NORMAL GENOMIC

• Kunzler M, Trueheart J, Sette C, Hurt E, Thorner J
• Mutations in the YRB1 gene encoding yeast ran-binding-protein-1 that impair nucleocytoplasmic transport and suppress yeast mating defects.
• Genetics. 2001; 157: 1089-105
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• We identified two temperature-sensitive (ts) mutations in the essential gene, YRB1, which encodes the yeast homolog of Ran-binding-protein-1 (RanBP1), a known coregulator of the Ran GTPase cycle. Both mutations result in single amino acid substitutions of evolutionarily conserved residues (A91D and R127K, respectively) in the Ran-binding domain of Yrb1. The altered proteins have reduced affinity for Ran (Gsp1) in vivo. After shift to restrictive temperature, both mutants display impaired nuclear protein import and one also reduces poly(A)+ RNA export, suggesting a primary defect in nucleocytoplasmic trafficking. Consistent with this conclusion, both yrb1ts mutations display deleterious genetic interactions with mutations in many other genes involved in nucleocytoplasmic transport, including SRP1 (alpha-importin) and several beta-importin family members. These yrb1ts alleles were isolated by their ability to suppress two different types of mating-defective mutants (respectively, fus1Delta and ste5ts), indicating that reduction in nucleocytoplasmic transport enhances mating proficiency. Indeed, in both yrb1ts mutants, Ste5 (scaffold protein for the pheromone response MAPK cascade) is mislocalized to the cytosol, even in the absence of pheromone. Also, both yrb1ts mutations suppress the mating defect of a null mutation in MSN5, which encodes the receptor for pheromone-stimulated nuclear export of Ste5. Our results suggest that reimport of Ste5 into the nucleus is important in downregulating mating response.

• Kutay U et al.
• Identification of two novel RanGTP-binding proteins belonging to the importin beta superfamily.
• J Biol Chem. 2000; 275: 40163-8
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• 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.

• Onuma Y, Nishihara R, Takahashi S, Tanegashima K, Fukui A, Asashima M
• Expression of the Xenopus GTP-binding protein gene Ran during embryogenesis.
• Dev Genes Evol. 2000; 210: 325-7
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• The Ran gene family encodes small GTP binding proteins that are associated with a variety of nuclear processes. We isolated a Xenopus Ran cDNA and analyzed the pattern of expression of this gene during embryogenesis. Ran is expressed maternally and later in the CNS, neural crest, mesenchyme, eyes, and otic vesicles. However, expression is not detected in the somites or the notochord.

• Steggerda SM, Paschal BM
• The mammalian Mog1 protein is a guanine nucleotide release factor for Ran.
• J Biol Chem. 2000; 275: 23175-80
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• Ran is a Ras-related GTPase that is essential for the transport of protein and RNA between the nucleus and the cytoplasm. Proteins that regulate the GTPase cycle and subcellular distribution of Ran include the cytoplasmic GTPase-activating protein (RanGAP) and its co-factors (RanBP1, RanBP2), the nuclear guanine nucleotide exchange factor (RanGEF), and the Ran import receptor (NTF2). The recent identification of the Saccharomyces cerevisiae protein Mog1p as a suppressor of temperature-sensitive Ran mutations suggests that additional regulatory proteins remain to be characterized. Here, we describe the identification and biochemical characterization of murine Mog1, which, like its yeast orthologue, is a nuclear protein that binds specifically to RanGTP. We show that Mog1 stimulates the release of GTP from Ran, indicating that Mog1 functions as a guanine nucleotide release factor in vitro. Following GTP release, Mog1 remains bound to nucleotide-free Ran in a conformation that prevents rebinding of the guanine nucleotide. These properties distinguish Mog1 from the well characterized RanGEF and suggest an unanticipated mechanism for modulating nuclear levels of RanGTP.

• Stewart M, Baker RP
• 1.9 A resolution crystal structure of the Saccharomyces cerevisiae Ran-binding protein Mog1p.
• J Mol Biol. 2000; 299: 213-23
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• The 1.9 A resolution X-ray crystal structure of Ran-binding protein Mog1p shows that it has a unique fold based on a six-stranded antiparallel beta-sheet backed on both sides by an extensive alpha-helix. The topology of some elements of Mog1p secondary structure resemble a portion of nuclear transport factor 2 (NTF2), but the hydrophobic cavity and surrounding negatively charged residues that are important in the NTF2-RanGDP interaction are not conserved in Mog1p. In addition to binding RanGTP, Mog1p forms a 1:1 complex with RanGDP and so binds Ran independent of its nucleotide state. Mog1p and NTF2 compete for binding to RanGDP indicating that their binding sites on RanGDP are sufficiently close to prevent both proteins binding simultaneously. Although there may be some overlap between the Mog1p and NTF2 binding sites on RanGDP, these sites are not identical. Sequence analysis of Mog1p homologues from Schizosaccharomyces pombe, human, and Caenorhabditis elegans in the context of the Mog1p crystal structure indicates the presence of a cluster of highly conserved surface residues consistent with an interaction site for Ran.

