Tissue-specific expression of Ran isoforms in the mouse

Ran and calcineurin can participate collaboratively in the regulation of spermatogenesis in scallop

Calcineurin is a calcium/calmodulin-dependent protein phosphatase that plays important roles in the transduction of calcium signals in a variety of tissues. In addition, calcineurin has been implicated in the process of spermatogenesis. A novel calcineurin-binding protein, CaNBP75, has been identified in scallop testis. The C-terminal region of CaNBP75 is homologous to the C-terminal region of RanBP3, a Ran-binding domain-containing protein. A small G protein Ran has been involved in spermiogenesis by virtue of the fact that its localization in spermatids changes during spermiogenesis. The current study was performed to investigate the functions of Ran and CaNBP75 in the regulation of calcineurin in testis to further understand the basic functions of calcineurin during spermatogenesis. First, cloning and sequencing of a scallop Ran cDNA isolated from testis revealed that scallop Ran is well-conserved at the amino acid level. Secondly, direct binding of Ran to CaNBP75 was demonstrated in an in vitro pull-down assay. Thirdly, analysis of the tissue distribution of Ran, CaNBP75, and calcineurin showed that these proteins are abundantly expressed in testis. Fourthly, comparison of the expression profiles of Ran and CaNBP75 with that of calcineurin in scallop testis during the maturation cycle revealed that Ran and CaNBP75 mRNA levels increase during meiosis and spermiogenesis, similar to calcineurin. Finally, co-immunoprecipitation analysis suggests that Ran, CaNBP75, and calcineurin interact in scallop testis during maturation. These results suggest that Ran, CaNBP75, and calcineurin may act in a coordinated manner to regulate spermatogenesis.

Regulated nucleocytoplasmic transport during gametogenesis

Gametogenesis is the process by which sperm or ova are produced in the gonads. It is governed by a tightly controlled series of gene expression events, with some common and others distinct for males and females. Nucleocytoplasmic transport is of central importance to the fidelity of gene regulation that is required to achieve the precisely regulated germ cell differentiation essential for fertility. In this review we discuss the physiological importance for gamete formation of the molecules involved in classical nucleocytoplasmic protein transport, including importins/karyopherins, Ran and nucleoporins. To address what functions/factors are conserved or specialized for these developmental processes between species, we compare knowledge from mice, flies and worms. The present analysis provides evidence of the necessity for and specificity of each nuclear transport factor and for nucleoporins during germ cell differentiation. This article is part of a Special Issue entitled: Nuclear Transport and RNA Processing.

Expression of nucleocytoplasmic transport machinery: clues to regulation of spermatogenic development

Spermatogenesis is one example of a developmental process which requires tight control of gene expression to achieve normal growth and sustain function. This review is based on the principle that events in spermatogenesis are controlled by changes in the distribution of proteins between the nuclear and cytoplasmic compartments. Through analysis of the regulated production of nucleocytoplasmic transport machinery in mammalian spermatogenesis, this review addresses the concept that access to the nucleus is tightly controlled to enable and prevent differentiation. A broad review of nuclear transport components is presented, outlining the different categories of machinery required for import, export and non-nuclear functions. In addition, the complexity of nomenclature is addressed by the provision of a concise yet comprehensive listing of information that will aid in comparative studies of different transport proteins and the genes which encode them. We review a suite of existing transcriptional analyses which identify common and distinct patterns of transport machinery expression, showing how these can be linked with key events in spermatogenic development. The additional importance of this for human fertility is considered, in light of data that identify which importin and nuclear transport machinery components are present in testicular cancer specimens, while also providing an indication of how their presence (and absence) may be considered as potential mediators of oncogenesis. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Identification and expression localization of a Ran homologue in mollusc abalone, Haliotis diversicolor supertexta

Ran protein is a central molecular in several key nuclear functions, including nucleocytoplasmic transport, cell-cycle progression and nuclear envelope assembly. In this study, we have isolated and characterized a Ran homologue from a gastropod abalone which we named ab-Ran. The full-length cDNA consists of 1239 bp with an ORF encoding a 220 amino acid protein. The deduced amino acid sequence of ab-Ran shows highly similar to that of other Ran members (84-88%). Moreover, the ab-Ran contains five conserved regions and four carboxy-terminal residues CAAX-box. RT-PCR analysis showed that the ab-Ran was ubiquitously expressed in abalone tissues. The intracellular localization examined by immunofluorescence and immunohistochemistry staining displayed that ab-Ran was largely concentrated in the nuclei and partially in the cytoplasm. To the best of our knowledge, this is the first identification and characterization of a Ran homologue in mollusk.

