Complex formation of Plk1 and INCENP required for metaphase-anaphase transition

Mitotic chromosomal dynamics is regulated by the coordinated activities of many mitotic kinases, such as cyclin-dependent kinase 1 (Cdk1), Aurora-B or Polo-like kinase 1 (Plk1), but the mechanisms of their coordination remain unknown. Here, we report that Cdk1 phosphorylates Thr 59 and Thr 388 on inner centromere protein (INCENP), which regulates the localization and kinase activity of Aurora-B from prophase to metaphase. INCENP depletion disrupts Plk1 localization specifically at the kinetochore. This phenotype is rescued by the exogenous expression of INCENP wild type and INCENP mutated at Thr 59 to Ala (T59A), but not at Thr 388 to Ala (T388A). The replacement of endogenous INCENP with T388A resulted in the delay of progression from metaphase to anaphase. We propose that INCENP phosphorylation by Cdk1 is necessary for the recruitment of Plk1 to the kinetochore, and that the complex formation of Plk1 and Aurora-B on INCENP may play crucial roles in the regulation of chromosomal dynamics.     

Plk1 negatively regulates PRC1 to prevent premature midzone formation before cytokinesis

To achieve mitosis and cytokinesis, microtubules must assemble into distinct structures at different stages of cell division-mitotic spindles to segregate the chromosomes before anaphase and midzones to keep sister genomes apart and guide the cleavage furrow after anaphase. This temporal regulation is believed to involve Cdk1 kinase, which is inactivated in a switch-like way after anaphase. We found that inhibiting Plk1 caused premature assembly of midzones in cells still in metaphase, breaking the temporal regulation of microtubules. The antiparallel microtubule-bundling protein PRC1 plays a key role in organizing the midzone complex. We found that Plk1 negatively regulates PRC1 through phosphorylation of a single site, Thr-602, near the C-terminus of PRC1. We also found that microtubules stimulated Thr-602 phosphorylation by Plk1. This creates a potential negative feedback loop controlling PRC1 activity. It also made the extent of Thr-602 phosphorylation during mitotic arrest dependent on the mechanism of the arresting drug. Unexpectedly, we could not detect a preanaphase regulatory role for Cdk1 sites on PRC1. We suggest that PRC1 is regulated by Plk1, rather than Cdk1 as previously proposed, because its activity must be spatiotemporally regulated both preanaphase and postanaphase, and Cdk1 activity is too binary for this purpose.     

Aurora-B regulates the cleavage furrow-specific vimentin phosphorylation in the cytokinetic process

Aurora-B is an evolutionally conserved protein kinase that regulates several mitotic events including cytokinesis. We previously demonstrated the possible existence of a protein kinase that phosphorylates at least Ser-72 on vimentin, the most widely expressed intermediate filament protein, in the cleavage furrow-specific manner. Here we showed that vimentin-Ser-72 phosphorylation occurred specifically at the border of the Aurora-B-localized area from anaphase to telophase. Expression of a dominant-negative mutant of Aurora-B led to a reduction of this vimentin-Ser-72 phosphorylation. In vitro analyses revealed that Aurora-B phosphorylates vimentin at approximately 2 mol phosphate/mol of substrate for 30 min and that this phosphorylation dramatically inhibits vimentin filament formation. We further identified eight Aurora-B phosphorylation sites, including Ser-72 on vimentin, and then constructed the mutant vimentin in which these identified sites are changed into Ala. Cells expressing this mutant formed an unusually long bridge-like intermediate filament structure between unseparated daughter cells. We then identified important phosphorylation sites for the bridge phenotype. Our findings indicate that Aurora-B regulates the cleavage furrow-specific vimentin phosphorylation and controls vimentin filament segregation in cytokinetic process.     

