The research, conducted by Juhong Xie, Jiayi Lei, Qinxue Zhu, Bin Gui, Yongming Li, Yaping Wang, and Rong Huang, began with an observation made in an earlier study. Using random mutagenesis in the rare minnow, Gobiocypris rarus, a small laboratory model fish widely used in Chinese aquatic disease research, the team had identified a gene whose variation was associated with susceptibility to hemorrhagic disease. That gene encoded a protein resembling an E3 ubiquitin ligase, the class of enzymes responsible for attaching ubiquitin tags to target proteins. In the new work, detailed domain and amino acid sequence analyses confirmed that the gene truly encodes a functional E3 ubiquitin ligase, and because close relatives of the gene are found only in fish, the team designated it NHRE3, a fish-specific member of this enzyme family.

Ubiquitination is one of the cell’s most versatile regulatory tools. By stringing chains of ubiquitin, a small regulatory protein, onto specific lysine residues of a target protein, E3 ligases determine that protein’s fate. K48-linked polyubiquitin chains, in particular, serve as a molecular kiss of death: they are recognized by the proteasome, the cell’s protein-shredding machinery, and the tagged protein is rapidly degraded. Viruses have learned to exploit this system extensively. In mammals, viral proteins frequently hijack the ubiquitin pathway to dismantle signaling molecules that would otherwise trigger interferon production, the body’s first chemical alarm against infection. The question the Chinese team posed was whether fish viruses use similar tricks, and if so, through what molecular intermediaries.

The first clue came from genetics. The researchers generated rare minnows in which the nhre3 gene was knocked out, producing nhre3-null animals. When these fish were challenged with GCRV, they displayed significantly enhanced resistance to the virus compared with normal fish. In other words, removing NHRE3 made the fish harder to infect, which strongly implied that the ligase normally works in the virus’s favor. This was a striking result, because it suggested that a host-encoded enzyme was acting as an accomplice to viral infection, suppressing the very defenses it might have been expected to support. Such pro-viral host factors are prized targets for intervention, since blocking them can bolster immunity without directly attacking the virus itself.

To find out what NHRE3 actually does at the molecular level, the team turned to immunoprecipitation followed by mass spectrometry, a technique that pulls a bait protein out of the cell along with everything it physically associates with. The screen identified interferon-stimulated gene 15, or ISG15, as an NHRE3-interacting protein. ISG15 is one of the most strongly induced genes during antiviral responses across vertebrates, and in mammals it functions both as a free intracellular protein and as a modifier conjugated onto other proteins in a process called ISGylation. Its precise antiviral role in fish, however, had remained poorly understood, which made the discovery of its interaction with NHRE3 doubly significant: it implicated ISG15 in fish antiviral defense while simultaneously suggesting a mechanism by which that defense could be undermined.

Subsequent biochemical experiments established the full pathway. NHRE3 possesses genuine E3 ubiquitin ligase activity, and it uses that activity to catalyze K48-linked polyubiquitination of ISG15. Once tagged with these destruction chains, ISG15 is routed to the proteasome and degraded. The logic of the system is therefore elegantly sinister: the host produces ISG15 to fight the virus, and NHRE3, a fish-specific enzyme, eliminates ISG15 before it can do its job, thereby promoting GCRV infection. The knockout fish, lacking NHRE3, retain their ISG15 and consequently mount a stronger antiviral response, which explains their enhanced resistance to hemorrhagic disease.

Because the antiviral mechanism of ISG15 in fish had not been clearly defined, the researchers went on to investigate what ISG15 actually does downstream. Their experiments revealed that fish ISG15 can interact with IRF3, interferon regulatory factor 3, a central transcription factor in the innate antiviral response. In mammals and other vertebrates, IRF3 is activated when viral nucleic acids are detected by pattern-recognition receptors; once activated, it translocates to the nucleus and switches on the transcription of interferons and a broad panel of antiviral effector genes. The new study found that fish ISG15 promotes the expression of innate immune factors downstream of IRF3, thereby amplifying the antiviral program and exerting its protective effect against GCRV. This positions ISG15 not merely as a passive marker of interferon activity but as an active participant in the signaling cascade that drives it.

The comparison with mammalian immune evasion strategies is where the study acquires its broader evolutionary resonance. In mammals, viruses that target the ISG15 system generally do so in one of two ways: they inhibit ISGylation, the conjugation of ISG15 onto host and viral proteins, or they induce deISGylation, the enzymatic removal of ISG15 from its targets. Both strategies tinker with the modification process itself. The mechanism described in the fish study is different in kind. Here, the virus benefits from a host-encoded, fish-specific ubiquitin ligase that removes ISG15 from the cell entirely by sending it to the proteasome. The authors suggest that this may represent a more primitive or more diverse form of innate immune regulation, one in which the balance of the antiviral response is governed by the stability of ISG15 itself rather than by its conjugation state.

