Regulation of a major microtubule-associated protein by MPF and MAP kinase.

The interphase-M phase transition of microtubule dynamics is thought to be induced by phosphorylation reactions mediated by MPF and by MAP kinase functioning downstream of MPF. We have now identified and purified from Xenopus eggs a major microtubule-associated protein, p220, that may be a target protein for these two M phase-activated kinases. p220, when purified from interphase cells, potently bound to microtubules and stimulated tubulin polymerization, whereas p220 purified from M phase cells showed little or no such activities. Cell staining with a monoclonal anti-p220 antibody revealed that p220 is localized on cytoplasmic microtubule networks during interphase, while it is distributed rather diffusely throughout the cell during M phase. We have further found that p220 is phosphorylated specifically in M phase. Moreover, p220 purified from interphase cells served as a good substrate for MAP kinase and MPF in vitro, and two-dimensional phosphopeptide mapping pattern of the p220 phosphorylated in vitro was very similar to that of p220 phosphorylated at M phase in vivo. These results suggest that the drastic change in p220 activity during the transition from interphase to M phase may be induced by its phosphorylation in M phase probably catalyzed by MAP kinase and MPF.     

Threonine phosphorylation is associated with mitosis in HeLa cells

Phosphorylation and dephosphorylation of proteins play an important role in the regulation of mitosis and meiosis. In our previous studies we have described mitosis-specific monoclonal antibody MPM-2 that recognizes a family of phosphopeptides in mitotic cells but not in interphase cells. These peptides are synthesized in S phase but modified by phosphorylation during G2/mitosis transition. The epitope for the MPM-2 is a phosphorylated site. In this study, we attempted to determine which amino acids are phosphorylated during the G2-mitosis (M) transition. We raised a polyclonal antibody against one of the antigens recognized by MPM-2, i.e. a protein of 55 kDa, that is present in interphase cells but modified by phosphorylation during mitosis. This antibody recognizes the p55 protein in both interphase and mitosis while it is recognized by the monoclonal antibody MPM-2 only in mitotic cells. Phosphoamino acid analysis of protein p55 from 32P-labeled S-phase and M-phase HeLa cell extracts after immunoprecipitation with anti-p55 antibodies revealed that threonine was extensively phosphorylated in p55 during G2-M but not in S phase, whereas serine was phosphorylated during both S and M phases. Tyrosine was not phosphorylated. Identical results were obtained when antigens recognized by MPM-2 were subjected to similar analysis. As cells completed mitosis and entered G1 phase phosphothreonine was completely dephosphorylated whereas phosphoserine was not. These results suggest that phosphorylation of threonine might be specific to some of the mitosis-related events.     

Regulation of Greatwall kinase during Xenopus oocyte maturation

Greatwall kinase has been identified as a key element in M phase initiation and maintenance in Drosophila, Xenopus oocytes/eggs, and mammalian cells. In M phase, Greatwall phosphorylates endosulfine and related proteins that bind to and inhibit protein phosphatase 2A/B55, the principal phosphatase for Cdk-phosphorylated substrates. We show that Greatwall binds active PP2A/B55 in G2 phase oocytes but dissociates from it when progesterone-treated oocytes reach M phase. This dissociation does not require Greatwall kinase activity or phosphorylation at T748 in the presumptive T loop of the kinase. A mutant K71M Greatwall, also known as Scant in Drosophila, induces M phase in the absence of progesterone when expressed in oocytes, despite its reduced stability and elevated degradation by the proteasome. M phase induction by Scant Greatwall requires protein synthesis but is not associated with altered binding or release of PP2A/B55 as compared to wild-type Greatwall. However, in vitro studies with Greatwall proteins purified from interphase cells indicate that Scant, but not wild-type Greatwall, has low but detectable activity against endosulfine. These results demonstrate progesterone-dependent regulation of the PP2A/B55-Greatwall interaction during oocyte maturation and suggest that the cognate Scant Greatwall mutation has sufficient constitutive kinase activity to promote M phase in Xenopus oocytes.     

