Characterization of centrosomal localization and dynamics of Cdc25C phosphatase in mitosis

In mammalian cells, three Cdc25 phosphatases A, B, C coordinate cell cycle progression through activating dephosphorylation of Cyclin-dependent kinases. Whereas Cdc25B is believed to trigger entry into mitosis, Cdc25C is thought to act at a later stage of mitosis and in the nucleus. We report that a fraction of Cdc25C localises to centrosomes in a cell cycle-dependent fashion, as of late S phase and throughout G(2) and mitosis. Moreover, Cdc25C colocalises with Cyclin B1 at centrosomes in G(2) and in prophase and Fluorescence Recovery after Photobleaching experiments reveal that they are both in dynamic exchange between the centrosome and the cytoplasm. The centrosomal localisation of Cdc25C is essentially mediated by its catalytic C-terminal domain, but does not require catalytic activity. In fact phosphatase-dead and substrate-binding hotspot mutants of Cdc25C accumulate at centrosomes together with phospho Tyr15-Cdk1 and behave as dominant negative forms that impair entry into mitosis. Taken together, our data suggest an unexpected function for Cdc25C at the G(2)/M transition, in dephosphorylation of Cdk1. We propose that Cdc25C may participate in amplification of Cdk1-Cyclin B1 activity following initial activation by Cdc25B, and that this process is initiated at the centrosome, then further propagated throughout the cytoplasm thanks to the dynamic behavior of both Cdc25C and Cyclin B1.