Mitotic kinases as regulators of cell division and its checkpoints

CSC-1: a subunit of the Aurora B kinase complex that binds to the survivin-like protein BIR-1 and the incenp-like protein ICP-1

The Aurora B kinase complex is a critical regulator of chromosome segregation and cytokinesis. In Caenorhabditis elegans, AIR-2 (Aurora B) function requires ICP-1 (Incenp) and BIR-1 (Survivin). In various systems, Aurora B binds to orthologues of these proteins. Through genetic analysis, we have identified a new subunit of the Aurora B kinase complex, CSC-1. C. elegans embryos depleted of CSC-1, AIR-2, ICP-1, or BIR-1 have identical phenotypes. CSC-1, BIR-1, and ICP-1 are interdependent for their localization, and all are required for AIR-2 localization. In vitro, CSC-1 binds directly to BIR-1. The CSC-1/BIR-1 complex, but not the individual subunits, associates with ICP-1. CSC-1 associates with ICP-1, BIR-1, and AIR-2 in vivo. ICP-1 dramatically stimulates AIR-2 kinase activity. This activity is not stimulated by CSC-1/BIR-1, suggesting that these two subunits function as targeting subunits for AIR-2 kinase.

The small molecule Hesperadin reveals a role for Aurora B in correcting kinetochore-microtubule attachment and in maintaining the spindle assembly checkpoint

The proper segregation of sister chromatids in mitosis depends on bipolar attachment of all chromosomes to the mitotic spindle. We have identified the small molecule Hesperadin as an inhibitor of chromosome alignment and segregation. Our data imply that Hesperadin causes this phenotype by inhibiting the function of the mitotic kinase Aurora B. Mammalian cells treated with Hesperadin enter anaphase in the presence of numerous monooriented chromosomes, many of which may have both sister kinetochores attached to one spindle pole (syntelic attachment). Hesperadin also causes cells arrested by taxol or monastrol to enter anaphase within <1 h, whereas cells in nocodazole stay arrested for 3-5 h. Together, our data suggest that Aurora B is required to generate unattached kinetochores on monooriented chromosomes, which in turn could promote bipolar attachment as well as maintain checkpoint signaling.

A novel mechanism for activation of the protein kinase Aurora A

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

Regulation of cell division: stop the SIN!

A novel mechanism, centered on the Polo-like kinase Plo1p and Dma1p - a protein with a RING finger and an FHA-domain - prevents cytokinesis as long as the spindle checkpoint is active.

Function and regulation of Aurora/Ipl1p kinase family in cell division

During mitosis, the parent cell distributes its genetic materials equally into two daughter cells through chromosome segregation, a complex movements orchestrated by mitotic kinases and its effector proteins. Faithful chromosome segregation and cytokinesis ensure that each daughter cell receives a full copy of genetic materials of parent cell. Defects in these processes can lead to aneuploidy or polyploidy. Aurora/Ipl1p family, a class of conserved serine/threonine kinases, plays key roles in chromosome segregation and cytokinesis. This article highlights the function and regulation of Aurora/Ipl1p family in mitosis and provides potential links between aberrant regulation of Aurora/Ipl1p kinases and pathogenesis of human cancer.

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.

The effect of activation of Mammalian oocytes on remodeling of donor nuclei after nuclear transfer

Activation of bovine oocytes by experimental procedures that closely mimic normal fertilization is essential both for intracytoplasmic sperm injection and for nuclear transfer (NT). Therefore, with the goal of producing haploid activated oocytes, we evaluated whether butyrolactone I and bohemine, either alone or in combination with ionomycin, are able to activate young matured mammalian oocytes. Furthermore, the effect on the patterns of DNA synthesis after pronuclear formation as well as changes in histone H1 kinase and MAP kinase activities during the process of activation were studied. Our results with bohemine show that the specific inhibition of cyclin-dependent kinases (CDKs) in metaphase II bovine oocytes induces parthenogenetic activation in a dose dependent manner (25, 50, and 100 microM, respectively), either alone (3%, 30%, and 50%) or in combination with ionomycin (30%, 70%, and 87.5%). The effect of two activation protocols on nuclear remodeling, DNA synthesis during the first cell cycle, chromosome segregation after first mitosis, and development to blastocyst of embryos produced by somatic nuclear transfer were studied. Pronuclear formation was significantly higher when activation lasted 5 h compared to 3 h for both ethanol-cycloheximide and ionomycin-bohemine treatment. Initiation of DNA synthesis was delayed in ethanol-cycloheximide group, however, after 12-h labeling 100% of embryos synthesized DNA in both groups. Analysis of two-cell embryos with DNA probes for chromosome 6, 7, and 15 by fluorescence in situ hybridization showed that at least 50% of NT embryos were of normal ploidy, independent of the activation protocol.

The appearance of truncated cyclin A2 correlates with differentiation of mouse embryonic stem cells

The presence of a form of cyclin A2 with an N-terminal truncation has recently been reported in various murine cell lines and tissues. The truncated cyclin A2 binds to and activates the cyclin-dependent kinase 2 (CDK2). However, CDK2 bound by the truncated cyclin A2 is located in the cytoplasm in contrast to CDK2 bound to full-length cyclin A2, which is in the nucleus. Here, we show that proliferating mouse embryonic stem cells (ES cells) contain very little truncated cyclin A2 but as the cells are induced to differentiate the amount of truncated cyclin A2 increases. The expression pattern of truncated cyclin A2 was the same in p27(Kip1) -/- differentiating ES cells as in the differentiating wild-type cells. We conclude that p27(Kip1) is not necessary for the proteolytic cleavage that gives rise to the truncated form of cyclin A2 in differentiating ES cells and that this post-translational modification is not a function of the cell density but is correlated with differentiation.

eIF4E/4E-BP dissociation and 4E-BP degradation in the first mitotic division of the sea urchin embryo

The mRNA's cap-binding protein eukaryotic translation initiation factor (eIF)4E is a major target for the regulation of translation initiation. eIF4E activity is controlled by a family of translation inhibitors, the eIF4E-binding proteins (4E-BPs). We have previously shown that a rapid dissociation of 4E-BP from eIF4E is related with the dramatic rise in protein synthesis that occurs following sea urchin fertilization. Here, we demonstrate that 4E-BP is destroyed shortly following fertilization and that 4E-BP degradation is sensitive to rapamycin, suggesting that proteolysis could be a novel means of regulating 4E-BP function. We also show that eIF4E/4E-BP dissociation following fertilization is sensitive to rapamycin. Furthermore, while rapamycin modestly affects global translation rates, the drug strongly inhibits cyclin B de novo synthesis and, consequently, precludes the completion of the first mitotic cleavage. These results demonstrate that, following sea urchin fertilization, cyclin B translation, and thus the onset of mitosis, are regulated by a rapamycin-sensitive pathway. These processes are effected at least in part through eIF4E/4E-BP complex dissociation and 4E-BP degradation.