That interpretation carries implications for how scientists think about the origins of antiviral immunity. The immunological arms race between hosts and pathogens is ancient, predating the divergence of the major vertebrate lineages by hundreds of millions of years. Fish occupy a pivotal position in that history, and discovering that a fish-specific enzyme mediates viral immune escape through protein degradation suggests that the repertoire of evasion strategies available to viruses may be broader, and older, than the mammalian-focused literature implies. It also underscores the degree to which lineage-specific innovations, in this case a ubiquitin ligase found only in fish, can shape the outcome of infection in ways that have no direct parallel in laboratory mammals.

For aquaculture, the practical implications are immediate. Grass carp hemorrhagic disease remains a serious threat to one of the world’s largest freshwater aquaculture industries, and GCRV has proven difficult to control through conventional means. The NHRE3-ISG15 axis offers a concrete molecular target: strategies that inhibit NHRE3 activity, stabilize ISG15, or enhance IRF3-driven antiviral gene expression could plausibly be developed into breeding markers, feed additives, or therapeutic approaches that strengthen fish resistance to GCRV. The nhre3 knockout minnows generated in the study provide a proof of principle that removing this single gene measurably improves survival in the face of viral challenge. More broadly, the work demonstrates the value of studying host-pathogen interactions in non-mammalian systems, where the molecular logic of immunity may differ in instructive ways. By tracing the fate of a single protein, ISG15, from induction to ubiquitination to proteasomal destruction, the researchers have assembled a complete evasion circuit and, in doing so, illuminated one of the more ancient strategies in the long war between viruses and vertebrate hosts.

Subject of Research: A fish-specific E3 ubiquitin ligase that suppresses antiviral immunity by ubiquitinating ISG15

Article Title: A fish-specific E3 ubiquitin ligase NHRE3 suppresses antiviral immunity by degrading ISG15 via ubiquitination

Article References: Xie, J., Lei, J., Zhu, Q., Gui, B., Li, Y., Wang, Y., & Huang, R. (2026). A fish-specific E3 ubiquitin ligase NHRE3 suppresses antiviral immunity by degrading ISG15 via ubiquitination. PLOS Pathogens, 22(9), e1014544. https://doi.org/10.1371/journal.ppat.1014544

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014544

Keywords: NHRE3, ISG15, E3 ubiquitin ligase, grass carp reovirus, antiviral immunity, ubiquitination, proteasome, innate immunity, aquaculture, rare minnow, IRF3, immune evasion

NSP14 is one of sixteen nonstructural proteins encoded by the SARS-CoV-2 genome, and it wears two enzymatic hats that are both essential for viral replication. Its N-terminal domain functions as an exoribonuclease, proofreading freshly synthesized viral RNA and excising mismatched nucleotides, while its C-terminal domain acts as an N7-methyltransferase, adding a methyl group to the 5′ cap of viral messenger RNA so that the RNA masquerades as a cellular transcript. This cap modification allows viral RNA to evade detection by innate immune receptors and to be translated efficiently by the host machinery. Previous research had implicated NSP14 in antagonizing interferon responses, but the precise route it takes through the signaling network remained incompletely mapped, particularly with respect to the RIG-I pathway that dominates the sensing of RNA viruses.

To dissect this question, the research team employed overexpression systems and reporter assays to test what happens to interferon production when NSP14 is present. The results were unambiguous. Elevated levels of NSP14 significantly suppressed both type I and type III interferon responses, along with the expression of downstream interferon-stimulated genes, the effector proteins that execute much of the antiviral program. The suppression extended to key signaling intermediates: activation of TBK1, the kinase that phosphorylates downstream transcription factors, and activation of IRF3, the transcription factor that drives interferon-beta transcription, were both dampened. In effect, NSP14 was acting upstream of the phosphorylation events that normally convert RNA sensing into gene expression, placing its point of interference early in the RIG-I signaling axis.

A central question in such experiments is which part of a viral protein carries out a given function, and the team addressed this by testing whether NSP14’s immunosuppressive effect depended on its catalytic activities or on its interactions with other viral components. NSP14 is known to form a complex with NSP10, a cofactor that stimulates its exoribonuclease activity, and the researchers examined whether this partnership was required. It was not. The inhibitory effect on RIG-I-mediated antiviral signaling persisted independently of complex formation with NSP10. What did matter was the C-terminal N7-methyltransferase activity. When that function was disabled, the ability of NSP14 to suppress interferon production was lost, indicating that the methyltransferase domain is the business end of the immune evasion strategy, even though the mechanism does not appear to operate through cap modification of viral RNA itself.

