Comparative study of the molecular mechanisms of oocyte maturation in amphibians.
Maturation-promoting factor (MPF), a complex of Cdc2 and cyclin B, is the final inducer of oocyte maturation. Its activity is controlled by inhibitory phosphorylation of Cdc2 on Tyr15/Thr14 and activating phosphorylation on Thr161. Full-grown immature oocytes of the African clawed frog Xenopus laevis contain inactive MPF (pre-MPF) that comprises cyclin B-bound Cdc2 phosphorylated on Tyr15/Thr14 and Thr161. The synthesis of Mos, but not cyclin B, after stimulation by the maturation-inducing steroid progesterone, is believed to be necessary for initiating Xenopus oocyte maturation through Tyr15/Thr14 dephosphorylation of pre-MPF. In contrast, amphibians other than Xenopus (and also fishes) employ a different mechanism. Full-grown immature oocytes of these species contain monomeric Cdc2 but not cyclin B. MPF is formed after hormonal stimulation by binding of the newly produced cyclin B to the pre-existing Cdc2 and is immediately activated through Thr161 phosphorylation. Mos/MAP kinase is neither necessary nor sufficient for initiating maturation in fishes and amphibians except for Xenopus. We propose a new model of MPF formation and activation during oocyte maturation that is applicable 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 maturation.
Yoshida
N
N
Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan.
Mita
K
K
Yamashita
M
M
D000663:Amphibians / Q000502:physiology*; D000818:Animals; D019926:Cyclin B / Q000378:metabolism; D020935:MAP Kinase Signaling System; D016200:Maturation-Promoting Factor / Q000378:metabolism; D008954:Models, Biological; D009865:Oocytes / Q000378:metabolism*; D009866:Oogenesis / Q000502:physiology*; D017231:Proto-Oncogene Proteins c-mos / Q000378:metabolism
D019926:Cyclin B; D017231:Proto-Oncogene Proteins c-mos; D016200:Maturation-Promoting Factor