Conserved histidine residues of RCC1 are essential for nucleotide exchange on Ran

Importazole, a small molecule inhibitor of the transport receptor importin-β

During interphase, the transport receptor importin-β carries cargoes into the nucleus, where RanGTP releases them. A similar mechanism operates in mitosis to generate a gradient of active spindle assembly factors around mitotic chromosomes. Importin-β and RanGTP have been implicated in additional cellular processes, but the precise roles of the Ran/importin-β pathway throughout the cell cycle remain poorly understood. We implemented a FRET-based, high-throughput small molecule screen for compounds that interfere with the interaction between RanGTP and importin-β and identified importazole, a 2,4-diaminoquinazoline. Importazole specifically blocks importin-β-mediated nuclear import both in Xenopus egg extracts and cultured cells, without disrupting transportin-mediated nuclear import or CRM1-mediated nuclear export. When added during mitosis, importazole impairs the release of an importin-β cargo FRET probe and causes both predicted and novel defects in spindle assembly. Together, these results indicate that importazole specifically inhibits the function of importin-β, likely by altering its interaction with RanGTP. Importazole is a valuable tool to evaluate the function of the importin-β/RanGTP pathway at specific stages during the cell cycle.

The design of Förster (fluorescence) resonance energy transfer (FRET)-based molecular sensors for Ran GTPase

The application of FRET-based molecular biosensors provided confirmation of the central model of Ran GTPase function and led to important new insights into its physiological role. In many fields of cell biology, methods employing FRET are a standard approach that is becoming increasingly accessible due to advances in instrumentation and available fluorophores. However, the optimal design of a FRET sensor remains to be the cornerstone of any successful FRET application. Utilizing the recent literature on FRET applications and our studies on Ran, we outline the basic considerations involved in designing molecular FRET sensors. We point to several broadly applicable principles that were used in many different FRET sensors that can detect a wide range of molecular events. Using the FRET sensors for Ran that we created as examples, we then focus on the practical aspects of FRET assays. We describe the preparation of a bipartite FRET sensor consisting of ECFP-Ran and EYFP-importin beta and its validation as a reporter for FRET-based high throughput screening in small molecule libraries. Finally, we review the design and optimization of monomolecular FRET sensors that monitor the RanGTP-RanBP1 interaction, and of sensors detecting the RanGTP-regulated importin beta cargo release.

The ins and outs of nuclear trafficking: unusual aspects in apicomplexan parasites

Apicomplexa is a phylum within the kingdom Protista that contains some of the most significant threats to public health. One of the members of this phylum, Toxoplasma gondii, is amenable to molecular genetic analyses allowing for the identification of factors critical for colonization and disease. A pathway found to be important for T. gondii pathogenesis is the Ran network of nuclear trafficking. Bioinformatics analysis of apicomplexan genomes shows that while Ran is well conserved, the key regulators of Ran--Regulator of Chromosome Condensation 1 and Ran GTPase activating protein--are either highly divergent or absent. Likewise, several import and export receptor molecules that are crucial for nuclear transport are either not present or have experienced genetic drift such that they are no longer recognizable by bioinformatics tools. In this minireview we describe the basics of nuclear trafficking and compare components within apicomplexans to defined systems in humans and yeast. A detailed analysis of the nuclear trafficking network in these eukaryotes is required to understand how this potentially unique cellular biological pathway contributes to host-parasite interactions.

