Aurora A phosphorylates MCAK to control ran-dependent spindle bipolarity

Mechanism and regulation of kinesin motors

Kinesins are a diverse superfamily of microtubule-based motors that perform fundamental roles in intracellular transport, cytoskeletal dynamics and cell division. These motors share a characteristic motor domain that powers unidirectional motility and force generation along microtubules, and they possess unique tail domains that recruit accessory proteins and facilitate oligomerization, regulation and cargo recognition. The location, direction and timing of kinesin-driven processes are tightly regulated by various cofactors, adaptors, microtubule tracks and microtubule-associated proteins. This Review focuses on recent structural and functional studies that reveal how members of the kinesin superfamily use the energy of ATP hydrolysis to transport cargoes, depolymerize microtubules and regulate microtubule dynamics. I also survey how accessory proteins and post-translational modifications regulate the autoinhibition, cargo binding and motility of some of the best-studied kinesins. Despite much progress, the mechanism and regulation of kinesins are still emerging, and unresolved questions can now be tackled using newly developed approaches in biophysics and structural biology.

KIF2C/MCAK a prognostic biomarker and its oncogenic potential in malignant progression, and prognosis of cancer patients: a systematic review and meta-analysis as biomarker

KIF2C/MCAK (KIF2C) is the most well-characterized member of the kinesin-13 family, which is critical in the regulation of microtubule (MT) dynamics during mitosis, as well as interphase. This systematic review briefly describes the important structural elements of KIF2C, its regulation by multiple molecular mechanisms, and its broad cellular functions. Furthermore, it systematically summarizes its oncogenic potential in malignant progression and performs a meta-analysis of its prognostic value in cancer patients. KIF2C was shown to be involved in multiple crucial cellular processes including cell migration and invasion, DNA repair, senescence induction and immune modulation, which are all known to be critical during the development of malignant tumors. Indeed, an increasing number of publications indicate that KIF2C is aberrantly expressed in multiple cancer entities. Consequently, we have highlighted its involvement in at least five hallmarks of cancer, namely: genome instability, resisting cell death, activating invasion and metastasis, avoiding immune destruction and cellular senescence. This was followed by a systematic search of KIF2C/MCAK's expression in various malignant tumor entities and its correlation with clinicopathologic features. Available data were pooled into multiple weighted meta-analyses for the correlation between KIF2Chigh protein or gene expression and the overall survival in breast cancer, non-small cell lung cancer and hepatocellular carcinoma patients. Furthermore, high expression of KIF2C was correlated to disease-free survival of hepatocellular carcinoma. All meta-analyses showed poor prognosis for cancer patients with KIF2Chigh expression, associated with a decreased overall survival and reduced disease-free survival, indicating KIF2C's oncogenic potential in malignant progression and as a prognostic marker. This work delineated the promising research perspective of KIF2C with modern in vivo and in vitro technologies to further decipher the function of KIF2C in malignant tumor development and progression. This might help to establish KIF2C as a biomarker for the diagnosis or evaluation of at least three cancer entities.

Unraveling the rhizobial infection thread

Most legumes can form an endosymbiotic association with soil bacteria called rhizobia, which colonize specialized root structures called nodules where they fix nitrogen. To colonize nodule cells, rhizobia must first traverse the epidermis and outer cortical cell layers of the root. In most legumes, this involves formation of the infection thread, an intracellular structure that becomes colonized by rhizobia, guiding their passage through the outer cell layers of the root and into the newly formed nodule cells. In this brief review, we recount the early research milestones relating to the rhizobial infection thread and highlight two relatively recent advances in the symbiotic infection mechanism, the eukaryotically conserved 'MYB-AUR1-MAP' mitotic module, which links cytokinesis mechanisms to intracellular infection, and the discovery of the 'infectosome' complex, which guides infection thread growth. We also discuss the potential intertwining of the two modules and the hypothesis that cytokinesis served as a foundation for intracellular infection of symbiotic microbes.

Nek2A prevents centrosome clustering and induces cell death in cancer cells via KIF2C interaction

Unlike normal cells, cancer cells frequently exhibit supernumerary centrosomes, leading to formation of multipolar spindles that can trigger cell death. Nevertheless, cancer cells with supernumerary centrosomes escape the deadly consequences of unequal segregation of genomic material by coalescing their centrosomes into two poles. This unique trait of cancer cells presents a promising target for cancer therapy, focusing on selectively attacking cells with supernumerary centrosomes. Nek2A is a kinase involved in mitotic regulation, including the centrosome cycle, where it phosphorylates linker proteins to separate centrosomes. In this study, we investigated if Nek2A also prevents clustering of supernumerary centrosomes, akin to its separation function. Reduction of Nek2A activity, achieved through knockout, silencing, or inhibition, promotes centrosome clustering, whereas its overexpression results in inhibition of clustering. Significantly, prevention of centrosome clustering induces cell death, but only in cancer cells with supernumerary centrosomes, both in vitro and in vivo. Notably, none of the known centrosomal (e.g., CNAP1, Rootletin, Gas2L1) or non-centrosomal (e.g., TRF1, HEC1) Nek2A targets were implicated in this machinery. Additionally, Nek2A operated via a pathway distinct from other proteins involved in centrosome clustering mechanisms, like HSET and NuMA. Through TurboID proximity labeling analysis, we identified novel proteins associated with the centrosome or microtubules, expanding the known interaction partners of Nek2A. KIF2C, in particular, emerged as a novel interactor, confirmed through coimmunoprecipitation and localization analysis. The silencing of KIF2C diminished the impact of Nek2A on centrosome clustering and rescued cell viability. Additionally, elevated Nek2A levels were indicative of better patient outcomes, specifically in those predicted to have excess centrosomes. Therefore, while Nek2A is a proposed target, its use must be specifically adapted to the broader cellular context, especially considering centrosome amplification. Discovering partners such as KIF2C offers fresh insights into cancer biology and new possibilities for targeted treatment.

KIF2C: An important factor involved in signaling pathways, immune infiltration, and DNA damage repair in tumorigenesis

BACKGROUNDS

Poorly regulated mitosis and chromosomal instability are common characteristics in malignant tumor cells. Kinesin family member 2 C (KIF2C), also known as mitotic centromere-associated kinesin (MCAK) is an essential component during mitotic regulation. In recent years, KIF2C was shown to be dysregulated in several tumors and was involved in many aspects of tumor self-regulation. Research on KIF2C may be a new direction and target for anti-tumor therapy.

OBJECT

The article aims at reviewing current literatures and summarizing the research status of KIF2C in malignant tumors as well as the oncogenic signaling pathways associated with KIF2C and its role in immune infiltration.

RESULT

In this review, we summarize the KIF2C mechanisms and signaling pathways in different malignant tumors, and briefly describe its involvement in pathways related to classical chemotherapeutic drug resistance, such as MEK/ERK, mTOR, Wnt/β-catenin, P53 and TGF-β1/Smad pathways. KIF2C upregulation was shown to promote tumor cell migration, invasion, chemotherapy resistance and inhibit DNA damage repair. It was also highly correlated with microRNAs, and CD4 +T cell and CD8 +T cell tumor immune infiltration.

CONCLUSION

This review shows that KIF2C may function as a new anticancer drug target with great potential for malignant tumor treatment and the mitigation of chemotherapy resistance.

