REVIEWSCONDENSIN AND COHESIN: MORE
THAN CHROMOSOME COMPACTOR
AND GLUE
Kirsten A. Hagstrom and Barbara J. Meyer
Two related protein complexes, cohesin and condensin, are essential for separating identical copiesof the genome into daughter cells during cell division. Cohesin glues replicated sister chromatids
together until they split at anaphase, whereas condensin reorganizes chromosomes into their highly
compact mitotic structure. Unexpectedly, mutations in the subunits of these complexes have been
uncovered in genetic screens that target completely different processes. Exciting new evidence isemerging that cohesin and condensin influence crucial processes during interphase, andunforeseen aspects of mitosis. Each complex can perform several roles, and individual subunits can
associate with different sets of proteins to achieve diverse functions, including the regulation of gene
expression, DNA repair, cell-cycle checkpoints and centromere organization.

Howard Hughes Medical
Institute and Department of
Molecular and Cell Biology,
University of California at
Berkeley, Berkeley,
California 94720-3204,USA.
e-mails: kirstenh@uclink.
berkeley.edu;
bjmeyer@uclink.berkeley.edu
doi:10.1038/nrg1110

Every time a cell divides, it must accurately distribute one
copy of each duplicated chromosome to its daughter
cells. To accomplish that task, chromosomes radicallychange their shape and undergo distinctive movements.
Replicated chromosomes are glued together, compacted,
untangled and then pulled to opposite sides of the cell
(BOX 1). These events must be coordinated and executedwith precision to avoid aneuploidy, which is a conditionof inappropriate chromosome number that is often
associated with birth defects and cancer. In the past
decade, the discovery of two multiprotein complexes,
cohesin and condensin, has provided important insights
into the molecular mechanisms of chromosome segregation. Although mitotic chromosome segregation mightseem unrelated to interphase functions, such as gene regulation and DNA repair, it is becoming increasingly clearthat cohesin and condensin subunits influence all of thesechromosomal processes.
Cytological observations of mitotic chromosome
dynamics originally indicated the existence of factorsthat condense chromatids or hold sister chromatids
together. In an exciting convergence of results, therecovery of mutants that were defective in theseprocesses, and the biochemical identification of mitotic

5 2 0 | JULY 2003 | VOLUME 4

chromosome-associated factors, led to the discovery of
condensin and cohesin. At their cores, condensin andcohesin contain chromosomal ATPases of the structural maintenance of chromosomes (SMC) protein
family1–3, but the two complexes are structurally andfunctionally distinct.
Although cohesin and condensin were named fortheir roles in sister-chromatid cohesion and condensation
during mitosis, genetic analysis has shown a wide range ofother chromosomal processes that rely on cohesin andcondensin subunits. Screens that were designed to understand processes as diverse as sex-specific gene regulation,
nerve-cell development, transposon-enhancer blocking
and DNA repair, have identified mutations in these genes
and have expanded our view of their functions. The
effects of mutations in cohesin and condensin subunits
are now being tested in specific functional assays to further explore their roles in gene regulation, DNA repair,cell-cycle checkpoints and centromere structure.
It is not yet known if a single molecular mechanism
underlies these different functions, or whether a single
complex can act through several mechanisms.
For example, a common change in chromosome structure might influence all these diverse processes.

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Alternatively, subunits of each complex could perform different functions depending on their chromosomal context, cell-cycle-dependent regulation andprotein partners. Because individual cohesin or condensin subunits can interact with different protein
partners to form complexes with different roles, thephenotypic consequences of mutating an individual
subunit might reflect not only the function of thatcomplex, but also other complexes that contain that
subunit.
Several excellent articles have covered the canonical roles of these SMC complexes in cohesion
and condensation, and should be consulted fora thorough review4–6. After providing a brief overview,
we focus here on the non-canonical roles of cohesinand condensin, discussing their influence on gene
regulation, DNA repair and centromere function. We
also highlight how insights into these roles have been
gained by investigating individual subunit mutations
in organisms that are amenable to genetic and cytological analysis. These studies provide a greaterappreciation of the versatility of cohesin andcondensin, and promise that insights into many cellular functions will be gained by examining theseimportant chromosomal proteins.

Cohesin and condensin — the canonical role

Cohesin is considered to be part of the glue between
sister chromatids, which is laid down at DNA replication so that sister chromatids are handled as a pairand subsequently segregated away from each other(BOXES 2,3). Mutations in cohesin subunits cause precocious sister-chromatid separation7–10, and mutations
that prevent cohesin from being removed inhibit
chromosome separation11–13. Once chromosomes areproperly aligned in the mitotic spindle, cohesin iscleaved to allow sister chromatids to segregate into thetwo daughter cells (BOX 2). Cohesin is also essential formeiosis, and its chromosomal location clearly parallels the differential cohesion events of meiosis I and II.
During meiosis I, in which homologues pair, cohesin
localizes all along the chromosome arms. It is released
along the arms when homologues segregate at the endof meiosis I, but remains at the centromeres until theend of meiosis II to keep the sister chromatids paired
until their segregation (BOX 2).
Condensin is required for the substantial reorganization of chromosome structure as chromosomes compact during mitosis (BOX 4). Condensin components
were identified in screens for chromosome-segregation
mutants in yeast14, and biochemically purified from

Box 1 | Chromosome events of mitosis

Preparing for mitosis: replication and establishment of cohesion
Preparations for mitotic chromosome segregation begin in the S phase of the cell cycle, as each chromosome is copied togenerate two identical sister chromatids and cohesion is established between them. Cohesion is thought to be mediatedboth by DNA connections (topological catenations) that are generated during DNA replication and by a proteinaceous
linkage (probably the cohesin complex) that hold sister chromatids together until anaphase.

Prophase
After replication and cohesin loading, chromosomes are in an extended configuration and appear as an amorphousmass in the nucleus. As cells enter prophase, chromosomes begin the process of condensation, undergoing a marked
change in structure that continues until they are fully compacted at metaphase. During prophase condensation, thesister chromatids begin to dissociate along their arms and organize along their individual axes — a process that is called
resolution. In many organisms this can be viewed cytologically as the transition from indistinguishable chromosomes to
two rod-shaped arms that are attached at the centromere.

Prometaphase
Prometaphase is defined as the stage during which the nuclear envelope breaks down. A specialized chromosome region,
the centromere, assembles a proteinaceous structure — the kinetochore — that mediates attachment to the microtubule
spindle. Sister chromatids are arranged so that each centromere interacts with microtubules from only one pole, aphenomenon that is called bi-orientation.

Metaphase
Chromosomes become fully condensed by metaphase. Condensation provides mechanical strength and reduces volume
so that chromosomes can withstand spindle forces as they are pulled to opposite sides of the cell. At metaphase, thechromosomes are completely aligned at the centre of the cell, along what is called the metaphase plate. A spindlecheckpoint mechanism monitors chromosome–spindle attachment and delays anaphase until the chromosomes areattached and under tension76.

Anaphase
At anaphase, the sister chromatids separate in a coordinated burst of movement. Once the spindle checkpoint is relieved,
the anaphase-promoting complex (APC) triggers the proteolysis of target proteins and chromosome segregation
follows 83. One target of the APC is a protein called securin that prevents the protease separase from cleaving a cohesin
subunit until the commitment is made to anaphase84. This mechanism ensures that the cohesin glue is not dissolved
until the cell is ready for division.

TelophaseIn telophase, the newly segregated chromosomes decondense; subsequently, cytokinesis divides the cytoplasm.

