Characterization of Mammalian eIF2A and Identification of the Yeast Homolog*

To begin the physical characterization of eukaryotic initiation factor (eIF) 2A, a translation initiation factor that binds Met-tRNAi, tryptic peptides from rabbit reticulocyte eIF2A were analyzed to obtain amino acid sequence information. Sequences for 8 peptides were matched to three different expressed sequence tag clones. The sequence predicted for eIF2A is 585 amino acids. Matching of the cDNA sequence to the human genome revealed that the eIF2A mRNA is made up of 15 or 16 exons, and the gene is contained on chromosome 3. A homolog in Saccharomyces cerevisiae was identified, YGR054W, which is a non-essential gene. Hemagglutinin-tagged yeast eIF2A localizes on both 40 S and 80 S ribosomes. A knockout of both eIF2A and eIF5B yielded a “synthetically sick” yeast strain with a severe slow growth phenotype. The phenotype of this double mutant and the biochemical localization suggest that eIF2A participates in translation initiation. eIF2A does not appear to participate in re-initiation as the ΔeIF2A strain shows the same level of GCN4 induction with amino acid starvation as seen in wild type yeast. The lack of any apparent phenotype in the ΔeIF2A strain suggests that eIF2A functions in a minor pathway, perhaps internal initiation or in the translation of a small number of specific mRNAs.