REVIEWS
Chromosomal passengers:
conducting cell division
Sandrine Ruchaud, Mar Carmena and William C. Earnshaw
Abstract | Mitosis and meiosis are remarkable processes during which cells undergo profound
changes in their structure and physiology. These events are orchestrated with a precision
that is worthy of a classical symphony, with different activities being switched on and off at
precise times and locations throughout the cell. One essential ‘conductor’ of this symphony
is the chromosomal passenger complex (CPC), which comprises Aurora-B protein kinase,
the inner centromere protein INCENP, survivin and borealin (also known as Dasra-B).
Studies of the CPC are providing insights into its functions, which range from
chromosome–microtubule interactions to sister chromatid cohesion to cytokinesis, and
constitute one of the most dynamic areas of ongoing mitosis and meiosis research.
Polo family
A family of Ser/Thr protein
kinases that have crucial roles
in cell-cycle regulation, in the
regulation of sister chromatid
pairing and in the assembly
and function of the mitotic
spindle.
Inner centromere
The heterochromatin-rich
region of the chromosome that
is situated in between the two
kinetochores of the paired
sister chromatids.
Spindle midzone
Organized bundles of
antiparallel microtubules at the
centre of the spindle that form
during anaphase and
telophase.
Wellcome Trust Centre for Cell
Biology, Institute of Cell and
Molecular Biology,
University of Edinburgh,
Swann Building, King’s
Buildings, Mayfield Road,
Edinburgh, EH9 3JR, UK.
Correspondence to W.C.E.
e-mail:
[email protected]
doi:10.1038/nrm2257
Published online
12 September 2007
Mitosis and meiosis are the most exciting and elaborate
processes that occur during the life of dividing cells. Over
the course of little more than an hour (for mitosis), macro­
molecular structures throughout the cell are reorganized,
signalling pathways are activated and silenced, proteins are
degraded and, at the end of each division, two daughter
cells are born. Not only are mitosis and meiosis wonder­
fully elaborate, it is also essential that they proceed without
error, as mistakes can result in the death of the organism.
How are all of these processes coordinated? The deci­
sion to divide is enforced by cyclin-dependent kinase-1
(CDK1)–cyclin A and CDK1–cyclin B. CDK activation is
complex, but is triggered in part by kinases of the Polo family, in particular Polo-like kinase-1 (PLK1) in vertebrates1.
However, these global controllers are not sufficient to run
the programme. Also needed are ‘hands-on’ controllers
that switch various activities on and off at specific times
and locations once the division programme has been trig­
gered. The best characterized of these controllers is the
chromosomal passenger complex (CPC).
Chromosomal passenger proteins were discovered
when a monoclonal antibody to a then-novel protein,
the inner centromere protein INCENP, was found to
label inner centromeres in metaphase, but subsequently
transferred to the spindle midzone and to the equatorial
cell cortex at the site of presumptive cleavage furrow for­
mation following the metaphase–anaphase transition2
(FIG. 1). Because other studies had suggested that mitotic
chromosome alignment at a metaphase plate was required
for spindle stability during anaphase, it was proposed that
the passenger proteins might regulate key mitotic proc­
esses by moving from place to place in the dividing cell3.
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This hypothesis has since been validated, and studies of
these proteins comprise a major area of ongoing mitosis
and meiosis research. For earlier reviews of chromosomal
passenger proteins, see Refs 4–6.
In this Review, we first introduce the four members of
the CPC, and then describe how their localization and act­
ivity change on the chromosomes, cytoskeleton and plas­ma
membrane as they perform different vital functions
as cells undergo mitotic and meiotic division.
Composition and activation of the CPC
In most organisms, the core CPC is composed of
Aurora-B kinase7 and three non-enzymatic subunits,
INCENP (Refs 2,8), survivin and borealin (also known
as Dasra‑B)9–12 (FIG. 2b,c; table 1). The non-enzymatic
members of the complex control the targeting, enzymatic
activity and stability of Aurora-B kinase13. Knockdown by
RNA interference (RNAi) of any member of the complex
delocalizes the others, disrupts mitotic progression and
may destabilize one or more of the other subunits9,11,14–17,
except in Caenorhabditis elegans, in which targeting of the
non-enzymatic subunits does not depend on Aurora-B18.
INCENP. INCENP was the first member of the complex
to be identified in a screen for new components of mitotic
chromosomes2 (FIG. 2a). INCENP appears to be a scaffold
that interacts with the three other members of the com­
plex8,11,19–22 (FIG. 1b). The C terminus of INCENP, which is
highly conserved from budding yeast to mammals8 (the
yeast homologue is called Sli15; hereafter referred to as
INCENP/Sli15), is involved in binding and regulation of
Aurora-B (see below).
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Early mitosis
Late mitosis
Prophase
Metaphase
Anaphase
Telophase
Aa
Ba
Ca
Da
Ab
Bb
Cb
Db
Figure 1 | Chromosomal passenger complex localization during mitosis. Indirect immunofluorescence (panels
Nature
Reviews
Biology
Aa–Da) and schematic representation (panels Ab–Db) of Aurora-B localization (green)
in HeLa
cells| Molecular
during theCell
main
phases of mitosis together with kinetochores (stained with anti-centromere autoantibodies, pink), α‑tubulin (red)
and DNA (blue). In prophase, Aurora-B is found on chromosome arms and starts to accumulate at centromeres
between kinetochores (panels Aa, Ab). In metaphase, chromosomes align on the spindle equator (panels Ba, Bb).
Tension at kinetochores is detected by the stretched centromeric Aurora-B staining (see enlargement in panel Ba).
In anaphase, Aurora-B leaves the centromere and relocates to the spindle midzone (panels Ca, Cb). In telophase,
Aurora-B concentrates at the midbody (panels Da, Db). Scale bar represents 5 µm.
Equatorial cell cortex
A region of the cell membrane
where actin and myosin fibres
assemble to form the
contractile ring during
anaphase.
Metaphase plate
A dynamic grouping of
chromosomes positioned on a
plane that is perpendicular to
the spindle axis midway
between spindle poles.
Inhibitor of apoptosis
protein (IAP) family
Proteins that are characterized
by their baculovirus IAP repeat
(BIR) domains. They suppress
apoptosis by interacting with
and inhibiting the enzymatic
activity of caspases.
Baculovirus IAP repeat
(BIR) domain
This Zn2+-finger domain is
important for protein–protein
interactions and is specific to
all proteins of the inhibitor of
apoptosis (IAP) family.
Aurora-B kinase. Aurora-B is a Ser/Thr kinase that is
conserved from yeast (in which it is known as Ipl1) to
mammals7,23. In vertebrates, three Aurora kinases, A, B
and C, all have different functions or tissue specificity (for
a review see Ref. 4). Budding yeast have a single Aurora
kinase, Ipl1 (hereafter referred to as Aurora/Ipl1)23, and
Drosophila melanogaster, in which the original aurora
gene (encoding an Aurora-A kinase) was identified24,
has two Aurora kinases.
