Emerging role of ubiquitination in antiviral RIG-I signaling

Abstract

Detection of viruses by the innate immune system involves the action of specialized pattern recognition receptors. Intracellular RIG-I receptors sense the presence of viral nucleic acids in infected cells and trigger signaling pathways that lead to the production of proinflammatory and antiviral proteins. Over the past few years, posttranslational modification of RIG-I and downstream signaling proteins by different types of ubiquitination has been found to be a key event in the regulation of RIG-I-induced NF-κB and interferon regulatory factor 3 (IRF3) activation. Multiple ubiquitin ligases, deubiquitinases, and ubiquitin binding scaffold proteins contribute to both positive and negative regulation of the RIG-I-induced antiviral immune response. A better understanding of the function and activity of these proteins might eventually lead to the development of novel therapeutic approaches for management of viral diseases.

Fig 1

RIG-I (de)activation. 5′-triphosphorylated dsRNA binds to the C-terminal domain of RIG-I. This causes dimerization of RIG-I and induces it to undergo a conformational change that exposes the two N-terminal CARDs. This allows the binding of Riplet and TRIM25 ubiquitin ligases, which attach K63-specific polyubiquitin chains to RIG-I. The functions of TRIM25 are antagonized by the influenza virus protein NS1 and by the HOIL-1L/HOIP complex. Additional unanchored K63-linked polyubiquitin chains bind to the CARDs. These ubiquitin modifications facilitate the binding of the mitochondrial adaptor protein MAVS. After signal transduction, RNF125 polymerizes K48-linked ubiquitin chains onto RIG-I, which leads to its degradation by the proteasome. CYLD constitutively removes the activating K63-linked polyubiquitin chains from RIG-I.

Fig 2

Similarities between signaling by RIG-I, TNFRI, CD40, and TLR4. Upon activation, RIG-I recruits the E3s TRAF6, TRAF2/5, cIAP1/2, and TRAF3 through the MAVS adaptor protein. When TRAF6 and RIP1 are K63 (auto)polyubiquitinated, they recruit the TAK1 and IKK complexes to the RIG-I receptor complex. TAK1 phosphorylates and activates the IKK complex, leading to activation of IKKβ and subsequent IκBα phosphorylation and degradation. This allows the translocation of NF-κB (p65/p50 dimer) to the nucleus. Similar mechanisms are involved in TNFRI-mediated NF-κB activation, which involves recruitment of TRAF2/5 via the TRADD adaptor protein. The RIG-I-induced TRAF3-mediated K63-linked autopolyubiquitination is involved in the activation of TBK1 and IKKε kinases, which phosphorylate IRF3, leading to its dimerization and nuclear translocation. Additionally, K27-linked polyubiquitination of NEMO by TRIM23 triggers NF-κB and IRF3 activation. The cIAP1/2-mediated K63-linked polyubiquitination of RIP1 and TRAF6/RIP1-mediated recruitment of the TAK1 complex (arrows) are hypothetical and are based on similarities between TNFRI and TLR4 signal transduction. Upon CD40 stimulation, TRAF2/5 targets cIAP1/2 for K63-linked polyubiquitination, which then mediate K48-linked polyubiquitination of TRAF3, which targets it for proteasomal degradation. This prevents the constitutive degradation of NIK by TRAF3, allowing NIK-mediated IKKα activation and IKKα-mediated phosphorylation of p100 NF-κB. This leads to partial processing of p100 into the p52 NF-κB subunit. The noncanonical NF-κB dimer p52/Rel-B then translocates to the nucleus. TLR4 stimulation induces the selective ubiquitination of TRAF3, which is either targeted for K48-linked polyubiquitination by cIAP1/2 (MyD88 dependent), leading to NF-κB activation, or subjected to K63-linked polyubiquitination, leading to IRF3 activation (TRIF dependent). It should be mentioned that specific signaling molecules have sometimes been reported to be modified by types of ubiquitination other than those that are indicated (e.g., linear polyubiquitination of NEMO in the case of TNF signaling or K63-linked polyubiquitination in case of TCR signaling), but for reasons of simplicity these have been omitted from the figure.