Cell, Vol. 109, 87–99, April 5, 2002, Copyright 2002 by Cell Press

Expansion of the Cell Plate in Plant Cytokinesis
Requires a Kinesin-like Protein/MAPKKK Complex
Ryuichi Nishihama,1,4 Takashi Soyano,1Masaki Ishikawa,1 Satoshi Araki,1,6
Hirokazu Tanaka,1 Tetsuhiro Asada,2 Kenji Irie,1Mayumi Ito,1 Mizuya Terada,1 Hiroharu Banno,1,5
Yoshiko Yamazaki,1 and Yasunori Machida1,3
1
Division of Biological Science
Graduate School of Science
Nagoya University
Chikusa-ku, Nagoya 464-8602
Japan
2
Department of Biology
Graduate School of Science
Osaka University
Toyonaka 560-0043
Japan

Summary
The tobacco mitogen-activated protein kinase kinasekinase NPK1 regulates lateral expansion of the cell
plate at cytokinesis. Here, we show that the kinesinlike proteins NACK1 and NACK2 act as activators of
NPK1. Biochemical analysis suggests that direct binding of NACK1 to NPK1 stimulates kinase activity.
NACK1 is accumulated specifically in M phase andcolocalized with NPK1 at the phragmoplast equator.
Overexpression of a truncated NACK1 protein thatlacks the motor domain disrupts NPK1 concentration
at the phragmoplast equator and cell plate formation.
Incomplete cytokinesis is also observed when expression of NACK1 and NACK2 is repressed by virusinduced gene silencing and in embryonic cells from
Arabidopsis mutants in which a NACK1 ortholog is
disrupted. Thus, we conclude that expansion of thecell plate requires NACK1/2 to regulate the activity
and localization of NPK1.
Introduction
Cell division in eukaryotes culminates in cytokinesis,
with appropriate distribution of nuclear and cytoplasmic
genetic information to daughter cells. Cell division in
animals and plants appears quite dissimilar: animal cells
divide by constriction of the cell cortex, whereas plant
cells divide by formation of new cross walls, known ascell plates, from the center to the periphery of eachdividing cell (Field et al., 1999).
In plant cytokinesis, formation of the cell plate occurs
in the phragmoplast, a complex structure composed
mainly of microtubules (MTs) and microfilaments (reviewed in Staehelin and Hepler, 1996; Heese et al., 1998;
Nishihama and Machida, 2001). At late anaphase, a3

Correspondence: yas@biol1.bio.nagoya-u.ac.jp
Present address: Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599.
5
Present address: College of Bioscience and Biotechnology, Chubu
University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan.
6
On leave from Central Research Institute, Ishihara Sangyo Kaisha,Ltd., 2-3-1 Nishi-shibukawa, Kusatsu, Shiga 525-0025, Japan.

phragmoplast appears midway between two daughter
nuclei, with two antiparallel bundles of MTs beingaligned such that the plus ends of the MTs overlap(Gunning, 1982). Golgi-derived membranous vesicles
carrying cell plate materials are then transported to theequatorial zone of the phragmoplast where they fuse to
create a membranous network, which matures into acell plate (Samuels et al., 1995). The continuous outward
redistribution of MTs transforms the phragmoplast intoa ring and increases its diameter, allowing the cell plate
to expand laterally until it reaches the cell wall (Yasuhara
et al., 1993; Yasuhara and Shibaoka, 2000). The KNOLLE
syntaxin homolog, the KEULE Sec1 homolog, and the
KORRIGAN cellulase are essential for such plant cytokinesis (Lukowitz et al., 1996; Zuo et al., 2000; Assaad etal., 2001).
Proteins related to vesicle trafficking and MT-associated proteins, including kinesin-like motor proteins(KLPs), are concentrated at the phragmoplast (Lloydand Hussey, 2001; Nishihama and Machida, 2001; Leeet al., 2001). Protein kinases, such as mitogen-activatedprotein kinases (MAPKs), p43Ntf6 in tobacco (Calderini
et al., 1998), and MMK3 in alfalfa (Bögre et al., 1999),
accumulate at the cell plate, while NPK1 (nucleus- andphragmoplast-localized protein kinase 1) MAPK kinasekinase (MAPKKK) accumulates in the equatorial regionof the tobacco phragmoplast (Nishihama et al., 2001).
Activation of these kinases at late M phase (Calderini
et al., 1998; Bögre et al., 1999; Nishihama et al., 2001)
suggests the involvement of the coordinated actionsof these enzymes in formation of the cell plate. Thishypothesis is supported by the observation that overexpression of a kinase-negative form of NPK1 in tobacco
cells inhibits expansion of both the phragmoplast andthe cell plate, with resultant generation of multinucleate
cells with incomplete cell plates (Nishihama et al., 2001).
It has been suggested that the noncatalytic region of
NPK1 might be involved in activation of this MAPKKK(Banno et al., 1993; Nishihama et al., 1997). Expressionof NPK1 or its Arabidopsis ortholog, ANP1, can suppress
mutations in genes for MAPKKKs in yeast. Removal of
noncatalytic regions enhanced suppression of the yeast
mutations, suggesting that these regions might havenegative regulatory functions.
We have identified novel tobacco KLPs (designated
NACK1 and NACK2) that are involved in the activationof NPK1 and shown that NACK1 regulates the activity
and the localization of NPK1 by association throughthe noncatalytic region of the kinase. Genetic analysis
revealed that disruption of NACK1 functions resulted in
defects in cytokinesis. Thus, the NACK1/NPK1 complex
plays an essential role in the intracellular events thatlead to cytokinesis.
Results4

Isolation of cDNAs for Activatorsof NPK1 MAPKKKTo isolate cDNAs for proteins that increased the activityof NPK1, we used a yeast system that exploits a modi-

