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Cell, Vol. 91, 357–366, October 31, 1997, Copyright 1997 by Cell Press
CENP-E Is a Plus End–Directed Kinetochore
Motor Required for Metaphase Chromosome Alignment
Kenneth W. Wood,* Roman Sakowicz,†
Lawrence S. B. Goldstein,†
and Don W. Cleveland*‡
* Laboratory of Cell Biology
Ludwig Institute for Cancer Research
† Howard Hughes Medical Institute
Division of Cellular and Molecular Medicine
University of California at San Diego
La Jolla, California 92093-0660
Summary
Mitosis requires dynamic attachment of chromosomes to spindle microtubules. This interaction is mediated largely by kinetochores. During prometaphase,
forces exerted at kinetochores, in combination with
polar ejection forces, drive congression of chromosomes to the metaphase plate. A major question has
been whether kinetochore-associated microtubule motors play an important role in congression. Using immunodepletion from and antibody addition to Xenopus
egg extracts, we show that the kinetochore-associated kinesin-like motor protein CENP-E is essential
for positioning chromosomes at the metaphase plate.
We further demonstrate that CENP-E powers movement toward microtubule plus ends in vitro. These
findings support a model in which CENP-E functions
in congression to tether kinetochores to dynamic microtubule plus ends.
Introduction
Accurate segregation of genetic material is mediated by
the microtubules of the mitotic spindle. Spindle microtubules have a defined polarity, with their slow-growing
minus ends anchored at or near the spindle pole, and
their dynamic, fast-growing plus ends interacting with
chromosomes and with microtubules emanating from
the opposite pole (McIntosh and Euteneur, 1984). Interaction of chromosomes with spindle microtubules is
mediated by kinetochores, specialized microtubule attachment structures located at the centromeric region
of each sister chromatid (reviewed in Rieder, 1982;
Mitchison, 1988). During prometaphase, kinetochores
capture and stabilize dynamically unstable microtubules
growing from the poles (Nicklas and Kubai, 1985; Mitchison et al., 1986). Once attached to microtubules, kinetochores and chromosomes exhibit oscillatory movements, switching between poleward and antipoleward
movement (Roos, 1976; Bajer, 1982; Rieder et al., 1986;
Skibbens et al., 1993; Khodjakov and Rieder, 1996; Waters et al., 1996) and culminating in alignment at the
metaphase plate. These movements are collectively referred to as congression, and are thought to be, at least
in part, a consequence of forces generated by microtubule motors localized to the kinetochore (Rieder
‡ To whom correspondence should be addressed.
and Salmon, 1994). Initially, the oscillatory movements
of bioriented chromosomes have a net bias toward
the spindle equator. Once an equatorial position is
achieved, chromosomes continue to oscillate much in
the same manner as during initial movement to the spindle midzone, except that these movements yield little
net displacement from the metaphase plate (Skibbens
et al., 1993; Khodjakov and Rieder, 1996). Stable metaphase chromosome positioning and the preceding prometaphase chromosome movements that establish
metaphase alignment are thus a consequence of the
balance or imbalance, respectively, of poleward and
antipoleward movements and are almost certainly temporally distinct halves of the same kinetochore-dependent process.
In many meiotic spindles, as well as early embryonic
mitotic spindles, such as in Drosophila (Theurkauf and
Hawley, 1992; Matthies et al., 1996; Wilson et al., 1997),
spindle assembly differs from the more typical mitotic
examples in that the spindles form by an “inside-out”
mechanism in which microtubules organize in the absence of centrosomes around a centrally located mass
of chromatin. Bipolarity is achieved by the action of
multivalent minus end–directed microtubule motor complexes, including NuMA, cytoplasmic dynein, and dynactin in frogs (Heald et al., 1996, 1997; Merdes et al.,
1996) and the kinesin-like motor Ncd in Drosophila (Matthies et al., 1996). These motor complexes tether parallel
microtubule bundles and stabilize converging microtubules into spindle poles on either side of the centrally
located chromatin. Compared to typical mitotic spindles, initial congression of chromosomes to the spindle
equator in these meiotic and early mitotic spindles
would seem to play at most a minor role in establishing
metaphase chromosome alignment. However, the second phase of congression, maintenance of alignment
prior to anaphase, is nonetheless of undiminished importance.
The two predominant and opposing forces currently
thought to be responsible for chromosome movement
during congression are an antipoleward polar ejection
force associated with regions of high microtubule density near the spindle poles and forces generated directly
at the kinetochore by microtubule-dependent motors.
Studies in vitro have demonstrated the presence of both
plus end– and minus end–directed microtubule motor
activities on prometaphase kinetochores (Mitchison and
Kirschner, 1985; Hyman and Mitchison, 1991). However,
microsurgical separation of sister kinetochores of congressing bioriented chromosomes results in continued
poleward movement of the kinetochore that was leading
at the time of severance (Khodjakov and Rieder, 1996).
The trailing kinetochore ceases to move. Laser ablation
of the leading kinetochore also causes the trailing kinetochore to halt. Furthermore, examination of the distance
separating sister kinetochores during congression reveals that pushing forces rarely result in compression
of the intrakinetochore distance (Waters et al., 1996).
Together these findings suggest that during congression kinetochore-associated minus end–directed pulling
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forces dominate, with plus end–directed pushing forces
playing at most a minor role in driving kinetochoremediated chromosome movement. The outstanding issue, however, has been the identity of the molecules at
the kinetochore that generate the force for initial chromosome movement and maintenance at the mitotic
midzone.
