Yrb4p, a yeast ran-GTP-binding protein involved in import of ribosomal protein L25 into the nucleus

CRM1-mediated recycling of snurportin 1 to the cytoplasm

Importin beta is a major mediator of import into the cell nucleus. Importin beta binds cargo molecules either directly or via two types of adapter molecules, importin alpha, for import of proteins with a classical nuclear localization signal (NLS), or snurportin 1, for import of m3G-capped U snRNPs. Both adapters have an NH2-terminal importin beta-binding domain for binding to, and import by, importin beta, and both need to be returned to the cytoplasm after having delivered their cargoes to the nucleus. We have shown previously that CAS mediates export of importin alpha. Here we show that snurportin 1 is exported by CRM1, the receptor for leucine-rich nuclear export signals (NESs). However, the interaction of CRM1 with snurportin 1 differs from that with previously characterized NESs. First, CRM1 binds snurportin 1 50-fold stronger than the Rev protein and 5,000-fold stronger than the minimum Rev activation domain. Second, snurportin 1 interacts with CRM1 not through a short peptide but rather via a large domain that allows regulation of affinity. Strikingly, snurportin 1 has a low affinity for CRM1 when bound to its m3G-capped import substrate, and a high affinity when substrate-free. This mechanism appears crucial for productive import cycles as it can ensure that CRM1 only exports snurportin 1 that has already released its import substrate in the nucleus.

Characterization of IkappaBalpha nuclear import pathway

IkappaBalpha controls the transcriptional activity of nuclear factor (NF)-kappaB by retaining it in the cytoplasm; but, when expressed in the nucleus, it can also inhibit the interaction of NF-kappaB with DNA and promote the export of NF-kappaB from the nucleus to the cytoplasm. Here, we report that IkappaBalpha, when not bound to NF-kappaB, is constitutively transported to the nucleus, and we confirm that the interaction of IkappaBalpha with NF-kappaB retains IkappaBalpha in the cytoplasm. Nuclear import of IkappaBalpha does not result from passive diffusion but from a specific energy-dependent transport process that requires the ankyrin repeats of IkappaBalpha. Nuclear accumulation of IkappaBalpha is dependent on importins alpha and beta as well as the small GTPase Ran, which are also responsible for the nuclear import mediated by basic nuclear localization sequences (NLS). However, these proteins are not sufficient to promote IkappaBalpha nuclear translocation. Factor(s) can be removed selectively from cell extracts with ankyrin repeats of IkappaBalpha which strongly reduce import of IkappaBalpha but not of proteins containing basic NLS. These findings indicate that IkappaBalpha is imported in the nucleus by a piggy-back mechanism that involves additional protein(s) containing a basic NLS and able to interact with ankyrin repeats of IkappaBalpha.

Overexpression of yeast karyopherin Pse1p/Kap121p stimulates the mitochondrial import of hydrophobic proteins in vivo

During evolution, cellular processes leading to the transfer of genetic information failed to send all the mitochondrial genes into the nuclear genome. Two mitochondrial genes are still exclusively located in the mitochondrial genome of all living organisms. They code for two highly hydrophobic proteins: the apocytochrome b and the subunit I of cytochrome oxidase. Assuming that the translocation machinery could not efficiently transport long hydrophobic fragments, we searched for multicopy suppressors of this physical blockage. We demonstrated that overexpression of Pse1p/Kap121p or Kap123p, which belong to the superfamily of karyopherin beta proteins, facilitates the translocation of chimeric proteins containing several stretches of apocytochrome b fused to a reporter mitochondrial gene. The effect of PSE1/KAP121 overexpression (in which PSE1 is protein secretion enhancer 1) on mitochondrial import of the chimera is correlated with an enrichment of the corresponding transcript in cytoplasmic ribosomes associated with mitochondria. PSE1/KAP121 overexpression also improves the import of the hydrophobic protein Atm1p, an ABC transporter of the mitochondrial inner membrane. These results suggest that in vivo PSE1/KAP121 overexpression facilitates, either directly or indirectly, the co-translational import of hydrophobic proteins into mitochondria.

