MPM-2 antigens, a discrete set of phosphoproteins that contain similar phosphoepitopes (the MPM-2 epitope), are associated with various mitotically important structures. The central mitotic regulator cdc2 kinase has been proposed to induce M-phase by phosphorylating many proteins which might include the MPM-2 antigens. To clarify the relationship of cdc2 kinase and the MPM-2 antigens, we developed an in vitro assay that enabled us to specifically detect the kinases that phosphorylate the MPM-2 epitope (ME kinases) in crude cell extracts. Two different ME kinase activities were identified in unfertilized Xenopus eggs, neither of which was cdc2 kinase, but both appeared to be activated by the introduction of cdc2 kinase into oocytes or oocyte extract. The two ME kinases differed in molecular size, substrate specificity, peptide components, and MPM-2 reactivity. The larger one, ME kinase-H, phosphorylated several MPM-2 antigens, while the smaller one, ME kinase-L, phosphorylated mainly one. We purified ME kinase-L to near homogeneity by sequential chromatography and showed that it has the characteristics of the 42-kD microtubule-associated protein (MAP) kinase. Our results support the previous finding that MAP kinase is activated during Xenopus oocyte maturation and suggest that MAP kinase may contribute to oocyte maturation induction by phosphorylating one subtype of MPM-2 epitope.

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