The ran GTPase regulates mitotic spindle assembly

Stathmin/Op18 phosphorylation is regulated by microtubule assembly

Stathmin/Op 18 is a microtubule (MT) dynamics-regulating protein that has been shown to have both catastrophe-promoting and tubulin-sequestering activities. The level of stathmin/Op18 phosphorylation was proved both in vitro and in vivo to be important in modulating its MT-destabilizing activity. To understand the in vivo regulation of stathmin/Op18 activity, we investigated whether MT assembly itself could control phosphorylation of stathmin/Op18 and thus its MT-destabilizing activity. We found that MT nucleation by centrosomes from Xenopus sperm or somatic cells and MT assembly promoted by dimethyl sulfoxide or paclitaxel induced stathmin/Op18 hyperphosphorylation in Xenopus egg extracts, leading to new stathmin/Op18 isoforms phosphorylated on Ser 16. The MT-dependent phosphorylation of stathmin/Op18 took place in interphase extracts as well, and was also observed in somatic cells. We show that the MT-dependent phosphorylation of stathmin/Op18 on Ser 16 is mediated by an activity associated to the MTs, and that it is responsible for the stathmin/Op18 hyperphosphorylation reported to be induced by the addition of "mitotic chromatin." Our results suggest the existence of a positive feedback loop, which could represent a novel mechanism contributing to MT network control.

Role of importin-beta in coupling Ran to downstream targets in microtubule assembly

The guanosine triphosphatase Ran stimulates assembly of microtubule asters and spindles in mitotic Xenopus egg extracts. A carboxyl-terminal region of the nuclear-mitotic apparatus protein (NuMA), a nuclear protein required for organizing mitotic spindle poles, mimics Ran's ability to induce asters. This NuMA fragment also specifically interacted with the nuclear transport factor, importin-beta. We show that importin-beta is an inhibitor of microtubule aster assembly in Xenopus egg extracts and that Ran regulates the interaction between importin-beta and NuMA. Importin-beta therefore links NuMA to regulation by Ran. This suggests that similar mechanisms regulate nuclear import during interphase and spindle assembly during mitosis.

Importin beta is a mitotic target of the small GTPase Ran in spindle assembly

The GTPase Ran has recently been shown to stimulate microtubule polymerization in mitotic extracts, but its mode of action is not understood. Here we show that the mitotic role of Ran is largely mediated by the nuclear transport factor importin beta. Importin beta inhibits spindle formation in vitro and in vivo and sequesters an aster promoting activity (APA) that consists of multiple, independent factors. One component of APA is the microtubule-associated protein NuMA. NuMA and other APA components are discharged from importin beta by RanGTP and induce spindle-like structures in the absence of centrosomes, chromatin, or Ran. We propose that RanGTP functions in mitosis as in interphase by locally releasing cargoes from transport factors. In mitosis, this promotes spindle assembly by organizing microtubules in the vicinity of chromosomes.

Ran induces spindle assembly by reversing the inhibitory effect of importin alpha on TPX2 activity

The small GTPase Ran, bound to GTP, is required for the induction of spindle formation by chromosomes in M phase. High concentrations of Ran.GTP are proposed to surround M phase chromatin. We show that the action of Ran.GTP in spindle formation requires TPX2, a microtubule-associated protein previously known to target a motor protein, Xklp2, to microtubules. TPX2 is normally inactivated by binding to the nuclear import factor, importin alpha, and is displaced from importin alpha by the action of Ran.GTP. TPX2 is required for Ran.GTP and chromatin-induced microtubule assembly in M phase extracts and mediates spontaneous microtubule assembly when present in excess over free importin alpha. Thus, components of the nuclear transport machinery serve to regulate spindle formation in M phase.