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Box 2 | Cohesin
Cohesin is a four-member protein complex that is required
Cohesinto hold together the sister chromatids of newly replicated
SMC1
SMC3
chromosomes. Cohesin contains a heterodimer of the SMC
proteins SMC1 (blue) and SMC3 (purple), which is
associated with the non-SMC proteins SCC1 (orange) and
SCC3 (yellow).
Cohesin proteins were identified by two Saccharomyces
cerevisiae laboratories in genetic screens that were designed
SCC1
to identify the ‘glue’ that is required for sister-chromatid
SCC3
attachment7,8. Both screens correctly presumed that
Mitosismutational disruption of the glue would cause precocious
sister-chromatid separation in metaphase-arrested cells.
Cytological proof of sister-chromatid separation defects
relied on clever approaches to mark a single locus on eachsister chromatid. Similar proteins have now been identified
Separase
as members of a cohesin complex in several
cleavesorganisms10,75,85–88 (TABLE 1).
SCC1
Cohesin has the cell-cycle-dependent chromatinlocalization pattern that would be predicted for a cohesion
factor. In yeast, cohesin loads onto chromatids at
Meiosis I
replication, and remains bound until its abrupt
dissociation at anaphase, when cohesion must be
released7,8,89,90. In higher organisms, the bulk of cohesin
dissociates at prophase, although a smaller amount persists
at centromeres and dissociates at anaphase85,91. This
Separaseprophase release of cohesin is necessary for sistercleaves
chromatid resolution92. If cohesin is not removed during
REC8 (meiotic SCC1)
11–13
along
arms,
not centromere
anaphase, sister chromatids fail to separate . Chromatin
immunoprecipitation and immunofluorescence
Meiosis II
experiments show cohesin along chromosome arms andconcentrated at the centromere, which is a site of tightcohesion, as would be expected for a complex that connects
Separase
sister chromatids40,42,64,93,94.
cleavesremaining
The localization pattern of cohesin during meiosis reveals
REC8
the most about its role in cohesion. During meiosis I whenhomologues are paired, cohesin localizes along chromosome arms, but when sister chromatids are paired during meiosis II,
cohesin is only found between centromeres88,90,95 –97. Most organisms contain a meiosis-specific cohesin complex, with ameiosis-specific SSC1 variant called REC8 in place of SCC1 (TABLE 1). Meiotic chromatids that lack REC8 cannot maintaincohesion. Reciprocally, meiotic chromatids on which cohesin inappropriately persists cannot lose cohesion and fail toseparate at meiosis I and meiosis II.
Dissociation of cohesin at anaphase is triggered by the proteolytic cleavage of its SCC1 or REC8 subunit by a proteasecalled separase84,98. Cleavage of SCC1 is necessary for sister-chromatid separation, and is an irreversible entry into
anaphase. Separase is inhibited by a protein called securin until the anaphase-promoting complex (APC) UBIQUITYLATES
and destroys securin83. The APC, in turn, is unable to perform its role until the spindle checkpoint has been cleared. Theseand other mechanisms ensure that cohesin remains between sister chromatids until the cell is ready to enter anaphase.

UBIQUITYLATIONThe covalent addition of thesmall protein ubiquitin toanother protein. Ubiquitin
conjugation generally targets
proteins for degradation byproteases.

condensed mitotic chromosomes in Xenopus egg
extracts15,16. In yeast, the mutational inactivation of condensin subunits increased the average distance between
two loci on a mitotic chromosome arm17–20. In Xenopus,
sperm DNA added to a mitotic egg extract assembled
into condensed mitotic chromosomes, and depletion ofcondensin disrupted condensation15,16. So, condensin was
proposed to drive mitotic chromosome condensation(BOXES 3,4). Condensin seems to be more than a simple
chromosome compactor, however, and is crucial for
resolving connections between sister chromatids. In a
Drosophila condensin subunit mutant called gluon, sister
chromatids are ‘fuzzy’ and fail to resolve into two distinct
rods at prophase, yet compaction along the longitudinal

5 2 2 | JULY 2003 | VOLUME 4axis seems normal at metaphase21. Similarly, depletion ofcondensin components in Caenorhabditis elegans causes
stringy rather than rod-shaped chromosomes atprometaphase, yet a high degree of compaction still
occurs by metaphase22. So, condensin might have a more
complex role in chromosome organization, which is necessary for sister-chromatid resolution. Consistent withthis idea, sister chromatids in condensin mutants remain
connected by chromatin bridges as they pull apart at
anaphase during mitosis14,21–24 and meiosis II (REF. 22).
Cohesin and condensin in gene regulation

Several mechanisms ensure that genes are expressed at
the proper time, place and level. Control mechanisms

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Table 1 | SMC protein complexes
Saccharomyces
cerevisiae

Schizosaccharomyces
pombe

Caenorhabditis
elegans

SMC1

Smc1

Psm1

HIM-1

SMC3

Smc3

Psm3

SMC-3

Drosophilamelanogaster

XenopuslaevisHomosapiens

SMC1

SMC1

SMC1α

SMC3

SMC3

SMC3

Cohesin

SCC1

Scc1/Mcd1

Rad21

SCC-1/COH-2

Rad21

RAD21

RAD21

SCC3

Scc3

Psc3

SCC-3

SA

SA1, SA2

SA1, SA2

REC8 (meiosisspecific SCC1)

Rec8

Rec8

REC-8

–

–

REC8

SMC2

Smc2

Cut14

MIX-1 (MIX-1)*

SMC2‡

CAP-E

CAP-E

SMC4

Smc4

Cut3

SMC-4 (DPY-27)*

SMC4/gluon

CAP-C

CAP-C

CAP-D2

Ycs4

Cnd1

HCP-6‡ (DPY-28)*

CG1911‡

CAP-D2/Eg7

CAP-D2/CNAP1

CAP-G

Ycs5/Ycg1

Cnd3–

CG17054‡

CAP-G

CAP-G

CAP-H

Brn1

Cnd2

(DPY-26)*

Barren

CAP-H

CAP-H

Smc5 (YOL034w)

Spr18

C27A2.1‡

CG3248‡SMC5‡ (AB103030.1)

SMC5

Rad18

‡

‡

CG5524

SMC6‡ (AB103031.1)

SMC6

–

–

NSE1‡

Condensin

DNA repair
SMC5
SMC6

Smc6/Rhc18

C23H4.6

F54D5.14‡
NSE1

Nse1

–

–

*Caenorhabditis elegans contains a second condensin-like complex that functions in X-chromosome dosage compensation. ‡Proteins that have been identified by homology,
but have not yet been shown to be members of the complex.

that modulate gene output can operate globally at a chromosome-wide level, regionally over a subchromosomal
domain or locally on an individual gene. Cis-regulatory
DNA elements, such as enhancers, silencers and insulators, mediate these levels of gene regulation by recruitingtrans-acting factors that influence transcription. Recent
studies have shown that either cohesin or condensin contributes to each of these levels of regulation, in part by
influencing enhancers, silencers and insulators.
Global and local gene repression. X-chromosome
dosage compensation is a regulatory process thatalters gene expression along an entire chromosome.
This form of regulation evolved in organisms that use
chromosomal mechanisms to determine sex (XX/XY
or XX/XO) and ensures equivalent levels of X-linked
gene products in males and females, despite the difference in X-chromosome number. Animals have solved
this problem in different ways: human females completely inactivate one X, male flies hypertranscribe
their single X and hermaphrodite nematodes partiallydownregulate both X chromosomes25. In these known
cases, dosage-compensation factors associate specifically with the X chromosome of one sex to superimpose chromosome-wide gene regulation on theunique spatial and temporal regulation of each gene.
Dosage-compensation factors in C. elegans resemble
condensin subunits, and form a complex that binds hermaphrodite X chromosomes and brings about a reduction in gene expression. Worm dosage-compensation
genes were discovered through a genetic analysis thatshowed a regulatory hierarchy that controls sex determination and dosage compensation. Both processes
respond to the primary sex-determining signal: the