Isolation and characterization of a calendic acid producing (8,11)-linoleoyl desaturase

Protein kinase C-theta is a member of the n-protein kinase C subfamily that in mitotic cells translocates to centrosomes and kinetochores. Although this kinase is expressed in comparable amounts in murine erythroleukaemia cells during the interphase or metaphase, when localized in the mitotic structures, it selectively phosphorylates a 66 kDa protein, also associated to chromosomes. Moreover, protein kinase C-theta immunoprecipitated from cells at the metaphase results four times more active in the absence of lipid cofactors as compared with the kinase obtained from cells in the interphase. This activation is accomplished by interaction of protein kinase C-theta with a protein factor which also promotes an increased autophosphorylation of the kinase. These findings indicate that in the mitotic phase of the cell cycle, protein kinase C-theta recognizes a protein factor which operates as a positive modulator of the kinase activity in the absence lipids.     

Oncostatin M induces tyrosine phosphorylation in endothelial cells and activation of p62yes tyrosine kinase.

Oncostatin M is a polypeptide cytokine produced by activated and transformed T lymphocytes that has diverse biologic effects, including growth inhibition of tumor cells and induction of IL-6 expression in cultured human endothelial cells (HEC). HEC are highly responsive to oncostatin M and express high levels of oncostatin M receptors relative to other cell types. Oncostatin M has previously been found to bind a specific receptor of 150 to 160 kDa. We have found through the use of anti-phosphotyrosine immunoblotting that oncostatin M induces tyrosine phosphorylation in HEC. Anti-phosphotyrosine antibodies specifically immunoprecipitated labeled oncostatin M cross-linked to its receptor, demonstrating that the oncostatin M receptor is either directly phosphorylated on tyrosine after ligand binding or is tightly associated with a phosphotyrosyl protein in these cells. The tyrosine kinase inhibitor herbimycin A blocked the induction of IL-6 by oncostatin M in HEC. In addition, immune complex kinase assays showed that oncostatin M markedly increased the activity of the p62yes tyrosine kinase with a small increase in p59fyn but no increase in p56lyn tyrosine kinase activity in HEC. We conclude that oncostatin M utilizes a tyrosine phosphorylation signal transduction pathway in HEC involving the activation of the p62yes tyrosine kinase, and that this tyrosine phosphorylation pathway leads to the induction of IL-6 expression.     

H1 subtype expression during cell proliferation and growth arrest

H1 histone subtype genes differ in their expression patterns during the different stages of the cell cycle interphase. While the group of replication-dependent H1 histone subtypes is synthesized during S phase, the replacement histone subtype H1.0 is also expressed replication-independently in non-proliferating cells. The present study is the first report about the analysis of the cell cycle-dependent expression of all five replication-dependent H1 subtypes, the replacement histone H1.0 and the ubiquitously expressed subtype H1x. The expression of these H1 histone subtypes in HeLa cells was analysed on mRNA level by quantitative real-time RT-PCR as well as on protein level by immunoblotting. We found that after arrest of HeLa cells in G1 phase by treatment with sodium butyrate, the mRNA levels of all replication-dependently expressed H1 subtypes decreased, but to very different extent. During S phase the individual replication-dependently expressed H1 subtypes show similar kinetics regarding their mRNA levels. However, the variations in their protein amounts partially differ from the respective RNA levels which especially applies to histone H1.3. In contrast, the mRNA as well as the protein level of H1x remained nearly unchanged in G1 as well as during S phase progression. The results of the present study demonstrate that the cell cycle-dependent mRNA and protein expression of various H1 subtypes is differentially regulated, supporting the hypothesis of a functional heterogeneity.     

Changes in a nuclear matrix antigen during the cell cycle: interphase and mitotic cells

We studied the behaviour in interphase and mitotic human cells of a 125 kDa (pI 6.5) antigen, associated with the nuclear matrix and detected in proliferating cells. Indirect immunofluorescence with a specific monoclonal antibody reveals that during interphase in WISH and Namalwa cells, as well as phytohaemagglutinin-stimulated lymphocytes, the antigen displays a speckled distribution in the nucleoplasm of all cells. At early prophase the fluorescence intensity of the coalesced speckles increases markedly. During metaphase and anaphase the antigen gives maximal fluorescence distributed diffusely in the nucleoplasm, while chromosomes remain negative. At anaphase and cytokinesis the antigen is still cytoplasmic, but fluorescence intensity decreases. Two-dimensional gel electrophoresis and immunoblotting reveal that the p125/6.5 antigen displays a net increase in isolated mitotic cells as compared to interphase cells. These results suggest that the p125/6.5 protein participates in late G2 phase and G2/M transition events preparing the cell for mitosis.     