Small GTP-binding proteins

Small GTP-binding proteins (G proteins) exist in eukaryotes from yeast to human and constitute a superfamily consisting of more than 100 members. This superfamily is structurally classified into at least five families: the Ras, Rho, Rab, Sar1/Arf, and Ran families. They regulate a wide variety of cell functions as biological timers (biotimers) that initiate and terminate specific cell functions and determine the periods of time for the continuation of the specific cell functions. They furthermore play key roles in not only temporal but also spatial determination of specific cell functions. The Ras family regulates gene expression, the Rho family regulates cytoskeletal reorganization and gene expression, the Rab and Sar1/Arf families regulate vesicle trafficking, and the Ran family regulates nucleocytoplasmic transport and microtubule organization. Many upstream regulators and downstream effectors of small G proteins have been isolated, and their modes of activation and action have gradually been elucidated. Cascades and cross-talks of small G proteins have also been clarified. In this review, functions of small G proteins and their modes of activation and action are described.

The Ran-GTPase and cell-cycle control

RCC1, the chromatin-bound guanine-nucleotide exchange factor (GEF) for the small nuclear GTPase, Ran, is required for coordinating the onset of mitosis with S-phase completion in mammalian cells. Other defects in the Ran-GTPase network also result in disruption of cell-cycle processes such as DNA replication, exit from mitosis and, at least in budding yeast, accurate chromosome segregation. However, the Ran system is now best known for its pivotal role in nucleocytoplasmic transport, where RanGTP is used as a positional flag for the nucleus during interphase. Ran's effectors are the shuttling transport factors, importins and exportins, which facilitate the transit of cargoes between the nucleus and cytoplasm: RanGTP regulates their cargo-binding properties so that they can move their cargo in the correct direction. RanGTP also plays a separate role during mitosis, influencing microtubule polymerisation, possibly specifically in the vicinity of chromosomes. Most recently, Ran has been shown to be crucial for the regeneration of a nuclear envelope after exit from mitosis. So, can the problems with cell-cycle progression and control induced by perturbing the Ran-system be attributed to defects in these three processes? This article examines this issue, concentrating on vertebrate systems. BioEssays 23:77-85, 2001.

The spindle: a dynamic assembly of microtubules and motors

In all eukaryotes, a microtubule-based structure known as the spindle is responsible for accurate chromosome segregation during cell division. Spindle assembly and function require localized regulation of microtubule dynamics and the activity of a variety of microtubule-based motor proteins. Recent work has begun to uncover the molecular mechanisms that underpin this process. Here we describe the structural and dynamic properties of the spindle, and introduce the current concepts regarding how a bipolar spindle is assembled and how it functions to segregate chromosomes.

Yeast Ran-binding protein Yrb1p is required for efficient proteolysis of cell cycle regulatory proteins Pds1p and Sic1p

Ubiquitin-dependent proteolysis of specific target proteins is required for several important steps during the cell cycle. Degradation of such proteins is strictly cell cycle-regulated and triggered by two large ubiquitin ligases, termed anaphase-promoting complex (APC) and Skp1/Cullin/F-box complex (SCF). Here we show that yeast Ran-binding protein 1 (Yrb1p), a predominantly cytoplasmic protein implicated in nucleocytoplasmic transport, is required for cell cycle regulated protein degradation. Depletion of Yrb1p results in the accumulation of unbudded G(1) cells and of cells arrested in mitosis implying a function of Yrb1p in the G(1)/S transition and in the progression through mitosis. Temperature-sensitive yrb1-51 mutants are defective in APC-mediated degradation of the anaphase inhibitor protein Pds1p and in degradation of the cyclin-dependent kinase inhibitor Sic1p, a target of SCF. Thus, Yrb1p is crucial for efficient APC- and SCF-mediated proteolysis of important cell cycle regulatory proteins. We have identified the UBS1 gene as a multicopy suppressor of yrb1-51 mutants. Ubs1p is a nuclear protein, and its deletion is synthetic lethal with a yrb1-51 mutation. Interestingly, UBS1 was previously identified as a multicopy suppressor of cdc34-2 mutants, which are defective in SCF activity. We suggest that Ubs1p may represent a link between nucleocytoplasmic transport and ubiquitin ligase activity.

