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

A Mant-GDP Dissociation Assay to Compare the Guanine Nucleotide Binding Preference of Small GTPases

Small GTPases are cellular switches that are switched on when bound to GTP and switched off when bound to GDP. Different small GTPase proteins or those with mutations may bind to GTP or GDP with different relative affinities. However, small GTPases generally have very high affinities for guanine nucleotides, rendering it difficult to compare the relative binding affinities for GTP and GDP. Here we developed a method for comparing the relative binding strength of a protein to GTP and GDP using a mant-GDP dissociation assay, whereby the abilities of GTP and GDP to induce the dissociation of bound mant-GDP are compared. This equilibrium type assay is simple, economic, and much faster than obtaining each protein's affinity for GDP and GTP. The GDP/GTP preference value obtained is useful for comparing the relative GTP/GDP binding preferences of different GTPases or different mutants, even though it is not the real GDP/GTP affinity ratio (but rather an estimation of the ratio).

Nicotiana benthamiana RanBP1-1 Is Involved in the Induction of Disease Resistance via Regulation of Nuclear-Cytoplasmic Transport of Small GTPase Ran

Plant cells enhance the tolerances to abiotic and biotic stresses via recognition of the stress, activation and nuclear import of signaling factors, up-regulation of defense genes, nuclear export of mRNA and translation of defense proteins. Nuclear pore-mediated transports should play critical roles in these processes, however, the regulatory mechanisms of nuclear-cytoplasmic transport during stress responses are largely unknown. In this study, a regulator of nuclear export of RNA and proteins, NbRanBP1-1 (Ran-binding protein1-1), was identified as an essential gene for the resistance of Nicotiana benthamiana to potato blight pathogen Phytophthora infestans. NbRanBP1-1-silenced plants showed delayed accumulation of capsidiol, a sesquiterpenoid phytoalexin, in response to elicitor treatment, and reduced resistance to P. infestans. Abnormal accumulation of mRNA was observed in NbRanBP1-1-silenced plants, indicating that NbRanBP1-1 is involved in the nuclear export of mRNA. In NbRanBP1-1-silenced plants, elicitor-induced expression of defense genes, NbEAS and NbWIPK, was not affected in the early stage of defense induction, but the accumulation of NbWIPK protein was reduced. Nuclear export of the small G-protein NbRan1a was activated during the induction of plant defense, whereas this process was compromised in NbRanBP1-1-silenced plants. Silencing of genes encoding the nuclear pore proteins, Nup75 and Nup160, also caused abnormal nuclear accumulation of mRNA, defects in the nuclear export of NbRan1a, and reduced production of capsidiol, resulting in decreased resistance to P. infestans. These results suggest that nuclear export of NbRan is a key event for defense induction in N. benthamiana, and both RanBP1-1 and nucleoporins play important roles in the process.

RanBP1 governs spindle assembly by defining mitotic Ran-GTP production

Accurate control of the Ras-related nuclear protein (Ran) GTPase cycle depends on the regulated activity of regulator of chromosome condensation 1 (RCC1), Ran's nucleotide exchange factor. RanBP1 has been characterized as a coactivator of the Ran GTPase-activating protein RanGAP1. RanBP1 can also form a stable complex with Ran and RCC1, although the dynamics and function of this complex remain poorly understood. Here, we show that formation of the heterotrimeric RCC1/Ran/RanBP1 complex in M phase Xenopus egg extracts controls both RCC1's enzymatic activity and partitioning between the chromatin-bound and soluble pools of RCC1. This mechanism is critical for spatial control of Ran-guanosine triphosphate (GTP) gradients that guide mitotic spindle assembly. Moreover, phosphorylation of RanBP1 drives changes in the dynamics of chromatin-bound RCC1 pools at the metaphase-anaphase transition. Our findings reveal an important mitotic role for RanBP1, controlling the spatial distribution and magnitude of mitotic Ran-GTP production and thereby ensuring accurate execution of Ran-dependent mitotic events.

