Mitotic kinases as regulators of cell division and its checkpoints

NEK2A interacts with MAD1 and possibly functions as a novel integrator of the spindle checkpoint signaling

Chromosome segregation in mitosis is orchestrated by protein kinase signaling cascades. A biochemical cascade named spindle checkpoint ensures the spatial and temporal order of chromosome segregation during mitosis. Here we report that spindle checkpoint protein MAD1 interacts with NEK2A, a human orthologue of the Aspergillus nidulans NIMA kinase. MAD1 interacts with NEK2A in vitro and in vivo via a leucine zipper-containing domain located at the C terminus of MAD1. Like MAD1, NEK2A is localized to HeLa cell kinetochore of mitotic cells. Elimination of NEK2A by small interfering RNA does not arrest cells in mitosis but causes aberrant premature chromosome segregation. NEK2A is required for MAD2 but not MAD1, BUB1, and HEC1 to associate with kinetochores. These NEK2A-eliminated or -suppressed cells display a chromosome bridge phenotype with sister chromatid inter-connected. Moreover, loss of NEK2A impairs mitotic checkpoint signaling in response to spindle damage by nocodazole, which affected mitotic escape and led to generation of cells with multiple nuclei. Our data demonstrate that NEK2A is a kinetochore-associated protein kinase essential for faithful chromosome segregation. We hypothesize that NEK2A links MAD2 molecular dynamics to spindle checkpoint signaling.

Cell cycle and no end

Our knowledge about the molecular circuits regulating the duplication of the genetic material and the subsequent division of a cell into two daughter cells has exploded over the last decade. Aberrations in the regulation of the cell cycle belong to the hallmarks of malignant transformation, leading, in turn, to the development of tumours. After introducing the basics of eukaryotic cell-cycle regulation and describing the four phases of the cell cycle (namely, G1, S, G2 and M) in more detail, alterations of key components of the cell-cycle machinery in human malignancies and their functional consequences are presented. Principally, deregulation of the cell cycle can be caused by unrestricted activity of cell-cycle promoting factors (many oncogenes fall into this class) or by inactivation of inhibitory factors (many tumour suppressor genes belong to this class). Both types of deregulation have been described in human tumours and are discussed in detail. Perspectives concerning the translation of this knowledge into daily routine practice and future applications are discussed at the end. The molecular mechanisms of actual cell division (sister chromatid segregation and cytokinesis) are mentioned only briefly.

Human papillomavirus type 16 E6 and E7 cause polyploidy in human keratinocytes and up-regulation of G2-M-phase proteins

Human papillomavirus type 16 proteins E6 and E7 have been shown to cause centrosome amplification and lagging chromosomes during mitosis. These abnormalities during mitosis can result in missegregation of the chromosomes, leading to chromosomal instability. Genomic instability is thought to be an essential part of the conversion of a normal cell to a cancer cell. We now show that E6 and E7 together cause polyploidy in primary human keratinocytes soon after these genes are introduced into the cells. Polyploidy seems to result from a spindle checkpoint failure arising from abrogation of the normal functions of p53 and retinoblastoma family members by E6 and E7, respectively. In addition, E6 and E7 cause deregulation of cellular genes such as Plk1, Aurora-A, cdk1, and Nek2, which are known to control the G(2)-M-phase transition and the ordered progression through mitosis.

Fission yeast Clp1p phosphatase affects G2/M transition and mitotic exit through Cdc25p inactivation

The Cdc14 family of phosphatases specifically reverses proline-directed phosphorylation events. In Saccharomyces cerevisiae, Cdc14p promotes Cdk1p inactivation at mitotic exit by reversing Cdk1p-dependent phosphorylations. Cdk1p is a proline-directed kinase whose activity is required in all eukaryotes for the transit into mitosis. At mitotic commitment, Cdk1p participates in its own regulation by activating the mitotic inducing phosphatase, Cdc25p, and inhibiting the opposing kinase, Wee1p. We have investigated the ability of Schizosaccharomyces pombe Clp1p, a Cdc14p homolog, to disrupt this auto-amplification loop. We show here that Clp1p is required to dephosphorylate, destabilize, and inactivate Cdc25p at the end of mitosis. Clp1p promotes recognition of Cdc25p by the anaphase-promoting complex/cyclosome, an E3 ubiquitin ligase. Failure to inactivate and destabilize Cdc25p in late mitosis delays progression through anaphase, interferes with septation initiation network signaling, and additionally advances the commitment to mitotic entry in the next cycle. This may be a widely conserved mechanism whereby Cdc14 proteins contribute to Cdk1p inactivation.

Aurora kinases link chromosome segregation and cell division to cancer susceptibility

Aurora kinases play critical roles in chromosome segregation and cell division. They are implicated in the centrosome cycle, spindle assembly, chromosome condensation, microtubule-kinetochore attachment, the spindle checkpoint and cytokinesis. Aurora kinases are regulated through phosphorylation, the binding of specific partners and ubiquitin-dependent proteolysis. Several Aurora substrates have been identified and their roles are being elucidated. The deregulation of Aurora kinases impairs spindle assembly, checkpoint function and cell division, causing missegregation of individual chromosomes or polyploidization accompanied by centrosome amplification. Aurora kinases are frequently overexpressed in cancers and the identification of Aurora A as a cancer-susceptibility gene provides a strong link between mitotic errors and carcinogenesis.

The human small glutamine-rich TPR-containing protein is required for progress through cell division

Eukaryotic organisms from yeast to human harbor genes encoding the small glutamine-rich tetratricopeptide repeat-containing (SGT) protein. Work presented here demonstrated the presence of human SGT (hSGT) protein in a panel of human cell lines and throughout the cell cycle. To identify cellular processes in which hSGT is involved, knock down populations were analyzed which were generated through transfection of hsgt-specific small interfering RNA. Most strikingly, depletion of hSGT led to reduced proliferation of the affected cell populations while the mitotic index was increased. Time-lapse video microscopy revealed that cells from hSGT-depleted populations were unable to complete cell division due to mitotic arrest which was frequently followed by cell death. Further evidence for a role in cell division was given by the accumulation of hSGT in the midzone and the midbody, and by a mitosis-specific migration pattern of hSGT as detected by Western blotting after SDS-PAGE or two-dimensional gel electrophoresis. In conclusion, results obtained in this study demonstrate that hSGT protein is a constitutive component of all human cell lines tested and that this protein is essential for successful completion of cell division.

