Ordered proteolysis in anaphase inactivates Plk1 to contribute to proper mitotic exit in human cells

Cdh1 regulates osteoblast function through an APC/C-independent modulation of Smurf1

The APC/Cdh1 E3 ubiquitin ligase plays an essential role in both mitotic exit and G1/S transition by targeting key cell-cycle regulators for destruction. There is mounting evidence indicating that Cdh1 has other functions in addition to cell-cycle regulation. However, it remains unclear whether these additional functions depend on its E3 ligase activity. Here, we report that Cdh1, but not Cdc20, promotes the E3 ligase activity of Smurf1. This is mediated by disruption of an autoinhibitory Smurf1 homodimer and is independent of APC/Cdh1 E3 ligase activity. As a result, depletion of Cdh1 leads to reduced Smurf1 activity and subsequent activation of multiple downstream targets, including the MEKK2 signaling pathway, inducing osteoblast differentiation. Our studies uncover a cell-cycle-independent function of Cdh1, establishing Cdh1 as an upstream component that governs Smurf1 activity. They further suggest that modulation of Cdh1 is a potential therapeutic option for treatment of osteoporosis.

Computational design of targeted inhibitors of polo-like kinase 1 (plk1)

Computational design of small molecule putative inhibitors of Polo-like kinase 1 (Plk1) is presented. Plk1, which regulates the cell cycle, is often over expressed in cancers. Down regulation of Plk1 has been shown to inhibit tumor progression. Most kinase inhibitors interact with the ATP binding site on Plk1, which is highly conserved. This makes the development of Plk1-specific inhibitors challenging, since different kinases have similar ATP sites. However, Plk1 also contains a unique region called the polo-box domain (PBD), which is absent from other kinases. In this study, the PBD site was used as a target for designed Plk1 putative inhibitors. Common structural features of several experimentally known Plk1 ligands were first identified. The findings were used to design small molecules that specifically bonded Plk1. Drug likeness and possible toxicities of the molecules were investigated. Molecules with no implied toxicities and optimal drug likeness values were used for docking studies. Several molecules were identified that made stable complexes only with Plk1 and LYN kinases, but not with other kinases. One molecule was found to bind exclusively the PBD site of Plk1. Possible utilization of the designed molecules in drugs against cancers with over expressed Plk1 is discussed.

The APC/C Ubiquitin Ligase: From Cell Biology to Tumorigenesis

The ubiquitin proteasome system (UPS) is required for normal cell proliferation, vertebrate development, and cancer cell transformation. The UPS consists of multiple proteins that work in concert to target a protein for degradation via the 26S proteasome. Chains of an 8.5-kDa protein called ubiquitin are attached to substrates, thus allowing recognition by the 26S proteasome. Enzymes called ubiquitin ligases or E3s mediate specific attachment to substrates. Although there are over 600 different ubiquitin ligases, the Skp1-Cullin-F-box (SCF) complexes and the anaphase promoting complex/cyclosome (APC/C) are the most studied. SCF involvement in cancer has been known for some time while APC/C's cancer role has recently emerged. In this review we will discuss the importance of APC/C to normal cell proliferation and development, underscoring its possible contribution to transformation. We will also examine the hypothesis that modulating a specific interaction of the APC/C may be therapeutically attractive in specific cancer subtypes. Finally, given that the APC/C pathway is relatively new as a cancer target, therapeutic interventions affecting APC/C activity may be beneficial in cancers that are resistant to classical chemotherapy.

The Anaphase-Promoting Complex activator Fizzy-Related-1 (FZR1) is involved in the establishment of a single mitotic spindle in 1-cell embryos and in the mitotic divisions of early mammalian embryos

In early embryos of a number of species the Anaphase-Promoting Complex (APC), an important cell cycle regulator, requires only CDC20 for cell division. In contrast FZR1, a non-essential gene in many cell types, is thought to play a role in APC activation at later cell cycles, and especially in endoreplication. In keeping with this, FZR1 knockout mouse embryos show normal preimplantation development but die due to a lack of endoreplication needed for placentation. However, interpretation of the role of FZR1 during this period is hindered by the presence of maternal stores. Here, therefore, we used an oocyte-specific knockout to examine FZR1 function in early mouse embryo development. Maternal FZR1 was not critical for completion of meiosis, and furthermore viable pups were born to these females mated with normal males. However, in early embryos the absence of both maternal and paternal FZR1 led to a dramatic loss in genome integrity, such that the majority of embryos arrested having undergone only a single mitotic division and contained many γ-H2AX foci, consistent with fragmented DNA. A prominent feature of such embryos was a the establishment of two independent spindles following pronuclear fusion and thus a failure of the chromosomes to mix (syngamy). These generated binucleate 2-cell embryos. In the 10% of embryos that progressed to the 4-cell stage, division was so slow that compaction occurred prematurely. No embryo development to the blastocyst stage was ever observed. We conclude that FZR1 is a surprisingly essential gene involved in the establishment of a single spindle from the two pronuclei in 1-cell embryos as well as being involved in the maintainence of genomic integrity during the mitotic divisions of early mammalian embryos.

Proteomic identification of Hsp70 as a new Plk1 substrate in arsenic trioxide-induced mitotically arrested cells

We previously demonstrated that when arsenic trioxide (ATO)-induced mitotically arrested HeLa S3 cells (AIMACs) were treated with staurosporine (SSP) the cells rapidly exited mitosis. To better define the cellular targets and the underlying mechanisms of AIMACs, we applied 2-D DIGE followed by LC-MS/MS analysis and showed that SSP induced a significant change in the phosphoproteome of AIMACs. Among the proteins whose phosphorylation was modulated by SSP, we identified Hsp70, Rad 23B, and eukaryotic translation initiation factor 4B as potentially new substrates of polo-like kinase 1 (Plk1), an essential serine/threonine kinase with versatile mitotic functions. Since Hsp70 is a stress protein responsible for ATO treatment, we further identified Thr(13) , Ser(362) , Ser(631) , and Ser(633) on Hsp70 intracellularly phosphorylated in AIMACs by combining TiO(2) phospho-peptides enrichment and MS/MS analysis. Using antibody specifically against phosph-Ser(631) Hsp70 and further aided by expression of kinase-dead Plk1 and pharmacological inhibition of Plk1, we concluded that Ser(631) on Hsp70 is phosphorylated by Plk1 in AIMACs. By immnuofluorescent staining, we found the colocalization of Hsp70 and Plk1 in AIMACs but not in interphase cells. In addition, Plk1-mediated phosphorylation of Hsp70 prevented AIMACs from mitotic death. Our results reveal that Hsp70 is a novel substrate of Plk1 and that its phosphorylation contributes to attenuation of ATO-induced mitotic abnormalities.

