Roles of multiple glucose transporters in Saccharomyces cerevisiae

Characterization of hyperactive mutations in the renal potassium channel ROMK uncovers unique effects on channel biogenesis and ion conductance

Hypertension affects one billion people worldwide and is the most common risk factor for cardiovascular disease, yet a comprehensive picture of its underlying genetic factors is incomplete. Amongst regulators of blood pressure is the renal outer medullary potassium (ROMK) channel. While select ROMK mutants are prone to premature degradation and lead to disease, heterozygous carriers of some of these same alleles are protected from hypertension. Therefore, we hypothesized that gain-of-function (GoF) ROMK variants which increase potassium flux may predispose people to hypertension. To begin to test this hypothesis, we employed genetic screens and a candidate-based approach to identify six GoF variants in yeast. Subsequent functional assays in higher cells revealed two variant classes. The first group exhibited greater stability in the endoplasmic reticulum, enhanced channel assembly, and/or increased protein at the cell surface. The second group of variants resided in the PIP2-binding pocket, and computational modeling coupled with patch-clamp studies demonstrated lower free energy for channel opening and slowed current rundown, consistent with an acquired PIP2-activated state. Together, these findings advance our understanding of ROMK structure-function, suggest the existence of hyperactive ROMK alleles in humans, and establish a system to facilitate the development of ROMK-targeted antihypertensives.

Genome mining yields new disease-associated ROMK variants with distinct defects

Bartter syndrome is a group of rare genetic disorders that compromise kidney function by impairing electrolyte reabsorption. Left untreated, the resulting hyponatremia, hypokalemia, and dehydration can be fatal. Although there is no cure for this disease, specific genes that lead to different Bartter syndrome subtypes have been identified. Bartter syndrome type II specifically arises from mutations in the KCNJ1 gene, which encodes the renal outer medullary potassium channel, ROMK. To date, over 40 Bartter syndrome-associated mutations in KCNJ1 have been identified. Yet, their molecular defects are mostly uncharacterized. Nevertheless, a subset of disease-linked mutations compromise ROMK folding in the endoplasmic reticulum (ER), which in turn results in premature degradation via the ER associated degradation (ERAD) pathway. To identify uncharacterized human variants that might similarly lead to premature degradation and thus disease, we mined three genomic databases. First, phenotypic data in the UK Biobank were analyzed using a recently developed computational platform to identify individuals carrying KCNJ1 variants with clinical features consistent with Bartter syndrome type II. In parallel, we examined ROMK genomic data in both the NIH TOPMed and ClinVar databases with the aid of a computational algorithm that predicts protein misfolding and disease severity. Subsequent phenotypic studies using a high throughput yeast screen to assess ROMK function-and analyses of ROMK biogenesis in yeast and human cells-identified four previously uncharacterized mutations. Among these, one mutation uncovered from the two parallel approaches (G228E) destabilized ROMK and targeted it for ERAD, resulting in reduced protein expression at the cell surface. Another ERAD-targeted ROMK mutant (L320P) was found in only one of the screens. In contrast, another mutation (T300R) was ERAD-resistant, but defects in ROMK activity were apparent after expression and two-electrode voltage clamp measurements in Xenopus oocytes. Together, our results outline a new computational and experimental pipeline that can be applied to identify disease-associated alleles linked to a range of other potassium channels, and further our understanding of the ROMK structure-function relationship that may aid future therapeutic strategies.

Author Summary

Bartter syndrome is a rare genetic disorder characterized by defective renal electrolyte handing, leading to debilitating symptoms and, in some patients, death in infancy. Currently, there is no cure for this disease. Bartter syndrome is divided into five types based on the causative gene. Bartter syndrome type II results from genetic variants in the gene encoding the ROMK protein, which is expressed in the kidney and assists in regulating sodium, potassium, and water homeostasis. Prior work established that some disease-associated ROMK mutants misfold and are destroyed soon after their synthesis in the endoplasmic reticulum (ER). Because a growing number of drugs have been identified that correct defective protein folding, we wished to identify an expanded cohort of similarly misshapen and unstable disease-associated ROMK variants. To this end, we developed a pipeline that employs computational analyses of human genome databases with genetic and biochemical assays. Next, we both confirmed the identity of known variants and uncovered previously uncharacterized ROMK variants associated with Bartter syndrome type II. Further analyses indicated that select mutants are targeted for ER-associated degradation, while another mutant compromises ROMK function. This work sets-the-stage for continued mining for ROMK loss of function alleles as well as other potassium channels, and positions select Bartter syndrome mutations for correction using emerging pharmaceuticals.

How Carbon Source and Degree of Oligosaccharide Polymerization Affect Production of Cellulase-Degrading Enzymes by Fusarium oxysporum f. sp. lycopersici

Cellulases are a group of enzymes responsible for the degradation of cellulose, which is one of the most abundant polymers on Earth. The three main groups of cellulases are endoglucosidases, exoglucosidases, and β-glucosidases; however, the mechanism of induction of these enzymes remains poorly characterized. Cellooligosaccharides are among the main inducers of these enzymes in filamentous fungi, yet it is not clear how their degree of polymerization may affect the strength of induction. In the present study, we investigated the effect of different carbohydrate-based inducers, such as lactose, sophorose, cellooligosaccharides, and xylooligosacharides, characterized by different concentrations and degree of polymerization, on cellulases production by the fungus Fusarium oxysporum f. sp. lycopersici, which is one of the most studied lignocellulose degrading fungi with the ability to consume both cellulose and hemicellulose. Moreover, the effect of carbon source on cellulase induction was assessed by growing the biomass on sucrose or glycerol. Results showed a correlation between induction efficiency and the cellooligosaccharides' concentration and size, as well as the carbon source available. Specifically, cellotetraose was a better inducer when sucrose was the carbon source, while cellobiose yielded a better result on glycerol. These findings can help optimize industrial cellulase production.

Regulation and metabolic engineering strategies for permeases of Saccharomyces cerevisiae

Microorganisms have evolved permeases to incorporate various essential nutrients and exclude harmful products, which assists in adaptation to different environmental conditions for survival. As permeases are directly involved in the utilization of and regulatory response to nutrient sources, metabolic engineering of microbial permeases can predictably influence nutrient metabolism and regulation. In this mini-review, we have summarized the mechanisms underlying the general regulation of permeases, and the current advancements and future prospects of metabolic engineering strategies targeting the permeases in Saccharomyces cerevisiae. The different types of permeases and their regulatory mechanisms have been discussed. Furthermore, methods for metabolic engineering of permeases have been highlighted. Understanding the mechanisms via which permeases are meticulously regulated and engineered will not only facilitate research on regulation of global nutrition and yeast metabolic engineering, but can also provide important insights for future studies on the synthesis of valuable products and elimination of harmful substances in S. cerevisiae.