NATURE REVIEWS | GENETICS

ratio of X chromosomes to autosomes25 (FIG. 1). An
X-chromosome counting mechanism translates thetwofold differences in this ratio into the on/off responseof the sexual-switch XO lethal gene (xol-1). When it isactive, xol-1 specifies male fate, whereas when it is inactive, it specifies both hermaphrodite sex determinationand dosage compensation by allowing the expression of
the sdc genes25 (FIG. 1). Two types of genetic screen wereused to identify dosage-compensation genes. First,
mutations in dosage-compensation genes were recognized by the XX-specific lethality and dumpy (short andfat) phenotypes that are associated with inappropriate
overexpression of X-linked genes. Second, mutations indosage-compensation genes rescued the XO lethality ofa xol-1 mutant by relieving the inappropriate halving ofgene expression from the single X.
The molecular similarity of dosage-compensation
genes to condensin genes was initially surprising, as it was
the first indication that condensin might function outside
the context of mitosis. mix-1 and dpy-27 encode SMC2and SMC4 homologues, respectively26,27, and DPY-26 andDPY-28 have limited homology to the non-SMC subunits CAP-H and CAP-D2, respectively28,29 (TABLE 1).
These dosage-compensation proteins form a complex
that resembles condensin, but this complex does notseem to affect mitotic chromosomes. Instead, this condensin-like complex assembles onto hermaphrodite X
chromosomes to downregulate X-linked gene expression.
Although the dosage-compensation complex is
specialized for regulating gene expression, phenotypic analysis indicates that some of its subunits haveother chromosomal functions. Mutations in dpy-26
and dpy-28, for example, cause defects in meiotic
chromosome segregation 30,31. Mutations in mix-1

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cause mitotic chromosome segregation defects that arelethal to both sexes26. These mix-1 phenotypes are reminiscent of condensin defects, but are not shared by othersubunits of the dosage-compensation complex, whichhas prompted a search for further MIX-1-interacting
proteins.
Biochemical and genomic approaches showed that
MIX-1 (SMC2) partners with the SMC4 variant
DPY-27 for its role in dosage compensation, but with
SMC-4 for its role in mitosis, in a complex that seems tobe the true mitotic condensin of C. elegans22 (FIG. 1). So,
MIX-1 is shared between two condensin complexes, onethat functions in mitosis and another that regulates
dosage compensation. The dosage-compensation complex is probably a recent evolutionary adaptation that
arose to fine-tune X-chromosome gene expression. As
dosage compensation evolved, proteins such as MIX-1
were apparently recruited for this new role and shared
between the mitotic and the dosage-compensation complexes. Others, such as DPY-27, might have arisen from

Box 3 | Mechanism of cohesin and condensin action

New models

Old models

The molecular mechanisms by
CohesinCondensin
which cohesin attaches sister
Inter-molecular crosslinker Intra-molecular crosslinker
chromatids and condensinreconfigures chromosome
structure remain mysterious,
but recent studies haveprovided new insight into theirmodes of action. Condensin
binds DNA and has ATPase
activity. When incubated withrelaxed circular DNA in thepresence of a type I
topoisomerase, condensin
causes ATP-dependent POSITIVE
22,99,100
SUPERCOILING. In thepresence of a type II
topoisomerase, condensin
converts nicked circular DNA'Embrace''Loop fastener''Orientated gyre'
into positive knots99. These
results led to the model thatcondensin introduces global
writhe into the DNA,promoting condensation bystabilizing large positively
supercoiled DNA loops.
Conversely, cohesin catenates
nicked circular DNA whenincubated with topoisomerase
II, and causes DNA–protein
aggregates in gel-shift
experiments101. So, condensin,
which compresses one sister
into itself, has intramolecular activites, whereas cohesin, which adheres two different sister chromatids, has intermolecular
activites. Electron microscopy (EM) shows that the arms of condensin are close together, whereas those of cohesin are
spread apart in a ‘V’ shape102. These activities and structures support the model that condensin acts as an intramolecularcrosslinker by grabbing sites on a single DNA strand and bringing them together, whereas cohesin acts as anintermolecular crosslinker by grabbing and holding two different sister chromatids.
New microscopic views of cohesin and condensin have inspired a revised view of their mechanism of action.
ATOMIC-FORCE MICROSCOPY shows that the SMCs of condensin form a globular head onto which the non-SMCs assemble,
and a coiled tail the end of which touches DNA103. These results suggest a ‘loop fastener’ model in which the condensin
hinge binds one region of DNA, then non-SMC proteins mediate an ATP-dependent opening and closing of the SMC ‘V’
to enclose a loop of DNA. Another study using electron spectroscopic imaging of condensin suggests an ‘orientated gyre’model rather than a ‘global writhe’ model, because a single condensin molecule seems to introduce two stacked supercoils
into closed plasmid DNA104. On the basis of this finding, it has been proposed that an ATP-hydrolysis cycle changes theconformation of condensin and allows it to trap two orientated positive supercoils in its coiled-coil arms105. A related‘embrace model’ has been proposed for cohesin. EM and interaction data indicate that SMC1 and SMC3 are linked at one
end by hinge interactions and at the other by interaction with SCC1 (REF. 106). Cohesin was, therefore, proposed to form alarge loop that encircles both sister chromatids, fastened by SCC1 at one end. Whereas earlier models envisioned cohesin
grabbing sister chromatids, this model indicates that the arms of cohesin might hold two sister chromatids in an‘embrace’ until proteolytic cleavage of SCC1 disrupts the cohesin loop106.

PARALOGUES

Homologous genes in the sameorganism that have evolved froma gene duplication and asubsequent divergence offunction.
POSITIVE SUPERCOILING

Twisting of the DNA about its
own axis. Twisting opposite to
the direction of the double-helix
turns produces negative
supercoils, whereas twisting inthe same direction produces
positive supercoils.
ATOMIC-FORCE MICROSCOPY

A method that maps
submicroscopic surfaces to giveinformation about their nature
at the atomic level. A mechanicalprobe with a sharp tip is tracked
over the surface of interest, andthe deflection of the cantilevered
tip is measured.

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duplication and divergence of the smc-4 gene to create a
PARALOGUE that is specialized for dosage compensation.
This example shows that an entire condensin-like complex can be used for functions other than mitosis, andthat a single condensin subunit can participate in twocomplexes with different functions. Several examples now
exist for such diversity in condensin and cohesin subunit
function — a recurring theme throughout this review.
The dosage-compensation complex is directed onto
the X chromosomes of hermaphrodites by specific targeting proteins. These X-targeting proteins, SDC-2 andSDC-3, are required for both sex determination anddosage compensation. The SDCs form a subcomplex
that associates with the condensin-like subunits and is
crucial for X recognition and assembly. As well as targeting SDC and dosage-compensation proteins to

the X chromosome, SDC-2 and SDC-3 promotehermaphrodite sexual development by targeting these
proteins to the male-determining autosomal gene her-1,
which becomes repressed 20-fold (REF. 32) (FIG. 1). So, thecondensin-like dosage-compensation complex (alongwith the associated SDC proteins) is versatile enough tocause both a twofold chromosome-wide repression and a
20-fold gene-specific repression.
Although the mechanism of X-chromosome repression is unknown, the involvement of condensin-related
proteins indicates that changes in chromatin structure
might contribute to X regulation. A slight condensationof each X might, ultimately, reduce gene expression.
Chromosome condensation during mitosis is correlatedwith gene inactivation and the displacement of RNApolymerase II and certain transcription factors33. It will

