Protein import into nuclei: association and dissociation reactions involving transport substrate, transport factors, and nucleoporins

Nuclear import of histone H2A and H2B is mediated by a network of karyopherins

The first step in the assembly of new chromatin is the cell cycle-regulated synthesis and nuclear import of core histones. The core histones include H2A and H2B, which are assembled into nucleosomes as heterodimers. We show here that the import of histone H2A and H2B is mediated by several members of the karyopherin (Kap; importin) family. An abundant complex of H2A, H2B, and Kap114p was detected in cytosol. In addition, two other Kaps, Kap121p and Kap123p, and the histone chaperone Nap1p were isolated with H2A and H2B. Nap1p is not necessary for the formation of the Kap114p-H2A/H2B complex or for import of H2A and H2B. We demonstrate that both histones contain a nuclear localization sequence (NLS) in the amino-terminal tail. Fusions of the NLSs to green fluorescent protein were specifically mislocalized to the cytoplasm in kap mutant strains. In addition, we detected a specific mislocalization in a kap95 temperature-sensitive strain, suggesting that this Kap is also involved in the import of H2A and H2B in vivo. Importantly, we show that Kap114p, Kap121p, and Kap95 interact directly with both histone NLSs and that RanGTP inhibits this association. These data suggest that the import of H2A and H2B is mediated by a network of Kaps, in which Kap114p may play the major role.

Dimerization of sterol regulatory element-binding protein 2 via the helix-loop-helix-leucine zipper domain is a prerequisite for its nuclear localization mediated by importin beta

The sterol regulatory element-binding protein 2 (SREBP-2), a transcription factor of the basic helix-loop-helix-leucine zipper (bHLH-Zip) family, is synthesized in the form of a membrane-attached precursor molecule. When cells are deprived of sterols, a two-step proteolytic processing releases the transcriptionally active N-terminal segment of SREBP-2, thereby allowing it to enter the nucleus. In previous studies, we showed that the nuclear import of SREBP-2 occurs via the direct interaction of importin beta with the HLH-Zip domain. In this study, in order to more completely understand the intracellular dynamics of SREBP-2, we focused on the manner by which importin beta recognizes SREBP-2 at the initial step of the import. It was found that the active form of SREBP-2 exists as a stable dimer in solution and that the substitution of leucine residues for alanine in the leucine zipper motif disrupted the dimerization. It was also demonstrated that this mutant protein did not enter the nucleus either in vivo or in vitro. Solution binding assays, which involved the chemical cross-linking of wild-type or mutated SREBP-2 with importin beta, revealed that the import-active complex appeared to be composed of a dimeric form of SREBP-2 and importin beta. In addition, the SREBP-2 binding domain of importin beta corresponded to an overlapping but not identical region for importin alpha binding, which may explain how importin beta is able to recognize the dimeric HLH-Zip directly. These results indicate that dimerization is a prerequisite process for the nuclear import of SREBP-2 mediated by importin beta.

Kinetic analysis of translocation through nuclear pore complexes

The mechanism of facilitated translocation through nuclear pore complexes (NPCs) is only poorly understood. Here, we present a kinetic analysis of the process using various model substrates. We find that the translocation capacity of NPCs is unexpectedly high, with a single NPC allowing a mass flow of nearly 100 MDa/s and rates in the order of 10(3) translocation events per second. Our data further indicate that high affinity interactions between the translocation substrate and NPC components are dispensable for translocation. We propose a 'selective phase model' that could explain how NPCs function as a permeability barrier for inert molecules and yet become selectively permeable for nuclear transport receptors and receptor-cargo complexes.

Disruption of the FG nucleoporin NUP98 causes selective changes in nuclear pore complex stoichiometry and function

The NUP98 gene encodes precursor proteins that generate two nucleoplasmically oriented nucleoporins, NUP98 and NUP96. By using gene targeting, we have selectively disrupted the murine NUP98 protein, leaving intact the expression and localization of NUP96. We show that NUP98 is essential for mouse gastrulation, a developmental stage that is associated with rapid cell proliferation, but dispensable for basal cell growth. NUP98-/- cells had an intact nuclear envelope with a normal number of embedded nuclear pore complexes. Typically, NUP98-deficient cells contained on average approximately 5-fold more cytoplasmic annulate lamellae than control cells. We found that a set of cytoplasmically oriented nucleoporins, including NUP358, NUP214, NUP88, and p62, assembled inefficiently into nuclear pores of NUP98-/- cells. Instead, these nucleoporins were prominently associated with the annulate lamellae. By contrast, a group of nucleoplasmically oriented nucleoporins, including NUP153, NUP50, NUP96, and NUP93, had no affinity for annulate lamellae and assembled normally into nuclear pores. Mutant pores were significantly impaired in transport receptor-mediated docking of proteins with a nuclear localization signal or M9 import signal and showed weak nuclear import of such substrates. In contrast, the ability of mutant pores to import ribosomal protein L23a and spliceosome protein U1A appeared intact. These observations show that NUP98 disruption selectively impairs discrete protein import pathways and support the idea that transport of distinct import complexes through the nuclear pore complex is mediated by specific subsets of nucleoporins.

The GLFG regions of Nup116p and Nup100p serve as binding sites for both Kap95p and Mex67p at the nuclear pore complex

Our previous studies have focused on a family of Saccharomyces cerevisiae nuclear pore complex (NPC) proteins that contain domains composed of repetitive tetrapeptide glycine-leucine-phenylalanine-glycine (GLFG) motifs. We have previously shown that the GLFG regions of Nup116p and Nup100p directly bind the karyopherin transport factor Kap95p during nuclear protein import. In this report, we have further investigated potential roles for the GLFG region in mRNA export. The subcellular localizations of green fluorescent protein (GFP)-tagged mRNA transport factors were individually examined in yeast cells overexpressing the Nup116-GLFG region. The essential mRNA export factors Mex67-GFP, Mtr2-GFP, and Dbp5-GFP accumulated in the nucleus. In contrast, the localizations of Gle1-GFP and Gle2-GFP remained predominantly associated with the NPC, as in wild type cells. The localization of Kap95p was also not perturbed with GLFG overexpression. Coimmunoprecipitation experiments from yeast cell lysates resulted in the isolation of a Mex67p-Nup116p complex. Soluble binding assays with bacterially expressed recombinant proteins confirmed a direct interaction between Mex67p and the Nup116-GLFG or Nup100-GLFG regions. Mtr2p was not required for in vitro binding of Mex67p to the GLFG region. To map the Nup116-GLFG subregion(s) required for Kap95p and/or Mex67p association, yeast two-hybrid analysis was used. Of the 33 Nup116-GLFG repeats that compose the domain, a central subregion of nine GLFG repeats was sufficient for binding either Kap95p or Mex67p. Interestingly, the first 12 repeats from the full-length region only had a positive interaction with Mex67p, whereas the last 12 were only positive with Kap95p. Thus, the GLFG domain may have the capacity to bind both karyopherins and an mRNA export factor simultaneously. Taken together, our in vivo and in vitro results define an essential role for a direct Mex67p-GLFG interaction during mRNA export.

