Download The TACC proteins: TACC-ling microtubule dynamics and

Review
The TACC proteins: TACC-ling
microtubule dynamics and centrosome
function
Isabel Peset1 and Isabelle Vernos1,2
1
Cell and Developmental Biology Program, Centre for Genomic Regulation (CRG), University Pompeu Fabra (UPF), Dr Aiguader 88,
Barcelona 08003, Spain
2
Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain
A major quest in cell biology is to understand the
molecular mechanisms underlying the high plasticity
of the microtubule network at different stages of the
cell cycle, and during and after differentiation. Initial
reports described the centrosomal localization of
proteins possessing transforming acidic coiled-coil
(TACC) domains. This discovery prompted several
groups to examine the role of TACC proteins during cell
division, leading to indications that they are important
players in this complex process in different organisms.
Here, we review the current understanding of the role of
TACC proteins in the regulation of microtubule
dynamics, and we highlight the complexity of centrosome function.
Introduction
Cell proliferation and differentiation require dramatic
rearrangements of the cytoskeleton that rely on the highly
dynamic nature of the cytoskeletal components. Microtubules are dynamic filaments with fundamental roles in
eukaryotic cell organization and function. During cell
division, they form the bipolar spindle, which segregates
the chromosomes into the two daughter cells. Microtubules
show prolonged states of polymerization and depolymerization that interconvert stochastically, exhibiting frequent transitions between growing and shrinking
phases, a property called ‘dynamic instability’ [1]. In the
cell, multiple factors modulate this property by acting
positively or negatively on the nucleation, elongation or
destabilization of microtubules [1–3]. The relative activity
of all these factors determines the steady-state length and
stability of microtubules, in addition to their organization,
and it is largely dictated by global and local phosphorylation–dephosphorylation reactions [2,3]. In addition, other
types of factors that have microtubule-severing and anchoring activities also influence the microtubule network. The main microtubule-organizing centre (MTOC)
of animal cells, the centrosome, acts as a platform upon
which the different factors and activities accumulate in a
regulated manner. It therefore exerts a tight local and
temporal control on the number, distribution and polarity
of microtubules [4,5].
Corresponding authors: Peset, I. ([email protected]);
Vernos, I. ([email protected]).
Transforming acidic coiled-coil (TACC) proteins emerged
initially as a group of proteins implicated in cancer. The
first member of the TACC family to be discovered was
identified in a search of genomic regions that are amplified
in breast cancer. It was named transforming acidic coiledcoil 1 (TACC1) because of its highly acidic nature, the
presence of a predicted coiled-coil domain at its C terminus
(now known as the TACC domain), and its ability to
promote cellular transformation [6]. TACC proteins are
present in different organisms, ranging from yeasts to
mammals. There is only one TACC protein in the nematode Caenorhabditis elegans (TAC-1), in Drosophila melanogaster (D-TACC), in Xenopus laevis (Maskin), and
Abbreviations
AINT: ARNT interacting protein
AKAP350: A kinase (PRKA) anchor protein
Alp7: Altered growth polarity 7
Ark1: aurora-related kinase
ARNT: aryl hydrocarbon nuclear translocator protein
AZU-1: anti-zuai-1
CBP: calcium-binding protein
CPEB: cytoplasmic polyadenylation element binding protein
DCLK: doublecortin-like kinase
ECTACC: endothelial cell TACC
E1F4E: eukaryotic initiation factor 4E
ERIC: erythropoietin-induced cDNA
FOG-1: Friend of Gatal
GAS41: glioma amplified sequence 41
GCN5L2: general control of amino-acid synthesis 5-like 2
g-TURC: g-tubulin related complex
HEAT: huntingtin, elongation factor 3, A subunit of protein
phosphatase 2A and TOR1
INI-1: SWI/SNF core subunit
Ipl1: Increase-in-ploidy 1
ISREC: Swiss Institute for Experimental Cancer Research
KIF2C: kinesin family member 2C
LIS1: Lissencephaly-1
LSM7: U6 small nuclear NRA associated
MBD2: methyl-CpG binding domain protein 2
Mial: melanoma inhibitory activity 1
Mps1: MonoPolar Spindle 1
NDEL1 and NUDEL: nude nuclear distribution gene E homolog
(A. nidulans)-like 1 pCAF, p300/CBP-associated factor
SmG: snRNP Sm protein G
TTK: TTK protein kinase
Zyg-8: ZYGote defective
Zyg-9: ZYGote defective
0962-8924/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2008.06.005 Available online 23 July 2008
379
Review
in the fission yeast Schizosaccharomyces pombe (Alp7
also known as Mia1p); by contrast, mammals have three
such proteins (TACC1, TACC2 [also known as AZU-1 and
ECTACC] and TACC3 [also known as AINT and ERIC1])
[7–11]. Alternative splicing further increases the complexity of the TACC protein family in mammals and flies
[12–16].
The three human genes encoding TACC proteins are all
in genomic regions that are rearranged in certain cancers,
and their expression is altered in cancers from different
tissues. TACC1 and TACC2 are located in chromosomes
8p11 and 10q26, respectively, two regions that are implicated in breast cancer and other tumors [6], and TACC3
maps to 4p16, within a translocation breakpoint region
associated with the disease multiple myeloma [17].
Trends in Cell Biology Vol.18 No.8
Although TACC1 was originally found to be upregulated
in breast cancer [6], subsequent studies found that its
expression is reduced in ovarian and breast cancer tissues
[18,19]. TACC3 is also upregulated in several cancer cell
lines, including lung cancer [17,20]; but, again, it was
reported as being absent or reduced in ovarian and thyroid
cancer tissues [21]. Initially, it was suggested that the
TACC2 splice variant AZU-1 is a tumor suppressor in
breast cancer. However, the lack of any tumor phenotype
in Tacc2-knockout mice did not support this idea [22]. It
therefore appears that these proteins can be upregulated
or downregulated in different types of cancer or, surprisingly, even in the same type [14,18–25]; as such, their
putative involvement in cancer development and/or progression is unclear.
Figure 1. The TACC family of proteins: structural organization and regions of interaction with binding partners. The figure shows alignment of the key structural features,
and the position of domains that interact with binding partners (underlined regions). TACC proteins have the conserved coiled-coil TACC domain at their C terminus (blue
box). In addition, some members have highly acidic, imperfect repeats of 33 amino acids (termed SPD repeats [28] owing to their specific amino acid composition [pale-blue
boxes]) or a Ser–Pro Azu-1 motif (SPAZ) [24] (dark-grey boxes). Yellow lines indicate the position of nuclear localization signals (NLSs). The conserved consensus sequences
for AurA phosphorylation are shown as orange bars. The conserved Ser residue is highlighted in orange, and additional consensus sites in Maskin are indicated in grey. The
position of the Leu residue, which is important for the C. elegans TAC-1–Zyg-9 interaction, is shown with a white line [44]. For the sake of simplicity, only TACC family
proteins that have mapped interactions are shown.
