Black carp RNF5 inhibits STING/IFN signaling through promoting K48-linked ubiquitination and degradation of STING

RNF5 inhibits HBV replication by mediating caspase-3-dependent degradation of core protein

The RING finger protein 5 (RNF5), an E3 ubiquitin ligase, has demonstrated significant antiviral activity against various viruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and Kaposi's sarcoma-associated herpesvirus (KSHV). However, its role in hepatitis B virus (HBV) replication has not been previously studied. In this study, we demonstrate that RNF5 effectively inhibits HBV replication by promoting the degradation of the HBV Core protein through a Caspase-3-dependent pathway. We first show that RNF5 expression is upregulated in HBV-infected cells and patient samples, suggesting a role in the host's antiviral response. Subsequently, we investigate the mechanism by which RNF5 mediates its antiviral effect, finding that RNF5 targets the Core protein for degradation independently of its E3 ubiquitin ligase activity. The degradation of Core protein is mediated through a Caspase-3-dependent mechanism rather than the proteasomal pathway. Interestingly, RNF5's antiviral function does not rely on ubiquitination, indicating an alternative pathway involving apoptosis-related processes. These findings highlight the multifunctional role of RNF5 and suggest that targeting RNF5 could serve as a novel therapeutic approach to control HBV replication, providing new insights into the development of antiviral therapies against HBV.

The role of cGAS-STING pathway ubiquitination in innate immunity and multiple diseases

The cGAS-STING pathway is essential in innate immunity, especially in antiviral responses and cellular stress management. cGAS acts as a cytoplasmic DNA sensor by initiating the synthesis of the second messenger cyclic GMP-AMP synthase (cGAMP), which subsequently activates the STING pathway, leading to the production of type I interferons and other cytokines, as well as the activation of inflammatory mediators. Recent studies have demonstrated that ubiquitination changes closely regulate the function of the cGAS-STING pathway. Ubiquitination modifications influence the stability and activity of cGAS and STING, while also influencing the accuracy of the immune response by adjusting their degradation and signal intensity. E3 ubiquitin ligase specifically facilitates the degradation or modulates the signaling of cGAS-STING-associated proteins via ubiquitination alterations. Furthermore, the ubiquitination of the cGAS-STING pathway serves distinct functions in various cell types and engages with NF-κB, IRF3/7, autophagy, and endoplasmic reticulum stress. This ubiquitin-mediated regulation is crucial for sustaining the balance of innate immunity, while excessive or inadequate ubiquitination can result in autoimmune disorders, cancers, and viral infections. An extensive examination of the ubiquitination process within the cGAS-STING pathway elucidates its specific regulatory mechanisms in innate immunity and identifies novel targets for the intervention of associated diseases.

Ring finger protein 5 mediates STING degradation through ubiquitinating K135 and K155 in a teleost fish

Stimulator of interferon genes (STING) is a key connector protein in interferon (IFN) signaling, crucial for IFN induction during the activation of antiviral innate immunity. In mammals, ring finger protein 5 (RNF5) functions as an E3 ubiquitin ligase, mediating STING regulation through K150 ubiquitylation to prevent excessive IFN production. However, the mechanisms underlying RNF5's regulation of STING in teleost fish remain unknown. This study investigated the regulatory role of the mandarin fish (Siniperca chuatsi) RNF5 (scRNF5) in the STING-mediated antiviral immune response and identified the specific regulatory sites on scSTING. Furthermore, an examination of scRNF5 expression patterns in virus-infected cells revealed its responsiveness to mandarin fish ranavirus (MRV) infection. The ectopic expression of scRNF5 suppressed scSTING-mediated IFN signaling and facilitated MRV replication. Co-immunoprecipitation experiments indicated an interaction between scRNF5 and scSTING. The further experiments demonstrated that scRNF5 exerted its inhibitory effect by promoting the degradation of scSTING, which was observed to be blocked by MG132 treatment. Ubiquitination assays with various scSTING mutants showed that scRNF5 catalyzed the ubiquitination of scSTING at K135 and K155 residues. Furthermore, we provided evidence that scRNF5 significantly attenuated scSTING-dependent antiviral immunity by targeting negative regulators within the scSTING signaling cascade. This study underscored that RNF5 negatively regulated the STING-mediated IFN signaling pathway in mandarin fish, attenuated STING's antiviral activity, and facilitated STING degradation via the ubiquitin-proteasome pathway at two novel lysine sites (K135 and K155). Our work offered valuable insights into the regulatory mechanisms of STING-mediated signaling in teleost fish, paving the way for further research.

RING finger protein 122-like (RNF122L) negatively regulates antiviral immune response by targeting STING in common carp (Cyprinus carpio L.)

Stimulator of interferon genes (STING), as an imperative adaptor protein in innate immune, responds to nucleic acid from invading pathogens to build antiviral responses in host cells. Aberrant activation of STING may trigger tissue damage and autoimmune diseases. Given the decisive role in initiating innate immune response, the activity of STING is intricately governed by several posttranslational modifications, including phosphorylation and ubiquitination. Here, we cloned and characterized a novel RNF122 homolog from common carp (named CcRNF122L). Expression analysis disclosed that the expression of CcRNF122L is up-regulated under spring viremia of carp virus (SVCV) stimulation in vivo and in vitro. Overexpression of CcRNF122L hampers SVCV- or poly(I:C)-mediated the expression of IFN-1 and ISGs in a dose-dependent way. Mechanistically, CcRNF122L interacts with STING and promotes the polyubiquitylation of STING. This polyubiquitylation event inhibits the aggregation of STING and the subsequent recruitment of TBK1 and IRF3 to the signaling complex. Additionally, the deletion of the TM domain abolishes the negative regulatory function of CcRNF122L. Collectively, our discoveries unveil a mechanism that governs the STING function and the precise adjustment of the innate immune response in teleost.

RNF5: inhibiting antiviral immunity and shaping virus life cycle

RNF5 is an E3 ubiquitin ligase involved in various physiological processes such as protein localization and cancer progression. Recent studies have shown that RNF5 significantly inhibits antiviral innate immunity by promoting the ubiquitination and degradation of STING and MAVS, which are essential adaptor proteins, as well as their downstream signal IRF3. The abundance of RNF5 is delicately regulated by both host factors and viruses. Host factors have been found to restrict RNF5-mediated ubiquitination, maintaining the stability of STING or MAVS through distinct mechanisms. Meanwhile, viruses have developed ingenious strategies to hijack RNF5 to ubiquitinate and degrade immune proteins. Moreover, recent studies have revealed the multifaceted roles of RNF5 in the life cycle of various viruses, including SARS-CoV-2 and KSHV. Based on these emerging discoveries, RNF5 represents a novel means of modulating antiviral immunity. In this review, we summarize the latest research on the roles of RNF5 in antiviral immunity and virus life cycle. This comprehensive understanding could offer valuable insights into exploring potential therapeutic applications focused on targeting RNF5 during viral infections.