Box 4 | Condensin
S phaseCondensin
SMC2

SMC4

DNA replication

MitosisProphase

Metaphase

Anaphase

Telophase

CAP-G CAP-H
CAP-D2Condensin is a five-member protein complex that is required for chromosome organization and segregation. It contains
a heterodimer of SMC proteins (SMC2, red; SMC4, orange) and three associated non-SMC proteins (CAP-D2, CAP-G
and CAP-H). In most organisms, condensin (red) only associates with chromosomes at times of the cell cycle when they
are condensed (prophase to anaphase).
Our understanding of condensin comes from both biochemical and genetic studies. Chicken SCII, later
determined to be SMC2, was identified as one of the proteins that remained after chromosomes were stripped
down to an insoluble ‘chromosome scaffold’, on which looped chromatin domains are thought to be organized107.
The complete known complex, containing CAP-E (SMC2), CAP-C (SMC4), CAP-D2, CAP-G and CAP-H, wasidentified by a sedimentation method for purifying Xenopus mitotic chromosome associated proteins (CAPs)
from egg extracts15,16. CAP-D2 was also identified independently108. The five-member protein complex was named
condensin because sperm chromosomes introduced into an egg extract that was depleted of any subunit formed a
diffuse mass rather than a condensed structure.
Genetic studies also uncovered condensin subunits. Mutations in Schizosaccharomyces pombe cut3 (SMC2) andcut14 (SMC4) cause a cell untimely torn (‘cut’) phenotype when the division septum cuts through unsegregatedchromosomes at the cell centre14. These mutants were proposed to affect chromosome condensation rather than theseparation of centromeres on sister chromatids, because the distance between two fluorescent in situ hybridization(FISH) probes on a chromosome arm increased in the mutants, but centromere probes separated normally14. The three
S. pombe non-SMC condensin subunits were later biochemically identified and shown to associate with the SMC
proteins24. In Saccharomyces cerevisiae, condensin subunit mutations were created and shown by a similar FISH assay
to have defects in chromosome segregation and condensation17,18.
In Drosophila and Caenorhabditis elegans, condensin subunits were identified fortuitously in genetic screens forunrelated processes, and were later shown to affect mitotic chromosome morphology and segregation21,23,26.
Observations of chromosomes in these condensin subunit mutants indicated that condensin might have an
important role in resolving sister chromatids at prophase, but might not be the only metaphase condensation factor.
Mutation of a Drosophila condensin subunit caused wider less-distinct prophase sister chromatids, yet thesechromatids had shortened along their longitudinal axis by metaphase21. Similarly, depletion of C. elegans condensin
components caused wispy instead of rigid prophase chromosomes that nevertheless align into a relatively compact
metaphase plate22,109. A common and notable phenotype of condensin mutants in many organisms is the failure tocompletely separate connections between sister chromatids as they pull apart at anaphase during mitosis14,21–24. A
similar anaphase segregation defect has also been observed during meiosis II (REF. 22).

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MIX-1

DPY-27

DPY-26
DPY-2820-fold repression

SDC-1 SDC-2 SDC-3

XX

XX:AA

xol-1

sdc

MIX-1

Autosomal her-1 gene

Development

DPY-27

DPY-26
DPY-28

twofold repression
SDC-1 SDC-2 SDC-3

Dosagecompensation

X-linked genes

b

XO

X:AA

xol-1

Autosomal her-1 gene

Developmentsdc
No dosage
compensation

X-linked genesc Mitosis

MIX-1
?
?
?
DPY-27

Figure 1 | A condensin-like complex mediates gene-specific and chromosome-wide repression in Caenorhabditis elegans.
a | Sex determination and X-chromosome dosage compensation are triggered in response to the ratio of X chromosomes
to sets of autosomes (A). In XX animals, the XX:AA ratio causes repression of the sexual-switch gene xol-1, which allows activity of the
sdc genes. The SDC proteins SDC-1, SDC-2 and SDC-3 recruit the condensin-like dosage-compensation complex to the autosomal
male-determining gene her-1 and all along the X chromosomes. Binding of SDC and dosage-compensation proteins leads to the
20-fold repression of her-1, promoting hermaphrodite development, and to the twofold repression of X-linked genes, thereby
equalizing X-linked gene expression between XX and XO animals during dosage compensation. The dosage-compensation complex
contains the condensin subunit homologues MIX-1 (SMC2), DPY-27 (SMC4), DPY-26 (CAP-H) and DPY-28 (CAP-D2). b | In XO
animals, the X:AA ratio leads to the activation of xol-1. High levels of xol-1 repress the sdc genes, and the SDC and dosagecompensation proteins are not recruited to her-1 or the X chromosome. Full expression of her-1 follows, which promotes maledevelopment. Full expression of X-linked genes occurs from the single male X chromosome. c | As well as the dosage-compensation
complex that regulates gene expression, C. elegans contains a second condensin complex with conserved functions in mitotic
chromosome structure and segregation. MIX-1 (SMC2) is found in both complexes. Although it interacts with the SMC4 homologue
DPY-27 in the dosage-compensation complex, it interacts with a different SMC4 homologue, called SMC-4, in the mitotic complex.
Other members of the mitotic complex are not yet known.

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be interesting to learn how the dosage-compensation
complex represses gene activity, and why the level of thisrepression differs so greatly between the X chromosome
and the her-1 locus.

GENE CONVERSIONA specific type ofrecombination, which results in
non-reciprocal genetic exchange,in which the sequence of one
DNA strand is used to alter thesequence of the other.

Gene regulation in subchromosomal domains.
Chromosomes are thought to be organized into domains— units of chromosome packaging and coordinatedgene regulation. In some cases, domains are bordered by
special insulator elements that prevent the spread of regulatory influences or actual chromatin structure intoneighbouring regions34. Insulators help restrict the activity of enhancers and silencers, which are elements thatwould otherwise be capable of contacting heterologous
promoters over long distances.
One of the best-studied domains is the silent matingtype locus in budding yeast. Saccharomyces cerevisiae has
two mating types, a and α, which are determined bywhether a- or α-genes are expressed from the active MAT(mating type) locus (FIG. 2). Genes that encode the opposite mating type exist in two different silent domains —HML (homothallic left) and HMR (homothallic right).
Mating-type switching occurs by intrachromosomal GENE
CONVERSION between the expressed genes at the MAT locus
and the alternative genes at one of two silent loci. Theseloci are repressed by the E and I silencers, which are essential and important DNA sequences that recruit silencing
proteins to modify histones and organize a domain ofrepressive chromatin structure (FIG. 2). The silent domain
is bordered by two insulator elements that prevent the
spread of repressive chromatin into the neighbouring
regions35. Reporter genes that are placed within theconfines of these insulators are silenced.
An unexpected link between cohesin and the ability toestablish silencing over the mating-type domain has
recently been uncovered. Previous evidence had indicatedHML
silent
E

I
α2

Left
insulator

MAT
expressed

α1HMR
silentE

CEN

E silencer

I
a2

a2
gene

a1
gene

a1

I silencer

Right
insulator

Figure 2 | Cohesin and condensin might influence silencing and insulation at the yeast
mating-type locus. Saccharomyces cerevisiae exists as an a- or α-mating type, and switchesmating type by intrachromosomal gene conversion between the active MAT (mating type) locus
and one of two silent loci — HML (homothallic left) and HMR (homothallic right). The silent locieach contain two genes (arrows), surrounded by two DNA-silencer elements that keep them
inactive (E and I, yellow triangles) and bracketed by insulator elements (brick walls). Cohesin (bluecomplex) seems to prevent the establishment of silencing at HMR, whereas condensin subunits(red complex) might be required for silencing. Cohesin also localizes to the insulators and might
assist them in preventing the spread of silencing into neighbouring regions. Although cohesin
localization to the locus has been shown by chromatin immunoprecipitation, it is not known
whether condensin is associated with HMR.

