Function of eukaryotic translation initiation factor 1A (eIF1A) (formerly called eIF-4C) in initiation of protein synthesis

We have used an efficient in vitro translation initiation system to show that the mammalian 17-kDa eukaryotic initiation factor, eIF1A (formerly designated eIF4C), is essential for transfer of the initiator Met-tRNA(f) (as Met-tRNA(f)εIF2·GTP ternary complex) to 40 S ribosomal subunits in the absence of mRNA to form the 40 S preinitiation complex (40 S·Met- tRNA(f)eIF2·GTP). Furthermore, eIF1A acted catalytically in this reaction to mediate highly efficient transfer of the Met-tRNA(f)·eIF2·GTP ternary complex to 40 S ribosomal subunits. The 40 S complex formed was free of eIF1A-indicating that its role in 40 S preinitiation complex formation is not to stabilize the binding of Met-tRNA(f) to 40 S ribosomes. Additionally, the eIF1A-mediated 40 S initiation complex formed in the presence of AUG codon efficiently joined 60 S ribosomal subunits in an eIF5-dependent reaction to form a functional 80 S initiation complex. In contrast to other reports, we found that eIF1A plays no role either in the subunit joining reaction or in the generation of ribosomal subunits from 80 S ribosomes. Our results indicate that the major function of eIF1A is to mediate the transfer of Met- tRNAφ to 40 S ribosomal subunits to form the 40 S preinitiation complex.