Hexokinase PII has a double cytosolic-nuclear localisation in Saccharomyces cerevisiae

Moonlighting Proteins: The Case of the Hexokinases

Moonlighting proteins are defined as proteins with two or more functions that are unrelated and independent to each other, so that inactivation of one of them should not affect the second one and vice versa. Intriguingly, all the glycolytic enzymes are described as moonlighting proteins in some organisms. Hexokinase (HXK) is a critical enzyme in the glycolytic pathway and displays a wide range of functions in different organisms such as fungi, parasites, mammals, and plants. This review discusses HXKs moonlighting functions in depth since they have a profound impact on the responses to nutritional, environmental, and disease challenges. HXKs’ activities can be as diverse as performing metabolic activities, as a gene repressor complexing with other proteins, as protein kinase, as immune receptor and regulating processes like autophagy, programmed cell death or immune system responses. However, most of those functions are particular for some organisms while the most common moonlighting HXK function in several kingdoms is being a glucose sensor. In this review, we also analyze how different regulation mechanisms cause HXK to change its subcellular localization, oligomeric or conformational state, the response to substrate and product concentration, and its interactions with membrane, proteins, or RNA, all of which might impact the HXK moonlighting functions.

Production of Glucose 6-Phosphate From a Cellulosic Feedstock in a One Pot Multi-Enzyme Synthesis

Glucose 6-phosphate is the phosphorylated form of glucose and is used as a reagent in enzymatic assays. Current production occurs via a multi-step chemical synthesis. In this study we established a fully enzymatic route for the synthesis of glucose 6-phosphate from cellulose. As the enzymatic phosphorylation requires ATP as phosphoryl donor, the use of a cofactor regeneration system is required. We evaluated Escherichia coli glucokinase and Saccharomyces cerevisiae hexokinase (HK) for the phosphorylation reaction and Pseudomonas aeruginosa polyphosphate kinase 2 (PPK2) for ATP regeneration. All three enzymes were characterized in terms of temperature and pH optimum and the effects of substrates and products concentrations on enzymatic activities. After optimization of the conditions, we achieved a 85% conversion of glucose into glucose 6-phosphate using the HK/PPK2 activities within a 24 h reaction resulting in 12.56 g/l of glucose 6-phosphate. Finally, we demonstrated the glucose 6-phosphate formation from microcrystalline cellulose in a one-pot reaction comprising Aspergillus niger cellulase for glucose release and HK/PPK2 activities. We achieved a 77% conversion of released glucose into glucose 6-phosphate, however at the expense of a lower glucose 6-phosphate yield of 1.17 g/l. Overall, our study shows an alternative approach for synthesis of glucose 6-phosphate that can be used to valorize biomass derived cellulose.

Candida albicans Hexokinase 2 Challenges the Saccharomyces cerevisiae Moonlight Protein Model

Survival of the pathogenic yeast Candida albicans depends upon assimilation of fermentable and non-fermentable carbon sources detected in host microenvironments. Among the various carbon sources encountered in a human body, glucose is the primary source of energy. Its effective detection, metabolism and prioritization via glucose repression are primordial for the metabolic adaptation of the pathogen. In C. albicans, glucose phosphorylation is mainly performed by the hexokinase 2 (CaHxk2). In addition, in the presence of glucose, CaHxK2 migrates in the nucleus and contributes to the glucose repression signaling pathway. Based on the known dual function of the Saccharomyces cerevisiae hexokinase 2 (ScHxk2), we intended to explore the impact of both enzymatic and regulatory functions of CaHxk2 on virulence, using a site-directed mutagenesis approach. We show that the conserved aspartate residue at position 210, implicated in the interaction with glucose, is essential for enzymatic and glucose repression functions but also for filamentation and virulence in macrophages. Point mutations and deletion into the N-terminal region known to specifically affect glucose repression in ScHxk2 proved to be ineffective in CaHxk2. These results clearly show that enzymatic and regulatory functions of the hexokinase 2 cannot be unlinked in C. albicans.

Cellular toxicity of the metabolic inhibitor 2-deoxyglucose and associated resistance mechanisms

Most malignant cells display increased glucose absorption and metabolism compared to surrounding tissues. This well-described phenomenon results from a metabolic reprogramming occurring during transformation, that provides the building blocks and supports the high energetic cost of proliferation by increasing glycolysis. These features led to the idea that drugs targeting glycolysis might prove efficient in the context of cancer treatment. One of these drugs, 2-deoxyglucose (2-DG), is a synthetic glucose analog that can be imported into cells and interfere with glycolysis and ATP generation. Its preferential targeting to sites of cell proliferation is supported by the observation that a derived molecule, 2-fluoro-2-deoxyglucose (FDG) accumulates in tumors and is used for cancer imaging. Here, we review the toxicity mechanisms of this drug, from the early-described effects on glycolysis to its other cellular consequences, including inhibition of protein glycosylation and endoplasmic reticulum stress, and its interference with signaling pathways. Then, we summarize the current data on the use of 2-DG as an anti-cancer agent, especially in the context of combination therapies, as novel 2-DG-derived drugs are being developed. We also show how the use of 2-DG helped to decipher glucose-signaling pathways in yeast and favored their engineering for biotechnologies. Finally, we discuss the resistance strategies to this inhibitor that have been identified in the course of these studies and which may have important implications regarding a medical use of this drug.

Hexokinase and Glucokinases Are Essential for Fitness and Virulence in the Pathogenic Yeast Candida albicans

The pathogenic yeast Candida albicans is both a powerful commensal and a pathogen of humans that can infect wide range of organs and body sites. Metabolic flexibility promotes infection and commensal colonization by this opportunistic pathogen. Yeast cell survival depends upon assimilation of fermentable and non-fermentable locally available carbon sources. Physiologically relevant sugars like glucose and fructose are present at low levels in host niches. However, because glucose is the preferred substrate for energy and biosynthesis of structural components, its efficient detection and metabolism are fundamental for the metabolic adaptation of the pathogen. We explored and characterized the C. albicans hexose kinase system composed of one hexokinase (CaHxk2) and two glucokinases (CaGlk1 and CaGlk4). Using a set of mutant strains, we found that hexose phosphorylation is mostly performed by CaHxk2, which sustains growth on hexoses. Our data on hexokinase and glucokinase expression point out an absence of cross regulation mechanisms at the transcription level and different regulatory pathways. In the presence of glucose, CaHxk2 migrates in the nucleus and contributes to the glucose repression signaling pathway. In addition, CaHxk2 participates in oxidative, osmotic and cell wall stress responses, while glucokinases are overexpressed under hypoxia. Hexose phosphorylation is a key step necessary for filamentation that is affected in the hexokinase mutant. Virulence of this mutant is clearly impacted in the Galleria mellonella and macrophage models. Filamentation, glucose phosphorylation and stress response defects of the hexokinase mutant prevent host killing by C. albicans. By contributing to metabolic flexibility, stress response and morphogenesis, hexose kinase enzymes play an essential role in the virulence of C. albicans.