• Macara IG
• Nuclear transport: randy couples.
• Curr Biol. 1999; 9: 4369-4369
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• The recently solved structures of the Ran GTPase with a Ran-binding domain and with karyopherin-beta2 have revealed unusually tight embraces that provide important insights into the mechanism of nuclear transport and the many ways in which common protein folds are adapted to perform very different functions.

• Vetter IR, Nowak C, Nishimoto T, Kuhlmann J, Wittinghofer A
• Structure of a Ran-binding domain complexed with Ran bound to a GTP analogue: implications for nuclear transport.
• Nature. 1999; 398: 39-46
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• The protein Ran is a small GTP-binding protein that binds to two types of effector inside the cell: Ran-binding proteins, which have a role in terminating export processes from the nucleus to the cytoplasm, and importin-beta-like molecules that bind cargo proteins during nuclear transport. The Ran-binding domain is a conserved sequence motif found in several proteins that participate in these transport processes. The Ran-binding protein RanBP2 contains four of these domains and constitutes a large part of the cytoplasmic fibrils that extend from the nuclear-pore complex. The structure of Ran bound to a non-hydrolysable GTP analogue (Ran x GppNHp) in complex with the first Ran-binding domain (RanBD1) of human RanBP2 reveals not only that RanBD1 has a pleckstrin-homology domain fold, but also that the switch-I region of Ran x GppNHp resembles the canonical Ras GppNHp structure and that the carboxy terminus of Ran is wrapped around RanBD1, contacting a basic patch on RanBD1 through its acidic end. This molecular 'embrace' enables RanBDs to sequester the Ran carboxy terminus, triggering the dissociation of Ran x GTP from importin-beta-related transport factors and facilitating GTP hydrolysis by the GTPase-activating protein ranGAP. Such a mechanism represents a new type of switch mechanism and regulatory protein-protein interaction for a Ras-related protein.

• Nachury MV, Weis K
• The direction of transport through the nuclear pore can be inverted.
• Proc Natl Acad Sci U S A. 1999; 96: 9622-7
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• Transport of macromolecules across the nuclear envelope is an active process that depends on soluble factors including the GTPase Ran. Ran-GTP is predominantly located in the nucleus and has been shown to regulate cargo binding and release of import and export receptors in their respective target compartments. Recently, it was shown that transport of receptor-cargo complexes across the nuclear pore complex (NPC) does not depend on GTP-hydrolysis by Ran; however, the mechanism of translocation is still poorly understood. Here, we show that the direction of transport through the NPC can be inverted in the presence of high concentrations of cytoplasmic Ran-GTP. Under these conditions, two different classes of export cargoes are transported into the nucleus in the absence of GTP hydrolysis. The inverted transport is very rapid and can be blocked by known inhibitors of nuclear protein export. These results suggest that the NPC functions as a facilitated transport channel, allowing the selective translocation of receptor-cargo complexes. We conclude that the directionality of nucleocytoplasmic transport is determined mainly by the compartmentalized distribution of Ran-GTP.

• Hillig RC, Renault L, Vetter IR, Drell T 4th, Wittinghofer A, Becker J
• The crystal structure of rna1p: a new fold for a GTPase-activating protein.
• Mol Cell. 1999; 3: 781-91
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• rna1p is the Schizosaccharomyces pombe ortholog of the mammalian GTPase-activating protein (GAP) of Ran. Both proteins are essential for nuclear transport. Here, we report the crystal structure of rna1p at 2.66 A resolution. It contains 11 leucine-rich repeats that adopt the nonglobular shape of a crescent, bearing no resemblance to RhoGAP or RasGAP. The invariant residues of RanGAP form a contiguous surface, strongly indicating the Ran-binding interface. Alanine mutations identify Arg-74 as a critical residue for GTP hydrolysis. In contrast to RasGAP and RhoGAP, Arg-74 could be substituted by lysine and contributed significantly to the binding of Ran. Therefore, we suggest a GAP mechanism for rna1p, which constitutes a variation of the arginine finger mechanism found for Ras GAP and RhoGAP.

• Black BE, Levesque L, Holaska JM, Wood TC, Paschal BM
• Identification of an NTF2-related factor that binds Ran-GTP and regulates nuclear protein export.
• Mol Cell Biol. 1999; 19: 8616-24
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• Active transport of macromolecules between the nucleus and cytoplasm requires signals for import and export and their recognition by shuttling receptors. Each class of macromolecule is thought to have a distinct receptor that mediates the transport reaction. Assembly and disassembly reactions of receptor-substrate complexes are coordinated by Ran, a GTP-binding protein whose nucleotide state is regulated catalytically by effector proteins. Ran function is modulated in a noncatalytic fashion by NTF2, a protein that mediates nuclear import of Ran-GDP. Here we characterize a novel component of the Ran system that is 26% identical to NTF2, which based on its function we refer to as NTF2-related export protein 1 (NXT1). In contrast to NTF2, NXT1 preferentially binds Ran-GTP, and it colocalizes with the nuclear pore complex (NPC) in mammalian cells. These properties, together with the fact that NXT1 shuttles between the nucleus and the cytoplasm, suggest an active role in nuclear transport. Indeed, NXT1 stimulates nuclear protein export of the NES-containing protein PKI in vitro. The export function of NXT1 is blocked by the addition of leptomycin B, a compound that selectively inhibits the NES receptor Crm1. Thus, NXT1 regulates the Crm1-dependent export pathway through its direct interaction with Ran-GTP.