Aurora-C can bind other members of the complex and
can rescue Aurora-B loss of function in several human
cell lines25,26. Aurora-C expression is highest in testis and
various cancer cell lines25,27, and knockout mice appear
nearly normal. Their sole reported defect is reduced
male fertility due to morphological abnormalities of
the sperm28. Recently, mutations in Aurora-C have been
shown to cause infertility in humans29 — homo­zygotes
have large-headed, multiflagellar sperm, which is a
stronger phenotype than that seen in the knockout mice.
These individuals do not have any overt somatic pheno­
type, suggesting that Aurora-C is not required for somatic
mitosis. Aurora-C will not be discussed further here.
nature reviews | molecular cell biology
Aurora-B binds to a region near the C terminus
of INCENP called the IN box (which corresponds to
amino acids 822–900 of human INCENP)8,30 (FIG. 2a,b).
INCENP binding activates Aurora-B in organisms
from yeast to mammals9,31–33. In turn, Aurora-B phos­
phorylates INCENP at two conserved adjacent Ser
residues close to the C terminus, which further activ­
ates the kinase in a positive feedback loop9,32,33 (FIG. 2d).
Clustering of the CPC by using an anti-INCENP anti­
body or by addition of chromatin to Xenopus laevis egg
extracts also stimulates Aurora-B activity34. In C. elegans,
Aurora-B phosphorylates Tousled-like kinase-1 (TLK‑1)
during prophase–prometaphase and this, in turn,
increases Aurora-B activity in an INCENP-dependent
manner35.
Survivin. Survivin is a conserved member of the inhibitor
of apoptosis protein (IAP) family and bears a single baculovirus
IAP repeat (BIR) domain that is responsible for dimerization
of survivin36–38. Survivin can bind the other three com­
plex members and is phosphorylated by Aurora-B15,39
(FIG. 2b). Survivin may21,22 or may not9 contribute to the
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REVIEWS
a
1
β-tubulin binding
HP1α binding
Putative coiled-coil
Aurora-B binding 918
IN box
TSS
Centromere
targeting
DAPI
b
TSS
ACA
INCENP
c
N-lobe
Aurora-B
Aurora-B
C-lobe
Survivin
INCENP
Borealin
Zn
Survivin
d
Partially active kinase
INCENP
Merged
Fully active kinase
C terminus of
INCENP
PP
PP
TSS
TSS
C terminus of
INCENP
N-lobe
N-lobe
P
Aurora-B
C-lobe
Aurora-B
C-lobe
Figure 2 | Interactions within the chromosomal passenger complex and Aurora-B activation. a | Functional domains
Nature Reviews | Molecular
Cellfollowed
Biology
of the inner centromere protein INCENP. The N terminus of the protein bears the centromere-targeting
domain
by a β‑tubulin-binding domain that is required for targeting to the spindle20,50. INCENP has been shown to bind to
heterochromatin protein-1α (HP1α) in vitro50 as well as to Aurora-B via a highly conserved C-terminal domain called the
IN box8,30. Inset: INCENP (green) localizes at the inner centromere between kinetochores (red) labelled with anticentromere autoantibodies (ACA); the metaphase chromosome (blue) is stained with 4′,6-diamidino-2-phenylindole
(DAPI). b | Schematic representation of the chromosomal passenger complex (CPC). Survivin and borealin interact with
the N terminus of INCENP (amino acids 1–58 of human INCENP)9,11,12,33, whereas the N terminus of Aurora-B binds to the
IN box (between the two black arrowheads; amino acids 822–900 of human INCENP)8,30. Borealin binds strongly to
survivin via its N terminus11. Interactions within the complex are represented by white arrowheads. The schematic
representation of Aurora-B and the associated region of the C-terminus of INCENP is derived from the X-ray structure
of the complex33 (Protein Data Bank ID: 2BFY). c | Distribution of INCENP (blue), survivin (green) and Aurora-B (red) in
the same prometaphase cell. Scale bar represents 5 µm. d | Two-step activation of Aurora-B by INCENP. Interaction with
INCENP partially activates Aurora-B, which autophosphorylates a residue in the T-loop (indicated by the black
arrowhead) and also phosphorylates a highly conserved Thr-Ser-Ser motif (TSS) in the IN box close to the C terminus
of INCENP. This results in a further stimulation of kinase activity (orange flashes).
regulation of Aurora-B activity. In recent years, a con­
troversy has raged over whether survivin functions
both in mitosis and as a protector against apoptosis (for
recent reviews, see Refs 40,41).
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Borealin/Dasra-B. Borealin was identified in a proteo­
mic screen for new components of the mitotic chromo­
some scaffold11 and, simultaneously, in a screen for novel
X. laevis chromosome-binding proteins, where it was
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Table 1 | Components of the chromosomal passenger complex in various species
Organism
INCENP
Aurora-B kinase
Survivin
Borealin/Dasra-B
Sc
Sli15
Ipl1
Bir1
–
Sp
Pic1
Ark1
Bir1/Cut17
–
Dm
Incenp
ial
Deterin
Borealin
Ce
ICP-1
AIR-2
BIR-1
CSC-1?
Xl
XINCENP
XAurora-B
XSurvivin
Dasra-A, Dasra-B
Gg
INCENP-I
Aurora-B
Survivin
Borealin-1, Borealin-2
Mm
INCENP-A
AIM-1
Survivin
Borealin-1, Borealin-2
Hs
INCENP
Aurora-B
Survivin
Borealin
Ce, Caenorhabditis elegans; Dm, Drosophila melanogaster; Gg, Gallus gallus; Hs, Homo sapiens; Mm, Mus musculus;
Sc, Saccharomyces cerevisiae; Sp, Schizosaccharomyces pombe; Xl, Xenopus laevis.
named Dasra-B10. X. laevis and many other vertebrates,
but not humans, have a second distantly related protein,
Dasra-A, which may have a similar function10. Borealin/
Dasra-B may be functionally related to the C. elegans
protein CSC‑1 (chromosome segregation and cyto­kinesis
defective‑1)18: the proteins share an 8‑amino-acid stretch
but are otherwise structurally unrelated. Borealin homo­
logues have yet to be identified in yeasts. Borealin is
phosphorylated by Aurora-B in vitro11 but the functional
consequences of this are not known.
CBF3 complex
A multisubunit protein complex
in Saccharomyces cerevisiae
that binds to centromeric DNA
and initiates kinetochore
assembly.
FRAP
(Fluorescence recovery after
photobleaching). A technique
that measures the dynamics of
fluorescently tagged
macromolecules within cellular
substructures.
FLIP
(Fluorescence loss in
photobleaching). A technique
that measures the mobility of
molecules by bleaching a
defined region of cytoplasm
and watching how this affects
the population of fluorescent
molecules in other regions of
the cell.
TD‑60, a passenger protein? Other proteins interact
with the complex to perform essential functions. TD‑60
(telophase disk 60 kDa; also known as regulator of
chromo­some condensation-2, RCC2) is not a member
of the core complex11 but has a typical chromosomal
passenger localization42 and is mislocalized if other
components of the complex are perturbed. Although
originally described as a putative guanosine nucleotide
exchange factor 43, more recent studies suggest that
TD‑60 works with microtubules to activate the kinase
activity of Aurora-B–INCENP (P.T. Stukenberg, personal
communication).
