GSP1 and GSP2, genetic suppressors of the prp20-1 mutant in Saccharomyces cerevisiae: GTP-binding proteins involved in the maintenance of nuclear organization

Analysis of the molecular basis of Saccharomyces cerevisiae mutant with high nucleic acid content by comparative transcriptomics

Ribonucleic acid (RNA) and its degradation products are important functional components widely used in the food industry. Transcription analysis was used to explore the genetic mechanism underlying nucleic acid synthesis in the chemical mutant Saccharomyces cerevisiae strain BY23-195 with high nucleic acid content. Results showed that ribosome biogenesis, meiosis, RNA transport, mitogen-activated protein kinase (MAPK) signaling pathway, tryptophan metabolism, carbon metabolism, and longevity regulating pathway are closely related to the high nucleic acid metabolism of S. cerevisiae. Fourteen most promising genes were selected to evaluate the effect of single-gene deletion or overexpression on the RNA synthesis of S. cerevisiae. Compared with the RNA content of the parent strain BY23, that of mutant strains BY23-HXT1, BY23-ΔGSP2 and BY23-ΔCTT1 increased by 8.19%, 11.60% and 14.00%, respectively. The possible reason why HXT1, GSP2, and CTT1 affect RNA content is by regulating cell fitness. This work was the first to report that regulating the transcription of HXT1, GSP2, and CTT1 could increase the RNA content of S. cerevisiae. This work also provides valuable knowledge on the genetic mechanism of high nucleic acid synthesis in S. cerevisiae and new strategies for increasing its RNA content.

A nuclear lamina-chromatin-Ran GTPase axis modulates nuclear import and DNA damage signaling

The Ran GTPase regulates nuclear import and export by controlling the assembly state of transport complexes. This involves the direct action of RanGTP, which is generated in the nucleus by the chromatin‐associated nucleotide exchange factor, RCC1. Ran interactions with RCC1 contribute to formation of a nuclear:cytoplasmic (N:C) Ran protein gradient in interphase cells. In previous work, we showed that the Ran protein gradient is disrupted in fibroblasts from Hutchinson–Gilford progeria syndrome (HGPS) patients. The Ran gradient disruption in these cells is caused by nuclear membrane association of a mutant form of Lamin A, which induces a global reduction in heterochromatin marked with Histone H3K9me3 and Histone H3K27me3. Here, we have tested the hypothesis that heterochromatin controls the Ran gradient. Chemical inhibition and depletion of the histone methyltransferases (HMTs) G9a and GLP in normal human fibroblasts reduced heterochromatin levels and caused disruption of the Ran gradient, comparable to that observed previously in HGPS fibroblasts. HMT inhibition caused a defect in nuclear localization of TPR, a high molecular weight protein that, owing to its large size, displays a Ran‐dependent import defect in HGPS. We reasoned that pathways dependent on nuclear import of large proteins might be compromised in HGPS. We found that nuclear import of ATM requires the Ran gradient, and disruption of the Ran gradient in HGPS causes a defect in generating nuclear γ‐H2AX in response to ionizing radiation. Our data suggest a lamina–chromatin–Ran axis is important for nuclear transport regulation and contributes to the DNA damage response.

Intertwined control of the cell cycle and nucleocytoplasmic transport by the cyclin-dependent kinase Pho85 and RanGTPase Gsp1 in Saccharomyces cerevisiae

Deciphering the molecular mechanisms that connect cell cycle progression and nucleocytoplasmic transport is of particular interest: this intertwined relationship, once understood, may provide useful insight on the diseases resulting from the malfunction of these processes. In the present study we report on findings that indicate a biochemical connection between the cell cycle regulator CDK Pho85 and Ran-GTPase Gsp1, an essential nucleocytoplasmic transport component. When Gsp1 cannot be phosphorylated by Pho85, the cell cycle progression is impaired. Accordingly, a nonphosphorylatable version of Gsp1 abnormally localizes to the nucleus, which impairs the nuclear transport of molecules, including key components of cell cycle progression. Furthermore, our results suggest that the physical interaction of Gsp1 and the Kap95 karyopherin, essential to the release of nuclear cargoes, is altered. Altogether, the present findings point to the involvement of a biochemical mechanism in the interlocked regulation of the cell cycle and nuclear transport.

Identification of Genes in Saccharomyces cerevisiae that Are Haploinsufficient for Overcoming Amino Acid Starvation

The yeast Saccharomyces cerevisiae responds to amino acid deprivation by activating a pathway conserved in eukaryotes to overcome the starvation stress. We have screened the entire yeast heterozygous deletion collection to identify strains haploinsufficient for growth in the presence of sulfometuron methyl, which causes starvation for isoleucine and valine. We have discovered that cells devoid of MET15 are sensitive to sulfometuron methyl, and loss of heterozygosity at the MET15 locus can complicate screening the heterozygous deletion collection. We identified 138 cases of loss of heterozygosity in this screen. After eliminating the issues of the MET15 loss of heterozygosity, strains isolated from the collection were retested on sulfometuron methyl. To determine the general effect of the mutations for a starvation response, SMM-sensitive strains were tested for the ability to grow in the presence of canavanine, which induces arginine starvation, and strains that were MET15 were also tested for growth in the presence of ethionine, which causes methionine starvation. Many of the genes identified in our study were not previously identified as starvation-responsive genes, including a number of essential genes that are not easily screened in a systematic way. The genes identified span a broad range of biological functions, including many involved in some level of gene expression. Several unnamed proteins have also been identified, giving a clue as to possible functions of the encoded proteins.

