The vertebrate antiviral innate immune system is often considered to be composed of two distinct groups of proteins: pattern recognition receptors (PRRs) that detect viral infection and activate the interferon (IFN) signaling pathway, and effectors that directly act against viral replication. Accordingly, previous studies on PRRs, such as RIG-I and MDA5, have been primarily focused on their functions in viral double-stranded RNA (dsRNA) detection and downstream signaling. Limited effort has been made in characterizing and dissecting their potential roles outside these canonical PRR functions. We have recently discovered that RIG-I and MDA5 utilize their helicase domains and ATP hydrolysis-driven receptor dynamics to displace viral proteins from dsRNA, and can perform signaling-independent antiviral functions against a broad spectrum of RNA viruses. This notion of the non-canonical, effector-like functions of RIG-I/MDA5 was also supported by publications from other laboratories (Weber, Takaoka and tenOever), although the deeper molecular mechanisms remain to be dissected. These findings challenge the conventional views that PRRs passively bind pre-exposed ligands and that their antiviral activities are solely dependent on their ability to induce immune signaling. In this proposal, we examine molecular mechanisms by which the protein displacement activity of RIG-I and MDA5 can exert signaling-independent antiviral functions. In particular, we examine the following two non- exclusive hypotheses using a combination of biochemical and cellular assays.
In Aims 1 -2, we test the hypothesis that displacement of viral dsRNA-binding proteins by RIG-I and MDA5 could lead to increase in overall accessibility of viral dsRNA, thereby promoting functions of other dsRNA-dependent antiviral proteins in the host, such as PKR, OAS1 and ADAR1.
In Aim 3, we explore the hypothesis that the protein displacement activity of RIG-I/MDA5 could be exerted on the viral replication or transcription complexes or capsid proteins, which may lead to direct inhibition of core viral life cycle steps. While this proposal explores the new function of RIG-I/MDA5, our previous work on these receptors, both their canonical and non-canonical functions, provide a solid foundation for the proposed research. In addition, our expertise on biochemical and cellular analysis of protein-RNA complexes will further help us achieve the goals outlined in this proposal. We believe that this investigation would not only re-define the functions of RIG-I and MDA5 as a novel class of receptors with ?effector-like? functions, but would also bring unique perspectives on functions of related proteins in antiviral defense.
Viral infection poses a major challenge to global health as demonstrated in the recent epidemics of Zika and Ebola viruses and pandemics of H1N1 influenza virus, SARS and HIV. The goal of the current proposal is to understand the molecule mechanism by which two viral RNA receptors, RIG-I and MDA5, perform the antiviral defense. We will particularly focus on the novel effector-like functions of these receptors, and how these non- canonical functions interplay with the canonical receptor function during viral infection.