Choice of Plk1 docking partners during mitosis and cytokinesis is controlled by the activation state of Cdk1

Spatial and temporal coordination of polo-like kinase 1 (Plk1) activity is necessary for mitosis and cytokinesis, and this is achieved through binding to phosphorylated docking proteins with distinct subcellular localizations. Although cyclin-dependent kinase 1 (Cdk1) creates these phosphorylated docking sites in metaphase, a general principle that explains how Plk1 activity is controlled in anaphase after Cdk1 inactivation is lacking. Here, we show that the microtubule-associated protein regulating cytokinesis (PRC1) is an anaphase-specific binding partner for Plk1, and that this interaction is required for cytokinesis. In anaphase, Plk1 creates its own docking site on PRC1, whereas in metaphase Cdk1 phosphorylates PRC1 adjacent to this docking site and thereby prevents binding of Plk1. Mutation of these Cdk1-sites results in a form of PRC1 that prematurely recruits Plk1 to the spindle during prometaphase and blocks mitotic progression. The activation state of Cdk1, therefore, controls the switch of Plk1 localization from centrosomes and kinetochores during metaphase, to the central spindle during anaphase.     

Inhibition of endocytic vesicle fusion by Plk1-mediated phosphorylation of vimentin during mitosis

Endocytic vesicle fusion is inhibited during mitosis, but the molecular pathways that mediate the inhibition remain unclear. Here we uncovered an essential role of Polo-like kinase 1 (Plk1) in this mechanism. Phosphoproteomic analysis revealed that Plk1 phosphorylates the intermediate filament protein vimentin on Ser459, which is dispensable for its filament formation but is necessary for the inhibition of endocytic vesicle fusion in mitosis. Furthermore, this mechanism is required for integrin trafficking toward the cleavage furrow during cytokinesis. Our results thus identify a novel mechanism for fusion inhibition in mitosis and implicate its role in vesicle trafficking after anaphase onset.     

Plk1 regulates activation of the anaphase promoting complex by phosphorylating and triggering SCFbetaTrCP-dependent destruction of the APC Inhibitor Emi1

Progression through mitosis requires activation of cyclin B/Cdk1 and its downstream targets, including Polo-like kinase and the anaphase-promoting complex (APC), the ubiquitin ligase directing degradation of cyclins A and B. Recent evidence shows that APC activation requires destruction of the APC inhibitor Emi1. In prophase, phosphorylation of Emi1 generates a D-pS-G-X-X-pS degron to recruit the SCF(betaTrCP) ubiquitin ligase, causing Emi1 destruction and allowing progression beyond prometaphase, but the kinases directing this phosphorylation remain undefined. We show here that the polo-like kinase Plk1 is strictly required for Emi1 destruction and that overexpression of Plk1 is sufficient to trigger Emi1 destruction. Plk1 stimulates Emi1 phosphorylation, betaTrCP binding, and ubiquitination in vitro and cyclin B/Cdk1 enhances these effects. Plk1 binds to Emi1 in mitosis and the two proteins colocalize on the mitotic spindle poles, suggesting that Plk1 may spatially control Emi1 destruction. These data support the hypothesis that Plk1 activates the APC by directing the SCF-dependent destruction of Emi1 in prophase.     

Phosphorylation-mediated stabilization of Bora in mitosis coordinates Plx1/Plk1 and Cdk1 oscillations

Cdk1 and Plk1/Plx1 activation leads to their inactivation through negative feedback loops. Cdk1 deactivates itself by activating the APC/C, consequently generating embryonic cell cycle oscillations. APC/C inhibition by the mitotic checkpoint in somatic cells and the cytostatic factor (CSF) in oocytes sustain the mitotic state. Plk1/Plx1 targets its co-activator Bora for degradation, but it remains unclear how embryonic oscillations in Plx1 activity are generated, and how Plk1/Plx1 activity is sustained during mitosis. We show that Plx1-mediated degradation of Bora in interphase generates oscillations in Plx1 activity and is essential for development. In CSF extracts, phosphorylation of Bora on the Cdk consensus site T52 blocks Bora degradation. Upon fertilization, Calcineurin dephosphorylates T52, triggering Plx1 oscillations. Similarly, we find that GFP-Bora is degraded when Plk1 activity spreads to somatic cell cytoplasm before mitosis. Interestingly, GFP–Bora degradation stops upon mitotic entry when Cdk1 activity is high. We hypothesize that Cdk1 controls Bora through an incoherent feedforward loop synchronizing the activities of mitotic kinases.     