Phosphorylation of Ran-binding protein-1 by Polo-like kinase-1 is required for interaction with Ran and early mitotic progression

Polo-like kinase-1 (Plk1) is essential for progression of mitosis and localizes to centrosomes, central spindles, midbody, and kinetochore. Ran, a small GTPase of the Ras superfamily, plays a role in microtubule dynamics and chromosome segregation during mitosis. Although Ran-binding protein-1 (RanBP1) has been reported as a regulator of RanGTPase for its mitotic functions, the action mechanism between Ran and RanBP1 during mitosis is still unknown. Here, we demonstrated in vitro and in vivo phosphorylation of RanBP1 by Plk1 as well as the importance of phosphorylation of RanBP1 in the interaction between Plk1 and Ran during early mitosis. Both phosphorylation-defective and N-terminal deletion mutant constructs of RanBP1 disrupted the interaction with Ran, and depletion of Plk1 also disrupted the formation of a complex between Ran and RanBP1. In addition, the results from both ectopic expression of phosphorylation-defective mutant construct and a functional complementation on RanBP1 deficiency with this mutant indicated that phosphorylation of RanBP1 by Plk1 might be crucial to microtubule nucleation and spindle assembly during mitosis.

Phosphatases: providing safe passage through mitotic exit

The mitosis-to-interphase transition involves dramatic cellular reorganization from a state that supports chromosome segregation to a state that complies with all functions of an interphase cell. This process, termed mitotic exit, depends on the removal of mitotic phosphorylations from a broad range of substrates. Mitotic exit regulation involves inactivation of mitotic kinases and activation of counteracting protein phosphatases. The key mitotic exit phosphatase in budding yeast, Cdc14, is now well understood. By contrast, in animal cells, it is now emerging that mitotic exit relies on distinct regulatory networks, including the protein phosphatases PP1 and PP2A.

How protein kinases co-ordinate mitosis in animal cells

Mitosis is associated with profound changes in cell physiology and a spectacular surge in protein phosphorylation. To accomplish these, a remarkably large portion of the kinome is involved in the process. In the present review, we will focus on classic mitotic kinases, such as cyclin-dependent kinases, Polo-like kinases and Aurora kinases, as well as more recently characterized players such as NIMA (never in mitosis in Aspergillus nidulans)-related kinases, Greatwall and Haspin. Together, these kinases co-ordinate the proper timing and fidelity of processes including centrosomal functions, spindle assembly and microtubule-kinetochore attachment, as well as sister chromatid separation and cytokinesis. A recurrent theme of the mitotic kinase network is the prevalence of elaborated feedback loops that ensure bistable conditions. Sequential phosphorylation and priming phosphorylation on substrates are also frequently employed. Another important concept is the role of scaffolds, such as centrosomes for protein kinases during mitosis. Elucidating the entire repertoire of mitotic kinases, their functions, regulation and interactions is critical for our understanding of normal cell growth and in diseases such as cancers.

Processive ubiquitin chain formation by the anaphase-promoting complex

Progression through mitosis requires the sequential ubiquitination of cell cycle regulators by the anaphase-promoting complex, resulting in their proteasomal degradation. Although several mechanisms contribute to APC/C regulation during mitosis, the APC/C is able to discriminate between its many substrates by exploiting differences in the processivity of ubiquitin chain assembly. Here, we discuss how the APC/C achieves processive ubiquitin chain formation to trigger the sequential degradation of cell cycle regulators during mitosis.

Non-mitotic functions of the Anaphase-Promoting Complex

The Anaphase-Promoting Complex or Cyclosome (APC/C) is an E3 ubiquitin ligase whose activation requires the binding of a cofactor, either Cdc20 or Cdh1. While APC/C-Cdc20 is a major player during mitotic exit, APC/C-Cdh1 plays a central role in maintaining quiescence and controlling the onset of DNA replication. In addition, APC/C-Cdh1 is essential for endoreduplication, a process in which several rounds of DNA synthesis occur without mitosis. Recent data suggest that the APC/C is also involved in differentiation and metabolism, and plays important roles in postmitotic cells such as neurons. Thus, the APC/C is not only critical for anaphase onset but also regulates many other cellular processes during G1/S or in quiescent cells.

On the cutting edge: post-translational modifications in cytokinesis

Cytokinesis represents the final stage in the cell cycle, in which two daughter cells, each with their complement of the duplicated genome, physically separate. At the core of this process sits highly conserved machinery responsible for specifying the plane of division, building a contractile apparatus and ultimately cleaving cells in two. Although the 'parts list' of contributing proteins has been well described, mechanisms by which these parts are spatially and temporally regulated are only beginning to be understood. With advancements in biochemical and proteomic analyses, recent work has uncovered multiple new roles for post-translational modifications in the regulation of cytokinesis. Here, we review these latest findings and interpret our current understanding of cytokinesis in light of relevant modifications.

Phosphorylation by polo-like kinase 1 induces the tumor-suppressing activity of FADD

Phosphorylation of the Fas-associated death domain (FADD) protein sensitizes cancer cells to various chemotherapeutics. However, the molecular mechanism underlying chemosensitization by phosphorylated FADD (P-FADD) is poorly understood. In this study, we describe the physical interactions and functional interplay between Polo-like kinase 1 (Plk1) and FADD. Plk1 phosphorylates FADD at Ser-194 in response to treatment with taxol. Overexpression of a phosphorylation-mimicking mutant, FADD S194D, caused degradation of Plk1 in an ubiquitin-independent manner, and delayed cytokinesis, consistent with the expected cellular phenotype of Plk1 deficiency. This demonstrates that Plk1 is regulated via a negative feedback loop by its substrate, FADD. Overexpression of FADD S194D sensitized HeLa cells to a low dose of taxol independently of caspase activation, whereas overexpression of FADD S194D resulted in caspase activation in response to a high dose of taxol. Therefore, we examined whether the death potential of P-FADD affected Plk1-mediated tumorigenesis. Transfection of FADD S194D inhibited colony formation by Plk1-overexpressing HeLa cells (HeLa-Plk1). Moreover, overexpression of FADD S194D suppressed tumorigenesis in nude mice xenografted with HeLa-Plk1. Therefore, this study reports the first in vivo validation of tumor-suppressing activity of P-FADD. Collectively, our data demonstrate that in response to taxol, Plk1 endows death-promoting and tumor-suppressor functions to its substrate, FADD.

