Release and recycling of eukaryotic initiation factor 2 in the formation of an 80 S ribosomal polypeptide chain initiation complex

The eukaryotic initiation factor (eIF)-5 mediates hydrolysis of GTP bound to the 40 S initiation complex in the absence of 60 S ribosomal subunits. The eIF-2·GDP formed under these conditions is released from the 40 S ribosomal subunit while initiator Met-tRNA(f) remains bound. The released eIF-2·GDP can participate in an eIF-2B-catalyzed GDP/GTP exchange reaction to reform the Met-tRNA(f)·eIF-2·GTP ternary complex. In contrast, when 60 S ribosomal subunits were also present in an eIF-5-catalyzed reaction, the eIF-2·GDP produced remained bound to the 60 S ribosomal subunit of the 80 S initiation complex. When such an 80 S initiation complex, containing bound eIF-2·GDP, was incubated with GTP and eIF-2B, GDP was released. However, eIF-2 still remained bound to the ribosomes and was unable to form a Met-tRNA(f)·eIF- 2·GTP ternary complex. In contrast, when 60 S ribosomal subunits were preincubated with either free eIF-2 or with eIF-2·eIF-2B complex and then added to a reaction containing both the 40 S initiation complex and eIF-5, the eIF-2·GDP produced did not bind to the 60 S ribosomal subunits but was released from the ribosomes. Thus, the 80 S initiation complex formed under these conditions did not contain bound eIF-2·GDP. Under similar experimental conditions, preincubation of 60 S ribosomal subunits with purified eIF-2B (free of eIF-2) failed to cause release of eIF-2·GDP from the ribosomal initiation complex. These results suggest that 60 S ribosome-bound eIF-2·GDP does not act as a direct substrate for eIF-2B-mediated release of eIF-2 from ribosomes. Rather, the affinity of 60 S ribosomal subunits for either eIF-2, or the eIF-2 moiety of the eIF-2·eIF-2B complex, prevents association of 60 S ribosomal subunits with eIF-2·GDP formed in the initiation reaction. This ensures release of eIF-2 from ribosomes following hydrolysis of GTP bound to the 40 S initiation complex.