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.

Proliferation of cancer cells despite CDK2 inhibition

We have investigated the contribution of CDK4 and CDK2 inhibition to G1 arrest in colon cancers following inhibition of the MEK/MAP kinase pathway. CDK4 inhibition is sufficient to cause arrest, but inhibition of CDK2 by p27 Kip1 redistribution or ectopic expression has no effect on proliferation. Likewise, inhibition of CDK2 through expression of dominant-negative (DN) CDK2 or antisense oligonucleotides did not prevent cell proliferation in these cells. We therefore tested whether CDK2 activity is dispensable in other cells. Surprisingly, osteosarcomas and Rb-negative cervical cancers continued to proliferate after depletion of CDK2 through antisense oligonucleotides or small interfering (si) RNA. Here we report of sustained cell proliferation in the absence of CDK2, and we suggest that CDK2 is not a suitable target for cancer therapy.

Biphasic activation of Aurora-A kinase during the meiosis I- meiosis II transition in Xenopus oocytes

Xenopus Aurora-A (also known as Eg2) is a member of the Aurora family of mitotic serine/threonine kinases. In Xenopus oocytes, Aurora-A phosphorylates and activates a cytoplasmic mRNA polyadenylation factor (CPEB) and therefore plays a pivotal role in MOS translation. However, hyperphosphorylation and activation of Aurora-A appear to be dependent on maturation-promoting factor (MPF) activation. To resolve this apparent paradox, we generated a constitutively activated Aurora-A by engineering a myristylation signal at its N terminus. Injection of Myr-Aurora-A mRNA induced germinal vesicle breakdown (GVBD) with the concomitant activation of MOS, mitogen-activated protein kinase, and MPF. Myr-Aurora-A-injected oocytes, however, appeared to arrest in meiosis I with high MPF activity and highly condensed, metaphase-like chromosomes but no organized microtubule spindles. No degradation of CPEB or cyclin B2 was observed following GVBD in Myr-Aurora-A-injected oocytes. In the presence of progesterone, the endogenous Aurora-A became hyperphosphorylated and activated at the time of MPF activation. Following GVBD, Aurora-A was gradually dephosphorylated and inactivated before it was hyperphosphorylated and activated again. This biphasic pattern of Aurora-A activation mirrored that of MPF activation and hence may explain meiosis I arrest by the constitutively activated Myr-Aurora-A.

A Ran signalling pathway mediated by the mitotic kinase Aurora A in spindle assembly

The activated form of Ran (Ran-GTP) stimulates spindle assembly in Xenopus laevis egg extracts, presumably by releasing spindle assembly factors, such as TPX2 (target protein for Xenopus kinesin-like protein 2) and NuMA (nuclear-mitotic apparatus protein) from the inhibitory binding of importin-alpha and -beta. We report here that Ran-GTP stimulates the interaction between TPX2 and the Xenopus Aurora A kinase, Eg2. This interaction causes TPX2 to stimulate both the phosphorylation and the kinase activity of Eg2 in a microtubule-dependent manner. We show that TPX2 and microtubules promote phosphorylation of Eg2 by preventing phosphatase I (PPI)-induced dephosphorylation. Activation of Eg2 by TPX2 and microtubules is inhibited by importin-alpha and -beta, although this inhibition is overcome by Ran-GTP both in the egg extracts and in vitro with purified proteins. As the phosphorylation of Eg2 stimulated by the Ran-GTP-TPX2 pathway is essential for spindle assembly, we hypothesize that the Ran-GTP gradient established by the condensed chromosomes is translated into the Aurora A kinase gradient on the microtubules to regulate spindle assembly and dynamics.

Mad2 phosphorylation regulates its association with Mad1 and the APC/C

Improper attachment of the mitotic spindle to the kinetochores of paired sister chromatids in mitosis is monitored by a checkpoint that leads to an arrest in early metaphase. This arrest requires the inhibitory association of Mad2 with the anaphase promoting complex/cyclosome (APC/C). It is not known how the association of Mad2 with the kinetochore and the APC/C is regulated in mitosis. Here, we demonstrate that human Mad2 is modified through phosphorylation on multiple serine residues in vivo in a cell cycle dependent manner and that only unphosphorylated Mad2 interacts with Mad1 or the APC/C in vivo. A Mad2 mutant containing serine to aspartic acid mutations mimicking the C-terminal phosphorylation events fails to interact with Mad1 or the APC/C and acts as a dominant-negative antagonist of wild-type Mad2. These data suggest that the phosphorylation state of Mad2 regulates its checkpoint activity by modulating its association with Mad1 and the APC/C.

A role for glycogen synthase kinase-3 in mitotic spindle dynamics and chromosome alignment

Glycogen synthase kinase-3 (GSK-3) is a conserved, multifunctional kinase that is constitutively active in resting cells, and inactivated through phosphorylation by protein kinase B (PKB). We have investigated the temporal and spatial control of GSK-3 phosphorylation during the cell cycle in mammalian cells. We show that GSK-3 is present along the length of spindle microtubules and that a fraction of GSK-3 is phosphorylated during mitosis. Phospho-GSK-3 is abundant at the centrosomes and spindle poles but absent from other areas of the spindle. GSK-3 phosphorylation occurs concomitantly with the appearance of phosphorylated and active PKB at the centrosome, which suggests that PKB is the kinase responsible for phosphorylating and inactivating GSK-3 at the centrosome during mitosis. We demonstrate that lithium and two structurally distinct inhibitors of GSK-3 promote defects in microtubule length and chromosomal alignment during prometaphase. Treated cells contain mono-oriented chromosomes concentrated at the plus ends of astral microtubules, which are longer than in untreated cells. Live microscopy of cells expressing Histone-2B-GFP confirms that the inhibition of GSK-3 suppresses mitotic chromosome movement and leads to a prometaphase-like arrest. We propose that GSK-3 is regulated in a temporal and spatial manner during mitosis and, through controlling microtubule dynamics, plays an important role in chromosomal alignment on the metaphase plate.