Reduced Ran expression in Ran(+/-) fibroblasts increases cytokine-stimulated nuclear abundance of the AP-1 subunits c-Fos and c-Jun

Ran (Ras-related nuclear protein), a Ras family GTPase, is involved in multiple cellular functions, including the regulation of DNA replication, cell cycle progression, nuclear structure formation, RNA processing-exportation, and nuclear protein importation. Ran(+/-) embryonic stem (ES) cells were produced in an attempt to generate Ran null mutant mice. Even after an extremely large number of blastocyst injections, no Ran(+/-) chimeric mice could be generated. Ran(+/-) ES cell-derived fibroblasts showed reduced Ran protein expression, and manifested augmented nuclear abundance of AP-1 factors (c-Jun and c-Fos) upon cytokine stimulation. Our experiments demonstrated that intracellular Ran protein levels controlled the nuclear presence of certain transcription factors, such as c-Fos and c-Jun.

Ran overexpression leads to diminished T cell responses and selectively modulates nuclear levels of c-Jun and c-Fos

Ras-related nuclear protein (Ran) is a Ras family GTPase, and its documented functions are the regulation of DNA replication, cell cycle progression, nuclear structure formation, RNA processing and exportation, and nuclear protein importation. In this study, we performed detailed mapping of Ran expression during mouse ontogeny using in situ hybridization. High Ran expression was found in various organs and tissues including the thymus cortex and spleen white pulp. Ran was induced in T cells 24 h after their activation. The function of Ran in the immune system was investigated using Ran transgenic (Tg) mice. In Ran Tg T cells, there was compromised activation marker expression, lymphokine secretion, and proliferation upon T cell receptor activation in vitro when compared with wild type T cells. Tg mice also manifested defective delayed type hypersensitivity in vivo. Upon PMA and ionomycin stimulation, Tg T cells were defective in nuclear accumulation of AP-1 factors (c-Jun and c-Fos) but not NF-kappaB family members. Our experiments showed that Ran had important regulatory function in T cell activation. One of the possible mechanisms is that intracellular Ran protein levels control the nuclear retention for selective transcription factors such as c-Jun and c-Fos of AP-1, which is known to be critical in T cell activation and proliferation and lymphokine secretion.

Identification of ventricular-side-enriched molecules regulated in a stage-dependent manner during cerebral cortical development

Radial glial cells are the main component of the embryonic cortical ventricular zone (VZ), producing deep-layer excitatory neurons in the early stage and upper-layer excitatory neurons in the late stage of development. Previous studies have suggested that the laminar fate of deep-layer neurons might be determined by early-stage-specific secretory or transmembrane molecules (S/TMs) in the VZ. However, the different properties required to produce the different types of neurons in early-stage and late-stage VZ cells are largely unknown. Herein, we investigated the stage-dependent transcriptional profiles of the ventricular side of the mouse cortex, which was manually dissected at embryonic day (E)12, E14 and E16, and identified 3985 'VZ-enriched' genes, regulated stage-dependently, by GeneChip analysis. These molecules were classified into nine types based on stage-dependent regulation patterns. Prediction programs for the S/TMs revealed 659 'VZ-enriched' S/TMs. In situ hybridization and real-time PCR analysis for several of these molecules showed results consistent with the statistical analysis of the GeneChip experiments. Moreover, we identified 17 cell cycle-related early-stage and 'VZ-enriched' molecules. These molecules included not only those involved in cell cycle progression, but also essential molecules for DNA double-strand break repair, such as Rad51 and Rpa1. These results suggest that the early stage-VZ cells, which produce both deep- and upper-layer neurons, and the late-stage VZ cells, which produce only upper-layer neurons, are intrinsically different. The gene lists presented here will be useful for the investigation of stage-dependent changes in VZ cells and their regulatory mechanisms in the developing cortex.

