A mutant form of the Ran/TC4 protein disrupts nuclear function in Xenopus laevis egg extracts by inhibiting the RCC1 protein, a regulator of chromosome condensation

A mechanism of coupling RCC1 mobility to RanGTP production on the chromatin in vivo

The RanGTP gradient across the interphase nuclear envelope and on the condensed mitotic chromosomes is essential for many cellular processes, including nucleocytoplasmic transport and spindle assembly. Although the chromosome-associated enzyme RCC1 is responsible for RanGTP production, the mechanism of generating and maintaining the RanGTP gradient in vivo remains unknown. Here, we report that regulator of chromosome condensation (RCC1) rapidly associates and dissociates with both interphase and mitotic chromosomes in living cells, and that this mobility is regulated during the cell cycle. Our kinetic modeling suggests that RCC1 couples its catalytic activity to chromosome binding to generate a RanGTP gradient. Indeed, we have demonstrated experimentally that the interaction of RCC1 with the chromatin is coupled to the nucleotide exchange on Ran in vivo. The coupling is due to the stable binding of the binary complex of RCC1-Ran to chromatin. Successful nucleotide exchange dissociates the binary complex, permitting the release of RCC1 and RanGTP from the chromatin and the production of RanGTP on the chromatin surface.

Chromosomal association of Ran during meiotic and mitotic divisions

Recent studies in Xenopus egg extracts indicate that the small G protein Ran has a central role in spindle assembly and nuclear envelope reformation. We determined Ran localization and dynamics in cells during M phase. By immunofluorescence, Ran is accumulated on the chromosomes of meiosis-II-arrested Xenopus eggs. In living cells, fluorescently labeled Ran associated with the chromosomes in Xenopus and remained associated during anaphase when eggs were artificially activated. Fluorescent Ran associated with chromosomes in mouse eggs, during meiotic maturation and early embryonic divisions in starfish, and to a lesser degree during mitosis of a cultured mammalian cell line. Chromosomal Ran undergoes constant flux. From photobleach experiments in immature starfish oocytes, chromosomal Ran has a k(off) of approximately 0.06 second(-1), and binding analysis suggests that there is a single major site. The chromosomal interactions may serve to keep Ran-GTP in the vicinity of the chromosomes for spindle assembly and nuclear envelope reformation.

Concentration of Ran on chromatin induces decondensation, nuclear envelope formation and nuclear pore complex assembly

Nuclear envelope (NE) formation can be studied in a cell-free system made from Xenopus eggs. In this system, NE formation involves the small GTPase Ran. Ran associates with chromatin early in nuclear assembly and concentration of Ran on inert beads is sufficient to induce NE formation. Here, we show that Ran binds to chromatin prior to NE formation and recruits RCC1, the nucleotide exchange factor that generates Ran-GTP. In extracts prepared by high-speed centrifugation, increased concentrations of Ran are sufficient to induce chromatin decondensation and NE assembly. Using field emission in-lens scanning electron microscopy (FEISEM), we show that Ran promotes the formation of smoothed membranes and the assembly of nuclear pore complexes (NPCs). In contrast, RanT24N, a mutant that fails to bind GTP and inhibits RCC1, does not support efficient NE assembly, whereas RanQ69L, a mutant locked in a GTP-bound state, permits some membrane vesicle recruitment to chromatin, but inhibits vesicle fusion and NPC assembly. Thus, binding of Ran to chromatin, followed by local generation of Ran-GTP and GTP hydrolysis by Ran, induces chromatin decondensation, membrane vesicle recruitment, membrane formation and NPC assembly. We propose that the biological activity of Ran is determined by its targeting to structures such as chromatin as well as its guanine nucleotide bound state.

Geminin becomes activated as an inhibitor of Cdt1/RLF-B following nuclear import

During late mitosis and early interphase, origins of replication become "licensed" for DNA replication by loading Mcm2-7 complexes. Mcm2-7 complexes are removed from origins as replication forks initiate replication, thus preventing rereplication of DNA in a single cell cycle. Premature origin licensing is prevented in metaphase by the action of geminin, which binds and inhibits Cdt1/RLF-B, a protein that is required for the loading of Mcm2-7. Recombinant geminin that is added to Xenopus egg extracts is efficiently degraded upon exit from metaphase. Here, we show that recombinant and endogenous forms of Xenopus geminin behave differently from one another, such that a significant proportion of endogenous geminin escapes proteolysis upon exit from metaphase. During late mitosis and early G1, the surviving population of endogenous geminin does not associate with Cdt1/RLF-B and does not inhibit licensing. Following nuclear assembly, geminin is imported into nuclei and becomes reactivated to bind Cdt1/RLF-B. This reactivated geminin provides the major nucleoplasmic inhibitor of origin relicensing during late interphase. Since the initiation of replication at licensed origins depends on nuclear assembly, our results suggest an elegant and novel mechanism for preventing rereplication of DNA in a single cell cycle.

Ran GTPase: a master regulator of nuclear structure and function during the eukaryotic cell division cycle?

Ran is an abundant GTPase that is highly conserved in eukaryotic cells and has been implicated in many aspects of nuclear structure and function, especially determining the directionality of nucleocytoplasmic transport during interphase. However, cell-free systems have recently shown that Ran plays distinct roles in mitotic spindle assembly and nuclear envelope (NE) formation in vitro. During spindle assembly, Ran controls the formation of complexes with importins, the same effectors that control nucleocytoplasmic transport. Here, we review these advances and discuss a general model for Ran in the coordination of nuclear processes throughout the cell division cycle via common biochemical mechanisms.

XMog1, a nuclear Ran-binding protein in Xenopus, is a functional homologue of Schizosaccharomyces pombe Mog1p that co-operates with RanBP1 to control generation of Ran-GTP

Ran is a multifunctional small GTPase of the Ras superfamily that plays roles in nucleocytoplasmic transport, mitotic spindle assembly and nuclear envelope formation. By screening a Xenopus oocyte cDNA library for Ran-GTP-binding proteins using the two-hybrid system of co-expression in yeast, we identified XMog1, a 20.4 kDa polypeptide related to Mog1p in Saccharomyces cerevisiae and similar gene products in Schizosaccharomyces pombe, Arabidopsis and mammals. We show that cDNAs encoding XMog1 and S. cerevisiae Mog1p rescue the growth defect of S. pombe cells lacking mog1, demonstrating conservation of their functions. In Xenopus somatic cells and transfected mammalian cells, XMog1 is localised to the nucleus. XMog1 alone does not stimulate Ran GTPase activity or nucleotide exchange, but causes nucleotide release from Ran-GTP and forms a complex with nucleotide-free Ran. However, in combination with Ran-binding protein 1 (RanBP1), XMog1 promotes the release of GDP and the selective binding of GTP to Ran. XMog1 and RanBP1 also promote selective GTP loading onto Ran catalysed by the nuclear guanine nucleotide exchange factor, RCC1. We propose that Mog1-related proteins, together with RanBP1, facilitate the generation of Ran-GTP from Ran-GDP in the nucleus.