The CPC can localize to centromeres after partial
depletion of TD‑60 from X. laevis extracts, but is not
fully active. TD‑60 activation of the CPC depends on
the phosphorylation state of the histone H3 tail. When
dephosphorylated, the histone H3 tail inhibits TD‑60
activation of the CPC. Phosphorylation of Thr3 of his­
tone H3 by haspin kinase44 at inner centromeres reverses
this inhibition. Thus, in prometaphase, the ‘histone code’
in the inner centromere is permissive for Aurora-B activ­
ation, whereas the code on the chromosome arms is
restrictive (P.T. Stukenberg, personal communication).
Targeting of the CPC in early mitosis
CPC localization is highly dynamic. The CPC is detected
initially along chromosome arms, but is progressively
concentrated in inner centromeres through prometa­
phase and metaphase45–49 (FIGS 1,3). A wealth of inform­
ation has sought to explain how the CPC is recruited to
centromeres.
Early studies revealed that the N terminus (amino
acids 1–58 of human INCENP) is sufficient for INCENP
targeting to the centromere50. Borealin and survivin bind
nature reviews | molecular cell biology
to the N terminus of INCENP in vitro and in vivo9,11,12,33
and, therefore, could be the primary targeting sub­units
of the complex. Indeed, survivin has been implicated in
targeting of the CPC to the centromere. The N‑terminal
targeting domain of INCENP does not target correctly
in survivin-depleted cells. Furthermore, although
N‑terminal deletions of INCENP fail to target to centro­
meres 51, a fusion protein comprising survivin and
amino acids 48–918 of INCENP can do so in borealindepleted cells and thereby restores CPC function 17.
The BIR domain of survivin is essential for centromere
targeting and spindle checkpoint function in human
cells13. In budding yeast, the centromere-binding factor-3
complex (CBF3 complex) was shown to interact with Bir1
(the yeast homologue of survivin; hereafter referred to
as survivin/Bir1)52. CBF3 appears not to be conserved in
higher eukaryotes, in which the centromeric receptor for
the CPC remains unknown.
Survivin and Aurora-B turn over rapidly at centromeres
until metaphase53,54. FRAP and FLIP experiments implicate
survivin ubiquitylation in regulating CPC dynamics at
centromeres55. Specifically, ubiquitylation of Lys63 medi­
ated by UFD1 (ubiquitin fusion degradation‑1) is required
for the association of survivin with centromeres, whereas
de-ubiquitylation of Lys63 by the hFAM enzyme promotes
its dissociation from centromeres.
Less is known about the role of the third non-enzymatic
subunit, borealin, in targeting the CPC to centromeres11,12.
Survivin does not bind INCENP in borealin-depleted
cells, which suggests a role for borealin in stabilizing the
survivin–INCENP interaction17. The borealin-related
protein Dasra-A is essential for loading the CPC onto
chromatin in X. laevis egg extracts34, but humans appear
to lack this protein. Borealin can bind DNA in vitro12,
and DNA methylation may be required for loading the
complex onto pericentromeric heterochromatin before
mitosis49. It remains to be determined whether borealin
shows any preference for binding to methylated DNA.
Histones may also be involved in CPC targeting to
centromeres. Phosphorylation of the kinetochore his­
tone CENP‑A on Ser7 by Aurora-A in early prophase
was suggested to recruit Aurora-B to the inner centro­
mere47,56. However, others found that CPC targeting to
centromeres is independent of CENP‑A12. The histone
variant H2A.Z can bind INCENP57 and could also have
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REVIEWS
On chromosome arms
and centromeres
•
•
•
•
Histone H3
CENP-A
Condensin
Topoisomerase-IIα
• Histone H3 Ser10 and Ser28
phosphorylation
• Mitotic chromosome
structure
• Release of arm cohesion
• Kinetochore maturation
Spindle
assembly
• MgcRacGAP/CYK-4
• MKLP1/ZEN-4
• Intermediate
filaments (vimentin,
desmin, GFAP)
• Myosin II regulatory
light chain
• EVI5
•
•
•
•
•
• MCAK
• Stathmin
G1/S/G2 phase
Dam1 complex
HEC1/Ndc80
MCAK
Shugoshin
Tousled-like kinase-1
Prophase
Cytokinesis
At centromeres
Prometaphase
At cleavage furrow
At centromeres
Telophase
Regulation of
kinetochore–
microtubule
attachment
Spindle
disassembly
At spindle midzone
and cortex
Central
spindle
formation
Anaphase
• Centromeric cohesion
Metaphase
• Chromosome alignment
• Control of spindle
checkpoint
• Stability of bipolar spindle
Figure 3 | Chromosomal passenger complex localization and function during mitosis. Schematic representation
Reviews
| Molecular
Cell Biology
of the chromosomal passenger complex (CPC) localization (green) correlated with itsNature
multiple
functions
(grey boxes)
and principal targets (red boxes) during the different phases of mitosis relative to tubulin and chromosome dynamics.
In prophase, the CPC is found on chromosome arms where it phosphorylates histone H3 on Ser10 and Ser28. It is involved
in the release of arm cohesion and mitotic chromosome structure. During this phase it accumulates at centromeres where
the maturation of kinetochores begins and continues through prometaphase. The CPC is required for the formation of a
bipolar spindle and its stability from prophase/prometaphase to anaphase. In metaphase, it localizes at centromeres,
where it has a central role in centromeric cohesion and the regulation of kinetochore–microtubule attachments.
It controls the correct alignment of chromosomes on the spindle equator and the spindle checkpoint. In anaphase, the
CPC translocates to the spindle midzone and appears at the cortex; it is involved in the formation of the central spindle.
In telophase, the CPC concentrates at the cleavage furrow and, subsequently, at the midbody, where it is required for
completion of cytokinesis. Chromosomes, blue; tubulin, red; nuclear envelope, grey. CENP-A, centromere protein-A;
CYK-4, CYtoKinesis defect (Caenorhabditis elegans MgcRacGAP homologue); EVI5, ecotropic viral integration site-5;
GFAP, glial fibrillary acidic protein; HEC1, highly expressed in cancer-1; MgcRacGAP, Rac GTPase activating protein-1;
MCAK, mitotic centromere-associated kinesin; MKLP1, mitotic kinesin-like protein-1; Ndc80, yeast homologue of HEC1;
ZEN-4, Zygotic epidermal ENclosure defective (C. elegans MKLP1 homologue).
a role in targeting the CPC to centromeres. Microscopy
of chromatin fibres and chromatin immunoprecipitation
assays have suggested an essential role for H2A.Z in the
structure of the inner centromere58. H2A.Z-depleted
cells exhibit chromosome segregation and cytokinesis
defects58,59. It will be interesting to determine whether
the defects that arise from H2A.Z depletion involve
perturbation of CPC function.
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Functions of the CPC in early mitosis
The mobile CPC performs and controls many aspects
of mitosis, ranging from chromosome and spindle
structure to the correction of kinetochore–microtubule
attachment errors, regulation of mitotic progression
and completion of cytokinesis (FIG. 3). Of these, the least
understood is its function in regulating the mitotic
chromo­some structure.
