Uncovering new substrates for Aurora A kinase

Kinase-catalyzed biotinylation for discovery and validation of substrates to multispecificity kinases NME1 and NME2

Protein phosphorylation by kinases regulates mammalian cell functions, such as growth, division, and signal transduction. Among human kinases, NME1 and NME2 are associated with metastatic tumor suppression but remain understudied due to the lack of tools to monitor their cellular substrates. In particular, NME1 and NME2 are multispecificity kinases phosphorylating serine, threonine, histidine, and aspartic acid residues of substrate proteins, and the heat and acid sensitivity of phosphohistidine and phosphoaspartate complicates substrate discovery and validation. To provide new substrate monitoring tools, we established the γ-phosphate-modified ATP analog, ATP-biotin, as a cosubstrate for phosphorylbiotinylation of NME1 and NME2 cellular substrates. Building upon this ATP-biotin compatibility, the Kinase-catalyzed Biotinylation with Inactivated Lysates for Discovery of Substrates method enabled validation of a known substrate and the discovery of seven NME1 and three NME2 substrates. Given the paucity of methods to study kinase substrates, ATP-biotin and the Kinase-catalyzed Biotinylation with Inactivated Lysates for Discovery of Substrates method are valuable tools to characterize the roles of NME1 and NME2 in human cell biology.

Temporally distinct roles of Aurora A in polarization of the C. elegans zygote

During asymmetric cell division, cell polarity is coordinated with the cell cycle to allow proper inheritance of cell fate determinants and the generation of cellular diversity. In the Caenorhabditis elegans zygote, polarity is governed by evolutionarily conserved Partitioning-defective (PAR) proteins that segregate to opposing cortical domains to specify asymmetric cell fates. Timely establishment of PAR domains requires a cell cycle kinase, Aurora A (AIR-1 in C. elegans). Aurora A depletion by RNAi causes a spectrum of phenotypes including reversed polarity, excess posterior domains and no posterior domain. How depletion of a single kinase can cause seemingly opposite phenotypes remains obscure. Using an auxin-inducible degradation system and drug treatments, we found that AIR-1 regulates polarity differently at different times of the cell cycle. During meiosis I, AIR-1 acts to prevent later formation of bipolar domains, whereas in meiosis II, AIR-1 is necessary to recruit PAR-2 onto the membrane. Together, these data clarify the origin of multiple polarization phenotypes in RNAi experiments and reveal multiple roles of AIR-1 in coordinating PAR protein localization with cell cycle progression.

NuSAP regulates microtubule flux and Kif2A localization to ensure accurate chromosome congression

Precise chromosome congression and segregation requires the proper assembly of a steady-state metaphase spindle, which is dynamic and maintained by continuous microtubule flux. NuSAP is a microtubule-stabilizing and -bundling protein that promotes chromosome-dependent spindle assembly. However, its function in spindle dynamics remains unclear. Here, we demonstrate that NuSAP regulates the metaphase spindle length control. Mechanistically, NuSAP facilitates kinetochore capture and spindle assembly by promoting Eg5 binding to microtubules. It also prevents excessive microtubule depolymerization through interaction with Kif2A, which reduces Kif2A spindle-pole localization. NuSAP is phosphorylated by Aurora A at Ser-240 during mitosis, and this phosphorylation promotes its interaction with Kif2A on the spindle body and reduces its localization with the spindle poles, thus maintaining proper spindle microtubule flux. NuSAP knockout resulted in the formation of shorter spindles with faster microtubule flux and chromosome misalignment. Taken together, we uncover that NuSAP participates in spindle assembly, dynamics, and metaphase spindle length control through the regulation of microtubule flux and Kif2A localization.

Distinct Mitotic Functions of Nucleolar and Spindle-Associated Protein 1 (NuSAP1) Are Controlled by Two Consensus SUMOylation Sites

Nucleolar and Spindle-Associated Protein 1 (NuSAP1) is an important mitotic regulator, implicated in control of mitotic microtubule stability and chromosome segregation. NuSAP1 regulates these processes by interacting with several protein partners. Its abundance, activity and interactions are therefore tightly regulated during mitosis. Protein conjugation with SUMO (Small Ubiquitin-like MOdifier peptide) is a reversible post-translational modification that modulates rapid changes in the structure, interaction(s) and localization of proteins. NuSAP1 was previously found to interact with RANBP2, a nucleoporin with SUMO ligase and SUMO-stabilizing activity, but how this interaction affects NuSAP1 activity has remained elusive. Here, we show that NuSAP1 interacts with RANBP2 and forms proximity ligation products with SUMO2/3 peptides in a RANBP2-dependent manner at key mitotic sites. A bioinformatic search identified two putative SUMO consensus sites in NuSAP1, within the DNA-binding and the microtubule-binding domains, respectively. Site-specific mutagenesis, and mitotic phenotyping in cell lines expressing each NuSAP1 mutant version, revealed selective roles of each individual site in control of NuSAP1 localization and in generation of specific mitotic defects and distinct fates in daughter cells. These results identify therefore two new regulatory sites for NuSAP1 functions and implicate RANBP2 in control of NuSAP1 activity.

Temporally distinct roles of Aurora A in polarization of the C. elegans zygote

During asymmetric cell division, coordination of cell polarity and the cell cycle is critical for proper inheritance of cell fate determinants and generation of cellular diversity. In Caenorhabditis elegans (C. elegans), polarity is established in the zygote and is governed by evolutionarily conserved Partitioning defective (PAR) proteins that localize to distinct cortical domains. At the time of polarity establishment, anterior and posterior PARs segregate to opposing cortical domains that specify asymmetric cell fates. Timely establishment of these PAR domains requires a cell cycle kinase, Aurora A (AIR-1 in C.elegans). Aurora A depletion by RNAi causes a spectrum of phenotypes including no posterior domain, reversed polarity, and excess posterior domains. How depletion of a single kinase can cause seemingly opposite phenotypes remains obscure. Using an auxin-inducible degradation system, drug treatments, and high-resolution microscopy, we found that AIR-1 regulates polarity via distinct mechanisms at different times of the cell cycle. During meiosis I, AIR-1 acts to prevent the formation of bipolar domains, while in meiosis II, AIR-1 is necessary to recruit PAR-2 onto the membrane. Together these data clarify the origin of the multiple polarization phenotypes observed in RNAi experiments and reveal multiple roles of AIR-1 in coordinating PAR protein localization with the progression of the cell cycle.

Analysis of Predicted Amino Acid Sequences of Diatom Microtubule Center Components

Diatoms synthesize species-specific exoskeletons inside cells under the control of the cytoskeleton and microtubule center. Previous studies have been conducted with the visualization of the microtubule center; however, its composition has not been studied and reliably established. In the present study, several components of MTOC in diatoms, GCP (gamma complex proteins), Aurora A, and centrins have been identified. Analysis of the predicted amino acid sequences of these proteins revealed structural features typical for diatoms. We analyzed the conserved amino acids and the motives necessary for the functioning of proteins. Phylogenetic analysis of GCP showed that all major groups of diatoms are distributed over phylogenetic trees according to their systematic position. This work is a theoretical study; however, it allows drawing some conclusions about the functioning of the studied components and possible ways to regulate them.