Regulation of microtubule organization during interphase and M phase

Microtubule (MT) dynamics and organization change markedly during interphase-M phase transition of the cell cycle. This mini review focuses first on p220, a ubiquitous MT-associated protein of Xenopus. p220 is phosphorylated by p34cdc2 kinase and MAP kinase in M phase, and concomitantly loses its MT-binding and MT-stabilizing activities. A cDNA encoding p220 was cloned, which identified p220 as a Xenopus homolog of MAP4, and p220 was therefore termed XMAP4. To examine the physiological relevance of XMAP4 phosphorylation during mitosis, Xenopus A6 cells were transfected with cDNA encoding wild-type or various XMAP4 mutants fused with a green fluorescent protein (GFP). Mutations of serine and threonine within potential phosphorylation sites for p34cdc2 kinase to nonphosphorylatable alanine interfered with mitosis-associated reduction in MT-affinity of XMAP4 and their overexpression affected chromosome movement during anaphase A. These results indicated that phosphorylation of XMAP4 by p34cdc2 kinase is responsible for the decrease in its MT-binding and MT-stabilizing activities during mitosis which are important for chromosome movement during anaphase A. The second focus is on a novel monoclonal antibody W8C3, which recognizes alpha-tubulin. W8C3 stained spindle MTs but not interphase MTs of Xenopus A6 cells, although tubulin dimers in M phase and interphase were equally recognized by this antibody. The difference in MT staining pattern may be because the W8C3-recognition site on alpha-tubulin is sterically hidden in interphase MTs but not in spindle MTs.     

Immunodetection of retinoblastoma-related protein and its phosphorylated form in interphase and mitotic alfalfa cells

Plant retinoblastoma-related (RBR) proteins are primarily considered as key regulators of G(1)/S phase transition, with functional roles in a variety of cellular events during plant growth and organ development. Polyclonal antibody against the C-terminal region of the Arabidopsis RBR1 protein also specifically recognizes the alfalfa 115?kDa MsRBR protein, as shown by the antigen competition assay. The MsRBR protein was detected in all cell cycle phases, with a moderate increase in samples representing G(2)/M cells. Antibody against the human phospho-pRb peptide (Ser807/811) cross-reacted with the same 115?kDa MsRBR protein and with the in vitro phosphorylated MsRBR protein C-terminal fragment. Phospho-MsRBR protein was low in G(1) cells. Its amount increased upon entry into the S phase and remained high during the G(2)/M phases. Roscovitine treatment abolished the activity of alfalfa MsCDKA1;1 and MsCDKB2;1, and the phospho-MsRBR protein level was significantly decreased in the treated cells. Colchicine block increased the detected levels of both forms of MsRBR protein. Reduced levels of the MsRBR protein in cells at stationary phase or grown in hormone-free medium can be a sign of the division-dependent presence of plant RBR proteins. Immunolocalization of the phospho-MsRBR protein indicated spots of variable number and size in the labelled interphase nuclei and high signal intensity of nuclear granules in prophase. Structures similar to phospho-MsRBR proteins cannot be recognized in later mitotic phases. Based on the presented western blot and immunolocalization data, the possible involvement of RBR proteins in G(2)/M phase regulation in plant cells is discussed.     