380
Review
Almost at the same time as the identification of
TACC1 in humans, Maskin was identified and extensively
characterized as a factor involved in the regulation of
mRNA translation during maturation of Xenopus oocytes
[26]. Other TACC family members have also been implicated in various events related to gene regulation, including the regulation of translation, RNA maturation
and gene expression (Figure 1, Table 1) [13,25,27–31].
However, to date, no major common role has emerged
for TACC proteins in these processes. By contrast, a major
breakthrough came with the identification of D-TACC as a
Drosophila microtubule-associated and centrosomal
protein required for centrosome activity and microtubule
assembly during mitosis [12]. Since then, the idea that
TACC proteins have a role in regulating microtubule
assembly has gained solid support through various studies
performed in different experimental systems. In the light
of these data, we review here our current understanding of
Trends in Cell Biology
Vol.18 No.8
the role of TACC proteins at the centrosome, and we
discuss some of the issues that still remain to be addressed.
The TACC proteins
The TACC domain is the signature of this protein family.
This coiled-coil domain is found at the C terminus of all
the family members, which have otherwise very diverse
N-terminal domains (Figure 1) [7,16]. The TACC domain
shows a high level of conservation throughout evolution,
and the shorter member of the family, C. elegans TAC-1,
consists of basically one TACC domain [8–10]. Together,
this suggests that the TACC domain carries most of
the common functional properties of this family of
proteins.
The temporal and tissue-specific expression patterns of
the three mammalian TACC proteins have been more
extensively studied. TACC1 can be detected in several
adult tissues, but relatively high levels of expression occur
Table 1. Partners of TACC proteins, and the putative functions of their interactions
The table summarizes all the interreactions described in the literature for some TACC proteins. Proteins involved in MT dynamics and centrosomal functions are indicated in
red; proteins involved in RNA regulation are indicated in green; proteins involved in gene regulation are indicated in blue and proteins involved in nucleo-cytoplasmic
transport are indicated in black.
Abbreviations: aa, amino acids; AKAP350, A kinase (PRKA) CPC, chromosomal passenger complex; CRC, chromatin-remodeling complex; CT, centrosome; HAT, histone
acetyltransferase; IF, immunofluorescence; IP, immunoprecipitation; MT, microtubule; n.d., not described; PCM, pericentriolar material; PD, pulldown; Y2H, Yeast two hybrid.
381
Review
only at the beginning of development, after which it
becomes dramatically downregulated [6,32]. TACC2 is also
widely expressed, showing the highest levels in heart and
muscle [32,33]. In mice, expression was detected at all
developmental stages [13]. By contrast, TACC3 is
expressed in relatively few adult tissues, but it shows
elevated levels in testis and ovary, and in the hematopoietic lineages [17,32,34]. During mouse development, TACC3
is present in all the embryonic stages and particularly in
proliferating tissues [15,32,35]. These data suggest that
TACC3 has a role during cell division, in particular during
development. Indeed, TACC3-deficient mice show embryonic lethality, associated with a greatly reduced cell number, widespread apoptotic cell death, and mitotic defects
[36,37]. Interestingly, this phenotype was partially rescued
in mice that had reduced levels of the tumor suppressor
protein p53 [36]. However, to date, no clear picture has
emerged to describe the molecular mechanism linking p53
activity and TACC3.
Intracellular localization of TACC proteins
Little information is available concerning the cellular
localization of TACC family members in interphase,
although studies have revealed that some of them – the
three human members and Maskin – are nuclear [38,39]. It
is during cell division that TACC proteins show their most
characteristic localization – within the centrosome
(Figure 2, Box 1) [8–12,38,40–43]. In humans, the three
family members show slightly different distribution patterns. TACC2 is strongly associated with the centrosome
throughout the cell cycle, whereas TACC1 and TACC3 only
localize to the centrosome during mitosis – TACC1 weakly,
and TACC3 covering a larger area [38]. These differences
in localizations suggest that the three human TACC
proteins have non-overlapping functions.
The centrosomal localization of TACC proteins is highly
dynamic. However, given that microtubules do not modulate the rapid exchange between centrosomal and cytoplasmic pools in C. elegans, and considering that the
centrosomal localization of TAC-1, D-TACC and human
TACC proteins is insensitive to microtubule-depolymerizing drugs, this class of proteins can be considered as core
components of the centrosome [8,9,12,38].
The characteristic centrosomal targeting of TACC
proteins relies on their conserved TACC domain. Indeed,
this domain alone was shown to localize strongly to the
spindle poles and to the centre of centrosomal asters
[12,42]. Recently, it was demonstrated that two residues
(L229 and M581) in the TACC domain of C. elegans TAC-1
are important for targeting TAC-1 to the centrosome [44].
It is unclear at the moment whether these residues are
important for structural reasons or for protein–protein
interactions.
However, the localization of TACC proteins is not
restricted to the centrosome, and most of them also associate with microtubules during cell division to various
extents (Figure 2). Human TACC3 shows only restricted
association with spindle microtubules, whereas Maskin
and C. elegans TAC-1 localize all along spindle microtubules, and D-TACC associates with both spindle and astral
microtubules (Figure 2a,b) [8,12,38,40,42,45–47]. These
382
Trends in Cell Biology Vol.18 No.8
microtubule localization patterns are therefore specific
for each protein and are determined by sequences outside
the TACC domain. It is interesting to note that truncated
proteins lacking the TACC domain do not localize to centrosomes or microtubules in Drosophila or Xenopus [12,42].
Interestingly, some data suggest that TACC proteins
bind to the ends of microtubules. On the one hand, the high
degree of accumulation to the spindle poles suggests that
TACC proteins bind to microtubule minus-ends. In fact,
this localization was directly observed for D-TACC in
Drosophila embryos, in which microtubule minus-ends
can be distinguished from the centrosomal aster at the
spindle poles (Figure 2a) [12]. Consistently, Maskin localizes to the centre of taxol-induced asters in Xenopus egg
extracts [42]. On the other hand, green fluorescent protein
(GFP)-labeled D-TACC proteins were visualized in living
Drosophila embryos as dots moving towards and away
from the centrosome, presumably associated to shrinking
or growing microtubule plus-ends [48]. Immunolocalization studies also suggest that TACC proteins are associated with microtubule plus-ends in the vicinity of the
chromosomes [8,9,11,12,48]. The mechanism underlying
the preferential microtubule end localization of TACC
proteins is still unclear. In vitro studies did not reveal
any preference for binding of Maskin to either the plusends or the minus-ends [40], suggesting that other proteins
mediate these end localizations.