A novel in vivo assay reveals inhibition of ribosomal nuclear export in ran-cycle and nucleoporin mutants

To identify components involved in the nuclear export of ribosomes in yeast, we developed an in vivo assay exploiting a green fluorescent protein (GFP)-tagged version of ribosomal protein L25. After its import into the nucleolus, L25-GFP assembles with 60S ribosomal subunits that are subsequently exported into the cytoplasm. In wild-type cells, GFP-labeled ribosomes are only detected by fluorescence in the cytoplasm. However, thermosensitive rna1-1 (Ran-GAP), prp20-1 (Ran-GEF), and nucleoporin nup49 and nsp1 mutants are impaired in ribosomal export as revealed by nuclear accumulation of L25-GFP. Furthermore, overexpression of dominant-negative RanGTP (Gsp1-G21V) and the tRNA exportin Los1p inhibits ribosomal export. The pattern of subnuclear accumulation of L25-GFP observed in different mutants is not identical, suggesting that transport can be blocked at different steps. Thus, nuclear export of ribosomes requires the nuclear/cytoplasmic Ran-cycle and distinct nucleoporins. This assay can be used to identify soluble transport factors required for nuclear exit of ribosomes.

Interactions between a nuclear transporter and a subset of nuclear pore complex proteins depend on Ran GTPase

Proteins to be transported into the nucleus are recognized by members of the importin-karyopherin nuclear transport receptor family. After docking at the nuclear pore complex (NPC), the cargo-receptor complex moves through the aqueous pore channel. Once cargo is released, the importin then moves back through the channel for new rounds of transport. Thus, importin and exportin, another member of this family involved in export, are thought to continuously shuttle between the nuclear interior and the cytoplasm. In order to understand how nuclear transporters traverse the NPC, we constructed functional protein fusions between several members of the yeast importin family, including Pse1p, Sxm1p, Xpo1p, and Kap95p, and the green fluorescent protein (GFP). Complexes containing nuclear transporters were isolated by using highly specific anti-GFP antibodies. Pse1-GFP was studied in the most detail. Pse1-GFP is in a complex with importin-alpha and -beta (Srp1p and Kap95p in yeast cells) that is sensitive to the nucleotide-bound state of the Ran GTPase. In addition, Pse1p associates with the nucleoporins Nsp1p, Nup159p, and Nup116p, while Sxm1p, Xpo1p, and Kap95p show different patterns of interaction with nucleoporins. Association of Pse1p with nucleoporins also depends on the nucleotide-bound state of Ran; when Ran is in the GTP-bound state, the nucleoporin association is lost. A mutant form of Pse1p that does not bind Ran also fails to interact with nucleoporins. These data indicate that transport receptors such as Pse1p interact in a Ran-dependent manner with certain nucleoporins. These nucleoporins may represent major docking sites for Pse1p as it moves in or out of the nucleus via the NPC.

Antagonistic effects of NES and NLS motifs determine S. cerevisiae Rna1p subcellular distribution

Nucleus/cytosol exchange requires a GTPase, Ran. In yeast Rna1p is the GTPase activating protein for Ran (RanGAP) and Prp20p is the Ran GDP/GTP exchange factor (GEF). RanGAP is primarily cytosolic and GEF is nuclear. Their subcellular distributions led to the prediction that Ran-GTP hydrolysis takes place solely in the cytosol and GDP/GTP exchange solely in the nucleus. Current models propose that the Ran-GTP/Ran-GDP gradient across the nuclear membrane determines the direction of exchange. We provide three lines of evidence that Rna1p enters and leaves the nuclear interior. (1) Rna1p possesses leucine-rich nuclear export sequences (NES) that are able to relocate a passenger karyophilic protein to the cytosol; alterations of consensus residues re-establish nuclear location. (2) Rna1p possesses other sequences that function as a novel nuclear localization sequence able to deliver a passenger cytosolic protein to the nucleus. (3) Endogenous Rna1p location is dependent upon Xpo1p/Crm1p, the yeast exportin for leucine-rich NES-containing proteins. The data support the hypothesis that Rna1p exists on both sides of the nuclear membrane, perhaps regulating the Ran-GTP/Ran-GDP gradient, participating in a complete RanGTPase nuclear cycle or serving a novel function.