A prominent candidate for powering one or more aspects of chromosome movement in mitosis is centromere-associated protein-E (CENP-E), a member of the
kinesin superfamily of microtubule motor proteins that
is an integral component of kinetochore corona fibers
that link centromeres to spindle microtubules (Yen et al.,
1992; Yao et al., 1997). CENP-E localizes to kinetochores
throughout all phases of mitotic chromosome movement (early prometaphase through anaphase A) (Yen et
al., 1992; Lombillo et al., 1995a; Brown et al., 1996).
Previous efforts have suggested a role for CENP-E in
mitosis and meiosis. Microinjection of a monoclonal antibody directed against CENP-E into cultured human
cells delays anaphase onset (Yen et al., 1991). AntiCENP-E antibody injection into maturing mouse oocytes
induces arrest at the first reductional division of meiosis
(Duesbery et al., 1997). Antibodies against CENP-E
block microtubule depolymerization-dependent minus
end–directed movement of purified chromosomes in
vitro (Lombillo et al., 1995a). Recently, CENP-E was reported to be associated with minus end–directed microtubule motor activity (Thrower et al., 1995), raising the
possibility that CENP-E might be responsible for poleward kinetochore movements in prometaphase and
anaphase A.
We report here that quantitative removal of Xenopus
CENP-E from Xenopus egg extracts, normally capable
of assembling metaphase mitotic spindles in vitro, disrupts metaphase chromosome alignment. We also demonstrate that the kinesin-like motor domain of Xenopus
CENP-E powers movement toward microtubule plus
ends. Together these findings suggest that CENP-E
functions in vivo at the trailing kinetochore to link antipoleward movement to microtubule growth and at the
leading kinetochore to couple microtubule disassembly
dynamics to minus end–directed movement.
Results
Identification of Xenopus CENP-E
To investigate the role of CENP-E in mitotic spindle
formation in vitro using extracts of Xenopus eggs, we
used low stringency hybridization followed by library
rescreening to clone the Xenopus homolog of CENP-E.
The full-length sequence encodes a protein of 2954
amino acids with a predicted molecular mass of 340
kDa (Figures 1A and 1C). The predicted structure of
Xenopus CENP-E (XCENP-E) is very similar to human
CENP-E (hCENP-E), consisting of an amino-terminal region containing a kinesin-like microtubule motor domain, linked to a globular tail domain by a region predicted to form a long, discontinuous a-helical coiled coil
(Lupas et al., 1991; Berger et al., 1995) (Figure 1A). Within
the core of the motor domain (residues 1–324), XCENP-E
and hCENP-E share 74% identity, significantly greater
than that shared between XCENP-E and its nearest phylogenetic neighbors (Moore and Endow, 1996). Outside
XCENP-E’s amino-terminal domain lie three additional
regions of greater than 25% identity with hCENP-E, but
not with other kinesin-like proteins (Figure 1A). On the
basis of these regions of identity and its comparably
large predicted size, we conclude that XCENP-E is the
Xenopus homolog of human CENP-E.
XCENP-E Associates with Xenopus Centromeres
In Vivo and In Vitro
To verify that Xenopus CENP-E exhibits a pattern of
cell cycle-dependent kinetochore association similar to
human CENP-E, polyclonal antibodies were raised against
two recombinant antigens, one spanning the tail and
C-terminal portion of the rod (a-XCENP-E TAIL, Figure 1B)
and the other corresponding to a portion of the N terminus of the rod domain (a-XCENP-E ROD; Figure 1B). Immunoblotting of Xenopus egg extract reveals that the
a-XCENP-ETAIL antibody specifically recognizes XCENP-E
as a single band of greater than 300 kDa (Figure 2B).
The a-XCENP-E ROD antibody specifically recognizes
XCENP-E (arrowhead, Figure 2B) and another protein
of slightly lower molecular weight (indicated by dot in
Figure 2B) that may be a distinct isoform of XCENP-E
lacking the tail domain or XCENP-E that has lost its tail
domain due to partial proteolysis. Immunostaining of
cultured Xenopus XTC cells using a-XCENP-ETAIL antibody (Figure 2C) revealed patterns of cell cycle–dependent localization similar to that observed for mammalian
CENP-E (Yen et al., 1992; Brown et al., 1996) with two
exceptions: (1) during interphase XCENP-E was localized to the nucleus (Figure 2C, panels 1 and 2), consistent with the presence of a nuclear localization signal
(Boulikas, 1993) at the C-terminal end of the rod domain
(Figures 1A and 1C), and (2) a proportion of XCENP-E
could occasionally be found localized to mitotic spindle
poles (e.g., Figure 2C, panels 5 and 6). a-XCENP-E TAIL
immunostaining of metaphase spindles assembled in
vitro using cytostatic factor (CSF)-arrested Xenopus egg
extracts cycled through interphase (Murray, 1991; Sawin
and Mitchison, 1991) revealed that XCENP-E was almost
exclusively associated with kinetochores (Figure 2A,
panel 2). Unlike in cultured cells, XCENP-E was infrequently found localized to poles of spindles assembled
in vitro.
XCENP-E Is Required for Metaphase
Chromosome Alignment
To determine the aspect(s) of mitosis for which
XCENP-E is required, a-XCENP-ETAIL antibody was used
to deplete XCENP-E from Xenopus egg extracts naturally arrested in metaphase (CSF-extract). Immunoblotting of control and XCENP-E-depleted CSF-extracts revealed that of XCENP-E could be quantitatively removed
by immunodepletion with this antibody (Figure 3A, lane
2). Unrelated antigens, such as XNuMA (Merdes et al.,
1996), were unaffected by depletion of XCENP-E (Figure
3A, lower panel). The specificity of depletion was further
confirmed by comparison of the proteins recovered in
the a-XCENP-ETAIL precipitate (Figure 3D, lane 2) with
those present in the control IgG precipitate (Figure 3D,
CENP-E Is Required for Chromosome Alignment
359
Figure 1. Identification of Xenopus CENP-E
(A) Structural comparison of Xenopus and human CENP-E. Hatched regions represent regions predicted to form a-helical coiled coils
(Lupas et al., 1991). Within the N-terminal regions of both hCENP-E and XCENP-E, there
is a domain of z324 amino acids corresponding to the kinesin-like motor domain.