Illumination of mitotic orchestra during cell division: a Polo view

Protein kinase and phosphatase signaling cascade, coupled with other post-translational modifications, orchestrates temporal order of various events during cell division. Among the many mitotic kinases, Polo-like kinase 1 (PLK1) as a key regulator, participates in regulating mitosis from mitotic entry to cytokinesis. The advancement in optical reporter engineering and the recent development of specific chemical probes enable us to visualize spatiotemporal gradient of kinase activity at nano-scale. One of such tools is FRET-based optic sensor that allows us to delineate the PLK1 activity in space and time. In this review, we address the inter-relationships between PLK1 and other protein kinases/phosphatases, as well as the crosstalk between PLK1 phosphorylation and ubiquitination during cell division. In particular, we discuss the molecular mechanisms and steps underlying PLK1 kinase priming, activation and turn-off during cell division.

Control of Aurora-A stability through interaction with TPX2

The Aurora-A kinase has well-established roles in spindle assembly and function and is frequently overexpressed in tumours. Its abundance is cell cycle regulated, with a peak in G2 and M phases, followed by regulated proteolysis at the end of mitosis. The microtubule-binding protein TPX2 plays a major role in regulating the activity and localisation of Aurora-A in mitotic cells. Here, we report a novel regulatory role of TPX2 and show that it protects Aurora-A from degradation both in interphase and in mitosis in human cells. Specifically, Aurora-A levels decrease in G2 and prometaphase cells silenced for TPX2, whereas degradation of Aurora-A is impaired in telophase cells overexpressing the Aurora-A-binding region of TPX2. The decrease in Aurora-A in TPX2-silenced prometaphases requires proteasome activity and the Cdh1 activator of the APC/C ubiquitin ligase. Reintroducing either full-length TPX2, or the Aurora-A-binding region of TPX2, but not a truncated TPX2 mutant lacking the Aurora-A-interaction domain, restores Aurora-A levels in TPX2-silenced prometaphases. The control by TPX2 of Aurora-A stability is independent of its ability to activate Aurora-A and to localise it to the spindle. These results highlight a novel regulatory level impinging on Aurora-A and provide further evidence for the central role of TPX2 in regulation of Aurora-A.

Plk1 negatively regulates Cep55 recruitment to the midbody to ensure orderly abscission

Cytokinesis requires a membrane-remodeling and fission event termed abscission that occurs after chromosome segregation, cleavage furrow formation, and contraction have completed. In this study, we show how abscission factor recruitment is controlled by the Polo-like kinase 1 (Plk1). At the metaphase-anaphase transition, Plk1 initiates cleavage furrow formation and is then progressively degraded during mitotic exit. During this period, Plk1 phosphorylates the abscission factor Cep55 in trans and prevents its untimely recruitment to the anaphase spindle. A Plk1 phosphorylation site mutant of Cep55 is prematurely recruited to the anaphase spindle and fails to support abscission. Endogenous Cep55 behaves similarly after Plk1 inhibition by the drugs BI2536 or GW842862. Only once Plk1 is degraded can Cep55 target to the midbody and promote abscission. Blocking Plk1 degradation leads to elevated levels of Plk1 at the midbody and the failure of Cep55 recruitment. Thus, Plk1 activity negatively regulates Cep55 to ensure orderly abscission factor recruitment and ensures that this occurs only once cell contraction has completed.

Anaphase-promoting complex/cyclosome participates in the acute response to protein-damaging stress

The ubiquitin E3 ligase anaphase-promoting complex/cyclosome (APC/C) drives degradation of cell cycle regulators in cycling cells by associating with the coactivators Cdc20 and Cdh1. Although a plethora of APC/C substrates have been identified, only a few transcriptional regulators are described as direct targets of APC/C-dependent ubiquitination. Here we show that APC/C, through substrate recognition by both Cdc20 and Cdh1, mediates ubiquitination and degradation of heat shock factor 2 (HSF2), a transcription factor that binds to the Hsp70 promoter. The interaction between HSF2 and the APC/C subunit Cdc27 and coactivator Cdc20 is enhanced by moderate heat stress, and the degradation of HSF2 is induced during the acute phase of the heat shock response, leading to clearance of HSF2 from the Hsp70 promoter. Remarkably, Cdc20 and the proteasome 20S core α2 subunit are recruited to the Hsp70 promoter in a heat shock-inducible manner. Moreover, the heat shock-induced expression of Hsp70 is increased when Cdc20 is silenced by a specific small interfering RNA (siRNA). Our results provide the first evidence for participation of APC/C in the acute response to protein-damaging stress.

Inhibition of Plk1 induces mitotic infidelity and embryonic growth defects in developing zebrafish embryos

Polo-like kinase 1 (Plk1) is central to cell division. Here, we report that Plk1 is critical for mitosis in the embryonic development of zebrafish. Using a combination of several cell biology tools, including single-cell live imaging applied to whole embryos, we show that Plk1 is essential for progression into mitosis during embryonic development. Plk1 morphant cells displayed mitotic infidelity, such as abnormal centrosomes, irregular spindle assembly, hypercondensed chromosomes, and a failure of chromosome arm separation. Consequently, depletion of Plk1 resulted in mitotic arrest and finally death by 6days post-fertilization. In comparison, Plk2 or Plk3 morphant embryos did not display any significant abnormalities. Treatment of embryos with the Plk1 inhibitor, BI 2536, caused a block in mitosis, which was more severe when used to treat plk1 morphants. Finally, using an assay to rescue the Plk1 morphant phenotype, we found that the kinase domain and PBD domains are both necessary for Plk1 function in zebrafish development. Our studies demonstrate that Plk1 is required for embryonic proliferation because its activity is crucial for mitotic integrity. Furthermore, our study suggests that zebrafish will be an efficient and economical in vivo system for the validation of anti-mitotic drugs.

