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 sh ow that the mammalian 17-kDa eukaryotic initiation factor, eIF1A (form erly designated eIF4C), is essential for transfer of the initiator Met -tRNA(f) (as Met-tRNA(f) . eIF2 . GTP ternary complex) to 40 S ribosom al subunits in the absence of mRNA to form the 40 S preinitiation comp lex (40 S . Met-tRNA(f) . eIF2 . GTP). Furthermore, eIF1A acted cataly tically in this reaction to mediate highly efficient transfer of the M et-tRNA(f) . eIF2 . GTP ternary complex to 40 S ribosomal subunits. Th e 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 i nitiation complex formed in the presence of AUG codon efficiently join ed 60 S ribosomal subunits in an eIF5-dependent reaction to form a fun ctional 80 S initiation complex. In contrast to other reports, we foun d that eIF1A plays no role either in the subunit joining reaction or i n the generation of ribosomal subunits from 80 S ribosomes, Our result s indicate that the major function of eIF1A is to mediate the transfer of Met-tRNA(f) to 40 S ribosomal subunits to form the 40 S preinitiat ion complex.