Multi-omics analysis of kinesin family member 2C in human tumors: novel prognostic biomarker and tumor microenvironment regulator

Kinesin family member 2C (KIF2C) is the best-characterized member of the kinesin-13 family and is involved in accurately fine-tuned dynamics of mitotic spindles. As KIF2C is involved in both spindle formation and regulation of DNA double-strand breaks, precise regulation of KIF2C is essential to prevent malignant transformation associated with gains and losses of DNA content. In the present study, we initially reviewed The Cancer Genome Atlas database and observed that KIF2C is abundantly expressed in most tumor types. We then analyzed the gene alteration profile, protein expression, prognosis, and immune reactivities of KIF2C in more than 10,000 samples from several well-established databases. In addition, we conducted a gene enrichment set analysis to investigate the potential mechanisms underlying the role of KIF2C in tumorigenesis. Multi-omics analysis of KIF2C demonstrated significant statistical correlations between KIF2C expression and clinical prognosis, oncogenic signature gene sets, myeloid-derived suppressor cell infiltration, ImmunoScore, immune checkpoints, microsatellite instability, and tumor mutational burden across multiple tumors. Single-cell data showed that KIF2C is abundantly expressed in malignant cells. The experimental validation demonstrated that KIF2C is highly expressed in gastric cancer cell lines, gastric adenocarcinoma, and hepatocelluar carcinoma. The findings of this study provide important insight for understanding the role and mechanisms of KIF2C in tumorigenesis and immunotherapy in a variety of cancers.

Kinesin-13, a Motor Protein, is Regulated by Polo-like Kinase in Giardia lamblia

Kinesin-13 (Kin-13), a depolymerizer of microtubule (MT), has been known to affect the length of Giardia. Giardia Kin-13 (GlKin-13) was localized to axoneme, flagellar tips, and centrosomes, where phosphorylated forms of Giardia polo-like kinase (GlPLK) were distributed. We observed the interaction between GlKin-13 and GlPLK via co-immunoprecipitation using transgenic Giardia cells expressing Myc-tagged GlKin-13, hemagglutinin-tagged GlPLK, and in vitro-synthesized GlKin-13 and GlPLK proteins. In vitro-synthesized GlPLK was demonstrated to auto-phosphorylate and phosphorylate GlKin-13 upon incubation with [γ-32P]ATP. Morpholino-mediated depletion of both GlKin-13 and GlPLK caused an extension of flagella and a decreased volume of median bodies in Giardia trophozoites. Our results suggest that GlPLK plays a pertinent role in formation of flagella and median bodies by modulating MT depolymerizing activity of GlKin-13.

Septins guide noncentrosomal microtubules to promote focal adhesion disassembly in migrating cells

Endothelial cell migration is critical for vascular angiogenesis and is compromised to facilitate tumor metastasis. The migratory process requires the coordinated assembly and disassembly of focal adhesions (FA), actin, and microtubules (MT). MT dynamics at FAs deliver vesicular cargoes and enhance actomyosin contractility to promote FA turnover and facilitate cell advance. Noncentrosomal (NC) MTs regulate FA dynamics and are sufficient to drive cell polarity, but how NC MTs target FAs to control FA turnover is not understood. Here, we show that Rac1 induces the assembly of FA-proximal septin filaments that promote NC MT growth into FAs and inhibit mitotic centromere-associated kinesin (MCAK)-associated MT disassembly, thereby maintaining intact MT plus ends proximal to FAs. Septin-associated MT rescue is coupled with accumulation of Aurora-A kinase and cytoplasmic linker-associated protein (CLASP) localization to the MT between septin and FAs. In this way, NC MTs are strategically positioned to undergo MCAK- and CLASP-regulated bouts of assembly and disassembly into FAs, thereby regulating FA turnover and cell migration.

Characterization of a recently synthesized microtubule-targeting compound that disrupts mitotic spindle poles in human cells

We reveal the effects of a new microtubule-destabilizing compound in human cells. C75 has a core thienoisoquinoline scaffold with several functional groups amenable to modification. Previously we found that sub micromolar concentrations of C75 caused cytotoxicity. We also found that C75 inhibited microtubule polymerization and competed with colchicine for tubulin-binding in vitro. However, here we found that the two compounds synergized suggesting differences in their mechanism of action. Indeed, live imaging revealed that C75 causes different spindle phenotypes compared to colchicine. Spindles remained bipolar and collapsed after colchicine treatment, while C75 caused bipolar spindles to become multipolar. Importantly, microtubules rapidly disappeared after C75-treatment, but then grew back unevenly and from multiple poles. The C75 spindle phenotype is reminiscent of phenotypes caused by depletion of ch-TOG, a microtubule polymerase, suggesting that C75 blocks microtubule polymerization in metaphase cells. C75 also caused an increase in the number of spindle poles in paclitaxel-treated cells, and combining low amounts of C75 and paclitaxel caused greater regression of multicellular tumour spheroids compared to each compound on their own. These findings warrant further exploration of C75’s anti-cancer potential.

The Dictyostelium Centrosome

The centrosome of Dictyostelium amoebae contains no centrioles and consists of a cylindrical layered core structure surrounded by a corona harboring microtubule-nucleating γ-tubulin complexes. It is the major centrosomal model beyond animals and yeasts. Proteomics, protein interaction studies by BioID and superresolution microscopy methods led to considerable progress in our understanding of the composition, structure and function of this centrosome type. We discuss all currently known components of the Dictyostelium centrosome in comparison to other centrosomes of animals and yeasts.

A mathematical model of kinetochore-microtubule attachment regulated by Aurora A activity gradient describes chromosome oscillation and correction of erroneous attachments

Chromosome oscillation during metaphase is attenuated in cancer cell lines, concomitant with the reduction of Aurora A activity on kinetochores, which results in reduced mitotic fidelity. To verify the correlation between Aurora A activity, chromosome oscillation, and error correction efficiency, we developed a mathematical model of kinetochore-microtubule dynamics, based on stochastic attachment/detachment events regulated by Aurora A activity gradient centered at spindle poles. The model accurately reproduced the oscillatory movements of chromosomes, which were suppressed not only when Aurora A activity was inhibited, but also when it was upregulated, mimicking the situation in cancer cells. Our simulation also predicted efficient correction of erroneous attachments through chromosome oscillation, which was hampered by both inhibition and upregulation of Aurora A activity. Our model provides a framework to understand the physiological role of chromosome oscillation in the correction of erroneous attachments that is intrinsically related to Aurora A activity.

Mechanisms underlying disruption of oocyte spindle stability by bisphenol compounds

Accurate chromosome segregation relies on correct chromosome-microtubule interactions within a stable bipolar spindle apparatus. Thus, exposure to spindle disrupting compounds can impair meiotic division and genomic stability in oocytes. The endocrine disrupting activity of bisphenols such as bisphenol A (BPA) is well recognized, yet their damaging effects on spindle microtubules (MTs) is poorly understood. Here, we tested the effect(s) of acute exposure to BPA and bisphenol F (BPF) on assembled spindle stability in ovulated oocytes. Brief (4 h) exposure to increasing concentrations (5, 25, and 50 µg/mL) of BPA or BPF disrupted spindle organization in a dose-dependent manner, resulting in significantly shorter spindles with highly unfocused poles and fragmented pericentrin. The chromosomes remained congressed in an abnormally elongated metaphase-like configuration, yet normal end-on chromosome-MT attachments were reduced in BPF-treated oocytes. Live-cell imaging revealed a rapid onset of bisphenol-mediated spindle MT disruption that was reversed upon compound removal. Moreover, MT stability and regrowth were impaired in BPA-exposed oocytes, with few cold-stable MTs and formation of multipolar spindles upon MT regrowth. MT-associated kinesin-14 motor protein (HSET/KIFC1) labeling along the spindle was also lower in BPA-treated oocytes. Conversely, cold stable MTs and HSET labeling persisted after BPF exposure. Notably, inhibition of Aurora Kinase A limited bisphenol-mediated spindle pole widening, revealing a potential interaction. These results demonstrate rapid MT disrupting activity by bisphenols, which is highly detrimental to meiotic spindle stability and organization. Moreover, we identify an important link between these defects and altered distribution of key spindle associated factors as well as Aurora Kinase A activity.

