Ubiquitin ligase RIPLET mediates polyubiquitination of RIG-I and LGP2 and regulates the innate immune responses to SARS-CoV-2 infection

RIG-I, a cytoplasmic viral RNA sensor, is crucial for innate antiviral immune responses; however, there are controversies about RIG-Is regulatory mechanism by several ubiquitin ligases and LGP2. Our genetic study revealed that the RIPLET ubiquitin ligase was a general activating factor for RIG-I signaling, whereas another ubiquitin ligase, TRIM25, activated RIG-I in a cell-type-specific manner. These RIPLET and TRIM25 functions were modulated by accessory factors, such as ZCCH3C and NLRP12. Interestingly, we found an additional role of RIPLET in innate immune responses. RIPLET induced delayed polyubiquitination of LGP, resulting in the attenuation of excessive cytokine expression at the late phase. Moreover, RIPLET was involved in the innate immune responses against SARS-CoV-2 infection, a cause of the recent COVID-19 pandemic. Our data indicate that RIPLET fine-tunes innate immune responses via polyubiquitination of RIG-I and LGP2 against virus infection, including SARS-CoV-2.