A Viral Deamidase Targets the Helicase Domain of RIG-I to Block RNA-Induced Activation

Abstract

SUMMARY RIG-I detects double-stranded RNA (dsRNA) to trigger antiviral cytokine production. Protein deamidation is emerging as a post-translational modification that chiefly regulates protein function. We report here that UL37 of herpes simplex virus 1 (HSV-1) is a protein deamidase that targets RIG-I to block RNA-induced activation. Mass spectrometry analysis identified two asparagine residues in the helicase 2i domain of RIG-I that were deamidated upon UL37 expression or HSV-1 infection. Deamidation rendered RIG-I unable to sense viral dsRNA, thus blocking its ability to trigger antiviral immune responses and restrict viral replication. Purified full-length UL37 and its carboxyl-terminal fragment were sufficient to deamidate RIG-I in vitro. Uncoupling RIG-I deamidation from HSV-1 infection, by engineering deamidation-resistant RIG-I or introducing deamidase-deficient UL37 into the HSV-1 genome, restored RIG-I activation and antiviral immune signaling. Our work identifies a viral deamidase and extends the paradigm of deamidation-mediated suppression of innate immunity by microbial pathogens.

Species-Specific Deamidation of cGAS by Herpes Simplex Virus UL37 Protein Facilitates Viral Replication

“…We previously reported that the UL37 tegument protein of HSV-1 deamidates RIG-I to avoid dsRNA-induced innate immune activation (Zhao et al, 2016b). Recombinant HSV-1 carrying the deamidase-inactivating UL37C819S mutation (HSV-1 UL37C819S) more robustly induced antiviral cytokines than HSV-1 containing UL37 wild-type (HSV-1 UL37WT), while they replicated comparatively in VERO cells that are defective in type I interferon production (Figure S1A).…”

“…We previously reported that UL37 deamidates RIG-I to avoid dsRNA-induced innate immune activation (Zhao et al, 2016b). To discern the contribution of RIG-I and cGAS to host defense against HSV-1, we generated primary bone marrow-derived macrophages (BMDMs) from WT, MAVS −/− and cGAS −/− mice and examined innate immune response upon infection.…”

“…Coevolving with their hosts, herpesviruses are expected to leave footprint on host immune system via an arms race between host immune defense and viral evasion thereof. We have characterized herpesvirus tegument proteins that function as either authentic deamidase or pseudo-enzymes to deamidate RIG-I, thus evading innate immune activation induced by double-stranded RNA (dsRNA) (He et al, 2015; Zhao et al, 2016b). Considering the pivotal role of cGAS in sensing viral DNA, we hypothesize that cGAS might be targeted by viral evasion mechanism.…”

Interleukin-36 cytokines may overcome microbial immune evasion strategies that inhibit interleukin-1 family signaling

“…4A), thus preventing activation of the inflammasome (129,130). In addition, the viral-encoded enzyme UL37 (unique long region 37) deamidates two asparagine residues in RIG-I that are critical for the ability of RIG-I to bind to double-stranded RNA (131). Consequently, this RLR-type sensor (Fig.…”

“…3A) is not activated, and production of antiviral IFN-b is inhibited (131). Although whether or not HSV-1 UL37 has an effect upon IL-1b and IL-18 activation was not examined in this study (131), the viral immune evasion strategy that was identified is likely to also affect these proinflammatory cytokines due to the known role of RIG-I in inducing their inflammasome-dependent processing (132). Finally, IL-1b that has been processed is retained within HSV-1-infected cells (Fig.…”