RBL2/DREAM-mediated repression of the Aurora kinase A/B pathway determines therapy responsiveness and outcome in p53 WT NSCLC

Wild-type p53 is a stress-responsive transcription factor and potent tumor suppressor. P53 activates or represses genes involved in cell cycle progression or apoptosis in order to arrest the cell cycle or induce cell death. Transcription repression by p53 is indirect and requires repressive members of the RB-family (RB1, RBL1, RBL2) and formation of repressor complexes of RB1-E2F and RBL1/RBL2-DREAM. Many aurora kinase A/B (AURKA/B) pathway genes are repressed in a p53-DREAM-dependent manner. We found heightened expression of RBL2 and reduced expression of AURKA/B pathway genes is associated with improved outcomes in p53 wild-type but not p53 mutant non-small cell lung cancer (NSCLC) patients. Knockdown of p53, RBL2, or the DREAM component LIN37 increased AURKA/B pathway gene expression and reduced paclitaxel and radiation toxicity in NSCLC cells. In contrast, pharmacologic inhibition of AURKA/B or knockdown of AURKA/B pathway components increased paclitaxel and IR sensitivity. The results support a model in which p53-RBL2-DREAM-mediated repression of the AURKA/B pathway contributes to tumor suppression, improved tumor therapy responses, and better outcomes in p53 wild-type NSCLCs.

A CLK1-KKT2 Signaling Pathway Regulating Kinetochore Assembly in Trypanosoma brucei

During mitosis, eukaryotic cells must duplicate and separate their chromosomes in a precise and timely manner. The apparatus responsible for this is the kinetochore, which is a large protein structure that links chromosomal DNA and spindle microtubules to facilitate chromosome alignment and segregation. The proteins that comprise the kinetochore in the protozoan parasite Trypanosoma brucei are divergent from yeast and mammals and comprise an inner kinetochore complex composed of 24 distinct proteins (KKT1 to KKT23, KKT25) that include four protein kinases, CLK1 (KKT10), CLK2 (KKT19), KKT2, and KKT3. We recently reported the identification of a specific trypanocidal inhibitor of T. brucei CLK1, an amidobenzimidazole, AB1. We now show that chemical inhibition of CLK1 with AB1 impairs inner kinetochore recruitment and compromises cell cycle progression, leading to cell death. Here, we show that KKT2 is a substrate for CLK1 and identify phosphorylation of S508 by CLK1 to be essential for KKT2 function and for kinetochore assembly. Additionally, KKT2 protein kinase activity is required for parasite proliferation but not for assembly of the inner kinetochore complex. We also show that chemical inhibition of the aurora kinase AUK1 does not affect CLK1 phosphorylation of KKT2, indicating that AUK1 and CLK1 are in separate regulatory pathways. We propose that CLK1 is part of a divergent signaling cascade that controls kinetochore function via phosphorylation of the inner kinetochore protein kinase KKT2. IMPORTANCE In eukaryotic cells, kinetochores are large protein complexes that link chromosomes to dynamic microtubule tips, ensuring proper segregation and genomic stability during cell division. Several proteins tightly coordinate kinetochore functions, including the protein kinase aurora kinase B. The kinetochore has diverse evolutionary roots. For example, trypanosomatids, single-cell parasitic protozoa that cause several neglected tropical diseases, possess a unique repertoire of kinetochore components whose regulation during the cell cycle remains unclear. Here, we shed light on trypanosomatid kinetochore biology by showing that the protein kinase CLK1 coordinates the assembly of the inner kinetochore by phosphorylating one of its components, KKT2, allowing the timely spatial recruitment of the rest of the kinetochore proteins and posterior attachment to microtubules in a process that is aurora kinase B independent.

PDS5A and PDS5B in Cohesin Function and Human Disease

Precocious dissociation of sisters 5 (PDS5) is an associate protein of cohesin that is conserved from yeast to humans. It acts as a regulator of the cohesin complex and plays important roles in various cellular processes, such as sister chromatid cohesion, DNA damage repair, gene transcription, and DNA replication. Vertebrates have two paralogs of PDS5, PDS5A and PDS5B, which have redundant and unique roles in regulating cohesin functions. Herein, we discuss the molecular characteristics and functions of PDS5, as well as the effects of its mutations in the development of diseases and their relevance for novel therapeutic strategies.

Use of the Polo-like kinase 4 (PLK4) inhibitor centrinone to investigate intracellular signalling networks using SILAC-based phosphoproteomics

Polo-like kinase 4 (PLK4) is the master regulator of centriole duplication in metazoan organisms. Catalytic activity and protein turnover of PLK4 are tightly coupled in human cells, since changes in PLK4 concentration and catalysis have profound effects on centriole duplication and supernumerary centrosomes, which are associated with aneuploidy and cancer. Recently, PLK4 has been targeted with a variety of small molecule kinase inhibitors exemplified by centrinone, which rapidly induces inhibitory effects on PLK4 and leads to on-target centrosome depletion. Despite this, relatively few PLK4 substrates have been identified unequivocally in human cells, and PLK4 signalling outside centriolar networks remains poorly characterised. We report an unbiased mass spectrometry (MS)-based quantitative analysis of cellular protein phosphorylation in stable PLK4-expressing U2OS human cells exposed to centrinone. PLK4 phosphorylation was itself sensitive to brief exposure to the compound, resulting in PLK4 stabilisation. Analysing asynchronous cell populations, we report hundreds of centrinone-regulated cellular phosphoproteins, including centrosomal and cell cycle proteins and a variety of likely 'non-canonical' substrates. Surprisingly, sequence interrogation of ∼300 significantly down-regulated phosphoproteins reveals an extensive network of centrinone-sensitive [Ser/Thr]Pro phosphorylation sequence motifs, which based on our analysis might be either direct or indirect targets of PLK4. In addition, we confirm that NMYC and PTPN12 are PLK4 substrates, both in vitro and in human cells. Our findings suggest that PLK4 catalytic output directly controls the phosphorylation of a diverse set of cellular proteins, including Pro-directed targets that are likely to be important in PLK4-mediated cell signalling.

