I-protein
2	I-protein
/	I-protein
3	I-protein
for	O
determining	O
GREB1	B-protein
levels	O
and	O
proliferation	O
of	O
breast	O
cancer	O
cells	O
.	O

AF	B-structure_element
‐	I-structure_element
2	I-structure_element
blockade	O
also	O
allows	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
to	O
function	O
independently	O
,	O
which	O
is	O
important	O
since	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
drives	O
tissue	O
‐	O
selective	O
effects	O
in	O
vivo	O
.	O

This	O
was	O
demonstrated	O
with	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
knockout	O
mice	O
that	O
show	O
E2	B-chemical
‐	O
dependent	O
vascular	O
protection	O
,	O
but	O
not	O
uterine	O
proliferation	O
,	O
thus	O
highlighting	O
the	O
role	O
of	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
in	O
tissue	O
‐	O
selective	O
or	O
cell	O
‐	O
specific	O
signaling	O
(	O
Billon	O
‐	O
Gales	O
et	O
al	O
,	O
2009	O
;	O
Abot	O
et	O
al	O
,	O
2013	O
).	O

Here	O
,	O
we	O
examined	O
many	O
LBD	B-structure_element
structures	B-evidence
and	O
tested	O
several	O
variables	O
that	O
were	O
not	O
predictive	O
,	O
including	O
ERβ	B-protein
activity	O
,	O
the	O
strength	O
of	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
signaling	O
,	O
and	O
NCOA3	B-protein
occupancy	O
at	O
the	O
GREB1	B-protein
gene	O
.	O

Similarly	O
,	O
we	O
visualized	O
structures	B-evidence
to	O
identify	O
patterns	O
.	O

For	O
example	O
,	O
phage	B-experimental_method
display	I-experimental_method
was	O
used	O
to	O
identify	O
the	O
androgen	O
receptor	O
interactome	O
,	O
which	O
was	O
cloned	O
into	O
an	O
M2H	B-experimental_method
library	O
and	O
used	O
to	O
identify	O
clusters	O
of	O
ligand	O
‐	O
selective	O
interactions	O
(	O
Norris	O
et	O
al	O
,	O
2009	O
).	O

Indeed	O
,	O
the	O
most	O
anti	O
‐	O
proliferative	O
compound	O
in	O
the	O
OBHS	B-chemical
‐	I-chemical
N	I-chemical
series	O
had	O
a	O
fulvestrant	O
‐	O
like	O
profile	O
across	O
a	O
battery	O
of	O
assays	O
(	O
S	O
.	O
Srinivasan	O
et	O
al	O
,	O
in	O
preparation	O
).	O

Secondly	O
,	O
our	O
finding	O
that	O
WAY	B-chemical
‐	I-chemical
C	I-chemical
compounds	O
do	O
not	O
rely	O
of	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
for	O
signaling	O
efficacy	O
may	O
derive	O
from	O
the	O
slight	O
contacts	O
with	O
h12	B-structure_element
observed	O
in	O
crystal	B-evidence
structures	I-evidence
(	O
Figs	O
3B	O
and	O
5H	O
),	O
unlike	O
other	O
compounds	O
in	O
cluster	O
1	O
that	O
dislocate	O
h11	B-structure_element
and	O
rely	O
on	O
AF	B-structure_element
‐	I-structure_element
1	I-structure_element
for	O
signaling	O
efficacy	O
(	O
Figs	O
3B	O
and	O
5C	O
,	O
and	O
EV5B	O
).	O

Some	O
of	O
these	O
ligands	O
altered	O
the	O
shape	O
of	O
the	O
AF	B-site
‐	I-site
2	I-site
surface	I-site
by	O
perturbing	O
the	O
h3	B-site
–	I-site
h12	I-site
interface	I-site
,	O
thus	O
providing	O
a	O
route	O
to	O
new	O
SERM	O
‐	O
like	O
activity	O
profiles	O
by	O
combining	O
indirect	O
and	O
direct	O
modulation	O
of	O
receptor	O
structure	O
.	O

Incorporation	O
of	O
statistical	O
approaches	O
to	O
understand	O
relationships	O
between	O
structure	O
and	O
signaling	O
variables	O
moves	O
us	O
toward	O
predictive	O
models	O
for	O
complex	O
ERα	B-protein
‐	O
mediated	O
responses	O
such	O
as	O
in	O
vivo	O
uterine	O
proliferation	O
or	O
tumor	O
growth	O
,	O
and	O
more	O
generally	O
toward	O
structure	O
‐	O
based	O
design	O
for	O
other	O
allosteric	O
drug	O
targets	O
including	O
GPCRs	B-protein_type
and	O
other	O
nuclear	B-protein_type
receptors	I-protein_type
.	O

Structure	B-evidence
of	O
a	O
quinolone	O
-	O
stabilized	O
cleavage	O
complex	O
of	O
topoisomerase	B-complex_assembly
IV	I-complex_assembly
from	O
Klebsiella	B-species
pneumoniae	I-species
and	O
comparison	O
with	O
a	O
related	O
Streptococcus	B-species
pneumoniae	I-species
complex	O

Crystal	B-evidence
structures	I-evidence
of	O
the	O
cleavage	O
complexes	O
of	O
topoisomerase	B-complex_assembly
IV	I-complex_assembly
from	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
negative	I-taxonomy_domain
(	O
K	B-species
.	I-species
pneumoniae	I-species
)	O
and	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
positive	I-taxonomy_domain
(	O
S	B-species
.	I-species
pneumoniae	I-species
)	O
bacterial	B-taxonomy_domain
pathogens	O
stabilized	O
by	O
the	O
clinically	O
important	O
antibacterial	O
drug	O
levofloxacin	B-chemical
are	O
presented	O
,	O
analysed	O
and	O
compared	O
.	O

Klebsiella	B-species
pneumoniae	I-species
is	O
a	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
negative	I-taxonomy_domain
bacterium	I-taxonomy_domain
that	O
is	O
responsible	O
for	O
a	O
range	O
of	O
common	O
infections	O
,	O
including	O
pulmonary	O
pneumonia	O
,	O
bloodstream	O
infections	O
and	O
meningitis	O
.	O

Certain	O
strains	O
of	O
Klebsiella	B-taxonomy_domain
have	O
become	O
highly	O
resistant	O
to	O
antibiotics	O
.	O

Despite	O
the	O
vast	O
amount	O
of	O
research	O
carried	O
out	O
on	O
this	O
class	O
of	O
bacteria	B-taxonomy_domain
,	O
the	O
molecular	O
structure	B-evidence
of	O
its	O
topoisomerase	B-complex_assembly
IV	I-complex_assembly
,	O
a	O
type	B-protein_type
II	I-protein_type
topoisomerase	I-protein_type
essential	O
for	O
catalysing	O
chromosomal	O
segregation	O
,	O
had	O
remained	O
unknown	O
.	O

In	O
this	O
paper	O
,	O
the	O
structure	B-evidence
of	O
its	O
DNA	B-chemical
-	O
cleavage	O
complex	O
is	O
reported	O
at	O
3	O
.	O
35	O
Å	O
resolution	O
.	O

The	O
complex	O
is	O
comprised	O
of	O
ParC	B-protein
breakage	B-structure_element
-	I-structure_element
reunion	I-structure_element
and	O
ParE	B-protein
TOPRIM	B-structure_element
domains	O
of	O
K	B-species
.	I-species
pneumoniae	I-species
topoisomerase	B-complex_assembly
IV	I-complex_assembly
with	O
DNA	B-chemical
stabilized	O
by	O
levofloxacin	B-chemical
,	O
a	O
broad	O
-	O
spectrum	O
fluoroquinolone	B-chemical
antimicrobial	O
agent	O
.	O

Klebsiella	B-taxonomy_domain
is	O
a	O
genus	O
belonging	O
to	O
the	O
Enterobacteriaceae	B-taxonomy_domain
family	O
of	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
negative	I-taxonomy_domain
bacilli	I-taxonomy_domain
,	O
which	O
is	O
divided	O
into	O
seven	O
species	O
with	O
demonstrated	O
similarities	O
in	O
DNA	B-chemical
homology	O
:	O
K	B-species
.	I-species
pneumoniae	I-species
,	O
K	B-species
.	I-species
ozaenae	I-species
,	O
K	B-species
.	I-species
rhinoscleromatis	I-species
,	O
K	B-species
.	I-species
oxytoca	I-species
,	O
K	B-species
.	I-species
planticola	I-species
,	O
K	B-species
.	I-species
terrigena	I-species
and	O
K	B-species
.	I-species
ornithinolytica	I-species
.	O

K	B-species
.	I-species
pneumoniae	I-species
is	O
the	O
most	O
medically	O
important	O
species	O
of	O
the	O
genus	O
owing	O
to	O
its	O
high	O
resistance	O
to	O
antibiotics	O
.	O

However	O
,	O
common	O
treatments	O
(	O
based	O
on	O
in	B-experimental_method
vitro	I-experimental_method
susceptibility	I-experimental_method
testing	I-experimental_method
)	O
are	O
the	O
polymyxins	B-chemical
,	O
tigecycline	B-chemical
and	O
,	O
less	O
frequently	O
,	O
aminoglycoside	B-chemical
antibiotics	O
(	O
Arnold	O
et	O
al	O
.,	O
2011	O
).	O

Another	O
effective	O
strategy	O
involves	O
the	O
limited	O
use	O
of	O
certain	O
antimicrobials	O
,	O
specifically	O
fluoroquinolones	B-chemical
and	O
cephalo	B-chemical
­	I-chemical
sporins	I-chemical
(	O
Gasink	O
et	O
al	O
.,	O
2009	O
).	O

These	O
include	O
combinations	O
of	O
existing	O
β	O
-	O
lactam	O
antibiotics	O
with	O
new	O
β	B-protein_type
-	I-protein_type
lactamase	I-protein_type
inhibitors	O
able	O
to	O
circumvent	O
KPC	O
resistance	O
.	O

Neoglycosides	B-chemical
are	O
novel	O
aminoglycosides	B-chemical
that	O
have	O
activity	O
against	O
KPC	O
-	O
producing	O
bacteria	B-taxonomy_domain
that	O
are	O
also	O
being	O
developed	O
(	O
Chen	O
et	O
al	O
.,	O
2012	O
).	O

Type	B-protein_type
II	I-protein_type
topoisomerase	I-protein_type
enzymes	I-protein_type
play	O
important	O
roles	O
in	O
prokaryotic	B-taxonomy_domain
and	O
eukaryotic	B-taxonomy_domain
DNA	B-chemical
replication	O
,	O
recombination	O
and	O
transcription	O
(	O
Drlica	O
et	O
al	O
.,	O
2008	O
;	O
Laponogov	O
et	O
al	O
.,	O
2013	O
;	O
Lee	O
et	O
al	O
.,	O
2013	O
;	O
Nitiss	O
,	O
2009a	O
,	O
b	O
;	O
Schoeffler	O
&	O
Berger	O
,	O
2008	O
;	O
Sissi	O
&	O
Palumbo	O
,	O
2009	O
;	O
Vos	O
et	O
al	O
.,	O
2011	O
;	O
Wendorff	O
et	O
al	O
.,	O
2012	O
;	O
Wu	O
et	O
al	O
.,	O
2011	O
,	O
2013	O
).	O

Both	O
enzymes	O
act	O
via	O
a	O
double	O
-	O
strand	O
DNA	B-chemical
break	O
involving	O
a	O
cleavage	O
complex	O
and	O
are	O
targets	O
for	O
quinolone	O
antimicrobials	O
that	O
act	O
by	O
trapping	O
these	O
enzymes	O
at	O
the	O
DNA	B-chemical
-	O
cleavage	O
stage	O
and	O
preventing	O
strand	O
re	O
-	O
joining	O
(	O
Drlica	O
et	O
al	O
.,	O
2008	O
).	O

It	O
is	O
active	O
against	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
positive	I-taxonomy_domain
and	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
negative	I-taxonomy_domain
bacteria	I-taxonomy_domain
and	O
functions	O
by	O
inhibiting	O
gyrase	B-protein_type
and	O
topoisomerase	B-complex_assembly
IV	I-complex_assembly
(	O
Drlica	O
&	O
Zhao	O
,	O
1997	O
;	O
Laponogov	O
et	O
al	O
.,	O
2010	O
).	O

Here	O
,	O
we	O
report	O
the	O
first	O
three	O
-	O
dimensional	O
X	B-evidence
-	I-evidence
ray	I-evidence
structure	I-evidence
of	O
a	O
K	B-species
.	I-species
pneumoniae	I-species
topoisomerase	B-complex_assembly
IV	I-complex_assembly
ParC	B-complex_assembly
/	I-complex_assembly
ParE	I-complex_assembly
cleavage	O
complex	O
with	O
DNA	B-chemical
stabilized	O
by	O
levofloxacin	B-chemical
.	O

The	O
crystal	B-evidence
structure	I-evidence
provides	O
structural	O
information	O
on	O
topoisomerase	B-complex_assembly
IV	I-complex_assembly
from	O
K	B-species
.	I-species
pneumoniae	I-species
,	O
a	O
pathogen	O
for	O
which	O
drug	O
resistance	O
is	O
a	O
serious	O
concern	O
.	O

The	O
structure	B-evidence
of	O
the	O
ParC	B-complex_assembly
/	I-complex_assembly
ParE	I-complex_assembly
–	O
DNA	B-site
–	I-site
levofloxacin	I-site
binding	I-site
site	I-site
highlights	O
the	O
details	O
of	O
the	O
cleavage	O
-	O
complex	O
assembly	O
that	O
are	O
essential	O
for	O
the	O
rational	O
design	O
of	O
Klebsiella	B-taxonomy_domain
topoisomerase	B-protein_type
inhibitors	O
.	O

We	O
have	O
co	B-experimental_method
-	I-experimental_method
crystallized	I-experimental_method
the	O
K	B-species
.	I-species
pneumoniae	I-species
topoisomerase	B-complex_assembly
IV	I-complex_assembly
ParC	B-complex_assembly
/	I-complex_assembly
ParE	I-complex_assembly
breakage	B-structure_element
-	I-structure_element
reunion	I-structure_element
domain	O
(	O
ParC55	B-protein
;	O
residues	O
1	B-residue_range
–	I-residue_range
490	I-residue_range
)	O
and	O
ParE	B-protein
TOPRIM	B-structure_element
domain	O
(	O
ParE30	B-protein
;	O
residues	O
390	B-residue_range
–	I-residue_range
631	I-residue_range
)	O
with	O
a	O
precut	O
34	O
bp	O
DNA	B-chemical
duplex	O
(	O
the	O
E	B-site
-	I-site
site	I-site
),	O
stabilized	O
by	O
levofloxacin	B-chemical
.	O

The	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystal	I-evidence
structure	I-evidence
of	O
the	O
complex	O
was	O
determined	O
to	O
3	O
.	O
35	O
Å	O
resolution	O
,	O
revealing	O
a	O
closed	B-protein_state
ParC55	B-protein
dimer	B-oligomeric_state
flanked	O
by	O
two	O
ParE30	B-protein
monomers	B-oligomeric_state
(	O
Figs	O
.	O
1	O
▸,	O
2	O
▸	O
and	O
3	O
▸).	O

The	O
overall	O
architecture	O
of	O
this	O
complex	O
is	O
similar	O
to	O
that	O
found	O
for	O
S	B-species
.	I-species
pneumoniae	I-species
topoisomerase	O
–	O
DNA	O
–	O
drug	O
complexes	O
(	O
Laponogov	O
et	O
al	O
.,	O
2009	O
,	O
2010	O
).	O

Residues	O
6	B-residue_range
–	I-residue_range
30	I-residue_range
of	O
the	O
N	O
-	O
terminal	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
α1	B-structure_element
of	O
the	O
ParC	B-protein
subunit	O
again	O
embrace	O
the	O
ParE	B-protein
subunit	O
,	O
‘	O
hugging	O
’	O
the	O
ParE	B-protein
subunits	O
close	O
to	O
either	O
side	O
of	O
the	O
ParC	B-protein
dimer	B-oligomeric_state
(	O
Laponogov	O
et	O
al	O
.,	O
2010	O
).	O

Deletion	B-experimental_method
of	I-experimental_method
this	O
‘	O
arm	B-structure_element
’	O
α1	B-structure_element
results	O
in	O
loss	B-protein_state
of	I-protein_state
DNA	I-protein_state
-	I-protein_state
cleavage	I-protein_state
activity	I-protein_state
(	O
Laponogov	O
et	O
al	O
.,	O
2007	O
)	O
and	O
is	O
clearly	O
very	O
important	O
in	O
complex	O
stability	O
(	O
Fig	O
.	O
3	O
▸).	O

This	O
structural	O
feature	O
was	O
absent	O
in	O
our	O
original	O
ParC55	B-protein
structure	B-evidence
(	O
Laponogov	O
et	O
al	O
.,	O
2007	O
;	O
Sohi	O
et	O
al	O
.,	O
2008	O
).	O

The	O
C	B-protein
-	I-protein
subunit	I-protein
provides	O
the	O
WHD	B-structure_element
(	O
winged	B-structure_element
-	I-structure_element
helix	I-structure_element
domain	I-structure_element
;	O
a	O
CAP	B-structure_element
-	I-structure_element
like	I-structure_element
structure	I-structure_element
;	O
McKay	O
&	O
Steitz	O
,	O
1981	O
)	O
and	O
the	O
‘	O
tower	B-structure_element
’	O
which	O
form	O
the	O
U	B-structure_element
groove	I-structure_element
-	O
shaped	O
protein	O
region	O
into	O
which	O
the	O
G	B-structure_element
-	I-structure_element
gate	I-structure_element
DNA	B-chemical
binds	O
with	O
an	O
induced	O
U	O
-	O
shaped	O
bend	O
.	O

The	O
lower	O
C	B-structure_element
-	I-structure_element
gate	I-structure_element
region	O
(	O
Fig	O
.	O
3	O
▸)	O
consists	O
of	O
the	O
same	O
disposition	O
of	O
pairs	O
of	O
two	O
long	B-structure_element
α	I-structure_element
-	I-structure_element
helices	I-structure_element
terminated	O
by	O
a	O
spanning	O
short	B-structure_element
α	I-structure_element
-	I-structure_element
helix	I-structure_element
forming	O
a	O
30	O
Å	O
wide	O
DNA	B-site
-	I-site
accommodating	I-site
cavity	I-site
through	O
which	O
the	O
T	B-structure_element
-	I-structure_element
gate	I-structure_element
DNA	B-chemical
passes	O
as	O
found	O
in	O
the	O
S	B-species
.	I-species
pneumoniae	I-species
complex	O
.	O

Owing	O
to	O
the	O
structural	O
similarity	O
,	O
it	O
appears	O
that	O
the	O
topo	B-complex_assembly
­	I-complex_assembly
isomerases	I-complex_assembly
IV	I-complex_assembly
from	O
K	B-species
.	I-species
pneumoniae	I-species
and	O
S	B-species
.	I-species
pneumoniae	I-species
are	O
likely	O
to	O
follow	O
a	O
similar	O
overall	O
topoisomerase	B-protein_type
catalytic	O
cycle	O
as	O
shown	O
in	O
Fig	O
.	O
4	O
▸;	O
we	O
have	O
confirmation	O
of	O
one	O
intermediate	O
from	O
our	O
recent	O
structure	B-evidence
of	O
the	O
full	B-protein_state
complex	I-protein_state
(	O
the	O
holoenzyme	B-protein_state
less	O
the	O
CTD	B-structure_element
β	I-structure_element
-	I-structure_element
pinwheel	I-structure_element
domain	O
)	O
with	O
the	O
ATPase	B-structure_element
domain	I-structure_element
in	O
the	O
open	B-protein_state
conformation	O
(	O
Laponogov	O
et	O
al	O
.,	O
2013	O
).	O

The	O
G	B-structure_element
-	I-structure_element
gate	I-structure_element
DNA	B-chemical
for	O
the	O
S	B-species
.	I-species
pneumoniae	I-species
complex	O
consists	O
of	O
an	O
18	O
-	O
base	O
-	O
pair	O
E	B-site
-	I-site
site	I-site
sequence	O
(	O
our	O
designation	O
for	O
a	O
DNA	B-site
site	I-site
which	O
we	O
first	O
found	O
from	O
DNA	B-experimental_method
-	I-experimental_method
mapping	I-experimental_method
studies	I-experimental_method
;	O
Leo	O
et	O
al	O
.,	O
2005	O
;	O
Arnoldi	O
et	O
al	O
.,	O
2013	O
;	O
Fig	O
.	O
1	O
▸).	O

The	O
crystallized	B-experimental_method
complex	O
was	O
formed	O
by	O
turning	O
over	O
the	O
topoisomerase	B-protein_type
tetramer	B-oligomeric_state
in	O
the	O
presence	B-protein_state
of	I-protein_state
DNA	B-chemical
and	O
levofloxacin	B-chemical
and	O
crystallizing	B-experimental_method
the	O
product	O
.	O

In	O
contrast	O
,	O
the	O
K	B-species
.	I-species
pneumoniae	I-species
complex	O
was	O
formed	O
by	O
co	B-experimental_method
-	I-experimental_method
crystallizing	I-experimental_method
the	O
topoisomerase	B-protein_type
tetramer	B-oligomeric_state
complex	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
a	O
34	O
-	O
base	O
-	O
pair	O
pre	B-protein_state
-	I-protein_state
cleaved	I-protein_state
DNA	B-chemical
in	O
the	O
presence	B-protein_state
of	I-protein_state
levofloxacin	B-chemical
.	O

We	O
have	O
been	O
able	O
to	O
unambiguously	O
read	O
off	O
the	O
DNA	B-chemical
sequences	O
in	O
the	O
electron	B-evidence
-	I-evidence
density	I-evidence
maps	I-evidence
.	O

For	O
the	O
ParC	B-protein
subunits	O
,	O
the	O
figures	O
are	O
40	O
.	O
8	O
identity	O
and	O
55	O
.	O
6	O
%	O
homology	O
between	O
the	O
two	O
organisms	O
.	O

The	O
binding	O
of	O
levofloxacin	B-chemical
in	O
the	O
K	B-species
.	I-species
pneumoniae	I-species
complex	O
is	O
shown	O
in	O
Figs	O
.	O
2	O
▸,	O
3	O
▸	O
and	O
5	O
▸	O
and	O
is	O
hemi	O
-	O
intercalated	O
into	O
the	O
DNA	B-chemical
and	O
stacked	O
against	O
the	O
DNA	B-chemical
bases	O
at	O
the	O
cleavage	B-site
site	I-site
(	O
positions	O
−	B-residue_number
1	I-residue_number
and	O
+	B-residue_number
1	I-residue_number
of	O
the	O
four	O
-	O
base	O
-	O
pair	O
staggered	O
cut	O
in	O
the	O
34	O
-	O
mer	O
DNA	B-chemical
)	O
which	O
is	O
similar	O
to	O
that	O
found	O
for	O
the	O
S	B-species
.	I-species
pneumoniae	I-species
complex	O
.	O

Fig	O
.	O
5	O
▸	O
presents	O
side	O
-	O
by	O
-	O
side	O
views	O
of	O
the	O
K	B-species
.	I-species
pneumoniae	I-species
and	O
S	B-species
.	I-species
pneumoniae	I-species
active	B-site
sites	I-site
which	O
shows	O
that	O
levofloxacin	B-chemical
binds	O
in	O
a	O
very	O
similar	O
manner	O
in	O
these	O
two	O
complexes	O
with	O
extensive	O
π	B-bond_interaction
–	I-bond_interaction
π	I-bond_interaction
stacking	I-bond_interaction
interaction	I-bond_interaction
between	O
the	O
bases	O
and	O
the	O
drug	O
.	O

The	O
methylpiperazine	B-chemical
at	O
C7	O
(	O
using	O
the	O
conventional	O
quinolone	B-chemical
numbering	O
;	O
C9	O
in	O
the	O
IUPAC	O
numbering	O
)	O
on	O
the	O
drug	O
extends	O
towards	O
residues	O
Glu474	B-residue_name_number
and	O
Glu475	B-residue_name_number
for	O
S	B-species
.	I-species
pneumoniae	I-species
and	O
towards	O
Gln460	B-residue_name_number
and	O
Glu461	B-residue_name_number
for	O
K	B-species
.	I-species
pneumoniae	I-species
,	O
where	O
the	O
glutamate	B-residue_name_number
at	I-residue_name_number
474	I-residue_name_number
is	O
substituted	O
by	O
a	O
glutamine	B-residue_name_number
at	I-residue_name_number
460	I-residue_name_number
in	O
the	O
Klebsiella	B-taxonomy_domain
strain	O
.	O

Obviously	O
,	O
the	O
drug	O
–	O
ParE	B-protein
interaction	O
in	O
this	O
region	O
is	O
less	O
strong	O
compared	O
with	O
PD	B-chemical
0305970	I-chemical
binding	O
to	O
the	O
S	B-species
.	I-species
pneumoniae	I-species
DNA	B-chemical
complex	O
,	O
where	O
PD	B-chemical
0305970	I-chemical
forms	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
to	O
ParE	B-protein
residue	O
Asp475	B-residue_name_number
and	O
can	O
form	O
one	O
to	O
Asp474	B-residue_name_number
if	O
the	O
bond	O
rotates	O
(	O
Laponogov	O
et	O
al	O
.,	O
2010	O
).	O

This	O
may	O
explain	O
why	O
drug	O
-	O
resistance	O
mutations	O
for	O
levofloxacin	B-chemical
are	O
more	O
likely	O
to	O
form	O
in	O
the	O
ParC	B-protein
subunits	O
rather	O
than	O
in	O
the	O
ParE	B-protein
subunits	O
.	O

For	O
both	O
complexes	O
there	O
is	O
a	O
Mg2	B-chemical
+	I-chemical
ion	O
bound	B-protein_state
to	I-protein_state
levofloxacin	B-chemical
between	O
the	O
carbonyl	O
group	O
at	O
position	O
4	O
of	O
the	O
quinolone	B-chemical
and	O
the	O
carboxyl	O
at	O
position	O
6	O
(	O
Figs	O
.	O
2	O
▸	O
and	O
5	O
▸	O
and	O
Supplementary	O
Fig	O
.	O
2	O
▸).	O

For	O
S	B-species
.	I-species
pneumoniae	I-species
topoisomerase	B-complex_assembly
IV	I-complex_assembly
,	O
one	O
of	O
the	O
O	O
atoms	O
of	O
the	O
carboxyl	O
of	O
Asp83	B-residue_name_number
points	O
towards	O
the	O
Mg2	B-chemical
+	I-chemical
ion	O
and	O
is	O
within	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonding	I-bond_interaction
distance	O
(	O
5	O
.	O
04	O
Å	O
)	O
through	O
an	O
Mg2	B-chemical
+-	I-chemical
coordinated	O
water	B-chemical
.	O

For	O
K	B-species
.	I-species
pneumoniae	I-species
both	O
of	O
the	O
carboxyl	O
O	O
atoms	O
are	O
pointing	O
towards	O
the	O
Mg2	B-chemical
+	I-chemical
ion	O
at	O
distances	O
of	O
4	O
.	O
86	O
and	O
4	O
.	O
23	O
Å	O
.	O
These	O
residues	O
are	O
ordered	O
in	O
only	O
one	O
of	O
the	O
two	O
dimers	B-oligomeric_state
in	O
the	O
K	B-species
.	I-species
pneumoniae	I-species
crystal	B-evidence
(	O
the	O
one	O
in	O
which	O
the	O
C7	O
group	O
is	O
pointing	O
towards	O
the	O
DNA	B-chemical
away	O
from	O
ParE	B-protein
,	O
although	O
the	O
conformations	O
of	O
these	O
two	O
groups	O
on	O
the	O
drug	O
are	O
probably	O
not	O
correlated	O
).	O

The	O
topoisomerase	B-complex_assembly
IV	I-complex_assembly
ParE27	B-complex_assembly
-	I-complex_assembly
ParC55	I-complex_assembly
fusion	O
protein	O
from	O
K	B-species
.	I-species
pneumoniae	I-species
was	O
fully	O
active	O
in	O
promoting	O
levofloxacin	B-chemical
-	O
mediated	O
cleavage	O
of	O
DNA	B-chemical
(	O
Fig	O
.	O
6	O
▸).	O

In	O
the	O
absence	B-protein_state
of	I-protein_state
the	O
drug	B-chemical
and	O
ATP	B-chemical
,	O
the	O
protein	O
converted	O
supercoiled	O
pBR322	O
into	O
a	O
ladder	O
of	O
bands	O
corresponding	O
to	O
relaxed	O
DNA	B-chemical
.	O

The	O
inclusion	O
of	O
levofloxacin	B-chemical
produced	O
linear	O
DNA	B-chemical
in	O
a	O
dose	O
-	O
dependent	O
and	O
ATP	B-chemical
-	O
independent	O
fashion	O
.	O

Interestingly	O
,	O
K	B-species
.	I-species
pneumoniae	I-species
strains	O
are	O
much	O
more	O
susceptible	O
to	O
levofloxacin	B-chemical
than	O
S	B-species
.	I-species
pneumoniae	I-species
,	O
with	O
typical	O
MIC	O
values	O
of	O
0	O
.	O
016	O
and	O
1	O
mg	O
l	O
−	O
1	O
,	O
respectively	O
(	O
Odenholt	O
&	O
Cars	O
,	O
2006	O
),	O
reflecting	O
differences	O
in	O
multiple	O
factors	O
(	O
in	O
addition	O
to	O
binding	B-evidence
affinity	I-evidence
)	O
that	O
influence	O
drug	O
responses	O
,	O
including	O
membrane	O
,	O
peptidoglycan	O
structure	O
,	O
drug	O
-	O
uptake	O
and	O
efflux	O
mechanisms	O
.	O

In	O
summary	O
,	O
we	O
have	O
determined	O
the	O
first	O
structure	B-evidence
of	O
a	O
quinolone	B-chemical
–	O
DNA	B-chemical
cleavage	O
complex	O
involving	O
a	O
type	B-protein_type
II	I-protein_type
topo	I-protein_type
­	I-protein_type
isomerase	I-protein_type
from	O
K	B-species
.	I-species
pneumoniae	I-species
.	O

Protein	O
and	O
DNA	B-chemical
used	O
in	O
the	O
co	B-experimental_method
-	I-experimental_method
crystallization	I-experimental_method
experiment	O
.	O

(	O
a	O
)	O
Coloured	O
diagram	O
of	O
the	O
protein	O
constructs	O
used	O
in	O
crystallization	B-experimental_method
.	O

(	O
b	O
)	O
DNA	B-chemical
sequences	O
used	O
in	O
crystallization	B-experimental_method
.	O

Chemical	O
structure	O
of	O
levofloxacin	B-chemical
(	O
a	O
)	O
and	O
its	O
conformations	O
observed	O
within	O
the	O
active	B-site
sites	I-site
of	O
S	B-species
.	I-species
pneumoniae	I-species
topoisomerase	B-complex_assembly
IV	I-complex_assembly
(	O
b	O
)	O
and	O
K	B-species
.	I-species
pneumoniae	I-species
topoisomerase	B-complex_assembly
IV	I-complex_assembly
(	O
c	O
,	O
d	O
).	O

Electron	B-evidence
-	I-evidence
density	I-evidence
maps	I-evidence
(	O
2F	O
obs	O
−	O
F	O
calc	O
)	O
are	O
shown	O
as	O
meshes	O
for	O
the	O
drug	O
molecules	O
contoured	O
at	O
1	O
.	O
5σ	O
and	O
are	O
limited	O
to	O
a	O
distance	O
of	O
2	O
.	O
3	O
Å	O
from	O
the	O
drug	O
atoms	O
.	O

Overall	O
orthogonal	O
views	O
of	O
the	O
cleavage	O
complex	O
of	O
topoisomerase	B-complex_assembly
IV	I-complex_assembly
from	O
K	B-species
.	I-species
pneumoniae	I-species
in	O
surface	O
(	O
left	O
)	O
and	O
cartoon	O
(	O
right	O
)	O
representations	O
.	O

The	O
ParC	B-protein
subunit	O
is	O
in	O
blue	O
,	O
ParE	B-protein
is	O
in	O
yellow	O
and	O
DNA	B-chemical
is	O
in	O
cyan	O
.	O

The	O
bound	B-protein_state
quinolone	B-chemical
molecules	O
(	O
levofloxacin	B-chemical
)	O
are	O
in	O
red	O
and	O
are	O
shown	O
using	O
van	O
der	O
Waals	O
representation	O
.	O

Schematic	O
representation	O
of	O
the	O
catalytic	O
cycle	O
of	O
type	B-protein_type
II	I-protein_type
topoisomerases	I-protein_type
.	O

The	O
ParC	B-protein
N	O
-	O
terminal	O
domain	O
(	O
ParC55	B-protein
)	O
is	O
in	O
grey	O
,	O
the	O
ParC	B-protein
C	O
-	O
terminal	O
β	B-structure_element
-­	I-structure_element
pinwheel	I-structure_element
domain	I-structure_element
is	O
in	O
silver	O
,	O
the	O
ParE	B-protein
N	O
-	O
terminal	O
ATPase	B-structure_element
domain	I-structure_element
is	O
in	O
red	O
,	O
the	O
ParE	B-protein
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
(	O
ParE30	B-protein
)	O
is	O
in	O
yellow	O
,	O
the	O
G	B-structure_element
-	I-structure_element
gate	I-structure_element
DNA	B-chemical
is	O
in	O
green	O
and	O
the	O
T	B-structure_element
-	I-structure_element
segment	I-structure_element
DNA	B-chemical
is	O
in	O
purple	O
.	O

Detailed	O
views	O
of	O
the	O
active	B-site
sites	I-site
of	O
topoisomerase	B-complex_assembly
IV	I-complex_assembly
from	O
S	B-species
.	I-species
pneumoniae	I-species
and	O
K	B-species
.	I-species
pneumoniae	I-species
with	O
quinolone	B-chemical
molecules	O
bound	B-protein_state
.	O

The	O
DNA	B-chemical
is	O
shown	O
in	O
silver	O
/	O
cyan	O
.	O

Supercoiled	O
plasmid	O
pBR322	O
(	O
400	O
ng	O
)	O
was	O
incubated	O
with	O
topoisomerase	B-complex_assembly
IV	I-complex_assembly
proteins	O
(	O
400	O
ng	O
)	O
in	O
the	O
absence	O
or	O
presence	B-protein_state
of	I-protein_state
levofloxacin	B-chemical
at	O
the	O
indicated	O
concentrations	O
.	O

Cryo	B-experimental_method
-	I-experimental_method
electron	I-experimental_method
microscopy	I-experimental_method
(	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
)	O
methods	O
are	O
now	O
being	O
used	O
to	O
determine	O
structures	B-evidence
at	O
near	O
-	O
atomic	O
resolution	O
and	O
have	O
great	O
promise	O
in	O
molecular	O
pharmacology	O
,	O
especially	O
in	O
the	O
context	O
of	O
mapping	O
the	O
binding	O
of	O
small	O
-	O
molecule	O
ligands	O
to	O
protein	O
complexes	O
that	O
display	O
conformational	O
flexibility	O
.	O

Dysregulation	O
of	O
GDH	B-protein_type
leads	O
to	O
a	O
variety	O
of	O
metabolic	O
and	O
neurologic	O
disorders	O
.	O

We	O
show	O
that	O
the	O
binding	O
of	O
the	O
coenzyme	O
NADH	B-chemical
alone	O
or	O
in	O
concert	O
with	O
GTP	B-chemical
results	O
in	O
a	O
binary	O
mixture	O
in	O
which	O
the	O
enzyme	O
is	O
in	O
either	O
an	O
“	O
open	B-protein_state
”	O
or	O
“	O
closed	B-protein_state
”	O
state	O
.	O

Whereas	O
the	O
structure	B-evidence
of	O
NADH	B-chemical
in	O
the	O
active	B-site
site	I-site
is	O
similar	O
between	O
the	O
open	B-protein_state
and	O
closed	B-protein_state
states	O
,	O
it	O
is	O
unexpectedly	O
different	O
at	O
the	O
regulatory	B-site
site	I-site
.	O

Our	O
studies	O
thus	O
demonstrate	O
that	O
even	O
in	O
instances	O
when	O
there	O
is	O
considerable	O
structural	O
information	O
available	O
from	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
,	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
methods	O
can	O
provide	O
useful	O
complementary	O
insights	O
into	O
regulatory	O
mechanisms	O
for	O
dynamic	O
protein	O
complexes	O
.	O

Recent	O
advances	O
in	O
cryo	B-experimental_method
-	I-experimental_method
electron	I-experimental_method
microscopy	I-experimental_method
(	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
)	O
allow	O
determination	O
of	O
structures	B-evidence
of	O
small	O
protein	O
complexes	O
and	O
membrane	O
proteins	O
at	O
near	O
-	O
atomic	O
resolution	O
,	O
marking	O
a	O
critical	O
shift	O
in	O
the	O
structural	O
biology	O
field	O
.	O

One	O
specific	O
area	O
of	O
broad	O
general	O
interest	O
in	O
drug	O
discovery	O
is	O
the	O
localization	O
of	O
bound	O
ligands	O
and	O
cofactors	O
under	O
conditions	O
in	O
which	O
efforts	O
at	O
crystallization	B-experimental_method
have	O
not	O
been	O
successful	O
because	O
of	O
structural	O
heterogeneity	O
.	O

Recent	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
analyses	O
have	O
already	O
demonstrated	O
that	O
it	O
is	O
now	O
possible	O
to	O
use	O
single	B-experimental_method
-	I-experimental_method
particle	I-experimental_method
cryo	I-experimental_method
-	I-experimental_method
EM	I-experimental_method
methods	O
to	O
localize	O
small	O
bound	O
ligands	O
or	O
inhibitors	O
on	O
target	O
proteins	O
.	O

Here	O
,	O
we	O
address	O
this	O
question	O
using	O
mammalian	B-taxonomy_domain
glutamate	B-protein_type
dehydrogenase	I-protein_type
as	O
an	O
example	O
.	O

Glutamate	B-protein_type
dehydrogenase	I-protein_type
(	O
GDH	B-protein_type
)	O
is	O
a	O
highly	B-protein_state
conserved	I-protein_state
enzyme	O
expressed	O
in	O
most	O
organisms	O
.	O

GDH	B-protein_type
plays	O
a	O
central	O
role	O
in	O
glutamate	B-chemical
metabolism	O
by	O
catalyzing	O
the	O
reversible	O
oxidative	O
deamination	O
of	O
glutamate	B-chemical
to	O
generate	O
α	B-chemical
-	I-chemical
ketoglutarate	I-chemical
and	O
ammonia	B-chemical
,	O
with	O
the	O
concomitant	O
transfer	O
of	O
a	O
pair	O
of	O
electrons	O
to	O
either	O
NAD	B-chemical
+	I-chemical
or	O
NADP	B-chemical
+.	I-chemical

Regulation	O
of	O
GDH	B-protein_type
is	O
tightly	O
controlled	O
through	O
multiple	O
allosteric	O
mechanisms	O
.	O

Extensive	O
biochemical	B-experimental_method
and	I-experimental_method
crystallographic	I-experimental_method
studies	I-experimental_method
have	O
characterized	O
the	O
enzymatic	O
activity	O
of	O
GDH	B-protein_type
and	O
its	O
modulation	O
by	O
a	O
chemically	O
diverse	O
group	O
of	O
compounds	O
such	O
as	O
nucleotides	O
,	O
amino	O
acids	O
,	O
steroid	O
hormones	O
,	O
antipsychotic	O
drugs	O
,	O
and	O
natural	O
products	O
.	O

The	O
second	O
,	O
a	O
nucleotide	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
(	O
NBD	B-structure_element
)	O
with	O
a	O
Rossmann	B-structure_element
fold	I-structure_element
,	O
defines	O
one	O
face	O
of	O
the	O
catalytic	B-site
cleft	I-site
bounded	O
by	O
the	O
core	O
domain	O
.	O

During	O
the	O
catalytic	O
cycle	O
,	O
the	O
NBD	B-structure_element
executes	O
a	O
large	O
movement	O
,	O
hinged	O
around	O
a	O
“	B-structure_element
pivot	I-structure_element
”	I-structure_element
helix	I-structure_element
,	O
that	O
closes	O
the	O
catalytic	B-site
cleft	I-site
,	O
and	O
drives	O
a	O
large	O
conformational	O
change	O
in	O
the	O
hexamer	B-oligomeric_state
from	O
open	B-protein_state
to	O
closed	B-protein_state
states	O
(	O
Fig	O
.	O
1B	O
).	O

The	O
third	O
domain	O
,	O
dubbed	O
the	O
“	O
antenna	B-structure_element
,”	O
is	O
an	O
evolutionary	O
acquisition	O
in	O
protista	B-taxonomy_domain
and	O
animals	B-taxonomy_domain
.	O

Antennae	B-structure_element
of	O
adjacent	O
protomers	B-oligomeric_state
in	O
each	O
trimer	B-oligomeric_state
intercalate	O
to	O
form	O
a	O
bundle	O
,	O
perpendicular	O
to	O
the	O
pivot	B-structure_element
helices	I-structure_element
,	O
that	O
protrudes	O
along	O
the	O
distal	O
extremes	O
of	O
the	O
3	O
-	O
fold	O
axis	O
.	O

When	O
a	O
protomer	B-oligomeric_state
undergoes	O
a	O
conformational	O
change	O
,	O
the	O
rotation	O
of	O
its	O
pivot	B-structure_element
helix	I-structure_element
is	O
transferred	O
through	O
the	O
antenna	B-structure_element
to	O
the	O
adjacent	O
subunit	B-structure_element
.	O

The	O
influence	O
of	O
the	O
antenna	B-structure_element
,	O
present	O
only	O
in	O
protozoan	B-taxonomy_domain
and	O
metazoan	B-taxonomy_domain
enzymes	O
,	O
has	O
been	O
proposed	O
to	O
explain	O
its	O
cooperative	O
behavior	O
,	O
which	O
is	O
absent	O
in	O
bacterial	B-taxonomy_domain
homologs	O
.	O

Only	O
three	O
protomers	B-oligomeric_state
are	O
shown	O
in	O
the	O
top	O
view	O
for	O
purposes	O
of	O
visual	O
clarity	O
.	O

The	O
transition	O
between	O
“	O
closed	B-protein_state
”	O
and	O
“	O
open	B-protein_state
”	O
states	O
of	O
GDH	B-protein_type
is	O
modulated	O
by	O
two	O
allosteric	B-site
sites	I-site
in	O
each	O
protomer	B-oligomeric_state
(	O
Fig	O
.	O
1A	O
),	O
which	O
are	O
differentially	O
bound	B-protein_state
by	I-protein_state
GTP	B-chemical
(	O
an	O
inhibitor	O
)	O
and	O
ADP	B-chemical
(	O
an	O
activator	O
).	O

In	O
the	O
first	O
site	O
,	O
which	O
sits	O
next	O
to	O
the	O
pivot	B-structure_element
helix	I-structure_element
at	O
the	O
base	O
of	O
the	O
antenna	B-structure_element
(	O
the	O
“	O
GTP	B-site
binding	I-site
site	I-site
”),	O
GTP	B-chemical
binding	O
is	O
known	O
to	O
act	O
as	O
an	O
inhibitor	O
,	O
preventing	O
release	O
of	O
the	O
reaction	O
product	O
from	O
the	O
catalytic	B-site
site	I-site
by	O
stabilizing	O
the	O
closed	B-protein_state
conformation	O
of	O
the	O
catalytic	B-site
cleft	I-site
.	O

In	O
the	O
second	O
“	O
regulatory	B-site
site	I-site
”,	O
which	O
is	O
situated	O
near	O
the	O
pivot	B-structure_element
helix	I-structure_element
between	O
adjacent	O
protomers	B-oligomeric_state
,	O
ADP	B-chemical
acts	O
as	O
an	O
activator	O
of	O
enzymatic	O
activity	O
,	O
presumably	O
by	O
hastening	O
the	O
opening	O
of	O
the	O
catalytic	B-site
cleft	I-site
that	O
leads	O
to	O
the	O
release	O
of	O
the	O
reaction	O
product	O
.	O

Interestingly	O
,	O
it	O
has	O
also	O
been	O
shown	O
that	O
the	O
coenzyme	O
NADH	B-chemical
can	O
bind	O
to	O
the	O
regulatory	B-site
site	I-site
(	O
also	O
bound	B-protein_state
by	I-protein_state
the	O
activator	O
ADP	B-chemical
),	O
exerting	O
a	O
converse	O
,	O
inhibitory	O
effect	O
on	O
GDH	B-protein_type
product	O
release	O
,	O
although	O
the	O
role	O
this	O
may	O
play	O
in	O
vivo	O
is	O
not	O
entirely	O
clear	O
.	O

Nearly	O
all	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
structures	B-evidence
of	O
mammalian	B-taxonomy_domain
GDH	B-protein_type
are	O
in	O
the	O
closed	B-protein_state
conformation	O
,	O
and	O
the	O
few	O
structures	B-evidence
that	O
are	O
in	O
the	O
open	B-protein_state
conformation	O
are	O
at	O
lower	O
resolution	O
(	O
Table	O
1	O
).	O

Of	O
those	O
structures	B-evidence
in	O
the	O
closed	B-protein_state
conformation	O
,	O
most	O
include	O
NAD	B-chemical
[	I-chemical
P	I-chemical
]	I-chemical
H	I-chemical
,	O
GTP	B-chemical
,	O
and	O
glutamate	B-chemical
(	O
or	O
,	O
alternately	O
,	O
NAD	B-chemical
+,	I-chemical
GTP	B-chemical
,	O
and	O
α	B-chemical
-	I-chemical
ketoglutarate	I-chemical
).	O

However	O
,	O
the	O
effects	O
of	O
coenzyme	O
and	O
GTP	B-chemical
,	O
bound	B-protein_state
alone	I-protein_state
or	O
in	O
concert	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
glutamate	B-chemical
,	O
have	O
not	O
been	O
analyzed	O
by	O
crystallographic	O
methods	O
.	O

Here	O
,	O
we	O
report	O
single	B-experimental_method
-	I-experimental_method
particle	I-experimental_method
cryo	I-experimental_method
-	I-experimental_method
electron	I-experimental_method
microscopy	I-experimental_method
(	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
)	O
studies	O
that	O
show	O
that	O
under	O
these	O
conditions	O
enzyme	O
complexes	O
coexist	O
in	O
both	O
closed	B-protein_state
and	O
open	B-protein_state
conformations	O
.	O

We	O
show	O
that	O
the	O
structures	B-evidence
in	O
both	O
states	O
can	O
be	O
resolved	O
at	O
near	O
-	O
atomic	O
resolution	O
,	O
suggesting	O
a	O
molecular	O
mechanism	O
for	O
synergistic	O
inhibition	O
of	O
GDH	B-protein_type
by	O
NADH	B-chemical
and	O
GTP	B-chemical
(	O
see	O
Table	O
2	O
for	O
detailed	O
information	O
on	O
all	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
-	O
derived	O
structures	B-evidence
that	O
we	O
report	O
in	O
this	O
work	O
).	O

X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
structures	B-evidence
of	O
mammalian	B-taxonomy_domain
GDH	B-protein_type
reported	O
in	O
both	O
the	O
open	B-protein_state
and	O
closed	B-protein_state
conformations	O

GDH	B-protein
Ligands	O
PDB	O
ID	O
Conformation	O
Resolution	O
WT	B-protein_state
NADH	B-chemical
+	O
GLU	B-chemical
+	O
GTP	B-chemical
3MW9	O
Closed	B-protein_state
2	O
.	O
4	O
WT	B-protein_state
Glu	B-chemical
,	O
GTP	B-chemical
,	O
NADPH	B-chemical
,	O
and	O
Bithionol	O
3ETD	O
Closed	B-protein_state
2	O
.	O
5	O
WT	B-protein_state
Glu	B-chemical
,	O
NADPH	B-chemical
,	O
GTP	B-chemical
+	O
GW5074	O
3ETG	O
Closed	B-protein_state
2	O
.	O
5	O
WT	B-protein_state
apo	B-protein_state
1L1F	O
Open	B-protein_state
2	O
.	O
7	O
WT	B-protein_state
NADPH	B-chemical
,	O
glutamate	B-chemical
,	O
and	O
GTP	B-chemical
1HWZ	O
Closed	B-protein_state
2	O
.	O
8	O
WT	B-protein_state
NADPH	B-chemical
+	O
GLU	B-chemical
+	O
GTP	B-chemical
+	O
Zinc	B-chemical
3MVQ	O
Closed	B-protein_state
2	O
.	O
94	O
WT	B-protein_state
NADPH	B-chemical
,	O
Glu	B-chemical
,	O
GTP	B-chemical
,	O
Hexachlorophene	O
3ETE	O
Closed	B-protein_state
3	O
WT	B-protein_state
NAD	B-chemical
,	O
PO4	B-chemical
,	O
and	O
2	B-chemical
-	I-chemical
oxoglutarate	I-chemical
1HWY	O
Closed	B-protein_state
3	O
.	O
2	O
WT	B-protein_state
NADPH	B-chemical
+	O
GLU	B-chemical
+	O
Eu	O
3MVO	O
Closed	B-protein_state
3	O
.	O
23	O
R463A	O
mutant	B-protein_state
apo	B-protein_state
1NR1	O
Open	B-protein_state
3	O
.	O
3	O
WT	B-protein_state
apo	B-protein_state
1NR7	O
Open	B-protein_state
3	O
.	O
3	O
WT	B-protein_state
ADP	B-chemical
1NQT	O
Open	B-protein_state
3	O
.	O
5	O
WT	B-protein_state
NADPH	B-chemical
and	O
Epicatechin	O
-	O
3	O
-	O
gallate	O
(	O
Ecg	O
)	O
3QMU	O
Open	B-protein_state
3	O
.	O
62	O

Cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
structures	B-evidence
of	O
mammalian	B-taxonomy_domain
GDH	B-protein_type
determined	O
for	O
this	O
study	O

GDH	B-protein_type
Ligands	O
EMDB	O
ID	O
PDB	O
ID	O
Conformation	O
Resolution	O
Particles	O
WT	B-protein_state
apo	B-protein_state
EMD	O
-	O
6630	O
3JCZ	O
Open	B-protein_state
3	O
.	O
26	O
22462	O
WT	B-protein_state
GTP	B-chemical
EMD	O
-	O
6631	O
3JD0	O
Open	B-protein_state
3	O
.	O
47	O
39439	O
WT	B-protein_state
NADH	B-chemical
EMD	O
-	O
6635	O
3JD2	O
Open	B-protein_state
3	O
.	O
27	O
34716	O
WT	B-protein_state
NADH	B-chemical
EMD	O
-	O
6634	O
3JD1	O
Closed	B-protein_state
3	O
.	O
27	O
34926	O
WT	B-protein_state
NADH	B-chemical
+	O
GTP	B-chemical
EMD	O
-	O
6632	O
3JD3	O
Open	B-protein_state
3	O
.	O
55	O
14793	O
WT	B-protein_state
NADH	B-chemical
+	O
GTP	B-chemical
EMD	O
-	O
6633	O
3JD4	O
Closed	B-protein_state
3	O
.	O
40	O
20429	O

To	O
explore	O
the	O
conformational	O
landscape	O
of	O
apo	B-protein_state
-	O
GDH	B-protein
,	O
we	O
first	O
determined	O
its	O
structure	B-evidence
in	O
the	O
absence	B-protein_state
of	I-protein_state
any	O
added	O
ligands	O
(	O
Supplemental	O
Fig	O
.	O
1	O
,	O
Fig	O
.	O
2	O
,	O
A	O
–	O
C	O
).	O

The	O
density	B-evidence
map	I-evidence
,	O
refined	O
to	O
an	O
average	O
resolution	O
of	O
∼	O
3	O
.	O
0	O
Å	O
(	O
Supplemental	O
Fig	O
.	O
2	O
),	O
is	O
in	O
the	O
open	B-protein_state
conformation	O
and	O
closely	O
matches	O
the	O
model	O
of	O
unliganded	B-protein_state
GDH	B-protein
derived	O
by	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
at	O
3	O
.	O
3	O
Å	O
resolution	O
(	O
PDB	O
ID	O
1NR7	O
).	O

The	O
variation	O
in	O
local	O
resolution	O
from	O
the	O
core	O
to	O
the	O
periphery	O
,	O
as	O
reported	O
by	O
ResMap	B-experimental_method
(	O
Supplemental	O
Fig	O
.	O
3D	O
),	O
is	O
consistent	O
with	O
the	O
B	B-evidence
-	I-evidence
factor	I-evidence
gradient	I-evidence
observed	O
in	O
the	O
crystal	B-evidence
structure	I-evidence
(	O
Supplemental	O
Fig	O
.	O
3A	O
).	O

Extensive	O
classification	O
without	O
imposing	O
symmetry	O
yielded	O
only	O
open	B-protein_state
structures	B-evidence
and	O
failed	O
to	O
detect	O
any	O
closed	B-protein_state
catalytic	B-site
cleft	I-site
in	O
the	O
unliganded	B-protein_state
enzyme	O
,	O
suggesting	O
that	O
all	O
six	O
protomers	B-oligomeric_state
are	O
in	O
the	O
open	B-protein_state
conformation	O
.	O

Consistent	O
with	O
this	O
conclusion	O
,	O
the	O
loops	B-structure_element
connecting	O
the	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
of	O
the	O
Rossmann	B-structure_element
fold	I-structure_element
are	O
well	O
-	O
defined	O
(	O
Fig	O
.	O
2B	O
),	O
implying	O
that	O
there	O
is	O
little	O
movement	O
at	O
the	O
NBD	B-structure_element
,	O
as	O
the	O
transition	O
between	O
closed	B-protein_state
and	O
open	B-protein_state
states	O
is	O
associated	O
with	O
NBD	B-structure_element
movement	O
(	O
Fig	O
.	O
1B	O
).	O

Cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
structures	B-evidence
of	O
GDH	B-protein
in	O
unliganded	B-protein_state
and	O
NADH	B-protein_state
-	I-protein_state
bound	I-protein_state
states	O
.	O
(	O
A	O
)	O
Refined	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
map	B-evidence
of	O
unliganded	B-protein_state
GDH	B-protein
at	O
∼	O
3	O
Å	O
resolution	O
.	O

(	O
B	O
,	O
C	O
)	O
Illustration	O
of	O
density	B-evidence
map	I-evidence
in	O
the	O
regions	O
that	O
contain	O
the	O
Rossmann	B-structure_element
nucleotide	I-structure_element
binding	I-structure_element
fold	I-structure_element
(	O
B	O
),	O
pivot	B-structure_element
and	I-structure_element
antenna	I-structure_element
helices	I-structure_element
(	O
C	O
)	O
in	O
the	O
unliganded	B-protein_state
GDH	B-protein
map	B-evidence
.	O
(	O
D	O
)	O
Cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
-	O
derived	O
density	B-evidence
maps	I-evidence
for	O
two	O
coexisting	O
conformations	O
that	O
are	O
present	O
when	O
GDH	B-protein
is	O
bound	B-protein_state
to	I-protein_state
the	O
cofactor	O
NADH	B-chemical
.	O

Each	O
protomer	B-oligomeric_state
is	O
shown	O
in	O
a	O
different	O
color	O
and	O
densities	B-evidence
for	O
NADH	B-chemical
bound	B-protein_state
in	I-protein_state
both	O
regulatory	B-site
(	O
red	O
)	O
and	O
catalytic	B-site
(	O
purple	O
)	O
sites	B-site
on	O
one	O
protomer	B-oligomeric_state
are	O
indicated	O
.	O

The	O
overall	O
quaternary	O
structures	O
of	O
the	O
two	O
conformations	O
are	O
essentially	O
the	O
same	O
as	O
that	O
of	O
the	O
open	B-protein_state
and	O
closed	B-protein_state
states	O
observed	O
by	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
.	O

To	O
test	O
the	O
effect	O
of	O
NADH	B-chemical
binding	O
on	O
GDH	B-protein
conformation	O
in	O
solution	O
,	O
we	O
determined	O
the	O
structure	B-evidence
of	O
this	O
binary	O
complex	O
using	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
methods	O
combined	O
with	O
three	B-experimental_method
-	I-experimental_method
dimensional	I-experimental_method
classification	I-experimental_method
.	O

Two	O
dominant	O
conformational	O
states	O
,	O
in	O
an	O
all	O
open	B-protein_state
or	O
all	O
closed	B-protein_state
conformation	O
were	O
detected	O
,	O
segregated	O
(	O
Fig	O
.	O
2D	O
),	O
and	O
further	O
refined	O
to	O
near	O
-	O
atomic	O
resolution	O
(∼	O
3	O
.	O
3	O
Å	O
;	O
Supplemental	O
Fig	O
.	O
2	O
).	O

Densities	B-evidence
for	O
12	O
molecules	O
of	O
bound	B-protein_state
NADH	B-chemical
were	O
identified	O
in	O
maps	B-evidence
of	O
both	O
open	B-protein_state
and	O
closed	B-protein_state
states	O
(	O
Supplemental	O
Fig	O
.	O
4	O
).	O

The	O
NADH	B-protein_state
-	I-protein_state
bound	I-protein_state
closed	B-protein_state
conformation	O
matches	O
the	O
structure	B-evidence
of	O
the	O
quaternary	O
complex	O
observed	O
by	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
,	O
with	O
the	O
exception	O
that	O
density	B-evidence
corresponding	O
to	O
GTP	B-chemical
and	O
glutamate	B-chemical
was	O
absent	O
in	O
the	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
-	O
derived	O
map	B-evidence
.	O

Thus	O
,	O
closure	O
of	O
the	O
catalytic	B-site
cleft	I-site
is	O
accompanied	O
by	O
a	O
quaternary	O
structural	O
change	O
that	O
can	O
be	O
described	O
as	O
a	O
global	O
bending	O
of	O
the	O
structure	B-evidence
about	O
an	O
axis	O
that	O
runs	O
parallel	O
to	O
the	O
pivot	B-structure_element
helix	I-structure_element
,	O
accompanied	O
by	O
an	O
expansion	O
of	O
the	O
core	O
(	O
Figs	O
.	O
1A	O
and	O
2D	O
).	O

Detailed	O
analysis	O
of	O
the	O
GDH	B-complex_assembly
/	I-complex_assembly
NADH	I-complex_assembly
structures	B-evidence
shows	O
that	O
both	O
the	O
adenosine	O
and	O
nicotinamide	O
moieties	O
of	O
NADH	B-chemical
bind	O
to	O
the	O
catalytic	B-site
site	I-site
within	O
the	O
NBD	B-structure_element
in	O
nearly	O
the	O
same	O
orientation	O
in	O
both	O
the	O
open	B-protein_state
and	O
the	O
closed	B-protein_state
states	O
,	O
and	O
display	O
closely	O
comparable	O
interactions	O
with	O
the	O
Rossmann	B-structure_element
fold	I-structure_element
(	O
Fig	O
.	O
3	O
,	O
A	O
and	O
B	O
).	O

At	O
the	O
regulatory	B-site
site	I-site
,	O
where	O
either	O
ADP	B-chemical
can	O
bind	O
as	O
an	O
activator	O
or	O
NADH	B-chemical
can	O
bind	O
as	O
an	O
inhibitor	O
,	O
the	O
binding	O
of	O
the	O
adenine	O
moiety	O
of	O
NADH	B-chemical
is	O
nearly	O
identical	O
between	O
the	O
two	O
conformers	O
.	O

In	O
the	O
closed	B-protein_state
state	O
,	O
the	O
nicotinamide	O
group	O
is	O
oriented	O
toward	O
the	O
center	O
of	O
the	O
hexamer	B-oligomeric_state
,	O
inserted	O
into	O
a	O
narrow	O
cavity	B-site
between	O
two	O
adjacent	O
subunits	B-structure_element
in	O
the	O
trimer	B-oligomeric_state
.	O

There	O
are	O
extensive	O
interactions	O
between	O
NADH	B-chemical
and	O
the	O
residues	O
lining	O
this	O
cavity	B-site
,	O
which	O
may	O
explain	O
the	O
well	O
-	O
defined	O
density	B-evidence
of	O
this	O
portion	O
of	O
NADH	B-chemical
in	O
the	O
closed	B-protein_state
state	O
.	O

Detailed	O
view	O
of	O
NADH	B-chemical
conformation	O
in	O
catalytic	B-site
and	I-site
regulatory	I-site
sites	I-site
.	O
(	O
A	O
,	O
B	O
)	O
NADH	B-chemical
density	B-evidence
(	O
purple	O
)	O
and	O
interactions	O
in	O
the	O
catalytic	B-site
sites	I-site
of	O
closed	B-protein_state
(	O
A	O
)	O
and	O
open	B-protein_state
(	O
B	O
)	O
states	O
.	O
(	O
C	O
,	O
D	O
)	O
NADH	B-chemical
density	B-evidence
(	O
red	O
)	O
and	O
interactions	O
in	O
the	O
regulatory	B-site
sites	I-site
of	O
closed	B-protein_state
(	O
C	O
)	O
and	O
open	B-protein_state
(	O
D	O
)	O
states	O
.	O

In	O
the	O
open	B-protein_state
state	O
,	O
the	O
binding	O
of	O
ADP	B-chemical
or	O
NADH	B-chemical
is	O
further	O
stabilized	O
by	O
His209	B-residue_name_number
,	O
a	O
residue	O
that	O
undergoes	O
a	O
large	O
movement	O
during	O
the	O
transition	O
from	O
open	B-protein_state
to	O
closed	B-protein_state
conformation	O
(	O
Fig	O
.	O
3	O
,	O
C	O
and	O
D	O
).	O

In	O
the	O
closed	B-protein_state
conformation	O
,	O
however	O
,	O
this	O
key	O
histidine	B-residue_name
residue	O
is	O
>	O
10	O
.	O
5	O
Å	O
away	O
from	O
the	O
nearest	O
phosphate	O
group	O
on	O
NADH	B-chemical
,	O
altering	O
a	O
critical	O
stabilization	O
point	O
within	O
the	O
regulatory	B-site
site	I-site
.	O

In	O
the	O
absence	B-protein_state
of	I-protein_state
NADH	B-chemical
,	O
GTP	B-chemical
binds	O
weakly	O
to	O
GDH	B-protein
with	O
a	O
dissociation	B-evidence
constant	I-evidence
of	O
∼	O
20	O
μM	O
.	O
Cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
analysis	O
of	O
GDH	B-protein
incubated	B-protein_state
with	I-protein_state
GTP	B-chemical
resulted	O
in	O
a	O
structure	B-evidence
at	O
an	O
overall	O
resolution	O
of	O
3	O
.	O
5	O
Å	O
,	O
showing	O
that	O
it	O
is	O
in	O
an	O
open	B-protein_state
conformation	O
(	O
Supplemental	O
Fig	O
.	O
6	O
),	O
with	O
all	O
NBDs	B-structure_element
in	O
the	O
open	B-protein_state
state	O
.	O

The	O
density	B-evidence
for	O
GTP	B-chemical
is	O
not	O
very	O
well	O
defined	O
,	O
suggesting	O
considerable	O
wobble	O
in	O
the	O
binding	B-site
site	I-site
.	O

Subtraction	B-experimental_method
of	O
the	O
GTP	B-protein_state
-	I-protein_state
bound	I-protein_state
map	B-evidence
with	O
that	O
of	O
the	O
apo	B-protein_state
state	O
shows	O
that	O
GTP	B-chemical
binding	O
can	O
nevertheless	O
be	O
visualized	O
specifically	O
in	O
the	O
GTP	B-site
binding	I-site
site	I-site
(	O
Supplemental	O
Fig	O
.	O
6	O
).	O

To	O
further	O
dissect	O
the	O
roles	O
of	O
NADH	B-chemical
and	O
GTP	B-chemical
in	O
the	O
transition	O
from	O
the	O
open	B-protein_state
to	O
closed	B-protein_state
conformations	O
,	O
we	O
next	O
determined	B-experimental_method
structures	B-evidence
of	O
GDH	B-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
both	O
NADH	B-chemical
and	O
GTP	B-chemical
,	O
but	O
without	B-protein_state
glutamate	B-chemical
.	O

Reconstruction	B-experimental_method
without	I-experimental_method
classification	I-experimental_method
,	O
however	O
,	O
yields	O
a	O
structure	B-evidence
clearly	O
in	O
the	O
closed	B-protein_state
conformation	O
,	O
suggesting	O
that	O
,	O
in	O
coordination	O
with	O
NADH	B-chemical
,	O
GTP	B-chemical
may	O
further	O
stabilize	O
the	O
closed	B-protein_state
conformation	O
.	O

The	O
location	O
of	O
GTP	B-chemical
in	O
the	O
open	B-protein_state
and	O
closed	B-protein_state
states	O
of	O
the	O
GDH	B-complex_assembly
/	I-complex_assembly
NADH	I-complex_assembly
/	I-complex_assembly
GTP	I-complex_assembly
complex	O
is	O
similar	O
to	O
that	O
in	O
the	O
crystal	B-evidence
structure	I-evidence
observed	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
NADH	B-chemical
,	O
GTP	B-chemical
,	O
and	O
glutamate	B-chemical
.	O

Likewise	O
,	O
the	O
position	O
of	O
NADH	B-chemical
in	O
the	O
open	B-protein_state
and	O
closed	B-protein_state
states	O
closely	O
resembles	O
the	O
position	O
of	O
NADH	B-chemical
in	O
the	O
GDH	B-complex_assembly
/	I-complex_assembly
NADH	I-complex_assembly
open	B-protein_state
and	O
closed	B-protein_state
structures	B-evidence
.	O

One	O
key	O
difference	O
between	O
the	O
open	B-protein_state
and	O
closed	B-protein_state
states	O
of	O
these	O
structures	B-evidence
is	O
the	O
position	O
of	O
the	O
His209	B-residue_name_number
residue	O
:	O
As	O
mentioned	O
above	O
,	O
His209	B-residue_name_number
swings	O
away	O
from	O
the	O
adenine	O
moiety	O
of	O
NADH	B-chemical
in	O
the	O
closed	B-protein_state
state	O
.	O

Thus	O
,	O
GTP	B-chemical
binding	O
to	O
GDH	B-protein
appears	O
synergistic	O
with	O
NADH	B-chemical
and	O
displaces	O
the	O
conformational	O
landscape	O
toward	O
the	O
closed	B-protein_state
state	O
.	O

(	O
A	O
,	O
B	O
)	O
Observation	O
of	O
co	O
-	O
existing	O
open	B-protein_state
(	O
A	O
)	O
and	O
closed	B-protein_state
(	O
B	O
)	O
conformations	O
in	O
the	O
GDH	B-complex_assembly
-	I-complex_assembly
NADH	I-complex_assembly
-	I-complex_assembly
GTP	I-complex_assembly
ternary	O
complex	O
.	O

Densities	B-evidence
for	O
GTP	B-chemical
(	O
yellow	O
)	O
as	O
well	O
as	O
NADH	B-chemical
bound	B-protein_state
to	I-protein_state
both	O
catalytic	B-site
(	O
purple	O
)	O
and	O
regulatory	B-site
(	O
red	O
)	O
sites	B-site
in	O
each	O
protomer	B-oligomeric_state
are	O
shown	O
.	O

Whereas	O
the	O
orientation	O
in	O
which	O
NADH	B-chemical
binds	O
at	O
the	O
catalytic	B-site
site	I-site
is	O
similar	O
for	O
both	O
conformations	O
,	O
the	O
orientation	O
of	O
the	O
nicotinamide	O
portion	O
of	O
NADH	B-chemical
in	O
the	O
regulatory	B-site
site	I-site
is	O
different	O
between	O
the	O
open	B-protein_state
and	O
closed	B-protein_state
conformations	O
(	O
Figs	O
.	O
3	O
and	O
4	O
).	O

In	O
the	O
closed	B-protein_state
state	O
,	O
the	O
nicotinamide	O
moiety	O
is	O
inserted	O
into	O
a	O
well	O
-	O
defined	O
cavity	B-site
at	O
the	O
interface	B-site
between	O
two	O
adjacent	O
protomers	B-oligomeric_state
in	O
the	O
trimer	B-oligomeric_state
.	O

As	O
mentioned	O
above	O
,	O
this	O
cavity	B-site
is	O
much	O
narrower	O
in	O
the	O
open	B-protein_state
state	O
,	O
suggesting	O
that	O
this	O
cavity	B-site
may	O
be	O
unavailable	O
to	O
the	O
NADH	B-chemical
nicotinamide	O
moiety	O
when	O
the	O
enzyme	O
is	O
in	O
the	O
open	B-protein_state
conformation	O
.	O

The	O
rapid	O
emergence	O
of	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
as	O
a	O
tool	O
for	O
near	O
-	O
atomic	O
resolution	O
structure	B-experimental_method
determination	I-experimental_method
provides	O
new	O
opportunities	O
for	O
complementing	O
atomic	O
resolution	O
information	O
from	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
,	O
as	O
illustrated	O
here	O
with	O
GDH	B-protein
.	O

Perhaps	O
the	O
most	O
important	O
contribution	O
of	O
these	O
methods	O
is	O
the	O
prospect	O
that	O
when	O
there	O
are	O
discrete	O
subpopulations	O
present	O
,	O
the	O
structure	B-evidence
of	O
each	O
state	O
can	O
be	O
determined	O
at	O
near	O
-	O
atomic	O
resolution	O
.	O
What	O
we	O
demonstrate	O
here	O
with	O
GDH	B-protein
is	O
that	O
by	O
employing	O
three	B-experimental_method
-	I-experimental_method
dimensional	I-experimental_method
image	I-experimental_method
classification	I-experimental_method
approaches	I-experimental_method
,	O
we	O
not	O
only	O
can	O
isolate	O
distinct	O
,	O
coexisting	O
conformations	O
,	O
but	O
we	O
can	O
also	O
localize	O
small	O
molecule	O
ligands	O
in	O
each	O
of	O
these	O
conformations	O
.	O

Investigation	O
of	O
the	O
Interaction	O
between	O
Cdc42	B-protein
and	O
Its	O
Effector	O
TOCA1	B-protein

Transducer	B-protein
of	I-protein
Cdc42	I-protein
-	I-protein
dependent	I-protein
actin	I-protein
assembly	I-protein
protein	I-protein
1	I-protein
(	O
TOCA1	B-protein
)	O
is	O
an	O
effector	O
of	O
the	O
Rho	B-protein_type
family	I-protein_type
small	I-protein_type
G	I-protein_type
protein	I-protein_type
Cdc42	B-protein
.	O

We	O
have	O
found	O
that	O
the	O
TOCA1	B-protein
HR1	B-structure_element
,	O
like	O
the	O
closely	O
related	O
CIP4	B-protein
HR1	B-structure_element
,	O
has	O
interesting	O
structural	O
features	O
that	O
are	O
not	O
observed	O
in	O
other	O
HR1	B-structure_element
domains	O
.	O

We	O
have	O
also	O
investigated	O
the	O
binding	O
of	O
the	O
TOCA	B-protein
HR1	B-structure_element
domain	O
to	O
Cdc42	B-protein
and	O
the	O
potential	O
ternary	O
complex	O
between	O
Cdc42	B-protein
and	O
the	O
G	B-site
protein	I-site
-	I-site
binding	I-site
regions	I-site
of	O
TOCA1	B-protein
and	O
a	O
member	O
of	O
the	O
Wiskott	B-protein_type
-	I-protein_type
Aldrich	I-protein_type
syndrome	I-protein_type
protein	I-protein_type
family	I-protein_type
,	O
N	B-protein
-	I-protein
WASP	I-protein
.	O

TOCA1	B-protein
binds	O
Cdc42	B-protein
with	O
micromolar	O
affinity	O
,	O
in	O
contrast	O
to	O
the	O
nanomolar	O
affinity	O
of	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
G	B-site
protein	I-site
-	I-site
binding	I-site
region	I-site
for	O
Cdc42	B-protein
.	O

NMR	B-experimental_method
experiments	O
show	O
that	O
the	O
Cdc42	B-site
-	I-site
binding	I-site
domain	I-site
from	O
N	B-protein
-	I-protein
WASP	I-protein
is	O
able	O
to	O
displace	O
TOCA1	B-protein
HR1	B-structure_element
from	O
Cdc42	B-protein
,	O
whereas	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
domain	O
but	O
not	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
inhibits	O
actin	O
polymerization	O
.	O

This	O
suggests	O
that	O
TOCA1	B-protein
binding	O
to	O
Cdc42	B-protein
is	O
an	O
early	O
step	O
in	O
the	O
Cdc42	B-protein
-	O
dependent	O
pathways	O
that	O
govern	O
actin	O
dynamics	O
,	O
and	O
the	O
differential	O
binding	B-evidence
affinities	I-evidence
of	O
the	O
effectors	O
facilitate	O
a	O
handover	O
from	O
TOCA1	B-protein
to	O
N	B-protein
-	I-protein
WASP	I-protein
,	O
which	O
can	O
then	O
drive	O
recruitment	O
of	O
the	O
actin	O
-	O
modifying	O
machinery	O
.	O

The	O
Ras	B-protein_type
superfamily	I-protein_type
of	O
small	B-protein_type
GTPases	I-protein_type
comprises	O
over	O
150	O
members	O
that	O
regulate	O
a	O
multitude	O
of	O
cellular	O
processes	O
in	O
eukaryotes	B-taxonomy_domain
.	O

The	O
superfamily	O
can	O
be	O
divided	O
into	O
five	O
families	O
based	O
on	O
structural	O
and	O
functional	O
similarities	O
:	O
Ras	B-protein_type
,	O
Rho	B-protein_type
,	O
Rab	B-protein_type
,	O
Arf	B-protein_type
,	O
and	O
Ran	B-protein_type
.	O

These	O
molecular	O
switches	O
cycle	O
between	O
active	B-protein_state
,	O
GTP	B-protein_state
-	I-protein_state
bound	I-protein_state
,	O
and	O
inactive	B-protein_state
,	O
GDP	B-protein_state
-	I-protein_state
bound	I-protein_state
,	O
states	O
with	O
the	O
help	O
of	O
auxiliary	O
proteins	O
.	O

The	O
guanine	B-protein_type
nucleotide	I-protein_type
exchange	I-protein_type
factors	I-protein_type
mediate	O
formation	O
of	O
the	O
active	B-protein_state
state	O
by	O
promoting	O
the	O
dissociation	O
of	O
GDP	B-chemical
,	O
allowing	O
GTP	B-chemical
to	O
bind	O
.	O

The	O
GTPase	B-protein_type
-	I-protein_type
activating	I-protein_type
proteins	I-protein_type
stimulate	O
the	O
rate	O
of	O
intrinsic	O
GTP	B-chemical
hydrolysis	O
,	O
mediating	O
the	O
return	O
to	O
the	O
inactive	B-protein_state
state	O
(	O
reviewed	O
in	O
Ref	O
.).	O

These	O
regions	O
are	O
responsible	O
for	O
“	O
sensing	O
”	O
the	O
nucleotide	O
state	O
,	O
with	O
the	O
GTP	B-protein_state
-	I-protein_state
bound	I-protein_state
state	O
showing	O
greater	O
rigidity	O
and	O
the	O
GDP	B-protein_state
-	I-protein_state
bound	I-protein_state
state	O
adopting	O
a	O
more	O
relaxed	O
conformation	O
(	O
reviewed	O
in	O
Ref	O
.).	O

In	O
the	O
active	B-protein_state
state	O
,	O
G	B-protein_type
proteins	I-protein_type
bind	O
to	O
an	O
array	O
of	O
downstream	O
effectors	O
,	O
through	O
which	O
they	O
exert	O
their	O
extensive	O
roles	O
within	O
the	O
cell	O
.	O

The	O
Rho	B-protein_type
family	I-protein_type
comprises	O
20	O
members	O
,	O
of	O
which	O
three	O
,	O
RhoA	B-protein
,	O
Rac1	B-protein
,	O
and	O
Cdc42	B-protein
,	O
have	O
been	O
relatively	O
well	O
studied	O
.	O

RhoA	B-protein
acts	O
to	O
rearrange	O
existing	O
actin	O
structures	O
to	O
form	O
stress	O
fibers	O
,	O
whereas	O
Rac1	B-protein
and	O
Cdc42	B-protein
promote	O
de	O
novo	O
actin	O
polymerization	O
to	O
form	O
lamellipodia	O
and	O
filopodia	O
,	O
respectively	O
.	O

A	O
number	O
of	O
RhoA	B-protein
and	O
Rac1	B-protein
effector	O
proteins	O
,	O
including	O
the	O
formins	O
and	O
members	O
of	O
the	O
protein	B-protein_type
kinase	I-protein_type
C	I-protein_type
-	I-protein_type
related	I-protein_type
kinase	I-protein_type
(	O
PRK	B-protein_type
)	O
6	B-protein_type
family	O
,	O
along	O
with	O
Cdc42	B-protein
effectors	O
,	O
including	O
the	O
Wiskott	B-protein_type
-	I-protein_type
Aldrich	I-protein_type
syndrome	I-protein_type
(	O
WASP	B-protein_type
)	O
family	O
and	O
the	O
transducer	O
of	O
Cdc42	B-protein_type
-	I-protein_type
dependent	I-protein_type
actin	I-protein_type
assembly	I-protein_type
(	O
TOCA	B-protein_type
)	O
family	O
,	O
have	O
also	O
been	O
linked	O
to	O
the	O
pathways	O
that	O
govern	O
cytoskeletal	O
dynamics	O
.	O

Cdc42	B-protein
effectors	O
,	O
TOCA1	B-protein
and	O
the	O
ubiquitously	O
expressed	O
member	O
of	O
the	O
WASP	B-protein_type
family	I-protein_type
,	O
N	B-protein
-	I-protein
WASP	I-protein
,	O
have	O
been	O
implicated	O
in	O
the	O
regulation	O
of	O
actin	O
polymerization	O
downstream	O
of	O
Cdc42	B-protein
and	O
phosphatidylinositol	B-chemical
4	I-chemical
,	I-chemical
5	I-chemical
-	I-chemical
bisphosphate	I-chemical
(	O
PI	B-chemical
(	I-chemical
4	I-chemical
,	I-chemical
5	I-chemical
)	I-chemical
P2	I-chemical
).	O

N	B-protein
-	I-protein
WASP	I-protein
exists	O
in	O
an	O
autoinhibited	B-protein_state
conformation	I-protein_state
,	O
which	O
is	O
released	O
upon	O
PI	B-chemical
(	I-chemical
4	I-chemical
,	I-chemical
5	I-chemical
)	I-chemical
P2	I-chemical
and	O
Cdc42	B-protein
binding	O
or	O
by	O
other	O
factors	O
,	O
such	O
as	O
phosphorylation	O
.	O

Following	O
their	O
release	O
,	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
regions	I-structure_element
of	O
N	B-protein
-	I-protein
WASP	I-protein
are	O
free	O
to	O
interact	O
with	O
G	B-protein_type
-	I-protein_type
actin	I-protein_type
and	O
a	O
known	O
nucleator	O
of	O
actin	O
assembly	O
,	O
the	O
Arp2	B-complex_assembly
/	I-complex_assembly
3	I-complex_assembly
complex	O
.	O

The	O
importance	O
of	O
TOCA1	B-protein
in	O
actin	O
polymerization	O
has	O
been	O
demonstrated	O
in	O
a	O
range	O
of	O
in	O
vitro	O
and	O
in	O
vivo	O
studies	O
,	O
but	O
the	O
exact	O
role	O
of	O
TOCA1	B-protein
in	O
the	O
many	O
pathways	O
involving	O
actin	O
assembly	O
remains	O
unclear	O
.	O

TOCA1	B-protein
comprises	O
an	O
N	O
-	O
terminal	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domain	O
,	O
a	O
central	B-structure_element
homology	I-structure_element
region	I-structure_element
1	I-structure_element
(	O
HR1	B-structure_element
)	O
domain	O
,	O
and	O
a	O
C	O
-	O
terminal	O
SH3	B-structure_element
domain	O
.	O

The	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domain	O
is	O
a	O
known	O
dimerization	O
,	O
membrane	O
-	O
binding	O
,	O
and	O
membrane	O
-	O
deforming	O
module	O
found	O
in	O
a	O
number	O
of	O
cell	O
signaling	O
proteins	O
.	O

The	O
HR1	B-structure_element
domain	O
has	O
been	O
directly	O
implicated	O
in	O
the	O
interaction	O
between	O
TOCA1	B-protein
and	O
Cdc42	B-protein
,	O
representing	O
the	O
first	O
Cdc42	B-protein
-	O
HR1	B-structure_element
domain	O
interaction	O
to	O
be	O
identified	O
.	O

Other	O
HR1	B-structure_element
domains	O
studied	O
so	O
far	O
,	O
including	O
those	O
from	O
the	O
PRK	B-protein_type
family	I-protein_type
,	O
have	O
been	O
found	O
to	O
bind	O
their	O
cognate	O
Rho	O
family	O
G	B-protein_type
protein	I-protein_type
-	O
binding	O
partner	O
with	O
high	O
specificity	O
and	O
affinities	B-evidence
in	O
the	O
nanomolar	O
range	O
.	O

These	O
HR1	B-structure_element
domains	O
,	O
however	O
,	O
show	O
specificity	O
for	O
Cdc42	B-protein
,	O
rather	O
than	O
RhoA	B-protein
or	O
Rac1	B-protein
.	O

Furthermore	O
,	O
the	O
biological	O
function	O
of	O
the	O
interaction	O
between	O
TOCA1	B-protein
and	O
Cdc42	B-protein
remains	O
poorly	O
understood	O
,	O
and	O
so	O
far	O
there	O
has	O
been	O
no	O
biophysical	O
or	O
structural	O
insight	O
.	O

The	O
interactions	O
of	O
TOCA1	B-protein
and	O
N	B-protein
-	I-protein
WASP	I-protein
with	O
Cdc42	B-protein
as	O
well	O
as	O
with	O
each	O
other	O
have	O
raised	O
questions	O
as	O
to	O
whether	O
the	O
two	O
Cdc42	B-protein
effectors	O
can	O
interact	O
with	O
a	O
single	O
molecule	O
of	O
Cdc42	B-protein
simultaneously	O
.	O

There	O
is	O
some	O
evidence	O
for	O
a	O
ternary	O
complex	O
between	O
Cdc42	B-protein
,	O
N	B-protein
-	I-protein
WASP	I-protein
,	O
and	O
TOCA1	B-protein
,	O
but	O
there	O
was	O
no	O
direct	O
demonstration	O
of	O
simultaneous	O
contacts	O
between	O
the	O
two	O
effectors	O
and	O
a	O
single	O
molecule	O
of	O
Cdc42	B-protein
.	O

Nonetheless	O
,	O
the	O
substantial	O
difference	O
between	O
the	O
structures	B-evidence
of	O
the	O
G	B-site
protein	I-site
-	I-site
binding	I-site
regions	I-site
of	O
the	O
two	O
effectors	O
is	O
intriguing	O
and	O
implies	O
that	O
they	O
bind	O
to	O
Cdc42	B-protein
quite	O
differently	O
,	O
providing	O
motivation	O
for	O
investigating	O
the	O
possibility	O
that	O
Cdc42	B-protein
can	O
bind	O
both	O
effectors	O
concurrently	O
.	O

WASP	B-protein_type
interacts	O
with	O
Cdc42	B-protein
via	O
a	O
conserved	B-protein_state
,	O
unstructured	B-structure_element
binding	I-structure_element
motif	I-structure_element
known	O
as	O
the	O
Cdc42	B-structure_element
-	I-structure_element
and	I-structure_element
Rac	I-structure_element
-	I-structure_element
interactive	I-structure_element
binding	I-structure_element
region	I-structure_element
(	O
CRIB	B-structure_element
),	O
which	O
forms	O
an	O
intermolecular	B-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
,	O
expanding	O
the	O
anti	O
-	O
parallel	O
β2	B-structure_element
and	I-structure_element
β3	I-structure_element
strands	I-structure_element
of	O
Cdc42	B-protein
.	O

In	O
contrast	O
,	O
the	O
TOCA	B-protein_type
family	I-protein_type
proteins	I-protein_type
are	O
thought	O
to	O
interact	O
via	O
the	O
HR1	B-structure_element
domain	O
,	O
which	O
may	O
form	O
a	O
triple	B-structure_element
coiled	I-structure_element
-	I-structure_element
coil	I-structure_element
with	O
switch	B-site
II	I-site
of	O
Rac1	B-protein
,	O
like	O
the	O
HR1b	B-structure_element
domain	O
of	O
PRK1	B-protein
.	O

Here	O
,	O
we	O
present	O
the	O
solution	B-experimental_method
NMR	I-experimental_method
structure	B-evidence
of	O
the	O
HR1	B-structure_element
domain	O
of	O
TOCA1	B-protein
,	O
providing	O
the	O
first	O
structural	B-evidence
data	I-evidence
for	O
this	O
protein	O
.	O

Finally	O
,	O
we	O
investigate	O
the	O
potential	O
ternary	O
complex	O
between	O
Cdc42	B-protein
and	O
the	O
G	B-site
protein	I-site
-	I-site
binding	I-site
regions	I-site
of	O
TOCA1	B-protein
and	O
N	B-protein
-	I-protein
WASP	I-protein
,	O
contributing	O
to	O
our	O
understanding	O
of	O
G	B-protein_type
protein	I-protein_type
-	O
effector	O
interactions	O
as	O
well	O
as	O
the	O
roles	O
of	O
Cdc42	B-protein
,	O
N	B-protein
-	I-protein
WASP	I-protein
,	O
and	O
TOCA1	B-protein
in	O
the	O
pathways	O
that	O
govern	O
actin	B-protein_type
dynamics	O
.	O

Cdc42	B-protein
-	O
TOCA1	B-protein
Binding	O

TOCA1	B-protein
was	O
identified	O
in	O
Xenopus	B-taxonomy_domain
extracts	O
as	O
a	O
protein	O
necessary	O
for	O
Cdc42	B-protein
-	O
dependent	O
actin	O
assembly	O
and	O
was	O
shown	O
to	O
bind	O
to	O
Cdc42	B-complex_assembly
·	I-complex_assembly
GTPγS	I-complex_assembly
but	O
not	O
to	O
Cdc42	B-complex_assembly
·	I-complex_assembly
GDP	I-complex_assembly
or	O
to	O
Rac1	B-protein
and	O
RhoA	B-protein
.	O
Given	O
its	O
homology	O
to	O
other	O
Rho	B-site
family	I-site
binding	I-site
modules	I-site
,	O
it	O
is	O
likely	O
that	O
the	O
HR1	B-structure_element
domain	O
of	O
TOCA1	B-protein
is	O
sufficient	O
to	O
bind	O
Cdc42	B-protein
.	O

The	O
C	B-species
.	I-species
elegans	I-species
TOCA1	B-protein
orthologues	O
also	O
bind	O
to	O
Cdc42	B-protein
via	O
their	O
consensus	O
HR1	B-structure_element
domain	O
.	O

The	O
HR1	B-structure_element
domains	O
from	O
the	O
PRK	B-protein_type
family	I-protein_type
bind	O
their	O
G	B-protein_type
protein	I-protein_type
partners	O
with	O
a	O
high	O
affinity	O
,	O
exhibiting	O
a	O
range	O
of	O
submicromolar	O
dissociation	B-evidence
constants	I-evidence
(	O
Kd	B-evidence
)	O
as	O
low	O
as	O
26	O
nm	O
.	O

A	O
Kd	B-evidence
in	O
the	O
nanomolar	O
range	O
was	O
therefore	O
expected	O
for	O
the	O
interaction	O
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
with	O
Cdc42	B-protein
.	O

We	O
generated	O
an	O
X	B-species
.	I-species
tropicalis	I-species
TOCA1	B-protein
HR1	B-structure_element
domain	O
construct	O
encompassing	O
residues	O
330	B-residue_range
–	I-residue_range
426	I-residue_range
.	O

This	O
region	O
comprises	O
the	O
complete	O
HR1	B-structure_element
domain	O
based	O
on	O
secondary	O
structure	O
predictions	O
and	O
sequence	B-experimental_method
alignments	I-experimental_method
with	O
another	O
TOCA	B-protein_type
family	I-protein_type
member	O
,	O
CIP4	B-protein
,	O
whose	O
structure	B-evidence
has	O
been	O
determined	O
.	O

The	O
interaction	O
between	O
[	B-complex_assembly
3H	I-complex_assembly
]	I-complex_assembly
GTP	I-complex_assembly
·	I-complex_assembly
Cdc42	I-complex_assembly
and	O
a	O
C	O
-	O
terminally	O
His	B-protein_state
-	I-protein_state
tagged	I-protein_state
TOCA1	B-protein
HR1	B-structure_element
domain	O
construct	O
was	O
investigated	O
using	O
SPA	B-experimental_method
.	O

The	O
binding	B-evidence
isotherm	I-evidence
for	O
the	O
interaction	O
is	O
shown	O
in	O
Fig	O
.	O
1A	O
,	O
together	O
with	O
the	O
Cdc42	B-protein
-	O
PAK	B-protein
interaction	O
as	O
a	O
positive	O
control	O
.	O

The	O
binding	O
of	O
TOCA1	B-protein
HR1	B-structure_element
to	O
Cdc42	B-protein
was	O
unexpectedly	O
weak	O
,	O
with	O
a	O
Kd	B-evidence
of	O
>	O
1	O
μm	O
.	O

It	O
was	O
not	O
possible	O
to	O
estimate	O
the	O
Kd	B-evidence
more	O
accurately	O
using	O
direct	O
SPA	B-experimental_method
experiments	O
,	O
because	O
saturation	O
could	O
not	O
be	O
reached	O
due	O
to	O
nonspecific	O
signal	O
at	O
higher	O
protein	O
concentrations	O
.	O

The	O
TOCA1	B-protein
HR1	B-structure_element
-	O
Cdc42	B-protein
interaction	O
is	O
low	O
affinity	O
.	O

A	O
,	O
curves	O
derived	O
from	O
direct	B-experimental_method
binding	I-experimental_method
assays	I-experimental_method
in	O
which	O
the	O
indicated	O
concentrations	O
of	O
Cdc42Δ7Q61L	B-complex_assembly
·[	I-complex_assembly
3H	I-complex_assembly
]	I-complex_assembly
GTP	I-complex_assembly
were	O
incubated	B-experimental_method
with	O
30	O
nm	O
GST	B-mutant
-	I-mutant
PAK	I-mutant
or	O
HR1	B-mutant
-	I-mutant
His6	I-mutant
in	O
SPAs	B-experimental_method
.	O

The	O
SPA	B-experimental_method
signal	O
was	O
corrected	O
by	O
subtraction	O
of	O
control	O
data	O
with	O
no	O
GST	B-mutant
-	I-mutant
PAK	I-mutant
or	O
HR1	B-mutant
-	I-mutant
His6	I-mutant
.	O

The	O
Kd	B-evidence
values	O
derived	O
for	O
the	O
ACK	B-protein
GBD	B-structure_element
with	O
Cdc42Δ7	B-mutant
and	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
Cdc42	B-protein
were	O
0	O
.	O
032	O
±	O
0	O
.	O
01	O
and	O
0	O
.	O
011	O
±	O
0	O
.	O
01	O
μm	O
,	O
respectively	O
.	O

Isothermal	B-experimental_method
titration	I-experimental_method
calorimetry	I-experimental_method
was	O
carried	O
out	O
,	O
but	O
no	O
heat	O
changes	O
were	O
observed	O
at	O
a	O
range	O
of	O
concentrations	O
and	O
temperatures	O
(	O
data	O
not	O
shown	O
),	O
suggesting	O
that	O
the	O
interaction	O
is	O
predominantly	O
entropically	O
driven	O
.	O

The	O
affinity	B-evidence
was	O
therefore	O
determined	O
using	O
competition	B-experimental_method
SPAs	I-experimental_method
.	O

A	O
complex	O
of	O
a	O
GST	B-experimental_method
fusion	I-experimental_method
of	O
the	O
GBD	B-structure_element
of	O
ACK	B-protein
,	O
which	O
binds	O
with	O
a	O
high	O
affinity	O
to	O
Cdc42	B-protein
,	O
with	O
radiolabeled	O
[	B-complex_assembly
3H	I-complex_assembly
]	I-complex_assembly
GTP	I-complex_assembly
·	I-complex_assembly
Cdc42	I-complex_assembly
was	O
preformed	O
,	O
and	O
the	O
effect	O
of	O
increasing	B-experimental_method
concentrations	I-experimental_method
of	O
untagged	B-protein_state
TOCA1	B-protein
HR1	B-structure_element
domain	O
was	O
examined	O
.	O

Competition	O
of	O
GST	B-mutant
-	I-mutant
ACK	I-mutant
GBD	B-structure_element
bound	B-protein_state
to	I-protein_state
[	B-complex_assembly
3H	I-complex_assembly
]	I-complex_assembly
GTP	I-complex_assembly
·	I-complex_assembly
Cdc42	I-complex_assembly
by	O
free	B-protein_state
ACK	B-protein
GBD	B-structure_element
was	O
used	O
as	O
a	O
control	O
and	O
to	O
establish	O
the	O
value	O
of	O
background	O
counts	O
when	O
Cdc42	B-protein
is	O
fully	O
displaced	O
.	O

Free	B-protein_state
ACK	B-protein
competed	O
with	O
itself	O
with	O
an	O
affinity	B-evidence
of	O
32	O
nm	O
,	O
similar	O
to	O
the	O
value	O
obtained	O
by	O
direct	B-experimental_method
binding	I-experimental_method
of	O
23	O
nm	O
.	O

The	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
also	O
fully	O
competed	O
with	O
the	O
GST	B-mutant
-	I-mutant
ACK	I-mutant
but	O
bound	B-protein_state
with	O
an	O
affinity	B-evidence
of	O
6	O
μm	O
(	O
Fig	O
.	O
1	O
,	O
B	O
and	O
C	O
),	O
in	O
agreement	O
with	O
the	O
low	O
affinity	B-evidence
observed	O
in	O
the	O
direct	B-experimental_method
binding	I-experimental_method
experiments	I-experimental_method
.	O

The	O
Cdc42	B-protein
construct	O
used	O
in	O
the	O
binding	B-experimental_method
assays	I-experimental_method
has	O
seven	B-residue_range
residues	I-residue_range
deleted	B-experimental_method
from	O
the	O
C	O
terminus	O
to	O
facilitate	O
purification	O
.	O

These	O
residues	O
are	O
not	O
generally	O
required	O
for	O
G	B-protein_type
protein	I-protein_type
-	O
effector	O
interactions	O
,	O
including	O
the	O
interaction	O
between	O
RhoA	B-protein
and	O
the	O
PRK1	B-protein
HR1a	B-structure_element
domain	O
.	O

In	O
contrast	O
,	O
the	O
C	O
terminus	O
of	O
Rac1	B-protein
contains	O
a	O
polybasic	O
sequence	O
,	O
which	O
is	O
crucial	O
for	O
Rac1	B-protein
binding	O
to	O
the	O
HR1b	B-structure_element
domain	O
from	O
PRK1	B-protein
.	O

The	O
binding	B-experimental_method
experiments	I-experimental_method
were	O
repeated	O
with	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
[	B-complex_assembly
3H	I-complex_assembly
]	I-complex_assembly
GTP	I-complex_assembly
·	I-complex_assembly
Cdc42	I-complex_assembly
,	O
but	O
the	O
affinity	B-evidence
of	O
the	O
HR1	B-structure_element
domain	O
for	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
Cdc42	B-protein
was	O
similar	O
to	O
its	O
affinity	B-evidence
for	O
truncated	B-protein_state
Cdc42	B-protein
(	O
Kd	B-evidence
≈	O
5	O
μm	O
;	O
Fig	O
.	O
1C	O
).	O

Another	O
possible	O
explanation	O
for	O
the	O
low	O
affinities	B-evidence
observed	O
was	O
that	O
the	O
HR1	B-structure_element
domain	O
alone	B-protein_state
is	O
not	O
sufficient	O
for	O
maximal	O
binding	O
of	O
the	O
TOCA	B-protein_type
proteins	I-protein_type
to	O
Cdc42	B-protein
and	O
that	O
the	O
other	O
domains	O
are	O
required	O
.	O

Indeed	O
,	O
GST	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
downs	I-experimental_method
performed	O
with	O
in	O
vitro	O
translated	O
human	B-species
TOCA1	B-protein
fragments	O
had	O
suggested	O
that	O
residues	O
N	O
-	O
terminal	O
to	O
the	O
HR1	B-structure_element
domain	O
may	O
be	O
required	O
to	O
stabilize	O
the	O
HR1	B-structure_element
domain	O
structure	O
.	O

Furthermore	O
,	O
both	O
BAR	B-structure_element
and	O
SH3	B-structure_element
domains	O
have	O
been	O
implicated	O
in	O
interactions	O
with	O
small	O
G	B-protein_type
proteins	I-protein_type
(	O
e	O
.	O
g	O
.	O
the	O
BAR	B-structure_element
domain	O
of	O
Arfaptin2	B-protein
binds	O
to	O
Rac1	B-protein
and	O
Arl1	B-protein
),	O
while	O
an	O
SH3	B-structure_element
domain	O
mediates	O
the	O
interaction	O
between	O
Rac1	B-protein
and	O
the	O
guanine	B-protein
nucleotide	I-protein
exchange	I-protein
factor	I-protein
,	O
β	B-protein
-	I-protein
PIX	I-protein
.	O

TOCA1	B-protein
dimerizes	B-oligomeric_state
via	O
its	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domain	O
,	O
which	O
could	O
also	O
affect	O
Cdc42	B-protein
binding	O
,	O
for	O
example	O
by	O
presenting	O
two	O
HR1	B-structure_element
domains	O
for	O
Cdc42	B-protein
interactions	O
.	O

Various	O
TOCA1	B-protein
fragments	O
(	O
Fig	O
.	O
2A	O
)	O
were	O
therefore	O
assessed	O
for	O
binding	O
to	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
Cdc42	B-protein
by	O
direct	O
SPA	B-experimental_method
.	O

The	O
isolated	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domain	O
showed	O
no	O
binding	O
to	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
Cdc42	B-protein
(	O
Fig	O
.	O
2B	O
).	O

The	O
HR1	B-mutant
-	I-mutant
SH3	I-mutant
protein	O
could	O
not	O
be	O
purified	O
to	O
homogeneity	O
as	O
a	O
fusion	O
protein	O
,	O
so	O
it	O
was	O
assayed	O
in	O
competition	B-experimental_method
assays	I-experimental_method
after	O
cleavage	O
of	O
the	O
His	O
tag	O
.	O

This	O
construct	O
competed	O
with	O
GST	B-mutant
-	I-mutant
ACK	I-mutant
GBD	B-structure_element
to	O
give	O
a	O
similar	O
affinity	O
to	O
the	O
HR1	B-structure_element
domain	O
alone	B-protein_state
(	O
Kd	B-evidence
=	O
4	O
.	O
6	O
±	O
4	O
μm	O
;	O
Fig	O
.	O
2C	O
).	O

Taken	O
together	O
,	O
these	O
data	O
suggest	O
that	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
is	O
sufficient	O
for	O
maximal	O
binding	O
and	O
that	O
this	O
binding	O
is	O
of	O
a	O
relatively	O
low	O
affinity	O
compared	O
with	O
many	O
other	O
Cdc42	B-protein
·	O
effector	O
complexes	O
.	O

The	O
Cdc42	B-complex_assembly
-	I-complex_assembly
HR1	I-complex_assembly
interaction	O
is	O
of	O
low	O
affinity	O
in	O
the	O
context	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
protein	O
and	O
in	O
TOCA1	B-protein
paralogues	O
.	O

A	O
,	O
diagram	O
illustrating	O
the	O
TOCA1	B-protein
constructs	O
assayed	O
for	O
Cdc42	B-protein
binding	O
.	O

Domain	O
boundaries	O
are	O
derived	O
from	O
secondary	O
structure	O
predictions	O
;	O
B	O
,	O
binding	B-evidence
curves	I-evidence
derived	O
from	O
direct	B-experimental_method
binding	I-experimental_method
assays	I-experimental_method
,	O
in	O
which	O
the	O
indicated	O
concentrations	O
of	O
Cdc42Δ7Q61L	B-complex_assembly
·[	I-complex_assembly
3H	I-complex_assembly
]	I-complex_assembly
GTP	I-complex_assembly
were	O
incubated	B-experimental_method
with	O
30	O
nm	O
GST	B-mutant
-	I-mutant
ACK	I-mutant
or	O
His	B-protein_state
-	I-protein_state
tagged	I-protein_state
TOCA1	B-protein
constructs	O
,	O
as	O
indicated	O
,	O
in	O
SPAs	B-experimental_method
.	O

C	O
–	O
E	O
,	O
representative	O
examples	O
of	O
competition	B-experimental_method
SPA	I-experimental_method
experiments	O
carried	O
out	O
with	O
the	O
indicated	O
concentrations	O
of	O
the	O
TOCA1	B-protein
HR1	B-mutant
-	I-mutant
SH3	I-mutant
construct	O
titrated	B-experimental_method
into	O
30	O
nm	O
GST	B-mutant
-	I-mutant
ACK	I-mutant
and	O
30	O
nm	O
Cdc42Δ7Q61L	B-complex_assembly
·[	I-complex_assembly
3H	I-complex_assembly
]	I-complex_assembly
GTP	I-complex_assembly
(	O
C	O
)	O
or	O
HR1CIP4	B-structure_element
(	O
D	O
)	O
or	O
HR1FBP17	B-structure_element
(	O
E	O
)	O
titrated	B-experimental_method
into	O
30	O
nm	O
GST	B-mutant
-	I-mutant
ACK	I-mutant
and	O
30	O
nm	O
Cdc42FLQ61L	B-complex_assembly
·[	I-complex_assembly
3H	I-complex_assembly
]	I-complex_assembly
GTP	I-complex_assembly
.	O

The	O
HR1	B-structure_element
domains	O
from	O
FBP17	B-protein
and	O
CIP4	B-protein
were	O
purified	B-experimental_method
and	O
assayed	O
for	O
Cdc42	B-protein
binding	O
in	O
competition	B-experimental_method
SPAs	I-experimental_method
,	O
analogous	O
to	O
those	O
carried	O
out	O
with	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
.	O

Structure	B-evidence
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
Domain	O

Because	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
was	O
sufficient	O
for	O
maximal	O
Cdc42	B-protein
-	O
binding	O
,	O
we	O
used	O
this	O
construct	O
for	O
structural	O
studies	O
.	O

2	O
,	O
778	O
non	O
-	O
degenerate	O
NOE	B-evidence
restraints	I-evidence
were	O
used	O
in	O
initial	O
structure	B-experimental_method
calculations	I-experimental_method
(	O
1	O
,	O
791	O
unambiguous	O
and	O
987	O
ambiguous	O
),	O
derived	O
from	O
three	O
-	O
dimensional	O
15N	B-experimental_method
-	I-experimental_method
separated	I-experimental_method
NOESY	I-experimental_method
and	O
13C	B-experimental_method
-	I-experimental_method
separated	I-experimental_method
NOESY	I-experimental_method
experiments	O
.	O

There	O
were	O
1	O
,	O
845	O
unambiguous	O
NOEs	B-evidence
and	O
757	O
ambiguous	O
NOEs	B-evidence
after	O
eight	O
iterations	O
.	O

Table	O
1	O
indicates	O
that	O
the	O
HR1	B-structure_element
domain	O
structure	B-evidence
is	O
well	O
defined	O
by	O
the	O
NMR	B-experimental_method
data	O
.	O

a	O
<	O
SA	O
>,	O
the	O
average	B-evidence
root	I-evidence
mean	I-evidence
square	I-evidence
deviations	I-evidence
for	O
the	O
ensemble	O
±	O
S	O
.	O
D	O
.	O

b	O
<	O
SA	O
>	O
c	O
,	O
values	O
for	O
the	O
structure	B-evidence
that	O
is	O
closest	O
to	O
the	O
mean	O
.	O

The	O
structure	B-evidence
closest	O
to	O
the	O
mean	O
is	O
shown	O
in	O
Fig	O
.	O
3A	O
.	O

A	O
sequence	B-experimental_method
alignment	I-experimental_method
illustrating	O
the	O
secondary	O
structure	O
elements	O
of	O
the	O
TOCA1	B-protein
and	O
CIP4	B-protein
HR1	B-structure_element
domains	O
and	O
the	O
HR1a	B-structure_element
and	O
HR1b	B-structure_element
domains	O
from	O
PRK1	B-protein
is	O
shown	O
in	O
Fig	O
.	O
3B	O
.	O

Flexible	O
regions	O
at	O
the	O
N	O
and	O
C	O
termini	O
(	O
residues	O
330	B-residue_range
–	I-residue_range
333	I-residue_range
and	O
421	B-residue_range
–	I-residue_range
426	I-residue_range
)	O
are	O
omitted	O
for	O
clarity	O
.	O

B	O
,	O
a	O
sequence	B-experimental_method
alignment	I-experimental_method
of	O
the	O
HR1	B-structure_element
domains	O
from	O
TOCA1	B-protein
,	O
CIP4	B-protein
,	O
and	O
PRK1	B-protein
.	O

The	O
secondary	O
structure	O
was	O
deduced	O
using	O
Stride	B-experimental_method
,	O
based	O
on	O
the	O
Ramachandran	B-evidence
angles	I-evidence
,	O
and	O
is	O
indicated	O
as	O
follows	O
:	O
gray	O
,	O
turn	O
;	O
yellow	O
,	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
;	O
blue	O
,	O
310	B-structure_element
helix	I-structure_element
;	O
white	O
,	O
coil	O
.	O

C	O
,	O
a	O
close	O
-	O
up	O
of	O
the	O
N	O
-	O
terminal	O
region	O
of	O
TOCA1	B-protein
HR1	B-structure_element
,	O
indicating	O
some	O
of	O
the	O
NOEs	B-evidence
defining	O
its	O
position	O
with	O
respect	O
to	O
the	O
two	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
.	O

Dotted	O
lines	O
,	O
NOE	B-evidence
restraints	I-evidence
.	O

In	O
the	O
HR1a	B-structure_element
domain	O
of	O
PRK1	B-protein
,	O
a	O
region	O
N	O
-	O
terminal	O
to	O
helix	B-structure_element
1	I-structure_element
forms	O
a	O
short	B-structure_element
α	I-structure_element
-	I-structure_element
helix	I-structure_element
,	O
which	O
packs	O
against	O
both	O
helices	O
of	O
the	O
HR1	B-structure_element
domain	O
.	O

This	O
region	O
of	O
TOCA1	B-protein
HR1	B-structure_element
(	O
residues	O
334	B-residue_range
–	I-residue_range
340	I-residue_range
)	O
is	O
well	O
defined	O
in	O
the	O
family	O
of	O
structures	B-evidence
(	O
Fig	O
.	O
3A	O
)	O
but	O
does	O
not	O
form	O
an	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
.	O

It	O
instead	O
forms	O
a	O
series	O
of	O
turns	O
,	O
defined	O
by	O
NOE	B-evidence
restraints	I-evidence
observed	O
between	O
residues	O
separated	O
by	O
one	O
(	O
residues	O
332	B-residue_range
–	I-residue_range
334	I-residue_range
,	O
333	B-residue_range
–	I-residue_range
335	I-residue_range
,	O
etc	O
.)	O
or	O
two	O
(	O
residues	O
337	B-residue_range
–	I-residue_range
340	I-residue_range
)	O
residues	O
in	O
the	O
sequence	O
and	O
the	O
φ	B-evidence
and	I-evidence
ψ	I-evidence
angles	I-evidence
,	O
assessed	O
using	O
Stride	B-experimental_method
.	O

These	O
turns	O
cause	O
the	O
chain	O
to	O
reverse	O
direction	O
,	O
allowing	O
the	O
N	O
-	O
terminal	O
segment	O
(	O
residues	O
334	B-residue_range
–	I-residue_range
340	I-residue_range
)	O
to	O
contact	O
both	O
helices	O
of	O
the	O
HR1	B-structure_element
domain	O
.	O

The	O
two	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
of	O
TOCA1	B-protein
HR1	B-structure_element
are	O
separated	O
by	O
a	O
long	O
loop	B-structure_element
of	O
10	O
residues	O
(	O
residues	O
380	B-residue_range
–	I-residue_range
389	I-residue_range
)	O
that	O
contains	O
two	O
short	B-structure_element
310	I-structure_element
helices	I-structure_element
(	O
residues	O
381	B-residue_range
–	I-residue_range
383	I-residue_range
and	O
386	B-residue_range
–	I-residue_range
389	I-residue_range
).	O

Interestingly	O
,	O
side	O
chains	O
of	O
residues	O
within	O
the	O
loop	B-structure_element
region	I-structure_element
point	O
back	O
toward	O
helix	B-structure_element
1	I-structure_element
;	O
for	O
example	O
,	O
there	O
are	O
numerous	O
distinct	O
NOEs	O
between	O
the	O
side	O
chains	O
of	O
Asn	B-residue_name_number
-	I-residue_name_number
380	I-residue_name_number
and	O
Met	B-residue_name_number
-	I-residue_name_number
383	I-residue_name_number
of	O
the	O
loop	B-structure_element
region	I-structure_element
and	O
Tyr	B-residue_name_number
-	I-residue_name_number
377	I-residue_name_number
and	O
Val	B-residue_name_number
-	I-residue_name_number
376	I-residue_name_number
of	O
helix	B-structure_element
1	I-structure_element
(	O
Fig	O
.	O
3D	O
).	O

The	O
backbone	O
NH	O
and	O
CHα	O
groups	O
of	O
Gly	B-residue_name_number
-	I-residue_name_number
384	I-residue_name_number
and	O
Asp	B-residue_name_number
-	I-residue_name_number
385	I-residue_name_number
also	O
show	O
NOEs	O
with	O
the	O
side	O
chain	O
of	O
Tyr	B-residue_name_number
-	I-residue_name_number
377	I-residue_name_number
.	O

Mapping	O
the	O
TOCA1	B-protein
and	O
Cdc42	B-site
Binding	I-site
Interfaces	I-site

The	O
HR1TOCA1	B-site
-	I-site
Cdc42	I-site
interface	I-site
was	O
investigated	O
using	O
NMR	B-experimental_method
spectroscopy	I-experimental_method
.	O

A	O
series	O
of	O
15N	B-experimental_method
HSQC	I-experimental_method
experiments	O
was	O
recorded	O
on	O
15N	B-chemical
-	O
labeled	B-protein_state
TOCA1	B-protein
HR1	B-structure_element
domain	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
increasing	B-experimental_method
concentrations	I-experimental_method
of	O
unlabeled	B-protein_state
Cdc42Δ7Q61L	B-complex_assembly
·	I-complex_assembly
GMPPNP	I-complex_assembly
to	O
map	O
the	O
Cdc42	B-site
-	I-site
binding	I-site
surface	I-site
.	O

A	O
comparison	O
of	O
the	O
15N	B-experimental_method
HSQC	I-experimental_method
spectra	B-evidence
of	O
free	B-protein_state
HR1	B-structure_element
and	O
HR1	B-structure_element
in	O
the	O
presence	B-protein_state
of	I-protein_state
excess	O
Cdc42	B-protein
shows	O
that	O
although	O
some	O
peaks	O
were	O
shifted	O
,	O
several	O
were	O
much	O
broader	O
in	O
the	O
complex	O
,	O
and	O
a	O
considerable	O
subset	O
had	O
disappeared	O
(	O
Fig	O
.	O
4A	O
).	O

This	O
behavior	O
cannot	O
be	O
explained	O
by	O
the	O
increase	O
in	O
molecular	O
mass	O
(	O
from	O
12	O
to	O
33	O
kDa	O
)	O
when	O
Cdc42	B-protein
binds	O
and	O
is	O
more	O
likely	O
to	O
be	O
due	O
to	O
conformational	O
exchange	O
.	O

A	O
peak	O
that	O
disappeared	O
or	O
had	O
a	O
CSP	B-experimental_method
above	O
the	O
mean	O
CSP	B-experimental_method
for	O
the	O
spectrum	O
was	O
considered	O
to	O
be	O
significantly	O
affected	O
.	O

A	O
,	O
the	O
15N	B-experimental_method
HSQC	I-experimental_method
of	O
200	O
μm	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
is	O
shown	O
in	O
the	O
free	B-protein_state
form	I-protein_state
(	O
black	O
)	O
and	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
a	O
4	O
-	O
fold	O
molar	O
excess	O
of	O
Cdc42Δ7Q61L	B-complex_assembly
·	I-complex_assembly
GMPPNP	I-complex_assembly
(	O
red	O
).	O

B	O
,	O
CSPs	B-experimental_method
were	O
calculated	O
as	O
described	O
under	O
“	O
Experimental	O
Procedures	O
”	O
and	O
are	O
shown	O
for	O
backbone	O
and	O
side	O
chain	O
NH	O
groups	O
.	O

The	O
mean	O
CSP	B-experimental_method
is	O
marked	O
with	O
a	O
red	O
line	O
.	O

Those	O
that	O
were	O
not	O
traceable	O
due	O
to	O
spectral	O
overlap	O
were	O
assigned	O
a	O
CSP	B-experimental_method
of	O
zero	O
and	O
are	O
marked	O
with	O
an	O
asterisk	O
below	O
the	O
bar	O
.	O

Residues	O
with	O
affected	O
side	O
chain	O
CSPs	B-experimental_method
derived	O
from	O
13C	B-experimental_method
HSQCs	I-experimental_method
are	O
marked	O
with	O
green	O
asterisks	O
above	O
the	O
bars	O
.	O

C	O
,	O
a	O
schematic	O
representation	O
of	O
the	O
HR1	B-structure_element
domain	O
.	O

Residues	O
with	O
significantly	O
affected	O
backbone	O
or	O
side	O
chain	O
chemical	O
shifts	O
when	O
Cdc42	B-protein_state
bound	I-protein_state
and	O
that	O
are	O
buried	O
are	O
colored	O
dark	O
blue	O
,	O
whereas	O
those	O
that	O
are	O
solvent	B-protein_state
-	I-protein_state
accessible	I-protein_state
are	O
colored	O
yellow	O
.	O

Residues	O
with	O
significantly	O
affected	O
backbone	O
and	O
side	O
chain	O
groups	O
that	O
are	O
solvent	B-protein_state
-	I-protein_state
accessible	I-protein_state
are	O
colored	O
red	O
.	O

A	O
close	O
-	O
up	O
of	O
the	O
binding	B-site
region	I-site
is	O
shown	O
,	O
with	O
affected	O
side	O
chain	O
heavy	O
atoms	O
shown	O
as	O
sticks	O
.	O

D	O
,	O
the	O
G	B-site
protein	I-site
-	I-site
binding	I-site
region	I-site
is	O
marked	O
in	O
red	O
onto	O
structures	B-evidence
of	O
the	O
HR1	B-structure_element
domains	O
as	O
indicated	O
.	O

15N	B-experimental_method
HSQC	I-experimental_method
shift	I-experimental_method
mapping	I-experimental_method
experiments	O
report	O
on	O
changes	O
to	O
amide	O
groups	O
,	O
which	O
are	O
mainly	O
inaccessible	O
because	O
they	O
are	O
buried	O
inside	O
the	O
helices	B-structure_element
and	O
are	O
involved	O
in	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
.	O

Therefore	O
,	O
13C	B-experimental_method
HSQC	I-experimental_method
and	O
methyl	B-experimental_method
-	I-experimental_method
selective	I-experimental_method
SOFAST	I-experimental_method
-	I-experimental_method
HMQC	I-experimental_method
experiments	O
were	O
also	O
recorded	O
on	O
15N	B-chemical
,	O
13C	B-chemical
-	O
labeled	B-protein_state
TOCA1	B-protein
HR1	B-structure_element
to	O
yield	O
more	O
information	O
on	O
side	O
chain	O
involvement	O
.	O

The	O
changes	O
were	O
localized	O
to	O
one	O
end	O
of	O
the	O
coiled	B-structure_element
-	I-structure_element
coil	I-structure_element
,	O
and	O
the	O
binding	B-site
site	I-site
appeared	O
to	O
include	O
residues	O
from	O
both	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
and	O
the	O
loop	B-structure_element
region	I-structure_element
that	O
joins	O
them	O
.	O

The	O
residues	O
in	O
the	O
interhelical	B-structure_element
loop	I-structure_element
and	O
helix	B-structure_element
1	I-structure_element
that	O
contact	O
each	O
other	O
(	O
Fig	O
.	O
3D	O
)	O
show	O
shift	O
changes	O
in	O
their	O
backbone	O
NH	O
and	O
side	O
chains	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
Cdc42	B-protein
.	O

For	O
example	O
,	O
the	O
side	O
chain	O
of	O
Asn	B-residue_name_number
-	I-residue_name_number
380	I-residue_name_number
and	O
the	O
backbones	O
of	O
Val	B-residue_name_number
-	I-residue_name_number
376	I-residue_name_number
and	O
Tyr	B-residue_name_number
-	I-residue_name_number
377	I-residue_name_number
were	O
significantly	O
affected	O
but	O
are	O
all	O
buried	O
in	O
the	O
free	B-protein_state
TOCA1	B-protein
HR1	B-structure_element
structure	B-evidence
,	O
indicating	O
that	O
local	O
conformational	O
changes	O
in	O
the	O
loop	B-structure_element
may	O
facilitate	O
complex	O
formation	O
.	O

The	O
chemical	B-experimental_method
shift	I-experimental_method
mapping	I-experimental_method
data	O
indicate	O
that	O
the	O
G	B-site
protein	I-site
-	I-site
binding	I-site
region	I-site
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
is	O
broadly	O
similar	O
to	O
that	O
of	O
the	O
CIP4	B-protein
and	O
PRK1	B-protein
HR1	B-structure_element
domains	O
(	O
Figs	O
.	O
3B	O
and	O
4D	O
).	O

The	O
overall	O
CSP	B-experimental_method
was	O
calculated	O
for	O
each	O
residue	O
.	O

Detailed	O
side	O
chain	O
data	O
could	O
not	O
be	O
obtained	O
for	O
all	O
residues	O
due	O
to	O
spectral	O
overlap	O
,	O
but	O
constant	B-experimental_method
time	I-experimental_method
13C	I-experimental_method
HSQC	I-experimental_method
and	O
methyl	B-experimental_method
-	I-experimental_method
selective	I-experimental_method
SOFAST	I-experimental_method
-	I-experimental_method
HMQC	I-experimental_method
experiments	O
provided	O
further	O
information	O
on	O
certain	O
well	O
resolved	O
side	O
chains	O
(	O
marked	O
with	O
green	O
asterisks	O
in	O
Fig	O
.	O
5B	O
).	O

B	O
,	O
CSPs	B-experimental_method
are	O
shown	O
for	O
backbone	O
NH	O
groups	O
.	O

The	O
red	O
line	O
indicates	O
the	O
mean	O
CSP	B-experimental_method
,	O
plus	O
one	O
S	O
.	O
D	O
.	O
Residues	O
that	O
disappeared	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
Cdc42	B-protein
were	O
assigned	O
a	O
CSP	B-experimental_method
of	O
0	O
.	O
1	O
and	O
are	O
indicated	O
with	O
open	O
bars	O
.	O

C	O
,	O
the	O
residues	O
with	O
significantly	O
affected	O
backbone	O
and	O
side	O
chain	O
groups	O
are	O
highlighted	O
on	O
an	O
NMR	B-experimental_method
structure	B-evidence
of	O
free	B-protein_state
Cdc42Δ7Q61L	B-complex_assembly
·	I-complex_assembly
GMPPNP	I-complex_assembly
;	O
those	O
that	O
are	O
buried	O
are	O
colored	O
dark	O
blue	O
,	O
whereas	O
those	O
that	O
are	O
solvent	B-protein_state
-	I-protein_state
accessible	I-protein_state
are	O
colored	O
red	O
.	O

Residues	O
without	O
information	O
from	O
shift	B-experimental_method
mapping	I-experimental_method
are	O
colored	O
gray	O
.	O

As	O
many	O
of	O
the	O
peaks	O
disappeared	O
,	O
the	O
mean	B-evidence
chemical	I-evidence
shift	I-evidence
change	I-evidence
was	O
relatively	O
low	O
,	O
so	O
a	O
threshold	O
of	O
the	O
mean	O
plus	O
one	O
S	O
.	O
D	O
.	O
value	O
was	O
used	O
to	O
define	O
a	O
significant	O
CSP	B-experimental_method
.	O

Parts	O
of	O
the	O
switch	B-site
regions	I-site
(	O
Fig	O
.	O
5	O
,	O
B	O
and	O
C	O
)	O
are	O
invisible	O
in	O
NMR	B-experimental_method
spectra	B-evidence
recorded	O
on	O
free	B-protein_state
Cdc42	B-protein
due	O
to	O
conformational	O
exchange	O
.	O

These	O
switch	B-site
regions	I-site
become	O
visible	O
in	O
Cdc42	B-protein
and	O
other	O
small	O
G	B-protein_type
protein	I-protein_type
·	O
effector	O
complexes	O
due	O
to	O
a	O
decrease	O
in	O
conformational	O
freedom	O
upon	O
complex	O
formation	O
.	O

Indeed	O
,	O
Ser	B-residue_name_number
-	I-residue_name_number
30	I-residue_name_number
of	O
switch	B-site
I	I-site
and	O
Arg	B-residue_name_number
-	I-residue_name_number
66	I-residue_name_number
,	O
Arg	B-residue_name_number
-	I-residue_name_number
68	I-residue_name_number
,	O
Leu	B-residue_name_number
-	I-residue_name_number
70	I-residue_name_number
,	O
and	O
Ser	B-residue_name_number
-	I-residue_name_number
71	I-residue_name_number
of	O
switch	B-site
II	I-site
are	O
visible	O
in	O
free	B-protein_state
Cdc42	B-protein
but	O
disappear	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
the	O
HR1	B-structure_element
domain	O
.	O

This	O
suggests	O
that	O
the	O
switch	B-site
regions	I-site
are	O
not	O
rigidified	O
in	O
the	O
HR1	B-structure_element
complex	O
and	O
are	O
still	O
in	O
conformational	O
exchange	O
.	O

Nevertheless	O
,	O
mapping	O
of	O
the	O
affected	O
residues	O
onto	O
the	O
NMR	B-experimental_method
structure	B-evidence
of	O
free	B-protein_state
Cdc42Δ7Q61L	B-complex_assembly
·	I-complex_assembly
GMPPNP	I-complex_assembly
(	O
Fig	O
.	O
5C	O
)	O
8	O
shows	O
that	O
,	O
although	O
they	O
are	O
relatively	O
widespread	O
compared	O
with	O
changes	O
in	O
the	O
HR1	B-structure_element
domain	O
,	O
in	O
general	O
,	O
they	O
are	O
on	O
the	O
face	O
of	O
the	O
protein	O
that	O
includes	O
the	O
switches	B-site
.	O

Modeling	O
the	O
Cdc42	B-complex_assembly
·	I-complex_assembly
TOCA1	I-complex_assembly
HR1	I-complex_assembly
Complex	O

HADDOCK	B-experimental_method
was	O
therefore	O
used	O
to	O
perform	O
rigid	B-experimental_method
body	I-experimental_method
docking	I-experimental_method
based	O
on	O
the	O
structures	B-evidence
of	O
free	B-protein_state
HR1	B-structure_element
domain	O
and	O
Cdc42	B-protein
and	O
ambiguous	O
interaction	O
restraints	O
derived	O
from	O
the	O
titration	B-experimental_method
experiments	I-experimental_method
described	O
above	O
.	O

The	O
orientation	O
of	O
the	O
HR1	B-structure_element
domain	O
with	O
respect	O
to	O
Cdc42	B-protein
cannot	O
be	O
definitively	O
concluded	O
in	O
the	O
absence	O
of	O
unambiguous	O
distance	O
restraints	O
;	O
hence	O
,	O
HADDOCK	B-experimental_method
produced	O
a	O
set	O
of	O
models	O
in	O
which	O
the	O
HR1	B-structure_element
domain	O
contacts	O
the	O
same	O
surface	O
on	O
Cdc42	B-protein
but	O
is	O
in	O
various	O
orientations	O
with	O
respect	O
to	O
Cdc42	B-protein
.	O

A	O
representative	O
model	O
from	O
this	O
cluster	O
is	O
shown	O
in	O
Fig	O
.	O
6A	O
alongside	O
the	O
Rac1	B-complex_assembly
-	I-complex_assembly
HR1b	I-complex_assembly
structure	B-evidence
(	O
PDB	O
code	O
2RMK	O
)	O
in	O
Fig	O
.	O
6B	O
.	O

A	O
,	O
a	O
representative	O
model	O
of	O
the	O
Cdc42	B-complex_assembly
·	I-complex_assembly
HR1	I-complex_assembly
complex	O
from	O
the	O
cluster	O
closest	O
to	O
the	O
lowest	O
energy	O
model	O
produced	O
using	O
HADDOCK	B-experimental_method
.	O

Residues	O
of	O
Cdc42	B-protein
that	O
are	O
affected	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
the	O
HR1	B-structure_element
domain	O
but	O
are	O
not	O
in	O
close	O
proximity	O
to	O
it	O
are	O
colored	O
in	O
red	O
and	O
labeled	O
.	O

B	O
,	O
structure	B-evidence
of	O
Rac1	B-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
the	O
HR1b	B-structure_element
domain	O
of	O
PRK1	B-protein
(	O
PDB	O
code	O
2RMK	O
).	O

Contact	O
residues	O
of	O
RhoA	B-protein
and	O
Rac1	B-protein
to	O
PRK1	B-protein
HR1a	B-structure_element
and	O
HR1b	B-structure_element
,	O
respectively	O
,	O
are	O
colored	O
cyan	O
.	O

Residues	O
equivalent	O
to	O
Rac1	B-protein
and	O
RhoA	B-protein
contact	B-site
sites	I-site
but	O
that	O
are	O
invisible	O
in	O
free	B-protein_state
Cdc42	B-protein
are	O
gray	O
.	O

The	O
four	O
lowest	O
energy	O
structures	B-evidence
in	O
the	O
chosen	O
HADDOCK	B-experimental_method
cluster	O
are	O
shown	O
overlaid	O
,	O
with	O
the	O
residues	O
of	O
interest	O
shown	O
as	O
sticks	O
and	O
labeled	O
.	O

Cdc42	O
is	O
shown	O
in	O
cyan	O
,	O
and	O
TOCA1	B-protein
is	O
shown	O
in	O
purple	O
.	O

A	O
sequence	B-experimental_method
alignment	I-experimental_method
of	O
RhoA	B-protein
,	O
Cdc42	B-protein
,	O
and	O
Rac1	B-protein
is	O
shown	O
in	O
Fig	O
.	O
6C	O
.	O

The	O
RhoA	B-protein
and	O
Rac1	B-protein
contact	O
residues	O
in	O
the	O
switch	B-site
regions	I-site
are	O
invisible	O
in	O
the	O
spectra	B-evidence
of	O
Cdc42	B-protein
,	O
but	O
they	O
are	O
generally	O
conserved	B-protein_state
between	O
all	O
three	O
G	B-protein_type
proteins	I-protein_type
.	O

Several	O
Cdc42	B-protein
residues	O
identified	O
by	O
chemical	B-experimental_method
shift	I-experimental_method
mapping	I-experimental_method
are	O
not	O
in	O
close	O
contact	O
in	O
the	O
Cdc42	B-complex_assembly
·	I-complex_assembly
TOCA1	I-complex_assembly
model	O
(	O
Fig	O
.	O
6A	O
).	O

Other	O
residues	O
that	O
are	O
affected	O
in	O
the	O
Cdc42	B-complex_assembly
·	I-complex_assembly
TOCA1	I-complex_assembly
complex	O
but	O
that	O
do	O
not	O
correspond	O
to	O
contact	O
residues	O
of	O
RhoA	B-protein
or	O
Rac1	B-protein
(	O
Fig	O
.	O
6C	O
)	O
include	O
Gln	B-residue_name_number
-	I-residue_name_number
2Cdc42	I-residue_name_number
,	O
Lys	B-residue_name_number
-	I-residue_name_number
16Cdc42	I-residue_name_number
,	O
Thr	B-residue_name_number
-	I-residue_name_number
52Cdc42	I-residue_name_number
,	O
and	O
Arg	B-residue_name_number
-	I-residue_name_number
68Cdc42	I-residue_name_number
.	O

Competition	O
between	O
N	B-protein
-	I-protein
WASP	I-protein
and	O
TOCA1	B-protein

From	O
the	O
known	O
interactions	O
and	O
effects	O
of	O
the	O
proteins	O
in	O
biological	O
systems	O
,	O
it	O
has	O
been	O
suggested	O
that	O
TOCA1	B-protein
and	O
N	B-protein
-	I-protein
WASP	I-protein
could	O
bind	O
Cdc42	B-protein
simultaneously	O
.	O

An	O
overlay	B-experimental_method
of	O
the	O
HADDOCK	B-experimental_method
model	B-evidence
of	O
the	O
Cdc42	B-complex_assembly
·	I-complex_assembly
HR1TOCA1	I-complex_assembly
complex	O
and	O
the	O
structure	B-evidence
of	O
Cdc42	B-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
the	O
GBD	B-structure_element
of	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
homologue	O
,	O
WASP	B-protein
(	O
PDB	O
code	O
1CEE	O
),	O
shows	O
that	O
the	O
HR1	B-structure_element
and	O
GBD	B-site
binding	I-site
sites	I-site
only	O
partly	O
overlap	O
,	O
and	O
,	O
therefore	O
,	O
a	O
ternary	O
complex	O
remained	O
possible	O
(	O
Fig	O
.	O
7A	O
).	O

Interestingly	O
,	O
the	O
presence	B-protein_state
of	I-protein_state
the	O
TOCA1	B-protein
HR1	B-structure_element
would	O
not	O
prevent	O
the	O
core	O
CRIB	B-structure_element
of	O
WASP	B-protein
from	O
binding	O
to	O
Cdc42	B-protein
,	O
although	O
the	O
regions	O
C	O
-	O
terminal	O
to	O
the	O
CRIB	B-structure_element
that	O
are	O
required	O
for	O
high	O
affinity	O
binding	O
of	O
WASP	B-protein
would	O
interfere	O
sterically	O
with	O
the	O
TOCA1	B-protein
HR1	B-structure_element
.	O

The	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
displaces	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
.	O

A	O
,	O
the	O
model	O
of	O
the	O
Cdc42	B-complex_assembly
·	I-complex_assembly
TOCA1	I-complex_assembly
HR1	B-structure_element
domain	O
complex	O
overlaid	O
with	O
the	O
Cdc42	B-complex_assembly
-	I-complex_assembly
WASP	I-complex_assembly
structure	B-evidence
.	O

Cdc42	O
is	O
shown	O
in	O
green	O
,	O
and	O
TOCA1	B-protein
is	O
shown	O
in	O
purple	O
.	O

A	O
semitransparent	O
surface	O
representation	O
of	O
Cdc42	B-protein
and	O
WASP	B-protein
is	O
shown	O
overlaid	O
with	O
the	O
schematic	O
.	O

B	O
,	O
competition	B-experimental_method
SPA	I-experimental_method
experiments	O
carried	O
out	O
with	O
indicated	O
concentrations	O
of	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
construct	O
titrated	B-experimental_method
into	O
30	O
nm	O
GST	B-mutant
-	I-mutant
ACK	I-mutant
or	O
GST	B-mutant
-	I-mutant
WASP	I-mutant
GBD	B-structure_element
and	O
30	O
nm	O
Cdc42Δ7Q61L	B-complex_assembly
·[	I-complex_assembly
3H	I-complex_assembly
]	I-complex_assembly
GTP	I-complex_assembly
.	O

C	O
,	O
Selected	O
regions	O
of	O
the	O
15N	B-experimental_method
HSQC	I-experimental_method
of	O
145	O
μm	O
Cdc42Δ7Q61L	B-complex_assembly
·	I-complex_assembly
GMPPNP	I-complex_assembly
with	O
the	O
indicated	O
ratios	O
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
,	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
,	O
or	O
both	O
,	O
showing	O
that	O
the	O
TOCA	B-protein
HR1	B-structure_element
domain	O
does	O
not	O
displace	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
.	O

The	O
Kd	B-evidence
that	O
was	O
determined	O
(	O
37	O
nm	O
)	O
is	O
consistent	O
with	O
the	O
previously	O
reported	O
affinity	B-evidence
.	O

Unlabeled	B-protein_state
HR1TOCA1	B-structure_element
was	O
then	O
added	O
to	O
the	O
Cdc42	B-complex_assembly
·	I-complex_assembly
N	I-complex_assembly
-	I-complex_assembly
WASP	I-complex_assembly
complex	O
,	O
and	O
no	O
changes	O
were	O
seen	O
,	O
suggesting	O
that	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
was	O
not	O
displaced	O
even	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
a	O
5	O
-	O
fold	O
excess	O
of	O
HR1TOCA1	B-structure_element
.	O

Furthermore	O
,	O
15N	B-chemical
-	O
TOCA1	B-protein
HR1	B-structure_element
was	O
monitored	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
unlabeled	B-protein_state
Cdc42Δ7Q61L	B-complex_assembly
·	I-complex_assembly
GMPPNP	I-complex_assembly
(	O
1	O
:	O
1	O
)	O
before	O
and	O
after	O
the	O
addition	O
of	O
0	O
.	O
25	O
and	O
1	O
.	O
0	O
eq	O
of	O
unlabeled	B-protein_state
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
.	O

When	O
in	O
fast	O
exchange	O
,	O
the	O
NMR	B-experimental_method
signal	O
represents	O
a	O
population	O
-	O
weighted	O
average	O
between	O
free	B-protein_state
and	O
bound	B-protein_state
states	O
,	O
so	O
the	O
intermediate	O
spectrum	B-evidence
indicates	O
that	O
the	O
population	O
comprises	O
a	O
mixture	O
of	O
free	B-protein_state
and	O
bound	B-protein_state
HR1	B-structure_element
domain	O
.	O

Again	O
,	O
the	O
experiments	O
were	O
recorded	O
on	O
protein	O
samples	O
far	O
in	O
excess	O
of	O
the	O
individual	O
Kd	B-evidence
values	O
(	O
600	O
μm	O
each	O
protein	O
).	O

These	O
data	O
indicate	O
that	O
the	O
HR1	B-structure_element
domain	O
is	O
displaced	O
from	O
Cdc42	B-protein
by	O
N	B-protein
-	I-protein
WASP	I-protein
and	O
that	O
a	O
ternary	O
complex	O
comprising	O
TOCA1	B-protein
HR1	B-structure_element
,	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
,	O
and	O
Cdc42	B-protein
is	O
not	O
formed	O
.	O

Taken	O
together	O
,	O
the	O
data	O
in	O
Fig	O
.	O
7	O
,	O
C	O
and	O
D	O
,	O
indicate	O
unidirectional	O
competition	O
for	O
Cdc42	B-protein
binding	O
in	O
which	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
displaces	O
TOCA1	B-protein
HR1	B-structure_element
but	O
not	O
vice	O
versa	O
.	O

To	O
extend	O
these	O
studies	O
to	O
a	O
more	O
complex	O
system	O
and	O
to	O
assess	O
the	O
ability	O
of	O
TOCA1	B-protein
HR1	B-structure_element
to	O
compete	O
with	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
N	B-protein
-	I-protein
WASP	I-protein
,	O
pyrene	B-experimental_method
actin	I-experimental_method
assays	I-experimental_method
were	O
employed	O
.	O

These	O
assays	O
,	O
described	O
in	O
detail	O
elsewhere	O
,	O
were	O
carried	O
out	O
using	O
pyrene	B-chemical
actin	I-chemical
-	O
supplemented	O
Xenopus	B-taxonomy_domain
extracts	O
into	O
which	O
exogenous	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
or	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
was	O
added	O
,	O
to	O
assess	O
their	O
effects	O
on	O
actin	B-protein_type
polymerization	O
.	O

Endogenous	O
N	B-protein
-	I-protein
WASP	I-protein
is	O
present	O
at	O
∼	O
100	O
nm	O
in	O
Xenopus	B-taxonomy_domain
extracts	O
,	O
whereas	O
TOCA1	B-protein
is	O
present	O
at	O
a	O
10	O
-	O
fold	O
lower	O
concentration	O
than	O
N	B-protein
-	I-protein
WASP	I-protein
.	O

The	O
addition	B-experimental_method
of	O
the	O
isolated	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
significantly	O
inhibited	O
the	O
polymerization	O
of	O
actin	B-protein_type
at	O
concentrations	O
as	O
low	O
as	O
100	O
nm	O
and	O
completely	O
abolished	O
polymerization	O
at	O
higher	O
concentrations	O
(	O
Fig	O
.	O
8	O
).	O

The	O
addition	B-experimental_method
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
to	O
100	O
μm	O
had	O
no	O
significant	O
effect	O
on	O
the	O
rate	O
of	O
actin	B-protein_type
polymerization	O
or	O
maximum	B-evidence
fluorescence	I-evidence
.	O

This	O
is	O
consistent	O
with	O
endogenous	B-protein_state
N	B-protein
-	I-protein
WASP	I-protein
,	O
activated	O
by	O
other	O
components	O
of	O
the	O
assay	O
,	O
outcompeting	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
for	O
Cdc42	B-protein
binding	O
.	O

Actin	O
polymerization	O
downstream	O
of	O
Cdc42	B-complex_assembly
·	I-complex_assembly
N	I-complex_assembly
-	I-complex_assembly
WASP	I-complex_assembly
·	I-complex_assembly
TOCA1	I-complex_assembly
is	O
inhibited	B-protein_state
by	O
excess	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
but	O
not	O
by	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
.	O

Fluorescence	B-evidence
curves	I-evidence
show	O
actin	O
polymerization	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
increasing	B-experimental_method
concentrations	I-experimental_method
of	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
or	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
as	O
indicated	O
.	O

The	O
Cdc42	B-protein
-	O
TOCA1	B-protein
Interaction	O

The	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
alone	B-protein_state
is	O
sufficient	O
for	O
Cdc42	B-protein
binding	O
in	O
vitro	O
,	O
yet	O
the	O
affinity	B-evidence
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
for	O
Cdc42	B-protein
is	O
remarkably	O
low	O
(	O
Kd	B-evidence
≈	O
5	O
μm	O
).	O

A	O
single	O
binding	B-site
interface	I-site
on	O
both	O
the	O
HR1	B-structure_element
domain	O
and	O
Cdc42	B-protein
can	O
be	O
concluded	O
from	O
the	O
data	O
presented	O
here	O
.	O

Furthermore	O
,	O
the	O
interfaces	B-site
are	O
comparable	O
with	O
those	O
of	O
other	O
G	B-protein_type
protein	I-protein_type
-	O
HR1	B-structure_element
interactions	O
(	O
Fig	O
.	O
4	O
),	O
and	O
the	O
lowest	O
energy	O
model	B-evidence
produced	O
in	O
rigid	B-experimental_method
body	I-experimental_method
docking	I-experimental_method
resembles	O
previously	O
studied	O
G	B-complex_assembly
protein	I-complex_assembly
·	I-complex_assembly
HR1	I-complex_assembly
complexes	O
(	O
Fig	O
.	O
6	O
).	O

It	O
seems	O
,	O
therefore	O
,	O
that	O
the	O
interaction	O
,	O
despite	O
its	O
relatively	O
low	O
affinity	O
,	O
is	O
specific	O
and	O
sterically	O
similar	O
to	O
other	O
HR1	B-structure_element
domain	O
-	O
G	B-protein_type
protein	I-protein_type
interactions	O
.	O

A	O
short	O
region	O
N	O
-	O
terminal	O
to	O
the	O
coiled	B-structure_element
-	I-structure_element
coil	I-structure_element
exhibits	O
a	O
series	O
of	O
turns	O
and	O
contacts	O
residues	O
of	O
both	O
helices	O
of	O
the	O
coiled	B-structure_element
-	I-structure_element
coil	I-structure_element
(	O
Fig	O
.	O
3	O
).	O

The	O
corresponding	O
sequence	O
in	O
CIP4	B-protein
also	O
includes	O
a	O
series	O
of	O
turns	O
but	O
is	O
flexible	O
,	O
whereas	O
in	O
the	O
HR1a	B-structure_element
domain	O
of	O
PRK1	B-protein
,	O
the	O
equivalent	O
region	O
adopts	O
an	O
α	B-structure_element
-	I-structure_element
helical	I-structure_element
structure	I-structure_element
that	O
packs	O
against	O
the	O
coiled	B-structure_element
-	I-structure_element
coil	I-structure_element
.	O

The	O
interhelical	B-structure_element
loops	I-structure_element
of	O
TOCA1	B-protein
and	O
CIP4	B-protein
differ	O
from	O
the	O
same	O
region	O
in	O
the	O
HR1	B-structure_element
domains	O
of	O
PRK1	B-protein
in	O
that	O
they	O
are	O
longer	O
and	O
contain	O
two	O
short	O
stretches	O
of	O
310	B-structure_element
-	I-structure_element
helix	I-structure_element
.	O

This	O
region	O
lies	O
within	O
the	O
G	B-site
protein	I-site
-	I-site
binding	I-site
surface	I-site
of	O
all	O
of	O
the	O
HR1	B-structure_element
domains	O
(	O
Fig	O
.	O
4D	O
).	O

TOCA1	B-protein
and	O
CIP4	B-protein
both	O
bind	O
weakly	O
to	O
Cdc42	B-protein
,	O
whereas	O
the	O
HR1a	B-structure_element
domain	O
of	O
PRK1	B-protein
binds	O
tightly	O
to	O
RhoA	B-protein
and	O
Rac1	B-protein
,	O
and	O
the	O
HR1b	B-structure_element
domain	O
binds	O
to	O
Rac1	B-protein
.	O

The	O
structural	O
features	O
shared	O
by	O
TOCA1	B-protein
and	O
CIP4	B-protein
may	O
therefore	O
be	O
related	O
to	O
Cdc42	B-protein
binding	O
specificity	O
and	O
the	O
low	O
affinities	O
.	O

In	O
free	B-protein_state
TOCA1	B-protein
,	O
the	O
side	O
chains	O
of	O
the	O
interhelical	B-structure_element
region	I-structure_element
make	O
extensive	O
contacts	O
with	O
residues	O
in	O
helix	B-structure_element
1	I-structure_element
.	O

Many	O
of	O
these	O
residues	O
are	O
significantly	O
affected	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
Cdc42	B-protein
,	O
so	O
it	O
is	O
likely	O
that	O
the	O
conformation	O
of	O
this	O
loop	B-structure_element
is	O
altered	O
in	O
the	O
Cdc42	B-protein
complex	O
.	O

These	O
observations	O
therefore	O
provide	O
a	O
molecular	O
mechanism	O
whereby	O
mutation	B-experimental_method
of	O
Met383	B-residue_name_number
-	O
Gly384	B-residue_name_number
-	O
Asp385	B-residue_name_number
to	O
Ile383	B-residue_name_number
-	O
Ser384	B-residue_name_number
-	O
Thr385	B-residue_name_number
abolishes	O
TOCA1	B-protein
binding	O
to	O
Cdc42	B-protein
.	O

The	O
lowest	O
energy	O
model	B-evidence
produced	O
by	O
HADDOCK	B-experimental_method
using	O
ambiguous	O
interaction	O
restraints	O
from	O
the	O
titration	B-evidence
data	O
resembled	O
the	O
NMR	B-experimental_method
structures	B-evidence
of	O
RhoA	B-protein
and	O
Rac1	B-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
their	O
HR1	B-structure_element
domain	O
partners	O
.	O

For	O
example	O
,	O
Phe	B-residue_name_number
-	I-residue_name_number
56Cdc42	I-residue_name_number
,	O
which	O
is	O
not	O
visible	O
in	O
free	B-protein_state
Cdc42	B-protein
or	O
Cdc42	B-complex_assembly
·	I-complex_assembly
HR1TOCA1	I-complex_assembly
,	O
is	O
close	O
to	O
the	O
TOCA1	B-protein
HR1	B-structure_element
(	O
Fig	O
.	O
6A	O
).	O

Phe	B-residue_name_number
-	I-residue_name_number
56Cdc42	I-residue_name_number
,	O
which	O
is	O
a	O
Trp	B-residue_name
in	O
both	O
Rac1	B-protein
and	O
RhoA	B-protein
(	O
Fig	O
.	O
6C	O
),	O
is	O
thought	O
to	O
pack	O
behind	O
switch	B-site
I	I-site
when	O
Cdc42	B-protein
interacts	O
with	O
ACK	B-protein
,	O
maintaining	O
the	O
switch	O
in	O
a	O
binding	O
-	O
competent	O
orientation	O
.	O

This	O
residue	O
has	O
also	O
been	O
identified	O
as	O
important	O
for	O
Cdc42	B-protein
-	O
WASP	B-protein
binding	O
.	O

Phe	B-residue_name_number
-	I-residue_name_number
56Cdc42	I-residue_name_number
is	O
therefore	O
likely	O
to	O
be	O
involved	O
in	O
the	O
Cdc42	B-protein
-	O
TOCA1	B-protein
interaction	O
,	O
probably	O
by	O
stabilizing	O
the	O
position	O
of	O
switch	B-site
I	I-site
.	O

Gln	B-residue_name_number
-	I-residue_name_number
2Cdc42	I-residue_name_number
,	O
which	O
has	O
also	O
been	O
identified	O
as	O
a	O
contact	O
residue	O
in	O
the	O
Cdc42	B-complex_assembly
·	I-complex_assembly
ACK	I-complex_assembly
complex	O
,	O
contacts	O
Val	B-residue_name_number
-	I-residue_name_number
376TOCA1	I-residue_name_number
and	O
Asn	B-residue_name_number
-	I-residue_name_number
380TOCA1	I-residue_name_number
in	O
the	O
model	O
and	O
disrupts	O
the	O
contacts	O
between	O
the	O
interhelical	B-structure_element
loop	I-structure_element
and	O
the	O
first	B-structure_element
helix	I-structure_element
of	O
the	O
TOCA1	B-protein
coiled	B-structure_element
-	I-structure_element
coil	I-structure_element
.	O

The	O
position	O
equivalent	O
to	O
Lys	B-residue_name_number
-	I-residue_name_number
372TOCA1	I-residue_name_number
in	O
PRK1	B-protein
is	O
Glu	B-residue_name_number
-	I-residue_name_number
58HR1a	I-residue_name_number
or	O
Gln	B-residue_name_number
-	I-residue_name_number
151HR1b	I-residue_name_number
.	O

Thr	B-residue_name_number
-	I-residue_name_number
52Cdc42	I-residue_name_number
-	O
Lys	B-residue_name_number
-	I-residue_name_number
372TOCA1	I-residue_name_number
may	O
therefore	O
represent	O
a	O
specific	O
Cdc42	B-protein
-	O
HR1TOCA1	B-structure_element
contact	O
.	O

Arg	B-residue_name_number
-	I-residue_name_number
68Cdc42	I-residue_name_number
of	O
switch	B-site
II	I-site
is	O
positioned	O
close	O
to	O
Glu	B-residue_name_number
-	I-residue_name_number
395TOCA1	I-residue_name_number
(	O
Fig	O
.	O
6D	O
),	O
suggesting	O
a	O
direct	O
electrostatic	O
contact	O
between	O
switch	B-site
II	I-site
of	O
Cdc42	B-protein
and	O
helix	B-structure_element
2	I-structure_element
of	O
the	O
HR1	B-structure_element
domain	O
.	O

The	O
equivalent	O
Arg	B-residue_name
in	O
Rac1	B-protein
and	O
RhoA	B-protein
is	O
pointing	O
away	O
from	O
the	O
HR1	B-structure_element
domains	O
of	O
PRK1	B-protein
.	O

The	O
solution	B-evidence
structure	I-evidence
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
presented	O
here	O
,	O
along	O
with	O
the	O
model	O
of	O
the	O
HR1TOCA1	B-complex_assembly
·	I-complex_assembly
Cdc42	I-complex_assembly
complex	O
is	O
consistent	O
with	O
a	O
conserved	O
mode	O
of	O
binding	O
across	O
the	O
known	O
HR1	B-structure_element
domain	O
-	O
Rho	O
family	O
interactions	O
,	O
despite	O
their	O
differing	O
affinities	O
.	O

The	O
weak	O
binding	O
prevented	O
detailed	O
structural	B-experimental_method
and	I-experimental_method
thermodynamic	I-experimental_method
studies	I-experimental_method
of	O
the	O
complex	O
.	O

We	O
have	O
previously	O
postulated	O
that	O
the	O
inherent	O
flexibility	O
of	O
HR1	B-structure_element
domains	O
contributes	O
to	O
their	O
ability	O
to	O
bind	O
to	O
different	O
Rho	B-protein_type
family	I-protein_type
G	I-protein_type
proteins	I-protein_type
,	O
with	O
Rho	O
-	O
binding	O
HR1	B-structure_element
domains	O
displaying	O
increased	O
flexibility	O
,	O
reflected	O
in	O
their	O
lower	O
melting	B-evidence
temperatures	I-evidence
(	O
Tm	B-evidence
)	O
and	O
Rac	B-protein_type
binders	O
being	O
more	O
rigid	O
.	O

The	O
Tm	B-evidence
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
is	O
61	O
.	O
9	O
°	O
C	O
(	O
data	O
not	O
shown	O
),	O
which	O
is	O
the	O
highest	O
Tm	B-evidence
that	O
we	O
have	O
measured	O
for	O
an	O
HR1	B-structure_element
domain	O
thus	O
far	O
.	O

As	O
such	O
,	O
the	O
ability	O
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
to	O
bind	O
to	O
Cdc42	B-protein
(	O
a	O
close	O
relative	O
of	O
Rac1	B-protein
rather	O
than	O
RhoA	B-protein
)	O
fits	O
this	O
trend	O
.	O

The	O
low	O
affinity	O
of	O
the	O
Cdc42	B-protein
-	O
HR1TOCA1	B-structure_element
interaction	O
is	O
consistent	O
with	O
a	O
tightly	O
spatially	O
and	O
temporally	O
regulated	O
pathway	O
,	O
requiring	O
combinatorial	O
signals	O
leading	O
to	O
a	O
series	O
of	O
coincident	O
weak	O
interactions	O
that	O
elicit	O
full	O
activation	O
.	O

The	O
HR1	B-structure_element
domains	O
from	O
other	O
TOCA	B-protein_type
family	I-protein_type
members	I-protein_type
,	O
CIP4	B-protein
and	O
FBP17	B-protein
,	O
also	O
bind	O
at	O
low	O
micromolar	O
affinities	O
to	O
Cdc42	B-protein
,	O
so	O
the	O
low	O
affinity	O
interaction	O
appears	O
to	O
be	O
commonplace	O
among	O
this	O
family	O
of	O
HR1	B-protein_type
domain	I-protein_type
proteins	I-protein_type
,	O
in	O
contrast	O
to	O
the	O
PRK	B-protein_type
family	I-protein_type
.	O

The	O
low	O
affinity	O
of	O
the	O
HR1TOCA1	B-structure_element
-	O
Cdc42	B-protein
interaction	O
in	O
the	O
context	O
of	O
the	O
physiological	O
concentration	O
of	O
TOCA1	B-protein
in	O
Xenopus	B-taxonomy_domain
extracts	O
(∼	O
10	O
nm	O
)	O
suggests	O
that	O
binding	O
between	O
TOCA1	B-protein
and	O
Cdc42	B-protein
is	O
likely	O
to	O
occur	O
in	O
vivo	O
only	O
when	O
TOCA1	B-protein
is	O
at	O
high	O
local	O
concentrations	O
and	O
membrane	O
-	O
localized	O
and	O
therefore	O
in	O
close	O
proximity	O
to	O
activated	B-protein_state
Cdc42	B-protein
.	O

For	O
example	O
,	O
electrostatic	B-bond_interaction
interactions	I-bond_interaction
between	O
the	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domain	O
and	O
the	O
membrane	O
are	O
required	O
for	O
TOCA1	B-protein
recruitment	O
to	O
membrane	O
vesicles	O
and	O
tubules	O
,	O
and	O
TOCA1	B-protein
-	O
dependent	O
actin	O
polymerization	O
is	O
known	O
to	O
depend	O
specifically	O
on	O
PI	B-chemical
(	I-chemical
4	I-chemical
,	I-chemical
5	I-chemical
)	I-chemical
P2	I-chemical
.	O

Furthermore	O
,	O
the	O
isolated	B-experimental_method
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domain	O
of	O
FBP17	B-protein
has	O
been	O
shown	O
to	O
induce	O
membrane	O
tubulation	O
of	O
brain	O
liposomes	O
and	O
BAR	B-structure_element
domain	O
proteins	O
that	O
promote	O
tubulation	O
cluster	O
on	O
membranes	O
at	O
high	O
densities	O
.	O

Once	O
at	O
the	O
membrane	O
,	O
high	O
local	O
concentrations	O
of	O
TOCA1	B-protein
could	O
exceed	O
the	O
Kd	B-evidence
of	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
dimerization	B-oligomeric_state
(	O
likely	O
to	O
be	O
comparable	O
with	O
that	O
of	O
the	O
FCHo2	B-protein
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domain	O
(	O
2	O
.	O
5	O
μm	O
))	O
and	O
that	O
of	O
the	O
Cdc42	B-protein
-	O
HR1TOCA1	B-structure_element
interaction	O
.	O

Cdc42	B-protein
-	O
HR1TOCA1	B-structure_element
binding	O
would	O
then	O
be	O
favorable	O
,	O
as	O
long	O
as	O
coincident	O
activation	O
of	O
Cdc42	B-protein
had	O
occurred	O
,	O
leading	O
to	O
stabilization	O
of	O
TOCA1	B-protein
at	O
the	O
membrane	O
and	O
downstream	O
activation	O
of	O
N	B-protein
-	I-protein
WASP	I-protein
.	O

It	O
has	O
been	O
postulated	O
that	O
WASP	B-protein_type
and	O
N	B-protein
-	I-protein
WASP	I-protein
exist	O
in	O
equilibrium	O
between	O
folded	B-protein_state
(	O
inactive	B-protein_state
)	O
and	O
unfolded	B-protein_state
(	O
active	B-protein_state
)	O
forms	O
,	O
and	O
the	O
affinity	B-evidence
of	O
Cdc42	B-protein
for	O
the	O
unfolded	B-protein_state
WASP	B-protein_type
proteins	O
is	O
significantly	O
enhanced	O
.	O

The	O
unfolded	B-protein_state
,	O
high	O
affinity	O
state	O
of	O
WASP	B-protein_type
is	O
represented	O
by	O
a	O
short	O
peptide	B-chemical
,	O
the	O
GBD	B-structure_element
,	O
which	O
binds	O
with	O
a	O
low	O
nanomolar	O
affinity	O
to	O
Cdc42	B-protein
.	O

In	O
contrast	O
,	O
the	O
best	O
estimate	O
of	O
the	O
affinity	B-evidence
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
WASP	B-protein_type
for	O
Cdc42	B-protein
is	O
low	O
micromolar	O
.	O

In	O
the	O
inactive	B-protein_state
state	O
of	O
WASP	B-protein_type
,	O
the	O
actin	O
-	O
and	O
Arp2	B-complex_assembly
/	I-complex_assembly
3	I-complex_assembly
-	O
binding	O
VCA	B-structure_element
domain	O
contacts	O
the	O
GBD	B-structure_element
,	O
competing	O
for	O
Cdc42	B-protein
binding	O
.	O

The	O
high	O
affinity	O
of	O
Cdc42	B-protein
for	O
the	O
unfolded	B-protein_state
,	O
active	B-protein_state
form	O
pushes	O
the	O
equilibrium	O
in	O
favor	O
of	O
(	B-protein
N	I-protein
-)	I-protein
WASP	I-protein
activation	O
.	O

Binding	O
of	O
PI	B-chemical
(	I-chemical
4	I-chemical
,	I-chemical
5	I-chemical
)	I-chemical
P2	I-chemical
to	O
the	O
basic	O
region	O
just	O
N	O
-	O
terminal	O
to	O
the	O
GBD	B-structure_element
further	O
favors	O
the	O
active	B-protein_state
conformation	O
.	O

A	O
substantial	O
body	O
of	O
data	O
has	O
illuminated	O
the	O
complex	O
regulation	O
of	O
WASP	B-protein_type
/	I-protein_type
N	I-protein_type
-	I-protein_type
WASP	I-protein_type
proteins	I-protein_type
,	O
and	O
current	O
evidence	O
suggests	O
that	O
these	O
allosteric	O
activation	O
mechanisms	O
and	O
oligomerization	O
combine	O
to	O
regulate	O
WASP	B-protein_type
activity	O
,	O
allowing	O
the	O
synchronization	O
and	O
integration	O
of	O
multiple	O
potential	O
activation	O
signals	O
(	O
reviewed	O
in	O
Ref	O
.).	O

We	O
envisage	O
that	O
TOCA1	B-protein
is	O
first	O
recruited	O
to	O
the	O
appropriate	O
membrane	O
in	O
response	O
to	O
PI	B-chemical
(	I-chemical
4	I-chemical
,	I-chemical
5	I-chemical
)	I-chemical
P2	I-chemical
via	O
its	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domain	O
,	O
where	O
the	O
local	O
increase	O
in	O
concentration	O
favors	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
-	O
mediated	O
dimerization	B-oligomeric_state
of	O
TOCA1	B-protein
.	O

Cdc42	B-protein
is	O
activated	O
in	O
response	O
to	O
co	O
-	O
incident	O
signals	O
and	O
can	O
then	O
bind	O
to	O
TOCA1	B-protein
,	O
further	O
stabilizing	O
TOCA1	B-protein
at	O
the	O
membrane	O
.	O

TOCA1	B-protein
can	O
then	O
recruit	O
N	B-protein
-	I-protein
WASP	I-protein
via	O
an	O
interaction	O
between	O
its	O
SH3	B-structure_element
domain	O
and	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
proline	B-structure_element
-	I-structure_element
rich	I-structure_element
region	I-structure_element
.	O

The	O
recruitment	O
of	O
N	B-protein
-	I-protein
WASP	I-protein
alone	B-protein_state
and	O
of	O
the	O
N	B-complex_assembly
-	I-complex_assembly
WASP	I-complex_assembly
·	I-complex_assembly
WIP	I-complex_assembly
complex	O
by	O
TOCA1	B-protein
and	O
FBP17	B-protein
has	O
been	O
demonstrated	O
.	O

It	O
may	O
therefore	O
be	O
envisaged	O
that	O
WIP	B-protein
and	O
TOCA1	B-protein
exert	O
opposing	O
allosteric	O
effects	O
on	O
N	B-protein
-	I-protein
WASP	I-protein
,	O
with	O
TOCA1	B-protein
favoring	O
the	O
unfolded	B-protein_state
,	O
active	B-protein_state
conformation	O
of	O
N	B-protein
-	I-protein
WASP	I-protein
and	O
increasing	O
its	O
affinity	O
for	O
Cdc42	B-protein
.	O

TOCA1	B-protein
may	O
also	O
activate	O
N	B-protein
-	I-protein
WASP	I-protein
by	O
effective	O
oligomerization	O
because	O
clustering	O
of	O
TOCA1	B-protein
at	O
the	O
membrane	O
following	O
coincident	O
interactions	O
with	O
PI	B-chemical
(	I-chemical
4	I-chemical
,	I-chemical
5	I-chemical
)	I-chemical
P2	I-chemical
and	O
Cdc42	B-protein
would	O
in	O
turn	O
lead	O
to	O
clustering	O
of	O
N	B-protein
-	I-protein
WASP	I-protein
,	O
in	O
addition	O
to	O
pushing	O
the	O
equilibrium	O
toward	O
the	O
unfolded	B-protein_state
,	O
active	B-protein_state
state	O
.	O

In	O
a	O
cellular	O
context	O
,	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
TOCA1	B-protein
and	O
N	B-protein
-	I-protein
WASP	I-protein
are	O
likely	O
to	O
have	O
similar	O
affinities	B-evidence
for	O
active	B-protein_state
Cdc42	B-protein
,	O
but	O
in	O
the	O
unfolded	B-protein_state
,	O
active	B-protein_state
conformation	O
,	O
the	O
affinity	B-evidence
of	O
N	B-protein
-	I-protein
WASP	I-protein
for	O
Cdc42	B-protein
dramatically	O
increases	O
.	O

Our	O
binding	B-evidence
data	I-evidence
suggest	O
that	O
TOCA1	B-protein
HR1	B-structure_element
binding	O
is	O
not	O
allosterically	O
regulated	O
,	O
and	O
our	O
NMR	B-experimental_method
data	O
,	O
along	O
with	O
the	O
high	O
stability	B-protein_state
of	O
TOCA1	B-protein
HR1	B-structure_element
,	O
suggest	O
that	O
there	O
is	O
no	O
widespread	O
conformational	O
change	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
Cdc42	B-protein
.	O

As	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
TOCA1	B-protein
and	O
the	O
isolated	B-protein_state
HR1	B-structure_element
domain	O
bind	O
Cdc42	B-protein
with	O
similar	O
affinities	O
,	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
-	O
Cdc42	B-protein
interaction	O
will	O
be	O
favored	O
because	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
can	O
easily	O
outcompete	O
the	O
TOCA1	B-protein
HR1	B-structure_element
for	O
Cdc42	B-protein
.	O

In	O
such	O
an	O
array	O
of	O
molecules	O
localized	O
to	O
a	O
discrete	O
region	O
of	O
the	O
membrane	O
,	O
it	O
is	O
plausible	O
that	O
WASP	B-protein
could	O
bind	O
to	O
a	O
second	O
Cdc42	B-protein
molecule	O
rather	O
than	O
displacing	O
TOCA1	B-protein
from	O
its	O
cognate	O
Cdc42	B-protein
.	O

Our	O
NMR	B-experimental_method
and	O
affinity	B-evidence
data	I-evidence
,	O
however	O
,	O
are	O
consistent	O
with	O
displacement	O
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
by	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
GBD	B-structure_element
.	O

The	O
commonly	O
used	O
MGD	B-mutant
→	I-mutant
IST	I-mutant
(	O
Cdc42	B-protein_state
-	I-protein_state
binding	I-protein_state
deficient	I-protein_state
)	O
mutant	O
of	O
TOCA1	B-protein
has	O
a	O
reduced	O
ability	O
to	O
activate	O
the	O
N	B-complex_assembly
-	I-complex_assembly
WASP	I-complex_assembly
·	I-complex_assembly
WIP	I-complex_assembly
complex	O
,	O
further	O
indicating	O
the	O
importance	O
of	O
the	O
Cdc42	B-protein
-	O
HR1TOCA1	B-structure_element
interaction	O
prior	O
to	O
downstream	O
activation	O
of	O
N	B-protein
-	I-protein
WASP	I-protein
.	O

In	O
light	O
of	O
this	O
,	O
we	O
favor	O
an	O
“	O
effector	O
handover	O
”	O
scheme	O
whereby	O
TOCA1	B-protein
interacts	O
with	O
Cdc42	B-protein
prior	O
to	O
N	B-protein
-	I-protein
WASP	I-protein
activation	O
,	O
after	O
which	O
N	B-protein
-	I-protein
WASP	I-protein
displaces	O
TOCA1	B-protein
from	O
its	O
bound	B-protein_state
Cdc42	B-protein
in	O
order	O
to	O
be	O
fully	B-protein_state
activated	I-protein_state
rather	O
than	O
binding	O
a	O
second	O
Cdc42	B-protein
molecule	O
.	O

Potentially	O
,	O
the	O
TOCA1	B-protein
-	O
Cdc42	B-protein
interaction	O
functions	O
to	O
position	O
N	B-protein
-	I-protein
WASP	I-protein
and	O
Cdc42	B-protein
such	O
that	O
they	O
are	O
poised	O
to	O
interact	O
with	O
high	O
affinity	O
.	O

The	O
concomitant	O
release	O
of	O
TOCA1	B-protein
from	O
Cdc42	B-protein
while	O
still	O
bound	B-protein_state
to	I-protein_state
N	B-protein
-	I-protein
WASP	I-protein
presumably	O
enhances	O
the	O
ability	O
of	O
TOCA1	B-protein
to	O
further	O
activate	O
N	B-complex_assembly
-	I-complex_assembly
WASP	I-complex_assembly
·	I-complex_assembly
WIP	I-complex_assembly
-	O
induced	O
actin	O
polymerization	O
.	O

Hence	O
,	O
actin	O
polymerization	O
cannot	O
occur	O
until	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domains	O
are	O
poised	O
for	O
membrane	O
distortion	O
.	O

Our	O
model	O
of	O
the	O
Cdc42	B-complex_assembly
·	I-complex_assembly
HR1TOCA1	I-complex_assembly
complex	O
indicates	O
a	O
mechanism	O
by	O
which	O
such	O
a	O
handover	O
could	O
take	O
place	O
(	O
Fig	O
.	O
9	O
)	O
because	O
it	O
shows	O
that	O
the	O
effector	B-site
binding	I-site
sites	I-site
only	O
partially	O
overlap	O
on	O
Cdc42	B-protein
.	O

The	O
lysine	B-residue_name
residues	O
thought	O
to	O
be	O
involved	O
in	O
an	O
electrostatic	O
steering	O
mechanism	O
in	O
WASP	B-protein
-	O
Cdc42	B-protein
binding	O
are	O
conserved	O
in	O
N	B-protein
-	I-protein
WASP	I-protein
and	O
would	O
be	O
able	O
to	O
interact	O
with	O
Cdc42	B-protein
even	O
when	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
is	O
already	O
bound	B-protein_state
.	O

It	O
has	O
been	O
postulated	O
that	O
the	O
initial	O
interactions	O
between	O
this	O
basic	O
region	O
and	O
Cdc42	B-protein
could	O
stabilize	O
the	O
active	B-protein_state
conformation	O
of	O
WASP	B-protein
,	O
leading	O
to	O
high	O
affinity	O
binding	O
between	O
the	O
core	O
CRIB	B-structure_element
and	O
Cdc42	B-protein
.	O

Step	O
1	O
,	O
TOCA1	B-protein
is	O
recruited	O
to	O
the	O
membrane	O
via	O
its	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domain	O
and	O
/	O
or	O
Cdc42	B-protein
interactions	O
.	O

F	O
-	O
BAR	O
oligomerization	O
is	O
expected	O
to	O
occur	O
following	O
membrane	O
binding	O
,	O
but	O
a	O
single	O
monomer	B-oligomeric_state
is	O
shown	O
for	O
clarity	O
.	O

Step	O
2	O
,	O
N	B-protein
-	I-protein
WASP	I-protein
exists	O
in	O
an	O
inactive	B-protein_state
,	O
folded	B-protein_state
conformation	O
.	O

The	O
TOCA1	B-protein
SH3	B-structure_element
domain	O
interacts	O
with	O
N	B-protein
-	I-protein
WASP	I-protein
,	O
causing	O
an	O
activatory	O
allosteric	O
effect	O
.	O

Step	O
3	O
,	O
electrostatic	B-bond_interaction
interactions	I-bond_interaction
between	O
Cdc42	B-protein
and	O
the	O
basic	O
region	O
upstream	O
of	O
the	O
CRIB	B-structure_element
initiate	O
Cdc42	B-complex_assembly
·	I-complex_assembly
N	I-complex_assembly
-	I-complex_assembly
WASP	I-complex_assembly
binding	O
.	O

Step	O
4	O
,	O
the	O
core	O
CRIB	B-structure_element
binds	O
with	O
high	O
affinity	O
while	O
the	O
region	O
C	O
-	O
terminal	O
to	O
the	O
CRIB	B-structure_element
displaces	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
and	O
increases	O
the	O
affinity	O
of	O
the	O
N	B-protein
-	I-protein
WASP	I-protein
-	O
Cdc42	O
interaction	O
further	O
.	O

The	O
VCA	B-structure_element
domain	O
is	O
released	O
for	O
downstream	O
interactions	O
,	O
and	O
actin	O
polymerization	O
proceeds	O
.	O

WH1	O
,	O
WASP	B-structure_element
homology	I-structure_element
1	I-structure_element
domain	I-structure_element
;	O
PP	B-structure_element
,	O
proline	B-structure_element
-	I-structure_element
rich	I-structure_element
region	I-structure_element
;	O
VCA	B-structure_element
,	O
verprolin	B-structure_element
homology	I-structure_element
,	I-structure_element
cofilin	I-structure_element
homology	I-structure_element
,	I-structure_element
acidic	I-structure_element
region	I-structure_element
.	O

The	O
analogous	O
HR1	B-structure_element
domains	O
from	O
other	O
TOCA1	B-protein_type
family	I-protein_type
members	O
,	O
FBP17	B-protein
and	O
CIP4	B-protein
,	O
also	O
exhibit	O
micromolar	O
affinity	O
for	O
Cdc42	B-protein
.	O

A	O
role	O
for	O
the	O
TOCA1	B-protein
-,	O
FBP17	B-protein
-,	O
and	O
CIP4	B-protein
-	O
Cdc42	B-protein
interactions	O
in	O
the	O
recruitment	O
of	O
these	O
proteins	O
to	O
the	O
membrane	O
therefore	O
appears	O
unlikely	O
.	O

Instead	O
,	O
our	O
findings	O
agree	O
with	O
earlier	O
suggestions	O
that	O
the	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
domain	O
is	O
responsible	O
for	O
membrane	O
recruitment	O
.	O

The	O
role	O
of	O
the	O
Cdc42	B-protein
-	O
TOCA1	B-protein
interaction	O
remains	O
somewhat	O
elusive	O
,	O
but	O
it	O
may	O
serve	O
to	O
position	O
activated	B-protein_state
Cdc42	B-protein
and	O
N	B-protein
-	I-protein
WASP	I-protein
to	O
allow	O
full	B-protein_state
activation	I-protein_state
of	O
N	B-protein
-	I-protein
WASP	I-protein
and	O
as	O
such	O
serve	O
to	O
couple	O
F	B-structure_element
-	I-structure_element
BAR	I-structure_element
-	O
mediated	O
membrane	O
deformation	O
with	O
N	B-protein
-	I-protein
WASP	I-protein
activation	O
.	O

It	O
is	O
clear	O
from	O
the	O
data	O
presented	O
here	O
that	O
TOCA1	B-protein
and	O
N	B-protein
-	I-protein
WASP	I-protein
do	O
not	O
bind	O
Cdc42	B-protein
simultaneously	O
and	O
that	O
N	B-protein
-	I-protein
WASP	I-protein
is	O
likely	O
to	O
outcompete	O
TOCA1	B-protein
for	O
Cdc42	B-protein
binding	O
.	O

We	O
therefore	O
postulate	O
an	O
effector	O
handover	O
mechanism	O
based	O
on	O
current	O
evidence	O
surrounding	O
WASP	B-protein
/	O
N	B-protein
-	I-protein
WASP	I-protein
activation	O
and	O
our	O
model	O
of	O
the	O
Cdc42	B-complex_assembly
·	I-complex_assembly
HR1TOCA1	I-complex_assembly
complex	O
.	O

The	O
displacement	O
of	O
the	O
TOCA1	B-protein
HR1	B-structure_element
domain	O
from	O
Cdc42	B-protein
by	O
N	B-protein
-	I-protein
WASP	I-protein
may	O
represent	O
a	O
unidirectional	O
step	O
in	O
the	O
pathway	O
of	O
Cdc42	B-complex_assembly
·	I-complex_assembly
N	I-complex_assembly
-	I-complex_assembly
WASP	I-complex_assembly
·	I-complex_assembly
TOCA1	I-complex_assembly
-	O
dependent	O
actin	O
assembly	O
.	O

The	O
dynamic	B-protein_state
organization	O
of	O
fungal	B-taxonomy_domain
acetyl	B-protein_type
-	I-protein_type
CoA	I-protein_type
carboxylase	I-protein_type

Here	O
we	O
report	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
yeast	B-taxonomy_domain
ACC	B-protein_type
CD	B-structure_element
,	O
revealing	O
a	O
unique	O
four	O
-	O
domain	O
organization	O
.	O

A	O
regulatory	B-structure_element
loop	I-structure_element
,	O
which	O
is	O
phosphorylated	B-protein_state
at	O
the	O
key	O
functional	O
phosphorylation	B-site
site	I-site
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type
,	O
wedges	O
into	O
a	O
crevice	O
between	O
two	O
domains	O
of	O
CD	B-structure_element
.	O

In	O
contrast	O
to	O
related	O
carboxylases	B-protein_type
,	O
large	O
-	O
scale	O
conformational	O
changes	O
are	O
required	O
for	O
substrate	O
turnover	O
,	O
and	O
are	O
mediated	O
by	O
the	O
CD	B-structure_element
under	O
phosphorylation	B-ptm
control	O
.	O

Acetyl	B-protein_type
-	I-protein_type
CoA	I-protein_type
carboxylases	I-protein_type
are	O
central	O
regulatory	O
hubs	O
of	O
fatty	O
acid	O
metabolism	O
and	O
are	O
important	O
targets	O
for	O
drug	O
development	O
in	O
obesity	O
and	O
cancer	O
.	O

Here	O
,	O
the	O
authors	O
demonstrate	O
that	O
the	O
regulation	O
of	O
these	O
highly	B-protein_state
dynamic	I-protein_state
enzymes	B-protein_type
in	O
fungi	B-taxonomy_domain
is	O
governed	O
by	O
a	O
mechanism	O
based	O
on	O
phosphorylation	B-ptm
-	O
dependent	O
conformational	O
variability	O
.	O

By	O
catalysing	O
this	O
rate	O
-	O
limiting	O
step	O
in	O
fatty	O
-	O
acid	O
biosynthesis	O
,	O
ACC	B-protein_type
plays	O
a	O
key	O
role	O
in	O
anabolic	O
metabolism	O
.	O

Furthermore	O
,	O
elevated	O
ACC	B-protein_type
activity	O
is	O
observed	O
in	O
malignant	O
tumours	O
.	O

A	O
direct	O
link	O
between	O
ACC	B-protein_type
and	O
cancer	O
is	O
provided	O
by	O
cancer	O
-	O
associated	O
mutations	B-mutant
in	O
the	O
breast	B-protein
cancer	I-protein
susceptibility	I-protein
gene	I-protein
1	I-protein
(	O
BRCA1	B-protein
),	O
which	O
relieve	O
inhibitory	O
interactions	O
of	O
BRCA1	B-protein
with	O
ACC	B-protein_type
.	O

Thus	O
,	O
ACC	B-protein_type
is	O
a	O
relevant	O
drug	O
target	O
for	O
type	O
2	O
diabetes	O
and	O
cancer	O
.	O

Microbial	B-taxonomy_domain
ACCs	B-protein_type
are	O
also	O
the	O
principal	O
target	O
of	O
antifungal	O
and	O
antibiotic	O
compounds	O
,	O
such	O
as	O
Soraphen	B-chemical
A	I-chemical
.	O

Carboxyltransferase	B-protein_type
(	O
CT	B-protein_type
)	O
transfers	O
the	O
activated	O
carboxyl	B-chemical
group	O
from	O
carboxybiotin	B-chemical
to	O
acetyl	B-chemical
-	I-chemical
CoA	I-chemical
to	O
yield	O
malonyl	B-chemical
-	I-chemical
CoA	I-chemical
.	O
Prokaryotic	B-taxonomy_domain
ACCs	B-protein_type
are	O
transient	B-protein_state
assemblies	O
of	O
individual	O
BC	B-protein_type
,	O
CT	B-protein_type
and	O
BCCP	B-protein_type
subunits	O
.	O

Eukaryotic	B-taxonomy_domain
ACCs	B-protein_type
,	O
instead	O
,	O
are	O
multienzymes	B-protein_type
,	O
which	O
integrate	O
all	O
functional	O
components	O
into	O
a	O
single	O
polypeptide	O
chain	O
of	O
∼	O
2	O
,	O
300	O
amino	O
acids	O
.	O

Human	B-species
ACC	B-protein_type
occurs	O
in	O
two	O
closely	O
related	O
isoforms	B-protein_state
,	O
ACC1	B-protein
and	O
2	B-protein
,	O
located	O
in	O
the	O
cytosol	O
and	O
at	O
the	O
outer	O
mitochondrial	O
membrane	O
,	O
respectively	O
.	O

In	O
addition	O
to	O
the	O
canonical	O
ACC	B-structure_element
components	I-structure_element
,	O
eukaryotic	B-taxonomy_domain
ACCs	B-protein_type
contain	O
two	O
non	B-protein_state
-	I-protein_state
catalytic	I-protein_state
regions	B-structure_element
,	O
the	O
large	O
central	B-structure_element
domain	I-structure_element
(	O
CD	B-structure_element
)	O
and	O
the	O
BC	B-structure_element
–	I-structure_element
CT	I-structure_element
interaction	I-structure_element
domain	I-structure_element
(	O
BT	B-structure_element
).	O

The	O
BT	B-structure_element
domain	O
has	O
been	O
visualized	O
in	O
bacterial	B-taxonomy_domain
carboxylases	B-protein_type
,	O
where	O
it	O
mediates	O
contacts	O
between	O
α	B-structure_element
-	I-structure_element
and	O
β	B-structure_element
-	I-structure_element
subunits	I-structure_element
.	O

Structural	B-experimental_method
studies	I-experimental_method
on	O
the	O
functional	O
architecture	O
of	O
intact	B-protein_state
ACCs	B-protein_type
have	O
been	O
hindered	O
by	O
their	O
huge	O
size	O
and	O
pronounced	O
dynamics	O
,	O
as	O
well	O
as	O
the	O
transient	B-protein_state
assembly	O
mode	O
of	O
bacterial	B-taxonomy_domain
ACCs	B-protein_type
.	O

However	O
,	O
crystal	B-evidence
structures	I-evidence
of	O
individual	O
components	O
or	O
domains	O
from	O
prokaryotic	B-taxonomy_domain
and	O
eukaryotic	B-taxonomy_domain
ACCs	B-protein_type
,	O
respectively	O
,	O
have	O
been	O
solved	O
.	O

The	O
structure	B-experimental_method
determination	I-experimental_method
of	O
the	O
holoenzymes	B-protein_state
of	O
bacterial	B-taxonomy_domain
biotin	B-protein_type
-	I-protein_type
dependent	I-protein_type
carboxylases	I-protein_type
,	O
which	O
lack	B-protein_state
the	O
characteristic	O
CD	B-structure_element
,	O
such	O
as	O
the	O
pyruvate	B-protein_type
carboxylase	I-protein_type
(	O
PC	B-protein_type
),	O
propionyl	B-protein_type
-	I-protein_type
CoA	I-protein_type
carboxylase	I-protein_type
,	O
3	B-protein_type
-	I-protein_type
methyl	I-protein_type
-	I-protein_type
crotonyl	I-protein_type
-	I-protein_type
CoA	I-protein_type
carboxylase	I-protein_type
and	O
a	O
long	B-protein_type
-	I-protein_type
chain	I-protein_type
acyl	I-protein_type
-	I-protein_type
CoA	I-protein_type
carboxylase	I-protein_type
revealed	O
strikingly	O
divergent	O
architectures	O
despite	O
a	O
general	O
conservation	O
of	O
all	O
functional	O
components	O
.	O

Human	B-species
ACC1	B-protein
is	O
regulated	B-protein_state
allosterically	I-protein_state
,	O
via	O
specific	O
protein	O
–	O
protein	O
interactions	O
,	O
and	O
by	O
reversible	O
phosphorylation	B-ptm
.	O

Dynamic	O
polymerization	O
of	O
human	B-species
ACC1	B-protein
is	O
linked	O
to	O
increased	O
activity	O
and	O
is	O
regulated	B-protein_state
allosterically	I-protein_state
by	O
the	O
activator	O
citrate	B-chemical
and	O
the	O
inhibitor	O
palmitate	B-chemical
,	O
or	O
by	O
binding	O
of	O
the	O
small	O
protein	O
MIG	B-protein
-	I-protein
12	I-protein
(	O
ref	O
.).	O

Human	B-species
ACC1	B-protein
is	O
further	O
regulated	O
by	O
specific	O
phosphorylation	B-ptm
-	O
dependent	O
binding	O
of	O
BRCA1	B-protein
to	O
Ser1263	B-residue_name_number
in	O
the	O
CD	B-structure_element
.	O

Furthermore	O
,	O
phosphorylation	B-ptm
by	O
AMP	B-protein
-	I-protein
activated	I-protein
protein	I-protein
kinase	I-protein
(	O
AMPK	B-protein
)	O
and	O
cAMP	B-protein
-	I-protein
dependent	I-protein
protein	I-protein
kinase	I-protein
(	O
PKA	B-protein
)	O
leads	O
to	O
a	O
decrease	O
in	O
ACC1	B-protein
activity	O
.	O

AMPK	B-protein
phosphorylates	O
ACC1	B-protein
in	O
vitro	O
at	O
Ser80	B-residue_name_number
,	O
Ser1201	B-residue_name_number
and	O
Ser1216	B-residue_name_number
and	O
PKA	B-protein
at	O
Ser78	B-residue_name_number
and	O
Ser1201	B-residue_name_number
.	O

However	O
,	O
regulatory	O
effects	O
on	O
ACC1	B-protein
activity	O
are	O
mainly	O
mediated	O
by	O
phosphorylation	B-ptm
of	O
Ser80	B-residue_name_number
and	O
Ser1201	B-residue_name_number
(	O
refs	O
).	O

Phosphorylated	B-protein_state
Ser80	B-residue_name_number
,	O
which	O
is	O
highly	B-protein_state
conserved	I-protein_state
only	O
in	O
higher	B-taxonomy_domain
eukaryotes	I-taxonomy_domain
,	O
presumably	O
binds	O
into	O
the	O
Soraphen	B-site
A	I-site
-	I-site
binding	I-site
pocket	I-site
.	O

The	O
regulatory	O
Ser1201	B-residue_name_number
shows	O
only	O
moderate	B-protein_state
conservation	I-protein_state
across	O
higher	B-taxonomy_domain
eukaryotes	I-taxonomy_domain
,	O
while	O
the	O
phosphorylated	B-protein_state
Ser1216	B-residue_name_number
is	O
highly	B-protein_state
conserved	I-protein_state
across	O
all	O
eukaryotes	B-taxonomy_domain
.	O

However	O
,	O
no	O
effect	O
of	O
Ser1216	B-residue_name_number
phosphorylation	B-ptm
on	O
ACC	B-protein_type
activity	O
has	O
been	O
reported	O
in	O
higher	B-taxonomy_domain
eukaryotes	I-taxonomy_domain
.	O

The	O
BRCA1	B-protein
-	O
interacting	O
phosphoserine	B-residue_name
position	O
is	O
not	B-protein_state
conserved	I-protein_state
in	O
fungal	B-taxonomy_domain
ACC	B-protein_type
,	O
and	O
no	O
other	O
phospho	O
-	O
dependent	O
protein	O
–	O
protein	O
interactions	O
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type
have	O
been	O
described	O
.	O

In	O
yeast	B-taxonomy_domain
ACC	B-protein_type
,	O
phosphorylation	B-site
sites	I-site
have	O
been	O
identified	O
at	O
Ser2	B-residue_name_number
,	O
Ser735	B-residue_name_number
,	O
Ser1148	B-residue_name_number
,	O
Ser1157	B-residue_name_number
and	O
Ser1162	B-residue_name_number
(	O
ref	O
.).	O

Of	O
these	O
,	O
only	O
Ser1157	B-residue_name_number
is	O
highly	B-protein_state
conserved	I-protein_state
in	O
fungal	B-taxonomy_domain
ACC	B-protein_type
and	O
aligns	B-experimental_method
to	I-experimental_method
Ser1216	B-residue_name_number
in	O
human	B-species
ACC1	B-protein
.	O

Its	O
phosphorylation	B-ptm
by	O
the	O
AMPK	B-protein
homologue	O
SNF1	B-protein
results	O
in	O
strongly	O
reduced	O
ACC	B-protein_type
activity	O
.	O

Here	O
we	O
provide	O
the	O
structure	B-evidence
of	O
Saccharomyces	B-species
cerevisiae	I-species
(	O
Sce	B-species
)	O
ACC	B-protein_type
CD	B-structure_element
,	O
intermediate	O
-	O
and	O
low	O
-	O
resolution	O
structures	B-evidence
of	O
human	B-species
(	O
Hsa	B-species
)	O
ACC	B-protein_type
CD	B-structure_element
and	O
larger	B-mutant
fragments	I-mutant
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type
from	O
Chaetomium	B-species
thermophilum	I-species
(	O
Cth	B-species
;	O
Fig	O
.	O
1a	O
).	O

The	O
organization	O
of	O
the	O
yeast	B-taxonomy_domain
ACC	B-protein_type
CD	B-structure_element

First	O
,	O
we	O
focused	O
on	O
structure	B-experimental_method
determination	I-experimental_method
of	O
the	O
82	O
-	O
kDa	O
CD	B-structure_element
.	O

The	O
overall	O
extent	O
of	O
the	O
SceCD	B-species
is	O
70	O
by	O
75	O
Å	O
(	O
Fig	O
.	O
1b	O
and	O
Supplementary	O
Fig	O
.	O
1a	O
,	O
b	O
),	O
and	O
the	O
attachment	O
points	O
of	O
the	O
N	O
-	O
terminal	O
26	B-structure_element
-	I-structure_element
residue	I-structure_element
linker	I-structure_element
to	O
the	O
BCCP	B-structure_element
domain	O
and	O
the	O
C	O
-	O
terminal	O
CT	B-structure_element
domain	O
are	O
separated	O
by	O
46	O
Å	O
(	O
the	O
N	O
-	O
and	O
C	O
termini	O
are	O
indicated	O
with	O
spheres	O
in	O
Fig	O
.	O
1b	O
).	O

SceCD	B-species
comprises	O
four	O
distinct	O
domains	O
,	O
an	O
N	O
-	O
terminal	O
α	B-structure_element
-	I-structure_element
helical	I-structure_element
domain	I-structure_element
(	O
CDN	B-structure_element
),	O
and	O
a	O
central	O
four	B-structure_element
-	I-structure_element
helix	I-structure_element
bundle	I-structure_element
linker	I-structure_element
domain	I-structure_element
(	O
CDL	B-structure_element
),	O
followed	O
by	O
two	O
α	B-structure_element
–	I-structure_element
β	I-structure_element
-	I-structure_element
fold	I-structure_element
C	I-structure_element
-	I-structure_element
terminal	I-structure_element
domains	I-structure_element
(	O
CDC1	B-structure_element
/	O
CDC2	B-structure_element
).	O

CDN	B-structure_element
adopts	O
a	O
letter	O
C	B-protein_state
shape	I-protein_state
,	O
where	O
one	O
of	O
the	O
ends	O
is	O
a	O
regular	B-structure_element
four	I-structure_element
-	I-structure_element
helix	I-structure_element
bundle	I-structure_element
(	O
Nα3	B-structure_element
-	I-structure_element
6	I-structure_element
),	O
the	O
other	O
end	O
is	O
a	O
helical	B-structure_element
hairpin	I-structure_element
(	O
Nα8	B-structure_element
,	I-structure_element
9	I-structure_element
)	O
and	O
the	O
bridging	B-structure_element
region	I-structure_element
comprises	O
six	O
helices	B-structure_element
(	O
Nα1	B-structure_element
,	I-structure_element
2	I-structure_element
,	I-structure_element
7	I-structure_element
,	I-structure_element
10	I-structure_element
–	I-structure_element
12	I-structure_element
).	O

CDL	B-structure_element
does	O
not	O
interact	O
with	O
CDN	B-structure_element
apart	O
from	O
the	O
covalent	O
linkage	O
and	O
forms	O
only	O
a	O
small	O
contact	O
to	O
CDC2	B-structure_element
via	O
a	O
loop	B-structure_element
between	O
Lα2	B-structure_element
/	I-structure_element
α3	I-structure_element
and	O
the	O
N	O
-	O
terminal	O
end	O
of	O
Lα1	B-structure_element
,	O
with	O
an	O
interface	B-site
area	O
of	O
400	O
Å2	O
.	O

CDC1	B-structure_element
/	O
CDC2	B-structure_element
share	O
a	O
common	O
fold	O
;	O
they	O
are	O
composed	O
of	O
six	B-structure_element
-	I-structure_element
stranded	I-structure_element
β	I-structure_element
-	I-structure_element
sheets	I-structure_element
flanked	O
on	O
one	O
side	O
by	O
two	O
long	B-structure_element
,	I-structure_element
bent	I-structure_element
helices	I-structure_element
inserted	O
between	O
strands	B-structure_element
β3	B-structure_element
/	I-structure_element
β4	I-structure_element
and	O
β4	B-structure_element
/	I-structure_element
β5	I-structure_element
.	O

CDC2	B-structure_element
is	O
extended	B-protein_state
at	O
its	O
C	O
terminus	O
by	O
an	O
additional	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
and	O
an	O
irregular	B-structure_element
β	I-structure_element
-	I-structure_element
hairpin	I-structure_element
.	O

On	O
the	O
basis	O
of	O
a	O
root	B-evidence
mean	I-evidence
square	I-evidence
deviation	I-evidence
of	O
main	O
chain	O
atom	O
positions	O
of	O
2	O
.	O
2	O
Å	O
,	O
CDC1	B-structure_element
/	O
CDC2	B-structure_element
are	O
structurally	O
more	O
closely	O
related	O
to	O
each	O
other	O
than	O
to	O
any	O
other	O
protein	O
(	O
Fig	O
.	O
1c	O
);	O
they	O
may	O
thus	O
have	O
evolved	O
by	O
duplication	O
.	O

Close	O
structural	O
homologues	O
could	O
not	O
be	O
found	O
for	O
the	O
CDN	B-structure_element
or	O
the	O
CDC	B-structure_element
domains	O
.	O

MS	B-experimental_method
analysis	O
of	O
dissolved	B-experimental_method
crystals	I-experimental_method
confirmed	O
the	O
phosphorylated	B-protein_state
state	O
of	O
Ser1157	B-residue_name_number
also	O
in	O
SceCD	B-species
crystals	B-evidence
.	O

In	O
the	O
SceCD	B-species
crystal	B-evidence
structure	I-evidence
,	O
the	O
phosphorylated	B-protein_state
Ser1157	B-residue_name_number
resides	O
in	O
a	O
regulatory	B-structure_element
36	I-structure_element
-	I-structure_element
amino	I-structure_element
-	I-structure_element
acid	I-structure_element
loop	I-structure_element
between	O
strands	B-structure_element
β2	B-structure_element
and	O
β3	B-structure_element
of	O
CDC1	B-structure_element
(	O
Fig	O
.	O
1b	O
,	O
d	O
),	O
which	O
contains	O
two	O
additional	O
less	B-protein_state
-	I-protein_state
conserved	I-protein_state
phosphorylation	B-site
sites	I-site
(	O
Ser1148	B-residue_name_number
and	O
Ser1162	B-residue_name_number
)	O
confirmed	O
in	O
yeast	B-taxonomy_domain
,	O
but	O
not	O
occupied	O
here	O
.	O

This	O
regulatory	B-structure_element
loop	I-structure_element
wedges	O
between	O
the	O
CDC1	B-structure_element
and	O
CDC2	B-structure_element
domains	O
and	O
provides	O
the	O
largest	O
contribution	O
to	O
the	O
interdomain	B-site
interface	I-site
.	O

The	O
N	O
-	O
terminal	O
region	O
of	O
the	O
regulatory	B-structure_element
loop	I-structure_element
also	O
directly	O
contacts	O
the	O
C	O
-	O
terminal	O
region	O
of	O
CDC2	B-structure_element
leading	O
into	O
CT	B-structure_element
.	O

Phosphoserine	B-residue_name_number
1157	I-residue_name_number
is	O
tightly	O
bound	O
by	O
two	O
highly	B-protein_state
conserved	I-protein_state
arginines	B-residue_name
(	O
Arg1173	B-residue_name_number
and	O
Arg1260	B-residue_name_number
)	O
of	O
CDC1	B-structure_element
(	O
Fig	O
.	O
1d	O
).	O

Phosphorylation	B-ptm
of	O
the	O
regulatory	B-structure_element
loop	I-structure_element
thus	O
determines	O
interdomain	O
interactions	O
of	O
CDC1	B-structure_element
and	O
CDC2	B-structure_element
,	O
suggesting	O
that	O
it	O
may	O
exert	O
its	O
regulatory	O
function	O
by	O
modifying	O
the	O
overall	O
structure	O
and	O
dynamics	O
of	O
the	O
CD	B-structure_element
.	O