Multilevel regulation of an α-arrestin by glucose depletion controls hexose transporter endocytosis

Changes in nutrient availability trigger massive rearrangements of the yeast plasma membrane proteome. This work shows that the arrestin-related protein Csr2/Art8 is regulated by glucose signaling at multiple levels, allowing control of hexose transporter ubiquitylation and endocytosis upon glucose depletion.

Nutrient availability controls the landscape of nutrient transporters present at the plasma membrane, notably by regulating their ubiquitylation and subsequent endocytosis. In yeast, this involves the Nedd4 ubiquitin ligase Rsp5 and arrestin-related trafficking adaptors (ARTs). ARTs are targeted by signaling pathways and warrant that cargo ubiquitylation and endocytosis appropriately respond to nutritional inputs. Here, we show that glucose deprivation regulates the ART protein Csr2/Art8 at multiple levels to trigger high-affinity glucose transporter endocytosis. Csr2 is transcriptionally induced in these conditions through the AMPK orthologue Snf1 and downstream transcriptional repressors. Upon synthesis, Csr2 becomes activated by ubiquitylation. In contrast, glucose replenishment induces CSR2 transcriptional shutdown and switches Csr2 to an inactive, deubiquitylated form. This glucose-induced deubiquitylation of Csr2 correlates with its phospho-dependent association with 14-3-3 proteins and involves protein kinase A. Thus, two glucose signaling pathways converge onto Csr2 to regulate hexose transporter endocytosis by glucose availability. These data illustrate novel mechanisms by which nutrients modulate ART activity and endocytosis.

Quantitative mass spectrometry-based multiplexing compares the abundance of 5000 S. cerevisiae proteins across 10 carbon sources

The budding yeast Saccharomyces cerevisiae is a model system for investigating biological processes. Cellular events are known to be dysregulated due to shifts in carbon sources. However, the comprehensive proteomic alterations thereof have not been fully investigated. Here we examined proteomic alterations in S. cerevisiae due to the adaptation of yeast from glucose to nine different carbon sources - maltose, trehalose, fructose, sucrose, glycerol, acetate, pyruvate, lactic acid, and oleate. Isobaric tag-based mass spectrometry techniques are at the forefront of global proteomic investigations. As such, we used a TMT10-plex strategy to study multiple growth conditions in a single experiment. The SPS-MS3 method on an Orbitrap Fusion Lumos mass spectrometer enabled the quantification of over 5000 yeast proteins across ten carbon sources at a 1% protein-level FDR. On average, the proteomes of yeast cultured in fructose and sucrose deviated the least from those cultured in glucose. As expected, gene ontology classification revealed the major alteration in protein abundances occurred in metabolic pathways and mitochondrial proteins. Our protocol lays the groundwork for further investigation of carbon source-induced protein alterations. Additionally, these data offer a hypothesis-generating resource for future studies aiming to investigate both characterized and uncharacterized genes.

BIOLOGICAL SIGNIFICANCE

We investigate the proteomic alterations in S. cerevisiae resulting from adaptation of yeast from glucose to nine different carbon sources - maltose, trehalose, fructose, sucrose, glycerol, acetate, pyruvate, lactic acid, and oleate. SPS-MS3 TMT10plex analysis is used for quantitative proteomic analysis. We showcase a technique that allows the quantification of over 5000 yeast proteins, the highest number to date in S. cerevisiae, across 10 growth conditions in a single experiment. As expected, gene ontology classification of proteins with the major alterations in abundances occurred in metabolic pathways and mitochondrial proteins, reflecting the degree of metabolic stress when cellular machinery shifts from growth on glucose to an alternative carbon source. Our protocol lays the groundwork for further investigation of carbon source-induced protein alterations. Improving depth of coverage - measuring abundance changes of over 5000 proteins - increases our understanding of difficult-to-study genes in the model system S. cerevisiae and by homology human cell biology. We submit this highly comprehensive dataset as a hypothesis generating resource for targeted studies on uncharacterized genes.

Interactions Between Monovalent Cations and Nutrient Homeostasis

Maintenance of appropriate fluxes of monovalent cation is a requirement for growth and survival. In the budding yeast Saccharomyces cerevisiae an electrochemical gradient of H(+) is fundamental for the uptake of diverse cations, such as K(+), and of many other nutrients. In spite of early work suggesting that alterations in monovalent cation fluxes impact on the uptake and utilization of nutrients, such as phosphate anions, only recently this important aspect of the yeast physiology has been addressed and characterized in some detail. This chapter provides a historical background and summarizes the latest findings.

Proteome-wide quantitative multiplexed profiling of protein expression: carbon-source dependency in Saccharomyces cerevisiae

A mass spectrometry–based tandem mass tag 9-plex strategy was used to determine alterations in relative protein abundance due to three carbon sources—glucose, galactose, and raffinose. More than 4700 proteins were quantified across all nine samples; 1003 demonstrated statistically significant differences in abundance in at least one condition.

The global proteomic alterations in the budding yeast Saccharomyces cerevisiae due to differences in carbon sources can be comprehensively examined using mass spectrometry–based multiplexing strategies. In this study, we investigate changes in the S. cerevisiae proteome resulting from cultures grown in minimal media using galactose, glucose, or raffinose as the carbon source. We used a tandem mass tag 9-plex strategy to determine alterations in relative protein abundance due to a particular carbon source, in triplicate, thereby permitting subsequent statistical analyses. We quantified more than 4700 proteins across all nine samples; 1003 proteins demonstrated statistically significant differences in abundance in at least one condition. The majority of altered proteins were classified as functioning in metabolic processes and as having cellular origins of plasma membrane and mitochondria. In contrast, proteins remaining relatively unchanged in abundance included those having nucleic acid–related processes, such as transcription and RNA processing. In addition, the comprehensiveness of the data set enabled the analysis of subsets of functionally related proteins, such as phosphatases, kinases, and transcription factors. As a resource, these data can be mined further in efforts to understand better the roles of carbon source fermentation in yeast metabolic pathways and the alterations observed therein, potentially for industrial applications, such as biofuel feedstock production.