The karyopherin Kap142p/Msn5p mediates nuclear import and nuclear export of different cargo proteins

We have identified a novel pathway for protein import into the nucleus. Although the product of Saccharomyces cerevisiae gene MSN5 was previously shown to function as a karyopherin (Kap) for nuclear export of various proteins, we discovered a nuclear import pathway mediated by Msn5p (also referred to as Kap142p). We have purified from yeast cytosol a complex containing Kap142p and the trimeric replication protein A (RPA), which is required for multiple aspects of DNA metabolism, including DNA replication, DNA repair, and recombination. In wild-type cells, RPA was localized primarily to the nucleus but, in a KAP142 deletion strain, RPA was mislocalized to the cytoplasm and the strain was highly sensitive to bleomycin (BLM). BLM causes DNA double-strand breaks and, in S. cerevisiae, the DNA damage is repaired predominantly by RPA-dependent homologous recombination. Therefore, our results indicate that in wild-type cells a critical portion of RPA was imported into the nucleus by Kap142p. Like several other import-related Kap-substrate complexes, the endogenous RPA-Kap142p complex was dissociated by RanGTP, but not by RanGDP. All three RPA genes are essential for viability, whereas KAP142 is not. Perhaps explaining this disparity, we observed an interaction between RPA and Kap95p in a strain lacking Kap142p. This interaction could provide a mechanism for import of RPA into the nucleus and cell viability in the absence of Kap142p. Together with published results (Kaffman, A., N.M. Rank, E.M. O'Neill, L.S. Huang, and E.K. O'Shea. 1998. Nature. 396:482-486; Blondel, M., P.M. Alepuz, L.S. Huang, S. Shaham, G. Ammerer, and M. Peter. 1999. Genes Dev. 13:2284-2300; DeVit, M.J., and M. Johnston. 1999. Curr. Biol. 9:1231-1241; Mahanty, S.K., Y. Wang, F.W. Farley, and E.A. Elion. 1999. Cell. 98:501-512) our data indicate that the karyopherin Kap142p is able to mediate nuclear import of one set of proteins and nuclear export of a different set of proteins.

Gradient of increasing affinity of importin beta for nucleoporins along the pathway of nuclear import

Nuclear import and export signals on macromolecules mediate directional, receptor-driven transport through the nuclear pore complex (NPC) by a process that is suggested to involve the sequential binding of transport complexes to different nucleoporins. The directionality of transport appears to be partly determined by the nucleocytoplasmic compartmentalization of components of the Ran GTPase system. We have analyzed whether the asymmetric localization of discrete nucleoporins can also contribute to transport directionality. To this end, we have used quantitative solid phase binding analysis to determine the affinity of an importin beta cargo complex for Nup358, the Nup62 complex, and Nup153, which are in the cytoplasmic, central, and nucleoplasmic regions of the NPC, respectively. These nucleoporins are proposed to provide progressively more distal binding sites for importin beta during import. Our results indicate that the importin beta transport complex binds to nucleoporins with progressively increasing affinity as the complex moves from Nup358 to the Nup62 complex and to Nup153. Antibody inhibition studies support the possibility that importin beta moves from Nup358 to Nup153 via the Nup62 complex during import. These results indicate that nucleoporins themselves, as well as the nucleocytoplasmic compartmentalization of the Ran system, are likely to play an important role in conferring directionality to nuclear protein import.

Dissection of a nuclear localization signal

The regulated process of protein import into the nucleus of a eukaryotic cell is mediated by specific nuclear localization signals (NLSs) that are recognized by protein import receptors. This study seeks to decipher the energetic details of NLS recognition by the receptor importin alpha through quantitative analysis of variant NLSs. The relative importance of each residue in two monopartite NLS sequences was determined using an alanine scanning approach. These measurements yield an energetic definition of a monopartite NLS sequence where a required lysine residue is followed by two other basic residues in the sequence K(K/R)X(K/R). In addition, the energetic contributions of the second basic cluster in a bipartite NLS ( approximately 3 kcal/mol) as well as the energy of inhibition of the importin alpha importin beta-binding domain ( approximately 3 kcal/mol) were also measured. These data allow the generation of an energetic scale of nuclear localization sequences based on a peptide's affinity for the importin alpha-importin beta complex. On this scale, a functional NLS has a binding constant of approximately 10 nm, whereas a nonfunctional NLS has a 100-fold weaker affinity of 1 microm. Further correlation between the current in vitro data and in vivo function will provide the foundation for a comprehensive quantitative model of protein import.

A role for the basic patch and the C terminus of RanGTP in regulating the dynamic interactions with importin beta, CRM1 and RanBP1

Transport of macromolecules between the nucleus and cytoplasm involves the recognition of intrinsic localization signals by either import or export receptors. The interaction of the receptors with their cargo is regulated by the small GTPase Ran in its GTP bound state. We have investigated the interaction of RanGTP with the import factor, importin beta, the export factor, CRM1, and the Ran binding protein, RanBP1, in solution. Importin beta specifically protected residues in the switch regions and basic patch region of Ran against proteolytic cleavage, whereas RanBP1 protected the C terminus. Moreover, the binding of importin beta induced a conformational change in the structure of Ran leading to an exposure of the C terminus and stimulated the binding of RanBP1. Mutating the basic patch (HRKK(142)) of Ran resulted in an increased binding of RanBP1 and weakened importin beta binding. In contrast to wild-type Ran, the mutant Ran could be released from importin beta independently of importin alpha. These data provide experimental support for a model in which the accessibility of the C terminus of Ran is influenced by an intramolecular interaction between the basic patch and the C-terminal acidic DEDDDL(216) motif. Binding of importin beta probably disrupts this interaction causing an exposure of the C-terminal extension, which is favorable for RanBP1 binding. Interestingly, basic patch mutations abolish CRM1 interaction, indicating that the determinants for RanGTP binding to the export factor, CRM1, is different from the import factor, importin beta.