The	O
functional	O
role	O
of	O
Ser1157	B-residue_name_number
was	O
confirmed	O
by	O
an	O
activity	B-experimental_method
assay	I-experimental_method
based	O
on	O
the	O
incorporation	O
of	O
radioactive	O
carbonate	O
into	O
acid	O
non	O
-	O
volatile	O
material	O
.	O

Phosphorylated	B-protein_state
SceACC	B-protein
shows	O
only	O
residual	O
activity	O
(	O
kcat	B-evidence
=	O
0	O
.	O
4	O
±	O
0	O
.	O
2	O
s	O
−	O
1	O
,	O
s	O
.	O
d	O
.	O
based	O
on	O
five	O
replicate	O
measurements	O
),	O
which	O
increases	O
16	O
-	O
fold	O
(	O
kcat	B-evidence
=	O
6	O
.	O
5	O
±	O
0	O
.	O
3	O
s	O
−	O
1	O
)	O
after	O
dephosphorylation	O
with	O
λ	B-protein_type
protein	I-protein_type
phosphatase	I-protein_type
.	O

The	O
values	O
obtained	O
for	O
dephosphorylated	B-protein_state
SceACC	B-protein
are	O
comparable	O
to	O
earlier	O
measurements	O
of	O
non	B-protein_state
-	I-protein_state
phosphorylated	I-protein_state
yeast	B-taxonomy_domain
ACC	B-protein_type
expressed	B-experimental_method
in	I-experimental_method
E	B-species
.	I-species
coli	I-species
.	O

The	O
variable	O
CD	B-structure_element
is	O
conserved	B-protein_state
between	O
yeast	B-taxonomy_domain
and	O
human	B-species

To	O
compare	O
the	O
organization	O
of	O
fungal	B-taxonomy_domain
and	O
human	B-species
ACC	B-protein_type
CD	B-structure_element
,	O
we	O
determined	B-experimental_method
the	I-experimental_method
structure	I-experimental_method
of	O
a	O
human	B-species
ACC1	B-mutant
fragment	I-mutant
that	O
comprises	O
the	O
BT	B-structure_element
and	O
CD	B-structure_element
domains	O
(	O
HsaBT	B-mutant
-	I-mutant
CD	I-mutant
),	O
but	O
lacks	B-protein_state
the	O
mobile	O
BCCP	B-structure_element
in	O
between	O
(	O
Fig	O
.	O
1a	O
).	O

An	O
experimentally	B-evidence
phased	I-evidence
map	I-evidence
was	O
obtained	O
at	O
3	O
.	O
7	O
Å	O
resolution	O
for	O
a	O
cadmium	B-chemical
-	O
derivatized	O
crystal	O
and	O
was	O
interpreted	O
by	O
a	O
poly	O
-	O
alanine	O
model	O
(	O
Fig	O
.	O
1e	O
and	O
Table	O
1	O
).	O

Each	O
of	O
the	O
four	O
CD	B-structure_element
domains	O
in	O
HsaBT	B-mutant
-	I-mutant
CD	I-mutant
individually	O
resembles	O
the	O
corresponding	O
SceCD	B-species
domain	O
;	O
however	O
,	O
human	B-species
and	O
yeast	B-taxonomy_domain
CDs	B-structure_element
exhibit	O
distinct	O
overall	O
structures	B-evidence
.	O

In	O
agreement	O
with	O
their	O
tight	O
interaction	O
in	O
SceCD	B-species
,	O
the	O
relative	O
spatial	O
arrangement	O
of	O
CDL	B-structure_element
and	O
CDC1	B-structure_element
is	O
preserved	O
in	O
HsaBT	B-mutant
-	I-mutant
CD	I-mutant
,	O
but	O
the	O
human	B-species
CDL	B-structure_element
/	O
CDC1	B-structure_element
didomain	O
is	O
tilted	O
by	O
30	O
°	O
based	O
on	O
a	O
superposition	B-experimental_method
of	O
human	B-species
and	O
yeast	B-taxonomy_domain
CDC2	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
1c	O
).	O

As	O
a	O
result	O
,	O
the	O
N	O
terminus	O
of	O
CDL	B-structure_element
at	O
helix	B-structure_element
Lα1	B-structure_element
,	O
which	O
connects	O
to	O
CDN	B-structure_element
,	O
is	O
shifted	O
by	O
12	O
Å	O
.	O
Remarkably	O
,	O
CDN	B-structure_element
of	O
HsaBT	B-mutant
-	I-mutant
CD	I-mutant
adopts	O
a	O
completely	O
different	O
orientation	O
compared	O
with	O
SceCD	B-species
.	O

With	O
CDL	B-structure_element
/	O
CDC1	B-structure_element
superposed	B-experimental_method
,	O
CDN	B-structure_element
in	O
HsaBT	B-mutant
-	I-mutant
CD	I-mutant
is	O
rotated	O
by	O
160	O
°	O
around	O
a	O
hinge	B-structure_element
at	O
the	O
connection	O
of	O
CDN	B-structure_element
/	O
CDL	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
1d	O
).	O

This	O
rotation	O
displaces	O
the	O
N	O
terminus	O
of	O
CDN	B-structure_element
in	O
HsaBT	B-mutant
-	I-mutant
CD	I-mutant
by	O
51	O
Å	O
compared	O
with	O
SceCD	B-species
,	O
resulting	O
in	O
a	O
separation	O
of	O
the	O
attachment	O
points	O
of	O
the	O
N	O
-	O
terminal	O
linker	B-structure_element
to	O
the	O
BCCP	B-structure_element
domain	I-structure_element
and	O
the	O
C	O
-	O
terminal	O
CT	B-structure_element
domain	O
by	O
67	O
Å	O
(	O
the	O
attachment	O
points	O
are	O
indicated	O
with	O
spheres	O
in	O
Fig	O
.	O
1e	O
).	O

The	O
BT	B-structure_element
domain	O
of	O
HsaBT	B-mutant
-	I-mutant
CD	I-mutant
consists	O
of	O
a	O
helix	B-structure_element
that	O
is	O
surrounded	O
at	O
its	O
N	O
terminus	O
by	O
an	O
antiparallel	B-structure_element
eight	I-structure_element
-	I-structure_element
stranded	I-structure_element
β	I-structure_element
-	I-structure_element
barrel	I-structure_element
.	O

It	O
resembles	O
the	O
BT	B-structure_element
of	O
propionyl	B-protein_type
-	I-protein_type
CoA	I-protein_type
carboxylase	I-protein_type
;	O
only	O
the	O
four	O
C	O
-	O
terminal	O
strands	B-structure_element
of	I-structure_element
the	I-structure_element
β	I-structure_element
-	I-structure_element
barrel	I-structure_element
are	O
slightly	O
tilted	O
.	O

The	O
highly	B-protein_state
conserved	I-protein_state
Ser1216	B-residue_name_number
(	O
corresponding	O
to	O
S	B-species
.	I-species
cerevisiae	I-species
Ser1157	B-residue_name_number
),	O
as	O
well	O
as	O
Ser1201	B-residue_name_number
,	O
both	O
in	O
the	O
regulatory	B-structure_element
loop	I-structure_element
discussed	O
above	O
,	O
are	O
not	B-protein_state
phosphorylated	I-protein_state
.	O

MS	B-experimental_method
analysis	O
of	O
the	O
HsaBT	B-mutant
-	I-mutant
CD	I-mutant
crystallization	B-evidence
sample	I-evidence
reveals	O
partial	O
proteolytic	O
digestion	O
of	O
the	O
regulatory	B-structure_element
loop	I-structure_element
.	O

The	O
absence	B-protein_state
of	I-protein_state
the	O
regulatory	B-structure_element
loop	I-structure_element
might	O
be	O
linked	O
to	O
the	O
less	B-protein_state
-	I-protein_state
restrained	I-protein_state
interface	B-site
of	O
CDL	B-structure_element
/	O
CDC1	B-structure_element
and	O
CDC2	B-structure_element
and	O
altered	O
relative	O
orientations	O
of	O
these	O
domains	B-structure_element
.	O

At	O
the	O
level	O
of	O
isolated	B-experimental_method
yeast	B-taxonomy_domain
and	O
human	B-species
CD	B-structure_element
,	O
the	O
structural	B-experimental_method
analysis	I-experimental_method
indicates	O
the	O
presence	O
of	O
at	O
least	O
two	O
hinges	B-structure_element
,	O
one	O
with	O
large	O
-	O
scale	O
flexibility	O
at	O
the	O
CDN	B-structure_element
/	I-structure_element
CDL	I-structure_element
connection	I-structure_element
,	O
and	O
one	O
with	O
tunable	O
plasticity	O
between	O
CDL	B-structure_element
/	O
CDC1	B-structure_element
and	O
CDC2	B-structure_element
,	O
plausibly	O
affected	O
by	O
phosphorylation	B-ptm
in	O
the	O
regulatory	B-structure_element
loop	I-structure_element
region	O
.	O

The	O
integration	O
of	O
CD	B-structure_element
into	O
the	O
fungal	B-taxonomy_domain
ACC	B-protein_type
multienzyme	I-protein_type

No	O
crystals	O
diffracting	O
to	O
sufficient	O
resolution	O
were	O
obtained	O
for	O
larger	B-mutant
BC	I-mutant
-	I-mutant
containing	I-mutant
fragments	I-mutant
,	O
or	O
for	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
Cth	B-species
or	O
SceACC	B-protein
.	O

To	O
improve	B-experimental_method
crystallizability	I-experimental_method
,	O
we	O
generated	B-experimental_method
ΔBCCP	B-mutant
variants	I-mutant
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
ACC	B-protein_type
,	O
which	O
,	O
based	O
on	O
SAXS	B-experimental_method
analysis	I-experimental_method
,	O
preserve	O
properties	O
of	O
intact	B-protein_state
ACC	B-protein_type
(	O
Supplementary	O
Table	O
1	O
and	O
Supplementary	O
Fig	O
.	O
2a	O
–	O
c	O
).	O

For	O
CthΔBCCP	B-mutant
,	O
crystals	B-evidence
diffracting	O
to	O
8	O
.	O
4	O
Å	O
resolution	O
were	O
obtained	O
.	O

However	O
,	O
molecular	B-experimental_method
replacement	I-experimental_method
did	O
not	O
reveal	O
a	O
unique	O
positioning	O
of	O
the	O
BC	B-structure_element
domain	O
.	O

Still	O
,	O
these	B-evidence
structures	I-evidence
contribute	O
considerably	O
to	O
the	O
visualization	O
of	O
an	O
intrinsically	O
dynamic	B-protein_state
fungal	B-taxonomy_domain
ACC	B-protein_type
.	O

In	O
all	O
these	O
crystal	B-evidence
structures	I-evidence
,	O
the	O
CT	B-structure_element
domains	O
build	O
a	O
canonical	O
head	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
tail	I-protein_state
dimer	B-oligomeric_state
,	O
with	O
active	B-site
sites	I-site
formed	O
by	O
contributions	O
from	O
both	O
protomers	B-oligomeric_state
(	O
Fig	O
.	O
2	O
and	O
Supplementary	O
Fig	O
.	O
3a	O
).	O

The	O
connection	B-structure_element
of	O
CD	B-structure_element
and	O
CT	B-structure_element
is	O
provided	O
by	O
a	O
10	B-residue_range
-	I-residue_range
residue	I-residue_range
peptide	I-residue_range
stretch	I-residue_range
,	O
which	O
links	O
the	O
N	O
terminus	O
of	O
CT	B-structure_element
to	O
the	O
irregular	B-structure_element
β	I-structure_element
-	I-structure_element
hairpin	I-structure_element
/	I-structure_element
β	I-structure_element
-	I-structure_element
strand	I-structure_element
extension	I-structure_element
of	O
CDC2	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
3b	O
).	O

The	O
connecting	B-structure_element
region	I-structure_element
is	O
remarkably	O
similar	O
in	O
isolated	B-protein_state
CD	B-structure_element
and	O
CthCD	B-mutant
-	I-mutant
CTCter	I-mutant
structures	B-evidence
,	O
indicating	O
inherent	O
conformational	O
stability	O
.	O

CD	B-structure_element
/	O
CT	B-structure_element
contacts	O
are	O
only	O
formed	O
in	O
direct	O
vicinity	O
of	O
the	O
covalent	O
linkage	O
and	O
involve	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
extension	I-structure_element
of	O
CDC2	B-structure_element
as	O
well	O
as	O
the	O
loop	B-structure_element
between	O
strands	B-structure_element
β2	I-structure_element
/	I-structure_element
β3	I-structure_element
of	O
the	O
CT	B-structure_element
N	I-structure_element
-	I-structure_element
lobe	I-structure_element
,	O
which	O
contains	O
a	O
conserved	B-protein_state
RxxGxN	B-structure_element
motif	I-structure_element
.	O

The	O
neighbouring	O
loop	B-structure_element
on	O
the	O
CT	B-structure_element
side	O
(	O
between	O
CT	B-structure_element
β1	B-structure_element
/	O
β2	B-structure_element
)	O
is	O
displaced	O
by	O
2	O
.	O
5	O
Å	O
compared	O
to	O
isolated	B-protein_state
CT	B-structure_element
structures	B-evidence
(	O
Supplementary	O
Fig	O
.	O
3c	O
).	O

Indeed	O
,	O
the	O
comparison	O
of	O
the	O
positioning	O
of	O
eight	O
instances	O
of	O
the	O
C	O
-	O
terminal	O
part	O
of	O
CD	B-structure_element
relative	O
to	O
CT	B-structure_element
in	O
crystal	B-evidence
structures	I-evidence
determined	B-experimental_method
here	O
,	O
reveals	O
flexible	O
interdomain	O
linking	O
(	O
Fig	O
.	O
3a	O
).	O

The	O
CDC2	B-site
/	I-site
CT	I-site
interface	I-site
acts	O
as	O
a	O
true	B-structure_element
hinge	I-structure_element
with	O
observed	O
rotation	O
up	O
to	O
16	O
°,	O
which	O
results	O
in	O
a	O
translocation	O
of	O
the	O
distal	O
end	O
of	O
CDC2	B-structure_element
by	O
8	O
Å	O
.	O

The	O
interface	B-site
between	O
CDC2	B-structure_element
and	O
CDL	B-structure_element
/	O
CDC1	B-structure_element
,	O
which	O
is	O
mediated	O
by	O
the	O
phosphorylated	B-protein_state
regulatory	B-structure_element
loop	I-structure_element
in	O
the	O
SceCD	B-species
structure	B-evidence
,	O
is	O
less	O
variable	O
than	O
the	O
CD	B-structure_element
–	I-structure_element
CT	I-structure_element
junction	I-structure_element
,	O
and	O
permits	O
only	O
limited	O
rotation	O
and	O
tilting	O
(	O
Fig	O
.	O
3b	O
).	O

Analysis	O
of	O
the	O
impact	O
of	O
phosphorylation	B-ptm
on	O
the	O
interface	B-site
between	O
CDC2	B-structure_element
and	O
CDL	B-structure_element
/	O
CDC1	B-structure_element
in	O
CthACC	B-mutant
variant	I-mutant
structures	B-evidence
is	O
precluded	O
by	O
the	O
limited	O
crystallographic	O
resolution	O
.	O

The	O
CDN	B-structure_element
domain	O
positioning	O
relative	O
to	O
CDL	B-structure_element
/	O
CDC1	B-structure_element
is	O
highly	O
variable	O
with	O
three	O
main	O
orientations	O
observed	O
in	O
the	O
structures	B-evidence
of	O
SceCD	B-species
and	O
the	O
larger	B-mutant
CthACC	I-mutant
fragments	I-mutant
:	O
CDN	B-structure_element
tilts	O
,	O
resulting	O
in	O
a	O
displacement	O
of	O
its	O
N	O
terminus	O
by	O
23	O
Å	O
(	O
Fig	O
.	O
4a	O
,	O
observed	O
in	O
both	O
protomers	B-oligomeric_state
of	O
CthCD	B-mutant
-	I-mutant
CT	I-mutant
and	O
one	O
protomer	B-oligomeric_state
of	O
CthΔBCCP	B-mutant
,	O
denoted	O
as	O
CthCD	B-mutant
-	I-mutant
CT1	I-mutant
/	I-mutant
2	I-mutant
and	O
CthΔBCCP1	B-mutant
,	O
respectively	O
).	O

In	O
addition	O
,	O
CDN	B-structure_element
can	O
rotate	O
around	O
hinges	B-structure_element
in	O
the	O
connection	O
between	O
CDN	B-structure_element
/	O
CDL	B-structure_element
by	O
70	O
°	O
(	O
Fig	O
.	O
4b	O
,	O
observed	O
in	O
the	O
second	O
protomer	B-oligomeric_state
of	O
CthΔBCCP	B-mutant
,	O
denoted	O
as	O
CthΔBCCP2	B-mutant
)	O
and	O
160	O
°	O
(	O
Fig	O
.	O
4c	O
,	O
observed	O
in	O
SceCD	B-species
)	O
leading	O
to	O
displacement	O
of	O
the	O
anchor	B-site
site	I-site
for	O
the	O
BCCP	B-structure_element
linker	I-structure_element
by	O
up	O
to	O
33	O
and	O
40	O
Å	O
,	O
respectively	O
.	O

Conformational	O
variability	O
in	O
the	O
CD	B-structure_element
thus	O
contributes	O
considerably	O
to	O
variations	O
in	O
the	O
spacing	O
between	O
the	O
BC	B-structure_element
and	O
CT	B-structure_element
domains	O
,	O
and	O
may	O
extend	O
to	O
distance	O
variations	O
beyond	O
the	O
mobility	O
range	O
of	O
the	O
flexibly	B-protein_state
tethered	I-protein_state
BCCP	B-structure_element
.	O

On	O
the	O
basis	O
of	O
the	O
occurrence	O
of	O
related	O
conformational	O
changes	O
between	O
fungal	B-taxonomy_domain
and	O
human	B-species
ACC	B-mutant
fragments	I-mutant
,	O
the	O
observed	O
set	O
of	O
conformations	O
may	O
well	O
represent	O
general	O
states	O
present	O
in	O
all	O
eukaryotic	B-taxonomy_domain
ACCs	B-protein_type
.	O

Large	O
-	O
scale	O
conformational	O
variability	O
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type

To	O
obtain	O
a	O
comprehensive	O
view	O
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type
dynamics	O
in	B-protein_state
solution	I-protein_state
,	O
we	O
employed	O
SAXS	B-experimental_method
and	O
EM	B-experimental_method
.	O

The	O
smooth	O
appearance	O
of	O
scattering	B-evidence
curves	I-evidence
and	O
derived	B-evidence
distance	I-evidence
distributions	I-evidence
might	O
indicate	O
substantial	O
interdomain	O
flexibility	O
(	O
Supplementary	O
Fig	O
.	O
2a	O
–	O
c	O
).	O

Direct	O
observation	O
of	O
individual	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
CthACC	B-protein
particles	B-evidence
,	O
according	O
to	O
MS	B-experimental_method
results	O
predominantly	O
in	O
a	O
phosphorylated	B-protein_state
low	B-protein_state
-	I-protein_state
activity	I-protein_state
state	I-protein_state
,	O
in	O
negative	B-experimental_method
stain	I-experimental_method
EM	I-experimental_method
reveals	O
a	O
large	O
set	O
of	O
conformations	O
from	O
rod	B-protein_state
-	I-protein_state
like	I-protein_state
extended	I-protein_state
to	O
U	B-protein_state
-	I-protein_state
shaped	I-protein_state
particles	B-evidence
.	O

The	O
flexibility	O
in	O
the	O
CDC2	B-structure_element
/	I-structure_element
CT	I-structure_element
hinge	I-structure_element
appears	O
substantially	O
larger	O
than	O
the	O
variations	O
observed	O
in	O
the	O
set	O
of	O
crystal	B-evidence
structures	I-evidence
.	O

The	O
BC	B-structure_element
domain	O
is	O
not	O
completely	O
disordered	O
,	O
but	O
laterally	O
attached	O
to	O
BT	B-structure_element
/	O
CDN	B-structure_element
in	O
a	O
generally	B-protein_state
conserved	I-protein_state
position	I-protein_state
,	O
albeit	O
with	O
increased	O
flexibility	O
.	O

Surprisingly	O
,	O
in	O
both	O
the	O
linear	B-protein_state
and	I-protein_state
U	I-protein_state
-	I-protein_state
shaped	I-protein_state
conformations	I-protein_state
,	O
the	O
approximate	O
distances	O
between	O
the	O
BC	B-structure_element
and	O
CT	B-structure_element
active	B-site
sites	I-site
would	O
remain	O
larger	O
than	O
110	O
Å	O
.	O
These	O
observed	O
distances	O
are	O
considerably	O
larger	O
than	O
in	O
static	B-protein_state
structures	B-evidence
of	O
any	O
other	O
related	O
biotin	B-protein_type
-	I-protein_type
dependent	I-protein_type
carboxylase	I-protein_type
.	O

Furthermore	O
,	O
based	O
on	O
an	O
average	O
length	O
of	O
the	O
BCCP	B-structure_element
–	I-structure_element
CD	I-structure_element
linker	I-structure_element
in	O
fungal	B-taxonomy_domain
ACC	B-protein_type
of	O
26	B-residue_range
amino	I-residue_range
acids	I-residue_range
,	O
mobility	O
of	O
the	O
BCCP	B-structure_element
alone	O
would	O
not	O
be	O
sufficient	O
to	O
bridge	O
the	O
active	B-site
sites	I-site
of	O
BC	B-structure_element
and	O
CT	B-structure_element
.	O

Altogether	O
,	O
the	O
architecture	O
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type
is	O
based	O
on	O
the	O
central	O
dimeric	B-oligomeric_state
CT	B-structure_element
domain	O
(	O
Fig	O
.	O
4d	O
).	O

In	O
fungal	B-taxonomy_domain
ACC	B-protein_type
,	O
however	O
,	O
Ser1157	B-residue_name_number
in	O
the	O
regulatory	B-structure_element
loop	I-structure_element
of	O
the	O
CD	B-structure_element
is	O
the	O
only	O
phosphorylation	B-site
site	I-site
that	O
has	O
been	O
demonstrated	O
to	O
be	O
both	O
phosphorylated	B-protein_state
in	O
vivo	O
and	O
involved	O
in	O
the	O
regulation	O
of	O
ACC	B-protein_type
activity	O
.	O

In	O
its	O
phosphorylated	B-protein_state
state	O
,	O
the	O
regulatory	B-structure_element
loop	I-structure_element
containing	O
Ser1157	B-residue_name_number
wedges	O
between	O
CDC1	B-structure_element
/	O
CDC2	B-structure_element
and	O
presumably	O
limits	O
the	O
conformational	B-protein_state
freedom	I-protein_state
at	O
this	O
interdomain	B-site
interface	I-site
.	O

The	O
current	O
data	O
thus	O
suggest	O
that	O
regulation	O
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type
is	O
mediated	O
by	O
controlling	O
the	O
dynamics	O
of	O
the	O
unique	B-protein_state
CD	B-structure_element
,	O
rather	O
than	O
directly	O
affecting	O
catalytic	O
turnover	O
at	O
the	O
active	B-site
sites	I-site
of	O
BC	B-structure_element
and	O
CT	B-structure_element
.	O

Most	O
recently	O
,	O
a	O
crystal	B-evidence
structure	I-evidence
of	O
near	B-protein_state
full	I-protein_state
-	I-protein_state
length	I-protein_state
non	B-protein_state
-	I-protein_state
phosphorylated	I-protein_state
ACC	B-protein_type
from	O
S	B-species
.	I-species
cerevisae	I-species
(	O
lacking	B-protein_state
only	I-protein_state
21	B-residue_range
N	O
-	O
terminal	O
amino	O
acids	O
,	O
here	O
denoted	O
as	O
flACC	B-protein
)	O
was	O
published	O
by	O
Wei	O
and	O
Tong	O
.	O

In	O
flACC	B-protein
,	O
the	O
ACC	B-protein_type
dimer	B-oligomeric_state
obeys	O
twofold	O
symmetry	O
and	O
assembles	O
in	O
a	O
triangular	B-protein_state
architecture	I-protein_state
with	O
dimeric	B-oligomeric_state
BC	B-structure_element
domains	O
(	O
Supplementary	O
Fig	O
.	O
5a	O
).	O

In	O
their	O
study	O
,	O
mutational	B-experimental_method
data	I-experimental_method
indicate	O
a	O
requirement	O
for	O
BC	O
dimerization	O
for	O
catalytic	O
activity	O
.	O

The	O
transition	O
from	O
the	O
elongated	B-protein_state
open	I-protein_state
shape	I-protein_state
,	O
observed	O
in	O
our	O
experiments	O
,	O
towards	O
a	O
compact	B-protein_state
triangular	I-protein_state
shape	I-protein_state
is	O
based	O
on	O
an	O
intricate	O
interplay	O
of	O
several	O
hinge	O
-	O
bending	O
motions	O
in	O
the	O
CD	B-structure_element
(	O
Fig	O
.	O
4d	O
).	O

Comparison	B-experimental_method
of	O
flACC	B-protein
with	O
our	O
CthΔBCCP	B-mutant
structure	B-evidence
reveals	O
the	O
CDC2	B-structure_element
/	I-structure_element
CT	I-structure_element
hinge	I-structure_element
as	O
a	O
major	O
contributor	O
to	O
conformational	O
flexibility	O
(	O
Supplementary	O
Fig	O
.	O
5b	O
,	O
c	O
).	O

The	O
only	O
bona	O
fide	O
regulatory	B-protein_state
phophorylation	B-site
site	I-site
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type
in	O
the	O
regulatory	B-structure_element
loop	I-structure_element
is	O
directly	O
participating	O
in	O
CDC1	B-structure_element
/	O
CDC2	B-structure_element
domain	O
interactions	O
and	O
thus	O
stabilizes	O
the	O
hinge	B-structure_element
conformation	I-structure_element
.	O

In	O
flACC	B-protein
,	O
the	O
regulatory	B-structure_element
loop	I-structure_element
is	O
mostly	B-protein_state
disordered	I-protein_state
,	O
illustrating	O
the	O
increased	O
flexibility	O
due	O
to	O
the	O
absence	O
of	O
the	O
phosphoryl	B-chemical
group	O
.	O

Only	O
in	O
three	O
out	O
of	O
eight	O
observed	O
protomers	B-oligomeric_state
a	O
short	B-structure_element
peptide	I-structure_element
stretch	O
(	O
including	O
Ser1157	B-residue_name_number
)	O
was	O
modelled	B-evidence
.	O

In	O
those	O
instances	O
the	O
Ser1157	B-residue_name_number
residue	O
is	O
located	O
at	O
a	O
distance	O
of	O
14	O
–	O
20	O
Å	O
away	O
from	O
the	O
location	O
of	O
the	O
phosphorylated	B-protein_state
serine	B-residue_name
observed	O
here	O
,	O
based	O
on	O
superposition	B-experimental_method
of	O
either	O
CDC1	B-structure_element
or	O
CDC2	B-structure_element
.	O

Applying	B-experimental_method
the	O
conformation	O
of	O
the	O
CDC1	B-structure_element
/	I-structure_element
CDC2	I-structure_element
hinge	I-structure_element
observed	O
in	O
SceCD	B-species
on	O
flACC	B-protein
leads	O
to	O
CDN	B-structure_element
sterically	O
clashing	O
with	O
CDC2	B-structure_element
and	O
BT	B-structure_element
/	O
CDN	B-structure_element
clashing	O
with	O
CT	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
6a	O
,	O
b	O
).	O

Thus	O
,	O
in	O
accordance	O
with	O
the	O
results	O
presented	O
here	O
,	O
phosphorylation	B-ptm
of	O
Ser1157	B-residue_name_number
in	O
SceACC	B-protein
most	O
likely	O
limits	O
flexibility	O
in	O
the	O
CDC1	B-structure_element
/	I-structure_element
CDC2	I-structure_element
hinge	I-structure_element
such	O
that	O
activation	O
through	O
BC	B-structure_element
dimerization	O
is	O
not	O
possible	O
(	O
Fig	O
.	O
4d	O
),	O
which	O
however	O
does	O
not	O
exclude	O
intermolecular	O
dimerization	O
.	O

In	O
addition	O
,	O
EM	B-experimental_method
micrographs	B-evidence
of	O
phosphorylated	B-protein_state
and	O
dephosphorylated	B-protein_state
SceACC	B-protein
display	O
for	O
both	O
samples	O
mainly	O
elongated	B-protein_state
and	I-protein_state
U	I-protein_state
-	I-protein_state
shaped	I-protein_state
conformations	I-protein_state
and	O
reveal	O
no	O
apparent	O
differences	O
in	O
particle	B-evidence
shape	I-evidence
distributions	I-evidence
(	O
Supplementary	O
Fig	O
.	O
7	O
).	O

This	O
implicates	O
that	O
the	O
triangular	B-protein_state
shape	I-protein_state
with	O
dimeric	B-oligomeric_state
BC	B-structure_element
domains	O
has	O
a	O
low	O
population	O
also	O
in	O
the	O
active	B-protein_state
form	I-protein_state
,	O
even	O
though	O
a	O
biasing	O
influence	O
of	O
grid	O
preparation	O
cannot	O
be	O
excluded	O
completely	O
.	O

Together	O
,	O
this	O
structural	B-evidence
information	I-evidence
suggests	O
that	O
variable	O
carrier	O
protein	O
tethering	O
is	O
not	O
sufficient	O
for	O
efficient	O
substrate	O
transfer	O
and	O
catalysis	O
in	O
any	O
of	O
these	O
systems	O
.	O

The	O
determination	B-experimental_method
of	I-experimental_method
a	I-experimental_method
set	I-experimental_method
of	I-experimental_method
crystal	B-evidence
structures	I-evidence
of	O
SceACC	B-protein
in	O
two	O
states	O
,	O
unphosphorylated	B-protein_state
and	O
phosphorylated	B-protein_state
at	O
the	O
major	B-site
regulatory	I-site
site	I-site
Ser1157	B-residue_name_number
,	O
provides	O
a	O
unique	O
depiction	O
of	O
multienzyme	O
regulation	O
by	O
post	O
-	O
translational	O
modification	O
(	O
Fig	O
.	O
4d	O
).	O

The	O
regulation	O
of	O
activity	O
thus	O
results	O
from	O
restrained	O
large	O
-	O
scale	O
conformational	O
dynamics	O
rather	O
than	O
a	O
direct	O
or	O
indirect	O
influence	O
on	O
active	B-site
site	I-site
structure	I-site
.	O

To	O
our	O
best	O
knowledge	O
,	O
ACC	B-protein_type
is	O
the	O
first	O
multienzyme	B-protein_type
for	O
which	O
such	O
a	O
phosphorylation	B-ptm
-	O
dependent	O
mechanical	O
control	O
mechanism	O
has	O
been	O
visualized	O
.	O

However	O
,	O
the	O
example	O
of	O
ACC	B-protein_type
now	O
demonstrates	O
the	O
possibility	O
of	O
regulating	O
activity	O
by	O
controlled	O
dynamics	O
of	O
non	B-structure_element
-	I-structure_element
enzymatic	I-structure_element
linker	I-structure_element
regions	I-structure_element
also	O
in	O
other	O
families	O
of	O
carrier	B-protein_type
-	I-protein_type
dependent	I-protein_type
multienzymes	I-protein_type
.	O

The	O
phosphorylated	B-protein_state
central	B-structure_element
domain	I-structure_element
of	O
yeast	B-taxonomy_domain
ACC	B-protein_type
.	O

(	O
a	O
)	O
Schematic	O
overview	O
of	O
the	O
domain	O
organization	O
of	O
eukaryotic	B-taxonomy_domain
ACCs	B-protein_type
.	O

Crystallized	B-evidence
constructs	I-evidence
are	O
indicated	O
.	O

(	O
b	O
)	O
Cartoon	O
representation	O
of	O
the	O
SceCD	B-species
crystal	B-evidence
structure	I-evidence
.	O

The	O
regulatory	B-structure_element
loop	I-structure_element
is	O
shown	O
as	O
bold	O
cartoon	O
,	O
and	O
the	O
phosphorylated	B-protein_state
Ser1157	B-residue_name_number
is	O
marked	O
by	O
a	O
red	O
triangle	O
.	O

The	O
attachment	O
points	O
to	O
the	O
N	O
-	O
terminal	O
BCCP	B-structure_element
domain	O
and	O
the	O
C	O
-	O
terminal	O
CT	B-structure_element
domain	O
are	O
indicated	O
with	O
spheres	O
.	O

Architecture	O
of	O
the	O
CD	B-structure_element
–	O
CT	B-structure_element
core	O
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type
.	O

One	O
protomer	B-oligomeric_state
is	O
shown	O
in	O
colour	O
and	O
one	O
in	O
grey	O
.	O

Individual	O
domains	O
are	O
labelled	O
;	O
the	O
active	B-site
site	I-site
of	O
CT	B-structure_element
and	O
the	O
position	O
of	O
the	O
conserved	B-protein_state
regulatory	B-protein_state
phosphoserine	B-site
site	I-site
based	O
on	O
SceCD	B-species
are	O
indicated	O
by	O
an	O
asterisk	O
and	O
a	O
triangle	O
,	O
respectively	O
.	O

(	O
a	O
)	O
Hinge	B-structure_element
properties	O
of	O
the	O
CDC2	B-structure_element
–	I-structure_element
CT	I-structure_element
connection	I-structure_element
analysed	O
by	O
a	O
CT	B-experimental_method
-	I-experimental_method
based	I-experimental_method
superposition	I-experimental_method
of	O
eight	O
instances	O
of	O
the	O
CDC2	B-mutant
-	I-mutant
CT	I-mutant
segment	I-mutant
.	O

For	O
clarity	O
,	O
only	O
one	O
protomer	B-oligomeric_state
of	O
CthCD	B-mutant
-	I-mutant
CTCter1	I-mutant
is	O
shown	O
in	O
full	O
colour	O
as	O
reference	O
.	O

The	O
range	O
of	O
hinge	O
bending	O
is	O
indicated	O
and	O
the	O
connection	O
points	O
between	O
CDC2	B-structure_element
and	O
CT	B-structure_element
(	O
blue	O
)	O
as	O
well	O
as	O
between	O
CDC1	B-structure_element
and	O
CDC2	B-structure_element
(	O
green	O
and	O
grey	O
)	O
are	O
marked	O
as	O
spheres	O
.	O

One	O
protomer	B-oligomeric_state
of	O
CthΔBCCP	B-mutant
is	O
shown	O
in	O
colour	O
,	O
the	O
CDL	B-structure_element
domains	O
are	O
omitted	O
for	O
clarity	O
and	O
the	O
position	O
of	O
the	O
phosphorylated	B-protein_state
serine	B-residue_name
based	O
on	O
SceCD	B-species
is	O
indicated	O
with	O
a	O
red	O
triangle	O
.	O

The	O
connection	O
points	O
from	O
CDC1	B-structure_element
to	O
CDC2	B-structure_element
and	O
to	O
CDL	B-structure_element
are	O
represented	O
by	O
green	O
spheres	O
.	O

The	O
conformational	O
dynamics	O
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type
.	O

Domains	O
other	O
than	O
CDN	B-structure_element
and	O
CDL	B-structure_element
/	O
CDC1	B-structure_element
are	O
omitted	O
for	O
clarity	O
.	O

The	O
domains	O
are	O
labelled	O
and	O
the	O
distances	O
between	O
the	O
N	O
termini	O
of	O
CDN	B-structure_element
(	O
spheres	O
)	O
in	O
the	O
compared	O
structures	O
are	O
indicated	O
.	O

(	O
d	O
)	O
Schematic	O
model	O
of	O
fungal	B-taxonomy_domain
ACC	B-protein_type
showing	O
the	O
intrinsic	O
,	O
regulated	O
flexibility	O
of	O
CD	B-structure_element
in	O
the	O
phosphorylated	B-protein_state
inhibited	B-protein_state
or	O
the	O
non	B-protein_state
-	I-protein_state
phosphorylated	I-protein_state
activated	B-protein_state
state	O
.	O

Flexibility	O
of	O
the	O
CDC2	B-structure_element
/	O
CT	B-structure_element
and	O
CDN	B-structure_element
/	O
CDL	B-structure_element
hinges	B-structure_element
is	O
illustrated	O
by	O
arrows	O
.	O

The	O
Ser1157	B-residue_name_number
phosphorylation	B-ptm
site	O
and	O
the	O
regulatory	B-structure_element
loop	I-structure_element
are	O
schematically	O
indicated	O
in	O
magenta	O
.	O

Crystal	B-evidence
structure	I-evidence
of	O
SEL1L	B-protein
:	O
Insight	O
into	O
the	O
roles	O
of	O
SLR	B-structure_element
motifs	O
in	O
ERAD	O
pathway	O

SEL1L	B-protein
,	O
a	O
component	O
of	O
the	O
ERAD	O
machinery	O
,	O
plays	O
an	O
important	O
role	O
in	O
selecting	O
and	O
transporting	O
ERAD	O
substrates	O
for	O
degradation	O
.	O

We	O
have	O
determined	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
mouse	B-taxonomy_domain
SEL1L	B-protein
central	B-structure_element
domain	I-structure_element
comprising	O
five	O
Sel1	B-structure_element
-	I-structure_element
Like	I-structure_element
Repeats	I-structure_element
(	O
SLR	B-structure_element
motifs	I-structure_element
5	I-structure_element
to	I-structure_element
9	I-structure_element
;	O
hereafter	O
called	O
SEL1Lcent	B-structure_element
).	O

Furthermore	O
,	O
we	O
discovered	O
that	O
the	O
SLR	B-structure_element
-	I-structure_element
C	I-structure_element
,	O
comprising	O
SLR	B-structure_element
motifs	I-structure_element
10	I-structure_element
and	I-structure_element
11	I-structure_element
,	O
of	O
SEL1L	B-protein
directly	O
interacts	O
with	O
the	O
N	O
-	O
terminus	O
luminal	B-structure_element
loops	I-structure_element
of	O
HRD1	B-protein
.	O

Therefore	O
,	O
we	O
propose	O
that	O
certain	O
SLR	B-structure_element
motifs	O
of	O
SEL1L	B-protein
play	O
a	O
unique	O
role	O
in	O
membrane	O
bound	O
ERAD	O
machinery	O
.	O

Protein	O
quality	O
control	O
in	O
the	O
endoplasmic	O
reticulum	O
(	O
ER	O
)	O
is	O
essential	O
for	O
maintenance	O
of	O
cellular	O
homeostasis	O
in	O
eukaryotes	B-taxonomy_domain
and	O
is	O
implicated	O
in	O
many	O
severe	O
diseases	O
.	O

Terminally	O
misfolded	O
proteins	O
in	O
the	O
lumen	O
or	O
membrane	O
of	O
the	O
ER	O
are	O
retrotranslocated	O
into	O
the	O
cytosol	O
,	O
polyubiquitinated	B-protein_state
,	O
and	O
degraded	O
by	O
the	O
proteasome	B-complex_assembly
.	O

Accumulating	O
studies	O
have	O
identified	O
key	O
components	O
for	O
ERAD	O
,	O
including	O
HRD1	B-protein
,	O
SEL1L	B-protein
(	O
Hrd3p	B-protein
),	O
Derlin	B-protein
-	I-protein
1	I-protein
,	I-protein
-	I-protein
2	I-protein
,	I-protein
-	I-protein
3	I-protein
(	O
Der1p	B-protein
),	O
HERP	B-protein
-	I-protein
1	I-protein
,	I-protein
-	I-protein
2	I-protein
(	O
Usa1p	B-protein
),	O
OS9	B-protein
(	O
Yos9	B-protein
),	O
XTP	B-protein
-	I-protein
B	I-protein
,	O
and	O
Grp94	B-protein
,	O
all	O
of	O
which	O
are	O
involved	O
in	O
the	O
recognition	O
and	O
translocation	O
of	O
the	O
ERAD	O
substrates	O
in	O
yeast	B-taxonomy_domain
and	O
metazoans	B-taxonomy_domain
.	O

Yeast	B-taxonomy_domain
ERAD	O
components	O
,	O
which	O
have	O
been	O
extensively	O
characterized	O
through	O
genetic	B-experimental_method
and	I-experimental_method
biochemical	I-experimental_method
studies	I-experimental_method
,	O
are	O
comparable	O
with	O
mammalian	B-taxonomy_domain
ERAD	O
components	O
,	O
sharing	O
similar	O
molecular	O
functions	O
and	O
structural	O
composition	O
.	O

The	O
HRD1	B-protein
E3	B-protein_type
ubiquitin	I-protein_type
ligase	I-protein_type
,	O
which	O
is	O
embedded	O
in	O
the	O
ER	O
membrane	O
,	O
is	O
involved	O
in	O
translocating	O
ERAD	O
substrates	O
across	O
the	O
ER	O
membrane	O
and	O
catalyzing	O
substrate	O
ubiquitination	O
via	O
its	O
cytosolic	O
RING	B-structure_element
finger	I-structure_element
domain	I-structure_element
.	O

SEL1L	B-protein
,	O
the	O
mammalian	B-taxonomy_domain
homolog	O
of	O
Hrd3p	B-protein
,	O
associates	O
with	O
HRD1	B-protein
,	O
mediates	O
HRD1	B-protein
interactions	O
with	O
the	O
ER	O
luminal	O
lectin	B-protein_type
OS9	B-protein
,	O
and	O
recognizes	O
substrates	O
to	O
be	O
degraded	O
.	O

Recent	O
research	O
based	O
on	O
the	O
inducible	O
Sel1l	B-gene
knockout	B-experimental_method
mouse	I-experimental_method
model	O
highlights	O
the	O
physiological	O
functions	O
of	O
SEL1L	B-protein
.	O

However	O
,	O
despite	O
the	O
functional	O
importance	O
of	O
SEL1L	B-protein
,	O
the	O
molecular	O
structure	B-evidence
of	O
SEL1L	B-protein
has	O
not	O
been	O
solved	O
.	O

Previous	O
biochemical	B-experimental_method
studies	I-experimental_method
reveal	O
that	O
SEL1L	B-protein
is	O
a	O
type	B-protein_type
I	I-protein_type
transmembrane	I-protein_type
protein	I-protein_type
and	O
has	O
a	O
large	O
luminal	B-structure_element
domain	I-structure_element
comprising	O
sets	O
of	O
repeated	B-structure_element
Sel1	I-structure_element
-	I-structure_element
like	I-structure_element
(	O
SLR	B-structure_element
)	O
motifs	O
.	O

The	O
SLR	B-structure_element
motif	O
is	O
a	O
structural	O
motif	O
that	O
closely	O
resembles	O
the	O
tetratricopeptide	B-structure_element
-	I-structure_element
repeat	I-structure_element
(	O
TPR	B-structure_element
)	O
motif	O
,	O
which	O
is	O
a	O
protein	O
-	O
protein	O
interaction	O
module	O
.	O

Furthermore	O
,	O
while	O
repeated	O
SLR	B-structure_element
motifs	O
are	O
commonly	O
found	O
in	O
tandem	O
arrays	O
,	O
the	O
SLR	B-structure_element
motifs	O
in	O
SEL1L	B-protein
are	O
,	O
according	O
to	O
the	O
primary	O
structure	O
prediction	O
of	O
SEL1L	B-protein
,	O
interspersed	O
among	O
other	O
sequences	O
in	O
the	O
luminal	B-structure_element
domain	I-structure_element
and	O
form	O
three	O
SLR	B-structure_element
domain	O
clusters	O
.	O

Therefore	O
,	O
the	O
way	O
in	O
which	O
these	O
unique	O
structural	O
features	O
of	O
SEL1L	B-protein
are	O
related	O
to	O
its	O
critical	O
function	O
in	O
ERAD	O
remains	O
to	O
be	O
elucidated	O
.	O

To	O
clearly	O
understand	O
the	O
biochemical	O
role	O
of	O
the	O
SLR	B-structure_element
domains	O
of	O
SEL1L	B-protein
in	O
ERAD	O
,	O
we	O
determined	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
central	O
SLR	B-structure_element
domain	O
of	O
SEL1L	B-protein
.	O

We	O
found	O
that	O
the	O
central	B-structure_element
domain	I-structure_element
of	O
SEL1L	B-protein
,	O
comprising	O
SLR	B-structure_element
motifs	I-structure_element
5	I-structure_element
through	I-structure_element
9	I-structure_element
(	O
SEL1Lcent	B-structure_element
),	O
forms	O
a	O
tight	O
dimer	B-oligomeric_state
with	O
two	O
-	O
fold	O
symmetry	O
due	O
to	O
domain	O
swapping	O
of	O
the	O
SLR	B-structure_element
motif	I-structure_element
9	I-structure_element
.	O

We	O
also	O
found	O
that	O
SLR	B-structure_element
-	I-structure_element
C	I-structure_element
,	O
consisting	O
of	O
SLR	B-structure_element
motifs	I-structure_element
10	I-structure_element
and	I-structure_element
11	I-structure_element
,	O
directly	O
interacts	O
with	O
the	O
N	O
-	O
terminus	O
luminal	B-structure_element
loop	I-structure_element
of	O
HRD1	B-protein
.	O

Structure	B-experimental_method
Determination	I-experimental_method
of	O
SEL1Lcent	B-structure_element

The	O
Mus	B-species
musculus	I-species
SEL1L	B-protein
protein	O
contains	O
790	O
amino	O
acids	O
and	O
has	O
17	O
%	O
sequence	O
identity	O
to	O
its	O
yeast	B-taxonomy_domain
homolog	O
,	O
Hrd3p	B-protein
.	O

Mouse	B-taxonomy_domain
SEL1L	B-protein
contains	O
a	O
fibronectin	B-structure_element
type	I-structure_element
II	I-structure_element
domain	I-structure_element
at	O
the	O
N	O
-	O
terminus	O
,	O
followed	O
by	O
11	O
SLR	B-structure_element
motifs	O
and	O
a	O
single	O
transmembrane	B-structure_element
domain	I-structure_element
at	O
the	O
C	O
-	O
terminus	O
(	O
Fig	O
.	O
1A	O
).	O

However	O
,	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
protein	O
aggregated	O
in	O
solution	O
and	O
produced	O
no	O
soluble	O
protein	O
.	O

However	O
,	O
the	O
central	B-structure_element
region	I-structure_element
of	O
SEL1L	B-protein
,	O
comprising	O
residues	O
337	B-residue_range
–	I-residue_range
554	I-residue_range
,	O
was	O
soluble	O
and	O
homogenous	O
in	O
size	O
,	O
as	O
determined	O
by	O
size	B-experimental_method
-	I-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
.	O

To	O
define	O
compact	O
domain	O
boundaries	O
for	O
the	O
central	B-structure_element
region	I-structure_element
of	O
SEL1L	B-protein
,	O
we	O
digested	B-experimental_method
the	I-experimental_method
protein	I-experimental_method
with	I-experimental_method
trypsin	I-experimental_method
and	O
analyzed	O
the	O
proteolysis	O
products	O
by	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
and	O
N	B-experimental_method
-	I-experimental_method
terminal	I-experimental_method
sequencing	I-experimental_method
.	O

The	O
results	O
of	O
this	O
preliminary	O
biochemical	O
analysis	O
suggested	O
that	O
SEL1L	B-protein
residues	O
348	B-residue_range
–	I-residue_range
533	I-residue_range
(	O
SEL1Lcent	B-structure_element
)	O
would	O
be	O
amenable	O
to	O
structural	B-experimental_method
analysis	I-experimental_method
(	O
Fig	O
.	O
1A	O
).	O

Crystals	B-evidence
of	O
SEL1Lcent	B-structure_element
grew	O
in	O
space	O
group	O
P21	O
with	O
four	O
copies	O
of	O
SEL1Lcent	B-structure_element
(	O
a	O
total	O
of	O
82	O
kDa	O
)	O
in	O
the	O
asymmetric	O
unit	O
.	O

The	O
structure	B-evidence
was	O
determined	O
by	O
the	O
single	B-experimental_method
-	I-experimental_method
wavelength	I-experimental_method
anomalous	I-experimental_method
diffraction	I-experimental_method
(	O
SAD	B-experimental_method
)	O
method	O
using	O
selenium	B-chemical
as	O
the	O
anomalous	O
scatterer	O
(	O
Table	O
1	O
and	O
Methods	O
).	O

The	O
assignment	O
of	O
residues	O
during	O
model	O
building	O
was	O
aided	O
by	O
the	O
selenium	B-chemical
atom	O
positions	O
,	O
and	O
the	O
structure	B-evidence
was	O
refined	O
with	O
native	O
data	O
to	O
2	O
.	O
6	O
Å	O
resolution	O
with	O
Rwork	B-evidence
/	I-evidence
Rfree	I-evidence
values	O
of	O
20	O
.	O
7	O
/	O
27	O
.	O
7	O
%.	O

Overall	O
Structure	B-evidence
of	O
SEL1Lcent	B-structure_element

The	O
mouse	B-taxonomy_domain
SEL1Lcent	B-structure_element
crystallized	B-experimental_method
as	O
a	O
homodimer	B-oligomeric_state
,	O
and	O
there	O
were	O
two	O
homodimers	B-oligomeric_state
in	O
the	O
crystal	O
asymmetric	O
unit	O
(	O
Fig	O
.	O
1B	O
,	O
C	O
,	O
Supplementary	O
Fig	O
.	O
1	O
).	O

The	O
two	O
SEL1Lcent	B-structure_element
molecules	O
dimerize	B-oligomeric_state
in	O
a	O
head	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
tail	I-protein_state
manner	O
through	O
a	O
two	B-site
-	I-site
fold	I-site
symmetry	I-site
interface	I-site
resulting	O
in	O
a	O
cosmos	O
-	O
like	O
shaped	O
structure	B-evidence
(	O
Fig	O
.	O
1B	O
).	O

The	O
resulting	O
structure	B-evidence
resembles	O
the	O
yin	O
-	O
yang	O
symbol	O
with	O
overall	O
dimensions	O
of	O
60	O
×	O
60	O
×	O
25	O
Å	O
,	O
where	O
a	O
SEL1Lcent	B-structure_element
monomer	B-oligomeric_state
corresponds	O
to	O
half	O
the	O
symbol	O
.	O

The	O
dimer	B-oligomeric_state
formation	O
buries	O
a	O
surface	O
area	O
of	O
1670	O
Å2	O
for	O
each	O
monomer	B-oligomeric_state
,	O
and	O
no	O
significant	O
differences	O
between	O
the	O
protomers	B-oligomeric_state
were	O
displayed	O
(	O
final	O
root	B-evidence
mean	I-evidence
square	I-evidence
deviation	I-evidence
(	O
RMSD	B-evidence
)	O
of	O
0	O
.	O
6	O
Å	O
for	O
all	O
Cα	O
atoms	O
).	O

Each	O
protomer	B-oligomeric_state
is	O
composed	O
of	O
ten	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
,	O
which	O
form	O
the	O
five	O
SLRs	B-structure_element
,	O
resulting	O
in	O
an	O
elongated	O
curved	O
structure	O
,	O
confirming	O
the	O
primary	O
structure	O
prediction	O
(	O
Fig	O
.	O
1D	O
).	O

The	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
subdivide	O
the	O
structure	B-evidence
into	O
five	O
pairs	O
(	O
A	B-structure_element
and	O
B	B-structure_element
)	O
as	O
shown	O
in	O
a	O
number	O
of	O
TPRs	B-structure_element
and	O
SLRs	B-structure_element
.	O

In	O
addition	O
,	O
a	O
longer	O
loop	B-structure_element
,	O
consisting	O
of	O
approximately	O
eight	O
amino	O
acids	O
,	O
is	O
inserted	O
between	O
helix	B-structure_element
B	I-structure_element
of	O
one	O
SLR	B-structure_element
and	O
helix	B-structure_element
A	I-structure_element
of	O
the	O
next	O
SLR	B-structure_element
.	O

This	O
arrangement	O
is	O
a	O
unique	O
feature	O
for	O
SLRs	B-structure_element
among	O
the	O
major	O
classes	O
of	O
repeats	O
containing	O
an	O
α	B-structure_element
-	I-structure_element
solenoid	I-structure_element
.	O

This	O
unique	O
conformation	O
of	O
helix	B-structure_element
9B	I-structure_element
most	O
likely	O
contributes	O
to	O
formation	O
of	O
the	O
dimer	B-oligomeric_state
structure	O
of	O
SEL1Lcent	B-structure_element
,	O
as	O
detailed	O
below	O
.	O

With	O
the	O
exception	O
of	O
the	O
last	O
SLR	B-structure_element
,	O
the	O
four	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
pairs	O
possess	O
similar	O
conformations	O
,	O
with	O
RMSD	B-evidence
values	O
of	O
0	O
.	O
7	O
Å	O
for	O
all	O
Cα	O
atoms	O
.	O

Although	O
the	O
sequence	O
similarity	O
for	O
the	O
pairwise	B-experimental_method
alignments	I-experimental_method
varies	O
between	O
25	O
%	O
and	O
35	O
%,	O
all	O
the	O
residues	O
present	O
in	O
the	O
SLR	B-structure_element
motifs	O
are	O
conserved	B-protein_state
among	O
the	O
five	O
pairs	O
.	O

The	O
SLR	B-structure_element
domain	O
of	O
SLR	B-structure_element
-	I-structure_element
M	I-structure_element
ends	O
at	O
residue	O
524	B-residue_number
,	O
and	O
C	O
-	O
terminal	O
amino	O
acids	O
525	B-residue_range
–	I-residue_range
533	I-residue_range
of	O
the	O
protein	O
are	O
not	O
visible	O
in	O
the	O
electron	B-evidence
density	I-evidence
map	I-evidence
,	O
suggesting	O
that	O
this	O
region	O
is	O
highly	B-protein_state
flexible	I-protein_state
.	O

Since	O
no	O
information	O
regarding	O
dimer	B-oligomeric_state
formation	O
by	O
SEL1L	B-protein
through	O
its	O
SLR	B-structure_element
motifs	O
is	O
available	O
,	O
we	O
tested	O
whether	O
the	O
SEL1Lcent	B-structure_element
dimer	B-oligomeric_state
shown	O
in	O
our	O
crystal	B-evidence
structure	I-evidence
is	O
a	O
biological	O
unit	O
.	O

First	O
,	O
we	O
cross	B-experimental_method
-	I-experimental_method
linked	I-experimental_method
SEL1Lcent	B-structure_element
or	O
a	O
longer	O
construct	O
of	O
SEL1L	B-protein
(	O
SEL1Llong	B-mutant
,	O
residues	O
337	B-residue_range
–	I-residue_range
554	I-residue_range
)	O
using	O
various	O
concentrations	O
of	O
glutaraldehyde	B-chemical
(	O
GA	B-chemical
)	O
or	O
dimethyl	B-chemical
suberimidate	I-chemical
(	O
DMS	B-chemical
)	O
and	O
analyzed	O
the	O
products	O
by	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
.	O

We	O
detected	O
bands	O
at	O
the	O
mass	O
of	O
a	O
dimer	B-oligomeric_state
for	O
both	O
SEL1Lcent	B-structure_element
and	O
SEL1Llong	B-mutant
when	O
cross	B-experimental_method
-	I-experimental_method
linked	I-experimental_method
with	O
low	O
concentrations	O
of	O
GA	B-chemical
(	O
0	O
.	O
005	O
%)	O
or	O
DMS	B-chemical
(	O
0	O
.	O
3	O
mM	O
)	O
(	O
Supplementary	O
Fig	O
.	O
2A	O
,	O
B	O
).	O

Next	O
,	O
we	O
conducted	O
analytical	B-experimental_method
ultracentrifugation	I-experimental_method
of	O
SEL1Lcent	B-structure_element
.	O

Consistent	O
with	O
the	O
cross	B-experimental_method
-	I-experimental_method
linking	I-experimental_method
data	O
,	O
analytical	B-experimental_method
ultracentrifugation	I-experimental_method
revealed	O
that	O
the	O
molecular	B-evidence
weight	I-evidence
of	O
SEL1Lcent	B-structure_element
corresponds	O
to	O
a	O
dimer	B-oligomeric_state
(	O
Supplementary	O
Fig	O
.	O
2C	O
).	O

Taken	O
together	O
,	O
these	O
data	O
indicate	O
that	O
some	O
kind	O
of	O
dimer	B-oligomeric_state
is	O
formed	O
in	O
solution	O
.	O

Dimer	B-site
Interface	I-site
of	O
SEL1Lcent	B-structure_element

In	O
contrast	O
to	O
a	O
previously	O
described	O
SLR	B-protein_type
motif	I-protein_type
containing	I-protein_type
proteins	I-protein_type
that	O
exist	O
as	O
monomers	B-oligomeric_state
in	O
solution	O
,	O
SEL1Lcent	B-structure_element
forms	O
an	O
intimate	O
two	B-site
-	I-site
fold	I-site
homotypic	I-site
dimer	I-site
interface	I-site
(	O
Figs	O
1B	O
and	O
2A	O
).	O

However	O
,	O
no	O
interactions	O
were	O
seen	O
between	O
the	O
two	O
-	O
fold	O
-	O
related	O
protomers	B-oligomeric_state
through	O
the	O
concave	B-site
inner	I-site
surfaces	I-site
themselves	O
.	O

Rather	O
,	O
the	O
unique	O
structure	O
of	O
SLR	B-structure_element
motif	I-structure_element
9	I-structure_element
,	O
consisting	O
of	O
two	O
parallel	O
helices	O
(	O
9A	B-structure_element
and	O
9B	B-structure_element
),	O
is	O
located	O
in	O
the	O
space	O
generated	O
by	O
the	O
concave	B-site
surface	I-site
and	O
provides	O
an	O
extensive	O
dimerization	B-site
interface	I-site
between	O
the	O
two	O
-	O
fold	O
-	O
related	O
molecules	O
(	O
Fig	O
.	O
2A	O
).	O

Three	O
major	O
contact	B-site
interfaces	I-site
are	O
involved	O
in	O
the	O
interactions	O
,	O
and	O
all	O
interfaces	B-site
are	O
symmetrically	O
related	O
between	O
the	O
dimer	B-oligomeric_state
subunits	O
(	O
Fig	O
.	O
2A	O
).	O

Structure	B-experimental_method
-	I-experimental_method
based	I-experimental_method
sequence	I-experimental_method
alignment	I-experimental_method
of	O
135	O
SEL1L	B-protein
phylogenetic	O
sequences	O
using	O
a	O
ConSurf	B-experimental_method
server	I-experimental_method
revealed	O
that	O
the	O
surface	O
residues	O
in	O
the	O
dimer	B-site
interfaces	I-site
were	O
highly	B-protein_state
conserved	I-protein_state
among	O
the	O
SEL1L	B-protein
orthologs	O
(	O
Fig	O
.	O
1E	O
).	O

First	O
,	O
helix	B-structure_element
9B	I-structure_element
of	O
each	O
SEL1Lcent	B-structure_element
subunit	O
interacts	O
with	O
residues	O
lining	O
the	O
inner	B-site
groove	I-site
from	O
the	O
SLR	B-structure_element
α	B-structure_element
-	I-structure_element
helices	I-structure_element
(	O
5B	B-structure_element
,	O
6B	B-structure_element
,	O
7B	B-structure_element
,	O
and	O
8B	B-structure_element
)	O
from	O
its	O
counterpart	O
.	O

In	O
addition	O
to	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
,	O
the	O
side	O
chain	O
hydroxyl	O
group	O
of	O
Tyr	B-residue_name_number
519	I-residue_name_number
and	O
the	O
main	O
-	O
chain	O
oxygen	O
of	O
Ile	B-residue_name_number
515	I-residue_name_number
form	O
H	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
to	O
the	O
side	O
chain	O
of	O
the	O
conserved	B-protein_state
Gln	B-residue_name_number
377	I-residue_name_number
and	O
His	B-residue_name_number
381	I-residue_name_number
on	O
helix	B-structure_element
5B	I-structure_element
of	O
the	O
two	O
-	O
fold	O
-	O
related	O
protomer	B-oligomeric_state
.	O

The	O
side	O
chain	O
of	O
Gln	B-residue_name_number
523	I-residue_name_number
forms	O
an	O
H	B-bond_interaction
-	I-bond_interaction
bond	I-bond_interaction
to	O
the	O
side	O
chain	O
of	O
Asp	B-residue_name_number
480	I-residue_name_number
on	O
the	O
two	O
-	O
fold	O
-	O
related	O
protomer	B-oligomeric_state
(	O
Fig	O
.	O
2A	O
,	O
Interface	B-site
1	I-site
detail	O
).	O

In	O
this	O
interface	B-site
,	O
the	O
interacting	O
residues	O
on	O
helix	B-structure_element
9A	I-structure_element
,	O
including	O
Leu	B-residue_name_number
503	I-residue_name_number
,	O
Tyr	B-residue_name_number
499	I-residue_name_number
,	O
and	O
the	O
aliphatic	O
side	O
chain	O
of	O
Lys	B-residue_name_number
500	I-residue_name_number
,	O
form	O
an	O
extensive	O
network	O
of	O
van	B-bond_interaction
der	I-bond_interaction
Waals	I-bond_interaction
contacts	I-bond_interaction
with	O
the	O
hydrophobic	O
residues	O
of	O
the	O
counterpart	O
helix	B-structure_element
5A	I-structure_element
,	O
including	O
Tyr	B-residue_name_number
360	I-residue_name_number
,	O
Leu	B-residue_name_number
356	I-residue_name_number
,	O
Tyr	B-residue_name_number
359	I-residue_name_number
,	O
and	O
Leu	B-residue_name_number
363	I-residue_name_number
.	O

In	O
addition	O
to	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
,	O
the	O
side	O
chains	O
of	O
Asn	B-residue_name_number
507	I-residue_name_number
and	O
Ser	B-residue_name_number
510	I-residue_name_number
on	O
helix	B-structure_element
9A	I-structure_element
make	O
H	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
with	O
highly	B-protein_state
conserved	I-protein_state
Arg	B-residue_name_number
384	I-residue_name_number
in	O
the	O
loop	B-structure_element
between	O
helix	B-structure_element
5B	I-structure_element
and	O
6A	B-structure_element
from	O
the	O
two	O
-	O
fold	O
-	O
related	O
protomer	B-oligomeric_state
(	O
Fig	O
.	O
2A	O
,	O
Interface	O
2	O
detail	O
).	O

Third	O
,	O
the	O
helix	B-structure_element
9B	I-structure_element
from	O
each	O
protomer	B-oligomeric_state
is	O
involved	O
in	O
the	O
dimer	B-oligomeric_state
interaction	O
by	O
forming	O
a	O
two	O
-	O
fold	O
antiparallel	O
symmetry	O
.	O

In	O
particular	O
,	O
the	O
side	O
chains	O
of	O
hydrophobic	O
residues	O
,	O
including	O
Phe	B-residue_name_number
518	I-residue_name_number
,	O
Leu	B-residue_name_number
521	I-residue_name_number
,	O
and	O
Met	B-residue_name_number
524	I-residue_name_number
,	O
are	O
directed	O
toward	O
each	O
other	O
,	O
where	O
they	O
make	O
both	O
inter	O
-	O
and	O
intramolecular	O
contacts	O
(	O
Fig	O
.	O
2A	O
,	O
Interface	B-site
3	I-site
detail	O
).	O

To	O
further	O
investigate	O
the	O
interactions	O
observed	O
in	O
our	O
crystal	B-evidence
structure	I-evidence
,	O
we	O
generated	O
a	O
C	O
-	O
terminal	O
deletion	B-protein_state
mutant	I-protein_state
(	O
SEL1L348	B-mutant
–	I-mutant
497	I-mutant
)	O
lacking	B-protein_state
SLR	B-structure_element
motif	I-structure_element
9	I-structure_element
(	O
helix	B-structure_element
9A	I-structure_element
and	O
9B	B-structure_element
)	O
from	O
SEL1Lcent	B-structure_element
for	O
comparative	O
analysis	O
.	O

The	O
deletion	B-protein_state
mutant	I-protein_state
and	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
SEL1Lcent	B-structure_element
showed	O
no	O
difference	O
in	O
spectra	B-evidence
by	O
CD	B-experimental_method
spectroscopy	I-experimental_method
,	O
indicating	O
that	O
the	O
deletion	B-experimental_method
of	O
the	O
SLR	B-structure_element
motif	I-structure_element
9	I-structure_element
did	O
not	O
affect	O
the	O
secondary	O
structure	O
of	O
SEL1Lcent	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
3	O
).	O

Additionally	O
,	O
to	O
further	O
validate	O
the	O
key	O
residues	O
involved	O
in	O
dimer	B-oligomeric_state
formation	O
,	O
we	O
generated	O
a	O
triple	B-protein_state
point	I-protein_state
mutant	I-protein_state
(	O
Interface	B-site
1	I-site
,	O
I515A	B-mutant
,	O
L516A	B-mutant
,	O
and	O
Y519A	B-mutant
)	O
of	O
the	O
hydrophobic	O
residues	O
that	O
are	O
involved	O
in	O
dimerization	B-oligomeric_state
.	O

The	O
triple	B-protein_state
point	I-protein_state
mutant	I-protein_state
eluted	O
at	O
the	O
monomer	B-oligomeric_state
position	O
upon	O
size	B-experimental_method
-	I-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
,	O
while	O
the	O
negative	O
control	O
point	B-protein_state
mutant	I-protein_state
(	O
Q460A	B-mutant
)	O
eluted	O
at	O
the	O
same	O
position	O
as	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
.	O

Notably	O
,	O
a	O
single	B-experimental_method
-	I-experimental_method
residue	I-experimental_method
mutation	I-experimental_method
(	O
L521A	B-mutant
in	O
interface	B-site
3	I-site
)	O
abolished	B-protein_state
the	I-protein_state
dimerization	I-protein_state
of	O
SEL1Lcent	B-structure_element
(	O
Fig	O
.	O
2B	O
).	O

Taken	O
together	O
,	O
these	O
structural	B-evidence
and	I-evidence
biochemical	I-evidence
data	I-evidence
demonstrate	O
that	O
SEL1Lcent	B-structure_element
exists	O
as	O
a	O
dimer	B-oligomeric_state
in	O
solution	O
and	O
that	O
SLR	B-structure_element
motif	I-structure_element
9	I-structure_element
in	O
SEL1Lcent	B-structure_element
plays	O
an	O
important	O
role	O
in	O
generating	O
a	O
two	O
-	O
fold	O
dimerization	B-site
interface	I-site
.	O

The	O
Two	O
Glycine	B-residue_name
Residues	O
(	O
G512	B-residue_name_number
and	O
G513	B-residue_name_number
)	O
Create	O
a	O
Hinge	B-structure_element
for	O
Domain	O
Swapping	O
of	O
SLR	B-structure_element
Motif	I-structure_element
9	I-structure_element

Based	O
on	O
the	O
prediction	O
,	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
contains	O
a	O
total	O
of	O
11	O
SLR	B-structure_element
motifs	O
,	O
and	O
our	O
construct	O
corresponds	O
to	O
SLR	B-structure_element
motifs	I-structure_element
5	I-structure_element
through	I-structure_element
9	I-structure_element
.	O

According	O
to	O
our	O
crystal	B-evidence
structure	I-evidence
,	O
the	O
central	O
axis	O
of	O
helix	B-structure_element
9B	I-structure_element
is	O
almost	O
parallel	O
to	O
that	O
of	O
helix	B-structure_element
9A	I-structure_element
(	O
Fig	O
.	O
3B	O
).	O

However	O
,	O
this	O
unusual	O
conformation	O
of	O
SLR	B-structure_element
motif	I-structure_element
9	I-structure_element
seems	O
to	O
be	O
essential	O
for	O
dimer	B-oligomeric_state
formation	O
,	O
as	O
described	O
earlier	O
.	O

The	O
phi	O
and	O
psi	O
dihedrals	O
are	O
100	O
°	O
and	O
20	O
°	O
for	O
Gly	B-residue_name_number
512	I-residue_name_number
,	O
and	O
110	O
°	O
and	O
−	O
20	O
°	O
for	O
Gly	B-residue_name_number
513	I-residue_name_number
,	O
respectively	O
(	O
Fig	O
.	O
3C	O
).	O

The	O
involvement	O
of	O
a	O
glycine	B-residue_name
residue	O
in	O
forming	O
a	O
hinge	B-structure_element
for	O
domain	O
swapping	O
has	O
been	O
reported	O
previously	O
.	O

The	O
significance	O
of	O
Gly	B-residue_name_number
513	I-residue_name_number
is	O
further	O
highlighted	O
by	O
its	O
absolute	B-protein_state
conservation	I-protein_state
among	O
different	O
species	O
,	O
including	O
the	O
budding	B-taxonomy_domain
yeast	I-taxonomy_domain
homolog	O
Hrd3p	B-protein
.	O

To	O
further	O
investigate	O
the	O
importance	O
of	O
Gly	B-residue_name_number
512	I-residue_name_number
and	O
Gly	B-residue_name_number
513	I-residue_name_number
in	O
the	O
unusual	O
SLR	B-structure_element
motif	O
geometry	O
,	O
we	O
generated	O
a	O
point	B-experimental_method
mutation	I-experimental_method
(	O
Gly	B-mutant
to	I-mutant
Ala	I-mutant
),	O
which	O
restricts	O
the	O
flexibility	O
.	O
Although	O
the	O
Gly	B-residue_name_number
512	I-residue_name_number
and	O
Gly	B-residue_name_number
513	I-residue_name_number
residues	O
are	O
closely	O
surrounded	O
by	O
helix	B-structure_element
9B	I-structure_element
from	O
the	O
counter	O
protomer	B-oligomeric_state
,	O
there	O
is	O
enough	O
space	O
for	O
the	O
side	O
chain	O
of	O
alanine	B-residue_name
,	O
suggesting	O
that	O
no	O
steric	O
hindrance	O
would	O
be	O
caused	O
by	O
the	O
mutation	B-experimental_method
(	O
Fig	O
.	O
3C	O
).	O

However	O
,	O
the	O
double	B-protein_state
mutant	I-protein_state
(	O
G512A	B-mutant
/	O
G513A	B-mutant
)	O
eluted	O
over	O
a	O
broad	O
range	O
and	O
much	O
earlier	O
than	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
,	O
suggesting	O
that	O
mutation	B-experimental_method
of	O
the	O
residues	O
involved	O
in	O
the	O
hinge	B-structure_element
linking	O
helix	B-structure_element
9A	I-structure_element
and	O
9B	B-structure_element
significantly	O
affected	O
the	O
geometry	O
of	O
helix	B-structure_element
9B	I-structure_element
in	O
generating	O
domain	O
swapping	O
,	O
and	O
eventually	O
altered	O
the	O
overall	O
oligomeric	O
state	O
of	O
SEL1Lcent	B-structure_element
into	O
a	O
polydisperse	O
pattern	O
(	O
Fig	O
.	O
3D	O
,	O
Supplementary	O
Fig	O
.	O
6	O
).	O

When	O
the	O
residues	O
were	O
mutated	B-experimental_method
to	I-experimental_method
lysine	B-residue_name
(	O
G512K	B-mutant
/	O
G513K	B-mutant
),	O
the	O
mutant	B-protein_state
not	O
only	O
restricted	O
the	O
geometry	O
of	O
residues	O
at	O
the	O
hinge	B-structure_element
but	O
also	O
generated	O
steric	O
hindrance	O
during	O
interaction	O
with	O
the	O
counter	O
protomer	B-oligomeric_state
of	O
SEL1Lcent	B-structure_element
,	O
thereby	O
inhibiting	O
self	O
-	O
association	O
of	O
SEL1Lcent	B-structure_element
completely	O
.	O

A	O
previous	O
study	O
shows	O
that	O
induction	O
of	O
steric	O
hindrance	O
by	O
mutation	B-experimental_method
destabilizes	O
the	O
dimerization	B-site
interface	I-site
of	O
a	O
different	O
protein	O
,	O
ClC	B-protein_type
transporter	I-protein_type
.	O

Collectively	O
,	O
these	O
data	O
suggest	O
that	O
the	O
Gly	B-residue_name_number
512	I-residue_name_number
and	O
Gly	B-residue_name_number
513	I-residue_name_number
at	O
the	O
connection	O
between	O
helix	B-structure_element
9A	I-structure_element
and	O
9B	B-structure_element
play	O
a	O
crucial	O
role	O
in	O
forming	O
the	O
domain	B-protein_state
-	I-protein_state
swapped	I-protein_state
conformation	O
that	O
enables	O
dimer	B-oligomeric_state
formation	O
.	O

SEL1L	B-protein
Forms	O
Self	B-oligomeric_state
-	I-oligomeric_state
oligomers	I-oligomeric_state
through	O
SEL1Lcent	B-structure_element
domain	O
in	O
vivo	O

Next	O
,	O
we	O
examined	O
if	O
SEL1L	B-protein
also	O
forms	O
self	B-oligomeric_state
-	I-oligomeric_state
oligomers	I-oligomeric_state
in	O
vivo	O
using	O
HEK293T	O
cells	O
.	O

A	O
co	B-experimental_method
-	I-experimental_method
immunoprecipitation	I-experimental_method
assay	I-experimental_method
using	O
an	O
anti	O
-	O
FLAG	B-experimental_method
antibody	O
followed	O
by	O
Western	B-experimental_method
blot	I-experimental_method
analysis	O
using	O
an	O
anti	O
-	O
HA	B-experimental_method
antibody	O
showed	O
that	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
forms	O
self	B-oligomeric_state
-	I-oligomeric_state
oligomers	I-oligomeric_state
in	O
vivo	O
(	O
Fig	O
.	O
4A	O
).	O

Co	B-experimental_method
-	I-experimental_method
immunoprecipitation	I-experimental_method
analysis	I-experimental_method
showed	O
that	O
the	O
SEL1Lcent	B-structure_element
was	O
sufficient	O
to	O
physically	O
interact	O
with	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
,	O
while	O
SEL1L348	B-mutant
–	I-mutant
497	I-mutant
failed	O
to	O
do	O
so	O
(	O
Fig	O
.	O
4A	O
).	O

Interestingly	O
,	O
however	O
,	O
the	O
expression	O
level	O
of	O
SEL1L348	B-mutant
–	I-mutant
497	I-mutant
was	O
consistently	O
lower	O
than	O
that	O
of	O
SEL1Lcent	B-structure_element
(	O
Fig	O
.	O
4A	O
,	O
B	O
).	O

Semi	B-experimental_method
-	I-experimental_method
quantitative	I-experimental_method
RT	I-experimental_method
-	I-experimental_method
PCR	I-experimental_method
revealed	O
no	O
significant	O
difference	O
in	O
transcriptional	O
levels	O
of	O
the	O
two	O
constructs	O
(	O
data	O
not	O
shown	O
).	O

We	O
speculated	O
that	O
SEL1L348	B-mutant
–	I-mutant
497	I-mutant
could	O
be	O
secreted	O
while	O
the	O
SEL1Lcent	B-structure_element
is	O
retained	O
in	O
the	O
ER	O
by	O
association	O
with	O
the	O
endogenous	O
ERAD	O
complex	O
.	O

We	O
next	O
examined	O
if	O
the	O
reason	O
why	O
SEL1L348	B-mutant
–	I-mutant
497	I-mutant
failed	O
to	O
bind	O
to	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
may	O
be	O
because	O
of	O
the	O
lower	O
level	O
of	O
SEL1L348	B-mutant
–	I-mutant
497	I-mutant
in	O
the	O
ER	O
lumen	O
compared	O
to	O
SEL1Lcent	B-structure_element
fragment	O
.	O

In	O
order	O
to	O
retain	O
two	O
SEL1L	B-protein
fragments	O
in	O
the	O
ER	O
lumen	O
,	O
we	O
added	O
KDEL	B-structure_element
ER	B-structure_element
retention	I-structure_element
sequence	I-structure_element
to	O
the	O
C	O
-	O
terminus	O
of	O
both	O
fragments	O
.	O

Indeed	O
,	O
the	O
addition	O
of	O
KDEL	B-structure_element
peptide	O
increased	O
the	O
level	O
of	O
SEL1L348	B-mutant
–	I-mutant
497	I-mutant
in	O
the	O
ER	O
lumen	O
(	O
Fig	O
.	O
4D	O
,	O
E	O
)	O
and	O
the	O
immunostaining	B-experimental_method
analysis	O
showed	O
both	O
constructs	O
were	O
well	O
localized	O
to	O
the	O
ER	O
(	O
Fig	O
.	O
4C	O
).	O

We	O
further	O
analyzed	O
whether	O
SEL1Lcent	B-structure_element
may	O
competitively	O
inhibit	O
the	O
self	O
-	O
oligomerization	O
of	O
SEL1L	B-protein
in	O
vivo	O
.	O

These	O
data	O
suggest	O
that	O
the	O
SEL1L	B-protein
forms	O
self	B-oligomeric_state
-	I-oligomeric_state
oligomers	I-oligomeric_state
and	O
the	O
oligomerization	O
is	O
mediated	O
by	O
the	O
SEL1Lcent	B-structure_element
domain	O
in	O
vivo	O
.	O

Structural	B-experimental_method
Comparison	I-experimental_method
of	O
SEL1L	B-protein
SLRs	B-structure_element
with	O
TPRs	B-structure_element
or	O
SLRs	B-structure_element
of	O
Other	O
Proteins	O

Previous	O
studies	O
reveal	O
that	O
TPRs	B-structure_element
and	O
SLRs	B-structure_element
have	O
similar	O
consensus	O
sequences	O
,	O
suggesting	O
that	O
their	O
three	O
-	O
dimensional	O
structures	O
are	O
also	O
similar	O
.	O

The	O
superposition	B-experimental_method
of	O
isolated	O
TPRs	B-structure_element
from	O
Cdc23	B-protein
(	O
S	B-species
.	I-species
pombe	I-species
,	O
cell	B-protein
division	I-protein
cycle	I-protein
23	I-protein
homolog	O
,	O
PDB	O
code	O
3ZN3	O
)	O
and	O
SLRs	B-structure_element
from	O
HcpC	B-protein
(	O
Helicobacter	B-protein
Cysteine	I-protein
-	I-protein
rich	I-protein
Protein	I-protein
C	I-protein
,	O
PDB	O
code	O
1OUV	O
)	O
yields	O
RMSDs	B-evidence
below	O
1	O
Å	O
,	O
confirming	O
that	O
the	O
isolated	O
repeats	O
are	O
indeed	O
similar	O
.	O

This	O
is	O
relevant	O
to	O
SLR	B-structure_element
motifs	O
in	O
SEL1L	B-protein
,	O
as	O
isolated	O
SLR	B-structure_element
motifs	O
from	O
SEL1Lcent	B-structure_element
showed	O
good	O
structural	B-experimental_method
alignment	I-experimental_method
with	O
isolated	O
TPRs	B-structure_element
(	O
RMSD	B-evidence
1	O
.	O
6	O
Å	O
for	O
all	O
Cα	O
chains	O
)	O
from	O
Cdc23N	B-protein
-	O
term	O
and	O
SLRs	B-structure_element
(	O
RMSD	B-evidence
0	O
.	O
6	O
Å	O
for	O
all	O
Cα	O
chains	O
)	O
from	O
HcpC	B-protein
(	O
Fig	O
.	O
5A	O
).	O

However	O
,	O
superimposing	B-experimental_method
the	O
structure	B-evidence
of	O
SLR	B-structure_element
motifs	I-structure_element
5	I-structure_element
to	I-structure_element
9	I-structure_element
from	O
SEL1Lcent	B-structure_element
onto	O
the	O
overall	O
Cdc23N	B-protein
-	O
term	O
or	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
HcpC	B-protein
structures	B-evidence
revealed	O
that	O
SLR	B-structure_element
motifs	I-structure_element
5	I-structure_element
to	I-structure_element
9	I-structure_element
in	O
SEL1Lcent	B-structure_element
have	O
a	O
different	O
superhelical	O
structure	O
than	O
either	O
Cdc23	B-protein
or	O
HcpC	B-protein
(	O
RMSD	B-evidence
values	O
of	O
>	O
2	O
.	O
5	O
Å	O
for	O
Cα	O
atoms	O
)	O
(	O
Fig	O
.	O
5B	O
).	O

The	O
differences	O
may	O
result	O
from	O
the	O
differing	O
numbers	O
of	O
residues	O
in	O
the	O
loops	B-structure_element
and	O
differences	O
in	O
antiparallel	B-structure_element
helix	I-structure_element
packing	O
.	O

Moreover	O
,	O
there	O
are	O
conserved	B-protein_state
disulfide	B-ptm
bonds	I-ptm
in	O
the	O
SLR	B-structure_element
motifs	O
of	O
HcpC	B-protein
and	O
HcpB	B-protein
,	O
but	O
no	O
such	O
bonds	O
are	O
observed	O
in	O
SEL1Lcent	B-structure_element
.	O

These	O
factors	O
contribute	O
to	O
the	O
differences	O
in	O
the	O
overall	O
conformation	O
of	O
the	O
SLR	B-structure_element
motifs	O
in	O
SEL1L	B-protein
and	O
other	O
SLR	B-protein_type
or	I-protein_type
TPR	I-protein_type
motif	I-protein_type
-	I-protein_type
containing	I-protein_type
proteins	I-protein_type
.	O

Another	O
major	O
difference	O
in	O
the	O
structure	B-evidence
of	O
SLR	B-structure_element
motifs	O
between	O
SEL1L	B-protein
and	O
HcpC	B-protein
is	O
the	O
oligomeric	O
state	O
of	O
proteins	O
.	O

The	O
TPR	B-structure_element
motif	O
is	O
involved	O
in	O
the	O
dimerization	B-oligomeric_state
of	O
proteins	O
such	O
as	O
Cdc23	B-protein
,	O
Cdc16	B-protein
,	O
and	O
Cdc27	B-protein
.	O

In	O
particular	O
,	O
the	O
N	O
-	O
terminal	O
domain	O
of	O
Cdc23	B-protein
(	O
Cdc23N	B-protein
-	O
term	O
)	O
has	O
a	O
TPR	B-structure_element
-	O
motif	O
organization	O
similar	O
to	O
that	O
of	O
the	O
SLR	B-structure_element
motif	O
in	O
SEL1Lcent	B-structure_element
.	O

The	O
seven	O
TPR	B-structure_element
motifs	O
of	O
Cdc23N	B-protein
-	O
term	O
are	O
assembled	O
into	O
a	O
superhelical	B-structure_element
structure	I-structure_element
,	O
generating	O
a	O
hollow	O
surface	O
and	O
encircling	O
its	O
dimer	B-oligomeric_state
counterpart	O
in	O
an	O
interlocking	O
clasp	O
-	O
like	O
arrangement	O
(	O
Fig	O
.	O
5C	O
).	O

The	O
TPR	B-structure_element
motif	I-structure_element
1	I-structure_element
(	O
TPR1	B-structure_element
)	O
of	O
each	O
Cdc23N	B-protein
-	O
term	O
subunit	O
is	O
located	O
in	O
the	O
hollow	O
surface	O
of	O
the	O
counter	O
subunit	O
and	O
interacts	O
with	O
residues	O
lining	O
the	O
inner	B-site
groove	I-site
TPR	B-structure_element
α	B-structure_element
-	I-structure_element
helices	I-structure_element
,	O
generating	O
two	O
-	O
fold	O
symmetry	O
homotype	O
interactions	O
.	O

However	O
,	O
in	O
this	O
structure	B-evidence
,	O
a	O
conformational	O
change	O
in	O
the	O
TPR	B-structure_element
motif	O
itself	O
is	O
not	O
observed	O
.	O

Self	O
-	O
association	O
of	O
HcpC	B-protein
has	O
not	O
been	O
reported	O
,	O
and	O
there	O
is	O
no	O
domain	B-protein_state
-	I-protein_state
swapped	I-protein_state
structure	O
in	O
the	O
SLR	B-structure_element
motifs	O
of	O
HcpC	B-protein
,	O
in	O
contrast	O
to	O
that	O
observed	O
in	O
SEL1Lcent	B-structure_element
.	O

Although	O
SEL1L	B-protein
contains	O
a	O
number	O
of	O
SLR	B-structure_element
motifs	O
comparable	O
to	O
HcpC	B-protein
,	O
the	O
SLR	B-structure_element
motifs	O
in	O
SEL1L	B-protein
are	O
interrupted	O
by	O
other	O
sequences	O
,	O
making	O
three	O
SLR	B-structure_element
motif	O
clusters	O
(	O
Fig	O
.	O
1A	O
).	O

The	O
interrupted	O
SLR	B-structure_element
motifs	O
may	O
be	O
required	O
for	O
dimerization	B-oligomeric_state
of	O
SEL1Lcent	B-structure_element
,	O
as	O
five	O
SLR	B-structure_element
motifs	O
are	O
more	O
than	O
enough	O
to	O
form	O
the	O
semicircle	B-structure_element
of	I-structure_element
the	I-structure_element
yin	I-structure_element
-	I-structure_element
yang	I-structure_element
symbol	O
(	O
Fig	O
.	O
1B	O
).	O

Helix	B-structure_element
5A	I-structure_element
from	O
SLR	B-structure_element
motif	I-structure_element
5	I-structure_element
meets	O
helix	B-structure_element
9A	I-structure_element
from	O
SLR	B-structure_element
motif	I-structure_element
9	I-structure_element
of	O
the	O
counterpart	O
SEL1L	B-protein
.	O

If	O
the	O
SLR	B-structure_element
motifs	I-structure_element
5	I-structure_element
to	I-structure_element
9	I-structure_element
were	O
not	O
isolated	O
from	O
other	O
SLR	B-structure_element
motifs	O
,	O
steric	O
hindrance	O
could	O
interfere	O
with	O
dimerization	B-oligomeric_state
of	O
SEL1L	B-protein
.	O

TPR	B-structure_element
and	O
SLR	B-structure_element
motifs	O
are	O
generally	O
involved	O
in	O
protein	O
-	O
protein	O
interaction	O
modules	O
,	O
and	O
the	O
sequences	O
between	O
the	O
SLR	B-structure_element
motifs	O
of	O
SEL1L	B-protein
might	O
actually	O
facilitate	O
the	O
self	O
-	O
association	O
of	O
this	O
protein	O
.	O

SLR	B-structure_element
-	I-structure_element
C	I-structure_element
of	O
SEL1L	B-protein
Binds	O
HRD1	B-protein
N	O
-	O
terminus	O
Luminal	B-structure_element
Loop	I-structure_element

Based	O
on	O
the	O
structural	B-evidence
data	I-evidence
presented	O
herein	O
,	O
a	O
possible	O
arrangement	O
of	O
membrane	O
-	O
associated	O
ERAD	O
components	O
in	O
mammals	B-taxonomy_domain
,	O
highlighting	O
the	O
molecular	O
functions	O
of	O
SLR	B-structure_element
domains	O
in	O
SEL1L	B-protein
,	O
is	O
shown	O
in	O
Fig	O
.	O
6C	O
.	O

We	O
suggest	O
that	O
the	O
middle	O
SLR	B-structure_element
domains	O
are	O
involved	O
in	O
the	O
dimerization	B-oligomeric_state
of	O
SEL1L	B-protein
based	O
on	O
the	O
crystal	B-evidence
structure	I-evidence
and	O
biochemical	O
data	O
.	O

SLR	B-structure_element
-	I-structure_element
C	I-structure_element
,	O
which	O
contains	O
SLR	B-structure_element
motifs	I-structure_element
10	I-structure_element
to	I-structure_element
11	I-structure_element
,	O
might	O
be	O
involved	O
in	O
the	O
interaction	O
with	O
HRD1	B-protein
.	O

The	O
Hrd3p	B-protein
residues	O
664	B-residue_range
–	I-residue_range
695	I-residue_range
correspond	O
to	O
mouse	B-taxonomy_domain
SEL1L	B-protein
residues	O
696	B-residue_range
–	I-residue_range
727	I-residue_range
,	O
which	O
include	O
the	O
entire	O
helix	B-structure_element
11B	I-structure_element
(	O
residue	O
697	B-residue_range
–	I-residue_range
709	I-residue_range
)	O
of	O
SLR	B-structure_element
motif	I-structure_element
11	I-structure_element
and	O
a	O
well	B-protein_state
-	I-protein_state
conserved	I-protein_state
adjacent	O
region	O
(	O
Supplementary	O
Fig	O
.	O
4	O
).	O

This	O
observation	O
is	O
supported	O
by	O
the	O
following	O
:	O
(	O
1	O
)	O
the	O
meticulous	O
range	O
of	O
SLR	B-structure_element
motif	I-structure_element
10	I-structure_element
to	I-structure_element
11	I-structure_element
is	O
newly	O
established	O
from	O
a	O
structure	B-experimental_method
-	I-experimental_method
guided	I-experimental_method
SLR	I-experimental_method
motif	I-experimental_method
alignment	I-experimental_method
,	O
based	O
on	O
the	O
present	O
structure	B-experimental_method
study	I-experimental_method
,	O
and	O
(	O
2	O
)	O
the	O
relatively	O
high	O
sequence	B-protein_state
conservation	I-protein_state
between	O
mammalian	B-taxonomy_domain
SEL1L	B-protein
and	O
yeast	B-taxonomy_domain
Hrd3p	B-protein
around	O
SLR	B-structure_element
motifs	I-structure_element
10	I-structure_element
to	I-structure_element
11	I-structure_element
,	O
which	O
contain	O
contact	O
regions	O
with	O
HRD1	B-protein
(	O
Hrd1p	B-protein
)	O
(	O
Supplementary	O
Figs	O
.	O
4	O
and	O
5	O
).	O

To	O
address	O
this	O
hypothesis	O
,	O
we	O
prepared	O
constructs	O
encoding	O
mouse	B-taxonomy_domain
HRD1	B-protein
luminal	O
fragments	O
fused	B-experimental_method
to	I-experimental_method
GST	I-experimental_method
as	O
shown	O
in	O
Fig	O
.	O
6A	O
,	O
and	O
tested	O
their	O
ability	O
to	O
bind	O
certain	O
SLR	B-structure_element
motifs	O
in	O
SEL1L	B-protein
.	O

Figure	O
6B	O
shows	O
that	O
the	O
SLR	B-structure_element
-	I-structure_element
C	I-structure_element
,	O
consisting	O
of	O
SLR	B-structure_element
motifs	I-structure_element
10	I-structure_element
and	I-structure_element
11	I-structure_element
,	O
exclusively	O
interacts	O
with	O
N	O
-	O
terminal	O
luminal	B-structure_element
loop	I-structure_element
(	O
residues	O
21	B-residue_range
–	I-residue_range
42	I-residue_range
)	O
of	O
HRD1	B-protein
.	O

The	O
molecular	O
functions	O
of	O
SLR	B-structure_element
-	I-structure_element
N	I-structure_element
are	O
unclear	O
.	O

SEL1Lcent	B-structure_element
contains	O
a	O
putative	O
N	B-site
-	I-site
glycosylation	I-site
site	I-site
,	O
Asn	B-residue_name_number
427	I-residue_name_number
,	O
which	O
is	O
highly	B-protein_state
conserved	I-protein_state
among	O
different	O
species	O
and	O
structurally	O
exposed	O
to	O
the	O
surface	O
of	O
the	O
SEL1L	B-protein
dimer	B-oligomeric_state
according	O
to	O
the	O
crystal	B-evidence
structure	I-evidence
(	O
Fig	O
.	O
6C	O
).	O

Many	O
reports	O
demonstrate	O
that	O
membrane	O
-	O
bound	O
ERAD	O
machinery	O
proteins	O
in	O
yeast	B-taxonomy_domain
,	O
such	O
as	O
Hrd1p	B-protein
,	O
Der1p	B-protein
,	O
and	O
Usa1p	B-protein
,	O
are	O
involved	O
in	O
oligomerization	O
of	O
ERAD	O
components	O
.	O

The	O
Hrd1p	B-protein
complex	O
forms	O
dimers	B-oligomeric_state
upon	O
sucrose	B-experimental_method
gradient	I-experimental_method
sedimentation	I-experimental_method
and	O
size	B-experimental_method
-	I-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
.	O

Previous	O
data	O
show	O
that	O
HA	B-protein_state
-	I-protein_state
epitope	I-protein_state
-	I-protein_state
tagged	I-protein_state
Hrd3p	B-protein
or	O
Hrd1p	B-protein
efficiently	O
co	O
-	O
precipitate	O
with	O
unmodified	B-protein_state
Hrd3p	B-protein
and	O
Hrd1p	B-protein
,	O
respectively	O
,	O
suggesting	O
that	O
both	O
Hrd1p	B-protein
and	O
Hrd3p	B-protein
homodimers	B-oligomeric_state
are	O
involved	O
in	O
self	O
-	O
association	O
of	O
the	O
Hrd	B-complex_assembly
complex	O
.	O

Considering	O
that	O
the	O
functional	O
and	O
structural	O
composition	O
of	O
ERAD	O
components	O
are	O
conserved	O
in	O
both	O
yeast	B-taxonomy_domain
and	O
mammals	B-taxonomy_domain
,	O
we	O
propose	O
that	O
the	O
mammalian	B-taxonomy_domain
ERAD	O
components	O
also	O
form	O
self	O
-	O
associating	O
oligomers	B-oligomeric_state
.	O

We	O
need	O
to	O
further	O
test	O
whether	O
there	O
are	O
contacts	O
involved	O
in	O
dimer	B-oligomeric_state
formation	O
in	O
SEL1L	B-protein
in	O
addition	O
to	O
those	O
in	O
the	O
SLR	B-structure_element
-	I-structure_element
M	I-structure_element
region	O
.	O

In	O
yeast	B-taxonomy_domain
,	O
Usa1p	B-protein
acts	O
as	O
a	O
scaffold	O
for	O
Hrd1p	B-protein
and	O
Der1p	B-protein
,	O
in	O
which	O
the	O
N	O
-	O
terminus	O
of	O
Usa1p	B-protein
interacts	O
with	O
the	O
C	O
-	O
terminal	O
34	O
amino	O
acids	O
of	O
Hrd1p	B-protein
in	O
the	O
cytosol	O
to	O
induce	O
oligomerization	O
of	O
Hrd1p	B-protein
,	O
which	O
is	O
essential	O
for	O
its	O
activity	O
.	O

Although	O
mammalian	B-taxonomy_domain
HERP	B-protein_type
has	O
sequences	O
and	O
domains	O
that	O
are	O
conserved	B-protein_state
in	I-protein_state
Usa1p	B-protein
,	O
the	O
molecular	O
function	O
of	O
HERP	B-protein_type
is	O
not	O
clearly	O
related	O
to	O
that	O
of	O
Usa1p	B-protein
.	O

This	O
is	O
further	O
supported	O
by	O
our	O
data	O
showing	O
that	O
the	O
SLR	B-structure_element
-	I-structure_element
C	I-structure_element
of	O
SEL1L	B-protein
directly	O
interacts	O
with	O
the	O
luminal	O
fragment	O
of	O
HRD1	B-protein
in	O
the	O
ER	O
lumen	O
.	O

Although	O
the	O
organization	O
of	O
membrane	O
-	O
bound	O
HRD	B-complex_assembly
complex	O
components	O
may	O
be	O
very	O
similar	O
between	O
metazoans	B-taxonomy_domain
and	O
yeast	B-taxonomy_domain
,	O
the	O
molecular	O
details	O
of	O
interactions	O
between	O
the	O
components	O
may	O
not	O
necessarily	O
be	O
conserved	O
.	O

Likewise	O
,	O
the	O
dimerization	B-oligomeric_state
of	O
SEL1L	B-protein
might	O
provide	O
stability	O
for	O
the	O
mammalian	B-taxonomy_domain
HRD	B-complex_assembly
oligomer	B-oligomeric_state
complex	O
.	O

Further	O
cell	O
biological	O
studies	O
are	O
required	O
to	O
clarify	O
whether	O
SEL1L	B-protein
(	O
Hrd3p	B-protein
)	O
dimerization	B-oligomeric_state
could	O
be	O
cooperative	O
with	O
the	O
oligomerization	O
of	O
the	O
HRD	B-complex_assembly
complex	O
.	O

Considering	O
that	O
it	O
is	O
very	O
important	O
for	O
the	O
function	O
of	O
the	O
HRD	B-complex_assembly
complex	O
that	O
the	O
components	O
assemble	O
as	O
oligomers	B-oligomeric_state
,	O
we	O
believe	O
that	O
the	O
self	O
-	O
association	O
of	O
SEL1L	B-protein
strongly	O
contributes	O
to	O
generating	O
active	B-protein_state
forms	O
of	O
the	O
HRD	B-complex_assembly
complex	O
,	O
even	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
Usa1p	B-protein
,	O
in	O
metazoans	B-taxonomy_domain
.	O

These	O
findings	O
should	O
provide	O
a	O
foundation	O
for	O
molecular	O
-	O
level	O
studies	O
to	O
understand	O
the	O
membrane	O
-	O
associated	O
HRD	B-complex_assembly
complex	O
assembly	O
in	O
ERAD	O
.	O

(	O
A	O
)	O
The	O
diagram	O
shows	O
the	O
domain	O
structure	O
of	O
Mus	B-species
musculus	I-species
SEL1L	B-protein
,	O
as	O
defined	O
by	O
proteolytic	B-experimental_method
mapping	I-experimental_method
and	O
sequence	B-experimental_method
/	I-experimental_method
structure	I-experimental_method
analysis	I-experimental_method
.	O

We	O
determined	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
SLR	B-structure_element
-	I-structure_element
M	I-structure_element
,	O
residues	O
348	B-residue_range
–	I-residue_range
533	I-residue_range
.	O
(	O
B	O
)	O
Ribbon	O
diagram	O
of	O
the	O
biological	O
unit	O
of	O
the	O
SEL1Lcent	B-structure_element
,	O
viewed	O
along	O
the	O
two	O
-	O
fold	O
NCS	O
axis	O
.	O

The	O
crystal	B-evidence
structure	I-evidence
was	O
determined	O
by	O
SAD	B-experimental_method
phasing	I-experimental_method
using	O
selenium	B-chemical
as	O
the	O
anomalous	O
scatterer	O
and	O
refined	O
to	O
2	O
.	O
6	O
Å	O
resolution	O
(	O
Table	O
1	O
).	O

(	O
C	O
)	O
SEL1Lcent	B-structure_element
ribbon	O
diagram	O
rotated	O
90	O
°	O
around	O
a	O
horizontal	O
axis	O
relative	O
to	O
(	O
B	O
).	O

(	O
D	O
)	O
One	O
protomer	B-oligomeric_state
of	O
the	O
SEL1Lcent	B-structure_element
dimer	B-oligomeric_state
.	O

Starting	O
from	O
the	O
N	O
-	O
terminus	O
,	O
SEL1Lcent	B-structure_element
has	O
five	O
SLR	B-structure_element
motifs	O
comprising	O
ten	O
α	B-structure_element
helices	I-structure_element
.	O

Each	O
SLR	B-structure_element
motif	O
(	O
from	O
5	O
to	O
9	O
)	O
is	O
indicated	O
in	O
a	O
different	O
color	O
.	O
(	O
E	O
)	O
Evolutionary	O
conservation	O
of	O
surface	O
residues	O
in	O
SEL1Lcent	B-structure_element
,	O
calculated	O
using	O
ConSurf	B-experimental_method
,	O
from	O
a	O
structure	B-experimental_method
-	I-experimental_method
based	I-experimental_method
alignment	I-experimental_method
of	O
135	O
SEL1L	B-protein
sequences	O
.	O

The	O
surface	O
is	O
colored	O
from	O
red	O
(	O
high	O
)	O
to	O
white	O
(	O
poor	O
)	O
according	O
to	O
the	O
degree	O
of	O
conservation	O
in	O
the	O
SEL1L	B-protein
phylogenetic	O
orthologs	O
.	O

The	O
ribbon	O
diagram	O
of	O
the	O
counterpart	O
protomer	B-oligomeric_state
is	O
drawn	O
to	O
show	O
the	O
orientation	O
of	O
the	O
SEL1Lcent	B-structure_element
dimer	B-oligomeric_state
.	O

Dimer	B-site
Interface	I-site
of	O
SEL1Lcent	B-structure_element
.	O

Three	O
distinct	O
contact	B-site
regions	I-site
are	O
indicated	O
with	O
labeled	O
boxes	O
.	O

The	O
close	O
-	O
up	O
view	O
on	O
the	O
right	O
shows	O
the	O
residues	O
of	O
SEL1Lcent	B-structure_element
that	O
contribute	O
to	O
dimer	B-oligomeric_state
formation	O
via	O
the	O
three	O
contact	B-site
interfaces	I-site
.	O

The	O
yellow	O
dotted	O
lines	O
indicate	O
intermolecular	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
between	O
two	O
protomers	B-oligomeric_state
of	O
SEL1Lcent	B-structure_element
.	O
(	O
B	O
)	O
Size	B-experimental_method
-	I-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
(	O
SEC	B-experimental_method
)	O
analysis	O
of	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
dimeric	B-site
interface	I-site
SEL1Lcent	B-structure_element
mutants	B-protein_state
to	O
compare	O
the	O
oligomeric	O
states	O
of	O
the	O
proteins	O
.	O

The	O
standard	O
molecular	O
masses	O
for	O
the	O
SEC	B-experimental_method
experiments	O
(	O
top	O
)	O
were	O
obtained	O
from	O
the	O
following	O
proteins	O
:	O
aldolase	O
,	O
158	O
kDa	O
;	O
cobalbumin	O
,	O
75	O
kDa	O
;	O
ovalbumin	O
,	O
44	O
kDa	O
;	O
and	O
carbonic	O
anhydrase	O
,	O
29	O
kDa	O
.	O

Domain	O
Swapping	O
for	O
Dimerization	B-oligomeric_state
of	O
SEL1Lcent	B-structure_element
.	O

The	O
11	O
SLR	B-structure_element
motifs	O
were	O
aligned	B-experimental_method
based	O
on	O
the	O
present	O
crystal	B-evidence
structure	I-evidence
of	O
SEL1Lcent	B-structure_element
.	O

The	O
secondary	O
structure	O
elements	O
are	O
indicated	O
above	O
the	O
sequences	O
,	O
with	O
helices	B-structure_element
depicted	O
as	O
cylinders	O
.	O

The	O
GG	B-structure_element
sequence	O
in	O
SLR	B-structure_element
motif	I-structure_element
9	I-structure_element
,	O
which	O
creates	O
the	O
hinge	B-structure_element
for	O
domain	O
swapping	O
(	O
see	O
text	O
),	O
is	O
shaded	O
yellow	O
.	O

Stars	O
below	O
the	O
sequences	O
indicate	O
the	O
specific	O
residues	O
that	O
commonly	O
appear	O
in	O
SLRs	B-structure_element
.	O

(	O
B	O
)	O
Structure	B-experimental_method
alignment	I-experimental_method
of	O
five	O
SLR	B-structure_element
motifs	O
in	O
SEL1Lcent	B-structure_element
is	O
shown	O
to	O
highlight	O
the	O
unusual	O
geometry	O
of	O
SLR	B-structure_element
motif	I-structure_element
9	I-structure_element
.	O

Each	O
SLR	B-structure_element
motif	O
is	O
shown	O
in	O
a	O
different	O
color	O
.	O

Size	B-experimental_method
-	I-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
was	O
conducted	O
as	O
described	O
in	O
Fig	O
.	O
2B	O
.	O

SEL1L	B-protein
forms	O
self	B-oligomeric_state
-	I-oligomeric_state
oligomer	I-oligomeric_state
mediated	O
by	O
the	O
SEL1Lcent	B-structure_element
domain	O
in	O
vivo	O
.	O

(	O
A	O
)	O
HEK293T	O
cells	O
were	O
transfected	O
with	O
the	O
indicated	O
plasmid	O
constructs	O
and	O
the	O
lysates	O
were	O
immunoprecipitated	B-experimental_method
with	O
an	O
anti	O
-	O
FLAG	B-experimental_method
antibody	O
followed	O
by	O
western	B-experimental_method
blot	I-experimental_method
analysis	O
using	O
an	O
anti	O
-	O
HA	B-experimental_method
antibody	O
.	O

The	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
-	O
FLAG	B-experimental_method
was	O
co	B-experimental_method
-	I-experimental_method
immunoprecipitated	I-experimental_method
with	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
-	O
HA	B-experimental_method
.	O

Also	O
,	O
SEL1Lcent	B-structure_element
was	O
co	B-experimental_method
-	I-experimental_method
immunoprecipitated	I-experimental_method
with	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
while	O
the	O
SLR	B-structure_element
motif	I-structure_element
9	I-structure_element
deletion	B-experimental_method
failed	O
to	O
do	O
so	O
.	O
(	O
B	O
)	O
The	O
HEK293T	O
cells	O
were	O
transfected	O
with	O
the	O
indicated	O
plasmid	O
constructs	O
and	O
the	O
cell	O
lysate	O
and	O
culture	O
media	O
were	O
analyzed	O
by	O
western	B-experimental_method
blot	I-experimental_method
analysis	O
and	O
immunoprecipitation	B-experimental_method
respectively	O
.	O

The	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
forms	O
self	B-oligomeric_state
-	I-oligomeric_state
oligomers	I-oligomeric_state
and	O
the	O
SEL1Lcent	B-mutant
-	I-mutant
FLAG	I-mutant
-	I-mutant
KDEL	I-mutant
was	O
co	B-experimental_method
-	I-experimental_method
immunoprecipitated	I-experimental_method
with	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
-	O
HA	B-experimental_method
.	O

SEL1L348	B-mutant
–	I-mutant
497	I-mutant
-	I-mutant
FLAG	I-mutant
-	I-mutant
KDEL	I-mutant
did	O
not	O
co	O
-	O
immunoprecipitate	O
with	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
-	O
HA	B-experimental_method
.	O

The	O
white	O
asterisks	O
indicate	O
non	O
-	O
specific	O
bands	O
.	O
(	O
E	O
)	O
SEL1Lcent	B-mutant
-	I-mutant
HA	I-mutant
-	I-mutant
KDEL	I-mutant
competitively	O
inhibited	O
self	O
-	O
oligomerization	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
SEL1L	B-protein
.	O

The	O
indicated	O
plasmid	O
constructs	O
were	O
transfected	O
and	O
immunoprecipitation	B-experimental_method
assay	I-experimental_method
was	O
performed	O
using	O
an	O
anti	O
-	O
FLAG	B-experimental_method
antibody	O
followed	O
by	O
western	B-experimental_method
blot	I-experimental_method
analysis	O
using	O
an	O
anti	O
-	O
HA	B-experimental_method
antibody	O
.	O

The	O
red	O
rectangle	O
indicates	O
competitively	O
inhibited	O
SEL1L	B-protein
self	O
-	O
oligomer	B-oligomeric_state
formation	O
by	O
the	O
increasing	O
doses	O
of	O
SEL1Lcent	B-mutant
-	I-mutant
HA	I-mutant
-	I-mutant
KDEL	I-mutant
.	O
(	O
F	O
)	O
L521A	B-mutant
point	B-protein_state
mutant	I-protein_state
in	O
SEL1Lcent	B-structure_element
did	O
not	O
inhibit	O
the	O
self	O
-	O
association	O
of	O
SEL1L	B-protein
.	O

Comparison	O
of	O
SLR	B-structure_element
in	O
SEL1L	B-protein
with	O
TPR	B-structure_element
or	O
Other	O
SLR	B-protein_type
-	I-protein_type
Containing	I-protein_type
Proteins	I-protein_type
.	O

The	O
SEL1L	B-protein
,	O
Cdc23	B-protein
,	O
and	O
HcpC	B-protein
are	O
colored	O
magenta	O
,	O
green	O
and	O
cyan	O
,	O
respectively	O
.	O

Both	O
SEL1Lcent	B-structure_element
schematics	O
are	O
identically	O
oriented	O
for	O
comparison	O
.	O

The	O
Cα	O
atoms	O
of	O
the	O
residues	O
in	O
each	O
α	B-structure_element
-	I-structure_element
solenoid	I-structure_element
domain	I-structure_element
are	O
superimposed	B-experimental_method
with	O
a	O
root	B-evidence
-	I-evidence
mean	I-evidence
-	I-evidence
squared	I-evidence
deviation	I-evidence
of	O
3	O
.	O
3	O
Å	O
for	O
Cdc23	B-protein
and	O
SEL1Lcent	B-structure_element
(	O
left	O
),	O
and	O
2	O
.	O
5	O
Å	O
for	O
HcpC	B-protein
and	O
SEL1Lcent	B-structure_element
(	O
right	O
).	O

SEL1Lcent	B-structure_element
,	O
Cdc23	B-protein
,	O
and	O
HcpC	B-protein
are	O
colored	O
as	O
in	O
(	O
A	O
).	O
(	O
C	O
)	O
Ribbon	O
diagram	O
showing	O
the	O
overall	O
structure	B-evidence
of	O
Cdc23N	B-protein
-	O
term	O
(	O
left	O
)	O
and	O
SEL1Lcent	B-structure_element
(	O
right	O
)	O
to	O
compare	O
their	O
similarities	O
regarding	O
dimer	B-oligomeric_state
formation	O
through	O
domain	O
swapping	O
.	O

The	O
Role	O
of	O
SLR	B-structure_element
-	I-structure_element
C	I-structure_element
in	O
ERAD	O
machinery	O
and	O
Model	O
for	O
the	O
Organization	O
of	O
Proteins	O
in	O
Membrane	O
-	O
Associated	O
ERAD	O
Components	O
.	O

(	O
A	O
)	O
Schematic	O
diagram	O
shows	O
three	O
HRD1	B-protein
fragment	O
constructs	O
used	O
in	O
the	O
GST	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
experiment	O
.	O
(	O
B	O
)	O
Pull	B-experimental_method
-	I-experimental_method
down	I-experimental_method
experiments	I-experimental_method
to	O
examine	O
the	O
interactions	O
between	O
HRD	B-complex_assembly
luminal	B-structure_element
loops	I-structure_element
and	O
certain	O
SLR	B-structure_element
motifs	O
of	O
SEL1L	B-protein
.	O

(	O
C	O
)	O
Schematic	O
representation	O
of	O
the	O
organization	O
of	O
metazoan	B-taxonomy_domain
ERAD	O
components	O
in	O
the	O
ER	O
membrane	O
.	O

We	O
hypothesized	O
that	O
the	O
interrupted	O
SLR	B-structure_element
motifs	O
of	O
SEL1L	B-protein
have	O
distinct	O
functions	O
such	O
that	O
the	O
SLR	B-structure_element
-	I-structure_element
M	I-structure_element
is	O
important	O
for	O
dimer	B-oligomeric_state
formation	O
of	O
the	O
protein	O
,	O
and	O
SLR	B-structure_element
-	I-structure_element
C	I-structure_element
is	O
involved	O
in	O
the	O
interaction	O
with	O
HRD1	B-protein
in	O
the	O
ER	O
lumen	O
.	O

The	O
surface	O
representation	O
of	O
SEL1Lcent	B-structure_element
is	O
placed	O
in	O
the	O
same	O
orientation	O
as	O
that	O
shown	O
in	O
the	O
schematic	O
model	O
to	O
show	O
that	O
the	O
putative	O
N	B-site
-	I-site
glycosylation	I-site
site	I-site
,	O
residue	O
N427	B-residue_name_number
(	O
indicated	O
in	O
yellow	O
),	O
is	O
exposed	O
on	O
the	O
surface	O
of	O
the	O
protein	O
.	O

Sequence	B-experimental_method
alignment	I-experimental_method
suggests	O
that	O
both	O
kinases	B-protein_type
belong	O
to	O
the	O
ribulokinase	B-protein_type
-	I-protein_type
like	I-protein_type
carbohydrate	I-protein_type
kinases	I-protein_type
,	O
a	O
sub	O
-	O
family	O
of	O
FGGY	B-protein_type
family	I-protein_type
carbohydrate	I-protein_type
kinases	I-protein_type
.	O

Here	O
we	O
solved	B-experimental_method
the	O
structures	B-evidence
of	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
in	O
both	O
their	O
apo	B-protein_state
forms	O
and	O
in	B-protein_state
complex	I-protein_state
with	I-protein_state
nucleotide	B-chemical
substrates	O
.	O

The	O
two	O
kinases	O
exhibit	O
nearly	O
identical	O
overall	O
architecture	O
,	O
with	O
both	O
kinases	B-protein_type
possessing	O
ATP	B-chemical
hydrolysis	O
activity	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
substrates	I-protein_state
.	O

In	O
addition	O
,	O
our	O
enzymatic	B-experimental_method
assays	I-experimental_method
suggested	O
that	O
SePSK	B-protein
has	O
the	O
capability	O
to	O
phosphorylate	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
.	O

Moreover	O
,	O
our	O
structural	B-experimental_method
comparison	I-experimental_method
with	O
other	O
family	O
members	O
suggests	O
that	O
there	O
are	O
major	O
conformational	O
changes	O
in	O
SePSK	B-protein
upon	O
substrate	O
binding	O
,	O
facilitating	O
the	O
catalytic	O
process	O
.	O

Phosphorylation	B-ptm
is	O
one	O
of	O
the	O
various	O
pivotal	O
modifications	O
of	O
carbohydrates	B-chemical
,	O
and	O
is	O
catalyzed	O
by	O
specific	O
sugar	B-protein_type
kinases	I-protein_type
.	O

These	O
kinases	B-protein_type
exhibit	O
considerable	O
differences	O
in	O
their	O
folding	O
pattern	O
and	O
substrate	O
specificity	O
.	O

Based	O
on	O
sequence	B-experimental_method
analysis	I-experimental_method
,	O
they	O
can	O
be	O
divided	O
into	O
four	O
families	O
,	O
namely	O
HSP	B-protein_type
70_NBD	I-protein_type
family	I-protein_type
,	O
FGGY	B-protein_type
family	I-protein_type
,	O
Mer_B	B-protein_type
like	I-protein_type
family	I-protein_type
and	O
Parm_like	B-protein_type
family	I-protein_type
.	O

These	O
sugar	B-chemical
substrates	O
include	O
L	B-chemical
-	I-chemical
ribulose	I-chemical
,	O
erythritol	B-chemical
,	O
L	B-chemical
-	I-chemical
fuculose	I-chemical
,	O
D	B-chemical
-	I-chemical
glycerol	I-chemical
,	O
D	B-chemical
-	I-chemical
gluconate	I-chemical
,	O
L	B-chemical
-	I-chemical
xylulose	I-chemical
,	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
,	O
L	B-chemical
-	I-chemical
rhamnulose	I-chemical
and	O
D	B-chemical
-	I-chemical
xylulose	I-chemical
.	O

Structures	B-evidence
reported	O
in	O
the	O
Protein	O
Data	O
Bank	O
of	O
the	O
FGGY	B-protein_type
family	I-protein_type
carbohydrate	I-protein_type
kinases	I-protein_type
exhibit	O
a	O
similar	O
overall	O
architecture	O
containing	O
two	O
protein	O
domains	O
,	O
one	O
of	O
which	O
is	O
responsible	O
for	O
the	O
binding	O
of	O
substrate	O
,	O
while	O
the	O
second	O
is	O
used	O
for	O
binding	O
cofactor	O
ATP	B-chemical
.	O

While	O
the	O
binding	B-site
pockets	I-site
for	O
substrates	O
are	O
at	O
the	O
same	O
position	O
,	O
each	O
FGGY	B-protein_type
family	I-protein_type
carbohydrate	I-protein_type
kinases	I-protein_type
uses	O
different	O
substrate	B-site
-	I-site
binding	I-site
residues	I-site
,	O
resulting	O
in	O
high	O
substrate	O
specificity	O
.	O

SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
display	O
a	O
sequence	O
identity	O
of	O
44	O
.	O
9	O
%,	O
and	O
belong	O
to	O
the	O
ribulokinase	B-protein_type
-	I-protein_type
like	I-protein_type
carbohydrate	I-protein_type
kinases	I-protein_type
,	O
a	O
sub	O
-	O
family	O
of	O
FGGY	B-protein_type
family	I-protein_type
carbohydrate	I-protein_type
kinases	I-protein_type
.	O

Members	O
of	O
this	O
sub	O
-	O
family	O
are	O
responsible	O
for	O
the	O
phosphorylation	B-ptm
of	O
sugars	B-chemical
similar	O
to	O
L	B-chemical
-	I-chemical
ribulose	I-chemical
and	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
.	O

The	O
sequence	O
and	O
the	O
substrate	O
specificity	O
of	O
ribulokinase	B-protein_type
-	I-protein_type
like	I-protein_type
carbohydrate	I-protein_type
kinases	I-protein_type
are	O
different	O
,	O
but	O
they	O
share	O
the	O
common	O
folding	O
feature	O
with	O
two	O
domains	O
.	O

Domain	B-structure_element
I	I-structure_element
exhibits	O
a	O
ribonuclease	B-structure_element
H	I-structure_element
-	I-structure_element
like	I-structure_element
folding	I-structure_element
pattern	I-structure_element
,	O
and	O
is	O
responsible	O
for	O
the	O
substrate	O
binding	O
,	O
while	O
domain	B-structure_element
II	I-structure_element
possesses	O
an	O
actin	B-structure_element
-	I-structure_element
like	I-structure_element
ATPase	I-structure_element
domain	I-structure_element
that	O
binds	O
cofactor	O
ATP	B-chemical
.	O

It	O
was	O
shown	O
that	O
XK	B-protein
-	I-protein
2	I-protein
(	O
At5g49650	B-gene
)	O
located	O
in	O
the	O
cytosol	O
is	O
indeed	O
xylulose	B-protein_type
kinase	I-protein_type
.	O

However	O
,	O
the	O
function	O
of	O
XK	B-protein
-	I-protein
1	I-protein
(	O
At2g21370	B-gene
)	O
inside	O
the	O
chloroplast	O
stroma	O
has	O
remained	O
unknown	O
.	O

SePSK	B-protein
from	O
Synechococcus	B-species
elongatus	I-species
strain	I-species
PCC	I-species
7942	I-species
is	O
the	O
homolog	O
of	O
AtXK	B-protein
-	I-protein
1	I-protein
,	O
though	O
its	O
physiological	O
function	O
and	O
substrates	O
remain	O
unclear	O
.	O

In	O
order	O
to	O
obtain	O
functional	O
and	O
structural	O
information	O
about	O
these	O
two	O
proteins	O
,	O
here	O
we	O
reported	O
the	O
crystal	B-evidence
structures	I-evidence
of	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
.	O

Our	O
findings	O
provide	O
new	O
details	O
of	O
the	O
catalytic	O
mechanism	O
of	O
SePSK	B-protein
and	O
lay	O
the	O
foundation	O
for	O
future	O
studies	O
into	O
its	O
homologs	O
in	O
eukaryotes	B-taxonomy_domain
.	O

Overall	O
structures	B-evidence
of	O
apo	B-protein_state
-	O
SePSK	B-protein
and	O
apo	B-protein_state
-	O
AtXK	B-protein
-	I-protein
1	I-protein

We	O
therefore	O
used	O
single	B-experimental_method
isomorphous	I-experimental_method
replacement	I-experimental_method
anomalous	I-experimental_method
scattering	I-experimental_method
method	I-experimental_method
(	O
SIRAS	B-experimental_method
)	O
for	O
successful	O
solution	O
of	O
the	O
apo	B-protein_state
-	O
SePSK	B-protein
structure	B-evidence
at	O
a	O
resolution	O
of	O
2	O
.	O
3	O
Å	O
.	O
Subsequently	O
,	O
the	O
apo	B-protein_state
-	O
SePSK	B-protein
structure	B-evidence
was	O
used	O
as	O
molecular	B-experimental_method
replacement	I-experimental_method
model	I-experimental_method
to	O
solve	O
all	O
other	O
structures	B-evidence
identified	O
in	O
this	O
study	O
.	O

The	O
amino	O
-	O
acid	O
residues	O
were	O
traced	O
from	O
Val2	B-residue_name_number
to	O
His419	B-residue_name_number
,	O
except	O
for	O
the	O
Met1	B-residue_name_number
residue	O
and	O
the	O
seven	O
residues	O
at	O
the	O
C	O
-	O
termini	O
.	O

Apo	B-protein_state
-	O
SePSK	B-protein
contains	O
two	O
domains	O
referred	O
to	O
further	O
on	O
as	O
domain	B-structure_element
I	I-structure_element
and	O
domain	B-structure_element
II	I-structure_element
(	O
Fig	O
1A	O
).	O

Domain	B-structure_element
I	I-structure_element
consists	O
of	O
non	O
-	O
contiguous	O
portions	O
of	O
the	O
polypeptide	O
chains	O
(	O
aa	O
.	O

2	B-residue_range
–	I-residue_range
228	I-residue_range
and	O
aa	O
.	O

402	B-residue_range
–	I-residue_range
419	I-residue_range
),	O
exhibiting	O
11	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
and	O
11	O
β	B-structure_element
-	I-structure_element
sheets	I-structure_element
.	O

In	O
addition	O
,	O
four	O
β	B-structure_element
-	I-structure_element
sheets	I-structure_element
(	O
β7	B-structure_element
,	O
β10	B-structure_element
,	O
β12	B-structure_element
and	O
β16	B-structure_element
)	O
and	O
five	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
(	O
α8	B-structure_element
,	O
α9	B-structure_element
,	O
α13	B-structure_element
,	O
α14	B-structure_element
and	O
α15	B-structure_element
)	O
flank	O
the	O
left	O
side	O
of	O
the	O
core	B-structure_element
region	I-structure_element
.	O

229	B-residue_range
–	I-residue_range
401	I-residue_range
and	O
classified	O
into	O
B2	B-structure_element
(	O
β31	B-structure_element
/	O
β29	B-structure_element
/	O
β22	B-structure_element
/	O
β23	B-structure_element
/	O
β25	B-structure_element
/	O
β24	B-structure_element
)	O
and	O
A3	B-structure_element
(	O
α26	B-structure_element
/	O
α27	B-structure_element
/	O
α28	B-structure_element
/	O
α30	B-structure_element
)	O
(	O
Fig	O
1A	O
and	O
S1	O
Fig	O
).	O

In	O
the	O
SePSK	B-protein
structure	B-evidence
,	O
B1	B-structure_element
and	O
B2	B-structure_element
are	O
sandwiched	O
by	O
A1	B-structure_element
,	O
A2	B-structure_element
and	O
A3	B-structure_element
,	O
and	O
the	O
whole	O
structure	B-evidence
shows	O
the	O
A1	B-structure_element
/	O
B1	B-structure_element
/	O
A2	B-structure_element
/	O
B2	B-structure_element
/	O
A3	B-structure_element
(	O
α	B-structure_element
/	O
β	B-structure_element
/	O
α	B-structure_element
/	O
β	B-structure_element
/	O
α	B-structure_element
)	O
folding	O
pattern	O
,	O
which	O
is	O
in	O
common	O
with	O
other	O
members	O
of	O
FGGY	B-protein_type
family	I-protein_type
carbohydrate	I-protein_type
kinases	I-protein_type
(	O
S2	O
Fig	O
).	O

The	O
overall	O
folding	O
of	O
SePSK	B-protein
resembles	O
a	O
clip	O
,	O
with	O
A2	B-structure_element
of	O
domain	B-structure_element
I	I-structure_element
acting	O
as	O
a	O
hinge	B-structure_element
region	I-structure_element
.	O

Overall	O
structures	B-evidence
of	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
.	O

The	O
secondary	O
structural	O
elements	O
are	O
indicated	O
(	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
:	O
cyan	O
,	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
:	O
yellow	O
).	O

The	O
secondary	O
structural	O
elements	O
are	O
indicated	O
(	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
:	O
green	O
,	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
:	O
wheat	O
).	O

Apo	B-protein_state
-	O
AtXK	B-protein
-	I-protein
1	I-protein
exhibits	O
a	O
folding	O
pattern	O
similar	O
to	O
that	O
of	O
SePSK	B-protein
in	O
line	O
with	O
their	O
high	O
sequence	O
identity	O
(	O
Fig	O
1B	O
and	O
S1	O
Fig	O
).	O

However	O
,	O
superposition	B-experimental_method
of	O
structures	B-evidence
of	O
AtXK	B-protein
-	I-protein
1	I-protein
and	O
SePSK	B-protein
shows	O
some	O
differences	O
,	O
especially	O
at	O
the	O
loop	B-structure_element
regions	I-structure_element
.	O

A	O
considerable	O
difference	O
is	O
found	O
in	O
the	O
loop3	B-structure_element
linking	O
β3	B-structure_element
and	O
α4	B-structure_element
,	O
which	O
is	O
stretched	O
out	O
in	O
the	O
AtXK	B-protein
-	I-protein
1	I-protein
structure	B-evidence
,	O
while	O
in	O
the	O
SePSK	B-protein
structure	B-evidence
,	O
it	O
is	O
bent	O
back	O
towards	O
the	O
inner	O
part	O
.	O

The	O
corresponding	O
residues	O
between	O
these	O
two	O
structures	B-evidence
(	O
SePSK	B-protein
-	O
Lys35	B-residue_name_number
and	O
AtXK	B-protein
-	I-protein
1	I-protein
-	O
Lys48	B-residue_name_number
)	O
have	O
a	O
distance	O
of	O
15	O
.	O
4	O
Å	O
(	O
S3	O
Fig	O
).	O

Activity	B-experimental_method
assays	I-experimental_method
of	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein

In	O
order	O
to	O
understand	O
the	O
function	O
of	O
these	O
two	O
kinases	O
,	O
we	O
performed	O
structural	B-experimental_method
comparison	I-experimental_method
using	O
Dali	B-experimental_method
server	I-experimental_method
.	O

The	O
structures	B-evidence
most	O
closely	O
related	O
to	O
SePSK	B-protein
are	O
xylulose	B-protein_type
kinase	I-protein_type
,	O
glycerol	B-protein_type
kinase	I-protein_type
and	O
ribulose	B-protein_type
kinase	I-protein_type
,	O
implying	O
that	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
might	O
function	O
similarly	O
to	O
these	O
kinases	B-protein_type
.	O

We	O
first	O
tested	O
whether	O
both	O
enzymes	O
possessed	O
ATP	B-chemical
hydrolysis	O
activity	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
substrates	O
.	O

As	O
shown	O
in	O
Fig	O
2A	O
,	O
both	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
exhibited	O
ATP	B-chemical
hydrolysis	O
activity	O
.	O

To	O
further	O
identify	O
the	O
actual	O
substrate	O
of	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
,	O
five	O
different	O
sugar	O
molecules	O
,	O
including	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
,	O
L	B-chemical
-	I-chemical
ribulose	I-chemical
,	O
D	B-chemical
-	I-chemical
xylulose	I-chemical
,	O
L	B-chemical
-	I-chemical
xylulose	I-chemical
and	O
Glycerol	B-chemical
,	O
were	O
used	O
in	O
enzymatic	B-experimental_method
activity	I-experimental_method
assays	I-experimental_method
.	O

As	O
shown	O
in	O
Fig	O
2B	O
,	O
the	O
ATP	B-chemical
hydrolysis	O
activity	O
of	O
SePSK	B-protein
greatly	O
increased	O
upon	O
adding	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
than	O
adding	O
other	O
potential	O
substrates	O
,	O
suggesting	O
that	O
it	O
has	O
D	B-protein_type
-	I-protein_type
ribulose	I-protein_type
kinase	I-protein_type
activity	O
.	O

The	O
enzymatic	B-experimental_method
activity	I-experimental_method
assays	I-experimental_method
of	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
.	O

(	O
A	O
)	O
The	O
ATP	B-chemical
hydrolysis	O
activity	O
of	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
.	O

(	O
B	O
)	O
The	O
ATP	B-chemical
hydrolysis	O
activity	O
of	O
SePSK	B-protein
with	O
addition	O
of	O
five	O
different	O
substrates	O
.	O

The	O
substrates	O
are	O
DR	B-chemical
(	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
),	O
LR	B-chemical
(	O
L	B-chemical
-	I-chemical
ribulose	I-chemical
),	O
DX	B-chemical
(	O
D	B-chemical
-	I-chemical
xylulose	I-chemical
),	O
LX	B-chemical
(	O
L	B-chemical
-	I-chemical
xylulose	I-chemical
)	O
and	O
GLY	B-chemical
(	O
Glycerol	B-chemical
).	O
(	O
C	O
)	O
The	O
ATP	B-chemical
hydrolysis	O
activity	O
of	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
with	O
or	O
without	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
.	O
(	O
D	O
)	O
The	O
ATP	B-chemical
hydrolysis	O
activity	O
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
WT	B-protein_state
)	O
and	O
single	O
-	O
site	O
mutants	O
of	O
SePSK	B-protein
.	O

Three	O
single	O
-	O
site	O
mutants	O
of	O
SePSK	B-protein
are	O
D8A	B-mutant
-	O
SePSK	B-protein
,	O
T11A	B-mutant
-	O
SePSK	B-protein
and	O
D221A	B-mutant
-	O
SePSK	B-protein
.	O

Mutations	B-experimental_method
of	O
the	O
corresponding	O
residue	O
in	O
xylulose	B-protein_type
kinase	I-protein_type
and	O
glycerol	B-protein_type
kinase	I-protein_type
from	O
Escherichia	B-species
coli	I-species
greatly	O
reduced	O
their	O
activity	O
.	O

To	O
identify	O
the	O
function	O
of	O
these	O
three	O
residues	O
of	O
SePSK	B-protein
,	O
we	O
constructed	O
D8A	B-mutant
,	O
T11A	B-mutant
and	O
D221A	B-mutant
mutants	B-protein_state
.	O

SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
possess	O
a	O
similar	O
ATP	B-site
binding	I-site
site	I-site

To	O
obtain	O
more	O
detailed	O
information	O
of	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
ATP	B-chemical
,	O
we	O
soaked	B-experimental_method
the	O
apo	B-protein_state
-	O
crystals	B-evidence
in	O
the	O
reservoir	O
adding	O
cofactor	O
ATP	B-chemical
,	O
and	O
obtained	O
the	O
structures	B-evidence
of	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
bound	B-protein_state
with	I-protein_state
ATP	B-chemical
at	O
the	O
resolution	O
of	O
2	O
.	O
3	O
Å	O
and	O
1	O
.	O
8	O
Å	O
,	O
respectively	O
.	O

In	O
both	O
structures	B-evidence
,	O
a	O
strong	O
electron	B-evidence
density	I-evidence
was	O
found	O
in	O
the	O
conserved	B-protein_state
ATP	B-site
binding	I-site
pocket	I-site
,	O
but	O
can	O
only	O
be	O
fitted	O
with	O
an	O
ADP	B-chemical
molecule	O
(	O
S4	O
Fig	O
).	O

This	O
result	O
was	O
consistent	O
with	O
our	O
enzymatic	B-experimental_method
activity	I-experimental_method
assays	I-experimental_method
where	O
SePSK	B-protein
and	O
AtXK	B-protein
-	I-protein
1	I-protein
showed	O
ATP	B-chemical
hydrolysis	O
activity	O
without	O
adding	O
any	O
substrates	O
(	O
Fig	O
2A	O
and	O
2C	O
).	O

To	O
avoid	O
hydrolysis	O
of	O
ATP	B-chemical
,	O
we	O
soaked	B-experimental_method
the	O
crystals	B-evidence
of	O
apo	B-protein_state
-	O
SePSK	B-protein
and	O
apo	B-protein_state
-	O
AtXK	B-protein
-	I-protein
1	I-protein
into	O
the	O
reservoir	O
adding	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
.	O

However	O
,	O
we	O
found	O
that	O
the	O
electron	B-evidence
densities	I-evidence
of	O
γ	O
-	O
phosphate	B-chemical
group	O
of	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
(	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
γ	O
-	O
phosphate	B-chemical
)	O
are	O
still	O
weak	O
in	O
the	O
AMP	B-complex_assembly
-	I-complex_assembly
PNP	I-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
and	O
AMP	B-complex_assembly
-	I-complex_assembly
PNP	I-complex_assembly
-	I-complex_assembly
AtXK	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
structures	B-evidence
,	O
suggesting	O
high	O
flexibility	O
of	O
ATP	B-chemical
-	O
γ	O
-	O
phosphate	B-chemical
.	O

The	O
γ	O
-	O
phosphate	B-chemical
group	O
of	O
ATP	B-chemical
is	O
transferred	O
to	O
the	O
sugar	B-chemical
substrate	O
during	O
the	O
reaction	O
process	O
,	O
so	O
this	O
flexibility	O
might	O
be	O
important	O
for	O
the	O
ability	O
of	O
these	O
kinases	B-protein_type
.	O

The	O
overall	O
structures	B-evidence
as	O
well	O
as	O
the	O
coordination	O
modes	O
of	O
ADP	B-chemical
and	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
in	O
the	O
AMP	B-complex_assembly
-	I-complex_assembly
PNP	I-complex_assembly
-	I-complex_assembly
AtXK	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
,	O
ADP	B-complex_assembly
-	I-complex_assembly
AtXK	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
,	O
ADP	B-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
and	O
AMP	B-complex_assembly
-	I-complex_assembly
PNP	I-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
structures	B-evidence
are	O
nearly	O
identical	O
(	O
S5	O
Fig	O
),	O
therefore	O
the	O
structure	B-evidence
of	O
AMP	B-complex_assembly
-	I-complex_assembly
PNP	I-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
is	O
used	O
here	O
to	O
describe	O
the	O
structural	O
details	O
and	O
to	O
compare	O
with	O
those	O
of	O
other	O
family	O
members	O
.	O

The	O
AMP	B-site
-	I-site
PNP	I-site
binding	I-site
pocket	I-site
consists	O
of	O
four	B-structure_element
α	I-structure_element
-	I-structure_element
helices	I-structure_element
(	O
α26	B-structure_element
,	O
α28	B-structure_element
,	O
α27	B-structure_element
and	O
α30	B-structure_element
)	O
and	O
forms	O
a	O
shape	B-protein_state
resembling	I-protein_state
a	I-protein_state
half	I-protein_state
-	I-protein_state
fist	I-protein_state
(	O
Fig	O
3A	O
and	O
3B	O
).	O

The	O
head	O
group	O
of	O
the	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
is	O
embedded	O
in	O
a	O
pocket	B-site
surrounded	O
by	O
Trp383	B-residue_name_number
,	O
Asn380	B-residue_name_number
,	O
Gly376	B-residue_name_number
and	O
Gly377	B-residue_name_number
.	O

The	O
purine	O
ring	O
of	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
is	O
positioned	O
in	O
parallel	O
to	O
the	O
indole	O
ring	O
of	O
Trp383	B-residue_name_number
.	O

In	O
addition	O
,	O
it	O
is	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
with	O
the	O
side	O
chain	O
amide	O
of	O
Asn380	B-residue_name_number
(	O
Fig	O
3B	O
).	O

The	O
tail	O
of	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
points	O
to	O
the	O
hinge	B-structure_element
region	I-structure_element
of	O
SePSK	B-protein
,	O
and	O
its	O
α	O
-	O
phosphate	B-chemical
and	O
β	O
-	O
phosphate	B-chemical
groups	O
are	O
stabilized	O
by	O
Gly376	B-residue_name_number
and	O
Ser243	B-residue_name_number
,	O
respectively	O
.	O

Together	O
,	O
this	O
structure	B-evidence
clearly	O
shows	O
that	O
the	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
-	O
β	O
-	O
phosphate	B-chemical
is	O
sticking	O
out	O
of	O
the	O
ATP	B-site
binding	I-site
pocket	I-site
,	O
thus	O
the	O
γ	O
-	O
phosphate	B-chemical
group	O
is	O
at	O
the	O
empty	O
space	O
between	O
domain	B-structure_element
I	I-structure_element
and	O
domain	B-structure_element
II	I-structure_element
and	O
is	O
unconstrained	O
in	O
its	O
movement	O
by	O
the	O
protein	O
.	O

Structure	B-evidence
of	O
SePSK	B-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
AMP	B-chemical
-	I-chemical
PNP	I-chemical
.	O

(	O
A	O
)	O
The	O
electron	B-evidence
density	I-evidence
of	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
.	O

The	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
is	O
depicted	O
as	O
sticks	O
with	O
its	O
ǀFoǀ	B-evidence
-	I-evidence
ǀFcǀ	I-evidence
map	I-evidence
contoured	O
at	O
3	O
σ	O
shown	O
as	O
cyan	O
mesh	O
.	O

The	O
head	O
of	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
is	O
sandwiched	B-bond_interaction
by	I-bond_interaction
four	O
residues	O
(	O
Leu293	B-residue_name_number
,	O
Gly376	B-residue_name_number
,	O
Gly377	B-residue_name_number
and	O
Trp383	B-residue_name_number
).	O

The	O
four	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
(	O
α26	B-structure_element
,	O
α28	B-structure_element
,	O
α27	B-structure_element
and	O
α30	B-structure_element
)	O
are	O
labeled	O
in	O
red	O
.	O

The	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
and	O
coordinated	O
residues	O
are	O
shown	O
as	O
sticks	O
.	O

The	O
potential	O
substrate	B-site
binding	I-site
site	I-site
in	O
SePSK	B-protein

The	O
results	O
from	O
our	O
activity	B-experimental_method
assays	I-experimental_method
suggested	O
that	O
SePSK	B-protein
has	O
D	B-protein_type
-	I-protein_type
ribulose	I-protein_type
kinase	I-protein_type
activity	O
.	O

To	O
better	O
understand	O
the	O
interaction	O
pattern	O
between	O
SePSK	B-protein
and	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
,	O
the	O
apo	B-protein_state
-	O
SePSK	B-protein
crystals	B-experimental_method
were	I-experimental_method
soaked	I-experimental_method
into	I-experimental_method
the	O
reservoir	B-experimental_method
with	O
10	O
mM	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
(	O
RBL	B-chemical
)	O
and	O
the	O
RBL	B-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
structure	B-evidence
was	O
solved	B-experimental_method
.	O

As	O
shown	O
in	O
S6	O
Fig	O
,	O
two	O
residual	O
electron	B-evidence
densities	I-evidence
are	O
visible	O
in	O
domain	B-structure_element
I	I-structure_element
,	O
which	O
can	O
be	O
interpreted	O
as	O
two	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
molecules	O
with	O
reasonable	O
fit	O
.	O

As	O
shown	O
in	O
Fig	O
4A	O
,	O
the	O
nearest	O
distance	O
between	O
the	O
carbon	O
skeleton	O
of	O
two	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
molecules	O
are	O
approx	O
.	O

RBL1	B-residue_name_number
is	O
located	O
in	O
the	O
pocket	B-site
consisting	O
of	O
α21	B-structure_element
and	O
the	O
loop	B-structure_element
between	O
β6	B-structure_element
and	I-structure_element
β7	I-structure_element
.	O

The	O
O4	O
and	O
O5	O
of	O
RBL1	B-residue_name_number
are	O
coordinated	B-bond_interaction
with	I-bond_interaction
the	O
side	O
chain	O
carboxyl	O
group	O
of	O
Asp221	B-residue_name_number
.	O

This	O
pocket	B-site
is	O
at	O
a	O
similar	O
position	O
of	O
substrate	B-site
binding	I-site
site	I-site
of	O
other	O
sugar	B-protein_type
kinase	I-protein_type
,	O
such	O
as	O
L	B-protein
-	I-protein
ribulokinase	I-protein
(	O
PDB	O
code	O
:	O
3QDK	O
)	O
(	O
S7	O
Fig	O
).	O

Glu329	B-residue_name_number
in	O
3QDK	O
has	O
no	O
counterpart	O
in	O
RBL	B-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
structure	B-evidence
.	O

In	O
addition	O
,	O
although	O
Lys208	B-residue_name_number
of	O
L	B-protein
-	I-protein
ribulokinase	I-protein
has	O
the	O
corresponding	O
residue	O
(	O
Lys163	B-residue_name_number
)	O
in	O
RBL	B-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
structure	B-evidence
,	O
the	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
of	O
Lys163	B-residue_name_number
is	O
broken	O
because	O
of	O
the	O
conformational	O
change	O
of	O
two	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
(	O
α9	B-structure_element
and	O
α13	B-structure_element
)	O
of	O
SePSK	B-protein
.	O

The	O
binding	O
of	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
(	O
RBL	B-chemical
)	O
with	O
SePSK	B-protein
.	O

(	O
A	O
)	O
The	O
electrostatic	B-evidence
potential	I-evidence
surface	I-evidence
map	I-evidence
of	O
RBL	B-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
and	O
a	O
zoom	O
-	O
in	O
view	O
of	O
RBL	B-site
binding	I-site
site	I-site
.	O

The	O
RBL1	B-residue_name_number
and	O
RBL2	B-residue_name_number
are	O
depicted	O
as	O
sticks	O
.	O
(	O
B	O
)	O
Interaction	O
of	O
two	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
molecules	O
(	O
RBL1	B-residue_name_number
and	O
RBL2	B-residue_name_number
)	O
with	O
SePSK	B-protein
.	O

The	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
are	O
indicated	O
by	O
the	O
black	O
dashed	O
lines	O
and	O
the	O
numbers	O
near	O
the	O
dashed	O
lines	O
are	O
the	O
distances	O
(	O
Å	O
).	O
(	O
C	O
)	O
The	O
binding	B-experimental_method
affinity	I-experimental_method
assays	I-experimental_method
of	O
SePSK	B-protein
with	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
.	O

Single	B-experimental_method
-	I-experimental_method
cycle	I-experimental_method
kinetic	I-experimental_method
data	I-experimental_method
are	O
reflecting	O
the	O
interaction	O
of	O
SePSK	B-protein
and	O
D8A	B-mutant
-	O
SePSK	B-protein
with	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
.	O

It	O
shows	O
two	O
experimental	O
sensorgrams	B-evidence
after	O
minus	O
the	O
empty	O
sensorgrams	B-evidence
.	O

Dissociation	B-evidence
rate	I-evidence
constant	I-evidence
of	O
wild	B-protein_state
type	I-protein_state
and	O
D8A	B-mutant
-	O
SePSK	B-protein
are	O
3	O
ms	O
-	O
1	O
and	O
9	O
ms	O
-	O
1	O
,	O
respectively	O
.	O

The	O
binding	B-site
pocket	I-site
of	O
RBL2	B-residue_name_number
with	O
relatively	O
weak	O
electron	B-evidence
density	I-evidence
is	O
near	O
the	O
N	O
-	O
terminal	O
region	O
of	O
SePSK	B-protein
and	O
is	O
negatively	O
charged	O
.	O

The	O
hydroxyl	O
group	O
of	O
Ser12	B-residue_name_number
coordinates	B-bond_interaction
with	I-bond_interaction
O2	O
of	O
RBL2	B-residue_name_number
.	O

The	O
backbone	O
amide	O
nitrogens	O
of	O
Gly13	B-residue_name_number
and	O
Arg15	B-residue_name_number
also	O
keep	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
RBL2	B-residue_name_number
(	O
Fig	O
4B	O
).	O

In	O
the	O
RBL	B-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
structure	B-evidence
,	O
a	O
2	O
.	O
6	O
Å	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
is	O
present	O
between	O
RBL2	B-residue_name_number
and	O
Ser12	B-residue_name_number
(	O
Fig	O
4B	O
),	O
while	O
in	O
the	O
AtXK	B-protein
-	I-protein
1	I-protein
structure	B-evidence
this	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
the	O
corresponding	O
residue	O
(	O
Ser22	B-residue_name_number
)	O
is	O
broken	O
.	O

In	O
addition	O
,	O
our	O
enzymatic	B-experimental_method
assays	I-experimental_method
indicated	O
that	O
Asp8	B-residue_name_number
is	O
important	O
for	O
the	O
activity	O
of	O
SePSK	B-protein
(	O
Fig	O
2D	O
).	O

To	O
further	O
verified	O
this	O
result	O
,	O
we	O
measured	O
the	O
binding	B-evidence
affinity	I-evidence
for	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
of	O
both	O
wild	B-protein_state
type	I-protein_state
(	O
WT	B-protein_state
)	O
and	O
D8A	B-mutant
mutant	B-protein_state
of	O
SePSK	B-protein
using	O
a	O
surface	B-experimental_method
plasmon	I-experimental_method
resonance	I-experimental_method
method	I-experimental_method
.	O

The	O
results	O
showed	O
that	O
the	O
affinity	B-evidence
of	O
D8A	B-mutant
-	O
SePSK	B-protein
with	O
D	B-chemical
-	I-chemical
ribulose	I-chemical
is	O
weaker	O
than	O
that	O
of	O
WT	B-protein_state
with	O
a	O
reduction	O
of	O
approx	O
.	O

Dissociation	B-evidence
rate	I-evidence
constant	I-evidence
(	O
Kd	B-evidence
)	O
of	O
wild	B-protein_state
type	I-protein_state
and	O
D8A	B-mutant
-	O
SePSK	B-protein
are	O
3	O
ms	O
-	O
1	O
and	O
9	O
ms	O
-	O
1	O
,	O
respectively	O
.	O

The	O
results	O
implied	O
that	O
the	O
second	B-site
RBL	I-site
binding	I-site
site	I-site
plays	O
a	O
role	O
in	O
the	O
D	B-protein_type
-	I-protein_type
ribulose	I-protein_type
kinase	I-protein_type
function	O
of	O
SePSK	B-protein
.	O

Simulated	O
conformational	O
change	O
of	O
SePSK	B-protein
during	O
the	O
catalytic	O
process	O

It	O
was	O
reported	O
earlier	O
that	O
the	O
crossing	O
angle	O
between	O
the	O
domain	B-structure_element
I	I-structure_element
and	O
domain	B-structure_element
II	I-structure_element
in	O
FGGY	B-protein_type
family	I-protein_type
carbohydrate	I-protein_type
kinases	I-protein_type
is	O
different	O
.	O

As	O
reported	O
previously	O
,	O
members	O
of	O
the	O
sugar	B-protein_type
kinase	I-protein_type
family	O
undergo	O
a	O
conformational	O
change	O
to	O
narrow	O
the	O
crossing	O
angle	O
between	O
two	O
domains	O
and	O
reduce	O
the	O
distance	O
between	O
substrate	O
and	O
ATP	B-chemical
in	O
order	O
to	O
facilitate	O
the	O
catalytic	O
reaction	O
of	O
phosphorylation	B-ptm
of	O
sugar	O
substrates	O
.	O

After	O
comparing	O
structures	B-evidence
of	O
apo	B-protein_state
-	O
SePSK	B-protein
,	O
RBL	B-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
and	O
AMP	B-complex_assembly
-	I-complex_assembly
PNP	I-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
,	O
we	O
noticed	O
that	O
these	O
structures	B-evidence
presented	O
here	O
are	O
similar	O
.	O

Since	O
the	O
two	O
domains	O
of	O
SePSK	B-protein
are	O
widely	O
separated	O
in	O
this	O
structure	B-evidence
,	O
we	O
hypothesize	O
that	O
our	O
structures	B-evidence
of	O
SePSK	B-protein
represent	O
its	O
open	B-protein_state
form	O
,	O
and	O
that	O
a	O
conformational	O
rearrangement	O
must	O
occur	O
to	O
switch	O
to	O
the	O
closed	B-protein_state
state	O
in	O
order	O
to	O
facilitate	O
the	O
catalytic	O
process	O
of	O
phosphorylation	B-ptm
of	O
sugar	O
substrates	O
.	O

For	O
studying	O
such	O
potential	O
conformational	O
change	O
,	O
a	O
simulation	B-experimental_method
on	O
the	O
Hingeprot	B-experimental_method
Server	I-experimental_method
was	O
performed	O
to	O
predict	O
the	O
movement	O
of	O
different	O
SePSK	B-protein
domains	O
.	O

Based	O
on	O
the	O
above	O
results	O
,	O
SePSK	B-protein
is	O
divided	O
into	O
two	O
rigid	O
parts	O
.	O

The	O
domain	B-structure_element
I	I-structure_element
of	O
RBL	B-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
(	O
aa	O
.	O
1	B-residue_range
–	I-residue_range
228	I-residue_range
,	O
aa	O
.	O
402	B-residue_range
–	I-residue_range
421	I-residue_range
)	O
and	O
the	O
domain	B-structure_element
II	I-structure_element
of	O
AMP	B-complex_assembly
-	I-complex_assembly
PNP	I-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
(	O
aa	O
.	O
229	B-residue_range
–	I-residue_range
401	I-residue_range
)	O
were	O
superposed	B-experimental_method
with	O
structures	B-evidence
,	O
including	O
apo	B-protein_state
-	O
AtXK	B-protein
-	I-protein
1	I-protein
,	O
apo	B-protein_state
-	O
SePSK	B-protein
,	O
xylulose	B-protein_type
kinase	I-protein_type
from	O
Lactobacillus	B-species
acidophilus	I-species
(	O
PDB	O
code	O
:	O
3LL3	O
)	O
and	O
the	O
S58W	B-mutant
mutant	B-protein_state
form	O
of	O
glycerol	B-protein_type
kinase	I-protein_type
from	O
Escherichia	B-species
coli	I-species
(	O
PDB	O
code	O
:	O
1GLJ	O
).	O

After	O
superposition	B-experimental_method
,	O
the	O
distances	O
of	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
γ	O
-	O
phosphate	B-chemical
and	O
the	O
fifth	O
hydroxyl	O
group	O
of	O
RBL1	B-residue_name_number
are	O
7	O
.	O
9	O
Å	O
(	O
superposed	B-experimental_method
with	O
AtXK	B-protein
-	I-protein
1	I-protein
),	O
7	O
.	O
4	O
Å	O
(	O
superposed	B-experimental_method
with	O
SePSK	B-protein
),	O
6	O
.	O
6	O
Å	O
(	O
superposed	B-experimental_method
with	O
3LL3	O
)	O
and	O
6	O
.	O
1	O
Å	O
(	O
superposed	B-experimental_method
with	O
1GLJ	O
).	O

This	O
distance	O
between	O
RBL2	B-residue_name_number
and	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
-	O
γ	O
-	O
phosphate	B-chemical
is	O
close	O
enough	O
to	O
facilitate	O
phosphate	B-chemical
transferring	O
.	O

Simulated	O
conformational	O
change	O
of	O
SePSK	B-protein
during	O
the	O
catalytic	O
process	O
.	O

The	O
structures	B-evidence
are	O
shown	O
as	O
cartoon	O
and	O
the	O
ligands	O
are	O
shown	O
as	O
sticks	O
.	O

Domain	B-structure_element
I	I-structure_element
from	O
D	B-complex_assembly
-	I-complex_assembly
ribulose	I-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
(	O
green	O
)	O
and	O
Domain	B-structure_element
II	I-structure_element
from	O
AMP	B-complex_assembly
-	I-complex_assembly
PNP	I-complex_assembly
-	I-complex_assembly
SePSK	I-complex_assembly
(	O
cyan	O
)	O
are	O
superposed	B-experimental_method
with	O
apo	B-protein_state
-	O
AtXK	B-protein
-	I-protein
1	I-protein
(	O
1st	O
),	O
apo	B-protein_state
-	O
SePSK	B-protein
(	O
2nd	O
),	O
3LL3	O
(	O
3rd	O
)	O
and	O
1GLJ	O
(	O
4th	O
),	O
respectively	O
.	O

The	O
numbers	O
near	O
the	O
black	O
dashed	O
lines	O
show	O
the	O
distances	O
(	O
Å	O
)	O
between	O
two	O
nearest	O
atoms	O
of	O
RBL	B-chemical
and	O
AMP	B-chemical
-	I-chemical
PNP	I-chemical
.	O

Our	O
results	O
provide	O
the	O
detailed	O
information	O
about	O
the	O
interaction	O
of	O
SePSK	B-protein
with	O
ATP	B-chemical
and	O
substrates	O
.	O

Moreover	O
,	O
structural	B-experimental_method
superposition	I-experimental_method
results	O
enable	O
us	O
to	O
visualize	O
the	O
conformational	O
change	O
of	O
SePSK	B-protein
during	O
the	O
catalytic	O
process	O
.	O

In	O
conclusion	O
,	O
our	O
results	O
provide	O
important	O
information	O
for	O
a	O
more	O
detailed	O
understanding	O
of	O
the	O
mechanisms	O
of	O
SePSK	B-protein
and	O
other	O
members	O
of	O
FGGY	B-protein_type
family	I-protein_type
carbohydrate	I-protein_type
kinases	I-protein_type
.	O

Structural	O
insights	O
into	O
the	O
Escherichia	B-species
coli	I-species
lysine	B-protein_type
decarboxylases	I-protein_type
and	O
molecular	O
determinants	O
of	O
interaction	O
with	O
the	O
AAA	B-protein_type
+	I-protein_type
ATPase	I-protein_type
RavA	B-protein

A	O
unique	O
macromolecular	O
cage	O
formed	O
by	O
two	O
decamers	B-oligomeric_state
of	O
the	O
Escherichia	B-species
coli	I-species
LdcI	B-protein
and	O
five	O
hexamers	B-oligomeric_state
of	O
the	O
AAA	B-protein_type
+	I-protein_type
ATPase	I-protein_type
RavA	B-protein
was	O
shown	O
to	O
counteract	O
acid	O
stress	O
under	O
starvation	O
.	O

Comparison	B-experimental_method
with	O
each	O
other	O
and	O
with	O
available	O
structures	B-evidence
uncovers	O
differences	O
between	O
LdcI	B-protein
and	O
LdcC	B-protein
explaining	O
why	O
only	O
the	O
acid	B-protein_type
stress	I-protein_type
response	I-protein_type
enzyme	I-protein_type
is	O
capable	O
of	O
binding	O
RavA	B-protein
.	O
We	O
identify	O
interdomain	O
movements	O
associated	O
with	O
the	O
pH	B-protein_state
-	I-protein_state
dependent	I-protein_state
enzyme	O
activation	O
and	O
with	O
the	O
RavA	B-protein
binding	O
.	O

Multiple	B-experimental_method
sequence	I-experimental_method
alignment	I-experimental_method
coupled	O
to	O
a	O
phylogenetic	B-experimental_method
analysis	I-experimental_method
reveals	O
that	O
certain	O
enterobacteria	B-taxonomy_domain
exert	O
evolutionary	O
pressure	O
on	O
the	O
lysine	B-protein_type
decarboxylase	I-protein_type
towards	O
the	O
cage	O
-	O
like	O
assembly	O
with	O
RavA	B-protein
,	O
implying	O
that	O
this	O
complex	O
may	O
have	O
an	O
important	O
function	O
under	O
particular	O
stress	O
conditions	O
.	O

Enterobacterial	B-taxonomy_domain
inducible	B-protein_state
decarboxylases	B-protein_type
of	O
basic	B-protein_state
amino	B-chemical
acids	I-chemical
lysine	B-residue_name
,	O
arginine	B-residue_name
and	O
ornithine	B-residue_name
have	O
a	O
common	O
evolutionary	O
origin	O
and	O
belong	O
to	O
the	O
α	B-protein_type
-	I-protein_type
family	I-protein_type
of	O
pyridoxal	B-chemical
-	I-chemical
5	I-chemical
′-	I-chemical
phosphate	I-chemical
(	O
PLP	B-chemical
)-	O
dependent	O
enzymes	O
.	O

Each	O
decarboxylase	B-protein_type
is	O
induced	O
by	O
an	O
excess	O
of	O
the	O
target	O
amino	B-chemical
acid	I-chemical
and	O
a	O
specific	O
range	O
of	O
extracellular	O
pH	O
,	O
and	O
works	O
in	O
conjunction	O
with	O
a	O
cognate	O
inner	B-protein_type
membrane	I-protein_type
antiporter	I-protein_type
.	O

Consequently	O
,	O
these	O
enzymes	O
buffer	O
both	O
the	O
bacterial	B-taxonomy_domain
cytoplasm	O
and	O
the	O
local	O
extracellular	O
environment	O
.	O

These	O
amino	B-protein_type
acid	I-protein_type
decarboxylases	I-protein_type
are	O
therefore	O
called	O
acid	O
stress	O
inducible	B-protein_state
or	O
biodegradative	B-protein_state
to	O
distinguish	O
them	O
from	O
their	O
biosynthetic	B-protein_state
lysine	B-protein_type
and	I-protein_type
ornithine	I-protein_type
decarboxylase	I-protein_type
paralogs	O
catalysing	O
the	O
same	O
reaction	O
but	O
responsible	O
for	O
the	O
polyamine	B-chemical
production	O
at	O
neutral	B-protein_state
pH	I-protein_state
.	O

Inducible	B-protein_state
enterobacterial	B-taxonomy_domain
amino	B-protein_type
acid	I-protein_type
decarboxylases	I-protein_type
have	O
been	O
intensively	O
studied	O
since	O
the	O
early	O
1940	O
because	O
the	O
ability	O
of	O
bacteria	B-taxonomy_domain
to	O
withstand	O
acid	O
stress	O
can	O
be	O
linked	O
to	O
their	O
pathogenicity	O
in	O
humans	B-species
.	O

In	O
particular	O
,	O
the	O
inducible	B-protein_state
lysine	B-protein_type
decarboxylase	I-protein_type
LdcI	B-protein
(	O
or	O
CadA	B-protein
)	O
attracts	O
attention	O
due	O
to	O
its	O
broad	B-protein_state
pH	I-protein_state
range	I-protein_state
of	O
activity	O
and	O
its	O
capacity	O
to	O
promote	O
survival	O
and	O
growth	O
of	O
pathogenic	O
enterobacteria	B-taxonomy_domain
such	O
as	O
Salmonella	B-species
enterica	I-species
serovar	I-species
Typhimurium	I-species
,	O
Vibrio	B-species
cholerae	I-species
and	O
Vibrio	B-species
vulnificus	I-species
under	O
acidic	O
conditions	O
.	O

Furthermore	O
,	O
both	O
LdcI	B-protein
and	O
the	O
biosynthetic	B-protein_state
lysine	B-protein_type
decarboxylase	I-protein_type
LdcC	B-protein
of	O
uropathogenic	B-species
Escherichia	I-species
coli	I-species
(	O
UPEC	B-species
)	O
appear	O
to	O
play	O
an	O
important	O
role	O
in	O
increased	O
resistance	O
of	O
this	O
pathogen	O
to	O
nitrosative	O
stress	O
produced	O
by	O
nitric	B-chemical
oxide	I-chemical
and	O
other	O
damaging	O
reactive	O
nitrogen	O
intermediates	O
accumulating	O
during	O
the	O
course	O
of	O
urinary	O
tract	O
infections	O
(	O
UTI	O
).	O

This	O
effect	O
is	O
attributed	O
to	O
cadaverine	B-chemical
,	O
the	O
diamine	O
produced	O
by	O
decarboxylation	O
of	O
lysine	B-residue_name
by	O
LdcI	B-protein
and	O
LdcC	B-protein
,	O
that	O
was	O
shown	O
to	O
enhance	O
UPEC	B-species
colonisation	O
of	O
the	O
bladder	O
.	O

Both	O
acid	B-protein_state
pH	I-protein_state
and	O
cadaverine	B-chemical
induce	O
closure	O
of	O
outer	O
membrane	O
porins	B-protein_type
thereby	O
contributing	O
to	O
bacterial	B-taxonomy_domain
protection	O
from	O
acid	O
stress	O
,	O
but	O
also	O
from	O
certain	O
antibiotics	O
,	O
by	O
reduction	O
in	O
membrane	O
permeability	O
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
E	B-species
.	I-species
coli	I-species
LdcI	B-protein
as	O
well	O
as	O
its	O
low	O
resolution	O
characterisation	O
by	O
electron	B-experimental_method
microscopy	I-experimental_method
(	O
EM	B-experimental_method
)	O
showed	O
that	O
it	O
is	O
a	O
decamer	B-oligomeric_state
made	O
of	O
two	O
pentameric	B-oligomeric_state
rings	B-structure_element
.	O

Ten	O
years	O
ago	O
we	O
showed	O
that	O
the	O
E	B-species
.	I-species
coli	I-species
AAA	B-protein_type
+	I-protein_type
ATPase	I-protein_type
RavA	B-protein
,	O
involved	O
in	O
multiple	O
stress	O
response	O
pathways	O
,	O
tightly	O
interacted	O
with	O
LdcI	B-protein
but	O
was	O
not	O
capable	O
of	O
binding	O
to	O
LdcC	B-protein
.	O
We	O
described	O
how	O
two	O
double	O
pentameric	B-oligomeric_state
rings	B-structure_element
of	O
the	O
LdcI	B-protein
tightly	O
associate	O
with	O
five	O
hexameric	B-oligomeric_state
rings	B-structure_element
of	O
RavA	B-protein
to	O
form	O
a	O
unique	O
cage	O
-	O
like	O
architecture	O
that	O
enables	O
the	O
bacterium	B-taxonomy_domain
to	O
withstand	O
acid	O
stress	O
even	O
under	O
conditions	O
of	O
nutrient	O
deprivation	O
eliciting	O
stringent	O
response	O
.	O

The	O
main	O
determinants	O
of	O
the	O
LdcI	B-complex_assembly
-	I-complex_assembly
RavA	I-complex_assembly
cage	O
assembly	O
appeared	O
to	O
be	O
the	O
N	O
-	O
terminal	O
loop	B-structure_element
of	O
the	O
LARA	B-structure_element
domain	I-structure_element
of	O
RavA	B-protein
and	O
the	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
of	O
LdcI	B-protein
.	O

In	O
spite	O
of	O
this	O
wealth	O
of	O
structural	B-evidence
information	I-evidence
,	O
the	O
fact	O
that	O
LdcC	B-protein
does	O
not	O
interact	O
with	O
RavA	B-protein
,	O
although	O
the	O
two	O
lysine	B-protein_type
decarboxylases	I-protein_type
are	O
69	O
%	O
identical	O
and	O
84	O
%	O
similar	O
,	O
and	O
the	O
physiological	O
significance	O
of	O
the	O
absence	O
of	O
this	O
interaction	O
remained	O
unexplored	O
.	O

Finally	O
,	O
we	O
performed	O
multiple	B-experimental_method
sequence	I-experimental_method
alignment	I-experimental_method
of	O
22	O
lysine	B-protein_type
decarboxylases	I-protein_type
from	O
Enterobacteriaceae	B-taxonomy_domain
containing	O
the	O
ravA	B-gene
-	I-gene
viaA	I-gene
operon	I-gene
in	O
their	O
genome	O
.	O

This	O
fascinating	O
parallelism	O
between	O
the	O
propensity	O
for	O
RavA	B-protein
binding	O
and	O
the	O
genetic	O
environment	O
of	O
an	O
enterobacterial	B-taxonomy_domain
lysine	B-protein_type
decarboxylase	I-protein_type
,	O
as	O
well	O
as	O
the	O
high	B-protein_state
degree	I-protein_state
of	I-protein_state
conservation	I-protein_state
of	O
this	O
small	B-structure_element
structural	I-structure_element
motif	I-structure_element
,	O
emphasize	O
the	O
functional	O
importance	O
of	O
the	O
interaction	O
between	O
biodegradative	B-protein_state
enterobacterial	B-taxonomy_domain
lysine	B-protein_type
decarboxylases	I-protein_type
and	O
the	O
AAA	B-protein_type
+	I-protein_type
ATPase	I-protein_type
RavA	B-protein
.	O

CryoEM	B-experimental_method
3D	B-evidence
reconstructions	I-evidence
of	O
LdcC	B-protein
,	O
LdcIa	B-protein
and	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly

In	O
the	O
frame	O
of	O
this	O
work	O
,	O
we	O
produced	O
two	O
novel	O
subnanometer	O
resolution	O
cryoEM	B-experimental_method
reconstructions	B-evidence
of	O
the	O
E	B-species
.	I-species
coli	I-species
lysine	B-protein_type
decarboxylases	I-protein_type
at	O
pH	B-protein_state
optimal	I-protein_state
for	O
their	O
enzymatic	O
activity	O
–	O
a	O
5	O
.	O
5	O
Å	O
resolution	O
cryoEM	B-experimental_method
map	B-evidence
of	O
the	O
LdcC	B-protein
(	O
pH	B-protein_state
7	I-protein_state
.	I-protein_state
5	I-protein_state
)	O
for	O
which	O
no	O
3D	O
structural	O
information	O
has	O
been	O
previously	O
available	O
(	O
Figs	O
1A	O
,	O
B	O
and	O
S1	O
),	O
and	O
a	O
6	O
.	O
1	O
Å	O
resolution	O
cryoEM	B-experimental_method
map	B-evidence
of	O
the	O
LdcIa	B-protein
,	O
(	O
pH	B-protein_state
6	I-protein_state
.	I-protein_state
2	I-protein_state
)	O
(	O
Figs	O
1C	O
,	O
D	O
and	O
S2	O
).	O

In	O
addition	O
,	O
we	O
improved	O
our	O
earlier	O
cryoEM	B-experimental_method
map	B-evidence
of	O
the	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
complex	O
from	O
7	O
.	O
5	O
Å	O
to	O
6	O
.	O
2	O
Å	O
resolution	O
(	O
Figs	O
1E	O
,	O
F	O
and	O
S3	O
).	O

Based	O
on	O
these	O
reconstructions	B-evidence
,	O
reliable	O
pseudoatomic	B-evidence
models	I-evidence
of	O
the	O
three	O
assemblies	O
were	O
obtained	O
by	O
flexible	B-experimental_method
fitting	I-experimental_method
of	I-experimental_method
either	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
LdcIi	B-protein
or	O
a	O
derived	O
structural	B-experimental_method
homology	I-experimental_method
model	I-experimental_method
of	O
LdcC	B-protein
(	O
Table	O
S1	O
).	O

Significant	O
differences	O
between	O
these	O
pseudoatomic	B-evidence
models	I-evidence
can	O
be	O
interpreted	O
as	O
movements	O
between	O
specific	O
biological	O
states	O
of	O
the	O
proteins	O
as	O
described	O
below	O
.	O

The	O
wing	B-structure_element
domains	I-structure_element
as	O
a	O
stable	O
anchor	O
at	O
the	O
center	O
of	O
the	O
double	B-structure_element
-	I-structure_element
ring	I-structure_element

As	O
a	O
first	O
step	O
of	O
a	O
comparative	O
analysis	O
,	O
we	O
superimposed	B-experimental_method
the	O
three	O
cryoEM	B-experimental_method
reconstructions	B-evidence
(	O
LdcIa	B-protein
,	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
and	O
LdcC	B-protein
)	O
and	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
LdcIi	B-protein
decamer	B-oligomeric_state
(	O
Fig	O
.	O
2	O
and	O
Movie	O
S1	O
).	O

This	O
superposition	B-experimental_method
reveals	O
that	O
the	O
densities	B-evidence
lining	O
the	O
central	B-structure_element
hole	I-structure_element
of	O
the	O
toroid	B-structure_element
are	O
roughly	O
at	O
the	O
same	O
location	O
,	O
while	O
the	O
rest	O
of	O
the	O
structure	B-evidence
exhibits	O
noticeable	O
changes	O
.	O

Specifically	O
,	O
at	O
the	O
center	O
of	O
the	O
double	B-structure_element
-	I-structure_element
ring	I-structure_element
the	O
wing	B-structure_element
domains	I-structure_element
of	O
the	O
subunits	O
provide	O
the	O
conserved	B-protein_state
basis	O
for	O
the	O
assembly	O
with	O
the	O
lowest	B-evidence
root	I-evidence
mean	I-evidence
square	I-evidence
deviation	I-evidence
(	O
RMSD	B-evidence
)	O
(	O
between	O
1	O
.	O
4	O
and	O
2	O
Å	O
for	O
the	O
Cα	O
atoms	O
only	O
),	O
whereas	O
the	O
peripheral	O
CTDs	B-structure_element
containing	O
the	O
RavA	B-site
binding	I-site
interface	I-site
manifest	O
the	O
highest	O
RMSD	B-evidence
(	O
up	O
to	O
4	O
.	O
2	O
Å	O
)	O
(	O
Table	O
S2	O
).	O

This	O
preservation	O
of	O
the	O
central	B-structure_element
part	I-structure_element
of	O
the	O
double	O
-	O
ring	O
assembly	O
may	O
help	O
the	O
enzymes	O
to	O
maintain	O
their	O
decameric	B-oligomeric_state
state	O
upon	O
activation	O
and	O
incorporation	O
into	O
the	O
LdcI	B-complex_assembly
-	I-complex_assembly
RavA	I-complex_assembly
cage	O
.	O

The	O
core	B-structure_element
domain	I-structure_element
and	O
the	O
active	B-site
site	I-site
rearrangements	O
upon	O
pH	B-protein_state
-	I-protein_state
dependent	I-protein_state
enzyme	O
activation	O
and	O
LARA	O
binding	O

Both	O
visual	B-experimental_method
inspection	I-experimental_method
(	O
Fig	O
.	O
2	O
)	O
and	O
RMSD	B-experimental_method
calculations	I-experimental_method
(	O
Table	O
S2	O
)	O
show	O
that	O
globally	O
the	O
three	O
structures	B-evidence
at	O
active	B-protein_state
pH	I-protein_state
(	O
LdcIa	B-protein
,	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
and	O
LdcC	B-protein
)	O
are	O
more	O
similar	O
to	O
each	O
other	O
than	O
to	O
the	O
structure	O
determined	O
at	O
high	B-protein_state
pH	I-protein_state
conditions	O
(	O
LdcIi	B-protein
).	O

The	O
decameric	B-oligomeric_state
enzyme	O
is	O
built	O
of	O
five	O
dimers	B-oligomeric_state
associating	O
into	O
a	O
5	B-structure_element
-	I-structure_element
fold	I-structure_element
symmetrical	I-structure_element
double	I-structure_element
-	I-structure_element
ring	I-structure_element
(	O
two	O
monomers	B-oligomeric_state
making	O
a	O
dimer	B-oligomeric_state
are	O
delineated	O
in	O
Fig	O
.	O
1	O
).	O

In	O
addition	O
,	O
our	O
earlier	O
biochemical	B-experimental_method
observation	I-experimental_method
that	O
the	O
enzymatic	O
activity	O
of	O
LdcIa	B-protein
is	O
unaffected	O
by	O
RavA	B-protein
binding	O
is	O
consistent	O
with	O
the	O
relatively	O
small	O
changes	O
undergone	O
by	O
the	O
active	B-site
site	I-site
upon	O
transition	O
from	O
LdcIa	B-protein
to	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
.	O

Worthy	O
of	O
note	O
,	O
our	O
previous	O
comparison	O
of	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
LdcIi	B-protein
with	O
that	O
of	O
the	O
inducible	B-protein_state
arginine	B-protein_type
decarboxylase	I-protein_type
AdiA	B-protein
revealed	O
high	B-protein_state
conservation	I-protein_state
of	O
the	O
PLP	B-site
-	I-site
coordinating	I-site
residues	I-site
and	O
identified	O
a	O
patch	B-site
of	I-site
negatively	I-site
charged	I-site
residues	I-site
lining	O
the	O
active	B-site
site	I-site
channel	I-site
as	O
a	O
potential	O
binding	B-site
site	I-site
for	O
the	O
target	O
amino	B-chemical
acid	I-chemical
substrate	O
(	O
Figs	O
S3	O
and	O
S4	O
in	O
ref	O
.).	O

Rearrangements	O
of	O
the	O
ppGpp	B-site
binding	I-site
pocket	I-site
upon	O
pH	B-protein_state
-	I-protein_state
dependent	I-protein_state
enzyme	O
activation	O
and	O
LARA	B-structure_element
binding	O

Whereas	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
ppGpp	B-complex_assembly
-	I-complex_assembly
LdcIi	I-complex_assembly
was	O
solved	B-experimental_method
to	O
2	O
Å	O
resolution	O
,	O
only	O
a	O
4	O
.	O
1	O
Å	O
resolution	O
structure	B-evidence
of	O
the	O
ppGpp	B-protein_state
-	I-protein_state
free	I-protein_state
LdcIi	B-protein
could	O
be	O
obtained	O
.	O

At	O
this	O
resolution	O
,	O
the	O
apo	B-protein_state
-	O
LdcIi	B-protein
and	O
ppGpp	B-complex_assembly
-	I-complex_assembly
LdcIi	I-complex_assembly
structures	B-evidence
(	O
both	O
solved	O
at	O
pH	B-protein_state
8	I-protein_state
.	I-protein_state
5	I-protein_state
)	O
appeared	O
indistinguishable	O
except	O
for	O
the	O
presence	O
of	O
ppGpp	B-chemical
(	O
Fig	O
.	O
S11	O
in	O
ref	O
.	O
).	O

Thus	O
,	O
we	O
speculated	O
that	O
inhibition	O
of	O
LdcI	B-protein
by	O
ppGpp	B-chemical
would	O
be	O
accompanied	O
by	O
a	O
transduction	O
of	O
subtle	O
structural	O
changes	O
at	O
the	O
level	O
of	O
individual	O
amino	B-chemical
acid	I-chemical
side	O
chains	O
between	O
the	O
ppGpp	B-site
binding	I-site
pocket	I-site
and	O
the	O
active	B-site
site	I-site
of	O
the	O
enzyme	O
.	O

While	O
differences	O
in	O
the	O
ppGpp	B-site
binding	I-site
site	I-site
could	O
indeed	O
be	O
visualized	O
(	O
Fig	O
.	O
S4	O
),	O
the	O
level	O
of	O
resolution	O
warns	O
against	O
speculations	O
about	O
their	O
significance	O
.	O

The	O
fact	O
that	O
interaction	O
with	O
RavA	B-protein
reduces	O
the	O
ppGpp	B-chemical
affinity	O
for	O
LdcI	B-protein
despite	O
the	O
long	O
distance	O
of	O
~	O
30	O
Å	O
between	O
the	O
LARA	B-site
domain	I-site
binding	I-site
site	I-site
and	O
the	O
closest	O
ppGpp	B-site
binding	I-site
pocket	I-site
(	O
Fig	O
.	O
S5	O
)	O
seems	O
to	O
favor	O
an	O
allosteric	O
regulation	O
mechanism	O
.	O

Swinging	O
and	O
stretching	O
of	O
the	O
CTDs	B-structure_element
upon	O
pH	B-protein_state
-	I-protein_state
dependent	I-protein_state
LdcI	B-protein
activation	O
and	O
LARA	B-structure_element
binding	O

Inspection	O
of	O
the	O
superimposed	B-experimental_method
decameric	B-oligomeric_state
structures	B-evidence
(	O
Figs	O
2	O
and	O
S6	O
)	O
suggests	O
a	O
depiction	O
of	O
the	O
wing	B-structure_element
domains	I-structure_element
as	O
an	O
anchor	O
around	O
which	O
the	O
peripheral	O
CTDs	B-structure_element
swing	O
.	O

Indeed	O
,	O
all	O
CTDs	B-structure_element
have	O
very	O
similar	O
structures	O
(	O
RMSDmin	B-evidence
<	O
1	O
Å	O
).	O

The	O
LdcIi	B-protein
monomer	B-oligomeric_state
is	O
the	O
most	B-protein_state
compact	I-protein_state
,	O
whereas	O
LdcIa	B-protein
and	O
especially	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
gradually	B-protein_state
extend	I-protein_state
their	O
CTDs	B-structure_element
towards	O
the	O
LARA	B-structure_element
domain	I-structure_element
of	O
RavA	B-protein
(	O
Figs	O
2	O
and	O
4	O
).	O

In	O
our	O
previous	O
contribution	O
,	O
based	O
on	O
the	O
fit	O
of	O
the	O
LdcIi	B-protein
and	O
the	O
LARA	B-structure_element
crystal	B-evidence
structures	I-evidence
into	O
the	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
cryoEM	B-experimental_method
density	B-evidence
,	O
we	O
predicted	O
that	O
the	O
LdcI	B-complex_assembly
-	I-complex_assembly
RavA	I-complex_assembly
interaction	O
should	O
involve	O
the	O
C	O
-	O
terminal	O
two	B-structure_element
-	I-structure_element
stranded	I-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
of	O
the	O
LdcI	B-protein
.	O
Our	O
present	O
cryoEM	B-experimental_method
maps	B-evidence
and	O
pseudoatomic	B-evidence
models	I-evidence
provide	O
first	O
structure	O
-	O
based	O
insights	O
into	O
the	O
differences	O
between	O
the	O
inducible	B-protein_state
and	O
the	O
constitutive	B-protein_state
lysine	B-protein_type
decarboxylases	I-protein_type
.	O

Therefore	O
,	O
we	O
wanted	O
to	O
check	O
the	O
influence	O
of	O
the	O
primary	O
sequence	O
of	O
the	O
two	O
proteins	O
in	O
this	O
region	O
on	O
their	O
ability	O
to	O
interact	O
with	O
RavA	B-protein
.	O
To	O
this	O
end	O
,	O
we	O
swapped	B-experimental_method
the	O
relevant	O
β	B-structure_element
-	I-structure_element
sheets	I-structure_element
of	O
the	O
two	O
proteins	O
and	O
produced	O
their	O
chimeras	B-mutant
,	O
namely	O
LdcIC	B-mutant
(	O
i	O
.	O
e	O
.	O
LdcI	B-protein
with	O
the	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
of	O
LdcC	B-protein
)	O
and	O
LdcCI	B-mutant
(	O
i	O
.	O
e	O
.	O
LdcC	B-protein
with	O
the	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
of	O
LdcI	B-protein
)	O
(	O
Fig	O
.	O
5A	O
–	O
C	O
).	O

Both	B-mutant
constructs	I-mutant
could	O
be	O
purified	O
and	O
could	O
form	O
decamers	B-oligomeric_state
visually	O
indistinguishable	O
from	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
proteins	O
.	O

As	O
expected	O
,	O
binding	O
of	O
LdcI	B-protein
to	O
RavA	B-protein
was	O
completely	O
abolished	O
by	O
this	O
procedure	O
and	O
no	O
LdcIC	B-complex_assembly
-	I-complex_assembly
RavA	I-complex_assembly
complex	O
could	O
be	O
detected	O
.	O

On	O
the	O
contrary	O
,	O
introduction	B-experimental_method
of	O
the	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
of	O
LdcI	B-protein
into	O
LdcC	B-protein
led	O
to	O
an	O
assembly	O
of	O
the	O
LdcCI	B-complex_assembly
-	I-complex_assembly
RavA	I-complex_assembly
complex	O
.	O

On	O
the	O
negative	B-experimental_method
stain	I-experimental_method
EM	I-experimental_method
grid	I-experimental_method
,	O
the	O
chimeric	B-protein_state
cages	O
appeared	O
less	O
rigid	O
than	O
the	O
native	B-protein_state
LdcI	B-complex_assembly
-	I-complex_assembly
RavA	I-complex_assembly
,	O
which	O
probably	O
means	O
that	O
the	O
environment	O
of	O
the	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
contributes	O
to	O
the	O
efficiency	O
of	O
the	O
interaction	O
and	O
the	O
stability	O
of	O
the	O
entire	O
architecture	O
(	O
Fig	O
.	O
5D	O
–	O
F	O
).	O

The	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
of	O
a	O
lysine	B-protein_type
decarboxylase	I-protein_type
is	O
a	O
highly	B-protein_state
conserved	I-protein_state
signature	O
allowing	O
to	O
distinguish	O
between	O
LdcI	B-protein
and	O
LdcC	B-protein

Alignment	B-experimental_method
of	I-experimental_method
the	I-experimental_method
primary	I-experimental_method
sequences	I-experimental_method
of	O
the	O
E	B-species
.	I-species
coli	I-species
LdcI	B-protein
and	O
LdcC	B-protein
shows	O
that	O
some	O
amino	O
acid	O
residues	O
of	O
the	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
are	O
the	O
same	O
in	O
the	O
two	O
proteins	O
,	O
whereas	O
others	O
are	O
notably	O
different	O
in	O
chemical	O
nature	O
.	O

Importantly	O
,	O
most	O
of	O
the	O
amino	O
acid	O
differences	O
between	O
the	O
two	O
enzymes	O
are	O
located	O
in	O
this	O
very	B-structure_element
region	I-structure_element
.	O

First	O
of	O
all	O
,	O
consensus	B-evidence
sequence	I-evidence
for	O
the	O
entire	O
lysine	B-protein_type
decarboxylase	I-protein_type
family	O
was	O
derived	O
.	O

Second	O
,	O
the	O
phylogenetic	B-experimental_method
analysis	I-experimental_method
clearly	O
split	O
the	O
lysine	B-protein_type
decarboxylases	I-protein_type
into	O
two	O
groups	O
(	O
Fig	O
.	O
6A	O
).	O

Thus	O
,	O
consensus	B-evidence
sequences	I-evidence
could	O
also	O
be	O
determined	O
for	O
each	O
of	O
the	O
two	O
groups	O
(	O
Figs	O
6B	O
,	O
C	O
and	O
S7	O
).	O

Inspection	O
of	O
these	O
consensus	B-evidence
sequences	I-evidence
revealed	O
important	O
differences	O
between	O
the	O
groups	O
regarding	O
charge	O
,	O
size	O
and	O
hydrophobicity	O
of	O
several	O
residues	O
precisely	O
at	O
the	O
level	O
of	O
the	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
that	O
is	O
responsible	O
for	O
the	O
interaction	O
with	O
RavA	B-protein
(	O
Fig	O
.	O
6B	O
–	O
D	O
).	O

For	O
example	O
,	O
in	O
our	O
previous	O
study	O
,	O
site	B-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutations	I-experimental_method
identified	O
Y697	B-residue_name_number
as	O
critically	O
required	O
for	O
the	O
RavA	B-protein
binding	O
.	O

Our	O
current	O
analysis	O
shows	O
that	O
Y697	B-residue_name_number
is	O
strictly	B-protein_state
conserved	I-protein_state
in	O
the	O
“	O
LdcI	B-protein_type
-	I-protein_type
like	I-protein_type
”	O
group	O
whereas	O
the	O
“	O
LdcC	B-protein_type
-	I-protein_type
like	I-protein_type
”	O
enzymes	O
always	B-protein_state
have	I-protein_state
a	O
lysine	B-residue_name
in	O
this	O
position	O
;	O
it	O
also	O
uncovers	O
several	O
other	O
residues	O
potentially	O
essential	O
for	O
the	O
interaction	O
with	O
RavA	B-protein
which	O
can	O
now	O
be	O
addressed	O
by	O
site	B-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
.	O

The	O
third	O
and	O
most	O
remarkable	O
finding	O
was	O
that	O
exactly	O
the	O
same	O
separation	O
into	O
“	O
LdcI	B-protein_type
-	I-protein_type
like	I-protein_type
”	O
and	O
“	O
LdcC	B-protein_type
”-	I-protein_type
like	I-protein_type
groups	O
can	O
be	O
obtained	O
based	O
on	O
a	O
comparison	O
of	O
the	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sheets	I-structure_element
only	O
,	O
without	O
taking	O
the	O
rest	O
of	O
the	O
primary	O
sequence	O
into	O
account	O
.	O

Therefore	O
the	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
emerges	O
as	O
being	O
a	O
highly	B-protein_state
conserved	I-protein_state
signature	B-structure_element
sequence	I-structure_element
,	O
sufficient	O
to	O
unambiguously	O
discriminate	O
between	O
the	O
“	O
LdcI	B-protein_type
-	I-protein_type
like	I-protein_type
”	O
and	O
“	O
LdcC	B-protein_type
-	I-protein_type
like	I-protein_type
”	O
enterobacterial	B-taxonomy_domain
lysine	B-protein_type
decarboxylases	I-protein_type
independently	O
of	O
any	O
other	O
information	O
(	O
Figs	O
6	O
and	O
S7	O
).	O

Thus	O
,	O
enterobacteria	B-taxonomy_domain
identified	O
here	O
(	O
Fig	O
.	O
6	O
,	O
Table	O
S4	O
)	O
appear	O
to	O
exert	O
evolutionary	O
pressure	O
on	O
the	O
biodegradative	B-protein_state
lysine	B-protein_type
decarboxylase	I-protein_type
towards	O
the	O
RavA	B-protein
binding	O
.	O

One	O
of	O
the	O
elucidated	O
roles	O
of	O
the	O
LdcI	B-complex_assembly
-	I-complex_assembly
RavA	I-complex_assembly
cage	O
is	O
to	O
maintain	O
LdcI	B-protein
activity	O
under	O
conditions	O
of	O
enterobacterial	B-taxonomy_domain
starvation	O
by	O
preventing	O
LdcI	B-protein
inhibition	O
by	O
the	O
stringent	B-chemical
response	I-chemical
alarmone	I-chemical
ppGpp	B-chemical
.	O

Furthermore	O
,	O
the	O
recently	O
documented	O
interaction	O
of	O
both	O
LdcI	B-protein
and	O
RavA	B-protein
with	O
specific	O
subunits	B-structure_element
of	O
the	O
respiratory	B-protein_type
complex	I-protein_type
I	I-protein_type
,	O
together	O
with	O
the	O
unanticipated	O
link	O
between	O
RavA	B-protein
and	O
maturation	O
of	O
numerous	O
iron	B-protein_type
-	I-protein_type
sulfur	I-protein_type
proteins	I-protein_type
,	O
tend	O
to	O
suggest	O
an	O
additional	O
intriguing	O
function	O
for	O
this	O
3	O
.	O
5	O
MDa	O
assembly	O
.	O

The	O
conformational	O
rearrangements	O
of	O
LdcI	B-protein
upon	O
enzyme	O
activation	O
and	O
RavA	B-protein
binding	O
revealed	O
in	O
this	O
work	O
,	O
and	O
our	O
amazing	O
finding	O
that	O
the	O
molecular	O
determinant	O
of	O
the	O
LdcI	B-complex_assembly
-	I-complex_assembly
RavA	I-complex_assembly
interaction	O
is	O
the	O
one	O
that	O
straightforwardly	O
determines	O
if	O
a	O
particular	O
enterobacterial	B-taxonomy_domain
lysine	B-protein_type
decarboxylase	I-protein_type
belongs	O
to	O
“	O
LdcI	B-protein_type
-	I-protein_type
like	I-protein_type
”	O
or	O
“	O
LdcC	B-protein_type
-	I-protein_type
like	I-protein_type
”	O
proteins	O
,	O
should	O
give	O
a	O
new	O
impetus	O
to	O
functional	O
studies	O
of	O
the	O
unique	O
LdcI	B-complex_assembly
-	I-complex_assembly
RavA	I-complex_assembly
cage	O
.	O

Besides	O
,	O
the	O
structures	B-evidence
and	O
the	O
pseudoatomic	B-evidence
models	I-evidence
of	O
the	O
active	B-protein_state
ppGpp	B-protein_state
-	I-protein_state
free	I-protein_state
states	O
of	O
both	O
the	O
biodegradative	B-protein_state
and	O
the	O
biosynthetic	B-protein_state
E	B-species
.	I-species
coli	I-species
lysine	B-protein_type
decarboxylases	I-protein_type
offer	O
an	O
additional	O
tool	O
for	O
analysis	O
of	O
their	O
role	O
in	O
UPEC	B-species
infectivity	O
.	O

Together	O
with	O
the	O
apo	B-protein_state
-	O
LdcI	B-protein
and	O
ppGpp	B-complex_assembly
-	I-complex_assembly
LdcIi	I-complex_assembly
crystal	B-evidence
structures	I-evidence
,	O
our	O
cryoEM	B-experimental_method
reconstructions	B-evidence
provide	O
a	O
structural	O
framework	O
for	O
future	O
studies	O
of	O
structure	O
-	O
function	O
relationships	O
of	O
lysine	B-protein_type
decarboxylases	I-protein_type
from	O
other	O
enterobacteria	B-taxonomy_domain
and	O
even	O
of	O
their	O
homologues	O
outside	O
Enterobacteriaceae	B-taxonomy_domain
.	O
For	O
example	O
,	O
the	O
lysine	B-protein_type
decarboxylase	I-protein_type
of	O
Eikenella	B-species
corrodens	I-species
is	O
thought	O
to	O
play	O
a	O
major	O
role	O
in	O
the	O
periodontal	O
disease	O
and	O
its	O
inhibitors	O
were	O
shown	O
to	O
retard	O
gingivitis	O
development	O
.	O

3D	O
cryoEM	B-experimental_method
reconstructions	B-evidence
of	O
LdcC	B-protein
,	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
and	O
LdcIa	B-protein
.	O

In	O
the	O
rest	O
of	O
the	O
protomers	B-oligomeric_state
,	O
the	O
wing	B-structure_element
,	O
core	B-structure_element
and	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domains	I-structure_element
are	O
colored	O
from	O
light	O
to	O
dark	O
in	O
shades	O
of	O
green	O
for	O
LdcC	B-protein
(	O
A	O
),	O
pink	O
for	O
LdcIa	B-protein
(	O
C	O
)	O
and	O
blue	O
for	O
LdcI	B-protein
in	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
(	O
E	O
).	O

In	O
(	O
E	O
),	O
the	O
LARA	B-structure_element
domain	I-structure_element
density	O
is	O
shown	O
in	O
dark	O
grey	O
.	O

Two	O
monomers	B-oligomeric_state
making	O
a	O
dimer	B-oligomeric_state
are	O
delineated	O
.	O

Scale	O
bar	O
50	O
Å	O
.	O
(	O
B	O
,	O
D	O
,	O
F	O
)	O
One	O
protomer	B-oligomeric_state
from	O
the	O
cryoEM	B-experimental_method
map	B-evidence
of	O
the	O
LdcC	B-protein
(	O
B	O
),	O
LdcIa	B-protein
(	O
D	O
)	O
and	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
(	O
F	O
)	O
in	O
light	O
grey	O
with	O
the	O
pseudoatomic	B-evidence
model	I-evidence
represented	O
as	O
cartoons	O
and	O
colored	O
as	O
the	O
densities	O
in	O
(	O
A	O
,	O
C	O
,	O
E	O
).	O

Superposition	B-experimental_method
of	O
the	O
pseudoatomic	B-evidence
models	I-evidence
of	O
LdcC	B-protein
,	O
LdcI	B-protein
from	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
and	O
LdcIa	B-protein
colored	O
as	O
in	O
Fig	O
.	O
1	O
,	O
and	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
LdcIi	B-protein
in	O
shades	O
of	O
yellow	O
.	O

Conformational	O
rearrangements	O
in	O
the	O
enzyme	O
active	B-site
site	I-site
.	O

(	O
A	O
)	O
LdcIi	B-protein
crystal	B-evidence
structure	I-evidence
,	O
with	O
one	O
ring	B-structure_element
represented	O
as	O
a	O
grey	O
surface	O
and	O
the	O
second	O
as	O
a	O
cartoon	O
.	O

A	O
monomer	B-oligomeric_state
with	O
its	O
PLP	B-chemical
cofactor	O
is	O
delineated	O
.	O

The	O
PLP	B-chemical
moieties	O
of	O
the	O
cartoon	O
ring	B-structure_element
are	O
shown	O
in	O
red	O
.	O

One	O
monomer	B-oligomeric_state
is	O
colored	O
in	O
shades	O
of	O
yellow	O
as	O
in	O
Figs	O
1	O
and	O
2	O
,	O
while	O
the	O
monomer	B-oligomeric_state
related	O
by	O
C2	O
symmetry	O
is	O
grey	O
.	O

The	O
active	B-site
site	I-site
is	O
boxed	O
.	O

Stretching	O
of	O
the	O
LdcI	B-protein
monomer	B-oligomeric_state
upon	O
pH	B-protein_state
-	I-protein_state
dependent	I-protein_state
enzyme	O
activation	O
and	O
LARA	B-structure_element
binding	O
.	O

(	O
A	O
–	O
C	O
)	O
A	O
slice	O
through	O
the	O
pseudoatomic	B-evidence
models	I-evidence
of	O
the	O
LdcI	B-protein
monomers	B-oligomeric_state
extracted	O
from	O
the	O
superimposed	B-experimental_method
decamers	B-oligomeric_state
(	O
Fig	O
.	O
2	O
)	O
The	O
rectangle	O
indicates	O
the	O
regions	O
enlarged	O
in	O
(	O
D	O
–	O
F	O
).	O

(	O
A	O
)	O
compares	O
LdcIi	B-protein
(	O
yellow	O
)	O
and	O
LdcIa	B-protein
(	O
pink	O
),	O
(	O
B	O
)	O
compares	O
LdcIa	B-protein
(	O
pink	O
)	O
and	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
(	O
blue	O
),	O
and	O
(	O
C	O
)	O
compares	O
LdcIi	B-protein
(	O
yellow	O
),	O
LdcIa	B-protein
(	O
pink	O
)	O
and	O
LdcI	B-complex_assembly
-	I-complex_assembly
LARA	I-complex_assembly
(	O
blue	O
)	O
simultaneously	O
in	O
order	O
to	O
show	O
the	O
progressive	O
stretching	O
described	O
in	O
the	O
text	O
.	O

