The non-catalytic domain of the Xenopus laevis auroraA kinase localises the protein to the centrosome

Selective targeting of non-centrosomal AURKA functions through use of a targeted protein degradation tool

Targeted protein degradation tools are becoming a new therapeutic modality, allowing small molecule ligands to be reformulated as heterobifunctional molecules (PROteolysis Targeting Chimeras, PROTACs) that recruit ubiquitin ligases to targets of interest, leading to ubiquitination and destruction of the targets. Several PROTACs against targets of clinical interest have been described, but detailed descriptions of the cell biology modulated by PROTACs are missing from the literature. Here we describe the functional characterization of a PROTAC derived from AURKA inhibitor MLN8237 (alisertib). We demonstrate efficient and specific destruction of both endogenous and overexpressed AURKA by Cereblon-directed PROTACs. At the subcellular level, we find differential targeting of AURKA on the mitotic spindle compared to centrosomes. The phenotypic consequences of PROTAC treatment are therefore distinct from those mediated by alisertib, and in mitotic cells differentially regulate centrosome- and chromatin- based microtubule spindle assembly pathways. In interphase cells PROTAC-mediated clearance of non-centrosomal AURKA modulates the cytoplasmic role played by AURKA in mitochondrial dynamics, whilst the centrosomal pool is refractory to PROTAC-mediated clearance. Our results point to differential sensitivity of subcellular pools of substrate, governed by substrate conformation or localization-dependent accessibility to PROTAC action, a phenomenon not previously described for this new class of degrader compounds.

Wang et al develop tools to target the mitotic regulator AURKA by synthesising PROTACs based on the inhibitor MLN8237. They find that the new PROTAC compound efficiently clears cytoplasmic and mitotic spindle-associated AURKA but does not eliminate AURKA activity from centrosomes, demonstrating the possibility of targeting subpopulations.

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 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.

Structure-based design, synthesis and crystallization of 2-arylquinazolines as lipid pocket ligands of p38α MAPK

In protein kinase research, identifying and addressing small molecule binding sites other than the highly conserved ATP-pocket are of intense interest because this line of investigation extends our understanding of kinase function beyond the catalytic phosphotransfer. Such alternative binding sites may be involved in altering the activation state through subtle conformational changes, control cellular enzyme localization, or in mediating and disrupting protein-protein interactions. Small organic molecules that target these less conserved regions might serve as tools for chemical biology research and to probe alternative strategies in targeting protein kinases in disease settings. Here, we present the structure-based design and synthesis of a focused library of 2-arylquinazoline derivatives to target the lipophilic C-terminal binding pocket in p38α MAPK, for which a clear biological function has yet to be identified. The interactions of the ligands with p38α MAPK was analyzed by SPR measurements and validated by protein X-ray crystallography.

Mechanisms for nonmitotic activation of Aurora-A at cilia

Overexpression of the Aurora kinase A (AURKA) is oncogenic in many tumors. Many studies of AURKA have focused on activities of this kinase in mitosis, and elucidated the mechanisms by which AURKA activity is induced at the G2/M boundary through interactions with proteins such as TPX2 and NEDD9. These studies have informed the development of small molecule inhibitors of AURKA, of which a number are currently under preclinical and clinical assessment. While the first activities defined for AURKA were its control of centrosomal maturation and organization of the mitotic spindle, an increasing number of studies over the past decade have recognized a separate biological function of AURKA, in controlling disassembly of the primary cilium, a small organelle protruding from the cell surface that serves as a signaling platform. Importantly, these activities require activation of AURKA in early G1, and the mechanisms of activation are much less well defined than those in mitosis. A better understanding of the control of AURKA activity and the role of AURKA at cilia are both important in optimizing the efficacy and interpreting potential downstream consequences of AURKA inhibitors in the clinic. We here provide a current overview of proteins and mechanisms that have been defined as activating AURKA in G1, based on the study of ciliary disassembly.