Human Cdc14A regulates Wee1 stability by counteracting CDK-mediated phosphorylation

The activity of Cdk1–cyclin B1 mitotic complexes is regulated by the balance between the counteracting activities of Wee1/Myt1 kinases and Cdc25 phosphatases. These kinases and phosphatases must be strictly regulated to ensure proper mitotic timing. One masterpiece of this regulatory network is Cdk1, which promotes Cdc25 activity and suppresses inhibitory Wee1/Myt1 kinases through direct phosphorylation. The Cdk1-dependent phosphorylation of Wee1 primes phosphorylation by additional kinases such as Plk1, triggering Wee1 degradation at the onset of mitosis. Here we report that Cdc14A plays an important role in the regulation of Wee1 stability. Depletion of Cdc14A results in a significant reduction in Wee1 protein levels. Cdc14A binds to Wee1 at its amino-terminal domain and reverses CDK-mediated Wee1 phosphorylation. In particular, we found that Cdc14A inhibits Wee1 degradation through the dephosphorylation of Ser-123 and Ser-139 residues. Thus the lack of phosphorylation of these two residues prevents the interaction with Plk1 and the consequent efficient Wee1 degradation at the onset of mitosis. These data support the hypothesis that Cdc14A counteracts Cdk1–cyclin B1 activity through Wee1 dephosphorylation.     

Cdk1 plays matchmaker for the Polo-like kinase and its activator SPAT-1/Bora

Mitosis is orchestrated by several protein kinases including Cdks, Plks and Aurora kinases. Despite considerable progress toward understanding the individual function of these protein kinases, how their activity is coordinated in space and time during mitosis is less well understood. In a recent article published in the Journal of Cell Biology, we show that CDK-1 regulates PLK-1 activity during mitosis in C. elegans embryos through multisite phosphorylation of the PLK-1 activator SPAT-1 (Aurora Borealis, Bora in human). SPAT-1 variants mutated on CDK-1 phosphorylation sites results in severe delays in mitotic entry, mimicking embryos lacking spat-1 or plk-1 function. We further show that SPAT-1 phosphorylation by CDK-1 promotes its binding to PLK-1 and stimulates PLK-1 phosphorylation on its activator T-loop by Aurora A kinase in vitro. Likewise, we find that phosphorylation of Bora by Cdk1 promotes phosphorylation of human Plk1 by Aurora A suggesting that this mechanism is conserved in humans. These results indicate that Cdk1 regulates Plk1 by boosting its kinase activity. Here we discuss these recent findings and open questions regarding the regulation of Plk1/PLK-1 by Cdk1/CDK-1 and Bora/SPAT-1.     

Functional significance of the specific sites phosphorylated in desmin at cleavage furrow: Aurora-B may phosphorylate and regulate type III intermediate filaments during cytokinesis coordinatedly with Rho-kinase

Aurora-B is a protein kinase required for chromosome segregation and the progression of cytokinesis during the cell cycle. We report here that Aurora-B phosphorylates GFAP and desmin in vitro, and this phosphorylation leads to a reduction in filament forming ability. The sites phosphorylated by Aurora-B; Thr-7/Ser-13/Ser-38 of GFAP, and Thr-16 of desmin are common with those related to Rho-associated kinase (Rho-kinase), which has been reported to phosphorylate GFAP and desmin at cleavage furrow during cytokinesis. We identified Ser-59 of desmin to be a specific site phosphorylated by Aurora-B in vitro. Use of an antibody that specifically recognized desmin phosphorylated at Ser-59 led to the finding that the site is also phosphorylated specifically at the cleavage furrow during cytokinesis in Saos-2 cells. Desmin mutants, in which in vitro phosphorylation sites by Aurora-B and/or Rho-kinase are changed to Ala or Gly, cause dramatic defects in filament separation between daughter cells in cytokinesis. The results presented here suggest the possibility that Aurora-B may regulate cleavage furrow-specific phosphorylation and segregation of type III IFs coordinatedly with Rho-kinase during cytokinesis.     