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fied version of the mating pheromone-responsive MAPK
cascade, which consists of STE11 MAPKKK, STE7
MAPKK, and FUS3 MAPK (Irie et al., 1994). Yeast strain
SY1984 has a disrupted STE11 gene and harbors a HIS3
reporter gene fused to the FUS1 promoter, which allows
examination of the activation of FUS3 MAPK by monitoring the growth of yeast cells on medium prepared without exogenous histidine (Stevenson et al., 1992). We
introduced into the SY1984 genome a construct for theconstitutive expression of Dsor1Su1, a gain-of-function
variant of the Dsor1 MAPKK of Drosophila (Tsuda etal., 1993), to enhance the sensitivity of the system to
MAPKKK activity. In the resultant strain 11Su1, expression of a truncated, active form of NPK1 that lacked thecarboxy-terminal regulatory region allowed cells to growwithout exogenous histidine; expression of full-length
NPK1 did not (data not shown).
We isolated RNA from actively dividing tobacco BY-2
cells and prepared a plasmid library of cDNAs, all ofwhich would be constitutively expressed in yeast cells.
The cDNA library was introduced into 11Su1 cells inwhich a fusion gene that encoded full-length NPK1 witha galactose-inducible promoter had been integrated.
We identified three transformants (#041, #051, and #081)
whose growth on histidine-free medium was galactose
dependent (Figure 1A; data for #081 not shown), suggesting that the modified yeast MAPK cascade was activated through NPK1 in the presence of the protein encoded by #041, #051, or #081 cDNA.
The 3.2 kb cDNA from clone #051 encoded a predicted
protein of 959 amino acid residues (ⵑ108 kDa), whose
amino-terminal region was homologous to the motor
domains that are conserved in members of the KLP
family and included amino acid residues that arestrongly conserved in various KLPs (Figure 1B). We designated this putative protein NPK1-activating kinesinlike protein 1 (NACK1).
The cDNA from clone #041 (1.3 kb) included a nucleotide sequence that was identical to that of the cDNAfrom clone #081 (1.0 kb). An open reading frame in cDNA
#041 encoded an amino acid sequence similar to thatof the carboxy-terminal region of NACK1. We amplified
a 2.2 kb fragment of DNA derived from the 5⬘ end of
cDNA #041 by PCR. These cDNAs encoded a predicted
protein of 955 amino acids that also had a putative motor
domain, and it was designated NACK2 (Figure 1B).
Analysis by the COILS program (Lupas et al., 1991)
predicted the presence of several potential coiled coils,
typical of the stalk domains in KLPs, in the middle regions of NACK1 and NACK2 (Figure 1C). We shall referto regions that include coiled coils and carboxy-terminal
noncoiled-coil regions as stalk and tail domains, respectively. At the amino acid level, identity between NACK1
and NACK2 was 59% overall, 68% for the amino-terminal putative motor domains, and 55% for the stalk/taildomains.
Two putative Arabidopsis KLPs (GenBank accession
numbers AAF25984 and CAB89042) exhibited the highest sequence similarities (77% and 65% identical) to
NACK1 and NACK2, respectively. We designated them
AtNACK1 (its gene maps to chromosome 1) and AtNACK2 (chromosome 3), respectively. Phylogenetic
analysis of motor domain sequences indicated that
NACK1, NACK2, AtNACK1, and AtNACK2 constitute a

subfamily that is closely related to human CENP-E (Yenet al., 1992; Figure 1D). The motor domains of membersof the NACK1 subfamily are 41%–44% identical to that of
CENP-E. Several other putative KLPs from Arabidopsis(e.g., BAA88112) also have motor domains with 45%–
51% identity to that of CENP-E. The stalk and tail domains of members of the NACK1 subfamily did not resemble domains of any KLPs in the database.
NACK1 Activates NPK1 Via Direct Interaction
Some MAPKKKs are activated via direct interactions
with their activator proteins (Shibuya et al., 1996; Posas
and Saito, 1998; Takekawa and Saito, 1998). Therefore,
we examined whether a similar mechanism might apply
to NPK1 and NACK1.
We performed coimmunoprecipitation analysis using
protein extracts from yeast cells that expressed cDNAs
for NACK1 and NPK1 separately and together. NPK1
was immunoprecipitated with NPK1-specific antibodies, and immunoprecipitates were analyzed by Western
blotting with NACK1-specific antibodies (see below). As
shown in Figure 2A, immunoprecipitates contained
NACK1 only after NACK1 and NPK1 had been coexpressed, indicating that NACK1 interacted physicallywith NPK1.
Using a yeast two-hybrid system, we identified theregions required for interactions between NACK1 and
NPK1 (data not shown). In NACK1, a 72 amino acidstretch (residues 685–756), corresponding to the fifth
putative coiled-coil region and strongly conservedin NACK1 and NACK2, was sufficient for interaction
with NPK1 (see Figures 1B and 1C). Conversely, NACK1
binding activity of NPK1 resided in 64 amino acids atthe carboxyl terminus (residues 627–690), which also
formed a coiled coil (Nishihama et al., 1997, 2001). Using
recombinant proteins synthesized in and purified from
Escherichia coli, we also observed direct interaction invitro between the stalk/tail region of NACK1 and theregulatory domain of NPK1 by coimmunoprecipitation(data not shown).
An immunocomplex kinase assay of NPK1 with yeast
cells expressing NACK1 and/or NPK1 (Figure 2B) revealed strong phosphorylation of myelin basic protein(MBP), as substrate, only when both NACK1 and NPK1
had been produced in yeast cells (lane 3). Very limited
phosphorylation was observed in the absence of NACK1(lane 1). When kinase-negative NPK1 (lanes 2 and 4) or
no NPK1 (lane 5) was expressed, similar limited phosphorylation occurred despite the presence of NACK1.
Thus, strong phosphorylation of MBP depended on thepresence of both NPK1 and NACK1, suggesting that
NACK1 might increase the activity of NPK1 via directinteractions between the two proteins. We cannot, however, exclude the possibility that the activation observedin yeast was due to combinatorial effects of some unidentified yeast protein(s).
We observed two additional bands (PB1 and PB2) ofphosphorylated proteins when both NACK1 and NPK1
were produced in yeast cells (Figure 2B, top). We concluded that PB1 and PB2 represented phosphorylated
forms of NACK1 and NPK1, respectively. When aminoterminally truncated NACK1 (which retained the ability
to increase the activity of NPK1) was expressed, weKinesin/MAPKKK Complex Required for Cytokinesis
89

Figure 1. Cloning and Characterization of cDNAs that Encode Activators of NPK1 MAPKKK(A) Selection of cDNAs for proteins that act as activators of NPK1 in yeast cells. Three yeast clones (#041, #051, and #081) were selected,
and plasmids were isolated from them. Yeast cells (11Su1N) were retransformed with the plasmids from clones #041 and #051, as well aswith the empty vector (control). Transformants were streaked on histidine-free medium that contained dextrose or galactose and grown at
30⬚C for 3 days.
(B) Comparison of the deduced amino acid sequences of NACK1 and NACK2. Identical residues are shown in black boxes. Motor domains
are underlined by thin lines. Asterisks mark residues conserved in most KLPs and also in NACK1 and NACK2. The NACK1 region responsible
for binding to NPK1 is underlined by thick lines.
(C) Prediction of coiled-coil structures. The COILS program (http://www.ch.embnet.org/software/COILS_form.html) was used to calculate
probabilities of formation of coiled coils in NACK1 and NACK2 with a window of 28 residues. The box above the graphs shows a schematic
representation of the structures of NACK1 and NACK2. A bar shows the region of NACK1 required for binding to NPK1.
(D) Phylogenetic analysis. Amino acid sequences of various KLP motor domains were aligned with the ClustalX program, and the resultingalignment was used to generate a phylogenetic tree with the PHYLIP program package v3.573c (http://evolution.genetics.washington.edu/
phylip.html). One thousand replicates of the data set were analyzed.