Protein kinase A-mediated phosphorylation of the RASSF1A tumour suppressor at Serine 203 and regulation of RASSF1A function

Epigenetic inactivation of the Ras-Association Domain Family 1A (RASSF1A) gene is one of the most frequent alterations detected in cancer. The tumour suppressor function of RASSF1A contributes to cell cycle progression, microtubule stabilisation and apoptotic signalling. Here we investigated the putative phosphorylation sites of RASSF1A and the functional consequences. RASSF1A is mainly phosphorylated at Serine 203 within its Ras association domain. Phosphorylation at this site is accomplished by protein kinase A (PKA) and is reduced and elevated by PKA-specific inhibitors and activators, respectively. Functionally, an alanine substitution of Serine 203 (S203A) slightly affected the microtubule stability mediated by RASSF1A (p<0.05). Interestingly, the inhibition of PKA and the S203A substitution of RASSF1A resulted in a reduced rate of apoptotic cells induced by RASSF1A. Moreover, RASSF1A-mediated upregulation of p21 and BAX was observed. This induction was reduced when the S203A substitution was present or when PKA activity was inhibited. In summary our data show that RASSF1A is phosphorylated by PKA and this phosphorylation may affect apoptotic signalling of RASSF1A. Thus epigenetic silencing of RASSF1A may counteract its proapoptotic function in cancer.

Cyclin E deregulation impairs mitotic progression through premature activation of Cdc25C

The cyclin E-cyclin-dependent kinase 2 (CDK2) complex accelerates entry into the S phase of the cell cycle and promotes polyploidy, which may contribute to genomic instability in cancer cells. The effect of low molecular weight isoforms of cyclin E (LMW-E) overexpression on mitotic progression and its link to genomic instability were the focus of this study. Here, we show that full-length cyclin E (EL) and LMW-E overexpression impairs the G(2)-M transition differently by targeting dual-specificity phosphatase Cdc25C activity. We identify Cdc25C as an interaction partner and substrate for cyclin E/CDK2 kinase. Specifically, the cyclin E/CDK2 complex phosphorylates Cdc25C on Ser(214), leading to its premature activation, which coincides with higher cyclin B/CDK1 and Polo-like kinase 1 (PLK1) activities in an S-phase-enriched population that result in faster mitotic entry. Whereas EL overexpression leads to hyperactivation of Cdc25C, cyclin B/CDK1, and PLK1 in a G(2)-M-enriched population, LMW-E overexpression causes premature inactivation of Cdc25C and PLK1, leading to faster mitotic exit. In addition, LMW-E-overexpressing cells showed a reduction in the mitotic index in the presence of a spindle poison and faster degradation of cyclin B, suggesting an increased rate of mitotic slippage and adaptation to the spindle checkpoint. Lastly, downregulation of Cdc25C inhibits LMW-E-mediated chromosome missegregation, anaphase bridges, and centrosome amplification. These results suggest that the high levels of LMW-E isoforms found in breast cancer may contribute to cellular transformation and genomic instability by impairing mitotic progression involving Cdc25C.

APC16 is a conserved subunit of the anaphase-promoting complex/cyclosome

Error-free chromosome segregation depends on timely activation of the multi-subunit E3 ubiquitin ligase APC/C. Activation of the APC/C initiates chromosome segregation and mitotic exit by targeting critical cell-cycle regulators for destruction. The APC/C is the principle target of the mitotic checkpoint, which prevents segregation while chromosomes are unattached to spindle microtubules. We now report the identification and characterization of APC16, a conserved subunit of the APC/C. APC16 was found in association with tandem-affinity-purified mitotic checkpoint complex protein complexes. APC16 is a bona fide subunit of human APC/C: it is present in APC/C complexes throughout the cell cycle, the phenotype of APC16-depleted cells copies depletion of other APC/C subunits, and APC16 is important for APC/C activity towards mitotic substrates. APC16 sequence homologues can be identified in metazoans, but not fungi, by four conserved primary sequence stretches. We provide evidence that the C. elegans gene K10D2.4 and the D. rerio gene zgc:110659 are functional equivalents of human APC16. Our findings show that APC/C is composed of previously undescribed subunits, and raise the question of why metazoan APC/C is molecularly different from unicellular APC/C.

Polo-like kinase 2-dependent phosphorylation of NPM/B23 on serine 4 triggers centriole duplication

Duplication of the centrosome is well controlled during faithful cell division while deregulation of this process leads to supernumary centrosomes, chromosome missegregation and aneuploidy, a hallmark of many cancer cells. We previously reported that Polo-like kinase 2 (Plk2) is activated near the G1/S phase transition, and regulates the reproduction of centrosomes. In search for Plk2 interacting proteins we have identified NPM/B23 (Nucleophosmin) as a novel Plk2 binding partner. We find that Plk2 and NPM/B23 interact in vitro in a Polo-box dependent manner. An association between both proteins was also observed in vivo. Moreover, we show that Plk2 phosphorylates NPM/B23 on serine 4 in vivo in S-phase. Notably, expression of a non-phosphorylatable NPM/B23 S4A mutant interferes with centriole reduplication in S-phase arrested cells and leads to a dilution of centriole numbers in unperturbed U2OS cells. The corresponding phospho-mimicking mutants have the opposite effect and their expression leads to the accumulation of centrioles. These findings suggest that NPM/B23 is a direct target of Plk2 in the regulation of centriole duplication and that phosphorylation on serine 4 can trigger this process.

Cyclin A and Nek2A: APC/C-Cdc20 substrates invisible to the mitotic spindle checkpoint

Active cyclin B1-Cdk1 (cyclin-dependent kinase 1) keeps cells in mitosis, allowing time for spindle microtubules to capture the chromosomes and for incorrect chromosome-spindle attachments to be repaired. Meanwhile, securin, an inhibitor of separase, secures cohesion between sister chromatids, preventing anaphase onset. The spindle checkpoint is a signalling pathway emerging from improperly attached chromosomes that inhibits Cdc20, the mitotic activator of the APC/C (anaphase-promoting complex/cyclosome) ubiquitin ligase. Blocking Cdc20 stabilizes cyclin B1 and securin to delay mitotic exit and anaphase until all chromosomes reach bipolar spindle attachments. Cells entering mitosis in the absence of a functional spindle checkpoint degrade cyclin B1 and securin right after nuclear-envelope breakdown, in prometaphase. Interestingly, two APC/C substrates, cyclin A and Nek2A, are normally degraded at nuclear-envelope breakdown, even when the spindle checkpoint is active. This indicates that the APC/C is activated early in mitosis, whereas cyclin B1 and securin are protected as long as the spindle checkpoint inhibits Cdc20. Remarkably, destruction of cyclin A and Nek2A also depends on Cdc20. The paradox of Cdc20 being both active and inhibited in prometaphase could be explained if cyclin A and Nek2A are either exceptionally efficient Cdc20 substrates, or if they are equipped with 'stealth' mechanisms to effectively escape detection by the spindle checkpoint. In the present paper, we discuss recently emerging models for spindle-checkpoint-independent APC/C-Cdc20 activity, which might even have implications for cancer therapy.