Using FRET to study RanGTP gradients in live mouse oocytes

Oocytes are extremely large cells that have to coordinate accurate chromosome segregation, asymmetric cytoplasm partitioning together with their own development as fertilizable gametes. For this, they undergo both global (cell cycle progression related) and local changes. It is therefore essential to be able to monitor local changes as they take place in live maturing oocytes. We describe here a method to follow RanGTP gradients using FRET technology in vivo.

Nuclear reformation after mitosis requires downregulation of the Ran GTPase effector RanBP1 in mammalian cells

The GTPase Ran regulates nucleocytoplasmic transport in interphase and spindle organisation in mitosis via effectors of the importin beta superfamily. Ran-binding protein 1 (RanBP1) regulates guanine nucleotide turnover on Ran, as well as its interactions with effectors. Unlike other Ran network members that are steadily expressed, RanBP1 abundance is modulated during the mammalian cell cycle, peaking in mitosis and declining at mitotic exit. Here, we show that RanBP1 downregulation takes place in mid to late telophase, concomitant with the reformation of nuclei. Mild RanBP1 overexpression in murine cells causes RanBP1 to persist in late mitosis and hinders a set of events underlying the telophase to interphase transition, including chromatin decondensation, nuclear expansion and nuclear lamina reorganisation. Moreover, the reorganisation of nuclear pores fails associated with defective nuclear relocalisation of NLS cargoes. Co-expression of importin beta, together with RanBP1, however mitigates these defects. Thus, RanBP1 downregulation is required for nuclear reorganisation pathways operated by importin beta after mitosis.

The nuclear transport machinery in Caenorhabditis elegans: A central role in morphogenesis

Our full understanding of the various roles for the nuclear transport machinery has come from a variety of model organisms including yeast, nematodes, fruit flies and vertebrates. Using the nematode Caenorhabditis elegans, it has been shown that the karyopherin family of nuclear transporters and the components of the Ran cycle have roles not only in nuclear protein transport, but also in mitotic spindle formation and regulation, and in nuclear envelope assembly. These studies have also demonstrated a role for nuclear transport factors in cellular differentiation and development, particularly for the formation of germ cells. This review highlights the small number of studies in C. elegans that have been critical to our understanding of this important cellular process.

Discovery of parasite virulence genes reveals a unique regulator of chromosome condensation 1 ortholog critical for efficient nuclear trafficking

Eukaryotic parasites are a leading cause of morbidity and mortality worldwide, yet little is known about the genetic basis of their virulence. Here, we present a forward genetic screen to study pathogenesis in the protozoan parasite Toxoplasma gondii. By using modified signature-tagged mutagenesis, the growth of 6,300 T. gondii insertional mutants was compared in cell culture and murine infection to identify genes required specifically in vivo. One of the 39 avirulent mutants is disrupted in a divergent ortholog of the regulator of chromosome condensation 1 (RCC1), which is critical for nuclear trafficking in model systems. Although this RCC1 mutant grows similar to wild type in standard tissue culture conditions, it is growth-impaired under nutrient limitation. Genetic complementation of mutant parasites with the T. gondii RCC1 gene fully restores both virulence in mice and growth under low-nutrient conditions. Further analysis shows that there is a significant defect in nuclear trafficking in the RCC1 mutant. These findings suggest that the rate of nuclear transport is a critical factor affecting growth in low-nutrient conditions in vivo and in vitro. Additionally, we observed that although RCC1 proteins are highly conserved in organisms from humans to yeast, no protozoan parasite encodes a characteristic RCC1. This protein divergence may represent a unique mechanism of nucleocytoplasmic transport. This study illustrates the power of this forward genetics approach to identify atypical virulence mechanisms.

