The first step in the decay of many eukaryotic mRNAs is shortening of the poly(A) tail. In yeast, deadenylation leads to mRNA decapping and subsequent 5' --> 3' exonucleolytic degradation of the transcript body. We have determined that the major poly(A)-binding protein Pab1p plays at least two critical roles in this pathway. First, mRNAs in pab1 delta strains were decapped prior to deadenylation. This observation defines a new function for Pab1p as an inhibitor of mRNA decapping. Moreover, mutations that inhibit mRNA turnover suppress the inviability of a pab1 delta mutation, suggesting that premature mRNA decapping in pab1 delta strains contributes to cell death. Second, we find that Pab1p is not required for deadenylation, although in its absence poly(A) tail shortening rates are significantly reduced. In addition, in the absence of Pab1p, newly synthesized mRNAs had poly(A) tails longer than those in wild-type strains and showed an unexpected temporal delay prior to the initiation of deadenylation and degradation. These results define new and critical functions for Pab1p in the regulation of mRNA decapping and deadenylation, two important control points in the specification of mRNA half-lives. Moreover, these results suggest that Pab1p functions in additional phases of mRNA metabolism such as mRNP maturation.