Function(s) of the TACC proteins during cell division
To date, all the phenotypes described for situations in
which the expression of TACC proteins is altered are
related to defects in microtubule stability. In C. elegans,
TAC-1-depleted embryos show defects in pronuclear
migration, shorter spindles and defective spindle
elongation in anaphase. They also have shorter astral
microtubules and, as a consequence, spindle-positioning
defects. Interestingly, microtubules do form in the cytoplasm of TAC-1-depleted embryos, suggesting that TAC-1
is required for microtubule assembly only at the centrosome. Consistently, the recovery of fluorescent tubulin at
the centrosome after photobleaching is slower in TAC-1depleted embryos [8–10]. In Drosophila, d-tacc mutants
are female-sterile and have failures of pronuclear fusion.
The majority of embryos appear to be arrested in the first
mitotic division, and those that develop have abnormally
short centrosomal microtubules at all stages of the cell
cycle and eventually die as a consequence of the accumulation of mitotic defects [12]. In Xenopus, spindles
assembled in egg extracts depleted of Maskin have
reduced size and microtubule content, and the centrosomes nucleate fewer and shorter microtubules
(Figure 2) [40–42]. In HeLa cells, depletion of TACC1 does
not affect the cell cycle, although multipolar spindles form
and the cells also show proliferation defects [18]. By contrast, the silencing of TACC3 results in partially destabilized microtubules, spindles with reduced microtubule
content, and defects in chromosome alignment, in addition
to a high mitotic index due to mitotic arrest [46,49].
Consistently, increasing the concentration of D-TACC
and Maskin results in the accumulation of these proteins
at the spindle poles and an increase in spindle microtubule
Review
Trends in Cell Biology
Vol.18 No.8
Figure 2. Localization and function of TACC proteins at the centrosome. (a) Localization of different members of the TACC family of proteins. Immunofluorescence images of a
one-cell stage C.elegans embryo (courtesy of P. Gönczy, Swiss Institute for Experimental Cancer Research, Ch. Des Boveresses 155, Case postale, CH – 1066 Epalinges), a D.
melanogaster early embryo (courtesy of J.Raff, Cancer Research UK Senior Group Leader, The Gordon Institute, Tennis Court Road, Cambridge, CB2 1QN) and a X. laevis tissue
culture cell, showing the localization of TAC-1, D-TACC and Maskin respectively during anaphase. The upper row shows overlay images with microtubules in green, DNA in blue
and the corresponding TACC proteins in red. The lower row shows the distribution of each TACC protein alone. (b) Model for TACC protein function at the spindle pole. (i)
Maskin (TACC3) localizes to the spindle poles and along spindle microtubules in Xenopus egg extracts. Maskin is shown in green, microtubules in red, and DNA in blue. Below,
the model shows that the targeting of TACC proteins to the spindle pole requires their phosphorylation by Aurora A (AurA), which is active at the centrosome (as indicated by its
phosphorylation). Phosphorylated TACC proteins accumulate at the centrosome and recruit ch-TOG/XMAP215. TACC–TOG complexes interact efficiently with nascent
microtubules and stabilize them, counteracting the microtubule-destabilizing activity of MCAK at the centrosome. TACC–TOG complexes also associate with microtubule plusends. (ii) Spindles assembled in Xenopus egg extracts depleted of Maskin are on average 30% smaller and have reduced microtubule mass. Immunofluorescence with
antibodies against Maskin verifies that Maskin is not associated with the spindle under these conditions, confirming the efficiency of the depletion. Below, the model shows that,
in the absence of TACC proteins, the recruitment of TOG to the centrosome is not as efficient. As a result, nascent microtubules are not efficiently protected from destabilization
by centrosomal MCAK, and therefore fewer microtubules elongate from the centrosome. (iii) Addition of GFP–TD (TACC Domain) to a Maskin-depleted extract rescues the size of
the spindle and its microtubule density. GFP–TD localizes to the spindle poles in the same way as Maskin but not along spindle microtubules. Below, the model shows that GFPTD localizes to the spindle poles and recruits TOG. The TD–TOG complex stabilizes nascent microtubules at the centrosome, protecting them against the destabilizing activity of
centrosomal MCAK. TD ensures the loading of TOG to the microtubule plus-end, promoting microtubule growth from the centrosome.
383
Review
Trends in Cell Biology Vol.18 No.8
Box 1. The centrosome
Box 2. The ch-TOG/XMAP215 family
The centrosome is the main microtubule-organizing centre (MTOC)
of animal cells [4,5]. Centrosomes are small cellular organelles with
diverse morphologies, but they each consist of a pair of centrioles
surrounded by pericentriolar material (PCM), an electron-dense
material comprising core resident proteins and several nonpermanent structural and regulatory proteins. One characteristic
component of the PCM is g-tubulin, which promotes efficient
nucleation of microtubules at the centrosome. This imposes a
natural polarity on the resulting microtubule network, with microtubule minus-ends focused at the PCM, and microtubule plus-ends
reaching out into the surrounding cytoplasm. In addition, centrosomes act as platforms for the recruitment of multiple structural and
regulatory proteins. These include various microtubule nucleation
factors (e.g. pericentrin and centrosomin), microtubule-stabilizing
factors (e.g. ch-TOG/XMAP215 and TACC proteins), and microtubule-destabilizing or -severing factors (e.g. the kinesin-like protein
MCAK, and katanin) [59]. The efficiency of microtubule elongation
from the centrosome is therefore determined by the relative
abundance and activity of all of these factors at any given time
[65]. The dynamic changes in centrosome morphology and activity
are tightly regulated by several centrosomal kinases, including
members of the Aurora, Polo-like kinase (Plk) and Never In Mitosis A
(NIMA) [4,5] families. Remarkably, many centrosomal proteins also
exist in a soluble, cytoplasmic pool, indicating that centrosomes are
highly dynamic organelles [5].
