Protein phosphatase-1 is involved in Xenopus oocyte maturation

Two key steps in meiotic maturation of the Xenopus oocyte involve protein phosphorylation-dephosphorylation. A burst of cyclic AMP-independent phosphorylation occurs at the time of the breakdown of the nuclear envelope, whether maturation is triggered by progesterone, by the inhibitor of cyclic AMP-dependent protein kinase (PKI)(2) or by the maturation promoting factor (MPF)(1,3). Also, an in ovo decrease in the level of the free catalytic (C) subunit of cyclic AMP-dependent protein kinase, induced by microinjection of pure PKI4, initiates meiotic maturation(1,2,4). This suggests that MPF appears only after dephosphorylation of a phosphorylated maturation protein Mp-P-4. We now show, by microinjection of pure inhibitor-1 of protein phosphatase-1(5), which blocks progesterone- and PKI-induced maturation but not the MPF-induced one, that protein phosphatase-1 catalyses the dephosphorylation of Mp-P.