Characterization of the nuclear import pathway for HIV-1 integrase

The karyophilic properties of the human immunodeficiency virus, type I (HIV-1) pre-integration complex (PIC) allow the virus to infect non-dividing cells. To better understand the mechanisms responsible for nuclear translocation of the PIC, we investigated nuclear import of HIV-1 integrase (IN), a PIC-associated viral enzyme involved in the integration of the viral genome in the host cell DNA. Accumulation of HIV-1 IN into nuclei of digitonin-permeabilized cells does not result from passive diffusion but rather from an active transport that occurs through the nuclear pore complexes. HIV-1 IN is imported by a saturable mechanism, implying that a limiting cellular factor is responsible for this process. Although IN has been previously proposed to contain classical basic nuclear localization signals, we found that nuclear accumulation of IN does not involve karyopherins alpha, beta1, and beta2-mediated pathways. Neither the non-hydrolyzable GTP analog, guanosine 5'-O-(thiotriphosphate), nor the GTP hydrolysis-deficient Ran mutant, RanQ69L, significantly affects nuclear import of IN, which depends instead on ATP hydrolysis. Therefore these results support the idea that IN import is not mediated by members of the karyopherin beta family. More generally, in vitro nuclear import of IN does not require addition of cytosolic factors, suggesting that cellular factor(s) involved in this active but atypical pathway process probably remain associated with the nuclear compartment or the nuclear pore complexes from permeabilized cells.

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.

The Ketel(D) dominant-negative mutations identify maternal function of the Drosophila importin-beta gene required for cleavage nuclei formation

The Ketel(D) dominant female-sterile mutations and their ketel(r) revertant alleles identify the Ketel gene, which encodes the importin-beta (karyopherin-beta) homologue of Drosophila melanogaster. Embryogenesis does not commence in the Ketel(D) eggs deposited by the Ketel(D)/+ females due to failure of cleavage nuclei formation. When injected into wild-type cleavage embryos, cytoplasm of the Ketel(D) eggs does not inhibit nuclear protein import but prevents cleavage nuclei formation following mitosis. The Ketel(+) transgenes slightly reduce effects of the Ketel(D) mutations. The paternally derived Ketel(D) alleles act as recessive zygotic lethal mutations: the Ketel(D)/- hemizygotes, like the ketel(r)/ketel(r) and the ketel(r)/- zygotes, perish during second larval instar. The Ketel maternal dowry supports their short life. The Ketel(D)-related defects originate most likely following association of the Ketel(D)-encoded mutant molecules with a maternally provided partner. As in the Ketel(D) eggs, embryogenesis does not commence in eggs of germline chimeras with ketel(r)/- germline cells and normal soma, underlining the dominant-negative nature of the Ketel(D) mutations. The ketel(r) homozygous clones are fully viable in the follicle epithelium in wings and tergites. The Ketel gene is not expressed in most larval tissues, as revealed by the expression pattern of a Ketel promoter-lacZ reporter gene.

RanGTP-binding protein NXT1 facilitates nuclear export of different classes of RNA in vitro

To better characterize the mechanisms responsible for RNA export from the nucleus, we developed an in vitro assay based on the use of permeabilized HeLa cells. This new assay supports nuclear export of U1 snRNA, tRNA, and mRNA in an energy- and Xenopus extract-dependent manner. U1 snRNA export requires a 5' monomethylated cap structure, the nuclear export signal receptor CRM1, and the small GTPase Ran. In contrast, mRNA export does not require the participation of CRM1. We show here that NXT1, an NTF2-related protein that binds directly to RanGTP, strongly stimulates export of U1 snRNA, tRNA, and mRNA. The ability of NXT1 to promote export is dependent on its capacity to bind RanGTP. These results support the emerging view that NXT1 is a general export factor, functioning on both CRM1-dependent and CRM1-independent pathways of RNA export.

GTP hydrolysis by Ran is required for nuclear envelope assembly

Nuclear formation in Xenopus egg extracts requires cytosol and is inhibited by GTP gamma S, indicating a requirement for GTPase activity. Nuclear envelope (NE) vesicle fusion is extensively inhibited by GTP gamma S and two mutant forms of the Ran GTPase, Q69L and T24N. Depletion of either Ran or RCC1, the exchange factor for Ran, from the assembly reaction also inhibits this step of NE formation. Ran depletion can be complemented by the addition of Ran loaded with either GTP or GDP but not with GTP gamma S. RCC1 depletion is only complemented by RCC1 itself or by RanGTP. Thus, generation of RanGTP by RCC1 and GTP hydrolysis by Ran are both required for the extensive membrane fusion events that lead to NE formation.

Chromatin-independent nuclear envelope assembly induced by Ran GTPase in Xenopus egg extracts

The nuclear envelope (NE) forms a controlled boundary between the cytoplasm and the nucleus of eukaryotic cells. To facilitate investigation of mechanisms controlling NE assembly, we developed a cell-free system made from Xenopus laevis eggs to study the process in the absence of chromatin. NEs incorporating nuclear pores were assembled around beads coated with the guanosine triphosphatase Ran, forming pseudo-nuclei that actively imported nuclear proteins. NE assembly required the cycling of guanine nucleotides on Ran and was promoted by RCC1, a nucleotide exchange factor recruited to beads by Ran-guanosine diphosphate (Ran-GDP). Thus, concentration of Ran-GDP followed by generation of Ran-GTP is sufficient to induce NE assembly.