Hexokinase 2 Is an Intracellular Glucose Sensor of Yeast Cells That Maintains the Structure and Activity of Mig1 Protein Repressor Complex

Hexokinase 2 (Hxk2) fromSaccharomyces cerevisiaeis a bi-functional enzyme, being both a catalyst in the cytosol and an important regulator of the glucose repression signal in the nucleus. Despite considerable recent progress, little is known about the regulatory mechanism that controls nuclear Hxk2 association with theSUC2promoter chromatin and how this association is necessary forSUC2gene repression. Our data indicate that in theSUC2promoter context, Hxk2 functions through a variety of structurally unrelated factors, mainly the DNA-binding Mig1 and Mig2 repressors and the regulatory Snf1 and Reg1 factors. Hxk2 sustains the repressor complex architecture maintaining transcriptional repression at theSUC2gene. Using chromatin immunoprecipitation assays, we discovered that the Hxk2 in its open configuration, at low glucose conditions, leaves the repressor complex that induces its dissociation and promotesSUC2gene expression. In high glucose conditions, Hxk2 adopts a close conformation that promotes Hxk2 binding to the Mig1 protein and the reassembly of theSUC2repressor complex. Additional findings highlight the possibility that Hxk2 constitutes an intracellular glucose sensor that operates by changing its conformation in response to cytoplasmic glucose levels that regulate its interaction with Mig1 and thus its recruitment to the repressor complex of theSUC2promoter. Thus, our data indicate that Hxk2 is more intimately involved in gene regulation than previously thought.

Network reconstruction and validation of the Snf1/AMPK pathway in baker's yeast based on a comprehensive literature review

BACKGROUND/OBJECTIVES

The SNF1/AMPK protein kinase has a central role in energy homeostasis in eukaryotic cells. It is activated by energy depletion and stimulates processes leading to the production of ATP while it downregulates ATP-consuming processes. The yeast SNF1 complex is best known for its role in glucose derepression.

METHODS

We performed a network reconstruction of the Snf1 pathway based on a comprehensive literature review. The network was formalised in the rxncon language, and we used the rxncon toolbox for model validation and gap filling.

RESULTS

We present a machine-readable network definition that summarises the mechanistic knowledge of the Snf1 pathway. Furthermore, we used the known input/output relationships in the network to identify and fill gaps in the information transfer through the pathway, to produce a functional network model. Finally, we convert the functional network model into a rule-based model as a proof-of-principle.

CONCLUSIONS

The workflow presented here enables large scale reconstruction, validation and gap filling of signal transduction networks. It is analogous to but distinct from that established for metabolic networks. We demonstrate the workflow capabilities, and the direct link between the reconstruction and dynamic modelling, with the Snf1 network. This network is a distillation of the knowledge from all previous publications on the Snf1/AMPK pathway. The network is a knowledge resource for modellers and experimentalists alike, and a template for similar efforts in higher eukaryotes. Finally, we envisage the workflow as an instrumental tool for reconstruction of large signalling networks across Eukaryota.

Evolution of moonlighting proteins: insight from yeasts

The present article addresses the possibilities offered by yeasts to study the problem of the evolution of moonlighting proteins. It focuses on data available on hexokinase from Saccharomyces cerevisiae that moonlights in catabolite repression and on galactokinase from Kluyveromyces lactis that moonlights controlling the induction of the GAL genes. Possible experimental approaches to studying the evolution of moonlighting hexose kinases are suggested.

Hexokinase and not glycogen synthase controls the flux through the glycogen synthesis pathway in frog oocytes

Here we set out to evaluate the role of hexokinase and glycogen synthase in the control of glycogen synthesis in vivo. We used metabolic control analysis (MCA) to determine the flux control coefficient for each of the enzymes involved in the pathway. Acute microinjection experiments in frog oocytes were specifically designed to change the endogenous activities of the enzymes, either by directly injecting increasing amounts of a given enzyme (HK, PGM and UGPase) or by microinjection of a positive allosteric effector (glc-6P for GS). Values of 0.61 ± 0.07, 0.19 ± 0.03, 0.13 ± 0.03, and -0.06 ± 0.08 were obtained for the flux control coefficients of hexokinase EC 2.7.1.1 (HK), phosphoglucomutase EC 5.4.2.1 (PGM), UDPglucose pyrophosphorylase EC 2.7.7.9 (UGPase) and glycogen synthase EC 2.4.1.11 (GS), respectively. These values satisfy the summation theorem since the sum of the control coefficients for all the enzymes of the pathway is 0.87. The results show that, in frog oocytes, glycogen synthesis through the direct pathway is under the control of hexokinase. Phosphoglucomutase and UDPG-pyrophosphorylase have a modest influence, while the control exerted by glycogen synthase is null.

Dissecting the molecular basis of the contribution of source strength to high fructan accumulation in wheat

Fructans represent the major component of water soluble carbohydrates (WSCs) in the maturing stem of temperate cereals and are an important temporary carbon reserve for grain filling. To investigate the importance of source carbon availability in fructan accumulation and its molecular basis, we performed comparative analyses of WSC components and the expression profiles of genes involved in major carbohydrate metabolism and photosynthesis in the flag leaves of recombinant inbred lines from wheat cultivars Seri M82 and Babax (SB lines). High sucrose levels in the mature flag leaf (source organ) were found to be positively associated with WSC and fructan concentrations in both the leaf and stem of SB lines in several field trials. Analysis of Affymetrix expression array data revealed that high leaf sucrose lines grown in abiotic-stress-prone environments had high expression levels of a number of genes in the leaf involved in the sucrose synthetic pathway and photosynthesis, such as Calvin cycle genes, antioxidant genes involved in chloroplast H(2)O(2) removal and genes involved in energy dissipation. The expression of the majority of genes involved in fructan and starch synthetic pathways were positively correlated with sucrose levels in the leaves of SB lines. The high level of leaf fructans in high leaf sucrose lines is likely attributed to the elevated expression levels of fructan synthetic enzymes, as the mRNA levels of three fructosyltransferase families were consistently correlated with leaf sucrose levels among SB lines. These data suggest that high source strength is one of the important genetic factors determining high levels of WSC in wheat.