NATURE REVIEWS | GENETICS

that the establishment of silencing requires passage
through the S phase of the cell cycle, but not DNA replication itself 36,37. A new study now shows that passage
through mitosis is also required to establish silencing,and indicates that the cleavage of cohesin (or at least its
Scc1/Mcd1 subunit) is necessary to establish this silencing38. Silencing proteins were found to localize to themating-type locus at G2/M, yet despite their presence
robust silencing was not established until cells progressed further through the cell cycle. In the absence of
Scc1/Mcd1, however, the silencing proteins present at
the locus during G2/M could fully repress transcription.
Conversely, expression of an uncleavable mutant formof Scc1/Mcd1 prevented silencing38. So, it seems that
Scc1/Mcd1 cleavage at the metaphase-to-anaphase transition is necessary to establish transcriptional silencing
over the yeast mating-type domain. Also, chromatin
immunoprecipitation experiments have localized
Scc1/Mcd1 to the mating-type locus and to other silent
heterochromatic domains in both S. cerevisiae andSchizosaccharomyces pombe, such as telomeres, centromeres and ribosomal DNA (rDNA) loci39–41. It is not
yet clear whether Scc1/Mcd1 affects the establishment ofsilencing at these other loci.
Cohesin might also contribute to gene regulation
by influencing the ability of insulators to separate
regions of active and inactive chromatin. Chromatin
immunoprecipitation experiments showed that sitesof Scc1/Mcd1 association along an S. cerevisiae chromosome are mainly AT-rich intergenic regions with aperiodicity of ~13 kb, similar to the proposed size ofchromatin domains42. The idea that cohesin localization might correspond to domain boundaries
prompted a subsequent study that showed the localization of Scc1/Mcd1 to several regions that borderrepressed and active chromatin 39. These regions
included the borders between copies of the repeatedrDNA genes, the HMR insulators but not the intervening silent domain, and the boundary between
active chromatin and a silent subtelomeric domain.
These results are interesting in light of genetic evidence that certain mutant alleles of SMC1 and SMC3
disrupt activity of the HMR insulators and allow
repressive chromatin to spread outside the domain43(FIG. 2) . It will be important to examine whether
cohesin is crucial for insulator function at the boundaries of other chromosomal domains.
A connection between condensin and silencing at theS. cerevisiae mating-type locus has also been reported.
This finding relies on the fact that inappropriate derepression of a silent locus allows cells to express both a- andα-genes, and the cells fail to arrest or form projections in
response to mating pheromone. Using this assay fordesilencing, it was shown that mutations in YCS4, which
encodes a CAP-D2 homologue, prevented arrest in
response to α-factor44. An allele of SMC4, but not of
SMC2, also had this effect. These results imply that condensin subunits help repress the silent mating-type locus(FIG. 2). However, derepression of a reporter gene in
the silent locus was not observed, perhaps because
only weak alleles could be used. The finding that

VOLUME 4 | JULY 2003 | 5 2 7© 2003 Nature Publishing GroupREVIEWS

Parasegment 10
regulation

Parasegment 11
enhancer

Parasegment 11
regulation

Fab-7
insulator

Parasegment 12
regulation

Fab-7 PREsilencer

Abd-B

Parasegment 13,14
regulation

Parasegment 12enhancer

Abd-B
gene

Figure 3 | Condensin might influence silencing or insulation at elements that regulate fly
body-segment identity. Segment-specific regulatory domains control Abdominal-B (Abd-B)
expression (arrow) to determine body-segment identity in Drosophila. Each domain contains
enhancers (green ovals) and silencers (yellow triangles) and is bordered by insulator elements (brickwalls). The Fab-7 insulator prevents cross-regulation between the PARASEGMENT 11 and 12 domains.
An adjacent silencer, the Fab-7 Polycomb response element (PRE), binds Polycomb-silencing
proteins to maintain the inactivity of the parasegment 12 domain in appropriate segments. A
condensin subunit was shown to localize to many PRE silencers by chromatin immunoprecipitation.
Mutations in this subunit enhance the segment-transformation phenotype that is caused by deleting
the Fab-7 insulator and PRE, which suggests a potential role in insulation or silencing.

some, but not all, condensin subunit mutations show
desilencing raises the possibility that condensin subunits associate with a complex other than condensin to
mediate this process.

PARASEGMENTIn Drosophila development,
body-patterning genes are
expressed in parasegmental units
that are out of register withmorphologically visible
segments. A parasegment
contains the posterior portion of
one segment and the anterior
section of the next segment.
TOPOISOMERASE IIAn ATP-dependent enzyme thatcreates transient breaks in bothstrands of the DNA sugarphosphate backbone, thenpasses one strand through theother and reseals the break. Such
enzymes can remove or create
supercoils in duplex DNA.

Maintaining repression of a regulatory region.
Condensin studies in Drosophila began with a genetic
screen for mutations that affected the development of
the peripheral nervous system. The barren (barr) gene,which was later recognized to encode a homologue ofthe condensin subunit CAP-H, was identified by amutation that causes embryonic lethality and defects inneuronal cell division23. Although barr phenotypes aremost easily recognized in the nervous system, barr
mutations affect general embryonic cell division andresult in aberrant anaphase chromosome segregation23.
Mutations in barr have been used to determine whether
condensin has a role in regulating gene expression
through subchromosomal domains.
barr influences a well-studied example of domainorganization and gene regulation, the Drosophila
Abdominal-B (Abd-B) gene. Abd-B specifies bodysegment identity in the posterior of the fly, and is regulated by a series of segment-specific regulatory domains.
Each domain is a collection of enhancers and silencers
that are surrounded by insulators (FIG. 3). Transient
embryonic-patterning proteins establish whether eachdomain is active or inactive in a particular body segment,
then a separate set of proteins maintains this activity
state. Domain inactivity is maintained by Polycombgroup proteins — factors that alter chromatin structure
to provide a ‘cellular memory’ of repression45. Polycombgroup proteins act through specific DNA regulatory

5 2 8 | JULY 2003 | VOLUME 4

elements in each domain, called Polycomb response
elements (PREs). For example, the regulatory domainsthat influence Abd-B expression in parasegments 11 and12 both include a PRE silencer and are separated by an
insulator called Fab-7 (FIG. 3). In mutant flies that carry a
Fab-7 deletion, these segments change their identity
because regulatory influences can inappropriately
spread between the domains46–49.
The suggestion has been made that condensin helps
Polycomb-group proteins maintain gene repression
over the Abd-B gene regulatory domains50. Mutations
in barr behave, in some respects, similarly to mutations
in Polycomb-group genes. Both relieve PRE-induced
silencing of a reporter transgene and enhance the phenotype of a Fab-7 insulator deletion50. Likewise, mutations in some Polycomb-group genes act similarly tobarr mutations, causing incomplete chromosome
segregation at anaphase. Moreover, Barren andPolycomb-group proteins co-localize at PRE-silencer
elements, according to chromatin immunoprecipitation studies50 (FIG. 3). These findings raise the possibility that Barren might work with Polycomb proteins tomaintain gene inactivation by altering the chromatin
structure of Abd-B regulatory domains. This possibility is intriguing, but further tests are necessary. First, it
is important to distinguish whether condensin has arole at PREs, insulator elements or both, as both classesof elements are often located in close proximity andthe genetic effect on the Fab-7 deletion does not differentiate between effects on either element. Second,
other available tests for Polycomb-group function,such as the ability of mutants to derepress Abd-B
reporter genes or to enhance Polycomb-group mutant
phenotypes, should be applied. Third, other condensin
subunit mutations should be tested to determine
whether this activity reflects condensin function or a
specific role for Barren.
Regulating enhancer–promoter communication. It
has been proposed that the mechanism used bycohesin to attach two different sister chromatids intrans might also be used in cis on the same sister
chromatid to help enhancers contact promoters overlong distances51. This idea stems from the identification of a cohesin regulatory-factor homologue in a
genetic screen for mutations that affect the ability ofan enhancer to overcome an insulator.
A Drosophila transposon called gypsy functions as
an insulator to block enhancer–promoter communication, and gypsy insertions cause loss-of-function
phenotypes when located between an enhancer and a
promoter34. For example, at cut, which is a gene thatis important for wing development, the insertion of
gypsy between a wing enhancer and the cut promoter
partially blocks their interaction (FIG. 4). The resulting
failure to properly activate cut in wing-margin cells
produces a ‘cut wing’ phenotype. A genetic screen formutations that exacerbate the gypsy-induced cutwing phenotype identified Nipped-B, which is a genewith similarity to the cohesin regulatory factor Scc2
(REF. 51) . It was, therefore, suggested that Nipped-B

www.nature.com/reviews/genetics© 2003 Nature Publishing GroupREVIEWSSYNTHETIC LETHALITY

A phenomenon that refers tolethality that is caused by the
combination of two mutations,
neither of which causes lethality
by itself. Synthetic lethality of
double mutants can indicate thattwo genes might function in
related processes.
DNA LIGASE III

Ligases are enzymes that seal
nicks in one strand of doublestranded DNA by creating anester bond between adjacent3′OH and 5′PO4 ends on thesame strand. DNA ligase III acts
in DNA repair to heal singlestranded DNA breaks.
DNA POLYMERASE-εPolymerases are enzymes thatsynthesize new DNA strands
using a DNA template. DNApolymerase-ε acts in DNA
repair when nucleotides havebeen excised, and also associateswith the replication fork as alagging-strand polymerase.

