Evidence for a dual role for TC4 protein in regulating nuclear structure and cell cycle progression

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.

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.

A RanBP1 mutation which does not visibly affect nuclear import may reveal additional functions of the ran GTPase system

Ran, a nuclear GTPase, and a number of interacting proteins, including regulators RanGEF1 and RanGAP1, are involved in nucleocytoplasmic transport. We have identified a new temperature-sensitive mutation in budding yeast YRB1 gene, which encodes Ran-binding protein-1 (RanBP1). In contrast to other yrb1 alleles, the new mutation (yrb1-21) does not cause visible defects in import of nuclear proteins Npl3p, histone H2B, or beta-galactosidase fused to a nuclear localization signal. We hypothesize that the inviability of mutant cells at the restrictive temperature is caused by an additional essential function of RanBP1 other than nuclear import. This function may be revealed by the terminal phenotypes of yrb1-21, which include failure of the mitotic spindles to properly align along the mother-bud axis and accumulation of cells in late mitosis or G1 phase of the cell cycle. These features are shared, in part, by a mutation in RanGEF1, but not in RanGAP1. The yrb1-21 allele suppresses a RanGEF1 mutation, indicating that RanGEF1 and RanBP1 may be involved in the same essential function.

Nuclear envelope disassembly in mitotic extract requires functional nuclear pores and a nuclear lamina

Using sea urchin embryonic and in-vitro-assembled nuclei incubated in sea urchin mitotic extract, I provide evidence for a requirement for functional nuclear pores and a nuclear lamina for nuclear envelope disassembly in vitro. In interphase gastrula nuclei, lamin B interacts with p56, an integral protein of inner nuclear membrane cross-reacting with antibodies to human lamin B receptor. Incubation of gastrula nuclei in mitotic cytosol containing an ATP-generating system rapidly induces hyperphosphorylation of p56 and lamin B. Subsequently, p56-lamin B interactions are weakened and the two proteins segregate into distinct nuclear envelope-derived vesicles upon disassembly of nuclear membranes and of the lamina. Nuclear disassembly is accompanied by chromatin condensation. Blocking nuclear pore function with wheat germ agglutinin or antibodies to nucleoporins prevents p56 and lamin B hyperphosphorylation, nuclear membrane breakdown and lamina solubilization. These events are not rescued by permeabilization of nuclear membranes to molecules of 150, 000 Mr with lysolecithin. In-vitro-assembled nuclei containing nuclear membranes with functional pores but no lamina do not disassemble in mitotic cytosol in spite of p56 hyperphosphorylation. Nuclear import of soluble lamin B and reformation of a lamina in interphase extract restores nuclear disassembly in mitotic cytosol. The data indicate a role for functional nuclear pores in nuclear disassembly in vitro. They show that p56 hyperphosphorylation is not sufficient for nuclear membrane disassembly in mitotic cytosol and argue that the nuclear lamina plays a critical role in nuclear disassembly at mitosis.

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.

Yrb2p is a nuclear protein that interacts with Prp20p, a yeast Rcc1 homologue

A conserved family of Ran binding proteins (RBPs) has been defined by their ability to bind to the Ran GTPase and the presence of a common region of approximately 100 amino acids (the Ran binding domain). The yeast Saccharomyces cerevisiae genome predicts only three proteins with canonical Ran binding domains. Mutation of one of these, YRB1, results in defects in transport of macromolecules across the nuclear envelope (Schlenstedt, G., Wong, D. H., Koepp, D. M., and Silver, P. A. (1995) EMBO J. 14, 5367-5378). The second one, encoded by YRB2, is a 327-amino acid protein with a Ran binding domain at its C terminus and an internal cluster of FXFG and FG repeats conserved in nucleoporins. Yrb2p is located inside the nucleus, and this localization relies on the N terminus. Results of both genetic and biochemical analyses show interactions of Yrb2p with the Ran nucleotide exchanger Prp20p/Rcc1. Yrb2p binding to Gsp1p (yeast Ran) as well as to a novel 150-kDa GTP-binding protein is also detected. The Ran binding domain of Yrb2p is essential for function and for its association with Prp20p and the GTP-binding proteins. Taken together, we suggest that Yrb2p may play a role in the Ran GTPase cycle distinct from nuclear transport.

Dynamic and equilibrium studies on the interaction of Ran with its effector, RanBP1

Ran, a small nuclear GTP-binding protein, is one of the most abundant Ras-related proteins in eucaryotic cells. Ran is essential for nucleo-cytoplasmatic transport and is primarily localized in the nucleus and at the nuclear pore complex. Here, we characterize the kinetics and equilibrium of the interaction between Ran and RanBP1 by two independent biophysical approaches: fluorescence spectroscopy using analogues of guanine nucleotides and surface plasmon resonance in the BIAcore system. Both approaches result in kinetic and equilibrium data which are in good agreement with each other. Affinities of RanBP1 for Ran in the GTP-bound state were in the nanomolar range, while Ran.GDP bound RanBP1 with a dissociation constant around 10 microM. Interestingly, the difference in affinity of RanBP1 for Ran.GDP was mostly due to a dramatic increase of the dissociation rate constant. Mutant Ran protein lacking the last five amino acids of the C-terminus (RanDeltaC) is unable to facilitate nuclear import in vitro and does not bind to RanBP1. Here, we show that RanBP1 binds RanDeltaC.mGppNHp with KD values around 10 microM, as is the case for its association with full-length Ran.GDP. The loss of affinity of RanBP1 for the triphosphate form of RanDeltaC was a result of both a decrease of the association rate and a moderately increased dissociation of the RanDeltaC.RanBP1 complex. Circular dichroism spectra indicate significant changes in the secondary structure of either Ran.GppNHp, RanBP1, or both proteins upon forming a stable complex with each other.

