Assembly of RIG-I Signaling Complexes in Antiviral Immunity

Abstract

RIG-I is a cytosolic sensor of viral dsRNA. RIG-I molecules are recruited to and translocate along the length of the viral dsRNA, forming filaments. Filament formation initiates a signaling cascade that leads to the aggregation of the adaptor MAVS, phosphorylation and dimerization of transcription factor IRF3, and expression of IFN-β and other antiviral genes. Ubiquitination, a form of post-translational modification, is critical for the activation of RIG-I. Although structural studies give us a picture of the ubiquitinated RIG-I, the identity of the ubiquitin ligase involved in this process remains debated. Furthermore, how RIG-I is recognized as a substrate for ubiquitination remains unknown. Our results showed that the RIPLET is the required ubiquitin ligase for RIG-I. RIPLET specifically ubiquitinated RIG-I in dsRNA-dependent manner. RIPLET bound to filamentous RIG-I in an RNA length-dependent manner and their interaction required bivalent RIPLET PRY/SPRY domains. Furthermore, RIPLET adopted an additional binding mode with long RIG-I filaments, leading to their clustering. In the absence of ubiquitination, the second binding mode of RIPLET was able to promote the aggregation of MAVS and the expression of antiviral genes in cells. These results show a previously un-recognized ubiquitin-independent role of RIPLET in antiviral signaling. Our work also highlights the importance of receptor clustering for the induction of IFN signaling. The clustering of RIG-I filaments, as well as the prion-like polymerization of MAVS, allows the rapid amplification of signaling. Yet, the process by which the macromolecular assemblies of RIG-I and MAVS are cleared following signaling remains poorly understood. In the last part of my work, we show that dsRNA stimulation induces the recruitment of RIG-I into atypical stress granules (SGs), which contained several IFN signaling molecules and autophagy markers. In the absence of SGs, cells are not able to degrade these signaling aggregates, leading to hyperactivation of the IFN pathway. In addition, cells lacking SGs showed pronounced caspase dependent death in response to dsRNA stimulation. We propose that IFN signaling aggregates are degraded through autophagy, and that SGs act as intermediates in this process. Thus, SGs play a key role preventing aberrant IFN signaling and cell death.