The	O
cryoEM	B-experimental_method
density	B-evidence
of	O
the	O
LARA	B-structure_element
domain	I-structure_element
is	O
represented	O
as	O
a	O
grey	O
surface	O
to	O
show	O
the	O
position	O
of	O
the	O
binding	B-site
site	I-site
and	O
the	O
direction	O
of	O
the	O
movement	O
.	O

Analysis	O
of	O
the	O
LdcIC	B-mutant
and	O
LdcCI	B-mutant
chimeras	B-mutant
.	O

(	O
A	O
)	O
A	O
slice	O
through	O
the	O
pseudoatomic	B-evidence
models	I-evidence
of	O
the	O
LdcIa	B-protein
(	O
purple	O
)	O
and	O
LdcC	B-protein
(	O
green	O
)	O
monomers	B-oligomeric_state
extracted	O
from	O
the	O
superimposed	B-experimental_method
decamers	B-oligomeric_state
(	O
Fig	O
.	O
2	O
).	O
(	O
B	O
)	O
The	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
in	O
LdcIa	B-protein
and	O
LdcC	B-protein
enlarged	O
from	O
(	O
A	O
,	O
C	O
)	O
Exchanged	O
primary	O
sequences	O
(	O
capital	O
letters	O
)	O
and	O
their	O
immediate	O
vicinity	O
(	O
lower	O
case	O
letters	O
)	O
colored	O
as	O
in	O
(	O
A	O
,	O
B	O
),	O
with	O
the	O
corresponding	O
secondary	O
structure	O
elements	O
and	O
the	O
amino	O
acid	O
numbering	O
shown	O
.	O

Sequence	B-experimental_method
analysis	I-experimental_method
of	O
enterobacterial	B-taxonomy_domain
lysine	B-protein_type
decarboxylases	I-protein_type
.	O

Numbering	O
as	O
in	O
E	B-species
.	I-species
coli	I-species
.	O

Structural	O
basis	O
for	O
Mep2	B-protein_type
ammonium	B-protein_type
transceptor	I-protein_type
activation	O
by	O
phosphorylation	B-ptm

Here	O
we	O
report	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystal	I-evidence
structures	I-evidence
of	O
the	O
Mep2	B-protein_type
orthologues	O
from	O
Saccharomyces	B-species
cerevisiae	I-species
and	O
Candida	B-species
albicans	I-species
and	O
show	O
that	O
under	O
nitrogen	O
-	O
sufficient	O
conditions	O
the	O
transporters	B-protein_type
are	O
not	B-protein_state
phosphorylated	I-protein_state
and	O
present	O
in	O
closed	B-protein_state
,	O
inactive	B-protein_state
conformations	O
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
phosphorylation	B-protein_state
-	I-protein_state
mimicking	I-protein_state
Mep2	B-mutant
variants	I-mutant
from	O
C	B-species
.	I-species
albicans	I-species
show	O
large	O
conformational	O
changes	O
in	O
a	O
conserved	B-protein_state
and	O
functionally	O
important	O
region	O
of	O
the	O
CTR	B-structure_element
.	O

The	O
results	O
allow	O
us	O
to	O
propose	O
a	O
model	O
for	O
regulation	O
of	O
eukaryotic	B-taxonomy_domain
ammonium	B-chemical
transport	O
by	O
phosphorylation	B-ptm
.	O

Mep2	B-protein_type
proteins	I-protein_type
are	O
tightly	O
regulated	O
fungal	B-taxonomy_domain
ammonium	B-protein_type
transporters	I-protein_type
.	O

Here	O
,	O
the	O
authors	O
report	O
the	O
crystal	B-evidence
structures	I-evidence
of	O
closed	B-protein_state
states	O
of	O
Mep2	B-protein_type
proteins	I-protein_type
and	O
propose	O
a	O
model	O
for	O
their	O
regulation	O
by	B-experimental_method
comparing	I-experimental_method
them	I-experimental_method
with	I-experimental_method
the	O
open	B-protein_state
ammonium	B-protein_type
transporters	I-protein_type
of	O
bacteria	B-taxonomy_domain
.	O

Transceptors	B-protein_type
are	O
membrane	B-protein_type
proteins	I-protein_type
that	O
function	O
not	O
only	O
as	O
transporters	O
but	O
also	O
as	O
receptors	O
/	O
sensors	O
during	O
nutrient	O
sensing	O
to	O
activate	O
downstream	O
signalling	O
pathways	O
.	O

While	O
most	O
studies	O
have	O
focused	O
on	O
the	O
Saccharomyces	B-species
cerevisiae	I-species
transceptors	B-protein_type
for	O
phosphate	B-chemical
(	O
Pho84	B-protein
),	O
amino	B-chemical
acids	I-chemical
(	O
Gap1	B-protein
)	O
and	O
ammonium	B-chemical
(	O
Mep2	B-protein
),	O
transceptors	B-protein_type
are	O
found	O
in	O
higher	B-taxonomy_domain
eukaryotes	I-taxonomy_domain
as	O
well	O
(	O
for	O
example	O
,	O
the	O
mammalian	B-taxonomy_domain
SNAT2	B-protein
amino	B-protein_type
-	I-protein_type
acid	I-protein_type
transporter	I-protein_type
and	O
the	O
GLUT2	B-protein
glucose	B-protein_type
transporter	I-protein_type
).	O

One	O
hypothesis	O
is	O
that	O
downstream	O
signalling	O
is	O
dependent	O
on	O
a	O
specific	O
conformation	O
of	O
the	O
transporter	B-protein_type
.	O

They	O
belong	O
to	O
the	O
Amt	B-protein_type
/	I-protein_type
Mep	I-protein_type
/	I-protein_type
Rh	I-protein_type
family	I-protein_type
of	I-protein_type
transporters	I-protein_type
that	O
are	O
present	O
in	O
all	B-taxonomy_domain
kingdoms	I-taxonomy_domain
of	I-taxonomy_domain
life	I-taxonomy_domain
and	O
they	O
take	O
up	O
ammonium	B-chemical
from	O
the	O
extracellular	O
environment	O
.	O

Of	O
these	O
,	O
only	O
Mep2	B-protein_type
proteins	I-protein_type
function	O
as	O
ammonium	B-chemical
receptors	O
/	O
sensors	O
in	O
fungal	B-taxonomy_domain
development	O
.	O

Under	O
conditions	O
of	O
nitrogen	O
limitation	O
,	O
Mep2	B-protein
initiates	O
a	O
signalling	O
cascade	O
that	O
results	O
in	O
a	O
switch	O
from	O
the	O
yeast	O
form	O
to	O
filamentous	O
(	O
pseudohyphal	O
)	O
growth	O
that	O
may	O
be	O
required	O
for	O
fungal	B-taxonomy_domain
pathogenicity	O
.	O

In	O
addition	O
,	O
Mep2	B-protein
is	O
also	O
important	O
for	O
uptake	O
of	O
ammonium	B-chemical
produced	O
by	O
growth	O
on	O
other	O
nitrogen	B-chemical
sources	O
.	O

With	O
the	O
exception	O
of	O
the	O
human	B-species
RhCG	B-protein
structure	B-evidence
,	O
no	O
structural	O
information	O
is	O
available	O
for	O
eukaryotic	B-taxonomy_domain
ammonium	B-protein_type
transporters	I-protein_type
.	O

The	O
proteins	O
form	O
stable	B-protein_state
trimers	B-oligomeric_state
,	O
with	O
each	O
monomer	B-oligomeric_state
having	O
11	O
transmembrane	B-structure_element
(	O
TM	B-structure_element
)	O
helices	B-structure_element
and	O
a	O
central	B-site
channel	I-site
for	O
the	O
transport	O
of	O
ammonium	B-chemical
.	O

All	O
structures	B-evidence
show	O
the	O
transporters	B-protein_type
in	O
open	B-protein_state
conformations	O
.	O

In	O
animals	B-taxonomy_domain
,	O
this	O
is	O
due	O
to	O
toxicity	O
of	O
elevated	O
intracellular	O
ammonium	B-chemical
levels	O
,	O
whereas	O
for	O
microorganisms	B-taxonomy_domain
ammonium	B-chemical
is	O
a	O
preferred	O
nitrogen	O
source	O
.	O

In	O
bacteria	B-taxonomy_domain
,	O
amt	B-gene
genes	O
are	O
present	O
in	O
an	O
operon	O
with	O
glnK	B-gene
,	O
encoding	O
a	O
PII	B-protein_type
-	I-protein_type
like	I-protein_type
signal	I-protein_type
transduction	I-protein_type
class	I-protein_type
protein	I-protein_type
.	O

By	O
binding	O
tightly	O
to	O
Amt	B-protein_type
proteins	I-protein_type
without	O
inducing	O
a	O
conformational	O
change	O
in	O
the	O
transporter	B-protein_type
,	O
GlnK	B-protein_type
sterically	O
blocks	O
ammonium	B-chemical
conductance	O
when	O
nitrogen	O
levels	O
are	O
sufficient	O
.	O

Under	O
conditions	O
of	O
nitrogen	B-chemical
limitation	O
,	O
GlnK	B-protein_type
becomes	O
uridylated	B-protein_state
,	O
blocking	O
its	O
ability	O
to	O
bind	O
and	O
inhibit	O
Amt	B-protein_type
proteins	I-protein_type
.	O

Importantly	O
,	O
eukaryotes	B-taxonomy_domain
do	O
not	O
have	O
GlnK	B-protein_type
orthologues	O
and	O
have	O
a	O
different	O
mechanism	O
for	O
regulation	O
of	O
ammonium	B-chemical
transport	O
activity	O
.	O

In	O
plants	B-taxonomy_domain
,	O
transporter	B-protein_type
phosphorylation	B-ptm
and	O
dephosphorylation	B-ptm
are	O
known	O
to	O
regulate	O
activity	O
.	O

In	O
S	B-species
.	I-species
cerevisiae	I-species
,	O
phosphorylation	B-ptm
of	O
Ser457	B-residue_name_number
within	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
region	I-structure_element
(	O
CTR	B-structure_element
)	O
in	O
the	O
cytoplasm	O
was	O
recently	O
proposed	O
to	O
cause	O
Mep2	B-protein_type
opening	O
,	O
possibly	O
via	O
inducing	O
a	O
conformational	O
change	O
.	O

To	O
elucidate	O
the	O
mechanism	O
of	O
Mep2	B-protein_type
transport	O
regulation	O
,	O
we	O
present	O
here	O
X	B-evidence
-	I-evidence
ray	I-evidence
crystal	I-evidence
structures	I-evidence
of	O
the	O
Mep2	B-protein_type
transceptors	I-protein_type
from	O
S	B-species
.	I-species
cerevisiae	I-species
and	O
C	B-species
.	I-species
albicans	I-species
.	O

The	O
most	O
striking	O
difference	O
is	O
the	O
fact	O
that	O
the	O
Mep2	B-protein_type
proteins	I-protein_type
have	O
closed	B-protein_state
conformations	O
.	O

The	O
putative	O
phosphorylation	B-site
site	I-site
is	O
solvent	B-protein_state
accessible	I-protein_state
and	O
located	O
in	O
a	O
negatively	B-site
charged	I-site
pocket	I-site
∼	O
30	O
Å	O
away	O
from	O
the	O
channel	B-site
exit	I-site
.	O

The	O
channels	B-site
of	O
phosphorylation	B-protein_state
-	I-protein_state
mimicking	I-protein_state
mutants	I-protein_state
of	O
C	B-species
.	I-species
albicans	I-species
Mep2	B-protein
are	O
still	O
closed	B-protein_state
but	O
show	O
large	O
conformational	O
changes	O
within	O
a	O
conserved	B-protein_state
part	O
of	O
the	O
CTR	B-structure_element
.	O

General	O
architecture	O
of	O
Mep2	B-protein_type
ammonium	B-protein_type
transceptors	I-protein_type

Of	O
these	O
,	O
Mep2	B-protein
from	O
C	B-species
.	I-species
albicans	I-species
(	O
CaMep2	B-protein
)	O
showed	O
superior	O
stability	O
in	O
relatively	O
harsh	O
detergents	O
such	O
as	O
nonyl	O
-	O
glucoside	O
,	O
allowing	O
structure	B-experimental_method
determination	I-experimental_method
in	O
two	O
different	O
crystal	B-evidence
forms	I-evidence
to	O
high	O
resolution	O
(	O
up	O
to	O
1	O
.	O
5	O
Å	O
).	O

Despite	O
different	O
crystal	O
packing	O
(	O
Supplementary	O
Table	O
1	O
),	O
the	O
two	O
CaMep2	B-protein
structures	B-evidence
are	O
identical	O
to	O
each	O
other	O
and	O
very	O
similar	O
to	O
ScMep2	B-protein
(	O
Cα	O
r	B-evidence
.	I-evidence
m	I-evidence
.	I-evidence
s	I-evidence
.	I-evidence
d	I-evidence
.	I-evidence

(	O
root	B-evidence
mean	I-evidence
square	I-evidence
deviation	I-evidence
)=	O
0	O
.	O
7	O
Å	O
for	O
434	O
residues	O
),	O
with	O
the	O
main	O
differences	O
confined	O
to	O
the	O
N	O
terminus	O
and	O
the	O
CTR	B-structure_element
(	O
Fig	O
.	O
1	O
).	O

Electron	B-evidence
density	I-evidence
is	O
visible	O
for	O
the	O
entire	O
polypeptide	O
chains	O
,	O
with	O
the	O
exception	O
of	O
the	O
C	O
-	O
terminal	O
43	B-residue_range
(	O
ScMep2	B-protein
)	O
and	O
25	B-residue_range
residues	O
(	O
CaMep2	B-protein
),	O
which	O
are	O
poorly	B-protein_state
conserved	I-protein_state
and	O
presumably	O
disordered	B-protein_state
.	O

Both	O
Mep2	B-protein_type
proteins	I-protein_type
show	O
the	O
archetypal	O
trimeric	B-oligomeric_state
assemblies	O
in	O
which	O
each	O
monomer	B-oligomeric_state
consists	O
of	O
11	O
TM	B-structure_element
helices	I-structure_element
surrounding	O
a	O
central	B-structure_element
pore	I-structure_element
.	O

Important	O
functional	O
features	O
such	O
as	O
the	O
extracellular	O
ammonium	B-site
binding	I-site
site	I-site
,	O
the	O
Phe	B-site
gate	I-site
and	O
the	O
twin	B-structure_element
-	I-structure_element
His	I-structure_element
motif	I-structure_element
within	O
the	O
hydrophobic	B-site
channel	I-site
are	O
all	O
very	O
similar	O
to	O
those	O
present	O
in	O
the	O
bacterial	B-taxonomy_domain
transporters	B-protein_type
and	O
RhCG	B-protein
.	O

In	O
the	O
remainder	O
of	O
the	O
manuscript	O
,	O
we	O
will	O
specifically	O
discuss	O
CaMep2	B-protein
due	O
to	O
the	O
superior	O
resolution	O
of	O
the	O
structure	B-evidence
.	O

Unless	O
specifically	O
stated	O
,	O
the	O
drawn	O
conclusions	O
also	O
apply	O
to	O
ScMep2	B-protein
.	O

While	O
the	O
overall	O
architecture	O
of	O
Mep2	B-protein
is	O
similar	O
to	O
that	O
of	O
the	O
prokaryotic	B-taxonomy_domain
transporters	B-protein_type
(	O
Cα	O
r	B-evidence
.	I-evidence
m	I-evidence
.	I-evidence
s	I-evidence
.	I-evidence
d	I-evidence
.	I-evidence
with	O
Amt	B-protein
-	I-protein
1	I-protein
=	O
1	O
.	O
4	O
Å	O
for	O
361	O
residues	O
),	O
there	O
are	O
large	O
differences	O
within	O
the	O
N	O
terminus	O
,	O
intracellular	B-structure_element
loops	I-structure_element
(	O
ICLs	B-structure_element
)	O
ICL1	B-structure_element
and	O
ICL3	B-structure_element
,	O
and	O
the	O
CTR	B-structure_element
.	O

Moreover	O
,	O
the	O
N	O
terminus	O
of	O
one	O
monomer	B-oligomeric_state
interacts	O
with	O
the	O
extended	O
extracellular	B-structure_element
loop	I-structure_element
ECL5	B-structure_element
of	O
a	O
neighbouring	O
monomer	B-oligomeric_state
.	O

Together	O
with	O
additional	O
,	O
smaller	O
differences	O
in	O
other	O
extracellular	B-structure_element
loops	I-structure_element
,	O
these	O
changes	O
generate	O
a	O
distinct	O
vestibule	B-structure_element
leading	O
to	O
the	O
ammonium	B-site
binding	I-site
site	I-site
that	O
is	O
much	O
more	O
pronounced	O
than	O
in	O
the	O
bacterial	B-taxonomy_domain
proteins	O
.	O

The	O
N	O
-	O
terminal	O
vestibule	B-structure_element
and	O
the	O
resulting	O
inter	O
-	O
monomer	B-oligomeric_state
interactions	O
likely	O
increase	O
the	O
stability	O
of	O
the	O
Mep2	B-protein
trimer	B-oligomeric_state
,	O
in	O
support	O
of	O
data	O
for	O
plant	B-taxonomy_domain
AMT	B-protein_type
proteins	I-protein_type
.	O

However	O
,	O
given	O
that	O
an	O
N	O
-	O
terminal	O
deletion	B-protein_state
mutant	I-protein_state
(	O
2	B-mutant
-	I-mutant
27Δ	I-mutant
)	O
grows	O
as	O
well	O
as	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
WT	B-protein_state
)	O
Mep2	B-protein
on	O
minimal	O
ammonium	B-chemical
medium	O
(	O
Fig	O
.	O
3	O
and	O
Supplementary	O
Fig	O
.	O
1	O
),	O
the	O
importance	O
of	O
the	O
N	O
terminus	O
for	O
Mep2	B-protein
activity	O
is	O
not	O
clear	O
.	O

In	O
the	O
vicinity	O
of	O
the	O
Mep2	B-protein
channel	B-site
exit	I-site
,	O
the	O
cytoplasmic	O
end	O
of	O
TM2	B-structure_element
has	O
unwound	O
,	O
generating	O
a	O
longer	O
ICL1	B-structure_element
even	O
though	O
there	O
are	O
no	O
insertions	O
in	O
this	O
region	O
compared	O
to	O
the	O
bacterial	B-taxonomy_domain
proteins	O
(	O
Figs	O
2	O
and	O
4	O
).	O

The	O
largest	O
backbone	O
movements	O
of	O
equivalent	O
residues	O
within	O
ICL1	B-structure_element
are	O
∼	O
10	O
Å	O
,	O
markedly	O
affecting	O
the	O
conserved	B-protein_state
basic	B-protein_state
RxK	B-structure_element
motif	I-structure_element
(	O
Fig	O
.	O
4	O
).	O

In	O
addition	O
to	O
changing	O
the	O
RxK	B-structure_element
motif	I-structure_element
,	O
the	O
movement	O
of	O
ICL1	B-structure_element
has	O
another	O
,	O
crucial	O
functional	O
consequence	O
.	O

At	O
the	O
C	O
-	O
terminal	O
end	O
of	O
TM1	B-structure_element
,	O
the	O
side	O
-	O
chain	O
hydroxyl	O
group	O
of	O
the	O
relatively	B-protein_state
conserved	I-protein_state
Tyr49	B-residue_name_number
(	O
Tyr53	B-residue_name_number
in	O
ScMep2	B-protein
)	O
makes	O
a	O
strong	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
the	O
ɛ2	O
nitrogen	O
atom	O
of	O
the	O
absolutely	B-protein_state
conserved	I-protein_state
His342	B-residue_name_number
of	O
the	O
twin	B-structure_element
-	I-structure_element
His	I-structure_element
motif	I-structure_element
(	O
His348	B-residue_name_number
in	O
ScMep2	B-protein
),	O
closing	O
the	O
channel	B-site
(	O
Figs	O
4	O
and	O
5	O
).	O

In	O
bacterial	B-taxonomy_domain
Amt	B-protein_type
proteins	I-protein_type
,	O
this	O
Tyr	B-residue_name
side	O
chain	O
is	O
rotated	O
∼	O
4	O
Å	O
away	O
as	O
a	O
result	O
of	O
the	O
different	O
conformation	O
of	O
TM1	B-structure_element
,	O
leaving	O
the	O
channel	B-site
open	B-protein_state
and	O
the	O
histidine	B-residue_name
available	O
for	O
its	O
putative	O
role	O
in	O
substrate	O
transport	O
(	O
Supplementary	O
Fig	O
.	O
2	O
).	O

Compared	O
with	O
ICL1	B-structure_element
,	O
the	O
backbone	O
conformational	O
changes	O
observed	O
for	O
the	O
neighbouring	O
ICL2	B-structure_element
are	O
smaller	O
,	O
but	O
large	O
shifts	O
are	O
nevertheless	O
observed	O
for	O
the	O
conserved	B-protein_state
residues	O
Glu140	B-residue_name_number
and	O
Arg141	B-residue_name_number
(	O
Fig	O
.	O
4	O
).	O

Finally	O
,	O
the	O
important	O
ICL3	B-structure_element
linking	O
the	O
pseudo	B-structure_element
-	I-structure_element
symmetrical	I-structure_element
halves	I-structure_element
(	O
TM1	B-structure_element
-	I-structure_element
5	I-structure_element
and	O
TM6	B-structure_element
-	I-structure_element
10	I-structure_element
)	O
of	O
the	O
transporter	B-protein_type
is	O
also	O
shifted	O
up	O
to	O
∼	O
10	O
Å	O
and	O
forms	O
an	O
additional	O
barrier	O
that	O
closes	O
the	O
channel	B-site
on	O
the	O
cytoplasmic	O
side	O
(	O
Fig	O
.	O
5	O
).	O

This	O
two	O
-	O
tier	O
channel	B-structure_element
block	I-structure_element
likely	O
ensures	O
that	O
very	O
little	O
ammonium	B-chemical
transport	O
will	O
take	O
place	O
under	O
nitrogen	B-chemical
-	O
sufficient	O
conditions	O
.	O

The	O
closed	B-protein_state
state	O
of	O
the	O
channel	B-site
might	O
also	O
explain	O
why	O
no	B-evidence
density	I-evidence
,	O
which	O
could	O
correspond	O
to	O
ammonium	B-chemical
(	O
or	O
water	B-chemical
),	O
is	O
observed	O
in	O
the	O
hydrophobic	O
part	O
of	O
the	O
Mep2	B-protein
channel	B-site
close	O
to	O
the	O
twin	B-structure_element
-	I-structure_element
His	I-structure_element
motif	I-structure_element
.	O

Significantly	O
,	O
this	O
is	O
also	O
true	O
for	O
ScMep2	B-protein
,	O
which	O
was	O
crystallized	B-experimental_method
in	O
the	O
presence	O
of	O
0	O
.	O
2	O
M	O
ammonium	B-chemical
ions	O
(	O
see	O
Methods	O
section	O
).	O

The	O
final	O
region	O
in	O
Mep2	B-protein
that	O
shows	O
large	O
differences	O
compared	O
with	O
the	O
bacterial	B-taxonomy_domain
transporters	B-protein_type
is	O
the	O
CTR	B-structure_element
.	O

By	O
contrast	O
,	O
in	O
the	O
structures	B-evidence
of	O
bacterial	B-taxonomy_domain
proteins	O
,	O
the	O
CTR	B-structure_element
is	O
docked	O
tightly	O
onto	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
half	I-structure_element
of	O
the	O
transporters	B-protein_type
(	O
corresponding	O
to	O
TM1	B-structure_element
-	I-structure_element
5	I-structure_element
),	O
resulting	O
in	O
a	O
more	O
compact	B-protein_state
structure	B-evidence
.	O

In	O
Amt	B-protein
-	I-protein
1	I-protein
and	O
other	O
bacterial	B-taxonomy_domain
ammonium	B-protein_type
transporters	I-protein_type
,	O
these	O
CTR	B-structure_element
residues	O
interact	O
with	O
residues	O
within	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
half	I-structure_element
of	O
the	O
protein	O
.	O

Similar	O
interactions	O
were	O
also	O
modelled	B-experimental_method
in	O
the	O
active	B-protein_state
,	O
non	B-protein_state
-	I-protein_state
phosphorylated	I-protein_state
plant	B-taxonomy_domain
AtAmt	B-protein
-	I-protein
1	I-protein
;	I-protein
1	I-protein
structure	B-evidence
(	O
for	O
example	O
,	O
Y467	B-residue_name_number
-	O
H239	B-residue_name_number
and	O
D458	B-residue_name_number
-	O
K71	B-residue_name_number
).	O

Also	O
noteworthy	O
is	O
Asp381	B-residue_name_number
,	O
the	O
side	O
chain	O
of	O
which	O
interacts	O
strongly	O
with	O
the	O
positive	O
dipole	O
on	O
the	O
N	O
-	O
terminal	O
end	O
of	O
TM2	B-structure_element
.	O

This	O
interaction	O
in	O
the	O
centre	O
of	O
the	O
protein	O
may	O
be	O
particularly	O
important	O
to	O
stabilize	O
the	O
open	B-protein_state
conformations	O
of	O
ammonium	B-protein_type
transporters	I-protein_type
.	O

In	O
the	O
Mep2	B-protein
structures	B-evidence
,	O
none	O
of	O
the	O
interactions	O
mentioned	O
above	O
are	O
present	O
.	O

Phosphorylation	B-site
target	I-site
site	I-site
is	O
at	O
the	O
periphery	O
of	O
Mep2	B-protein

Recently	O
Boeckstaens	O
et	O
al	O
.	O
provided	O
evidence	O
that	O
Ser457	B-residue_name_number
in	O
ScMep2	B-protein
(	O
corresponding	O
to	O
Ser453	B-residue_name_number
in	O
CaMep2	B-protein
)	O
is	O
phosphorylated	B-protein_state
by	O
the	O
TORC1	B-protein_type
effector	I-protein_type
kinase	I-protein_type
Npr1	B-protein
under	O
nitrogen	B-chemical
-	O
limiting	O
conditions	O
.	O

In	O
the	O
absence	B-protein_state
of	I-protein_state
Npr1	B-protein
,	O
plasmid	B-experimental_method
-	I-experimental_method
encoded	I-experimental_method
WT	B-protein_state
Mep2	B-protein
in	O
a	O
S	B-species
.	I-species
cerevisiae	I-species
mep1	B-mutant
-	I-mutant
3Δ	I-mutant
strain	O
(	O
triple	B-mutant
mepΔ	I-mutant
)	O
does	O
not	O
allow	O
growth	O
on	O
low	O
concentrations	O
of	O
ammonium	B-chemical
,	O
suggesting	O
that	O
the	O
transporter	B-protein_type
is	O
inactive	B-protein_state
(	O
Fig	O
.	O
3	O
and	O
Supplementary	O
Fig	O
.	O
1	O
).	O

Collectively	O
,	O
these	O
data	O
suggest	O
that	O
phosphorylation	B-ptm
of	O
Ser457	B-residue_name_number
opens	O
the	O
Mep2	B-protein
channel	B-site
to	O
allow	O
ammonium	B-chemical
uptake	O
.	O

Ser457	B-residue_name_number
is	O
located	O
in	O
a	O
part	O
of	O
the	O
CTR	B-structure_element
that	O
is	O
conserved	B-protein_state
in	O
a	O
subgroup	O
of	O
Mep2	B-protein_type
proteins	I-protein_type
,	O
but	O
which	O
is	O
not	O
present	O
in	O
bacterial	B-taxonomy_domain
proteins	O
(	O
Fig	O
.	O
2	O
).	O

Where	O
is	O
the	O
AI	B-structure_element
region	I-structure_element
and	O
the	O
Npr1	B-protein
phosphorylation	B-site
site	I-site
located	O
?	O
Our	O
structures	B-evidence
reveal	O
that	O
surprisingly	O
,	O
the	O
AI	B-structure_element
region	I-structure_element
is	O
folded	O
back	O
onto	O
the	O
CTR	B-structure_element
and	O
is	O
not	O
located	O
near	O
the	O
centre	O
of	O
the	O
trimer	B-oligomeric_state
as	O
expected	O
from	O
the	O
bacterial	B-taxonomy_domain
structures	B-evidence
(	O
Fig	O
.	O
4	O
).	O

The	O
AI	B-structure_element
regions	I-structure_element
have	O
very	O
similar	O
conformations	O
in	O
CaMep2	B-protein
and	O
ScMep2	B-protein
,	O
despite	O
considerable	O
differences	O
in	O
the	O
rest	O
of	O
the	O
CTR	B-structure_element
(	O
Fig	O
.	O
6	O
).	O

Strikingly	O
,	O
the	O
Npr1	B-site
target	I-site
serine	I-site
residue	O
is	O
located	O
at	O
the	O
periphery	O
of	O
the	O
trimer	B-oligomeric_state
,	O
far	O
away	O
(∼	O
30	O
Å	O
)	O
from	O
any	O
channel	B-site
exit	I-site
(	O
Fig	O
.	O
6	O
).	O

This	O
makes	O
sense	O
since	O
the	O
proteins	O
were	O
expressed	O
in	O
rich	O
medium	O
and	O
confirms	O
the	O
recent	O
suggestion	O
by	O
Boeckstaens	O
et	O
al	O
.	O
that	O
the	O
non	B-protein_state
-	I-protein_state
phosphorylated	I-protein_state
form	O
of	O
Mep2	B-protein
corresponds	O
to	O
the	O
inactive	B-protein_state
state	O
.	O

For	O
ScMep2	B-protein
,	O
Ser457	B-residue_name_number
is	O
the	O
most	O
C	O
-	O
terminal	O
residue	O
for	O
which	O
electron	B-evidence
density	I-evidence
is	O
visible	O
,	O
indicating	O
that	O
the	O
region	O
beyond	O
Ser457	B-residue_name_number
is	O
disordered	B-protein_state
.	O

In	O
CaMep2	B-protein
,	O
the	O
visible	O
part	O
of	O
the	O
sequence	O
extends	O
for	O
two	O
residues	O
beyond	O
Ser453	B-residue_name_number
(	O
Fig	O
.	O
6	O
).	O

Interestingly	O
,	O
a	O
ScMep2	B-protein
457Δ	B-mutant
truncation	B-protein_state
mutant	I-protein_state
in	O
which	O
a	O
His	O
-	O
tag	O
directly	O
follows	O
Ser457	B-residue_name_number
is	O
highly	O
expressed	O
but	O
has	O
low	B-protein_state
activity	I-protein_state
(	O
Fig	O
.	O
3	O
and	O
Supplementary	O
Fig	O
.	O
1b	O
),	O
suggesting	O
that	O
the	O
His	O
-	O
tag	O
interferes	O
with	O
phosphorylation	B-ptm
by	O
Npr1	B-protein
.	O

Given	O
that	O
Ser457	B-residue_name_number
/	O
453	B-residue_number
is	O
far	O
from	O
any	O
channel	B-site
exit	I-site
(	O
Fig	O
.	O
6	O
),	O
the	O
crucial	O
question	O
is	O
how	O
phosphorylation	B-ptm
opens	O
the	O
Mep2	B-protein
channel	B-site
to	O
generate	O
an	O
active	B-protein_state
transporter	B-protein_type
.	O

Boeckstaens	O
et	O
al	O
.	O
proposed	O
that	O
phosphorylation	B-ptm
does	O
not	O
affect	O
channel	O
activity	O
directly	O
,	O
but	O
instead	O
relieves	O
inhibition	O
by	O
the	O
AI	B-structure_element
region	I-structure_element
.	O

We	O
obtained	O
a	O
similar	O
result	O
for	O
ammonium	O
uptake	O
by	O
the	O
446Δ	B-mutant
mutant	B-protein_state
(	O
Fig	O
.	O
3	O
),	O
supporting	O
the	O
data	O
from	O
Marini	O
et	O
al	O
.	O
We	O
then	O
constructed	B-experimental_method
and	I-experimental_method
purified	I-experimental_method
the	O
analogous	O
CaMep2	B-protein
442Δ	B-mutant
truncation	B-protein_state
mutant	I-protein_state
and	O
determined	B-experimental_method
the	O
crystal	B-evidence
structure	I-evidence
using	O
data	O
to	O
3	O
.	O
4	O
Å	O
resolution	O
.	O

The	O
structure	B-evidence
shows	O
that	O
removal	B-experimental_method
of	I-experimental_method
the	O
AI	B-structure_element
region	I-structure_element
markedly	O
increases	O
the	O
dynamics	O
of	O
the	O
cytoplasmic	B-structure_element
parts	I-structure_element
of	O
the	O
transporter	B-protein_type
.	O

Density	B-evidence
for	O
ICL3	B-structure_element
and	O
the	O
CTR	B-structure_element
beyond	O
residue	O
Arg415	B-residue_name_number
is	O
missing	O
in	O
the	O
442Δ	B-mutant
mutant	B-protein_state
,	O
and	O
the	O
density	B-evidence
for	O
the	O
other	O
ICLs	B-structure_element
including	O
ICL1	B-structure_element
is	O
generally	O
poor	O
with	O
visible	O
parts	O
of	O
the	O
structure	B-evidence
having	O
high	O
B	O
-	O
factors	O
(	O
Fig	O
.	O
7	O
).	O

Why	O
then	O
does	O
this	O
mutant	O
appear	O
to	O
be	O
constitutively	O
active	B-protein_state
?	O
We	O
propose	O
two	O
possibilities	O
.	O

The	O
first	O
one	O
is	O
that	O
the	O
open	B-protein_state
state	O
is	O
disfavoured	O
by	O
crystallization	B-experimental_method
because	O
of	O
lower	O
stability	O
or	O
due	O
to	O
crystal	O
packing	O
constraints	O
.	O

The	O
second	O
possibility	O
is	O
that	O
the	O
Tyr	B-site
–	I-site
His	I-site
hydrogen	I-site
bond	I-site
has	O
to	O
be	O
disrupted	O
by	O
the	O
incoming	O
substrate	O
to	O
open	B-protein_state
the	O
channel	O
.	O

The	O
latter	O
model	O
would	O
fit	O
well	O
with	O
the	O
NH3	B-chemical
/	O
H	B-chemical
+	I-chemical
symport	O
model	O
in	O
which	O
the	O
proton	O
is	O
relayed	O
by	O
the	O
twin	B-structure_element
-	I-structure_element
His	I-structure_element
motif	I-structure_element
.	O

The	O
importance	O
of	O
the	O
Tyr	B-site
–	I-site
His	I-site
hydrogen	I-site
bond	I-site
is	O
underscored	O
by	O
the	O
fact	O
that	O
its	O
removal	B-experimental_method
in	O
the	O
ScMep2	B-protein
Y53A	B-mutant
mutant	B-protein_state
results	O
in	O
a	O
constitutively	B-protein_state
active	I-protein_state
transporter	B-protein_type
(	O
Fig	O
.	O
3	O
).	O

Phosphorylation	B-ptm
causes	O
a	O
conformational	O
change	O
in	O
the	O
CTR	B-structure_element

Do	O
the	O
Mep2	B-protein
structures	B-evidence
provide	O
any	O
clues	O
regarding	O
the	O
potential	O
effect	O
of	O
phosphorylation	B-ptm
?	O

The	O
side	O
-	O
chain	O
hydroxyl	O
of	O
Ser457	B-residue_name_number
/	O
453	B-residue_number
is	O
located	O
in	O
a	O
well	O
-	O
defined	O
electronegative	B-site
pocket	I-site
that	O
is	O
solvent	B-protein_state
accessible	I-protein_state
(	O
Fig	O
.	O
6	O
).	O

The	O
closest	O
atoms	O
to	O
the	O
serine	B-residue_name
hydroxyl	O
group	O
are	O
the	O
backbone	O
carbonyl	O
atoms	O
of	O
Asp419	B-residue_name_number
,	O
Glu420	B-residue_name_number
and	O
Glu421	B-residue_name_number
,	O
which	O
are	O
3	O
–	O
4	O
Å	O
away	O
.	O

To	O
test	O
this	O
hypothesis	O
,	O
we	O
determined	B-experimental_method
the	O
structure	B-evidence
of	O
the	O
phosphorylation	B-protein_state
-	I-protein_state
mimicking	I-protein_state
R452D	B-mutant
/	I-mutant
S453D	I-mutant
protein	O
(	O
hereafter	O
termed	O
‘	O
DD	B-mutant
mutant	I-mutant
'),	O
using	O
data	O
to	O
a	O
resolution	O
of	O
2	O
.	O
4	O
Å	O
.	O
The	O
additional	B-experimental_method
mutation	I-experimental_method
of	I-experimental_method
the	O
arginine	B-residue_name
preceding	O
the	O
phosphorylation	B-site
site	I-site
was	O
introduced	O
(	O
i	O
)	O
to	O
increase	O
the	O
negative	O
charge	O
density	O
and	O
make	O
it	O
more	O
comparable	O
to	O
a	O
phosphate	B-chemical
at	O
neutral	O
pH	O
,	O
and	O
(	O
ii	O
)	O
to	O
further	O
destabilize	O
the	O
interactions	O
of	O
the	O
AI	B-structure_element
region	I-structure_element
with	O
the	O
main	B-structure_element
body	I-structure_element
of	O
the	O
transporter	B-protein_type
(	O
Fig	O
.	O
6	O
).	O

The	O
ammonium	B-chemical
uptake	O
activity	O
of	O
the	O
S	B-species
.	I-species
cerevisiae	I-species
version	O
of	O
the	O
DD	B-mutant
mutant	I-mutant
is	O
the	O
same	O
as	O
that	O
of	O
WT	B-protein_state
Mep2	B-protein
and	O
the	O
S453D	B-mutant
mutant	B-protein_state
,	O
indicating	O
that	O
the	O
mutations	O
do	O
not	O
affect	O
transporter	O
functionality	O
in	O
the	O
triple	B-mutant
mepΔ	I-mutant
background	O
(	O
Fig	O
.	O
3	O
).	O

Unexpectedly	O
,	O
the	O
AI	B-structure_element
segment	I-structure_element
containing	O
the	O
mutated	O
residues	O
has	O
only	O
undergone	O
a	O
slight	O
shift	O
compared	O
with	O
the	O
WT	B-protein_state
protein	O
(	O
Fig	O
.	O
8	O
and	O
Supplementary	O
Fig	O
.	O
3	O
).	O

In	O
addition	O
,	O
residues	O
Glu420	B-residue_range
-	I-residue_range
Leu423	I-residue_range
including	O
Glu421	B-residue_name_number
of	O
the	O
ExxGxD	B-structure_element
motif	I-structure_element
are	O
now	O
disordered	B-protein_state
(	O
Fig	O
.	O
8	O
and	O
Supplementary	O
Fig	O
.	O
3	O
).	O

To	O
exclude	O
the	O
possibility	O
that	O
the	O
additional	O
R452D	B-mutant
mutation	O
is	O
responsible	O
for	O
the	O
observed	O
changes	O
,	O
we	O
also	O
determined	B-experimental_method
the	O
structure	B-evidence
of	O
the	O
‘	O
single	B-mutant
D	I-mutant
'	O
S453D	B-mutant
mutant	B-protein_state
.	O

To	O
supplement	O
the	O
crystal	B-evidence
structures	I-evidence
,	O
we	O
also	O
performed	O
modelling	B-experimental_method
and	O
MD	B-experimental_method
studies	O
of	O
WT	B-protein_state
CaMep2	B-protein
,	O
the	O
DD	B-mutant
mutant	I-mutant
and	O
phosphorylated	B-protein_state
protein	O
(	O
S453J	B-mutant
).	O

In	O
the	O
WT	B-protein_state
structure	B-evidence
,	O
the	O
acidic	O
residues	O
Asp419	B-residue_name_number
,	O
Glu420	B-residue_name_number
and	O
Glu421	B-residue_name_number
are	O
within	O
hydrogen	B-bond_interaction
bonding	I-bond_interaction
distance	O
of	O
Ser453	B-residue_name_number
.	O

The	O
protein	O
backbone	O
has	O
an	O
average	O
r	B-evidence
.	I-evidence
m	I-evidence
.	I-evidence
s	I-evidence
.	I-evidence
d	I-evidence
.	I-evidence
of	O
only	O
∼	O
3	O
Å	O
during	O
the	O
200	O
-	O
ns	O
simulation	B-experimental_method
,	O
indicating	O
that	O
the	O
protein	O
is	O
stable	B-protein_state
.	O

There	O
is	O
flexibility	O
in	O
the	O
side	O
chains	O
of	O
the	O
acidic	O
residues	O
so	O
that	O
they	O
are	O
able	O
to	O
form	O
stable	B-protein_state
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
Ser453	B-residue_name_number
.	O

In	O
particular	O
,	O
persistent	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
are	O
observed	O
between	O
the	O
Ser453	B-residue_name_number
hydroxyl	O
group	O
and	O
the	O
acidic	O
group	O
of	O
Glu420	B-residue_name_number
,	O
and	O
also	O
between	O
the	O
amine	O
group	O
of	O
Ser453	B-residue_name_number
and	O
the	O
backbone	O
carbonyl	O
of	O
Glu420	B-residue_name_number
(	O
Supplementary	O
Fig	O
.	O
5	O
).	O

The	O
DD	B-mutant
mutant	I-mutant
is	O
also	O
stable	B-protein_state
during	O
the	O
simulations	B-experimental_method
,	O
but	O
the	O
average	O
backbone	O
r	B-evidence
.	I-evidence
m	I-evidence
.	I-evidence
s	I-evidence
.	I-evidence
d	I-evidence
of	O
∼	O
3	O
.	O
6	O
Å	O
suggests	O
slightly	O
more	O
conformational	O
flexibility	O
than	O
WT	B-protein_state
.	O

For	O
example	O
,	O
the	O
distance	B-evidence
between	O
the	O
Asp453	B-residue_name_number
acidic	O
oxygens	O
and	O
the	O
Glu420	B-residue_name_number
acidic	O
oxygens	O
increases	O
from	O
∼	O
7	O
to	O
>	O
22	O
Å	O
after	O
200	O
ns	O
simulations	B-experimental_method
,	O
and	O
thus	O
these	O
residues	O
are	O
not	O
interacting	O
.	O

The	O
protein	O
is	O
structurally	B-protein_state
stable	I-protein_state
throughout	O
the	O
simulation	B-experimental_method
with	O
little	O
deviation	O
in	O
the	O
other	O
parts	O
of	O
the	O
protein	O
.	O

The	O
movement	O
of	O
the	O
acidic	O
residues	O
away	O
from	O
Arg452	B-residue_name_number
and	O
Sep453	B-residue_name_number
is	O
more	O
pronounced	O
in	O
this	O
simulation	B-experimental_method
in	O
comparison	O
with	O
the	O
movement	O
away	O
from	O
Asp452	B-residue_name_number
and	O
Asp453	B-residue_name_number
in	O
the	O
DD	B-mutant
mutant	I-mutant
.	O

The	O
short	B-structure_element
helix	I-structure_element
formed	O
by	O
residues	O
Leu427	B-residue_range
to	I-residue_range
Asp438	I-residue_range
unravels	O
during	O
the	O
simulations	B-experimental_method
to	O
a	O
disordered	B-protein_state
state	O
.	O

However	O
,	O
the	O
conformational	O
changes	O
for	O
the	O
phosphomimetic	B-mutant
mutants	I-mutant
in	O
the	O
crystals	B-evidence
are	O
confined	O
to	O
the	O
CTR	B-structure_element
(	O
Fig	O
.	O
8	O
),	O
and	O
the	O
channels	B-site
are	O
still	O
closed	B-protein_state
(	O
Supplementary	O
Fig	O
.	O
2	O
).	O

The	O
fact	O
that	O
the	O
S453D	B-mutant
structure	B-evidence
was	O
obtained	O
in	O
the	O
presence	O
of	O
10	O
mM	O
ammonium	B-chemical
ions	O
suggests	O
that	O
the	O
crystallization	B-experimental_method
process	O
favours	O
closed	B-protein_state
states	O
of	O
the	O
Mep2	B-protein
channels	B-site
.	O

Knowledge	O
about	O
ammonium	B-protein_type
transporter	I-protein_type
structure	B-evidence
has	O
been	O
obtained	O
from	O
experimental	O
and	O
theoretical	O
studies	O
on	O
bacterial	B-taxonomy_domain
family	O
members	O
.	O

These	O
efforts	O
have	O
advanced	O
our	O
knowledge	O
considerably	O
but	O
have	O
not	O
yet	O
yielded	O
atomic	O
-	O
level	O
answers	O
to	O
several	O
important	O
mechanistic	O
questions	O
,	O
including	O
how	O
ammonium	B-chemical
transport	O
is	O
regulated	O
in	O
eukaryotes	B-taxonomy_domain
and	O
the	O
mechanism	O
of	O
ammonium	B-chemical
signalling	O
.	O

Interestingly	O
,	O
phosphomimetic	B-mutant
mutations	I-mutant
introduced	O
into	O
one	O
monomer	B-oligomeric_state
inactivate	O
the	O
entire	O
trimer	B-oligomeric_state
,	O
indicating	O
that	O
(	O
i	O
)	O
heterotrimerization	O
occurs	O
and	O
(	O
ii	O
)	O
the	O
CTR	B-structure_element
mediates	O
allosteric	O
regulation	O
of	O
ammonium	B-chemical
transport	O
activity	O
via	O
phosphorylation	B-ptm
.	O

Contrasting	O
with	O
the	O
plant	B-taxonomy_domain
transporters	B-protein_type
,	O
the	O
inactive	B-protein_state
states	O
of	O
Mep2	B-protein_type
proteins	I-protein_type
under	O
conditions	O
of	O
high	O
ammonium	B-chemical
are	O
non	B-protein_state
-	I-protein_state
phosphorylated	I-protein_state
,	O
with	O
channels	B-site
that	O
are	O
closed	B-protein_state
on	O
the	O
cytoplasmic	O
side	O
.	O

The	O
reason	O
why	O
similar	O
transporters	B-protein_type
such	O
as	O
A	B-species
.	I-species
thaliana	I-species
Amt	B-protein
-	I-protein
1	I-protein
;	I-protein
1	I-protein
and	O
Mep2	B-protein
are	O
regulated	O
in	O
opposite	O
ways	O
by	O
phosphorylation	B-ptm
(	O
inactivation	B-protein_state
in	O
plants	B-taxonomy_domain
and	O
activation	B-protein_state
in	O
fungi	B-taxonomy_domain
)	O
is	O
not	O
known	O
.	O

In	O
fungi	B-taxonomy_domain
,	O
preventing	O
ammonium	B-chemical
entry	O
via	O
channel	O
closure	O
in	O
ammonium	B-protein_type
transporters	I-protein_type
would	O
be	O
one	O
way	O
to	O
alleviate	O
ammonium	B-chemical
toxicity	O
,	O
in	O
addition	O
to	O
ammonium	B-chemical
excretion	O
via	O
Ato	B-protein_type
transporters	B-protein_type
and	O
amino	O
-	O
acid	O
secretion	O
.	O

By	O
determining	O
the	O
first	O
structures	B-evidence
of	O
closed	B-protein_state
ammonium	B-protein_type
transporters	I-protein_type
and	O
comparing	B-experimental_method
those	O
structures	B-evidence
with	O
the	O
permanently	B-protein_state
open	I-protein_state
bacterial	B-taxonomy_domain
proteins	O
,	O
we	O
demonstrate	O
that	O
Mep2	B-protein_type
channel	B-site
closure	O
is	O
likely	O
due	O
to	O
movements	O
of	O
the	O
CTR	B-structure_element
and	O
ICL1	B-structure_element
and	O
ICL3	B-structure_element
.	O

More	O
specifically	O
,	O
the	O
close	O
interactions	O
between	O
the	O
CTR	B-structure_element
and	O
ICL1	B-structure_element
/	O
ICL3	B-structure_element
present	O
in	O
open	B-protein_state
transporters	B-protein_type
are	O
disrupted	O
,	O
causing	O
ICL3	B-structure_element
to	O
move	O
outwards	O
and	O
block	O
the	O
channel	B-site
(	O
Figs	O
4	O
and	O
9a	O
).	O

In	O
addition	O
,	O
ICL1	B-structure_element
has	O
shifted	O
inwards	O
to	O
contribute	O
to	O
the	O
channel	B-site
closure	O
by	O
engaging	O
His2	B-residue_name_number
from	O
the	O
twin	B-structure_element
-	I-structure_element
His	I-structure_element
motif	I-structure_element
via	O
hydrogen	B-bond_interaction
bonding	I-bond_interaction
with	O
a	O
highly	B-protein_state
conserved	I-protein_state
tyrosine	B-residue_name
hydroxyl	O
group	O
.	O

Importantly	O
,	O
the	O
structural	B-evidence
similarities	I-evidence
in	O
the	O
TM	B-structure_element
parts	I-structure_element
of	O
Mep2	B-protein
and	O
AfAmt	B-protein
-	I-protein
1	I-protein
(	O
Fig	O
.	O
5a	O
)	O
suggest	O
that	O
channel	B-site
opening	O
/	O
closure	O
does	O
not	O
require	O
substantial	O
changes	O
in	O
the	O
residues	O
lining	O
the	O
channel	B-site
.	O

How	O
exactly	O
the	O
channel	B-site
opens	O
and	O
whether	O
opening	O
is	O
intra	O
-	O
monomeric	O
are	O
still	O
open	B-protein_state
questions	O
;	O
it	O
is	O
possible	O
that	O
the	O
change	O
in	O
the	O
CTR	B-structure_element
may	O
disrupt	O
its	O
interactions	O
with	O
ICL3	B-structure_element
of	O
the	O
neighbouring	O
monomer	B-oligomeric_state
(	O
Fig	O
.	O
9b	O
),	O
which	O
could	O
result	O
in	O
opening	O
of	O
the	O
neighbouring	O
channel	B-site
via	O
inward	O
movement	O
of	O
its	O
ICL3	B-structure_element
.	O

Owing	O
to	O
the	O
crosstalk	O
between	O
monomers	B-oligomeric_state
,	O
a	O
single	O
phosphorylation	B-ptm
event	O
might	O
lead	O
to	O
opening	O
of	O
the	O
entire	O
trimer	B-oligomeric_state
,	O
although	O
this	O
has	O
not	O
yet	O
been	O
tested	O
(	O
Fig	O
.	O
9b	O
).	O

Whether	O
or	O
not	O
Mep2	B-protein_type
channel	B-site
opening	O
requires	O
,	O
in	O
addition	O
to	O
phosphorylation	B-ptm
,	O
disruption	O
of	O
the	O
Tyr	B-site
–	I-site
His2	I-site
interaction	I-site
by	O
the	O
ammonium	B-chemical
substrate	O
is	O
not	O
yet	O
clear	O
.	O

Is	O
our	O
model	O
for	O
opening	O
and	O
closing	O
of	O
Mep2	B-protein
channels	B-site
valid	O
for	O
other	O
eukaryotic	B-taxonomy_domain
ammonium	B-protein_type
transporters	I-protein_type
?	O
Our	O
structural	B-evidence
data	I-evidence
support	O
previous	O
studies	O
and	O
clarify	O
the	O
central	O
role	O
of	O
the	O
CTR	B-structure_element
and	O
cytoplasmic	B-structure_element
loops	I-structure_element
in	O
the	O
transition	O
between	O
closed	B-protein_state
and	O
open	B-protein_state
states	O
.	O

Nevertheless	O
,	O
given	O
the	O
central	O
role	O
of	O
absolutely	B-protein_state
conserved	I-protein_state
residues	O
within	O
the	O
ICL1	B-site
-	I-site
ICL3	I-site
-	I-site
CTR	I-site
interaction	I-site
network	I-site
(	O
Fig	O
.	O
4	O
),	O
we	O
propose	O
that	O
the	O
structural	O
basics	O
of	O
fungal	B-taxonomy_domain
ammonium	B-chemical
transporter	O
activation	O
are	O
conserved	B-protein_state
.	O

The	O
fact	O
that	O
Mep2	B-protein_type
orthologues	O
of	O
distantly	O
related	O
fungi	B-taxonomy_domain
are	O
fully	O
functional	O
in	O
ammonium	B-chemical
transport	O
and	O
signalling	O
in	O
S	B-species
.	I-species
cerevisiae	I-species
supports	O
this	O
notion	O
.	O

It	O
should	O
also	O
be	O
noted	O
that	O
the	O
tyrosine	B-residue_name
residue	O
interacting	O
with	O
His2	B-residue_name_number
is	O
highly	B-protein_state
conserved	I-protein_state
in	O
fungal	B-taxonomy_domain
Mep2	B-protein_type
orthologues	O
,	O
suggesting	O
that	O
the	O
Tyr	B-site
–	I-site
His2	I-site
hydrogen	I-site
bond	I-site
might	O
be	O
a	O
general	O
way	O
to	O
close	B-protein_state
Mep2	B-protein_type
proteins	I-protein_type
.	O

With	O
regards	O
to	O
plant	B-taxonomy_domain
AMTs	B-protein_type
,	O
it	O
has	O
been	O
proposed	O
that	O
phosphorylation	B-ptm
at	O
T460	B-residue_name_number
generates	O
conformational	O
changes	O
that	O
would	O
close	O
the	O
neighbouring	O
pore	B-site
via	O
the	O
C	B-structure_element
terminus	I-structure_element
.	O

This	O
assumption	O
was	O
based	O
partly	O
on	O
a	O
homology	B-experimental_method
model	I-experimental_method
for	O
Amt	B-protein
-	I-protein
1	I-protein
;	I-protein
1	I-protein
based	O
on	O
the	O
(	O
open	B-protein_state
)	O
archaebacterial	B-taxonomy_domain
AfAmt	B-protein
-	I-protein
1	I-protein
structure	B-evidence
,	O
which	O
suggested	O
that	O
the	O
C	B-structure_element
terminus	I-structure_element
of	O
Amt	B-protein
-	I-protein
1	I-protein
;	I-protein
1	I-protein
would	O
extend	O
further	O
to	O
the	O
neighbouring	O
monomer	B-oligomeric_state
.	O

Based	O
on	O
the	O
available	O
structural	B-evidence
information	I-evidence
,	O
we	O
consider	O
it	O
more	O
likely	O
that	O
phosphorylation	O
-	O
mediated	O
pore	O
closure	O
in	O
Amt	B-protein
-	I-protein
1	I-protein
;	I-protein
1	I-protein
is	O
intra	O
-	O
monomeric	O
,	O
via	O
disruption	O
of	O
the	O
interactions	O
between	O
the	O
CTR	B-structure_element
and	O
ICL1	B-structure_element
/	O
ICL3	B-structure_element
(	O
for	O
example	O
,	O
Y467	B-residue_name_number
-	O
H239	B-residue_name_number
and	O
D458	B-residue_name_number
-	O
K71	B-residue_name_number
).	O

There	O
is	O
generally	O
no	O
equivalent	O
for	O
CaMep2	B-protein
Tyr49	B-residue_name_number
in	O
plant	B-taxonomy_domain
AMTs	B-protein_type
,	O
indicating	O
that	O
a	O
Tyr	B-site
–	I-site
His2	I-site
hydrogen	I-site
bond	I-site
as	O
observed	O
in	O
Mep2	B-protein
may	O
not	O
contribute	O
to	O
the	O
closed	B-protein_state
state	O
in	O
plant	B-taxonomy_domain
transporters	B-protein_type
.	O

We	O
propose	O
that	O
intra	B-site
-	I-site
monomeric	I-site
CTR	I-site
-	I-site
ICL1	I-site
/	I-site
ICL3	I-site
interactions	I-site
lie	O
at	O
the	O
basis	O
of	O
regulation	O
of	O
both	O
fungal	B-taxonomy_domain
and	O
plant	B-taxonomy_domain
ammonium	B-protein_type
transporters	I-protein_type
;	O
close	O
interactions	O
generate	O
open	B-protein_state
channels	B-site
,	O
whereas	O
the	O
lack	B-protein_state
of	I-protein_state
‘	O
intra	O
-'	O
interactions	O
leads	O
to	O
inactive	B-protein_state
states	O
.	O

The	O
need	O
to	O
regulate	O
in	O
opposite	O
ways	O
may	O
be	O
the	O
reason	O
why	O
the	O
phosphorylation	B-site
sites	I-site
are	O
in	O
different	O
parts	O
of	O
the	O
CTR	B-structure_element
,	O
that	O
is	O
,	O
centrally	O
located	O
close	O
to	O
the	O
ExxGxD	B-structure_element
motif	I-structure_element
in	O
AMTs	B-protein_type
and	O
peripherally	O
in	O
Mep2	B-protein
.	O

Our	O
model	O
also	O
provides	O
an	O
explanation	O
for	O
the	O
observation	O
that	O
certain	B-mutant
mutations	I-mutant
within	O
the	O
CTR	B-structure_element
completely	O
abolish	O
transport	O
activity	O
.	O

An	O
example	O
of	O
an	O
inactivating	O
residue	O
is	O
the	O
glycine	B-residue_name
of	O
the	O
ExxGxD	B-structure_element
motif	I-structure_element
of	O
the	O
CTR	B-structure_element
.	O

Mutation	B-experimental_method
of	O
this	O
residue	O
(	O
G393	B-residue_name_number
in	O
EcAmtB	B-protein
;	O
G456	B-residue_name_number
in	O
AtAmt	B-protein
-	I-protein
1	I-protein
;	I-protein
1	I-protein
)	O
inactivates	O
transporters	B-protein_type
as	O
diverse	O
as	O
Escherichia	B-species
coli	I-species
AmtB	B-protein
and	O
A	B-species
.	I-species
thaliana	I-species
Amt	B-protein
-	I-protein
1	I-protein
;	I-protein
1	I-protein
(	O
refs	O
).	O

Regulation	O
and	O
modulation	O
of	O
membrane	O
transport	O
by	O
phosphorylation	B-ptm
is	O
known	O
to	O
occur	O
in	O
,	O
for	O
example	O
,	O
aquaporins	B-protein_type
and	O
urea	B-protein_type
transporters	I-protein_type
,	O
and	O
is	O
likely	O
to	O
be	O
a	O
common	O
theme	O
for	O
eukaryotic	B-taxonomy_domain
channels	B-protein_type
and	O
transporters	B-protein_type
.	O

With	O
respect	O
to	O
ammonium	B-chemical
transport	O
,	O
phosphorylation	B-ptm
has	O
thus	O
far	O
only	O
been	O
shown	O
for	O
A	B-species
.	I-species
thaliana	I-species
AMTs	B-protein_type
and	O
for	O
S	B-species
.	I-species
cerevisiae	I-species
Mep2	B-protein
(	O
refs	O
).	O

However	O
,	O
the	O
absence	B-protein_state
of	I-protein_state
GlnK	B-protein_type
proteins	I-protein_type
in	O
eukaryotes	B-taxonomy_domain
suggests	O
that	O
phosphorylation	B-ptm
-	O
based	O
regulation	O
of	O
ammonium	B-chemical
transport	O
may	O
be	O
widespread	O
.	O

With	O
respect	O
to	O
Mep2	B-protein_type
-	O
mediated	O
signalling	O
to	O
induce	O
pseudohyphal	O
growth	O
,	O
two	O
models	O
have	O
been	O
put	O
forward	O
as	O
to	O
how	O
this	O
occurs	O
and	O
why	O
it	O
is	O
specific	O
to	O
Mep2	B-protein_type
proteins	I-protein_type
.	O

In	O
one	O
model	O
,	O
signalling	O
is	O
proposed	O
to	O
depend	O
on	O
the	O
nature	O
of	O
the	O
transported	O
substrate	O
,	O
which	O
might	O
be	O
different	O
in	O
certain	O
subfamilies	O
of	O
ammonium	B-protein_type
transporters	I-protein_type
(	O
for	O
example	O
,	O
Mep1	B-protein
/	O
Mep3	B-protein
versus	O
Mep2	B-protein
).	O

For	O
example	O
,	O
NH3	B-chemical
uniport	O
or	O
symport	O
of	O
NH3	B-chemical
/	O
H	B-chemical
+	I-chemical
might	O
result	O
in	O
changes	O
in	O
local	O
pH	O
,	O
but	O
NH4	B-chemical
+	I-chemical
uniport	O
might	O
not	O
,	O
and	O
this	O
difference	O
might	O
determine	O
signalling	O
.	O

In	O
the	O
other	O
model	O
,	O
signalling	O
is	O
thought	O
to	O
require	O
a	O
distinct	O
conformation	O
of	O
the	O
Mep2	B-protein
transporter	B-protein_type
occurring	O
during	O
the	O
transport	O
cycle	O
.	O

While	O
the	O
current	O
study	O
does	O
not	O
specifically	O
address	O
the	O
mechanism	O
of	O
signalling	O
underlying	O
pseudohyphal	O
growth	O
,	O
our	O
structures	B-evidence
do	O
show	O
that	O
Mep2	B-protein_type
proteins	I-protein_type
can	O
assume	O
different	O
conformations	O
.	O

It	O
is	O
clear	O
that	O
ammonium	B-chemical
transport	O
across	O
biomembranes	O
remains	O
a	O
fascinating	O
and	O
challenging	O
field	O
in	O
large	O
part	O
due	O
to	O
the	O
unique	O
properties	O
of	O
the	O
substrate	O
.	O

Our	O
Mep2	B-protein
structural	O
work	O
now	O
provides	O
a	O
foundation	O
for	O
future	O
studies	O
to	O
uncover	O
the	O
details	O
of	O
the	O
structural	O
changes	O
that	O
occur	O
during	O
eukaryotic	B-taxonomy_domain
ammonium	B-chemical
transport	O
and	O
signaling	O
,	O
and	O
to	O
assess	O
the	O
possibility	O
to	O
utilize	O
small	O
molecules	O
to	O
shut	O
down	O
ammonium	B-chemical
sensing	O
and	O
downstream	O
signalling	O
pathways	O
in	O
pathogenic	O
fungi	B-taxonomy_domain
.	O

The	O
region	O
showing	O
ICL1	B-structure_element
(	O
blue	O
),	O
ICL3	B-structure_element
(	O
green	O
)	O
and	O
the	O
CTR	B-structure_element
(	O
red	O
)	O
is	O
boxed	O
for	O
comparison	O
.	O

(	O
b	O
)	O
CaMep2	B-protein
trimer	B-oligomeric_state
viewed	O
from	O
the	O
intracellular	O
side	O
(	O
right	O
).	O

The	O
CTR	B-structure_element
is	O
boxed	O
.	O

(	O
c	O
)	O
Overlay	B-experimental_method
of	O
ScMep2	B-protein
(	O
grey	O
)	O
and	O
CaMep2	B-protein
(	O
rainbow	O
),	O
illustrating	O
the	O
differences	O
in	O
the	O
CTRs	B-structure_element
.	O

Sequence	B-evidence
conservation	I-evidence
in	O
ammonium	B-protein_type
transporters	I-protein_type
.	O

The	O
secondary	O
structure	O
elements	O
observed	O
for	O
CaMep2	B-protein
are	O
indicated	O
,	O
with	O
the	O
numbers	O
corresponding	O
to	O
the	O
centre	O
of	O
the	O
TM	B-structure_element
segment	I-structure_element
.	O

Coloured	O
residues	O
are	O
functionally	O
important	O
and	O
correspond	O
to	O
those	O
of	O
the	O
Phe	B-site
gate	I-site
(	O
blue	O
),	O
the	O
binding	B-site
site	I-site
Trp	B-residue_name
residue	O
(	O
magenta	O
)	O
and	O
the	O
twin	O
-	O
His	O
motif	O
(	O
red	O
).	O

Growth	B-experimental_method
of	O
ScMep2	B-mutant
variants	I-mutant
on	O
low	O
ammonium	O
medium	O
.	O

(	O
a	O
)	O
The	O
triple	B-mutant
mepΔ	I-mutant
strain	O
(	O
black	O
)	O
and	O
triple	O
mepΔ	O
npr1Δ	O
strain	O
(	O
grey	O
)	O
containing	O
plasmids	O
expressing	O
WT	B-protein_state
and	O
variant	B-mutant
ScMep2	I-mutant
were	O
grown	B-experimental_method
on	I-experimental_method
minimal	I-experimental_method
medium	I-experimental_method
containing	O
1	O
mM	O
ammonium	B-chemical
sulphate	I-chemical
.	O

The	O
quantified	O
cell	B-evidence
density	I-evidence
reflects	O
logarithmic	O
growth	O
after	O
24	O
h	O
.	O
Error	O
bars	O
are	O
the	O
s	O
.	O
d	O
.	O
for	O
three	O
replicates	O
of	O
each	O
strain	O
(	O
b	O
)	O
The	O
strains	O
used	O
in	O
a	O
were	O
also	O
serially	O
diluted	O
and	O
spotted	O
onto	O
minimal	O
agar	O
plates	O
containing	O
glutamate	B-chemical
(	O
0	O
.	O
1	O
%)	O
or	O
ammonium	B-chemical
sulphate	I-chemical
(	O
1	O
mM	O
),	O
and	O
grown	O
for	O
3	O
days	O
at	O
30	O
°	O
C	O
.	O

Structural	O
differences	O
between	O
Mep2	B-protein
and	O
bacterial	B-taxonomy_domain
ammonium	O
transporters	O
.	O

The	O
numbering	O
is	O
for	O
CaMep2	B-protein
.	O

(	O
c	O
)	O
Conserved	B-protein_state
residues	O
in	O
ICL1	B-structure_element
-	I-structure_element
3	I-structure_element
and	O
the	O
CTR	B-structure_element
.	O

Views	O
from	O
the	O
cytosol	O
for	O
CaMep2	B-protein
(	O
left	O
)	O
and	O
AfAmt	B-protein
-	I-protein
1	I-protein
,	O
highlighting	O
the	O
large	O
differences	O
in	O
conformation	O
of	O
the	O
conserved	B-protein_state
residues	O
in	O
ICL1	B-structure_element
(	O
RxK	O
motif	O
;	O
blue	O
),	O
ICL2	B-structure_element
(	O
ER	B-structure_element
motif	I-structure_element
;	O
cyan	O
),	O
ICL3	B-structure_element
(	O
green	O
)	O
and	O
the	O
CTR	B-structure_element
(	O
red	O
).	O

The	O
labelled	O
residues	O
are	O
analogous	O
within	O
both	O
structures	B-evidence
.	O

Channel	O
closures	O
in	O
Mep2	B-protein
.	O

(	O
a	O
)	O
Stereo	O
superposition	B-experimental_method
of	O
AfAmt	B-protein
-	I-protein
1	I-protein
and	O
CaMep2	B-protein
showing	O
the	O
residues	O
of	O
the	O
Phe	B-site
gate	I-site
,	O
His2	B-residue_name_number
of	O
the	O
twin	B-structure_element
-	I-structure_element
His	I-structure_element
motif	I-structure_element
and	O
the	O
tyrosine	B-residue_name
residue	O
Y49	B-residue_name_number
in	O
TM1	B-structure_element
that	O
forms	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
His2	B-residue_name_number
in	O
CaMep2	B-protein
.	O
(	O
b	O
)	O
Surface	O
views	O
from	O
the	O
side	O
in	O
rainbow	O
colouring	O
,	O
showing	O
the	O
two	O
-	O
tier	O
channel	B-structure_element
block	I-structure_element
(	O
indicated	O
by	O
the	O
arrows	O
)	O
in	O
CaMep2	B-protein
.	O

The	O
Npr1	B-protein
kinase	B-protein_type
target	O
Ser453	B-residue_name_number
is	O
dephosphorylated	B-protein_state
and	O
located	O
in	O
an	O
electronegative	B-site
pocket	I-site
.	O

The	O
phosphorylation	B-ptm
target	O
residue	O
Ser453	B-residue_name_number
is	O
labelled	O
in	O
bold	O
.	O

(	O
b	O
)	O
Overlay	B-experimental_method
of	O
the	O
CTRs	B-structure_element
of	O
ScMep2	B-protein
(	O
grey	O
)	O
and	O
CaMep2	B-protein
(	O
green	O
),	O
showing	O
the	O
similar	O
electronegative	O
environment	O
surrounding	O
the	O
phosphorylation	B-site
site	I-site
(	O
P	O
).	O

The	O
AI	B-structure_element
regions	I-structure_element
are	O
coloured	O
magenta	O
.	O

(	O
c	O
)	O
Cytoplasmic	O
view	O
of	O
the	O
Mep2	B-protein
trimer	B-oligomeric_state
indicating	O
the	O
large	O
distance	O
between	O
Ser453	B-residue_name_number
and	O
the	O
channel	B-site
exits	I-site
(	O
circles	O
;	O
Ile52	B-residue_name_number
lining	O
the	O
channel	B-site
exit	I-site
is	O
shown	O
).	O

Effect	O
of	O
removal	B-experimental_method
of	O
the	O
AI	B-structure_element
region	I-structure_element
on	O
Mep2	B-protein
structure	B-evidence
.	O

(	O
a	O
)	O
Side	O
views	O
for	O
WT	B-protein_state
CaMep2	B-protein
(	O
left	O
)	O
and	O
the	O
truncation	B-protein_state
mutant	I-protein_state
442Δ	B-mutant
(	O
right	O
).	O

The	O
latter	O
is	O
shown	O
as	O
a	O
putty	O
model	O
according	O
to	O
B	O
-	O
factors	O
to	O
illustrate	O
the	O
disorder	B-protein_state
in	O
the	O
protein	O
on	O
the	O
cytoplasmic	O
side	O
.	O

Missing	O
regions	O
are	O
labelled	O
.	O
(	O
b	O
)	O
Stereo	O
superpositions	B-experimental_method
of	O
WT	B-protein_state
CaMep2	B-protein
and	O
the	O
truncation	B-protein_state
mutant	I-protein_state
.	O

2Fo	O
–	O
Fc	O
electron	O
density	O
(	O
contoured	O
at	O
1	O
.	O
0	O
σ	O
)	O
for	O
residues	O
Tyr49	B-residue_name_number
and	O
His342	B-residue_name_number
is	O
shown	O
for	O
the	O
truncation	B-protein_state
mutant	I-protein_state
.	O

(	O
a	O
)	O
Cytoplasmic	O
view	O
of	O
the	O
DD	B-mutant
mutant	I-mutant
trimer	B-oligomeric_state
,	O
with	O
WT	B-protein_state
CaMep2	B-protein
superposed	B-experimental_method
in	O
grey	O
for	O
one	O
of	O
the	O
monomers	B-oligomeric_state
.	O

The	O
AI	B-structure_element
region	I-structure_element
is	O
coloured	O
magenta	O
.	O

(	O
b	O
)	O
Monomer	B-oligomeric_state
side	O
-	O
view	O
superposition	B-experimental_method
of	O
WT	B-protein_state
CaMep2	B-protein
and	O
the	O
DD	B-mutant
mutant	I-mutant
,	O
showing	O
the	O
conformational	O
change	O
and	O
disorder	O
around	O
the	O
ExxGxD	B-structure_element
motif	I-structure_element
.	O

Side	O
chains	O
for	O
residues	O
452	B-residue_number
and	O
453	B-residue_number
are	O
shown	O
as	O
stick	O
models	O
.	O

Schematic	O
model	O
for	O
phosphorylation	O
-	O
based	O
regulation	O
of	O
Mep2	B-protein
ammonium	O
transporters	O
.	O

(	O
a	O
)	O
In	O
the	O
closed	B-protein_state
,	O
non	B-protein_state
-	I-protein_state
phosphorylated	I-protein_state
state	O
(	O
i	O
),	O
the	O
CTR	B-structure_element
(	O
magenta	O
)	O
and	O
ICL3	B-structure_element
(	O
green	O
)	O
are	O
far	O
apart	O
with	O
the	O
latter	O
blocking	O
the	O
intracellular	O
channel	B-site
exit	I-site
(	O
indicated	O
with	O
a	O
hatched	O
circle	O
).	O

Upon	O
phosphorylation	B-ptm
and	O
mimicked	B-protein_state
by	O
the	O
CaMep2	B-protein
S453D	B-mutant
and	O
DD	B-mutant
mutants	I-mutant
(	O
ii	O
),	O
the	O
region	O
around	O
the	O
ExxGxD	B-structure_element
motif	I-structure_element
undergoes	O
a	O
conformational	O
change	O
that	O
results	O
in	O
the	O
CTR	B-structure_element
interacting	O
with	O
the	O
inward	O
-	O
moving	O
ICL3	B-structure_element
,	O
opening	O
the	O
channel	B-site
(	O
full	O
circle	O
)	O
(	O
iii	O
).	O

The	O
open	B-protein_state
-	O
channel	B-site
Mep2	B-protein
structure	B-evidence
is	O
represented	O
by	O
archaebacterial	B-taxonomy_domain
Amt	B-protein
-	I-protein
1	I-protein
and	O
shown	O
in	O
lighter	O
colours	O
consistent	O
with	O
Fig	O
.	O
4	O
.	O

As	O
discussed	O
in	O
the	O
text	O
,	O
similar	O
structural	O
arrangements	O
may	O
occur	O
in	O
plant	B-taxonomy_domain
AMTs	B-protein_type
.	O

Template	O
-	O
dependent	O
nucleotide	O
addition	O
in	O
the	O
reverse	O
(	O
3	O
′-	O
5	O
′)	O
direction	O
by	O
Thg1	B-protein_type
-	I-protein_type
like	I-protein_type
protein	I-protein_type

Structures	B-evidence
of	O
Thg1	B-protein_type
-	I-protein_type
like	I-protein_type
proteins	I-protein_type
provide	O
insight	O
into	O
the	O
template	O
-	O
dependent	O
nucleotide	O
addition	O
in	O
the	O
reverse	O
(	O
3	O
′-	O
5	O
′)	O
direction	O
.	O

Thg1	B-protein_type
-	I-protein_type
like	I-protein_type
protein	I-protein_type
(	O
TLP	B-protein_type
)	O
catalyzes	O
the	O
addition	O
of	O
a	O
nucleotide	O
to	O
the	O
5	O
′-	O
end	O
of	O
truncated	O
transfer	B-chemical
RNA	I-chemical
(	O
tRNA	B-chemical
)	O
species	O
in	O
a	O
Watson	O
-	O
Crick	O
template	O
–	O
dependent	O
manner	O
.	O

The	O
reaction	O
proceeds	O
in	O
two	O
steps	O
:	O
the	O
activation	O
of	O
the	O
5	O
′-	O
end	O
by	O
adenosine	B-chemical
5	I-chemical
′-	I-chemical
triphosphate	I-chemical
(	O
ATP	B-chemical
)/	O
guanosine	B-chemical
5	I-chemical
′-	I-chemical
triphosphate	I-chemical
(	O
GTP	B-chemical
),	O
followed	O
by	O
nucleotide	O
addition	O
.	O

Structural	B-experimental_method
analyses	I-experimental_method
of	O
the	O
TLP	B-protein_type
and	O
its	O
reaction	O
intermediates	O
have	O
revealed	O
the	O
atomic	O
detail	O
of	O
the	O
template	O
-	O
dependent	O
elongation	O
reaction	O
in	O
the	O
3	O
′-	O
5	O
′	O
direction	O
.	O

The	O
enzyme	O
creates	O
two	O
substrate	B-site
binding	I-site
sites	I-site
for	O
the	O
first	O
-	O
and	O
second	O
-	O
step	O
reactions	O
in	O
the	O
vicinity	O
of	O
one	O
reaction	B-site
center	I-site
consisting	O
of	O
two	O
Mg2	B-chemical
+	I-chemical
ions	O
,	O
and	O
the	O
two	O
reactions	O
are	O
executed	O
at	O
the	O
same	O
reaction	B-site
center	I-site
in	O
a	O
stepwise	O
fashion	O
.	O

When	O
the	O
incoming	O
nucleotide	B-chemical
is	O
bound	B-protein_state
to	I-protein_state
the	O
second	B-site
binding	I-site
site	I-site
with	O
Watson	B-bond_interaction
-	I-bond_interaction
Crick	I-bond_interaction
hydrogen	I-bond_interaction
bonds	I-bond_interaction
,	O
the	O
3	O
′-	O
OH	O
of	O
the	O
incoming	O
nucleotide	O
and	O
the	O
5	B-chemical
′-	I-chemical
triphosphate	I-chemical
of	O
the	O
tRNA	B-chemical
are	O
moved	O
to	O
the	O
reaction	B-site
center	I-site
where	O
the	O
first	O
reaction	O
has	O
occurred	O
.	O

Although	O
TLP	B-protein_type
and	O
Thg1	B-protein
have	O
similar	O
tetrameric	B-oligomeric_state
organization	O
,	O
the	O
tRNA	B-chemical
binding	O
mode	O
of	O
TLP	B-protein_type
is	O
different	O
from	O
that	O
of	O
Thg1	B-protein
,	O
a	O
tRNAHis	B-protein_type
-	I-protein_type
specific	I-protein_type
G	I-protein_type
−	I-protein_type
1	I-protein_type
addition	I-protein_type
enzyme	I-protein_type
.	O

Each	O
tRNAHis	B-chemical
binds	O
to	O
three	O
of	O
the	O
four	O
Thg1	B-protein
tetramer	B-oligomeric_state
subunits	B-structure_element
,	O
whereas	O
in	O
TLP	B-protein_type
,	O
tRNA	B-chemical
only	O
binds	O
to	O
a	O
dimer	B-site
interface	I-site
and	O
the	O
elongation	O
reaction	O
is	O
terminated	O
by	O
measuring	O
the	O
accepter	B-structure_element
stem	I-structure_element
length	O
through	O
the	O
flexible	B-protein_state
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
.	O

All	O
polynucleotide	O
chain	O
elongation	O
reactions	O
,	O
whether	O
with	O
DNA	B-chemical
or	O
RNA	B-chemical
,	O
proceed	O
in	O
the	O
5	O
′-	O
3	O
′	O
direction	O
.	O

This	O
reaction	O
involves	O
the	O
nucleophilic	O
attack	O
of	O
a	O
3	O
′-	O
OH	O
of	O
the	O
terminal	O
nucleotide	O
in	O
the	O
elongating	O
chain	O
on	O
the	O
α	O
-	O
phosphate	B-chemical
of	O
an	O
incoming	O
nucleotide	O
.	O

This	O
elongation	O
reaction	O
of	O
DNA	B-chemical
/	O
RNA	B-chemical
chains	O
is	O
in	O
clear	O
contrast	O
to	O
the	O
elongation	O
of	O
protein	O
chains	O
in	O
which	O
the	O
high	O
energy	O
of	O
the	O
incoming	O
aminoacyl	B-chemical
-	I-chemical
tRNA	I-chemical
is	O
not	O
used	O
for	O
its	O
own	O
addition	O
but	O
for	O
the	O
addition	O
of	O
the	O
next	O
monomer	B-oligomeric_state
(	O
termed	O
head	O
polymerization	O
).	O

In	O
this	O
case	O
,	O
the	O
5	O
′-	O
end	O
of	O
tRNA	B-chemical
is	O
first	O
activated	O
using	O
adenosine	B-chemical
5	I-chemical
′-	I-chemical
triphosphate	I-chemical
(	O
ATP	B-chemical
)/	O
guanosine	B-chemical
5	I-chemical
′-	I-chemical
triphosphate	I-chemical
(	O
GTP	B-chemical
),	O
followed	O
by	O
nucleophilic	O
attack	O
of	O
a	O
3	O
′-	O
OH	O
on	O
the	O
incoming	O
nucleotide	O
[	O
nucleoside	B-chemical
5	I-chemical
′-	I-chemical
triphosphate	I-chemical
(	O
NTP	B-chemical
)]	O
to	O
yield	O
pppN	B-chemical
-	I-chemical
tRNA	I-chemical
.	O

Thus	O
,	O
the	O
energy	O
in	O
the	O
triphosphate	B-chemical
bond	O
of	O
the	O
incoming	O
nucleotide	O
is	O
not	O
used	O
for	O
its	O
own	O
addition	O
but	O
is	O
reserved	O
for	O
subsequent	O
polymerization	O
(	O
that	O
is	O
,	O
head	O
polymerization	O
)	O
(	O
Fig	O
.	O
1	O
).	O

Top	O
:	O
Reaction	O
scheme	O
of	O
3	O
′-	O
5	O
′	O
elongation	O
by	O
Thg1	B-protein
/	O
TLP	B-protein_type
family	O
proteins	O
.	O

Bottom	O
:	O
Reaction	O
scheme	O
of	O
5	O
′-	O
3	O
′	O
elongation	O
by	O
DNA	B-protein_type
/	I-protein_type
RNA	I-protein_type
polymerases	I-protein_type
.	O

The	O
best	O
-	O
characterized	O
member	O
of	O
this	O
family	O
of	O
proteins	O
is	O
eukaryotic	B-taxonomy_domain
Thg1	B-protein
(	O
tRNAHis	B-protein_type
guanylyltransferase	I-protein_type
),	O
which	O
catalyzes	O
the	O
nontemplated	O
addition	O
of	O
a	O
guanylate	O
to	O
the	O
5	O
′-	O
end	O
of	O
immature	O
tRNAHis	B-chemical
.	O

Therefore	O
,	O
Thg1	B-protein
is	O
essential	O
to	O
the	O
fidelity	O
of	O
protein	O
synthesis	O
in	O
eukaryotes	B-taxonomy_domain
.	O

However	O
,	O
Thg1	B-protein
homologs	O
or	O
TLPs	B-protein_type
are	O
found	O
in	O
organisms	O
in	O
which	O
G	B-residue_name_number
−	I-residue_name_number
1	I-residue_name_number
is	O
genetically	O
encoded	O
,	O
and	O
thus	O
,	O
posttranscriptional	O
modification	O
is	O
not	O
required	O
.	O

TLPs	B-protein_type
have	O
been	O
shown	O
to	O
catalyze	O
5	O
′-	O
end	O
nucleotide	O
addition	O
to	O
truncated	O
tRNA	B-chemical
species	O
in	O
vitro	O
in	O
a	O
Watson	O
-	O
Crick	O
template	O
–	O
dependent	O
manner	O
.	O

This	O
function	O
of	O
TLPs	B-protein_type
is	O
not	O
limited	O
to	O
tRNAHis	B-chemical
but	O
occurs	O
efficiently	O
with	O
other	O
tRNAs	B-chemical
.	O

Furthermore	O
,	O
the	O
yeast	B-taxonomy_domain
homolog	O
(	O
Thg1p	B-protein
)	O
has	O
been	O
shown	O
to	O
interact	O
with	O
the	O
replication	O
origin	O
recognition	O
complex	O
for	O
DNA	B-chemical
replication	O
,	O
and	O
the	O
plant	B-taxonomy_domain
homolog	O
(	O
ICA1	B-protein
)	O
was	O
identified	O
as	O
a	O
protein	O
affecting	O
the	O
capacity	O
to	O
repair	O
DNA	B-chemical
damage	O
.	O

These	O
observations	O
suggest	O
that	O
TLPs	B-protein_type
may	O
have	O
more	O
general	O
functions	O
in	O
DNA	B-chemical
/	O
RNA	B-chemical
repair	O
.	O

The	O
3	O
′-	O
5	O
′	O
addition	O
reaction	O
catalyzed	O
by	O
Thg1	B-protein
occurs	O
through	O
three	O
reaction	O
steps	O
.	O

In	O
the	O
first	O
step	O
,	O
the	O
5	O
′-	O
monophosphorylated	O
tRNAHis	B-chemical
,	O
which	O
is	O
cleaved	O
by	O
ribonuclease	B-protein_type
P	I-protein_type
from	O
pre	B-chemical
-	I-chemical
tRNAHis	I-chemical
,	O
is	O
activated	O
by	O
ATP	B-chemical
,	O
creating	O
a	O
5	O
′-	O
adenylylated	O
tRNAHis	B-chemical
intermediate	O
.	O

In	O
the	O
second	O
step	O
,	O
the	O
3	O
′-	O
OH	O
of	O
the	O
incoming	O
GTP	B-chemical
attacks	O
the	O
activated	O
intermediate	O
,	O
yielding	O
pppG	B-chemical
−	I-chemical
1	I-chemical
-	I-chemical
tRNAHis	I-chemical
.	O

Finally	O
,	O
the	O
pyrophosphate	B-chemical
is	O
removed	O
,	O
and	O
mature	O
pG	B-chemical
−	I-chemical
1	I-chemical
-	I-chemical
tRNAHis	I-chemical
is	O
created	O
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
human	B-species
Thg1	B-protein
(	O
HsThg1	B-protein
)	O
shows	O
that	O
its	O
catalytic	B-site
core	I-site
shares	O
structural	O
homology	O
with	O
canonical	O
5	B-protein_type
′-	I-protein_type
3	I-protein_type
′	I-protein_type
nucleotide	I-protein_type
polymerases	I-protein_type
,	O
such	O
as	O
T7	B-protein_type
DNA	I-protein_type
/	I-protein_type
RNA	I-protein_type
polymerases	I-protein_type
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
TLP	B-protein_type
from	O
Bacillus	B-species
thuringiensis	I-species
shows	O
that	O
it	O
shares	O
a	O
similar	O
tetrameric	B-oligomeric_state
assembly	O
and	O
active	B-site
-	I-site
site	I-site
architecture	O
with	O
HsThg1	B-protein
.	O

However	O
,	O
in	O
this	O
structural	B-experimental_method
analysis	I-experimental_method
,	O
the	O
5	O
′-	O
end	O
of	O
tRNAHis	B-chemical
was	O
not	O
activated	O
and	O
the	O
second	O
substrate	O
(	O
GTP	B-chemical
)	O
was	O
not	B-protein_state
bound	I-protein_state
.	O

Here	O
,	O
we	O
successfully	O
solved	B-experimental_method
the	O
structure	B-evidence
of	O
TLP	B-protein_type
from	O
the	O
methanogenic	B-taxonomy_domain
archaeon	I-taxonomy_domain
Methanosarcina	B-species
acetivorans	I-species
(	O
MaTLP	B-protein
)	O
in	B-protein_state
complex	I-protein_state
with	I-protein_state
ppptRNAPheΔ1	B-chemical
,	O
which	O
mimics	O
the	O
activated	O
intermediate	O
of	O
the	O
repair	O
substrate	O
.	O

Although	O
TLP	B-protein_type
and	O
Thg1	B-protein
have	O
similar	O
tetrameric	B-oligomeric_state
organization	O
,	O
the	O
mode	O
of	O
tRNA	B-chemical
binding	O
is	O
different	O
in	O
TLP	B-protein_type
.	O

Furthermore	O
,	O
we	O
obtained	O
the	O
structure	B-evidence
in	O
which	O
the	O
GTP	B-chemical
analog	O
(	O
GDPNP	B-chemical
)	O
was	O
inserted	O
into	O
this	O
complex	O
to	O
form	O
a	O
Watson	B-bond_interaction
-	I-bond_interaction
Crick	I-bond_interaction
base	I-bond_interaction
pair	I-bond_interaction
with	O
C72	B-residue_name_number
at	O
the	O
3	O
′-	O
end	O
region	O
of	O
the	O
tRNA	B-chemical
.	O

On	O
the	O
basis	O
of	O
these	O
structures	B-evidence
,	O
we	O
discuss	O
the	O
reaction	O
mechanism	O
of	O
template	O
-	O
dependent	O
reverse	O
(	O
3	O
′-	O
5	O
′)	O
polymerization	O
in	O
comparison	O
with	O
canonical	O
5	O
′-	O
3	O
′	O
polymerization	O
.	O

Anticodon	O
-	O
independent	O
binding	O
of	O
ppptRNAPheΔ1	B-chemical
to	O
MaTLP	B-protein

Previous	O
biochemical	B-experimental_method
experiments	I-experimental_method
have	O
suggested	O
that	O
ppptRNAPheΔ1	B-chemical
,	O
in	O
which	O
the	O
5	O
′-	O
end	O
of	O
tRNAPhe	B-chemical
was	O
triphosphorylated	O
and	O
G1	B-residue_name_number
was	O
deleted	B-experimental_method
,	O
can	O
be	O
an	O
efficient	O
substrate	O
for	O
the	O
repair	O
reaction	O
(	O
guanylyl	O
transfer	O
)	O
of	O
Thg1	B-protein
/	O
TLP	B-protein_type
.	O

The	O
crystal	B-evidence
contained	O
a	O
dimer	B-oligomeric_state
of	O
TLP	B-protein_type
(	O
A	B-structure_element
and	O
B	B-structure_element
molecules	O
)	O
and	O
one	O
tRNA	B-chemical
in	O
an	O
asymmetric	O
unit	O
.	O

Two	O
dimers	B-oligomeric_state
in	O
the	O
crystal	B-evidence
further	O
assembled	O
as	O
a	O
dimer	B-oligomeric_state
of	O
dimers	B-oligomeric_state
by	O
the	O
crystallographic	O
twofold	O
axis	O
(	O
Fig	O
.	O
2	O
).	O

This	O
tetrameric	B-oligomeric_state
structure	B-evidence
and	O
4	O
:	O
2	O
stoichiometry	O
of	O
the	O
TLP	B-complex_assembly
-	I-complex_assembly
tRNA	I-complex_assembly
complex	O
are	O
the	O
same	O
as	O
those	O
of	O
the	O
CaThg1	B-complex_assembly
-	I-complex_assembly
tRNA	I-complex_assembly
complex	O
.	O

However	O
,	O
whereas	O
the	O
AB	B-structure_element
and	O
CD	B-structure_element
dimers	B-oligomeric_state
of	O
tetrameric	B-oligomeric_state
CaThg1	B-protein
play	O
different	O
roles	O
,	O
respectively	O
recognizing	O
the	O
accepter	B-structure_element
stem	I-structure_element
and	O
anticodon	O
of	O
tRNAHis	B-chemical
,	O
the	O
AB	B-structure_element
dimer	B-oligomeric_state
and	O
its	O
symmetry	O
mate	O
(	O
CD	B-structure_element
dimer	B-oligomeric_state
)	O
on	O
tetrameric	B-oligomeric_state
MaTLP	B-protein
independently	O
bind	O
one	O
molecule	O
of	O
tRNA	B-chemical
(	O
fig	O
.	O
S2	O
),	O
recognizing	O
the	O
tRNA	B-chemical
accepter	B-structure_element
stem	I-structure_element
and	O
elbow	B-structure_element
region	I-structure_element
.	O

Thus	O
,	O
consistent	O
with	O
the	O
notion	O
that	O
MaTLP	B-protein
is	O
an	O
anticodon	B-protein_type
-	I-protein_type
independent	I-protein_type
repair	I-protein_type
enzyme	I-protein_type
,	O
the	O
anticodon	O
was	O
not	O
recognized	O
in	O
the	O
MaTLP	B-complex_assembly
-	I-complex_assembly
tRNA	I-complex_assembly
complex	O
,	O
whereas	O
the	O
binding	O
mode	O
of	O
CaThg1	B-protein
is	O
for	O
the	O
G	B-residue_name_number
−	I-residue_name_number
1	I-residue_name_number
addition	O
reaction	O
,	O
therefore	O
the	O
His	B-residue_name
anticodon	O
has	O
to	O
be	O
recognized	O
(	O
see	O
“	O
Dual	O
binding	O
mode	O
for	O
tRNA	B-chemical
repair	O
”).	O

Structure	B-evidence
of	O
the	O
MaTLP	B-protein
complex	B-protein_state
with	I-protein_state
ppptRNAPheΔ1	B-chemical
.	O

Left	O
:	O
One	O
molecule	O
of	O
the	O
tRNA	B-chemical
substrate	O
(	O
ppptRNAPheΔ1	B-chemical
)	O
is	O
bound	B-protein_state
to	I-protein_state
the	O
MaTLP	B-protein
dimer	B-oligomeric_state
.	O

The	O
AB	B-structure_element
and	O
CD	B-structure_element
dimers	B-oligomeric_state
are	O
further	O
dimerized	B-oligomeric_state
by	O
the	O
crystallographic	O
twofold	O
axis	O
to	O
form	O
a	O
tetrameric	B-oligomeric_state
structure	B-evidence
(	O
dimer	B-oligomeric_state
of	O
dimers	B-oligomeric_state
).	O

The	O
CD	B-structure_element
dimer	B-oligomeric_state
is	O
omitted	O
for	O
clarity	O
.	O

The	O
accepter	B-structure_element
stem	I-structure_element
of	O
the	O
tRNA	B-chemical
is	O
recognized	O
by	O
molecule	O
A	O
(	O
yellow	O
),	O
and	O
the	O
elbow	B-structure_element
region	I-structure_element
by	O
molecule	O
B	O
(	O
blue	O
).	O

The	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
region	O
of	O
molecule	O
B	O
is	O
shown	O
in	O
red	O
.	O

The	O
elbow	B-structure_element
region	I-structure_element
of	O
the	O
tRNA	B-chemical
substrate	O
was	O
recognized	O
by	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
of	O
molecule	O
B	O
of	O
MaTLP	B-protein
.	O

The	O
N	O
atoms	O
in	O
the	O
side	O
chain	O
of	O
R215	B-residue_name_number
in	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
region	O
of	O
MaTLP	B-protein
were	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
to	O
the	O
phosphate	B-chemical
groups	O
of	O
U55	B-residue_name_number
and	O
G57	B-residue_name_number
.	O

This	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
region	O
was	O
disordered	B-protein_state
in	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
CaThg1	B-complex_assembly
-	I-complex_assembly
tRNA	I-complex_assembly
complex	O
.	O

The	O
accepter	B-structure_element
stem	I-structure_element
of	O
the	O
tRNA	B-chemical
substrate	O
was	O
recognized	O
by	O
molecule	O
A	O
of	O
MaTLP	B-protein
.	O

R136	B-residue_name_number
was	O
also	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
to	O
the	O
base	O
of	O
C72	B-residue_name_number
(	O
the	O
Watson	O
-	O
Crick	O
bond	O
partner	O
of	O
ΔG1	O
).	O

The	O
triphosphate	B-chemical
moiety	O
at	O
the	O
5	O
′-	O
end	O
of	O
the	O
tRNA	B-chemical
was	O
bonded	O
to	O
the	O
D21	B-residue_range
-	I-residue_range
K26	I-residue_range
region	O
.	O

These	O
phosphates	B-chemical
were	O
also	O
coordinated	B-bond_interaction
to	I-bond_interaction
two	O
metal	O
ions	O
,	O
presumably	O
Mg2	B-chemical
+	I-chemical
(	O
Mg2	B-chemical
+	I-chemical
A	O
and	O
Mg2	B-chemical
+	I-chemical
B	O
)	O
because	O
they	O
were	O
observed	O
at	O
the	O
same	O
positions	O
(	O
figs	O
.	O
S3	O
and	O
S4	O
)	O
previously	O
identified	O
by	O
CaThg1	B-protein
and	O
HsThg1	B-protein
structures	B-evidence
.	O

These	O
ions	O
were	O
in	O
turn	O
coordinated	B-bond_interaction
by	I-bond_interaction
the	O
O	O
atoms	O
of	O
the	O
side	O
chains	O
of	O
D21	B-residue_name_number
and	O
D69	B-residue_name_number
and	O
the	O
main	O
-	O
chain	O
O	O
of	O
G22	B-residue_name_number
(	O
fig	O
.	O
S3A	O
).	O

Mutation	B-experimental_method
of	O
D29	B-residue_name_number
and	O
D76	B-residue_name_number
in	O
HsThg1	B-protein
(	O
corresponding	O
to	O
D21	B-residue_name_number
and	O
D69	B-residue_name_number
of	O
MaTLP	B-protein
)	O
has	O
been	O
shown	O
to	O
markedly	O
decrease	O
G	B-residue_name_number
−	I-residue_name_number
1	I-residue_name_number
addition	O
activity	O
.	O

Template	O
-	O
dependent	O
binding	O
of	O
the	O
GTP	B-chemical
analog	O
to	O
the	O
MaTLP	B-complex_assembly
-	I-complex_assembly
ppptRNAPheΔ1	I-complex_assembly
complex	O

Here	O
,	O
we	O
successfully	O
obtained	O
the	O
structure	B-evidence
of	O
the	O
ternary	O
complex	O
of	O
MaTLP	B-protein
,	O
5	O
′-	O
activated	O
tRNA	B-chemical
(	O
ppptRNAPheΔ1	B-chemical
),	O
and	O
the	O
GTP	B-chemical
analog	O
(	O
GDPNP	B-chemical
)	O
(	O
Fig	O
.	O
3	O
and	O
fig	O
.	O
S4	O
)	O
by	O
soaking	B-experimental_method
the	O
MaTLP	B-complex_assembly
-	I-complex_assembly
ppptRNAPheΔ1	I-complex_assembly
complex	O
crystal	B-evidence
in	O
a	O
solution	O
containing	O
GDPNP	B-chemical
.	O

The	O
5	O
′-	O
end	O
(	O
position	O
2	O
)	O
of	O
the	O
tRNA	B-chemical
moved	O
significantly	O
(	O
Fig	O
.	O
3C	O
)	O
due	O
to	O
the	O
insertion	O
of	O
GDPNP	B-chemical
.	O

Together	O
with	O
the	O
observation	O
that	O
the	O
3	O
′-	O
OH	O
of	O
the	O
incoming	O
GTP	B-chemical
analog	O
was	O
within	O
coordination	O
distance	O
(	O
2	O
.	O
8	O
Å	O
)	O
to	O
Mg2	B-chemical
+	I-chemical
A	O
(	O
fig	O
.	O
S3B	O
)	O
and	O
was	O
able	O
to	O
execute	O
a	O
nucleophilic	O
attack	O
on	O
the	O
α	O
-	O
phosphate	B-chemical
of	O
the	O
5	O
′-	O
end	O
,	O
this	O
structure	B-evidence
indicates	O
that	O
the	O
elongation	O
reaction	O
(	O
second	O
reaction	O
)	O
takes	O
place	O
at	O
the	O
same	O
reaction	B-site
center	I-site
where	O
the	O
activation	O
reaction	O
(	O
first	O
reaction	O
)	O
occurs	O
.	O

Structural	O
change	O
of	O
the	O
tRNA	B-chemical
(	O
ppptRNAPheΔ1	B-chemical
).	O

Structural	O
change	O
of	O
the	O
tRNA	B-chemical
(	O
ppptRNAPheΔ1	B-chemical
)	O
accepter	B-structure_element
stem	I-structure_element
in	O
MaTLP	B-protein
caused	O
by	O
insertion	O
of	O
GDPNP	B-chemical
.	O
(	O
A	O
)	O
Structure	B-evidence
before	O
GDPNP	B-chemical
binding	O
.	O

The	O
triphosphate	B-chemical
of	O
the	O
GDPNP	B-chemical
was	O
also	O
bonded	O
to	O
the	O
third	O
Mg2	B-chemical
+	I-chemical
(	O
Mg2	B-chemical
+	I-chemical
C	O
),	O
which	O
,	O
unlike	O
Mg2	B-chemical
+	I-chemical
A	O
and	O
Mg2	B-chemical
+	I-chemical
B	O
,	O
is	O
not	O
coordinated	B-bond_interaction
by	I-bond_interaction
the	O
TLP	B-protein_type
molecule	O
(	O
fig	O
.	O
S3B	O
).	O

This	O
triphosphate	B-chemical
binding	O
mode	O
is	O
the	O
same	O
as	O
that	O
for	O
the	O
second	B-site
nucleotide	I-site
binding	I-site
site	I-site
in	O
Thg1	B-protein
.	O

However	O
,	O
in	O
previous	O
analyses	O
,	O
the	O
base	O
moiety	O
at	O
the	O
second	B-site
site	I-site
was	O
either	O
invisible	O
or	O
far	O
beyond	O
the	O
reaction	O
distance	O
of	O
the	O
phosphate	B-chemical
,	O
and	O
therefore	O
,	O
flipping	O
of	O
the	O
base	O
was	O
expected	O
to	O
occur	O
.	O

tRNA	B-experimental_method
binding	I-experimental_method
and	I-experimental_method
repair	I-experimental_method
experiments	I-experimental_method
of	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
mutants	B-protein_state

To	O
confirm	O
tRNA	B-chemical
recognition	O
by	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
,	O
we	O
created	B-experimental_method
mutation	I-experimental_method
variants	I-experimental_method
with	O
altered	O
residues	O
in	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
region	O
.	O

Then	O
,	O
tRNA	B-experimental_method
binding	I-experimental_method
and	I-experimental_method
enzymatic	I-experimental_method
activities	I-experimental_method
were	I-experimental_method
measured	I-experimental_method
.	O

β	B-structure_element
-	I-structure_element
Hairpin	I-structure_element
deletion	B-protein_state
variant	I-protein_state
delR198	B-mutant
-	I-mutant
R215	I-mutant
almost	O
completely	O
abolished	O
the	O
binding	O
of	O
tRNAPheΔ1	B-chemical
(	O
fig	O
.	O
S6	O
).	O

Furthermore	O
,	O
the	O
enzymatic	O
activities	O
of	O
delR198	B-mutant
-	I-mutant
R215	I-mutant
and	O
delG202	B-mutant
-	I-mutant
E210	I-mutant
were	O
very	O
weak	O
(	O
5	O
.	O
2	O
and	O
13	O
.	O
5	O
%,	O
respectively	O
)	O
compared	O
with	O
wild	B-protein_state
type	I-protein_state
,	O
whereas	O
mutations	B-experimental_method
(	O
N179A	B-mutant
and	O
F174A	B-mutant
/	O
N179A	B-mutant
/	O
R188A	B-mutant
)	O
on	O
the	O
anticodon	B-site
recognition	I-site
site	I-site
[	O
deduced	O
from	O
the	O
Thg1	B-complex_assembly
-	I-complex_assembly
tRNAHis	I-complex_assembly
complex	O
structure	B-evidence
]	O
had	O
no	O
effect	O
on	O
the	O
catalytic	O
activity	O
(	O
Fig	O
.	O
4A	O
).	O

Experiments	O
using	O
the	O
tRNAHisΔ1	B-chemical
substrate	O
gave	O
similar	O
results	O
(	O
Fig	O
.	O
4A	O
).	O

All	O
these	O
results	O
are	O
consistent	O
with	O
the	O
crystal	B-evidence
structure	I-evidence
and	O
suggest	O
that	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
plays	O
an	O
important	O
role	O
in	O
anticodon	O
-	O
independent	O
binding	O
of	O
the	O
tRNA	B-chemical
substrate	O
.	O

Residues	O
in	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
are	O
not	B-protein_state
well	I-protein_state
conserved	I-protein_state
,	O
except	O
for	O
R215	B-residue_name_number
(	O
fig	O
.	O
S5	O
).	O

Mutants	B-protein_state
R215A	B-mutant
and	O
R215A	B-mutant
/	O
S213A	B-mutant
,	O
in	O
which	O
the	O
completely	B-protein_state
conserved	I-protein_state
R215	B-residue_name_number
was	O
changed	B-experimental_method
to	O
alanine	B-residue_name
,	O
showed	O
a	O
moderate	O
effect	O
on	O
the	O
activity	O
(	O
27	O
.	O
3	O
and	O
16	O
.	O
3	O
%,	O
respectively	O
).	O

Thus	O
,	O
specific	O
interactions	O
with	O
the	O
conserved	B-protein_state
R215	B-residue_name_number
and	O
van	B-bond_interaction
der	I-bond_interaction
Waals	I-bond_interaction
contacts	I-bond_interaction
to	O
residues	O
in	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
would	O
be	O
important	O
for	O
tRNA	B-chemical
recognition	O
.	O

Mutational	B-experimental_method
analysis	I-experimental_method
of	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
and	O
anticodon	B-site
binding	I-site
region	I-site
.	O

The	O
activity	O
to	O
tRNAPheΔ1	B-chemical
is	O
about	O
10	O
%	O
of	O
ppptRNAPheΔ1	B-chemical
.	O

Termination	O
of	O
the	O
elongation	O
reaction	O
by	O
measuring	O
the	O
accepter	B-structure_element
stem	I-structure_element

TLPs	B-protein_type
catalyze	O
the	O
Watson	O
-	O
Crick	O
template	O
–	O
dependent	O
elongation	O
or	O
repair	O
reaction	O
for	O
5	O
′-	O
end	O
truncated	O
tRNAPhe	B-chemical
substrates	O
lacking	O
G1	B-residue_name_number
only	O
(	O
tRNAPheΔ1	B-chemical
),	O
or	O
lacking	O
both	O
G1	B-residue_name_number
and	O
G2	B-residue_name_number
(	O
tRNAPheΔ1	B-chemical
,	I-chemical
2	I-chemical
),	O
whereas	O
they	O
do	O
not	O
show	O
any	O
activity	O
with	O
intact	O
tRNAPhe	B-chemical
(	O
thus	O
,	O
repair	O
is	O
unnecessary	O
).	O

How	O
TLP	B-protein_type
distinguishes	O
between	O
tRNAs	B-chemical
that	O
need	O
5	O
′-	O
end	O
repair	O
from	O
ones	O
that	O
do	O
not	O
,	O
or	O
in	O
other	O
words	O
,	O
how	O
the	O
elongation	O
reaction	O
is	O
properly	O
terminated	O
,	O
remains	O
unknown	O
.	O

The	O
present	O
structure	B-evidence
of	O
the	O
MaTLP	B-complex_assembly
-	I-complex_assembly
ppptRNAPheΔ1	I-complex_assembly
complex	O
shows	O
that	O
,	O
unlike	O
Thg1	B-protein
,	O
the	O
TLP	B-protein_type
dimer	B-oligomeric_state
binds	O
one	O
molecule	O
of	O
tRNA	B-chemical
by	O
recognizing	O
the	O
elbow	B-structure_element
region	I-structure_element
by	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
of	O
molecule	O
B	B-structure_element
and	O
the	O
5	O
′-	O
end	O
by	O
molecule	O
A	B-structure_element
.	O
Therefore	O
,	O
we	O
speculated	O
that	O
the	O
flexible	B-protein_state
nature	O
of	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
enables	O
the	O
recognition	O
of	O
tRNA	B-chemical
substrates	O
with	O
different	O
accepter	B-structure_element
stem	I-structure_element
lengths	O
.	O

To	O
confirm	O
this	O
speculation	O
,	O
we	O
used	B-experimental_method
computer	I-experimental_method
graphics	I-experimental_method
to	I-experimental_method
examine	I-experimental_method
whether	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
region	O
was	O
able	O
to	O
bind	O
tRNA	B-chemical
substrates	O
with	O
different	O
accepter	B-structure_element
stem	I-structure_element
lengths	O
when	O
the	O
5	O
′-	O
end	O
was	O
properly	O
placed	O
in	O
the	O
reaction	B-site
site	I-site
.	O

When	O
the	O
5	O
′-	O
end	O
was	O
placed	O
in	O
the	O
reaction	O
site	O
,	O
the	O
body	O
of	O
the	O
tRNA	B-chemical
molecule	O
shifted	O
in	O
a	O
manner	O
dependent	O
on	O
the	O
accepter	B-structure_element
stem	I-structure_element
length	O
.	O

The	O
tRNA	B-chemical
body	O
also	O
rotated	O
because	O
of	O
the	O
helical	O
nature	O
of	O
the	O
accepter	B-structure_element
stem	I-structure_element
(	O
fig	O
.	O
S7	O
).	O

This	O
model	O
structure	B-evidence
showed	O
that	O
the	O
accepter	B-structure_element
stem	I-structure_element
of	O
intact	O
tRNAPhe	B-chemical
was	O
too	O
long	O
for	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
to	O
recognize	O
its	O
elbow	B-structure_element
region	I-structure_element
,	O
whereas	O
tRNAPheΔ1	B-chemical
and	O
tRNAPheΔ1	B-chemical
,	I-chemical
2	I-chemical
were	O
recognized	O
by	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
region	O
(	O
fig	O
.	O
S7	O
),	O
which	O
is	O
consistent	O
with	O
previous	O
experiments	O
.	O

On	O
the	O
basis	O
of	O
these	O
model	O
structures	B-evidence
,	O
we	O
concluded	O
that	O
the	O
TLP	B-protein_type
molecule	O
can	O
properly	O
terminate	O
elongation	O
by	O
measuring	O
the	O
accepter	B-structure_element
stem	I-structure_element
length	O
of	O
tRNA	B-chemical
substrates	O
.	O

Dual	O
binding	O
mode	O
for	O
tRNA	B-chemical
repair	O

The	O
present	O
structural	B-experimental_method
analysis	I-experimental_method
revealed	O
that	O
although	O
TLP	B-protein_type
and	O
Thg1	B-protein
have	O
a	O
similar	O
tetrameric	B-oligomeric_state
architecture	O
,	O
they	O
have	O
different	O
binding	O
modes	O
for	O
tRNAs	B-chemical
:	O
Thg1	B-protein
is	O
bound	B-protein_state
to	I-protein_state
tRNAHis	B-chemical
as	O
a	O
tetramer	B-oligomeric_state
,	O
whereas	O
TLP	B-protein_type
is	O
bound	B-protein_state
to	I-protein_state
tRNAPhe	B-chemical
as	O
a	O
dimer	B-oligomeric_state
.	O

This	O
difference	O
in	O
the	O
tRNA	B-chemical
binding	O
modes	O
is	O
closely	O
related	O
to	O
their	O
enzymatic	O
functions	O
.	O

The	O
tetrameric	B-oligomeric_state
architecture	O
of	O
the	O
Thg1	B-protein
molecule	O
allows	O
it	O
to	O
access	O
both	O
regions	O
located	O
at	O
the	O
opposite	O
side	O
of	O
the	O
tRNA	B-chemical
molecule	O
[	O
the	O
AB	B-structure_element
dimer	B-oligomeric_state
recognizes	O
the	O
accepter	B-structure_element
stem	I-structure_element
and	O
CD	B-structure_element
dimer	B-oligomeric_state
anticodon	O
].	O

In	O
contrast	O
,	O
the	O
binding	O
mode	O
of	O
TLP	B-protein_type
corresponds	O
to	O
the	O
anticodon	O
-	O
independent	O
repair	O
reactions	O
of	O
5	O
′-	O
truncated	O
general	O
tRNAs	B-chemical
.	O

This	O
binding	O
mode	O
is	O
also	O
suitable	O
for	O
the	O
correct	O
termination	O
of	O
the	O
elongation	O
or	O
repair	O
reaction	O
by	O
measuring	O
the	O
length	O
of	O
the	O
accepter	B-structure_element
stem	I-structure_element
by	O
the	O
flexible	B-protein_state
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
.	O

Because	O
tRNAHis	B-chemical
requires	O
an	O
extra	O
guanosine	B-chemical
(	O
G	B-residue_name_number
−	I-residue_name_number
1	I-residue_name_number
)	O
at	O
the	O
5	O
′-	O
end	O
,	O
the	O
repair	O
enzyme	O
has	O
to	O
extend	O
the	O
5	O
′-	O
end	O
by	O
one	O
more	O
nucleotide	O
than	O
other	O
tRNAs	B-chemical
.	O

Here	O
,	O
we	O
showed	O
that	O
the	O
TLP	B-protein_type
mutants	B-protein_state
,	O
wherein	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
is	O
truncated	B-protein_state
and	O
tRNAPheΔ1	B-chemical
binding	O
ability	O
is	O
lost	O
,	O
can	O
still	O
bind	O
to	O
tRNAPhe	B-chemical
(	O
GUG	B-chemical
)	O
whose	O
anticodon	O
is	O
changed	O
to	O
that	O
for	O
His	B-residue_name
(	O
fig	O
.	O
S6	O
,	O
C	O
,	O
H	O
,	O
and	O
I	O
).	O

Also	O
,	O
the	O
intact	O
tRNAPhe	B-chemical
,	O
which	O
is	O
not	O
recognized	O
by	O
TLP	B-protein_type
(	O
Fig	O
.	O
4B	O
and	O
fig	O
.	O
S6E	O
),	O
can	O
be	O
recognized	O
when	O
its	O
anticodon	O
is	O
changed	O
to	O
that	O
for	O
His	B-residue_name
(	O
fig	O
.	O
S6D	O
).	O

Furthermore	O
,	O
the	O
TLP	B-protein_type
variant	B-protein_state
(	O
F174A	B-mutant
/	O
N179A	B-mutant
/	O
R188A	B-mutant
)	O
whose	O
anticodon	B-site
recognition	I-site
site	I-site
[	O
deduced	O
from	O
the	O
Thg1	B-complex_assembly
-	I-complex_assembly
tRNAHis	I-complex_assembly
complex	O
structure	B-evidence
]	O
is	O
disrupted	O
has	O
been	O
shown	O
to	O
have	O
a	O
reduced	O
catalytic	O
activity	O
to	O
tRNAHisΔ	B-chemical
−	I-chemical
1	I-chemical
(	O
Fig	O
.	O
4B	O
).	O

All	O
these	O
experimental	O
results	O
indicate	O
that	O
TLP	B-protein_type
recognizes	O
and	O
binds	O
tRNAs	B-chemical
carrying	O
the	O
His	B-residue_name
anticodon	O
in	O
the	O
same	O
way	O
that	O
Thg1	B-protein
recognizes	O
tRNAHis	B-chemical
.	O

The	O
elongation	O
or	O
repair	O
reaction	O
normally	O
terminates	O
when	O
the	O
5	O
′-	O
end	O
reaches	O
position	O
1	O
,	O
but	O
when	O
the	O
His	B-residue_name
anticodon	O
is	O
present	O
,	O
TLP	B-protein_type
binds	O
the	O
tRNA	B-chemical
in	O
the	O
second	O
mode	O
by	O
recognizing	O
the	O
anticodon	O
to	O
execute	O
the	O
G	B-residue_name_number
−	I-residue_name_number
1	I-residue_name_number
addition	O
reaction	O
.	O

By	O
having	O
two	O
different	O
binding	O
modes	O
,	O
TLP	B-protein_type
can	O
manage	O
this	O
special	O
feature	O
of	O
tRNAHis	B-chemical
.	O

The	O
Thg1	B-protein
/	O
TLP	B-protein_type
family	O
of	O
proteins	O
extends	O
tRNA	B-chemical
chains	O
in	O
the	O
3	O
′-	O
5	O
′	O
direction	O
.	O

First	O
,	O
the	O
5	B-chemical
′-	I-chemical
phosphate	I-chemical
is	O
activated	O
by	O
GTP	B-chemical
/	O
ATP	B-chemical
.	O

Then	O
,	O
the	O
activated	O
phosphate	B-chemical
is	O
attacked	O
by	O
the	O
incoming	O
nucleotide	O
,	O
resulting	O
in	O
an	O
extension	O
by	O
one	O
nucleotide	O
at	O
the	O
5	O
′-	O
end	O
.	O

Here	O
,	O
we	O
successfully	O
solved	B-experimental_method
for	O
the	O
first	O
time	O
the	O
intermediate	O
structures	B-evidence
of	O
the	O
template	O
-	O
dependent	O
3	O
′-	O
5	O
′	O
elongation	O
complex	O
of	O
MaTLP	B-protein
.	O

Figure	O
5	O
is	O
a	O
schematic	O
diagram	O
of	O
the	O
3	O
′-	O
5	O
′	O
addition	O
reaction	O
of	O
TLP	B-protein_type
.	O

The	O
structure	B-evidence
of	O
the	O
MaTLP	B-complex_assembly
-	I-complex_assembly
ppptRNAPheΔ1	I-complex_assembly
complex	O
,	O
wherein	O
β	O
-	O
and	O
γ	O
-	O
phosphates	B-chemical
coordinate	B-bond_interaction
with	I-bond_interaction
Mg2	B-chemical
+	I-chemical
A	O
and	O
Mg2	B-chemical
+	I-chemical
B	O
,	O
respectively	O
(	O
Figs	O
.	O
3A	O
and	O
5C	O
′),	O
may	O
represent	O
this	O
activated	O
intermediate	O
.	O

Subsequent	O
binding	O
of	O
an	O
incoming	O
nucleotide	B-chemical
to	O
site	B-site
2	I-site
followed	O
by	O
formation	O
of	O
the	O
Watson	B-bond_interaction
-	I-bond_interaction
Crick	I-bond_interaction
base	I-bond_interaction
pair	I-bond_interaction
with	O
a	O
nucleotide	B-chemical
in	O
the	O
template	O
strand	O
conveys	O
the	O
3	O
′-	O
OH	O
of	O
the	O
incoming	O
nucleotide	B-chemical
to	O
the	O
position	O
of	O
deprotonation	O
by	O
Mg2	B-chemical
+	I-chemical
A	O
and	O
the	O
5	B-chemical
′-	I-chemical
triphosphate	I-chemical
of	O
the	O
tRNA	B-chemical
to	O
the	O
reaction	B-site
center	I-site
(	O
Figs	O
.	O
3B	O
and	O
5D	O
).	O

Thus	O
,	O
the	O
present	O
structure	B-evidence
shows	O
that	O
this	O
3	B-protein_type
′-	I-protein_type
5	I-protein_type
′	I-protein_type
elongation	I-protein_type
enzyme	I-protein_type
utilizes	O
a	O
reaction	B-site
center	I-site
homologous	O
to	O
that	O
of	O
5	B-protein_type
′-	I-protein_type
3	I-protein_type
′	I-protein_type
elongation	I-protein_type
enzymes	I-protein_type
for	O
both	O
activation	O
and	O
elongation	O
in	O
a	O
stepwise	O
fashion	O
.	O

It	O
should	O
be	O
noted	O
that	O
TLP	B-protein_type
has	O
evolved	O
to	O
allow	O
the	O
occurrence	O
of	O
these	O
two	O
elaborate	O
reaction	O
steps	O
within	O
one	O
reaction	B-site
center	I-site
.	O

A	O
,	O
B	O
,	O
and	O
C	O
(	O
in	O
green	O
)	O
represent	O
binding	B-site
sites	I-site
for	O
Mg2	B-chemical
+	I-chemical
A	O
,	O
Mg2	B-chemical
+	I-chemical
B	O
,	O
and	O
Mg2	B-chemical
+	I-chemical
C	O
.	O
P	B-site
(	O
in	O
blue	O
)	O
represents	O
the	O
phosphate	B-site
binding	I-site
sites	I-site
;	O
O	O
−	O
(	O
in	O
red	O
)	O
is	O
the	O
binding	B-site
site	I-site
for	O
the	O
deprotonated	O
OH	O
group	O
.	O

Important	O
TLP	B-protein_type
residues	O
for	O
tRNA	B-chemical
and	O
Mg2	B-chemical
+	I-chemical
binding	O
are	O
also	O
shown	O
.	O
(	O
B	O
)	O
Structure	B-evidence
of	O
the	O
activation	O
complex	O
(	O
corresponding	O
to	O
fig	O
.	O
S8	O
).	O

GTP	B-chemical
/	O
ATP	B-chemical
binds	O
to	O
triphosphate	B-site
binding	I-site
site	I-site
1	I-site
;	O
the	O
deprotonated	O
OH	O
group	O
of	O
the	O
5	B-chemical
′-	I-chemical
phosphate	I-chemical
attacks	O
the	O
α	O
-	O
phosphate	B-chemical
of	O
GTP	B-chemical
/	O
ATP	B-chemical
,	O
and	O
PPi	B-chemical
(	O
inorganic	B-chemical
pyrophosphate	I-chemical
)	O
is	O
released	O
.	O
(	O
C	O
)	O
Possible	O
structure	B-evidence
after	O
the	O
activation	O
step	O
as	O
suggested	O
from	O
the	O
structure	B-evidence
of	O
(	O
C	O
′).	O

(	O
C	O
′)	O
Structure	B-evidence
before	O
the	O
elongation	O
reaction	O
(	O
corresponding	O
to	O
Fig	O
.	O
3A	O
).	O

The	O
5	B-chemical
′-	I-chemical
triphosphate	I-chemical
of	O
the	O
tRNA	B-chemical
binds	O
to	O
the	O
same	O
site	O
as	O
for	O
activation	O
of	O
the	O
5	O
′-	O
terminus	O
of	O
the	O
tRNA	B-chemical
in	O
(	O
B	O
).	O

The	O
base	O
of	O
the	O
incoming	O
GTP	B-chemical
forms	O
a	O
Watson	B-bond_interaction
-	I-bond_interaction
Crick	I-bond_interaction
hydrogen	I-bond_interaction
bond	I-bond_interaction
with	O
the	O
nucleotide	B-chemical
at	O
position	O
72	B-residue_number
in	O
the	O
template	O
chain	O
and	O
a	O
base	B-bond_interaction
-	I-bond_interaction
stacking	I-bond_interaction
interaction	I-bond_interaction
with	O
a	O
neighboring	O
base	O
(	O
G2	B-residue_name_number
).	O

The	O
3	O
′-	O
OH	O
of	O
the	O
incoming	O
GTP	B-chemical
is	O
deprotonated	O
by	O
Mg2	B-chemical
+	I-chemical
A	O
and	O
attacks	O
the	O
α	O
-	O
phosphate	B-chemical
to	O
form	O
a	O
covalent	O
bond	O
.	O
(	O
E	O
)	O
After	O
the	O
elongation	O
reaction	O
,	O
the	O
triphosphate	B-chemical
of	O
the	O
new	O
nucleotide	B-chemical
is	O
placed	O
on	O
site	B-site
1	I-site
,	O
as	O
in	O
(	O
C	O
′),	O
and	O
is	O
ready	O
for	O
the	O
next	O
reaction	O
.	O

Figure	O
6	O
compares	O
the	O
3	O
′-	O
5	O
′	O
and	O
5	O
′-	O
3	O
′	O
elongation	O
mechanisms	O
,	O
showing	O
the	O
symmetrical	O
nature	O
of	O
both	O
elongation	O
reactions	O
using	O
a	O
similar	O
reaction	B-site
center	I-site
composed	O
of	O
Mg2	B-chemical
+	I-chemical
A	O
and	O
Mg2	B-chemical
+	I-chemical
B	O
in	O
the	O
conserved	B-protein_state
catalytic	B-site
core	I-site
.	O

In	O
TLP	B-protein_type
,	O
which	O
carries	O
out	O
3	O
′-	O
5	O
′	O
elongation	O
,	O
the	O
3	O
′-	O
OH	O
of	O
the	O
incoming	O
nucleotide	O
attacks	O
the	O
5	O
′-	O
activated	O
phosphate	B-chemical
of	O
the	O
tRNA	B-chemical
to	O
form	O
a	O
phosphodiester	O
bond	O
,	O
whereas	O
in	O
the	O
T7	B-protein
RNA	I-protein
polymerase	I-protein
,	O
a	O
representative	O
5	B-protein_type
′-	I-protein_type
3	I-protein_type
′	I-protein_type
DNA	I-protein_type
/	I-protein_type
RNA	I-protein_type
polymerase	I-protein_type
,	O
the	O
3	O
′-	O
OH	O
of	O
the	O
3	O
′-	O
terminal	O
nucleotide	O
of	O
the	O
RNA	B-chemical
attacks	O
the	O
activated	O
phosphate	B-chemical
of	O
the	O
incoming	O
nucleotide	O
to	O
form	O
a	O
phosphodiester	O
bond	O
.	O

Mg2	B-chemical
+	I-chemical
A	O
activates	O
the	O
3	O
′-	O
OH	O
of	O
the	O
incoming	O
nucleotide	O
in	O
TLP	B-protein_type
and	O
the	O
3	O
′-	O
OH	O
of	O
the	O
3	O
′-	O
end	O
of	O
the	O
RNA	B-chemical
chain	O
in	O
T7	B-protein
RNA	I-protein
polymerase	I-protein
.	O

Symmetrical	O
relationship	O
between	O
3	O
′-	O
5	O
′	O
elongation	O
by	O
TLP	B-protein_type
(	O
this	O
study	O
)	O
(	O
left	O
)	O
and	O
5	O
′-	O
3	O
′	O
elongation	O
by	O
T7	B-protein
RNA	I-protein
polymerase	I-protein
[	O
Protein	O
Data	O
Bank	O
(	O
PDB	O
)	O
ID	O
:	O
1S76	O
]	O
(	O
right	O
).	O

In	O
the	O
3	O
′-	O
5	O
′	O
elongation	O
reaction	O
,	O
the	O
3	O
′-	O
OH	O
of	O
the	O
incoming	O
nucleotide	O
attacks	O
the	O
5	O
′-	O
activated	O
phosphate	B-chemical
of	O
the	O
tRNA	B-chemical
to	O
form	O
a	O
phosphodiester	O
bond	O
,	O
whereas	O
in	O
the	O
5	O
′-	O
3	O
′	O
elongation	O
reaction	O
,	O
the	O
3	O
′-	O
OH	O
of	O
the	O
3	O
′-	O
terminal	O
nucleotide	O
of	O
the	O
RNA	B-chemical
attacks	O
the	O
activated	O
phosphate	B-chemical
of	O
the	O
incoming	O
nucleotide	O
to	O
form	O
a	O
phosphodiester	O
bond	O
.	O

Green	O
spheres	O
represent	O
Mg2	B-chemical
+	I-chemical
ions	O
.	O

Because	O
the	O
chemical	O
roles	O
of	O
tRNA	B-chemical
and	O
the	O
incoming	O
nucleotide	O
are	O
reversed	O
in	O
these	O
two	O
reactions	O
,	O
these	O
two	O
substrates	O
are	O
inserted	O
into	O
a	O
similar	O
reaction	B-site
center	I-site
from	O
opposite	O
directions	O
(	O
Fig	O
.	O
6	O
).	O

For	O
this	O
reason	O
,	O
TLP	B-protein_type
requires	O
a	O
mechanism	O
that	O
activates	O
the	O
5	O
′-	O
terminus	O
of	O
the	O
tRNA	B-chemical
during	O
the	O
initial	O
step	O
of	O
the	O
reaction	O
.	O

Our	O
analysis	O
showed	O
that	O
the	O
initial	O
activation	O
and	O
subsequent	O
elongation	O
reactions	O
occur	O
sequentially	O
at	O
one	O
reaction	B-site
center	I-site
.	O

In	O
this	O
case	O
,	O
the	O
enzyme	O
needs	O
to	O
create	O
two	O
substrate	B-site
binding	I-site
sites	I-site
for	O
two	O
different	O
reactions	O
in	O
the	O
vicinities	O
of	O
one	O
reaction	B-site
center	I-site
.	O

These	O
structural	O
features	O
of	O
the	O
TLP	B-protein_type
molecule	O
suggest	O
that	O
development	O
of	O
an	O
activation	B-site
reaction	I-site
site	I-site
is	O
a	O
prerequisite	O
for	O
developing	O
the	O
3	B-protein_type
′-	I-protein_type
5	I-protein_type
′	I-protein_type
elongation	I-protein_type
enzyme	I-protein_type
.	O

This	O
is	O
clearly	O
more	O
difficult	O
than	O
developing	O
the	O
5	B-protein_type
′-	I-protein_type
3	I-protein_type
′	I-protein_type
elongation	I-protein_type
enzyme	I-protein_type
,	O
wherein	O
the	O
activation	B-site
reaction	I-site
site	I-site
is	O
not	O
necessary	O
,	O
and	O
which	O
may	O
be	O
the	O
primary	O
reason	O
why	O
the	O
5	B-protein_type
′-	I-protein_type
3	I-protein_type
′	I-protein_type
elongation	I-protein_type
enzyme	I-protein_type
has	O
been	O
exclusively	O
developed	O
.	O

Here	O
,	O
we	O
established	O
a	O
structural	O
basis	O
for	O
3	O
′-	O
5	O
′	O
nucleotide	O
elongation	O
and	O
showed	O
that	O
TLP	B-protein_type
has	O
evolved	O
to	O
acquire	O
a	O
two	O
-	O
step	O
Watson	O
-	O
Crick	O
template	O
–	O
dependent	O
3	O
′-	O
5	O
′	O
elongation	O
reaction	O
using	O
the	O
catalytic	B-site
center	I-site
homologous	O
to	O
5	B-protein_type
′-	I-protein_type
3	I-protein_type
′	I-protein_type
elongation	I-protein_type
enzymes	I-protein_type
.	O

The	O
active	B-site
site	I-site
of	O
this	O
enzyme	O
is	O
created	O
at	O
the	O
dimerization	B-site
interface	I-site
.	O

The	O
dimerization	O
also	O
endows	O
this	O
protein	O
with	O
the	O
ability	O
to	O
measure	O
the	O
length	O
of	O
the	O
accepter	B-structure_element
stem	I-structure_element
of	O
the	O
tRNA	B-chemical
substrate	O
,	O
so	O
that	O
the	O
enzyme	O
can	O
properly	O
terminate	O
the	O
elongation	O
reaction	O
.	O

The	O
detailed	O
molecular	O
mechanism	O
of	O
the	O
Thg1	B-protein
/	O
TLP	B-protein_type
family	O
established	O
by	O
our	O
analysis	O
will	O
open	O
up	O
new	O
perspectives	O
in	O
our	O
understanding	O
of	O
3	O
′-	O
5	O
′	O
versus	O
5	O
′-	O
3	O
′	O
polymerization	O
and	O
the	O
molecular	O
evolution	O
of	O
template	B-protein_type
-	I-protein_type
dependent	I-protein_type
polymerases	I-protein_type
.	O

Transcribed	O
tRNAs	B-chemical
were	O
purified	O
by	O
a	O
HiTrap	O
DEAE	O
FF	O
column	O
(	O
GE	O
Healthcare	O
)	O
as	O
previously	O
described	O
.	O

The	O
highly	O
conserved	O
tRNAHis	O
guanylyltransferase	O
Thg1p	B-protein
interacts	O
with	O
the	O
origin	O
recognition	O
complex	O
and	O
is	O
required	O
for	O
the	O
G2	O
/	O
M	O
phase	O
transition	O
in	O
the	O
yeast	O
Saccharomyces	O
cerevisiae	O

To	O
support	O
antibody	B-protein_type
therapeutic	O
development	O
,	O
the	O
crystal	B-evidence
structures	I-evidence
of	O
a	O
set	O
of	O
16	O
germline	O
variants	O
composed	O
of	O
4	O
different	O
kappa	B-structure_element
light	I-structure_element
chains	I-structure_element
paired	O
with	O
4	O
different	O
heavy	B-structure_element
chains	I-structure_element
have	O
been	O
determined	O
.	O

All	O
four	O
heavy	B-structure_element
chains	I-structure_element
of	O
the	O
antigen	B-structure_element
-	I-structure_element
binding	I-structure_element
fragments	I-structure_element
(	O
Fabs	B-structure_element
)	O
have	O
the	O
same	O
complementarity	B-structure_element
-	I-structure_element
determining	I-structure_element
region	I-structure_element
(	O
CDR	B-structure_element
)	O
H3	B-structure_element
that	O
was	O
reported	O
in	O
an	O
earlier	O
Fab	B-structure_element
structure	B-evidence
.	O

The	O
structure	B-experimental_method
analyses	I-experimental_method
include	O
comparisons	O
of	O
the	O
overall	O
structures	B-evidence
,	O
canonical	O
structures	B-evidence
of	O
the	O
CDRs	B-structure_element
and	O
the	O
VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
packing	B-bond_interaction
interactions	I-bond_interaction
.	O

The	O
CDR	B-structure_element
conformations	O
for	O
the	O
most	O
part	O
are	O
tightly	O
clustered	O
,	O
especially	O
for	O
the	O
ones	O
with	O
shorter	O
lengths	O
.	O

CDR	B-structure_element
H3	B-structure_element
,	O
despite	O
having	O
the	O
same	O
amino	O
acid	O
sequence	O
,	O
exhibits	O
the	O
largest	O
conformational	O
diversity	O
.	O

About	O
half	O
of	O
the	O
structures	B-evidence
have	O
CDR	B-structure_element
H3	B-structure_element
conformations	O
similar	O
to	O
that	O
of	O
the	O
parent	O
;	O
the	O
others	O
diverge	O
significantly	O
.	O

The	O
stem	B-structure_element
regions	I-structure_element
of	O
14	O
of	O
the	O
variant	O
pairs	O
are	O
in	O
the	O
‘	O
kinked	B-protein_state
’	O
conformation	O
,	O
and	O
only	O
2	O
are	O
in	O
the	O
extended	B-protein_state
conformation	O
.	O

The	O
packing	O
of	O
the	O
VH	B-structure_element
and	O
VL	B-structure_element
domains	O
is	O
consistent	O
with	O
our	O
knowledge	O
of	O
antibody	B-protein_type
structure	B-evidence
,	O
and	O
the	O
tilt	B-evidence
angles	I-evidence
between	O
these	O
domains	O
cover	O
a	O
range	O
of	O
11	O
degrees	O
.	O

Two	O
of	O
16	O
structures	B-evidence
showed	O
particularly	O
large	O
variations	O
in	O
the	O
tilt	B-evidence
angles	I-evidence
when	O
compared	O
with	O
the	O
other	O
pairings	O
.	O

At	O
present	O
,	O
therapeutic	O
antibodies	B-protein_type
are	O
the	O
largest	O
class	O
of	O
biotherapeutic	O
proteins	O
that	O
are	O
in	O
clinical	O
trials	O
.	O

The	O
use	O
of	O
monoclonal	O
antibodies	B-protein_type
as	O
therapeutics	O
began	O
in	O
the	O
early	O
1980s	O
,	O
and	O
their	O
composition	O
has	O
transitioned	O
from	O
murine	B-taxonomy_domain
antibodies	B-protein_type
to	O
generally	O
less	O
immunogenic	O
humanized	O
and	O
human	B-species
antibodies	B-protein_type
.	O

The	O
technologies	O
currently	O
used	O
to	O
obtain	O
human	B-species
antibodies	B-protein_type
include	O
transgenic	O
mice	B-taxonomy_domain
containing	O
human	B-species
antibody	B-protein_type
repertoires	O
,	O
cloning	O
directly	O
from	O
human	B-species
B	O
cells	O
,	O
and	O
in	B-experimental_method
vitro	I-experimental_method
selection	I-experimental_method
from	O
antibody	B-experimental_method
libraries	I-experimental_method
using	O
various	O
display	O
technologies	O
.	O

Once	O
a	O
candidate	O
antibody	B-protein_type
is	O
identified	O
,	O
protein	B-experimental_method
engineering	I-experimental_method
is	O
usually	O
required	O
to	O
produce	O
a	O
molecule	O
with	O
the	O
right	O
biophysical	O
and	O
functional	O
properties	O
.	O

All	O
engineering	O
efforts	O
are	O
guided	O
by	O
our	O
understanding	O
of	O
the	O
atomic	B-evidence
structures	I-evidence
of	O
antibodies	B-protein_type
.	O

Today	O
'	O
s	O
antibody	B-protein_type
modeling	O
approaches	O
,	O
which	O
normally	O
focus	O
on	O
the	O
variable	B-structure_element
region	I-structure_element
,	O
are	O
being	O
developed	O
by	O
the	O
application	O
of	O
structural	O
principles	O
and	O
insights	O
that	O
are	O
evolving	O
as	O
our	O
knowledge	O
of	O
antibody	B-protein_type
structures	B-evidence
continues	O
to	O
expand	O
.	O

Five	O
different	O
antibody	B-protein_type
isotypes	O
occur	O
,	O
IgG	B-protein
,	O
IgD	B-protein
,	O
IgE	B-protein
,	O
IgA	B-protein
and	O
IgM	B-protein
,	O
and	O
each	O
isotype	O
has	O
a	O
unique	O
role	O
in	O
the	O
adaptive	O
immune	O
system	O
.	O

Isotypes	O
IgG	B-protein
,	O
IgD	B-protein
and	O
IgA	B-protein
each	O
have	O
4	O
domains	O
,	O
one	O
variable	B-structure_element
(	O
V	B-structure_element
)	O
and	O
3	O
constant	B-structure_element
(	O
C	B-structure_element
)	O
domains	O
,	O
while	O
IgE	B-protein
and	O
IgM	B-protein
each	O
have	O
the	O
same	O
4	O
domains	O
along	O
with	O
an	O
additional	O
C	B-structure_element
domain	I-structure_element
.	O

Both	O
κ	B-structure_element
and	O
λ	B-structure_element
polypeptide	O
chains	O
are	O
composed	O
of	O
a	O
single	O
V	B-structure_element
domain	I-structure_element
and	O
a	O
single	O
C	B-structure_element
domain	I-structure_element
.	O

All	O
immunoglobulin	B-protein_type
chains	I-protein_type
have	O
an	O
N	O
-	O
terminal	O
V	B-structure_element
domain	I-structure_element
followed	O
by	O
1	O
to	O
4	O
C	B-structure_element
domains	I-structure_element
,	O
depending	O
upon	O
the	O
chain	O
type	O
.	O

In	O
antibodies	B-protein_type
,	O
the	O
heavy	B-structure_element
and	I-structure_element
light	I-structure_element
chain	I-structure_element
V	B-structure_element
domains	I-structure_element
pack	O
together	O
forming	O
the	O
antigen	B-site
combining	I-site
site	I-site
.	O

The	O
sequence	O
diversity	O
of	O
the	O
CDR	B-structure_element
regions	I-structure_element
presents	O
a	O
substantial	O
challenge	O
to	O
antibody	B-protein_type
modeling	O
.	O

However	O
,	O
an	O
initial	O
structural	B-experimental_method
analysis	I-experimental_method
of	O
the	O
combining	B-site
sites	I-site
of	O
the	O
small	O
set	O
of	O
structures	B-evidence
of	O
immunoglobulin	O
fragments	O
available	O
in	O
the	O
1980s	O
found	O
that	O
5	O
of	O
the	O
6	O
hypervariable	B-structure_element
loops	I-structure_element
or	O
CDRs	B-structure_element
had	O
canonical	O
structures	O
(	O
a	O
limited	O
set	O
of	O
main	O
-	O
chain	O
conformations	O
).	O

Furthermore	O
,	O
studies	O
of	O
antibody	B-protein_type
sequences	O
revealed	O
that	O
the	O
total	O
number	O
of	O
canonical	O
structures	O
are	O
limited	O
for	O
each	O
CDR	B-structure_element
,	O
indicating	O
possibly	O
that	O
antigen	O
recognition	O
may	O
be	O
affected	O
by	O
structural	O
restrictions	O
at	O
the	O
antigen	B-site
-	I-site
binding	I-site
site	I-site
.	O

Later	O
studies	O
found	O
that	O
the	O
CDR	B-structure_element
loop	I-structure_element
length	O
is	O
the	O
primary	O
determining	O
factor	O
of	O
antigen	B-site
-	I-site
binding	I-site
site	I-site
topography	O
because	O
it	O
is	O
the	O
primary	O
factor	O
for	O
determining	O
a	O
canonical	O
structure	O
.	O

Additional	O
efforts	O
have	O
led	O
to	O
our	O
current	O
understanding	O
that	O
the	O
LC	B-structure_element
CDRs	B-structure_element
L1	B-structure_element
,	O
L2	B-structure_element
,	O
and	O
L3	B-structure_element
have	O
preferred	O
sets	O
of	O
canonical	O
structures	O
based	O
on	O
length	O
and	O
amino	O
acid	O
sequence	O
composition	O
.	O

This	O
was	O
also	O
found	O
to	O
be	O
the	O
case	O
for	O
the	O
H1	B-structure_element
and	O
H2	B-structure_element
CDRs	B-structure_element
.	O

Recently	O
,	O
a	O
comprehensive	O
CDR	B-structure_element
classification	O
scheme	O
was	O
reported	O
identifying	O
72	O
clusters	O
of	O
conformations	O
observed	O
in	O
antibody	B-protein_type
structures	B-evidence
.	O

The	O
knowledge	O
and	O
predictability	O
of	O
these	O
CDR	B-structure_element
canonical	O
structures	B-evidence
have	O
greatly	O
advanced	O
antibody	B-protein_type
modeling	O
efforts	O
.	O

In	O
contrast	O
to	O
CDRs	B-structure_element
L1	B-structure_element
,	O
L2	B-structure_element
,	O
L3	B-structure_element
,	O
H1	B-structure_element
and	O
H2	B-structure_element
,	O
no	O
canonical	O
structures	B-evidence
have	O
been	O
observed	O
for	O
CDR	B-structure_element
H3	B-structure_element
,	O
which	O
is	O
the	O
most	O
variable	O
in	O
length	O
and	O
amino	O
acid	O
sequence	O
.	O

In	O
the	O
torso	B-structure_element
region	I-structure_element
,	O
2	O
primary	O
groups	O
could	O
be	O
identified	O
,	O
which	O
led	O
to	O
sequence	O
-	O
based	O
rules	O
that	O
can	O
predict	O
with	O
some	O
degree	O
of	O
reliability	O
the	O
conformation	O
of	O
the	O
stem	B-structure_element
region	I-structure_element
.	O

The	O
cataloging	O
and	O
development	O
of	O
the	O
rules	O
for	O
predicting	O
the	O
conformation	O
of	O
the	O
anchor	B-structure_element
region	I-structure_element
of	O
CDR	B-structure_element
H3	B-structure_element
continue	O
to	O
be	O
refined	O
,	O
producing	O
new	O
insight	O
into	O
the	O
CDR	B-structure_element
H3	B-structure_element
conformations	O
and	O
new	O
tools	O
for	O
antibody	B-protein_type
engineering	O
.	O

Although	O
antibody	B-protein_type
modeling	O
is	O
improving	O
,	O
the	O
latest	O
assessment	O
revealed	O
a	O
number	O
of	O
challenges	O
that	O
need	O
to	O
be	O
overcome	O
to	O
provide	O
accurate	O
3	O
-	O
dimensional	O
models	O
of	O
antibody	B-protein_type
V	B-structure_element
regions	I-structure_element
,	O
including	O
accuracies	O
in	O
the	O
modeling	O
of	O
CDR	B-structure_element
H3	B-structure_element
.	O

To	O
support	O
antibody	B-protein_type
engineering	O
and	O
therapeutic	O
development	O
efforts	O
,	O
a	O
phage	B-experimental_method
library	I-experimental_method
was	O
designed	O
and	O
constructed	O
based	O
on	O
a	O
limited	O
number	O
of	O
scaffolds	O
built	O
with	O
frequently	O
used	O
human	B-species
germ	O
-	O
line	O
IGV	B-structure_element
and	O
IGJ	B-structure_element
gene	O
segments	O
that	O
encode	O
antigen	B-site
combining	I-site
sites	I-site
suitable	O
for	O
recognition	O
of	O
peptides	O
and	O
proteins	O
.	O

Selection	O
of	O
these	O
genes	O
was	O
based	O
on	O
the	O
high	O
frequency	O
of	O
their	O
use	O
and	O
their	O
cognate	O
canonical	O
structures	B-evidence
that	O
were	O
found	O
binding	O
to	O
peptides	O
and	O
proteins	O
,	O
as	O
well	O
as	O
their	O
ability	O
to	O
be	O
expressed	B-experimental_method
in	I-experimental_method
bacteria	I-experimental_method
and	O
displayed	B-experimental_method
on	I-experimental_method
filamentous	I-experimental_method
phage	I-experimental_method
.	O

The	O
implementation	O
of	O
the	O
library	O
involves	O
the	O
diversification	O
of	O
the	O
human	B-species
germline	O
genes	O
to	O
mimic	O
that	O
found	O
in	O
natural	O
human	B-species
libraries	O
.	O

The	O
crystal	B-experimental_method
structure	I-experimental_method
determinations	I-experimental_method
and	O
structural	B-experimental_method
analyses	I-experimental_method
of	O
all	O
germline	O
Fabs	B-structure_element
in	O
the	O
library	O
described	O
above	O
along	O
with	O
the	O
structures	B-evidence
of	O
a	O
fourth	O
HC	B-structure_element
germline	O
,	O
IGHV3	B-mutant
-	I-mutant
53	I-mutant
(	O
H3	B-mutant
-	I-mutant
53	I-mutant
),	O
paired	O
with	O
the	O
4	O
LCs	B-structure_element
of	O
the	O
library	O
have	O
been	O
carried	O
out	O
to	O
support	O
antibody	B-protein_type
therapeutic	O
development	O
.	O

The	O
structures	B-evidence
and	O
their	O
analyses	O
provide	O
a	O
foundation	O
for	O
future	O
antibody	B-protein_type
engineering	O
and	O
structure	O
determination	O
efforts	O
.	O

Crystal	B-evidence
structures	I-evidence

Crystal	B-evidence
data	I-evidence
,	O
X	B-evidence
-	I-evidence
ray	I-evidence
data	I-evidence
,	O
and	O
refinement	B-evidence
statistics	I-evidence
.	O

(	O
Continued	O
)	O
Crystal	B-evidence
data	I-evidence
,	O
X	B-evidence
-	I-evidence
ray	I-evidence
data	I-evidence
,	O
and	O
refinement	B-evidence
statistics	I-evidence
.	O

The	O
crystal	B-evidence
structures	I-evidence
of	O
a	O
germline	B-experimental_method
library	I-experimental_method
composed	O
of	O
16	O
Fabs	B-structure_element
generated	O
by	O
combining	O
4	O
HCs	B-structure_element
(	O
H1	B-mutant
-	I-mutant
69	I-mutant
,	O
H3	B-mutant
-	I-mutant
23	I-mutant
,	O
H3	B-mutant
-	I-mutant
53	I-mutant
and	O
H5	B-mutant
-	I-mutant
51	I-mutant
)	O
and	O
4	O
LCs	B-structure_element
(	O
L1	B-mutant
-	I-mutant
39	I-mutant
,	O
L3	B-mutant
-	I-mutant
11	I-mutant
,	O
L3	B-mutant
-	I-mutant
20	I-mutant
and	O
L4	B-mutant
-	I-mutant
1	I-mutant
)	O
have	O
been	O
determined	O
.	O

The	O
Fab	B-structure_element
heavy	O
and	O
light	B-structure_element
chain	I-structure_element
sequences	O
for	O
the	O
variants	O
numbered	O
according	O
to	O
Chothia	O
are	O
shown	O
in	O
Fig	O
.	O
S1	O
.	O

The	O
four	O
different	O
HCs	B-structure_element
all	O
have	O
the	O
same	O
CDR	B-structure_element
H3	B-structure_element
sequence	O
,	O
ARYDGIYGELDF	B-structure_element
.	O

Variations	O
occur	O
in	O
the	O
pH	O
(	O
buffer	O
)	O
and	O
the	O
additives	O
,	O
and	O
,	O
in	O
group	O
3	O
,	O
PEG	B-chemical
3350	I-chemical
is	O
the	O
precipitant	O
for	O
one	O
variants	O
while	O
ammonium	B-chemical
sulfate	I-chemical
is	O
the	O
precipitant	O
for	O
the	O
other	O
two	O
.	O

The	O
similarity	O
in	O
the	O
crystal	B-evidence
forms	I-evidence
is	O
attributed	O
in	O
part	O
to	O
cross	O
-	O
seeding	O
using	O
the	O
microseed	B-experimental_method
matrix	I-experimental_method
screening	I-experimental_method
for	O
groups	O
2	O
and	O
3	O
.	O

The	O
crystal	B-evidence
structures	I-evidence
of	O
the	O
16	O
Fabs	B-structure_element
have	O
been	O
determined	O
at	O
resolutions	O
ranging	O
from	O
3	O
.	O
3	O
Å	O
to	O
1	O
.	O
65	O
Å	O
(	O
Table	O
1	O
).	O

The	O
number	O
of	O
Fab	B-structure_element
molecules	O
in	O
the	O
crystallographic	O
asymmetric	O
unit	O
varies	O
from	O
1	O
(	O
for	O
12	O
Fabs	B-structure_element
)	O
to	O
2	O
(	O
for	O
4	O
Fabs	B-structure_element
).	O

Overall	O
the	O
structures	B-evidence
are	O
fairly	O
complete	O
,	O
and	O
,	O
as	O
can	O
be	O
expected	O
,	O
the	O
models	O
for	O
the	O
higher	O
resolution	O
structures	B-evidence
are	O
more	O
complete	O
than	O
those	O
for	O
the	O
lower	O
resolution	O
structures	B-evidence
(	O
Table	O
S1	O
).	O

Invariably	O
,	O
the	O
HCs	B-structure_element
have	O
more	O
disorder	B-protein_state
than	O
the	O
LCs	B-structure_element
.	O

For	O
the	O
LC	B-structure_element
,	O
the	O
disorder	B-protein_state
is	O
observed	O
at	O
2	O
of	O
the	O
C	O
-	O
terminal	O
residues	O
with	O
few	O
exceptions	O
.	O

Apart	O
from	O
the	O
C	O
-	O
terminus	O
,	O
only	O
a	O
few	O
surface	O
residues	O
in	O
LC	B-structure_element
are	O
disordered	B-protein_state
.	O

The	O
HCs	B-structure_element
feature	O
the	O
largest	O
number	O
of	O
disordered	B-protein_state
residues	O
,	O
with	O
the	O
lower	O
resolution	O
structures	B-evidence
having	O
the	O
most	O
.	O

The	O
C	O
-	O
terminal	O
residues	O
including	O
the	O
6xHis	O
tags	O
are	O
disordered	B-protein_state
in	O
all	O
16	O
structures	B-evidence
.	O

One	O
involves	O
the	O
loop	B-structure_element
connecting	O
the	O
first	O
2	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
of	O
the	O
constant	B-structure_element
domain	I-structure_element
(	O
in	O
all	O
Fabs	B-structure_element
except	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
,	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
11	I-complex_assembly
and	O
H3	B-complex_assembly
-	I-complex_assembly
53	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
).	O

CDR	B-structure_element
H1	B-structure_element
and	O
CDR	B-structure_element
H2	B-structure_element
also	O
show	O
some	O
degree	O
of	O
disorder	B-protein_state
,	O
but	O
to	O
a	O
lesser	O
extent	O
.	O

CDR	B-structure_element
canonical	O
structures	B-evidence

Several	O
CDR	B-structure_element
definitions	O
have	O
evolved	O
over	O
decades	O
of	O
antibody	B-protein_type
research	O
.	O

Depending	O
on	O
the	O
focus	O
of	O
the	O
study	O
,	O
the	O
CDR	B-structure_element
boundaries	O
differ	O
slightly	O
between	O
various	O
definitions	O
.	O

In	O
this	O
work	O
,	O
we	O
use	O
the	O
CDR	B-structure_element
definition	O
of	O
North	O
et	O
al	O
.,	O
which	O
is	O
similar	O
to	O
that	O
of	O
Martin	O
with	O
the	O
following	O
exceptions	O
:	O
1	O
)	O
CDRs	B-structure_element
H1	B-structure_element
and	O
H3	B-structure_element
begin	O
immediately	O
after	O
the	O
Cys	B-residue_name
;	O
and	O
2	O
)	O
CDR	B-structure_element
L2	B-structure_element
includes	O
an	O
additional	O
residue	O
at	O
the	O
N	O
-	O
terminal	O
side	O
,	O
typically	O
Tyr	B-residue_name
.	O

CDR	B-structure_element
H1	B-structure_element

The	O
superposition	B-experimental_method
of	O
CDR	B-structure_element
H1	B-structure_element
backbones	O
for	O
all	O
HC	B-complex_assembly
:	I-complex_assembly
LC	I-complex_assembly
pairs	O
with	O
heavy	B-structure_element
chains	I-structure_element
:	O
(	O
A	O
)	O
H1	B-mutant
-	I-mutant
69	I-mutant
,	O
(	O
B	O
)	O
H3	B-mutant
-	I-mutant
23	I-mutant
,	O
(	O
C	O
)	O
H3	B-mutant
-	I-mutant
53	I-mutant
and	O
(	O
D	O
)	O
H5	B-mutant
-	I-mutant
51	I-mutant
.	O

CDRs	B-structure_element
are	O
defined	O
using	O
the	O
Dunbrack	O
convention	O
[	O
12	O
].	O

Assignments	O
for	O
2	O
copies	O
of	O
the	O
Fab	B-structure_element
in	O
the	O
asymmetric	O
unit	O
are	O
given	O
for	O
5	O
structures	B-evidence
.	O

The	O
four	O
HCs	B-structure_element
feature	O
CDR	B-structure_element
H1	B-structure_element
of	O
the	O
same	O
length	O
,	O
and	O
their	O
sequences	O
are	O
highly	O
similar	O
(	O
Table	O
2	O
).	O

Three	O
of	O
the	O
HCs	B-structure_element
,	O
H3	B-mutant
-	I-mutant
23	I-mutant
,	O
H3	B-mutant
-	I-mutant
53	I-mutant
and	O
H5	B-mutant
-	I-mutant
51	I-mutant
,	O
have	O
the	O
same	O
canonical	O
structure	O
,	O
H1	B-mutant
-	I-mutant
13	I-mutant
-	I-mutant
1	I-mutant
,	O
and	O
the	O
backbone	O
conformations	O
are	O
tightly	O
clustered	O
for	O
each	O
set	O
of	O
Fab	B-structure_element
structures	B-evidence
as	O
reflected	O
in	O
the	O
rmsd	B-evidence
values	I-evidence
(	O
Fig	O
.	O
1B	O
-	O
D	O
).	O

Some	O
deviation	O
is	O
observed	O
for	O
H3	B-mutant
-	I-mutant
53	I-mutant
,	O
mostly	O
due	O
to	O
H3	B-complex_assembly
-	I-complex_assembly
53	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
,	O
which	O
exhibits	O
a	O
significant	O
degree	O
of	O
disorder	O
in	O
CDR	B-structure_element
H1	B-structure_element
.	O

The	O
electron	B-evidence
density	I-evidence
for	O
the	O
backbone	O
is	O
weak	O
and	O
discontinuous	O
,	O
and	O
completely	O
missing	O
for	O
several	O
side	O
chains	O
.	O

The	O
CDR	B-structure_element
H1	B-structure_element
structures	B-evidence
with	O
H1	B-mutant
-	I-mutant
69	I-mutant
shown	O
in	O
Fig	O
.	O
1A	O
are	O
quite	O
variable	O
,	O
both	O
for	O
the	O
structures	B-evidence
with	O
different	O
LCs	B-structure_element
and	O
for	O
the	O
copies	O
of	O
the	O
same	O
Fab	B-structure_element
in	O
the	O
asymmetric	O
unit	O
,	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
11	I-complex_assembly
and	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
.	O

In	O
total	O
,	O
6	O
independent	O
Fab	B-structure_element
structures	B-evidence
produce	O
5	O
different	O
canonical	O
structures	B-evidence
,	O
namely	O
H1	B-mutant
-	I-mutant
13	I-mutant
-	I-mutant
1	I-mutant
,	O
H1	B-mutant
-	I-mutant
13	I-mutant
-	I-mutant
3	I-mutant
,	O
H1	B-mutant
-	I-mutant
13	I-mutant
-	I-mutant
4	I-mutant
,	O
H1	B-mutant
-	I-mutant
13	I-mutant
-	I-mutant
6	I-mutant
and	O
H1	B-mutant
-	I-mutant
13	I-mutant
-	I-mutant
10	I-mutant
.	O

Glycine	B-residue_name
introduces	O
the	O
possibility	O
of	O
a	O
higher	O
degree	O
of	O
conformational	O
flexibility	O
that	O
undoubtedly	O
translates	O
to	O
the	O
differences	O
observed	O
,	O
and	O
contributes	O
to	O
the	O
elevated	O
thermal	O
parameters	O
for	O
the	O
atoms	O
in	O
the	O
amino	O
acid	O
residues	O
in	O
this	O
region	O
.	O

CDR	B-structure_element
H2	B-structure_element

The	O
canonical	O
structures	O
of	O
CDR	B-structure_element
H2	B-structure_element
have	O
fairly	O
consistent	O
conformations	O
(	O
Table	O
2	O
,	O
Fig	O
.	O
2	O
).	O

Each	O
of	O
the	O
4	O
HCs	B-structure_element
adopts	O
only	O
one	O
canonical	O
structure	O
regardless	O
of	O
the	O
pairing	O
LC	B-structure_element
.	O

Germlines	O
H1	B-mutant
-	I-mutant
69	I-mutant
and	O
H5	B-mutant
-	I-mutant
51	I-mutant
have	O
the	O
same	O
canonical	O
structure	O
assignment	O
H2	B-mutant
-	I-mutant
10	I-mutant
-	I-mutant
1	I-mutant
,	O
H3	B-mutant
-	I-mutant
23	I-mutant
has	O
H2	B-mutant
-	I-mutant
10	I-mutant
-	I-mutant
2	I-mutant
,	O
and	O
H3	B-mutant
-	I-mutant
53	I-mutant
has	O
H2	B-mutant
-	I-mutant
9	I-mutant
-	I-mutant
3	I-mutant
.	O

The	O
conformations	O
for	O
all	O
of	O
these	O
CDR	B-structure_element
H2s	B-structure_element
are	O
tightly	O
clustered	O
(	O
Fig	O
.	O
2	O
).	O

In	O
one	O
case	O
,	O
in	O
the	O
second	O
Fab	B-structure_element
of	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
,	O
CDR	B-structure_element
H2	B-structure_element
is	O
partially	B-protein_state
disordered	I-protein_state
(	O
Δ55	B-mutant
-	I-mutant
60	I-mutant
).	O

Although	O
three	O
of	O
the	O
germlines	O
have	O
CDR	B-structure_element
H2	B-structure_element
of	O
the	O
same	O
length	O
,	O
10	B-residue_range
residues	I-residue_range
,	O
they	O
adopt	O
2	O
distinctively	O
different	O
conformations	O
depending	O
mostly	O
on	O
the	O
residue	O
at	O
position	O
71	B-residue_number
from	O
the	O
so	O
-	O
called	O
CDR	B-structure_element
H4	B-structure_element
.	O

Germlines	O
H1	B-mutant
-	I-mutant
69	I-mutant
and	O
H5	B-mutant
-	I-mutant
51	I-mutant
are	O
unique	O
in	O
the	O
human	B-species
repertoire	O
in	O
having	O
an	O
Ala	B-residue_name
at	O
position	O
71	B-residue_number
that	O
leaves	O
enough	O
space	O
for	O
H	B-structure_element
-	O
Pro52a	B-residue_name_number
to	O
pack	O
deeper	O
against	O
CDR	B-structure_element
H4	B-structure_element
so	O
that	O
the	O
following	O
residues	O
53	B-residue_number
and	O
54	B-residue_number
point	O
toward	O
the	O
putative	O
antigen	O
.	O

Conformations	O
of	O
CDR	B-structure_element
H2	B-structure_element
in	O
H1	B-mutant
-	I-mutant
69	I-mutant
and	O
H5	B-mutant
-	I-mutant
51	I-mutant
,	O
both	O
of	O
which	O
have	O
canonical	O
structure	O
H2	B-mutant
-	I-mutant
10	I-mutant
-	I-mutant
1	I-mutant
,	O
show	O
little	O
deviation	O
within	O
each	O
set	O
of	O
4	O
structures	B-evidence
.	O

However	O
,	O
there	O
is	O
a	O
significant	O
shift	O
of	O
the	O
CDR	B-structure_element
as	O
a	O
rigid	O
body	O
when	O
the	O
2	O
sets	O
are	O
superimposed	B-experimental_method
.	O

Most	O
likely	O
this	O
is	O
the	O
result	O
of	O
interaction	O
of	O
CDR	B-structure_element
H2	B-structure_element
with	O
CDR	B-structure_element
H1	B-structure_element
,	O
namely	O
with	O
the	O
residue	O
at	O
position	O
33	B-residue_number
(	O
residue	O
11	O
of	O
13	O
in	O
CDR	B-structure_element
H1	B-structure_element
).	O

Germline	O
H1	B-mutant
-	I-mutant
69	I-mutant
has	O
Ala	B-residue_name
at	O
position	O
33	B-residue_number
whereas	O
in	O
H5	B-mutant
-	I-mutant
51	I-mutant
position	O
33	B-residue_number
is	O
occupied	O
by	O
a	O
bulky	O
Trp	B-residue_name
,	O
which	O
stacks	O
against	O
H	B-structure_element
-	O
Tyr52	B-residue_name_number
and	O
drives	O
CDR	B-structure_element
H2	B-structure_element
away	O
from	O
the	O
center	O
.	O

CDR	B-structure_element
L1	B-structure_element

The	O
four	O
LC	B-structure_element
CDRs	B-structure_element
L1	B-structure_element
feature	O
3	O
different	O
lengths	O
(	O
11	B-residue_range
,	O
12	B-residue_range
and	O
17	B-residue_range
residues	O
)	O
having	O
a	O
total	O
of	O
4	O
different	O
canonical	O
structure	O
assignments	O
.	O

Of	O
these	O
LCs	B-structure_element
,	O
L1	B-mutant
-	I-mutant
39	I-mutant
and	O
L3	B-mutant
-	I-mutant
11	I-mutant
have	O
the	O
same	O
canonical	O
structure	O
,	O
L1	B-mutant
-	I-mutant
11	I-mutant
-	I-mutant
1	I-mutant
,	O
and	O
superimpose	B-experimental_method
very	O
well	O
(	O
Fig	O
.	O
3A	O
,	O
B	O
).	O

L4	B-mutant
-	I-mutant
1	I-mutant
has	O
the	O
longest	O
CDR	B-structure_element
L1	B-structure_element
,	O
composed	O
of	O
17	B-residue_range
amino	I-residue_range
acid	I-residue_range
residues	I-residue_range
(	O
Fig	O
.	O
3D	O
).	O

Despite	O
this	O
,	O
the	O
conformations	O
are	O
tightly	O
clustered	O
(	O
rmsd	B-evidence
is	O
0	O
.	O
20	O
Å	O
).	O

The	O
backbone	O
conformations	O
of	O
the	O
stem	B-structure_element
regions	I-structure_element
superimpose	O
well	O
.	O

This	O
is	O
the	O
tip	O
of	O
the	O
loop	B-structure_element
region	I-structure_element
,	O
which	O
appears	O
to	O
have	O
similar	O
conformations	O
that	O
fan	O
out	O
the	O
structures	B-evidence
because	O
of	O
the	O
slight	O
differences	O
in	O
torsion	O
angles	O
in	O
the	O
backbone	O
near	O
Tyr30a	B-residue_name_number
and	O
Lys30f	B-residue_name_number
.	O

L3	B-mutant
-	I-mutant
20	I-mutant
is	O
the	O
most	O
variable	O
in	O
CDR	B-structure_element
L1	B-structure_element
among	O
the	O
4	O
germlines	O
as	O
indicated	O
by	O
an	O
rmsd	B-evidence
of	O
0	O
.	O
54	O
Å	O
(	O
Fig	O
.	O
3C	O
).	O

The	O
fourth	O
member	O
of	O
the	O
set	O
,	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
,	O
was	O
crystallized	B-experimental_method
with	O
2	O
Fabs	B-structure_element
in	O
the	O
asymmetric	O
unit	O
.	O

This	O
reflects	O
the	O
lack	O
of	O
accuracy	O
in	O
the	O
structure	B-evidence
due	O
to	O
low	O
resolution	O
of	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
data	I-evidence
(	O
3	O
.	O
3	O
Å	O
).	O

CDR	B-structure_element
L2	B-structure_element

The	O
superposition	B-experimental_method
of	O
CDR	B-structure_element
L2	B-structure_element
backbones	O
for	O
all	O
HC	B-complex_assembly
:	I-complex_assembly
LC	I-complex_assembly
pairs	O
with	O
light	B-structure_element
chains	I-structure_element
:	O
(	O
A	O
)	O
L1	B-mutant
-	I-mutant
39	I-mutant
,	O
(	O
B	O
)	O
L3	B-mutant
-	I-mutant
11	I-mutant
,	O
(	O
C	O
)	O
L3	B-mutant
-	I-mutant
20	I-mutant
and	O
(	O
D	O
)	O
L4	B-mutant
-	I-mutant
1	I-mutant
.	O

All	O
four	O
LCs	B-structure_element
have	O
CDR	B-structure_element
L2	B-structure_element
of	O
the	O
same	O
length	O
and	O
canonical	O
structure	O
,	O
L2	B-mutant
-	I-mutant
8	I-mutant
-	I-mutant
1	I-mutant
(	O
Table	O
2	O
).	O

The	O
CDR	B-structure_element
L2	B-structure_element
conformations	O
for	O
each	O
of	O
the	O
LCs	B-structure_element
paired	O
with	O
the	O
4	O
HCs	B-structure_element
are	O
clustered	O
more	O
tightly	O
than	O
any	O
of	O
the	O
other	O
CDRs	B-structure_element
(	O
rmsd	B-evidence
values	O
are	O
in	O
the	O
range	O
0	O
.	O
09	O
-	O
0	O
.	O
16	O
Å	O
),	O
and	O
all	O
4	O
sets	O
have	O
virtually	O
the	O
same	O
conformation	O
despite	O
the	O
sequence	O
diversity	O
of	O
the	O
loop	B-structure_element
.	O

CDR	B-structure_element
L3	B-structure_element

The	O
superposition	B-experimental_method
of	O
CDR	B-structure_element
L3	B-structure_element
backbones	O
for	O
all	O
HC	B-complex_assembly
:	I-complex_assembly
LC	I-complex_assembly
pairs	O
with	O
light	B-structure_element
chains	I-structure_element
:	O
(	O
A	O
)	O
L1	B-mutant
-	I-mutant
39	I-mutant
,	O
(	O
B	O
)	O
L3	B-mutant
-	I-mutant
11	I-mutant
,	O
(	O
C	O
)	O
L3	B-mutant
-	I-mutant
20	I-mutant
and	O
(	O
D	O
)	O
L4	B-mutant
-	I-mutant
1	I-mutant
.	O

The	O
conformations	O
of	O
CDR	B-structure_element
L3	B-structure_element
for	O
L1	B-mutant
-	I-mutant
39	I-mutant
,	O
L3	B-mutant
-	I-mutant
11	I-mutant
,	O
and	O
particularly	O
for	O
L320	O
,	O
are	O
not	O
as	O
tightly	O
clustered	O
as	O
those	O
of	O
L4	B-mutant
-	I-mutant
1	I-mutant
(	O
Fig	O
.	O
5	O
).	O

The	O
slight	O
conformational	O
variability	O
occurs	O
in	O
the	O
region	O
of	O
amino	O
acid	O
residues	O
90	B-residue_range
-	I-residue_range
92	I-residue_range
,	O
which	O
is	O
in	O
contact	O
with	O
CDR	B-structure_element
H3	B-structure_element
.	O

CDR	B-structure_element
H3	B-structure_element
conformational	O
diversity	O

The	O
loop	B-structure_element
and	O
the	O
2	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
of	O
the	O
CDR	B-structure_element
H3	B-structure_element
in	O
this	O
‘	O
parent	O
’	O
structure	B-evidence
are	O
stabilized	O
by	O
H	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
between	O
the	O
carbonyl	O
oxygen	O
and	O
peptide	O
nitrogen	O
atoms	O
in	O
the	O
2	O
strands	O
.	O

An	O
interesting	O
feature	O
of	O
these	O
CDR	B-structure_element
H3	B-structure_element
structures	B-evidence
is	O
the	O
presence	O
of	O
a	O
water	B-chemical
molecule	O
that	O
interacts	O
with	O
the	O
peptide	O
nitrogens	O
and	O
carbonyl	O
oxygens	O
near	O
the	O
bridging	O
loop	B-structure_element
connecting	O
the	O
2	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
.	O

This	O
water	B-chemical
is	O
present	O
in	O
both	O
the	O
bound	B-protein_state
(	O
4DN4	O
)	O
and	O
unbound	B-protein_state
(	O
4DN3	O
)	O
forms	O
of	O
CNTO	B-chemical
888	I-chemical
.	O

The	O
carboxyl	O
group	O
of	O
Asp101	B-residue_name_number
forms	O
a	O
salt	B-bond_interaction
bridge	I-bond_interaction
with	O
Arg94	B-residue_name_number
.	O

Three	O
of	O
the	O
21	O
Fab	B-structure_element
structures	B-evidence
(	O
including	O
multiple	O
copies	O
in	O
the	O
asymmetric	O
unit	O
),	O
H5	B-complex_assembly
-	I-complex_assembly
51	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
11	I-complex_assembly
,	O
H551	B-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
and	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
(	O
one	O
of	O
the	O
2	O
Fabs	B-structure_element
),	O
have	O
missing	B-protein_state
(	O
disordered	B-protein_state
)	O
residues	O
at	O
the	O
apex	O
of	O
the	O
CDR	B-structure_element
loop	I-structure_element
.	O

Another	O
four	O
of	O
the	O
Fabs	B-structure_element
,	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
,	O
H3	B-complex_assembly
-	I-complex_assembly
53	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
,	O
H3	B-complex_assembly
-	I-complex_assembly
53	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
11	I-complex_assembly
and	O
H3	B-complex_assembly
-	I-complex_assembly
53	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
have	O
missing	O
side	O
-	O
chain	O
atoms	O
.	O

The	O
variations	O
in	O
CDR	B-structure_element
H3	B-structure_element
conformation	O
are	O
illustrated	O
in	O
Fig	O
.	O
6	O
for	O
the	O
18	O
Fab	B-structure_element
structures	B-evidence
that	O
have	O
ordered	O
backbone	O
atoms	O
.	O

(	O
B	O
)	O
The	O
“	O
extended	B-protein_state
”	O
CDR	B-structure_element
H3	B-structure_element
of	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
with	O
green	O
carbon	O
atoms	O
and	O
yellow	O
dashed	O
lines	O
connecting	O
the	O
H	O
-	O
bond	O
pairs	O
for	O
Asp101	B-residue_name_number
OD1	O
and	O
OD2	O
and	O
Trp103	B-residue_name_number
NE1	O
.	O

The	O
bases	O
of	O
these	O
structures	B-evidence
have	O
the	O
‘	O
kinked	B-protein_state
’	O
conformation	O
with	O
the	O
H	O
-	O
bond	O
between	O
Trp103	B-residue_name_number
and	O
Leu100b	B-residue_name_number
.	O

A	O
representative	O
CDR	B-structure_element
H3	B-structure_element
structure	B-evidence
for	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
illustrating	O
this	O
is	O
shown	O
in	O
Fig	O
.	O
7A	O
.	O

The	O
largest	O
backbone	O
conformational	O
deviation	O
for	O
the	O
set	O
is	O
at	O
Tyr99	B-residue_name_number
,	O
where	O
the	O
C	O
=	O
O	O
is	O
rotated	O
by	O
90	O
°	O
relative	O
to	O
that	O
observed	O
in	O
4DN3	O
.	O

Also	O
,	O
it	O
is	O
worth	O
noting	O
that	O
only	O
one	O
of	O
these	O
structures	B-evidence
,	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
,	O
has	O
the	O
conserved	B-protein_state
water	B-chemical
molecule	O
in	O
CDR	B-structure_element
H3	B-structure_element
observed	O
in	O
the	O
4DN3	O
and	O
4DN4	O
structures	B-evidence
.	O

In	O
fact	O
,	O
it	O
is	O
the	O
only	O
Fab	B-structure_element
in	O
the	O
set	O
that	O
has	O
a	O
water	B-chemical
molecule	O
present	O
at	O
this	O
site	O
.	O

The	O
CDR	B-structure_element
H3	B-structure_element
for	O
this	O
structure	B-evidence
is	O
shown	O
in	O
Fig	O
.	O
S3	O
.	O

The	O
remaining	O
8	O
Fabs	B-structure_element
can	O
be	O
grouped	O
into	O
5	O
different	O
conformational	O
classes	O
.	O

Three	O
of	O
the	O
Fabs	B-structure_element
,	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
,	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
and	O
H3	B-complex_assembly
-	I-complex_assembly
53	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
,	O
have	O
distinctive	O
conformations	O
.	O

The	O
five	O
remaining	O
Fabs	B-structure_element
,	O
H5	B-complex_assembly
-	I-complex_assembly
51	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
(	O
2	O
copies	O
),	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
(	O
2	O
copies	O
)	O
and	O
H3	B-complex_assembly
-	I-complex_assembly
53	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
,	O
have	O
3	O
different	O
CDR	B-structure_element
H3	B-structure_element
conformations	O
(	O
Fig	O
.	O
S4	O
).	O

The	O
stem	B-structure_element
regions	I-structure_element
of	O
CDR	B-structure_element
H3	B-structure_element
for	O
the	O
H5	B-complex_assembly
-	I-complex_assembly
51	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
Fabs	B-structure_element
are	O
in	O
the	O
‘	O
kinked	B-protein_state
’	O
conformation	O
while	O
,	O
surprisingly	O
,	O
those	O
of	O
the	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
pair	O
and	O
H3	B-complex_assembly
-	I-complex_assembly
53	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
are	O
in	O
the	O
‘	O
extended	B-protein_state
’	O
conformation	O
(	O
Fig	O
.	O
7B	O
).	O

VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
domain	O
packing	O

The	O
two	O
domains	O
pack	O
together	O
such	O
that	O
the	O
5	B-structure_element
-	I-structure_element
stranded	I-structure_element
β	I-structure_element
-	I-structure_element
sheets	I-structure_element
,	O
which	O
have	O
hydrophobic	O
surfaces	O
,	O
interact	O
with	O
each	O
other	O
bringing	O
the	O
CDRs	B-structure_element
from	O
both	O
the	O
VH	B-structure_element
and	O
VL	B-structure_element
domains	O
into	O
close	O
proximity	O
.	O

The	O
conserved	B-protein_state
VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
interactions	O
as	O
viewed	O
along	O
the	O
VH	B-structure_element
/	O
VL	B-structure_element
axis	O
.	O

There	O
are	O
a	O
few	O
principal	O
inter	O
-	O
domain	O
interactions	O
that	O
are	O
conserved	O
not	O
only	O
in	O
the	O
experimental	O
set	O
of	O
16	O
Fabs	B-structure_element
,	O
but	O
in	O
all	O
human	B-species
antibodies	B-protein_type
.	O

With	O
the	O
exception	O
of	O
L	B-structure_element
-	O
Ala43	B-residue_name_number
,	O
all	O
other	O
residues	O
are	O
conserved	O
in	O
human	B-species
germlines	O
.	O

Position	O
43	B-residue_number
may	O
be	O
alternatively	O
occupied	O
by	O
Ser	B-residue_name
,	O
Val	B-residue_name
or	O
Pro	B-residue_name
(	O
as	O
in	O
L4	B-mutant
-	I-mutant
1	I-mutant
),	O
but	O
the	O
hydrophobic	B-bond_interaction
interaction	I-bond_interaction
with	O
H	B-structure_element
-	O
Tyr91	B-residue_name_number
is	O
preserved	O
.	O

In	O
total	O
,	O
about	O
20	B-residue_range
residues	I-residue_range
are	O
involved	O
in	O
the	O
VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
interactions	O
on	O
each	O
side	O
(	O
Fig	O
.	O
S5	O
).	O

Half	O
of	O
them	O
are	O
in	O
the	O
framework	B-structure_element
regions	I-structure_element
and	O
those	O
residues	O
(	O
except	O
residue	O
61	B-residue_number
in	O
HC	B-structure_element
,	O
which	O
is	O
actually	O
in	O
CDR2	B-structure_element
in	O
Kabat	O
'	O
s	O
definition	O
)	O
are	O
conserved	O
in	O
the	O
set	O
of	O
16	O
Fabs	B-structure_element
.	O

One	O
notable	O
exception	O
is	O
H	B-structure_element
-	O
Trp47	B-residue_name_number
,	O
which	O
exhibits	O
2	O
conformations	O
of	O
the	O
indole	O
ring	O
.	O

Interestingly	O
,	O
these	O
are	O
the	O
only	O
2	O
structures	B-evidence
with	O
residues	O
missing	B-protein_state
in	O
CDR	B-structure_element
H3	B-structure_element
because	O
of	O
disorder	O
,	O
although	O
both	O
structures	B-evidence
are	O
determined	O
at	O
high	O
resolution	O
and	O
the	O
rest	O
of	O
the	O
structure	B-evidence
is	O
well	O
defined	O
.	O

VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
tilt	B-evidence
angles	I-evidence

The	O
relative	O
orientation	O
of	O
VH	B-structure_element
and	O
VL	B-structure_element
has	O
been	O
measured	O
in	O
a	O
number	O
of	O
different	O
ways	O
.	O

The	O
first	O
approach	O
uses	O
ABangles	B-experimental_method
,	O
the	O
results	O
of	O
which	O
are	O
shown	O
in	O
Table	O
S2	O
.	O

The	O
four	O
LCs	B-structure_element
all	O
are	O
classified	O
as	O
Type	O
A	O
because	O
they	O
have	O
a	O
proline	B-residue_name
at	O
position	O
44	B-residue_number
,	O
and	O
the	O
results	O
for	O
each	O
orientation	B-evidence
parameter	I-evidence
are	O
within	O
the	O
range	O
of	O
values	O
of	O
this	O
type	O
reported	O
by	O
Dunbar	O
and	O
co	O
-	O
workers	O
.	O

In	O
fact	O
,	O
the	O
parameter	O
values	O
for	O
the	O
set	O
of	O
16	O
Fabs	B-structure_element
are	O
in	O
the	O
middle	O
of	O
the	O
distribution	O
observed	O
for	O
351	O
non	O
-	O
redundant	O
antibody	B-protein_type
structures	B-evidence
determined	O
at	O
3	O
.	O
0	O
Å	O
resolution	O
or	O
better	O
.	O

The	O
only	O
exception	O
is	O
HC1	B-structure_element
,	O
which	O
is	O
shifted	O
toward	O
smaller	O
angles	O
with	O
the	O
mean	O
value	O
of	O
70	O
.	O
8	O
°	O
as	O
compared	O
to	O
the	O
distribution	O
centered	O
at	O
72	O
°	O
for	O
the	O
entire	O
PDB	O
.	O

This	O
probably	O
reflects	O
the	O
invariance	O
of	O
CDR	B-structure_element
H3	B-structure_element
in	O
the	O
current	O
set	O
as	O
opposed	O
to	O
the	O
CDR	B-structure_element
H3	B-structure_element
diversity	O
in	O
the	O
PDB	O
.	O

For	O
structures	B-evidence
with	O
2	O
copies	O
of	O
the	O
Fab	B-structure_element
in	O
the	O
asymmetric	O
unit	O
,	O
only	O
one	O
structure	B-evidence
was	O
used	O
.	O

The	O
differences	O
between	O
independent	O
Fabs	B-structure_element
in	O
the	O
same	O
structure	B-evidence
are	O
4	O
.	O
9	O
°	O
for	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
,	O
1	O
.	O
6	O
°	O
for	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
11	I-complex_assembly
,	O
1	O
.	O
4	O
°	O
for	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
,	O
3	O
.	O
3	O
°	O
for	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
11	I-complex_assembly
,	O
and	O
2	O
.	O
5	O
°	O
for	O
H5	B-complex_assembly
-	I-complex_assembly
51	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
.	O

With	O
the	O
exception	O
of	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
,	O
the	O
angles	O
are	O
within	O
the	O
range	O
of	O
2	O
-	O
3	O
°	O
as	O
are	O
observed	O
in	O
the	O
identical	O
structures	B-evidence
in	O
the	O
PDB	O
.	O

In	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
,	O
one	O
of	O
the	O
Fabs	B-structure_element
is	O
substantially	O
disordered	B-protein_state
so	O
that	O
part	O
of	O
CDR	B-structure_element
H2	B-structure_element
(	O
the	O
outer	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
,	O
residues	O
55	B-residue_range
-	I-residue_range
60	I-residue_range
)	O
is	O
completely	O
missing	O
.	O

This	O
kind	O
of	O
disorder	O
may	O
compromise	O
the	O
integrity	O
of	O
the	O
VH	B-structure_element
domain	O
and	O
its	O
interaction	O
with	O
the	O
VL	B-structure_element
.	O

An	O
illustration	O
of	O
the	O
difference	O
in	O
tilt	O
angle	O
for	O
2	O
pairs	O
of	O
variants	O
by	O
the	O
superposition	B-experimental_method
of	O
the	O
VH	B-structure_element
domains	O
of	O
(	O
A	O
)	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
on	O
that	O
of	O
H5	B-complex_assembly
-	I-complex_assembly
51	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
(	O
the	O
VL	B-structure_element
domain	O
is	O
off	O
by	O
a	O
rigid	O
-	O
body	O
roatation	O
of	O
10	O
.	O
5	O
°)	O
and	O
(	O
B	O
)	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
on	O
that	O
of	O
H5	B-complex_assembly
-	I-complex_assembly
51	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
(	O
the	O
VL	B-structure_element
domain	O
is	O
off	O
by	O
a	O
rigid	O
-	O
body	O
roatation	O
of	O
1	O
.	O
6	O
°).	O

Differences	O
in	O
VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
tilt	B-evidence
angles	I-evidence
.	O

The	O
differences	B-evidence
in	O
the	O
tilt	B-evidence
angle	I-evidence
are	O
shown	O
for	O
all	O
pairs	O
of	O
V	B-structure_element
regions	I-structure_element
in	O
Table	O
3	O
.	O

The	O
smallest	O
differences	O
in	O
the	O
tilt	B-evidence
angle	I-evidence
are	O
between	O
the	O
Fabs	B-structure_element
in	O
isomorphous	O
crystal	B-evidence
forms	I-evidence
.	O

The	O
largest	O
deviations	O
in	O
the	O
tilt	B-evidence
angle	I-evidence
,	O
up	O
to	O
11	O
.	O
0	O
°,	O
are	O
found	O
for	O
2	O
structures	B-evidence
,	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
and	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
,	O
that	O
stand	O
out	O
from	O
the	O
other	O
Fabs	B-structure_element
.	O

Two	O
examples	O
illustrating	O
large	O
(	O
10	O
.	O
5	O
°)	O
and	O
small	O
(	O
1	O
.	O
6	O
°)	O
differences	O
in	O
the	O
tilt	B-evidence
angles	I-evidence
are	O
shown	O
in	O
Fig	O
.	O
9	O
.	O

VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
buried	O
surface	O
area	O
and	O
complementarity	O

VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
surface	O
areas	O
and	O
surface	O
complementarity	O
.	O

The	O
results	O
of	O
the	O
PISA	B-experimental_method
contact	B-experimental_method
surface	I-experimental_method
calculation	I-experimental_method
and	O
surface	B-experimental_method
complementarity	I-experimental_method
calculation	I-experimental_method
are	O
shown	O
in	O
Table	O
4	O
.	O

The	O
interface	B-site
areas	O
are	O
calculated	O
as	O
the	O
average	O
of	O
the	O
VH	B-site
and	I-site
VL	I-site
contact	I-site
surfaces	I-site
.	O

Six	O
of	O
the	O
16	O
structures	B-evidence
have	O
CDR	B-structure_element
H3	B-structure_element
side	O
chains	O
or	O
complete	O
residues	O
missing	B-protein_state
,	O
and	O
therefore	O
their	O
interfaces	B-site
are	O
much	O
smaller	O
than	O
in	O
the	O
other	O
10	O
structures	B-evidence
with	O
complete	B-protein_state
CDRs	B-structure_element
(	O
the	O
results	O
are	O
provided	O
for	O
all	O
Fabs	B-structure_element
for	O
completeness	O
).	O

Among	O
the	O
complete	B-protein_state
structures	B-evidence
,	O
the	O
interface	B-site
areas	O
range	O
from	O
684	O
to	O
836	O
Å2	O
.	O

Interestingly	O
,	O
the	O
2	O
structures	B-evidence
that	O
have	O
the	O
largest	O
tilt	B-evidence
angle	I-evidence
differences	I-evidence
with	O
the	O
other	O
variants	O
,	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
and	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
,	O
have	O
the	O
smallest	O
VH	B-site
:	I-site
VL	I-site
interfaces	I-site
,	O
684	O
and	O
725	O
Å2	O
,	O
respectively	O
.	O

H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
is	O
also	O
unique	O
in	O
that	O
it	O
has	O
the	O
lowest	O
value	O
(	O
0	O
.	O
676	O
)	O
of	O
surface	B-evidence
complementarity	I-evidence
.	O

Melting	B-evidence
temperatures	I-evidence
(	O
Tm	B-evidence
)	O
were	O
measured	O
for	O
all	O
Fabs	B-structure_element
using	O
differential	B-experimental_method
scanning	I-experimental_method
calorimetry	I-experimental_method
(	O
Table	O
5	O
).	O

In	O
addition	O
,	O
L1	B-mutant
-	I-mutant
39	I-mutant
provides	O
a	O
much	O
higher	O
degree	O
of	O
stabilization	O
than	O
the	O
other	O
3	O
LC	B-structure_element
germlines	O
when	O
combined	O
with	O
any	O
of	O
the	O
HCs	B-structure_element
.	O

As	O
a	O
result	O
,	O
the	O
Tm	B-evidence
for	O
pairs	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
and	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L1	I-complex_assembly
-	I-complex_assembly
39	I-complex_assembly
is	O
12	O
-	O
13	O
°	O
higher	O
than	O
for	O
pairs	O
H3	B-complex_assembly
-	I-complex_assembly
53	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
,	O
H3	B-complex_assembly
-	I-complex_assembly
53	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
,	O
H5	B-complex_assembly
-	I-complex_assembly
51	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
and	O
H5	B-complex_assembly
-	I-complex_assembly
51	I-complex_assembly
:	I-complex_assembly
L4	I-complex_assembly
-	I-complex_assembly
1	I-complex_assembly
.	O

These	O
findings	O
correlate	O
well	O
with	O
the	O
degree	O
of	O
conformational	O
disorder	O
observed	O
in	O
the	O
crystal	B-evidence
structures	I-evidence
.	O

Parts	O
of	O
CDR	B-structure_element
H3	B-structure_element
main	O
chain	O
are	O
completely	O
disordered	B-protein_state
,	O
and	O
were	O
not	O
modeled	O
in	O
Fabs	B-structure_element
H5	B-complex_assembly
-	I-complex_assembly
51	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
and	O
H5	B-complex_assembly
-	I-complex_assembly
51	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
11	I-complex_assembly
that	O
have	O
the	O
lowest	O
Tms	B-evidence
in	O
the	O
set	O
.	O

All	O
those	O
molecules	O
are	O
relatively	O
unstable	O
,	O
as	O
is	O
reflected	O
in	O
their	O
low	O
Tms	B-evidence
.	O

This	O
is	O
the	O
first	O
report	O
of	O
a	O
systematic	B-experimental_method
structural	I-experimental_method
investigation	I-experimental_method
of	O
a	O
phage	B-experimental_method
germline	I-experimental_method
library	I-experimental_method
.	O

The	O
16	O
Fab	B-structure_element
structures	B-evidence
offer	O
a	O
unique	O
look	O
at	O
all	O
pairings	O
of	O
4	O
different	O
HCs	B-structure_element
(	O
H1	B-mutant
-	I-mutant
69	I-mutant
,	O
H3	B-mutant
-	I-mutant
23	I-mutant
,	O
H3	B-mutant
-	I-mutant
53	I-mutant
,	O
and	O
H5	B-mutant
-	I-mutant
51	I-mutant
)	O
and	O
4	O
different	O
LCs	B-structure_element
(	O
L1	B-mutant
-	I-mutant
39	I-mutant
,	O
L3	B-mutant
-	I-mutant
11	I-mutant
,	O
L3	B-mutant
-	I-mutant
20	I-mutant
and	O
L4	B-mutant
-	I-mutant
1	I-mutant
),	O
all	O
with	O
the	O
same	O
CDR	B-structure_element
H3	B-structure_element
.	O

The	O
structural	B-evidence
data	I-evidence
set	O
taken	O
as	O
a	O
whole	O
provides	O
insight	O
into	O
how	O
the	O
backbone	O
conformations	O
of	O
the	O
CDRs	B-structure_element
of	O
a	O
specific	O
heavy	O
or	O
light	B-structure_element
chain	I-structure_element
vary	O
when	O
it	O
is	O
paired	O
with	O
4	O
different	O
light	O
or	O
heavy	B-structure_element
chains	I-structure_element
,	O
respectively	O
.	O

A	O
large	O
variability	O
in	O
the	O
CDR	B-structure_element
conformations	O
for	O
the	O
sets	O
of	O
HCs	B-structure_element
and	O
LCs	B-structure_element
is	O
observed	O
.	O

In	O
some	O
cases	O
the	O
CDR	B-structure_element
conformations	O
for	O
all	O
members	O
of	O
a	O
set	O
are	O
virtually	O
identical	O
,	O
for	O
others	O
subtle	O
changes	O
occur	O
in	O
a	O
few	O
members	O
of	O
a	O
set	O
,	O
and	O
in	O
some	O
cases	O
larger	O
deviations	O
are	O
observed	O
within	O
a	O
set	O
.	O

The	O
five	O
variants	O
that	O
crystallized	B-experimental_method
with	O
2	O
copies	O
of	O
the	O
Fab	B-structure_element
in	O
the	O
asymmetric	O
unit	O
serve	O
somewhat	O
as	O
controls	O
for	O
the	O
influence	O
of	O
crystal	O
packing	O
on	O
the	O
conformations	O
of	O
the	O
CDRs	B-structure_element
.	O

In	O
four	O
of	O
the	O
5	O
structures	B-evidence
the	O
CDR	B-structure_element
conformations	O
are	O
consistent	O
.	O

In	O
only	O
one	O
case	O
,	O
that	O
of	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
(	O
the	O
lowest	O
resolution	O
structure	B-evidence
),	O
do	O
we	O
see	O
differences	O
in	O
the	O
conformations	O
of	O
the	O
2	O
copies	O
of	O
CDRs	B-structure_element
H1	B-structure_element
and	O
L1	B-structure_element
.	O

This	O
variability	O
is	O
likely	O
a	O
result	O
of	O
2	O
factors	O
,	O
crystal	O
packing	O
interactions	O
and	O
internal	O
instability	O
of	O
the	O
variable	B-structure_element
domain	I-structure_element
.	O

For	O
the	O
CDRs	B-structure_element
with	O
canonical	O
structures	O
,	O
the	O
largest	O
changes	O
in	O
conformation	O
occur	O
for	O
CDR	B-structure_element
H1	B-structure_element
of	O
H1	B-mutant
-	I-mutant
69	I-mutant
and	O
H3	B-mutant
-	I-mutant
53	I-mutant
.	O

The	O
other	O
2	O
HCs	B-structure_element
,	O
H3	B-mutant
-	I-mutant
23	I-mutant
and	O
H5	B-mutant
-	I-mutant
51	I-mutant
,	O
have	O
canonical	O
structures	O
that	O
are	O
remarkably	B-protein_state
well	I-protein_state
conserved	I-protein_state
(	O
Fig	O
.	O
1	O
).	O

H1	B-mutant
-	I-mutant
69	I-mutant
is	O
unique	O
in	O
having	O
a	O
pair	O
of	O
glycine	B-residue_name
residues	O
at	O
positions	O
26	B-residue_number
and	O
27	B-residue_number
,	O
which	O
provide	O
more	O
conformational	B-protein_state
freedom	I-protein_state
in	O
CDR	B-structure_element
H1	B-structure_element
.	O

Besides	O
IGHV1	B-mutant
-	I-mutant
69	I-mutant
,	O
only	O
the	O
germlines	O
of	O
the	O
VH4	B-structure_element
family	O
possess	O
double	O
glycines	B-residue_name
in	O
CDR	B-structure_element
H1	B-structure_element
,	O
and	O
it	O
will	O
be	O
interesting	O
to	O
see	O
if	O
they	O
are	O
also	O
conformationally	B-protein_state
unstable	I-protein_state
.	O

More	O
than	O
half	O
of	O
the	O
variants	O
retain	O
the	O
conformation	O
of	O
the	O
parent	O
despite	O
having	O
differences	O
in	O
the	O
VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
pairing	O
.	O

This	O
subset	O
includes	O
2	O
structures	B-evidence
with	O
2	O
copies	O
of	O
the	O
Fab	B-structure_element
in	O
the	O
asymmetric	O
unit	O
,	O
all	O
of	O
which	O
are	O
nearly	O
identical	O
in	O
conformation	O
.	O

The	O
remaining	O
8	O
structures	B-evidence
exhibit	O
“	O
non	O
-	O
parental	O
”	O
conformations	O
,	O
indicating	O
that	O
the	O
VH	B-structure_element
and	O
VL	B-structure_element
context	O
can	O
also	O
be	O
a	O
dominating	O
factor	O
influencing	O
CDR	B-structure_element
H3	B-structure_element
.	O

In	O
looking	O
at	O
a	O
possible	O
correlation	O
between	O
the	O
tilt	B-evidence
angle	I-evidence
and	O
the	O
conformation	O
of	O
CDR	B-structure_element
H3	B-structure_element
,	O
no	O
clear	O
trends	O
are	O
observed	O
.	O

The	O
absolute	O
VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
orientation	B-evidence
parameters	I-evidence
for	O
the	O
2	O
Fabs	B-structure_element
(	O
Table	O
S2	O
)	O
show	O
significant	O
deviation	B-evidence
in	O
HL	B-structure_element
,	O
LC1	B-structure_element
and	O
HC2	B-structure_element
values	O
(	O
2	O
-	O
3	O
standard	O
deviations	O
from	O
the	O
mean	O
).	O