detected PB2 but not PB1. Instead, we detected a newphosphorylated protein with the greater mobility expected of the truncated NACK1 (data not shown). Moreover, the mobility of a significant fraction of wild-type
NPK1 in extracts was reduced when NACK1 had been
coexpressed in cells (Figure 2B, bottom, lane 3), while nosuch mobility shift was observed when kinase-negative
NPK1 was expressed, suggesting the catalytic activitydependent phosphorylation of NPK1. Thus, we proposethat NPK1, bound to NACK1, phosphorylates MBP,
NACK1, and itself.
NACK1 Is a Mitotic KLP that Associateswith NPK1 When Its Activity Is HighWe examined the accumulation of NACK1 and NACK2transcripts during the cell cycle. We synchronized thegrowth of BY-2 cells at the G1/S boundary using aphidicolin. Northern blotting analysis revealed that high levelsof transcripts of both NACK1 and NACK2 accumulated
in cells that had been cultured for 6 to 10 hr after theremoval of aphidicolin, with almost the same time course
as the change in mitotic indices (MIs; Figure 3A).
We performed Western blotting to examine the accumulation of NACK1 proteins. Affinity-purified antibodies
against amino acid residues 385–402 of NACK1 (a synthetic oligopeptide) recognized proteins only in extracts
from M phase cells with a mobility that corresponded
approximately to that predicted for NACK1 (Figure 3B).
After removal of aphidicolin, NACK1 became detectable
at 8 hr, reached its highest level at 10 hr, and became
undetectable at 14 hr. In the same extracts, we detected
NPK1 at a constant level from 0 to 6 hr; the level was

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mitosis during the first hour after removal of propyzamide. Then, MIs fell rapidly to the basal value within 3 hr.
In these cells, NACK1 was detectable at 0 hr; its level
increased during the first hr and then fell rapidly during
the next 2 hr (Figure 3C). We showed previously that
NPK1 activity is high 1 to 2 hr after the start of synchronization (late M phase; Nishihama et al., 2001). Thus,
changes in NPK1 activity were closely correlated withlevels of NACK1.
We examined whether NACK1 was associated with
NPK1 at the late M phase. We prepared an extract of
BY-2 cells that had been harvested 1.5 hr after two-stepsynchronization. Proteins were immunoprecipitatedwith NACK1-specific antibodies that had or had notbeen incubated with the antigenic oligopeptide. Western
blotting revealed not only the recovery of NACK1, butalso of NPK1 in the precipitate only if antibodies had
not been incubated with the oligopeptide (Figure 3D).
These data revealed the physical interaction between
NACK1 and NPK1 at late M phase in tobacco cells at
the time when NPK1 is activated.

Figure 2. Interaction between NACK1 and NPK1 and Activation of
NPK1 by NACK1(A) Coimmunoprecipitation of NACK1 with NPK1 from extracts ofyeast cells. Yeast 11Su1N cells with a galactose-inducible NPK1
construct were transformed with either NACK1-expressing vector(lanes 1 and 2) or the control vector (lanes 3 and 4). Cells were
cultured in the presence of dextrose (lanes 1 and 3) or galactose(lanes 2 and 4). Proteins were extracted from cells, and NPK1 was
immunoprecipitated with NPK1-specific antibodies. Aliquots of immunoprecipitates were analyzed by Western blotting with NPK1specific or NACK1-specific antibodies.
(B) Kinase activity in immunocomplexes. Proteins were extracted
from yeast SY1984 cells that had been transformed with two plasmids: one was either a NACK1-expressing vector (lanes 3–5) or thecontrol vector (lanes 1 and 2); the other was a wild-type NPK1expressing vector (lanes 1 and 3), a kinase-negative NPK1-expressing vector (KW; lanes 2 and 4), or the control vector (lane 5). Immunocomplex kinase assays were performed with NPK1-specificantibodies and MBP as substrate. Aliquots of reaction mixtures were
subjected to SDS-PAGE (8% [H] and 12% [L] polyacrylamide), andphosphorylated proteins were detected by autoradiography (autorad.). Two major bands of phosphorylated protein are indicated (PB1
and PB2). Yeast protein extracts were also analyzed by Western
blotting with NPK1-specific antibodies.

highest at 8 hr, and NPK1 became undetectable at 14hr, as described previously (Nishihama et al., 2001).
We examined the details of NACK1 accumulation during M phase. BY-2 cells were cultured in synchrony
from prometaphase by two-step synchronization withaphidicolin and propyzamide (Kakimoto and Shibaoka,1988). In the synchronized culture for which results areshown in Figure 3C, more than 50% of cells were in

NACK1 Is Probably a Plus End-Directed KLP
A protein fraction (E1), purified biochemically fromphragmoplasts by following the activity of MT-associated motor proteins, was shown by Asada and Shibaoka(1994) to have plus end-directed motor activity and to
consist of polypeptides of 125 kDa and 120 kDa. The
125 kDa protein was the KLP encoded by the TKRP125
gene (Asada et al., 1997). When we analyzed an E1
fraction by Western blotting with NACK1-specific antibodies, we detected a band at the same position as thatof a 120 kDa polypeptide (Figure 3E, lane 2). We repeatedWestern blotting of the same membrane with TKRP125specific antibodies without removing the NACK1-specific antibodies and detected a doublet band that corresponded to polypeptides of 125 kDa and 120 kDa (Figure
3E, lane 3). These observations suggested that NACK1
KLP might be a phragmoplast-associated motor proteinwith plus end-directed motor activity.
NACK1 Is Colocalized with NPK1
at the Phragmoplast Equator
We examined the subcellular localization of NACK1 during M phase by immunofluorescence (Figure 4A). In prophase cells, no NACK1 was clearly detectable. From
prometaphase to early anaphase, NACK1 was dispersedin the cytoplasm, yielding patchy signals. At late anaphase, NACK1 accumulated as a band in the spindle
midzone. From early to late telophase, NACK1 was consistently localized in the equatorial zone of the phragmoplast; it was colocalized with phragmoplast MTs, but
was not found in the inner region of the phragmoplast
where the cell plate matures (see late telophase). Incubation of antibodies with the antigenic oligopeptide prior
to use abolished the patchy cytoplasmic signals andthe signals from the phragmoplast equator (data notshown), confirming that signals of interest were due to
NACK1 that was recognized by the antibodies.
We demonstrated previously that green fluorescentprotein-fused NPK1 protein (GFP-NPK1) is localized at
the equatorial zone of the phragmoplast (Nishihama etal., 2001). When BY-2 cells that expressed GFP-NPK1