Spatial regulation of MCAK promotes cell polarization and focal adhesion turnover to drive robust cell migration

The asymmetric distribution of microtubule (MT) dynamics in migrating cells is important for cell polarization, yet the underlying regulatory mechanisms remain underexplored. Here, we addressed this question by studying the role of the MT depolymerase, MCAK (mitotic centromere-associated kinesin), in the highly persistent migration of RPE-1 cells. MCAK knockdown leads to slowed migration and poor directional movement. Fixed and live cell imaging revealed that MCAK knockdown results in excessive membrane ruffling as well as defects in cell polarization and the maintenance of a major protrusive front. Additionally, loss of MCAK increases the lifetime of focal adhesions by decreasing their disassembly rate. These functions correlate with a spatial distribution of MCAK activity, wherein activity is higher in the trailing edge of cells compared with the leading edge. Overexpression of Rac1 has a dominant effect over MCAK activity, placing it downstream of or in a parallel pathway to MCAK function in migration. Together, our data support a model in which the polarized distribution of MCAK activity and subsequent differential regulation of MT dynamics contribute to cell polarity, centrosome positioning, and focal adhesion dynamics, which all help facilitate robust directional migration.

CRISPR/Cas9 screening identifies a kinetochore-microtubule dependent mechanism for Aurora-A inhibitor resistance in breast cancer

Background

Overexpression of Aurora‐A (AURKA) is a feature of breast cancer and associates with adverse prognosis. The selective Aurora‐A inhibitor alisertib (MLN8237) has recently demonstrated promising antitumor responses as a single agent in various cancer types but its phase III clinical trial was reported as a failure since MLN8237 did not show an apparent effect in prolonging the survival of patients. Thus, identification of potential targets that could enhance the activity of MLN8237 would provide a rationale for drug combination to achieve better therapeutic outcome.

Methods

Here, we conducted a systematic synthetic lethality CRISPR/Cas9 screening of 507 kinases using MLN8237 in breast cancer cells and identified a number of targetable kinases that displayed synthetic lethality interactions with MLN8237. Then, we performed competitive growth assays, colony formation assays, cell viability assays, apoptosis assays, and xenograft murine model to evaluate the synergistic therapeutic effects of Haspin (GSG2) depletion or inhibition with MLN8237. For mechanistic studies, immunofluorescence was used to detect the state of microtubules and the localization of Aurora‐B and mitotic centromere‐associated kinesin (MCAK).

Results

Among the hits, we observed that Haspin depletion or inhibition marginally inhibited breast cancer cell growth but could substantially enhance the killing effects of MLN8237. Mechanistic studies showed that co‐treatment with Aurora‐A and Haspin inhibitors abolished the recruitment of Aurora‐B and mitotic centromere‐associated kinesin (MCAK) to centromeres which were associated with excessive microtubule depolymerization, kinetochore‐microtubule (KT‐MT) attachment failure, and severe mitotic catastrophe. We further showed that the combination of MLN8237 and the Haspin inhibitor CHR‐6494 synergistically reduced breast cancer cell viability and significantly inhibited both in vitro and in vivo tumor growth.

Conclusions

These findings establish Haspin as a synthetic lethal target and demonstrate CHR‐6494 as a potential combinational drug for promoting the therapeutic effects of MLN8237 on breast cancer.

Centrosome Aurora A regulates RhoGEF ECT-2 localisation and ensures a single PAR-2 polarity axis in C. elegans embryos

Proper establishment of cell polarity is essential for development. In the one-cell C. elegans embryo, a centrosome-localised signal provides spatial information for polarity establishment. It is hypothesised that this signal causes local inhibition of the cortical actomyosin network, and breaks symmetry to direct partitioning of the PAR proteins. However, the molecular nature of the centrosomal signal that triggers cortical anisotropy in the actomyosin network to promote polarity establishment remains elusive. Here, we discover that depletion of Aurora A kinase (AIR-1 in C. elegans) causes pronounced cortical contractions on the embryo surface, and this creates more than one PAR-2 polarity axis. This function of AIR-1 appears to be independent of its role in microtubule nucleation. Importantly, upon AIR-1 depletion, centrosome positioning becomes dispensable in dictating the PAR-2 axis. Moreover, we uncovered that a Rho GEF, ECT-2, acts downstream of AIR-1 in regulating contractility and PAR-2 localisation, and, notably, AIR-1 depletion influences ECT-2 cortical localisation. Overall, this study provides a novel insight into how an evolutionarily conserved centrosome Aurora A kinase inhibits promiscuous PAR-2 domain formation to ensure singularity in the polarity establishment axis.

Induction of a Spindle-Assembly-Competent M Phase in Xenopus Egg Extracts

Normal mitotic spindle assembly is a prerequisite for faithful chromosome segregation and unperturbed cell-cycle progression. Precise functioning of the spindle machinery relies on conserved architectural features, such as focused poles, chromosome alignment at the metaphase plate, and proper spindle length. These morphological requirements can be achieved only within a compositionally distinct cytoplasm that results from cell-cycle-dependent regulation of specific protein levels and specific post-translational modifications. Here, we used cell-free extracts derived from Xenopus laevis eggs to recapitulate different phases of the cell cycle in vitro and to determine which components are required to render interphase cytoplasm spindle-assembly competent in the absence of protein translation. We found that addition of a nondegradable form of the master cell-cycle regulator cyclin B1 can indeed induce some biochemical and phenomenological characteristics of mitosis, but cyclin B1 alone is insufficient and actually deleterious at high levels for normal spindle assembly. In contrast, addition of a phosphomimetic form of the Greatwall-kinase effector Arpp19 with a specific concentration of nondegradable cyclin B1 rescued spindle bipolarity but resulted in larger-than-normal bipolar spindles with a misalignment of chromosomes. Both were corrected by the addition of exogenous Xkid (Xenopus homolog of human Kid/KIF22), indicating a role for this chromokinesin in regulating spindle length. These observations suggest that, of the many components degraded at mitotic exit and then replenished during the subsequent interphase, only a few are required to induce a cell-cycle transition that produces a spindle-assembly-competent cytoplasm.

Aurora A promotes chromosome congression by activating the condensin-dependent pool of KIF4A

Aurora kinases create phosphorylation gradients within the spindle during prometaphase and anaphase, thereby locally regulating factors that promote spindle organization, chromosome condensation and movement, and cytokinesis. We show that one such factor is the kinesin KIF4A, which is present along the chromosome axes throughout mitosis and the central spindle in anaphase. These two pools of KIF4A depend on condensin I and PRC1, respectively. Previous work has shown KIF4A is activated by Aurora B at the anaphase central spindle. However, whether or not chromosome-associated KIF4A bound to condensin I is regulated by Aurora kinases remain unclear. To determine the roles of the two different pools of KIF4A, we generated specific point mutants that are unable to interact with either condensin I or PRC1 or are deficient for Aurora kinase regulation. By analyzing these mutants, we show that Aurora A phosphorylates the condensin I-dependent pool of KIF4A and thus actively promotes chromosome congression from the spindle poles to the metaphase plate.