With the functional requirement mapped to the methyltransferase domain, the investigators turned to the question of which host protein NSP14 might be targeting. Using immunoprecipitation followed by mass spectrometry, they searched for cellular interaction partners of NSP14 and identified DHX15, a DEAH-box helicase that had previously been characterized as a co-receptor of RIG-I. DHX15 belongs to the large family of DExD/C-box helicases, enzymes that remodel RNA structures and, in several cases, contribute to innate immune signaling. The finding that a viral enzyme with methyltransferase activity physically contacts DHX15 suggested a plausible bridge between the protein’s known biochemistry and its observed immunological effect.

The significance of DHX15 as a target becomes clear when its normal role is considered. Earlier studies had established that DHX15 cooperates with RIG-I to enhance signaling triggered by RNA viruses, functioning as a co-receptor that strengthens the antiviral response. In other words, DHX15 is not merely an accessory protein; it is part of the amplification system that allows the RIG-I pathway to generate a robust interferon signal. A virus that could disrupt the DHX15-RIG-I partnership would effectively turn down the volume of the alarm before it ever reaches the nucleus. The research team set out to test whether NSP14 does exactly that, and their experiments confirmed the hypothesis.

Using co-immunoprecipitation assays, the researchers demonstrated that NSP14 dampens the interaction between DHX15 and RIG-I. By binding to DHX15, the viral protein occupies a partner that would otherwise assist RIG-I in propagating the activation signal downstream to MAVS, the mitochondrial antiviral-signaling protein that serves as the platform assembling TBK1 and other signaling components. The consequences of this interference follow a coherent logic: with fewer productive DHX15-RIG-I complexes, reduced activation of TBK1 and IRF3 follows, and the transcription of interferon genes falters. The pathway from viral methyltransferase to blunted cytokine output is thus traced step by step, from protein-protein contact at the level of the sensors to diminished phosphorylation at the level of the kinases to reduced gene expression at the level of the interferon loci.

These findings carry several implications for how scientists understand the immune battlefield inside SARS-CoV-2-infected cells. First, they illustrate the economy of viral genomes, in which a single protein performs multiple jobs: the same NSP14 molecule that proofreads and caps the viral RNA also moonlights as an immune antagonist, using its methyltransferase domain for a second, noncanonical purpose. Second, they highlight DHX15 as a newly appreciated node of viral attack, expanding the list of host factors that coronaviruses manipulate and suggesting that the partnership between helicase co-receptors and RIG-I-like receptors is a vulnerability that pathogens have evolved to exploit. Third, they reinforce the picture of SARS-CoV-2 as a virus with a formidable arsenal of evasion strategies, in which early innate immune suppression buys time for viral replication before adaptive immunity can mount an effective response.

The study also opens avenues for therapeutic thinking. If NSP14’s N7-methyltransferase activity is required for suppressing interferon production, then inhibitors of that enzymatic function might do double duty, impairing viral RNA capping while simultaneously relieving the brake on innate immunity. Small molecules targeting the methyltransferase domain are already of interest in antiviral drug development, and this work adds an immunological rationale to the enzymatic one. Similarly, approaches that stabilize or mimic the DHX15-RIG-I interaction could, in principle, restore signaling amplitude in infected tissues, although such strategies remain far from clinical application. As with all mechanistic cell culture studies, important caveats apply: the experiments relied on overexpression systems, and confirming that the same interference operates during authentic SARS-CoV-2 infection at physiological protein levels will be an important next step.

What the research ultimately delivers is a clearer molecular narrative of how SARS-CoV-2 mutes the body’s first response to infection. The virus does not merely hide its RNA; it actively dismantles the cooperative architecture of the sensing machinery, using NSP14 to pry apart DHX15 and RIG-I and thereby weakening the signal before it can cascade into interferon production. Every element of that cascade, from the helicase partnership at the membrane-associated sensor level to the phosphorylation of TBK1 and IRF3 to the expression of interferon-stimulated genes, is diminished as a result. In mapping this route with biochemical precision, the team at Guizhou Medical University has not only illuminated a specific evasion tactic of a globally significant pathogen but has also underscored a broader lesson of the COVID-19 era: the proteins viruses bring with them are often multitaskers, and understanding their secondary functions may reveal the most consequential battlegrounds of all.

Subject of Research: SARS-CoV-2 NSP14-mediated evasion of RIG-I-mediated innate antiviral immunity through disruption of the DHX15-RIG-I interaction

Article Title: SARS-CoV-2 NSP14 suppresses RIG-I-mediated interferon production by dampening the interaction between DHX15 and RIG-I

Article References: Liu, H., Hu, R., Lei, X., Han, Y., & Nie, Y. (2026). SARS-CoV-2 NSP14 suppresses RIG-I-mediated interferon production by dampening the interaction between DHX15 and RIG-I. Virology Journal. https://doi.org/10.1186/s12985-026-03314-y

Image Credits: AI Generated

DOI: 10.1186/s12985-026-03314-y

Keywords: SARS-CoV-2, NSP14, RIG-I, DHX15, innate immunity, interferon, immune evasion, N7-methyltransferase, TBK1, IRF3, virology, pattern recognition receptors