Characterization of a Ran gene from Puccinia striiformis f. sp. tritici involved in fungal growth and anti-cell death

Ran, an important family of small GTP-binding proteins, has been shown to regulate a variety of important cellular processes in many eukaryotes. However, little is known about Ran function in pathogenic fungi. In this study, we report the identification and functional analysis of a Ran gene (designated PsRan) from Puccinia striiformis f. sp. tritici (Pst), an important fungal pathogen affecting wheat production worldwide. The PsRan protein contains all conserved domains of Ran GTPases and shares more than 70% identity with Ran proteins from other organisms, indicating that Ran proteins are conserved in different organisms. PsRan shows a low level of intra-species polymorphism and is localized to the nucleus. qRT-PCR analysis showed that transcript level of PsRan was induced in planta during Pst infection. Silencing of PsRan did not alter Pst virulence phenotype but impeded fungal growth of Pst. In addition, heterologous overexpression of PsRan in plant failed to induce cell death but suppressed cell death triggered by a mouse BAX gene or a Pst Ras gene. Our results suggest that PsRan is involved in the regulation of fungal growth and anti-cell death, which provides significant insight into Ran function in pathogenic fungi.

Altering nuclear pore complex function impacts longevity and mitochondrial function in S. cerevisiae

Specific nucleoporins and nuclear pore complex–dependent transport events directly influence aging in yeast.

The eukaryotic nuclear permeability barrier and selective nucleocytoplasmic transport are maintained by nuclear pore complexes (NPCs), large structures composed of ∼30 proteins (nucleoporins [Nups]). NPC structure and function are disrupted in aged nondividing metazoan cells, although it is unclear whether these changes are a cause or consequence of aging. Using the replicative life span (RLS) of Saccharomyces cerevisiae as a model, we find that specific Nups and transport events regulate longevity independent of changes in NPC permeability. Mutants lacking the GLFG domain of Nup116 displayed decreased RLSs, whereas longevity was increased in nup100-null mutants. We show that Nup116 mediates nuclear import of the karyopherin Kap121, and each protein is required for mitochondrial function. Both Kap121-dependent transport and Nup116 levels decrease in replicatively aged yeast. Overexpression of GSP1, the small GTPase that powers karyopherin-mediated transport, rescued mitochondrial and RLS defects in nup116 mutants and increased longevity in wild-type cells. Together, these studies reveal that specific NPC nuclear transport events directly influence aging.

Targeting of the SUN protein Mps3 to the inner nuclear membrane by the histone variant H2A.Z

Binding of histone H2A.Z to the SUN family member Mps3 is chromatin independent.

Understanding the relationship between chromatin and proteins at the nuclear periphery, such as the conserved SUN family of inner nuclear membrane (INM) proteins, is necessary to elucidate how three-dimensional nuclear architecture is established and maintained. We found that the budding yeast SUN protein Mps3 directly binds to the histone variant H2A.Z but not other histones. Biochemical and genetic data indicate that the interaction between Mps3 and H2A.Z requires the Mps3 N-terminal acidic domain and unique sequences in the H2A.Z N terminus and histone-fold domain. Analysis of binding-defective mutants showed that the Mps3–H2A.Z interaction is not essential for any previously described role for either protein in nuclear organization, and multiple lines of evidence suggest that Mps3–H2A.Z binding occurs independently of H2A.Z incorporation into chromatin. We demonstrate that H2A.Z is required to target a soluble Mps3 fragment to the nucleus and to localize full-length Mps3 in the INM, indicating that H2A.Z has a novel chromatin-independent function in INM targeting of SUN proteins.

Importin β-type nuclear transport receptors have distinct binding affinities for Ran-GTP

Cargos destined to enter or leave the cell nucleus are typically transported by receptors of the importin β family to pass the nuclear pore complex. The yeast Saccharomyces cerevisiae comprises 14 members of this protein family, which can be divided in importins and exportins. The Ran GTPase regulates the association and dissociation of receptors and cargos as well as the transport direction through the nuclear pore. All receptors bind to Ran exclusively in its GTP-bound state and this event is restricted to the nuclear compartment. We determined the Ran-GTP binding properties of all yeast transport receptors by biosensor measurements and observed that the affinity of importins for Ran-GTP differs significantly. The dissociation constants range from 230 pM to 270 nM, which is mostly based on a variability of the off-rate constants. The divergent affinity of importins for Ran-GTP suggests the existence of a novel mode of nucleocytoplasmic transport regulation. Furthermore, the cellular concentration of β-receptors and of other Ran-binding proteins was determined. We found that the number of β-receptors altogether about equals the amounts of yeast Ran, but Ran-GTP is not limiting in the nucleus. The implications of our results for nucleocytoplasmic transport mechanisms are discussed.

Glycinebetaine protects plants against abiotic stress: mechanisms and biotechnological applications

Various compatible solutes enable plants to tolerate abiotic stress, and glycinebetaine (GB) is one of the most-studied among such solutes. Early research on GB focused on the maintenance of cellular osmotic potential in plant cells. Subsequent genetically engineered synthesis of GB-biosynthetic enzymes and studies of transgenic plants demonstrated that accumulation of GB increases tolerance of plants to various abiotic stresses at all stages of their life cycle. Such GB-accumulating plants exhibit various advantageous traits, such as enlarged fruits and flowers and/or increased seed number under non-stress conditions. However, levels of GB in transgenic GB-accumulating plants are relatively low being, generally, in the millimolar range. Nonetheless, these low levels of GB confer considerable tolerance to various stresses, without necessarily contributing significantly to cellular osmotic potential. Moreover, low levels of GB, applied exogenously or generated by transgenes for GB biosynthesis, can induce the expression of certain stress-responsive genes, including those for enzymes that scavenge reactive oxygen species. Thus, transgenic approaches that increase tolerance to abiotic stress have enhanced our understanding of mechanisms that protect plants against such stress.

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.

