The yeast nucleoporin Nsp1 binds nuclear localization sequences in vitro

The N-terminal domain of the mammalian nucleoporin p62 interacts with other nucleoporins of the FXFG family during interphase

Nuclear pore complexes (NPCs) provide the only sites for macromolecular transport between nucleus and cytoplasm. The nucleoporin p62, a component of higher eukaryotic NPCs, is located at the central gated channel and involved in nuclear trafficking of various cargos. p62 is organized into an N-terminal segment that contains FXFG repeats and binds the soluble transport factor NTF2, whereas the C-terminal portion associates with other nucleoporins and importin-beta1. We have now identified new components that interact specifically with the p62 N-terminal domain. Using the p62 N-terminal segment as bait, we affinity-purified nucleoporins Nup358, Nup214 and Nup153 from crude cell extracts. In ligand binding assays, the N-terminal p62 segment associated with Nup358 and p62, suggesting their direct binding to the p62 N-terminal portion. Furthermore, p62 was isolated in complex with Nup358, Nup214 and Nup153 from growing HeLa cells, indicating that the interactions Nup358/p62, Nup214/p62 and p62/Nup153 also occur in vivo. The formation of Nup358/p62 and p62/Nup153 complexes was restricted to interphase cells, whereas Nup214/p62 binding was detected in interphase as well as during mitosis. Our results support a model of complex interactions between FXFG containing nucleoporins, and we propose that some of these interactions may contribute to the movement of cargo across the NPC.

Nuclear accumulation of the small GTPase Gsp1p depends on nucleoporins Nup133p, Rat2p/Nup120p, Nup85p, Nic96p, and the acetyl-CoA carboxylase Acc1p

The small GTPase Ran/Gsp1p plays an essential role in nuclear trafficking of macromolecules, as Ran/Gsp1p regulates many transport processes across the nuclear pore complex (NPC). To determine the role of nucleoporins in the generation of the nucleocytoplasmic Gsp1p concentration gradient, mutations in various nucleoporin genes were analyzed in the yeast Saccharomyces cerevisiae. We show that the nucleoporins Nup133p, Rat2p/Nup120p, Nup85p, Nic96p, and the enzyme acetyl-CoA carboxylase (MTR7) control the distribution and cellular concentration of Gsp1p. At the restrictive temperature the reporter protein GFP-Gsp1p, which is too large to diffuse across the nuclear envelope, fails to concentrate in nuclei of nup133delta, rat2-1, nup85delta, nic96deltaC, and mtr7-1 cells, demonstrating that GFP-Gsp1p nuclear import is deficient. In addition, the concentration of Gsp1p is severely reduced in mutants nup133Delta and mtr7-1 under these conditions. We have now identified the molecular mechanisms that contribute to the dissipation of the Gsp1p concentration gradient in these mutants. Loss of the Gsp1p gradient in nup133delta and rat2-1 can be explained by reduced binding of the Gsp1p nuclear carrier Ntf2p to NPCs. Likewise, nup85delta cells that mislocalize GFP-Gsp1p at the permissive as well as non-permissive temperature have a diminished association of Ntf2p-GFP with nuclear envelopes under both conditions. Moreover, under restrictive conditions Prp20p, the guanine nucleotide exchange factor for Gsp1p, mislocalizes to the cytoplasm in nup85delta, nic96deltaC, and mtr7-1 cells, thereby contributing to a collapse of the Gsp1p gradient. Taken together, components of the NPC subcomplex containing Rat2p/Nup120p, Nup133p, and Nup85p, in addition to proteins Nic96p and Mtr7p, are shown to be crucial for the formation of a nucleocytoplasmic Gsp1p gradient.

In vivo administration of 15-deoxyspergulin inhibits antigen-presenting cell stimulation of T cells and NF-kappaB activation

15-Deoxyspergulin (DSG), a synthetic derivative of spergulin, was initially characterized for its antibiotic and antitumor effects. Recent studies have described the immunosuppressive properties of this molecule, but its mechanism of action is not clearly understood. In the study reported here, mice were treated in vivo with DSG prior to the measurement of IL-2 production and proliferation in an in vitro antigen presentation assay. At suboptimal antigen concentrations, elicited peritoneal macrophages or percoll isolated B cells from DSG-treated mice showed a 50-96% reduction in their ability to present chicken ovalbumin (cOva), cOva peptide, or superantigen (SAg) to MHC class II-matched antigen-specific primary T cells. No significant changes could be found in the cell surface expression of CD80, CD86, MHC I, MHC II, CD18, CD11b, CD40, CD25, and CD54 in antigen-presenting cells (APC) from control or DSG-treated animals. Activation with SAg of macrophages or splenocytes from DSG-treated mice revealed that there was a significant reduction in nuclear NF-kappaB levels compared to cells from untreated animals. Additionally, analysis of cytokines showed that production of TNF-alpha and IL-1beta was inhibited in cultures where macrophages from DSG-mice were used to present cOva to T cells. These results indicate that the effects of DSG in mice are not simply due to altered antigen processing or from any marked changes in cell surface antigen expression. Rather, the immunosuppression may arise from alterations in the release of one or more soluble factor from DSG-treated APC, which prevents effective antigen presentation and T cell activation.

