CPEB phosphorylation and cytoplasmic polyadenylation are catalyzed by the kinase IAK1/Eg2 in maturing mouse oocytes

In both vertebrates and invertebrates, the expression of several maternal m RNAs is regulated by cytoplasmic polyadenylation. In Xenopus oocytes, where most of the biochemical details of this process have been examined, polyad enylation is controlled by CPEB, a sequence-specific RNA binding protein. T he activity of CPEB, which is to recruit cleavage and polyadenylation speci ficity factor (CPSF) and poly(A) polymerase (PAP) into an active cytoplasmi c polyadenylation complex, is controlled by Eg2-catalyzed phosphorylation. Soon after CPEB phosphorylation and resulting polyadenylation take place, t he interaction between maskin, a CPEB-associated factor, and eIF4E, the cap -binding protein, is destroyed, which results in the recruitment of mRNA in to polysomes. Polyadenylation also occurs in maturing mouse oocytes, althou gh the biochemical events that govern the reaction in these cells are not k nown. In this study, we have examined the phosphorylation of CPEB and have assessed the necessity of this protein for polyadenylation in maturing mous e oocytes. Immunobistochemistry has revealed that all the factors that cont rol polyadenylation and translation in Xenopus oocytes (CPEB, CPSF, PAP, ma skin, and IAK1, the murine homologue of Eg2) are also present in the cytopl asm of mouse oocytes. After the induction of maturation, a kinase is activa ted that phosphorylates CPEB on a critical regulatory residue, an event tha t is essential for CPEB activity. A peptide that competitively inhibits the activity of IAK1/Eg2 blocks the progression of meiosis in injected oocytes . Finally, a CPEB protein that acts as a dominant negative mutation because it cannot be phosphorylated by IAK1/Eg2, prevents cytoplasmic polyadenylat ion. These data indicate that cytoplasmic polyadenylation in mouse oocytes is mediated by IAK1/Eg2-catalyzed phosphorylation of CPEB.