MERS-CoV 4b protein interferes with the NFκB-dependent innate immune response during infection

Abstract

Middle East respiratory syndrome coronavirus (MERS-CoV) is a novel human coronavirus that emerged in 2012 in the Middle East, causing severe pneumonia and acute respiratory distress syndrome (ARDS), with a case fatality rate of ~36%. CoV accessory proteins are not essential for replication in cell culture, although they have been involved in virus pathogenicity in vivo. When these proteins are expressed in isolation, they interfere with innate antiviral signaling pathways. However, there is limited information on the specific contribution of MERS-CoV accessory protein 4b to the repression of the innate antiviral response in the context of infection. We found that MERS-CoV 4b was required to prevent a robust NF-kB-dependent response during infection. In wild-type virus infected cells, 4b localized to the nucleus, while NF-kB was retained in the cytoplasm. In contrast, in the absence of 4b or in the presence of cytoplasmic 4b proteins mutated in its nuclear localization signal (NLS), NF-kB was translocated to the nucleus leading to a significant high expression of pro-inflammatory cytokines, as compared to the wild-type infection. This indicates that NF-kB repression required the nuclear import of 4b mediated by a specific NLS. Interestingly, we also found that both in isolation and during infection, 4b interacted with α-karyopherin proteins in an NLS-dependent manner. In particular, 4b had a strong preference for binding karyopherin-α4 (KPNA4), which is known to translocate the NF-kB protein complex into the nucleus. Binding of 4b to KPNA4 during infection inhibited its interaction with NF-kB-p65 subunit. Thereby we propose a model where 4b outcompetes NF-kB for KPNA4 binding and translocation into the nucleus as a mechanism of interference with the NFkB-mediated innate immune response.