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Activation of Cdc2/cyclin B kinase and entry into mitosis requires dephosphorylation of inhibitory sites on Cdc2 by Cdc25 phosphatase. In vertebrates, Cdc25C is inhibited by phosphorylation at a single site targeted by the checkpoint kinases Chk1 and Cds1/Chk2 in response to DNA damage or replication arrest. In Xenopus early embryos, the inhibitory site on Cdc25C (S287) is also phosphorylated by a distinct protein kinase that may determine the intrinsic timing of the cell cycle. We show that S287-kinase activity is repressed in extracts of unfertilized Xenopus eggs arrested in M phase but is rapidly stimulated upon release into interphase by addition of Ca2+, which mimics fertilization. S287-kinase activity is not dependent on cyclin B degradation or inactivation of Cdc2/cyclin B kinase, indicating a direct mechanism of activation by Ca2+. Indeed, inhibitor studies identify the predominant S287-kinase as Ca2+/calmodulin-dependent protein kinase II (CaMKII). CaMKII phosphorylates Cdc25C efficiently on S287 in vitro and, like Chk1, is inhibited by 7-hydroxystaurosporine (UCN-01) and debromohymenialdisine, compounds that abrogate G2 arrest in somatic cells. CaMKII delays Cdc2/cyclin B activation via phosphorylation of Cdc25C at S287 in egg extracts, indicating that this pathway regulates the timing of mitosis during the early embryonic cell cycle.

molecular biology of the cell

Regulation of Cdc2/Cyclin B Activation in<i>Xenopus</i>Egg Extracts via Inhibitory Phosphorylation of Cdc25C Phosphatase by Ca<sup>2+</sup>/Calmodium-dependent Kinase II

Regulation of Cdc2/Cyclin B Activation inXenopusEgg Extracts via Inhibitory Phosphorylation of Cdc25C Phosphatase by Ca2+/Calmodium-dependent Kinase II

Regulation of Cdc2/Cyclin B Activation inXenopusEgg Extracts via Inhibitory Phosphorylation of Cdc25C Phosphatase by Ca²⁺/Calmodium-dependent Kinase II