• Vetter IR, Arndt A, Kutay U, Gorlich D, Wittinghofer A
• Structural view of the Ran-Importin beta interaction at 2.3 A resolution.
• Cell. 1999; 97: 635-46
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• Transport receptors of the Importin beta family shuttle between the nucleus and cytoplasm and mediate transport of macromolecules through nuclear pore complexes. They interact specifically with the GTP-binding protein Ran, which in turn regulates their interaction with cargo. Here, we report the three-dimensional structure of a complex between Ran bound to the nonhydrolyzable GTP analog GppNHp and a 462-residue fragment from Importin beta. The structure of Importin beta shows 10 tandem repeats resembling HEAT and Armadillo motifs. They form an irregular crescent, the concave site of which forms the interface with Ran-triphosphate. The importin-binding site of Ran does not overlap with that of the Ran-binding domain of RanBP2.

• Melchior F, Gerace L
• Two-way trafficking with Ran.
• Trends Cell Biol. 1998; 8: 175-9
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• The small Ras-related GTPase Ran is directly involved in nuclear protein import and export. However, the question of how Ran functions in transport is highly controversial. Here, we suggest that Ran is important for the formation, vectorial movement and disassembly of many different classes of transport complexes that traverse the nuclear pore complex during import and export processes. Comparison of Ran with the translation elongation factor Ef-Tu raises the possibility that Ran might also be involved in a proofreading function related to the assembly of import complexes. Although aspects of this model are hypothetical and challenge some current dogma in the field, we believe that it can integrate most of the current data into a coherent picture of the import process.

• Dasso M, Pu RT
• Nuclear transport: run by Ran?
• Am J Hum Genet. 1998; 63: 311-6

• Senger B, Simos G, Bischoff FR, Podtelejnikov A, Mann M, Hurt E
• Mtr10p functions as a nuclear import receptor for the mRNA-binding protein Npl3p.
• EMBO J. 1998; 17: 2196-207
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• MTR10, previously shown to be involved in mRNA export, was found in a synthetic lethal relationship with nucleoporin NUP85. Green fluorescent protein (GFP)-tagged Mtr10p localizes preferentially inside the nucleus, but a nuclear pore and cytoplasmic distribution is also evident. Purified Mtr10p forms a complex with Npl3p, an RNA-binding protein that shuttles in and out of the nucleus. In mtr10 mutants, nuclear uptake of Npl3p is strongly impaired at the restrictive temperature, while import of a classic nuclear localization signal (NLS)-containing protein is not. Accordingly, the NLS within Npl3p is extended and consists of the RGG box plus a short and non-repetitive C-terminal tail. Mtr10p interacts in vitro with Gsp1p-GTP, but with low affinity. Interestingly, Npl3p dissociates from Mtr10p only by incubation with Ran-GTP plus RNA. This suggests that Npl3p follows a distinct nuclear import pathway and that intranuclear release from its specific import receptor Mtr10p requires the cooperative action of both Ran-GTP and newly synthesized mRNA.

• Schlenstedt G et al.
• Yrb4p, a yeast ran-GTP-binding protein involved in import of ribosomal protein L25 into the nucleus.
• EMBO J. 1997; 16: 6237-49
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• Gsp1p, the essential yeast Ran homologue, is a key regulator of transport across the nuclear pore complex (NPC). We report the identification of Yrb4p, a novel Gsp1p binding protein. The 123 kDa protein was isolated from Saccharomyces cerevisiae cells and found to be related to importin-beta, the mediator of nuclear localization signal (NLS)-dependent import into the nucleus, and to Pse1p. Like importin-beta, Yrb4p and Pse1p specifically bind to Gsp1p-GTP, protecting it from GTP hydrolysis and nucleotide exchange. The GTPase block of Gsp1p complexed to Yrb4p or Pse1p is released by Yrb1p, which contains a Gsp1p binding domain distinct from that of Yrb4p. This might reflect an in vivo function for Yrb1p. Cells disrupted for YRB4 are defective in nuclear import of ribosomal protein L25, but show no defect in the import of proteins containing classical NLSs. Expression of a Yrb4p mutant deficient in Gsp1p-binding is dominant-lethal and blocks bidirectional traffic across the NPC in wild-type cells. L25 binds to Yrb4p and Pse1p and is released by Gsp1p-GTP. Consistent with its putative role as an import receptor for L25-like proteins, Yrb4p localizes to the cytoplasm, the nucleoplasm and the NPC.

