Normal Mitotic Progression Research Articles

The Cul3/Klhdc5 E3 Ligase Regulates p60/Katanin and Is Required for Normal Mitosis in Mammalian Cells

The proper regulation of factors involved in mitosis is crucial to ensure normal cell division. Levels and activities of proteins are regulated in many ways, one of which is ubiquitin-mediated protein degradation. E3 ubiquitin ligases are involved in targeting specific substrates for degradation by facilitating their ubiquitination. In seeking to elucidate additional biological roles for Cul3 we performed a two-hybrid screen and identified Ctb9/KLHDC5 as a Cul3-interacting protein. Overexpression of Ctb9/KLHDC5 resulted in an increase in microtubule density as well as persistent microtubule bridges between post-mitotic cells. Conversely, down-regulation of Ctb9/KLHDC5 showed a pronounced reduction in microtubule density. Based on these observations, we examined the interactions between Cul3, Ctb9/KLHDC5, and the microtubule-severing protein, p60/katanin. Here we show that p60/katanin interacts with a complex consisting of Cul3 and Ctb9/KLHDC5, which results in ubiquitin laddering of p60/katanin. Also, Cul3-deficient cells or Ctb9/KLHDC5-deficient cells show an increase in p60/katanin levels, indicating that Cul3/Ctb9/KLHDC5 is required for efficient p60/katanin removal. We demonstrate a novel regulatory mechanism for p60/katanin that occurs at the level of targeted proteolysis to allow normal mitotic progression in mammalian cells.

Plk1- and β-TrCP–dependent degradation of Bora controls mitotic progression

Through a convergence of functional genomic and proteomic studies, we identify Bora as a previously unknown cell cycle protein that interacts with the Plk1 kinase and the SCF–β-TrCP ubiquitin ligase. We show that the Bora protein peaks in G2 and is degraded by proteasomes in mitosis. Proteolysis of Bora requires the Plk1 kinase activity and is mediated by SCF–β-TrCP. Plk1 phosphorylates a conserved DSGxxT degron in Bora and promotes its interaction with β-TrCP. Mutations in this degron stabilize Bora. Expression of a nondegradable Bora variant prolongs the metaphase and delays anaphase onset, indicating a physiological requirement of Bora degradation. Interestingly, the activity of Bora is also required for normal mitotic progression, as knockdown of Bora activates the spindle checkpoint and delays sister chromatid segregation. Mechanistically, Bora regulates spindle stability and microtubule polymerization and promotes tension across sister kinetochores during mitosis. We conclude that tight regulation of the Bora protein by its synthesis and degradation is critical for cell cycle progression.

PAR proteins direct asymmetry of the cell cycle regulators Polo-like kinase and Cdc25.

Cell cycle lengths vary widely among different cells within an animal, yet mechanisms of cell cycle length regulation are poorly understood. In the Caenorhabditis elegans embryo, the first cell division produces two cells with different cell cycle lengths, which are dependent on the conserved partitioning-defective (PAR) polarity proteins. We show that two key cell cycle regulators, the Polo-like kinase PLK-1 and the cyclin-dependent kinase phosphatase CDC-25.1, are asymmetrically distributed in early embryos. PLK-1 shows anterior cytoplasmic enrichment and CDC-25.1 shows PLK-1–dependent enrichment in the anterior nucleus. Both proteins are required for normal mitotic progression. Furthermore, these asymmetries are controlled by PAR proteins and the muscle excess (MEX) proteins MEX-5/MEX-6, and the latter is linked to protein degradation. Our results support a model whereby the PAR and MEX-5/MEX-6 proteins asymmetrically control PLK-1 levels, which asymmetrically regulates CDC-25.1 to promote differences in cell cycle lengths. We suggest that control of Plk1 and Cdc25 may be relevant to regulation of cell cycle length in other developmental contexts.

