Specific inhibition of types 1 and 2A protein phosphatases by microinjection of okadaic acid (OA) into starfish oocytes induced germinal vesicle breakdown and activation of M phase-promoting factor (MPF) and histone H1 kinase. The effects were evident in immature oocytes arrested at first meiotic prophase as well as in fully mature oocytes arrested at the pronucleus stage. In addition, MPF and histone H1 kinase were stabilized for several hours and protected from inactivation by inhibition of type 1 protein phosphatases with either OA or specific anti-phosphatase antibodies. Microinjection of okadaic acid was associated with unusual changes of the microtubule network, including the disappearance of spindles and extension of the cytoplasmic array of microtubules. MPF activation after OA injection was associated with dephosphorylation of phosphothreonine and phosphoserine residues in cdc2, showing that neither type 1 nor 2A protein phosphatases catalyzes these dephosphorylations. The effects of OA on MPF activation and inactivation appeared to involve the cyclin subunit. OA did not induce MPF activation in the absence of protein synthesis and it prevented degradation of cyclin. Therefore protein phosphatases types 1 and 2A appear to be involved in activation and inactivation of MPF involving mechanisms that operate after cyclin synthesis and before its degradation.

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