Phosphorylation of yeast hexokinase 2 regulates its nucleocytoplasmic shuttling

Nucleocytoplasmic shuttling of Hxk2 induced by glucose levels has been reported recently. Here we present evidence that indicates that Hxk2 nucleocytoplasmic traffic is regulated by phosphorylation and dephosphorylation at serine 14. Moreover, we identified the protein kinase Snf1 and the protein phosphatase Glc7-Reg1 as novel regulatory partners for the nucleocytoplasmic shuttling of Hxk2. Functional studies revealed that, in contrast to the wild-type protein, the dephosphorylation-mimicking mutant of Hxk2 retains its nuclear localization in low glucose conditions, and the phosphomimetic mutant of Hxk2 retains its cytoplasmic localization in high glucose conditions. Interaction experiments of Hxk2 with Kap60 and Xpo1 indicated that nuclear import of the S14D mutant of Hxk2 is severely decreased but that the export is significantly enhanced. Conversely, nuclear import of the S14A mutant of Hxk2 was significantly enhanced, although the export was severely decreased. The interaction of Hxk2 with Kap60 and Xpo1 was found to occur in the dephosphorylated and phosphorylated states of the protein, respectively. In addition, we found that Hxk2 is a substrate for Snf1. Mutational analysis indicated that serine 14 is a major in vitro and in vivo phosphorylation site for Snf1. We also provide evidence that dephosphorylation of Hxk2 at serine 14 is a protein phosphatase Glc7-Reg1-dependent process. Taken together, this study establishes a functional link between Hxk2, Reg1, and Snf1 signaling, which involves the regulation of Hxk2 nucleocytoplasmic shuttling by phosphorylation-dephosphorylation of serine 14.

Glucose levels regulate the nucleo-mitochondrial distribution of Mig2

Mig2 has been described as a transcriptional factor that in the absence of Mig1 protein is required for glucose repression of the SUC2 gene. Thus, until now, the main role assigned to Mig2 has been the functional redundancy to Mig1. In this study, we report that Mig2 has a double subcellular localization. As expected, in high-glucose conditions it is accumulated in the nucleus but in low-glucose conditions Mig2 has an unexpected mitochondrial localization and role in mitochondrial morphology. We describe that Mig2 physically interacts with the mitochondrial protein Ups1 in a glucose-dependent manner. We also show that Δmig2 mutant cells exhibit a fragmented network of mitochondrial tubules, a phenotype similarly observed in cells lacking Fzo1 and Ups1. Furthermore, Mig2 acts antagonistically with respect to the fission-promoting components, because mitochondrial aggregation induced by DNM1 deletion was rescued in the Δdnm1Δmig2 double mutant. Thus, our studies have revealed an additional role for Mig2 as a novel factor required for the maintenance of fusion-competent mitochondria in Saccharomyces cerevisiae and strongly suggest that Mig2 could be involved in the cross talk between the nucleus and the mitochondria through Ups1 to regulate mitochondrial morphology in a glucose dependent manner.

Saccharomyces cerevisiae gene YMR291W/TDA1 mediates the in vivo phosphorylation of hexokinase isoenzyme 2 at serine-15

Hxk2 is the predominant hexokinase of Saccharomyces cerevisiae during growth on glucose. In addition to its role in glycolysis, the enzyme is involved in glucose sensing and regulation of gene expression. Glucose limitation causes the phosphorylation of Hxk2 at serine-15 which affects the nucleo-cytoplasmic distribution and dimer stability of the enzyme. In order to identify the responsible kinase, we screened selected protein kinase single-gene deletion mutants by high resolution clear native PAGE. Deletion of YMR291W/TDA1 resulted in the absence of the Hxk2 phosphomonomer, indicating an indispensable role of the corresponding protein in Hxk2 phosphorylation.

Nuclear import of the yeast hexokinase 2 protein requires α/β-importin-dependent pathway

Hexokinase 2 (Hxk2) from Saccharomyces cerevisiae was one of the first metabolic enzymes described as a multifunctional protein. Hxk2 has a double subcellular localization and role, it functions as a glycolytic enzyme in the cytoplasm and as a regulator of gene transcription of several Mig1-regulated genes in the nucleus. However, the mechanism by which Hxk2 enters in the nucleus was unknown until now. Here, we report that the Hxk2 protein is an import substrate of the carriers α-importin (Kap60 in yeast) and β-importin (Kap95 in yeast). We also show that the Hxk2 nuclear import and the binding of Hxk2 with Kap60 are glucose-dependent and involve one lysine-rich nuclear localization sequence (NLS), located between lysine 6 and lysine 12. Moreover, Kap95 facilitates the recognition of the Hxk2 NLS1 motif by Kap60 and both importins are essential for Hxk2 nuclear import. It is also demonstrated that Hxk2 nuclear import and its binding to Kap95 and Kap60 depend on the Gsp1-GTP/GDP protein levels. Thus, our study uncovers Hxk2 as a new cargo for the α/β-importin pathway of S. cerevisiae.

Dynamic analysis of cytosolic glucose and ATP levels in yeast using optical sensors

Precise and dynamic measurement of intracellular metabolite levels has been hampered by difficulties in differentiating between adsorbed and imported fractions and the subcellular distribution between cytosol, endomembrane compartments and mitochondria. In the present study, genetically encoded FRET (Förster resonance energy transfer)-based sensors were deployed for dynamic measurements of free cytosolic glucose and ATP with varying external supply and in glucose-transport mutants. Moreover, by using the FRET sensors in a microfluidic platform, we were able to monitor in vivo changes of intracellular free glucose in individual yeast cells. We demonstrate the suitability of the FRET sensors for gaining physiological insight by demonstrating that free intracellular glucose and ATP levels are reduced in a hxt5Δ hexose-transporter mutant compared with wild-type and other hxtΔ strains.

Functional domains of yeast hexokinase 2

Hkx2 (hexokinase 2) from Saccharomyces cerevisiae was one of the first metabolic enzymes described as a multifunctional protein. Hxk2 has a double subcellular localization: it functions as a glycolytic enzyme in the cytoplasm and as a regulator of gene transcription of several Mig1-regulated genes in the nucleus. To get more insights into the structure–function relationships of the Hxk2 protein, we followed two different approaches. In the first, we deleted the last eight amino acids of Hxk2 and replaced Ser³⁰⁴ with phenylalanine to generate Hxk2_wca. Analysis of this mutant demonstrated that these domains play an essential role in the catalytic activity of yeast Hxk2, but has no effect on the regulatory function of this protein. In the second, we analysed whether amino acids from Lys⁶ to Met¹⁵ of Hxk2 (Hxk2_wrf) are essential for the regulatory role of Hxk2 and whether there is an effect on the hexose kinase activity of this protein. In the present paper, we report that the Hxk2_wca mutant protein interacts with the Mig1 transcriptional repressor and the Snf1 protein kinase in the nucleus at the level of the SUC2–Mig1 repressor complex. We have demonstrated that Hxk2_wca maintained full regulatory function because the glucose-repression signalling of the wild-type machinery is maintained. We also report that the Hxk2_wrf mutant allele is incapable of glucose repression signalling because it does not interact with Mig1 at the level of the SUC2–Mig1 repressor complex. The two mutants, Hxk2_wca and Hxk2_wrf retain single functions, as a transcriptional factor or as an enzyme with hexose-phosphorylating activity, but have lost the original bifunctionality of Hxk2.