Kinesin/MAPKKK Complex Required for Cytokinesis
91

Figure 3. Expression and Immunochemical
Analysis of NACK1 in BY-2 Cells(A) Patterns of accumulation of NACK1 andNACK2 transcripts during the cell cycle. The
cell cycle was arrested at the G1/S boundarywith aphidicolin. At the indicated times (hr)after removal of aphidicolin, cells were harvested and total RNA was extracted. Northernblot analysis was performed with 32P-labeledprobes specific to the NACK1 or NACK2
gene. MIs are shown in the top panel. Stainingof ribosomal RNAs with ethidium bromide isshown in the bottom panel.
(B) Patterns of accumulation of NACK1 during
the cell cycle. Proteins were extracted from
cells prepared as described in the legend topanel (A). Western blotting was performed
with affinity-purified NACK1-specific and
NPK1-specific antibodies. MIs are shown in
the top panel.
(C) Patterns of accumulation of NACK1 proteins from the M to G1 phase. The cell cycle
was arrested at prometaphase with propyzamide after release from the aphidicolin
block. At the indicated times (hr) after removalof propyzamide, cells were harvested andproteins were extracted. Western blotting
was performed with NACK1-specific antibodies. MIs are shown in the top panel.
(D) Physical interaction of NACK1 with NPK1
at late M phase. Proteins were extracted from
BY-2 cells harvested 1.5 hr after release fromthe propyzamide block. Immunoprecipitation
was performed with NACK1-specific antibodies with (lane 2) or without (lane 1) preincubation with the antigenic oligopeptide. Precipitates were analyzed by Western blotting with
NACK1-specific and NPK1-specific antibodies.
(E) Putative identification of NACK1 as a plus
end-directed motor protein associated withphragmoplasts. A fraction (E1 fraction), containing polypeptides of 125 kDa and 120 kDa
with plus end-directed MT motor activity, wasisolated from phragmoplasts (Asada and Shibaoka, 1994). Proteins in this fraction wereseparated by SDS-PAGE and transferred toa nitrocellulose membrane. The membrane was probed with NACK1-specific antibodies (lane 2) and then with TKRP125-specific antibodies(Asada et al., 1997; lane 3). Signals were detected on X-ray film with a chemiluminescence detection kit. Amido black staining of transferred
proteins is shown in lane 1. Note the doublet was detected after probing with the two antibodies.

at telophase were double-stained with NACK1-specificantibodies and 4⬘,6-diamidino-2-phenylindole (DAPI),fluorescent signals derived from GFP and NACK1 weredetected as sharp bands at the same position that corresponded to the equatorial zone of the phragmoplast(Figure 4B, left), demonstrating colocalization of the two
proteins. Upon expression of a DNA construct that encoded a GFP-fused variant of NPK1 (GFP-NPK1:1–627),
which lacked the carboxy-terminal region (residue 628
to the carboxyl terminus) that contains the NACK1 binding site (see above), GFP-NPK1:1–627 was not localized
as a sharp band but appeared to be dispersed throughout the cytoplasm, as was GFP alone, while NACK1 was
localized as a sharp band in the equatorial zone (Figure
4B, middle and right). These results suggested that
NPK1 is recruited to the phragmoplast equator throughthe carboxy-terminal region, probably via association
with NACK1.

Effects of Overexpression of Dominant-Negative
NACK1 Mutant cDNA on Cytokinesis
We postulated that overexpression of truncated NACK1that lacked the motor domain (NACK1:ST) might disrupt
cytokinesis by dominant-negative disturbance of the localization of NPK1. Therefore, we transformed BY-2
cells with a DNA construct for hemagglutinin-tagged
NACK1:ST (HA-NACK1:ST), driven by a dexamethasone(DEX)-inducible transcription system (Aoyama and Chua,
1997). We confirmed that overexpressed HA-NACK1:ST
was coimmunoprecipitated with endogenous NPK1
(data not shown).
Double staining with Calcofluor and propidium iodide(PI) revealed that approximately 10% of cells became
multinucleate upon DEX treatment (Figure 5A, ⫹DEX),
with aberrant cell plates of irregular size that resembled
those generated upon overexpression of kinase-negative NPK1 (Nishihama et al., 2001). This abnormality was

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Figure 4. Subcellular Localization of NACK1and Its Colocalization with NPK1 in BY-2 Cells(A) Localization of NACK1. BY-2 cells at various phases of the cell cycle were triple
stained with NACK1-specific antibodies (red),␣-tubulin-specific antibodies (green), and
DAPI (blue). Merged images are shown at thebottom.
(B) Colocalization of NACK1 with NPK1 at telophase. BY-2 cells harboring constructs for
DEX-inducible GFP-NPK1, GFP-NPK1:1–627,
or GFP were treated with aphidicolin. DEX(0.1 ␮M) was added 12 hr after the start ofaphidicolin treatment, and cells were cultured
for another 12 hr. After removal of aphidicolin,cells were cultured in fresh medium plus 0.1
␮M DEX for 10 hr and double stained with
NACK1-specific antibodies (red) and DAPI
(blue). Fluorescence of GFP (green) was monitored simultaneously. Merged images areshown at the bottom. Bars are 20 ␮m.

not observed when transformed cells were not exposed
to DEX (Figure 5A, –DEX) or when BY-2 cells were transformed with the control empty vector (Nishihama et al.,2001).
We followed progression of cell plate formation inliving cells that had the HA-NACK1:ST construct (Figure
5B). In DEX-untreated cells (–DEX), the cell plate visualized just after chromosome separation (3 min) started

lateral expansion immediately, reaching parental cell
walls at 23 min. By contrast, a significant proportion of
DEX-treated cells exhibited different profiles. In suchcells (⫹DEX), although initial formation of a cell plate (2min) and its slight enlargement (7 min) occurred, nofurther expansion was observed for 52 min. Thus, overexpression of HA-NACK1:ST blocked expansion of thecell plate, suggesting that NACK1, together with NPK1

Figure 5. Cytokinetic Defects and NPK1 Mislocalization upon Overexpression of Truncated NACK1(A) Generation of incomplete cell plates uponoverexpression of motor domain-truncated
NACK1. BY-2 cells, transformed with a DEXinducible HA-NACK1:ST construct, were cultured with (⫹) or without (–) 0.1 ␮M DEX for
3 days. Cells were fixed and double stainedwith Calcofluor (blue) and PI (red). Arrows indicate incomplete cell plates. N1 through N4
indicate nuclei found in individual multinucleate cells. Bar is 50 ␮m.
(B) Time-lapse micrographs of cell-plate formation in living cells. BY-2 cells with DEXinducible HA-NACK1:ST were cultured with(⫹) or without (–) 0.1 ␮M DEX for 2 days. Nomarski images were recorded at indicatedtimes (min) after the start of observation ofanaphase cells.
(C) Inhibition of concentration of NPK1 at thephragmoplast upon overexpression of HANACK1:ST. Cells, prepared as described in
the legend to (A), were triple stained with
␣-tubulin-specific antibodies (green), DAPI
(blue), and NPK1-specific antibodies (red).
Merged images are shown on the right. Nomarski images (DIC) are shown on the left.
Note the multinucleate cell in the panellabeled ⫹DEX. Arrows indicate multiple nuclei in which NPK1 was concentrated. Barsfor (B) and (C) are 20 ␮m.
(D and E) Generation of multinucleate cells in the epidermis of cotyledons upon expression of HA-NACK1:ST. Seeds of a plant transformed
with the DEX-inducible HA-NACK1:ST construct were allowed to germinate on solid medium supplemented with 1 ␮M DEX (⫹DEX) or without
DEX (–DEX).
(D) Surface views of cotyledons of 10-day-old seedlings. Bar is 1 mm.
(E) Normarski images of guard cells and a pavement cell (bottom right) in orcein-stained cotyledons. Bar is 10 ␮m.