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.

RCC1 and nuclear organization

We have examined the effect of RCC1 function on the nuclear organization of pre-mRNA splicing factors and poly(A)+ RNA in the tsBN2 cells, a RCC1 temperature-sensitive mutant cell line. We have found that at 4-6 h after shifting cells from the permissive temperature (32.5 degrees C) to the restrictive temperature (39.5 degrees C), both small nuclear ribonucleoprotein particles and a general splicing factor SC35 reorganized into 4-10 large round clusters in the nucleus, as compared with the typical speckled distribution seen in cells at the permissive temperature. In situ hybridization to poly(A)+ RNA resulted in a similar pattern. Examination by double labeling demonstrated that the redistribution of splicing factors coincides with that of poly(A)+ RNA. Such changes in the nuclear organization of splicing factors and poly(A)+ RNA were not the result of the temperature shift or of chromatin condensation. Cellular transcription was not significantly altered in these cells and extracts made from both the permissive and restrictive temperature were splicing competent. Electron microscopic examination demonstrated that the large clusters containing both splicing factors and poly(A)+ RNA were fused interchromatin granule clusters. In addition, small electron-dense dot-like structures measuring approximately 80 nm in diameter were also observed, most of which are accumulated in enlarged interchromatin granule clusters in the nucleoplasm of RCC1- cells. In spite of the significant changes observed in the nucleoplasm, relatively little alteration was observed in nucleolar structure by both light and electron microscopic examination. The above observations suggest that the RCC1 protein directly or indirectly regulates the organization of splicing components and poly(A)+ RNA in the cell nucleus and that RCC1 may play a role in nuclear organization.

Nucleocytoplasmic transport of macromolecules

Nucleocytoplasmic transport is a complex process that consists of the movement of numerous macromolecules back and forth across the nuclear envelope. All macromolecules that move in and out of the nucleus do so via nuclear pore complexes that form large proteinaceous channels in the nuclear envelope. In addition to nuclear pores, nuclear transport of macromolecules requires a number of soluble factors that are found both in the cytoplasm and in the nucleus. A combination of biochemical, genetic, and cell biological approaches have been used to identify and characterize the various components of the nuclear transport machinery. Recent studies have shown that both import to and export from the nucleus are mediated by signals found within the transport substrates. Several studies have demonstrated that these signals are recognized by soluble factors that target these substrates to the nuclear pore. Once substrates have been directed to the pore, most transport events depend on a cycle of GTP hydrolysis mediated by the small Ras-like GTPase, Ran, as well as other proteins that regulate the guanine nucleotide-bound state of Ran. Many of the essential factors have been identified, and the challenge that remains is to determine the exact mechanism by which transport occurs. This review attempts to present an integrated view of our current understanding of nuclear transport while highlighting the contributions that have been made through studies with genetic organisms such as the budding yeast, Saccharomyces cerevisiae.

RanBP2 associates with Ubc9p and a modified form of RanGAP1

Ran is a small GTPase required for nuclear transport in eukaryotic cells [Gorlich, D. & Mattaj, I. W. (1996) Science 271, 1513-1518]. Mutants in Ran also show defects in mRNA processing, cell cycle regulation, and other aspects of nuclear function [Rush, M. G., Drivas, G. & D'Eustachio, P. (1996) BioEssays 18, 103-112; Sazer, S. (1996) Trends Cell Biol. 6, 81-85]. In an effort to understand the role of Ran in these diverse processes, we previously characterized 10 Ran interacting proteins (Rips) from Xenopus egg extracts. In this report, we present further characterization of a complex containing three of these Rips: p340(RanBP2), p88, and p18. We have cloned the Xenopus homologue of RanGAP1, and we show here that p88 is a modified form of this protein. In RanGAP assays, the p340(RanBP2)-p88-p18 complex contains GTPase-activating protein activity, indicating that RanGAP1 is not inactivated by modification. Rather, modification of RanGAP1 appears to be linked to its association with p340(RanBP2) because we did not observe unmodified RanGAP1 in p340(RanBP2) immunoprecipitates. We have also characterized p18, and we found that it is the Xenopus homologue of Ubc9p, an E2 ubiquitin-conjugating enzyme that is required for cell cycle regulation [Seufert, W., Futcher, B. & Jentsch, S. (1995) Nature (London) 373, 78-81]. Using antibodies directed against Xenopus Ubc9p, we have confirmed that Ubc9p associates with p340(RanBP2) in Xenopus extracts. These results suggest Ubc9p's role in cell cycle regulation may involve either modification of nuclear transport substrates or the nuclear transport machinery.

Nuclear envelope assembly after mitosis

In higher eukaryotes, the entire nucleus disassembles during prometaphase of the cell cycle and later reassembles around daughter chromosomes. Remarkably, the complex events that occur to create a functional nucleus in vivo can be duplicated in vitro by using cell-free extracts. Current experiments are aimed at understanding the molecular mechanisms of assembly and disassembly of the nuclear pore complexes and nuclear membranes, and the functional roles of four identified inner membrane proteins, two of which bind to both chromatin and the nuclear lamina.