Receptor-mediated substrate translocation through the nuclear pore complex without nucleotide triphosphate hydrolysis

BACKGROUND

The transport of macromolecules between the nucleus and cytoplasm is an energy-dependent process. Substrates are translocated across the nuclear envelope through nuclear pore complexes (NPCs). Translocation requires nucleocytoplasmic transport receptors of the importin beta family, which interact both with the NPC and, either directly or via an adaptor, with the transport substrate. Although certain receptors have recently been shown to cross the NPC in an energy-independent manner, translocation of substrate-receptor complexes through the NPC has generally been regarded as an energy-requiring step.

RESULTS

We describe an in vitro system that is based on permeabilised cells and supports nuclear export mediated by leucine-rich nuclear export signals. In this system, export is dependent on exogenous CRM1/Exportin1 - a nuclear export receptor - the GTPase Ran and nucleotide triphosphates (NTPs), and is further stimulated by Ran-binding protein 1 (RanBP1) and nuclear transport factor 2 (NTF2). Unexpectedly, non-hydrolysable NTP analogues completely satisfy the NTP requirements for a single-round of CRM1-mediated translocation of protein substrates across the NPC. Similarly, single transportin-mediated nuclear protein import events are shown not to require hydrolysable NTPs and to occur in the absence of the Ran GTPase.

CONCLUSIONS

Our data show that, contrary to expectation and prior conclusions, the translocation of substrate-receptor complexes across the NPC in either direction occurs in the absence of NTP hydrolysis and is thus energy independent. The energy needed to drive substrate transport against a concentration gradient is supplied at the step of receptor recycling in the cytoplasm.

Specific binding of the karyopherin Kap121p to a subunit of the nuclear pore complex containing Nup53p, Nup59p, and Nup170p

We have identified a specific karyopherin docking complex within the yeast nuclear pore complex (NPC) that contains two novel, structurally related nucleoporins, Nup53p and Nup59p, and the NPC core protein Nup170p. This complex was affinity purified from cells expressing a functional Nup53p-protein A chimera. The localization of Nup53p, Nup59p, and Nup170p within the NPC by immunoelectron microscopy suggests that the Nup53p-containing complex is positioned on both the cytoplasmic and nucleoplasmic faces of the NPC core. In association with the isolated complex, we have also identified the nuclear transport factor Kap121p (Pse1p). Using in vitro binding assays, we showed that each of the nucleoporins interacts with one another. However, the association of Kap121p with the complex is mediated by its interaction with Nup53p. Moreover, Kap121p is the only beta-type karyopherin that binds Nup53p suggesting that Nup53p acts as a specific Kap121p docking site. Kap121p can be released from Nup53p by the GTP bound form of the small GTPase Ran. The physiological relevance of the interaction between Nup53p and Kap121p was further underscored by the observation that NUP53 mutations alter the subcellular distribution of Kap121p and the Kap121p- mediated import of a ribosomal L25 reporter protein. Interestingly, Nup53p is specifically phosphorylated during mitosis. This phenomenon is correlated with a transient decrease in perinuclear-associated Kap121p.