Functional analysis of key nuclear trafficking components reveals an atypical Ran network required for parasite pathogenesis

Protozoan parasites represent major public health challenges. Many aspects of their cell biology are distinct from their animal hosts, providing potential therapeutic targets. Toxoplasma gondii is a protozoan parasite that contains a divergent regulator of chromosome condensation 1 (TgRCC1) that is required for virulence and efficient nuclear trafficking. RCC1 proteins function as a guanine exchange factor for Ras-related nuclear protein (Ran), an abundant GTPase responsible for the majority of nucleocytoplasmic transport. Here we show that while there are dramatic differences from well-conserved RCC1 proteins, TgRCC1 associates with chromatin, interacts with Ran and complements a mammalian temperature-sensitive RCC1 mutant cell line. During the investigation of TgRCC1, we observed several unprecedented phenotypes for TgRan, despite a high level of sequence conservation. The cellular distribution of TgRan is found throughout the parasite cell, whereas Ran in late branching eukaryotes is predominantly nuclear. Additionally, T. gondii tolerates at least low-level expression of dominant lethal Ran mutants. Wild type parasites expressing dominant negative TgRan grew similarly to wild type in standard tissue culture conditions, but were attenuated in serum-starved host cells and mice. These growth characteristics paralleled the TgRCC1 mutant and highlight the importance of the nuclear transport pathway for virulence of eukaryotic pathogens.

Ran GTPase guanine nucleotide exchange factor RCC1 is phosphorylated on serine 11 by cdc2 kinase in vitro

RCC1, a guanine nucleotide exchange factor for Ran GTPase, plays essential roles in the growth and viability of mammalian cells. Here, we examined the phosphorylation of specific serine and threonine residues of RCC1 in vivo and showed that RCC1 is indeed phosphorylated. Analysis by two-dimensional (2D) gel electrophoresis suggested that serine 11 (S11) of hamster RCC1 is phosphorylated in vivo. A point mutation of S11 of hamster RCC1 resulted in a decrease in the number of 2D gel spots, indicating a lack of phosphorylation at the mutant residue. S11 phosphorylation in vitro depended on cyclin B-cdc2 kinase. An RCC1 mutant in which all N-terminal serine and threonine residues were substituted with glutamate residues to mimic phosphorylation at these residues showed decreased binding to the karyopherin, KPNA4, compared with wild type RCC1. We conclude that RCC1 undergoes post-translational phosphorylation.

Atomic resolution structures in nuclear transport

There are currently at least 53 structures of components of nuclear transport in the Protein Databank. In addition to providing critical insights into molecular mechanisms of nuclear transport, these atomic resolution structures provide a large body of information that could guide biochemical and cell biological analyses involving nuclear transport proteins. This paper catalogs 53 crystal and NMR structures of nuclear transport proteins, with the emphasis on providing information useful for mutagenesis and overexpression of recombinant proteins.

The importance of P-loop and domain movements in EF-Tu for guanine nucleotide exchange

Elongation factor Ts (EF-Ts) is the guanine nucleotide exchange factor for elongation factor Tu (EF-Tu). An important feature of the nucleotide exchange is the structural rearrangement of EF-Tu in the EF-Tu.EF-Ts complex caused by insertion of Phe-81 of EF-Ts between His-84 and His-118 of EF-Tu. In this study, the contribution of His-118 to nucleotide release was studied by pre-steady state kinetic analysis of nucleotide exchange in EF-Tu mutants in which His-118 was replaced by Ala or Glu. Intrinsic as well as EF-Ts-catalyzed release of GDP/GTP was affected by the mutations, resulting in an approximately 10-fold faster spontaneous nucleotide release and a 10-50-fold slower EF-Ts-catalyzed nucleotide release. The effects are attributed to the interference of the mutations with the EF-Ts-induced movements of the P-loop of EF-Tu and changes at the domain 1/3 interface, leading to the release of the beta-phosphate group of GTP/GDP. The K(d) for GTP is increased by more than 40 times when His-118 is replaced with Glu, which may explain the inhibition by His-118 mutations of aminoacyl-tRNA binding to EF-Tu. The mutations had no effect on EF-Tu-dependent delivery of aminoacyl-tRNA to the ribosome.