The budding yeast Ipl1/Aurora protein kinase regulates mitotic spindle disassembly

Ipl1p is the budding yeast member of the Aurora family of protein kinases, critical regulators of genomic stability that are required for chromosome segregation, the spindle checkpoint, and cytokinesis. Using time-lapse microscopy, we found that Ipl1p also has a function in mitotic spindle disassembly that is separable from its previously identified roles. Ipl1-GFP localizes to kinetochores from G1 to metaphase, transfers to the spindle after metaphase, and accumulates at the spindle midzone late in anaphase. Ipl1p kinase activity increases at anaphase, and ipl1 mutants can stabilize fragile spindles. As the spindle disassembles, Ipl1p follows the plus ends of the depolymerizing spindle microtubules. Many Ipl1p substrates colocalize with Ipl1p to the spindle midzone, identifying additional proteins that may regulate spindle disassembly. We propose that Ipl1p regulates both the kinetochore and interpolar microtubule plus ends to regulate its various mitotic functions.

Rae1 is an essential mitotic checkpoint regulator that cooperates with Bub3 to prevent chromosome missegregation

The WD-repeat proteins Rae1 and Bub3 show extensive sequence homology, indicative of functional similarity. However, previous studies have suggested that Rae1 is involved in the mRNA export pathway and Bub3 in the mitotic checkpoint. To determine the in vivo roles of Rae1 and Bub3 in mammals, we generated knockout mice that have these genes deleted individually or in combination. Here we show that haplo-insufficiency of either Rae1 or Bub3 results in a similar phenotype involving mitotic checkpoint defects and chromosome missegregation. We also show that overexpression of Rae1 can correct for Rae1 haplo-insufficiency and, surprisingly, Bub3 haplo-insufficiency. Rae1-null and Bub3-null mice are embryonic lethal, although cells from these mice did not have a detectable defect in nuclear export of mRNA. Unlike null mice, compound haplo-insufficient Rae1/Bub3 mice are viable. However, cells from these mice exhibit much greater rates of premature sister chromatid separation and chromosome missegregation than single haplo-insufficient cells. Finally, we show that mice with mitotic checkpoint defects are more susceptible to dimethylbenzanthrene-induced tumorigenesis than wild-type mice. Thus, our data demonstrate a novel function for Rae1 and characterize Rae1 and Bub3 as related proteins with essential, overlapping, and cooperating roles in the mitotic checkpoint.

Polo-like kinase 1 (PLK1) is overexpressed in primary colorectal cancers

PLK (polo-like kinase), the human counterpart of polo in Drosophila melanogaster and of CDC5 in Saccharomyces cerevisiae, belongs to a family of serine/threonine kinases. It is intimately involved in spindle formation and chromosome segregation during mitosis. The purpose of this study was to determine whether PLK1 is overexpressed in primary colorectal cancer specimens as compared with normal colon mucosa and to assess its relation to other kinases as a potential new tumor marker. In the present study, immunohistochemical analyses were performed of PLK1 expression in 78 primary colorectal cancers as well as 15 normal colorectal specimens. Furthermore, we examined the relationship between other kinases, Aurora-A and Aurora-C, and PLK1 expression. In normal colon mucosa, some crypt cells showed weakly positive staining for PLK1 in 13 out of 15 cases, the remaining cases being negative. Elevated expression of PLK1 was observed in 57 (73.1%) of the colorectal cancers, statistically significant associations being evident with pT (primary tumor invasion) (P=0.0006, Mann-Whitney U test), pN (regional lymph nodes) (P=0.008, chi2 test) and the Dukes' classification (P=0.0005, Mann-Whitney U test). Mean proliferating cell nuclear antigen-labeling index was 52.3%, with a range of 24.1% to 77.3%. Values for lesions with high and low PLK1 expression were 54.7+/-10.3% (mean+/-SD) and 45.9+/-11.9% (P=0.002, Student's t test). PLK1 was significantly associated with Aurora-A, but PLK1 staining was more diffuse and extensive than for Aurora-A or Aurora-C. Interestingly, PLK1 overexpression was significantly associated with p53 accumulation in colorectal cancers. Our results suggest overexpression of PLK1 might be of pathogenic, prognostic and proliferative importance, so that this kinase might have potential as a new tumor marker for colorectal cancers.

Novel mitosis-specific phosphorylation of histone H3 at Thr11 mediated by Dlk/ZIP kinase

Death-associated protein (DAP)-like kinase (Dlk), also known as Zipper interacting protein (ZIP) kinase, is a nuclear serine/threonine-specific kinase that phosphorylates core histones H3 and H4, and myosine light chain in vitro. It interacts with transcription and splicing factors as well as with pro-apoptotic protein Par-4 suggesting that it participates in multiple cellular processes. To explore the significance of histone phosphorylation by Dlk, we determined the phosphorylation site in H3 and generated phosphospecific antibodies for in vivo analyses. Interestingly, Dlk/ZIP kinase phosphorylated histone H3 at a novel site, Thr11, rather than Ser10, which is characteristic of mitotic chromosomes. Immunoblotting and confocal immunofluorescence analyses demonstrated that phosphorylation of H3 at Thr11 occurred in vivo and was restricted to mitosis as well. It was discernable from prophase to early anaphase and particularly enriched at centromeres. Strikingly, during this time interval, Dlk was associated with centromeres too, as revealed by stable expression of a green fluorescent protein (GFP)-Dlk fusion protein. These findings strongly suggest that Dlk is a centromere-specific histone kinase that might play a role in labeling centromere-specific chromatin for subsequent mitotic processes.