Cse1p is required for export of Srp1p/importin-alpha from the nucleus in Saccharomyces cerevisiae

In metazoan cells, the CAS protein has been shown to function as a recycling factor for the importin-alpha subunit of the classical nuclear localization signal receptor, exporting importin-alpha from the nucleus to allow its participation in multiple rounds of nuclear import. CAS is a member of a family of proteins that bear homology to the larger subunit of the nuclear localization signal receptor, importin-beta, and that are found in all eukaryotes from yeast to humans. Sequence similarity identifies the product of the Saccharomyces cerevisiae CSE1 gene as a potential CAS homologue. Here we present evidence that Cse1p is the functional homologue of CAS: Cse1p is required to prevent accumulation of Srp1p/importin-alpha in the nucleus, it localizes to the nuclear envelope in a pattern typical of nuclear transport receptors, and it associates in vivo with Srp1p in a nucleotide-specific manner. We show further that mutations in CSE1 and SRP1 have specific effects on their association and on the intracellular localization of Cse1p.

NTF2 mediates nuclear import of Ran

Importin beta family transport receptors shuttle between the nucleus and the cytoplasm and mediate transport of macromolecules through nuclear pore complexes (NPCs). The interactions between these receptors and their cargoes are regulated by binding RanGTP; all receptors probably exit the nucleus complexed with RanGTP, and so should deplete RanGTP continuously from the nucleus. We describe here the development of an in vitro system to study how nuclear Ran is replenished. Nuclear import of Ran does not rely on simple diffusion as Ran's small size would permit, but instead is stimulated by soluble transport factors. This facilitated import is specific for cytoplasmic RanGDP and employs nuclear transport factor 2 (NTF2) as the actual carrier. NTF2 binds RanGDP initially to NPCs and probably also mediates translocation of the NTF2-RanGDP complex to the nuclear side of the NPCs. A direct NTF2-RanGDP interaction is crucial for this process, since point mutations that disturb the RanGDP-NTF2 interaction also interfere with Ran import. The subsequent nuclear accumulation of Ran also requires GTP, but not GTP hydrolysis. The release of Ran from NTF2 into the nucleus, and thus the directionality of Ran import, probably involves nucleotide exchange to generate RanGTP, for which NTF2 has no detectable affinity, followed by binding of the RanGTP to an importin beta family transport receptor.

Characterization of HIV-1 vpr nuclear import: analysis of signals and pathways

While the Vpr protein of HIV-1 has been implicated in import of the viral preintegration complex across the nuclear pore complex (NPC) of nondividing cellular hosts, the mechanism by which Vpr enters the nucleus remains unknown. We now demonstrate that Vpr contains two discrete nuclear targeting signals that use two different import pathways, both of which are distinct from the classical nuclear localization signal (NLS)- and the M9-dependent pathways. Vpr import does not appear to require Ran-mediated GTP hydrolysis and persists under conditions of low energy. Competition experiments further suggest that Vpr directly engages the NPC at two discrete sites. These sites appear to form distal components of a common import pathway used by NLS- and M9-containing proteins. Together, our data suggest that Vpr bypasses many of the soluble receptors involved in import of cellular cargoes. Rather, this viral protein appears to directly access the NPC, a property that may help to ensure the capacity of HIV to replicate in nondividing cellular hosts.

Cse1p is involved in export of yeast importin alpha from the nucleus

Proteins bearing a nuclear localization signal (NLS) are targeted to the nucleus by the heterodimeric transporter importin. Importin alpha binds to the NLS and to importin beta, which carries it through the nuclear pore complex (NPC). Importin disassembles in the nucleus, evidently by binding of RanGTP to importin beta. The importin subunits are exported separately. We investigated the role of Cse1p, the Saccharomyces cerevisiae homologue of human CAS, in nuclear export of Srp1p (yeast importin alpha). Cse1p is located predominantly in the nucleus but also is present in the cytoplasm and at the NPC. We analyzed the in vivo localization of the importin subunits fused to the green fluorescent protein in wild-type and cse1-1 mutant cells. Srp1p but not importin beta accumulated in nuclei of cse1-1 mutants, which are defective in NLS import but not defective in NLS-independent import pathways. Purified Cse1p binds with high affinity to Srp1p only in the presence of RanGTP. The complex is dissociated by the cytoplasmic RanGTP-binding protein Yrb1p. Combined with the in vivo results, this suggests that a complex containing Srp1p, Cse1p, and RanGTP is exported from the nucleus and is subsequently disassembled in the cytoplasm by Yrb1p. The formation of the trimeric Srp1p-Cse1p-RanGTP complex is inhibited by NLS peptides, indicating that only NLS-free Srp1p will be exported to the cytoplasm.