Ubiquitin-Mediated Degradation of Aurora Kinases

The Aurora kinases are essential regulators of mitosis in eukaryotes. In somatic cell divisions of higher eukaryotes, the paralogs Aurora kinase A (AurA) and Aurora kinase B (AurB) play non-overlapping roles that depend on their distinct spatiotemporal activities. These mitotic roles of Aurora kinases depend on their interactions with different partners that direct them to different mitotic destinations and different substrates: AurB is a component of the chromosome passenger complex that orchestrates the tasks of chromosome segregation and cytokinesis, while AurA has many known binding partners and mitotic roles, including a well-characterized interaction with TPX2 that mediates its role in mitotic spindle assembly. Beyond the spatial control conferred by different binding partners, Aurora kinases are subject to temporal control of their activation and inactivation. Ubiquitin-mediated proteolysis is a critical route to irreversible inactivation of these kinases, which must occur for ordered transition from mitosis back to interphase. Both AurA and AurB undergo targeted proteolysis after anaphase onset as substrates of the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase, even while they continue to regulate steps during mitotic exit. Temporal control of Aurora kinase destruction ensures that AurB remains active at the midbody during cytokinesis long after AurA activity has been largely eliminated from the cell. Differential destruction of Aurora kinases is achieved despite the fact that they are targeted at the same time and by the same ubiquitin ligase, making these substrates an interesting case study for investigating molecular determinants of ubiquitin-mediated proteolysis in higher eukaryotes. The prevalence of Aurora overexpression in cancers and their potential as therapeutic targets add importance to the task of understanding the molecular determinants of Aurora kinase stability. Here, we review what is known about ubiquitin-mediated targeting of these critical mitotic regulators and discuss the different factors that contribute to proteolytic control of Aurora kinase activity in the cell.

Spatial Compartmentalization Specializes the Function of Aurora A and Aurora B

Aurora kinase A and B share great similarity in sequences, structures, and phosphorylation motif, yet they show different localizations and play distinct crucial roles. The factors that determine such differences are largely unknown. Here we targeted Aurora A to the localization of Aurora B and found that Aurora A phosphorylates the substrate of Aurora B and substitutes its function in spindle checkpoint. In return, the centrosome targeting of Aurora B substitutes the function of Aurora A in the mitotic entry. Expressing the chimera proteins of the Auroras with exchanged N termini in cells indicates that the divergent N termini are also important for their spatiotemporal localizations and functions. Collectively, we demonstrate that functional divergence of Aurora kinases is determined by spatial compartmentalization, and their divergent N termini also contribute to their spatial and functional differentiation.

Mitotic Kinases and p53 Signaling

Mitosis is tightly regulated and any errors in this process often lead to aneuploidy, genomic instability, and tumorigenesis. Deregulation of mitotic kinases is significantly associated with improper cell division and aneuploidy. Because of their importance during mitosis and the relevance to cancer, mitotic kinase signaling has been extensively studied over the past few decades and, as a result, several mitotic kinase inhibitors have been developed. Despite promising preclinical results, targeting mitotic kinases for cancer therapy faces numerous challenges, including safety and patient selection issues. Therefore, there is an urgent need to better understand the molecular mechanisms underlying mitotic kinase signaling and its interactive network. Increasing evidence suggests that tumor suppressor p53 functions at the center of the mitotic kinase signaling network. In response to mitotic spindle damage, multiple mitotic kinases phosphorylate p53 to either activate or deactivate p53-mediated signaling. p53 can also regulate the expression and function of mitotic kinases, suggesting the existence of a network of mutual regulation, which can be positive or negative, between mitotic kinases and p53 signaling. Therefore, deciphering this regulatory network will provide knowledge to overcome current limitations of targeting mitotic kinases and further improve the results of targeted therapy.

Calmodulin activation of Aurora-A kinase (AURKA) is required during ciliary disassembly and in mitosis

This study demonstrates for the first time that binding of calcium-activated calmodulin to a minimal interaction site within the disordered N-terminal domain is required for the essential Aurora-A activity in mitosis and in regulation of ciliary disassembly.

