CYTOPLASMIC ACCUMULATION OF CDC25B PHOSPHATASE IN MITOSIS TRIGGERS CENTROSOMAL MICROTUBULE NUCLEATION IN HELA-CELLS

The formation of the mitotic spindle is an essential prerequisite for successful mitosis. The dramatic changes in the level of microtubule ( Mt) nucleation at the centrosomes and Mt dynamics that occur in propha se are presumed to be initiated through the activity of cdc2/cyclin B, Here we present data that the cdc25B isoform functions to activate th e cytoplasmic pool of cdc2/cyclin B responsible for these events. In c ontrast to cdc25C, cdc25B is present at low levels in HeLa cells durin g interphase, but sharply increases in prophase, when cdc25B accumulat ion in the cytoplasm correlates with prophase spindle formation. Overe xpression of wild type and dominant negative mutants of cdc25B and cdc 25C shows that prophase Mt nucleation is a consequence of cytoplasmic cdc25B activity, and that cdc25C regulates nuclear G(2)/M events, Our data also suggest that the functional status of the centrosome can reg ulate nuclear mitotic events.