The Xenopus TACC homologue, maskin, functions in mitotic spindle assembly

The TOG5 domain of CKAP5 is required to interact with F-actin and promote microtubule advancement in neurons

Microtubule (MT) and F-actin cytoskeletal cross-talk and organization are important aspects of axon guidance mechanisms, but how associated proteins facilitate this function remains largely unknown. While the MT-associated protein, CKAP5 (XMAP215/ch-TOG), has been best characterized as a MT polymerase, we have recently highlighted a novel role for CKAP5 in facilitating interactions between MT and F-actin in vitro and in embryonic Xenopus laevis neuronal growth cones. However, the mechanism by which it does so is unclear. Here, using in vitro reconstitution assays coupled with total internal reflection fluorescence microscopy, we report that the TOG5 domain of CKAP5 is necessary for its ability to bind to and bundle actin filaments, as well as to cross-link MTs and F-actin in vitro. Additionally, we show that this novel MT/F-actin cross-linking function of CKAP5 is possible even in MT polymerase-incompetent mutants of CKAP5 in vivo. Indeed, CKAP5 requires both MT and F-actin binding, but not MT polymerization, to promote MT-F-actin alignment in growth cones and axon outgrowth. Taken together, our findings provide mechanistic insights into how MT populations penetrate the growth cone periphery through CKAP5-facilitated interaction with F-actin during axon outgrowth and guidance.

Microtubule remodelling as a driving force of axon guidance and pruning

The establishment of neuronal connectivity relies on the microtubule (MT) cytoskeleton, which provides mechanical support, roads for axonal transport and mediates signalling events. Fine-tuned spatiotemporal regulation of MT functions by tubulin post-translational modifications and MT-associated proteins is critical for the coarse wiring and subsequent refinement of neuronal connectivity. The defective regulation of these processes causes a wide range of neurodevelopmental disorders associated with connectivity defects. This review focuses on recent studies unravelling how MT composition, post-translational modifications and associated proteins influence MT functions in axon guidance and/or pruning to build functional neuronal circuits. We here summarise experimental evidence supporting the key role of this network as a driving force for growth cone steering and branch-specific axon elimination. We further provide a global overview of the MT-interactors that tune developing axon behaviours, with a special emphasis on their emerging versatility in the regulation of MT dynamics/structure. Recent studies establishing the key and highly selective role of the tubulin code in the regulation of MT functions in axon pathfinding are also reported. Finally, our review highlights the emerging molecular links between these MT regulation processes and guidance signals that wire the nervous system.

TACC3-ch-TOG interaction regulates spindle microtubule assembly by controlling centrosomal recruitment of γ-TuRC

γ-Tubulin ring complex (γ-TuRC), composed of γ-tubulin and multiple γ-tubulin complex proteins (GCPs), serves as the major microtubule nucleating complex in animal cells. However, several γ-TuRC-associated proteins have been shown to control its function. Centrosomal adaptor protein, TACC3, is one such γ-TuRC-interacting factor that is essential for proper mitotic spindle assembly across organisms. ch-TOG is another microtubule assembly promoting protein, which interacts with TACC3 and cooperates in mitotic spindle assembly. However, the mechanism how TACC3-ch-TOG interaction regulates microtubule assembly and the γ-TuRC functions at the centrosomes remain unclear. Here, we show that deletion of the ch-TOG-binding region in TACC3 enhances recruitment of the γ-TuRC proteins to centrosomes and aggravates spindle microtubule assembly in human cells. Loss of TACC3-ch-TOG binding imparts stabilization on TACC3 interaction with the γ-TuRC proteins and it does so by stimulating TACC3 phosphorylation and thereby enhancing phospho-TACC3 recruitment to the centrosomes. We also show that localization of ch-TOG at the centrosomes is substantially reduced and the same on the spindle microtubules is increased in its TACC3-unbound condition. Additional results reveal that ch-TOG depletion stimulates γ-tubulin localization on the spindles without significantly affecting the centrosomal γ-tubulin level. The results indicate that ch-TOG binding to TACC3 controls TACC3 phosphorylation and TACC3-mediated stabilization of the γ-TuRCs at the centrosomes. They also implicate that the spatio-temporal control of TACC3 phosphorylation via ch-TOG-binding ensures mitotic spindle assembly to the optimal level.

Functions and mechanisms of RNA tailing by metazoan terminal nucleotidyltransferases

Termini often determine the fate of RNA molecules. In recent years, 3' ends of almost all classes of RNA species have been shown to acquire nontemplated nucleotides that are added by terminal nucleotidyltransferases (TENTs). The best-described role of 3' tailing is the bulk polyadenylation of messenger RNAs in the cell nucleus that is catalyzed by canonical poly(A) polymerases (PAPs). However, many other enzymes that add adenosines, uridines, or even more complex combinations of nucleotides have recently been described. This review focuses on metazoan TENTs, which are either noncanonical PAPs or terminal uridylyltransferases with varying processivity. These enzymes regulate RNA stability and RNA functions and are crucial in early development, gamete production, and somatic tissues. TENTs regulate gene expression at the posttranscriptional level, participate in the maturation of many transcripts, and protect cells against viral invasion and the transposition of repetitive sequences. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Processing > 3' End Processing RNA Turnover and Surveillance > Regulation of RNA Stability.

Aurora A site specific TACC3 phosphorylation regulates astral microtubule assembly by stabilizing γ-tubulin ring complex

Background

Astral microtubules emanating from the mitotic centrosomes play pivotal roles in defining cell division axis and tissue morphogenesis. Previous studies have demonstrated that human transforming acidic coiled-coil 3 (TACC3), the most conserved TACC family protein, regulates formation of astral microtubules at centrosomes in vertebrate cells by affecting γ-tubulin ring complex (γ-TuRC) assembly. However, the molecular mechanisms underlying such function were not completely understood.

Results

Here, we show that Aurora A site-specific phosphorylation in TACC3 regulates formation of astral microtubules by stabilizing γ-TuRC assembly in human cells. Mutation of the most conserved Aurora A targeting site, Ser 558 to alanine (S558A) in TACC3 results in robust loss of astral microtubules and disrupts localization of the γ-tubulin ring complex (γ-TuRC) proteins at the spindle poles. Under similar condition, phospho-mimicking S558D mutation retains astral microtubules and the γ-TuRC proteins in a manner similar to control cells expressed with wild type TACC3. Time-lapse imaging reveals that S558A mutation leads to defects in positioning of the spindle-poles and thereby causes delay in metaphase to anaphase transition. Biochemical results determine that the Ser 558- phosphorylated TACC3 interacts with the γ-TuRC proteins and further, S558A mutation impairs the interaction. We further reveal that the mutation affects the assembly of γ-TuRC from the small complex components.

