FIGURE 5-24 Initiation of translation in eukaryotes. The current model of eukaryotic initiation involves eight steps. Step 1: An eIF2 ternary complex forms when eIF2·GTP binds a tRNA_i^Met. Step 2: When a ribosome dissociates at the termination of translation, the 40S subunit is bound by eIF1, eIF1A, and eIF3. A 43S preinitiation complex forms when this subunit associates with an eIF2 ternary complex and eIF5. Step 3: An mRNA is activated when a multisubunit eIF4 complex binds: subunit eIF4E binds to the 5′ cap, and subunit eIF4G binds multiple copies of the cytoplasmic poly(A)-binding protein (PABPC) bound to the mRNA poly(A) tail. For simplicity, binding of only one PABPC to eIF4G is shown. Then eIF4B, which stimulates eIF4A helicase activity, also joins this circular complex in which both the mRNA 5′ cap and poly(A) tail are associated with the eIF4 complex. Step 4: The 43S preinitiation complex binds an eIF4-mRNA complex. Step 5: The RNA helicase activity of subunit eIF4A unwinds any RNA secondary structure at the 5′ end of the mRNA as the 40S subunit scans in the 5′→3′ direction until it recognizes the initiation codon. For simplicity, eIF4E is diagrammed as releasing from the remainder of the eIF4 complex, but in reality, it remains associated, forming a loop in the mRNA between the 5′ cap and the scanning eIF4 complex. Step 6: Recognition of the initiation codon causes eIF5 to stimulate hydrolysis of eIF2-bound GTP. This switches the conformation of the scanning complex to a 48S initiation complex with the anticodon of tRNA_i^Met base-paired to the initiator AUG in the 40S-subunit P site. Step 7: The 60S subunit joins the 40S subunit, leading to the release of most of the earlier-acting eIFs as eIF5B-GTP binds to eIF1A in the ribosomal A site. The released eIF4 complex and eIF4B associate with the 5′ cap and PABPC as shown in step 3 to prepare for interaction with another 43S preinitiation complex. For simplicity, this association is not shown. Step 8: Correct association of the 40S and 60S subunits results in hydrolysis of eIF5B-bound GTP, release of eIF5B-GDP and eIF1A, and formation of the 80S initiation complex with tRNA_i^Metbase-paired to the initiation codon in the ribosomal P site. See R. J. Jackson et al., 2010, Nature Rev. Mol. Cell Biol. 11:113.