Figure 2. Architecture of ribosomal initiation complexes.
(A) Model of a 40S subunit with eIF3 on its exterior (solvent) surface and eIF4G bound to eIF3 near the E-site, based on cryoelectron microscopy analysis, and showing positions of mRNA (red line) and eIF1 on the subunit interface. Binding of eIF3 to the solvent surface is compatible with its potential partial retention on ribosomes during translation of short upstream open reading frames (uORFs). Adapted with permission from Ref. 8. (B) Positions of eIF1 (magenta) and eIF1A (with its structured domain in light blue , its carboxy-terminal tail in dark blue and its amino-terminal tail in green) on the 40S subunit relative to mRNA (red) and P-site tRNA (yellow), based on directed hydroxyl radical probing data10, 13 and modeled using T. thermophilus 30S subunit crystal structures (PDB codes 1JGO and 1JGP). (C) Cryoelectron microscopy reconstructions of yeast apo 40S subunits (left panel) and 40S–eIF1–eIF1A complexes (right panel), labelled to indicate the A-, P- and E-sites in the mRNA-binding channel, and the positions of rRNA helices h16, h18 and h34, which are involved in forming the mRNA entry-channel (h18-h34) and the eIF1- and eIF1A–induced head–shoulder connection (h16-rpS3) (indicated by an asterisk). Adapted from Ref. 14 with permission.