Global control of histone modification by the anaphase-promoting complex

Acetylation and phosphorylation of the amino-terminal tails of the core histones fluctuate on a global scale in concert with other major events in chromosome metabolism. A ubiquitin ligase, the anaphase-promoting complex (APC), controls events in chromosome metabolism such as sister chromatid cohesion and may regulate H3 phosphorylation by targeting Aurora A, one of several S10-directed H3 kinases in vertebrate cells, for destruction by the proteasome. Our analysis of apc10Delta and apc11(ts) loss-of-function mutants reveals that the APC controls the global level of H3 S10 phosphorylation in cycling yeast cells. Surprisingly, it also regulates dephosphorylation of H3 and global deacetylation of H2B, H3, and H4 during exit from the cell cycle into G(0). Genetic, biochemical, and microarray analyses suggest that APC-dependent cell cycle control of H3 phosphorylation is exerted at the level of an Aurora H3 kinase, Ipl1p, while APC-dependent transcriptional induction of GLC7, an essential H3 phosphatase, contributes to sustained H3 dephosphorylation upon cell cycle withdrawal. Collectively, our results establish that core histone acetylation state and H3 phosphorylation are physiologically regulated by the APC and suggest a model in which global reconfiguration of H3 phosphorylation state involves APC-dependent control of both an H3 kinase and a conserved phosphatase.

AuroraA overexpression overrides the mitotic spindle checkpoint triggered by nocodazole, a microtubule destabilizer

AuroraA, a mitotic kinase, is reported to be amplified and overexpressed in a variety of human tumors. Active mutants of AuroraA can transform mouse fibroblasts and form tumors in nude mice. However, the mechanism behind this oncogenic potential remains elusive. In this study, we investigated the consequences of AuroraA overexpression and showed that increased AuroraA levels compromise the mitotic spindle checkpoint triggered by nocodazole, a microtubule polymerization inhibitor. This is accomplished by disrupting the proper assembly of the mitotic checkpoint complex at the level of the Cdc20-BubR1 interaction. As a result, the spindle checkpoint complex fails to form and cells progress through mitosis without proper arrest in response to nocodazole. This ability to override the mitotic spindle checkpoint was found to be independent of AuroraA kinase activity. We conclude that maintenance of a functional balance between AuroraA and mitotic checkpoint proteins is essential for the proper progression through mitosis. This study therefore offers a possible explanation of how deregulation of AuroraA can contribute to genetic instability and tumorigenesis.

Survivin regulates hematopoietic progenitor cell proliferation through p21WAF1/Cip1-dependent and -independent pathways

The cyclin-dependent kinase inhibitor p21WAF1/Cip1 and Survivin enhance granulocyte macrophage colony-forming unit (CFU-GM) cell cycle and proliferation and have been implicated as antiapoptotic proteins. We investigated the relationships between p21 and Survivin in primary CFU-GM and c-kit+, lineage-negative (Lin-) cells and demonstrate p21-dependent and -independent pathways whereby Survivin regulates progenitor cell proliferation. Ectopic Survivin enhanced p21+/+ CFU-GM formation and expansion of c-kit+, Lin- cells, whereas p21 gene loss abrogated these effects, indicating a p21 requirement. A dominant-negative form of Survivin and p21 gene deletion accelerated the loss of CFU-GM upon growth factor deprivation, and wild-type Survivin overexpression inhibited apoptosis of p21+/+ CFU-GM and c-kit+, Lin- cells but not p21-/- cells, suggesting that both Survivin and p21 block apoptosis of progenitors and that Survivin-mediated antiapoptosis requires p21. In contrast to the p21-dependent antiapoptotic effects, Survivin increased the proportion of CFU-GM in S-phase in both p21+/+ and p21-/- cells. Furthermore, modulating Survivin expression increased polyploidy in c-kit+, Lin- cells, which was accentuated by p21 deficiency. These results suggest that the Survivin-p21 axis plays an important role in the proliferation of normal hematopoietic cells and that Survivin regulates apoptosis through a p21 WAF1/Cip1-dependent pathway but may control S-phase entry independent of p21.

Actin-binding proteins required for reliable chromosome segregation in mitosis

While studying mitosis in Dictyostelium mutants with deficiencies in actin-binding proteins, we found that two of these proteins, cortexillin and Aip1, are required for the precise segregation of chromosomes. Atypical spindles and nuclei with varying DNA content indicate that mutants lacking cortexillin or Aip1 are genetically unstable. These aberrations are caused by the detachment and irregular reattachment of centrosomes to the nuclear surface. Live imaging showed how coalescing mitotic complexes give rise to a multipolar spindle, and how excess centrosomes can be eliminated by mitotic cleavage between anucleate and nucleated portions of a cell. We hypothesize that mutations in regulatory proteins of the actin network might be one cause of genetic instability of malignant tumor cells.

Cell-cycle dysregulation in the immunopathogenesis of AIDS

For a number of years the pathogenesis of AIDS was thought to be essentially related to direct human immunodeficiency virus (HIV)-mediated killing of CD4+ T cells. More recently, attention has shifted to pathogenic models that emphasize the role of generalized immune system activation and the excess apoptosis of uninfected T cells in inducing HIV-associated CD4+ T-cell depletion. The main focus of our research is to better define the determinants and the consequences of these "indirect" mechanisms of immunodeficiency by studying both HIV-infected patients and nonhuman primates infected with simian immunodeficiency virus (SIV). We have discovered that pathogenic models of retroviral infections of primates (i.e., HIV infection in humans and SIV infection in rhesus macaques) are associated with the presence of a set of perturbations of normal cell-cycle control in T lymphocytes. These perturbations, to which we collectively refer to as cell-cycle dysregulation, or CCD, may represent an important biological link between chronic immune activation and excess apoptosis and therefore may play a significant role in the pathogenesis of AIDS. A better understanding of the determinants and consequences of CCD may pave the way for the introduction of new therapeutic strategies to be used in addition to standard antiretroviral therapy in HIV-infected patients.

Identification of the Substrates and Interaction Proteins of Aurora Kinases from a Protein-Protein Interaction Model

The increasing use of high-throughput and large-scale bioinformatics-based studies has generated a massive amount of data stored in a number of different databases. The major need now is to explore this disparate data to find biologically relevant interactions and pathways. Thus, in the post-genomic era, there is clearly a need for the development of algorithms that can accurately predict novel protein-protein interaction networks in silico. The evolutionarily conserved Aurora family kinases have been chosen as a model for the development of a method to identify novel biological networks by a comparison of human and various model organisms. Our search methodology was designed to predict and prioritize molecular targets for Aurora family kinases, so that only the most promising are subjected to empirical testing. Four potential Aurora substrates and/or interacting proteins, TACC3, survivin, Hec1, and hsNuf2, were identified and empirically validated. Together, these results justify the timely implementation of in silico biology in routine wet-lab studies and have also allowed the application of a new approach to the elucidation of protein function in the post-genomic era.