Ran induces spindle assembly by reversing the inhibitory effect of importin alpha on TPX2 activity

The small GTPase Ran, bound to GTP, is required for the induction of spindle formation by chromosomes in M phase. High concentrations of Ran.GTP are proposed to surround M phase chromatin. We show that the action of Ran.GTP in spindle formation requires TPX2, a microtubule-associated protein previously known to target a motor protein, Xklp2, to microtubules. TPX2 is normally inactivated by binding to the nuclear import factor, importin alpha, and is displaced from importin alpha by the action of Ran.GTP. TPX2 is required for Ran.GTP and chromatin-induced microtubule assembly in M phase extracts and mediates spontaneous microtubule assembly when present in excess over free importin alpha. Thus, components of the nuclear transport machinery serve to regulate spindle formation in M phase.

Enhancement of MSH receptor- and GAL4-mediated gene transfer by switching the nuclear import pathway

Efficient nuclear delivery of plasmid DNA represents a major barrier in nonviral gene transfer. One approach has been to use DNA-binding proteins such as GAL4 from yeast as DNA carriers with nuclear targeting properties. We recently showed, however, that GAL4 is inefficient in targeting DNA to the nucleus because its DNA-binding and nuclear targeting activities are mutually exclusive, which relates to the fact that GAL4 nuclear import occurs via a novel pathway. Here, we 'switch' this pathway to a more conventional one by adding a modified poly-lysine to which an optimized nuclear targeting signal, based on that of the SV40 large T-antigen, is linked. We also use a chimeric GAL4-alpha-melanocyte stimulating hormone (MSH) fusion protein to enable gene transfer to cells expressing the MSH receptor. Switching the nuclear import pathway of the transfecting complex significantly enhances receptor-mediated gene transfer through enabling interaction with desired components of the cellular nuclear import machinery. The present study represents the first demonstration that nuclear targeting signals can enhance receptor-mediated gene delivery, the approaches having important relevance to research and clinical applications, such as in generating transgenic or knock-out animals, or in gene therapy.

The nuclear pore complex

Nuclear pore complexes, the conduits for information exchange between the nucleus and cytoplasm, appear broadly similar in eukaryotes from yeast to human. Precisely how nuclear pore complexes regulate macromolecular and ionic traffic remains unknown, but recent advances in the identification and characterization of components of the complex by proteomics and genomics have provided new insights.

The Ran-GTPase and cell-cycle control

RCC1, the chromatin-bound guanine-nucleotide exchange factor (GEF) for the small nuclear GTPase, Ran, is required for coordinating the onset of mitosis with S-phase completion in mammalian cells. Other defects in the Ran-GTPase network also result in disruption of cell-cycle processes such as DNA replication, exit from mitosis and, at least in budding yeast, accurate chromosome segregation. However, the Ran system is now best known for its pivotal role in nucleocytoplasmic transport, where RanGTP is used as a positional flag for the nucleus during interphase. Ran's effectors are the shuttling transport factors, importins and exportins, which facilitate the transit of cargoes between the nucleus and cytoplasm: RanGTP regulates their cargo-binding properties so that they can move their cargo in the correct direction. RanGTP also plays a separate role during mitosis, influencing microtubule polymerisation, possibly specifically in the vicinity of chromosomes. Most recently, Ran has been shown to be crucial for the regeneration of a nuclear envelope after exit from mitosis. So, can the problems with cell-cycle progression and control induced by perturbing the Ran-system be attributed to defects in these three processes? This article examines this issue, concentrating on vertebrate systems. BioEssays 23:77-85, 2001.

Importin-beta-like nuclear transport receptors

SUMMARY

In recent years, our understanding of macromolecular transport processes across the nuclear envelope has grown dramatically, and a large number of soluble transport receptors mediating either nuclear import or nuclear export have been identified. Most of these receptors belong to one large family of proteins, all of which share homology with the protein import receptor importin beta (also named karyopherin beta). Members of this family have been classified as importins or exportins on the basis of the direction they carry their cargo. To date, the family includes 14 members in the yeast Saccharomyces cerevisiae and at least 22 members in humans. Importins and exportins are regulated by the small GTPase Ran, which is thought to be highly enriched in the nucleus in its GTP-bound form. Importins recognize their substrates in the cytoplasm and transport them through nuclear pores into the nucleus. In the nucleoplasm, RanGTP binds to importins, inducing the release of import cargoes. In contrast, exportins interact with their substrates only in the nucleus in the presence of RanGTP and release them after GTP hydrolysis in the cytoplasm, causing disassembly of the export complex. Thus, common features of all importin-beta-like transport factors are their ability to shuttle between the nucleus and the cytoplasm, their interaction with RanGTP as well as their ability to recognize specific transport substrates.

Identification of two novel RanGTP-binding proteins belonging to the importin beta superfamily

Nucleo-cytoplasmic transport comprises a large number of distinct pathways, many of which are defined by members of the importin beta superfamily of nuclear transport receptors. These transport receptors all directly interact with RanGTP to modulate the compartment-specific binding of their transport substrates. To identify new members of the importin beta family, we used affinity chromatography on immobilized RanGTP and isolated Ran-binding protein (RanBP) 16 from HeLa cell extracts. RanBP16 and its close human homologue, RanBP17, are distant members of the importin beta family. Like the other members of the transport receptor superfamily, RanBP16 interacts with the nuclear pore complex and is able to enter the nucleus independent of energy and additional nuclear transport receptors.

Yeast Ran-binding protein Yrb1p is required for efficient proteolysis of cell cycle regulatory proteins Pds1p and Sic1p

Ubiquitin-dependent proteolysis of specific target proteins is required for several important steps during the cell cycle. Degradation of such proteins is strictly cell cycle-regulated and triggered by two large ubiquitin ligases, termed anaphase-promoting complex (APC) and Skp1/Cullin/F-box complex (SCF). Here we show that yeast Ran-binding protein 1 (Yrb1p), a predominantly cytoplasmic protein implicated in nucleocytoplasmic transport, is required for cell cycle regulated protein degradation. Depletion of Yrb1p results in the accumulation of unbudded G(1) cells and of cells arrested in mitosis implying a function of Yrb1p in the G(1)/S transition and in the progression through mitosis. Temperature-sensitive yrb1-51 mutants are defective in APC-mediated degradation of the anaphase inhibitor protein Pds1p and in degradation of the cyclin-dependent kinase inhibitor Sic1p, a target of SCF. Thus, Yrb1p is crucial for efficient APC- and SCF-mediated proteolysis of important cell cycle regulatory proteins. We have identified the UBS1 gene as a multicopy suppressor of yrb1-51 mutants. Ubs1p is a nuclear protein, and its deletion is synthetic lethal with a yrb1-51 mutation. Interestingly, UBS1 was previously identified as a multicopy suppressor of cdc34-2 mutants, which are defective in SCF activity. We suggest that Ubs1p may represent a link between nucleocytoplasmic transport and ubiquitin ligase activity.

