Full activation of the rat oocyte by protein synthesis inhibition requires protein phosphatase activity

The rat oocyte provides an interesting system in which to dissect the control mechanisms involved in the transition between a meiotic M phase and a mitotic interphase. In this study, we show that in rat oocytes activated parthenogenetically by puromycin, okadaic acid (a potent inhibitor of protein phosphatases 1 and 2A) induced an increase in histone H1 kinase activity suggesting that MPF was reactivated. However, the inhibition of phosphatases 1 and 2A shortly after second polar body extrusion did not allow the formation of a metaphase-like spindle, although microtubule polymerization was not inhibited. Instead, the chromatin remained condensed as a single mass and a large aster formed around it