The kinase activity of aurora B is required for kinetochore-microtubule interactions during mitosis.
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As a component of the "chromosomal passenger protein complex," the aurora B kinase is associated with centromeres during prometaphase and with midzone microtubules during anaphase and is required for both mitosis and cytokinesis. Ablation of aurora B causes defects in both prometaphase chromosomal congression and the spindle checkpoint; however, the mechanisms underlying these defects are unclear. To address this question, we have examined chromosomal movement, spindle organization, and microtubule motor distribution in NRK cells transfected with a kinase-inactive, dominant-negative mutant of aurora B, aurora B(K-R). In cells overexpressing aurora B(K-R) fused with GFP, centromeres moved in a synchronized and predominantly unidirectional manner, as opposed to the independent, bidirectional movement in control cells expressing a similar level of wild-type aurora B-GFP. In addition, most kinetochores became physically separated from spindle microtubules, which appeared as a striking bundle between the spindle poles. These defects were associated with a microtubule-dependent depletion of motor proteins dynein and CENP-E from kinetochores. Our observations suggest that aurora B regulates the association of motor proteins with kinetochores during prometaphase. Interactions of kinetochore motors with microtubules may in turn regulate the organization of microtubules, the movement of prometaphase chromosomes, and the release of the spindle checkpoint. (+info)
Inhibition of aurora B kinase blocks chromosome segregation, overrides the spindle checkpoint, and perturbs microtubule dynamics in mitosis.
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How kinetochores correct improper microtubule attachments and regulate the spindle checkpoint signal is unclear. In budding yeast, kinetochores harboring mutations in the mitotic kinase Ipl1 fail to bind chromosomes in a bipolar fashion. In C. elegans and Drosophila, inhibition of the Ipl1 homolog, Aurora B kinase, induces aberrant anaphase and cytokinesis. To study Aurora B kinase in vertebrates, we microinjected mitotic XTC cells with inhibitory antibody and found several related effects. After injection of the antibody, some chromosomes failed to congress to the metaphase plate, consistent with a conserved role for Aurora B in bipolar attachment of chromosomes. Injected cells exited mitosis with no evidence of anaphase or cytokinesis. Injection of anti-Xaurora B antibody also altered the microtubule network in mitotic cells with an extension of the astral microtubules and a reduction of kinetochore microtubules. Finally, inhibition of Aurora B in cultured cells and in cycling Xenopus egg extracts caused escape from the spindle checkpoint arrest induced by microtubule drugs. Our findings implicate Aurora B as a critical coordinator relating changes in microtubule dynamics in mitosis, chromosome movement in prometaphase and anaphase, signaling of the spindle checkpoint, and cytokinesis. (+info)
Aurora-B associated protein phosphatases as negative regulators of kinase activation.
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The human serine/threonine kinase Aurora-B is structurally related to the protein kinase Ipl1p from S cerevisiae and aurora from Drosophila melanogaster, which are key regulators of mitosis. The present study shows that human Aurora-B is activated by okadaic acid and forms complexes with the protein serine/threonine phosphatase type 1 (PP1) or PP2A, but not with PP5. These data identified Aurora-B associated protein phosphatases as negative regulators of kinase activation. We then used a series of substrates based on a histone H3 phosphorylation site (residues 5-15) to determine the substrate specificity of human Aurora-B. We found that this enzyme is an arginine-directed kinase that can phosphorylate histone H3 at serines 10 and 28 in vitro, suggesting that human Aurora-B is a mitotic histone H3 kinase. (+info)
Translational control of the embryonic cell cycle.
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The synthesis and destruction of cyclin B drives mitosis in eukaryotic cells. Cell cycle progression is also regulated at the level of cyclin B translation. In cycling extracts from Xenopus embryos, progression into M phase requires the polyadenylation-induced translation of cyclin B1 mRNA. Polyadenylation is mediated by the phosphorylation of CPEB by Aurora, a kinase whose activity oscillates with the cell cycle. Exit from M phase seems to require deadenylation and subsequent translational silencing of cyclin B1 mRNA by Maskin, a CPEB and eIF4E binding factor, whose expression is cell cycle regulated. These observations suggest that regulated cyclin B1 mRNA translation is essential for the embryonic cell cycle. Mammalian cells also display a cell cycle-dependent cytoplasmic polyadenylation, suggesting that translational control by polyadenylation might be a general feature of mitosis in animal cells. (+info)
Chromosome dynamics: new light on Aurora B kinase function.