The early region 1B 55-kilodalton oncoprotein of adenovirus relieves growth restrictions imposed on viral replication by the cell cycle

The E1B 55-kDa oncoprotein of adenovirus enables the virus to overcome restrictions imposed on viral replication by the cell cycle. Approximately 20% of HeLa cells infected with an E1B 55-kDa mutant adenovirus produced virus when evaluated by electron microscopy or by assays for infectious centers. By contrast, all HeLa cells infected with a wild-type adenovirus produced virus. The yield of E1B mutant virus from randomly cycling HeLa cells correlated with the fraction of cells in S phase at the time of infection. In synchronously growing HeLa cells, approximately 75% of the cells infected during S phase with the E1B mutant virus produced virus, whereas only 10% of the cells infected during G1 produced virus. The yield of E1B mutant virus from HeLa cells infected during S phase was sevenfold greater than that of cells infected during G1 and threefold greater than that of cells infected during asynchronous growth. Cells infected during S phase with the E1B mutant virus exhibited severe cytopathic effects, whereas cells infected with the E1B mutant virus during G1 exhibited a mild cytopathic effect. Viral DNA synthesis appeared independent of the cell cycle because equivalent amounts of viral DNA were synthesized in cells infected with either wild-type or E1B mutant virus. The inability of the E1B mutant virus to replicate was not mediated by the status of p53. These results define a novel property of the large tumor antigen of adenovirus in relieving growth restrictions imposed on viral replication by the cell cycle.

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.

A novel ubiquitin-like modification modulates the partitioning of the Ran-GTPase-activating protein RanGAP1 between the cytosol and the nuclear pore complex

Ran is a nuclear Ras-like GTPase that is required for the bidirectional transport of proteins and ribnucleoproteins across the nuclear pore complex (NPC). A key regulator of the Ran GTP/GDP cycle is the 70-kD Ran-GTPase-activating protein RanGAP1. Here, we report the identification and localization of a novel form of RanGAP1. Using peptide sequence analysis and specific mAbs, RanGAP1 was found to be modified by conjugation to a ubiquitin-like protein. Immunoblot analysis and immunolocalization by light and EM demonstrated that the 70-kD unmodified from of RanGAP1 is exclusively cytoplasmic, whereas the 90-kD modified form of RanGAP1 is associated with the cytoplasmic fibers of the NPC. The modified form of RanGAP1 also appeared to associated with the mitotic spindle apparatus during mitosis. These findings have specific implications for Ran function and broad implications for protein regulation by ubiquitin-like modifications. Moreover, the variety and function of ubiquitin-like protein modifications in the cell may be more diverse than previously realized.

Ubiquitous expression and testis-specific alternative polyadenylation of mRNA for the human Ran GTPase activator RanGAP1

RanGAP1 is the activator of the Ras-related nuclear GTPase Ran, which is involved in the nucleo-cytoplasmic transport of both, proteins and mRNAs, and also in cell cycle regulation. Here, we report a 2970-bp cDNA clone of RanGAP1 isolated from a HeLa lambda gt11 cDNA library. It contains a 215-bp 5' untranslated region (UTR) with a G + C-content of 68%, followed by a 1764-bp open reading frame and a 989-bp 3' UTR preceding a 77-bp poly(A)+ tail. RanGAP1 shows differential patterns of expression in human tissues. In addition to the 3.5-kb transcript present in all tissues and highly expressed in brain, thymus and testis, we found a second transcript of 2.8 kb in testis. In order to analyze this shorter transcript, we screened a human testis lambda gt10 cDNA library and cloned an alternatively polyadenylated RanGAP1 transcript. Taking the 3' UTR of RanGAP1, which lies downstream of the first polyadenylation signal, as a probe in Northern blot analysis, we confirmed that this second transcript found in testis results from a distinct 3' UTR.

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.

Ran, a GTPase involved in nuclear processes: its regulators and effectors

Ran is a small GTPase that has been implicated in a variety of nuclear processes, including the maintainance of nuclear structure, protein import, mRNA processing and export, and cell cycle regulation. There has been significant progress in determining the role of Ran in nuclear protein import. However, it has been unclear whether this role is sufficient to account for the diverse effects of disrupting Ran functions. Recently, several proteins have been identified that bind specifically to Ran and are, therefore, possible effectors. Other experiments using dominant mutants of Ran that block its GTP/GDP cycle have suggested that Ran may have multiple roles. Here, these results are summarised and discussed with respect to the action of Ran.

