Deletion of the PAT1 gene affects translation initiation and suppresses a PAB1 gene deletion in yeast

The yeast poly(A) binding protein Pab1p mediates the interactions between t he 5' cap structure and the 3' poly(A) tail of mRNA, whose structures syner gistically activate translation in vivo and in vitro. We found that deletio n of the PAT1 (YCR077c) gene suppresses a PAB1 gene deletion and that Pat1p is required for the normal initiation of translation. A fraction of Pat1p cosediments with free 40S ribosomal subunits on sucrose gradients. The PAT1 gene is not essential for viability, although disruption of the gene sever ely impairs translation initiation in vivo, resulting in the accumulation o f 80S ribosomes and in a large decrease in the amounts of heavier polysomes . Pat1p contributes to the efficiency of translation in a yeast cell-free s ystem. However, the synergy between the cap structure and the poly(A) tail is maintained in vitro in the absence of Pat1p, Analysis of translation ini tiation intermediates on gradients indicates that Pat1p acts at a step befo re or during the recruitment of the 40S ribosomal subunit by the mRNA, a st ep which may be independent of that involving Pab1p. We conclude that Pat1p is a new factor involved in protein synthesis and that Pat1p might be requ ired for promoting the formation or the stabilization of the preinitiation translation complexes.