Synthetic lethality of RB1 and aurora A is driven by stathmin-mediated disruption of microtubule dynamics

RB1 mutational inactivation is a cancer driver in various types of cancer including lung cancer, making it an important target for therapeutic exploitation. We performed chemical and genetic vulnerability screens in RB1-isogenic lung cancer pair and herein report that aurora kinase A (AURKA) inhibition is synthetic lethal in RB1-deficient lung cancer. Mechanistically, RB1^−/− cells show unbalanced microtubule dynamics through E2F-mediated upregulation of the microtubule destabilizer stathmin and are hypersensitive to agents targeting microtubule stability. Inhibition of AURKA activity activates stathmin function via reduced phosphorylation and facilitates microtubule destabilization in RB1^−/− cells, heavily impacting the bipolar spindle formation and inducing mitotic cell death selectively in RB1^−/− cells. This study shows that stathmin-mediated disruption of microtubule dynamics is critical to induce synthetic lethality in RB1-deficient cancer and suggests that upstream factors regulating microtubule dynamics, such as AURKA, can be potential therapeutic targets in RB1-deficient cancer.

Retinoblastoma susceptibility gene (RB1) is frequently mutated in lung cancers. Here the authors perform chemical and genetic vulnerability screens and identify aurora A kinase (AURKA) as a synthetic lethal candidate for RB1-deficient lung cancer cells and that AURKA inhibition sensitizes these cells to mitotic cell death.

Epigenome-wide DNA methylation analysis of small cell lung cancer cell lines suggests potential chemotherapy targets

BACKGROUND

Small cell lung cancer (SCLC) is an aggressive neuroendocrine lung cancer. SCLC progression and treatment resistance involve epigenetic processes. However, links between SCLC DNA methylation and drug response remain unclear. We performed an epigenome-wide study of 66 human SCLC cell lines using the Illumina Infinium MethylationEPIC BeadChip array. Correlations of SCLC DNA methylation and gene expression with in vitro response to 526 antitumor agents were examined.

RESULTS

We found multiple significant correlations between DNA methylation and chemosensitivity. A potentially important association was observed for TREX1, which encodes the 3' exonuclease I that serves as a STING antagonist in the regulation of a cytosolic DNA-sensing pathway. Increased methylation and low expression of TREX1 were associated with the sensitivity to Aurora kinase inhibitors AZD-1152, SCH-1473759, SNS-314, and TAK-901; the CDK inhibitor R-547; the Vertex ATR inhibitor Cpd 45; and the mitotic spindle disruptor vinorelbine. Compared with cell lines of other cancer types, TREX1 had low mRNA expression and increased upstream region methylation in SCLC, suggesting a possible relationship with SCLC sensitivity to Aurora kinase inhibitors. We also identified multiple additional correlations indicative of potential mechanisms of chemosensitivity. Methylation of the 3'UTR of CEP350 and MLPH, involved in centrosome machinery and microtubule tracking, respectively, was associated with response to Aurora kinase inhibitors and other agents. EPAS1 methylation was associated with response to Aurora kinase inhibitors, a PLK-1 inhibitor and a Bcl-2 inhibitor. KDM1A methylation was associated with PLK-1 inhibitors and a KSP inhibitor. Increased promoter methylation of SLFN11 was correlated with resistance to DNA damaging agents, as a result of low or no SLFN11 expression. The 5' UTR of the epigenetic modifier EZH2 was associated with response to Aurora kinase inhibitors and a FGFR inhibitor. Methylation and expression of YAP1 were correlated with response to an mTOR inhibitor. Among non-neuroendocrine markers, EPHA2 was associated with response to Aurora kinase inhibitors and a PLK-1 inhibitor and CD151 with Bcl-2 inhibitors.

CONCLUSIONS

Multiple associations indicate potential epigenetic mechanisms affecting SCLC response to chemotherapy and suggest targets for combination therapies. While many correlations were not specific to SCLC lineages, several lineage markers were associated with specific agents.

Covalent Aurora A regulation by the metabolic integrator coenzyme A

Aurora A kinase is a master mitotic regulator whose functions are controlled by several regulatory interactions and post-translational modifications. It is frequently dysregulated in cancer, making Aurora A inhibition a very attractive antitumor target. However, recently uncovered links between Aurora A, cellular metabolism and redox regulation are not well understood. In this study, we report a novel mechanism of Aurora A regulation in the cellular response to oxidative stress through CoAlation. A combination of biochemical, biophysical, crystallographic and cell biology approaches revealed a new and, to our knowledge, unique mode of Aurora A inhibition by CoA, involving selective binding of the ADP moiety of CoA to the ATP binding pocket and covalent modification of Cys290 in the activation loop by the thiol group of the pantetheine tail. We provide evidence that covalent CoA modification (CoAlation) of Aurora A is specific, and that it can be induced by oxidative stress in human cells. Oxidising agents, such as diamide, hydrogen peroxide and menadione were found to induce Thr 288 phosphorylation and DTT-dependent dimerization of Aurora A. Moreover, microinjection of CoA into fertilized mouse embryos disrupts bipolar spindle formation and the alignment of chromosomes, consistent with Aurora A inhibition. Altogether, our data reveal CoA as a new, rather selective, inhibitor of Aurora A, which locks this kinase in an inactive state via a "dual anchor" mechanism of inhibition that might also operate in cellular response to oxidative stress. Finally and most importantly, we believe that these novel findings provide a new rationale for developing effective and irreversible inhibitors of Aurora A, and perhaps other protein kinases containing appropriately conserved Cys residues.

New type of interaction between the SARAH domain of the tumour suppressor RASSF1A and its mitotic kinase Aurora A

The tumour suppressor protein RASSF1A is phosphorylated by Aurora A kinase, thereby impairing its tumour suppressor function. Consequently, inhibiting the interaction between Aurora A and RASSF1A may be used for anti-tumour therapy. We used recombinant variants of RASSF1A to map the sites of interaction with Aurora A. The phosphorylation kinetics of three truncated RASSF1A variants has been analysed. Compared to the RASSF1A form lacking the 120 residue long N-terminal part, the Km value of the phosphorylation is increased from 10 to 45 μM upon additional deletion of the C-terminal SARAH domain. On the other hand, deletion of the flexible loop (Δ177-197) that precedes the phosphorylation site/s (T202/S203) results in a reduction of the kcat value from about 40 to 7 min-1. Direct physical interaction between the isolated SARAH domain and Aurora A was revealed by SPR. These data demonstrate that the SARAH domain of RASSF1A is involved in the binding to Aurora A kinase. Structural modelling confirms that a novel complex is feasible between the SARAH domain and the kinase domain of Aurora A. In addition, a regulatory role of the loop in the catalytic phosphorylation reaction has been demonstrated both experimentally and by structural modelling.

