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

Nuclear pores and nuclear assembly

Communication between the nucleus and cytoplasm occurs through large macromolecular structures, the nuclear pores. Quantitative scanning transmission electron microscopy has estimated the mass of a nuclear pore to be 60 million Daltons in yeast and 120 million Daltons in vertebrates. The past two years were noteworthy in that they saw: 1) the purification of both the yeast and vertebrate nuclear pores, 2) the initial description of routes through the pore for specific transport receptors, 3) glimpses of intranuclear organization imposed by the nuclear pores and envelope and 4) the revelation of new and pivotal roles for the small GTPase Ran not only in nuclear import but in spindle assembly and nuclear membrane fusion.

Nuclear import of the U1A splicesome protein is mediated by importin alpha /beta and Ran in living mammalian cells

U1A is a component of the uracil-rich small nuclear ribonucleoprotein. The molecular mechanism of nuclear import of U1A was investigated in vivo and in vitro. When recombinant deletion mutants of U1A are injected into the BHK21 cell cytoplasm, the nuclear localization signal (NLS) of U1A is found in the N-terminal half of the central domain (residues 100-144 in mouse U1A). In an in vitro assay, it was found that the U1A-NLS accumulated in only a portion of the nuclei in the absence of cytosolic extract. In contrast, the addition of importin alpha/beta and Ran induced the uniform nuclear accumulation of U1A-NLS in all cells. Furthermore, U1A was found to bind the C-terminal portion of importin alpha. In addition, the in vitro nuclear migration of full-length U1A was found to be exclusively dependent on importin alpha/beta and Ran. Moreover, in living cells, the full-length U1A accumulated in the nucleus in a Ran-dependent manner, and nuclear accumulation was inhibited by the importin beta binding domain of importin alpha. These results suggest that the nuclear import of U1A is mediated by at least two distinct pathways, an importin alpha/beta and Ran-dependent and an -independent pathway in permeabilized cells, and that the latter pathway may be suppressed in intact cells.

Specific association of Type I c-Abl with Ran GTPase in lipopolysaccharide-mediated differentiation

Each of several isoforms of c-Abl may be involved in different biological functions. Type I c-Abl has been shown to be involved in LPS-induced differentiation and Type IV c-Abl, apoptosis. Ran has recently been shown to be involved in LPS endotoxin signal transduction. Here we show that Type I c-Abl associates with Ran. Formation of this complex is specific, as Ran did not associate with the highly homologous Type IV c-Abl isoform. In non-stimulated lymphoid B cells, Type I c-Abl tyrosine kinase is inactive, whereas Type IV kinase is active. Formation of Type I c-Abl/Ran complex and activation of Type I c-Abl kinase activity are LPS dose-dependent. This complex is detectable in B cells of endotoxin-sensitive inbred mice but absent in B cells of endotoxin-resistant mice. These findings therefore suggest that Type I c-Abl and Ran are important targets in lipopolysaccharide-induced biological responses of hematopoietic cells.

Structural basis for guanine nucleotide exchange on Ran by the regulator of chromosome condensation (RCC1)

RCC1 (regulator of chromosome condensation), a beta propeller chromatin-bound protein, is the guanine nucleotide exchange factor (GEF) for the nuclear GTP binding protein Ran. We report here the 1.8 A crystal structure of a Ran*RCC1 complex in the absence of nucleotide, an intermediate in the multistep GEF reaction. In contrast to previous structures, the phosphate binding region of the nucleotide binding site is perturbed only marginally, possibly due to the presence of a polyvalent anion in the P loop. Biochemical experiments show that a sulfate ion stabilizes the Ran*RCC1 complex and inhibits dissociation by guanine nucleotides. Based on the available structural and biochemical evidence, we present a unified scenario for the GEF mechanism where interaction of the P loop lysine with an acidic residue is a crucial element for the overall reaction.

Novel G proteins, Rag C and Rag D, interact with GTP-binding proteins, Rag A and Rag B

Rag A/Gtr1p are G proteins and are known to be involved in the RCC1-Ran pathway. We employed the two-hybrid method using Rag A as the bait to identify proteins binding to Rag A, and we isolated two novel human G proteins, Rag C and Rag D. Rag C demonstrates homology with Rag D (81.1% identity) and with Gtr2p of Saccharomyces cerevisiae (46.1% identity), and it belongs to the Rag A subfamily of the Ras family. Rag C and Rag D contain conserved GTP-binding motifs (PM-1, -2, and -3) in their N-terminal regions. Recombinant glutathione S-transferase fusion protein of Rag C efficiently bound to both [(3)H]GTP and [(3)H]GDP. Rag A was associated with both Rag C and Rag D in their C-terminal regions where a potential leucine zipper motif and a coiled-coil structure were found. Rag C and D were associated with both the GDP and GTP forms of Rag A. Both Rag C and Rag D changed their subcellular localization, depending on the nucleotide-bound state of Rag A. In a similar way, the disruption of S. cerevisiae GTR1 resulted in a change in the localization of Gtr2p.

