Septins are a family of conserved proteins implicated in a variety of cellular functions such as cytokinesis and vesicle trafficking, but their properties and modes of action are largely unknown. Here we now report findings of immunocytochemical and biochemical characterization of a mammalian septin, MSF-A. Using an antibody specific for MSF subfamily proteins, MSF-A was found to be expressed predominantly in mammary human mammary epithelial cells (HMEC). MSF-A was associated with microtubules in interphase HMEC cells as it localized with the mitotic spindle and the bundle of microtubule at midzone during mitosis. Biochemical analysis revealed direct binding of MSF-A with polymerized tubulin through its central region containing guanine nucleotide-interactive motifs. GTPase activity, however, was not required for the association. Conditions that disrupt the microtubule network also disrupted the MSF-A-containing filament structure, resulting in a punctate cytoplasmic pattern. Depletion of MSF-A using small interfering RNAs caused incomplete cell division and resulted in the accumulation of binucleated cells. Unlike Nedd5, an MSF mutant deficient in GTPase activity forms filament indistinguishable from that of the wild type in COS cells. These results strongly suggest that septin filaments may interact not only with actin filaments but also with microtubule networks and that GTPase activity of MSF-A is not indispensable to incorporation of MSF-A into septin filaments.Septins, a family of heteropolymeric filament-forming proteins, were originally discovered in yeast to be essential for budding, and have since been identified in most eukaryotic organisms, with the exception of plants (for review, see Refs.

Mitotic kinases regulate cell division and its checkpoints, errors of which can lead to aneuploidy or genetic instability. One of these is Aurora-B, a key kinase that is required for chromosome alignment at the metaphase plate and for cytokinesis in mammalian cells. We report here that human Aurora-B is phosphorylated at Thr-232 through interaction with the inner centromere protein (INCENP) in vivo. The phosphorylation of Thr-232 occurs by means of an autophosphorylation mechanism, which is indispensable for the Aurora-B kinase activity. The activation of Aurora-B spatio-temporally correlated with the site-specific phosphorylation of its physiological substrates, histone H3 and vimentin. Overexpression of the TA mutant of Aurora-B, in which Thr-232 was changed into alanine, frequently induced multinuclearity in cells. These results indicate that the phosphorylation of Thr-232 is an essential regulatory mechanism for Aurora-B activation.

Aurora-B is an evolutionally conserved protein kinase that regulates several mitotic events including cytokinesis. We previously demonstrated the possible existence of a protein kinase that phosphorylates at least Ser-72 on vimentin, the most widely expressed intermediate filament protein, in the cleavage furrow-specific manner. Here we showed that vimentin-Ser-72 phosphorylation occurred specifically at the border of the Aurora-B-localized area from anaphase to telophase. Expression of a dominant-negative mutant of Aurora-B led to a reduction of this vimentin-Ser-72 phosphorylation. In vitro analyses revealed that Aurora-B phosphorylates vimentin at ϳ2 mol phosphate/mol of substrate for 30 min and that this phosphorylation dramatically inhibits vimentin filament formation. We further identified eight Aurora-B phosphorylation sites, including Ser-72 on vimentin, and then constructed the mutant vimentin in which these identified sites are changed into Ala. Cells expressing this mutant formed an unusually long bridge-like intermediate filament structure between unseparated daughter cells. We then identified important phosphorylation sites for the bridge phenotype. Our findings indicate that Aurora-B regulates the cleavage furrow-specific vimentin phosphorylation and controls vimentin filament segregation in cytokinetic process.

Rho-associated kinase (Rho-kinase), which is activated by the small GTPase Rho, regulates formation of stress fibers and focal adhesions, myosin fiber organization, and neurite retraction through the phosphorylation of cytoskeletal proteins, including myosin light chain, the ERM family proteins (ezrin, radixin, and moesin) and adducin. Rho-kinase was found to phosphorylate a type III intermediate filament (IF) protein, glial fibrillary acidic protein (GFAP), exclusively at the cleavage furrow during cytokinesis. In the present study, we examined the roles of Rho-kinase in cytokinesis, in particular organization of glial filaments during cytokinesis. Expression of the dominant-negative form of Rho-kinase inhibited the cytokinesis of Xenopus embryo and mammalian cells, the result being production of multinuclei. We then constructed a series of mutant GFAPs, where Rho-kinase phosphorylation sites were variously mutated, and expressed them in type III IF-negative cells. The mutations induced impaired segregation of glial filament (GFAP filament) into postmitotic daughter cells. As a result, an unusually long bridge-like cytoplasmic structure formed between the unseparated daughter cells. Alteration of other sites, including the cdc2 kinase phosphorylation site, led to no remarkable defect in glial filament separation. These results suggest that Rho-kinase is essential not only for actomyosin regulation but also for segregation of glial filaments into daughter cells which in turn ensures correct cytokinetic processes.