In resting mammalian cells, the centrosome migrates to the cell
surface, and one of the centrioles differentiates into the basal body
of a cilium [4], which functions as a sensory organelle or as a fluid
propeller. The presence of a cilium is transient in proliferating cells
in which the activity and number of centrosomes varies in tight
coordination with the cell cycle. The primary cilium present in G1
disassembles before the cell progresses into the cell cycle, and the
two centrioles duplicate during S phase. During G2, the two newly
formed centrosomes undergo a process called maturation, and they
integrate the control of entry into M phase with an increase in their
microtubule-nucleation capacity. This generates a robust aster of
highly dynamic microtubules, which are involved in centrosome
separation and spindle assembly. During mitosis, the centrosomes
are positioned at each spindle pole and have an important role in
determining spindle orientation and the plane of cell division [4].
XMAP215 is the founding member of a large family of microtubulebinding proteins. It was originally purified in the late 1980s from
Xenopus egg extracts. It was characterized as a factor that increases
the elongation rate of microtubules in vitro [66], and was found to
be related to chTOG (for colonic and hepatic tumor overexpressed
protein), a human protein overexpressed in tumor cells.
XMAP215 shows very different effects on microtubule dynamics
in vitro compared with other microtubule-associated proteins
(MAPs) such as tau and MAP2. Indeed, although these brain MAPs
promote a strong reduction in the frequency of microtubule
catastrophe without substantially altering the growth rate,
XMAP215 stimulates the growth rate of microtubules without
changing the catastrophe frequency. XMAP215 has also been found
to increase the depolymerization rate and to reduce the frequency of
rescues (i.e. switching between depolymerization and polymerization phases). As a result, XMAP215 promotes an increase in
microtubule length and mass but, in so doing, promotes the
formation of microtubules of a highly dynamic nature, a property
that can be particularly important during M phase. XMAP215 has
also been shown to counteract the activity of the microtubuledestabilizing motor MCAK [51]. Consistently, all of the phenotypes
associated with the disruption of ch-TOG/XMAP215 family members
are related to changes in microtubule stability, including decreased
microtubule growth and defects in spindle function [67].
ch-TOG/XMAP215 family proteins have a C-terminal domain
involved in MT binding and a N-terminal domain consisting of a
variable number of TOG domains. Each TOG domain contains six
HEAT repeats, which fold into a paddle-like domain, and wrap itself
around one tubulin dimer [68]. Recently, it was proposed that
XMAP215 acts as a processive polymerase, catalyzing the addition
of 25 tubulin dimers while moving with the assembling microtubule
tip. Under some circumstances, XMAP215 can also catalyze the
reverse reaction, therefore modulating microtubule dynamics [69].
Although, Drosophila Msps also localizes to the acentrosomal
spindle poles during female meiosis [50,70], a universal feature of this
family of proteins is their localization to the centrosome of metazoan
cells and to the spindle pole bodies of yeast. The centrosomal
localization of XMAP215 is mediated by its C-terminal microtubulebinding domain [67]. In some organisms, this domain interacts with
TACC proteins [11,18]. Given that the localization of Msps is also
dependent on AurA in Drosophila [47], these data suggest that the
localization of XMAP215 family members to the centrosome relies on
their interaction with members of the TACC family.
length and number, which are effects opposite to those
caused by depletion. A similar phenotype – accumulation
at spindle poles and increase in microtubule length and
number – arises upon overexpression of human TACC3
but not TACC1 or TACC2 [38,42,48]. Altogether, these
data clearly indicate that the TACC proteins have a conserved function in promoting centrosomal microtubule
assembly.
How do TACC proteins participate in microtubule
stabilization?
Several observations strongly suggest that TACC proteins
function not at the level of nucleation of microtubules but,
rather, in the stabilization of microtubules. Experiments
performed in Xenopus egg extracts have clearly shown that
Maskin has no role in centrosomal microtubule nucleation
activity [42]. In C. elegans, TAC-1 mutant embryos do not
show defects in the distribution of the microtubule-nucleator g-tubulin [8]. In Drosophila d-tacc mutant embryos, the
localization of g-tubulin and the centrosomal proteins
CP190 and CP60 to the centrosome is normal, suggesting
that microtubule nucleation at centrosomes is also unaffected [12]. Therefore, TACC proteins promote microtubule
growth from the centrosome without altering the nucleation of microtubules.
384
The TACC proteins only interact very weakly with
polymerized microtubules in vitro, but they do co-pellet
very efficiently with microtubules in Drosophila embryos
and Xenopus egg extracts, suggesting that other factor(s)
are involved [12,42,48]. The first clue to shed some light
upon this issue was a report describing an interaction
between D-TACC and Minispindles (Msps), the Drosophila
member of the colonic and hepatic tumor-overexpressed
gene (ch-TOG; also known as XMAP215) family of microtubule-associated proteins (Box 2) [48]. The functional
relevance of this interaction is underscored by the observation that TACC and ch-TOG/XMAP215 proteins have
similar localizations. Furthermore, perturbing any of these
proteins produces similar phenotypes [8,49,50]. Interestingly, this interaction turned out to be conserved throughout evolution (Table 1) and to be mediated by the TACC
domain (Figure 1) [8,10,11,18,40–42,48]. This is consistent
with the idea that this domain has a crucial role in promoting microtubule assembly, and it also agrees with experimental data showing that the TACC domain is sufficient
to rescue the phenotype of Maskin depletion in Xenopus
egg extracts (Figure 2b) [40,42]. Overexpression studies
Review
have also provided some additional insights into the functional role of this domain. In HeLa cells, the overexpression
of any of the three TACC domains results in the formation
of highly ordered, cytoplasmic polymers that interact with
bundled microtubules but not with tubulin oligomers [38].
In Drosophila embryos, overexpression of the C-terminal
part of D-TACC results in the formation of microtubule
asters in the cytoplasm – but only if Msps is also present
[48]. All these data support the idea that the function of the
TACC domain in promoting microtubule assembly is
highly dependent on its interaction with ch-TOG/
XMAP215.
So what is the underlying mechanism promoting the
assembly of microtubules? Experiments performed in
different systems have shown that reducing TACC protein
levels impairs the correct localization of ch-TOG/XMAP125
to the centrosome [8–11,41,48,50]. One exception concerns
TACC3 in HeLa cells, but this might have been due to
incomplete TACC3 depletion or because any of the other
TACC proteins were compensating for the lack of TACC3
by targeting ch-TOG to the centrosome [49]. In any case,
increasing the concentration of TACC proteins results in
an increase in the recruitment of ch-TOG/XMAP215 to the
spindle poles [38,42,48]. By contrast, ch-TOG/XMAP215 is
not required for the localization of TACC proteins to the
centrosome [11,44,49].