Factors affecting nuclear export of the 60S ribosomal subunit in vivo

In Saccharomyces cerevisiae, the 60S ribosomal subunit assembles in the nucleolus and then is exported to the cytoplasm, where it joins the 40S subunit for translation. Export of the 60S subunit from the nucleus is known to be an energy-dependent and factor-mediated process, but very little is known about the specifics of its transport. To begin to address this problem, an assay was developed to follow the localization of the 60S ribosomal subunit in S. cerevisiae. Ribosomal protein L11b (Rpl11b), one of the approximately 45 ribosomal proteins of the 60S subunit, was tagged at its carboxyl terminus with the green fluorescent protein (GFP) to enable visualization of the 60S subunit in living cells. A panel of mutant yeast strains was screened for their accumulation of Rpl11b-GFP in the nucleus as an indicator of their involvement in ribosome synthesis and/or transport. This panel included conditional alleles of several rRNA-processing factors, nucleoporins, general transport factors, and karyopherins. As predicted, conditional alleles of rRNA-processing factors that affect 60S ribosomal subunit assembly accumulated Rpl11b-GFP in the nucleus. In addition, several of the nucleoporin mutants as well as a few of the karyopherin and transport factor mutants also mislocalized Rpl11b-GFP. In particular, deletion of the previously uncharacterized karyopherin KAP120 caused accumulation of Rpl11b-GFP in the nucleus, whereas ribosomal protein import was not impaired. Together, these data further define the requirements for ribosomal subunit export and suggest a biological function for KAP120.

Nup2p, a yeast nucleoporin, functions in bidirectional transport of importin alpha

Import of proteins containing a classical nuclear localization signal (NLS) into the nucleus is mediated by importin alpha and importin beta. Srp1p, the Saccharomyces cerevisiae homologue of importin alpha, returns from the nucleus in a complex with its export factor Cse1p and with Gsp1p (yeast Ran) in its GTP-bound state. We studied the role of the nucleoporin Nup2p in the transport cycle of Srp1p. Cells lacking NUP2 show a specific defect in both NLS import and Srp1p export, indicating that Nup2p is required for efficient bidirectional transport of Srp1p across the nuclear pore complex (NPC). Nup2p is located at the nuclear side of the central gated channel of the NPC and provides a binding site for Srp1p via its amino-terminal domain. We show that Nup2p effectively releases the NLS protein from importin alpha-importin and beta and strongly binds to the importin heterodimer via Srp1p. Kap95p (importin beta) is released from this complex by a direct interaction with Gsp1p-GTP. These data suggest that besides Gsp1p, which disassembles the NLS-importin alpha-importin beta complex upon binding to Kap95p in the nucleus, Nup2p can also dissociate the import complex by binding to Srp1p. We also show data indicating that Nup1p, a relative of Nup2p, plays a similar role in termination of NLS import. Cse1p and Gsp1p-GTP release Srp1p from Nup2p, which suggests that the Srp1p export complex can be formed directly at the NPC. The changed distribution of Cse1p at the NPC in nup2 mutants also supports a role for Nup2p in Srp1p export from the nucleus.

Selective disruption of nuclear import by a functional mutant nuclear transport carrier

p10/NTF2 is a nuclear transport carrier that mediates the uptake of cytoplasmic RanGDP into the nucleus. We constructed a point mutant of p10, D23A, that exhibited unexpected behavior both in digitonin-permeabilized and microinjected mammalian cells. D23A p10 was markedly more efficient than wild-type (wt) p10 at supporting Ran import, but simultaneously acted as a dominant-negative inhibitor of classical nuclear localization sequence (cNLS)-mediated nuclear import supported by karyopherins (Kaps) alpha and beta1. Binding studies indicated that these two nuclear transport carriers of different classes, p10 and Kap-beta1, compete for identical and/or overlapping binding sites at the nuclear pore complex (NPC) and that D23A p10 has an increased affinity relative to wt p10 and Kap-beta1 for these shared binding sites. Because of this increased affinity, D23A p10 is able to import its own cargo (RanGDP) more efficiently than wt p10, but Kap-beta1 can no longer compete efficiently for shared NPC docking sites, thus the import of cNLS cargo is inhibited. The competition of different nuclear carriers for shared NPC docking sites observed here predicts a dynamic equilibrium between multiple nuclear transport pathways inside the cell that could be easily shifted by a transient modification of one of the carriers.

The nucleoporin Nup98 is a site for GDP/GTP exchange on ran and termination of karyopherin beta 2-mediated nuclear import

Karyopherin beta2 (Kapbeta2, transportin) binds the M9 sequence of human ribonucleoprotein A1 and mediates its nuclear import. Here we show a role for the nucleoporin Nup98 in the disassembly of Kapbeta2 import complexes at the nuclear side of the nuclear pore complex (NPC). Kapbeta2 bound to a region at the N terminus of Nup98 that contains an M9-like sequence. The human ribonucleoprotein A1 M9 sequence competed with Nup98 for binding to Kapbeta2, indicating that Nup98 can dissociate Kapbeta2 from its substrate. Binding of Kapbeta2 to Nup98 was inhibited by Ran loaded with guanylyl imidophosphate, suggesting that RanGTP dissociates Kapbeta2 from Nup98. RanGTP is produced from RanGDP through nucleotide exchange mediated by RanGEF (RCC1). Immunoelectron microscopy and nucleotide exchange assays revealed functional RanGEF on both sides of the NPC. On the nuclear side, the localization of RanGEF coincided with that of Nup98. RanGEF bound to Nup98 at a region adjacent to the Kapbeta2-binding site. These findings suggest a model where 1) import substrate is released from Kapbeta2 at the nucleoplasmic side of the NPC by competition with the Nup98 M9-like site, 2) Nup98-bound RanGEF catalyzes the formation of RanGTP, and 3) RanGTP dissociates Kapbeta2 from Nup98 allowing repeated cycles of import.