The centrosomal Aurora-A kinase (AURKA) regulates mitotic progression, and overexpression and hyperactivation of AURKA commonly promotes genomic instability in many tumors. Although most studies of AURKA focus on its role in mitosis, some recent work identified unexpected nonmitotic activities of AURKA. Among these, a role for basal body–localized AURKA in regulating ciliary disassembly in interphase cells has highlighted a role in regulating cellular responsiveness to growth factors and mechanical cues. The mechanism of AURKA activation involves interactions with multiple partner proteins and is not well understood, particularly in interphase cells. We show here that AURKA activation at the basal body in ciliary disassembly requires interactions with Ca²⁺ and calmodulin (CaM) and that Ca²⁺/CaM are important mediators of the ciliary disassembly process. We also show that Ca²⁺/CaM binding is required for AURKA activation in mitosis and that inhibition of CaM activity reduces interaction between AURKA and its activator, NEDD9. Finally, mutated derivatives of AURKA impaired for CaM binding and/or CaM-dependent activation cause defects in mitotic progression, cytokinesis, and ciliary resorption. These results define Ca²⁺/CaM as important regulators of AURKA activation in mitotic and nonmitotic signaling.

Molecular distinctions between Aurora A and B: a single residue change transforms Aurora A into correctly localized and functional Aurora B

Aurora A and Aurora B, paralogue mitotic kinases, share highly similar primary sequence. Both are important to mitotic progression, but their localizations and functions are distinct. We have combined shRNA suppression with overexpression of Aurora mutants to address the cause of the distinction between Aurora A and Aurora B. Aurora A residue glycine 198 (G198), mutated to asparagine to mimic the aligned asparagine 142 (N142) of Aurora B, causes Aurora A to bind the Aurora B binding partner INCENP but not the Aurora A binding partner TPX2. The mutant Aurora A rescues Aurora B mitotic function. We conclude that binding to INCENP is alone critical to the distinct function of Aurora B. Although G198 of Aurora A is required for TPX2 binding, N142G Aurora B retains INCENP binding and Aurora B function. Thus, although a single residue change transforms Aurora A, the reciprocal mutation of Aurora B does not create Aurora A function. An Aurora A-Delta120 N-terminal truncation construct reinforces Aurora A similarity to Aurora B, because it does not associate with centrosomes but instead associates with kinetochores.

Antitumor activity of MLN8054, an orally active small-molecule inhibitor of Aurora A kinase

Increased Aurora A expression occurs in a variety of human cancers and induces chromosomal abnormalities during mitosis associated with tumor initiation and progression. MLN8054 is a selective small-molecule Aurora A kinase inhibitor that has entered Phase I clinical trials for advanced solid tumors. MLN8054 inhibits recombinant Aurora A kinase activity in vitro and is selective for Aurora A over the family member Aurora B in cultured cells. MLN8054 treatment results in G(2)/M accumulation and spindle defects and inhibits proliferation in multiple cultured human tumor cells lines. Growth of human tumor xenografts in nude mice was dramatically inhibited after oral administration of MLN8054 at well tolerated doses. Moreover, the tumor growth inhibition was sustained after discontinuing MLN8054 treatment. In human tumor xenografts, MLN8054 induced mitotic accumulation and apoptosis, phenotypes consistent with inhibition of Aurora A. MLN8054 is a selective inhibitor of Aurora A kinase that robustly inhibits growth of human tumor xenografts and represents an attractive modality for therapeutic intervention of human cancers.

Characterization of a splicing variant of plant Aurora kinase

Aurora kinases play a key role in chromosome segregation and cytokinesis. In plants, three Aurora kinases (AtAUR1-AtAUR3) have been identified in Arabidopsis thaliana. Here, we report an AtAUR2 splicing variant (AtAUR2S), which lacks the fourth exon encoding a part of the kinase domain of AtAUR2. AtAUR2S was shown to have lost its kinase activity to phosphorylate histone H3 at Ser10; however, it maintained its ability to bind to histone H3. The localization pattern of AtAUR2S was the same as that of AtAUR2. The findings suggest that AtAUR2S affects cell division by competing with AtAUR2.

