INDUCTION OF METAPHASE ARREST IN CLEAVING XENOPUS-EMBRYOS BY MAP KINASE

The natural arrest of vertebrate unfertilized eggs in second meiotic m etaphase results from the activity of cytostatic factor (CSF). The pro duct of the c-mos(xe) proto-oncogene is thought to be a component of C SF and can induce metaphase arrest when injected into blastomeres of t wo-cell embryos. The c-Mos(xe) protein can directly activate the mitog en-activated protein kinase kinase (MAP kinase kinase) in vitro, leadi ng to activation of MAP kinase. MAP kinase and c-Mos(xe) are active in unfertilized eggs and are rapidly inactivated after fertilization. Mi croinjection of thiophosphorylated MAP kinase into one blastomere of a two-cell embryo induced metaphase arrest similar to that induced by c -Mos(xe). However, only arrest with c-Mos(xe) was associated with acti vation of endogenous MAP kinase. These results indicate that active MA P kinase is a component of CSF in Xenopus and suggest that the CSF act ivity of C-Mos(xe) is mediated by MAP kinase.