Regulatory Mechanism of M-phase by MPF,M-phase Promoting Factor

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MPF (M-phase promoting factor), which is composed of the p34^<cdc2>/cyclin B complex, cdc2 kinase, governs M-phase in all eukaryotic cells. In this project, we studied how M-phase is executed by cdc2 kinase, and obtained following results.1.We developed a new method to purify cdc2 kinase at high performance by modifying p13^<sucl>-affinity chromatography. This enabled us to analyze the function of cdc2 kinase in inducing various M-phase events.2.At G2/M-phase border, the inactive form of p34^<cdc2>/cyclin B complex localizes in the cytoplasm. The complex is transformed to active form in the cytoplasm at the transition to M-phase. Thereafter, a part of the complex moves into nucleus, while other associates with mitotic apparatus. These specific localizations of the complex might support its specific function to execute M-phase within a cell.3.In contrast to the previous belief that histone H1 phosphorylation by cdc2 kinase plays a key role in chromosome condensation, condensation occurs normally even in the chromatins lacking histone H1. This suggests the presence of an unknown major target of cdc2 kinase for chromosome condensation other than histone H1.4.The p34^<cdc2/>cyclin B complex associates with a microtubule via the binding between cyclin B component and the prolin-rich region of MAP4. In such a complex, cdc2 kinase phosphorylates MAP4 to decrease its microtubule-stabilizing ability, resulting in increased dynamic instability of an individual microtubule. This effect of cdc2 kinase appears to contribute to form metaphase spindle.