The GIT-associated kinase PAK targets to the centrosome and regulates Aurora-A

Previously, we showed PAK-PIX-GIT targets and regulates focal adhesions; here, we uncover a different function for the complex at the centrosome. Active PAK1 is particularly evident in mitosis and phosphorylates the centrosomal adaptor GIT1 on serine 517. Interestingly, direct centrosome targeting activates the kinase via a process not requiring Rho GTPases; excision of the centrosome prevents this activation. Once activated, PAK1 dissociates from PIX/GIT but can bind to and phosphorylate the important centrosomal kinase Aurora-A. PAK1 promotes phosphorylation of Aurora-A on Thr288 and Ser342, which are key sites for kinase activation in mitosis. In vivo PAK activation causes an accumulation of activated Aurora-A; conversely, when betaPIX is depleted or PAK is inhibited, there is a delay in centrosome maturation. These observations may underlie reported effects of active PAK on cells, including histone H3 phosphorylation, alterations in centrosome number, and progression through mitosis.

The centrosome cycle

Centrosomes are dynamic organelles involved in many aspects of cell function and growth. Centrosomes act as microtubule organizing centers, and provide a site for concerted regulation of cell cycle progression. While there is diversity in microtubule organizing center structure among eukaryotes, many centrosome components, such as centrin, are conserved. Experimental analysis has provided an outline to describe centrosome duplication, and numerous centrosome components have been identified. Even so, more work is needed to provide a detailed understanding of the interactions between centrosome components and their roles in centrosome function and duplication. Precise duplication of centrosomes once during each cell cycle ensures proper mitotic spindle formation and chromosome segregation. Defects in centrosome duplication or function are linked to human diseases including cancer. Here we provide a multifaceted look at centrosomes with a detailed summary of the centrosome cycle.

Cloning and characterization of a novel human Aurora C splicing variant

In the last 10 years, Aurora kinases have emerged as the key proteins regulating many events during cell mitosis. Despite the wealth of studies on human Aurora A and B, little is known about human Aurora C. Here we report a novel splicing variant of Aurora C, named as Aurora C-SV (Aurora C splicing variant), which encodes a 290-amino-acid protein. By RT-PCR analysis in various tissues, Aurora C-SV, like Aurora C, was found to be expressed at the highest level in human testis. The in vitro kinase assay showed that this Aurora C-SV phosphorylated MBP, and its T179A mutant lost the kinase activity. During cell mitosis, Aurora C-SV-EGFP associated with chromosomes in prophase and metaphase, and then transferred to the central spindle midzone and the cortex where the contract ring formed during the transition from anaphase to telophase. It then remained in the midbody during cytokinesis. Therefore, we speculated that Aurora C-SV might also contribute to the regulation of chromosome segregation and cytokinesis.

A small C-terminal sequence of Aurora B is responsible for localization and function

Aurora B, a protein kinase required in mitosis, localizes to inner centromeres at metaphase and the spindle midzone in anaphase and is required for proper chromosome segregation and cytokinesis. Aurora A, a paralogue of Aurora B, localizes instead to centrosomes and spindle microtubules. Except for distinct N termini, Aurora B and Aurora A have highly similar sequences. We have combined small interfering RNA (siRNA) ablation of Aurora B with overexpression of truncation mutants to investigate the role of Aurora B sequence in its function. Reintroduction of Aurora B during siRNA treatment restored its localization and function. This permitted a restoration of function test to determine the sequence requirements for Aurora B targeting and function. Using this rescue protocol, neither N-terminal truncation of Aurora B unique sequence nor substitution with Aurora A N-terminal sequence affected Aurora B localization or function. Truncation of unique Aurora B C-terminal sequence from terminal residue 344 to residue 333 was without effect, but truncation to 326 abolished localization and function. Deletion of residues 326-333 completely abolished localization and blocked cells at prometaphase, establishing this sequence as critical to Aurora B function. Our findings thus establish a small sequence as essential for the distinct localization and function of Aurora B.