Binding partner switching on microtubules and aurora-B in the mitosis to cytokinesis transition

The cytoskeleton globally reorganizes between mitosis (M phase) and cytokinesis (C phase), which presumably requires extensive regulatory changes. To reveal these changes, we undertook a comparative proteomics analysis of cells tightly drug-synchronized in each phase. We identified 25 proteins that bind selectively to microtubules in C phase and identified several novel binding partners including nucleolar and spindle-associated protein. C phase-selective microtubule binding of many of these proteins depended on activity of Aurora kinases as assayed by treatment with the drug VX680. Aurora-B binding partners switched dramatically between M phase to C phase, and we identified several novel C phase-selective Aurora-B binding partners including PRC1, KIF4, and anaphase-promoting complex/cyclosome. Our approach can be extended to other cellular compartments and cell states, and our data provide the first broad biochemical framework for understanding C phase. Concretely, we report a central role for Aurora-B in regulating the C phase cytoskeleton.

A mitotic phosphorylation feedback network connects Cdk1, Plk1, 53BP1, and Chk2 to inactivate the G(2)/M DNA damage checkpoint

DNA damage checkpoints arrest cell cycle progression to facilitate DNA repair. The ability to survive genotoxic insults depends not only on the initiation of cell cycle checkpoints but also on checkpoint maintenance. While activation of DNA damage checkpoints has been studied extensively, molecular mechanisms involved in sustaining and ultimately inactivating cell cycle checkpoints are largely unknown. Here, we explored feedback mechanisms that control the maintenance and termination of checkpoint function by computationally identifying an evolutionary conserved mitotic phosphorylation network within the DNA damage response. We demonstrate that the non-enzymatic checkpoint adaptor protein 53BP1 is an in vivo target of the cell cycle kinases Cyclin-dependent kinase-1 and Polo-like kinase-1 (Plk1). We show that Plk1 binds 53BP1 during mitosis and that this interaction is required for proper inactivation of the DNA damage checkpoint. 53BP1 mutants that are unable to bind Plk1 fail to restart the cell cycle after ionizing radiation-mediated cell cycle arrest. Importantly, we show that Plk1 also phosphorylates the 53BP1-binding checkpoint kinase Chk2 to inactivate its FHA domain and inhibit its kinase activity in mammalian cells. Thus, a mitotic kinase-mediated negative feedback loop regulates the ATM-Chk2 branch of the DNA damage signaling network by phosphorylating conserved sites in 53BP1 and Chk2 to inactivate checkpoint signaling and control checkpoint duration.

Cdc20 is required for the post-anaphase, KEN-dependent degradation of centromere protein F

Progression through mitosis and cytokinesis requires the sequential proteolysis of several cell-cycle regulators. This proteolysis is mediated by the ubiquitin-proteasome system, with the E3 ligase being the anaphase-promoting complex, also known as the cyclosome (APC/C). The APC/C is regulated by two activators, namely Cdc20 and Cdh1. The current view is that prior to anaphase, the APC/C is activated by Cdc20, but that following anaphase, APC/C switches to Cdh1-dependent activation. However, here we present an analysis of the kinetochore protein Cenp-F that is inconsistent with this notion. Although it has long been appreciated that Cenp-F is degraded sometime during or after mitosis, exactly when and how has not been clear. Here we show that degradation of Cenp-F initiates about six minutes after anaphase, and that this is dependent on a C-terminal KEN-box. Although these two observations are consistent with Cenp-F being a substrate of Cdh1-activated APC/C, Cenp-F is degraded normally in Cdh1-null cells. By contrast, RNAi-mediated repression of APC/C subunits or Cdc20 does inhibit Cenp-F degradation. These findings therefore suggest that the APC/C does not simply 'switch' upon anaphase onset; rather, our observations indicate that Cdc20 also contributes to post-anaphase activation of the APC/C. We also show that the post-anaphase, KEN-box-dependent degradation of Cenp-F requires it to be farnesylated, a post-translational modification usually linked to membrane association. Because so many of the behaviours of Cenp-F are farnesylation-dependent, we suggest that this modification plays a more global role in Cenp-F function.

Understanding cytokinesis failure

Cytokinesis is the final step in cell division. The process begins during chromosome segregation, when the ingressing cleavage furrow begins to partition the cytoplasm between the nascent daughter cells. The process is not completed until much later, however, when the final cytoplasmic bridge connecting the two daughter cells is severed. Cytokinesis is a highly ordered process, requiring an intricate interplay between cytoskeletal, chromosomal and cell cycle regulatory pathways. A surprisingly broad range of additional cellular processes are also important for cytokinesis, including protein and membrane trafficking, lipid metabolism, protein synthesis and signaling pathways. As a highly regulated, complex process, it is not surprising that cytokinesis can sometimes fail. Cytokinesis failure leads to both centrosome amplification and production of tetraploid cells, which may set the stage for the development of tumor cells. However, tetraploid cells are abundant components of some normal tissues including liver and heart, indicating that cytokinesis is physiologically regulated. In this chapter, we summarize our current understanding of the mechanisms of cytokinesis, emphasizing steps in the pathway that may be regulated or prone to failure. Our discussion emphasizes findings in vertebrate cells although we have attempted to highlight important contributions from other model systems.

The emerging role of APC/CCdh1 in controlling differentiation, genomic stability and tumor suppression

Deregulation of the G1/G0 phase of the cell cycle can lead to cancer. During G1, most cells commit alternatively to DNA replication and division, or to cell-cycle exit and differentiation. The anaphase-promoting complex or cyclosome (APC/C) activated by Cdh1 coordinately eliminates positive cell-cycle regulators as well as inhibitors of differentiation, thereby coupling cell-cycle exit and differentiation. Misregulation of Cdh1 thus has the potential to promote both cell-cycle re-entry and either perturbed differentiation or dedifferentiation. In addition, APC/C(Cdh1) is required to maintain genomic stability. As a result, loss of Cdh1 can contribute to tumorigenesis in the form of proliferation of poorly differentiated and genetically unstable cells.

Cytokinetic abscission: cellular dynamics at the midbody

The intercellular canal containing the midbody is one of the most prominent structures in dividing animal cells, yet its function in the completion of cytokinesis by abscission remains largely unknown. This is because of its small size, which makes it difficult to investigate the cytoskeletal and membrane dynamics underlying abscission by standard light microscopy. The advent of new fluorescent probes and imaging technologies, along with sophisticated perturbation tools, provides new possibilities to elucidate the molecular control of this essential cell biological process. Here we discuss the control of midbody assembly and current models for the mechanism of abscission in animal cells. We highlight new methodologies that will facilitate testing and refining of these models.