Hxt1, a monosaccharide transporter and sensor required for virulence of the maize pathogen Ustilago maydis

The smut Ustilago maydis, a ubiquitous pest of corn, is highly adapted to its host to parasitize on its organic carbon sources. We have identified a hexose transporter, Hxt1, as important for fungal development during both the saprophytic and the pathogenic stage of the fungus. Hxt1 was characterized as a high-affinity transporter for glucose, fructose, and mannose; ∆hxt1 strains show significantly reduced growth on these substrates, setting Hxt1 as the main hexose transporter during saprophytic growth. After plant infection, ∆hxt1 strains show decreased symptom development. However, expression of a Hxt1 protein with a mutation leading to constitutively active signaling in the yeast glucose sensors Snf3p and Rgt2p results in completely apathogenic strains. Fungal development is stalled immediately after plant penetration, implying a dual function of Hxt1 as transporter and sensor. As glucose sensors are only known for yeasts, 'transceptor' as Hxt1 may constitute a general mechanism for sensing of glucose in fungi. In U. maydis, Hxt1 links a nutrient-dependent environmental signal to the developmental program during pathogenic development.

Allelic variation, aneuploidy, and nongenetic mechanisms suppress a monogenic trait in yeast

Clinically relevant features of monogenic diseases, including severity of symptoms and age of onset, can vary widely in response to environmental differences as well as to the presence of genetic modifiers affecting the trait's penetrance and expressivity. While a better understanding of modifier loci could lead to treatments for Mendelian diseases, the rarity of individuals harboring both a disease-causing allele and a modifying genotype hinders their study in human populations. We examined the genetic architecture of monogenic trait modifiers using a well-characterized yeast model of the human Mendelian disease classic galactosemia. Yeast strains with loss-of-function mutations in the yeast ortholog (GAL7) of the human disease gene (GALT) fail to grow in the presence of even small amounts of galactose due to accumulation of the same toxic intermediates that poison human cells. To isolate and individually genotype large numbers of the very rare (∼0.1%) galactose-tolerant recombinant progeny from a cross between two gal7Δ parents, we developed a new method, called "FACS-QTL." FACS-QTL improves upon the currently used approaches of bulk segregant analysis and extreme QTL mapping by requiring less genome engineering and strain manipulation as well as maintaining individual genotype information. Our results identified multiple distinct solutions by which the monogenic trait could be suppressed, including genetic and nongenetic mechanisms as well as frequent aneuploidy. Taken together, our results imply that the modifiers of monogenic traits are likely to be genetically complex and heterogeneous.

Electrochemical regulation of budding yeast polarity

Cells are naturally surrounded by organized electrical signals in the form of local ion fluxes, membrane potential, and electric fields (EFs) at their surface. Although the contribution of electrochemical elements to cell polarity and migration is beginning to be appreciated, underlying mechanisms are not known. Here we show that an exogenous EF can orient cell polarization in budding yeast (Saccharomyces cerevisiae) cells, directing the growth of mating projections towards sites of hyperpolarized membrane potential, while directing bud emergence in the opposite direction, towards sites of depolarized potential. Using an optogenetic approach, we demonstrate that a local change in membrane potential triggered by light is sufficient to direct cell polarization. Screens for mutants with altered EF responses identify genes involved in transducing electrochemical signals to the polarity machinery. Membrane potential, which is regulated by the potassium transporter Trk1p, is required for polarity orientation during mating and EF response. Membrane potential may regulate membrane charges through negatively charged phosphatidylserines (PSs), which act to position the Cdc42p-based polarity machinery. These studies thus define an electrochemical pathway that directs the orientation of cell polarization.

Identification of glucose transporters in Aspergillus nidulans

To characterize the mechanisms involved in glucose transport, in the filamentous fungus Aspergillus nidulans, we have identified four glucose transporter encoding genes hxtB-E. We evaluated the ability of hxtB-E to functionally complement the Saccharomyces cerevisiae EBY.VW4000 strain that is unable to grow on glucose, fructose, mannose or galactose as single carbon source. In S. cerevisiae HxtB-E were targeted to the plasma membrane. The expression of HxtB, HxtC and HxtE was able to restore growth on glucose, fructose, mannose or galactose, indicating that these transporters accept multiple sugars as a substrate through an energy dependent process. A tenfold excess of unlabeled maltose, galactose, fructose, and mannose were able to inhibit glucose uptake to different levels (50 to 80 %) in these s. cerevisiae complemented strains. Moreover, experiments with cyanide-m-chlorophenylhydrazone (CCCP), strongly suggest that hxtB, -C, and -E mediate glucose transport via active proton symport. The A. nidulans ΔhxtB, ΔhxtC or ΔhxtE null mutants showed ~2.5-fold reduction in the affinity for glucose, while ΔhxtB and -C also showed a 2-fold reduction in the capacity for glucose uptake. The ΔhxtD mutant had a 7.8-fold reduction in affinity, but a 3-fold increase in the capacity for glucose uptake. However, only the ΔhxtB mutant strain showed a detectable decreased rate of glucose consumption at low concentrations and an increased resistance to 2-deoxyglucose.

Mth1 regulates the interaction between the Rgt1 repressor and the Ssn6-Tup1 corepressor complex by modulating PKA-dependent phosphorylation of Rgt1

The yeast glucose transporter gene (HXT) repressor Rgt1 recruits the general corepressor complex Ssn6-Tup1 to bring about repression. The glucose-responsive transcription factor Mth1 is a transcriptional corepressor that mediates the interaction of Rgt1 with Ssn6-Tup1 by blocking the PKA-dependent phosphorylation of Rgt1.

Glucose uptake, the first, rate-limiting step of its utilization, is facilitated by glucose transporters. Expression of several glucose transporter (HXT) genes in yeast is repressed by the Rgt1 repressor, which recruits the glucose-responsive transcription factor Mth1 and the general corepressor complex Ssn6-Tup1 in the absence of glucose; however, it is derepressed when Mth1 is inactivated by glucose. Here we show that Ssn6-Tup1 interferes with the DNA-binding ability of Rgt1 in the absence of Mth1 and that the Rgt1 function abrogated by Ssn6 overexpression is restored by co-overexpression of Mth1. Thus Mth1 likely regulates Rgt1 function not by modulating its DNA-binding activity directly but by functionally antagonizing Ssn6-Tup1. Mth1 does so by acting as a scaffold-like protein to recruit Ssn6-Tup1 to Rgt1. Supporting evidence shows that Mth1 blocks the protein kinase A–dependent phosphorylation of Rgt1 that impairs the ability of Rgt1 to interact with Ssn6-Tup1. Of note, Rgt1 can bind DNA in the absence of Ssn6-Tup1 but does not inhibit transcription, suggesting that dissociation of Rgt1 from Ssn6-Tup1, but not from DNA, is necessary and sufficient for the expression of its target genes. Taken together, these findings show that Mth1 is a transcriptional corepressor that facilitates the recruitment of Ssn6-Tup1 by Rgt1.