Plk1 docking to GRASP65 phosphorylated by Cdk1 suggests a mechanism for Golgi checkpoint signalling

GRASP65, a structural protein of the Golgi apparatus, has been linked to the sensing of Golgi structure and the integration of this information with the control of mitotic entry in the form of a Golgi checkpoint. We show that Cdk1-cyclin B is the major kinase phosphorylating GRASP65 in mitosis, and that phosphorylated GRASP65 interacts with the polo box domain of the polo-like kinase Plk1. GRASP65 is phosphorylated in its C-terminal domain at four consensus sites by Cdk1-cyclin B, and mutation of these residues to alanine essentially abolishes both mitotic phosphorylation and Plk1 binding. Expression of the wild-type GRASP65 C-terminus but not the phosphorylation defective mutant in normal rat kidney cells causes a delay but not the block in mitotic entry expected if this were a true cell cycle checkpoint. These findings identify a Plk1-dependent signalling mechanism potentially linking Golgi structure and cell cycle control, but suggest that this may not be a cell cycle checkpoint in the classical sense.     

Mitotic entry: The interplay between Cdk1, Plk1 and Bora

Polo-like kinase 1 (Plk1) is an important mitotic kinase that is crucial for entry into mitosis after recovery from DNA damage-induced cell cycle arrest. Plk1 activation is promoted by the conserved protein Bora (SPAT-1 in C. elegans), which stimulates the phosphorylation of a conserved residue in the activation loop by the Aurora A kinase. In a recent article published in Cell Reports, we show that the master mitotic kinase Cdk1 contributes to Plk1 activation through SPAT-1/Bora phosphorylation. We identified 3 conserved Sp/Tp residues that are located in the N-terminal, most conserved part, of SPAT-1/Bora. Phosphorylation of these sites by Cdk1 is essential for Plk1 function in mitotic entry in C. elegans embryos and during DNA damage checkpoint recovery in mammalian cells. Here, using an untargeted Förster Resonance Energy Transfer (FRET) biosensor to monitor Plk1 activation, we provide additional experimental evidence supporting the importance of these phosphorylation sites for Plk1 activation and subsequent mitotic entry after DNA damage. We also briefly discuss the mechanism of Plk1 activation and the potential role of Bora phosphorylation by Cdk1 in this process. As Plk1 is overexpressed in cancer cells and this correlates with poor prognosis, understanding how Bora contributes to Plk1 activation is paramount for the development of innovative therapeutical approaches.     

Polo‐like kinase‐1 triggers histone phosphorylation by Haspin in mitosis

Histone modifications coordinate the chromatin localization of key regulatory factors in mitosis. For example, mitotic phosphorylation of Histone H3 threonine‐3 (H3T3ph) by Haspin creates a binding site for the chromosomal passenger complex (CPC). However, how these histone modifications are spatiotemporally controlled during the cell cycle is unclear. Here we show that Plk1 binds to Haspin in a Cdk1‐phosphorylation‐dependent manner. Reducing Plk1 activity decreases the phosphorylation of Haspin and inhibits H3T3ph, particularly in prophase, suggesting that Plk1 is required for initial activation of Haspin in early mitosis. These studies demonstrate that Plk1 can positively regulate CPC recruitment in mitosis.     

Mitotic phosphorylation of the peripheral Golgi protein Nir2 by Cdk1 provides a docking mechanism for Plk1 and affects cytokinesis completion

The rearrangement of the Golgi apparatus during mitosis is regulated by several protein kinases, including Cdk1 and Plk1. Several peripheral Golgi proteins that dissociate from the Golgi during mitosis are implicated in regulation of cytokinesis or chromosome segregation, thereby coordinating mitotic and cytokinetic events to Golgi rearrangement. Here we show that, at the onset of mitosis, Cdk1 phosphorylates the peripheral Golgi protein Nir2 at multiple sites; of these, S382 is the most prominent. Phosphorylation of Nir2 by Cdk1 facilitates its dissociation from the Golgi apparatus, and phospho-Nir2(pS382) is localized in the cleavage furrow and midbody during cytokinesis. Mitotic phosphorylation of Nir2 is required for docking of the phospho-Ser/Thr binding module, the Polo box domain of Plk1, and overexpression of a Nir2 mutant, which fails to interact with Plk1, affects the completion of cytokinesis. These results demonstrate a mechanism for coordinating mitotic and cytokinetic events with Golgi rearrangement during cell division.     