Nuclear localization signal peptides induce molecular delivery along microtubules

Many essential processes in eukaryotic cells depend on regulated molecular exchange between its two major compartments, the cytoplasm and the nucleus. In general, nuclear import of macromolecular complexes is dependent on specific peptide signals and their recognition by receptors that mediate translocation through the nuclear pores. Here we address the question of how protein products bearing such nuclear localization signals arrive at the nuclear membrane before import, i.e., by simple diffusion or perhaps with assistance of cytoskeletal elements or cytoskeleton-associated motor proteins. Using direct single-particle tracking and detailed statistical analysis, we show that the presence of nuclear localization signals invokes active transport along microtubules in a cell-free Xenopus egg extract. Chemical and antibody inhibition of minus-end directed cytoplasmic dynein blocks this active movement. In the intact cell, where microtubules project radially from the centrosome, such an interaction would effectively deliver nuclear-targeted cargo to the nuclear envelope in preparation for import.

Importin beta: conducting a much larger cellular symphony

Importin beta, once thought to be exclusively a nuclear transport receptor, is emerging as a global regulator of diverse cellular functions. Importin beta acts positively in multiple interphase roles: in nuclear import, as a chaperone for highly charged nuclear proteins, and as a potential motor adaptor for movement along microtubules. In contrast, importin beta plays a negative regulatory role in mitotic spindle assembly, centrosome dynamics, nuclear membrane formation, and nuclear pore assembly. In most of these, importin beta is counteracted by its regulator, Ran-GTP. In light of this, the recent discovery of Ran's involvement in spindle checkpoint control suggested a potential new arena for importin beta action, although it is also possible that one of importin beta's relatives, the karyopherin family of proteins, manages this checkpoint. Lastly, importin beta plays a role in transducing damage signals from the axons of injured neurons back to the cell body.

Importin beta negatively regulates nuclear membrane fusion and nuclear pore complex assembly

Assembly of a eukaryotic nucleus involves three distinct events: membrane recruitment, fusion to form a double nuclear membrane, and nuclear pore complex (NPC) assembly. We report that importin beta negatively regulates two of these events, membrane fusion and NPC assembly. When excess importin beta is added to a full Xenopus nuclear reconstitution reaction, vesicles are recruited to chromatin but their fusion is blocked. The importin beta down-regulation of membrane fusion is Ran-GTP reversible. Indeed, excess RanGTP (RanQ69L) alone stimulates excessive membrane fusion, leading to intranuclear membrane tubules and cytoplasmic annulate lamellae-like structures. We propose that a precise balance of importin beta to Ran is required to create a correct double nuclear membrane and simultaneously to repress undesirable fusion events. Interestingly, truncated importin beta 45-462 allows membrane fusion but produces nuclei lacking any NPCs. This reveals distinct importin beta-regulation of NPC assembly. Excess full-length importin beta and beta 45-462 act similarly when added to prefused nuclear intermediates, i.e., both block NPC assembly. The importin beta NPC block, which maps downstream of GTPgammaS and BAPTA-sensitive steps in NPC assembly, is reversible by cytosol. Remarkably, it is not reversible by 25 microM RanGTP, a concentration that easily reverses fusion inhibition. This report, using a full reconstitution system and natural chromatin substrates, significantly expands the repertoire of importin beta. Its roles now encompass negative regulation of two of the major events of nuclear assembly: membrane fusion and NPC assembly.

The Ran GTPase regulates kinetochore function

The Ran GTPase is required for nuclear assembly, nuclear transport, spindle assembly, and mitotic regulation. While the first three processes are relatively well understood, details of Ran's role in mitotic progression remain obscure. We have found that elevated levels of Ran's exchange factor (RCC1) abrogate the spindle assembly checkpoint in Xenopus egg extracts, restore APC/C activity, and disrupt the kinetochore localization of checkpoint regulators, including Mad2, CENP-E, Bub1, and Bub3. Depletion of Ran's GTPase activating protein (RanGAP1) and its accessory factor (RanBP1) similarly abrogates checkpoint arrest. By contrast, the addition of RanGAP1 and RanBP1 to extracts with exogenous RCC1 restores the spindle checkpoint. Together, these observations suggest that the spindle checkpoint is directly responsive to Ran-GTP levels. Finally, we observe a clear wave of RCC1 association to mitotic chromosomes at the metaphase-anaphase transition in normal cycling extracts, suggesting that this mechanism has an important role in unperturbed cell cycles.