Insights from the fungus Fusarium oxysporum point to high affinity glucose transporters as targets for enhancing ethanol production from lignocellulose

Ethanol is the most-widely used biofuel in the world today. Lignocellulosic plant biomass derived from agricultural residue can be converted to ethanol via microbial bioprocessing. Fungi such as Fusarium oxysporum can simultaneously saccharify straw to sugars and ferment sugars to ethanol. But there are many bottlenecks that need to be overcome to increase the efficacy of microbial production of ethanol from straw, not least enhancement of the rate of fermentation of both hexose and pentose sugars. This research tested the hypothesis that the rate of sugar uptake by F. oxysporum would enhance the ethanol yields from lignocellulosic straw and that high affinity glucose transporters can enhance ethanol yields from this substrate. We characterized a novel hexose transporter (Hxt) from this fungus. The F. oxysporum Hxt represents a novel transporter with homology to yeast glucose signaling/transporter proteins Rgt2 and Snf3, but it lacks their C-terminal domain which is necessary for glucose signalling. Its expression level decreased with increasing glucose concentration in the medium and in a glucose uptake study the Km_(glucose) was 0.9 mM, which indicated that the protein is a high affinity glucose transporter. Post-translational gene silencing or over expression of the Hxt in F. oxysporum directly affected the glucose and xylose transport capacity and ethanol yielded by F. oxysporum from straw, glucose and xylose. Thus we conclude that this Hxt has the capacity to transport both C5 and C6 sugars and to enhance ethanol yields from lignocellulosic material. This study has confirmed that high affinity glucose transporters are ideal candidates for improving ethanol yields from lignocellulose because their activity and level of expression is high in low glucose concentrations, which is very common during the process of consolidated processing.

Role of glucose signaling in yeast metabolism

In this article, knowledge concerning the relation between uptake of and signaling by glucose in the yeast Saccharomyces cerevisiae is reviewed and compared to the analogous process in prokaryotes. It is concluded that (much) more fundamental knowledge concerning these processes is required before rational redesign of metabolic fluxes from glucose in yeast can be achieved. (c) 1996 John Wiley & Sons, Inc.

Bioconversion of lignocellulose-derived sugars to ethanol by engineered Saccharomyces cerevisiae

Lignocellulosic biomass from agricultural and agro-industrial residues represents one of the most important renewable resources that can be utilized for the biological production of ethanol. The yeast Saccharomyces cerevisiae is widely used for the commercial production of bioethanol from sucrose or starch-derived glucose. While glucose and other hexose sugars like galactose and mannose can be fermented to ethanol by S. cerevisiae, the major pentose sugars D-xylose and L-arabinose remain unutilized. Nevertheless, D-xylulose, the keto isomer of xylose, can be fermented slowly by the yeast and thus, the incorporation of functional routes for the conversion of xylose and arabinose to xylulose or xylulose-5-phosphate in Saccharomyces cerevisiae can help to improve the ethanol productivity and make the fermentation process more cost-effective. Other crucial bottlenecks in pentose fermentation include low activity of the pentose phosphate pathway enzymes and competitive inhibition of xylose and arabinose transport into the cell cytoplasm by glucose and other hexose sugars. Along with a brief introduction of the pretreatment of lignocellulose and detoxification of the hydrolysate, this review provides an updated overview of (a) the key steps involved in the uptake and metabolism of the hexose sugars: glucose, galactose, and mannose, together with the pentose sugars: xylose and arabinose, (b) various factors that play a major role in the efficient fermentation of pentose sugars along with hexose sugars, and (c) the approaches used to overcome the metabolic constraints in the production of bioethanol from lignocellulose-derived sugars by developing recombinant S. cerevisiae strains.

Early development of Moniliophthora perniciosa basidiomata and developmentally regulated genes

BACKGROUND

The hemibiotrophic fungus Moniliophthora perniciosa is the causal agent of Witches' broom, a disease of Theobroma cacao. The pathogen life cycle ends with the production of basidiocarps in dead tissues of the infected host. This structure generates millions of basidiospores that reinfect young tissues of the same or other plants. A deeper understanding of the mechanisms underlying the sexual phase of this fungus may help develop chemical, biological or genetic strategies to control the disease.

RESULTS

Mycelium was morphologically analyzed prior to emergence of basidiomata by stereomicroscopy, light microscopy and scanning electron microscopy. The morphological changes in the mycelium before fructification show a pattern similar to other members of the order Agaricales. Changes and appearance of hyphae forming a surface layer by fusion were correlated with primordia emergence. The stages of hyphal nodules, aggregation, initial primordium and differentiated primordium were detected. The morphological analysis also allowed conclusions on morphogenetic aspects. To analyze the genes involved in basidiomata development, the expression of some selected EST genes from a non-normalized cDNA library, representative of the fruiting stage of M. perniciosa, was evaluated. A macroarray analysis was performed with 192 selected clones and hybridized with two distinct RNA pools extracted from mycelium in different phases of basidiomata formation. This analysis showed two groups of up and down-regulated genes in primordial phases of mycelia. Hydrophobin coding, glucose transporter, Rho-GEF, Rheb, extensin precursor and cytochrome p450 monooxygenase genes were grouped among the up-regulated. In the down-regulated group relevant genes clustered coding calmodulin, lanosterol 14 alpha demethylase and PIM1. In addition, 12 genes with more detailed expression profiles were analyzed by RT-qPCR. One aegerolysin gene had a peak of expression in mycelium with primordia and a second in basidiomata, confirming their distinctiveness. The number of transcripts of the gene for plerototolysin B increased in reddish-pink mycelium and indicated an activation of the initial basidiomata production even at this culturing stage. Expression of the glucose transporter gene increased in mycelium after the stress, coinciding with a decrease of adenylate cyclase gene transcription. This indicated that nutrient uptake can be an important signal to trigger fruiting in this fungus.

CONCLUSION

The identification of genes with increased expression in this phase of the life cycle of M. perniciosa opens up new possibilities of controlling fungus spread as well as of genetic studies of biological processes that lead to basidiomycete fruiting. This is the first comparative morphologic study of the early development both in vivo and in vitro of M. perniciosa basidiomata and the first description of genes expressed at this stage of the fungal life cycle.

Degradation of the hexose transporter Hxt5p in Saccharomyces cerevisiae

BACKGROUND INFORMATION

Hxt5p is a member of a multigene family of hexose transporter proteins which translocate glucose across the plasma membrane of the yeast Saccharomyces cerevisiae. In contrast with other major hexose transporters of this family, Hxt5p expression is regulated by the growth rate of the cells and not by the external glucose concentration. Furthermore, Hxt5p is the only glucose transporter expressed during stationary phase. These observations suggest a different role for Hxt5p in S. cerevisiae. Therefore we studied the metabolism and localization of Hxt5p in more detail.

