An in vitro system that recapitulates the in vivo effect of AU-rich elements (AREs) on mRNA deadenylation has been developed from
Xenopusactivated egg extracts. ARE-mediated deadenylation is uncoupled from mRNA body decay, and the rate of deadenylation increases with the number of tandem AUUUAs. A novel ARE-binding protein called ePAB (for embryonic poly( A)- binding protein) has been purified from this extract by ARE affinity selection. ePAB exhibits 72% identity to mammalian and XenopusPABP1 and is the predominant poly(A)-binding protein expressed in the stage VI oocyte and during Xenopusearly development. Immunodepletion of ePAB increases the rate of both ARE-mediated and default deadenylation in vitro. In contrast, addition of even a small excess of ePAB inhibits deadenylation, demonstrating that the ePAB concentration is critical for determining the rate of ARE-mediated deadenylation. These data argue that ePAB is the poly(A)-binding protein responsible for stabilization of poly(A) tails and is thus a potential regulator of mRNA deadenylation and translation during early development.