tRNA nuclear export in saccharomyces cerevisiae: in situ hybridization analysis

To understand the factors specifically affecting tRNA nuclear export, we adapted in situ hybridization procedures to locate endogenous levels of individual tRNA families in wild-type and mutant yeast cells. Our studies of tRNAs encoded by genes lacking introns show that nucleoporin Nup116p affects both poly(A) RNA and tRNA export, whereas Nup159p affects only poly(A) RNA export. Los1p is similar to exportin-t, which facilitates vertebrate tRNA export. A los1 deletion mutation affects tRNA but not poly(A) RNA export. The data support the notion that Los1p and exportin-t are functional homologues. Because LOS1 is nonessential, tRNA export in vertebrate and yeast cells likely involves factors in addition to exportin-t. Mutation of RNA1, which encodes RanGAP, causes nuclear accumulation of tRNAs and poly(A) RNA. Many yeast mutants, including those with the rna1-1 mutation, affect both pre-tRNA splicing and RNA export. Our studies of the location of intron-containing pre-tRNAs in the rna1-1 mutant rule out the possibility that this results from tRNA export occurring before splicing. Our results also argue against inappropriate subnuclear compartmentalization causing defects in pre-tRNA splicing. Rather, the data support "feedback" of nucleus/cytosol exchange to the pre-tRNA splicing machinery.

Yeast Los1p has properties of an exportin-like nucleocytoplasmic transport factor for tRNA

Saccharomyces cerevisiae Los1p, which is genetically linked to the nuclear pore protein Nsp1p and several tRNA biogenesis factors, was recently grouped into the family of importin/karyopherin-beta-like proteins on the basis of its sequence similarity. In a two-hybrid screen, we identified Nup2p as a nucleoporin interacting with Los1p. Subsequent purification of Los1p from yeast demonstrates its physical association not only with Nup2p but also with Nsp1p. By the use of the Gsp1p-G21V mutant, Los1p was shown to preferentially bind to the GTP-bound form of yeast Ran. Furthermore, overexpression of full-length or N-terminally truncated Los1p was shown to have dominant-negative effects on cell growth and different nuclear export pathways. Finally, Los1p could interact with Gsp1p-GTP, but only in the presence of tRNA, as revealed in an indirect in vitro binding assay. These data confirm the homology between Los1p and the recently identified human exportin for tRNA and reinforce the possibility of a role for Los1p in nuclear export of tRNA in yeast.

Nucleocytoplasmic transport: the soluble phase

Active transport between the nucleus and cytoplasm involves primarily three classes of macromolecules: substrates, adaptors, and receptors. Some transport substrates bind directly to an import or an export receptor while others require one or more adaptors to mediate formation of a receptor-substrate complex. Once assembled, these transport complexes are transferred in one direction across the nuclear envelope through aqueous channels that are part of the nuclear pore complexes (NPCs). Dissociation of the transport complex must then take place, and both adaptors and receptors must be recycled through the NPC to allow another round of transport to occur. Directionality of either import or export therefore depends on association between a substrate and its receptor on one side of the nuclear envelope and dissociation on the other. The Ran GTPase is critical in generating this asymmetry. Regulation of nucleocytoplasmic transport generally involves specific inhibition of the formation of a transport complex; however, more global forms of regulation also occur.