RESULTS AND CONCLUSIONS

Inhibition of HXT5 expression in stationary-phase cells by the addition of glucose, which increases the growth rate, led to a decrease in the amount of Hxt5 protein within a few hours. Addition of glucose to stationary-phase cells resulted in a transient phosphorylation of Hxt5p on serine residues, but no ubiquitination was detected. The decrease in Hxt5p levels is caused by internalization of the protein, as observed by immunofluorescence microscopy. In stationary-phase cells, Hxt5p was localized predominantly at the cell periphery and upon addition of glucose to the cells the protein translocated to the cell interior. Electron microscopy demonstrated that the internalized Hxt5p-HA (haemagglutinin) protein was localized to small vesicles, multivesicular bodies and the vacuole. These results suggest that internalization and degradation of Hxt5p in the vacuole occur in an ubiquitination-independent manner via the endocytic pathway.

Different signalling pathways mediate glucose induction of SUC2, HXT1 and pyruvate decarboxylase in yeast

The glucose sensors Gpr1, Snf3 and Rgt2 generate the earliest signals produced by glucose in yeast. We showed that a lack of Gpr1 or Snf3/Rgt2 decreased by twofold the glucose induction of SUC2, but had no effect on the induction of pyruvate decarboxylase (Pdc). The induction of HXT1 was not affected by the absence of Gpr1. In an hxk1 hxk2 glk1 strain, high glucose fully induced SUC2, caused partial induction of HXT1 and had no effect on Pdc. In this strain, SUC2 induction was dependent on Gpr1, but HXT1 induction was not. Hxk2, required for the high expression of HXT1, was dispensable for the full induction of SUC2 or Pdc. These results indicate that glucose does not induce transcription through a single signalling pathway, but that several pathways may, in different combinations, regulate the transcription of different genes.

Loss of IRA2 suppresses the growth defect on low glucose caused by the snf3 mutation in Saccharomyces cerevisiae

SNF3 encodes a low-glucose sensor in Saccharomyces cerevisiae that regulates the expression of a subset of hexose transporter genes. Deletion of SNF3 prevents rapid adaptation to low glucose concentration. Novel spontaneous suppressor mutants of the snf3Delta phenotype were isolated. The mutations isolated fell into one of two groups: those that increase the expression of transporters regulated by Snf3p, and those that show no detectable effect on the regulation of these genes. The physiologic role of one mutation, rgg2 (restoration of growth on glucose), that did not affect HXT gene expression was assessed by transcriptome analysis. Genes involved in glycogen metabolism and cAMP pathways were affected by the rgg2 mutation, suggesting a cellular role as a regulatory protein. Attempts to clone the wild-type RGG2 allele were unsuccessful. The glycogen phenotype and genetic crossing allowed rgg2 to be identified as an allele of the IRA2 gene. Suppression of the snf3 mutant phenotype by deletion of IRA2 was confirmed. A possible mechanism of the suppression of the snf3 growth defect by mutation of ira2 is discussed.

Expression of the HXT13, HXT15 and HXT17 genes in Saccharomyces cerevisiae and stabilization of the HXT1 gene transcript by sugar-induced osmotic stress

Saccharomyces cerevisiae contains a family of 17 hexose transporter (HXT) genes; only nine have assigned functions, some of which are still poorly defined. Despite extensive efforts to characterize the hexose transporters, the expression of HXT6 and HXT8-17 remains an enigma. In nature, S. cerevisiae finds itself under extreme nutritional conditions including sugars in excess of 40% (w/v), depletion of nutrients and extremes of both temperature and pH. Using HXT promoter-lacZ fusions, we have identified novel conditions under which the HXT17 gene is expressed; HXT17 promoter activity is up-regulated in media containing raffinose and galactose at pH 7.7 versus pH 4.7. We demonstrated that HXT5, HXT13 and, to a lesser extent, HXT15 were all induced in the presence of non-fermentable carbon sources. HXT1 encodes a low-affinity transporter and in short-term osmotic shock experiments, HXT1 promoter activity was reduced when cells were exposed to media containing 40% glucose. However, we found that the HXT1 mRNA transcript was stabilized under conditions of osmotic stress. Furthermore, the stabilization of HXT1 mRNA does not appear to be gene specific because 30 min after transcriptional arrest there is a fourfold more mRNA in osmotically stressed versus non-stressed yeast cells. A large portion of S. cerevisiae mRNA molecules may, therefore, have a decreased rate of turnover during exposure to osmotic stress indicating that post-transcriptional regulation plays an important role in the adaptation of S. cerevisiae to osmotic stress.

A novel pathway determining multidrug sensitivity in Schizosaccharomyces pombe

In this study, we show that a mutation isolated during a screen for determinants of chemosensitivity in S. pombe results in loss of function of a previously uncharacterized protein kinase now named Hal4. Hal4 shares sequence homology to Hal4 and Hal5 in S. cerevisiae, and previous evidence indicates that these kinases positively regulate the major potassium transporter Trk1,2 and thereby maintain the plasma membrane potential. Disruption of this ion homeostasis pathway results in a hyperpolarized membrane and a concomitant increased sensitivity to cations. We demonstrate that a mutation in hal4+ results in hyperpolarization of the plasma membrane. In addition to the original selection agent, the hal4-1 mutant is sensitive to a variety of chemotherapeutic agents and stress-inducing compounds. Furthermore, this wider chemosensitive phenotype is also displayed by corresponding mutants in S. cerevisiae, and in a trk1deltatrk2delta double deletion mutant in S. pombe. We propose that this pathway and its role in regulating the plasma membrane potential may act as a pleiotropic determinant of sensitivity to chemotherapeutic agents.

Analysis of Saccharomyces cerevisiae hexose carrier expression during wine fermentation: both low- and high-affinity Hxt transporters are expressed

The transport of glucose and fructose into yeast cells is a critical step in the utilization of sugars during wine fermentation. Hexose uptake can be carried out by various Hxt carriers, each possessing distinct regulatory and transport-kinetic properties capable of influencing yeast fermentation capacity. We investigated the expression pattern of the hexose transporters Hxt1 to 7 at the promoter and protein levels in Saccharomyces cerevisiae during wine fermentation. The Hxt1p carrier was expressed only at the beginning of fermentation, and had no role during stationary phase. The Hxt3p carrier was the only one to be expressed throughout fermentation, displaying maximal expression at growth arrest and slowly decreasing in abundance over the course of the stationary phase. The high-affinity carriers Hxt6p and Hxt7p displayed similar expression profiles, with expression induced at entry into stationary phase and persisting throughout the phase. The expression of these two carriers occurred despite the presence of high amounts of hexoses, and the proteins were stably expressed when the cells were starved for nitrogen. The Hxt2p transporter was only transiently expressed during lag phase, which suggests a role for the protein in growth initiation. Characterization of glucose transport kinetics indicated the presence of a shift in the low-affinity component that is consistent with a predominant expression of Hxt1p during growth phase and of Hxt3p during stationary phase. In addition, a high-affinity uptake component consistent with functional expression of Hxt6p/Hxt7p was identified during stationary phase.

