Dynamic Phosphorylation of NudC by Aurora B in Cytokinesis

Nuclear distribution protein C (NudC) is a mitotic regulator that plays a role in cytokinesis. However, how NudC is regulated during cytokinesis remains unclear. Here, we show that NudC is phosphorylated by Aurora B, a kinase critical for cell abscission. NudC is co-localized with Aurora B at the midbody and co-immunoprecipitated with Aurora B in mitosis. Inhibition of Aurora B by ZM447439 reduced NudC phosphorylation, suggesting that NudC is an Aurora B substrate in vivo. We identified T40 on NudC as an Aurora B phosphorylation site. NudC depletion resulted in cytokinesis failure with a dramatic elongation of the intercellular bridge between daughter cells, sustained Aurora B activity at the midbody, and reduced cell abscission. These cytokinetic defects can be rescued by the ectopic expression of wild-type NudC. Reconstitution with T40A phospho-defective NudC was found to rescue the cytokinesis defect. In contrast, reconstitution with the T40D phospho-mimetic NudC was inefficient in supporting the completion of cytokinesis. These results suggest that that dynamic phosphorylation of NudC by Aurora B regulates cytokinesis.     

Late mitotic functions of Aurora kinases

The coordination between late mitotic events such as poleward chromosome motion, spindle elongation, DNA decondensation, and nuclear envelope reformation (NER) is crucial for the completion of chromosome segregation at the anaphase-telophase transition. Mitotic exit is driven by a decrease of Cdk1 kinase activity and an increase of PP1/PP2A phosphatase activities. More recently, Aurora kinases have also emerged as master regulators of late mitotic events and cytokinesis. Aurora A is mainly associated with spindle poles throughout mitosis and midbody during telophase, whereas Aurora B re-localizes from centromeres in early mitosis to the spindle midzone and midbody as cells progress from anaphase to the completion of cytokinesis. Functional studies, together with the identification of a phosphorylation gradient during anaphase, established Aurora B as a major player in the organization of the spindle midzone and in the spatiotemporal coordination between chromosome segregation and NER. Aurora A has been less explored, but a cooperative role in spindle midzone stability has also been proposed, implying that both Aurora A and B contribute to accurate chromosome segregation during mitotic exit. Here, we review the roles of the Aurora kinases in the regulation of late mitotic events and discuss how they work together with other mitotic players to ensure an error-free mitosis.     

Role of chromosomal passenger complex in chromosome segregation and cytokinesis

Chromosomal passenger proteins associate with chromosomes early in mitosis and transfer to the spindle at ana/telophase. Recent results show that aurora B/AIM-1 (aurora and Ipl1-like midbody-associated protein kinase), which is responsible for mitotic histone H3 phosphorylation, INCENP (Inner Centromere protein) and Survivin/BIR are in a macromolecular complex as novel chromosomal passenger proteins. Aurora B/AIM-1 can bind to Survivin and the C-terminal region of INCENP, respectively, and colocalizes with both proteins to the centromeres, midzone and midbody. Disruption of either aurora B/AIM-1 or INCENP function leads to sever defects in chromosome segregation and cytokinesis. Moreover, the formation of the central spindle through anaphase to cytokinesis is also disrupted severely. These data suggest that chromosomal passenger complex is required for proper chromosome segregation by phosphorylating histone H3, and cytokinesis by ensuring the correct assembly of the midzone and midbody microtubule. Chromosomal passenger protein complex may couple chromosome segregation with cytokinesis.     