The NPK1 mitogen-activated protein kinase kinase kinase contains a functional nuclear localization signal at the binding site for the NACK1 kinesin-like protein

The tobacco mitogen-activated protein kinase kinase kinase NPK1 localizes to the equatorial region of phragmoplasts by interacting with kinesin-like protein NACK1. This leads to activation of NPK1 kinase at late M phase, which is necessary for cell plate formation. Until now, its localization during interphase has not been reported. We investigated the subcellular localization of NPK1 in tobacco-cultured BY-2 cells at interphase using indirect immunofluorescence microscopy and fusion to green fluorescent protein (GFP). Fluorescence of anti-NPK1 antibodies and GFP-fused NPK1 were detected only in the nuclei of BY-2 cells at interphase. Examination of the amino acid sequence of NPK1 showed that at the carboxyl-terminal region in the regulatory domain, which contains the binding site of NACK1, NPK1 contained a cluster of basic amino acids that resemble a bipartite nuclear localization signal (NLS). Amino acid substitution mutations in the critical residues in putative NLS caused a marked reduction in nuclear localization of NPK1 in BY-2 cells, indicating that this sequence is functional in tobacco BY-2 cells. We also found that the 64-amino acid sequence at the carboxyl terminus that contains NLS sequence is essential for interaction with NACK1, and that mutations in the NLS sequence prevented NPK1 from interacting with NACK1. Thus, the amino acid sequence at the carboxyl-terminal region of NPK1 has dual functions for nuclear localization during interphase and binding NACK1 in M phase.     

Cell cycle function of a Medicago sativa A2-type cyclin interacting with a PSTAIRE-type cyclin-dependent kinase and a retinoblastoma protein

In plants multiple A-type cyclins with distinct expression patterns have been isolated and classified into three subgroups (A1-A3), while in animal somatic cells a single type of cyclin A is required for cell-cycle regulation from the S to M phases. We studied the function of an A2-type cyclin from Medicago sativa (Medsa;cycA2) which, in contrast to animal and most plant A-type cyclins, was expressed in all phases of the cell cycle. Using synchronized alfalfa cell cultures and anti-Medsa;CycA2 polyclonal antibodies, we showed that while the mRNA level increased steadily from the late G1 to the G2-M phase, the protein level after a rapid increase in S-phase reached a plateau during the G2 phase. In the yeast two-hybrid system, the Medsa;CycA2 protein interacted with the PSTAIRE-motif-containing cyclin-dependent kinase Cdc2MsA and with the maize retinoblastoma protein. Unexpectedly, the CycA2-associated kinase activity was biphasic: a first activity peak occurred in the S phase while the major one occurred during the G2/M transition, with no apparent dependence upon the actual levels of the Medsa;CycA2 and Cdc2MsA proteins. Immunohistological localization of the cyclin A2 protein by immunofluorescence and immunogold labelling revealed the presence of Medsa;CycA2 in the nucleus of the interphase and prophase cells, while it was undetectable thereafter during mitosis. Together these data suggest that Medsa;CycA2 plays a role both in the S phase and at the G2/M transition.     

The cdc2 kinase is a nuclear protein that is essential for mitosis in mammalian cells

A homolog of the fission yeast cdc2-encoded protein kinase (p34) is a component of M phase promoting factor in Xenopus oocytes. The homologous kinase in human HeLa cells is maximally active during mitosis, suggesting a mitotic role in mammalian somatic cells. This has been directly investigated by microinjection of anti-p34 antibodies into serum-stimulated rat fibroblasts. DNA synthesis was unaffected but cell division was quantitatively blocked in injected cells. Injection of antibodies against p13suc1, a component of the p34 kinase complex, did not block mitosis but caused mitotic abnormalities resulting in cells containing multiple micronuclei in the subsequent interphase. p34 localized in the nucleus during interphase. During mitosis, a fraction tightly associated with centrosomes. p13 was more evenly distributed between the nucleus and cytoplasm. These observations demonstrate that cdc2 is a nuclear and centrosomal protein that is required for mitosis in mammalian cells.     

Two distinct monokines, interleukin 1 and tumor necrosis factor, each independently induce biosynthesis and transient expression of the same antigen on the surface of cultured human vascular endothelial cells