Plk1 phosphorylation of TRF1 is essential for its binding to telomeres

In a search for Polo-like kinase 1 (Plk1) interaction proteins, we have identified TRF1 (telomeric repeat binding factor 1) as a potential Plk1 target. In this communication we report further characterization of the interaction. We show that Plk1 associates with TRF1, and Plk1 phosphorylates TRF1 at Ser-435 in vivo. Moreover, Cdk1, serving as a priming kinase, phosphorylates TRF1 to generate a docking site for Plk1 toward TRF1. In the presence of nocodazole, ectopic expression of wild type TRF1 but not TRF1 with alanine mutation in the Plk1 phosphorylation site induces apoptosis in cells containing short telomeres but not in cells containing long telomeres. Unexpectedly, down-regulation of TRF1 by RNA interference affects cell proliferation and results in obvious apoptosis in cells with short telomeres but not in cells with long telomeres. Importantly, we observe that telomeric DNA binding ability of TRF1 is cell cycle-regulated and reaches a peak during mitosis. Upon phosphorylation by Plk1 in vivo and in vitro, the ability of TRF1 to bind telomeric DNA is dramatically increased. These results demonstrate that Plk1 interacts with and phosphorylates TRF1 and suggest that Plk1-mediated phosphorylation is involved in both TRF1 overexpression-induced apoptosis and its telomeric DNA binding ability.     

Dynamic regulation of Emi2 by Emi2-bound Cdk1/Plk1/CK1 and PP2A-B56 in meiotic arrest of Xenopus eggs

In vertebrates, unfertilized eggs are arrested at metaphase of meiosis II by Mos and Emi2, an inhibitor of the APC/C ubiquitin ligase. In Xenopus, Cdk1 phosphorylates Emi2 and both destabilizes and inactivates it, whereas Mos recruits PP2A phosphatase to antagonize the Cdk1 phosphorylation. However, how Cdk1 phosphorylation inhibits Emi2 is largely unknown. Here we show that multiple N-terminal Cdk1 phosphorylation motifs bind cyclin B1-Cdk1 itself, Plk1, and CK1δ/ε to inhibit Emi2. Plk1, after rebinding to other sites by self-priming phosphorylation, partially destabilizes Emi2. Cdk1 and CK1δ/ε sequentially phosphorylate the C-terminal APC/C-docking site, thereby cooperatively inhibiting Emi2 from binding the APC/C. In the presence of Mos, however, PP2A-B56β/ε bind to Emi2 and keep dephosphorylating it, particularly at the APC/C-docking site. Thus, Emi2 stability and activity are dynamically regulated by Emi2-bound multiple kinases and PP2A phosphatase. Our data also suggest a general role for Cdk1 substrate phosphorylation motifs in M phase regulation.     

Tex14, a Plk1-regulated protein, is required for kinetochore-microtubule attachment and regulation of the spindle assembly checkpoint

Proper assembly of kinetochores (KTs) during mitosis is required for bipolar attachment of spindle microtubules (MTs) and the accumulation of spindle assembly checkpoint (SAC) components. Here we show that testis-expressed protein 14 (Tex14), which has been implicated in midbody function, is recruited to KTs by Plk1 in a Cdk1-dependent manner during early mitosis. Exclusion of Tex14 from kinetochores results in an inability to efficiently localize outer KT components, impaired KT-MT attachment, chromosome congression defects, and whole-chromosome instability. In addition, we demonstrate that phosphorylation of Tex14 by Plk1 during metaphase promotes APC(Cdc20)-mediated Tex14 degradation. Inhibition of this phosphorylation event causes retention of Tex14 at KTs and results in delayed metaphase-to-anaphase transition and chromosome segregation defects. Our findings identify Tex14 as an important mediator of KT structure and function and the fidelity of chromosome separation.     

