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

Influenza A virus belongs to the orthomyxovirus family, containing eight negative-sense RNA segments in an enveloped viral particle encoding 14 or 17 proteins (9) . This array of proteins contributes to virulence; including the proteins associated with viral RNA-dependent RNA polymerase (10) and the nonstructural protein 1 (NS1). NS1 consists of 215-237 amino acids and comprises two functional domains: an N-terminal RNAbinding domain (RBD) (AA1 to 73) and a C-terminal effector domain (ED) (AA74-end) (11) . The NS1 protein plays a crucial role in regulating the host antiviral response through various mechanisms. One important function of the NS1 protein involves inhibition of IFN production. The mechanism of this inhibition includes activation of the transcription factors IRF3 (12) , NF-κB (13) , and AP-1 (14) , thus blocking IFN production. This efficient inhibitory effect is associated with an RIG-I signaling pathway through the NS1-RIG-I complex (15) (16) (17) . Previous studies have indicated that NS1 is also related to two positive factors of RIG-I, the E3 ligases TRIM25 (18) and RIPLET (19) . The residues E96/ E97 of NS1 mediate their interaction with the coiled-coil domain of TRIM25, thus blocking both TRIM25 multimerization and RIG-I CARD domain ubiquitination. This subsequently induces lower levels of IFN-β (18) . NS1 can also interact with RIPLET preventing the activation of RIG-I, although E96/E97 are not involved in that inhibition (19) . The dsRNA binding ability of NS1 could also be playing a role in the pre-transcriptional inhibition of the interferon pathway by sequestering the pathogenassociated molecular patterns (PAMPs) that RIG-I recognizes. Two residues, R38 and K41, are required for the dsRNA binding activity of NS1 (20) , thus highly impairing its ability to block interferon production.