Conclusions

The results demonstrate that TACC3 phosphorylation stabilizes γ- tubulin ring complex assembly and thereby regulates formation of centrosomal asters. They also implicate a potential role of TACC3 phosphorylation in the functional integrity of centrosomes/spindle poles.

XMAP215 promotes microtubule-F-actin interactions to regulate growth cone microtubules during axon guidance in Xenopus laevis

It has long been established that neuronal growth cone navigation depends on changes in microtubule (MT) and F-actin architecture downstream of guidance cues. However, the mechanisms by which MTs and F-actin are dually coordinated remain a fundamentally unresolved question. Here, we report that the well-characterized MT polymerase, XMAP215 (also known as CKAP5), plays an important role in mediating MT-F-actin interaction within the growth cone. We demonstrate that XMAP215 regulates MT-F-actin alignment through its N-terminal TOG 1-5 domains. Additionally, we show that XMAP215 directly binds to F-actin in vitro and co-localizes with F-actin in the growth cone periphery. We also find that XMAP215 is required for regulation of growth cone morphology and response to the guidance cue, Ephrin A5. Our findings provide the first strong evidence that XMAP215 coordinates MT and F-actin interaction in vivo We suggest a model in which XMAP215 regulates MT extension along F-actin bundles into the growth cone periphery and that these interactions may be important to control cytoskeletal dynamics downstream of guidance cues. This article has an associated First Person interview with the first author of the paper.

Structural insight into the XTACC3/XMAP215 interaction from CD and NMR studies on model peptides

TACC3 is a centrosomal adaptor protein that plays important roles during mitotic spindle assembly. It interacts with chTOG/XMAP215, which catalyzes the addition of tubulin dimers during microtubule growth. A 3D coiled-coil model for this interaction is available but the structural details are not well described. To characterize this interaction at atomic resolution, we have designed a simplified version of the system based on small peptides. Four different peptides have been studied by circular dichroism and nuclear magnetic resonance both singly and in all possible combinations; namely, five peptide pairs and two trios. In cosolvents, all single peptides tend to adopt helical conformations resembling those of the full-length protein. However, neither the single peptides nor pairs of peptides form coiled coils. We show that the simultaneous presence of all preformed helices is a prerequisite for binding. The simplest 3D model for the interaction, based on the NMR results, is proposed. Interestingly, the peptide's structure remains unaffected by mutations at essential positions for TACC3 activity. This suggests that the lack of interaction of this TACC3 mutant with XMAP does not correlate with changes in the protein structure and that specific interactions are likely responsible for the interaction and stability of the complex.

The role of TACC3 in mitotic spindle organization

TACC3 regulates spindle organization during mitosis and also regulates centrosome-mediated microtubule nucleation by affecting γ-Tubulin ring complexes. In addition, it interacts with different proteins (such as ch-TOG, clathrin and Aurora-A) to function in mitotic spindle assembly and stability. By forming the TACC3/ch-TOG complex, TACC3 acts as a plus end-tracking protein to promote microtubule elongation. The TACC3/ch-TOG/clathrin complex is formed to stabilize kinetochore fibers by crosslinking adjacent microtubules. Furthermore, the phosphorylation of TACC3 by Aurora-A is important for the formation of TACC3/ch-TOG/clathrin and its recruitment to kinetochore fibers. Recently, the aberrant expression of TACC3 in a variety of human cancers has been linked with mitotic defects. Thus, in this review, we will discuss our current understanding of the biological roles of TACC3 in mitotic spindle organization.

Xenopus as a model for studies in mechanical stress and cell division

We exist in a physical world, and cells within biological tissues must respond appropriately to both environmental forces and forces generated within the tissue to ensure normal development and homeostasis. Cell division is required for normal tissue growth and maintenance, but both the direction and rate of cell division must be tightly controlled to avoid diseases of over-proliferation such as cancer. Recent studies have shown that mechanical cues can cause mitotic entry and orient the mitotic spindle, suggesting that physical force could play a role in patterning tissue growth. However, to fully understand how mechanics guides cells in vivo, it is necessary to assess the interaction of mechanical strain and cell division in a whole tissue context. In this mini-review we first summarise the body of work linking mechanics and cell division, before looking at the advantages that the Xenopus embryo can offer as a model organism for understanding: (1) the mechanical environment during embryogenesis, and (2) factors important for cell division. Finally, we introduce a novel method for applying a reproducible strain to Xenopus embryonic tissue and assessing subsequent cell divisions.

Knockdown of TACC3 inhibits trophoblast cell migration and invasion through the PI3K/Akt signaling pathway

The insufficient invasion of trophoblasts is known to be correlated with the development of preeclampsia. Transforming acidic coiled‑coil protein 3 (TACC3), a member of the TACC domain family, is important in the regulation of cell differentiation, migration and invasion. However, the role of TACC3 in trophoblast function during placental development remains to be fully elucidated. The present study aimed to determine the expression and function of TACC3 in human placenta and to examine the underlying mechanisms. TACC3 expression was analyzed in preeclamptic placentas using reverse transcription‑quantitative polymerase chain reaction and western blotting. Cell proliferation was determined by the MTT assay, and cell migration and invasion were measured using Transwell assays. The expression levels of TACC3, matrix metalloproteinase (MMP)‑2, MMP‑9, tissue inhibitor of metalloproteinase (TIMP)‑1, TIMP‑2, phosphoinositide 3‑kinase (PI3K), phosphorylated (p)‑PI3K, AKT and p‑AKT were detected by western blotting. The results showed that the expression of TACC3 was downregulated in preeclamptic placentas. The knockdown of TACC3 significantly inhibited HTR8/SVneo cell proliferation, migration and invasion, and inhibited the expression of matrix metalloproteinases. In addition, the knockdown of TACC3 significantly reduced the levels of p‑PI3K and Akt in the HTR8/SVneo cells. Taken together, the results of the present study demonstrated that the knockdown of TACC3 inhibited the migration and invasion of HTR8/SVneo cells through suppression of the PI3K/Akt signaling pathway. Therefore, TACC3 may serve as a novel potential target for treating preeclampsia.