Spindles get the ran around

Despite its fundamental role in cell division, the mitotic spindle remains an enigmatic figure in cell biology. This is due to the complex dynamic behaviour of microtubules, which form the spindle fibres responsible for segregating chromosomes to opposite ends of the cell during mitosis. Recent reports indicate that the small GTPase Ran, which plays a key role in nuclear transport, also has a role in mitosis by regulating microtubule nucleation and/or growth. The race is now on to determine how Ran exerts its effects on spindle assembly.

Domains of Crm1 involved in the formation of the Crm1, RanGTP, and leucine-rich nuclear export sequences trimeric complex

Nuclear export of proteins containing a leucine-rich nuclear export sequence (NES) is mediated by a specific NES receptor known as Crm1. This protein, which is related to the karyopherin beta family, interacts directly with NES in a RanGTP-dependent manner. To characterize the domains of Crm1 involved in formation of the trimeric Crm1-NES-RanGTP complex, N- and C-terminal deletion mutants of Crm1 were generated and their ability to bind NES and RanGTP in vitro was analyzed. Our results indicate that two regions of Crm1 are required for the formation of the trimeric Crm1-NES-RanGTP complex, the N-terminal domain of Crm1 and the central domain of the receptor, starting after residue 160 with an essential region between 566 and 720. The N-terminal domain is homologous to the RanGTP-binding domain of karyopherin beta and therefore is likely involved in the interaction with RanGTP. Consequently, the central domain likely corresponds to the NES-binding site of Crm1.

A new role of ran GTPase

Ran is a G protein similar to Ras, but it has no membrane binding site. RanGEF, RCC1, is on chromatin and RanGAP, RanGAP1/Rna1p is in cytoplasm. Ran, thus, shuttles between the nucleus and the cytoplasm to complete its GTPase cycle, carrying out nucleocytoplasmic transport of macromolecules. A majority of Ran binding proteins, thus far found, are required for this process. A recently found novel Ran-binding protein, RanBPM, however, is localized in the centrosome. Subsequently, four groups reported that RanGTP, but not RanGDP, can induce microtubule self-organization in Xenopus egg extracts where no nuclear membrane is present. Thus, Ran is suggested to have a new role beyond the nucleocytoplasmic transport of macromolecules. In both microtubule assembly and nucleocytoplasmic transport, chromosomal localization of RCC1 is important to carry out the functions of RanGTPase. In this regard, a future intriguing question is how RCC1 interacts with chromatin DNA.

Plant importin alpha binds nuclear localization sequences with high affinity and can mediate nuclear import independent of importin beta

Nuclear import of conventional nuclear localization sequence (NLS)-containing proteins initially involves recognition by the importin (IMP) alpha/beta heterodimer, where IMPalpha binds the NLS and IMPbeta targets the IMPalpha/NLS-containing protein complex to the nuclear pore. Here we examine IMPalpha from the plant Arabidopsis thaliana (At-IMPalpha), which exhibits nuclear envelope localization typical of IMPbeta rather than IMPalpha in other eukaryotic cell systems. We show that At-IMPalpha recognizes conventional NLSs of two different types with high affinity (K(d) of 5-10 nM), in contrast to mouse IMPalpha (m-IMPalpha), which exhibits much lower affinity (K(d) of 50-70 nM) and only achieves high affinity in the presence of m-IMPbeta. Unlike m-IMPalpha, At-IMPalpha is thus a high affinity NLS receptor in the absence of IMPbeta. Interestingly, At-IMPalpha was also able to bind with high affinity to NLSs recognized specifically by m-IMPbeta and not m-IMPalpha, including that of the maize transcription factor Opaque-2. Reconstitution of nuclear import in vitro indicated that in the absence of exogenous IMPbeta subunit but dependent on RanGDP and NTF2, At-IMPalpha was able to mediate nuclear accumulation to levels comparable with those mediated by m-IMPalpha/beta. Neither m-IMPalpha nor -beta was able to mediate nuclear import in the absence of the other subunit. At-IMPalpha's novel NLS recognition and nuclear transport properties imply that plants may possess an IMPalpha-mediated nuclear import pathway independent of IMPbeta in addition to that mediated by IMPalpha/beta.

Generation of GTP-bound Ran by RCC1 is required for chromatin-induced mitotic spindle formation

Chromosomes are segregated by two antiparallel arrays of microtubules arranged to form the spindle apparatus. During cell division, the nucleation of cytosolic microtubules is prevented and spindle microtubules nucleate from centrosomes (in mitotic animal cells) or around chromosomes (in plants and some meiotic cells). The molecular mechanism by which chromosomes induce local microtubule nucleation in the absence of centrosomes is unknown, but it can be studied by adding chromatin beads to Xenopus egg extracts. The beads nucleate microtubules that eventually reorganize into a bipolar spindle. RCC1, the guanine-nucleotide-exchange factor for the GTPase protein Ran, is a component of chromatin. Using the chromatin bead assay, we show here that the activity of chromosome-associated RCC1 protein is required for spindle formation. Ran itself, when in the GTP-bound state (Ran-GTP), induces microtubule nucleation and spindle-like structures in M-phase extract. We propose that RCC1 generates a high local concentration of Ran-GTP around chromatin which in turn induces the local nucleation of microtubules.

Saccharomyces cerevisiae putative G protein, Gtr1p, which forms complexes with itself and a novel protein designated as Gtr2p, negatively regulates the Ran/Gsp1p G protein cycle through Gtr2p

Prp20p and Rna1p are GDP/GTP exchanging and GTPase-activating factors of Gsp1p, respectively, and their mutations, prp20-1 and rna1-1, can both be suppressed by Saccharomyces cerevisiae gtr1-11. We found that gtr1-11 caused a single amino acid substitution in Gtr1p, forming S20L, which is a putative GDP-bound mutant protein, while Gtr1p has been reported to bind to GTP alone. Consistently, gtr1-S20N, another putative GDP-bound mutant, suppressed both prp20-1 and rna1-1. On the other hand, gtr1-Q65L, a putative GTP-bound mutant, was inhibitory to prp20-1 and rna1-1. Thus, the role that Gtr1p plays in vivo appears to depend upon the nucleotide bound to it. Our data suggested that the GTP-bound Gtr1p, but not the GDP-bound Gtr1p, interacts with itself through its C-terminal tail. S. cerevisiae possesses a novel gene, GTR2, which is homologous to GTR1. Gtr2p interacts with itself in the presence of Gtr1p. The disruption of GTR2 suppressed prp20-1 and abolished the inhibitory effect of gtr1-Q65L on prp20-1. This finding, taken together with the fact that Gtr1p-S20L is a putative, inactive GDP-bound mutant, implies that Gtr1p negatively regulates the Ran/Gsp1p GTPase cycle through Gtr2p.