Human papillomavirus 16 E7 oncoprotein attenuates DNA damage checkpoint control by increasing the proteolytic turnover of claspin

The human papillomavirus (HPV) 16 E7 oncoprotein has been reported previously to stimulate DNA damage and to activate host cell DNA damage checkpoints. How HPV-16 E7 maintains proliferation despite activated DNA damage checkpoints is incompletely understood. Here, we provide evidence that cells expressing the HPV-16 E7 oncoprotein can enter mitosis in the presence of DNA damage. We show that this activity of HPV-16 E7 involves attenuation of DNA damage checkpoint control by accelerating the proteolytic turnover of claspin. Claspin mediates the activation of CHK1 by ATR in response to replication stress, and its degradation plays a critical role in DNA damage checkpoint recovery. Expression of a nondegradable mutant of claspin was shown to inhibit mitotic entry in HPV-16 E7-expressing cells. Multiple components of the SCF(beta-TrCP)-based claspin degradation machinery were found deregulated in the presence of HPV-16 E7, including cullin 1, beta-TrCP, Aurora A, and Polo-like kinase-1 (PLK1). In contrast, no difference in the expression level of the claspin deubiquitinating enzyme USP7 was detected. Levels of Aurora A and PLK1 as well as phosphorylated PLK1 at threonine 210, a prerequisite for DNA damage checkpoint recovery, remained detectable following replication stress in HPV-16 E7-expressing cells but not in control cells. In summary, our results suggest that the HPV-16 E7 oncoprotein alleviates DNA damage checkpoint responses and promotes mitotic entry by accelerating claspin degradation through a mechanism that involves deregulation of components of the SCF(beta-TrCP)-based claspin degradation machinery.

Degradation of activated protein kinases by ubiquitination

Protein kinases are important regulators of intracellular signal transduction pathways and play critical roles in diverse cellular functions. Once a protein kinase is activated, its activity is subsequently downregulated through a variety of mechanisms. Accumulating evidence indicates that the activation of protein kinases commonly initiates their downregulation via the ubiquitin/proteasome pathway. Failure to regulate protein kinase activity or expression levels can cause human diseases.

Regulation of the endothelial cell cycle by the ubiquitin-proteasome system

Degradation of poly-ubiquitinated proteins by the 26S-proteasome complex represents a crucial quantitative control mechanism. The ubiquitin-proteasome system (UPS) plays a pivotal role in the complex molecular network regulating the progression both between and within each cell-cycle phase. Two major complexes are involved: the SKP1-CUL1-F-box-protein complex (SCF) and the anaphase-promoting complex/cyclosome (APC/C). Notwithstanding structural similarities, SCF and APC/C display different cellular functions and mechanisms of action. SCF modulates all cell-cycle stages and plays a prominent role at G1/S transition mainly through three regulatory subunits: Skp2, Fbw7, and beta-TRCP. APC/C, regulated by Cdc20 or Cdh1 subunits, has a crucial role in mitosis. In this review, we will describe how the endothelial cell cycle is regulated by the UPS. We will illustrate the principal SCF- and APC/C-dependent molecular mechanisms that modulate cell growth, allowing a unidirectional cell-cycle progression. Then, we will focus our attention on UPS modulation by oxidative stress, a pathogenic stimulus that causes endothelial dysfunction and is involved in numerous cardiovascular diseases.

Alternative functions of core cell cycle regulators in neuronal migration, neuronal maturation, and synaptic plasticity

Recent studies have demonstrated that boundaries separating a cycling cell from a postmitotic neuron are not as concrete as expected. Novel and unique physiological functions in neurons have been ascribed for proteins fundamentally required for cell cycle progression and control. These "core" cell cycle regulators serve diverse postmitotic functions that span various developmental stages of a neuron, including neuronal migration, axonal elongation, axon pruning, dendrite morphogenesis, and synaptic maturation and plasticity. In this review, we detail the nonproliferative postmitotic roles that these cell cycle proteins have recently been reported to play, the significance of their expression in neurons, mechanistic insight when available, and future prospects.

Functional dynamics of Polo-like kinase 1 at the centrosome

Polo-like kinase 1 (Plk1) functions as a key regulator of mitotic events by phosphorylating substrate proteins on centrosomes, kinetochores, the mitotic spindle, and the midbody. Through mechanisms that are incompletely understood, Plk1 is released from and relocalizes to different mitotic structures as cells proceed through mitosis. We used fluorescence recovery after photobleaching to examine the kinetics of this process in more detail. We observed that Plk1 displayed a range of different recovery rates that differ at each mitotic substructure and depend on both the Polo-box domain and a functional kinase domain. Upon mitotic entry, centrosomal Plk1 becomes more dynamic, a process that is directly enhanced by Plk1 kinase activity. In contrast, Plk1 displays little dynamic exchange at the midbody, a process that again is modulated by the kinase activity of Plk1. Our findings suggest that the intrinsic kinase activity of Plk1 triggers its release from early mitotic structures and its relocalization to late mitotic structures. To assess the importance of Plk1 dynamic relocalization, Plk1 was persistently tethered to the centrosome. This resulted in a G(2) delay, followed by a prominent prometaphase arrest, as a consequence of defective spindle formation and activation of the spindle checkpoint. The dynamic release of Plk1 from early mitotic structures is thus crucial for mid- to late-stage mitotic events and demonstrates the importance of a fully dynamic Plk1 at the centrosome for proper cell cycle progression. This dependence on dynamic Plk1 was further observed during the mitotic reentry of cells after a DNA damage G(2) checkpoint, as this process was significantly delayed upon centrosomal tethering of Plk1. These results indicate that mitotic progression and control of mitotic reentry after DNA damage resides, at least in part, on the dynamic behavior of Plk1.

Measuring proteolysis in mitosis

The targeted destruction of key regulators helps to drive the cell cycle. Here we describe a quantitative assay to measure destruction of different regulators in mitotic cells. This assay uses GFP-tagged substrates and time-lapse fluorescence microscopy of single cells to pinpoint the timing of destruction of different substrates at different stages in mitosis.