Regulation of Xenopus Aurora A Activation by TPX2

The oncogenic protein kinase Aurora A is a critical regulator of meiotic and mitotic cell cycles in eukaryotic cells. Aurora A autoactivation by autophosphorylation is promoted by specific non-catalytic binding proteins. One such protein is TPX2, a required spindle assembly factor in higher eukaryotes whose ability to activate Aurora A by direct binding to the kinase catalytic domain has been established by biochemical and structural analysis. In this report we clarify the autoactivation mechanism of Aurora A by demonstrating that of seven amino acids which become autophosphorylated by Aurora A, only Thr-295 is required for activity. Association of Aurora A with TPX2 leads to activation of the kinase, in parallel with phosphorylation of TPX2. We identify the sites as three Ser residues in the N terminus of TPX2; however, mutation of these residues does not affect Aurora A activation by TPX2. In contrast, the mutation of a putative Aurora A-binding motif in TPX2 abolishes both phosphorylation of TPX2 and activation of Aurora A. We have also investigated the interaction between Xenopus p53 and Xenopus Aurora A. p53 blocks the activity of either full-length Aurora A or the isolated catalytic domain. Interestingly, inhibition is blocked by TPX2, suggesting that the ability of Aurora A to transform cells could be regulated by p53, TPX2, or other binding proteins.

The Centrosome in Higher Organisms: Structure, Composition, and Duplication

The centrosome found in higher organisms is an organelle with a complex and dynamic architecture and composition. This organelle not only functions as a microtubule-organizing center, but also is integrated with or impacts a number of cellular processes. Defects associated with this organelle have been linked to a variety of human diseases including several forms of cancer. Here we review the emerging picture of how the structure, composition, duplication, and function of the centrosome found in higher organisms are interrelated.

Dynamic association of a tumor amplified kinase, Aurora-A, with the centrosome and mitotic spindle

Aurora-A kinase, also known as STK15/BTAK kinase, is a member of a serine/threonine kinase superfamily that includes the prototypic yeast Ipl1 and Drosophila aurora kinases as well as other mammalian and non-mammalian aurora kinases involved in the regulation of centrosomes and chromosome segregation. The Aurora-A gene is amplified and overexpressed in a wide variety of human tumors. Aurora-A is centrosome-associated during interphase, and binds the poles and half-spindle during mitosis; its over-expression has been associated with centrosome amplification and multipolar spindles. GFP-Aurora-A was used to mark centrosomes and spindles, and monitor their movements in living cells. Centrosome pairs labeled with GFP-Aurora-A are motile throughout interphase undergoing oscillations and tumbling motions requiring intact microtubules and ATP. Fluorescence recovery after photobleaching (FRAP) was used to examine the relative molecular mobility of GFP-Aurora-A, and GFP-labeled alpha-tubulin, gamma-tubulin, and NuMA. GFP-Aurora-A rapidly exchanges in and out of the centrosome and mitotic spindle (t(1/2) approximately 3 sec); in contrast, both tubulins are relatively immobile indicative of a structural role. GFP-NuMA mobility was intermediate in both interphase nuclei and at the mitotic spindle (t(1/2) approximately 23-30 sec). Deletion mapping identifies a central domain of Aurora-A as essential for its centrosomal localization that is augmented by both the amino and the carboxyl terminal ends of the protein. Interestingly, amino or carboxy terminal deletion mutants that maintained centrosomal targeting exhibited significantly slower molecular exchange. Collectively, these studies contrast the relative cellular dynamics of Aurora-A with other cytoskeletal proteins that share its micro-domains, and identify essential regions required for targeting and dynamics.

