Middle East respiratory syndrome coronavirus nucleocapsid protein suppresses type I and type III interferon induction by targeting RIG-I signaling.

Type I and type III interferons (IFNs) are the frontline of antiviral de-fense mechanisms that trigger hundreds of downstream antiviral genes. In this study, we observed that MERS-CoV nucleocapsid (N) protein suppresses type I and type III IFN gene expression. The N protein suppresses Sendai virus-induced IFN-beta and IFN-lambda 1 by reducing their promoter activity and mRNA levels, as well as down-stream IFN-stimulated genes (ISGs). Retinoic acid-inducible gene I (RIG-I) is known to recognize viral RNA and induce IFN expression through tripartite motif-containing protein 25 (TRIM25)-mediated ubiquitination of RIG-I caspase activation and recruit-ment domains (CARDs). We discovered that MERS-CoV N protein suppresses RIG-I-CARD-induced, but not MDA5-CARD-induced, IFN-beta and IFN-lambda 1 promoter activity. By interacting with TRIM25, N protein impedes RIG-I ubiquitination and activation and inhibits the phosphorylation of transcription factors IFN-regulatory factor 3 (IRF3) and NF-kappa B that are known to be important for IFN gene activation. By employing a recombinant Sindbis virus-EGFP replication system, we showed that viral N protein downregulated the production of not only IFN mRNA but also bioactive IFN pro-teins. Taken together, MERS-CoV N protein functions as an IFN antagonist. It sup-presses RIG-I-induced type I and type III IFN production by interfering with TRIM25-mediated RIG-I ubiquitination. Our study sheds light on the pathogenic mechanism of how MERS-CoV causes disease. IMPORTANCE MERS-CoV causes death of about 35% of patients. Published studies showed that some coronaviruses are capable of suppressing interferon (IFN) expres-sion in the early phase of infection and MERS-CoV proteins can modulate host im-mune response. In this study, we demonstrated that MERS-CoV nucleocapsid (N) protein suppresses the production of both type I and type III IFNs via sequestering TRIM25, an E3 ubiquitin ligase that is essential for activating the RIG-I signaling pathway. Ectopic expression of TRIM25 rescues the suppressive effect of the N pro-tein. In addition, the C-terminal domain of the viral N protein plays a pivotal role in the suppression of IFN-beta promoter activity. Our findings reveal how MERS-CoV evades innate immunity and provide insights into the interplay between host im-mune response and viral pathogenicity.