Aurora-A is a critical regulator of microtubule assembly and nuclear activity in mouse oocytes, fertilized eggs, and early embryos

Aurora-A is a serine/threonine protein kinase that plays a role in cell-cycle regulation. The activity of this kinase has been shown to be required for regulating multiple stages of mitotic progression in somatic cells. In this study, the changes in aurora-;A expression were revealed in mouse oocytes using Western blotting. The subcellular localization of aurora-A during oocyte meiotic maturation, fertilization, and early cleavages as well as after antibody microinjection or microtubule assembly perturbance was studied with confocal microscopy. The quantity of aurora-A protein was high in the germinal vesicle (GV) and metaphase II (MII) oocytes and remained stable during other meiotic maturation stages. Aurora-A concentrated in the GV before meiosis resumption, in the pronuclei of fertilized eggs, and in the nuclei of early embryo blastomeres. Aurora-A was localized to the spindle poles of the meiotic spindle from the metaphase I (MI) stage to metaphase II stage. During early embryo development, aurora-A was found in association with the mitotic spindle poles. Aurora-A was not found in the spindle region when colchicine or staurosporine was used to inhibit microtubule organization, while it accumulated as several dots in the cytoplasm after taxol treatment. Aurora-A antibody microinjection decreased the rate of germinal vesicle breakdown (GVBD) and distorted MI spindle organization. Our results indicate that aurora-A is a critical regulator of cell-cycle progression and microtubule organization during mouse oocyte meiotic maturation, fertilization, and early embryo cleavage.

Inhibition of prostate tumor growth by overexpression of NudC, a microtubule motor-associated protein

Microtubules play a central role in coordinating various cellular functions that are orchestrated by their interaction with molecular motors. Anticancer drugs that target microtubule dynamics have been shown to be effective in cancer treatment. However, the effect of microtubule motor-associated molecules on cancer cell proliferation is not clear. Here, we investigated the role of NudC, a nuclear movement protein associated with the microtubule motor dynein, on prostate tumorigenesis. Recombinant adenovirus expressing NudC (Ad-NudC) was used to examine the effects of NudC on the tumorigenicity of prostate cancer cells. Expression of NudC in LNCaP cells inhibited their anchorage-independent growth in a soft agar colony assay. Expression of NudC in DU145 or PC-3 cells inhibited tumor growth in a subcutaneous xenograft model. At the cellular level, expression of NudC in DU145 and PC-3 cells inhibited cell proliferation at 48 h after Ad-NudC infection. FACS analysis of cell cycle distribution showed that 50-60% of Ad-NudC-infected PC-3 cells have a G2/M-phase DNA content compared to about 16-19% in Ad-Luciferase (Ad-Luc)-infected control cells, suggesting that NudC overexpression resulted in aberrant cell cycle progression. Immunofluorescence microscopy revealed a significant increase in cells with a single enlarged nucleus and cells exhibiting multiple nuclei, along with a concomitant increase in cell size in Ad-NudC-infected cells. These results suggest that NudC overexpression led to a block in cell division of prostate cancer cells, and that Ad-NudC may provide a new anticancer drug approach targeting the function of a microtubule motor-associated protein.

Phosphorylation by aurora kinase A induces Mdm2-mediated destabilization and inhibition of p53

Aurora kinase A (also called STK15 and BTAK) is overexpressed in many human cancers. Ectopic overexpression of aurora kinase A in mammalian cells induces centrosome amplification, chromosome instability and oncogenic transformation, a phenotype characteristic of loss-of-function mutations of p53. Here we show that aurora kinase A phosphorylates p53 at Ser315, leading to its ubiquitination by Mdm2 and proteolysis. p53 is not degraded in the presence of inactive aurora kinase A or ubiquitination-defective Mdm2. Destabilization of p53 by aurora kinase A is abrogated in the presence of mutant Mdm2 that is unable to bind p53 and after repression of Mdm2 by RNA interference. Silencing of aurora kinase A results in less phosphorylation of p53 at Ser315, greater stability of p53 and cell-cycle arrest at G2-M. Cells depleted of aurora kinase A are more sensitive to cisplatin-induced apoptosis, and elevated expression of aurora kinase A abolishes this response. In a sample of bladder tumors with wild-type p53, elevated expression of aurora kinase A was correlated with low p53 concentration. We conclude that aurora kinase A is a key regulatory component of the p53 pathway and that overexpression of aurora kinase A leads to increased degradation of p53, causing downregulation of checkpoint-response pathways and facilitating oncogenic transformation of cells.

TACC1-chTOG-Aurora A protein complex in breast cancer

The three human TACC (transforming acidic coiled-coil) genes encode a family of proteins with poorly defined functions that are suspected to play a role in oncogenesis. A Xenopus TACC homolog called Maskin is involved in translational control, while Drosophila D-TACC interacts with the microtubule-associated protein MSPS (Mini SPindleS) to ensure proper dynamics of spindle pole microtubules during cell division. We have delineated here the interactions of TACC1 with four proteins, namely the microtubule-associated chTOG (colonic and hepatic tumor-overexpressed gene) protein (ortholog of Drosophila MSPS), the adaptor protein TRAP (tudor repeat associator with PCTAIRE2), the mitotic serine/threonine kinase Aurora A and the mRNA regulator LSM7 (Like-Sm protein 7). To measure the relevance of the TACC1-associated complex in human cancer we have examined the expression of the three TACC, chTOG and Aurora A in breast cancer using immunohistochemistry on tissue microarrays. We show that expressions of TACC1, TACC2, TACC3 and Aurora A are significantly correlated and downregulated in a subset of breast tumors. Using siRNAs, we further show that depletion of chTOG and, to a lesser extent of TACC1, perturbates cell division. We propose that TACC proteins, which we also named 'Taxins', control mRNA translation and cell division in conjunction with microtubule organization and in association with chTOG and Aurora A, and that these complexes and cell processes may be affected during mammary gland oncogenesis.

Repression of mRNA for the PLK cell cycle gene after DNA damage requires BRCA1

DNA damage activates the G2 cell cycle checkpoint to allow time for DNA repair before mitotic entry. The mechanism involves inhibition of the enzymatic activity for polo-like kinase 1 (Plk1), rendering Cdc25C with a basal phosphatase activity that is insufficient for converting Cdc2 to the fully active G2/M transition kinase. We found that cell cycle arrest at the G2/M boundary after ionizing radiation (IR) of breast carcinoma cells may involve repression of the gene for Plk1, PLK, mediated by the tumor-suppressor protein BRCA1. The p53-defective MT-1 cell line had an apparent accumulation of G2/M phase cells 12 h after irradiation. This response was preceded by a transient downregulation of PLK mRNA expression with a barely detectable level 6 h after exposure to IR but recovered after 12 h. A significantly lower fraction of irradiated BRCA1(-/-) HCC1937 cells arrested in the G2/M phase after 12 h, and the transient response of PLK mRNA was also considerably impaired. After reconstitution of wild-type BRCA1 in the HCC1937 cells however, downregulation of PLK mRNA as well as Plk1 protein expression after IR was restored. Moreover, the suppression of PLK mRNA expression 6 h after irradiation was completely abolished by the specific CHEK1 kinase inhibitor UCN-01, further indicating that the effector mechanism of DNA damage on PLK signals through BRCA1 and its downstream CHEK1. Our observations provide new information about the diversity of regulatory mechanisms governed by BRCA1 in DNA damage checkpoint control.