Kinesin/MAPKKK Complex Required for Cytokinesis
93(Nishihama et al., 2001), is directly involved in expansionof cell plates.
We triple stained cells that expressed HA-NACK1:ST
with NPK1-specific antibodies, tubulin-specific antibodies, and DAPI. In multinucleate cells, phragmoplasts
formed at telophase and, in a significant proportion ofcells, NPK1 was not concentrated at the phragmoplast
equator (Figure 5C, ⫹DEX). Cells with such NPK1-negative phragmoplasts were never found in cultures of DEXuntreated cells at telophase (Figure 5C, –DEX). We
noted, however, that NPK1 concentrated in pairs ofdaughter nuclei even when cells were treated with DEX(arrows in Figure 5C). These results indicated that
NACK1 functions by recruiting NPK1 to the phragmoplast equator, as proposed above. Inappropriate localization of NPK1 might have been one of the causes ofimperfect formation of a cell plate in cells that expressed
HA-NACK1:ST.
To examine whether overexpression of HA-NACK1:ST
can disturb NACK1 functions in plants, we preparedtransgenic tobacco plants that harbored DEX-inducible
HA-NACK1:ST. After germination on DEX-containing
medium, cotyledons of such plants developed withrough surfaces and were sometimes etiolated (Figure
5D, ⫹DEX). Epidermal cells in cotyledons were most
severely affected (Figure 5E, ⫹DEX): 50% of pavement
cells were multinucleate; 25% of guard cells were binucleate; some cells were anucleate; and, in some cells,
cytokinesis was incomplete or absent. Such phenotypes
can be explained by the arrest or delay of cytokinesis
during the development of pavement and guard cells.
No abnormalities were seen in cotyledons of transgenic
plants in the absence of DEX (Figures 5D and 5E, –DEX).
Similar defects were seen in cotyledons of transgenic
tobacco plants that expressed the kinase-negative mutant of NPK1 (Nishihama et al., 2001).

Loss-of-Function Phenotypes Causedby Silencing of NACK1 and NACK2
To analyze phenotypes caused by loss of function of
NACK1, we exploited a virus-induced gene-silencing(VIGS) system using potato virus X (PVX; Angell and
Baulcombe, 1999). Since possible functional redundancy between NACK1 and NACK2 might attenuate theseverity of defects, we attempted to repress expressionof both genes simultaneously. Fragments of NACK1 andNACK2 cDNAs were introduced downstream of the T7
promoter in a PVX vector (Baulcombe et al., 1995), in
tandem, in the sense orientation. GFP cDNA was introduced into the same site in the vector as a control. PVXNACK1/NACK2 or PVX-GFP transcripts, synthesized invitro, were inoculated onto leaves of Nicotiana benthamiana plants. Fourteen days postinoculation (DPI),
mild mosaic symptoms appeared on the upper leaves
in both series of inoculated plants. PVX-NACK1/NACK2
RNA was detected by reverse transcription-PCR (RTPCR) in such leaves on PVX-NACK1/NACK2 RNA-inoculated plants, and fluorescence due to GFP was detectedin leaves of the control plants (data not shown), indicating successful infection.
By 21 DPI with PVX-NACK1/NACK2 RNA, retardationof growth was apparent in five out of nine infected

plants, which finally stopped growing (Figure 6A; plants
at 30 DPI). All such arrested plants generated small,
humped leaves at a position above inoculated leaves,and these malformed leaves remained small (Figure 6A,part 1). Their one or two abortive flower buds had morphologically normal sepals but prematurely terminatedpetals, stamens, and gynoecia, which eventually underwent necrosis (Figure 6A, part 2; aborted stamens not
visible). By contrast, all of the nine PVX-GFP RNAinfected plants grew normally and generated five to six
flower buds by 30 DPI (Figure 6A). Therefore, the growthdefect seemed to be induced by infection with PVXNACK1/NACK2 RNA and not by disease due to PVX.
We investigated cytokinesis in plants with a growthdefect by examining guard cells in leaves. As shown in
Figure 6B, 20% to 50% of guard mother cells did notdivide to generate guard cells and had two nuclei (Figure
6B, parts 2–4). Some guard cells had incomplete cell
walls (Figure 6B, parts 2 and 3). PVX-GFP RNA-infected
plants developed guard cells normally (Figure 6B, part1). Thus, cytokinesis during development of guard cells
was inhibited in malformed leaves after infection with
PVX-NACK1/NACK2 RNA. Using RNA isolated from tissues in the shoot apex (including small leaves) as template for RT-PCR, we examined levels of NACK1 andNACK2 transcripts derived from the respective endogenous genes. Levels of NACK1 and NACK2 transcriptsof the endogenous genes in PVX-NACK1/NACK2 RNAinfected plants with the phenotypes fell to the limit ofdetection, while such transcripts in PVX-NACK1/NACK2
RNA-infected plants without the phenotypes, PVX-GFP
RNA-infected plants or uninfected plants were clearly
detected (Figure 6C).
Our results suggested that the infection with PVXNACK1/NACK2 RNA silenced expression of NACK1 andNACK2 genes, with resultant cytokinetic defects duringleaf production and, probably, in floral organs. Phenotypes after NACK1/NACK2 VIGS were compatible withthose observed after overexpression of dominant-negative NACK1 cDNA, suggesting that abnormal phenotypes were not caused by artifacts due to overexpression. However, it remains to be determined whether thegrowth defect of lateral organs and plant bodies would
be due to direct effects of the cytokinetic defect caused
by VIGS.
Disruption of AtNACK1 Causes Cytokinetic
Defects in Arabidopsis Embryos
Using PCR-based screening, we identified two mutant
alleles of the AtNACK1 gene (see Figure 1D), atnack1-1
and atnack1-2, with T-DNA insertions in the putative
5th and 12th exons that encode parts of the motor domain
and the stalk region, respectively (Figure 7A).
Homozygous atnack1-1 mutants exhibited growthdefects, and plants were dwarfed with small, humped
cotyledons and leaves (Figure 7B). Development of
atnack1-1 plants ceased at the vegetative stage, andthey died without any traces of production of reproductive organs (data not shown). The atnack1-2 plants had
essentially identical defects (data not shown). Therefore,both alleles were maintained in heterozygous plants,
which were morphologically indistinguishable from wildtype plants.

Cell
94

When heterozygous atnack1-1 and atnack1-2 plants
were self-pollinated, 18.8% (n ⫽ 213) and 20.7% (n ⫽285) of their progeny, respectively, exhibited a mutant
phenotype 10 days after germination on solid medium,indicating the recessive nature of the mutation. Genetic
linkage analysis of atnack1-1 (no recombinant/140
meioses) indicated that the mutation was closely linked
to the T-DNA insertion in the AtNACK1 gene. In addition,a normal phenotype was restored by a transgene that
contained the AtNACK1 genomic region (data notshown). Thus, the T-DNA inserted in the AtNACK1 gene
was responsible for the observed phenotype.
The morphological defects in cotyledons of atnack1
mutants were apparent immediately after germination,so we examined the morphology of embryonic cells inseeds collected from plants heterozygous for the atnack1-1 or atnack1-2 mutation. Some seeds (20.1% [n ⫽199] for atnack1-1; 22.7% [n ⫽ 229] for atnack1-2) contained malformed embryos composed of cells with single nuclei and some multinucleate cells with incomplete
cell walls (Figure 7C), indicating the occasional failure
in cytokinesis during embryogenesis. It is possible thatsuccessful cytokinetic events were due to redundantfunctions of the AtNACK2 gene. Our observations suggested that mutations in the AtNACK1 gene resulted ina defect in the production of cell walls, which was, atleast in part, responsible for the phenotypes of mutant
seedlings. Thus, the AtNACK1 gene is required forproper cytokinesis, and the functions of NACK1-related
genes are conserved in different plant species.