A murine monoclonal antibody (H4/18) raised against cultured human endothelial cells (HEC) prestimulated by the monokine interleukin 1 (IL 1) recognizes a cell surface molecule inducible by IL 1 or by the distinct monokine tumor necrosis factor (TNF) in primary or serially passaged HEC. H4/18 binding is not basally expressed or inducible by IL 1 in an SV-40 transformed HEC line, in human dermal fibroblasts, or in blood leukocytes. Expression of this molecule by HEC in response to IL 1 can be blocked by protein and RNA synthesis inhibitors but not by cyclooxygenase inhibitors. In addition, H4/18 can immunoprecipitate two biosynthetically labeled polypeptides (Mr 100,000 and 120,000) from HEC stimulated with IL 1 but not from control HEC. Thus, the H4/18 binding site appears to be an inducible surface protein specific for HEC. The majority of HEC in a culture can be induced to express the H4/18 binding protein, but expression is transient (peak 4 to 6 hr) and over the next 24 hr declines to near basal levels either in the continued presence of or upon removal of IL 1. The magnitude of the peak response depends upon IL 1 concentration (peak 5 to 10 U/ml), and the response is optimized by the continued presence of IL 1 during the initial 4- to 6-hr induction period. The time of peak H4/18 binding does not appear to be a function of IL 1 concentration. The decline of H4/18 binding from peak levels is prevented by cycloheximide, a protein synthesis inhibitor. HEC maintained in the presence of IL 1 for 24 hr become refractory to restimulation by IL 1; however, IL 1-stimulated cells rested in the absence of IL 1 for 20 hr can be stimulated by fresh IL 1. HEC expression of the H4/18 binding protein is not induced by interleukin 2 or by interferon-alpha, -beta, or -gamma. Induction of H4/18 binding by TNF is also concentration dependent, transient, and dependent upon protein and RNA synthesis. Several observations suggest that IL1 and TNF act independently on HEC. Our TNF is a recombinant protein, expressed from a cloned cDNA and thus free of IL 1 contamination; it also has no activity in a highly sensitive IL 1 assay. Our standard IL 1 preparation is affinity purified and lacks TNF activity on L929 cells. Thus, our monokine preparations are not cross-contaminated. Most interestingly, HEC incubated with IL 1 and refractory to IL1 restimulation can be restimulated by TNF to express H4/18 binding and vice versa.(ABSTRACT TRUNCATED AT 400 WORDS)     

Canonical Non-Homologous End Joining in Mitosis Induces Genome Instability and Is Suppressed by M-phase-Specific Phosphorylation of XRCC4

DNA double-strand breaks (DSBs) can be repaired by one of two major pathways—non-homologous end-joining (NHEJ) and homologous recombination (HR)—depending on whether cells are in G1 or S/G2 phase, respectively. However, the mechanisms of DSB repair during M phase remain largely unclear. In this study, we demonstrate that transient treatment of M-phase cells with the chemotherapeutic topoisomerase inhibitor etoposide induced DSBs that were often associated with anaphase bridge formation and genome instability such as dicentric chromosomes. Although most of the DSBs were carried over into the next G1 phase, some were repaired during M phase. Both NHEJ and HR, in particular NHEJ, promoted anaphase-bridge formation, suggesting that these repair pathways can induce genome instability during M phase. On the other hand, C-terminal-binding protein interacting protein (CtIP) suppressed anaphase bridge formation, implying that CtIP function prevents genome instability during mitosis. We also observed M-phase-specific phosphorylation of XRCC4, a regulatory subunit of the ligase IV complex specialized for NHEJ. This phosphorylation required cyclin-dependent kinase (CDK) activity as well as polo-like kinase 1 (Plk1). A phosphorylation-defective XRCC4 mutant showed more efficient M-phase DSB repair accompanied with an increase in anaphase bridge formation. These results suggest that phosphorylation of XRCC4 suppresses DSB repair by modulating ligase IV function to prevent genome instability during M phase. Taken together, our results indicate that XRCC4 is required not only for the promotion of NHEJ during interphase but also for its M-phase-specific suppression of DSB repair.     

蛋白激酶A对小鼠受精卵早期发育过程中M期促进因子活性的影响

为研究小鼠体内 1 细胞期受精卵M期蛋白激酶A(PKA)对M期促进因子 (MPF)活性的影响 ,应用PKA激动剂cAMP及热稳定性抑制剂PKI显微注射入 1 细胞期受精卵内 ,观察MPF及PKA活性变化 .未经注射的对照组MPF活性在分裂期增高 ,分裂间期下降 ;而PKA活性在进入分裂期下降 ,分裂间期升高 .cAMP组PKA活性维持高峰值 ,直至注射HCG后 2 8h ,MPF活性高峰延迟 30min出现 ;PKI显微注射组PKA活性低 ,而MPF活性在注射HCG后 2 7 5h即达高峰 ,且维持高峰时间达1 5h .结果表明 ,PKA活性在细胞周期中也呈波动性 ,间期活性高 ,分裂期活性低 ;PKA高活性抑制MPF活性 ,而抑制PKA活性则MPF活性高峰提前出现 .     