Overexpression of OsRAN2 in rice and Arabidopsis renders transgenic plants hypersensitive to salinity and osmotic stress

Nucleo-cytoplasmic partitioning of regulatory proteins is increasingly being recognized as a major control mechanism for the regulation of signalling in plants. Ras-related nuclear protein (Ran) GTPase is required for regulating transport of proteins and RNA across the nuclear envelope and also has roles in mitotic spindle assembly and nuclear envelope (NE) assembly. However, thus far little is known of any Ran functions in the signalling pathways in plants in response to changing environmental stimuli. The OsRAN2 gene, which has high homology (77% at the amino acid level) with its human counterpart, was isolated here. Subcellular localization results showed that OsRan2 is mainly localized in the nucleus, with some in the cytoplasm. Transcription of OsRAN2 was reduced by salt, osmotic, and exogenous abscisic acid (ABA) treatments, as determined by real-time PCR. Overexpression of OsRAN2 in rice resulted in enhanced sensitivity to salinity, osmotic stress, and ABA. Seedlings of transgenic Arabidopsis thaliana plants overexpressing OsRAN2 were overly sensitive to salinity stress and exogenous ABA treatment. Furthermore, three ABA- or stress-responsive genes, AtNCED3, AtPLC1, and AtMYB2, encoding a key enzyme in ABA synthesis, a phospholipase C homologue, and a putative transcriptional factor, respectively, were shown to have differentially induced expression under salinity and ABA treatments in transgenic and wild-type Arabidopsis plants. OsRAN2 overexpression in tobacco epidermal leaf cells disturbed the nuclear import of a maize (Zea mays L.) leaf colour transcription factor (Lc). In addition, gene-silenced rice plants generated via RNA interference (RNAi) displayed pleiotropic developmental abnormalities and were male sterile.

Lte1, Cdc14 and MEN-controlled Cdk inactivation in yeast coordinate rDNA decompaction with late telophase progression

The mechanism of chromatin compaction in mitosis has been well studied, while little is known about what controls chromatin decompaction in early G1 phase. We have localized the Condensin subunit Brn1 to a compact spiral of rDNA in mitotic budding yeast cells. Brn1 release and the resulting rDNA decompaction in late telophase coincided with mitotic spindle dissociation, and occurred asymmetrically (daughter cells first). We immunoprecipitated the GTP-exchange factor Lte1, which helps activate the mitotic exit network (MEN) in anaphase, with mitotic Brn1. In lteDelta cells Brn1 release was delayed, even at temperatures that do not impair mitotic exit. Mutations in MEN pathway components that act downstream of Lte1 similarly delayed rDNA decompaction. We found that Brn1 release in wild-type cells coincided with the release of Cdc14 phosphatase from the nucleolus and with mitotic CDK inactivation, yet it could be selectively delayed by perturbation of the MEN pathway. This may argue that different levels of Cdk inactivation control spindle disassembly and chromatin decompaction. Mutation of lte1 also impaired rotation of the nucleus in early G1.

RanGTPase: A Key Regulator of Nucleocytoplasmic Trafficking

RanGTPase belongs to the Ras superfamily of small GTPases. It possesses a distinctive acidic C-terminal DEDDDL motif and predominantly localizes to the nucleus. RanGTPase is known to regulate nucleocytoplasmic trafficking as well as mitotic spindle and nuclear envelope formation. Ran-directed nucleocytoplasmic trafficking is an energy-dependent directional process that also depends on nuclear import or export signals. Ran-directed nucleocytoplasmic trafficking is also facilitated by several cellular components, including RanGTPase, karyopherins, NTF2 and nucleoporins. GTP-bound Ran is asymmetrically distributed in the nucleus, while GDP-bound Ran is predominantly cytoplasmic. Controlled by RanGEF and RanGAP, RanGTPase cycles between the GDP- and GTP-bound states enabling it to shuttle cargoes in an accurate spatial and temporal manner. RanGTPase plays a role in the nuclear import in such a way that GTP-bound Ran dissociates importin:cargo complex in the nucleus and recycles importin back to cytoplasm. Likewise, RanGTPase plays a role in the nuclear export in such a way that nuclear GTP-bound Ran triggers the aggregation of Ran:exportin:cargo trimeric complex which is then transported to cytoplasm while hydrolysis of RanGTP to RanGDP releases the export cargoes in cytoplasm. RanGTPase has been reported to be essential for cell viability and its over-expression is linked to tumorigenesis. Thus, RanGTPase plays a crucial role in regulating key cellular events and alterations in its expression may lead to cancer development and/or progression.

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.

Identification of novel suppressors for Mog1 implies its involvement in RNA metabolism, lipid metabolism and signal transduction

Mog1 is conserved from yeast to mammal, but its function is obscure. We isolated yeast genes that rescued a temperature-sensitive death of S. cerevisiae Scmog1Delta, and of S. pombe Spmog1(ts). Scmog1Delta was rescued by Opi3p, a phospholipid N-methyltransferase, in addition to S. cerevisiae Ran-homologue Gsp1p, and a RanGDP binding protein Ntf2p. On the other hand, Spmog1(ts) was rescued by Cid13 that is a poly (A) polymerase specific for suc22(+) mRNA encoding a subunit of ribonucleotide reductase, Ssp1 that is a protein kinase involved in stress response pathway, and Crp79 that is required for mRNA export, in addition to Spi1, S. pombe Ran-homologue, and Nxt2, S. pombe homologue of Ntf2p. Consistent with the identification of those suppressors, lack of ScMog1p dislocates Opi3p from the nuclear membrane and all of Spmog1(ts) showed the nuclear accumulation of mRNA. Furthermore, SpMog1 was co-precipitated with Nxt2 and Cid13.

The nuclear GTPase Gsp1p can affect proper telomeric function through the Sir4 protein inSaccharomyces cerevisiae

The small Ras-like GTPase Ran/Gsp1p is a highly conserved nuclear protein required for the nucleocytoplasmic trafficking of macromolecules. Recent findings suggest that the Ran/Gsp1p pathway may have additional roles in several aspects of nuclear structure and function, including spindle assembly, nuclear envelope formation, nuclear pore complex assembly and RNA processing. Here, we provide evidence that Gsp1p can regulate telomeric function in Saccharomyces cerevisiae. We show that overexpression of PRP20, encoding the Gsp1p GDP/GTP nuclear exchange factor, specifically weakens telomeric silencing without detectably affecting nucleocytoplasmic transport. In addition to this silencing defect, we show that Rap1p and Sir3p delocalize from their normal telomeric foci. Interestingly, Gsp1p was found to interact genetically and physically with the telomeric component Sir4p. Taken together, these results suggest that the GSP1 pathway could regulate proper telomeric function in yeast through Sir4p.