A novel mechanism for activation of the protein kinase Aurora A

Segregation of chromosomes during mitosis requires interplay between several classes of protein on the spindle, including protein kinases, protein phosphatases, and microtubule binding motor proteins [1-4]. Aurora A is an oncogenic cell cycle-regulated protein kinase that is subject to phosphorylation-dependent activation [5-11]. Aurora A localization to the mitotic spindle depends on the motor binding protein TPX2 (Targeting Protein for Xenopus kinesin-like protein 2), but the protein(s) involved in Aurora A activation are unknown [11-13]. Here, we purify an activator of Aurora A from Xenopus eggs and identify it as TPX2. Remarkably, Aurora A that has been fully deactivated by Protein Phosphatase 2A (PP2A) becomes phosphorylated and reactivated by recombinant TPX2 in an ATP-dependent manner. Increased phosphorylation and activation of Aurora A requires its own kinase activity, suggesting that TPX2 stimulates autophosphorylation and autoactivation of the enzyme. Consistently, wild-type Aurora A, but not a kinase inactive mutant, becomes autophosphorylated on the regulatory T loop residue (Thr 295) after TPX2 treatment. Active Aurora A from bacteria is further activated at least 7-fold by recombinant TPX2, and TPX2 also impairs the ability of protein phosphatases to inactivate Aurora A in vitro. This concerted mechanism of stimulation of activation and inhibition of deactivation implies that TPX2 is the likely regulator of Aurora A activity at the mitotic spindle and may explain why loss of TPX2 in model systems perturbs spindle assembly [14-16]. Our finding that a known binding protein, and not a conventional protein kinase, is the relevant activator for Aurora A suggests a biochemical model in which the dynamic localization of TPX2 on mitotic structures directly modulates the activity of Aurora A for spindle assembly.

Survivin enhances Aurora-B kinase activity and localizes Aurora-B in human cells

Survivin, one of the most tumor-specific gene products, has been implicated in both anti-apoptosis and cytokinesis. However, the mechanism by which survivin regulates these two different processes is still elusive. Here, we show that survivin binds to the catalytic domain of Aurora-B. We demonstrate that in the presence of survivin, Aurora-B phosphorylates histone H3 much more efficiently than in the absence of survivin in a cell-free system. Furthermore, we confirm that cells lacking survivin due to survivin antisense oligonucleotide-treatment have lower Aurora-B kinase activity, whereas cells overexpressing survivin have higher Aurora-B kinase activity. We also provide evidence that depletion of survivin by survivin antisense oligonucleotide treatment causes significant reduction of endogenous phosphorylated histone H3 and mislocalization of Aurora-B. These results indicate that survivin stimulates Aurora-B kinase activity and helps correctly target Aurora-B to its substrates during the cell cycle, thus providing a mechanism as to how survivin exerts its function in human cells.

Identification of phosphorylated residues that affect the activity of the mitotic kinase Aurora-A

The activity of the kinase Aurora-A (Aur-A) peaks during mitosis and depends on phosphorylation by one or more unknown kinases. Mitotic phosphorylation sites were mapped by mass spec sequencing of recombinant Aur-A protein that had been activated by incubation in extracts of metaphase-arrested Xenopus eggs. Three sites were identified: serine 53 (Ser-53), threonine 295 (Thr-295), and serine 349 (Ser-349), which are equivalent to Ser-51, Thr-288, and Ser-342, respectively, in human Aur-A. To ask how phosphorylation of these residues might affect kinase activity, each was mutated to either alanine or aspartic acid, and the recombinant proteins were then tested for their ability to be activated by M phase extract. Mutation of Thr-295, which resides in the activation loop of the kinase, to either alanine or aspartic acid abolished activity. The S349A mutant had slightly reduced activity, indicating that phosphorylation is not required for activity. The S349D mutation completely blocked activation, suggesting that Ser-349 is important for either the structure or regulation of Aur-A. Finally, like human Aur-A, overexpression of Xenopus Aur-A transformed NIH 3T3 cells and led to tumors in nude mice. These results provide further evidence that Xenopus Aur-A is a functional ortholog of human Aur-A and, along with the recently described crystal structure of human Aur-A, should help in future studies of the mechanisms that regulate Aur-A activity during mitotic progression.

