SARS-CoV-2 Nsp5 Demonstrates Two Distinct Mechanisms Targeting RIG-I and MAVS To Evade the Innate Immune Response

Newly emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) caused a global pandemic with astonishing mortality and morbidity. The high replication and transmission of SARS-CoV-2 are remarkably distinct from those of previous closely related coronaviruses, and the underlying molecular mechanisms remain unclear. The innate immune defense is a physical barrier that restricts viral replication. We report here that the SARS-CoV-2 Nsp5 main protease targets RIG-I and mitochondrial antiviral signaling (MAVS) protein via two distinct mechanisms for inhibition. Specifically, Nsp5 cleaves off the 10 most-N-terminal amino acids from RIG-I and deprives it of the ability to activate MAVS, whereas Nsp5 promotes the ubiquitination and proteosome-mediated degradation of MAVS. As such, Nsp5 potently inhibits interferon (IFN) induction by double-stranded RNA (dsRNA) in an enzyme-dependent manner. A synthetic small-molecule inhibitor blunts the Nsp5mediated destruction of cellular RIG-I and MAVS and processing of SARS-CoV-2 nonstructural proteins, thus restoring the innate immune response and impeding SARSCoV-2 replication. This work offers new insight into the immune evasion strategy of SARS-CoV-2 and provides a potential antiviral agent to treat CoV disease 2019 (COVID-19) patients. IMPORTANCE The ongoing COVID-19 pandemic is caused by SARS-CoV-2, which is rapidly evolving with better transmissibility. Understanding the molecular basis of the SARS-CoV-2 interaction with host cells is of paramount significance, and development of antiviral agents provides new avenues to prevent and treat COVID-19 diseases. This study describes a molecular characterization of innate immune evasion mediated by the SARS-CoV-2 Nsp5 main protease and subsequent development of a small-molecule inhibitor.