Figure 1. Model of the canonical pathway of eukaryotic translation initiation.

This pathway is divided into eight stages (2–9), which follow (1) recycling of post-termination complexes to yield separated 40S and 60S ribosomal subunits, and result in formation of an 80S ribosomal initiation complex in which Met-tRNA^Met_i is base-paired with the initiation codon in the ribosomal P-site and which is competent to start the elongation stage of translation. These stages are: (2) formation of the eIF2•GTP/Met-tRNA^Met_i ternary complex; (3) formation of a 43S preinitiation complex comprising a 40S subunit, eIF1, eIF1A, eIF3, eIF2•GTP/Met-tRNA^Met_i and probably eIF5; (4) mRNA activation, during which the mRNA cap-proximal region is unwound in an ATP-dependent manner by eIF4F with eIF4B; (5) attachment of the 43S complex to this mRNA region; (6) scanning of the 5’UTR in a 5’→3’direction by 43S complexes; (7) recognition of the initiation codon and 48S initiation complex formation, which switches the scanning complex to a ‘closed’ conformation and leads to displacement of eIF1, permitting eIF5-mediated hydrolysis of eIF2-bound GTP and Pi release; (8) joining of 60S subunits to 48S complexes and concomitant displacement of eIF2•GDP and other factors (eIF1, eIF3, eIF4B, eIF4F and eIF5) mediated by eIF5B; and (9) GTP hydrolysis by eIF5B and release of eIF1A and eIF5B•GDP from assembled elongation-competent 80S ribosomes. Translation is a cyclical process in which termination follows elongation, and leads to recycling (1) which generates separated ribosomal subunits. The model omits potential ‘closed-loop’ interactions involving poly(A) binding protein (PABP), eukaryotic release facto 3 (eRF3) and eIF4F during recycling (Supplementary information S5), and recycling of eIF2•GDP by eIF2B. Whether eRF3 is still present on ribosomes at the recycling stage is unknown.