SUMMARY
Hepatitis C virus mRNA contains an internal ribosome entry site (IRES) that mediates end-independent translation initiation, requiring a subset of eukaryotic initiation factors (eIFs). Direct binding of the IRES to the 40S subunit places the initiation codon into the P site, where it base-pairs with eIF2-bound Met-tRNAiMet forming a 48S initiation complex. Then, eIF5 and eIF5B mediate subunit joining. Initiation can also proceed without eIF2, in which case Met-tRNAiMet is recruited directly by eIF5B. Here, we present cryo-EM structures of IRES initiation complexes at resolutions up to 3.5 Å that cover all major stages from initial ribosomal association, through eIF2-containing 48S initiation complexes, to eIF5B-containing complexes immediately prior to subunit joining. These structures provide insights into the dynamic network of 40S/IRES contacts, highlight the role for IRES domain II, and reveal conformational changes that occur during the transition from eIF2- to eIF5B-containing 48S complexes that prepare them for subunit joining.
Competing Interest Statement
The authors have declared no competing interest.