Direct and indirect association of the small GTPase ran with nuclear pore proteins and soluble transport factors: studies in Xenopus laevis egg extracts

Ran is a small GTPase that is required for protein import, mRNA export, and the maintenance of nuclear structures. To gain a better understanding of Ran's role in the nucleus, we have sought to use Xenopus egg extracts for the purification and characterization of proteins from egg extracts bound with a high affinity to a glutathione-S-transferase-Ran fusion protein (GST-Ran). We found that GST-Ran associates specifically with at least 10 extract proteins. We determined the identifies of six Ran-interacting proteins (Rips), and found that they include RanBP2/Nup358, Nup153, Importin beta, hsc70, RCC1, and RanBP1. On the basis of peptide sequence, a seventh Rip (p88) seems to be similar but not identical to Fug1/RanGAP1, the mammalian Ran-GTPase-activating protein. Gel filtration analysis of endogenous extract proteins suggests that Importin beta acts as a primary GTP-Ran effector. Both Ran and Importin beta are coimmunoprecipitated by anti-p340RanBP2 antibodies in the presence of nonhydrolyzable GTP analogues, suggesting that Ran-Importin beta complexes interact with p340RanBP2. Two other Rips, p18 and p88, are coprecipitated with p340RanBP2 in a nucleotide-independent manner. Analysis of the Ran-GTPase pathway in Xenopus extracts allows the examination of interactions between Ran-associated proteins under conditions that resemble in vivo conditions more closely than in assays with purified components, and it thereby allows additional insights into the molecular mechanism of nuclear transport.

A nuclear export signal is essential for the cytosolic localization of the Ran binding protein, RanBP1

RanBP1 is a Ran/TC4 binding protein that can promote the interaction between Ran and beta-importin /beta-karyopherin, a component of the docking complex for nuclear protein cargo. This interaction occurs through a Ran binding domain (RBD). Here we show that RanBP1 is primarily cytoplasmic, but the isolated RBD accumulates in the nucleus. A region COOH-terminal to the RBD is responsible for this cytoplasmic localization. This domain acts heterologously, localizing a nuclear cyclin B1 mutant to the cytoplasm. The domain contains a nuclear export signal that is necessary but not sufficient for the nuclear export of a functional RBD In transiently transfected cells, epitope-tagged RanBP1 promotes dexamethasone-dependent nuclear accumulation of a glucocorticoid receptor-green fluorescent protein fusion, but the isolated RBD potently inhibits this accumulation. The cytosolic location of RanBP1 may therefore be important for nuclear protein import. RanBP1 may provide a key link between the nuclear import and export pathways.

Nucleus-associated pools of Rna1p, the Saccharomyces cerevisiae Ran/TC4 GTPse activating protein involved in nucleus/cytosol transit

Rna1p is the GTPase activating enzyme for Ran/TC4, a Ras-like GTPase necessary for nuclear/cytosolic exchange. Although most wild-type Rna1p is located in the cytosol, we found that the vast majority of the mutant Rna1-1p and, under appropriate physiological conditions, a small portion of the wild-type Rna1p cofractionate with yeast nuclei. Subnuclear fractionation studies show that most of the Rna1p is tightly associated with nuclear components, and that a portion of the active protein can be solubilized by treatments that fail to solubilize inactive Rna1-1p. To learn the precise nuclear locations of the Rna1 proteins, we studied their subcellular distributions in HeLa cells. By indirect immuno-fluorescence we show that wild-type Rna1p has three subcellular locations. The majority of the protein is distributed throughout the cytosol, but a portion of the protein is nucleus-associated, located at both the cytosolic surface and within the nucleoplasm. Mutant Rna1-1p is found at the outer nuclear surface and in the cytosol. We propose that a small pool of the wild-type Rna1p is located in the nuclear interior, supporting the model that the same components of the Ran/TC4 GTPase cycle exist on both sides of the nuclear membrane.

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.

Dynamic localization of the nuclear import receptor and its interactions with transport factors

Characterization of the interactions between soluble factors required for nuclear transport is key to understanding the process of nuclear trafficking. Using a synthetic lethal screen with the rna1-1 strain, we have identified a genetic interaction between Rna1p, a GTPase activating protein required for nuclear transport, and yeast importin-beta, a component of the nuclear localization signal receptor. By the use of fusion proteins, we demonstrate that Rna1p physically interacts with importin-beta. Mutants in importin-beta exhibit in vivo nuclear protein import defects, and importin-beta localizes to the nuclear envelope along with other proteins associated with the nuclear pore complex. In addition, we present evidence that importin-alpha, but not importin-beta, mislocalizes to the nucleus in cells where the GTPase Ran is likely to be in the GDP-bound state. We suggest a model of nuclear transport in which Ran-mediated hydrolysis of GTP is necessary for the import of importin-alpha and the nuclear localization signal-bearing substrate into the nucleus, while exchange of GDP for GTP on Ran is required for the export of both mRNA and importin-alpha from the nucleus.

Evidence using a green fluorescent protein-glucocorticoid receptor chimera that the Ran/TC4 GTPase mediates an essential function independent of nuclear protein import

