RIG-I and MDA5 are cytoplasmic sensors of viral RNA that initiate innate antiviral immune response. These DExD/H-box RNA helicases share structural similarity with Dicer nuclease which mediates RNA interference. It is noteworthy that optimal function of Dicer requires double-stranded RNA (dsRNA)-binding protein activators including PACT. In this study, we demonstrated PACT to be a novel cellular binding partner and activator of RIG-I and MDA5. PACT physically binds to the C-terminal regulatory domain of RIG-I and MDA5. Functionally, PACT acts as a potent activator of RIG-I- and MDA5-dependent production of type I interferons in normal human lung fibroblasts, mouse embryonic fibroblasts and other types of cultured mammalian cells. PACT augments the activation of RIG-I and MDA5 by poly (I:C) of both intermediate and long length. PACT also synergizes with RIG-I and MDA5 to amplify interferon-induced antiviral immune response in virus-infected and uninfected cells. Silencing of PACT by RNAi attenuated Sendai virus- and vesicular stomatitis virus-induced activation of type I interferon production. The activation of RIG-I and MDA5 by PACT requires neither dsRNA-dependent protein kinase PKR nor Dicer. Direct interaction of RIG-I and MDA5 with PACT results in the activation of their ATPase activity. Intriguingly, the RIG-I- and MDA5-activating activity of PACT is counteracted by well-known virus-encoded antagonists of interferon such as influenza A virus NS1 and herpes simplex type I Us11 proteins. Our work reveals a novel component and a new layer of regulation in RIG-I- and MDA5-dependent innate antiviral immune response. Our findings have implications in rationale design and development of new antiviral and immunomodulatory agents.
This work was supported by Hong Kong Research Fund for the Control of Infectious Diseases (HSI/Lab-17 and 10091202) and Hong Kong Research Grants Council (HKU 7677/10M, HKU7/CRF/09 and HKU1/CRF/11G).