The C-Terminal Effector Domain of Non-Structural Protein 1 of Influenza A Virus Blocks IFN-ß Production by Targeting TNF Receptor-Associated Factor 3

Qian, Wei; Wei, Xiaoqin; Guo, Kelei; Li, Yongtao; Lin, Xian; Zou, Zhong; Zhou, Hongbo; Jin, Meilin

The MAVS-TRAF3 complex is a focal point of RLR-directed signaling response (42, 43) . TRAF3 localizes to the endoplasmic reticulum (ER) and needs to be recruited to mitochondrial MAVS in order to activate TBK1 complexes (44) . Many viral proteins, accessory and non-structural proteins in particular, hijack TRAF3 or the TRAF3 complex to mediate immune evasion. SARS coronavirus M protein or Open Reading Frame-9b prevents the formation of TRAF3-TANK-TBK1/IKKε complex or MAVS-TRAF3/TRAF6 signalosome to evade host innate immunity (45, 46) . SARS-CoV papain-like protease interacts with and disrupts STING-TRAF3-TBK1 complex, it also inhibits the TLR7-mediated innate immunity through removing Lys63linked ubiquitin chains of TRAF3 and TRAF6 (47, 48) . Herpes simplex virus 1 ubiquitin-specific protease UL36 deubiquitinates TRAF3 then counteracts the IFN-β pathway (49) . Over the past 10 years, there have been major advances in understanding how influenza A viruses successfully escape the surveillance of the immune system. The current report furthers this research revealing the surprising finding that NS1/126-225 acts by targeting TRAF3; specifically, NS1/126-225 targets the TRAF domain of TRAF3. TRAF3 links the upstream IFN signaling responses of MAVS to TBK1 relying on the TRAF domain. This report also shows that a specific interaction between TRAF3 and MAVS was observed when TRAF3 and MAVS were co-expressed in 293T cells. However, the interaction between TRAF3 and MAVS was disrupted in the presence of NS1/126-225. Interestingly, the interaction between MAVS and IKKε was markedly increased in NS1/126-225-expressing cells. It has been previously demonstrated that, after Sev infection, K63-linked polyubiquitination at Lys500 of MAVS recruits IKKε to the mitochondria, functionally causes release of TRAF3 from MAVS initiating the signal to shutdown the IFN response (31) . The MAVS-IKKε complex was enhanced when NS1/126-225 was present, indicating that NS1/126-225 can utilize this process to shut down further activation of IFN pathway. Taken together, these data indicate that NS1/126-225 impedes the interactions between components of MAVS-TRAF3 complex, preventing the phosphorylation of IRF3, where it would activate the IFN-β response.