Life in the midst of scarcity: adaptations to nutrient availability in Saccharomyces cerevisiae

Cells of all living organisms contain complex signal transduction networks to ensure that a wide range of physiological properties are properly adapted to the environmental conditions. The fundamental concepts and individual building blocks of these signalling networks are generally well-conserved from yeast to man; yet, the central role that growth factors and hormones play in the regulation of signalling cascades in higher eukaryotes is executed by nutrients in yeast. Several nutrient-controlled pathways, which regulate cell growth and proliferation, metabolism and stress resistance, have been defined in yeast. These pathways are integrated into a signalling network, which ensures that yeast cells enter a quiescent, resting phase (G0) to survive periods of nutrient scarceness and that they rapidly resume growth and cell proliferation when nutrient conditions become favourable again. A series of well-conserved nutrient-sensory protein kinases perform key roles in this signalling network: i.e. Snf1, PKA, Tor1 and Tor2, Sch9 and Pho85-Pho80. In this review, we provide a comprehensive overview on the current understanding of the signalling processes mediated via these kinases with a particular focus on how these individual pathways converge to signalling networks that ultimately ensure the dynamic translation of extracellular nutrient signals into appropriate physiological responses.

Evidence for a role of hexokinases as conserved glucose sensors in both monocot and dicot plant species

The role of the hexokinases (HXKs) as glucose (Glc) sensors has been mainly demonstrated for Arabidopsis (Arabidopsis thaliana) HXK1 (AtHXK1) but has yet to be shown in other plant species. In our recent publication, we reported that two rice (Oryza sativa) HXKs, OsHXK5 and OsHXK6, also function as Glc sensors. These two enzymes harbor both mitochondrial targeting peptides (mTPs) and nuclear localization signals (NLSs), and we confirmed their dual-targeting ability to nuclei and mitochondria using GFP fusion experiments. Consistently, it has been previously known that AtHXK1 is predominantly associated with mitochondria but is also present in nuclei in vivo at appreciable levels. Notably, the expression of OsHXK5, OsHXK6, or their catalytically inactive mutant alleles complemented the Arabidopsis glucose insensitive2 (gin2) mutant. In addition, transgenic rice plants overexpressing OsHXK5 or OsHXK6 exhibited hypersensitive plant growth retardation and enhanced repression of the Rubisco small subunit (RbcS) gene in response to glucose treatment. Our results thus provided evidence that OsHXK5 and OsHXK6 can function as glucose sensors in rice. Hence, the available current data suggest that the role of the HXKs as Glc sensors may be conserved in both monocot and dicot plant species, and that the nuclear localization of AtHXK1, OsHXK5 and OsHXK6 may be critical for Glc sensing and signaling.

Nuclear export of the yeast hexokinase 2 protein requires the Xpo1 (Crm1)-dependent pathway

Hexokinase 2 (Hxk2) from Saccharomyces cerevisiae was one of the first metabolic enzymes described as a multifunctional protein. Hxk2 has a double subcellular localization; it functions as a glycolytic enzyme in the cytoplasm and as a regulator of gene transcription of several Mig1-regulated genes in the nucleus. However, the mechanism by which Hxk2 enters and leaves the nucleus is still unknown. In low glucose conditions, Hxk2 is phosphorylated at serine 14, but how this phosphorylation may affect glucose signaling is also unknown at the moment. Here we report that the Hxk2 protein is an export substrate of the carrier protein Xpo1 (Crm1). We also show that the Hxk2 nuclear export and the binding of Hxk2 and Xpo1 involve two leucine-rich nuclear export signals (NES) located between leucine 23 and isoleucine 33 (NES1) and between leucine 310 and leucine 318 (NES2). We also show that the Hxk2 phosphorylation at serine 14 promotes Hxk2 export by facilitating the association of Hxk2 with Xpo1. Our study uncovers a new cargo for the Xpo1 yeast exportin and identifies Hxk2 phosphorylation at serine 14 as a regulatory mechanism that controls its nuclear exit in function of the glucose levels.

Role of the rice hexokinases OsHXK5 and OsHXK6 as glucose sensors

The Arabidopsis (Arabidopsis thaliana) hexokinase 1 (AtHXK1) is recognized as an important glucose (Glc) sensor. However, the function of hexokinases as Glc sensors has not been clearly demonstrated in other plant species, including rice (Oryza sativa). To investigate the functions of rice hexokinase isoforms, we characterized OsHXK5 and OsHXK6, which are evolutionarily related to AtHXK1. Transient expression analyses using GFP fusion constructs revealed that OsHXK5 and OsHXK6 are associated with mitochondria. Interestingly, the OsHXK5DeltamTP-GFP and OsHXK6DeltamTP-GFP fusion proteins, which lack N-terminal mitochondrial targeting peptides, were present mainly in the nucleus with a small amount of the proteins seen in the cytosol. In addition, the OsHXK5NLS-GFP and OsHXK6NLS-GFP fusion proteins harboring nuclear localization signals were targeted predominantly in the nucleus, suggesting that these OsHXKs retain a dual-targeting ability to mitochondria and nuclei. In transient expression assays using promoterluciferase fusion constructs, these two OsHXKs and their catalytically inactive alleles dramatically enhanced the Glc-dependent repression of the maize (Zea mays) Rubisco small subunit (RbcS) and rice alpha-amylase genes in mesophyll protoplasts of maize and rice. Notably, the expression of OsHXK5, OsHXK6, or their mutant alleles complemented the Arabidopsis glucose insensitive2-1 mutant, thereby resulting in wild-type characteristics in seedling development, Glc-dependent gene expression, and plant growth. Furthermore, transgenic rice plants overexpressing OsHXK5 or OsHXK6 exhibited hypersensitive plant growth retardation and enhanced repression of the photosynthetic gene RbcS in response to Glc treatment. These results provide evidence that rice OsHXK5 and OsHXK6 can function as Glc sensors.

Human pancreatic beta-cell glucokinase: subcellular localization and glucose repression signalling function in the yeast cell

Human GK(beta) (pancreatic beta-cell glucokinase) is the main glucose-phosphorylating enzyme in pancreatic beta-cells. It shares several structural, catalytic and regulatory properties with Hxk2 (hexokinase 2) from Saccharomyces cerevisiae. In fact, it has been previously described that expression of GK(beta) in yeast could replace Hxk2 in the glucose signalling pathway of S. cerevisiae. In the present study we report that GK(beta) exerts its regulatory role by association with the yeast transcriptional repressor Mig1 (multicopy inhibitor of GAL gene expression 1); the presence of Mig1 allows GK(beta) to bind to the SUC2 (sucrose fermentation 2) promoter, helping in this way in the maintenance of the repression of the SUC2 gene under high-glucose conditions. Since a similar mechanism has been described for the yeast Hxk2, the findings of the present study suggest that the function of the regulatory domain present in these two proteins has been conserved throughout evolution. In addition, we report that GK(beta) is enriched in the yeast nucleus of high-glucose growing cells, whereas it shows a mitochondrial localization upon removal of the sugar. However, GK(beta) does not exit the nucleus in the absence of Mig1, suggesting that Mig1 regulates the nuclear exit of GK(beta) under low-glucose conditions. We also report that binding of GK(beta) to Mig1 allows the latter protein to be located at the mitochondrial network under low-glucose conditions.

