Dual anti-influenza virus activity of PACT through activation of RIG-I and suppression of viral RNA polymerase

Abstract

PACT is a double-stranded RNA-binding protein that interacts with RIG-I as well as influenza A virus (IAV) proteins NS1 and PA. On one hand, PACT functions as an essential partner and activator of RIG-I in the activation of viral RNA-induced innate antiviral response. On the other hand, the interaction of PACT with NS1 and PA might also have an impact on viral replication. Exactly how PACT exerts its anti-IAV activity to inhibit IAV replication is incompletely understood. Particularly, it remains to be determined whether the interaction of PACT with RIG-I or IAV protein is more important in infected cells. In this study we demonstrated the role of PACT in the sensing of IAV RNA by RIG-I as well as the interaction of PACT with IAV polymerase and suppression of viral RNA synthesis by PACT. RIG-I-induced activation of type I interferon (IFN) response by IAV ribonucleoprotein complex was most robust in the presence of PACT. PACT-dependent activation of IFN-β production was suppressed not only by NS1 but also by viral polymerase subunits PA, PB1 and PB2. PACT interacted with PA, PB1 and PB2 in an RNase A-insensitive manner. Knockdown of PACT in IAV-infected A549 cells boosted viral RNA accumulation and consequent IFN-β induction. Viral RNA replication was enhanced when PACT was compromised not only in IFN-proficient A549 cells but also in IFN-deficient Vero cells, suggesting an IFN-independent mechanism in the suppression of IAV replication. Thus, the antiviral activity of PACT is ascribed to its activation of RIG-I when IAV RNA is sensed in the early stage of infection. This activity is effectively inhibited by NS1 and viral polymerase. More importantly, the anti-IAV property of PACT seen in later stage of infection is primarily attributed to its interaction with and suppression of viral polymerase. Taken together, our work unravels new mechanistic details of IAV-host interaction in which the outcome of viral infection and antiviral response is dictated by PACT through its interaction with RIG-I and IAV polymerase. Supported by RGC-CRF C7011-15R.