Characterization of potassium transport in wild-type and isogenic yeast strains carrying all combinations of trk1, trk2 and tok1 null mutations

Saccharomyces cerevisiae cells express three defined potassium-specific transport systems en-coded by TRK1, TRK2 and TOK1. To gain a more complete understanding of the physiological function of these transport proteins, we have constructed a set of isogenic yeast strains carrying all combinations of trk1delta, trk2delta and tok1delta null mutations. The in vivo K+ transport characteristics of each strain have been documented using growth-based assays, and the in vitro biochemical and electrophysiological properties associated with K+ transport have been determined. As has been reported previously, Trk1p and Trk2p facilitate high-affinity potassium uptake and appear to be functionally redundant under a wide range of environmental conditions. In the absence of TRK1 and TRK2, strains lack the ability specifically to take up K+, and trk1deltatrk2delta double mutant cells depend upon poorly understood non-specific cation uptake mechanisms for growth. Under conditions that impair the activity of the non-specific uptake system, termed NSC1, we have found that the presence of functional Tok1p renders cells sensitive to Cs+. Based on this finding, we have established a growth-based assay that monitors the in vivo activity of Tok1p.

Deletions of SKY1 or PTK2 in the Saccharomyces cerevisiae trk1Deltatrk2Delta mutant cells exert dual effect on ion homeostasis

Sky1p and Ptk2p are protein kinases that regulate ion transport across the plasma membrane of Saccharomyces cerevisiae. We show here that deletion of SKY1 or PTK2 in trk1,2Delta cells increase spermine tolerance, implying Trk1,2p independent activity. Unexpectedly, trk1,2Deltasky1Delta and trk1,2Deltaptk2Delta cells display hypersensitivity to LiCl. These cells also show increased tolerance to low pH and improved growth in low K(+), as demonstrated for deletion of PMP3 in trk1,2Delta cells. We show that Sky1p and Pmp3p act in different pathways. Hypersensitivity to LiCl and improved growth in low K(+) are partly dependent on the Nha1p and Kha1p antiporters and on the Tok1p channel. Finally, Dhh1p, a RNA helicase was demonstrated to improve growth of trk1,2Deltasky1Delta cells in low K(+). Overexpression of Dhh1p improves the ability of trk1,2Delta cells to grow in low K(+) while dhh1Delta cells are sensitive to spermine and salt ions. A model that integrates these results to explain the mechanism of ion transport across the plasma membrane is proposed.

The hexose transporters of Saccharomyces cerevisiae play different roles during enological fermentation

We investigated the role of hexose transporters in a Saccharomyces cerevisiae strain derived from an industrial wine strain by carrying out a functional analysis of HXT genes 1-7 under enological conditions. A strain in which the sugar carrier genes HXT1-HXT7 were deleted was constructed and the HXT genes were expressed individually or in combination to evaluate their role under wine alcoholic fermentation conditions. No growth or fermentation was observed in winemaking conditions for the hxt1-7 delta strain. The low-affinity carriers Hxt1 and Hxt3 were the only carriers giving complete fermentation of sugars when expressed alone, indicating that these carriers play a predominant role in wine fermentation. However, these two carriers have different functions. The Hxt3 transporter is thought to play a major role, as it was the only carrier that gave an almost normal fermentation profile when produced alone. The hxt1 carrier was much less effective during the stationary phase and its role is thought to be restricted to the beginning of fermentation. The high-affinity carriers Hxt2, Hxt6 and/or Hxt7 were also required for normal fermentation. These high-affinity transporters have different functions: hxt2 is involved in growth initiation, whereas Hxt6 and/or Hxt7 are required at the end of alcoholic fermentation. This work shows that the successful alcoholic fermentation of wine involves at least four or five hexose carriers, playing different roles at various stages in the fermentation cycle.

The hexokinase 2-dependent glucose signal transduction pathway of Saccharomyces cerevisiae

Sugars, predominantly glucose, evoke a variety of responses in Saccharomyces cerevisiae. These responses are elicited through a complex network of regulatory mechanisms that transduce the signal of presence of external glucose to their final intracellular targets. The HXK2 gene, encoding hexokinase 2 (Hxk2), the enzyme that initiates glucose metabolism, is highly expressed during growth in glucose and plays a pivotal role in the control of the expression of numerous genes, including itself. The mechanism of this autocontrol of expression is not completely understood. Hxk2 is found both in the nucleus and in the cytoplasm of S. cerevisiae; the nuclear localization is dependent on the presence of a stretch of amino acids located from lysine-6 to methionine-15. Although serine-14, within this stretch, can be phosphorylated in the absence of glucose, it is still unsettled whether this phosphorylation plays a role in the cellular localization of Hxk2. The elucidation of the mechanism of transport of Hxk2 to and from the nucleus, the influence of the oligomeric state of the protein on the nuclear transport and the fine mechanism of regulation of transcription of HXK2 are among the important unanswered questions in relation with the regulatory role of Hxk2.

Expression and activity of the Hxt7 high-affinity hexose transporter of Saccharomyces cerevisiae

High-affinity hexose transport is required for efficient utilization of low hexose concentrations by the baker's yeast Saccharomyces cerevisiae. These low concentrations occur during the late exponential phase of batch growth on hexoses, during hexose-limited chemostat or fed-batch culture, or during growth on sugars such as sucrose and raffinose that are hydrolysed to hexoses outside the cell. The expression of the Hxt7 high-affinity glucose transporter of S. cerevisiae was examined during batch growth on glucose medium in a wild-type strain and a strain expressing only HXT7 (i.e. with null mutations in HXT1-HXT6). In the wild-type strain, HXT7 transcription was repressed at high glucose and was detected when the glucose in the culture approached depletion. In the HXT7-only strain, transcription of HXT7 was constitutive throughout the glucose growth phase and was increased further at low glucose concentrations. After glucose depletion, the levels of HXT7 mRNA declined rapidly in both strains. In contrast, the Hxt7 protein was relatively stable after glucose depletion. By monitoring the subcellular localization of an Hxt7::GFP fusion protein it was observed that Hxt7 was localized in the plasma membrane, even when expressed at high glucose concentrations in the HXT7-only strain. After glucose depletion Hxt7 was gradually endocytosed and targeted to the vacuole for degradation. The Hxt7::GFP fusion protein was a fully functional hexose transporter with a catalytic centre activity of approximately 200/sec. It is concluded that repression of HXT7 and degradation of Hxt7 at high glucose concentrations is dependent on a high glucose transport capacity.