Wild typenormally facilitates long-range enhancer–promoter
communication, and when mutated, reduces the ability of the enhancer to overcome the gypsy insulator51(FIG. 4). In support of this idea, Nipped-B mutations
also enhance the phenotype caused by a gypsy insertion between a distant enhancer and promoter at
another locus. Also, Nipped-B mutations cause a weak
cut-wing phenotype even in the absence of gypsy,
which confirms a normal role in gene regulation at
the cut locus. To determine whether the effect ofNipped-B on gene regulation results from a disruptionof cohesin activity, mutations in cohesin subunits
should be analysed. The speculation that similar factors adhere sister chromatids, and adhere enhancers totheir promoters along a single sister chromatid, is an
attractive idea that awaits further testing.
Together, these examples illustrate how genetic
screens that had been designed to understand developmental processes as varied as X-chromosome
dosage compensation, nerve-cell formation and wing
patterning, have exposed gene regulatory functions
for condensin and cohesin subunits. Once identified,these mutations allowed the continued exploration offurther gene regulatory functions.

cut gene

Wing enhancer

cut gypsy

Wing enhancer

gypsy transposon
insulator

cut genecut gypsy; Nipped-B–

Wing enhancer

gypsy transposon
insulator

cut gene

Figure 4 | A cohesin regulatory protein might affect enhancer–promoter communication in
Drosophila. A wing enhancer (green oval) regulates the Drosophila cut gene promoter (straight arrow)from a distance of 85 kb, to promote normal wing development (top). A transposon called gypsy
contains an insulator element (brick wall) and partially blocks this enhancer–promoter interaction wheninserted between them, causing a ‘cut wing’ phenotype (middle). This gypsy-induced wing defect iseven more severe when Nipped-B, a gene encoding a homologue of the cohesin regulatory protein
Scc2, is mutated (bottom). This finding supports the model that Nipped-B (and perhaps cohesin, blue
complexes) normally facilitates long-range enhancer–promoter communication (curved arrows).
Cohesin localization to this locus has not been proven, but is shown to illustrate the model.

NATURE REVIEWS | GENETICS

Recombination and repair

As well as cohesin and condensin, eukaryotes contain athird SMC complex that specializes in DNA repair.
Recent evidence indicates that all three SMC complexes might help cells cope with DNA damage.
Cohesin and condensin subunits have been implicatedboth in the actual repair of DNA damage and in a
DNA-damage-checkpoint pathway that senses damage
and delays replication until the damage is fixed.
The multiprotein SMC complex that specializes in
the repair of DNA damage includes an SMC5–SMC6
heterodimer52–54. Long before it was recognized to
encode the SMC6 protein, rad18 was identified in
S. pombe in a screen for radiation-sensitive mutations55.
This repair function of Rad18 was later shown to be distinct from the standard nucleotide-excision repair pathway, and analysis of rad18 mutants showed further roles
in mitotic growth and the DNA-damage-checkpoint
response52,56. Also, mutations that affect Rad18 and
TOPOISOMERASE II show SYNTHETIC LETHALITY, which indicatesthat this SMC complex might influence chromosome
organization56.
The first indication that cohesin subunits
were involved in DNA repair was the discovery thatmammalian SMC1 and SMC3 are part of a biochemically purified recombinational repair complex, RC-1
(REF. 57) . Unlike mitotic cohesin, this complex does
not contain SCC1 and SCC3, but instead contains
DNA LIGASE III and DNA POLYMERASE- ε , and promotes therepair of DNA gaps and deletions. Genetic studies
also connected cohesin subunits to repair.
SCC1/MCD1 was first identified in S. pombe as rad21,
a gene that is involved in double-strand breakrepair58. This function is conserved, as deletion of thechicken SCC1 or mutation of S. cerevisiae
SCC1/MCD1 or SMC1 hinders double-strand breakrepair and reduces sister-chromatid exchange 59,60.
Mutations in S. cerevisiae SMC1, SMC3, SCC3 and
PDS5 are hypersensitive to damage by γ-irradiation,and cohesin must be present in S phase for efficient
repair of double-strand breaks 60. It is likely thatcohesin facilitates repair by promoting cohesion
between sisters, so ensuring that the correct sequenceis used as a donor template for repair3.
As well as DNA repair, the cohesin subunit Smc1
participates in a signal-transduction pathway thatelicits a checkpoint response to DNA damage. The
Atm kinase, which is mutant in the human disorderataxia-telangiectasia, is activated in response to irradiation and phosphorylates important cell-cycle targets. In response to radiation, two serines of Smc1 arephosphorylated by Atm, and this modification isessential for the S-phase DNA-damage checkpoint61,62(FIG. 5). Biochemical studies indicate that both Smc1
and Smc3 might associate in a complex that contains
DNA-damage-response proteins such as Atm, Nbs1,
the Bloom syndrome factor Blm and the breast
cancer tumour suppressor Brca1 (REF. 61) (FIG. 5).
The mechanism by which phosphorylated Smc1
helps prevent DNA synthesis in response to damage
is unknown.

VOLUME 4 | JULY 2003 | 5 2 9© 2003 Nature Publishing GroupREVIEWS

ATM
kinaseBRCA 1NBS1BLM

S-phasecheckpoint

DNA damage
?
SMC1

SMC3

Figure 5 | Cohesin subunits participate in a DNA-damage-checkpoint response pathway. DNA damage produced byirradiation (red arrow) activates the ATM kinase, which phosphorylates (P) the cohesin subunit SMC1. SMC1 is found in a complex
distinct from cohesin, and some evidence indicates that this complex might also contain SMC3 and the DNA-damage-response
proteins NBS1, BLM and BRCA1. Phosphorylation of SMC1 is essential for eliciting the S-phase DNA-damage checkpoint thatwill delay DNA replication until damage is repaired. Although not illustrated here, a condensin subunit has also been implicated in
the ATM-checkpoint pathway.

A condensin subunit is also required for DNA repair
and for the DNA-damage checkpoint. In S. pombe, mutations in the gene that encodes the non-SMC protein
Cnd2 (CAP-H), but not in genes that encode the SMC
proteins Cut14 (SMC2) and Cut3 (SMC4), result in
hypersensitivity to DNA-damaging agents63. Mutations in
cnd2 cause accumulation of damage such as THYMINEDIMERS, and in combination with mutations in excisionrepair genes they cause increased sensitivity to irradiation.
Although only Cnd2 seems to be required for repair, allsubunits of the condensin complex seem to influence theDNA-damage checkpoint. Indicative of this role, mutation of any gene that encodes a condensin subunit fails toactivate the DNA-damage-checkpoint kinase Cds1. Also,
cnd2 mutations cause a cell-cycle delay that is relieved
when combined with mutations in the checkpoint regulators crb2 (BRCA1) and the rad3 (ATM) kinase63. So,
condensin, as well as cohesin, might have a role during
interphase as part of the ATM-checkpoint pathway.
The mechanisms by which these complexes or theirsubunits promote repair and checkpoint function are
not yet clear. Several models have been suggested3. One
possibility is that cohesin and condensin organize chromosome structure, even during interphase, in a way thatfacilitates recombinational repair. Alternatively, certain
subunits might interact with DNA recombination andrepair proteins to form distinct complexes. These studies again illustrate that careful analysis of cohesin andcondensin subunit mutations has the potential to reveal
new roles for cohesin and condensin.