Ubiquitin-mediated degradation of the formin mDia2 upon completion of cell division

Formins assemble non-branched actin filaments and modulate microtubule dynamics during cell migration and cell division. At the end of mitosis formins contribute to the generation of actin filaments that form the contractile ring. Rho small GTP-binding proteins activate mammalian diaphanous-related (mDia) formins by directly binding and disrupting an intramolecular autoinhibitory mechanism. Although the Rho-regulated activation mechanism is well characterized, little is known about how formins are switched off. Here we reveal a novel mechanism of formin regulation during cytokinesis based on the following observations; 1) mDia2 is degraded at the end of mitosis, 2) mDia2 is targeted for disposal by post-translational ubiquitin modification, 3) forced expression of activated mDia2 yields binucleate cells due to failed cytokinesis, and 4) the cytokinesis block is dependent upon mDia2-mediated actin assembly as versions of mDia2 incapable of nucleating actin but that still stabilize microtubules have no effect on cytokinesis. We propose that the tight control of mDia2 expression and ubiquitin-mediated degradation is essential for the completion of cell division. Because of the many roles for formins in cell morphology, we discuss the relevance of mDia protein turnover in other processes where ubiquitin-mediated proteolysis is an essential component.

BubR1 acetylation at prometaphase is required for modulating APC/C activity and timing of mitosis

Regulation of BubR1 is central to the control of APC/C activity. We have found that BubR1 forms a complex with PCAF and is acetylated at lysine 250. Using mass spectrometry and acetylated BubR1-specific antibodies, we have confirmed that BubR1 acetylation occurs at prometaphase. Importantly, BubR1 acetylation was required for checkpoint function, through the inhibition of ubiquitin-dependent BubR1 degradation. BubR1 degradation began before the onset of anaphase. It was noted that the pre-anaphase degradation was regulated by BubR1 acetylation. Degradation of an acetylation-mimetic form, BubR1–K250Q, was inhibited and chromosome segregation in cells expressing BubR1–K250Q was markedly delayed. By contrast, the acetylation-deficient mutant, BubR1–K250R, was unstable, and mitosis was accelerated in BubR1–K250R-expressing cells. Furthermore, we found that APC/C–Cdc20 was responsible for BubR1 degradation during mitosis. On the basis of our collective results, we propose that the acetylation status of BubR1 is a molecular switch that converts BubR1 from an inhibitor to a substrate of the APC/C complex, thus providing an efficient way to modulate APC/C activity and mitotic timing.

Polo-like kinase-1 controls Aurora A destruction by activating APC/C-Cdh1

Polo-like kinase-1 (Plk1) is activated before mitosis by Aurora A and its cofactor Bora. In mitosis, Bora is degraded in a manner dependent on Plk1 kinase activity and the E3 ubiquitin ligase SCF-betaTrCP. Here, we show that Plk1 is also required for the timely destruction of its activator Aurora A in late anaphase. It has been shown that Aurora A destruction is controlled by the auxiliary subunit Cdh1 of the Anaphase-Promoting Complex/Cyclosome (APC/C). Remarkably, we found that Plk1-depletion prevented the efficient dephosphorylation of Cdh1 during mitotic exit. Plk1 mediated its effect on Cdh1, at least in part, through direct phosphorylation of the human phosphatase Cdc14A, controlling the phosphorylation state of Cdh1. We conclude that Plk1 facilitates efficient Aurora A degradation through APC/C-Cdh1 activation after mitosis, with a potential role for hCdc14A.

Polo-like kinases: conservation and divergence in their functions and regulation

Polo-like kinases (Plks) are potent regulators of M phase that are conserved from yeasts to humans. Their roles in mitotic entry, spindle pole functions and cytokinesis are broadly conserved despite physical and molecular differences in these processes in disparate organisms. Plks are characterized by their Polo-box domain, which mediates protein interactions. They are additionally controlled by phosphorylation, proteolysis and transcription, depending on the biological context. Plks are now recognized to link cell division to developmental processes and to function in differentiated cells. A comparison of Plk function and regulation between organisms offers insight into the rich variations of cell division.

The adaptor protein of the anaphase promoting complex Cdh1 is essential in maintaining replicative lifespan and in learning and memory

The anaphase promoting complex (APC) or cyclosome is a multisubunit E3 ubiquitin ligase. Cdc20 (fizzy (fzy)) or p55CDC, and Cdh1 (Hct1, srw1 or fizzy-related 1 (fzr1)) encode two adaptor proteins that bring substrates to the APC. Both APC-Cdc20 and APC-Cdh1 have been implicated in the control of mitosis through mediating ubiquitination of mitotic regulators, such as cyclin B1 and securin. However, the importance of Cdh1 function in vivo and whether its function is redundant with that of Cdc20 are unclear. Here we have analysed mice lacking Cdh1. We show that Cdh1 is essential for placental development and that its deficiency causes early lethality. Cdhl-deficient mouse embryonic fibroblasts (MEFs) entered replicative senescence prematurely because of stabilization of Ets2 and subsequent activation of p6(Ink4a) expression. These results have uncovered an unexpected role of the APC in maintaining replicative lifespan of MEFs. Further, Cdh1 heterozygous mice show defects in late-phase long-term potentiation (L-LTP) in the hippocampus and are deficient in contextual fear-conditioning, suggesting that Cdh1 has a role in learning and memory.

Mitosis: Cdh1 clears the way for anaphase spindle assembly

The two co-activators of the anaphase-promoting complex/cyclosome (APC/C), Cdc20 and Cdh1, facilitate ubiquitination by the core complex, but their role in conferring substrate specificity has been contentious. A new report reveals that Cdc20-bound APC/C initiates the timely proteolysis of many Cdh1 substrates with the exception of the Aurora kinases. Failure to degrade Aurora kinases results in abnormal anaphase microtubule organisation and premature cytokinesis.

Fine tuning the cell cycle: activation of the Cdk1 inhibitory phosphorylation pathway during mitotic exit

Inactivation of cyclin-dependent kinase (Cdk) 1 promotes exit from mitosis and establishes G1. Proteolysis of cyclin B is the major known mechanism that turns off Cdk1 during mitotic exit. Here, we show that mitotic exit also activates pathways that catalyze inhibitory phosphorylation of Cdk1, a mechanism previously known to repress Cdk1 only during S and G2 phases of the cell cycle. We present evidence that down-regulation of Cdk1 activates Wee1 and Myt1 kinases and inhibits Cdc25 phosphatase during the M to G1 transition. If cyclin B/Cdk1 complex is present in G1, the inhibitory sites on Cdk1 become phosphorylated. Exit from mitosis induced by chemical Cdk inhibition can be reversed if cyclin B is preserved. However, this reversibility decreases with time after mitotic exit despite the continued presence of the cyclin. We show that this G1 block is due to phosphorylation of Cdk1 on inhibitory residues T14 and Y15. Chemical inhibition of Wee1 and Myt1 or expression of Cdk1 phosphorylation site mutants allows reversal to M phase even from late G1. This late Cdk1 reactivation often results in caspase-dependent cell death. Thus, in G1, the Cdk inhibitory phosphorylation pathway is functional and can lock Cdk1 in the inactive state.