Identification of novel MAP kinase pathway signaling targets by functional proteomics and mass spectrometry

Functional proteomics provides a powerful method for monitoring global molecular responses following activation of signal transduction pathways, reporting altered protein posttranslational modification and expression. Here we combine functional proteomics with selective activation and inhibition of MKK1/2, in order to identify cellular targets regulated by the MKK/ERK cascade. Twenty-five targets of this signaling pathway were identified, of which only five were previously characterized as MKK/ERK effectors. The remaining targets suggest novel roles for this signaling cascade in cellular processes of nuclear transport, nucleotide excision repair, nucleosome assembly, membrane trafficking, and cytoskeletal regulation. This study represents an application of functional proteomics toward identifying regulated targets of a discrete signal transduction pathway and demonstrates the utility of this discovery-based strategy in elucidating novel MAP kinase pathway effectors.

The fission yeast ran GTPase is required for microtubule integrity

The microtubule cytoskeleton plays a pivotal role in cytoplasmic organization, cell division, and the correct transmission of genetic information. In a screen designed to identify fission yeast genes required for chromosome segregation, we identified a strain that carries a point mutation in the SpRan GTPase. Ran is an evolutionarily conserved eukaryotic GTPase that directly participates in nucleocytoplasmic transport and whose loss affects many biological processes. Recently a transport-independent effect of Ran on spindle formation in vitro was demonstrated, but the in vivo relevance of these findings was unclear. Here, we report the characterization of a Schizosaccharomyces pombe Ran GTPase partial loss of function mutant in which nucleocytoplasmic protein transport is normal, but the microtubule cytoskeleton is defective, resulting in chromosome missegregation and abnormal cell shape. These abnormalities are exacerbated by microtubule destabilizing drugs, by loss of the spindle checkpoint protein Mph1p, and by mutations in the spindle pole body component Cut11p, indicating that SpRan influences microtubule integrity. As the SpRan mutant phenotype can be partially suppressed by the presence of extra Mal3p, we suggest that SpRan plays a role in microtubule stability.

Functional genomic analysis of cell division in C. elegans using RNAi of genes on chromosome III

Genome sequencing projects generate a wealth of information; however, the ultimate goal of such projects is to accelerate the identification of the biological function of genes. This creates a need for comprehensive studies to fill the gap between sequence and function. Here we report the results of a functional genomic screen to identify genes required for cell division in Caenorhabditis elegans. We inhibited the expression of approximately 96% of the approximately 2,300 predicted open reading frames on chromosome III using RNA-mediated interference (RNAi). By using an in vivo time-lapse differential interference contrast microscopy assay, we identified 133 genes (approximately 6%) necessary for distinct cellular processes in early embryos. Our results indicate that these genes represent most of the genes on chromosome III that are required for proper cell division in C. elegans embryos. The complete data set, including sample time-lapse recordings, has been deposited in an open access database. We found that approximately 47% of the genes associated with a differential interference contrast phenotype have clear orthologues in other eukaryotes, indicating that this screen provides putative gene functions for other species as well.

The interaction between Ran and NTF2 is required for cell cycle progression

The small GTPase Ran is required for the trafficking of macromolecules into and out of the nucleus. Ran also has been implicated in cell cycle control, specifically in mitotic spindle assembly. In interphase cells, Ran is predominately nuclear and thought to be GTP bound, but it is also present in the cytoplasm, probably in the GDP-bound state. Nuclear transport factor 2 (NTF2) has been shown to import RanGDP into the nucleus. Here, we examine the in vivo role of NTF2 in Ran import and the effect that disruption of Ran imported into the nucleus has on the cell cycle. A temperature-sensitive (ts) mutant of Saccharomyces cerevisiae NTF2 that does not bind to Ran is unable to import Ran into the nucleus at the nonpermissive temperature. Moreover, when Ran is inefficiently imported into the nucleus, cells arrest in G(2) in a MAD2 checkpoint-dependent manner. These findings demonstrate that NTF2 is required to transport Ran into the nucleus in vivo. Furthermore, we present data that suggest that depletion of nuclear Ran triggers a spindle-assembly checkpoint-dependent cell cycle arrest.