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.

CENP-A Phosphorylation by Aurora-A in Prophase Is Required for Enrichment of Aurora-B at Inner Centromeres and for Kinetochore Function

The Aurora (Ipl1)-related kinases are universal regulators of mitosis. We now show that Aurora-A, in addition to Aurora-B, regulates kinetochore function in human cells. A two-hybrid screen identified the kinetochore component CENP-A as a protein that interacts with Aurora-A. Aurora-A phosphorylated CENP-A in vitro on Ser-7, a residue also known to be targeted by Aurora-B. Depletion of Aurora-A or Aurora-B by RNA interference revealed that CENP-A is initially phosphorylated in prophase in a manner dependent on Aurora-A, and that this reaction appears to be required for the subsequent Aurora-B-dependent phosphorylation of CENP-A as well as for the restriction of Aurora-B to the inner centromere in prometaphase. Prevention of CENP-A phosphorylation also led to chromosome misalignment during mitosis as a result of a defect in kinetochore attachment to microtubules. Our observations suggest that phosphorylation of CENP-A on Ser-7 by Aurora-A in prophase is essential for kinetochore function.

Cell-cycle control during meiotic maturation

The meiotic cell cycle, which is comprised of two consecutive M-phases, is crucial for the production of haploid germ cells. Although both mitotic and meiotic M-phases share cyclin-B-Cdc2/CDK1 as a key controller, there are meiosis-specific modulations in the regulation of cyclin-B-Cdc2. Recent insights indicate that a common pattern in these modulations can be found by considering the particular activities of mitogen-activated protein kinase (MAPK) during meiosis. The G(2)-phase arrest of meiosis I is released via specific, MAPK-independent signalling that leads to cyclin-B-Cdc2 activation; thereafter, however, the meiotic process is under the control of interplay between MAPK and cyclin-B-Cdc2.

Targets of the cyclin-dependent kinase Cdk1

The events of cell reproduction are governed by oscillations in the activities of cyclin-dependent kinases (Cdks). Cdks control the cell cycle by catalysing the transfer of phosphate from ATP to specific protein substrates. Despite their importance in cell-cycle control, few Cdk substrates have been identified. Here, we screened a budding yeast proteomic library for proteins that are directly phosphorylated by Cdk1 in whole-cell extracts. We identified about 200 Cdk1 substrates, several of which are phosphorylated in vivo in a Cdk1-dependent manner. The identities of these substrates reveal that Cdk1 employs a global regulatory strategy involving phosphorylation of other regulatory molecules as well as phosphorylation of the molecular machines that drive cell-cycle events. Detailed analysis of these substrates is likely to yield important insights into cell-cycle regulation.

Pin1 colocalization with phosphorylated tau in Alzheimer's disease and other tauopathies

Pin1, a peptidyl-prolyl isomerase binds to mitotic serine or threonine phosphoproteins. In Alzheimer's disease (AD) evidence points to the reactivation of mitosis in vulnerable neurons. Tangles composed of hyperphosphorylated tau contain phosphorylated Thr231 (pThr231 tau), which occurs to a greater extent in the AD brain than in the normal brain, and Pin1 has been shown to bind pThr231 tau. Here, Pin1 distribution in AD, and its colocalization with pThr231 tau in AD, FTDP-17 (P301L), Pick's disease (PiD), and PSP was investigated using TG-3, a monoclonal antibody to conformationally altered pThr231 tau. The Pin1 antibody A-20 detected granular Pin1 staining in AD brains, but not in normal brains. A-20 immunoreactive granules colocalized with TG-3-stained granules but not with TG-3-stained pretangles, tangles, or Pick bodies in AD, PiD, and FTDP-17 (P301L). Pin1 granules were sparse in PSP, and rarely did A-20 colocalize with TG-3. The appearance of Pin1 granules in the early stages of AD, PiD, and FTDP-17 (P301L) implicates Pin1 in their pathogenesis but not in PSP.

Aurora-A and an interacting activator, the LIM protein Ajuba, are required for mitotic commitment in human cells

Aurora family kinases contribute to regulation of mitosis. Using RNA interference in synchronized HeLa cells, we now show that Aurora-A is required for mitotic entry. We found that initial activation of Aurora-A in late G2 phase of the cell cycle is essential for recruitment of the cyclin B1-Cdk1 complex to centrosomes, where it becomes activated and commits cells to mitosis. A two-hybrid screen identified the LIM protein Ajuba as an Aurora-A binding protein. Ajuba and Aurora-A interact in mitotic cells and become phosphorylated as they do so. In vitro analyses revealed that Ajuba induces the autophosphorylation and consequent activation of Aurora-A. Depletion of Ajuba prevented activation of Aurora-A at centrosomes in late G2 phase and inhibited mitotic entry. Overall, our data suggest that Ajuba is an essential activator of Aurora-A in mitotic commitment.

Distinct regulators for Plk1 activation in starfish meiotic and early embryonic cycles

The Polo-like kinase, Plk, has multiple roles in regulating mitosis. In particular, Plk1 has been postulated to function as a trigger kinase that phosphorylates and activates Cdc25C prior to the activation of cyclin B-Cdc2 and thereby initiates its activation. However, the upstream regulation of Plk1 activation remains unclear. Here we have studied the interplay between Plk1 and Cdc2 through meiotic and early embryonic cycles in starfish. Distinct kinases, cyclin B-Cdc2, MAPK along with cyclin B- and/or cyclin A-Cdc2 and cyclin A-Cdc2, were unique upstream regulators for Plk1 activation at meiosis I, meiosis II and embryonic M-phase, respectively, indicating that Plk1 is not the trigger kinase at meiotic reinitiation. When Plk1 was required for cyclin B-Cdc2 activation, the action of Plk1 was mediated primarily through suppression of Myt1 rather than through activation of Cdc25. We propose that Plk1 can be activated by either cyclin A- or cyclin B-Cdc2, and its primary target is Myt1.