Model of the ran-RCC1 interaction using biochemical and docking experiments

RCC1, the regulator of chromosome condensation, is the guanine nucleotide exchange factor (GEF) for the nuclear Ras-like GTP-binding protein Ran. Its structure was solved by X-ray crystallography and revealed a seven-bladed beta-propeller, one side of which was proposed to be the interaction site with Ran. To gain more insight into this interaction, alanine mutagenesis studies were performed on conserved residues on the surface of the structure. Purified mutant proteins were analysed by steady-state kinetic analysis of their GEF activities towards Ran. A number of residues were identified whose mutation affected either the KMor kcatof the overall reaction, or had no effect. Mutants were further analysed by plasmon surface resonance in order to get more information on individual steps of the complex reaction pathway. Ran-GDP was coupled to the sensor chip and reacted with RCC1 mutants to categorise them into different groups, demonstrating the usefulness of plasmon surface resonance in the study of complex multi-step kinetic processes. A docking solution of Ran-RCC1 structures in combination with sequence analysis allows prediction of the site of interaction between RCC1 and Ran and proposes a model for the Ran-RCC1 structure which corresponds to and extends the biochemical data. Three invariant residues which most severely affect the kcatof the reaction, D128, D182 and H304, are located in the centre of the Ran-RCC1 interface and interfere with switch II and the phosphate binding area. The structural model suggests that different guanine nucleotide exchange factors use a similar interaction site on their respective GTP-binding proteins, but that the molecular mechanisms for the release of nucleotides are likely to be different.

Stimulation of microtubule aster formation and spindle assembly by the small GTPase Ran

Ran, a small guanosine triphosphatase, is suggested to have additional functions beyond its well-characterized role in nuclear trafficking. Guanosine triphosphate-bound Ran, but not guanosine diphosphate-bound Ran, stimulated polymerization of astral microtubules from centrosomes assembled on Xenopus sperm. Moreover, a Ran allele with a mutation in the effector domain (RanL43E) induced the formation of microtubule asters and spindle assembly, in the absence of sperm nuclei, in a gammaTuRC (gamma-tubulin ring complex)- and XMAP215 (Xenopus microtubule associated protein)-dependent manner. Therefore, Ran could be a key signaling molecule regulating microtubule polymerization during mitosis.

Self-organization of microtubule asters induced in Xenopus egg extracts by GTP-bound Ran

The nucleotide exchange activity of RCC1, the only known nucleotide exchange factor for Ran, a Ras-like small guanosine triphosphatase, was required for microtubule aster formation with or without demembranated sperm in Xenopus egg extracts arrested in meiosis II. Consistently, in the RCC1-depleted egg extracts, Ran guanosine triphosphate (RanGTP), but not Ran guanosine diphosphate (RanGDP), induced self-organization of microtubule asters, and the process required the activity of dynein. Thus, Ran was shown to regulate formation of the microtubule network.

The ran GTPase regulates mitotic spindle assembly

Ran is an abundant nuclear GTPase with a clear role in nuclear transport during interphase but with roles in mitotic regulation that are less well understood. The nucleotide-binding state of Ran is regulated by a GTPase activating protein, RanGAP1, and by a guanine nucleotide exchange factor, RCC1. Ran also interacts with a guanine nucleotide dissociation inhibitor, RanBP1. RanBP1 has a high affinity for GTP-bound Ran, and it acts as a cofactor for RanGAP1, increasing the rate of GAP-mediated GTP hydrolysis on Ran approximately tenfold. RanBP1 levels oscillate during the cell cycle [4], and increased concentrations of RanBP1 prolong mitosis in mammalian cells and in Xenopus egg extracts (our unpublished observations). We investigated how increased concentrations of RanBP1 disturb mitosis. We found that spindle assembly is dramatically disrupted when exogenous RanBP1 is added to M phase Xenopus egg extracts. We present evidence that the role of Ran in spindle assembly is independent of nuclear transport and is probably mediated through changes in microtubule dynamics.

Importin beta recognizes parathyroid hormone-related protein with high affinity and mediates its nuclear import in the absence of importin alpha

Parathyroid hormone-related protein (PTHrP), expressed in a range of tumors, has endocrine, autocrine/paracrine, and intracrine actions, some of which relate to its ability to localize in the nucleus. Here we show for the first time that extracellularly added human PTHrP (amino acids 1-108) can be taken up specifically by receptor-expressing UMR106.01 osteogenic sarcoma cells and accumulate to quite high levels in the nucleus and nucleolus within 40 min. Quantitation of recognition by the nuclear localization sequence (NLS)-binding importin subunits indicated that in contrast to proteins containing conventional NLSs, PTHrP is recognized exclusively by importin beta and not by importin alpha. The sequence of PTHrP responsible for binding was mapped to amino acids 66-94, which includes an SV40 large tumor-antigen NLS-like sequence, although sequence determinants amino-terminal to this region were also necessary for high affinity binding (apparent dissociation constant of approximately 2 nM for importin beta). Nuclear import of PTHrP was assessed in vitro using purified components, demonstrating that importin beta, together with the GTP-binding protein Ran, was able to mediate efficient nuclear accumulation in the absence of importin alpha, whereas the addition of nuclear transport factor NTF2 reduced transport. The polypeptide ligand PTHrP thus appears to be accumulated in the nucleus/nucleolus through a novel, NLS-dependent nuclear import pathway independent of importin alpha and perhaps also of NTF2.

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.

Ran-dependent signal-mediated nuclear import does not require GTP hydrolysis by Ran

Nuclear import of classical nuclear localization sequence-containing proteins involves the assembly of an import complex at the cytoplasmic face of the nuclear pore complex (NPC) followed by movement of this complex through the NPC and release of the import substrate into the nuclear interior. This process has historically been thought to require nucleotide hydrolysis as a source of energy. We found, using hydrolysis-resistant GTP analogs and a mutant Ran unable to hydrolyze GTP, that transport of classical nuclear localization sequence containing substrate through the NPC and release of the substrate into the nucleus did not require hydrolysis of GTP by Ran. In fact, for movement of this type of import substrate into the nuclear interior we did not observe a requirement for hydrolysis of any nucleotide triphosphate. We did, however, find that a pool of free GTP (or its structural equivalent) must be added, probably because the GDP Ran that is added must be converted to GTP Ran during the import process. We found that a requirement for GTP hydrolysis can be restored to an import mixture consisting of recombinant import factors by the addition of RCC1, the Ran guanine nucleotide exchange factor.