Although all of these data strongly support the idea that
TACC proteins are required for the efficient recruitment of
ch-TOG/XMAP215 proteins to the centrosome, it is still
unclear whether this targeting function is sufficient to
explain the function of TACC proteins. It is also possible
that a functional relationship exists between TACC
proteins and ch-TOG/XMAP215. In this context, it is interesting to recall that Msps is required for the formation of
microtubule asters in Drosophila embryo extracts containing the C-terminal part of D-TACC [48]. Although there are
only a few clues concerning the mechanism involved at the
molecular level, gel filtration experiments have shown that
XMAP215 and Maskin form a one-to-one complex in vitro,
and this complex possesses a higher affinity for microtubules than do each protein on its own [41,42]. TACC
proteins might therefore promote a conformational change
in ch-TOG/XMAP215 that renders the molecule more efficient for microtubule binding. Interestingly, under these
conditions, microtubules are more resistant to depolymerization by a destabilizing factor, the mitotic centromereassociated kinesin MCAK (a kinesin-13 also known as
KIF2C) [41,42].
The model that emerges is one in which TACC proteins
recruit ch-TOG/XMAP215 to the centrosome and enhances
its microtubule binding and stabilizing activity. This counteracts the destabilizing activity of MCAK and thereby
promotes microtubule growth from the centrosome
(Figure 2) [41,42,51].
Regulation of TACC proteins by Aurora A
Another conserved partner of TACC proteins is the Ser–
Thr kinase Aurora A (AurA–STK6) (Table 1, Box 3)
[18,41,42,45,47,52]. In vitro pull-down experiments have
shown that Maskin interacts directly with AurA [42].
Moreover, TACC proteins are good substrates for this
Trends in Cell Biology
Vol.18 No.8
Box 3. The Aurora kinase family
The Aurora kinase family is an evolutionarily conserved family of
serine–threonine kinases. Although there is only one Aurora kinase
in yeasts (Ipl1p in S. cerevisiae, and Ark1 in S. pombe), metazoans
have three Aurora kinases: Aurora A (also known as STK6), Aurora B
(STK12) and Aurora C (STK13). Each of these proteins shows its
highest levels and activity during the G2 and M phases of the cell
cycle. The initial discovery of the Aurora mutation in Drosophila
implicated the Aurora protein in spindle assembly, but extensive
studies have shown that these kinases have more functions [71].
AurA and AurB have been more extensively studied. During cell
division, they have non-overlapping roles related to their distinctive
localizations. AurB is a chromosomal passenger protein required for
phosphorylation of histone H3, chromosome bi-orientation, the
spindle assembly checkpoint, and cytokinesis. The centrosomal
AurA, by contrast, has emerged as a major regulator of centrosome
activity, participating in centrosome maturation and separation, and
in spindle assembly. In addition, AurA has been implicated in entry
to M phase, and in mRNA translation, cilia disassembly, and
asymmetric cell division [72].
One clear function of AurA is the recruitment and regulation of
proteins at the centrosome, including centrosomin, g-TURC and
TACC proteins [71]. Relatively few substrates of the AurA kinase
have been identified, but several are spindle-assembly factors, such
as TACC proteins and NDEL1. Although AurA kinase can selfactivate by autophosphorylation, several activators have been
reported. One of them is TPX2, a RanGTP-regulated factor involved
in spindle assembly. TPX2 also targets AurA to spindle microtubules
[73,74].
In humans, the three Aurora kinases are overexpressed in a
variety of human cancers and are believed to have multiple roles in
the development and progression of cancer. Moreover, the gene
encoding the Aurora A kinase (AURKA) maps to the chromosome
region 20q13, which is frequently amplified in many human cancers.
The overexpression of AURKA induces centrosome amplification
and aneuploidy. It also confers resistance to taxol-dependent
apoptosis in cancer cells. In this context, it is also interesting to
note that AurA interacts with and inactivates the tumor suppressor
p53, and that it also interacts with the breast cancer susceptibility
gene BRCA1 and colocalizes with it at the centrosome. Given that
the overexpression of AurA has been shown to cause tumorigenic
transformation of human and rodent cells in vitro and in vivo [72], it
has been proposed that AurA acts as an oncogene.
kinase in vitro, and most of them have one or more
sequences that conform to a consensus motif for phosphorylation by AurA. In all cases, these sites are located
outside the TACC domain (Figure 1, Table 1)
[41,42,45,47,52,53]. One of them is conserved in several
TACC orthologs, indicating that it is functionally important [45,52,53]. There is strong experimental support in
several systems (i.e. nematode, fly, frog and human cells)
indicating that phosphorylation has a role in TACC function at the centrosome [8,41,42,47,53].
Experiments with purified proteins have shown that
Maskin can bind simultaneously to AurA and XMAP215,
suggesting that the binding sites for these two proteins do
not overlap (Figure 1) [42]. Moreover, phosphorylation
does not appear to have any positive or negative influence
on these interactions. Indeed, in Xenopus egg extract,
XMAP215 is pulled down as efficiently by phosphorylated
wild type Maskin as it is by an unphosphorylatable,
mutated Maskin protein [41]. Finally, AurA does not
enhance the microtubule binding affinity achieved by
the Maskin–XMAP215 complex in comparison with
Maskin or XMAP215 proteins in isolation [41]. All of these
data suggest that phosphorylation does not regulate the
385
Review
interaction or functionality of TACC–XMAP215 complexes. It also seems unlikely that phosphorylation induces
a change in the oligomerization state of Maskin, because
Maskin and unphosphorylatable mutants of Maskin are
found at the same position on sucrose gradients of various
protein combinations [41].
What then does phosphorylation regulate? One possibility is that it regulates the turnover of the protein at the
centrosome, favoring the retention of the phosphorylated
form. Another possibility is that a direct interaction with
the AurA is involved, resulting in the phosphorylation of
the TACC protein at the centrosome. In fact, in Drosophila, phosphorylation of D-TACC seems to be required
not for its localization to the centrosome but, rather, to
enable it to load at the microtubule minus-ends [53].
