INDUCTION OF A G(2)-PHASE ARREST IN XENOPUS EGG EXTRACTS BY ACTIVATION OF P42 MITOGEN-ACTIVATED PROTEIN-KINASE

Previous work has established that activation of Mos, Mek, and p42 mit ogen-activated protein (MAP) kinase can trigger release from G(2)-phas e arrest in Xenopus oocytes and oocyte extracts and can cause Xenopus embryos and extracts to arrest in mitosis. Herein we have found that a ctivation of the MAP kinase cascade can also bring about an interphase arrest in cycling extracts. Activation of the cascade early in the cy cle was found to bring about the interphase arrest, which was characte rized by an intact nuclear envelope, partially condensed chromatin, an d interphase levels of H1 kinase activity, whereas activation of the c ascade just before mitosis brought about the mitotic arrest, with a di ssolved nuclear envelope, condensed chromatin, and high levels of H1 k inase activity. Early MAP kinase activation did not interfere signific antly with DNA replication, cyclin synthesis, or association of cyclin s with Cdc2, but it did prevent hyperphosphorylation of Cdc25 and Wee1 and activation of Cdc2/cyclin complexes. Thus, the extracts were arre sted in a G(2)-like state, unable to activate Cdc2/cyclin complexes. T he MAP kinase-induced G(2) arrest appeared not to be related to the DN A replication checkpoint and not to be mediated through inhibition of Cdk2/cyclin E; evidently a novel mechanism underlies this arrest. Fina lly, we found that by delaying the inactivation of MAP kinase during r elease of a cytostatic factor-arrested extract from its arrest state, we could delay the subsequent entry into mitosis. This finding suggest s that it is the persistence of activated MAP kinase after fertilizati on that allows the occurrence of a G(2)-phase during the first mitotic cell cycle.