Analysis of an essential requirement for the poly(A) binding protein function using cross-species complementation

Poly(A) binding protein (PABP) is an essential, well-conserved, multifuncti onal protein involved in translational initiation, mRNA biogenesis, and deg radation [1-5]. We have used a cross-species complementation approach to ad dress the nature of the essential requirement for PABP in yeast. The expres sion of Pab3p, a member of the Arabidopsis thaliana PABP multigene family, rescues the lethal phenotype associated with the loss of the yeast Pab1p. H owever, Pab3p neither protects the mRNA 5' cap from premature removal, nor does it support poly(A)-dependent translational initiation or the synergist ic enhancement of translation by the poly(A) tail and 5' cap in yeast. Howe ver, Pab3p corrects the temporal lag prior to the entry of the mRNA into th e degradation pathway characteristic of pab1 Delta yeast strains. Furthermo re, this lag correction by Pab3p requires Pan3p, a subunit of poly(A) nucle ase, an enzyme involved in the mRNA 3'-end processing. Importantly, the sub stitution of! Pab3p for the yeast Pab1p is synthetically lethal with the PA N3 gene deletion. These results show that the function of PABP in mRNA biog enesis alone could be sufficient to support cell viability in yeast.