Conformational Differences between Open and Closed States of the Eukaryotic Translation Initiation Complex

Translation initiation in eukaryotes begins with the formation of a pre-initiation complex (PIC) containing the 40S ribosomal subunit, eIF1, eIF1A, eIF3, ternary complex (eIF2-GTP-Met-tRNAi), and eIF5. The PIC, in an open conformation, attaches to the 5′ end of the mRNA and scans to locate the start codon, whereupon it closes to arrest scanning. We present single particle cryo-electron microscopy (cryo-EM) reconstructions of 48S PICs from yeast in these open and closed states, at 6.0 Å and 4.9 Å, respectively. These reconstructions show eIF2β as well as a configuration of eIF3 that appears to encircle the 40S, occupying part of the subunit interface. Comparison of the complexes reveals a large conformational change in the 40S head from an open mRNA latch conformation to a closed one that constricts the mRNA entry channel and narrows the P site to enclose tRNAi, thus elucidating key events in start codon recognition. [Display omitted] •Structures of eukaryotic translation initiation complexes in open and closed states•In the open complex the 40S head moves upward to open the mRNA entry channel latch•Transition to closed state locks initiator tRNA in the P site base-paired with AUG•The structures show how eIF3 contacts eIF2 and eIF1 on the 40S subunit interface The small ribosomal subunit latch has to open to allow mRNA loading. Llacer et al. report cryo-EM reconstructions of yeast translation pre-initiation complexes in both open and closed conformations, highlighting structural differences and elucidating key events in start codon recognition.