OVEREXPRESSION OF POLY(A) BINDING-PROTEIN PREVENTS MATURATION-SPECIFIC DEADENYLATION AND TRANSLATIONAL INACTIVATION IN XENOPUS OOCYTES

The translational regulation of maternal mRNAs is the primary mechanis m by which stage-specific programs of protein synthesis are executed d uring early development. Translation of a variety of maternal mRNAs re quires either the maintenance or cytoplasmic elongation of a 3' poly(A ) tail. Conversely, deadenylation results in translational inactivatio n. Although its precise function remains to be elucidated, the highly conserved poly(A) binding protein I (PABP) mediates poly(A)-dependent events in translation initiation and mRNA stability. Xenopus oocytes c ontain less than one PABP per poly(A) binding site suggesting that the translation of maternal mRNAs could be either limited by or independe nt of PARR In this report, we have analyzed the effects of overexpress ing PABP on the regulation of mRNAs during Xenopus oocyte maturation. Increased levels of PABP prevent the maturation-specific deadenylation and translational inactivation of maternal mRNAs that lack cytoplasmi c polyadenylation elements. Overexpression of PABP does not interfere with maturation-specific polyadenylation, but reduces the recruitment of some mRNAs onto polysomes. Deletion of the C-terminal basic region and a single RNP motif from PABP significantly reduces both its bindin g to polyadenylated RNA in vivo and its ability to prevent deadenylati on. In contrast to a yeast PABP-dependent poly(A) nuclease, PABP inhib its Xenopus oocyte deadenylase in vitro. These results indicate that m aturation-specific deadenylation in Xenopus oocytes is facilitated by a low level of PABP consistent with a primary function for PABP to con fer poly(A) stability.