13q deletion in chronic lymphocytic leukemia: characterization of E4.5, a novel chromosome condensation regulator-like guanine nucleotide exchange factor

We report the characterization of a new gene (E4.5) that maps at chromosome band 13q14.3, a chromosomal area frequently deleted in chronic lymphocytic leukemia (CLL) and in other lymphoid malignancies. E4.5 gene encodes for a 4 kb mRNA expressed in various tissues and has an open reading frame of 531 amino acids. The predicted E4.5 protein shows strong homology with the human regulator of chromosome condensation (RCC1) protein, the principal GTP exchange factor for Ran protein. The E4.5 protein contains a BTB domain in its N-terminus, a protein-protein interaction motif. Therefore, we propose that E4.5 is a new member of the RCC1-related guanine nucleotide exchange factor (GEF) family with potent interaction with other proteins and unknown function. Until now, no tumor suppressor genes have been mapped in the 13q14.3 minimal deleted region (MDR) in patients with CLL. It has been proposed that loss of the 13q14.3 MDR may contribute to lymphoid neoplasia by altering the expression/function of genes located on 13q14.3 outside the MDR. The E4.5 is one of these genes with a potential role in the pathogenesis of CLL.

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.

Caffeine mimics adenine and 2'-deoxyadenosine, both of which inhibit the guanine-nucleotide exchange activity of RCC1 and the kinase activity of ATR

BACKGROUND

Both caffeine and the inactivation of RCC1, the guanine-nucleotide exchange factor (GEF) of Ran, induce premature chromatin condensation (PCC) in hamster BHK21 cells arrested in the S-phase, suggesting that RCC1 is a target for caffeine.

RESULTS

Caffeine inhibited the Ran-GEF activity of RCC1 by preventing the binary complex formation of Ran-RCC1. Inhibition of the Ran-GEF activity of RCC1 by caffeine and its derivatives was correlated with their ability to induce PCC. Since caffeine is a derivative of xanthine, the bases and nucleosides were screened for their ability to inhibit RCC1. Adenine, adenosine, and all of the 2'-deoxynucleosides inhibited the Ran-GEF activity of RCC1; however, only adenine and 2'-deoxyadenosine (2'-dA) induced PCC. A factor(s) other than RCC1, should therefore be involved in PCC-induction. We found that both adenine and 2'-dA, but none of the other 2'-deoxynucleosides, inhibited the kinase activity of ATR, similar to that of caffeine. The ATR pathway was also abrogated by the inactivation of RCC1 in tsBN2 cells.

CONCLUSION

The effect of caffeine on cell-cycle control mimics the biological effect of adenine and 2'-dA, both of which inhibit ATR. dATP, a final metabolite of adenine and 2'-dA, is suggested to inhibit ATR, resulting in PCC.

Arabidopsis UVR8 regulates ultraviolet-B signal transduction and tolerance and contains sequence similarity to human regulator of chromatin condensation 1

To further our understanding of how plants defend against the harmful effects of ultraviolet (UV) light, we characterized an Arabidopsis mutant hypersensitive to UV-B. This mutant, UV resistance locus 8-1 (uvr8-1), contains a single recessive mutation at the bottom of chromosome 5. Fine-scale mapping localized uvr8-1 to a 21-kb locus containing five predicted open reading frames. Sequencing of this entire region revealed that the uvr8-1 allele contains a 15-nucleotide deletion in a gene similar to the human guanine nucleotide exchange factor regulator of chromatin condensation 1. This mutation reduces the UV-B-mediated induction of flavonoids and blocks chalcone synthase mRNA and protein induction. In contrast, uvr8-1 has enhanced induction of PR1 and PR5 proteins in response to UV-B, an indication of increased UV-B injury. These results suggest that UVR8 acts in a UV-B signal transduction pathway leading to induction of flavonoid biosynthesis.