As	O
noted	O
in	O
the	O
Results	O
section	O
,	O
the	O
2	O
variants	O
,	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
and	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
,	O
are	O
outliers	O
in	O
terms	O
of	O
the	O
tilt	B-evidence
angle	I-evidence
;	O
at	O
the	O
same	O
time	O
,	O
both	O
have	O
the	O
smallest	O
VH	B-site
:	I-site
VL	I-site
interface	I-site
.	O

These	O
smaller	O
interfaces	B-site
may	O
perhaps	O
translate	O
to	O
a	O
significant	O
deviation	O
in	O
how	O
VH	B-structure_element
is	O
oriented	O
relative	O
to	O
VL	B-structure_element
than	O
the	O
other	O
variants	O
.	O

These	O
deviations	O
from	O
the	O
other	O
variants	O
can	O
also	O
be	O
seen	O
to	O
some	O
extent	O
in	O
VH	B-complex_assembly
:	I-complex_assembly
VL	I-complex_assembly
orientation	O
parameters	O
in	O
Table	O
S2	O
,	O
as	O
well	O
as	O
in	O
the	O
smaller	O
number	O
of	O
residues	O
involved	O
in	O
the	O
VH	B-site
:	I-site
VL	I-site
interfaces	I-site
of	O
these	O
2	O
variants	O
(	O
Fig	O
.	O
S5	O
).	O

These	O
differences	O
undoubtedly	O
influence	O
the	O
conformation	O
of	O
the	O
CDRs	B-structure_element
,	O
in	O
particular	O
CDR	B-structure_element
H1	B-structure_element
(	O
Fig	O
.	O
1A	O
)	O
and	O
CDR	B-structure_element
L1	B-structure_element
(	O
Fig	O
.	O
3C	O
),	O
especially	O
with	O
the	O
tandem	O
glycines	B-residue_name
and	O
multiple	O
serines	B-residue_name
present	O
,	O
respectively	O
.	O

As	O
indicated	O
by	O
the	O
melting	B-evidence
temperatures	I-evidence
,	O
germlines	O
H1	B-mutant
-	I-mutant
69	I-mutant
and	O
H3	B-mutant
-	I-mutant
23	I-mutant
for	O
HC	B-structure_element
and	O
germline	O
L1	B-mutant
-	I-mutant
39	I-mutant
for	O
LC	B-structure_element
produce	O
more	O
stable	B-protein_state
Fabs	B-structure_element
compared	O
to	O
the	O
other	O
germlines	O
in	O
the	O
experimental	O
set	O
.	O

One	O
possible	O
explanation	O
of	O
the	O
clear	O
preference	O
of	O
LC	B-structure_element
germline	O
L1	B-mutant
-	I-mutant
39	I-mutant
is	O
that	O
CDR	B-structure_element
L3	B-structure_element
has	O
smaller	O
residues	O
at	O
positions	O
91	B-residue_number
and	O
94	B-residue_number
,	O
allowing	O
for	O
more	O
room	O
to	O
accommodate	O
CDR	B-structure_element
H3	B-structure_element
.	O

Other	O
germlines	O
have	O
bulky	O
residues	O
,	O
Tyr	B-residue_name
,	O
Arg	B-residue_name
and	O
Trp	B-residue_name
,	O
at	O
these	O
positions	O
,	O
whereas	O
L1	B-mutant
-	I-mutant
39	I-mutant
has	O
Ser	B-residue_name
and	O
Thr	B-residue_name
.	O

Various	O
combinations	O
of	O
germline	O
sequences	O
for	O
VL	B-structure_element
and	O
VH	B-structure_element
impose	O
certain	O
constraints	O
on	O
CDR	B-structure_element
H3	B-structure_element
,	O
which	O
has	O
to	O
adapt	O
to	O
the	O
environment	O
.	O

A	O
more	O
compact	B-protein_state
CDR	B-structure_element
L3	B-structure_element
may	O
be	O
beneficial	O
in	O
this	O
situation	O
.	O

While	O
pairings	O
with	O
H3	B-mutant
-	I-mutant
53	I-mutant
and	O
H5	B-mutant
-	I-mutant
51	I-mutant
may	O
be	O
safely	O
called	O
a	O
mismatch	O
,	O
those	O
with	O
H1	B-mutant
-	I-mutant
69	I-mutant
and	O
H3	B-mutant
-	I-mutant
23	I-mutant
have	O
Tms	B-evidence
about	O
5	O
-	O
6	O
°	O
higher	O
.	O

Curiously	O
,	O
the	O
2	O
Fabs	B-structure_element
,	O
H1	B-complex_assembly
-	I-complex_assembly
69	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
and	O
H3	B-complex_assembly
-	I-complex_assembly
23	I-complex_assembly
:	I-complex_assembly
L3	I-complex_assembly
-	I-complex_assembly
20	I-complex_assembly
,	O
deviate	O
markedly	O
in	O
their	O
tilt	B-evidence
angles	I-evidence
from	O
the	O
rest	O
of	O
the	O
panel	O
.	O

It	O
is	O
possible	O
that	O
by	O
adopting	O
extreme	O
tilt	B-evidence
angles	I-evidence
the	O
structure	B-evidence
modulates	O
CDR	B-structure_element
H3	B-structure_element
and	O
its	O
environment	O
,	O
which	O
apparently	O
cannot	O
be	O
achieved	O
solely	O
by	O
conformational	O
rearrangement	O
of	O
the	O
CDR	B-structure_element
.	O

Overall	O
,	O
the	O
stability	O
of	O
the	O
Fab	B-structure_element
,	O
as	O
measured	O
by	O
Tm	B-evidence
,	O
is	O
a	O
result	O
of	O
the	O
mutual	O
adjustment	O
of	O
the	O
HC	B-structure_element
and	O
LC	B-structure_element
variable	B-structure_element
domains	I-structure_element
and	O
adjustment	O
of	O
CDR	B-structure_element
H3	B-structure_element
to	O
the	O
VH	B-site
:	I-site
VL	I-site
cleft	I-site
.	O

In	O
summary	O
,	O
the	O
analysis	O
of	O
this	O
structural	B-experimental_method
library	I-experimental_method
of	O
germline	O
variants	O
composed	O
of	O
all	O
pairs	O
of	O
4	O
HCs	B-structure_element
and	O
4LCs	O
,	O
all	O
with	O
the	O
same	O
CDR	B-structure_element
H3	B-structure_element
,	O
offers	O
some	O
unique	O
insights	O
into	O
antibody	B-protein_type
structure	B-evidence
and	O
how	O
pairing	O
and	O
sequence	O
may	O
influence	O
,	O
or	O
not	O
,	O
the	O
canonical	O
structures	O
of	O
the	O
L1	B-structure_element
,	O
L2	B-structure_element
,	O
L3	B-structure_element
,	O
H1	B-structure_element
and	O
H2	B-structure_element
CDRs	B-structure_element
.	O

Comparison	O
of	O
the	O
CDR	B-structure_element
H3s	B-structure_element
reveals	O
a	O
large	O
set	O
of	O
variants	O
with	O
conformations	O
similar	O
to	O
the	O
parent	O
,	O
while	O
a	O
second	O
set	O
has	O
significant	O
conformational	O
variability	O
,	O
indicating	O
that	O
both	O
the	O
sequence	O
and	O
the	O
structural	O
context	O
define	O
the	O
CDR	B-structure_element
H3	B-structure_element
conformation	O
.	O

Furthermore	O
,	O
antibody	B-protein_type
CDRs	B-structure_element
,	O
H3	B-structure_element
in	O
particular	O
,	O
may	O
go	O
through	O
conformational	O
changes	O
upon	O
binding	O
their	O
targets	O
,	O
making	O
structural	O
prediction	O
for	O
docking	O
purposes	O
an	O
even	O
more	O
difficult	O
task	O
.	O

Fortunately	O
,	O
for	O
most	O
applications	O
of	O
antibody	B-protein_type
modeling	O
,	O
such	O
as	O
engineering	O
affinity	O
and	O
biophysical	O
properties	O
,	O
an	O
accurate	O
CDR	B-structure_element
H3	B-structure_element
structure	B-evidence
is	O
not	O
always	O
necessary	O
.	O

The	O
set	O
of	O
16	O
germline	O
Fab	B-structure_element
structures	B-evidence
offers	O
a	O
unique	O
dataset	O
to	O
facilitate	O
software	O
development	O
for	O
antibody	B-protein_type
modeling	O
.	O

The	O
results	O
essentially	O
support	O
the	O
underlying	O
idea	O
of	O
canonical	O
structures	B-evidence
,	O
indicating	O
that	O
most	O
CDRs	B-structure_element
with	O
germline	O
sequences	O
tend	O
to	O
adopt	O
predefined	O
conformations	O
.	O

From	O
this	O
point	O
of	O
view	O
,	O
a	O
novel	O
approach	O
to	O
design	O
combinatorial	O
antibody	B-protein_type
libraries	O
would	O
be	O
to	O
cover	O
the	O
range	O
of	O
CDR	B-structure_element
conformations	O
that	O
may	O
not	O
necessarily	O
coincide	O
with	O
the	O
germline	O
usage	O
in	O
the	O
human	B-species
repertoire	O
.	O

This	O
would	O
insure	O
more	O
structural	O
diversity	O
,	O
leading	O
to	O
a	O
more	O
diverse	O
panel	O
of	O
antibodies	B-protein_type
that	O
would	O
bind	O
to	O
a	O
broad	O
spectrum	O
of	O
targets	O
.	O

Structure	B-evidence
of	O
the	O
Response	B-protein_type
Regulator	I-protein_type
NsrR	B-protein
from	O
Streptococcus	B-species
agalactiae	I-species
,	O
Which	O
Is	O
Involved	O
in	O
Lantibiotic	B-chemical
Resistance	O

Lantibiotics	B-chemical
are	O
antimicrobial	B-chemical
peptides	I-chemical
produced	O
by	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
positive	I-taxonomy_domain
bacteria	I-taxonomy_domain
.	O

The	O
expression	O
of	O
the	O
genes	O
responsible	O
for	O
lantibiotic	B-chemical
resistance	O
is	O
regulated	O
by	O
a	O
specific	O
two	B-complex_assembly
-	I-complex_assembly
component	I-complex_assembly
system	I-complex_assembly
consisting	O
of	O
a	O
histidine	B-protein_type
kinase	I-protein_type
and	O
a	O
response	B-protein_type
regulator	I-protein_type
.	O

Here	O
,	O
we	O
focused	O
on	O
a	O
response	B-protein_type
regulator	I-protein_type
involved	O
in	O
lantibiotic	B-chemical
resistance	O
,	O
NsrR	B-protein
from	O
Streptococcus	B-species
agalactiae	I-species
,	O
and	O
determined	O
the	O
crystal	B-evidence
structures	I-evidence
of	O
its	O
N	O
-	O
terminal	O
receiver	B-structure_element
domain	I-structure_element
and	O
C	O
-	O
terminal	O
DNA	B-structure_element
-	I-structure_element
binding	I-structure_element
effector	I-structure_element
domain	I-structure_element
.	O

Amino	O
acids	O
involved	O
in	O
phosphorylation	B-ptm
,	O
dimerization	O
,	O
and	O
DNA	B-chemical
-	O
binding	O
were	O
identified	O
and	O
demonstrated	O
to	O
be	O
conserved	B-protein_state
in	O
lantibiotic	B-protein_type
resistance	I-protein_type
regulators	I-protein_type
.	O

This	O
has	O
led	O
to	O
the	O
search	O
for	O
novel	O
antibiotics	O
that	O
can	O
be	O
used	O
as	O
pharmaceuticals	O
against	O
human	B-species
pathogenic	O
bacteria	B-taxonomy_domain
.	O

Lantibiotics	B-chemical
are	O
small	O
antimicrobial	B-chemical
peptides	I-chemical
(	O
30	O
–	O
50	O
amino	O
acids	O
in	O
length	O
),	O
which	O
are	O
produced	O
by	O
several	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
positive	I-taxonomy_domain
bacterial	I-taxonomy_domain
strains	O
.	O

Lantibiotics	B-chemical
are	O
for	O
example	O
highly	O
effective	O
against	O
various	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
positive	I-taxonomy_domain
,	O
human	B-species
pathogenic	O
bacteria	B-taxonomy_domain
including	O
Streptococcus	B-species
pneumoniae	I-species
and	O
several	O
methicillin	B-species
-	I-species
resistant	I-species
Staphylococcus	I-species
aureus	I-species
(	O
MRSA	B-species
)	O
strains	O
.	O

The	O
high	O
potency	O
of	O
lantibiotics	B-chemical
for	O
medical	O
usage	O
has	O
already	O
been	O
noticed	O
,	O
and	O
several	O
lantibiotics	B-chemical
are	O
already	O
included	O
in	O
clinical	O
trials	O
.	O

Nisin	B-chemical
is	O
the	O
most	O
prominent	O
member	O
of	O
the	O
lantibiotic	B-chemical
family	O
and	O
is	O
able	O
to	O
inhibit	O
cell	O
growth	O
,	O
penetrates	O
the	O
membranes	O
of	O
various	O
Gram	B-taxonomy_domain
-	I-taxonomy_domain
positive	I-taxonomy_domain
bacteria	I-taxonomy_domain
,	O
and	O
is	O
characterized	O
by	O
five	O
specific	O
(	O
methyl	O
-)	O
lanthionine	O
rings	O
,	O
which	O
are	O
crucial	O
for	O
stability	O
and	O
activity	O
in	O
the	O
nanomolar	O
range	O
.	O

This	O
immunity	O
system	O
consists	O
of	O
a	O
membrane	B-protein_type
–	I-protein_type
associated	I-protein_type
lipoprotein	I-protein_type
(	O
usually	O
referred	O
to	O
as	O
LanI	B-protein_type
)	O
and	O
/	O
or	O
an	O
ABC	B-protein_type
transporter	I-protein_type
(	O
termed	O
as	O
LanFEG	B-protein_type
and	O
comprising	O
three	O
subunits	O
).	O

Although	O
some	O
lantibiotics	B-chemical
such	O
as	O
Pep5	B-chemical
,	O
epicidin	B-chemical
,	O
epilancin	B-chemical
,	O
and	O
lactocin	B-chemical
S	I-chemical
only	O
require	O
LanI	B-protein_type
for	O
immunity	O
,	O
other	O
lantibiotics	B-chemical
with	O
a	O
dual	O
mode	O
of	O
action	O
involving	O
pore	O
formation	O
and	O
lipid	O
II	O
binding	O
such	O
as	O
nisin	B-chemical
,	O
subtilin	B-chemical
,	O
epidermin	B-chemical
,	O
gallidermin	B-chemical
,	O
and	O
lacticin	B-chemical
3147	I-chemical
require	O
additionally	O
the	O
presence	O
of	O
LanFEG	B-protein_type
.	O

Structural	B-evidence
data	I-evidence
are	O
reported	O
for	O
the	O
immunity	B-protein_type
proteins	I-protein_type
NisI	B-protein
from	O
Lactococcus	B-species
lactis	I-species
,	O
SpaI	B-protein
from	O
Bacillus	B-species
subtilis	I-species
and	O
MlbQ	B-protein
from	O
the	O
lantibiotic	B-chemical
NAI	B-chemical
-	I-chemical
107	I-chemical
producer	O
strain	O
Microbispora	B-species
ATCC	I-species
PTA	I-species
-	I-species
5024	I-species
.	O

Recently	O
,	O
gene	O
clusters	O
were	O
identified	O
in	O
certain	O
clinically	O
relevant	O
human	B-species
pathogenic	O
strains	O
such	O
as	O
Streptococcus	B-species
agalactiae	I-species
,	O
S	B-species
.	I-species
aureus	I-species
,	O
and	O
others	O
that	O
confer	O
inherent	O
resistance	O
against	O
specific	O
lantibiotics	B-chemical
such	O
as	O
nisin	B-chemical
and	O
resemble	O
the	O
genetic	O
architecture	O
of	O
the	O
lantibiotic	O
immunity	O
genes	O
found	O
in	O
the	O
producing	O
strains	O
.	O

Within	O
these	O
resistance	O
operons	O
,	O
genes	O
encoding	O
for	O
a	O
membrane	B-protein_type
-	I-protein_type
associated	I-protein_type
protease	I-protein_type
and	O
an	O
ABC	B-protein_type
transporter	I-protein_type
were	O
identified	O
.	O

Expression	O
of	O
these	O
proteins	O
provides	O
resistance	O
against	O
lantibiotics	B-chemical
.	O

Bacteria	B-taxonomy_domain
have	O
the	O
ability	O
to	O
sense	O
and	O
survive	O
various	O
environmental	O
stimuli	O
through	O
adaptive	O
responses	O
,	O
which	O
are	O
regulated	O
by	O
TCSs	B-complex_assembly
.	O

The	O
absence	B-protein_state
of	I-protein_state
TCSs	B-complex_assembly
within	O
mammals	B-taxonomy_domain
makes	O
them	O
unique	O
targets	O
for	O
novel	O
antimicrobial	O
drugs	O
.	O

Some	O
examples	O
of	O
TCS	B-complex_assembly
are	O
:	O
BraRS	B-protein
in	O
S	B-species
.	I-species
aureus	I-species
which	O
is	O
induced	O
by	O
bacitracin	B-chemical
,	O
nisin	B-chemical
and	O
nukacin	B-chemical
-	I-chemical
ISK	I-chemical
-	I-chemical
1	I-chemical
resistance	O
,	O
BceRS	B-protein
in	O
Bacillus	B-taxonomy_domain
spp	I-taxonomy_domain
.	O
that	O
is	O
induced	O
by	O
actagardine	B-chemical
and	O
mersacidin	B-chemical
resistance	O
,	O
LcrRS	B-protein
in	O
Streptococcus	B-species
mutans	I-species
induced	O
by	O
nukacin	B-chemical
-	I-chemical
ISK	I-chemical
-	I-chemical
1	I-chemical
and	O
lacticin	B-chemical
481	I-chemical
and	O
LisRK	B-protein
of	O
Listeria	B-species
monocytogenes	I-species
induced	O
by	O
nisin	B-chemical
resistance	O
.	O

Furthermore	O
,	O
multiple	O
lantibiotics	B-chemical
can	O
induce	O
the	O
TCS	B-complex_assembly
CprRK	B-protein
from	O
Clostridium	B-species
difficile	I-species
,	O
leading	O
to	O
the	O
expression	O
of	O
the	O
genes	O
localized	O
on	O
the	O
cpr	B-gene
operon	O
,	O
resulting	O
in	O
resistance	O
against	O
several	O
lantibiotics	B-chemical
of	O
which	O
nisin	B-chemical
,	O
gallidermin	B-chemical
,	O
subtilin	B-chemical
,	O
and	O
mutacin	B-chemical
1140	I-chemical
are	O
some	O
examples	O
.	O

It	O
has	O
been	O
suggested	O
that	O
in	O
addition	O
to	O
conferring	O
general	O
resistance	O
against	O
lantibiotics	B-chemical
,	O
the	O
BceAB	B-protein_type
-	I-protein_type
type	I-protein_type
transporters	I-protein_type
assist	O
in	O
signalling	O
as	O
via	O
the	O
presence	O
of	O
a	O
large	O
extracellular	B-structure_element
domain	I-structure_element
within	O
the	O
transmembrane	B-structure_element
segment	I-structure_element
indicated	O
by	O
experimental	O
evidence	O
from	O
various	O
systems	O
.	O

Homologous	O
operons	O
have	O
been	O
identified	O
in	O
various	O
human	B-species
pathogenic	O
strains	O
such	O
as	O
Staphylococcus	B-species
epidermis	I-species
and	O
Streptococcus	B-species
ictaluri	I-species
based	O
on	O
the	O
high	O
sequence	O
identity	O
of	O
NSR	B-protein
and	O
NsrFP	B-protein
.	O

In	O
this	O
gene	O
cluster	O
,	O
the	O
TCS	B-complex_assembly
NsrRK	B-protein
is	O
responsible	O
for	O
the	O
expression	O
of	O
the	O
nsr	B-gene
and	O
nsrFP	B-gene
genes	O
.	O

The	O
similarity	O
of	O
the	O
TCS	B-complex_assembly
within	O
all	O
the	O
described	O
nisin	B-chemical
resistance	O
operons	O
suggests	O
an	O
expression	O
specifically	O
induced	O
by	O
nisin	B-chemical
.	O

Thus	O
,	O
NsrRK	B-protein
might	O
be	O
a	O
useful	O
target	O
to	O
combat	O
inherently	O
pathogenic	O
lantibiotic	B-chemical
-	O
resistant	O
strains	O
.	O

Generally	O
,	O
RRs	B-protein_type
consist	O
of	O
two	O
distinct	O
structural	O
domains	O
,	O
a	O
receiver	B-structure_element
domain	I-structure_element
(	O
RD	B-structure_element
)	O
and	O
an	O
effector	B-structure_element
domain	I-structure_element
(	O
ED	B-structure_element
),	O
that	O
are	O
separated	O
from	O
each	O
other	O
by	O
a	O
flexible	B-protein_state
linker	B-structure_element
.	O

RDs	B-structure_element
contain	O
a	O
highly	B-protein_state
conserved	I-protein_state
aspartate	B-residue_name
residue	O
,	O
which	O
acts	O
as	O
a	O
phosphoryl	O
acceptor	O
that	O
becomes	O
phosphorylated	B-protein_state
by	O
the	O
kinase	B-structure_element
domain	I-structure_element
of	O
the	O
histidine	B-protein_type
kinase	I-protein_type
upon	O
reception	O
of	O
an	O
external	O
signal	O
.	O

The	O
RRs	B-protein_type
are	O
classified	O
into	O
different	O
subfamilies	O
depending	O
on	O
the	O
three	O
-	O
dimensional	O
structure	O
of	O
their	O
EDs	B-structure_element
.	O

The	O
OmpR	B-protein_type
/	I-protein_type
PhoB	I-protein_type
subfamily	I-protein_type
is	O
the	O
largest	O
subgroup	O
of	O
RRs	B-protein_type
and	O
comprises	O
approximately	O
40	O
%	O
of	O
all	O
response	B-protein_type
regulators	I-protein_type
in	O
bacteria	B-taxonomy_domain
.	O

The	O
various	O
structures	B-evidence
of	O
RRs	B-protein_type
reveal	O
that	O
in	O
addition	O
to	O
being	O
in	O
either	O
“	O
inactive	B-protein_state
”	O
or	O
“	O
active	B-protein_state
”	O
state	O
,	O
the	O
RRs	B-protein_type
can	O
also	O
exist	O
in	O
two	O
distinct	O
conformations	O
:	O
“	O
open	B-protein_state
”	O
and	O
“	O
closed	B-protein_state
”.	O

MtrA	B-protein
and	O
PrrA	B-protein
exhibit	O
a	O
very	B-protein_state
compact	I-protein_state
,	O
closed	B-protein_state
structure	B-evidence
with	O
the	O
DNA	B-structure_element
-	I-structure_element
binding	I-structure_element
sequence	I-structure_element
,	O
called	O
recognition	B-structure_element
helix	I-structure_element
,	O
of	O
the	O
ED	B-structure_element
being	O
inaccessible	O
to	O
DNA	B-chemical
.	O

Here	O
,	O
we	O
describe	O
the	O
crystal	B-evidence
structures	I-evidence
of	O
the	O
N	O
-	O
terminal	O
RD	B-structure_element
and	O
the	O
C	O
-	O
terminal	O
ED	B-structure_element
of	O
the	O
lantibiotic	B-protein_type
resistance	I-protein_type
-	I-protein_type
associated	I-protein_type
RR	I-protein_type
NsrR	B-protein
from	O
S	B-species
.	I-species
agalactiae	I-species
.	O

NsrR	B-protein
is	O
part	O
of	O
the	O
nisin	B-chemical
resistance	O
operon	O
.	O

The	O
expression	O
of	O
the	O
genes	O
of	O
this	O
operon	O
is	O
induced	O
by	O
a	O
TCS	B-complex_assembly
consisting	O
of	O
the	O
HK	B-protein_type
NsrK	B-protein
and	O
the	O
RR	B-protein_type
NsrR	B-protein
.	O
Based	O
on	O
the	O
crystal	B-evidence
structures	I-evidence
of	O
both	O
the	O
domains	O
,	O
modeling	O
was	O
employed	O
to	O
shed	O
light	O
on	O
the	O
putative	O
DNA	B-protein_state
-	I-protein_state
bound	I-protein_state
state	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
NsrR	B-protein
.	O

Both	O
bands	O
were	O
subjected	O
to	O
mass	B-experimental_method
spectrometry	I-experimental_method
analysis	I-experimental_method
.	O

The	O
analysis	O
revealed	O
that	O
the	O
larger	O
fragment	O
(**)	O
represents	O
the	O
N	O
-	O
terminal	O
receiver	B-structure_element
domain	I-structure_element
(	O
residues	O
1	B-residue_range
–	I-residue_range
119	I-residue_range
;	O
referred	O
to	O
as	O
NsrR	B-protein
-	O
RD	B-structure_element
)	O
whereas	O
the	O
smaller	O
fragment	O
(***)	O
contained	O
the	O
C	O
-	O
terminal	O
DNA	B-structure_element
-	I-structure_element
binding	I-structure_element
effector	I-structure_element
domain	I-structure_element
of	O
NsrR	B-protein
(	O
residues	O
129	B-residue_range
–	I-residue_range
243	I-residue_range
including	O
21	O
amino	O
acids	O
derived	O
from	O
the	O
expression	O
tag	O
;	O
referred	O
to	O
as	O
NsrR	B-protein
-	O
ED	B-structure_element
)	O
(	O
Fig	O
1C	O
).	O

Residues	O
120	B-residue_range
–	I-residue_range
128	I-residue_range
form	O
the	O
linker	B-structure_element
connecting	O
the	O
RD	B-structure_element
and	O
ED	B-structure_element
.	O

Mass	B-experimental_method
spectrometry	I-experimental_method
analysis	I-experimental_method
did	O
not	O
reveal	O
the	O
presence	O
of	O
any	O
specific	O
protease	O
in	O
the	O
purified	O
NsrR	B-protein
sample	O
.	O

Furthermore	O
,	O
addition	O
of	O
a	O
protease	O
inhibitor	O
,	O
such	O
as	O
PMSF	B-chemical
(	O
Phenylmethylsulfonyl	B-chemical
fluoride	I-chemical
)	O
and	O
AEBSF	B-chemical
{	O
4	B-chemical
-(	I-chemical
2	I-chemical
-	I-chemical
Aminoethyl	I-chemical
)	I-chemical
benzenesulfonyl	I-chemical
fluoride	I-chemical
hydrochloride	I-chemical
},	O
even	O
at	O
high	O
concentrations	O
,	O
did	O
not	O
inhibit	O
proteolysis	O
(	O
data	O
not	O
shown	O
).	O

Purification	B-experimental_method
of	O
NsrR	B-protein
and	O
SDS	B-experimental_method
PAGE	I-experimental_method
analysis	O
of	O
purified	O
NsrR	B-protein
directly	O
and	O
one	O
week	O
after	O
purification	O
.	O

(	O
a	O
)	O
Elution	B-evidence
profile	I-evidence
of	O
size	B-experimental_method
-	I-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
step	O
of	O
NsrR	B-protein
.	O
The	O
y	O
-	O
axis	O
represents	O
the	O
UV	O
absorption	O
of	O
the	O
protein	O
at	O
280	O
nm	O
,	O
while	O
the	O
x	O
-	O
axis	O
represents	O
the	O
elution	O
volume	O
.	O

(	O
b	O
)	O
Freshly	O
purified	O
NsrR	B-protein
protein	O
,	O
and	O
(	O
c	O
)	O
NsrR	B-protein
protein	O
after	O
one	O
week	O
.	O

Lanes	O
:	O
M	O
represents	O
the	O
PAGE	O
Ruler	O
Unstained	O
Ladder	O
;	O
1	O
:	O
NsrR	B-protein
after	O
a	O
two	O
-	O
step	O
purification	O
;	O
2	O
:	O
NsrR	B-protein
one	O
week	O
after	O
purification	O
.	O

*	O
corresponds	O
to	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
NsrR	B-protein
protein	O
at	O
27	O
kDa	O
,	O
while	O
**	O
and	O
***	O
correspond	O
to	O
the	O
NsrR	B-protein
-	O
RD	B-structure_element
and	O
NsrR	B-protein
-	O
ED	B-structure_element
domain	O
at	O
around	O
13	O
kDa	O
,	O
respectively	O
.	O

Since	O
formation	O
of	O
the	O
crystals	B-evidence
took	O
around	O
one	O
month	O
,	O
it	O
is	O
not	O
surprising	O
that	O
this	O
cleavage	O
also	O
occurred	O
in	O
the	O
crystallization	O
drop	O
.	O

Initially	O
,	O
we	O
tried	O
to	O
solve	O
the	O
structure	B-evidence
of	O
NsrR	B-protein
by	O
molecular	B-experimental_method
replacement	I-experimental_method
,	O
which	O
was	O
not	O
successful	O
.	O

Therefore	O
,	O
we	O
tried	O
heavy	B-experimental_method
atom	I-experimental_method
phasing	I-experimental_method
using	O
a	O
platinum	B-chemical
compound	O
.	O

This	O
succeeded	O
for	O
the	O
rectangular	O
plate	O
-	O
shaped	O
crystals	B-evidence
.	O

Therefore	O
,	O
we	O
thought	O
that	O
these	O
crystals	B-evidence
contained	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
of	O
NsrR	B-protein
and	O
successfully	O
phased	O
this	O
dataset	O
using	O
molecular	B-experimental_method
replacement	I-experimental_method
with	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
of	O
PhoB	B-protein
(	O
PDB	O
code	O
:	O
1B00	O
;	O
as	O
a	O
template	O
.	O

This	O
approach	O
revealed	O
that	O
this	O
crystal	O
form	O
indeed	O
contained	O
two	O
monomers	B-oligomeric_state
of	O
the	O
RD	B-structure_element
of	O
NsrR	B-protein
in	O
the	O
asymmetric	O
unit	O
.	O

Since	O
both	O
crystals	O
forms	O
were	O
obtained	O
in	O
the	O
same	O
drop	O
it	O
is	O
not	O
surprising	O
that	O
,	O
when	O
dissolving	O
several	O
crystals	B-evidence
and	O
performing	O
subsequent	O
mass	B-experimental_method
-	I-experimental_method
spectrometry	I-experimental_method
to	O
identify	O
the	O
protein	O
in	O
the	O
crystals	B-evidence
,	O
it	O
yielded	O
peptide	O
fragments	O
throughout	O
the	O
NsrR	B-protein
sequence	O
.	O

In	O
summary	O
,	O
the	O
two	O
crystal	B-evidence
forms	I-evidence
contained	O
one	O
of	O
the	O
two	O
domains	O
,	O
respectively	O
,	O
such	O
that	O
both	O
domains	O
were	O
successfully	O
crystallized	B-experimental_method
.	O

However	O
,	O
a	O
part	O
of	O
the	O
linker	B-structure_element
region	I-structure_element
(	O
residues	O
120	B-residue_range
–	I-residue_range
128	I-residue_range
;	O
120RRSQQFIQQ128	B-structure_element
;	O
underlined	O
are	O
the	O
amino	O
acid	O
residues	O
not	O
visible	O
in	O
either	O
domain	O
)	O
could	O
not	O
be	O
traced	O
in	O
the	O
electron	B-evidence
density	I-evidence
.	O

Overall	O
structure	B-evidence
of	O
the	O
N	O
-	O
terminal	O
NsrR	B-protein
receiver	B-structure_element
domain	I-structure_element
(	O
NsrR	B-protein
-	O
RD	B-structure_element
)	O

The	O
structure	B-evidence
of	O
the	O
NsrR	B-protein
-	O
RD	B-structure_element
was	O
determined	O
at	O
a	O
resolution	O
of	O
1	O
.	O
8	O
Å	O
(	O
Table	O
1	O
).	O

The	O
structure	B-evidence
contained	O
many	O
ethylene	B-chemical
glycol	I-chemical
molecules	O
arising	O
from	O
the	O
cryo	O
-	O
protecting	O
procedure	O
.	O

The	O
asymmetric	O
unit	O
contains	O
two	O
copies	O
of	O
NsrR	B-protein
-	O
RD	B-structure_element
.	O

For	O
Asn85	B-residue_name_number
,	O
Asp86	B-residue_name_number
,	O
and	O
Glu87	B-residue_name_number
of	O
chain	B-structure_element
A	I-structure_element
,	O
poor	O
electron	B-evidence
density	I-evidence
was	O
observed	O
for	O
the	O
side	O
chains	O
and	O
,	O
thus	O
,	O
these	O
side	O
chains	O
were	O
deleted	O
during	O
refinement	O
and	O
are	O
not	O
present	O
in	O
the	O
final	O
structure	B-evidence
.	O

Since	O
the	O
two	O
monomers	B-oligomeric_state
of	O
NsrR	B-protein
-	O
RD	B-structure_element
were	O
virtually	O
identical	O
(	O
rmsd	B-evidence
of	O
0	O
.	O
6	O
Å	O
over	O
116	O
Cα	O
atoms	O
for	O
the	O
two	O
monomers	B-oligomeric_state
).	O

Therefore	O
,	O
the	O
overall	O
structure	B-evidence
is	O
described	O
for	O
monomer	B-oligomeric_state
A	B-structure_element
only	O
.	O

NsrR	B-protein
-	O
RD	B-structure_element
structurally	O
adopts	O
a	O
αβ	B-structure_element
doubly	I-structure_element
-	I-structure_element
wound	I-structure_element
fold	I-structure_element
previously	O
observed	O
in	O
OmpR	B-protein_type
/	I-protein_type
PhoB	I-protein_type
type	I-protein_type
regulators	I-protein_type
.	O

Five	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
(	O
β1	B-structure_element
-	I-structure_element
β5	I-structure_element
)	O
are	O
arranged	O
in	O
a	O
parallel	O
fashion	O
constituting	O
the	O
central	O
core	O
of	O
the	O
structure	B-evidence
,	O
which	O
is	O
surrounded	O
by	O
two	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
(	O
α1	B-structure_element
and	O
α5	B-structure_element
)	O
on	O
one	O
and	O
three	O
helices	B-structure_element
(	O
α2	B-structure_element
,	O
α3	B-structure_element
,	O
α4	B-structure_element
)	O
on	O
the	O
other	O
side	O
(	O
Fig	O
2	O
).	O

Structure	B-evidence
of	O
NsrR	B-protein
-	O
RD	B-structure_element
.	O

Comparison	B-experimental_method
with	O
structures	B-evidence
of	O
other	O
receiver	B-structure_element
domains	I-structure_element

NsrR	B-protein
belongs	O
to	O
the	O
OmpR	B-protein_type
/	I-protein_type
PhoB	I-protein_type
family	I-protein_type
of	O
RRs	B-protein_type
.	O

Superimposition	B-experimental_method
of	O
the	O
structures	B-evidence
revealed	O
that	O
helix	B-structure_element
α4	B-structure_element
is	O
slightly	O
rotated	O
outward	O
in	O
NsrR	B-protein
-	O
RD	B-structure_element
(	O
Fig	O
2	O
).	O

In	O
receiver	B-structure_element
domains	I-structure_element
of	O
response	B-protein_type
regulators	I-protein_type
,	O
helix	B-structure_element
α4	B-structure_element
has	O
been	O
shown	O
to	O
be	O
a	O
crucial	O
part	O
of	O
the	O
dimerization	B-site
interface	I-site
.	O

Furthermore	O
,	O
helix	B-structure_element
α4	B-structure_element
in	O
NsrR	B-protein
is	O
shorter	O
than	O
in	O
other	O
RRs	B-protein_type
.	O

The	O
first	B-structure_element
helical	I-structure_element
turn	I-structure_element
is	O
unwound	B-protein_state
and	O
adopts	O
an	O
unstructured	B-protein_state
region	O
(	O
see	O
Fig	O
2	O
).	O

A	O
slightly	O
outward	O
rotation	O
or	O
unwinding	O
of	O
helix	B-structure_element
α4	B-structure_element
has	O
been	O
observed	O
in	O
the	O
structures	B-evidence
of	O
other	O
RD	B-structure_element
of	O
regulators	O
.	O

For	O
example	O
,	O
the	O
structure	B-evidence
of	O
BaeR	B-protein
and	O
RegX3	B-protein
displayed	O
a	O
completely	O
unwound	B-protein_state
helix	B-structure_element
α4	B-structure_element
.	O

In	O
the	O
structure	B-evidence
of	O
DrrD	B-protein
,	O
helix	B-structure_element
α4	B-structure_element
is	O
only	O
partially	O
displaced	O
.	O

In	O
the	O
receiver	B-structure_element
domain	I-structure_element
of	O
NsrR	B-protein
,	O
helix	B-structure_element
α4	B-structure_element
is	O
also	O
partially	O
displaced	O
but	O
in	O
a	O
different	O
direction	O
(	O
S1	O
Fig	O
).	O

Inspection	O
of	O
the	O
crystal	O
contacts	O
revealed	O
no	O
major	O
interactions	O
in	O
this	O
region	O
that	O
could	O
have	O
influenced	O
the	O
orientation	O
of	O
helix	B-structure_element
α4	B-structure_element
.	O

The	O
structures	B-evidence
of	O
the	O
RD	B-structure_element
and	O
ED	B-structure_element
domains	O
of	O
NsrR	B-protein
aligned	B-experimental_method
to	O
other	O
response	B-protein_type
regulators	I-protein_type
.	O

This	O
structural	O
homology	O
is	O
also	O
reflected	O
by	O
the	O
low	O
rmsd	B-evidence
of	O
1	O
.	O
9	O
Å	O
over	O
117	O
Cα	O
atoms	O
after	O
superimposition	B-experimental_method
of	O
the	O
receiver	B-structure_element
domains	I-structure_element
of	O
NsrR	B-protein
and	O
KdpE	B-protein
(	O
Table	O
2	O
).	O

Furthermore	O
,	O
the	O
orientation	O
of	O
the	O
helix	B-structure_element
α4	B-structure_element
in	O
NsrR	B-protein
is	O
close	O
to	O
that	O
present	O
in	O
KdpE	B-protein
(	O
S1	O
Fig	O
).	O

Active	B-site
site	I-site
residues	O
and	O
dimerization	O

All	O
RRs	B-protein_type
contain	O
a	O
highly	B-protein_state
conserved	I-protein_state
aspartate	B-residue_name
residue	O
in	O
the	O
active	B-site
site	I-site
(	O
Fig	O
3	O
;	O
shown	O
in	O
red	O
).	O

Phosphorylation	B-ptm
of	O
this	O
aspartate	B-residue_name
residue	O
induces	O
a	O
conformational	O
change	O
leading	O
to	O
the	O
activation	O
of	O
the	O
effector	B-structure_element
domain	I-structure_element
that	O
binds	O
DNA	B-chemical
and	O
regulates	O
the	O
transcription	O
of	O
target	O
genes	O
.	O

Sequence	B-experimental_method
alignment	I-experimental_method
of	O
NsrR	B-protein
protein	O
with	O
other	O
response	B-protein_type
regulators	I-protein_type
.	O

A	O
sequence	B-experimental_method
alignment	I-experimental_method
of	O
NsrR	B-protein
with	O
RRs	B-protein_type
belonging	O
to	O
the	O
OmpR	B-protein_type
/	I-protein_type
PhoB	I-protein_type
subfamily	I-protein_type
(	O
marked	O
in	O
grey	O
)	O
and	O
RRs	B-protein_type
involved	O
in	O
lantibiotic	B-chemical
resistance	O
(	O
black	O
)	O
is	O
shown	O
.	O

The	O
active	B-site
site	I-site
aspartate	B-residue_name
residue	O
(	O
highlighted	O
in	O
red	O
),	O
the	O
residues	O
forming	O
the	O
acidic	B-site
pocket	I-site
surrounding	O
it	O
(	O
highlighted	O
in	O
pink	O
),	O
the	O
switch	B-site
residues	I-site
(	O
highlighted	O
in	O
blue	O
),	O
the	O
conserved	B-protein_state
lysine	B-residue_name
residue	O
(	O
highlighted	O
in	O
green	O
),	O
the	O
highly	B-protein_state
conserved	I-protein_state
residues	O
of	O
the	O
linker	B-structure_element
region	I-structure_element
(	O
colored	O
in	O
purple	O
),	O
the	O
residues	O
involved	O
in	O
dimer	B-site
interface	I-site
of	O
receiver	B-structure_element
domain	I-structure_element
(	O
highlighted	O
in	O
yellow	O
),	O
residues	O
involved	O
in	O
interdomain	O
interactions	O
(	O
shown	O
in	O
orange	O
boxes	O
and	O
in	O
cyan	O
)	O
and	O
the	O
residues	O
involved	O
in	O
interaction	O
with	O
DNA	B-chemical
(	O
colored	O
in	O
blue	O
)	O
are	O
shown	O
.	O

In	O
NsrR	B-protein
,	O
Glu12	B-residue_name_number
,	O
Asp13	B-residue_name_number
,	O
and	O
Asp55	B-residue_name_number
are	O
in	O
close	O
proximity	O
of	O
a	O
highly	B-protein_state
conserved	I-protein_state
Lys104	B-residue_name_number
residue	O
(	O
highlighted	O
in	O
green	O
in	O
Fig	O
3	O
).	O

Location	O
of	O
the	O
highly	B-protein_state
conserved	I-protein_state
Asp55	B-residue_name_number
and	O
inactive	B-protein_state
state	O
conformation	O
of	O
the	O
key	O
switch	B-site
residues	I-site
,	O
Ser82	B-residue_name_number
and	O
Phe101	B-residue_name_number
in	O
NsrR	B-protein
-	O
RD	B-structure_element
.	O

NsrR	B-protein
(	O
represented	O
in	O
yellow	O
)	O
displays	O
a	O
geometry	O
representing	O
the	O
inactive	B-protein_state
state	O
as	O
deduced	O
from	O
the	O
inactive	B-protein_state
state	O
structure	B-evidence
of	O
PhoB	B-protein
(	O
shown	O
in	O
brown	O
,	O
PDB	O
code	O
1B00	O
)	O
(	O
a	O
).	O

However	O
,	O
the	O
structure	B-evidence
of	O
NsrR	B-protein
-	O
RD	B-structure_element
did	O
not	O
contain	O
any	O
divalent	O
ion	O
.	O

Instead	O
,	O
a	O
water	B-chemical
molecule	O
is	O
present	O
,	O
which	O
interacts	O
with	O
Glu12	B-residue_name_number
of	O
the	O
acidic	B-site
pocket	I-site
,	O
Lys104	B-residue_name_number
,	O
and	O
another	O
water	B-chemical
molecule	O
in	O
the	O
vicinity	O
.	O

As	O
seen	O
in	O
the	O
alignment	B-experimental_method
(	O
Fig	O
3	O
,	O
highlighted	O
in	O
blue	O
),	O
these	O
signature	O
residues	O
(	O
Ser	B-residue_name
/	O
Thr	B-residue_name
and	O
Phe	B-residue_name
/	O
Tyr	B-residue_name
)	O
are	O
highly	B-protein_state
conserved	I-protein_state
in	O
the	O
lantibiotic	B-protein_type
resistance	I-protein_type
-	I-protein_type
associated	I-protein_type
RRs	I-protein_type
.	O

The	O
orientation	O
of	O
the	O
side	O
chains	O
of	O
these	O
residues	O
determines	O
whether	O
the	O
RD	B-structure_element
is	O
in	O
an	O
active	B-protein_state
or	O
inactive	B-protein_state
state	O
.	O

In	O
the	O
inactive	B-protein_state
state	O
,	O
the	O
phenylalanine	B-residue_name
or	O
tyrosine	B-residue_name
residue	O
faces	O
away	O
from	O
the	O
active	B-site
site	I-site
,	O
and	O
the	O
corresponding	O
serine	B-residue_name
or	O
threonine	B-residue_name
residue	O
adopts	O
an	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
conformation	O
as	O
well	O
(	O
Fig	O
4A	O
).	O

By	O
sequence	B-experimental_method
alignment	I-experimental_method
with	O
other	O
lantibiotic	B-protein_type
resistance	I-protein_type
-	I-protein_type
associated	I-protein_type
RRs	I-protein_type
,	O
these	O
“	O
signature	B-site
switch	I-site
residues	I-site
”	O
are	O
identified	O
as	O
Ser82	B-residue_name_number
and	O
Phe101	B-residue_name_number
in	O
NsrR	B-protein
(	O
see	O
above	O
).	O

Furthermore	O
,	O
the	O
second	B-site
switch	I-site
residue	I-site
is	O
mostly	O
a	O
tyrosine	B-residue_name
,	O
with	O
NsrR	B-protein
,	O
BraR	B-protein
,	O
and	O
BceR	B-protein
being	O
the	O
only	O
exceptions	O
containing	O
a	O
phenylalanine	B-residue_name
at	O
that	O
position	O
.	O

As	O
mentioned	O
above	O
,	O
RRs	B-protein_type
contain	O
a	O
phosphorylation	B-protein_state
-	I-protein_state
activated	I-protein_state
switch	B-site
and	O
normally	O
exist	O
in	O
equilibrium	O
between	O
the	O
active	B-protein_state
and	O
inactive	B-protein_state
conformations	O
.	O

Therefore	O
,	O
we	O
performed	O
a	O
DALI	B-experimental_method
search	I-experimental_method
and	O
focused	O
on	O
RD	B-structure_element
domains	O
that	O
were	O
structurally	O
determined	O
as	O
functional	B-protein_state
dimers	B-oligomeric_state
.	O

In	O
this	O
context	O
,	O
the	O
dimer	B-oligomeric_state
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
KdpE	B-protein
from	O
E	B-species
.	I-species
coli	I-species
(	O
Z	B-evidence
-	I-evidence
score	I-evidence
18	O
.	O
8	O
,	O
rmsd	B-evidence
1	O
.	O
9	O
Å	O
over	O
117	O
Cα	O
atoms	O
)	O
(	O
PDB	O
code	O
:	O
4KNY	O
)	O
and	O
the	O
structure	B-evidence
of	O
the	O
functional	B-protein_state
dimer	B-oligomeric_state
of	O
the	O
RD	B-structure_element
of	O
KdpE	B-protein
from	O
E	B-species
.	I-species
coli	I-species
(	O
PDB	O
code	O
:	O
1ZH2	O
)	O
represent	O
the	O
most	O
structurally	O
related	O
structures	B-evidence
.	O

We	O
aligned	B-experimental_method
NsrR	B-protein
-	O
RD	B-structure_element
on	O
both	O
monomers	B-oligomeric_state
of	O
the	O
RD	B-structure_element
of	O
KdpE	B-protein
.	O
Since	O
helix	B-structure_element
α4	B-structure_element
of	O
NsrR	B-protein
-	O
RD	B-structure_element
is	O
orientated	O
slightly	O
different	O
when	O
compared	O
with	O
other	O
structures	B-evidence
of	O
RDs	B-structure_element
(	O
Fig	O
2	O
),	O
helix	B-structure_element
α4	B-structure_element
and	O
the	O
N	O
-	O
terminal	O
loop	B-structure_element
of	O
one	O
monomer	B-oligomeric_state
were	O
clashing	O
with	O
the	O
second	O
monomer	B-oligomeric_state
(	O
S2A	O
Fig	O
).	O

Therefore	O
,	O
helix	B-structure_element
α4	B-structure_element
and	O
the	O
N	O
-	O
terminal	O
loop	B-structure_element
were	O
shifted	O
to	O
the	O
position	O
of	O
KdpE	B-protein
by	O
primarily	O
modifying	O
backbone	O
torsion	O
angles	O
in	O
the	O
region	O
immediately	O
C	O
-	O
terminal	O
to	O
helix	B-structure_element
α4	B-structure_element
.	O

The	O
dimeric	B-site
interface	I-site
is	O
formed	O
by	O
α4	B-structure_element
-	I-structure_element
β5	I-structure_element
-	I-structure_element
α5	I-structure_element
of	O
RD	B-structure_element
(	O
Fig	O
5A	O
),	O
as	O
previously	O
observed	O
in	O
other	O
RRs	B-protein_type
.	O

In	O
KdpE	B-protein
,	O
a	O
network	O
of	O
salt	B-bond_interaction
bridges	I-bond_interaction
and	O
other	O
electrostatic	B-bond_interaction
interactions	I-bond_interaction
stabilize	O
the	O
interface	B-site
within	O
a	O
single	O
monomer	B-oligomeric_state
as	O
well	O
as	O
between	O
the	O
monomers	B-oligomeric_state
.	O

Functional	O
dimer	B-oligomeric_state
orientation	O
of	O
the	O
RDs	B-structure_element
of	O
NsrR	B-protein
.	O

(	O
b	O
)	O
Zoom	O
-	O
in	O
of	O
the	O
dimeric	B-site
interface	I-site
mediated	O
by	O
α4	B-structure_element
-	I-structure_element
β5	I-structure_element
-	I-structure_element
α5	I-structure_element
.	O

The	O
monomer	B-oligomeric_state
-	O
monomer	B-oligomeric_state
interactions	O
are	O
facilitated	O
by	O
hydrophobic	O
residues	O
(	O
displayed	O
as	O
spheres	O
),	O
inter	O
-	O
and	O
intra	O
-	O
domain	O
interactions	O
(	O
displayed	O
as	O
sticks	O
).	O

Conserved	O
intermolecular	O
electrostatic	B-bond_interaction
interactions	I-bond_interaction
further	O
stabilize	O
the	O
monomer	B-oligomeric_state
-	O
monomer	B-oligomeric_state
interaction	O
of	O
KdpE	B-protein
and	O
are	O
formed	O
between	O
Asp97	B-residue_name_number
(	O
β5	B-structure_element
)	O
and	O
Arg111	B-residue_name_number
(	O
α5	B-structure_element
),	O
Asp96	B-residue_name_number
(	O
α4	B-structure_element
–	I-structure_element
β5	I-structure_element
loop	I-structure_element
)	O
and	O
Arg118	B-residue_name_number
(	O
α5	B-structure_element
),	O
and	O
Asp92	B-residue_name_number
(	O
α4	B-structure_element
)	O
and	O
Arg113	B-residue_name_number
(	O
α5	B-structure_element
).	O

Some	O
of	O
these	O
interactions	O
can	O
also	O
be	O
identified	O
in	O
the	O
dimeric	B-oligomeric_state
model	O
of	O
NsrR	B-protein
-	O
RD	B-structure_element
.	O

Asp99	B-residue_name_number
(	O
α4	B-structure_element
–	I-structure_element
β5	I-structure_element
loop	I-structure_element
;	O
Fig	O
3	O
,	O
shown	O
in	O
cyan	O
)	O
points	O
toward	O
the	O
side	O
chain	O
of	O
Arg121	B-residue_name_number
.	O

This	O
interaction	O
is	O
also	O
observed	O
in	O
KdpE	B-protein
(	O
Asp96	B-residue_name_number
(	O
α4	B-structure_element
–	I-structure_element
β5	I-structure_element
loop	I-structure_element
)	O
and	O
Arg118	B-residue_name_number
(	O
α5	B-structure_element
)).	O

In	O
KdpE	B-protein
,	O
Arg111	B-residue_name_number
is	O
additionally	O
stabilized	O
by	O
another	O
intra	O
-	O
molecular	O
salt	B-bond_interaction
bridge	I-bond_interaction
with	O
Glu107	B-residue_name_number
(	O
α5	B-structure_element
).	O

Interestingly	O
,	O
in	O
NsrR	B-protein
-	O
RD	B-structure_element
this	O
amino	O
acid	O
corresponds	O
to	O
Val110	B-residue_name_number
(	O
highlighted	O
in	O
yellow	O
in	O
Fig	O
3	O
).	O

As	O
observed	O
in	O
this	O
alignment	B-experimental_method
,	O
the	O
above	O
-	O
mentioned	O
arginine	B-residue_name
residue	O
(	O
Arg111	B-residue_name_number
in	O
KdpE	B-protein
)	O
is	O
either	O
an	O
arginine	B-residue_name
or	O
a	O
lysine	B-residue_name
residue	O
(	O
Lys114	B-residue_name_number
in	O
NsrR	B-protein
)	O
in	O
all	O
RRs	B-protein_type
used	O
in	O
the	O
alignment	B-experimental_method
(	O
Fig	O
3	O
,	O
shown	O
in	O
cyan	O
).	O

Interestingly	O
,	O
whenever	O
an	O
arginine	B-residue_name
is	O
present	O
at	O
this	O
position	O
(	O
Arg111	B-residue_name_number
in	O
KdpE	B-protein
),	O
a	O
glutamate	B-residue_name
(	O
Glu107	B-residue_name_number
in	O
KdpE	B-protein
)	O
is	O
present	O
as	O
well	O
,	O
presumably	O
stabilizing	O
the	O
arginine	B-residue_name
side	O
chain	O
.	O

However	O
,	O
when	O
a	O
lysine	B-residue_name
is	O
present	O
at	O
this	O
position	O
,	O
the	O
glutamate	B-residue_name
is	O
exchanged	O
to	O
a	O
hydrophobic	O
residue	O
contributing	O
to	O
the	O
hydrophobic	B-site
patch	I-site
described	O
above	O
.	O

Additionally	O
,	O
it	O
has	O
been	O
shown	O
for	O
PhoB	B-protein
from	O
E	B-species
.	I-species
coli	I-species
and	O
PhoP	B-protein
from	O
B	B-species
.	I-species
subtilis	I-species
that	O
mutating	B-experimental_method
the	O
corresponding	O
residues	O
involved	O
in	O
dimerisation	O
(	O
residues	O
Asp100	B-residue_name_number
,	O
Val110	B-residue_name_number
and	O
Lys114	B-residue_name_number
in	O
NsrR	B-protein
)	O
results	O
in	O
monomeric	B-oligomeric_state
form	O
of	O
response	B-protein_type
regulator	I-protein_type
which	O
has	O
lost	B-protein_state
the	I-protein_state
ability	I-protein_state
to	I-protein_state
dimerize	I-protein_state
as	O
well	O
as	O
display	O
reduced	O
DNA	B-chemical
binding	O
capabilities	O
.	O

The	O
structure	B-evidence
of	O
NsrR	B-protein
-	O
ED	B-structure_element
from	O
S	B-species
.	I-species
agalactiae	I-species
was	O
determined	O
using	O
experimental	O
phases	O
from	O
a	O
single	B-experimental_method
-	I-experimental_method
wavelength	I-experimental_method
anomalous	I-experimental_method
dispersion	I-experimental_method
dataset	I-experimental_method
from	O
the	O
rectangular	O
plate	O
-	O
shaped	O
crystal	O
derivatized	O
with	O
platinum	B-chemical
at	O
a	O
resolution	O
of	O
1	O
.	O
6	O
Å	O
in	O
space	O
group	O
P21212	O
.	O

The	O
Rwork	B-evidence
and	O
Rfree	B-evidence
values	O
after	O
refinement	O
were	O
0	O
.	O
18	O
and	O
0	O
.	O
22	O
,	O
respectively	O
.	O

The	O
latter	O
is	O
Glu128	B-residue_name_number
(	O
last	O
residue	O
of	O
the	O
linker	B-structure_element
region	I-structure_element
)	O
of	O
chain	B-structure_element
B	I-structure_element
that	O
is	O
involved	O
in	O
crystal	O
contacts	O
and	O
,	O
therefore	O
,	O
likely	O
adopts	O
an	O
unfavorable	O
conformation	O
.	O

The	O
structure	B-evidence
contained	O
a	O
few	O
ethylene	B-chemical
glycol	I-chemical
molecules	O
introduced	O
by	O
the	O
cryo	O
-	O
protecting	O
procedure	O
.	O

The	O
C	O
-	O
terminal	O
effector	B-structure_element
DNA	I-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
of	O
NsrR	B-protein
is	O
about	O
13	O
kDa	O
in	O
size	O
and	O
consists	O
of	O
residues	O
129	B-residue_range
–	I-residue_range
243	I-residue_range
(	O
including	O
21	O
amino	O
acid	O
residues	O
of	O
the	O
expression	O
tag	O
).	O

For	O
Asp147	B-residue_name_number
of	O
chain	B-structure_element
A	I-structure_element
and	O
Glu174	B-residue_name_number
of	O
chain	B-structure_element
B	I-structure_element
,	O
poor	O
electron	B-evidence
density	I-evidence
was	O
observed	O
for	O
the	O
side	O
chains	O
and	O
,	O
thus	O
,	O
these	O
side	O
chains	O
were	O
removed	O
during	O
refinement	O
.	O

An	O
overlay	B-experimental_method
revealed	O
that	O
both	O
monomers	B-oligomeric_state
display	O
high	O
similarity	O
in	O
their	O
overall	O
structure	B-evidence
with	O
an	O
rmsd	B-evidence
of	O
0	O
.	O
5	O
Å	O
over	O
95	O
Cα	O
atoms	O
.	O

We	O
therefore	O
describe	O
for	O
the	O
overall	O
structure	B-evidence
only	O
monomer	B-oligomeric_state
A	B-structure_element
.	O

The	O
ED	B-structure_element
domain	O
of	O
NsrR	B-protein
consists	O
of	O
six	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
and	O
three	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
in	O
a	O
β6	B-structure_element
-	I-structure_element
β7	I-structure_element
-	I-structure_element
β8	I-structure_element
-	I-structure_element
β9	I-structure_element
-	I-structure_element
α6	I-structure_element
-	I-structure_element
α7	I-structure_element
-	I-structure_element
α8	I-structure_element
-	I-structure_element
β10	I-structure_element
-	I-structure_element
β11	I-structure_element
topology	O
(	O
the	O
secondary	O
structure	O
elements	O
are	O
counted	O
in	O
continuation	O
of	O
those	O
of	O
the	O
RD	B-structure_element
).	O

The	O
effector	B-structure_element
domain	I-structure_element
starts	O
with	O
a	O
4	B-structure_element
-	I-structure_element
stranded	I-structure_element
antiparallel	I-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
,	O
followed	O
by	O
three	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
and	O
eventually	O
ends	O
in	O
a	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
(	O
Fig	O
6	O
).	O

Cartoon	O
representation	O
of	O
the	O
C	O
-	O
terminal	O
effector	B-structure_element
domain	I-structure_element
of	O
NsrR	B-protein
(	O
green	O
;	O
recognition	B-structure_element
helix	I-structure_element
in	O
cyan	O
).	O

The	O
structural	O
areas	O
with	O
the	O
highest	O
variations	O
compared	O
to	O
the	O
effector	B-structure_element
domains	I-structure_element
of	O
DrrB	B-protein
(	O
pink	O
,	O
1P2F	O
),	O
MtrA	B-protein
(	O
grey	O
,	O
2GWR	O
),	O
and	O
PhoB	B-protein
(	O
blue	O
,	O
1GXQ	O
)	O
are	O
marked	O
.	O

The	O
transactivation	B-structure_element
loop	I-structure_element
of	O
MtrA	B-protein
is	O
missing	B-protein_state
in	O
the	O
structure	B-evidence
,	O
therefore	O
,	O
the	O
two	O
termini	O
are	O
connected	O
by	O
a	O
dashed	O
line	O
.	O

The	O
characteristic	O
feature	O
of	O
the	O
OmpR	B-protein_type
/	I-protein_type
PhoB	I-protein_type
subfamily	I-protein_type
of	O
RRs	B-protein_type
is	O
a	O
winged	B-structure_element
helix	I-structure_element
-	I-structure_element
turn	I-structure_element
-	I-structure_element
helix	I-structure_element
(	O
wHTH	B-structure_element
)	O
fold	O
that	O
is	O
adopted	O
by	O
the	O
α7	B-structure_element
-	I-structure_element
loop	I-structure_element
-	I-structure_element
α8	I-structure_element
segment	I-structure_element
in	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
and	O
single	O
effector	B-structure_element
domain	I-structure_element
structures	B-evidence
of	O
RRs	B-protein_type
of	O
this	O
subfamily	O
.	O

The	O
second	O
helix	B-structure_element
of	O
the	O
wHTH	B-structure_element
motif	O
is	O
important	O
for	O
DNA	B-chemical
-	O
binding	O
and	O
,	O
therefore	O
,	O
is	O
termed	O
“	O
recognition	B-structure_element
helix	I-structure_element
”	O
(	O
shown	O
in	O
cyan	O
in	O
Fig	O
6	O
).	O

Furthermore	O
,	O
a	O
helix	B-structure_element
within	O
the	O
HTH	B-structure_element
motif	O
,	O
named	O
“	O
positioning	B-structure_element
helix	I-structure_element
”,	O
is	O
important	O
for	O
proper	O
orientation	O
and	O
positioning	O
of	O
the	O
loop	B-structure_element
between	O
these	O
two	O
helices	O
and	O
is	O
referred	O
to	O
as	O
“	O
transactivation	B-structure_element
loop	I-structure_element
”	O
(	O
also	O
called	O
α	B-structure_element
-	I-structure_element
loop	I-structure_element
;	O
Fig	O
6	O
).	O

The	O
16	O
-	O
residue	O
long	O
,	O
solvent	B-protein_state
-	I-protein_state
exposed	I-protein_state
recognition	B-structure_element
helix	I-structure_element
α8	B-structure_element
of	O
NsrR	B-protein
-	O
ED	B-structure_element
contains	O
four	O
positively	O
charged	O
residues	O
that	O
can	O
potentially	O
interact	O
with	O
DNA	B-chemical
.	O

These	O
are	O
Arg198	B-residue_name_number
,	O
Arg200	B-residue_name_number
,	O
Lys201	B-residue_name_number
,	O
and	O
Lys202	B-residue_name_number
.	O

When	O
comparing	O
the	O
sequence	O
of	O
NsrR	B-protein
with	O
PhoB	B-protein
,	O
KdpE	B-protein
,	O
and	O
MtrA	B-protein
,	O
the	O
alignment	B-experimental_method
(	O
Fig	O
3	O
,	O
colored	O
in	O
blue	O
)	O
emphasizes	O
the	O
variations	O
at	O
these	O
positions	O
,	O
except	O
for	O
Arg200	B-residue_name_number
,	O
which	O
is	O
conserved	B-protein_state
throughout	O
the	O
lantibiotic	B-protein_type
resistance	I-protein_type
RRs	I-protein_type
.	O

Additionally	O
,	O
Lys202	B-residue_name_number
is	O
also	O
highly	B-protein_state
conserved	I-protein_state
throughout	O
the	O
family	O
of	O
RRs	B-protein_type
except	O
PhoB	B-protein
,	O
clearly	O
reflecting	O
differences	O
in	O
the	O
sequences	O
of	O
DNA	B-chemical
to	O
be	O
bound	O
.	O

Comparison	O
with	O
structures	B-evidence
of	O
other	O
effector	B-structure_element
domains	I-structure_element

We	O
performed	O
a	O
DALI	B-experimental_method
search	I-experimental_method
to	O
identify	O
structurally	O
related	O
proteins	O
to	O
NsrR	B-protein
-	O
ED	B-structure_element
.	O

Here	O
the	O
structure	B-evidence
of	O
the	O
effector	B-structure_element
domain	I-structure_element
of	O
PhoB	B-protein
from	O
E	B-species
.	I-species
coli	I-species
(	O
PDB	O
code	O
:	O
1GXQ	O
)	O
(	O
Z	B-evidence
-	I-evidence
score	I-evidence
of	O
13	O
.	O
7	O
)	O
is	O
structurally	O
the	O
most	O
closely	O
related	O
.	O

Similar	O
to	O
the	O
PhoB	B-protein
effector	B-structure_element
domain	I-structure_element
,	O
a	O
9	O
-	O
residues	O
long	O
loop	B-structure_element
(	O
amino	O
acid	O
182	B-residue_range
–	I-residue_range
189	I-residue_range
)	O
is	O
also	O
present	O
in	O
the	O
structure	B-evidence
of	O
NsrR	B-protein
-	O
ED	B-structure_element
that	O
connects	O
helices	B-structure_element
α7	B-structure_element
and	O
α8	B-structure_element
.	O

Therefore	O
,	O
we	O
superimposed	B-experimental_method
the	O
Cα	O
traces	O
of	O
the	O
effector	B-structure_element
domain	I-structure_element
of	O
NsrR	B-protein
(	O
NsrR	B-protein
-	O
ED	B-structure_element
)	O
with	O
other	O
previously	O
determined	O
effector	B-structure_element
domains	I-structure_element
from	O
the	O
OmpR	B-protein_type
/	I-protein_type
PhoB	I-protein_type
family	I-protein_type
such	O
as	O
DrrB	B-protein
,	O
MtrA	B-protein
and	O
of	O
only	O
the	O
effector	B-structure_element
domain	I-structure_element
structure	B-evidence
of	O
PhoB	B-protein
from	O
E	B-species
.	I-species
coli	I-species
.	O

The	O
highest	O
variations	O
(	O
Fig	O
6	O
)	O
are	O
visible	O
in	O
in	O
the	O
loop	B-structure_element
regions	O
α7	B-structure_element
-	I-structure_element
α8	I-structure_element
,	O
which	O
corresponds	O
to	O
the	O
transactivation	B-structure_element
loop	I-structure_element
.	O

In	O
many	O
RRs	B-protein_type
this	O
transactivation	B-structure_element
loop	I-structure_element
along	O
with	O
the	O
recognition	B-structure_element
helix	I-structure_element
α8	B-structure_element
,	O
form	O
inter	O
-	O
domain	O
contacts	O
in	O
the	O
inactive	B-protein_state
state	O
and	O
are	O
only	O
exposed	O
upon	O
activation	O
of	O
the	O
RRs	B-protein_type
via	O
a	O
conformational	O
change	O
where	O
the	O
N	O
-	O
and	O
C	O
-	O
terminal	O
domains	O
move	O
away	O
from	O
each	O
other	O
.	O

Linker	B-structure_element
region	I-structure_element

The	O
linkers	B-structure_element
that	O
connect	O
the	O
RDs	B-structure_element
and	O
EDs	B-structure_element
in	O
response	B-protein_type
regulators	I-protein_type
are	O
highly	B-protein_state
variable	I-protein_state
with	O
respect	O
to	O
both	O
length	O
and	O
sequence	O
.	O

Similar	O
to	O
the	O
OmpR	B-protein_type
/	I-protein_type
PhoB	I-protein_type
family	I-protein_type
,	O
the	O
lantibiotic	B-protein_type
resistance	I-protein_type
-	I-protein_type
associated	I-protein_type
family	I-protein_type
of	I-protein_type
response	I-protein_type
regulators	I-protein_type
also	O
displays	O
diverse	O
linker	B-structure_element
regions	I-structure_element
,	O
which	O
are	O
recognized	O
in	O
sequence	B-experimental_method
alignments	I-experimental_method
by	O
the	O
introduction	O
of	O
gaps	O
(	O
Fig	O
3	O
).	O

Interestingly	O
,	O
two	O
arginine	B-residue_name
residues	O
(	O
Arg120	B-residue_name_number
and	O
Arg121	B-residue_name_number
in	O
NsrR	B-protein
;	O
Fig	O
3	O
,	O
shown	O
in	O
purple	O
)	O
at	O
the	O
end	O
of	O
the	O
RDs	B-structure_element
seem	O
to	O
be	O
strictly	B-protein_state
conserved	I-protein_state
throughout	O
the	O
family	O
of	O
response	B-protein_type
regulators	I-protein_type
in	O
both	O
the	O
OmpR	B-protein_type
/	I-protein_type
PhoB	I-protein_type
and	I-protein_type
lantibiotic	I-protein_type
resistance	I-protein_type
-	I-protein_type
associated	I-protein_type
RRs	I-protein_type
,	O
indicating	O
a	O
conserved	B-protein_state
similarity	O
.	O

This	O
aspartate	B-residue_name
residue	O
is	O
identified	O
in	O
NsrR	B-protein
as	O
Asp99	B-residue_name_number
(	O
see	O
above	O
).	O

Arginine	B-residue_name_number
121	I-residue_name_number
of	O
NsrR	B-protein
points	O
towards	O
this	O
Asp99	B-residue_name_number
residue	O
however	O
,	O
the	O
distance	O
for	O
a	O
salt	B-bond_interaction
bridge	I-bond_interaction
interaction	O
is	O
too	O
large	O
.	O

Nonetheless	O
,	O
the	O
structures	B-evidence
of	O
NsrR	B-protein
-	O
RD	B-structure_element
and	O
NsrR	B-protein
-	O
ED	B-structure_element
together	O
provide	O
the	O
required	O
structural	O
knowledge	O
to	O
predict	O
the	O
linker	B-structure_element
region	I-structure_element
that	O
joins	O
the	O
receiver	B-structure_element
and	I-structure_element
effector	I-structure_element
domains	I-structure_element
.	O

The	O
linker	B-structure_element
region	I-structure_element
of	O
NsrR	B-protein
consists	O
of	O
approximately	O
nine	O
residues	O
(	O
Fig	O
3	O
),	O
comprising	O
120RRSQQFIQQ128	B-structure_element
(	O
underlined	O
residues	O
are	O
neither	O
present	O
in	O
the	O
structure	B-evidence
of	O
RD	B-structure_element
nor	O
in	O
ED	B-structure_element
of	O
NsrR	B-protein
)	O
and	O
contains	O
two	O
positively	O
charged	O
amino	O
acids	O
.	O

To	O
achieve	O
this	O
,	O
we	O
first	O
carefully	O
analyzed	O
the	O
outcome	O
of	O
the	O
Dali	B-experimental_method
search	I-experimental_method
for	O
each	O
domain	O
and	O
identified	O
structurally	O
highly	O
similar	O
proteins	O
(	O
based	O
on	O
Z	B-evidence
-	I-evidence
scores	I-evidence
and	O
rmsd	B-evidence
values	O
)	O
and	O
choose	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
structures	B-evidence
previously	O
reported	O
.	O

In	O
solution	O
,	O
RRs	B-protein_type
exist	O
in	O
equilibrium	O
between	O
the	O
active	B-protein_state
and	O
inactive	B-protein_state
state	O
,	O
which	O
is	O
shifted	O
towards	O
the	O
active	B-protein_state
state	O
upon	O
phosphorylation	B-ptm
of	O
the	O
ED	B-structure_element
.	O

This	O
results	O
in	O
oligomerization	O
of	O
the	O
RR	B-protein_type
and	O
a	O
higher	O
affinity	O
towards	O
DNA	B-chemical
.	O

Based	O
on	O
the	O
above	O
-	O
mentioned	O
criteria	O
,	O
the	O
structure	B-evidence
of	O
MtrA	B-protein
from	O
M	B-species
.	I-species
tuberculosis	I-species
,	O
crystallized	B-experimental_method
in	O
an	O
inactive	B-protein_state
and	O
non	B-protein_state
-	I-protein_state
phosphorylated	I-protein_state
state	O
,	O
seemed	O
best	O
suited	O
for	O
modeling	O
purposes	O
.	O

Furthermore	O
,	O
the	O
linker	B-structure_element
between	O
the	O
two	O
domains	O
of	O
MtrA	B-protein
contains	O
nine	O
amino	O
acids	O
and	O
is	O
of	O
similar	O
length	O
as	O
the	O
linker	B-structure_element
of	O
NsrR	B-protein
.	O
We	O
aligned	B-experimental_method
the	O
NsrR	B-protein
-	O
RD	B-structure_element
and	O
-	O
ED	B-structure_element
to	O
the	O
corresponding	O
MtrA	B-protein
domains	O
and	O
evaluated	O
the	O
structure	B-evidence
.	O

The	O
missing	B-protein_state
linker	B-structure_element
is	O
represented	O
by	O
a	O
dotted	O
line	O
.	O

(	O
b	O
)	O
Active	B-protein_state
state	O
conformation	O
:	O
A	O
model	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
NsrR	B-protein
in	O
active	B-protein_state
conformation	O
based	O
on	O
the	O
alignment	B-experimental_method
of	O
both	O
the	O
domains	O
of	O
NsrR	B-protein
to	O
the	O
structure	B-evidence
of	O
DNA	B-protein_state
bound	I-protein_state
structure	B-evidence
of	O
KdpE	B-protein
(	O
PDB	O
code	O
:	O
4KNY	O
),	O
adopting	O
an	O
active	B-protein_state
open	B-protein_state
conformation	O
,	O
where	O
the	O
other	O
molecule	O
of	O
NsrR	B-protein
is	O
shown	O
in	O
shades	O
of	O
blue	O
with	O
the	O
recognition	B-structure_element
helix	I-structure_element
colored	O
in	O
green	O
.	O

In	O
MtrA	B-protein
,	O
the	O
two	O
domains	O
interact	O
via	O
the	O
α4	B-site
-	I-site
β5	I-site
-	I-site
α5	I-site
interface	I-site
of	O
the	O
receiver	B-structure_element
domain	I-structure_element
and	O
the	O
end	O
of	O
α7	B-structure_element
,	O
α7	B-structure_element
-	I-structure_element
α8	I-structure_element
loop	I-structure_element
and	O
α8	B-structure_element
of	O
the	O
effector	B-structure_element
domain	I-structure_element
.	O

Both	O
interfaces	B-site
have	O
been	O
shown	O
to	O
form	O
functionally	O
important	O
contact	O
areas	O
in	O
the	O
active	B-protein_state
state	O
within	O
members	O
of	O
the	O
OmpR	B-protein_type
/	I-protein_type
PhoB	I-protein_type
subfamily	I-protein_type
.	O

In	O
our	O
model	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
NsrR	B-protein
,	O
a	O
similar	O
orientation	O
between	O
the	O
domains	O
is	O
observed	O
,	O
contributing	O
to	O
the	O
inter	O
-	O
domain	O
interactions	O
.	O

The	O
inactive	B-protein_state
conformation	O
of	O
MtrA	B-protein
is	O
supported	O
by	O
the	O
orientation	O
of	O
the	O
side	O
chain	O
of	O
Tyr102	B-residue_name_number
,	O
which	O
points	O
away	O
from	O
the	O
active	B-protein_state
Asp56	B-residue_name_number
residue	O
,	O
while	O
the	O
side	O
chain	O
of	O
Tyr102	B-residue_name_number
interacts	O
with	O
Asp190	B-residue_name_number
of	O
the	O
RD	B-structure_element
of	O
MtrA	B-protein
,	O
thereby	O
stabilizing	O
its	O
closed	B-protein_state
conformation	O
.	O

In	O
the	O
model	O
of	O
NsrR	B-protein
,	O
similar	O
amino	O
acids	O
are	O
present	O
,	O
Phe101	B-residue_name_number
(	O
switch	B-site
residue	I-site
)	O
and	O
Asp188	B-residue_name_number
(	O
Fig	O
3	O
,	O
represented	O
by	O
orange	O
boxes	O
)	O
forming	O
a	O
likewise	O
similar	O
network	O
of	O
interaction	O
.	O

Next	O
,	O
we	O
were	O
interested	O
in	O
the	O
active	B-protein_state
conformation	O
of	O
the	O
NsrR	B-protein
protein	O
adopting	O
an	O
active	B-protein_state
“	O
open	B-protein_state
”	O
conformation	O
in	O
the	O
dimeric	B-oligomeric_state
state	O
.	O

We	O
compared	B-experimental_method
and	I-experimental_method
aligned	I-experimental_method
the	O
NsrR	B-protein
-	O
RD	B-structure_element
and	O
ED	B-structure_element
on	O
the	O
dimeric	B-oligomeric_state
structure	B-evidence
of	O
KdpE	B-protein
that	O
was	O
solved	B-experimental_method
in	O
the	O
DNA	B-protein_state
-	I-protein_state
bound	I-protein_state
state	O
(	O
Fig	O
7B	O
).	O

Also	O
the	O
linker	B-structure_element
region	I-structure_element
of	O
KdpE	B-protein
is	O
of	O
similar	O
length	O
as	O
of	O
NsrR	B-protein
,	O
which	O
suggests	O
that	O
the	O
distance	O
in	O
the	O
DNA	B-protein_state
-	I-protein_state
bound	I-protein_state
state	O
between	O
the	O
RD	B-structure_element
and	O
ED	B-structure_element
of	O
NsrR	B-protein
will	O
be	O
similar	O
to	O
that	O
in	O
the	O
KdpE	B-protein
active	B-protein_state
dimer	B-oligomeric_state
.	O

We	O
superimposed	B-experimental_method
the	O
ED	B-structure_element
of	O
NsrR	B-protein
with	O
the	O
DNA	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
of	O
KdpE	B-protein
resulting	O
in	O
a	O
reasonably	O
well	O
-	O
aligned	O
structure	B-evidence
(	O
rmsd	B-evidence
of	O
2	O
.	O
6Å	O
over	O
86	O
Cα	O
atoms	O
;	O
Table	O
2	O
).	O

As	O
a	O
result	O
,	O
a	O
highly	B-site
positive	I-site
groove	I-site
is	O
created	O
by	O
the	O
two	O
ED	B-structure_element
domains	O
of	O
NsrR	B-protein
which	O
likely	O
represents	O
the	O
DNA	B-site
binding	I-site
site	I-site
as	O
observed	O
in	O
KdpE	B-protein
.	O
A	O
prediction	O
of	O
the	O
putative	O
promoter	O
sequence	O
that	O
NsrR	B-protein
binds	O
via	O
the	O
BPROM	O
online	O
server	O
was	O
performed	O
(	O
S3	O
Fig	O
).	O

A	O
promoter	O
region	O
was	O
identified	O
upstream	O
of	O
the	O
nsr	B-gene
operon	O
.	O

However	O
,	O
the	O
regulation	O
of	O
the	O
predicted	O
promoter	O
and	O
the	O
DNA	B-chemical
binding	O
by	O
NsrR	B-protein
has	O
to	O
be	O
confirmed	O
.	O

The	O
structure	B-evidence
of	O
the	O
response	B-protein_type
regulator	I-protein_type
NsrR	B-protein
from	O
S	B-species
.	I-species
agalactiae	I-species
presented	O
in	O
this	O
study	O
is	O
the	O
first	O
structural	O
information	O
available	O
for	O
the	O
subgroup	O
of	O
lantibiotic	B-protein_type
resistance	I-protein_type
-	I-protein_type
associated	I-protein_type
RRs	I-protein_type
.	O

Visualizing	O
chaperone	B-protein_type
-	O
assisted	O
protein	O
folding	O

Challenges	O
in	O
determining	O
the	O
structures	B-evidence
of	O
heterogeneous	O
and	O
dynamic	O
protein	O
complexes	O
have	O
greatly	O
hampered	O
past	O
efforts	O
to	O
obtain	O
a	O
mechanistic	O
understanding	O
of	O
many	O
important	O
biological	O
processes	O
.	O

One	O
such	O
process	O
is	O
chaperone	B-protein_type
-	O
assisted	O
protein	O
folding	O
,	O
where	O
obtaining	O
structural	O
ensembles	O
of	O
chaperone	B-protein_type
:	O
substrate	O
complexes	O
would	O
ultimately	O
reveal	O
how	O
chaperones	B-protein_type
help	O
proteins	O
fold	O
into	O
their	O
native	O
state	O
.	O

To	O
address	O
this	O
problem	O
,	O
we	O
devised	O
a	O
novel	O
structural	O
biology	O
approach	O
based	O
on	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
,	O
termed	O
Residual	B-experimental_method
Electron	I-experimental_method
and	I-experimental_method
Anomalous	I-experimental_method
Density	I-experimental_method
(	O
READ	B-experimental_method
).	O

The	O
ensemble	O
shows	O
that	O
Spy	B-protein_state
-	I-protein_state
associated	I-protein_state
Im7	B-protein
samples	O
conformations	O
ranging	O
from	O
unfolded	B-protein_state
to	O
partially	O
folded	B-protein_state
and	O
native	B-protein_state
-	O
like	O
states	O
,	O
and	O
reveals	O
how	O
a	O
substrate	O
can	O
explore	O
its	O
folding	O
landscape	O
while	O
bound	B-protein_state
to	I-protein_state
a	O
chaperone	B-protein_type
.	O

High	O
-	O
resolution	O
structural	B-evidence
models	I-evidence
of	O
protein	O
-	O
protein	O
interactions	O
are	O
critical	O
for	O
obtaining	O
mechanistic	O
insights	O
into	O
biological	O
processes	O
.	O

However	O
,	O
many	O
protein	O
-	O
protein	O
interactions	O
are	O
highly	B-protein_state
dynamic	I-protein_state
,	O
making	O
it	O
difficult	O
to	O
obtain	O
high	O
-	O
resolution	O
data	O
.	O

Recent	O
advances	O
in	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
and	O
NMR	B-experimental_method
spectroscopy	I-experimental_method
continue	O
to	O
improve	O
our	O
ability	O
to	O
analyze	O
biomolecules	O
that	O
exist	O
in	O
multiple	O
conformations	O
.	O

X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
has	O
historically	O
provided	O
valuable	O
information	O
on	O
small	O
-	O
scale	O
conformational	O
changes	O
,	O
but	O
observing	O
large	O
-	O
amplitude	O
heterogeneous	O
conformational	O
changes	O
often	O
falls	O
beyond	O
the	O
reach	O
of	O
current	O
crystallographic	O
techniques	O
.	O

It	O
is	O
clear	O
that	O
molecular	O
chaperones	B-protein_type
aid	O
in	O
protein	O
folding	O
.	O

Structural	B-evidence
models	I-evidence
of	O
chaperone	B-protein_type
-	O
substrate	O
complexes	O
have	O
recently	O
begun	O
to	O
provide	O
information	O
as	O
to	O
how	O
a	O
chaperone	B-protein_type
can	O
recognize	O
its	O
substrate	O
.	O

For	O
most	O
chaperones	B-protein_type
,	O
it	O
is	O
still	O
unclear	O
whether	O
the	O
chaperone	B-protein_type
actively	O
participates	O
in	O
and	O
affects	O
the	O
folding	O
of	O
the	O
substrate	O
proteins	O
,	O
or	O
merely	O
provides	O
a	O
suitable	O
microenvironment	O
enabling	O
the	O
substrate	O
to	O
fold	O
on	O
its	O
own	O
.	O

This	O
is	O
a	O
truly	O
fundamental	O
question	O
in	O
the	O
chaperone	B-protein_type
field	O
,	O
and	O
one	O
that	O
has	O
eluded	O
the	O
community	O
largely	O
because	O
of	O
the	O
highly	B-protein_state
dynamic	I-protein_state
nature	O
of	O
the	O
chaperone	B-protein_type
-	O
substrate	O
complexes	O
.	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
Spy	B-protein
revealed	O
that	O
it	O
forms	O
a	O
thin	O
α	O
-	O
helical	O
homodimeric	B-oligomeric_state
cradle	B-site
.	O

Crosslinking	B-experimental_method
and	I-experimental_method
genetic	I-experimental_method
experiments	I-experimental_method
suggested	O
that	O
Spy	B-protein
interacts	O
with	O
substrates	O
somewhere	O
on	O
its	O
concave	O
side	O
.	O

By	O
using	O
a	O
novel	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
-	O
based	O
approach	O
to	O
model	O
disorder	O
in	O
crystal	B-evidence
structures	I-evidence
,	O
we	O
have	O
now	O
determined	O
the	O
high	O
-	O
resolution	O
ensemble	B-evidence
of	O
the	O
dynamic	B-protein_state
Spy	B-complex_assembly
:	I-complex_assembly
Im7	I-complex_assembly
complex	O
.	O

This	O
work	O
provides	O
a	O
detailed	O
view	O
of	O
chaperone	B-protein_type
-	O
mediated	O
protein	O
folding	O
and	O
shows	O
how	O
substrates	O
like	O
Im7	B-protein
find	O
their	O
native	O
fold	O
while	O
bound	B-protein_state
to	I-protein_state
their	O
chaperones	B-protein_type
.	O

Crystallizing	B-experimental_method
the	O
Spy	B-complex_assembly
:	I-complex_assembly
Im7	I-complex_assembly
complex	O

We	O
reasoned	O
that	O
to	O
obtain	O
crystals	B-evidence
of	O
complexes	O
between	O
Spy	B-protein
(	O
domain	O
boundaries	O
in	O
Supplementary	O
Fig	O
.	O
1	O
)	O
and	O
its	O
substrate	O
proteins	O
,	O
our	O
best	O
approach	O
was	O
to	O
identify	O
crystallization	B-experimental_method
conditions	I-experimental_method
that	O
yielded	O
Spy	B-protein
crystals	B-evidence
in	O
the	O
presence	B-protein_state
of	I-protein_state
protein	O
substrates	O
but	O
not	O
in	O
their	O
absence	B-protein_state
.	O

We	O
therefore	O
screened	B-experimental_method
crystallization	B-experimental_method
conditions	I-experimental_method
for	O
Spy	B-protein
with	O
four	O
different	O
substrate	O
proteins	O
:	O
a	O
fragment	O
of	O
the	O
largely	O
unfolded	B-protein_state
bovine	B-taxonomy_domain
α	B-chemical
-	I-chemical
casein	I-chemical
protein	O
,	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
WT	B-protein_state
)	O
E	B-species
.	I-species
coli	I-species
Im7	B-protein
,	O
an	O
unfolded	B-protein_state
variant	O
of	O
Im7	B-protein
(	O
L18A	B-mutant
L19A	B-mutant
L37A	B-mutant
),	O
and	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
half	I-structure_element
of	O
Im7	B-protein
(	O
Im76	B-mutant
-	I-mutant
45	I-mutant
),	O
which	O
encompasses	O
the	O
entire	O
Spy	B-structure_element
-	I-structure_element
binding	I-structure_element
portion	I-structure_element
of	O
Im7	B-protein
.	O

We	O
found	O
conditions	O
in	O
which	O
all	O
four	O
substrates	O
co	B-experimental_method
-	I-experimental_method
crystallized	I-experimental_method
with	B-protein_state
Spy	B-protein
,	O
but	O
in	O
which	O
Spy	B-protein
alone	B-protein_state
did	O
not	O
yield	O
crystals	B-evidence
.	O

Subsequent	O
crystal	B-experimental_method
washing	I-experimental_method
and	I-experimental_method
dissolution	I-experimental_method
experiments	O
confirmed	O
the	O
presence	O
of	O
the	O
substrates	O
in	O
the	O
co	B-experimental_method
-	I-experimental_method
crystals	I-experimental_method
(	O
Supplementary	O
Fig	O
.	O
2	O
).	O

The	O
crystals	B-evidence
diffracted	O
to	O
~	O
1	O
.	O
8	O
Å	O
resolution	O
.	O

However	O
,	O
modeling	O
of	O
the	O
substrate	O
in	O
the	O
complex	O
proved	O
to	O
be	O
a	O
substantial	O
challenge	O
,	O
as	O
the	O
electron	B-evidence
density	I-evidence
of	O
the	O
substrate	O
was	O
discontinuous	O
and	O
fragmented	O
.	O

Even	O
the	O
minimal	B-structure_element
binding	I-structure_element
portion	I-structure_element
of	O
Im7	B-protein
(	O
Im76	B-mutant
-	I-mutant
45	I-mutant
)	O
showed	O
highly	O
dispersed	O
electron	B-evidence
density	I-evidence
(	O
Fig	O
.	O
1a	O
).	O

We	O
hypothesized	O
that	O
the	O
fragmented	O
density	B-evidence
was	O
due	O
to	O
multiple	O
,	O
partially	O
occupied	O
conformations	O
of	O
the	O
substrate	O
bound	O
within	O
the	O
crystal	B-evidence
.	O

Thus	O
,	O
we	O
developed	O
a	O
new	O
approach	O
to	O
interpret	O
the	O
chaperone	B-protein_state
-	I-protein_state
bound	I-protein_state
substrate	O
in	O
multiple	O
conformations	O
.	O

READ	B-experimental_method
:	O
a	O
strategy	O
to	O
visualize	O
heterogeneous	O
and	O
dynamic	O
biomolecules	O

(	O
2	O
)	O
We	O
then	O
labeled	O
individual	O
residues	O
in	O
the	O
flexible	B-protein_state
regions	O
of	O
the	O
substrate	O
with	O
the	O
strong	O
anomalous	O
scatterer	O
iodine	B-chemical
,	O
which	O
serves	O
to	O
locate	O
these	O
residues	O
in	O
three	O
-	O
dimensional	O
space	O
using	O
their	O
anomalous	B-evidence
density	I-evidence
.	O

(	O
3	O
)	O
We	O
performed	O
molecular	B-experimental_method
dynamics	I-experimental_method
(	O
MD	B-experimental_method
)	O
simulations	B-experimental_method
to	O
generate	O
a	O
pool	O
of	O
energetically	O
reasonable	O
conformations	O
of	O
the	O
dynamic	B-protein_state
complex	O
and	O
(	O
4	O
)	O
applied	O
a	O
sample	B-experimental_method
-	I-experimental_method
and	I-experimental_method
-	I-experimental_method
select	I-experimental_method
algorithm	I-experimental_method
to	O
determine	O
the	O
minimal	O
set	O
of	O
substrate	O
conformations	O
that	O
fit	O
both	O
the	O
residual	B-evidence
and	I-evidence
anomalous	I-evidence
density	I-evidence
.	O

Importantly	O
,	O
even	O
though	O
we	O
only	O
labeled	O
a	O
subset	O
of	O
the	O
residues	O
in	O
the	O
flexible	B-protein_state
regions	O
of	O
the	O
substrate	O
with	O
iodine	B-chemical
,	O
the	O
residual	B-evidence
electron	I-evidence
density	I-evidence
can	O
provide	O
spatial	O
information	O
on	O
many	O
of	O
the	O
other	O
flexible	B-protein_state
residues	O
.	O

The	O
electron	B-evidence
density	I-evidence
then	O
allowed	O
us	O
to	O
connect	O
the	O
labeled	O
residues	O
of	O
the	O
substrate	O
by	O
confining	O
the	O
protein	O
chain	O
within	O
regions	O
of	O
detectable	O
density	B-evidence
.	O

In	O
this	O
way	O
,	O
the	O
two	O
forms	O
of	O
data	O
together	O
were	O
able	O
to	O
describe	O
multiple	O
conformations	O
of	O
the	O
substrate	O
within	O
the	O
crystal	B-evidence
.	O

As	O
described	O
in	O
detail	O
below	O
,	O
we	O
developed	O
the	O
READ	B-experimental_method
method	O
to	O
uncover	O
the	O
ensemble	O
of	O
conformations	O
that	O
the	O
Spy	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
of	O
Im7	B-protein
(	O
i	O
.	O
e	O
.,	O
Im76	B-mutant
-	I-mutant
45	I-mutant
)	O
adopts	O
while	O
bound	B-protein_state
to	I-protein_state
Spy	B-protein
.	O

However	O
,	O
we	O
believe	O
that	O
READ	B-experimental_method
will	O
prove	O
generally	O
applicable	O
to	O
visualizing	O
heterogeneous	O
and	O
dynamic	O
complexes	O
that	O
have	O
previously	O
escaped	O
detailed	O
structural	O
analysis	O
.	O

Collecting	O
READ	B-experimental_method
data	O
for	O
the	O
Spy	B-complex_assembly
:	I-complex_assembly
Im76	I-complex_assembly
-	I-complex_assembly
45	I-complex_assembly
complex	O

To	O
apply	O
the	O
READ	B-experimental_method
technique	I-experimental_method
to	O
the	O
folding	O
mechanism	O
employed	O
by	O
the	O
chaperone	B-protein_type
Spy	B-protein
,	O
we	O
selected	O
Im76	B-mutant
-	I-mutant
45	I-mutant
for	O
further	O
investigation	O
because	O
NMR	B-experimental_method
data	O
suggested	O
that	O
Im76	B-mutant
-	I-mutant
45	I-mutant
could	O
recapitulate	O
unfolded	B-protein_state
,	O
partially	O
folded	B-protein_state
,	O
and	O
native	O
-	O
like	O
states	O
of	O
Im7	B-protein
(	O
Supplementary	O
Fig	O
.	O
3	O
).	O

To	O
introduce	O
the	O
anomalous	O
scatterer	O
iodine	B-chemical
,	O
we	O
replaced	B-experimental_method
eight	O
Im76	B-mutant
-	I-mutant
45	I-mutant
residues	O
with	O
the	O
non	O
-	O
canonical	O
amino	O
acid	O
4	B-chemical
-	I-chemical
iodophenylalanine	I-chemical
(	O
pI	B-chemical
-	I-chemical
Phe	I-chemical
).	O

Its	O
strong	O
anomalous	B-evidence
scattering	I-evidence
allowed	O
us	O
to	O
track	O
the	O
positions	O
of	O
these	O
individual	O
Im76	B-mutant
-	I-mutant
45	I-mutant
residues	O
one	O
at	O
a	O
time	O
,	O
potentially	O
even	O
if	O
the	O
residue	O
was	O
found	O
in	O
several	O
locations	O
in	O
the	O
same	O
crystal	B-evidence
.	O

Consistent	O
with	O
our	O
electron	B-evidence
density	I-evidence
map	I-evidence
,	O
we	O
found	O
that	O
the	O
majority	O
of	O
anomalous	B-evidence
signals	I-evidence
emerged	O
in	O
the	O
cradle	B-site
of	O
Spy	B-protein
,	O
implying	O
that	O
this	O
is	O
the	O
likely	O
Im7	B-protein
substrate	B-site
binding	I-site
site	I-site
.	O

Together	O
,	O
these	O
results	O
indicated	O
that	O
the	O
Im7	B-protein
substrate	O
binds	O
Spy	B-protein
in	O
multiple	O
conformations	O
.	O

READ	B-experimental_method
sample	B-experimental_method
-	I-experimental_method
and	I-experimental_method
-	I-experimental_method
select	I-experimental_method
procedure	O

To	O
determine	O
the	O
structural	O
ensemble	O
that	O
Im76	B-mutant
-	I-mutant
45	I-mutant
adopts	O
while	O
bound	B-protein_state
to	I-protein_state
Spy	B-protein
,	O
we	O
combined	O
the	O
residual	B-evidence
electron	I-evidence
density	I-evidence
and	O
the	O
anomalous	B-evidence
signals	I-evidence
from	O
our	O
pI	B-chemical
-	I-chemical
Phe	I-chemical
substituted	O
Spy	B-complex_assembly
:	I-complex_assembly
Im76	I-complex_assembly
-	I-complex_assembly
45	I-complex_assembly
complexes	O
.	O

To	O
generate	O
an	O
accurate	O
depiction	O
of	O
the	O
chaperone	B-protein_type
-	O
substrate	O
interactions	O
,	O
we	O
devised	O
a	O
selection	O
protocol	O
based	O
on	O
a	O
sample	B-experimental_method
-	I-experimental_method
and	I-experimental_method
-	I-experimental_method
select	I-experimental_method
procedure	O
employed	O
in	O
NMR	B-experimental_method
spectroscopy	I-experimental_method
.	O

If	O
successful	O
,	O
the	O
selection	O
identifies	O
the	O
smallest	O
group	O
of	O
specific	O
conformations	O
that	O
best	O
fits	O
the	O
residual	B-evidence
electron	I-evidence
density	I-evidence
and	O
anomalous	B-evidence
signals	I-evidence
.	O

The	O
READ	B-experimental_method
sample	B-experimental_method
-	I-experimental_method
and	I-experimental_method
-	I-experimental_method
select	I-experimental_method
algorithm	I-experimental_method
is	O
diagrammed	O
in	O
Fig	O
.	O
2	O
.	O

Prior	O
to	O
performing	O
the	O
selection	O
,	O
we	O
generated	O
a	O
large	O
and	O
diverse	O
pool	O
of	O
chaperone	B-protein_type
-	O
substrate	O
complexes	O
using	O
coarse	B-experimental_method
-	I-experimental_method
grained	I-experimental_method
MD	I-experimental_method
simulations	I-experimental_method
in	O
a	O
pseudo	B-experimental_method
-	I-experimental_method
crystal	I-experimental_method
environment	I-experimental_method
(	O
Fig	O
.	O
2	O
and	O
Supplementary	O
Fig	O
.	O
4	O
).	O

The	O
coarse	B-experimental_method
-	I-experimental_method
grained	I-experimental_method
simulations	I-experimental_method
are	O
based	O
on	O
a	O
single	O
-	O
residue	O
resolution	O
model	O
for	O
protein	O
folding	O
and	O
were	O
extended	O
here	O
to	O
describe	O
Spy	B-complex_assembly
-	I-complex_assembly
Im76	I-complex_assembly
-	I-complex_assembly
45	I-complex_assembly
binding	O
events	O
(	O
Online	O
Methods	O
).	O

The	O
initial	O
conditions	O
of	O
the	O
binding	B-experimental_method
simulations	I-experimental_method
are	O
not	O
biased	O
toward	O
a	O
particular	O
conformation	O
of	O
the	O
substrate	O
or	O
any	O
specific	O
chaperone	B-protein_type
-	O
substrate	O
interaction	O
(	O
Online	O
Methods	O
).	O

Im76	B-mutant
-	I-mutant
45	I-mutant
binds	O
and	O
unbinds	O
to	O
Spy	B-protein
throughout	O
the	O
simulations	B-experimental_method
.	O

From	O
the	O
MD	B-experimental_method
simulations	B-experimental_method
,	O
we	O
extracted	O
~	O
10	O
,	O
000	O
diverse	O
Spy	B-complex_assembly
:	I-complex_assembly
Im76	I-complex_assembly
-	I-complex_assembly
45	I-complex_assembly
complexes	O
to	O
be	O
used	O
by	O
the	O
ensuing	O
selection	O
.	O

Each	O
complex	O
within	O
this	O
pool	O
comprises	O
one	O
Spy	B-protein
dimer	B-oligomeric_state
bound	B-protein_state
to	I-protein_state
a	O
single	O
Im76	B-mutant
-	I-mutant
45	I-mutant
substrate	O
.	O

This	O
pool	O
was	O
then	O
used	O
by	O
the	O
selection	O
algorithm	O
to	O
identify	O
the	O
minimal	O
ensemble	O
that	O
best	O
satisfies	O
both	O
the	O
residual	B-evidence
electron	I-evidence
and	I-evidence
anomalous	I-evidence
crystallographic	I-evidence
data	I-evidence
.	O

The	O
anomalous	B-evidence
scattering	I-evidence
portion	O
of	O
the	O
selection	O
uses	O
our	O
basic	O
knowledge	O
of	O
pI	B-chemical
-	I-chemical
Phe	I-chemical
geometry	O
:	O
the	O
iodine	B-chemical
is	O
separated	O
from	O
its	O
respective	O
Cα	O
atom	O
in	O
each	O
coarse	O
-	O
grained	O
conformer	O
by	O
6	O
.	O
5	O
Å	O
.	O
The	O
selection	O
then	O
picks	O
ensembles	O
that	O
best	O
reproduce	O
the	O
collection	O
of	O
iodine	B-chemical
anomalous	B-evidence
signals	I-evidence
.	O

To	O
make	O
the	O
electron	B-experimental_method
density	I-experimental_method
selection	I-experimental_method
practical	O
,	O
we	O
needed	O
to	O
develop	O
a	O
method	O
to	O
rapidly	O
evaluate	O
the	O
agreement	O
between	O
the	O
selected	O
sub	O
-	O
ensembles	O
and	O
the	O
experimental	O
electron	B-evidence
density	I-evidence
on	O
-	O
the	O
-	O
fly	O
during	O
the	O
selection	O
procedure	O
.	O

To	O
accomplish	O
this	O
task	O
,	O
we	O
generated	O
a	O
compressed	O
version	O
of	O
the	O
experimental	O
2mFo	B-evidence
−	I-evidence
DFc	I-evidence
electron	I-evidence
density	I-evidence
map	I-evidence
for	O
use	O
in	O
the	O
selection	O
.	O

This	O
process	O
provided	O
us	O
with	O
a	O
target	O
map	B-evidence
that	O
the	O
ensuing	O
selection	O
tried	O
to	O
recapitulate	O
.	O

To	O
reduce	O
the	O
extent	O
of	O
3D	O
space	O
to	O
be	O
explored	O
,	O
this	O
compressed	O
map	B-evidence
was	O
created	O
by	O
only	O
using	O
density	B-evidence
from	O
regions	O
of	O
space	O
significantly	O
sampled	O
by	O
Im76	B-mutant
-	I-mutant
45	I-mutant
in	O
the	O
Spy	B-complex_assembly
:	I-complex_assembly
Im76	I-complex_assembly
-	I-complex_assembly
45	I-complex_assembly
MD	B-experimental_method
simulations	B-experimental_method
.	O

For	O
each	O
of	O
the	O
~	O
10	O
,	O
000	O
complexes	O
in	O
the	O
coarse	B-experimental_method
-	I-experimental_method
grained	I-experimental_method
MD	B-experimental_method
pool	O
,	O
the	O
electron	B-evidence
density	I-evidence
at	O
the	O
Cα	O
positions	O
of	O
Im76	B-mutant
-	I-mutant
45	I-mutant
was	O
extracted	O
and	O
used	O
to	O
construct	O
an	O
electron	B-evidence
density	I-evidence
map	I-evidence
(	O
Online	O
Methods	O
).	O

These	O
individual	O
electron	B-evidence
density	I-evidence
maps	I-evidence
from	O
the	O
separate	O
conformers	O
could	O
then	O
be	O
combined	O
(	O
Fig	O
.	O
2	O
)	O
and	O
compared	O
to	O
the	O
averaged	O
experimental	O
electron	B-evidence
density	I-evidence
map	I-evidence
as	O
part	O
of	O
the	O
selection	O
algorithm	O
.	O

This	O
approach	O
allowed	O
us	O
to	O
simultaneously	O
use	O
both	O
the	O
iodine	B-chemical
anomalous	B-evidence
signals	I-evidence
and	O
the	O
residual	B-evidence
electron	I-evidence
density	I-evidence
in	O
the	O
selection	O
procedure	O
.	O

The	O
selection	O
resulted	O
in	O
small	O
ensembles	O
from	O
the	O
MD	B-experimental_method
pool	O
that	O
best	O
fit	O
the	O
READ	B-experimental_method
data	O
(	O
Fig	O
.	O
1c	O
,	O
d	O
).	O

Before	O
analyzing	O
the	O
details	O
of	O
the	O
Spy	B-complex_assembly
:	I-complex_assembly
Im76	I-complex_assembly
-	I-complex_assembly
45	I-complex_assembly
complex	O
,	O
we	O
first	O
engaged	O
in	O
a	O
series	O
of	O
validation	O
tests	O
to	O
verify	O
the	O
ensemble	O
and	O
selection	O
procedure	O
(	O
Supplementary	O
Note	O
1	O
,	O
Figures	O
1c	O
,	O
d	O
,	O
Supplemental	O
Figures	O
5	O
-	O
7	O
).	O

Of	O
note	O
,	O
the	O
final	O
six	O
-	O
membered	O
ensemble	O
was	O
the	O
largest	O
ensemble	O
that	O
could	O
simultaneously	O
decrease	O
the	O
RFree	B-evidence
and	O
pass	O
the	O
10	B-experimental_method
-	I-experimental_method
fold	I-experimental_method
cross	I-experimental_method
-	I-experimental_method
validation	I-experimental_method
test	I-experimental_method
.	O

This	O
ensemble	O
depicts	O
the	O
conformations	O
that	O
the	O
substrate	O
Im76	B-mutant
-	I-mutant
45	I-mutant
adopts	O
while	O
bound	B-protein_state
to	I-protein_state
the	O
chaperone	B-protein_type
Spy	B-protein
(	O
Fig	O
.	O
3	O
Supplementary	O
Movie	O
1	O
,	O
and	O
Table	O
1	O
).	O

Folding	O
and	O
interactions	O
of	O
Im7	B-protein
while	O
bound	B-protein_state
to	I-protein_state
Spy	B-protein

Our	O
results	O
showed	O
that	O
by	O
using	O
this	O
novel	O
READ	B-experimental_method
approach	O
,	O
we	O
were	O
able	O
to	O
obtain	O
structural	O
information	O
about	O
the	O
dynamic	O
interaction	O
of	O
a	O
chaperone	B-protein_type
with	O
its	O
substrate	O
protein	O
.	O

We	O
were	O
particularly	O
interested	O
in	O
finding	O
answers	O
to	O
one	O
of	O
the	O
most	O
fundamental	O
questions	O
in	O
chaperone	B-protein_type
biology	O
—	O
how	O
does	O
chaperone	B-protein_type
binding	O
affect	O
substrate	O
structure	O
and	O
vice	O
versa	O
.	O

We	O
found	O
these	O
conformations	O
to	O
be	O
highly	O
heterogeneous	O
and	O
to	O
include	O
unfolded	B-protein_state
,	O
partially	B-protein_state
folded	I-protein_state
,	O
and	O
native	B-protein_state
-	I-protein_state
like	I-protein_state
states	O
(	O
Fig	O
.	O
3	O
).	O

We	O
constructed	O
a	O
contact	B-evidence
map	I-evidence
of	O
the	O
complex	O
,	O
which	O
shows	O
the	O
frequency	O
of	O
interactions	O
for	O
chaperone	B-protein_type
-	O
substrate	O
residue	O
pairs	O
(	O
Fig	O
.	O
4	O
).	O

We	O
found	O
that	O
the	O
primary	O
interaction	B-site
sites	I-site
on	O
Spy	B-protein
reside	O
at	O
the	O
N	O
and	O
C	O
termini	O
(	O
Arg122	B-residue_name_number
,	O
Thr124	B-residue_name_number
,	O
and	O
Phe29	B-residue_name_number
)	O
as	O
well	O
as	O
on	O
the	O
concave	O
face	O
of	O
the	O
chaperone	B-protein_type
(	O
Arg61	B-residue_name_number
,	O
Arg43	B-residue_name_number
,	O
Lys47	B-residue_name_number
,	O
His96	B-residue_name_number
,	O
and	O
Met46	B-residue_name_number
).	O

With	O
respect	O
to	O
the	O
substrate	O
,	O
we	O
observed	O
that	O
nearly	O
every	O
residue	O
in	O
Im76	B-mutant
-	I-mutant
45	I-mutant
is	O
in	O
contact	O
with	O
Spy	B-protein
(	O
Fig	O
.	O
4a	O
).	O

However	O
,	O
we	O
did	O
notice	O
that	O
despite	O
this	O
uniformity	O
,	O
regions	O
of	O
Im76	B-mutant
-	I-mutant
45	I-mutant
preferentially	O
interact	O
with	O
different	O
regions	O
in	O
Spy	B-protein
(	O
Fig	O
.	O
4b	O
).	O

Not	O
unexpectedly	O
,	O
we	O
found	O
that	O
as	O
Im76	B-mutant
-	I-mutant
45	I-mutant
progresses	O
from	O
the	O
unfolded	B-protein_state
to	O
the	O
native	B-protein_state
state	O
,	O
its	O
interactions	O
with	O
Spy	B-protein
shift	O
accordingly	O
.	O

This	O
shift	O
in	O
contacts	O
is	O
likely	O
due	O
to	O
hydrophobic	O
residues	O
of	O
Im76	B-mutant
-	I-mutant
45	I-mutant
preferentially	O
forming	O
intra	O
-	O
molecular	O
contacts	O
upon	O
folding	O
(	O
i	O
.	O
e	O
.,	O
hydrophobic	O
collapse	O
),	O
effectively	O
removing	O
themselves	O
from	O
the	O
interaction	B-site
sites	I-site
.	O

The	O
diversity	O
of	O
conformations	O
and	O
binding	B-site
sites	I-site
observed	O
here	O
emphasizes	O
the	O
dynamic	O
and	O
heterogeneous	O
nature	O
of	O
the	O
chaperone	B-protein_type
-	O
substrate	O
ensemble	O
.	O

Comparing	O
the	O
structure	B-evidence
of	O
Spy	B-protein
in	O
its	O
substrate	B-protein_state
-	I-protein_state
bound	I-protein_state
and	O
apo	B-protein_state
states	O
revealed	O
that	O
the	O
Spy	B-protein
dimer	B-oligomeric_state
also	O
undergoes	O
significant	O
conformational	O
changes	O
upon	O
substrate	O
binding	O
(	O
Fig	O
.	O
5a	O
and	O
Supplementary	O
Movie	O
2	O
).	O

Upon	O
substrate	O
binding	O
,	O
the	O
Spy	B-protein
dimer	B-oligomeric_state
twists	O
9	O
°	O
about	O
its	O
center	O
relative	O
to	O
its	O
apo	B-protein_state
form	O
.	O

This	O
twist	O
yields	O
asymmetry	O
and	O
results	O
in	O
substantially	O
different	O
interaction	O
patterns	O
in	O
the	O
two	O
Spy	B-protein
monomers	B-oligomeric_state
(	O
Fig	O
.	O
4b	O
).	O

It	O
is	O
possible	O
that	O
this	O
twist	O
serves	O
to	O
increase	O
heterogeneity	O
in	O
Spy	B-protein
by	O
providing	O
more	O
binding	O
poses	O
.	O

This	O
increased	O
disorder	O
might	O
explain	O
how	O
Spy	B-protein
is	O
able	O
to	O
recognize	O
and	O
bind	O
different	O
substrates	O
and	O
/	O
or	O
differing	O
conformations	O
of	O
the	O
same	O
substrate	O
.	O

Importantly	O
,	O
we	O
observed	O
the	O
same	O
structural	O
changes	O
in	O
Spy	B-protein
regardless	O
of	O
which	O
of	O
the	O
four	O
substrates	O
was	O
bound	O
(	O
Fig	O
.	O
5b	O
,	O
Table	O
1	O
).	O

The	O
RMSD	B-evidence
between	O
the	O
well	B-protein_state
-	I-protein_state
folded	I-protein_state
sections	O
of	O
Spy	B-protein
in	O
the	O
four	O
chaperone	B-protein_type
-	O
substrate	O
complexes	O
was	O
very	O
small	O
,	O
less	O
than	O
0	O
.	O
3	O
Å	O
.	O
Combined	O
with	O
competition	B-experimental_method
experiments	I-experimental_method
showing	O
that	O
the	O
substrates	O
compete	O
in	O
solution	O
for	O
Spy	B-protein
binding	O
(	O
Fig	O
.	O
5c	O
and	O
Supplementary	O
Fig	O
.	O
8	O
),	O
we	O
conclude	O
that	O
all	O
the	O
tested	O
substrates	O
share	O
the	O
same	O
overall	O
Spy	B-site
binding	I-site
site	I-site
.	O

To	O
shed	O
light	O
on	O
how	O
chaperones	B-protein_type
interact	O
with	O
their	O
substrates	O
,	O
we	O
developed	O
a	O
novel	O
structural	O
biology	O
method	O
(	O
READ	B-experimental_method
)	O
and	O
applied	O
it	O
to	O
determine	O
a	O
conformational	B-evidence
ensemble	I-evidence
of	O
the	O
chaperone	B-protein_type
Spy	B-protein
bound	B-protein_state
to	I-protein_state
substrate	I-protein_state
.	O

As	O
a	O
substrate	O
,	O
we	O
used	O
Im76	B-mutant
-	I-mutant
45	I-mutant
,	O
the	O
chaperone	B-structure_element
-	I-structure_element
interacting	I-structure_element
portion	I-structure_element
of	O
the	O
protein	O
-	O
folding	O
model	O
protein	O
Im7	B-protein
.	O

This	O
substrate	O
-	O
chaperone	B-protein_type
ensemble	O
helps	O
accomplish	O
the	O
longstanding	O
goal	O
of	O
obtaining	O
a	O
detailed	O
view	O
of	O
how	O
a	O
chaperone	B-protein_type
aids	O
protein	O
folding	O
.	O

We	O
recently	O
showed	O
that	O
Im7	B-protein
can	O
fold	O
while	O
remaining	O
continuously	B-protein_state
bound	I-protein_state
to	I-protein_state
Spy	B-protein
.	O

The	O
structures	B-evidence
of	O
our	O
ensemble	B-evidence
agree	O
well	O
with	O
lower	O
-	O
resolution	O
crosslinking	O
data	O
,	O
which	O
indicate	O
that	O
chaperone	B-protein_type
-	O
substrate	O
interactions	O
primarily	O
occur	O
on	O
the	O
concave	B-site
surface	I-site
of	O
Spy	B-protein
.	O

This	O
model	O
is	O
consistent	O
with	O
previous	O
studies	O
postulating	O
that	O
the	O
flexible	O
binding	O
of	O
chaperones	B-protein_type
allows	O
for	O
substrate	O
protein	O
folding	O
.	O

The	O
amphipathic	O
concave	B-site
surface	I-site
of	O
Spy	B-protein
likely	O
facilitates	O
this	O
flexible	O
binding	O
and	O
may	O
be	O
a	O
crucial	O
feature	O
for	O
Spy	B-protein
and	O
potentially	O
other	O
chaperones	B-protein_type
,	O
allowing	O
them	O
to	O
bind	O
multiple	O
conformations	O
of	O
many	O
different	O
substrates	O
.	O

In	O
contrast	O
to	O
Spy	B-protein
’	O
s	O
binding	B-site
hotspots	I-site
,	O
Im76	B-mutant
-	I-mutant
45	I-mutant
displays	O
substantially	O
less	O
specificity	O
in	O
its	O
binding	B-site
sites	I-site
.	O

Nearly	O
all	O
Im76	B-mutant
-	I-mutant
45	I-mutant
residues	O
come	O
in	O
contact	O
with	O
Spy	B-protein
.	O

Unfolded	B-protein_state
substrate	O
conformers	O
interact	O
with	O
Spy	B-protein
through	O
both	O
hydrophobic	B-bond_interaction
and	I-bond_interaction
hydrophilic	I-bond_interaction
interactions	I-bond_interaction
,	O
whereas	O
the	O
binding	O
of	O
native	B-protein_state
-	I-protein_state
like	I-protein_state
states	O
is	O
mainly	O
hydrophilic	O
.	O

This	O
trend	O
suggests	O
that	O
complex	O
formation	O
between	O
an	O
ATP	B-protein_state
-	I-protein_state
independent	I-protein_state
chaperone	B-protein_type
and	O
its	O
unfolded	B-protein_state
substrate	O
may	O
initially	O
involve	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
,	O
effectively	O
shielding	O
the	O
exposed	O
aggregation	O
-	O
sensitive	O
hydrophobic	B-site
regions	I-site
in	O
the	O
substrate	O
.	O

Once	O
the	O
substrate	O
begins	O
to	O
fold	O
within	O
this	O
protected	O
environment	O
,	O
it	O
progressively	O
buries	O
its	O
own	O
hydrophobic	O
residues	O
,	O
and	O
its	O
interactions	O
with	O
the	O
chaperone	B-protein_type
shift	O
towards	O
becoming	O
more	O
electrostatic	O
.	O

Notably	O
,	O
the	O
most	O
frequent	O
contacts	O
between	O
Spy	B-protein
and	O
Im76	B-mutant
-	I-mutant
45	I-mutant
are	O
charge	B-bond_interaction
-	I-bond_interaction
charge	I-bond_interaction
interactions	I-bond_interaction
.	O

The	O
negatively	O
charged	O
Im7	B-protein
residues	O
Glu21	B-residue_name_number
,	O
Asp32	B-residue_name_number
,	O
and	O
Asp35	B-residue_name_number
reside	O
on	O
the	O
surface	O
of	O
Im7	B-protein
and	O
form	O
interactions	O
with	O
Spy	B-protein
’	O
s	O
positively	O
charged	O
cradle	B-site
in	O
both	O
the	O
unfolded	B-protein_state
and	O
native	B-protein_state
-	I-protein_state
like	I-protein_state
states	O
.	O

This	O
proximity	O
likely	O
causes	O
electrostatic	O
repulsion	O
that	O
destabilizes	O
Im7	B-protein
’	O
s	O
native	B-protein_state
state	O
.	O

Interaction	O
with	O
Spy	B-protein
’	O
s	O
positively	O
-	O
charged	O
residues	O
likely	O
relieves	O
the	O
charge	O
repulsion	O
between	O
Asp32	B-residue_name_number
and	O
Asp35	B-residue_name_number
,	O
promoting	O
their	O
compaction	O
into	O
a	O
helical	B-protein_state
conformation	I-protein_state
.	O

Recently	O
,	O
we	O
employed	O
a	O
genetic	B-experimental_method
selection	I-experimental_method
system	I-experimental_method
to	O
improve	O
the	O
chaperone	B-protein_type
activity	O
of	O
Spy	B-protein
.	O

This	O
selection	O
resulted	O
in	O
“	O
Super	O
Spy	B-protein
”	O
variants	B-protein_state
that	O
were	O
more	O
effective	O
at	O
both	O
preventing	O
aggregation	O
and	O
promoting	O
protein	O
folding	O
.	O

In	O
conjunction	O
with	O
our	O
bound	B-protein_state
Im76	B-mutant
-	I-mutant
45	I-mutant
ensemble	B-evidence
,	O
these	O
mutants	O
now	O
allowed	O
us	O
to	O
investigate	O
structural	O
features	O
important	O
to	O
chaperone	B-protein_type
function	O
.	O

Previous	O
analysis	O
revealed	O
that	O
the	O
Super	O
Spy	B-protein
variants	B-protein_state
either	O
bound	B-protein_state
Im7	B-protein
tighter	O
than	O
WT	B-protein_state
Spy	B-protein
,	O
increased	O
chaperone	B-protein_type
flexibility	O
as	O
measured	O
via	O
H	B-experimental_method
/	I-experimental_method
D	I-experimental_method
exchange	I-experimental_method
,	O
or	O
both	O
.	O

Our	O
ensemble	B-evidence
revealed	O
that	O
two	O
of	O
the	O
Super	O
Spy	B-protein
mutations	B-protein_state
(	O
H96L	B-mutant
and	O
Q100L	B-mutant
)	O
form	O
part	O
of	O
the	O
chaperone	B-site
contact	I-site
surface	I-site
that	O
binds	O
to	O
Im76	B-mutant
-	I-mutant
45	I-mutant
(	O
Fig	O
.	O
4a	O
).	O

Moreover	O
,	O
our	O
co	B-evidence
-	I-evidence
structure	I-evidence
suggests	O
that	O
the	O
L32P	B-mutant
substitution	O
,	O
which	O
increases	O
Spy	B-protein
’	O
s	O
flexibility	O
,	O
could	O
operate	O
by	O
unhinging	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
helix	I-structure_element
and	O
effectively	O
expanding	O
the	O
size	O
of	O
the	O
disordered	B-protein_state
linker	B-structure_element
.	O

This	O
possibility	O
is	O
supported	O
by	O
the	O
Spy	B-protein
:	O
substrate	O
structures	B-evidence
,	O
in	O
which	O
the	O
linker	B-structure_element
region	I-structure_element
becomes	O
more	O
flexible	O
compared	O
to	O
the	O
apo	B-protein_state
state	O
(	O
Fig	O
.	O
6a	O
).	O

By	O
sampling	O
multiple	O
conformations	O
,	O
this	O
linker	B-structure_element
region	I-structure_element
may	O
allow	O
diverse	O
substrate	O
conformations	O
to	O
be	O
accommodated	O
.	O

Other	O
Super	O
Spy	B-protein
mutations	B-protein_state
(	O
F115I	B-mutant
and	O
F115L	B-mutant
)	O
caused	O
increased	O
flexibility	O
but	O
not	O
tighter	O
substrate	O
binding	O
.	O

This	O
residue	O
does	O
not	O
directly	O
contact	O
Im76	B-mutant
-	I-mutant
45	I-mutant
in	O
our	O
READ	B-experimental_method
-	O
derived	O
ensemble	B-evidence
.	O

Instead	O
,	O
when	O
Spy	B-protein
is	O
bound	B-protein_state
to	I-protein_state
substrate	O
,	O
F115	B-residue_name_number
engages	O
in	O
close	O
CH	O
⋯	O
π	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
Tyr104	B-residue_name_number
(	O
Fig	O
.	O
6b	O
).	O

Overall	O
,	O
comparison	O
of	O
our	O
ensemble	B-evidence
to	O
the	O
Super	O
Spy	B-protein
variants	B-protein_state
provides	O
specific	O
examples	O
to	O
corroborate	O
the	O
importance	O
of	O
conformational	O
flexibility	O
in	O
chaperone	B-protein_type
-	O
substrate	O
interactions	O
.	O

Despite	O
extensive	O
studies	O
,	O
exactly	O
how	O
complex	O
chaperone	B-protein_type
machines	O
help	O
proteins	O
fold	O
remains	O
controversial	O
.	O

We	O
speculate	O
that	O
many	O
other	O
chaperones	B-protein_type
could	O
utilize	O
a	O
similar	O
strategy	O
.	O

ATP	B-chemical
and	O
co	O
-	O
chaperone	B-protein_type
dependencies	O
may	O
have	O
emerged	O
later	O
through	O
evolution	O
to	O
better	O
modulate	O
and	O
control	O
chaperone	B-protein_type
action	O
.	O

In	O
addition	O
to	O
insights	O
into	O
chaperone	B-protein_type
function	O
,	O
this	O
work	O
presents	O
a	O
new	O
method	O
for	O
determining	O
heterogeneous	O
structural	O
ensembles	O
via	O
a	O
hybrid	O
methodology	O
of	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
and	O
computational	B-experimental_method
modeling	I-experimental_method
.	O

Heterogeneous	O
dynamic	O
complexes	O
or	O
disordered	B-protein_state
regions	O
of	O
single	O
proteins	O
,	O
once	O
considered	O
solely	O
approachable	O
by	O
NMR	B-experimental_method
spectroscopy	I-experimental_method
,	O
can	O
now	O
be	O
visualized	O
through	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
.	O

This	O
is	O
the	O
residual	O
density	B-evidence
that	O
is	O
used	O
in	O
the	O
READ	B-experimental_method
selection	O
.	O

Multiple	O
iodine	B-chemical
positions	O
were	O
detected	O
for	O
most	O
residues	O
.	O

Agreement	O
to	O
the	O
residual	O
Im76	B-mutant
-	I-mutant
45	I-mutant
electron	B-evidence
density	I-evidence
(	O
c	O
)	O
and	O
anomalous	B-evidence
iodine	I-evidence
signals	I-evidence
(	O
d	O
)	O
for	O
ensembles	O
of	O
varying	O
size	O
generated	O
by	O
randomly	O
choosing	O
from	O
the	O
MD	B-experimental_method
pool	O
(	O
blue	O
)	O
and	O
from	O
the	O
selection	O
procedure	O
(	O
black	O
).	O

The	O
cost	B-evidence
function	I-evidence
,	O
χ2	B-evidence
,	O
decreases	O
as	O
the	O
agreement	O
to	O
the	O
experimental	O
data	O
increases	O
and	O
is	O
defined	O
in	O
the	O
Online	O
Methods	O
.	O

Flowchart	O
of	O
the	O
READ	B-experimental_method
sample	B-experimental_method
-	I-experimental_method
and	I-experimental_method
-	I-experimental_method
select	I-experimental_method
process	O
.	O

Spy	B-protein
is	O
depicted	O
as	O
a	O
gray	O
surface	O
and	O
the	O
Im76	B-mutant
-	I-mutant
45	I-mutant
conformer	O
is	O
shown	O
as	O
orange	O
balls	O
.	O

Atoms	O
that	O
were	O
either	O
not	O
directly	O
selected	O
in	O
the	O
READ	B-experimental_method
procedure	O
,	O
or	O
whose	O
position	O
could	O
not	O
be	O
justified	O
based	O
on	O
agreement	O
with	O
the	O
residual	B-evidence
electron	I-evidence
density	I-evidence
were	O
removed	O
,	O
leading	O
to	O
non	O
-	O
contiguous	O
sections	O
.	O

Residues	O
of	O
the	O
Spy	B-protein
flexible	O
linker	B-structure_element
region	I-structure_element
that	O
fit	O
the	O
residual	B-evidence
electron	I-evidence
density	I-evidence
are	O
shown	O
as	O
larger	O
gray	O
spheres	O
.	O

Shown	O
below	O
each	O
ensemble	O
member	O
is	O
the	O
RMSD	B-evidence
of	O
each	O
conformer	O
to	O
the	O
native	B-protein_state
state	O
of	O
Im76	B-mutant
-	I-mutant
45	I-mutant
,	O
as	O
well	O
as	O
the	O
percentage	O
of	O
contacts	O
between	O
Im76	B-mutant
-	I-mutant
45	I-mutant
and	O
Spy	B-protein
that	O
are	O
hydrophobic	O
.	O

Contact	B-evidence
maps	I-evidence
of	O
Spy	B-complex_assembly
:	I-complex_assembly
Im76	I-complex_assembly
-	I-complex_assembly
45	I-complex_assembly
complex	O
.	O

Contacts	O
to	O
the	O
two	O
Spy	B-protein
monomers	B-oligomeric_state
are	O
depicted	O
separately	O
.	O

Note	O
that	O
the	O
flexible	B-protein_state
linker	B-structure_element
region	I-structure_element
of	O
Spy	B-protein
(	O
residues	O
47	B-residue_range
–	I-residue_range
57	I-residue_range
)	O
is	O
not	O
represented	O
in	O
the	O
2D	O
contact	B-evidence
maps	I-evidence
.	O

Spy	B-protein
conformation	O
changes	O
upon	O
substrate	O
binding	O
.	O

The	O
Super	O
Spy	B-protein
mutants	O
F115L	B-mutant
,	O
F115I	B-mutant
,	O
and	O
L32P	B-mutant
are	O
proposed	O
to	O
gain	O
activity	O
by	O
increasing	O
the	O
flexibility	O
or	O
size	O
of	O
this	O
linker	B-structure_element
region	I-structure_element
.	O

Tdp2	B-protein
deficiencies	O
are	O
linked	O
to	O
neurological	O
disease	O
and	O
cellular	O
sensitivity	O
to	O
Top2	B-protein_type
poisons	O
.	O

Modeling	O
of	O
a	O
proposed	O
Tdp2	B-protein
reaction	O
coordinate	O
,	O
combined	O
with	O
mutagenesis	B-experimental_method
and	O
biochemical	B-experimental_method
studies	I-experimental_method
support	O
a	O
single	O
Mg2	B-chemical
+-	I-chemical
ion	O
mechanism	O
assisted	O
by	O
a	O
phosphotyrosyl	B-site
-	I-site
arginine	I-site
cation	I-site
-	I-site
π	I-site
interface	I-site
.	O

We	O
further	O
identify	O
a	O
Tdp2	B-protein
active	B-site
site	I-site
SNP	O
that	O
ablates	B-protein_state
Tdp2	B-protein
Mg2	B-chemical
+	I-chemical
binding	O
and	O
catalytic	O
activity	O
,	O
impairs	O
Tdp2	B-protein
mediated	O
NHEJ	O
of	O
tyrosine	B-residue_name
blocked	O
termini	O
,	O
and	O
renders	O
cells	O
sensitive	O
to	O
the	O
anticancer	O
agent	O
etoposide	B-chemical
.	O

Collectively	O
,	O
our	O
results	O
provide	O
a	O
structural	O
mechanism	O
for	O
Tdp2	B-protein
engagement	O
of	O
heterogeneous	O
DNA	B-chemical
damage	O
that	O
causes	O
Top2	B-protein_type
poisoning	O
,	O
and	O
indicate	O
that	O
evaluation	O
of	O
Tdp2	B-protein
status	O
may	O
be	O
an	O
important	O
personalized	O
medicine	O
biomarker	O
informing	O
on	O
individual	O
sensitivities	O
to	O
chemotherapeutic	O
Top2	B-protein_type
poisons	O
.	O

Topoisomerases	B-protein_type
relieve	O
topological	O
DNA	B-chemical
strain	O
and	O
entanglement	O
to	O
facilitate	O
critical	O
nuclear	O
DNA	B-chemical
transactions	O
including	O
DNA	B-chemical
replication	O
,	O
transcription	O
and	O
cell	O
division	O
.	O

The	O
mammalian	B-taxonomy_domain
type	B-protein_type
II	I-protein_type
topoisomerases	I-protein_type
Top2α	B-protein
and	O
Top2β	B-protein
enzymes	O
generate	O
transient	O
,	O
reversible	O
DNA	B-chemical
double	O
strand	O
breaks	O
(	O
DSBs	O
)	O
to	O
drive	O
topological	O
transactions	O
.	O

Reversibility	O
of	O
Top2	B-protein_type
DNA	B-chemical
cleavage	O
reactions	O
is	O
facilitated	O
by	O
formation	O
of	O
covalent	O
enzyme	O
phosphotyrosyl	B-ptm
linkages	I-ptm
between	O
the	O
5	B-chemical
′-	I-chemical
phosphate	I-chemical
ends	O
of	O
the	O
incised	O
duplex	O
and	O
an	O
active	B-site
site	I-site
Top2	B-protein_type
tyrosine	B-residue_name
,	O
resulting	O
in	O
Top2	B-protein_type
cleavage	O
complexes	O
(	O
Top2cc	B-complex_assembly
).	O

The	O
Top2cc	B-complex_assembly
protein	O
–	O
DNA	B-chemical
adduct	O
is	O
a	O
unique	O
threat	O
to	O
genomic	O
integrity	O
which	O
must	O
be	O
resolved	O
to	O
prevent	O
catastrophic	O
Top2cc	B-complex_assembly
collisions	O
with	O
the	O
cellular	O
replication	O
and	O
transcription	O
machineries	O
.	O

Importantly	O
,	O
Top2	B-protein_type
is	O
also	O
poisoned	O
when	O
it	O
engages	O
abundant	O
endogenous	O
DNA	B-chemical
damage	O
not	O
limited	O
to	O
but	O
including	O
ribonucleotides	O
,	O
abasic	O
sites	O
and	O
alkylation	O
damage	O
such	O
as	O
exocyclic	O
DNA	B-chemical
adducts	O
arising	O
from	O
bioactivation	O
of	O
the	O
vinyl	O
chloride	O
carcinogen	O
(	O
Figure	O
1A	O
).	O

In	O
the	O
case	O
of	O
DNA	B-chemical
damage	O
-	O
triggered	O
Top2cc	B-complex_assembly
,	O
compound	O
DNA	B-chemical
lesions	O
arise	O
that	O
consist	O
of	O
the	O
instigating	O
lesion	O
,	O
and	O
a	O
DNA	B-chemical
DSB	O
bearing	O
a	O
bulky	O
terminal	O
5	O
′-	O
linked	O
Top2	B-protein_type
DNA	B-chemical
–	O
protein	O
crosslink	O
.	O

Precisely	O
how	O
the	O
cellular	O
DNA	B-chemical
repair	O
machinery	O
navigates	O
these	O
complex	O
lesions	O
is	O
an	O
important	O
aspect	O
of	O
Top2cc	B-complex_assembly
repair	O
that	O
has	O
not	O
yet	O
been	O
explored	O
.	O

(	O
A	O
)	O
Unrepaired	O
DNA	B-chemical
damage	O
and	O
repair	O
intermediates	O
such	O
as	O
bulky	O
DNA	B-chemical
adducts	O
,	O
ribonucleotides	O
or	O
abasic	O
sites	O
can	O
poison	O
Top2	B-protein_type
and	O
trap	O
Top2	B-protein_type
cleavage	O
complex	O
(	O
Top2cc	B-complex_assembly
),	O
resulting	O
in	O
a	O
DSB	O
with	O
a	O
5	O
′–	O
Top2	B-protein_type
protein	O
adduct	O
linked	O
by	O
a	O
phosphotyrosine	B-residue_name
bond	O
.	O

PDB	O
entry	O
5HT2	O
is	O
displayed	O
,	O
also	O
see	O
Table	O
1	O
.	O
(	O
E	O
)	O
Structure	B-evidence
of	O
mTdp2cat	B-structure_element
bound	B-protein_state
to	I-protein_state
5	B-chemical
′-	I-chemical
phosphate	I-chemical
DNA	I-chemical
(	O
product	O
complex	O
)	O
containing	O
THF	B-chemical
(	O
yellow	O
).	O

PDB	O
entry	O
5INK	O
is	O
displayed	O
,	O
also	O
see	O
Table	O
1	O
.	O
(	O
F	O
)	O
Structure	B-evidence
of	O
mTdp2cat	B-structure_element
in	O
the	O
absence	B-protein_state
of	I-protein_state
DNA	B-chemical
showing	O
the	O
extended	B-protein_state
3	B-structure_element
-	I-structure_element
helix	I-structure_element
loop	I-structure_element
(	O
tan	O
)	O
open	B-protein_state
-	O
conformation	O
of	O
the	O
DNA	B-site
-	I-site
binding	I-site
grasp	I-site
as	O
seen	O
in	O
monomer	B-oligomeric_state
E	B-structure_element
of	O
the	O
apo	B-protein_state
structure	B-evidence
.	O

Tyrosyl	B-protein
DNA	I-protein
phosphodiesterase	I-protein
2	I-protein
(	O
Tdp2	B-protein
)	O
directly	O
hydrolyzes	O
5	B-ptm
′-	I-ptm
phosphotyrosyl	I-ptm
(	O
5	B-ptm
′-	I-ptm
Y	I-ptm
)	O
linkages	B-ptm
,	O
and	O
is	O
a	O
key	O
modulator	O
of	O
cellular	O
resistance	O
to	O
chemotherapeutic	O
Top2	B-protein_type
poisons	O
.	O

Tdp2	B-protein
knockdown	B-experimental_method
sensitizes	O
A549	O
lung	O
cancer	O
cells	O
to	O
etoposide	B-chemical
,	O
and	O
increases	O
formation	O
of	O
nuclear	O
γH2AX	O
foci	O
,	O
a	O
marker	O
of	O
DSBs	O
,	O
underlining	O
the	O
importance	O
of	O
Tdp2	B-protein
in	O
cellular	O
Top2cc	B-complex_assembly
repair	O
.	O

Tdp2	B-protein
is	O
overexpressed	O
in	O
lung	O
cancers	O
,	O
is	O
transcriptionally	O
up	O
-	O
regulated	O
in	O
mutant	B-protein_state
p53	B-protein
cells	O
and	O
mediates	O
mutant	B-protein_state
p53	B-protein
gain	O
of	O
function	O
phenotypes	O
,	O
which	O
can	O
lead	O
to	O
acquisition	O
of	O
therapy	O
resistance	O
during	O
cancer	O
progression	O
.	O

The	O
importance	O
of	O
Tdp2	B-protein
in	O
mediating	O
topoisomerase	B-protein_type
biology	O
is	O
further	O
underlined	O
by	O
the	O
facts	O
that	O
human	B-species
TDP2	B-protein
inactivating	O
mutations	O
are	O
found	O
in	O
individuals	O
with	O
intellectual	O
disabilities	O
,	O
seizures	O
and	O
ataxia	O
,	O
and	O
at	O
the	O
cellular	O
level	O
,	O
loss	B-protein_state
of	I-protein_state
Tdp2	B-protein
inhibits	O
Top2β	B-protein
-	O
dependent	O
transcription	O
.	O

It	O
is	O
possible	O
that	O
TDP2	B-protein
single	O
nucleotide	O
polymorphisms	O
(	O
SNPs	O
)	O
encode	O
mutations	O
that	O
impact	O
Tdp2	B-protein
function	O
,	O
but	O
the	O
molecular	O
underpinnings	O
for	O
such	O
Tdp2	B-protein
deficiencies	O
are	O
not	O
understood	O
.	O

Previously	O
we	O
reported	O
high	O
-	O
resolution	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystal	B-evidence
structures	I-evidence
of	O
the	O
minimal	B-protein_state
catalytically	I-protein_state
active	I-protein_state
endonuclease	B-structure_element
/	I-structure_element
exonuclease	I-structure_element
/	I-structure_element
phosphatase	I-structure_element
(	O
EEP	B-structure_element
)	O
domain	O
of	O
mouse	B-taxonomy_domain
Tdp2	B-protein
(	O
mTdp2cat	B-structure_element
)	O
bound	B-protein_state
to	I-protein_state
a	O
DNA	B-chemical
substrate	O
mimic	O
,	O
and	O
a	O
5	B-protein_state
′-	I-protein_state
phosphorylated	I-protein_state
reaction	O
product	O
.	O

First	O
,	O
it	O
is	O
unclear	O
if	O
Tdp2	B-protein
processes	O
phosphotyrosyl	B-ptm
linkages	I-ptm
in	O
the	O
context	O
of	O
DNA	B-chemical
damage	O
that	O
triggers	O
Top2cc	B-complex_assembly
,	O
and	O
if	O
so	O
,	O
how	O
the	O
enzyme	O
can	O
accommodate	O
such	O
complex	O
DNA	B-chemical
damage	O
within	O
its	O
active	B-site
site	I-site
.	O

Based	O
on	O
metal	B-protein_state
-	I-protein_state
bound	I-protein_state
Tdp2	B-protein
structures	B-evidence
,	O
we	O
also	O
proposed	O
a	O
single	O
Mg2	B-chemical
+	I-chemical
mediated	O
catalytic	O
mechanism	O
,	O
but	O
this	O
mechanism	O
requires	O
further	O
scrutiny	O
and	O
characterization	O
.	O

Herein	O
,	O
we	O
report	O
an	O
integrated	O
structure	B-experimental_method
-	I-experimental_method
function	I-experimental_method
study	I-experimental_method
of	O
the	O
Tdp2	B-protein
reaction	O
mechanism	O
,	O
including	O
a	O
description	O
of	O
new	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
structures	B-evidence
of	O
ligand	B-protein_state
-	I-protein_state
free	I-protein_state
Tdp2	B-protein
,	O
and	O
Tdp2	B-protein
bound	B-protein_state
to	I-protein_state
abasic	O
and	O
alkylated	O
(	O
1	B-chemical
-	I-chemical
N6	I-chemical
-	I-chemical
etheno	I-chemical
-	I-chemical
adenine	I-chemical
)	O
DNA	B-chemical
damage	O
.	O

We	O
further	O
establish	O
that	O
DNA	B-chemical
damage	O
binding	O
in	O
the	O
Tdp2	B-protein
active	B-site
site	I-site
is	O
linked	O
to	O
conformational	O
change	O
and	O
binding	O
of	O
metal	O
cofactor	O
.	O

Tdp2	B-protein
processing	O
of	O
compound	O
DNA	B-chemical
damage	O

Two	O
potent	O
Top2	B-protein_type
poisons	O
include	O
bulky	O
alkylated	O
DNA	B-chemical
helix	O
-	O
distorting	O
DNA	B-chemical
base	O
adducts	O
(	O
e	O
.	O
g	O
.	O
1	B-chemical
-	I-chemical
N6	I-chemical
-	I-chemical
ethenoadenine	I-chemical
,	O
ϵA	B-chemical
)	O
and	O
abundant	O
abasic	O
sites	O
(	O
Figure	O
1A	O
).	O

Whether	O
Tdp2	B-protein
processes	O
phosphotyrosyl	B-ptm
linkages	I-ptm
within	O
these	O
diverse	O
structural	O
contexts	O
is	O
not	O
known	O
.	O

We	O
then	O
evaluated	O
the	O
ability	O
of	O
a	O
recombinant	O
purified	O
mouse	B-taxonomy_domain
Tdp2	B-protein
catalytic	B-structure_element
domain	I-structure_element
(	O
mTdp2cat	B-structure_element
)	O
to	O
release	O
PNP	B-chemical
(	O
a	O
structural	O
mimic	O
of	O
a	O
topoisomerase	B-protein_type
tyrosine	B-residue_name
)	O
from	O
the	O
5	O
′-	O
terminus	O
of	O
compound	O
damaged	O
DNA	B-chemical
substrates	O
using	O
a	O
colorimetric	B-experimental_method
assay	I-experimental_method
(	O
Figure	O
1B	O
).	O

We	O
observe	O
robust	O
Tdp2	B-protein
-	O
dependent	O
release	O
of	O
PNP	B-chemical
from	O
5	O
′-	O
modified	O
oligonucleotides	O
in	O
the	O
context	O
of	O
dA	B-chemical
-	I-chemical
PNP	I-chemical
,	O
ϵA	B-chemical
-	I-chemical
PNP	I-chemical
or	O
the	O
abasic	O
-	O
site	O
analog	O
tetrahydrofuran	B-chemical
spacer	I-chemical
(	O
THF	B-chemical
)	O
(	O
Figure	O
1C	O
).	O

Thus	O
,	O
Tdp2	B-protein
efficiently	O
cleaves	O
phosphotyrosyl	B-ptm
linkages	I-ptm
in	O
the	O
context	O
of	O
a	O
compound	O
5	O
′	O
lesions	O
composed	O
of	O
abasic	O
or	O
bulky	O
DNA	B-chemical
base	O
adduct	O
DNA	B-chemical
damage	O
.	O

In	O
these	O
Tdp2	B-complex_assembly
-	I-complex_assembly
DNA	I-complex_assembly
complex	O
structures	B-evidence
,	O
mTdp2cat	B-structure_element
adopts	O
a	O
mixed	B-structure_element
α	I-structure_element
-	I-structure_element
β	I-structure_element
fold	I-structure_element
typified	O
by	O
a	O
central	O
12	B-structure_element
-	I-structure_element
stranded	I-structure_element
anti	I-structure_element
-	I-structure_element
parallel	I-structure_element
β	I-structure_element
-	I-structure_element
sandwich	I-structure_element
enveloped	O
by	O
several	O
helical	O
elements	O
that	O
mold	O
the	O
Tdp2	B-protein
active	B-site
site	I-site
.	O

One	O
half	O
of	O
the	O
molecule	O
contributes	O
to	O
formation	O
of	O
the	O
walls	O
of	O
the	O
DNA	B-site
-	I-site
binding	I-site
cleft	I-site
that	O
embraces	O
the	O
terminal	O
position	O
of	O
the	O
damaged	O
DNA	B-chemical
substrate	O
.	O

In	O
the	O
DNA	B-protein_state
lesion	I-protein_state
-	I-protein_state
bound	I-protein_state
state	O
,	O
two	O
key	O
DNA	B-chemical
binding	O
elements	O
,	O
the	O
β	B-structure_element
-	I-structure_element
2	I-structure_element
-	I-structure_element
helix	I-structure_element
-	I-structure_element
β	I-structure_element
(	O
β2Hβ	B-structure_element
)	O
‘	O
grasp	B-structure_element
’,	O
and	O
‘	O
helical	B-structure_element
cap	I-structure_element
’	O
mold	O
the	O
substrate	B-site
binding	I-site
trench	I-site
and	O
direct	O
the	O
ssDNA	B-chemical
of	O
a	O
5	O
′-	O
overhang	O
substrate	O
into	O
the	O
active	B-site
site	I-site
.	O

A	O
comparison	O
to	O
an	O
additional	O
new	O
structure	B-evidence
of	O
DNA	B-protein_state
-	I-protein_state
free	I-protein_state
Tdp2	B-protein
(	O
apo	B-protein_state
state	O
,	O
Figure	O
1F	O
)	O
shows	O
that	O
this	O
loop	B-structure_element
is	O
conformationally	B-protein_state
mobile	I-protein_state
and	O
important	O
for	O
engaging	O
DNA	B-chemical
substrates	O
.	O

The	O
mode	O
of	O
engagement	O
of	O
the	O
5	O
′-	O
nucleobase	O
of	O
the	O
bulky	O
ϵA	B-chemical
adduct	O
describes	O
a	O
mechanism	O
for	O
Tdp2	B-protein
to	O
bind	O
5	B-protein_state
′-	I-protein_state
tyrosylated	I-protein_state
substrates	O
that	O
contain	O
diverse	O
forms	O
of	O
DNA	B-chemical
damage	O
.	O

The	O
5	B-chemical
′-	I-chemical
ϵA	I-chemical
nucleobase	O
is	O
recognized	O
by	O
an	O
extended	O
Tdp2	B-protein
van	B-site
Der	I-site
Waals	I-site
interaction	I-site
surface	I-site
,	O
referred	O
to	O
here	O
as	O
the	O
‘	O
hydrophobic	B-site
wall	I-site
’	O
that	O
is	O
assembled	O
with	O
the	O
sidechains	O
of	O
residues	O
Leu315	B-residue_name_number
and	O
Ile317	B-residue_name_number
(	O
Figure	O
2A	O
and	O
B	O
).	O

Structures	B-evidence
of	O
mTdp2cat	B-structure_element
bound	B-protein_state
to	I-protein_state
DNA	B-chemical
damage	O
that	O
triggers	O
Top2	B-protein_type
poisoning	O
.	O

mTdp2cat	B-structure_element
is	O
colored	O
by	O
electrostatic	O
surface	O
potential	O
(	O
red	O
=	O
negative	O
,	O
blue	O
=	O
positive	O
,	O
gray	O
=	O
neutral	O
/	O
hydrophobic	O
).	O

(	O
B	O
)	O
σ	B-evidence
-	I-evidence
A	I-evidence
weighted	I-evidence
2Fo	I-evidence
-	I-evidence
Fc	I-evidence
electron	I-evidence
density	I-evidence
map	I-evidence
(	O
at	O
1	O
.	O
43	O
Å	O
resolution	O
,	O
contoured	O
at	O
2	O
.	O
0	O
σ	O
)	O
for	O
the	O
ϵA	B-chemical
DNA	I-chemical
complex	O
.	O

The	O
ϵA	B-chemical
nucleotide	O
is	O
shown	O
in	O
yellow	O
and	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
from	O
the	O
ϵA	B-chemical
O4	O
′	O
to	O
inner	O
-	O
sphere	O
water	B-chemical
is	O
shown	O
as	O
gray	O
dashes	O
.	O
(	O
C	O
)	O
Structure	B-evidence
of	O
mTdp2cat	B-structure_element
bound	B-protein_state
to	I-protein_state
5	B-chemical
′-	I-chemical
phosphate	I-chemical
DNA	I-chemical
(	O
product	O
complex	O
)	O
containing	O
THF	B-chemical
(	O
yellow	O
),	O
Mg2	B-chemical
+	I-chemical
(	O
magenta	O
)	O
and	O
its	O
inner	O
-	O
sphere	O
waters	B-chemical
(	O
gray	O
).	O

PDB	O
entry	O
5INK	O
is	O
displayed	O
.	O
(	O
D	O
)	O
σ	B-evidence
-	I-evidence
A	I-evidence
weighted	I-evidence
2Fo	I-evidence
-	I-evidence
Fc	I-evidence
electron	I-evidence
density	I-evidence
map	I-evidence
(	O
at	O
2	O
.	O
15	O
Å	O
resolution	O
,	O
contoured	O
at	O
2	O
.	O
0	O
σ	O
)	O
for	O
THF	B-complex_assembly
-	I-complex_assembly
DNA	I-complex_assembly
complex	O
.	O

For	O
comparison	O
,	O
we	O
also	O
determined	B-experimental_method
a	O
structure	B-evidence
of	O
an	O
undamaged	O
5	B-chemical
′-	I-chemical
adenine	I-chemical
(	O
5	B-chemical
′-	I-chemical
dA	I-chemical
)	O
bound	B-protein_state
to	I-protein_state
Tdp2	B-protein
at	O
1	O
.	O
55	O
Å	O
(	O
PDB	O
entry	O
5INL	O
).	O

Therefore	O
,	O
structurally	O
diverse	O
undamaged	O
or	O
alkylated	O
bases	O
(	O
e	O
.	O
g	O
.	O
ϵG	B-chemical
,	O
ϵT	B-chemical
)	O
could	O
likely	O
be	O
accommodated	O
in	O
the	O
Tdp2	B-protein
active	B-site
site	I-site
via	O
planar	B-bond_interaction
base	I-bond_interaction
stacking	I-bond_interaction
with	O
the	O
active	B-site
site	I-site
facing	O
hydrophobic	B-site
wall	I-site
of	O
the	O
β2Hβ	B-structure_element
motif	O
.	O

Interestingly	O
,	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
a	O
nucleobase	O
,	O
O4	O
′	O
of	O
the	O
THF	B-chemical
ring	O
adopts	O
a	O
close	O
approach	O
(	O
2	O
.	O
8	O
Å	O
)	O
to	O
a	O
water	B-chemical
molecule	O
that	O
directly	O
participates	O
in	O
the	O
outer	O
sphere	O
single	O
Mg2	B-chemical
+	I-chemical
ion	B-bond_interaction
coordination	I-bond_interaction
shell	I-bond_interaction
(	O
Figure	O
2D	O
).	O

These	O
collective	O
differences	O
may	O
explain	O
the	O
slight	O
,	O
but	O
statistically	O
significant	O
elevated	O
activity	O
on	O
the	O
THF	B-chemical
substrate	O
(	O
Figure	O
1C	O
).	O

In	O
this	O
arrangement	O
,	O
six	O
solvent	O
molecules	O
form	O
a	O
channel	O
under	O
the	O
β2Hβ	B-site
-	I-site
grasp	I-site
,	O
ending	O
with	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
to	O
the	O
peptide	O
backbone	O
of	O
the	O
Mg2	B-chemical
+	I-chemical
ligand	O
Asp358	B-residue_name_number
.	O

To	O
test	O
this	O
hypothesis	O
,	O
we	O
crystallized	B-experimental_method
Tdp2	B-protein
in	O
the	O
absence	B-protein_state
of	I-protein_state
DNA	B-chemical
and	O
determined	O
a	O
DNA	B-protein_state
free	I-protein_state
Tdp2	B-protein
structure	B-evidence
to	O
2	O
.	O
4	O
Å	O
resolution	O
(	O
PDB	O
entry	O
5INM	O
;	O
Figures	O
1F	O
and	O
3A	O
).	O

(	O
A	O
)	O
The	O
open	B-protein_state
,	O
3	B-structure_element
-	I-structure_element
helix	I-structure_element
conformation	O
(	O
tan	O
)	O
of	O
flexible	B-protein_state
active	B-structure_element
-	I-structure_element
site	I-structure_element
loop	I-structure_element
observed	O
in	O
monomer	B-oligomeric_state
E	B-structure_element
of	O
the	O
DNA	B-protein_state
-	I-protein_state
free	I-protein_state
mTdp2cat	B-structure_element
structure	B-evidence
(	O
PDB	O
entry	O
5INM	O
)	O
is	O
supported	O
by	O
T309	B-residue_name_number
(	O
green	O
),	O
which	O
packs	O
against	O
the	O
EEP	B-structure_element
core	O
.	O

mTdp2cat	B-structure_element
WT	B-protein_state
(	O
lanes	O
1	O
–	O
13	O
)	O
or	O
mTdp2cat	B-structure_element
D358N	B-mutant
(	O
lanes	O
14	O
–	O
26	O
)	O
were	O
incubated	O
in	O
the	O
presence	B-protein_state
or	O
absence	B-protein_state
of	I-protein_state
Mg2	B-chemical
+	I-chemical
and	O
/	O
or	O
a	O
12	O
nt	O
self	O
annealing	O
,	O
5	O
′-	O
phosphorylated	O
DNA	B-chemical
(	O
substrate	O
‘	O
12	O
nt	O
’	O
in	O
Supplementary	O
Table	O
S1	O
),	O
then	O
reacted	O
with	O
0	O
.	O
6	O
,	O
1	O
.	O
7	O
or	O
5	O
ng	O
μl	O
−	O
1	O
of	O
trypsin	O
.	O

This	O
crystal	O
form	O
contains	O
5	O
Tdp2	B-protein
protein	O
molecules	O
in	O
the	O
asymmetric	O
unit	O
,	O
with	O
variations	O
in	O
active	B-site
site	I-site
Mg2	B-chemical
+	I-chemical
occupancy	O
and	O
substrate	B-structure_element
binding	I-structure_element
loops	I-structure_element
observed	O
for	O
the	O
individual	O
protomers	B-oligomeric_state
.	O

The	O
most	O
striking	O
feature	O
of	O
the	O
DNA	B-protein_state
ligand	I-protein_state
-	I-protein_state
free	I-protein_state
state	O
is	O
that	O
the	O
active	B-site
site	I-site
β2Hβ	I-site
-	I-site
grasp	I-site
can	O
adopt	O
alternative	O
structures	O
that	O
are	O
distinct	O
from	O
the	O
DNA	B-protein_state
-	I-protein_state
bound	I-protein_state
,	O
closed	B-protein_state
β2Hβ	B-site
DNA	I-site
binding	I-site
grasp	I-site
(	O
Figure	O
3A	O
and	O
B	O
).	O

In	O
one	O
monomer	B-oligomeric_state
(	O
chain	B-structure_element
‘	I-structure_element
E	I-structure_element
’),	I-structure_element
the	O
grasp	B-structure_element
adopts	O
an	O
‘	O
open	B-protein_state
’	O
3	B-structure_element
-	I-structure_element
helix	I-structure_element
loop	I-structure_element
conformation	O
that	O
projects	O
away	O
from	O
the	O
EEP	B-structure_element
catalytic	B-site
core	I-site
.	O

Two	O
monomers	B-oligomeric_state
have	O
variable	O
disordered	B-protein_state
states	O
for	O
which	O
much	O
of	O
the	O
DNA	B-structure_element
binding	I-structure_element
loop	I-structure_element
is	O
not	O
visible	O
in	O
the	O
electron	B-evidence
density	I-evidence
.	O

The	O
remaining	O
two	O
molecules	O
in	O
the	O
DNA	B-protein_state
-	I-protein_state
free	I-protein_state
crystal	B-evidence
form	I-evidence
are	O
closed	B-protein_state
β2Hβ	B-structure_element
conformers	O
similar	O
to	O
the	O
DNA	B-protein_state
bound	I-protein_state
structures	B-evidence
(	O
Figure	O
3C	O
).	O

Thus	O
,	O
we	O
posit	O
that	O
Tdp2	B-protein
DNA	B-chemical
binding	O
conformationally	O
selects	O
the	O
closed	B-protein_state
form	O
of	O
the	O
β2Hβ	B-site
grasp	I-site
,	O
rather	O
than	O
inducing	O
closure	O
upon	O
binding	O
.	O

A	O
detailed	O
analysis	O
of	O
the	O
extended	B-protein_state
3	B-structure_element
-	I-structure_element
helix	I-structure_element
conformation	O
shows	O
that	O
the	O
substrate	B-structure_element
-	I-structure_element
binding	I-structure_element
loop	I-structure_element
is	O
able	O
to	O
undergo	O
metamorphic	O
structural	O
changes	O
.	O

In	O
this	O
open	B-protein_state
form	O
,	O
residues	O
Asn312	B-residue_range
-	I-residue_range
Leu315	I-residue_range
are	O
distal	O
from	O
the	O
active	B-site
site	I-site
and	O
solvent	B-protein_state
-	I-protein_state
exposed	I-protein_state
(	O
orange	O
sticks	O
,	O
Figure	O
3A	O
),	O
while	O
Thr309	B-residue_name_number
(	O
green	O
surface	O
,	O
Figure	O
3A	O
)	O
packs	O
into	O
a	O
shallow	O
pocket	B-site
of	O
the	O
EEP	B-structure_element
core	O
to	O
anchor	O
the	O
loop	B-structure_element
.	O

By	O
comparison	O
,	O
the	O
closed	B-protein_state
β2Hβ	B-site
grasp	I-site
conformer	O
is	O
stabilized	O
by	O
Asn312	B-residue_name_number
and	O
Asn314	B-residue_name_number
binding	O
into	O
two	O
β2Hβ	B-site
docking	I-site
pockets	I-site
,	O
and	O
Leu315	B-residue_name_number
engagement	O
of	O
the	O
5	O
′-	O
terminal	O
nucleobase	O
(	O
Figure	O
3B	O
).	O

Stabilization	O
of	O
the	O
closed	B-protein_state
β2Hβ	B-site
-	I-site
grasp	I-site
conformation	O
is	O
linked	O
to	O
the	O
active	B-site
site	I-site
through	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
between	O
Trp307	B-residue_name_number
and	O
the	O
Mg2	B-site
+	I-site
coordinating	I-site
residue	I-site
Asp358	B-residue_name_number
.	O

To	O
evaluate	O
Mg2	B-chemical
+	I-chemical
and	O
DNA	B-chemical
-	O
dependent	O
Tdp2	B-protein
structural	O
states	O
in	O
solution	O
,	O
we	O
probed	O
mTdp2cat	B-structure_element
conformations	O
using	O
limited	B-experimental_method
trypsin	I-experimental_method
and	I-experimental_method
chymotrypsin	I-experimental_method
proteolysis	I-experimental_method
(	O
Figure	O
3C	O
–	O
E	O
).	O

In	B-protein_state
the	I-protein_state
absence	I-protein_state
of	I-protein_state
DNA	B-chemical
or	O
Mg2	B-chemical
+,	I-chemical
mTdp2cat	B-structure_element
is	O
efficiently	O
cleaved	O
in	O
the	O
metamorphic	O
DNA	B-site
binding	I-site
grasp	I-site
at	O
one	O
site	O
by	O
trypsin	B-experimental_method
(	O
Arg316	B-residue_name_number
),	O
or	O
at	O
two	O
positions	O
by	O
chymotrypsin	B-experimental_method
(	O
Trp307	B-residue_name_number
and	O
Leu315	B-residue_name_number
).	O

By	O
comparison	O
,	O
Mg2	B-chemical
+,	I-chemical
and	O
to	O
a	O
greater	O
extent	O
Mg2	B-chemical
+/	I-chemical
DNA	B-chemical
mixtures	O
(	O
compare	O
Figure	O
3	O
,	O
lanes	O
4	O
,	O
7	O
and	O
13	O
)	O
protect	O
mTdp2cat	B-structure_element
from	O
proteolytic	O
cleavage	O
.	O

Interestingly	O
,	O
addition	O
of	O
Mg2	B-chemical
+	I-chemical
alone	O
protects	O
against	O
proteolysis	O
as	O
well	O
.	O

This	O
is	O
consistent	O
with	O
Mg2	B-chemical
+	I-chemical
stabilizing	O
the	O
closed	B-protein_state
conformation	O
of	O
the	O
β2Hβ	B-site
-	I-site
grasp	I-site
through	O
an	O
extended	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonding	I-bond_interaction
network	I-bond_interaction
with	O
Asp358	B-residue_name_number
and	O
the	O
indole	O
ring	O
of	O
the	O
β2Hβ	B-site
-	I-site
grasp	I-site
residue	O
Trp307	B-residue_name_number
(	O
also	O
discussion	O
below	O
on	O
Tdp2	B-protein
active	B-site
site	I-site
SNPs	O
).	O

To	O
assess	O
structural	O
conservation	O
of	O
Tdp2	B-protein
conformational	O
changes	O
between	O
human	B-species
and	O
mouse	B-taxonomy_domain
Tdp2	B-protein
,	O
we	O
also	O
determined	B-experimental_method
a	O
3	O
.	O
2	O
Å	O
resolution	O
structure	B-evidence
of	O
the	O
human	B-species
Tdp2cat	B-structure_element
domain	O
bound	B-protein_state
to	I-protein_state
a	O
DNA	B-chemical
5	I-chemical
′-	I-chemical
PO4	I-chemical
terminus	O
product	O
complex	O
(	O
PDB	O
entry	O
5INO	O
).	O

Comparisons	O
of	O
the	O
human	B-species
hTdp2cat	B-complex_assembly
-	I-complex_assembly
DNA	I-complex_assembly
complex	O
structure	B-evidence
to	O
the	O
mTdp2cat	B-structure_element
DNA	B-protein_state
bound	I-protein_state
state	O
show	O
a	O
high	O
level	O
of	O
conservation	O
of	O
the	O
DNA	B-protein_state
-	I-protein_state
bound	I-protein_state
conformations	O
(	O
Supplementary	O
Figure	O
S3A	O
).	O

Moreover	O
,	O
similar	O
to	O
mTdp2cat	B-structure_element
,	O
proteolytic	O
protection	O
of	O
the	O
hTdp2cat	B-structure_element
substrate	B-structure_element
binding	I-structure_element
loop	I-structure_element
occurs	O
with	O
addition	O
of	O
Mg2	B-chemical
+	I-chemical
and	O
DNA	B-chemical
(	O
Supplementary	O
Figure	O
S3B	O
).	O

Tdp2	B-protein
metal	O
ion	O
dependence	O

Consistently	O
in	O
high	O
-	O
resolution	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
structural	I-experimental_method
analyses	I-experimental_method
we	O
,	O
and	O
others	O
observe	O
a	O
single	O
Mg2	B-chemical
+	I-chemical
metal	O
bound	B-protein_state
in	I-protein_state
the	O
Tdp2	B-protein
active	B-site
site	I-site
.	O

This	O
includes	O
the	O
DNA	B-protein_state
-	I-protein_state
free	I-protein_state
(	O
Figure	O
3A	O
),	O
DNA	B-protein_state
damage	I-protein_state
bound	I-protein_state
(	O
Figure	O
3B	O
)	O
and	O
reaction	B-protein_state
product	I-protein_state
-	I-protein_state
bound	I-protein_state
crystal	B-evidence
forms	I-evidence
of	O
mouse	B-taxonomy_domain
,	O
(	O
PDB	O
entry	O
4GZ1	O
),	O
D	B-species
.	I-species
rerio	I-species
(	O
PDB	O
entry	O
4FPV	O
)	O
and	O
C	B-species
.	I-species
elegans	I-species
Tdp2	B-protein
(	O
PDB	O
entry	O
4FVA	O
).	O

In	O
these	O
experiments	O
,	O
at	O
limiting	O
Mg2	B-chemical
+	I-chemical
concentrations	O
,	O
Ca2	B-chemical
+	I-chemical
addition	O
to	O
Tdp2	B-protein
reactions	O
stimulated	O
activity	O
.	O

In	O
fact	O
,	O
divalent	B-chemical
metals	I-chemical
have	O
been	O
observed	O
in	O
the	O
Tdp2	B-protein
protein	O
–	O
DNA	B-chemical
complexes	O
(	O
PDB	O
entry	O
4GZ2	O
)	O
distal	O
to	O
the	O
active	B-site
center	I-site
,	O
and	O
we	O
propose	O
this	O
might	O
account	O
for	O
varied	O
results	O
in	O
different	O
studies	O
.	O

To	O
further	O
probe	O
the	O
metal	O
ion	O
dependence	O
of	O
the	O
Tdp2	B-protein
phosphodiesterase	B-protein_type
reaction	O
,	O
we	O
performed	O
metal	B-experimental_method
ion	I-experimental_method
binding	I-experimental_method
assays	I-experimental_method
,	O
determined	O
crystal	B-evidence
structures	I-evidence
in	O
the	O
presence	B-protein_state
of	I-protein_state
varied	O
divalent	B-chemical
metals	I-chemical
(	O
Mn2	B-chemical
+	I-chemical
and	O
Ca2	B-chemical
+),	I-chemical
and	O
analyzed	O
metal	O
ion	O
dependence	O
of	O
the	O
Tdp2	B-protein
phosphotyrosyl	B-protein_type
phosphodiesterase	I-protein_type
reaction	O
(	O
Figure	O
4	O
).	O

Metal	O
cofactor	O
interactions	O
with	O
Tdp2	B-protein
.	O
(	O
A	O
)	O
Intrinsic	B-evidence
tryptophan	I-evidence
fluorescence	I-evidence
of	O
mTdp2cat	B-structure_element
was	O
used	O
to	O
monitor	O
a	O
conformational	O
response	O
to	O
divalent	O
metal	O
ion	O
binding	O
.	O

Both	O
Mg2	B-chemical
+	I-chemical
and	O
Ca2	B-chemical
+	I-chemical
induce	O
a	O
conformational	O
change	O
which	O
elicits	O
an	O
increase	O
in	O
tryptophan	B-evidence
fluorescence	I-evidence
of	O
mTdp2cat	B-structure_element
in	O
the	O
presence	B-protein_state
and	O
absence	B-protein_state
of	I-protein_state
DNA	B-chemical
,	O
while	O
D358N	B-mutant
active	B-site
site	I-site
mutant	B-protein_state
of	O
mTdp2cat	B-structure_element
is	O
unresponsive	B-protein_state
to	O
Mg2	B-chemical
+.	I-chemical
(	O
B	O
)	O
mTdp2cat	B-structure_element
activity	O
assayed	O
on	O
a	O
T5PNP	B-chemical
substrate	O
as	O
a	O
function	O
of	O
Mg2	B-chemical
+	I-chemical
and	O
Ca2	B-chemical
+	I-chemical
concentration	O
.	O

A	O
53σ	O
peak	O
in	O
the	O
anomalous	B-evidence
difference	I-evidence
Fourier	I-evidence
map	I-evidence
(	O
data	O
collected	O
at	O
λ	O
=	O
1	O
.	O
5418	O
Å	O
)	O
supports	O
Mn2	B-chemical
+	I-chemical
as	O
the	O
identity	O
of	O
this	O
atom	O
.	O

(	O
D	O
)	O
Comparison	O
of	O
Ca2	B-chemical
+	I-chemical
(	O
green	O
Ca2	B-chemical
+	I-chemical
ion	O
,	O
orange	O
DNA	B-chemical
)	O
(	O
PDB	O
entry	O
5INQ	O
),	O
and	O
Mg2	B-chemical
+	I-chemical
(	O
magenta	O
Mg2	B-chemical
+	I-chemical
ion	O
,	O
yellow	O
DNA	B-chemical
)	O
(	O
PDB	O
entry	O
4GZ1	O
)	O
mTdp2cat	B-complex_assembly
–	I-complex_assembly
DNA	I-complex_assembly
structures	B-evidence
shows	O
that	O
Ca2	B-chemical
+	I-chemical
distorts	O
the	O
5	B-site
′-	I-site
phosphate	I-site
binding	I-site
mode	I-site
.	O

Our	O
proteolysis	B-experimental_method
results	O
indicate	O
a	O
Mg2	B-chemical
+-	I-chemical
dependent	O
Tdp2	B-protein
conformational	O
response	O
to	O
metal	O
binding	O
.	O

The	O
Tdp2	B-protein
active	B-site
site	I-site
has	O
three	O
tryptophan	B-residue_name
residues	O
within	O
10	O
Å	O
of	O
the	O
metal	B-site
binding	I-site
center	I-site
,	O
so	O
we	O
assayed	O
intrinsic	B-evidence
tryptophan	I-evidence
fluorescence	I-evidence
to	O
detect	O
metal	O
-	O
induced	O
conformational	O
changes	O
in	O
mTdp2cat	B-structure_element
.	O

We	O
then	O
measured	O
effects	O
of	O
metal	O
ion	O
concentrations	O
on	O
Tdp2	B-protein
cleavage	O
of	O
p	B-chemical
-	I-chemical
nitrophenyl	I-chemical
-	I-chemical
thymidine	I-chemical
-	I-chemical
5	I-chemical
′-	I-chemical
phosphate	I-chemical
by	O
mTdp2cat	B-structure_element
.	O

This	O
small	O
molecule	O
substrate	O
is	O
not	O
expected	O
to	O
be	O
influenced	O
by	O
metal	O
–	O
DNA	B-chemical
coordination	O
outside	O
of	O
the	O
active	B-site
site	I-site
.	O

Inclusion	O
of	O
ultrapure	O
Ca2	B-chemical
+	I-chemical
(	O
1	O
mM	O
or	O
10	O
mM	O
)	O
results	O
in	O
a	O
dose	O
-	O
dependent	O
inhibition	O
but	O
not	O
stimulation	O
Tdp2	B-protein
activity	O
,	O
even	O
in	O
conditions	O
of	O
limiting	O
Mg2	B-chemical
+	I-chemical
(	O
Figure	O
4B	O
).	O

We	O
performed	O
the	O
same	O
titrations	B-experimental_method
with	O
human	B-species
hTdp2FL	B-protein
and	O
hTdp2cat	B-structure_element
(	O
Supplementary	O
Figure	O
S4	O
),	O
and	O
find	O
similar	O
stimulation	O
of	O
activity	O
by	O
Mg2	B-chemical
+	I-chemical
and	O
inhibition	O
by	O
Ca2	B-chemical
+.	I-chemical

To	O
further	O
evaluate	O
the	O
structural	O
influence	O
of	O
divalent	O
cations	O
on	O
the	O
Tdp2	B-protein
active	B-site
site	I-site
,	O
we	O
determined	O
crystal	B-evidence
structures	I-evidence
by	O
soaking	B-experimental_method
crystals	I-experimental_method
with	O
metal	O
cofactors	O
that	O
either	O
support	B-protein_state
(	O
Mn2	B-chemical
+)	I-chemical
or	O
inhibit	B-protein_state
(	O
Ca2	B-chemical
+,	I-chemical
Figure	O
4B	O
)	O
the	O
Tdp2	B-protein
reaction	O
(	O
PDB	O
entries	O
5INP	O
and	O
5INQ	O
).	O

Anomalous	B-evidence
difference	I-evidence
Fourier	I-evidence
maps	I-evidence
of	O
the	O
Tdp2	B-complex_assembly
–	I-complex_assembly
DNA	I-complex_assembly
–	I-complex_assembly
Mn2	I-complex_assembly
+	I-complex_assembly
complex	O
show	O
a	O
single	O
binding	B-site
site	I-site
for	O
Mn2	B-chemical
+	I-chemical
in	O
each	O
Tdp2	B-protein
active	B-site
site	I-site
(	O
Figure	O
4C	O
),	O
with	O
octahedral	B-bond_interaction
coordination	I-bond_interaction
and	O
bond	O
lengths	O
typical	O
for	O
Mn2	B-chemical
+	I-chemical
ligands	O
(	O
Supplementary	O
Table	O
S3	O
).	O

The	O
Mn2	B-chemical
+	I-chemical
ion	O
is	O
positioned	O
in	O
the	O
Tdp2	B-protein
active	B-site
site	I-site
similar	O
to	O
the	O
Mg2	B-protein_state
+-	I-protein_state
bound	I-protein_state
complex	O
(	O
Figure	O
2C	O
),	O
which	O
is	O
consistent	O
with	O
the	O
ability	O
of	O
Mn2	B-chemical
+	I-chemical
to	O
support	O
robust	O
Tdp2	B-protein
catalytic	O
activity	O
.	O

Although	O
Ca2	B-chemical
+	I-chemical
is	O
also	O
octahedrally	B-bond_interaction
coordinated	I-bond_interaction
,	O
longer	O
bond	O
lengths	O
for	O
the	O
Ca2	B-chemical
+	I-chemical
ligands	O
(	O
Supplementary	O
Table	O
S3	O
)	O
shift	O
the	O
Ca2	B-chemical
+	I-chemical
ion	O
relative	O
to	O
the	O
Mg2	B-site
+	I-site
ion	I-site
site	I-site
.	O

Interestingly	O
,	O
bi	B-bond_interaction
-	I-bond_interaction
dentate	I-bond_interaction
inner	I-bond_interaction
sphere	I-bond_interaction
metal	I-bond_interaction
contacts	I-bond_interaction
from	O
the	O
Ca2	B-chemical
+	I-chemical
ion	O
to	O
Glu162	B-residue_name_number
distort	O
the	O
active	B-site
site	I-site
phosphate	I-site
-	I-site
binding	I-site
mode	I-site
,	O
and	O
dislodge	O
the	O
5	B-chemical
′-	I-chemical
PO4	I-chemical
out	O
of	O
the	O
Tdp2	B-protein
active	B-site
site	I-site
(	O
Figure	O
4D	O
).	O

Together	O
with	O
results	O
showing	O
that	O
under	O
the	O
conditions	O
examined	O
here	O
,	O
Ca2	B-chemical
+	I-chemical
inhibits	O
rather	O
than	O
stimulates	O
the	O
Tdp2	B-protein
reaction	O
,	O
the	O
divalent	B-protein_state
metal	I-protein_state
bound	I-protein_state
Tdp2	B-protein
structures	B-evidence
provide	O
a	O
mechanism	O
for	O
Ca2	B-chemical
+-	I-chemical
mediated	O
inhibition	O
of	O
the	O
Tdp2	B-protein
reaction	O
.	O

Modeling	O
the	O
Tdp2	B-protein
reaction	O
coordinate	O

Next	O
,	O
to	O
examine	O
the	O
feasibility	O
of	O
our	O
proposed	O
single	O
Mg2	B-chemical
+	I-chemical
mechanism	O
,	O
we	O
simulated	B-experimental_method
the	O
Tdp2	B-protein
reaction	O
coordinate	O
with	O
hybrid	B-experimental_method
QM	I-experimental_method
/	I-experimental_method
MM	I-experimental_method
modeling	I-experimental_method
using	O
Tdp2	B-protein
substrate	B-protein_state
analog	I-protein_state
-	I-protein_state
and	O
product	B-protein_state
-	I-protein_state
bound	I-protein_state
structures	B-evidence
as	O
guides	O
.	O

Previous	O
structural	B-experimental_method
analyses	I-experimental_method
showed	O
that	O
the	O
superposition	B-experimental_method
of	O
a	O
DNA	B-chemical
substrate	O
mimic	O
(	O
5	B-chemical
′-	I-chemical
aminohexanol	I-chemical
)	O
and	O
product	O
(	O
5	B-chemical
′-	I-chemical
PO4	I-chemical
)	O
complexes	O
delineates	O
a	O
probable	O
Tdp2	B-protein
reaction	O
trajectory	O
characterized	O
by	O
inversion	O
of	O
stereochemistry	O
about	O
the	O
adducted	O
5	O
′-	O
phosphorus	O
.	O

In	O
this	O
scheme	O
(	O
Figure	O
5A	O
),	O
a	O
candidate	O
nucleophilic	O
water	B-chemical
that	O
is	O
strongly	O
hydrogen	B-bond_interaction
bonded	I-bond_interaction
to	O
Asp272	B-residue_name_number
and	O
Asn274	B-residue_name_number
,	O
is	O
well	O
positioned	O
for	O
the	O
in	O
-	O
line	O
nucleophilic	O
attack	O
∼	O
180	O
°	O
opposite	O
of	O
the	O
P	O
–	O
O	O
bond	O
of	O
the	O
5	O
′-	O
Tyr	O
adduct	O
.	O

Structure	O
-	O
function	O
analysis	O
of	O
the	O
Tdp2	B-protein
reaction	O
mechanism	O
.	O

(	O
A	O
)	O
Proposed	O
mechanism	O
for	O
hydrolysis	O
of	O
phosphotyrosine	B-residue_name
bond	O
by	O
Tdp2	B-protein
.	O

Residues	O
in	O
green	O
form	O
the	O
binding	B-site
-	I-site
site	I-site
for	O
the	O
5	B-residue_name
′-	I-residue_name
tyrosine	I-residue_name
(	O
red	O
)	O
and	O
phosphate	B-chemical
,	O
yellow	O
bind	O
the	O
5	O
′	O
nucleotide	O
and	O
blue	O
bind	O
nucleotides	O
2	O
–	O
3	O
.	O

Residue	O
numbers	O
shown	O
are	O
for	O
the	O
mTdp2	B-protein
homolog	O
.	O
(	O
B	O
)	O
Free	B-evidence
energy	I-evidence
during	O
the	O
QM	B-experimental_method
/	I-experimental_method
MM	I-experimental_method
simulation	I-experimental_method
as	O
a	O
function	O
of	O
distance	O
between	O
the	O
nucleophilic	O
water	B-chemical
and	O
5	O
′-	O
phosphorus	O
atom	O
.	O

Reaction	O
proceeds	O
from	O
right	O
to	O
left	O
.	O
(	O
C	O
)	O
Models	O
for	O
the	O
mTdp2cat	B-complex_assembly
-	I-complex_assembly
DNA	I-complex_assembly
complex	O
during	O
the	O
QM	B-experimental_method
/	I-experimental_method
MM	I-experimental_method
reaction	I-experimental_method
path	I-experimental_method
simulation	I-experimental_method
showing	O
the	O
substrate	O
(	O
left	O
,	O
tan	O
),	O
transition	O
state	O
intermediate	O
(	O
center	O
,	O
cyan	O
)	O
and	O
product	O
(	O
right	O
,	O
pink	O
)	O
states	O
.	O

Electrostatic	B-evidence
potential	I-evidence
color	O
gradient	O
extends	O
from	O
positive	O
(	O
red	O
)	O
through	O
neutral	O
(	O
gray	O
),	O
to	O
negative	O
(	O
blue	O
).	O
(	O
E	O
)	O
Bar	O
graph	O
displaying	O
the	O
relative	O
activity	O
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
mutant	B-protein_state
human	B-species
MBP	B-experimental_method
-	O
hTdp2cat	B-structure_element
fusion	B-experimental_method
proteins	I-experimental_method
on	O
the	O
three	O
substrates	O
.	O

Release	O
of	O
PNP	B-chemical
from	O
PNP	B-chemical
phosphate	B-chemical
and	O
T5PNP	B-chemical
was	O
detected	O
as	O
an	O
increase	O
in	O
absorbance	O
at	O
415	O
nm	O
.	O

Reaction	B-evidence
rates	I-evidence
are	O
expressed	O
as	O
the	O
percent	O
of	O
activity	O
relative	O
to	O
wildtype	B-protein_state
MBP	B-experimental_method
-	O
hTdp2cat	B-structure_element
;	O
error	O
bars	O
,	O
s	O
.	O
d	O
.	O

Mutants	O
of	O
hTdp2	B-protein
(	O
black	O
)	O
and	O
the	O
equivalent	O
residue	O
in	O
mTdp2	B-protein
(	O
tan	O
)	O
are	O
indicated	O
.	O

We	O
examined	O
the	O
energy	O
profile	O
of	O
the	O
nucleophilic	O
attack	O
of	O
the	O
water	B-chemical
molecule	O
by	O
using	O
the	O
distance	O
between	O
the	O
water	B-chemical
oxygen	O
and	O
the	O
P	O
atom	O
on	O
the	O
phosphate	O
moiety	O
as	O
the	O
sole	O
reaction	O
coordinate	O
in	O
the	O
present	O
calculation	O
(	O
Figure	O
5B	O
and	O
C	O
).	O

A	O
starting	O
model	O
was	O
generated	O
from	O
atomic	O
coordinates	O
of	O
the	O
mTdp2cat	B-structure_element
5	B-chemical
′–	I-chemical
aminohexanol	I-chemical
substrate	O
analog	O
structure	B-evidence
(	O
PDB	O
4GZ0	O
)	O
with	O
a	O
tyrosine	B-residue_name
replacing	O
the	O
5	B-chemical
′-	I-chemical
aminohexanol	I-chemical
then	O
adding	O
the	O
Mg2	B-chemical
+	I-chemical
and	O
inner	O
-	O
sphere	O
waters	B-chemical
from	O
the	O
mTdp2	B-complex_assembly
-	I-complex_assembly
DNA	I-complex_assembly
product	O
structure	B-evidence
(	O
PDB	O
,	O
4GZ1	O
),	O
and	O
running	O
an	O
initial	O
round	O
of	O
molecular	B-experimental_method
dynamics	I-experimental_method
simulation	I-experimental_method
(	O
10	O
ns	O
)	O
to	O
allow	O
the	O
system	O
to	O
reach	O
an	O
equilibrium	O
.	O

After	O
QM	B-experimental_method
/	I-experimental_method
MM	I-experimental_method
optimization	I-experimental_method
of	O
this	O
model	O
(	O
Figure	O
5C	O
,	O
‘	O
i	O
-	O
substrate	O
’),	O
the	O
O	O
–	O
P	O
distance	O
is	O
3	O
.	O
4	O
Å	O
,	O
which	O
is	O
in	O
agreement	O
with	O
the	O
range	O
of	O
distances	O
observed	O
in	O
the	O
mTdp2cat	B-structure_element
5	B-chemical
′-	I-chemical
aminohexanol	I-chemical
substrate	O
analog	O
structure	B-evidence
(	O
3	O
.	O
2	O
–	O
3	O
.	O
4	O
Å	O
).	O

In	O
the	O
subsequent	O
two	O
steps	O
of	O
the	O
simulation	B-experimental_method
,	O
as	O
the	O
water	B-chemical
-	O
phosphate	O
O	O
–	O
P	O
distance	O
reduces	O
to	O
1	O
.	O
98	O
Å	O
,	O
a	O
key	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
between	O
the	O
nucleophilic	O
water	B-chemical
and	O
Asp272	B-residue_name_number
shortens	O
to	O
1	O
.	O
38	O
Å	O
as	O
the	O
water	B-chemical
H	O
–	O
O	O
bond	O
approaches	O
the	O
point	O
of	O
dissociation	O
.	O

The	O
second	O
proton	O
on	O
the	O
water	B-chemical
nucleophile	O
maintains	O
a	O
strong	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
Asn274	B-residue_name_number
throughout	O
the	O
reaction	O
,	O
implicating	O
this	O
residue	O
in	O
orienting	O
the	O
water	B-chemical
nucleophile	O
during	O
the	O
reaction	O
.	O

Concomitant	O
with	O
this	O
,	O
the	O
phosphotyrosyl	B-ptm
O	O
–	O
P	O
bond	O
weakens	O
(	O
d	O
=	O
1	O
.	O
89	O
Å	O
),	O
and	O
the	O
formation	O
of	O
the	O
penta	O
-	O
covalent	O
transition	O
state	O
(	O
Figure	O
5C	O
‘	O
ii	O
-	O
transition	O
state	O
’)	O
is	O
observed	O
.	O

The	O
final	O
steps	O
show	O
inversion	O
of	O
stereochemistry	O
at	O
the	O
phosphate	B-chemical
,	O
along	O
with	O
lengthening	O
and	O
breaking	O
of	O
the	O
phosphotyrosyl	B-ptm
O	O
–	O
P	O
bond	O
.	O

Of	O
note	O
,	O
both	O
nitrogens	O
of	O
the	O
imidazole	O
side	O
chain	O
of	O
His	B-residue_name_number
359	I-residue_name_number
require	O
protonation	O
for	O
stability	O
of	O
the	O
simulation	B-experimental_method
.	O

Asp	B-residue_name_number
326	I-residue_name_number
makes	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
to	O
N	O
∂	O
1	O
of	O
His359	B-residue_name_number
,	O
suggesting	O
that	O
this	O
salt	B-bond_interaction
bridge	I-bond_interaction
could	O
stabilize	O
the	O
protonated	B-protein_state
form	O
of	O
His359	B-residue_name_number
as	O
has	O
been	O
demonstrated	O
for	O
the	O
analogous	O
Asp	B-residue_name
-	O
His	B-residue_name
pair	O
in	O
the	O
EEP	B-structure_element
domain	O
of	O
APE1	B-protein
,	O
which	O
elevates	O
the	O
pKa	B-evidence
of	O
this	O
His	B-residue_name
above	O
8	O
.	O
0	O
.	O
In	O
our	O
model	O
,	O
the	O
transition	O
state	O
contains	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
between	O
the	O
doubly	B-protein_state
protonated	I-protein_state
His359	B-residue_name_number
and	O
the	O
phosphate	B-chemical
oxygen	O
that	O
also	O
coordinates	O
with	O
the	O
single	O
catalytic	O
Mg2	B-chemical
+,	I-chemical
while	O
the	O
second	O
His359	B-residue_name_number
imidazole	O
proton	O
maintains	O
a	O
H	B-bond_interaction
-	I-bond_interaction
bond	I-bond_interaction
with	O
the	O
Asp326	B-residue_name_number
residue	O
throughout	O
the	O
reaction	O
.	O

In	O
the	O
final	O
optimized	O
structure	B-evidence
,	O
the	O
observed	O
product	O
state	O
(	O
Figure	O
5C	O
,	O
‘	O
iii	O
-	O
product	O
’)	O
is	O
found	O
in	O
a	O
conformation	O
that	O
is	O
7	O
.	O
4	O
kcal	O
mol	O
−	O
1	O
more	O
stable	O
than	O
the	O
initial	O
reactive	O
state	O
(	O
Figure	O
5B	O
).	O

The	O
tyrosine	B-residue_name
oxy	O
-	O
anion	O
product	O
is	O
coordinated	B-bond_interaction
to	I-bond_interaction
the	O
Mg2	B-chemical
+	I-chemical
ion	O
with	O
a	O
2	O
.	O
0	O
Å	O
distance	O
,	O
which	O
is	O
the	O
shortest	O
of	O
the	O
six	O
Mg2	B-chemical
+	I-chemical
ligands	O
(	O
including	O
three	O
water	B-chemical
molecules	O
,	O
one	O
of	O
the	O
free	O
oxygens	O
on	O
the	O
phosphate	B-chemical
group	O
and	O
the	O
Glu162	B-residue_name_number
residue	O
),	O
indicating	O
the	O
single	O
Mg2	B-chemical
+	I-chemical
greatly	O
stabilizes	O
the	O
product	O
oxy	O
-	O
anion	O
.	O

An	O
additional	O
striking	O
feature	O
gleaned	O
from	O
the	O
QM	B-experimental_method
/	I-experimental_method
MM	I-experimental_method
modeling	I-experimental_method
is	O
the	O
putative	O
binding	O
mode	O
of	O
the	O
Top2	B-protein_type
tyrosine	B-residue_name
-	O
leaving	O
group	O
.	O

A	O
trio	O
of	O
conserved	B-protein_state
residues	O
(	O
Tyr	B-residue_name_number
188	I-residue_name_number
,	O
Arg	B-residue_name_number
216	I-residue_name_number
and	O
Ser	B-residue_name_number
239	I-residue_name_number
)	O
forms	O
the	O
walls	O
of	O
a	O
conserved	B-protein_state
Top2	B-protein_type
tyrosine	B-site
binding	I-site
pocket	I-site
.	O

We	O
propose	O
this	O
cation	B-bond_interaction
–	I-bond_interaction
π	I-bond_interaction
interaction	I-bond_interaction
further	O
contributes	O
to	O
tuned	O
stabilization	O
of	O
the	O
negatively	O
charged	O
phenolate	O
reaction	O
product	O
.	O

Consistent	O
with	O
this	O
,	O
analysis	O
of	O
electrostatic	B-evidence
potential	I-evidence
of	O
the	O
phosphotyrosyl	B-ptm
moiety	O
using	O
Gaussian	O
09	O
.	O
D01	O
in	O
the	O
presence	B-protein_state
and	O
absence	B-protein_state
of	I-protein_state
the	O
Arg216	B-residue_name_number
guanidinium	O
reveals	O
Arg216	B-residue_name_number
is	O
strongly	O
electron	O
withdrawing	O
(	O
Figure	O
5D	O
).	O

We	O
further	O
examined	O
the	O
contribution	O
of	O
this	O
cation	B-bond_interaction
–	I-bond_interaction
π	I-bond_interaction
interaction	I-bond_interaction
to	O
the	O
reaction	O
chemistry	O
by	O
moving	O
the	O
guanidinium	O
group	O
of	O
Arg216	B-residue_name_number
from	O
the	O
QM	B-experimental_method
system	O
to	O
the	O
MM	B-experimental_method
system	O
as	O
either	O
a	O
+	O
1	O
or	O
∼	O
0	O
charge	O
species	O
,	O
and	O
re	O
-	O
computed	O
energy	B-evidence
penalties	I-evidence
for	O
each	O
step	O
in	O
the	O
reaction	O
coordinate	O
(	O
Supplementary	O
Figure	O
S5A	O
).	O

Removing	O
Arg216	B-residue_name_number
from	O
the	O
quantum	O
subsystem	O
incurs	O
an	O
∼	O
2	O
kcal	O
mol	O
−	O
1	O
penalty	O
in	O
the	O
transition	O
state	O
and	O
product	O
complex	O
.	O

Removing	O
the	O
+	O
1	O
charge	O
on	O
the	O
Arg216	B-residue_name_number
has	O
a	O
minimal	O
impact	O
on	O
the	O
transition	O
state	O
,	O
but	O
incurs	O
an	O
additional	O
∼	O
2	O
kcal	O
mol	O
−	O
1	O
penalty	O
in	O
the	O
product	O
complex	O
.	O

Tdp2	B-protein
mutational	B-experimental_method
analysis	I-experimental_method

For	O
example	O
,	O
mutations	B-experimental_method
impacting	O
Tdp2	B-protein
active	B-site
site	I-site
chemistry	O
and	O
phosphotyrosyl	B-ptm
bond	O
cleavage	O
should	O
similarly	O
affect	O
catalysis	O
on	O
all	O
three	O
substrates	O
,	O
but	O
mutants	O
impacting	O
DNA	B-chemical
damage	O
binding	O
might	O
only	O
impair	O
catalysis	O
on	O
5	B-ptm
′-	I-ptm
Y	I-ptm
and	O
T5PNP	B-chemical
but	O
not	O
PNPP	B-chemical
that	O
lacks	O
a	O
nucleobase	O
.	O

To	O
test	O
if	O
this	O
proposed	O
Lewis	O
base	O
is	O
critical	O
for	O
reaction	O
chemistry	O
we	O
mutated	B-experimental_method
it	O
to	B-experimental_method
a	O
His	B-residue_name
,	O
which	O
could	O
alternatively	O
support	O
metal	O
binding	O
,	O
as	O
well	O
as	O
bulky	O
hydrophobic	O
residues	O
(	O
Leu	B-residue_name
and	O
Met	B-residue_name
)	O
that	O
we	O
predict	O
would	O
block	O
the	O
water	B-site
-	I-site
binding	I-site
site	I-site
.	O

Similar	O
to	O
a	O
previously	O
characterized	O
hD262N	B-mutant
mutation	O
,	O
all	O
three	O
substitutions	B-experimental_method
ablate	O
activity	O
,	O
supporting	O
essential	O
roles	O
for	O
hAsp262	B-residue_name_number
(	O
mAsp272	B-residue_name_number
)	O
in	O
catalysis	O
.	O

Next	O
,	O
we	O
mutated	B-experimental_method
key	O
elements	O
of	O
the	O
mobile	O
loop	O
(	O
β2Hβ	B-site
hydrophobic	I-site
wall	I-site
,	O
Figure	O
2A	O
and	O
C	O
).	O

The	O
hL305W	B-mutant
substitution	O
that	O
we	O
expect	O
to	O
have	O
the	O
most	O
distorting	O
impact	O
on	O
conformation	O
of	O
the	O
β2Hβ	B-site
hydrophobic	I-site
wall	I-site
also	O
has	O
the	O
largest	O
impact	O
on	O
catalysis	O
of	O
the	O
DNA	B-chemical
substrate	O
5	B-ptm
′-	I-ptm
Y	I-ptm
.	O
By	O
comparison	O
,	O
as	O
predicted	O
by	O
our	O
model	O
where	O
β2Hβ	B-structure_element
dictates	O
key	O
interactions	O
with	O
undamaged	O
and	O
damaged	O
nucleobases	O
,	O
all	O
of	O
these	O
substitutions	B-experimental_method
have	O
little	O
impact	O
on	O
PNPP	B-chemical
(>	O
90	O
%	O
activity	O
).	O

Third	O
,	O
we	O
altered	O
properties	O
of	O
the	O
proposed	O
enzyme	B-site
substrate	I-site
cation	I-site
–	I-site
π	I-site
interface	I-site
.	O

No	O
activity	O
was	O
detected	O
for	O
a	O
mutant	B-protein_state
that	O
removes	O
the	O
positive	O
charge	O
at	O
this	O
position	O
(	O
hR206A	B-mutant
).	O

The	O
precise	O
geometry	O
of	O
this	O
pocket	B-site
is	O
also	O
critical	O
for	O
catalysis	O
as	O
replacement	B-experimental_method
of	O
hArg206	B-residue_name_number
(	O
mArg216	B-residue_name_number
)	O
with	O
a	O
lysine	B-residue_name
also	O
results	O
in	O
a	O
profound	O
decrease	O
in	O
catalysis	O
(<	O
5	O
%	O
activity	O
on	O
5	B-ptm
′-	I-ptm
Y	I-ptm
,	O
no	O
detectable	O
activity	O
on	O
T5PNP	B-chemical
or	O
PNPP	B-chemical
).	O

Similarly	O
,	O
mutation	B-experimental_method
of	O
hTyr178	B-residue_name_number
that	O
structurally	O
scaffolds	O
the	O
hArg206	B-residue_name_number
(	O
mArg216	B-residue_name_number
)	O
guanidinium	O
also	O
significantly	O
impacts	O
activity	O
,	O
with	O
Y178F	B-mutant
and	O
Y178W	B-mutant
having	O
<	O
25	O
%	O
activity	O
on	O
all	O
substrates	O
.	O

Fourth	O
,	O
we	O
evaluated	O
roles	O
for	O
the	O
hHis351	B-residue_name_number
–	O
hAsp316	B-residue_name_number
(	O
mAsp326	B-residue_name_number
–	O
mHis359	B-residue_name_number
)	O
transition	O
state	O
stabilization	O
charge	O
pair	O
.	O

