Porcine Deltacoronavirus Accessory Protein NS7a Antagonizes IFN-β Production by Competing With TRAF3 and IRF3 for Binding to IKKε.

Puxian Fang,Shuhong Zhao,Jiansong Zhang,Liurong Fang,Shaobo Xiao,Dongcheng Bai,Jie Ren,Guiqing Peng,Sijin Xia,Yanrong Zhou

doi:10.3389/fcimb.2020.00257

Abstract

As an emerging swine enteropathogenic coronavirus, porcine deltacoronavirus (PDCoV) not only causes serious diarrhea in suckling piglets but also possesses the potential for cross-species transmission, which has sparked growing interest when studying this emerging virus. We previously identified a novel accessory protein NS7a encoded by PDCoV; however, the function of NS7a was not resolved. In this study, we demonstrated that PDCoV NS7a is an interferon antagonist. Overexpression of NS7a notably inhibited Sendai virus (SeV)-induced interferon-β (IFN-β) production and the activation of IRF3 rather than NF-κB. NS7a also inhibited IFN-β promoter activity induced by RIG-I, MDA5, MAVS, TBK1, and IKKε, which are key components of the RIG-I-like receptor (RLR) signaling pathway but not IRF3, the transcription factor downstream of TBK1/IKKε. Surprisingly, NS7a specifically interacts with IKKε but not with the closely related TBK1. Furthermore, NS7a interacts simultaneously with the kinase domain (KD) and the scaffold dimerization domain (SDD) of IKKε, competing with TRAF3, and IRF3 for binding to IKKε, leading to the reduction of RLR-mediated IFN-β production. The interactions of TRAF3-IKKε and IKKε-IRF3 are also attenuated in PDCoV-infected cells. Taken together, our results demonstrate that PDCoV NS7a inhibits IFN-β production by disrupting the association of IKKε with both TRAF3 and IRF3, revealing a new mechanism utilized by a PDCoV accessory protein to evade the host antiviral innate immune response.

Highlights

Porcine deltacoronavirus (PDCoV), a newly emerging swine enteropathogenic coronavirus, belongs to the genus Deltacoronavirus in the family Coronaviridae of the order Nidovirales (Wang et al, 2019)
To determine whether porcine deltacoronavirus (PDCoV) NS7a inhibits IFN-β production, HEK-293T cells, LLC-PK1 cells, or IPI-2I were co-transfected with increasing amounts of pCAGGS-HA-NS7a or empty vector along with the firefly luciferase reporter plasmid IFN-β-Luc and Renilla luciferase reporter plasmid pRL-TK for 24 h, and this was followed by treatment with Sendai virus (SeV) for 12 h
The results showed that SeV significantly induced the activation of the IFN-β-Luc promoter, but upregulation of IFN-β promoter activation was impaired by NS7a protein expression in all three tested cell lines (Figures 1A–C, Supplementary Tables 1–3)

Summary

Introduction

Porcine deltacoronavirus (PDCoV), a newly emerging swine enteropathogenic coronavirus, belongs to the genus Deltacoronavirus in the family Coronaviridae of the order Nidovirales (Wang et al, 2019). PDCoV infection causes typical clinical symptoms characterized by acute diarrhea and vomiting—even mortality—in piglets, leading to economic losses for the swine industry (Jung et al, 2015; Ma et al, 2015; Zhang, 2016). Recent studies have demonstrated that chicken and calves are susceptible to PDCoV infection (Jung et al, 2017; Liang et al, 2019) and that PDCoV possesses the potential to infect humans (Li et al, 2018), which has sparked growing interest in studying this emerging coronavirus

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Results

Conclusion