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. This process is initiated at prophase when changes in microtubule dynamics and an increase in the number of microtubules nucleated from the centrosome, results in the conversion of the centrosome from an interphase to a mitotic microtubule organising centre (MTOC). This dramatic change in microtubule nucleation and dynamics is presumed to be initiated through the activity of cdc2/cyclin complexes. We have shown that the cdc25B isoform functions to activate the cytoplasmic pool of cdc2/cyclin complexes responsible for these events [1]. In contrast to cdc25C, cdc25B is present at low levels in HeLa cells during interphase, but sharply increases in prophase, when cdc25B accumulation in the cytoplasm correlates with spindle formation. The overexpression of catalytically inactive cdc25B in HeLa cells resulted in an accumulation of cells in G2/M, which appeared to be due to a dominant negative effect on prophase microtubule nucleation. HeLa cells overexpressing wild type cdc25B often displayed striking "mini-spindles" which were likely to have formed as a consequence of the inappropriate activation of cdc2/cyclin complexes. In contrast, overexpression of mutant or wild type cdc25C did not influence prophase microtubule nucleation. The data also suggests that the status of the centrosome can regulate nuclear mitotic events