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Retinoic-acid-inducible gene-I (RIG-I; also called DDX58) is a cytosolic viral RNA receptor that interacts with MAVS (also called VISA, IPS-1 or Cardif) to induce type I interferon-mediated host protective innate immunity against viral infection. Furthermore, members of the tripartite motif (TRIM) protein family, which contain a cluster of a RING-finger domain, a B box/coiled-coil domain and a SPRY domain, are involved in various cellular processes, including cell proliferation and antiviral activity. Here we report that the amino-terminal caspase recruitment domains (CARDs) of RIG-I undergo robust ubiquitination induced by TRIM25 in mammalian cells. The carboxy-terminal SPRY domain of TRIM25 interacts with the N-terminal CARDs of RIG-I; this interaction effectively delivers the Lys 63-linked ubiquitin moiety to the N-terminal CARDs of RIG-I, resulting in a marked increase in RIG-I downstream signalling activity. The Lys 172 residue of RIG-I is critical for efficient TRIM25-mediated ubiquitination and for MAVS binding, as well as the ability of RIG-I to induce antiviral signal transduction. Furthermore, gene targeting demonstrates that TRIM25 is essential not only for RIG-I ubiquitination but also for RIG-I-mediated interferon-β production and antiviral activity in response to RNA virus infection. Thus, we demonstrate that TRIM25 E3 ubiquitin ligase induces the Lys 63-linked ubiquitination of RIG-I, which is crucial for the cytosolic RIG-I signalling pathway to elicit host antiviral innate immunity. [ABSTRACT FROM AUTHOR]

Upon viral infection, the major defense mounted by the host immune system is the activation of the interferon (IFN)-mediated antiviral pathway. In order to complete their life cycle, viruses that are obligatory intracellular parasites must modulate the host IFN-mediated immune response. Murine gammaherpesvirus 68 (γHV68) infects a wide range of cell types and establishes latent infections in mice. Here we demonstrate that the γHV68 latency-associated M2 protein has a cell-type-dependent localization pattern: M2 is present in the cytoplasm and plasma membrane in lymphocytes, whereas it is present primarily in the nucleus in epithelial and fibroblast cells. A mutational analysis indicated that the internal positively charged amino acids of M2 are required for its nuclear localization in fibroblasts. Purification of the M2 complex showed that M2 specifically interacts with the cellular p32 acidic protein through its central positively charged region and that this interaction recruits the cellular p32 protein to the nucleus in fibroblasts. Regardless of its localization, M2 expression effectively induced the downregulation of STAT1 and/or STAT2 in both A20 B lymphocytes and NIH 3T3 fibroblasts, resulting in the inhibition of IFN-α/β- and IFN-γ-mediated transcriptional activation. Finally, the M2 interaction with the p32 protein appeared to contribute to its ability to inhibit IFN-mediated transcriptional activation. These results indicate that γHV68 harbors a latency-associated M2 gene that antagonizes IFN-mediated host innate immunity and thus could play an important role in the establishment and maintenance of viral latency in infected animals. [ABSTRACT FROM AUTHOR]

Upon viral infection, the major defense mounted by the host immune system is activation of the interferon (IFN)-mediated antiviral pathway that is mediated by IFN regulatory factors (IRFs). In order to complete their life cycle, viruses must modulate the host IFN-mediated immune response. Kaposi's sarcoma-associated herpesvirus (KSHV), a human tumor-inducing herpesvirus, has developed a unique mechanism for antagonizing cellular IFN-mediated antiviral activity by incorporating viral homologs of the cellular IRFs, called vIRFs. Here, we report a novel immune evasion mechanism of KSHV vIRF3 to block cellular IRF7-mediated innate immunity in response to viral infection. KSHV vIRF3 specifically interacts with either the DNA binding domain or the central IRF association domain of IRF7, and this interaction leads to the inhibition of IRF7 DNA binding activity and, therefore, suppression of alpha interferon (IFN-α) production and IFN-mediated immunity. Remarkably, the central 40 amino acids of vIRF3, containing the double α helix motifs, are sufficient not only for binding to IRF7, but also for inhibiting IRF7 DNA binding activity. Consequently, the expression of the double α helix motif-containing peptide effectively suppresses IRF7-mediated IFN-α production. This demonstrates a remarkably efficient means of viral avoidance of host antiviral activity. [ABSTRACT FROM AUTHOR]

Cells infected by viruses utilize interferon (IFN)-mediated and p53-mediated irreversible cell cycle arrest and apoptosis as part of the overall host surveillance mechanism to ultimately block viral replication and dissemination. Viruses, in turn, have evolved elaborate mechanisms to subvert IFN- and p53-mediated host innate immune responses. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes several viral IFN regulatory factors (vIRF1 to vIRF4) within a cluster of loci, their functions being primarily to inhibit host IFN-mediated innate immunity and deregulate p53-mediated cell growth control. Despite its significant homology and similar genomic location to other vIRFs, vIRF4 is distinctive, as it does not target and antagonize host IFN-mediated signal transduction. Here, we show that KSHV vIRF4 interacts with the murine double minute 2 (MDM2) E3 ubiquitin ligase, leading to the reduction of p53, a tumor suppressor, via proteasome-mediated degradation. The central region of vIRF4 is required for its interaction with MDM2, which led to the suppression of MDM2 autoubiquitination and, thereby, a dramatic increase in MDM2 stability. Consequently, vIRF4 expression markedly enhanced p53 ubiquitination and degradation, effectively suppressing p53-mediated apoptosis. These results indicate that KSHV vIRF4 targets and stabilizes the MDM2 E3 ubiquitin ligase to facilitate the proteasome-mediated degradation of p53, perhaps to circumvent host growth surveillance and facilitate viral replication in infected cells. Taken together, the indications are that the downregulation of p53-mediated cell growth control is a common characteristic of the four KSHV vIRFs and that p53 is indeed a key factor in the host's immune surveillance program against viral infections. [ABSTRACT FROM AUTHOR]