A rapid computational approach identifies SPICE1 as an Aurora kinase substrate

Aurora kinases play a major role in mitosis by regulating diverse substrates. Defining their critical downstream targets is important in understanding Aurora kinase function. Here we have developed an unbiased computational approach to identify new Aurora kinase substrates based on phosphorylation site clustering, protein localization, protein structure, and species conservation. We validate the microtubule-associated proteins Clasp2, Elys, tubulin tyrosine ligase-like polyglutamylase residues 330-624 and spindle and centriole associated protein 1, residues 549-855 (SPICE1), as Aurora A and B kinases substrates in vitro. We also demonstrate that SPICE1 localization is regulated by Aurora kinases during mitosis. In the absence of Aurora kinase activity, SPICE1 remains at centrioles but does not target to the spindle. Similarly, a nonphosphorylatable SPICE1 mutant no longer localizes to the spindle. Finally, we show that misregulating SPICE1 phosphorylation results in abnormal centriole number, spindle multipolarity, and chromosome alignment defects. Overall, our work indicates that temporal and spatial Aurora kinase-mediated regulation of SPICE1 is important for correct chromosome segregation. In addition, our work provides a database-search tool that enables rapid identification of Aurora kinase substrates.

Potential involvement of RITA in the activation of Aurora A at spindle poles during mitosis

The mitotic kinase Aurora A is crucial for various mitotic events. Its activation has been intensively investigated and is not yet completely understood. RITA, the RBP-J interacting and tubulin-associated protein, has been shown to modulate microtubule dynamics in mitosis. We asked if RITA could be related to the activation of Aurora A. We show here that RITA is colocalized with Aurora A and its activator TPX2 at spindle poles during mitosis. FLAG-RITA is precipitated with the complex of Aurora A, TPX2 and tubulin. Depletion of RITA increases exclusively active Aurora A and TPX2 at spindle poles in diverse cancer cell lines and in RITA knockout mouse embryonic fibroblasts. The enhanced active Aurora A, its substrate p-TACC3 and TPX2 are restored by adding back of RITA but not its Δtub mutant with an impaired tubulin-binding capability, indicating that RITA's role as Aurora A's modulator is mediated through its interaction with tubulin. Also, the mitotic failures in cells depleted of RITA are rescued by the inhibition of Aurora A. RITA itself does not directly interfere with the catalytic activity of Aurora A, instead, affects the microtubule binding of its activator TPX2. Moreover, Aurora A's activation correlates with microtubule stabilization induced by the microtubule stabilizer paclitaxel, implicating that stabilized microtubules caused by RITA depletion could also account for increased active Aurora A. Our data suggest a potential role for RITA in the activation of Aurora A at spindle poles by modulating the microtubule binding of TPX2 and the microtubule stability during mitosis.

Aurora A Protein Kinase: To the Centrosome and Beyond

Accurate chromosome segregation requires the perfect spatiotemporal rearrangement of the cellular cytoskeleton. Isolated more than two decades ago from Drosophila, Aurora A is a widespread protein kinase that plays key roles during cell division. Numerous studies have described the localisation of Aurora A at centrosomes, the mitotic spindle, and, more recently, at mitotic centromeres. In this review, we will summarise the cytoskeletal rearrangements regulated by Aurora A during cell division. We will also discuss the recent discoveries showing that Aurora A also controls not only the dynamics of the cortical proteins but also regulates the centromeric proteins, revealing new roles for this kinase during cell division.

The depolymerase activity of MCAK shows a graded response to Aurora B kinase phosphorylation through allosteric regulation

Kinesin-13 motors regulate precise microtubule dynamics and limit microtubule length throughout metazoans by depolymerizing microtubule ends. Recently, the kinesin-13 motor family member MCAK (also known Kif2C) has been proposed to undergo large conformational changes during its catalytic cycle, as it switches from being in solution to being bound to microtubules. Here, we reveal that MCAK has a compact conformation in solution through crosslinking and electron microscopy experiments. When MCAK is bound to the microtubule ends, it adopts an extended conformation with the N-terminus and neck region of MCAK interacting with the microtubule. Interestingly, the region of MCAK that interacts with the microtubule is the region phosphorylated by Aurora B and contains an end binding (EB) protein-binding motif. The level of phosphorylation of the N-terminus results in a graded microtubule depolymerase activity. Here, we show that the N-terminus of MCAK forms a platform to integrate Aurora B kinase downstream signals and in response fine-tunes its depolymerase activity during mitosis. We propose that this allosteric control mechanism allows decoupling of the N-terminus from the motor domain of MCAK to allow MCAK depolymerase activity at kinetochores.

Summary: The kinesin-13 MCAK has a compact conformation in solution but is extended when bound to microtubules. Aurora B phosphorylation of MCAK inhibits depolymerase activity by disrupting its extended conformation.

The functional diversity of Aurora kinases: a comprehensive review

Aurora kinases are serine/threonine kinases essential for the onset and progression of mitosis. Aurora members share a similar protein structure and kinase activity, but exhibit distinct cellular and subcellular localization. AurA favors the G2/M transition by promoting centrosome maturation and mitotic spindle assembly. AurB and AurC are chromosome-passenger complex proteins, crucial for chromosome binding to kinetochores and segregation of chromosomes. Cellular distribution of AurB is ubiquitous, while AurC expression is mainly restricted to meiotically-active germ cells. In human tumors, all Aurora kinase members play oncogenic roles related to their mitotic activity and promote cancer cell survival and proliferation. Furthermore, AurA plays tumor-promoting roles unrelated to mitosis, including tumor stemness, epithelial-to-mesenchymal transition and invasion. In this review, we aim to understand the functional interplay of Aurora kinases in various types of human cells, including tumor cells. The understanding of the functional diversity of Aurora kinases could help to evaluate their relevance as potential therapeutic targets in cancer.

Proteomic Profiling of Microtubule Self-organization in M-phase

Microtubules (MTs) and associated proteins can self-organize into complex structures such as the bipolar spindle, a process in which RanGTP plays a major role. Addition of RanGTP to M-phase Xenopus egg extracts promotes the nucleation and self-organization of MTs into asters and bipolar-like structures in the absence of centrosomes or chromosomes. We show here that the complex proteome of these RanGTP-induced MT assemblies is similar to that of mitotic spindles. Using proteomic profiling we show that MT self-organization in the M-phase cytoplasm involves the non-linear and non-stoichiometric recruitment of proteins from specific functional groups. Our study provides for the first time a temporal understanding of the protein dynamics driving MT self-organization in M-phase.

Aurora A kinase activity is required to maintain an active spindle assembly checkpoint during prometaphase

During the prometaphase stage of mitosis, the cell builds a bipolar spindle of microtubules that mechanically segregates sister chromatids between two daughter cells in anaphase. The spindle assembly checkpoint (SAC) is a quality control mechanism that monitors proper attachment of microtubules to chromosome kinetochores during prometaphase. Segregation occurs only when each chromosome is bi-oriented with each kinetochore pair attached to microtubules emanating from opposite spindle poles. Overexpression of the protein kinase Aurora A is a feature of various cancers and is thought to enable tumour cells to bypass the SAC, leading to aneuploidy. Here, we took advantage of a chemical and chemical-genetic approach to specifically inhibit Aurora A kinase activity in late prometaphase. We observed that a loss of Aurora A activity directly affects SAC function, that Aurora A is essential for maintaining the checkpoint protein Mad2 on unattached kinetochores and that inhibition of Aurora A leads to loss of the SAC, even in the presence of nocodazole or Taxol. This is a new finding that should affect the way Aurora A inhibitors are used in cancer treatments.This article has an associated First Person interview with the first authors of the paper.

