Abstract
Hepatitis C virus (HCV), a widespread human pathogen, is dependent on a highly structured 5′-untranslated region of its mRNA, referred to as internal ribosome entry site (IRES), for the translation of all of its proteins. The HCV IRES initiates translation by directly binding to the small ribosomal subunit (40S), circumventing the need for many eukaryotic translation initiation factors required for mRNA scanning. Here we present the cryo-EM structure of the human 40S ribosomal subunit in complex with the HCV IRES at 3.9 Å resolution, determined by focused refinement of an 80S ribosome–HCV IRES complex. The structure reveals the molecular details of the interactions between the IRES and the 40S, showing that expansion segment 7 (ES7) of the 18S rRNA acts as a central anchor point for the HCV IRES. The structural data rationalizes previous biochemical and genetic evidence regarding the initiation mechanism of the HCV and other related IRESs.
Highlights
Hepatitis C virus (HCV), a widespread human pathogen, is dependent on a highly structured 50-untranslated region of its mRNA, referred to as internal ribosome entry site (IRES), for the translation of all of its proteins
Our complex contained both ribosomal subunits, initial single particle cryo-Electron microscopic (EM) reconstructions indicated that the IRES in the 80S complex contacts exclusively the 40S subunit and adopts a conformation previously observed in 40S–HCV IRES complexes[7] (Fig. 1c)
The general features of the structure are very similar to a previous structure of the 80S ribosome stalled in initiation by cycloheximide bound to HCV IRES8 except that the stalled
Summary
Hepatitis C virus (HCV), a widespread human pathogen, is dependent on a highly structured 50-untranslated region of its mRNA, referred to as internal ribosome entry site (IRES), for the translation of all of its proteins. We present a 3.9 Å cryo-EM structure of the homologous complex of the HCV IRES bound to the human 40S ribosome, which was solved by focused refinement of the HCV IRES bound to 80S ribosomes. This represents the first near-atomic structure of the HCV IRES that allows visualization of the network of molecular interactions formed between the 40S subunit and the HCV IRES. We show that the HCV IRES and the Cricket paralysis virus (CrPV) IRES, structurally unrelated, share interactions with ribosomal proteins
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