Xeroderma pigmentosum group C protein possesses a high affinity binding site to human centrin 2 and calmodulin

Human centrin 2 (HsCen2), a member of the EF-hand superfamily of Ca2+-binding proteins, is commonly associated with centrosome-related structures. The protein is organized in two domains, each containing two EF-hand motifs, but only the C-terminal half exhibits Ca2+ sensor properties. A significant fraction of HsCen2 is localized in the nucleus, where it was recently found associated with the xeroderma pigmentosum group C protein (XPC), a component of the nuclear excision repair pathway. Analysis of the XPC sequence (940 residues), using a calmodulin target recognition software, enabled us to predict two putative binding sites. The binding properties of the two corresponding peptides were investigated by isothermal titration calorimetry. Only one of the peptides (P1-XPC) interacts strongly (Ka = 2.2 x 10(8) m-1, stoichiometry 1:1) with HsCen2 in a Ca2+-dependent manner. This peptide also binds, with a similar affinity (Ka = 1.1 x 10(8) m-1) to a C-terminal construct of HsCen2, indicating that the interaction with the integral protein is mainly the result of the contribution of the C-terminal half. The second peptide (P2-XPC) failed to show any detectable binding either to HsCen2 or to its C-terminal lobe. The two peptides interact with different affinities and mechanisms with calmodulin. Circular dichroism and nuclear magnetic resonance were used to structurally characterize the complex formed by the C-terminal domain of HsCen2 with P1-XPC.

Cell cycle-regulated phosphorylation of hamartin, the product of the tuberous sclerosis complex 1 gene, by cyclin-dependent kinase 1/cyclin B

Tuberous sclerosis complex is a tumor suppressor gene syndrome whose manifestations can include seizures, mental retardation, and benign tumors of the brain, skin, heart, and kidneys. Hamartin and tuberin, the products of the TSC1 and TSC2 genes, respectively, form a complex and inhibit signaling by the mammalian target of rapamycin. Here, we demonstrate that endogenous hamartin is threonine-phosphorylated during nocodazole-induced G2/M arrest and during the G2/M phase of a normal cell cycle. In vitro assays showed that cyclin-dependent kinase 1 phosphorylates hamartin at three sites, one of which (Thr417) is in the hamartin-tuberin interaction domain. Tuberin interacts with phosphohamartin, and tuberin expression attenuates the phosphorylation of exogenous hamartin. Hamartin with alanine mutations in the three cyclin-dependent kinase 1 phosphorylation sites increased the inhibition of p70S6 kinase by the hamartin-tuberin complex. These findings support a model in which phosphorylation of hamartin regulates the function of the hamartin-tuberin complex during the G2/M phase of the cell cycle.

A polo match for Plk1

New work by Elia et al. in this issue of Cell reveals the molecular basis of phosphopeptide recognition by the polo domain and the domain's dual function to promote substrate recognition by targeting the kinase to subcellular structures and to autoregulate the adjacent protein kinase catalytic domain.

Structural Basis of Aurora-A Activation by TPX2 at the Mitotic Spindle

Aurora-A is an oncogenic kinase essential for mitotic spindle assembly. It is activated by phosphorylation and by the microtubule-associated protein TPX2, which also localizes the kinase to spindle microtubules. We have uncovered the molecular mechanism of Aurora-A activation by determining crystal structures of its phosphorylated form both with and without a 43 residue long domain of TPX2 that we identified as fully functional for kinase activation and protection from dephosphorylation. In the absence of TPX2, the Aurora-A activation segment is in an inactive conformation, with the crucial phosphothreonine exposed and accessible for deactivation. Binding of TPX2 triggers no global conformational changes in the kinase but pulls on the activation segment, swinging the phosphothreonine into a buried position and locking the active conformation. The recognition between Aurora-A and TPX2 resembles that between the cAPK catalytic core and its flanking regions, suggesting this molecular mechanism may be a recurring theme in kinase regulation.

Aurora-A kinase maintains the fidelity of early and late mitotic events in HeLa cells

Aurora-A, a member of the Aurora/Ipl1-related kinase family, is overexpressed in various types of cancer and considered to play critical roles in tumorigenesis. To better understand the pathological effect of Aurora-A activation, it is first necessary to elucidate the physiological functions of Aurora-A. Here, we have investigated the roles of Aurora-A in mitotic progression with the small interfering RNA, antibody microinjection, and time lapse microscopy using human cells. We demonstrated that suppression of Aurora-A by small interfering RNA caused multiple events to fail in mitosis, such as incorrect separation of centriole pairs, misalignment of chromosomes on the metaphase plate, and incomplete cytokinesis. Antibody microinjection of Aurora-A into late G2 cells induced dose-dependent failure in separation of centriole pairs at prophase, indicating that Aurora-A is essential for proper separation of centriole pairs. When we injected anti-Aurora-A antibodies into prometaphase cells that had separated their centriole pairs, chromosomes were severely misaligned on the metaphase plate, indicating that Aurora-A is required for proper movement of chromosomes on the metaphase plate. Furthermore, inhibition of Aurora-A at metaphase by microinjected antibodies prevented cells from completing cytokinesis, suggesting that Aurora-A also has important functions in late mitosis. These results strongly suggest that Aurora-A is essential for many crucial events during mitosis and that the phosphorylation of a series of substrates by Aurora-A at different stages of mitosis may promote diverse critical events in mitosis to maintain chromosome integrity in human cells.

Identification of novel phosphorylation sites on Xenopus laevis Aurora A and analysis of phosphopeptide enrichment by immobilized metal-affinity chromatography

Mass spectrometric analysis of proteolytically derived phosphopeptides has developed into a widespread technique for the identification of phosphorylated amino acids. Using liquid chromatography-electrospray ionization tandem mass spectrometry, 14 phosphorylation sites were identified on Xenopus laevis His6-Aurora A, a highly conserved regulator of centrosome maturation and cell division. These included seven novel phosphorylation sites, Ser-12, Thr-21, Thr-103, Ser-116, Thr-122, Tyr-155, and Thr-294, as well as the previously identified regulatory sites, Ser-53, Thr-295, and Ser-349. The identification of these novel phosphorylation sites will be important for future studies aimed at elucidating the mechanisms of Aurora A regulation by phosphorylation. Furthermore, we demonstrate that a "kinase-inactive" mutant of Aurora A, K169R, still retains 10% of activity of the wild-type enzyme in vitro along with occupancy of Thr-295 and Ser-12. However, mutation of Asp-281 to Ala completely abolishes activity of the enzyme and should therefore be used preferentially as a genuine kinase-dead construct. Because of the abundance of phosphorylated residues on His6-Aurora A, we found this protein to be an ideal tool for the characterization of immobilized metal-affinity chromatography (IMAC) as a method for phosphopeptide enrichment from complex mixtures. We present a detailed analysis of the binding and elution properties of both the phosphopeptides and unphosphorylated peptides of His6-Aurora A to Fe3+-IMAC before and after methyl esterification. Moreover, we demonstrate a significant difference in enrichment of phosphopeptides when different resins are used for Fe3+-IMAC and characterize the strengths and limitations of this methodology for the study of phosphoproteomics.

