Multiple portions of poly(A)-binding protein stimulate translation in vivo

Translational stimulation of mRNAs during early development is often accomp anied by increases in poly(A) tail length. Poly(A)-binding protein (PAB) is an evolutionarily conserved protein that binds to the poly(A) tails of euk aryotic mRNAs. We examined PAB's role in living cells, using both Xenopus l aevis oocytes and Saccharomyces cerevisiae, by tethering it to the 3'-untra nslated region of reporter mRNAs, Tethered PAB stimulates translation in vi vo. Neither a poly(A) tail nor PAB's poly(A)-binding activity is required. Multiple domains of PAB act redundantly in oocytes to stimulate translation : the interaction of RNA recognition motifs (RRMs) 1 and 2 with eukaryotic initiation factor-4G correlates with translational stimulation. Interaction with Paip-1 is insufficient for stimulation. RRMs 3 and 4 also stimulate, but bind neither factor. The regions of tethered PAB required in yeast to s timulate translation and stabilize mRNAs differ, implying that the two func tions are distinct. Our results establish that oocytes contain the machiner y necessary to support PAB-mediated translation and suggest that PAB may be an important participant in translational regulation during early developm ent.