Phosphorylation regulates association of the transcription factor Pho4 with its import receptor Pse1/Kap121

The transcription factor Pho4 is phosphorylated and localized predominantly to the cytoplasm when budding yeast are grown in phosphate-rich medium and is unphosphorylated and localized to the nucleus upon phosphate starvation. We have investigated the requirements for nuclear import of Pho4 and find that Pho4 enters the nucleus via a nonclassical import pathway that utilizes the importin beta family member Pse1/Kap121. Pse1 binds directly to Pho4 and is required for its import in vivo. We have defined the nuclear localization signal on Pho4 and demonstrate that it is required for Pse1 binding in vitro and is sufficient for PSE1-dependent import in vivo. Phosphorylation of Pho4 inhibits its interaction with Pse1, providing a mechanism by which phosphorylation may regulate import of Pho4 in vivo.

Cse1p functions as the nuclear export receptor for importin alpha in yeast

CSE1 is essential for yeast cell viability and has been implicated in chromosome segregation. Based on its sequence similarity, Cse1p has been grouped into the family of importin beta-like nucleocytoplasmic transport receptors with highest homology to the recently identified human nuclear export receptor for importin alpha, CAS. We demonstrate here that Cse1p physically interacts with yeast Ran and yeast importin alpha (Srp1p) in the yeast two-hybrid system and that recombinant Cse1p, Srp1p and Ran-GTP form a trimeric complex in vitro. Re-export of Srp1p from the nucleus into the cytoplasm and nuclear uptake of a reporter protein containing a classical NLS are inhibited in a cse1 mutant strain. These findings suggest that Cse1p is the exportin of importin alpha in yeast.

Importin beta, transportin, RanBP5 and RanBP7 mediate nuclear import of ribosomal proteins in mammalian cells

The assembly of eukaryotic ribosomal subunits takes place in the nucleolus and requires nuclear import of ribosomal proteins. We have studied this import in a mammalian system and found that the classical nuclear import pathway using the importin alpha/beta heterodimer apparently plays only a minor role. Instead, at least four importin beta-like transport receptors, namely importin beta itself, transportin, RanBP5 and RanBP7, directly bind and import ribosomal proteins. We found that the ribosomal proteins L23a, S7 and L5 can each be imported alternatively by any of the four receptors. We have studied rpL23a in detail and identified a very basic region to which each of the four import receptors bind avidly. This domain might be considered as an archetypal import signal that evolved before import receptors diverged in evolution. The presence of distinct binding sites for rpL23a and the M9 import signal in transportin, and for rpL23a and importin alpha in importin beta might explain how a single receptor can recognize very different import signals.

Snurportin1, an m3G-cap-specific nuclear import receptor with a novel domain structure

The nuclear import of the spliceosomal snRNPs U1, U2, U4 and U5, is dependent on the presence of a complex nuclear localization signal (NLS). The latter is composed of the 5'-2,2,7-terminal trimethylguanosine (m3G) cap structure of the U snRNA and the Sm core domain. Here, we describe the isolation and cDNA cloning of a 45 kDa protein, termed snurportin1, which interacts specifically with m3G-cap but not m7G-cap structures. Snurportin1 enhances the m3G-capdependent nuclear import of U snRNPs in both Xenopus laevis oocytes and digitonin-permeabilized HeLa cells, demonstrating that it functions as an snRNP-specific nuclear import receptor. Interestingly, solely the m3G-cap and not the Sm core NLS appears to be recognized by snurportin1, indicating that at least two distinct import receptors interact with the complex snRNP NLS. Snurportin1 represents a novel nuclear import receptor which contains an N-terminal importin beta binding (IBB) domain, essential for function, and a C-terminal m3G-cap-binding region with no structural similarity to the arm repeat domain of importin alpha.

