Ser149 is another potential PKA phosphorylation target of Cdc25B in G2/M transition of fertilized mouse eggs.

It is well documented that protein kinase A (PKA) acts as a negative regulator of M phase promoting factor (MPF) by phosphorylating cell division cycle 25 homolog B (Cdc25B) in mammals. However, the molecular mechanism remains unclear. In this study, we identified PKA phosphorylation sites in vitro by LC-MS/MS analysis, including Ser(149), Ser(229), and Ser(321) of Cdc25B, and explored the role of Ser(149) in G(2)/M transition of fertilized mouse eggs. The results showed that the overexpressed Cdc25B-S149A mutant initiated efficient MPF activation by direct dephosphorylation of Cdc2-Tyr(15), resulting in triggering mitosis prior to Cdc25B-WT. Conversely, overexpression of the phosphomimic Cdc25B-S149D mutant showed no significant difference in comparison with the control groups. Furthermore, we found that Cdc25B-Ser(149) was phosphorylated at G(1) and S phases, whereas dephosphorylated at G(2) and M phases, and the phosphorylation of Cdc25B-Ser(149) was modulated by PKA in vivo. In addition, we examined endogenous and exogenous Cdc25B, which were expressed mostly in the cytoplasm at the G(1) and S phases and translocated to the nucleus at the G(2) phase. Collectively, our findings provide evidence that Ser(149) may be another potential PKA phosphorylation target of Cdc25B in G(2)/M transition of fertilized mouse eggs and Cdc25B as a direct downstream substrate of PKA in mammals, which plays important roles in the regulation of early development of mouse embryos.