Regulation of nuclear import and export by the GTPase Ran

This review focuses on the control of nuclear import and export pathways by the small GTPase Ran. Transport of signal-containing cargo substrates is mediated by receptors that bind to the cargo proteins and RNAs and deliver them to the appropriate cellular compartment. Ran is an evolutionarily conserved member of the Ras superfamily that regulates all receptor-mediated transport between the nucleus and the cytoplasm. We describe the identification and characterization of the RanGTPase and its binding partners: the guanine nucleotide exchange factor, RanGEF; the GTPase activating protein, RanGAP; the soluble import and export receptors; Ran-binding domain-(RBD) containing proteins; and NTF2 and related factors.

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.

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.

Ran binding protein RanBP1 in zebrafish embryonic development

Ran, which functions in nucleocytoplasmic transport and mitosis, binds to and is regulated in part by RanBP1. We have identified a zebrafish RanBP1 cDNA and report that it encodes for a polypeptide of 233 amino acids with considerable similarity to human and Xenopus RanBP1, despite the fact that it is 10% longer due to an extension at its carboxy terminus. RanBP1 mRNA is present as a maternal transcript and is expressed ubiquitously throughout the developing embryo. At the protein level, RanBP1 is present at all embryonic stages. Surprisingly, the ectopic overexpression of the protein had no obvious effect on embryogenesis. Attempts were also made to down-regulate RanBP1 activity by RNA interference. Injecting double-stranded RNA augmented both the mortality rate and the frequency of induced defects. Specific defects accompanied by changes in RanBP1 expression were not seen, leading us to propose that RNAi is not a reliable method for deregulating the activity of constitutively expressed genes, like RanBP1, in zebrafish. Mol. Reprod. Dev. 59:235-248, 2001.

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.

Yeast Ran-binding protein 1 (Yrb1) shuttles between the nucleus and cytoplasm and is exported from the nucleus via a CRM1 (XPO1)-dependent pathway

The RanGTP-binding protein RanBP1, which is located in the cytoplasm, has been implicated in release of nuclear export complexes from the cytoplasmic side of the nuclear pore complex. Here we show that Yrb1 (the yeast homolog of RanBP1) shuttles between the nucleus and the cytoplasm. Nuclear import of Yrb1 is a facilitated process that requires a short basic sequence within the Ran-binding domain (RBD). By contrast, nuclear export of Yrb1 requires an intact RBD, which forms a ternary complex with the Xpo1 (Crm1) NES receptor in the presence of RanGTP. Nuclear export of Yrb1, however, is insensitive towards leptomycin B, suggesting a novel type of substrate recognition between Yrb1 and Xpo1. Taken together, these data suggest that ongoing nuclear import and export is an important feature of Yrb1 function in vivo.

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.

The role of Ran in nuclear function

There have been three major advances in the understanding of the Ran pathway during the past two years: first, a general model for Ran's function in nuclear transport has been proposed and extensively tested. Second, crystal structures for many proteins that regulate or interact with Ran have been reported, which provide molecular details of how Ran works. Third, it has been documented that Ran regulates mitotic spindle assembly in a transport-independent fashion.

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.

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.

Isolated mammalian and Schizosaccharomyces pombe ran-binding domains rescue S. pombe sbp1 (RanBP1) genomic mutants

Mammalian Ran-binding protein-1 (RanBP1) and its fission yeast homologue, sbp1p, are cytosolic proteins that interact with the GTP-charged form of Ran GTPase through a conserved Ran-binding domain (RBD). In vitro, this interaction can accelerate the Ran GTPase-activating protein-mediated hydrolysis of GTP on Ran and the turnover of nuclear import and export complexes. To analyze RanBP1 function in vivo, we expressed exogenous RanBP1, sbp1p, and the RBD of each in mammalian cells, in wild-type fission yeast, and in yeast whose endogenous sbp1 gene was disrupted. Mammalian cells and wild-type yeast expressing moderate levels of each protein were viable and displayed normal nuclear protein import. sbp1(-) yeast were inviable but could be rescued by all four exogenous proteins. Two RBDs of the mammalian nucleoporin RanBP2 also rescued sbp1(-) yeast. In mammalian cells, wild-type yeast, and rescued mutant yeast, exogenous full-length RanBP1 and sbp1p localized predominantly to the cytosol, whereas exogenous RBDs localized predominantly to the cell nucleus. These results suggest that only the RBD of sbp1p is required for its function in fission yeast, and that this function may not require confinement of the RBD to the cytosol. The results also indicate that the polar amino-terminal portion of sbp1p mediates cytosolic localization of the protein in both yeast and mammalian cells.