The role of hexose transport and phosphorylation in cAMP signalling in the yeast Saccharomyces cerevisiae

Glucose-induced cAMP signalling in Saccharomyces cerevisiae requires extracellular glucose detection via the Gpr1-Gpa2 G-protein coupled receptor system and intracellular glucose-sensing that depends on glucose uptake and phosphorylation. The glucose uptake requirement can be fulfilled by any glucose carrier including the Gal2 permease or by intracellular hydrolysis of maltose. Hence, the glucose carriers do not seem to play a regulatory role in cAMP signalling. Also the glucose carrier homologues, Snf3 and Rgt2, are not required for glucose-induced cAMP synthesis. Although no further metabolism beyond glucose phosphorylation is required, neither Glu6P nor ATP appears to act as metabolic trigger for cAMP signalling. This indicates that a regulatory function may be associated with the hexose kinases. Consistently, intracellular acidification, another known trigger of cAMP synthesis, can bypass the glucose uptake requirement but not the absence of a functional hexose kinase. This may indicate that intracellular acidification can boost a downstream effect that amplifies the residual signal transmitted via the hexose kinases when glucose uptake is too low.

Cloning of a sugar utilization gene cluster in Aspergillus parasiticus

At one end of the 70 kb aflatoxin biosynthetic pathway gene cluster in Aspergillus parasiticus and Aspergillus flavus reported earlier, we have cloned a group of four genes that constitute a well-defined gene cluster related to sugar utilization in A. parasiticus: (1) sugR, (2) hxtA, (3) glcA and (4) nadA. No similar well-defined sugar gene cluster has been reported so far in any other related Aspergillus species such as A. flavus, A. nidulans, A. sojae, A. niger, A. oryzae and A. fumigatus. The expression of the hxtA gene, encoding a hexose transporter protein, was found to be concurrent with the aflatoxin pathway cluster genes, in aflatoxin-conducive medium. This is significant since a close linkage between the two gene clusters could potentially explain the induction of aflatoxin biosynthesis by simple sugars such as glucose or sucrose.

A glucose transporter chimera confers a dominant negative glucose starvation phenotype in Saccharomyces cerevisiae

A family of glucose transporters mediates glucose uptake in Saccharomyces cerevisiae. We show that the dominant mutation GSF4-1, which impairs glucose repression of SUC2, results in a nonfunctional chimera of the transporters Hxt1p and Hxt4p. Hxt1/4p inhibits the function of wild-type glucose transporters. Similar mutations may facilitate analysis of the major facilitator superfamily.

The large intracytoplasmic loop of the glucose transporter GLUT2 is involved in glucose signaling in hepatic cells

The hypothesis that the glucose transporter GLUT2 can function as a protein mediating transcriptional glucose signaling was addressed. To divert the putative interacting proteins from a glucose signaling pathway, two intracytoplasmic domains of GLUT2, the C terminus and the large loop located between transmembrane domains 6 and 7, were transfected into mhAT3F hepatoma cells. Glucose-induced accumulation of two hepatic gene mRNAs (GLUT2 and L-pyruvate kinase) was specifically inhibited in cells transfected with the GLUT2 loop and not with the GLUT2 C terminus. The dual effects of glucose were dissociated in cells expressing the GLUT2 loop; in fact a normal glucose metabolism into glycogen occurred concomitantly with the inhibition of the glucose-induced transcription. This inhibition by the GLUT2 loop could be due to competitive binding of a protein that normally interacts with endogenous GLUT2. In addition, the GLUT2 loop, tagged with green fluorescent protein (GFP), was located within the nucleus, whereas the GFP and GFP-GLUT2 C-terminal proteins remained in the cytoplasm. In living cells, a fraction (50%) of the expressed GFP-GLUT2 loop translocated rapidly from the cytoplasm to the nucleus in response to high glucose concentration and conversely in the absence of glucose. We conclude that, via protein interactions with its large loop, GLUT2 may transduce a glucose signal from the plasma membrane to the nucleus.

Tpr1, a Schizosaccharomyces pombe protein involved in potassium transport

The Schizosaccharomyces pombe Tpr1 was isolated as suppressor of the Saccharomyces cerevisiae Delta trk1,2 potassium uptake deficient phenotype. Tpr1, for tetratrico peptide repeat, encodes a 1039 amino acid residues protein with several reiterated TPR units displaying significant homology to p150(TSP), a recently identified phosphoprotein of mouse, to S. cerevisiae CTR9 and to related sequences of human, Caenorhabditis elegans, Methanoccocus jannaschii and Arabidopsis thaliana. Expression of Tpr1 restored growth on 0.2 mM K(+) media, induced K(+) transport with a K(T) of 4.6 mM and resumed inward currents of -90 pA at -250 mV (pH 7.2) conducting K(+) and other alkali-metal ions. The tetratrico peptide repeat is a degenerate motif of 34 amino acids that is repeated several times within TPR-containing proteins and has been suggested to mediate protein-protein interactions. The sequence and putative binding properties of Tpr1 suggest the protein unlikely as transporter but involved in the enhancement of K(+) uptake via conventional carriers.

Transcription of the HXT4 gene is regulated by Gcr1p and Gcr2p in the yeast S. cerevisiae

Glucose transport and glycolysis are two sequential events which are regulated by both physiological and environmental signals in the yeast Saccharomyces cerevisiae. Transcription of the HXT4 gene was found to be regulated by Gcr1p and Gcr2p, transcription factors that are required for the regulated high level transcriptions of glycolytic genes. Transcription of HXT4 decreased about 35-fold in gcr1 mutant and two-fold in gcr2 mutant yeast cells. However, transcription of other HXT genes was not affected at a significant level by gcr1 or gcr2 mutations. Overproduction of Gcr1p from an inducible promoter resulted in a 15-64% increase in transcription of HXT4, depending on the growth conditions. Gel mobility shift assays performed with the purified DNA binding domain of Gcr1p and the UAS region of the HXT4 gene showed that Gcr1p interacts directly with multiple sites on the HXT4 UAS region. These results indicate that Gcr1p and Gcr2p coordinate the transcription of HXT4 and glycolytic genes.

