S Subunit

Figure 15.17. A model of the cyclic use of the eukaryotic initiation factors eIF2 and eIF2B (M. Salimans, H. Goumans, H. Amesz & H.O. Voorma, Eur. J. Biochem. 145, 91-98, 1984).

Met-tRNAi:eIF2:GTP:eIF2B. The ternary complex Met-tRNAi:eIF2:GTP can be transferred directly from this big complex to the 40S ribosomal subunit. Thus eIF2B appears both as a GDP/GTP exchange factor and as a carrier of eIF2. The phosphorylation of eIF2 mentioned above (see Section 14.3.5) makes the complex between eIF2 and eIF2B firmer, thus making the complexed eIF2 and eIF2B unavailable for further functional interactions. The schematic representation of the eIF2 cycling through the complex formation with eIF2B is given in Fig. 15.17.

Step 5. The vacant A site of the 80S initiation complex accepts the first elongator aminoacyl-tRNA in the form of its ternary complex with eEF1A and GTP. The subsequent GTP cleavage and eEF1A release are followed by the transpeptidation reaction between the initiator Met-tRNAi and the elongator aminoacyl-tRNA. The formation of the first peptide bond completes the initiation and starts the elongation.

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