The gluconeogenic enzyme fructose-1,6-bisphosphatase is dispensable for growth of the yeast Yarrowia lipolytica in gluconeogenic substrates

The genes encoding gluconeogenic enzymes in the nonconventional yeast Yarrowia lipolytica were found to be differentially regulated. The expression of Y. lipolytica FBP1 (YlFBP1) encoding the key enzyme fructose-1,6-bisphosphatase was not repressed by glucose in contrast with the situation in other yeasts; however, this sugar markedly repressed the expression of YlPCK1, encoding phosphoenolpyruvate carboxykinase, and YlICL1, encoding isocitrate lyase. We constructed Y. lipolytica strains with two different disrupted versions of YlFBP1 and found that they grew much slower than the wild type in gluconeogenic carbon sources but that growth was not abolished as happens in most microorganisms. We attribute this growth to the existence of an alternative phosphatase with a high K(m) (2.3 mM) for fructose-1,6-bisphosphate. The gene YlFBP1 restored fructose-1,6-bisphosphatase activity and growth in gluconeogenic carbon sources to a Saccharomyces cerevisiae fbp1 mutant, but the introduction of the FBP1 gene from S. cerevisiae in the Ylfbp1 mutant did not produce fructose-1,6-bisphosphatase activity or growth complementation. Subcellular fractionation revealed the presence of fructose-1,6-bisphosphatase both in the cytoplasm and in the nucleus.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

Regulation of xylose metabolism in recombinant Saccharomyces cerevisiae

Background

Considerable interest in the bioconversion of lignocellulosic biomass into ethanol has led to metabolic engineering of Saccharomyces cerevisiae for fermentation of xylose. In the present study, the transcriptome and proteome of recombinant, xylose-utilising S. cerevisiae grown in aerobic batch cultures on xylose were compared with those of glucose-grown cells both in glucose repressed and derepressed states. The aim was to study at the genome-wide level how signalling and carbon catabolite repression differ in cells grown on either glucose or xylose. The more detailed knowledge whether xylose is sensed as a fermentable carbon source, capable of catabolite repression like glucose, or is rather recognised as a non-fermentable carbon source is important for further engineering this yeast for more efficient anaerobic fermentation of xylose.

Results

Genes encoding respiratory proteins, proteins of the tricarboxylic acid and glyoxylate cycles, and gluconeogenesis were only partially repressed by xylose, similar to the genes encoding their transcriptional regulators HAP4, CAT8 and SIP1-2 and 4. Several genes that are repressed via the Snf1p/Mig1p-pathway during growth on glucose had higher expression in the cells grown on xylose than in the glucose repressed cells but lower than in the glucose derepressed cells. The observed expression profiles of the transcription repressor RGT1 and its target genes HXT2-3, encoding hexose transporters suggested that extracellular xylose was sensed by the glucose sensors Rgt2p and Snf3p. Proteome analyses revealed distinct patterns in phosphorylation of hexokinase 2, glucokinase and enolase isoenzymes in the xylose- and glucose-grown cells.

Conclusion

The results indicate that the metabolism of yeast growing on xylose corresponds neither to that of fully glucose repressed cells nor that of derepressed cells. This may be one of the major reasons for the suboptimal fermentation of xylose by recombinant S. cerevisiae strains. Phosphorylation of different isoforms of glycolytic enzymes suggests that regulation of glycolysis also occurred at a post-translational level, supporting prior findings.

Moonlighting proteins in yeasts

Proteins able to participate in unrelated biological processes have been grouped under the generic name of moonlighting proteins. Work with different yeast species has uncovered a great number of moonlighting proteins and shown their importance for adequate functioning of the yeast cell. Moonlighting activities in yeasts include such diverse functions as control of gene expression, organelle assembly, and modification of the activity of metabolic pathways. In this review, we consider several well-studied moonlighting proteins in different yeast species, paying attention to the experimental approaches used to identify them and the evidence that supports their participation in the unexpected function. Usually, moonlighting activities have been uncovered unexpectedly, and up to now, no satisfactory way to predict moonlighting activities has been found. Among the well-characterized moonlighting proteins in yeasts, enzymes from the glycolytic pathway appear to be prominent. For some cases, it is shown that despite close phylogenetic relationships, moonlighting activities are not necessarily conserved among yeast species. Organisms may utilize moonlighting to add a new layer of regulation to conventional regulatory networks. The existence of this type of proteins in yeasts should be taken into account when designing mutant screens or in attempts to model or modify yeast metabolism.

Molecular dissection of variation in carbohydrate metabolism related to water-soluble carbohydrate accumulation in stems of wheat

Water-soluble carbohydrates (WSCs; composed of mainly fructans, sucrose [Suc], glucose [Glc], and fructose) deposited in wheat (Triticum aestivum) stems are important carbon sources for grain filling. Variation in stem WSC concentrations among wheat genotypes is one of the genetic factors influencing grain weight and yield under water-limited environments. Here, we describe the molecular dissection of carbohydrate metabolism in stems, at the WSC accumulation phase, of recombinant inbred Seri/Babax lines of wheat differing in stem WSC concentrations. Affymetrix GeneChip analysis of carbohydrate metabolic enzymes revealed that the mRNA levels of two fructan synthetic enzyme families (Suc:Suc 1-fructosyltransferase and Suc:fructan 6-fructosyltransferase) in the stem were positively correlated with stem WSC and fructan concentrations, whereas the mRNA levels of enzyme families involved in Suc hydrolysis (Suc synthase and soluble acid invertase) were inversely correlated with WSC concentrations. Differential regulation of the mRNA levels of these Suc hydrolytic enzymes in Seri/Babax lines resulted in genotypic differences in these enzyme activities. Down-regulation of Suc synthase and soluble acid invertase in high WSC lines was accompanied by significant decreases in the mRNA levels of enzyme families related to sugar catabolic pathways (fructokinase and mitochondrion pyruvate dehydrogenase complex) and enzyme families involved in diverting UDP-Glc to cell wall synthesis (UDP-Glc 6-dehydrogenase, UDP-glucuronate decarboxylase, and cellulose synthase), resulting in a reduction in cell wall polysaccharide contents (mainly hemicellulose) in the stem of high WSC lines. These data suggest that differential carbon partitioning in the wheat stem is one mechanism that contributes to genotypic variation in WSC accumulation.

