Cdc25C phosphorylation on serine 191 by Plk3 promotes its nuclear translocation.

Mitosis in human cells is initiated at the end of G2 by activation of the Cdc2/cyclin B complex. Activation occurs by dephosphorylation of the inhibitory residues, threonine 14 (T14) and tyrosine 15 (Y15), on Cdc2 by the Cdc25C phosphatase. Entry into mitosis is regulated by the subcellular relocalization of Cdc2/cyclin B, which is rapidly imported into the nucleus at the end of G2. Here, we show that polo-like kinase 3 (Plk3) is able to phosphorylate Cdc25C primarily on S191, and to a lesser extent on S198 in vitro, both of which are within a nuclear exclusion motif. Following transfection, the S191D Cdc25C mutant leads to an enhanced accumulation of Cdc25C in the nucleus, while the S191A mutant facilitated the Cdc25C nuclear exclusion. Furthermore, translocation of Cdc25C to the nucleus was accompanied by a decrease in Cdc2 phosphorylation on Y15. Plk3-WT overexpression led to a sharp increase in Cdc25C nuclear accumulation, while Plk3-KD overexpression failed to do so. The effect of Plk3 overexpression on Cdc25C was reversed by coexpression of a Plk3 siRNA. These results support a role for the polo kinases in coordinating the translocation and perhaps the timing of both Cdc25C and its target Cdc2/cyclin B to the nucleus upon entry into mitosis.