Within these 324 amino acids, XCENP-E
and hCENP-E are 74% identical. Overall
XCENP-E and human CENP-E are 37% identical and, with conservative substitutions,
57% similar. Unshaded regions within the rod
are those regions sharing 47% identity with
human CENP-E. One cDNA clone encoded a
protein with a 9 amino acid insertion relative
to other cDNAs isolated (marked by the arrowhead, see Figure 1C). XCENP-E contains
a putative nuclear localization signal (NLS)
located at the C-terminal end of the rod domain not present in hCENP-E.
(B) XCENP-E fusion proteins used for polyclonal antibody production.
(C) Deduced amino acid sequence of Xenopus CENP-E. cDNA sequence was compiled
from 6 overlapping cDNA clones. Residues
identical in hCENP-E and XCENP-E are
shaded. The boxed region is the portion of
XCENP-E used to assay motility in vitro (see
Figure 6). The underlined sequence NSREHSINA
at position 599 is the 9 amino acid relative
insertion encoded by one of the cDNAs isolated (see Figure 1A). The putative NLS,
RKKTK, is also underlined. Nucleotide sequence is available from EMBL/GenBank (see
end of this article).
lane 1). XCENP-E was specifically recovered in precipitates prepared both with a-XCENP-E TAIL antibody and
with a-XCENP-E ROD antibody (Figure 3D, arrowhead,
lanes 3 and 2, respectively).
To examine the effects of immunodepletion on spindle assembly and chromosome movement, demembranated Xenopus sperm nuclei were added to undepleted,
mock depleted, and XCENP-E-depleted CSF-extracts.
Extracts were cycled through interphase and into the
subsequent M phase, whereupon an additional aliquot
of the appropriate uncycled, metaphase-arrested extract was added to reimpose a metaphase arrest, thus
allowing the accumulation of M-phase structures. While
mock depleted and undepleted extracts yielded predominantly bipolar spindles with chromosomes aligned
at the metaphase plate (Figure 3B, panel 1; Figure 3C),
depletion of XCENP-E produced a 5-fold increase in the
number of bipolar spindles with misaligned chromosomes (Figure 3B, panels 4 and 5; Figure 3C), as well
as a smaller increase in the percentage of monopolar
structures, including radial asters, half-spindles, and
chromosomes associated with microtubules with indeterminate organization (Figure 3B, panels 2 and 3; Figure
3C). Extended incubation failed to alter the proportion
of bipolar spindles with properly aligned chromosomes.
Removal of 85% or more of XCENP-E was sufficient
to perturb metaphase chromosome alignment. Three
independent depletion experiments revealed in every
case a decrease in the percentage of metaphase spindles accompanied by an increased percentage of bipolar/misaligned and monopolar structures, although the
distribution of the aberrant structures between the
monopolar and bipolar/misaligned classes was variable.
Addition of a-XCENP-E Antibody Disrupts
Metaphase Chromosome Alignment
As a further test of the requirement for CENP-E in mediating chromosome congression we perturbed XCENP-E
function in situ by addition of the monospecific a-XCENPETAIL antibody to CSF-arrested Xenopus egg extracts.
These extracts were cycled through interphase and arrested at the subsequent M phase. As observed upon
immunodepletion of XCENP-E, addition of a-XCENPETAIL antibody resulted in almost total elimination of bipolar spindles with properly aligned chromosomes (Figure
4A; Figure 4B, left panel). This defect was apparent as
early as metaphase spindles became visible in control
extracts. Loss of bipolar structures with metaphase
aligned chromosomes was accompanied by an increase
in the percentage of bipolar spindles with misaligned
chromosomes (Figure 4A, panels 4–6; Figure 4B, left
panel) indicating a role for XCENP-E in congression or
maintenance of chromosome alignment, or both. We
also observed a smaller increase in the proportion of
monopolar structures (Figure 4A, panels 4–6, Figure 4B,
left panel). Similar results were obtained in seven experiments, and also using a-XCENP-E ROD antibody, though
the distribution of aberrant structures between the
monopolar and bipolar misaligned classes was variable,
with monopoles sometimes predominating.
To examine the possibility that a-XCENP-E antibody
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of beads, followed by tethering of the distal microtubule
minus ends by a complex of cytoplasmic dynein and
NuMA (Heald et al., 1996, 1997; Merdes et al., 1996).
While a-XCENP-ETAIL antibody addition induced chromosome misalignment in spindles assembled around
sperm nuclei (Figure 4B, left panel), this antibody did
not perturb assembly of chromatin-bead spindles (Figure 4B, right panel; Figure 4C). Thus, the disruption of
spindle bipolarity and chromosome alignment observed
upon addition of a-XCENP-E antibody is not due to a
defect in spindle assembly unrelated to XCENP-E function at kinetochores.
Figure 2. XCENP-E Localizes to Kinetochores In Vivo and In Vitro
(A) Localization of XCENP-E on cycled mitotic spindles assemble
in vitro. Panel 1, rhodamine-tubulin (red), DAPI-stained chromatin
(blue); Panel 2, a-XCENP-ETAIL staining. Scalebar, 10 mm.
(B) Xenopus egg extract (z60 mg/lane) resolved on a 4% gel, immunoblotted and probed with affinity-purified a-XCENP-E TAIL or
a-XCENP-E ROD antibody. XCENP-E is indicated by the arrowhead,
and the XCENP-E-related protein by the dot.