Aurora A depletion reveals centrosome-independent polarization mechanism in Caenorhabditis elegans

How living systems break symmetry in an organized manner is a fundamental question in biology. In wild-type Caenorhabditis elegans zygotes, symmetry breaking during anterior-posterior axis specification is guided by centrosomes, resulting in anterior-directed cortical flows and a single posterior PAR-2 domain. We uncover that C. elegans zygotes depleted of the Aurora A kinase AIR-1 or lacking centrosomes entirely usually establish two posterior PAR-2 domains, one at each pole. We demonstrate that AIR-1 prevents symmetry breaking early in the cell cycle, whereas centrosomal AIR-1 instructs polarity initiation thereafter. Using triangular microfabricated chambers, we establish that bipolarity of air-1(RNAi) embryos occurs effectively in a cell-shape and curvature-dependent manner. Furthermore, we develop an integrated physical description of symmetry breaking, wherein local PAR-2-dependent weakening of the actin cortex, together with mutual inhibition of anterior and posterior PAR proteins, provides a mechanism for spontaneous symmetry breaking without centrosomes.

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.

Centrosome Amplification and Tumorigenesis: Cause or Effect?

Centrosome amplification is a feature of multiple tumour types and has been postulated to contribute to both tumour initiation and tumour progression. This chapter focuses on the mechanisms by which an increase in centrosome number might lead to an increase or decrease in tumour progression and the role of proteins that regulate centrosome number in driving tumorigenesis.

Unraveling the role of aurora A beyond centrosomes and spindle assembly: implications in muscle differentiation

Aurora kinases are critical mitotic serine/threonine kinases and are often implicated in tumorigenesis. Recent studies of the interphase functions for aurora kinase (Aurk)A have considerably expanded our understanding of its role beyond mitosis. To identify the unknown targets of AurkA, we used peptide array-based screening and found E2F4 to be a novel substrate. Phosphorylation of E2F4 by AurkA at Ser75 regulates its DNA binding and subcellular localization. Because E2F4 plays an important role in skeletal muscle differentiation, we attempted to gain insight into E2F4 phosphorylation in this context. We observed that a block in E2F4 phosphorylation retained it better within the nucleus and inhibited muscle differentiation. RNA sequencing analysis revealed a perturbation of the gene network involved in the process of muscle differentiation and mitochondrial biogenesis. Collectively, our findings establish a novel role of AurkA in the process of skeletal muscle differentiation.-Dhanasekaran, K., Bose, A., Rao, V. J., Boopathi, R., Shankar, S. R., Rao, V. K., Swaminathan, A., Vasudevan, M., Taneja, R., Kundu, T. K. Unravelling the role of aurora A beyond centrosomes and spindle assembly: implications in muscle differentiation.

Mechanisms of Mitotic Kinase Regulation: A Structural Perspective

Protein kinases are major regulators of mitosis, with over 30% of the mitotic proteome phosphorylated on serines, threonines and tyrosines. The human genome encodes for 518 kinases that have a structurally conserved catalytic domain and includes about a dozen of cell division specific ones. Yet each kinase has unique structural features that allow their distinct substrate recognition and modes of regulation. These unique regulatory features determine their accurate spatio-temporal activation critical for correct progression through mitosis and are exploited for therapeutic purposes. In this review, we will discuss the principles of mitotic kinase activation and the structural determinants that underlie functional specificity.

TACC3 transcriptionally upregulates E2F1 to promote cell growth and confer sensitivity to cisplatin in bladder cancer

Accumulating evidence has shown that transforming acidic coiled-coil 3 (TACC3) is deregulated in a broad spectrum of cancers. In the present study, we reported that TACC3 was markedly elevated in bladder cancer, especially in muscle-invasive bladder cancers (MIBCs). The upregulation of TACC3 was positively associated with tumor invasiveness, grade, T stage, and progression in patients with bladder cancer. Furthermore, a Kaplan-Meier survival analysis showed that patients with bladder cancer whose tumors had high TACC3 expression experienced a dismal prognosis compared with patients whose tumors had low TACC3 expression. Functional studies have found that TACC3 is a prerequisite for the development of malignant characteristics of bladder cancer cells, including cell proliferation and invasion. Moreover, TACC3 promoted G1/S transition, which was mediated via activation of the transcription of E2F1, eventually enhancing cell proliferation. Notably, the overexpression of TACC3 or E2F1 indicates a high sensitivity to cisplatin. Taken together, these findings define a tumor-supportive role for TACC3, which may also serve as a prognostic and therapeutic indicator in bladder cancers.

Characterization of Three Druggable Hot-Spots in the Aurora-A/TPX2 Interaction Using Biochemical, Biophysical, and Fragment-Based Approaches

The mitotic kinase Aurora-A and its partner protein TPX2 (Targeting Protein for Xenopus kinesin-like protein 2) are overexpressed in cancers, and it has been proposed that they work together as an oncogenic holoenzyme. TPX2 is responsible for activating Aurora-A during mitosis, ensuring proper cell division. Disruption of the interface with TPX2 is therefore a potential target for novel anticancer drugs that exploit the increased sensitivity of cancer cells to mitotic stress. Here, we investigate the interface using coprecipitation assays and isothermal titration calorimetry to quantify the energetic contribution of individual residues of TPX2. Residues Tyr8, Tyr10, Phe16, and Trp34 of TPX2 are shown to be crucial for robust complex formation, suggesting that the interaction could be abrogated through blocking any of the three pockets on Aurora-A that complement these residues. Phosphorylation of Aurora-A on Thr288 is also necessary for high-affinity binding, and here we identify arginine residues that communicate the phosphorylation of Thr288 to the TPX2 binding site. With these findings in mind, we conducted a high-throughput X-ray crystallography-based screen of 1255 fragments against Aurora-A and identified 59 hits. Over three-quarters of these hits bound to the pockets described above, both validating our identification of hotspots and demonstrating the druggability of this protein-protein interaction. Our study exemplifies the potential of high-throughput crystallography facilities such as XChem to aid drug discovery. These results will accelerate the development of chemical inhibitors of the Aurora-A/TPX2 interaction.

Aurora A Phosphorylation of YY1 during Mitosis Inactivates its DNA Binding Activity

Successful execution of mitotic cell division requires the tight synchronisation of numerous biochemical pathways. The underlying mechanisms that govern chromosome segregation have been thoroughly investigated. However, the mechanisms that regulate transcription factors in coordination with mitotic progression remain poorly understood. In this report, we identify the transcription factor YY1 as a novel mitotic substrate for the Aurora A kinase, a key regulator of critical mitotic events, like centrosome maturation and spindle formation. Using in vitro kinase assays, we show that Aurora A directly phosphorylates YY1 at serine 365 in the DNA-binding domain. Using a new phospho-specific antibody, we show that YY1 phosphorylation at serine 365 occurs during mitosis, and that this phosphorylation is significantly reduced upon inhibition of Aurora A. Furthermore, we show, using electrophoretic mobility shift and chromatin immunoprecipitation assays, that phosphorylation of YY1 at this site abolishes its DNA binding activity in vitro and in vivo. In conformity with this loss of binding activity, phosphorylated YY1 also loses its transctivation ability as demonstrated by a luciferase reporter assay. These results uncover a novel mechanism that implicates Aurora A in the mitotic inactivation of transcription factors.

