Middle East respiratory syndrome coronavirus ORF4b protein inhibits TLR7-and TLR9-dependent alpha interferon induction

The Toll-like receptor (TLR)7- and TLR9-dependent signalling cascade is responsible for production of a large amount of alpha interferon by plasmacytoid dendritic cells upon viral infection. Here, we show that Middle East respiratory syndrome coronavirus (MERS-CoV) accessory protein ORF4b has the most potential among the MERS-CoV accessory proteins to inhibit the TLR7/9-signaling-dependent alpha interferon production. ORF4b protein, which has a bipartite nuclear localization signal, was found to bind to IKK alpha, a kinase responsible for phosphorylation of interferon regulatory factor (IRF) 7. This interaction caused relocation of a large proportion of IKK alpha from the cytoplasm to the nucleus. Studies using ORF4b and IKK alpha mutants demonstrated that ORF4b protein inhibited IKK alpha-mediated IRF7 phosphorylation by sequestering IKK alpha in the nucleus and by impeding the phosphorylation process of cytoplasmic IKK alpha.