Riplet Ubiquitin Ligase Regulates Innate Antiviral Immune Responses Via Lysine 63-Linked Polyubiquitination of RIG-I and LGP2 and is Essential for Protection Against SARS-CoV-2

Abstract

RIG-I, a cytoplasmic viral RNA sensor, is crucial for innate antiviral immune responses; however, there are controversies about regulatory mechanism of RIG-I by several ubiquitin ligases and LGP2. In this study, our genetic data showed that Riplet was a primary ubiquitin ligase for RIG-I activation and could compensate for defective TRIM25 in RIG-I activation, whereas TRIM25 activated RIG-I in a cell-type-specific manner with several accessory proteins, such as ZCCHC3. Interestingly, viral RNA of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was recognized by cytoplasmic innate immune system, and type I IFN expression was induced in a Riplet-dependent manner. However, several viral proteins of SARS-CoV-2 attenuated Riplet-dependent cytokine expression. Additionally, we revealed that Riplet mediated Lys63-linked polyubiquitination of LGP2 and attenuated excessive cytokine expression at the late phase of viral infection. Our data indicate that Riplet is an essential host factor for the innate immune response against SARS-CoV-2.Funding: This work was supported in part by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) and Japan Agency for Medical Research and Development (AMED).Conflict of Interest: The authors declare no competing interests.Ethical Approval: The experiments using SARS-CoV-2 were performed in BSL3 level facilities according to approved protocols in Kumamoto University.