[Oncogenes and mitotic regulators: a change in perspective of our view of neoplastic processes]

Our vision of the cancer cell has dramatically changed since the discovery of proto-oncogenes, whose deregulation was proposed to mimic normal growth signalling. This notion, linking cancer to cell signalling pathways, has progressively led the way to the concept of the mutator phenotype, in which genetic instability plays an essential role in the onset of cancer. This then transformed cancer into a DNA repair disease. However, as foreseen decades ago by cytogeneticists, point mutations are not sufficient to give a full picture of the whole process. As a result, aneuploidy, rather than gene mutation, has been proposed as the explanation for the complex changes observed in cancer cells. The culprits were found among genes involved in the control of the cell division cycle, and work aimed at understanding the regulation of S phase and mitosis have yielded new insights into our understanding of cancer.

Involvement of Aurora A kinase during meiosis I-II transition in Xenopus oocytes

The Aurora kinase family has been involved both in vivo and in vitro in the stability of the metaphase plate and chromosome segregation. However, to date only one member of this family, the protein kinase Aurora B, has been implicated in the regulation of meiotic division in Caenorhabditis elegans. In this species, disruption of Aurora B results in the failure of polar body extrusion. To investigate whether Aurora A is also required in meiosis, we microinjected highly specific alpha-Aurora A antibodies in Xenopus oocytes. We demonstrated that microinjected oocytes fail to extrude the first polar body and are arrested with condensed chromosomes on a typical metaphase I plate, which has not performed its normal 90 degrees rotation. We additionally found that, although the failure of first polar body extrusion observed in alpha-Aurora A-microinjected oocytes is likely mediated by Eg5, the impairment of the metaphase plate rotation does not involve this kinesin-like protein. Surprisingly, although chromosomes remain condensed at a metaphase I stage in alpha-Aurora A-microinjected oocytes, the cytoplasmic cell cycle events progress normally through meiosis until metaphase II arrest. Moreover, these oocytes are able to undergo parthenogenetic activation. We conclude that Aurora A and Eg5 are involved in meiosis I to meiosis II transition in Xenopus oocytes.

Cell cycle control, DNA damage checkpoints and cancer

DNA damage checkpoints are essential control points in the cell cycle ensuring effective damage repair. The loss of checkpoint functions leads to loss of genomic integrity and allows accumulation of genetic damage in the daughter cells. Checkpoint deficiency is one of the main causes of DNA aberrations in cancer.

Time-lapse imaging reveals dynamic relocalization of PP1gamma throughout the mammalian cell cycle

Protein phosphatase 1 (PP1) is a ubiquitous serine/threonine phosphatase that regulates many cellular processes, including cell division. When transiently expressed as fluorescent protein (FP) fusions, the three PP1 isoforms, alpha, beta/delta, and gamma1, are active phosphatases with distinct localization patterns. We report here the establishment and characterization of HeLa cell lines stably expressing either FP-PP1gamma or FP alone. Time-lapse imaging reveals dynamic targeting of FP-PP1gamma to specific sites throughout the cell cycle, contrasting with the diffuse pattern observed for FP alone. FP-PP1gamma shows a nucleolar accumulation during interphase. On entry into mitosis, it localizes initially at kinetochores, where it exchanges rapidly with the diffuse cytoplasmic pool. A dramatic relocalization of PP1 to the chromosome-containing regions occurs at the transition from early to late anaphase, and by telophase FP-PP1gamma also accumulates at the cleavage furrow and midbody. The changing spatio-temporal distribution of PP1gamma revealed using the stable PP1 cell lines implicates it in multiple processes, including nucleolar function, the regulation of chromosome segregation and cytokinesis.

Preparation and characterization of a human aurora-A kinase monoclonal antibody

We have developed monoclonal antibodies against the human aurora-A serine/threonine kinase. After immunization of a mouse, a fusion was performed to obtain hybridomas that were selected because they produced immunoglobulin positively reacting against the protein used for immunization. We isolated one particular monoclonal that we named 35C1 using a series of selective assays. The first criteria of the screen for monoclonals was an ELISA (Enzyme Linked Immunosorbant Assay) assay performed in 96-well plates against the purified recombinant histidine-tagged aurora-A. The second was a positive Western blot against the same recombinant protein. The third criteria was a positive western blot against an HeLa cell extract, the selected monoclonal should detect only one protein migrating at 46 kDa (kiloDalton) on SDS (Sodium Dodecyl Sulfate)-polyacrylamide gel electrophoresis. Finally, the monoclonal had to bind to duplicated centrosomes and spindle poles in human MCF7 cultured cells by indirect immunofluorescence. At this stage several monoclonals were still positive. We then increased the selectivity by searching for antibodies that were able to cross-react with the mouse aurora-A kinase both by western blot and indirect immunofluorescence. We selected and cloned the 35C1 hybridoma to produce the antibody. Further characterization of the 35C1 antibody revealed that it was able to immunoprecipitate the kinase, that it did not inhibit the aurora-A kinase activity and consequently could be used to measure the aurora-A kinase activity in vivo after immunoprecipitation.

STK15/Aurora-A expression in primary breast tumors is correlated with nuclear grade but not with prognosis

BACKGROUND

DNA amplification on chromosome 20q13 is commonly detected in breast carcinoma and is correlated with poor prognosis. STK15 maps to this amplicon. The objective of the current study was to use immunohistochemistry to determine STK15 expression in primary breast tumors. The authors also explored whether STK15 was a prognostic factor for breast carcinoma by comparing the level of STK15 gene expression with clinical parameters that are known prognostic factors for the disease.

METHODS

Archival mastectomy and lumpectomy specimens, randomly selected, were immunohistochemically stained to determine the STK15 gene expression level. The clinical parameters of these same patients were reviewed retrospectively and analyzed for correlations with STK15 expression level, based on a positive-versus-negative scoring system.

