Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the ongoing coronavirus disease 2019 (COVID-19) pandemic that is a serious global health problem. Evasion of IFN-mediated antiviral signaling is a common defense strategy that pathogenic viruses use to replicate and propagate in their host. In this study, we show that SARS-CoV-2 is able to efficiently block STAT1 and STAT2 nuclear translocation in order to impair transcriptional induction of IFN-stimulated genes (ISGs). Our results demonstrate that the viral accessory protein Orf6 exerts this anti-IFN activity. We found that SARS-CoV-2 Orf6 localizes at the nuclear pore complex (NPC) and directly interacts with Nup98-Rae1 via its C-terminal domain to impair docking of cargo-receptor (karyopherin/importin) complex and disrupt nuclear import. In addition, we show that a methionine-to-arginine substitution at residue 58 impairs Orf6 binding to the Nup98-Rae1 complex and abolishes its IFN antagonistic function. All together our data unravel a mechanism of viral antagonism in which a virus hijacks the Nup98-Rae1 complex to overcome the antiviral action of IFN.

The ongoing unprecedented severe acute respiratory syndrome caused by the SARS-CoV-2 outbreak worldwide has highlighted the need for understanding viral-host interactions involved in mechanisms of virulence. Here, we show that the virulence factor Nsp1 protein of SARS-CoV-2 interacts with the host messenger RNA (mRNA) export receptor heterodimer NXF1-NXT1, which is responsible for nuclear export of cellular mRNAs. Nsp1 prevents proper binding of NXF1 to mRNA export adaptors and NXF1 docking at the nuclear pore complex. As a result, a significant number of cellular mRNAs are retained in the nucleus during infection. Increased levels of NXF1 rescues the Nsp1-mediated mRNA export block and inhibits SARS-CoV-2 infection. Thus, antagonizing the Nsp1 inhibitory function on mRNA export may represent a strategy to restoring proper antiviral host gene expression in infected cells.

The nuclear export of the influenza A virus ribonucleoprotein (vRNP) is crucial for virus replication. As a major component of the vRNP, nucleoprotein (NP) alone can also be shuttled out of the nucleus by interacting with chromosome region maintenance 1 (CRM1) and is therefore hypothesized to promote the nuclear export of the vRNP. In the present study, three novel nuclear export signals (NESs) of the NP-NES1, NES2, and NES3-were identified as being responsible for mediating its nuclear export. The nuclear export of NES3 was CRM1 dependent, whereas that of NES1 or NES2 was CRM1 independent. Inactivation of these NESs led to an overall nuclear accumulation of NP. Mutation of all three NP-NESs significantly impaired viral replication. Based on structures of influenza virus NP oligomers, these three hydrophobic NESs are found present on the surface of oligomeric NPs. Functional studies indicated that oligomerization is also required for nuclear export of NP. Together, these results suggest that the nuclear export of NP is important for virus replication and relies on its NESs and oligomerization.T he influenza A virus genome consists of eight negative-sense single-stranded RNA segments (vRNA) (17). Each vRNA segment is associated with multiple copies of the viral nucleoprotein (NP) and three polymerase subunits (PA, PB1, and PB2), forming the viral ribonucleoprotein (vRNP) complex. During an early stage of infection, the vRNPs are released into the cytoplasm from virions following fusion of the viral membrane and endosomal membrane (41). Subsequently, the incoming vRNPs are transported into the nucleus, where viral genome replication and transcription occur (41).One of the determinants for vRNP nuclear import is NP (8, 28, 44, 49, 52, 53), the major protein in the vRNP structure. Thus far, two nuclear localization signals (NLSs) and a nuclear accumulation signal (NAS) have been identified in NP. The stronger NLS is an unconventional signal located in the N-terminal basic region (between residues 3 and 13) of NP (28, 44), and the weaker signal, a classical bipartite NLS, is located between residues 198 and 216 (49). The NAS-spanning residues (327 to 345) were identified through analyses of mutants that lacked both of the NLSs but still exhibited partial nuclear distribution (9).At a late stage of virus infection, the vRNPs exit the nucleus to assemble and bud from the apical plasma membrane of polarized cells (3). The trafficking of the vRNPs into and out of the nucleus is a tightly regulated process (7). The nuclear export of progeny vRNPs is mediated by the CRM1 cellular export receptor (12,23,29,48). Nuclear export protein (NEP; formerly referred to as the NS2 protein), which possesses a nuclear export signal (NES), and the matrix protein (M1) of influenza A virus are deemed responsible for directing export of the vRNPs (29, 31). This process is regulated by the Raf/MEK/ERK pathway (32), which is stimulated by the membrane association of influenza virus hemagglutinin (HA) (25). Interestingly, exogenously expresse...