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Mitotic chromosome structure. Histone H3 phospho­
rylation on Ser10 (and Ser28), a conserved hallmark
of Aurora-B activity48,60–62, is first detected in early G2
phase near centromeres and then spreads over the
entire chromosome as chromosomes condense during
prophase49 (FIG. 3). The role of this phosphorylation, if
any, in mitotic chromosome structure is unclear14,60,63.
It can negatively regulate binding of the chromodomain
of heterochromatin protein-1 (HP1) to the adjacent tri­
methylated Lys9 (Refs 64,65), but cannot alone account
for HP1 displacement from chromosome arms during
mitosis because acetylation of Lys14 is also required66,67.
Furthermore, the HP1α isoform remains associated
with mitotic centromeres, where it has a crucial role in
chromatid cohesion68.
The role of Aurora-B in loading the condensin
complex onto chromosomes is similarly controversial.
Condensin complexes are involved in the maintenance
of chromosome architecture throughout mitosis 69.
In D. melanogaster S2 cells and C. elegans, depletion
of Aurora-B causes a failure to recruit the condensin
proteins Barren62 (CAP‑H; also known as kleisin‑γ),
SMC‑4 and SMC‑2 (also known as CAP‑E; MIX‑1 is
the C. elegans homologue)70 to mitotic chromosomes.
Recently, Aurora-B was found to phosphorylate the con­
densin I subunits CAP‑D2, CAP‑G and CAP‑H in vitro
and to increase the loading of condensin I, but not of
condensin II, onto mitotic chromosomes71,72. However,
in a previous study, depletion of the CPC had no effect
on condensin association with mitotic chromosomes or
chromosome condensation in X. laevis egg extracts73.
Clearly, the role of the CPC in mitotic chromosome
structure merits further study.
Chromodomain
A conserved motif that is
present in various chromatin
proteins and is involved in
binding methylated histone
tails.
Condensin complex
A complex of two structural
maintenance of chromosomes
(SMC) subunits and three
auxiliary non-SMC subunits. It
is essential for the structural
integrity of chromosomes.
Taxol
Also known as paclitaxel. A
drug of major importance in
cancer chemotherapy that
suppresses microtubule
dynamic instability, thereby
stabilizing microtubules.
Dam1 complex
A multiprotein complex in
budding yeast that encircles
the plus ends of microtubules
proximal to the kinetochore
and which is important for
kinetochore–microtubule
interactions.
Spindle formation. The CPC is essential both for the
assembly and stability of a bipolar mitotic spindle.
Mitotic spindle assembly requires at least two pathways74:
first, the canonical pathway, which involves microtubule
nucleation at spindle poles and capture plus stabilization
by kinetochores; and second, the chromatin-driven path­
way, in which microtubules that assemble in the cytosol
are preferentially stabilized in the vicinity of mitotic
chromosomes by Ran-GTP, and then subsequently
organized into fusiform spindles through the action of
motor proteins74.
The CPC may be involved in both of these pathways.
If CPC function is perturbed, bipolar spindle formation
is disrupted in D. melanogaster S2 cells14. In human cells
that are depleted of borealin, bipolar spindles do form
but appear to ‘unravel’ during metaphase, giving rise
to ectopic poles that ultimately disrupt chromosome
segregation and cytokinesis11. The targets of the CPC in
centrosome-driven spindle assembly and organization
— if any — are not known.
The chromatin-driven pathway of spindle assembly
requires CPC function, at least in X. laevis egg extracts.
Immunodepletion of the CPC or addition of the Aurora
inhibitor ZM447439 blocks spindle assembly in this
system 10,75. Although this was initially assumed to
involve the interplay between the CPC and the RanGTP-regulated pathway of spindle assembly, recent
nature reviews | molecular cell biology
studies have focussed on stathmin (also known as onco­
protein-18; OP18), a microtubule-destabilizing protein
that has previously been implicated in spindle assem­
bly76. Stathmin activity is negatively regulated by phos­
phorylation on Ser16, which is induced in X. laevis egg
extracts by the addition of mitotic chromatin and stim­
ulated by the presence of taxol-stabilized microtubules76.
Stathmin is highly phosphorylated by Aurora-B on
Ser16 in vitro, and depletion of INCENP or Aurora‑B
in X. laevis egg extracts completely blocks stathmin
hyperphosphorylation34,77. It has been suggested that
Aurora-B activation in early mitotic X. laevis extracts
is a response to the increased level of the CPC on chrom­
atin, and is not driven by the widely studied Ran-GTP
pathway34. Consistent with this, clustering of the CPC
with an anti-INCENP antibody stimulates Aurora-B
activity and stathmin phosphorylation.
The kinesin-13 microtubule depolymerase MCAK
(mitotic centromere-associated kinesin) is also involved
in bipolar spindle assembly in X. laevis egg extracts.
MCAK mutants that are resistant to Aurora-B phospho­
rylation are unable to support bipolar spindle formation
or to localize to inner centromeres78. MCAK depletion
stabilizes microtubules in CPC-depleted cells, which
suggests that the requirement for the CPC in chromatininduced microtubule assembly reflects the activity of the
complex in inhibiting MCAK10,79.
In budding yeast, Aurora/Ipl1 is required for spindle
disassembly following the completion of anaphase 80.
This function of Ipl1 appears to be independent of its
other roles in chromosome segregation and the spindle
checkpoint.
Kinetochore attachment and chromosome segregation.
The CPC is required for proper chromosome bi-orienta­
tion at metaphase in all of the eukaryotes that have been
studied11,14,30,62,81–85 (BOX 1). This reflects a requirement
for the CPC in forming the trilaminar kinetochore86,
as well as in the regulation of kinetochore–microtubule
attachments.
Studies in budding yeast first revealed that Aurora/
Ipl1 is required to release spindle microtubules from
kinetochores82,87,88. It is now accepted that the CPC is
a key component in detecting aberrant kinetochore–
microtubule attachments and, particularly, a lack of
tension89 (FIG. 3). This role of the CPC appears to involve
the phosphorylation of key kinetochore–centromere
components by Aurora-B, although INCENP and sur­
vivin/Bir1 may also have important parts to play (see
below).
In budding yeast, Aurora/Ipl1 phosphorylates several
kinetochore targets, including components of the Dam1
complex31,90, which encircles kinetochore microtubules
near their plus ends. This phosphorylation was impli­
cated in the regulation of kinetochore–microtubule
interactions, but remained rather enigmatic because
vertebrate homologues of the target proteins could not
be identified.
A key target for Aurora-B within the vertebrate
kinetochore has been identified in recent studies. This
protein, HEC1 (highly expressed in cancer-1; known
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Box 1 | Chromosome bi-orientation and the spindle checkpoint
a Amphitelic or bi-orientated
b Syntelic
c Merotelic
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Accurate chromosome segregation requires that kinetochores from each sister chromatid
bind
microtubules
that
emanate
from opposing spindle poles (amphitelic attachment; see figure part a). This is achieved by a process called chromosome
bi-orientation. During prometaphase, various modes of kinetochore–microtubule attachment are observed. Most
attachments are monotelic at the beginning of prometaphase, with one kinetochore bound to microtubules from one
spindle pole and the other kinetochore unbound. Syntelic attachments, in which both kinetochores bind to microtubules
that emanate from the same pole, are also likely to be common during the earliest stages of chromosome attachment to
the spindle (see figure part b). Merotelic attachments, with a single kinetochore binding to microtubules from both spindle
poles, are likewise seen early in mitosis, but are clearly aberrant (see figure part c).