Saccharomyces cerevisiae GTPase complex: Gtr1p-Gtr2p regulates cell-proliferation through Saccharomyces cerevisiae Ran-binding protein, Yrb2p

A Gtr1p GTPase, the GDP mutant of which suppresses both temperature-sensitive mutants of Saccharomyces cerevisiae RanGEF/Prp20p and RanGAP/Rna1p, was presently found to interact with Yrb2p, the S. cerevisiae homologue of mammalian Ran-binding protein 3. Gtr1p bound the Ran-binding domain of Yrb2p. In contrast, Gtr2p, a partner of Gtr1p, did not bind Yrb2p, although it bound Gtr1p. A triple mutant: yrb2delta gtr1delta gtr2delta was lethal, while a double mutant: gtr1delta gtr2delta survived well, indicating that Yrb2p protected cells from the killing effect of gtr1delta gtr2delta. Recombinant Gtr1p and Gtr2p were purified as a complex from Escherichia coli. The resulting Gtr1p-Gtr2p complex was comprised of an equal amount of Gtr1p and Gtr2p, which inhibited the Rna1p/Yrb2 dependent RanGAP activity. Thus, the Gtr1p-Gtr2p cycle was suggested to regulate the Ran cycle through Yrb2p.

Differentiating mechanisms of toxicity using global gene expression analysis in Saccharomyces cerevisiae

Genotoxic stress triggers a variety of biological responses including the transcriptional activation of genes regulating DNA repair, cell survival and cell death. Genomic approaches, which monitor gene expressions across large numbers of genes, can serve as a powerful tool for exploring mechanisms of toxicity. Here, using five different agents, we investigated whether the analysis of genome-wide expression profiles in Saccharomyces cerevisiae could provide insights into mechanisms of genotoxicity versus cytotoxicity. To differentiate the genotoxic stress-associated expression signatures from that of a general cytotoxic stress, we compared gene expression profiles following the treatment with DNA-reactive (cisplatin, MMS, bleomycin) and DNA non-reactive (ethanol and sodium chloride) compounds. Although each of the tested chemicals produced a distinct gene expression profile, we were able to identify a gene expression signature consisting of a relatively small number of biologically relevant genes capable of differentiating genotoxic and cytotoxic stress. The gene set includes such upregulated genes as HUG1, ECM4 and previously uncharacterized gene, YLR297W in the genotoxic and GAP1, CGR1 in the cytotoxic group. Our results indicate the potential of gene expression profile analysis for elucidating mechanism of action of genotoxic agents.

Brl1p -- a novel nuclear envelope protein required for nuclear transport

In this article, we identify a cold-sensitive mutant of Xpo1p designated as xop1-2 (but will be referred to from here on as xpo1-ok) that is synthetically lethal with srm1-1, a Saccharomyces cerevisiae RCC1 homolog. xpo1-ok was a novel mutated allele with a single point mutation, T283P. Suppressors of xpo1-ok were isolated, and one of them was found to encode a novel nuclear envelope integral membrane protein designated as Brl1p (Brr6 like protein no. 1). Brl1p is homologous with Brr6p at the C-terminal domain, which is well conserved in the Brr6/Brl1 family. To characterize the function of Brl1p, a series of temperature-sensitive mutants of Brl1p were isolated. All of brl1 mutations were localized to the conserved C-terminal domain that is essential for a function of Brl1p. Some brl1 alleles showed defects in nuclear export of either mRNA or protein, and nuclear pore clustering, similar to brr6-1. The cellular localization of Brl1p is also similar to that of Brr6p. The genetic analysis suggested that Brl1p functionally interacts with Brr6p. An interaction of Brl1p with Brr6p was shown by the two-hybrid method. We hypothesize that Brl1p functions for nuclear export as a complex with Brr6p.

Differential gene expression in filamentous cells of Ustilago maydis

When fungi interact with plants as pathogens or as symbionts, there are often changes in fungal cell morphology and nuclear state. This study establishes the use of cDNA microarrays to detect gene expression changes in Ustilago maydis cells that differ in structure and nuclear content. Categorizing differentially expressed genes on the basis of function indicated that U. maydis cell types vary most in the expression of genes related to metabolism. We also observed that more genes are up-regulated in the filamentous dikaryon than in the filamentous diploid, relative to non-pathogenic budding cells. Our comparison of pathogenic development indicated that the dikaryon is more virulent than the diploid. Other identified expression patterns suggest a cell-specific difference in nutrient acquisition, cell metabolism and signal transduction. The relevance of gene expression change to cell type biology is discussed.

Proteasome inhibition alters the transcription of multiple yeast genes

The 26S proteasome degrades denatured proteins and other proteins targeted for destruction by covalent modification. Here we show that impaired proteasome function influences the transcription of numerous yeast genes. Of 6144 genes present on the macroarray filters used in this study, approximately 5% showed measurable mRNA decreases and 2% showed mRNA increases following 30 min of proteasome inhibition. Northern blot hybridization shows that this response is time- and dose-dependent and occurs with either pharmacological or genetic impairment of the proteasome. A number of splicing factors possess the PEST motif found in certain proteasome substrates. However, mRNA levels drop with proteasome inhibition without obvious increases in intron-bearing pre-mRNA, indicating that splicing is not generally impaired when proteome activity is blocked. Chimeric gene constructs, nuclear run-on experiments, and transcriptional inhibition studies show that for members of three functional groups (i.e., ribosomal protein genes, histone genes, and heat shock protein genes) changes in mRNA levels occur largely by transcriptional modulation. In addition, these studies reveal a possible new means of modulating kinetochore levels through CEP3 expression. Together these data strongly support the view that proteasome activity plays a significant role in the regulation of eukaryotic gene expression.