Characterization of human thioredoxin-like 2. A novel microtubule-binding thioredoxin expressed predominantly in the cilia of lung airway epithelium and spermatid manchette and axoneme

We describe here the cloning and characterization of a novel member of the thioredoxin family, thioredoxin-like protein 2 (Txl-2). The Txl-2 open reading frame codes for a protein of 330 amino acids consisting of two distinct domains: an N-terminal domain typical of thioredoxins and a C-terminal domain belonging to the nucleoside-diphosphate kinase family, separated by a small interface domain. The Txl-2 gene spans approximately 28 kb, is organized into 11 exons, and maps at locus 3q22.3-q23. A splicing variant lacking exon 5 (Delta 5Txl-2) has also been isolated. By quantitative real time PCR we demonstrate that Txl-2 mRNA is ubiquitously expressed, with testis and lung having the highest levels of expression. Unexpectedly, light and electron microscopy analyses show that the protein is associated with microtubular structures such as lung airway epithelium cilia and the manchette and axoneme of spermatids. Using in vitro translated proteins, we demonstrate that full-length Txl-2 weakly associates with microtubules. In contrast, Delta 5Txl-2 specifically binds with very high affinity brain microtubule preparations containing microtubule-binding proteins. Importantly, Delta 5Txl-2 also binds to pure microtubules, proving that it possesses intrinsic microtubule binding capability. Taken together, Delta 5Txl-2 is the first thioredoxin reported to bind microtubules and might therefore be a novel regulator of microtubule physiology.

Ran, a GTP-binding protein involved in nucleocytoplasmic transport and microtubule nucleation, relocates from the manchette to the centrosome region during rat spermiogenesis

Ran, a Ras-related GTPase, is required for transporting proteins in and out of the nucleus during interphase and for regulating the assembly of microtubules. cDNA cloning shows that rat testis, like mouse testis, expresses both somatic and testis-specific forms of Ran-GTPase. The presence of a homologous testis-specific form of Ran-GTPase in rodents implies that the Ran-GTPase pathway plays a significant role during sperm development. This suggestions is supported by distinct Ran-GTPase immunolocalization sites identified in developing spermatids. Confocal microscopy demonstrates that Ran-GTPase localizes in the nucleus of round spermatids and along the microtubules of the manchette in elongating spermatids. When the manchette disassembles, Ran-GTPase immunoreactivity is visualized in the centrosome region of maturing spermatids. The circumstantial observation that fractionated manchettes, containing copurified centrin-immunoreactive centrosomes, can organize a three-dimensional lattice in the presence of taxol and GTP, points to the role of Ran-GTPase and associated factors in microtubule nucleation as well as the potential nucleating function of spermatid centrosomes undergoing a reduction process. Electron microscopy demonstrates the presence in manchette preparations of spermatid centrosomes, recognized as such by their association with remnants of the implantation fossa, a dense plate observed only at the basal surface of developing spermatid and sperm nuclei. In addition, we have found importin beta1 immunoreactivity in the nucleus of elongating spermatids, a finding that, together with the presence of Ran-GTPase in the nucleus of round spermatids and the manchette, suggest a potential role of Ran-GTPase machinery in nucleocytoplasmic transport. Our expression and localization analysis, correlated with functional observations in other cell systems, suggest that Ran-GTPase may be involved in both nucleocytoplasmic transport and microtubules assembly, two critical events during the development of functional sperm. In addition, the manchette-to-centrosome Ran-GTPase relocation, together with the similar redistribution of various proteins associated to the manchette, suggest the existence of an intramanchette molecular transport mechanism, which may share molecular analogies with intraflagellar transport.

Ran GTPase expression during early development of the mouse embryo

Ran is a small GTP-binding protein involved in several essential roles for cell viability. This observation implies Ran might be ubiquitously expressed during development. However, Ran shows a differentiated expression pattern that is restricted to specific tissues from embryo to adult. At early embryonic stages of mouse development we found persistent Ran expression in proliferating neural tissue, neural crest derived dorsal root ganglions and sensory pits. We also showed an accumulation of Ran transcripts in main embryonic haematopoietic tissues: blood islands first and then hepatic bud. In advanced stages of development Ran is also expressed in other tissues showing a high cell turnover.

