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Influenza viruses antagonize key immune defence mechanisms via the virulence factor non-structural protein 1 (NS1). A key mechanism of virulence by NS1 is blocking nuclear export of host messenger RNAs, including those encoding immune factors; however, the direct cellular target of NS1 and the mechanism of host mRNA export inhibition are not known. Here, we identify the target of NS1 as the mRNA export receptor complex, nuclear RNA export factor 1-nuclear transport factor 2-related export protein 1 (NXF1-NXT1), which is the principal receptor mediating docking and translocation of mRNAs through the nuclear pore complex via interactions with nucleoporins. We determined the crystal structure of NS1 in complex with NXF1-NXT1 at 3.8 Å resolution. The structure reveals that NS1 prevents binding of NXF1-NXT1 to nucleoporins, thereby inhibiting mRNA export through the nuclear pore complex into the cytoplasm for translation. We demonstrate that a mutant influenza virus deficient in binding NXF1-NXT1 does not block host mRNA export and is attenuated. This attenuation is marked by the release of mRNAs encoding immune factors from the nucleus. In sum, our study uncovers the molecular basis of a major nuclear function of influenza NS1 protein that causes potent blockage of host gene expression and contributes to inhibition of host immunity.
The mRNA lifecycle is driven through spatiotemporal changes in the protein composition of mRNA particles (mRNPs) that are triggered by RNA-dependent DEAD-box protein (Dbp) ATPases. As mRNPs exit the nuclear pore complex (NPC) in Saccharomyces cerevisiae, this remodeling occurs through activation of Dbp5 by inositol hexakisphosphate (IP )-bound Gle1. At the NPC, Gle1 also binds Nup42, but Nup42's molecular function is unclear. Here we employ the power of structure-function analysis in S. cerevisiae and human (h) cells, and find that the high-affinity Nup42-Gle1 interaction is integral to Dbp5 (hDDX19B) activation and efficient mRNA export. The Nup42 carboxy-terminal domain (CTD) binds Gle1/hGle1B at an interface distinct from the Gle1-Dbp5/hDDX19B interaction site. A nup42-CTD/gle1-CTD/Dbp5 trimeric complex forms in the presence of IP . Deletion of NUP42 abrogates Gle1-Dbp5 interaction, and disruption of the Nup42 or IP binding interfaces on Gle1/hGle1B leads to defective mRNA export in S. cerevisiae and human cells. In vitro, Nup42-CTD and IP stimulate Gle1/hGle1B activation of Dbp5 and DDX19B recombinant proteins in similar, nonadditive manners, demonstrating complete functional conservation between humans and S. cerevisiae. Together, a highly conserved mechanism governs spatial coordination of mRNP remodeling during export. This has implications for understanding human disease mutations that perturb the Nup42-hGle1B interaction.
© 2017 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.
Control of organellar assembly and function is critical to eukaryotic homeostasis and survival. Gle1 is a highly conserved regulator of RNA-dependent DEAD-box ATPase proteins, with critical roles in both mRNA export and translation. In addition to its well-defined interaction with nuclear pore complexes, here we find that Gle1 is enriched at the centrosome and basal body. Gle1 assembles into the toroid-shaped pericentriolar material around the mother centriole. Reduced Gle1 levels are correlated with decreased pericentrin localization at the centrosome and microtubule organization defects. Of importance, these alterations in centrosome integrity do not result from loss of mRNA export. Examination of the Kupffer's vesicle in Gle1-depleted zebrafish revealed compromised ciliary beating and developmental defects. We propose that Gle1 assembly into the pericentriolar material positions the DEAD-box protein regulator to function in localized mRNA metabolism required for proper centrosome function.
© 2017 Jao et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).
Nuclear pore complexes (NPCs), which are composed of nucleoporins (Nups) and regulate transport between the nucleus and cytoplasm, significantly impact the replicative life span (RLS) of We previously reported that deletion of the nonessential gene increases RLS, although the molecular basis for this effect was unknown. In this study, we find that nuclear tRNA accumulation contributes to increased longevity in Δ cells. Fluorescence in situ hybridization (FISH) experiments demonstrate that several specific tRNAs accumulate in the nuclei of Δ mutants. Protein levels of the transcription factor Gcn4 are increased when is deleted, and is required for the elevated life spans of Δ mutants, similar to other previously described tRNA export and ribosomal mutants. Northern blots indicate that tRNA splicing and aminoacylation are not significantly affected in Δ cells, suggesting that Nup100 is largely required for nuclear export of mature, processed tRNAs. Distinct tRNAs accumulate in the nuclei of Δ and Δ mutants, while Los1-GFP nucleocytoplasmic shuttling is unaffected by Nup100. Thus, we conclude that Nup100 regulates tRNA export in a manner distinct from Los1 or Msn5. Together, these experiments reveal a novel Nup100 role in the tRNA life cycle that impacts the life span.