When overexpressed, a novel centrosomal protein, RanBPM, causes ectopic microtubule nucleation similar to gamma-tubulin

A novel human protein with a molecular mass of 55 kD, designated RanBPM, was isolated with the two-hybrid method using Ran as a bait. Mouse and hamster RanBPM possessed a polypeptide identical to the human one. Furthermore, Saccharomyces cerevisiae was found to have a gene, YGL227w, the COOH-terminal half of which is 30% identical to RanBPM. Anti-RanBPM antibodies revealed that RanBPM was localized within the centrosome throughout the cell cycle. Overexpression of RanBPM produced multiple spots which were colocalized with gamma-tubulin and acted as ectopic microtubule nucleation sites, resulting in a reorganization of microtubule network. RanBPM cosedimented with the centrosomal fractions by sucrose- density gradient centrifugation. The formation of microtubule asters was inhibited not only by anti- RanBPM antibodies, but also by nonhydrolyzable GTP-Ran. Indeed, RanBPM specifically interacted with GTP-Ran in two-hybrid assay. The central part of asters stained by anti-RanBPM antibodies or by the mAb to gamma-tubulin was faded by the addition of GTPgammaS-Ran, but not by the addition of anti-RanBPM anti- bodies. These results provide evidence that the Ran-binding protein, RanBPM, is involved in microtubule nucleation, thereby suggesting that Ran regulates the centrosome through RanBPM.

Atomic structure of clathrin: a beta propeller terminal domain joins an alpha zigzag linker

Clathrin triskelions form the lattice that organizes recruitment of proteins to coated pits and helps drive vesiculation of the lipid bilayer. We report the crystal structure at 2.6 A resolution of a 55 kDa N-terminal fragment from the 190 kDa clathrin heavy chain. The structure comprises the globular "terminal domain" and the linker that joins it to the end of a triskelion leg. The terminal domain is a seven-blade beta propeller, a structure well adapted to interaction with multiple partners, such as the AP-1 and AP-2 sorting adaptor complexes and the nonvisual arrestins. The linker is an alpha-helical zigzag emanating from the propeller domain. We propose that this simple motif may extend into the rest of the clathrin leg.

The role of the ran GTPase in nuclear assembly and DNA replication: characterisation of the effects of Ran mutants

The Ran GTPase plays a critical role in nucleocytoplasmic transport and has been implicated in the maintenance of nuclear structure and cell cycle control. Here, we have investigated its role in nuclear assembly and DNA replication using recombinant wild-type and mutant Ran proteins added to a cell-free system of Xenopus egg extracts. RanQ69L and RanT24N prevent lamina assembly, PCNA accumulation and DNA replication. These effects may be due to the disruption of nucleocytoplasmic transport, since both mutants inhibit nuclear import of a protein carrying a nuclear localisation signal (NLS). RanQ69L, which is deficient in GTPase activity, sequesters importins in stable complexes that are unable to support the docking of NLS-proteins at the nuclear pore complex (NPC). RanT24N, in contrast to wild-type Ran-GDP, interacts only weakly with importin alpha and nucleoporins, and not at all with the import factor p10, consistent with its poor activity in nuclear import. However, RanT24N does interact stably with importin beta, Ran binding protein 1 and RCC1, an exchange factor for Ran. We show that Ran-GDP is essential for proper nuclear assembly and DNA replication, the requirement being primarily before the initiation of DNA replication. Ran-GDP therefore mediates the active transport of necessary factors or otherwise controls the onset of S-phase in this system.

The role of the Ran GTPase pathway in cell cycle control and interphase nuclear functions

Ran is a small, highly abundant, nuclear GTPase. Mutants in Ran and in proteins that interact with it disrupt the normal checkpoint control of mitosis with respect to the completion of DNA synthesis. Ran and other components of this pathway are also required for numerous nuclear functions such as RNA export, protein import, RNA processing and DNA replication. It will be important to understand how these facets of Ran's activities are linked and how they promote correct control of the cell cycle. This review examines recent progress in discovering other components of the Ran GTPase pathway and considers how this pathway may be required for the control of the cell cycle.

The identification of cDNAs that affect the mitosis-to-interphase transition in Schizosaccharomyces pombe, including sbp1, which encodes a spi1p-GTP-binding protein

Perturbations of the spi1p GTPase system in fission yeast, caused by mutation or overexpression of several regulatory proteins, result in a unique terminal phenotype that includes condensed chromosomes, a wide medial septum, and a fragmented nuclear envelope. To identify potential regulators or targets of the spi1p GTPase system, a screen for cDNAs whose overexpression results in this terminal phenotype was conducted, and seven clones that represent three genes, named med1, med2, and med3 (mitotic exit defect), were identified. Their genetic interaction with the spi1p GTPase system was established by showing that the spi1p guanine nucleotide exchange factor mutant pim1-d1ts was hypersensitive to their overexpression. med1 encodes a homologue of the human Ran-binding protein, RanBP1, and has been renamed sbp1 (spi1-binding protein). sbp1p binds to spi1p-GTP and costimulates the GTPase-activating protein (GAP)-catalyzed GTPase activity. Cells in which sbp1p is depleted or overproduced phenocopy cells in which the balance between spi1p-GTP and spi1p-GDP is perturbed by other means. Therefore, sbp1p mediates and/or regulates the essential functions of the spi1p GTPase system. med2 and med3 encode novel fission yeast proteins that, based on our phenotypic analyses, are likely to identify additional regulators or effectors of the spi1p GTPase system.

