Comparative study of the molecular mechanisms of oocyte maturation in amphibians

Maturation-promoting factor (MPF), a complex of Cdc2 and cyclin B, is the f inal inducer of oocyte maturation. Its activity is controlled by inhibitory phosphorylation of Cdc2 on Tyr15/Thr14 and activating phosphorylation on T hr161. Full-grown immature oocytes of the African clawed frog Xenopus laevi s contain inactive MPF (pre-MPF) that comprises cyclin B-bound Cdc2 phospho rylated on Tyr15/Thr14 and Thr161. The synthesis of Mos, but not cyclin B, after stimulation by the maturation-inducing steroid progesterone, is belie ved to be necessary for initiating Xenopus oocyte maturation through Tyr15/ Thr14 dephosphorylation of pre-MPF. In contrast, amphibians other than Xeno pus (and also fishes) employ a different mechanism. Full-grown immature ooc ytes of these species contain monomeric Cdc2 but not cyclin B. MPF is forme d after hormonal stimulation by binding of the newly produced cyclin B to t he pre-existing Cdc2 and is immediately activated through Thr161 phosphoryl ation. Mos/MAP kinase is neither necessary nor sufficient for initiating ma turation in fishes and amphibians except for Xenopus. We propose a new mode l of MPF formation and activation during oocyte maturation that is applicab le to all amphibians (as well as fishes), based on a novel concept that pre -MPF is an artificial molecule that is not essential for inducing oocyte ma turation. (C) 2000 Elsevier Science Inc. All rights reserved.