Small GTP-binding proteins in the nuclei of human placenta

Mitochondria and crude nuclei containing fractions from human placenta have been shown to contain proteins which bind [alpha(32)P]-GTP. Prior to this study the number of GTP-binding proteins in placental nuclei and their nucleotide specificity was not known. Also unknown was the identity of any of the GTP-binding proteins in mitochondria of human placenta. Nuclei and mitochondria were purified from human placental extracts by sedimentation. Proteins were separated by electrophoresis and transferred to nitrocellulose membranes. Overlay blot with [alpha(32)P]-GTP identified two nuclei proteins with approximate molecular weights of 24 and 27 kDa. Binding of [alpha(32)P]-GTP to the 27 and 24 kDa proteins was significantly displaced by guanine nucleotides but not by adenine, thymine or cytosine nucleotides or deoxy (d) GTP. Western blot with a specific antibody to Ran identified a band at 27 kDa in nuclei and in mitochondrial fractions. These data indicate that both nuclei and mitochondria contain 24 and 27 kDa GTP-binding proteins. The GTP-binding proteins in nuclei display binding specificity for guanine nucleotides and the hydroxylated carbon 2 on the ribose ring of GTP appears essential for binding. It will be important in future studies to determine the functions of these small GTP-binding proteins in the development and physiology of the placenta.

Changes in gene expression profiles in developing B cells of murine bone marrow

Gene expression profiles of five consecutive stages of mouse B cell development were generated with high-density oligonucleotide arrays from as few as 2 x 10(4) ex vivo isolated and flow-cytometrically purified cells. Between 2.8% and 6.8% of all genes change on differentiation from one cellular stage to the next by at least twofold. The entire pathway involves differential expression of 10.7% of all genes. Previously known expression patterns of 15 genes (like surrogate light chain, RAG-1/2, MHC class II, mel-14 antigen) are confirmed. The gene expression patterns of the proliferating pre-BI and large pre-BII cells on the one hand, and the resting immature and mature B cells on the other hand, are most similar to each other. Small pre-BII cells display a pattern that is transitional between these two groups. Most of the genes expressed in early precursors are involved in general processes, like protein folding or cell cycle regulation, whereas more mature precursors express genes involved in more specific molecular programs (cell surface receptors, secreted factors, and adhesion molecules, among others). Between 19 and 139 genes share a given expression pattern. Combining knowledge about gene function and expression pattern allows identification of novel candidate genes potentially involved in self-maintenance of pre-BI cells, allelic exclusion and pre-B cell receptor signaling in large pre BII cells, cell-cycle arrest of small pre-BII cells, propensity toward apoptosis or anergization in immature B cells, propensity toward cell division and activation in mature B cells, and stage-specific interactions with stromal cells in the bone marrow.

A single point mutation at the 3'-untranslated region of Ran mRNA leads to profound changes in lipopolysaccharide endotoxin-mediated responses

By functional cDNA expression cloning, we have previously established that Ran is important in lipopolysaccharide (LPS) signaling. This was achieved by functional comparison between two cDNAs, differing by a single base substitution within the 3'-untranslated region of the cDNA. This point mutation results in a striking RNA conformational change. No dramatic difference in total RNA at steady state could be found between the two molecules. However, at the protein level, RanC/d (from 870C mRNA) was 5-10-fold higher than RanT/n (from 870T mRNA) and this difference was not observed in non-hematopoietic cells transduced with the same vectors. This tissue-specific difference correlated with a difference in LPS endotoxin responses in corresponding hematopoietic cells. Importantly, the amounts of Ran- C/d and RanT/n proteins were similar initially but the difference became obvious with time. Both Ran proteins migrated from the cytoplasm to the nucleus, but Ran from RanC/d migrated faster than that of RanT/n. RanT/n protein preferentially remained in the cytoplasm and its overall amount was reduced at steady state, consistent with its degradation by intracellular proteases known to be involved in LPS-mediated signal transduction. As the two proteins are identical, the faster RanC/d nuclear localization and a preferred initial cytoplasmic RanT/n distribution suggest a difference in mRNA intracellular localization between the two molecules, as dictated by their RNA structural difference. By pulse-chase experiments, RanC/d proteins are more resistant to degradation than RanT/n protein; there also appear to have two populations of RanT/n proteins, one may reside in the cytoplasm and the other, in the nucleus. More RanC/d GTPase accumulated in the nuclei would conceivably alter the potency of signal transduction and therefore down-modulate LPS-mediated biological responses.

