Tyrosine phosphorylation of p34and p42 during meiotic maturation of Xenopus oocyte Antagonistic action of okadaic acid and 6-DMAP

The tyrosine phosphorylation/dephosphorylation of p34 was estimated by immunoblotting with antiphosphotyrosine antibody during meiotic maturation of Xenopus oocytes. At the time of germinal vesicle breakdown (GVBD), p34 is tyrosine dephosphorylated whereas a p42 protein, which might correspond to a MAP2 kinase, becomes tyrosine phosphorylated. No modification in the level of tyrosine phosphorylation of either proteins was noticed during the whole maturation process from GVBD until metaphase II. When added to prophase oocytes, 6-DMAP (6-dimethyl-aminopurine) blocks GVBD, M-phase-promoting factor (MPF) activation and Hl-histone, kinase activation induced by either progesterone, MPF transfer or okadaic acid microinjection. In each case, the tyrosine dephosphorylation reaction of p34 is inhibited. In meiosis I oocytes (just after the initiation of GVBD), 6-DMAP provokes the rephosphorylatlon of p34 on tyrosine residue(s), inactivation of MPF and Hl-histone kinase and re-entry of the cell into an interphase-like state. These processes are reversible by simply removing the agent. In contrast to the observations in prophase oocytes, okadaic acid is able to reverse the inhibitory effect of 6-DMAP hi meiosis I oocytes on MPF and Hl-histone kinase activities and to initiate dephosphorylation pf p34 on tyrosyl residue(s) even in the presence of 6-DMAP. Altogether, our results show that 6-DMAP and okadaic acid antagonistically control in vivo the level of tyrosine phosphorylation of p34.