Oncogenic Ras triggers the activation of 42-kDa mitogen-activated protein kinase in extracts of quiescent Xenopus oocytes

Quiescent, full-grown Xenopus oocytes, which are arrested at the G₂/M border of meiosis, contain an inactive 42-kDa mitogen-activated protein kinase (p42^MAPK) that is activated when oocytes are stimulated to resume the meiotic cell cycle. We have made extracts from these oocytes that respond to four cell cycle activators: oncogenic [Val¹²]Ras protein, clam cyclins AΔ60 and BΔ97, and the phosphatase inhibitor okadaic acid. All four induce the tyrosine phosphorylation and activation of p42^MAPK. Both cyclins and okadaic acid, but not [Val¹²]Ras, also lead to activation of the endogenous cyclin B/cdc2 kinase complexes in extracts of quiescent oocytes. Using extracts prepared from cycloheximide-arrested interphase cells, we show that although p42^MAPK activation can occur in response to cyclin-activated cdc2, the Ras-induced activation of p42^MAPK occurs without intervening cdc2 activation. Neither the nononcogenic [Gly¹²]Ras nor [Val¹²,Arg¹⁸⁶]-Ras, a mutant that lacks the C-terminal consensus sequence directing prenylation and subsequent membrane association, is an effective activator of p42^MAPK in vitro.