Ran-binding protein 3 links Crm1 to the Ran guanine nucleotide exchange factor

Ran-binding protein 3 (RanBP3) is an approximately 55-kDa protein that functions as a cofactor for Crm1-mediated nuclear export. RanBP3 stimulates export by enhancing the affinity of Crm1 for Ran.GTP and cargo. However, important additional functions for this cofactor may exist. We now report that RanBP3 associates with the Ran-specific guanine nucleotide exchange factor, regulator of chromosome condensation 1 (RCC1). This interaction was stimulated by the addition of Ran; moreover, Ran.GDP, Ran.GTP, and Ran without nucleotide could all stimulate complex formation between RanBP3 and RCC1 even though binding of Ran.GDP to RanBP3 alone was undetectable. RanBP3 could also promote binding of Crm1 to RCC1 in the presence of Ran. Binding of RanBP3 to RCC1 increased the catalytic activity of RCC1 toward Ran, and importantly, the ability of RanBP3 to stimulate RCC1 was not affected by the presence of Crm1. These data indicate that RanBP3 acts as a scaffold protein to promote the efficient assembly of export complexes. By tethering Crm1 to catalytically enhanced RCC1, RanBP3 may lower the entropic barrier for the loading of Ran.GTP onto Crm1. We propose that this provides an additional mechanism by which RanBP3 facilitates export.

Mutations of RPGR in X-linked retinitis pigmentosa (RP3)

Mutations in RPGR, retinitis pigmentosa GTPase regulator, are associated with RP3 type of X-linked retinitis pigmentosa, a severe, non-syndromic form of retinal degeneration. In the majority of subjects RPGR mutations are associated with a typical rod-cone degeneration, but in a small number, cone-rod dystrophy, deafness, and abnormalities in respiratory cilia have been noted. Alternative splicing of RPGR is complex in all species examined. In RP3 patients, mutations have been found in exons 1-14 and ORF15, thus delineating a transcript necessary for normal retinal function in humans. The great majority of mutations are predicted to result in premature termination of translation. These mutations are scattered over exons 1-14 and ORF15, while most missense mutations occur in a domain with homology to the protein RCC1, encoded by exons 1-10. Exon ORF15 is a "hot spot" for mutation, at least in the British population, in which it harbors 80% of the mutations found within a sample of 47 X-linked retinitis pigmentosa patients. Most RPGR mutations are unique to single families, which makes it difficult to demonstrate phenotype-genotype correlations.

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.

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.

Regulation of presynaptic terminal organization by C. elegans RPM-1, a putative guanine nucleotide exchanger with a RING-H2 finger domain

Presynaptic terminals contain highly organized subcellular structures to facilitate neurotransmitter release. In C. elegans, the typical presynaptic terminal has an electron-dense active zone surrounded by synaptic vesicles. Loss-of-function mutations in the rpm-1 gene result in abnormally structured presynaptic terminals in GABAergic neuromuscular junctions (NMJs), most often manifested as a single presynaptic terminal containing multiple active zones. The RPM-1 protein has an RCC1-like guanine nucleotide exchange factor (GEF) domain and a RING-H2 finger. RPM-1 is most similar to the Drosophila presynaptic protein Highwire (HIW) and the mammalian Myc binding protein Pam. RPM-1 is localized to the presynaptic region independent of synaptic vesicles and functions cell autonomously. The temperature-sensitive period of rpm-1 coincides with the time of synaptogenesis. rpm-1 may regulate the spatial arrangement, or restrict the formation, of presynaptic structures.

DelGEF, an RCC1-related protein encoded by a gene on chromosome 11p14 critical for two forms of hereditary deafness

We have cloned a human cDNA, DELGEF (deafness locus associated putative guanine nucleotide exchange factor), derived from a 225 kb genomic sequence of chromosome 11p14, critical for the Usher 1C syndrome and for DFNB18, a locus for non-syndromic sensorineural deafness. The amino acid sequence of the protein hDelGEF1 is homologous to the nucleotide exchange factor RCCI for the small GTPase Ran. hDelGEF2 is derived from the same DELGEF gene by alternative splicing. In addition, we have identified a murine homologue, mDelGEF. The ubiquitously expressed soluble protein hDelGEF1 is found both in the cyytoplasm and in the nucleus. Overexpressed hDelGEF2 colocalizes with mitochondria.