From meiosis to mitosis - the sperm centrosome defines the kinetics of spindle assembly after fertilization in Xenopus

Bipolar spindle assembly in the vertebrate oocyte relies on a self-organization chromosome-dependent pathway. Upon fertilization, the male gamete provides a centrosome, and the first and subsequent embryonic divisions occur in the presence of duplicated centrosomes that act as dominant microtubule organizing centres (MTOCs). The transition from meiosis to embryonic mitosis involves a necessary adaptation to integrate the dominant chromosome-dependent pathway with the centrosomes to form the bipolar spindle. Here, we took advantage of the Xenopus laevis egg extract system to mimic in vitro the assembly of the first embryonic spindle and investigate the respective contributions of the centrosome and the chromosome-dependent pathway to the kinetics of the spindle bipolarization. We found that centrosomes control the transition from the meiotic to the mitotic spindle assembly mechanism. By defining the kinetics of spindle bipolarization, the centrosomes ensure their own positioning to each spindle pole and thereby their essential correct inheritance to the two first daughter cells of the embryo for the development of a healthy organism.

Pulling it together: The mitotic function of TACC3

Transforming acidic coiled coil 3 (TACC3) is a non-motor microtubule-associated protein (MAP) that is important for mitotic spindle stability and organization. The exact mechanism by which TACC3 acts at microtubules to stabilize the spindle has been unclear. However, several recent studies identified that the TACC3 complex at microtubules contains clathrin in addition to its previously identified binding partner, colonic and hepatic tumor overexpressed gene (ch-TOG). In this complex, phosphorylated TACC3 interacts directly with both ch-TOG and clathrin heavy chain, promoting accumulation of all complex members at the mitotic spindle. This complex stabilizes kinetochore fibers within the spindle by forming cross-bridges that link adjacent microtubules in these bundles. So, TACC3 is an adaptor that recruits ch-TOG and clathrin to mitotic microtubules, in an Aurora A kinase-regulated manner. In this mini-review we will describe the recent advances in the understanding of TACC 3 function and present a model that pulls together these new data with previous observations.

XTACC3-XMAP215 association reveals an asymmetric interaction promoting microtubule elongation

chTOG is a conserved microtubule polymerase that catalyses the addition of tubulin dimers to promote microtubule growth. chTOG interacts with TACC3, a member of the transforming acidic coiled-coil (TACC) family. Here we analyse their association using the Xenopus homologues, XTACC3 (TACC3) and XMAP215 (chTOG), dissecting the mechanism by which their interaction promotes microtubule elongation during spindle assembly. Using SAXS, we show that the TACC domain (TD) is an elongated structure that mediates the interaction with the C terminus of XMAP215. Our data suggest that one TD and two XMAP215 molecules associate to form a four-helix coiled-coil complex. A hybrid methods approach was used to define the precise regions of the TACC heptad repeat and the XMAP215 C terminus required for assembly and functioning of the complex. We show that XTACC3 can induce the recruitment of larger amounts of XMAP215 by increasing its local concentration, thereby promoting efficient microtubule elongation during mitosis.

TACC3 is a microtubule plus end-tracking protein that promotes axon elongation and also regulates microtubule plus end dynamics in multiple embryonic cell types

TACC3 is a microtubule plus end–tracking protein in vertebrates. TACC3 localizes to the extreme microtubule plus end, where it interacts with XMAP215 to regulate microtubule polymerization. TACC3 is also required to promote normal axon outgrowth, likely through its regulation of microtubule dynamics within the growth cone.

Microtubule plus end dynamics are regulated by a conserved family of proteins called plus end–tracking proteins (+TIPs). It is unclear how various +TIPs interact with each other and with plus ends to control microtubule behavior. The centrosome-associated protein TACC3, a member of the transforming acidic coiled-coil (TACC) domain family, has been implicated in regulating several aspects of microtubule dynamics. However, TACC3 has not been shown to function as a +TIP in vertebrates. Here we show that TACC3 promotes axon outgrowth and regulates microtubule dynamics by increasing microtubule plus end velocities in vivo. We also demonstrate that TACC3 acts as a +TIP in multiple embryonic cell types and that this requires the conserved C-terminal TACC domain. Using high-resolution live-imaging data on tagged +TIPs, we show that TACC3 localizes to the extreme microtubule plus end, where it lies distal to the microtubule polymerization marker EB1 and directly overlaps with the microtubule polymerase XMAP215. TACC3 also plays a role in regulating XMAP215 stability and localizing XMAP215 to microtubule plus ends. Taken together, our results implicate TACC3 as a +TIP that functions with XMAP215 to regulate microtubule plus end dynamics.

Csi1p recruits alp7p/TACC to the spindle pole bodies for bipolar spindle formation

The spindle pole body (SPB) localization of the fission yeast Schizosaccharomyces pombe TACC orthologue alp7p depends on the SPB protein csi1p. Compromised interaction between csi1p and alp7p delays bipolar spindle formation and leads to abnormal chromosome segregation.

Accurate chromosome segregation requires timely bipolar spindle formation during mitosis. The transforming acidic coiled-coil (TACC) family proteins and the ch-TOG family proteins are key players in bipolar spindle formation. They form a complex to stabilize spindle microtubules, mainly dependent on their localization to the centrosome (the spindle pole body [SPB] in yeast). The molecular mechanism underlying the targeting of the TACC–ch-TOG complex to the centrosome remains unclear. Here we show that the fission yeast Schizosaccharomyces pombe TACC orthologue alp7p is recruited to the SPB by csi1p. The csi1p-interacting region lies within the conserved TACC domain of alp7p, and the carboxyl-terminal domain of csi1p is responsible for interacting with alp7p. Compromised interaction between csi1p and alp7p impairs the localization of alp7p to the SPB during mitosis, thus delaying bipolar spindle formation and leading to anaphase B lagging chromosomes. Hence our study establishes that csi1p serves as a linking molecule tethering spindle-stabilizing factors to the SPB for promoting bipolar spindle assembly.

Role of centrosomal adaptor proteins of the TACC family in the regulation of microtubule dynamics during mitotic cell division

During the mitotic division cycle, cells pass through an extensive microtubule rearrangement process where microtubules forming the mitotic spindle apparatus are dynamically instable. Several centrosomal- and microtubule-associated proteins are involved in the regulation of microtubule dynamics and stability during mitosis. Here, we focus on members of the transforming acidic coiled coil (TACC) family of centrosomal adaptor proteins, in particular TACC3, in which their subcellular localization at the mitotic spindle apparatus is controlled by Aurora-A kinase-mediated phosphorylation. At the effector level, several TACC-binding partners have been identified and characterized in greater detail, in particular, the microtubule polymerase XMAP215/ch-TOG/CKAP5 and clathrin heavy chain (CHC). We summarize the recent progress in the molecular understanding of these TACC3 protein complexes, which are crucial for proper mitotic spindle assembly and dynamics to prevent faulty cell division and aneuploidy. In this regard, the (patho)biological role of TACC3 in development and cancer will be discussed.