M phase-specific phosphorylation of BRCA2 by Polo-like kinase 1 correlates with the dissociation of the BRCA2-P/CAF complex

BRCA2 is a breast tumor susceptibility gene encoding a 390-kDa protein with functions in maintaining genomic stability and cell cycle progression. Evidence has been accumulated to support the concept that BRCA2 has a critical role in homologous recombination of DNA double-stranded breaks by interacting with RAD51. In addition, BRCA2 may have chromatin modifying activity through interaction with a histone acetyltransferase protein, p300/CBP-associated factor (P/CAF). To explore how the functions of BRCA2 may be regulated, the post-translational modifications of BRCA2 throughout the cell cycle were examined. We found that BRCA2 is hyperphosphorylated specifically in M phase and becomes dephosphorylated as cells exit M phase and enter interphase. This specific phosphorylation of BRCA2 was not observed in cells treated with DNA-damaging agents. Systematic mapping of the potential mitosis specific phosphorylation sites revealed the N-terminal 284 amino acids of BRCA2 (BR-N1) as the major region of phosphorylation and mass spectrometric analysis identified two phosphopeptides that contain "phosphorylation consensus motifs" for Polo-like kinase 1 (Plk1). Phosphorylation of BR-N1 with Plk1 recapitulated the electrophoretic mobility change as seen in BR-N1 isolated from M phase cells. Plk1 interacts with BRCA2 in vivo, and mutation of Ser193, Ser205/206, and Thr203/207 to Ala in BR-N1 abolished Plk1 phosphorylation, suggesting that BRCA2 is the substrate of Plk1. Furthermore, both the hyperphosphorylated and hypophosphorylated forms of BRCA2 bind to RAD51, whereas the M phase hyperphosphorylated form of BRCA2 no longer associates with the P/CAF, suggesting that the dissociation of P/CAF-BRCA2 complex is regulated by phosphorylation. Taken together, these results implicate a potential role of BRCA2 in modulating M phase progression.

Molecular characterization of human ninein protein: two distinct subdomains required for centrosomal targeting and regulating signals in cell cycle

The centrosomal protein ninein has been identified as a microtubules minus end capping, centriole position, and anchoring protein, but the true physiological function remains to be determined. In this report, using immunofluorescence analysis and GFP-fusions we show that coiled-coil II domain (CCII domain, 1303-2096) co-localized with gamma-tubulin and centrin at the centrosome. We further narrow down within 83 amino acids and classify a new centrosomal targeting signal. Interestingly, antibodies raised against CCII domain reveal that ninein protein declines from spindle poles during mitosis, but reaccumulates at centrosomes at the end of cell division. Moreover, the data also suggest that fragment 1783-1866 may be attributed to declined signal of ninein. In kinase assay, we show that CCII domain could readily be phosphorylated by AIK and PKA. Taken together, our results suggest that ninein protein contains two distinct subdomains which are required for targeting and regulating asymmetry centrosomes. Importantly, the decline of ninein during mitosis implies that this centrosomal protein may play a role to regulate the process of chromosome segregation without discrimination. The model we propose here will foster a clearer picture of how two asymmetric centrosomes could direct and ensure the correct segregation of chromosomes during the mitotic stage.

Phosphorylation of mitotic kinesin-like protein 2 by polo-like kinase 1 is required for cytokinesis

We have investigated the function of mitotic kinesin-like protein (MKlp) 2, a kinesin localized to the central spindle, and demonstrate that its depletion results in a failure of cleavage furrow ingression and cytokinesis, and disrupts localization of polo-like kinase 1 (Plk1). MKlp2 is a target for Plk1, and phosphorylated MKlp2 binds to the polo box domain of Plk1. Plk1 also binds directly to microtubules and targets to the central spindle via its polo box domain, and this interaction controls the activity of Plk1 toward MKlp2. An antibody to the neck region of MKlp2 that prevents phosphorylation of MKlp2 by Plk1 causes a cytokinesis defect when introduced into cells. We propose that phosphorylation of MKlp2 by Plk1 is necessary for the spatial restriction of Plk1 to the central spindle during anaphase and telophase, and the complex of these two proteins is required for cytokinesis.

Aurora A and Mitotic Commitment

A remarkable study published in this issue of Cell reveals a key role of Aurora A protein kinase in G2/M progression. To achieve this role, Aurora A acts in conjunction with the LIM protein Ajuba, which functions as an activating factor.

The minus end in sight

Microtubules are intrinsically polar structures. A consequence of this polarity is that the two ends of the microtubule polymer exhibit different properties. The more dynamic plus ends and the mechanisms that regulate their behavior have been the focus of much recent attention. Here, we concentrate on the dynamics and regulation of minus ends, which play distinct but equally critical roles in microtubule function. In the first part of this review, we compare the in vitro and in vivo behavior of microtubules from a minus end perspective. This comparison suggests that cells possess conserved mechanisms to specifically inhibit minus end polymerization, and perhaps also to actively promote depolymerization. In the second part, we focus on the spatial positioning of minus ends, which is achieved by localized microtubule nucleation, minus end capping and minus end anchoring as well as by motor-dependent sorting. These mechanisms are used in different biological contexts to generate the diversity of organized microtubule arrays in cells.

Exploring the functional interactions between Aurora B, INCENP, and survivin in mitosis

The function of the Aurora B kinase at centromeres and the central spindle is crucial for chromosome segregation and cytokinesis, respectively. Herein, we have investigated the regulation of human Aurora B by its complex partners inner centromere protein (INCENP) and survivin. We found that overexpression of a catalytically inactive, dominant-negative mutant of Aurora B impaired the localization of the entire Aurora B/INCENP/survivin complex to centromeres and the central spindle and severely disturbed mitotic progression. Similar results were also observed after depletion, by RNA interference, of either Aurora B, INCENP, or survivin. These data suggest that Aurora B kinase activity and the formation of the Aurora B/INCENP/survivin complex both contribute to its proper localization. Using recombinant proteins, we found that Aurora B kinase activity was stimulated by INCENP and that the C-terminal region of INCENP was sufficient for activation. Under identical assay conditions, survivin did not detectably influence kinase activity. Human INCENP was a substrate of Aurora B and mass spectrometry identified three consecutive residues (threonine 893, serine 894, and serine 895) containing at least two phosphorylation sites. A nonphosphorylatable mutant (TSS893-895AAA) was a poor activator of Aurora B, demonstrating that INCENP phosphorylation is important for kinase activation.