The mechanism of ran import into the nucleus by nuclear transport factor 2

The small GTPase Ran is essential for virtually all nucleocytoplasmic transport events. It is hypothesized that Ran drives vectorial transport of macromolecules into and out of the nucleus via the establishment of a Ran gradient between the cytoplasm and nucleoplasm. Although Ran shuttles between the nucleus and cytoplasm, it is concentrated in the nucleus at steady state. We show that nuclear transport factor 2 (NTF2) is required to concentrate Ran in the nucleus in the budding yeast, Saccharomyces cerevisiae. To analyze the mechanism of Ran import into the nucleus by NTF2, we use mutants in a variety of nuclear transport factors along with biochemical analyses of NTF2 complexes. We find that Ran remains concentrated in the nucleus when importin-mediated protein import is disrupted and demonstrate that NTF2 does not form a stable complex with the transport receptor, importin-beta. Consistent with a critical role for NTF2 in establishing and maintaining the Ran gradient, we show that NTF2 is required for early embryogenesis in Caenorhabditis elegans. Our data distinguish between two possible mechanisms for Ran import by NTF2 and demonstrate that Ran import is independent from importin-beta-mediated protein import.

Crystallization and initial X-ray diffraction characterization of complexes of FxFG nucleoporin repeats with nuclear transport factors

NTF2 and importin-beta are transport factors that mediate nuclear protein import and which interact with nuclear pore proteins (nucleoporins) during translocation from the cytoplasm to the nucleus through nuclear pore complexes. We employed a native gel electrophoresis method to assess the interaction of nucleoporin constructs that contain FxFG sequence repeats with NTF2 and truncation mutants of importin-beta to determine suitable fragments for crystallization. Based on these data, we obtained crystals of complexes between yeast NTF2 and a construct containing five FxFG nucleoporin repeats from the yeast nucleoporin Nsp1p and between a construct containing residues 1-442 of human importin-beta and the same nucleoporin construct. The yeast NTF2-nucleoporin crystals have trigonal symmetry and diffract past 2.8 A resolution using synchrotron radiation, whereas the importin-beta-nucleoporin complex crystals have P2(1)2(1)2 orthorhombic symmetry and diffract past 3.2 A resolution.

Binding of the Mex67p/Mtr2p heterodimer to FXFG, GLFG, and FG repeat nucleoporins is essential for nuclear mRNA export

It is not known how Mex67p and Mtr2p, which form a heterodimer essential for mRNA export, transport mRNPs through the nuclear pore. Here, we show that the Mex67p/Mtr2p complex binds to all of the repeat types (GLFG, FXFG, and FG) found in nucleoporins. For this interaction, complex formation between Mex67p and Mtr2p has to occur. MEX67 and MTR2 also genetically interact with different types of repeat nucleoporins, such as Nup116p, Nup159p, Nsp1p, and Rip1p/Nup40p. These data suggest a model in which nuclear mRNA export requires the Mex67p/Mtr2p heterodimeric complex to directly contact several repeat nucleoporins, organized in different nuclear pore complex subcomplexes, as it carries the mRNP cargo through the nuclear pore.

Exportin 4: a mediator of a novel nuclear export pathway in higher eukaryotes

Transport receptors of the importin beta superfamily account for many of the nuclear import and export events in eukaryotic cells. They mediate translocation through nuclear pore complexes, shuttle between nucleus and cytoplasm and co-operate with the RanGTPase system to regulate their interactions with cargo molecules in a compartment-specific manner. We used affinity chromatography on immobilized RanGTP to isolate further candidate nuclear transport receptors and thereby identified exportin 4 as the most distant member of the importin beta family so far. Exportin 4 appears to be conserved amongst higher eukaryotes, but lacks obvious orthologues in yeast. It mediates nuclear export of eIF-5A (eukaryotic translation initiation factor 5A) and possibly that of other cargoes. The export signal in eIF-5A appears to be complex and to involve the hypusine modification that is unique to eIF-5A. We discuss possible cellular roles for nuclear export of eIF-5A.

The molecular mechanisms of messenger RNA nuclear export

In eukaryotic cells, the nuclear membrane creates a barrier between the nucleus and the cytoplasm. Whereas RNA synthesis occurs in the nucleus, they mostly function in the cytoplasm; thus export of RNA molecules from the nucleus to the cytoplasm is indispensable for normal function of the cells. The molecular mechanisms involved in each kind of cellular RNA export is gradually understood. The focus of this review will be mRNA export. mRNAs are multiformed. In order to ensure that this variety of mRNA molecules are all exported, cells are probably equipped with multiple export pathways. A number of proteins is predicted to be involved in mRNA export. Ascertaining which proteins play crucial roles in the pathways is the key point in the study of mRNA export.

Insights into the molecular mechanism of nuclear trafficking using nuclear transport factor 2 (NTF2)

Nuclear transport factor 2 (NTF2) mediates the nuclear import of RanGDP. The simplicity and specialization of this system, combined with the availability of crystal structures of NTF2, RanGDP and their complex, has facilitated the investigation of the molecular mechanism of its trafficking. NTF2 binds to both RanGDP and FxFG repeat-containing nucleoporins. Mutants engineered on the basis of structural information together with determination of binding constants have been used to dissect the roles of these interactions in transport. Thus, NTF2 binds to RanGDP sufficiently strongly for the complex to remain intact during transport through NPCs, but the interaction between NTF2 and FxFG nucleoporins is much more transient, which would enable NTF2 to move through the NPC by hopping from one repeat to another. An analogous nucleoporin hopping mechanism may also be used by carrier molecules of the importin-beta family to move through NPCs.

Evidence for separable functions of Srp1p, the yeast homolog of importin alpha (Karyopherin alpha): role for Srp1p and Sts1p in protein degradation

Srp1p (importin alpha) functions as the nuclear localization signal (NLS) receptor in Saccharomyces cerevisiae. The srp1-31 mutant is defective in this nuclear localization function, whereas an srp1-49 mutant exhibits defects that are unrelated to this localization function, as was confirmed by intragenic complementation between the two mutants. RPN11 and STS1 (DBF8) were identified as high-dosage suppressors of the srp1-49 mutation but not of the srp1-31 mutation. We found that Sts1p interacts directly with Srp1p in vitro and also in vivo, as judged by coimmunoprecipitation and two-hybrid analyses. Mutants of Sts1p that cannot interact with Srp1p are incapable of suppressing srp1-49 defects, strongly suggesting that Sts1p functions in a complex with Srp1p. STS1 also interacted with the second suppressor, RPN11, a subunit of the 26S proteasome, in the two-hybrid system. Further, degradation of Ub-Pro-beta-galactosidase, a test substrate for the ubiquitin-proteasome system, was defective in srp1-49 but not in srp1-31. This defect in protein degradation was alleviated by overexpression of either RPN11 or STS1 in srp1-49. These results suggest a role for Srp1p in regulation of protein degradation separate from its well-established role as the NLS receptor.