Coordination of adjacent domains mediates TACC3-ch-TOG-clathrin assembly and mitotic spindle binding

A complex of transforming acidic coiled-coil protein 3 (TACC3), colonic and hepatic tumor overexpressed gene (ch-TOG), and clathrin has been implicated in mitotic spindle assembly and in the stabilization of kinetochore fibers by cross-linking microtubules. It is unclear how this complex binds microtubules and how the proteins in the complex interact with one another. TACC3 and clathrin have each been proposed to be the spindle recruitment factor. We have mapped the interactions within the complex and show that TACC3 and clathrin were interdependent for spindle recruitment, having to interact in order for either to be recruited to the spindle. The N-terminal domain of clathrin and the TACC domain of TACC3 in tandem made a microtubule interaction surface, coordinated by TACC3-clathrin binding. A dileucine motif and Aurora A-phosphorylated serine 558 on TACC3 bound to the "ankle" of clathrin. The other interaction within the complex involved a stutter in the TACC3 coiled-coil and a proposed novel sixth TOG domain in ch-TOG, which was required for microtubule localization of ch-TOG but not TACC3-clathrin.

TACC3 is essential for EGF-mediated EMT in cervical cancer

The third member of transforming acidic coiled-coil protein (TACC) family, TACC3, has been shown to be an important player in the regulation of centrosome/microtubule dynamics during mitosis and found to be deregulated in a variety of human malignancies. Our previous studies have suggested that TACC3 may be involved in cervical cancer progression and chemoresistance, and its overexpression can induce epithelial-mesenchymal transition (EMT) by activating the phosphatidylinositol 3-kinase (PI3K)/Akt and extracellular signal-regulated protein kinases (ERKs) signal transduction pathways. However, the upstream mechanisms of TACC3-mediated EMT and its functional/clinical importance in human cervical cancer remain elusive. Epidermal growth factor (EGF) has been shown to be a potent inducer of EMT in cervical cancer and associated with tumor invasion and metastasis. In this study, we found that TACC3 is overexpressed in cervical cancer and can be induced upon EGF stimulation. The induction of TACC3 by EGF is dependent on the tyrosine kinase activity of the EGF receptor (EGFR). Intriguingly, depletion of TACC3 abolishes EGF-mediated EMT, suggesting that TACC3 is required for EGF/EGFR-driven EMT process. Moreover, Snail, a key player in EGF-mediated EMT, is found to be correlated with the expression of TACC3 in cervical cancer. Collectively, our study highlights a novel function for TACC3 in EGF-mediated EMT process and suggests that targeting of TACC3 may be an attractive strategy to treat cervical cancers driven by EGF/EGFR signaling pathways.

A role of CPEB1 in the modulation of proliferation and neuronal maturation of rat primary neural progenitor cells

Cytoplasmic polyadenylation binding protein 1 (CPEB1) is a RNA binding protein, which regulates translation of target mRNAs by regulating polyadenylation status. CPEB1 plays important roles in the regulation of germline cell development by modulating cell cycle progression through the polyadenylation of target mRNAs such as cyclin B1. Similar mechanism is reported in proliferating astrocytes by us, although CPEB1 is involved in the transport of target mRNAs as well as local translation at dendritic spines. In this study, we found the expression of CPEB1 in cultured rat primary neural progenitor cells (NPCs). EGF stimulation of cultured NPCs induced rapid phosphorylation of CPEB1, a hallmark of CPEB1-dependent translational control along with cyclin B1 polyadenylation and translation. EGF-induced activation of ERK1/2 and Aurora A kinase was responsible for CPEB1 phosphorylation. Pharmacological inhibition studies suggested that ERK1/2 is involved in the activation of Aurora A kinase and regulation of CPEB1 phosphorylation in cultured NPCs. Long-term incubation in EGF resulted in the down-regulation of CPEB1 expression, which further increased expression of cyclin B1 and cell cycle progression. When we down-regulated the expression of CPEB1 in NPCs by siRNA transfection, the proliferation of NPCs was increased. Increased NPCs proliferation by down-regulation of CPEB1 resulted in eventual up-regulation of neuronal differentiation with increase in both pre- and post-synaptic proteins. The results from the present study may suggest the importance of translational control in the regulation of neuronal development, an emerging concept in many neurodevelopmental and psychiatric disorders such as autism spectrum disorder.

The role of clathrin in mitotic spindle organisation

Clathrin, a protein best known for its role in membrane trafficking, has been recognised for many years as localising to the spindle apparatus during mitosis, but its function at the spindle remained unclear. Recent work has better defined the role of clathrin in the function of the mitotic spindle and proposed that clathrin crosslinks the microtubules (MTs) comprising the kinetochore fibres (K-fibres) in the mitotic spindle. This mitotic function is unrelated to the role of clathrin in membrane trafficking and occurs in partnership with two other spindle proteins: transforming acidic coiled-coil protein 3 (TACC3) and colonic hepatic tumour overexpressed gene (ch-TOG; also known as cytoskeleton-associated protein 5, CKAP5). This review summarises the role of clathrin in mitotic spindle organisation with an emphasis on the recent discovery of the TACC3-ch-TOG-clathrin complex.

Transforming acidic coiled-coil proteins (TACCs) in human cancer

Fine-tuned regulation of the centrosome/microtubule dynamics during mitosis is essential for faithful cell division. Thus, it is not surprising that deregulations in this dynamic network can contribute to genomic instability and tumorigenesis. Indeed, centrosome loss or amplification, spindle multipolarity and aneuploidy are often found in a majority of human malignancies, suggesting that defects in centrosome and associated microtubules may be directly or indirectly linked to cancer. Therefore, future research to identify and characterize genes required for the normal centrosome function and microtubule dynamics may help us gain insight into the complexity of cancer, and further provide new avenues for prognostic, diagnostics and therapeutic interventions. Members of the transforming acidic coiled-coil proteins (TACCs) family are emerging as important players of centrosome and microtubule-associated functions. Growing evidence indicates that TACCs are involved in the progression of certain solid tumors. Here, we will discuss our current understanding of the biological function of TACCs, their relevance to human cancer and possible implications for cancer management.