© 2017 Lord et al.; Published by Cold Spring Harbor Laboratory Press for the RNA Society.
MAP1LC3B (microtubule-associated protein 1 light chain 3, LC3) is a key component of the autophagy pathway, contributing to both cargo selection and autophagosome formation in the cytoplasm. Emerging evidence suggests that nuclear forms of LC3 are also functionally important; however, the mechanisms that facilitate the nuclear targeting and trafficking of LC3 between the nucleus and cytoplasm under steady-state conditions are poorly understood. In this study, we examine how residues known to regulate the interactions between LC3 and other proteins or RNA (F52 L53, R68-R70 and G120) contribute to its nuclear targeting, nucleocytoplasmic transport and association with nucleoli and other nuclear components. We find that residues F52 L53 and R68-70, but not G120, regulate targeting of LC3 to the nucleus, its rates of nucleocytoplasmic transport and the apparent sizes of LC3-associated complexes in the nucleus inferred from fluorescence recovery after photobleaching (FRAP) measurements. We also show that LC3 is enriched in nucleoli and its triple arginine motif is especially important for nucleolar targeting. Finally, we identify a series of candidate nuclear LC3-interacting proteins using mass spectrometry, including MAP1B, tubulin and several 40S ribosomal proteins. These findings suggest LC3 is retained in the nucleus in association with high-molecular weight complexes that continuously scan the nucleolus.
© 2016 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.
Nuclear LASP-1 (LIM and SH3 protein-1) has a direct correlation with overall survival of breast cancer patients. In this study, immunohistochemical analysis of a human breast TMA showed that LASP-1 is absent in normal human breast epithelium but the expression increases with malignancy and is highly nuclear in aggressive breast cancer. We investigated whether the chemokines and growth factors present in the tumor microenvironment could trigger nuclear translocation of LASP-1.Treatment of human breast cancer cells with CXCL12, EGF and HRG, and HMEC-CXCR2 cells with CXCL8 facilitated nuclear shuttling of LASP-1. Data from the biochemical analysis of the nuclear and cytosolic fractions further confirmed the nuclear translocation of LASP-1 upon chemokine and growth factor treatment. CXCL12-dependent nuclear import of LASP-1 could be blocked by CXCR4 antagonist, AMD-3100. Knock down of LASP-1 resulted in alterations in gene expression leading to an increased level of cell-junction and extracellular matrix proteins and an altered cytokine secretory profile. Three-dimensional cultures of human breast cancer cells on Matrigel revealed an altered colony growth, morphology and arborization pattern in LASP-1 knockdown cells. Functional analysis of the LASP-1 knockdown cells revealed increased adhesion to collagen IV and decreased invasion through the Matrigel. Proteomic analysis of immunoprecipitates of LASP-1 and subsequent validation approaches revealed that LASP-1 associated with the epigenetic machinery especially UHRF1, DNMT1, G9a and the transcription factor Snail1. Interestingly, LASP-1 associated with UHRF1, G9a, Snail1 and di- and tri-methylated histoneH3 in a CXCL12-dependent manner based on immunoprecipitation and proximity ligation assays. LASP-1 also directly bound to Snail1 which may stabilize Snail1. Thus, nuclear LASP-1 appears to functionally serve as a hub for the epigenetic machinery.
Using in vitro and in vivo models, we investigated the role of TFF1 in suppressing H. pylori-mediated activation of oncogenic β-catenin in gastric tumorigenesis. A reconstitution of TFF1 expression in gastric cancer cells decreased H. pylori-induced β-catenin nuclear translocation, as compared to control (p < 0.001). These cells exhibited significantly lower β-catenin transcriptional activity, measured by pTopFlash reporter, and induction of its target genes (CCND1 and c-MYC), as compared to control. Because of the role of AKT in regulating β-catenin, we performed Western blot analysis and demonstrated that TFF1 reconstitution abrogates H. pylori-induced p-AKT (Ser473), p-β-catenin (Ser552), c-MYC, and CCND1 protein levels. For in vivo validation, we utilized the Tff1-KO gastric neoplasm mouse model. Following infection with PMSS1 H. pylori strain, we detected an increase in the nuclear staining for β-catenin and Ki-67 with a significant induction in the levels of Ccnd1 and c-Myc in the stomach of the Tff1-KO, as compared to Tff1-WT mice (p < 0.05). Only 10% of uninfected Tff1-KO mice, as opposed to one-third of H. pylori-infected Tff1-KO mice, developed invasive adenocarcinoma (p = 0.03). These findings suggest that loss of TFF1 could be a critical step in promoting the H. pylori-mediated oncogenic activation of β-catenin and gastric tumorigenesis.