(C) Localization of XCENP-E in cultured Xenopus XTC cells. XTC
cells were fixed and stained with mouse monoclonal anti-a-tubulin
antibody (red) and affinity-purified rabbit a-XCENP-ETAIL antibody
(green). Chromatin was visualized by staining with DAPI (blue). Cells
at progressive stages of the cell cycle are displayed. Similar staining
was observed using a-XCENP-EROD antibody. Scalebar, 5 mm.
disruption of chromosome alignment and spindle bipolarity could be due to an antibody-induced aberrant action of XCENP-E unrelated to its action at kinetochores,
we examined the effect of a-XCENP-E TAIL antibody addition on bipolar spindle formation around chromatin
bound to magnetic beads. When incubated in mitotic
Xenopus egg extract, these chromatin beads promote
the formation of bipolar spindles in the absence of both
kinetochores and centrosomes (Heald et al., 1996). Under these conditions, spindle formation occurs by coalescence of microtubules around a central collection
Chromosome Misalignment Is Caused by Failure
of Congression or Its Maintenance
Since spindle formation in these extracts may initially
occur at least in part though an “inside out” pathway
with centrally aligned chromosomes, the misaligned
chromosomes seen on bipolar spindles after XCENP-E
depletion or XCENP-E antibody addition could reflect
either a requirement for XCENP-E in maintenance of
chromosome congression or premature entry into an
aberrant anaphase. Consistent with the latter possibility, monopoles arising following antibody addition or
XCENP-E depletion could be produced by degeneration
of bipolar structures induced by disruption of sister
chromatid cohesion. To test whether misaligned chromosomes arising from reduction of XCENP-E remained
in duplicated chromosome pairs, we examined the kinetochore localization of residual XCENP-E on chromosomes arrayed on bipolar spindles incompletely depleted
of XCENP-E (Figure 5). Images aquired at exposures
yielding saturating XCENP-E fluorescence in undepleted samples (e.g., Figure 2A) revealed no XCENP-E
signal on spindles assembled in extract 85%–90% depleted of XCENP-E (Figure 5A). However, overexposure
revealed low amounts of residual XCENP-E localized to
paired centromeres characteristic of nondisjoined sister
chromatids (Figure 5B, arrowheads). While we cannot
exclude that some misaligned chromosomes, on which
paired kinetochores cannot be resolved, may arise from
an aberrant anaphase-like sister chromatid disjunction, for those chromosomes displaying paired kinetochores these data indicate that XCENP-E is required for
either achieving or maintaining metaphase chromosome
alignment.
XCENP-E Is a Plus End–Directed
Microtubule Motor
Localization of CENP-E to kinetochores of mitotic chromosomes places CENP-E in a position to mediate attachment of chromosomes to microtubules, movement
of chromosomes during congression, and movement of
chromosomes toward the spindle poles during anaphase A. In addition, localization of XCENP-E and human
CENP-E to the spindle midzone during anaphase B (Yen
et al., 1992) indicates that if CENP-E were a plus end–
directed motor it might slide anti-parallel interzonal microtubules during spindle elongation. To test directly
if XCENP-E is a microtubule motor and to determine
the directionality of XCENP-E movement, a region
of XCENP-E containing the motor domain (Figure 1C,
CENP-E Is Required for Chromosome Alignment
361
Figure 4. Addition of a-XENP-E Antibody Disrupts Metaphase
Chromosome Alignment, but Not Bipolar Spindle Assembly around
Chromatin Beads
Figure 3. XCENP-E Is Required for Metaphase Chromosome
Alignment
(A) Immunodepletion of XCENP-E. An immunoblot of 1 ml each of
Xenopus egg extract depleted of XCENP-E using a-XCENP-ETAIL
antibody (lane 2), mock depleted with nonimmune rabbit IgG (lane
3), and unmanipulated extract (lane 1) was probed with a-XCENPETAIL antibody (upper panel). To control for specificify of depletion
and loading of the gel, the blot was subsequently probed with
a-XNuMA tail antibody (Merdes et al., 1996) (lower panel).
(B) Representative structures formed in control extracts (panel 1),
and in XCENP-E-depleted extract (panels 2–5). Tubulin is shown in
red, chromatin in blue. Sperm nuclei were added, and extracts were
cycled through interphase and arrested at the following metaphase.
Arrowheads in panels 2 and 5 indicate apparently nondisjoined sister
chromatids. Scalebar, 10 mm.
(C) Quantitation of structures formed in undepleted extract (n 5
123), extract mock depleted with nonimmune rabbit IgG (n 5 156)
and extract depleted of XCENP-E (n 5 98) scored 80 min after
exit from interphase. Data are presented from one representative
experiment. Structures were scored as bipolar spindles with chromatin aligned at the metaphase plate, bipolar spindles with misaligned chromosomes, monopolar spindles, including radial asters,
half spindles, and chromosomes associated with microtubules with
indeterminant organization, and other, including multipolar structures and groups of chromosomes apparently unassociated with
microtubules.
(D) Coomassie blue staining of a-XCENP-ETAIL and a-XCENP-E ROD
immunoprecipitates. Immunoprecipitates were prepared from 100
ml CSF-arrested extract using affinity-purified a-XCENP-ETAIL anti-
(A) Representative structures formed in the presence of 0.5 mg/ml
rabbit IgG and in the absence of added antibody (panels 1–3), and
in the presence of 0.5 mg/ml a-XCENP-ETAIL antibody (panels 4–8).
Tubulin is shown in red; chromosomes, in blue. Extracts containing
demebranated sperm nuclei and antibody were cycled through interphase and arrested at the following metaphase.