Thus	O
altogether	O
,	O
our	O
mutational	O
data	O
support	O
key	O
roles	O
for	O
the	O
active	B-site
site	I-site
Lewis	O
base	O
aspartate	B-residue_name
,	O
mobile	B-protein_state
substrate	B-structure_element
engagement	I-structure_element
loops	I-structure_element
,	O
enzyme	O
–	O
substrate	O
cation	B-bond_interaction
–	I-bond_interaction
π	I-bond_interaction
interactions	I-bond_interaction
,	O
and	O
active	B-site
site	I-site
transition	O
state	O
stabilizing	O
charge	B-bond_interaction
interaction	I-bond_interaction
in	O
supporting	O
Tdp2	B-protein
catalysis	O
.	O

A	O
Tdp2	B-protein
active	B-site
site	I-site
single	O
nucleotide	O
polymorphism	O
impairs	O
Tdp2	B-protein
function	O

Recently	O
,	O
it	O
was	O
found	O
that	O
inactivation	O
of	O
TDP2	B-protein
by	O
a	O
splice	O
-	O
site	O
mutation	O
is	O
associated	O
with	O
neurological	O
disease	O
and	O
confers	O
hypersensitivity	O
to	O
Top2	B-protein_type
poisons	O
.	O

We	O
considered	O
whether	O
human	B-species
SNPs	O
causing	O
missense	O
mutations	O
might	O
also	O
impact	O
Tdp2	B-protein
DNA	B-chemical
–	O
protein	O
crosslink	O
repair	O
functions	O
established	O
here	O
as	O
well	O
as	O
Tdp2	B-protein
-	O
mediated	O
NHEJ	O
of	O
blocked	O
DNA	B-chemical
termini	O
.	O

We	O
identified	O
two	O
SNPs	O
in	O
human	B-species
TDP2	B-protein
curated	O
in	O
the	O
NCBI	O
SNP	O
database	O
that	O
result	O
in	O
missense	O
mutations	O
within	O
the	O
DNA	B-site
processing	I-site
active	I-site
site	I-site
:	O
rs199602263	B-gene
(	O
minor	O
allele	O
frequency	O
0	O
.	O
0002	O
),	O
which	O
substitutes	O
hAsp350	B-residue_name_number
for	O
Asn	B-residue_name
,	O
and	O
rs77273535	B-gene
(	O
minor	O
allele	O
frequency	O
0	O
.	O
004	O
,	O
which	O
substitutes	O
hIle307	B-residue_name_number
for	O
Val	B-residue_name
)	O
(	O
Figure	O
6A	O
).	O

We	O
show	O
the	O
hD350N	B-mutant
substitution	B-experimental_method
severely	O
impairs	O
activity	O
on	O
all	O
substrates	O
tested	O
in	O
vitro	O
,	O
whereas	O
hI307V	B-mutant
only	O
has	O
a	O
mild	O
impact	O
on	O
catalysis	O
(	O
Figure	O
6B	O
–	O
D	O
).	O

To	O
better	O
understand	O
the	O
basis	O
for	O
the	O
D350N	B-mutant
catalytic	O
defect	O
,	O
we	O
analyzed	O
the	O
structural	O
environment	O
of	O
this	O
substitution	O
based	O
on	O
the	O
high	O
-	O
resolution	O
structures	B-evidence
of	O
mTdp2cat	B-structure_element
(	O
Figure	O
6A	O
).	O

Here	O
,	O
hAsp350	B-residue_name_number
(	O
mAsp358	B-residue_name_number
)	O
serves	O
as	O
a	O
structural	O
nexus	O
linking	O
active	B-site
site	I-site
metal	O
binding	O
to	O
substrate	B-structure_element
binding	I-structure_element
loop	I-structure_element
conformations	O
.	O

Tdp2	B-protein
SNPs	O
impair	O
function	O
.	O
(	O
A	O
)	O
Active	B-site
site	I-site
residues	O
mutated	O
by	O
TDP2	B-protein
SNPs	O
.	O

D350N	B-mutant
(	O
mTdp2	B-protein
D358N	B-mutant
)	O
and	O
I307V	B-mutant
(	O
mTdp2	B-protein
I317V	B-mutant
)	O
substitutions	B-experimental_method
are	O
mapped	O
onto	O
the	O
Tdp2	B-protein
active	B-site
site	I-site
of	O
the	O
high	O
-	O
resolution	O
mTdp2cat	B-structure_element
structure	B-evidence
(	O
4GZ1	O
).	O

(	O
B	O
)	O
Coomassie	O
blue	O
stained	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
gel	O
of	O
purified	O
WT	B-protein_state
and	O
mutant	B-protein_state
MBP	B-experimental_method
-	O
hTdp2cat	B-structure_element
proteins	O
used	O
for	O
assays	O
in	O
panels	O
C	O
and	O
D	O
.	O
(	O
C	O
)	O
Activity	O
of	O
WT	B-protein_state
and	O
mutant	B-protein_state
MBP	B-experimental_method
-	O
hTdp2cat	B-structure_element
proteins	O
on	O
a	O
5	B-chemical
′–	I-chemical
phosphotyrosyl	I-chemical
–	I-chemical
DNA	I-chemical
oligonucleotides	I-chemical
with	O
3	O
′-	O
fluorescein	B-chemical
label	O
.	O

Release	O
of	O
PNP	B-chemical
from	O
PNP	B-chemical
phosphate	I-chemical
(	O
PNPP	B-chemical
)	O
and	O
was	O
detected	O
as	O
an	O
increase	O
in	O
absorbance	O
at	O
415	O
nm	O
,	O
whereas	O
the	O
5	B-ptm
′-	I-ptm
Y	I-ptm
substrate	O
is	O
quantification	O
of	O
activity	O
in	O
a	O
gel	B-experimental_method
based	I-experimental_method
assay	I-experimental_method
shown	O
in	O
Figure	O
6C	O
.	O

To	O
define	O
the	O
molecular	O
basis	O
for	O
the	O
hD350N	B-mutant
(	O
mD358N	B-mutant
)	O
defect	O
,	O
we	O
crystallized	B-experimental_method
and	I-experimental_method
determined	I-experimental_method
the	O
structure	B-evidence
of	O
the	O
DNA	B-protein_state
-	I-protein_state
free	I-protein_state
form	O
of	O
the	O
mD358N	B-mutant
protein	O
to	O
2	O
.	O
8Å	O
resolution	O
(	O
PDB	O
entry	O
5INN	O
).	O

This	O
structure	B-evidence
shows	O
the	O
D358N	B-mutant
mutation	B-experimental_method
disrupts	O
the	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
between	O
Asp358	B-residue_name_number
and	O
Trp307	B-residue_name_number
,	O
shifts	O
the	O
position	O
of	O
Asn358	B-residue_name_number
and	O
destabilizes	O
Trp307	B-residue_name_number
.	O

Consequently	O
,	O
poor	O
electron	B-evidence
density	I-evidence
is	O
visible	O
for	O
the	O
β2Hβ	B-structure_element
loop	I-structure_element
which	O
is	O
mostly	O
disordered	B-protein_state
(	O
Supplementary	O
Figure	O
S6	O
).	O

Tdp2	B-protein
facilitates	O
NHEJ	O
repair	O
of	O
5	B-residue_name
′-	I-residue_name
phosphotyrosine	I-residue_name
adducted	O
DSBs	O

Overall	O
,	O
our	O
Tdp2	B-protein
structure	B-experimental_method
/	I-experimental_method
activity	I-experimental_method
studies	I-experimental_method
reveal	O
a	O
tuned	O
,	O
5	B-ptm
′-	I-ptm
detyrosylation	I-ptm
DNA	B-chemical
end	O
processing	O
activity	O
and	O
it	O
has	O
been	O
demonstrated	O
that	O
Tdp2	B-protein
could	O
enable	O
repair	O
of	O
Top2	B-protein_type
damage	O
by	O
the	O
non	O
-	O
homologous	O
end	O
-	O
joining	O
(	O
NHEJ	O
)	O
pathway	O
.	O

Interestingly	O
,	O
hTdp2cat	B-structure_element
is	O
slightly	O
more	O
effective	O
than	O
hTdp2FL	B-protein
in	O
promoting	O
NHEJ	O
of	O
adducted	O
ends	O
,	O
while	O
a	O
catalytically	B-protein_state
deficient	I-protein_state
E152Q	B-mutant
mutant	B-protein_state
was	O
inactive	B-protein_state
in	O
this	O
assay	O
,	O
supporting	O
the	O
notion	O
that	O
Tdp2	B-protein
catalytic	O
activity	O
is	O
required	O
to	O
support	O
NHEJ	O
of	O
phosphotyrosyl	B-ptm
blocked	O
DSBs	O
(	O
Supplementary	O
Figure	O
S7A	O
).	O

We	O
confirmed	O
that	O
efficient	O
joining	O
of	O
the	O
same	O
tyrosine	B-residue_name
-	O
adducted	O
substrate	O
in	O
cells	O
(	O
Figure	O
7B	O
)	O
was	O
dependent	O
on	O
both	O
NHEJ	O
(	O
reduced	O
over	O
10	O
-	O
fold	O
in	O
ligase	O
IV	O
deficient	O
HCT	O
116	O
cells	O
;	O
Supplementary	O
Figure	O
S7B	O
),	O
and	O
Tdp2	B-protein
(	O
reduced	O
5	O
-	O
fold	O
in	O
Tdp2	B-protein
deficient	O
MEFs	O
;	O
Figure	O
7C	O
).	O

Moreover	O
,	O
products	O
with	O
error	O
(	O
i	O
.	O
e	O
.	O
junctions	O
have	O
missing	O
sequence	O
flanking	O
the	O
adducted	O
terminus	O
)	O
are	O
twice	O
as	O
frequent	O
in	O
cells	O
deficient	O
in	O
Tdp2	B-protein
(	O
Figure	O
7D	O
).	O

Therefore	O
,	O
in	O
accord	O
with	O
previous	O
work	O
,	O
joining	O
of	O
tyrosine	B-residue_name
adducted	O
ends	O
after	O
Tdp2	B-protein
-	O
mediated	O
detyrosylation	B-ptm
is	O
both	O
more	O
efficient	O
and	O
more	O
accurate	O
than	O
joining	O
after	O
endonucleolytic	O
excision	O
(	O
e	O
.	O
g	O
.	O
mediated	O
by	O
Artemis	B-protein
or	O
the	O
Mre11	B-complex_assembly
/	I-complex_assembly
Rad50	I-complex_assembly
/	I-complex_assembly
Nbs1	I-complex_assembly
complex	O
).	O

Effects	O
of	O
Tdp2	B-protein
active	B-site
site	I-site
SNP	O
-	O
encoded	O
mutants	O
on	O
cellular	O
Tdp2	B-protein
functions	O
.	O
(	O
A	O
)	O
Cy5	B-chemical
labeled	O
substrates	O
with	O
5	B-chemical
′-	I-chemical
phosphate	I-chemical
termini	O
(	O
Lanes	O
1	O
–	O
4	O
)	O
or	O
5	B-protein_state
′-	I-protein_state
tyrosylated	I-protein_state
termini	O
(	O
Lanes	O
5	O
–	O
9	O
)	O
were	O
incubated	O
with	O
Ku	B-protein
,	O
the	O
NHEJ	B-protein_type
ligase	I-protein_type
(	O
XRCC4	B-protein
,	O
ligase	B-protein
IV	I-protein
and	O
XLF	B-protein
;	O
X	O
-	O
L	O
-	O
X	O
)	O
and	O
1	O
nM	O
hTdp2FL	B-protein
as	O
indicated	O
(+)	O
for	O
5	O
min	O
at	O
37	O
°	O
C	O
.	O

Concatemer	O
ligation	O
products	O
were	O
detected	O
by	O
5	O
%	O
native	B-experimental_method
PAGE	I-experimental_method
.	O

(	O
B	O
)	O
Workflow	O
diagram	O
of	O
cellular	B-experimental_method
end	I-experimental_method
joining	I-experimental_method
assays	I-experimental_method
.	O

After	O
1	O
h	O
,	O
DNA	B-chemical
was	O
recovered	O
from	O
cells	O
and	O
repair	O
efficiency	O
by	O
qPCR	B-experimental_method
or	O
sequencing	B-experimental_method
as	O
indicated	O
.	O
(	O
C	O
)	O
qPCR	B-experimental_method
assessment	O
of	O
cellular	O
end	O
joining	O
efficiency	O
of	O
the	O
tyrosylated	B-protein_state
substrate	O
comparing	O
results	O
from	O
wildtype	B-protein_state
MEF	O
cells	O
to	O
Tdp2	B-protein
−/−	O
cells	O
and	O
Tdp2	B-protein
−/−	O
cells	O
complemented	O
with	O
wildtype	B-protein_state
or	O
the	O
noted	O
hTDP2FL	B-protein
variants	O
;	O
Joining	O
efficiency	O
shown	O
is	O
the	O
ratio	O
of	O
junctions	O
recovered	O
relative	O
to	O
WT	B-protein_state
cells	O
.	O

We	O
next	O
compared	O
the	O
ability	O
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
mutant	B-protein_state
hTdp2FL	B-protein
variants	O
to	O
complement	O
Tdp2	B-protein
deficient	O
mouse	B-taxonomy_domain
embryonic	O
fibroblasts	O
(	O
Supplementary	O
Figure	O
S7C	O
).	O

Joining	O
of	O
extrachromosomal	O
DNA	B-chemical
with	O
phosphotyrosine	B-residue_name
blocked	O
ends	O
,	O
both	O
in	O
terms	O
of	O
efficiency	O
(	O
Figure	O
7C	O
)	O
and	O
fidelity	O
(	O
Figure	O
7D	O
),	O
was	O
indistinguishable	O
comparing	O
MEFs	O
from	O
a	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
mouse	B-taxonomy_domain
,	O
MEFs	O
from	O
a	O
Tdp2	B-protein
-/-	O
mouse	B-taxonomy_domain
overexpressing	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
human	B-species
Tdp2	B-protein
,	O
and	O
Tdp2	B-protein
-/-	O
MEFs	O
overexpressing	O
the	O
I307V	B-mutant
variant	B-protein_state
human	B-species
Tdp2	B-protein
.	O

In	O
contrast	O
,	O
joining	O
of	O
5	O
′	O
phosphotyrosine	O
-	O
blocked	O
ends	O
was	O
reduced	O
5	O
-	O
fold	O
in	O
Tdp2	B-protein
-/-	O
MEFs	O
,	O
and	O
an	O
equivalent	O
defect	O
was	O
observed	O
in	O
Tdp2	B-protein
-/-	O
MEFs	O
overexpressing	O
Tdp2	B-protein
D350N	B-mutant
.	O

Moreover	O
,	O
the	O
frequency	O
of	O
inaccurate	O
repair	O
was	O
2	O
-	O
fold	O
higher	O
in	O
both	O
Tdp2	B-protein
deficient	O
cells	O
and	O
Tdp2	B-protein
deficient	O
cells	O
overexpressing	O
D350N	B-mutant
,	O
relative	O
to	O
cells	O
expressing	O
wild	B-protein_state
type	I-protein_state
Tdp2	B-protein
or	O
hTdp2	B-protein
I307V	B-mutant
(	O
Figure	O
7D	O
).	O

Expression	O
of	O
wild	B-protein_state
type	I-protein_state
or	O
I307V	B-mutant
human	B-species
Tdp2	B-protein
in	O
Tdp2	B-protein
-/-	O
MEFs	O
was	O
also	O
sufficient	O
to	O
confer	O
levels	O
of	O
resistance	O
to	O
etoposide	B-chemical
comparable	O
to	O
the	O
matched	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
MEF	O
line	O
,	O
while	O
overexpression	B-experimental_method
of	O
human	B-species
D350N	B-mutant
Tdp2	B-protein
had	O
no	O
apparent	O
complementation	O
activity	O
(	O
Figure	O
7E	O
).	O

The	O
rare	O
D350N	B-mutant
variant	B-protein_state
is	O
thus	O
inactive	B-protein_state
by	O
all	O
metrics	O
analyzed	O
.	O

Understanding	O
how	O
cells	O
cope	O
with	O
complex	O
DNA	B-chemical
breaks	O
bearing	O
topoisomerase	O
–	O
DNA	B-chemical
protein	O
crosslinks	O
is	O
key	O
to	O
deciphering	O
individual	O
responses	O
to	O
chemotherapeutic	O
outcomes	O
and	O
genotoxic	O
agents	O
that	O
poison	O
Top2	B-protein_type
.	O

This	O
mechanistic	O
dissection	O
of	O
Tdp2	B-protein
interactions	O
with	O
damaged	O
DNA	B-chemical
and	O
metal	O
cofactor	O
provides	O
a	O
detailed	O
molecular	O
understanding	O
of	O
the	O
mechanism	O
of	O
Tdp2	B-protein
DNA	B-chemical
protein	O
crosslink	O
processing	O
.	O

The	O
properties	O
of	O
complex	O
DNA	B-chemical
strand	O
breaks	O
bearing	O
Top2	B-protein_type
-	O
DNA	B-chemical
protein	O
crosslinks	O
necessitate	O
that	O
Tdp2	B-protein
accommodates	O
both	O
damaged	O
nucleic	O
acid	O
as	O
well	O
as	O
the	O
topoisomerase	B-protein_type
protein	O
in	O
its	O
active	B-site
site	I-site
for	O
catalysis	O
.	O

The	O
Tdp2	B-protein
substrate	B-site
interaction	I-site
groove	I-site
facilitates	O
DNA	B-chemical
-	O
protein	O
conjugate	O
recognition	O
in	O
two	O
important	O
ways	O
.	O

First	O
,	O
the	O
nucleic	B-site
acid	I-site
binding	I-site
trench	I-site
is	O
assembled	O
by	O
a	O
dynamic	B-protein_state
β2Hβ	B-structure_element
DNA	I-structure_element
damage	I-structure_element
-	I-structure_element
binding	I-structure_element
loop	I-structure_element
that	O
is	O
capable	O
of	O
recognizing	O
and	O
processing	O
diverse	O
phosphotyrosyl	B-ptm
linkages	I-ptm
even	O
in	O
the	O
context	O
of	O
bulky	O
adducts	O
such	O
as	O
ϵA	B-chemical
.	O
This	O
is	O
achieved	O
by	O
binding	O
of	O
nucleic	O
acid	O
‘	O
bases	O
out	O
’	O
by	O
an	O
extended	O
base	B-bond_interaction
-	I-bond_interaction
stacking	I-bond_interaction
hydrophobic	B-site
wall	I-site
of	O
the	O
β2Hβ	B-structure_element
-	I-structure_element
loop	I-structure_element
.	O

Secondly	O
,	O
our	O
QM	B-experimental_method
/	I-experimental_method
MM	I-experimental_method
analysis	O
further	O
highlights	O
an	O
enzyme	O
–	O
substrate	O
cation	B-bond_interaction
–	I-bond_interaction
π	I-bond_interaction
interaction	I-bond_interaction
as	O
an	O
additional	O
key	O
feature	O
of	O
the	O
Tdp2	B-protein
protein	O
–	O
DNA	B-chemical
crosslink	O
binding	O
and	O
reversal	O
.	O

The	O
strictly	B-protein_state
conserved	I-protein_state
active	B-site
site	I-site
Arg216	B-residue_name_number
appears	O
optimally	O
positioned	O
to	O
stabilize	O
a	O
delocalized	O
charge	O
on	O
the	O
phenolate	O
product	O
of	O
the	O
phosphotyrosyl	B-ptm
cleavage	O
reaction	O
through	O
molecular	O
orbital	O
overlap	O
and	O
polarization	O
of	O
the	O
leaving	O
group	O
.	O

To	O
our	O
knowledge	O
,	O
this	O
is	O
the	O
first	O
proposed	O
example	O
of	O
a	O
substrate	B-site
cation	I-site
–	I-site
π	I-site
interface	I-site
exploited	O
to	O
promote	O
a	O
phosphoryl	O
-	O
transfer	O
reaction	O
.	O

This	O
unique	O
feature	O
likely	O
provides	O
an	O
additional	O
level	O
of	O
substrate	O
-	O
specificity	O
for	O
Tdp2	B-protein
by	O
restricting	O
activity	O
to	O
hydrolysis	O
of	O
aromatic	O
adducts	O
characteristic	O
of	O
Top2cc	B-complex_assembly
,	O
picornaviral	B-taxonomy_domain
protein	O
–	O
RNA	B-chemical
and	O
Hepatitis	B-taxonomy_domain
B	I-taxonomy_domain
Virus	I-taxonomy_domain
(	O
HBV	B-taxonomy_domain
)	O
protein	O
–	O
DNA	B-chemical
processing	O
intermediates	O
.	O

By	O
comparison	O
,	O
other	O
EEP	B-structure_element
nucleases	B-protein_type
such	O
as	O
Ape1	B-protein
and	O
Ape2	B-protein
have	O
evolved	O
robust	O
DNA	B-chemical
damage	O
specific	O
endonucleolytic	O
and	O
exonucleolytic	O
activities	O
not	O
shared	O
with	O
Tdp2	B-protein
.	O

The	O
dynamic	O
nature	O
of	O
the	O
Tdp2	B-protein
active	B-site
site	I-site
presents	O
opportunities	O
for	O
enzyme	O
regulation	O
.	O

However	O
,	O
whether	O
additional	O
protein	O
factors	O
can	O
bind	O
to	O
Tdp2	B-protein
and	O
modulate	O
assembly	O
/	O
disassembly	O
of	O
the	O
Tdp2	B-protein
β2Hβ	B-structure_element
-	I-structure_element
loop	I-structure_element
is	O
unknown	O
.	O

Furthermore	O
,	O
high	O
-	O
resolution	O
structures	B-evidence
of	O
mouse	B-taxonomy_domain
(	O
Figures	O
3	O
and	O
4	O
)	O
and	O
C	B-species
.	I-species
elegans	I-species
Tdp2	B-protein
show	O
that	O
a	O
single	O
metal	O
ion	O
typifies	O
the	O
Tdp2	B-protein
active	B-site
site	I-site
from	O
worms	B-taxonomy_domain
to	O
man	B-taxonomy_domain
.	O

Herein	O
,	O
we	O
report	O
five	O
additional	O
lines	O
of	O
evidence	O
from	O
metal	O
binding	O
detected	O
by	O
intrinsic	B-experimental_method
tryptophan	I-experimental_method
fluorescence	I-experimental_method
,	O
crystallographic	B-experimental_method
analysis	I-experimental_method
of	O
varied	O
metal	O
cofactor	O
complexes	O
,	O
mutagenesis	B-experimental_method
,	O
Ca2	B-experimental_method
+	I-experimental_method
inhibition	I-experimental_method
studies	I-experimental_method
and	O
QM	B-experimental_method
/	I-experimental_method
MM	I-experimental_method
analysis	I-experimental_method
that	O
all	O
support	O
a	O
feasible	O
single	O
Mg2	B-chemical
+	I-chemical
mediated	O
Tdp2	B-protein
catalytic	O
mechanism	O
.	O

Given	O
Tdp2	B-protein
variation	O
in	O
the	O
human	B-species
population	O
,	O
links	O
to	O
neurological	O
disease	O
and	O
viral	O
pathogenesis	O
,	O
our	O
finding	O
that	O
TDP2	B-protein
SNPs	O
ablate	O
catalytic	O
activity	O
has	O
probable	O
implications	O
for	O
modulation	O
of	O
cancer	O
chemotherapy	O
,	O
susceptibility	O
to	O
environmentally	O
linked	O
Top2	B-protein_type
poisons	O
,	O
and	O
viral	B-taxonomy_domain
infection	O
.	O

Lastly	O
,	O
Tdp2	B-protein
inhibitors	O
may	O
synergize	O
or	O
potentiate	O
cytotoxic	O
effects	O
of	O
current	O
anticancer	O
treatments	O
that	O
target	O
Tdp2	B-protein
.	O

Thus	O
,	O
we	O
anticipate	O
this	O
atomic	O
-	O
level	O
and	O
mechanistic	O
definition	O
of	O
the	O
molecular	O
determinants	O
of	O
Tdp2	B-protein
catalysis	O
and	O
conformational	O
changes	O
driven	O
by	O
DNA	B-chemical
–	O
protein	O
and	O
protein	O
–	O
protein	O
interactions	O
will	O
foster	O
unique	O
strategies	O
for	O
the	O
development	O
of	O
Tdp2	B-protein
targeted	O
small	O
molecule	O
interventions	O
.	O

Mechanism	O
of	O
extracellular	O
ion	O
exchange	O
and	O
binding	B-site
-	I-site
site	I-site
occlusion	O
in	O
the	O
sodium	B-protein_type
-	I-protein_type
calcium	I-protein_type
exchanger	I-protein_type

Na	B-protein_type
+/	I-protein_type
Ca2	I-protein_type
+	I-protein_type
exchangers	I-protein_type
utilize	O
the	O
Na	B-chemical
+	I-chemical
electrochemical	O
gradient	O
across	O
the	O
plasma	O
membrane	O
to	O
extrude	O
intracellular	O
Ca2	B-chemical
+,	I-chemical
and	O
play	O
a	O
central	O
role	O
in	O
Ca2	B-chemical
+	I-chemical
homeostasis	O
.	O

This	O
analysis	O
defines	O
the	O
binding	O
mode	O
and	O
relative	O
affinity	O
of	O
these	O
ions	O
,	O
establishes	O
the	O
structural	O
basis	O
for	O
the	O
anticipated	O
3Na	O
+:	B-chemical
1Ca2	O
+	B-chemical
exchange	O
stoichiometry	O
,	O
and	O
reveals	O
the	O
conformational	O
changes	O
at	O
the	O
onset	O
of	O
the	O
alternating	O
-	O
access	O
transport	O
mechanism	O
.	O

An	O
independent	O
analysis	O
of	O
the	O
dynamics	O
and	O
conformational	B-evidence
free	I-evidence
-	I-evidence
energy	I-evidence
landscape	I-evidence
of	O
NCX_Mj	B-protein
in	O
different	O
ion	B-protein_state
-	I-protein_state
occupancy	I-protein_state
states	O
,	O
based	O
on	O
enhanced	B-experimental_method
-	I-experimental_method
sampling	I-experimental_method
molecular	I-experimental_method
-	I-experimental_method
dynamics	I-experimental_method
simulations	I-experimental_method
,	O
demonstrates	O
that	O
the	O
crystal	B-evidence
structures	I-evidence
reflect	O
mechanistically	O
relevant	O
,	O
interconverting	O
conformations	O
.	O

These	O
calculations	B-experimental_method
also	O
reveal	O
the	O
mechanism	O
by	O
which	O
the	O
outward	B-protein_state
-	O
to	O
-	O
inward	B-protein_state
transition	O
is	O
controlled	O
by	O
the	O
ion	O
-	O
occupancy	O
state	O
,	O
thereby	O
explaining	O
the	O
emergence	O
of	O
strictly	O
-	O
coupled	O
Na	B-chemical
+/	I-chemical
Ca2	B-chemical
+	I-chemical
antiport	O
.	O

Na	B-protein_type
+/	I-protein_type
Ca2	I-protein_type
+	I-protein_type
exchangers	I-protein_type
(	O
NCX	B-protein_type
)	O
play	O
physiologically	O
essential	O
roles	O
in	O
Ca2	B-chemical
+	I-chemical
signaling	O
and	O
homeostasis	O
.	O

NCX	B-protein_type
catalyzes	O
the	O
uphill	O
extrusion	O
of	O
intracellular	O
Ca2	B-chemical
+	I-chemical
across	O
the	O
cell	O
membrane	O
,	O
by	O
coupling	O
this	O
process	O
to	O
the	O
downhill	O
permeation	O
of	O
Na	B-chemical
+	I-chemical
into	O
the	O
cell	O
,	O
with	O
a	O
3	O
Na	B-chemical
+	I-chemical
to	O
1	O
Ca2	B-chemical
+	I-chemical
stoichiometry	O
.	O

Each	O
of	O
these	O
halves	B-structure_element
contains	O
a	O
highly	B-protein_state
conserved	I-protein_state
region	O
,	O
referred	O
to	O
as	O
α	B-structure_element
-	I-structure_element
repeat	I-structure_element
,	O
known	O
to	O
be	O
important	O
for	O
ion	O
binding	O
and	O
translocation	O
;	O
in	O
eukaryotic	B-taxonomy_domain
NCX	B-protein_type
,	O
the	O
two	O
halves	B-structure_element
are	O
connected	O
by	O
a	O
large	O
intracellular	B-structure_element
regulatory	I-structure_element
domain	I-structure_element
,	O
which	O
is	O
absent	B-protein_state
in	O
microbial	B-taxonomy_domain
NCX	B-protein_type
(	O
Supplementary	O
Fig	O
.	O
1	O
).	O

Despite	O
a	O
long	O
history	O
of	O
physiological	O
and	O
functional	O
studies	O
,	O
the	O
molecular	O
mechanism	O
of	O
NCX	B-protein_type
has	O
been	O
elusive	O
,	O
owing	O
to	O
the	O
lack	O
of	O
structural	O
information	O
.	O

This	O
structure	B-evidence
shows	O
the	O
exchanger	B-protein_type
in	O
an	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
conformation	O
and	O
reveals	O
four	O
putative	O
ion	B-site
-	I-site
binding	I-site
sites	I-site
,	O
denominated	O
internal	B-site
(	O
Sint	B-site
),	O
external	B-site
(	O
Sext	B-site
),	O
Ca2	B-site
+-	I-site
binding	I-site
(	O
SCa	B-site
)	O
and	O
middle	B-site
(	O
Smid	B-site
),	O
clustered	O
in	O
the	O
center	O
of	O
the	O
protein	O
and	O
occluded	B-protein_state
from	I-protein_state
the	O
solvent	O
(	O
Fig	O
.	O
1a	O
-	O
b	O
).	O

In	O
this	O
study	O
,	O
we	O
set	O
out	O
to	O
determine	O
the	O
structures	B-evidence
of	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
wild	B-protein_state
-	I-protein_state
type	I-protein_state
NCX_Mj	B-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
Na	B-chemical
+,	I-chemical
Ca2	B-chemical
+	I-chemical
and	O
Sr2	B-chemical
+,	I-chemical
at	O
various	O
concentrations	O
.	O

These	O
calculations	B-experimental_method
also	O
reveal	O
how	O
the	O
ion	O
occupancy	O
state	O
of	O
the	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
exchanger	B-protein_type
determines	O
the	O
feasibility	O
of	O
the	O
transition	O
to	O
the	O
inward	B-protein_state
-	I-protein_state
facing	I-protein_state
conformation	O
,	O
thereby	O
addressing	O
a	O
key	O
outstanding	O
question	O
in	O
secondary	O
-	O
active	O
transport	O
,	O
namely	O
how	O
the	O
transported	O
substrates	O
control	O
the	O
alternating	O
-	O
access	O
mechanism	O
.	O

The	O
assignment	O
of	O
the	O
four	O
central	B-site
binding	I-site
sites	I-site
identified	O
in	O
the	O
previously	O
reported	O
NCX_Mj	B-protein
structure	B-evidence
was	O
hampered	O
by	O
the	O
presence	O
of	O
both	O
Na	B-chemical
+	I-chemical
and	O
Ca2	B-chemical
+	I-chemical
in	O
the	O
protein	O
crystals	B-evidence
.	O

To	O
conclusively	O
clarify	O
this	O
assignment	O
,	O
we	O
first	O
set	O
out	O
to	O
examine	O
the	O
Na	B-chemical
+	I-chemical
occupancy	O
of	O
these	O
sites	O
without	O
Ca2	B-chemical
+.	I-chemical

Crystals	B-evidence
were	O
grown	O
in	O
150	O
mM	O
NaCl	B-chemical
using	O
the	O
lipidic	B-experimental_method
cubic	I-experimental_method
phase	I-experimental_method
(	O
LCP	B-experimental_method
)	O
technique	O
.	O

The	O
crystallization	O
solutions	O
around	O
the	O
LCP	B-experimental_method
droplets	O
were	O
then	O
slowly	O
replaced	O
by	O
solutions	O
containing	O
different	O
concentrations	O
of	O
NaCl	B-chemical
and	O
EGTA	B-chemical
(	O
Methods	O
).	O

X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
diffraction	I-experimental_method
of	O
these	O
soaked	O
crystals	B-evidence
revealed	O
a	O
Na	B-chemical
+-	I-chemical
dependent	O
variation	O
in	O
the	O
electron	B-evidence
-	I-evidence
density	I-evidence
distribution	I-evidence
at	O
sites	O
Sext	B-site
,	O
SCa	B-site
and	O
Sint	B-site
,	O
indicating	O
a	O
Na	B-chemical
+	I-chemical
occupancy	O
change	O
(	O
Fig	O
.	O
1c	O
).	O

Occupancy	B-experimental_method
refinement	I-experimental_method
indicated	O
two	O
Na	B-chemical
+	I-chemical
ions	O
bind	O
to	O
Sint	B-site
and	O
SCa	B-site
at	O
low	O
Na	B-chemical
+	I-chemical
concentrations	O
(	O
Fig	O
.	O
1c	O
),	O
with	O
a	O
slight	O
preference	O
for	O
Sint	B-site
(	O
Table	O
1	O
).	O

Binding	O
of	O
a	O
third	O
Na	B-chemical
+	I-chemical
to	O
Sext	B-site
occurs	O
at	O
higher	O
concentrations	O
,	O
as	O
no	O
density	B-evidence
was	O
observed	O
there	O
at	O
10	O
mM	O
Na	B-chemical
+	I-chemical
or	O
lower	O
(	O
Fig	O
.	O
1c	O
);	O
Sext	B-site
is	O
however	O
partially	O
occupied	O
at	O
20	O
mM	O
Na	B-chemical
+,	I-chemical
and	O
fully	O
occupied	O
at	O
150	O
mM	O
(	O
Fig	O
.	O
1c	O
).	O

Indeed	O
,	O
two	O
observations	O
indicate	O
that	O
a	O
water	B-chemical
molecule	O
rather	O
than	O
a	O
Na	B-chemical
+	I-chemical
ion	O
occupies	O
Smid	B-site
,	O
as	O
was	O
predicted	O
in	O
a	O
recent	O
simulation	B-experimental_method
study	O
.	O

First	O
,	O
the	O
electron	B-evidence
density	I-evidence
at	O
Smid	B-site
does	O
not	O
depend	O
significantly	O
on	O
the	O
Na	B-chemical
+	I-chemical
concentration	O
.	O

Second	O
,	O
the	O
protein	O
coordination	O
geometry	O
at	O
Smid	B-site
is	O
clearly	O
suboptimal	O
for	O
Na	B-chemical
+	I-chemical
(	O
Supplementary	O
Fig	O
.	O
1d	O
).	O

It	O
can	O
be	O
inferred	O
from	O
this	O
assignment	O
that	O
Glu54	B-residue_name_number
and	O
Glu213	B-residue_name_number
are	O
ionized	O
,	O
while	O
Asp240	B-residue_name_number
,	O
which	O
flanks	O
Smid	B-site
(	O
and	O
is	O
replaced	O
by	O
Asn	B-residue_name
in	O
eukaryotic	B-taxonomy_domain
NCX	B-protein_type
)	O
would	O
be	O
protonated	B-protein_state
,	O
as	O
indicated	O
by	O
the	O
abovementioned	O
simulation	B-experimental_method
study	O
.	O

Na	B-chemical
+-	I-chemical
dependent	O
conformational	O
change	O

The	O
NCX_Mj	B-protein
structures	B-evidence
in	O
various	O
Na	B-chemical
+	I-chemical
concentrations	O
also	O
reveal	O
that	O
Na	B-chemical
+	I-chemical
binding	O
to	O
Sext	B-site
is	O
coupled	O
to	O
a	O
subtle	O
but	O
important	O
conformational	O
change	O
(	O
Fig	O
.	O
2	O
).	O

TM7b	B-structure_element
occludes	O
the	O
four	O
central	B-site
binding	I-site
sites	I-site
from	O
the	O
external	O
solution	O
,	O
with	O
the	O
backbone	O
carbonyl	O
of	O
Ala206	B-residue_name_number
coordinating	B-bond_interaction
the	O
Na	B-chemical
+	I-chemical
ion	O
(	O
Fig	O
.	O
2b	O
-	O
d	O
).	O

TM7a	B-structure_element
also	O
forms	O
hydrophobic	B-bond_interaction
contacts	I-bond_interaction
with	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
half	I-structure_element
of	O
TM6	B-structure_element
.	O

The	O
straightening	O
of	O
TM7ab	B-structure_element
also	O
opens	O
up	O
a	O
passageway	O
from	O
the	O
external	O
solution	O
to	O
Sext	B-site
and	O
Smid	B-site
,	O
while	O
SCa	B-site
and	O
Sint	B-site
remain	O
occluded	B-protein_state
(	O
Fig	O
.	O
2d	O
).	O

Thus	O
,	O
the	O
structures	B-evidence
at	O
high	B-protein_state
and	O
low	B-protein_state
Na	B-chemical
+	I-chemical
concentrations	O
represent	O
the	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
occluded	B-protein_state
and	O
partially	B-protein_state
open	I-protein_state
states	O
,	O
respectively	O
.	O

This	O
conformational	O
change	O
is	O
dependent	O
on	O
the	O
Na	B-chemical
+	I-chemical
occupancy	O
of	O
Sext	B-site
and	O
occurs	O
when	O
Na	B-chemical
+	I-chemical
already	O
occupies	O
Sint	B-site
and	O
SCa	B-site
.	O

Our	O
crystallographic	B-experimental_method
titration	I-experimental_method
experiment	I-experimental_method
indicates	O
that	O
the	O
K1	B-evidence
/	I-evidence
2	I-evidence
of	O
this	O
Na	B-chemical
+-	I-chemical
driven	O
conformational	O
transition	O
is	O
~	O
20	O
mM	O
.	O
At	O
this	O
concentration	O
,	O
Sext	B-site
is	O
partially	B-protein_state
occupied	I-protein_state
and	O
the	O
NCX_Mj	B-protein
crystal	B-evidence
is	O
a	O
mixture	O
of	O
both	O
the	O
occluded	B-protein_state
and	O
partially	B-protein_state
open	I-protein_state
conformations	O
.	O

The	O
finding	O
that	O
the	O
Na	B-chemical
+	I-chemical
occupancy	O
change	O
from	O
2	O
to	O
3	O
ions	O
coincides	O
with	O
a	O
conformational	O
change	O
of	O
the	O
transporter	B-protein_type
also	O
provides	O
a	O
rationale	O
to	O
the	O
Hill	B-evidence
coefficient	I-evidence
of	O
the	O
Na	B-chemical
+-	I-chemical
dependent	O
activation	O
process	O
in	O
eukaryotic	B-taxonomy_domain
NCX	B-protein_type
.	O

Extracellular	O
Ca2	B-chemical
+	I-chemical
and	O
Sr2	B-chemical
+	I-chemical
binding	O
and	O
their	O
competition	O
with	O
Na	B-chemical
+	I-chemical

To	O
determine	O
how	O
Ca2	B-chemical
+	I-chemical
binds	O
to	O
NCX_Mj	B-protein
and	O
competes	O
with	O
Na	B-chemical
+,	I-chemical
we	O
first	O
titrated	B-experimental_method
the	I-experimental_method
crystals	I-experimental_method
with	O
Sr2	B-chemical
+	I-chemical
(	O
Methods	O
).	O

Protein	B-experimental_method
crystals	I-experimental_method
soaked	I-experimental_method
with	O
10	O
mM	O
Sr2	B-chemical
+	I-chemical
and	O
2	O
.	O
5	O
mM	O
Na	B-chemical
+	I-chemical
revealed	O
a	O
strong	O
electron	B-evidence
-	I-evidence
density	I-evidence
peak	I-evidence
at	O
site	O
SCa	B-site
,	O
indicating	O
binding	O
of	O
a	O
single	O
Sr2	B-chemical
+	I-chemical
ion	O
(	O
Fig	O
.	O
3a	O
).	O

Binding	O
of	O
Sr2	B-chemical
+,	I-chemical
however	O
,	O
excludes	O
Na	B-chemical
+	I-chemical
entirely	O
.	O

Crystal	B-experimental_method
titrations	I-experimental_method
with	O
decreasing	B-experimental_method
Sr2	B-chemical
+	I-chemical
or	O
increasing	B-experimental_method
Na	B-chemical
+	I-chemical
demonstrated	O
that	O
Sr2	B-chemical
+	I-chemical
binds	O
to	O
the	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
NCX_Mj	B-protein
with	O
low	O
affinity	O
,	O
and	O
that	O
it	O
can	O
be	O
out	O
-	O
competed	O
by	O
Na	B-chemical
+	I-chemical
even	O
at	O
low	O
concentrations	O
(	O
Supplementary	O
Note	O
1	O
and	O
Supplementary	O
Fig	O
.	O
2a	O
-	O
b	O
).	O

Binding	O
of	O
Ca2	B-chemical
+	I-chemical
to	O
both	O
sites	O
simultaneously	O
is	O
highly	O
improbable	O
due	O
to	O
their	O
close	O
proximity	O
,	O
and	O
at	O
least	O
one	O
water	B-chemical
molecule	O
can	O
be	O
discerned	O
coordinating	B-bond_interaction
the	O
ion	O
(	O
Fig	O
.	O
3b	O
).	O

The	O
partial	B-protein_state
Ca2	B-chemical
+	I-chemical
occupancy	B-protein_state
at	O
Smid	B-site
is	O
likely	O
caused	O
by	O
Asp240	B-residue_name_number
,	O
which	O
flanks	O
this	O
site	O
and	O
can	O
in	O
principle	O
coordinate	B-bond_interaction
Ca2	B-chemical
+.	I-chemical

Previous	O
functional	B-experimental_method
and	I-experimental_method
computational	I-experimental_method
studies	I-experimental_method
,	O
however	O
,	O
indicate	O
Asp240	B-residue_name_number
becomes	O
protonated	B-protein_state
during	O
transport	O
.	O

Indeed	O
,	O
in	O
most	O
NCX	B-protein_type
proteins	O
Asp240	B-residue_name_number
is	O
substituted	B-experimental_method
by	O
Asn	B-residue_name
,	O
which	O
would	O
likely	O
weaken	O
or	O
abrogate	O
Ca2	B-chemical
+	I-chemical
binding	O
to	O
Smid	B-site
.	O

SCa	B-site
is	O
therefore	O
the	O
functional	O
Ca2	B-site
+	I-site
site	I-site
.	O

Similarly	O
to	O
Sr2	B-chemical
+,	I-chemical
Ca2	B-chemical
+	I-chemical
binds	O
with	O
low	O
affinity	B-evidence
to	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
NCX_Mj	B-protein
and	O
can	O
be	O
readily	O
displaced	O
by	O
Na	B-chemical
+	I-chemical
(	O
Supplementary	O
Note	O
1	O
and	O
Supplementary	O
Fig	O
.	O
2c	O
).	O

Specifically	O
,	O
our	O
crystallographic	B-experimental_method
titration	I-experimental_method
assay	I-experimental_method
indicates	O
Ca2	B-chemical
+	I-chemical
binds	O
with	O
sub	O
-	O
millimolar	O
affinity	B-evidence
,	O
in	O
good	O
agreement	O
with	O
the	O
external	O
apparent	O
Ca2	B-evidence
+	I-evidence
affinities	I-evidence
deduced	O
functionally	O
for	O
cardiac	O
NCX	B-protein_type
(	O
Km	B-evidence
~	O
0	O
.	O
32	O
mM	O
)	O
and	O
NCX_Mj	B-protein
(	O
Km	B-evidence
~	O
0	O
.	O
175	O
mM	O
).	O

Taken	O
together	O
,	O
these	O
crystal	B-experimental_method
titration	I-experimental_method
experiments	I-experimental_method
demonstrate	O
that	O
the	O
four	O
binding	B-site
sites	I-site
in	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
NCX_Mj	B-protein
exhibit	O
different	O
specificity	O
:	O
Sint	B-site
and	O
Sext	B-site
are	O
Na	B-chemical
+	I-chemical
specific	O
whereas	O
SCa	B-site
,	O
previously	O
hypothesized	O
to	O
be	O
Ca2	B-chemical
+	I-chemical
specific	O
,	O
can	O
also	O
bind	O
Na	B-chemical
+,	I-chemical
confirming	O
our	O
earlier	O
simulation	B-experimental_method
study	O
,	O
as	O
well	O
as	O
Sr2	B-chemical
+;	I-chemical
Smid	B-site
can	O
also	O
transiently	O
accommodate	O
Ca2	B-chemical
+	I-chemical
but	O
during	O
transport	O
Smid	B-site
is	O
most	O
likely	O
occupied	O
by	O
water	B-chemical
.	O

The	O
ion	B-site
-	I-site
binding	I-site
sites	I-site
in	O
NCX_Mj	B-protein
can	O
therefore	O
accommodate	O
up	O
to	O
three	O
Na	B-chemical
+	I-chemical
ions	O
or	O
a	O
single	O
divalent	O
ion	O
,	O
and	O
occupancy	O
by	O
Na	B-chemical
+	I-chemical
and	O
Ca2	B-chemical
+	I-chemical
(	O
or	O
Sr2	B-chemical
+)	I-chemical
are	O
mutually	O
exclusive	O
,	O
as	O
was	O
deduced	O
for	O
eukaryotic	B-taxonomy_domain
exchangers	B-protein_type
.	O

A	O
structure	B-evidence
of	O
NCX_Mj	B-protein
without	B-protein_state
Na	B-chemical
+	I-chemical
or	O
Ca2	B-chemical
+	I-chemical
bound	B-protein_state

We	O
were	O
able	O
to	O
determine	O
an	O
apo	B-protein_state
-	O
state	O
structure	B-evidence
of	O
NCX_Mj	B-protein
,	O
by	O
crystallizing	B-experimental_method
the	O
protein	O
at	O
lower	B-protein_state
pH	I-protein_state
and	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
Na	B-chemical
+	I-chemical
(	O
Methods	O
).	O

This	O
structure	B-evidence
is	O
similar	O
to	O
the	O
partially	B-protein_state
open	I-protein_state
structure	B-evidence
with	O
two	O
Na	B-chemical
+	I-chemical
or	O
either	O
one	O
Ca2	B-chemical
+	I-chemical
or	O
one	O
Sr2	B-chemical
+	I-chemical
ion	O
,	O
with	O
two	O
noticeable	O
differences	O
.	O

First	O
,	O
TM7ab	B-structure_element
along	O
with	O
the	O
extracellular	B-structure_element
half	I-structure_element
of	O
the	O
TM6	B-structure_element
and	O
TM1	B-structure_element
swing	O
further	O
away	O
from	O
the	O
protein	O
core	O
(	O
Fig	O
.	O
3c	O
),	O
resulting	O
in	O
a	O
slightly	O
wider	O
passageway	O
into	O
the	O
binding	B-site
sites	I-site
.	O

Second	O
,	O
Glu54	B-residue_name_number
and	O
Glu213	B-residue_name_number
side	O
chains	O
rotate	O
away	O
from	O
the	O
binding	B-site
sites	I-site
and	O
appear	O
to	O
form	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
with	O
residues	O
involved	O
in	O
ion	B-bond_interaction
coordination	I-bond_interaction
in	O
the	O
fully	B-protein_state
Na	I-protein_state
+-	I-protein_state
loaded	I-protein_state
structure	B-evidence
(	O
Fig	O
.	O
3d	O
).	O

This	O
apo	B-protein_state
structure	B-evidence
might	O
therefore	O
represent	O
the	O
unloaded	B-protein_state
,	O
open	B-protein_state
state	O
of	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
NCX_Mj	B-protein
.	O

Such	O
interpretation	O
would	O
be	O
consistent	O
with	O
the	O
computer	B-experimental_method
simulations	I-experimental_method
reported	O
below	O
.	O

Indeed	O
,	O
transport	B-experimental_method
assays	I-experimental_method
of	O
NCX_Mj	B-protein
have	O
shown	O
that	O
even	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
Na	B-chemical
+	I-chemical
or	O
Ca2	B-chemical
+,	I-chemical
low	B-protein_state
pH	I-protein_state
inactivates	B-protein_state
the	O
transport	O
cycle	O
.	O

NCX	B-protein_type
must	O
be	O
loaded	O
either	O
with	O
3	O
Na	B-chemical
+	I-chemical
or	O
1	O
Ca2	B-chemical
+,	I-chemical
and	O
therefore	O
functions	O
as	O
an	O
antiporter	B-protein_type
;	O
symporters	B-protein_type
,	O
by	O
contrast	O
,	O
undergo	O
the	O
alternating	O
-	O
access	O
transition	O
only	O
when	O
all	O
substrates	O
and	O
coupling	O
ions	O
are	O
concurrently	O
bound	B-protein_state
,	O
or	O
in	O
the	O
apo	B-protein_state
state	O
.	O

This	O
computational	O
analysis	O
was	O
based	O
solely	O
on	O
the	O
published	O
structure	B-evidence
of	O
NCX_Mj	B-protein
,	O
independently	O
of	O
the	O
crystallographic	B-experimental_method
studies	I-experimental_method
described	O
above	O
.	O

A	O
series	O
of	O
exploratory	O
MD	B-experimental_method
simulations	I-experimental_method
was	O
initially	O
carried	O
out	O
to	O
examine	O
what	O
features	O
of	O
the	O
NCX_Mj	B-protein
structure	B-evidence
might	O
depend	O
on	O
the	O
ion	B-site
-	I-site
binding	I-site
sites	I-site
occupancy	O
.	O

The	O
Na	B-chemical
+	I-chemical
ions	O
at	O
SCa	B-site
and	O
Sint	B-site
were	O
displaced	O
subsequently	O
,	O
and	O
an	O
analogous	O
simulation	B-experimental_method
was	O
then	O
carried	O
out	O
.	O

These	O
initial	O
simulations	B-experimental_method
revealed	O
noticeable	O
changes	O
in	O
the	O
transporter	B-protein_type
,	O
consistent	O
with	O
those	O
observed	O
in	O
the	O
new	O
crystal	B-evidence
structures	I-evidence
.	O

The	O
most	O
notable	O
change	O
upon	O
displacement	O
of	O
Na	B-chemical
+	I-chemical
from	O
Sext	B-site
was	O
the	O
straightening	O
of	O
TM7ab	B-structure_element
(	O
Fig	O
.	O
4a	O
).	O

When	O
3	O
Na	B-chemical
+	I-chemical
ions	O
are	O
bound	B-protein_state
,	O
TM7ab	B-structure_element
primarily	O
folds	O
as	O
two	O
distinct	O
,	O
non	O
-	O
collinear	O
α	B-structure_element
-	I-structure_element
helical	I-structure_element
fragments	I-structure_element
,	O
owing	O
to	O
the	O
loss	O
of	O
the	O
backbone	O
carbonyl	O
-	O
amide	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonds	I-bond_interaction
between	O
F202	B-residue_name_number
and	O
A206	B-residue_name_number
,	O
and	O
T203	B-residue_name_number
and	O
F207	B-residue_name_number
(	O
Fig	O
.	O
4b	O
).	O

This	O
distortion	O
occludes	O
Sext	B-site
from	O
the	O
exterior	O
(	O
Fig	O
.	O
4d	O
,	O
4h	O
-	O
i	O
)	O
and	O
appears	O
to	O
be	O
induced	O
by	O
the	O
Na	B-chemical
+	I-chemical
ion	O
itself	O
,	O
which	O
pulls	O
the	O
carbonyl	O
group	O
of	O
A206	B-residue_name_number
into	O
its	O
coordination	O
sphere	O
(	O
Fig	O
.	O
4g	O
).	O

Together	O
,	O
these	O
changes	O
open	O
a	O
second	O
aqueous	B-site
channel	I-site
leading	O
directly	O
into	O
SCa	B-site
and	O
Sint	B-site
(	O
Fig	O
.	O
4f	O
,	O
Fig	O
.	O
4h	O
-	O
i	O
).	O

The	O
transporter	B-protein_type
thus	O
becomes	O
fully	B-protein_state
outward	I-protein_state
-	I-protein_state
open	I-protein_state
.	O

To	O
more	O
rigorously	O
characterize	O
the	O
influence	O
of	O
the	O
ion	O
-	O
occupancy	O
state	O
on	O
the	O
conformational	O
dynamics	O
of	O
the	O
exchanger	B-protein_type
,	O
we	O
carried	O
out	O
a	O
series	O
of	O
enhanced	O
-	O
sampling	O
MD	B-experimental_method
calculations	I-experimental_method
designed	O
to	O
reversibly	O
simulate	O
the	O
transition	O
between	O
the	O
outward	B-protein_state
-	I-protein_state
occluded	I-protein_state
and	O
fully	B-protein_state
outward	I-protein_state
-	I-protein_state
open	I-protein_state
states	O
,	O
and	O
thus	O
quantify	O
the	O
free	B-evidence
-	I-evidence
energy	I-evidence
landscape	I-evidence
encompassing	O
these	O
states	O
(	O
Methods	O
).	O

As	O
above	O
,	O
we	O
initially	O
examined	O
three	O
occupancy	O
states	O
,	O
namely	O
with	O
Na	B-chemical
+	I-chemical
in	O
Sext	B-site
,	O
SCa	B-site
and	O
Sint	B-site
,	O
with	O
Na	B-chemical
+	I-chemical
only	O
at	O
SCa	B-site
and	O
Sint	B-site
,	O
and	O
without	B-protein_state
Na	B-chemical
+.	I-chemical

When	O
all	O
Na	B-site
+	I-site
sites	I-site
are	O
occupied	O
,	O
the	O
global	O
free	B-evidence
-	I-evidence
energy	I-evidence
minimum	I-evidence
corresponds	O
to	O
a	O
conformation	O
in	O
which	O
the	O
ions	O
are	O
maximally	O
coordinated	O
by	O
the	O
protein	O
(	O
Fig	O
.	O
5a	O
,	O
5c	O
);	O
TM7ab	B-structure_element
is	O
bent	O
and	O
packs	O
closely	O
with	O
TM2	B-structure_element
and	O
TM3	B-structure_element
,	O
and	O
so	O
the	O
binding	B-site
sites	I-site
are	O
occluded	O
from	O
the	O
solvent	O
(	O
Fig	O
.	O
5b	O
).	O

The	O
Na	B-chemical
+	I-chemical
ion	O
at	O
Sext	B-site
remains	O
fully	B-protein_state
coordinated	I-protein_state
,	O
but	O
an	O
ordered	O
water	B-chemical
molecule	O
now	O
mediates	O
its	O
interaction	O
with	O
A206	B-residue_name_number
:	O
O	O
,	O
relieving	O
the	O
strain	O
on	O
the	O
F202	B-residue_name_number
:	O
O	O
–	O
A206	B-residue_name_number
:	O
N	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bond	I-bond_interaction
(	O
Fig	O
.	O
5c	O
).	O

Interestingly	O
,	O
this	O
doubly	O
occupied	O
state	O
can	O
also	O
access	O
conformations	O
in	O
which	O
the	O
second	O
aqueous	B-site
channel	I-site
mentioned	O
above	O
,	O
i	O
.	O
e	O
.	O
leading	O
to	O
SCa	B-site
between	O
TM7	B-structure_element
and	O
TM2	B-structure_element
and	O
over	O
the	O
gating	B-structure_element
helices	I-structure_element
TM1	B-structure_element
and	O
TM6	B-structure_element
,	O
also	O
becomes	O
open	B-protein_state
(	O
Fig	O
.	O
5b	O
-	O
c	O
).	O

Crucially	O
,	O
though	O
,	O
the	O
free	B-evidence
-	I-evidence
energy	I-evidence
landscape	I-evidence
for	O
this	O
partially	B-protein_state
occupied	I-protein_state
state	O
demonstrates	O
that	O
the	O
occluded	B-protein_state
conformation	O
is	O
no	O
longer	O
energetically	O
feasible	O
(	O
Fig	O
.	O
5a	O
).	O

Displacement	O
of	O
the	O
two	O
remaining	O
Na	B-chemical
+	I-chemical
ions	O
from	O
SCa	B-site
and	O
Sint	B-site
further	O
reshapes	O
the	O
free	B-evidence
-	I-evidence
energy	I-evidence
landscape	I-evidence
of	O
the	O
transporter	B-protein_type
(	O
No	O
ions	O
,	O
Fig	O
.	O
5a	O
),	O
which	O
now	O
can	O
only	O
adopt	O
a	O
fully	B-protein_state
open	I-protein_state
state	O
featuring	O
the	O
two	O
aqueous	B-site
channels	I-site
(	O
Fig	O
.	O
5b	O
-	O
c	O
).	O

The	O
transition	O
to	O
the	O
occluded	B-protein_state
state	O
in	O
this	O
apo	B-protein_state
state	O
is	O
again	O
energetically	O
unfeasible	O
.	O

From	O
a	O
mechanistic	O
standpoint	O
,	O
it	O
is	O
satisfying	O
to	O
observe	O
how	O
the	O
open	B-protein_state
and	O
semi	B-protein_state
-	I-protein_state
open	I-protein_state
states	O
are	O
each	O
compatible	O
with	O
two	O
different	O
Na	B-chemical
+	I-chemical
occupancies	O
,	O
explaining	O
how	O
sequential	O
Na	B-chemical
+	I-chemical
binding	O
to	O
energetically	O
accessible	O
conformations	O
(	O
prior	O
to	O
those	O
binding	O
events	O
)	O
progressively	O
reshape	O
the	O
free	B-evidence
-	I-evidence
energy	I-evidence
landscape	I-evidence
of	O
the	O
transporter	B-protein_type
;	O
by	O
contrast	O
,	O
the	O
occluded	B-protein_state
conformation	O
is	O
forbidden	O
unless	O
the	O
Na	B-protein_state
+	I-protein_state
occupancy	I-protein_state
is	I-protein_state
complete	I-protein_state
.	O

This	O
processivity	O
is	O
logical	O
since	O
three	O
Na	B-chemical
+	I-chemical
ions	O
are	O
involved	O
,	O
but	O
also	O
implies	O
that	O
in	O
the	O
Ca2	B-protein_state
+-	I-protein_state
bound	I-protein_state
state	O
,	O
which	O
includes	O
a	O
single	O
ion	O
,	O
the	O
transporter	B-protein_type
ought	O
to	O
be	O
able	O
to	O
access	O
all	O
three	O
major	O
conformations	O
,	O
i	O
.	O
e	O
.	O
the	O
outward	B-protein_state
-	I-protein_state
open	I-protein_state
state	O
,	O
in	O
order	O
to	O
release	O
(	O
or	O
re	O
-	O
bind	O
)	O
Ca2	B-chemical
+,	I-chemical
but	O
also	O
the	O
occluded	B-protein_state
conformation	O
,	O
and	O
thus	O
the	O
semi	B-protein_state
-	I-protein_state
open	I-protein_state
intermediate	O
,	O
in	O
order	O
to	O
transition	O
to	O
the	O
inward	B-protein_state
-	I-protein_state
open	I-protein_state
state	O
.	O

By	O
contrast	O
,	O
occupancy	O
by	O
H	B-chemical
+,	I-chemical
which	O
as	O
mentioned	O
are	O
not	O
transported	O
,	O
might	O
be	O
compatible	O
with	O
a	O
semi	B-protein_state
-	I-protein_state
open	I-protein_state
state	O
as	O
well	O
as	O
with	O
the	O
fully	B-protein_state
open	I-protein_state
conformation	O
,	O
but	O
should	O
not	O
be	O
conducive	O
to	O
occlusion	O
.	O

To	O
assess	O
this	O
hypothesis	O
,	O
we	O
carried	O
out	O
enhanced	B-experimental_method
-	I-experimental_method
sampling	I-experimental_method
simulations	I-experimental_method
for	O
the	O
Ca2	B-protein_state
+	I-protein_state
and	O
H	B-protein_state
+-	I-protein_state
bound	I-protein_state
states	O
of	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
NCX_Mj	B-protein
analogous	O
to	O
those	O
described	O
above	O
for	O
Na	B-chemical
+	I-chemical
(	O
see	O
Supplementary	O
Note	O
2	O
and	O
Supplementary	O
Fig	O
.	O
3	O
-	O
4	O
for	O
details	O
on	O
how	O
the	O
structures	B-evidence
of	O
the	O
Ca2	B-protein_state
+-	I-protein_state
bound	I-protein_state
state	O
was	O
predicted	O
).	O

The	O
calculated	B-experimental_method
free	B-evidence
-	I-evidence
energy	I-evidence
landscape	I-evidence
for	O
Ca2	B-protein_state
+-	I-protein_state
bound	I-protein_state
NCX_Mj	B-protein
confirms	O
the	O
hypothesis	O
outlined	O
above	O
(	O
1	O
×	O
Ca2	B-chemical
+,	I-chemical
Fig	O
.	O
6a	O
):	O
consistent	O
with	O
the	O
fact	O
that	O
NCX_Mj	B-protein
transports	O
a	O
single	O
Ca2	B-chemical
+,	I-chemical
the	O
occluded	B-protein_state
,	O
dehydrated	B-protein_state
conformation	O
is	O
one	O
of	O
the	O
major	O
energetic	O
minima	O
,	O
but	O
clearly	O
the	O
exchanger	B-protein_type
can	O
also	O
adopt	O
the	O
semi	B-protein_state
-	I-protein_state
open	I-protein_state
and	O
open	B-protein_state
states	O
that	O
would	O
be	O
required	O
for	O
Ca2	B-chemical
+	I-chemical
release	O
and	O
Na	B-chemical
+	I-chemical
entry	O
,	O
via	O
either	O
of	O
the	O
aqueous	B-site
access	I-site
channels	I-site
that	O
lead	O
to	O
Sext	B-site
and	O
SCa	B-site
(	O
Fig	O
.	O
6b	O
-	O
c	O
).	O

By	O
contrast	O
,	O
protonation	B-protein_state
of	O
Glu54	B-residue_name_number
and	O
Glu213	B-residue_name_number
makes	O
the	O
occluded	B-protein_state
conformation	O
energetically	O
unfeasible	O
,	O
consistent	O
with	O
the	O
fact	O
that	O
NCX_Mj	B-protein
does	O
not	O
transport	O
protons	B-chemical
;	O
in	O
this	O
H	B-protein_state
+-	I-protein_state
bound	I-protein_state
state	O
,	O
though	O
,	O
the	O
exchanger	B-protein_type
can	O
adopt	O
the	O
semi	B-protein_state
-	I-protein_state
open	I-protein_state
conformation	O
captured	O
in	O
the	O
low	B-protein_state
pH	I-protein_state
,	O
apo	B-protein_state
crystal	B-evidence
structure	I-evidence
(	O
2	O
×	O
H	B-chemical
+,	I-chemical
Fig	O
.	O
6a	O
-	O
c	O
).	O

Taken	O
together	O
,	O
this	O
systematic	B-experimental_method
computational	I-experimental_method
analysis	I-experimental_method
of	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
NCX_Mj	B-protein
clearly	O
demonstrates	O
that	O
the	O
alternating	O
-	O
access	O
and	O
ion	O
-	O
recognition	O
mechanisms	O
in	O
this	O
Na	B-protein_type
+/	I-protein_type
Ca2	I-protein_type
+	I-protein_type
exchanger	I-protein_type
are	O
coupled	O
through	O
the	O
influence	O
that	O
the	O
bound	O
ions	O
have	O
on	O
the	O
free	B-evidence
-	I-evidence
energy	I-evidence
landscape	I-evidence
of	O
the	O
protein	O
,	O
which	O
in	O
turn	O
determines	O
whether	O
or	O
not	O
the	O
occluded	B-protein_state
conformation	O
is	O
energetically	O
feasible	O
.	O

The	O
alternating	O
-	O
access	O
hypothesis	O
implicitly	O
dictates	O
that	O
the	O
switch	O
between	O
outward	B-protein_state
-	O
and	O
inward	B-protein_state
-	I-protein_state
open	I-protein_state
conformations	O
of	O
a	O
given	O
secondary	B-protein_state
-	I-protein_state
active	I-protein_state
transporter	B-protein_type
must	O
not	O
occur	O
unless	O
the	O
appropriate	O
type	O
and	O
number	O
of	O
substrates	O
are	O
recognized	O
.	O

It	O
is	O
however	O
also	O
non	O
-	O
trivial	O
:	O
antiporters	B-protein_type
,	O
for	O
example	O
,	O
do	O
not	O
undergo	O
the	O
alternating	O
-	O
access	O
transition	O
without	O
a	O
cargo	O
,	O
but	O
this	O
is	O
precisely	O
how	O
membrane	B-protein_type
symporters	I-protein_type
reset	O
their	O
transport	O
cycles	O
.	O

Here	O
,	O
we	O
have	O
provided	O
novel	O
insights	O
into	O
this	O
intriguing	O
mechanism	O
of	O
conformational	O
control	O
through	O
structural	B-experimental_method
studies	I-experimental_method
and	O
quantitative	B-experimental_method
molecular	I-experimental_method
simulations	I-experimental_method
of	O
a	O
Na	B-protein_type
+/	I-protein_type
Ca2	I-protein_type
+	I-protein_type
exchanger	I-protein_type
.	O

Specifically	O
,	O
our	O
studies	O
of	O
NCX_Mj	B-protein
reveal	O
the	O
mechanism	O
of	O
forward	O
ion	O
exchange	O
(	O
Fig	O
.	O
7	O
).	O

The	O
internal	O
symmetry	O
of	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
NCX_Mj	B-protein
and	O
the	O
inward	B-protein_state
-	I-protein_state
facing	I-protein_state
crystal	B-evidence
structures	I-evidence
of	O
several	O
Ca2	B-protein_type
+/	I-protein_type
H	I-protein_type
+	I-protein_type
exchangers	I-protein_type
indicate	O
that	O
the	O
alternating	O
-	O
access	O
mechanism	O
of	O
NCX	B-protein_type
proteins	O
entails	O
a	O
sliding	O
motion	O
of	O
TM1	B-structure_element
and	O
TM6	B-structure_element
relative	O
to	O
the	O
rest	O
of	O
the	O
transporter	B-protein_type
.	O

Here	O
,	O
we	O
demonstrate	O
that	O
conformational	O
changes	O
in	O
the	O
extracellular	B-structure_element
region	I-structure_element
of	O
the	O
TM2	B-structure_element
-	I-structure_element
TM3	I-structure_element
and	O
TM7	B-structure_element
-	I-structure_element
TM8	I-structure_element
bundle	I-structure_element
precede	O
and	O
are	O
necessary	O
for	O
the	O
transition	O
,	O
and	O
are	O
associated	O
with	O
ion	O
recognition	O
and	O
/	O
or	O
release	O
.	O

The	O
most	O
apparent	O
of	O
these	O
changes	O
involves	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
half	I-structure_element
of	O
TM7	B-structure_element
(	O
TM7ab	B-structure_element
);	O
together	O
with	O
more	O
subtle	O
displacements	O
in	O
TM2	B-structure_element
and	O
TM3	B-structure_element
,	O
this	O
change	O
in	O
TM7ab	B-structure_element
correlates	O
with	O
the	O
opening	O
and	O
closing	O
of	O
two	O
distinct	O
aqueous	B-site
channels	I-site
leading	O
into	O
the	O
ion	B-site
-	I-site
binding	I-site
sites	I-site
from	O
the	O
extracellular	O
solution	O
.	O

Interestingly	O
,	O
the	O
bending	O
of	O
TM7	B-structure_element
associated	O
with	O
the	O
occlusion	O
of	O
the	O
ion	B-site
-	I-site
binding	I-site
sites	I-site
also	O
unlocks	O
its	O
interaction	O
with	O
TM6	B-structure_element
,	O
and	O
thus	O
enables	O
TM6	B-structure_element
and	O
TM1	B-structure_element
to	O
freely	O
slide	O
to	O
the	O
inward	B-protein_state
-	I-protein_state
facing	I-protein_state
conformation	O
.	O

The	O
crystal	B-evidence
structures	I-evidence
of	O
NCX_Mj	B-protein
reported	O
here	O
,	O
with	O
either	O
Na	B-chemical
+,	I-chemical
Ca2	B-chemical
+,	I-chemical
Sr2	B-chemical
+	I-chemical
or	O
H	B-chemical
+	I-chemical
bound	B-protein_state
,	O
capture	O
the	O
exchanger	B-protein_type
in	O
different	O
conformational	O
states	O
.	O

These	O
states	O
can	O
only	O
represent	O
a	O
subset	O
among	O
all	O
possible	O
,	O
but	O
they	O
ought	O
to	O
reflect	O
inherent	O
preferences	O
of	O
the	O
transporter	B-protein_type
,	O
modulated	O
by	O
the	O
experimental	O
conditions	O
.	O

Indeed	O
,	O
we	O
show	O
that	O
it	O
is	O
the	O
presence	O
or	O
absence	O
of	O
the	O
occluded	B-protein_state
state	O
in	O
this	O
landscape	O
that	O
explains	O
the	O
antiport	O
function	O
of	O
NCX_Mj	B-protein
and	O
its	O
3Na	O
+:	B-chemical
1Ca2	O
+	B-chemical
stoichiometry	O
.	O

In	O
multiple	O
ways	O
,	O
our	O
findings	O
provide	O
an	O
explanation	O
for	O
,	O
existing	O
functional	O
,	O
biochemical	O
and	O
biophysical	O
data	O
for	O
both	O
NCX_Mj	B-protein
and	O
its	O
eukaryotic	B-taxonomy_domain
homologues	O
.	O

The	O
striking	O
quantitative	O
agreement	O
between	O
the	O
ion	B-evidence
-	I-evidence
binding	I-evidence
affinities	I-evidence
inferred	O
from	O
our	O
crystallographic	B-experimental_method
titrations	I-experimental_method
and	O
the	O
Km	B-evidence
and	O
K1	B-evidence
/	I-evidence
2	I-evidence
values	I-evidence
previously	O
deduced	O
from	O
functional	B-experimental_method
assays	I-experimental_method
has	O
been	O
discussed	O
above	O
.	O

Consistent	O
with	O
that	O
finding	O
,	O
mutations	O
that	O
have	O
been	O
shown	O
to	O
inactivate	O
or	O
diminish	O
the	O
transport	O
activity	O
of	O
NCX_Mj	B-protein
and	O
cardiac	O
NCX	B-protein_type
perfectly	O
map	O
to	O
the	O
first	O
ion	O
-	O
coordination	O
shell	O
in	O
our	O
NCX_Mj	B-protein
structures	B-evidence
(	O
Supplementary	O
Fig	O
.	O
4c	O
-	O
d	O
).	O

The	O
crystallographic	B-evidence
data	I-evidence
also	O
provides	O
the	O
long	O
-	O
sought	O
structural	O
basis	O
for	O
the	O
‘	O
two	O
-	O
site	O
’	O
model	O
proposed	O
to	O
describe	O
competitive	O
cation	O
binding	O
in	O
eukaryotic	B-taxonomy_domain
NCX	B-protein_type
,	O
underscoring	O
the	O
relevance	O
of	O
these	O
studies	O
of	O
NCX_Mj	B-protein
as	O
a	O
prototypical	O
Na	B-protein_type
+/	I-protein_type
Ca2	I-protein_type
+	I-protein_type
exchanger	I-protein_type
.	O

The	O
Sext	B-site
site	O
,	O
by	O
contrast	O
,	O
might	O
be	O
thought	O
as	O
an	O
activation	B-site
site	I-site
for	O
inward	O
Na	B-chemical
+	I-chemical
translocation	O
,	O
since	O
this	O
is	O
where	O
the	O
third	O
Na	B-chemical
+	I-chemical
ion	O
binds	O
at	O
high	O
Na	B-chemical
+	I-chemical
concentration	O
,	O
enabling	O
the	O
transition	O
to	O
the	O
occluded	B-protein_state
state	O
.	O

Indeed	O
,	O
structures	B-evidence
of	O
NCX_Mj	B-protein
bound	B-protein_state
to	I-protein_state
Cd2	B-chemical
+	I-chemical
or	O
Mn2	B-chemical
+,	I-chemical
both	O
of	O
which	O
inhibit	O
transport	O
,	O
show	O
these	O
ions	O
at	O
Smid	B-site
;	O
by	O
contrast	O
,	O
Sr2	B-chemical
+	I-chemical
binds	O
only	O
to	O
SCa	B-site
,	O
and	O
accordingly	O
,	O
is	O
transported	O
by	O
NCX	B-protein_type
similarly	O
to	O
calcium	B-chemical
.	O

Lastly	O
,	O
our	O
theory	O
that	O
occlusion	O
of	O
NCX_Mj	B-protein
is	O
selectively	O
induced	O
upon	O
Ca2	B-chemical
+	I-chemical
or	O
Na	B-chemical
+	I-chemical
recognition	O
is	O
consonant	O
with	O
a	O
recent	O
analysis	O
of	O
the	O
rate	O
of	O
hydrogen	B-experimental_method
-	I-experimental_method
deuterium	I-experimental_method
exchange	I-experimental_method
(	O
HDX	B-experimental_method
)	O
in	O
NCX_Mj	B-protein
,	O
in	O
the	O
presence	B-protein_state
or	O
absence	B-protein_state
of	I-protein_state
these	O
ions	O
,	O
in	O
conditions	O
that	O
favor	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
conformations	O
.	O

We	O
interpret	O
these	O
observations	O
as	O
reflecting	O
that	O
the	O
solvent	O
accessibility	O
of	O
the	O
protein	O
interior	O
is	O
diminished	O
upon	O
ion	O
recognition	O
,	O
consistent	O
with	O
our	O
finding	O
that	O
opening	O
and	O
closing	O
of	O
extracellular	O
aqueous	O
pathways	O
to	O
the	O
ion	B-site
-	I-site
binding	I-site
sites	I-site
depend	O
on	O
ion	O
occupancy	O
state	O
.	O

In	O
addition	O
,	O
the	O
increased	O
compactness	O
of	O
the	O
protein	O
tertiary	O
structure	O
in	O
the	O
occluded	B-protein_state
state	O
would	O
also	O
slow	O
down	O
the	O
dynamics	O
of	O
the	O
secondary	O
-	O
structure	O
elements	O
,	O
and	O
thus	O
further	O
reduce	O
the	O
HDX	B-evidence
rate	I-evidence
.	O

Our	O
data	O
would	O
also	O
explain	O
the	O
observation	O
that	O
the	O
reduction	O
in	O
the	O
HDX	B-evidence
rate	I-evidence
is	O
comparable	O
for	O
Na	B-chemical
+	I-chemical
and	O
Ca2	B-chemical
+,	I-chemical
as	O
well	O
as	O
the	O
finding	O
that	O
the	O
degree	O
of	O
deuterium	O
incorporation	O
remains	O
non	O
-	O
negligible	O
even	O
under	O
saturating	O
ion	O
concentrations	O
.	O

As	O
the	O
calculated	B-evidence
free	I-evidence
-	I-evidence
energy	I-evidence
landscapes	I-evidence
show	O
,	O
Na	B-chemical
+	I-chemical
and	O
Ca2	B-chemical
+	I-chemical
induce	O
the	O
occlusion	O
of	O
the	O
transporter	B-protein_type
in	O
a	O
comparable	O
manner	O
,	O
and	O
yet	O
the	O
ion	B-protein_state
-	I-protein_state
bound	I-protein_state
states	O
retain	O
the	O
ability	O
to	O
explore	O
conformations	O
that	O
are	O
partially	O
or	O
fully	B-protein_state
open	I-protein_state
to	O
the	O
extracellular	O
solution	O
,	O
precisely	O
so	O
as	O
to	O
be	O
able	O
to	O
unload	O
and	O
re	O
-	O
load	O
the	O
substrates	O
.	O

Colored	O
spheres	O
represent	O
the	O
bound	O
Na	B-chemical
+	I-chemical
(	O
green	O
)	O
and	O
water	B-chemical
(	O
red	O
).	O

(	O
b	O
)	O
Structural	O
details	O
and	O
definition	O
of	O
the	O
four	O
central	B-site
binding	I-site
sites	I-site
.	O

Further	O
details	O
are	O
shown	O
in	O
Supplementary	O
Fig	O
.	O
1	O
.	O
(	O
c	O
)	O
Concentration	O
-	O
dependent	O
change	O
in	O
Na	B-chemical
+	I-chemical
occupancy	O
(	O
see	O
also	O
Table	O
1	O
).	O

All	O
Fo	B-evidence
–	I-evidence
Fc	I-evidence
ion	I-evidence
-	I-evidence
omit	I-evidence
maps	I-evidence
are	O
calculated	O
to	O
2	O
.	O
4	O
Å	O
and	O
contoured	O
at	O
3σ	O
for	O
comparison	O
.	O

At	O
20	O
mM	O
Na	B-chemical
+,	I-chemical
both	O
conformations	O
co	O
-	O
exist	O
.	O

Na	B-chemical
+-	I-chemical
occupancy	O
dependent	O
conformational	O
change	O
in	O
NCX_Mj	B-protein
.	O

(	O
a	O
)	O
Superimposition	B-experimental_method
of	O
the	O
NCX_Mj	B-protein
crystal	B-evidence
structures	I-evidence
obtained	O
in	O
high	O
(	O
100	O
mM	O
,	O
cyan	O
cylinders	O
)	O
and	O
low	O
(	O
10	O
mM	O
,	O
brown	O
cylinders	O
)	O
Na	B-chemical
+	I-chemical
concentrations	O
.	O

(	O
b	O
)	O
Close	O
-	O
up	O
view	O
of	O
the	O
interface	B-site
between	O
TM6	B-structure_element
and	O
TM7ab	B-structure_element
in	O
the	O
NCX_Mj	B-protein
structures	B-evidence
obtained	O
at	O
high	O
and	O
low	O
Na	B-chemical
+	I-chemical
concentrations	O
(	O
top	O
and	O
lower	O
panels	O
,	O
respectively	O
).	O

(	O
c	O
)	O
Close	O
-	O
up	O
view	O
of	O
the	O
Na	B-site
+-	I-site
binding	I-site
sites	I-site
.	O

Residues	O
surrounding	O
this	O
site	O
are	O
also	O
indicated	O
;	O
note	O
A206	B-residue_name_number
(	O
labeled	O
in	O
red	O
)	O
coordinates	B-bond_interaction
Na	B-chemical
+	I-chemical
at	O
Sext	B-site
via	O
its	O
backbone	O
carbonyl	O
oxygen	O
.	O

(	O
d	O
)	O
Extracellular	O
solvent	O
accessibility	O
of	O
the	O
ion	B-site
binding	I-site
sites	I-site
in	O
the	O
structures	B-evidence
at	O
high	B-protein_state
and	O
low	B-protein_state
[	O
Na	B-chemical
+].	I-chemical

Putative	O
solvent	B-site
channels	I-site
are	O
represented	O
as	O
light	O
-	O
purple	O
surfaces	O
.	O

Divalent	O
cation	O
binding	O
and	O
apo	B-protein_state
structure	B-evidence
of	O
NCX_Mj	B-protein
.	O
(	O
a	O
)	O
A	O
single	O
Sr2	B-chemical
+	I-chemical
(	O
dark	O
blue	O
sphere	O
)	O
binds	O
at	O
SCa	B-site
in	O
crystals	B-experimental_method
titrated	I-experimental_method
with	O
10	O
mM	O
Sr2	B-chemical
+	I-chemical
and	O
2	O
.	O
5	O
mM	O
Na	B-chemical
+	I-chemical
(	O
see	O
also	O
Supplementary	O
Fig	O
.	O
2	O
).	O

There	O
are	O
no	O
significant	O
changes	O
in	O
the	O
side	O
-	O
chains	O
involved	O
in	O
ion	O
coordination	O
,	O
relative	O
to	O
the	O
Na	B-protein_state
+-	I-protein_state
bound	I-protein_state
state	O
.	O

T50	B-residue_name_number
and	O
T209	B-residue_name_number
(	O
labeled	O
in	O
red	O
)	O
coordinate	B-bond_interaction
Sr2	B-chemical
+	I-chemical
through	O
their	O
backbone	O
carbonyl	O
-	O
oxygen	O
atoms	O
.	O

High	O
Na	B-chemical
+	I-chemical
concentration	O
(	O
100	O
mM	O
)	O
completely	O
displaces	O
Sr2	B-chemical
+	I-chemical
and	O
reverts	O
NCX_Mj	B-protein
to	O
the	O
occluded	B-protein_state
state	O
(	O
right	O
panel	O
).	O

The	O
contour	O
level	O
of	O
the	O
Fo	B-evidence
–	I-evidence
Fc	I-evidence
omit	I-evidence
map	I-evidence
of	O
the	O
structure	B-evidence
at	O
high	O
Na	B-chemical
+	I-chemical
concentration	O
was	O
lowered	O
(	O
to	O
4σ	O
)	O
so	O
as	O
to	O
visualize	O
the	O
density	B-evidence
from	O
Na	B-chemical
+	I-chemical
ions	O
and	O
H2O	B-chemical
.	O

(	O
b	O
)	O
Ca2	B-chemical
+	I-chemical
(	O
tanned	O
spheres	O
)	O
binds	O
either	O
to	O
SCa	B-site
or	O
Smid	B-site
in	O
crystals	B-experimental_method
titrated	I-experimental_method
with	O
10	O
mM	O
Ca2	B-chemical
+	I-chemical
and	O
2	O
.	O
5	O
mM	O
Na	B-chemical
+	I-chemical
(	O
see	O
also	O
Supplementary	O
Fig	O
.	O
2	O
).	O

The	O
relative	O
occupancies	O
are	O
55	O
%	O
and	O
45	O
%,	O
respectively	O
.	O
(	O
c	O
)	O
Superimposition	B-experimental_method
of	O
NCX_Mj	B-protein
structures	B-evidence
obtained	O
at	O
low	O
Na	B-chemical
+	I-chemical
concentration	O
(	O
10	O
mM	O
)	O
and	O
pH	O
6	O
.	O
5	O
(	O
brown	O
)	O
and	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
Na	B-chemical
+	I-chemical
and	O
pH	B-protein_state
4	I-protein_state
(	O
light	O
green	O
),	O
referred	O
to	O
as	O
apo	B-protein_state
state	O
.	O
(	O
d	O
)	O
Close	O
-	O
up	O
view	O
of	O
the	O
ion	B-site
-	I-site
binding	I-site
sites	I-site
in	O
the	O
apo	B-protein_state
(	O
or	O
high	B-protein_state
H	I-protein_state
+)	I-protein_state
state	O
.	O

The	O
side	O
chains	O
of	O
E54	B-residue_name_number
and	O
E213	B-residue_name_number
from	O
the	O
low	B-protein_state
Na	I-protein_state
+	I-protein_state
structure	B-evidence
are	O
also	O
shown	O
(	O
light	O
brown	O
)	O
for	O
comparison	O
.	O

White	O
spheres	O
indicate	O
the	O
location	O
Sint	B-site
,	O
Smid	B-site
SCa	B-site
.	O
(	O
e	O
)	O
Extracellular	O
solvent	O
accessibility	O
of	O
the	O
ion	B-site
-	I-site
binding	I-site
sites	I-site
in	O
apo	B-protein_state
NCX_Mj	B-protein
.	O

Spontaneous	O
changes	O
in	O
the	O
structure	B-evidence
of	O
outward	B-protein_state
-	I-protein_state
occluded	I-protein_state
,	O
fully	B-protein_state
Na	I-protein_state
+-	I-protein_state
occupied	I-protein_state
NCX_Mj	B-protein
(	O
PDB	O
code	O
3V5U	O
)	O
upon	O
sequential	O
displacement	O
of	O
Na	B-chemical
+.	I-chemical

(	O
a	O
)	O
Representative	O
simulation	B-experimental_method
snapshots	O
of	O
NCX_Mj	B-protein
(	O
Methods	O
)	O
with	O
Na	B-chemical
+	I-chemical
bound	B-protein_state
at	I-protein_state
Sext	B-site
,	O
SCa	B-site
and	O
Sint	B-site
(	O
orange	O
cartoons	O
,	O
green	O
spheres	O
)	O
and	O
with	O
Na	B-chemical
+	I-chemical
bound	B-protein_state
only	I-protein_state
at	I-protein_state
SCa	B-site
and	O
Sint	B-site
(	O
marine	O
cartoons	O
,	O
yellow	O
spheres	O
)	O
(	O
b	O
)	O
Close	O
-	O
up	O
of	O
the	O
backbone	O
of	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
half	I-structure_element
of	O
TM7	B-structure_element
(	O
TM7ab	B-structure_element
),	O
in	O
the	O
same	O
Na	B-chemical
+	I-chemical
occupancy	O
states	O
depicted	O
in	O
(	O
a	O
).	O

(	O
c	O
)	O
Representative	O
simulation	B-evidence
snapshots	I-evidence
(	O
same	O
as	O
above	O
)	O
with	O
Na	B-chemical
+	I-chemical
bound	B-protein_state
at	I-protein_state
SCa	B-site
and	O
Sint	B-site
(	O
marine	O
cartoons	O
,	O
yellow	O
spheres	O
)	O
and	O
without	B-protein_state
any	O
Na	B-chemical
+	I-chemical
bound	B-protein_state
(	O
grey	O
cartoons	O
).	O

Approximate	O
distances	O
between	O
TM2	B-structure_element
,	O
TM3	B-structure_element
and	O
TM7	B-structure_element
are	O
indicated	O
in	O
Å	O
.	O
(	O
e	O
)	O
Close	O
-	O
up	O
of	O
the	O
ion	B-site
-	I-site
binding	I-site
region	I-site
in	O
the	O
partially	B-protein_state
Na	I-protein_state
+-	I-protein_state
occupied	I-protein_state
state	O
.	O

(	O
f	O
)	O
Close	O
-	O
up	O
of	O
the	O
ion	B-site
-	I-site
binding	I-site
region	I-site
in	O
the	O
Na	B-protein_state
+-	I-protein_state
free	I-protein_state
state	O
.	O
(	O
g	O
-	O
i	O
)	O
Analytical	O
descriptors	O
of	O
the	O
changes	O
just	O
described	O
,	O
calculated	O
from	O
the	O
simulations	B-experimental_method
of	O
each	O
Na	B-protein_state
+-	I-protein_state
occupancy	I-protein_state
state	O
depicted	O
in	O
panels	O
(	O
a	O
-	O
f	O
).	O

(	O
g	O
)	O
Probability	B-evidence
distributions	I-evidence
of	O
an	O
analytical	O
descriptor	O
of	O
the	O
backbone	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonding	I-bond_interaction
pattern	O
in	O
TM7ab	B-structure_element
(	O
Eq	O
.	O
2	O
).	O
(	O
h	O
)	O
Mean	O
value	O
(	O
with	O
standard	O
deviation	O
)	O
of	O
a	O
quantitative	O
descriptor	O
of	O
the	O
solvent	O
accessibility	O
of	O
the	O
Sext	B-site
site	O
(	O
Eq	O
.	O
1	O
).	O
(	O
i	O
)	O
Mean	O
value	O
(	O
with	O
standard	O
deviation	O
)	O
of	O
a	O
quantitative	O
descriptor	O
of	O
the	O
solvent	O
accessibility	O
of	O
the	O
SCa	B-site
site	O
(	O
Eq	O
.	O
1	O
).	O

The	O
free	B-evidence
energy	I-evidence
is	O
plotted	O
as	O
a	O
function	O
of	O
two	O
coordinates	O
,	O
each	O
describing	O
the	O
degree	O
of	O
opening	O
of	O
the	O
aqueous	B-site
channels	I-site
leading	O
to	O
the	O
Sext	B-site
and	O
SCa	B-site
sites	O
,	O
respectively	O
(	O
see	O
Methods	O
).	O

Black	O
circles	O
map	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
structures	I-evidence
of	O
NCX_Mj	B-protein
obtained	O
at	O
high	B-protein_state
and	O
low	B-protein_state
Na	B-chemical
+	I-chemical
concentration	O
,	O
as	O
well	O
as	O
that	O
at	O
low	B-protein_state
pH	I-protein_state
,	O
reported	O
in	O
this	O
study	O
.	O

(	O
b	O
)	O
Density	B-evidence
isosurfaces	I-evidence
for	O
water	B-chemical
molecules	O
within	O
12	O
Å	O
of	O
the	O
ion	B-site
-	I-site
binding	I-site
region	I-site
(	O
grey	O
volumes	O
),	O
for	O
each	O
of	O
the	O
major	O
conformational	B-evidence
free	I-evidence
-	I-evidence
energy	I-evidence
minima	I-evidence
in	O
each	O
ion	O
-	O
occupancy	O
state	O
.	O

Na	B-chemical
+	I-chemical
ions	O
are	O
shown	O
as	O
green	O
spheres	O
.	O

The	O
two	O
inverted	B-structure_element
-	I-structure_element
topology	I-structure_element
repeats	I-structure_element
in	O
the	O
transporter	B-protein_type
structure	B-evidence
(	O
transparent	O
cartoons	O
)	O
are	O
colored	O
differently	O
(	O
TM1	B-structure_element
-	I-structure_element
5	I-structure_element
,	O
orange	O
;	O
TM6	B-structure_element
-	I-structure_element
10	I-structure_element
,	O
marine	O
).	O

(	O
c	O
)	O
Close	O
-	O
up	O
views	O
of	O
the	O
ion	B-site
-	I-site
binding	I-site
region	I-site
in	O
the	O
same	O
conformational	B-evidence
free	I-evidence
-	I-evidence
energy	I-evidence
minima	I-evidence
.	O

Key	O
residues	O
involved	O
in	O
Na	B-chemical
+	I-chemical
and	O
water	B-chemical
coordination	O
(	O
W	O
)	O
are	O
highlighted	O
(	O
sticks	O
,	O
black	O
lines	O
).	O

The	O
water	B-evidence
-	I-evidence
density	I-evidence
maps	I-evidence
in	O
(	O
b	O
)	O
is	O
shown	O
here	O
as	O
a	O
grey	O
mesh	O
.	O

Note	O
D240	B-residue_name_number
is	O
protonated	O
,	O
while	O
E54	B-residue_name_number
and	O
E213	B-residue_name_number
are	O
ionized	O
.	O

Thermodynamic	O
basis	O
for	O
the	O
proposed	O
mechanism	O
of	O
substrate	O
control	O
of	O
the	O
alternating	O
-	O
access	O
transition	O
of	O
NCX	B-protein_type
.	O
(	O
a	O
)	O
Calculated	B-evidence
free	I-evidence
-	I-evidence
energy	I-evidence
landscapes	I-evidence
for	O
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
NCX_Mj	B-protein
,	O
for	O
the	O
Ca2	B-chemical
+	I-chemical
and	O
the	O
fully	B-protein_state
protonated	I-protein_state
state	O
.	O

The	O
free	B-evidence
energy	I-evidence
is	O
plotted	O
as	O
in	O
Fig	O
.	O
5	O
.	O

Black	O
circles	O
map	O
the	O
crystal	B-evidence
structures	I-evidence
obtained	O
at	O
high	O
Ca2	B-chemical
+	I-chemical
concentration	O
and	O
at	O
low	B-protein_state
pH	I-protein_state
(	O
or	O
high	B-protein_state
H	I-protein_state
+)	I-protein_state
reported	O
in	O
this	O
study	O
.	O

(	O
b	O
)	O
Water	B-evidence
-	I-evidence
density	I-evidence
isosurfaces	I-evidence
analogous	O
to	O
those	O
in	O
Fig	O
.	O
5	O
are	O
shown	O
for	O
each	O
of	O
the	O
major	O
conformational	O
free	B-evidence
-	I-evidence
energy	I-evidence
minima	I-evidence
in	O
the	O
free	B-evidence
-	I-evidence
energy	I-evidence
maps	I-evidence
.	O

The	O
Ca2	B-chemical
+	I-chemical
ion	O
is	O
shown	O
as	O
a	O
red	O
sphere	O
;	O
the	O
protein	O
is	O
shown	O
as	O
in	O
Fig	O
.	O
5	O
.	O
(	O
c	O
)	O
Close	O
-	O
up	O
views	O
of	O
the	O
ion	B-site
-	I-site
binding	I-site
region	I-site
in	O
the	O
same	O
conformational	B-evidence
free	I-evidence
-	I-evidence
energy	I-evidence
minima	I-evidence
.	O

Key	O
residues	O
involved	O
in	O
Ca2	B-chemical
+	I-chemical
and	O
water	B-chemical
coordination	O
(	O
W	O
)	O
are	O
highlighted	O
(	O
sticks	O
,	O
black	O
lines	O
).	O

The	O
water	B-evidence
-	I-evidence
density	I-evidence
maps	I-evidence
in	O
(	O
b	O
)	O
are	O
shown	O
here	O
as	O
a	O
grey	O
mesh	O
.	O

In	O
the	O
occluded	B-protein_state
state	O
with	O
Ca2	B-chemical
+	I-chemical
bound	B-protein_state
,	O
helix	B-structure_element
TM7ab	B-structure_element
bends	O
in	O
the	O
same	O
way	O
as	O
in	O
the	O
fully	B-protein_state
occupied	I-protein_state
Na	B-chemical
+	I-chemical
state	O
,	O
as	O
the	O
carbonyl	O
of	O
Ala206	B-residue_name_number
forms	O
a	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonding	I-bond_interaction
interaction	I-bond_interaction
with	O
Ser210	B-residue_name_number
.	O

Structural	O
mechanism	O
of	O
extracellular	O
forward	O
ion	O
exchange	O
in	O
NCX	B-protein_type
.	O

The	O
carbonyl	O
groups	O
of	O
Ala47	B-residue_name_number
(	O
on	O
TM2b	B-structure_element
)	O
and	O
Ala206	B-residue_name_number
(	O
on	O
TM7b	B-structure_element
),	O
and	O
the	O
side	O
chains	O
of	O
Glu54	B-residue_name_number
(	O
on	O
TM2c	B-structure_element
)	O
and	O
Glu213	B-residue_name_number
(	O
on	O
TM7c	B-structure_element
)	O
are	O
highlighted	O
;	O
these	O
are	O
four	O
of	O
the	O
key	O
residues	O
for	O
ion	O
chelation	O
and	O
conformational	O
changes	O
.	O

The	O
green	O
open	O
cylinders	O
represent	O
the	O
gating	B-structure_element
helices	I-structure_element
TM1	B-structure_element
and	O
TM6	B-structure_element
.	O

These	O
states	O
and	O
their	O
connectivity	O
can	O
also	O
be	O
deduced	O
from	O
the	O
calculated	B-evidence
free	I-evidence
-	I-evidence
energy	I-evidence
landscapes	I-evidence
,	O
which	O
also	O
reveal	O
a	O
Ca2	B-protein_state
+-	I-protein_state
loaded	I-protein_state
outward	B-protein_state
-	I-protein_state
facing	I-protein_state
occluded	B-protein_state
state	O
,	O
and	O
an	O
unloaded	B-protein_state
,	O
fully	B-protein_state
open	I-protein_state
state	O
.	O

An	O
extended	B-protein_state
U2AF65	B-structure_element
–	I-structure_element
RNA	I-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
recognizes	O
the	O
3	B-site
′	I-site
splice	I-site
site	I-site
signal	O

We	O
determined	B-experimental_method
four	I-experimental_method
structures	I-experimental_method
of	O
an	O
extended	B-protein_state
U2AF65	B-structure_element
–	I-structure_element
RNA	I-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
bound	B-protein_state
to	I-protein_state
Py	B-chemical
-	I-chemical
tract	I-chemical
oligonucleotides	I-chemical
at	O
resolutions	O
between	O
2	O
.	O
0	O
and	O
1	O
.	O
5	O
Å	O
.	O
These	O
structures	B-evidence
together	O
with	O
RNA	B-experimental_method
binding	I-experimental_method
and	I-experimental_method
splicing	I-experimental_method
assays	I-experimental_method
reveal	O
unforeseen	O
roles	O
for	O
U2AF65	B-protein
inter	B-site
-	I-site
domain	I-site
residues	I-site
in	O
recognizing	O
a	O
contiguous	B-structure_element
,	O
nine	O
-	O
nucleotide	B-chemical
Py	B-chemical
tract	I-chemical
.	O

The	O
U2AF65	B-protein
linker	B-structure_element
residues	O
between	O
the	O
dual	O
RNA	B-structure_element
recognition	I-structure_element
motifs	I-structure_element
(	O
RRMs	B-structure_element
)	O
recognize	O
the	O
central	O
nucleotide	B-chemical
,	O
whereas	O
the	O
N	O
-	O
and	O
C	O
-	O
terminal	O
RRM	B-structure_element
extensions	I-structure_element
recognize	O
the	O
3	B-site
′	I-site
terminus	I-site
and	O
third	B-residue_number
nucleotide	B-chemical
.	O

Altogether	O
,	O
these	O
results	O
advance	O
the	O
mechanistic	O
understanding	O
of	O
molecular	O
recognition	O
for	O
a	O
major	O
class	O
of	O
splice	B-site
site	I-site
signals	O
.	O

Here	O
,	O
the	O
authors	O
report	O
U2AF65	B-protein
structures	B-evidence
and	O
single	B-experimental_method
molecule	I-experimental_method
FRET	I-experimental_method
that	O
reveal	O
mechanistic	O
insights	O
into	O
splice	B-site
site	I-site
recognition	O
.	O

The	O
differential	O
skipping	O
or	O
inclusion	O
of	O
alternatively	O
spliced	O
pre	B-structure_element
-	I-structure_element
mRNA	I-structure_element
regions	I-structure_element
is	O
a	O
major	O
source	O
of	O
diversity	O
for	O
nearly	O
all	O
human	B-species
gene	O
transcripts	O
.	O

The	O
splice	B-site
sites	I-site
are	O
marked	O
by	O
relatively	O
short	B-structure_element
consensus	I-structure_element
sequences	I-structure_element
and	O
are	O
regulated	O
by	O
additional	O
pre	B-structure_element
-	I-structure_element
mRNA	I-structure_element
motifs	I-structure_element
(	O
reviewed	O
in	O
ref	O
.).	O

At	O
the	O
3	B-site
′	I-site
splice	I-site
site	I-site
of	O
the	O
major	O
intron	O
class	O
,	O
these	O
include	O
a	O
polypyrimidine	B-chemical
(	I-chemical
Py	I-chemical
)	I-chemical
tract	I-chemical
comprising	O
primarily	O
Us	B-residue_name
or	O
Cs	B-residue_name
,	O
which	O
is	O
preceded	O
by	O
a	O
branch	B-site
point	I-site
sequence	I-site
(	O
BPS	B-site
)	O
that	O
ultimately	O
serves	O
as	O
the	O
nucleophile	O
in	O
the	O
splicing	O
reaction	O
and	O
an	O
AG	B-chemical
-	I-chemical
dinucleotide	I-chemical
at	O
the	O
3	B-site
′	I-site
splice	I-site
site	I-site
junction	O
.	O

Disease	O
-	O
causing	O
mutations	O
often	O
compromise	O
pre	B-chemical
-	I-chemical
mRNA	I-chemical
splicing	O
(	O
reviewed	O
in	O
refs	O
),	O
yet	O
a	O
priori	O
predictions	O
of	O
splice	B-site
sites	I-site
and	O
the	O
consequences	O
of	O
their	O
mutations	O
are	O
challenged	O
by	O
the	O
brevity	O
and	O
degeneracy	O
of	O
known	O
splice	B-site
site	I-site
sequences	O
.	O

High	O
-	O
resolution	O
structures	B-evidence
of	O
intact	B-protein_state
splicing	B-complex_assembly
factor	I-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
complexes	O
would	O
offer	O
key	O
insights	O
regarding	O
the	O
juxtaposition	O
of	O
the	O
distinct	O
splice	B-site
site	I-site
consensus	O
sequences	O
and	O
their	O
relationship	O
to	O
disease	O
-	O
causing	O
point	O
mutations	O
.	O

The	O
early	O
-	O
stage	O
pre	B-protein_type
-	I-protein_type
mRNA	I-protein_type
splicing	I-protein_type
factor	I-protein_type
U2AF65	B-protein
is	O
essential	O
for	O
viability	O
in	O
vertebrates	B-taxonomy_domain
and	O
other	O
model	O
organisms	O
(	O
for	O
example	O
,	O
ref	O
.).	O

A	O
tightly	O
controlled	O
assembly	B-complex_assembly
among	O
U2AF65	B-protein
,	O
the	O
pre	B-chemical
-	I-chemical
mRNA	I-chemical
,	O
and	O
partner	O
proteins	O
sequentially	O
identifies	O
the	O
3	B-site
′	I-site
splice	I-site
site	I-site
and	O
promotes	O
association	O
of	O
the	O
spliceosome	B-complex_assembly
,	O
which	O
ultimately	O
accomplishes	O
the	O
task	O
of	O
splicing	O
.	O

Initially	O
U2AF65	B-protein
recognizes	O
the	O
Py	B-chemical
-	I-chemical
tract	I-chemical
splice	B-site
site	I-site
signal	O
.	O

In	O
turn	O
,	O
the	O
ternary	B-complex_assembly
complex	I-complex_assembly
of	O
U2AF65	B-protein
with	O
SF1	B-protein
and	O
U2AF35	B-protein
identifies	O
the	O
surrounding	O
BPS	B-site
and	O
3	B-site
′	I-site
splice	I-site
site	I-site
junctions	O
.	O

Subsequently	O
U2AF65	B-protein
recruits	O
the	O
U2	B-complex_assembly
small	I-complex_assembly
nuclear	I-complex_assembly
ribonucleoprotein	I-complex_assembly
particle	I-complex_assembly
(	O
snRNP	B-complex_assembly
)	O
and	O
ultimately	O
dissociates	O
from	O
the	O
active	B-protein_state
spliceosome	B-complex_assembly
.	O

Biochemical	B-experimental_method
characterizations	I-experimental_method
of	O
U2AF65	B-protein
demonstrated	O
that	O
tandem	O
RNA	B-structure_element
recognition	I-structure_element
motifs	I-structure_element
(	O
RRM1	B-structure_element
and	O
RRM2	B-structure_element
)	O
recognize	O
the	O
Py	B-chemical
tract	I-chemical
(	O
Fig	O
.	O
1a	O
).	O

A	O
subsequent	O
NMR	B-experimental_method
structure	B-evidence
characterized	O
the	O
side	B-protein_state
-	I-protein_state
by	I-protein_state
-	I-protein_state
side	I-protein_state
arrangement	O
of	O
the	O
minimal	B-protein_state
U2AF65	B-protein
RRM1	B-structure_element
and	O
RRM2	B-structure_element
connected	O
by	O
a	O
linker	B-structure_element
of	O
natural	B-protein_state
length	I-protein_state
(	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
),	O
yet	O
depended	O
on	O
the	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
crystal	B-evidence
structures	I-evidence
for	O
RNA	B-chemical
interactions	O
and	O
an	O
ab	O
initio	O
model	O
for	O
the	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
conformation	O
.	O

As	O
such	O
,	O
the	O
molecular	O
mechanisms	O
for	O
Py	B-chemical
-	I-chemical
tract	I-chemical
recognition	O
by	O
the	O
intact	B-protein_state
U2AF65	B-structure_element
–	I-structure_element
RNA	I-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
remained	O
unknown	O
.	O

Here	O
,	O
we	O
use	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
and	O
biochemical	B-experimental_method
studies	I-experimental_method
to	O
reveal	O
new	O
roles	O
in	O
Py	B-chemical
-	I-chemical
tract	I-chemical
recognition	O
for	O
the	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
and	O
key	O
residues	O
surrounding	O
the	O
core	B-protein_state
U2AF65	B-protein
RRMs	B-structure_element
.	O

Cognate	O
U2AF65	B-protein
–	O
Py	B-chemical
-	I-chemical
tract	I-chemical
recognition	O
requires	O
RRM	B-structure_element
extensions	I-structure_element

The	O
RNA	B-evidence
affinity	I-evidence
of	O
the	O
minimal	B-protein_state
U2AF651	B-mutant
,	I-mutant
2	I-mutant
domain	O
comprising	O
the	O
core	B-protein_state
RRM1	B-structure_element
–	O
RRM2	B-structure_element
folds	B-structure_element
(	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
,	O
residues	O
148	B-residue_range
–	I-residue_range
336	I-residue_range
)	O
is	O
relatively	O
weak	O
compared	O
with	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
U2AF65	B-protein
(	O
Fig	O
.	O
1a	O
,	O
b	O
;	O
Supplementary	O
Fig	O
.	O
1	O
).	O

Historically	O
,	O
this	O
difference	O
was	O
attributed	O
to	O
the	O
U2AF65	B-protein
arginine	B-structure_element
–	I-structure_element
serine	I-structure_element
rich	I-structure_element
domain	I-structure_element
,	O
which	O
contacts	O
pre	B-complex_assembly
-	I-complex_assembly
mRNA	I-complex_assembly
–	I-complex_assembly
U2	I-complex_assembly
snRNA	I-complex_assembly
duplexes	I-complex_assembly
outside	O
of	O
the	O
Py	B-chemical
tract	I-chemical
.	O

We	O
noticed	O
that	O
the	O
RNA	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
the	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
domain	O
was	O
greatly	O
enhanced	O
by	O
the	O
addition	B-experimental_method
of	I-experimental_method
seven	I-experimental_method
and	I-experimental_method
six	I-experimental_method
residues	I-experimental_method
at	O
the	O
respective	O
N	O
and	O
C	O
termini	O
of	O
the	O
minimal	B-protein_state
RRM1	B-structure_element
and	O
RRM2	B-structure_element
(	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
,	O
residues	O
141	B-residue_range
–	I-residue_range
342	I-residue_range
;	O
Fig	O
.	O
1a	O
).	O

Likewise	O
,	O
both	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
and	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
U2AF65	B-protein
showed	O
similar	O
sequence	B-evidence
specificity	I-evidence
for	O
U	B-structure_element
-	I-structure_element
rich	I-structure_element
stretches	I-structure_element
in	O
the	O
5	B-site
′-	I-site
region	I-site
of	O
the	O
Py	B-chemical
tract	I-chemical
and	O
promiscuity	O
for	O
C	B-structure_element
-	I-structure_element
rich	I-structure_element
regions	I-structure_element
in	O
the	O
3	B-site
′-	I-site
region	I-site
(	O
Fig	O
.	O
1c	O
,	O
Supplementary	O
Fig	O
.	O
1e	O
–	O
h	O
).	O

U2AF65	B-protein_state
-	I-protein_state
bound	I-protein_state
Py	B-chemical
tract	I-chemical
comprises	O
nine	O
contiguous	B-structure_element
nucleotides	B-chemical

To	O
investigate	O
the	O
structural	O
basis	O
for	O
cognate	O
U2AF65	B-protein
recognition	O
of	O
a	O
contiguous	B-structure_element
Py	B-chemical
tract	I-chemical
,	O
we	O
determined	B-experimental_method
four	O
crystal	B-evidence
structures	I-evidence
of	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
bound	B-protein_state
to	I-protein_state
Py	B-chemical
-	I-chemical
tract	I-chemical
oligonucleotides	I-chemical
(	O
Fig	O
.	O
2a	O
;	O
Table	O
1	O
).	O

By	O
sequential	B-experimental_method
boot	I-experimental_method
strapping	I-experimental_method
(	O
Methods	O
),	O
we	O
optimized	O
the	O
oligonucleotide	B-chemical
length	O
,	O
the	O
position	O
of	O
a	O
Br	B-chemical
-	I-chemical
dU	I-chemical
,	O
and	O
the	O
identity	O
of	O
the	O
terminal	O
nucleotide	B-chemical
(	O
rU	B-residue_name
,	O
dU	B-residue_name
and	O
rC	B-residue_name
)	O
to	O
achieve	O
full	O
views	O
of	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
bound	B-protein_state
to	I-protein_state
contiguous	B-structure_element
Py	B-chemical
tracts	I-chemical
at	O
up	O
to	O
1	O
.	O
5	O
Å	O
resolution	O
.	O

The	O
protein	O
and	O
oligonucleotide	B-chemical
conformations	O
are	O
nearly	O
identical	O
among	O
the	O
four	O
new	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
structures	B-evidence
(	O
Supplementary	O
Fig	O
.	O
2a	O
).	O

The	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
RRM1	B-structure_element
and	O
RRM2	B-structure_element
associate	O
with	O
the	O
Py	B-chemical
tract	I-chemical
in	O
a	O
parallel	B-protein_state
,	O
side	B-protein_state
-	I-protein_state
by	I-protein_state
-	I-protein_state
side	I-protein_state
arrangement	O
(	O
shown	O
for	O
representative	O
structure	O
iv	O
in	O
Fig	O
.	O
2b	O
,	O
c	O
;	O
Supplementary	O
Movie	O
1	O
).	O

An	O
extended	B-protein_state
conformation	I-protein_state
of	O
the	O
U2AF65	B-protein
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
traverses	O
across	O
the	O
α	B-structure_element
-	I-structure_element
helical	I-structure_element
surface	I-structure_element
of	O
RRM1	B-structure_element
and	O
the	O
central	O
β	B-structure_element
-	I-structure_element
strands	I-structure_element
of	O
RRM2	B-structure_element
and	O
is	O
well	O
defined	O
in	O
the	O
electron	B-evidence
density	I-evidence
(	O
Fig	O
.	O
2b	O
).	O

The	O
extensions	B-structure_element
at	O
the	O
N	O
terminus	O
of	O
RRM1	B-structure_element
and	O
C	O
terminus	O
of	O
RRM2	B-structure_element
adopt	O
well	O
-	O
ordered	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
.	O

We	O
compare	O
the	O
global	O
conformation	O
of	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
structures	B-evidence
with	O
the	O
prior	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
crystal	B-evidence
structure	I-evidence
and	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
NMR	B-experimental_method
structure	B-evidence
in	O
the	O
Supplementary	O
Discussion	O
and	O
Supplementary	O
Fig	O
.	O
2	O
.	O

Based	O
on	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
structures	B-evidence
,	O
we	O
originally	O
hypothesized	O
that	O
the	O
U2AF65	B-protein
RRMs	B-structure_element
would	O
bind	O
the	O
minimal	B-protein_state
seven	O
nucleotides	B-chemical
observed	O
in	O
these	O
structures	B-evidence
.	O

Qualitatively	O
,	O
a	O
subset	O
of	O
the	O
U2AF651	B-site
,	I-site
2L	I-site
-	I-site
nucleotide	I-site
-	I-site
binding	I-site
sites	I-site
(	O
sites	B-site
1	I-site
–	I-site
3	I-site
and	O
7	B-site
–	I-site
9	I-site
)	O
share	O
similar	O
locations	O
to	O
those	O
of	O
the	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
structures	B-evidence
(	O
Supplementary	O
Figs	O
2c	O
,	O
d	O
and	O
3	O
).	O

Yet	O
,	O
only	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
interactions	O
at	O
sites	B-site
1	I-site
and	I-site
7	I-site
are	O
nearly	O
identical	O
to	O
those	O
of	O
the	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
structures	B-evidence
(	O
Supplementary	O
Fig	O
.	O
3a	O
,	O
f	O
).	O

In	O
striking	O
departures	O
from	O
prior	O
partial	O
views	O
,	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
structures	B-evidence
reveal	O
three	O
unanticipated	O
nucleotide	B-site
-	I-site
binding	I-site
sites	I-site
at	O
the	O
centre	O
of	O
the	O
Py	B-chemical
tract	I-chemical
,	O
as	O
well	O
as	O
numerous	O
new	O
interactions	O
that	O
underlie	O
cognate	O
recognition	O
of	O
the	O
Py	B-chemical
tract	I-chemical
(	O
Fig	O
.	O
3a	O
–	O
h	O
).	O

The	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
RRM2	B-structure_element
,	O
the	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
and	O
RRM1	B-structure_element
concomitantly	O
recognize	O
the	O
three	O
central	O
nucleotides	B-chemical
of	O
the	O
Py	B-chemical
tract	I-chemical
,	O
which	O
are	O
likely	O
to	O
coordinate	O
the	O
conformational	O
arrangement	O
of	O
these	O
disparate	O
portions	O
of	O
the	O
protein	O
.	O

Residues	O
in	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
region	I-structure_element
of	O
the	O
U2AF65	B-protein
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
comprise	O
a	O
centrally	O
located	O
binding	B-site
site	I-site
for	O
the	O
fifth	B-residue_number
nucleotide	B-chemical
on	O
the	O
RRM2	B-site
surface	I-site
and	O
abutting	O
the	O
RRM1	B-site
/	I-site
RRM2	I-site
interface	I-site
(	O
Fig	O
.	O
3d	O
).	O

The	O
backbone	O
amide	O
of	O
the	O
linker	B-structure_element
V254	B-residue_name_number
and	O
the	O
carbonyl	O
of	O
T252	B-residue_name_number
engage	O
in	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
the	O
rU5	B-residue_name_number
-	O
O4	O
and	O
-	O
N3H	O
atoms	O
.	O

In	O
the	O
C	O
-	O
terminal	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
of	O
RRM1	B-structure_element
,	O
the	O
side	O
chains	O
of	O
K225	B-residue_name_number
and	O
R227	B-residue_name_number
donate	O
additional	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
to	O
the	O
rU5	B-residue_name_number
-	O
O2	O
lone	O
pair	O
electrons	O
.	O

The	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
region	I-structure_element
of	O
the	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
also	O
participates	O
in	O
the	O
preceding	O
rU4	B-site
-	I-site
binding	I-site
site	I-site
,	O
where	O
the	O
V254	B-residue_name_number
backbone	O
carbonyl	O
and	O
D256	B-residue_name_number
carboxylate	O
position	O
the	O
K260	B-residue_name_number
side	O
chain	O
to	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
the	O
rU4	B-residue_name_number
-	O
O4	O
(	O
Fig	O
.	O
3c	O
).	O

This	O
nucleotide	B-chemical
twists	O
to	O
face	O
away	O
from	O
the	O
U2AF65	B-protein
linker	B-structure_element
and	O
instead	O
inserts	O
the	O
rU6	B-residue_name_number
-	O
uracil	B-residue_name
into	O
a	O
sandwich	O
between	O
the	O
β2	B-structure_element
/	I-structure_element
β3	I-structure_element
loops	I-structure_element
of	O
RRM1	B-structure_element
and	O
RRM2	B-structure_element
.	O

The	O
rU6	B-residue_name_number
base	O
edge	O
is	O
relatively	O
solvent	B-protein_state
exposed	I-protein_state
;	O
accordingly	O
,	O
the	O
rU6	B-residue_name_number
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
U2AF65	B-protein
are	O
water	B-chemical
mediated	O
apart	O
from	O
a	O
single	O
direct	O
interaction	O
by	O
the	O
RRM1	B-structure_element
-	O
N196	B-residue_name_number
side	O
chain	O
.	O

Mutagenesis	B-experimental_method
of	O
either	O
V254	B-residue_name_number
in	O
the	O
U2AF65	B-protein
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
to	O
proline	B-residue_name
or	O
RRM1	B-structure_element
–	O
R227	B-residue_name_number
to	O
alanine	B-residue_name
,	O
which	O
remove	O
the	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
the	O
fifth	B-residue_number
uracil	B-residue_name
-	O
O4	O
or	O
-	O
O2	O
,	O
reduced	O
the	O
affinities	B-evidence
of	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
for	O
the	O
representative	O
AdML	B-gene
Py	B-chemical
tract	I-chemical
by	O
four	O
-	O
or	O
five	O
-	O
fold	O
,	O
respectively	O
.	O

The	O
energetic	O
penalties	O
due	O
to	O
these	O
mutations	O
(	O
ΔΔG	B-evidence
0	O
.	O
8	O
–	O
0	O
.	O
9	O
kcal	O
mol	O
−	O
1	O
)	O
are	O
consistent	O
with	O
the	O
loss	O
of	O
each	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
the	O
rU5	B-residue_name_number
base	O
and	O
support	O
the	O
relevance	O
of	O
the	O
central	O
nucleotide	O
interactions	O
observed	O
in	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
structures	B-evidence
.	O

The	O
N	B-structure_element
-	I-structure_element
and	I-structure_element
C	I-structure_element
-	I-structure_element
terminal	I-structure_element
extensions	I-structure_element
of	O
the	O
U2AF65	B-protein
RRM1	B-structure_element
and	O
RRM2	B-structure_element
directly	O
contact	O
the	O
bound	B-protein_state
Py	B-chemical
tract	I-chemical
.	O

Rather	O
than	O
interacting	O
with	O
a	O
new	O
5	O
′-	O
terminal	O
nucleotide	B-chemical
as	O
we	O
had	O
hypothesized	O
,	O
the	O
C	O
-	O
terminal	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
of	O
RRM2	B-structure_element
instead	O
folds	O
across	O
one	O
surface	O
of	O
rU3	B-residue_name_number
in	O
the	O
third	B-site
binding	I-site
site	I-site
(	O
Fig	O
.	O
3b	O
).	O

There	O
,	O
a	O
salt	B-bond_interaction
bridge	I-bond_interaction
between	O
the	O
K340	B-residue_name_number
side	O
chain	O
and	O
nucleotide	B-chemical
phosphate	O
,	O
as	O
well	O
as	O
G338	B-residue_name_number
-	O
base	O
stacking	B-bond_interaction
and	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
between	O
the	O
backbone	O
amide	O
of	O
G338	B-residue_name_number
and	O
the	O
rU3	B-residue_name_number
-	O
O4	O
,	O
secure	O
the	O
RRM2	B-structure_element
extension	I-structure_element
.	O

Consequently	O
,	O
the	O
U2AF651	B-protein_state
,	I-protein_state
2L	I-protein_state
-	I-protein_state
bound	I-protein_state
rU2	B-residue_name_number
-	O
O4	O
and	O
-	O
N3H	O
form	O
dual	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
the	O
K329	B-residue_name_number
backbone	O
atoms	O
(	O
Fig	O
.	O
3a	O
),	O
rather	O
than	O
a	O
single	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
the	O
K329	B-residue_name_number
side	O
chain	O
as	O
in	O
the	O
prior	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
structure	B-evidence
(	O
Supplementary	O
Fig	O
.	O
3b	O
).	O

At	O
the	O
N	O
terminus	O
,	O
the	O
α	B-structure_element
-	I-structure_element
helical	I-structure_element
extension	I-structure_element
of	O
U2AF65	B-protein
RRM1	B-structure_element
positions	O
the	O
Q147	B-residue_name_number
side	O
chain	O
to	O
bridge	O
the	O
eighth	B-residue_number
and	O
ninth	B-residue_number
nucleotides	B-chemical
at	O
the	O
3	B-site
′	I-site
terminus	I-site
of	O
the	O
Py	B-chemical
tract	I-chemical
(	O
Fig	O
.	O
3f	O
–	O
h	O
).	O

The	O
Q147	B-residue_name_number
residue	O
participates	O
in	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
the	O
-	O
N3H	O
of	O
the	O
eighth	B-residue_number
uracil	B-residue_name
and	O
-	O
O2	O
of	O
the	O
ninth	B-residue_number
pyrimidine	B-chemical
.	O

The	O
adjacent	O
R146	B-residue_name_number
guanidinium	O
group	O
donates	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
to	O
the	O
3	O
′-	O
terminal	O
ribose	B-chemical
-	O
O2	O
′	O
and	O
O3	O
′	O
atoms	O
,	O
where	O
it	O
could	O
form	O
a	O
salt	B-bond_interaction
bridge	I-bond_interaction
with	O
a	O
phospho	O
-	O
diester	O
group	O
in	O
the	O
context	O
of	O
a	O
longer	O
pre	B-chemical
-	I-chemical
mRNA	I-chemical
.	O

Consistent	O
with	O
loss	O
of	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
the	O
ninth	B-residue_number
pyrimidine	B-chemical
-	O
O2	O
(	O
ΔΔG	B-evidence
1	O
.	O
0	O
kcal	O
mol	O
−	O
1	O
),	O
mutation	B-experimental_method
of	O
the	O
Q147	B-residue_name_number
to	O
an	O
alanine	B-residue_name
reduced	O
U2AF651	B-evidence
,	I-evidence
2L	I-evidence
affinity	I-evidence
for	O
the	O
AdML	B-gene
Py	B-chemical
tract	I-chemical
by	O
five	O
-	O
fold	O
(	O
Fig	O
.	O
3i	O
;	O
Supplementary	O
Fig	O
.	O
4c	O
).	O

We	O
compare	B-experimental_method
U2AF65	B-protein
interactions	O
with	O
uracil	B-residue_name
relative	O
to	O
cytosine	B-residue_name
pyrimidines	B-chemical
at	O
the	O
ninth	B-site
binding	I-site
site	I-site
in	O
Fig	O
.	O
3g	O
,	O
h	O
and	O
the	O
Supplementary	O
Discussion	O
.	O

The	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
structures	B-evidence
reveal	O
that	O
the	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
mediates	O
an	O
extensive	B-site
interface	I-site
with	O
the	O
second	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
of	O
RRM1	B-structure_element
,	O
the	O
β2	B-structure_element
/	I-structure_element
β3	I-structure_element
strands	I-structure_element
of	O
RRM2	B-structure_element
and	O
the	O
N	O
-	O
terminal	O
α	B-structure_element
-	I-structure_element
helical	I-structure_element
extension	I-structure_element
of	O
RRM1	B-structure_element
.	O

Altogether	O
,	O
the	O
U2AF65	B-protein
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
residues	O
(	O
R228	B-residue_range
–	I-residue_range
K260	I-residue_range
)	O
bury	O
2	O
,	O
800	O
Å2	O
of	O
surface	O
area	O
in	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
holo	B-protein_state
-	I-protein_state
protein	I-protein_state
,	O
suggestive	O
of	O
a	O
cognate	B-site
interface	I-site
compared	O
with	O
1	O
,	O
900	O
Å2	O
for	O
a	O
typical	O
protein	O
–	O
protein	O
complex	O
.	O

The	O
path	O
of	O
the	O
linker	B-structure_element
initiates	O
at	O
P229	B-residue_name_number
following	O
the	O
core	B-protein_state
RRM1	B-structure_element
β	B-structure_element
-	I-structure_element
strand	I-structure_element
,	O
in	O
a	O
kink	B-structure_element
that	O
is	O
positioned	O
by	O
intra	B-bond_interaction
-	I-bond_interaction
molecular	I-bond_interaction
stacking	I-bond_interaction
among	O
the	O
consecutive	O
R228	B-residue_name_number
,	O
Y232	B-residue_name_number
and	O
P234	B-residue_name_number
side	O
chains	O
(	O
Fig	O
.	O
4a	O
,	O
lower	O
right	O
).	O

In	O
the	O
neighbouring	O
apical	O
region	O
of	O
the	O
linker	B-structure_element
,	O
the	O
V244	B-residue_name_number
and	O
V246	B-residue_name_number
side	O
chains	O
pack	O
in	O
a	O
hydrophobic	B-site
pocket	I-site
between	O
two	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
of	O
the	O
core	B-protein_state
RRM1	B-structure_element
.	O

The	O
adjacent	O
V249	B-residue_name_number
and	O
V250	B-residue_name_number
are	O
notable	O
for	O
their	O
respective	O
interactions	O
that	O
connect	O
RRM1	B-structure_element
and	O
RRM2	B-structure_element
at	O
this	O
distal	O
interface	B-site
from	O
the	O
RNA	B-site
-	I-site
binding	I-site
site	I-site
(	O
Fig	O
.	O
4a	O
,	O
top	O
).	O

A	O
third	B-structure_element
kink	I-structure_element
stacks	B-bond_interaction
P247	B-residue_name_number
and	O
G248	B-residue_name_number
with	O
Y245	B-residue_name_number
and	O
re	O
-	O
orients	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
region	I-structure_element
of	O
the	O
linker	B-structure_element
towards	O
the	O
RRM2	B-structure_element
and	O
bound	B-protein_state
RNA	B-chemical
.	O

Few	O
direct	O
contacts	O
are	O
made	O
between	O
the	O
remaining	O
residues	O
of	O
the	O
linker	B-structure_element
and	O
the	O
U2AF65	B-protein
RRM2	B-structure_element
;	O
instead	O
,	O
the	O
C	O
-	O
terminal	O
conformation	O
of	O
the	O
linker	B-structure_element
appears	O
primarily	O
RNA	B-chemical
mediated	O
(	O
Fig	O
.	O
3c	O
,	O
d	O
).	O

We	O
investigated	O
whether	O
the	O
observed	O
contacts	O
between	O
the	O
RRMs	B-structure_element
and	O
linker	B-structure_element
were	O
critical	O
for	O
RNA	O
binding	O
by	O
structure	B-experimental_method
-	I-experimental_method
guided	I-experimental_method
mutagenesis	I-experimental_method
(	O
Fig	O
.	O
4b	O
).	O

We	O
titrated	B-experimental_method
these	O
mutant	B-protein_state
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
proteins	O
into	O
fluorescein	B-chemical
-	O
labelled	O
AdML	B-gene
Py	B-chemical
-	I-chemical
tract	I-chemical
RNA	I-chemical
and	O
fit	O
the	O
fluorescence	B-evidence
anisotropy	I-evidence
changes	I-evidence
to	O
obtain	O
the	O
apparent	O
equilibrium	B-evidence
affinities	I-evidence
(	O
Supplementary	O
Fig	O
.	O
4d	O
–	O
h	O
).	O

We	O
introduced	O
glycine	B-residue_name
substitutions	B-experimental_method
to	O
maximally	O
reduce	O
the	O
buried	O
surface	O
area	O
without	O
directly	O
interfering	O
with	O
its	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
between	O
backbone	O
atoms	O
and	O
the	O
base	O
.	O

First	O
,	O
we	O
replaced	B-experimental_method
V249	B-residue_name_number
and	O
V250	B-residue_name_number
at	O
the	O
RRM1	B-site
/	I-site
RRM2	I-site
interface	I-site
and	O
V254	B-residue_name_number
at	O
the	O
bound	B-protein_state
RNA	B-chemical
site	O
with	O
glycine	B-residue_name
(	O
3Gly	B-mutant
).	O

However	O
,	O
the	O
resulting	O
decrease	O
in	O
the	O
AdML	B-gene
RNA	B-evidence
affinity	I-evidence
of	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
-	I-mutant
3Gly	I-mutant
mutant	B-protein_state
relative	O
to	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
protein	B-protein
was	O
not	O
significant	O
(	O
Fig	O
.	O
4b	O
).	O

In	O
parallel	O
,	O
we	O
replaced	B-experimental_method
five	O
linker	B-structure_element
residues	I-structure_element
(	O
S251	B-residue_name_number
,	O
T252	B-residue_name_number
,	O
V253	B-residue_name_number
,	O
V254	B-residue_name_number
and	O
P255	B-residue_name_number
)	O
at	O
the	O
fifth	B-site
nucleotide	I-site
-	I-site
binding	I-site
site	I-site
with	O
glycines	B-residue_name
(	O
5Gly	B-mutant
)	O
and	O
also	O
found	O
that	O
the	O
RNA	B-evidence
affinity	I-evidence
of	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
-	I-mutant
5Gly	I-mutant
mutant	B-protein_state
likewise	O
decreased	O
only	O
slightly	O
relative	O
to	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
protein	B-protein
.	O

A	O
more	O
conservative	B-experimental_method
substitution	I-experimental_method
of	O
these	O
five	O
residues	O
(	O
251	B-residue_range
–	I-residue_range
255	I-residue_range
)	O
with	O
an	O
unrelated	O
sequence	O
capable	O
of	O
backbone	O
-	O
mediated	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
(	O
STVVP	B-mutant
>	I-mutant
NLALA	I-mutant
)	O
confirmed	O
the	O
subtle	O
impact	O
of	O
this	O
versatile	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
sequence	I-structure_element
on	O
affinity	B-evidence
for	O
the	O
AdML	B-gene
Py	B-chemical
tract	I-chemical
.	O

Finally	O
,	O
to	O
ensure	O
that	O
these	O
selective	O
mutations	O
were	O
sufficient	O
to	O
disrupt	O
the	O
linker	B-structure_element
/	O
RRM	B-structure_element
contacts	O
,	O
we	O
substituted	B-experimental_method
glycine	B-residue_name
for	O
the	O
majority	O
of	O
buried	O
hydrophobic	O
residues	O
in	O
the	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
(	O
including	O
M144	B-residue_name_number
,	O
L235	B-residue_name_number
,	O
M238	B-residue_name_number
,	O
V244	B-residue_name_number
,	O
V246	B-residue_name_number
,	O
V249	B-residue_name_number
,	O
V250	B-residue_name_number
,	O
S251	B-residue_name_number
,	O
T252	B-residue_name_number
,	O
V253	B-residue_name_number
,	O
V254	B-residue_name_number
,	O
P255	B-residue_name_number
;	O
called	O
12Gly	B-mutant
).	O

To	O
test	O
the	O
interplay	O
of	O
the	O
U2AF65	B-protein
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
with	O
its	O
N	O
-	O
and	O
C	O
-	O
terminal	O
RRM	B-structure_element
extensions	I-structure_element
,	O
we	O
constructed	B-experimental_method
an	O
internal	O
linker	B-experimental_method
deletion	I-experimental_method
of	O
20	B-residue_range
-	I-residue_range
residues	I-residue_range
within	O
the	O
extended	B-protein_state
RNA	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
(	O
dU2AF651	B-mutant
,	I-mutant
2L	I-mutant
).	O

We	O
found	O
that	O
the	O
affinity	B-evidence
of	O
dU2AF651	B-mutant
,	I-mutant
2L	I-mutant
for	O
the	O
AdML	B-gene
RNA	B-chemical
was	O
significantly	O
reduced	O
relative	O
to	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
(	O
four	O
-	O
fold	O
,	O
Figs	O
1b	O
and	O
4b	O
;	O
Supplementary	O
Fig	O
.	O
4i	O
).	O

Yet	O
,	O
it	O
is	O
well	O
known	O
that	O
the	O
linker	B-experimental_method
deletion	I-experimental_method
in	O
the	O
context	O
of	O
the	O
minimal	B-protein_state
RRM1	B-structure_element
–	O
RRM2	B-structure_element
boundaries	O
has	O
no	O
detectable	O
effect	O
on	O
the	O
RNA	B-evidence
affinities	I-evidence
of	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
compared	O
with	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
(	O
refs	O
;	O
Figs	O
1b	O
and	O
4b	O
;	O
Supplementary	O
Fig	O
.	O
4j	O
).	O

The	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
structures	B-evidence
suggest	O
that	O
an	O
extended	B-protein_state
conformation	I-protein_state
of	O
the	O
truncated	B-protein_state
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
would	O
suffice	O
to	O
connect	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
RRM1	B-structure_element
C	O
terminus	O
to	O
the	O
N	O
terminus	O
of	O
RRM2	B-structure_element
(	O
24	O
Å	O
distance	O
between	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
R227	B-residue_name_number
-	O
Cα	O
–	O
H259	B-residue_name_number
-	O
Cα	O
atoms	O
),	O
which	O
agrees	O
with	O
the	O
greater	O
RNA	B-evidence
affinities	I-evidence
of	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
and	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
dual	B-protein_state
RRMs	B-structure_element
compared	O
with	O
the	O
individual	B-protein_state
U2AF65	B-protein
RRMs	B-structure_element
.	O

This	O
difference	O
indicates	O
that	O
the	O
linearly	B-protein_state
distant	I-protein_state
regions	B-structure_element
of	O
the	O
U2AF65	B-protein
primary	O
sequence	O
,	O
including	O
Q147	B-residue_name_number
in	O
the	O
N	O
-	O
terminal	O
RRM1	B-structure_element
extension	I-structure_element
and	O
R227	B-residue_name_number
/	O
V254	B-residue_name_number
in	O
the	O
N	O
-/	O
C	O
-	O
terminal	O
linker	B-structure_element
regions	I-structure_element
at	O
the	O
fifth	B-site
nucleotide	I-site
site	I-site
,	O
cooperatively	O
recognize	O
the	O
Py	B-chemical
tract	I-chemical
.	O

Altogether	O
,	O
we	O
conclude	O
that	O
the	O
conformation	O
of	O
the	O
U2AF65	B-protein
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
is	O
key	O
for	O
recognizing	O
RNA	B-chemical
and	O
is	O
positioned	O
by	O
the	O
RRM	B-structure_element
extension	I-structure_element
but	O
otherwise	O
relatively	O
independent	O
of	O
the	O
side	O
chain	O
composition	O
.	O

Importance	O
of	O
U2AF65	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
contacts	O
for	O
pre	B-chemical
-	I-chemical
mRNA	I-chemical
splicing	O

We	O
proceeded	O
to	O
test	O
the	O
importance	O
of	O
new	O
U2AF65	B-complex_assembly
–	I-complex_assembly
Py	I-complex_assembly
-	I-complex_assembly
tract	I-complex_assembly
interactions	O
for	O
splicing	O
of	O
a	O
model	O
pre	B-chemical
-	I-chemical
mRNA	I-chemical
substrate	O
in	O
a	O
human	B-species
cell	O
line	O
(	O
Fig	O
.	O
5	O
;	O
Supplementary	O
Fig	O
.	O
5	O
).	O

When	O
transfected	B-experimental_method
into	O
HEK293T	O
cells	O
containing	O
only	O
endogenous	B-protein_state
U2AF65	B-protein
,	O
the	O
PY	B-site
splice	I-site
site	I-site
is	O
used	O
and	O
the	O
remaining	O
transcript	O
remains	O
unspliced	O
.	O

When	O
co	B-experimental_method
-	I-experimental_method
transfected	I-experimental_method
with	O
an	O
expression	B-experimental_method
plasmid	I-experimental_method
for	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
U2AF65	B-protein
,	O
use	O
of	O
the	O
py	B-site
splice	I-site
site	I-site
significantly	O
increases	O
(	O
by	O
more	O
than	O
five	O
-	O
fold	O
)	O
and	O
as	O
documented	O
converts	O
a	O
fraction	O
of	O
the	O
unspliced	O
to	O
spliced	O
transcript	O
.	O

The	O
strong	O
PY	B-site
splice	I-site
site	I-site
is	O
insensitive	O
to	O
added	O
U2AF65	B-protein
,	O
suggesting	O
that	O
endogenous	B-protein_state
U2AF65	B-protein
levels	O
are	O
sufficient	O
to	O
saturate	O
this	O
site	O
(	O
Supplementary	O
Fig	O
.	O
5b	O
).	O

We	O
introduced	O
the	O
triple	B-experimental_method
mutation	I-experimental_method
(	O
V254P	B-mutant
/	O
R227A	B-mutant
/	O
Q147A	B-mutant
)	O
that	O
significantly	O
reduced	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
association	O
with	O
the	O
Py	B-chemical
tract	I-chemical
(	O
Fig	O
.	O
4b	O
)	O
in	O
the	O
context	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
U2AF65	B-protein
(	O
U2AF65	B-mutant
-	I-mutant
3Mut	I-mutant
).	O

Co	B-experimental_method
-	I-experimental_method
transfection	I-experimental_method
of	O
the	O
U2AF65	B-mutant
-	I-mutant
3Mut	I-mutant
with	O
the	O
pyPY	B-chemical
splicing	O
substrate	O
significantly	O
reduced	O
splicing	O
of	O
the	O
weak	O
‘	B-site
py	I-site
'	I-site
splice	I-site
site	I-site
relative	O
to	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
U2AF65	B-protein
(	O
Fig	O
.	O
5b	O
,	O
c	O
).	O

We	O
conclude	O
that	O
the	O
Py	B-chemical
-	I-chemical
tract	I-chemical
interactions	O
with	O
these	O
residues	O
of	O
the	O
U2AF65	B-protein
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
and	O
RRM	B-structure_element
extensions	I-structure_element
are	O
important	O
for	O
splicing	O
as	O
well	O
as	O
for	O
binding	O
a	O
representative	O
of	O
the	O
major	B-structure_element
U2	I-structure_element
-	I-structure_element
class	I-structure_element
of	I-structure_element
splice	I-structure_element
sites	I-structure_element
.	O

This	O
minor	O
U2AF65	B-protein
RRM1	B-site
/	I-site
RRM2	I-site
interface	I-site
,	O
coupled	O
with	O
the	O
versatile	O
sequence	O
of	O
the	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
,	O
highlighted	O
the	O
potential	O
role	O
for	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
conformational	O
dynamics	O
in	O
U2AF65	B-protein
-	O
splice	O
site	O
recognition	O
.	O

Yet	O
,	O
small	B-experimental_method
-	I-experimental_method
angle	I-experimental_method
X	I-experimental_method
-	I-experimental_method
ray	I-experimental_method
scattering	I-experimental_method
(	O
SAXS	B-experimental_method
)	O
data	O
indicated	O
that	O
both	O
the	O
minimal	B-protein_state
U2AF651	B-mutant
,	I-mutant
2	I-mutant
and	O
longer	O
constructs	O
comprise	O
a	O
highly	B-protein_state
diverse	I-protein_state
continuum	I-protein_state
of	I-protein_state
conformations	I-protein_state
in	O
the	O
absence	B-protein_state
of	I-protein_state
RNA	B-chemical
that	O
includes	O
the	O
‘	O
closed	B-protein_state
'	O
and	O
‘	O
open	B-protein_state
'	O
conformations	O
.	O

To	O
complement	O
the	O
static	O
portraits	O
of	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
structure	B-evidence
that	O
we	O
had	O
determined	O
by	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
,	O
we	O
used	O
smFRET	B-experimental_method
to	O
characterize	O
the	O
probability	B-evidence
distribution	I-evidence
functions	I-evidence
and	O
time	O
dependence	O
of	O
U2AF65	B-protein
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
conformational	O
dynamics	O
in	O
solution	O
.	O

The	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
dynamics	O
of	O
U2AF65	B-protein
were	O
followed	O
using	O
FRET	B-experimental_method
between	O
fluorophores	B-chemical
attached	O
to	O
RRM1	B-structure_element
and	O
RRM2	B-structure_element
(	O
Fig	O
.	O
6a	O
,	O
b	O
,	O
Methods	O
).	O

The	O
positions	O
of	O
single	O
cysteine	B-residue_name
mutations	B-experimental_method
for	O
fluorophore	B-chemical
attachment	O
(	O
A181C	B-mutant
in	O
RRM1	B-structure_element
and	O
Q324C	B-mutant
in	O
RRM2	B-structure_element
)	O
were	O
chosen	O
based	O
on	O
inspection	O
of	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
structures	B-evidence
and	O
the	O
‘	O
closed	B-protein_state
'	O
model	O
of	O
apo	B-protein_state
-	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
.	O

Criteria	O
included	O
(	O
i	O
)	O
residue	O
locations	O
that	O
are	O
distant	O
from	O
and	O
hence	O
not	O
expected	O
to	O
interfere	O
with	O
the	O
RRM	B-complex_assembly
/	I-complex_assembly
RNA	I-complex_assembly
or	O
inter	B-site
-	I-site
RRM	I-site
interfaces	I-site
,	O
(	O
ii	O
)	O
inter	O
-	O
dye	O
distances	O
(	O
50	O
Å	O
for	O
U2AF651	B-complex_assembly
,	I-complex_assembly
2L	I-complex_assembly
–	I-complex_assembly
Py	I-complex_assembly
tract	I-complex_assembly
and	O
30	O
Å	O
for	O
the	O
closed	B-protein_state
apo	B-protein_state
-	O
model	O
)	O
that	O
are	O
expected	O
to	O
be	O
near	O
the	O
Förster	B-experimental_method
radius	I-experimental_method
(	I-experimental_method
Ro	I-experimental_method
)	I-experimental_method
for	O
the	O
Cy3	B-chemical
/	O
Cy5	B-chemical
pair	O
(	O
56	O
Å	O
),	O
where	O
changes	O
in	O
the	O
efficiency	O
of	O
energy	O
transfer	O
are	O
most	O
sensitive	O
to	O
distance	O
,	O
and	O
(	O
iii	O
)	O
FRET	B-evidence
efficiencies	I-evidence
that	O
are	O
calculated	O
to	O
be	O
significantly	O
greater	O
for	O
the	O
‘	O
closed	B-protein_state
'	O
apo	B-protein_state
-	O
model	O
as	O
opposed	O
to	O
the	O
‘	O
open	B-protein_state
'	O
RNA	B-protein_state
-	I-protein_state
bound	I-protein_state
structures	B-evidence
(	O
by	O
∼	O
30	O
%).	O

The	O
FRET	B-evidence
efficiencies	I-evidence
of	O
either	O
of	O
these	O
structurally	O
characterized	O
conformations	O
also	O
are	O
expected	O
to	O
be	O
significantly	O
greater	O
than	O
elongated	B-protein_state
U2AF65	B-protein
conformations	O
that	O
lack	B-protein_state
inter	O
-	O
RRM	B-structure_element
contacts	O
.	O

Double	O
-	O
cysteine	B-residue_name
variant	B-protein_state
of	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
was	O
modified	B-experimental_method
with	O
equimolar	O
amount	O
of	O
Cy3	B-chemical
and	O
Cy5	B-chemical
.	O

Only	O
traces	B-evidence
that	O
showed	O
single	O
photobleaching	O
events	O
for	O
both	O
donor	O
and	O
acceptor	O
dyes	O
and	O
anti	O
-	O
correlated	O
changes	O
in	O
acceptor	O
and	O
donor	O
fluorescence	O
were	O
included	O
in	O
smFRET	B-experimental_method
data	O
analysis	O
.	O

The	O
double	O
-	O
labelled	O
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
protein	O
was	O
tethered	B-protein_state
to	O
a	O
slide	O
via	O
biotin	B-chemical
-	I-chemical
NTA	I-chemical
/	I-chemical
Ni	I-chemical
+	I-chemical
2	I-chemical
resin	I-chemical
.	O

Virtually	O
no	O
fluorescent	O
molecules	O
were	O
detected	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
biotin	B-chemical
-	I-chemical
NTA	I-chemical
/	I-chemical
Ni	I-chemical
+	I-chemical
2	I-chemical
,	O
which	O
demonstrates	O
the	O
absence	B-protein_state
of	I-protein_state
detectable	O
non	O
-	O
specific	O
binding	O
of	O
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
to	O
the	O
slide	O
.	O

The	O
FRET	B-evidence
distribution	I-evidence
histogram	I-evidence
built	O
from	O
more	O
than	O
a	O
thousand	O
traces	B-evidence
of	O
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
ligand	B-chemical
showed	O
an	O
extremely	O
broad	O
distribution	O
centred	O
at	O
a	O
FRET	B-evidence
efficiency	I-evidence
of	O
∼	O
0	O
.	O
4	O
(	O
Fig	O
.	O
6d	O
).	O

Despite	O
the	O
large	O
width	O
of	O
the	O
FRET	B-evidence
-	I-evidence
distribution	I-evidence
histogram	I-evidence
,	O
the	O
majority	O
(	O
80	O
%)	O
of	O
traces	B-evidence
that	O
showed	O
fluctuations	O
sampled	O
only	O
two	O
distinct	O
FRET	B-evidence
states	I-evidence
(	O
for	O
example	O
,	O
Supplementary	O
Fig	O
.	O
7a	O
).	O

Approximately	O
70	O
%	O
of	O
observed	O
fluctuations	O
were	O
interchanges	O
between	O
the	O
∼	O
0	O
.	O
65	O
and	O
∼	O
0	O
.	O
45	O
FRET	B-evidence
values	I-evidence
(	O
Supplementary	O
Fig	O
.	O
7b	O
).	O

We	O
cannot	O
exclude	O
a	O
possibility	O
that	O
tethering	O
of	O
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
to	O
the	O
microscope	O
slide	O
introduces	O
structural	O
heterogeneity	O
into	O
the	O
protein	O
and	O
,	O
thus	O
,	O
contributes	O
to	O
the	O
breadth	O
of	O
the	O
FRET	B-evidence
distribution	I-evidence
histogram	I-evidence
.	O

However	O
,	O
the	O
presence	O
of	O
repetitive	O
fluctuations	O
between	O
particular	O
FRET	B-evidence
values	I-evidence
supports	O
the	O
hypothesis	O
that	O
RNA	B-protein_state
-	I-protein_state
free	I-protein_state
U2AF65	B-protein
samples	O
several	O
distinct	O
conformations	O
.	O

This	O
result	O
is	O
consistent	O
with	O
the	O
broad	O
ensembles	O
of	O
extended	B-protein_state
solution	O
conformations	O
that	O
best	O
fit	O
the	O
SAXS	B-experimental_method
data	O
collected	O
for	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
as	O
well	O
as	O
for	O
a	O
longer	O
construct	O
(	O
residues	O
136	B-residue_range
–	I-residue_range
347	I-residue_range
).	O

We	O
conclude	O
that	O
weak	O
contacts	O
between	O
the	O
U2AF65	B-protein
RRM1	B-structure_element
and	O
RRM2	B-structure_element
permit	O
dissociation	O
of	O
these	O
RRMs	B-structure_element
in	O
the	O
absence	B-protein_state
of	I-protein_state
RNA	B-chemical
.	O

U2AF65	B-protein
conformational	O
selection	O
and	O
induced	O
fit	O
by	O
bound	B-protein_state
RNA	B-chemical

We	O
next	O
used	O
smFRET	B-experimental_method
to	O
probe	O
the	O
conformational	O
selection	O
of	O
distinct	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
arrangements	O
following	O
association	O
of	O
U2AF65	B-protein
with	O
the	O
AdML	B-gene
Py	B-chemical
-	I-chemical
tract	I-chemical
prototype	O
.	O

Addition	O
of	O
the	O
AdML	B-gene
RNA	B-chemical
to	O
tethered	B-protein_state
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
selectively	O
increases	O
a	O
fraction	O
of	O
molecules	O
showing	O
an	O
∼	O
0	O
.	O
45	O
apparent	O
FRET	B-evidence
efficiency	I-evidence
,	O
suggesting	O
that	O
RNA	O
binding	O
stabilizes	O
a	O
single	O
conformation	O
,	O
which	O
corresponds	O
to	O
the	O
0	O
.	O
45	O
FRET	B-evidence
state	I-evidence
(	O
Fig	O
.	O
6e	O
,	O
f	O
).	O

To	O
assess	O
the	O
possible	O
contributions	O
of	O
RNA	B-protein_state
-	I-protein_state
free	I-protein_state
conformations	O
of	O
U2AF65	B-protein
and	O
/	O
or	O
structural	O
heterogeneity	O
introduced	O
by	O
tethering	B-experimental_method
of	O
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
to	O
the	O
slide	O
to	O
the	O
observed	O
distribution	B-evidence
of	I-evidence
FRET	I-evidence
values	I-evidence
,	O
we	O
reversed	B-experimental_method
the	I-experimental_method
immobilization	I-experimental_method
scheme	I-experimental_method
.	O

We	O
tethered	B-protein_state
the	O
AdML	B-gene
RNA	B-chemical
to	O
the	O
slide	O
via	O
a	O
biotinylated	B-chemical
oligonucleotide	I-chemical
DNA	I-chemical
handle	O
and	O
added	B-experimental_method
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
biotin	B-chemical
-	I-chemical
NTA	I-chemical
resin	I-chemical
(	O
Fig	O
.	O
6g	O
,	O
h	O
;	O
Supplementary	O
Fig	O
.	O
7c	O
–	O
g	O
).	O

We	O
examined	O
the	O
effect	O
on	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
conformations	O
of	O
purine	B-experimental_method
interruptions	I-experimental_method
that	O
often	O
occur	O
in	O
relatively	O
degenerate	O
human	B-species
Py	B-chemical
tracts	I-chemical
.	O

We	O
introduced	B-experimental_method
an	O
rArA	B-chemical
purine	B-chemical
dinucleotide	I-chemical
within	O
a	O
variant	O
of	O
the	O
AdML	B-gene
Py	B-chemical
tract	I-chemical
(	O
detailed	O
in	O
Methods	O
).	O

Insertion	B-experimental_method
of	O
adenine	B-chemical
nucleotides	I-chemical
decreased	O
binding	B-evidence
affinity	I-evidence
of	O
U2AF65	B-protein
to	O
RNA	B-chemical
by	O
approximately	O
five	O
-	O
fold	O
.	O

Nevertheless	O
,	O
in	O
the	O
presence	O
of	O
saturating	O
concentrations	O
of	O
rArA	B-chemical
-	O
interrupted	O
RNA	B-chemical
slide	B-protein_state
-	I-protein_state
tethered	I-protein_state
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
showed	O
a	O
prevalent	O
∼	O
0	O
.	O
45	O
apparent	O
FRET	B-evidence
value	I-evidence
(	O
Fig	O
.	O
6i	O
,	O
j	O
),	O
which	O
was	O
also	O
predominant	O
in	O
the	O
presence	O
of	O
continuous	O
Py	B-chemical
tract	I-chemical
.	O

It	O
should	O
be	O
noted	O
that	O
inferring	O
distances	O
from	O
FRET	B-evidence
values	I-evidence
is	O
prone	O
to	O
significant	O
error	O
because	O
of	O
uncertainties	O
in	O
the	O
determination	O
of	O
fluorophore	O
orientation	O
factor	O
κ2	O
and	O
Förster	O
radius	O
R0	O
,	O
the	O
parameters	O
used	O
in	O
distance	O
calculations	O
.	O

Importantly	O
,	O
the	O
majority	O
of	O
traces	B-evidence
(∼	O
70	O
%)	O
of	O
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
bound	B-protein_state
to	I-protein_state
the	O
slide	O
-	O
tethered	O
RNA	B-chemical
lacked	O
FRET	O
fluctuations	O
and	O
predominately	O
exhibited	O
a	O
∼	O
0	O
.	O
45	O
FRET	B-evidence
value	I-evidence
(	O
for	O
example	O
,	O
Fig	O
.	O
6g	O
).	O

The	O
remaining	O
∼	O
30	O
%	O
of	O
traces	B-evidence
for	O
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
bound	B-protein_state
to	I-protein_state
the	O
slide	O
-	O
tethered	O
RNA	B-chemical
showed	O
fluctuations	O
between	O
distinct	O
FRET	B-evidence
values	I-evidence
.	O

Hidden	B-experimental_method
Markov	I-experimental_method
modelling	I-experimental_method
analysis	I-experimental_method
of	O
smFRET	B-experimental_method
traces	B-evidence
suggests	O
that	O
RNA	B-protein_state
-	I-protein_state
bound	I-protein_state
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
can	O
sample	O
at	O
least	O
two	O
other	O
conformations	O
corresponding	O
to	O
∼	O
0	O
.	O
7	O
–	O
0	O
.	O
8	O
and	O
∼	O
0	O
.	O
3	O
FRET	B-evidence
values	I-evidence
in	O
addition	O
to	O
the	O
predominant	O
conformation	O
corresponding	O
to	O
the	O
0	O
.	O
45	O
FRET	B-evidence
state	I-evidence
.	O

Although	O
a	O
compact	O
conformation	O
(	O
or	O
multiple	O
conformations	O
)	O
of	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
corresponding	O
to	O
∼	O
0	O
.	O
7	O
–	O
0	O
.	O
8	O
FRET	B-evidence
values	I-evidence
can	O
bind	O
RNA	B-chemical
,	O
on	O
RNA	B-chemical
binding	O
,	O
these	O
compact	B-protein_state
conformations	O
of	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
transition	O
into	O
a	O
more	O
stable	O
structural	O
state	O
that	O
corresponds	O
to	O
∼	O
0	O
.	O
45	O
FRET	B-evidence
value	I-evidence
and	O
is	O
likely	O
similar	O
to	O
the	O
side	B-protein_state
-	I-protein_state
by	I-protein_state
-	I-protein_state
side	I-protein_state
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
-	O
arrangement	O
of	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
crystal	B-evidence
structures	I-evidence
.	O

The	O
U2AF65	B-protein
structures	B-evidence
and	O
analyses	B-evidence
presented	O
here	O
represent	O
a	O
successful	O
step	O
towards	O
defining	O
a	O
molecular	O
map	O
of	O
the	O
3	B-site
′	I-site
splice	I-site
site	I-site
.	O

Truncation	B-experimental_method
of	O
U2AF65	B-protein
to	O
the	O
core	B-protein_state
RRM1	B-structure_element
–	I-structure_element
RRM2	I-structure_element
region	I-structure_element
reduces	O
its	O
RNA	B-evidence
affinity	I-evidence
by	O
100	O
-	O
fold	O
.	O

Likewise	O
,	O
deletion	B-experimental_method
of	O
20	B-residue_range
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
residues	I-structure_element
significantly	O
reduces	O
U2AF65	B-protein
–	O
RNA	B-chemical
binding	O
only	O
when	O
introduced	O
in	O
the	O
context	O
of	O
the	O
longer	B-protein_state
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
construct	O
comprising	O
the	O
RRM	B-structure_element
extensions	I-structure_element
,	O
which	O
in	O
turn	O
position	O
the	O
linker	B-structure_element
for	O
RNA	B-chemical
interactions	O
.	O

Notably	O
,	O
a	O
triple	B-protein_state
mutation	I-protein_state
of	O
three	O
residues	O
(	O
V254P	B-mutant
,	O
Q147A	B-mutant
and	O
R227A	B-mutant
)	O
in	O
the	O
respective	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
,	O
N	B-structure_element
-	I-structure_element
and	I-structure_element
C	I-structure_element
-	I-structure_element
terminal	I-structure_element
extensions	I-structure_element
non	O
-	O
additively	O
reduce	O
RNA	B-evidence
binding	I-evidence
by	O
150	O
-	O
fold	O
.	O

The	O
implications	O
of	O
this	O
finding	O
for	O
U2AF65	B-protein
conservation	O
and	O
Py	B-chemical
-	I-chemical
tract	I-chemical
recognition	O
are	O
detailed	O
in	O
the	O
Supplementary	O
Discussion	O
.	O

Recently	O
,	O
high	B-experimental_method
-	I-experimental_method
throughput	I-experimental_method
sequencing	I-experimental_method
studies	I-experimental_method
have	O
shown	O
that	O
somatic	O
mutations	O
in	O
pre	B-protein_type
-	I-protein_type
mRNA	I-protein_type
splicing	I-protein_type
factors	I-protein_type
occur	O
in	O
the	O
majority	O
of	O
patients	O
with	O
myelodysplastic	O
syndrome	O
(	O
MDS	O
).	O

MDS	O
-	O
relevant	O
mutations	O
are	O
common	O
in	O
the	O
small	B-protein_state
U2AF	B-protein_type
subunit	I-protein_type
(	O
U2AF35	B-protein
,	O
or	O
U2AF1	B-protein
),	O
yet	O
such	O
mutations	O
are	O
rare	O
in	O
the	O
large	B-protein_state
U2AF65	B-protein
subunit	O
(	O
also	O
called	O
U2AF2	B-protein
)—	O
possibly	O
due	O
to	O
the	O
selective	O
versus	O
nearly	O
universal	O
requirements	O
of	O
these	O
factors	O
for	O
splicing	O
.	O

A	O
confirmed	O
somatic	O
mutation	O
of	O
U2AF65	B-protein
in	O
patients	O
with	O
MDS	O
,	O
L187V	B-mutant
,	O
is	O
located	O
on	O
a	O
solvent	B-site
-	I-site
exposed	I-site
surface	I-site
of	O
RRM1	B-structure_element
that	O
is	O
distinct	O
from	O
the	O
RNA	B-site
interface	I-site
(	O
Fig	O
.	O
7a	O
).	O

This	O
L187	B-residue_name_number
surface	O
is	O
oriented	O
towards	O
the	O
N	O
terminus	O
of	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
construct	O
,	O
where	O
it	O
is	O
expected	O
to	O
abut	O
the	O
U2AF35	B-site
-	I-site
binding	I-site
site	I-site
in	O
the	O
context	O
of	O
the	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
U2AF	B-protein
heterodimer	B-oligomeric_state
.	O

Likewise	O
,	O
an	O
unconfirmed	O
M144I	B-mutant
mutation	O
reported	O
by	O
the	O
same	O
group	O
corresponds	O
to	O
the	O
N	O
-	O
terminal	O
residue	O
of	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
,	O
which	O
is	O
separated	O
by	O
only	O
∼	O
20	O
residues	O
from	O
the	O
U2AF35	B-site
-	I-site
binding	I-site
site	I-site
.	O

As	O
such	O
,	O
we	O
suggest	O
that	O
the	O
MDS	O
-	O
relevant	O
U2AF65	B-protein
mutations	O
contribute	O
to	O
MDS	O
progression	O
indirectly	O
,	O
by	O
destabilizing	O
a	O
relevant	O
conformation	O
of	O
the	O
conjoined	O
U2AF35	B-protein
subunit	O
rather	O
than	O
affecting	O
U2AF65	B-protein
functions	O
in	O
RNA	B-chemical
binding	O
or	O
spliceosome	B-complex_assembly
recruitment	O
per	O
se	O
.	O

An	O
increased	O
prevalence	O
of	O
the	O
∼	O
0	O
.	O
45	O
FRET	B-evidence
value	I-evidence
following	O
U2AF65	B-protein
–	O
RNA	B-chemical
binding	O
,	O
coupled	O
with	O
the	O
apparent	O
absence	B-protein_state
of	I-protein_state
transitions	O
in	O
many	O
∼	O
0	O
.	O
45	O
-	O
value	O
single	O
molecule	O
traces	B-evidence
(	O
for	O
example	O
,	O
Fig	O
.	O
6e	O
),	O
suggests	O
a	O
population	O
shift	O
in	O
which	O
RNA	B-chemical
binds	O
to	O
(	O
and	O
draws	O
the	O
equilibrium	O
towards	O
)	O
a	O
pre	B-protein_state
-	I-protein_state
configured	I-protein_state
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
proximity	O
that	O
most	O
often	O
corresponds	O
to	O
the	O
∼	O
0	O
.	O
45	O
FRET	B-evidence
value	I-evidence
.	O

Examples	O
of	O
‘	O
extended	B-protein_state
conformational	O
selection	O
'	O
during	O
ligand	O
binding	O
have	O
been	O
characterized	O
for	O
a	O
growing	O
number	O
of	O
macromolecules	O
(	O
for	O
example	O
,	O
adenylate	B-protein_type
kinase	I-protein_type
,	O
LAO	B-protein_type
-	I-protein_type
binding	I-protein_type
protein	I-protein_type
,	O
poly	B-protein_type
-	I-protein_type
ubiquitin	I-protein_type
,	O
maltose	B-protein_type
-	I-protein_type
binding	I-protein_type
protein	I-protein_type
and	O
the	O
preQ1	B-protein_type
riboswitch	I-protein_type
,	O
among	O
others	O
).	O

Here	O
,	O
the	O
majority	O
of	O
changes	O
in	O
smFRET	B-experimental_method
traces	B-evidence
for	O
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
bound	B-protein_state
to	I-protein_state
slide	O
-	O
tethered	O
RNA	B-chemical
began	O
at	O
high	O
(	O
0	O
.	O
65	O
–	O
0	O
.	O
8	O
)	O
FRET	B-evidence
value	I-evidence
and	O
transition	O
to	O
the	O
predominant	O
0	O
.	O
45	O
FRET	B-evidence
value	I-evidence
(	O
Supplementary	O
Fig	O
.	O
7c	O
–	O
g	O
).	O

These	O
transitions	O
could	O
correspond	O
to	O
rearrangement	O
from	O
the	O
‘	O
closed	B-protein_state
'	O
NMR	B-experimental_method
/	O
PRE	B-experimental_method
-	O
based	O
U2AF65	B-protein
conformation	O
in	O
which	O
the	O
RNA	B-site
-	I-site
binding	I-site
surface	I-site
of	O
only	O
a	O
single	B-protein_state
RRM	B-structure_element
is	O
exposed	O
and	O
available	O
for	O
RNA	O
binding	O
,	O
to	O
the	O
structural	O
state	O
seen	O
in	O
the	O
side	B-protein_state
-	I-protein_state
by	I-protein_state
-	I-protein_state
side	I-protein_state
,	O
RNA	B-protein_state
-	I-protein_state
bound	I-protein_state
crystal	B-evidence
structure	I-evidence
.	O

The	O
finding	O
that	O
U2AF65	B-protein
recognizes	O
a	O
nine	O
base	O
pair	O
Py	B-chemical
tract	I-chemical
contributes	O
to	O
an	O
elusive	O
‘	O
code	O
'	O
for	O
predicting	O
splicing	O
patterns	O
from	O
primary	O
sequences	O
in	O
the	O
post	O
-	O
genomic	O
era	O
(	O
reviewed	O
in	O
ref	O
.).	O

Further	O
research	O
will	O
be	O
needed	O
to	O
understand	O
the	O
roles	O
of	O
SF1	B-protein
and	O
U2AF35	B-protein
subunits	O
in	O
the	O
conformational	O
equilibria	O
underlying	O
U2AF65	B-protein
association	O
with	O
Py	B-chemical
tracts	I-chemical
.	O

Moreover	O
,	O
structural	O
differences	O
among	O
U2AF65	B-protein
homologues	O
and	O
paralogues	O
may	O
regulate	O
splice	B-site
site	I-site
selection	O
.	O

Ultimately	O
,	O
these	O
guidelines	O
will	O
assist	O
the	O
identification	O
of	O
3	B-site
′	I-site
splice	I-site
sites	I-site
and	O
the	O
relationship	O
of	O
disease	O
-	O
causing	O
mutations	O
to	O
penalties	O
for	O
U2AF65	B-protein
association	O
.	O

The	O
intact	B-protein_state
U2AF65	B-protein
RRM1	B-structure_element
/	O
RRM2	B-structure_element
-	O
containing	O
domain	O
and	O
flanking	O
residues	O
are	O
required	O
for	O
binding	O
contiguous	B-structure_element
Py	B-chemical
tracts	I-chemical
.	O

(	O
a	O
)	O
Domain	O
organization	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
(	O
fl	B-protein_state
)	O
U2AF65	B-protein
and	O
constructs	O
used	O
for	O
RNA	B-chemical
binding	O
and	O
structural	O
experiments	O
.	O

An	O
internal	O
deletion	O
(	O
d	B-mutant
,	O
Δ	B-mutant
)	O
of	O
residues	O
238	B-residue_range
–	I-residue_range
257	I-residue_range
removes	O
a	O
portion	O
of	O
the	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
from	O
the	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
and	O
dU2AF651	B-mutant
,	I-mutant
2L	I-mutant
constructs	O
.	O

(	O
b	O
)	O
Comparison	O
of	O
the	O
apparent	O
equilibrium	B-evidence
affinities	I-evidence
of	O
various	O
U2AF65	B-protein
constructs	O
for	O
binding	O
the	O
prototypical	O
AdML	B-gene
Py	B-chemical
tract	I-chemical
(	O
5	B-chemical
′-	I-chemical
CCCUUUUUUUUCC	I-chemical
-	I-chemical
3	I-chemical
′).	I-chemical

The	O
flU2AF65	B-protein
protein	O
includes	O
a	O
heterodimerization	B-structure_element
domain	I-structure_element
of	O
the	O
U2AF35	B-protein
subunit	O
to	O
promote	O
solubility	O
and	O
folding	O
.	O

The	O
apparent	O
equilibrium	B-evidence
dissociation	I-evidence
constants	I-evidence
(	O
KD	B-evidence
)	O
for	O
binding	O
the	O
AdML	B-gene
13mer	O
are	O
as	O
follows	O
:	O
flU2AF65	B-protein
,	O
30	O
±	O
3	O
nM	O
;	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
,	O
35	O
±	O
6	O
nM	O
;	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
,	O
3	O
,	O
600	O
±	O
300	O
nM	O
.	O
(	O
c	O
)	O
Comparison	O
of	O
the	O
RNA	B-evidence
sequence	I-evidence
specificities	I-evidence
of	O
flU2AF65	B-protein
and	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
constructs	O
binding	O
C	B-structure_element
-	I-structure_element
rich	I-structure_element
Py	B-chemical
tracts	I-chemical
with	O
4U	O
'	O
s	O
embedded	O
in	O
either	O
the	O
5	O
′-	O
(	O
light	O
grey	O
fill	O
)	O
or	O
3	O
′-	O
(	O
dark	O
grey	O
fill	O
)	O
regions	O
.	O

The	O
KD	B-evidence
'	O
s	O
for	O
binding	O
5	B-chemical
′-	I-chemical
CCUUUUCCCCCCC	I-chemical
-	I-chemical
3	I-chemical
′	I-chemical
are	O
:	O
flU2AF65	B-protein
,	O
41	O
±	O
2	O
nM	O
;	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
,	O
31	O
±	O
3	O
nM	O
.	O
The	O
KD	B-evidence
'	O
s	O
for	O
binding	O
5	B-chemical
′-	I-chemical
CCCCCCCUUUUCC	I-chemical
-	I-chemical
3	I-chemical
′	I-chemical
are	O
:	O
flU2AF65	B-protein
,	O
414	O
±	O
12	O
nM	O
;	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
,	O
417	O
±	O
10	O
nM	O
.	O
Bar	O
graphs	O
are	O
hatched	O
to	O
match	O
the	O
constructs	O
shown	O
in	O
a	O
.	O
The	O
average	B-evidence
apparent	I-evidence
equilibrium	I-evidence
affinity	I-evidence
(	O
KA	B-evidence
)	O
and	O
s	O
.	O
e	O
.	O
m	O
.	O
for	O
three	O
independent	O
titrations	O
are	O
plotted	O
.	O

The	O
purified	O
protein	O
and	O
average	B-evidence
fitted	I-evidence
fluorescence	I-evidence
anisotropy	I-evidence
RNA	I-evidence
-	I-evidence
binding	I-evidence
curves	I-evidence
are	O
shown	O
in	O
Supplementary	O
Fig	O
.	O
1	O
.	O

Structures	B-evidence
of	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
recognizing	O
a	O
contiguous	B-structure_element
Py	B-chemical
tract	I-chemical
.	O

(	O
a	O
)	O
Alignment	B-experimental_method
of	O
oligonucleotide	B-chemical
sequences	O
that	O
were	O
co	B-experimental_method
-	I-experimental_method
crystallized	I-experimental_method
in	O
the	O
indicated	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
structures	B-evidence
.	O

The	O
regions	O
of	O
RRM1	B-structure_element
,	O
RRM2	B-structure_element
and	O
linker	B-structure_element
contacts	O
are	O
indicated	O
above	O
by	O
respective	O
black	O
and	O
blue	O
arrows	O
from	O
N	O
-	O
to	O
C	O
-	O
terminus	O
.	O

For	O
clarity	O
,	O
we	O
consistently	O
number	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
nucleotide	B-site
-	I-site
binding	I-site
sites	I-site
from	O
one	O
to	O
nine	O
,	O
although	O
in	O
some	O
cases	O
the	O
co	B-experimental_method
-	I-experimental_method
crystallized	I-experimental_method
oligonucleotide	B-chemical
comprises	O
eight	O
nucleotides	B-chemical
and	O
as	O
such	O
leaves	O
the	O
first	B-site
binding	I-site
site	I-site
empty	O
.	O

Crystallographic	O
statistics	O
are	O
given	O
in	O
Table	O
1	O
and	O
the	O
overall	O
conformations	O
of	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
and	O
prior	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
/	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
structures	B-evidence
are	O
compared	O
in	O
Supplementary	O
Fig	O
.	O
2	O
.	O

BrdU	B-chemical
,	O
5	B-chemical
-	I-chemical
bromo	I-chemical
-	I-chemical
deoxy	I-chemical
-	I-chemical
uridine	I-chemical
;	O
d	B-chemical
,	O
deoxy	B-chemical
-	I-chemical
ribose	I-chemical
;	O
P	B-chemical
-,	I-chemical
5	B-chemical
′-	I-chemical
phosphorylation	I-chemical
;	O
r	B-chemical
,	O
ribose	B-chemical
.	O

Representative	O
views	O
of	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
interactions	O
with	O
each	O
new	O
nucleotide	B-chemical
of	O
the	O
bound	B-protein_state
Py	B-chemical
tract	I-chemical
.	O

The	O
apparent	O
equilibrium	B-evidence
dissociation	I-evidence
constants	I-evidence
(	O
KD	B-evidence
)	O
of	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
mutant	B-protein_state
proteins	O
are	O
:	O
wild	B-protein_state
type	I-protein_state
(	O
WT	B-protein_state
),	O
35	O
±	O
6	O
nM	O
;	O
R227A	B-mutant
,	O
166	O
±	O
2	O
nM	O
;	O
V254P	B-mutant
,	O
137	O
±	O
10	O
nM	O
;	O
Q147A	B-mutant
,	O
171	O
±	O
21	O
nM	O
.	O
The	O
average	O
KA	B-evidence
and	O
s	O
.	O
e	O
.	O
m	O
.	O
for	O
three	O
independent	O
titrations	O
are	O
plotted	O
.	O

The	O
U2AF65	B-protein
linker	B-structure_element
/	O
RRM	B-structure_element
and	O
inter	O
-	O
RRM	B-structure_element
interactions	O
.	O

(	O
a	O
)	O
Contacts	O
of	O
the	O
U2AF65	B-protein
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
linker	I-structure_element
with	O
the	O
RRMs	B-structure_element
.	O

A	O
semi	O
-	O
transparent	O
space	O
-	O
filling	O
surface	O
is	O
shown	O
for	O
the	O
RRM1	B-structure_element
(	O
green	O
)	O
and	O
RRM2	B-structure_element
(	O
light	O
blue	O
).	O

Residues	O
V249	B-residue_name_number
,	O
V250	B-residue_name_number
,	O
V254	B-residue_name_number
(	O
yellow	O
)	O
are	O
mutated	B-experimental_method
to	O
V249G	B-mutant
/	O
V250G	B-mutant
/	O
V254G	B-mutant
in	O
the	O
3Gly	B-mutant
mutant	I-mutant
;	O
residues	O
S251	B-residue_name_number
,	O
T252	B-residue_name_number
,	O
V253	B-residue_name_number
,	O
P255	B-residue_name_number
(	O
red	O
)	O
along	O
with	O
V254	B-residue_name_number
are	O
mutated	B-experimental_method
to	O
S251G	B-mutant
/	O
T252G	B-mutant
/	O
V253G	B-mutant
/	O
V254G	B-mutant
/	O
P255G	B-mutant
in	O
the	O
5Gly	B-mutant
mutant	I-mutant
or	O
to	O
S251N	B-mutant
/	O
T252L	B-mutant
/	O
V253A	B-mutant
/	O
V254L	B-mutant
/	O
P255A	B-mutant
in	O
the	O
NLALA	B-mutant
mutant	I-mutant
;	O
residues	O
M144	B-residue_name_number
,	O
L235	B-residue_name_number
,	O
M238	B-residue_name_number
,	O
V244	B-residue_name_number
,	O
V246	B-residue_name_number
(	O
orange	O
)	O
along	O
with	O
V249	B-residue_name_number
,	O
V250	B-residue_name_number
,	O
S251	B-residue_name_number
,	O
T252	B-residue_name_number
,	O
V253	B-residue_name_number
,	O
V254	B-residue_name_number
,	O
P255	B-residue_name_number
are	O
mutated	B-experimental_method
to	O
M144G	B-mutant
/	O
L235G	B-mutant
/	O
M238G	B-mutant
/	O
V244G	B-mutant
/	O
V246G	B-mutant
/	O
V249G	B-mutant
/	O
V250G	B-mutant
/	O
S251G	B-mutant
/	O
T252G	B-mutant
/	O
V253G	B-mutant
/	O
V254G	B-mutant
/	O
P255G	B-mutant
in	O
the	O
12Gly	B-mutant
mutant	I-mutant
.	O

The	O
central	O
panel	O
shows	O
an	O
overall	O
view	O
with	O
stick	O
diagrams	O
for	O
mutated	O
residues	O
;	O
boxed	O
regions	O
are	O
expanded	O
to	O
show	O
the	O
C	O
-	O
terminal	O
(	O
bottom	O
left	O
)	O
and	O
central	B-structure_element
linker	I-structure_element
regions	I-structure_element
(	O
top	O
)	O
at	O
the	O
inter	B-structure_element
-	I-structure_element
RRM	I-structure_element
interfaces	I-structure_element
,	O
and	O
N	O
-	O
terminal	O
linker	O
region	O
contacts	O
with	O
RRM1	B-structure_element
(	O
bottom	O
right	O
).	O

(	O
b	O
)	O
Bar	O
graph	O
of	O
apparent	O
equilibrium	B-evidence
affinities	I-evidence
(	O
KA	B-evidence
)	O
for	O
the	O
AdML	B-gene
Py	B-chemical
tract	I-chemical
(	O
5	B-chemical
′-	I-chemical
CCCUUUUUUUUCC	I-chemical
-	I-chemical
3	I-chemical
′)	I-chemical
of	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
blue	O
)	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
protein	O
compared	O
with	O
mutations	O
of	O
the	O
residues	O
shown	O
in	O
a	O
:	O
3Gly	B-mutant
(	O
yellow	O
),	O
5Gly	B-mutant
(	O
red	O
),	O
NLALA	B-mutant
(	O
hatched	O
red	O
),	O
12Gly	B-mutant
(	O
orange	O
)	O
and	O
the	O
linker	B-experimental_method
deletions	I-experimental_method
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
in	O
the	O
minimal	B-protein_state
RRM1	B-structure_element
–	I-structure_element
RRM2	I-structure_element
region	I-structure_element
(	O
residues	O
148	B-residue_range
–	I-residue_range
237	I-residue_range
,	O
258	B-residue_range
–	I-residue_range
336	I-residue_range
)	O
or	O
dU2AF651	B-mutant
,	I-mutant
2L	I-mutant
(	O
residues	O
141	B-residue_range
–	I-residue_range
237	I-residue_range
,	O
258	B-residue_range
–	I-residue_range
342	I-residue_range
).	O

The	O
apparent	O
equilibrium	B-evidence
dissociation	I-evidence
constants	I-evidence
(	O
KD	B-evidence
)	O
of	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
mutant	B-protein_state
proteins	O
are	O
:	O
wild	B-protein_state
type	I-protein_state
(	O
WT	B-protein_state
),	O
35	O
±	O
6	O
nM	O
;	O
3Gly	B-mutant
,	O
47	O
±	O
4	O
nM	O
;	O
5Gly	B-mutant
,	O
61	O
±	O
3	O
nM	O
;	O
12Gly	B-mutant
,	O
88	O
±	O
21	O
nM	O
;	O
NLALA	B-mutant
,	O
45	O
±	O
3	O
nM	O
;	O
dU2AF651	B-mutant
,	I-mutant
2L	I-mutant
,	O
123	O
±	O
5	O
nM	O
;	O
dU2AF651	B-mutant
,	I-mutant
2	I-mutant
,	O
5000	O
±	O
100	O
nM	O
;	O
3Mut	B-mutant
,	O
5630	O
±	O
70	O
nM	O
.	O
The	O
average	O
KA	B-evidence
and	O
s	O
.	O
e	O
.	O
m	O
.	O
for	O
three	O
independent	O
titrations	O
are	O
plotted	O
.	O

The	O
fitted	O
fluorescence	O
anisotropy	O
RNA	B-evidence
-	I-evidence
binding	I-evidence
curves	I-evidence
are	O
shown	O
in	O
Supplementary	O
Fig	O
.	O
4d	O
–	O
j	O
.	O
(	O
c	O
)	O
Close	O
view	O
of	O
the	O
U2AF65	B-protein
RRM1	B-site
/	I-site
RRM2	I-site
interface	I-site
following	O
a	O
two	O
-	O
fold	O
rotation	O
about	O
the	O
x	O
-	O
axis	O
relative	O
to	O
a	O
.	O

U2AF65	B-protein
inter	O
-	O
domain	O
residues	O
are	O
important	O
for	O
splicing	O
a	O
representative	O
pre	B-chemical
-	I-chemical
mRNA	I-chemical
substrate	O
in	O
human	B-species
cells	O
.	O

(	O
a	O
)	O
Schematic	O
diagram	O
of	O
the	O
pyPY	B-chemical
reporter	O
minigene	O
construct	O
comprising	O
two	O
alternative	O
splice	B-site
sites	I-site
preceded	O
by	O
either	O
the	O
weak	O
IgM	O
Py	B-chemical
tract	I-chemical
(	O
py	B-chemical
)	O
or	O
the	O
strong	O
AdML	B-gene
Py	B-chemical
tract	I-chemical
(	O
PY	B-chemical
)	O
(	O
sequences	O
inset	O
).	O

RNA	O
binding	O
stabilizes	O
the	O
side	B-protein_state
-	I-protein_state
by	I-protein_state
-	I-protein_state
side	I-protein_state
conformation	O
of	O
U2AF65	B-protein
RRMs	B-structure_element
.	O

(	O
a	O
,	O
b	O
)	O
Views	O
of	O
FRET	B-experimental_method
pairs	O
chosen	O
to	O
follow	O
the	O
relative	O
movement	O
of	O
RRM1	B-structure_element
and	O
RRM2	B-structure_element
on	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
‘	O
side	B-protein_state
-	I-protein_state
by	I-protein_state
-	I-protein_state
side	I-protein_state
'	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
RRMs	B-structure_element
bound	B-protein_state
to	I-protein_state
a	O
Py	B-chemical
-	I-chemical
tract	I-chemical
oligonucleotide	I-chemical
(	O
a	O
,	O
representative	O
structure	O
iv	O
)	O
or	O
‘	O
closed	B-protein_state
'	O
NMR	B-experimental_method
/	O
PRE	B-experimental_method
-	O
based	O
model	O
of	O
U2AF651	B-mutant
,	I-mutant
2	I-mutant
(	O
b	O
,	O
PDB	O
ID	O
2YH0	O
)	O
in	O
identical	O
orientations	O
of	O
RRM2	B-structure_element
.	O

The	O
untethered	B-protein_state
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
protein	O
(	O
1	O
nM	O
)	O
was	O
added	O
to	O
AdML	B-gene
RNA	B-chemical
–	I-chemical
polyethylene	I-chemical
-	I-chemical
glycol	I-chemical
-	I-chemical
linker	I-chemical
–	I-chemical
DNA	I-chemical
oligonucleotide	I-chemical
(	O
10	O
nM	O
),	O
which	O
was	O
immobilized	O
on	O
the	O
microscope	O
slide	O
by	O
annealing	O
with	O
a	O
complementary	O
biotinyl	B-chemical
-	I-chemical
DNA	I-chemical
oligonucleotide	I-chemical
(	O
black	O
vertical	O
line	O
).	O

Additional	O
traces	B-evidence
for	O
untethered	B-protein_state
,	O
RNA	B-protein_state
-	I-protein_state
bound	I-protein_state
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
are	O
shown	O
in	O
Supplementary	O
Fig	O
.	O
7c	O
,	O
d	O
.	O
Histograms	B-evidence
(	O
d	O
,	O
f	O
,	O
h	O
,	O
j	O
)	O
show	O
the	O
distribution	B-evidence
of	I-evidence
FRET	I-evidence
values	I-evidence
in	O
RNA	B-protein_state
-	I-protein_state
free	I-protein_state
,	O
slide	B-protein_state
-	I-protein_state
tethered	I-protein_state
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
(	O
d	O
);	O
AdML	B-gene
RNA	B-protein_state
-	I-protein_state
bound	I-protein_state
,	O
slide	B-protein_state
-	I-protein_state
tethered	I-protein_state
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
(	O
f	O
);	O
AdML	B-gene
RNA	B-protein_state
-	I-protein_state
bound	I-protein_state
,	O
untethered	B-protein_state
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
(	O
h	O
)	O
and	O
adenosine	O
-	O
interrupted	O
RNA	B-protein_state
-	I-protein_state
bound	I-protein_state
,	O
slide	B-protein_state
-	I-protein_state
tethered	I-protein_state
U2AF651	B-mutant
,	I-mutant
2LFRET	I-mutant
(	O
Cy3	B-chemical
/	O
Cy5	B-chemical
)	O
(	O
j	O
).	O

MDS	O
-	O
relevant	O
mutated	O
residues	O
of	O
U2AF65	B-protein
are	O
shown	O
as	O
yellow	O
spheres	O
(	O
L187	B-residue_name_number
and	O
M144	B-residue_name_number
).	O

(	O
b	O
)	O
Following	O
binding	O
to	O
the	O
Py	B-chemical
-	I-chemical
tract	I-chemical
RNA	I-chemical
,	O
a	O
conformation	O
corresponding	O
to	O
high	B-evidence
FRET	I-evidence
and	O
consistent	O
with	O
the	O
‘	O
closed	B-protein_state
',	O
back	B-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
back	I-protein_state
apo	B-protein_state
-	O
U2AF65	B-protein
model	O
resulting	O
from	O
PRE	B-experimental_method
/	O
NMR	B-experimental_method
characterization	O
(	O
PDB	O
ID	O
2YH0	O
)	O
often	O
transitions	O
to	O
a	O
conformation	O
corresponding	O
to	O
∼	O
0	O
.	O
45	O
FRET	B-evidence
value	I-evidence
,	O
which	O
is	O
consistent	O
with	O
‘	O
open	B-protein_state
',	O
side	B-protein_state
-	I-protein_state
by	I-protein_state
-	I-protein_state
side	I-protein_state
RRMs	B-structure_element
such	O
as	O
the	O
U2AF651	B-mutant
,	I-mutant
2L	I-mutant
crystal	B-evidence
structures	I-evidence
.	O

RRM1	B-structure_element
,	O
green	O
;	O
RRM2	B-structure_element
,	O
pale	O
blue	O
;	O
RRM	B-structure_element
extensions	I-structure_element
/	O
linker	B-structure_element
,	O
blue	O
.	O

RNA	B-chemical
protects	O
a	O
nucleoprotein	B-complex_assembly
complex	O
against	O
radiation	O
damage	O

Systematic	O
analysis	O
of	O
radiation	O
damage	O
within	O
a	O
protein	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
complex	O
over	O
a	O
large	O
dose	O
range	O
(	O
1	O
.	O
3	O
–	O
25	O
MGy	O
)	O
reveals	O
significant	O
differential	O
susceptibility	O
of	O
RNA	B-chemical
and	O
protein	O
.	O

A	O
new	O
method	O
of	O
difference	B-experimental_method
electron	I-experimental_method
-	I-experimental_method
density	I-experimental_method
quantification	I-experimental_method
is	O
presented	O
.	O

Radiation	O
damage	O
during	O
macromolecular	B-experimental_method
X	I-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallographic	I-experimental_method
data	I-experimental_method
collection	I-experimental_method
is	O
still	O
the	O
main	O
impediment	O
for	O
many	O
macromolecular	B-experimental_method
structure	I-experimental_method
determinations	I-experimental_method
.	O

Although	O
this	O
has	O
been	O
well	O
characterized	O
within	O
protein	O
crystals	B-evidence
,	O
far	O
less	O
is	O
known	O
about	O
specific	O
damage	O
effects	O
within	O
the	O
larger	O
class	O
of	O
nucleoprotein	O
complexes	O
.	O

Here	O
,	O
a	O
methodology	O
has	O
been	O
developed	O
whereby	O
per	B-evidence
-	I-evidence
atom	I-evidence
density	I-evidence
changes	I-evidence
could	O
be	O
quantified	O
with	O
increasing	O
dose	O
over	O
a	O
wide	O
(	O
1	O
.	O
3	O
–	O
25	O
.	O
0	O
MGy	O
)	O
range	O
and	O
at	O
higher	O
resolution	O
(	O
1	O
.	O
98	O
Å	O
)	O
than	O
the	O
previous	O
systematic	O
specific	O
damage	O
study	O
on	O
a	O
protein	O
–	O
DNA	B-chemical
complex	O
.	O

Additionally	O
,	O
the	O
method	O
enabled	O
a	O
quantification	O
of	O
the	O
reduction	O
in	O
radiation	O
-	O
induced	O
Lys	B-residue_name
and	O
Phe	B-residue_name
disordering	O
upon	O
RNA	B-chemical
binding	O
directly	O
from	O
the	O
electron	B-evidence
density	I-evidence
.	O

With	O
the	O
wide	O
use	O
of	O
high	O
-	O
flux	O
third	O
-	O
generation	O
synchrotron	O
sources	O
,	O
radiation	O
damage	O
(	O
RD	O
)	O
has	O
once	O
again	O
become	O
a	O
dominant	O
reason	O
for	O
the	O
failure	O
of	O
structure	B-experimental_method
determination	I-experimental_method
using	O
macromolecular	B-experimental_method
crystallography	I-experimental_method
(	O
MX	B-experimental_method
)	O
in	O
experiments	O
conducted	O
both	O
at	O
room	O
temperature	O
and	O
under	O
cryocooled	O
conditions	O
(	O
100	O
K	O
).	O

Significant	O
progress	O
has	O
been	O
made	O
in	O
recent	O
years	O
in	O
understanding	O
the	O
inevitable	O
manifestations	O
of	O
X	O
-	O
ray	O
-	O
induced	O
RD	O
within	O
protein	O
crystals	B-evidence
,	O
and	O
there	O
is	O
now	O
a	O
body	O
of	O
literature	O
on	O
possible	O
strategies	O
to	O
mitigate	O
the	O
effects	O
of	O
RD	O
(	O
e	O
.	O
g	O
.	O
Zeldin	O
,	O
Brockhauser	O
et	O
al	O
.,	O
2013	O
;	O
Bourenkov	O
&	O
Popov	O
,	O
2010	O
).	O

Dose	O
is	O
defined	O
as	O
the	O
absorbed	O
energy	O
per	O
unit	O
mass	O
of	O
crystal	O
in	O
grays	O
(	O
Gy	O
;	O
1	O
Gy	O
=	O
1	O
J	O
kg	O
−	O
1	O
),	O
and	O
is	O
the	O
metric	O
against	O
which	O
damage	O
progression	O
should	O
be	O
monitored	O
during	O
MX	B-experimental_method
data	O
collection	O
,	O
as	O
opposed	O
to	O
time	O
.	O

SRD	O
has	O
been	O
well	O
characterized	O
in	O
a	O
large	O
range	O
of	O
proteins	O
,	O
and	O
is	O
seen	O
to	O
follow	O
a	O
reproducible	O
order	O
:	O
metallo	O
-	O
centre	O
reduction	O
,	O
disulfide	B-ptm
-	I-ptm
bond	I-ptm
cleavage	O
,	O
acidic	O
residue	O
decarboxylation	O
and	O
methionine	O
methylthio	O
cleavage	O
(	O
Ravelli	O
&	O
McSweeney	O
,	O
2000	O
;	O
Burmeister	O
,	O
2000	O
;	O
Weik	O
et	O
al	O
.,	O
2000	O
;	O
Yano	O
et	O
al	O
.,	O
2005	O
).	O

There	O
are	O
a	O
number	O
of	O
cases	O
where	O
SRD	O
manifestations	O
have	O
compromised	O
the	O
biological	O
information	O
extracted	O
from	O
MX	B-experimental_method
-	I-experimental_method
determined	I-experimental_method
structures	B-evidence
at	O
much	O
lower	O
doses	O
than	O
the	O
recommended	O
30	O
MGy	O
limit	O
,	O
leading	O
to	O
false	O
structural	O
interpretations	O
of	O
protein	O
mechanisms	O
.	O

Active	B-site
-	I-site
site	I-site
residues	I-site
appear	O
to	O
be	O
particularly	O
susceptible	O
,	O
particularly	O
for	O
photosensitive	O
proteins	O
and	O
in	O
instances	O
where	O
chemical	O
strain	O
is	O
an	O
intrinsic	O
feature	O
of	O
the	O
reaction	O
mechanism	O
.	O

Since	O
the	O
majority	O
of	O
SRD	B-experimental_method
studies	I-experimental_method
to	O
date	O
have	O
focused	O
on	O
proteins	O
,	O
much	O
less	O
is	O
known	O
about	O
the	O
effects	O
of	O
X	O
-	O
ray	O
irradiation	O
on	O
the	O
wider	O
class	O
of	O
crystalline	O
nucleoprotein	B-complex_assembly
complexes	O
or	O
how	O
to	O
correct	O
for	O
such	O
radiation	O
-	O
induced	O
structural	O
changes	O
.	O

Understanding	O
RD	O
to	O
such	O
complexes	O
is	O
crucial	O
,	O
since	O
DNA	B-chemical
is	O
rarely	O
naked	O
within	O
a	O
cell	O
,	O
instead	O
dynamically	O
interacting	O
with	O
proteins	O
,	O
facilitating	O
replication	O
,	O
transcription	O
,	O
modification	O
and	O
DNA	B-chemical
repair	O
.	O

It	O
is	O
essential	O
to	O
understand	O
how	O
these	O
increasingly	O
complex	O
macromolecular	O
structures	B-evidence
are	O
affected	O
by	O
the	O
radiation	O
used	O
to	O
solve	O
them	O
.	O

Investigations	O
on	O
sub	O
-	O
ionization	O
-	O
level	O
LEEs	O
(	O
0	O
–	O
15	O
eV	O
)	O
interacting	O
with	O
both	O
dried	O
and	O
aqueous	O
oligonucleotides	O
(	O
Alizadeh	O
&	O
Sanche	O
,	O
2014	O
;	O
Simons	O
,	O
2006	O
)	O
concluded	O
that	O
resonant	O
electron	O
attachment	O
to	O
DNA	B-chemical
bases	O
and	O
the	O
sugar	O
-	O
phosphate	O
backbone	O
could	O
lead	O
to	O
the	O
preferential	O
cleavage	O
of	O
strong	O
(∼	O
4	O
eV	O
,	O
385	O
kJ	O
mol	O
−	O
1	O
)	O
sugar	O
-	O
phosphate	O
C	O
—	O
O	O
covalent	O
bonds	O
within	O
the	O
DNA	B-chemical
backbone	O
and	O
then	O
base	O
-	O
sugar	O
N1	O
—	O
C	O
bonds	O
,	O
eventually	O
leading	O
to	O
single	O
-	O
strand	O
breakages	O
(	O
SSBs	O
;	O
Ptasińska	O
&	O
Sanche	O
,	O
2007	O
).	O

Electrons	O
have	O
been	O
shown	O
to	O
be	O
mobile	O
at	O
77	O
K	O
by	O
electron	B-experimental_method
spin	I-experimental_method
resonance	I-experimental_method
spectroscopy	I-experimental_method
studies	O
(	O
Symons	O
,	O
1997	O
;	O
Jones	O
et	O
al	O
.,	O
1987	O
),	O
with	O
rapid	O
electron	O
quantum	O
tunnelling	O
and	O
positive	O
hole	O
migration	O
along	O
the	O
protein	O
backbone	O
and	O
through	O
stacked	O
DNA	B-chemical
bases	O
indicated	O
as	O
a	O
dominant	O
mechanism	O
by	O
which	O
oxidative	O
and	O
reductive	O
damage	O
localizes	O
at	O
distances	O
from	O
initial	O
ionization	B-site
sites	I-site
at	O
100	O
K	O
(	O
O	O
’	O
Neill	O
et	O
al	O
.,	O
2002	O
).	O

The	O
investigation	O
of	O
naturally	O
forming	O
nucleoprotein	O
complexes	O
circumvents	O
the	O
inherent	O
challenges	O
in	O
making	O
controlled	O
comparisons	O
of	O
damage	O
mechanisms	O
between	O
protein	O
and	O
nucleic	O
acids	O
crystallized	B-experimental_method
separately	O
.	O
Recently	O
,	O
for	O
a	O
well	O
characterized	O
bacterial	B-taxonomy_domain
protein	O
–	O
DNA	B-chemical
complex	O
(	O
C	B-complex_assembly
.	I-complex_assembly
Esp1396I	I-complex_assembly
;	O
PDB	O
entry	O
3clc	O
;	O
resolution	O
2	O
.	O
8	O
Å	O
;	O
McGeehan	O
et	O
al	O
.,	O
2008	O
)	O
it	O
was	O
concluded	O
that	O
over	O
a	O
wide	O
dose	O
range	O
(	O
2	O
.	O
1	O
–	O
44	O
.	O
6	O
MGy	O
)	O
the	O
protein	O
was	O
far	O
more	O
susceptible	O
to	O
SRD	O
than	O
the	O
DNA	B-chemical
within	O
the	O
crystal	B-evidence
(	O
Bury	O
et	O
al	O
.,	O
2015	O
).	O

Only	O
at	O
doses	O
above	O
20	O
MGy	O
were	O
precursors	O
of	O
phosphodiester	O
-	O
bond	O
cleavage	O
observed	O
within	O
AT	B-structure_element
-	I-structure_element
rich	I-structure_element
regions	I-structure_element
of	O
the	O
35	O
-	O
mer	O
DNA	B-chemical
.	O

For	O
crystalline	O
complexes	O
such	O
as	O
C	B-complex_assembly
.	I-complex_assembly
Esp1396I	I-complex_assembly
,	O
whether	O
the	O
protein	O
is	O
intrinsically	O
more	O
susceptible	O
to	O
X	O
-	O
ray	O
-	O
induced	O
damage	O
or	O
whether	O
the	O
protein	O
scavenges	O
electrons	O
to	O
protect	O
the	O
DNA	B-chemical
remains	O
unclear	O
in	O
the	O
absence	O
of	O
a	O
non	O
-	O
nucleic	O
acid	O
-	O
bound	O
protein	O
control	O
obtained	O
under	O
exactly	O
the	O
same	O
crystallization	O
and	O
data	O
-	O
collection	O
conditions	O
.	O