Cross-regulation between Aurora B and Citron kinase controls midbody architecture in cytokinesis

Cytokinesis culminates in the final separation, or abscission, of the two daughter cells at the end of cell division. Abscission relies on an organelle, the midbody, which forms at the intercellular bridge and is composed of various proteins arranged in a precise stereotypic pattern. The molecular mechanisms controlling midbody organization and function, however, are obscure. Here we show that proper midbody architecture requires cross-regulation between two cell division kinases, Citron kinase (CIT-K) and Aurora B, the kinase component of the chromosomal passenger complex (CPC). CIT-K interacts directly with three CPC components and is required for proper midbody architecture and the orderly arrangement of midbody proteins, including the CPC. In addition, we show that CIT-K promotes Aurora B activity through phosphorylation of the INCENP CPC subunit at the TSS motif. In turn, Aurora B controls CIT-K localization and association with its central spindle partners through phosphorylation of CIT-K''s coiled coil domain. Our results identify, for the first time, a cross-regulatory mechanism between two kinases during cytokinesis, which is crucial for establishing the stereotyped organization of midbody proteins.     

A mitotic GlcNAcylation/phosphorylation signaling complex alters the posttranslational state of the cytoskeletal protein vimentin

O-linked β-N-acetylglucosamine (O-GlcNAc) is a highly dynamic intracellular protein modification responsive to stress, hormones, nutrients, and cell cycle stage. Alterations in O-GlcNAc addition or removal (cycling) impair cell cycle progression and cytokinesis, but the mechanisms are not well understood. Here, we demonstrate that the enzymes responsible for O-GlcNAc cycling, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) are in a transient complex at M phase with the mitotic kinase Aurora B and protein phosphatase 1. OGT colocalized to the midbody during telophase with Aurora B. Furthermore, these proteins coprecipitated with each other in a late mitotic extract. The complex was stable under Aurora inhibition; however, the total cellular levels of O-GlcNAc were increased and the localization of OGT was decreased at the midbody after Aurora inhibition. Vimentin, an intermediate filament protein, is an M phase substrate for both Aurora B and OGT. Overexpression of OGT or OGA led to defects in mitotic phosphorylation on multiple sites, whereas OGT overexpression increased mitotic GlcNAcylation of vimentin. OGA inhibition caused a decrease in vimentin late mitotic phosphorylation but increased GlcNAcylation. Together, these data demonstrate that the O-GlcNAc cycling enzymes associate with kinases and phosphatases at M phase to regulate the posttranslational status of vimentin.     

Aurora B Interaction of Centrosomal Nlp Regulates Cytokinesis

Cytokinesis is a fundamental cellular process, which ensures equal abscission and fosters diploid progenies. Aberrant cytokinesis may result in genomic instability and cell transformation. However, the underlying regulatory machinery of cytokinesis is largely undefined. Here, we demonstrate that Nlp (Ninein-like protein), a recently identified BRCA1-associated centrosomal protein that is required for centrosomes maturation at interphase and spindle formation in mitosis, also contributes to the accomplishment of cytokinesis. Through immunofluorescent analysis, Nlp is found to localize at midbody during cytokinesis. Depletion of endogenous Nlp triggers aborted division and subsequently leads to multinucleated phenotypes. Nlp can be recruited by Aurora B to the midbody apparatus via their physical association at the late stage of mitosis. Disruption of their interaction induces aborted cytokinesis. Importantly, Nlp is characterized as a novel substrate of Aurora B and can be phosphorylated by Aurora B. The specific phosphorylation sites are mapped at Ser-185, Ser-448, and Ser-585. The phosphorylation at Ser-448 and Ser-585 is likely required for Nlp association with Aurora B and localization at midbody. Meanwhile, the phosphorylation at Ser-185 is vital to Nlp protein stability. Disruptions of these phosphorylation sites abolish cytokinesis and lead to chromosomal instability. Collectively, these observations demonstrate that regulation of Nlp by Aurora B is critical for the completion of cytokinesis, providing novel insights into understanding the machinery of cell cycle progression.     