Phosphorylation of MAP65-1 by Arabidopsis Aurora Kinases Is Required for Efficient Cell Cycle Progression

Aurora kinases are key effectors of mitosis. Plant Auroras are functionally divided into two clades. The alpha Auroras (Aurora1 and Aurora2) associate with the spindle and the cell plate and are implicated in controlling formative divisions throughout plant development. The beta Aurora (Aurora3) localizes to centromeres and likely functions in chromosome separation. In contrast to the wealth of data available on the role of Aurora in other kingdoms, knowledge on their function in plants is merely emerging. This is exemplified by the fact that only histone H3 and the plant homolog of TPX2 have been identified as Aurora substrates in plants. Here we provide biochemical, genetic, and cell biological evidence that the microtubule-bundling protein MAP65-1-a member of the MAP65/Ase1/PRC1 protein family, implicated in central spindle formation and cytokinesis in animals, yeasts, and plants-is a genuine substrate of alpha Aurora kinases. MAP65-1 interacts with Aurora1 in vivo and is phosphorylated on two residues at its unfolded tail domain. Its overexpression and down-regulation antagonistically affect the alpha Aurora double mutant phenotypes. Phospho-mutant analysis shows that Aurora contributes to the microtubule bundling capacity of MAP65-1 in concert with other mitotic kinases.

Aurora-A shines on T cell activation through the regulation of Lck

Different protein kinases control signaling emanating from the T cell receptor (TCR) during antigen-specific T cell activation. Mitotic kinases, e.g. Aurora-A, have been widely studied in the context of mitosis due to their role during microtubule (MT) nucleation, becoming critical regulators of cell cycle progression. We have recently described a specific role for Aurora-A kinase in antigenic T cell activation. Blockade of Aurora-A in T cells severely disrupts the dynamics of MTs and CD3ζ-bearing signaling vesicles during T cell activation. Furthermore, Aurora-A deletion impairs the activation of signaling molecules downstream of the TCR. Targeting Aurora-A disturbs the activation of Lck, which is one of the first signals that drive T cell activation in an antigen-dependent manner. This work describes possible models of regulation of Lck by Aurora-A during T cell activation. We also discuss possible roles for Aurora-A in other systems similar to the IS, and its putative functions in cell polarization.

TPX2 phosphorylation maintains metaphase spindle length by regulating microtubule flux

TPX2 is phosphorylated by Aurora A and is essential for normal microtubule flux on the metaphase spindle.

A steady-state metaphase spindle maintains constant length, although the microtubules undergo intensive dynamics. Tubulin dimers are incorporated at plus ends of spindle microtubules while they are removed from the minus ends, resulting in poleward movement. Such microtubule flux is regulated by the microtubule rescue factors CLASPs at kinetochores and depolymerizing protein Kif2a at the poles, along with other regulators of microtubule dynamics. How microtubule polymerization and depolymerization are coordinated remains unclear. Here we show that TPX2, a microtubule-bundling protein and activator of Aurora A, plays an important role. TPX2 was phosphorylated by Aurora A during mitosis. Its phospho-null mutant caused short metaphase spindles coupled with low microtubule flux rate. Interestingly, phosphorylation of TPX2 regulated its interaction with CLASP1 but not Kif2a. The effect of its mutant in shortening the spindle could be rescued by codepletion of CLASP1 and Kif2a that abolished microtubule flux. Together we propose that Aurora A–dependent TPX2 phosphorylation controls mitotic spindle length through regulating microtubule flux.

NuSAP modulates the dynamics of kinetochore microtubules by attenuating MCAK depolymerisation activity

Nucleolar and spindle-associated protein (NuSAP) is a microtubule-associated protein that functions as a microtubule stabiliser. Depletion of NuSAP leads to severe mitotic defects, however the mechanism by which NuSAP regulates mitosis remains elusive. In this study, we identify the microtubule depolymeriser, mitotic centromere-associated kinesin (MCAK), as a novel binding partner of NuSAP. We show that NuSAP regulates the dynamics and depolymerisation activity of MCAK. Phosphorylation of MCAK by Aurora B kinase, a component of the chromosomal passenger complex, significantly enhances the interaction of NuSAP with MCAK and modulates the effects of NuSAP on the depolymerisation activity of MCAK. Our results reveal an underlying mechanism by which NuSAP controls kinetochore microtubule dynamics spatially and temporally by modulating the depolymerisation function of MCAK in an Aurora B kinase-dependent manner. Hence, this study provides new insights into the function of NuSAP in spindle formation during mitosis.

Aurora Kinases Throughout Plant Development

Aurora kinases are evolutionarily conserved key mitotic determinants in all eukaryotes. Yeasts contain a single Aurora kinase, whereas multicellular eukaryotes have at least two functionally diverged members. The involvement of Aurora kinases in human cancers has provided an in-depth mechanistic understanding of their roles throughout cell division in animal and yeast models. By contrast, understanding Aurora kinase function in plants is only starting to emerge. Nevertheless, genetic, cell biological, and biochemical approaches have revealed functional diversification between the plant Aurora kinases and suggest a role in formative (asymmetric) divisions, chromatin modification, and genome stability. This review provides an overview of the accumulated knowledge on the function of plant Aurora kinases as well as some major challenges for the future.

Prediction of Functionally Important Phospho-Regulatory Events in Xenopus laevis Oocytes

The African clawed frog Xenopus laevis is an important model organism for studies in developmental and cell biology, including cell-signaling. However, our knowledge of X. laevis protein post-translational modifications remains scarce. Here, we used a mass spectrometry-based approach to survey the phosphoproteome of this species, compiling a list of 2636 phosphosites. We used structural information and phosphoproteomic data for 13 other species in order to predict functionally important phospho-regulatory events. We found that the degree of conservation of phosphosites across species is predictive of sites with known molecular function. In addition, we predicted kinase-protein interactions for a set of cell-cycle kinases across all species. The degree of conservation of kinase-protein interactions was found to be predictive of functionally relevant regulatory interactions. Finally, using comparative protein structure models, we find that phosphosites within structured domains tend to be located at positions with high conformational flexibility. Our analysis suggests that a small class of phosphosites occurs in positions that have the potential to regulate protein conformation.

Phosphoproteomic Analysis of Aurora Kinase Inhibition in Monopolar Cytokinesis

Cytokinesis is the last step of the cell cycle that requires coordinated activities of the microtubule cytoskeleton, actin cytoskeleton, and membrane compartments. Aurora B kinase is one of the master regulatory kinases that orchestrate multiple events during cytokinesis. To reveal targets of the Aurora B kinase, we combined quantitative mass spectrometry with chemical genetics. Using the quantitative proteomic approach, SILAC (stable isotope labeling with amino acids in cell culture), we analyzed the phosphoproteome of monopolar cytokinesis upon VX680- or AZD1152-mediated aurora kinase inhibition. In total, our analysis quantified over 20 000 phosphopeptides in response to the Aurora-B kinase inhibition; 246 unique phosphopeptides were significantly down-regulated and 74 were up-regulated. Our data provide a broad analysis of downstream effectors of Aurora kinase and offer insights into how Aurora kinase regulates cytokinesis.

