Inhibition of mos-induced oocyte maturation by protein kinase A

The relationship between the mos protooncogene protein and cAMP-dependent protein kinase (PKA) during the maturation of Xenopus oocytes was investigated. Microinjection of the PKA catalytic subunit (PKAc) into Xenopus oocytes inhibited oocyte maturation induced by the mos product but did not markedly affect the autophosphorylation activity of injected mos protein. By contrast, PKAc did not inhibit maturation promoting factor (MPF) activation or germinal vesicle breakdown (GVBD) that was initiated by injecting crude MPF preparations. In addition, inhibiting endogenous PKA activity by microinjecting the PKA regulatory subunit (PKAr) induced oocyte maturation that was dependent upon the presence of the endogenous mos product. Moreover, PKAr potentiated mos protein-induced MPF activation in the absence of progesterone and protein synthesis. These data are consistent with the hypothesis that progesterone-induced release from G2/M is regulated via PKAc and that PKAc negatively regulates a downstream target that is positively regulated by mos.