Short 5’ UTRs serve as a marker for mRNA translation inhibition by the IFIT2-IFIT3 antiviral complex

Recognition of non-self nucleic acids, including cytoplasmic dsDNA, dsRNA, or mRNAs lacking proper 5' cap structures, is critical for the induction of innate immunity against viruses. Here, we demonstrate that short 5’ untranslated regions (UTRs), a characteristic of many viral mRNAs, can also serve as molecular pattern for innate immune recognition via the interferon-induced proteins, IFIT2 and IFIT3. The IFIT2-IFIT3 heterodimer, formed through an intricate domain swap structure, mediates viral mRNA 5' end recognition, translation inhibition, and ultimately antiviral activity. Critically, 5' UTR lengths <50 nucleotides are necessary and sufficient to sensitize an mRNA to translation inhibition by the IFIT2-IFIT3 complex. Thus, diverse viruses whose mRNAs contain short 5’ UTRs, such as vesicular stomatitis virus and parainfluenza virus 3, are sensitive to IFIT2-IFIT3-mediated antiviral activity. Our work reveals a new size-restricted pattern of nucleic acid innate immune recognition for the selective repression of viral replication. eCLIP was performed as previously described (Blue SB, et al. Nature Protocols 2022), using antibodies against FLAG (M2 / F1804, Sigma) and HA (HA.11 / 901502, Biolegend) for 293T experiments, and IFIT2 (PA3-845, ThermoFisher) and IFIT3 (ABF1048, Millipore) for MEF experiments. Most experiments were performed in biological duplicate, except for uninfected 293T samples (which were single replicates).