Physical and functional interaction between the eukaryotic orthologs of prokaryotic translation initiation factors IF1 and IF2

To initiate protein synthesis, a ribosome with bound initiator methionyl-tR NA must be assembled at the start codon of an mRNA. This process requires t he coordinated activities of three translation initiation factors (IF) in p rokaryotes and at least 12 translation initiation factors in eukaryotes (eI F). The factors eIF1A and eIF5B front eukaryotes show extensive amino acid sequence similarity to the factors IF1 and IF2 from prokaryotes. By a combi nation of two-hybrid, coimmunoprecipitation, and in vitro binding assays eI F1A and eIF5B were found to interact directly, and the eIF1A binding site w as mapped to the C-terminal region of eIF5B. This portion of eIF5B was foun d to be critical for growth in vivo and for translation in vitro. Overexpre ssion of eIF1A exacerbated the slow-growth phenotype of yeast strains expre ssing C-terminally truncated eIF5B. These findings indicate that the physic al interaction between the evolutionarily conserved factors eIF1A and eIF5B plays an important role in translation initiation, perhaps to direct or st abilize the binding of methionyl-tRNA to the ribosomal P site.