The Mechanisms Of VISA-Mediated Innate Immune Response By TARBP2

The innate immune system is the first line of defense against the invasion of pathogenic microorganism.Host cells sense the invasion of viruses and bacterias through pattern recognition receptors(PRRs)and bind to their pathogen associated molecular patterns(PAMPs)to induce a variety of transcription factors entering the nucleus,including interferon regulatory factors 3/7 and NF-?B,and initiate transcription of type I interferons,inflammatory cytokines,etc.,thereby enabling cells to establish antiviral immune defense mechanismsThe RIG-I-like receptors(RLRs)family,as an important cellular pattern recognition receptor,after recognizing and binding to the dsRNA of the virus,it changes from a closed circular conformation to an activated linear conformation.After a series of modifications,RIG-I is recruited by its exposed CARD domain to the mitochondrial and transmits antiviral signals by interacting with VISA anchoring the outer membrane of the mitochondria.The adaptor protein VISA is further oligomerized,and the activated VISA continues to recruit members of the downstream TRAF family to form the VISA signalsome.On the one hand,TRAFs transmit signals to downstream kinases such as TBK1/IKK?;on the other hand,the signalsome continues to recruit proteins including TANK,TRADD,etc.to amplify the casade.Ultimately,the composed signal protein complex transmits antiviral signals and finally induces the production of type I interferon and the innate antiviral immune response.The formation mechanism of the VISA signal complex is not fully understood for now,and RIG-I,as a representative of the RLRs family,is indispensable for the activation of VISA.Based on the high density of VISA interaction protein reported by previous researchers,we used RIG-I,the upstream of VISA,as a bait protein to screen for potential associated proteins to enrich the signal transduction mechanism of VISA by using the yeast two-hybrid system.By screening and identification,we obtained prey protein TARBP2(transactivation response element RNA-binding protein),which interacts with both RIG-I and VISA.Our results show that the interaction between endogenous TARBP2 and RIG-I and VISA only exists in resting state rather than upon viral stimulus,but the presence of TARBP2 can attenuate the interaction between RIG-I and VISA in infected cells.Further studies have shown that knockdown of intracellular TARBP2 expression by RNAi interference can inhibit viral-induced RIG-I expression.Furthermore,TARBP2 affects the activation or protein stability of TRAF3 and TRAF6 by inhibiting the ubiquitination of TRAF3 and TRAF6,and hinders the formation of the VISA signaling protein complex and the activation of the downstream transcription factor IRF7 and the expression of type I interferon.