A member of the Ran-binding protein family, Yrb2p, is involved in nuclear protein export

Yeast cells mutated in YRB2, which encodes a nuclear protein with similarity to other Ran-binding proteins, fail to export nuclear export signal (NES)-containing proteins including HIV Rev out of the nucleus. Unlike Xpo1p/Crm1p/exportin, an NES receptor, Yrb2p does not shuttle between the nucleus and the cytoplasm but instead remains inside the nucleus. However, by both biochemical and genetic criteria, Yrb2p interacts with Xpo1p and not with other members of the importin/karyopherin beta superfamily. Moreover, the Yrb2p region containing nucleoporin-like FG repeats is important for NES-mediated protein export. Taken together, these data suggest that Yrb2p acts inside the nucleus to mediate the action of Xpo1p in at least one of several nuclear export pathways.

Nuclear import and export of viruses and virus genomes

Many viruses replicate in the nucleus of their animal and plant host cells. Nuclear import, export, and nucleo-cytoplasmic shuttling play a central role in their replication cycle. Although the trafficking of individual virus proteins into and out of the nucleus has been well studied for some virus systems, the nuclear transport of larger entities such as viral genomes and capsids has only recently become a subject of molecular analysis. In this review, the general concepts emerging are discussed and a survey is provided of current information on both plant and animal viruses. Summarizing the main findings in this emerging field, it is evident that most viruses that enter or exit the nucleus take advantage of the cell's nuclear import and export machinery. With a few exceptions, viruses seem to cross the nuclear envelope through the nuclear pore complexes, making use of cellular nuclear import and export signals, receptors, and transport factors. In many cases, they capitalize on subtle control systems such as phosphorylation that regulate traffic of cellular components into and out of the nucleus. The large size of viral capsids and their composition (they contain large RNA and DNA molecules for which there are few precedents in normal nuclear transport) make the processes unique and complicated. Prior capsid disassembly (or deformation) is required before entry of viral genomes and accessory proteins can occur through nuclear pores. Capsids of different virus families display diverse uncoating programs which culminate in genome transfer through the nuclear pores.

Transport routes through the nuclear pore complex

The nuclear pore complex can be considered to be the stationary phase of bidirectional traffic between the nucleus and the cytoplasm. The mobile phase consists of karyopherins, transport substrates, and the small GTPase Ran and its modulators. Recently, the family of karyopherins was expanded with the recognition of numerous open reading frames with limited homology to karyopherin beta 1. In several cases, the specific substrates transported by the new karyopherins have been identified, allowing the characterization of new pathways into and out of the nucleus. However, the mechanisms of transport, particularly the role of Ran, remain poorly understood.

Human RanBP3, a group of nuclear RanGTP binding proteins

A group of novel human Ran-binding proteins, RanBP3, was identified using the yeast two-hybrid system via Ran-mediated interaction with the nucleotide exchange factor RCC1. Several open reading frames, representing putative alternatively spliced products, were established by cDNA cloning. Two of them, RanBP3-a and RanBP3-b, encode nuclear hydrophilic proteins of 499 and 562 amino acid residues. The sequences contain FXFG motifs, characteristic of a subgroup of nucleoporins, and a C-terminal domain showing similarity to the Ran-binding protein RanBP1. These proteins are localized in the nucleus, preferentially bind RanGTP and may be nuclear effectors of the Ran pathway.

Karyopherins and kissing cousins

In eukaryotic cells, a regulated flux of molecules between the cytoplasm and the nucleus maintains two very different environments while allowing the controlled exchange of macromolecules necessary for their individual functions. Molecules entering or leaving the nucleus use nuclear localization signals or nuclear export signals to pass through selective channels in the nuclear envelope formed by nuclear pore complexes. The recognition of signal-bearing cargo, its interaction with the nuclear pore complex and its translocation through the pore complex are mediated by soluble transport factors. Recently, the list of potential transport factors has grown rapidly, suggesting a previously unanticipated level of complexity for nuclear transport.