All of these modes of attachment, if not corrected, can lead to improper chromosome segregation and aneuploidy.
To avoid this, all cycling cells enter mitosis ‘with the brakes on’ as far as mitotic progression is concerned. In this case, the
‘brakes’ are applied by the spindle checkpoint network, which inhibits the activity of the anaphase promoting complex/
cyclosome (APC/C)149 until all chromosomes have achieved a proper bi-orientation. The APC/C is an E3 ubiquitin ligase
that targets securin, an inhibitor of the protease separase, for degradation by the 26S proteasome. Separase cleavage of
cohesin component RAD21 (also known as Scc1 or Mcd1) triggers sister chromatid separation and anaphase onset150.
The spindle checkpoint is activated both by unoccupied kinetochores and by kinetochores in which the microtubule
attachments are not under the correct amount of tension. The relationship between these two checkpoint pathways
remains controversial because a lack of tension may promote a lack of attachment. Although the workings of the
checkpoint remain under active investigation, most evidence suggests that the conserved checkpoint component MAD2
directly or indirectly inhibits the activity of the APC/C-associated factor CDC20, and that this somehow keeps the APC/C
inactive. This checkpoint is extremely important: the aneuploidy that would result from segregation of incorrectly
attached chromosomes is widely believed to be a factor that strongly predisposes cells to cancerous transformation.
Aneuploidy
A condition in which the
number of chromosomes is not
an exact multiple of the
haploid set.
Anaphase promoting
complex/cyclosome
(APC/C). A multisubunit E3
ubiquitin ligase that targets
proteins for proteasomemediated degradation by
attaching polyubiquitin chains
to them. It has a key role in
regulating the eukaryotic cell
cycle.
Chromosome congression
The movement of correctly
attached chromosomes to form
a metaphase plate at the
midplane of the mitotic
spindle.
as Ndc80 in yeast and hereafter referred to as HEC1/
Ndc80), is required for kinetochore–microtubule
interactions and chromosome segregation 91. Elegant
biochemical fractionation experiments revealed it to be
a subunit of the KMN complex (KNL‑1 (Kinetochore
NuL-1), Mis12 complex and Ndc80/HEC1 complex;
FIG. 4; BOX 2), which is essential for microtubule binding
by the kinetochore92. Phosphorylation of the N terminus
of HEC1/Ndc80 by Aurora-B negatively regulates micro­
tubule binding by the KMN network in vitro92 (FIG. 4a).
Interference with HEC1/Ndc80 phosphorylation in vivo
by antibody microinjection produced abnormally robust
kinetochore fibres and a high frequency of merotelic
attachments (BOX 1), whereas transfection with mutants
of the six putative Aurora-B phosphorylation sites had
a dominant-negative effect, suggesting that this domain
is required for microtubule turnover at kinetochores93.
Together, these studies suggest that Aurora-B phospho­
rylation of HEC1/Ndc80 might be one way in which
the kinetochore releases microtubules to which it has
formed inappropriate attachments.
MCAK contributes to proper anaphase chromo­
some segregation, and its depletion leads to chromosome
congression and segregation defects due to improp­
erly attached microtubules at kinetochores94. MCAK
is phosphorylated by Aurora-B at centromeres95,96.
Aurora-B phosphorylation of X. laevis MCAK on Ser196
inhibits its microtubule depolymerization activity95,96.
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The CPC is enriched specifically at non-attached5 and
merotelically attached kinetochores97. It has been sug­
gested that Aurora-B promotes MCAK accumulation
and somehow regulates its activity at these aberrant
attachments95,96. MCAK at merotelic attachments could
be regulated by a balance between Aurora-B activity
and that of a counteracting phosphatase, such as pro­
tein phosphatase-1, the γ-isoform of which localizes to
kinetochores in early mitosis61,98. MCAK may also be
activated at incorrectly attached kinetochores by the
inner centromere protein ICIS99.
How does the kinetochore know when microtubule
attachments are aberrant and should be relinquished?
One characteristic of many defective attachments is that
local tension within the kinetochore and its attached
microtubules is aberrant. A surprising study in budding
yeast has revealed a possible role for two CPC com­
ponents, INCENP/Sli15 and survivin/Bir1, in tension
detection by kinetochores100.
It had been known for several years that INCENP can
associate with microtubules101, but the significance of this
binding remained unclear. A recent study has now shed
light on this: an assay was used that measured the ability
of the basal yeast kinetochore (centromeric DNA with the
CBF3 complex bound to it) to attach to microtubules100.
It was known that CBF3–DNA complexes could bind
to microtubules if they were pre-incubated in yeast cell
extracts, but the crucial factor contributed by the extract
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a
b
Mis12
complex
HEC1/Ndc80 complex
KMN network
KNL-1
Microtubule
P
P P
Microtubule
CPC
H3K4me2-containing
nucleosomes
CENP-A-containing
nucleosomes
Centromeric
heterochromatin
Microtubule
CENP-C–H–I complex
Figure 4 | Regulation of kinetochore binding to microtubules by the chromosomal passenger complex (CPC).
a | The KMN network, composed of KNL-1 (Kinetochore NuL-1) protein, plus the Mis12
and highly
expressed
in cancer-1
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(HEC1)/Ndc80 complexes, has two microtubule-binding activities: one mediated by KNL-1 and the other by HEC1/Ndc80
(Ref. 92). The interaction between HEC1/Ndc80 and microtubules is negatively regulated by Aurora-B‑mediated
phosphorylation of the N terminus of HEC1/Ndc80 (Refs 92,93). b | Electron microscopy of a chromosome showing the
localization of the centromere (top). Inset (middle): magnification of a kinetochore attached to microtubules. On the
right is a diagram of the kinetochore surface, with centromeric nucleosomes that contain the histone variant CENP‑A
and histone H3 dimethylated on residue Lys4 (H3K4me2), on which the CENP-C–H–I complex assembles beneath
microtubules that are tethered to the kinetochore via the KMN network92.
had long resisted identification. This factor has now been
identified as a Sli15–Bir1 (INCENP–survivin) complex100.
Other kinetochore components are not required.
In summary, it now appears that microtubule bind­
ing by kinetochores is extremely complex, involving a
network of redundant weak interactions. HEC1/Ndc80
and the KMN network may be important for creating
multiple low-affinity links that can be released through
the action of Aurora-B in the CPC92. Links that involve
INCENP/Sli15 and survivin/Bir1 may enable kineto­
chores to sense tension within the spindle. MCAK may
use its ability to destabilize microtubules to release mero­
telic kinetochores where the attachment is undesirable,
but local tension within the spindle is normal. Regardless
of the exact mechanism, it is now clear that the CPC,
with its active kinase Aurora-B, is a master regulator
of kinetochore–microtubule attachments that control
different essential mechanisms of microtubule release.
