| The innate immune response is the first line of defense against viruses and is the foundation of adaptive immune systems. Initially, host sensor molecules detect specific pathogen-associated molecular patterns (PAMPs) and activate the innate immune system through pathogen recognition receptors (PRRs). Among those PRRs, Retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) can detect viral RNA in cytoplasm and induce interferons and proinflammatory cytokines, thus playing a pivotal role in host defense. Specificly, RLRs selectively detect the non-self-RNA, and induce ATP-dependent conformational change to form a dimmer or oligomer, which allows its interaction with the downstream adapter protein IPS-1 (IFN-βpromoter stimulator 1). IPS-1 resides on the outer membrane of mitochondria, and recruitment of RIG-I to IPS-1 leads to the activation the two IKK-related kinases, TANK(TRAF family member-associated NF-κB activator) binding kinase 1(TBK1) and inducible inhibitor-KB kinase (IKKi), both of which phosphorylate interferon refulatory factor 3/7, in turn, induce the expression of type I interferon. In regards to the importance of innate immune signaling regulation, extensive study is required to identify novel immune regulating molecules.The tripartite motif (TRIM) protein family was originally described as the RBCC family because members belonging to this family contain a RING finger, one or two B-boxes, followed by a coiled-coil domain. There are now more than 70 TRIM proteins known in humans. Members of the TRIM protein family are involved in various cellular processes, including cell proliferation, differentiation, oncogenesis and apoptosis. Importantly, some TRIM proteins involve in innate immune signaling regulation and/or display antiviral properties. However, only few TRIM proteins were fully elucidated.TRIM38 located in the chromosome 6p21.3. In comparison of the TRIM38 sequence with other TRIM proteins, we found that TRIM38 contains several evolutionary conserved molecular domains, including a RING finger, two B-boxes, and one pry/spry domain. Previous studies have demonstrated that the activity of most E3 ligases is specified by a RIGN domain. Accordingly, our observations reveal that forced expression of TRIM38 protein results in a remarkable increase in the level of ubiquitin-protein conjugates. Further studies demostrated that TRIM38 was conjugated with K48-linked poly-ubiquitin chains, which suggests that TRIM38 targets itself for proteasomal degradation through the K48-linked poly-ubiquitylation, and which indicates that TRIM38 may function as an E3 ubiquitin ligase.To explore the role of TRIM38 in the regulation of innate immune signaling, the interferon-βpromoter activity assays were conducted. Results showed that overexpression of TRIM38 inhibits Sendai virus(SeV) triggered type I interferon induction. To investigate the mechanism by which TRIM38 inhibits the innate immune signaling,293T cells were transfected with plasmids encoding TRIM38 together with RIG-1, MDA5, IPS1, TBK1, IKKi, and interferon-βreporter plasmids. Results showed that TRIM38 inhibits RIG-I-mediated IFN-P promoter activity in a dose dependent manner, wheares MDA5, IPS1, TBK1 or IKKi mediated activation of IFN-P promoter remain unattenuated. Immunoprecipitation studies suggest that TRIM38 associates with RIG-I, and TRIM38 could disrupt the interaction between RIG-I and IPSl/TBKl.These findings showed that TRIM38 may function as an E3 ubiquitin ligase and is a negative regulator of RIG-I-mediated innate antiviral response. The investigations on the function and mechanism of TRIM38 will facilitate the development of new strategies to control immune injury, and also will contribute to the better understanding of immune recognition and immune regulation. |