Discussion

Figure 6. Phenotypes Generated by Suppression of NACK1 and
NACK2(A) Leaves of 1-month-old plants of N. benthamiana were inoculatedwith PVX-NACK1/NACK2 or PVX-GFP RNA. Gross morphology oftypical plants, 30 DPI, is shown. The insets show magnified viewsof the shoot apex (part 1) and an aborted flower (part 2) of a PVXNACK1/NACK2 RNA-inoculated plant. Necrotic petals (brown) andsmall green carpels are visible.
(B) Nomarski images of orcein-stained guard cells on the second
highest leaves of a PVX-GFP-infected plant (part 1) and a PVXNACK1/NACK2 RNA-infected plant (parts 2–4), 30 DPI. These are
the same plants as shown in (A).
(C) RT-PCR analysis. RNA was prepared from shoot apices containing small leaves, of an uninfected plant (lane 1), of a plant witha growth defect (⫹) 35 DPI with PVX-NACK1/NACK2 RNA (lane 2),of a plant with a normal phenotype (–) 35 DPI with PVX-NACK1/
NACK2 RNA (lane 3), or of a PVX-GFP RNA-inoculated plant (lane4). RT-PCR was performed for NACK1 (top), and NACK2 (middle)
with primers designed to amplify cDNAs derived only from endogenous genes and for EF1␣ as control (bottom). Numbers of cyclesof PCR are indicated on the right.

Molecular Mechanism of Activationof NPK1 by NACK1
Our genetic (Figure 1A) and biochemical (Figure 2B) evidence indicates that NACK1 is essential for activationof NPK1 in yeast cells, and considerable evidence indicates that NACK1 binds to NPK1 in vivo and in vitro.
Therefore, NACK1 probably activates NPK1 via directinteraction.
NACK1 binds to the carboxy-terminal negative-regulatory domain of NPK1, perhaps via interaction between
coiled coils formed by 72 amino acids in the stalk regionof NACK1 (Figure 1C) and 64 amino acids at the carboxyl
terminus of NPK1 (Nishihama et al., 1997). Association of
NPK1 with NACK1 might release NPK1 from an inactive
conformation. However, the extent of suppression of
mutations in yeast MAPKKK genes by coexpression of
wild-type NPK1 and NACK1 was greater than that observed upon expression exclusively of the truncated
NPK1 (data not shown), suggesting that the release,alone, of the catalytic domain from auto-inhibition conferred by the regulatory domain is insufficient for full
activation of NPK1. Autophosphorylation of NPK1 andphosphorylation of NACK1 by NPK1 might occur whenthese proteins have already formed a complex (Figure 2). Hence, the activity of NPK1 might be further
enhanced by autophosphorylation and the action(s)of phosphorylated NACK1, as proposed for other
MAP-KKKs (Posas and Saito, 1998; Kishimoto et al.,
2000; Sakurai et al., 2000).

Kinesin/MAPKKK Complex Required for Cytokinesis
95

Figure 7. Mutations in the AtNACK1 Gene Affect Cytokinesis(A) Schematic diagram of the AtNACK1 gene.
Boxes represent exons. The putative motor
domain, ATP binding site (asterisk), and
NPK1 binding site are indicated. T-DNA insertion sites in atnack1-1 and atnack1-2 are also
shown. T-DNA inserts are not drawn to scale.
(B) Gross morphology of wild-type (ecotype
Wassilewskija) and atnack1-1 plants 23 days
after germination. Inset: magnified view of themutant. Bar is 1 mm.
(C) Sections of wild-type and atnack1 mature
seeds stained with toluidine blue. Top: Whole
views of single seeds. Bottom: Magnified
views of boxed regions in corresponding upper images. Longitudinal sections of hypocotyls (wild-type and atnack1-2 [left]) and a
cotyledon (atnack1-1) and a transverse section of a hypocotyl (atnack1-2 [right]) are
shown. Arrows indicate incomplete cell walls.
Arrowheads indicate nuclei in multinucleate
cells. Bar for (Top) is 50 ␮m. Bar for (Bottom)
is 10 ␮m.

M Phase-Specific Expression of NACK1
and NACK2
It is concluded that NACK1-mediated activation of NPK1
occurs at the end of M phase since, at this time, theactivity of NPK1 (Nishihama et al., 2001) and the levelof NACK1 (Figure 3C) are both highest and moreover,these proteins associate (Figure 3D). The M phase-specific accumulation of NACK1 proteins is, at least in part,
attributed to that of NACK1 transcripts (Figure 3A). Transcription of the NACK1 and NACK2 genes is regulated
by cis elements known as M-specific activators (MSAs;
Ito et al., 1998). Recently, MSA-dependent transcription
has been shown to be controlled by novel c-Myb-like
transcription factors (Ito et al., 2001). Detailed mechanisms for the regulation of MSA-dependent transcription
by such proteins must be clarified if we are to understand the temporal regulation of the expression of
NACK1 and NACK2.
NACK1-Related KLPs Unique to Plants
A few KLPs, such as KIF3X and kinesin-I, are involved
in regulation of the transport or localization of components of MAPK cascades (Nagata et al., 1998; Verhey
et al., 2001). However, no KLPs have been reported, to
our knowledge, to activate associated protein kinases.
Animal KLPs in the MKLP1 family concentrate at thespindle midzone and the midbody; they are essential
for organization of midzone MTs and subsequent constriction of dividing cells during cytokinesis (Adams etal., 1998; Raich et al., 1998). Some members of thisfamily colocalize with Polo-like kinases or Aurora kinases, which are also required for cytokinesis (Lee etal., 1995; Adams et al., 1998; Severson et al., 2000).
These features of the MKLP1 family are similar to
those of NACK1. However, amino acid sequences ofmotor domains indicate that NACK1 does not belong to
the MKLP1 family but is most closely related to CENP-E(Figure 1D), which concentrates at the spindle midzone
and the midbody during late M phase (Yen et al., 1991)
and associates with a MAPK (Zecevic et al., 1998). It
remains unclear whether CENP-E associates with a
MAPKKK and/or whether it plays a role in cytokinesis.
We were unable to demonstrate the centromeric localization of NACK1. NACK1-related KLPs in plants might
participate in a process unique to plant cytokinesis.
The Role of the NACK1/NPK1 Complex
in Cell Plate Formation
The key site for cell plate formation is the equatorialzone of the phragmoplast, where two bundles of MTs
are arranged with interdigitating plus ends. Lateralexpansion of the cell plate is accompanied by MT disas-