Tektin 2 is required for central spindle microtubule organization and the completion of cytokinesis

During anaphase, the nonkinetochore microtubules in the spindle midzone become compacted into the central spindle, a structure which is required to both initiate and complete cytokinesis. We show that Tektin 2 (Tek2) associates with the spindle poles throughout mitosis, organizes the spindle midzone microtubules during anaphase, and assembles into the midbody matrix surrounding the compacted midzone microtubules during cytokinesis. Tek2 small interfering RNA (siRNA) disrupts central spindle organization and proper localization of MKLP1, PRC1, and Aurora B to the midzone and prevents the formation of a midbody matrix. Video microscopy revealed that loss of Tek2 results in binucleate cell formation by aberrant fusion of daughter cells after cytokinesis. Although a myosin II inhibitor, blebbistatin, prevents actin-myosin contractility, the microtubules of the central spindle are compacted. Strikingly, Tek2 siRNA abolishes this actin-myosin-independent midzone microtubule compaction. Thus, Tek2-dependent organization of the central spindle during anaphase is essential for proper midbody formation and the segregation of daughter cells after cytokinesis.     

GEF-H1 modulates localized RhoA activation during cytokinesis under the control of mitotic kinases

Formation of the mitotic cleavage furrow is dependent upon both microtubules and activity of the small GTPase RhoA. GEF-H1 is a microtubule-regulated exchange factor that couples microtubule dynamics to RhoA activation. GEF-H1 localized to the mitotic apparatus in HeLa cells, particularly at the tips of cortical microtubules and the midbody, and perturbation of GEF-H1 function induced mitotic aberrations, including asymmetric furrowing, membrane blebbing, and impaired cytokinesis. The mitotic kinases Aurora A/B and Cdk1/Cyclin B phosphorylate GEF-H1, thereby inhibiting GEF-H1 catalytic activity. Dephosphorylation of GEF-H1 occurs just prior to cytokinesis, accompanied by GEF-H1-dependent GTP loading on RhoA. Using a live cell biosensor, we demonstrate distinct roles for GEF-H1 and Ect2 in regulating Rho activity in the cleavage furrow, with GEF-H1 catalyzing Rho activation in response to Ect2-dependent localization and initiation of cell cleavage. Our results identify a GEF-H1-dependent mechanism to modulate localized RhoA activation during cytokinesis under the control of mitotic kinases.     

Mitotic Regulation of the Stability of Microtubule Plus-end Tracking Protein EB3 by Ubiquitin Ligase SIAH-1 and Aurora Mitotic Kinases

