The matrix protein of respiratory syncytial virus suppresses interferon signaling via RACK1 association

Respiratory syncytial virus (RSV) is a frequent cause of respiratory illness among pediatric and elderly populations. The severity of the respiratory disease is determined, in part, by RSV virulence and the host immune response, especially type I interferon (IFN) production. Using proteomics for the identification of partner proteins associated with RSV-encoded matrix (M) protein in transiently M-expressed and RSV-infected cells, we identified many M-interacting proteins involved in diverse biological processes including cell stress and innate immune response, highlighting M protein as a novel antagonist of IFN-β potentially accounting for limited IFN production in RSV-infected epithelial cells. To clarify the M antagonistic mechanism, we focused on M-interacting receptor of activated C kinase 1 (RACK1), which is an adaptor protein and a negative regulator of IRF3/7. Knockdown of RACK1 with small-interfering RNA attenuated the M-suppressed IFN-β response leading to increased IFN-β production and reduced RSV genome replication, confirming RACK1 as a critical host factor for efficient RSV infection. Our finding unravels an immune-intervening effect of RSV M protein on IFN response and further identifies RACK1 as a promising target for antiviral therapy. IMPORTANCE Respiratory syncytial virus (RSV) matrix (M) protein is indispensable for virion assembly and release. It is localized to the nucleus during early infection to perturb host transcription. However, the function of RSV M protein in other cellular activities remains poorly understood. In this study, several interferon response-associated host factors, including RACK1, were identified by proteomic analysis as RSV M interactors. Knockdown of RACK1 attenuates RSV-restricted IFN signaling leading to enhanced host defense against RSV infection, unraveling a role of M protein in antagonizing IFN response via association with RACK1. Our study uncovers a previously unrecognized mechanism of immune evasion by RSV M protein and identifies RACK1 as a novel host factor recruited by RSV, highlighting RACK1 as a potential new target for RSV therapeutics development.