Detection of protein kinase activity specifically activated at metaphase-anaphase transition

We have previously reported that Ser13 and Ser34 on glial fibrillary acidic protein (GFAP) in the cleavage furrow of glioma cells are phosphorylated during late mitotic phase (Matsuoka, Y., K. Nishizawa, T. Yano, M. Shibata, S. Ando, T. Takahashi, and M. Inagaki. 1992, EMBO (Eur. Mol. Biol. Organ.) J. 11:2895-2902). This observation implies a possibility that there is a protein kinase specifically activated at metaphase-anaphase transition. To further analyze the cell cycle- dependent GFAP phosphorylation, we prepared monoclonal antibodies KT13 and KT34 which recognize the phosphorylation of GFAP at Ser13 and Ser34, respectively. Immunocytochemical studies with KT13 and KT34 revealed that the GFAP phosphorylation in the cleavage furrow during late mitotic phase occurred not only in glioma cells but also in human SW-13 and mouse Ltk- cells in which GFAP was ectopically expressed, thus the phosphorylation can be monitored in a wide range of cell types. Furthermore, we detected kinase activity which phosphorylates GFAP at Ser13 and Ser34 in the lysates of late mitotic cells but not in those of interphase cells or early mitotic cells. These results suggest that there exists a protein kinase which is specifically activated at the transition of metaphase to anaphase not only in GFAP-expressing cells but also in cells without GFAP.     

The human kinesin-like protein RB6K is under tight cell cycle control and is essential for cytokinesis

Several members of the kinesin superfamily are known to play a prominent role in the motor-driven transport processes that occur in mitotic cells. Here we describe a new mitotic human kinesin-like protein, RB6K (Rabkinesin 6), distantly related to MKLP-1. Expression of RB6K is regulated during the cell cycle at both the mRNA and protein level and, similar to cyclin B, shows a maximum during M phase. Isolation of the RB6K promoter allowed identification of a CDE-CHR element and promoter activity was shown to be maximal during M phase. Immunofluorescence microscopy using antibodies raised against RB6K showed a weak signal in interphase Golgi but a 10-fold higher signal in prophase nuclei. During M phase, the newly synthesized RB6K does not colocalise with Rab6. In later stages of mitosis RB6K localized to the spindle midzone and appeared on the midbodies during cytokinesis. The functional significance of this localization during M phase was revealed by antibody microinjection studies which resulted exclusively in binucleate cells, showing a complete failure of cytokinesis. These results substantiate a crucial role for RB6K in late anaphase B and/or cytokinesis, clearly distinct from the role of MKLP-1.     

Induction and amplification of T-lymphocyte proliferative responses to periodate and soybean agglutinin by human adult vascular endothelial cells

Human mononuclear phagocyte (M phi) populations were compared to adult human endothelial cells (HEC) for their respective abilities to influence the proliferative responses of purified human T lymphocytes to the mitogenic agents Na-m-periodate (IO-4), soybean agglutinin (SBA), or allogeneic cells. HEC and M phi were both capable of inducing proliferative responses of allogeneic T lymphocytes in mixed-lymphocyte culture. Under low cell density culture conditions, purified T-lymphocyte proliferative responses to IO-4 or SBA could be restored by addition of syngeneic M phi or HEC. At higher cell density culture conditions, proliferation of T cells to IO-4 could be amplified more by HEC than M phi. T-lymphocyte proliferative responses to SBA were amplified by addition of HEC but were suppressed by addition of M phi. These findings indicate that human adult HEC are unique and potent accessory cells for T lymphocytes. Furthermore, these findings demonstrate that accessory cell functions of HEC can be discriminated from those of M phi.