Genome-wide analysis of signal transducers and regulators of mitochondrial dysfunction in Saccharomyces cerevisiae

Mitochondrial dysfunction is a hallmark of cancer cells. However, genetic response to mitochondrial dysfunction during carcinogenesis is unknown. To elucidate genetic response to mitochondrial dysfunction we used Saccharomyces cerevisiae as a model system. We analyzed genome-wide expression of nuclear genes involved in signal transduction and transcriptional regulation in a wild-type yeast and a yeast strain lacking the mitochondrial genome (rho(0)). Our analysis revealed that the gene encoding cAMP-dependent protein kinase subunit 3 (PKA3) was upregulated. However, the gene encoding cAMP-dependent protein kinase subunit 2 (PKA2) and the VTC1, PTK2, TFS1, CMK1, and CMK2 genes, involved in signal transduction, were downregulated. Among the known transcriptional factors, OPI1, MIG2, INO2, and ROX1 belonged to the upregulated genes, whereas MSN4, MBR1, ZMS1, ZAP1, TFC3, GAT1, ADR1, CAT8, and YAP4 including RFA1 were downregulated. RFA1 regulates DNA repair genes at the transcriptional level. RFA is also involved directly in DNA recombination, DNA replication, and DNA base excision repair. Downregulation of RFA1 in rho(0) cells is consistent with our finding that mitochondrial dysfunction leads to instability of the nuclear genome. Together, our data suggest that gene(s) involved in mitochondria-to-nucleus communication play a role in mutagenesis and may be implicated in carcinogenesis.

Genome-wide transcriptional changes during the lag phase of Saccharomyces cerevisiae

The set of physiological and metabolic changes occurring immediately after inoculation and during the lag phase is thought to be of vital importance for optimal offset of fermentation. The transcriptional changes taking place during the lag phase after inoculation of a late-respiratory-phase yeast culture into fresh, minimal medium were investigated by use of Yeast GeneFilters. In response to the nutritional up-shift, 240 open reading frames were at least five-fold induced and 122 were at least five-fold repressed. These genes were hierarchically clustered according to their lag-phase expression patterns. The majority of the induced genes were most highly induced early in the lag phase, whereas strong repression generally occurred later. Clustering of the genes showed that many genes with similar roles had similar expression patterns. Repressed genes, however, did not cluster as tightly according to function as induced genes. Genes involved in RNA and protein synthesis and processing showed a peak in expression early in the lag phase, except most ribosomal protein genes, which were induced early and whose expression was sustained. Genes involved in chromatin/chromosome structure showed late induction. The correlation between function and expression pattern for these genes indicates regulation by similar mechanisms. Much of the transcriptional response observed appeared to be due to the presence of glucose in the new medium.

The Ran GTPase cycle is required for yeast nuclear pore complex assembly

Here, we report the first evidence that the Ran GTPase cycle is required for nuclear pore complex (NPC) assembly. Using a genetic approach, factors required for NPC assembly were identified in Saccharomyces cerevisiae. Four mutant complementation groups were characterized that correspond to respective mutations in genes encoding Ran (gsp1), and essential Ran regulatory factors Ran GTPase-activating protein (rna1), Ran guanine nucleotide exchange factor (prp20), and the RanGDP import factor (ntf2). All the mutants showed temperature-dependent mislocalization of green fluorescence protein (GFP)-tagged nucleoporins (nups) and the pore-membrane protein Pom152. A decrease in GFP fluorescence associated with the nuclear envelope was observed along with an increase in the diffuse, cytoplasmic signal with GFP foci. The defects did not affect the stability of existing NPCs, and nup mislocalization was dependent on de novo protein synthesis and continued cell growth. Electron microscopy analysis revealed striking membrane perturbations and the accumulation of vesicles in arrested mutants. Using both biochemical fractionation and immunoelectron microscopy methods, these vesicles were shown to contain nups. We propose a model wherein a Ran-mediated vesicular fusion step is required for NPC assembly into intact nuclear envelopes.

Expression of testicular germ cell genes identified by differential display analysis

Using differential display reverse transcriptase-polymerase chain reaction (DDRT-PCR) we identified transcripts encoding for the RNA helicase mDEAH9, Ran binding protein 5 (RanBP5), and 3 novel complementary DNAs designated GC3, GC12, and GC14 in developing testicular germ cells. Sources of RNA for the initial DDRT-PCR screen were purified mouse type A spermatogonia, adult mouse wild-type testis, and W/W(v) mutant mouse testis. We identified cDNA fragments for mDEAH9, RanBP5, GC3, GC12, and GC14 in testis and type A spermatogonia samples from wild-type mice, but not in samples from the W/W(v) mouse testis. These same transcripts were absent in Northern blots of testis RNA from mice treated with busulfan 30 days prior, but were present in testis RNA from wild-type mice at 5, 15, 25, and 40 days of age. The mDEAH9 gene was expressed in many tissues, whereas RanBP5 and GC12 genes were expressed predominantly in the testis with much lower expression in other tissues. The expression of GC3 and GC14 were limited to the testis as evidenced by Northern blot and RT-PCR analyses. The mDEAH9 transcript was not detected in cultured interstitial cells but was found at low levels in cultured immature Sertoli cells, whereas the RanBP5, GC3, GC12, and GC14 transcripts were not detected in either cultured testicular interstitial cells or cultured Sertoli cells. RT-PCR analyses of isolated spermatogonia, pachytene spermatocytes, and round spermatids revealed that mDEAH9, RanBP5, GC3, GC12, and GC14 genes were expressed in all 3 cellular populations. In situ hybridization analyses of testis samples from 40-day-old mice localized expression of mDEAH9, RanBP5, GC3, GC12, and GC14 to the seminiferous tubules. RanBP5 expression appeared to be regulated during the cycle of the seminiferous epithelium, with the highest expression in stages III through VII. Expression of GC14 was greatest in the meiotic germ cell populations.

Regulation of nuclear import and export by the GTPase Ran

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

Chromatin boundaries in budding yeast: the nuclear pore connection

Chromatin boundary activities (BAs) were identified in Saccharomyces cerevisiae by genetic screening. Such BAs bound to sites flanking a reporter gene establish a nonsilenced domain within the silent mating-type locus HML. Interestingly, various proteins involved in nuclear-cytoplasmic traffic, such as exportins Cse1p, Mex67p, and Los1p, exhibit a robust BA. Genetic studies, immunolocalization, live imaging, and chromatin immunoprecipitation experiments show that these transport proteins block spreading of heterochromatin by physical tethering of the HML locus to the Nup2p receptor of the nuclear pore complex. Genetic deletion of NUP2 abolishes the BA of all transport proteins, while direct targeting of Nup2p to the bracketing DNA elements restores activity. The data demonstrate that physical tethering of genomic loci to the NPC can dramatically alter their epigenetic activity.