Importin-alpha3 is required at multiple stages of Drosophila development and has a role in the completion of oogenesis

The Drosophila importin-alpha3 gene was isolated through its interaction with the large subunit of the DNA polymerase alpha in a two-hybrid screen. The predicted protein sequence of Importin-alpha3 is 65-66% identical to those of the human and mouse importin-alpha3 and alpha4 and 42.7% identical to that of Importin-alpha2 (Oho31/Pendulin), the previously reported Drosophila homologue. Both Importin-alpha3 and Importin-alpha2 interact with similar subsets of proteins in vitro, one of which is Ketel, the importin-beta homologue of Drosophila. importin-alpha3 is an essential gene, whose encoded protein is expressed throughout development. During early embryogenesis, Importin-alpha3 accumulates at the nuclear membrane of cleavage nuclei, whereas after blastoderm formation it is characteristically found within the interphase nuclei. Nuclear localisation is seen in several tissues throughout subsequent development. During oogenesis its concentration within the nurse cell nuclei increases during stages 7-10, concomitant with a decline in levels in the oocyte nucleus. Mutation of importin-alpha3 results in lethality throughout pupal development. Surviving females are sterile and show arrest of oogenesis at stages 7-10. Thus, Importin-alpha3-mediated nuclear transport is essential for completion of oogenesis and becomes limiting during pupal development. Since they have different expression patterns and subcellular localisation profiles, we suggest that the two importin-alpha homologues are not redundant in the context of normal Drosophila development.

Quantitative analysis of nuclear localization signal (NLS)-importin alpha interaction through fluorescence depolarization. Evidence for auto-inhibitory regulation of NLS binding

We have developed a quantitative in vitro steady-state fluorescence depolarization assay to measure the interaction of a nuclear localization signal (NLS) substrate with its receptors. This assay relies on the change in fluorescence depolarization of an NLS fused to the green fluorescent protein upon binding to receptor. No binding is observed in the absence of a functional NLS, and binding affinities measured correlate with previous in vivo studies of NLS function. We have used this assay to test an auto-inhibitory model for the interaction of an NLS with the NLS receptor complex. This model suggests that NLS binding to importin alpha is modulated by an auto-inhibitory sequence within the N terminus of importin alpha, which is displaced by importin beta binding. Consistent with this model, NLS substrates bind tightly to an N-terminally truncated importin alpha lacking the auto-inhibitory domain (K(d) approximately 10 nm), but measurable binding to full-length importin alpha is only observed upon addition of importin beta. Our quantitative results support the auto-inhibitory model and suggest a mechanism for a switch between a cytoplasmic, high affinity and a nuclear, low affinity NLS receptor. This predicted mode of interaction would facilitate binding of substrate in the cytoplasm and its subsequent release into the nucleus.

Structural basis for the interaction between FxFG nucleoporin repeats and importin-beta in nuclear trafficking

We describe the crystal structure of a complex between importin-beta residues 1-442 (Ib442) and five FxFG nucleoporin repeats from Nsp1p. Nucleoporin FxFG cores bind on the convex face of Ib442 to a primary site between the A helices of HEAT repeats 5 and 6, and to a secondary site between HEAT repeats 6 and 7. Mutations at importin-beta Ile178 in the primary FxFG binding site reduce both binding and nuclear protein import, providing direct evidence for the functional significance of the importin-beta-FxFG interaction. The FxFG binding sites on importin-beta do not overlap with the RanGTP binding site. Instead, RanGTP may release importin-beta from FxFG nucleoporins by generating a conformational change that alters the structure of the FxFG binding site.

The C-terminal proline-rich tail of human immunodeficiency virus type 2 Vpx is necessary for nuclear localization of the viral preintegration complex in nondividing cells

Human immunodeficiency virus type 2 (HIV-2), like other lentiviruses, is capable of infecting nondividing T cells and macrophages. The present work shows that in HIV-2-infected cells, Vpx is necessary for efficient nuclear import of the preintegration complex. In agreement with this finding, the subcellular localization of a GFP-Vpx fusion protein was found to be predominantly nuclear. However, deletion of the proline-rich C-terminal 11 residues of Vpx resulted in a shift of the fusion protein to the cytoplasm. Furthermore, the same deletion in the context of the provirus resulted in a decrease in nuclear import of the preintegration complex and attenuated replication in macrophages.

Phosphorylation of the nuclear transport machinery down-regulates nuclear protein import in vitro

We have examined whether signal-mediated nucleocytoplasmic transport can be regulated by phosphorylation of the nuclear transport machinery. Using digitonin-permeabilized cell assays to measure nuclear import and export, we found that the phosphatase inhibitors okadaic acid and microcystin inhibit transport mediated by the import receptors importin beta and transportin, but not by the export receptor CRM1. Several lines of evidence, including the finding that transport inhibition is partially reversed by the broad specificity protein kinase inhibitor staurosporine, indicate that transport inhibition is due to elevated phosphorylation of a component of the nuclear transport machinery. The kinases and phosphatases involved in this regulation are present in the permeabilized cells. A phosphorylation-sensitive component of the nuclear transport machinery also is present in permeabilized cells and is most likely a component of the nuclear pore complex. Substrate binding by the importin alpha.beta complex and the association of the complex with the nucleoporins Nup358/RanBP2 and Nup153 are not affected by phosphatase inhibitors, suggesting that transport inhibition by protein phosphorylation does not involve these steps. These results suggest that cells have mechanisms to negatively regulate entire nuclear transport pathways, thus providing a means to globally control cellular activity through effects on nucleocytoplasmic trafficking.

The nuclear pore complex: a protein machine bridging the nucleus and cytoplasm

Compositional analysis of nuclear pore complexes (NPCs) is nearing completion, and efforts are now focused on understanding how these protein machines work. Recent analysis of soluble transport factor interactions with NPC proteins reveals distinct and overlapping pathways for movement between the nucleus and cytoplasm. New fluorescence- and microscopy-based strategies have been used to monitor the pathway of NPC assembly and to reveal the dynamics of the NPC during transport.

Nuclear targeting signal recognition: a key control point in nuclear transport?

Recent progress indicates that there are multiple pathways of nucleocytoplasmic transport which involve specific targeting sequences, such as nuclear localization sequences (NLSs), and cytosolic receptor molecules of the importin/karyopherin superfamily which recognise and dock the NLS-containing proteins at the nuclear pore. This first step of nuclear import/export is of central importance, with the affinity of the importin-targeting sequence interaction a critical parameter in determining transport efficiency. Different importins possess distinct NLS-binding specificities, which allows the system to be modulated through differential expression of the importins themselves, as well as through competition between different importins for the same protein, and between different proteins for the same importin. The targeting sequence-importin interaction can also be influenced directly by phosphorylation increasing the affinity of the interaction with importins or by targeting sequence masking through phosphorylation or specific protein binding. Targeting sequence recognition thus appears to represent a key control point in the regulation of nuclear transport. BioEssays 22:532-544, 2000.