GEFs: structural basis for their activation of small GTP-binding proteins

Small GTP-binding proteins of the Ras superfamily function as molecular switches in fundamental events such as signal transduction, cytoskeleton dynamics and intracellular trafficking. Guanine-nucleotide-exchange factors (GEFs) positively regulate these GTP-binding proteins in response to a variety of signals. GEFs catalyze the dissociation of GDP from the inactive GTP-binding proteins. GTP can then bind and induce structural changes that allow interaction with effectors. Representative structures of four main classes of exchange factors have been described recently and, in two cases, structures of the GTP-binding protein-GEF complex have been solved. These structures, together with biochemical studies, have allowed a deeper understanding of the mechanisms of activation of Ras-like GTP-binding proteins and suggested how they might represent targets for therapeutic intervention.

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

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

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

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

A glutamic finger in the guanine nucleotide exchange factor ARNO displaces Mg2+ and the beta-phosphate to destabilize GDP on ARF1

The Sec7 domain of the guanine nucleotide exchange factor ARNO (ARNO-Sec7) is responsible for the exchange activity on the small GTP-binding protein ARF1. ARNO-Sec7 forms a stable complex with the nucleotide-free form of [Delta17]ARF1, a soluble truncated form of ARF1. The crystal structure of ARNO-Sec7 has been solved recently, and a site-directed mutagenesis approach identified a hydrophobic groove and an adjacent hydrophilic loop as the ARF1-binding site. We show that Glu156 in the hydrophilic loop of ARNO-Sec7 is involved in the destabilization of Mg2+ and GDP from ARF1. The conservative mutation E156D and the charge reversal mutation E156K reduce the exchange activity of ARNO-Sec7 by several orders of magnitude. Moreover, [E156K]ARNO-Sec7 forms a complex with the Mg2+-free form of [Delta17]ARF1-GDP without inducing the release of GDP. Other mutations in ARNO-Sec7 and in [Delta17]ARF1 suggest that prominent hydrophobic residues of the switch I region of ARF1 insert into the groove of the Sec7 domain, and that Lys73 of the switch II region of ARF1 forms an ion pair with Asp183 of ARNO-Sec7.

Human RanBP3, a group of nuclear RanGTP binding proteins

A group of novel human Ran-binding proteins, RanBP3, was identified using the yeast two-hybrid system via Ran-mediated interaction with the nucleotide exchange factor RCC1. Several open reading frames, representing putative alternatively spliced products, were established by cDNA cloning. Two of them, RanBP3-a and RanBP3-b, encode nuclear hydrophilic proteins of 499 and 562 amino acid residues. The sequences contain FXFG motifs, characteristic of a subgroup of nucleoporins, and a C-terminal domain showing similarity to the Ran-binding protein RanBP1. These proteins are localized in the nucleus, preferentially bind RanGTP and may be nuclear effectors of the Ran pathway.

The alpha and beta of turning on a molecular switch

The crystal structures of RCC1 and the Sec7 domain of human Arno, nucleotide exchange factors for the Ras-related GTPases Ran and ARF, reveal two very different folds, the former a seven-bladed beta-propeller, the latter a capped right-handed superhelix. Both are also unrelated to the folds of Mss4 and elongation factor Ts, nucleotide exchange factors for Rab and elongation factor Tu.