Mixed and diverse metabolic and gene-expression regulation of the glycolytic and fermentative pathways in response to a HXK2 deletion in Saccharomyces cerevisiae

In Saccharomyces cerevisiae the HXK2 gene, which encodes the glycolytic enzyme hexokinase II, is involved in the regulatory mechanism known as 'glucose repression'. Its deletion leads to fully respiratory growth at high glucose concentrations where the wild type ferments profusely. Here we describe that deletion of the HXK2 gene resulted in a 75% reduction in fermentative capacity. Using regulation analysis we found that the fluxes through most glycolytic and fermentative enzymes were regulated cooperatively by changes in their capacities (Vmax) and by changes in the way they interacted with the rest of the metabolism. Glucose transport and phosphofructokinase were regulated purely at the metabolic level. The reduction of fermentative capacity in the mutant was accompanied by a remarkable resilience of the remaining capacity to nutrient starvation. After starvation, the fermentative capacity of the hxk2Delta mutant was similar to that of the wild type. Based on our results and previous reports, we suggest an inverse correlation between glucose repression and the resilience of fermentative capacity towards nutrient starvation. Only a limited number of glycolytic enzyme activities changed upon starvation of the hxk2Delta mutant and we discuss to what extent this could explain the stability of the fermentative capacity.

Glucose signaling in Saccharomyces cerevisiae

Eukaryotic cells possess an exquisitely interwoven and fine-tuned series of signal transduction mechanisms with which to sense and respond to the ubiquitous fermentable carbon source glucose. The budding yeast Saccharomyces cerevisiae has proven to be a fertile model system with which to identify glucose signaling factors, determine the relevant functional and physical interrelationships, and characterize the corresponding metabolic, transcriptomic, and proteomic readouts. The early events in glucose signaling appear to require both extracellular sensing by transmembrane proteins and intracellular sensing by G proteins. Intermediate steps involve cAMP-dependent stimulation of protein kinase A (PKA) as well as one or more redundant PKA-independent pathways. The final steps are mediated by a relatively small collection of transcriptional regulators that collaborate closely to maximize the cellular rates of energy generation and growth. Understanding the nuclear events in this process may necessitate the further elaboration of a new model for eukaryotic gene regulation, called "reverse recruitment." An essential feature of this idea is that fine-structure mapping of nuclear architecture will be required to understand the reception of regulatory signals that emanate from the plasma membrane and cytoplasm. Completion of this task should result in a much improved understanding of eukaryotic growth, differentiation, and carcinogenesis.

Membrane Binding Modulates the Quaternary Structure of CTP:Phosphocholine Cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCT), a key enzyme that controls phosphatidylcholine synthesis, is regulated by reversible interactions with membranes containing anionic lipids. Previous work demonstrated that CCT is a homodimer. In this work we show that the structure of the dimer interface is altered upon encountering membranes that activate CCT. Chemical cross-linking reactions were established which captured intradimeric interactions but not random CCT dimer collisions. The efficiency of capturing covalent cross-links with four different reagents was diminished markedly upon presentation of activating anionic lipid vesicles but not zwitterionic vesicles. Experiments were conducted to show that the anionic vesicles did not interfere with the chemistry of the cross-linking reactions and did not sequester available cysteine sites on CCT for reaction with the cysteine-directed cross-linking reagent. Thus, the loss of cross-linking efficiency suggested that contact sites at the dimer interface had increased distance or reduced flexibility upon binding of CCT to membranes. The regions of the enzyme involved in dimerization were mapped using three approaches: 1) limited proteolysis followed by cross-linking of fragments, 2) yeast two-hybrid analysis of interactions between select domains, and 3) disulfide bonding potential of CCTs with individual cysteine to serine substitutions for the seven native cysteines. We found that the N-terminal domain (amino acids 1-72) is an important participant in forming the dimer interface, in addition to the catalytic domain (amino acids 73-236). We mapped the intersubunit disulfide bond to the cystine 37 pair in domain N and showed that this disulfide is sensitive to anionic vesicles, implicating this specific region in the membrane-sensitive dimer interface.

The glucose-regulated nuclear localization of hexokinase 2 in Saccharomyces cerevisiae is Mig1-dependent

Two major mediators of glucose repression in Saccharomyces cerevisiae are the proteins Mig1 and Hxk2. The mechanism of Hxk2-dependent glucose repression pathway is not well understood, but the Mig1-dependent part of the pathway has been elucidated in great detail. Here we report that Hxk2 has a glucose-regulated nuclear localization and that Mig1, a transcriptional repressor responsible for glucose repression of many genes, is required to sequester Hxk2 into the nucleus. Mig1 and Hxk2 interacted in vivo in a yeast two-hybrid assay and in vitro in immunoprecipitation and glutathione S-transferase pull-down experiments. We found that the Lys(6)-Met(15) decapeptide of Hxk2, which is necessary for nuclear localization of the protein, is also essential for interaction with the Mig1 protein. Our results also show that the Hxk2-Mig1 interaction is of physiological significance because both proteins have been found interacting together in a cluster with DNA fragments containing the MIG1 site of SUC2 promoter. We conclude that Hxk2 operates by interacting with Mig1 to generate a repressor complex located in the nucleus of S. cerevisiae during growth in glucose medium.

Carbon metabolite sensing and signalling

The regulation of carbon metabolism in plant cells responds sensitively to the levels of carbon metabolites that are available. The sensing and signalling systems that are involved in this process form a complex web that comprises metabolites, transporters, enzymes, transcription factors and hormones. Exactly which metabolites are sensed is not yet known, but candidates include sucrose, glucose and other hexoses, glucose-6-phosphate, trehalose-6-phosphate, trehalose and adenosine monophosphate. Important components of the signalling pathways include sucrose non-fermenting-1-related protein kinase-1 (SnRK1) and hexokinase; sugar transporters are also implicated. A battery of genes and enzymes involved in carbohydrate metabolism, secondary metabolism, nitrogen assimilation and photosynthesis are under the control of these pathways and fundamental developmental processes such as germination, sprouting, pollen development and senescence are affected by them. Here we review the current knowledge of carbon metabolite sensing and signalling in plants, drawing comparisons with homologous and analogous systems in animals and fungi. We also review the evidence for cross-talk between carbon metabolite and other major signalling systems in plant cells and the prospects for manipulating this fundamentally important aspect of metabolic regulation for crop improvement.

The unique hexokinase of Kluyveromyces lactis. Molecular and functional characterization and evaluation of a role in glucose signaling

The Crabtree-negative yeast Kluyveromyces lactis is capable of adjusting its glycolytic flux to the requirements of respiration by tightly regulating glucose uptake. RAG5 encoding the only glucose and fructose phosphorylating enzyme present in K. lactis is required for the up-regulation of glucose transport and also for glucose repression. To understand the significance of the molecular identity and specific function(s) of the corresponding kinase to glucose signaling, RAG5 was overexpressed and its gene product KlHxk1 (Rag5p) isolated and characterized. Stopped-flow kinetics and sedimentation analysis indicated a monomer-homodimer equilibrium of KlHxk1 in a condition of catalysis, i.e. in the presence of substrates and products. The kinetic constants of ATP-dependent glucose phosphorylation identified a 53-kDa monomer as the high affinity/high activity form of the novel enzyme for both glycolytic substrates suggesting a control of glucose phosphorylation at the level of dimer formation and dissociation. In contrast to the highly homologous hexokinase isoenzyme 2 of Saccharomyces cerevisiae (ScHxk2), KlHxk1 was not inhibited by free ATP in a physiological range of nucleotide concentration. Mass spectrometric sequencing of tryptic peptides of KlHxk1 identified unmodified serine at amino acid position 156. The corresponding amino acid in ScHxk2 is serine 157, which represents the autophosphorylation-inactivation site. KlHxk1 did not display, however, the typical pattern of inactivation under the respective in vitro conditions and maintained a high residual glucose phosphorylating activity. The biophysical and functional data are discussed with respect to a possible regulatory role of KlHxk1 in glucose metabolism and signaling in K. lactis.