THYMINE DIMERS

A pair of abnormal covalently
bonded adjacent thymine
residues in DNA that are caused
by DNA damage.
SPINDLE MIDZONE

A region of overlapping
microtubules at the centre of thespindle-microtubule apparatus.
The midzone forms whenchromosomes segregate, and isrequired for proper spindle bipolarity and elongation,
chromosome movement andcytokinesis.

impact on centromere function during mitosis, which
influences the orientation of centromeres towards thepoles and their attachment to spindle microtubules.

Cohesin and condensin at the centromere

Interplay of cohesin and centromere proteins. Centromere
chromatin proteins help recruit cohesin to the centromere. Chromatin immunoprecipitation experiments
in budding yeast indicate that Cse4(CENP-A) is required
to localize cohesin components to the centromere64.
When CENP-A is directed to ectopic sites along humanchromosome arms by overexpression, it recruits thecohesin subunit SMC1 as well as kinetochore proteins65.
At the centromere in S. pombe, histone H3 is methylated
at lysine 9 (REF. 66). This modification is required torecruit the heterochromatin protein Swi6(HP1),
which in turn is required for cohesin association withcentromeres, but not arms40,41 (FIG. 6a). However,cohesin does not seem to be required to localize centromeric chromatin proteins to the centromere. Cohesinmutations, for example, do not disrupt Swi6 (HP1)binding40,41 or Cse4 (CENP-A) binding67.
Cohesin also requires kinetochore proteins for its
proper localization. The budding yeast kinetochore
protein Ndc80/Hec1 interacts with Smc1 in a twohybrid assay and its overexpression suppresses thelethality of mutations in SMC1 (REF. 68). Chromatin
immunoprecipitation experiments also indicate thatthe budding yeast kinetochore proteins Ndc10 and
Mif2 (CENP-C) are required to localize cohesin components to the centromere64. So, centromere and kinetochore proteins are instrumental in recruiting cohesinto this chromosomal location.

Cohesin at the centromere. The centromere is where
cohesin performs its best-described role: adhering sister
chromatids until anaphase. The centromere is a chromosomal region with a distinct chromatin structurethat contains modified histones and a unique histone
H3 variant called CENP-A. A proteinaceous structure
called the kinetochore assembles onto this site andmediates microtubule attachment. As well as forming
the microtubule-attachment site, the centromere is also
an important site of sister-chromatid cohesion. New
studies of cohesin mutants indicate that the cohesin
complex is not simply a structural bridge that links sister
centromeres. Instead, cohesin might have a broader

Sister chromatid bi-orientation and tension. Accurate
chromosome segregation requires chromosome‘bi-orientation’ in which each kinetochore on a pair ofattached sister chromatids faces outwards and presents a microtubule-binding face towards only one
spindle pole. This arrangement ensures that a pair ofkinetochores attaches to two different poles (bi-polar
attachment) rather than to the same pole (monopolar attachment) (FIG. 6b). Proper segregation also
requires tension between kinetochores as the microtubule spindle aligns the chromosomes. In theabsence of proper attachment and tension, the spindle

5 3 0 | JULY 2003 | VOLUME 4

www.nature.com/reviews/genetics© 2003 Nature Publishing GroupREVIEWS
checkpoint engages and delays anaphase until the
problem is corrected.
Cohesin is crucial for bi-orientation. In both yeast
and vertebrate SCC1 mutants, monopolar spindle
attachment is observed59,69. Bi-orientation in budding
yeast has been visualized by the transient separation andreassociation of green fluorescent protein (GFP)marked centromeres. A mutation in the cohesin subunit
a
Swi6
Me

Swi6

Swi6
CENP-A

Me

Centromeric heterochromatinb

Ipl1
cohesin
Bi-oriented
monocentricchromosome

Elicit checkpoint until correctedcBi-oriented
holocentricchromosome

Condensin-depleted
holocentricchromosomeDNA
CENP-A
SMC-4
MERGE
Caenorhabditis elegans holocentric chromosome

Figure 6 | Cohesin and condensin at the centromere. a | Centromere proteins recruit
cohesin. Chromatin at the centromere in Schizosaccharomyces pombe contains nucleosomes(turquoise ovals) and histone H3, which is methylated (Me) on lysine 9 (red flag). This
modification, which is characteristic of heterochromatin, recruits the heterochromatin protein
Swi6 (HP1) (green box) to this site. Swi6 (HP1), in turn, is required to recruit cohesin (bluecomplex) to centromeric heterochromatin, where it performs important functions. In manyorganisms, the histone H3 variant CENP-A (red), which is found in centromeric nucleosomes,
is also required to recruit cohesin. b | Cohesin is important for bi-polar spindle attachment. At
mitosis, paired sister chromatids (green) assemble kinetochores (red) onto their centromeres(green circle) to mediate attachment to microtubules (lines). A spindle-checkpoint mechanism
delays sister-chromatid separation if chromosomes are improperly attached to microtubules.
Cohesin is required for bi-polar microtubule attachment, and mutations in cohesin subunits
trigger the spindle checkpoint. A budding yeast kinase, Ipl1, is required for cohesin mutants
to trigger the checkpoint. Ipl1 has been proposed to correct mono-polar attachment bydestabilizing inappropriate microtubule–kinetochore interactions, or to sense tension between
properly attached sister chromatids. c | Condensin might organize centromere structure.
Caenorhabditis elegans condensin (SMC-4, green) co-localizes with centromeric proteins(CENP-A, red) on mitotic chromosomes (DNA, blue). C. elegans chromosomes (drawn ingreen) are holocentric, meaning that the centromere and kinetochore (red) extend along
the length of the chromosome, rather than forming a single site as for the monocentricchromosomes drawn in part b. Depletion of condensin subunits disrupts centromere
organization (right), and can result in lagging chromosomes at anaphase.

NATURE REVIEWS | GENETICS

gene SCC1 disrupts this transient centromere splitting69,70. One possibility is that cohesin not only glues
sister chromatids together, but might also impose a‘back-to-back’ structural organization so that kinetochores are more likely to be captured by microtubulesfrom opposite poles.
Cohesin might also promote bi-orientation and tension by assisting the mitotic kinase Aurora B. This kinase,along with INCENP (inner centromere protein) andBIR1(Survivin), forms a complex that is essential for
chromosome segregation. These proteins are known as‘chromosome passengers’ because they ‘ride’ metaphase
chromosomes until anaphase, when they redistribute to
the SPINDLE MIDZONE. Mutations in budding yeast
IPL1(Aurora B) were shown to cause mono-polar
attachment that occurs preferentially with the spindle
pole inherited from the previous division. The same
preference was observed for mutations in SCC1 or ingenes that encode the Dam/Duo kinetochore protein
complex71,72. It was, therefore, proposed that sister chromatids initially associate with microtubules from the oldspindle-pole body, and in the absence of Ipl1 cannot
break this association to allow connection to the otherspindle pole. According to this model, Ipl1 is part of themechanism to correct mono-polar attachment by destabilizing microtubule-kinetochore interactions until
chromatids re-orientate, form bi-polar attachments andestablish tension72. Ipl1 is also required for spindlecheckpoint activation by sister chromatids that are
not under tension73. Ipl1 is, therefore, proposed to be a‘tensiometer’ that promotes bi-orientation by monitoring
tension at the kinetochore.
Cohesin could be important for the tension-sensing
and attachment-correcting properties of Ipl1 in several
ways that are not mutually exclusive. As cohesin adheres
sister chromatids it might orientate them so that the kinetochores are more likely to be captured by microtubulesfrom opposite poles. Alternatively, the tension thatcohesin generates by gluing sister chromatids together
might stabilize kinetochore–microtubule interactions
and be sensed by Ipl1. Finally, cohesin might simply
be required to localize the passenger complex that
contains Ipl1. Mutations in S. pombe rad21 (SCC1),
deletion of chicken SCC1 and depletion of Drosophila
Rad21(Scc1) all disrupt the localization of passenger
proteins59,74,75.
Cohesin and the spindle checkpoint. Mutations incohesin subunits trigger the spindle checkpoint67. This
checkpoint monitors proper kinetochore–spindle
attachment, delaying anaphase if sister chromatids areunattached, attached only at one kinetochore, or both
attached to the same pole (FIG. 6b). How proper attachment is signalled is not clear, but the checkpoint isthought to detect a lack of tension at the kinetochoreor unattached kinetochores 76. The finding thatcohesin subunit mutations elicit this checkpoint
response extends the evidence that cohesin influencesbi-orientation and/or tension. Mutations in S. pombemis4 (SCC2) and rad21(SCC1) genes cause mitotic
delay and hypersensitivity to spindle poisons such as