(B) Left panel: Quantitation of structures formed from sperm nuclei
in extract containing no antibody (n 5 125), extract containing 0.5
mg/ml nonimmune rabbit IgG (rIgG, n 5 172), and extract containing
0.5 mg/ml a-XCENP-ETAIL (n 5 101) scored 80 min after exit from
interphase. Data are presented from one representative experiment.
Structures were scored as described in Figure 3C. Right panel:
Quantitation of microtubule-containing structures formed around
bead-bound chromatin in the presence and absence of 0.5 mg/ml
a-XCENP-E TAIL antibody.
(C) Representative spindles assembled around bead-bound plasmid
DNA in the presence (panel 2) and absence (panel 1) of 0.5 mg/ml
a-XCENP-E TAIL antibody (Heald et al., 1996).
boxed region) was fused at the C terminus to a c-Myc
peptide epitope followed by a hexahistidine tag, expressed in E. coli, and purified over Ni-NTA-agarose
yielding the expected 57 kDa polypeptide as the major
product (Figure 6A, lane 1). Immunoblotting with a-Myc
body (lane 3), affinity purified a-XCENP-E ROD antibody (lane 2), or
nonimmune rabbit IgG (lane 1). Immunoprecipitates were washed
and resolved by SDS–PAGE on a 5% gel. Proteins were visualized by
staining with Coomassie blue. The arrowhead indicates XCENP-E.
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362
of microtubules polarity marked with brightly fluorescent
rhodamine-labeled seeds near their minus ends was
recorded by time-lapse digital fluorescence microscopy. Selected frames demonstrating three examples
of plus end–directed movement are presented in Figure
6B. Microtubules moved at a velocity of 5.1 6 1.7
mm/min (n 5 49) with brightly fluorescent seeds leading,
indicating that the immobilized XCENP-E fusion protein
was moving toward microtubule plus ends. When assayed in the absence of a-Myc antibody the XCENP-E
fusion protein also supported microtubule gliding, albeit
less robustly.
Discussion
Figure 5. Disruption of XCENP-E Function Yields Misaligned Chromosomes that Have Not Undergone Premature Anaphase
Spindles assembled in the extracts incompletely depleted of
XCENP-E (85–90% depletion), stained with a-XCENP-ETAIL antibody.
Images collected with exposure times sufficient to yield saturated
kinetochore staining of control spindles (e.g., Figure 2A) yielded no
visible kinetochore staining (A). However, longer exposure revealed
residual XCENP-E, present in paired spots characteristic of nondisjoined prometaphase sister chromatids ([B], panels 1 and 2, corresponding to boxes 1 and 2 in [A]). From the fluorescence intensity,
we estimate that kinetochore-associated XCENP-E has been reduced 6-fold or greater, consistent with the extent of depletion.
monoclonal antibody (9E10; Evans and Bishop, 1985)
confirmed the 57 kDa band as the XCENP-E fusion protein (Figure 6A, lane 2). This protein was tethered to a
glass coverslip using the a-Myc antibody, and gliding
Kinetochores and Mitotic Spindle Formation
Misalignment of chromosomes may result from a failure
to reach the spindle equator, that is, failure to congress,
or from failure to maintain an equatorial position once
initial alignment is achieved. While congression has
classically referred to the process that establishes metaphase alignment, rather than the subsequent maintenance of metaphase chromosome position at the spindle equator, the separation of these two processes is
almost certainly temporal, rather than mechanistic. The
forces that establish equatorial chromosome positioning also likely function to maintain that position. Thus,
the defect in chromosome alignment that we observe
upon perturbation of XCENP-E function, combined with
the presence of paired sister chromatids on misaligned
chromosomes, indicates a defect in the mechanics of
congression.
Recent models for spindle assembly in Xenopus egg
extracts have highlighted their ability to assemble bipolar spindles in the absence of kinetochores and centrosomes (Heald et al., 1996, 1997; Hyman and Karsenti,
Figure 6. XCENP-E Is a Plus End–Directed Microtubule Motor
(A) XCENP-E residues 1–473 fused at the C terminus to a c-Myc epitope and a hexahistidine tag (see schematic). Coomassie blue stain of
purified XCENP-E fusion protein (arrowhead) used for motility (lane 1); immunoblot of XCENP-E fusion protein probed with a-Myc monoclonal
antibody (lane 2).
(B) XCENP-E motility assay. Microtubules marked near their minus ends with brightly fluorescent seeds were added with ATP to a flow
chamber containing purified XCENP-E fusion protein tethered to the coverslip with a-Myc monoclonal antibody. Gliding of microtubules was
monitored by time-lapse digital fluorescence microscopy. Selected frames from one time lapse series, spaced 90 sec apart, are presented.
The positions of the plus ends of microtubules numbered 1, 2, and 3 at the start of continuous gliding are marked with solid white dots, and
the position of a stationary microtubule end is marked by the arrowhead. The bright seed of microtubule 3 enters the plane of focus at 1.5
min, and glides 13.6 mm downward with the bright seed leading over the following 3 min. Microtubule 2 moves continuously during the first
3 min, after which point it detaches and reattaches further toward the bottom of the frame. Microtubule 1 glides minus-end leading throughout
the entire time course. The average microtubule velocity of all microtubules was 5.1 mm/min 6 1.7 (n 5 49). Of those, 33 microtubules were
unambiguously polarity marked, and all glided with their bright seeds leading. Scalebar, 5 mm.
CENP-E Is Required for Chromosome Alignment
363
1996). However, the presence of centrosomes dramatically alters the pattern of microtubule nucleation: centrosomes, not chromatin, become the predominant sites
for microtubule nucleation (Heald et al., 1997). These
findings, and the data we present here, indicate that
while Xenopus egg extracts are competent to assemble
bipolar spindles in the absence of centrosomes and
kinetochores, when present, both centrosomes and kinetochores play dominant roles in normal mitotic spindle
assembly in vitro.