Tumor suppressor protein p53-mediated repression of human mitotic centromere-associated kinesin gene expression is exerted via down-regulation of Sp1 level

The repressive role of p53 on the human mitotic centromere-associated kinesin (MCAK) core promoter from ‒266 to +54, relative to the transcription start site, has been determined. The MCAK mRNA and protein levels were 2.1- and 3.0-fold higher, respectively, in HCT116 (p53^‒/‒) than in HCT116 (p53^+/+) cells. Enforced down-regulation of p53 levels either in HCT116 (p53^+/+) cells by p53 RNAi treatment or in MCF-7 cells using shRNA for p53 (shp53) resulted in a remarkable increase in the MCAK protein level. Site-directed mutagenesis and ChIP analyses showed that p53-mediated repression of the MCAK core promoter activity was not directly exerted by p53-binding to putative p53-response elements (p53-RE1 at −173/−166 and p53-RE2 at −245/−238), but indirectly by attenuating Sp1 binding to GC-motifs (GC1 at −93/−84 and GC2 at −119/−110). Treatment of HEK-293 cells bearing the MCAK core promoter-reporter (pGL2-320-Luc) with mithramycin A, which down-regulates Sp1 gene expression, reduced the promoter activity as well as endogenous MCAK levels. Exposure of HCT116 (p53^+/+) cells to nutlin-3a, a validated activator of p53, caused a simultaneous reduction in Sp1 and MCAK protein levels, but not in HCT116 (p53^−/−) cells. In contrast to wild-type (wt)-p53, tumor-derived p53 mutants (p53^V143A, p53^R248W, and p53^R273H) failed to repress the Sp1-dependent activation of the MCAK promoter and to down-regulate endogenous levels of Sp1 and MCAK proteins. Collectively, these findings demonstrate that p53 can repress MCAK promoter activity indirectly via down-regulation of Sp1 expression level, and suggest that MCAK elevation in human tumor cells might be due to p53 mutation.

A Cdk1 phosphomimic mutant of MCAK impairs microtubule end recognition

The microtubule depolymerising kinesin-13, MCAK, is phosphorylated at residue T537 by Cdk1. This is the only known phosphorylation site within MCAK’s motor domain. To understand the impact of phosphorylation by Cdk1 on microtubule depolymerisation activity, we have investigated the molecular mechanism of the phosphomimic mutant T537E. This mutant significantly impairs microtubule depolymerisation activity and when transfected into cells causes metaphase arrest and misaligned chromosomes. We show that the molecular mechanism underlying the reduced depolymerisation activity of this phosphomimic mutant is an inability to recognise the microtubule end. The microtubule-end residence time is reduced relative to wild-type MCAK, whereas the lattice residence time is unchanged by the phosphomimic mutation. Further, the microtubule-end specific stimulation of ADP dissociation, characteristic of MCAK, is abolished by this mutation. Our data shows that T537E is unable to distinguish between the microtubule end and the microtubule lattice.

Aurora A activation in mitosis promoted by BuGZ

Mitotic spindle component BuGZ is known to undergo phase separation. Huang et al. show that BuGZ promotes Aurora A phosphorylation and activation and that this is inhibited when BuGZ phase separation is disrupted.

Protein phase separation or coacervation has emerged as a potential mechanism to regulate biological functions. We have shown that coacervation of a mostly unstructured protein, BuGZ, promotes assembly of spindle and its matrix. BuGZ in the spindle matrix binds and concentrates tubulin to promote microtubule (MT) assembly. It remains unclear, however, whether BuGZ could regulate additional proteins to promote spindle assembly. In this study, we report that BuGZ promotes Aurora A (AurA) activation in vitro. Depletion of BuGZ in cells reduces the amount of phosphorylated AurA on spindle MTs. BuGZ also enhances MCAK phosphorylation. The two zinc fingers in BuGZ directly bind to the kinase domain of AurA, which allows AurA to incorporate into the coacervates formed by BuGZ in vitro. Importantly, mutant BuGZ that disrupts the coacervation activity in vitro fails to promote AurA phosphorylation in Xenopus laevis egg extracts. These results suggest that BuGZ coacervation promotes AurA activation in mitosis.

Three Cdk1 sites in the kinesin-5 Cin8 catalytic domain coordinate motor localization and activity during anaphase

The bipolar kinesin-5 motors perform essential functions in mitotic spindle dynamics. We previously demonstrated that phosphorylation of at least one of the Cdk1 sites in the catalytic domain of the Saccharomyces cerevisiae kinesin-5 Cin8 (S277, T285, S493) regulates its localization to the anaphase spindle. The contribution of these three sites to phospho-regulation of Cin8, as well as the timing of such contributions, remains unknown. Here, we examined the function and spindle localization of phospho-deficient (serine/threonine to alanine) and phospho-mimic (serine/threonine to aspartic acid) Cin8 mutants. In vitro, the three Cdk1 sites undergo phosphorylation by Clb2-Cdk1. In cells, phosphorylation of Cin8 affects two aspects of its localization to the anaphase spindle, translocation from the spindle-pole bodies (SPBs) region to spindle microtubules (MTs) and the midzone, and detachment from the mitotic spindle. We found that phosphorylation of S277 is essential for the translocation of Cin8 from SPBs to spindle MTs and the subsequent detachment from the spindle. Phosphorylation of T285 mainly affects the detachment of Cin8 from spindle MTs during anaphase, while phosphorylation at S493 affects both the translocation of Cin8 from SPBs to the spindle and detachment from the spindle. Only S493 phosphorylation affected the anaphase spindle elongation rate. We conclude that each phosphorylation site plays a unique role in regulating Cin8 functions and postulate a model in which the timing and extent of phosphorylation of the three sites orchestrates the anaphase function of Cin8.

MCAK-mediated regulation of endothelial cell microtubule dynamics is mechanosensitive to myosin-II contractility

This study indicates that MCAK contributes to the mechanosensing-mediated regulation of MT dynamics through a myosin-II–dependent mechanism that becomes uncoupled in response to 3D ECM engagement specifically within EC branches.

Compliance and dimensionality mechanosensing, the processes by which cells sense the physical attributes of the extracellular matrix (ECM), are known to drive cell branching and shape change largely through a myosin-II–mediated reorganization of the actin and microtubule (MT) cytoskeletons. Subcellular regulation of MT dynamics is spatially controlled through a Rac1–Aurora-A kinase pathway that locally inhibits the MT depolymerizing activity of mitotic centromere–associated kinesin (MCAK), thereby promoting leading-edge MT growth and cell polarization. These results suggest that the regulation of MT growth dynamics is intimately linked to physical engagement of the cell with the ECM. Here, we tested the hypothesis that MCAK contributes to compliance and dimensionality mechanosensing-mediated regulation of MT growth dynamics through a myosin-II–dependent signaling pathway. We cultured endothelial cells (ECs) on collagen-coupled stiff or compliant polyacrylamide ECMs to examine the effects of MCAK expression on MT growth dynamics and EC branching morphology. Our results identify that MCAK promotes fast MT growth speeds in ECs cultured on compliant 2D ECMs but promotes slow MT growth speeds in ECs cultured on compliant 3D ECMs, and these effects are myosin-II dependent. Furthermore, we find that 3D ECM engagement uncouples MCAK-mediated regulation of MT growth persistence from myosin-II–mediated regulation of growth persistence specifically within EC branched protrusions.

