Cyclin dependent kinase cascade.

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Mitotic chromosomal dynamics is regulated by the coordinated activities of many mitotic kinases, such as Cdc2 (Cdkl), Aurora-B or Polo-like kinase 1 (P1k1), but mechanisms of its coordination remain unknown. We demonstrated two novel Cdc2 kinase cascades. 1) We demonstrated a direct interaction between Plkl and vimentin-Ser55 phosphorylated by Cdc2, an event which led to Plkl activation and further vimentin phosphorylation by Plkl. In vitro analyses revealed that Plkl phosphorylated vimentin at,・1 mol phosphate/mol substrate, which partly inhibited its filament forming ability. Plkl phosphorylated Ser82 on vimentin in vitro, and this phosphorylation was elevated from metaphase and was maintained until the end of mitosis. This elevation thus followed the Cdc2-induced vimentin-Ser55 phosphorylation. Depletion of Plkl by RNA interference impaired the elevation of vimentin-Ser82 phosphorylation in mitosis. These results indicated a novel mechanism that Cdc2 regulated mitotic vimentin phosphorylation via not only a direct enzyme reaction but also Plkl recruitment to vimentin. 2) We found that Cdc2 phosphorylates Thr59 and Thr388 on inner centromere protein (INCENP), which regulates localization and kinase activity of Aurora-B, from prophase to metaphase. INCENP depletion disrupts Plkl localization specifically at the kinetochore. This phenotype is rescued by the exogenous expression of INCENP wild type (WT) and INCENP mutated at Thr59 to Ala (T59A), but not at Thr388 to Ala (T388A). The replacement of endogenous INCENP with T388A resulted in the delay of progression from metaphase to anaphage. Our findings suggested that INCENP phosphorylation by Cdc2 is necessary for the recruitment of Plkl to the kinetochore, and the complex formation of Plkl and Aurora-B on INCENP may play critical roles in the regulation of chromosomal dynamics.