The structure and expression of the Salmo salar Ran gene

The Ran protein is a highly conserved GTPase factor of the Ras-family of proteins. It is involved in the transport of proteins and RNA through the nuclear pores. cDNA sequences of Ran genes from different organisms are available but no vertebrate genomic sequence as yet. We have isolated a genomic cosmid clone of the Ran1 gene of Atlantic salmon (Salmo salar) and found that the gene is about 2.2 kb and has got 5 introns. Its gene structure was compared to those of Caenorhabditis elegans and Arabidopsis thaliana and one intron was found to be conserved in position. Two different Ran transcripts were identified in salmon, indicating that there are two Ran genes, Ran1 and Ran2. Both transcripts were found in all ten salmon tissues tested, though Ran1 to a higher degree.

Antisperm antibodies associated with infertility: properties and encoding genes of target antigens

Infertility among couples of reproductive age is a perplexing condition when the cause is indeterminate. These cases are classified as unexplained infertility. In a subset of subjects, antisperm antibodies with sperm agglutinating and/or immobilizing activities have been detected in the blood or fluids of the reproductive tract. These cases are designated as immunologic infertility although a cause and effect relationship of the antibodies to infertility has not been established. In this review, seven target sperm antigens to antibodies associated with infertility and their encoding genes are described. The antisperm antibodies (ASAs) examined were obtained from infertile women or were monoclonal antibodies (mAb) raised against human sperm proteins. All the ASAs studied possessed potent sperm agglutinating and/or immobilizing activities. The target antigens were isolated from human and other mammalian sperm, and the encoding genes identified. The seven antigens are YWK-II, BE-20, rSMP-B, BS-63 (nucleoporin-related), BS-17 (calpastatin), HED-2 (zyxin), and 75- kDa. Each antigen is a distinct and separate entity and is produced by different cells of the reproductive tract, (e.g., germ cells, epididymal epithelial cells, and Sertoli cells). No single predominant target component has been found to interact with the ASAs. It is proposed that immunologic infertility is the consequence of the combined actions of multiple ASAs in immobilizing and/or agglutinating spermatozoa, blocking spermegg interaction, preventing implantation, and/or arresting embryo development.

Dominant negative down-regulation of endotoxin-induced tumor necrosis factor alpha production by Lps(d)/Ran

We recently showed that adenoviral transfer and expression of the Lps(d)/Ran gene isolated from endotoxin-resistant C3H/HeJ mice could protect endotoxin-sensitive mice from endotoxic shock. Elevation of proinflammatory cytokines, such as tumor necrosis factor alpha (TNF-alpha), is thought to be essential for the development of septic shock. To investigate the extent to which Lps(d)/Ran affects TNF-alpha production, we transduced primary macrophages from endotoxin-sensitive and -resistant mice with adenoviral vectors expressing the wild-type and the mutant Lps/Ran cDNAs and other control genes, and compared the amount of TNF-alpha produced by these various transduced macrophages. Successful transfer and expression of Lps(d)/Ran cDNA in endotoxin-sensitive C3H/HeOuJ macrophages reduced TNF-alpha production upon lipopolysaccharide (LPS) stimulation, as compared with macrophages transduced with vectors expressing the wild-type Lps(n)/Ran cDNA, the green fluorescent protein gene, or the lacZ gene. On the other hand, successful transfer and expression of the wild-type Lps(n)/Ran cDNA in primary macrophages from endotoxin-resistant C3H/HeJ mice failed to induce TNF-alpha production to any significant extent unless a very high LPS concentration was used. Given our previous demonstration that Lps(n)/Ran functions effectively in restoring LPS responsiveness in B cells from C3H/HeJ mice, we conclude that Lps/Ran is involved in a CD14-independent signal transduction pathway. This dominant negative down-regulation by Lps(d)/Ran on TNF-alpha production by macrophages and probably other innate immune responses may be key to the development of an effective gene therapy for endotoxic or septic shock.