Identification of a consensus motif for Plk (Polo-like kinase) phosphorylation reveals Myt1 as a Plk1 substrate

Plk1 (Polo-like kinase 1), an evolutionarily conserved serine/threonine kinase, is crucially involved in multiple events during the M phase. Here we have identified a consensus phosphorylation sequence for Plk1, by testing the ability of systematically mutated peptides derived from human Cdc25C to serve as a substrate for Plk1. The obtained results show that a hydrophobic amino acid at position +1 carboxyl-terminal of phosphorylated Ser/Thr and an acidic amino acid at position -2 are important for optimal phosphorylation by Plk1. We have then found that Myt1, an inhibitory kinase for MPF, has a number of putative phosphorylation sites for Plk1 in its COOH-terminal portion. While wild-type Myt1 (Myt1-WT) served as a good substrate for Plk1 in vitro, a mutant Myt1 (Myt1-4A), in which the four putative phosphorylation sites are replaced by alanines, did not. In nocodazole-treated cells, Myt1-WT, but not Myt1-4A, displayed its mobility shift in gel electrophoresis, due to phosphorylation. These results suggest that Plk1 phosphorylates Myt1 during M phase. Thus, this study identifies a novel substrate for Plk1 by determining a consensus phosphorylation sequence by Plk1.

Nek2A kinase stimulates centrosome disjunction and is required for formation of bipolar mitotic spindles

Nek2A is a cell cycle-regulated kinase of the never in mitosis A (NIMA) family that is highly enriched at the centrosome. One model for Nek2A function proposes that it regulates cohesion between the mother and daughter centriole through phosphorylation of C-Nap1, a large coiled-coil protein that localizes to centriolar ends. Phosphorylation of C-Nap1 at the G2/M transition may trigger its displacement from centrioles, promoting their separation and subsequent bipolar spindle formation. To test this model, we generated tetracycline-inducible cell lines overexpressing wild-type and kinase-dead versions of Nek2A. Live cell imaging revealed that active Nek2A stimulates the sustained splitting of interphase centrioles indicative of loss of cohesion. However, this splitting is accompanied by only a partial reduction in centriolar C-Nap1. Strikingly, induction of kinase-dead Nek2A led to formation of monopolar spindles with unseparated spindle poles that lack C-Nap1. Furthermore, kinase-dead Nek2A interfered with chromosome segregation and cytokinesis and led to an overall change in the DNA content of the cell population. These results provide the first direct evidence in human cells that Nek2A function is required for the correct execution of mitosis, most likely through promotion of centrosome disjunction. However, they suggest that loss of centriole cohesion and C-Nap1 displacement may be distinct mitotic events.

Cell cycle dependent expression of Plk1 in synchronized porcine fetal fibroblasts

Enzymes of the Polo-like kinase (Plk) family are active in the pathways controlling mitosis in several species. We have cloned cDNA fragments of the porcine homologues of Plk1, Plk2, and Plk3 employing fetal fibroblasts as source. All three partial cDNAs showed high sequence homology with their mouse and human counterparts and contained the Polo box, a domain characteristic for all Polo kinases. The expression levels of Plk1 mRNA at various points of the cell cycle in synchronized porcine fetal fibroblasts were analyzed by both RT-PCR and the ribonuclease protection assay. Plk1 mRNA was barely detectable in G0 and G1, increased during S phase and peaked after the G2/M transition. A monoclonal antibody was generated against an in vitro expressed porcine Plk1-protein fragment and used to detect changes in Plk1 expression at the protein level. Plk1 protein was first detected by immunoblotting at the beginning of S phase and was highest after the G2/M transition. In summary, the Plk1 expression pattern in the pig is similar to that reported for other species. The absence of Plk1 mRNA and protein appears to be a good marker for G0/G1 and thus for the selection of donor cells for nuclear transfer based somatic cloning.

A requirement for MAP kinase in the assembly and maintenance of the mitotic spindle

Circumstantial evidence has suggested the possibility of microtubule-associated protein (MAP) kinase's involvement in spindle regulation. To test this directly, we asked whether MAP kinase was required for spindle assembly in Xenopus egg extracts. Either the inhibition or the depletion of endogenous p42 MAP kinase resulted in defective spindle structures resembling asters or half-spindles. Likewise, an increase in the length and polymerization of microtubules was measured in aster assays suggesting a role for MAP kinase in regulating microtubule dynamics. Consistent with this, treatment of extracts with either a specific MAP kinase kinase inhibitor or a MAP kinase phosphatase resulted in the rapid disassembly of bipolar spindles into large asters. Finally, we report that mitotic progression in the absence of MAP kinase signaling led to multiple spindle abnormalities in NIH 3T3 cells. We therefore propose that MAP kinase is a key regulator of the mitotic spindle.

Cdc2-cyclin B triggers H3 kinase activation of Aurora-A in Xenopus oocytes

Xenopus oocytes are arrested in meiotic prophase I and resume meiotic divisions in response to progesterone. Progesterone triggers activation of M-phase promoting factor (MPF) or Cdc2-cyclin B complex and neosynthesis of Mos kinase, responsible for MAPK activation. Both Cdc2 and MAPK activities are required for the success of meiotic maturation. However, the signaling pathway induced by progesterone and leading to MPF activation is poorly understood, and most of the targets of both Cdc2 and MAPK in the oocyte remain to be determined. Aurora-A is a Ser/Thr kinase involved in separation of centrosomes and in spindle assembly during mitosis. It has been proposed that in Xenopus oocytes Aurora-A could be an early component of the progesterone-transduction pathway, acting through the regulation of Mos synthesis upstream Cdc2 activation. We addressed here the question of Aurora-A regulation during meiotic maturation by using new in vitro and in vivo experimental approaches. We demonstrate that Cdc2 kinase activity is necessary and sufficient to trigger both Aurora-A phosphorylation and kinase activation in Xenopus oocyte. In contrast, these events are independent of the Mos/MAPK pathway. Aurora-A is phosphorylated in vivo at least on three residues that regulate differentially its kinase activity. Therefore, Aurora-A is under the control of Cdc2 in the Xenopus oocyte and could be involved in meiotic spindle establishment.