A role for RanBP1 in the release of CRM1 from the nuclear pore complex in a terminal step of nuclear export

We recently developed an assay in which nuclear export of the shuttling transcription factor NFAT (nuclear factor of activated T cells) can be reconstituted in permeabilized cells with the GTPase Ran and the nuclear export receptor CRM1. We have now used this assay to identify another export factor. After preincubation of permeabilized cells with a Ran mutant that cannot hydrolyze GTP (RanQ69L), cytosol supports NFAT export, but CRM1 and Ran alone do not. The RanQ69L preincubation leads to accumulation of CRM1 at the cytoplasmic periphery of the nuclear pore complex (NPC) in association with the p62 complex and Can/Nup214. RanGTP-dependent association of CRM1 with these nucleoporins was reconstituted in vitro. By biochemical fractionation and reconstitution, we showed that RanBP1 restores nuclear export after the RanQ69L preincubation. It also stimulates nuclear export in cells that have not been preincubated with RanQ69L. RanBP1 as well as Ran-binding domains of the cytoplasmic nucleoporin RanBP2 promote the release of CRM1 from the NPC. Taken together, our results indicate that RanGTP is important for the targeting of export complexes to the cytoplasmic side of the NPC and that RanBP1 and probably RanBP2 are involved in the dissociation of nuclear export complexes from the NPC in a terminal step of transport.

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.

Two E2F sites control growth-regulated and cell cycle-regulated transcription of the Htf9-a/RanBP1 gene through functionally distinct mechanisms

The gene encoding Ran-binding protein 1 (RanBP1) is transcribed in a cell cycle-dependent manner. The RanBP1 promoter contains two binding sites for E2F factors, named E2F-c, located proximal to the transcription start, and E2F-b, falling in a more distal promoter region. We have now induced site-directed mutagenesis in both sites. We have found that the distal E2F-b site, together with a neighboring Sp1 element, actively controls up-regulation of transcription in S phase. The proximal E2F-c site plays no apparent role in cycling cells yet is required for transcriptional repression upon growth arrest. Protein binding studies suggest that each E2F site mediates specific interactions with individual E2F family members. In addition, transient expression assays with mutagenized promoter constructs indicate that the functional role of each site is also dependent on its position relative to other regulatory elements in the promoter context. Thus, the two E2F sites play opposite genetic functions and control RanBP1 transcription through distinct molecular mechanisms.

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.

Transport routes through the nuclear pore complex

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

Functions of the GTPase Ran in RNA export from the nucleus

Significant and exciting advances in the field of RNA and protein export have been made recently, due in large part to discovery of the roles played by Ran, a small, soluble GTPase present in both the nucleus and cytoplasm of all eukaryotic cells. Ran is thought to be primarily bound to GTP in the nucleus and to GDP in the cytoplasm, as a result of the assymetric distribution of factors that interact with Ran to promote guanine nucleotide exchange (in the nucleus) and GTP hydrolysis (in the cytoplasm). A key function of the nuclear Ran.GTP is to support formation of complexes containing an export receptor (an exportin) and cargos such as RNAs, RNPs or proteins that are destined for export. In the cytoplasm, removal of the Ran.GTP from the complex results in its destabilization and release of the export cargo. Although Ran.GTP is required for formation of the export complex, GTP hydrolysis does not appear to be necessary for translocation through the nuclear pore complex or cytoplasmic release. Nevertheless, the GTPase of Ran does appear to be required in as yet unidentified intranuclear steps prior to export of some, but not all, RNAs.