Loss of ATRX leads to chromosome cohesion and congression defects

αThalassemia/mental retardation X linked (ATRX) is a switch/sucrose nonfermenting-type ATPase localized at pericentromeric heterochromatin in mouse and human cells. Human ATRX mutations give rise to mental retardation syndromes characterized by developmental delay, facial dysmorphisms, cognitive deficits, and microcephaly and the loss of ATRX in the mouse brain leads to reduced cortical size. We find that ATRX is required for normal mitotic progression in human cultured cells and in neuroprogenitors. Using live cell imaging, we show that the transition from prometaphase to metaphase is prolonged in ATRX-depleted cells and is accompanied by defective sister chromatid cohesion and congression at the metaphase plate. We also demonstrate that loss of ATRX in the embryonic mouse brain induces mitotic defects in neuroprogenitors in vivo with evidence of abnormal chromosome congression and segregation. These findings reveal that ATRX contributes to chromosome dynamics during mitosis and provide a possible cellular explanation for reduced cortical size and abnormal brain development associated with ATRX deficiency.

A role for IκB kinase 2 in bipolar spindle assembly

IkappaB kinase 2 (IKK2 or IKKbeta) is a component of the IKK complex that coordinates the cellular response to a diverse set of extracellular stimuli, including cytokines, microbial infection, and stress. In response to an external stimulus, the complex is activated, resulting in the phosphorylation and subsequent proteasome-mediated degradation of IkappaB proteins. This event triggers the nuclear import of the NF-kappaB transcription factor, which activates the transcription of genes that regulate a variety of fundamental biological processes, including immune response, cell survival, and development. Here, we define an essential role for IKK2 in normal mitotic progression and the maintenance of spindle bipolarity. Chemical and genetic perturbation of IKK2 promotes the formation of multipolar spindles and chromosome missegregation. Depletion of IKK2 results in the deregulation of Aurora A protein stability and coincident hyperactivation of a putative Aurora A substrate, the mitotic motor KIF11. These data support a function for IKK2 as an antagonist of Aurora A signaling during mitosis. Additionally, our results indicate a direct role for IKK2 in the maintenance of genome stability and underscore the potential for oncogenic consequences in targeting this kinase for therapeutic intervention.

Histone deacetylase inhibitors induce mitotic slippage

Chromosomal passenger proteins have emerged as key players in the regulation of mitosis and cytokinesis. Histone deacetylase inhibitors (HDACi) are a class of anticancer drugs that induce aberrant mitosis and can overcome the spindle assembly checkpoint. Here, we investigate the mechanism by which HDACi disrupt normal mitotic progression and checkpoint function. We demonstrate that HDACi treatment temporarily delays mitotic progression through prometaphase due to activation of the spindle assembly checkpoint. Despite failing to congress chromosomes to the metaphase plate, cells aberrantly segregate their chromosomes and exit mitosis to undergo a failed cytokinesis. We show that this premature exit from mitosis is a form of mitotic slippage. Chromosomal passenger proteins fail to accumulate at the centromere following HDACi treatment. This results in inadequate concentrations of chromosomal passenger proteins at the centromere, which are insufficient to regulate the phosphorylation of the kinetochore checkpoint component BubR1, and an inability to maintain the mitotic arrest. Thus, the centromeric accumulation of chromosomal passenger complex components is critical for regulating kinetochore but not centromeric processes, and failure of this accumulation underlies the HDACi-induced mitotic slippage.

Activation of Mps1 Promotes Transforming Growth Factor-β-independent Smad Signaling

The primary intracellular mediators of TGF-beta signaling are the Smad proteins. Phosphorylation of R-Smad at the C-terminal SSXS motif by the activated TGF-beta type I receptor kinase triggers a conformation change in R-Smad and facilitates complex formation between R-Smad and Smad4, which shuttle into the nucleus where they interact with DNA and other transcription factors to regulate gene expression. In an attempt to identify proteins interacting with activated Smad signaling complex, we discovered that Mps1, a protein kinase that plays important roles in normal mitotic progression and mitotic checkpoint signaling, co-purifies with this complex. We demonstrated that Smad2 and Smad3 but not Smad4 are substrates of Mps1 in vitro and in vivo. Mps1 phosphorylates Smad2 and Smad3 at the SSXS motif in their C-terminal regions in vitro and in vivo. Disruption of microtubule networks by nocodazole activates Mps1 and promotes TGF-beta-independent activation of Smad signaling. We found that Mps1 is involved in turning on Smad signaling by phosphorylating R-Smads. Our results reveal a novel functional link between Mps1 and Smads in a non-canonical Smad signaling pathway.

