Tethering of Poly(A)-binding Protein Interferes with Non-translated MRNA Decay from the 5' End in Yeast

The decay of eukaryotic mRNA is triggered mainly by deadenylation, which leads to decapping and degradation from the 5' end of an mRNA. Poly(A)-binding protein has been proposed to inhibit the decapping process and to stabilize mRNA by blocking the recruitment of mRNA to the P-bodies where mRNA degradation takes place after stimulation of translation initiation. In contrast, several lines of evidence show that poly(A)-binding protein (Pab1p) has distinct functions in mRNA decay and translation in yeast. To address the translation-independent function of Pab1p in inhibition of decapping, we examined the contribution of Pab1p to the stability of non-translated mRNAs, an AUG codon-less mRNA or an mRNA containing a stable stem-loop structure at the 5'-UTR. Tethering of Pab1p stabilized non-translated mRNAs, and this stabilization did not require either the eIF4G-interacting domain of Pab1p or the Pab1p-interacting domain of eIF4G. In a ski2Δ mutant in which 3' to 5' mRNA degradation activity is defective, stabilization of non-translated mRNAs by the tethering of Pab1p lacking an eIF4G-interacting domain (Pab1-34Cp) requires a cap structure but not a poly(A) tail. In wild type cells, stabilization of non-translated mRNA by tethered Pab1-34Cp results in the accumulation of deadenylated mRNA. These results strongly suggest that tethering of Pab1p may inhibit the decapping reaction after deadenylation, independent of translation. We propose that Pab1p inhibits the decapping reaction in a translation-independent manner in vivo.