Importins and exportins: how to get in and out of the nucleus

Every minute, several million protein and RNA molecules must be transported between the cytoplasm and the nucleus of a eukaryotic cell. The characterization of mediators, receptors and accessory factors for different nuclear import and export pathways has provided a glimpse at the molecular machinery that is responsible for these trafficking events. It appears that both inbound and outbound traffic is mediated by a protein family of related transport factors that can be classified as importins and exportins.

Mtr10p functions as a nuclear import receptor for the mRNA-binding protein Npl3p

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.

Transport of macromolecules between the nucleus and the cytoplasm

Nuclear transport is an energy-dependent process mediated by saturable receptors. Import and export receptors are thought to recognize and bind to nuclear localization signals or nuclear export signals, respectively, in the transported molecules. The receptor-substrate interaction can be direct or mediated by an additional adapter protein. The transport receptors dock their cargoes to the nuclear pore complexes (NPC) and facilitate their translocation through the NPC. After delivering their cargoes, the receptors are recycled to initiate additional rounds of transport. Because a transport event for a cargo molecule is unidirectional, the transport receptors engage in asymmetric cycles of translocation across the NPC. The GTPase Ran acts as a molecular switch for receptor-cargo interaction and imparts directionality to the transport process. Recently, the combined use of different in vitro and in vivo approaches has led to the characterization of novel import and export signals and to the identification of the first nuclear import and export receptors.

Identification of a nuclear export receptor for tRNA

BACKGROUND

Transport of macromolecules between the nucleus and cytoplasm of eukaryotic cells is mediated by nuclear import and export receptors. The receptors identified to date are members of a family of Ran GTPase-binding proteins whose founding member is importin-beta. Interaction between these receptors and their cargo is regulated by the GTP-bound form of Ran. Export complexes form and import complexes disassemble on binding of RanGTP to the receptor. Yeast Los 1 p is a member of the importin-beta family with a poorly defined role in tRNA production.

RESULTS

A human member of the importin-beta family that is distantly related to Los 1 p (21% identity) has been characterized. The protein shuttled between the nucleus and cytoplasm and interacts with tRNA in a RanGTP-dependent manner. Injection of the protein into the nuclei of Xenopus oocytes resulted in a specific stimulation of the export of tRNA from the nucleus and in relief of the competitive inhibition of tRNA export caused by the introduction of saturating amounts of nuclear tRNA.

CONCLUSIONS

The human protein has the functional properties expected of a transport receptor that mediates export of tRNA from the nucleus. We therefore name the protein Exportin(tRNA).

Identification of a tRNA-specific nuclear export receptor

In eukaryotes, tRNAs are synthesized in the nucleus and after several maturation steps exported to the cytoplasm. Here, we identify exportin-t as a specific mediator of tRNA export. It is a RanGTP-binding, importin beta-related factor with predominantly nuclear localization. It shuttles rapidly between nucleus and cytoplasm and interacts with nuclear pore complexes. Exportin-t binds tRNA directly and with high affinity. Its cellular concentration in Xenopus oocytes was found to be rate-limiting for export of all tRNAs tested, as judged by microinjection experiments. RanGTP regulates the substrate-exportin-t interaction such that tRNA can be preferentially bound in the nucleus and released in the cytoplasm.

RanBP1 is crucial for the release of RanGTP from importin beta-related nuclear transport factors

Nucleocytoplasmic transport appears mediated by shuttling transport receptors that bind RanGTP as a means to regulate interactions with their cargoes. The receptor-RanGTP complexes are kinetically very stable with nucleotide exchange and GTP hydrolysis being blocked, predicting that a specific disassembly mechanism exists. Here we show in three cases receptor RanGTP x RanBP1 complexes to be the key disassembly intermediates, where RanBP1 stimulates the off-rate at the receptor/RanGTP interface by more than two orders of magnitude. The transiently released RanGTP x RanBP1 complex is then induced by RanGAP to hydrolyse GTP, preventing the receptor to rebind RanGTP. The efficient release of importin beta from RanGTP requires importin alpha, in addition to RanBP1.