Isolation and characterization of fission yeast sns mutants defective at the mitosis-to-interphase transition

pim1-d1ts was previously identified in a visual screen for fission yeast mutants unable to complete the mitosis-to-interphase transition. pim1+ encodes the guanine nucleotide exchange factor (GEF) for the spi1 GTPase. Perturbations of this GTPase system by either mutation or overproduction of its regulatory proteins cause cells to arrest with postmitotic condensed chromosomes, an unreplicated genome, and a wide medial septum. The septation phenotype of pim1-d1ts was used as the basis for a more extensive screen for this novel class of sns (septated, not in S-phase) mutants. Seventeen mutants representing 14 complementation groups were isolated. Three strains, sns-A3, sns-A5, and sns-A6, representing two different alleles, are mutated in the pim1+ gene. Of the 13 non-pim1ts sns complementation groups, 11 showed genetic interactions with the spi1 GTPase system. The genes mutated in 10 sns strains were synthetically lethal with pim1-d1, and six sns strains were hypersensitive to overexpression of one or more of the known components of the spil GTPase system. Epistasis analysis places the action of the genes mutated in nine of these strains downstream of pim1+ and the action of one gene upstream of pim1+. Three strains, sns-A2, sns-B1, and sns-B9, showed genetic interaction with the spil GTPase system in every test performed. sns-B1 and sns-B9 are likely to identify downstream targets, whereas sns-A2 is likely to identify upstream regulators of the spi1 GTPase system that are required for the mitosis-to-interphase transition.

The 1.7 A crystal structure of the regulator of chromosome condensation (RCC1) reveals a seven-bladed propeller

The gene encoding the regulator of chromosome condensation (RCC1) was cloned by virtue of its ability to complement the temperature-sensitive phenotype of the hamster cell line tsBN2, which undergoes premature chromosome condensation or arrest in the G1 phase of the cell cycle at non-permissive temperatures. RCC1 homologues have been identified in many eukaryotes, including budding and fission yeast. Mutations in the gene affect pre-messenger RNA processing and transport, mating, initiation of mitosis and chromatin decondensation, suggesting that RCC1 is important in the control of nucleo-cytoplasmic transport and the cell cycle. Biochemically, RCC1 is a guanine-nucleotide-exchange factor for the nuclear Ras homologue Ran; it increases the dissociation of Ran-bound GDP by 10(5)-fold. It may also bind to DNAvia a protein-protein complex. Here we show that the structure of human RCC1, solved to 1.7-A resolution by X-ray crystallography, consists of a seven-bladed propeller formed from internal repeats of 51-68 residues per blade. The sequence and structure of the repeats differ from those of WD40-domain proteins, which also form seven-bladed propellers and include the beta-subunits of G proteins. The nature of the structure explains the consequences of a wide range of known mutations. The region of the protein that is involved in guanine-nucleotide exchange is located opposite the region that is thought to be involved in chromosome binding.

RCC1 and nuclear organization

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

Putative GTPase Gtr1p genetically interacts with the RanGTPase cycle in Saccharomyces cerevisiae

In order to identify a protein interacting with RCC1, a guanine nucleotide-exchange factor for the nuclear GTPase Ran, we isolated a series of cold-sensitive suppressors of mtr1-2, a temperature-sensitive mutant of the Saccharomyces cerevisiae RCC1 homologue. One of the isolated suppressor mutants was mutated in the putative GTPase Gtr1p, being designated as gtr1-11. It also suppressed other alleles of mtr1-2, srm1-1 and prp20-1 in contrast to overexpression of the S. cerevisiae Ran/TC4 homologue Gsp1p, previously reported to suppress prp20-1, but not mtr1-2 or srm1-1. Furthermore, gtr1-11 suppressed the rna1-1, temperature-sensitive mutant of the Gsp1p GTPase-activating protein, but not the srp1-31, temperature-sensitive mutant of the S. cerevisiae importin alpha homologue. mtr1-2, srm1-1 and prp20-1 were also suppressed by overexpression of the mutated Gtr1p, Gtr1-11p. In summary, Gtr1p that was localized in the cytoplasm by immunofluoresence staining was suggested to function as a negative regulator for the Ran/TC4 GTPase cycle.