RESULTS

Of the 112 human breast tumor specimens analyzed, 26% stained positively for STK15 by immunohistochemistry. Of the tumors, that stained positively 62.1% had a well-to-moderately differentiated nuclear grade. The correlation between STK15 staining and nuclear grade was nearly statistically significant (P = 0.05). No association was found between STK15 staining and tumor size, lymph node status, or hormone receptor status. Analysis of recurrence-free survival and overall survival rates also failed to reveal a statistically significant difference between the two groups.

CONCLUSIONS

STK15 expression by immunohistochemistry was noted in approximately one-fourth of primary breast tumors. STK15 expression was associated with nuclear grade, but no correlation was found between the other clinical parameters evaluated. Furthermore, no differences were found in survival rates when they were analyzed by level of STK15 staining.

AURORA-A amplification overrides the mitotic spindle assembly checkpoint, inducing resistance to Taxol

The serine-threonine kinase gene AURORA-A is commonly amplified in epithelial malignancies. Here we show that elevated Aurora-A expression at levels that reflect cancer-associated gene amplification overrides the checkpoint mechanism that monitors mitotic spindle assembly, inducing resistance to the chemotherapeutic agent paclitaxel (Taxol). Cells overexpressing Aurora-A inappropriately enter anaphase despite defective spindle formation, and the persistence of Mad2 at the kinetochores, marking continued activation of the spindle assembly checkpoint. Mitosis is subsequently arrested by failure to complete cytokinesis, resulting in multinucleation. This abnormality is relieved by an inhibitory mutant of BUB1, linking the mitotic abnormalities provoked by Aurora-A overexpression to spindle checkpoint activity. Consistent with this conclusion, elevated Aurora-A expression causes resistance to apoptosis induced by Taxol in a human cancer cell line.

MBD3 and HDAC1, two components of the NuRD complex, are localized at Aurora-A-positive centrosomes in M phase

MBD3, a component of the histone deacetylase NuRD complex, contains the methyl-CpG-binding domain (MBD), yet does not possess appreciable mCpG-specific binding activity. The functional significance of MBD3 in the NuRD complex remains enigmatic, partly because of the limited availability of biochemical approaches, such as immunoprecipitation, to analyze MBD3. In this study, we stably expressed the FLAG-tagged version of MBD3 in HeLa cells. We found that MBD3-FLAG was incorporated into the NuRD complex, and the MBD3-FLAG-containing NuRD complex was efficiently immunoprecipitated by anti-FLAG antibodies. By exploiting this system, we found that MBD3 is phosphorylated in vivo in the late G(2) and early M phases. Moreover, we found that Aurora-A, a serine/threonine kinase active specifically in the late G(2) and early M phases, phosphorylates MBD3 in vitro, physically associates with MBD3 in vivo, and co-localizes with MBD3 at the centrosomes in the early M phase. Interestingly, HDAC1 is distributed at the centrosomes in a manner similar to MBD3. These results suggest the highly dynamic nature of the temporal and spatial distributions, as well as the biochemical modification, of the NuRD complex in M phase, probably through an interaction with kinases, including Aurora-A. These observations will contribute significantly to the elucidation of the yet-uncharacterized cell cycle-controlled functions of the NuRD complex.

Nuclear translocation of plk1 mediated by its bipartite nuclear localization signal

Polo-like kinase 1 (Plk1), a mammalian ortholog of Drosophila Polo, is a serine-threonine protein kinase implicated in the regulation of multiple aspects of mitosis. The protein level, activity, and localization of Plk1 change during the cell cycle, and its proper subcellular localization is thought to be crucial for its function. Although localization of Plk1 to the centrosome has been established, nuclear localization or nucleocytoplasmic translocation of Plk1 has not been fully addressed. Here we show that Plk1 accumulates in both the nucleus and the cytoplasm in addition to its localization to the centrosome during S and G(2) phases. Our results identify a conserved region in the kinase domain of Plk1 (residues 134-146) as a functional bipartite nuclear localization signal (NLS) sequence that regulates nuclear translocation of Plk1. The identified NLS is necessary and sufficient for directing nuclear localization of Plk1. This bipartite NLS has an unusually short spacer sequence between two clusters of basic amino acids but is sensitive to RanQ69L, a dominant negative form of Ran, similar to ordinary bipartite NLS. Remarkably, the expression of an NLS-disrupted mutant of Plk1 during S phase was found to arrest the cells in G(2) phase. These results suggest that the bipartite NLS-dependent nuclear localization of Plk1 before mitosis is important for ensuring normal cell cycle progression.

Aurora-A in cell fate control

Key cells divide asymmetrically during the development of multicellular organisms to give rise to offspring with different fates. In the Drosophila external sensory organ, asymmetrical division depends on polarization of the precursor cells during interphase and the consequent unequal distribution during mitosis of the protein Numb, which determines cell fate. Gonzalez discusses recent research implicating the mitotic kinase Aurora-A in the asymmetric localization of Numb in sensory organ pI precursor cells, a new function that appears to be independent of Aurora-A's known roles in regulating centrosomal maturation and the organization of mitotic spindle microtubules.

Caenorhabditis elegans Aurora A kinase AIR-1 is required for postembryonic cell divisions and germline development

Many kinases are required for progression through the eukaryotic cell cycle. The Aurora kinases comprise a highly conserved family of serine/threonine kinases that have been implicated in chromosome segregation and cytokinesis in several organisms. We have isolated a sterile Caenorhabditis elegans mutant in which the majority of the locus encoding the Aurora A kinase air-1 has been deleted. Complementation tests with previously isolated sterile mutations in the air-1 genetic interval demonstrate that the air-1 and let-412 loci are identical. Previous analysis of AIR-1 function by RNA-mediated interference (RNAi) has shown that AIR-1 is required for embryonic survival. The characterization of the three sterile air-1 mutant alleles described here extends these studies by revealing an allelic series that differentially affects postembryonic cell divisions and germline development.