• Lounsbury KM, Macara IG
• Ran-binding protein 1 (RanBP1) forms a ternary complex with Ran and karyopherin beta and reduces Ran GTPase-activating protein (RanGAP) inhibition by karyopherin beta.
• J Biol Chem. 1997; 272: 551-5
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• The nuclear accumulation of proteins containing nuclear localization signals requires the Ran GTPase and a complex of proteins assembled at the nuclear pore. RanBP1 is a cytosolic Ran-binding protein that inhibits RCC1-stimulated release of GTP from Ran. RanBP1 also promotes the binding of Ran to karyopherin beta (also called importin beta and p97) and is a co-stimulator of RanGAP activity. Yeast karyopherin beta inhibits the GTP hydrolysis by Ran catalyzed by RanGAP. To further define the roles of RanBP1 and karyopherin beta in Ran function, we explored the effects of RanBP1 and karyopherin beta on mammalian proteins known to regulate Ran. Like RanBP1, karyopherin beta prevented the release of GTP from Ran stimulated by RCC1 or EDTA. As with the yeast protein, mammalian karyopherin beta completely blocked RanGAP activity. However, the addition of RanBP1 to this assay partially rescued the inhibited RanGAP activity. Kinetic analysis of the effects on RanGAP activity by karyopherin beta and RanBP1 revealed a combination of competitive and noncompetitive interactions. Solution binding assays confirmed the ability of RanBP1 to associate with Ran and karyopherin beta in a ternary complex, and RanBP1 binding was not competed out by the addition of karyopherin beta. These results demonstrate that RanBP1 and karyopherin beta interact with distinct sites of Ran and suggest that RanBP1 plays an essential role in nuclear transport by permitting RanGAP-mediated hydrolysis of GTP on Ran complexed to karyopherin beta.

• Haberland J, Becker J, Gerke V
• The acidic C-terminal domain of rna1p is required for the binding of Ran.GTP and for RanGAP activity.
• J Biol Chem. 1997; 272: 24717-26
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• The small GTP binding protein Ran is an essential component of the nuclear protein import machinery whose GTPase cycle is regulated by the nuclear guanosine nucleotide exchange factor RCC1 and by the cytosolic GTPase activating protein RanGAP. In the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae the RanGAP activity is encoded by the RNA1 genes which are essential for cell viability and nucleocytoplasmic transport in vivo. Although of limited sequence identity the two yeast proteins show a conserved structural organization characterized by an N-terminal domain of eight leucine-rich repeats, motifs implicated in protein-protein interactions, and a C-terminal domain rich in acidic amino acid residues. By analyzing the RanGAP activity of a series of recombinantly expressed rna1p mutant derivatives, we show that the highly acidic sequence in the C-terminal domain of both yeast proteins is indispensable for activating Ran-mediated GTP hydrolysis. Chemical cross-linking reveals that the same sequence in rna1p is required for rna1p.Ran complex formation indicating that the loss of GAP activity in the C-terminally truncated rna1p mutants results from an impaired interaction with Ran. The predominant species stabilized through the covalent cross-link is a rna1p.Ran heterodimer whose formation requires the GTP-bound conformation of Ran. As the acidic C-terminal domain of rna1p is required for establishing the interaction with Ran, the leucine-rich repeats domain in rna1p is potentially available for additional protein interactions perhaps required for directing a fraction of rna1p to the nuclear pore.

• Deane R et al.
• Ran-binding protein 5 (RanBP5) is related to the nuclear transport factor importin-beta but interacts differently with RanBP1.
• Mol Cell Biol. 1997; 17: 5087-96
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• We report the identification and characterization of a novel 124-kDa Ran binding protein, RanBP5. This protein is related to importin-beta, the key mediator of nuclear localization signal (NLS)-dependent nuclear transport. RanBP5 was identified by two independent methods: it was isolated from HeLa cells by using its interaction with RanGTP in an overlay assay to monitor enrichment, and it was also found by the yeast two-hybrid selection method with RanBP1 as bait. RanBP5 binds to RanBP1 as part of a trimeric RanBP1-Ran-RanBP5 complex. Like importin-beta, RanBP5 strongly binds the GTP-bound form of Ran, stabilizing it against both intrinsic and RanGAP1-induced GTP hydrolysis and also against nucleotide exchange. The GAP resistance of the RanBP5-RanGTP complex can be relieved by RanBP1, which might reflect an in vivo role for RanBP1. RanBP5 is a predominantly cytoplasmic protein that can bind to nuclear pore complexes. We propose that RanBP5 is a mediator of a nucleocytoplasmic transport pathway that is distinct from the importin-alpha-dependent import of proteins with a classical NLS.

• Nakielny S, Dreyfuss G
• Nuclear export of proteins and RNAs.
• Curr Opin Cell Biol. 1997; 9: 420-9
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• Our understanding of protein export from the nucleus to the cytoplasm has been advanced recently by the discovery of active, signal-mediated export pathways. Nuclear export signals have been identified in several proteins, the majority of which are RNA-binding proteins. Nuclear export of RNA molecules is likely to be driven by protein-based nuclear export signals.