RagA is a functional homologue of S. cerevisiae Gtr1p involved in the Ran/Gsp1-GTPase pathway

Human RagA and RagB is reported to be 52% identical to a putative GTPase of Saccharomyces cerevisiae, Gtr1p. According to the reported nucleotide sequence, we amplified human RagA and RagBs cDNAs from the human B cell cDNA library with PCR. Both cDNAs rescued a cold sensitivity of S. cerevisiae, gtr1-11. Furthermore, we introduced into the cloned human RagA cDNA, the mutation ‘T21L’ corresponding to the gtr1-11 mutation which has been reported to suppress not only all of rcc1-, temperature-sensitive mutants of Ran/Gsp1p GTPase GDP/GTP-exchanging factor, but also rna1-1, a temperature-sensitive mutant of Ran/Gsp1p GTPase-activating protein. The resulting RagAgtr1-11 cDNA partially, but significantly, suppressed both rcc1- and rna1-1 mutations. These results indicated that RagA and RagBs are functional homologues of S. cervisiae Gtr1p. Interestingly, while wild-type human RagA and RagBs were localized within the cytoplasm, similar to S. cerevisiae Gtr1p, the mutated human RagAgtr1-11 corresponding to a dominant negative form of RagA was distributed in discrete speckles in the nucleus, being localized side by side with SC-35, a non-snRNP of the splicing complex. In contrast, a dominant positive form of RagA, Q66L was localized in the cytoplasm. Thus, RagA was suggested to shuttle between the cytoplasm and the nucleus, depending on the bound nucleotide state.

A T42A Ran mutation: differential interactions with effectors and regulators, and defect in nuclear protein import

Ran, the small, predominantly nuclear GTPase, has been implicated in the regulation of a variety of cellular processes including cell cycle progression, nuclear-cytoplasmic trafficking of RNA and protein, nuclear structure, and DNA synthesis. It is not known whether Ran functions directly in each process or whether many of its roles may be secondary to a direct role in only one, for example, nuclear protein import. To identify biochemical links between Ran and its functional target(s), we have generated and examined the properties of a putative Ran effector mutation, T42A-Ran. T42A-Ran binds guanine nucleotides as well as wild-type Ran and responds as well as wild-type Ran to GTP or GDP exchange stimulated by the Ran-specific guanine nucleotide exchange factor, RCC1. T42A-Ran.GDP also retains the ability to bind p10/NTF2, a component of the nuclear import pathway. In contrast to wild-type Ran, T42A-Ran.GTP binds very weakly or not detectably to three proposed Ran effectors, Ran-binding protein 1 (RanBP1), Ran-binding protein 2 (RanBP2, a nucleoporin), and karyopherin beta (a component of the nuclear protein import pathway), and is not stimulated to hydrolyze bound GTP by Ran GTPase-activating protein, RanGAP1. Also in contrast to wild-type Ran, T42A-Ran does not stimulate nuclear protein import in a digitonin permeabilized cell assay and also inhibits wild-type Ran function in this system. However, the T42A mutation does not block the docking of karyophilic substrates at the nuclear pore. These properties of T42A-Ran are consistent with its classification as an effector mutant and define the exposed region of Ran containing the mutation as a probable effector loop.

Ran-unassisted nuclear migration of a 97-kD component of nuclear pore-targeting complex

A 97-kD component of nuclear pore-targeting complex (the beta-subunit of nuclear pore-targeting complex [PTAC]/importin/karyopherin) mediates the import of nuclear localization signal (NLS)-containing proteins by anchoring the NLS receptor protein (the alpha-subunit of PTAC/importin/karyopherin) to the nuclear pore complex (NPC). The import requires a small GTPase Ran, which interacts directly with the beta-subunit. The present study describes an examination of the behavior of the beta-subunit in living cells and in digitonin-permeabilized cells. In living cells, cytoplasmically injected beta-subunit rapidly migrates into the nucleus. The use of deletion mutants reveals that nuclear migration of the beta-subunit requires neither Ran- nor alpha-subunit-binding but only the NPC-binding domain of this molecule, which is also involved in NLS-mediated import. Furthermore, unlike NLS-mediated import, a dominant-negative Ran, defective in GTP-hydrolysis, did not inhibit nuclear migration of the beta-subunit. In the digitonin-permeabilized cell-free import assay, the beta-subunit transits rapidly through the NPC into the nucleus in a saturating manner in the absence of exogenous addition of soluble factors. These results show that the beta-subunit undergoes translocation at the NPC in a Ran-unassisted manner when it does not carry alpha-subunit/NLS substrate. Therefore, a requirement for Ran arises only when the beta-subunit undergoes a translocation reaction together with the alpha-subunit/NLS substrate. The results provide an insight to the yet unsolved question regarding the mechanism by which proteins are directionally transported through the NPC, and the role of Ran in this process.

The balance of RanBP1 and RCC1 is critical for nuclear assembly and nuclear transport

Ran is a small GTPase that is essential for nuclear transport, mRNA processing, maintenance of structural integrity of nuclei, and cell cycle control. RanBP1 is a highly conserved Ran guanine nucleotide dissociation inhibitor. We sought to use Xenopus egg extracts for the development of an in vitro assay for RanBP1 activity in nuclear assembly, protein import, and DNA replication. Surprisingly, when we used anti-RanBP1 antibodies to immunodeplete RanBP1 from Xenopus egg extracts, we found that the extracts were also depleted of RCC1, Ran's guanine nucleotide exchange factor, suggesting that these proteins form a stable complex. In contrast to previous observations using extracts that had been depleted of RCC1 only, extracts lacking both RanBP1 and RCC1 (codepleted extracts) did not exhibit defects in assays of nuclear assembly, nuclear transport, or DNA replication. Addition of either recombinant RanBP1 or RCC1 to codepleted extracts to restore only one of the depleted proteins caused abnormal nuclear assembly and inhibited nuclear transport and DNA replication in a manner that could be rescued be further addition of RCC1 or RanBP1, respectively. Exogenous mutant Ran proteins could partially rescue nuclear function in extracts without RanBP1 or without RCC1, in a manner that was correlated with their nucleotide binding state. These results suggest that little RanBP1 or RCC1 is required for nuclear assembly, nuclear import, or DNA replication in the absence of the other protein. The results further suggest that the balance of GTP- and GDP-Ran is critical for proper nuclear assembly and function in vitro.

Requirement of guanosine triphosphate-bound ran for signal-mediated nuclear protein export

A leucine-rich nuclear export signal (NES) allows rapid export of proteins from cell nuclei. Microinjection studies revealed a role for the guanosine triphosphatase (GTPase) Ran in NES-mediated export. Nuclear injection of a Ran mutant (Thr24 --> Asn) blocked protein export but not import, whereas depletion of the Ran nucleotide exchange factor RCC1 blocked protein import but not export. However, injection of Ran GTPase-activating protein (RanGAP) into RCC1-depleted cell nuclei inhibited export. Coinjection with Ran mutants insensitive to RanGAP prevented this inhibition. Therefore, NES-mediated protein export appears to require a Ran-GTP complex but does not require Ran-dependent GTP hydrolysis.

