CDC25B Phosphorylated by pEg3 Localizes to the Centrosome and the Spindle Poles at Mitosis

The phosphatase CDC25B is one of the key regulators that control entry into mitosis through the dephosphorylation and subsequent activation of the cyclin-dependent kinases. Here we study the phosphorylation of CDC25B at mitosis by the kinase pEg3, a member of the KIN1/PAR-1/MARK family. Using mass spectrometry analysis we demonstrate that CDC25B is phosphorylated in vitro by pEg3 on serine 169, a residue that lies within the B domain. Moreover, using phosphoepitope-specific antibodies we show that serine 169 is phosphorylated in vivo, that this phosphorylated form of CDC25B accumulates during mitosis, and is localized to the centrosomes. This labelling is abrogated when pEg3 expression is repressed by RNA interference. Taken together, these results support a model in which pEg3 contributes to the control of progression through mitosis by phosphorylation of the CDC25 phosphatases.