VOLUME 4 | JULY 2003 | 5 3 1© 2003 Nature Publishing GroupREVIEWS
thiabendazole. This mitotic delay seems to result fromengaging the spindle checkpoint, as the combination ofthese mutations and checkpoint-gene mutations prevents delay67. It will be important to determine the generality of these findings. Although it has been suggested
that depletion of cohesin from human cells impairs invitro SPINDLE-ASTER assembly77, the effect of cohesin depletion on spindle assembly has not yet been addressed in
the well-characterized Xenopus egg-extract system. In
budding yeast, mutations in genes that encode thecohesin loading factor Ctf7/Eco1 (REF. 78) or the replication protein Ctf18 (REF. 79) disrupt cohesion and trigger
the spindle checkpoint. However, the exact relationshipamong the cohesin complex, the localization of passenger proteins and the spindle checkpoint has not yet been
determined.
Condensin and the centromere. Recent evidence from
C. elegans indicates that condensin might influencecentromere organization. Most model organisms havemonocentric chromosomes, that is, the centromere is asingle discrete region (FIG. 6b). C. elegans, however, has
holocentric chromosomes — the centromere extends
along the length of the chromosome (FIG. 6c). C. elegans
condensin components co-localize with centromeric
proteins, such as CENP-A, along the chromosome
length22 (FIG. 6c). Mutation or depletion of a condensin
subunit disrupts centromere bi-orientation, and centromere proteins fail to show their normal restrictedorientation towards the spindle poles22. Moreover,mutation of the C. elegans CAP-D2 homologue causes
lagging anaphase chromosomes that seem to result
from attachment of each sister chromatid to bothpoles80. So, the organization imposed by condensin
might help build and orientate the centromere onthese holocentric chromosomes.
Whether condensin organizes centromeres on
monocentric chromosomes is unknown, but circumstantial evidence indicates that it might. Chromatin
immunoprecipitation experiments in S. pombe haveshown that condensin localizes to the centromere63.
In S. cerevisiae, chromosomes in brn1 (CAP-H)
mutants are displaced from the spindle, perhaps
owing to defective kinetochore–spindle attachment19,20. In Xenopus egg extracts, immunodepletionof condensin causes disorganized localizationof a kinetochore protein, which indicates a role inkinetochore morphology81.
Perspectives and future directions

SPINDLE ASTER

A star-shaped cluster ofmicrotubules that emanate
towards the cell cortex from themicrotubule-organizing centres
at the poles of the spindle. Astral
microtubules help position the
mitotic apparatus.

Genetic analysis of cohesin and condensin has provided a better understanding of how chromosomes
adhere, compact, resolve and separate during cell
division. Concerted investigation of cohesin and condensin mutants, using an expanded set of functional
assays, has also shown new roles for these proteins ina wide range of chromosomal processes throughout
the cell cycle, including gene silencing and insulator
function, DNA-damage sensing and repair, and centromere orientation and function. Also, genetic
screens that were designed to understand processes as

5 3 2 | JULY 2003 | VOLUME 4

diverse as X-gene regulation, enhancer–promoter communication, nerve-cell proliferation and DNA repair
have unexpectedly identified cohesin and condensin
mutations.
One concept that emerges from these studies is that
individual subunits of cohesin and condensin might
associate with different sets of proteins to performdifferent functions. For example, SMC1 and SMC3
associate with distinct sets of proteins in the cohesin
complex, the RC-1 recombination repair complex and
the ATM DNA-damage-response complex. Similarly in
C. elegans, MIX-1(SMC2) is a component of both
mitotic condensin and a specialized condensin-like complex that is central to X-chromosome gene regulation.
Several protein partners and roles should be kept inmind as the functions of cohesin and condensin subunits are investigated. Mutant phenotypes could represent the action of a single complex in more than one
process, or the participation of a single subunit in different complexes with different functions. Althoughchromosome segregation is probably the ancestral roleof these complexes, certain subunits might have been
recruited during evolution for more specialized functions such as coordinating the cell cycle or fine-tuning
gene expression.
It remains to be seen whether these seemingly
unrelated activities primarily result from the influence that condensin and cohesin exert on chromosome structure. Cohesin and condensin subunits
impact several loci that are organized into silent
chromatin domains, such as the centromere, therDNA repeats, the yeast silent mating-type loci andthe fly Abd-B regulatory elements. Even on interphase
chromosomes, cohesin and condensin might define
domains of chromatin folding that impact processes
such as gene regulation. In one model, cohesin forms
the boundaries of chromatin-loop domains that areformed by condensin82. This model stems from theperiodicity of cohesin localization sites and therequirement for cohesin in re-establishing condensation after temperature-sensitive condensin mutants
are returned to a permissive temperature82. Further
studies will be necessary to understand whether
cohesin and condensin influence these domains bydirecting higher levels of chromosome structure.
The finding that cohesin and condensin participate
in a growing assortment of chromosome processes
provides the perspective that new roles for these proteins will probably be discovered. Even before suchnew roles might be detected, evaluating the evolutionary conservation of the known non-canonical functions will help elucidate the biological context andpossible mechanisms of these roles. Moreover, in cases
in which a single subunit is known to function outsideof the classical condensin and cohesin activities, thequestion remains as to whether the observed phenotype reflects the role of a subunit in these definedcomplexes or in another complex. Many fascinating
questions remain to be answered; studying these two
crucial protein complexes should continue to provide
important insights into chromosome biology.

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Acknowledgements
We thank R. Heald, S. Wignall, N. Bhalla and A. Severson for
critical reading of the manuscript.

Online links
DATABASES
The following terms in this article are linked online to:
FlyBase: http://flybase.bio.indiana.edu
Abd-B | barr | cut | Fab-7 | gluon | gypsy | Nipped-B
GeneDB: http://www.genedb.org
cnd2 | cut3 | cut14 | mis4 | rad18 | rad21
OMIM: http://www.ncbi.nlm.nih.gov/omim
ataxia-telangiectasia | Bloom syndrome
Saccharomyces Genome Database:
http://www.yeastgenome.org
MCD1 | PDS5 | SCC3 | SMC1 | SMC2 | SMC3 | SMC4 | YCS4
WormBase: http://www.wormbase.org
dpy-26 | dpy-27 | dpy-28 | her-1 | mix-1 | smc-4 | xol-1
FURTHER INFORMATION
Chromatin Structure and Function page:
http://www.cstone.net/~jrb7q/chrom.html
Chromosome Passenger homepage:
http://homepages.ed.ac.uk/rradams/passengers.html
Mitosis World:
http://www.bio.unc.edu/faculty/salmon/lab/mitosis/mitosis.html
Nature Cell Division web sites:
http://www.nature.com/celldivision/links
Access to this interactive links box is free online.

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