Xp38γ/SAPK3 promotes meiotic G2/M transition in Xenopus oocytes and activates Cdc25C

We have studied the role of p38 mitogen-activated protein kinases (MAPKs) in the meiotic maturation of Xenopus oocytes. Overexpression of a constitutively active mutant of the p38 activator MKK6 accelerates progesterone-induced maturation. Immunoprecipit ation experiments indicate that p38γ/SAPK3 is the major p38 activated by MKK6 in the oocytes. We have cloned Xenopus p38γ (Xp38γ) and show that co-expression of active MKK6 with Xp38γ induces oocyte maturation in the absence of progesterone. The maturation induced by Xp38γ requires neither protein synthesis nor activation of the p42 MAPK–p90Rsk pathway, but it is blocked by cAMP-dependent protein kinase. A role for the endogenous Xp38γ in progesterone-induced maturation is supported by the inhibitory effect of kinase-dead mutants of MKK6 and Xp38γ. Furthermore, MKK6 can rescue the inhibition of oocyte maturation by anthrax lethal factor, a protease that inactivates MAPK kinases. We also show that Xp38γ can activate the phosphatase XCdc25C, and we identified Ser205 of XCdc25C as a major phosphorylation site for Xp38γ. Our results indicate that phosphorylation of XCdc25C by Xp38γ/SAPK3 is important for the meiotic G2/M progression of Xenopus oocytes.