TPX2, A novel xenopus MAP involved in spindle pole organization

TPX2, the targeting protein for Xenopus kinesin-like protein 2 (Xklp2), was identified as a microtubule-associated protein that mediates the binding of the COOH-terminal domain of Xklp2 to microtubules (Wittmann, T., H. Boleti, C. Antony, E. Karsenti, and I. Vernos. 1998. J. Cell Biol. 143:673-685). Here, we report the cloning and functional characterization of Xenopus TPX2. TPX2 is a novel, basic 82.4-kD protein that is phosphorylated during mitosis in a microtubule-dependent way. TPX2 is nuclear during interphase and becomes localized to spindle poles in mitosis. Spindle pole localization of TPX2 requires the activity of the dynein-dynactin complex. In late anaphase TPX2 becomes relocalized from the spindle poles to the midbody. TPX2 is highly homologous to a human protein of unknown function and thus defines a new family of vertebrate spindle pole components. We investigated the function of TPX2 using spindle assembly in Xenopus egg extracts. Immunodepletion of TPX2 from mitotic egg extracts resulted in bipolar structures with disintegrating poles and a decreased microtubule density. Addition of an excess of TPX2 to spindle assembly reactions gave rise to monopolar structures with abnormally enlarged poles. We conclude that, in addition to its function in targeting Xklp2 to microtubule minus ends during mitosis, TPX2 also participates in the organization of spindle poles.

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.

1.9 A resolution crystal structure of the Saccharomyces cerevisiae Ran-binding protein Mog1p

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

The nuclear import of RCC1 requires a specific nuclear localization sequence receptor, karyopherin alpha3/Qip

RCC1 is the only known guanine nucleotide exchange factor for the small GTPase Ran and is normally found inside the nucleus bound to chromatin. In order to analyze in more detail the nuclear import of RCC1, we created a fusion construct in which four IgG binding domains of protein A were fused to the amino terminus of human RCC1 (pA-RCC1). Surprisingly, we found that neither Xenopus ovarian cytosol nor a mixture of recombinant import factors (karyopherin alpha2, karyopherin beta1, Ran, and p10/NTF2) were able to support the import of pA-RCC1 into the nuclei of digitonin-permeabilized cells. Both, in contrast, were capable of supporting the import of a construct containing another classical nuclear localization sequence (NLS), glutathione S-transferase-green fluorescent protein-NLS. Subsequently, we found that only one of the NLS receptors, karyopherin alpha3 (Kapalpha3/Qip), would support significant nuclear import of pA-RCC1 in permeabilized cells, while members of the other two main classes, Kapalpha1 and Kapalpha2, would not. Accordingly, in vitro binding studies revealed that only Kapalpha3 showed significant binding to RCC1 (unlike Kapalpha1 and Kapalpha2) and that this binding was dependent on the basic amino acids present in the RCC1 NLS. In addition to Kapalpha3, we found that the nuclear import of pA-RCC1 also required both karyopherin beta1 and Ran.