The eukaryotic nuclear permeability barrier and selective nucleocytoplasmic transport are maintained by nuclear pore complexes (NPCs), large structures composed of ∼ 30 proteins (nucleoporins [Nups]). NPC structure and function are disrupted in aged nondividing metazoan cells, although it is unclear whether these changes are a cause or consequence of aging. Using the replicative life span (RLS) of Saccharomyces cerevisiae as a model, we find that specific Nups and transport events regulate longevity independent of changes in NPC permeability. Mutants lacking the GLFG domain of Nup116 displayed decreased RLSs, whereas longevity was increased in nup100-null mutants. We show that Nup116 mediates nuclear import of the karyopherin Kap121, and each protein is required for mitochondrial function. Both Kap121-dependent transport and Nup116 levels decrease in replicatively aged yeast. Overexpression of GSP1, the small GTPase that powers karyopherin-mediated transport, rescued mitochondrial and RLS defects in nup116 mutants and increased longevity in wild-type cells. Together, these studies reveal that specific NPC nuclear transport events directly influence aging.
© 2015 Lord et al.
Lipopolysaccharide (LPS) is a potent microbial virulence factor that can trigger production of proinflammatory mediators involved in the pathogenesis of localized and systemic inflammation. Importantly, the role of nuclear transport of stress responsive transcription factors in this LPS-generated "genomic storm" remains largely undefined. We developed a new nuclear transport modifier (NTM) peptide, cell-penetrating cSN50.1, which targets nuclear transport shuttles importin α5 and importin β1, to analyze its effect in LPS-induced localized (acute lung injury) and systemic (lethal endotoxic shock) murine inflammation models. We analyzed a human genome database to match 46 genes that encode cytokines, chemokines and their receptors with transcription factors whose nuclear transport is known to be modulated by NTM. We then tested the effect of cSN50.1 peptide on proinflammatory gene expression in murine bone marrow-derived macrophages stimulated with LPS. This NTM suppressed a proinflammatory transcriptome of 37 out of 84 genes analyzed, without altering expression of housekeeping genes or being cytotoxic. Consistent with gene expression analysis in primary macrophages, plasma levels of 23 out of 26 LPS-induced proinflammatory cytokines, chemokines, and growth factors were significantly attenuated in a murine model of LPS-induced systemic inflammation (lethal endotoxic shock) while the anti-inflammatory cytokine, interleukin 10, was enhanced. This anti-inflammatory reprogramming of the endotoxin-induced genomic response was accompanied by complete protection against lethal endotoxic shock with prophylactic NTM treatment, and 75% protection when NTM was first administered after LPS exposure. In a murine model of localized lung inflammation caused by direct airway exposure to LPS, expression of cytokines and chemokines in the bronchoalveolar space was suppressed with a concomitant reduction of neutrophil trafficking. Thus, calming the LPS-triggered "genomic storm" by modulating nuclear transport with cSN50.1 peptide attenuates the systemic inflammatory response associated with lethal shock as well as localized lung inflammation.
The Est1 (ever shorter telomeres 1) protein is an essential component of yeast telomerase, a ribonucleoprotein complex that restores the repetitive sequences at chromosome ends (telomeres) that would otherwise be lost during DNA replication. Previous work has shown that the telomerase RNA component (TLC1) transits through the cytoplasm during telomerase biogenesis, but mechanisms of protein import have not been addressed. Here we identify three nuclear localization sequences (NLSs) in Est1p. Mutation of the most N-terminal NLS in the context of full-length Est1p reduces Est1p nuclear localization and causes telomere shortening-phenotypes that are rescued by fusion with the NLS from the simian virus 40 (SV40) large-T antigen. In contrast to that of the TLC1 RNA, Est1p nuclear import is facilitated by Srp1p, the yeast homolog of importin α. The reduction in telomere length observed at the semipermissive temperature in a srp1 mutant strain is rescued by increased Est1p expression, consistent with a defect in Est1p nuclear import. These studies suggest that at least two nuclear import pathways are required to achieve normal telomere length homeostasis in yeast.
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