Centrosome maturation

In the past, centrosome maturation has been described as the change in microtubule nucleation potential that occurs as cells pass through specific phases of the cell cycle. It is suggested that the idea of centrosome maturation be expanded to include gain of functions that are not necessarily related to microtubule nucleation. Some of these functions could be transient and dependent on the temporary association of molecules with the centrosome as cells progress through the cell cycle. Thus, the centrosome may best be viewed as a site for mediating macromolecular interactions, perhaps as a central processing station within the cell. The centromatrix, a relatively stable lattice of polymers within the centrosome's PCM, could serve as a scaffold for the transient binding of mediator molecules, as well as allow the dynamic exchange of centrosome constituents with a soluble cytoplasmic pool. New evidence adds support to the idea that centrioles are crucial for the maintenance of PCM structure. However, significant evidence indicates that aspects of centrosome structure and function can be maintained in the absence of centrioles. In the case of paternal centrosome maturation, sperm centrioles may not contain an associated centromatrix. It is proposed that regulation of paternal centrioles or centriole associated proteins could mediate centriole-dependent centromatrix assembly following fertilization. Thus, regulation of centromatrix-centriole interactions could be involved in maintaining the integrity of the centrosome's PCM and play an important role in centrosome disassembly during cell differentiation and morphogenesis.

Ran stimulates spindle assembly by altering microtubule dynamics and the balance of motor activities

The guanosine tri-phosphatase Ran stimulates assembly of microtubule spindles. However, it is not known what aspects of the microtubule cytoskeleton are subject to regulation by Ran in mitosis. Here we show that Ran-GTP stimulates microtubule assembly by increasing the rescue frequency of microtubules three- to eightfold. In addition to changing microtubule dynamics, Ran-GTP also alters the balance of motor activities, partly as a result of an increase in the amount of motile Eg5, a plus-end-directed microtubule motor that is essential for spindle formation. Thus, Ran regulates multiple processes that are involved in spindle assembly.

Ran-GTP coordinates regulation of microtubule nucleation and dynamics during mitotic-spindle assembly

It was recently reported that GTP-bound Ran induces microtubule and pseudo-spindle assembly in mitotic egg extracts in the absence of chromosomes and centrosomes, and that chromosomes induce the assembly of spindle microtubules in these extracts through generation of Ran-GTP. Here we examine the effects of Ran-GTP on microtubule nucleation and dynamics and show that Ran-GTP has independent effects on both the nucleation activity of centrosomes and the stability of centrosomal microtubules. We also show that inhibition of Ran-GTP production, even in the presence of duplicated centrosomes and kinetochores, prevents assembly of a bipolar spindle in M-phase extracts.

Motile plant cell body: a "bug" within a "cage"

Analysis of the cytoskeleton in morphogenetically active plant cells allows us to propose a unified concept for the structural organization of eukaryotic cells. Their cytoarchitecture is determined by two principal structural complexes: nucleus-microtubule-based cell bodies ("bugs") and plasma-membrane-F-actin-based cell periphery complexes ("cages"). There are dynamic interactions between each of these entities in response to extracellular and intracellular signals. In the case of the cell body, these signals determine its polarization, rotation and migration. Interactions between cell body and cell periphery complexes determine cell growth polarity and morphogenesis throughout the eukaryotic kingdom.

Roles of Ran-GTP and Ran-GDP in precursor vesicle recruitment and fusion during nuclear envelope assembly in a human cell-free system

The molecular mechanism of nuclear envelope (NE) assembly is poorly understood, but in a cell-free system made from Xenopus eggs NE assembly is controlled by the small GTPase Ran [1,2]. In this system, Sepharose beads coated with Ran induce the formation of functional NEs in the absence of chromatin [1]. Both generation of Ran-GTP by the guanine nucleotide exchange factor RCC1 and GTP hydrolysis by Ran are required for NE assembly, although the roles of the GDP- and GTP-bound forms of Ran in the recruitment of precursor vesicles and their fusion have been unclear. We now show that beads coated with either Ran-GDP or Ran-GTP assemble functional nuclear envelopes in a cell-free system derived from mitotic human cells, forming pseudo-nuclei that actively transport proteins across the NE. Both RCC1 and the GTPase-activating protein RanGAP1 are recruited to the beads, allowing interconversion between Ran-GDP and Ran-GTP. However, addition of antibodies to RCC1 and RanGAP1 shows that Ran-GDP must be converted to Ran-GTP by RCC1 before precursor vesicles are recruited, whereas GTP hydrolysis by Ran stimulated by RanGAP1 promotes vesicle recruitment and is necessary for vesicle fusion to form an intact envelope. Thus, the GTP-GDP cycle of Ran controls both the recruitment of vesicles and their fusion to form NEs.