To	O
monitor	O
the	O
effects	O
of	O
nucleic	O
acid	O
binding	O
on	O
protein	O
damage	O
susceptibility	O
,	O
a	O
crystal	B-evidence
containing	O
two	O
protein	O
molecules	O
per	O
asymmetric	O
unit	O
,	O
only	O
one	O
of	O
which	O
was	O
bound	B-protein_state
to	I-protein_state
RNA	B-chemical
,	O
is	O
reported	O
here	O
(	O
Fig	O
.	O
1	O
▸).	O

Using	O
newly	O
developed	O
methodology	O
,	O
we	O
present	O
a	O
controlled	B-experimental_method
SRD	I-experimental_method
investigation	O
at	O
1	O
.	O
98	O
Å	O
resolution	O
using	O
a	O
large	O
(∼	O
91	O
kDa	O
)	O
crystalline	O
protein	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
complex	O
:	O
trp	B-protein_type
RNA	I-protein_type
-	I-protein_type
binding	I-protein_type
attenuation	I-protein_type
protein	I-protein_type
(	O
TRAP	B-complex_assembly
)	O
bound	B-protein_state
to	I-protein_state
a	O
53	O
bp	O
RNA	B-chemical
sequence	O
(	B-chemical
GAGUU	I-chemical
)	I-chemical
10GAG	I-chemical
(	O
PDB	O
entry	O
1gtf	O
;	O
Hopcroft	O
et	O
al	O
.,	O
2002	O
).	O

It	O
binds	O
with	O
high	O
affinity	O
(	O
K	B-evidence
d	I-evidence
≃	O
1	O
.	O
0	O
nM	O
)	O
to	O
RNA	B-chemical
segments	O
containing	O
11	O
GAG	B-structure_element
/	I-structure_element
UAG	I-structure_element
triplets	I-structure_element
separated	O
by	O
two	O
or	O
three	O
spacer	B-structure_element
nucleotides	I-structure_element
(	O
Elliott	O
et	O
al	O
.,	O
2001	O
)	O
to	O
regulate	O
the	O
transcription	O
of	O
tryptophan	B-chemical
biosynthetic	O
genes	O
in	O
Bacillus	B-species
subtilis	I-species
(	O
Antson	O
et	O
al	O
.,	O
1999	O
).	O

In	O
this	O
structure	B-evidence
,	O
the	O
bases	O
of	O
the	O
G1	B-chemical
-	I-chemical
A2	I-chemical
-	I-chemical
G3	I-chemical
nucleotides	O
form	O
direct	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
to	O
the	O
protein	O
,	O
unlike	O
the	O
U4	B-chemical
-	I-chemical
U5	I-chemical
nucleotides	O
,	O
which	O
appear	O
to	O
be	O
more	O
flexible	O
.	O

Ten	O
successive	O
1	O
.	O
98	O
Å	O
resolution	O
MX	B-experimental_method
data	O
sets	O
were	O
collected	O
from	O
the	O
same	O
TRAP	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
crystal	B-evidence
to	O
analyse	O
X	O
-	O
ray	O
-	O
induced	O
structural	O
changes	O
over	O
a	O
large	O
dose	O
range	O
(	O
d	O
1	O
=	O
1	O
.	O
3	O
MGy	O
to	O
d	O
10	O
=	O
25	O
.	O
0	O
MGy	O
).	O

To	O
avoid	O
the	O
previous	O
necessity	O
for	O
visual	O
inspection	O
of	O
electron	B-evidence
-	I-evidence
density	I-evidence
maps	I-evidence
to	O
detect	O
SRD	B-site
sites	I-site
,	O
a	O
computational	O
approach	O
was	O
designed	O
to	O
quantify	O
the	O
electron	B-evidence
-	I-evidence
density	I-evidence
change	I-evidence
for	O
each	O
refined	O
atom	O
with	O
increasing	O
dose	O
,	O
thus	O
providing	O
a	O
rapid	O
systematic	O
method	O
for	O
SRD	O
study	O
on	O
such	O
large	O
multimeric	O
complexes	O
.	O

By	O
employing	O
the	O
high	O
11	O
-	O
fold	O
structural	O
symmetry	O
within	O
each	O
TRAP	B-complex_assembly
macromolecule	O
,	O
this	O
approach	O
permitted	O
a	O
thorough	O
statistical	O
quantification	O
of	O
the	O
RD	O
effects	O
of	O
RNA	B-chemical
binding	O
to	O
TRAP	B-complex_assembly
.	O

Per	B-experimental_method
-	I-experimental_method
atom	I-experimental_method
quantification	I-experimental_method
of	I-experimental_method
electron	I-experimental_method
density	I-experimental_method

To	O
quantify	O
the	O
exact	O
effects	O
of	O
nucleic	O
acid	O
binding	O
to	O
a	O
protein	O
on	O
SRD	O
susceptibility	O
,	O
a	O
high	O
-	O
throughput	O
and	O
automated	O
pipeline	O
was	O
created	O
to	O
systematically	O
calculate	O
the	O
electron	B-evidence
-	I-evidence
density	I-evidence
change	I-evidence
for	O
every	O
refined	O
atom	O
within	O
the	O
TRAP	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
structure	B-evidence
as	O
a	O
function	O
of	O
dose	O
.	O

This	O
provides	O
an	O
atom	O
-	O
specific	O
quantification	O
of	O
density	B-evidence
–	I-evidence
dose	I-evidence
dynamics	I-evidence
,	O
which	O
was	O
previously	O
lacking	O
within	O
the	O
field	O
.	O

Previous	O
studies	O
have	O
characterized	O
SRD	B-site
sites	I-site
by	O
reporting	O
magnitudes	O
of	O
F	B-evidence
obs	I-evidence
(	I-evidence
d	I-evidence
n	I-evidence
)	I-evidence
−	I-evidence
F	I-evidence
obs	I-evidence
(	I-evidence
d	I-evidence
1	I-evidence
)	I-evidence
Fourier	I-evidence
difference	I-evidence
map	I-evidence
peaks	I-evidence
in	O
terms	O
of	O
the	O
sigma	B-evidence
(	O
σ	B-evidence
)	O
contour	O
level	O
(	O
the	O
number	O
of	O
standard	B-evidence
deviations	I-evidence
from	O
the	O
mean	B-evidence
map	I-evidence
electron	I-evidence
-	I-evidence
density	I-evidence
value	I-evidence
)	O
at	O
which	O
peaks	O
become	O
visible	O
.	O

However	O
,	O
these	O
σ	B-evidence
levels	O
depend	O
on	O
the	O
standard	B-evidence
deviation	I-evidence
values	O
of	O
the	O
map	B-evidence
,	O
which	O
can	O
deviate	O
between	O
data	O
sets	O
,	O
and	O
are	O
thus	O
unsuitable	O
for	O
quantitative	O
comparison	O
of	O
density	B-evidence
between	O
different	O
dose	O
data	O
sets	O
.	O

Large	O
positive	O
D	B-evidence
loss	I-evidence
values	O
indicate	O
radiation	O
-	O
induced	O
atomic	O
disordering	O
reproducibly	O
throughout	O
the	O
unit	O
cells	O
with	O
respect	O
to	O
the	O
initial	O
low	O
-	O
dose	O
data	O
set	O
.	O

For	O
each	O
TRAP	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
data	O
set	O
,	O
the	O
D	B-evidence
loss	I-evidence
metric	I-evidence
successfully	O
identified	O
the	O
recognized	O
forms	O
of	O
protein	O
SRD	B-experimental_method
(	O
Fig	O
.	O
2	O
▸	O
a	O
),	O
with	O
clear	O
Glu	B-residue_name
and	O
Asp	B-residue_name
side	O
-	O
chain	O
decarboxylation	O
even	O
in	O
the	O
first	O
difference	B-evidence
map	I-evidence
calculated	O
(	O
3	O
.	O
9	O
MGy	O
;	O
Fig	O
.	O
3	O
▸	O
a	O
).	O

The	O
main	O
sequence	O
of	O
TRAP	B-complex_assembly
does	O
not	O
contain	O
any	O
Trp	B-residue_name
and	O
Cys	B-residue_name
residues	O
(	O
and	O
thus	O
contains	O
no	O
disulfide	O
bonds	O
).	O

Even	O
for	O
radiation	O
-	O
insensitive	O
residues	O
(	O
e	O
.	O
g	O
.	O
Gly	B-residue_name
)	O
the	O
average	O
D	B-evidence
loss	I-evidence
increases	O
with	O
dose	O
:	O
this	O
is	O
the	O
effect	O
of	O
global	O
radiation	O
damage	O
,	O
since	O
as	O
dose	O
increases	O
the	O
electron	B-evidence
density	I-evidence
associated	O
with	O
each	O
refined	O
atom	O
becomes	O
weaker	O
as	O
the	O
atomic	O
occupancy	O
decreases	O
(	O
Fig	O
.	O
2	O
▸	O
b	O
).	O

Only	O
Glu	B-residue_name
and	O
Asp	B-residue_name
residues	O
exhibit	O
a	O
rate	O
of	O
D	B-evidence
loss	I-evidence
increase	O
that	O
consistently	O
exceeds	O
the	O
average	O
decay	O
(	O
Fig	O
.	O
2	O
▸	O
b	O
,	O
dashed	O
line	O
)	O
at	O
each	O
dose	O
.	O

RNA	B-chemical
is	O
less	O
susceptible	O
to	O
electron	B-evidence
-	I-evidence
density	I-evidence
loss	O
than	O
protein	O
within	O
the	O
TRAP	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
complex	O

Visual	B-experimental_method
inspection	I-experimental_method
of	I-experimental_method
Fourier	B-evidence
difference	I-evidence
maps	I-evidence
illustrated	O
the	O
clear	O
lack	O
of	O
RNA	B-chemical
electron	B-evidence
-	I-evidence
density	I-evidence
degradation	I-evidence
with	O
increasing	O
dose	O
compared	O
with	O
the	O
obvious	O
protein	O
damage	O
manifestations	O
(	O
Figs	O
.	O
3	O
▸	O
b	O
and	O
3	O
▸	O
c	O
).	O

Only	O
at	O
the	O
highest	O
doses	O
investigated	O
(>	O
20	O
MGy	O
)	O
was	O
density	O
loss	O
observed	O
at	O
the	O
RNA	B-chemical
phosphate	O
and	O
C	O
—	O
O	O
bonds	O
of	O
the	O
phosphodiester	O
backbone	O
.	O

However	O
,	O
the	O
median	O
D	B-evidence
loss	I-evidence
was	O
lower	O
by	O
a	O
factor	O
of	O
>	O
2	O
for	O
RNA	B-chemical
P	O
atoms	O
than	O
for	O
Glu	B-residue_name
and	O
Asp	B-residue_name
side	O
-	O
chain	O
groups	O
at	O
25	O
.	O
0	O
MGy	O
(	O
Supplementary	O
Fig	O
.	O
S4	O
),	O
and	O
furthermore	O
could	O
not	O
be	O
numerically	O
distinguished	O
from	O
Gly	B-residue_name
Cα	O
atoms	O
within	O
TRAP	B-complex_assembly
,	O
which	O
are	O
not	O
radiation	O
-	O
sensitive	O
at	O
the	O
doses	O
tested	O
here	O
(	O
Supplementary	O
Fig	O
.	O
S3	O
).	O

RNA	B-chemical
binding	O
protects	O
radiation	O
-	O
sensitive	O
residues	O

Hotelling	B-experimental_method
’	I-experimental_method
s	I-experimental_method
T	I-experimental_method
-	I-experimental_method
squared	I-experimental_method
test	I-experimental_method
(	O
the	O
multivariate	O
counterpart	O
of	O
Student	B-experimental_method
’	I-experimental_method
s	I-experimental_method
t	I-experimental_method
-	I-experimental_method
test	I-experimental_method
)	O
was	O
used	O
to	O
reject	O
the	O
null	O
hypothesis	O
that	O
the	O
means	O
of	O
the	O
D	B-evidence
loss	I-evidence
metric	I-evidence
were	O
equal	O
for	O
the	O
bound	B-protein_state
and	O
nonbound	B-protein_state
groups	O
in	O
Fig	O
.	O
5	O
▸.	O

A	O
significant	O
reduction	O
in	O
D	B-evidence
loss	I-evidence
is	O
seen	O
for	O
Glu36	B-residue_name_number
in	O
RNA	B-protein_state
-	I-protein_state
bound	I-protein_state
compared	O
with	O
nonbound	B-protein_state
TRAP	B-complex_assembly
,	O
indicative	O
of	O
a	O
lower	O
rate	O
of	O
side	O
-	O
chain	O
decarboxylation	O
(	O
Fig	O
.	O
5	O
▸	O
a	O
;	O
p	O
=	O
6	O
.	O
06	O
×	O
10	O
−	O
5	O
).	O

For	O
each	O
TRAP	B-complex_assembly
ring	B-structure_element
subunit	B-structure_element
,	O
the	O
Glu36	B-residue_name_number
side	O
-	O
chain	O
carboxyl	O
group	O
accepts	O
a	O
pair	O
of	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
from	O
the	O
two	O
N	O
atoms	O
of	O
the	O
G3	B-residue_name_number
RNA	B-chemical
base	O
.	O

In	O
our	O
analysis	O
,	O
Asp39	B-residue_name_number
in	O
the	O
TRAP	B-complex_assembly
–(	I-complex_assembly
GAGUU	I-complex_assembly
)	I-complex_assembly
10GAG	I-complex_assembly
structure	B-evidence
appears	O
to	O
exhibit	O
two	O
distinct	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
to	O
the	O
G1	B-residue_name_number
base	O
within	O
each	O
of	O
the	O
11	O
TRAP	B-site
–	I-site
RNA	I-site
interfaces	I-site
,	O
as	O
does	O
Glu36	B-residue_name_number
to	O
G3	B-residue_name_number
;	O
however	O
,	O
the	O
reduction	O
in	O
density	B-evidence
disordering	O
upon	O
RNA	B-chemical
binding	O
is	O
far	O
less	O
significant	O
for	O
Asp39	B-residue_name_number
than	O
for	O
Glu36	B-residue_name_number
(	O
Fig	O
.	O
5	O
▸	O
b	O
,	O
p	O
=	O
0	O
.	O
0925	O
).	O

RNA	B-chemical
binding	O
reduces	O
radiation	O
-	O
induced	O
disorder	O
on	O
the	O
atomic	O
scale	O

One	O
oxygen	O
(	O
O	O
∊	O
1	O
)	O
of	O
Glu42	B-residue_name_number
appears	O
to	O
form	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
to	O
a	O
nearby	O
water	B-chemical
within	O
each	O
TRAP	B-site
RNA	I-site
-	I-site
binding	I-site
pocket	I-site
,	O
with	O
the	O
other	O
(	O
O	O
∊	O
2	O
)	O
being	O
involved	O
in	O
a	O
salt	B-bond_interaction
-	I-bond_interaction
bridge	I-bond_interaction
interaction	O
with	O
Arg58	B-residue_name_number
(	O
Hopcroft	O
et	O
al	O
.,	O
2002	O
;	O
Antson	O
et	O
al	O
.,	O
1999	O
).	O

The	O
density	B-evidence
-	I-evidence
change	I-evidence
dynamics	I-evidence
were	O
statistically	O
indistinguishable	O
between	O
bound	B-protein_state
and	O
nonbound	B-protein_state
TRAP	B-complex_assembly
for	O
each	O
Glu42	B-residue_name_number
carboxyl	O
group	O
Cδ	O
atom	O
(	O
p	O
=	O
0	O
.	O
435	O
),	O
indicating	O
that	O
upon	O
RNA	B-chemical
binding	O
the	O
conserved	O
salt	B-bond_interaction
-	I-bond_interaction
bridge	I-bond_interaction
interaction	O
ultimately	O
dictated	O
the	O
overall	O
Glu42	B-residue_name_number
decarboxylation	O
rate	O
.	O

With	O
increasing	O
dose	O
,	O
the	O
D	B-evidence
loss	I-evidence
associated	O
with	O
the	O
Phe32	B-residue_name_number
side	O
chain	O
was	O
significantly	O
reduced	O
upon	O
RNA	B-chemical
binding	O
(	O
Fig	O
.	O
5	O
▸	O
e	O
;	O
Phe32	B-residue_name_number
Cζ	O
;	O
p	O
=	O
0	O
.	O
0014	O
),	O
an	O
indication	O
that	O
radiation	O
-	O
induced	O
conformation	O
disordering	O
of	O
Phe32	B-residue_name_number
had	O
been	O
reduced	O
.	O

The	O
extended	O
aliphatic	O
Lys37	B-residue_name_number
side	O
chain	O
stacks	O
against	O
the	O
nearby	O
G1	B-residue_name_number
base	O
,	O
making	O
a	O
series	O
of	O
nonpolar	B-bond_interaction
contacts	I-bond_interaction
within	O
each	O
RNA	B-site
-	I-site
binding	I-site
interface	I-site
.	O

The	O
D	B-evidence
loss	I-evidence
for	O
Lys37	B-residue_name_number
side	O
-	O
chain	O
atoms	O
was	O
also	O
reduced	O
when	O
stacked	B-bond_interaction
against	O
the	O
G1	B-residue_name_number
base	O
(	O
Fig	O
.	O
5	O
▸	O
f	O
;	O
p	O
=	O
0	O
.	O
0243	O
for	O
Lys37	B-residue_name_number
C	O
∊	O
atoms	O
).	O

Representative	O
Phe32	B-residue_name_number
and	O
Lys37	B-residue_name_number
atoms	O
were	O
selected	O
to	O
illustrate	O
these	O
trends	O
.	O

Compared	O
with	O
previous	O
studies	O
,	O
the	O
results	O
provide	O
a	O
further	O
step	O
in	O
the	O
detailed	O
characterization	O
of	O
SRD	O
effects	O
in	O
MX	B-experimental_method
.	O

Our	O
method	O
­	O
ology	O
,	O
which	O
eliminated	O
tedious	O
and	O
error	O
-	O
prone	O
visual	O
inspection	O
,	O
permitted	O
the	O
determination	O
on	O
a	O
per	O
-	O
atom	O
basis	O
of	O
the	O
most	O
damaged	O
sites	O
,	O
as	O
characterized	O
by	O
F	B-evidence
obs	I-evidence
(	I-evidence
d	I-evidence
n	I-evidence
)	I-evidence
−	I-evidence
F	I-evidence
obs	I-evidence
(	I-evidence
d	I-evidence
1	I-evidence
)	I-evidence
Fourier	I-evidence
difference	I-evidence
map	I-evidence
peaks	I-evidence
between	O
successive	O
data	O
sets	O
collected	O
from	O
the	O
same	O
crystal	B-evidence
.	O

The	O
RNA	B-chemical
was	O
found	O
to	O
be	O
substantially	O
more	O
radiation	B-protein_state
-	I-protein_state
resistant	I-protein_state
than	O
the	O
protein	O
,	O
even	O
at	O
the	O
highest	O
doses	O
investigated	O
(∼	O
25	O
.	O
0	O
MGy	O
),	O
which	O
is	O
in	O
strong	O
concurrence	O
with	O
our	O
previous	O
SRD	B-experimental_method
investigation	I-experimental_method
of	O
the	O
C	B-complex_assembly
.	I-complex_assembly
Esp1396I	I-complex_assembly
protein	O
–	O
DNA	B-chemical
complex	O
(	O
Bury	O
et	O
al	O
.,	O
2015	O
).	O

RNA	B-chemical
backbone	O
disordering	O
thus	O
appears	O
to	O
be	O
the	O
main	O
radiation	O
-	O
induced	O
effect	O
in	O
RNA	B-chemical
,	O
with	O
the	O
protein	O
–	O
base	O
interactions	O
maintained	O
even	O
at	O
high	O
doses	O
(>	O
20	O
MGy	O
).	O

The	O
U4	B-residue_name_number
phosphate	B-chemical
exhibited	O
marginally	O
larger	O
D	B-evidence
loss	I-evidence
values	O
above	O
20	O
MGy	O
than	O
G1	B-residue_name_number
,	O
A2	B-residue_name_number
and	O
G3	B-residue_name_number
(	O
Supplementary	O
Fig	O
.	O
S4	O
).	O

Consequently	O
,	O
no	O
clear	O
single	O
-	O
strand	O
breaks	O
could	O
be	O
located	O
,	O
and	O
since	O
RNA	B-chemical
-	O
binding	O
within	O
the	O
current	O
TRAP	B-complex_assembly
–(	I-complex_assembly
GAGUU	I-complex_assembly
)	I-complex_assembly
10GAG	I-complex_assembly
complex	O
is	O
mediated	O
predominantly	O
through	O
base	O
–	O
protein	O
interactions	O
,	O
the	O
biological	O
integrity	O
of	O
the	O
RNA	B-chemical
complex	O
was	O
dictated	O
by	O
the	O
rate	O
at	O
which	O
protein	O
decarboxylation	O
occurred	O
.	O

RNA	B-chemical
interacting	O
with	O
TRAP	B-complex_assembly
was	O
shown	O
to	O
offer	O
significant	O
protection	O
against	O
radiation	O
-	O
induced	O
structural	O
changes	O
.	O

Both	O
Glu36	B-residue_name_number
and	O
Asp39	B-residue_name_number
bind	O
directly	O
to	O
RNA	B-chemical
,	O
each	O
through	O
two	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
to	O
guanine	B-chemical
bases	O
(	O
G3	B-residue_name_number
and	O
G1	B-residue_name_number
,	O
respectively	O
).	O

However	O
,	O
compared	O
with	O
Asp39	B-residue_name_number
,	O
Glu36	B-residue_name_number
is	O
strikingly	O
less	O
decarboxylated	O
when	O
bound	B-protein_state
to	I-protein_state
RNA	B-chemical
(	O
Fig	O
.	O
4	O
▸).	O

This	O
is	O
in	O
good	O
agreement	O
with	O
previous	O
mutagenesis	B-experimental_method
and	I-experimental_method
nucleoside	I-experimental_method
analogue	I-experimental_method
studies	I-experimental_method
(	O
Elliott	O
et	O
al	O
.,	O
2001	O
),	O
which	O
indicated	O
that	O
the	O
G1	B-residue_name_number
nucleotide	O
does	O
not	O
bind	O
to	O
TRAP	B-complex_assembly
as	O
strongly	O
as	O
do	O
A2	B-residue_name_number
and	O
G3	B-residue_name_number
,	O
and	O
plays	O
little	O
role	O
in	O
the	O
high	O
RNA	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
TRAP	B-complex_assembly
(	O
K	B-evidence
d	I-evidence
≃	O
1	O
.	O
1	O
±	O
0	O
.	O
4	O
nM	O
).	O

Thus	O
,	O
another	O
factor	O
must	O
be	O
responsible	O
for	O
this	O
clear	O
reduction	O
in	O
Glu36	B-residue_name_number
CO2	O
decarboxyl	O
­	O
ation	O
in	O
RNA	B-protein_state
-	I-protein_state
bound	I-protein_state
TRAP	B-complex_assembly
.	O

When	O
bound	B-protein_state
to	I-protein_state
RNA	B-chemical
,	O
the	O
average	O
solvent	O
-	O
accessible	O
area	O
of	O
the	O
Glu36	B-residue_name_number
side	O
-	O
chain	O
O	O
atoms	O
is	O
reduced	O
from	O
∼	O
15	O
to	O
0	O
Å2	O
.	O

The	O
electron	B-evidence
-	I-evidence
recombination	I-evidence
rate	I-evidence
K	I-evidence
−	I-evidence
1	I-evidence
remains	O
high	O
,	O
however	O
,	O
owing	O
to	O
rapid	O
electron	O
migration	O
through	O
the	O
protein	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
complex	O
to	O
refill	O
the	O
Glu36	B-residue_name_number
positive	B-site
hole	I-site
(	O
the	O
precursor	O
for	O
Glu	B-residue_name
decarboxylation	O
).	O

The	O
prevalence	O
of	O
radical	O
attack	O
from	O
solvent	O
channels	O
surrounding	O
the	O
protein	O
in	O
the	O
crystal	B-evidence
is	O
a	O
questionable	O
cause	O
,	O
considering	O
previous	O
observations	O
indicating	O
that	O
the	O
strongly	O
oxidizing	O
hydroxyl	O
radical	O
is	O
immobile	O
at	O
100	O
K	O
(	O
Allan	O
et	O
al	O
.,	O
2013	O
;	O
Owen	O
et	O
al	O
.,	O
2012	O
).	O

By	O
comparing	O
equivalent	O
acidic	O
residues	O
with	B-protein_state
and	O
without	B-protein_state
RNA	B-chemical
,	O
we	O
have	O
now	O
deconvoluted	O
the	O
role	O
of	O
solvent	O
accessibility	O
from	O
other	O
local	O
protein	O
environment	O
factors	O
,	O
and	O
thus	O
propose	O
a	O
suitable	O
mechanism	O
by	O
which	O
exceptionally	O
low	O
solvent	O
accessibility	O
can	O
reduce	O
the	O
rate	O
of	O
decarboxylation	O
.	O

Apart	O
from	O
these	O
RNA	B-site
-	I-site
binding	I-site
interfaces	I-site
,	O
RNA	B-chemical
binding	O
was	O
seen	O
to	O
enhance	O
decarboxylation	O
for	O
residues	O
Glu50	B-residue_name_number
,	O
Glu71	B-residue_name_number
and	O
Glu73	B-residue_name_number
,	O
all	O
of	O
which	O
are	O
involved	O
in	O
crystal	O
contacts	O
between	O
TRAP	B-complex_assembly
rings	B-structure_element
(	O
Fig	O
.	O
4	O
▸	O
c	O
).	O

Radiation	O
-	O
induced	O
side	O
-	O
chain	O
conformational	O
changes	O
have	O
been	O
poorly	O
characterized	O
in	O
previous	O
SRD	B-experimental_method
investigations	I-experimental_method
owing	O
to	O
their	O
strong	O
dependence	O
on	O
packing	O
density	O
and	O
geometric	O
strain	O
.	O

Such	O
structural	O
changes	O
are	O
known	O
to	O
have	O
significant	O
roles	O
within	O
enzymatic	O
pathways	O
,	O
and	O
experimenters	O
must	O
be	O
aware	O
of	O
these	O
possible	O
confounding	O
factors	O
when	O
assigning	O
true	O
functional	O
mechanisms	O
using	O
MX	B-experimental_method
.	O

Our	O
results	O
show	O
that	O
RNA	B-chemical
binding	O
to	O
TRAP	B-complex_assembly
physically	O
stabilizes	O
non	O
-	O
acidic	O
residues	O
within	O
the	O
TRAP	B-complex_assembly
macromolecule	O
,	O
most	O
notably	O
Lys37	B-residue_name_number
and	O
Phe32	B-residue_name_number
,	O
which	O
stack	O
against	O
the	O
G1	B-residue_name_number
and	O
G3	B-residue_name_number
bases	O
,	O
respectively	O
.	O

In	O
TRAP	B-complex_assembly
,	O
D	B-evidence
loss	I-evidence
increased	O
at	O
a	O
similar	O
rate	O
for	O
both	O
the	O
Tyr	B-residue_name
O	O
atoms	O
and	O
aromatic	O
ring	B-structure_element
atoms	O
,	O
suggesting	O
that	O
full	O
ring	B-structure_element
conformational	O
disordering	O
is	O
more	O
likely	O
.	O

The	O
RNA	B-chemical
-	O
stabilization	O
effects	O
on	O
protein	O
are	O
observed	O
at	O
short	O
ranges	O
and	O
are	O
restricted	O
to	O
within	O
the	O
RNA	B-site
-	I-site
binding	I-site
interfaces	I-site
around	O
the	O
TRAP	B-complex_assembly
ring	B-structure_element
.	O

For	O
example	O
,	O
Asp17	B-residue_name_number
is	O
located	O
∼	O
6	O
.	O
8	O
Å	O
from	O
the	O
G1	B-residue_name_number
base	O
,	O
outside	O
the	O
RNA	B-site
-	I-site
binding	I-site
interfaces	I-site
,	O
and	O
has	O
indistinguishable	O
Cγ	O
atom	O
D	O
loss	B-evidence
dose	I-evidence
-	I-evidence
dynamics	I-evidence
between	O
RNA	B-protein_state
-	I-protein_state
bound	I-protein_state
and	O
nonbound	B-protein_state
TRAP	B-complex_assembly
(	O
p	O
>	O
0	O
.	O
9	O
).	O

An	O
increase	O
in	O
the	O
dose	O
at	O
which	O
functionally	O
important	O
residues	O
remain	O
intact	O
has	O
biological	O
ramifications	O
for	O
understanding	O
the	O
mechanisms	O
at	O
which	O
ionizing	O
radiation	O
damage	O
is	O
mitigated	O
within	O
naturally	O
forming	O
DNA	B-complex_assembly
–	I-complex_assembly
protein	I-complex_assembly
and	O
RNA	B-complex_assembly
–	I-complex_assembly
protein	I-complex_assembly
complexes	O
.	O

Observations	O
of	O
lower	O
protein	O
radiation	O
-	O
sensitivity	O
in	O
DNA	B-protein_state
-	I-protein_state
bound	I-protein_state
forms	O
have	O
been	O
recorded	O
in	O
solution	O
at	O
RT	O
at	O
much	O
lower	O
doses	O
(∼	O
1	O
kGy	O
)	O
than	O
those	O
used	O
for	O
typical	O
MX	B-experimental_method
experiments	O
[	O
e	O
.	O
g	O
.	O
an	O
oestrogen	O
response	O
element	O
–	O
receptor	O
complex	O
(	O
Stísová	O
et	O
al	O
.,	O
2006	O
)	O
and	O
a	O
DNA	B-protein_type
glycosylase	I-protein_type
and	O
its	O
abasic	B-site
DNA	I-site
target	I-site
site	I-site
(	O
Gillard	O
et	O
al	O
.,	O
2004	O
)].	O

In	O
these	O
studies	O
,	O
the	O
main	O
damaging	O
species	O
is	O
predicted	O
to	O
be	O
the	O
oxidizing	O
hydroxyl	O
radical	O
produced	O
through	O
solvent	O
irradiation	O
,	O
which	O
is	O
known	O
to	O
add	O
to	O
double	O
covalent	O
bonds	O
within	O
both	O
DNA	B-chemical
and	O
RNA	B-chemical
bases	O
to	O
induce	O
strand	O
breaks	O
and	O
base	O
modification	O
(	O
Spotheim	O
-	O
Maurizot	O
&	O
Davídková	O
,	O
2011	O
;	O
Chance	O
et	O
al	O
.,	O
1997	O
).	O

It	O
was	O
suggested	O
that	O
physical	O
screening	O
of	O
DNA	B-chemical
by	O
protein	O
shielded	O
the	O
DNA	B-site
–	I-site
protein	I-site
interaction	I-site
sites	I-site
from	O
radical	O
damage	O
,	O
yielding	O
an	O
extended	O
life	O
-	O
dose	O
for	O
the	O
nucleoprotein	O
complex	O
compared	O
with	O
separate	O
protein	O
and	O
DNA	B-chemical
constituents	O
at	O
RT	O
.	O

However	O
,	O
in	O
the	O
current	O
MX	B-experimental_method
study	O
at	O
100	O
K	O
,	O
the	O
main	O
damaging	O
species	O
are	O
believed	O
to	O
be	O
migrating	O
LEEs	O
and	O
holes	O
produced	O
directly	O
within	O
the	O
protein	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
components	O
or	O
in	O
closely	O
associated	O
solvent	O
.	O

The	O
results	O
presented	O
here	O
suggest	O
that	O
biologically	O
relevant	O
nucleoprotein	B-complex_assembly
complexes	O
also	O
exhibit	O
prolonged	O
life	O
-	O
doses	O
under	O
the	O
effect	O
of	O
LEE	O
-	O
induced	O
structural	O
changes	O
,	O
involving	O
direct	O
physical	O
protection	O
of	O
key	O
RNA	B-site
-	I-site
binding	I-site
residues	I-site
.	O

When	O
exposed	O
to	O
X	O
-	O
rays	O
,	O
these	O
residues	O
will	O
be	O
preferentially	O
damaged	O
by	O
X	O
-	O
rays	O
and	O
subsequently	O
reduce	O
the	O
affinity	O
with	O
which	O
TRAP	B-complex_assembly
binds	O
to	O
RNA	B-chemical
.	O

Within	O
the	O
cellular	O
environment	O
,	O
this	O
mechanism	O
could	O
reduce	O
the	O
risk	O
that	O
radiation	O
-	O
damaged	O
proteins	O
might	O
bind	O
to	O
RNA	B-chemical
,	O
thus	O
avoiding	O
the	O
detrimental	O
introduction	O
of	O
incorrect	O
DNA	B-chemical
-	O
repair	O
,	O
transcriptional	O
and	O
base	O
-	O
modification	O
pathways	O
.	O

The	O
TRAP	B-complex_assembly
–(	I-complex_assembly
GAGUU	I-complex_assembly
)	I-complex_assembly
10GAG	I-complex_assembly
complex	O
asymmetric	O
unit	O
(	O
PDB	O
entry	O
1gtf	O
;	O
Hopcroft	O
et	O
al	O
.,	O
2002	O
).	O

Bound	B-protein_state
tryptophan	B-chemical
ligands	O
are	O
represented	O
as	O
coloured	O
spheres	O
.	O

(	O
a	O
)	O
Electron	O
-	O
density	O
loss	O
sites	O
as	O
indicated	O
by	O
D	O
loss	O
in	O
the	O
TRAP	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
complex	O
crystal	B-evidence
by	O
residue	O
/	O
nucleotide	O
type	O
for	O
five	O
doses	O
[	O
sites	O
determined	O
above	O
the	O
4	O
×	O
average	O
D	O
loss	O
threshold	O
,	O
calculated	O
over	O
the	O
TRAP	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
structure	B-evidence
for	O
the	O
first	O
difference	B-evidence
map	I-evidence
:	O
F	O
obs	O
(	O
d	O
2	O
)	O
−	O
F	O
obs	O
(	O
d	O
1	O
)].	O

The	O
average	O
D	O
loss	O
(	O
calculated	O
over	O
the	O
whole	O
TRAP	B-complex_assembly
asymmetric	O
unit	O
)	O
is	O
shown	O
at	O
each	O
dose	O
(	O
dashed	O
line	O
).	O

Radiation	O
-	O
induced	O
protein	O
disordering	O
is	O
evident	O
across	O
the	O
large	O
dose	O
range	O
(	O
b	O
,	O
c	O
);	O
in	O
comparison	O
,	O
no	O
clear	O
deterioration	O
of	O
the	O
RNA	B-chemical
density	B-evidence
was	O
observed	O
.	O

D	O
loss	O
calculated	O
for	O
all	O
side	O
-	O
chain	O
carboxyl	O
group	O
Glu	B-residue_name
Cδ	O
and	O
Asp	B-residue_name
Cγ	O
atoms	O
within	O
the	O
TRAP	B-complex_assembly
–	I-complex_assembly
RNA	I-complex_assembly
complex	O
for	O
a	O
dose	O
of	O
19	O
.	O
3	O
MGy	O
(	O
d	O
8	O
).	O

Residues	O
have	O
been	O
grouped	O
by	O
amino	O
-	O
acid	O
number	O
,	O
and	O
split	O
into	O
bound	B-protein_state
and	O
nonbound	B-protein_state
groupings	O
,	O
with	O
each	O
bar	O
representing	O
the	O
mean	O
calculated	O
over	O
11	O
equivalent	O
atoms	O
around	O
a	O
TRAP	B-complex_assembly
ring	B-structure_element
.	O

D	O
loss	O
against	O
dose	O
for	O
(	O
a	O
)	O
Glu36	B-residue_name_number
Cδ	O
,	O
(	O
b	O
)	O
Asp39	B-residue_name_number
Cγ	O
,	O
(	O
c	O
)	O
Glu42	B-residue_name_number
O	O
∊	O
1	O
,	O
(	O
d	O
)	O
Glu42	B-residue_name_number
O	O
∊	O
2	O
,	O
(	O
e	O
)	O
Phe32	B-residue_name_number
Cζ	O
and	O
(	O
f	O
)	O
Lys37	B-residue_name_number
C	O
∊	O
atoms	O
.	O

95	O
%	O
CI	O
are	O
included	O
for	O
each	O
set	O
of	O
11	O
equivalent	O
atoms	O
grouped	O
as	O
bound	B-protein_state
/	O
nonbound	B-protein_state
.	O

RNA	B-site
-	I-site
binding	I-site
interface	I-site
interactions	O
are	O
shown	O
for	O
TRAP	B-complex_assembly
chain	O
N	O
,	O
with	O
the	O
F	O
obs	O
(	O
d	O
7	O
)	O
−	O
F	O
obs	O
(	O
d	O
1	O
)	O
Fourier	O
difference	O
map	O
(	O
dose	O
16	O
.	O
7	O
MGy	O
)	O
overlaid	O
and	O
contoured	O
at	O
a	O
±	O
4σ	O
level	O
.	O

A	O
conserved	O
motif	O
in	O
JNK	B-protein_type
/	I-protein_type
p38	I-protein_type
-	I-protein_type
specific	I-protein_type
MAPK	I-protein_type
phosphatases	I-protein_type
as	O
a	O
determinant	O
for	O
JNK1	B-protein
recognition	O
and	O
inactivation	O

MAPK	B-protein_type
signalling	O
efficiency	O
and	O
specificity	O
is	O
modulated	O
by	O
protein	O
–	O
protein	O
interactions	O
between	O
individual	O
MAPKs	B-protein_type
and	O
the	O
docking	B-structure_element
motifs	I-structure_element
in	O
cognate	O
binding	O
partners	O
.	O

Here	O
we	O
report	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
JNK1	B-protein
bound	B-protein_state
to	I-protein_state
the	O
catalytic	B-structure_element
domain	I-structure_element
of	O
MKP7	B-protein
at	O
2	O
.	O
4	O
-	O
Å	O
resolution	O
,	O
providing	O
high	O
-	O
resolution	O
structural	O
insight	O
into	O
the	O
FXF	B-site
-	I-site
docking	I-site
interaction	I-site
.	O

Here	O
,	O
the	O
authors	O
report	O
the	O
structure	B-evidence
of	O
MKP7	B-protein
bound	B-protein_state
to	I-protein_state
JNK1	B-protein
and	O
characterise	O
the	O
conserved	B-protein_state
MKP	B-protein_type
-	O
MAPK	B-protein_type
interaction	O
.	O

The	O
mitogen	B-protein_type
-	I-protein_type
activated	I-protein_type
protein	I-protein_type
kinases	I-protein_type
(	O
MAPKs	B-protein_type
)	O
are	O
central	O
components	O
of	O
the	O
signal	O
-	O
transduction	O
pathways	O
,	O
which	O
mediate	O
the	O
cellular	O
response	O
to	O
a	O
variety	O
of	O
extracellular	O
stimuli	O
,	O
ranging	O
from	O
growth	O
factors	O
to	O
environmental	O
stresses	O
.	O

The	O
MAPK	B-protein_type
signalling	O
pathways	O
are	O
evolutionally	O
highly	O
conserved	O
.	O

The	O
basic	O
assembly	O
of	O
MAPK	B-protein_type
pathways	O
is	O
a	O
three	O
-	O
tier	O
kinase	B-protein_type
module	O
that	O
establishes	O
a	O
sequential	O
activation	O
cascade	O
:	O
a	O
MAPK	B-protein_type
kinase	I-protein_type
kinase	I-protein_type
activates	O
a	O
MAPK	B-protein_type
kinase	I-protein_type
,	O
which	O
in	O
turn	O
activates	O
a	O
MAPK	B-protein_type
.	O

The	O
three	O
best	O
-	O
characterized	O
MAPK	B-protein_type
signalling	O
pathways	O
are	O
mediated	O
by	O
the	O
kinases	B-protein_type
extracellular	B-protein_type
signal	I-protein_type
-	I-protein_type
regulated	I-protein_type
kinase	I-protein_type
(	O
ERK	B-protein_type
),	O
c	B-protein_type
-	I-protein_type
Jun	I-protein_type
N	I-protein_type
-	I-protein_type
terminal	I-protein_type
kinase	I-protein_type
(	O
JNK	B-protein_type
)	O
and	O
p38	B-protein_type
.	O

The	O
ERK	B-protein_type
pathway	O
is	O
activated	O
by	O
various	O
mitogens	O
and	O
phorbol	O
esters	O
,	O
whereas	O
the	O
JNK	B-protein_type
and	O
p38	B-protein_type
pathways	O
are	O
stimulated	O
mainly	O
by	O
environmental	O
stress	O
and	O
inflammatory	O
cytokines	B-protein_type
.	O

After	O
activation	O
,	O
each	O
MAPK	B-protein_type
phosphorylates	O
a	O
distinct	O
set	O
of	O
protein	O
substrates	O
,	O
which	O
act	O
as	O
the	O
critical	O
effectors	O
that	O
enable	O
cells	O
to	O
mount	O
the	O
appropriate	O
responses	O
to	O
varied	O
stimuli	O
.	O

Two	O
types	O
of	O
docking	B-structure_element
motifs	I-structure_element
have	O
been	O
identified	O
in	O
MAPK	B-protein_type
substrates	O
and	O
cognate	O
proteins	O
:	O
kinase	B-structure_element
-	I-structure_element
interacting	I-structure_element
motif	I-structure_element
(	O
D	B-structure_element
-	I-structure_element
motif	I-structure_element
)	O
and	O
FXF	B-structure_element
-	I-structure_element
motif	I-structure_element
(	O
also	O
called	O
DEF	B-structure_element
motif	I-structure_element
,	O
docking	B-site
site	I-site
for	O
ERK	B-protein_type
FXF	B-structure_element
).	O

The	O
D	B-site
-	I-site
motif	I-site
-	I-site
docking	I-site
site	I-site
(	O
D	B-site
-	I-site
site	I-site
)	O
in	O
MAPKs	B-protein_type
is	O
situated	O
in	O
a	O
noncatalytic	B-site
region	I-site
opposite	O
of	O
the	O
kinase	B-protein_type
catalytic	B-site
pocket	I-site
and	O
is	O
comprised	O
of	O
a	O
highly	B-site
acidic	I-site
patch	I-site
and	O
a	O
hydrophobic	B-site
groove	I-site
.	O

D	B-structure_element
-	I-structure_element
motifs	I-structure_element
are	O
found	O
in	O
many	O
MAPK	B-protein_type
-	I-protein_type
interacting	I-protein_type
proteins	I-protein_type
,	O
including	O
substrates	O
,	O
activating	O
kinases	B-protein_type
and	O
inactivating	O
phosphatases	B-protein_type
,	O
as	O
well	O
as	O
scaffolding	O
proteins	O
.	O

A	O
second	B-structure_element
docking	I-structure_element
motif	I-structure_element
for	O
MAPKs	B-protein_type
consists	O
of	O
two	O
Phe	B-residue_name
residues	O
separated	O
by	O
one	O
residue	O
(	O
FXF	B-structure_element
-	I-structure_element
motif	I-structure_element
).	O

This	O
motif	O
has	O
been	O
observed	O
in	O
several	O
MAPK	B-protein_type
substrates	O
.	O

The	O
FXF	B-site
-	I-site
motif	I-site
-	I-site
binding	I-site
site	I-site
of	O
ERK2	B-protein
has	O
been	O
mapped	O
to	O
a	O
hydrophobic	B-site
pocket	I-site
formed	O
between	O
the	O
P	B-site
+	I-site
1	I-site
site	I-site
,	O
αG	B-structure_element
helix	I-structure_element
and	O
the	O
MAPK	B-structure_element
insert	I-structure_element
.	O

However	O
,	O
the	O
generality	O
and	O
mechanism	O
of	O
the	O
FXF	B-structure_element
-	O
mediated	O
interaction	O
is	O
unclear	O
.	O

The	O
physiological	O
outcome	O
of	O
MAPK	B-protein_type
signalling	O
depends	O
on	O
both	O
the	O
magnitude	O
and	O
the	O
duration	O
of	O
kinase	O
activation	O
.	O

MKPs	B-protein_type
constitute	O
a	O
group	O
of	O
DUSPs	B-protein_type
that	O
are	O
characterized	O
by	O
their	O
ability	O
to	O
dephosphorylate	O
both	O
phosphotyrosine	B-residue_name
and	O
phosphoserine	B-residue_name
/	O
phospho	B-residue_name
-	I-residue_name
threonine	I-residue_name
residues	O
within	O
a	O
substrate	O
.	O

Dysregulated	O
expression	O
of	O
MKPs	B-protein_type
has	O
been	O
associated	O
with	O
pathogenesis	O
of	O
various	O
diseases	O
,	O
and	O
understanding	O
their	O
precise	O
recognition	O
mechanism	O
presents	O
an	O
important	O
challenge	O
and	O
opportunity	O
for	O
drug	O
development	O
.	O

This	O
structure	B-evidence
reveals	O
the	O
molecular	O
mechanism	O
underlying	O
the	O
docking	O
interaction	O
between	O
MKP7	B-protein
and	O
JNK1	B-protein
.	O

In	O
the	O
JNK1	B-complex_assembly
–	I-complex_assembly
MKP7	I-complex_assembly
complex	O
,	O
a	O
hydrophobic	B-structure_element
motif	I-structure_element
(	O
285FNFL288	B-structure_element
)	O
that	O
initiates	O
the	O
helix	B-structure_element
α5	B-structure_element
in	O
the	O
MKP7	B-protein
catalytic	B-structure_element
domain	I-structure_element
directly	O
binds	O
to	O
the	O
FXF	B-site
-	I-site
motif	I-site
-	I-site
binding	I-site
site	I-site
on	O
JNK1	B-protein
,	O
providing	O
the	O
structural	O
insight	O
into	O
the	O
classic	O
FXF	B-site
-	I-site
type	I-site
docking	I-site
interaction	I-site
.	O

Biochemical	B-experimental_method
and	I-experimental_method
modelling	I-experimental_method
studies	I-experimental_method
further	O
demonstrate	O
that	O
the	O
molecular	O
interactions	O
mediate	O
this	O
key	O
element	O
for	O
substrate	O
recognition	O
are	O
highly	O
conserved	O
among	O
all	O
MKP	B-protein_type
-	I-protein_type
family	I-protein_type
members	I-protein_type
.	O

Thus	O
,	O
our	O
study	O
reveals	O
a	O
hitherto	O
unrecognized	O
interaction	O
mode	O
for	O
encoding	O
complex	O
target	O
specificity	O
among	O
MAPK	B-protein_type
isoforms	I-protein_type
.	O

Interaction	O
of	O
JNK1	B-protein
with	O
the	O
MKP7	B-protein
catalytic	B-structure_element
domain	I-structure_element

MKPs	B-protein_type
represent	O
a	O
distinct	O
subfamily	O
within	O
a	O
larger	O
group	O
of	O
DUSPs	B-protein_type
.	O

In	O
mammalian	B-taxonomy_domain
cells	O
,	O
the	O
MKP	B-protein_type
subfamily	I-protein_type
includes	O
10	O
distinct	O
catalytically	B-protein_state
active	I-protein_state
MKPs	B-protein_type
.	O

All	O
MKPs	B-protein_type
contain	O
a	O
highly	B-protein_state
conserved	I-protein_state
C	O
-	O
terminal	O
catalytic	B-structure_element
domain	I-structure_element
(	O
CD	B-structure_element
)	O
and	O
an	O
N	O
-	O
terminal	O
kinase	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
(	O
KBD	B-structure_element
).	O

On	O
the	O
basis	O
of	O
sequence	O
similarity	O
,	O
substrate	O
specificity	O
and	O
predominant	O
subcellular	O
localization	O
,	O
the	O
MKP	B-protein_type
family	I-protein_type
can	O
be	O
further	O
divided	O
into	O
three	O
groups	O
(	O
Fig	O
.	O
1	O
).	O

Biochemical	B-experimental_method
and	I-experimental_method
structural	I-experimental_method
studies	I-experimental_method
have	O
revealed	O
that	O
the	O
KBD	B-structure_element
of	O
MKPs	B-protein_type
is	O
critical	O
for	O
MKP3	B-protein
docking	O
to	O
ERK2	B-protein
,	O
and	O
MKP5	B-protein
binding	O
to	O
p38α	B-protein
,	O
although	O
their	O
binding	O
mechanisms	O
are	O
completely	O
different	O
.	O

However	O
,	O
it	O
is	O
unknown	O
if	O
other	O
MAPKs	B-protein_type
can	O
interact	O
with	O
the	O
KBD	B-structure_element
of	O
their	O
cognate	O
phosphatases	B-protein_type
in	O
the	O
same	O
manner	O
as	O
observed	O
for	O
recognition	O
of	O
ERK2	B-protein
and	O
p38α	B-protein
by	O
their	O
MKPs	B-protein_type
,	O
or	O
whether	O
they	O
recognize	O
distinct	O
docking	B-structure_element
motifs	I-structure_element
of	O
MKPs	B-protein_type
.	O

MKP7	B-protein
,	O
the	O
biggest	O
molecule	O
in	O
the	O
MKP	B-protein_type
family	I-protein_type
,	O
selectively	O
inactivates	O
JNK	B-protein_type
and	O
p38	B-protein_type
following	O
stress	O
activation	O
.	O

In	O
addition	O
to	O
the	O
CD	B-structure_element
and	O
KBD	B-structure_element
,	O
MKP7	B-protein
has	O
a	O
long	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
region	I-structure_element
that	O
contains	O
both	O
nuclear	O
localization	O
and	O
export	O
sequences	O
by	O
which	O
MKP7	B-protein
shuttles	O
between	O
the	O
nucleus	O
and	O
the	O
cytoplasm	O
(	O
Fig	O
.	O
2a	O
).	O

To	O
quantitatively	O
assess	O
the	O
contribution	O
of	O
the	O
N	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
to	O
the	O
MKP7	B-protein
-	O
catalysed	O
JNK1	B-protein
dephosphorylation	B-ptm
,	O
we	O
first	O
measured	O
the	O
kinetic	B-evidence
parameters	O
of	O
the	O
C	O
-	O
terminal	O
truncation	B-experimental_method
of	O
MKP7	B-protein
(	O
MKP7ΔC304	B-mutant
,	O
residues	O
5	B-residue_range
–	I-residue_range
303	I-residue_range
)	O
and	O
MKP7	B-protein
-	O
CD	B-structure_element
(	O
residues	O
156	B-residue_range
–	I-residue_range
301	I-residue_range
)	O
towards	O
phosphorylated	B-protein_state
JNK1	B-protein
(	O
pJNK1	B-protein_state
).	O

Figure	O
2b	O
shows	O
the	O
variation	B-evidence
of	I-evidence
initial	I-evidence
rates	I-evidence
of	O
the	O
MKP7ΔC304	B-mutant
and	O
MKP7	B-protein
-	O
CD	B-structure_element
-	O
catalysed	O
reaction	O
with	O
the	O
concentration	O
of	O
phospho	B-protein_state
-	O
JNK1	B-protein
.	O
Because	O
the	O
concentrations	O
of	O
MKP7	B-protein
and	O
pJNK1	B-protein_state
were	O
comparable	O
in	O
the	O
reaction	O
,	O
the	O
assumption	O
that	O
the	O
free	O
-	O
substrate	O
concentration	O
is	O
equal	O
to	O
the	O
total	O
substrate	O
concentration	O
is	O
not	O
valid	O
.	O

The	O
kcat	B-evidence
and	O
Km	B-evidence
of	O
the	O
MKP7	B-protein
-	O
CD	B-structure_element
(	O
0	O
.	O
028	O
s	O
−	O
1	O
and	O
0	O
.	O
26	O
μM	O
)	O
so	O
determined	O
were	O
nearly	O
identical	O
to	O
those	O
of	O
MKP7ΔC304	B-mutant
(	O
0	O
.	O
029	O
s	O
−	O
1	O
and	O
0	O
.	O
27	O
μM	O
),	O
indicating	O
that	O
the	O
MKP7	B-protein
-	O
KBD	B-structure_element
has	O
no	O
effect	O
on	O
enzyme	O
catalysis	O
.	O

We	O
next	O
examined	O
the	O
interaction	O
of	O
JNK1	B-protein
with	O
the	O
CD	B-structure_element
and	O
KBD	B-structure_element
of	O
MKP7	B-protein
by	O
gel	B-experimental_method
filtration	I-experimental_method
analysis	I-experimental_method
.	O

When	O
3	O
molar	O
equivalents	O
of	O
CD	B-structure_element
were	O
mixed	O
with	O
1	O
molar	O
equivalent	O
of	O
JNK1	B-protein
,	O
a	O
significant	O
amount	O
of	O
CD	B-structure_element
co	O
-	O
migrated	O
with	O
JNK1	B-protein
to	O
earlier	O
fractions	O
,	O
and	O
the	O
excess	O
amount	O
of	O
CD	B-structure_element
was	O
eluted	O
from	O
the	O
size	O
exclusion	O
column	O
as	O
a	O
monomer	B-oligomeric_state
,	O
indicating	O
stable	O
complex	O
formation	O
(	O
Fig	O
.	O
2c	O
).	O

In	O
contrast	O
,	O
no	O
KBD	B-complex_assembly
–	I-complex_assembly
JNK1	I-complex_assembly
complex	O
was	O
detected	O
when	O
3	O
molar	O
equivalents	O
of	O
KBD	B-structure_element
were	O
mixed	O
with	O
1	O
molar	O
equivalent	O
of	O
JNK1	B-protein
.	O

Taken	O
together	O
,	O
our	O
data	O
indicate	O
that	O
the	O
CD	B-structure_element
of	O
MKP7	B-protein
,	O
but	O
not	O
the	O
KBD	B-structure_element
domain	O
,	O
is	O
responsible	O
for	O
JNK	B-protein_type
substrate	O
-	O
binding	O
and	O
enzymatic	O
specificity	O
.	O

Crystal	B-evidence
structure	I-evidence
of	O
JNK1	B-protein
in	B-protein_state
complex	I-protein_state
with	I-protein_state
the	O
MKP7	B-protein
-	O
CD	B-structure_element

The	O
overall	O
folding	O
of	O
MKP7	B-protein
-	O
CD	B-structure_element
is	O
typical	O
of	O
DUSPs	B-protein_type
,	O
with	O
a	O
central	O
twisted	B-structure_element
five	I-structure_element
-	I-structure_element
stranded	I-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
surrounded	O
by	O
six	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
.	O

One	O
side	O
of	O
the	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
is	O
covered	O
with	O
two	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
and	O
the	O
other	O
is	O
covered	O
with	O
four	O
α	B-structure_element
-	I-structure_element
helices	I-structure_element
(	O
Fig	O
.	O
3b	O
).	O

The	O
catalytic	B-structure_element
domain	I-structure_element
of	O
MKP7	B-protein
interacts	O
with	O
JNK1	B-protein
through	O
a	O
contiguous	O
surface	O
area	O
that	O
is	O
remote	O
from	O
the	O
active	B-site
site	I-site
.	O

MKP7	B-protein
-	O
CD	B-structure_element
is	O
positioned	O
onto	O
the	O
JNK1	B-protein
molecule	O
so	O
that	O
the	O
active	B-site
site	I-site
of	O
the	O
phosphatase	B-protein_type
faces	O
towards	O
the	O
activation	B-structure_element
segment	I-structure_element
.	O

The	O
most	O
striking	O
difference	O
is	O
that	O
helix	B-structure_element
α0	B-structure_element
and	O
loop	B-structure_element
α0	B-structure_element
–	I-structure_element
β1	I-structure_element
of	O
VHR	B-protein
are	O
absent	O
in	O
MKP7	B-protein
-	O
CD	B-structure_element
.	O

Another	O
region	O
that	O
cannot	O
be	O
aligned	O
with	O
VHR	B-protein
is	O
found	O
in	O
loop	B-structure_element
β3	B-structure_element
–	I-structure_element
β4	I-structure_element
.	O

The	O
active	B-site
site	I-site
of	O
MKP7	B-protein
consists	O
of	O
the	O
phosphate	B-structure_element
-	I-structure_element
binding	I-structure_element
loop	I-structure_element
(	O
P	B-structure_element
-	I-structure_element
loop	I-structure_element
,	O
Cys244	B-residue_name_number
-	O
Leu245	B-residue_name_number
-	O
Ala246	B-residue_name_number
-	O
Gly247	B-residue_name_number
-	O
Ile248	B-residue_name_number
-	O
Ser249	B-residue_name_number
-	O
Arg250	B-residue_name_number
),	O
and	O
Asp213	B-residue_name_number
in	O
the	O
general	B-structure_element
acid	I-structure_element
loop	I-structure_element
(	O
Fig	O
.	O
3b	O
and	O
Supplementary	O
Fig	O
.	O
1b	O
).	O

The	O
MKP7	B-protein
-	O
CD	B-structure_element
structure	B-evidence
near	O
the	O
active	B-site
site	I-site
exhibits	O
a	O
typical	O
active	B-protein_state
conformation	I-protein_state
as	O
found	O
in	O
VHR	B-protein
and	O
other	O
PTPs	B-protein_type
.	O

The	O
catalytic	B-site
residue	I-site
,	O
Cys244	B-residue_name_number
,	O
is	O
located	O
just	O
after	O
strand	B-structure_element
β5	B-structure_element
and	O
optimally	O
positioned	O
for	O
nucleophilic	O
attack	O
.	O

Asp213	B-residue_name_number
in	O
MKP7	B-protein
also	O
adopts	O
a	O
position	O
similar	O
to	O
that	O
of	O
Asp92	B-residue_name_number
in	O
VHR	B-protein
(	O
Supplementary	O
Fig	O
.	O
1c	O
),	O
indicating	O
that	O
Asp213	B-residue_name_number
is	O
likely	O
to	O
function	O
as	O
the	O
general	O
acid	O
in	O
MKP7	B-protein
.	O

It	O
is	O
located	O
3	O
.	O
36	O
Å	O
from	O
the	O
Cys244	B-residue_name_number
side	O
chain	O
and	O
makes	O
electrostatic	B-bond_interaction
interactions	I-bond_interaction
with	O
the	O
dipole	O
moment	O
of	O
helix	B-structure_element
α3	B-structure_element
and	O
with	O
several	O
main	O
-	O
chain	O
amide	O
groups	O
.	O

The	O
side	O
chain	O
of	O
strictly	B-protein_state
conserved	I-protein_state
Arg250	B-residue_name_number
is	O
oriented	O
towards	O
the	O
negatively	O
charged	O
chloride	B-chemical
,	O
similar	O
to	O
the	O
canonical	O
phosphate	B-structure_element
-	I-structure_element
coordinating	I-structure_element
conformation	I-structure_element
.	O

The	O
difference	O
in	O
the	O
polarity	O
/	O
hydrophobicity	O
of	O
the	O
surface	O
may	O
also	O
point	O
to	O
the	O
origin	O
of	O
the	O
differences	O
in	O
the	O
substrate	O
-	O
recognition	O
mechanism	O
for	O
these	O
two	O
phosphatases	B-protein_type
(	O
Supplementary	O
Fig	O
.	O
1e	O
,	O
f	O
).	O

As	O
a	O
result	O
,	O
the	O
buried	O
solvent	O
-	O
accessible	O
surface	O
area	O
is	O
∼	O
1	O
,	O
315	O
Å	O
.	O
In	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
domain	I-structure_element
,	O
JNK1	B-protein
has	O
an	O
insertion	O
after	O
the	O
helix	B-structure_element
αG	B-structure_element
.	O
This	O
insertion	O
consists	O
of	O
two	O
helices	B-structure_element
(	O
α1L14	B-structure_element
and	O
α2L14	B-structure_element
)	O
that	O
are	O
common	O
to	O
all	O
members	O
of	O
the	O
MAPK	B-protein_type
family	I-protein_type
.	O

The	O
interactive	B-site
surface	I-site
in	O
JNK1	B-protein
,	O
formed	O
by	O
the	O
helices	B-structure_element
αG	B-structure_element
and	O
α2L14	B-structure_element
,	O
displays	O
a	O
hydrophobic	B-site
region	I-site
,	O
centred	O
at	O
Trp234	B-residue_name_number
(	O
Fig	O
.	O
3d	O
).	O

The	O
MKP7	B-site
-	I-site
docking	I-site
region	I-site
includes	O
two	O
helices	B-structure_element
,	O
α4	B-structure_element
and	O
α5	B-structure_element
,	O
and	O
the	O
general	B-structure_element
acid	I-structure_element
loop	I-structure_element
.	O

In	O
addition	O
,	O
there	O
are	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
between	O
Ser282	B-residue_name_number
and	O
Asn286	B-residue_name_number
of	O
MKP7	B-protein
and	O
His230	B-residue_name_number
and	O
Thr255	B-residue_name_number
of	O
JNK1	B-protein
,	O
and	O
the	O
main	O
chain	O
of	O
Phe215	B-residue_name_number
in	O
the	O
general	B-structure_element
acid	I-structure_element
loop	I-structure_element
of	O
MKP7	B-protein
is	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
to	O
the	O
side	O
chain	O
of	O
Gln253	B-residue_name_number
in	O
JNK1	B-protein
.	O

The	O
second	B-site
interactive	I-site
area	I-site
involves	O
the	O
α4	B-structure_element
helix	I-structure_element
of	O
MKP7	B-protein
and	O
charged	O
/	O
polar	O
residues	O
of	O
JNK1	B-protein
(	O
Fig	O
.	O
3e	O
).	O

The	O
carboxylate	O
of	O
Asp268	B-residue_name_number
in	O
MKP7	B-protein
forms	O
a	O
salt	B-bond_interaction
bridge	I-bond_interaction
with	O
side	O
chain	O
of	O
Arg263	B-residue_name_number
in	O
JNK1	B-protein
,	O
and	O
Lys275	B-residue_name_number
of	O
MKP7	B-protein
forms	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
and	O
a	O
salt	B-bond_interaction
bridge	I-bond_interaction
with	O
Thr228	B-residue_name_number
and	O
Asp229	B-residue_name_number
of	O
JNK1	B-protein
,	O
respectively	O
.	O

Mutational	B-experimental_method
analysis	I-experimental_method
of	O
the	O
JNK1	B-site
–	I-site
MKP7	I-site
docking	I-site
interface	I-site

To	O
assess	O
the	O
importance	O
of	O
the	O
aforementioned	O
interactions	O
,	O
we	O
generated	O
a	O
series	O
of	O
point	B-experimental_method
mutations	I-experimental_method
on	O
the	O
MKP7	B-protein
-	O
CD	B-structure_element
and	O
examined	O
their	O
effect	O
on	O
the	O
MKP7	B-protein
-	O
catalysed	O
JNK1	B-protein
dephosphorylation	B-ptm
(	O
Fig	O
.	O
4a	O
).	O

When	O
the	O
hydrophobic	O
residues	O
Phe285	B-residue_name_number
and	O
Phe287	B-residue_name_number
on	O
the	O
α5	B-structure_element
of	O
MKP7	B-protein
-	O
CD	B-structure_element
were	O
replaced	B-experimental_method
by	O
Asp	B-residue_name
or	O
Ala	B-residue_name
,	O
their	O
phosphatase	O
activities	O
for	O
JNK1	B-protein
dephosphorylation	B-ptm
decreased	O
∼	O
10	O
-	O
fold	O
.	O

In	O
comparison	O
,	O
replacement	B-experimental_method
of	O
the	O
other	O
residues	O
(	O
Phe215	B-residue_name_number
,	O
Asp268	B-residue_name_number
,	O
Lys275	B-residue_name_number
,	O
Ser282	B-residue_name_number
,	O
Asn286	B-residue_name_number
and	O
Leu292	B-residue_name_number
)	O
with	O
an	O
Ala	B-residue_name
or	O
Asp	B-residue_name
individually	O
led	O
to	O
a	O
modest	O
decrease	O
in	O
catalytic	O
efficiencies	O
,	O
suggesting	O
that	O
this	O
position	O
may	O
only	O
affect	O
some	O
selectivity	O
of	O
MKP	B-protein_type
.	O

Mutation	B-experimental_method
of	O
Leu288	B-residue_name_number
markedly	O
reduced	O
its	O
solubility	O
when	O
expressed	O
in	O
Escherichia	B-species
coli	I-species
,	O
resulting	O
in	O
the	O
insoluble	O
aggregation	O
of	O
the	O
mutant	B-protein_state
protein	O
.	O

Gel	B-experimental_method
filtration	I-experimental_method
analysis	I-experimental_method
further	O
confirmed	O
the	O
key	O
role	O
of	O
Phe285	B-residue_name_number
in	O
the	O
MKP7	B-protein
–	O
JNK1	B-protein
interaction	O
:	O
no	O
F285D	B-complex_assembly
–	I-complex_assembly
JNK1	I-complex_assembly
complex	O
was	O
detected	O
when	O
3	O
molar	O
equivalents	O
of	O
MKP7	B-protein
-	O
CD	B-structure_element
(	O
F285D	B-mutant
)	O
were	O
mixed	O
with	O
1	O
molar	O
equivalent	O
of	O
JNK1	B-protein
(	O
Fig	O
.	O
4b	O
).	O

We	O
also	O
generated	O
a	O
series	O
of	O
point	B-experimental_method
mutations	I-experimental_method
in	O
the	O
JNK1	B-protein
and	O
assessed	O
the	O
effect	O
on	O
JNK1	B-protein
binding	O
using	O
the	O
GST	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assay	I-experimental_method
(	O
Fig	O
.	O
4c	O
).	O

Substitution	B-experimental_method
at	O
Asp229	B-residue_name_number
,	O
Trp234	B-residue_name_number
,	O
Thr255	B-residue_name_number
,	O
Val256	B-residue_name_number
,	O
Tyr259	B-residue_name_number
and	O
Val260	B-residue_name_number
significantly	O
reduced	O
the	O
binding	B-evidence
affinity	I-evidence
of	O
MKP7	B-protein
-	O
CD	B-structure_element
for	O
JNK	B-protein_type
.	O

To	O
determine	O
whether	O
the	O
deficiencies	O
in	O
their	O
abilities	O
to	O
bind	O
partner	O
proteins	O
or	O
carry	O
out	O
catalytic	O
function	O
are	O
owing	O
to	O
misfolding	O
of	O
the	O
purified	O
mutant	B-protein_state
proteins	O
,	O
we	O
also	O
examined	O
the	O
folding	O
properties	O
of	O
the	O
JNK1	B-protein
and	O
MKP7	B-protein
mutants	B-protein_state
with	O
circular	B-experimental_method
dichroism	I-experimental_method
.	O

The	O
spectra	B-evidence
of	O
these	O
mutants	B-protein_state
are	O
similar	O
to	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
proteins	O
,	O
indicating	O
that	O
these	O
mutants	B-protein_state
fold	O
as	O
well	O
as	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
proteins	O
(	O
Fig	O
.	O
4d	O
,	O
e	O
).	O

Taken	O
together	O
,	O
these	O
results	O
are	O
consistent	O
with	O
the	O
present	O
crystallographic	B-evidence
model	I-evidence
,	O
which	O
reveal	O
the	O
hydrophobic	B-bond_interaction
contacts	I-bond_interaction
between	O
the	O
MKP7	B-protein
catalytic	B-structure_element
domain	I-structure_element
and	O
JNK1	B-protein
have	O
a	O
predominant	O
role	O
in	O
the	O
enzyme	O
–	O
substrate	O
interaction	O
,	O
and	O
hydrophobic	O
residue	O
Phe285	B-residue_name_number
in	O
the	O
MKP7	B-protein
-	O
CD	B-structure_element
is	O
a	O
key	O
residue	O
for	O
its	O
high	O
-	O
affinity	O
binding	O
to	O
JNK1	B-protein
.	O

It	O
has	O
previously	O
been	O
reported	O
that	O
several	O
cytosolic	O
and	O
inducible	O
nuclear	O
MKPs	B-protein_type
undergo	O
catalytic	O
activation	O
upon	O
interaction	O
with	O
the	O
MAPK	B-protein_type
substrates	O
.	O

This	O
allosteric	O
activation	O
of	O
MKP3	B-protein
has	O
been	O
well	O
-	O
documented	O
in	O
vitro	O
using	O
pNPP	B-chemical
,	O
a	O
small	O
-	O
molecule	O
phosphotyrosine	B-residue_name
analogue	O
of	O
its	O
normal	O
substrate	O
.	O

We	O
then	O
assayed	O
pNPPase	B-protein_type
activities	O
of	O
MKP7ΔC304	B-mutant
and	O
MKP7	B-protein
-	O
CD	B-structure_element
in	O
the	O
presence	B-protein_state
of	I-protein_state
JNK1	B-protein
.	O

The	O
small	O
pNPP	B-chemical
molecule	O
binds	O
directly	O
at	O
the	O
enzyme	O
active	B-site
site	I-site
and	O
can	O
be	O
used	O
to	O
probe	O
the	O
reaction	O
mechanism	O
of	O
protein	B-protein_type
phosphatases	I-protein_type
.	O

We	O
therefore	O
examined	O
the	O
effects	O
of	O
the	O
MKP7	B-protein
-	O
CD	B-structure_element
mutants	B-protein_state
on	O
their	O
pNPPase	B-protein_type
activities	O
.	O

In	O
the	O
JNK1	B-complex_assembly
/	I-complex_assembly
MKP7	I-complex_assembly
-	I-complex_assembly
CD	I-complex_assembly
complex	O
structure	B-evidence
,	O
Phe287	B-residue_name_number
of	O
MKP7	B-protein
does	O
not	O
make	O
contacts	O
with	O
JNK1	B-protein
substrate	O
.	O

It	O
penetrates	O
into	O
a	O
pocket	B-site
formed	O
by	O
residues	O
from	O
the	O
P	B-structure_element
-	I-structure_element
loop	I-structure_element
and	O
general	B-structure_element
acid	I-structure_element
loop	I-structure_element
and	O
forms	O
hydrophobic	B-bond_interaction
contacts	I-bond_interaction
with	O
the	O
aliphatic	O
portions	O
of	O
side	O
chains	O
of	O
Arg250	B-residue_name_number
,	O
Glu217	B-residue_name_number
and	O
Ile219	B-residue_name_number
,	O
suggesting	O
that	O
Phe287	B-residue_name_number
in	O
MKP7	B-protein
would	O
play	O
a	O
similar	O
role	O
to	O
that	O
of	O
its	O
structural	O
counterpart	O
in	O
the	O
PTPs	B-protein_type
(	O
Gln266	B-residue_name_number
in	O
PTP1B	B-protein
)	O
and	O
VHR	B-protein
(	O
Phe166	B-residue_name_number
in	O
VHR	B-protein
)	O
in	O
the	O
precise	O
alignment	O
of	O
active	B-site
-	I-site
site	I-site
residues	I-site
in	O
MKP7	B-protein
with	O
respect	O
to	O
the	O
substrate	O
for	O
efficient	O
catalysis	O
(	O
Supplementary	O
Fig	O
.	O
2c	O
).	O

Kinase	B-protein
-	I-protein
associated	I-protein
phosphatase	I-protein
(	O
KAP	B-protein
),	O
a	O
member	O
of	O
the	O
DUSP	B-protein_type
family	I-protein_type
,	O
plays	O
a	O
crucial	O
role	O
in	O
cell	O
cycle	O
regulation	O
by	O
dephosphorylating	O
the	O
pThr160	B-ptm
residue	O
of	O
CDK2	B-protein
(	O
cyclin	B-protein
-	I-protein
dependent	I-protein
kinase	I-protein
2	I-protein
).	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
CDK2	B-complex_assembly
/	I-complex_assembly
KAP	I-complex_assembly
complex	O
has	O
been	O
determined	O
at	O
3	O
.	O
0	O
Å	O
(	O
Fig	O
.	O
5a	O
).	O

The	O
interface	B-site
between	O
these	O
two	O
proteins	O
consists	O
of	O
three	O
discontinuous	O
contact	O
regions	O
.	O

Structural	B-experimental_method
analysis	I-experimental_method
and	O
sequence	B-experimental_method
alignment	I-experimental_method
reveal	O
that	O
one	O
of	O
the	O
few	O
differences	O
between	O
MKP7	B-protein
-	O
CD	B-structure_element
and	O
KAP	B-protein
in	O
the	O
substrate	B-site
-	I-site
binding	I-site
region	I-site
is	O
the	O
presence	O
of	O
the	O
motif	O
FNFL	B-structure_element
in	O
MKP7	B-protein
-	O
CD	B-structure_element
,	O
which	O
corresponds	O
to	O
IKQY	B-structure_element
in	O
KAP	B-protein
(	O
Fig	O
.	O
5c	O
).	O

The	O
substitution	B-experimental_method
of	O
the	O
two	O
hydrophobic	O
residues	O
with	O
charged	O
/	O
polar	O
residues	O
(	O
F285I	B-mutant
/	O
N286K	B-mutant
)	O
seriously	O
disrupts	O
the	O
hydrophobic	B-bond_interaction
interaction	I-bond_interaction
required	O
for	O
MKP7	B-protein
binding	O
on	O
JNK1	B-protein
(	O
Fig	O
.	O
4a	O
).	O

These	O
data	O
indicated	O
that	O
a	O
unique	O
hydrophobic	B-site
pocket	I-site
formed	O
between	O
the	O
MAPK	B-structure_element
insert	I-structure_element
and	O
αG	B-structure_element
helix	I-structure_element
plays	O
a	O
major	O
role	O
in	O
the	O
substrate	O
recognition	O
by	O
MKPs	B-protein_type
.	O

F	B-site
-	I-site
site	I-site
interaction	O
is	O
crucial	O
for	O
JNK1	B-protein
inactivation	O
in	O
vivo	O

To	O
assess	O
the	O
effects	O
of	O
MKP7	B-protein
and	O
its	O
mutants	B-protein_state
on	O
the	O
activation	O
of	O
endogenous	O
JNK	B-protein_type
in	O
vivo	O
,	O
HEK293T	O
cells	O
were	O
transfected	O
with	O
blank	O
vector	O
or	O
with	O
HA	B-protein_state
-	I-protein_state
tagged	I-protein_state
constructs	O
for	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
MKP7	B-protein
,	O
MKP7ΔC304	B-mutant
and	O
MKP7	B-protein
-	O
CD	B-structure_element
or	O
MKP7	B-protein
mutants	B-protein_state
,	O
and	O
stimulated	O
with	O
ultraviolet	O
or	O
etoposide	B-chemical
treatment	O
.	O

As	O
shown	O
in	O
Fig	O
.	O
6a	O
–	O
c	O
,	O
immunobloting	B-experimental_method
showed	O
similar	O
expression	O
levels	O
for	O
the	O
different	O
MKP7	B-protein
constructs	O
in	O
all	O
the	O
cells	O
.	O

Overexpressed	B-experimental_method
full	B-protein_state
-	I-protein_state
length	I-protein_state
MKP7	B-protein
,	O
MKP7ΔC304	B-mutant
and	O
MKP7	B-protein
-	O
CD	B-structure_element
significantly	O
reduced	O
the	O
endogenous	O
level	O
of	O
phosphorylated	B-protein_state
JNK	B-protein_type
compared	O
with	O
vector	O
-	O
transfected	O
cells	O
.	O

In	O
agreement	O
with	O
the	O
in	B-experimental_method
vitro	I-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
results	O
,	O
the	O
mutants	B-protein_state
D229A	B-mutant
,	O
W234D	B-mutant
and	O
Y259D	B-mutant
were	O
not	O
co	O
-	O
precipitated	O
with	O
MKP7	B-protein
,	O
and	O
the	O
I231D	B-mutant
mutant	B-protein_state
had	O
only	O
little	O
effect	O
on	O
the	O
JNK1	B-complex_assembly
–	I-complex_assembly
MKP7	I-complex_assembly
interaction	O
(	O
Fig	O
.	O
6d	O
and	O
Supplementary	O
Fig	O
.	O
3a	O
).	O

Activation	O
of	O
the	O
JNK	B-protein_type
signalling	O
pathway	O
is	O
frequently	O
associated	O
with	O
apoptotic	O
cell	O
death	O
,	O
and	O
inhibition	O
of	O
JNK	B-protein_type
can	O
prevent	O
apoptotic	O
death	O
of	O
multiple	O
cells	O
.	O

To	O
examine	O
whether	O
the	O
inhibition	O
of	O
JNK	B-protein_type
activity	O
by	O
MKP7	B-protein
would	O
provide	O
protections	O
against	O
the	O
apoptosis	O
,	O
we	O
analysed	O
the	O
rate	O
of	O
apoptosis	O
in	O
ultraviolet	O
-	O
irradiated	O
cells	O
transfected	O
with	O
MKP7	B-protein
(	O
wild	B-protein_state
type	I-protein_state
or	O
mutants	B-protein_state
)	O
by	O
flow	B-experimental_method
cytometry	I-experimental_method
.	O

The	O
results	O
showed	O
similar	O
apoptotic	O
rates	O
between	O
cells	O
transfected	O
with	O
blank	O
vector	O
or	O
with	O
MKP7	B-protein
(	O
wild	B-protein_state
type	I-protein_state
or	O
mutants	B-protein_state
)	O
under	O
unstimulated	O
conditions	O
(	O
Supplementary	O
Fig	O
.	O
3b	O
),	O
while	O
ultraviolet	O
-	O
irradiation	O
significantly	O
increased	O
apoptotic	O
rate	O
in	O
cells	O
transfected	O
with	O
blank	O
vector	O
(	O
Fig	O
.	O
6e	O
).	O

Moreover	O
,	O
treatment	O
of	O
cells	O
expressing	O
MKP7	B-protein
-	O
KBD	B-structure_element
mutants	B-protein_state
(	O
R56A	B-mutant
/	O
R57A	B-mutant
and	O
V63A	B-mutant
/	O
I65A	B-mutant
)	O
decreased	O
the	O
apoptosis	O
rates	O
to	O
a	O
similar	O
extent	O
as	O
MKP7	B-protein
wild	B-protein_state
type	I-protein_state
did	O
.	O

Taken	O
together	O
,	O
our	O
results	O
suggested	O
that	O
FXF	B-structure_element
-	I-structure_element
motif	I-structure_element
-	O
mediated	O
,	O
rather	O
than	O
KBD	B-structure_element
-	O
mediated	O
,	O
interaction	O
is	O
essential	O
for	O
MKP7	B-protein
to	O
block	O
ultraviolet	O
-	O
induced	O
apoptosis	O
.	O

A	O
similar	O
docking	O
mechanism	O
for	O
JNK1	B-protein
recognition	O
by	O
MKP5	B-protein

MKP5	B-protein
belongs	O
to	O
the	O
same	O
subfamily	O
as	O
MKP7	B-protein
.	O

The	O
KBD	B-structure_element
of	O
MKP5	B-protein
interacts	O
with	O
the	O
D	B-site
-	I-site
site	I-site
of	O
p38α	B-protein
to	O
mediate	O
the	O
enzyme	O
–	O
substrate	O
interaction	O
.	O

In	O
contrast	O
to	O
p38α	B-protein
substrate	O
,	O
deletion	B-experimental_method
of	I-experimental_method
the	O
MKP5	B-protein
-	O
KBD	B-structure_element
had	O
little	O
effects	O
on	O
the	O
kinetic	O
parameters	O
for	O
the	O
JNK1	B-protein
dephosphorylation	O
,	O
indicating	O
that	O
the	O
KBD	B-structure_element
of	O
MKP5	B-protein
is	O
not	O
required	O
for	O
the	O
JNK1	B-protein
dephosphorylation	O
(	O
Fig	O
.	O
7b	O
).	O

The	O
substrate	B-evidence
specificity	I-evidence
constant	I-evidence
kcat	B-evidence
/	I-evidence
Km	I-evidence
value	O
for	O
MKP5	B-protein
-	O
CD	B-structure_element
was	O
calculated	O
as	O
1	O
.	O
0	O
×	O
105	O
M	O
−	O
1	O
s	O
−	O
1	O
,	O
which	O
is	O
very	O
close	O
to	O
that	O
of	O
MKP7	B-protein
-	O
CD	B-structure_element
(	O
1	O
.	O
07	O
×	O
105	O
M	O
−	O
1	O
s	O
−	O
1	O
).	O

The	O
crystal	B-evidence
structure	I-evidence
of	O
human	B-species
MKP5	B-protein
-	O
CD	B-structure_element
has	O
been	O
determined	O
.	O

Given	O
the	O
distinct	O
interaction	O
mode	O
revealed	O
by	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
JNK1	B-complex_assembly
–	I-complex_assembly
MKP7	I-complex_assembly
-	I-complex_assembly
CD	I-complex_assembly
,	O
one	O
obvious	O
question	O
is	O
whether	O
this	O
is	O
a	O
general	O
mechanism	O
used	O
by	O
all	O
members	O
of	O
the	O
JNK	B-protein_type
-	I-protein_type
specific	I-protein_type
MKPs	I-protein_type
.	O

To	O
address	O
this	O
issue	O
,	O
we	O
first	O
examined	O
the	O
docking	O
ability	O
of	O
JNK1	B-protein
to	O
the	O
KBD	B-structure_element
and	O
CD	B-structure_element
of	O
MKP5	B-protein
using	O
gel	B-experimental_method
filtration	I-experimental_method
analysis	I-experimental_method
and	O
pull	B-experimental_method
-	I-experimental_method
down	I-experimental_method
assays	I-experimental_method
.	O

It	O
can	O
be	O
seen	O
from	O
gel	B-experimental_method
filtration	I-experimental_method
experiments	I-experimental_method
that	O
JNK1	B-protein
can	O
forms	O
a	O
stable	B-protein_state
heterodimer	B-oligomeric_state
with	O
MKP5	B-protein
-	O
CD	B-structure_element
in	O
solution	O
,	O
but	O
no	O
detectable	O
interaction	O
was	O
found	O
with	O
the	O
KBD	B-structure_element
domain	O
(	O
Fig	O
.	O
7d	O
).	O

The	O
catalytic	B-structure_element
domain	I-structure_element
of	O
MKP5	B-protein
,	O
but	O
not	O
its	O
KBD	B-structure_element
,	O
was	O
able	O
to	O
pull	O
-	O
down	O
a	O
detectable	O
amount	O
of	O
JNK1	B-protein
(	O
Fig	O
.	O
7e	O
),	O
implicating	O
a	O
different	O
substrate	O
-	O
recognition	O
mechanisms	O
for	O
p38	B-protein_type
and	O
JNK	B-protein_type
MAPKs	B-protein_type
.	O

To	O
further	O
test	O
our	O
hypothesis	O
,	O
we	O
generated	O
forms	O
of	O
MKP5	B-protein
-	O
CD	B-structure_element
bearing	O
mutations	B-experimental_method
corresponding	O
to	O
the	O
changes	O
we	O
made	O
on	O
MKP7	B-protein
-	O
CD	B-structure_element
on	O
the	O
basis	O
of	O
sequence	B-experimental_method
and	I-experimental_method
structural	I-experimental_method
alignment	I-experimental_method
and	O
examined	O
their	O
effects	O
on	O
the	O
phosphatase	B-protein_type
activity	O
.	O

In	O
addition	O
,	O
there	O
were	O
no	O
significant	O
differences	O
in	O
the	O
CD	B-evidence
spectra	I-evidence
between	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
mutant	B-protein_state
proteins	O
,	O
indicating	O
that	O
the	O
overall	O
structures	B-evidence
of	O
these	O
mutants	B-protein_state
did	O
not	O
change	O
significantly	O
from	O
that	O
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
MKP5	B-protein
protein	O
(	O
Supplementary	O
Fig	O
.	O
4a	O
).	O

In	O
particular	O
,	O
Leu449	B-residue_name_number
of	O
MKP5	B-protein
,	O
which	O
is	O
equivalent	O
to	O
the	O
key	O
residue	O
Phe285	B-residue_name_number
of	O
MKP7	B-protein
,	O
buried	O
deeply	O
within	O
the	O
hydrophobic	B-site
pocket	I-site
of	O
JNK1	B-protein
in	O
the	O
same	O
way	O
as	O
Phe285	B-residue_name_number
in	O
the	O
JNK1	B-complex_assembly
–	I-complex_assembly
MKP7	I-complex_assembly
-	I-complex_assembly
CD	I-complex_assembly
complex	O
(	O
Supplementary	O
Fig	O
.	O
4d	O
).	O

Despite	O
the	O
strong	O
similarities	O
between	O
JNK1	B-protein
–	O
MKP5	B-protein
-	O
CD	B-structure_element
and	O
JNK1	B-complex_assembly
–	I-complex_assembly
MKP7	I-complex_assembly
-	I-complex_assembly
CD	I-complex_assembly
,	O
however	O
,	O
there	O
are	O
differences	O
.	O

Asp268	B-residue_name_number
of	O
MKP7	B-protein
-	O
CD	B-structure_element
forms	O
salt	B-bond_interaction
bridge	I-bond_interaction
with	O
JNK1	B-protein
Arg263	B-residue_name_number
,	O
whereas	O
the	O
corresponding	O
residue	O
Thr432	B-residue_name_number
in	O
MKP5	B-protein
-	O
CD	B-structure_element
may	O
not	O
interact	O
with	O
JNK1	B-protein
.	O

In	O
addition	O
,	O
the	O
key	O
interacting	O
residues	O
of	O
MKP7	B-protein
-	O
CD	B-structure_element
,	O
Phe215	B-residue_name_number
,	O
Leu267	B-residue_name_number
and	O
Leu288	B-residue_name_number
,	O
are	O
replaced	O
by	O
less	O
hydrophobic	O
residues	O
,	O
Asn379	B-residue_name_number
,	O
Met431	B-residue_name_number
and	O
Met452	B-residue_name_number
in	O
MKP5	B-protein
-	O
CD	B-structure_element
(	O
Fig	O
.	O
5c	O
),	O
respectively	O
,	O
which	O
may	O
result	O
in	O
weaker	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
between	O
MKP5	B-protein
-	O
CD	B-structure_element
and	O
JNK1	B-protein
.	O

This	O
is	O
consistent	O
with	O
the	O
experimental	O
observation	O
showing	O
that	O
JNK1	B-protein
binds	O
to	O
MKP7	B-protein
-	O
CD	B-structure_element
much	O
more	O
tightly	O
than	O
MKP5	B-protein
-	O
CD	B-structure_element
(	O
Km	O
value	O
of	O
MKP5	B-protein
-	O
CD	B-structure_element
for	O
pJNK1	B-protein_state
substrate	O
is	O
∼	O
20	O
-	O
fold	O
higher	O
than	O
that	O
of	O
MKP7	B-protein
-	O
CD	B-structure_element
).	O

The	O
MAPKs	B-protein_type
p38	B-protein_type
,	O
ERK	B-protein_type
and	O
JNK	B-protein_type
,	O
are	O
central	O
to	O
evolutionarily	O
conserved	O
signalling	O
pathways	O
that	O
are	O
present	O
in	O
all	O
eukaryotic	B-taxonomy_domain
cells	O
.	O

Each	O
MAPK	B-protein_type
cascade	O
is	O
activated	O
in	O
response	O
to	O
a	O
diverse	O
array	O
of	O
extracellular	O
signals	O
and	O
culminates	O
in	O
the	O
dual	B-ptm
-	I-ptm
phosphorylation	I-ptm
of	O
a	O
threonine	B-residue_name
and	O
a	O
tyrosine	B-residue_name
residue	O
in	O
the	O
MAPK	B-structure_element
-	I-structure_element
activation	I-structure_element
loop	I-structure_element
.	O

The	O
propagation	O
of	O
MAPK	B-protein_type
signals	O
is	O
attenuated	O
through	O
the	O
actions	O
of	O
the	O
MKPs	B-protein_type
.	O

Most	O
studies	O
have	O
focused	O
on	O
the	O
dephosphorylation	O
of	O
MAPKs	B-protein_type
by	O
phosphatases	B-protein_type
containing	O
the	O
‘	O
kinase	B-structure_element
-	I-structure_element
interaction	I-structure_element
motif	I-structure_element
'	O
(	O
D	B-structure_element
-	I-structure_element
motif	I-structure_element
),	O
including	O
a	O
group	O
of	O
DUSPs	B-protein_type
(	O
MKPs	B-protein_type
)	O
and	O
a	O
distinct	O
subfamily	O
of	O
tyrosine	B-protein_type
phosphatases	I-protein_type
(	O
HePTP	B-protein
,	O
STEP	B-protein
and	O
PTP	B-protein
-	I-protein
SL	I-protein
).	O

Crystal	B-evidence
structures	I-evidence
of	O
ERK2	B-protein
bound	B-protein_state
with	I-protein_state
the	O
D	B-structure_element
-	I-structure_element
motif	I-structure_element
sequences	O
derived	O
from	O
MKP3	B-protein
and	O
HePTP	B-protein
have	O
been	O
reported	O
.	O

The	O
particular	O
amino	O
acids	O
and	O
their	O
spacing	O
within	O
D	B-structure_element
-	I-structure_element
motif	I-structure_element
sequences	O
and	O
amino	O
acid	O
composition	O
of	O
the	O
docking	B-site
sites	I-site
on	O
MAPKs	B-protein_type
appear	O
to	O
determine	O
the	O
specificity	O
of	O
D	B-structure_element
-	I-structure_element
motifs	I-structure_element
for	O
individual	O
MAPKs	B-protein_type
.	O

Recently	O
,	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
a	O
complex	O
between	O
the	O
KBD	B-structure_element
of	O
MKP5	B-protein
and	O
p38α	B-protein
has	O
been	O
obtained	O
.	O

This	O
complex	O
has	O
revealed	O
a	O
distinct	O
interaction	O
mode	O
for	O
MKP5	B-protein
.	O

The	O
KBD	B-structure_element
of	O
MKP5	B-protein
binds	O
to	O
p38α	B-protein
in	O
the	O
opposite	O
polypeptide	O
direction	O
compared	O
with	O
how	O
the	O
D	B-structure_element
-	I-structure_element
motif	I-structure_element
of	O
MKP3	B-protein
binds	O
to	O
ERK2	B-protein
.	O

Further	O
structural	B-experimental_method
and	I-experimental_method
biochemical	I-experimental_method
studies	I-experimental_method
indicate	O
that	O
KBD	B-structure_element
of	O
MKP7	B-protein
may	O
interact	O
with	O
p38α	B-protein
in	O
a	O
similar	O
manner	O
to	O
that	O
of	O
MKP5	B-protein
.	O

In	O
contrast	O
to	O
MKP5	B-protein
,	O
removal	B-experimental_method
of	I-experimental_method
the	O
KBD	B-structure_element
domain	O
from	O
MKP7	B-protein
does	O
not	O
drastically	O
affect	O
enzyme	O
catalysis	O
,	O
and	O
the	O
kinetic	O
parameters	O
of	O
MKP7	B-protein
-	O
CD	B-structure_element
for	O
p38α	B-protein
substrate	O
are	O
very	O
similar	O
to	O
those	O
for	O
JNK1	B-protein
substrate	O
.	O

Taken	O
together	O
,	O
these	O
results	O
suggest	O
that	O
MKP7	B-protein
utilizes	O
a	O
bipartite	O
recognition	O
mechanism	O
to	O
achieve	O
the	O
efficiency	O
and	O
fidelity	O
of	O
p38α	B-protein
signalling	O
.	O

This	O
hydrophobic	B-site
site	I-site
was	O
first	O
identified	O
by	O
changes	B-evidence
in	I-evidence
deuterium	I-evidence
exchange	I-evidence
profiles	I-evidence
,	O
and	O
is	O
near	O
the	O
MAPK	B-structure_element
insertion	I-structure_element
and	O
helix	B-structure_element
αG	B-structure_element
.	O
Interestingly	O
,	O
many	O
of	O
the	O
equivalent	O
residues	O
in	O
JNK1	B-protein
,	O
important	O
for	O
MKP7	B-protein
-	O
CD	B-structure_element
recognition	O
,	O
are	O
also	O
used	O
for	O
substrate	O
binding	O
by	O
ERK2	B-protein
(	O
ref	O
.),	O
indicating	O
that	O
this	O
site	O
is	O
overlapped	O
with	O
the	O
DEF	B-site
-	I-site
site	I-site
previously	O
identified	O
in	O
ERK2	B-protein
(	O
Fig	O
.	O
5d	O
).	O

MKP3	B-protein
is	O
highly	O
specific	O
in	O
dephosphorylating	O
and	O
inactivating	O
ERK2	B-protein
,	O
and	O
the	O
phosphatase	O
activity	O
of	O
the	O
MKP3	B-protein
-	O
catalysed	O
pNPP	B-chemical
reaction	O
can	O
be	O
markedly	O
increased	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
ERK2	B-protein
(	O
refs	O
).	O

Therefore	O
,	O
it	O
is	O
tempting	O
to	O
speculate	O
that	O
the	O
catalytic	B-structure_element
domain	I-structure_element
of	O
MKP3	B-protein
may	O
bind	O
to	O
ERK2	B-protein
in	O
a	O
manner	O
analogous	O
to	O
the	O
way	O
by	O
which	O
MKP7	B-protein
-	O
CD	B-structure_element
binds	O
to	O
JNK1	B-protein
.	O

A	O
comprehensive	O
examination	O
of	O
the	O
molecular	O
basis	O
of	O
the	O
specific	O
ERK2	B-protein
recognition	O
by	O
MKP3	B-protein
is	O
underway	O
.	O

The	O
ongoing	O
work	O
demonstrates	O
that	O
although	O
the	O
overall	O
interaction	O
modes	O
are	O
similar	O
between	O
the	O
JNK1	B-complex_assembly
–	I-complex_assembly
MKP7	I-complex_assembly
-	I-complex_assembly
CD	I-complex_assembly
and	O
ERK2	B-complex_assembly
–	I-complex_assembly
MKP3	I-complex_assembly
-	I-complex_assembly
CD	I-complex_assembly
complexes	O
,	O
the	O
ERK2	B-complex_assembly
–	I-complex_assembly
MKP3	I-complex_assembly
-	I-complex_assembly
CD	I-complex_assembly
interaction	O
is	O
less	O
extensive	O
and	O
helix	B-structure_element
α4	B-structure_element
from	O
MKP3	B-protein
-	O
CD	B-structure_element
does	O
not	O
interact	O
directly	O
with	O
ERK2	B-protein
.	O

The	O
FXF	B-structure_element
-	I-structure_element
motif	I-structure_element
-	O
mediated	O
interaction	O
is	O
critical	O
for	O
both	O
pERK2	B-protein_state
inactivation	O
and	O
ERK2	B-protein
-	O
induced	O
MKP3	B-protein
activation	O
(	O
manuscript	O
in	O
preparation	O
).	O

This	O
structure	B-evidence
reveals	O
an	O
FXF	B-site
-	I-site
docking	I-site
interaction	I-site
mode	I-site
between	O
MAPK	B-protein_type
and	O
MKP	B-protein_type
.	O

Results	O
from	O
biochemical	B-experimental_method
characterization	I-experimental_method
of	O
the	O
Phe285	B-residue_name_number
and	O
Phe287	B-residue_name_number
MKP7	B-protein
mutants	B-protein_state
combined	O
with	O
structural	B-evidence
information	I-evidence
support	O
the	O
conclusion	O
that	O
the	O
two	O
Phe	B-residue_name
residues	O
serve	O
different	O
roles	O
in	O
the	O
catalytic	O
reaction	O
.	O

Phe285	B-residue_name_number
is	O
essential	O
for	O
JNK1	B-protein
substrate	O
binding	O
,	O
whereas	O
Phe287	B-residue_name_number
plays	O
a	O
role	O
for	O
the	O
precise	O
alignment	O
of	O
active	B-site
-	I-site
site	I-site
residues	I-site
,	O
which	O
are	O
important	O
for	O
transition	O
-	O
state	O
stabilization	O
.	O

This	O
newly	O
identified	O
FXF	B-structure_element
-	I-structure_element
type	I-structure_element
motif	I-structure_element
is	O
present	O
in	O
all	O
MKPs	B-protein_type
,	O
except	O
that	O
the	O
residue	O
at	O
the	O
first	O
position	O
in	O
MKP5	B-protein
is	O
an	O
equivalent	O
hydrophobic	O
leucine	B-residue_name
residue	O
(	O
see	O
also	O
Fig	O
.	O
7f	O
,	O
g	O
),	O
suggesting	O
that	O
these	O
two	O
Phe	B-residue_name
residues	O
would	O
play	O
a	O
similar	O
role	O
in	O
facilitating	O
substrate	O
recognition	O
and	O
catalysis	O
,	O
respectively	O
.	O

An	O
important	O
feature	O
of	O
MKP	B-protein_type
–	O
JNK1	B-protein
interactions	O
is	O
that	O
MKP7	B-protein
or	O
MKP5	B-protein
only	O
interact	O
with	O
the	O
F	B-site
-	I-site
site	I-site
of	O
JNK1	B-protein
.	O

One	O
possible	O
explanation	O
is	O
that	O
JNK1	B-protein
needs	O
to	O
use	O
the	O
D	B-site
-	I-site
site	I-site
to	O
interact	O
with	O
JIP	B-protein
-	I-protein
1	I-protein
,	O
a	O
scaffold	O
protein	O
for	O
JNK	B-protein_type
signalling	O
.	O

The	O
N	O
-	O
terminal	O
JNK	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
of	O
JIP	B-protein
-	I-protein
1	I-protein
interacts	O
with	O
the	O
D	B-site
-	I-site
site	I-site
on	O
JNK	B-protein_type
and	O
this	O
interaction	O
is	O
required	O
for	O
JIP	B-protein
-	I-protein
1	I-protein
-	O
mediated	O
enhancement	O
of	O
JNK	B-protein_type
activation	O
.	O

Domain	O
structures	B-evidence
of	O
ten	O
human	B-species
MKPs	B-protein_type
and	O
the	O
atypical	O
VHR	B-protein
.	O

On	O
the	O
basis	O
of	O
sequence	O
similarity	O
,	O
protein	O
structure	B-evidence
,	O
substrate	O
specificity	O
and	O
subcellular	O
localization	O
,	O
the	O
ten	O
members	O
of	O
MKP	B-protein_type
family	I-protein_type
can	O
be	O
divided	O
into	O
three	O
groups	O
.	O

The	O
first	O
subfamily	O
comprises	O
MKP1	B-protein
,	O
MKP2	B-protein
,	O
PAC1	B-protein
and	O
hVH3	B-protein
,	O
which	O
are	O
inducible	B-protein_state
nuclear	B-protein_type
phosphatases	I-protein_type
and	O
can	O
dephosphorylate	O
ERK	B-protein_type
(	O
and	O
JNK	B-protein_type
,	O
p38	B-protein_type
)	O
MAPKs	B-protein_type
.	O

The	O
second	O
subfamily	O
contains	O
MKP3	B-protein
,	O
MKP4	B-protein
and	O
MKPX	B-protein
,	O
which	O
are	O
cytoplasmic	O
ERK	B-protein_type
-	I-protein_type
specific	I-protein_type
MKPs	I-protein_type
.	O

The	O
third	O
subfamily	O
comprises	O
MKP5	B-protein
,	O
MKP7	B-protein
and	O
hVH5	B-protein
,	O
which	O
were	O
located	O
in	O
both	O
nucleus	O
and	O
cytoplasm	O
,	O
and	O
selectively	O
inactivate	O
JNK	B-protein_type
and	O
p38	B-protein_type
.	O

All	O
MKPs	B-protein_type
contain	O
both	O
the	O
CD	B-structure_element
and	O
KBD	B-structure_element
domains	O
,	O
whereas	O
VHR	B-protein
,	O
an	O
atypical	O
MKP	B-protein_type
,	O
only	O
contains	O
a	O
highly	B-protein_state
conserved	I-protein_state
catalytic	B-structure_element
domain	I-structure_element
.	O

In	O
addition	O
to	O
the	O
CD	B-structure_element
and	O
KBD	B-structure_element
,	O
MKP7	B-protein
contains	O
a	O
unique	O
long	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
region	I-structure_element
that	O
contains	O
NES	B-structure_element
,	O
NLS	B-structure_element
and	O
PEST	B-structure_element
motifs	I-structure_element
,	O
which	O
has	O
no	O
effect	O
on	O
the	O
binding	O
ability	O
and	O
phosphatase	O
activity	O
of	O
MKP7	B-protein
toward	O
MAPKs	B-protein_type
.	O

NES	B-structure_element
,	O
nuclear	B-structure_element
export	I-structure_element
signal	I-structure_element
;	O
NLS	B-structure_element
,	O
nuclear	B-structure_element
localization	I-structure_element
signal	I-structure_element
;	O
PEST	B-structure_element
,	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
sequence	I-structure_element
rich	I-structure_element
in	O
prolines	B-residue_name
,	O
glutamates	B-residue_name
,	O
serines	B-residue_name
and	O
threonines	B-residue_name
.	O

MKP7	B-protein
-	O
CD	B-structure_element
is	O
crucial	O
for	O
JNK1	B-protein
binding	O
and	O
enzyme	O
catalysis	O
.	O

(	O
a	O
)	O
Domain	O
organization	O
of	O
human	B-species
MKP7	B-protein
and	O
JNK1	B-protein
.	O

The	O
KBD	B-structure_element
and	O
CD	B-structure_element
of	O
MKP7	B-protein
are	O
shown	O
in	O
green	O
and	O
blue	O
,	O
and	O
the	O
N	B-structure_element
-	I-structure_element
lobe	I-structure_element
and	O
C	B-structure_element
-	I-structure_element
lobe	I-structure_element
of	O
JNK1	B-protein
are	O
coloured	O
in	O
lemon	O
and	O
yellow	O
,	O
respectively	O
.	O

The	O
error	O
bars	O
represent	O
s	O
.	O
e	O
.	O
m	O
.	O
(	O
c	O
)	O
Gel	B-experimental_method
filtration	I-experimental_method
analysis	I-experimental_method
for	O
interaction	O
of	O
JNK1	B-protein
with	O
MKP7	B-protein
-	O
CD	B-structure_element
and	O
MKP7	B-protein
-	O
KBD	B-structure_element
.	O

The	O
top	O
panel	O
shows	O
the	O
relative	O
affinities	B-evidence
of	O
MKP7	B-protein
-	O
CD	B-structure_element
and	O
MKP7	B-protein
-	O
KBD	B-structure_element
to	O
JNK1	B-protein
,	O
with	O
the	O
affinity	B-evidence
of	O
MKP7	B-protein
-	O
CD	B-structure_element
defined	O
as	O
100	O
%;	O
the	O
middle	O
panel	O
is	O
the	O
electrophoretic	O
pattern	O
of	O
MKP7	B-protein
and	O
JNK1	B-protein
after	O
GST	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assays	I-experimental_method
.	O

The	O
protein	O
amounts	O
of	O
MKP7	B-protein
used	O
are	O
shown	O
at	O
the	O
bottom	O
.	O

The	O
JNK1	B-protein
is	O
shown	O
in	O
surface	O
representation	O
coloured	O
according	O
to	O
electrostatic	O
potential	O
(	O
positive	O
,	O
blue	O
;	O
negative	O
,	O
red	O
).	O

(	O
e	O
)	O
Interaction	B-site
networks	I-site
mainly	O
involving	O
helices	B-structure_element
α4	B-structure_element
and	O
α5	B-structure_element
from	O
MKP7	B-protein
-	O
CD	B-structure_element
,	O
and	O
αG	B-structure_element
and	O
α2L14	B-structure_element
of	O
JNK1	B-protein
.	O

Blue	O
dashed	O
lines	O
represent	O
polar	B-bond_interaction
interactions	I-bond_interaction
.	O

(	O
f	O
)	O
The	O
2Fo	B-evidence
−	I-evidence
Fc	I-evidence
omit	I-evidence
map	I-evidence
(	O
contoured	O
at	O
1	O
.	O
5σ	O
)	O
clearly	O
shows	O
electron	B-evidence
density	I-evidence
for	O
the	O
285FNFL288	B-structure_element
segment	I-structure_element
of	O
MKP7	B-protein
-	O
CD	B-structure_element
.	O

Mutational	B-experimental_method
analysis	I-experimental_method
on	O
interactions	O
between	O
MKP7	B-protein
-	O
CD	B-structure_element
and	O
JNK1	B-protein
.	O

Residues	O
involved	O
in	O
hydrophobic	B-bond_interaction
and	I-bond_interaction
hydrophilic	I-bond_interaction
contacts	I-bond_interaction
are	O
coloured	O
in	O
red	O
and	O
blue	O
,	O
respectively	O
.	O
(	O
b	O
)	O
Gel	B-experimental_method
filtration	I-experimental_method
analysis	I-experimental_method
for	O
interaction	O
of	O
JNK1	B-protein
with	O
MKP7	B-protein
-	O
CD	B-structure_element
mutant	B-protein_state
F285D	B-mutant
.	O

(	O
c	O
)	O
Pull	B-experimental_method
-	I-experimental_method
down	I-experimental_method
assays	I-experimental_method
of	O
MKP7	B-protein
-	O
CD	B-structure_element
by	O
GST	B-protein_state
-	I-protein_state
tagged	I-protein_state
JNK1	B-protein
mutants	B-protein_state
.	O

The	O
top	O
panel	O
shows	O
the	O
relative	O
affinities	B-evidence
of	O
MKP7	B-protein
-	O
CD	B-structure_element
to	O
JNK1	B-protein
mutants	B-protein_state
,	O
with	O
the	O
affinity	B-evidence
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
JNK1	B-protein
defined	O
as	O
100	O
%,	O
the	O
middle	O
panel	O
is	O
the	O
electrophoretic	O
pattern	O
of	O
MKP7	B-protein
-	O
CD	B-structure_element
and	O
JNK1	B-protein
mutants	B-protein_state
after	O
GST	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assays	I-experimental_method
.	O

The	O
protein	O
amounts	O
of	O
MKP7	B-protein
-	O
CD	B-structure_element
used	O
are	O
shown	O
at	O
the	O
bottom	O
.	O
(	O
d	O
)	O
Circular	B-experimental_method
dichroism	I-experimental_method
spectra	B-evidence
for	O
MKP7	B-protein
-	O
CD	B-structure_element
wild	B-protein_state
type	I-protein_state
and	O
mutants	B-protein_state
.	O

Measurements	O
were	O
averaged	O
for	O
three	O
scans	O
.	O
(	O
e	O
)	O
Circular	B-experimental_method
dichroism	I-experimental_method
spectra	B-evidence
for	O
JNK1	B-protein
wild	B-protein_state
type	I-protein_state
and	O
mutants	B-protein_state
.	O

Comparison	O
of	O
CDK2	B-complex_assembly
-	I-complex_assembly
KAP	I-complex_assembly
and	O
JNK1	B-complex_assembly
–	I-complex_assembly
MKP7	I-complex_assembly
-	I-complex_assembly
CD	I-complex_assembly
.	O

(	O
a	O
)	O
Superposition	B-experimental_method
of	O
the	O
complex	O
structures	B-evidence
of	O
CDK2	B-complex_assembly
-	I-complex_assembly
KAP	I-complex_assembly
(	O
PDB	O
1FQ1	O
)	O
and	O
JNK1	B-complex_assembly
–	I-complex_assembly
MKP7	I-complex_assembly
-	I-complex_assembly
CD	I-complex_assembly
.	O

The	O
N	B-structure_element
-	I-structure_element
lobe	I-structure_element
and	O
C	B-structure_element
-	I-structure_element
lobe	I-structure_element
of	O
CDK2	B-protein
are	O
coloured	O
in	O
grey	O
and	O
pink	O
,	O
respectively	O
,	O
and	O
KAP	B-protein
is	O
coloured	O
in	O
green	O
.	O

The	O
interactions	O
between	O
these	O
two	O
proteins	O
consist	O
of	O
three	O
discontinuous	O
contact	B-site
regions	I-site
,	O
centred	O
at	O
the	O
multiple	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
between	O
the	O
pThr160	B-ptm
of	O
CDK2	B-protein
and	O
the	O
active	B-site
site	I-site
of	O
KAP	B-protein
(	O
region	B-structure_element
I	I-structure_element
).	O

Interestingly	O
,	O
the	O
recognition	O
of	O
CDK2	B-protein
by	O
KAP	B-protein
is	O
augmented	O
by	O
a	O
similar	O
interface	B-site
as	O
that	O
observed	O
in	O
the	O
complex	O
of	O
JNK1	B-protein
and	O
MKP7	B-protein
-	O
CD	B-structure_element
(	O
region	B-structure_element
II	I-structure_element
).	O

(	O
b	O
)	O
Interactions	O
networks	O
at	O
the	O
auxiliary	B-structure_element
region	I-structure_element
II	I-structure_element
mainly	O
involving	O
helix	B-structure_element
α7	B-structure_element
from	O
KAP	B-protein
and	O
the	O
αG	B-structure_element
helix	I-structure_element
and	O
following	O
L14	B-structure_element
loop	I-structure_element
of	O
CDK2	B-protein
.	O

The	O
CDK2	B-protein
is	O
shown	O
in	O
surface	O
representation	O
coloured	O
according	O
to	O
the	O
electrostatic	O
potential	O
(	O
positive	O
,	O
blue	O
;	O
negative	O
,	O
red	O
).	O

Residues	O
of	O
KAP	B-protein
and	O
CDK2	B-protein
are	O
highlighted	O
as	O
green	O
and	O
red	O
sticks	O
,	O
respectively	O
.	O

Residues	O
of	O
MKP7	B-protein
-	O
CD	B-structure_element
involved	O
in	O
JNK1	B-protein
recognition	O
are	O
indicated	O
by	O
cyan	O
asterisks	O
,	O
and	O
the	O
conserved	B-protein_state
FXF	B-structure_element
-	I-structure_element
motif	I-structure_element
is	O
highlighted	O
in	O
cyan	O
.	O

The	O
secondary	O
structure	O
assignments	O
of	O
MKP7	B-protein
-	O
CD	B-structure_element
and	O
KAP	B-protein
are	O
shown	O
above	O
and	O
below	O
each	O
sequence	O
.	O

Residues	O
of	O
JNK1	B-protein
involved	O
in	O
recognition	O
of	O
MKP7	B-protein
are	O
indicated	O
by	O
orange	O
asterisks	O
,	O
and	O
those	O
forming	O
the	O
F	B-site
-	I-site
site	I-site
are	O
highlighted	O
in	O
yellow	O
.	O

(	O
a	O
–	O
c	O
)	O
FXF	B-structure_element
-	I-structure_element
motif	I-structure_element
is	O
essential	O
for	O
the	O
dephosphorylation	O
of	O
JNK	B-protein_type
by	O
MKP7	B-protein
.	O

Shown	O
is	O
a	O
typical	O
immunoblot	O
for	O
phosphorylated	B-protein_state
JNK	B-protein_type
from	O
three	O
independent	O
experiments	O
.	O

(	O
d	O
)	O
F	B-site
-	I-site
site	I-site
is	O
required	O
for	O
JNK1	B-protein
to	O
interact	O
with	O
MKP7	B-protein
.	O

Whole	O
-	O
cell	O
extracts	O
were	O
then	O
immunoprecipitated	B-experimental_method
with	O
antibody	O
against	O
Myc	O
for	O
MKP7	B-protein
;	O
immunobloting	O
was	O
carried	O
out	O
with	O
antibodies	O
indicated	O
.	O

IP	B-experimental_method
,	O
immunoprecipitation	B-experimental_method
;	O
TCL	O
,	O
total	O
cell	O
lysate	O
.	O

(	O
e	O
)	O
Effect	O
of	O
MKP7	B-protein
(	O
wild	B-protein_state
type	I-protein_state
or	O
mutants	B-protein_state
)	O
expression	O
on	O
ultraviolet	O
-	O
induced	O
apoptosis	O
.	O

HeLa	O
cells	O
were	O
infected	O
with	O
lentiviruses	B-taxonomy_domain
expressing	O
MKP7	B-protein
full	B-protein_state
-	I-protein_state
length	I-protein_state
and	O
its	O
mutants	B-protein_state
.	O

Cells	O
were	O
then	O
subjected	O
to	O
flow	B-experimental_method
cytometry	I-experimental_method
analysis	O
.	O

The	O
results	O
using	O
Annexin	B-chemical
-	I-chemical
V	I-chemical
stain	O
for	O
membrane	O
phosphatidylserine	O
eversion	O
,	O
combined	O
with	O
propidium	B-chemical
iodide	I-chemical
(	O
PI	B-chemical
)	O
uptake	O
to	O
evaluate	O
cells	O
whose	O
membranes	O
had	O
been	O
compromised	O
.	O

Staining	O
with	O
both	O
Annexin	B-chemical
-	I-chemical
V	I-chemical
and	O
PI	B-chemical
indicate	O
apoptosis	O
(	O
upper	O
right	O
quadrant	O
).	O

(	O
f	O
)	O
Statistical	O
analysis	O
of	O
apoptotic	O
cells	O
(	O
mean	O
±	O
s	O
.	O
e	O
.	O
m	O
.,	O
n	O
=	O
3	O
),	O
*	B-evidence
P	I-evidence
<	O
0	O
.	O
05	O
,	O
***	B-evidence
P	I-evidence
<	O
0	O
.	O
001	O
(	O
ANOVA	B-experimental_method
followed	O
by	O
Tukey	B-experimental_method
'	I-experimental_method
s	I-experimental_method
test	I-experimental_method
).	O

(	O
a	O
)	O
Domain	O
organization	O
of	O
human	B-species
MKP5	B-protein
.	O

The	O
KBD	B-structure_element
and	O
CD	B-structure_element
of	O
MKP5	B-protein
are	O
shown	O
in	O
brown	O
and	O
grey	O
,	O
respectively	O
.	O
(	O
b	O
)	O
Plots	B-evidence
of	I-evidence
initial	I-evidence
velocity	I-evidence
of	O
the	O
MKP5	B-protein
-	O
catalysed	O
reaction	O
versus	O
phospho	B-protein_state
-	O
JNK1	B-protein
concentration	O
.	O

The	O
error	O
bars	O
represent	O
s	O
.	O
e	O
.	O
m	O
.	O
(	O
c	O
)	O
Structural	B-experimental_method
comparison	I-experimental_method
of	O
the	O
JNK	B-site
-	I-site
interacting	I-site
residues	I-site
on	O
MKP5	B-protein
-	O
CD	B-structure_element
(	O
PDB	O
1ZZW	O
)	O
and	O
MKP7	B-protein
-	O
CD	B-structure_element
.	O

The	O
corresponding	O
residues	O
on	O
MKP5	B-protein
are	O
depicted	O
as	O
orange	O
sticks	O
,	O
and	O
MKP5	B-protein
residues	O
numbers	O
are	O
in	O
parentheses	O
.	O

(	O
d	O
)	O
Gel	B-experimental_method
filtration	I-experimental_method
analysis	I-experimental_method
for	O
interaction	O
of	O
JNK1	B-protein
with	O
MKP5	B-protein
-	O
CD	B-structure_element
and	O
MKP5	B-protein
-	O
KBD	B-structure_element
.	O
(	O
e	O
)	O
GST	B-experimental_method
-	I-experimental_method
mediated	I-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assays	I-experimental_method
for	O
interaction	O
of	O
JNK1	B-protein
with	O
MKP5	B-protein
-	O
CD	B-structure_element
and	O
MKP5	B-protein
-	O
KBD	B-structure_element
.	O

The	O
panels	O
are	O
arranged	O
the	O
same	O
as	O
in	O
Fig	O
.	O
2d	O
.	O
(	O
f	O
)	O
Effects	O
of	O
mutations	B-experimental_method
in	O
MKP5	B-protein
-	O
CD	B-structure_element
on	O
the	O
JNK1	B-protein
dephosphorylation	O
(	O
mean	O
±	O
s	O
.	O
e	O
.	O
m	O
.,	O
n	O
=	O
3	O
).	O

(	O
g	O
)	O
Effects	O
of	O
mutations	B-experimental_method
in	O
MKP5	B-protein
-	O
CD	B-structure_element
on	O
the	O
pNPP	B-chemical
hydrolysis	O
reaction	O
(	O
mean	O
±	O
s	O
.	O
e	O
.	O
m	O
.,	O
n	O
=	O
3	O
).	O

Mechanistic	O
insight	O
into	O
a	O
peptide	B-protein_type
hormone	I-protein_type
signaling	O
complex	O
mediating	O
floral	O
organ	O
abscission	O

Plants	B-taxonomy_domain
constantly	O
renew	O
during	O
their	O
life	O
cycle	O
and	O
thus	O
require	O
to	O
shed	O
senescent	O
and	O
damaged	O
organs	O
.	O

It	O
is	O
unknown	O
how	O
expression	O
of	O
IDA	B-protein
in	O
the	O
abscission	O
zone	O
leads	O
to	O
HAESA	B-protein
activation	O
.	O

Here	O
we	O
show	O
that	O
IDA	B-protein
is	O
sensed	O
directly	O
by	O
the	O
HAESA	B-protein
ectodomain	B-structure_element
.	O

A	O
central	O
hydroxyproline	B-residue_name
residue	O
anchors	O
IDA	B-protein
to	O
the	O
receptor	O
.	O

Plants	B-taxonomy_domain
can	O
shed	O
their	O
leaves	O
,	O
flowers	O
or	O
other	O
organs	O
when	O
they	O
no	O
longer	O
need	O
them	O
.	O
But	O
how	O
does	O
a	O
leaf	O
or	O
a	O
flower	O
know	O
when	O
to	O
let	O
go	O
?	O
A	O
receptor	B-protein_type
protein	I-protein_type
called	O
HAESA	B-protein
is	O
found	O
on	O
the	O
surface	O
of	O
the	O
cells	O
that	O
surround	O
a	O
future	O
break	O
point	O
on	O
the	O
plant	O
.	O
When	O
its	O
time	O
to	O
shed	O
an	O
organ	O
,	O
a	O
hormone	B-chemical
called	O
IDA	B-protein
instructs	O
HAESA	B-protein
to	O
trigger	O
the	O
shedding	O
process	O
.	O

However	O
,	O
the	O
molecular	O
details	O
of	O
how	O
IDA	B-protein
triggers	O
organ	O
shedding	O
are	O
not	O
clear	O
.	O

The	O
shedding	O
of	O
floral	O
organs	O
(	O
or	O
leaves	O
)	O
can	O
be	O
easily	O
studied	O
in	O
a	O
model	O
plant	B-taxonomy_domain
called	O
Arabidopsis	B-taxonomy_domain
.	O

Santiago	O
et	O
al	O
.	O
used	O
protein	B-experimental_method
biochemistry	I-experimental_method
,	O
structural	B-experimental_method
biology	I-experimental_method
and	O
genetics	B-experimental_method
to	O
uncover	O
how	O
the	O
IDA	B-protein
hormone	B-chemical
activates	O
HAESA	B-protein
.	O

The	O
experiments	O
show	O
that	O
IDA	B-protein
binds	B-protein_state
directly	I-protein_state
to	I-protein_state
a	O
canyon	B-protein_state
shaped	I-protein_state
pocket	B-site
in	O
HAESA	B-protein
that	O
extends	O
out	O
from	O
the	O
surface	O
of	O
the	O
cell	O
.	O

IDA	B-protein
binding	O
to	O
HAESA	B-protein
allows	O
another	O
receptor	B-protein_type
protein	I-protein_type
called	O
SERK1	B-protein
to	B-protein_state
bind	I-protein_state
to	I-protein_state
HAESA	B-protein
,	O
which	O
results	O
in	O
the	O
release	O
of	O
signals	O
inside	O
the	O
cell	O
that	O
trigger	O
the	O
shedding	O
of	O
organs	O
.	O

The	O
HAESA	B-protein
ectodomain	B-structure_element
folds	O
into	O
a	O
superhelical	B-structure_element
assembly	I-structure_element
of	O
21	O
leucine	B-structure_element
-	I-structure_element
rich	I-structure_element
repeats	I-structure_element
.	O

The	O
calculated	O
molecular	O
mass	O
is	O
65	O
.	O
7	O
kDa	O
,	O
the	O
actual	O
molecular	O
mass	O
obtained	O
by	O
mass	B-experimental_method
spectrometry	I-experimental_method
is	O
74	O
,	O
896	O
Da	O
,	O
accounting	O
for	O
the	O
N	B-chemical
-	I-chemical
glycans	I-chemical
.	O
(	O
B	O
)	O
Ribbon	O
diagrams	O
showing	O
front	O
(	O
left	O
panel	O
)	O
and	O
side	O
views	O
(	O
right	O
panel	O
)	O
of	O
the	O
isolated	O
HAESA	B-protein
LRR	B-structure_element
domain	I-structure_element
.	O

The	O
N	O
-	O
(	O
residues	O
20	B-residue_range
–	I-residue_range
88	I-residue_range
)	O
and	O
C	O
-	O
terminal	O
(	O
residues	O
593	B-residue_range
–	I-residue_range
615	I-residue_range
)	O
capping	B-structure_element
domains	I-structure_element
are	O
shown	O
in	O
yellow	O
,	O
the	O
central	O
21	O
LRR	B-structure_element
motifs	I-structure_element
are	O
in	O
blue	O
and	O
disulphide	B-ptm
bonds	I-ptm
are	O
highlighted	O
in	O
green	O
(	O
in	O
bonds	O
representation	O
).	O
(	O
C	O
)	O
Structure	B-experimental_method
based	I-experimental_method
sequence	I-experimental_method
alignment	I-experimental_method
of	O
the	O
21	O
leucine	B-structure_element
-	I-structure_element
rich	I-structure_element
repeats	I-structure_element
in	O
HAESA	B-protein
with	O
the	O
plant	B-taxonomy_domain
LRR	B-structure_element
consensus	O
sequence	O
shown	O
for	O
comparison	O
.	O

Conserved	B-protein_state
hydrophobic	B-protein_state
residues	B-structure_element
are	O
shaded	O
in	O
gray	O
,	O
N	B-site
-	I-site
glycosylation	I-site
sites	I-site
visible	O
in	O
our	O
structures	B-evidence
are	O
highlighted	O
in	O
blue	O
,	O
cysteine	B-residue_name
residues	O
involved	O
in	O
disulphide	B-ptm
bridge	I-ptm
formation	O
in	O
green	O
.	O
(	O
D	O
)	O
Asn	B-ptm
-	I-ptm
linked	I-ptm
glycans	I-ptm
mask	O
the	O
N	O
-	O
terminal	O
portion	O
of	O
the	O
HAESA	B-protein
ectodomain	B-structure_element
.	O

Oligomannose	B-chemical
core	O
structures	O
(	O
containing	O
two	O
N	B-chemical
-	I-chemical
actylglucosamines	I-chemical
and	O
three	O
terminal	O
mannose	B-chemical
units	O
)	O
as	O
found	O
in	O
Trichoplusia	B-species
ni	I-species
cells	O
and	O
in	O
plants	B-taxonomy_domain
were	O
modeled	O
onto	O
the	O
seven	O
glycosylation	B-site
sites	I-site
observed	O
in	O
our	O
HAESA	B-protein
structures	B-evidence
,	O
to	O
visualize	O
the	O
surface	O
areas	O
potentially	O
not	O
masked	O
by	O
carbohydrate	B-chemical
.	O

The	O
HAESA	B-protein
ectodomain	B-structure_element
is	O
shown	O
in	O
blue	O
(	O
in	O
surface	O
representation	O
),	O
the	O
glycan	B-chemical
structures	O
are	O
shown	O
in	O
yellow	O
.	O

Hydrophobic	B-bond_interaction
contacts	I-bond_interaction
and	O
a	O
hydrogen	B-site
-	I-site
bond	I-site
network	I-site
mediate	O
the	O
interaction	O
between	O
HAESA	B-protein
and	O
the	O
peptide	B-protein_type
hormone	I-protein_type
IDA	B-protein
.	O

(	O
A	O
)	O
Details	O
of	O
the	O
IDA	B-site
binding	I-site
pocket	I-site
.	O

HAESA	B-protein
is	O
shown	O
in	O
blue	O
(	O
ribbon	O
diagram	O
),	O
the	O
C	O
-	O
terminal	O
Arg	B-structure_element
-	I-structure_element
His	I-structure_element
-	I-structure_element
Asn	I-structure_element
motif	I-structure_element
(	O
left	O
panel	O
),	O
the	O
central	O
Hyp	B-structure_element
anchor	I-structure_element
(	O
center	O
)	O
and	O
the	O
N	O
-	O
terminal	O
Pro	B-structure_element
-	I-structure_element
rich	I-structure_element
motif	I-structure_element
in	O
IDA	B-protein
(	O
right	O
panel	O
)	O
are	O
shown	O
in	O
yellow	O
(	O
in	O
bonds	O
representation	O
).	O

HAESA	B-site
interface	I-site
residues	I-site
are	O
shown	O
as	O
sticks	O
,	O
selected	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
interactions	I-bond_interaction
are	O
denoted	O
as	O
dotted	O
lines	O
(	O
in	O
magenta	O
).	O
(	O
B	O
)	O
View	O
of	O
the	O
complete	O
IDA	B-protein
(	O
in	O
bonds	O
representation	O
,	O
in	O
yellow	O
)	O
binding	B-site
pocket	I-site
in	O
HAESA	B-protein
(	O
surface	O
view	O
,	O
in	O
blue	O
).	O

Orientation	O
as	O
in	O
(	O
A	O
).	O
(	O
C	O
)	O
Structure	B-experimental_method
based	I-experimental_method
sequence	I-experimental_method
alignment	I-experimental_method
of	O
leucine	B-structure_element
-	I-structure_element
rich	I-structure_element
repeats	I-structure_element
in	O
HAESA	B-protein
with	O
the	O
plant	B-taxonomy_domain
LRR	B-structure_element
consensus	B-evidence
sequence	I-evidence
shown	O
for	O
comparison	O
.	O

The	O
IDA	B-complex_assembly
-	I-complex_assembly
HAESA	I-complex_assembly
and	O
SERK1	B-complex_assembly
-	I-complex_assembly
HAESA	I-complex_assembly
complex	O
interfaces	B-site
are	O
conserved	B-protein_state
among	O
HAESA	B-protein
and	O
HAESA	B-protein_type
-	I-protein_type
like	I-protein_type
proteins	I-protein_type
from	O
different	O
plant	B-taxonomy_domain
species	O
.	O

Structure	B-experimental_method
-	I-experimental_method
based	I-experimental_method
sequence	I-experimental_method
alignment	I-experimental_method
of	O
the	O
HAESA	B-protein_type
family	I-protein_type
members	I-protein_type
:	O
Arabidopsis	B-species
thaliana	I-species
HAESA	B-protein
(	O
Uniprot	O
(	O
http	O
://	O
www	O
.	O
uniprot	O
.	O
org	O
)	O
ID	O
P47735	O
),	O
Arabidopsis	B-species
thaliana	I-species
HSL2	B-protein
(	O
Uniprot	O
ID	O
C0LGX3	O
),	O
Capsella	B-species
rubella	I-species
HAESA	B-protein
(	O
Uniprot	O
ID	O
R0F2U6	O
),	O
Citrus	B-species
clementina	I-species
HSL2	B-protein
(	O
Uniprot	O
ID	O
V4U227	O
),	O
Vitis	B-species
vinifera	I-species
HAESA	B-protein
(	O
Uniprot	O
ID	O
F6HM39	O
).	O

The	O
peptide	B-protein_type
hormone	I-protein_type
IDA	B-protein
binds	O
to	O
the	O
HAESA	B-protein
LRR	B-structure_element
ectodomain	I-structure_element
.	O

(	O
A	O
)	O
Multiple	B-experimental_method
sequence	I-experimental_method
alignment	I-experimental_method
of	O
selected	O
IDA	B-protein_type
family	I-protein_type
members	I-protein_type
.	O

Details	O
of	O
the	O
interactions	O
between	O
the	O
central	O
Hyp	B-structure_element
anchor	I-structure_element
in	O
IDA	B-protein
and	O
the	O
C	O
-	O
terminal	O
Arg	B-structure_element
-	I-structure_element
His	I-structure_element
-	I-structure_element
Asn	I-structure_element
motif	I-structure_element
with	O
HAESA	B-protein
are	O
highlighted	O
in	O
(	O
E	O
)	O
and	O
(	O
F	O
),	O
respectively	O
.	O

Hydrogren	O
bonds	O
are	O
depicted	O
as	O
dotted	O
lines	O
(	O
in	O
magenta	O
),	O
a	O
water	B-chemical
molecule	O
is	O
shown	O
as	O
a	O
red	O
sphere	O
.	O

During	O
their	O
growth	O
,	O
development	O
and	O
reproduction	O
plants	B-taxonomy_domain
use	O
cell	O
separation	O
processes	O
to	O
detach	O
no	O
-	O
longer	O
required	O
,	O
damaged	O
or	O
senescent	O
organs	O
.	O

Abscission	O
of	O
floral	O
organs	O
in	O
Arabidopsis	B-taxonomy_domain
is	O
a	O
model	O
system	O
to	O
study	O
these	O
cell	O
separation	O
processes	O
in	O
molecular	O
detail	O
.	O

The	O
LRR	B-structure_element
-	I-structure_element
RKs	I-structure_element
HAESA	B-protein
(	O
greek	O
:	O
to	O
adhere	O
to	O
)	O
and	O
HAESA	B-protein
-	I-protein
LIKE	I-protein
2	I-protein
(	O
HSL2	B-protein
)	O
redundantly	O
control	O
floral	O
abscission	O
.	O

Loss	O
-	O
of	O
-	O
function	O
of	O
the	O
secreted	O
small	O
protein	O
INFLORESCENCE	B-protein
DEFICIENT	I-protein
IN	I-protein
ABSCISSION	I-protein
(	O
IDA	B-protein
)	O
causes	O
floral	O
organs	O
to	O
remain	O
attached	O
while	O
its	O
over	O
-	O
expression	O
leads	O
to	O
premature	O
shedding	O
.	O

It	O
has	O
been	O
demonstrated	O
that	O
a	O
dodecamer	B-structure_element
peptide	B-chemical
within	O
EPIP	B-structure_element
is	O
able	O
to	O
activate	O
HAESA	B-protein
and	O
HSL2	B-protein
in	O
transient	B-experimental_method
assays	I-experimental_method
in	O
tobacco	B-taxonomy_domain
cells	O
.	O

This	B-structure_element
sequence	I-structure_element
motif	I-structure_element
is	O
highly	B-protein_state
conserved	I-protein_state
among	O
IDA	B-protein_type
family	I-protein_type
members	I-protein_type
(	O
IDA	B-protein_type
-	I-protein_type
LIKE	I-protein_type
PROTEINS	I-protein_type
,	O
IDLs	B-protein_type
)	O
and	O
contains	O
a	O
central	O
Pro	B-residue_name
residue	O
,	O
presumed	O
to	O
be	O
post	B-protein_state
-	I-protein_state
translationally	I-protein_state
modified	I-protein_state
to	O
hydroxyproline	B-residue_name
(	O
Hyp	B-residue_name
;	O
Figure	O
1A	O
).	O

The	O
available	O
genetic	O
and	O
biochemical	O
evidence	O
suggests	O
that	O
IDA	B-protein
and	O
HAESA	B-protein
together	O
control	O
floral	O
abscission	O
,	O
but	O
it	O
is	O
poorly	O
understood	O
if	O
IDA	B-protein
is	O
directly	O
sensed	O
by	O
the	O
receptor	B-protein_type
kinase	I-protein_type
HAESA	B-protein
and	O
how	O
IDA	B-protein
binding	O
at	O
the	O
cell	O
surface	O
would	O
activate	O
the	O
receptor	O
.	O

IDA	B-protein
directly	O
binds	O
to	O
the	O
LRR	B-structure_element
domain	I-structure_element
of	O
HAESA	B-protein

Active	B-protein_state
IDA	B-protein_type
-	I-protein_type
family	I-protein_type
peptide	I-protein_type
hormones	I-protein_type
are	O
hydroxyprolinated	B-protein_state
dodecamers	B-structure_element
.	O

Note	O
that	O
Pro58IDA	B-residue_name_number
and	O
Leu67IDA	B-residue_name_number
are	O
the	O
first	O
residues	O
defined	O
by	O
electron	B-evidence
density	I-evidence
when	O
bound	B-protein_state
to	I-protein_state
the	O
HAESA	B-protein
ectodomain	B-structure_element
.	O
(	O
D	O
)	O
Table	O
summaries	O
for	O
equilibrium	B-evidence
dissociation	I-evidence
constants	I-evidence
(	O
Kd	B-evidence
),	O
binding	B-evidence
enthalpies	I-evidence
(	O
ΔH	B-evidence
),	O
binding	B-evidence
entropies	I-evidence
(	O
ΔS	B-evidence
)	O
and	O
stoichoimetries	O
(	O
N	O
)	O
for	O
different	O
IDA	B-chemical
peptides	I-chemical
binding	O
to	O
the	O
HAESA	B-protein
ectodomain	B-structure_element
(	O
±	O
fitting	O
errors	O
;	O
n	O
.	O
d	O
.	O

Root	B-evidence
mean	I-evidence
square	I-evidence
deviation	I-evidence
(	O
r	B-evidence
.	I-evidence
m	I-evidence
.	I-evidence
s	I-evidence
.	I-evidence
d	I-evidence
.)	I-evidence
is	O
1	O
.	O
0	O
Å	O
comparing	O
100	O
corresponding	O
atoms	O
.	O

The	O
receptor	B-protein_type
kinase	I-protein_type
SERK1	B-protein
acts	O
as	O
a	O
HAESA	B-protein_type
co	I-protein_type
-	I-protein_type
receptor	I-protein_type
and	O
promotes	O
high	O
-	O
affinity	O
IDA	B-protein
sensing	O
.	O

(	O
A	O
)	O
Petal	B-experimental_method
break	I-experimental_method
-	I-experimental_method
strength	I-experimental_method
assays	I-experimental_method
measure	O
the	O
force	O
(	O
expressed	O
in	O
gram	O
equivalents	O
)	O
required	O
to	O
remove	O
the	O
petals	O
from	O
the	O
flower	O
of	O
serk	B-gene
mutant	B-protein_state
plants	B-taxonomy_domain
compared	O
to	O
haesa	B-gene
/	O
hsl2	B-gene
mutant	B-protein_state
and	O
Col	O
-	O
0	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
flowers	O
.	O

Petal	O
break	O
-	O
strength	O
was	O
found	O
significantly	O
increased	O
in	O
almost	O
all	O
positions	O
(	O
indicated	O
with	O
a	O
*)	O
for	O
haesa	B-gene
/	O
hsl2	B-gene
and	O
serk1	B-gene
-	I-gene
1	I-gene
mutant	B-protein_state
plants	B-taxonomy_domain
with	O
respect	O
to	O
the	O
Col	O
-	O
0	O
control	O
.	O

(	O
B	O
)	O
Analytical	B-experimental_method
size	I-experimental_method
-	I-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
.	O

The	O
HAESA	B-protein
LRR	B-structure_element
domain	I-structure_element
elutes	O
as	O
a	O
monomer	B-oligomeric_state
(	O
black	O
dotted	O
line	O
),	O
as	O
does	O
the	O
isolated	O
SERK1	B-protein
ectodomain	B-structure_element
(	O
blue	O
dotted	O
line	O
).	O

A	O
HAESA	B-complex_assembly
–	I-complex_assembly
IDA	I-complex_assembly
–	I-complex_assembly
SERK1	I-complex_assembly
complex	O
elutes	O
as	O
an	O
apparent	O
heterodimer	B-oligomeric_state
(	O
red	O
line	O
),	O
while	O
a	O
mixture	O
of	O
HAESA	B-protein
and	O
SERK1	B-protein
yields	O
two	O
isolated	O
peaks	O
that	O
correspond	O
to	O
monomeric	B-oligomeric_state
HAESA	B-protein
and	O
SERK1	B-protein
,	O
respectively	O
(	O
black	O
line	O
).	O

Void	O
(	O
V0	O
)	O
volume	O
and	O
total	O
volume	O
(	O
Vt	O
)	O
are	O
shown	O
,	O
together	O
with	O
elution	O
volumes	O
for	O
molecular	O
mass	O
standards	O
(	O
A	O
,	O
Thyroglobulin	B-protein
,	O
669	O
,	O
000	O
Da	O
;	O
B	O
,	O
Ferritin	B-protein
,	O
440	O
,	O
00	O
Da	O
,	O
C	O
,	O
Aldolase	B-protein
,	O
158	O
,	O
000	O
Da	O
;	O
D	O
,	O
Conalbumin	B-protein
,	O
75	O
,	O
000	O
Da	O
;	O
E	O
,	O
Ovalbumin	B-protein
,	O
44	O
,	O
000	O
Da	O
;	O
F	O
,	O
Carbonic	B-protein
anhydrase	I-protein
,	O
29	O
,	O
000	O
Da	O
).	O

A	O
SDS	B-experimental_method
PAGE	I-experimental_method
of	O
the	O
peak	O
fractions	O
is	O
shown	O
alongside	O
.	O

Purified	O
HAESA	B-protein
and	O
SERK1	B-protein
are	O
~	O
75	O
and	O
~	O
28	O
kDa	O
,	O
respectively	O
.	O
(	O
C	O
)	O
Isothermal	B-experimental_method
titration	I-experimental_method
calorimetry	I-experimental_method
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
Hyp64	B-ptm
→	I-ptm
Pro	I-ptm
IDA	B-protein
versus	O
the	O
HAESA	B-protein
and	O
SERK1	B-protein
ectodomains	B-structure_element
.	O

no	O
detectable	O
binding	O
)	O
(	O
D	O
)	O
Analytical	B-experimental_method
size	I-experimental_method
-	I-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
in	O
the	O
presence	B-protein_state
of	I-protein_state
the	O
IDA	B-protein
Hyp64	B-ptm
→	I-ptm
Pro	I-ptm
mutant	B-protein_state
peptide	B-chemical
reveals	O
no	O
complex	O
formation	O
between	O
HAESA	B-protein
and	O
SERK1	B-protein
ectodomains	B-structure_element
.	O

(	O
E	O
)	O
In	B-experimental_method
vitro	I-experimental_method
kinase	I-experimental_method
assays	I-experimental_method
of	O
the	O
HAESA	B-protein
and	O
SERK1	B-protein
kinase	B-structure_element
domains	I-structure_element
.	O

Wild	B-protein_state
-	I-protein_state
type	I-protein_state
HAESA	B-protein
and	O
SERK1	B-protein
kinase	B-structure_element
domains	I-structure_element
(	O
KDs	B-structure_element
)	O
exhibit	O
auto	O
-	O
phosphorylation	O
activities	O
(	O
lanes	O
1	O
+	O
3	O
).	O

Transphosphorylation	O
activity	O
from	O
the	O
active	B-protein_state
kinase	O
to	O
the	O
mutated	B-protein_state
form	O
can	O
be	O
observed	O
in	O
both	O
directions	O
(	O
lanes	O
5	O
+	O
6	O
).	O

A	O
Hyp	B-protein_state
-	I-protein_state
modified	I-protein_state
dodecamer	B-structure_element
comprising	O
the	O
highly	B-protein_state
conserved	I-protein_state
PIP	B-structure_element
motif	I-structure_element
in	O
IDA	B-protein
(	O
Figure	O
1A	O
)	O
interacts	O
with	O
HAESA	B-protein
with	O
1	O
:	O
1	O
stoichiometry	O
(	O
N	O
)	O
and	O
with	O
a	O
dissociation	B-evidence
constant	I-evidence
(	O
Kd	B-evidence
)	O
of	O
~	O
20	O
μM	O
(	O
Figure	O
1B	O
).	O

We	O
next	O
determined	O
crystal	B-evidence
structures	I-evidence
of	O
the	O
apo	B-protein_state
HAESA	B-protein
ectodomain	B-structure_element
and	O
of	O
a	O
HAESA	B-complex_assembly
-	I-complex_assembly
IDA	I-complex_assembly
complex	O
,	O
at	O
1	O
.	O
74	O
and	O
1	O
.	O
86	O
Å	O
resolution	O
,	O
respectively	O
(	O
Figure	O
1C	O
;	O
Figure	O
1	O
—	O
figure	O
supplement	O
1B	O
–	O
D	O
;	O
Tables	O
1	O
,	O
2	O
).	O

IDA	B-protein
binds	O
in	O
a	O
completely	B-protein_state
extended	I-protein_state
conformation	I-protein_state
along	O
the	O
inner	O
surface	O
of	O
the	O
HAESA	B-protein
ectodomain	B-structure_element
,	O
covering	O
LRRs	B-structure_element
2	I-structure_element
–	I-structure_element
14	I-structure_element
(	O
Figure	O
1C	O
,	O
D	O
,	O
Figure	O
1	O
—	O
figure	O
supplement	O
2	O
).	O

The	O
central	O
Hyp64IDA	B-ptm
is	O
buried	O
in	O
a	O
specific	O
pocket	B-site
formed	O
by	O
HAESA	B-protein
LRRs	B-structure_element
8	I-structure_element
–	I-structure_element
10	I-structure_element
,	O
with	O
its	O
hydroxyl	O
group	O
establishing	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
the	O
strictly	B-protein_state
conserved	I-protein_state
Glu266HAESA	B-residue_name_number
and	O
with	O
a	O
water	B-chemical
molecule	O
,	O
which	O
in	O
turn	O
is	O
coordinated	O
by	O
the	O
main	O
chain	O
oxygens	O
of	O
Phe289HAESA	B-residue_name_number
and	O
Ser311HAESA	B-residue_name_number
(	O
Figure	O
1E	O
;	O
Figure	O
1	O
—	O
figure	O
supplement	O
3	O
).	O

The	O
restricted	O
size	O
of	O
the	O
Hyp	B-site
pocket	I-site
suggests	O
that	O
IDA	B-protein
does	O
not	O
require	O
arabinosylation	B-ptm
of	O
Hyp64IDA	B-ptm
for	O
activity	O
in	O
vivo	O
,	O
a	O
modification	O
that	O
has	O
been	O
reported	O
for	O
Hyp	B-residue_name
residues	O
in	O
plant	B-taxonomy_domain
CLE	B-protein_type
peptide	I-protein_type
hormones	I-protein_type
.	O

The	O
C	O
-	O
terminal	O
Arg	B-structure_element
-	I-structure_element
His	I-structure_element
-	I-structure_element
Asn	I-structure_element
motif	I-structure_element
in	O
IDA	B-protein
maps	O
to	O
a	O
cavity	B-site
formed	O
by	O
HAESA	B-protein
LRRs	B-structure_element
11	I-structure_element
–	I-structure_element
14	I-structure_element
(	O
Figure	O
1D	O
,	O
F	O
).	O

Mutation	B-experimental_method
of	O
Arg417HSL2	B-residue_name_number
(	O
which	O
corresponds	O
to	O
Arg409HAESA	B-residue_name_number
)	O
causes	O
a	O
loss	O
-	O
of	O
-	O
function	O
phenotype	O
in	O
HSL2	B-protein
,	O
which	O
indicates	O
that	O
the	O
peptide	B-site
binding	I-site
pockets	I-site
in	O
different	O
HAESA	B-protein_type
receptors	I-protein_type
have	O
common	O
structural	O
and	O
sequence	O
features	O
.	O

Indeed	O
,	O
we	O
find	O
many	O
of	O
the	O
residues	O
contributing	O
to	O
the	O
formation	O
of	O
the	O
IDA	B-site
binding	I-site
surface	I-site
in	O
HAESA	B-protein
to	O
be	O
conserved	B-protein_state
in	O
HSL2	B-protein
and	O
in	O
other	O
HAESA	B-protein_type
-	I-protein_type
type	I-protein_type
receptors	I-protein_type
in	O
different	O
plant	B-taxonomy_domain
species	O
(	O
Figure	O
1	O
—	O
figure	O
supplement	O
3	O
).	O

A	O
N	O
-	O
terminal	O
Pro	B-structure_element
-	I-structure_element
rich	I-structure_element
motif	I-structure_element
in	O
IDA	B-protein
makes	O
contacts	O
with	O
LRRs	B-structure_element
2	I-structure_element
–	I-structure_element
6	I-structure_element
of	O
the	O
receptor	O
(	O
Figure	O
1D	O
,	O
Figure	O
1	O
—	O
figure	O
supplement	O
2A	O
–	O
C	O
).	O

Other	O
hydrophobic	B-bond_interaction
and	I-bond_interaction
polar	I-bond_interaction
interactions	I-bond_interaction
are	O
mediated	O
by	O
Ser62IDA	B-residue_name_number
,	O
Ser65IDA	B-residue_name_number
and	O
by	O
backbone	O
atoms	O
along	O
the	O
IDA	B-chemical
peptide	I-chemical
(	O
Figure	O
1D	O
,	O
Figure	O
1	O
—	O
figure	O
supplement	O
2A	O
–	O
C	O
).	O

HAESA	B-protein
specifically	O
senses	O
IDA	B-protein_type
-	I-protein_type
family	I-protein_type
dodecamer	B-structure_element
peptides	B-chemical

We	O
next	O
investigated	O
whether	O
HAESA	B-protein
binds	O
N	B-protein_state
-	I-protein_state
terminally	I-protein_state
extended	I-protein_state
versions	O
of	O
IDA	B-protein
.	O

In	O
this	O
structure	B-evidence
,	O
no	O
additional	O
electron	B-evidence
density	I-evidence
accounts	O
for	O
the	O
PKGV	B-structure_element
motif	I-structure_element
at	O
the	O
IDA	B-protein
N	O
-	O
terminus	O
(	O
Figure	O
2A	O
,	O
B	O
).	O

Consistently	O
,	O
PKGV	B-mutant
-	I-mutant
IDA	I-mutant
and	O
IDA	B-protein
have	O
similar	O
binding	B-evidence
affinities	I-evidence
in	O
our	O
ITC	B-experimental_method
assays	I-experimental_method
,	O
further	O
indicating	O
that	O
HAESA	B-protein
senses	O
a	O
dodecamer	B-structure_element
peptide	B-chemical
comprising	O
residues	O
58	B-residue_range
-	I-residue_range
69IDA	I-residue_range
(	O
Figure	O
2D	O
).	O

IDL1	B-protein
,	O
which	O
can	O
rescue	O
IDA	B-protein
loss	O
-	O
of	O
-	O
function	O
mutants	O
when	O
introduced	O
in	O
abscission	O
zone	O
cells	O
,	O
can	O
also	O
be	O
sensed	O
by	O
HAESA	B-protein
,	O
albeit	O
with	O
lower	O
affinity	B-evidence
(	O
Figure	O
2D	O
).	O

A	O
2	O
.	O
56	O
Å	O
co	B-evidence
-	I-evidence
crystal	I-evidence
structure	I-evidence
with	O
IDL1	B-protein
reveals	O
that	O
different	O
IDA	B-protein_type
family	I-protein_type
members	I-protein_type
use	O
a	O
common	O
binding	O
mode	O
to	O
interact	O
with	O
HAESA	B-protein_type
-	I-protein_type
type	I-protein_type
receptors	I-protein_type
(	O
Figure	O
2A	O
–	O
C	O
,	O
E	O
,	O
Table	O
2	O
).	O

Notably	O
,	O
HAESA	B-protein
can	O
discriminate	O
between	O
IDLs	B-protein_type
and	O
functionally	B-protein_state
unrelated	I-protein_state
dodecamer	B-structure_element
peptides	B-chemical
with	O
Hyp	B-ptm
modifications	I-ptm
,	O
such	O
as	O
CLV3	B-protein
(	O
Figures	O
2D	O
,	O
7	O
).	O

The	O
co	B-protein_type
-	I-protein_type
receptor	I-protein_type
kinase	I-protein_type
SERK1	B-protein
allows	O
for	O
high	O
-	O
affinity	O
IDA	O
sensing	O

As	O
all	O
five	O
SERK	B-protein_type
family	I-protein_type
members	I-protein_type
appear	O
to	O
be	O
expressed	O
in	O
the	O
Arabidopsis	B-taxonomy_domain
abscission	O
zone	O
,	O
we	O
quantified	O
their	O
relative	O
contribution	O
to	O
floral	O
abscission	O
in	O
Arabidopsis	B-taxonomy_domain
using	O
a	O
petal	B-experimental_method
break	I-experimental_method
-	I-experimental_method
strength	I-experimental_method
assay	I-experimental_method
.	O

Our	O
experiments	O
suggest	O
that	O
among	O
the	O
SERK	B-protein_type
family	I-protein_type
members	I-protein_type
,	O
SERK1	B-protein
is	O
a	O
positive	O
regulator	O
of	O
floral	O
abscission	O
.	O

Possibly	O
because	O
SERKs	B-protein_type
have	O
additional	O
roles	O
in	O
plant	O
development	O
such	O
as	O
in	O
pollen	O
formation	O
and	O
brassinosteroid	O
signaling	O
,	O
we	O
found	O
that	O
higher	O
-	O
order	O
SERK	O
mutants	O
exhibit	O
pleiotropic	O
phenotypes	O
in	O
the	O
flower	O
,	O
rendering	O
their	O
analysis	O
and	O
comparison	O
by	O
quantitative	B-experimental_method
petal	I-experimental_method
break	I-experimental_method
-	I-experimental_method
strength	I-experimental_method
assays	I-experimental_method
difficult	O
.	O

We	O
thus	O
focused	O
on	O
analyzing	O
the	O
contribution	O
of	O
SERK1	B-protein
to	O
HAESA	B-protein
ligand	O
sensing	O
and	O
receptor	O
activation	O
.	O

We	O
next	O
quantified	O
the	O
contribution	O
of	O
SERK1	B-protein
to	O
IDA	B-protein
recognition	O
by	O
HAESA	B-protein
.	O

We	O
found	O
that	O
HAESA	B-protein
senses	O
IDA	B-protein
with	O
a	O
~	O
60	O
fold	O
higher	O
binding	B-evidence
affinity	I-evidence
in	O
the	O
presence	B-protein_state
of	I-protein_state
SERK1	B-protein
,	O
suggesting	O
that	O
SERK1	B-protein
is	O
involved	O
in	O
the	O
specific	O
recognition	O
of	O
the	O
peptide	B-protein_type
hormone	I-protein_type
(	O
Figure	O
3C	O
).	O

This	O
suggests	O
that	O
IDA	B-protein
itself	O
promotes	O
receptor	O
–	O
co	O
-	O
receptor	O
association	O
,	O
as	O
previously	O
described	O
for	O
the	O
steroid	B-chemical
hormone	I-chemical
brassinolide	B-chemical
and	O
for	O
other	O
LRR	B-complex_assembly
-	I-complex_assembly
RK	I-complex_assembly
complexes	O
.	O

Importantly	O
,	O
hydroxyprolination	B-ptm
of	O
IDA	B-protein
is	O
critical	O
for	O
HAESA	B-complex_assembly
-	I-complex_assembly
IDA	I-complex_assembly
-	I-complex_assembly
SERK1	I-complex_assembly
complex	O
formation	O
(	O
Figure	O
3C	O
,	O
D	O
).	O

Upon	O
IDA	B-protein
binding	O
at	O
the	O
cell	O
surface	O
,	O
the	O
kinase	B-structure_element
domains	I-structure_element
of	O
HAESA	B-protein
and	O
SERK1	B-protein
,	O
which	O
have	O
been	O
shown	O
to	O
be	O
active	B-protein_state
protein	B-protein_type
kinases	I-protein_type
,	O
may	O
interact	O
in	O
the	O
cytoplasm	O
to	O
activate	O
each	O
other	O
.	O

Consistently	O
,	O
the	O
HAESA	B-protein
kinase	B-structure_element
domain	I-structure_element
can	O
transphosphorylate	O
SERK1	B-protein
and	O
vice	O
versa	O
in	O
in	O
vitro	O
transphosphorylation	B-experimental_method
assays	I-experimental_method
(	O
Figure	O
3E	O
).	O

Crystal	B-evidence
structure	I-evidence
of	O
a	O
HAESA	B-complex_assembly
–	I-complex_assembly
IDA	I-complex_assembly
–	I-complex_assembly
SERK1	I-complex_assembly
signaling	O
complex	O
.	O

(	O
A	O
)	O
Overview	O
of	O
the	O
ternary	O
complex	O
with	O
HAESA	B-protein
in	O
blue	O
(	O
surface	O
representation	O
),	O
IDA	B-protein
in	O
yellow	O
(	O
bonds	O
representation	O
)	O
and	O
SERK1	B-protein
in	O
orange	O
(	O
surface	O
view	O
).	O
(	O
B	O
)	O
The	O
HAESA	B-protein
ectodomain	B-structure_element
undergoes	O
a	O
conformational	O
change	O
upon	O
SERK1	B-protein
co	O
-	O
receptor	O
binding	O
.	O

Shown	O
are	O
Cα	O
traces	O
of	O
a	O
structural	B-experimental_method
superposition	I-experimental_method
of	O
the	O
unbound	B-protein_state
(	O
yellow	O
)	O
and	O
SERK1	B-protein_state
-	I-protein_state
bound	I-protein_state
(	O
blue	O
)	O
HAESA	B-protein
ectodomains	B-structure_element
(	O
r	B-evidence
.	I-evidence
m	I-evidence
.	I-evidence
s	I-evidence
.	I-evidence
d	I-evidence
.	I-evidence
is	O
1	O
.	O
5	O
Å	O
between	O
572	O
corresponding	O
Cα	O
atoms	O
).	O

SERK1	B-protein
(	O
in	O
orange	O
)	O
and	O
IDA	B-protein
(	O
in	O
red	O
)	O
are	O
shown	O
alongside	O
.	O

The	O
N	O
-	O
terminal	O
capping	B-structure_element
domain	I-structure_element
of	O
SERK1	B-protein
(	O
in	O
orange	O
)	O
directly	O
contacts	O
the	O
C	O
-	O
terminal	O
part	O
of	O
IDA	B-protein
(	O
in	O
yellow	O
,	O
in	O
bonds	O
representation	O
)	O
and	O
the	O
receptor	B-protein_type
HAESA	B-protein
(	O
in	O
blue	O
).	O

Polar	B-bond_interaction
contacts	I-bond_interaction
of	O
SERK1	B-protein
with	O
IDA	B-protein
are	O
shown	O
in	O
magenta	O
,	O
with	O
the	O
HAESA	B-protein
LRR	B-structure_element
domain	I-structure_element
in	O
gray	O
.	O
(	O
D	O
)	O
Details	O
of	O
the	O
zipper	B-structure_element
-	I-structure_element
like	I-structure_element
SERK1	B-site
-	I-site
HAESA	I-site
interface	I-site
.	O

Polar	B-bond_interaction
interactions	I-bond_interaction
are	O
highlighted	O
as	O
dotted	O
lines	O
(	O
in	O
magenta	O
).	O

HAESA	B-protein
LRRs	B-structure_element
16	I-structure_element
–	I-structure_element
21	I-structure_element
and	O
its	O
C	O
-	O
terminal	O
capping	B-structure_element
domain	I-structure_element
undergo	O
a	O
conformational	O
change	O
upon	O
SERK1	B-protein
binding	O
(	O
Figure	O
4B	O
).	O

The	O
SERK1	B-protein
ectodomain	B-structure_element
interacts	O
with	O
the	O
IDA	B-site
peptide	I-site
binding	I-site
site	I-site
using	O
a	O
loop	B-structure_element
region	I-structure_element
(	O
residues	O
51	B-residue_range
-	I-residue_range
59SERK1	I-residue_range
)	O
from	O
its	O
N	O
-	O
terminal	O
cap	B-structure_element
(	O
Figure	O
4A	O
,	O
C	O
).	O

SERK1	B-protein
binds	O
HAESA	B-protein
using	O
these	O
two	O
distinct	O
interaction	B-site
surfaces	I-site
(	O
Figure	O
1	O
—	O
figure	O
supplement	O
3	O
),	O
with	O
the	O
N	B-structure_element
-	I-structure_element
cap	I-structure_element
of	O
the	O
SERK1	B-protein
LRR	B-structure_element
domain	I-structure_element
partially	O
covering	O
the	O
IDA	B-site
peptide	I-site
binding	I-site
cleft	I-site
.	O

The	O
IDA	B-protein
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
motif	I-structure_element
is	O
required	O
for	O
HAESA	B-complex_assembly
-	I-complex_assembly
SERK1	I-complex_assembly
complex	O
formation	O
and	O
for	O
IDA	O
bioactivity	O
.	O

(	O
A	O
)	O
Size	B-experimental_method
exclusion	I-experimental_method
chromatography	I-experimental_method
experiments	O
similar	O
to	O
Figure	O
3B	O
,	O
D	O
reveal	O
that	O
IDA	B-protein
mutant	B-protein_state
peptides	B-chemical
targeting	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
motif	I-structure_element
do	O
not	O
form	O
biochemically	B-protein_state
stable	I-protein_state
HAESA	B-complex_assembly
-	I-complex_assembly
IDA	I-complex_assembly
-	I-complex_assembly
SERK1	I-complex_assembly
complexes	O
.	O

Purified	B-experimental_method
HAESA	B-protein
and	O
SERK1	B-protein
are	O
~	O
75	O
and	O
~	O
28	O
kDa	O
,	O
respectively	O
.	O

Left	O
panel	O
:	O
IDA	B-mutant
K66A	I-mutant
/	I-mutant
R67A	I-mutant
;	O
center	O
:	O
IDA	B-mutant
ΔN69	I-mutant
,	O
right	O
panel	O
:	O
SDS	B-experimental_method
-	I-experimental_method
PAGE	I-experimental_method
of	O
peak	O
fractions	O
.	O

Note	O
that	O
the	O
HAESA	B-protein
and	O
SERK1	B-protein
input	O
lanes	O
have	O
already	O
been	O
shown	O
in	O
Figure	O
3D	O
.	O
(	O
B	O
)	O
Isothermal	B-evidence
titration	I-evidence
thermographs	I-evidence
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
mutant	B-protein_state
IDA	B-chemical
peptides	I-chemical
titrated	B-experimental_method
into	O
a	O
HAESA	B-protein
-	O
SERK1	B-protein
mixture	O
in	O
the	O
cell	O
.	O

Table	O
summaries	O
for	O
calorimetric	B-evidence
binding	I-evidence
constants	I-evidence
and	O
stoichoimetries	O
for	O
different	O
IDA	B-chemical
peptides	I-chemical
binding	O
to	O
the	O
HAESA	B-protein
–	O
SERK1	B-protein
ectodomain	B-structure_element
mixture	O
(	O
±	O
fitting	O
errors	O
;	O
n	O
.	O
d	O
.	O

(	O
C	O
)	O
Quantitative	O
petal	B-experimental_method
break	I-experimental_method
-	I-experimental_method
strength	I-experimental_method
assay	I-experimental_method
for	O
Col	O
-	O
0	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
flowers	O
and	O
35S	B-gene
::	O
IDA	B-protein
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
35S	B-gene
::	O
IDA	B-mutant
K66A	I-mutant
/	I-mutant
R67A	I-mutant
mutant	B-protein_state
flowers	O
.	O

Up	O
to	O
inflorescence	O
position	O
4	O
,	O
petal	O
break	O
in	O
35S	B-gene
::	O
IDA	B-mutant
K66A	I-mutant
/	I-mutant
R67A	I-mutant
mutant	B-protein_state
plants	B-taxonomy_domain
was	O
significantly	O
increased	O
compared	O
to	O
both	O
Col	O
-	O
0	O
control	O
plants	B-taxonomy_domain
(	O
b	O
)	O
and	O
35S	B-gene
::	O
IDA	B-protein
plants	B-taxonomy_domain
(	O
c	O
)	O
(	O
D	O
)	O
Normalized	O
expression	O
levels	O
(	O
relative	O
expression	O
±	O
standard	O
error	O
;	O
ida	O
:	O
-	O
0	O
.	O
02	O
±	O
0	O
.	O
001	O
;	O
Col	O
-	O
0	O
:	O
1	O
±	O
0	O
.	O
11	O
;	O
35S	B-gene
::	O
IDA	B-protein
124	O
±	O
0	O
.	O
75	O
;	O
35S	B-gene
::	O
IDA	B-mutant
K66A	I-mutant
/	I-mutant
R67A	I-mutant
:	O
159	O
±	O
0	O
.	O
58	O
)	O
of	O
IDA	B-protein
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
mutant	B-protein_state
transcripts	O
in	O
the	O
35S	B-experimental_method
promoter	I-experimental_method
over	I-experimental_method
-	I-experimental_method
expression	I-experimental_method
lines	I-experimental_method
analyzed	O
in	O
(	O
C	O
).	O
(	O
E	O
)	O
Magnified	O
view	O
of	O
representative	O
abscission	O
zones	O
from	O
35S	B-gene
::	O
IDA	B-protein
,	O
Col	O
-	O
0	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
35S	B-gene
::	O
IDA	B-mutant
K66A	I-mutant
/	I-mutant
R67A	I-mutant
double	B-protein_state
-	I-protein_state
mutant	I-protein_state
T3	B-experimental_method
transgenic	I-experimental_method
lines	I-experimental_method
.	O

15	O
out	O
of	O
15	O
35S	B-gene
::	O
IDA	B-protein
plants	B-taxonomy_domain
,	O
0	O
out	O
of	O
15	O
Col	O
-	O
0	O
plants	B-taxonomy_domain
and	O
0	O
out	O
of	O
15	O
35S	B-gene
::	O
IDA	B-mutant
K66A	I-mutant
/	I-mutant
R67A	I-mutant
double	B-protein_state
-	I-protein_state
mutant	I-protein_state
plants	B-taxonomy_domain
,	O
showed	O
an	O
enlarged	O
abscission	O
zone	O
,	O
respectively	O
(	O
3	O
independent	O
lines	O
were	O
analyzed	O
).	O

The	O
four	O
C	O
-	O
terminal	O
residues	O
in	O
IDA	B-protein
(	O
Lys66IDA	B-residue_range
-	I-residue_range
Asn69IDA	I-residue_range
)	O
are	O
conserved	B-protein_state
among	O
IDA	B-protein_type
family	I-protein_type
members	I-protein_type
and	O
are	O
in	O
direct	O
contact	O
with	O
SERK1	B-protein
(	O
Figures	O
1A	O
,	O
4C	O
).	O

We	O
thus	O
assessed	O
their	O
contribution	O
to	O
HAESA	B-complex_assembly
–	I-complex_assembly
SERK1	I-complex_assembly
complex	O
formation	O
.	O

A	O
synthetic	B-protein_state
Lys66IDA	B-mutant
/	I-mutant
Arg67IDA	I-mutant
→	I-mutant
Ala	I-mutant
mutant	B-protein_state
peptide	B-chemical
(	O
IDA	B-mutant
K66A	I-mutant
/	I-mutant
R66A	I-mutant
)	O
showed	O
a	O
10	O
fold	O
reduced	O
binding	B-evidence
affinity	I-evidence
when	O
titrated	B-experimental_method
in	O
a	O
HAESA	B-protein
/	O
SERK1	B-protein
protein	O
solution	O
(	O
Figures	O
5A	O
,	O
B	O
,	O
2D	O
).	O

We	O
over	B-experimental_method
-	I-experimental_method
expressed	I-experimental_method
full	B-protein_state
-	I-protein_state
length	I-protein_state
wild	B-protein_state
-	I-protein_state
type	I-protein_state
IDA	B-protein
or	O
this	O
Lys66IDA	B-mutant
/	I-mutant
Arg67IDA	I-mutant
→	I-mutant
Ala	I-mutant
double	B-protein_state
-	I-protein_state
mutant	I-protein_state
to	O
similar	O
levels	O
in	O
Col	O
-	O
0	O
Arabidopsis	B-taxonomy_domain
plants	B-taxonomy_domain
(	O
Figure	O
5D	O
).	O

We	O
found	O
that	O
over	B-experimental_method
-	I-experimental_method
expression	I-experimental_method
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
IDA	B-protein
leads	O
to	O
early	O
floral	O
abscission	O
and	O
an	O
enlargement	O
of	O
the	O
abscission	O
zone	O
(	O
Figure	O
5C	O
–	O
E	O
).	O

In	O
contrast	O
,	O
over	B-experimental_method
-	I-experimental_method
expression	I-experimental_method
of	O
the	O
IDA	B-mutant
Lys66IDA	I-mutant
/	I-mutant
Arg67IDA	I-mutant
→	I-mutant
Ala	I-mutant
double	B-protein_state
mutant	I-protein_state
significantly	O
delays	O
floral	O
abscission	O
when	O
compared	O
to	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
control	O
plants	B-taxonomy_domain
,	O
suggesting	O
that	O
the	O
mutant	B-protein_state
IDA	B-chemical
peptide	I-chemical
has	O
reduced	O
activity	O
in	O
planta	B-taxonomy_domain
(	O
Figure	O
5C	O
–	O
E	O
).	O

In	O
agreement	O
with	O
our	O
structures	B-evidence
and	O
biochemical	B-experimental_method
assays	I-experimental_method
,	O
this	O
experiment	O
suggests	O
a	O
role	O
of	O
the	O
conserved	B-protein_state
IDA	B-protein
C	O
-	O
terminus	O
in	O
the	O
control	O
of	O
floral	O
abscission	O
.	O

For	O
a	O
rapidly	O
growing	O
number	O
of	O
plant	B-taxonomy_domain
signaling	O
pathways	O
,	O
SERK	B-protein_type
proteins	I-protein_type
act	O
as	O
these	O
essential	O
co	B-protein_type
-	I-protein_type
receptors	I-protein_type
(;	O
).	O

SERK1	O
has	O
been	O
previously	O
reported	O
as	O
a	O
positive	O
regulator	O
in	O
plant	B-taxonomy_domain
embryogenesis	O
,	O
male	O
sporogenesis	O
,	O
brassinosteroid	O
signaling	O
and	O
in	O
phytosulfokine	O
perception	O
.	O

Recent	O
findings	O
by	O
and	O
our	O
mechanistic	O
studies	O
now	O
also	O
support	O
a	O
positive	O
role	O
for	O
SERK1	B-protein
in	O
floral	O
abscission	O
.	O

It	O
has	O
been	O
previously	O
suggested	O
that	O
SERK1	B-protein
can	O
inhibit	O
cell	O
separation	O
.	O

However	O
our	O
results	O
show	O
that	O
SERK1	B-protein
also	O
can	O
activate	O
this	O
process	O
upon	O
IDA	B-protein
sensing	O
,	O
indicating	O
that	O
SERKs	B-protein_type
may	O
fulfill	O
several	O
different	O
functions	O
in	O
the	O
course	O
of	O
the	O
abscission	O
process	O
.	O

While	O
the	O
sequence	O
of	O
the	O
mature	B-protein_state
IDA	B-chemical
peptide	I-chemical
has	O
not	O
been	O
experimentally	O
determined	O
in	O
planta	B-taxonomy_domain
,	O
our	O
HAESA	B-complex_assembly
-	I-complex_assembly
IDA	I-complex_assembly
complex	O
structures	B-evidence
and	O
calorimetry	B-evidence
assays	I-evidence
suggest	O
that	O
active	B-protein_state
IDLs	B-protein_type
are	O
hydroxyprolinated	B-protein_state
dodecamers	B-structure_element
.	O

It	O
will	O
be	O
thus	O
interesting	O
to	O
see	O
if	O
proteolytic	O
processing	O
of	O
full	B-protein_state
-	I-protein_state
length	I-protein_state
IDA	B-protein
in	O
vivo	O
is	O
regulated	O
in	O
a	O
cell	O
-	O
type	O
or	O
tissue	O
-	O
specific	O
manner	O
.	O

The	O
central	O
Hyp	B-residue_name
residue	O
in	O
IDA	B-protein
is	O
found	O
buried	O
in	O
the	O
HAESA	B-protein
peptide	B-site
binding	I-site
surface	I-site
and	O
thus	O
this	O
post	O
-	O
translational	O
modification	O
may	O
regulate	O
IDA	B-protein
bioactivity	O
.	O

In	O
our	O
quantitative	B-experimental_method
biochemical	I-experimental_method
assays	I-experimental_method
,	O
the	O
presence	B-protein_state
of	I-protein_state
SERK1	B-protein
dramatically	O
increases	O
the	O
HAESA	B-protein
binding	O
specificity	O
and	O
affinity	O
for	O
IDA	B-protein
.	O

This	O
observation	O
is	O
consistent	O
with	O
our	O
complex	O
structure	B-evidence
in	O
which	O
receptor	O
and	O
co	O
-	O
receptor	O
together	O
form	O
the	O
IDA	B-site
binding	I-site
pocket	I-site
.	O

The	O
fact	O
that	O
SERK1	B-protein
specifically	O
interacts	O
with	O
the	O
very	O
C	O
-	O
terminus	O
of	O
IDLs	B-protein_type
may	O
allow	O
for	O
the	O
rational	O
design	O
of	O
peptide	B-chemical
hormone	I-chemical
antagonists	I-chemical
,	O
as	O
previously	O
demonstrated	O
for	O
the	O
brassinosteroid	O
pathway	O
.	O

Importantly	O
,	O
our	O
calorimetry	B-experimental_method
assays	I-experimental_method
reveal	O
that	O
the	O
SERK1	B-protein
ectodomain	B-structure_element
binds	B-protein_state
HAESA	B-protein
with	O
nanomolar	O
affinity	O
,	O
but	O
only	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
IDA	B-protein
(	O
Figure	O
3C	O
).	O

This	O
ligand	O
-	O
induced	O
formation	O
of	O
a	O
receptor	O
–	O
co	O
-	O
receptor	O
complex	O
may	O
allow	O
the	O
HAESA	B-protein
and	O
SERK1	B-protein
kinase	B-structure_element
domains	I-structure_element
to	O
efficiently	O
trans	O
-	O
phosphorylate	O
and	O
activate	O
each	O
other	O
in	O
the	O
cytoplasm	O
.	O

SERK1	B-protein
uses	O
partially	O
overlapping	O
surface	O
areas	O
to	O
activate	O
different	O
plant	B-taxonomy_domain
signaling	B-protein_type
receptors	I-protein_type
.	O

Left	O
panel	O
:	O
Ribbon	O
diagram	O
of	O
HAESA	B-protein
(	O
in	O
blue	O
),	O
SERK1	B-protein
(	O
in	O
orange	O
)	O
and	O
IDA	B-protein
(	O
in	O
bonds	O
and	O
surface	O
represention	O
).	O

Right	O
panel	O
:	O
Ribbon	O
diagram	O
of	O
the	O
plant	B-taxonomy_domain
steroid	B-protein_type
receptor	I-protein_type
BRI1	B-protein
(	O
in	O
blue	O
)	O
bound	B-protein_state
to	I-protein_state
brassinolide	B-chemical
(	O
in	O
gray	O
,	O
in	O
bonds	O
representation	O
)	O
and	O
to	O
SERK1	B-protein
,	O
shown	O
in	O
the	O
same	O
orientation	O
(	O
PDB	O
-	O
ID	O
.	O
4lsx	O
).	O

(	O
B	O
)	O
View	O
of	O
the	O
inner	O
surface	O
of	O
the	O
SERK1	B-protein
LRR	B-structure_element
domain	I-structure_element
(	O
PDB	O
-	O
ID	O
4lsc	O
,	O
surface	O
representation	O
,	O
in	O
gray	O
).	O

A	O
ribbon	O
diagram	O
of	O
SERK1	B-protein
in	O
the	O
same	O
orientation	O
is	O
shown	O
alongside	O
.	O

Residues	O
interacting	O
with	O
the	O
HAESA	B-protein
or	O
BRI1	B-protein
LRR	B-structure_element
domains	I-structure_element
are	O
shown	O
in	O
orange	O
or	O
magenta	O
,	O
respectively	O
.	O

Comparison	B-experimental_method
of	O
our	O
HAESA	B-complex_assembly
–	I-complex_assembly
IDA	I-complex_assembly
–	I-complex_assembly
SERK1	I-complex_assembly
structure	B-evidence
with	O
the	O
brassinosteroid	O
receptor	O
signaling	O
complex	O
,	O
where	O
SERK1	B-protein
also	O
acts	O
as	O
co	B-protein_type
-	I-protein_type
receptor	I-protein_type
,	O
reveals	O
an	O
overall	O
conserved	B-protein_state
mode	O
of	O
SERK1	B-protein
binding	O
,	O
while	O
the	O
ligand	B-site
binding	I-site
pockets	I-site
map	O
to	O
very	O
different	O
areas	O
in	O
the	O
corresponding	O
receptors	O
(	O
LRRs	B-structure_element
2	I-structure_element
–	I-structure_element
14	I-structure_element
;	O
HAESA	B-protein
;	O
LRRs	B-structure_element
21	I-structure_element
–	I-structure_element
25	I-structure_element
,	O
BRI1	B-protein
)	O
and	O
may	O
involve	O
an	O
island	O
domain	O
(	O
BRI1	B-protein
)	O
or	O
not	O
(	O
HAESA	B-protein
)	O
(	O
Figure	O
6A	O
).	O

These	O
residues	O
are	O
not	O
involved	O
in	O
the	O
sensing	O
of	O
the	O
steroid	B-chemical
hormone	I-chemical
brassinolide	B-chemical
.	O

In	O
both	O
cases	O
however	O
,	O
the	O
co	O
-	O
receptor	O
completes	O
the	O
hormone	B-site
binding	I-site
pocket	I-site
.	O

Different	O
plant	B-taxonomy_domain
peptide	B-protein_type
hormone	I-protein_type
families	I-protein_type
contain	O
a	O
C	O
-	O
terminal	O
(	B-structure_element
Arg	I-structure_element
)-	I-structure_element
His	I-structure_element
-	I-structure_element
Asn	I-structure_element
motif	I-structure_element
,	O
which	O
in	O
IDA	B-protein
represents	O
the	O
co	B-site
-	I-site
receptor	I-site
recognition	I-site
site	I-site
.	O

Our	O
experiments	O
reveal	O
that	O
SERK1	B-protein
recognizes	O
a	O
C	O
-	O
terminal	O
Arg	B-structure_element
-	I-structure_element
His	I-structure_element
-	I-structure_element
Asn	I-structure_element
motif	I-structure_element
in	O
IDA	B-protein
.	O

Importantly	O
,	O
this	B-structure_element
motif	I-structure_element
can	O
also	O
be	O
found	O
in	O
other	O
peptide	B-protein_type
hormone	I-protein_type
families	I-protein_type
(	O
Figure	O
7	O
).	O

Internal	B-site
ribosome	I-site
entry	I-site
sites	I-site
(	O
IRESs	B-site
)	O
mediate	O
cap	O
-	O
independent	O
translation	O
of	O
viral	B-taxonomy_domain
mRNAs	B-chemical
.	O

Using	O
electron	B-experimental_method
cryo	I-experimental_method
-	I-experimental_method
microscopy	I-experimental_method
of	O
a	O
single	O
specimen	O
,	O
we	O
present	O
five	O
ribosome	B-complex_assembly
structures	B-evidence
formed	O
with	O
the	O
Taura	B-species
syndrome	I-species
virus	I-species
IRES	B-site
and	O
translocase	B-protein_type
eEF2	B-complex_assembly
•	I-complex_assembly
GTP	I-complex_assembly
bound	B-protein_state
with	I-protein_state
sordarin	B-chemical
.	O

The	O
structures	B-evidence
suggest	O
a	O
trajectory	O
of	O
IRES	B-site
translocation	O
,	O
required	O
for	O
translation	O
initiation	B-protein_state
,	O
and	O
provide	O
an	O
unprecedented	O
view	O
of	O
eEF2	B-protein
dynamics	O
.	O

The	O
IRES	B-site
rearranges	O
from	O
extended	B-protein_state
to	O
bent	B-protein_state
to	O
extended	B-protein_state
conformations	O
.	O

This	O
inchworm	B-protein_state
-	O
like	O
movement	O
is	O
coupled	O
with	O
ribosomal	O
inter	O
-	O
subunit	O
rotation	O
and	O
40S	B-complex_assembly
head	B-structure_element
swivel	O
.	O

eEF2	B-protein
,	O
attached	O
to	O
the	O
60S	B-complex_assembly
subunit	B-structure_element
,	O
slides	O
along	O
the	O
rotating	O
40S	B-complex_assembly
subunit	B-structure_element
to	O
enter	O
the	O
A	B-site
site	I-site
.	O

Its	O
diphthamide	B-ptm
-	O
bearing	O
tip	O
at	O
domain	O
IV	B-structure_element
separates	O
the	O
tRNA	B-structure_element
-	I-structure_element
mRNA	I-structure_element
-	I-structure_element
like	I-structure_element
pseudoknot	I-structure_element
I	I-structure_element
(	O
PKI	B-structure_element
)	O
of	O
the	O
IRES	B-site
from	O
the	O
decoding	B-site
center	I-site
.	O

To	O
efficiently	O
compete	O
with	O
host	O
mRNAs	B-chemical
and	O
engage	O
in	O
translation	O
under	O
stress	O
,	O
some	O
viral	B-taxonomy_domain
mRNAs	B-chemical
undergo	O
cap	O
-	O
independent	O
translation	O
.	O

To	O
this	O
end	O
,	O
internal	B-site
ribosome	I-site
entry	I-site
site	I-site
(	O
IRES	B-site
)	O
RNAs	B-chemical
are	O
employed	O
(	O
reviewed	O
in	O
.	O

An	O
IRES	B-site
is	O
located	O
at	O
the	O
5	B-structure_element
’	I-structure_element
untranslated	I-structure_element
region	I-structure_element
of	O
the	O
viral	B-taxonomy_domain
mRNA	B-chemical
,	O
preceding	O
an	O
open	B-structure_element
reading	I-structure_element
frame	I-structure_element
(	O
ORF	B-structure_element
).	O

To	O
initiate	O
translation	O
,	O
a	O
structured	B-protein_state
IRES	B-site
RNA	B-chemical
interacts	O
with	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
or	O
the	O
80S	B-complex_assembly
ribosome	I-complex_assembly
,	O
resulting	O
in	O
precise	O
positioning	O
of	O
the	O
downstream	O
start	O
codon	O
in	O
the	O
small	B-protein_state
40S	B-complex_assembly
subunit	B-structure_element
.	O

Upon	O
peptide	O
bond	O
formation	O
,	O
the	O
two	O
tRNAs	B-chemical
and	O
their	O
respective	O
mRNA	B-chemical
codons	O
translocate	O
from	O
the	O
A	B-site
and	I-site
P	I-site
to	O
P	B-site
and	I-site
E	I-site
(	I-site
exit	I-site
)	I-site
sites	I-site
,	O
freeing	O
the	O
A	B-site
site	I-site
for	O
the	O
next	O
elongator	O
tRNA	B-chemical
.	O

An	O
unusual	O
strategy	O
of	O
initiation	B-protein_state
is	O
used	O
by	O
intergenic	B-structure_element
-	I-structure_element
region	I-structure_element
(	O
IGR	B-structure_element
)	O
IRESs	B-site
found	O
in	O
Dicistroviridae	B-species
arthropod	I-species
-	O
infecting	O
viruses	B-taxonomy_domain
.	O

These	O
include	O
shrimp	B-taxonomy_domain
-	O
infecting	O
Taura	B-species
syndrome	I-species
virus	I-species
(	O
TSV	B-species
),	O
and	O
insect	B-taxonomy_domain
viruses	O
Plautia	B-species
stali	I-species
intestine	I-species
virus	I-species
(	O
PSIV	B-species
)	O
and	O
Cricket	B-species
paralysis	I-species
virus	I-species
(	O
CrPV	B-species
).	O

The	O
IGR	B-structure_element
IRES	B-site
mRNAs	B-chemical
do	O
not	O
contain	O
an	O
AUG	O
start	O
codon	O
.	O

As	O
such	O
,	O
this	O
group	O
of	O
IRESs	B-site
represents	O
the	O
most	O
streamlined	O
mechanism	O
of	O
eukaryotic	B-taxonomy_domain
translation	O
initiation	B-protein_state
.	O

Early	O
electron	B-experimental_method
cryo	I-experimental_method
-	I-experimental_method
microscopy	I-experimental_method
(	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
)	O
studies	O
have	O
found	O
that	O
the	O
CrPV	B-species
IRES	B-site
packs	O
in	O
the	O
ribosome	B-complex_assembly
intersubunit	B-site
space	I-site
.	O

Recent	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
structures	B-evidence
of	O
ribosome	B-protein_state
-	I-protein_state
bound	I-protein_state
TSV	B-species
IRES	B-site
and	O
CrPV	B-species
IRES	B-site
revealed	O
that	O
IGR	B-structure_element
IRESs	B-site
position	O
the	O
ORF	B-structure_element
by	O
mimicking	O
a	O
translating	O
ribosome	B-complex_assembly
bound	B-protein_state
with	I-protein_state
tRNA	B-chemical
and	O
mRNA	B-chemical
.	O

The	O
~	O
200	O
-	O
nt	O
IRES	B-site
RNAs	B-chemical
span	O
from	O
the	O
A	B-site
site	I-site
beyond	O
the	O
E	B-site
site	I-site
.	O

A	O
conserved	B-protein_state
tRNA	B-structure_element
-	I-structure_element
mRNA	I-structure_element
–	I-structure_element
like	I-structure_element
structural	I-structure_element
element	I-structure_element
of	O
pseudoknot	B-structure_element
I	I-structure_element
(	O
PKI	B-structure_element
)	O
interacts	O
with	O
the	O
decoding	B-site
center	I-site
in	O
the	O
A	B-site
site	I-site
of	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
.	O

The	O
downstream	O
initiation	O
codon	O
—	O
coding	O
for	O
alanine	B-residue_name
—	O
is	O
placed	O
in	O
the	O
mRNA	B-site
tunnel	I-site
,	O
preceding	O
the	O
decoding	B-site
center	I-site
.	O

PKI	B-structure_element
of	O
IGR	B-structure_element
IRESs	B-site
therefore	O
mimics	O
an	O
A	B-site
-	I-site
site	I-site
elongator	O
tRNA	B-chemical
interacting	O
with	O
an	O
mRNA	B-chemical
sense	O
codon	O
,	O
but	O
not	O
a	O
P	B-site
-	I-site
site	I-site
initiator	O
tRNAMet	B-chemical
and	O
the	O
AUG	O
start	O
codon	O
.	O

How	O
this	O
non	O
-	O
canonical	O
initiation	B-protein_state
complex	O
transitions	O
to	O
the	O
elongation	O
step	O
is	O
not	O
fully	O
understood	O
.	O

A	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
structure	B-evidence
of	O
the	O
ribosome	B-complex_assembly
bound	B-protein_state
with	I-protein_state
a	O
CrPV	B-species
IRES	B-site
and	O
release	B-protein_type
factor	I-protein_type
eRF1	B-protein
occupying	O
the	O
A	B-site
site	I-site
provided	O
insight	O
into	O
the	O
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
state	O
.	O

In	O
this	O
structure	B-evidence
,	O
PKI	B-structure_element
is	O
positioned	O
in	O
the	O
P	B-site
site	I-site
and	O
the	O
first	O
mRNA	B-chemical
codon	O
is	O
located	O
in	O
the	O
A	B-site
site	I-site
.	O

How	O
the	O
large	B-protein_state
IRES	B-site
RNA	B-chemical
translocates	O
within	O
the	O
ribosome	B-complex_assembly
,	O
allowing	O
PKI	B-structure_element
translocation	O
from	O
the	O
A	B-site
to	I-site
P	I-site
site	I-site
is	O
not	O
known	O
.	O

The	O
structural	O
similarity	O
of	O
PKI	B-structure_element
and	O
the	O
tRNA	B-chemical
anticodon	B-structure_element
stem	I-structure_element
loop	I-structure_element
(	O
ASL	B-structure_element
)	O
bound	B-protein_state
to	I-protein_state
a	O
codon	O
suggests	O
that	O
their	O
mechanisms	O
of	O
translocation	O
are	O
similar	O
to	O
some	O
extent	O
.	O

Translocation	O
of	O
the	O
IRES	B-site
or	O
tRNA	B-complex_assembly
-	I-complex_assembly
mRNA	I-complex_assembly
requires	O
eukaryotic	B-taxonomy_domain
elongation	B-protein
factor	I-protein
2	I-protein
(	O
eEF2	B-protein
),	O
a	O
structural	O
and	O
functional	O
homolog	O
of	O
the	O
well	O
-	O
studied	O
bacterial	B-taxonomy_domain
EF	B-protein
-	I-protein
G	I-protein
.	O
Pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
tRNA	B-protein_state
-	I-protein_state
bound	I-protein_state
ribosomes	B-complex_assembly
contain	O
a	O
peptidyl	B-chemical
-	I-chemical
and	I-chemical
deacyl	I-chemical
-	I-chemical
tRNA	I-chemical
,	O
both	O
base	O
-	O
paired	O
to	O
mRNA	B-chemical
codons	O
in	O
the	O
A	B-site
and	I-site
P	I-site
sites	I-site
(	O
termed	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
complex	O
).	O

Translocation	O
of	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
involves	O
two	O
major	O
large	O
-	O
scale	O
ribosome	B-complex_assembly
rearrangements	O
(	O
Figure	O
1	O
—	O
figure	O
supplement	O
1	O
)	O
(	O
reviewed	O
in	O
).	O

First	O
,	O
studies	O
of	O
bacterial	B-taxonomy_domain
ribosomes	B-complex_assembly
showed	O
that	O
a	O
~	O
10	O
°	O
rotation	O
of	O
the	O
small	B-structure_element
subunit	I-structure_element
relative	O
to	O
the	O
large	B-structure_element
subunit	I-structure_element
,	O
known	O
as	O
intersubunit	O
rotation	O
,	O
or	O
ratcheting	O
,	O
is	O
required	O
for	O
translocation	O
.	O

In	O
the	O
rotated	B-protein_state
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
ribosome	B-complex_assembly
,	O
the	O
peptidyl	B-chemical
-	I-chemical
tRNA	I-chemical
binds	O
the	O
A	B-site
site	I-site
of	O
the	O
small	B-structure_element
subunit	I-structure_element
with	O
its	O
ASL	B-structure_element
and	O
the	O
P	B-site
site	I-site
of	O
the	O
large	B-structure_element
subunit	I-structure_element
with	O
the	O
CCA	O
3	O
’	O
end	O
(	O
A	B-protein_state
/	I-protein_state
P	I-protein_state
hybrid	I-protein_state
state	O
).	O

Concurrently	O
,	O
the	O
deacyl	B-chemical
-	I-chemical
tRNA	I-chemical
interacts	O
with	O
the	O
P	B-site
site	I-site
of	O
the	O
small	B-structure_element
subunit	I-structure_element
and	O
the	O
E	B-site
site	I-site
of	O
the	O
large	B-structure_element
subunit	I-structure_element
(	O
P	B-protein_state
/	I-protein_state
E	I-protein_state
hybrid	I-protein_state
state	O
).	O

The	O
ribosome	B-complex_assembly
can	O
undergo	O
spontaneous	O
,	O
thermally	O
-	O
driven	O
forward	O
-	O
reverse	O
rotation	O
that	O
shifts	O
the	O
two	O
tRNAs	B-chemical
between	O
the	O
hybrid	B-protein_state
and	O
'	O
classical	B-protein_state
'	O
states	O
while	O
the	O
anticodon	B-structure_element
stem	I-structure_element
loops	I-structure_element
remain	O
non	B-protein_state
-	I-protein_state
translocated	I-protein_state
.	O

Binding	O
of	O
EF	B-protein
-	I-protein
G	I-protein
next	O
to	O
the	O
A	B-site
site	I-site
and	O
reverse	O
rotation	O
of	O
the	O
small	B-structure_element
subunit	I-structure_element
results	O
in	O
translocation	O
of	O
both	O
ASLs	B-structure_element
on	O
the	O
small	B-structure_element
subunit	I-structure_element
.	O

EF	B-protein
-	I-protein
G	I-protein
is	O
thought	O
to	O
'	O
unlock	O
'	O
the	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
ribosome	B-complex_assembly
,	O
allowing	O
movement	O
of	O
the	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
complex	O
,	O
however	O
the	O
structural	O
details	O
of	O
this	O
unlocking	O
are	O
not	O
known	O
.	O

The	O
second	O
large	O
-	O
scale	O
rearrangement	O
involves	O
rotation	O
,	O
or	O
swiveling	O
,	O
of	O
the	O
head	B-structure_element
of	O
the	O
small	B-structure_element
subunit	I-structure_element
relative	O
to	O
the	O
body	B-structure_element
.	O

The	O
head	B-structure_element
can	O
rotate	O
by	O
up	O
to	O
~	O
20	O
°	O
around	O
the	O
axis	O
nearly	O
orthogonal	O
to	O
that	O
of	O
intersubunit	O
rotation	O
,	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
tRNA	B-chemical
or	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
a	O
single	O
P	B-site
/	O
E	B-site
tRNA	B-chemical
and	O
eEF2	B-protein
or	O
EF	B-protein
-	I-protein
G	I-protein
.	O
Förster	B-experimental_method
resonance	I-experimental_method
energy	I-experimental_method
transfer	I-experimental_method
(	O
FRET	B-experimental_method
)	O
data	O
suggest	O
that	O
head	B-structure_element
swivel	O
of	O
the	O
rotated	B-protein_state
small	B-structure_element
subunit	I-structure_element
facilitates	O
EF	B-protein
-	I-protein
G	I-protein
-	O
mediated	O
movement	O
of	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
.	O

Structures	B-evidence
of	O
the	O
70S	B-complex_assembly
•	I-complex_assembly
EF	I-complex_assembly
-	I-complex_assembly
G	I-complex_assembly
complex	O
bound	B-protein_state
with	I-protein_state
two	O
nearly	B-protein_state
translocated	I-protein_state
tRNAs	B-chemical
,	O
exhibit	O
a	O
large	O
18	O
°	O
to	O
21	O
°	O
head	B-structure_element
swivel	O
in	O
a	O
mid	B-protein_state
-	I-protein_state
rotated	I-protein_state
subunit	B-structure_element
,	O
whereas	O
no	O
head	B-structure_element
swivel	O
is	O
observed	O
in	O
the	O
fully	B-protein_state
rotated	I-protein_state
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
or	O
in	O
the	O
non	B-protein_state
-	I-protein_state
rotated	I-protein_state
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
70S	B-complex_assembly
•	I-complex_assembly
2tRNA	I-complex_assembly
•	I-complex_assembly
EF	I-complex_assembly
-	I-complex_assembly
G	I-complex_assembly
structures	B-evidence
.	O

The	O
structural	O
role	O
of	O
head	B-structure_element
swivel	O
is	O
not	O
fully	O
understood	O
.	O

The	O
head	B-structure_element
swivel	O
was	O
proposed	O
to	O
facilitate	O
transition	O
of	O
the	O
tRNA	B-chemical
from	O
the	O
P	B-site
to	I-site
E	I-site
site	I-site
by	O
widening	O
a	O
constriction	B-site
between	O
these	O
sites	O
on	O
the	O
30S	B-complex_assembly
subunit	B-structure_element
.	O

This	O
widening	O
allows	O
the	O
ASL	B-structure_element
to	O
sample	O
positions	O
between	O
the	O
P	B-site
and	I-site
E	I-site
sites	I-site
.	O

Whether	O
and	O
how	O
the	O
head	B-structure_element
swivel	O
mediates	O
tRNA	B-chemical
transition	O
from	O
the	O
A	B-site
to	I-site
P	I-site
site	I-site
remains	O
unknown	O
.	O

Comparison	O
of	O
70S	B-complex_assembly
•	I-complex_assembly
2tRNA	I-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
and	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
translocation	O
complexes	O
.	O

The	O
large	O
ribosomal	O
subunit	B-structure_element
is	O
shown	O
in	O
cyan	O
;	O
the	O
small	B-structure_element
subunit	I-structure_element
in	O
light	O
yellow	O
(	O
head	B-structure_element
)	O
and	O
wheat	O
-	O
yellow	O
(	O
body	B-structure_element
),	O
elongation	B-protein
factor	I-protein
G	I-protein
(	O
EF	B-protein
-	I-protein
G	I-protein
)	O
is	O
shown	O
in	O
green	O
.	O

Nucleotides	O
C1054	B-residue_name_number
,	O
G966	B-residue_name_number
and	O
G693	B-residue_name_number
of	O
16S	B-chemical
rRNA	I-chemical
are	O
shown	O
in	O
black	O
to	O
denote	O
the	O
A	B-site
,	I-site
P	I-site
and	I-site
E	I-site
sites	I-site
,	O
respectively	O
.	O

