RINGO/cdk1 and CPEB mediate poly(A) tail stabilization and translational regulation by ePAB

One activity that controls mRNA translation in vertebrate oocytes, embryos, and neurons is cytoplasmic polyadenylation. In Xenopus oocytes, where much of the biochemistry of this process has been elucidated, nuclear pre-mRNAs containing a cytoplasmic polyadenylation element (CPE) in their 3' untranslated regions (UTRs) have long poly(A) tails; once the RNAs are spliced and transported to the cytoplasm, the tails are shortened. Following the resumption of meiosis, the poly(A) tails are lengthened and translation ensues. CPEB is a sequence-specific RNA-binding protein that coordinates these events and does so by binding to the CPE as well as several factors including Gld2, a poly(A) polymerase, and PARN [poly(A)-specific ribonuclease], a deadenylase. Here, we show that ePAB, embryonic poly(A)-binding protein, transiently associates with the polyadenylation complex; it initially interacts with CPEB, but after polyadenylation, it binds the poly(A) tail. ePAB dissociation from CPEB is regulated by RINGO (Rapid Inducer of G2/ M progression in Oocytes), a cyclin B1-like cofactor that activates cdk1, a protein kinase that phosphorylates CPEB. Subsequent ePAB binding to the poly(A) tail is necessary to protect the homopolymer from degradation by deadenylating enzymes. Poly(A)-bound ePAB also interacts with eIF4G, which instigates translation initiation of CPEB-bound mRNAs.