Suppression of the dual antiviral activity of PACT against influenza A virus by NS1

Abstract

Influenza A virus (IAV) confronts host innate immune responses including type I interferon (IFN) production using multiple viral proteins, among which non-structural protein 1 (NS1) is a key virulence factor that suppresses multiple innate signaling pathways. Double stranded RNA-binding protein PACT is an essential coactivator of RIG-I in innate antiviral response. One recent report has demonstrated that the IFN antagonism of NS1 is attributed to its inhibition of PACT. On the other we have further shown that PACT has dual antiviral activity mediated through not only the inhibition of RIG-I and IFN production but also the direct suppression of IAV polymerase. In this study we reported on the suppression of the dual antiviral activity of PACT by NS1. NS1 physically interacted with PACT and potently suppressed the IFN-inducing activity of PACT in a PKR- or TRIM25-independent manner. On the other hand, NS1 was also capable of counteracting the suppressive effect of PACT on IAV polymerase. Overexpression of NS1 reversed the inhibition of viral polymerase by PACT in the ribonucleoprotein assay. This was also observed in the IFN-deficient Vero cells, suggesting that NS1 also antagonizes the IFN-independent suppression of IAV by PACT. Consistent with this, the antiviral activity of PACT against an NS1-deficient IAV was more potent and was also seen in Vero cells. Taken together, our findings indicate the suppression of the IFN-dependent and IFN-independent anti- IAV activity of PACT by NS1. Our work provides a new mechanism by which NS1 counteracts innate antiviral response. Supported by RGC (HKU1/CRF/11G, N-HKU712/12, T11-707/15-R and C7011-15R), HMRF (12111312, 14130862, HKM-15-M01 and 15140662) and S. K. Yee MRF (2011).