Controlling the spindle checkpoint. The spindle check­
point (BOX 1) is a quality-control circuit that blocks
anaphase onset until all chromosomes have achieved
a bipolar attachment to the mitotic spindle 102. The
checkpoint can detect kinetochores that lack bound
microtubules as well as kinetochore–microtubule
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attachments that are not under proper tension. Whether
this represents two independent arms of the pathway
or whether the loss of tension results in microtubule
release (and hence, free kinetochores) remains under
intense debate. For example, in budding yeast, Aurora/
Ipl1 was proposed to activate the checkpoint under con­
ditions in which spindle tension is aberrant by creating
unattached kinetochores103.
Whatever the detailed mechanism, the CPC is
required for spindle checkpoint function when tension
is lost, but not in response to agents that disassemble
microtubules88 (for a review see Ref. 103). Using RNAi
technology, survivin and INCENP were shown to be
essential for checkpoint function in the presence of
taxol (which perturbs spindle tension by dampening
microtubule dynamics), and for the recruitment of the
checkpoint protein BUBR1 to the kinetochore15–17. The
use of the selective inhibitors ZM447438 and hesperadin,
or microinjection of inhibitory antibodies, revealed
that Aurora-B is similarly required for the checkpoint
response in the presence of taxol75,83,84,104. Aurora-B also
appears to cooperate with the checkpoint kinase BUB1
in maintaining the spindle checkpoint by promoting the
association of BUBR1 with the anaphase promoting
complex/cyclosome (APC/C)105.
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Box 2 | The KMN network
Years of searching for a single microtubule-binding ‘receptor’ on kinetochores have given way to models in which this
binding involves a network of cooperating interactions through several different protein subcomplexes151,152 (reviewed in
Ref. 153). Functional and structural investigation of kinetochore proteins in Caenorhabditis elegans led to the
identification of the KMN network (KNL‑1 (Kinetochore NuL-1), Mis12 complex and Ndc80/HEC1 (highly expressed in
cancer-1) complex, which consists of a single protein, KNL‑1, plus two protein complexes (reviewed in Ref. 154). The
interactions of the KMN complex with microtubules are regulated by the chromosomal passenger complex (CPC)
(FIG. 4a).
In C. elegans, depletion of either KNL‑1 or KNL‑3 shows a severe ‘kinetochore-null’ phenotype that is reminiscent of the
phenotypes that are seen after depletion of the key kinetochore proteins CENP‑A or CENP‑C151,155–157. KNL‑1 is required
for the localization of multiple kinetochore components, including the HEC1/Ndc80 complex (the ‘N’ in KMN) and the
checkpoint protein kinase BUB-1. KNL‑3 is essential for targeting the Mis12 complex156, the third component of the KMN
network. Mis12, a protein that was first discovered in Schizosaccharomyces pombe158, is conserved from yeast to humans.
It forms a complex in yeast with three other proteins, Dsn1, Nnf1 and Nsl1 (Ref. 153). Mis12 depletion by RNA
interference leads to chromosome misalignment and mis-segregation159 as a consequence of disrupted kinetochore
assembly151. Mis12 is required for recruitment of the HEC1/Ndc80 complex to kinetochores160 (FIG. 4b). The HEC1/Ndc80
complex is composed of four subunits, HEC1/Ndc80, Nuf2, Spc24 and Spc25, in organisms ranging from budding yeast to
humans. The HEC1/Ndc80 complex is involved in kinetochore–microtubule attachments and is also required to recruit
checkpoint proteins to incorrectly attached kinetochores (reviewed in Ref. 161).
The HEC1/Ndc80 complex is apparently required for
spindle checkpoint activity, possibly by recruiting the
essential checkpoint proteins MPS1 (a protein kinase),
MAD1 (a scaffolding protein) and MAD2 (BOX 1) to
kinetochores106,107. Knowing that Ndc80 phosphorylation
by Aurora-B regulates microtubule binding to the kine­
tochore92,93, it will be interesting to determine whether
this modification is involved in signalling to the spindle
checkpoint.
INCENP appears to function with CDK1–cyclin B to
regulate the timing of anaphase onset. CDK1 phospho­
rylation of INCENP on Thr388 creates a binding site
for PLK1 (Ref. 108). This interaction is required for the
recruitment of PLK1 to kinetochores in mitosis. It also
regulates the normal timing of the metaphase-to-ana­
phase transition by an as-yet-unknown mechanism108
that could involve regulation of the spindle checkpoint.
Shugoshin family
A family of proteins that
protect centromeric cohesin
from cleavage by separase,
possibly by recruiting protein
phosphatase-2A to
centromeres.
Midbody
A dense structure that is
derived from the remains of
the central spindle during late
telophase and is present at the
intercellular bridge during
cytokinesis.
Regulation of centromeric cohesion. Chromosome seg­
regation in mitosis requires the establishment of cohe­
sion between sister chromatids during or after S phase,
and its release at the onset of anaphase. In vertebrates,
this cohesion is released in two stages. During prophase,
phosphorylation of the cohesin subunit SA2 by PLK1 on
the chromosome arms triggers the release of the bulk
of the complex and the relaxation of arm cohesion109. In
X. laevis extracts, depletion of Aurora-B decreases the
efficiency of this cohesin release in prophase110. In addi­
tion, cohesin release is prevented by Aurora-B inhibi­
tors in prometaphase-arrested cultured human cells.
The underlying mechanism is not clear because there is
no in vivo evidence that cohesin is a direct substrate of
Aurora-B (which appears to cooperate with PLK1 in the
relaxation of arm cohesion)110,111.
Following the release of arm cohesion, sister chroma­
tids are held together by cohesion at centromeres, and are
protected from release during prophase by members of
the Shugoshin family (for a review see Ref. 112). Shugoshin
proteins function in both mitosis and meiosis, at least in
part, through interactions with protein phosphatase-2A
806 | october 2007 | volume 8
(PP2A)113–115, which has been proposed to locally reverse
the phosphorylation of key substrates by enzymes such
as PLK1, thereby preventing the dissociation of cohesin.
INCENP is required for the correct centromeric localiza­
tion of Shugoshin proteins in both mitosis116 and meio­
sis116 (see below). INCENP can bind to both MEI‑S332
(D. melanogaster Shugoshin) and Aurora-B, and may
function by bringing the two together116. Aurora-B
phosphorylation is required for the stable centromeric
localization of MEI‑S332 in D. melanogaster116.
When Aurora-B is depleted by RNAi, MEI‑S332/
SGO1 (human Shugoshin) associates diffusely with
chromatin and does not accumulate on centromeres116,117.
This may be partly mediated by the checkpoint kinase
BUB1, which is also required for localization of SGO1
(Refs 115,118). The ectopic presence of SGO1 along the
chromatid arms could provide an explanation for defects
in arm cohesion release during mitotic prophase when
Aurora-B function is compromised117. It is worth not­
ing that other members of the CPC, including INCENP
and survivin, are also dispersed along chromatid arms
when Aurora-B is depleted by RNAi9,14. Therefore, the
relocalization of MEI‑S332/SGO1 could be mediated
by INCENP, which binds MEI‑S332 in vitro116, or by
survivin. Further evidence of the interplay between
the CPC and Shugoshin at centromeres is also seen in
S. pombe, in which the Shugoshin-family member Sgo2
interacts directly with survivin/Bir1 and is essential for
maintaining the CPC on centromeres upon checkpoint
activation119,120.
