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 pore complex (NPC) is a protein assembly that contains several distinct subcomplexes. The mammalian nucleoporin (Nup)-107 is part of a hetero-oligomeric complex, that also contains Nup160, Nup133, Nup96, and the mammalian homolog of yeast Sec13p. We used transfection of HeLa cells with small interfering RNAs to specifically deplete mRNA for Nup107. In a domino effect, Nup107 depletion caused codepletion of a subset of other Nups on their protein but not on their mRNA level. Among the affected Nups was a member of the Nup107 subcomplex, Nup133, whereas two other tested members of this complex, Nup96 and Sec13, were unaffected and assembled into Nup107͞Nup133-deficient NPCs. We also tested several phenylalanine-glycine repeat-containing Nups that serve as docking sites for karyopherins. Some of these, such as Nup358, Nup214 on the cytoplasmic, and Nup153 on the nucleoplasmic side of the NPC, failed to assemble into Nup107͞ Nup133-depleted NPCs, whereas p62, a Nup at the center of the NPC, was unaffected. Interestingly, the filamentous, NPC-associated protein Tpr also failed to assemble into the NPCs of Nup107-depleted cells. These data indicate that Nup107 functions as a keystone Nup that is required for the assembly of a subset of Nups into the NPC. Despite the depletion of Nup107 and the accompanying effects on other Nups, there was no significant effect on the growth rate of these cells and only a partial inhibition of mRNA export. These data indicate redundancy of Nups in the function of the mammalian NPC.nucleoporin stability ͉ cell viability ͉ mRNA export

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The Nup98 gene codes for several alternatively spliced protein precursors. Two in vitro translated and autoproteolytically cleaved precursors yielded heterodimers of Nup98-6kDa peptide and Nup98-Nup96. TPR (translocated promoter region) is a protein that forms filamentous structures extending from nuclear pore complexes (NPCs) to intranuclear sites. We found that in vitro translated TPR bound to in vitro translated Nup98 and, via Nup98, to Nup96. Double-immunofluorescence microscopy with antibodies to TPR and Nup98 showed colocalization. In confocal sections the nucleolus itself was only weakly stained but there was intensive perinucleolar staining. Striking spike-like structures emanated from this perinucleolar ring and attenuated into thinner structures as they extended to the nuclear periphery. This characteristic staining pattern of the TPR network was considerably enhanced when a myc-tagged pyruvate kinase-6kDa fusion protein was overexpressed in HeLa cells. Double-immunoelectron microscopy of these cells using anti-myc and anti-TPR antibodies and secondary gold-coupled antibodies yielded row-like arrangements of gold particles. Taken together, the immunolocalization data support previous electron microscopical data, suggesting that TPR forms filaments that extend from the NPC to the nucleolus. We discuss the possible implications of the association of Nup98 with this intranuclear TPR network for an intranuclear phase of transport. One of the most intriguing candidate proteins that might participate in the formation of paths for intranuclear diffusion is the large protein, TPR (for translocated promoter region). TPR forms filaments that emanate from the nuclear basket of the nuclear pore complex (NPC) and extend to the nucleolus (1, 2). The full-length mammalian TPR is about 270 kDa. Its aminoterminal region of about 200 kDa predicts a coiled-coil structure that is likely to be involved in filament formation (1, 3-7).Field-emission scanning electron microscopy of isolated amphibian oocyte nuclear envelopes has revealed striking hollow cables that emanate from the NPC toward the nuclear interior. Interestingly, many of the NPC-attached hollow cables anastomose with each other after projecting only a short distance into the nucleus (8). These ''branching hollow cables'' are most likely formed by TPR, either alone or together with other proteins. In support of the scanning electron microscopy data from amphibian oocytes are immunofluorescence localization studies of TPR in Drosophila cells. Here, TPR has been shown to delineate a chromatin-free network that extends from the nuclear envelope to the perinucleolar region (2).Together these structural and localization data suggest at least two functions for TPR. One is that TPR plays an important role in the organization of interphase chromatin. In support for such a function are recent data on yeast TPR showing that gene deletion disrupts the clustering of perinuclear telomeres and results in a severe deficiency in the repair of DNA double-strand breaks (9). Anot...