MERS-CoV 4b protein interferes with the NF-κB-dependent innate immune response during infection
Middle East respiratory syndrome coronavirus (MERS-CoV) is a novel human coronavirus that emerged in 2012, causing severe pneumonia and acute respiratory distress syndrome (ARDS), with a case fatality rate of~36%. When expressed in isolation, CoV accessory proteins have been shown to 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
... xt of infection. We found that MERS-CoV 4b was required to prevent a robust NF-κB dependent response during infection. In wild-type virus infected cells, 4b localized to the nucleus, while NF-κB was retained in the cytoplasm. In contrast, in the absence of 4b or in the presence of cytoplasmic 4b mutants lacking a nuclear localization signal (NLS), NF-κB was translocated to the nucleus leading to the expression of pro-inflammatory cytokines. This indicates that NF-κB 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-κB protein complex into the nucleus. Binding of 4b to KPNA4 during infection inhibited its interaction with NF-κB-p65 subunit. Thereby we propose a model where 4b outcompetes NF-κB for KPNA4 binding and translocation into the nucleus as a mechanism of interference with the NF-κB-mediated innate immune response. Middle East respiratory syndrome coronavirus (MERS-CoV) is a highly pathogenic human CoV that continues to cause lethal human infections, primarily in the Middle East. Virus accessory genes are potential contributors to pathology, possibly by interfering with the innate immune response. However, understanding their interactions with host proteins in the context of infection is rudimentary. Here, we provide evidence that the MERS-CoV accessory protein 4b functioned, at least in part, to prevent a robust NF-κB dependent response during infection. This effect depended on the nuclear localization of 4b, which was associated with the cytoplasmic retention of NF-κB. We show that 4b interacted with α-karyopherin proteins (importins) involved in the nuclear import of NF-κB, inhibiting the binding of α-karyopherin to NF-κB-p65 subunit. We propose that 4b contributes to the evasion of the innate immune response by binding α-karyopherin proteins, leading to the inhibition of NF-κB nuclear transport. NF-κB inhibition by MERS-CoV 4b protein during infection PLOS Pathogens | https://doi.