The Ran/TC4 GTPase is required for the nuclear accumulation of artificial karyophiles in permeabilized cell assays. To investigate Ran function in a physiologically intact setting using mammalian cells, we examined the effects of several Ran mutants on cell growth and on the nuclear translocation of a glucocorticoid receptor-green fluorescent protein fusion (GR-GFP). Glucocorticoid receptor is cytosolic in the absence of ligand, but translocates to the nucleus on binding the agonist dexamethasone. After transfection into baby hamster kidney cells (BHK21), GR-GFP was detectable in living cells by direct fluorescence microscopy. Addition of dexamethasone caused a rapid translocation of the chimeric protein from the cytosol into the nucleus (t1/2 approximately 5 min). Cotransfection with epitope-tagged, wild-type Ran led to expression of HA1-Ran that was approximately 1.6-fold higher than the level of the endogenous protein, but it had no deleterious effect on nuclear import of the GR-GFP. However, expression of the Ran mutants G19V, T24N, or a COOH-terminal deletion (delta C) mutant dramatically reduced the accumulation of GR-GFP in the nuclei. An L43E mutant of Ran was without significant effect on nuclear GR-GFP import. Identical results were obtained following micro-injection of recombinant Ran mutants into cells expressing GR-GFP. Significantly, all of the Ran mutants, including L43E, strongly inhibited cell growth. These results demonstrate the use of GR-GFP in real-time imaging of nuclear transport. They also show that multiple types of Ran mutant exert dominant effects on this process, and that normal Ran function requires cycling between the GTP- and GDP-bound states of the protein. Most importantly, the results with the L43E Ran mutant provide strong evidence that Ran mediates a function essential to cell viability that is independent of nuclear protein import.

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.

Nuclear proteins of quiescent Xenopus laevis cells inhibit DNA replication in intact and permeabilized nuclei

Quiescent cells from adult vertebrate liver and contact-inhibited or serum-deprived tissue cultures are active metabolically but do not carry out nuclear DNA replication and cell division. Replication of intact nuclei isolated from either quiescent Xenopus liver or cultured Xenopus A6 cells in quiescence was barely detectable in interphase extracts of Xenopus laevis eggs, although Xenopus sperm chromatin was replicated with approximately 100% efficiency in the same extracts. Permeabilization of nuclei from quiescent Xenopus liver or cultured Xenopus epithelial A6 cells did not facilitate efficient replication in egg extracts. Moreover, replication of Xenopus sperm chromatin in egg extracts was strongly inhibited by a soluble extract of isolated Xenopus liver nuclei; in contrast, complementary-strand synthesis on single-stranded DNA templates in egg extracts was not affected. Inhibition was specific to endogenous molecules localized preferentially in quiescent as opposed to proliferating cell nuclei, and was not due to suppression of cdk2 kinase activity. Extracts of Xenopus liver nuclei also inhibited growth of sperm nuclei formed in egg extracts. However, the rate and extent of decondensation of sperm chromatin in egg extracts were not affected. The formation of prereplication centers detected by anti-RP-A antibody was not affected by extracts of liver nuclei, but formation of active replication foci was blocked by the same extracts. Inhibition of DNA replication was alleviated when liver nuclear extracts were added to metaphase egg extracts before or immediately after Ca++ ion-induced transition to interphase. A plausible interpretation of our data is that endogenous inhibitors of DNA replication play an important role in establishing and maintaining a quiescent state in Xenopus cells, both in vivo and in cultured cells, perhaps by negatively regulating positive modulators of the replication machinery.

RAN/TC4 mutants identify a common requirement for snRNP and protein import into the nucleus

Kinetic competition experiments have demonstrated that at least some factors required for the nuclear import of proteins and U snRNPs are distinct. Both import processes require energy, and in the case of protein import, the energy requirement is known to be at least partly met by GTP hydrolysis by the Ran GTPase. We have compared the effects of nonhydrolyzable GTP analogues and two mutant Ran proteins on the nuclear import of proteins and U snRNPs in vitro. The mutant Ran proteins have different defects; Q69L (glutamine 69 changed to leucine) is defective in GTP hydrolysis while T24N (threonine 24 changed to asparagine) is defective in binding GTP. Both protein and snRNP import are sensitive either to the presence of the two mutant Ran proteins, which act as dominant negative inhibitors of nuclear import, or to incubation with nonhydrolyzable GTP analogues. This demonstrates that there is a requirement for a GTPase activity for the import of U snRNPs, as well as proteins, into the nucleus. The dominant negative effects of the two mutant Ran proteins indicate that the pathways of protein and snRNP import share at lease one common component.

Small G proteins of two green algae are localized to exocytic compartments and to flagella

The Ypt/Rab proteins are small GTPases, which belong to the Ras superfamily and have been shown to be involved in endo- and exocytosis in mammalian cells and yeast. Using affinity-purified antibodies specific for four Ypt proteins, namely Ypt1p, Ypt4p, Ypt5p and Ypt6p, of the multicellular green alga Volvox carteri (YptVp) and its close unicellular relative Chlamydomonas reinhardtii (YptCp), we examined the abundance of the corresponding antigens during the asexual life cycle of Volvox, and their intracellular localization. The YptV proteins were found in all stages throughout the asexual life cycle and are tightly associated with intracellular membranes. Indirect immunofluorescence revealed that YptV4p, YptV5p and YptV6p are present in perinuclear regions of the cell, indicating an association with the Golgi region. Golgi localization of YptV4p and YptV6p in Volvox was confirmed by immunogold electron microscopy. In contrast, we found Ypt1p associated with the contractile vacuole in both V. carteri and C. reinhardtii. Furthermore, the YptV proteins were also detected along the entire length of the flagella of somatic Volvox cells. This flagellar location was substantiated by western blot analysis of extracts prepared from isolated flagella of both algae. While localization to exocytic compartments is in agreement with the established Ypt/Rab function in intracellular vesicle transport of eukaryotic cells, presence in the algal flagellum is the first hint of a possible role for small G proteins also in motility organelles.