Identification of the nuclear localization signal in Xenopus cyclin E and analysis of its role in replication and mitosis

Cyclin-dependent kinase (Cdk)2/cyclin E is imported into nuclei assembled in Xenopus egg extracts by a pathway that requires importin-alpha and -beta. Here, we identify a basic nuclear localization sequence (NLS) in the N-terminus of Xenopus cyclin E. Mutation of the NLS eliminated nuclear accumulation of both cyclin E and Cdk2, and such versions of cyclin E were unable to trigger DNA replication. Addition of a heterologous NLS from SV40 large T antigen restored both nuclear targeting of Cdk2/cyclin E and DNA replication. We present evidence indicating that Cdk2/cyclin E complexes must become highly concentrated within nuclei to support replication and find that cyclin A can trigger replication at much lower intranuclear concentrations. We confirmed that depletion of endogenous cyclin E increases the concentration of cyclin B necessary to promote entry into mitosis. In contrast to its inability to promote DNA replication, cyclin E lacking its NLS was able to cooperate with cyclin B in promoting mitotic entry.

The D-Box-activating domain (DAD) is a new proteolysis signal that stimulates the silent D-Box sequence of Aurora-A

We have demonstrated previously that Xenopus Aurora-A is degraded at late mitosis by the APC/Fizzy-Related in a D-Box-dependent manner. Here we demonstrate that, although Aurora-B possesses the same D-Box as Aurora-A, Aurora-B is not degraded by this ubiquitin ligase. We have constructed a chimera Aurora-A/B with the N-terminus of Aurora-A and the C-terminus of Aurora-B and we have examined its degradation by APC/Fizzy-Related. We demonstrate that the N-terminus of Aurora-A confers degradation capacity on the C-terminus of Aurora-B and that this feature is blocked by mutation of the conserved D-Box sequence. We characterize the minimal degradation signal at the N-terminus of Aurora-A and demonstrate that its deletion blocks the degradation of this protein by APC/Fizzy-Related. Thus, we conclude that two different degradation signals are required for proteolysis of Aurora-A. The first one, which we designated D-Box-activating domain, within the N-terminal domain of Aurora-A confers the functionality to the second, a silent D-Box, present within the C-terminus of the kinase.

Cyclin-dependent kinase-1: linking apoptosis to cell cycle and mitotic catastrophe

The cyclin-dependent kinase 1 (Cdk1), formerly called Cdc2 (or p34(Cdc2)), interacts with cyclin B1 to form an active heterodimer. The activity of Cdk1 is subjected to a complex spatiotemporary regulation, required to guarantee its scheduled contribution to the mitotic prophase and metaphase. Moreover, the activation of Cdk1 may be required for apoptosis induction in some particular pathways of cell killing. This applies to several clinically important settings, for instance to paclitaxel-induced killing of breast cancer cells, in which the ErbB2 receptor kinase can mediate apoptosis inhibition through inactivation of Cdk1. The activation of Cdk1 participates also in HIV-1-induced apoptosis, upstream of the p53-dependent mitochondrial permeabilization step. An unscheduled Cdk1 activation may contribute to neuronal apoptosis occurring in neurodegenerative diseases. Finally, the premature activation of Cdk1 can lead to mitotic catastrophe, for instance after irradiation-induced DNA damage. Thus, a cell type-specific modulation of Cdk1 might be taken advantage of for the therapeutic correction of pathogenic imbalances in apoptosis control.

Proteomic analysis of human metaphase chromosomes reveals topoisomerase II alpha as an Aurora B substrate

The essential Aurora B kinase is a chromosomal passenger protein that is required for mitotic chromosome alignment and segregation. Aurora B function is dependent on the chromosome passenger, INCENP. INCENP, in turn, requires sister chromatid cohesion for its appropriate behaviour. Relatively few substrates have been identified for Aurora B, so that the precise role it plays in controlling mitosis remains to be elucidated. To identify potential novel mitotic substrates of Aurora B, extracted chromosomes were prepared from mitotically-arrested HeLa S3 cells and incubated with recombinant human Aurora B in the presence of radioactive ATP. Immunoblot analysis confirmed the HeLa scaffold fraction to be enriched for known chromosomal proteins including CENP-A, CENP-B, CENP-C, ScII and INCENP. Mass spectrometry of bands excised from one-dimensional polyacrylamide gels further defined the protein composition of the extracted chromosome fraction. Cloning, fluorescent tagging and expression in HeLa cells of the putative GTP-binding protein NGB/CRFG demonstrated it to be a novel mitotic chromosome protein, with a perichromosomal localisation. Identi fication of the protein bands corresponding to those phosphorylated by Aurora B revealed topoisomerase II alpha (topo IIalpha) as a potential Aurora B substrate. Purified recombinant human topo IIalpha was phosphorylated by Aurora B in vitro, confirming this proteomic approach as a valid method for the initial definition of candidate substrates of key mitotic kinases.

Molecular dynamics of Aurora-A kinase in living mitotic cells simultaneously visualized with histone H3 and nuclear membrane protein importinalpha

Aurora-A is known to be a mitotic kinase required for spindle assembly. We constructed a human stable cell-line in which Aurora-A, histone H3 and importinalpha were differentially expressed as fusions to green, cyan, and red fluorescent proteins (GFP, CFP and DsRed). In interphase cells, GFP-Aurora-A was localized in the centrosome. Its molecular behavior in living mitotic cells was extensively analyzed by an advanced timelapse image analyzing system. In G2 phase, duplicated centrosomal dots of Aurora-A separated and moved to the opposite poles, a process requiring 18 min. In prophase, the Aurora-A dots approached closer and the nuclear membrane of DsRed-importinalpha beneath them became thick and invaginated, resulting in a "dumb-bell" shaped nucleus with condensed chromatin. As the importinalpha membrane further shrank and disappeared, the condensed chromatin was excluded from the nucleus and the Aurora-A dots grew rapidly into a spindle-like structure. Congression of mitotic chromosomes continued for 20-50 min until they were properly aligned at the spindle equator and then the sister chromatids started to segregate, taking 4-6 min for them to reach the poles. An importinalpha membrane reappeared around the surface of chromatin 10 min after anaphase onset. Aurora-A gradually decreased in size in telophase and returned to the surface of the newly formed small sister nuclei. These observations showed that the morphological change of Aurora-A was cooperated with the breakdown and reformation of nuclear membrane. Immunostaining with anti-alpha or gamma-tubulin further indicated that Aurora-A was involved in the formation of mitotic spindle in metaphase as well as the subsequent chromosome movement in anaphase.

