Figure 15.16. Sequence of events during initiation of translation in Eukaryotes: the model of the initiating 40S ribosomal subunit with pre-bound initiation factors. The consecutive steps from (1) to (6) are explained in the text.

initiator AUG codon in the initiation complex.

Step 4. The 48S initiation complex formed joins the 60S ribosomal subunit. The process is promoted by eIF5 which seems to react with the 48S complex first and induce the hydrolysis of the eIF2-bound GTP. The GDP-form of eIF2, as well as other initiation factors, are released from the 40S subunit, and the 80S initiation particle with Met-tRNAi in the P site is assembled at the initiation codon of mRNA (Fig. 15.16).

The eIF2:GDP complex released presents a special problem for consideration. It is a stable but inactive form of eIF2. Its reactivation and recycling requires the involvement of eIF2B, a large multisubunit protein. eIF2B interacts with eIF2:GDP resulting in the replacement of GDP by GTP (GDP/ GTP exchange reaction). The complex eIF2:GTP:eIF2B may bind Met-tRNAi to form a heavier complex

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