Cdk1 and Plk1 mediate a CLASP2 phospho-switch that stabilizes kinetochore–microtubule attachments

Accurate chromosome segregation during mitosis relies on a dynamic kinetochore (KT)–microtubule (MT) interface that switches from a labile to a stable condition in response to correct MT attachments. This transition is essential to satisfy the spindle-assembly checkpoint (SAC) and couple MT-generated force with chromosome movements, but the underlying regulatory mechanism remains unclear. In this study, we show that during mitosis the MT- and KT-associated protein CLASP2 is progressively and distinctively phosphorylated by Cdk1 and Plk1 kinases, concomitant with the establishment of KT–MT attachments. CLASP2 S1234 was phosphorylated by Cdk1, which primed CLASP2 for association with Plk1. Plk1 recruitment to KTs was enhanced by CLASP2 phosphorylation on S1234. This was specifically required to stabilize KT–MT attachments important for chromosome alignment and to coordinate KT and non-KT MT dynamics necessary to maintain spindle bipolarity. CLASP2 C-terminal phosphorylation by Plk1 was also required for chromosome alignment and timely satisfaction of the SAC. We propose that Cdk1 and Plk1 mediate a fine CLASP2 “phospho-switch” that temporally regulates KT–MT attachment stability.     

Downregulation of PP2A(Cdc55) phosphatase by separase initiates mitotic exit in budding yeast

After anaphase, the high mitotic cyclin-dependent kinase (Cdk) activity is downregulated to promote exit from mitosis. To this end, in the budding yeast S. cerevisiae, the Cdk counteracting phosphatase Cdc14 is activated. In metaphase, Cdc14 is kept inactive in the nucleolus by its inhibitor Net1. During anaphase, Cdk- and Polo-dependent phosphorylation of Net1 is thought to release active Cdc14. How Net1 is phosphorylated specifically in anaphase, when mitotic kinase activity starts to decline, has remained unexplained. Here, we show that PP2A(Cdc55) phosphatase keeps Net1 underphosphorylated in metaphase. The sister chromatid-separating protease separase, activated at anaphase onset, interacts with and downregulates PP2A(Cdc55), thereby facilitating Cdk-dependent Net1 phosphorylation. PP2A(Cdc55) downregulation also promotes phosphorylation of Bfa1, contributing to activation of the "mitotic exit network" that sustains Cdc14 as Cdk activity declines. These findings allow us to present a new quantitative model for mitotic exit in budding yeast.     

Cdk1 phosphorylates SPAT-1/Bora to trigger PLK-1 activation and drive mitotic entry in C. elegans embryos

The molecular mechanisms governing mitotic entry during animal development are incompletely understood. Here, we show that the mitotic kinase CDK-1 phosphorylates Suppressor of Par-Two 1 (SPAT-1)/Bora to regulate its interaction with PLK-1 and to trigger mitotic entry in early Caenorhabditis elegans embryos. Embryos expressing a SPAT-1 version that is nonphosphorylatable by CDK-1 and that is defective in PLK-1 binding in vitro present delays in mitotic entry, mimicking embryos lacking SPAT-1 or PLK-1 functions. We further show that phospho–SPAT-1 activates PLK-1 by triggering phosphorylation on its activator T loop in vitro by Aurora A. Likewise, we show that phosphorylation of human Bora by Cdk1 promotes phosphorylation of human Plk1 by Aurora A, suggesting that this mechanism is conserved in humans. Our results suggest that CDK-1 activates PLK-1 via SPAT-1 phosphorylation to promote entry into mitosis. We propose the existence of a positive feedback loop that connects Cdk1 and Plk1 activation to ensure a robust control of mitotic entry and cell division timing.