The molecular mechanism of transport of macromolecules through nuclear pore complexes

Trafficking of macromolecules between nuclear and cytoplasmic compartments takes place through the nuclear pore complexes (NPCs) of the nuclear envelope. Nuclear trafficking involves a complex series of interactions between cargo, soluble transport factors (carriers) and nuclear pore proteins (nucleoporins) that are orchestrated by the Ras-family GTPase Ran. The primary role of Ran is probably to establish directionality and to sort molecules to be transported by controlling the interaction between carriers and cargoes, so that they bind in one compartment but dissociate in the other. Translocation of carriers and cargo-carrier complexes through NPCs requires interactions between the carriers and nucleoporins that contain distinctive tandem sequence repeats based on cores rich in glycine and phenylalanine residues that are separated by hydrophilic linkers. Much recent work has focused on these interactions and, in particular, their specificity, regulation and function. Evidence is accumulating that carriers move through the NPC by distinct but overlapping routes using specific subsets of nucleoporins.

Nuclear import of the ran exchange factor, RCC1, is mediated by at least two distinct mechanisms

RCC1, the only known guanine-nucleotide exchange factor for the Ran GTPase, is an approximately 45-kD nuclear protein that can bind chromatin. An important question concerns how RCC1 traverses the nuclear envelope. We now show that nuclear RCC1 is not exported readily in interphase cells and that the import of RCC1 into the nucleoplasm is extremely rapid. Import can proceed by at least two distinct mechanisms. The first is a classic import pathway mediated by basic residues within the NH(2)-terminal domain (NTD) of RCC1. This pathway is dependent upon both a preexisting Ran gradient and energy, and preferentially uses the importin-alpha3 isoform of importin-alpha. The second pathway is not mediated by the NTD of RCC1. This novel pathway does not require importin-alpha or importin-beta or the addition of any other soluble factor in vitro; however, this pathway is saturable and sensitive only to a subset of inhibitors of classical import pathways. Furthermore, the nuclear import of RCC1 does not require a preexisting Ran gradient or energy. We speculate that this second import pathway evolved to ensure that RCC1 never accumulates in the cytoplasm.

Facilitated nucleocytoplasmic shuttling of the Ran binding protein RanBP1

The Ran binding protein RanBP1 is localized to the cytosol of interphase cells. A leucine-rich nuclear export signal (NES) near the C terminus of RanBP1 is essential to maintain this distribution. We now show that RanBP1 accumulates in nuclei of cells treated with the export inhibitor, leptomycin B, and collapse of the nucleocytoplasmic Ran:GTP gradient leads to equilibration of RanBP1 across the nuclear envelope. Low temperature prevents nuclear accumulation of RanBP1, suggesting that import does not occur via simple diffusion. Glutathione S-transferase (GST)-RanBP1(1-161), which lacks the NES, accumulates in the nucleus after cytoplasmic microinjection. In permeabilized cells, nuclear accumulation of GST-RanBP1(1-161) requires nuclear Ran:GTP but is not inhibited by a dominant interfering G19V mutant of Ran. Nuclear accumulation is enhanced by addition of exogenous karyopherins/importins or RCC1, both of which also enhance nuclear Ran accumulation. Import correlates with Ran concentration. Remarkably, an E37K mutant of RanBP1 does not import into the nuclei under any conditions tested despite the fact that it can form a ternary complex with Ran and importin beta. These data indicate that RanBP1 translocates through the pores by an active, nonclassical mechanism and requires Ran:GTP for nuclear accumulation. Shuttling of RanBP1 may function to clear nuclear pores of Ran:GTP, to prevent premature release of import cargo from transport receptors.

Nup2p is located on the nuclear side of the nuclear pore complex and coordinates Srp1p/importin-alpha export

Proteins bearing canonical nuclear localization sequences are imported into the nucleus by the importin/karyopherin-alpha/beta heterodimer. Recycling of the importin-alpha subunit to the cytoplasm requires the action of Cas, a member of the importin-beta superfamily. In the yeast Saccharomyces ceresivisiae, the essential gene CSE1 encodes a Cas homologue that exports the yeast importin-alpha protein Srp1p/Kap60p from the nucleus. In this report, we describe a role for the FXFG nucleoporin Nup2p, and possibly the related Nup1p, in the Cse1p-mediated nuclear export pathway. Yeast cells lacking Nup2p or containing a particular temperature-sensitive mutation in NUP1 accumulate Srp1p in the nucleus. Similarly, Cse1p is displaced from the nuclear rim to the nuclear interior in deltanup2 cells. We do not observe any biochemical interaction between Cse1p and Nup2p. Instead, we find that Nup2p binds directly to Srp1p. We have localized Nup2p to the interior face of the nuclear pore complex, and have shown that its N terminus is sufficient for targeting Nup2p to the pore, as well as for binding to Srp1p. Taken together, these data suggest that Nup2p is an important NPC docking site in the Srp1p export pathway.

The nuclear import of RCC1 requires a specific nuclear localization sequence receptor, karyopherin alpha3/Qip

RCC1 is the only known guanine nucleotide exchange factor for the small GTPase Ran and is normally found inside the nucleus bound to chromatin. In order to analyze in more detail the nuclear import of RCC1, we created a fusion construct in which four IgG binding domains of protein A were fused to the amino terminus of human RCC1 (pA-RCC1). Surprisingly, we found that neither Xenopus ovarian cytosol nor a mixture of recombinant import factors (karyopherin alpha2, karyopherin beta1, Ran, and p10/NTF2) were able to support the import of pA-RCC1 into the nuclei of digitonin-permeabilized cells. Both, in contrast, were capable of supporting the import of a construct containing another classical nuclear localization sequence (NLS), glutathione S-transferase-green fluorescent protein-NLS. Subsequently, we found that only one of the NLS receptors, karyopherin alpha3 (Kapalpha3/Qip), would support significant nuclear import of pA-RCC1 in permeabilized cells, while members of the other two main classes, Kapalpha1 and Kapalpha2, would not. Accordingly, in vitro binding studies revealed that only Kapalpha3 showed significant binding to RCC1 (unlike Kapalpha1 and Kapalpha2) and that this binding was dependent on the basic amino acids present in the RCC1 NLS. In addition to Kapalpha3, we found that the nuclear import of pA-RCC1 also required both karyopherin beta1 and Ran.