Structure, biochemistry, and biology of PAK kinases

PAKs, p21-activated kinases, play central roles and act as converging junctions for discrete signals elicited on the cell surface and for a number of intracellular signaling cascades. PAKs phosphorylate a vast number of substrates and act by remodeling cytoskeleton, employing scaffolding, and relocating to distinct subcellular compartments. PAKs affect wide range of processes that are crucial to the cell from regulation of cell motility, survival, redox, metabolism, cell cycle, proliferation, transformation, stress, inflammation, to gene expression. Understandably, their dysregulation disrupts cellular homeostasis and severely impacts key cell functions, and many of those are implicated in a number of human diseases including cancers, neurological disorders, and cardiac disorders. Here we provide an overview of the members of the PAK family and their current status. We give special emphasis to PAK1 and PAK4, the prototypes of groups I and II, for their profound roles in cancer, the nervous system, and the heart. We also highlight other family members. We provide our perspective on the current advancements, their growing importance as strategic therapeutic targets, and our vision on the future of PAKs.

The far C-terminus of MCAK regulates its conformation and spindle pole focusing

Spatial regulation of microtubule dynamics is critical for proper spindle assembly. The far C-terminus of the microtubule-depolymerizing kinesin-13 MCAK regulates MCAK localization at spindle poles, which is needed for proper pole focusing.

To ensure proper spindle assembly, microtubule (MT) dynamics needs to be spatially regulated within the cell. The kinesin-13 MCAK is a potent MT depolymerase with a complex subcellular localization, yet how MCAK spatial regulation contributes to spindle assembly is not understood. Here we show that the far C-terminus of MCAK plays a critical role in regulating MCAK conformation, subspindle localization, and spindle assembly in Xenopus egg extracts. Alteration of MCAK conformation by the point mutation E715A/E716A in the far C-terminus increased MCAK targeting to the poles and reduced MT lifetimes, which induced spindles with unfocused poles. These effects were phenocopied by the Aurora A phosphomimetic mutation, S719E. Furthermore, addition of the kinesin-14 XCTK2 to spindle assembly reactions rescued the unfocused-pole phenotype. Collectively our work shows how the regional targeting of MCAK regulates MT dynamics, highlighting the idea that multiple phosphorylation pathways of MCAK cooperate to spatially control MT dynamics to maintain spindle architecture.

PAKs in Human Cancer Progression: From Inception to Cancer Therapeutic to Future Oncobiology

Since the initial recognition of a mechanistic role of p21-activated kinase 1 (PAK1) in breast cancer invasion, PAK1 has emerged as one of the widely overexpressed or hyperactivated kinases in human cancer at-large, allowing the PAK family to make in-roads in cancer biology, tumorigenesis, and cancer therapeutics. Much of our current understanding of the PAK family in cancer progression relates to a central role of the PAK family in the integration of cancer-promoting signals from cell membrane receptors as well as function as a key nexus-modifier of complex, cytoplasmic signaling network. Another core aspect of PAK signaling that highlights its importance in cancer progression is through PAK's central role in the cross talk with signaling and interacting proteins, as well as PAK's position as a key player in the phosphorylation of effector substrates to engage downstream components that ultimately leads to the development cancerous phenotypes. Here we provide a comprehensive review of the recent advances in PAK cancer research and its downstream substrates in the context of invasion, nuclear signaling and localization, gene expression, and DNA damage response. We discuss how a deeper understanding of PAK1's pathobiology over the years has widened research interest to the PAK family and human cancer, and positioning the PAK family as a promising cancer therapeutic target either alone or in combination with other therapies. With many landmark findings and leaps in the progress of PAK cancer research since the infancy of this field nearly 20 years ago, we also discuss postulated advances in the coming decade as the PAK family continues to shape the future of oncobiology.

KLP-7 acts through the Ndc80 complex to limit pole number in C. elegans oocyte meiotic spindle assembly

KLP-7/MCAK regulates kinetochore–microtubule attachment and spindle tension to promote the coalescence of early spindle pole foci, which produces a bipolar structure during the acentrosomal process of oocyte meiotic spindle assembly in C. elegans.

During oocyte meiotic cell division in many animals, bipolar spindles assemble in the absence of centrosomes, but the mechanisms that restrict pole assembly to a bipolar state are unknown. We show that KLP-7, the single mitotic centromere–associated kinesin (MCAK)/kinesin-13 in Caenorhabditis elegans, is required for bipolar oocyte meiotic spindle assembly. In klp-7(−) mutants, extra microtubules accumulated, extra functional spindle poles assembled, and chromosomes frequently segregated as three distinct masses during meiosis I anaphase. Moreover, reducing KLP-7 function in monopolar klp-18(−) mutants often restored spindle bipolarity and chromosome segregation. MCAKs act at kinetochores to correct improper kinetochore–microtubule (k–MT) attachments, and depletion of the Ndc-80 kinetochore complex, which binds microtubules to mediate kinetochore attachment, restored bipolarity in klp-7(−) mutant oocytes. We propose a model in which KLP-7/MCAK regulates k–MT attachment and spindle tension to promote the coalescence of early spindle pole foci that produces a bipolar structure during the acentrosomal process of oocyte meiotic spindle assembly.

Aurora A's Functions During Mitotic Exit: The Guess Who Game

Until recently, the knowledge of Aurora A kinase functions during mitosis was limited to pre-metaphase events, particularly centrosome maturation, G2/M transition, and mitotic spindle assembly. However, an involvement of Aurora A in post-metaphase events was also suspected, but not clearly demonstrated due to the technical difficulty to perform the appropriate experiments. Recent developments of both an analog-specific version of Aurora A and small molecule inhibitors have led to the first demonstration that Aurora A is required for the early steps of cytokinesis. As in pre-metaphase, Aurora A plays diverse functions during anaphase, essentially participating in astral microtubules dynamics and central spindle assembly and functioning. The present review describes the experimental systems used to decipher new functions of Aurora A during late mitosis and situate these functions into the context of cytokinesis mechanisms.

The nucleoporin ALADIN regulates Aurora A localization to ensure robust mitotic spindle formation

The nucleoporin ALADIN, which is mutated in patients with triple A syndrome, is necessary for proper spindle formation. Without ALADIN, active Aurora A moves away from centrosomes. The relocalization of active Aurora A leads to a redistribution of specific spindle assembly factors that make spindles less stable and slows their formation.

The formation of the mitotic spindle is a complex process that requires massive cellular reorganization. Regulation by mitotic kinases controls this entire process. One of these mitotic controllers is Aurora A kinase, which is itself highly regulated. In this study, we show that the nuclear pore protein ALADIN is a novel spatial regulator of Aurora A. Without ALADIN, Aurora A spreads from centrosomes onto spindle microtubules, which affects the distribution of a subset of microtubule regulators and slows spindle assembly and chromosome alignment. ALADIN interacts with inactive Aurora A and is recruited to the spindle pole after Aurora A inhibition. Of interest, mutations in ALADIN cause triple A syndrome. We find that some of the mitotic phenotypes that we observe after ALADIN depletion also occur in cells from triple A syndrome patients, which raises the possibility that mitotic errors may underlie part of the etiology of this syndrome.

Spatiotemporal dynamics of Aurora B-PLK1-MCAK signaling axis orchestrates kinetochore bi-orientation and faithful chromosome segregation

Chromosome segregation in mitosis is orchestrated by the dynamic interactions between the kinetochore and spindle microtubules. The microtubule depolymerase mitotic centromere-associated kinesin (MCAK) is a key regulator for an accurate kinetochore-microtubule attachment. However, the regulatory mechanism underlying precise MCAK depolymerase activity control during mitosis remains elusive. Here, we describe a novel pathway involving an Aurora B-PLK1 axis for regulation of MCAK activity in mitosis. Aurora B phosphorylates PLK1 on Thr210 to activate its kinase activity at the kinetochores during mitosis. Aurora B-orchestrated PLK1 kinase activity was examined in real-time mitosis using a fluorescence resonance energy transfer-based reporter and quantitative analysis of native PLK1 substrate phosphorylation. Active PLK1, in turn, phosphorylates MCAK at Ser715 which promotes its microtubule depolymerase activity essential for faithful chromosome segregation. Importantly, inhibition of PLK1 kinase activity or expression of a non-phosphorylatable MCAK mutant prevents correct kinetochore-microtubule attachment, resulting in abnormal anaphase with chromosome bridges. We reason that the Aurora B-PLK1 signaling at the kinetochore orchestrates MCAK activity, which is essential for timely correction of aberrant kinetochore attachment to ensure accurate chromosome segregation during mitosis.

