TRIM38 Induced in Respiratory Syncytial Virus-infected Cells Downregulates Type I Interferon Expression by Competing with TRIM25 to Bind RIG-I.

Innate immune response is the first line of defense for the host against virus invasion. One important response is the synthesis and secretion of type I interferon (IFN-I) in the virus-infected host cells. Here, we found that respiratory syncytial virus (RSV) infection induced high expression of TRIM25, which belongs to the tripartite motif-containing (TRIM) family of proteins. TRIM25 bound and activated retinoic acid-inducible gene I (RIG-I) by K63-linked ubiquitination. Accordingly, RIG-I mediated the production of IFN-I mainly through the nuclear factor kappa-B (NF-κB) pathway in respiratory epithelial cells. Interestingly, IFN-I, in turn, promoted a high expression of TRIM38 which downregulated the expression of IFN-I by reducing the protein level of RIG-I by K48-linked ubiquitination. More importantly, the binding site of TRIM25 to RIG-I was found in the narrow 25th-43rd amino acid (aa) region of RIG-I N-terminus. In contrast, the binding sites of TRIM38 to RIG-I were found in a much wider amino acid region, which included the binding site of TRIM25 on RIG-I. As a result, TRIM38 inhibits the production of IFN-I by competing with TRIM25 for RIG-I binding. Thus, TRIM38 negatively regulates RIG-I activation to, in turn, downregulate IFN-I expression, thus interfering with host immune response. A negative feedback loop effectively "puts the brakes" on the reaction once host immune response is overactivated and homeostasis is unbalanced. We also discovered that TRIM25 bound RIG-I by a new K63-linked ubiquitination located at K-45 of the first caspase recruitment domain (CARD). Collectively, these results confirm an antagonism between TRIM38 and TRIM25 in regulating IFN-I production by affecting RIG-I activity following RNA virus infection.