A TACC3/ch-TOG/clathrin complex stabilises kinetochore fibres by inter-microtubule bridging

Kinetochore fibres (K-fibres) of the spindle apparatus move chromosomes during mitosis. These fibres are discrete bundles of parallel microtubules (MTs) that are crosslinked by inter-MT 'bridges' that are thought to improve fibre stability during chromosomal movement. The identity of these bridges is unknown. Clathrin is a multimeric protein that has been shown to stabilise K-fibres during early mitosis by a mechanism independent of its role in membrane trafficking. In this study, we show that clathrin at the mitotic spindle is in a transforming acidic colied-coil protein 3 (TACC3)/colonic, hepatic tumour overexpressed gene (ch-TOG)/clathrin complex. The complex is anchored to the spindle by TACC3 and ch-TOG. Ultrastructural analysis of clathrin-depleted K-fibres revealed a selective loss of a population of short inter-MT bridges and a general loss of MTs. A similar loss of short inter-MT bridges was observed in TACC3-depleted K-fibres. Finally, immunogold labelling confirmed that inter-MT bridges in K-fibres contain clathrin. Our results suggest that the TACC3/ch-TOG/clathrin complex is an inter-MT bridge that stabilises K-fibres by physical crosslinking and by reducing rates of MT catastrophe.

Nup98 regulates bipolar spindle assembly through association with microtubules and opposition of MCAK

During mitosis, the nuclear pore complex is disassembled and, increasingly, nucleoporins are proving to have mitotic functions when released from the pore. We find a contribution of the nucleoporin Nup98 to mitotic spindle assembly through regulation of microtubule dynamics. When added to Xenopus extract spindle assembly assays, the C-terminal domain of Nup98 stimulates uncontrolled growth of microtubules. Conversely, inhibition or depletion of Nup98 leads to formation of stable monopolar spindles. Spindle bipolarity is restored by addition of purified, recombinant Nup98 C-terminus. The minimal required region of Nup98 corresponds to a portion of the C-terminal domain lacking a previously characterized function. We show association between this region of the C-terminus of Nup98 and both Taxol-stabilized microtubules and the microtubule-depolymerizing mitotic centromere-associated kinesin (MCAK). Importantly, we demonstrate that this domain of Nup98 inhibits MCAK depolymerization activity in vitro. These data support a model in which Nup98 interacts with microtubules and antagonizes MCAK activity, thus promoting bipolar spindle assembly.

Clathrin recruits phosphorylated TACC3 to spindle poles for bipolar spindle assembly and chromosome alignment

Transforming acidic coiled-coil-containing protein 3 (TACC3) has been implicated in mitotic spindle assembly, although the mechanisms involved are largely unknown. Here we identify that clathrin heavy chain (CHC) binds specifically to phosphorylated TACC3 and recruits it to spindle poles for proper spindle assembly and chromosome alignment. Phosphorylation of Xenopus TACC3 at serine 620 (S620) and S626, but not S33, is required for its binding with CHC. Knockdown of CHC by RNA interference (RNAi) abolishes the targeting of TACC3 to spindle poles and results in abnormal spindle assembly and chromosome misalignment, similar to the defects caused by TACC3 knockdown. Furthermore, the binding of CHC with phosphorylated TACC3 is inhibited by importin β and this inhibition is reversed by the presence of the GTP-binding nuclear protein Ran in the GTP-bound state. Together, these results indicate that the recruitment of phosphorylated TACC3 to spindle poles by CHC ensures proper spindle assembly and chromosome alignment, and is regulated by Ran.

Clathrin heavy chain mediates TACC3 targeting to mitotic spindles to ensure spindle stability

CHC binds specifically to aurora A–phosphorylated TACC3, recruiting it to the spindle during mitosis.

Mitotic spindles play essential roles in chromosome congression and segregation during mitosis. Aurora A regulates spindle assembly in part via phosphorylating human TACC3 on S558, which triggers TACC3 relocalization to mitotic spindles and stabilizes microtubules (MTs). In this study, we identified clathrin heavy chain (CHC) as an adaptor protein to recruit S558-phosphorylated TACC3 onto the spindle during mitosis for MT stabilization. CHC binds phospho-S558 TACC3 via its linker domain and first CHC repeat. CHC depletion or mutation on phospho-TACC3 binding abrogates TACC3 spindle relocalization. Depletion of either or both CHC and TACC3 yields similar defective phenotypes: loss of ch-TOG on spindles, disorganized spindles, and chromosome misalignment with comparable mitotic delay. Our findings elucidate the association between aurora A phosphorylation and spindle apparatus and demonstrate that regulation from aurora A is mediated by CHC in recruiting phospho-TACC3 and subsequently ch-TOG to mitotic spindles.

Learning about cancer from frogs: analysis of mitotic spindles in Xenopus egg extracts

The mitotic spindle is responsible for correctly segregating chromosomes during cellular division. Disruption of this process leads to genomic instability in the form of aneuploidy, which can contribute to the development of cancer. Therefore, identification and characterization of factors that are responsible for the assembly and regulation of the spindle are crucial. Not only are these factors often altered in cancer, but they also serve as potential therapeutic targets. Xenopus egg extract is a powerful tool for studying spindle assembly and other cell cycle-related events owing, in large part, to the ease with which protein function can be manipulated in the extract. Importantly, the spindle factors that have been characterized in egg extract are conserved in human spindle assembly. In this review, we explain how the extract is prepared and manipulated to study the function of individual factors in spindle assembly and the spindle checkpoint. Furthermore, we provide examples of several spindle factors that have been defined functionally using the extract system and discuss how these factors are altered in human cancer.

Ranking genes by their co-expression to subsets of pathway members

Cellular processes are often carried out by intricate systems of interacting genes and proteins. Some of these systems are rather well studied and described in pathway databases, while the roles and functions of the majority of genes are poorly understood. A large compendium of public microarray data is available that covers a variety of conditions, samples, and tissues and provides a rich source for genome-scale information. We focus our study on the analysis of 35 curated biological pathways in the context of gene co-expression over a large variety of biological conditions. By defining a global co-expression similarity rank for each gene and pathway, we perform exhaustive leave-one-out computations to describe existing pathway memberships using other members of the corresponding pathway as reference. We demonstrate that while successful in recovering biological base processes such as metabolism and translation, the global correlation measure fails to detect gene memberships in signaling pathways where co-expression is less evident. Our results also show that pathway membership detection is more effective when using only a subset of corresponding pathway members as reference, supporting the existence of more tightly co-expressed subsets of genes within pathways. Our study assesses the predictive power of global gene expression correlation measures in reconstructing biological systems of various functions and specificity. The developed computational network has immediate applications in detecting dubious pathway members and predicting novel member candidates.

The conserved protein SZY-20 opposes the Plk4-related kinase ZYG-1 to limit centrosome size

Microtubules are organized by the centrosome, a dynamic organelle that exhibits changes in both size and number during the cell cycle. Here we show that SZY-20, a putative RNA-binding protein, plays a critical role in limiting centrosome size in C. elegans. SZY-20 localizes in part to centrosomes and in its absence centrosomes possess increased levels of centriolar and pericentriolar components including gamma-tubulin and the centriole duplication factors ZYG-1 and SPD-2. These enlarged centrosomes possess normal centrioles, nucleate more microtubules, and fail to properly direct a number of microtubule-dependent processes. Depletion of ZYG-1 restores normal centrosome size and function to szy-20 mutants, whereas loss of szy-20 suppresses the centrosome duplication defects in both zyg-1 and spd-2 mutants. Our results describe a pathway that determines centrosome size and implicate centriole duplication factors in this process.