The CPC during late mitosis
Once the spindle checkpoint has been inactivated and
sister chromatids have separated, cells proceed through
anaphase, telophase and cytokinesis as they exit mitosis.
CPC targeting in anaphase. The CPC transfers from
centromeres to the spindle midzone during late meta­
phase–early anaphase, to the equatorial cortex later in
anaphase and, finally, concentrates in the midbody during
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telophase and cytokinesis45 (FIGS 1,3). During mitotic exit,
the CPC appears to associate with material that coats
antiparallel microtubules in the spindle midzone. This
correlates with a dramatic loss of dynamic behaviour,
and the complex becomes largely static as it associates
with the central spindle53,54.
The transfer of INCENP to the central spindle
requires kinesin-related motor proteins, and may be
regulated by dephosphorylation of key subunits. In
budding yeast, the transfer is triggered by separase
activation of Cdc14 phosphatase121. Although the roles
of CDC14 paralogues in vertebrate cells are less well
defined, a kinesin superfamily member, MKLP2 (mitotic
kinesin-like protein-2), which can bind both Aurora-B
and CDC14, is required to localize the CPC and CDC14
to the spindle midzone at anaphase122. Intriguingly, the
binding of phosphorylated MKLP2 to PLK1 is required
for targeting PLK1 to the midzone123. Whether this
involves the motor activity of MKLP2 or the associated
phosphatase activity of CDC14, which could release
PLK1 from INCENP, remains to be determined.
Genetic analysis in D. melanogaster reveals a similar
interaction between Subito (the MKLP2 homologue in
D. melanogaster), INCENP, Aurora-B and Polo kinase124.
Mutants of subito also fail to localize Aurora-B and
Polo kinases to the spindle midzone, with the outcome
being defective spindle assembly and chromosome
segregation124.
Bivalent
A pair of homologous
(maternal plus paternal)
chromosomes that are linked
together after prophase of
meiosis I.
Chiasmata
Chromosomal structures that
interlink homologous
chromosomes at the site of
mature crossovers during
meiosis I.
An essential and universal role in cytokinesis. Even
though the mechanisms that regulate cytokinesis have
proven to be surprisingly difficult to elucidate, it is now
clear that members of the CPC are absolutely essential
for this process. INCENP concentrates at the site of
presumptive cleavage furrow formation even before
myosin II (Ref. 125), and it associates intimately with the
inner surface of the plasma membrane in the contracting
furrow45. Although normal CPC localization is not
required for the initiation of furrowing51, studies in many
different organisms reveal that CPC components are
essential for the completion of cytokinesis7,9,11,14,15,126,127.
Defects in cytokinesis may also involve interactions
between the CPC and kinesin superfamily proteins.
MKLP2 is not the only kinesin superfamily member to
interact with the CPC during anaphase. In C. elegans, the
Aurora-B kinase AIR‑2 is required for localization of the
MKLP1 homologue ZEN‑4 (Zygotic epidermal ENclosure
defective) to the spindle midzone128. In D. melanogaster
cells, Aurora-B small interfering RNA (siRNA) results
in cytokinesis failure14 and, in one study, in improper
localization of the MKLP1 homologue Pavarotti62. Loss
of MKLP1 and MKLP2 results in spindle midzone and
cytokinesis defects122,128. Aurora‑B can phosphorylate the
MKLP1 homologue ZEN‑4 in C. elegans, and human
and non-phosphorylatable MKLP1/ZEN‑4 mutants are
impaired in the completion of cytokinesis129.
MKLP1/ZEN‑4 forms a complex with Rac GTPase
activating protein-1 (MgcRacGAP)/CYK‑4 (CYK‑4 is
the C. elegans homologue)130, which is also important
for spindle assembly and cytokinesis131. This complex
has been termed centralspindlin130. Aurora-B-mediated
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phosphorylation of MgcRacGAP/CYK‑4 appears to be
required for the completion of cytokinesis132. Other com­
ponents that are required for cytokinesis that interact
with the CPC include vimentin, the phosphorylation of
which by Aurora-B appears to be required for cleavage
furrow formation133,134, and the protein EVI5 (ecotropic
viral integration site-5), which has an unknown role in
cytokinesis135.
Aurora-B may also be involved in the negative con­
trol of cytokinesis. In budding yeast, the NoCut pathway
has been proposed as an Aurora/Ipl1-dependent mecha­
nism for ensuring that cytokinesis occurs only after the
segregating chromatids have cleared the midzone. This
pathway may function by regulating the localization of
anillin-like proteins136.
The CPC in meiosis
Studies of CPC function in meiosis in most multi­cellular
model organisms are hindered by the fact that null muta­
tions in genes encoding the complex subunits are embry­
onic lethal137–140 and, therefore, do not enable study of
the meiotic divisions. Depletion of CPC components
by RNAi in C. elegans first revealed meiosis-specific
functions of the complex141,142 (see below). Chemical
inhibition of Aurora kinases in meiotic cells produces
phenotypes with similar effects to those seen in somatic
cells, and causes defects in chromosome condensation143,
chromosome alignment, spindle assembly and spindle
checkpoint signalling144.
The requirement to segregate paired maternal and
paternal homologous chromosomes rather than sister
chromatids during meiosis I necessitates several impor­
tant differences from mitosis (FIG. 5). During metaphase I,
bivalents are kept at the spindle midzone by chiasmata that
hold the recombined chromosome arms together. At
anaphase I, the resolution of sister chromatid cohesion
distal to the chiasmata allows chromosome segregation.
Experiments in C. elegans showed that this release requires
the CPC141,142. Remarkably, CPC localization in metaphase
of meiosis I in C. elegans is restricted to chromosome arms
that are distal to chiasmata. Depletion of the Aurora-B
homologue AIR‑2 by RNAi causes defective chiasmata
resolution and homologue segregation141,142.
Because sister chromatids remain paired and move
to the same spindle pole during anaphase I of meiosis
(FIGS 5,6), the regulation of centromeric cohesion must
also differ from that in mitosis. Sister centromere cohe­
sion needs to be protected until the metaphase–anaphase
transition of the second meiotic division, when sister
chromatids segregate from each other. The CPC has a
dual role in the protection of meiotic centromeric cohe­
sion. Similar to what happens in mitosis (see above),
D. melanogaster Incenp mutants are defective in the
regulation of MEI‑S332/Shugoshin behaviour in meiosis,
and sister chromatids separate prematurely during the
first meiotic division116. Aurora/Ipl1 is not only required
for correct Sgo1 localization145,146, but it is also vital to
maintain the PP2A subunit Rts1 on yeast centromeres
after anaphase I until metaphase II146, thereby facilitat­
ing the dephosphorylation of centromeric cohesin and
protecting it from Polo-dependent cleavage by separase.