Recycling of importin alpha from the nucleus is suppressed by loss of RCC1 function in living mammalian cells

We previously reported that the nuclear import of substrates containing SV40 T antigen nuclear localization signal (NLS) was suppressed in a temperature-sensitive RCC1 mutant cell line, tsBN2, at nonpermissive temperature. Moreover, it was shown that import into wild type BHK21 cell-derived nuclei gradually decreased in heterokaryons between the tsBN2 and BHK21 cells, although the BHK21 nuclei retained wild type RCC1 and should contain RanGTP (Tachibana et al., 1994). In this study, it was found that in the heterokaryons cultured at non-permissive temperature, endogenous importin alpha was not detected immunocytochemically in the cytoplasm or BHK21 nuclei but only in the tsBN2 nuclei, suggesting that importin alpha cannot be exported from the RCC1-depleted nuclei. In fact, importin alpha microinjected into the nucleus of tsBN2 cells at non-permissive temperature remained in the nucleus. These results strongly support the hypothesis that the recycling of importin alpha from the nucleus requires nuclear RanGTP. Moreover, it was found that cytoplasmic injection of importin alpha restored the import of SV40 T-NLS substrates in the BHK21 nuclei but not the tsBN2 nuclei in the heterokaryons. This indicates that the decrease of importin alpha from the cytoplasm in the heterokaryons leads to a suppression of the efficiency of nuclear import of the T-NLS substrate and provides support for the view that nuclear RanGTP is essential for the nuclear entry of the substrates.

Role of molecular chaperones in subnuclear trafficking of glucocorticoid receptors

The delivery of activated steroid receptors to high-affinity genomic sites must be efficient enough to account for the rapidity and selectivity of many transcriptional responses to steroid hormones. Thus, the signal transduction capacity of steroid hormone receptors will be influenced by the efficiency of receptor trafficking both between different subcellular compartments (that is, the cytoplasm and nucleus) and within a specific compartment (that is, the nucleus). Molecular chaperones, such as heat shock proteins, have long been recognized to play important roles in the management of protein folding in both stressed and nonstressed cells. In recent years, the participation of these proteins in various signal transduction pathways (for example, steroid hormone responses) has also been recognized. In this review, recent results that implicate a role for distinct heat shock proteins in subnuclear trafficking of glucocorticoid receptors are discussed. These studies also highlight the importance of mobilizing the cellular chaperone machinery for managing steroid receptor folding within the nucleus.

Crystallographic analysis of the specific yet versatile recognition of distinct nuclear localization signals by karyopherin alpha

BACKGROUND

Karyopherin alpha (importin alpha) is an adaptor molecule that recognizes proteins containing nuclear localization signals (NLSs). The prototypical NLS that is able to bind to karyopherin alpha is that of the SV40 T antigen, and consists of a short positively charged sequence motif. Distinct classes of NLSs (monopartite and bipartite) have been identified that are only partly conserved with respect to one another but are nevertheless recognized by the same receptor.

RESULTS

We report the crystal structures of two peptide complexes of yeast karyopherin alpha (Kapalpha): one with a human c-myc NLS peptide, determined at 2.1 A resolution, and one with a Xenopus nucleoplasmin NLS peptide, determined at 2.4 A resolution. Analysis of these structures reveals the determinants of specificity for the binding of a relatively hydrophobic monopartite NLS and of a bipartite NLS peptide. The peptides bind Kapalpha in its extended surface groove, which presents a modular array of tandem binding pockets for amino acid residues.

CONCLUSIONS

Monopartite and bipartite NLSs bind to a different number of amino acid binding pockets and make different interactions within them. The relatively hydrophobic monopartite c-myc NLS binds extensively at a few binding pockets in a similar manner to that of the SV40 T antigen NLS. In contrast, the bipartite nucleoplasmin NLS engages the whole array of pockets with individually more limited but overall more abundant interactions, which include the NLS two basic clusters and the backbone of its non-conserved linker region. Versatility in the specific recognition of NLSs relies on the modular.

Nuclear import of IkappaBalpha is accomplished by a ran-independent transport pathway

The inhibitor of kappa B alpha (IkappaBalpha) protein is able to shuttle between the cytoplasm and the nucleus. We have utilized a combination of in vivo and in vitro approaches to provide mechanistic insight into nucleocytoplasmic shuttling by IkappaBalpha. IkappaBalpha contains multiple functional domains that contribute to shuttling of IkappaBalpha between the cytoplasm and the nucleus. Nuclear import of IkappaBalpha is mediated by the central ankyrin repeat domain. Similar to previously described nuclear import pathways, nuclear import of IkappaBalpha is temperature and ATP dependent and is blocked by a dominant-negative mutant of importin beta. However, in contrast to classical nuclear import pathways, nuclear import of IkappaBalpha is independent of soluble cytosolic factors and is not blocked by the dominant-negative RanQ69L protein. Nuclear export of IkappaBalpha is mediated by an N-terminal nuclear export sequence. Nuclear export of IkappaBalpha requires the CRM1 nuclear export receptor and is blocked by the dominant-negative RanQ69L protein. Our results are consistent with a model in which nuclear import of IkappaBalpha is mediated through direct interactions with components of the nuclear pore complex, while nuclear export of IkappaBalpha is mediated via a CRM1-dependent pathway.

The yeast nuclear pore complex: composition, architecture, and transport mechanism

An understanding of how the nuclear pore complex (NPC) mediates nucleocytoplasmic exchange requires a comprehensive inventory of the molecular components of the NPC and a knowledge of how each component contributes to the overall structure of this large molecular translocation machine. Therefore, we have taken a comprehensive approach to classify all components of the yeast NPC (nucleoporins). This involved identifying all the proteins present in a highly enriched NPC fraction, determining which of these proteins were nucleoporins, and localizing each nucleoporin within the NPC. Using these data, we present a map of the molecular architecture of the yeast NPC and provide evidence for a Brownian affinity gating mechanism for nucleocytoplasmic transport.

Distinct importin recognition properties of histones and chromatin assembly factors

Synthesis of the protein components of nuclear chromatin occurs in the cytoplasm, necessitating specific import into the nucleus. Here, we report the binding affinities of the nuclear localisation sequence (NLS)-binding importin subunits for a range of histones and chromatin assembly factors. The results suggest that import of histones to the nucleus may be mediated predominantly by importin beta1, whereas the import of the other components probably relies on the conventional alpha/beta1 import pathway. Differences in recognition by importin beta1 were observed between histone H2A and the variant H2AZ, as well as between histone H3/4 with or without acetylation. The results imply that different histone variants may possess distinct nuclear import properties, with acetylation possibly playing an inhibitory role through NLS masking.