Localization of Human TACC3 to Mitotic Spindles Is Mediated by Phosphorylation on Ser558 by Aurora A: A Novel Pharmacodynamic Method for Measuring Aurora A Activity

Aurora A is a serine/threonine protein kinase essential for normal mitotic progression. Aberrant increased expression of Aurora A, which occurs frequently in human cancers, results in abnormal mitoses leading to chromosome instability and possibly tumorigenesis. Consequently, Aurora A has received considerable attention as a potential target for anticancer therapeutic intervention. Aurora A coordinates several essential mitotic activities through phosphorylation of a variety of proteins, including TACC3, which modulates microtubule stabilization of the mitotic spindle. Recent studies identified a conserved serine in Xenopus (Ser(626)) and Drosophila (Ser(863)) TACC3 orthologues that is phosphorylated by Aurora A. We show that this conserved serine on human TACC3 (Ser(558)) is also phosphorylated by Aurora A. Moreover, phosphorylation of TACC3 by Aurora A in human cells is essential for its proper localization to centrosomes and proximal mitotic spindles. Inhibition of Aurora A with the selective small molecule inhibitor MLN8054 in cultured human tumor cells resulted in mislocalization of TACC3 away from mitotic spindles in a concentration-dependent manner. Furthermore, oral administration of MLN8054 to nude mice bearing HCT-116 human tumor xenografts caused a dose-dependent mislocalization of TACC3 away from spindle poles that correlated with tumor growth inhibition. As TACC3 localization to mitotic spindles depends on Aurora A-mediated phosphorylation, quantifying TACC3 mislocalization represents a novel pharmacodynamic approach for measuring Aurora A activity in cancer patients treated with inhibitors of Aurora A kinase.

CKAP2 Is a Spindle-associated Protein Degraded by APC/C-Cdh1 during Mitotic Exit

We reported here an efficient and generally applicable genomic analysis that uses transcriptional profiling to identify candidate substrates of regulatory enzymes, such as kinases and ubiquitin ligases. We applied this strategy to the anaphase-promoting complex/cyclosome (APC/C), a ubiquitin ligase that controls sister chromatid separation and exit from mitosis. We found that a microtubule-associated protein, CKAP2, is a substrate of APC/C and demonstrated that ubiquitination and degradation of CKAP2 in vitro require a KEN-box and is mediated by Cdh1, an activator of APC/C. We showed that the levels of CKAP2 fluctuated across the cell cycle in culture cells, high in mitosis and low during mitotic exit. Overexpression of Cdh1 reduced the levels of CKAP2 in a KEN-box-dependent manner, while knockdown of Cdh1 increased the half-life of CKAP2. CKAP2 associated with centrosomal microtubules in late G(2), but only after the separation of the duplicated centrosomes. During mitosis, CKAP2 associated with spindle poles and with spindle microtubules from prophase through anaphase and dis-appeared from microtubules during cytokinesis. The function of CKAP2 during mitosis does not seem essential, as efficient knockdown of CKAP2 neither altered the cell cycle distribution of the cells, nor generated observable mitotic defects. On the other hand, ectopic expression of either the wild-type or a non-degradable CKAP2 led to a mitotic arrest with monopolar spindles containing highly bundled microtubules. We concluded that CKAP2 is a physiological substrate of APC/C during mitotic exit and that a tight regulation of the CKAP2 protein level is critical for the normal mitotic progression.

The DNA helicase ChlR1 is required for sister chromatid cohesion in mammalian cells

It has recently been suggested that the Saccharomyces cerevisiae protein Chl1p plays a role in cohesion establishment. Here, we show that the human ATP-dependent DNA helicase ChlR1 is required for sister chromatid cohesion in mammalian cells. Localization studies show that ChlR1 diffusely coats mitotic chromatin in prophase and then translocates from the chromatids to concentrate at the spindle poles during the transition to metaphase. Depletion of ChlR1 protein by RNA interference results in mitotic failure with replicated chromosomes failing to segregate after a pro-metaphase arrest. We show that depletion also results in abnormal sister chromatid cohesion, determined by increased separation of chromatid pairs at the centromere. Furthermore, biochemical studies show that ChlR1 is in complex with cohesin factors Scc1, Smc1 and Smc3. We conclude that human ChlR1 is required for sister chromatid cohesion and, hence, normal mitotic progression. These functions are important to maintain genetic fidelity.