• Haizel T, Merkle T, Pay A, Fejes E, Nagy F
• Characterization of proteins that interact with the GTP-bound form of the regulatory GTPase Ran in Arabidopsis.
• Plant J. 1997; 11: 93-103
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• Ran, a small soluble GTP-binding protein, has been shown to be essential for the nuclear translocation of proteins and it is also thought to be involved in regulating cell cycle progression in mammalian and yeast cells. Genes encoding Ran-like proteins have been isolated from different higher plant species. Overexpression of plant Ran cDNAs, similarly to their mammalian/yeast homologues, suppresses the phenotype of the pim46-1 cell cycle mutant in yeast cells. The mammalian/yeast Ran proteins have been shown to interact with a battery of Ran-binding proteins, including the guanidine nucleotide exchange factor RCC1, the GTPase-activating Ran-GAP, nucleoporins and other Ran-binding proteins (RanBPs) specific for Ran-GTP. Here, the characterization of the first Ran-binding proteins from higher plants is reported. The yeast two-hybrid system was used to isolate cDNA clones encoding proteins of approximately 28 kDa (At-RanBP1a, At-RanBP1b) that interact with the GTP-bound forms of the Ran1, Ran2 and Ran3 proteins of Arabidopsis thaliana. The deduced amino acid sequences of the At-RanBP1s display high similarity (60%) to mammalian/yeast RanBP1 proteins and contain the characteristic Ran-binding domains. Furthermore, interaction of the plant Ran and RanBP1 proteins, is shown to require the acidic C-terminal domain (-DEDDDL) of Ran proteins in addition to the presence of an intact Ran-binding domain. In whole cell extracts, the GST-RanBP1a fusion protein binds specifically to GTP-Ran and will not interact with Rab/Ypt-type small GTP-binding proteins. Finally, in good agreement with their proposed biological function, the At-Ran and the At-RanBP genes are expressed coordinately and show the highest level of expression in meristematic tissues.

• Paschal BM, Fritze C, Guan T, Gerace L
• High levels of the GTPase Ran/TC4 relieve the requirement for nuclear protein transport factor 2.
• J Biol Chem. 1997; 272: 21534-9
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• The GTPase Ran/TC4 and the 14-kDa protein nuclear transport factor 2 (NTF2) are two of the cytosolic factors that mediate nuclear protein import in vertebrates. Previous biochemical studies have shown that NTF2 binds directly to the GDP-bound form of Ran/TC4 and to proteins of the nuclear pore complex that contain phenylalanine-glycine repeats. In the present study we have used molecular genetic approaches to study the Saccharomyces cerevisiae homologue of NTF2. The scNTF2 gene encodes a protein that is 44% identical to the human protein. We found that deletion of the scNTF2 gene is lethal and that repression of NTF2p expression by a regulatable promoter results in gross structural distortions of the nuclear envelope. In a screen for high copy number suppressors of a scNTF2 deletion, the only gene we isolated other than scNTF2 itself was GSP1, the S. cerevisiae homologue of Ran/TC4. Furthermore, we found that high levels of Ran/TC4 can relieve the requirement for NTF2 in a mammalian-permeabilized cell assay for nuclear protein import. These data suggest that certain of the nuclear protein import functions of NTF2 and Ran/TC4 are closely linked and that NTF2 may serve to modulate a transport step involving Ran/TC4.

• Lounsbury KM, Richards SA, Perlungher RR, Macara IG
• Ran binding domains promote the interaction of Ran with p97/beta-karyopherin, linking the docking and translocation steps of nuclear import.
• J Biol Chem. 1996; 271: 2357-60
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• Nuclear protein import is accomplished by two sequential events; docking at the nuclear pore complex followed by ATP-dependent translocation across the nuclear envelope. Docking of nuclear targeted proteins requires a 56-kDa nuclear localization signal receptor (alpha-karyopherin, importin-alpha, SRP1 alpha) and a 97-kDa protein (beta-karyopherin, importin-beta). Components necessary for translocation include the Ran/TC4 GTPase and NTF2/B-2. The functions of these factors at a molecular level remain unclear. We have now found that a complex of Ran, in the GTP-bound state, with either the Ran binding protein, RanBP1, or an isolated Ran binding domain binds with high affinity and specificity to beta-karyopherin to form a ternary complex. We find that a C-terminal truncation mutant of Ran, delta-DE Ran, also binds to beta-karyopherin and that delta-DE Ran can associate with a cytosolic, multiprotein complex that contains beta-karyopherin and another delta-DE Ran binding protein of 115/120 kDa. These data suggest a physical link between docking and translocation mediated by a Ran GTPase-Ran binding protein complex.