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.

Targeting of Ran: variation on a common theme?

The Ran GTPase plays a key role in nucleocytoplasmic transport. In its GTP-bound form, it directly interacts with members of the importin β family of nuclear transport receptors and modulates their association with cargo. Work in cell-free higher-eukaryote systems has demonstrated additional roles for Ran in spindle and nuclear envelope formation during mitosis. However, until recently, no Ran-target proteins in these cellular processes were known. Several groups have now identified importin β as one important target of Ran during mitotic spindle formation. This finding suggests that Ran uses the same effectors to regulate different cellular processes.

Nuclear localization of the Saccharomyces cerevisiae HMG protein NHP6A occurs by a Ran-independent nonclassical pathway

The Saccharomyces cerevisiae non-histone protein 6-A (NHP6A) is a member of the high-mobility group 1/2 protein family that bind and bend DNA of mixed sequence. NHP6A has only one high-mobility group 1/2 DNA binding domain and also requires a 16-amino-acid basic tail at its N-terminus for DNA binding. We show in this report that nuclear accumulation of NHP6A is strictly correlated with its DNA binding properties since only nonhistone protein 6 A-green fluorescent protein chimeras that were competent for DNA binding were localized to the nucleus. Despite the requirement for basic residues within the N-terminal segment for DNA binding and nuclear accumulation, this region does not appear to contain a nuclear localization signal. Moreover, NHP6A does not bind to the yeast nuclear localization signal receptor SRP1 and nuclear targeting of NHP6A does not require the function of the 14 different importins. Unlike histone H2B1 which contains a classical nuclear localization signal, entry of NHP6A into the nucleus was found to be independent of Ran as judged by coexpression of Ran GTPase mutants and was shown to occur at 0 degrees C after a 15-min induction. These unusual properties lead us to suggest that NHP6A entry into the nucleus proceeds by a nonclassical Ran-independent pathway.

Mutations in the YRB1 gene encoding yeast ran-binding-protein-1 that impair nucleocytoplasmic transport and suppress yeast mating defects

We identified two temperature-sensitive (ts) mutations in the essential gene, YRB1, which encodes the yeast homolog of Ran-binding-protein-1 (RanBP1), a known coregulator of the Ran GTPase cycle. Both mutations result in single amino acid substitutions of evolutionarily conserved residues (A91D and R127K, respectively) in the Ran-binding domain of Yrb1. The altered proteins have reduced affinity for Ran (Gsp1) in vivo. After shift to restrictive temperature, both mutants display impaired nuclear protein import and one also reduces poly(A)+ RNA export, suggesting a primary defect in nucleocytoplasmic trafficking. Consistent with this conclusion, both yrb1ts mutations display deleterious genetic interactions with mutations in many other genes involved in nucleocytoplasmic transport, including SRP1 (alpha-importin) and several beta-importin family members. These yrb1ts alleles were isolated by their ability to suppress two different types of mating-defective mutants (respectively, fus1Delta and ste5ts), indicating that reduction in nucleocytoplasmic transport enhances mating proficiency. Indeed, in both yrb1ts mutants, Ste5 (scaffold protein for the pheromone response MAPK cascade) is mislocalized to the cytosol, even in the absence of pheromone. Also, both yrb1ts mutations suppress the mating defect of a null mutation in MSN5, which encodes the receptor for pheromone-stimulated nuclear export of Ste5. Our results suggest that reimport of Ste5 into the nucleus is important in downregulating mating response.

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.

Yrb1p interaction with the gsp1p C terminus blocks Mog1p stimulation of GTP release from Gsp1p

Mog1p, a multicopy suppressor of gsp1, the temperature-sensitive mutant of the Saccharomyces cerevisiae Ran homologue, binds to GTP-Gsp1p but not to GDP-Gsp1p. The function of Mog1p in the Ran cycle is as yet unknown. This study found that Mog1p releases a nucleotide from GTP-Gsp1p but not from GDP-Gsp1p. Yrb1p, the S. cerevisiae homologue of RanBP1, which is a strong inhibitor of RCC1-stimulated nucleotide release, also inhibited the Mog1p-stimulated nucleotide release from GTP-Gsp1p. At a concentration corresponding to the molar concentration of GTP-Gsp1p, Yrb1p completely inhibited the Mog1p-stimulated nucleotide release. Consistently, the Yrb1p.GTP-Gsp1p complex was more stable than the Mog1p.GTP-Gsp1p complex. Yrb1p did not inhibit the Mog1p-stimulated nucleotide release from GTP-Gsp1DeltaC. The Gsp1DeltaC protein lacks the final eight amino acids of the C terminus, and for this reason, the interaction between GTP-Gsp1DeltaC and Yrb1p was strongly reduced. On the other hand, Mog1p binds to GTP-Gsp1DeltaC more efficiently than to GTP-Gsp1p.

Upregulation of a raf kinase and a DP-1 family transcription factor in epidermal growth factor (EGF) stimulated filarial parasites

Previously we have shown that in the filarial parasite Brugia malayi, stimulation with murine epidermal growth factor (EGF) upregulated the expression of the nuclear GTPase, Ran. In this paper we provide further evidence that filarial parasites possess the ability to respond to mammalian EGF. Stimulation of B. malayi microfilariae with EGF increased transcription of a Raf kinase, increased the physical interaction between Ran and at least eight unidentified proteins, abolished the association of a putative EGF receptor with the nuclear GTPase Ran and enhanced phosphorylation of native microfilarial proteins. In the cattle filarial parasite Setaria digitata, stimulation of adult worms with EGF was probably responsible for up-regulation of a DP-1 family transcription factor. These data suggest that filarial parasites possess the ability to respond to mammalian EGF and that mammalian growth factors may regulate developmental maturation of filarial parasites.