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Therefore, the CPC performs differing roles in
mitosis and meiosis; that is, preserving centromere
cohesion through the onset of anaphase of meiosis I,
while allowing its release at the metaphase–anaphase
transition of mitosis and meiosis II (FIG. 5). How it does
this is not known, but the complex itself exhibits sig­
nificant differences in localization between mitosis and
meiosis116,147.
Synaptonemal complex
A proteinaceous complex that
links pairs of homologous
chromosomes during
pachytene of meiosis I. It forms
during zygotene and
disassembles during diplotene
stages of meiosis I prophase.
During meiotic prophase, INCENP localizes to
the central element of the synaptonemal complex and
relocal­izes to chromocenters by late pachytene 147.
During prometaphase and metaphase I, both INCENP
and Aurora-B accumulate at centromeres. However,
in meiosis I, the complex does not transfer to central
spindle microtubules in early anaphase I as it does in
mitosis. Instead, it remains at the centromeres of the
Meiosis I
Metaphase I
Early anaphase I
Anaphase I
Telophase I
Aa
Ba
Ca
Da
Ab
Bb
Cb
Db
Meiosis II
Metaphase II
Anaphase II
Ea
Fa
Eb
Fb
Pachytene
A stage of the prophase of
meiosis I during which the
homologous chromosomes are
paired lengthwise, forming
thick threads, and are linked by
the synaptonemal complex.
Figure 5 | Chromosomal passenger complex
localization during meiosis. Distribution of
chromosomal passenger proteins in Drosophila
melanogaster male meiosis. Indirect immunofluorescence
labelling of the inner centromere protein INCENP (green),
microtubules (red) and DNA (blue) (panels Aa–Fa) and a
general schematic representation (panels Ab–Fb) of
INCENP localization during the different phases of
meiosis I (parts A–D) and meiosis II (parts E–F). In
metaphase I, chromosomes align at the spindle equator
and INCENP is localized at centromeres (Aa, Ab). In early
anaphase I, INCENP remains at centromeres; a distinct
subpopulation of INCENP is present at the cell cortex
where the cleavage furrow later forms (Ba, Bb). In
anaphase, some INCENP spreads from the centromere
onto the chromosome arms and it also appears at the
spindle midzone (Ca, Cb). In telophase I, INCENP remains
diffusely distributed throughout the condensed nuclei, as
well as locating to the midbody (Da, Db). In metaphase of
meiosis II, INCENP is located at centromeres (Ea, Eb). As
in meiosis I, a portion of INCENP remains diffusely
distributed on the chromosomes at anaphase II (Fa, Fb),
whereas a subpopulation transfers to the central spindle
and ends up in the midbody in telophase II (not shown).
Scale bars represent 5 µm.
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REVIEWS
At centromeres
and cortex
Sister centromere cohesion
At centromeres
• Kinetochore orientation
• Correction of chromosome
attachment errors
Metaphase I
Early anaphase I
At centromeres,
chromosome arms,
cortex and spindle midzone
• Release of arm cohesion
distal to chiasmata
(C. elegans)
At cleavage furrow
and reforming nucleus
Cytokinesis
Telophase I
Late anaphase I
Figure 6 | Localization and function of the chromosomal passenger complex during meiosis I. Schematic
representation of the localization of the chromosomal passenger complex (CPC; green)
correlated
its meiotic
Nature
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| Molecular
Cell Biology
specific functions (grey boxes) and relative to the dynamics of tubulin (red) and chromosomes (blue and grey). During
prometaphase I, the CPC accumulates at centromeres where it has a central role in the protection of centromeric
cohesion until the metaphase I–anaphase I transition of the second meiotic division. The CPC is required for the stable
centromeric localization of the protector protein MEI‑S332/Shugoshin directly through binding to INCENP and
phosphorylation by Aurora-B kinase. Aurora-B also regulates Shugoshin by ensuring the stable localization to centromeres
of Rts1, the budding yeast regulatory subunit of protein phosphatase-2A146. The CPC collaborates with the monopolin
protein complex to promote sister kinetochore co-orientation. C. elegans, Caenorhabditis elegans .
segregating bivalents until late anaphase I, when it
redistributes along the chromosome arms and starts
to accumulate at the spindle midzone116,147. At the meta­
phase II–anaphase II transition, the complex transfers
to microtubules, as in mitosis (FIGS 5,6).
How does Aurora-B regulate kinetochore–microtubule
interactions in the first meiotic division when both sister
kinetochores are orientated towards the same spindle pole
(referred to as co-orientation) and there is no local ten­
sion across the centromere? Studies of the distribution of
MCAK and Aurora-B in mouse spermatocytes show that
these proteins form a unique ring-like structure that sur­
rounds both sister kinetochores in meiosis I148. Aurora-B
could somehow recognize each pair of sister kinetochores
as a unit and interact with it in a similar fashion as it does
with a single sister kinetochore in mitosis. This seems to be
the case in budding yeast, in which the monopolin protein
complex promotes sister kinetochore co-orientation and,
thus, ensures that the yeast Aurora/Ipl1 acts upon sister
kinetochore pairs145. Homologues of the components of
the monopolin complex have yet to be found in higher
eukaryotes, so the identification of factors that promote
co-orientation remains an important priority.
nature reviews | molecular cell biology
Conclusions and future directions
We now understand that the CPC orchestrates mito­
sis and meiosis at several different levels to ensure
that two daughter cells are generated with an accurate
distri­bution of genetic material. The regulation of
kinetochore–microtubule attachments in a bipolar
spindle, the delay of anaphase onset when spindle tension
is aberrant, the regulation of sister chromatid cohesion
and the completion of cytokinesis are among the crucial
mitotic functions that require CPC activity. So far, a
few of the key substrates of Aurora-B kinase have been
identified, although many more are clearly waiting to
be discovered, and the functional consequences of sub­
strate phosphorylation remain to be elucidated. Other
key questions to be investigated in the future include
the possible role of the CPC in integrating signalling
through multiple protein kinase pathways (for example,
integrating Aurora-B and PLK1 signalling), and the pos­
sibility that there are multiple CPC subcomplexes that
specialize to control particular mitotic functions. The
score for the elaborate and wonderful symphonies that
are mitosis and meiosis therefore remains unfinished,
with much more to be written.
volume 8 | october 2007 | 809
© 2007 Nature Publishing Group
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Competing interests statement
The authors declare no competing financial interests.
DATABASES
FlyBase: http://flybase.bio.indiana.edu
MEI‑S332
Protein Data Bank: http://www.pdb.org/pdb/home/home.do
2BFY
UniProtKB: http://ca.expasy.org/sprot
Aurora-B | borealin | CENP‑A | HEC1 | INCENP | MCAK | PLK1 |
stathmin | survivin | TD‑60
FURTHER INFORMATION
William C. Earnshaw’s homepage:
http://www.wcb.ed.ac.uk/earnshaw.htm
Cell Biology (2nd edition), a textbook by T.D. Pollard
and W.C. Earnshaw with J. Lippincott-Schwartz and
G. T. Johnson: http://www.us.elsevierhealth.com/product.
jsp?isbn=9781416022558
All links are active in the online pdf
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