Roles for CENP-E in Chromosome Alignment
Like all other kinesin-related motor proteins with aminoterminal motor domains for which movement in vitro has
been demonstrated, CENP-E moves toward microtubule
plus ends. Hyman and Mitchison (1991) have shown
that the kinetochores of prometaphase Chinese hamster
(CHO) chromosomes harbor a plus end–directed microtubule motor activity that can move chromosomes in
vitro at a velocity similar to that of CENP-E (Figure 6).
This activity was revealed only following incubation of
the chromosomes with ATP-g-S, presumably resulting
in the irreversible thiophosphorylation of kinetochore
proteins. Prior to incubation with ATP-g-S, kinetochores
exhibited only minus end–directed motility. As it is the
only plus end–directed motor known to associate with
prometaphase kinetochores, CENP-E is the best candidate for this kinetochore-associated, plus end–directed
motor activity.
Previously, antibodies to human CENP-E were found
to immunodeplete a minus end–directed motor activity
from biochemical fractions enriched in CENP-E (Thrower
et al., 1995). While our findings demonstrate unambiguously that the CENP-E motor moves toward microtubule
plus ends in the presence of ATP, the intriguing possibility arises that CENP-E might participate in a complex
of microtubule motors with opposite directionalities.
Furthermore, the two motors in that complex, CENP-E
and an as-yet-unidentified minus end–directed motor,
might be regulated, perhaps by phosphorylation, so as
to ensure unidirectionality of movement during specific
phases of mitosis. Although XCENP-E is the major protein present in XCENP-E immunoprecipitates (Figure
3D), given the size of XCENP-E, we cannot rule out the
possible presence of additional motor proteins, either
smaller than XCENP-E, or present in substoichiometric
amounts relative to XCENP-E. An alternative possibility
that cannot yet be excluded is that the movement of
CENP-E itself could be redirected toward microtubule
minus ends by interaction with some regulatory factor
or by posttranslational modification. While such an activity is without precedent, the structural features that
determine the direction of movement of kinesin superfamily members remain obscure (Kull et al., 1996; Sablin
et al., 1996; Case et al., 1997; Henningsen and Schliwa,
1997).
What function might a plus end–directed motor perform at the kinetochore during congression? Pushing
forces at the kinetochore apparently play at most a minor
a role in driving chromosome movement during congression (Khodjakov and Rieder, 1996; Waters et al.,
1996). This does not mean, however, that plus end–
Figure 7. A Model for CENP-E Function in Chromosome Alignment:
Tethering of Kinetochores to Microtubule Plus Ends
At both leading and trailing kinetochores, CENP-E (yellow ovals)
binds to microtubules and moves toward the plus ends, where it
remains attached. At the leading kinetochore, CENP-E molecules
detach and reattach as kinetochore microtubules disassemble (perhaps induced by XKCM1, depicted as orange ovals), thus moving
the kinetochore poleward by maintaining kinetochore attachment
to microtubules. At the trailing kinetochore, the plus end–directed
CENP-E motor pushes the kinetochore to the newly assembled
microtubule plus end. Plus end growth at the trailing kinetochore
might occur in response to a promoter of plus end assembly (lavender circles). CENP-E would push only to the extent that microtubule
plus ends grew, and as long as the rate of plus end growth did not
exceed the rate of CENP-E movement, trailing kinetochore attachment would be maintained. A CENP-E-dependent tether to microtubule plus ends, in combination with regulators of microtubule plus
end dynamics functionally linked by tension-sensors, would be sufficient to mediate oscillatory movement of congressing chromosomes.
directed kinetochore forces play no role in congression,
only that the trailing kinetochore is pushed only to the
extent that its leading sister is pulled. During congression the trailing kinetochore is not towed passively
along by the leading sister, but rather appears to actively
follow (Skibbens et al., 1993, 1995; Waters et al., 1996).
As a kinetochore-associated plus end–directed motor,
CENP-E could mediate this movement. In addition,
CENP-E could act as a tether, coupling both leading
and trailing kinetochores to the dynamic plus ends of
kinetochore microtubules. This is depicted in the model
presented in Figure 7. Precedent for such a proposal
is provided by the findings that the microtubule motor
kinesin can mediate ATP-dependent motility of latex
beads toward microtubule plus ends where the beads
remain attached (Lombillo et al., 1995b). Kinesin-mediated attachment of beads to microtubule plus ends persists in the absence of ATP, even when conditions are
altered to induce microtubule plus end disassembly,
thus resulting in displacement of the beads toward microtubule minus ends. These findings demonstrate both
that a plus end–directed motor can be sufficient to mediate attachment to dynamic microtubule plus ends, and
that a motor classified by conventional motility assays
as plus end–directed can mediate minus end–directed
movement by following shrinking microtubule plus ends.
Direct support for a role for CENP-E in such a process
is provided by the finding that antibodies recognizing
CENP-E inhibit microtubule-depolymerization driven,
ATP-independent, minus end–directed movement of
Cell
364
isolated CHO chromosomes, while antibodies recognizing two other kinetochore motors, cytoplasmic dynein
and MCAK, have no effect (Lombillo et al., 1995a).