Structures of the cancer-related Aurora-A, FAK, and EphA2 protein kinases from nanovolume crystallography

Protein kinases are important drug targets in human cancers, inflammation, and metabolic diseases. This report presents the structures of kinase domains for three cancer-associated protein kinases: ephrin receptor A2 (EphA2), focal adhesion kinase (FAK), and Aurora-A. The expression profiles of EphA2, FAK, and Aurora-A in carcinomas suggest that inhibitors of these kinases may have inherent potential as therapeutic agents. The structures were determined from crystals grown in nanovolume droplets, which produced high-resolution diffraction data at 1.7, 1.9, and 2.3 A for FAK, Aurora-A, and EphA2, respectively. The FAK and Aurora-A structures are the first determined within two unique subfamilies of human kinases, and all three structures provide new insights into kinase regulation and the design of selective inhibitors.

Dma1 prevents mitotic exit and cytokinesis by inhibiting the septation initiation network (SIN)

In the fission yeast Schizosaccharomyces pombe, the septation initiation network (SIN) triggers cytokinesis after mitosis. We investigated the relationship between Dma1p, a spindle checkpoint protein and cytokinesis inhibitor, and the SIN. Deletion of dma1 inactivates the spindle checkpoint and allows precocious SIN activation, while overexpressing Dma1p reduces SIN signaling. Dma1p seems to function by inhibiting the SIN activator, Plo1p kinase, since dma1 overexpression and deletion phenotypes suggest that Dma1p antagonizes Plo1p localization. Furthermore, failure to maintain high cyclin-dependent kinase (CDK) activity during spindle checkpoint activation in dma1 deletion cells requires Plo1p. Dma1p itself localizes to spindle pole bodies through interaction with Sid4p. Our observations suggest that Dma1p functions to prevent mitotic exit and cytokinesis during spindle checkpoint arrest by inhibiting SIN signaling.

Cooperative phosphorylation including the activity of polo-like kinase 1 regulates the subcellular localization of cyclin B1

The cyclin-dependent kinase 1 (Cdc2)/cyclin B1 complex performs cardinal roles for eukaryotic mitotic progression. Phosphorylation of four serine residues within cyclin B1 promotes the rapid nuclear translocation of Cdc2/cyclin B1 at the G(2)/M transition. Still, the role of individual phosphorylation sites and their corresponding kinases remain to be elucidated. Polo-like kinase 1 (Plk1) shows a spatial and temporal distribution which makes it a candidate kinase for the phosphorylation of cyclin B1. We could demonstrate the interaction of both proteins in mammalian cells. Plk1 phosphorylated wild-type cyclin B1 expressed in bacteria and in mammalian cells. Ser-133 within the cytoplasmic retention signal (CRS) of cyclin B1, which regulates the nuclear entry of the heterodimeric complex during prophase, is a target of Plk1. In contrast, MAPK (Erk2) and MPF phosphorylate Ser-126 and Ser-128 within the CRS. Phosphorylation of CRS by MAPK (Erk2) prior to Plk1 treatment induced enhanced phosphorylation of cyclin B1 by Plk 1 suggesting a synergistic action of both enzymes towards cyclin B1. In addition, pretreatment of cyclin B1 by MAPK (Erk2) altered the phosphorylation pattern of Plk 1. Mutation of Ser-133 to Ala decreased the phosphorylation of cyclin B1 in vivo. An immunofluorescence study revealed that a mutation of Ser-133 reduced the nuclear import rate of cyclin B1. Still, multiple serine mutations are required to prevent nuclear translocation completely indicating that orchestrated phosphorylation within the CRS triggers rapid import of cyclin B1.

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

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

Human testis/sperm-specific histone H2B (hTSH2B). Molecular cloning and characterization

Human sperm, unlike the sperm of other mammals, contain replacement histones with unknown biological functions. Here, we report the identification of the novel human gene coding for a testis/sperm-specific histone H2B (hTSH2B). This variant histone is 85% homologous to somatic H2B and has over 93% homology with the testis H2B of rodents. Using genomic PCR, two genetic alleles of hTSH2B were found in the human population. The hTSH2B gene is transcribed exclusively in testis, and the corresponding protein is also present in mature sperm. We expressed recombinant hTSH2B and identified this protein with a particular H2B subtype expressed in vivo. The subnuclear distribution of H2B variants in sperm was determined using biochemical fractionation and immunoblotting. The H2B variant associated with telomere-binding activity () was solubilized by Triton X-100 or micrococcal nuclease extraction, whereas hTSH2B was relatively tightly bound in nuclei. Immunofluorescence showed that hTSH2B was concentrated in spots located at the basal nuclear area of a subpopulation (20% of cells) of mature sperm. This fact may be of particular importance, because the hTSH2B "positive" and "negative" sperm cells may undergo significantly different decondensation processes following fertilization.

Roles of aurora-A kinase in mitotic entry and G2 checkpoint in mammalian cells

BACKGROUND

Various mitotic events are controlled by Cdc2-cyclin B and other mitotic kinases. Aurora/Ipl1-related mitotic kinases were proved to play key roles in mitotic progression in diverse lower organisms. Aurora-A is a mammalian counterpart of aurora/Ipl1-related kinases and is thought to be a potential oncogene. However, the regulation of aurora-A activation and the commitment of aurora-A in the progression of G2-M phase are largely unknown in mammalian cells.

RESULTS

We demonstrated that aurora-A is activated depending on the activation of Cdc2-cyclin B in mammalian cells. Since Cdc2-cyclin B does not directly phosphorylate aurora-A, indirect pathways such as the inhibition of PP1 by Cdc2-cyclin B may act for the activation of aurora-A kinase. Microinjection of anti-aurora-A antibodies into HeLa cells at late G2 phase caused a significant delay in mitotic entry. Furthermore, aurora-A activation at G2-M transition was inhibited by DNA damage, and the over-expression of aurora-A induced the abrogation of the DNA damage-induced G2 checkpoint.

