eIF3b and eIF3i relocate together to the ribosomal subunit interface during translation initiation and modulate start codon selection

During eukaryotic translational initiation, the 48S ribosomal pre-initiation complex (PIC) scans the 5 untranslated region of mRNA until it encounters a start codon. We present a single particle electron cryomicroscopy (cryo-EM) reconstruction of a yeast 48S PIC in an open scanning-competent state in which eIF3b is observed bound on the 40S subunit interface. eIF3b is re-located with eIF3i from their solvent-interface locations observed in other PIC structures; however, eIF3i is not in contact with the 40S. Re-processing of micrographs of our previous 48S PIC in a closed state using currently available tools reveal a similar re-location of eIF3b and eIF3i from the solvent to subunit interface. Genetic analysis indicates that high fidelity initiation in vivo depends strongly on eIF3b interactions at the subunit interface that either promote the closed conformation of the PIC on start codon selection or facilitate subsequent relocation back to the solvent side of the 40S subunit.