Two distinct mechanisms control the accumulation of cyclin B1 and Mos in Xenopus oocytes in response to progesterone

Progesterone-induced meiotic maturation of Xenopus oocytes requires the syn thesis of new proteins, such as Mos and cyclin B. Synthesis of Mos is thoug ht to be necessary and sufficient for meiotic maturation; however, it has r ecently been proposed that newly synthesized proteins binding to p34(cdc2) could be involved in a signaling pathway that triggers the activation of ma turation-promoting factor. We focused our attention on cyclin B proteins be cause they are synthesized in response to progesterone, they bind to p34(cd c2), and their microinjection into resting oocytes induces meiotic maturati on. We investigated cyclin B accumulation in response to progesterone in th e absence of maturation-promoting factor-induced feedback. We report here t hat the cdk inhibitor p21(cip1), when microinjected into immature Xenopus o ocytes, blocks germinal vesicle breakdown induced by progesterone, by matur ation-promoting factor transfer, or by injection of okadaic acid. After mic roinjection of p21(cip1), progesterone fails to induce the activation of MA PK or p34(cdc2), and Mos does not accumulate. In contrast, the revel of cyc lin B1 increases normally in a manner dependent on down-regulation of cAMP- dependent protein kinase but independent of cap-ribose methylation of mRNA.