The Antiviral Evolutionary Arms Race between IFITs and RNA Viruses

The mammalian innate immune system serves as a first line of defense against pathogens. Innate immunity molecules are critical for initial recognition and inhibition of non- self targets. However, viruses rapidly evolve to evade these innate defenses. Therefore, to successfully mitigate pathogenesis, host proteins must adapt against evolving viruses. The effects of this antagonistic coevolution are evident in the genetic footprints of many innate immunity proteins such as interferon-induced with tetratricopeptide repeats (IFIT) proteins 2 and 3. Despite the ancestral gene duplication that gave rise to IFIT2 and IFIT3, there is one region of high similarity between these two genes. Interestingly, this genetic similarity is due to recurrent recombination between IFIT2 and IFIT3. This co-evolutionary signature suggests that IFIT2 and IFIT3 may interact and share a common antiviral function. To understand how potential IFIT2/3 heterodimers inhibit infection, the antiviral activity of IFIT2 and IFIT3 were analyzed in Semliki forest virus (SFV), Thogoto virus (THOV), influenza A virus (IAV), and Ebola virus (EBOV) reporter assays. Our findings indicate that IFIT2/3 antiviral activity is mediated by two components: RNA binding and heterodimerization. We propose that key acidic and basic residues (IFIT2 E156, IFIT2 K158, IFIT3 E153, IFIT3 K156) in subdomain 2 of IFIT2 and IFIT3 are critical for viral RNA recognition. Consistent with our hypothesis that the recombining region may mediate heterodimerization, we find that IFIT2/3 interaction is disrupted by a single change (S111A) in the recombination region of subdomain 1 in IFIT2. Further analysis of the unique antiviral mechanism of IFIT2/3 will help predict which pathogens IFIT2/3 inhibit and provide insight into the host innate immune response