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 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.

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.

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.

BSAP (Pax5)-importin alpha 1 (Rch1) interaction identifies a nuclear localization sequence

BSAP (Pax5) is an essential transcription factor for early B cell and central nervous system development. In later B cell development, BSAP has been implicated in the regulation of 3' Ig enhancers and a number of B cell-specific genes. Previous studies have suggested a role for BSAP-interacting proteins in the regulation of the function of BSAP. Using the yeast two-hybrid system, we identified importin alpha1 (Rch1) as a BSAP-interacting protein. Importin alpha proteins have been shown to escort proteins into the nucleus through interaction with a nuclear localization signal (NLS), composed of short stretches of basic amino acids. A predicted NLS in BSAP (NKRKRDE, located at amino acids 195-201 in the central domain) was confirmed to be essential for interaction with importin alpha1 by the yeast two-hybrid assay. Physical interaction between BSAP and importin alpha1 was detected in vitro by a glutathione S-transferase (GST) pulldown assay. The NLS sequence in BSAP conferred nuclear localization to green fluorescent protein (GFP)-BSAP fusion proteins. Although the N-terminal paired (DNA-binding) domain of BSAP also conferred nuclear localization when coupled to green fluorescent protein, this domain did not bind to importin alpha1 in the yeast two-hybrid assay. The NLS sequence in the central domain of BSAP binds to the C-terminal 98-amino acid fragment of importin alpha1.

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.

Nuclear import of the ran exchange factor, RCC1, is mediated by at least two distinct mechanisms

RCC1, the only known guanine-nucleotide exchange factor for the Ran GTPase, is an approximately 45-kD nuclear protein that can bind chromatin. An important question concerns how RCC1 traverses the nuclear envelope. We now show that nuclear RCC1 is not exported readily in interphase cells and that the import of RCC1 into the nucleoplasm is extremely rapid. Import can proceed by at least two distinct mechanisms. The first is a classic import pathway mediated by basic residues within the NH(2)-terminal domain (NTD) of RCC1. This pathway is dependent upon both a preexisting Ran gradient and energy, and preferentially uses the importin-alpha3 isoform of importin-alpha. The second pathway is not mediated by the NTD of RCC1. This novel pathway does not require importin-alpha or importin-beta or the addition of any other soluble factor in vitro; however, this pathway is saturable and sensitive only to a subset of inhibitors of classical import pathways. Furthermore, the nuclear import of RCC1 does not require a preexisting Ran gradient or energy. We speculate that this second import pathway evolved to ensure that RCC1 never accumulates in the cytoplasm.

Expression analysis of human HL60 cells exposed to 60 Hz square- or sine-wave magnetic fields

A total of 960 complementary DNA (cDNA) clones from an HL60 cell cDNA library were screened to discover genes that were differentially expressed in HL60 cells exposed to 60 Hz square-wave magnetic fields (MFs) compared to sham-exposed cells. Square-wave fields are rich in odd harmonic frequency content. We used a two-gel cDNA library screening method (BIGEL) to identify treatment-induced alterations in gene expression. Four cDNA clones were tentatively identified as differentially expressed after exposure to square-wave MFs at 2 mT for 24 h. BIGEL-identified genes (GenBank accession number) corresponding to these clones were: TI227H (D50525), EST Homo sapiens partial cDNA (Z17814), human ribosomal protein S13 (L01124), and AICAR transformylase mRNAs (D82348). The differences in mRNA levels were not confirmed in test compared to experimental cells by Northern analysis. In other experiments, we used concurrent exposure to 60 Hz sine- or square-wave MFs (0 or 2 mT, duration of 3 or 24 h, no postexposure delay). In addition to the four BIGEL genes, we also investigated MYC, HSP70, RAN and SOD1. In the case of MYC and HSP70, square-wave MFs appeared to exhibit more marked alterations when compared to sinusoidal waveforms, but the overall results indicated no effect of possible differential magnetic-field-induced expression of all eight genes. In contrast, alterations of mRNA levels were observed for seven genes after exposure to X irradiation, hyperthermia and TPA. These results are contrary to previously proposed similarities between the action of these agents and MF effects on gene transcription.