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.

Aurora-A-Dependent Control of TACC3 Influences the Rate of Mitotic Spindle Assembly

The essential mammalian gene TACC3 is frequently mutated and amplified in cancers and its fusion products exhibit oncogenic activity in glioblastomas. TACC3 functions in mitotic spindle assembly and chromosome segregation. In particular, phosphorylation on S558 by the mitotic kinase, Aurora-A, promotes spindle recruitment of TACC3 and triggers the formation of a complex with ch-TOG-clathrin that crosslinks and stabilises kinetochore microtubules. Here we map the Aurora-A-binding interface in TACC3 and show that TACC3 potently activates Aurora-A through a domain centered on F525. Vertebrate cells carrying homozygous F525A mutation in the endogenous TACC3 loci exhibit defects in TACC3 function, namely perturbed localization, reduced phosphorylation and weakened interaction with clathrin. The most striking feature of the F525A cells however is a marked shortening of mitosis, at least in part due to rapid spindle assembly. F525A cells do not exhibit chromosome missegregation, indicating that they undergo fast yet apparently faithful mitosis. By contrast, mutating the phosphorylation site S558 to alanine in TACC3 causes aneuploidy without a significant change in mitotic duration. Our work has therefore defined a regulatory role for the Aurora-A-TACC3 interaction beyond the act of phosphorylation at S558. We propose that the regulatory relationship between Aurora-A and TACC3 enables the transition from the microtubule-polymerase activity of TACC3-ch-TOG to the microtubule-crosslinking activity of TACC3-ch-TOG-clathrin complexes as mitosis progresses. Aurora-A-dependent control of TACC3 could determine the balance between these activities, thereby influencing not only spindle length and stability but also the speed of spindle formation with vital consequences for chromosome alignment and segregation.

The structure of C290A:C393A Aurora A provides structural insights into kinase regulation

Aurora A is a Ser/Thr protein kinase that functions in cell-cycle regulation and is implicated in cancer development. During mitosis, Aurora A is activated by autophosphorylation on its activation loop at Thr288. The Aurora A catalytic domain (amino acids 122-403) expressed in Escherichia coli autophosphorylates on two activation-loop threonine residues (Thr288 and Thr287), whereas a C290A,C393A double point mutant of the Aurora A catalytic domain autophosphorylates only on Thr288. The structure of the complex of this mutant with ADP and magnesium was determined to 2.1 Å resolution using molecular replacement. This is an improvement on the existing 2.75 Å resolution structure of the equivalent wild-type complex. The structure confirms that single phosphorylation of the activation loop on Thr288 is insufficient to stabilize a `fully active' conformation of the activation loop in the absence of binding to TPX2.

TPX2: of spindle assembly, DNA damage response, and cancer

For more than 15 years, TPX2 has been studied as a factor critical for mitosis and spindle assembly. These functions of TPX2 are attributed to its Ran-regulated microtubule-associated protein properties and to its control of the Aurora A kinase. Overexpressed in cancers, TPX2 is being established as marker for the diagnosis and prognosis of malignancies. During interphase, TPX2 resides preferentially in the nucleus where its function had remained elusive until recently. The latest finding that TPX2 plays a role in amplification of the DNA damage response, combined with the characterization of TPX2 knockout mice, open new perspectives to understand the biology of this protein. This review provides an historic overview of the discovery of TPX2 and summarizes its cytoskeletal and signaling roles with relevance to cancer therapies. Finally, the review aims to reconcile discrepancies between the experimental and pathological effects of TPX2 overexpression and advances new roles for compartmentalized TPX2.

Molecular mechanism of Aurora A kinase autophosphorylation and its allosteric activation by TPX2

We elucidate the molecular mechanisms of two distinct activation strategies (autophosphorylation and TPX2-mediated activation) in human Aurora A kinase. Classic allosteric activation is in play where either activation loop phosphorylation or TPX2 binding to a conserved hydrophobic groove shifts the equilibrium far towards the active conformation. We resolve the controversy about the mechanism of autophosphorylation by demonstrating intermolecular autophosphorylation in a long-lived dimer by combining X-ray crystallography with functional assays. We then address the allosteric activation by TPX2 through activity assays and the crystal structure of a domain-swapped dimer of dephosphorylated Aurora A and TPX2¹⁻²⁵. While autophosphorylation is the key regulatory mechanism in the centrosomes in the early stages of mitosis, allosteric activation by TPX2 of dephosphorylated Aurora A could be at play in the spindle microtubules. The mechanistic insights into autophosphorylation and allosteric activation by TPX2 binding proposed here, may have implications for understanding regulation of other protein kinases.

DOI:http://dx.doi.org/10.7554/eLife.02667.001

The kinase, Aurora A, is a human protein that is needed for cells to divide normally. Kinases are enzymes that control other proteins by adding phosphate groups to these proteins; however, like other kinases, Aurora A must first be activated or ‘switched on’ before it can do this. Aurora A kinase can be switched on in two ways: by having a phosphate group added to its ‘activation loop’; or by binding to another protein called TPX2.

Also like other kinases, Aurora A can self-activate, but the details of this process are not understood. Does a single Aurora A kinase add a phosphate group to its own activation loop, or does one Aurora A kinase activate a second? Furthermore, it is not clear how binding to TPX2 can activate an Aurora A kinase without adding a phosphate group to the activation loop.

Zorba, Buosi et al. now show that Aurora A kinases that have been activated in different ways—via the addition of a phosphate group or binding to TPX2—are equally good at adding phosphate groups to other proteins. Zorba, Buosi et al. also worked out the three-dimensional shapes of the kinases activated in these two ways—since many proteins change shape when they are switched on—and found that they were also the same. Finally, it was shown that self-activation involves two Aurora A kinases binding to each other, and one kinase adding a phosphate group to the other, rather than a single kinase adding a phosphate group to itself.

Since other protein kinases can be activated in similar ways to Aurora A, the findings of Zorba, Buosi et al. might also help us to understand how other protein kinases can be switched ‘on’ or ‘off’. And, as mutations in Aurora A have been linked to the development of cancer, uncovering how this kinase is controlled could help efforts to design new drugs to treat this disease.

DOI:http://dx.doi.org/10.7554/eLife.02667.002

Anticancer activity of the Aurora A kinase inhibitor MK-5108 in non-small-cell lung cancer (NSCLC) in vitro as monotherapy and in combination with chemotherapies

PURPOSE

Aurora kinases are key regulators of mitotic events. Dysfunction of these kinases can cause polyploidy and chromosomal instability, a contributor to tumorigenesis. MK-5108 is a potent inhibitor of Aurora A kinase that has shown preclinical potent activity in malignancies of breast, cervical, colon, ovarian, and pancreatic origin. We sought to assess the preclinical efficacy of MK-5108 in a panel of non-small-cell lung cancer cell lines as a single agent and in combination with cisplatin and docetaxel.