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Aurora B family kinases play an essential role in chromosome segregation and cytokinesis. Recent work suggests that the kinase activity is required for bipolar chromosome orientation, kinetochore assembly, spindle checkpoint and microtubule dynamics. Aurora B also has additional functions in chromosome condensation and cohesion. (+info)
Centrosome amplification and overexpression of aurora A are early events in rat mammary carcinogenesis.
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The cells of many solid tumors have been found to contain supernumerary centrosomes, a condition known as centrosome amplification. Centrosome amplification, accompanied by the overexpression of an associated kinase, Aurora A (AurA), has been implicated in mechanisms leading to mitotic spindle aberrations, aneuploidy, and genomic instability. Using a well-established rat mammary model favorable for experimental carcinogenesis, we analyzed centrosome amplification as a cellular marker for early stages of transformation and its regulation by the kinase ratAurA. Parity or treatment with estrogen and progesterone conferred resistance to tumorigenesis, as well as to overexpression of ratAurA and to centrosome amplification. ratAurA, cloned from a rat mammary gland cDNA library, is a bona fide Ser/Thr kinase, and sequence comparison demonstrated high homology to members of the entire AurA kinase family. Using immunocytochemical localization with confocal microscopy, we found ratAurA to be localized at the centrosome in normal and neoplastic tissues of the rat mammary gland. Normal ductal epithelium and stromal cells displayed an expected complement of one to two centrosomes/cell, whereas comparable cells in methylnitrosourea-treated animals displayed significantly elevated centrosome numbers. In tumors, 46% of cells showed more than two centrosomes/cell, and ratAurA expression levels coincided with higher centrosome numbers. Both centrosome numbers and ratAurA expression were permanently elevated. Centrosome amplification was found to occur at a very early, premalignant stage prior to detectable lesions after treatment with methylnitrosourea, a condition that was not detected in mammary glands of rats made refractory to the carcinogen via pregnancy or estrogen and progesterone treatment. Our results indicate that hormones influence kinase expression, and progesterone had the major effect on ratAurA expression levels. Cumulatively, these results suggest that ratAurA overexpression and centrosome amplification were linked to tumor development and progression and may serve as early markers in tumorigenesis. (+info)
Human TPX2 is required for targeting Aurora-A kinase to the spindle.
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Aurora-A is a serine-threonine kinase implicated in the assembly and maintenance of the mitotic spindle. Here we show that human Aurora-A binds to TPX2, a prominent component of the spindle apparatus. TPX2 was identified by mass spectrometry as a major protein coimmunoprecipitating specifically with Aurora-A from mitotic HeLa cell extracts. Conversely, Aurora-A could be detected in TPX2 immunoprecipitates. This indicates that subpopulations of these two proteins undergo complex formation in vivo. Binding studies demonstrated that the NH2 terminus of TPX2 can directly interact with the COOH-terminal catalytic domain of Aurora-A. Although kinase activity was not required for this interaction, TPX2 was readily phosphorylated by Aurora-A. Upon siRNA-mediated elimination of TPX2 from cells, the association of Aurora-A with the spindle microtubules was abolished, although its association with spindle poles was unaffected. Conversely, depletion of Aurora-A by siRNA had no detectable influence on the localization of TPX2. We propose that human TPX2 is required for targeting Aurora-A kinase to the spindle apparatus. In turn, Aurora-A might regulate the function of TPX2 during spindle assembly. (+info)
Suppression of the STK15 oncogenic activity requires a transactivation-independent p53 function.
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Using a transactivation-defective p53 derivative as bait, STK15, a centrosome-associated oncogenic serine/threonine kinase, was isolated as a p53 partner. The p53-STK15 interaction was confirmed further by co-immunoprecipitation and GST pull-down studies. In co-transfection experiments, p53 suppressed STK15-induced centrosome amplification and cellular transformation in a transactivation-independent manner. The suppression of STK15 oncogenic activity by p53 might be explained in part by the finding that p53 inhibited STK15 kinase activity via direct interaction with the latter's Aurora box. Taken together, these findings revealed a novel mechanism for the tumor suppressor function of p53. (+info)