• Noguchi E et al.
• Dis3, implicated in mitotic control, binds directly to Ran and enhances the GEF activity of RCC1.
• EMBO J. 1996; 15: 5595-605
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• Using the two-hybrid method, we isolated a Saccharomyces cerevisiae cDNA encoding a protein homologous to Schizosaccharomyces pombe protein Dis3sp, using as bait, human GTPase Ran. The DIS3 gene is essential for viability and complements S.pombe mutant dis3-54 which is defective in mitosis. Although Dis3sc has no homology to RanBP1, it bound directly to Ran and the S.cerevisiae Ran homologue Cnr1, but not to the S.cerevisiae RCC1 homologue Srm1. Upon binding to Ran with a 1:1 molar ratio, Dis3sc enhanced a nucleotide-releasing activity of RCC1 on Ran. In the presence of Dis3sc, the K(m) of RCC1 on Ran decreased by half, while the kcat was unchanged. In vivo, Dis3sp was present as oligomers of M(r) 670-200 kDa as previously reported, and the 200 kDa oligomer of Dis3sp was found to include Spi1 and Pim1, the S.pombe homologues of Ran and RCC1, respectively. Although the biological function of the heterotrimeric oligomer consisting of Dis3, Spi1 and Pim1 is unknown, our results indicate that Dis3 is a component of the RCC1-Ran pathway.

• Moore MS
• Generation of GTP-Ran for nuclear protein import.
• Science. 1996; 272: 47-47

• Seki T, Hayashi N, Nishimoto T
• RCC1 in the Ran pathway.
• J Biochem (Tokyo). 1996; 120: 207-14
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• RCC1 is a chromosomal protein that functions as a GEF of the nuclear G protein Ran, which GTPase activity is enhanced by RNA1 located in the cytoplasm. RCC1 has no preference for GTP or GDP-bound Ran, so that GTP-Ran formation in vivo is regulated by relative concentrations of GTP/GDP and regulatory proteins interacting with RCC1, Ran, and RNA1. Proteins possessing the special Ran-binding motif have been found to be conserved in species ranging from yeasts to mammalians. The finding of RanBP2/NUP358 clearly indicates the involvement of the Ran pathway in the nuclear pore transport function, in agreement with the finding that both rcc1- and rna1- show defects in this process. However, loss of RCC1 induces premature initiation of mitosis, resulting in G1 arrest with the micronuclei possessing mitotic condensed chromosomes. How both the cell cycle and nucleocytoplasmic transport are regulated by the RCC1-Ran pathway is a major question.

• Gorlich D, Mattaj IW
• Nucleocytoplasmic transport.
• Science. 1996; 271: 1513-8
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• Active transport of proteins and RNAs between the nucleus and cytoplasm is a major process in eukaryotic cells. Recently, factors that recognize transport substrates and mediate nuclear import or export have been characterized, revealing interactions that target substrates to the nuclear pore complexes, through which translocation occurs. Translocation requires energy, and for the import process this energy is at least partly consumed by the action of the small guanosine triphosphatase Ran. In the first half of the review, some of the well-established general background information on nucleocytoplasmic transport is discussed. The second half describes recent information on the mechanistic details of nuclear import and export as well as major unresolved issues such as how directionality is conferred on either import or export. The whole review is slanted toward discussion of metazoan cells.

• Melchior F, Guan T, Yokoyama N, Nishimoto T, Gerace L
• GTP hydrolysis by Ran occurs at the nuclear pore complex in an early step of protein import.
• J Cell Biol. 1995; 131: 571-81
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• Mediated import of proteins into the nucleus involves multiple cytosolic factors, including the small GTPase Ran. Whether Ran functions by interacting with other cytosolic proteins or components of the nuclear pore complex has been unclear. Furthermore, the precise transport step where Ran acts has not been determined. To address these questions, we have analyzed the binding interactions of Ran using permeabilized cells and isolated nuclear envelopes. By light and electron microscope immunolocalization, we have found that Ran accumulates specifically at the cytoplasmic surface of the nuclear pore complex when nuclear import in permeabilized cells is inhibited by nonhydrolyzable analogs of GTP. Ran associates with a peripheral pore complex region that is similar to the area where transport ligands accumulate by depletion of ATP, which arrests an early step of transport. Binding studies with isolated nuclear envelopes in the absence of added cytosol indicate that Ran-GTP directly interacts with a pore complex protein. Using blot overlay techniques, we detected a single prominent polypeptide of isolated nuclear envelopes that binds Ran-GTP. This corresponds to the 358-kD protein RanBP2, a Ran binding pore complex protein recently identified by two-hybrid screening. Thus, RanBP2 is likely to constitute the Ran-GTP-binding site detected at the cytoplasmic periphery of the pore complex. These data support a model in which initial ligand binding to the nuclear pore complex occurs at or near RanBP2, and that hydrolysis of GTP by Ran at this site serves to define commitment to the nuclear import pathway.

