LUBAC is required for the RIG-I mediated antiviral response
RNA viruses are responsible for many of the most recent threats to human health by emerging infectious diseases, including epidemics caused by Influenza, Zika, Dengue and novel Coronaviruses. The body’s first line of defence against viruses is the innate immune system. Pattern recognition receptors (PRR) detect pattern associated molecular patterns (PAMPs), such as incoming viral genomes. The RIG-I like receptor family sense viral RNA in the host cell cytoplasm, with RIG-I sensing double stranded regions of RNA with an exposed 5’ triphosphate group, present in viruses such as Influenza and Zika. The resulting signalling platform activates both IRF3 and NF-κB, resulting in induction of an antiviral response. Early sensing of infection is essential for effective clearance of virus infection with minimal damage to the host. In this study we set out to understand the contribution of the linear ubiquitin chain assembly complex (LUBAC) to the sensing of RNA and RNA viruses by RIG-I. By using synthetic RNAs, 3p-hpRNA and Poly(I:C), and RNA viruses to specifically stimulate RIG-I, we confirmed the requirement of signalling proteins MAVS and NEMO for activation of IRF3 and NF-κB in this context. TBK1 and IKKε were shown to function redundantly to activate IRF3 but not NF-kB. Linear ubiquitin chains are produced exclusively by the tripartite E3 ligase LUBAC, made up of HOIP, HOIL and SHARPIN, and are required for NF-κB activation by PRR such as TLR3 and MDA5 signalling. Here we show that the catalytic component HOIP, and HOIL-1, are required for activation of both IRF3 and NF-κB-dependent antiviral signalling during RIG-I activation. Conversely, SHARPIN is not required for either IRF3 or NF-κB-driven responses, an observation that helps rectify previous inconsistencies in the literature. Expression of a catalytic dead mutant of HOIP, and the resultant loss of M1 chains, only partially abrogated activation of IRF3 and NF-κB, suggesting LUBAC has an additional mechanism for regulating RIG-I signalling. HOIP and SHARPIN were shown interact with both NEMO and TBK1 after stimulation of RIG-I, suggesting LUBAC acts as a scaffold to recruit proteins to the signalling complex. We have therefore defined the different roles of individual LUBAC components in RIG-I signalling and established that this complex is essential for innate immune sensing of multiple RNA viruses.