A small ubiquitin-related polypeptide involved in targeting RanGAP1 to nuclear pore complex protein RanBP2

We have found that the mammalian Ran GTPase-activating protein RanGAP1 is highly concentrated at the cytoplasmic periphery of the nuclear pore complex (NPC), where it associates with the 358-kDa Ran-GTP-binding protein RanBP2. This interaction requires the ATP-dependent posttranslational conjugation of RanGAP1 with SUMO-1 (for small ubiquitin-related modifier), a novel protein of 101 amino acids that contains low but significant homology to ubiquitin. SUMO-1 appears to represent the prototype for a novel family of ubiquitin-related protein modifiers. Inhibition of nuclear protein import resulting from antibodies directed at NPC-associated RanGAP1 cannot be overcome by soluble cytosolic RanGAP1, indicating that GTP hydrolysis by Ran at RanBP2 is required for nuclear protein import.

Mutations within the Ran/TC4 GTPase. Effects on regulatory factor interactions and subcellular localization

Ran, a member of the Ras superfamily of GTPases, is predominantly localized in the nucleus and is a necessary component in the active transport of proteins through nuclear pores. Disruption of Ran function affects the regulation of mitosis, DNA synthesis, and RNA processing and export. To explore the mechanisms of Ran function, mutants of the Ran GTPase were characterized, several of which are capable of dominantly interfering with nuclear protein import. Unlike wild-type Ran, the putative gain-of-function mutant (G19V Ran) was not sensitive to the exchange factor, RCC1. In addition the G19V Ran and effector domain mutants (L43E and E46G Ran) were not sensitive to the GTPase-activating protein, Fug1. Epitope-tagged G19V Ran and L43E Ran isolated from transfected BHK21 cells were each about 50% GTP-bound, whereas the wild-type and a C-terminal deletion mutant (Delta-DE Ran) were primarily bound to GDP. While G19V Ran interacted with known Ran-binding proteins and with an isolated Ran-binding domain, the T24N Ran did not, and binding by L43E Ran was substantially reduced. Wild-type HA1-tagged Ran expressed in BHK21 cells was nuclear, whereas the G19V, T24N, L43E, and E46G forms of Ran were predominantly localized at the nuclear envelope, and Delta-DE Ran was primarily cytosolic. Similar results were observed when permeabilized BHK21 cells were incubated with extracts of COS cells expressing the mutants. Thus mutations that affect the interaction of Ran with regulatory proteins and effectors can disrupt the normal subcellular localization of Ran, lending support for the current model of Ran-mediated nuclear import.

Dis3, implicated in mitotic control, binds directly to Ran and enhances the GEF activity of RCC1

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.

Premature chromatin condensation induced by loss of RCC1 is inhibited by GTP- and GTPgammaS-Ran, but not GDP-Ran

RCC1 is a guanine nucleotide exchanging factor acting on nuclear G protein Ran. Premature chromatin condensation occurs in the temperature-sensitive rcc1- mutant of the BHK21 cell line, tsBN2, at the restrictive temperature. This observation can be explained if the premature activation of MPF is normally inhibited by GTP-Ran. In the absence of RCC1, GDP-Ran predominates, resulting in MPF activation. However, experiments with Ran mutants to determine whether GTP- or GDP-Ran prevents activation of MPF have yielded conflicting results. In order to clarify this point, we have microinjected nucleotide-bound Ran, instead of mutated Ran, into the nuclei of tsBN2 cells treated to reduce RCC1-mediated guanine nucleotide exchange. GTP-Ran, GTPgammaS-Ran, and GDP-Ran all inhibited chromatin condensation. However, the inhibition of chromatin condensation by GDP-Ran could be completely abolished by co-injection with GDP, but not GTP. Thus, we conclude that GTP-Ran blocks the activation of MPF and that hydrolysis of GTP is not required to prevent MPF activation.

The thermolability of nuclear protein import in tsBN2 cells is suppressed by microinjected Ran-GTP or Ran-GDP, but not by RanQ69L or RanT24N

The nuclear protein regulator of chromosome condensation 1 (RCC1) stimulates guanine nucleotide exchange on a protein, Ran, that is required for nuclear protein import. In the present report, we confirm that RCC1 is also required for nuclear protein import in tsBN2 hamster cells in vivo. The thermolability of nuclear protein import in tsBN2 cells was suppressed by microinjection of purified Ran-GTP into the cytoplasm, but Ran-GDP also relieved the import deficiency, suggesting either that both forms of Ran are active in import in vivo or that tsBN2 cells at restrictive temperature retain a mechanism to convert Ran-GDP to Ran-GTP. To distinguish between these possibilities, nuclear protein import in tsBN2 cells was tested in the presence of Ran mutants, one deficient in GTP hydrolysis (RanQ69L), and one with weak binding to GDP and little or no binding to GTP (RanT24N). Microinjection of the mutant RanQ69L inhibited import in vivo in either the GTP- or GDP-bound form at both the permissive and nonpermissive temperatures. RanT24N-GDP inhibited import in vivo at the permissive temperature and failed to stimulate nuclear protein import at the nonpermissive temperature. The implications of these results for the roles of RCC1 and Ran in nuclear protein import in vivo are discussed.

Partial purification and characterization of a protein kinase that is activated by nuclear localization signal peptides

A nuclear localization signal (NLS) is required for the transport of karyophilic proteins from the cytoplasm to the nucleus. In this study, NLS was examined in terms of its effect on diverse cellular functions such as protein phosphorylation reactions. When synthetic peptides containing the NLS of SV40 T-antigen were injected into the cytoplasm of Xenopus oocytes, and the oocytes incubated with [32P]phosphorous-containing medium, a 32 kDa protein was found to be preferentially phosphorylated in an NLS-dependent manner. The incubation of fractionated cytosolic extracts prepared from mouse Ehrlich ascites tumor cells with [gamma-32P]ATP in the presence of the NLS peptides, results in the stimulation of the phosphorylation of several proteins. Similar in vitro stimulation was observed by other functional NLS peptides such as those of polyoma virus T-antigen and nucleoplasmin. Little or no stimulation, however, was detected for peptides of mutant type and reverse type NLS of SV40 T-antigen, and the C-terminal portion of lamin B. Using an in vitro assay, the phosphorylation activity was fractionated chromatographically and a fraction was obtained which contained a high level of activity. The fraction was found to contain three major proteins having molecular masses of 95, 70, and 43 kDa. The in vivo and in vitro results are consistent with the existence of a protein kinase, called NLS kinase, that is specifically activated by NLS peptides.

