Interferon-Inducible Protein IFI35 Negatively Regulates RIG-I Antiviral Signaling and Supports Vesicular Stomatitis Virus Replication

In a genome-wide small interfering RNA (siRNA) screen, we recently identified the interferon (IFN)-inducible protein 35 (IF135; also known as IFP35) as a factor required for vesicular stomatitis virus (VSV) infection. Studies reported here were conducted to further understand the role and requirement of IF135 in VSV infection. Consistent with the siRNA screening data, we found that depletion of IF135 led to reduced VSV replication at the level of viral gene expression. Although no direct interaction of IF135 with the viral replication machinery was observed, we found that IF135 negatively regulated the host innate immune response and rescued poly(I center dot C)-induced inhibition of VSV replication. Promoter-driven reporter gene assays demonstrated that IF135 overexpression suppressed the activation of IFN-beta and ISG56 promoters, whereas its depletion had the opposite effect. Further investigation revealed that IF135 specifically interacted with retinoic acid-inducible gene I (RIG-I) and negatively regulated its activation through mechanisms that included (i) suppression of dephosphorylation (activation) of RIG-I and (ii) proteasome-mediated degradation of RIG-I via K48-linked ubiquitination Overall, the results presented here suggest a novel role for IF135 in negative regulation of RIG-I-mediated antiviral signaling, which will have implications for diseases associated with excessive immune signaling.