B-Raf Is Critical For MAPK Activation during Mitosis and Is Regulated in an M Phase-dependent Manner in Xenopus Egg Extracts

Activation of the MAPK cascade during mitosis is critical for spindle assembly and normal mitotic progression. The underlying regulatory mechanisms that control activation of the MEK/MAPK cascade during mitosis are poorly understood. Here we purified and characterized the MEK kinase activity present in Xenopus M phase-arrested egg extracts. Our results show that B-Raf was the critical MEK kinase required for M phase activation of the MAPK pathway. Consistent with this, B-Raf was activated and underwent hyperphosphorylation in an M phase-dependent manner. Interestingly B-Raf hyperphosphorylation at mitosis occurred, at least in part, as a consequence of a feedback loop involving MAPK-mediated phosphorylation within a conserved C-terminal SPKTP motif. The kinase activity of a B-Raf mutant defective at both phosphorylation sites was substantially greater than its wild type counterpart when incubated in Xenopus M phase egg extracts. Furthermore suppression of MAPK feedback at mitosis enhanced B-Raf activity, whereas constitutive activation of MAPK at mitosis strongly suppressed B-Raf activity. These results suggest that feedback phosphorylation by MAPK negatively regulates B-Raf activity at mitosis. Collectively our data demonstrate for the first time a role for B-Raf at mitosis and provide new insight into understanding the regulation and function of B-Raf during cell proliferation.

TheDrosophilaBub3 protein is required for the mitotic checkpoint and for normal accumulation of cyclins during G2 and early stages of mitosis

During mitosis, a checkpoint mechanism delays metaphase-anaphase transition in the presence of unattached and/or unaligned chromosomes. This delay is achieved through inhibition of the anaphase promoting complex/cyclosome (APC/C) preventing sister chromatid separation and cyclin degradation. In the present study, we show that Bub3 is an essential protein required during normal mitotic progression to prevent premature sister chromatid separation, missegreation and aneuploidy. We also found that Bub3 is required during G2 and early stages of mitosis to promote normal mitotic entry. We show that loss of Bub3 function by mutation or RNAi depletion causes cells to progress slowly through prophase, a delay that appears to result from a failure to accumulate mitotic cyclins A and B. Defective accumulation of mitotic cyclins results from inappropriate APC/C activity, as mutations in the gene encoding the APC/C subunit cdc27 partially rescue this phenotype. Furthermore, analysis of mitotic progression in cells carrying mutations for cdc27 and bub3 suggest the existence of differentially activated APC/C complexes. Altogether, our data support the hypothesis that the mitotic checkpoint protein Bub3 is also required to regulate entry and progression through early stages of mitosis.

Human Mps1 protein kinase is required for centrosome duplication and normal mitotic progression.

The mitotic spindle is essential for the maintenance of genetic stability, and in budding yeast its assembly and function depend on the Mps1 protein kinase. Mps1p is required for centrosome duplication and the spindle checkpoint. Several recent reports demonstrate that vertebrate Mps1 proteins regulate the spindle checkpoint, but reports conflict regarding their role in centrosome duplication. Here we provide multiple lines of evidence that the human Mps1 protein (hMps1) is required for centrosome duplication. A recently described rabbit polyclonal antibody against hMps1 specifically recognizes centrosomes in a variety of human cell types. Overexpression of a dominant-negative version of hMps1 (hMps1KD) can prevent centrosome duplication in a variety of cell types, and active hMps1 accelerates centrosome reduplication in U2OS cells. Finally, we demonstrate that disruption of hMps1 function with pools of hMps1-specific small interfering RNAs causes a pleiotropic phenotype resulting from the combination of severe mitotic abnormalities and failures in centrosome duplication. This approach demonstrates that hMps1 is required for centrosome duplication and for the normal progression of mitosis, and suggests that the threshold level of hMps1 function required for centrosome duplication is lower than that required for hMps1 mitotic functions.

Cell damage and reactive oxygen species production induced by fluorescence microscopy: effect on mitosis and guidelines for non-invasive fluorescence microscopy.