Lps(d)/Ran of endotoxin-resistant C3H/HeJ mice is defective in mediating lipopolysaccharide endotoxin responses

C3H/HeJ inbred mice are defective in that they are highly resistant to endotoxic shock as compared with normal responder mice. Their B cells and macrophages do not respond significantly when exposed to lipopolysaccharide (LPS), whereas cells from the responder mice do. Using a functional assay, we previously isolated a cDNA, which encodes for Ran/TC4 GTPase. We now show that this gene is mutated in C3H/HeJ mice, which accounts for their resistance to endotoxin stimulation. Sequence analysis of independent mutant Lps(d)/Ran cDNAs isolated from splenic B cells of C3H/HeJ mice reveals a consistent single base substitution at position 870, where a thymidine is replaced with a cytidine. In situ hybridization maps the Lps(d)/Ran cDNA to mouse chromosome 4. By retroviral gene transfer, the wild-type Lps(n)/Ran cDNA but not the mutant Lps(d)/Ran cDNA can restore LPS responsiveness of C3H/HeJ cells. Adenoviral gene transfer in vivo with the mutant Lps(d)/Ran cDNA but not the wild-type Lps(n)/Ran cDNA rescues endotoxin-sensitive mice from septic shock. Thus Lps/Ran is an important target for LPS-mediated signal transduction, and the Lps(d)/Ran gene may be useful as a therapeutic sequence in gene therapy for endotoxemia and septic shock.

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

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

Molecular cloning and characterization of a novel testis-specific nucleoporin-related gene

A 20-kDa sperm membrane protein cDNA, designated as RSD-1, was isolated by epitope selection from a rat testis lambda gtll expression library. RSD-1 was used as a probe to screen a human testis lambda ZAPII cDNA expression library. A cDNA designated as BS-63 was isolated and found to consist of 1933 bp with an open reading frame of 1824 bp and assigned the accession number U64675 by GenBank. The deduced polypeptide consisted of 608 amino acid residues containing XFXFG or FG motifs that are characteristic of nuclear pore complex (NPC) proteins and act as potential binding sites for Ran. The N-terminal region has high homology with RanBP2/Nup358, a nucleoporin component, showing that BS-63 is a member of the NPC family. Northern blot analysis of mRNAs prepared from various human tissues shows that BS-63 is transcribed in two forms: 6.0 and 8.5 kb. The 8.5-kb transcript was present in low amounts in several somatic tissues, whereas the 6.0-kb transcript is expressed only in testis. In situ hybridization analysis of human testis sections showed that BS-63 mRNA is expressed only in germ cells at all stages of spermatogenesis. Sertoli cells did not transcribe the gene.

Highly conserved RNA sequences that are sensors of environmental stress

The putative function of highly conserved regions (HCRs) within 3' untranslated regions (3'UTRs) as regulatory RNA sequences was efficiently and quantitatively assessed by using modular retroviral vectors. This strategy led to the identification of HCRs that alter gene expression in response to oxidative or mitogenic stress. Databases were screened for UTR sequences of >100 nucleotides that had retained 70% identity over more than 300 million years of evolution. The effects of 10 such HCRs on a standard reporter mRNA or protein were studied. To this end, we developed a modular retroviral vector that can allow for a direct comparison of the effects of different HCRs on gene expression independent of their gene-intrinsic 5'UTR, promoter, protein coding region, or poly(A) sequence. Five of the HCRs tested decreased mRNA steady-state levels 2- to 10-fold relative to controls, presumably by altering mRNA stability. One HCR increased translation, and one decreased translation. Elevated mitogen levels caused four HCRs to increase protein levels twofold. One HCR increased protein levels fourfold in response to hypoxia. Although nonconserved UTR sequences may also have a role, these results provide evidence that sequences that are highly conserved during evolution are good candidates for RNA motifs with posttranscriptional regulatory functions in gene expression.