• Ouspenski II, Mueller UW, Matynia A, Sazer S, Elledge SJ, Brinkley BR
• Ran-binding protein-1 is an essential component of the Ran/RCC1 molecular switch system in budding yeast.
• J Biol Chem. 1995; 270: 1975-8
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• We have performed a screen for genes that affect chromosome stability when overexpressed in the budding yeast Saccharomyces cerevisiae. Two of the genes recovered in the screen, CST17 and CST20, share a number of phenotypic properties, suggesting their involvement in the same cellular process. DNA sequence analysis of these genes revealed that they encode components of the Ran/RCC1 molecular switch system: CST17 is Ran itself (Ras-like nuclear protein) and CST20 is a novel yeast protein with a high degree of similarity to mammalian RanBP1, which is known to interact with Ran-GTP in vitro. We demonstrate that the CST20 protein can interact with Ran-GTP in vitro under similar conditions, indicating that it is the functional yeast homolog of mammalian RanBP1. The results of immunoprecipitation experiments show that the two yeast proteins form a complex in vivo. Deletion of the gene encoding RanBP1 revealed that it is essential for viability, as are Ran and RCC1. Similar phenotypic consequences of overproduction of either Ran or RanBP1 indicate that the latter protein is a functional component of the Ran/RCC1 molecular switch system, which is implicated in the control of a number of nuclear functions. Our finding that overproduction of two components of this system results in mitotic chromosome nondisjunction and sensitivity to an anti-microtubule drug benomyl suggest their involvement in mitosis as well. Thus RanBP1 is a functional component of a highly conserved molecular system that affects diverse cellular processes. The availability of this gene in S. cerevisiae provides a genetic system for the analysis of RanBP1 function in vivo.

• Hayashi N, Yokoyama N, Seki T, Azuma Y, Ohba T, Nishimoto T
• RanBP1, a Ras-like nuclear G protein binding to Ran/TC4, inhibits RCC1 via Ran/TC4.
• Mol Gen Genet. 1995; 247: 661-9
• Display abstract

• A human protein that is 92% identical and 97% homologous at the amino acid level to RanBP1 from mouse was identified by the two-hybrid method, using two types of target cDNAs fused to sequences encoding the GAL4 DNA-binding domain. The target cDNAs encoded the human Ran/TC4 and human RCC1 proteins, respectively. An in vitro binding experiment showed that RanBP1 binds to RCC1 with the aid of Ran. Partially purified, GST-fused RanBP1 inhibited RCC1-stimulated guanine nucleotide release from Ran in vitro. Consistent with this in vitro finding, overproduction of human RanBP1 was detrimental to growth of tsBN2, a temperature-sensitive BHK21 hamster cell line defective in the RCC1 gene, and inhibited the growth of the Saccharomyces cerevisiae rcc1 mutants prp20, mtr1 and srm1. The specific effect of RanBP1 on rcc1- cells was confirmed by the finding that overproduction of RanBP1 induces significant levels of expression of a FUS1-lacZ gene and an increase in mating efficiencies in a ste3, pheromone receptor-deficient yeast mutant. This phenotype is similar to the srm1, a mutant isolated as a suppressor that restores mating to receptorless mutants. These findings indicate that RanBP1 negatively regulates RCC1.

• Yokoyama N et al.
• A giant nucleopore protein that binds Ran/TC4.
• Nature. 1995; 376: 184-8
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• Ran/TC4 is a small nuclear G protein that forms a complex with the chromatin-bound guanine nucleotide release factor RCC1 (ref. 2). Loss of RCC1 causes defects in cell cycle progression, RNA export and nuclear protein import. Some of these can be suppressed by overexpression of Ran/TC4 (ref. 1), suggesting that Ran/TC4 functions downstream of RCC1. We have searched for proteins that bind Ran/TC4 by using a two-hybrid screen, and here we report the identification of RanBP2, a novel protein of 3,224 residues. This giant protein comprises an amino-terminal 700-residue leucine-rich region, four RanBP1-homologous (refs 9, 10) domains, eight zinc-finger motifs similar to those of NUP153 (refs 11, 12), and a carboxy terminus with high homology to cyclophilin. The molecule contains the XFXFG pentapeptide motif characteristic of nuclear pore complex (NPC) proteins, and immunolocalization suggests that RanBP2 is a constituent of the NPC. The fact that NLS-mediated nuclear import can be inhibited by an antibody directed against RanBP2 supports a functional role in protein import through the NPC.

• Schlenstedt G, Saavedra C, Loeb JD, Cole CN, Silver PA
• The GTP-bound form of the yeast Ran/TC4 homologue blocks nuclear protein import and appearance of poly(A)+ RNA in the cytoplasm.
• Proc Natl Acad Sci U S A. 1995; 92: 225-9
• Display abstract

• Ran/TC4, a Ras-like GTP-binding protein, and its nucleotide exchanger, RCC1, have been implicated in control of protein movement into the nucleus and cytoplasmic accumulation of mRNA. Saccharomyces cerevisiae contains two homologues of the mammalian Ran/TC4, encoded by the GSP1 and GSP2 genes. We have constructed yeast strains that overproduce either wild-type Gsp1 or a form of Gsp1 with glycine-21 converted to valine (Gsp1-G21V), which we show stabilizes the GTP-bound form. Cells producing Gsp1-G21V have defects in localization of nuclear proteins; nuclear proteins accumulate in the cytoplasm following galactose induction of Gsp1-G21V. Similarly, cells producing Gsp1-G21V retain poly(A)+ RNA in their nuclei. These findings suggest that hydrolysis of GTP by Ran/TC4 is necessary for proper import of proteins into the nucleus and appearance of poly(A)+ RNA in the cytoplasm.