Control of mRNA turnover as a mechanism of glucose repression in Saccharomyces cerevisiae

The phenomenon of glucose repression in yeast is concerned with the repression of a large number of genes when glucose is an abundant carbon source and almost all of the energy requirements of the cell can be satisfied from glycolysis. Prominent among the repressed genes are those encoding mitochondrial proteins required for respiration and oxidative phosphorylation. Past studies have characterized a pathway by which a signal generated from extracellular glucose is transmitted to the nucleus. The ultimate outcome is the repression of transcription of numerous genes, but also the induction of a limited number of others. The emphasis has been almost exclusively on transcriptional control mechanisms. A discovery made originally with the transcript of the SDH2 gene prompted an investigation of post-transcriptional mechanisms, and more specifically a study of the turnover rate of this mRNA in the absence and presence of glucose. SDH2 mRNA has a very short half-life in medium with glucose (YPD) and a significantly longer half-life in medium with glycerol (YPG). Experimental evidence and recent progress in understanding of (1) mRNA turnover in yeast and (2) initiation of translation on the 5' untranslated region of mRNAs, lead to a working hypothesis with the following major features: the carbon source, via a signaling pathway involving kinase/phosphatase activities, controls the rate of initiation, and thus influences a competition between eukaryotic initiation factors (prominently eIF4E, eIF4G, eIF3) binding to the capped mRNA and a decapping activity (DCP1) which is one of the rate limiting activities in the turnover of such mRNAs.

Glucose repression in Saccharomyces cerevisiae is related to the glucose concentration rather than the glucose flux

Glucose plays an important regulatory role in the yeast Saccharomyces cerevisiae, which is mostly reflected at the transcriptional level by glucose repression. The signal that initiates glucose repression is unknown, but data indicate that it is located at or above the level of glucose 6-phosphate, suggesting the involvement of either the intracellular or extracellular glucose concentration or the glucose flux in triggering glucose repression. We have investigated the role of the glucose flux and the extracellular glucose concentration in glucose repression by growing the cells in continuous culture under nitrogen limitation. By a step-wise increase in the glucose feed concentration, the glucose flux and extracellular glucose concentrations were modulated in an accurate way. Furthermore, the glucose flux and glucose concentrations were modulated independently of each other by increasing the dilution rate or by the use of fructose as a substrate. Using these approaches we demonstrate that glucose repression is related to the extracellular (or intracellular) glucose concentration rather than the glucose flux. At external glucose concentrations lower than 14 mM, glucose repression of SUC2 gene transcription was not triggered, whereas glucose repression of this gene was activated when the glucose concentration exceeded 18 mM. A comparable effect was observed for the glucose-repressible carbon source fructose.

Ectopic potassium uptake in trk1 trk2 mutants of Saccharomyces cerevisiae correlates with a highly hyperpolarized membrane potential

Null trk1 trk2 mutants of Saccharomyces cerevisiae exhibit a low-affinity uptake of K+ and Rb+. We show that this low-affinity Rb+ uptake is mediated by several independent transporters, and that trk1Delta cells and especially trk1Delta trk2Delta cells are highly hyperpolarized. Differences in the membrane potentials were assessed for sensitivity to hygromycin B and by flow cytometric analyses of cellular DiOC6(3) fluorescence. On the basis of the latter analyses, it is proposed that Trk1p and Trk2p are involved in the control of the membrane potential, preventing excessive hyperpolarizations. K+ starvation and nitrogen starvation hyperpolarize both TRK1 TRK2 and trk1Delta trk2Delta cells, thus suggesting that other proteins, in addition to Trk1p and Trk2p, participate in the control of the membrane potential. The HAK1 K+ transporter from Schwanniomyces occidentalis suppresses the K+-defective transport of trk1Delta trk2Delta cells but not the high hyperpolarization, and the HKT1 K+ transporter from wheat suppresses both defects, in the presence of Na+. We discuss the mechanism involved in the control of the membrane potential by Trk1p and Trk2p and the causal relationship between the high membrane potential (negative inside) of trk1Delta trk2Delta cells and its ectopic transport of alkali cations.

The SKS1 gene of Saccharomyces cerevisiae is required for long-term adaptation of snf3 null strains to low glucose

The SKS1 gene was originally identified as a multicopy suppressor of the growth defect of snf3 null mutations on low glucose concentrations. Snf3p is required for the rapid induction of HXT2 during growth on low substrate concentrations. Loss of Snf3p leads to a dramatic delay in expression of HXT2. Adaptation to low substrate concentrations does not occur in snf3 sks1 double null mutant strains, suggesting that SKS1 is required for the glucose-dependent expression of HXT2 in the absence of Snf3p activity. Over-expression of SKS1 leads to over-expression of Hxt2p, thus explaining the mechanism of suppression of the snf3 defect. SKS1 defines a novel, Snf3p-independent pathway for the expression of Hxt2p. Under certain growth conditions, over-expression of SKS1 itself leads to a growth defect which is diminished in snf3 hxt2 double mutants. This suggests that over-expression of Hxt2p at physiologically inappropriate times is detrimental to the cells.

Mutations in GSF1 and GSF2 alter glucose signaling in Saccharomyces cerevisiae

One function of the Saccharomyces cerevisiae Snf1 protein kinase is to relieve glucose repression of SUC, GAL, and other genes in response to glucose depletion. To identify genes that regulate Snf1 kinase activity, we have selected mutants that inappropriately express a SUC2promoter::HIS3 gene fusion when grown in glucose and that require Snf1 function for this phenotype. Mutations representing two new complementation groups (gsf1 and gsf2) were isolated. gsf1 mutations affect two distinct responses to glucose: the Snf1-regulated glucose repression of SUC2 and GAL10 transcription and the Snf1-independent induction by glucose of HXT1 transcription. gsf2 mutations relieve glucose repression of SUC2 and GAL10 transcription and, in combination with snf1 delta, cause an extreme slow growth phenotype. The GSF2 gene was cloned by complementation of the gsf2-1 snf1 delta slow growth phenotype and encodes a previously uncharacterized 46kD protein.

The molecular genetics of hexose transport in yeasts

Transport across the plasma membrane is the first, obligatory step of hexose utilization. In yeast cells the uptake of hexoses is mediated by a large family of related transporter proteins. In baker's yeast Saccharomyces cerevisiae the genes of 20 different hexose transporter-related proteins have been identified. Six of these transmembrane proteins mediate the metabolically relevant uptake of glucose, fructose and mannose for growth, two others catalyze the transport of only small amounts of these sugars, one protein is a galactose transporter but also able to transport glucose, two transporters act as glucose sensors, two others are involved in the pleiotropic drug resistance process, and the functions of the remaining hexose transporter-related proteins are not yet known. The catabolic hexose transporters exhibit different affinities for their substrates, and expression of their corresponding genes is controlled by the glucose sensors according to the availability of carbon sources. In contrast, milk yeast Kluyveromyces lactis contains only a few different hexose transporters. Genes of other monosaccharide transporter-related proteins have been found in fission yeast Schizosaccharomyces pombe and in the xylose-fermenting yeast Pichia stipitis. However, the molecular genetics of hexose transport in many other yeasts remains to be established. The further characterization of this multigene family of hexose transporters should help to elucidate the role of transport in yeast sugar metabolism.