Premature chromatin condensation caused by loss of RCC1

Hamster rcc1 mutant, tsBN2, prematurely enter mitosis during S phase. RCC1 is a guanine nucleotide exchanging factor for a small G protein Ran and localised on the chromatin, whereas RanGTPase activating protein is in the cytoplasm. Consistently, Ran shuttles between the nucleus and the cytoplasm, carrying out nucleus-cytosol exchange of macromolecules, which regulates the cell cycle. The finding that loss of RCC1 which disturbs nuclear protein export due to loss of RanGTP, abrogates the check point control suggests that RCC1 senses the status of the chromatin, such as replication, and couples it to the cell cycle progression through Ran.

Characterization of CaGSP1, the Candida albicans RAN/GSP1 homologue

Gsp1p is a small nuclear-located GTP binding protein from the yeast Saccharomyces cerevisiae. It is highly conserved among eucaryotic cells and is involved in numerous cellular processes, including nucleocytoplasmic trafficking of macromolecules. To learn more about the GSP1 structure/function, we have characterized its Candida albicans homologue. CaGsp1p is 214 amino acids long and displays 91% identity to the ScGsp1p. There is functional complementation in S. cerevisiae, and its mRNA is constitutively expressed in the diploid C. albicans grown under various physiological conditions. Disruption of both alleles was not possible, suggesting that it could be an essential gene, but heterozygous mutants exhibited genomic instability.

Ribosome synthesis in Saccharomyces cerevisiae

The synthesis of ribosomes is one of the major metabolic pathways in all cells. In addition to around 75 individual ribosomal proteins and 4 ribosomal RNAs, synthesis of a functional eukaryotic ribosome requires a remarkable number of trans-acting factors. Here, we will discuss the recent, and often surprising, advances in our understanding of ribosome synthesis in the yeast Saccharomyces cerevisiae. These will underscore the unexpected complexity of eukaryotic ribosome synthesis.

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.

Transport between the cell nucleus and the cytoplasm

The compartmentation of eukaryotic cells requires all nuclear proteins to be imported from the cytoplasm, whereas, for example, transfer RNAs, messenger RNAs, and ribosomes are made in the nucleus and need to be exported to the cytoplasm. Nuclear import and export proceed through nuclear pore complexes and can occur along a great number of distinct pathways, many of which are mediated by importin beta-related nuclear transport receptors. These receptors shuttle between nucleus and cytoplasm, and they bind transport substrates either directly or via adapter molecules. They all cooperate with the RanGTPase system to regulate the interactions with their cargoes. Another focus of our review is nuclear export of messenger RNA, which apparently largely relies on export mediators distinct from importin beta-related factors. We discuss mechanistic aspects and the energetics of transport receptor function and describe a number of pathways in detail.

Saccharomyces cerevisiae putative G protein, Gtr1p, which forms complexes with itself and a novel protein designated as Gtr2p, negatively regulates the Ran/Gsp1p G protein cycle through Gtr2p

Prp20p and Rna1p are GDP/GTP exchanging and GTPase-activating factors of Gsp1p, respectively, and their mutations, prp20-1 and rna1-1, can both be suppressed by Saccharomyces cerevisiae gtr1-11. We found that gtr1-11 caused a single amino acid substitution in Gtr1p, forming S20L, which is a putative GDP-bound mutant protein, while Gtr1p has been reported to bind to GTP alone. Consistently, gtr1-S20N, another putative GDP-bound mutant, suppressed both prp20-1 and rna1-1. On the other hand, gtr1-Q65L, a putative GTP-bound mutant, was inhibitory to prp20-1 and rna1-1. Thus, the role that Gtr1p plays in vivo appears to depend upon the nucleotide bound to it. Our data suggested that the GTP-bound Gtr1p, but not the GDP-bound Gtr1p, interacts with itself through its C-terminal tail. S. cerevisiae possesses a novel gene, GTR2, which is homologous to GTR1. Gtr2p interacts with itself in the presence of Gtr1p. The disruption of GTR2 suppressed prp20-1 and abolished the inhibitory effect of gtr1-Q65L on prp20-1. This finding, taken together with the fact that Gtr1p-S20L is a putative, inactive GDP-bound mutant, implies that Gtr1p negatively regulates the Ran/Gsp1p GTPase cycle through Gtr2p.

Antagonistic effects of NES and NLS motifs determine S. cerevisiae Rna1p subcellular distribution

Nucleus/cytosol exchange requires a GTPase, Ran. In yeast Rna1p is the GTPase activating protein for Ran (RanGAP) and Prp20p is the Ran GDP/GTP exchange factor (GEF). RanGAP is primarily cytosolic and GEF is nuclear. Their subcellular distributions led to the prediction that Ran-GTP hydrolysis takes place solely in the cytosol and GDP/GTP exchange solely in the nucleus. Current models propose that the Ran-GTP/Ran-GDP gradient across the nuclear membrane determines the direction of exchange. We provide three lines of evidence that Rna1p enters and leaves the nuclear interior. (1) Rna1p possesses leucine-rich nuclear export sequences (NES) that are able to relocate a passenger karyophilic protein to the cytosol; alterations of consensus residues re-establish nuclear location. (2) Rna1p possesses other sequences that function as a novel nuclear localization sequence able to deliver a passenger cytosolic protein to the nucleus. (3) Endogenous Rna1p location is dependent upon Xpo1p/Crm1p, the yeast exportin for leucine-rich NES-containing proteins. The data support the hypothesis that Rna1p exists on both sides of the nuclear membrane, perhaps regulating the Ran-GTP/Ran-GDP gradient, participating in a complete RanGTPase nuclear cycle or serving a novel function.