The green fluorescent protein (GFP) and other intrinsically fluorescent proteins (IFPs) are popular reporters because they allow visualization of cellular constituents in living specimens. IFP technology makes it possible to view dynamic processes in living cells, but extended observation, using fluorescence microscopy (both wide-field and confocal), can result in significant light energy exposure. Therefore, it is possible that cells experience light-induced damage that alters cell physiology and confounds observations. To understand the impact that extended viewing has on cells, we obtained quantitative information about the effect of light energy dose and observation conditions on tobacco BY-2 cell physiology. Our results show a non-linear relationship between the excitation light intensity and mitotic arrest, and the frequency of mitotic arrest is dependent on the presence of an IFP that absorbs the excitation light. Moreover, fluorescence microscopy induces the production of reactive oxygen species (ROS), as assayed using BY-2 cells loaded with oxidation-sensitive dyes, and the level of ROS production increases if the cells express an IFP that absorbs the excitation light energy. The dye oxidation follows sigmoidal kinetics and is reversible if the cells are exposed to low irradiation levels. In addition, the dye oxidation rate shows a non-linear relationship to the excitation light intensity, and a good correlation exists between photobleaching, mitotic arrest, and dye oxidation. The data highlight the importance of ROS scavenging for normal mitotic progression, and provide a reference for judiciously choosing conditions that avoid photobleaching that can lead to ROS accumulation and physiological damage.

Replication protein A is sequentially phosphorylated during meiosis.

Phosphorylation of the cellular single-stranded DNA-binding protein, replication protein A (RPA), occurs during normal mitotic cell cycle progression and also in response to genotoxic stress. In budding yeast, these reactions require the ATM homolog Mec1, a central regulator of the DNA replication and DNA damage checkpoint responses. We now demonstrate that the middle subunit of yeast RPA (Rfa2) becomes phosphorylated in two discrete steps during meiosis. Primary Rfa2 phosphorylation occurs early in meiotic progression and is independent of DNA replication, recombination and Mec1. In contrast, secondary Rfa2 phosphorylation is activated upon initiation of recombination and requires Mec1. While the primary Rfa2 phosphoisomer is detectable throughout most of meiosis, the secondary Rfa2 phosphoisomer is only transiently generated and begins to disappear soon after recombination is complete. Extensive secondary Rfa2 phosphorylation is observed in a recombination mutant defective for the pachytene checkpoint, indicating that Mec1-dependent Rfa2 phosphorylation does not function to maintain meiotic delay in response to DNA double-strand breaks. Our results suggest that Mec1-dependent RPA phosphorylation could be involved in regulating recombination rather than cell cycle or meiotic progression.

Inhibition of serine/threonine-specific protein phosphatases causes premature activation of cdc2MsF kinase at G2/M transition and early mitotic microtubule organisation in alfalfa.

Reversible phosphorylation of serine/threonine residues of cell cycle-regulatory proteins is one of the key molecular mechanisms controlling eukaryotic cell division. In plants, the protein kinase partners (i.e. p34cdc2/CDC28-related kinases) have been extensively studied, while the role of counter-acting protein phosphatases is less well understood. We used endothall (ET) as a cell-permeable inhibitor of serine/threonine-specific protein phosphatases to alter cytological and biochemical characteristics of cell division in cultured alfalfa cells. A high concentration of ET (10 and 50 microM) inhibited both protein phosphatases 1 and 2 (PP1 and PP2A), while a low concentration (1 microM) of ET-treatment primarily reduced the PP2A activity. High concentrations of the inhibitor increased the frequency of hypercondensed early and late prophase chromosomes that could not enter metaphase. In contrast, a low concentration of ET did not interfere with chromosomal events but caused significant alterations in the organisation of microtubules. Exposure of cells to 1 microM ET resulted in disturbance of preprophase band formation, increase in the number of nuclei with prophase microtubule assembly, premature polarisation of the spindle, and abnormal phragmoplast maturation. Under the same conditions, the ET-treated cells exhibited an early increase in cdc2MsF kinase activity. These results suggest that PP2A contributes to the control of mitotic kinase activities and microtubule organisation. Normal chromosome condensation and mitotic progression are dependent on both PP1 and PP2A activities. The presented data support the functional role of protein phosphatases in the co-ordination of chromosomal and microtubule events in dividing plant cells.