Transcriptional control of nonfermentative metabolism in the yeast Saccharomyces cerevisiae

Although sugars are clearly the preferred carbon sources of the yeast Saccharomyces cerevisiae, nonfermentable substrates such as ethanol, glycerol, lactate, acetate or oleate can also be used for the generation of energy and cellular biomass. Several regulatory networks of glucose repression (carbon catabolite repression) are involved in the coordinate biosynthesis of enzymes required for the utilization of nonfermentable substrates. Positively and negatively acting complexes of pleiotropic regulatory proteins have been characterized. The Snf1 (Cat1) protein kinase complex, together with its regulatory subunit Snf4 (Cat3) and alternative beta-subunits Sip1, Sip2 or Gal83, plays an outstanding role for the derepression of structural genes which are repressed in the presence of a high glucose concentration. One molecular function of the Snf1 complex is deactivation by phosphorylation of the general glucose repressor Mig1. In addition to regulation of alternative sugar fermentation, Mig1 also influences activators of respiration and gluconeogenesis, although to a lesser extent. Snf1 is also required for conversion of specific regulatory factors into transcriptional activators. This review summarizes regulatory cis-acting elements of structural genes of the nonfermentative metabolism, together with the corresponding DNA-binding proteins (Hap2-5, Rtg1-3, Cat8, Sip4, Adr1, Oaf1, Pip2), and describes the molecular interactions among general regulators and pathway-specific factors. In addition to the influence of the carbon source at the transcriptional level, mechanisms of post-transcriptional control such as glucose-regulated stability of mRNA are also discussed briefly.

A Saccharomyces cerevisiae mutant unable to convert glucose to glucose-6-phosphate accumulates excessive glucose in the endoplasmic reticulum due to core oligosaccharide trimming

D-Glucose is the preferred carbon and energy source for most eukaryotic cells. Immediately following its uptake, glucose is rapidly phosphorylated to glucose-6-phosphate (Glc-6-P). The yeast Saccharomyces cerevisiae has three enzymes (Hxk1p, Hxk2p, and Glk1p) that convert glucose to Glc-6-P. In the present study, we found that yeast mutants lacking any two of these enzymes retain the ability to efficiently convert glucose to Glc-6-P and thus maintain a low level of cellular glucose. However, a mutant strain lacking all three glucose-phosphorylating enzymes contained up to 225-fold more intracellular glucose than normal. Drugs that inhibit the synthesis or the trimming of the lipid-linked core oligosaccharide Glu(3)Man(9)GlcNac(2) effectively reduced the accumulation of glucose. Similarly, mutations that block the addition of glucose residues to the core oligosaccharide moiety, such as alg5Delta or alg6Delta, also diminished glucose accumulation. These results indicate that the intracellular glucose accumulation observed in the glucose phosphorylation mutant results primarily from the trimming of glucose residues from core oligosaccharide chains within the endoplasmic reticulum (ER). Consistent with this conclusion, both [(14)C]glucose exchange and subcellular fractionation experiments indicate that much of the accumulated glucose is retained within an intracellular compartment, suggesting that the efficient transport of glucose from the ER to the cytosol in yeast may be coupled to its rephosphorylation to Glc-6-P. The high level of cellular glucose was associated with an increased level of protein glycation and the release of glucose into the culture medium via its transit through the secretory pathway. Finally, we also found that the accumulation of glucose may lead to a subtle alteration in ion homeostasis, particularly Ca(2+) uptake. This suggests that this mutant strain may serve as a useful model to study the consequences of excessive glucose accumulation and protein glycation.

Calorimetric determination of thermodynamic parameters of reaction reveals different enthalpic compensations of the yeast hexokinase isozymes

The change in enthalpy and rate constants for the reactions of yeast hexokinase isozymes, PI (Hxk1) and PII (Hxk2), was determined at pH 7.6 and 25 degrees C by isothermal titration calorimetry. The reactions were done in five buffer systems with enthalpy of protonation varying from -1.22 kcal/mol (phosphate) to -11.51 kcal/mol (Tris), allowing the determination of the number of protons released during glucose phosphorylation. The reaction is exothermic for both isozymes with a small, but significant (p < 0.0001), difference in the enthalpy of reaction (Delta HR), with an Delta HR of -5.1 +/- 0.2 (mean +/- S.D.) kcal/mol for Hxk1, and an Delta HR of -3.3 +/- 0.3 (mean +/- S.D.) kcal/mol for Hxk2. The Km for ATP determined by ITC was very similar to those reported in the literature for both isozymes. The effect of NaCl and KCl, from 0 to 200 mM, showed that although the rate of reaction decreases with increasing ionic strength, no change in the Delta HR was observed suggesting an entropic nature for the ionic strength. The differences in Delta HR obtained here for both isozymes strongly suggest that, besides glucose phosphorylation, another side reaction such as ATP hydrolysis and/or enzyme phosphorylation is taking place.

Exportin 1-independent nuclear export of GAPDH

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key enzyme of the glycolytic pathway. Recent studies have demonstrated an additional role in apoptosis: GAPDH is targeted to the nucleus during apoptotic signalling. This nuclear transport has also been observed in serum-depleted cells, but it is reversible in fibroblasts, in contrast to apoptotic-induced transport (Eur J Cell Biol 80 (2001) 419). Here, we analyse the serum depletion-induced transport processes of GAPDH in NIH 3T3 cells. Prolonged serum depletion did not cause cell death, nuclear fragmentation (hoechst staining) or a significant increase in DNA strand-breaks (comet assay). Using cells expressing green fluorescent protein (GFP)-tagged GAPDH allowed us to monitor its intracellular localisation by confocal laser scanning microscopy (CLSM). Treatment of cells with the exportin1 inhibitor leptomycin B (LMB) did not influence cytoplasmic localisation of GFP-GAPDH, indicating that nuclear targeting of GAPDH is not constitutive and may be altered via a serum-dependent regulatory export process. Suprisingly, the export of nuclear GFP-GAPDH after re-addition of serum to starved cells was not prevented by LMB. Thus, nuclear export of GAPDH upon serum depletion is not mediated by exportin1.