CONCLUSIONS

Aurora-A is activated downstream of Cdc2-cyclin B and plays crucial roles in proper mitotic entry and G2 checkpoint control. Dysregulation of aurora-A induces abnormal G2-M transition in mammalian cells and may lead to chromosome instability, which results in the development and progression of malignant tumours.

Regulation of G(2)/M events by Cdc25A through phosphorylation-dependent modulation of its stability

DNA replication in higher eukaryotes requires activation of a Cdk2 kinase by Cdc25A, a labile phosphatase subject to further destabilization upon genotoxic stress. We describe a distinct, markedly stable form of Cdc25A, which plays a previously unrecognized role in mitosis. Mitotic stabilization of Cdc25A reflects its phosphorylation on Ser17 and Ser115 by cyclin B-Cdk1, modifications required to uncouple Cdc25A from its ubiquitin-proteasome-mediated turnover. Cdc25A binds and activates cyclin B-Cdk1, accelerates cell division when overexpressed, and its downregulation by RNA interference (RNAi) delays mitotic entry. DNA damage-induced G(2) arrest, in contrast, is accompanied by proteasome-dependent destruction of Cdc25A, and ectopic Cdc25A abrogates the G(2) checkpoint. Thus, phosphorylation-mediated switches among three differentially stable forms ensure distinct thresholds, and thereby distinct roles for Cdc25A in multiple cell cycle transitions and checkpoints.

Nek11, a new member of the NIMA family of kinases, involved in DNA replication and genotoxic stress responses

DNA replication and genotoxic stresses activate various checkpoint-associated protein kinases, and checkpoint dysfunction often leads to cell lethality. Here, we have identified new members of the mammalian NIMA family of kinases, termed Nek11L and Nek11S (NIMA-related kinase 11 Long and Short isoform) as novel DNA replication/damage stresses-responsive kinases. Molecular cloning and biochemical studies showed that the catalytic domain of Nek11 is most similar to Nek4 and Nek3, and substrate specificity of Nek11L is distinguishable from those of NIMA and Nek2. The expression of nek11L mRNA increased through S to G(2)/M phase, and subcellular localization of Nek11 protein altered between interphase and prometaphase, suggesting multiple roles of Nek11. We found an activation of Nek11 kinase activity when cells were treated with various DNA-damaging agents and replication inhibitors, and this activation of Nek11 was suppressed by caffeine in HeLaS3 cells. The transient expression of wild-type Nek11L enhanced the aphidicolin-induced S-phase arrest, whereas the aphidicolin-induced S-phase arrest was reduced in the U2OS cell lines expressing kinase-negative Nek11L (K61R), and these cells were more sensitive to aphidicolin-induced cell lethality. Collectively, these results suggest that Nek11 has a role in the S-phase checkpoint downstream of the caffeine-sensitive pathway.

Growth retardation, polyploidy, and multinucleation induced by Clast3, a novel cell cycle-regulated protein

We have identified a novel gene, Clast3, by subtraction of cDNAs derived from activated and naive B lymphocytes. Clast3 expression is elevated in cycling cells and down-regulated in cells undergoing growth arrest, indicating that its expression is controlled in a cell cycle-dependent manner. The deduced amino acid sequence of Clast3 cDNA exhibits no significant homology to the known proteins in mammalian and other species. Immunofluorescence staining revealed that Clast3 localizes into discrete nuclear foci. Forced expression of Clast3 results in growth retardation, polyploidy, and generation of multinucleated cells. Treatment of Clast3 transfectants with nocodazole, a spindle-damaging agent, greatly enhances the incidence of the multinucleated cells, suggesting that Clast3 overexpression impairs the same checkpoint activated by nocodazole. Down-regulation of Clast3 expression by antisense oligonucleotides results in a decrease of cells at G(2)-M phase and a concomitant increase of apoptotic cells. These findings indicate that Clast3 is a novel cell cycle-regulated protein and that its constitutive overexpression induces polyploidy and multinucleation by interfering with the mitotic spindle checkpoint.

Cdc37 is essential for chromosome segregation and cytokinesis in higher eukaryotes

Cdc37 has been shown to be required for the activity and stability of protein kinases that regulate different stages of cell cycle progression. However, little is known so far regarding interactions of Cdc37 with kinases that play a role in cell division. Here we show that the loss of function of Cdc37 in Drosophila leads to defects in mitosis and male meiosis, and that these phenotypes closely resemble those brought about by the inactivation of Aurora B. We provide evidence that Aurora B interacts with and requires the Cdc37/Hsp90 complex for its stability. We conclude that the Cdc37/Hsp90 complex modulates the function of Aurora B and that most of the phenotypes brought about by the loss of Cdc37 function can be explained by the inactivation of this kinase. These observations substantiate the role of Cdc37 as an upstream regulatory element of key cell cycle kinases.

Interaction of p58(PITSLRE), a G2/M-specific protein kinase, with cyclin D3

The p58(PITSLRE) is a p34(cdc2)-related protein kinase that plays an important role in normal cell cycle progression. Elevated expression of p58(PITSLRE) in eukaryotic cells prevents them from undergoing normal cytokinesis and appears to delay them in late telophase. To investigate the molecular mechanism of p58(PITSLRE) action, we used the yeast two-hybrid system, screened a human fetal liver cDNA library, and identified cyclin D3 as an interacting partner of p58(PITSLRE). In vitro binding assay, in vivo coimmunoprecipitation, and immunofluorescence cell staining further confirmed the association of p58(PITSLRE) with cyclin D3. This binding was observed only in the G(2)/M phase but not in the G(1)/S phase of the cell cycle; meanwhile, no interaction between p110(PITSLRE) and cyclin D3 was observed in all the cell cycle. The overexpression of cyclin D3 in 7721 cells leads to an exclusively accumulation of p58(PITSLRE) in the nuclear region, affecting its cellular distribution. Histone H1 kinase activity of p58(PITSLRE) was greatly enhanced upon interaction with cyclin D3. Furthermore, kinase activity of p58(PITSLRE) was found to increase greatly in the presence of cyclin D3 using a specific substrate, beta-1,4-galactosyltransferase 1. These data provide a new clue to our understanding of the cellular function of p58(PITSLRE) and cyclin D3.