The TACC proteins: TACC-ling microtubule dynamics and centrosome function

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.

Myosin-10 and actin filaments are essential for mitotic spindle function

Mitotic spindles are microtubule-based structures responsible for chromosome partitioning during cell division. Although the roles of microtubules and microtubule-based motors in mitotic spindles are well established, whether or not actin filaments (F-actin) and F-actin-based motors (myosins) are required components of mitotic spindles has long been controversial. Based on the demonstration that myosin-10 (Myo10) is important for assembly of meiotic spindles, we assessed the role of this unconventional myosin, as well as F-actin, in mitotic spindles. We find that Myo10 localizes to mitotic spindle poles and is essential for proper spindle anchoring, normal spindle length, spindle pole integrity, and progression through metaphase. Furthermore, we show that F-actin localizes to mitotic spindles in dynamic cables that surround the spindle and extend between the spindle and the cortex. Remarkably, although proper anchoring depends on both F-actin and Myo10, the requirement for Myo10 in spindle pole integrity is F-actin independent, whereas F-actin and Myo10 actually play antagonistic roles in maintenance of spindle length.

Xenopus TACC3/maskin is not required for microtubule stability but is required for anchoring microtubules at the centrosome

Members of the transforming acidic coiled coil (TACC) protein family are emerging as important mitotic spindle assembly proteins in a variety of organisms. The molecular details of how TACC proteins function are unknown, but TACC proteins have been proposed to recruit microtubule-stabilizing proteins of the tumor overexpressed gene (TOG) family to the centrosome and to facilitate their loading onto newly emerging microtubules. Using Xenopus egg extracts and in vitro assays, we show that the Xenopus TACC protein maskin is required for centrosome function beyond recruiting the Xenopus TOG protein XMAP215. The conserved C-terminal TACC domain of maskin is both necessary and sufficient to restore centrosome function in maskin-depleted extracts, and we provide evidence that the N terminus of maskin inhibits the function of the TACC domain. Time-lapse video microscopy reveals that microtubule dynamics in Xenopus egg extracts are unaffected by maskin depletion. Our results provide direct experimental evidence of a role for maskin in centrosome function and suggest that maskin is required for microtubule anchoring at the centrosome.

The RanGTP gradient - a GPS for the mitotic spindle

The GTPase Ran has a key role in nuclear import and export, mitotic spindle assembly and nuclear envelope formation. The cycling of Ran between its GTP- and GDP-bound forms is catalyzed by the chromatin-bound guanine nucleotide exchange factor RCC1 and the cytoplasmic Ran GTPase-activating protein RanGAP. The result is an intracellular concentration gradient of RanGTP that equips eukaryotic cells with a ;genome-positioning system' (GPS). The binding of RanGTP to nuclear transport receptors (NTRs) of the importin beta superfamily mediates the effects of the gradient and generates further downstream gradients, which have been elucidated by fluorescence resonance energy transfer (FRET) imaging and computational modeling. The Ran-dependent GPS spatially directs many functions required for genome segregation by the mitotic spindle during mitosis. Through exportin 1, RanGTP recruits essential centrosome and kinetochore components, whereas the RanGTP-induced release of spindle assembly factors (SAFs) from importins activates SAFs to nucleate, bind and organize nascent spindle microtubules. Although a considerable fraction of cytoplasmic SAFs is active and RanGTP induces only partial further activation near chromatin, bipolar spindle assembly is robustly induced by cooperativity and positive-feedback mechanisms within the network of Ran-activated SAFs. The RanGTP gradient is conserved, although its roles vary among different cell types and species, and much remains to be learned regarding its functions.

Aurora A phosphorylates MCAK to control ran-dependent spindle bipolarity

During mitosis, mitotic centromere-associated kinesin (MCAK) localizes to chromatin/kinetochores, a cytoplasmic pool, and spindle poles. Its localization and activity in the chromatin region are regulated by Aurora B kinase; however, how the cytoplasmic- and pole-localized MCAK are regulated is currently not clear. In this study, we used Xenopus egg extracts to form spindles in the absence of chromatin and centrosomes and found that MCAK localization and activity are tightly regulated by Aurora A. This regulation is important to focus microtubules at aster centers and to facilitate the transition from asters to bipolar spindles. In particular, we found that MCAK colocalized with NuMA and XMAP215 at the center of Ran asters where its activity is regulated by Aurora A-dependent phosphorylation of S196, which contributes to proper pole focusing. In addition, we found that MCAK localization at spindle poles was regulated through another Aurora A phosphorylation site (S719), which positively enhances bipolar spindle formation. This is the first study that clearly defines a role for MCAK at the spindle poles as well as identifies another key Aurora A substrate that contributes to spindle bipolarity.

Xenopus NEDD1 is required for microtubule organization in Xenopus egg extracts

The centrosome serves as the major microtubule-nucleating and -organizing center in animal cells. It is composed of hundreds of proteins. The molecular details of how centrosomal proteins contribute to centrotome function are only beginning to emerge. Members of the neuron-precursor-cell-expressed developmentally downregulated protein 1 (NEDD1) family of conserved proteins have recently been implicated in recruiting γ-tubulin and its associated proteins, which together make up the γ-tubulin ring complex (γTuRC), to the centrosome. Human NEDD1 and its Drosophila ortholog Dgp71WD are WD-repeat proteins that interact with the γTuRC. Experimental knockdown of human NEDD1 was recently shown to result in loss of γ-tubulin from the centrosome. By contrast, however, Dgp71WD knockdown has no effect on targeting the γTuRC to the centrosome in flies. Using Xenopus egg extracts, we show that Xenopus NEDD1 is mostly dispensable for targeting γ-tubulin to centrosomes, but that microtubule organization is disrupted in NEDD1-depleted extracts. We show that NEDD1 exists in a complex that is distinct from the γTuRC, suggesting that NEDD1 may not be a bona fide subunit of the Xenopus γTuRC. We propose that the main function of NEDD1 in Xenopus is in microtubule organization.

Translational control by cytoplasmic polyadenylation in Xenopus oocytes

Elongation of the poly(A) tails of specific mRNAs in the cytoplasm is a crucial regulatory step in oogenesis and early development of many animal species. The best studied example is the regulation of translation by cytoplasmic polyadenylation elements (CPEs) in the 3′ untranslated region of mRNAs involved in Xenopus oocyte maturation. In this review we discuss the mechanism of translational control by the CPE binding protein (CPEB) in Xenopus oocytes as follows:1.