METHODS

Eleven lung cancer cell lines were studied. Growth inhibition by MK-5108 was assessed with short- and long-term MTT assays. Cell cycling was measured by flow cytometry. Immunoblotting was used to determine targeted activity of MK-5108 on Aurora A and downstream effects (TACC3 and Plk1). Efficacy of combination studies performed with cisplatin and docetaxel was evaluated by median effect analysis.

RESULTS

All cell lines demonstrated sustained growth inhibition following MK-5108 at varying nanomolar concentrations. MK-5108 induced G2/M accumulation, polyploidy, and apoptosis (increased sub-G1/PARP cleavage). Levels of Aurora A, TACC3, and Plk1 diminished. Concurrent treatment of MK-5108 with cisplatin or docetaxel synergistically inhibited cell growth with the docetaxel combination performing better. When administered sequentially, treatment with docetaxel first followed by MK-5108 exhibited greater growth inhibition than the inverse; yet concurrent treatment remained superior.

CONCLUSIONS

MK-5108 has potent anti-proliferative activity in lung cancer cell lines alone and in combination with chemotherapies. Determining how best to integrate Aurora inhibitors into current lung cancer treatment regimens would be beneficial.

Posttranslationally modified tubulins and other cytoskeletal proteins: their role in gametogenesis, oocyte maturation, fertilization and Pre-implantation embryo development

The cytoskeleton, mainly consisting of microtubules, intermediate filaments and microfilaments, along with cytoskeleton associated and interconnecting proteins as well as the centrosome, plays enormously important roles in all stages of embryogenesis and undergoes significant changes to accommodate a diversity of cellular functions during gametogenesis, oocyte maturation, fertilization and pre-implantation embryo development. The varied functions of the cytoskeleton can be accomplished on many different levels, among which are a diversity of different posttranslational modifications (PTMs), chemical modifications that regulate activity, localization and interactions with other cellular molecules. PTMs of the cytoskeleton, including phosphorylation, glycosylation, ubiquitination, detyrosination/tyrosination, (poly)glutamylation and (poly)glycylation, acetylation, sumoylation, and palmitoylation, will be addressed in this chapter. Focus will be on (1) Microtubules, microtubule organizing centers (centrosomes), intermediate filaments, microfilaments and their PTMs; (2) Cytoskeletal functions and cytoskeletal PTMs during gametogenesis and oocyte maturation; and (3) Cytoskeletal functions and cytoskeletal PTMs during fertilization and pre-implantation embryo development.

Stathmin and microtubules regulate mitotic entry in HeLa cells by controlling activation of both Aurora kinase A and Plk1

Depletion of stathmin, a microtubule (MT) destabilizer, delays mitotic entry by ∼4 h in HeLa cells. Stathmin depletion reduced the activity of CDC25 and its upstream activators, Aurora A and Plk1. Chemical inhibition of both Aurora A and Plk1 was sufficient to delay mitotic entry by 4 h, while inhibiting either kinase alone did not cause a delay. Aurora A and Plk1 are likely regulated downstream of stathmin, because the combination of stathmin knockdown and inhibition of Aurora A and Plk1 was not additive and again delayed mitotic entry by 4 h. Aurora A localization to the centrosome required MTs, while stathmin depletion spread its localization beyond that of γ-tubulin, indicating an MT-dependent regulation of Aurora A activation. Plk1 was inhibited by excess stathmin, detected in in vitro assays and cells overexpressing stathmin-cyan fluorescent protein. Recruitment of Plk1 to the centrosome was delayed in stathmin-depleted cells, independent of MTs. It has been shown that depolymerizing MTs with nocodazole abrogates the stathmin-depletion induced cell cycle delay; in this study, depolymerization with nocodazole restored Plk1 activity to near normal levels, demonstrating that MTs also contribute to Plk1 activation. These data demonstrate that stathmin regulates mitotic entry, partially via MTs, to control localization and activation of both Aurora A and Plk1.

The centrosomal adaptor TACC3 and the microtubule polymerase chTOG interact via defined C-terminal subdomains in an Aurora-A kinase-independent manner

The cancer-associated, centrosomal adaptor protein TACC3 (transforming acidic coiled-coil 3) and its direct effector, the microtubule polymerase chTOG (colonic and hepatic tumor overexpressed gene), play a crucial function in centrosome-driven mitotic spindle assembly. It is unclear how TACC3 interacts with chTOG. Here, we show that the C-terminal TACC domain of TACC3 and a C-terminal fragment adjacent to the TOG domains of chTOG mediate the interaction between these two proteins. Interestingly, the TACC domain consists of two functionally distinct subdomains, CC1 (amino acids (aa) 414-530) and CC2 (aa 530-630). Whereas CC1 is responsible for the interaction with chTOG, CC2 performs an intradomain interaction with the central repeat region of TACC3, thereby masking the TACC domain before effector binding. Contrary to previous findings, our data clearly demonstrate that Aurora-A kinase does not regulate TACC3-chTOG complex formation, indicating that Aurora-A solely functions as a recruitment factor for the TACC3-chTOG complex to centrosomes and proximal mitotic spindles. We identified with CC1 and CC2, two functionally diverse modules within the TACC domain of TACC3 that modulate and mediate, respectively, TACC3 interaction with chTOG required for spindle assembly and microtubule dynamics during mitotic cell division.

Functional impact of Aurora A-mediated phosphorylation of HP1γ at serine 83 during cell cycle progression

Background

Previous elegant studies performed in the fission yeast Schizosaccharomyces pombe have identified a requirement for heterochromatin protein 1 (HP1) for spindle pole formation and appropriate cell division. In mammalian cells, HP1γ has been implicated in both somatic and germ cell proliferation. High levels of HP1γ protein associate with enhanced cell proliferation and oncogenesis, while its genetic inactivation results in meiotic and mitotic failure. However, the regulation of HP1γ by kinases, critical for supporting mitotic progression, remains to be fully characterized.

Results

We report for the first time that during mitotic cell division, HP1γ colocalizes and is phosphorylated at serine 83 (Ser⁸³) in G₂/M phase by Aurora A. Since Aurora A regulates both cell proliferation and mitotic aberrations, we evaluated the role of HP1γ in the regulation of these phenomena using siRNA-mediated knockdown, as well as phosphomimetic and nonphosphorylatable site-directed mutants. We found that genetic downregulation of HP1γ, which decreases the levels of phosphorylation of HP1γ at Ser⁸³ (P-Ser⁸³-HP1γ), results in mitotic aberrations that can be rescued by reintroducing wild type HP1γ, but not the nonphosphorylatable S83A-HP1γ mutant. In addition, proliferation assays showed that the phosphomimetic S83D-HP1γ increases 5-ethynyl-2´-deoxyuridine (EdU) incorporation, whereas the nonphosphorylatable S83A-HP1γ mutant abrogates this effect. Genome-wide expression profiling revealed that the effects of these mutants on mitotic functions are congruently reflected in G₂/M gene expression networks in a manner that mimics the on and off states for P-Ser⁸³-HP1γ.