Upstream and downstream of ran GTPase

Among the Ras family, Ran is a unique small G protein. It does not have a lipid modification motif at the C-terminus to bind to the membrane, which is often observed within the Ras family. Ran may therefore interact with a wide range of proteins in various intracellular locations. This means that Ran could play many different roles like nucleocytoplasmic transport, microtubule assembly and so on. All of the Ran functions should be regulated by RanGEF and RanGAP. It is an interesting issue why RCC1, a RanGEF, is localized in the nucleus and RanGAP1/Ran1p in the cytoplasm. It is possible that RCC1 checks the state of chromosomal DNA replication and transfers it to the downstream events through Ran; thereby, RCC1 would be involved in coupling the spatial localization of cellular macromolecules with the cell cycle progression. In this context, Ran will be a very important cell cycle mediator. There is yet another G protein cascade, Gtr1-Gtr2, which interacts with the Ran cycle.

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.

Biochemical and genetic conservation of fission yeast Dsk1 and human SR protein-specific kinase 1

Arginine/serine-rich (RS) domain-containing proteins and their phosphorylation by specific protein kinases constitute control circuits to regulate pre-mRNA splicing and coordinate splicing with transcription in mammalian cells. We present here the finding that similar SR networks exist in Schizosaccharomyces pombe. We previously showed that Dsk1 protein, originally described as a mitotic regulator, displays high activity in phosphorylating S. pombe Prp2 protein (spU2AF59), a homologue of human U2AF65. We now demonstrate that Dsk1 also phosphorylates two recently identified fission yeast proteins with RS repeats, Srp1 and Srp2, in vitro. The phosphorylated proteins bear the same phosphoepitope found in mammalian SR proteins. Consistent with its substrate specificity, Dsk1 forms kinase-competent complexes with those proteins. Furthermore, dsk1(+) gene determines the phenotype of prp2(+) overexpression, providing in vivo evidence that Prp2 is a target for Dsk1. The dsk1-null mutant strain became severely sick with the additional deletion of a related kinase gene. Significantly, human SR protein-specific kinase 1 (SRPK1) complements the growth defect of the double-deletion mutant. In conjunction with the resemblance of dsk1(+) and SRPK1 in sequence homology, biochemical properties, and overexpression phenotypes, the complementation result indicates that SRPK1 is a functional homologue of Dsk1. Collectively, our studies illustrate the conserved SR networks in S. pombe consisting of RS domain-containing proteins and SR protein-specific kinases and thus establish the importance of the networks in eucaryotic organisms.

Genes, receptors, signals and responses to lipopolysaccharide endotoxin

C3H/HeJ inbred mice have been very useful for identifying genetic elements responsible for endotoxin mediated responses. Depending on the type of assays employed, Tlr-2, Tlr-4 and Lps/Ran have been shown to be important in lipopolysaccharide (LPS)-mediated responses. The concept of a single LPS gene being responsible for the genetic defect found in C3H/HeJ mice should therefore be re-examined more closely. Given the most recent discoveries, it is probable that more than one signal transduction pathway is involved. One is a CD14-dependent pathway, the other a CD14-independent pathway. Identification of the genetic elements involved in these pathways will be beneficial in designing therapeutic strategies for treating patients with endotoxic or septic shock.

Regulation and regulatory activities of centrosomes

The centrosome functions in the organization of the cytoskeleton, in specification of cell polarity, and in the assembly of the bipolar spindle during mitosis. These activities are largely the result of microtubule nucleation activity and the centrosome's structural influence on the form of the microtubule array that it anchors. Centrosome duplication and microtubule nucleation activity are precisely regulated during development and the cell cycle. Loss of normal centrosome regulation and function may lead to alterations in cell polarity and to chromosomal instability through mitotic defects resulting in aneuploidy. This is particularly true for many malignant tumors. Here, we review the regulation and regulatory activities of centrosomes and consider some of the questions of current interest in this area. J. Cell. Biochem. Suppls. 32/33:192-199, 1999.

Purification of protein A-tagged yeast ran reveals association with a novel karyopherin beta family member, Pdr6p

The small GTPase Ran (encoded by GSP1 and GSP2 in yeast) plays a central role in nucleocytoplasmic transport. GSP1 and GSP2 were tagged with protein A and functionally expressed in a gsp1 null mutant. After affinity purification of protein A-tagged Gsp1p or Gsp2p by IgG-Sepharose chromatography, known karyopherin beta transport receptors (e.g. Kap121p and Kap123p) and a novel member of this protein family, Pdr6p, were found to be associated with yeast Ran. Subsequent tagging of Pdr6p with green fluorescent protein revealed association with the nuclear pore complexes in vivo. Thus, functional tagging of yeast Ran allowed the study of its in vivo distribution and interaction with known and novel Ran-binding proteins.

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.