Microtubule plus-end tracking proteins (+TIPs) control microtubule dynamics in fundamental processes such as cell cycle, intracellular transport, and cell motility, but how +TIPs are regulated during mitosis remains largely unclear. Here we show that the endogenous end-binding protein family EB3 is stable during mitosis, facilitates cell cycle progression at prometaphase, and then is down-regulated during the transition to G₁ phase. The ubiquitin-protein isopeptide ligase SIAH-1 facilitates EB3 polyubiquitination and subsequent proteasome-mediated degradation, whereas SIAH-1 knockdown increases EB3 stability and steady-state levels. Two mitotic kinases, Aurora-A and Aurora-B, phosphorylate endogenous EB3 at Ser-176, and the phosphorylation triggers disruption of the EB3-SIAH-1 complex, resulting in EB3 stabilization during mitosis. Our results provide new insight into a regulatory mechanism of +TIPs in cell cycle transition.Microtubule dynamics are essential in many cellular processes, including cell motility, intracellular transport, accurate mitosis, and cytokinesis in all eukaryotes. The regulatory factors for microtubule dynamics can be classified into two main types as follows: microtubule-destabilizing proteins, such as stathmin/Op18 (1) and the Kinesin-13 family (also known as MCAK/KIF2 family) (2), and microtubule-stabilizing proteins, the classic superfamily of microtubule-associated proteins (3). Additionally, the plus-end tracking proteins (+TIPs)³ have recently been identified; this family specifically accumulates at the ends of growing microtubules and regulates the microtubule plus-end targeting to the cell cortex or mitotic kinetochores (4, 5).The EB1 family is a member of the +TIPs family and consists of three homologs in mammals: EB1, EB2/RP1 (henceforth, EB2), and EB3 (6). As EB1 was originally identified as a protein that interacts with the well characterized tumor suppressor adenomatous polyposis coli (APC) protein (7), the function of EB1 has been investigated extensively. EB1 interacts with other +TIPs, including APC, p150^glued, CLIPs, and CLASP1/2, and the interaction network controls microtubule orientation and microtubule-cortex interaction during cell migration (5, 8, 9). EB1 functions not only in the regulation of interphase microtubule dynamics but also in mitotic spindle regulation. For accurate chromosomal segregation, sister chromatids become aligned to the metaphase plate during metaphase, and the alignment requires spindle-kinetochore attachment. Two models have been proposed; in the first, termed the “search-and-capture” model, EB1 localized at the growing microtubule plus-ends searches for binding partners located on kinetochores (10, 11). In the second model proposed recently, EB1 makes kinetochore fibers and centrosomal microtubules connect, and it is essential for the formation of a functional bipolar spindle (12). Thus, EB1 is thought to be a master controller of microtubule plus-ends; however, little is known about other EB1 family members. Given that EB3 is localized on the microtubule network and binds to APC and CLIPs identically to EB1, it is possible that EB3 acts as an EB1 analog in cells (13–15).Cell division is precisely regulated by several post-translational modifications of proteins, mainly reversible phosphorylation and ubiquitination, which is followed by degradation. Accurate mitotic phase progression requires the appropriate phosphorylation of various proteins by mitotic kinases (16, 17). One of the key mitotic kinases is the Aurora family that has been highly conserved from yeast to humans. There are three homologs (Aurora-A, -B, and -C) in human and mouse (18). Although their homology at the protein level is more than 84%, their functions and subcellular localizations are distinct. Aurora-A is located in the centrosomes and spindle and is required for mitotic entry, centrosome maturation/separation, and spindle assembly (19). Aurora-B is a chromosomal passenger protein that localizes on the inner centromere of the chromosomes until metaphase to regulate the spindle-kinetochore attachment, and from anaphase, it translocates to the central spindle and then accumulates in the midbody for cytokinesis (20, 21). The numerous substrates of the Aurora family include regulatory factors for microtubule dynamics, such as the microtubule-destabilizing proteins MCAK and stathmin, which help to establish the bipolar attachment and spindle assembly, respectively (22–24). It is possible that the Aurora family regulates the EB1 family by phosphorylation.In this study, we performed yeast two-hybrid screening and obtained the EB1 yeast homolog Bim1 as a protein that interacts with Ipl1, a yeast counterpart of Aurora. Here we demonstrate the novel regulatory mechanisms of EB3 by two cell cycle-dependent post-translational modifications, phosphorylation and ubiquitin-proteasome-mediated degradation.     

The role of pre- and post-anaphase microtubules in the cytokinesis phase of the cell cycle

The cytokinesis phase, or C phase, of the cell cycle results in the separation of one cell into two daughter cells after the completion of mitosis. Although it is known that microtubules are required for proper positioning of the cytokinetic furrow [1] [2], the role of pre-anaphase microtubules in cytokinesis has not been clearly defined for three key reasons. First, inducing microtubule depolymerization or stabilization before the onset of anaphase blocks entry into anaphase and cytokinesis via the spindle checkpoint [3]. Second, microtubule organization changes rapidly at anaphase onset as the mitotic kinase, Cdc2-cyclin B, is inactivated [4]. Third, the time between the onset of anaphase and the initiation of cytokinesis is very short, making it difficult to unambiguously alter microtubule polymer levels before cytokinesis, but after inactivation of the spindle checkpoint. Here, we have taken advantage of the discovery that microinjection of antibodies to the spindle checkpoint protein Mad2 (mitotic arrest deficient) in prometaphase abrogates the spindle checkpoint, producing premature chromosome separation, segregation, and normal cytokinesis [5] [6]. To test the role of pre-anaphase microtubules in cytokinesis, microtubules were disassembled in prophase and prometaphase cells, the cells were then injected with anti-Mad2 antibodies and recorded through C phase. The results show that exit from mitosis in the absence of microtubules triggered a 50 minute period of cortical contractility that was independent of microtubules. Furthermore, upon microtubule reassembly during this contractile C-phase period, approximately 30% of the cells underwent chromosome poleward movement, formed a midzone microtubule complex, and completed cytokinesis.     