On the role of aurora-A in centrosome function

Mammalian aurora-A belongs to a multigenic family of mitotic serine/threonine kinases comprising two other members: aurora-B and aurora-C. In this review we will focus on aurora-A that starts to localize to centrosomes only in S phase as soon as centrioles have been duplicated, the protein is then degraded in early G1. Works in various organisms have revealed that the kinase is involved in centrosome separation, duplication and maturation as well as in bipolar spindle assembly and stability. Aurora kinases are found in all organisms in which their function has been conserved throughout evolution, namely the control of chromosome segregation. In human, aurora-A has focused a lot of attention, since its overexpression has been found to be correlated with the grade of various solid tumours. Ectopic kinase overexpression in any culture cell line leads to polyploidy and centrosome amplification. However, overexpression of aurora-A in particular cell lines such as NIH3T3 is sufficient to induce growth on soft agar. Those transformed cells form tumours when implanted in immunodeficient mice, indicating that the kinase is an oncogene.

Identification of a new APC/C recognition domain, the A box, which is required for the Cdh1-dependent destruction of the kinase Aurora-A during mitotic exit

The mitotic kinase Aurora A (Aur-A) is required for formation of a bipolar mitotic spindle and accurate chromosome segregation. In somatic cells, Aur-A protein and kinase activity levels peak during mitosis, and Aur-A is degraded during mitotic exit. Here, we investigated how Aur-A protein and kinase activity levels are regulated, taking advantage of the rapid synchronous cell division cycles of Xenopus eggs and cell-free systems derived from them. Aur-A kinase activity oscillates in the early embryonic cell cycles, just as in somatic cells, but Aur-A protein levels are constant, indicating that regulated activation and inactivation, instead of periodic proteolysis, is the dominant mode of Aur-A regulation in these cell cycles. Cdh1, the APC/C activator that targets many mitotic proteins for ubiquitin-dependent proteolysis during late mitosis and G1 in somatic cells, is missing in Xenopus eggs and early embryos. We find that addition of Cdh1 to egg extracts undergoing M phase exit is sufficient to induce rapid degradation of Aur-A. Aur-A contains both of the two known APC/C recognition signals, (1) a C-terminal D box similar to those required for ubiquitin-dependent destruction of cyclin B and several other mitotic proteins, and (2) an N-terminal KEN box similar to that found on cdc20, which is ubiquitinated in response to APC/C(Cdh1). The D box is required for Cdh1-induced destruction of Aur-A but the KEN box is not. Destruction also requires a short region in the N terminus, which contains a newly identified recognition signal, the A box. The A box is conserved in vertebrate Aur-As and contains serine 53, which is phosphorylated during M phase. Mutation of serine 53 to aspartic acid, which can mimic the effect of phosphorylation, completely blocks Cdh1-dependent destruction of Aur-A. These results suggest that dephosphorylation of serine 53 during mitotic exit could control the timing of Aur-A destruction, allowing recognition of both the A box and D box by Cdh1-activated APC/C.

The Schizosaccharomyces pombe aurora-related kinase Ark1 interacts with the inner centromere protein Pic1 and mediates chromosome segregation and cytokinesis

The chromosomal passenger proteins aurora-B, survivin, and inner centromere protein (INCENP) have been implicated in coordinating chromosome segregation with cell division. This work describes the interplay between aurora, survivin, and INCENP orthologs in the fission yeast Schizosaccharomyces pombe and defines their roles in regulating chromosome segregation and cytokinesis. We describe the cloning and characterization of the aurora-related kinase gene ark1(+), demonstrating that it is an essential gene required for sister chromatid segregation. Cells lacking Ark1p exhibit the cut phenotype, DNA fragmentation, and other defects in chromosome segregation. Overexpression of a kinase-defective version of Ark1, Ark1-K147R, inhibits cytokinesis, with cells exhibiting an elongated, multiseptate phenotype. Ark1p interacts physically and/or genetically with the survivin and INCENP orthologs Bir1p and Pic1p. We identified Pic1p in a two-hybrid screen for Ark1-K147R interacting partners and went on to map domains in both proteins that mediate their binding. Pic1p residues 925-972 are necessary and sufficient for Ark1p binding, which occurs through the kinase domain. As with Ark1-K147R, overexpression of Ark1p-binding fragments of Pic1p leads to multiseptate phenotypes. We also provide evidence that the dominant-negative effect of Ark1-K147R requires Pic1p binding, indicating that the formation of Ark1p-Pic1p complexes is required for the execution of cytokinesis.