Allele-specific suppression of a Saccharomyces cerevisiae prp20 mutation by overexpression of a nuclear serine/threonine protein kinase

The yeast PRP20 protein is homologous to the RCC1 protein of higher eukaryotes and is required for mRNA export and maintenance of nuclear structure. RCC1/PRP20 act as guanine nucleotide exchange factors for the nuclear Ras-like Ran/GSP1 proteins. In a search for prp20-10 allele-specific high-copy-number suppressors, the KSP1 locus, encoding a serine/threonine protein kinase was isolated. Ksp1p is a nuclear protein that is not essential for vegetative growth of yeast. Inactivation of the kinase activity by a mutation affecting the catalytic center of the Ksp1p eliminated the suppressing activity. Based on the isolation of a protein kinase as a high-copy-number suppressor, the phosphorylation of Prp20p was examined. In vivo labeling experiments showed that Prp20p is a phosphoprotein; however, deletion of the KSP1 kinase did not affect Prp20p phosphorylation.

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.

Influence of Mg2+ on the structure and function of Rab5

Mg2+ inhibits GDP release from Rab5WT but not from Rab5S34N, a mutant lacking Ser34 critical for Mg2+ coordination in the nucleotide binding pocket. Thus, inhibition of GDP release is apparently exerted via coordination of Mg2+ between Rab5 and GDP. Mg2+ also induces conformational changes in Rab5WT, demonstrated by increased tryptophan fluorescence intensity and a red shift in lambda max for the GDP-bound protein. Mg(2+)-induced fluorescence changes are not observed for Rab5S34N. The correlation between Mg2+ effects on nucleotide exchange and the fluorescence properties of Rab5 suggests that a conformation promoted through Mg2+ coordination with Ser34 also contributes to inhibition of GDP release. The role of structural changes in GDP release was investigated using C- and N-terminal truncation mutants. Similar to Rab5WT, Mg2+ inhibits GDP release and alters the fluorescence of Rab5(1-198) but only partially inhibits release from Rab5(23-198) and fails to induce changes in the latter's fluorescence properties. Since Rab5(23-198) maintains Ser34 necessary for Mg2+ coordination, the lack of Mg(2+)-induced fluorescence changes suggests a requirement for the N-terminal domain to promote a conformation blocking GDP release. A model for mechanisms of interaction between Ras-like proteins and their exchange factors is proposed.

Regulatory GTPases

The past year has witnessed a tremendous increase in our understanding of the structures and interactions of the GTPases. The highlights include crystal structures of G alpha subunits, as well as the first complex between a GTPase (Rap1A) and an effector molecule (c-Raf1 Ras-binding domain). In the field of elongation factors (EFs), three very important structures have been determined: EF-G, the ternary complex of EF-Tu.GTP with aminoacyl-tRNA, and the EF-Tu.EF-Ts complex.

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.

GTP hydrolysis by Ran occurs at the nuclear pore complex in an early step of protein import

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

Requirement for Src family protein tyrosine kinases in G2 for fibroblast cell division

The protein tyrosine kinase c-Src is transiently activated at the transition from the G2 phase to mitosis in the cell cycle of mammalian fibroblasts. Fyn and Yes, the other members of the Src family present in fibroblasts, were also found to be activated at mitosis. In cells microinjected with a neutralizing antibody specific for Src, Fyn, and Yes (anti-cst.1) during G2, cell division was inhibited by 75 percent. The block occurred before nuclear envelope breakdown. Antibodies specific for phosphatidylinositol-3 kinase alpha and phospholipase C-gamma 1 had no effect. Microinjection of the Src homology 2 (SH2) domain of Fyn was also inhibitory. Functional redundancy between members of the Src family was observed; a Src-specific antibody had no effect in NIH 3T3 cells but inhibited cell division in fibroblasts in which the only functional Src family kinase was Src itself. Thus, Src family kinases and proteins associating with their SH2 domains are required for entry into mitosis.

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.

The C terminus of the nuclear RAN/TC4 GTPase stabilizes the GDP-bound state and mediates interactions with RCC1, RAN-GAP, and HTF9A/RANBP1

Ran/TC4 is a member of the Ras superfamily of GTPases. It is unusual in being predominantly nuclear and because it possesses an acidic -DEDDDL sequence instead of a consensus prenylation domain at the C terminus. Ran is required for nuclear protein import and cell cycle progression, and has been implicated in mRNA processing and export and DNA replication. The inhibition of cell cycle progression by a dominant gain-of-function mutant of Ran has been shown to be abrogated by removal of the -DEDDDL sequence, suggesting that this domain is essential for Ran function. We demonstrate here that the -DEDDDL sequence stabilizes GDP binding to Ran, and that the domain is required for high affinity interaction with a Ran-binding protein, HTF9A/RanBP1. HTF9A functions as a co-stimulator of Ran-GAP (GTPase activating protein) activity on wild-type Ran, but in the absence of the acidic C terminus of Ran, HTF9A behaves as a Ran-GAP inhibitor. An antibody directed against the C-terminal region preferentially recognizes the GTP-bound form of Ran, suggesting that this domain undergoes a nucleotide-dependent conformational change. The results suggest that the acidic C-terminal domain is important in modulating the interaction of Ran with regulatory factors, and implicate Ran-binding proteins in mediating the effects of Ran on cell cycle progression.