New Role for Zyxin

Although the serine-threonine kinase h-warts/LATS1 has been implicated in regulating mitosis, its cellular targets have remained elusive. Hirota et al . have now found that it binds to zyxin, a regulator of actin assembly. Interaction required two of the three LIM protein interaction domains of zyxin. During mitosis, a fraction of zyxin, which normally resides at focal adhesion plaques of adherent cells during interphase, localized to the mitotic spindle and poles. Its colocalization at the mitotic apparatus with h-warts/LATS1 required that zyxin be phosphorylated, possibly by the cdc2 kinase. Disruption of the interaction between h-warts/LATS1 and zyxin delayed normal mitotic progression. Hence, zyxin may also regulate actin dynamics during cell division. Hirota, T., Morisaki, T., Nishiyama, Y., Matumoto, T., Tada, K., Hara, T., Masuko, N., Inagaki, M., Hatakeyama, K., and Saya, H. (2000) Zyxin, a regulator of actin filament assembly, targets the mitotic apparatus by interacting with h-warts/LATS1 tumor suppressor. J. Cell Biol. 149 : 1073-1086. [Abstract] [Full Text]

Pleiotropic cell-division defects and apoptosis induced by interference with survivin function.

Here we investigate the role of the control of apoptosis in normal cell division. We show that interference with the expression or function of the apoptosis inhibitor survivin causes caspase-dependent cell death in the G2/M phase of the cell cycle, and a cell-division defect characterized by centrosome dysregulation, multipolar mitotic spindles and multinucleated, polyploid cells. Use of a dominant-negative survivin mutant or antisense survivin complementary DNA disrupts a supramolecular assembly of survivin, caspase-3 and the cyclin-dependent-kinase inhibitor p21Waf1/Cip1 within centrosomes, and results in caspase-dependent cleavage of p21. Polyploidy induced by survivin antagonists is accentuated in p21-deficient cells, and corrected by exogenous expression of p21. These findings show that control of apoptosis and preservation of p21 integrity within centrosomes by survivin are required for normal mitotic progression.

Human BUBR1 is a mitotic checkpoint kinase that monitors CENP-E functions at kinetochores and binds the cyclosome/APC.

Human cells express two kinases that are related to the yeast mitotic checkpoint kinase BUB1. hBUB1 and hBUBR1 bind to kinetochores where they are postulated to be components of the mitotic checkpoint that monitors kinetochore activities to determine if chromosomes have achieved alignment at the spindle equator (Jablonski, S.A., G.K.T. Chan, C.A. Cooke, W.C. Earnshaw, and T.J. Yen. 1998. Chromosoma. 107:386-396). In support of this, hBUB1 and the homologous mouse BUB1 have been shown to be important for the mitotic checkpoint (Cahill, D.P., C. Lengauer, J. Yu, G.J. Riggins, J.K. Willson, S.D. Markowitz, K.W. Kinzler, and B. Vogelstein. 1998. Nature. 392:300-303; Taylor, S.S., and F. McKeon. 1997. Cell. 89:727-735). We now demonstrate that hBUBR1 is also an essential component of the mitotic checkpoint. hBUBR1 is required by cells that are exposed to microtubule inhibitors to arrest in mitosis. Additionally, hBUBR1 is essential for normal mitotic progression as it prevents cells from prematurely entering anaphase. We establish that one of hBUBR1's checkpoint functions is to monitor kinetochore activities that depend on the kinetochore motor CENP-E. hBUBR1 is expressed throughout the cell cycle, but its kinase activity is detected after cells have entered mitosis. hBUBR1 kinase activity was rapidly stimulated when the spindle was disrupted in mitotic cells. Finally, hBUBR1 was associated with the cyclosome/anaphase-promoting complex (APC) in mitotically arrested cells but not in interphase cells. The combined data indicate that hBUBR1 can potentially provide two checkpoint functions by monitoring CENP-E-dependent activities at the kinetochore and regulating cyclosome/APC activity.

Requirement for MAPK activation for normal mitotic progression in Xenopus egg extracts.

The p42 mitogen-activated protein kinase (MAPK) is required for progression through meiotic M phase in Xenopus oocytes. This report examines whether it also plays a role in normal mitotic progression. MAPK was transiently activated during mitosis in cycling Xenopus egg extracts after activation of the cyclin-dependent kinase Cdc2-cyclin B. Interference with MAPK activation by immunodepletion of its activator MEK, or by addition of the MEK inhibitor PD98059, caused precocious termination of mitosis and interfered with production of normal mitotic microtubules. Sustained activation of MAPK arrested extracts in mitosis in the absence of active Cdc2-cyclin B. These findings identify a role for MEK and MAPK in maintaining the mitotic state.