The	O
extents	O
of	O
the	O
30S	B-complex_assembly
subunit	B-structure_element
rotation	O
and	O
head	B-structure_element
swivel	O
relative	O
to	O
their	O
positions	O
in	O
the	O
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
structure	B-evidence
are	O
shown	O
with	O
arrows	O
.	O

References	O
and	O
PDB	O
codes	O
of	O
the	O
structures	B-evidence
are	O
shown	O
.	O

(	O
b	O
)	O
Structures	B-evidence
of	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
complexes	O
in	O
the	O
absence	B-protein_state
and	O
presence	B-protein_state
of	I-protein_state
eEF2	B-protein
(	O
this	O
work	O
).	O

Unresolved	O
regions	O
of	O
the	O
IRES	B-site
in	O
densities	B-evidence
for	O
Structures	B-evidence
III	I-evidence
and	I-evidence
V	I-evidence
are	O
shown	O
in	O
gray	O
.	O

Schematic	O
of	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
refinement	O
and	O
classification	O
procedures	O
.	O

All	O
particles	B-experimental_method
were	O
initially	O
aligned	O
to	O
a	O
single	O
model	O
.	O

3D	B-experimental_method
classification	I-experimental_method
using	O
a	O
3D	B-evidence
mask	I-evidence
around	O
the	O
40S	B-complex_assembly
head	B-structure_element
,	O
TSV	B-species
IRES	B-site
and	O
eEF2	B-protein
,	O
of	O
the	O
4x	O
binned	O
stack	B-bond_interaction
was	O
used	O
to	O
identify	O
particles	B-experimental_method
containing	O
both	O
the	O
IRES	B-site
and	O
eEF2	B-protein
.	O

Subsequent	O
3D	B-experimental_method
classification	I-experimental_method
using	O
a	O
2D	B-evidence
mask	I-evidence
comprising	O
PKI	B-structure_element
and	O
domain	O
IV	B-structure_element
of	O
eEF2	B-protein
yielded	O
5	O
'	O
purified	O
'	O
classes	O
representing	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
.	O
Sub	B-experimental_method
-	I-experimental_method
classification	I-experimental_method
of	O
each	O
class	O
did	O
not	O
yield	O
additional	O
classes	O
,	O
but	O
helped	O
improve	O
density	B-evidence
in	O
the	O
PKI	B-structure_element
region	O
of	O
class	O
III	O
(	O
estimated	O
resolution	O
and	O
percentage	O
of	O
particles	B-experimental_method
in	O
the	O
sub	B-experimental_method
-	I-experimental_method
classified	I-experimental_method
reconstruction	B-evidence
are	O
shown	O
in	O
parentheses	O
).	O

Cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
density	B-evidence
of	O
Structures	B-evidence
I	I-evidence
-	I-evidence
V	I-evidence
.	O

The	O
maps	B-evidence
in	O
all	O
panels	O
were	O
B	O
-	O
softened	O
by	O
applying	O
a	O
B	O
-	O
factor	O
of	O
30	O
Å2	O
.	O

(	O
a	O
-	O
e	O
)	O
Cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
map	B-evidence
of	O
Structures	B-evidence
I	I-evidence
,	I-evidence
II	I-evidence
,	I-evidence
III	I-evidence
,	I-evidence
IV	I-evidence
and	I-evidence
V	I-evidence
.	O
(	O
f	O
-	O
j	O
)	O
Local	O
resolution	O
of	O
unfiltered	O
and	O
unmasked	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
reconstructions	B-evidence
,	O
assessed	O
using	O
Blocres	B-experimental_method
from	O
the	O
BSoft	O
package	O
,	O
for	O
Structures	B-evidence
I	I-evidence
,	I-evidence
II	I-evidence
,	I-evidence
III	I-evidence
,	I-evidence
IV	I-evidence
and	I-evidence
V	I-evidence
.	O
(	O
k	O
-	O
o	O
)	O
Cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
density	B-evidence
for	O
the	O
TSV	B-species
IRES	B-site
(	O
red	O
model	O
)	O
and	O
eEF2	B-protein
(	O
green	O
model	O
)	O
in	O
Structures	B-evidence
I	I-evidence
,	I-evidence
II	I-evidence
,	I-evidence
III	I-evidence
,	I-evidence
IV	I-evidence
and	I-evidence
V	I-evidence
.	O
(	O
p	O
)	O
Fourier	B-evidence
shell	I-evidence
correlation	I-evidence
(	O
FSC	B-evidence
)	O
curves	B-evidence
for	O
Structures	B-evidence
I	I-evidence
-	I-evidence
V	I-evidence
.	O
The	O
horizontal	O
axis	O
is	O
labeled	O
with	O
spatial	O
frequency	O
Å	O
-	O
1	O
and	O
with	O
Å	O
.	O
The	O
resolutions	O
stated	O
in	O
the	O
text	O
correspond	O
to	O
an	O
FSC	B-evidence
threshold	O
value	O
of	O
0	O
.	O
143	O
,	O
shown	O
as	O
a	O
dotted	O
line	O
,	O
for	O
the	O
FREALIGN	B-experimental_method
-	O
derived	O
FSC	B-evidence
('	O
Part_FSC	O
').	O

Nucleotides	O
C1274	B-residue_name_number
,	O
U1191	B-residue_name_number
of	O
the	O
40S	B-complex_assembly
head	B-structure_element
and	O
G904	B-residue_name_number
of	O
the	O
platform	B-site
(	O
C1054	B-residue_name_number
,	O
G966	B-residue_name_number
and	O
G693	B-residue_name_number
in	O
E	B-species
.	I-species
coli	I-species
16S	B-chemical
rRNA	I-chemical
)	O
are	O
shown	O
in	O
black	O
to	O
denote	O
the	O
A	B-site
,	I-site
P	I-site
and	I-site
E	I-site
sites	I-site
,	O
respectively	O
.	O

(	O
b	O
)	O
Schematic	O
representation	O
of	O
the	O
structures	B-evidence
shown	O
in	O
panel	O
a	O
,	O
denoting	O
the	O
conformations	O
of	O
the	O
small	B-structure_element
subunit	I-structure_element
relative	O
to	O
the	O
large	B-structure_element
subunit	I-structure_element
.	O

A	B-site
,	I-site
P	I-site
and	I-site
E	I-site
sites	I-site
are	O
shown	O
as	O
rectangles	O
.	O

All	O
measurements	O
are	O
relative	O
to	O
the	O
non	B-protein_state
-	I-protein_state
rotated	I-protein_state
80S	B-complex_assembly
•	I-complex_assembly
2tRNA	I-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
structure	B-evidence
.	O

We	O
sought	O
to	O
address	O
the	O
following	O
questions	O
by	O
structural	B-experimental_method
visualization	I-experimental_method
of	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
•	I-complex_assembly
eEF2	I-complex_assembly
translocation	O
complexes	O
:	O
(	O
1	O
)	O
How	O
does	O
a	O
large	O
IRES	B-site
RNA	B-chemical
move	O
through	O
the	O
restricted	O
intersubunit	O
space	O
,	O
bringing	O
PKI	B-structure_element
from	O
the	O
A	B-site
to	I-site
P	I-site
site	I-site
of	O
the	O
small	B-structure_element
subunit	I-structure_element
?	O
(	O
2	O
)	O
How	O
does	O
eEF2	B-protein
mediate	O
IRES	B-site
translocation	O
?	O
(	O
3	O
)	O
Does	O
IRES	B-site
translocation	O
involve	O
large	O
rearrangements	O
in	O
the	O
ribosome	B-complex_assembly
,	O
similar	O
to	O
tRNA	B-chemical
translocation	O
?	O
(	O
4	O
)	O
What	O
,	O
if	O
any	O
,	O
is	O
the	O
mechanistic	O
role	O
of	O
40S	B-complex_assembly
head	B-structure_element
rotation	O
in	O
IRES	B-site
translocation	O
?	O

We	O
used	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
to	O
visualize	O
80S	B-complex_assembly
•	I-complex_assembly
TSV	I-complex_assembly
IRES	I-complex_assembly
complexes	O
formed	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
eEF2	B-complex_assembly
•	I-complex_assembly
GTP	I-complex_assembly
and	O
the	O
translation	O
inhibitor	O
sordarin	B-chemical
,	O
which	O
stabilizes	O
eEF2	B-protein
on	O
the	O
ribosome	B-complex_assembly
.	O

Maximum	B-experimental_method
-	I-experimental_method
likelihood	I-experimental_method
classification	I-experimental_method
using	O
FREALIGN	B-experimental_method
identified	O
five	O
IRES	B-protein_state
-	I-protein_state
eEF2	I-protein_state
-	I-protein_state
bound	I-protein_state
ribosome	B-complex_assembly
structures	B-evidence
within	O
a	O
single	O
sample	O
(	O
Figures	O
1	O
and	O
2	O
).	O

The	O
structures	B-evidence
differ	O
in	O
the	O
positions	O
and	O
conformations	O
of	O
ribosomal	O
subunits	O
(	O
Figures	O
1b	O
and	O
2	O
),	O
IRES	B-site
RNA	B-chemical
(	O
Figures	O
3	O
and	O
4	O
)	O
and	O
eEF2	B-protein
(	O
Figures	O
5	O
and	O
6	O
).	O

This	O
ensemble	O
of	O
structures	B-evidence
allowed	O
us	O
to	O
reconstruct	O
a	O
sequence	O
of	O
steps	O
in	O
IRES	B-site
translocation	O
induced	O
by	O
eEF2	B-protein
.	O

We	O
used	O
single	B-experimental_method
-	I-experimental_method
particle	I-experimental_method
cryo	I-experimental_method
-	I-experimental_method
EM	I-experimental_method
and	O
maximum	B-experimental_method
-	I-experimental_method
likelihood	I-experimental_method
image	I-experimental_method
classification	I-experimental_method
in	O
FREALIGN	B-experimental_method
to	O
obtain	O
three	O
-	O
dimensional	O
density	B-evidence
maps	I-evidence
from	O
a	O
single	O
specimen	O
.	O

The	O
translocation	O
complex	O
was	O
formed	O
using	O
S	B-species
.	I-species
cerevisiae	I-species
80S	B-complex_assembly
ribosomes	I-complex_assembly
,	O
Taura	B-species
syndrome	I-species
virus	I-species
IRES	B-site
,	O
and	O
S	B-species
.	I-species
cerevisiae	I-species
eEF2	B-protein
in	O
the	O
presence	B-protein_state
of	I-protein_state
GTP	B-chemical
and	O
the	O
eEF2	B-protein
-	O
binding	O
translation	O
inhibitor	O
sordarin	B-chemical
.	O

This	O
approach	O
revealed	O
five	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
•	I-complex_assembly
eEF2	I-complex_assembly
•	I-complex_assembly
GDP	I-complex_assembly
structures	B-evidence
at	O
average	O
resolutions	O
of	O
3	O
.	O
5	O
to	O
4	O
.	O
2	O
Å	O
,	O
sufficient	O
to	O
locate	O
IRES	B-site
domains	O
and	O
to	O
resolve	O
individual	O
residues	O
in	O
the	O
core	O
regions	O
of	O
the	O
ribosome	B-complex_assembly
and	O
eEF2	B-protein
(	O
Figures	O
3c	O
,	O
d	O
,	O
and	O
5f	O
,	O
h	O
;	O
see	O
also	O
Figure	O
1	O
—	O
figure	O
supplement	O
2	O
and	O
Figure	O
5	O
—	O
figure	O
supplement	O
2	O
),	O
including	O
the	O
post	O
-	O
translational	O
modification	O
diphthamide	B-ptm
699	I-ptm
(	O
Figure	O
3c	O
).	O

Large	O
-	O
scale	O
rearrangements	O
in	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
,	O
coupled	O
with	O
the	O
movement	O
of	O
PKI	B-structure_element
from	O
the	O
A	B-site
to	I-site
P	I-site
site	I-site
and	O
eEF2	B-protein
entry	O
into	O
the	O
A	B-site
site	I-site
.	O

For	O
each	O
structure	B-evidence
,	O
the	O
triangle	O
outlines	O
the	O
contours	O
of	O
the	O
40S	B-complex_assembly
body	B-structure_element
;	O
the	O
lower	O
angle	O
illustrates	O
the	O
extent	O
of	O
intersubunit	O
(	O
body	B-structure_element
)	O
rotation	O
.	O

(	O
b	O
)	O
Solvent	O
view	O
(	O
opposite	O
from	O
that	O
shown	O
in	O
(	O
a	O
))	O
of	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
in	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
structure	B-evidence
(	O
INIT	B-complex_assembly
;	O
PDB	O
3J6Y	O
)	O
and	O
in	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
•	I-complex_assembly
eEF2	I-complex_assembly
Structures	B-evidence
I	I-evidence
,	I-evidence
II	I-evidence
,	I-evidence
III	I-evidence
,	I-evidence
IV	I-evidence
and	I-evidence
V	I-evidence
(	O
this	O
work	O
).	O

The	O
structures	B-evidence
are	O
colored	O
as	O
in	O
Figure	O
1	O
.	O

(	O
a	O
)	O
Comparison	O
of	O
the	O
40S	B-complex_assembly
-	O
subunit	B-structure_element
rotational	O
states	O
in	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
,	O
sampling	O
a	O
~	O
10	O
°	O
range	O
between	O
Structure	B-evidence
I	I-evidence
(	O
fully	B-protein_state
rotated	I-protein_state
)	O
and	O
Structure	B-evidence
V	I-evidence
(	O
non	B-protein_state
-	I-protein_state
rotated	I-protein_state
).	O

18S	B-chemical
ribosomal	I-chemical
RNA	I-chemical
is	O
shown	O
and	O
ribosomal	O
proteins	O
are	O
omitted	O
for	O
clarity	O
.	O

The	O
superpositions	B-experimental_method
of	O
Structures	B-evidence
I	I-evidence
-	I-evidence
V	I-evidence
were	O
performed	O
by	O
structural	B-experimental_method
alignments	I-experimental_method
of	O
the	O
25S	B-chemical
ribosomal	I-chemical
RNAs	I-chemical
.	O

(	O
b	O
)	O
Bar	O
graph	O
of	O
the	O
angles	O
characterizing	O
the	O
40S	B-complex_assembly
rotational	O
and	O
40S	B-complex_assembly
head	B-structure_element
swiveling	O
states	O
in	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
.	O
Measurements	O
for	O
the	O
two	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
(	O
INIT	B-complex_assembly
)	O
structures	B-evidence
are	O
included	O
for	O
comparison	O
.	O

(	O
c	O
)	O
Comparison	O
of	O
the	O
40S	B-complex_assembly
conformations	O
in	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
shows	O
distinct	O
positions	O
of	O
the	O
head	B-structure_element
relative	O
to	O
the	O
body	B-structure_element
of	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
(	O
head	B-structure_element
swivel	O
).	O

(	O
d	O
)	O
Comparison	O
of	O
conformations	O
of	O
the	O
L1	B-structure_element
and	O
P	B-structure_element
stalks	I-structure_element
of	O
the	O
large	B-structure_element
subunit	I-structure_element
in	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
with	O
those	O
in	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
and	O
tRNA	B-protein_state
-	I-protein_state
bound	I-protein_state
80S	B-complex_assembly
structures	B-evidence
.	O

Superpositions	B-experimental_method
were	O
performed	O
by	O
structural	B-experimental_method
alignments	I-experimental_method
of	O
25S	B-chemical
ribosomal	I-chemical
RNAs	I-chemical
.	O

The	O
central	B-structure_element
protuberance	I-structure_element
(	O
CP	B-structure_element
)	O
is	O
labeled	O
.	O

(	O
e	O
)	O
Bar	O
graph	O
of	O
the	O
positions	O
of	O
PKI	B-structure_element
and	O
domain	O
IV	B-structure_element
of	O
eEF2	B-protein
relative	O
to	O
the	O
P	B-site
site	I-site
residues	O
of	O
the	O
head	B-structure_element
(	O
U1191	B-residue_name_number
)	O
and	O
body	B-structure_element
(	O
C1637	B-residue_name_number
)	O
in	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
.	O
(	O
f	O
and	O
g	O
)	O
Close	O
-	O
up	O
view	O
of	O
rearrangements	O
in	O
the	O
A	B-site
and	I-site
P	I-site
sites	I-site
from	O
the	O
initiation	B-protein_state
state	O
(	O
INIT	B-complex_assembly
:	O
PDB	O
ID	O
3J6Y	O
)	O
to	O
the	O
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
Structure	B-evidence
V	I-evidence
.	O
The	O
fragment	O
shown	O
within	O
a	O
rectangle	O
in	O
panel	O
f	O
is	O
magnified	O
in	O
panel	O
g	O
.	O
Nucleotides	O
of	O
the	O
40S	B-complex_assembly
body	B-structure_element
are	O
shown	O
in	O
orange	O
,	O
40S	B-complex_assembly
head	B-structure_element
in	O
yellow	O
.	O

The	O
superpositions	B-experimental_method
of	O
structures	B-evidence
were	O
performed	O
by	O
structural	B-experimental_method
alignments	I-experimental_method
of	O
the	O
18S	B-chemical
ribosomal	I-chemical
RNAs	I-chemical
excluding	O
the	O
head	B-structure_element
region	O
(	O
nt	O
1150	B-residue_range
–	I-residue_range
1620	I-residue_range
).	O

We	O
numbered	O
the	O
structures	B-evidence
from	I-evidence
I	I-evidence
to	I-evidence
V	I-evidence
,	O
according	O
to	O
the	O
position	O
of	O
the	O
tRNA	B-complex_assembly
-	I-complex_assembly
mRNA	I-complex_assembly
-	O
like	O
PKI	B-structure_element
on	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
(	O
Figure	O
2	O
—	O
source	O
data	O
1	O
).	O

Specifically	O
,	O
PKI	B-structure_element
is	O
partially	O
withdrawn	O
from	O
the	O
A	B-site
site	I-site
in	O
Structure	B-evidence
I	I-evidence
,	O
and	O
fully	B-protein_state
translocated	I-protein_state
to	O
the	O
P	B-site
site	I-site
in	O
Structure	B-evidence
V	I-evidence
(	O
Figure	O
4	O
;	O
see	O
also	O
Figure	O
3	O
—	O
figure	O
supplement	O
1	O
).	O

Thus	O
Structures	B-evidence
I	I-evidence
to	I-evidence
IV	I-evidence
represent	O
different	O
positions	O
of	O
PKI	B-structure_element
between	O
the	O
A	B-site
and	I-site
P	I-site
sites	I-site
(	O
Figure	O
2	O
—	O
source	O
data	O
1	O
),	O
suggesting	O
that	O
these	O
structures	B-evidence
describe	O
intermediate	O
states	O
of	O
translocation	O
.	O

Structure	B-evidence
V	I-evidence
corresponds	O
to	O
the	O
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
state	O
.	O

From	O
Structure	B-evidence
I	I-evidence
to	I-evidence
V	I-evidence
,	O
the	O
body	B-structure_element
of	O
the	O
small	B-structure_element
subunit	I-structure_element
undergoes	O
backward	O
(	O
reverse	O
)	O
rotation	O
(	O
Figure	O
2b	O
;	O
see	O
also	O
Figure	O
1	O
—	O
figure	O
supplement	O
2	O
and	O
Figure	O
2	O
—	O
figure	O
supplement	O
1	O
).	O

Structures	B-evidence
II	I-evidence
and	I-evidence
III	I-evidence
are	O
in	O
mid	B-protein_state
-	I-protein_state
rotation	I-protein_state
conformations	O
(~	O
5	O
°).	O

The	O
pattern	O
of	O
40S	B-complex_assembly
head	B-structure_element
swivel	O
between	O
the	O
structures	B-evidence
is	O
different	O
from	O
that	O
of	O
intersubunit	O
rotation	O
(	O
Figures	O
2c	O
and	O
d	O
;	O
see	O
also	O
Figure	O
2	O
—	O
source	O
data	O
1	O
).	O

The	O
maximum	O
head	B-structure_element
swivel	O
is	O
observed	O
in	O
the	O
mid	B-protein_state
-	I-protein_state
rotated	I-protein_state
complexes	O
II	B-evidence
and	I-evidence
III	I-evidence
,	O
in	O
which	O
PKI	B-structure_element
transitions	O
from	O
the	O
A	B-site
to	I-site
P	I-site
site	I-site
,	O
while	O
eEF2	B-protein
occupies	O
the	O
A	B-site
site	I-site
partially	O
.	O

These	O
observations	O
suggest	O
that	O
eEF2	B-protein
is	O
necessary	O
for	O
inducing	O
or	O
stabilizing	O
the	O
large	O
head	B-structure_element
swivel	O
of	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
characteristic	O
for	O
IRES	B-site
translocation	O
intermediates	O
.	O

IRES	B-site
rearrangements	O

Comparison	O
of	O
the	O
TSV	B-species
IRES	B-site
and	O
eEF2	B-protein
positions	O
in	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
.	O

(	O
a	O
)	O
Positions	O
of	O
the	O
IRES	B-site
and	O
eEF2	B-protein
in	O
the	O
initiation	B-protein_state
,	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
(	O
I	B-evidence
)	O
and	O
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
(	O
V	B-evidence
)	O
states	O
,	O
relative	O
to	O
the	O
body	B-structure_element
of	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
(	O
not	O
shown	O
)	O
(	O
b	O
)	O
Positions	O
of	O
the	O
IRES	B-site
and	O
eEF2	B-protein
in	O
the	O
initiation	B-protein_state
state	O
(	O
INIT	B-complex_assembly
)	O
and	O
intermediate	O
steps	O
of	O
translocation	O
(	O
II	B-evidence
,	I-evidence
III	I-evidence
and	I-evidence
IV	I-evidence
),	O
relative	O
to	O
the	O
body	B-structure_element
of	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
(	O
not	O
shown	O
).	O

Positions	O
of	O
the	O
IRES	B-site
relative	O
to	O
proteins	O
uS7	B-protein
,	O
uS11	B-protein
and	O
eS25	B-protein
.	O

(	O
a	O
)	O
Intra	O
-	O
IRES	B-site
rearrangements	O
from	O
the	O
80S	B-complex_assembly
*	I-complex_assembly
IRES	I-complex_assembly
initiation	B-protein_state
structure	B-evidence
(	O
INIT	B-complex_assembly
;	O
PDB	O
3J6Y	O
,)	O
to	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
.	O
For	O
each	O
structure	B-evidence
(	O
shown	O
in	O
red	O
),	O
the	O
conformation	O
from	O
a	O
preceding	O
structure	B-evidence
is	O
shown	O
in	O
light	O
red	O
for	O
comparison	O
.	O

Superpositions	B-experimental_method
were	O
obtained	O
by	O
structural	B-experimental_method
alignments	I-experimental_method
of	O
18S	B-chemical
rRNA	I-chemical
.	O

(	O
b	O
)	O
Positions	O
of	O
the	O
IRES	B-site
and	O
eEF2	B-protein
relative	O
to	O
those	O
of	O
classical	O
P	B-site
-	I-site
and	I-site
E	I-site
-	I-site
site	I-site
tRNAs	B-chemical
in	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
tRNA	I-complex_assembly
complex	O
.	O
(	O
c	O
)	O
Positions	O
of	O
the	O
IRES	B-site
relative	O
to	O
proteins	O
uS11	B-protein
(	O
40S	B-site
platform	I-site
)	O
and	O
uS7	B-protein
and	O
eS25	B-protein
(	O
40S	B-complex_assembly
head	B-structure_element
),	O
which	O
interact	O
with	O
the	O
5	B-structure_element
′	I-structure_element
domain	I-structure_element
of	O
the	O
IRES	B-site
in	O
the	O
initiation	B-protein_state
state	O
(	O
left	O
panel	O
).	O

In	O
all	O
panels	O
,	O
superpositions	B-experimental_method
were	O
obtained	O
by	O
structural	B-experimental_method
alignments	I-experimental_method
of	O
the	O
18S	B-chemical
rRNAs	I-chemical
.	O

Ribosomal	O
proteins	O
of	O
the	O
initiation	B-protein_state
state	O
are	O
shown	O
in	O
gray	O
for	O
comparison	O
.	O

Positions	O
of	O
the	O
L1stalk	B-structure_element
,	O
tRNA	B-chemical
and	O
TSV	B-species
IRES	B-site
relative	O
to	O
proteins	O
uS7	B-protein
and	O
eS25	B-protein
,	O
in	O
80S	B-complex_assembly
•	I-complex_assembly
tRNA	I-complex_assembly
structures	B-evidence
and	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
structures	B-evidence
I	I-evidence
and	I-evidence
V	I-evidence
(	O
this	O
work	O
).	O

Loop	B-structure_element
1	I-structure_element
.	I-structure_element
1	I-structure_element
and	O
stem	B-structure_element
loops	I-structure_element
4	I-structure_element
and	I-structure_element
5	I-structure_element
of	O
the	O
IRES	B-site
are	O
labeled	O
.	O

2668	B-residue_range
–	I-residue_range
2687	I-residue_range
)	O
and	O
protein	O
uL5	B-protein
(	O
collectively	O
labeled	O
as	O
central	B-structure_element
protuberance	I-structure_element
,	O
CP	B-structure_element
,	O
in	O
the	O
upper	O
-	O
row	O
first	O
figure	O
,	O
and	O
individually	O
labeled	O
in	O
the	O
lower	O
-	O
row	O
first	O
figure	O
).	O

Structures	B-evidence
of	O
translocation	O
complexes	O
of	O
the	O
bacterial	B-taxonomy_domain
70S	B-complex_assembly
ribosome	I-complex_assembly
bound	B-protein_state
with	I-protein_state
two	O
tRNAs	B-chemical
and	O
yeast	B-taxonomy_domain
80S	B-complex_assembly
complexes	B-protein_state
with	I-protein_state
tRNAs	B-chemical
are	O
shown	O
in	O
the	O
upper	O
row	O
and	O
labeled	O
.	O

Structures	B-evidence
of	O
the	O
80S	B-complex_assembly
complexes	B-protein_state
with	I-protein_state
tRNAs	B-chemical
are	O
shown	O
in	O
the	O
lower	O
row	O
in	O
a	O
view	O
similar	O
to	O
that	O
for	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
complex	O
.	O

(	O
a	O
)	O
Secondary	O
structure	B-evidence
of	O
the	O
TSV	B-species
IRES	B-site
.	O

The	O
TSV	B-species
IRES	B-site
comprises	O
two	O
domains	O
:	O
the	O
5	B-structure_element
'	I-structure_element
domain	I-structure_element
(	O
blue	O
)	O
and	O
the	O
PKI	B-structure_element
domain	O
(	O
red	O
).	O

The	O
open	B-structure_element
reading	I-structure_element
frame	I-structure_element
(	O
gray	O
)	O
is	O
immediately	O
following	O
pseudoknot	B-structure_element
I	I-structure_element
(	O
PKI	B-structure_element
).	O

(	O
b	O
)	O
Three	O
-	O
dimensional	O
structure	B-evidence
of	O
the	O
TSV	B-species
IRES	B-site
(	O
Structure	B-evidence
II	I-evidence
).	O

(	O
c	O
)	O
Positions	O
of	O
the	O
IRES	B-site
and	O
eEF2	B-protein
on	O
the	O
small	B-structure_element
subunit	I-structure_element
in	O
Structures	B-evidence
I	I-evidence
to	I-evidence
V	I-evidence
.	O
The	O
initiation	B-protein_state
-	O
state	O
IRES	B-site
is	O
shown	O
in	O
gray	O
.	O

The	O
insert	O
shows	O
density	O
for	O
interaction	O
of	O
diphthamide	O
699	O
(	B-protein
eEF2	O
;	O
green	O
)	O
with	O
the	O
codon	O
-	O
anticodon	O
-	O
like	O
helix	O
(	B-structure_element
PKI	O
;	O
red	O
)	O
in	O
Structure	O
V	O
.	O
(	O
d	O
and	O
e	O
)	O
Density	O
of	O
the	O
P	O
site	O
in	O
Structure	O
V	O
shows	O
that	O
interactions	O
of	O
PKI	O
with	O
the	O
18S	O
rRNA	O
nucleotides	O
(	O
c	O
)	O
are	O
nearly	O
identical	O
to	O
those	O
in	O
the	O
P	O
site	O
of	O
the	O
2tRNA	O
•	O
mRNA	O
-	O
bound	O
70S	O
ribosome	O
(	O
d	O
).	O

In	O
each	O
structure	B-evidence
,	O
the	O
TSV	B-species
IRES	B-site
adopts	O
a	O
distinct	O
conformation	O
in	O
the	O
intersubunit	O
space	O
of	O
the	O
ribosome	B-complex_assembly
(	O
Figures	O
3	O
and	O
4	O
).	O

The	O
IRES	B-site
(	O
nt	O
6758	B-residue_range
–	I-residue_range
6952	I-residue_range
)	O
consists	O
of	O
two	O
globular	O
parts	O
(	O
Figure	O
3a	O
):	O
the	O
5	B-structure_element
’-	I-structure_element
region	I-structure_element
(	O
domains	O
I	B-structure_element
and	O
II	B-structure_element
,	O
nt	O
6758	B-residue_range
–	I-residue_range
6888	I-residue_range
)	O
and	O
the	O
PKI	B-structure_element
domain	O
(	O
domain	O
III	B-structure_element
,	O
nt	O
6889	B-residue_range
–	I-residue_range
6952	I-residue_range
).	O

The	O
PKI	B-structure_element
domain	O
comprises	O
PKI	B-structure_element
and	O
stem	B-structure_element
loop	I-structure_element
3	I-structure_element
(	O
SL3	B-structure_element
),	O
which	O
stacks	O
on	O
top	O
of	O
the	O
stem	O
of	O
PKI	B-structure_element
.	O

The	O
6953GCU	O
triplet	O
immediately	O
following	O
the	O
PKI	B-structure_element
domain	O
is	O
the	O
first	O
codon	O
of	O
the	O
open	B-structure_element
reading	I-structure_element
frame	I-structure_element
.	O

In	O
the	O
eEF2	B-protein_state
-	I-protein_state
free	I-protein_state
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
initiation	B-protein_state
complex	O
(	O
INIT	B-complex_assembly
),	O
the	O
bulk	O
of	O
the	O
5	B-structure_element
’-	I-structure_element
domain	I-structure_element
(	O
nt	O
.	O

In	O
Structures	B-evidence
I	I-evidence
to	I-evidence
IV	I-evidence
,	O
these	O
contacts	O
remain	O
as	O
in	O
the	O
initiation	B-complex_assembly
complex	I-complex_assembly
(	O
Figure	O
1a	O
).	O

Specifically	O
,	O
the	O
L1	B-structure_element
.	I-structure_element
1	I-structure_element
region	I-structure_element
interacts	O
with	O
the	O
L1	B-structure_element
stalk	I-structure_element
of	O
the	O
large	B-structure_element
subunit	I-structure_element
,	O
while	O
SL4	B-structure_element
and	O
SL5	B-structure_element
bind	O
at	O
the	O
side	O
of	O
the	O
40S	B-complex_assembly
head	B-structure_element
and	O
interact	O
with	O
proteins	O
uS7	B-protein
,	O
uS11	B-protein
and	O
eS25	B-protein
(	O
Figure	O
3	O
—	O
figure	O
supplement	O
2	O
and	O
Figure	O
3	O
—	O
figure	O
supplement	O
3	O
;	O
ribosomal	O
proteins	O
are	O
termed	O
according	O
to	O
).	O

In	O
Structures	B-evidence
I	I-evidence
-	I-evidence
IV	I-evidence
,	O
the	O
minor	B-site
groove	I-site
of	O
SL4	B-structure_element
(	O
at	O
nt	O
6840	B-residue_range
–	I-residue_range
6846	I-residue_range
)	O
binds	O
next	O
to	O
an	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
of	O
uS7	B-protein
,	O
which	O
is	O
rich	O
in	O
positively	O
charged	O
residues	O
(	O
K212	B-residue_name_number
,	O
K213	B-residue_name_number
,	O
R219	B-residue_name_number
and	O
K222	B-residue_name_number
).	O

The	O
tip	O
of	O
SL4	B-structure_element
binds	O
in	O
the	O
vicinity	O
of	O
R157	B-residue_name_number
in	O
the	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
of	O
uS7	B-protein
and	O
of	O
Y58	B-residue_name_number
in	O
uS11	B-protein
.	O

In	O
Structure	B-evidence
V	I-evidence
,	O
however	O
,	O
the	O
density	B-evidence
for	O
SL5	B-structure_element
is	O
missing	O
suggesting	O
that	O
SL5	B-structure_element
is	O
mobile	B-protein_state
,	O
while	O
weak	O
SL4	B-structure_element
density	B-evidence
suggests	O
that	O
SL4	B-structure_element
is	O
shifted	O
along	O
the	O
surface	O
of	O
uS7	B-protein
,	O
~	O
20	O
Å	O
away	O
from	O
its	O
initial	O
position	O
(	O
Figure	O
3	O
—	O
figure	O
supplement	O
2c	O
).	O

The	O
L1	B-structure_element
.	I-structure_element
1	I-structure_element
region	I-structure_element
remains	O
in	O
contact	O
with	O
the	O
L1	B-structure_element
stalk	I-structure_element
(	O
Figure	O
3	O
—	O
figure	O
supplement	O
3	O
).	O

Inchworm	B-protein_state
-	O
like	O
translocation	O
of	O
the	O
TSV	B-species
IRES	B-site
.	O

The	O
shape	O
of	O
the	O
IRES	B-site
changes	O
considerably	O
from	O
the	O
initiation	B-protein_state
state	O
to	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
,	O
from	O
an	O
extended	B-protein_state
to	O
compact	B-protein_state
to	O
extended	B-protein_state
conformation	O
(	O
Figure	O
4	O
;	O
see	O
also	O
Figure	O
3	O
—	O
figure	O
supplement	O
2a	O
).	O

Because	O
in	O
Structures	B-evidence
I	I-evidence
to	I-evidence
IV	I-evidence
the	O
PKI	B-structure_element
domain	O
shifts	O
toward	O
the	O
P	B-site
site	I-site
,	O
while	O
the	O
5	O
’	O
remains	O
unchanged	O
near	O
the	O
E	B-site
site	I-site
,	O
the	O
distance	O
between	O
the	O
domains	O
shortens	O
(	O
Figure	O
4	O
).	O

In	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
initiation	B-protein_state
state	O
,	O
the	O
A	B-protein_state
-	I-protein_state
site	I-protein_state
-	I-protein_state
bound	I-protein_state
PKI	B-structure_element
is	O
separated	O
from	O
SL4	B-structure_element
by	O
almost	O
50	O
Å	O
(	O
Figure	O
4	O
).	O

In	O
Structures	B-evidence
I	I-evidence
and	I-evidence
II	I-evidence
,	O
the	O
PKI	B-structure_element
is	O
partially	O
retracted	O
from	O
the	O
A	B-site
site	I-site
and	O
the	O
distance	O
from	O
SL4	B-structure_element
shortens	O
to	O
~	O
35	O
Å	O
.	O
As	O
PKI	B-structure_element
moves	O
toward	O
the	O
P	B-site
site	I-site
in	O
Structures	B-evidence
III	I-evidence
and	I-evidence
IV	I-evidence
,	O
the	O
PKI	B-structure_element
domain	O
approaches	O
to	O
within	O
~	O
25	O
Å	O
of	O
SL4	B-structure_element
.	O

Because	O
the	O
5	B-structure_element
’-	I-structure_element
domain	I-structure_element
in	O
the	O
following	O
structure	B-evidence
(	I-evidence
V	I-evidence
)	I-evidence
moves	O
by	O
~	O
20	O
Å	O
along	O
the	O
40S	B-complex_assembly
head	B-structure_element
,	O
the	O
IRES	B-site
returns	O
to	O
an	O
extended	B-protein_state
conformation	O
(~	O
45	O
Å	O
)	O
that	O
is	O
similar	O
to	O
that	O
in	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
initiation	B-protein_state
complex	O
.	O

2668	B-residue_range
–	I-residue_range
2687	I-residue_range
)	O
and	O
protein	O
uL5	B-protein
(	O
Figure	O
3	O
—	O
figure	O
supplement	O
6	O
).	O

This	O
position	O
of	O
SL3	B-structure_element
is	O
~	O
25	O
Å	O
away	O
from	O
that	O
in	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
initiation	B-protein_state
state	O
,	O
in	O
which	O
PKI	B-structure_element
and	O
SL3	B-structure_element
closely	O
mimic	O
the	O
ASL	B-structure_element
and	O
elbow	B-structure_element
of	O
the	O
A	B-site
-	I-site
site	I-site
tRNA	B-chemical
,	O
respectively	O
.	O

As	O
such	O
,	O
the	O
transition	O
from	O
the	O
initiation	B-protein_state
state	O
to	O
Structure	B-evidence
I	I-evidence
involves	O
repositioning	O
of	O
SL3	B-structure_element
around	O
the	O
A	B-structure_element
-	I-structure_element
site	I-structure_element
finger	I-structure_element
,	O
resembling	O
the	O
transition	O
between	O
the	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
A	B-site
/	I-site
P	I-site
and	O
A	B-site
/	I-site
P	I-site
*	I-site
tRNA	B-chemical
.	O

The	O
second	O
set	O
of	O
major	O
structural	O
changes	O
involves	O
interaction	O
of	O
the	O
P	B-site
site	I-site
region	I-site
of	O
the	O
large	B-structure_element
subunit	I-structure_element
with	O
the	O
hinge	B-structure_element
point	I-structure_element
of	O
the	O
IRES	B-site
bending	O
between	O
the	O
5	B-structure_element
´	I-structure_element
domain	I-structure_element
and	O
the	O
PKI	B-structure_element
domain	O
(	O
nt	O
.	O
6886	B-residue_range
–	I-residue_range
6890	I-residue_range
).	O

In	O
the	O
highly	B-protein_state
bent	I-protein_state
Structures	B-evidence
III	I-evidence
and	I-evidence
IV	I-evidence
,	O
the	O
hinge	B-structure_element
region	I-structure_element
interacts	O
with	O
the	O
universally	B-protein_state
conserved	I-protein_state
uL5	B-protein
and	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
tail	I-structure_element
of	O
eL42	B-protein
(	O
Figure	O
3	O
—	O
figure	O
supplement	O
7	O
).	O

Another	O
local	O
rearrangement	O
concerns	O
loop	B-structure_element
3	I-structure_element
,	O
also	O
known	O
as	O
the	O
variable	B-structure_element
loop	I-structure_element
region	I-structure_element
,	O
which	O
connects	O
the	O
ASL	B-structure_element
-	I-structure_element
and	I-structure_element
mRNA	I-structure_element
-	I-structure_element
like	I-structure_element
parts	I-structure_element
of	O
PKI	B-structure_element
.	O

This	O
loop	B-structure_element
is	O
poorly	O
resolved	O
in	O
Structures	B-evidence
I	I-evidence
through	I-evidence
IV	I-evidence
,	O
suggesting	O
conformational	O
flexibility	O
in	O
agreement	O
with	O
structural	B-experimental_method
studies	I-experimental_method
of	O
the	O
isolated	B-protein_state
PKI	B-structure_element
and	O
biochemical	B-experimental_method
studies	I-experimental_method
of	O
unbound	B-protein_state
IRESs	B-site
.	O

6945	B-residue_range
–	I-residue_range
6946	I-residue_range
)	O
interacts	O
with	O
R148	B-residue_name_number
and	O
R157	B-residue_name_number
in	O
β	B-structure_element
-	I-structure_element
hairpin	I-structure_element
of	O
uS7	B-protein
.	O

This	O
interpretation	O
is	O
consistent	O
with	O
the	O
recent	O
observation	O
that	O
alterations	O
in	O
loop	B-structure_element
3	I-structure_element
of	O
the	O
CrPV	B-species
IRES	B-site
result	O
in	O
decreased	O
efficiency	O
of	O
translocation	O
.	O

eEF2	B-protein
structures	B-evidence

Elements	O
of	O
the	O
80S	B-complex_assembly
ribosome	I-complex_assembly
that	O
contact	O
eEF2	B-protein
in	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
.	O

The	O
view	O
and	O
colors	O
are	O
as	O
in	O
Figure	O
5b	O
:	O
eEF2	B-protein
is	O
shown	O
in	O
green	O
,	O
IRES	B-site
RNA	B-chemical
in	O
red	O
,	O
40S	B-complex_assembly
subunit	B-structure_element
elements	O
in	O
orange	O
,	O
60S	B-complex_assembly
in	O
cyan	O
/	O
teal	O
.	O

The	O
switch	B-structure_element
loop	I-structure_element
I	I-structure_element
in	O
Structure	B-evidence
I	I-evidence
is	O
shown	O
in	O
blue	O
.	O

The	O
putative	O
position	O
of	O
the	O
switch	B-structure_element
loop	I-structure_element
I	I-structure_element
,	O
unresolved	O
in	O
the	O
density	B-evidence
of	O
Structure	B-evidence
II	I-evidence
,	O
is	O
shown	O
with	O
a	O
dashed	O
line	O
.	O

Conformations	O
and	O
interactions	O
of	O
eEF2	B-protein
.	O

(	O
a	O
)	O
Conformations	O
of	O
eEF2	B-protein
in	O
Structures	B-evidence
I	I-evidence
-	I-evidence
V	I-evidence
and	O
domain	O
organization	O
of	O
eEF2	B-protein
are	O
shown	O
.	O

Roman	O
numerals	O
denote	O
eEF2	B-protein
domains	O
.	O

Superposition	B-experimental_method
was	O
obtained	O
by	O
structural	B-experimental_method
alignment	I-experimental_method
of	O
domains	O
I	B-structure_element
and	O
II	B-structure_element
.	O

(	O
b	O
)	O
Elements	O
of	O
the	O
80S	B-complex_assembly
ribosome	I-complex_assembly
in	O
Structures	B-evidence
I	I-evidence
and	I-evidence
V	I-evidence
that	O
contact	O
eEF2	B-protein
.	O

eEF2	B-protein
is	O
shown	O
in	O
green	O
,	O
IRES	B-site
RNA	B-chemical
in	O
red	O
,	O
40S	B-complex_assembly
subunit	B-structure_element
elements	O
in	O
orange	O
,	O
60S	B-complex_assembly
in	O
cyan	O
/	O
teal	O
.	O

(	O
c	O
)	O
Comparison	O
of	O
conformations	O
of	O
eEF2	B-complex_assembly
•	I-complex_assembly
sordarin	I-complex_assembly
in	O
Structure	B-evidence
I	I-evidence
(	O
light	O
green	O
)	O
with	O
those	O
of	O
free	B-protein_state
apo	B-protein_state
-	O
eEF2	B-protein
(	O
magenta	O
)	O
and	O
eEF2	B-complex_assembly
•	I-complex_assembly
sordarin	I-complex_assembly
(	O
teal	O
).	O

(	O
d	O
)	O
Interactions	O
of	O
the	O
GTPase	B-structure_element
domains	I-structure_element
with	O
the	O
40S	B-complex_assembly
and	O
60S	B-complex_assembly
subunits	B-structure_element
in	O
Structure	B-evidence
I	I-evidence
(	O
colored	O
in	O
green	O
/	O
blue	O
,	O
eEF2	B-protein
;	O
orange	O
,	O
40S	B-complex_assembly
;	O
cyan	O
/	O
teal	O
,	O
60S	B-complex_assembly
)	O
and	O
in	O
Structure	B-evidence
II	I-evidence
(	O
gray	O
).	O

(	O
e	O
)	O
Comparison	O
of	O
the	O
GTP	B-protein_state
-	I-protein_state
like	I-protein_state
conformation	O
of	O
eEF2	B-complex_assembly
•	I-complex_assembly
GDP	I-complex_assembly
in	O
Structure	B-evidence
I	I-evidence
(	O
light	O
green	O
)	O
with	O
those	O
of	O
70S	B-protein_state
-	I-protein_state
bound	I-protein_state
elongation	B-protein_type
factors	I-protein_type
EF	B-complex_assembly
-	I-complex_assembly
Tu	I-complex_assembly
•	I-complex_assembly
GDPCP	I-complex_assembly
(	O
teal	O
)	O
and	O
EF	B-complex_assembly
-	I-complex_assembly
G	I-complex_assembly
•	I-complex_assembly
GDP	I-complex_assembly
•	I-complex_assembly
fusidic	I-complex_assembly
acid	I-complex_assembly
(	O
magenta	O
;	O
fusidic	O
acid	O
not	O
shown	O
).	O
(	O
f	O
)	O
Cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
density	B-evidence
showing	O
guanosine	B-chemical
diphosphate	I-chemical
bound	B-protein_state
in	I-protein_state
the	O
GTPase	B-site
center	I-site
(	O
green	O
)	O
next	O
to	O
the	O
sarcin	B-structure_element
-	I-structure_element
ricin	I-structure_element
loop	I-structure_element
of	O
25S	B-chemical
rRNA	I-chemical
(	O
cyan	O
)	O
of	O
Structure	B-evidence
II	I-evidence
.	O
(	O
g	O
)	O
Comparison	O
of	O
the	O
sordarin	B-site
-	I-site
binding	I-site
sites	I-site
in	O
the	O
ribosome	B-protein_state
-	I-protein_state
bound	I-protein_state
(	O
light	O
green	O
;	O
Structure	B-evidence
II	I-evidence
)	O
and	O
isolated	O
eEF2	B-protein
(	O
teal	O
).	O

(	O
h	O
)	O
Cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
density	B-evidence
showing	O
the	O
sordarin	B-site
-	I-site
binding	I-site
pocket	I-site
of	O
eEF2	B-protein
(	O
Structure	B-evidence
II	I-evidence
).	O

The	O
elongation	B-protein_type
factor	I-protein_type
consists	O
of	O
three	O
dynamic	O
superdomains	B-structure_element
:	O
an	O
N	O
-	O
terminal	O
globular	O
superdomain	B-structure_element
formed	O
by	O
the	O
G	B-structure_element
(	I-structure_element
GTPase	I-structure_element
)	I-structure_element
domain	I-structure_element
(	O
domain	O
I	B-structure_element
)	O
and	O
domain	O
II	B-structure_element
;	O
a	O
linker	B-structure_element
domain	I-structure_element
III	I-structure_element
;	O
and	O
a	O
C	O
-	O
terminal	O
superdomain	B-structure_element
comprising	O
domains	O
IV	B-structure_element
and	O
V	B-structure_element
(	O
Figure	O
5a	O
).	O

Domain	O
IV	B-structure_element
extends	O
from	O
the	O
main	O
body	B-structure_element
and	O
is	O
critical	O
for	O
translocation	O
catalyzed	O
by	O
eEF2	B-protein
or	O
EF	B-protein
-	I-protein
G	I-protein
.	O
ADP	B-ptm
-	I-ptm
ribosylation	I-ptm
of	O
eEF2	B-protein
at	O
the	O
tip	O
of	O
domain	O
IV	B-structure_element
or	O
deletion	B-experimental_method
of	O
domain	O
IV	B-structure_element
from	O
EF	B-protein
-	I-protein
G	I-protein
abrogate	O
translocation	O
.	O

In	O
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
-	O
like	O
80S	B-complex_assembly
•	I-complex_assembly
tRNA	I-complex_assembly
•	I-complex_assembly
eEF2	I-complex_assembly
complexes	O
,	O
domain	O
IV	B-structure_element
binds	O
in	O
the	O
40S	B-complex_assembly
A	B-site
site	I-site
,	O
suggesting	O
direct	O
involvement	O
of	O
domain	O
IV	B-structure_element
in	O
translocation	O
of	O
tRNA	B-chemical
from	O
the	O
A	B-site
to	I-site
P	I-site
site	I-site
.	O

The	O
global	O
conformations	O
of	O
eEF2	B-protein
(	O
Figure	O
5a	O
)	O
are	O
similar	O
in	O
these	O
structures	B-evidence
(	O
all	O
-	O
atom	O
RMSD	B-evidence
≤	O
2	O
Å	O
),	O
but	O
the	O
positions	O
of	O
eEF2	B-protein
relative	O
to	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
differ	O
substantially	O
as	O
a	O
result	O
of	O
40S	B-complex_assembly
subunit	B-structure_element
rotation	O
(	O
Figure	O
2	O
—	O
source	O
data	O
1	O
).	O

Although	O
the	O
P	B-structure_element
/	I-structure_element
L11	I-structure_element
stalk	I-structure_element
is	O
known	O
to	O
be	O
dynamic	O
,	O
its	O
position	O
remains	O
unchanged	O
from	O
Structure	B-evidence
I	I-evidence
to	I-evidence
V	I-evidence
:	O
all	O
-	O
atom	O
root	B-evidence
-	I-evidence
mean	I-evidence
-	I-evidence
square	I-evidence
differences	I-evidence
for	O
the	O
25S	B-chemical
rRNA	I-chemical
of	O
the	O
P	B-structure_element
stalk	I-structure_element
(	O
nt	O
1223	B-residue_range
–	I-residue_range
1286	I-residue_range
)	O
are	O
within	O
2	O
.	O
5	O
Å	O
.	O
However	O
,	O
with	O
respect	O
to	O
its	O
position	O
in	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
complex	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
eEF2	B-protein
and	O
in	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
2tRNA	I-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
complex	O
,	O
the	O
P	B-structure_element
stalk	I-structure_element
is	O
shifted	O
by	O
~	O
13	O
Å	O
toward	O
the	O
A	B-site
site	I-site
(	O
Figure	O
2d	O
).	O

The	O
sarcin	B-structure_element
-	I-structure_element
ricin	I-structure_element
loop	I-structure_element
interacts	O
with	O
the	O
GTP	B-site
-	I-site
binding	I-site
site	I-site
of	O
eEF2	B-protein
(	O
Figures	O
5d	O
and	O
f	O
).	O

Repositioning	O
(	O
sliding	O
)	O
of	O
the	O
positively	B-site
-	I-site
charged	I-site
cluster	I-site
of	O
domain	O
IV	B-structure_element
of	O
eEF2	B-protein
over	O
the	O
phosphate	O
backbone	O
(	O
red	O
)	O
of	O
the	O
18S	B-structure_element
helices	I-structure_element
33	I-structure_element
and	I-structure_element
34	I-structure_element
.	O

Electrostatic	O
surface	O
of	O
eEF2	B-protein
is	O
shown	O
;	O
negatively	O
and	O
positively	O
charged	O
regions	O
are	O
shown	O
in	O
red	O
and	O
blue	O
,	O
respectively	O
.	O

Interactions	O
of	O
eEF2	B-protein
with	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
.	O

(	O
c	O
)	O
Rearrangements	O
,	O
from	O
Structure	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
,	O
of	O
a	O
positively	O
charged	O
cluster	O
of	O
eEF2	B-protein
(	O
K613	B-residue_name_number
,	O
R617	B-residue_name_number
and	O
R631	B-residue_name_number
)	O
positioned	O
over	O
the	O
phosphate	O
backbone	O
of	O
18S	B-structure_element
helices	I-structure_element
33	I-structure_element
and	I-structure_element
34	I-structure_element
,	O
suggesting	O
a	O
role	O
of	O
electrostatic	B-bond_interaction
interactions	I-bond_interaction
in	O
eEF2	B-protein
diffusion	O
over	O
the	O
40S	B-complex_assembly
surface	O
.	O

(	O
d	O
)	O
Shift	O
of	O
the	O
tip	O
of	O
domain	O
III	B-structure_element
of	O
eEF2	B-protein
,	O
interacting	O
with	O
uS12	B-protein
upon	O
reverse	O
subunit	B-structure_element
rotation	O
from	O
Structure	B-evidence
I	I-evidence
to	I-evidence
Structure	I-evidence
V	I-evidence
.	O
Structure	B-evidence
I	I-evidence
colored	O
as	O
in	O
Figure	O
1	O
,	O
except	O
uS12	B-protein
,	O
which	O
is	O
in	O
purple	O
;	O
Structure	B-evidence
V	I-evidence
is	O
in	O
gray	O
.	O

This	O
limited	O
flexibility	O
of	O
the	O
ribosome	B-protein_state
-	I-protein_state
bound	I-protein_state
eEF2	B-protein
is	O
likely	O
the	O
result	O
of	O
simultaneous	O
fixation	O
of	O
eEF2	B-protein
superdomains	B-structure_element
,	O
via	O
domains	O
I	B-structure_element
and	O
V	B-structure_element
,	O
by	O
the	O
GTPase	B-site
-	I-site
associated	I-site
center	I-site
of	O
the	O
large	B-structure_element
subunit	I-structure_element
.	O

eEF2	B-protein
settles	O
into	O
the	O
A	B-site
site	I-site
from	O
Structure	B-evidence
I	I-evidence
to	I-evidence
V	I-evidence
,	O
as	O
the	O
tip	O
of	O
domain	O
IV	B-structure_element
shifts	O
by	O
~	O
10	O
Å	O
relative	O
to	O
the	O
body	B-structure_element
and	O
by	O
~	O
20	O
Å	O
relative	O
to	O
the	O
swiveling	O
head	B-structure_element
.	O

At	O
the	O
central	O
region	O
of	O
eEF2	B-protein
,	O
domains	O
II	B-structure_element
and	O
III	B-structure_element
contact	O
the	O
40S	B-complex_assembly
body	B-structure_element
(	O
mainly	O
at	O
nucleotides	O
48	B-residue_range
–	I-residue_range
52	I-residue_range
and	O
429	B-residue_range
–	I-residue_range
432	I-residue_range
of	O
18S	B-chemical
rRNA	I-chemical
helix	B-structure_element
5	I-structure_element
and	O
uS12	B-protein
).	O

From	O
Structure	B-evidence
I	I-evidence
to	I-evidence
V	I-evidence
,	O
these	O
central	O
domains	O
migrate	O
by	O
~	O
10	O
Å	O
along	O
the	O
40S	B-complex_assembly
surface	O
(	O
Figure	O
6c	O
).	O

Comparison	O
of	O
eEF2	B-protein
conformations	O
reveals	O
that	O
in	O
Structure	B-evidence
V	I-evidence
,	O
domain	O
III	B-structure_element
is	O
displaced	O
as	O
a	O
result	O
of	O
interaction	O
with	O
uS12	B-protein
,	O
as	O
discussed	O
below	O
.	O

In	O
summary	O
,	O
between	O
Structures	B-evidence
I	I-evidence
and	I-evidence
V	I-evidence
,	O
a	O
step	O
-	O
wise	O
translocation	O
of	O
PKI	B-structure_element
by	O
~	O
15	O
Å	O
from	O
the	O
A	B-site
to	I-site
P	I-site
site	I-site
-	O
within	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
–	O
occurs	O
simultaneously	O
with	O
the	O
~	O
11	O
Å	O
side	O
-	O
way	O
entry	O
of	O
domain	O
IV	B-structure_element
into	O
the	O
A	B-site
site	I-site
coupled	O
with	O
~	O
3	O
to	O
5	O
Å	O
inter	O
-	O
domain	O
rearrangements	O
in	O
eEF2	B-protein
.	O

These	O
shifts	O
occur	O
during	O
the	O
reverse	O
rotation	O
of	O
the	O
40S	B-complex_assembly
body	B-structure_element
coupled	O
with	O
the	O
forward	O
-	O
then	O
-	O
reverse	O
head	B-structure_element
swivel	O
.	O

To	O
elucidate	O
the	O
detailed	O
structural	O
mechanism	O
of	O
IRES	B-site
translocation	O
and	O
the	O
roles	O
of	O
eEF2	B-protein
and	O
ribosome	B-complex_assembly
rearrangements	O
,	O
we	O
describe	O
in	O
the	O
following	O
sections	O
the	O
interactions	O
of	O
PKI	B-structure_element
and	O
eEF2	B-protein
with	O
the	O
ribosomal	O
A	B-site
and	I-site
P	I-site
sites	I-site
in	O
Structures	B-evidence
I	I-evidence
through	I-evidence
V	I-evidence
(	O
Figure	O
2g	O
;	O
see	O
also	O
Figure	O
1	O
—	O
figure	O
supplement	O
1	O
).	O

Structure	B-evidence
I	I-evidence
represents	O
a	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
IRES	B-site
and	O
initial	O
entry	O
of	O
eEF2	B-protein
in	O
a	O
GTP	B-chemical
-	O
like	O
state	O

In	O
the	O
fully	B-protein_state
rotated	I-protein_state
Structure	B-evidence
I	I-evidence
,	O
PKI	B-structure_element
is	O
shifted	O
toward	O
the	O
P	B-site
site	I-site
by	O
~	O
3	O
Å	O
relative	O
to	O
its	O
position	O
in	O
the	O
initiation	B-complex_assembly
complex	I-complex_assembly
but	O
maintains	O
interactions	O
with	O
the	O
partially	B-protein_state
swiveled	I-protein_state
head	B-structure_element
.	O

The	O
C1274	B-residue_name_number
:	O
G6953	B-residue_name_number
base	O
pair	O
provides	O
a	O
stacking	B-site
platform	I-site
for	O
the	O
codon	B-structure_element
-	I-structure_element
anticodon	I-structure_element
–	I-structure_element
like	I-structure_element
helix	I-structure_element
of	O
PKI	B-structure_element
.	O

Interactions	O
of	O
the	O
residues	O
at	O
the	O
eEF2	B-protein
tip	O
with	O
the	O
decoding	B-site
center	I-site
of	O
the	O
IRES	B-protein_state
-	I-protein_state
bound	I-protein_state
ribosome	B-complex_assembly
.	O

The	O
histidine	B-site
-	I-site
diphthamide	I-site
tip	I-site
of	O
eEF2	B-protein
is	O
shown	O
in	O
green	O
.	O

A	B-site
and	I-site
P	I-site
sites	I-site
are	O
schematically	O
demarcated	O
by	O
dotted	O
lines	O
.	O

In	O
Structure	B-evidence
I	I-evidence
,	O
PKI	B-structure_element
does	O
not	O
contact	O
these	O
nucleotides	O
(	O
Figures	O
2g	O
and	O
7	O
).	O

The	O
position	O
of	O
eEF2	B-protein
on	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
of	O
Structure	B-evidence
I	I-evidence
is	O
markedly	O
distinct	O
from	O
those	O
in	O
Structures	B-evidence
II	I-evidence
to	I-evidence
V	I-evidence
.	O
The	O
translocase	B-protein_type
interacts	O
with	O
the	O
40S	B-complex_assembly
body	B-structure_element
but	O
does	O
not	O
contact	O
the	O
head	B-structure_element
(	O
Figures	O
5b	O
and	O
6a	O
;	O
Figure	O
5	O
—	O
figure	O
supplement	O
1	O
).	O

Domain	O
IV	B-structure_element
is	O
partially	O
engaged	O
with	O
the	O
body	B-structure_element
A	B-site
site	I-site
.	O

The	O
tip	O
of	O
domain	O
IV	B-structure_element
is	O
wedged	O
between	O
PKI	B-structure_element
and	O
decoding	B-site
-	I-site
center	I-site
nucleotides	O
A1755	B-residue_name_number
and	O
A1756	B-residue_name_number
,	O
which	O
are	O
bulged	O
out	O
of	O
h44	O
.	O

This	O
tip	O
contains	O
the	O
histidine	B-site
-	I-site
diphthamide	I-site
triad	I-site
(	O
H583	B-residue_name_number
,	O
H694	B-residue_name_number
and	O
Diph699	B-ptm
),	O
which	O
interacts	O
with	O
the	O
codon	B-structure_element
-	I-structure_element
anticodon	I-structure_element
-	I-structure_element
like	I-structure_element
helix	I-structure_element
of	O
PKI	B-structure_element
and	O
A1756	B-residue_name_number
(	O
Figure	O
7	O
).	O

The	O
trimethylamino	O
end	O
of	O
Diph699	B-ptm
packs	O
over	O
A1756	B-residue_name_number
(	O
Figure	O
7	O
).	O

The	O
opposite	O
surface	O
of	O
the	O
tail	O
is	O
oriented	O
toward	O
the	O
minor	B-site
-	I-site
groove	I-site
side	O
of	O
the	O
second	O
base	O
pair	O
of	O
the	O
codon	B-structure_element
-	I-structure_element
anticodon	I-structure_element
helix	I-structure_element
(	O
G6906	B-residue_name_number
:	O
C6951	B-residue_name_number
).	O

Thus	O
,	O
in	O
comparison	O
with	O
the	O
initiation	B-protein_state
state	O
,	O
the	O
histidine	B-site
-	I-site
diphthamide	I-site
tip	I-site
of	O
eEF2	B-protein
replaces	O
the	O
codon	B-structure_element
-	I-structure_element
anticodon	I-structure_element
–	I-structure_element
like	I-structure_element
helix	I-structure_element
of	O
PKI	B-structure_element
.	O

The	O
splitting	O
of	O
the	O
interaction	O
of	O
A1755	B-residue_name_number
-	O
A1756	B-residue_name_number
and	O
PKI	B-structure_element
is	O
achieved	O
by	O
providing	O
the	O
histidine	B-site
-	I-site
diphthamine	I-site
tip	I-site
as	O
a	O
binding	O
partner	O
for	O
both	O
A1756	B-residue_name_number
and	O
the	O
minor	B-site
groove	I-site
of	O
the	O
codon	B-structure_element
-	I-structure_element
anticodon	I-structure_element
helix	I-structure_element
(	O
Figure	O
7	O
).	O

Unlike	O
in	O
Structures	B-evidence
II	I-evidence
to	I-evidence
V	I-evidence
,	O
the	O
conformation	O
of	O
the	O
eEF2	B-protein
GTPase	B-site
center	I-site
in	O
Structure	B-evidence
I	I-evidence
resembles	O
that	O
of	O
a	O
GTP	B-protein_state
-	I-protein_state
bound	I-protein_state
translocase	B-protein_type
(	O
Figure	O
5e	O
).	O

In	O
translational	B-protein_type
GTPases	I-protein_type
,	O
switch	B-structure_element
loops	I-structure_element
I	I-structure_element
and	I-structure_element
II	I-structure_element
are	O
involved	O
in	O
the	O
GTPase	B-protein_type
activity	O
(	O
reviewed	O
in	O
).	O

The	O
histidine	B-residue_name
resides	O
next	O
to	O
the	O
backbone	O
of	O
G3028	B-residue_name_number
of	O
the	O
sarcin	B-structure_element
-	I-structure_element
ricin	I-structure_element
loop	I-structure_element
and	O
near	O
the	O
diphosphate	O
of	O
GDP	B-chemical
(	O
Figure	O
5e	O
).	O

By	O
contrast	O
,	O
switch	B-structure_element
loop	I-structure_element
I	I-structure_element
(	O
aa	O
50	B-residue_range
–	I-residue_range
70	I-residue_range
in	O
S	B-species
.	I-species
cerevisiae	I-species
eEF2	B-protein
)	O
is	O
resolved	O
only	O
in	O
Structure	B-evidence
I	I-evidence
(	O
Figure	O
5	O
—	O
figure	O
supplement	O
2	O
).	O

Structure	B-evidence
II	I-evidence
reveals	O
PKI	B-structure_element
between	O
the	O
body	B-structure_element
A	B-site
and	I-site
P	I-site
sites	I-site
and	O
eEF2	B-protein
partially	O
advanced	O
into	O
the	O
A	B-site
site	I-site

In	O
Structure	B-evidence
II	I-evidence
,	O
relative	O
to	O
Structure	B-evidence
I	I-evidence
,	O
PKI	B-structure_element
is	O
further	O
shifted	O
along	O
the	O
40S	B-complex_assembly
body	B-structure_element
,	O
traversing	O
~	O
4	O
Å	O
toward	O
the	O
P	B-site
site	I-site
(	O
Figures	O
2e	O
,	O
f	O
,	O
and	O
g	O
),	O
while	O
stacking	B-bond_interaction
on	O
C1274	B-residue_name_number
at	O
the	O
head	B-structure_element
A	B-site
site	I-site
.	O

Thus	O
,	O
the	O
intermediate	O
position	O
of	O
PKI	B-structure_element
is	O
possible	O
due	O
to	O
a	O
large	O
swivel	O
of	O
the	O
head	B-structure_element
relative	O
to	O
the	O
body	B-structure_element
,	O
which	O
brings	O
the	O
head	B-structure_element
A	B-site
site	I-site
close	O
to	O
the	O
body	B-structure_element
P	B-site
site	I-site
.	O

Domain	O
IV	B-structure_element
of	O
eEF2	B-protein
is	O
further	O
entrenched	O
in	O
the	O
A	B-site
site	I-site
by	O
~	O
3	O
Å	O
relative	O
to	O
the	O
body	B-structure_element
and	O
~	O
8	O
Å	O
relative	O
to	O
the	O
head	B-structure_element
,	O
preserving	O
its	O
interactions	O
with	O
PKI	B-structure_element
.	O

The	O
decoding	B-site
center	I-site
residues	O
A1755	B-residue_name_number
and	O
A1756	B-residue_name_number
are	O
rearranged	O
to	O
pack	O
inside	O
helix	B-structure_element
44	I-structure_element
,	O
making	O
room	O
for	O
eEF2	B-protein
.	O

This	O
conformation	O
of	O
decoding	B-site
center	I-site
residues	O
is	O
also	O
observed	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
A	B-site
-	I-site
site	I-site
ligands	O
.	O

Here	O
,	O
a	O
positively	B-site
charged	I-site
surface	I-site
of	O
eEF2	B-protein
,	O
formed	O
by	O
K613	B-residue_name_number
,	O
R617	B-residue_name_number
and	O
R631	B-residue_name_number
contacts	O
the	O
phosphate	O
backbone	O
of	O
helix	B-structure_element
33	I-structure_element
(	O
Figures	O
6c	O
;	O
see	O
also	O
Figure	O
6	O
—	O
figure	O
supplement	O
1	O
).	O

Consistent	O
with	O
the	O
similar	O
head	B-structure_element
swivels	O
in	O
Structure	B-evidence
III	I-evidence
and	O
Structure	B-evidence
II	I-evidence
,	O
relative	O
positions	O
of	O
the	O
40S	B-complex_assembly
head	B-structure_element
A	B-site
site	I-site
and	O
body	B-structure_element
P	B-site
site	I-site
remain	O
as	O
in	O
Structure	B-evidence
II	I-evidence
.	O

The	O
map	B-evidence
allows	O
placement	O
of	O
PKI	B-structure_element
at	O
the	O
body	B-structure_element
P	B-site
site	I-site
(	O
Figure	O
1	O
—	O
figure	O
supplement	O
3	O
).	O

Lower	O
resolution	O
of	O
the	O
map	B-evidence
in	O
this	O
region	O
suggests	O
that	O
PKI	B-structure_element
is	O
somewhat	O
destabilized	O
in	O
the	O
vicinity	O
of	O
the	O
body	B-structure_element
P	B-site
site	I-site
in	O
the	O
absence	B-protein_state
of	I-protein_state
stacking	B-bond_interaction
with	O
the	O
foundations	O
of	O
the	O
head	B-structure_element
A	B-site
site	I-site
(	O
C1274	B-residue_name_number
)	O
or	O
P	B-site
site	I-site
(	O
U1191	B-residue_name_number
).	O

Structure	B-evidence
IV	I-evidence
represents	O
a	O
highly	B-protein_state
bent	I-protein_state
IRES	B-site
with	O
PKI	B-structure_element
partially	O
accommodated	O
in	O
the	O
P	B-site
site	I-site

In	O
Structure	B-evidence
IV	I-evidence
,	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
is	O
almost	O
non	B-protein_state
-	I-protein_state
rotated	I-protein_state
relative	O
to	O
the	O
60S	B-complex_assembly
subunit	B-structure_element
,	O
and	O
the	O
40S	B-complex_assembly
head	B-structure_element
is	O
mid	B-protein_state
-	I-protein_state
swiveled	I-protein_state
.	O

Unwinding	O
of	O
the	O
head	B-structure_element
moves	O
the	O
head	B-structure_element
P	B-site
-	I-site
site	I-site
residue	O
U1191	B-residue_name_number
and	O
body	B-structure_element
P	B-site
-	I-site
site	I-site
residue	O
C1637	B-residue_name_number
closer	O
together	O
,	O
resulting	O
in	O
a	O
partially	O
restored	O
40S	B-complex_assembly
P	B-site
site	I-site
.	O

Whereas	O
C1637	B-residue_name_number
forms	O
a	O
stacking	B-site
platform	I-site
for	O
the	O
last	O
base	O
pair	O
of	O
PKI	B-structure_element
,	O
U1191	B-residue_name_number
does	O
not	O
yet	O
stack	B-bond_interaction
on	O
PKI	B-structure_element
because	O
the	O
head	B-structure_element
remains	O
partially	O
swiveled	O
.	O

This	O
renders	O
PKI	B-structure_element
partially	O
accommodated	O
in	O
the	O
P	B-site
site	I-site
(	O
Figure	O
2g	O
).	O

Unwinding	O
of	O
the	O
40S	B-complex_assembly
head	B-structure_element
also	O
positions	O
the	O
head	B-structure_element
A	B-site
site	I-site
closer	O
to	O
the	O
body	B-structure_element
A	B-site
site	I-site
.	O

This	O
results	O
in	O
rearrangements	O
of	O
eEF2	B-protein
interactions	O
with	O
the	O
head	B-structure_element
,	O
allowing	O
eEF2	B-protein
to	O
advance	O
further	O
into	O
the	O
A	B-site
site	I-site
.	O

Structure	B-evidence
V	I-evidence
represents	O
an	O
extended	B-protein_state
IRES	B-site
with	O
PKI	B-structure_element
fully	O
accommodated	O
in	O
the	O
P	B-site
site	I-site
and	O
domain	O
IV	B-structure_element
of	O
eEF2	B-protein
in	O
the	O
A	B-site
site	I-site

A	O
notable	O
conformational	O
change	O
in	O
eEF2	B-protein
from	O
that	O
in	O
the	O
preceding	O
Structures	B-evidence
is	O
visible	O
in	O
the	O
position	O
of	O
domain	O
III	B-structure_element
,	O
which	O
contacts	O
uS12	B-protein
(	O
Figure	O
6d	O
).	O

In	O
Structure	B-evidence
V	I-evidence
,	O
protein	O
uS12	B-protein
is	O
shifted	O
along	O
with	O
the	O
40S	B-complex_assembly
body	B-structure_element
as	O
a	O
result	O
of	O
intersubunit	O
rotation	O
.	O

Specifically	O
,	O
the	O
C	B-structure_element
-	I-structure_element
terminal	I-structure_element
tail	I-structure_element
of	O
uS12	B-protein
packs	O
against	O
the	O
β	B-structure_element
-	I-structure_element
barrel	I-structure_element
of	O
domain	O
II	B-structure_element
,	O
while	O
the	O
β	B-structure_element
-	I-structure_element
barrel	I-structure_element
of	O
uS12	B-protein
packs	O
against	O
helix	B-structure_element
A	I-structure_element
of	O
domain	O
III	B-structure_element
.	O

This	O
shifts	O
the	O
tip	O
of	O
helix	B-structure_element
A	I-structure_element
of	O
domain	O
III	B-structure_element
(	O
at	O
aa	O
500	B-residue_number
)	O
by	O
~	O
5	O
Å	O
(	O
relative	O
to	O
that	O
in	O
Structure	B-evidence
I	I-evidence
)	O
toward	O
domain	O
I	B-structure_element
.	O
Although	O
domain	O
III	B-structure_element
remains	O
in	O
contact	O
with	O
domain	O
V	B-structure_element
,	O
the	O
shift	O
occurs	O
in	O
the	O
direction	O
that	O
could	O
eventually	O
disconnect	O
the	O
β	B-structure_element
-	I-structure_element
platforms	I-structure_element
of	O
these	O
domains	O
.	O

The	O
amide	O
at	O
the	O
diphthamide	B-ptm
end	O
interacts	O
with	O
N2	O
of	O
G6906	B-residue_name_number
and	O
O2	O
and	O
O2	O
’	O
of	O
C6951	B-residue_name_number
(	O
corresponding	O
to	O
nt	O
2	O
of	O
the	O
codon	O
).	O

The	O
trimethylamino	O
-	O
group	O
is	O
positioned	O
over	O
the	O
ribose	O
of	O
C6952	B-residue_name_number
(	O
codon	O
nt	O
3	O
).	O

IRES	B-site
translocation	O
mechanism	O

Four	O
views	O
(	O
scenes	O
)	O
are	O
shown	O
:	O
(	O
1	O
)	O
A	O
view	O
down	O
the	O
intersubunit	O
space	O
,	O
with	O
the	O
head	B-structure_element
of	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
oriented	O
toward	O
a	O
viewer	O
,	O
as	O
in	O
Figure	O
1a	O
;	O
(	O
2	O
)	O
A	O
view	O
at	O
the	O
solvent	O
side	O
of	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
,	O
with	O
the	O
40S	B-complex_assembly
head	B-structure_element
shown	O
at	O
the	O
top	O
,	O
as	O
in	O
Figure	O
2	O
—	O
figure	O
supplement	O
1	O
;	O
(	O
3	O
)	O
A	O
view	O
down	O
at	O
the	O
subunit	O
interface	O
of	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
;	O
(	O
4	O
)	O
A	O
close	O
-	O
up	O
view	O
of	O
the	O
decoding	B-site
center	I-site
(	O
A	B-site
site	I-site
)	O
and	O
the	O
P	B-site
site	I-site
,	O
as	O
in	O
Figure	O
2g	O
.	O
Each	O
scene	O
is	O
shown	O
twice	O
.	O

We	O
propose	O
that	O
together	O
with	O
the	O
previously	O
reported	O
initiation	B-protein_state
state	O
,	O
these	O
structures	B-evidence
represent	O
the	O
trajectory	O
of	O
eEF2	B-protein
-	O
induced	O
IRES	B-site
translocation	O
(	O
shown	O
as	O
an	O
animation	O
in	O
http	O
://	O
labs	O
.	O
umassmed	O
.	O
edu	O
/	O
korostelevlab	O
/	O
msc	O
/	O
iresmovie	O
.	O
gif	O
and	O
Video	O
1	O
).	O

Our	O
structures	B-evidence
reveal	O
previously	O
unseen	O
intermediate	O
states	O
of	O
eEF2	B-protein
or	O
EF	B-protein
-	I-protein
G	I-protein
engagement	O
with	O
the	O
A	B-site
site	I-site
,	O
providing	O
the	O
structural	O
basis	O
for	O
the	O
mechanism	O
of	O
translocase	B-protein_type
action	O
.	O

Furthermore	O
,	O
they	O
provide	O
insight	O
into	O
the	O
mechanism	O
of	O
eEF2	B-complex_assembly
•	I-complex_assembly
GTP	I-complex_assembly
association	O
with	O
the	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
ribosome	B-complex_assembly
and	O
eEF2	B-complex_assembly
•	I-complex_assembly
GDP	I-complex_assembly
dissociation	O
from	O
the	O
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
ribosome	B-complex_assembly
,	O
also	O
delineating	O
the	O
mechanism	O
of	O
translation	O
inhibition	O
by	O
the	O
antifungal	O
drug	O
sordarin	B-chemical
.	O

At	O
the	O
start	O
(	O
initiation	B-protein_state
state	O
),	O
the	O
IRES	B-site
adopts	O
an	O
extended	B-protein_state
conformation	O
(	O
extended	B-protein_state
inchworm	I-protein_state
).	O

The	O
front	B-structure_element
'	I-structure_element
legs	I-structure_element
'	O
(	O
SL4	B-structure_element
and	O
SL5	B-structure_element
)	O
of	O
the	O
5	B-structure_element
’-	I-structure_element
domain	I-structure_element
(	O
front	B-structure_element
end	I-structure_element
)	O
are	O
attached	O
to	O
the	O
40S	B-complex_assembly
head	B-structure_element
proteins	O
uS7	B-protein
,	O
uS11	B-protein
and	O
eS25	B-protein
(	O
Figure	O
3	O
—	O
figure	O
supplement	O
2	O
).	O

PKI	B-structure_element
,	O
representing	O
the	O
hind	B-structure_element
end	I-structure_element
,	O
is	O
bound	B-protein_state
in	I-protein_state
the	O
A	B-site
site	I-site
.	O

This	O
shortens	O
the	O
distance	O
between	O
PKI	B-structure_element
and	O
SL4	B-structure_element
by	O
up	O
to	O
20	O
Å	O
relative	O
to	O
the	O
initiating	O
IRES	B-site
structure	B-evidence
,	O
resulting	O
in	O
a	O
bent	B-protein_state
IRES	B-site
conformation	O
(	O
bent	B-protein_state
inchworm	I-protein_state
).	O

In	O
the	O
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
CrPV	B-species
IRES	B-site
structure	B-evidence
,	O
the	O
5	B-structure_element
’-	I-structure_element
domain	I-structure_element
similarly	O
protrudes	O
between	O
the	O
subunits	O
and	O
interacts	O
with	O
the	O
L1	B-structure_element
stalk	I-structure_element
,	O
as	O
in	O
the	O
initiation	B-protein_state
state	O
for	O
this	O
IRES	B-site
.	O

This	O
underlines	O
structural	O
similarity	O
for	O
the	O
TSV	B-species
and	O
CrPV	B-species
IRES	B-site
translocation	O
mechanisms	O
.	O

One	O
of	O
the	O
mechanistic	O
scenarios	O
(	O
discussed	O
in	O
)	O
involves	O
binding	O
of	O
the	O
first	O
aminoacyl	B-chemical
-	I-chemical
tRNA	I-chemical
to	O
the	O
post	B-protein_state
-	I-protein_state
translocated	I-protein_state
IRES	B-site
mRNA	B-chemical
frame	O
shifted	O
by	O
one	O
nucleotide	O
(	O
predominantly	O
a	O
+	O
1	O
frame	O
shift	O
).	O

It	O
is	O
likely	O
that	O
alternative	O
frame	O
setting	O
occurs	O
following	O
eEF2	B-protein
release	O
and	O
that	O
this	O
depends	O
on	O
transient	O
displacement	O
of	O
the	O
start	O
codon	O
in	O
the	O
decoding	B-site
center	I-site
,	O
allowing	O
binding	O
of	O
the	O
corresponding	O
amino	B-chemical
acyl	I-chemical
-	I-chemical
tRNA	I-chemical
to	O
an	O
off	O
-	O
frame	O
codon	O
.	O

Further	O
structural	B-experimental_method
studies	I-experimental_method
involving	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
•	I-complex_assembly
tRNA	I-complex_assembly
complexes	O
are	O
necessary	O
to	O
understand	O
the	O
mechanisms	O
underlying	O
alternative	O
reading	O
frame	O
selection	O
.	O

This	O
is	O
consistent	O
with	O
the	O
observations	O
that	O
the	O
intergenic	O
IRESs	B-site
are	O
prone	O
to	O
reverse	O
translocation	O
.	O

This	O
contrasts	O
with	O
the	O
post	B-protein_state
-	I-protein_state
translocated	I-protein_state
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
complex	O
,	O
in	O
which	O
the	O
classical	O
P	B-site
and	I-site
E	I-site
-	I-site
site	I-site
tRNAs	B-chemical
are	O
stabilized	O
in	O
the	O
non	B-protein_state
-	I-protein_state
rotated	I-protein_state
ribosome	B-complex_assembly
after	O
translocase	B-protein_type
release	O
.	O

In	O
the	O
initiation	B-protein_state
state	O
,	O
the	O
IRES	B-site
resembles	O
a	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
complex	O
reduced	O
to	O
the	O
A	B-site
/	I-site
P	I-site
-	O
tRNA	B-chemical
anticodon	B-structure_element
-	I-structure_element
stem	I-structure_element
loop	I-structure_element
and	O
elbow	B-structure_element
in	O
the	O
A	B-site
site	I-site
and	O
the	O
P	B-site
/	I-site
E	I-site
-	O
tRNA	B-chemical
elbow	B-structure_element
contacting	O
the	O
L1	B-structure_element
stalk	I-structure_element
.	O

Because	O
the	O
anticodon	B-structure_element
-	I-structure_element
stem	I-structure_element
loop	I-structure_element
of	O
the	O
A	B-site
-	O
tRNA	B-chemical
is	O
sufficient	O
for	O
translocation	O
completion	O
,	O
we	O
ascribe	O
the	O
meta	O
-	O
stability	O
of	O
the	O
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
IRES	B-site
to	O
the	O
absence	B-protein_state
of	I-protein_state
the	O
P	B-site
/	I-site
E	I-site
-	O
tRNA	B-chemical
elements	O
,	O
either	O
the	O
ASL	B-structure_element
or	O
the	O
acceptor	O
arm	O
,	O
or	O
both	O
.	O

Furthermore	O
,	O
interactions	O
of	O
SL4	B-structure_element
and	O
SL5	B-structure_element
with	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
likely	O
contribute	O
to	O
stabilization	O
of	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
structures	B-evidence
.	O

Partitioned	O
roles	O
of	O
40S	B-complex_assembly
subunit	B-structure_element
rearrangements	O

Our	O
structures	B-evidence
delineate	O
the	O
mechanistic	O
functions	O
for	O
intersubunit	O
rotation	O
and	O
head	B-structure_element
swivel	O
in	O
translocation	O
.	O

Specifically	O
,	O
intersubunit	O
rotation	O
allows	O
eEF2	B-protein
entry	O
into	O
the	O
A	B-site
site	I-site
,	O
while	O
the	O
head	B-structure_element
swivel	O
mediates	O
PKI	B-structure_element
translocation	O
.	O

This	O
suggests	O
that	O
the	O
subunits	B-structure_element
are	O
capable	O
of	O
spontaneous	O
rotation	O
,	O
as	O
is	O
the	O
case	O
for	O
tRNA	B-protein_state
-	I-protein_state
bound	I-protein_state
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
complexes	O
.	O

This	O
allows	O
eEF2	B-protein
to	O
move	O
into	O
the	O
A	B-site
site	I-site
.	O

As	O
such	O
,	O
reverse	O
intersubunit	O
rotation	O
facilitates	O
full	O
docking	O
of	O
eEF2	B-protein
in	O
the	O
A	B-site
site	I-site
.	O

Because	O
the	O
histidine	B-site
-	I-site
diphthamide	I-site
tip	I-site
of	O
eEF2	B-protein
(	O
H583	B-residue_name_number
,	O
H694	B-residue_name_number
and	O
Diph699	B-ptm
)	O
attaches	O
to	O
the	O
codon	B-structure_element
-	I-structure_element
anticodon	I-structure_element
-	I-structure_element
like	I-structure_element
helix	I-structure_element
of	O
PKI	B-structure_element
,	O
eEF2	B-protein
appears	O
to	O
directly	O
force	O
PKI	B-structure_element
out	O
of	O
the	O
A	B-site
site	I-site
.	O

The	O
fully	B-protein_state
swiveled	I-protein_state
conformations	O
of	O
Structures	B-evidence
II	I-evidence
and	I-evidence
III	I-evidence
represent	O
the	O
mid	O
-	O
point	O
of	O
translocation	O
,	O
in	O
which	O
PKI	B-structure_element
relocates	O
between	O
the	O
head	B-structure_element
A	B-site
site	I-site
and	O
body	B-structure_element
P	B-site
site	I-site
.	O

We	O
note	O
that	O
such	O
mid	O
-	O
states	O
have	O
not	O
been	O
observed	O
for	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
,	O
but	O
their	O
formation	O
can	O
explain	O
the	O
formation	O
of	O
subsequent	O
pe	B-protein_state
/	I-protein_state
E	I-protein_state
hybrid	I-protein_state
and	O
ap	B-protein_state
/	I-protein_state
P	I-protein_state
chimeric	I-protein_state
structures	B-evidence
(	O
Figure	O
1	O
—	O
figure	O
supplement	O
1	O
).	O

Reverse	O
swivel	O
from	O
Structure	B-evidence
III	I-evidence
to	I-evidence
V	I-evidence
brings	O
the	O
head	B-structure_element
to	O
the	O
non	B-protein_state
-	I-protein_state
swiveled	I-protein_state
position	O
,	O
restoring	O
the	O
A	B-site
and	I-site
P	I-site
sites	I-site
on	O
the	O
small	B-structure_element
subunit	I-structure_element
.	O

The	O
functions	O
of	O
eEF2	B-protein
in	O
translocation	O

To	O
our	O
knowledge	O
,	O
our	O
work	O
provides	O
the	O
first	O
high	O
-	O
resolution	O
view	O
of	O
the	O
dynamics	O
of	O
a	O
ribosomal	B-protein_type
translocase	I-protein_type
that	O
is	O
inferred	O
from	O
an	O
ensemble	O
of	O
structures	B-evidence
sampled	O
under	O
uniform	O
conditions	O
.	O

The	O
structures	B-evidence
,	O
therefore	O
,	O
offer	O
a	O
unique	O
opportunity	O
to	O
address	O
the	O
role	O
of	O
the	O
elongation	B-protein_type
factors	I-protein_type
during	O
translocation	O
.	O

While	O
the	O
ribosome	B-complex_assembly
itself	O
has	O
the	O
capacity	O
to	O
translocate	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
the	O
translocase	B-protein_type
,	O
spontaneous	O
translocation	O
is	O
slow	O
.	O

EF	B-protein
-	I-protein
G	I-protein
enhances	O
the	O
translocation	O
rate	O
by	O
several	O
orders	O
of	O
magnitude	O
,	O
aided	O
by	O
an	O
additional	O
2	O
-	O
to	O
50	O
-	O
fold	O
boost	O
from	O
GTP	B-chemical
hydrolysis	O
.	O

Due	O
to	O
the	O
lack	O
of	O
structures	B-evidence
of	O
translocation	O
intermediates	O
,	O
the	O
mechanistic	O
role	O
of	O
eEF2	B-protein
/	O
EF	B-protein
-	I-protein
G	I-protein
is	O
not	O
fully	O
understood	O
.	O

The	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
•	I-complex_assembly
eEF2	I-complex_assembly
structures	B-evidence
reported	O
here	O
suggest	O
two	O
main	O
roles	O
for	O
eEF2	B-protein
in	O
translocation	O
.	O

As	O
discussed	O
above	O
,	O
the	O
first	O
role	O
is	O
to	O
directly	O
shift	O
PKI	B-structure_element
out	O
of	O
the	O
A	B-site
site	I-site
upon	O
spontaneous	O
reverse	O
intersubunit	O
rotation	O
.	O

In	O
our	O
structures	B-evidence
,	O
the	O
tip	O
of	O
domain	O
IV	B-structure_element
docks	O
next	O
to	O
PKI	B-structure_element
,	O
with	O
diphthamide	B-ptm
699	I-ptm
fit	O
into	O
the	O
minor	B-site
groove	I-site
of	O
the	O
codon	B-structure_element
-	I-structure_element
anticodon	I-structure_element
-	I-structure_element
like	I-structure_element
helix	I-structure_element
of	O
PKI	B-structure_element
(	O
Figure	O
7	O
).	O

This	O
arrangement	O
rationalizes	O
inactivation	O
of	O
eEF2	B-protein
by	O
diphtheria	B-protein_type
toxin	I-protein_type
,	O
which	O
catalyzes	O
ADP	B-ptm
-	I-ptm
ribosylation	I-ptm
of	O
the	O
diphthamide	B-ptm
(	O
reviewed	O
in	O
).	O

The	O
enzyme	O
ADP	B-ptm
-	I-ptm
ribosylates	I-ptm
the	O
NE2	O
atom	O
of	O
the	O
imidazole	O
ring	O
,	O
which	O
in	O
our	O
structures	B-evidence
interacts	O
with	O
the	O
first	O
two	O
residues	O
of	O
the	O
anticodon	B-structure_element
-	I-structure_element
like	I-structure_element
strand	I-structure_element
of	O
PKI	B-structure_element
.	O

As	O
eEF2	B-protein
shifts	O
PKI	B-structure_element
toward	O
the	O
P	B-site
site	I-site
in	O
the	O
course	O
of	O
reverse	O
intersubunit	O
rotation	O
,	O
the	O
60S	B-protein_state
-	I-protein_state
attached	I-protein_state
translocase	B-protein_type
migrates	O
along	O
the	O
surface	O
of	O
the	O
40S	B-complex_assembly
subunit	B-structure_element
,	O
guided	O
by	O
electrostatic	B-bond_interaction
interactions	I-bond_interaction
.	O

Positively	B-site
-	I-site
charged	I-site
patches	I-site
of	O
domains	O
II	B-structure_element
and	O
III	B-structure_element
(	O
R391	B-residue_name_number
,	O
K394	B-residue_name_number
,	O
R433	B-residue_name_number
,	O
R510	B-residue_name_number
)	O
and	O
IV	B-structure_element
(	O
K613	B-residue_name_number
,	O
R617	B-residue_name_number
,	O
R609	B-residue_name_number
,	O
R631	B-residue_name_number
,	O
K651	B-residue_name_number
)	O
slide	O
over	O
rRNA	B-chemical
of	O
the	O
40S	B-complex_assembly
body	B-structure_element
(	O
h5	B-structure_element
)	O
and	O
head	B-structure_element
(	O
h18	B-structure_element
and	O
h33	B-structure_element
/	O
h34	B-structure_element
),	O
respectively	O
.	O

For	O
example	O
,	O
between	O
Structures	B-evidence
II	I-evidence
and	I-evidence
V	I-evidence
,	O
the	O
K613	B-residue_name_number
/	O
R617	B-residue_name_number
/	O
R631	B-residue_name_number
cluster	O
of	O
domain	O
IV	B-structure_element
hops	O
by	O
~	O
19	O
Å	O
(	O
for	O
Cα	O
of	O
R617	B-residue_name_number
)	O
from	O
the	O
phosphate	O
backbone	O
of	O
h33	B-structure_element
(	O
at	O
nt	O
1261	B-residue_range
–	I-residue_range
1264	I-residue_range
)	O
to	O
that	O
of	O
the	O
neighboring	O
h34	B-structure_element
(	O
at	O
nt	O
1442	B-residue_range
–	I-residue_range
1445	I-residue_range
).	O

Thus	O
,	O
sliding	O
of	O
eEF2	B-protein
involves	O
reorganization	O
of	O
electrostatic	B-bond_interaction
,	I-bond_interaction
perhaps	I-bond_interaction
isoenergetic	I-bond_interaction
interactions	I-bond_interaction
,	O
echoing	O
those	O
implied	O
in	O
extraordinarily	O
fast	O
ribosome	B-complex_assembly
inactivation	O
rates	O
by	O
the	O
small	O
-	O
protein	O
ribotoxins	O
and	O
in	O
fast	O
protein	O
association	O
and	O
diffusion	O
along	O
DNA	O
.	O

Comparison	B-experimental_method
of	O
our	O
structures	B-evidence
with	O
the	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
initiation	B-protein_state
structure	B-evidence
reveals	O
the	O
structural	O
basis	O
for	O
the	O
second	O
key	O
function	O
of	O
the	O
translocase	B-protein_type
:	O
'	O
unlocking	O
'	O
of	O
intrasubunit	O
rearrangements	O
that	O
are	O
required	O
for	O
step	O
-	O
wise	O
translocation	O
of	O
PKI	B-structure_element
on	O
the	O
small	B-structure_element
subunit	I-structure_element
.	O

The	O
unlocking	O
model	O
of	O
the	O
ribosome	B-complex_assembly
•	I-complex_assembly
2tRNA	I-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
complex	O
has	O
been	O
proposed	O
decades	O
ago	O
and	O
functional	O
requirement	O
of	O
the	O
translocase	B-protein_type
in	O
this	O
process	O
has	O
been	O
implicated	O
.	O

FRET	B-evidence
data	I-evidence
indicate	O
that	O
translocation	O
of	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
on	O
the	O
70S	B-complex_assembly
ribosome	I-complex_assembly
requires	O
a	O
forward	O
-	O
and	O
-	O
reverse	O
head	B-structure_element
swivel	O
,	O
which	O
may	O
be	O
related	O
to	O
the	O
unlocking	O
phenomenon	O
.	O

Our	O
structures	B-evidence
suggest	O
that	O
eEF2	B-protein
induces	O
head	B-structure_element
swivel	O
by	O
'	O
unlocking	O
'	O
the	O
head	B-structure_element
-	O
body	B-structure_element
interactions	O
(	O
Figure	O
7	O
).	O

Binding	O
of	O
the	O
ASL	B-structure_element
to	O
the	O
A	B-site
site	I-site
is	O
known	O
from	O
structural	B-experimental_method
studies	I-experimental_method
of	O
bacterial	B-taxonomy_domain
ribosomes	B-complex_assembly
to	O
result	O
in	O
'	O
domain	B-protein_state
closure	I-protein_state
'	O
of	O
the	O
small	B-structure_element
subunit	I-structure_element
,	O
i	O
.	O
e	O
.	O
closer	O
association	O
of	O
the	O
head	B-structure_element
,	O
shoulder	B-structure_element
and	O
body	B-structure_element
domains	O
.	O

The	O
domain	O
closure	O
'	O
locks	O
'	O
cognate	O
tRNA	B-chemical
in	O
the	O
A	B-site
site	I-site
via	O
stacking	B-bond_interaction
on	O
the	O
head	B-structure_element
A	B-site
site	I-site
(	O
C1274	B-residue_name_number
in	O
S	B-species
.	I-species
cerevisiae	I-species
or	O
C1054	B-residue_name_number
in	O
E	B-species
.	I-species
coli	I-species
)	O
and	O
interactions	O
with	O
the	O
body	B-structure_element
A	B-site
-	I-site
site	I-site
nucleotides	O
A1755	B-residue_name_number
and	O
A1756	B-residue_name_number
(	O
A1492	B-residue_name_number
and	O
A1493	B-residue_name_number
in	O
E	B-species
.	I-species
coli	I-species
).	O

The	O
histidine	B-ptm
-	I-ptm
diphthamide	I-ptm
-	O
induced	O
disengagement	O
of	O
PKI	B-structure_element
from	O
A1755	B-residue_name_number
and	O
A1756	B-residue_name_number
therefore	O
provides	O
the	O
structural	O
definition	O
for	O
the	O
'	O
unlocking	O
'	O
mode	O
of	O
eEF2	B-protein
action	O
.	O

In	O
summary	O
,	O
our	O
structures	B-evidence
are	O
consistent	O
with	O
a	O
model	O
of	O
eEF2	B-protein
-	O
induced	O
translocation	O
in	O
which	O
both	O
PKI	B-structure_element
and	O
eEF2	B-protein
passively	O
migrate	O
into	O
the	O
P	B-site
and	I-site
A	I-site
site	I-site
,	O
respectively	O
,	O
during	O
spontaneous	O
40S	B-complex_assembly
body	B-structure_element
rotation	O
and	O
head	B-structure_element
swivel	O
,	O
the	O
latter	O
being	O
allowed	O
by	O
'	O
unlocking	O
'	O
of	O
the	O
A	B-site
site	I-site
by	O
eEF2	B-protein
.	O

Insights	O
into	O
eEF2	B-protein
association	O
with	O
and	O
dissociation	O
from	O
the	O
ribosome	B-complex_assembly

The	O
conformational	O
rearrangements	O
in	O
eEF2	B-protein
from	O
Structure	B-evidence
I	I-evidence
through	O
Structure	B-evidence
V	I-evidence
provide	O
insights	O
into	O
the	O
mechanisms	O
of	O
eEF2	B-protein
association	O
with	O
the	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
ribosome	B-complex_assembly
and	O
dissociation	O
from	O
the	O
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
ribosome	B-complex_assembly
.	O

In	O
the	O
fully	B-protein_state
-	I-protein_state
rotated	I-protein_state
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
-	O
like	O
Structure	B-evidence
I	I-evidence
,	O
an	O
additional	O
interaction	O
exists	O
.	O

The	O
interaction	O
between	O
h14	B-structure_element
and	O
switch	B-structure_element
loop	I-structure_element
I	I-structure_element
is	O
not	O
resolved	O
in	O
Structures	B-evidence
II	I-evidence
to	I-evidence
V	I-evidence
,	O
in	O
all	O
of	O
which	O
the	O
small	B-structure_element
subunit	I-structure_element
is	O
partially	B-protein_state
rotated	I-protein_state
or	O
non	B-protein_state
-	I-protein_state
rotated	I-protein_state
,	O
so	O
that	O
helix	B-structure_element
14	I-structure_element
is	O
placed	O
at	O
least	O
6	O
Å	O
farther	O
from	O
eEF2	B-protein
(	O
Figure	O
5d	O
).	O

We	O
conclude	O
that	O
unlike	O
other	O
conformations	O
of	O
the	O
ribosome	B-complex_assembly
,	O
the	O
fully	B-protein_state
rotated	I-protein_state
40S	B-complex_assembly
subunit	B-structure_element
of	O
the	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
ribosome	B-complex_assembly
provides	O
an	O
interaction	B-site
surface	I-site
,	O
complementing	O
the	O
P	B-structure_element
stalk	I-structure_element
and	O
SRL	B-structure_element
,	O
for	O
binding	O
of	O
the	O
GTP	B-protein_state
-	I-protein_state
bound	I-protein_state
translocase	B-protein_type
.	O

This	O
structural	O
basis	O
rationalizes	O
the	O
observation	O
of	O
transient	O
stabilization	O
of	O
the	O
rotated	B-protein_state
70S	B-complex_assembly
ribosome	I-complex_assembly
upon	O
EF	B-complex_assembly
-	I-complex_assembly
G	I-complex_assembly
•	I-complex_assembly
GTP	I-complex_assembly
binding	O
and	O
prior	O
to	O
translocation	O
.	O

The	O
most	O
pronounced	O
inter	O
-	O
domain	O
rearrangement	O
in	O
eEF2	B-protein
involves	O
movement	O
of	O
domain	O
III	B-structure_element
.	O

In	O
the	O
rotated	B-protein_state
or	O
mid	B-protein_state
-	I-protein_state
rotated	I-protein_state
Structures	B-evidence
I	I-evidence
through	I-evidence
III	I-evidence
,	O
this	O
domain	O
remains	O
rigidly	O
associated	O
with	O
domain	O
V	B-structure_element
and	O
the	O
N	O
-	O
terminal	O
superdomain	B-structure_element
and	O
does	O
not	O
undergo	O
noticeable	O
rearrangements	O
.	O

In	O
Structure	B-evidence
V	I-evidence
,	O
however	O
,	O
the	O
tip	O
of	O
helix	B-structure_element
A	I-structure_element
of	O
domain	O
III	B-structure_element
is	O
displaced	O
toward	O
domain	O
I	B-structure_element
by	O
~	O
5	O
Å	O
relative	O
to	O
that	O
in	O
mid	B-protein_state
-	I-protein_state
rotated	I-protein_state
or	O
fully	B-protein_state
rotated	I-protein_state
structures	B-evidence
.	O

This	O
displacement	O
is	O
caused	O
by	O
the	O
8	O
Å	O
movement	O
of	O
the	O
40S	B-complex_assembly
body	B-structure_element
protein	O
uS12	B-protein
upon	O
reverse	O
intersubunit	O
rotation	O
from	O
Structure	B-evidence
I	I-evidence
to	I-evidence
V	I-evidence
(	O
Figure	O
6d	O
).	O

We	O
propose	O
that	O
the	O
shift	O
of	O
domain	O
III	B-structure_element
by	O
uS12	B-protein
initiates	O
intra	O
-	O
domain	O
rearrangements	O
in	O
eEF2	B-protein
,	O
which	O
unstack	O
the	O
β	B-structure_element
-	I-structure_element
platform	I-structure_element
of	O
domain	O
III	B-structure_element
from	O
that	O
of	O
domain	O
V	B-structure_element
.	O
This	O
would	O
result	O
in	O
a	O
conformation	O
characteristic	O
of	O
free	B-protein_state
eEF2	B-protein
and	O
EF	B-protein
-	I-protein
G	I-protein
in	O
which	O
the	O
β	B-structure_element
-	I-structure_element
platforms	I-structure_element
are	O
nearly	O
perpendicular	O
.	O

As	O
we	O
discuss	O
below	O
,	O
Structure	B-evidence
V	I-evidence
captures	O
a	O
'	O
pre	B-protein_state
-	I-protein_state
unstacking	I-protein_state
'	O
state	O
due	O
to	O
stabilization	O
of	O
the	O
interface	B-site
between	O
domains	O
III	B-structure_element
and	O
V	B-structure_element
by	O
sordarin	B-chemical
.	O

Sordarin	B-chemical
stabilizes	O
GDP	B-protein_state
-	I-protein_state
bound	I-protein_state
eEF2	B-protein
on	O
the	O
ribosome	B-complex_assembly

Our	O
structures	B-evidence
therefore	O
indicate	O
that	O
sordarin	B-chemical
stalls	O
eEF2	B-protein
on	O
the	O
ribosome	B-complex_assembly
in	O
the	O
GDP	B-protein_state
-	I-protein_state
bound	I-protein_state
form	O
,	O
i	O
.	O
e	O
.	O
following	O
GTP	B-chemical
hydrolysis	O
and	O
phosphate	O
release	O
.	O

In	O
all	O
five	O
structures	B-evidence
,	O
sordarin	B-chemical
is	O
bound	B-protein_state
between	O
domains	O
III	B-structure_element
and	O
V	B-structure_element
of	O
eEF2	B-protein
,	O
stabilized	O
by	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
identical	O
to	O
those	O
in	O
the	O
isolated	B-protein_state
eEF2	B-complex_assembly
•	I-complex_assembly
sordarin	I-complex_assembly
complex	O
(	O
Figures	O
5g	O
and	O
h	O
).	O

In	O
the	O
nearly	B-protein_state
non	I-protein_state
-	I-protein_state
rotated	I-protein_state
post	B-protein_state
-	I-protein_state
translocation	I-protein_state
Structure	B-evidence
V	I-evidence
,	O
the	O
tip	O
of	O
domain	O
III	B-structure_element
is	O
shifted	O
,	O
however	O
the	O
interface	B-site
between	O
domains	O
III	B-structure_element
and	O
V	B-structure_element
remains	O
unchanged	O
,	O
suggesting	O
strong	O
stabilization	O
of	O
this	O
interface	B-site
by	O
sordarin	B-chemical
.	O

We	O
propose	O
that	O
sordarin	B-chemical
acts	O
to	O
prevent	O
full	O
reverse	O
rotation	O
and	O
release	O
of	O
eEF2	B-complex_assembly
•	I-complex_assembly
GDP	I-complex_assembly
by	O
stabilizing	O
the	O
interdomain	B-site
interface	I-site
and	O
thus	O
blocking	O
uS12	B-protein
-	O
induced	O
disengagement	O
of	O
domain	O
III	B-structure_element
from	O
domain	O
V	B-structure_element
.	O

Implications	O
for	O
tRNA	B-chemical
and	O
mRNA	B-chemical
translocation	O
during	O
translation	O

Despite	O
an	O
impressive	O
body	B-structure_element
of	O
biochemical	B-evidence
,	I-evidence
fluorescence	I-evidence
and	I-evidence
structural	I-evidence
data	I-evidence
accumulated	O
since	O
then	O
,	O
translocation	O
remains	O
the	O
least	O
understood	O
step	O
of	O
elongation	O
.	O

However	O
,	O
visualization	O
of	O
the	O
eEF2	B-protein
/	O
EF	B-protein
-	I-protein
G	I-protein
-	O
induced	O
translocation	O
is	O
confined	O
to	O
very	O
early	O
pre	B-protein_state
-	I-protein_state
EF	I-protein_state
-	I-protein_state
G	I-protein_state
-	I-protein_state
entry	I-protein_state
states	O
and	O
late	O
(	O
almost	B-protein_state
translocated	I-protein_state
or	O
fully	B-protein_state
translocated	I-protein_state
)	O
states	O
,	O
leaving	O
most	O
of	O
the	O
path	O
from	O
the	O
A	B-site
to	I-site
the	I-site
P	I-site
site	I-site
uncharacterized	O
(	O
Figure	O
1	O
—	O
figure	O
supplement	O
1	O
).	O

First	O
,	O
we	O
propose	O
that	O
tRNA	B-chemical
and	O
IRES	B-site
translocations	O
occur	O
via	O
the	O
same	O
general	O
trajectory	O
.	O

This	O
is	O
evident	O
from	O
the	O
fact	O
that	O
ribosome	B-complex_assembly
rearrangements	O
in	O
translocation	O
are	O
inherent	O
to	O
the	O
ribosome	B-complex_assembly
and	O
likely	O
occur	O
in	O
similar	O
ways	O
in	O
both	O
cases	O
.	O

Furthermore	O
,	O
the	O
step	O
-	O
wise	O
coupling	O
of	O
ribosome	B-complex_assembly
dynamics	O
with	O
IRES	B-site
translocation	O
is	O
overall	O
consistent	O
with	O
that	O
observed	O
for	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
translocation	O
in	O
solution	O
.	O

For	O
example	O
,	O
fluorescence	B-experimental_method
and	I-experimental_method
biochemical	I-experimental_method
studies	I-experimental_method
revealed	O
that	O
the	O
early	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
EF	B-protein_state
-	I-protein_state
G	I-protein_state
-	I-protein_state
bound	I-protein_state
ribosomes	B-complex_assembly
are	O
fully	B-protein_state
rotated	I-protein_state
and	O
translocation	O
of	O
the	O
tRNA	B-complex_assembly
-	I-complex_assembly
mRNA	I-complex_assembly
complex	O
occurs	O
during	O
reverse	O
rotation	O
of	O
the	O
small	B-structure_element
subunit	I-structure_element
,	O
coupled	O
with	O
head	B-structure_element
swivel	O
.	O

The	O
sequence	O
of	O
ribosome	B-complex_assembly
rearrangements	O
during	O
IRES	B-site
translocation	O
also	O
agrees	O
with	O
that	O
inferred	O
from	O
70S	B-complex_assembly
•	I-complex_assembly
EF	I-complex_assembly
-	I-complex_assembly
G	I-complex_assembly
structures	B-evidence
,	O
including	O
those	O
in	O
which	O
the	O
A	B-site
-	I-site
to	I-site
-	I-site
P	I-site
-	I-site
site	I-site
translocating	O
tRNA	B-chemical
was	O
not	O
present	O
.	O

Specifically	O
,	O
an	O
earlier	O
translocation	O
intermediate	O
ribosome	B-complex_assembly
(	O
TIpre	O
)	O
was	O
proposed	O
to	O
adopt	O
a	O
rotated	B-protein_state
(	O
7	O
–	O
9	O
°)	O
body	B-structure_element
and	O
a	O
partly	B-protein_state
rotated	I-protein_state
head	B-structure_element
(	O
5	O
–	O
7	O
.	O
5	O
°),	O
in	O
agreement	O
with	O
the	O
conformation	O
of	O
our	O
Structure	B-evidence
I	I-evidence
.	O
The	O
most	B-protein_state
swiveled	I-protein_state
head	B-structure_element
(	O
18	O
–	O
21	O
°)	O
was	O
observed	O
in	O
a	O
mid	B-protein_state
-	I-protein_state
rotated	I-protein_state
ribosome	B-complex_assembly
(	O
3	O
–	O
5	O
°)	O
of	O
a	O
later	O
translocation	O
intermediate	O
TIpost	O
,	O
similar	O
to	O
the	O
conformation	O
of	O
our	O
Structure	B-evidence
III	I-evidence
.	O

Overall	O
,	O
these	O
correlations	O
suggest	O
that	O
the	O
intermediate	O
locations	O
of	O
the	O
elusive	O
A	B-site
-	I-site
to	I-site
-	I-site
P	I-site
-	I-site
site	I-site
translocating	O
tRNA	B-chemical
are	O
similar	O
to	O
those	O
of	O
PKI	B-structure_element
in	O
our	O
structures	B-evidence
.	O

We	O
deem	O
the	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
complex	O
locked	B-protein_state
,	O
because	O
the	O
A	B-protein_state
-	I-protein_state
site	I-protein_state
bound	I-protein_state
ASL	O
-	O
mRNA	B-chemical
is	O
stabilized	O
by	O
interactions	O
with	O
the	O
decoding	B-site
center	I-site
.	O

Unlocking	O
involves	O
separation	O
of	O
the	O
codon	B-structure_element
-	I-structure_element
anticodon	I-structure_element
helix	I-structure_element
from	O
the	O
decoding	B-site
center	I-site
residues	O
by	O
the	O
protruding	O
tip	O
of	O
eEF2	B-protein
/	O
EF	B-protein
-	I-protein
G	I-protein
(	O
Figure	O
7	O
),	O
occurring	O
in	O
the	O
fully	B-protein_state
rotated	I-protein_state
ribosome	B-complex_assembly
at	O
an	O
early	O
pre	B-protein_state
-	I-protein_state
translocation	I-protein_state
step	O
.	O

Third	O
,	O
our	O
findings	O
uncover	O
a	O
new	O
role	O
of	O
the	O
head	B-structure_element
swivel	O
.	O

Finally	O
,	O
the	O
similar	O
populations	O
of	O
particles	B-experimental_method
(	O
within	O
a	O
2X	O
range	O
)	O
in	O
our	O
80S	B-complex_assembly
•	I-complex_assembly
IRES	I-complex_assembly
•	I-complex_assembly
eEF2	I-complex_assembly
reconstructions	B-evidence
(	O
Figure	O
1	O
—	O
figure	O
supplement	O
2	O
)	O
suggest	O
that	O
the	O
intermediate	O
translocation	O
states	O
sample	O
several	O
energetically	O
similar	O
and	O
interconverting	O
conformations	O
.	O

This	O
is	O
consistent	O
with	O
the	O
idea	O
of	O
a	O
rather	O
flat	O
energy	O
landscape	O
of	O
translocation	O
,	O
suggested	O
by	O
recent	O
work	O
that	O
measured	O
mechanical	O
work	O
produced	O
by	O
the	O
ribosome	B-complex_assembly
during	O
translocation	O
.	O

Our	O
findings	O
implicate	O
,	O
however	O
,	O
that	O
the	O
energy	O
landscape	O
is	O
not	O
completely	O
flat	O
and	O
contains	O
local	O
minima	O
for	O
transient	O
positions	O
of	O
the	O
codon	B-structure_element
-	I-structure_element
anticodon	I-structure_element
helix	I-structure_element
between	O
the	O
A	B-site
and	I-site
P	I-site
sites	I-site
.	O

The	O
shift	O
of	O
the	O
PKI	B-structure_element
with	O
respect	O
to	O
the	O
body	B-structure_element
occurs	O
during	O
forward	O
head	B-structure_element
swivel	O
in	O
two	O
major	O
sub	O
-	O
steps	O
of	O
~	O
4	O
Å	O
each	O
(	O
initiation	B-complex_assembly
complex	I-complex_assembly
to	O
I	B-evidence
,	O
and	O
I	B-evidence
to	O
II	B-evidence
),	O
after	O
which	O
PKI	B-structure_element
undergoes	O
small	O
shifts	O
to	O
settle	O
in	O
the	O
body	B-structure_element
P	B-site
site	I-site
in	O
Structures	B-evidence
III	I-evidence
,	I-evidence
IV	I-evidence
and	I-evidence
V	I-evidence
(	O
Figure	O
2	O
—	O
source	O
data	O
1	O
).	O

We	O
note	O
that	O
four	O
of	O
our	O
near	O
-	O
atomic	O
resolution	O
maps	B-evidence
comprised	O
~	O
30	O
,	O
000	O
particles	B-experimental_method
each	O
,	O
the	O
minimum	O
number	O
required	O
for	O
a	O
near	B-evidence
-	I-evidence
atomic	I-evidence
-	I-evidence
resolution	I-evidence
reconstruction	I-evidence
of	O
the	O
ribosome	B-complex_assembly
.	O

Translation	O
of	O
viral	B-taxonomy_domain
mRNA	B-chemical

The	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
structures	B-evidence
demonstrate	O
that	O
the	O
TSV	B-species
IRES	B-site
structurally	O
and	O
dynamically	O
represents	O
a	O
chimera	O
of	O
the	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
translocating	O
complex	O
(	O
A	B-complex_assembly
/	I-complex_assembly
P	I-complex_assembly
-	I-complex_assembly
tRNA	I-complex_assembly
•	I-complex_assembly
P	I-complex_assembly
/	I-complex_assembly
E	I-complex_assembly
-	I-complex_assembly
tRNA	I-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
).	O

Like	O
in	O
the	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
translocating	O
complex	O
in	O
which	O
the	O
two	O
tRNAs	B-chemical
move	O
independently	O
of	O
each	O
other	O
,	O
the	O
PKI	B-structure_element
domain	O
moves	O
relative	O
to	O
the	O
5	B-structure_element
´-	I-structure_element
domain	I-structure_element
,	O
causing	O
the	O
IRES	B-site
to	O
undergo	O
an	O
inchworm	B-protein_state
-	O
walk	O
translocation	O
.	O

A	O
large	O
structural	O
difference	O
between	O
the	O
IRES	B-site
and	O
the	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
complex	O
exists	O
,	O
however	O
,	O
in	O
that	O
the	O
IRES	B-site
lacks	B-protein_state
three	O
out	O
of	O
six	O
tRNA	B-structure_element
-	I-structure_element
like	I-structure_element
domains	I-structure_element
involved	O
in	O
tRNA	B-chemical
translocation	O
.	O

Although	O
structurally	O
handicapped	O
,	O
the	O
TSV	B-species
IRES	B-site
manages	O
to	O
translocate	O
by	O
employing	O
ribosome	B-complex_assembly
dynamics	O
that	O
are	O
remarkably	O
similar	O
to	O
that	O
in	O
2tRNA	B-complex_assembly
•	I-complex_assembly
mRNA	I-complex_assembly
translocation	O
.	O

The	O
uniformity	O
of	O
ribosome	B-complex_assembly
dynamics	O
underscores	O
the	O
idea	O
that	O
translocation	O
is	O
an	O
inherent	O
and	O
structurally	O
-	O
optimized	O
property	O
of	O
the	O
ribosome	B-complex_assembly
,	O
supported	O
also	O
by	O
translocation	O
activity	O
in	O
the	O
absence	B-protein_state
of	I-protein_state
the	O
elongation	B-protein_type
factor	I-protein_type
.	O

This	O
property	O
is	O
rendered	O
by	O
the	O
relative	O
mobility	O
of	O
the	O
three	O
major	O
building	O
blocks	O
,	O
the	O
60S	B-complex_assembly
subunit	B-structure_element
and	O
the	O
40S	B-complex_assembly
head	B-structure_element
and	O
body	B-structure_element
,	O
assisted	O
by	O
ligand	B-structure_element
-	I-structure_element
interacting	I-structure_element
extensions	I-structure_element
including	O
the	O
L1	B-structure_element
stalk	I-structure_element
and	O
the	O
P	B-structure_element
stalk	I-structure_element
.	O

Viral	B-taxonomy_domain
mRNAs	B-chemical
have	O
evolved	O
to	O
adopt	O
an	O
atypical	O
structure	B-evidence
to	O
employ	O
the	O
inherent	O
ribosome	B-complex_assembly
dynamics	O
,	O
to	O
be	O
able	O
to	O
hijack	O
the	O
host	O
translational	O
machinery	O
in	O
a	O
simple	O
fashion	O
.	O

Our	O
current	O
understanding	O
of	O
macromolecular	O
machines	O
,	O
such	O
as	O
the	O
ribosome	B-complex_assembly
,	O
is	O
often	O
limited	O
by	O
a	O
gap	O
between	O
biophysical	B-experimental_method
/	I-experimental_method
biochemical	I-experimental_method
studies	I-experimental_method
and	O
structural	B-experimental_method
studies	I-experimental_method
.	O

High	O
-	O
resolution	O
crystal	B-evidence
structures	I-evidence
,	O
on	O
the	O
other	O
hand	O
,	O
can	O
provide	O
static	O
images	O
of	O
an	O
assembly	O
,	O
and	O
the	O
structural	O
dynamics	O
can	O
only	O
be	O
inferred	O
by	O
comparing	O
structures	B-evidence
that	O
are	O
usually	O
obtained	O
in	O
different	O
experiments	O
and	O
under	O
different	O
,	O
often	O
non	O
-	O
native	O
,	O
conditions	O
.	O

Cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
offers	O
the	O
possibility	O
of	O
obtaining	O
integrated	O
information	O
of	O
both	O
structure	B-evidence
and	O
dynamics	O
as	O
demonstrated	O
in	O
lower	O
-	O
resolution	O
studies	O
of	O
bacterial	B-taxonomy_domain
ribosome	B-complex_assembly
complexes	O
.	O

This	O
is	O
presumably	O
one	O
of	O
the	O
reasons	O
why	O
most	O
recent	O
studies	O
of	O
ribosome	B-complex_assembly
complexes	O
have	O
focused	O
on	O
a	O
single	O
high	O
-	O
resolution	O
structure	B-evidence
despite	O
the	O
non	O
-	O
uniform	O
local	O
resolution	O
of	O
the	O
maps	B-evidence
that	O
likely	O
reflects	O
structural	O
heterogeneity	O
.	O

The	O
computational	O
efficiency	O
of	O
FREALIGN	B-experimental_method
has	O
allowed	O
us	O
to	O
classify	O
a	O
relatively	O
large	O
dataset	O
(	O
1	O
.	O
1	O
million	O
particles	B-experimental_method
)	O
into	O
15	O
classes	O
(	O
Figure	O
1	O
—	O
figure	O
supplement	O
2	O
)	O
and	O
obtain	O
eight	O
near	O
-	O
atomic	O
-	O
resolution	O
structures	B-evidence
from	O
it	O
.	O

The	O
classification	O
,	O
which	O
followed	O
an	O
initial	O
alignment	O
of	O
all	O
particles	B-experimental_method
to	O
a	O
single	O
reference	O
,	O
required	O
about	O
130	O
,	O
000	O
CPU	O
hours	O
or	O
about	O
five	O
to	O
six	O
full	O
days	O
on	O
a	O
1000	O
-	O
CPU	O
cluster	O
.	O

Therefore	O
,	O
cryo	B-experimental_method
-	I-experimental_method
EM	I-experimental_method
has	O
the	O
potential	O
to	O
become	O
a	O
standard	O
tool	O
for	O
uncovering	O
detailed	O
dynamic	O
pathways	O
of	O
complex	O
macromolecular	O
machines	O
.	O

The	O
N	O
-	O
terminal	O
propeptides	B-structure_element
protecting	O
the	O
active	B-site
-	I-site
site	I-site
threonines	B-residue_name
are	O
autocatalytically	B-ptm
released	O
only	O
on	O
completion	O
of	O
assembly	O
.	O

However	O
,	O
the	O
trigger	O
for	O
the	O
self	O
-	O
activation	O
and	O
the	O
reason	O
for	O
the	O
strict	B-protein_state
conservation	I-protein_state
of	O
threonine	B-residue_name
as	O
the	O
active	O
site	O
nucleophile	O
remain	O
enigmatic	O
.	O

Here	O
we	O
use	O
mutagenesis	B-experimental_method
,	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
and	O
biochemical	B-experimental_method
assays	I-experimental_method
to	O
suggest	O
that	O
Lys33	B-residue_name_number
initiates	O
nucleophilic	O
attack	O
of	O
the	O
propeptide	B-structure_element
by	O
deprotonating	O
the	O
Thr1	B-residue_name_number
hydroxyl	O
group	O
and	O
that	O
both	O
residues	O
together	O
with	O
Asp17	B-residue_name_number
are	O
part	O
of	O
a	O
catalytic	B-site
triad	I-site
.	O

Substitution	B-experimental_method
of	O
Thr1	B-residue_name_number
by	O
Cys	B-residue_name
disrupts	O
the	O
interaction	O
with	O
Lys33	B-residue_name_number
and	O
inactivates	B-protein_state
the	O
proteasome	B-complex_assembly
.	O

Although	O
a	O
Thr1Ser	B-mutant
mutant	B-protein_state
is	O
active	B-protein_state
,	O
it	O
is	O
less	O
efficient	O
compared	O
with	O
wild	B-protein_state
type	I-protein_state
because	O
of	O
the	O
unfavourable	O
orientation	O
of	O
Ser1	B-residue_name_number
towards	O
incoming	O
substrates	O
.	O

The	O
proteasome	B-complex_assembly
,	O
an	O
essential	O
molecular	O
machine	O
,	O
is	O
a	O
threonine	B-protein_type
protease	I-protein_type
,	O
but	O
the	O
evolution	O
and	O
the	O
components	O
of	O
its	O
proteolytic	O
centre	O
are	O
unclear	O
.	O

Here	O
,	O
the	O
authors	O
use	O
structural	O
biology	O
and	O
biochemistry	O
to	O
investigate	O
the	O
role	O
of	O
proteasome	B-complex_assembly
active	B-site
site	I-site
residues	O
on	O
maturation	O
and	O
activity	O
.	O

The	O
20S	B-complex_assembly
proteasome	I-complex_assembly
core	I-complex_assembly
particle	I-complex_assembly
(	O
CP	B-complex_assembly
)	O
is	O
the	O
key	O
non	B-protein_type
-	I-protein_type
lysosomal	I-protein_type
protease	I-protein_type
of	O
eukaryotic	B-taxonomy_domain
cells	O
.	O

Its	O
seven	O
different	O
α	B-protein
and	O
seven	O
different	O
β	B-protein
subunits	I-protein
assemble	O
into	O
four	O
heptameric	B-oligomeric_state
rings	B-structure_element
that	O
are	O
stacked	O
on	O
each	O
other	O
to	O
form	O
a	O
hollow	B-structure_element
cylinder	I-structure_element
.	O

Release	O
of	O
the	O
propeptides	B-structure_element
creates	O
a	O
functionally	O
active	B-protein_state
CP	B-complex_assembly
that	O
cleaves	O
proteins	O
into	O
short	O
peptides	O
.	O

Nucleophilic	O
attack	O
of	O
Thr1Oγ	B-residue_name_number
on	O
the	O
carbonyl	O
carbon	O
atom	O
of	O
the	O
scissile	O
peptide	O
bond	O
creates	O
a	O
first	O
cleavage	O
product	O
and	O
a	O
covalent	O
acyl	O
-	O
enzyme	O
intermediate	O
.	O

Hydrolysis	O
of	O
this	O
complex	B-complex_assembly
by	O
the	O
addition	O
of	O
a	O
nucleophilic	O
water	B-chemical
molecule	O
regenerates	O
the	O
enzyme	B-complex_assembly
and	O
releases	O
the	O
second	O
peptide	B-chemical
fragment	O
.	O

The	O
proteasome	B-complex_assembly
belongs	O
to	O
the	O
family	O
of	O
N	B-protein_type
-	I-protein_type
terminal	I-protein_type
nucleophilic	I-protein_type
(	I-protein_type
Ntn	I-protein_type
)	I-protein_type
hydrolases	I-protein_type
,	O
and	O
the	O
free	B-protein_state
N	O
-	O
terminal	O
amine	O
group	O
of	O
Thr1	B-residue_name_number
was	O
proposed	O
to	O
deprotonate	O
the	O
Thr1	B-residue_name_number
hydroxyl	O
group	O
to	O
generate	O
a	O
nucleophilic	O
Thr1Oγ	B-residue_name_number
for	O
peptide	O
-	O
bond	O
cleavage	O
.	O

An	O
alternative	O
candidate	O
for	O
deprotonating	O
the	O
Thr1	B-residue_name_number
hydroxyl	O
group	O
is	O
the	O
side	O
chain	O
of	O
Lys33	B-residue_name_number
as	O
it	O
is	O
within	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonding	I-bond_interaction
distance	O
to	O
Thr1OH	B-residue_name_number
(	O
2	O
.	O
7	O
Å	O
).	O

In	O
principle	O
it	O
could	O
function	O
as	O
the	O
general	O
base	O
during	O
both	O
autocatalytic	B-ptm
removal	I-ptm
of	O
the	O
propeptide	B-structure_element
and	O
protein	O
substrate	O
cleavage	O
.	O

Here	O
we	O
provide	O
experimental	O
evidences	O
for	O
this	O
distinct	O
view	O
of	O
the	O
proteasome	B-complex_assembly
active	B-site
-	I-site
site	I-site
mechanism	O
.	O

Furthermore	O
,	O
we	O
determine	O
the	O
advantages	O
of	O
Thr	B-residue_name
over	O
Cys	B-residue_name
or	O
Ser	B-residue_name
as	O
the	O
active	O
-	O
site	O
nucleophile	O
using	O
X	B-experimental_method
-	I-experimental_method
ray	I-experimental_method
crystallography	I-experimental_method
together	O
with	O
activity	B-experimental_method
and	I-experimental_method
inhibition	I-experimental_method
assays	I-experimental_method
.	O

Proteasome	B-complex_assembly
-	O
mediated	O
degradation	O
of	O
cell	O
-	O
cycle	O
regulators	O
and	O
potentially	O
toxic	O
misfolded	O
proteins	O
is	O
required	O
for	O
the	O
viability	O
of	O
eukaryotic	B-taxonomy_domain
cells	O
.	O

Viability	O
is	O
restored	O
if	O
the	O
β5	B-mutant
-	I-mutant
T1A	I-mutant
subunit	O
has	O
its	O
propeptide	B-structure_element
(	O
pp	B-chemical
)	O
deleted	B-experimental_method
but	I-experimental_method
expressed	I-experimental_method
separately	I-experimental_method
in	O
trans	B-protein_state
(	O
β5	B-mutant
-	I-mutant
T1A	I-mutant
pp	B-chemical
trans	B-protein_state
),	O
although	O
substantial	O
phenotypic	O
impairment	O
remains	O
(	O
Table	O
1	O
).	O

Sequencing	B-experimental_method
of	I-experimental_method
the	I-experimental_method
plasmids	I-experimental_method
,	O
testing	O
them	O
in	O
both	O
published	O
yeast	B-taxonomy_domain
strain	O
backgrounds	O
and	O
site	B-experimental_method
-	I-experimental_method
directed	I-experimental_method
mutagenesis	I-experimental_method
revealed	O
that	O
the	O
β5	B-mutant
-	I-mutant
T1A	I-mutant
mutant	B-protein_state
pp	B-chemical
cis	B-protein_state
is	O
viable	O
,	O
but	O
suffers	O
from	O
a	O
marked	O
growth	O
defect	O
that	O
requires	O
extended	O
incubation	O
of	O
4	O
–	O
5	O
days	O
for	O
initial	O
colony	O
formation	O
(	O
Table	O
1	O
and	O
Supplementary	O
Methods	O
).	O

We	O
also	O
identified	O
an	O
additional	O
point	O
mutation	O
K81R	B-mutant
in	O
subunit	O
β5	B-protein
that	O
was	O
present	O
in	O
the	O
allele	O
used	O
in	O
ref	O
..	O
This	B-experimental_method
single	I-experimental_method
amino	I-experimental_method
-	I-experimental_method
acid	I-experimental_method
exchange	I-experimental_method
is	O
located	O
at	O
the	O
interface	B-site
of	O
the	O
subunits	O
α4	B-protein
,	O
β4	B-protein
and	O
β5	B-protein
(	O
Supplementary	O
Fig	O
.	O
1b	O
)	O
and	O
might	O
weakly	O
promote	O
CP	B-complex_assembly
assembly	O
by	O
enhancing	O
inter	O
-	O
subunit	O
contacts	O
.	O

The	O
slightly	O
better	O
growth	O
of	O
the	O
β5	B-mutant
-	I-mutant
T1A	I-mutant
-	I-mutant
K81R	I-mutant
mutant	B-protein_state
allowed	O
us	O
to	O
solve	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
a	O
yeast	B-taxonomy_domain
proteasome	B-complex_assembly
(	O
yCP	B-complex_assembly
)	O
with	O
the	O
β5	B-mutant
-	I-mutant
T1A	I-mutant
mutation	O
,	O
which	O
is	O
discussed	O
in	O
the	O
following	O
section	O
(	O
for	O
details	O
see	O
Supplementary	O
Note	O
1	O
).	O

Propeptide	B-structure_element
conformation	O
and	O
triggering	O
of	O
autolysis	B-ptm

In	O
the	O
final	O
steps	O
of	O
proteasome	B-complex_assembly
biogenesis	O
,	O
the	O
propeptides	B-structure_element
are	O
autocatalytically	B-ptm
cleaved	I-ptm
from	O
the	O
mature	B-protein_state
β	B-protein
-	I-protein
subunit	I-protein
domains	I-protein
.	O

For	O
subunit	O
β1	B-protein
,	O
this	O
process	O
was	O
previously	O
inferred	O
to	O
require	O
that	O
the	O
propeptide	B-structure_element
residue	O
at	O
position	O
(-	B-residue_number
2	I-residue_number
)	I-residue_number
of	O
the	O
subunit	O
precursor	O
occupies	O
the	O
S1	B-site
specificity	I-site
pocket	I-site
of	O
the	O
substrate	B-site
-	I-site
binding	I-site
channel	I-site
formed	O
by	O
amino	O
acid	O
45	B-residue_number
(	O
for	O
details	O
see	O
Supplementary	O
Note	O
2	O
).	O

Furthermore	O
,	O
it	O
was	O
observed	O
that	O
the	O
prosegment	B-structure_element
forms	O
an	O
antiparallel	B-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
in	O
the	O
active	B-site
site	I-site
,	O
and	O
that	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
adopts	O
a	O
γ	B-structure_element
-	I-structure_element
turn	I-structure_element
conformation	I-structure_element
,	O
which	O
by	O
definition	O
is	O
characterized	O
by	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
between	O
Leu	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
O	O
and	O
Thr1NH	B-residue_name_number
(	O
ref	O
.).	O

Here	O
we	O
again	O
analysed	O
the	O
β1	B-mutant
-	I-mutant
T1A	I-mutant
mutant	B-protein_state
crystallographically	B-experimental_method
but	O
in	O
addition	O
determined	O
the	O
structures	B-evidence
of	O
the	O
β2	B-mutant
-	I-mutant
T1A	I-mutant
single	O
and	O
β1	B-mutant
-	I-mutant
T1A	I-mutant
-	I-mutant
β2	I-mutant
-	I-mutant
T1A	I-mutant
double	O
mutants	O
(	O
Protein	O
Data	O
Bank	O
(	O
PDB	O
)	O
entry	O
codes	O
are	O
provided	O
in	O
Supplementary	O
Table	O
1	O
).	O

However	O
,	O
the	O
γ	B-structure_element
-	I-structure_element
turn	I-structure_element
conformation	I-structure_element
and	O
the	O
associated	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
initially	O
proposed	O
is	O
for	O
geometric	O
and	O
chemical	O
reasons	O
inappropriate	O
and	O
would	O
not	O
perfectly	O
position	O
the	O
carbonyl	O
carbon	O
atom	O
of	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
for	O
nucleophilic	O
attack	O
by	O
Thr1	B-residue_name_number
.	O

Surprisingly	O
,	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
is	O
completely	O
extended	O
and	O
forces	O
the	O
histidine	B-residue_name
side	O
chain	O
at	O
position	O
(-	B-residue_number
2	I-residue_number
)	I-residue_number
to	O
occupy	O
the	O
S2	B-site
instead	O
of	O
the	O
S1	B-site
pocket	I-site
,	O
thereby	O
disrupting	O
the	O
antiparallel	B-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
.	O

Nonetheless	O
,	O
the	O
carbonyl	O
carbon	O
of	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
would	O
be	O
ideally	O
placed	O
for	O
nucleophilic	O
attack	O
by	O
Thr1Oγ	B-residue_name_number
(	O
Fig	O
.	O
1c	O
and	O
Supplementary	O
Fig	O
.	O
2c	O
,	O
d	O
).	O

As	O
the	O
K81R	B-mutant
mutation	O
is	O
located	O
far	O
from	O
the	O
active	B-site
site	I-site
(	O
Thr1Cα	B-residue_name_number
–	O
Arg81Cα	B-residue_name_number
:	O
24	O
Å	O
),	O
any	O
influence	O
on	O
propeptide	B-structure_element
conformation	O
can	O
be	O
excluded	O
.	O

Instead	O
,	O
the	O
plasticity	O
of	O
the	O
β5	B-protein
S1	B-site
pocket	I-site
caused	O
by	O
the	O
rotational	O
flexibility	O
of	O
Met45	B-residue_name_number
might	O
prevent	O
stable	O
accommodation	O
of	O
His	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
in	O
the	O
S1	B-site
site	I-site
and	O
thus	O
also	O
promote	O
its	O
immediate	O
release	O
after	O
autolysis	B-ptm
.	O

Remarkably	O
,	O
eukaryotic	B-taxonomy_domain
proteasomal	O
β5	B-protein
subunits	O
bear	O
a	O
His	B-residue_name
residue	O
in	O
position	O
(-	B-residue_number
2	I-residue_number
)	I-residue_number
of	O
the	O
propeptide	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
3a	O
).	O

In	O
agreement	O
,	O
the	O
chymotrypsin	O
-	O
like	O
(	O
ChT	O
-	O
L	O
)	O
activity	O
of	O
H	B-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
N	I-mutant
and	O
H	B-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
F	I-mutant
mutant	B-protein_state
yCPs	B-complex_assembly
was	O
impaired	O
in	O
situ	O
and	O
in	O
vitro	O
(	O
Supplementary	O
Fig	O
.	O
3c	O
).	O

Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
and	O
Phe	B-residue_name
/	O
Lys	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
were	O
visualized	O
at	O
low	O
occupancy	O
,	O
while	O
Ala	B-residue_name
/	O
Asn	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
could	O
not	O
be	O
assigned	O
.	O

This	O
observation	O
indicates	O
a	O
mixture	O
of	O
processed	B-protein_state
and	O
unprocessed	B-protein_state
β5	B-protein
subunits	O
and	O
partially	O
impaired	O
autolysis	B-ptm
,	O
thereby	O
excluding	O
any	O
essential	O
role	O
of	O
residue	O
(-	B-residue_number
2	I-residue_number
)	I-residue_number
as	O
the	O
general	O
base	O
.	O

Next	O
,	O
we	O
examined	O
the	O
effect	O
of	O
residue	O
(-	B-residue_number
2	I-residue_number
)	I-residue_number
on	O
the	O
orientation	O
of	O
the	O
propeptide	B-structure_element
by	O
creating	B-experimental_method
mutants	I-experimental_method
that	I-experimental_method
combine	I-experimental_method
the	O
T1A	B-mutant
(	O
K81R	B-mutant
)	O
mutation	B-experimental_method
(	I-experimental_method
s	I-experimental_method
)	I-experimental_method
with	O
H	B-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
L	I-mutant
,	O
H	B-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
T	I-mutant
or	O
H	B-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
A	I-mutant
substitutions	B-experimental_method
.	O

Leu	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
is	O
encoded	O
in	O
the	O
yeast	B-taxonomy_domain
β1	B-protein
subunit	O
precursor	O
(	O
Supplementary	O
Fig	O
.	O
3a	O
);	O
Thr	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
is	O
generally	O
part	O
of	O
β2	B-protein
-	O
propeptides	B-structure_element
(	O
Supplementary	O
Fig	O
.	O
3a	O
);	O
and	O
Ala	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
was	O
expected	O
to	O
fit	O
the	O
β5	B-protein
-	O
S1	B-site
pocket	I-site
without	O
inducing	O
conformational	O
changes	O
of	O
Met45	B-residue_name_number
,	O
allowing	O
it	O
to	O
accommodate	O
‘	O
β1	O
-	O
like	O
'	O
propeptide	O
positioning	O
.	O

As	O
expected	O
from	O
β5	B-mutant
-	I-mutant
T1A	I-mutant
mutants	O
,	O
the	O
yeasts	B-taxonomy_domain
show	O
severe	O
growth	O
phenotypes	O
,	O
with	O
minor	O
variations	O
(	O
Supplementary	O
Fig	O
.	O
4a	O
and	O
Table	O
1	O
).	O

We	O
determined	O
crystal	B-evidence
structures	I-evidence
of	O
the	O
β5	B-mutant
-	I-mutant
H	I-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
L	I-mutant
-	I-mutant
T1A	I-mutant
,	O
β5	B-mutant
-	I-mutant
H	I-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
T	I-mutant
-	I-mutant
T1A	I-mutant
and	O
the	O
β5	B-mutant
-	I-mutant
H	I-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
A	I-mutant
-	I-mutant
T1A	I-mutant
-	I-mutant
K81R	I-mutant
mutants	O
(	O
Supplementary	O
Table	O
1	O
).	O

For	O
the	O
β5	B-mutant
-	I-mutant
H	I-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
A	I-mutant
-	I-mutant
T1A	I-mutant
-	I-mutant
K81R	I-mutant
variant	O
,	O
only	O
the	O
residues	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
and	O
Ala	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
could	O
be	O
visualized	O
,	O
indicating	O
that	O
Ala	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
leads	O
to	O
insufficient	O
stabilization	O
of	O
the	O
propeptide	B-structure_element
in	O
the	O
substrate	B-site
-	I-site
binding	I-site
channel	I-site
(	O
Supplementary	O
Fig	O
.	O
4d	O
).	O

By	O
contrast	O
,	O
the	O
prosegments	B-structure_element
of	O
the	O
β5	B-mutant
-	I-mutant
H	I-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
L	I-mutant
-	I-mutant
T1A	I-mutant
and	O
the	O
β5	B-mutant
-	I-mutant
H	I-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
T	I-mutant
-	I-mutant
T1A	I-mutant
mutants	O
were	O
significantly	O
better	O
resolved	O
in	O
the	O
2FO	B-evidence
–	I-evidence
FC	I-evidence
electron	I-evidence
-	I-evidence
density	I-evidence
maps	I-evidence
yet	O
not	O
at	O
full	O
occupancy	O
(	O
Supplementary	O
Fig	O
.	O
4b	O
,	O
c	O
and	O
Supplementary	O
Table	O
1	O
),	O
suggesting	O
that	O
the	O
natural	O
propeptide	B-structure_element
bearing	O
His	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
is	O
most	O
favourable	O
.	O

This	O
result	O
proves	O
that	O
the	O
naturally	O
occurring	O
His	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
of	O
the	O
β5	B-protein
propeptide	B-structure_element
does	O
not	O
stably	O
fit	O
into	O
the	O
S1	B-site
site	I-site
.	O

Since	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
adopts	O
the	O
same	O
position	O
in	O
both	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
WT	B-protein_state
)	O
and	O
mutant	B-protein_state
β5	B-protein
propeptides	B-structure_element
,	O
and	O
since	O
in	O
all	O
cases	O
its	O
carbonyl	O
carbon	O
is	O
perfectly	O
placed	O
for	O
nucleophilic	O
attack	O
by	O
Thr1Oγ	B-residue_name_number
(	O
Fig	O
.	O
2b	O
),	O
we	O
propose	O
that	O
neither	O
binding	O
of	O
residue	O
(-	B-residue_number
2	I-residue_number
)	I-residue_number
to	O
the	O
S1	B-site
pocket	I-site
nor	O
formation	O
of	O
the	O
antiparallel	B-structure_element
β	I-structure_element
-	I-structure_element
sheet	I-structure_element
is	O
essential	O
for	O
autolysis	B-ptm
of	O
the	O
propeptide	B-structure_element
.	O

Next	O
,	O
we	O
determined	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
a	O
chimeric	B-protein_state
yCP	B-complex_assembly
having	O
the	O
yeast	B-taxonomy_domain
β1	B-protein
-	O
propeptide	B-structure_element
replaced	B-experimental_method
by	I-experimental_method
its	O
β5	B-protein
counterpart	B-structure_element
.	O

As	O
proven	O
by	O
the	O
β2	B-mutant
-	I-mutant
T1A	I-mutant
crystal	B-evidence
structures	I-evidence
,	O
Thr	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
hydrogen	B-bond_interaction
bonds	I-bond_interaction
to	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
O	O
.	O
Although	O
this	O
interaction	O
was	O
not	O
observed	O
for	O
the	O
β5	B-mutant
-	I-mutant
H	I-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
T	I-mutant
-	I-mutant
T1A	I-mutant
mutant	B-protein_state
(	O
Fig	O
.	O
2c	O
and	O
Supplementary	O
Fig	O
.	O
4c	O
,	O
i	O
),	O
exchange	B-experimental_method
of	O
Thr	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
by	O
Val	B-residue_name
in	O
β2	B-protein
,	O
a	O
conservative	O
mutation	O
regarding	O
size	O
but	O
drastic	O
with	O
respect	O
to	O
polarity	O
,	O
was	O
found	O
to	O
inhibit	O
maturation	O
of	O
this	O
subunit	O
(	O
Fig	O
.	O
2d	O
and	O
Supplementary	O
Fig	O
.	O
4e	O
,	O
j	O
).	O

In	O
particular	O
,	O
Val	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
is	O
displaced	O
from	O
the	O
S1	B-site
site	I-site
and	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
is	O
severely	O
shifted	O
(	O
movement	O
of	O
the	O
carbonyl	O
oxygen	O
atom	O
of	O
3	O
.	O
8	O
Å	O
),	O
thereby	O
preventing	O
nucleophilic	O
attack	O
of	O
Thr1	B-residue_name_number
(	O
Fig	O
.	O
2d	O
and	O
Supplementary	O
Fig	O
.	O
4j	O
,	O
k	O
).	O

These	O
results	O
further	O
confirm	O
that	O
correct	O
positioning	O
of	O
the	O
active	B-site
-	I-site
site	I-site
residues	I-site
and	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
is	O
decisive	O
for	O
the	O
maturation	O
of	O
the	O
proteasome	B-complex_assembly
.	O

Proton	O
shuttling	O
from	O
the	O
proteasomal	O
active	B-site
site	I-site
Thr1OH	B-residue_name_number
to	O
Thr1NH2	B-residue_name_number
via	O
a	O
nucleophilic	O
water	B-chemical
molecule	O
was	O
suggested	O
to	O
initiate	O
peptide	O
-	O
bond	O
hydrolysis	O
.	O

However	O
,	O
in	O
the	O
immature	B-protein_state
particle	B-complex_assembly
Thr1NH2	B-residue_name_number
is	O
blocked	O
by	O
the	O
propeptide	B-structure_element
and	O
cannot	O
activate	O
Thr1Oγ	B-residue_name_number
.	O

A	O
proposed	O
catalytic	B-site
tetrad	I-site
model	O
involving	O
Thr1OH	B-residue_name_number
,	O
Thr1NH2	B-residue_name_number
,	O
Lys33NH2	B-residue_name_number
and	O
Asp17Oδ	B-residue_name_number
,	O
as	O
well	O
as	O
a	O
nucleophilic	O
water	B-chemical
molecule	O
as	O
the	O
proton	O
shuttle	O
appeared	O
to	O
accommodate	O
all	O
possible	O
views	O
of	O
the	O
proteasomal	O
active	B-site
site	I-site
.	O

Twenty	O
years	O
later	O
,	O
with	O
a	O
plethora	O
of	O
yCP	B-complex_assembly
X	B-evidence
-	I-evidence
ray	I-evidence
structures	I-evidence
in	O
hand	O
,	O
we	O
decided	O
to	O
re	O
-	O
analyse	O
the	O
active	B-site
site	I-site
of	O
the	O
proteasome	B-complex_assembly
and	O
to	O
resolve	O
the	O
uncertainty	O
regarding	O
the	O
nature	O
of	O
the	O
general	O
base	O
.	O

This	O
discrepancy	O
in	O
growth	O
was	O
traced	O
to	O
an	O
additional	O
point	O
mutation	O
L	B-mutant
(-	I-mutant
49	I-mutant
)	I-mutant
S	I-mutant
in	O
the	O
β5	B-protein
-	O
propeptide	B-structure_element
of	O
the	O
β5	B-mutant
-	I-mutant
K33A	I-mutant
mutant	B-protein_state
(	O
see	O
also	O
Supplementary	O
Note	O
1	O
).	O

This	O
structural	O
alteration	O
destroys	O
active	B-site
-	I-site
site	I-site
integrity	O
and	O
abolishes	O
catalytic	O
activity	O
of	O
the	O
β5	B-protein
active	B-site
site	I-site
(	O
Supplementary	O
Fig	O
.	O
5a	O
).	O

Additional	O
proof	O
for	O
the	O
key	O
function	O
of	O
Lys33	B-residue_name_number
was	O
obtained	O
from	O
the	O
β5	B-mutant
-	I-mutant
K33A	I-mutant
mutant	B-protein_state
,	O
with	O
the	O
propeptide	B-structure_element
expressed	B-experimental_method
separately	I-experimental_method
from	O
the	O
main	O
subunit	O
(	O
pp	B-chemical
trans	B-protein_state
).	O

The	O
Thr1	B-residue_name_number
N	O
terminus	O
of	O
this	O
mutant	B-protein_state
is	O
not	O
blocked	O
by	O
the	O
propeptide	B-structure_element
,	O
yet	O
its	O
catalytic	O
activity	O
is	O
reduced	O
by	O
∼	O
83	O
%	O
(	O
Supplementary	O
Fig	O
.	O
6b	O
).	O

Consistent	O
with	O
this	O
,	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
β5	B-mutant
-	I-mutant
K33A	I-mutant
pp	B-chemical
trans	B-protein_state
mutant	B-protein_state
in	B-protein_state
complex	I-protein_state
with	I-protein_state
carfilzomib	B-chemical
only	O
showed	O
partial	O
occupancy	O
of	O
the	O
ligand	O
at	O
the	O
β5	B-protein
active	B-site
sites	I-site
(	O
Supplementary	O
Fig	O
.	O
5b	O
and	O
Supplementary	O
Table	O
1	O
).	O

Since	O
no	O
acetylation	B-ptm
of	O
the	O
Thr1	B-residue_name_number
N	O
terminus	O
was	O
observed	O
for	O
the	O
β5	B-mutant
-	I-mutant
K33A	I-mutant
pp	B-chemical
trans	B-protein_state
apo	B-protein_state
crystal	B-evidence
structure	I-evidence
,	O
the	O
reduced	O
reactivity	O
towards	O
substrates	O
and	O
inhibitors	O
indicates	O
that	O
Lys33NH2	B-residue_name_number
,	O
rather	O
than	O
Thr1NH2	B-residue_name_number
,	O
deprotonates	O
and	O
activates	O
Thr1OH	B-residue_name_number
.	O

Remarkably	O
,	O
the	O
solvent	O
molecule	O
occupies	O
the	O
position	O
normally	O
taken	O
by	O
Lys33NH2	B-residue_name_number
in	O
the	O
WT	B-protein_state
proteasome	B-complex_assembly
structure	B-evidence
(	O
Fig	O
.	O
3c	O
),	O
further	O
corroborating	O
the	O
essential	O
role	O
of	O
Lys33	B-residue_name_number
as	O
the	O
general	O
base	O
for	O
autolysis	B-ptm
and	O
proteolysis	O
.	O

Conservative	B-experimental_method
substitution	I-experimental_method
of	O
Lys33	B-residue_name_number
by	O
Arg	B-residue_name
delays	O
autolysis	B-ptm
of	O
the	O
β5	B-protein
precursor	O
and	O
impairs	O
yeast	B-taxonomy_domain
growth	O
(	O
for	O
details	O
see	O
Supplementary	O
Note	O
1	O
).	O

While	O
Thr1	B-residue_name_number
occupies	O
the	O
same	O
position	O
as	O
in	O
WT	B-protein_state
yCPs	B-complex_assembly
,	O
Arg33	B-residue_name_number
is	O
unable	O
to	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
to	O
Asp17	B-residue_name_number
,	O
thereby	O
inactivating	O
the	O
β5	B-protein
active	B-site
site	I-site
(	O
Supplementary	O
Fig	O
.	O
5e	O
).	O

The	O
conservative	B-experimental_method
mutation	I-experimental_method
of	O
Asp17	B-residue_name_number
to	O
Asn	B-residue_name
in	O
subunit	O
β5	B-protein
of	O
the	O
yCP	B-complex_assembly
also	O
provokes	O
a	O
severe	O
growth	O
defect	O
(	O
Supplementary	O
Note	O
1	O
,	O
Supplementary	O
Fig	O
.	O
6a	O
and	O
Table	O
1	O
).	O

Notably	O
,	O
only	O
with	O
the	O
additional	O
point	O
mutation	O
L	B-mutant
(-	I-mutant
49	I-mutant
)	I-mutant
S	I-mutant
present	O
in	O
the	O
β5	B-protein
propeptide	B-structure_element
could	O
we	O
purify	O
a	O
small	O
amount	O
of	O
the	O
β5	B-mutant
-	I-mutant
D17N	I-mutant
mutant	B-protein_state
yCP	B-complex_assembly
.	O

As	O
determined	O
by	O
crystallographic	B-experimental_method
analysis	I-experimental_method
,	O
this	O
mutant	B-protein_state
β5	B-protein
subunit	O
was	O
partially	B-protein_state
processed	I-protein_state
(	O
Table	O
1	O
)	O
but	O
displayed	O
impaired	O
reactivity	O
towards	O
the	O
proteasome	B-complex_assembly
inhibitor	O
carfilzomib	B-chemical
compared	O
with	O
the	O
subunits	O
β1	B-protein
and	O
β2	B-protein
,	O
and	O
with	O
WT	B-protein_state
β5	B-protein
(	O
Supplementary	O
Fig	O
.	O
7a	O
).	O

Autolysis	B-ptm
and	O
residual	O
catalytic	O
activity	O
of	O
the	O
β5	B-mutant
-	I-mutant
D17N	I-mutant
mutants	O
may	O
originate	O
from	O
the	O
carbonyl	O
group	O
of	O
Asn17	B-residue_name_number
,	O
which	O
albeit	O
to	O
a	O
lower	O
degree	O
still	O
can	O
polarize	O
Lys33	B-residue_name_number
for	O
the	O
activation	O
of	O
Thr1	B-residue_name_number
.	O

Strikingly	O
,	O
although	O
the	O
X	B-evidence
-	I-evidence
ray	I-evidence
data	I-evidence
on	O
the	O
β5	B-mutant
-	I-mutant
D17N	I-mutant
mutant	B-protein_state
with	O
the	O
propeptide	B-structure_element
expressed	B-experimental_method
in	O
cis	B-protein_state
and	O
in	O
trans	B-protein_state
looked	O
similar	O
,	O
there	O
was	O
a	O
pronounced	O
difference	O
in	O
their	O
growth	O
phenotypes	O
observed	O
(	O
Supplementary	O
Fig	O
.	O
6a	O
and	O
Supplementary	O
Fig	O
.	O
7b	O
).	O

This	O
model	O
is	O
also	O
consistent	O
with	O
the	O
fact	O
that	O
no	O
defined	O
water	B-chemical
molecule	O
is	O
observed	O
in	O
the	O
mature	B-protein_state
WT	B-protein_state
proteasomal	O
active	B-site
site	I-site
that	O
could	O
shuttle	O
the	O
proton	O
from	O
Thr1Oγ	B-residue_name_number
to	O
Thr1NH2	B-residue_name_number
.	O

To	O
explore	O
this	O
active	B-site
-	I-site
site	I-site
model	O
further	O
,	O
we	O
exchanged	B-experimental_method
the	I-experimental_method
conserved	I-experimental_method
Asp166	B-residue_name_number
residue	O
for	O
Asn	B-residue_name
in	O
the	O
yeast	B-taxonomy_domain
β5	B-protein
subunit	O
.	O

Asp166Oδ	B-residue_name_number
is	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
to	O
Thr1NH2	B-residue_name_number
via	O
Ser129OH	B-residue_name_number
and	O
Ser169OH	B-residue_name_number
,	O
and	O
therefore	O
was	O
proposed	O
to	O
be	O
involved	O
in	O
catalysis	O
.	O

Instead	O
,	O
a	O
water	B-chemical
molecule	O
is	O
bound	B-protein_state
to	I-protein_state
Ser129OH	B-residue_name_number
and	O
Thr1NH2	B-residue_name_number
(	O
Supplementary	O
Fig	O
.	O
8b	O
),	O
which	O
may	O
enable	O
precursor	B-ptm
processing	I-ptm
.	O

The	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
involving	O
Ser169OH	B-residue_name_number
are	O
intact	O
and	O
may	O
account	O
for	O
residual	O
substrate	O
turnover	O
.	O

In	O
the	O
carfilzomib	B-complex_assembly
complex	I-complex_assembly
structure	B-evidence
,	O
Thr1Oγ	B-residue_name_number
and	O
Thr1N	B-residue_name_number
incorporate	O
into	O
a	O
morpholine	O
ring	O
structure	O
and	O
Ser129	B-residue_name_number
adopts	O
its	O
WT	B-protein_state
-	O
like	O
orientation	O
.	O

In	O
the	O
MG132	B-protein_state
-	I-protein_state
bound	I-protein_state
state	I-protein_state
,	O
Thr1N	B-residue_name_number
is	O
unmodified	B-protein_state
,	O
and	O
we	O
again	O
observe	O
that	O
Ser129	B-residue_name_number
is	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
to	O
a	O
water	B-chemical
molecule	O
instead	O
of	O
Asn166	B-residue_name_number
.	O

Whereas	O
Asn	B-residue_name
can	O
to	O
some	O
degree	O
replace	O
Asp166	B-residue_name_number
due	O
to	O
its	O
carbonyl	O
group	O
in	O
the	O
side	O
chain	O
,	O
Ala	B-residue_name
at	O
this	O
position	O
was	O
found	O
to	O
prevent	O
both	O
autolysis	B-ptm
and	O
catalysis	O
.	O

Substitution	B-experimental_method
of	O
the	O
active	B-site
-	I-site
site	I-site
Thr1	B-residue_name_number
by	O
Cys	B-residue_name

Mutation	B-experimental_method
of	O
Thr1	B-residue_name_number
to	O
Cys	B-residue_name
inactivates	O
the	O
20S	B-complex_assembly
proteasome	I-complex_assembly
from	O
the	O
archaeon	B-taxonomy_domain
T	B-species
.	I-species
acidophilum	I-species
.	O

In	O
yeast	B-taxonomy_domain
,	O
this	O
mutation	B-experimental_method
causes	O
a	O
strong	O
growth	O
defect	O
(	O
Fig	O
.	O
4a	O
and	O
Table	O
1	O
),	O
although	O
the	O
propeptide	B-structure_element
is	O
hydrolysed	O
,	O
as	O
shown	O
here	O
by	O
its	O
X	B-evidence
-	I-evidence
ray	I-evidence
structure	I-evidence
.	O

In	O
one	O
of	O
the	O
two	O
β5	B-protein
subunits	O
,	O
however	O
,	O
we	O
found	O
the	O
cleaved	B-protein_state
propeptide	B-structure_element
still	B-protein_state
bound	I-protein_state
in	O
the	O
substrate	B-site
-	I-site
binding	I-site
channel	I-site
(	O
Fig	O
.	O
4c	O
).	O

On	O
the	O
basis	O
of	O
the	O
phenotype	O
of	O
the	O
T1C	B-mutant
mutant	B-protein_state
and	O
the	O
propeptide	B-structure_element
remnant	O
identified	O
in	O
its	O
active	B-site
site	I-site
,	O
we	O
suppose	O
that	O
autolysis	B-ptm
is	O
retarded	O
and	O
may	O
not	O
have	O
been	O
completed	O
before	O
crystallization	B-experimental_method
.	O

In	O
agreement	O
,	O
soaking	B-experimental_method
crystals	I-experimental_method
with	O
the	O
CP	B-complex_assembly
inhibitors	O
bortezomib	B-chemical
or	O
carfilzomib	B-chemical
modifies	O
only	O
the	O
β1	B-protein
and	O
β2	B-protein
active	B-site
sites	I-site
,	O
while	O
leaving	O
the	O
β5	B-mutant
-	I-mutant
T1C	I-mutant
proteolytic	B-site
centres	I-site
unmodified	B-protein_state
even	O
though	O
they	O
are	O
only	O
partially	O
occupied	O
by	O
the	O
cleaved	B-protein_state
propeptide	B-structure_element
remnant	O
.	O

Consequently	O
,	O
the	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
bridging	O
the	O
active	O
-	O
site	O
nucleophile	O
and	O
Lys33	B-residue_name_number
in	O
WT	B-protein_state
CPs	B-complex_assembly
is	O
broken	O
with	O
Cys1	B-residue_name_number
.	O

Together	O
,	O
these	O
observations	O
suggest	O
that	O
efficient	O
peptide	O
-	O
bond	O
hydrolysis	O
requires	O
that	O
Lys33NH2	B-residue_name_number
hydrogen	B-bond_interaction
bonds	I-bond_interaction
to	O
the	O
active	O
site	O
nucleophile	O
.	O

The	O
benefit	O
of	O
Thr	B-residue_name
over	O
Ser	B-residue_name
as	O
the	O
active	O
-	O
site	O
nucleophile	O

All	O
proteasomes	B-complex_assembly
strictly	B-protein_state
employ	I-protein_state
threonine	B-residue_name
as	O
the	O
active	B-site
-	I-site
site	I-site
residue	I-site
instead	O
of	O
serine	B-residue_name
.	O

To	O
investigate	O
the	O
reason	O
for	O
this	O
singularity	O
,	O
we	O
analysed	O
a	O
β5	B-mutant
-	I-mutant
T1S	I-mutant
mutant	B-protein_state
,	O
which	O
is	O
viable	O
but	O
suffers	O
from	O
growth	O
defects	O
(	O
Fig	O
.	O
4a	O
and	O
Table	O
1	O
).	O

Activity	B-experimental_method
assays	I-experimental_method
with	O
the	O
β5	B-protein
-	O
specific	O
substrate	O
Suc	B-chemical
-	I-chemical
LLVY	I-chemical
-	I-chemical
AMC	I-chemical
demonstrated	O
that	O
the	O
ChT	O
-	O
L	O
activity	O
of	O
the	O
T1S	B-mutant
mutant	B-protein_state
is	O
reduced	O
by	O
40	O
–	O
45	O
%	O
compared	O
with	O
WT	B-protein_state
proteasomes	B-complex_assembly
depending	O
on	O
the	O
incubation	O
temperature	O
(	O
Fig	O
.	O
4b	O
and	O
Supplementary	O
Fig	O
.	O
9c	O
).	O

By	O
contrast	O
,	O
turnover	O
of	O
the	O
substrate	O
Z	B-chemical
-	I-chemical
GGL	I-chemical
-	I-chemical
pNA	I-chemical
,	O
used	O
to	O
monitor	O
ChT	O
-	O
L	O
activity	O
in	O
situ	O
but	O
in	O
a	O
less	O
quantitative	O
fashion	O
,	O
is	O
not	O
detectably	O
impaired	O
(	O
Supplementary	O
Fig	O
.	O
9a	O
).	O

Compared	O
with	O
Thr1Oγ	B-residue_name_number
in	O
WT	B-protein_state
CP	B-complex_assembly
structures	B-evidence
,	O
Ser1Oγ	B-residue_name_number
is	O
rotated	O
by	O
60	O
°.	O

The	O
active	B-site
-	I-site
site	I-site
residue	I-site
Thr1	B-residue_name_number
is	O
fixed	O
in	O
its	O
position	O
,	O
as	O
its	O
methyl	O
group	O
is	O
engaged	O
in	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
with	O
Thr3	B-residue_name_number
and	O
Ala46	B-residue_name_number
(	O
Fig	O
.	O
4h	O
).	O

Consequently	O
,	O
the	O
hydroxyl	O
group	O
of	O
Thr1	B-residue_name_number
requires	O
no	O
reorientation	O
before	O
substrate	O
cleavage	O
and	O
is	O
thus	O
more	O
catalytically	O
efficient	O
than	O
Ser1	B-residue_name_number
.	O

In	O
vitro	O
,	O
the	O
mutant	B-protein_state
proteasome	B-complex_assembly
is	O
less	O
susceptible	O
to	O
proteasome	B-complex_assembly
inhibition	O
by	O
bortezomib	B-chemical
(	O
3	O
.	O
7	O
-	O
fold	O
)	O
and	O
carfilzomib	B-chemical
(	O
1	O
.	O
8	O
-	O
fold	O
;	O
Fig	O
.	O
5	O
).	O

Nevertheless	O
,	O
inhibitor	B-complex_assembly
complex	I-complex_assembly
structures	B-evidence
indicate	O
identical	O
binding	O
modes	O
compared	O
with	O
the	O
WT	B-protein_state
yCP	B-complex_assembly
structures	B-evidence
,	O
with	B-protein_state
the	I-protein_state
same	I-protein_state
inhibitors	I-protein_state
.	O

Notably	O
,	O
the	O
affinity	B-evidence
of	O
the	O
tetrapeptide	O
carfilzomib	B-chemical
is	O
less	O
impaired	O
,	O
as	O
it	O
is	O
better	O
stabilized	O
in	O
the	O
substrate	B-site
-	I-site
binding	I-site
channel	I-site
than	O
the	O
dipeptide	O
bortezomib	B-chemical
,	O
which	O
lacks	O
a	O
defined	O
P3	O
site	O
and	O
has	O
only	O
a	O
few	O
interactions	O
with	O
the	O
surrounding	O
protein	O
.	O

Hence	O
,	O
the	O
mean	B-evidence
residence	I-evidence
time	I-evidence
of	O
carfilzomib	B-chemical
at	O
the	O
active	B-site
site	I-site
is	O
prolonged	O
and	O
the	O
probability	O
to	O
covalently	O
react	O
with	O
Ser1	B-residue_name_number
is	O
increased	O
.	O

Considered	O
together	O
,	O
these	O
results	O
provide	O
a	O
plausible	O
explanation	O
for	O
the	O
invariance	O
of	O
threonine	B-residue_name
as	O
the	O
active	O
-	O
site	O
nucleophile	O
in	O
proteasomes	B-complex_assembly
in	O
all	O
three	O
domains	O
of	O
life	O
,	O
as	O
well	O
as	O
in	O
proteasome	B-protein_type
-	I-protein_type
like	I-protein_type
proteases	I-protein_type
such	O
as	O
HslV	B-protein
(	O
ref	O
.).	O

The	O
β	B-protein
-	I-protein
subunit	I-protein
propeptides	B-structure_element
,	O
particularly	O
that	O
of	O
β5	B-protein
,	O
are	O
key	O
factors	O
that	O
help	O
drive	O
proper	O
assembly	O
of	O
the	O
CP	B-complex_assembly
complex	O
.	O

In	O
addition	O
,	O
they	O
prevent	O
irreversible	O
inactivation	O
of	O
the	O
Thr1	B-residue_name_number
N	O
terminus	O
by	O
N	B-ptm
-	I-ptm
acetylation	I-ptm
.	O

In	O
eukaryotes	B-taxonomy_domain
,	O
deletion	O
of	O
or	O
failure	O
to	O
cleave	O
the	O
β1	B-protein
and	O
β2	B-protein
propeptides	B-structure_element
is	O
well	O
tolerated	O
.	O

However	O
,	O
removal	B-experimental_method
of	I-experimental_method
the	O
β5	B-protein
prosegment	B-structure_element
or	O
any	O
interference	O
with	O
its	O
cleavage	O
causes	O
severe	O
phenotypic	O
defects	O
.	O

Depending	O
on	O
the	O
(-	B-residue_number
2	I-residue_number
)	I-residue_number
residue	O
we	O
observed	O
various	O
propeptide	B-structure_element
conformations	O
,	O
but	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
is	O
in	O
all	O
structures	B-evidence
perfectly	O
located	O
for	O
the	O
nucleophilic	O
attack	O
by	O
Thr1Oγ	B-residue_name_number
,	O
although	O
it	O
does	O
not	O
adopt	O
the	O
tight	B-structure_element
turn	I-structure_element
observed	O
for	O
the	O
prosegment	B-structure_element
of	O
subunit	O
β1	B-protein
.	O

In	O
this	O
regard	O
,	O
inappropriate	O
N	B-ptm
-	I-ptm
acetylation	I-ptm
of	O
the	O
Thr1	B-residue_name_number
N	O
terminus	O
cannot	O
be	O
removed	O
by	O
Thr1Oγ	B-residue_name_number
due	O
to	O
the	O
rotational	O
freedom	O
and	O
flexibility	O
of	O
the	O
acetyl	O
group	O
.	O

Autolytic	O
activation	O
of	O
the	O
CP	B-complex_assembly
constitutes	O
one	O
of	O
the	O
final	O
steps	O
of	O
proteasome	O
biogenesis	O
,	O
but	O
the	O
trigger	O
for	O
propeptide	B-ptm
cleavage	I-ptm
had	O
remained	O
enigmatic	O
.	O

We	O
propose	O
a	O
catalytic	B-site
triad	I-site
for	O
the	O
active	B-site
site	I-site
of	O
the	O
CP	B-complex_assembly
consisting	O
of	O
residues	O
Thr1	B-residue_name_number
,	O
Lys33	B-residue_name_number
and	O
Asp	B-residue_name
/	O
Glu17	B-residue_name_number
,	O
which	O
are	O
conserved	O
among	O
all	O
proteolytically	O
active	O
eukaryotic	B-taxonomy_domain
,	O
bacterial	B-taxonomy_domain
and	O
archaeal	B-taxonomy_domain
proteasome	B-complex_assembly
subunits	O
.	O

Analogously	O
to	O
the	O
proteasome	B-complex_assembly
,	O
a	O
Thr	B-site
–	I-site
Lys	I-site
–	I-site
Asp	I-site
triad	I-site
is	O
also	O
found	O
in	O
L	B-protein_type
-	I-protein_type
asparaginase	I-protein_type
.	O

Thus	O
,	O
specific	O
protein	O
surroundings	O
can	O
significantly	O
alter	O
the	O
chemical	O
properties	O
of	O
amino	O
acids	O
such	O
as	O
Lys	B-residue_name
to	O
function	O
as	O
an	O
acid	O
–	O
base	O
catalyst	O
.	O

The	O
resulting	O
uncharged	O
Thr1NH2	B-residue_name_number
is	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bridged	I-bond_interaction
to	O
the	O
C3	O
-	O
OH	O
group	O
.	O

Cleavage	O
of	O
the	O
scissile	O
peptide	O
bond	O
requires	O
protonation	O
of	O
the	O
emerging	O
free	O
amine	O
,	O
and	O
in	O
the	O
proteasome	B-complex_assembly
,	O
the	O
Thr1	B-residue_name_number
amine	O
group	O
is	O
likely	O
to	O
assume	O
this	O
function	O
.	O

Analogously	O
,	O
Thr1NH3	B-residue_name_number
+	O
might	O
promote	O
the	O
bivalent	O
reaction	O
mode	O
of	O
epoxyketone	O
inhibitors	O
by	O
protonating	O
the	O
epoxide	O
moiety	O
to	O
create	O
a	O
positively	O
charged	O
trivalent	O
oxygen	O
atom	O
that	O
is	O
subsequently	O
nucleophilically	O
attacked	O
by	O
Thr1NH2	B-residue_name_number
.	O

Breakdown	O
of	O
this	O
tetrahedral	O
transition	O
state	O
releases	O
the	O
Thr1	B-residue_name_number
N	O
terminus	O
that	O
is	O
protonated	O
by	O
aspartic	B-residue_name_number
acid	I-residue_name_number
166	I-residue_name_number
via	O
Ser129OH	B-residue_name_number
to	O
yield	O
Thr1NH3	B-residue_name_number
+.	O

The	O
residues	O
Ser129	B-residue_name_number
and	O
Asp166	B-residue_name_number
are	O
expected	O
to	O
increase	O
the	O
pKa	O
value	O
of	O
Thr1N	B-residue_name_number
,	O
thereby	O
favouring	O
its	O
charged	O
state	O
.	O

Consistent	O
with	O
playing	O
an	O
essential	O
role	O
in	O
proton	O
shuttling	O
,	O
the	O
mutation	B-experimental_method
D166A	B-mutant
prevents	O
autolysis	B-ptm
of	O
the	O
archaeal	B-taxonomy_domain
CP	B-complex_assembly
and	O
the	O
exchange	B-experimental_method
D166N	B-mutant
impairs	O
catalytic	O
activity	O
of	O
the	O
yeast	B-taxonomy_domain
CP	B-complex_assembly
about	O
60	O
%.	O

This	O
interpretation	O
agrees	O
with	O
the	O
strongly	O
reduced	O
catalytic	O
activity	O
of	O
the	O
β5	B-mutant
-	I-mutant
D166N	I-mutant
mutant	B-protein_state
on	O
the	O
one	O
hand	O
,	O
and	O
the	O
ability	O
to	O
react	O
readily	O
with	O
carfilzomib	B-chemical
on	O
the	O
other	O
.	O

Hence	O
,	O
the	O
proteasome	B-complex_assembly
can	O
be	O
viewed	O
as	O
having	O
a	O
second	B-site
triad	I-site
that	O
is	O
essential	O
for	O
efficient	O
proteolysis	O
.	O

While	O
Lys33NH2	B-residue_name_number
and	O
Asp17Oδ	B-residue_name_number
are	O
required	O
to	O
deprotonate	O
the	O
Thr1	B-residue_name_number
hydroxyl	O
side	O
chain	O
,	O
Ser129OH	B-residue_name_number
and	O
Asp166OH	B-residue_name_number
serve	O
to	O
protonate	O
the	O
N	O
-	O
terminal	O
amine	O
group	O
of	O
Thr1	B-residue_name_number
.	O

In	O
accord	O
with	O
the	O
proposed	O
Thr1	B-residue_name_number
–	O
Lys33	B-residue_name_number
–	O
Asp17	B-residue_name_number
catalytic	B-site
triad	I-site
,	O
crystallographic	B-evidence
data	I-evidence
on	O
the	O
proteolytically	B-protein_state
inactive	I-protein_state
β5	B-mutant
-	I-mutant
T1C	I-mutant
mutant	B-protein_state
demonstrate	O
that	O
the	O
interaction	O
of	O
Lys33NH2	B-residue_name_number
and	O
Cys1	B-residue_name_number
is	O
broken	O
.	O

However	O
,	O
owing	O
to	O
Cys	B-residue_name
being	O
a	O
strong	O
nucleophile	O
,	O
the	O
propeptide	B-structure_element
can	O
still	O
be	O
cleaved	B-protein_state
off	O
over	O
time	O
.	O

While	O
only	O
one	O
single	O
turnover	O
is	O
necessary	O
for	O
autolysis	B-ptm
,	O
continuous	O
enzymatic	O
activity	O
is	O
required	O
for	O
significant	O
and	O
detectable	O
substrate	O
hydrolysis	O
.	O

Notably	O
,	O
in	O
the	O
Ntn	B-protein_type
hydrolase	I-protein_type
penicillin	B-protein_type
acylase	I-protein_type
,	O
substitution	B-experimental_method
of	O
the	O
catalytic	B-protein_state
N	O
-	O
terminal	O
Ser	B-residue_name
residue	O
by	O
Cys	B-residue_name
also	O
inactivates	B-protein_state
the	O
enzyme	B-protein_type
but	O
still	O
enables	O
precursor	B-ptm
processing	I-ptm
.	O

To	O
investigate	O
why	O
the	O
CP	B-complex_assembly
specifically	O
employs	O
threonine	B-residue_name
as	O
its	O
active	B-site
-	I-site
site	I-site
residue	I-site
,	O
we	O
used	O
a	O
β5	B-mutant
-	I-mutant
T1S	I-mutant
mutant	B-protein_state
of	O
the	O
yCP	B-complex_assembly
and	O
characterized	O
it	O
biochemically	B-experimental_method
and	I-experimental_method
structurally	I-experimental_method
.	O

Structural	B-evidence
analyses	I-evidence
support	O
these	O
findings	O
with	O
the	O
T1S	B-mutant
mutant	B-protein_state
and	O
provide	O
an	O
explanation	O
for	O
the	O
strict	B-protein_state
use	I-protein_state
of	I-protein_state
Thr	B-residue_name
residues	O
in	O
proteasomes	B-complex_assembly
.	O

Notably	O
,	O
proteolytically	B-protein_state
active	I-protein_state
proteasome	B-complex_assembly
subunits	O
from	O
archaea	B-taxonomy_domain
,	O
yeast	B-taxonomy_domain
and	O
mammals	B-taxonomy_domain
,	O
including	O
constitutive	O
,	O
immuno	O
-	O
and	O
thymoproteasome	O
subunits	O
,	O
either	O
encode	O
Thr	B-residue_name
or	O
Ile	B-residue_name
at	O
position	O
3	B-residue_number
,	O
indicating	O
the	O
importance	O
of	O
the	O
Cγ	O
for	O
fixing	O
the	O
position	O
of	O
the	O
nucleophilic	O
Thr1	B-residue_name_number
.	O

Similarly	O
,	O
although	O
the	O
serine	B-residue_name
mutant	B-protein_state
is	O
active	B-protein_state
,	O
threonine	B-residue_name
is	O
more	O
efficient	O
in	O
the	O
context	O
of	O
the	O
proteasome	B-complex_assembly
active	B-site
site	I-site
.	O

(	O
a	O
)	O
Structural	B-experimental_method
superposition	I-experimental_method
of	O
the	O
β1	B-mutant
-	I-mutant
T1A	I-mutant
propeptide	B-structure_element
and	O
the	O
matured	B-protein_state
WT	B-protein_state
β1	B-protein
active	B-site
-	I-site
site	I-site
Thr1	B-residue_name_number
.	O

The	O
major	O
determinant	O
of	O
the	O
S1	B-site
specificity	I-site
pocket	I-site
,	O
residue	O
45	B-residue_number
,	O
is	O
depicted	O
.	O

The	O
black	O
arrow	O
indicates	O
the	O
attack	O
of	O
Thr1Oγ	B-residue_name_number
onto	O
the	O
carbonyl	O
carbon	O
atom	O
of	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
).	I-residue_name_number

(	O
b	O
)	O
Structural	B-experimental_method
superposition	I-experimental_method
of	O
the	O
β1	B-mutant
-	I-mutant
T1A	I-mutant
propeptide	B-structure_element
and	O
the	O
β2	B-mutant
-	I-mutant
T1A	I-mutant
propeptide	B-structure_element
highlights	O
subtle	O
differences	O
in	O
their	O
conformations	O
,	O
but	O
illustrates	O
that	O
Ala1	B-residue_name_number
and	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
match	O
well	O
.	O

(	O
c	O
)	O
Structural	B-experimental_method
superposition	I-experimental_method
of	O
the	O
β1	B-mutant
-	I-mutant
T1A	I-mutant
,	O
the	O
β2	B-mutant
-	I-mutant
T1A	I-mutant
and	O
the	O
β5	B-mutant
-	I-mutant
T1A	I-mutant
-	I-mutant
K81R	I-mutant
propeptide	B-structure_element
remnants	O
depict	O
their	O
differences	O
in	O
conformation	O
.	O

While	O
residue	O
(-	B-residue_number
2	I-residue_number
)	I-residue_number
of	O
the	O
β1	B-protein
and	O
β2	B-protein
prosegments	B-structure_element
fit	O
the	O
S1	B-site
pocket	I-site
,	O
His	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
of	O
the	O
β5	B-protein
propeptide	B-structure_element
occupies	O
the	O
S2	B-site
pocket	I-site
.	O

Nonetheless	O
,	O
in	O
all	O
mutants	O
the	O
carbonyl	O
carbon	O
atom	O
of	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
is	O
ideally	O
placed	O
for	O
the	O
nucleophilic	O
attack	O
by	O
Thr1Oγ	B-residue_name_number
.	O

The	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
between	O
Thr	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
OH	O
and	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
O	O
(∼	O
2	O
.	O
8	O
Å	O
)	O
is	O
indicated	O
by	O
a	O
black	O
dashed	O
line	O
.	O

(	O
a	O
)	O
Structural	B-experimental_method
superposition	I-experimental_method
of	O
the	O
β1	B-mutant
-	I-mutant
T1A	I-mutant
propeptide	B-structure_element
and	O
the	O
β5	B-mutant
-	I-mutant
H	I-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
L	I-mutant
-	I-mutant
T1A	I-mutant
mutant	B-protein_state
propeptide	B-structure_element
.	O

The	O
(-	B-residue_number
2	I-residue_number
)	I-residue_number
residues	O
of	O
both	O
prosegments	B-structure_element
point	O
into	O
the	O
S1	B-site
pocket	I-site
.	O

While	O
the	O
residues	O
(-	B-residue_range
2	I-residue_range
)	I-residue_range
to	I-residue_range
(-	I-residue_range
4	I-residue_range
)	I-residue_range
vary	O
in	O
their	O
conformation	O
,	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
and	O
Ala1	B-residue_name_number
are	O
located	O
in	O
all	O
structures	B-evidence
at	O
the	O
same	O
positions	O
.	O

(	O
c	O
)	O
Structural	B-experimental_method
superposition	I-experimental_method
of	O
the	O
β2	B-mutant
-	I-mutant
T1A	I-mutant
propeptide	B-structure_element
and	O
the	O
β5	B-mutant
-	I-mutant
H	I-mutant
(-	I-mutant
2	I-mutant
)	I-mutant
T	I-mutant
-	I-mutant
T1A	I-mutant
mutant	B-protein_state
propeptide	B-structure_element
.	O

The	O
(-	B-residue_number
2	I-residue_number
)	I-residue_number
residues	O
of	O
both	O
prosegments	B-structure_element
point	O
into	O
the	O
S1	B-site
pocket	I-site
,	O
but	O
only	O
Thr	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
OH	O
of	O
β2	B-protein
forms	O
a	O
hydrogen	B-bond_interaction
bridge	I-bond_interaction
to	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
O	O
(	O
black	O
dashed	O
line	O
).	O

Notably	O
,	O
Val	B-residue_name_number
(-	I-residue_name_number
2	I-residue_name_number
)	I-residue_name_number
of	O
the	O
latter	O
does	O
not	O
occupy	O
the	O
S1	B-site
pocket	I-site
,	O
thereby	O
changing	O
the	O
orientation	O
of	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
)	I-residue_name_number
and	O
preventing	O
nucleophilic	O
attack	O
of	O
Thr1Oγ	B-residue_name_number
on	O
the	O
carbonyl	O
carbon	O
atom	O
of	O
Gly	B-residue_name_number
(-	I-residue_name_number
1	I-residue_name_number
).	I-residue_name_number

Architecture	O
and	O
proposed	O
reaction	O
mechanism	O
of	O
the	O
proteasomal	O
active	B-site
site	I-site
.	O

Thr1OH	B-residue_name_number
is	O
hydrogen	B-bond_interaction
-	I-bond_interaction
bonded	I-bond_interaction
to	O
Lys33NH2	B-residue_name_number
(	O
2	O
.	O
7	O
Å	O
),	O
which	O
in	O
turn	O
interacts	O
with	O
Asp17Oδ	B-residue_name_number
.	O

The	O
Thr1	B-residue_name_number
N	O
terminus	O
is	O
engaged	O
in	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
Ser129Oγ	B-residue_name_number
,	O
the	O
carbonyl	O
oxygen	O
of	O
residue	O
168	B-residue_number
,	O
Ser169Oγ	B-residue_name_number
and	O
Asp166Oδ	B-residue_name_number
.	O
(	O
b	O
)	O
The	O
orientations	O
of	O
the	O
active	B-site
-	I-site
site	I-site
residues	I-site
involved	O
in	O
hydrogen	B-bond_interaction
bonding	I-bond_interaction
are	O
strictly	B-protein_state
conserved	I-protein_state
in	O
each	O
proteolytic	B-site
centre	I-site
,	O
as	O
shown	O
by	O
superposition	B-experimental_method
of	O
the	O
β	B-protein
subunits	I-protein
.	O

In	O
the	O
latter	O
,	O
a	O
water	B-chemical
molecule	O
(	O
red	O
sphere	O
)	O
is	O
found	O
at	O
the	O
position	O
where	O
in	O
the	O
WT	B-protein_state
structure	O
the	O
side	O
chain	O
amine	O
group	O
of	O
Lys33	B-residue_name_number
is	O
located	O
.	O

Similarly	O
to	O
Lys33	B-residue_name_number
,	O
the	O
water	B-chemical
molecule	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
to	O
Arg19O	B-residue_name_number
,	O
Asp17Oδ	B-residue_name_number
and	O
Thr1OH	B-residue_name_number
.	O

(	O
d	O
)	O
Proposed	O
chemical	O
reaction	O
mechanism	O
for	O
autocatalytic	B-ptm
precursor	I-ptm
processing	I-ptm
and	O
proteolysis	O
in	O
the	O
proteasome	B-complex_assembly
.	O

The	O
active	B-site
-	I-site
site	I-site
Thr1	B-residue_name_number
is	O
depicted	O
in	O
blue	O
,	O
the	O
propeptide	B-structure_element
segment	O
and	O
the	O
peptide	O
substrate	O
are	O
coloured	O
in	O
green	O
,	O
whereas	O
the	O
scissile	O
peptide	O
bond	O
is	O
highlighted	O
in	O
red	O
.	O

Autolysis	B-ptm
(	O
left	O
set	O
of	O
structures	O
)	O
is	O
initiated	O
by	O
deprotonation	O
of	O
Thr1OH	B-residue_name_number
via	O
Lys33NH2	B-residue_name_number
and	O
the	O
formation	O
of	O
a	O
tetrahedral	O
transition	O
state	O
.	O

The	O
strictly	B-protein_state
conserved	I-protein_state
oxyanion	O
hole	O
Gly47NH	B-residue_name_number
stabilizing	O
the	O
negatively	O
charged	O
intermediate	O
is	O
illustrated	O
as	O
a	O
semicircle	O
.	O

Collapse	O
of	O
the	O
transition	O
state	O
frees	O
the	O
Thr1	B-residue_name_number
N	O
terminus	O
(	O
by	O
completing	O
an	O
N	O
-	O
to	O
-	O
O	O
acyl	O
shift	O
of	O
the	O
propeptide	B-structure_element
),	O
which	O
is	O
subsequently	O
protonated	O
by	O
Asp166OH	B-residue_name_number
via	O
Ser129OH	B-residue_name_number
.	O

Next	O
,	O
Thr1NH2	B-residue_name_number
polarizes	O
a	O
water	B-chemical
molecule	O
for	O
the	O
nucleophilic	O
attack	O
of	O
the	O
acyl	O
-	O
enzyme	O
intermediate	O
.	O

On	O
hydrolysis	O
of	O
the	O
latter	O
,	O
the	O
active	B-site
-	I-site
site	I-site
Thr1	B-residue_name_number
is	O
ready	O
for	O
catalysis	O
(	O
right	O
set	O
of	O
structures	O
).	O

The	O
charged	O
Thr1	B-residue_name_number
N	O
terminus	O
may	O
engage	O
in	O
the	O
orientation	O
of	O
the	O
amide	O
moiety	O
and	O
donate	O
a	O
proton	O
to	O
the	O
emerging	O
N	O
terminus	O
of	O
the	O
C	O
-	O
terminal	O
cleavage	O
product	O
.	O

The	O
resulting	O
deprotonated	O
Thr1NH2	B-residue_name_number
finally	O
activates	O
a	O
water	B-chemical
molecule	O
for	O
hydrolysis	O
of	O
the	O
acyl	O
-	O
enzyme	O
.	O

The	O
proteasome	B-complex_assembly
favours	O
threonine	B-residue_name
as	O
the	O
active	O
-	O
site	O
nucleophile	O
.	O

(	O
a	O
)	O
Growth	B-experimental_method
tests	I-experimental_method
by	I-experimental_method
serial	I-experimental_method
dilution	I-experimental_method
of	O
WT	B-protein_state
and	O
pre2	O
(	O
β5	B-protein
)	O
mutant	B-protein_state
yeast	B-taxonomy_domain
cultures	O
reveal	O
growth	O
defects	O
of	O
the	O
active	B-site
-	I-site
site	I-site
mutants	B-experimental_method
under	O
the	O
indicated	O
conditions	O
after	O
2	O
days	O
(	O
2	O
d	O
)	O
of	O
incubation	O
.	O

(	O
b	O
)	O
Purified	O
WT	B-protein_state
and	O
mutant	B-protein_state
proteasomes	B-complex_assembly
were	O
tested	O
for	O
their	O
chymotrypsin	O
-	O
like	O
activity	O
(	O
β5	B-protein
)	O
using	O
the	O
substrate	O
Suc	B-chemical
-	I-chemical
LLVY	I-chemical
-	I-chemical
AMC	I-chemical
.	O

The	O
prosegment	B-structure_element
is	O
cleaved	B-protein_state
but	O
still	B-protein_state
bound	I-protein_state
in	O
the	O
substrate	B-site
-	I-site
binding	I-site
channel	I-site
.	O

(	O
d	O
)	O
Structural	B-experimental_method
superposition	I-experimental_method
of	O
the	O
β5	B-mutant
-	I-mutant
T1A	I-mutant
-	I-mutant
K81R	I-mutant
and	O
the	O
β5	B-mutant
-	I-mutant
T1C	I-mutant
mutant	B-protein_state
subunits	O
onto	O
the	O
WT	B-protein_state
β5	B-protein
subunit	O
.	O
(	O
e	O
)	O
Structural	B-experimental_method
superposition	I-experimental_method
of	O
the	O
β5	B-mutant
-	I-mutant
T1C	I-mutant
propeptide	B-structure_element
onto	O
the	O
β1	B-mutant
-	I-mutant
T1A	I-mutant
active	B-site
site	I-site
(	O
blue	O
)	O
and	O
the	O
WT	B-protein_state
β5	B-protein
active	B-site
site	I-site
in	B-protein_state
complex	I-protein_state
with	I-protein_state
the	O
proteasome	B-complex_assembly
inhibitor	O
MG132	B-chemical
(	O
ref	O
.).	O

The	O
inhibitor	B-chemical
as	O
well	O
as	O
the	O
propeptides	B-structure_element
adopt	O
similar	O
conformations	O
in	O
the	O
substrate	B-site
-	I-site
binding	I-site
channel	I-site
.	O

The	O
2FO	B-evidence
–	I-evidence
FC	I-evidence
electron	I-evidence
-	I-evidence
density	I-evidence
map	I-evidence
for	O
Ser1	B-residue_name_number
(	O
blue	O
mesh	O
contoured	O
at	O
1σ	O
)	O
is	O
illustrated	O
.	O

(	O
h	O
)	O
The	O
methyl	O
group	O
of	O
Thr1	B-residue_name_number
is	O
anchored	O
by	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
with	O
Ala46Cβ	B-residue_name_number
and	O
Thr3Cγ	B-residue_name_number
.	O

Inhibition	O
of	O
WT	B-protein_state
and	O
mutant	B-protein_state
β5	B-mutant
-	I-mutant
T1S	I-mutant
proteasomes	B-complex_assembly
by	O
bortezomib	B-chemical
and	O
carfilzomib	B-chemical
.	O

Inhibition	B-experimental_method
assays	I-experimental_method
(	O
left	O
panel	O
).	O

Purified	O
yeast	B-taxonomy_domain
proteasomes	B-complex_assembly
were	O
tested	O
for	O
the	O
susceptibility	O
of	O
their	O
ChT	O
-	O
L	O
(	O
β5	B-protein
)	O
activity	O
to	O
inhibition	O
by	O
bortezomib	B-chemical
and	O
carfilzomib	B-chemical
using	O
the	O
substrate	O
Suc	B-chemical
-	I-chemical
LLVY	I-chemical
-	I-chemical
AMC	I-chemical
.	O

IC50	B-evidence
values	I-evidence
were	O
determined	O
in	O
triplicate	O
;	O
s	O
.	O
d	O
.'	O
s	O
are	O
indicated	O
by	O
error	O
bars	O
.	O

Note	O
that	O
IC50	B-evidence
values	I-evidence
depend	O
on	O
time	O
and	O
enzyme	O
concentration	O
.	O

Proteasomes	B-complex_assembly
(	O
final	O
concentration	O
:	O
66	O
nM	O
)	O
were	O
incubated	O
with	O
inhibitor	O
for	O
45	O
min	O
before	O
substrate	O
addition	O
(	O
final	O
concentration	O
:	O
200	O
μM	O
).	O

Structures	B-evidence
of	O
the	O
β5	B-mutant
-	I-mutant
T1S	I-mutant
mutant	B-protein_state
in	O
complex	B-complex_assembly
with	I-complex_assembly
both	I-complex_assembly
ligands	I-complex_assembly
(	O
green	O
)	O
prove	O
the	O
reactivity	O
of	O
Ser1	B-residue_name_number
(	O
right	O
panel	O
).	O

The	O
WT	B-protein_state
proteasome	B-complex_assembly
:	I-complex_assembly
inhibitor	I-complex_assembly
complex	I-complex_assembly
structures	B-evidence
(	O
inhibitor	O
in	O
grey	O
;	O
Thr1	B-residue_name_number
in	O
black	O
)	O
are	O
superimposed	B-experimental_method
and	O
demonstrate	O
that	O
mutation	B-experimental_method
of	O
Thr1	B-residue_name_number
to	O
Ser	B-residue_name
does	O
not	O
affect	O
the	O
binding	O
mode	O
of	O
bortezomib	B-chemical
or	O
carfilzomib	B-chemical
.	O

The	O
Taf14	B-protein
YEATS	B-structure_element
domain	I-structure_element
is	O
a	O
reader	O
of	O
histone	B-protein_type
crotonylation	B-ptm

The	O
discovery	O
of	O
new	O
histone	B-protein_type
modifications	O
is	O
unfolding	O
at	O
startling	O
rates	O
,	O
however	O
,	O
the	O
identification	O
of	O
effectors	O
capable	O
of	O
interpreting	O
these	O
modifications	O
has	O
lagged	O
behind	O
.	O

Here	O
we	O
report	O
the	O
YEATS	B-structure_element
domain	I-structure_element
as	O
an	O
effective	O
reader	O
of	O
histone	B-protein_type
lysine	B-residue_name
crotonylation	B-ptm
–	O
an	O
epigenetic	O
signature	O
associated	O
with	O
active	O
transcription	O
.	O

We	O
show	O
that	O
the	O
Taf14	B-protein
YEATS	B-structure_element
domain	I-structure_element
engages	O
crotonyllysine	B-residue_name
via	O
a	O
unique	O
π	B-bond_interaction
-	I-bond_interaction
π	I-bond_interaction
-	I-bond_interaction
π	I-bond_interaction
-	I-bond_interaction
stacking	I-bond_interaction
mechanism	O
and	O
that	O
other	O
YEATS	B-structure_element
domains	I-structure_element
have	O
crotonyllysine	B-residue_name
binding	O
activity	O
.	O

The	O
crotonyllysine	B-residue_name
mark	O
on	O
histone	B-protein_type
H3K18	B-protein_type
is	O
produced	O
by	O
p300	B-protein
,	O
a	O
histone	B-protein_type
acetyltransferase	I-protein_type
also	O
responsible	O
for	O
acetylation	B-ptm
of	O
histones	O
.	O

Owing	O
to	O
some	O
differences	O
in	O
their	O
genomic	O
distribution	O
,	O
the	O
crotonyllysine	B-residue_name
and	O
acetyllysine	B-residue_name
(	O
Kac	B-residue_name
)	O
modifications	O
have	O
been	O
linked	O
to	O
distinct	O
functional	O
outcomes	O
.	O

p300	B-protein
-	O
catalyzed	O
histone	B-protein_type
crotonylation	B-ptm
,	O
which	O
is	O
likely	O
metabolically	O
regulated	O
,	O
stimulates	O
transcription	O
to	O
a	O
greater	O
degree	O
than	O
p300	B-protein
-	O
catalyzed	O
acetylation	B-ptm
.	O

The	O
discovery	O
of	O
individual	O
biological	O
roles	O
for	O
the	O
crotonyllysine	B-residue_name
and	O
acetyllysine	B-residue_name
marks	O
suggests	O
that	O
these	O
PTMs	O
can	O
be	O
read	O
by	O
distinct	O
readers	O
.	O

The	O
family	O
of	O
acetyllysine	B-residue_name
readers	O
has	O
been	O
expanded	O
with	O
the	O
discovery	O
that	O
the	O
YEATS	B-structure_element
(	O
Yaf9	B-protein
,	O
ENL	B-protein
,	O
AF9	B-protein
,	O
Taf14	B-protein
,	O
Sas5	B-protein
)	O
domains	O
of	O
human	B-species
AF9	B-protein
and	O
yeast	B-taxonomy_domain
Taf14	B-protein
are	O
capable	O
of	O
recognizing	O
the	O
histone	B-protein_type
mark	O
H3K9ac	B-protein_type
.	O

Similarly	O
,	O
activation	O
of	O
a	O
subset	O
of	O
genes	O
and	O
DNA	O
damage	O
repair	O
in	O
yeast	B-taxonomy_domain
require	O
the	O
acetyllysine	B-residue_name
binding	O
activity	O
of	O
the	O
Taf14	B-protein
YEATS	B-structure_element
domain	I-structure_element
.	O

Consistent	O
with	O
its	O
role	O
in	O
gene	O
regulation	O
,	O
Taf14	B-protein
was	O
identified	O
as	O
a	O
core	O
component	O
of	O
the	O
transcription	O
factor	O
complexes	O
TFIID	B-complex_assembly
and	O
TFIIF	B-complex_assembly
.	O