The function of APC/CCdh1 in cell cycle and beyond

The anaphase promoting complex/cyclosome (APC/C) is a multi-subunit E3 ubiquitin ligase playing essential functions in mitosis. It is conserved from yeast to human and relies on two adaptor proteins, Cdc20 and Cdh1, to bring in substrates. Both APCCdc20 and APCCdh1 are implicated in the control of mitosis through mediating ubiquitination and degradation of important mitotic regulators such as cyclin B1, securin, and Plk1. In addition, APCCdh1 is thought to prevent premature S phase entry by limiting the accumulation of mitotic cyclins in G1 and to regulate processes unrelated to cell cycle. In this review, we will summarize our current understanding of APCCdh1 function in cell cycle and beyond.

Polo-like kinase 1 reaches beyond mitosis--cytokinesis, DNA damage response, and development

Polo-like kinase 1 (Plk1) is a key regulator of cell division in eukaryotic cells. In this review we focus on recent leaps in our understanding of how Plk1 controls cytokinesis, the final stage of cell division. Furthermore, we will go beyond mitosis to highlight unexpected roles of Plk1 during interphase and during animal development. In vertebrate cells, Plk1 has emerged as a novel player in maintaining genomic stability during DNA replication and as an important modulator of the DNA damage checkpoint. Plk1 functions extend past the 'core' cell cycle. Plk1 acts as a link between developmental processes and the cell cycle machinery during asymmetric cell divisions in flies and worms. The term 'mitotic kinase' might not do justice to Plk1 in the light of these recent results.

Sequestration of Polo kinase to microtubules by phosphopriming-independent binding to Map205 is relieved by phosphorylation at a CDK site in mitosis

The conserved Polo kinase controls multiple events in mitosis and cytokinesis. Although Polo-like kinases are regulated by phosphorylation and proteolysis, control of subcellular localization plays a major role in coordinating their mitotic functions. This is achieved largely by the Polo-Box Domain, which binds prephosphorylated targets. However, it remains unclear whether and how Polo might interact with partner proteins when priming mitotic kinases are inactive. Here we show that Polo associates with microtubules in interphase and cytokinesis, through a strong interaction with the microtubule-associated protein Map205. Surprisingly, this interaction does not require priming phosphorylation of Map205, and the Polo-Box Domain of Polo is required but not sufficient for this interaction. Moreover, phosphorylation of Map205 at a CDK site relieves this interaction. Map205 can stabilize Polo and inhibit its cellular activity in vivo. In syncytial embryos, the centrosome defects observed in polo hypomorphs are enhanced by overexpression of Map205 and suppressed by its deletion. We propose that Map205-dependent targeting of Polo to microtubules provides a stable reservoir of Polo that can be rapidly mobilized by the activity of Cdk1 at mitotic entry.

Phosphorylation of Plk1 at Ser326 regulates its functions during mitotic progression

Polo-like kinase 1 (Plk1), the best characterized member of the mammalian polo-like kinase family, is well regulated throughout the cell cycle at the protein expression level. Moreover, it is known that Plk1 kinase activity is also regulated at the post-translational level through phosphorylation. However, the upstream kinases of Plk1 have not been identified. Although the involvement of the p38 MAP kinase pathway in cellular responses to stress has been well documented, the role of this pathway in normal cell cycle progression is unclear. Here, we show that phosphorylated p38 and MAP kinase-activated protein kinase 2 (MK2) are colocalized with Plk1 to the spindle poles during prophase and metaphase. Specific depletion of various members of the p38 MAP kinase pathway by the use of RNA interference revealed that the pathway is required for mitotic progression under normal growth conditions. Furthermore, MK2 directly phosphorylates Ser326 of Plk1. Ectopic expression of Plk1-S326A completely blocked cells at mitosis, likely due to the defect of bipolar spindle formation and subsequent activation of the spindle checkpoint. Only Plk1-S326E, but not the Plk1-S326A, efficiently rescued the p38 or MK2-depletion-induced mitotic defects, further solidifying the requirement of S326 phosphorylation during mitotic progression.

Control of mitotic exit and cytokinesis by the APC/C

Inactivation of key substrates by ubiquitin-mediated proteolysis controls the passage of cells through mitosis. The APC/C (anaphase-promoting complex/cyclosome) targets a large number of substrates for proteolysis during the final steps of mitosis and cytokinesis, but the significance of these targeting events, particularly in mammalian cells, is largely unknown. In this review, I summarize what is known about how the APC/C selects its targets during mitotic exit and review the evidence that substrate targeting after anaphase onset may be required for the correct execution of events at this time in the cell cycle.

APC/C Cdh1 targets aurora kinase to control reorganization of the mitotic spindle at anaphase

BACKGROUND

Control of mitotic cell cycles by the anaphase-promoting complex or cyclosome (APC/C) ubiquitin ligase depends on its coactivators Cdc20 and Cdh1. APC/C(Cdc20) is active during mitosis and promotes anaphase onset by targeting mitotic cyclins and securin. APC/C(Cdh1) becomes active during mitotic exit and has essential targets in G1 phase. It is not known whether targeting of substrates by APC/C(Cdh1) plays any role in the final stages of mitosis. Here, we have investigated the role of APC/C(Cdh1) at this time in the cell cycle by using siRNA-mediated depletion of Cdh1 in human cells.

RESULTS

In contrast to the current view that Cdh1 takes over from Cdc20 at anaphase, we show that reduced Cdh1 levels have no effect on destruction of many APC/C substrates during mitotic exit but strongly and specifically stabilize Aurora kinases. We find that APC/C(Cdh1) is required for assembly of a robust spindle midzone at anaphase and for normal timings of spindle elongation and cytokinesis. The effect of Cdh1 siRNA on anaphase spindle dynamics requires Aurora A, and its effect can be mimicked by nondegradable Aurora kinase.

CONCLUSIONS

Targeting of Aurora kinases at anaphase by APC/C(Cdh1) participates in the control of mitotic exit and cytokinesis.