Tyrosine phosphorylation of p34cdc2 is regulated by protein phosphatase 2A in growing immature Xenopus oocytes.

Growing stage IV Xenopus oocytes are unresponsive to progesterone treatment. They contain a store of preMPF composed of tyrosine phosphorylated p34cdc2 and cyclin B2. The endogenous store of preMPF cannot be recruited by cdc25 protein phosphatase or cyclin protein microinjections. This is in contrast with full-grown stage VI oocytes where microinjections of these proteins are known to activate the autoamplification of MPF. When cyclins are microinjected into stage IV oocytes, they associate with endogenous free p34cdc2 and the illegitimate complexes undergo phosphorylation on tyrosine 15. High doses of human cyclin A allow, however, part of the neoformed complexes to be activated as an histone-H1 kinase; this partial activation of p34cdc2 is sufficient to induce germinal vesicle breakdown in these small oocytes. Co-injections of cyclin A or cyclin B together with okadaic acid (10 microM in the microinjection solution), an inhibitor of protein phosphatase 2A (PP2A), lead to the full activation of neoformed p34cdc2/cyclin complexes. These results indicate that small oocytes possess an active tyrosine kinase that inactivates new p34cdc2/cyclin complexes. Inhibition of PP2A by okadaic acid prevents this inactivation reaction and conversely allows the illegitimate complex to be activated. Neither the activating phosphorylation on threonine 161 nor the inactivating phosphorylation on tyrosine 15 take place in stage IV enucleated oocytes. Altogether, our results show that the accumulation of inactive p34cdc2/cyclin B2 during the long-lasting prophase of the oocyte is positively controlled by PP2A through the tyrosine phosphorylation of p34cdc2.