Cell
96

sembly at the inside and MT reassembly at the outsideof the phragmoplast. When depolymerization of phragmoplast MTs is blocked by taxol, phragmoplasts nolonger expand laterally and, coincidentally, incomplete
cell plates are formed, indicating that phragmoplast
expansion requires disassembly of MTs and allows thecell plate to expand (Yasuhara et al., 1993).
We showed previously that overexpression of kinasenegative NPK1 inhibits expansion of both the phragmoplast and the cell plate, as does taxol, suggesting arole for NPK1 in regulation of the dynamics of phragmoplast MTs (Nishihama and Machida, 2001; Nishihama
et al., 2001). We demonstrated here that overexpressionof dominant-negative NACK1 inhibits expansion of thecell plate (Figure 5B), a phenomenon that suggests that
NACK1 is involved in the same process as NPK1. Thus,
NACK1, complexed with NPK1, might regulate depolymerization of MTs during phragmoplast expansion. Thishypothesis is supported by the observation that N. benthamiana plants in which NACK1 and NACK2 expression
was repressed by VIGS developed multinucleate guard
cells (Figure 6) and that Arabidopsis atnack1 mutant
embryos contained cells with multiple nuclei and cellwall stubs (Figure 7).
Recently, Strompen et al. (2002) have reported thatplant cytokinesis requires HINKEL (HIK) KLP of Arabidopsis, which is identical to AtNACK1 we identified.
Phenotypes of hik mutants are consistent with our results, including those of atnack1-1 and atnack1-2 (e.g.,
the generation of multinucleate cells and incomplete cell
walls), except that only fully expanded phragmoplasts
were found in embryo cells of hik. The presence of suchphragmoplasts seems to be somewhat different fromthe results of the arrest of cell plate and phragmoplast
expansion by kinetic analyses upon expression of thedominant inhibitory NACK1:ST (Figure 5B) and NPK1KW(Nishihama et al., 2001), respectively. This might be due
to different experimental systems; in our experimentswith tobacco cells and plants, functions of both NACK1
and NACK2 may have been interfered with by the dominant-negative forms and VIGS, while AtNACK2 may have
replaced at least in part functions of HIK and AtNACK1
in Arabidopsis.
During telophase, both NACK1 and NPK1 are concentrated at the equator of the phragmoplast. We proposethat NPK1 is transported by NACK1 for the following
reasons. First, truncated NPK1 proteins lacking the
NACK1 binding site were incapable of equatorial localization (Figure 4B). Second, truncated NACK1 proteinsthat lacked the motor domain prevented localization of
NPK1 to the phragmoplast equator (Figure 5C). Movement of NACK1 to the plus ends of phragmoplast MTs
appears to involve its motor activity and, indeed, NACK1
was purified from phragmoplasts in a fraction with plus
end-directed motor activity (Figure 3E).
The activity and positioning of NPK1 appear to be
critical for cytokinesis. Activation of NPK1 by binding
to KLPs, such as NACK1 that is localized to the phragmoplast equator could provide a simple and robust
mechanism for precise regulation of NPK1. How might
NPK1 signaling be involved in cell plate formation? We
recently identified a MAPKK (NQK1) and a MAPK (NRK1)
as putative factors that act downstream of NPK1 (T.S.,
R.N., and Y.M., unpublished data). We shall try to deter-mine whether these kinases, and also p43Ntf6 (Calderini
et al., 1998), are involved in cell plate formation andfunction downstream of NPK1. However, one target of
NPK1 might be a regulator of MT dynamics. Some KLPs
have MT-depolymerizing activity (Hunter and Wordeman, 2000). Since NPK1 can phosphorylate NACK1, it
is possible that NACK1, phosphorylated by NPK1, might
control MT dynamics. Further investigation on functionsof NACK1 and NPK1 should help to clarify mechanisms
for the inside-out nature of cell plate expansion.
Experimental ProceduresPlant and Yeast Materials
Maintenance and transformation of Nicotiana plants (N. tabacum
SR1 and N. benthamiana), BY-2 tobacco cultured cells, and Arabidopsis thaliana plants are described elsewhere (Nishihama et al.,
2001; Tanaka et al., 2001). Saccharomyces cerevisiae SY1984(MAT␣ leu2 ura3 trp1 his3⌬::ura3 pep4⌬::ura3 can1 FUS1::HIS3
ste11⌬::ura3; Stevenson et al., 1992) was used for preparation of
11Su1 cells, in which CYC1 promoter-fused Dsor1Su1 cDNA (Tsudaet al., 1993) was integrated at the trp1 locus. GAL1 promoter-fused
NPK1 cDNA was integrated at the leu2 locus of 11Su1 to generate
11Su1N cells.
Isolation of Tobacco cDNAs for Activators of NPK1
Using Yeast Cells
Total RNA from BY-2 cells, 3 days after subculture, was isolated,and poly(A)⫹ RNA was purified as described elsewhere (Banno etal., 1993). cDNAs were synthesized from the poly(A)⫹ RNA andcloned into plasmid pKT11 (URA3 marker; see below). Yeast 11Su1N
cells were transformed with the cDNA library and grown on synthetic
galactose (SG) medium without tryptophan (Trp), leucine (Leu), uracil(Ura), and histidine (His) at 30⬚C. Of 3 ⫻ 105 transformants, 150
clones formed colonies within 2 weeks. These clones were further
selected for failure to grow on synthetic dextrose (SD) medium without Trp, Leu, Ura, and His and for failure to grow on SG medium
without Trp, Leu, and His after cells had been cured of plasmids bytreatment with 5-fluoroorotic acid.
Construction of Plasmids
A linker fragment, prepared by annealing the oligonucleotides
5⬘-AATTGGTCGACGAATTCTCTAGAGCGGCCGC-3⬘ and 5⬘-TCGAGCGGCCGCTCTAGAGAATTCGTCGACC-3⬘, was inserted into pKT10(Tanaka et al., 1989) that had been digested with EcoRI and SalI toyield pKT11. YCpGT (TRP1 marker) was generated by inserting a
SalI-NdeI (filled-in) fragment of pGT5 (Miyajima et al., 1987) into the
SalI and filled-in EcoRI sites of YCplac22 (Gietz and Sugino, 1988).
YCpGT-NPK1 and YCpGT-NPK1KW were generated by inserting an
XbaI-StuI fragment of pNPK1-#6 (Banno et al., 1993) or pNPK1KW(Nishihama et al., 2001), respectively, into YCpGT that had beendigested with XbaI and SmaI. For DEX-inducible expression of proteins in BY-2 cells, DNA encoding HA-NACK1:ST or GFP-NPK1:1–
627 was cloned into pTA7001 (Aoyama and Chua, 1997) to yield
pTA71-HA-NACK1:ST and pTA71-GFP-NPK1:1–627, respectively.
To generate PVX constructs using the pP2C2S vector (Baulcombe
et al., 1995), we amplified a 380 bp fragment of NACK1 cDNA anda 358 bp fragment of NACK2 cDNA, which both include regions
encoding the NPK1 binding sites, by PCR, using ologonucleotide primers N1F2194 (5⬘-GCCGTTCCAGTTCTGTGAAC-3⬘) andN1R2573 (5⬘-AGTTCAACTTCCAGATATATTTC-3⬘) for NACK1 andN2F2175 (5⬘-GGCAGTCAAACTCTGTTAAT-3⬘) and N2R2532(5⬘-AGCTCAACTTCCAAGTATATC-3⬘) for NACK2. Both were inserted into pP2C2S such that they were arranged in the sense orientation downstream of the T7 promoter to yield pP2C2S-NACK1/
NACK2. GFP cDNA (Chiu et al., 1996) was introduced into pP2C2S
to yield pP2C2S-GFP. Details of construction of these plasmids areavailable on request.
Production and Purification of Antibodies
Antibodies against NACK1 were prepared by MBL (Nagoya, Japan)
with a synthetic peptide, CTPDPANEKDWKIQQMEME (residues

