CPEB IS A SPECIFICITY FACTOR THAT MEDIATES CYTOPLASMIC POLYADENYLATION DURING XENOPUS OOCYTE MATURATION

The translational activation of several maternal mRNAs during Xenopus oocyte maturation is stimulated by cytoplasmic poly(A) elongation, whi ch requires the uridine-rich cytoplasmic polyadenylation element (CPE) and the hexanucleotide AAUAAA. Here, we have enriched a CPE-binding p rotein (CPEB) by single-step RNA affinity chromatography, have obtaine d a CPEB cDNA, and have assessed the role of CPEB in cytoplasmic polya denylation. The 62 kDa CPEB contains two RNA recognition motifs, and w ithin this region, it is 62% identical to orb, an oocyte-specific RNA- binding protein from Drosophila. CPEB mRNA and protein are abundant in oocytes and are not detected in embryos beyond the gastrula stage. Du ring oocyte maturation, CPEB is phosphorylated at a time that correspo nds with the induction of polyadenylation. Immunodepletion of CPEB fro m polyadenylation-proficient egg extracts renders them incapable of ad enylating exogenous RNA. Partial restoration of polyadenylation in dep leted extracts is achieved by the addition of CPEB, thus demonstrating that this protein is required for cytoplasmic polyadenylation.