cDNA cloning, gene characterization and 13q14.3 chromosomal assignment of CHC1-L, a chromosome condensation regulator-like guanine nucleotide exchange factor

We report the characterization of a new gene mapped at chromosome band 13q14.3 telomeric to the retinoblastoma gene. This gene, designated CHC1L (for chromosome condensation 1-like), is composed of 14 exons spanning 30 kb of genomic DNA and encodes a ubiquitously expressed 3-kb mRNA. The N-terminal half of the deduced amino acid sequence shows strong homology with the seven tandem repeat structure of the regulator of chromosome condensation RCC1, which acts as a guanine nucleotide exchange factor (GEF) protein for the Ras-related GTPase Ran. CHC1L appears to be a new member of the RCC1-related GEF family.

Nucleocytoplasmic transport of macromolecules

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

An abundant zebrafish cDNA clone encodes a ras-like protein which is expressed ubiquitously

An abundant clone from a zebrafish embryonic cDNA library was identified and sequenced. It encodes a ras-like protein, being 94-95% identical to vertebrate Ran, ras-related nuclear protein. The zebrafish Ran mRNA is expressed in all tissues examined and throughout embryogenesis; therefore, it is likely that the Ran protein functions in all types of cells.

A gene (RPGR) with homology to the RCC1 guanine nucleotide exchange factor is mutated in X-linked retinitis pigmentosa (RP3)

X-linked retinitis pigmentosa (xlRP) is a severe progressive retinal degeneration which affects about 1 in 25,000 of the population. The most common form of xlRP, RP3, has been localised to the interval between CYBB and OTC in Xp21.1 by linkage analysis and deletion mapping. Identification of microdeletions within this region has now led to the positional cloning of a gene, RPGR, that spans 60 kg of genomic DNA and is ubiquitously expressed. The predicted 90 kD protein contains in its N-terminal half a tandem repeat structure highly similar to RCC1 (regulator of chromosome condensation), suggesting an interaction with a small GTPase. The C-terminal half contains a domain, rich in acidic residues, and ends in a potential isoprenylation anchorage site. The two intragenic deletions, two nonsense and three missense mutations within conserved domains provide evidence that RPGR (retinitis pigmentosa GTPase regulator) is the RP3 gene.

Multiplex messenger assay: simultaneous, quantitative measurement of expression of many genes in the context of T cell activation

The hybridization signature approach, using colony filters and labeled complex probes, can provide high throughput measurement of gene activity. We describe here the implementation of this method to follow the expression levels of 47 genes in resting and activated T cells, as well as in epithelial cells. Using 4-fold spotting of colonies, imaging plate detection and various correction and normalization procedures, the technique is sensitive enough to quantify expression levels for sequences present at 0.005% abundance in the probe. Comparison with Northern blotting shows good consistency between the two methods. Upon activation of a T cell clone by an anti-CD3 antibody variations ranging from 2- to 20-fold are measured, some of which had not been reported previously. This 'multiplex messenger assay' method, performed using available commercial apparatus, can be used in many cases where simultaneous assessment of mRNA levels for many genes is of interest.

The small nuclear GTPase Ran: how much does it run?

Ran is one of the most abundant and best conserved of the small GTP binding and hydrolyzing proteins of eukaryotes. It is located predominantly in cell nuclei. Ran is a member of the Ras family of GTPases, which includes the Ras and Ras-like proteins that regulate cell growth and division, the Rho and Rac proteins that regulate cytoskeletal organization and the Rab proteins that regulate vesicular sorting. Ran differs most obviously from other members of the Ras family in both its nuclear localization, and its lack of sites required for post-translational lipid modification. Ran is, however, similar to other Ras family members in requiring a specific guanine nucleotide exchange factor (GEF) and a specific GTPase activating protein (GAP) as stimulators of overall GTPase activity. In this review, the multiple cellular functions of Ran are evaluated with respect to its known biochemistry and molecular interactions.

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

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