Random mutagenesis and functional analysis of the Ran-binding protein, RanBP1

Ran GTPase is required for nucleocytoplasmic transport of many types of cargo. Several proteins that recognize Ran in its GTP-bound state (Ran x GTP) possess a conserved Ran-binding domain (RanBD). Ran-binding protein-1 (RanBP1) has a single RanBD and is required for RanGAP-mediated GTP hydrolysis and release of Ran from nuclear transport receptors (karyopherins). In budding yeast (Saccharomyces cerevisiae), RanBP1 is encoded by the essential YRB1 gene; expression of mouse RanBP1 cDNA rescues the lethality of Yrb1-deficient cells. We generated libraries of mouse RanBP1 mutants and examined 11 mutants in vitro and for their ability to complement a temperature-sensitive yrb1 mutant (yrb1-51(ts)) in vivo. In 9 of the mutants, the alteration was a change in a residue (or 2 residues) that is conserved in all known RanBDs. However, 4 of these 9 mutants displayed biochemical properties indistinguishable from that of wild-type RanBP1. These mutants bound to Ran x GTP, stimulated RanGAP, inhibited the exchange activity of RCC1, and rescued growth of the yrb1-51(ts) yeast cells. Two of the 9 mutants altered in residues thought to be essential for interaction with Ran were unable to rescue growth of the yrb1(ts) mutant and did not bind detectably to Ran in vitro. However, one of these 2 mutants (and 2 others that were crippled in other RanBP1 functions) retained some ability to co-activate RanGAP. A truncated form of RanBP1 (lacking its nuclear export signal) was able to complement the yrb1(ts) mutation. When driven from the YRB1 promoter, 4 of the 5 mutants most impaired for Ran binding were unable to rescue growth of the yrb1(ts) cells; remarkably, these mutants could nevertheless form ternary complexes with importin-5 or importin-beta and Ran-GTP. The same mutants stimulated only inefficiently RanGAP-mediated GTP hydrolysis of the Ran x GTP x importin-5 complex. Thus, the essential biological activity of RanBP1 in budding yeast correlates not with Ran x GTP binding per se or with the ability to form ternary complexes with karyopherins, but with the capacity to potentiate RanGAP activity toward GTP-bound Ran in these complexes.

gamma-tubulin complexes: binding to the centrosome, regulation and microtubule nucleation

Microtubule assembly is initiated in vivo by gamma-tubulin complexes. Cytoplasmic gamma-tubulin complexes are targeted to centrosomes or to other microtubule organizing centers (MTOCs) via a set of so called gamma-tubulin complex binding proteins (GTBPs) that probably interact with the conserved Spc97p/Spc98p protein family of gamma-tubulin complexes. In other cell types, gamma-tubulin complexes may initiate microtubule formation near chromosomes in a MTOC-independent manner. Recently, major advances have been achieved through the finding that gamma-tubulin, Spc97p and Spc98p form a conserved core that is probably responsible for microtubule nucleation, and by the discovery that a yeast GTBP is regulated in a cell-cycle-dependent manner and in response to an external signal. Furthermore, it was found that the small GTPase Ran in its GDP-bound state may promote spindle assembly. In addition, an essential function of gamma-tubulin in basal body duplication has been demonstrated in Paramecium.

Spindles get the ran around

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

A novel human gene encoding HECT domain and RCC1-like repeats interacts with cyclins and is potentially regulated by the tumor suppressor proteins

Cyclin E-Cdk2 is an evolutionary conserved cyclin-dependent kinase (CDK) complex that drives the G1 to S phase transition of the cell cycle. A novel cDNA encoding a HECT family protein also containing RCC1-like repeats was isolated by a yeast two-hybrid screening using both cyclin E and its inhibitor p21. The protein product of this cDNA, Ceb1, interacts with various cyclin subunits of CDKs in mammalian cells. Expression of Ceb1 is specifically detected in testis and ovary and is highly elevated when the functions of the tumor suppressor proteins, p53 and RB, are compromised by mutations or viral oncoproteins. The present results suggest that Ceb1 may play a critical role when its expression and the CDK activity are upregulated by inactivation of p53 and RB.

Microtubule cytoskeleton: No longer an also Ran

The Ran GTPase cycle has been extensively studied in the context of nuclear transport. Recent work indicates that this GTPase cycle also plays an important role in regulating the microtubule cytoskeleton.

A new role of ran GTPase

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