A Model for Chromosome Congression
Kinetochores of congressing chromosomes oscillate
between poleward (P) and antipoleward (AP) motility
states, exhibiting what has been called directional instability (Skibbens et al., 1993). Switching of kinetochores
between P and AP motility states is regulated in a manner that appears to be influenced by tension (Skibbens et
al., 1993, 1995; Cassimeris et al., 1994) and may involve
phosphorylation of kinetochore proteins (Nicklas et al.,
1995). Poleward movement of leading kinetochores is
accompanied by loss of tubulin subunits from kinetochore-bound microtubule plus ends, while antipoleward
movement of trailing kinetochores is accompanied by
microtubule growth (Figure 7; reviewed in Inoue and
Salmon, 1996). Chromosome movement during congression could be driven by a tension-sensitive, leading
kinetochore-induced destabilization of the attached microtubule plus ends, rather than by the regulated activity
of kinetochore-associated, minus end–directed microtubule motors. As a plus end–directed motor, CENP-E
could tether kinetochores to both growing and shrinking
microtubule plus ends, thus coupling chromosome
movement directly to the dynamics of kinetochore microtubule plus ends (yellow ovals, Figure 7). This scheme
requires that a molecule capable of inducing microtubule depolymerization, such as XKCM1 (Walczak et al.,
1996), also be localized to kinetochores (orange ovals,
Figure 7), perhaps complemented by an activity promoting microtubule growth at the trailing kinetochore (lavender circles, Figure 7). Disruption of the CENP-E linkage
would be expected to result in defective alignment, as
we have reported here, and could potentially cause dissociation of chromosomes from the mitotic spindle.
While we do not observe a dramatic reduction in the
association of chromosomes with microtubules upon
XCENP-E depletion or addition of anti-XCENP-E antibodies (see Figures 3B and 4A), the ability of chromatin
to associate with microtubules in the apparent absence
of kinetochores in this system (Heald et al., 1996)
might mask a substantial defect in kinetochore-microtubule association, resulting in chromosome misalignment rather than dissociation. Our findings offer compelling evidence that CENP-E mediates chromosome
movement during metaphase alignment, consistent with
CENP-E powering plus end–directed movement of trailing kinetochores and transducing forces generated by
kinetochore microtubule plus end dynamics.
Experimental Procedures
Isolation of XCENP-E cDNA and DNA Constructs
Fragments spanning nucleotides 1–1707 and 6376–8080 of
hCENP-E cDNA (Yen et al., 1992) were used to screen a lgt10
Xenopus cDNA library (Rebagliati et al., 1985), hybridizing at 428C
(Church and Gilbert, 1984). Multiple cDNA clones spanning the full
length of XCENP-E were isolated and DNA sequence determined by
a combination of automated cycle sequencing (Applied Biosystems,
Perkin Elmer) and manual sequencing using Sequenase version 2.0
(USB). Overlapping sequence was compiled using MacVector software (Kodak Scientific Imaging Systems, Rochester, NY).
Fusion Protein Expression, Antibody Production,
and Immunoblotting
XCENP-ETAIL (aa 2396–2954) and XCENP-E ROD (aa 826–1106) antigens
were produced as hexahistidine fusion proteins using the pRSETB
expression plasmid (Invitrogen). Following induction with IPTG inclusion bodies were purified and solubilized in 8 M urea, 0.1 M
sodium phosphate (pH 8.0). XCENP-EROD fusion protein was further
purified over Ni-NTA agarose (Qiagen). XCENP-ETAIL protein was
isolated from preparative SDS–PAGE gels. These antigens were
used to raise polyclonal antibodies in rabbits.
Antibodies were affinity purified over antigen coupled to cyanogen
bromide–activated Sepharose (Pharmacia), eluting with 0.2 M glycine (pH 2.5), dialyzed into 10 mM K-HEPES (pH 7.8), 100 mM KCl,
1 mM MgCl2 and concentrated using Nanospin filter concentrators
(Gelman Sciences). Immunoblots were blocked with TBS (20 mM
Tris [pH 7.6], 150 mM NaCl) containing 5% w/v nonfat dried milk
(TBS/NFDM) and probed with 2 mg/ml affinity-purified antibody overnight in TBS/NFDM 0.05% Tween. Primary antibody was visualized
using 125I-Protein A (Amersham) or alkaline phosphatase–conjugated
goat anti-rabbit antibody (Promega). Quantitative phosphorimaging
was performed using a Molecular Dynamics model 445 SI phosphorimager.
Spindle Assembly In Vitro
CSF-arrested extract was prepared from Xenopus eggs essentially
as described in (Murray, 1991; Sawin and Mitchison, 1991). Rhodamine-labeled bovine brain tubulin (Hyman et al., 1992) was added
at 1 ml/300 ml of extract. For immunodepletion, 100 mg of affinitypurified a-XCENP-E antibody or nonimmune rabbit IgG (Calbiochem) was bound for 1 hr to 30 ml slurry of protein A Affiprep beads
(BioRad), the beads sedimented and washed with CSF-XB (Murray,
1991), CSF-XB containing 0.5 M NaCl, and CSF-XB. CSF extract
(100 ml) was added to the bead-bound antibody and rocked 1 hr at
48C. Beads were sedimented by brief centrifugation, and the depleted extract recovered. The extent of XCENP-E depletion was
variable. Immunoprecipitates were washed with CSF-XB/1% Triton
X-100 and CSF-XB/0.5 M NaCl, then examined by SDS–PAGE and
Coomassie blue staining. Demembranated sperm were added to a
portion of each extract, and exit from metaphase arrest induced by
addition of CaCl2 . Progress of extracts through the cell cycle was
monitored by fluorescence microscopic examination of 1 ml aliquots
squashed under a coverslip (Murray, 1991). At 80 min following exit
from metaphase, one half volume of the appropriate extract was
added and the reaction incubated for an additional 80–120 min.
M-phase structures accumulating in extracts were scored in
squashed samples taken after 160–200 min total elapsed time.
