Identification of ribosome-bound eukaryotic initiation factor 2·GDP binary complex as an intermediate in polypeptide chain initiation reaction

Studies on the formation and release of the eukaryotic initiation factor (eIF)-2·GDP binary complex formed during eIF-5-mediated assembly of an 80 S initiation complex have been carried out. Incubation of a 40 S initiation complex with eIF-5, in the presence or absence of 60 S ribosomal subunits at 25°C, causes rapid and quantitative hydrolysis of ribosome-bound GTP to form an eIF-2·GDP binary complex and P(i). Analysis of both reaction products by Sephadex G-200 gel filtration reveals that while P(i) is released from ribosomes, the eIF-2·GDP complex remains bound to the ribosomal initiation complex. The eIF-2·GDP binary complex can however be released from ribosome by subjecting the eIF-5-catalyzed reaction products to either longer periods of incubation at 37°C or sucrose gradient centrifugation. Furthermore, addition of a high molar excess of isolated eIF-2·GDP binary complex to a 40 S initiation reaction mixture does not cause exchange of ribosome-bound eIF-2·GDP complex formed by eIF-5-catalyzed hydrolysis of GTP. These results indicate that eIF-2·GDP complex is directly formed on the surface of ribosomes following hydrolysis of GTP bound to a 40 S initiation complex, and that ribosome-bound eIF-2·GDP complex is an intermediate in polypeptide chain initiation reaction.