• Becker J, Melchior F, Gerke V, Bischoff FR, Ponstingl H, Wittinghofer A
• RNA1 encodes a GTPase-activating protein specific for Gsp1p, the Ran/TC4 homologue of Saccharomyces cerevisiae.
• J Biol Chem. 1995; 270: 11860-5
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• Ran/TC4 is a ras-related GTP-binding protein predominantly located in the nucleus. Ran/TC4 is essential for nuclear transport and is involved in mitotic control. In Saccharomyces cerevisiae a gene highly homologous to Ran/TC4 has been identified and named GSP1. Like all ras-related GTP-binding proteins, Gsp1p undergoes cycles of GTP hydrolysis and GDP/GTP exchange. The switching between the two different nucleotide bound states regulates the function of these GTP-binding proteins. Here we identify the product of the yeast RNA1 gene as the GTPase-activating protein (GAP) of Gsp1p. RNA1 belongs to a group of genes which are conserved in a variety of different organisms. We have expressed and purified recombinant Gsp1p and Rna1p from Escherichia coli. The GTPase activity of Gsp1p is stimulated 10(7)-fold by Rna1p. In addition, we find that the previously identified human RanGAP1 and rna1p from Schizosaccharomyces pombe are also able to induce GTPase activity of Gsp1p. The GTP hydrolysis of Ran is induced by RanGAP1 and rna1p but not by Rna1p. Implications for the suggested functions of Ran/TC4/Gsp1p in nuclear transport and mitotic control are discussed.

• Goldfarb DS
• Protein translocation. GTPase cycle for nuclear transport.
• Curr Biol. 1994; 4: 57-60
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• The discovery that the small Ras-like GTPase, Ran, is required for the transport of proteins into the nucleus suggests a general mechanism for protein import.

• Merkle T, Haizel T, Matsumoto T, Harter K, Dallmann G, Nagy F
• Phenotype of the fission yeast cell cycle regulatory mutant pim1-46 is suppressed by a tobacco cDNA encoding a small, Ran-like GTP-binding protein.
• Plant J. 1994; 6: 555-65
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• Mutations in which the onset of mitosis is uncoupled from the completion of DNA replication has recently been described. Characterization of these mutants led to the identification of Pim1/Spi1 in fission yeast and RCC1/Ran proteins in mammalian cells. Their Saccharomyces cerevisae homologues, the MTR1/CNR1 proteins, appear to be involved in controlling RNA metabolism and transport. Here the isolation and partial characterization of plant cDNA clones which encode proteins homologous to the mammalian/fission yeast/budding yeast Ran/Spi/CNR proteins are reported. Higher plants appear to contain more than one gene per haploid genome which codes for Ran proteins. These genes are expressed in different plant tissues, including root tips and stems, known to contain mitotically active cells. The tobacco Ran-like proteins, like their mammalian and yeast homologues, are soluble proteins which are found in the cytoplasm and in the nucleus. In addition, it has been shown that overexpression of the tobacco Nt-Ran-A1 cDNA suppressed the phenotype of the temperature-sensitive fission yeast pim1-46 mutant. These results suggest that the plant Ran genes can be functionally equivalent to the mammalian/fission yeast/budding yeast Ran/Spi/CNR genes and that they may play a role: (i) in maintaining a coordinated cell cycle; (ii) in controlling RNA metabolism and transport in higher plants; and/or (iii) in protein import into the nucleus.

• Bush J, Cardelli J
• Molecular cloning and DNA sequence of a Dictyostelium cDNA encoding a Ran/TC4 related GTP binding protein belonging to the ras superfamily.
• Nucleic Acids Res. 1993; 21: 1675-1675

• Dupree P, Olkkonen VM, Chavrier P
• Sequence of a canine cDNA clone encoding a Ran/TC4 GTP-binding protein.
• Gene. 1992; 120: 325-6
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• We report the isolation and characterization of a canine cDNA encoding a 216-amino acid GTP-binding protein of the Ras superfamily. The protein is almost identical to the human TC4 [Drivas et al., Mol. Cell. Biol. 10 (1990) 1793-1798] and Ran [Bischoff and Ponstingl, Proc. Natl. Acad. Sci. USA 88 (1991) 10830-10834; Nature 354 (1991) 80-82] proteins, the latter of which has been found to be involved in cell cycle control. Furthermore, the protein is highly similar to the fission yeast spi1 gene product [Matsumoto and Beach, Cell 66 (1991) 347-360]. The high degree of evolutionary conservation in this protein suggests that it plays a vital role in the eukaryotic cell.