Phosphorylation of CPE binding factor by Eg2 regulates translation of c-mos mRNA

Full-grown Xenopus oocytes arrest at the G2/M border of meiosis. I. Progest erone breaks this arrest, leading to the resumption of the meiotic cell cyc les and maturation of the oocyte into a fertilizable egg. In these oocytes, progesterone interacts with an unidentified surface-associated receptor, w hich induces a non-transcriptional signalling pathway that stimulates the t ranslation of dormant c-mos messenger RNA. Mos, a mitogen-activated protein (MAP) kinase kinase kinase, indirectly activates MAP kinase, which in turn leads to oocyte maturation. The translational recruitment of c-mos and sev eral other mRNAs is regulated by cytoplasmic polyadenylation, a process tha t requires two 3' untranslated regions, the cytoplasmic polyadenylation ele ment (CPE) and the polyadenylation hexanucleotide AAUAAA(1-4). Although the signalling events that trigger c-mos mRNA polyadenylation and translation are unclear, they probably involve the activation of CPEB, the CPE binding factor(5,6), Here we show that an early site-specific phosphorylation of CP EB is essential for the polyadenylation of c-mos mRNA and its subsequent tr anslation, and for oocyte maturation. In addition, we show that this select ive, early phosphorylation of CPEB is catalysed by Eg2, a member of the Aur ora family of serine/threonine protein kinases.