The KLP-7 Residue S546 Is a Putative Aurora Kinase Site Required for Microtubule Regulation at the Centrosome in C. elegans

Regulation of microtubule dynamics is essential for many cellular processes, including proper assembly and function of the mitotic spindle. The kinesin-13 microtubule-depolymerizing enzymes provide one mechanism to regulate microtubule behaviour temporally and spatially. Vertebrate MCAK locates to chromatin, kinetochores, spindle poles, microtubule tips, and the cytoplasm, implying that the regulation of kinesin-13 activity and subcellular targeting is complex. Phosphorylation of kinesin-13 by Aurora kinase inhibits microtubule depolymerization activity and some Aurora phosphorylation sites on kinesin-13 are required for subcellular localization. Herein, we determine that a C. elegans deletion mutant klp-7(tm2143) causes meiotic and mitotic defects that are consistent with an increase in the amount of microtubules in the cytoplasmic and spindle regions of meiotic embryos, and an increase in microtubules emanating from centrosomes. We show that KLP-7 is phosphorylated by Aurora A and Aurora B kinases in vitro, and that the phosphorylation by Aurora A is stimulated by TPXL-1. Using a structure-function approach, we establish that one putative Aurora kinase site, S546, within the C-terminal part of the core domain is required for the function, but not subcellular localization, of KLP-7 in vivo. Furthermore, FRAP analysis reveals microtubule-dependent differences in the turnover of KLP-7(S546A) and KLP-7(S546E) mutant proteins at the centrosome, suggesting a possible mechanism for the regulation of KLP-7 by Aurora kinase.

Functional analysis of phosphorylation of the mitotic centromere-associated kinesin by Aurora B kinase in human tumor cells

Mitotic centromere-associated kinesin (MCAK) is the best characterized member of the kinesin-13 family and plays important roles in microtubule dynamics during mitosis. Its activity and subcellular localization is tightly regulated by an orchestra of mitotic kinases, such as Aurora B. It is well known that serine 196 of MCAK is the major phosphorylation site of Aurora B in Xenopus leavis extracts and that this phosphorylation regulates its catalytic activity and subcellular localization. In the current study, we have addressed the conserved phosphorylation site serine 192 in human MCAK to characterize its function in more depth in human cancer cells. Our data confirm that S192 is the major phosphorylation site of Aurora B in human MCAK and that this phosphorylation has crucial roles in regulating its catalytic activity and localization at the kinetochore/centromere region in mitosis. Interfering with this phosphorylation leads to a delayed progression through prometa- and metaphase associated with mitotic defects in chromosome alignment and segregation. We show further that MCAK is involved in directional migration and invasion of tumor cells, and interestingly, interference with the S192 phosphorylation affects this capability of MCAK. These data provide the first molecular explanation for clinical observation, where an overexpression of MCAK was associated with lymphatic invasion and lymph node metastasis in gastric and colorectal cancer patients.

Mio depletion links mTOR regulation to Aurora A and Plk1 activation at mitotic centrosomes

Coordination of cell growth and proliferation in response to nutrient supply is mediated by mammalian target of rapamycin (mTOR) signaling. In this study, we report that Mio, a highly conserved member of the SEACAT/GATOR2 complex necessary for the activation of mTORC1 kinase, plays a critical role in mitotic spindle formation and subsequent chromosome segregation by regulating the proper concentration of active key mitotic kinases Plk1 and Aurora A at centrosomes and spindle poles. Mio-depleted cells showed reduced activation of Plk1 and Aurora A kinase at spindle poles and an impaired localization of MCAK and HURP, two key regulators of mitotic spindle formation and known substrates of Aurora A kinase, resulting in spindle assembly and cytokinesis defects. Our results indicate that a major function of Mio in mitosis is to regulate the activation/deactivation of Plk1 and Aurora A, possibly by linking them to mTOR signaling in a pathway to promote faithful mitotic progression.

The C-terminal region of the motor protein MCAK controls its structure and activity through a conformational switch

The precise regulation of microtubule dynamics is essential during cell division. The kinesin-13 motor protein MCAK is a potent microtubule depolymerase. The divergent non-motor regions flanking the ATPase domain are critical in regulating its targeting and activity. However, the molecular basis for the function of the non-motor regions within the context of full-length MCAK is unknown. Here, we determine the structure of MCAK motor domain bound to its regulatory C-terminus. Our analysis reveals that the MCAK C-terminus binds to two motor domains in solution and is displaced allosterically upon microtubule binding, which allows its robust accumulation at microtubule ends. These results demonstrate that MCAK undergoes long-range conformational changes involving its C-terminus during the soluble to microtubule-bound transition and that the C-terminus-motor interaction represents a structural intermediate in the MCAK catalytic cycle. Together, our work reveals intrinsic molecular mechanisms underlying the regulation of kinesin-13 activity.

DOI:http://dx.doi.org/10.7554/eLife.06421.001

Within a cell, there is a scaffold-like structure called the cytoskeleton that provides shape and structural support, and acts as a transport network for the movement of molecules around the cell. This scaffold contains highly dynamic polymers called microtubules that are made from a protein called tubulin. The constant growth and shrinking of the ends of the microtubules is essential to rebuild and adapt the cytoskeleton according to the needs of the cell.

A protein called MCAK belongs to a family of motor proteins that can move along microtubules. It generally binds to the ends of the microtubules to shorten them. Previous studies have found that a single MCAK protein binds to another MCAK protein to form a larger molecule known as a dimer. Part of the MCAK protein forms a so-called motor domain, which enables this protein to bind to the microtubules. One end of the protein, known as the C-terminus, controls the activity of this motor domain. However, it is not clear how this works.

Talapatra et al. have now revealed the three-dimensional structure of MCAK's motor domain with the C-terminus using a technique called X-ray crystallography. The experiments show that the C-terminus binds to the motor domain, which promotes the formation of the dimers. A short stretch of amino acids—the building blocks of proteins—in the C-terminus interacts with two motor molecules. This ‘motif’ is also found in other similar proteins from a variety of animals. However, once MCAK binds to a microtubule, the microtubule triggers the release of the C-terminus from the motor domain. This allows MCAK to bind more strongly to the microtubule.

The experiments also show that the binding of the C-terminus to the motor domain alters the ability of MCAK to associate with microtubules, which encourages the protein to reach the ends of the polymers. Future work is required to see whether other motor proteins work in a similar way.