However	O
,	O
Taf14	B-protein
is	O
also	O
found	O
in	O
a	O
number	O
of	O
chromatin	O
-	O
remodeling	O
complexes	O
(	O
i	O
.	O
e	O
.,	O
INO80	B-complex_assembly
,	O
SWI	B-complex_assembly
/	I-complex_assembly
SNF	I-complex_assembly
and	O
RSC	B-complex_assembly
)	O
and	O
the	O
histone	B-protein_type
acetyltransferase	I-protein_type
complex	O
NuA3	B-complex_assembly
,	O
indicating	O
a	O
multifaceted	O
role	O
of	O
Taf14	B-protein
in	O
transcriptional	O
regulation	O
and	O
chromatin	O
biology	O
.	O

We	O
found	O
that	O
H3K9cr	B-protein_type
is	O
present	O
in	O
yeast	B-taxonomy_domain
and	O
is	O
dynamically	O
regulated	O
.	O

The	O
Taf14	B-protein
YEATS	B-structure_element
domain	I-structure_element
adopts	O
an	O
immunoglobin	B-structure_element
-	I-structure_element
like	I-structure_element
β	I-structure_element
sandwich	I-structure_element
fold	I-structure_element
containing	O
eight	O
anti	B-structure_element
-	I-structure_element
parallel	I-structure_element
β	I-structure_element
strands	I-structure_element
linked	O
by	O
short	O
loops	B-structure_element
that	O
form	O
a	O
binding	B-site
site	I-site
for	O
H3K9cr	B-protein_type
(	O
Fig	O
.	O
1b	O
).	O

The	O
most	O
striking	O
feature	O
of	O
the	O
crotonyllysine	B-residue_name
recognition	O
mechanism	O
is	O
the	O
unique	O
coordination	O
of	O
crotonylated	B-protein_state
lysine	B-residue_name
residue	O
.	O

The	O
fully	O
extended	O
side	O
chain	O
of	O
K9cr	B-ptm
transverses	O
the	O
narrow	O
tunnel	O
,	O
crossing	O
the	O
β	B-structure_element
sandwich	I-structure_element
at	O
right	O
angle	O
in	O
a	O
corkscrew	O
-	O
like	O
manner	O
(	O
Fig	O
.	O
1b	O
and	O
Supplementary	O
Figure	O
1b	O
).	O

The	O
side	O
chain	O
of	O
Trp81	B-residue_name_number
appears	O
to	O
adopt	O
two	O
conformations	O
,	O
one	O
of	O
which	O
provides	O
maximum	O
π	B-bond_interaction
-	I-bond_interaction
stacking	I-bond_interaction
with	O
the	O
alkene	O
functional	O
group	O
while	O
the	O
other	O
rotamer	O
affords	O
maximum	O
π	B-bond_interaction
-	I-bond_interaction
stacking	I-bond_interaction
with	O
the	O
amide	O
π	O
electrons	O
(	O
Supplementary	O
Fig	O
.	O
1c	O
).	O

The	O
dual	O
conformation	O
of	O
Trp81	B-residue_name_number
is	O
likely	O
due	O
to	O
the	O
conjugated	O
nature	O
of	O
the	O
C	O
=	O
C	O
and	O
C	O
=	O
O	O
π	O
-	O
orbitals	O
within	O
the	O
crotonyl	B-chemical
functional	O
group	O
.	O

The	O
π	B-bond_interaction
bond	I-bond_interaction
conjugation	O
of	O
the	O
crotonyl	B-chemical
group	O
gives	O
rise	O
to	O
a	O
dipole	O
moment	O
of	O
the	O
alkene	O
moiety	O
,	O
resulting	O
in	O
a	O
partial	O
positive	O
charge	O
on	O
the	O
β	O
-	O
carbon	O
(	O
Cβ	O
)	O
and	O
a	O
partial	O
negative	O
charge	O
on	O
the	O
α	O
-	O
carbon	O
(	O
Cα	O
).	O

This	O
provides	O
the	O
capability	O
for	O
the	O
alkene	O
moiety	O
to	O
form	O
electrostatic	B-bond_interaction
contacts	I-bond_interaction
,	O
as	O
Cα	O
and	O
Cβ	O
lay	O
within	O
electrostatic	B-bond_interaction
interaction	I-bond_interaction
distances	O
of	O
the	O
carbonyl	O
oxygen	O
of	O
Gln79	B-residue_name_number
and	O
of	O
the	O
hydroxyl	O
group	O
of	O
Thr61	B-residue_name_number
,	O
respectively	O
.	O

Extra	O
stabilization	O
of	O
K9cr	B-ptm
is	O
attained	O
by	O
a	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
formed	O
between	O
its	O
carbonyl	O
oxygen	O
and	O
the	O
backbone	O
nitrogen	O
of	O
Trp81	B-residue_name_number
,	O
as	O
well	O
as	O
a	O
water	B-chemical
-	O
mediated	O
hydrogen	B-bond_interaction
bond	I-bond_interaction
with	O
the	O
backbone	O
carbonyl	O
group	O
of	O
Gly82	B-residue_name_number
(	O
Fig	O
1d	O
).	O

This	O
distinctive	O
mechanism	O
was	O
corroborated	O
through	O
mapping	O
the	O
Taf14	B-protein
YEATS	B-site
-	I-site
H3K9cr	I-site
binding	I-site
interface	I-site
in	O
solution	O
using	O
NMR	B-experimental_method
chemical	I-experimental_method
shift	I-experimental_method
perturbation	I-experimental_method
analysis	I-experimental_method
(	O
Supplementary	O
Fig	O
.	O
2a	O
,	O
b	O
).	O

The	O
dissociation	B-evidence
constant	I-evidence
(	O
Kd	B-evidence
)	O
for	O
the	O
Taf14	B-complex_assembly
YEATS	I-complex_assembly
-	I-complex_assembly
H3K9cr5	I-complex_assembly
-	I-complex_assembly
13	I-complex_assembly
complex	O
was	O
found	O
to	O
be	O
9	O
.	O
5	O
μM	O
,	O
as	O
measured	O
by	O
fluorescence	B-experimental_method
spectroscopy	I-experimental_method
(	O
Supplementary	O
Fig	O
.	O
2c	O
).	O

This	O
value	O
is	O
in	O
the	O
range	O
of	O
binding	B-evidence
affinities	I-evidence
exhibited	O
by	O
the	O
majority	O
of	O
histone	O
readers	O
,	O
thus	O
attesting	O
to	O
the	O
physiological	O
relevance	O
of	O
the	O
H3K9cr	B-protein_type
recognition	O
by	O
Taf14	B-protein
.	O

Both	O
H3K9cr	B-protein_type
and	O
H3K9ac	B-protein_type
were	O
detected	O
in	O
yeast	B-taxonomy_domain
histones	B-protein_type
;	O
to	O
our	O
knowledge	O
,	O
this	O
is	O
the	O
first	O
report	O
of	O
H3K9cr	B-protein_type
occurring	O
in	O
yeast	B-taxonomy_domain
.	O

Towards	O
this	O
end	O
,	O
we	O
probed	O
extracts	O
derived	O
from	O
yeast	B-taxonomy_domain
cells	O
in	O
which	O
major	O
yeast	B-taxonomy_domain
HATs	B-protein_type
(	O
HAT1	B-protein
,	O
Gcn5	B-protein
,	O
and	O
Rtt109	B-protein
)	O
or	O
HDACs	B-protein_type
(	O
Rpd3	B-protein
,	O
Hos1	B-protein
,	O
and	O
Hos2	B-protein
)	O
were	O
deleted	B-experimental_method
.	O

As	O
shown	O
in	O
Figure	O
2a	O
,	O
b	O
and	O
Supplementary	O
Fig	O
.	O
3e	O
,	O
H3K9cr	B-protein_type
levels	O
were	O
abolished	O
or	O
reduced	O
considerably	O
in	O
the	O
HAT	B-protein_type
deletion	B-experimental_method
strains	O
,	O
whereas	O
they	O
were	O
dramatically	O
increased	O
in	O
the	O
HDAC	B-protein_type
deletion	B-experimental_method
strains	O
.	O

Furthermore	O
,	O
fluctuations	O
in	O
the	O
H3K9cr	B-protein_type
levels	O
were	O
more	O
substantial	O
than	O
fluctuations	O
in	O
the	O
corresponding	O
H3K9ac	B-protein_type
levels	O
.	O

Together	O
,	O
these	O
results	O
reveal	O
that	O
H3K9cr	B-protein_type
is	O
a	O
dynamic	O
mark	O
of	O
chromatin	O
in	O
yeast	B-taxonomy_domain
and	O
suggest	O
an	O
important	O
role	O
for	O
this	O
modification	O
in	O
transcription	O
as	O
it	O
is	O
regulated	O
by	O
HATs	B-protein_type
and	O
HDACs	B-protein_type
.	O

We	O
have	O
previously	O
shown	O
that	O
among	O
acetylated	B-protein_state
histone	B-protein_type
marks	O
,	O
the	O
Taf14	B-protein
YEATS	B-structure_element
domain	I-structure_element
prefers	O
acetylated	B-protein_state
H3K9	B-protein_type
(	O
also	O
see	O
Supplementary	O
Fig	O
.	O
3b	O
),	O
however	O
it	O
binds	O
to	O
H3K9cr	B-protein_type
tighter	O
.	O

The	O
selectivity	O
of	O
Taf14	B-protein
towards	O
crotonyllysine	B-residue_name
was	O
substantiated	O
by	O
1H	B-experimental_method
,	I-experimental_method
15N	I-experimental_method
HSQC	I-experimental_method
experiments	O
,	O
in	O
which	O
either	O
H3K9cr5	B-chemical
-	I-chemical
13	I-chemical
or	O
H3K9ac5	B-chemical
-	I-chemical
13	I-chemical
peptide	O
was	O
titrated	B-experimental_method
into	O
the	O
15N	B-protein_state
-	I-protein_state
labeled	I-protein_state
Taf14	B-protein
YEATS	B-structure_element
domain	I-structure_element
(	O
Fig	O
.	O
2c	O
and	O
Supplementary	O
Fig	O
.	O
4a	O
,	O
b	O
).	O

In	O
contrast	O
,	O
binding	O
of	O
H3K9ac	B-protein_type
resulted	O
in	O
an	O
intermediate	O
exchange	O
,	O
which	O
is	O
characteristic	O
of	O
a	O
weaker	O
association	O
.	O

Furthermore	O
,	O
crosspeaks	B-evidence
of	O
Gly80	B-residue_name_number
and	O
Trp81	B-residue_name_number
of	O
the	O
YEATS	B-structure_element
domain	I-structure_element
were	O
uniquely	O
perturbed	O
by	O
H3K9cr	B-protein_type
and	O
H3K9ac	B-protein_type
,	O
indicating	O
a	O
different	O
chemical	O
environment	O
in	O
the	O
respective	O
crotonyllysine	B-site
and	I-site
acetyllysine	I-site
binding	I-site
pockets	I-site
(	O
Supplementary	O
Fig	O
.	O
4a	O
).	O

These	O
differences	O
support	O
our	O
model	O
that	O
Trp81	B-residue_name_number
adopts	O
two	O
conformations	O
upon	O
complex	O
formation	O
with	O
the	O
H3K9cr	B-protein_type
mark	O
as	O
compared	O
to	O
H3K9ac	B-protein_type
(	O
Supplementary	O
Figs	O
.	O
1c	O
,	O
d	O
and	O
4c	O
).	O

One	O
of	O
the	O
conformations	O
,	O
characterized	O
by	O
the	O
π	O
stacking	O
involving	O
two	O
aromatic	O
residues	O
and	O
the	O
alkene	O
group	O
,	O
is	O
observed	O
only	O
in	O
the	O
YEATS	B-complex_assembly
-	I-complex_assembly
H3K9cr	I-complex_assembly
complex	O
.	O

As	O
shown	O
in	O
Figure	O
2d	O
and	O
Supplementary	O
Fig	O
.	O
5a	O
,	O
the	O
Taf14	B-protein
YEATS	B-structure_element
domain	I-structure_element
binds	O
more	O
strongly	O
to	O
H3K9cr1	B-chemical
-	I-chemical
20	I-chemical
,	O
as	O
compared	O
to	O
other	O
acylated	B-protein_state
histone	O
peptides	O
.	O

The	O
preference	O
for	O
H3K9cr	B-protein_type
over	O
H3K9ac	B-protein_type
,	O
H3K9pr	B-protein_type
and	O
H3K9bu	B-protein_type
was	O
supported	O
by	O
1H	B-experimental_method
,	I-experimental_method
15N	I-experimental_method
HSQC	I-experimental_method
titration	I-experimental_method
experiments	I-experimental_method
.	O

Addition	O
of	O
H3K9ac1	B-chemical
-	I-chemical
20	I-chemical
,	O
H3K9pr1	B-chemical
-	I-chemical
20	I-chemical
,	O
and	O
H3K9bu1	B-chemical
-	I-chemical
20	I-chemical
peptides	O
caused	O
chemical	B-evidence
shift	I-evidence
perturbations	I-evidence
in	O
the	O
Taf14	B-protein
YEATS	B-structure_element
domain	I-structure_element
in	O
intermediate	O
exchange	O
regime	O
,	O
implying	O
that	O
these	O
interactions	O
are	O
weaker	O
compared	O
to	O
the	O
interaction	O
with	O
the	O
H3K9cr1	B-chemical
-	I-chemical
20	I-chemical
peptide	O
(	O
Supplementary	O
Fig	O
.	O
5b	O
).	O

From	O
comparative	B-experimental_method
structural	I-experimental_method
analysis	I-experimental_method
of	O
the	O
YEATS	O
complexes	O
,	O
Gly80	B-residue_name_number
emerged	O
as	O
candidate	O
residue	O
potentially	O
responsible	O
for	O
the	O
preference	O
for	O
crotonyllysine	B-residue_name
.	O

In	O
attempt	O
to	O
generate	O
a	O
mutant	O
capable	O
of	O
accommodating	O
a	O
short	O
acetyl	O
moiety	O
but	O
discriminating	O
against	O
a	O
longer	O
,	O
planar	O
crotonyl	B-chemical
moiety	O
,	O
we	O
mutated	B-protein_state
Gly80	B-residue_name_number
to	O
more	O
bulky	O
residues	O
,	O
however	O
all	O
mutants	B-protein_state
of	I-protein_state
Gly80	B-residue_name_number
lost	O
their	O
binding	O
activities	O
towards	O
either	O
acylated	B-protein_state
peptide	O
,	O
suggesting	O
that	O
Gly80	B-residue_name_number
is	O
absolutely	O
required	O
for	O
the	O
interaction	O
.	O

To	O
determine	O
if	O
the	O
binding	O
to	O
crotonyllysine	B-residue_name
is	O
conserved	B-protein_state
,	O
we	O
tested	O
human	B-species
YEATS	B-structure_element
domains	I-structure_element
by	O
pull	B-experimental_method
-	I-experimental_method
down	I-experimental_method
experiments	I-experimental_method
using	O
singly	O
and	O
multiply	O
acetylated	B-protein_state
,	O
propionylated	B-protein_state
,	O
butyrylated	B-protein_state
,	O
and	O
crotonylated	B-protein_state
histone	B-protein_type
peptides	O
(	O
Supplementary	O
Fig	O
.	O
6	O
).	O

We	O
found	O
that	O
all	O
YEATS	B-structure_element
domains	I-structure_element
tested	O
are	O
capable	O
of	O
binding	O
to	O
crotonyllysine	B-residue_name
peptides	O
,	O
though	O
they	O
display	O
variable	O
preferences	O
for	O
the	O
acyl	O
moieties	O
.	O

While	O
YEATS2	B-protein
and	O
ENL	B-protein
showed	O
selectivity	O
for	O
the	O
crotonylated	B-protein_state
peptides	O
,	O
GAS41	B-protein
and	O
AF9	B-protein
bound	O
acylated	B-protein_state
peptides	O
almost	O
equally	O
well	O
.	O

We	O
assayed	O
a	O
large	O
set	O
of	O
BDs	B-structure_element
in	O
pull	B-experimental_method
-	I-experimental_method
down	I-experimental_method
experiments	I-experimental_method
and	O
found	O
that	O
this	O
module	O
is	O
highly	O
specific	O
for	O
acetyllysine	B-residue_name
and	O
propionyllysine	B-residue_name
containing	O
peptides	O
(	O
Supplementary	O
Fig	O
.	O
7	O
).	O

However	O
,	O
bromodomains	B-structure_element
did	O
not	O
interact	O
(	O
or	O
associated	O
very	O
weakly	O
)	O
with	O
longer	O
acyl	O
modifications	O
,	O
including	O
crotonyllysine	B-residue_name
,	O
as	O
in	O
the	O
case	O
of	O
BDs	B-structure_element
of	O
TAF1	B-protein
and	O
BRD2	B-protein
,	O
supporting	O
recent	O
reports	O
.	O

These	O
results	O
demonstrate	O
that	O
the	O
YEATS	B-structure_element
domain	I-structure_element
is	O
currently	O
the	O
sole	O
reader	O
of	O
crotonyllysine	B-residue_name
.	O

In	O
conclusion	O
,	O
we	O
have	O
identified	O
the	O
YEATS	B-structure_element
domain	I-structure_element
of	O
Taf14	B-protein
as	O
the	O
first	O
reader	O
of	O
histone	B-protein_type
crotonylation	B-ptm
.	O

We	O
further	O
demonstrate	O
that	O
H3K9cr	B-protein_type
exists	O
in	O
yeast	B-taxonomy_domain
and	O
is	O
dynamically	O
regulated	O
by	O
HATs	B-protein_type
and	O
HDACs	B-protein_type
.	O

Furthermore	O
,	O
the	O
functional	O
significance	O
of	O
crotonyllysine	B-residue_name
recognition	O
by	O
other	O
YEATS	B-protein_type
proteins	O
will	O
be	O
of	O
great	O
importance	O
to	O
elucidate	O
and	O
compare	O
.	O

The	O
structural	O
mechanism	O
for	O
the	O
recognition	O
of	O
H3K9cr	B-protein_type

(	O
a	O
)	O
Chemical	O
structure	O
of	O
crotonyllysine	B-residue_name
.	O
(	O
b	O
)	O
The	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
Taf14	B-protein
YEATS	B-structure_element
domain	I-structure_element
(	O
wheat	O
)	O
in	B-protein_state
complex	I-protein_state
with	I-protein_state
the	O
H3K9cr5	B-chemical
-	I-chemical
13	I-chemical
peptide	O
(	O
green	O
).	O
(	O
c	O
)	O
H3K9cr	B-protein_type
is	O
stabilized	O
via	O
an	O
extensive	O
network	O
of	O
intermolecular	O
electrostatic	B-bond_interaction
and	I-bond_interaction
polar	I-bond_interaction
interactions	I-bond_interaction
with	O
the	O
Taf14	B-protein
YEATS	B-structure_element
domain	I-structure_element
.	O

Total	O
H3	B-protein_type
was	O
used	O
as	O
a	O
loading	O
control	O
.	O

(	O
c	O
)	O
Superimposed	O
1H	B-experimental_method
,	I-experimental_method
15N	I-experimental_method
HSQC	I-experimental_method
spectra	B-evidence
of	O
Taf14	B-protein
YEATS	B-structure_element
recorded	O
as	O
H3K9cr5	B-chemical
-	I-chemical
13	I-chemical
and	O
H3K9ac5	B-chemical
-	I-chemical
13	I-chemical
peptides	O
were	O
titrated	B-experimental_method
in	O
.	O

Spectra	B-evidence
are	O
color	O
coded	O
according	O
to	O
the	O
protein	O
:	O
peptide	O
molar	O
ratio	O
.	O

(	O
d	O
)	O
Western	B-experimental_method
blot	I-experimental_method
analyses	O
of	O
peptide	B-experimental_method
pull	I-experimental_method
-	I-experimental_method
down	I-experimental_method
assays	I-experimental_method
using	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
mutated	B-protein_state
Taf14	B-protein
YEATS	B-structure_element
domains	I-structure_element
and	O
indicated	O
peptides	O
.	O

Cellular	O
homeostasis	O
requires	O
correct	O
delivery	O
of	O
cell	B-protein_type
-	I-protein_type
surface	I-protein_type
receptor	I-protein_type
proteins	O
(	O
cargo	O
)	O
to	O
their	O
target	O
subcellular	O
compartments	O
.	O

The	O
adapter	B-protein_type
proteins	I-protein_type
Tom1	B-protein
and	O
Tollip	B-protein
are	O
involved	O
in	O
sorting	O
of	O
ubiquitinated	B-ptm
cargo	O
in	O
endosomal	O
compartments	O
.	O

Recruitment	O
of	O
Tom1	B-protein
to	O
the	O
endosomal	O
compartments	O
is	O
mediated	O
by	O
its	O
GAT	B-structure_element
domain	O
’	O
s	O
association	O
to	O
Tollip	B-protein
’	O
s	O
Tom1	B-structure_element
-	I-structure_element
binding	I-structure_element
domain	I-structure_element
(	O
TBD	B-structure_element
).	O

In	O
this	O
data	O
article	O
,	O
we	O
report	O
the	O
solution	B-experimental_method
NMR	I-experimental_method
-	O
derived	O
structure	B-evidence
of	O
the	O
Tom1	B-protein
GAT	B-structure_element
domain	O
.	O

The	O
estimated	O
protein	O
structure	B-evidence
exhibits	O
a	O
bundle	O
of	O
three	O
helical	O
elements	O
.	O

We	O
compare	B-experimental_method
the	O
Tom1	B-protein
GAT	B-structure_element
structure	B-evidence
with	O
those	O
structures	B-evidence
corresponding	O
to	O
the	O
Tollip	B-protein
TBD	B-protein_state
-	I-protein_state
and	O
ubiquitin	B-protein_state
-	I-protein_state
bound	I-protein_state
states	O
.	O

Subject	O
area	O
Biology	O
More	O
specific	O
subject	O
area	O
Structural	O
biology	O
Type	O
of	O
data	O
Table	O
,	O
text	O
file	O
,	O
graph	O
,	O
figures	O
How	O
data	O
was	O
acquired	O
Circular	B-experimental_method
dichroism	I-experimental_method
and	O
NMR	B-experimental_method
.	O

The	O
Tom1	B-protein
GAT	B-structure_element
domain	O
solution	B-evidence
structure	I-evidence
will	O
provide	O
additional	O
tools	O
for	O
modulating	O
its	O
biological	O
function	O
.	O

Tom1	B-protein
GAT	B-structure_element
can	O
adopt	O
distinct	O
conformations	O
upon	O
ligand	O
binding	O
.	O

A	O
conformational	O
response	O
of	O
the	O
Tom1	B-protein
GAT	B-structure_element
domain	O
upon	O
Tollip	B-protein
TBD	B-structure_element
binding	O
can	O
serve	O
as	O
an	O
example	O
to	O
explain	O
mutually	O
exclusive	O
ligand	O
binding	O
events	O
.	O

Analysis	O
of	O
the	O
far	B-experimental_method
-	I-experimental_method
UV	I-experimental_method
circular	I-experimental_method
dichroism	I-experimental_method
(	O
CD	B-experimental_method
)	O
spectrum	B-evidence
of	O
the	O
Tom	B-protein
1	I-protein
GAT	B-structure_element
domain	O
(	O
Fig	O
.	O
1	O
)	O
predicts	O
58	O
.	O
7	O
%	O
α	B-structure_element
-	I-structure_element
helix	I-structure_element
,	O
3	O
%	O
β	B-structure_element
-	I-structure_element
strand	I-structure_element
,	O
15	O
.	O
5	O
%	O
turn	O
,	O
and	O
22	O
.	O
8	O
%	O
disordered	O
regions	O
.	O

Representative	O
far	B-experimental_method
-	I-experimental_method
UV	I-experimental_method
CD	I-experimental_method
spectrum	B-evidence
of	O
the	O
His	B-experimental_method
-	I-experimental_method
Tom1	B-protein
GAT	B-structure_element
domain	O
.	O

(	O
A	O
)	O
Stereo	O
view	O
displaying	O
the	O
best	O
-	O
fit	O
backbone	B-experimental_method
superposition	I-experimental_method
of	O
the	O
refined	O
structures	B-evidence
for	O
the	O
Tom1	B-protein
GAT	B-structure_element
domain	O
.	O

Helices	O
are	O
shown	O
in	O
orange	O
,	O
whereas	O
loops	O
are	O
colored	O
in	O
green	O
.	O
(	O
B	O
)	O
Ribbon	O
illustration	O
of	O
the	O
Tom1	B-protein
GAT	B-structure_element
domain	O
.	O

(	O
A	O
)	O
Two	O
views	O
of	O
the	O
superimposed	B-experimental_method
structures	I-experimental_method
of	O
the	O
Tom1	B-protein
GAT	B-structure_element
domain	O
in	O
the	O
free	B-protein_state
state	O
(	O
gray	O
)	O
with	O
that	O
in	O
the	O
Tollip	B-protein
TBD	B-protein_state
-	I-protein_state
bound	I-protein_state
state	O
(	O
red	O
).	O
(	O
B	O
)	O
Two	O
views	O
of	O
the	O
superimposed	B-experimental_method
structures	I-experimental_method
of	O
the	O
Tom1	B-protein
GAT	B-structure_element
domain	O
(	O
gray	O
)	O
with	O
that	O
in	O
the	O
Ub	B-protein_state
-	I-protein_state
bound	I-protein_state
state	O
(	O
green	O
).	O

deviations	O
were	O
obtained	O
by	O
superimposing	B-experimental_method
residues	O
215	B-residue_range
–	I-residue_range
309	I-residue_range
of	O
Tom1	B-protein
GAT	B-structure_element
among	O
10	O
lowest	O
energy	O
refined	O
structures	B-evidence
.	O

Haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
/	O
Sigma	B-protein
-	I-protein
2	I-protein
receptor	O
facilitates	O
cancer	O
proliferation	O
and	O
chemoresistance	O

Progesterone	B-protein
-	I-protein
receptor	I-protein
membrane	I-protein
component	I-protein
1	I-protein
(	O
PGRMC1	B-protein
/	O
Sigma	B-protein
-	I-protein
2	I-protein
receptor	I-protein
)	O
is	O
a	O
haem	B-protein_type
-	I-protein_type
containing	I-protein_type
protein	I-protein_type
that	O
interacts	O
with	O
epidermal	B-protein_type
growth	I-protein_type
factor	I-protein_type
receptor	I-protein_type
(	O
EGFR	B-protein_type
)	O
and	O
cytochromes	B-protein_type
P450	I-protein_type
to	O
regulate	O
cancer	O
proliferation	O
and	O
chemoresistance	O
;	O
its	O
structural	O
basis	O
remains	O
unknown	O
.	O

Here	O
crystallographic	B-experimental_method
analyses	I-experimental_method
of	O
the	O
PGRMC1	B-protein
cytosolic	B-structure_element
domain	I-structure_element
at	O
1	O
.	O
95	O
Å	O
resolution	O
reveal	O
that	O
it	O
forms	O
a	O
stable	B-protein_state
dimer	B-oligomeric_state
through	O
stacking	B-bond_interaction
interactions	I-bond_interaction
of	O
two	O
protruding	O
haem	B-chemical
molecules	O
.	O

The	O
haem	B-chemical
iron	B-chemical
is	O
five	B-bond_interaction
-	I-bond_interaction
coordinated	I-bond_interaction
by	I-bond_interaction
Tyr113	B-residue_name_number
,	O
and	O
the	O
open	O
surface	B-site
of	O
the	O
haem	B-chemical
mediates	O
dimerization	B-oligomeric_state
.	O

Carbon	B-chemical
monoxide	I-chemical
(	O
CO	B-chemical
)	O
interferes	O
with	O
PGRMC1	B-protein
dimerization	B-oligomeric_state
by	O
binding	O
to	O
the	O
sixth	B-site
coordination	I-site
site	I-site
of	O
the	O
haem	B-chemical
.	O

Haem	B-chemical
-	O
mediated	O
PGRMC1	B-protein
dimerization	B-oligomeric_state
is	O
required	O
for	O
interactions	O
with	O
EGFR	B-protein_type
and	O
cytochromes	B-protein_type
P450	I-protein_type
,	O
cancer	O
proliferation	O
and	O
chemoresistance	O
against	O
anti	O
-	O
cancer	O
drugs	O
;	O
these	O
events	O
are	O
attenuated	O
by	O
either	O
CO	B-chemical
or	O
haem	B-chemical
deprivation	O
in	O
cancer	O
cells	O
.	O

This	O
study	O
demonstrates	O
protein	O
dimerization	B-oligomeric_state
via	O
haem	B-bond_interaction
–	I-bond_interaction
haem	I-bond_interaction
stacking	I-bond_interaction
,	O
which	O
has	O
not	O
been	O
seen	O
in	O
eukaryotes	B-taxonomy_domain
,	O
and	O
provides	O
insights	O
into	O
its	O
functional	O
significance	O
in	O
cancer	O
.	O

Here	O
,	O
the	O
authors	O
determine	O
the	O
structure	B-evidence
of	O
the	O
cytosolic	B-structure_element
domain	I-structure_element
of	O
PGRMC1	B-protein
,	O
which	O
forms	O
a	O
dimer	B-oligomeric_state
via	O
haem	B-bond_interaction
–	I-bond_interaction
haem	I-bond_interaction
stacking	I-bond_interaction
,	O
and	O
propose	O
how	O
this	O
interaction	O
could	O
be	O
involved	O
in	O
its	O
function	O
.	O

Much	O
attention	O
has	O
been	O
paid	O
to	O
the	O
roles	O
of	O
haem	B-chemical
-	O
iron	B-chemical
in	O
cancer	O
development	O
.	O

Increased	O
dietary	O
intake	O
of	O
haem	B-chemical
is	O
a	O
risk	O
factor	O
for	O
several	O
types	O
of	O
cancer	O
.	O

On	O
the	O
other	O
hand	O
,	O
carbon	B-chemical
monoxide	I-chemical
(	O
CO	B-chemical
),	O
the	O
gaseous	O
mediator	O
generated	O
by	O
oxidative	O
degradation	O
of	O
haem	B-chemical
via	O
haem	B-protein_type
oxygenase	I-protein_type
(	O
HO	B-protein_type
),	O
inhibits	O
tumour	O
growth	O
.	O

Thus	O
,	O
a	O
tenuous	O
balance	O
between	O
free	O
haem	B-chemical
and	O
CO	B-chemical
plays	O
key	O
roles	O
in	O
cancer	O
development	O
and	O
chemoresistance	O
,	O
although	O
the	O
underlying	O
mechanisms	O
are	O
not	O
fully	O
understood	O
.	O

To	O
gain	O
insight	O
into	O
the	O
underlying	O
mechanisms	O
,	O
we	O
took	O
chemical	O
biological	O
approaches	O
using	O
affinity	B-experimental_method
nanobeads	I-experimental_method
carrying	O
haem	B-chemical
and	O
identified	O
progesterone	B-protein
-	I-protein
receptor	I-protein
membrane	I-protein
component	I-protein
1	I-protein
(	O
PGRMC1	B-protein
)	O
as	O
a	O
haem	B-chemical
-	O
binding	O
protein	O
from	O
mouse	B-taxonomy_domain
liver	O
extracts	O
(	O
Supplementary	O
Fig	O
.	O
1	O
).	O

PGRMC1	B-protein
is	O
a	O
member	O
of	O
the	O
membrane	B-protein_type
-	I-protein_type
associated	I-protein_type
progesterone	I-protein_type
receptor	I-protein_type
(	O
MAPR	B-protein_type
)	O
family	O
with	O
a	O
cytochrome	B-structure_element
b5	I-structure_element
-	I-structure_element
like	I-structure_element
haem	B-site
-	I-site
binding	I-site
region	I-site
,	O
and	O
is	O
known	O
to	O
be	O
highly	B-protein_state
expressed	I-protein_state
in	O
various	O
types	O
of	O
cancers	O
.	O

PGRMC1	B-protein
is	O
anchored	O
to	O
the	O
cell	O
membrane	O
through	O
the	O
N	O
-	O
terminal	O
transmembrane	B-structure_element
helix	I-structure_element
and	O
interacts	O
with	O
epidermal	B-protein_type
growth	I-protein_type
factor	I-protein_type
receptor	I-protein_type
(	O
EGFR	B-protein_type
)	O
and	O
cytochromes	B-protein_type
P450	I-protein_type
(	O
ref	O
).	O

While	O
PGRMC1	B-protein
is	O
implicated	O
in	O
cell	O
proliferation	O
and	O
cholesterol	O
biosynthesis	O
,	O
the	O
structural	O
basis	O
on	O
which	O
PGRMC1	B-protein
exerts	O
its	O
function	O
remains	O
largely	O
unknown	O
.	O

Here	O
we	O
show	O
that	O
PGRMC1	B-protein
exhibits	O
a	O
unique	O
haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
.	O

X	B-evidence
-	I-evidence
ray	I-evidence
crystal	I-evidence
structure	I-evidence
of	O
PGRMC1	B-protein

We	O
solved	B-experimental_method
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
haem	B-protein_state
-	I-protein_state
bound	I-protein_state
PGRMC1	B-protein
cytosolic	B-structure_element
domain	I-structure_element
(	O
a	O
.	O
a	O
.	O
72	B-residue_range
–	I-residue_range
195	I-residue_range
)	O
at	O
1	O
.	O
95	O
Å	O
resolution	O
(	O
Supplementary	O
Fig	O
.	O
2	O
).	O

In	O
the	O
presence	B-protein_state
of	I-protein_state
haem	B-chemical
,	O
PGRMC1	B-protein
forms	O
a	O
dimeric	B-oligomeric_state
structure	O
largely	O
through	O
hydrophobic	B-bond_interaction
interactions	I-bond_interaction
between	O
the	O
haem	B-chemical
moieties	O
of	O
two	O
monomers	B-oligomeric_state
(	O
Fig	O
.	O
1a	O
,	O
Table	O
1	O
and	O
Supplementary	O
Fig	O
.	O
3	O
;	O
a	O
stereo	O
-	O
structural	O
image	O
is	O
shown	O
in	O
Supplementary	O
Fig	O
4	O
).	O

In	O
cytochrome	B-protein_type
b5	I-protein_type
,	O
the	O
haem	B-chemical
iron	B-chemical
is	O
six	B-bond_interaction
-	I-bond_interaction
coordinated	I-bond_interaction
by	I-bond_interaction
two	O
axial	O
histidine	B-residue_name
residues	O
.	O

These	O
histidines	B-residue_name
are	O
missing	B-protein_state
in	O
PGRMC1	B-protein
,	O
and	O
the	O
haem	B-chemical
iron	B-chemical
is	O
five	B-bond_interaction
-	I-bond_interaction
coordinated	I-bond_interaction
by	I-bond_interaction
Tyr113	B-residue_name_number
(	O
Y113	B-residue_name_number
)	O
alone	B-protein_state
(	O
Fig	O
.	O
1b	O
and	O
Supplementary	O
Fig	O
.	O
3	O
).	O

A	O
homologous	B-structure_element
helix	I-structure_element
that	O
holds	O
haem	B-chemical
in	O
cytochrome	B-protein_type
b5	I-protein_type
is	O
longer	O
,	O
shifts	O
away	O
from	O
haem	B-chemical
,	O
and	O
does	O
not	O
form	O
a	O
coordinate	O
bond	O
in	O
PGRMC1	B-protein
(	O
Fig	O
.	O
1c	O
).	O

Consequently	O
,	O
the	O
five	O
-	O
coordinated	O
haem	B-chemical
of	O
PGRMC1	B-protein
has	O
an	O
open	O
surface	B-site
that	O
allows	O
its	O
dimerization	B-oligomeric_state
through	O
hydrophobic	B-bond_interaction
haem	I-bond_interaction
–	I-bond_interaction
haem	I-bond_interaction
stacking	I-bond_interaction
.	O

Contrary	O
to	O
our	O
finding	O
,	O
Kaluka	O
et	O
al	O
.	O
recently	O
reported	O
that	O
Tyr164	B-residue_name_number
of	O
PGRMC1	B-protein
is	O
the	O
axial	O
ligand	O
of	O
haem	B-chemical
because	O
mutation	B-experimental_method
of	O
this	O
residue	O
impairs	O
haem	B-chemical
binding	O
.	O

This	O
is	O
consistent	O
with	O
observations	O
by	O
Min	O
et	O
al	O
.	O
that	O
Tyr	B-residue_name_number
107	I-residue_name_number
and	O
Tyr113	B-residue_name_number
of	O
PGRMC1	B-protein
are	O
involved	O
in	O
binding	O
with	O
haem	B-chemical
.	O

These	O
amino	O
acid	O
residues	O
are	O
conserved	B-protein_state
among	O
MAPR	B-protein_type
family	O
members	O
(	O
Supplementary	O
Fig	O
.	O
5a	O
),	O
suggesting	O
that	O
these	O
proteins	O
share	O
the	O
ability	O
to	O
exhibit	O
haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
.	O

PGRMC1	B-protein
exhibits	O
haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
in	O
solution	O

In	O
the	O
PGRMC1	B-protein
crystal	B-evidence
,	O
two	O
different	O
types	O
of	O
crystal	O
contacts	O
(	O
chain	O
A	O
–	O
A	O
″	O
and	O
A	O
–	O
B	O
)	O
were	O
observed	O
in	O
addition	O
to	O
the	O
haem	B-chemical
-	O
mediated	O
dimer	B-oligomeric_state
(	O
chain	O
A	O
–	O
A	O
′)	O
(	O
Supplementary	O
Figs	O
3	O
and	O
6a	O
).	O

To	O
confirm	O
that	O
haem	B-chemical
-	O
assisted	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
occurs	O
in	O
solution	O
,	O
we	O
analysed	O
the	O
structure	B-evidence
of	O
apo	B-protein_state
-	O
and	O
haem	B-protein_state
-	I-protein_state
bound	I-protein_state
PGMRC1	B-protein
by	O
two	B-experimental_method
-	I-experimental_method
dimensional	I-experimental_method
nuclear	I-experimental_method
magnetic	I-experimental_method
resonance	I-experimental_method
(	O
NMR	B-experimental_method
)	O
using	O
heteronuclear	B-experimental_method
single	I-experimental_method
-	I-experimental_method
quantum	I-experimental_method
coherence	I-experimental_method
and	I-experimental_method
transverse	I-experimental_method
relaxation	I-experimental_method
-	I-experimental_method
optimized	I-experimental_method
spectroscopy	I-experimental_method
(	O
Supplementary	O
Figs	O
6b	O
and	O
7	O
).	O

NMR	B-experimental_method
signals	O
from	O
some	O
amino	O
acid	O
residues	O
of	O
PGRMC1	B-protein
disappeared	O
due	O
to	O
the	O
paramagnetic	O
relaxation	O
effect	O
of	O
haem	B-chemical
(	O
Supplementary	O
Figs	O
6b	O
);	O
these	O
residues	O
were	O
located	O
in	O
the	O
haem	B-site
-	I-site
binding	I-site
region	I-site
.	O

Furthermore	O
,	O
free	B-evidence
energy	I-evidence
of	I-evidence
dissociation	I-evidence
predicted	O
by	O
PISA	B-experimental_method
suggested	O
that	O
the	O
haem	B-chemical
-	O
mediated	O
dimer	B-oligomeric_state
is	O
stable	B-protein_state
in	O
solution	O
while	O
the	O
other	O
potential	O
interactions	O
are	O
not	O
.	O

We	O
also	O
attempted	O
to	O
predict	O
the	O
secondary	O
structure	O
of	O
PGRMC1	B-protein
through	O
NMR	B-experimental_method
data	O
by	O
calculating	O
with	O
TALOS	B-experimental_method
+	I-experimental_method
program	I-experimental_method
(	O
Supplementary	O
Fig	O
.	O
8	O
);	O
the	O
prediction	O
suggested	O
that	O
the	O
overall	O
secondary	O
structure	O
is	O
comparable	O
between	O
apo	B-protein_state
-	O
and	O
haem	B-protein_state
-	I-protein_state
bound	I-protein_state
forms	O
of	O
PGRMC1	B-protein
in	O
solution	O
.	O

Mass	B-experimental_method
spectrometry	I-experimental_method
(	O
MS	B-experimental_method
)	O
analyses	O
under	O
non	B-experimental_method
-	I-experimental_method
denaturing	I-experimental_method
condition	I-experimental_method
demonstrated	O
that	O
the	O
apo	B-protein_state
-	O
monomer	B-oligomeric_state
PGRMC1	B-protein
resulted	O
in	O
dimerization	B-oligomeric_state
by	O
binding	O
with	O
haem	B-chemical
(	O
Fig	O
.	O
2a	O
).	O

It	O
should	O
be	O
noted	O
that	O
a	O
disulfide	B-ptm
bond	I-ptm
between	O
two	O
Cys129	B-residue_name_number
residues	O
is	O
observed	O
in	O
the	O
crystal	B-evidence
of	O
PGRMC1	B-protein
(	O
Fig	O
.	O
1a	O
),	O
while	O
Cys129	B-residue_name_number
is	O
not	B-protein_state
conserved	I-protein_state
among	O
the	O
MAPR	B-protein_type
family	O
proteins	O
(	O
Supplementary	O
Fig	O
.	O
5a	O
).	O

This	O
observation	O
led	O
us	O
to	O
examine	O
whether	O
or	O
not	O
the	O
disulfide	B-ptm
bond	I-ptm
contributes	O
to	O
PGRMC1	B-protein
dimerization	B-oligomeric_state
.	O

MS	B-experimental_method
analyses	O
under	O
non	B-experimental_method
-	I-experimental_method
denaturing	I-experimental_method
conditions	I-experimental_method
clearly	O
showed	O
that	O
the	O
Cys129Ser	B-mutant
(	O
C129S	B-mutant
)	O
mutant	B-protein_state
is	O
dimerized	B-protein_state
in	O
the	O
presence	B-protein_state
of	I-protein_state
haem	B-chemical
,	O
indicating	O
that	O
the	O
haem	B-chemical
-	O
mediated	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
occurs	O
independently	O
of	O
the	O
disulfide	B-ptm
bond	I-ptm
formation	O
via	O
Cys129	B-residue_name_number
(	O
Fig	O
.	O
2a	O
).	O

Supporting	O
this	O
,	O
MS	B-experimental_method
analyses	O
under	O
denaturing	B-experimental_method
conditions	I-experimental_method
showed	O
that	O
haem	B-chemical
-	O
mediated	O
PGRMC1	B-protein
dimer	B-oligomeric_state
is	O
completely	O
dissociated	O
into	O
monomer	B-oligomeric_state
,	O
indicating	O
that	O
dimerization	B-oligomeric_state
of	O
this	O
kind	O
is	O
not	O
mediated	O
by	O
any	O
covalent	O
bond	O
such	O
as	O
disulfide	B-ptm
bond	I-ptm
(	O
Supplementary	O
Fig	O
.	O
9	O
).	O

We	O
also	O
analysed	O
the	O
haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
by	O
diffusion	B-experimental_method
-	I-experimental_method
ordered	I-experimental_method
NMR	I-experimental_method
spectroscopy	I-experimental_method
(	O
DOSY	B-experimental_method
)	O
analyses	O
(	O
Table	O
2	O
,	O
Supplementary	O
Fig	O
.	O
10	O
).	O

The	O
results	O
suggested	O
that	O
the	O
hydrodynamic	B-evidence
radius	I-evidence
of	O
haem	B-protein_state
-	I-protein_state
bound	I-protein_state
PGRMC1	B-protein
is	O
larger	O
than	O
that	O
of	O
apo	B-protein_state
-	O
PGRMC1	B-protein
.	O

To	O
further	O
evaluate	O
changes	O
in	O
molecular	O
weights	O
in	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
,	O
sedimentation	B-experimental_method
velocity	I-experimental_method
analytical	I-experimental_method
ultracentrifugation	I-experimental_method
(	O
SV	B-experimental_method
-	I-experimental_method
AUC	I-experimental_method
)	O
analysis	O
was	O
carried	O
out	O
.	O

Whereas	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
wt	B-protein_state
)	O
apo	B-protein_state
-	O
PGRMC1	B-protein
appeared	O
at	O
a	O
1	O
.	O
9	O
S	O
peak	O
as	O
monomer	B-oligomeric_state
,	O
the	O
haem	B-chemical
-	O
binding	O
PGRMC1	B-protein
was	O
converted	O
into	O
dimer	B-oligomeric_state
at	O
a	O
3	O
.	O
1	O
S	O
peak	O
(	O
Fig	O
.	O
2b	O
).	O

The	O
sedimentation	B-evidence
coefficients	I-evidence
calculated	O
on	O
the	O
basis	O
of	O
the	O
crystal	B-evidence
structure	I-evidence
were	O
1	O
.	O
71	O
S	O
for	O
monomer	B-oligomeric_state
and	O
2	O
.	O
56	O
S	O
for	O
dimer	B-oligomeric_state
(	O
Supplementary	O
Fig	O
.	O
11	O
,	O
upper	O
panel	O
).	O

The	O
results	O
showed	O
that	O
the	O
PGRMC1	B-protein
dimer	B-oligomeric_state
is	O
not	O
dissociated	O
into	O
monomer	B-oligomeric_state
at	O
all	O
concentrations	O
examined	O
(	O
Supplementary	O
Fig	O
.	O
11	O
,	O
lower	O
panel	O
),	O
suggesting	O
that	O
the	O
Kd	B-evidence
value	O
of	O
haem	B-chemical
-	O
mediated	O
dimer	B-oligomeric_state
of	O
PGRMC1	B-protein
is	O
under	O
3	O
.	O
5	O
μmol	O
l	O
−	O
1	O
.	O

A	O
value	O
of	O
this	O
kind	O
implies	O
that	O
the	O
PGRMC1	B-protein
dimer	B-oligomeric_state
is	O
more	O
stable	O
than	O
other	O
dimers	B-oligomeric_state
of	O
extracellular	B-structure_element
domain	I-structure_element
of	O
membrane	B-protein_type
proteins	I-protein_type
such	O
as	O
Toll	B-protein
like	I-protein
receptor	I-protein
9	I-protein
(	O
dimerization	B-oligomeric_state
Kd	B-evidence
of	O
20	O
μmol	O
l	O
−	O
1	O
)	O
(	O
ref	O
.)	O
and	O
plexin	B-protein
A2	I-protein
receptor	I-protein
(	O
dimerization	B-oligomeric_state
Kd	B-evidence
higher	O
than	O
300	O
μmol	O
l	O
−	O
1	O
)	O
(	O
ref	O
.).	O

The	O
current	O
analytical	O
data	O
confirmed	O
that	O
apo	B-protein_state
-	O
PGRMC1	B-protein
monomer	B-oligomeric_state
converts	O
into	O
dimer	B-oligomeric_state
by	O
binding	O
to	O
haem	B-chemical
in	O
solution	O
(	O
Table	O
2	O
).	O

These	O
results	O
raised	O
the	O
possibility	O
that	O
the	O
function	O
of	O
PGRMC1	B-protein
is	O
regulated	O
by	O
intracellular	O
haem	B-chemical
concentrations	O
.	O

CO	B-chemical
inhibits	O
haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein

Crystallographic	B-experimental_method
analyses	I-experimental_method
revealed	O
that	O
Tyr113	B-residue_name_number
of	O
PGRMC1	B-protein
is	O
an	O
axial	O
ligand	O
for	O
haem	B-chemical
and	O
contributes	O
to	O
haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
(	O
Fig	O
.	O
1a	O
).	O

Analysis	O
of	O
UV	B-evidence
-	I-evidence
visible	I-evidence
spectra	I-evidence
revealed	O
that	O
the	O
heme	B-chemical
of	O
PGRMC1	B-protein
is	O
reducible	O
from	O
ferric	B-protein_state
to	O
ferrous	B-protein_state
state	O
,	O
thus	O
allowing	O
CO	B-chemical
binding	O
(	O
Fig	O
.	O
3a	O
).	O

Furthermore	O
,	O
the	O
UV	B-evidence
-	I-evidence
visible	I-evidence
spectrum	I-evidence
of	O
the	O
wild	B-protein_state
type	I-protein_state
PGRMC1	B-protein
was	O
the	O
same	O
as	O
that	O
of	O
the	O
C129S	B-mutant
mutant	B-protein_state
of	O
PGRMC1	B-protein
,	O
and	O
the	O
R	B-evidence
/	I-evidence
Z	I-evidence
ratio	I-evidence
determined	O
by	O
the	O
intensities	O
between	O
the	O
Soret	O
band	O
(	O
394	O
nm	O
)	O
peak	O
and	O
the	O
274	O
-	O
nm	O
peak	O
showed	O
that	O
these	O
proteins	O
were	O
fully	B-protein_state
loaded	I-protein_state
with	I-protein_state
haem	B-chemical
(	O
Supplementary	O
Fig	O
.	O
12	O
).	O

Analysis	O
of	O
the	O
ferric	B-protein_state
form	O
of	O
PGRMC1	B-protein
using	O
resonance	B-experimental_method
Raman	I-experimental_method
spectroscopy	I-experimental_method
(	O
Supplementary	O
Fig	O
.	O
13	O
)	O
showed	O
that	O
the	O
relative	O
intensity	O
of	O
oxidation	O
and	O
spin	O
state	O
marker	O
bands	O
(	O
ν4	O
and	O
ν3	O
)	O
is	O
close	O
to	O
1	O
.	O
0	O
,	O
which	O
is	O
consistent	O
with	O
it	O
being	O
a	O
haem	B-chemical
protein	O
with	O
a	O
proximal	O
Tyr	B-residue_name
coordination	O
.	O

A	O
specific	O
Raman	B-evidence
shift	I-evidence
peaking	O
at	O
vFe	O
–	O
CO	O
=	O
500	O
cm	O
−	O
1	O
demonstrated	O
that	O
the	O
CO	B-protein_state
-	I-protein_state
bound	I-protein_state
haem	B-chemical
of	O
PGRMC1	B-protein
is	O
six	O
-	O
coordinated	O
(	O
Supplementary	O
Fig	O
.	O
13	O
).	O

Since	O
PGRMC1	B-protein
dimerization	B-oligomeric_state
involves	O
the	O
open	O
surface	B-site
of	O
haem	B-chemical
on	O
the	O
opposite	O
side	O
of	O
the	O
axial	O
Tyr113	B-residue_name_number
,	O
no	O
space	O
for	O
CO	B-chemical
binding	O
is	O
available	O
in	O
the	O
dimeric	B-oligomeric_state
structure	B-evidence
(	O
Fig	O
.	O
3b	O
).	O

This	O
prompted	O
us	O
to	O
ask	O
if	O
CO	B-chemical
binding	O
to	O
haem	B-chemical
causes	O
dissociation	O
of	O
the	O
PGRMC1	B-protein
dimer	B-oligomeric_state
.	O

Analysis	O
by	O
gel	B-experimental_method
filtration	I-experimental_method
chromatography	I-experimental_method
revealed	O
that	O
the	O
relative	O
molecular	O
sizes	O
of	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
and	O
the	O
C129S	B-mutant
mutant	B-protein_state
of	O
PGRMC1	B-protein
are	O
increased	O
by	O
adding	O
haem	B-chemical
to	O
apo	B-protein_state
-	O
PGRMC1	B-protein
regardless	O
of	O
the	O
oxidation	O
state	O
of	O
the	O
iron	B-chemical
(	O
Fig	O
.	O
3c	O
),	O
which	O
is	O
in	O
agreement	O
with	O
the	O
results	O
in	O
Table	O
1	O
.	O

CO	B-chemical
application	O
to	O
ferrous	B-protein_state
PGRMC1	B-protein
abolished	O
the	O
haem	B-chemical
-	O
dependent	O
increase	O
in	O
its	O
molecular	O
size	O
.	O

Under	O
this	O
reducing	O
condition	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
dithionite	B-chemical
,	O
analyses	O
of	O
UV	B-evidence
-	I-evidence
visible	I-evidence
spectra	I-evidence
indicated	O
that	O
CO	B-chemical
-	O
binding	O
with	O
haem	B-complex_assembly
-	I-complex_assembly
PGRMC1	I-complex_assembly
is	O
stable	B-protein_state
,	O
showing	O
only	O
20	O
%	O
reduction	O
of	O
the	O
absorbance	O
at	O
412	O
nm	O
within	O
2	O
h	O
(	O
Supplementary	O
Fig	O
.	O
14	O
).	O

Furthermore	O
,	O
the	O
Tyr113Phe	B-mutant
(	O
Y113F	B-mutant
)	O
mutant	B-protein_state
of	O
PGRMC1	B-protein
was	O
not	O
responsive	O
to	O
haem	B-chemical
.	O

These	O
results	O
suggest	O
that	O
CO	B-chemical
favours	O
the	O
six	O
-	O
coordinate	O
form	O
of	O
haem	B-chemical
and	O
interferes	O
with	O
the	O
haem	B-chemical
-	O
mediated	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
.	O

To	O
examine	O
the	O
inhibitory	O
effects	O
of	O
CO	B-chemical
on	O
haem	B-chemical
-	O
mediated	O
PGRMC1	B-protein
dimerization	B-oligomeric_state
,	O
SV	B-experimental_method
-	I-experimental_method
AUC	I-experimental_method
analysis	O
was	O
carried	O
out	O
.	O

The	O
peak	O
corresponding	O
to	O
the	O
haem	B-chemical
/	O
PGRMC1	B-protein
dimer	B-oligomeric_state
was	O
detected	O
under	O
reducing	O
conditions	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
dithionite	B-chemical
(	O
Supplementary	O
Fig	O
.	O
15	O
,	O
middle	O
panel	O
).	O

These	O
observations	O
raised	O
the	O
transition	O
model	O
for	O
structural	O
regulation	O
of	O
PGRMC1	B-protein
in	O
response	O
to	O
haem	B-chemical
(	O
Fig	O
.	O
3d	O
).	O

As	O
mentioned	O
above	O
,	O
apo	B-protein_state
-	O
PGRMC1	B-protein
exists	O
as	O
monomer	B-oligomeric_state
.	O

By	O
binding	O
with	O
haem	B-chemical
(	O
binding	O
Kd	B-evidence
=	O
50	O
nmol	O
l	O
−	O
1	O
),	O
PGRMC1	B-protein
forms	O
a	O
stable	B-protein_state
dimer	B-oligomeric_state
(	O
dimerization	B-oligomeric_state
Kd	B-evidence
<<	O
3	O
.	O
5	O
μmol	O
l	O
−	O
1	O
)	O
through	O
stacking	B-bond_interaction
of	O
the	O
two	O
open	O
surfaces	B-site
of	O
the	O
five	O
-	O
coordinated	O
haem	B-chemical
molecules	O
in	O
each	O
monomer	B-oligomeric_state
.	O

CO	B-chemical
induces	O
the	O
dissociation	O
of	O
the	O
haem	B-chemical
-	O
mediated	O
dimer	B-oligomeric_state
of	O
PGRMC1	B-protein
by	O
interfering	O
with	O
the	O
haem	B-site
-	I-site
stacking	I-site
interface	I-site
via	O
formation	O
of	O
the	O
six	O
-	O
coordinated	O
CO	B-complex_assembly
-	I-complex_assembly
haem	I-complex_assembly
-	I-complex_assembly
PGRMC1	I-complex_assembly
complex	O
.	O

Such	O
a	O
dynamic	O
structural	O
regulation	O
led	O
us	O
to	O
further	O
examine	O
the	O
regulation	O
of	O
PGRMC1	B-protein
functions	O
in	O
cancer	O
cells	O
.	O

PGRMC1	B-protein
dimerization	B-oligomeric_state
is	O
required	O
for	O
binding	O
to	O
EGFR	B-protein_type

Because	O
PGRMC1	B-protein
is	O
known	O
to	O
interact	O
with	O
EGFR	B-protein_type
and	O
to	O
accelerate	O
tumour	O
progression	O
,	O
we	O
examined	O
the	O
effect	O
of	O
haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
on	O
its	O
interaction	O
with	O
EGFR	B-protein_type
by	O
using	O
purified	O
proteins	O
.	O

As	O
shown	O
in	O
Fig	O
.	O
4a	O
,	O
the	O
cytosolic	B-structure_element
domain	I-structure_element
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
PGRMC1	B-protein
,	O
but	O
not	O
the	O
Y113F	B-mutant
mutant	B-protein_state
,	O
interacted	O
with	O
purified	O
EGFR	B-protein_type
in	O
a	O
haem	B-chemical
-	O
dependent	O
manner	O
.	O

We	O
further	O
analysed	O
the	O
intracellular	O
interaction	O
between	O
PGRMC1	B-protein
and	O
EGFR	B-protein_type
.	O

FLAG	B-protein_state
-	I-protein_state
tagged	I-protein_state
PGRMC1	B-protein
ectopically	B-experimental_method
expressed	I-experimental_method
in	O
human	B-species
colon	O
cancer	O
HCT116	O
cells	O
was	O
immunoprecipitated	B-experimental_method
with	O
anti	O
-	O
FLAG	O
antibody	O
,	O
and	O
co	B-experimental_method
-	I-experimental_method
immunoprecipitated	I-experimental_method
EGFR	B-protein_type
and	O
endogenous	B-protein_state
PGRMC1	B-protein
binding	O
to	O
FLAG	O
-	O
PGRMC1	B-protein
were	O
detected	O
by	O
Western	B-experimental_method
blotting	I-experimental_method
(	O
Fig	O
.	O
4c	O
).	O

Whereas	O
FLAG	B-protein_state
-	I-protein_state
tagged	I-protein_state
wild	B-protein_state
-	I-protein_state
type	I-protein_state
PGRMC1	B-protein
interacted	O
with	O
endogenous	B-protein_state
PGRMC1	B-protein
and	O
EGFR	B-protein_type
,	O
the	O
Y113F	B-mutant
mutant	B-protein_state
did	O
not	O
.	O

We	O
also	O
examined	O
the	O
effect	O
of	O
succinylacetone	B-chemical
(	O
SA	B-chemical
),	O
an	O
inhibitor	O
of	O
haem	B-chemical
biosynthesis	O
(	O
Fig	O
.	O
4d	O
).	O

As	O
expected	O
,	O
SA	B-chemical
significantly	O
reduced	B-protein_state
PGRMC1	B-protein
dimerization	B-oligomeric_state
and	O
its	O
interaction	O
with	O
EGFR	B-protein_type
(	O
Fig	O
.	O
4e	O
),	O
indicating	O
that	O
haem	B-chemical
-	O
mediated	O
dimerization	B-oligomeric_state
of	O
PGMRC1	B-protein
is	O
critical	O
for	O
its	O
binding	O
to	O
EGFR	B-protein_type
.	O

PGRMC1	B-protein
dimer	B-oligomeric_state
facilitates	O
EGFR	B-protein_type
-	O
mediated	O
cancer	O
growth	O

Next	O
,	O
we	O
investigated	O
the	O
functional	O
significance	O
of	O
PGRMC1	B-protein
dimerization	B-oligomeric_state
in	O
EGFR	B-protein_type
signaling	O
.	O

EGF	B-protein_type
-	O
induced	O
phosphorylations	B-ptm
of	O
EGFR	B-protein_type
and	O
its	O
downstream	O
targets	O
AKT	B-protein_type
and	O
ERK	B-protein_type
were	O
decreased	O
by	O
PGRMC1	B-protein
knockdown	B-protein_state
(	O
PGRMC1	B-mutant
-	I-mutant
KD	I-mutant
)	O
(	O
Fig	O
.	O
4f	O
).	O

These	O
results	O
suggested	O
that	O
haem	B-chemical
-	O
mediated	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
is	O
critical	O
for	O
EGFR	B-protein_type
signaling	O
.	O

Chemosensitivity	O
enhancement	O
by	O
two	O
different	O
shRNAs	B-chemical
to	O
PGRMC1	B-protein
was	O
seen	O
also	O
in	O
HCT116	O
cells	O
and	O
human	B-species
hepatoma	O
HuH7	O
cells	O
(	O
Supplementary	O
Fig	O
.	O
17	O
).	O

Furthermore	O
,	O
PGRMC1	B-mutant
-	I-mutant
KD	I-mutant
inhibited	O
spheroid	O
formation	O
of	O
HCT116	O
cells	O
in	O
culture	O
,	O
and	O
this	O
inhibition	O
was	O
reversed	O
by	O
co	B-experimental_method
-	I-experimental_method
expression	I-experimental_method
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
PGRMC1	B-protein
but	O
not	O
of	O
the	O
Y113F	B-mutant
mutant	B-protein_state
(	O
Fig	O
.	O
5c	O
and	O
Supplementary	O
Fig	O
.	O
18	O
).	O

Thus	O
,	O
PGRMC1	B-protein
dimerization	B-oligomeric_state
is	O
important	O
for	O
cancer	O
cell	O
proliferation	O
and	O
chemoresistance	O
.	O

Ten	O
days	O
after	O
intra	B-experimental_method
-	I-experimental_method
splenic	I-experimental_method
implantation	I-experimental_method
of	O
HCT116	O
cells	O
that	O
were	O
genetically	O
tagged	O
with	O
a	O
fluorescent	O
protein	O
Venus	O
,	O
the	O
group	O
implanted	O
with	O
PGRMC1	B-mutant
-	I-mutant
KD	I-mutant
cells	O
showed	O
a	O
significant	O
decrease	O
of	O
liver	O
metastasis	O
in	O
comparison	O
with	O
the	O
control	O
group	O
(	O
Fig	O
.	O
5d	O
).	O

Since	O
PGRMC1	B-protein
has	O
been	O
shown	O
to	O
interact	O
with	O
cytochromes	B-protein_type
P450	I-protein_type
(	O
ref	O
),	O
we	O
investigated	O
whether	O
the	O
haem	B-chemical
-	O
mediated	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
is	O
necessary	O
for	O
their	O
interactions	O
.	O

Recombinant	O
CYP1A2	B-protein
and	O
CYP3A4	B-protein
including	O
a	O
microsomal	O
formulation	O
containing	O
cytochrome	B-protein_type
b5	I-protein_type
and	O
cytochrome	B-protein
P450	I-protein
reductase	I-protein
,	O
drug	O
-	O
metabolizing	O
cytochromes	B-protein_type
P450	I-protein_type
,	O
interacted	O
with	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
PGRMC1	B-protein
,	O
but	O
not	O
with	O
the	O
Y113F	B-mutant
mutant	B-protein_state
,	O
in	O
a	O
haem	B-chemical
-	O
dependent	O
manner	O
(	O
Fig	O
.	O
6a	O
,	O
b	O
).	O

Moreover	O
,	O
the	O
interaction	O
of	O
PGRMC1	B-protein
with	O
CYP1A2	B-protein
was	O
blocked	O
by	O
CORM3	B-chemical
under	O
reducing	O
conditions	O
(	O
Fig	O
.	O
6c	O
),	O
indicating	O
that	O
PGRMC1	B-protein
dimerization	B-oligomeric_state
is	O
necessary	O
for	O
its	O
interaction	O
with	O
cytochromes	B-protein_type
P450	I-protein_type
.	O

Doxorubicin	B-chemical
is	O
an	O
anti	O
-	O
cancer	O
reagent	O
that	O
is	O
metabolized	O
into	O
inactive	O
doxorubicinol	B-chemical
by	O
CYP2D6	B-protein
and	O
CYP3A4	B-protein
(	O
Fig	O
.	O
6d	O
).	O

PGRMC1	B-mutant
-	I-mutant
KD	I-mutant
significantly	O
suppressed	O
the	O
conversion	O
of	O
doxorubicin	B-chemical
to	O
doxorubicinol	B-chemical
(	O
Fig	O
.	O
6d	O
)	O
and	O
increased	O
sensitivity	O
to	O
doxorubicin	B-chemical
(	O
Fig	O
.	O
6e	O
).	O

Enhanced	O
doxorubicin	B-chemical
sensitivity	O
was	O
modestly	O
but	O
significantly	O
induced	O
by	O
PGRMC1	B-mutant
-	I-mutant
KD	I-mutant
.	O

To	O
gain	O
further	O
insight	O
into	O
the	O
interaction	O
between	O
PGRMC1	B-protein
and	O
cytochromes	B-protein_type
P450	I-protein_type
,	O
surface	B-experimental_method
plasmon	I-experimental_method
resonance	I-experimental_method
analyses	I-experimental_method
were	O
conducted	O
using	O
recombinant	O
CYP51	B-protein
and	O
PGRMC1	B-protein
.	O

This	O
was	O
based	O
on	O
a	O
previous	O
study	O
showing	O
that	O
PGRMC1	B-protein
binds	O
to	O
CYP51	B-protein
and	O
enhances	O
cholesterol	O
biosynthesis	O
by	O
CYP51	B-protein
(	O
refs	O
).	O

CYP51	B-protein
interacted	O
with	O
PGRMC1	B-protein
in	O
a	O
concentration	O
-	O
dependent	O
manner	O
in	O
the	O
presence	B-protein_state
of	I-protein_state
haem	B-chemical
,	O
but	O
not	O
in	O
its	O
absence	B-protein_state
(	O
Supplementary	O
Fig	O
.	O
19	O
),	O
suggesting	O
the	O
requirement	O
for	O
the	O
haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
.	O

The	O
Kd	B-evidence
value	O
of	O
PGRMC1	B-protein
binding	O
to	O
CYP51	B-protein
was	O
in	O
a	O
micromolar	O
range	O
and	O
comparable	O
with	O
those	O
of	O
other	O
haem	B-chemical
proteins	O
,	O
such	O
as	O
cytochrome	B-protein
P450	I-protein
reductase	I-protein
and	O
neuroglobin	B-protein
/	O
Gαi1	B-protein
(	O
ref	O
.),	O
suggesting	O
that	O
haem	B-chemical
-	O
dependent	O
PGRMC1	B-protein
interaction	O
with	O
CYP51	B-protein
is	O
biologically	O
relevant	O
.	O

In	O
this	O
study	O
,	O
we	O
showed	O
that	O
PGRMC1	B-protein
dimerizes	B-oligomeric_state
by	O
stacking	B-bond_interaction
interactions	I-bond_interaction
of	O
haem	B-chemical
molecules	O
from	O
each	O
monomer	B-oligomeric_state
.	O

Recently	O
,	O
Lucas	O
et	O
al	O
.	O
reported	O
that	O
translationally	B-protein_type
-	I-protein_type
controlled	I-protein_type
tumour	I-protein_type
protein	I-protein_type
was	O
dimerized	B-protein_state
by	O
binding	O
with	O
haem	B-chemical
,	O
but	O
its	O
structural	O
basis	O
remains	O
unclear	O
.	O

This	O
is	O
the	O
report	O
showing	O
crystallographic	O
evidence	O
that	O
indicates	O
roles	O
of	O
the	O
direct	O
haem	B-bond_interaction
–	I-bond_interaction
haem	I-bond_interaction
stacking	I-bond_interaction
in	O
haem	B-chemical
-	O
mediated	O
dimerization	B-oligomeric_state
in	O
eukaryotes	B-taxonomy_domain
,	O
although	O
a	O
few	O
examples	O
are	O
known	O
in	O
bacteria	B-taxonomy_domain
.	O

Sequence	B-experimental_method
alignments	I-experimental_method
show	O
that	O
haem	B-site
-	I-site
binding	I-site
residues	I-site
(	O
Tyr113	B-residue_name_number
,	O
Tyr107	B-residue_name_number
,	O
Lys163	B-residue_name_number
and	O
Tyr164	B-residue_name_number
)	O
in	O
PGRMC1	B-protein
are	O
conserved	B-protein_state
among	O
MAPR	B-protein_type
proteins	O
(	O
Supplementary	O
Fig	O
.	O
5	O
).	O

Since	O
the	O
Y113	B-residue_name_number
residue	O
is	O
involved	O
in	O
the	O
putative	O
consensus	B-structure_element
motif	I-structure_element
of	O
phosphorylation	B-ptm
by	O
tyrosine	B-protein_type
kinases	I-protein_type
such	O
as	O
Abl	B-protein_type
and	O
Lck	B-protein_type
,	O
we	O
investigated	O
whether	O
phosphorylated	B-protein_state
Y113	B-residue_name_number
is	O
present	O
in	O
HCT116	O
cells	O
by	O
ESI	B-experimental_method
-	I-experimental_method
MS	I-experimental_method
analysis	O
.	O

While	O
the	O
effects	O
of	O
PGRMC1	B-protein
on	O
cholesterol	O
synthesis	O
mediated	O
by	O
CYP51	B-protein
have	O
been	O
well	O
documented	O
in	O
yeast	B-taxonomy_domain
and	O
human	B-species
cells	O
,	O
it	O
has	O
not	O
been	O
clear	O
whether	O
drug	O
-	O
metabolizing	O
CYP	B-protein_type
activities	O
are	O
regulated	O
by	O
PGRMC1	B-protein
.	O

Szczesna	O
-	O
Skorupa	O
and	O
Kemper	O
reported	O
that	O
PGRMC1	B-protein
exhibited	O
an	O
inhibitory	O
effect	O
on	O
CYP3A4	B-protein
drug	O
metabolizing	O
activity	O
by	O
competitively	O
binding	O
with	O
cytochrome	B-protein
P450	I-protein
reductase	I-protein
(	O
CPR	B-protein
)	O
in	O
HEK293	O
or	O
HepG2	O
cells	O
.	O

On	O
the	O
other	O
hand	O
,	O
Oda	O
et	O
al	O
.	O
reported	O
that	O
PGRMC1	B-protein
had	O
no	O
effect	O
to	O
CYP2E1	B-protein
and	O
CYP3A4	B-protein
activities	O
in	O
HepG2	O
cell	O
.	O

Several	O
other	O
groups	O
showed	O
that	O
PGRMC1	B-protein
enhanced	O
chemoresistance	O
in	O
several	O
cancer	O
cells	O
such	O
as	O
uterine	O
sarcoma	O
,	O
breast	O
cancer	O
,	O
endometrial	O
tumour	O
and	O
ovarian	O
cancer	O
;	O
however	O
,	O
no	O
evidence	O
of	O
PGRMC1	B-protein
-	O
dependent	O
regulation	O
of	O
CYP	B-protein_type
activity	O
was	O
provided	O
.	O

Our	O
results	O
showed	O
that	O
PGRMC1	B-protein
contributes	O
to	O
enhancement	O
of	O
the	O
doxorubicin	B-chemical
metabolism	O
,	O
which	O
is	O
mediated	O
by	O
CYP2D6	B-protein
or	O
CYP3A4	B-protein
in	O
human	B-species
colon	O
cancer	O
HCT116	O
cells	O
(	O
Fig	O
.	O
6d	O
).	O

We	O
showed	O
that	O
haem	B-chemical
-	O
mediated	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
enhances	O
proliferation	O
and	O
chemoresistance	O
of	O
cancer	O
cells	O
through	O
binding	O
to	O
and	O
regulating	O
EGFR	B-protein_type
and	O
cytochromes	B-protein_type
P450	I-protein_type
(	O
illustrated	O
in	O
Fig	O
.	O
7	O
).	O

Since	O
the	O
haem	B-evidence
-	I-evidence
binding	I-evidence
affinity	I-evidence
of	O
PGRMC1	B-protein
is	O
lower	O
than	O
those	O
of	O
constitutive	B-protein_state
haem	B-protein_type
-	I-protein_type
binding	I-protein_type
proteins	I-protein_type
such	O
as	O
myoglobin	B-protein
,	O
PGMRC1	B-protein
is	O
probably	O
interconverted	O
between	O
apo	B-protein_state
-	O
monomer	B-oligomeric_state
and	O
haem	B-protein_state
-	I-protein_state
bound	I-protein_state
dimer	B-oligomeric_state
forms	O
in	O
response	O
to	O
changes	O
in	O
the	O
intracellular	O
haem	B-chemical
concentration	O
.	O

Moreover	O
,	O
exposure	O
of	O
cancer	O
cells	O
to	O
stimuli	O
such	O
as	O
hypoxia	O
,	O
radiation	O
and	O
chemotherapy	O
causes	O
cell	O
damages	O
and	O
leads	O
to	O
protein	O
degradation	O
,	O
resulting	O
in	O
increased	O
levels	O
of	O
TCA	O
cycle	O
intermediates	O
and	O
in	O
an	O
enhanced	O
haem	B-chemical
biosynthesis	O
.	O

On	O
the	O
other	O
hand	O
,	O
excessive	O
haem	B-chemical
induces	O
HO	B-protein
-	I-protein
1	I-protein
,	O
the	O
enzyme	O
that	O
oxidatively	O
degrades	O
haem	B-chemical
and	O
generates	O
CO	B-chemical
.	O

This	O
idea	O
is	O
consistent	O
with	O
the	O
observation	O
that	O
HO	B-protein
-	I-protein
1	I-protein
induction	O
or	O
CO	B-chemical
inhibits	O
tumour	O
growth	O
.	O

Besides	O
the	O
regulatory	O
roles	O
of	O
PGRMC1	B-protein
/	O
Sigma	B-protein
-	I-protein
2	I-protein
receptor	O
in	O
proliferation	O
and	O
chemoresistance	O
in	O
cancer	O
cells	O
(	O
ref	O
.),	O
recent	O
reports	O
show	O
that	O
PGRMC1	B-protein
is	O
able	O
to	O
bind	O
to	O
amyloid	B-protein
beta	I-protein
oligomer	B-oligomeric_state
to	O
enhance	O
its	O
neurotoxicity	O
.	O

The	O
roles	O
of	O
haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
in	O
the	O
functional	O
regulation	O
of	O
its	O
target	O
proteins	O
deserve	O
further	O
studies	O
to	O
find	O
evidence	O
that	O
therapeutic	O
interventions	O
to	O
interfere	O
with	O
the	O
function	O
of	O
the	O
dimer	B-oligomeric_state
may	O
control	O
varied	O
disease	O
conditions	O
.	O

Alzheimer	O
'	O
s	O
therapeutics	O
targeting	O
amyloid	O
beta	O
1	O
-	O
42	O
oligomers	O
II	O
:	O
Sigma	O
-	O
2	O
/	O
PGRMC1	O
receptors	O
mediate	O
Abeta	O
42	O
oligomer	B-oligomeric_state
binding	O
and	O
synaptotoxicity	O

X	B-evidence
-	I-evidence
ray	I-evidence
crystal	I-evidence
structure	I-evidence
of	O
PGRMC1	B-protein
.	O

(	O
a	O
)	O
Structure	O
of	O
the	O
PGRMC1	B-protein
dimer	B-oligomeric_state
formed	O
through	O
stacked	O
haems	B-chemical
.	O

Two	O
PGRMC1	B-protein
subunits	B-structure_element
(	O
blue	O
and	O
green	O
ribbons	O
)	O
dimerize	B-oligomeric_state
via	O
stacking	B-bond_interaction
of	O
the	O
haem	B-chemical
molecules	O
.	O

PGRCM1	B-protein
is	O
dimerized	B-protein_state
by	O
binding	O
with	O
haem	B-chemical
.	O

(	O
a	O
)	O
Mass	B-experimental_method
spectrometric	I-experimental_method
analyses	O
of	O
the	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
wt	B-protein_state
)	O
PGRMC1	B-protein
or	O
the	O
C129S	B-mutant
mutant	B-protein_state
in	O
the	O
presence	B-protein_state
or	O
absence	B-protein_state
of	I-protein_state
haem	B-chemical
under	O
non	O
-	O
denaturing	O
condition	O
.	O

Both	O
proteins	O
had	O
identical	O
lengths	O
(	O
a	O
.	O
a	O
.	O
44	B-residue_range
–	I-residue_range
195	I-residue_range
).	O

(	O
b	O
)	O
SV	B-experimental_method
-	I-experimental_method
AUC	I-experimental_method
analyses	O
of	O
the	O
wt	B-protein_state
-	O
PGRMC1	B-protein
and	O
the	O
C129S	B-mutant
mutant	B-protein_state
(	O
a	O
.	O
a	O
.	O
44	B-residue_range
–	I-residue_range
195	I-residue_range
)	O
in	O
the	O
presence	B-protein_state
or	O
absence	B-protein_state
of	I-protein_state
haem	B-chemical
.	O

(	O
c	O
)	O
Difference	B-evidence
absorption	I-evidence
spectra	I-evidence
of	O
PGRMC1	B-protein
(	O
a	O
.	O
a	O
.	O
44	B-residue_range
–	I-residue_range
195	I-residue_range
)	O
titrated	B-experimental_method
with	I-experimental_method
haem	B-chemical
(	O
left	O
panel	O
).	O

The	O
titration	B-evidence
curve	I-evidence
of	O
haem	B-chemical
to	O
PGRMC1	B-protein
(	O
right	O
panel	O
).	O

The	O
absorbance	B-evidence
difference	I-evidence
at	O
400	O
nm	O
is	O
plotted	O
against	O
the	O
haem	B-chemical
concentration	O
.	O

Carbon	B-chemical
monoxide	I-chemical
inhibits	O
haem	B-chemical
-	O
dependent	O
PGRMC1	B-protein
dimerization	B-oligomeric_state
.	O

Haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
is	O
necessary	O
for	O
tumour	O
proliferation	O
mediated	O
by	O
EGFR	B-protein_type
signalling	O
.	O

(	O
c	O
)	O
FLAG	O
-	O
PGRMC1	B-protein
wt	B-protein_state
or	O
Y113F	B-mutant
(	O
full	B-protein_state
length	I-protein_state
)	O
was	O
over	B-experimental_method
-	I-experimental_method
expressed	I-experimental_method
in	O
HCT116	O
cells	O
and	O
immunoprecipitated	B-experimental_method
with	O
anti	O
-	O
FLAG	O
antibody	O
-	O
conjugated	O
beads	O
.	O

(	O
d	O
)	O
HCT116	O
cells	O
were	O
treated	O
with	O
or	O
without	O
250	O
μmol	O
l	O
−	O
1	O
of	O
succinylacetone	B-chemical
(	O
SA	B-chemical
)	O
for	O
48	O
h	O
.	O
The	O
intracellular	O
haem	B-chemical
was	O
extracted	O
and	O
quantified	O
by	O
reverse	B-experimental_method
-	I-experimental_method
phase	I-experimental_method
HPLC	I-experimental_method
.	O

of	O
four	O
separate	O
experiments	O
.	O
**	O
P	O
<	O
0	O
.	O
01	O
using	O
unpaired	O
Student	B-experimental_method
'	I-experimental_method
s	I-experimental_method
t	I-experimental_method
-	I-experimental_method
test	I-experimental_method
.	O
(	O
e	O
)	O
Co	B-experimental_method
-	I-experimental_method
immunoprecipitation	I-experimental_method
assay	I-experimental_method
was	O
performed	O
as	O
in	O
(	O
c	O
)	O
with	O
or	O
without	O
SA	B-chemical
treatment	O
in	O
HCT116	O
cells	O
.	O

(	O
g	O
,	O
h	O
)	O
HCT116	O
cells	O
were	O
treated	O
with	O
or	O
without	O
EGF	B-protein_type
,	O
SA	B-chemical
,	O
RuCl3	B-chemical
and	O
CORM3	B-chemical
as	O
indicated	O
,	O
and	O
components	O
of	O
the	O
EGFR	B-protein_type
signaling	O
pathway	O
were	O
detected	O
by	O
Western	B-experimental_method
blotting	I-experimental_method
.	O

Haem	B-chemical
-	O
dependent	O
dimerization	B-oligomeric_state
of	O
PGRMC1	B-protein
accelerates	O
tumour	O
growth	O
through	O
the	O
EGFR	B-protein_type
signaling	O
pathway	O
.	O

(	O
a	O
)	O
Nucleotide	O
sequences	O
of	O
PGRMC1	B-protein
targeted	O
by	O
shRNA	B-chemical
and	O
of	O
the	O
shRNA	B-protein_state
-	I-protein_state
resistant	I-protein_state
full	B-protein_state
length	I-protein_state
PGRMC1	B-protein
expression	O
vector	O
.	O

Stable	O
PGRMC1	B-mutant
-	I-mutant
knockdown	I-mutant
(	O
PGRMC1	B-mutant
-	I-mutant
KD	I-mutant
)	O
HCT116	O
cells	O
were	O
transiently	B-experimental_method
transfected	I-experimental_method
with	O
the	O
shRNA	B-protein_state
-	I-protein_state
resistant	I-protein_state
expression	B-experimental_method
vector	I-experimental_method
of	O
wild	B-protein_state
-	I-protein_state
type	I-protein_state
PGRMC1	B-protein
(	O
wt	B-protein_state
)	O
or	O
the	O
Y113F	B-mutant
mutant	B-protein_state
(	O
Y113F	B-mutant
).	O

(	O
b	O
)	O
Erlotinib	B-chemical
was	O
added	O
to	O
HCT116	O
(	O
control	O
)	O
cells	O
,	O
PGRMC1	B-mutant
-	I-mutant
KD	I-mutant
cells	O
or	O
PGRMC1	B-mutant
-	I-mutant
KD	I-mutant
cells	O
expressing	O
shRNA	B-protein_state
-	I-protein_state
resistant	I-protein_state
PGRMC1	B-protein
wt	B-protein_state
or	O
Y113F	B-mutant
,	O
and	O
cell	O
viability	O
was	O
examined	O
by	O
MTT	B-experimental_method
assay	I-experimental_method
.	O

of	O
four	O
separate	O
experiments	O
.	O
*	B-evidence
P	I-evidence
<	O
0	O
.	O
01	O
using	O
ANOVA	B-experimental_method
with	O
Fischer	B-experimental_method
'	I-experimental_method
s	I-experimental_method
LSD	I-experimental_method
test	I-experimental_method
.	O

(	O
c	O
)	O
Spheroid	O
formation	O
in	O
control	O
and	O
PGRMC1	B-mutant
-	I-mutant
KD	I-mutant
HCT116	O
cells	O
.	O

The	O
graph	O
represents	O
mean	O
±	O
s	O
.	O
e	O
.	O
of	O
each	O
spheroid	O
size	O
.	O
*	B-evidence
P	I-evidence
<	O
0	O
.	O
01	O
using	O
ANOVA	B-experimental_method
with	O
Fischer	B-experimental_method
'	I-experimental_method
s	I-experimental_method
LSD	I-experimental_method
test	I-experimental_method
.	O

Scale	O
bar	O
:	O
0	O
.	O
1	O
mm	O
.	O
(	O
d	O
)	O
Tumour	O
-	O
bearing	O
livers	O
of	O
NOG	O
mice	O
at	O
10	O
days	O
after	O
intrasplenic	B-experimental_method
injection	I-experimental_method
of	O
HCT116	O
(	O
control	O
)	O
or	O
PGRMC1	B-mutant
-	I-mutant
KD	I-mutant
cells	O
.	O

(	O
a	O
,	O
b	O
)	O
FLAG	O
-	O
PGRMC1	B-protein
wild	B-protein_state
-	I-protein_state
type	I-protein_state
(	O
wt	B-protein_state
)	O
and	O
Y113F	B-mutant
mutant	B-protein_state
proteins	O
(	O
a	O
.	O
a	O
.	O
44	B-residue_range
–	I-residue_range
195	I-residue_range
),	O
in	O
either	O
apo	B-protein_state
or	O
haem	B-protein_state
-	I-protein_state
bound	I-protein_state
form	O
,	O
were	O
incubated	B-experimental_method
with	O
CYP1A2	B-protein
(	O
a	O
)	O
or	O
CYP3A4	B-protein
(	O
b	O
)	O
and	O
immunoprecipitated	B-experimental_method
with	O
anti	O
-	O
FLAG	O
antibody	O
-	O
conjugated	O
beads	O
.	O

(	O
c	O
)	O
Binding	B-experimental_method
assay	I-experimental_method
was	O
performed	O
as	O
in	O
(	O
a	O
)	O
using	O
haem	B-protein_state
-	I-protein_state
bound	I-protein_state
FLAG	O
-	O
PGRMC1	B-protein
wt	B-protein_state
and	O
CYP1A2	B-protein
with	O
or	O
without	O
RuCl3	B-chemical
and	O
CORM3	B-chemical
.	O

(	O
d	O
)	O
Schematic	O
illustration	O
of	O
doxorubicin	B-chemical
metabolism	O
is	O
shown	O
on	O
the	O
left	O
.	O

Schematic	O
diagram	O
for	O
the	O
regulation	O
of	O
PGRMC1	B-protein
functions	O
.	O

Apo	B-protein_state
-	O
PGRMC1	B-protein
exists	O
as	O
an	O
inactive	B-protein_state
monomer	B-oligomeric_state
.	O

On	O
binding	B-protein_state
to	I-protein_state
haem	B-chemical
,	O
PGRMC1	B-protein
forms	O
a	O
dimer	B-oligomeric_state
through	O
stacking	B-bond_interaction
interactions	I-bond_interaction
between	O
the	O
haem	B-chemical
moieties	O
,	O
which	O
enables	O
PGRMC1	B-protein
to	O
interact	O
with	O
EGFR	B-protein_type
and	O
cytochromes	B-protein_type
P450	I-protein_type
,	O
leading	O
to	O
an	O
enhanced	O
proliferation	O
and	O
chemoresistance	O
of	O
cancer	O
cells	O
.	O

Apo	O
form	O
Haem	O
-	O
bound	O
form	O
Mass	O
(	O
Da	O
)	O
Mass	O
(	O
Da	O
)	O
aPGRMC1	O
wt	O
(	O
a	O
.	O
a	O
.	O
44	O
–	O
195	O
)	O
ESI	O
-	O
MS	O
—	O
17	O
,	O
844	O
.	O
14	O
—	O
36	O
,	O
920	O
.	O
19	O
Theoretical	O
17	O
,	O
843	O
.	O
65	O
36	O
,	O
918	O
.	O
06	O
Hydrodynamic	O
radius	O
10	O
−	O
9	O
(	O
m	O
)	O
MW	O
(	O
kDa	O
)	O
Hydrodynamic	O
radius	O
10	O
−	O
9	O
(	O
m	O
)	O
MW	O
(	O
kDa	O
)	O
DOSY	O
2	O
.	O
04	O
–	O
2	O
.	O
15	O
20	O
2	O
.	O
94	O
–	O
3	O
.	O
02	O
42	O
S20	O
,	O
w	O
(	O
S	O
)	O
MW	O
(	O
kDa	O
)	O
S20	O
,	O
w	O
(	O
S	O
)	O
MW	O
(	O
kDa	O
)	O
SV	O
-	O
AUC	O
1	O
.	O
9	O
17	O
.	O
6	O
3	O
.	O
1	O
35	O
.	O
5	O
bPGRMC1	O
C129S	B-mutant
(	O
a	O
.	O
a	O
.	O
44	O
–	O
195	O
)	O
ESI	O
-	O
MS	O
—	O
17	O
,	O
827	O
.	O
91	O
—	O
36	O
,	O
887	O
.	O
07	O
Theoretical	O
17	O
,	O
827	O
.	O
59	O
36	O
,	O
885	O
.	O
6	O
S20	O
,	O
w	O
(	O
S	O
)	O
MW	O
(	O
kDa	O
)	O
S20	O
,	O
w	O
(	O
S	O
)	O
MW	O
(	O
kDa	O
)	O
SV	O
-	O
AUC	O
2	O
.	O
0	O
18	O
.	O
1	O
3	O
.	O
1	O
35	O
.	O
8	O

Differences	O
in	O
molecular	O
weights	O
of	O
the	O
wild	O
-	O
type	O
(	O
wt	O
;	O
a	O
)	O
and	O
the	O
C129S	B-mutant
mutant	O
(	O
b	O
)	O
PGRMC1	O
proteins	O
in	O
the	O
absence	O
(	O
apo	O
form	O
)	O
or	O
the	O
presence	O
of	O
haem	O
(	O
haem	O
-	O
bound	O
form	O
).	O

The	O
protein	O
sizes	O
of	O
the	O
wt	O
and	O
C129S	B-mutant
PGRMC1	O
cytosolic	O
domains	O
(	O
a	O
.	O
a	O
.	O
44	O
–	O
195	O
)	O
in	O
the	O
presence	O
or	O
absence	O
of	O
haem	O
were	O
estimated	O
by	O
ESI	O
-	O
MS	O
,	O
DOSY	O
and	O
SV	O
-	O
AUC	O
.	O

Hotspot	O
autoimmune	O
T	B-protein_type
cell	I-protein_type
receptor	I-protein_type
binding	O
underlies	O
pathogen	O
and	O
insulin	B-chemical
peptide	O
cross	O
-	O
reactivity	O

However	O
,	O
the	O
mechanisms	O
that	O
allow	O
the	O
clonal	O
T	B-complex_assembly
cell	I-complex_assembly
antigen	I-complex_assembly
receptor	I-complex_assembly
(	O
TCR	B-complex_assembly
)	O
to	O
functionally	O
engage	O
multiple	O
peptide	B-complex_assembly
–	I-complex_assembly
major	I-complex_assembly
histocompatibility	I-complex_assembly
complexes	I-complex_assembly
(	O
pMHC	B-complex_assembly
)	O
are	O
unclear	O
.	O

Here	O
,	O
we	O
studied	O
multiligand	O
discrimination	O
by	O
a	O
human	B-species
,	O
preproinsulin	B-protein
reactive	O
,	O
MHC	B-complex_assembly
class	O
-	O
I	O
–	O
restricted	O
CD8	O
+	O
T	O
cell	O
clone	O
(	O
1E6	O
)	O
that	O
can	O
recognize	O
over	O
1	O
million	O
different	O
peptides	O
.	O

We	O
generated	O
high	O
-	O
resolution	O
structures	B-evidence
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
bound	B-protein_state
to	I-protein_state
7	O
altered	B-chemical
peptide	I-chemical
ligands	I-chemical
,	O
including	O
a	O
pathogen	O
-	O
derived	O
peptide	O
that	O
was	O
an	O
order	O
of	O
magnitude	O
more	O
potent	O
than	O
the	O
natural	O
self	O
-	O
peptide	O
.	O

Evaluation	O
of	O
these	O
structures	B-evidence
demonstrated	O
that	O
binding	O
was	O
stabilized	O
through	O
a	O
conserved	O
lock	O
-	O
and	O
-	O
key	O
–	O
like	O
minimal	O
binding	O
footprint	O
that	O
enables	O
1E6	B-complex_assembly
TCR	I-complex_assembly
to	O
tolerate	O
vast	O
numbers	O
of	O
substitutions	O
outside	O
of	O
this	O
so	O
-	O
called	O
hotspot	O
.	O

Highly	O
potent	O
antigens	O
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
engaged	O
with	O
a	O
strong	O
antipathogen	B-evidence
-	I-evidence
like	I-evidence
binding	I-evidence
affinity	I-evidence
;	O
this	O
engagement	O
was	O
governed	O
though	O
an	O
energetic	O
switch	O
from	O
an	O
enthalpically	O
to	O
entropically	O
driven	O
interaction	O
compared	O
with	O
the	O
natural	O
autoimmune	O
ligand	O
.	O

This	O
ability	O
is	O
required	O
to	O
enable	O
the	O
estimated	O
25	O
million	O
distinct	O
TCRs	B-complex_assembly
expressed	O
in	O
humans	B-species
to	O
provide	O
effective	O
immune	O
coverage	O
against	O
all	O
possible	O
foreign	O
peptide	O
antigens	O
.	O

Several	O
mechanisms	O
,	O
by	O
which	O
TCRs	B-complex_assembly
could	O
bind	O
to	O
a	O
large	O
number	O
of	O
different	O
peptide	B-complex_assembly
-	I-complex_assembly
MHC	I-complex_assembly
(	O
pMHC	B-complex_assembly
),	O
have	O
been	O
proposed	O
.	O

Both	O
MHC	B-complex_assembly
and	O
peptide	B-chemical
have	O
also	O
been	O
shown	O
to	O
undergo	O
structural	O
changes	O
upon	O
TCR	B-complex_assembly
binding	O
,	O
mediating	O
an	O
induced	O
fit	O
between	O
the	O
TCR	B-complex_assembly
and	O
pMHC	B-complex_assembly
.	O

Other	O
studies	O
,	O
mainly	O
in	O
the	O
murine	B-taxonomy_domain
system	O
,	O
have	O
demonstrated	O
that	O
the	O
same	O
TCR	B-complex_assembly
can	O
interact	O
with	O
different	O
pMHCs	B-complex_assembly
using	O
a	O
common	O
or	O
divergent	O
modality	O
.	O

Recent	O
studies	O
in	O
model	O
murine	B-taxonomy_domain
systems	O
demonstrate	O
that	O
TCR	B-complex_assembly
cross	O
-	O
reactivity	O
can	O
be	O
governed	O
by	O
recognition	O
of	O
a	O
conserved	O
region	O
in	O
the	O
peptide	O
that	O
allows	O
tolerance	O
of	O
peptide	O
sequence	O
variation	O
outside	O
of	O
this	O
hotspot	O
.	O

CD8	O
+	O
T	O
cells	O
that	O
recognize	O
HLA	B-complex_assembly
-	I-complex_assembly
A	I-complex_assembly
*	I-complex_assembly
0201	I-complex_assembly
–	I-complex_assembly
ALWGPDPAAA	I-complex_assembly
have	O
been	O
shown	O
to	O
populate	O
insulitic	O
lesions	O
in	O
patients	O
with	O
type	O
1	O
diabetes	O
(	O
T1D	O
).	O

We	O
demonstrated	O
that	O
the	O
TCR	B-complex_assembly
from	O
the	O
1E6	O
T	O
cell	O
clone	O
bound	B-protein_state
to	I-protein_state
HLA	B-complex_assembly
-	I-complex_assembly
A	I-complex_assembly
*	I-complex_assembly
0201	I-complex_assembly
–	I-complex_assembly
ALWGPDPAAA	I-complex_assembly
using	O
a	O
limited	O
footprint	O
and	O
very	O
weak	O
binding	B-evidence
affinity	I-evidence
.	O

This	O
first	O
experimental	O
evidence	O
of	O
a	O
high	O
level	O
of	O
CD8	O
+	O
T	O
cell	O
cross	O
-	O
reactivity	O
in	O
a	O
human	B-species
autoimmune	O
disease	O
system	O
hinted	O
toward	O
molecular	O
mimicry	O
by	O
a	O
more	O
potent	O
pathogenic	O
peptide	O
as	O
a	O
potential	O
mechanism	O
leading	O
to	O
β	O
cell	O
destruction	O
.	O

Here	O
,	O
we	O
solved	B-experimental_method
the	O
structure	B-evidence
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
with	O
7	O
altered	B-chemical
peptide	I-chemical
ligands	I-chemical
(	O
APLs	B-chemical
)	O
determined	O
by	O
our	O
previously	O
published	O
combinatorial	B-experimental_method
peptide	I-experimental_method
library	I-experimental_method
(	I-experimental_method
CPL	I-experimental_method
)	I-experimental_method
screening	I-experimental_method
,	O
2	O
of	O
which	O
mapped	O
within	O
human	B-species
pathogens	O
.	O

These	O
APLs	B-chemical
differed	O
from	O
the	O
natural	O
preproinsulin	B-protein
peptide	O
by	O
up	O
to	O
7	O
of	O
10	O
residues	O
.	O

We	O
also	O
solved	B-experimental_method
the	O
structure	B-evidence
of	O
each	O
unligated	B-protein_state
APL	B-chemical
to	O
investigate	O
whether	O
structural	O
changes	O
occurred	O
before	O
or	O
after	O
binding	O
—	O
which	O
,	O
combined	O
with	O
an	O
in	O
-	O
depth	O
cellular	B-experimental_method
and	I-experimental_method
biophysical	I-experimental_method
analysis	I-experimental_method
of	O
the	O
1E6	O
interaction	O
with	O
each	O
APL	B-chemical
,	O
demonstrated	O
the	O
molecular	O
mechanism	O
mediating	O
the	O
high	O
level	O
of	O
cross	O
-	O
reactivity	O
exhibited	O
by	O
this	O
preproinsulin	B-protein
-	O
reactive	O
human	B-species
CD8	O
+	O
T	O
cell	O
clone	O
.	O

The	O
1E6	O
T	O
cell	O
clone	O
recognizes	O
APLs	B-chemical
across	O
a	O
large	O
dynamic	O
range	O
.	O

We	O
have	O
previously	O
demonstrated	O
that	O
the	O
1E6	O
T	O
cell	O
clone	O
can	O
recognize	O
over	O
1	O
million	O
different	O
peptides	O
with	O
a	O
potency	O
comparable	O
with	O
,	O
or	O
better	O
than	O
,	O
the	O
cognate	O
preproinsulin	B-protein
peptide	O
ALWGPDPAAA	B-chemical
.	O

Two	O
of	O
these	O
peptides	O
,	O
MVWGPDPLYV	B-chemical
and	O
RQFGPDWIVA	B-chemical
(	O
bold	O
text	O
signifies	O
amino	O
acids	O
that	O
are	O
different	O
from	O
the	O
index	O
preproinsulin	B-protein
–	O
derived	O
sequence	O
),	O
are	O
contained	O
within	O
the	O
proteomes	O
of	O
the	O
human	B-species
pathogens	O
Bacteroides	B-species
fragilis	I-species
/	I-species
thetaiotaomicron	I-species
and	O
Clostridium	B-species
asparagiforme	I-species
,	O
respectively	O
.	O

Competitive	B-experimental_method
functional	I-experimental_method
testing	I-experimental_method
revealed	O
that	O
the	O
preproinsulin	B-protein
-	O
derived	O
sequence	O
ALWGPDPAAA	B-chemical
was	O
one	O
of	O
the	O
least	O
potent	O
targets	O
for	O
1E6	O
,	O
with	O
only	O
the	O
MVWGPDPLYV	B-chemical
and	O
YLGGPDFPTI	B-chemical
demonstrating	O
a	O
similar	O
low	O
-	O
activity	O
profile	O
in	O
MIP	B-protein
-	I-protein
1β	I-protein
secretion	O
and	O
target	O
killing	O
assays	O
(	O
Figure	O
1	O
,	O
A	O
and	O
B	O
).	O

The	O
RQFGPDWIVA	B-chemical
sequence	O
(	O
present	O
in	O
C	B-species
.	I-species
asparagiforme	I-species
)	O
activated	O
the	O
1E6	O
T	O
cell	O
with	O
around	O
1	O
log	O
–	O
greater	O
potency	O
compared	O
with	O
ALWGPDPAAA	B-chemical
.	O

At	O
the	O
other	O
end	O
of	O
the	O
spectrum	O
,	O
the	O
RQFGPDFPTI	B-chemical
peptide	O
stimulated	O
MIP	B-protein
-	I-protein
1β	I-protein
release	O
and	O
killing	O
by	O
1E6	O
at	O
an	O
exogenous	O
peptide	O
concentration	O
2	O
–	O
3	O
logs	O
lower	O
compared	O
with	O
ALWGPDPAAA	B-chemical
.	O

The	O
pattern	O
of	O
peptide	O
potency	O
was	O
closely	O
mirrored	O
by	O
pMHC	B-complex_assembly
tetramer	B-experimental_method
staining	I-experimental_method
experiments	O
(	O
Figure	O
1C	O
and	O
plots	O
shown	O
in	O
Supplemental	O
Figure	O
1	O
;	O
supplemental	O
material	O
available	O
online	O
with	O
this	O
article	O
;	O
doi	O
:	O
10	O
.	O
1172	O
/	O
JCI85679DS1	O
).	O

To	O
parallel	O
the	O
functional	O
analysis	O
,	O
we	O
also	O
performed	O
thermal	B-experimental_method
melt	I-experimental_method
(	O
Tm	B-evidence
)	O
experiments	O
using	O
synchrotron	B-experimental_method
radiation	I-experimental_method
circular	I-experimental_method
dichroism	I-experimental_method
(	O
SRCD	B-experimental_method
)	O
to	O
investigate	O
the	O
stability	O
of	O
each	O
APL	B-chemical
(	O
Figure	O
1D	O
).	O

This	O
pattern	O
of	O
stability	O
did	O
not	O
correlate	O
with	O
the	O
T	O
cell	O
activation	O
or	O
tetramer	B-experimental_method
staining	I-experimental_method
experiments	O
,	O
indicating	O
that	O
peptide	O
binding	O
to	O
the	O
MHC	B-complex_assembly
do	O
not	O
explain	O
ligand	O
potency	O
.	O

The	O
1E6	B-complex_assembly
TCR	I-complex_assembly
can	O
bind	O
peptides	O
with	O
strong	O
antipathogen	O
-	O
like	O
affinities	O
.	O

We	O
,	O
and	O
others	O
,	O
have	O
previously	O
demonstrated	O
that	O
antipathogenic	O
TCRs	B-complex_assembly
tend	O
to	O
bind	O
with	O
stronger	O
affinity	B-evidence
compared	O
with	O
self	O
-	O
reactive	O
TCRs	B-complex_assembly
,	O
likely	O
a	O
consequence	O
of	O
the	O
deletion	O
of	O
T	O
cells	O
with	O
high	O
-	O
affinity	B-evidence
self	O
-	O
reactive	O
TCR	B-complex_assembly
during	O
thymic	O
selection	O
.	O

First	O
,	O
the	O
1E6	O
T	O
cell	O
could	O
still	O
functionally	O
respond	O
to	O
peptide	O
when	O
the	O
TCR	B-evidence
binding	I-evidence
affinity	I-evidence
was	O
extremely	O
weak	O
,	O
e	O
.	O
g	O
.,	O
the	O
1E6	B-evidence
TCR	I-evidence
binding	I-evidence
affinity	I-evidence
for	O
the	O
A2	B-chemical
-	I-chemical
MVWGPDPLYV	I-chemical
peptide	O
was	O
KD	B-evidence
=	O
~	O
600	O
μM	O
.	O
Second	O
,	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
bound	B-protein_state
to	I-protein_state
A2	B-chemical
-	I-chemical
RQFGPDFPTI	I-chemical
with	O
KD	B-evidence
=	O
0	O
.	O
5	O
μM	O
,	O
equivalent	O
to	O
the	O
binding	B-evidence
affinity	I-evidence
of	O
the	O
very	O
strongest	O
antipathogen	O
TCRs	B-complex_assembly
.	O

Third	O
,	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
bound	B-protein_state
to	I-protein_state
A2	B-chemical
-	I-chemical
RQFGPDWIVA	I-chemical
peptide	O
,	O
within	O
the	O
C	B-species
.	I-species
asparagiforme	I-species
proteome	O
,	O
with	O
approximately	O
4	O
-	O
fold	O
stronger	O
affinity	B-evidence
than	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
,	O
demonstrating	O
the	O
potential	O
for	O
a	O
pathogen	O
-	O
derived	O
antigen	O
to	O
initiate	O
a	O
response	O
to	O
the	O
self	O
-	O
derived	O
sequence	O
.	O

To	O
confirm	O
the	O
affinity	B-evidence
spread	O
detected	O
by	O
SPR	B-experimental_method
,	O
and	O
to	O
evaluate	O
whether	O
experiments	O
performed	O
using	O
soluble	O
molecules	O
were	O
biologically	O
relevant	O
to	O
events	O
at	O
the	O
T	O
cell	O
surface	O
,	O
we	O
determined	O
the	O
effective	O
2D	B-evidence
affinity	I-evidence
of	O
each	O
APL	B-chemical
using	O
an	O
adhesion	B-experimental_method
frequency	I-experimental_method
assay	I-experimental_method
in	O
which	O
a	O
human	B-species
rbc	O
coated	O
in	O
pMHC	B-complex_assembly
acted	O
as	O
an	O
adhesion	O
sensor	O
.	O

In	O
agreement	O
with	O
SPR	B-experimental_method
experiments	O
,	O
the	O
range	O
of	O
2D	B-evidence
affinities	I-evidence
we	O
detected	O
differed	O
by	O
around	O
3	O
logs	O
,	O
with	O
the	O
A2	B-chemical
-	I-chemical
MVWGPDPLYV	I-chemical
generating	O
the	O
weakest	O
2D	B-evidence
affinity	I-evidence
(	O
2	O
.	O
6	O
×	O
10	O
–	O
5	O
AcKa	B-evidence
μm4	O
)	O
and	O
A2	B-chemical
-	I-chemical
RQFGPDFPTI	I-chemical
the	O
strongest	O
(	O
4	O
.	O
5	O
×	O
10	O
–	O
2	O
AcKa	B-evidence
μm4	O
)	O
(	O
Figure	O
2J	O
).	O

Of	O
note	O
,	O
these	O
data	O
demonstrate	O
a	O
close	O
agreement	O
between	O
the	O
3D	B-evidence
affinity	I-evidence
values	O
generated	O
using	O
SPR	B-experimental_method
and	O
2D	B-evidence
affinity	I-evidence
values	O
generated	O
using	O
adhesion	O
frequency	O
assays	O
.	O

All	O
structures	B-evidence
were	O
solved	B-experimental_method
in	O
space	O
group	O
P1	O
to	O
2	O
–	O
3	O
Å	O
resolution	O
with	O
crystallographic	O
Rwork	B-evidence
/	I-evidence
Rfree	I-evidence
ratios	I-evidence
within	O
accepted	O
limits	O
as	O
shown	O
in	O
the	O
theoretically	O
expected	O
distribution	O
(	O
ref	O
.	O
and	O
Supplemental	O
Table	O
1	O
).	O

The	O
1E6	B-complex_assembly
TCR	I-complex_assembly
used	O
a	O
very	O
similar	O
overall	O
binding	O
modality	O
to	O
engage	O
all	O
of	O
the	O
APLs	B-chemical
,	O
with	O
root	B-evidence
mean	I-evidence
square	I-evidence
deviation	I-evidence
ranging	O
between	O
0	O
.	O
81	O
and	O
1	O
.	O
12	O
Å2	O
(	O
compared	O
with	O
1E6	B-complex_assembly
-	I-complex_assembly
A2	I-complex_assembly
-	I-complex_assembly
ALWGPDPAAA	I-complex_assembly
).	O

Overall	O
,	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
used	O
a	O
canonical	O
binding	O
mode	O
to	O
engage	O
each	O
APL	B-chemical
with	O
the	O
TCR	B-complex_assembly
α	B-structure_element
-	I-structure_element
chain	I-structure_element
positioned	O
over	O
the	O
MHC	B-complex_assembly
class	I-complex_assembly
I	I-complex_assembly
(	O
MHCI	B-complex_assembly
)	O
α2	B-structure_element
-	I-structure_element
helix	I-structure_element
and	O
the	O
TCR	B-complex_assembly
β	B-structure_element
-	I-structure_element
chain	I-structure_element
over	O
the	O
MHCI	B-complex_assembly
α	B-structure_element
-	I-structure_element
1	I-structure_element
helix	I-structure_element
,	O
straddling	O
the	O
peptide	O
cargo	O
.	O

However	O
,	O
subtle	O
differences	O
in	O
the	O
respective	O
interfaces	B-site
were	O
apparent	O
(	O
discussed	O
below	O
)	O
and	O
resulted	O
in	O
altered	O
binding	B-evidence
affinities	I-evidence
of	O
the	O
respective	O
complexes	O
.	O

Interactions	O
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
and	O
different	O
APLs	B-chemical
are	O
focused	O
around	O
a	O
conserved	B-protein_state
GPD	B-structure_element
peptide	I-structure_element
motif	I-structure_element
.	O

We	O
next	O
performed	O
an	O
in	O
-	O
depth	O
atomic	B-experimental_method
analysis	I-experimental_method
of	O
the	O
contacts	O
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
and	O
each	O
APL	B-chemical
to	O
determine	O
the	O
structural	O
basis	O
for	O
the	O
altered	O
T	O
cell	O
peptide	O
sensitivities	O
and	O
TCR	B-evidence
binding	I-evidence
affinities	I-evidence
(	O
Table	O
2	O
).	O

Concomitant	O
with	O
our	O
global	O
analysis	O
of	O
1E6	B-complex_assembly
TCR	I-complex_assembly
binding	O
to	O
the	O
APLs	B-chemical
,	O
we	O
observed	O
a	O
common	O
interaction	O
element	O
,	O
consistent	O
with	O
our	O
previous	O
findings	O
,	O
that	O
utilized	O
TCR	B-complex_assembly
residues	O
Tyr97α	B-residue_name_number
and	O
Trp97β	B-residue_name_number
,	O
forming	O
an	O
aromatic	B-structure_element
cap	I-structure_element
over	O
a	O
central	O
GPD	B-structure_element
motif	I-structure_element
that	O
was	O
present	O
in	O
all	O
of	O
the	O
APLs	B-chemical
(	O
Figure	O
4	O
).	O

Interactions	O
between	O
these	O
2	O
TCR	B-complex_assembly
and	O
3	O
peptide	O
residues	O
accounted	O
for	O
41	O
%–	O
50	O
%	O
of	O
the	O
total	O
contacts	O
across	O
all	O
complexes	O
(	O
Table	O
2	O
),	O
demonstrating	O
the	O
conserved	B-protein_state
peptide	O
centric	O
binding	O
mode	O
utilized	O
by	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
.	O

This	O
fixed	O
anchoring	O
between	O
the	O
2	O
molecules	O
was	O
important	O
for	O
stabilization	O
of	O
the	O
TCR	B-complex_assembly
-	I-complex_assembly
pMHC	I-complex_assembly
complex	O
,	O
as	O
—	O
although	O
other	O
peptides	O
without	O
the	O
‘	B-structure_element
GDP	I-structure_element
’	I-structure_element
motif	I-structure_element
were	O
tested	O
and	O
shown	O
to	O
activate	O
the	O
1E6	O
T	O
cell	O
clone	O
—	O
we	O
were	O
unable	O
to	O
measure	O
robust	O
affinities	B-evidence
using	O
SPR	B-experimental_method
(	O
data	O
not	O
shown	O
).	O

These	O
data	O
support	O
the	O
requirement	O
for	O
a	O
conserved	B-protein_state
interaction	O
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
and	O
the	O
GPD	B-structure_element
motif	I-structure_element
,	O
as	O
we	O
observed	O
in	O
our	O
previously	O
published	O
1E6	B-complex_assembly
-	I-complex_assembly
A2	I-complex_assembly
-	I-complex_assembly
ALWGPDPAAA	I-complex_assembly
structure	B-evidence
.	O

Although	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
formed	O
a	O
similar	O
overall	O
interaction	O
with	O
each	O
APL	B-chemical
,	O
the	O
stabilization	O
between	O
the	O
TCR	B-complex_assembly
and	O
the	O
GPD	B-structure_element
motif	I-structure_element
enabled	O
fine	O
differences	O
in	O
the	O
contact	B-site
network	I-site
with	O
both	O
the	O
peptide	B-chemical
and	O
MHC	B-site
surface	I-site
that	O
allowed	O
discrimination	O
between	O
each	O
ligand	O
(	O
Figure	O
5	O
).	O

For	O
example	O
,	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
made	O
only	O
47	O
peptide	O
contacts	O
with	O
A2	B-chemical
-	I-chemical
MVWGPDPLYV	I-chemical
(	O
KD	B-evidence
=	O
~	O
600	O
μM	O
)	O
compared	O
with	O
63	O
and	O
57	O
contacts	O
with	O
A2	B-chemical
-	I-chemical
YQFGPDFPIA	I-chemical
(	O
KD	B-evidence
=	O
7	O
.	O
4	O
μM	O
)	O
and	O
A2	B-chemical
-	I-chemical
RQFGPDFPTI	I-chemical
(	O
KD	B-evidence
=	O
0	O
.	O
5	O
μM	O
),	O
respectively	O
.	O

For	O
example	O
,	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
made	O
64	O
peptide	O
contacts	O
with	O
A2	B-chemical
-	I-chemical
YLGGPDFPTI	I-chemical
(	O
KD	B-evidence
=	O
~	O
400	O
μM	O
)	O
compared	O
with	O
43	O
contacts	O
with	O
A2	B-chemical
-	I-chemical
RQWGPDPAAV	I-chemical
(	O
KD	B-evidence
=	O
7	O
.	O
8	O
μM	O
).	O

The	O
most	O
important	O
peptide	O
modification	O
in	O
terms	O
of	O
generating	O
new	O
contacts	O
was	O
peptide	O
position	O
1	B-residue_number
.	O

We	O
have	O
previously	O
shown	O
that	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
uses	O
a	O
rigid	O
lock	O
-	O
and	O
-	O
key	O
mechanism	O
during	O
binding	O
to	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
.	O

In	O
order	O
to	O
determine	O
whether	O
any	O
of	O
the	O
APLs	B-chemical
required	O
an	O
induced	O
fit	O
mechanism	O
during	O
binding	O
that	O
could	O
explain	O
the	O
difference	O
in	O
free	B-evidence
binding	I-evidence
energy	I-evidence
(	O
ΔG	B-evidence
)	O
between	O
each	O
complex	O
(	O
Table	O
2	O
),	O
we	O
solved	B-experimental_method
the	O
unligated	B-protein_state
structures	B-evidence
of	O
all	O
7	O
APLs	B-chemical
(	O
the	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
structure	B-evidence
has	O
been	O
previously	O
published	O
and	O
was	O
used	O
in	O
this	O
comparison	O
,	O
ref	O
.)	O
(	O
Figure	O
6	O
and	O
Supplemental	O
Table	O
2	O
).	O

The	O
unligated	B-protein_state
A2	B-chemical
-	I-chemical
MVWGPDPLYV	I-chemical
(	O
KD	B-evidence
=	O
~	O
600	O
μM	O
)	O
structure	B-evidence
revealed	O
that	O
the	O
side	O
chain	O
Tyr9	B-residue_name_number
swung	O
around	O
8	O
Å	O
in	O
the	O
complex	O
structure	B-evidence
,	O
subsequently	O
making	O
contacts	O
with	O
TCR	B-complex_assembly
residues	O
Asp30β	B-residue_name_number
and	O
Asn51β	B-residue_name_number
(	O
Figure	O
6A	O
and	O
Figure	O
5A	O
,	O
respectively	O
).	O

This	O
movement	O
could	O
result	O
in	O
an	O
entropic	O
penalty	O
contributing	O
to	O
the	O
weak	O
TCR	B-evidence
binding	I-evidence
affinity	I-evidence
we	O
observed	O
for	O
this	O
ligand	O
.	O

Additional	O
small	O
movements	O
in	O
the	O
Cα	O
backbone	O
of	O
the	O
peptide	O
around	O
peptide	O
residue	O
Asp6	B-residue_name_number
were	O
apparent	O
in	O
the	O
A2	B-chemical
-	I-chemical
YLGGPDFPTI	I-chemical
(	O
KD	B-evidence
=	O
~	O
400	O
μM	O
),	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
(	O
KD	B-evidence
=	O
~	O
208	O
μM	O
),	O
and	O
A2	B-chemical
-	I-chemical
RQFGPDWIVA	I-chemical
(	O
KD	B-evidence
=	O
44	O
.	O
4	O
μM	O
)	O
structures	B-evidence
(	O
Figure	O
6	O
,	O
B	O
,	O
C	O
,	O
and	O
E	O
).	O

Apart	O
from	O
the	O
case	O
of	O
A2	B-chemical
-	I-chemical
AQWGPDAAA	I-chemical
(	O
KD	B-evidence
=	O
61	O
.	O
9	O
μM	O
),	O
these	O
observations	O
support	O
the	O
conclusion	O
that	O
the	O
higher	O
-	O
affinity	B-evidence
ligands	O
required	O
less	O
conformational	O
melding	O
during	O
binding	O
,	O
which	O
could	O
be	O
energetically	O
beneficial	O
(	O
lower	O
entopic	O
cost	O
)	O
during	O
ligation	O
with	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
.	O

MHC	B-complex_assembly
residue	O
Arg65	B-residue_name_number
that	O
forms	O
part	O
of	O
the	O
MHC	B-site
restriction	I-site
triad	I-site
(	O
Arg65	B-residue_name_number
,	O
Ala69	B-residue_name_number
,	O
and	O
Gln155	B-residue_name_number
)	O
played	O
a	O
central	O
role	O
in	O
TCR	B-complex_assembly
-	O
MHC	B-complex_assembly
contacts	O
,	O
with	O
Gln155	B-residue_name_number
playing	O
a	O
less	O
important	O
role	O
and	O
Ala69	B-residue_name_number
playing	O
no	O
role	O
in	O
binding	O
at	O
the	O
interface	B-site
(	O
Figure	O
7	O
).	O

For	O
instance	O
,	O
contacts	O
were	O
made	O
between	O
TCR	B-complex_assembly
residue	O
Val53β	B-residue_name_number
and	O
MHC	B-complex_assembly
residue	O
Gln72	B-residue_name_number
in	O
all	O
APLs	B-chemical
except	O
for	O
in	O
the	O
weakest	O
affinity	B-evidence
ligand	O
pair	O
,	O
1E6	B-complex_assembly
-	I-complex_assembly
A2	I-complex_assembly
-	I-complex_assembly
MVWGPDPLYV	I-complex_assembly
,	O
in	O
which	O
a	O
subtle	O
change	O
in	O
TCR	B-complex_assembly
conformation	O
—	O
probably	O
mediated	O
by	O
different	O
peptide	O
contacts	O
—	O
abrogated	O
this	O
interaction	O
(	O
Figure	O
7A	O
).	O

Our	O
analysis	O
of	O
the	O
contact	B-site
network	I-site
provided	O
some	O
clues	O
that	O
could	O
explain	O
the	O
different	O
antigen	O
potencies	O
and	O
binding	B-evidence
affinities	I-evidence
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
and	O
the	O
different	O
APLs	B-chemical
.	O

The	O
overall	O
free	B-evidence
binding	I-evidence
energies	I-evidence
(	O
ΔG	B-evidence
°)	I-evidence
were	O
between	O
–	O
4	O
.	O
4	O
and	O
–	O
8	O
.	O
6	O
kcal	O
/	O
mol	O
,	O
reflecting	O
the	O
wide	O
range	O
of	O
TCR	B-evidence
binding	I-evidence
affinities	I-evidence
we	O
observed	O
for	O
the	O
different	O
APLs	B-chemical
.	O

The	O
enthalpic	O
contribution	O
in	O
each	O
complex	O
did	O
not	O
follow	O
a	O
clear	O
trend	O
with	O
affinity	B-evidence
,	O
with	O
all	O
but	O
the	O
1E6	B-complex_assembly
-	I-complex_assembly
A2	I-complex_assembly
-	I-complex_assembly
RQFGPDFPTI	I-complex_assembly
interaction	O
(	O
ΔH	B-evidence
°	I-evidence
=	O
6	O
.	O
3	O
kcal	O
/	O
mol	O
)	O
generating	O
an	O
energetically	O
favorable	O
enthalpy	B-evidence
value	O
(	O
ΔH	B-evidence
°	I-evidence
=	O
–	O
3	O
.	O
7	O
to	O
–	O
11	O
.	O
4	O
kcal	O
/	O
mol	O
);	O
this	O
indicated	O
a	O
net	O
gain	O
in	O
electrostatic	O
interactions	O
during	O
complex	O
formation	O
.	O

For	O
instance	O
,	O
the	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
,	O
A2	B-chemical
-	I-chemical
AQWGPDAAA	I-chemical
,	O
and	O
A2	B-chemical
-	I-chemical
RQFGPDWIVA	I-chemical
(	O
KD	B-evidence
=	O
~	O
208	O
μM	O
,	O
KD	B-evidence
=	O
61	O
.	O
9	O
μM	O
,	O
and	O
KD	B-evidence
=	O
44	O
.	O
4	O
μM	O
,	O
respectively	O
)	O
were	O
all	O
entropically	O
unfavorable	O
(	O
TΔS	B-evidence
°	I-evidence
=	O
–	O
2	O
.	O
9	O
to	O
–	O
5	O
.	O
6	O
kcal	O
/	O
mol	O
),	O
indicating	O
a	O
net	O
change	O
from	O
disorder	O
to	O
order	O
.	O

Furthermore	O
,	O
the	O
structures	O
of	O
the	O
unligated	B-protein_state
pMHCs	B-complex_assembly
demonstrated	O
that	O
,	O
for	O
these	O
stronger	O
-	O
affinity	B-evidence
ligands	O
,	O
there	O
was	O
less	O
conformational	O
difference	O
between	O
the	O
TCR	B-complex_assembly
ligated	B-protein_state
pMHCs	B-complex_assembly
compared	O
with	O
the	O
weaker	O
-	O
affinity	B-evidence
ligands	O
(	O
Figure	O
6	O
).	O

The	O
potential	O
requirement	O
for	O
a	O
larger	O
degree	O
of	O
induced	O
fit	O
during	O
binding	O
to	O
these	O
weaker	O
-	O
affinity	B-evidence
ligands	O
is	O
consistent	O
with	O
the	O
larger	O
entropic	O
penalties	O
observed	O
for	O
these	O
interactions	O
.	O

Potential	O
epitopes	O
for	O
1E6	B-complex_assembly
TCR	I-complex_assembly
occur	O
commonly	O
in	O
the	O
viral	B-taxonomy_domain
proteome	O
.	O

We	O
searched	O
a	O
database	O
of	O
over	O
1	O
,	O
924	O
,	O
572	O
unique	O
decamer	O
peptides	B-chemical
from	O
the	O
proteome	O
of	O
viral	B-taxonomy_domain
pathogens	O
that	O
are	O
known	O
,	O
or	O
strongly	O
suspected	O
,	O
to	O
infect	O
humans	B-species
.	O

Three	O
hundred	O
forty	O
-	O
two	O
of	O
these	O
decamers	O
conformed	O
to	O
the	O
motif	O
xxxGPDxxxx	B-structure_element
.	O

Of	O
these	O
,	O
53	O
peptides	O
contained	O
the	O
motif	O
xOxGPDxxxO	B-structure_element
,	O
where	O
O	O
is	O
one	O
of	O
the	O
hydrophobic	O
amino	O
acid	O
residues	O
A	B-residue_name
,	O
V	B-residue_name
,	O
I	B-residue_name
,	O
L	B-residue_name
,	O
M	B-residue_name
,	O
Y	B-residue_name
,	O
F	B-residue_name
,	O
and	O
W	B-residue_name
that	O
might	O
allow	O
binding	O
to	O
HLA	B-protein
-	I-protein
A	I-protein
*	I-protein
0201	I-protein
(	O
Supplemental	O
Table	O
4	O
).	O

Thus	O
,	O
there	O
are	O
many	O
pathogen	O
-	O
encoded	O
peptides	O
that	O
could	O
act	O
as	O
agonists	O
for	O
the	O
1E6	O
T	O
cell	O
beyond	O
the	O
MVWGPDPLYV	B-chemical
and	O
RQFGPDWIVA	B-chemical
sequences	O
studied	O
here	O
.	O

Extension	O
of	O
these	O
analyses	O
to	O
include	O
the	O
larger	O
genomes	O
of	O
bacterial	B-taxonomy_domain
pathogens	O
would	O
be	O
expected	O
to	O
considerably	O
increase	O
these	O
numbers	O
.	O

T	O
cell	O
antigen	O
discrimination	O
is	O
governed	O
by	O
an	O
interaction	O
between	O
the	O
clonally	O
expressed	O
TCR	B-complex_assembly
and	O
pMHC	B-complex_assembly
,	O
mediated	O
by	O
the	O
chemical	O
characteristics	O
of	O
the	O
interacting	O
molecules	O
.	O

It	O
has	O
recently	O
become	O
clear	O
that	O
TCR	B-complex_assembly
cross	O
-	O
reactivity	O
with	O
large	O
numbers	O
of	O
different	O
pMHC	B-complex_assembly
ligands	O
is	O
essential	O
to	O
plug	O
holes	O
in	O
T	O
cell	O
immune	O
coverage	O
that	O
pathogens	O
could	O
exploit	O
.	O

This	O
notion	O
is	O
attractive	O
because	O
the	O
CDR	B-structure_element
loops	I-structure_element
,	O
which	O
form	O
the	O
TCR	B-site
antigen	I-site
-	I-site
binding	I-site
site	I-site
,	O
are	O
usually	O
the	O
most	O
flexible	O
part	O
of	O
the	O
TCR	B-complex_assembly
and	O
have	O
the	O
ability	O
to	O
mold	O
around	O
differently	O
shaped	O
ligands	O
.	O

Notably	O
among	O
these	O
studies	O
,	O
Garcia	O
and	O
colleagues	O
recently	O
used	O
the	O
alloreactive	B-protein_state
murine	B-taxonomy_domain
TCR	B-complex_assembly
-	O
MHC	B-complex_assembly
pair	O
of	O
the	O
42F3	B-protein
TCR	B-complex_assembly
and	O
H2	B-protein
-	I-protein
Ld	I-protein
to	O
demonstrate	O
recognition	O
of	O
a	O
large	O
number	O
of	O
different	O
peptides	O
via	O
conserved	B-protein_state
hotspot	B-site
contacts	O
with	O
prominent	O
up	O
-	O
facing	O
peptide	O
residues	O
.	O

Sethi	O
and	O
colleagues	O
recently	O
demonstrated	O
that	O
the	O
MHCII	B-protein_type
-	O
restricted	O
Hy	B-protein
.	I-protein
1B11	I-protein
TCR	B-complex_assembly
,	O
which	O
was	O
isolated	O
from	O
a	O
patient	O
with	O
multiple	O
sclerosis	O
,	O
could	O
anchor	O
into	O
a	O
deep	B-site
pocket	I-site
formed	O
from	O
peptide	O
residues	O
2	B-residue_number
,	O
3	B-residue_number
,	O
and	O
5	B-residue_number
(	O
from	O
MBP85	B-protein
–	I-protein
99	I-protein
bound	B-protein_state
to	I-protein_state
HLA	B-protein
-	I-protein
DQ1	I-protein
).	O

First	O
,	O
we	O
currently	O
know	O
nothing	O
about	O
how	O
human	B-species
MHCI	B-complex_assembly
–	O
restricted	O
TCRs	B-complex_assembly
mediate	O
cross	O
-	O
reactivity	O
in	O
the	O
context	O
of	O
a	O
clinically	O
relevant	O
model	O
of	O
autoimmunity	O
,	O
thought	O
to	O
be	O
a	O
major	O
pathway	O
of	O
disease	O
initiation	O
in	O
several	O
autoimmune	O
diseases	O
.	O

Here	O
,	O
we	O
investigated	O
a	O
highly	O
cross	O
-	O
reactive	O
MHCI	B-complex_assembly
-	O
restricted	O
TCR	B-complex_assembly
isolated	O
from	O
a	O
patient	O
with	O
T1D	O
that	O
recognizes	O
an	O
HLA	B-protein
-	I-protein
A	I-protein
*	I-protein
0201	I-protein
–	O
restricted	O
preproinsulin	B-protein
signal	B-structure_element
peptide	I-structure_element
(	O
ALWGPDPAAA15	B-chemical
–	I-chemical
24	I-chemical
).	O

Human	B-species
CD8	O
+	O
T	O
cell	O
clones	O
expressing	O
TCRs	B-complex_assembly
with	O
this	O
specificity	O
mediate	O
the	O
destruction	O
of	O
β	O
cells	O
,	O
have	O
been	O
found	O
in	O
islets	O
early	O
in	O
infection	O
,	O
and	O
are	O
proposed	O
to	O
be	O
a	O
major	O
driver	O
of	O
disease	O
.	O

We	O
solved	B-experimental_method
the	O
structure	B-evidence
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
with	O
7	O
APLs	B-chemical
to	O
enable	O
a	O
comprehensive	O
analysis	O
of	O
the	O
molecular	O
basis	O
of	O
TCR	B-complex_assembly
degeneracy	O
.	O

Overall	O
,	O
the	O
difference	O
in	O
antigen	O
potency	O
correlated	O
well	O
with	O
the	O
binding	B-evidence
energy	I-evidence
(	O
ΔG	B-evidence
°	I-evidence
kcal	O
/	O
mol	O
)	O
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
for	O
the	O
different	O
epitopes	O
,	O
which	O
ranged	O
from	O
values	O
of	O
ΔG	B-evidence
°	I-evidence
=	O
~–	O
4	O
.	O
4	O
to	O
–	O
8	O
.	O
6	O
kcal	O
/	O
mol	O
(	O
calculated	O
from	O
3D	B-evidence
affinity	I-evidence
data	O
)	O
or	O
2D	B-evidence
affinity	I-evidence
values	O
of	O
AcKa	B-evidence
=	O
2	O
.	O
5	O
×	O
10	O
–	O
5	O
to	O
4	O
.	O
4	O
×	O
10	O
–	O
2	O
μm4	O
.	O

The	O
weaker	O
end	O
of	O
this	O
spectrum	O
extends	O
our	O
understanding	O
of	O
the	O
limits	O
in	O
which	O
T	O
cells	O
can	O
functionally	O
operate	O
in	O
terms	O
of	O
TCR	B-evidence
3D	I-evidence
binding	I-evidence
affinity	I-evidence
and	O
is	O
in	O
line	O
with	O
the	O
types	O
of	O
very	O
low	O
affinity	B-evidence
,	O
yet	O
fully	O
functional	O
self	O
-	O
reactive	O
CD8	O
+	O
T	O
cells	O
we	O
have	O
observed	O
in	O
tumor	O
-	O
infiltrating	O
lymphocytes	O
.	O

Previous	O
studies	O
of	O
autoreactive	O
TCRs	B-complex_assembly
have	O
shown	O
that	O
their	O
binding	O
mode	O
is	O
generally	O
atypical	O
,	O
either	O
due	O
to	O
an	O
unusual	O
binding	O
manner	O
,	O
weak	O
TCR	B-evidence
binding	I-evidence
affinity	I-evidence
,	O
an	O
unstable	B-protein_state
pMHC	B-complex_assembly
,	O
or	O
a	O
combination	O
of	O
these	O
factors	O
.	O

Our	O
data	O
demonstrate	O
the	O
potential	O
for	O
an	O
autoreactive	O
TCR	B-complex_assembly
to	O
bind	O
with	O
a	O
conventional	O
binding	O
mode	O
to	O
a	O
stable	B-protein_state
pMHC	B-complex_assembly
with	O
antipathogen	O
-	O
like	O
affinity	B-evidence
(	O
KD	B-evidence
=	O
0	O
.	O
5	O
μM	O
)	O
depending	O
on	O
the	O
peptide	O
sequence	O
.	O

Our	O
structural	B-experimental_method
analysis	I-experimental_method
revealed	O
that	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
bound	B-protein_state
with	O
a	O
conserved	B-protein_state
conformation	I-protein_state
across	O
all	O
APLs	B-chemical
investigated	O
.	O

This	O
binding	O
orientation	O
was	O
mediated	O
through	O
a	O
focused	O
interaction	O
with	O
TCR	B-complex_assembly
residues	O
Tyr97α	B-residue_name_number
and	O
Trp97β	B-residue_name_number
that	O
formed	O
an	O
aromatic	B-structure_element
cap	I-structure_element
over	O
a	O
central	O
‘	B-structure_element
GDP	I-structure_element
’	I-structure_element
motif	I-structure_element
that	O
was	O
common	O
to	O
all	O
APLs	B-chemical
.	O

We	O
have	O
previously	O
demonstrated	O
the	O
importance	O
of	O
the	O
GPD	B-structure_element
motif	I-structure_element
using	O
a	O
peptide	B-experimental_method
library	I-experimental_method
scan	I-experimental_method
,	O
as	O
well	O
as	O
a	O
CPL	B-experimental_method
scan	I-experimental_method
approach	O
.	O

This	O
hotspot	O
binding	O
,	O
defined	O
as	O
a	O
localized	O
cluster	O
of	O
interactions	O
that	O
dominate	O
binding	O
energy	O
during	O
protein	O
-	O
protein	O
interactions	O
,	O
has	O
been	O
previously	O
shown	O
to	O
contribute	O
to	O
TCR	B-complex_assembly
recognition	O
of	O
MHC	B-complex_assembly
as	O
a	O
mechanism	O
that	O
tunes	O
T	O
cell	O
cross	O
-	O
reactivity	O
by	O
providing	O
fixed	O
anchor	O
points	O
that	O
enable	O
TCRs	B-complex_assembly
to	O
tolerate	O
a	O
variable	O
peptide	O
cargo	O
.	O

Alternatively	O
,	O
interactions	O
between	O
the	O
TCR	B-complex_assembly
and	O
peptide	O
have	O
been	O
shown	O
to	O
dominate	O
the	O
energetic	O
landscape	O
during	O
ligand	O
engagement	O
,	O
ensuring	O
that	O
T	O
cells	O
retain	O
peptide	O
specificity	O
.	O

These	O
findings	O
are	O
also	O
analogous	O
to	O
the	O
observed	O
binding	O
mode	O
of	O
the	O
Hy	B-protein
.	I-protein
1B11	I-protein
TCR	B-complex_assembly
,	O
in	O
which	O
one	O
aromatic	O
residue	O
of	O
the	O
TCR	B-complex_assembly
CDR3α	B-structure_element
loop	I-structure_element
anchored	O
into	O
a	O
pocket	O
created	O
by	O
a	O
conserved	O
peptide	O
motif	O
.	O

In	O
both	O
of	O
these	O
examples	O
,	O
self	O
-	O
recognition	O
is	O
mediated	O
by	O
TCR	B-complex_assembly
residues	O
with	O
aromatic	O
side	O
chains	O
.	O

Despite	O
some	O
weak	O
statistical	O
correlation	O
between	O
the	O
surface	B-evidence
complementarity	I-evidence
(	O
SC	B-evidence
)	O
and	O
affinity	B-evidence
,	O
closer	O
inspection	O
of	O
the	O
interface	B-site
revealed	O
no	O
obvious	O
structural	O
signature	O
that	O
could	O
definitively	O
explain	O
the	O
differences	O
in	O
antigen	B-evidence
potency	I-evidence
and	O
TCR	B-evidence
binding	I-evidence
strength	I-evidence
between	O
the	O
different	O
ligands	O
.	O

However	O
,	O
similar	O
to	O
our	O
findings	O
in	O
other	O
systems	O
,	O
modifications	O
to	O
residues	O
outside	O
of	O
the	O
canonical	O
central	B-structure_element
peptide	I-structure_element
bulge	I-structure_element
were	O
important	O
for	O
generating	O
new	O
interactions	O
.	O

For	O
example	O
,	O
all	O
of	O
the	O
stronger	O
ligands	O
encoded	O
larger	O
side	O
chains	O
(	O
Arg	B-residue_name
or	O
Tyr	B-residue_name
)	O
at	O
peptide	O
position	O
1	B-residue_number
that	O
enabled	O
new	O
interactions	O
with	O
1E6	O
not	O
present	O
with	O
the	O
Ala	B-residue_name
at	O
this	O
position	O
in	O
the	O
natural	O
preproinsulin	B-protein
peptide	B-chemical
.	O

We	O
have	O
recently	O
demonstrated	O
how	O
a	O
suboptimal	O
position	O
2	B-residue_number
anchor	B-structure_element
in	O
a	O
melanoma	O
-	O
derived	O
antigen	O
can	O
improve	O
TCR	B-complex_assembly
binding	O
through	O
a	O
similar	O
mechanism	O
.	O

These	O
results	O
challenge	O
the	O
notion	O
that	O
the	O
most	O
potent	O
peptide	O
antigens	O
exhibit	O
the	O
greatest	O
pMHC	B-complex_assembly
stability	O
and	O
have	O
implications	O
for	O
the	O
design	O
of	O
anchor	O
residue	O
–	O
modified	O
heteroclitic	O
peptides	O
for	O
vaccination	O
.	O

These	O
parameters	O
aligned	O
well	O
with	O
structural	B-evidence
data	I-evidence
,	O
demonstrating	O
that	O
TCRs	B-complex_assembly
engaged	O
pMHC	B-complex_assembly
using	O
an	O
induced	O
fit	O
binding	O
mode	O
.	O

However	O
,	O
more	O
recent	O
data	O
have	O
shown	O
that	O
TCRs	B-complex_assembly
can	O
utilize	O
a	O
range	O
of	O
energetic	O
strategies	O
during	O
pMHC	B-complex_assembly
binding	O
,	O
currently	O
with	O
no	O
obvious	O
pattern	O
in	O
terms	O
of	O
TCR	B-evidence
affinity	I-evidence
,	O
binding	O
mechanism	O
,	O
or	O
specificity	O
(	O
pathogen	O
,	O
cancer	O
,	O
or	O
self	O
-	O
ligands	O
).	O

Although	O
no	O
energetic	O
signature	O
appears	O
to	O
exist	O
for	O
different	O
TCRs	B-complex_assembly
,	O
we	O
used	O
thermodynamic	B-experimental_method
analysis	I-experimental_method
here	O
to	O
explore	O
whether	O
changes	O
in	O
energetics	O
could	O
help	O
explain	O
ligand	O
discrimination	O
by	O
a	O
single	O
TCR	B-complex_assembly
.	O

The	O
weaker	O
APL	B-chemical
ligands	O
were	O
characterized	O
by	O
favorable	O
enthalpy	B-evidence
and	O
unfavorable	O
entropy	B-evidence
,	O
whereas	O
the	O
stronger	O
ligands	O
progressively	O
shifted	O
to	O
favorable	O
entropy	B-evidence
.	O

Thus	O
,	O
the	O
enhanced	O
antigen	O
potency	O
was	O
probably	O
mediated	O
through	O
a	O
shift	O
from	O
an	O
induced	O
fit	O
to	O
a	O
lock	O
-	O
and	O
-	O
key	O
interaction	O
between	O
the	O
stronger	O
ligands	O
(	O
less	O
requirement	O
for	O
energetically	O
unfavorable	O
disorder	O
-	O
to	O
-	O
order	O
changes	O
),	O
resulting	O
in	O
a	O
more	O
energetically	O
favorable	O
ΔG	B-evidence
value	I-evidence
.	O

The	O
RQFGPDWIVA	B-chemical
peptide	O
,	O
which	O
was	O
substantially	O
more	O
potent	O
than	O
the	O
preproinsulin	B-protein
peptide	O
,	O
is	O
within	O
the	O
proteome	O
of	O
a	O
common	O
human	B-species
pathogen	O
(	O
C	B-species
.	I-species
asparagiforme	I-species
),	O
demonstrating	O
the	O
potential	O
for	O
an	O
encounter	O
between	O
a	O
naive	O
1E6	O
-	O
like	O
T	O
cell	O
and	O
a	O
foreign	O
peptide	O
with	O
a	O
more	O
potent	O
ligand	O
that	O
might	O
then	O
break	O
self	O
-	O
tolerance	O
.	O

Indeed	O
,	O
we	O
found	O
over	O
50	O
decamer	O
peptides	O
from	O
the	O
proteome	O
of	O
likely	O
,	O
or	O
known	O
,	O
human	B-species
viral	B-taxonomy_domain
pathogens	O
alone	O
that	O
contained	O
both	O
the	O
conserved	B-protein_state
central	O
GPD	B-structure_element
motif	I-structure_element
and	O
anchor	B-structure_element
residues	I-structure_element
at	O
positions	O
2	B-residue_number
and	O
10	B-residue_number
that	O
would	O
enable	O
binding	O
to	O
HLA	B-protein
-	I-protein
A	I-protein
*	I-protein
02	I-protein
:	I-protein
01	I-protein
.	O

In	O
summary	O
,	O
this	O
investigation	O
into	O
the	O
molecular	O
basis	O
of	O
T	O
cell	O
cross	O
-	O
reactivity	O
using	O
a	O
clinically	O
relevant	O
cytotoxic	O
CD8	O
+	O
T	O
cell	O
clone	O
that	O
kills	O
human	B-species
pancreatic	O
β	O
cells	O
provides	O
answers	O
to	O
a	O
number	O
of	O
previously	O
outstanding	O
questions	O
.	O

First	O
,	O
our	O
data	O
shows	O
that	O
a	O
single	O
TCR	B-complex_assembly
has	O
the	O
potential	O
to	O
functionally	O
(	O
assessed	O
through	O
T	O
cell	O
activation	O
)	O
bind	O
to	O
different	O
ligands	O
with	O
affinities	B-evidence
ranging	O
across	O
3	O
orders	O
of	O
magnitude	O
.	O

Second	O
,	O
this	O
is	O
the	O
first	O
example	O
in	O
which	O
ligands	O
have	O
been	O
identified	O
and	O
characterized	O
for	O
a	O
human	B-species
autoreactive	O
TCR	B-complex_assembly
that	O
are	O
substantially	O
more	O
potent	O
than	O
the	O
natural	O
self	O
-	O
ligand	O
,	O
demonstrating	O
the	O
potential	O
for	O
a	O
pathogenic	O
ligand	O
to	O
break	O
self	O
-	O
tolerance	O
and	O
prime	O
self	O
-	O
reactive	O
T	O
cells	O
.	O

Third	O
,	O
this	O
first	O
structural	B-experimental_method
analysis	I-experimental_method
of	O
a	O
cross	O
-	O
reactive	O
human	B-species
MHCI	B-complex_assembly
–	O
restricted	O
autoimmune	O
TCR	B-complex_assembly
showed	O
that	O
degeneracy	O
was	O
mediated	O
through	O
TCR	B-complex_assembly
-	I-complex_assembly
pMHC	I-complex_assembly
anchoring	O
by	O
a	O
conserved	B-protein_state
minimal	B-structure_element
binding	I-structure_element
peptide	I-structure_element
motif	I-structure_element
.	O

Our	O
demonstration	O
of	O
the	O
molecular	O
mechanism	O
governing	O
cross	O
-	O
reactivity	O
by	O
this	O
preproinsulin	B-protein
reactive	O
human	B-species
CD8	O
+	O
T	O
cell	O
clone	O
supports	O
the	O
notion	O
first	O
put	O
forward	O
by	O
Wucherpfennig	O
and	O
Strominger	O
that	O
molecular	O
mimicry	O
could	O
mediate	O
autoimmunity	O
and	O
has	O
far	O
-	O
reaching	O
implications	O
for	O
the	O
complex	O
nature	O
of	O
T	O
cell	O
antigen	O
discrimination	O
.	O

The	O
1E6	O
T	O
cell	O
clone	O
reacts	O
with	O
a	O
broad	O
sensitivity	O
range	O
to	O
APLs	B-chemical
.	O

(	O
A	O
and	O
B	O
)	O
The	O
1E6	O
T	O
cell	O
clone	O
was	O
tested	O
in	O
a	O
peptide	B-experimental_method
dilution	I-experimental_method
assay	I-experimental_method
,	O
in	O
triplicate	O
,	O
with	O
MVWGPDPLYV	B-chemical
(	O
gray	O
),	O
YLGGPDFPTI	B-chemical
(	O
red	O
),	O
ALWGPDPAAA	B-chemical
(	O
blue	O
),	O
AQWGPDPAAA	B-chemical
(	O
green	O
),	O
RQFGPDWIVA	B-chemical
(	O
dark	O
blue	O
),	O
RQWGPDPAAV	B-chemical
(	O
purple	O
),	O
YQFGPDFPTA	B-chemical
(	O
yellow	O
),	O
and	O
RQFGPDFPTI	B-chemical
(	O
cyan	O
)	O
peptides	O
presented	O
by	O
HLA	B-protein
-	I-protein
A	I-protein
*	I-protein
0201	I-protein
–	O
expressing	O
C1R	O
cells	O
for	O
release	O
of	O
MIP	B-protein
-	I-protein
1β	I-protein
(	O
A	O
)	O
and	O
killing	O
(	O
B	O
).	O

Tm	B-evidence
values	O
were	O
calculated	O
using	O
a	O
Boltzmann	B-experimental_method
fit	I-experimental_method
to	I-experimental_method
each	I-experimental_method
set	I-experimental_method
of	I-experimental_method
data	I-experimental_method
.	O

(	O
A	O
–	O
H	O
)	O
Binding	B-evidence
affinity	I-evidence
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
interaction	O
at	O
25	O
°	O
C	O
using	O
SPR	B-experimental_method
.	O

Eight	O
serial	O
dilutions	O
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
were	O
measured	O
(	O
shown	O
in	O
the	O
inset	O
);	O
representative	O
data	O
from	O
3	O
independent	O
experiments	O
are	O
plotted	O
.	O

In	O
order	O
to	O
calculate	O
each	O
response	O
,	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
was	O
also	O
injected	O
over	O
a	O
control	O
sample	O
(	O
HLA	B-complex_assembly
-	I-complex_assembly
A	I-complex_assembly
*	I-complex_assembly
0201	I-complex_assembly
–	I-complex_assembly
ILAKFLHWL	I-complex_assembly
)	O
that	O
was	O
deducted	O
from	O
the	O
experimental	O
data	O
.	O

(	O
J	O
)	O
Effective	B-evidence
2D	I-evidence
affinity	I-evidence
(	O
AcKa	B-evidence
)	O
calculated	O
using	O
adhesion	B-experimental_method
frequency	I-experimental_method
assays	I-experimental_method
,	O
using	O
at	O
least	O
5	O
cell	O
pairs	O
,	O
and	O
calculated	O
as	O
an	O
average	O
of	O
100	O
cell	O
cell	O
contacts	O
.	O

(	O
K	O
)	O
Effective	B-evidence
2D	I-evidence
affinity	I-evidence
plotted	O
against	O
1	O
/	O
EC50	B-evidence
showing	O
Pearson	B-experimental_method
’	I-experimental_method
s	I-experimental_method
coefficient	I-experimental_method
analysis	I-experimental_method
(	O
r	B-evidence
)	O
and	O
P	B-evidence
value	I-evidence
.	O

The	O
1E6	B-complex_assembly
TCR	I-complex_assembly
uses	O
a	O
conserved	O
binding	O
mode	O
to	O
engage	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
and	O
the	O
APLs	B-chemical
.	O

(	O
A	O
)	O
Superposition	B-experimental_method
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
(	O
multicolored	O
illustration	O
)	O
in	B-protein_state
complex	I-protein_state
with	I-protein_state
all	O
7	O
APLs	B-chemical
(	O
multicolored	O
sticks	O
)	O
and	O
the	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
ligand	O
using	O
the	O
HLA	B-protein
-	I-protein
A	I-protein
*	I-protein
0201	I-protein
(	O
gray	O
illustration	O
)	O
molecule	O
to	O
align	B-experimental_method
all	O
of	O
the	O
structures	B-evidence
.	O

The	O
1E6	B-complex_assembly
TCR	I-complex_assembly
and	O
each	O
peptide	O
are	O
colored	O
according	O
to	O
the	O
APL	B-chemical
used	O
in	O
the	O
complex	O
as	O
in	O
Figure	O
1	O
.	O
(	O
B	O
)	O
Position	O
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
CDR	B-structure_element
loops	I-structure_element
(	O
multicolored	O
lines	O
)	O
in	O
each	O
complex	O
.	O

The	O
ALWGPDPAAA	B-chemical
peptide	O
(	O
green	O
sticks	O
)	O
is	O
shown	O
in	O
the	O
HLA	B-site
-	I-site
A	I-site
*	I-site
0201	I-site
binding	I-site
groove	I-site
(	O
gray	O
surface	O
).	O
(	O
C	O
)	O
The	O
Cα	O
backbone	O
conformation	O
of	O
each	O
APL	B-chemical
(	O
multicolored	O
illustration	O
)	O
in	O
the	O
context	O
of	O
the	O
HLA	B-protein
-	I-protein
A	I-protein
*	I-protein
0201	I-protein
α1	B-structure_element
helices	I-structure_element
(	O
gray	O
illustration	O
).	O
(	O
D	O
)	O
Crossing	B-evidence
angle	I-evidence
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
(	O
multicolored	O
lines	O
)	O
calculated	O
using	O
previously	O
published	O
parameters	O
in	O
the	O
context	O
of	O
the	O
ALWGPDPAAA	B-chemical
peptide	O
(	O
green	O
sticks	O
)	O
bound	B-protein_state
in	I-protein_state
the	O
HLA	B-site
-	I-site
A	I-site
*	I-site
0201	I-site
binding	I-site
groove	I-site
(	O
gray	O
surface	O
).	O

A	O
conserved	O
interaction	O
with	O
a	O
GPD	B-structure_element
motif	I-structure_element
underpins	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
interaction	O
with	O
the	O
APLs	B-chemical
.	O

Interaction	O
between	O
1E6	B-complex_assembly
TCR	I-complex_assembly
(	O
gray	O
illustration	O
)	O
residues	O
Tyr97α	B-residue_name_number
and	O
Tyr97β	B-residue_name_number
(	O
the	O
position	O
of	O
these	O
side	O
chains	O
in	O
the	O
TCR	B-complex_assembly
in	B-protein_state
complex	I-protein_state
with	I-protein_state
all	O
7	O
APLs	B-chemical
,	O
and	O
the	O
previously	O
reported	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
epitope	O
,	O
is	O
shown	O
in	O
multicolored	O
sticks	O
;	O
ref	O
.)	O
and	O
the	O
GPD	B-structure_element
peptide	I-structure_element
motif	I-structure_element
(	O
the	O
position	O
of	O
these	O
side	O
chains	O
in	O
all	O
7	O
APLs	B-chemical
and	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
in	B-protein_state
complex	I-protein_state
with	I-protein_state
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
is	O
shown	O
in	O
multicolored	O
sticks	O
).	O

The	O
rest	O
of	O
the	O
peptide	O
,	O
and	O
the	O
MHCα1	B-complex_assembly
helix	B-structure_element
,	O
are	O
shown	O
as	O
a	O
gray	O
illustration	O
.	O

The	O
1E6	B-complex_assembly
TCR	I-complex_assembly
makes	O
distinct	O
peptide	O
contacts	O
with	O
peripheral	O
APL	B-chemical
residues	O
.	O

Interactions	O
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
and	O
peptide	O
residues	O
outside	O
of	O
the	O
conserved	B-protein_state
GPD	B-structure_element
motif	I-structure_element
.	O

The	O
MHCα1	B-complex_assembly
helix	B-structure_element
is	O
shown	O
in	O
gray	O
illustrations	O
.	O

Hydrogen	B-bond_interaction
bonds	I-bond_interaction
are	O
shown	O
as	O
red	O
dotted	O
lines	O
;	O
van	B-bond_interaction
der	I-bond_interaction
Waals	I-bond_interaction
(	I-bond_interaction
vdW	I-bond_interaction
)	I-bond_interaction
contacts	I-bond_interaction
are	O
shown	O
as	O
black	O
dotted	O
lines	O
.	O

Boxes	O
show	O
total	O
contacts	O
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
and	O
each	O
peptide	O
ligand	O
.	O

(	O
D	O
)	O
Interaction	O
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
(	O
green	O
illustration	O
and	O
sticks	O
)	O
and	O
A2	B-chemical
-	I-chemical
AQWGPDPAAA	I-chemical
(	O
green	O
illustration	O
and	O
sticks	O
).	O
(	O
E	O
)	O
Interaction	O
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
(	O
dark	O
blue	O
illustration	O
and	O
sticks	O
)	O
and	O
A2	B-chemical
-	I-chemical
RQFGPDWIVA	I-chemical
(	O
dark	O
blue	O
illustration	O
and	O
sticks	O
).	O
(	O
F	O
)	O
Interaction	O
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
(	O
purple	O
illustration	O
and	O
sticks	O
)	O
and	O
A2	B-chemical
-	I-chemical
RQWGPDPAAV	I-chemical
(	O
purple	O
illustration	O
and	O
sticks	O
).	O
(	O
G	O
)	O
Interaction	O
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
(	O
yellow	O
illustration	O
and	O
sticks	O
)	O
and	O
A2	B-chemical
-	I-chemical
YQFGPDFPTA	I-chemical
(	O
yellow	O
illustration	O
and	O
sticks	O
).	O
(	O
H	O
)	O
Interaction	O
between	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
(	O
cyan	O
illustration	O
and	O
sticks	O
)	O
and	O
A2	B-chemical
-	I-chemical
RQFGPDFPTI	I-chemical
(	O
cyan	O
illustration	O
and	O
sticks	O
).	O

Comparison	O
of	O
ligated	B-protein_state
and	O
unligated	B-protein_state
APLs	B-chemical
.	O

Peptide	O
sequences	O
are	O
shown	O
underneath	O
each	O
structure	B-evidence
aligned	O
with	O
the	O
peptide	O
structure	B-evidence
.	O

(	O
A	O
)	O
A2	B-chemical
-	I-chemical
MVWGPDPLYV	I-chemical
(	O
black	O
sticks	O
).	O

A	O
large	O
conformational	O
shift	O
was	O
observed	O
for	O
Tyr8	B-residue_name_number
in	O
the	O
ligated	B-protein_state
versus	O
unligated	B-protein_state
states	O
(	O
black	O
circle	O
).	O
(	O
B	O
)	O
A2	B-chemical
-	I-chemical
YLGGPDFPTI	I-chemical
(	O
red	O
sticks	O
).	O
(	O
C	O
)	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
(	O
blue	O
sticks	O
)	O
reproduced	O
from	O
previous	O
published	O
data	O
.	O
(	O
D	O
)	O
A2	B-chemical
-	I-chemical
AQWGPDPAAA	I-chemical
(	O
green	O
sticks	O
).	O
(	O
E	O
)	O
A2	B-chemical
-	I-chemical
RQFGPDWIVA	I-chemical
(	O
dark	O
blue	O
sticks	O
).	O
(	O
F	O
)	O
A2	B-chemical
-	I-chemical
RQWGPDPAAV	I-chemical
(	O
purple	O
sticks	O
).	O
(	O
G	O
)	O
A2	B-chemical
-	I-chemical
YQFGPDFPTA	I-chemical
(	O
yellow	O
sticks	O
).	O
(	O
H	O
)	O
A2	B-chemical
-	I-chemical
RQFGPDFPTI	I-chemical
(	O
cyan	O
sticks	O
).	O

The	O
1E6	B-complex_assembly
TCR	I-complex_assembly
makes	O
distinct	O
peptide	O
contacts	O
with	O
the	O
MHC	B-site
surface	I-site
depending	O
on	O
the	O
peptide	O
cargo	O
.	O

MHCα1	B-complex_assembly
helix	B-structure_element
are	O
shown	O
in	O
gray	O
illustrations	O
.	O

Thermodynamic	B-experimental_method
analysis	I-experimental_method
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
with	O
A2	B-chemical
-	I-chemical
ALWGPDPAAA	I-chemical
and	O
the	O
APLs	B-chemical
.	O

Eight	O
serial	O
dilutions	O
of	O
the	O
1E6	B-complex_assembly
TCR	I-complex_assembly
were	O
injected	O
,	O
in	O
duplicate	O
,	O
over	O
each	O
immobilized	O
APL	B-chemical
and	O
A2	B-chemical
-	I-chemical
ALW	I-chemical
at	O
5	O
°	O
C	O
,	O
13	O
°	O
C	O
,	O
18	O
°	O
C	O
,	O
25	O
°	O
C	O
,	O
30	O
°	O
C	O
,	O
and	O
37	O
°	O
C	O
.	O

The	O
equilibrium	B-evidence
binding	I-evidence
constant	I-evidence
(	O
KD	B-evidence
)	O
values	O
were	O
calculated	O
using	O
a	O
nonlinear	B-experimental_method
curve	I-experimental_method
fit	I-experimental_method
(	O
y	O
=	O
[	O
P1x	O
]/[	O
P2	O
+	O
X	O
]),	O
and	O
thermodynamic	O
parameters	O
were	O
calculated	O
according	O
to	O
the	O
Gibbs	B-experimental_method
-	I-experimental_method
Helmholtz	I-experimental_method
equation	I-experimental_method
(	O
ΔG	B-evidence
°	I-evidence
=	O
ΔH	B-evidence
−	O
TΔS	B-evidence
°).	I-evidence

The	O
binding	B-evidence
free	I-evidence
energies	I-evidence
,	O
ΔG	B-evidence
°	I-evidence
(	O
ΔG	B-evidence
°	I-evidence
=	O
RTlnKD	O
),	O
were	O
plotted	O
against	O
temperature	O
(	O
K	O
)	O
using	O
nonlinear	B-experimental_method
regression	I-experimental_method
to	O
fit	O
the	O
3	O
-	O
parameters	O
van	B-experimental_method
’	I-experimental_method
t	I-experimental_method
Hoff	I-experimental_method
equation	I-experimental_method
(	O
RT	B-evidence
ln	I-evidence
KD	I-evidence
=	O
ΔH	B-evidence
°	I-evidence
–	O
TΔS	B-evidence
°	I-evidence
+	O
ΔCp	B-evidence
°[	I-evidence
T	O
-	O
T0	O
]	O
–	O
TΔCp	B-evidence
°	I-evidence
ln	O
[	O
T	O
/	O
T0	O
]	O
with	O
T0	O
=	O
298	O
K	O
).	O

(	O
A	O
)	O
1E6	B-complex_assembly
-	I-complex_assembly
A2	I-complex_assembly
-	I-complex_assembly
ALWGPDPAAA	I-complex_assembly
;	O
(	O
B	O
)	O
1E6	B-complex_assembly
-	I-complex_assembly
A2	I-complex_assembly
-	I-complex_assembly
AQWGPDPAAA	I-complex_assembly
;	O
(	O
C	O
)	O
1E6	B-complex_assembly
-	I-complex_assembly
A2	I-complex_assembly
-	I-complex_assembly
RQFGPDWIVA	I-complex_assembly
;	O
(	O
D	O
)	O
1E6	B-complex_assembly
-	I-complex_assembly
A2	I-complex_assembly
-	I-complex_assembly
RQWGPDPAAV	I-complex_assembly
,	O
(	O
E	O
)	O
1E6	B-complex_assembly
-	I-complex_assembly
A2	I-complex_assembly
-	I-complex_assembly
YQFGPDFPTA	I-complex_assembly
;	O
and	O
(	O
F	O
)	O
1E6	B-complex_assembly
-	I-complex_assembly
A2	I-complex_assembly
-	I-complex_assembly
RQFGPDFPTI	I-complex_assembly
.	O

1E6	B-complex_assembly
TCR	I-complex_assembly
-	I-complex_assembly
pMHC	I-complex_assembly
contacts	O
,	O
affinity	B-experimental_method
measurements	I-experimental_method
and	O
thermodynamics	B-experimental_method

The	O
immunity	B-protein_type
-	I-protein_type
related	I-protein_type
GTPase	I-protein_type
Irga6	B-protein
dimerizes	B-oligomeric_state
in	O
a	O
parallel	B-protein_state
head	I-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
head	I-protein_state
fashion	O

The	O
immunity	B-protein_type
-	I-protein_type
related	I-protein_type
GTPases	I-protein_type
(	O
IRGs	B-protein_type
)	O
constitute	O
a	O
powerful	O
cell	O
-	O
autonomous	O
resistance	O
system	O
against	O
several	O
intracellular	O
pathogens	O
.	O

Irga6	B-protein
is	O
a	O
dynamin	B-protein_type
-	I-protein_type
like	I-protein_type
protein	I-protein_type
that	O
oligomerizes	O
at	O
the	O
parasitophorous	O
vacuolar	O
membrane	O
(	O
PVM	O
)	O
of	O
Toxoplasma	B-species
gondii	I-species
leading	O
to	O
its	O
vesiculation	O
.	O

Based	O
on	O
a	O
previous	O
biochemical	B-experimental_method
analysis	I-experimental_method
,	O
it	O
has	O
been	O
proposed	O
that	O
the	O
GTPase	B-structure_element
domains	I-structure_element
of	O
Irga6	B-protein
dimerize	B-oligomeric_state
in	O
an	O
antiparallel	B-protein_state
fashion	O
during	O
oligomerization	O
.	O

Contrary	O
to	O
the	O
previous	O
model	O
,	O
the	O
structure	B-evidence
shows	O
that	O
the	O
GTPase	B-structure_element
domains	I-structure_element
dimerize	B-oligomeric_state
in	O
a	O
parallel	B-protein_state
fashion	O
.	O

The	O
nucleotides	B-chemical
in	O
the	O
center	O
of	O
the	O
interface	B-site
participate	O
in	O
dimerization	O
by	O
forming	O
symmetric	O
contacts	O
with	O
each	O
other	O
and	O
with	O
the	O
switch	B-site
I	I-site
region	O
of	O
the	O
opposing	O
Irga6	B-protein
molecule	O
.	O

The	O
latter	O
contact	O
appears	O
to	O
activate	O
GTP	B-chemical
hydrolysis	O
by	O
stabilizing	O
the	O
position	O
of	O
the	O
catalytic	B-protein_state
glutamate	B-residue_name_number
106	I-residue_name_number
in	O
switch	B-site
I	I-site
close	O
to	O
the	O
active	B-site
site	I-site
.	O

The	O
Irga6	B-protein
structure	B-evidence
features	O
a	O
parallel	B-protein_state
GTPase	B-structure_element
domain	I-structure_element
dimer	B-oligomeric_state
,	O
which	O
appears	O
to	O
be	O
a	O
unifying	O
feature	O
of	O
all	O
dynamin	B-protein_type
and	O
septin	B-protein_type
superfamily	O
members	O
.	O

Immunity	B-protein_type
-	I-protein_type
related	I-protein_type
GTPases	I-protein_type
(	O
IRGs	B-protein_type
)	O
comprise	O
a	O
family	O
of	O
dynamin	B-protein_type
-	I-protein_type
related	I-protein_type
cell	I-protein_type
-	I-protein_type
autonomous	I-protein_type
resistance	I-protein_type
proteins	I-protein_type
targeting	O
intracellular	O
pathogens	O
,	O
such	O
as	O
Mycobacterium	B-species
tuberculosis	I-species
,	O
Mycobacterium	B-species
avium	I-species
,	O
Listeria	B-species
monocytogenes	I-species
,	O
Trypanosoma	B-species
cruzi	I-species
,	O
and	O
Toxoplasma	B-species
gondii	I-species
.	O

In	O
mice	B-taxonomy_domain
,	O
the	O
23	O
IRG	B-protein_type
members	O
are	O
induced	O
by	O
interferons	B-protein_type
,	O
whereas	O
the	O
single	O
human	B-species
homologue	O
is	O
constitutively	O
expressed	O
in	O
some	O
tissues	O
,	O
especially	O
in	O
testis	O
.	O

In	O
non	O
-	O
infected	O
cells	O
,	O
most	O
IRGs	B-protein_type
are	O
largely	O
cytosolic	O
.	O

In	O
this	O
way	O
,	O
IRGs	B-protein_type
contribute	O
to	O
the	O
release	O
of	O
the	O
pathogen	O
into	O
the	O
cytoplasm	O
and	O
its	O
subsequent	O
destruction	O
.	O

Irga6	B-protein
,	O
one	O
of	O
the	O
effector	O
IRG	B-protein_type
proteins	O
,	O
localizes	O
to	O
the	O
intact	O
parasitophorous	O
vacuole	O
membrane	O
(	O
PVM	O
)	O
and	O
,	O
after	O
disruption	O
of	O
the	O
PVM	O
,	O
is	O
found	O
associated	O
with	O
vesicular	O
accumulations	O
,	O
presumably	O
derived	O
from	O
the	O
PVM	O
.	O

A	O
myristoylation	B-site
site	I-site
at	O
Gly2	B-residue_name_number
is	O
necessary	O
for	O
the	O
recruitment	O
to	O
the	O
PVM	O
but	O
not	O
for	O
the	O
weak	O
constitutive	O
binding	O
to	O
the	O
ER	O
membrane	O
.	O

An	O
internally	O
oriented	O
antibody	O
epitope	O
on	O
helix	B-structure_element
A	I-structure_element
between	O
positions	O
20	B-residue_range
and	I-residue_range
24	I-residue_range
was	O
demonstrated	O
to	O
be	O
accessible	O
in	O
the	O
GTP	B-protein_state
-,	I-protein_state
but	O
not	O
in	O
the	O
GDP	B-protein_state
-	I-protein_state
bound	I-protein_state
state	O
.	O

This	O
indicates	O
large	O
-	O
scale	O
structural	O
changes	O
upon	O
GTP	B-chemical
binding	O
that	O
probably	O
include	O
exposure	O
of	O
the	O
myristoyl	O
group	O
,	O
enhancing	O
binding	O
to	O
the	O
PVM	O
.	O

Biochemical	B-experimental_method
studies	I-experimental_method
indicated	O
that	O
Irga6	B-protein
hydrolyses	O
GTP	B-chemical
in	O
a	O
cooperative	O
manner	O
and	O
forms	O
GTP	B-protein_state
-	I-protein_state
dependent	I-protein_state
oligomers	B-oligomeric_state
in	O
vitro	O
and	O
in	O
vivo	O
.	O

Crystal	B-evidence
structures	I-evidence
of	O
Irga6	B-protein
in	O
various	O
nucleotide	B-protein_state
-	I-protein_state
loaded	I-protein_state
states	O
revealed	O
the	O
basic	O
architecture	O
of	O
IRG	B-protein_type
proteins	O
,	O
including	O
a	O
GTPase	B-structure_element
domain	I-structure_element
and	O
a	O
composite	B-structure_element
helical	I-structure_element
domain	I-structure_element
.	O

These	O
studies	O
additionally	O
showed	O
a	O
dimerization	B-site
interface	I-site
in	O
the	O
nucleotide	B-protein_state
-	I-protein_state
free	I-protein_state
protein	O
as	O
well	O
as	O
in	O
all	O
nucleotide	B-protein_state
-	I-protein_state
bound	I-protein_state
states	O
.	O

It	O
involves	O
a	O
GTPase	B-site
domain	I-site
surface	I-site
,	O
which	O
is	O
located	O
at	O
the	O
opposite	O
side	O
of	O
the	O
nucleotide	O
,	O
and	O
an	O
interface	B-site
in	O
the	O
helical	B-structure_element
domain	I-structure_element
,	O
with	O
a	O
water	B-chemical
-	O
filled	O
gap	O
between	O
the	O
two	O
contact	B-site
surfaces	I-site
.	O

Recent	O
structural	B-experimental_method
studies	I-experimental_method
indicated	O
that	O
a	O
'	O
G	B-site
interface	I-site
'	O
is	O
typical	O
of	O
dynamin	B-protein_type
superfamily	O
members	O
,	O
such	O
as	O
dynamin	B-protein_type
,	O
MxA	B-protein
,	O
the	O
guanylate	B-protein
binding	I-protein
protein	I-protein
-	I-protein
1	I-protein
(	O
GBP	B-protein
-	I-protein
1	I-protein
),	O
atlastin	B-protein_type
and	O
the	O
bacterial	B-taxonomy_domain
dynamin	B-protein_type
-	I-protein_type
like	I-protein_type
proteins	I-protein_type
(	O
BDLP	B-protein_type
).	O

In	O
dynamin	B-protein_type
,	O
the	O
G	B-site
interface	I-site
includes	O
residues	O
in	O
the	O
phosphate	B-structure_element
binding	I-structure_element
loop	I-structure_element
,	O
the	O
two	O
switch	B-site
regions	I-site
,	O
the	O
'	O
trans	B-structure_element
stabilizing	I-structure_element
loop	I-structure_element
'	O
and	O
the	O
'	O
G4	B-structure_element
loop	I-structure_element
'.	O

For	O
Irga6	B-protein
,	O
it	O
was	O
demonstrated	O
that	O
besides	O
residues	O
in	O
the	O
switch	B-site
I	I-site
and	O
switch	B-site
II	I-site
regions	O
,	O
the	O
3	O
'-	O
OH	O
group	O
of	O
the	O
ribose	O
participates	O
in	O
this	O
interface	B-site
.	O

This	O
structure	B-evidence
was	O
obtained	O
by	O
soaking	B-experimental_method
GMPPNP	B-chemical
in	O
nucleotide	B-protein_state
-	I-protein_state
free	I-protein_state
crystals	B-evidence
of	O
Irga6	B-protein
,	O
an	O
approach	O
which	O
may	O
have	O
interfered	O
with	O
nucleotide	O
-	O
induced	O
domain	O
rearrangements	O
.	O

To	O
clarify	O
the	O
dimerization	O
mode	O
via	O
the	O
G	B-site
interface	I-site
,	O
we	O
determined	B-experimental_method
the	O
GMPPNP	B-protein_state
-	I-protein_state
bound	I-protein_state
crystal	B-evidence
structure	I-evidence
of	O
a	O
non	B-protein_state
-	I-protein_state
oligomerizing	I-protein_state
Irga6	B-protein
variant	B-protein_state
.	O

Previous	O
results	O
indicated	O
that	O
Irga6	B-protein
mutations	B-experimental_method
in	O
a	O
loosely	O
defined	O
surface	B-site
region	I-site
(	O
the	O
""""	O
secondary	B-site
patch	I-site
"""),"	O
which	O
is	O
distant	O
from	O
the	O
G	B-site
-	I-site
interface	I-site
and	O
only	O
slightly	O
overlapping	O
with	O
the	O
backside	B-site
interface	I-site
(	O
see	O
below	O
),	O
individually	O
reduced	O
GTP	B-chemical
-	O
dependent	O
oligomerization	O
.	O

A	O
combination	B-experimental_method
of	I-experimental_method
four	I-experimental_method
of	I-experimental_method
these	I-experimental_method
mutations	I-experimental_method
(	O
R31E	B-mutant
,	O
K32E	B-mutant
,	O
K176E	B-mutant
,	O
and	O
K246E	B-mutant
)	O
essentially	O
eliminated	B-protein_state
GTP	B-chemical
-	O
dependent	O
assembly	O
(	O
Additional	O
file	O
1	O
:	O
Figure	O
S1	O
)	O
and	O
allowed	O
crystallization	B-experimental_method
of	O
Irga6	B-protein
in	O
the	O
presence	B-protein_state
of	I-protein_state
GMPPNP	B-chemical
.	O

The	O
asymmetric	O
unit	O
contained	O
seven	O
Irga6	B-protein
molecules	O
that	O
were	O
arranged	O
in	O
a	O
helical	O
pattern	O
along	O
the	O
long	O
cell	O
axis	O
(	O
Additional	O
file	O
1	O
:	O
Figure	O
S2	O
).	O

a	O
Schematic	O
view	O
of	O
the	O
domain	O
architecture	O
of	O
mouse	B-taxonomy_domain
Irga6	B-protein
.	O

The	O
nucleotide	O
and	O
Mg2	B-chemical
+	I-chemical
ion	O
(	O
green	O
)	O
are	O
shown	O
in	O
sphere	O
representation	O
.	O

The	O
GTPase	B-structure_element
domain	I-structure_element
dimer	B-oligomeric_state
is	O
boxed	O
.	O

d	O
Magnification	O
of	O
the	O
contact	B-site
sites	I-site
.	O

e	O
Superposition	B-experimental_method
of	O
different	O
switch	B-site
I	I-site
conformations	O
in	O
the	O
asymmetric	O
unit	O
;	O
the	O
same	O
colors	O
as	O
in	O
Additional	O
file	O
1	O
:	O
Figure	O
S2	O
are	O
used	O
for	O
the	O
switch	B-site
I	I-site
regions	O
of	O
the	O
individual	O
subunits	O
.	O

Switch	B-site
I	I-site
residues	O
of	O
subunit	O
A	B-structure_element
(	O
yellow	O
)	O
involved	O
in	O
ribose	O
binding	O
are	O
labelled	O
and	O
shown	O
in	O
stick	O
representation	O
.	O

Like	O
other	O
dynamin	B-protein_type
superfamily	O
members	O
,	O
the	O
GTPase	B-structure_element
domain	I-structure_element
of	O
Irga6	B-protein
comprises	O
a	O
canonical	O
GTPase	B-structure_element
domain	I-structure_element
fold	O
,	O
with	O
a	O
central	O
β	B-structure_element
-	I-structure_element
sheet	I-structure_element
surrounded	O
by	O
helices	B-structure_element
on	O
both	O
sides	O
(	O
Fig	O
.	O
1a	O
-	O
c	O
).	O

The	O
helical	B-structure_element
domain	I-structure_element
is	O
a	O
bipartite	O
structure	O
composed	O
of	O
helices	B-structure_element
αA	B-structure_element
-	I-structure_element
C	I-structure_element
at	O
the	O
N	O
-	O
terminus	O
and	O
helix	B-structure_element
αF	B-structure_element
-	I-structure_element
L	I-structure_element
at	O
the	O
C	O
-	O
terminus	O
of	O
the	O
GTPase	B-structure_element
domain	I-structure_element
.	O

Overall	O
,	O
the	O
seven	O
molecules	O
in	O
the	O
asymmetric	O
unit	O
are	O
very	O
similar	O
to	O
each	O
other	O
,	O
with	O
root	B-evidence
mean	I-evidence
square	I-evidence
deviations	I-evidence
(	O
rmsd	B-evidence
)	O
ranging	O
from	O
0	O
.	O
32	O
–	O
0	O
.	O
45	O
Å	O
over	O
all	O
Cα	O
atoms	O
.	O

The	O
seven	O
Irga6	B-protein
molecules	O
in	O
the	O
asymmetric	O
unit	O
form	O
various	O
higher	O
order	O
contacts	O
in	O
the	O
crystals	B-evidence
.	O

Another	O
assembly	B-site
interface	I-site
with	O
a	O
buried	O
surface	O
area	O
of	O
450	O
Å2	O
,	O
which	O
we	O
call	O
the	O
“	O
tertiary	B-site
patch	I-site
”,	O
was	O
formed	O
via	O
two	O
interaction	B-site
sites	I-site
in	O
the	O
helical	B-structure_element
domains	I-structure_element
(	O
Additional	O
file	O
1	O
:	O
Figure	O
S2c	O
,	O
d	O
,	O
S3	O
).	O

In	O
this	O
interface	B-site
,	O
helices	B-structure_element
αK	B-structure_element
from	O
two	O
adjacent	O
molecules	O
form	O
a	O
hydrogen	B-bond_interaction
bonding	I-bond_interaction
network	I-bond_interaction
involving	O
residues	O
373	B-residue_range
-	I-residue_range
376	I-residue_range
.	O

Furthermore	O
,	O
two	O
adjacent	O
helices	B-structure_element
αA	B-structure_element
form	O
hydrophobic	B-bond_interaction
contacts	I-bond_interaction
.	O

It	O
was	O
previously	O
shown	O
that	O
the	O
double	B-protein_state
mutation	I-protein_state
L372R	B-mutant
/	O
A373R	B-mutant
did	O
not	O
prevent	O
GTP	B-chemical
-	O
induced	O
assembly	O
,	O
so	O
there	O
is	O
currently	O
no	O
evidence	O
supporting	O
an	O
involvement	O
of	O
this	O
interface	B-site
in	O
higher	O
-	O
order	O
oligomerization	O
.	O

This	O
assembly	O
results	O
in	O
a	O
butterfly	B-protein_state
-	I-protein_state
shaped	I-protein_state
Irga6	B-protein
dimer	B-oligomeric_state
in	O
which	O
the	O
helical	B-structure_element
domains	I-structure_element
protrude	O
in	O
parallel	B-protein_state
orientations	O
(	O
Fig	O
.	O
1b	O
,	O
Additional	O
file	O
1	O
:	O
Figure	O
S3	O
).	O

In	O
contrast	O
,	O
the	O
other	O
six	O
molecules	O
in	O
the	O
asymmetric	O
unit	O
do	O
not	O
assemble	O
via	O
the	O
G	B-site
interface	I-site
.	O

The	O
G	B-site
interface	I-site
in	O
molecule	O
A	O
can	O
be	O
subdivided	O
into	O
three	O
distinct	O
contact	B-site
sites	I-site
(	O
Fig	O
.	O
1c	O
,	O
d	O
).	O

Contact	B-site
site	I-site
I	I-site
is	O
formed	O
between	O
R159	B-residue_name_number
and	O
K161	B-residue_name_number
in	O
the	O
trans	B-structure_element
stabilizing	I-structure_element
loops	I-structure_element
,	O
and	O
S132	B-residue_name_number
in	O
the	O
switch	B-site
II	I-site
regions	O
of	O
the	O
opposing	O
molecules	O
.	O

In	O
contact	B-site
site	I-site
III	I-site
,	O
G103	B-residue_name_number
of	O
switch	B-site
I	I-site
interacts	O
via	O
its	O
main	O
chain	O
nitrogen	O
with	O
the	O
exocyclic	O
2	O
’-	O
OH	O
and	O
3	O
’-	O
OH	O
groups	O
of	O
the	O
opposing	O
ribose	B-chemical
in	O
trans	O
,	O
whereas	O
the	O
two	O
opposing	O
exocyclic	O
3	O
’-	O
OH	O
group	O
of	O
the	O
ribose	B-chemical
form	O
hydrogen	B-bond_interaction
bonds	I-bond_interaction
with	O
each	O
other	O
.	O

Via	O
the	O
ribose	B-chemical
contact	O
,	O
switch	B-site
I	I-site
is	O
pulled	O
towards	O
the	O
opposing	O
nucleotide	B-chemical
(	O
Fig	O
.	O
1e	O
).	O

E106	B-residue_name_number
was	O
previously	O
shown	O
to	O
be	O
essential	O
for	O
catalysis	O
,	O
and	O
the	O
observed	O
interactions	O
in	O
contact	B-site
site	I-site
III	I-site
explain	O
how	O
dimerization	O
via	O
the	O
ribose	B-chemical
is	O
directly	O
coupled	O
to	O
the	O
activation	O
of	O
GTP	B-chemical
hydrolysis	O
.	O

The	O
G	B-site
interface	I-site
is	O
in	O
full	O
agreement	O
with	O
previously	O
published	O
biochemical	O
data	O
that	O
indicate	O
crucial	O
roles	O
of	O
E77	B-residue_name_number
,	O
G103	B-residue_name_number
,	O
E106	B-residue_name_number
,	O
S132	B-residue_name_number
,	O
R159	B-residue_name_number
,	O
K161	B-residue_name_number
,	O
K162	B-residue_name_number
,	O
D164	B-residue_name_number
,	O
N191	B-residue_name_number
,	O
and	O
K196	B-residue_name_number
for	O
oligomerization	O
and	O
oligomerization	O
-	O
induced	O
GTP	B-chemical
hydrolysis	O
.	O

All	O
of	O
these	O
residues	O
directly	O
participate	O
in	O
contacts	O
(	O
G103	B-residue_name_number
,	O
S132	B-residue_name_number
,	O
R159	B-residue_name_number
,	O
and	O
K161	B-residue_name_number
)	O
or	O
are	O
in	O
direct	O
vicinity	O
to	O
the	O
interface	B-site
(	O
E77	B-residue_name_number
,	O
E106	B-residue_name_number
,	O
K162	B-residue_name_number
,	O
D164	B-residue_name_number
,	O
and	O
N191	B-residue_name_number
).	O

Residues	O
E77	B-residue_name_number
,	O
K162	B-residue_name_number
,	O
and	O
D164	B-residue_name_number
appear	O
to	O
orient	O
the	O
trans	B-structure_element
stabilizing	I-structure_element
loop	I-structure_element
which	O
is	O
involved	O
in	O
interface	B-site
formation	O
in	O
contact	B-site
site	I-site
II	I-site
.	O

In	O
the	O
earlier	O
model	O
of	O
an	O
anti	B-protein_state
-	I-protein_state
parallel	I-protein_state
G	B-site
interface	I-site
,	O
it	O
was	O
not	O
possible	O
to	O
position	O
the	O
side	O
chain	O
of	O
R159	B-residue_name_number
to	O
avoid	O
steric	O
conflict	O
.	O

In	O
the	O
present	O
structure	B-evidence
,	O
the	O
side	O
-	O
chain	O
of	O
R159	B-residue_name_number
projects	O
laterally	O
along	O
the	O
G	B-site
interface	I-site
and	O
,	O
therefore	O
,	O
does	O
not	O
cause	O
a	O
steric	O
conflict	O
.	O

A	O
conserved	O
dimerization	O
mode	O
via	O
the	O
G	B-site
interface	I-site
in	O
dynamin	B-protein_type
and	O
septin	B-protein_type
GTPases	I-protein_type
.	O

The	O
overall	O
architecture	O
of	O
the	O
parallel	B-protein_state
GTPase	B-structure_element
domain	I-structure_element
dimer	B-oligomeric_state
of	O
Irga6	B-protein
is	O
related	O
to	O
that	O
of	O
other	O
dynamin	B-protein_type
and	O
septin	B-protein_type
superfamily	O
proteins	O
.	O

Irga6	B-protein
immunity	B-protein
-	I-protein
related	I-protein
GTPase	I-protein
6	I-protein
,	O
GMPPNP	B-chemical
5	B-chemical
'-	I-chemical
guanylyl	I-chemical
imidodiphosphate	I-chemical
,	O
GTP	B-chemical
guanosine	B-chemical
-	I-chemical
triphosphate	I-chemical
,	O
BDLP	B-protein_type
bacterial	B-taxonomy_domain
dynamin	B-protein_type
like	I-protein_type
protein	I-protein_type
,	O
GIMAP2	B-protein
,	O
GTPase	B-protein
of	I-protein
immunity	I-protein
associated	I-protein
protein	I-protein
2	I-protein

The	O
buried	O
surface	O
area	O
per	O
molecule	O
(	O
BSA	O
)	O
of	O
the	O
G	B-site
interface	I-site
in	O
Irga6	B-protein
is	O
relatively	O
small	O
(	O
470	O
Å2	O
)	O
compared	O
to	O
that	O
of	O
other	O
dynamin	B-protein_type
superfamily	O
members	O
,	O
such	O
as	O
dynamin	B-protein_type
(	O
BSA	O
:	O
1400	O
Å2	O
),	O
atlastin	B-protein_type
(	O
BSA	O
:	O
820	O
Å2	O
),	O
GBP	B-protein
-	I-protein
1	I-protein
(	O
BSA	O
:	O
2060	O
Å2	O
),	O
BDLP	B-protein_type
(	O
BSA	O
:	O
2300	O
Å2	O
)	O
or	O
the	O
septin	B-protein
-	I-protein
related	I-protein
GTPase	I-protein
of	I-protein
immunity	I-protein
associated	I-protein
protein	I-protein
2	I-protein
(	O
GIMAP2	B-protein
)	O
(	O
BSA	O
:	O
590	O
Å2	O
)	O
(	O
Fig	O
.	O
2	O
).	O

However	O
,	O
the	O
relative	O
orientations	O
of	O
the	O
GTPase	B-structure_element
domains	I-structure_element
in	O
these	O
dimers	B-oligomeric_state
are	O
strikingly	O
similar	O
,	O
and	O
the	O
same	O
elements	O
,	O
such	O
as	O
switch	B-site
I	I-site
,	O
switch	B-site
II	I-site
,	O
the	O
trans	B-structure_element
activating	I-structure_element
and	I-structure_element
G4	I-structure_element
loops	I-structure_element
are	O
involved	O
in	O
the	O
parallel	B-protein_state
dimerization	O
mode	O
in	O
all	O
of	O
these	O
GTPase	B-protein_type
families	O
.	O

IRG	B-protein_type
proteins	O
are	O
crucial	O
mediators	O
of	O
the	O
innate	O
immune	O
response	O
in	O
mice	B-taxonomy_domain
against	O
a	O
specific	O
subset	O
of	O
intracellular	O
pathogens	O
,	O
all	O
of	O
which	O
enter	O
the	O
cell	O
to	O
form	O
a	O
membrane	O
-	O
bounded	O
vacuole	O
without	O
engagement	O
of	O
the	O
phagocytic	O
machinery	O
.	O

As	O
members	O
of	O
the	O
dynamin	B-protein_type
superfamily	O
,	O
IRGs	B-protein_type
oligomerize	O
at	O
cellular	O
membranes	O
in	O
response	O
to	O
GTP	B-chemical
binding	O
.	O

Oligomerization	O
and	O
oligomerization	O
-	O
induced	O
GTP	B-chemical
hydrolysis	O
are	O
thought	O
to	O
induce	O
membrane	O
remodeling	O
events	O
ultimately	O
leading	O
to	O
disruption	O
of	O
the	O
PVM	O
.	O

Recent	O
structural	B-experimental_method
and	I-experimental_method
mechanistic	I-experimental_method
analyses	I-experimental_method
have	O
begun	O
to	O
unravel	O
the	O
molecular	O
basis	O
for	O
the	O
membrane	O
-	O
remodeling	O
activity	O
and	O
mechano	O
-	O
chemical	O
function	O
of	O
some	O
members	O
(	O
reviewed	O
in	O
).	O

However	O
,	O
for	O
other	O
dynamin	B-protein_type
superfamily	O
members	O
such	O
as	O
IRGs	B-protein_type
,	O
the	O
molecular	O
basis	O
for	O
GTP	B-chemical
hydrolysis	O
and	O
the	O
exact	O
role	O
of	O
the	O
mechano	O
-	O
chemical	O
function	O
are	O
still	O
unclear	O
.	O

Such	O
a	O
low	O
affinity	O
interaction	O
mode	O
may	O
allow	O
reversibility	O
of	O
oligomerization	O
following	O
GTP	B-chemical
hydrolysis	O
.	O

Similar	O
low	O
affinity	O
G	B-site
interface	I-site
interactions	O
were	O
reported	O
for	O
dynamin	B-protein_type
and	O
MxA	B-protein
.	O

The	O
dimerization	O
mode	O
is	O
strikingly	O
different	O
from	O
the	O
previously	O
proposed	O
anti	B-protein_state
-	I-protein_state
parallel	I-protein_state
model	O
that	O
was	O
based	O
on	O
the	O
crystal	B-evidence
structure	I-evidence
of	O
the	O
signal	B-protein_type
recognition	I-protein_type
particle	I-protein_type
GTPase	I-protein_type
,	O
SRP54	B-protein
and	O
its	O
homologous	O
receptor	O
.	O

However	O
,	O
the	O
G	B-site
dimer	I-site
interface	I-site
is	O
reminiscent	O
of	O
the	O
GTPase	B-structure_element
domain	I-structure_element
dimers	B-oligomeric_state
observed	O
for	O
several	O
other	O
dynamin	B-protein_type
superfamily	O
members	O
,	O
such	O
as	O
dynamin	B-protein_type
,	O
GBP1	B-protein
,	O
atlastin	B-protein_type
,	O
and	O
BDLP	B-protein_type
.	O

It	O
was	O
recently	O
shown	O
that	O
septin	B-protein_type
and	O
septin	B-protein_type
-	I-protein_type
related	I-protein_type
GTPases	I-protein_type
,	O
such	O
as	O
the	O
Tocs	B-protein_type
GTPases	I-protein_type
or	O
GTPases	B-protein_type
of	I-protein_type
immunity	I-protein_type
related	I-protein_type
proteins	I-protein_type
(	O
GIMAPs	B-protein_type
),	O
also	O
employ	O
a	O
GTP	B-chemical
-	O
dependent	O
parallel	B-protein_state
dimerization	O
mode	O
.	O

Importantly	O
,	O
our	O
analysis	O
indicates	O
that	O
IRGs	B-protein_type
are	O
not	O
outliers	O
,	O
but	O
bona	O
-	O
fide	O
representatives	O
of	O
the	O
dynamin	B-protein_type
superfamily	O
.	O

Whereas	O
the	O
overall	O
dimerization	O
mode	O
is	O
similar	O
in	O
septin	B-protein_type
and	O
dynamin	B-protein_type
GTPases	I-protein_type
,	O
family	O
-	O
specific	O
differences	O
in	O
the	O
G	B-site
interface	I-site
and	O
the	O
oligomerization	B-site
interfaces	I-site
exist	O
.	O

For	O
example	O
,	O
the	O
involvement	O
of	O
the	O
2	O
’	O
and	O
3	O
’-	O
OH	O
groups	O
of	O
the	O
ribose	B-chemical
in	O
the	O
dimerization	B-site
interface	I-site
of	O
Irga6	B-protein
has	O
not	O
been	O
observed	O
for	O
other	O
dynamin	B-protein_type
and	O
septin	B-protein_type
superfamily	O
members	O
.	O

The	O
surface	O
-	O
exposed	O
location	O
of	O
the	O
ribose	O
in	O
the	O
Irga6	B-protein
structure	B-evidence
,	O
with	O
a	O
wide	B-protein_state
-	I-protein_state
open	I-protein_state
nucleotide	B-site
-	I-site
binding	I-site
pocket	I-site
,	O
facilitates	O
its	O
engagement	O
in	O
the	O
dimerization	B-site
interface	I-site
.	O

This	O
contact	O
,	O
in	O
turn	O
,	O
appears	O
to	O
activate	O
GTP	B-chemical
hydrolysis	O
by	O
inducing	O
rearrangements	O
in	O
switch	B-site
I	I-site
and	O
the	O
positioning	O
of	O
the	O
catalytic	B-protein_state
E106	B-residue_name_number
.	O

In	O
dynamin	B-protein_type
,	O
the	O
corresponding	O
serine	B-residue_name
residue	O
coordinates	O
a	O
sodium	B-chemical
ion	O
that	O
is	O
crucial	O
for	O
GTP	B-chemical
hydrolysis	O
.	O

Higher	O
resolution	O
structures	B-evidence
of	O
the	O
Irga6	B-protein
dimer	B-oligomeric_state
in	O
the	O
presence	B-protein_state
of	I-protein_state
a	O
transition	O
state	O
analogue	O
are	O
required	O
to	O
show	O
whether	O
Gly79	B-residue_name_number
directly	O
participates	O
in	O
GTP	B-chemical
hydrolysis	O
or	O
whether	O
it	O
may	O
also	O
position	O
a	O
catalytic	O
cation	O
.	O

In	O
dynamin	B-protein_type
,	O
further	O
assembly	B-site
sites	I-site
are	O
provided	O
by	O
the	O
helical	B-structure_element
domains	I-structure_element
which	O
assemble	O
in	O
a	O
criss	O
-	O
cross	O
fashion	O
to	O
form	O
a	O
helical	B-structure_element
filament	I-structure_element
.	O

In	O
dynamin	B-protein_type
-	I-protein_type
related	I-protein_type
Eps15	I-protein_type
homology	I-protein_type
domain	I-protein_type
containing	I-protein_type
proteins	I-protein_type
(	O
EHDs	B-protein_type
),	O
a	O
second	B-site
assembly	I-site
interface	I-site
is	O
present	O
in	O
the	O
GTPase	B-structure_element
domain	I-structure_element
.	O

Further	O
structural	B-experimental_method
studies	I-experimental_method
,	O
especially	O
electron	B-experimental_method
microscopy	I-experimental_method
analysis	I-experimental_method
of	O
the	O
Irga6	B-protein
oligomers	B-oligomeric_state
,	O
are	O
required	O
to	O
clarify	O
the	O
assembly	O
mode	O
via	O
the	O
helical	B-structure_element
domains	I-structure_element
and	O
to	O
show	O
how	O
these	O
interfaces	B-site
cooperate	O
with	O
the	O
G	B-site
interface	I-site
to	O
mediate	O
the	O
regulated	O
assembly	O
on	O
a	O
membrane	O
surface	O
.	O

Notably	O
,	O
we	O
did	O
not	O
observe	O
major	O
rearrangements	O
of	O
the	O
helical	B-structure_element
domain	I-structure_element
versus	O
the	O
GTPase	B-structure_element
domain	I-structure_element
in	O
the	O
Irga6	B-protein
molecules	O
that	O
dimerized	B-protein_state
via	O
the	O
G	B-site
interface	I-site
.	O

In	O
a	O
manner	O
similar	O
to	O
BDLP	B-protein_type
,	O
such	O
large	O
-	O
scale	O
conformational	O
changes	O
may	O
be	O
induced	O
by	O
membrane	O
binding	O
.	O

Our	O
structural	B-experimental_method
analysis	I-experimental_method
and	O
the	O
identification	O
of	O
the	O
G	B-site
-	I-site
interface	I-site
paves	O
the	O
way	O
for	O
determining	O
the	O
specific	O
assembly	O
of	O
Irga6	B-protein
into	O
a	O
membrane	O
-	O
associated	O
scaffold	O
as	O
the	O
prerequisite	O
to	O
understand	O
its	O
action	O
as	O
an	O
anti	O
-	O
parasitic	O
machine	O
.	O

Our	O
study	O
indicates	O
that	O
Irg	B-protein_type
proteins	O
dimerize	B-oligomeric_state
via	O
the	O
G	B-site
interface	I-site
in	O
a	O
parallel	B-protein_state
head	I-protein_state
-	I-protein_state
to	I-protein_state
-	I-protein_state
head	I-protein_state
fashion	O
thereby	O
facilitating	O
GTPase	B-protein_type
activation	O
.	O

These	O
findings	O
contribute	O
to	O
a	O
molecular	O
understanding	O
of	O
the	O
anti	O
-	O
parasitic	O
action	O
of	O
the	O
Irg	B-protein_type
protein	O
family	O
and	O
suggest	O
that	O
Irgs	B-protein_type
are	O
bona	O
-	O
fide	O
members	O
of	O
the	O
dynamin	B-protein_type
superfamily	O
.	O