Cse1p is required for export of Srp1p/importin-alpha from the nucleus in Saccharomyces cerevisiae

In metazoan cells, the CAS protein has been shown to function as a recycling factor for the importin-alpha subunit of the classical nuclear localization signal receptor, exporting importin-alpha from the nucleus to allow its participation in multiple rounds of nuclear import. CAS is a member of a family of proteins that bear homology to the larger subunit of the nuclear localization signal receptor, importin-beta, and that are found in all eukaryotes from yeast to humans. Sequence similarity identifies the product of the Saccharomyces cerevisiae CSE1 gene as a potential CAS homologue. Here we present evidence that Cse1p is the functional homologue of CAS: Cse1p is required to prevent accumulation of Srp1p/importin-alpha in the nucleus, it localizes to the nuclear envelope in a pattern typical of nuclear transport receptors, and it associates in vivo with Srp1p in a nucleotide-specific manner. We show further that mutations in CSE1 and SRP1 have specific effects on their association and on the intracellular localization of Cse1p.

Distribution and functional diversification of the ras superfamily in Saccharomyces cerevisiae

The recent availability of the full Saccharomyces cerevisiae genome sequence offers a first opportunity to analyze the composition, function and evolution of GTPases in the ras-p21 superfamily. This superfamily in yeast is composed of 29 proteins divided into five families: ras with four sequences implicated in cell signalling; rho, six genes related to the cell shape machinery; ypt-rab, ten proteins with different roles in intracellular trafficking; arf-sar, seven proteins related to vesicular trafficking in secretory pathways; and ran, two proteins acting as components of the nuclear transport system. The superfamily covers a wide range of cellular functions from signalling to intracellular trafficking, while conserving the structural framework and a common mechanism of GTP hydrolysis.

Cse1p functions as the nuclear export receptor for importin alpha in yeast

CSE1 is essential for yeast cell viability and has been implicated in chromosome segregation. Based on its sequence similarity, Cse1p has been grouped into the family of importin beta-like nucleocytoplasmic transport receptors with highest homology to the recently identified human nuclear export receptor for importin alpha, CAS. We demonstrate here that Cse1p physically interacts with yeast Ran and yeast importin alpha (Srp1p) in the yeast two-hybrid system and that recombinant Cse1p, Srp1p and Ran-GTP form a trimeric complex in vitro. Re-export of Srp1p from the nucleus into the cytoplasm and nuclear uptake of a reporter protein containing a classical NLS are inhibited in a cse1 mutant strain. These findings suggest that Cse1p is the exportin of importin alpha in yeast.

Overexpression of Candida albicans secretory aspartyl proteinase 2 and its expression in Saccharomyces cerevisiae do not augment virulence in mice

To elucidate the implications of secreted aspartyl proteinase (Sap)2p in the pathogenesis of Candida infections, the SAP2 gene was expressed in Saccharomyces cerevisiae and overexpressed in Candida albicans. The coding region of SAP2, including its signal sequence and propeptide, was amplified by PCR and cloned downstream of the S. cerevisiae or C. albicans ADH1 promoter. Plasmid expression of SAP2 in S. cerevisiae showed that the signal peptide was functional. Integrative transformation of S. cerevisiae and C. albicans was accomplished by homologous recombination within the URA3 locus for S. cerevisiae and the SAP2 locus for C. albicans. Negative control transformants carried plasmids either without the SAP2 insert or with mutated sap2. S. cerevisiae and C. albicans transformants showed similar growth rates to their parental strains or negative controls, when grown in medium containing amino acids. However, in medium with BSA as sole nitrogen source, constitutive expression of SAP2 enabled S. cerevisiae to grow and increased the growth rate of C. albicans. In both media, only S. cerevisiae transformants harbouring SAP2 secreted the enzyme, as confirmed by proteinase activity assays and immunoblotting. When C. albicans was grown in amino acids medium, the enzyme was detected exclusively in transformants constitutively expressing SAP2. However, in BSA medium these strains secreted enzyme earlier and secreted higher amounts of enzyme and total proteinase activity. In pathogenicity studies in intact mice, expression of Sap2p as a sole putative virulence factor did not cause S. cerevisiae to become virulent and constitutive overexpression of SAP2 did not augment virulence of C. albicans in experimental oral or systemic infection.

Isolation and characterization of the Candida albicans SEC4 gene

The SEC4 gene product is a major component of the protein secretion machinery. More specifically, it is believed to play a pivotal role in targeting and fusion of secretory vesicles to the plasma membrane. Its recently described implication with the Saccharomyces cerevisiae Rho3p, which is required for directing growing points during bud formation, has prompted us to investigate the role and function of Sec4p in the morphological changes of the yeast pathogen Candida albicans. We have therefore cloned the C. albicans SEC4 gene. It encodes a 210 amino acids long protein sharing up to 75% homology to the S. cerevisiae homolog, when conserved changes are allowed. Its RNA is constitutively expressed in C. albicans grown under various physiological conditions. We also show that it can functionally complement a S. cerevisiae sec4 thermosensitive mutant.

The role of the Ran GTPase pathway in cell cycle control and interphase nuclear functions

Ran is a small, highly abundant, nuclear GTPase. Mutants in Ran and in proteins that interact with it disrupt the normal checkpoint control of mitosis with respect to the completion of DNA synthesis. Ran and other components of this pathway are also required for numerous nuclear functions such as RNA export, protein import, RNA processing and DNA replication. It will be important to understand how these facets of Ran's activities are linked and how they promote correct control of the cell cycle. This review examines recent progress in discovering other components of the Ran GTPase pathway and considers how this pathway may be required for the control of the cell cycle.

Yeast genetics to dissect the nuclear pore complex and nucleocytoplasmic trafficking

Eukaryotic cells evolved when their genetic information was packed into the cell nucleus. DNA replication and RNA biogenesis occur inside the nucleus while protein synthesis takes place in the cytoplasm. Bi-directional trafficking between these two compartments is mediated by a single supramolecular assembly, the nuclear pore complex. Nucleocytoplasmic transport is signal mediated, energy dependent, and requires, besides nuclear pore proteins (nucleoporins), a number of soluble transport factors. We review here our current knowledge on the role of nucleoporins, and on the mechanism of nucleocytoplasmic transport, with emphasis on the yeast Saccharomyces cerevisiae.