DOI:http://dx.doi.org/10.7554/eLife.06421.002

The activity regulation of the mitotic centromere-associated kinesin by Polo-like kinase 1

The mitotic centromere-associated kinesin (MCAK), a potent microtubule depolymerase, is involved in regulating microtubule dynamics. The activity and subcellular localization of MCAK are tightly regulated by key mitotic kinases, such as Polo-like kinase 1 (Plk1) by phosphorylating multiple residues in MCAK. Since Plk1 phosphorylates very often different residues of substrates at different stages, we have dissected individual phosphorylation of MCAK by Plk1 and characterized its function in more depth. We have recently shown that S621 in MCAK is the major phosphorylation site of Plk1, which is responsible for regulating MCAK's degradation by promoting the association of MCAK with APC/CCdc20. In the present study, we have addressed another two residues phosphorylated by Plk1, namely S632/S633 in the C-terminus of MCAK. Our data suggest that Plk1 phosphorylates S632/S633 and regulates its catalytic activity in mitosis. This phosphorylation is required for proper spindle assembly during early phases of mitosis. The subsequent dephosphorylation of S632/S633 might be necessary to timely align the chromosomes onto the metaphase plate. Therefore, our studies suggest new mechanisms by which Plk1 regulates MCAK: the degradation of MCAK is controlled by Plk1 phosphorylation on S621, whereas its activity is modulated by Plk1 phosphorylation on S632/S633 in mitosis.

Aurora A inhibition by MNL8054 promotes centriole elongation during Drosophila male meiosis

Aurora A kinase plays an important role in several aspects of cell division, including centrosome maturation and separation, a crucial step for the correct organization of the bipolar spindle. Although it has long been showed that this kinase accumulates at the centrosome throughout mitosis its precise contribution to centriole biogenesis and structure has until now not been reported. It is not surprising that so little is known, due to the small size of somatic centrioles, where only dramatic structural changes may be identified by careful electron microscopy analysis. Conversely, centrioles of Drosophila primary spermatocytes increase tenfold in length during the first prophase, thus making any change easily detectable. Therefore, we examined the consequence of the pharmacological inhibition of Aurora A by MLN8054 on centriole biogenesis during early Drosophila gametogenesis. Here, we show that depletion of this kinase results in longer centrioles, mainly during transition from prophase to prometaphase of the first meiosis. We also found abnormal ciliogenesis characterized by irregularly growing axonemal doublets. Our results represent the first documentation of a potential requirement of Aurora A in centriole integrity and elongation.

Regulatory mechanisms that control mitotic kinesins

During mitosis, the mitotic spindle is assembled to align chromosomes at the spindle equator in metaphase, and to separate the genetic material equally to daughter cells in anaphase. The spindle itself is a macromolecular machine composed of an array of dynamic microtubules and associated proteins that coordinate the diverse events of mitosis. Among the microtubule associated proteins are a plethora of molecular motor proteins that couple the energy of ATP hydrolysis to force production. These motors, including members of the kinesin superfamily, must function at the right time and in the right place to insure the fidelity of mitosis. Misregulation of mitotic motors in disease states, such as cancer, underlies their potential utility as targets for antitumor drug development and highlights the importance of understanding the molecular mechanisms for regulating their function. Here, we focus on recent progress about regulatory mechanisms that control the proper function of mitotic kinesins and highlight new findings that lay the path for future studies.

Regulation of localization and activity of the microtubule depolymerase MCAK

Mitotic Centromere Associated Kinesin (MCAK) is a potent microtubule depolymerizing and catastrophe-inducing factor, which uses the energy of ATP hydrolysis to destabilize microtubule ends. MCAK is localized to inner centromeres, kinetochores and spindle poles of mitotic cells, and is also present in the cytoplasm. Both in interphase and in mitosis, MCAK can specifically accumulate at the growing microtubule ends. Here we discuss the mechanisms, which modulate subcellular localization and activity of MCAK through the interaction with the End Binding (EB) proteins and phosphorylation.

Microtubule plus-end tracking proteins and their roles in cell division

Microtubules are cellular components that are required for a variety of essential processes such as cell motility, mitosis, and intracellular transport. This is possible because of the inherent dynamic properties of microtubules. Many of these properties are tightly regulated by a number of microtubule plus-end-binding proteins or +TIPs. These proteins recognize the distal end of microtubules and are thus in the right context to control microtubule dynamics. In this review, we address how microtubule dynamics are regulated by different +TIP families, focusing on how functionally diverse +TIPs spatially and temporally regulate microtubule dynamics during animal cell division.

Rac1 and Aurora A regulate MCAK to polarize microtubule growth in migrating endothelial cells

A Rac1–Aurora A–MCAK signaling pathway mediates endothelial cell polarization and directional migration by promoting regional differences in microtubule dynamics in the leading and trailing cell edges.

Endothelial cells (ECs) migrate directionally during angiogenesis and wound healing by polarizing to extracellular cues to guide directional movement. EC polarization is controlled by microtubule (MT) growth dynamics, which are regulated by MT-associated proteins (MAPs) that alter MT stability. Mitotic centromere-associated kinesin (MCAK) is a MAP that promotes MT disassembly within the mitotic spindle, yet its function in regulating MT dynamics to promote EC polarity and migration has not been investigated. We used high-resolution fluorescence microscopy coupled with computational image analysis to elucidate the role of MCAK in regulating MT growth dynamics, morphology, and directional migration of ECs. Our results show that MCAK-mediated depolymerization of MTs is specifically targeted to the trailing edge of polarized wound-edge ECs. Regulation of MCAK function is dependent on Aurora A kinase, which is regionally enhanced by signaling from the small guanosine triphosphatase, Rac1. Thus, a Rac1–Aurora A–MCAK signaling pathway mediates EC polarization and directional migration by promoting regional differences in MT dynamics in the leading and trailing cell edges.

Aurora A orchestrates entosis by regulating a dynamic MCAK-TIP150 interaction

Entosis, a cell-in-cell process, has been implicated in the formation of aneuploidy associated with an aberrant cell division control. Microtubule plus-end-tracking protein TIP150 facilitates the loading of MCAK onto the microtubule plus ends and orchestrates microtubule plus-end dynamics during cell division. Here we show that TIP150 cooperates with MCAK to govern entosis via a regulatory circuitry that involves Aurora A-mediated phosphorylation of MCAK. Our biochemical analyses show that MCAK forms an intra-molecular association, which is essential for TIP150 binding. Interestingly, Aurora A-mediated phosphorylation of MCAK modulates its intra-molecular association, which perturbs the MCAK-TIP150 interaction in vitro and inhibits entosis in vivo. To probe if MCAK-TIP150 interaction regulates microtubule plasticity to affect the mechanical properties of cells during entosis, we used an optical trap to measure the mechanical rigidity of live MCF7 cells. We find that the MCAK cooperates with TIP150 to promote microtubule dynamics and modulate the mechanical rigidity of the cells during entosis. Our results show that a dynamic interaction of MCAK-TIP150 orchestrated by Aurora A-mediated phosphorylation governs entosis via regulating microtubule plus-end dynamics and cell rigidity. These data reveal a previously unknown mechanism of Aurora A regulation in the control of microtubule plasticity during cell-in-cell processes.

Polo-like kinase 1 regulates the stability of the mitotic centromere-associated kinesin in mitosis

Proper bi-orientation of chromosomes is critical for the accurate segregation of chromosomes in mitosis. A key regulator of this process is MCAK, the mitotic centromere-associated kinesin. During mitosis the activity and localization of MCAK are regulated by mitotic key kinases including Plk1 and Aurora B. We show here that S621 in the MCAK's C-terminal domain is the major phosphorylation site for Plk1. This phosphorylation regulates MCAK's stability and facilitates its recognition by the ubiquitin/proteasome dependent APC/C(Cdc20) pathway leading to its D-box dependent degradation in mitosis. While phosphorylation of S621 does not directly affect its microtubule depolymerising activity, loss of Plk1 phosphorylation on S621 indirectly enhances its depolymerization activity in vivo by stabilizing MCAK, leading to an increased level of protein. Interfering with phosphorylation at S621 causes spindle formation defects and chromosome misalignments. Therefore, this study suggests a new mechanism by which Plk1 regulates MCAK: by regulating its degradation and hence controlling its turnover in mitosis.