However, the inhibition of AurA kinase activity by a small
molecule results in TACC3 failing to localize to centrosomes in human cells [45], suggesting that kinase activity
rather than the protein itself is required. In any case,
various data support the idea that AurA regulates the
centrosomal recruitment of TACC. Immunofluorescence
studies performed with an antibody against the phosphorylated form of TACC3 have shown that phosphorylated TACC proteins are mostly found at the centrosome in
HeLa cells and Drosophila embryos [41,53,54]. Consistent
with this, a non-phosphorylatable mutant of TACC3 does
not localize efficiently to the centrosome in transfected
HeLa cells or in Xenopus egg extracts [41,53,54]. In Drosophila embryos, a similar, although less pronounced,
effect is observed for the localization of a non-phosphorylatable mutant of D-TACC [53]. In further agreement,
TACC proteins do not localize to the centrosome in AurA
mutants in C. elegans or Drosophila, and there is a strong
reduction in the association of Maskin with spindles
assembled in Xenopus egg extracts depleted of AurA
[8,42,47]. However, these observations cannot exclude
the possibility that these mislocalizations are attributable
to indirect effects on either centrosome content or functionality due to the absence of this important centrosomal
kinase. Overall, AurA phosphorylation of TACC proteins
appears to contribute substantially to their recruitment to
or accumulation at the centrosome, and, as a consequence,
to the recruitment of ch-TOG/XMAP215 (Figure 2) [41].
The precise molecular mechanism involved remains to be
elucidated. In this context, it is noteworthy that alternative pathways to target TACC proteins to the centrosome
have been described recently. Indeed, the centrosomal
localization of D-TACC relies on its interaction with motif
1 of the Drosophila protein centrosomin [55], and the dualspecificity kinase TTK is essential for the centrosomal
localization of human TACC2 [56]. Recently, it was also
reported that the centrosomal protein NDEL1 (also known
as NUDEL), another substrate of AurA, is required for the
targeting of TACC3 to the centrosome in human cells [57].
Interestingly, NDEL1 is a binding partner of LIS1, a
protein that participates in the regulation of cytoplasmic
dynein function and microtubule organization during cell
division and neuronal migration [58]. NDEL1 is also
related to microtubule-remodeling mechanisms through
the recruitment of the microtubule-severing factor katanin [59].
386
Trends in Cell Biology Vol.18 No.8
Clearly, additional work is needed to address some key
questions and to understand how TACC proteins function
to promote the assembly of microtubules.
Conclusions
TACC proteins have recently emerged as important
players in the complex process of regulating microtubule
dynamics during cell division. Although it is now clearly
established that they have a major role at the centrosome –
promoting microtubule elongation together with ch-TOG/
XMAP215 proteins – the molecular mechanism underlying
their activity is still unclear. Solid data support the fact
that they interact with, and are substrates of, the kinase
AurA; but, again, although phosphorylation is essential for
their localization to and function at the centrosome, the
molecular mechanism involved is far from being understood. In fact, it is noteworthy that TACC proteins also
function in pathways that do not involve the centrosome.
Indeed, D-TACC and Msps both localize at the acentrosomal poles of Drosophila female meiotic spindles, and both
are required for maintaining the bipolarity of these spindles [50]. In addition, some data from frog and fission yeast
support the idea that TACC proteins participate in the
RanGTP-dependent spindle assembly pathway [39,40,60].
Furthermore, in the fission yeast S. pombe, Alp7 functions
in the maintenance of microtubule organization during
interphase [61]. It is therefore very likely that TACC
proteins have a more general role in microtubule stabilization than currently appreciated.
Finally, it is still unclear whether the quite distinct roles
of TACC proteins in different aspects of gene regulation
and in microtubule assembly are connected in some way. In
this context, it is noteworthy that recent studies have
revealed that different classes of RNAs are associated with
the mitotic spindle [62,63]. Interestingly, both Maskin and
AurA are involved in the regulation of mRNA translation
in Xenopus oocytes [26].
It is quite apparent that much remains to be done to
understand more clearly how TACC proteins function. In a
wider context, it will be interesting to elucidate the functional connections between their roles in microtubule
stabilization and in RNA control. In addition, future work
should reveal whether their role is only restricted to events
occurring during cell division.
As a final remark, it should be noted that the study of
TACC proteins is starting to offer a variety of promising
applications related to cancer therapies. Recently, it was
shown that monitoring phospho-TACC3 is an efficient way
of evaluating the effectiveness of AurA inhibitors that are
promising anti-cancer drugs, some of which have recently
entered clinical trials [45]. The observation that TACC3
depletion sensitizes cells to paclitaxel-induced cell death
also suggests that TACC3 itself is a promising target for
the treatment of the tumors resistant to this widely used
therapy [64]. Finally, TACC3 is also emerging as a good
prognostic marker for some cancers [20].
Acknowledgements
We thank all members of the Vernos Laboratory, especially Luis
Bejarano, Teresa Sardon and Roser Pinyol, for critically reading the
manuscript and providing helpful comments. We thank P. Gönczy
(ISREC, CH) and J. Raff (The Gurdon Institute, UK) for the kind gift of
Review
the immunofluorescence images of C. elegans and D. melanogaster
embryos, respectively. We also thank Christoph Spinzig for creative
suggestions. Work in the I.V. laboratory is supported by the CRG, the
European Union MRTN/CT 2004 512348 and the Spanish Ministry
grants BFU2006–04694 and CSD2006–0023.
References
1 Desai, A. and Mitchison, T.J. (1997) Microtubule polymerization
dynamics. Annu. Rev. Cell Dev. Biol. 13, 83–117
2 Kline-Smith, S.L. and Walczak, C.E. (2004) Mitotic spindle assembly
and chromosome segregation: refocusing on microtubule dynamics.
Mol. Cell 15, 317–327
3 Gadde, S. and Heald, R. (2004) Mechanisms and molecules of the
mitotic spindle. Curr. Biol. 14, R797–R805
4 Bettencourt-Dias, M. and Glover, D.M. (2007) Centrosome biogenesis
and function: centrosomics brings new understanding. Nat. Rev. Mol.
Cell Biol. 8, 451–463
5 Nigg, E.A. (2002) Centrosome aberrations: cause or consequence of
cancer progression? Nat. Rev. Cancer 2, 815–825
6 Still, I.H. et al. (1999) Cloning of TACC1, an embryonically expressed,
potentially transforming coiled coil containing gene, from the 8p11
breast cancer amplicon. Oncogene 18, 4032–4038
7 Gergely, F. (2002) Centrosomal TACCtics. Bioessays 24, 915–925
8 Bellanger, J.M. and Gonczy, P. (2003) TAC-1 and ZYG-9 form a complex
that promotes microtubule assembly in C. elegans embryos. Curr. Biol.