The small nuclear GTPase Ran: how much does it run?

Ran is one of the most abundant and best conserved of the small GTP binding and hydrolyzing proteins of eukaryotes. It is located predominantly in cell nuclei. Ran is a member of the Ras family of GTPases, which includes the Ras and Ras-like proteins that regulate cell growth and division, the Rho and Rac proteins that regulate cytoskeletal organization and the Rab proteins that regulate vesicular sorting. Ran differs most obviously from other members of the Ras family in both its nuclear localization, and its lack of sites required for post-translational lipid modification. Ran is, however, similar to other Ras family members in requiring a specific guanine nucleotide exchange factor (GEF) and a specific GTPase activating protein (GAP) as stimulators of overall GTPase activity. In this review, the multiple cellular functions of Ran are evaluated with respect to its known biochemistry and molecular interactions.

Xenopus lamin B3 has a direct role in the assembly of a replication competent nucleus: evidence from cell-free egg extracts

Xenopus egg extracts which assemble replication competent nuclei in vitro were depleted of lamin B3 using monoclonal antibody L6 5D5 linked to paramagnetic beads. After depletion, the extracts were still capable of assembling nuclei around demembranated sperm heads. Using field emission in lens scanning electron microscopy (FEISEM) we show that most nuclei assembled in lamin B3-depleted extracts have continuous nuclear envelopes and well formed nuclear pores. However, several consistent differences were observed. Most nuclei were small and only attained diameters which were half the size of controls. In a small number of nuclei, nuclear pore baskets, normally present on the inner aspect of the nuclear envelope, appeared on its outer surface. Finally, the assembly of nuclear pores was slower in lamin B3-depleted extracts, indicating a slower overall rate of nuclear envelope assembly. The results of FEISEM were confirmed using conventional TEM thin sections, where again the majority of nuclei assembled in lamin B3-depleted extracts had well formed double unit membranes containing a high density of nuclear pores. Since nuclear envelope assembly was mostly normal but slow in these nuclei, the lamin content of 'depleted' extracts was investigated. While lamin B3 was recovered efficiently from cytosolic and membrane fractions by our procedure, a second minor lamin isoform, which has characteristics similar to those of the somatic lamin B2, remained in the extract. Thus it is likely that this lamin is necessary for nuclear envelope assembly. However, while lamin B2 did not co-precipitate with lamin B3 during immunodepletion experiments, several protein species did specifically associate with lamin B3 on paramagnetic immunobeads. The major protein species associated with lamin B3 migrated with molecular masses of 102 kDa and 57 kDa, respectively, on one-dimensional polyacrylamide gels. On two-dimensional O'Farrell gels the mobility of the 102 kDa protein was identical to the mobility of a major nuclear matrix protein, indicating a specific association between lamin B3 and other nuclear matrix proteins. Nuclei assembled in lamin B3-depleted extracts did not assemble a lamina, judged by indirect immunofluorescence, and failed to initiate semi-conservative DNA replication. However, by reinoculating depleted extracts with purified lamin B3, nuclear lamina assembly and DNA replication could both be rescued. Thus it seems likely that the inability of lamin-depleted extracts to assemble a replication competent nucleus is a direct consequence of a failure to assemble a lamina.

A giant nucleopore protein that binds Ran/TC4

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.

RanBP1, a Ras-like nuclear G protein binding to Ran/TC4, inhibits RCC1 via Ran/TC4

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.

Mechanisms of nuclear protein import

The past two years have seen a significant increase in our understanding of nuclear protein import. Five cytosolic import factors have been identified, two of which have been shown to directly interact with components of the nuclear pore complex. These findings enable refinement of previous models for steps in the nuclear import pathway, and provide a framework for future research.

The RCC1 protein interacts with Ran, RanBP1, hsc70, and a 340-kDa protein in Xenopus extracts

RCC1 is an abundant, highly conserved, chromatin-associated protein whose function is necessary for the preservation of a properly ordered cell cycle. RCC1 is also necessary for numerous nuclear processes, including nuclear transport and RNA metabolism; and it functions enzymatically as a guanine nucleotide exchange factor for a small, ras-related GTPase called Ran. Studies in several organisms suggest that RCC1 may be part of a large complex containing multiple proteins. There is also evidence that RCC1 associates with chromatin through other proteins and that the binding of the complex to chromatin varies within the cell cycle. In order to characterize this putative complex, we have identified a number of other proteins as candidate components of the complex by their association with a GST-RCC1 fusion protein. Three of these proteins have previously been identified (Ran, RanBP1, and hsc70). The fourth protein is novel and has a molecular mass of 340 kDa. In this report, we discuss a preliminary characterization of the interactions between these proteins.

Co-activation of RanGTPase and inhibition of GTP dissociation by Ran-GTP binding protein RanBP1

RCC1 (the regulator of chromosome condensation) stimulates guanine nucleotide dissociation on the Ras-related nuclear protein Ran. Both polypeptides are components of a regulatory pathway that has been implicated in regulating DNA replication, onset of and exit from mitosis, mRNA processing and transport, and import of proteins into the nucleus. In a search for further members of the RCC1-Ran signal pathway, we have identified proteins of 23, 45 and 300 kDa which tightly bind to Ran-GTP but not Ran-GDP. The purified soluble 23 kDa Ran binding protein RanBP1 does not activate RanGTPase, but increases GTP hydrolysis induced by the RanGTPase-activating protein RanGAP1 by an order of magnitude. In the absence of RanGAP, it strongly inhibits RCC1-induced exchange of Ran-bound GTP. In addition, it forms a stable complex with nucleotide-free RCC1-Ran. With these properties, it differs markedly from guanine diphosphate dissociation inhibitors which preferentially prevent the exchange of protein-bound GDP and in some cases were shown to inhibit GAP-induced GTP hydrolysis. RanBP1 is the first member of a new class of proteins regulating the binding and hydrolysis of GTP by Ras-related proteins.