MPF is activated in growing immature Xenopus oocytes in the absence of detectable tyrosine dephosphorylation of p34 cdc2

Abstract

Tyrosine-phosphorylated p34 cdc2 and cyclin B2 are present and physically associated in small growing stage IV oocytes (800 μm in diameter) of Xenopus laevis. Microinjection of M-phase promoting factor (MPF) into stage IV oocytes induces germinal vesicle breakdown and the activation of the kinase activity of the p34 cdc2 /cyclin B2 complex measured on p13 suc1 beads. During the in vivo activation of MPF in stage IV oocytes, p34 cdc2 tyrosine dephosphorylation is not detectable, in contrast to stage VI oocytes. Addition of cycloheximide in MPF-injected stage IV oocytes induces neither the inhibition of histone H1 kinase activity nor the cyclin B2 degradation. Therefore, the activation mechanism of histone H1 kinase in stage IV oocytes does not require detectable tyrosine dephosphorylation of p34 cdc2 . It is suggested rather that the tyrosine phosphorylation of p34 cdc2 plays a role in inhibiting cyclin B2 degradation.