Cdk1/Erk2- and Plk1-dependent phosphorylation of a centrosome protein, Cep55, is required for its recruitment to midbody and cytokinesis

Centrosomes in mammalian cells have recently been implicated in cytokinesis; however, their role in this process is poorly defined. Here, we describe a human coiled-coil protein, Cep55 (centrosome protein 55 kDa), that localizes to the mother centriole during interphase. Despite its association with gamma-TuRC anchoring proteins CG-NAP and Kendrin, Cep55 is not required for microtubule nucleation. Upon mitotic entry, centrosome dissociation of Cep55 is triggered by Erk2/Cdk1-dependent phosphorylation at S425 and S428. Furthermore, Cep55 locates to the midbody and plays a role in cytokinesis, as its depletion by siRNA results in failure of this process. S425/428 phosphorylation is required for interaction with Plk1, enabling phosphorylation of Cep55 at S436. Cells expressing phosphorylation-deficient mutant forms of Cep55 undergo cytokinesis failure. These results highlight the centrosome as a site to organize phosphorylation of Cep55, enabling it to relocate to the midbody to function in mitotic exit and cytokinesis.     

A Cul3-based E3 ligase removes Aurora B from mitotic chromosomes, regulating mitotic progression and completion of cytokinesis in human cells

Faithful cell-cycle progression is tightly controlled by the ubiquitin-proteasome system. Here we identify a human Cullin 3-based E3 ligase (Cul3) which is essential for mitotic division. In a complex with the substrate-specific adaptors KLHL9 and KLHL13, Cul3 is required for correct chromosome alignment in metaphase, proper midzone and midbody formation, and completion of cytokinesis. This Cul3-based E3 ligase removes components of the chromosomal passenger complex from mitotic chromosomes and allows their accumulation on the central spindle during anaphase. Aurora B directly binds to the substrate-recognition domain of KLHL9 and KLHL13 in vitro, and coimmunoprecipitates with the Cul3 complex during mitosis. Moreover, Aurora B is ubiquitylated in a Cul3-dependent manner in vivo, and by reconstituted Cul3/KLHL9/KLHL13 ligase in vitro. We thus propose that the Cul3/KLHL9/KLHL13 E3 ligase controls the dynamic behavior of Aurora B on mitotic chromosomes, and thereby coordinates faithful mitotic progression and completion of cytokinesis.     

Calmodulin protects Aurora B on the midbody to regulate the fidelity of cytokinesis

Aurora B kinase is an integral regulator of cytokinesis as it stabilizes the intercellular canal within the midbody to ensure proper chromosomal segregation during cell division. Here we identified an E3 ligase subunit, F box protein FBXL2, that by recognizing a calmodulin binding signature within Aurora B, ubiquitinates and removes the kinase from the midbody. Calmodulin, by competing with the F box protein for access to the calmodulin binding signature, protected Aurora B from FBXL2. Calmodulin co-localized with Aurora B on the midbody, preserved Aurora B levels in cells, and stabilized intercellular canals during delayed abscission. Genetic or pharmaceutical depletion of endogenous calmodulin significantly reduced Aurora B protein levels at the midbody resulting in tetraploidy and multi-spindle formation. The calmodulin inhibitor, calmidazolium, reduced Aurora B protein levels resulting in tetraploidy, mitotic arrest, and apoptosis of tumorigenic cells and profoundly inhibiting tumor formation in athymic nude mice. These observations indicate molecular interplay between Aurora B and calmodulin in telophase and suggest that calmodulin acts as a checkpoint sensor for chromosomal segregation errors during mitosis.     