For antibody addition experiments, purified anti-XCENP-E antibody or nonimmune rabbit IgG at 10 mg/ml was added to extract
at a 1:20 dilution, followed by sperm nuclei and CaCl2 . Eighty minutes
later a half volume of CSF-arrested extract was added, containing
a proportional amount of the appropriate antibody. Structures were
scored as describe above.
Chromatin bead spindles were assembled as described by Heald
et al. (1996), using plasmid DNA and a kilobaseBINDER kit (Dynal).
Microtubule-containing structures were scored following sedimentation onto coverslips as described below.
Immunofluorescence Microscopy
Extract containing mitotic spindles was diluted z30-fold in BRB80
(80 mM K-PIPES [pH 6.8], 6 mM MgCl2 , 1 mM EGTA) containing
0.5% Triton X-100 and 30% glycerol, and spindles sedimented onto
a coverslip through a cushion of BRB80 containing 0.5% Triton
X-100 and 40% glycerol at 7000 rpm in a Sorvall HS4 rotor. Coverslips were fixed in 2208C methanol, rehydrated in TBS-Tx (TBS,
0.1 % Triton X-100), blocked for 1 hr with 1% BSA in TBS-Tx, and
probed with 5 mg/ml affinity-purified antibody in 1% BSA in TBSTx. Primary antibody was visualized using by FITC-conjugated secondary goat anti-rabbit antibody (Cappel). Chromatin was visualized
by staining with DAPI.
Xenopus XTC cells cultured on coverslips in 60% L15 medium
containing 5% FBS were rinsed in TBS, fixed, and stained with
affinity-purified antibody as described above. Tubulin was stained
CENP-E Is Required for Chromosome Alignment
365
with monoclonal anti-alpha tubulin antibody DM1A (Sigma), followed
by Texas red goat anti-mouse secondary antibody (Cappel).
Fluorescent images were collected using a Princeton Instruments
cooled CCD mounted on a Zeiss Axioplan microscope and controlled by Metamorph software (Universal Imaging). Image processing was performed using both Metamorph and Adobe Photoshop software.
Expression and Purification of XCENP-E Motor Domain
E. coli strain BL21(DE3) pLysS, carrying a plasmid consisting of
pEt23d expression vector (Novagen) containing a fragment of the
XCENP-E cDNA encoding amino acids 1–473 fused at the C terminus
to a c-Myc epitope tag followed by a hexahistidine tag, was grown
at 378C in a modified LB medium. At OD600nm of z1, expression of
fusion protein was induced at room temperature for 4 hr with 0.5
mM IPTG. Bacterial pellets were suspended in lysis buffer (50 mM
Tris-HCl [pH 7.5], 50 mM NaCl, 1 mM PMSF, 0.1 mM ATP), and
lysed by 3 passages through a french press. Insoluble debris was
removed by a ultracentrifugation, and soluble protein bound in batch
at 48C to 0.5 ml of Ni-NTI-agarose resin (Qiagen). The resin was
washed with 5 ml of lysis buffer containing 20 mM Imidazole, and
XCENP-E fusion protein eluted in lysis buffer containing 100 mM
Imidazole and 1 mM DTT. Freshly prepared protein was used to
assay motility.
Preparation of Polarity-Marked Microtubules
and Motility Assay
Fluorescent, taxol stabilized, polarity-marked microtubules were
prepared essentially as described (Howard and Hyman, 1993) using
unlabeled bovine brain tubulin, rhodamine-labeled tubulin and
N-ethyl maleimide modified tubulin (Hyman et al., 1992). Flow chambers prepared from coverslips sealed with an Apiezon grease were
preadsorbed with a 1:10 dilution of anti-Myc monoclonal antibody
9E10 ascities fluid (Evans and Bishop, 1985), washed with PEM80
(80 mM PIPES pH 6.9, 1 mM EGTA, 1 mM MgCl2 ), incubated with
0.1 mg/ml XCENP-E motor protein, and unbound protein removed
by rinsing with PEM80. Polarity-marked microtubules in PEM80 containing 10 mM taxol, 2 mM MgATP, and an oxygen scavenging system (0.1 mg/ml catalase, 0.03 mg/ml glucose oxidase, 10 mM glucose, 0.1% b-mercapoethanol [Kishino and Yanagida, 1989]) were
then flowed into the chamber. Time-lapse image acquisition and
data analysis were performed at room temperature using a Zeiss
Axioplan fluorescence microscope, a Princeton Instruments cooled
CCD and the MetaMorph software package (Universal Imaging,
West Chester, PA).
Acknowledgments
We thank members of the Cleveland laboratory mitosis group for
stimulating discussions and helpful comments on the manuscript,
especially Andreas Merdes, Jennifer Yucel, and Xuebiao Yao.
Thanks also to Sandra Holloway, Ken Sawin, Andrew Murray, and
Tim Mitchison for help with Xenopus extracts and to John Newport
for advice on frogs. Special thanks to Rebecca Heald for advice
and reagents for making bead spindles. K. W. W. and R. S. were
supported by postdoctoral fellowships from the Cancer Research
Fund of the Damon Runyon-Walter Winchell Foundation. This work
was supported by the Ludwig Institute for Cancer Research and a
grant from the NIH to D. W. C. L. S. B. G. is an investigator of the
Howard Hughes Medical Institute.
Received June 30, 1997; revised September 26, 1997.
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EMBL/GenBank Accession Number
The nucleotide sequence of Xenopus CENP-E is available from
EMBL/GenBank under accession number AF027728.
Note Added in Proof
Schaar et al. have obtained similar results upon perturbation of
CENP-E function in cultured human cells: Schaar, B.T., Chan, G.K.T.,
Maddox, P., Salmon, E.D., and Yen, T.J. (1997). CENP-E function
at kinetochores is essential for chromosome alignment. J. Cell Biol.,
in press.