Starvation promotes nuclear accumulation of the hsp70 Ssa4p in yeast cells

Nuclear import of proteins that are too large to passively enter the nucleus requires soluble factors, energy, and a nuclear localization signal (NLS). Nuclear protein transport can be regulated, and different forms of stress affect nucleocytoplasmic trafficking. As such, import of proteins containing a classical NLS is inhibited in starving yeast cells. In contrast, the hsp70 Ssa4p concentrates in nuclei upon starvation. Nuclear concentration of Ssa4p in starving cells is reversible, and transfer of stationary phase cells to fresh medium induces Ssa4p nuclear export. This export reaction represents an active process that is sensitive to oxidative stress. In starving cells, the N-terminal domain of Ssa4p mediates Ssa4p nuclear accumulation, and a short hydrophobic sequence, termed Star (for starvation), is sufficient to localize the reporter proteins green fluorescent protein or beta-galactosidase to nuclei. To determine whether nuclear accumulation of Star-beta-galactosidase depends on a specific nuclear carrier, we have analyzed its distribution in mutant yeast strains that carry a deletion of a single beta-importin gene. With this assay we have identified Nmd5p as a beta-importin required to concentrate Star-beta-galactosidase in nuclei when cells enter stationary phase.

Targeting of the mammalian nucleoporin p62 to the nuclear envelope in the yeast Saccharomyces cerevisiae and HeLa cells

We have analyzed the sorting of the mammalian nucleoporin p62 in human culture cells and in the yeast Saccharomyces cerevisiae. To this end, gene fusions were generated that carry Aequorea victoria green fluorescence protein and defined portions of p62. Upon transient gene expression fluorescent fusion proteins were localized in HeLa cells. Likewise, fusion proteins were studied in S. cerevisiae using wild-type as well as mutant cells that cluster nuclear pore complexes. Our results demonstrate that evolutionarily distant organisms, such as humans and yeasts, recognize the same sequence elements of p62 for sorting to the nuclear envelope. Specifically, the entire sequence of p62 or its complete C-terminal domain targeted fusion proteins to the nuclear membranes. In contrast, truncations of the C-terminal domain or the N-terminal segment of p62 failed to associate with the nuclear envelope in either organism. In HeLa cells overexpression of several p62-containing fusion proteins resulted in nuclear fragmentation. The C-terminal domain of p62 caused this effect, and amino acid residues 477 to 525 were sufficient to induce aberrant nuclei. Thus, overexpression of 49 amino acid residues located at the C-terminal tail of p62 interferes with the nuclear integrity in human culture cells.Key words: nuclear pore complex, nuclear transport, nucleoporin, green fluorescent protein, transfection.

Molecular architecture of the yeast nuclear pore complex: localization of Nsp1p subcomplexes

The nuclear pore complex (NPC), a supramolecular assembly of approximately 100 different proteins (nucleoporins), mediates bidirectional transport of molecules between the cytoplasm and the cell nucleus. Extensive structural studies have revealed the three- dimensional (3D) architecture of Xenopus NPCs, and eight of the approximately 12 cloned and characterized vertebrate nucleoporins have been localized within the NPC. Thanks to the power of yeast genetics, 30 yeast nucleoporins have recently been cloned and characterized at the molecular level. However, the localization of these nucleoporins within the 3D structure of the NPC has remain elusive, mainly due to limitations of preparing yeast cells for electron microscopy (EM). We have developed a new protocol for preparing yeast cells for EM that yielded structurally well-preserved yeast NPCs. A direct comparison of yeast and Xenopus NPCs revealed that the NPC structure is evolutionarily conserved, although yeast NPCs are 15% smaller in their linear dimensions. With this preparation protocol and yeast strains expressing nucleoporins tagged with protein A, we have localized Nsp1p and its interacting partners Nup49p, Nup57p, Nup82p, and Nic96p by immuno-EM. Accordingly, Nsp1p resides in three distinct subcomplexes which are located at the entry and exit of the central gated channel and at the terminal ring of the nuclear basket.

The small GTPase Gsp1p binds to the repeat domain of the nucleoporin Nsp1p

The small GTPase Gsp1p of Saccharomyces cerevisiae and its homologue Ran play essential roles in several nuclear processes, such as cell-cycle progression, nuclear organization and nucleocytoplasmic traffic of RNA and proteins. Gsp1p/Ran is an abundant nuclear protein that interacts with different cytoplasmic and nuclear factors. Several of the previously identified Ran-binding proteins located at the nuclear-pore complex carry a specific Ran-binding domain. So far, direct interactions between the GTPase and other proteins have been mostly characterized in higher eukaryotes. Here we report that the yeast protein Gsp1p can directly bind to the nucleoporin Nsp1p in vitro. Nsp1p does not contain a Ran-binding domain and therefore represents a distinct type of nucleoporin that associates with Gsp1p. We demonstrate that the middle domain of Nsp1p is sufficient to mediate this interaction. Importantly, we show that a conserved cluster of positively charged amino acid residues of Gsp1p located at positions 142-144 is essential for the binding reaction. Thus we have identified Nsp1p as a new candidate protein located at the nuclear pore complex of the yeast S. cerevisiae that interacts directly with Gsp1p. We further demonstrate that both Gsp1p and Nsp1p are components of larger protein complexes in vivo, supporting the idea that the association between both proteins takes place in growing cells.

In vivo analysis of nuclear protein traffic in mammalian cells

We have developed an experimental system to study nucleocytoplasmic traffic of proteins in living mammalian cells. Toward this goal, substrates were generated that contain several copies of Aequorea victoria green fluorescent protein (GFP). To follow facilitated transport across the nuclear envelope we created reporter proteins that carry different nuclear localization sequences (NLSs). The expression of reporter genes was controlled by an inducible promoter. Transiently transfected HeLa cells were employed to follow the sorting of fluorescent reporter proteins. When NLS-GFP fusions were located in HeLa cells, we found that direct fusion of the NLS derived from SV40 T-antigen to GFP prevented nuclear accumulation of the protein. However, insertion between NLS and GFP of different linkers encoding small amino acid residues produced reporter proteins that were competent for nuclear import. Taken together, we have generated unique tools for the characterization of nuclear protein import in dividing mammalian cells.