Conclusions

This is the first description of a mitotic Aurora A-HP1γ pathway, whose integrity is necessary for the execution of proper somatic cell division, providing insight into specific types of posttranslational modifications that associate to distinct functional outcomes of this important chromatin protein.

The role of NEDD1 phosphorylation by Aurora A in chromosomal microtubule nucleation and spindle function

Chromatin directs de novo microtubule (MT) nucleation in dividing cells by generating a gradient of GTP-bound Ran protein (RanGTP) that controls the activity of a number of spindle assembly factors (SAFs). It is now well established that these MTs are essential for the assembly of a functional bipolar spindle. Although it has been shown that RanGTP-dependent MT nucleation requires γ-tubulin and a number of RanGTP-regulated proteins, the mechanism involved is still poorly understood. We previously showed that the mitotic kinase Aurora A, which is activated in a RanGTP-dependent manner in mitotic cells, has a role in this pathway. Here we show that Aurora A interacts with and phosphorylates the γTURC adaptor protein NEDD1 at a single residue, Ser405. Ser405 phosphorylation is not required for centrosomal MT nucleation but is critical for MT nucleation in the vicinity of the chromosomes in mitotic cells. Moreover, it is essential for RanGTP aster formation and chromatin-driven MT assembly in Xenopus egg extracts. Our data suggest that one important function of Aurora A in mitotic cells is to promote MT nucleation around the chromatin by phosphorylating NEDD1, and thereby to promote functional spindle assembly.

New insights into subcomplex assembly and modifications of centrosomal proteins

This review provides a brief overview of the recent work on centrosome proteomics, protein complex identification and functional characterization with an emphasis on the literature of the last three years. Proteomics, genetic screens and comparative genomics studies in different model organisms have almost exhaustively identified the molecular components of the centrosome. However, much knowledge is still missing on the protein-protein interactions, protein modifications and molecular changes the centrosome undergoes throughout the cell cycle and development. The dynamic nature of this large multi-protein complex is reflected in the variety of annotated subcellular locations and biological processes of its proposed components. Some centrosomal proteins and complexes have been studied intensively in different organisms and provided detailed insight into centrosome functions. For example, the molecular, structural and functional characterization of the γ-Tubulin ring complex (γ-TuRC) and the the discovery of the Augmin/HAUS complex has advanced our understanding of microtubule (MT) capture, nucleation and organization. Surprising findings revealed new functions and localizations of proteins that were previously regarded as bona fide centriolar or centrosome components, e.g. at the kinetochore or in the nuclear pore complex regulating MT plus end capture or mRNA processing. Many centrosome components undergo posttranslational modifications such as phosphorylation, SUMOylation and ubiquitylation that are critical in modulating centrosome function and biology. A wealth of information has recently become available driven by new developments in technologies such as mass spectrometry, light and electron microscopy providing more detailed molecular and structural definition of the centrosome and particular roles of proteins throughout the cell cycle and development.

A functional cooperativity between Aurora A kinase and LIM kinase1: implication in the mitotic process

Aurora kinase A (Aur-A), a mitotic kinase, regulates initiation of mitosis through centrosome separation and proper assembly of bipolar spindles. LIM kinase 1 (LIMK1), a modulator of actin and microtubule dynamics, is involved in the mitotic process through inactivating phosphorylation of cofilin. Phosphorylated LIMK1 is recruited to the centrosomes during early prophase, where it colocalizes with γ-tubulin. Here, we report a novel functional cooperativity between Aur-A and LIMK1 through mutual phosphorylation. LIMK1 is recruited to the centrosomes during early prophase and then to the spindle poles, where it colocalizes with Aur-A. Aur-A physically associates with LIMK1 and activates it through phosphorylation, which is important for its centrosomal and spindle pole localization. Aur-A also acts as a substrate of LIMK1, and the function of LIMK1 is important for its specific localization and regulation of spindle morphology. Taken together, the novel molecular interaction between these two kinases and their regulatory roles on one another's function may provide new insight on the role of Aur-A in manipulation of actin and microtubular structures during spindle formation.

Aurora-A inactivation causes mitotic spindle pole fragmentation by unbalancing microtubule-generated forces

BACKGROUND

Aurora-A is an oncogenic kinase playing well-documented roles in mitotic spindle organisation. We previously found that Aurora-A inactivation yields the formation of spindles with fragmented poles that can drive chromosome mis-segregation. Here we have addressed the mechanism through which Aurora-A activity regulates the structure and cohesion of spindle poles.

RESULTS

We inactivated Aurora-A in human U2OS osteosarcoma cells either by RNA-interference-mediated silencing or treating cultures with the specific inhibitor MLN8237. We show that mitotic spindle pole fragmentation induced by Aurora-A inactivation is associated with microtubule hyperstabilisation. Silencing of the microtubule-stabilising factor ch-TOG prevents spindle pole fragmentation caused by inactivation of Aurora-A alone and concomitantly reduces the hyperstabilisation of microtubules. Furthermore, decreasing pole-directed spindle forces by inhibition of the Eg5 kinesin, or by destabilisation of microtubule-kinetochore attachments, also prevents pole fragmentation in Aurora-A-inactivated mitoses.

CONCLUSIONS

Our findings indicate that microtubule-generated forces are imbalanced in Aurora-A-defective cells and exert abnormal pressure at the level of spindle poles, ultimately causing their fragmentation. This study therefore highlights a novel role of the Aurora-A kinase in regulating the balance between microtubule forces during bipolar spindle assembly.

Aurora A kinase activity is required for localization of TACC3/ch-TOG/clathrin inter-microtubule bridges

Accurate chromosome segregation during mitosis is achieved by the kinetochore fibers (K-fibers) of the spindle apparatus. These fibers are bundles of microtubules (MTs) connected by non-motor bridges. We recently identified a TACC3/ch-TOG/clathrin complex that constitutes the shortest class of inter-MT bridge in K-fibers. TACC3 anchors the complex to MTs and this is dependent on phosphorylation by Aurora A kinase. Here we show that inhibition of Aurora A kinase using MLN8237 results in (1) loss of clathrin and TACC3 from spindles, (2) destabilization of K-fibers and (3) loss of inter-MT bridges. These results are similar to those in cells depleted of clathrin or TACC3; suggesting that TACC3/ch-TOG/clathrin bridges are the major class of bridge that is regulated by this kinase.