Kinesin/MAPKKK Complex Required for Cytokinesis
97

385–402 of NACK1), as antigen. Specific antibodies were affinitypurified with the antigenic oligopeptide (Nishihama et al., 2001).
Coimmunoprecipitation and Immunocomplex Kinase Assays
We cultured yeast 11Su1N cells that harbored pKT11-051 or pKT11
in 50 ml of liquid SD or SG medium without Trp, Leu, and Ura until
absorbance at 600 nm reached 1.0. Cells were washed twice withchilled water and resuspended in 500 ␮l of TG250Y buffer (TG250
buffer [Nishihama et al., 2001] prepared with 15 mM, instead of 10
mM, EGTA, and without EDTA and bovine serum albumin [BSA]).
Cells were disrupted with glass beads. Cleared lysates containing
400 ␮g of protein were diluted with 500 ␮l of T150Y buffer (TG250Y
buffer prepared with 150 mM NaCl instead of 250 mM NaCl andwithout glycerol) and incubated with 1 ␮g of NPK1-specific antibodies (P557-2; Nishihama et al., 2001) on ice for 90 min, followed by
60 min incubation at 4⬚C with 20 ␮l of a 50% slurry of protein
A-Sepharose (Amersham Pharmacia Biotech, Piscataway, NJ). The
resin was washed four times in T150Y buffer, resuspended in 80 ␮lof 1⫻ sample buffer for SDS-PAGE, and boiled.
For assays of kinase activity, yeast SY1984 cells harboring
YCpGT, YCpGT-NPK1, or YCpGT-NPK1KW plus either pKT11 or
pKT11-051 were cultured in 50 ml of SG medium without Trp and Ura.
NPK1 was immunoprecipitated from extracts (500 ␮g of protein), as
described above. The resin was washed four times in T150Y buffer
and twice in kinase buffer (Nishihama et al., 2001). Precipitates were
resuspended in a reaction cocktail (20 ␮l) that included 1 ␮g of MBP(Sigma, St. Louis, MO), 50 ␮M ATP, and 10 ␮Ci of [␥-32P]ATP inkinase buffer and incubated for 30 min at 25⬚C.
Coimmunoprecipitation analysis with BY-2 cells was performed
as follows. Frozen BY-2 cells were ground to a powder in liquid
nitrogen. The powder was suspended in an equal volume of PG150
buffer (TG250 buffer [Nishihama et al., 2001] prepared with 20 mM
PIPES [pH 6.2] instead of 25 mM Tris-HCl [pH 7.5] and 150 mM NaCl
instead of 250 mM NaCl, without BSA). Cleared lysate containing 6
mg of protein was incubated on ice for 2 hr with 2 ␮g of NACK1specific antibodies that had or had not been incubated with a 70fold molar excess of antigenic oligopeptide. Precipitates wereprepared as described above with PG150 buffer for washing andresuspension in 30 ␮l of 1⫻ sample buffer for SDS-PAGE.
Northern Blotting Analysis
Northern blotting was performed with total RNA (10 ␮g) from synchronized BY-2 cells using 32P-labeled gene-specific probes for
NACK1 and NACK2 (SacI-BstPI and StyI-BsmI segments of the respective cDNAs) as described elsewhere (Banno et al., 1993).
VIGS Technology
VIGS technology is described in detail on the Internet (http://
www.jic.bbsrc.ac.uk/Sainsbury-Lab/david-baulcombe/objectives/
gstech.htm). Infectious PVX RNAs were transcribed in vitro with
pP2C2S-NACK1/NACK2 or pP2C2S-GFP DNA as template. Leavesof nine N. benthamiana plants were inoculated with transcribed
RNAs. Inoculated plants were grown with 16 hr of light daily at 23⬚C
in a growth chamber. Levels of transcripts of endogenous NACK1
and NACK2 genes were examined by quantitative RT-PCR (Semiarti
et al., 2001) using primers NAK1-2091F (5⬘-TAGAGCTCCAGATTCG
GATG-3⬘) and NAK1-3081R (5⬘-CCAAAATTGGGAGGCTCCAG-3⬘)
for NACK1 and NAK2-1810F (5⬘-TGCACGGCTGAGATCTCAGG-3⬘)
and NAK2-2740R (5⬘-TCCATCAGGTGGAATGTCCC-3⬘) for NACK2.
To detect levels of control EF1␣ transcripts, we used primers EFF(5⬘-AGACCACCAAGTACTACTGC-3⬘) and EFR (5⬘-GTCAAGAGCCTCAAGGAGAG-3⬘) (Ingram et al., 1999).
Isolation of T-DNA-Tagged atnack1 Mutants
We screened 133,440 T-DNA-tagged Arabidopsis plants generated
at the Arabidopsis Knockout Facility in the University of Wisconsin(60,480 lines from the ALPHA population; Krysan et al., 1999; 72,960
lines from the BASTA population) for atnack1 mutations by PCR
according to the instructions of www.biotech.wisc.edu/Arabidopsis.
Primers specific for the T-DNA left-border (5⬘-CATTTTATAATAACG
CTGCGGACATCTAC-3⬘) and AtNACK1 (AtNA1-1F, 5⬘-ACCTTTT
TATGGATTAAGTCTGCACAATG-3⬘ and AtNA1-1R, 5⬘-CATAATCAGACGAGTCTGGTAAAGCTTAT-3⬘) were used.

Microscopy
All techniques for microscopic analysis were described previously(Nishihama et al., 2001; Tanaka et al., 2001). Sections of Arabidopsis
seeds were made from siliques that were collected and fixed 7 days
after flowering.
Acknowledgments
The authors thank Kunihiro Matsumoto for the yeast screening system; David Baulcombe for the PVX vector (pP2C2S); Shoji Sonoda
and Masamichi Nishiguchi for providing N. benthamiana plants andhelpful discussions; Hiroki Yasuhara, Takahide Yokoi, Keisuke Matsui, and Toshiya Muranaka for technical assistance and helpful
discussions; and Sangho Jeong for critical reading of the manuscript. This work was supported by Grants-in-Aid for Scientific Research on Priority Areas (0678101-3 and 10182101-2) from the Japanese Ministry of Education, Science, Culture, and Sports and by agrant from the Research for the Future Program of the Japan Society
for the Promotion of Science (JSPS). R.N. was supported by Research Fellowships from JSPS.
Received: September 17, 2001
Revised: February 28, 2002
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Accession Numbers
Sequences of NACK1, NACK2, and AtNACK1 cDNAs have been
deposited in GenBank under accession numbers AB071435,
AB071436, and AB081599, respectively.