13, 1488–1498
9 Le Bot, N. et al. (2003) TAC-1, a regulator of microtubule length in the
C. elegans embryo. Curr. Biol. 13, 1499–1505
10 Srayko, M. et al. (2003) Caenorhabditis elegans TAC-1 and ZYG-9 form
a complex that is essential for long astral and spindle microtubules.
Curr. Biol. 13, 1506–1511
11 Sato, M. et al. (2004) Interdependency of fission yeast Alp14/TOG and
coiled coil protein Alp7 in microtubule localization and bipolar spindle
formation. Mol. Biol. Cell 15, 1609–1622
12 Gergely, F. et al. (2000) D-TACC: a novel centrosomal protein required
for normal spindle function in the early Drosophila embryo. EMBO J.
19, 241–252
13 Lauffart, B. et al. (2003) Molecular cloning, genomic structure and
interactions of the putative breast tumor suppressor TACC2. Genomics
81, 192–201
14 Line, A. et al. (2002) Altered splicing pattern of TACC1 mRNA in
gastric cancer. Cancer Genet. Cytogenet. 139, 78–83
15 Hao, Z. et al. (2002) TACC3 expression and localization in the murine
egg and ovary. Mol. Reprod. Dev. 63, 291–299
16 Still, I.H. et al. (2004) Structure–function evolution of the transforming
acidic coiled coil genes revealed by analysis of phylogenetically diverse
organisms. BMC Evol. Biol. 4, 16
17 Still, I.H. et al. (1999) The third member of the transforming acidic
coiled coil-containing gene family, TACC3, maps in 4p16, close to
translocation breakpoints in multiple myeloma, and is upregulated
in various cancer cell lines. Genomics 58, 165–170
18 Conte, N. et al. (2003) TACC1–chTOG–Aurora A protein complex in
breast cancer. Oncogene 22, 8102–8116
19 Lauffart, B. et al. (2005) Aberrations of TACC1 and TACC3 are
associated with ovarian cancer. BMC Womens Health 5, 8
20 Jung, C.K. et al. (2006) Expression of transforming acidic coiled-coil
containing protein 3 is a novel independent prognostic marker in nonsmall cell lung cancer. Pathol. Int. 56, 503–509
21 Ulisse, S. et al. (2007) Transforming acidic coiled-coil 3 and Aurora-A
interact in human thyrocytes and their expression is deregulated in
thyroid cancer tissues. Endocr. Relat. Cancer 14, 827–837
22 Schuendeln, M.M. et al. (2004) The centrosomal, putative tumor
suppressor protein TACC2 is dispensable for normal development,
and deficiency does not lead to cancer. Mol. Cell. Biol. 24, 6403–6409
23 Raff, J.W. (2002) Centrosomes and cancer: lessons from a TACC.
Trends Cell Biol. 12, 222–225
24 Chen, H.M. et al. (2000) AZU-1: a candidate breast tumor suppressor
and biomarker for tumor progression. Mol. Biol. Cell 11, 1357–1367
25 Conte, N. et al. (2002) Carcinogenesis and translational controls:
TACC1 is down-regulated in human cancers and associates with
mRNA regulators. Oncogene 21, 5619–5630
26 Richter, J.D. (2007) CPEB: a life in translation. Trends Biochem. Sci.
32, 279–285
Trends in Cell Biology
Vol.18 No.8
27 Garriga-Canut, M. and Orkin, S.H. (2004) Transforming acidic coiledcoil protein 3 (TACC3) controls friend of GATA-1 (FOG-1) subcellular
localization and regulates the association between GATA-1 and FOG-1
during hematopoiesis. J. Biol. Chem. 279, 23597–23605
28 Lauffart, B. et al. (2002) Interaction of the transforming acidic coiledcoil 1 (TACC1) protein with ch-TOG and GAS41/NuBI1 suggests
multiple TACC1-containing protein complexes in human cells.
Biochem. J. 363, 195–200
29 Sadek, C.M. et al. (2000) Isolation and characterization of AINT: a
novel ARNT interacting protein expressed during murine embryonic
development. Mech. Dev. 97, 13–26
30 Gangisetty, O. et al. (2004) The transforming acidic coiled coil proteins
interact with nuclear histone acetyltransferases. Oncogene 23, 2559–
2563
31 Angrisano, T. et al. (2006) TACC3 mediates the association of MBD2
with histone acetyltransferases and relieves transcriptional repression
of methylated promoters. Nucleic Acids Res. 34, 364–372
32 Sadek, C.M. et al. (2003) TACC3 expression is tightly regulated during
early differentiation. Gene Expr. Patterns 3, 203–211
33 Pu, J.J. et al. (2001) Cloning and structural characterization of
ECTACC, a new member of the transforming acidic coiled coil
(TACC) gene family: cDNA sequence and expression analysis in
human microvascular endothelial cells. Cytokine 13, 129–137
34 McKeveney, P.J. et al. (2001) Characterization and localization of
expression of an erythropoietin-induced gene, ERIC-1/TACC3,
identified in erythroid precursor cells. Br. J. Haematol. 112, 1016–1024
35 Aitola, M. et al. (2003) Aint/Tacc3 is highly expressed in proliferating
mouse tissues during development, spermatogenesis, and oogenesis.
J. Histochem. Cytochem. 51, 455–469
36 Piekorz, R.P. et al. (2002) The centrosomal protein TACC3 is essential
for hematopoietic stem cell function and genetically interfaces with
p53-regulated apoptosis. EMBO J. 21, 653–664
37 Yao, R. et al. (2007) TACC3 is required for the proper mitosis of
sclerotome mesenchymal cells during formation of the axial
skeleton. Cancer Sci. 98, 555–562
38 Gergely, F. et al. (2000) The TACC domain identifies a family of
centrosomal proteins that can interact with microtubules. Proc.
Natl. Acad. Sci. U. S. A. 97, 14352–14357
39 Albee, A.J. et al. (2006) Phosphorylation of Maskin by Aurora-A is
regulated by RanGTP and importin beta. J. Biol. Chem. 281, 38293–
38301
40 O’Brien, L.L. et al. (2005) The Xenopus TACC homologue, Maskin,
functions in mitotic spindle assembly. Mol. Biol. Cell 16, 2836–2847
41 Kinoshita, K. et al. (2005) Aurora A phosphorylation of TACC3/Maskin
is required for centrosome-dependent microtubule assembly in mitosis.