Nuclear envelope breakdown is under nuclear not cytoplasmic control in sea urchin zygotes

Nuclear envelope breakdown (NEB) and entry into mitosis are though to be driven by the activation of the p34cdc2-cyclin B kinase complex or mitosis promoting factor (MPF). Checkpoint control mechanisms that monitor essential preparatory events for mitosis, such as DNA replication, are thought to prevent entry into mitosis by downregulating MPF activation until these events are completed. Thus, we were surprised to find that when pronuclear fusion in sea urchin zygotes is blocked with Colcemid, the female pronucleus consistently breaks down before the male pronucleus. This is not due to regional differences in the time of MPF activation, because pronuclei touching each other break down asynchronously to the same extent. To test whether NEB is controlled at the nuclear or cytoplasmic level, we activated the checkpoint for the completion of DNA synthesis separately in female and male pronuclei by treating either eggs or sperm before fertilization with psoralen to covalently cross-link base-paired strands of DNA. When only the maternal DNA is cross-linked, the male pronucleus breaks down first. When the sperm DNA is cross-linked, male pronuclear breakdown is substantially delayed relative to female pronuclear breakdown and sometimes does not occur. Inactivation of the Colcemid after female NEB in such zygotes with touching pronuclei yields a functional spindle composed of maternal chromosomes and paternal centrosomes. The intact male pronucleus remains located at one aster throughout mitosis. In other experiments, when psoralen-treated sperm nuclei, over 90% of the zygote nuclei do not break down for at least 2 h after the controls even though H1 histone kinase activity gradually rises close to, or higher than, control mitotic levels. The same is true for normal zygotes treated with aphidicolin to block DNA synthesis. From these results, we conclude that NEB in sea urchin zygotes is controlled at the nuclear, not cytoplasmic, level, and that mitotic levels of cytoplasmic MPF activity are not sufficient to drive NEB for a nucleus that is under checkpoint control. Our results also demonstrate that the checkpoint for the completion of DNA synthesis inhibits NEB by acting primarily within the nucleus, not by downregulating the activity of cytoplasmic MPF.

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.

Isolation of a yeast protein kinase that is activated by the protein encoded by SRP1 (Srp1p) and phosphorylates Srp1p complexed with nuclear localization signal peptides

Srp1p, the protein encoded by SRP1 of Saccharomyces cerevisiae, is a nuclear-pore-associated protein. Its Xenopus homolog, importin, was recently shown to be an essential component required for nuclear localization signal (NLS)-dependent binding of karyophilic proteins to the nuclear envelope [Gorlich, D., Prehn, S., Laskey, R. A. & Hartman, E. (1994) Cell 79, 767-778]. We have discovered a protein kinase whose activity is stimulated by Srp1p (Srp1p fused to glutathione S-transferase and expressed in Escherichia coli) and is detected by phosphorylation of Srp1p and of a 36-kDa protein, a component of the protein kinase complex. The enzyme, called Srp1p kinase, is a protein-serine kinase and was found in extracts in two related complexes of approximately 180 kDa and 220 kDa. The second complex, when purified, contained four protein components including the 36-kDa protein. We observed that, upon purification of the kinase, phosphorylation of Srp1p became very weak, while activation of phosphorylation of the 36-kDa protein by Srp1p remained unaltered. Significantly, NLS peptides and the nuclear proteins we have tested greatly stimulated phosphorylation of Srp1p, suggesting that Srp1p, complexed with karyophilic proteins carrying an NLS, is the in vivo substrate of this protein kinase.

RNA1 encodes a GTPase-activating protein specific for Gsp1p, the Ran/TC4 homologue of Saccharomyces cerevisiae

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

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.

The mechanism of nuclear transport of natural or artificial transport substrates in digitonin-permeabilized cells

Characterization of nuclear protein transport in digitonin-permeabilized cells revealed that the number of the nuclear localization signal sequences (NLS) within the transport substrate basically influences the mechanism of the transport reaction. Phycoerythrine-NLS transport substrate carrying a maximum of 4–5 conjugated NLSs/subunit, or Bsp methyltransferase-NLS fusion protein were efficiently transported into the nuclei of digitonin-permeabilized cultured cells without any exogenously added cytosolic protein. All the characteristic properties of in vivo nuclear transport are faithfully reproduced with these transport substrates: (i) the transport requires a functional NLS in the transported protein, a transport-incompetent mutant NLS being ineffective; (ii) the transport is energy dependent; (iii) the wild type nuclear localization peptide efficiently competes for transport, while the transport-incompetent mutant peptide does not; and (iv) wheat germ agglutinin inhibits this transport reaction. Nuclear transport observed with these substrates was not due to any damage of the nuclear membrane or inefficient extraction of the cytosolic proteins during the permeabilization of the cells. The nuclear transport was proportional to the number of conjugated NLSs. Nuclear transport of phycoerythrine carrying 7–8 conjugated NLSs/subunit required the addition of exogenous cytosolic proteins. This transport also fulfilled all the characteristic properties of an authentic nuclear transport. Nuclear transport with different combinations of transport substrates further supported the assumption that distinct transport mechanisms operate for different substrates. From a mixture of PE-NLS7-8 and Bsp methyltransferase-NLS, the highly conjugated substrate was completely retained in the cytoplasm in the absence of exogenous cytosol, while Bsp methyltransferase-NLS was efficiently transported. Exogenous cytosol promoted the nuclear transport of the highly conjugated substrate.