Exploration of the cell-cycle genes found within the RIKEN FANTOM2 data set

The cell cycle is one of the most fundamental processes within a cell. Phase-dependent expression and cell-cycle checkpoints require a high level of control. A large number of genes with varying functions and modes of action are responsible for this biology. In a targeted exploration of the FANTOM2-Variable Protein Set, a number of mouse homologs to known cell-cycle regulators as well as novel members of cell-cycle families were identified. Focusing on two prototype cell-cycle families, the cyclins and the NIMA-related kinases (NEKs), we believe we have identified all of the mouse members of these families, 24 cyclins and 10 NEKs, and mapped them to ENSEMBL transcripts. To attempt to globally identify all potential cell cycle-related genes within mouse, the MGI (Mouse Genome Database) assignments for the RIKEN Representative Set (RPS) and the results from two homology-based queries were merged. We identified 1415 genes with possible cell-cycle roles, and 1758 potential paralogs. We comment on the genes identified in this screen and evaluate the merits of each approach.

Genomics and proteomics in cancer

Cancer development is driven by the accumulation of DNA changes in the approximately 40000 chromosomal genes. In solid tumours, chromosomal numerical/structural aberrations are common. DNA repair defects may lead to genome-wide genetic instability, which can drive further cancer progression. The genes code the actual players in the cellular processes, the 100000-10 million proteins, which in (pre)malignant cells can also be altered in a variety of ways. Over the past decade, our knowledge of the human genome and Genomics (the study of the human genome) in (pre)malignancies has increased enormously and Proteomics (the analysis of the protein complement of the genome) has taken off as well. Both will play an increasingly important role. In this article, a short description of the essential molecular biological cell processes is given. Important genomic and proteomic research methods are described and illustrated. Applications are still limited, but the evidence so far is exciting. Will genomics replace classical diagnostic or prognostic procedures? In breast cancers, the gene expression array is stronger than classical criteria, but in endometrial hyperplasia, quantitative morphological features are more cost-effective than genetic testing. It is still too early to make strong statements, the more so because it is expected that genomics and proteomics will expand rapidly. However, it is likely that they will take a central place in the understanding, diagnosis, monitoring and treatment of (pre)cancers of many different sites.

Regulation of p27 in the process of neuroblastoma N2A differentiation

Neuronal differentiation implies morphological and biochemical changes to generate a specialized neuron. N2A neuroblastoma cells can be promoted to undergo differentiation associated to neurites outgrowth, a process linked to the arrest of cell division. Using N2A cells as a model, we investigated the detailed molecular aspects on the involvement of p27 in dibutyryl cAMP-induced neuronal differentiation. In the undifferentiated N2A phenotype, an unusually high level of accumulated p27 protein mass was evidenced. Data suggest that in proliferating cells, p27 could be sequestered by direct interaction with cyclin D1, thus preventing its inhibitory action on cell cycle Cdks. Studies also indicate that p27 is functionally active and that its loss of action on Cdks in proliferating cells is due to its strong association with cyclin D1. Therefore, when cell differentiation is triggered, the action of p27 on Cdks seems to depend on both p27 and cyclin D1 degradation during the early steps of differentiation followed by late events of re-synthesis of active p27. In this context, an overexpression of p27 after N2A transfection with a mouse p27 clone induces the outgrowth of neurites associated with a decrease in cyclin D1 expression. On the other hand, treatment of N2A undifferentiated cells with c-myc antisense oligonucleotides led to a decrease in p27 and cyclin D1 levels, similar events as those in early stages of cell differentiation. Studies suggest that blockage in c-myc expression triggers early events in neuronal differentiation. These studies are of the utmost importance to elucidate regulatory mechanisms of molecules that play a critical role in the transition from a proliferating phenotype to differentiated cells.

Cells in G2/M phase increased in human nasopharyngeal carcinoma cell line by EBV-LMP1 through activation of NF-kappaB and AP-1

Although previous studies showed that the principal oncoprotein encoded by Epstein-Barr virus, latent membrane protein 1(LMP1), could induce the nasopharyngeal carcinoma cells in G2/M phase increased, little is known about the target molecules and mechanisms. The present study demonstrated that LMP1 could induce the accumulation of p53 protein and upregulate its transactivity in a dose dependent manner, which resulted in the decrease of the kinase activity of cdc2/cyclin B complex and inducing arrest at G2/M phase through the activation of NF-kappaB and AP-1 signaling pathways, and the effect of NF-kappaB was more obvious than that of AP-1. This study provided some significant evidence for further elucidating the molecular mechanisms that LMP1 had effects on the surveillance mechanism of cell cycle and promoting the survival of transformed cells and tumorigenesis.

Phosphoregulators: protein kinases and protein phosphatases of mouse

With the completion of the human and mouse genome sequences, the task now turns to identifying their encoded transcripts and assigning gene function. In this study, we have undertaken a computational approach to identify and classify all of the protein kinases and phosphatases present in the mouse gene complement. A nonredundant set of these sequences was produced by mining Ensembl gene predictions and publicly available cDNA sequences with a panel of InterPro domains. This approach identified 561 candidate protein kinases and 162 candidate protein phosphatases. This cohort was then analyzed using TribeMCL protein sequence similarity clustering followed by CLUSTALV alignment and hierarchical tree generation. This approach allowed us to (1) distinguish between true members of the protein kinase and phosphatase families and enzymes of related biochemistry, (2) determine the structure of the families, and (3) suggest functions for previously uncharacterized members. The classifications obtained by this approach were in good agreement with previous schemes and allowed us to demonstrate domain associations with a number of clusters. Finally, we comment on the complementary nature of cDNA and genome-based gene detection and the impact of the FANTOM2 transcriptome project.

Golgi architecture and inheritance

Golgi inheritance proceeds via sequential biogenesis and partitioning phases. Although little is known about Golgi growth and replication (biogenesis), ultrastructural and fluorescence analyses have provided a detailed, though still controversial, perspective of Golgi partitioning during mitosis in mammalian cells. Partitioning requires the fragmentation of the juxtanuclear ribbon of interconnected Golgi stacks into a multitude of tubulovesicular clusters. This process is choreographed by a cohort of mitotic kinases and an inhibition of heterotypic and homotypic Golgi membrane-fusion events. Our model posits that accurate partitioning occurs early in mitosis by the equilibration of Golgi components on either side of the metaphase plate. Disseminated Golgi components then coalesce to regenerate Golgi stacks during telophase. Semi-intact cell and cell-free assays have accurately recreated these processes and allowed their molecular dissection. This review attempts to integrate recent findings to depict a more coherent, synthetic molecular picture of mitotic Golgi fragmentation and reassembly. Of particular importance is the emerging concept of a highly regulated and dynamic Golgi structural matrix or template that interfaces with cargo receptors, Golgi enzymes, Rab-GTPases, and SNAREs to tightly couple biosynthetic transport to Golgi architecture. This structural framework may be instructive for Golgi biogenesis and may encode sufficient information to ensure accurate Golgi inheritance, thereby helping to resolve some of the current discrepancies between different workers.