Phosphorylation of ZEN-4/MKLP1 by aurora B regulates completion of cytokinesis

The central spindle regulates the formation and positioning of the contractile ring and is essential for completion of cytokinesis [1]. Central spindle assembly begins in early anaphase with the bundling of overlapping, antiparallel, nonkinetochore microtubules [2, 3], and these bundles become compacted and mature into the midbody. Prominent components of the central spindle include aurora B kinase and centralspindlin, a complex containing a Kinesin-6 protein (ZEN-4/MKLP1) and a Rho family GAP (CYK-4/MgcRacGAP) that is essential for central spindle assembly [4]. Centralspindlin localization depends on aurora B kinase [5]. Aurora B concentrates in the midbody and persists between daughter cells. Here, we show that in C. elegans embryos and in cultured human cells, respectively, ZEN-4 and MKLP1 are phosphorylated by aurora B in vitro and in vivo on conserved C-terminal serine residues. In C. elegans embryos, a nonphosphorylatable mutant of ZEN-4 localizes properly but does not efficiently support completion of cytokinesis. In mammalian cells, an inhibitor of aurora kinase acutely attenuates phosphorylation of MKLP1. Inhibition of aurora B in late anaphase causes cytokinesis defects without disrupting the central spindle. These data indicate a conserved role for aurora-B-mediated phosphorylation of ZEN-4/MKLP1 in the completion of cytokinesis.     

有丝分裂激酶Aurora B的研究进展

真核生物细胞通过有丝分裂将遗传物质均等地分配到两个子细胞中,从而维持基因组的稳定性。有丝分裂的每一环节都需要精准而细致的调控,这依赖于一系列调节机制,尤其需要多个相关激酶的共同协调。Aurora B是一个关键的有丝分裂调控激酶,伴随有丝分裂的进行,其先后在染色体臂、内着丝粒、中央纺锤体、中体上动态分布。与其高度时空动态性相一致的是,Aurora B在有丝分裂的多个环节,如姐妹染色体粘连、动粒微管连接、纺锤体检验点和胞质分裂过程中都发挥着一系列重要功能。本文将概述近年来Aurora B激酶功能与调控方面的研究进展。     

EB1 enables spindle microtubules to regulate centromeric recruitment of Aurora B

The Aurora B kinase coordinates kinetochore–microtubule attachments with spindle checkpoint signaling on each mitotic chromosome. We find that EB1, a microtubule plus end–tracking protein, is required to enrich Aurora B at inner centromeres in a microtubule-dependent manner. This regulates phosphorylation of both kinetochore and chromatin substrates. EB1 regulates the histone phosphorylation marks (histone H2A phospho-Thr120 and histone H3 phospho-Thr3) that localize Aurora B. The chromosomal passenger complex containing Aurora B can be found on a subset of spindle microtubules that exist near prometaphase kinetochores, known as preformed K-fibers (kinetochore fibers). Our data suggest that EB1 enables the spindle microtubules to regulate the phosphorylation of kinetochores through recruitment of the Aurora B kinase.     

Phosphorylation by aurora B converts MgcRacGAP to a RhoGAP during cytokinesis

Cell division is finely controlled by various molecules including small G proteins and kinases/phosphatases. Among these, Aurora B, RhoA, and the GAP MgcRacGAP have been implicated in cytokinesis, but their underlying mechanisms of action have remained unclear. Here, we show that MgcRacGAP colocalizes with Aurora B and RhoA, but not Rac1/Cdc42, at the midbody. We also report that Aurora B phosphorylates MgcRacGAP on serine residues and that this modification induces latent GAP activity toward RhoA in vitro. Expression of a kinase-defective mutant of Aurora B disrupts cytokinesis and inhibits phosphorylation of MgcRacGAP at Ser387, but not its localization to the midbody. Overexpression of a phosphorylation-deficient MgcRacGAP-S387A mutant, but not phosphorylation-mimic MgcRacGAP-S387D mutant, arrests cytokinesis at a late stage and induces polyploidy. Together, these findings indicate that during cytokinesis, MgcRacGAP, previously known as a GAP for Rac/Cdc42, is functionally converted to a RhoGAP through phosphorylation by Aurora B.     