The cytoplasmic polyadenylation machinery such as CPEB, the subunits of cleavage and polyadenylation specificity factor (CPSF), symplekin, Gld-2 and poly(A) polymerase (PAP).

2.

The signal transduction that leads to the activation of CPE-mediated polyadenylation during oocyte maturation, including the potential roles of kinases such as MAPK, Aurora A, CamKII, cdk1/Ringo and cdk1/cyclin B.

3.

The role of deadenylation and translational repression, including the potential involvement of PARN, CCR4/NOT, maskin, pumilio, Xp54 (Ddx6, Rck), other P-body components and isoforms of the cap binding initiation factor eIF4E.

Finally we discuss some of the remaining questions regarding the mechanisms of translational regulation by cytoplasmic polyadenylation and give our view on where our knowledge is likely to be expanded in the near future.

Cep120 and TACCs control interkinetic nuclear migration and the neural progenitor pool

Centrosome- and microtubule-associated proteins have been shown to be important for maintaining the neural progenitor pool during neocortical development by regulating the mitotic spindle. It remains unclear whether these proteins may control neurogenesis by regulating other microtubule-dependent processes such as nuclear migration. Here, we identify Cep120, a centrosomal protein preferentially expressed in neural progenitors during neocortical development. We demonstrate that silencing Cep120 in the developing neocortex impairs both interkinetic nuclear migration (INM), a characteristic pattern of nuclear movement in neural progenitors, and neural progenitor self-renewal. Furthermore, we show that Cep120 interacts with transforming acidic coiled-coil proteins (TACCs) and that silencing TACCs also causes defects in INM and neural progenitor self-renewal. Our data suggest a critical role for Cep120 and TACCs in both INM and neurogenesis. We propose that sustaining INM may be a mechanism by which microtubule-regulating proteins maintain the neural progenitor pool during neocortical development.

Mechanisms of mitotic spindle assembly and function

The mitotic spindle is the macromolecular machine that segregates chromosomes to two daughter cells during mitosis. The major structural elements of the spindle are microtubule polymers, whose intrinsic polarity and dynamic properties are critical for bipolar spindle organization and function. In most cell types, spindle microtubule nucleation occurs primarily at two centrosomes, which define the spindle poles, but microtubules can also be generated by the chromosomes and within the spindle itself. Many associated factors help organize the spindle, including molecular motors and regulators of microtubule dynamics. The past decade has provided a wealth of information on the molecular players that are critical for spindle assembly as well as a high-resolution view of the intricate movements and dynamics of the spindle microtubules and the chromosomes. In this chapter we provide a historical account of the key observations leading to current models of spindle assembly, as well as an up-to-date status report on this exciting field.

Brinkley-fest of mitosis

"Mitosis: Spindle Assembly and Function," a conference in honor of Dr. Bill R. Brinkley, brought together many researchers to discuss progress in the field and celebrate the many contributions that Dr. Brinkley has made.

The microtubule-associated protein tumor overexpressed gene/cytoskeleton-associated protein 5 is necessary for myelin basic protein expression in oligodendrocytes

Tumor overexpressed gene (TOG) protein, encoded by cytoskeleton-associated protein CKAP5, is a microtubule-associated protein that binds to heterogeneous nuclear ribonucleoprotein (hnRNP) A2. hnRNP A2 is an RNA trafficking factor that associates with myelin basic protein (MBP) mRNA. In oligodendrocytes, TOG, hnRNP A2, and MBP mRNA colocalize in granules that assemble in the perikaryon and are transported to the peripheral network of processes that extends from it. MBP accumulates preferentially in the membrane of the medial and distal portions of these cellular processes. MBP expression was reduced when TOG level was lowered by short-hairpin (sh) RNA. The reduction in TOG did not affect overall cell morphology or the assembly, transport, localization, or number of MBP mRNA-containing granules. Reduced levels of TOG did not affect another oligodendrocyte-specific component, myelin oligodendrocyte glycoprotein, which is expressed at the same time as MBP but translated from mRNA localized in the cell body. Expression in a neural cell line of a green fluorescent protein (GFP)-MBP fusion protein derived from a construct containing GFP and the full-length cDNA for the rat 14 kDa MBP was reduced when TOG level was lowered by shRNA treatment. Expression of GFP, derived from GFP mRNA containing the hnRNP A2 binding element of MBP mRNA, was similarly reduced in cells with low TOG levels. These data indicate that TOG is necessary for efficient translation of MBP mRNA and suggest that this role is mediated by its interaction with hnRNP A2.

ZYG-9, TAC-1 and ZYG-8 together ensure correct microtubule function throughout the cell cycle of C. elegans embryos

The early Caenorhabditis elegans embryo is well suited for investigating microtubule-dependent cell division processes. In the one-cell stage, the XMAP215 homologue ZYG-9, associated with the TACC protein TAC-1, promotes microtubule growth during interphase and mitosis, whereas the doublecortin domain protein ZYG-8 is required for anaphase spindle positioning. How ZYG-9, TAC-1 and ZYG-8 together ensure correct microtubule-dependent processes throughout the cell cycle is not fully understood. Here, we identify new temperature-sensitive alleles of zyg-9 and tac-1. Analysis of ZYG-9 and TAC-1 distribution in these mutants identifies amino acids important for centrosomal targeting and for stability of the two proteins. This analysis also reveals that TAC-1 is needed for correct ZYG-9 centrosomal enrichment. Moreover, we find that ZYG-9, but not TAC-1, is limiting for microtubule-dependent processes in one-cell-stage embryos. Using two of these alleles to rapidly inactivate ZYG-9-TAC-1 function, we establish that this complex is required for correct anaphase spindle positioning. Furthermore, we uncover that ZYG-9-TAC-1 and ZYG-8 function together during meiosis, interphase and mitosis. We also find that TAC-1 physically interacts with ZYG-8 through its doublecortin domain, and that in vivo TAC-1 and ZYG-8 are part of a complex that does not contain ZYG-9. Taken together, these findings indicate that ZYG-9-TAC-1 and ZYG-8 act in a partially redundant manner to ensure correct microtubule assembly throughout the cell cycle of early C. elegans embryos.