J. Cell Biol. 170, 1047–1055
42 Peset, I. et al. (2005) Function and regulation of Maskin, a TACC family
protein, in microtubule growth during mitosis. J. Cell Biol. 170, 1057–
1066
43 Steadman, B.T. et al. (2002) Transforming acidic coiled-coil-containing
protein 4 interacts with centrosomal AKAP350 and the mitotic spindle
apparatus. J. Biol. Chem. 277, 30165–30176
44 Bellanger, J.M. et al. (2007) ZYG-9, TAC-1 and ZYG-8 together ensure
correct microtubule function throughout the cell cycle of C. elegans
embryos. J. Cell Sci. 120, 2963–2973
45 LeRoy, P.J. et al. (2007) Localization of human TACC3 to mitotic
spindles is mediated by phosphorylation on Ser558 by Aurora A: a
novel pharmacodynamic method for measuring Aurora A activity.
Cancer Res. 67, 5362–5370
46 Schneider, L. et al. (2007) The transforming acidic coiled coil 3 protein
is essential for spindle-dependent chromosome alignment and mitotic
survival. J. Biol. Chem. 282, 29273–29283
47 Giet, R. et al. (2002) Drosophila Aurora A kinase is required to localize
D-TACC to centrosomes and to regulate astral microtubules. J. Cell
Biol. 156, 437–451
48 Lee, M.J. et al. (2001) Msps/XMAP215 interacts with the centrosomal
protein D-TACC to regulate microtubule behaviour. Nat. Cell Biol. 3,
643–649
49 Gergely, F. et al. (2003) The ch-TOG/XMAP215 protein is essential for
spindle pole organization in human somatic cells. Genes Dev. 17, 336–341
50 Cullen, C.F. and Ohkura, H. (2001) Msps protein is localized to
acentrosomal poles to ensure bipolarity of Drosophila meiotic
spindles. Nat. Cell Biol. 3, 637–642
387
Review
51 Tournebize, R. et al. (2000) Control of microtubule dynamics by the
antagonistic activities of XMAP215 and XKCM1 in Xenopus egg
extracts. Nat. Cell Biol. 2, 13–19
52 Pascreau, G. et al. (2005) Phosphorylation of Maskin by Aurora-A
participates in the control of sequential protein synthesis during
Xenopus laevis oocyte maturation. J. Biol. Chem. 280, 13415–13423
53 Barros, T.P. et al. (2005) Aurora A activates D-TACC–Msps complexes
exclusively at centrosomes to stabilize centrosomal microtubules.
J. Cell Biol. 170, 1039–1046
54 Barnard, D.C. et al. (2005) Differential phosphorylation controls Maskin
association with eukaryotic translation initiation factor 4E and
localization on the mitotic apparatus. Mol. Cell. Biol. 25, 7605–7615
55 Zhang, J. and Megraw, T.L. (2007) Proper recruitment of gammatubulin and D-TACC/Msps to embryonic Drosophila centrosomes
requires Centrosomin Motif 1. Mol. Biol. Cell 18, 4037–4049
56 Dou, Z. et al. (2004) TTK kinase is essential for the centrosomal
localization of TACC2. FEBS Lett. 572, 51–56
57 Mori, D. et al. (2007) NDEL1 phosphorylation by Aurora-A kinase is
essential for centrosomal maturation, separation, and TACC3
recruitment. Mol. Cell. Biol. 27, 352–367
58 Wynshaw-Boris, A. (2007) Lissencephaly and LIS1: insights into the
molecular mechanisms of neuronal migration and development. Clin.
Genet. 72, 296–304
59 Toyo-Oka, K. et al. (2005) Recruitment of katanin p60 by
phosphorylated NDEL1, an LIS1 interacting protein, is essential for
mitotic cell division and neuronal migration. Hum. Mol. Genet. 14,
3113–3128
60 Sato, M. and Toda, T. (2007) Alp7/TACC is a crucial target in RanGTPase-dependent spindle formation in fission yeast. Nature 447, 334–
337
61 Zheng, L. et al. (2006) The fission yeast transforming acidic coiled coilrelated protein Mia1p/Alp7p is required for formation and
maintenance of persistent microtubule-organizing centers at the
nuclear envelope. Mol. Biol. Cell 17, 2212–2222
388
Trends in Cell Biology Vol.18 No.8
62 Blower, M.D. et al. (2007) Genome-wide analysis demonstrates
conserved localization of messenger RNAs to mitotic microtubules.
J. Cell Biol. 179, 1365–1373
63 Blower, M.D. et al. (2005) A Rae1-containing ribonucleoprotein
complex is required for mitotic spindle assembly. Cell 121,
223–234
64 Schneider, L. et al. (2008) TACC3 depletion sensitizes to paclitaxelinduced cell death and overrides p21(WAF)-mediated cell cycle arrest.
Oncogene 27, 116–125
65 Howard, J. and Hyman, A.A. (2007) Microtubule polymerases and
depolymerases. Curr. Opin. Cell Biol. 19, 31–35
66 Gard, D.L. and Kirschner, M.W. (1987) A microtubule-associated
protein from Xenopus eggs that specifically promotes assembly at
the plus-end. J. Cell Biol. 105, 2203–2215
67 Kinoshita, K. et al. (2002) XMAP215: a key component of the dynamic
microtubule cytoskeleton. Trends Cell Biol. 12, 267–273
68 Al-Bassam, J. et al. (2007) Crystal structure of a TOG domain:
conserved features of XMAP215/Dis1-family TOG domains and
implications for tubulin binding. Structure 15, 355–362
69 Brouhard, G.J. et al. (2008) XMAP215 is a processive microtubule
polymerase. Cell 132, 79–88
70 Cullen, C.F. et al. (1999) mini spindles: A gene encoding a conserved
microtubule-associated protein required for the integrity of the mitotic
spindle in Drosophila. J. Cell Biol. 146, 1005–1018
71 Barr, A.R. and Gergely, F. (2007) Aurora-A: the maker and breaker of
spindle poles. J. Cell Sci. 120, 2987–2996
72 Giet, R. et al. (2005) Aurora kinases, aneuploidy and cancer, a
coincidence or a real link? Trends Cell Biol. 15, 241–250
73 Kufer, T.A. et al. (2002) Human TPX2 is required for targeting AuroraA kinase to the spindle. J. Cell Biol. 158, 617–623
74 Bayliss, R. et al. (2003) Structural basis of Aurora-A activation by TPX2
at the mitotic spindle. Mol. Cell 12, 851–862
75 Delaval, B. et al. (2004) Aurora B -TACC1 protein complex in
cytokinesis. Oncogene 23, 4516–4522