A Novel Radical SAM mechanism mediated by the Interferon‐Inducible Protein Viperin

Abstract

To survive infection the immune system deploys an arsenal of defensive measures to combat invading pathogens. The type I interferon (IFN) response presents an early host defense by generating an intracellular environment that restricts virus replication and signals the presence of a viral pathogen to the adaptive arm of the immune response. Viperin (virus inhibitory protein, endoplasmic reticulum‐associated, interferon‐inducible) is an interferon stimulated gene (ISG), that is induced by type I, II, and III IFNs, or after infection by a broad range of DNA and RNA viruses. Viperin contains an S‐adenosyl‐L‐methionine (SAM) domain and an N‐terminal amphipathic helix that contributes to its antiviral activity by localizing viperin to ER membranes or lipid droplets. Viperin has been found to require ER localization for full antiviral activity and mutation of the cysteine residues (CX3CX2C motif) ligating the Fe‐S cluster in the central radical SAM domain entirely abolished antiviral activity. Viperin induces a cascade of signaling events to mount an immune response against the viral replication; however, the exact molecular basis for viperin's cellular activity and the role of SAM‐dependent reactions in inhibiting viral synthesis remains unknown. Our studies suggest that the radical SAM activity of viperin is critical for activation of innate immune signaling. The enzyme localizes the E3 ubiquitin ligase TRAF6 and the interleukin 1 receptor‐associated kinase (IRAK1) to the endoplasmic reticulum and activates TRAF6 to catalyze polyubiquitylation of IRAK1, key step in the Toll‐like receptor‐7 (TLR) and TLR‐9 signaling pathways. Our data implicates the radical SAM activity of viperin is essential in this activation step. We found that in presence of IRAK1 and TRAF6 the reductive cleavage of SAM to form 5′‐deoxyadenosine is increased ~ 4‐fold. Thus, our data provides the first evidence that viperin's radical SAM activity is linked to activation of a ubiquitin ligase. This observation has implications for how the enzyme may exert its multifaceted roles in the cellular antiviral response.Support or Funding InformationThis work was supported in part by NIH grants GM 093088 to E.N.G.M. and DK 046960 to R.T.K.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.