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

Aurora A is a serine/threonine protein kinase essential for normal mitotic progression. Aberrant increased expression of Aurora A, which occurs frequently in human cancers, results in abnormal mitoses leading to chromosome instability and possibly tumorigenesis. Consequently, Aurora A has received considerable attention as a potential target for anticancer therapeutic intervention. Aurora A coordinates several essential mitotic activities through phosphorylation of a variety of proteins, including TACC3, which modulates microtubule stabilization of the mitotic spindle. Recent studies identified a conserved serine in Xenopus (Ser(626)) and Drosophila (Ser(863)) TACC3 orthologues that is phosphorylated by Aurora A. We show that this conserved serine on human TACC3 (Ser(558)) is also phosphorylated by Aurora A. Moreover, phosphorylation of TACC3 by Aurora A in human cells is essential for its proper localization to centrosomes and proximal mitotic spindles. Inhibition of Aurora A with the selective small molecule inhibitor MLN8054 in cultured human tumor cells resulted in mislocalization of TACC3 away from mitotic spindles in a concentration-dependent manner. Furthermore, oral administration of MLN8054 to nude mice bearing HCT-116 human tumor xenografts caused a dose-dependent mislocalization of TACC3 away from spindle poles that correlated with tumor growth inhibition. As TACC3 localization to mitotic spindles depends on Aurora A-mediated phosphorylation, quantifying TACC3 mislocalization represents a novel pharmacodynamic approach for measuring Aurora A activity in cancer patients treated with inhibitors of Aurora A kinase.

Translational control of maskin mRNA by its 3' untranslated region

BACKGROUND INFORMATION

Maskin is a member of the TACC (transforming acidic coiled-coil) domain proteins found in Xenopus laevis oocytes and embryos. It has been implicated in the co-ordination of the spindle and has been reported to mediate translational repression of cyclin B1 mRNA.

RESULTS

In the present study, we report that maskin mRNA is translationally repressed at the level of initiation in stage 4 oocytes and becomes activated in stage 6 oocytes. The translational repression of maskin mRNA correlates with the presence of a short poly(A) tail on this mRNA in stage 4 oocytes. The 3'-UTR (untranslated region) of maskin can confer the translational regulation to a reporter mRNA, and so can the 3'-UTR of human TACC3. A conserved GUCU repeat element was found to repress translation in both stage 4 and stage 6 oocytes, but deletion of this element did not abrogate repression in stage 4 oocytes. UV cross-linking experiments indicated that overlapping sets of proteins bind efficiently to both the maskin and the cyclin B1 3'-UTRs. As reported previously, CPEB [CPE (cytoplasmic polyadenylation element)-binding protein] binds to the cyclin B1 3'-UTR, but its binding to the maskin 3'-UTR is minimal. By RNA affinity chromatography and MS, we identified the EDEN-BP [EDEN (embryonic deadenylation element)-binding protein] as one of the proteins binding to both the maskin and the cyclin B1 3'-UTRs.

CONCLUSIONS

Maskin mRNA is translationally regulated by at least two repressor elements and an activation element. One of the repessor elements is the evolutionarily conserved GUCU repeat. EDEN-BP binds to both the maskin and cyclin B1 3'-UTRs, indicating it may be involved in the deadenylation of these mRNAs.

TACC3 is required for the proper mitosis of sclerotome mesenchymal cells during formation of the axial skeleton

Transforming acidic coiled-coil-containing (TACC) family members regulate mitotic spindles and have essential roles in embryogenesis. However, the functions of TACC3 in mitosis during mammalian development are not known. We have generated and characterized three mutant alleles of mouse Tacc3 including a conditional allele. Homozygous mutants of a hypomorphic allele exhibited malformations of the axial skeleton. The primary cause of this defect was the failure of mitosis in mesenchymal sclerotome cells. In vitro, 36% of primary mouse embryo fibroblasts (MEF) obtained from mutants homozygous for the hypomorphic allele and 67% of MEF from Tacc3 null mutants failed mitosis. In cloned immortalized MEF, Tacc3 depletion destabilized spindles and prevented chromosomes from aligning properly. Furthermore, chromosome separation and cytokinesis were also severely impaired. Chromosomes were moved randomly and cytokinesis initiated but the cleavage furrow eventually regressed, resulting in binucleate cells that then yielded aneuploid cells in the next cell division. Thus, in addition to spindle assembly, Tacc3 has critical roles in chromosome separation and cytokinesis, and is essential for the mitosis of sclerotome mesenchymal cells during axial formation in mammals.

Microtubule nucleation: gamma-tubulin and beyond

Centrosomes and their fungal equivalents, spindle pole bodies (SPBs), are the main microtubule (MT)-organizing centers in eukaryotic cells. Several proteins have been implicated in microtubule formation by centrosomes and SPBs, including microtubule-minus-end-binding proteins and proteins that bind along the length or stabilize the plus ends of microtubules. Recent work has improved our understanding of the molecular mechanisms of MT formation. In particular, it has shown that gamma-tubulin and its associated proteins play key roles in microtubule nucleation and spindle assembly in evolutionarily distant species ranging from fungi to mammals. Other work indicates that gamma-tubulin-mediated microtubule nucleation, although necessary, is not sufficient for mitotic spindle assembly but requires additional proteins that regulate microtubule nucleation independently of centrosomes.

Cell and Molecular Biology of the Spindle Matrix

The concept of a spindle matrix has long been proposed to account for incompletely understood features of microtubule spindle dynamics and force production during mitosis. In its simplest formulation, the spindle matrix is hypothesized to provide a stationary or elastic molecular matrix that can provide a substrate for motor molecules to interact with during microtubule sliding and which can stabilize the spindle during force production. Although this is an attractive concept with the potential to greatly simplify current models of microtubule spindle behavior, definitive evidence for the molecular nature of a spindle matrix or for its direct role in microtubule spindle function has been lagging. However, as reviewed here multiple studies spanning the evolutionary spectrum from lower eukaryotes to vertebrates have provided new and intriguing evidence that a spindle matrix may be a general feature of mitosis.

Phosphorylation of maskin by Aurora-A is regulated by RanGTP and importin beta

Mitotic spindle assembly in Xenopus egg extracts is regulated at least in part by importin beta and its regulator, the small GTPase, Ran. RanGTP stabilizes microtubules near the chromosomes during spindle assembly by selectively releasing spindle assembly factors from inhibition by importin alpha/beta in the vicinity of the chromosomes. Several spindle assembly factors are regulated in this manner. We identified maskin, the Xenopus member of the transforming acidic coiled coil family of proteins, as a potential candidate in a two-step affinity chromatography approach designed to uncover additional downstream targets of importin alpha/beta in mitosis. Here, we show that although maskin lacks a canonical nuclear localization sequence, it binds importin beta in a RanGTP-regulated manner. We further show that importin beta inhibits the regulatory phosphorylation of maskin by Aurora-A. This suggests a novel mechanism by which importin beta regulates the activity of a spindle assembly factor.