The Ran/TC4 GTPase-binding domain: identification by expression cloning and characterization of a conserved sequence motif

Ran/TC4 is an essential, nuclear GTPase implicated in the initiation of DNA replication, entry into and exit from mitosis, and in nuclear RNA and protein transport through the nuclear pore complex. This diversity of functions suggests that Ran interacts with a large number of down-stream targets. Using an overlay assay, we detected a family of putative target proteins that associate with GTP-bound Ran. The sequence of only one such protein, HTF9a/RanBP1, is known. We have now cloned two additional Ran-binding proteins, allowing identification of a distinctive, highly conserved sequence motif of approximately 150 residues. This motif represents a minimal Ran-binding domain that stabilizes the GTP-bound state of Ran. The isolated domain also functions as a coactivator of Ran-GTPase-activating protein. Mutation of a conserved residue within the Ran-binding domain of HTF9a protein drastically reduced Ran binding. Ran-binding proteins coimmunoprecipitated with epitope-tagged Ran from cell lysates, suggesting that these proteins may associate in vivo. A previously uncharacterized Caenorhabditis elegans gene could encode a protein (96 kDa) possessing two Ran-binding domains. This open reading frame also contains similarities to nucleoporins, suggesting a functional link between Ran and nuclear pore complexes.

Separate domains of the Ran GTPase interact with different factors to regulate nuclear protein import and RNA processing

The small Ras-related GTP binding and hydrolyzing protein Ran has been implicated in a variety of processes, including cell cycle progression, DNA synthesis, RNA processing, and nuclear-cytosolic trafficking of both RNA and proteins. Like other small GTPases, Ran appears to function as a switch: Ran-GTP and Ran-GDP levels are regulated both by guanine nucleotide exchange factors and GTPase activating proteins, and Ran-GTP and Ran-GDP interact differentially with one or more effectors. One such putative effector, Ran-binding protein 1 (RanBP1), interacts selectively with Ran-GTP. Ran proteins contain a diagnostic short, acidic, carboxyl-terminal domain, DEDDDL, which, at least in the case of human Ran, is required for its role in cell cycle regulation. We show here that this domain is required for the interaction between Ran and RanBP1 but not for the interaction between Ran and a Ran guanine nucleotide exchange factor or between Ran and a Ran GTPase activating protein. In addition, Ran lacking this carboxyl-terminal domain functions normally in an in vitro nuclear protein import assay. We also show that RanBP1 interacts with the mammalian homolog of yeast protein RNA1, a protein involved in RNA transport and processing. These results are consistent with the hypothesis that Ran functions directly in at least two pathways, one, dependent on RanBP1, that affects cell cycle progression and RNA export, and another, independent of RanBP1, that affects nuclear protein import.

Crystal structure of the nuclear Ras-related protein Ran in its GDP-bound form

The Ran proteins constitute a distinct branch of the superfamily of Ras-related GTP-binding proteins which function as molecular switches cycling between GTP-bound 'on' and GDP-bound 'off' states. Ran is located predominantly in the nucleus of eukaryotic cells and is involved in the nuclear import of proteins as well as in control of DNA synthesis and of cell-cycle progression. We report here the crystal structure at 2.3 A resolution of human Ran (Mr 24K) complexed with GDP and Mg2+. This structure reveals a similarity with the Ras core (G-domain) but with significant variations in regions involved in GDP and Mg2+ coordination (switch I and switch II regions in Ras), suggesting that there could be major conformational changes upon GTP binding. In addition to the G-domain, an extended chain and an alpha-helix were identified at the carboxy terminus. The amino-terminal (amino-acid residues MAAQGEP) stretch and the acidic tail (DEDDDL) appear to be flexible in the crystal structure.

Regulation of Cdc2/cyclin B activation by Ran, a Ras-related GTPase

During the cell cycle, a checkpoint prevents the initiation of mitosis until S-phase is completed. The molecular mechanism may involve the RCC1 protein, which catalyses guanine nucleotide exchange on the Ras-related nuclear protein, Ran (or TC4). Genetic studies have suggested that RCC1 may be involved in sensing the replication state of DNA and controlling the activation of Cdc2/cyclin B protein kinase through Ran. In this report, we present direct biochemical evidence for the post-translational control of Cdc2/cyclin B activation by Ran. In a cell-free system of concentrated Xenopus egg extracts supplemented with nuclei, a mutant form of Ran (T24N) analogous to dominant inactive mutants of other Ras-related GTPases inhibits Cdc2/cyclin B activation in the presence of replicating nuclear DNA. This role for Ran is mediated through control of the tyrosine phosphorylation state of Cdc2 and appears to be distinct from other effects on nuclear import, nuclear formation and DNA replication. When extracts were supplemented with RCC1 protein prior to addition of Ran T24N, inhibition of Cdc2/cyclin B by Ran T24N was relieved. This suggests that Ran T24N may act in a dominant manner by sequestering RCC1 in an inactive form. In contrast to Ran T24N, a mutant of Ran (Q69L) defective in GTPase activity and hence locked in the GTP-bound state has no inhibitory effect on Cdc2/cyclin B activation. In the light of these results, we propose that generation of the GTP-bound form of Ran is required for Cdc2/cyclin B activation and entry into mitosis when this process is coupled to the progression of S-phase.