Protein phosphorylation/dephosphorylation in the meiotic cell cycle of mammalian oocytes.

The meiotic cell cycle in mammalian oocytes commences in the fetal ovary, proceeds up to the diplotene stage of the first prophase, and is arrested around birth at a G2-like phase. Reinitiation of meiosis, which occurs immediately before ovulation, represents the transition from G2 to M phase of the cell cycle. Resumption of meiosis is associated with a reduction in cAMP concentrations within the oocyte and is negatively regulated by an activated cAMP-dependent protein kinase (PKA). These findings led to the hypothesis that meiotic arrest is mediated by a PKA-phosphoprotein substrate, which undergoes dephosphorylation under conditions of decreasing intra-oocyte concentrations of cAMP. Thus far, a phosphoprotein that serves as a substrate for PKA and maintains meiotic arrest has not been identified. Nevertheless, the idea that regulation of enzyme activities of a series of protein kinases and phosphatases governs the progression through the meiotic cell cycle is now commonly accepted. The present knowledge of the specific phosphorylation/dephosphorylation biochemical events that participate in the control of meiosis in mammalian oocytes is discussed in this review.