Abstract

During the growth of the ovarian follicle, mammalian oocytes are arrested in the late G2 phase of meiosis through ill-defined mechanisms until shortly before ovulation. The molecular machinery controlling the meiotic, as well as mitotic, cell cycle is centered around the regulation of the activity of MPF, a complex composed of a catalytic Cdc2 and the cyclin B regulatory subunit. Cdc2 kinase is inactive as long as oocytes remain in a germinal vesicle state. Its activation is the molecular event that triggers germinal vesicle breakdown and oocyte reentry into the cell cycle. Countless studies have indicated that levels of the second messenger cAMP in the oocyte play a critical role in maintaining meiotic arrest. High cyclic AMP levels in the oocyte maintain protein kinase A (PKA) in an active/dissociated state, which in turn leads to the phosphorylation of unknown protein substrates. The biochemical steps linking a decrease in cAMP levels and MPF activation have been explored only recently. Here we will review the data supporting a simple scenario whereby Cdc25 and Wee1 kinase are substrates of the PKA in oocytes. As as result of these regulatory loops, the Cdc2/cyclin B complex is maintained in an inactive state by the two-way PKA-dependent activation of Wee1 and inactivation of Cdc25.