NAT10, a nucleolar protein, localizes to the midbody and regulates cytokinesis and acetylation of microtubules

The midbody is a structural organelle formed in late phase mitosis which is responsible for completion of cytokinesis. Although various kinds of proteins have been found to distribute or immigrate to this organelle, their functions have still not been completely worked out. In this study, we demonstrated that NAT10 (N-acetyltransferase 10, NAT10) is not only predominantly distributed in the nucleolus in interphase, but is also concentrated in the mitotic midbody during telophase. The domain in N-terminal residues 549-834 of NAT10 specifically mediated its subcellular localization. Treatment with genotoxic agents or irradiation increased concentration of NAT10 in both the nucleolus and midbody. Moreover, DNA damage induced increase of NAT10 in the midbody apparently accompanied by in situ elevation of the level of acetylated α-tubulin, suggesting that it plays a role in maintaining or enhancing stability of α-tubulin. The depletion of NAT10 induced defects in nucleolar assembly, cytokinesis and decreased acetylated α-tubulin, leading to G2/M cell cycle arrest or delay of mitotic exit. In addition, over-expression of NAT10 was found in a variety of soft tissue sarcomas, and correlated with tumor histological grading. These results indicate that NAT10 may play an important role in cell division through facilitating reformation of the nucleolus and midbody in the late phase of cell mitosis, and stabilization of microtubules.     

Phosphorylation of multifunctional nucleolar protein nucleophosmin (NPM1) by aurora kinase B is critical for mitotic progression

The functional association of NPM1 with Aurora kinases is well documented. Surprisingly, although NPM1 is a well characterized phosphoprotein, it is unknown whether it is a substrate of Aurora kinases. We have found that Aurora kinases A and B can phosphorylate NPM1 at a single serine residue, Ser125, in vitro and in vivo. Phosphorylated-S125-NPM1 (pS125-NPM1) localizes to the midbody region during late cytokinesis where it colocalizes with Aurora B. The overexpression of mutant (S125A) NPM1 resulted in the deregulation of centrosome duplication and mitotic defects possibly due to cytokinesis failure. These data suggest that Aurora kinase B-mediated phosphorylation of NPM1 plays a critical role during mitosis, which could have wider implications in oncogenesis.     

AIR-2: An Aurora/Ipl1-related Protein Kinase Associated with Chromosomes and Midbody Microtubules Is Required for Polar Body Extrusion and Cytokinesis in Caenorhabditis elegans Embryos

An emerging family of kinases related to the Drosophila Aurora and budding yeast Ipl1 proteins has been implicated in chromosome segregation and mitotic spindle formation in a number of organisms. Unlike other Aurora/Ipl1-related kinases, the Caenorhabditis elegans orthologue, AIR-2, is associated with meiotic and mitotic chromosomes. AIR-2 is initially localized to the chromosomes of the most mature prophase I–arrested oocyte residing next to the spermatheca. This localization is dependent on the presence of sperm in the spermatheca. After fertilization, AIR-2 remains associated with chromosomes during each meiotic division. However, during both meiotic anaphases, AIR-2 is present between the separating chromosomes. AIR-2 also remains associated with both extruded polar bodies. In the embryo, AIR-2 is found on metaphase chromosomes, moves to midbody microtubules at anaphase, and then persists at the cytokinesis remnant. Disruption of AIR-2 expression by RNA- mediated interference produces entire broods of one-cell embryos that have executed multiple cell cycles in the complete absence of cytokinesis. The embryos accumulate large amounts of DNA and microtubule asters. Polar bodies are not extruded, but remain in the embryo where they continue to replicate. The cytokinesis defect appears to be late in the cell cycle because transient cleavage furrows initiate at the proper location, but regress before the division is complete. Additionally, staining with a marker of midbody microtubules revealed that at least some of the components of the midbody are not well localized in the absence of AIR-2 activity. Our results suggest that during each meiotic and mitotic division, AIR-2 may coordinate the congression of metaphase chromosomes with the subsequent events of polar body extrusion and cytokinesis.