Relationship between protein kinase C and derepression of different enzymes

The PKC1 gene in the yeast Saccharomyces cerevisiae encodes for protein kinase C which is known to control a MAP kinase cascade consisting of different kinases: Bck1, Mkk1 and Mkk2, and Mpk1. This cascade affects the cell wall integrity but the phenotype of pkc1Delta mutants suggests additional targets that have not yet been identified [Heinisch et al., Mol. Microbiol. 32 (1999) 671-680]. The pkc1Delta mutant, as opposed to other mutants in the MAP kinase cascade, displays defects in the control of carbon metabolism. One of them occurs in the derepression of SUC2 gene after exhaustion of glucose from the medium, suggesting an involvement of Pkc1p in the derepression process that is not shared by the downstream MAP kinase cascade. In this work, we demonstrate that Pkc1p is required for the increase of the activity of enzymatic systems during the derepression process. We observed that Pkc1p is involved in the derepression of invertase and alcohol dehydrogenase activities. On the other hand, it seems not to be necessary for the derepression of the enzymes of the GAL system. Our results suggest that Pkc1p is acting through the main glucose repression pathway, since introduction of an additional mutation in the PKC1 gene in yeast strains already presenting mutations in the HXKII or MIG1 genes does not interfere with the typical derepressed phenotype observed in these single mutants. Moreover, our data indicate that Pkc1p participates in this process through the control of the cellular localization of the Mig1 transcriptional factor.

The sensing of nutritional status and the relationship to filamentous growth in Saccharomyces cerevisiae

Heterotrophic organisms rely on the ingestion of organic molecules or nutrients from the environment to sustain energy and biomass production. Non-motile, unicellular organisms have a limited ability to store nutrients or to take evasive action, and are therefore most directly dependent on the availability of nutrients in their immediate surrounding. Such organisms have evolved numerous developmental options in order to adapt to and to survive the permanently changing nutritional status of the environment. The phenotypical, physiological and molecular nature of nutrient-induced cellular adaptations has been most extensively studied in the yeast Saccharomyces cerevisiae. These studies have revealed a network of sensing mechanisms and of signalling pathways that generate and transmit the information on the nutritional status of the environment to the cellular machinery that implements specific developmental programmes. This review integrates our current knowledge on nutrient sensing and signalling in S. cerevisiae, and suggests how an integrated signalling network may lead to the establishment of a specific developmental programme, namely pseudohyphal differentiation and invasive growth.

Integrated bioprocessing in Saccharomyces cerevisiae using green fluorescent protein as a fusion partner

In this study, we examine the use of green fluorescent protein (GFP) for monitoring a hexokinase (HXK)-GFP fusion protein in Saccharomyces cerevisiae for various events including expression, degradation, purification, and localization. The fusion, HXK-EK-GFP-6 x His, was constructed where the histidine tag (6 x His) would allow for convenient affinity purification, and the enterokinase (EK) cleavage site would be used for separation of HXK from GFP after affinity purification. Our results showed that both HXK and GFP remained active in the fusion and, more importantly, that there was a linear correlation between HXK activity and GFP fluorescence. Enterokinase cleavage studies revealed that both GFP fluorescence intensity and HXK activity remained unchanged after separation of the fusion proteins, which indicated that fusion of GFP did not cause structural alteration of HXK and thus did not affect the enzymatic activity of HXK. We also found that degradation of the fusion protein occurred, and that degradation was limited to HXK with GFP remaining intact in the fusion. Confocal microscopy studies showed that while GFP was distributed evenly in the yeast cytosol, HXK-GFP fusion followed the correct localization of HXK, which resulted in a di-localization of both cytosol and the nucleus. GFP proved to be a useful fusion partner that may lead to the possibility of integrating the bioprocesses by quantitatively following the entire process visually.

Mediator factor Med8p interacts with the hexokinase 2: implication in the glucose signalling pathway of Saccharomyces cerevisiae

In the presence of glucose the protein hexokinase 2 (Hxk2p), normally resident in the cytosol, is translocated to the nucleus where it impairs the activation of transcription of the glucose-repressed genes HXK1, GLK1 and SUC2, and promotes the activation of transcription of the glucose-induced genes HXK2 and HXT1. Here, we demonstrate the involvement of an heptameric motif, named the MED8 site, in the direct binding of the mediator protein Med8p, either as a monomer or as a homodimer. Because this site was previously involved in the Hxk2p-dependent glucose-induced regulation of gene transcription, we tested whether Hxk2p interacts with Med8p. Our results show that Hxk2 and Med8 proteins are physically associated and that this Hxk2p-Med8p interaction is of physiological significance because both proteins have been found interacting together in a cluster with DNA fragments containing the MED8 site. We conclude that Hxk2p operates through the MED8 site, by interacting with Med8p, in the glucose signal transduction pathway of Saccharomyces cerevisiae.

Hexose phosphorylation and the putative calcium channel component Mid1p are required for the hexose-induced transient elevation of cytosolic calcium response in Saccharomyces cerevisiae

Saccharomyces cerevisiae responds to environ-mental stimuli such as an exposure to pheromone or to hexoses after carbon source limitation with a transient elevation of cytosolic calcium (TECC) response. In this study, we examined whether hexose transport and phosphorylation are necessary for the TECC response. We found that a mutant strain lacking most of the known hexose transporters was unable to carry out the TECC response when exposed to glucose. A mutant strain that lacked the ability to phosphorylate glucose was unable to respond to glucose addition, but displayed a normal TECC response after the addition of galactose. These results indicate that hexose uptake and phosphorylation are required to trigger the hexose-induced TECC response. We also found that the TECC response was significantly smaller than normal when the level of environmental calcium was reduced, and was abolished in a mid1 mutant that lacked a subunit of the high-affinity calcium channel of the yeast plasma membrane. These results indicate that most or all of the TECC response is mediated by an influx of calcium from the extracellular space. Our results indicate that this transient increase in plasma membrane calcium permeability may be linked to the accumulation of Glc-1-P (or a related glucose metabolite) in yeast.

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.

Sugar repression in the methylotrophic yeast Hansenula polymorpha studied by using hexokinase-negative, glucokinase-negative and double kinase-negative mutants

Two glucose-phosphorylating enzymes, a hexokinase phosphorylating both glucose and fructose, and a glucose-specific glucokinase were electrophoretically separated in the methylotrophic yeast Hansenula polymorpha. Hexokinase-negative, glucokinase-negative and double kinase-negative mutants were isolated in H. polymorpha by using mutagenesis, selection and genetic crosses. Regulation of synthesis of the sugar-repressed alcohol oxidase, catalase and maltase was studied in different hexose kinase mutants. In the wild type and in mutants possessing either hexokinase or glucokinase, glucose repressed the synthesis of maltase, alcohol oxidase and catalase. Glucose repression of alcohol oxidase and catalase was abolished in mutants lacking both glucose-phosphorylating enzymes (i.e. in double kinase-negative mutants). Thus, glucose repression in H. polymorpha cells requires a glucose-phosphorylating enzyme, either hexokinase or glucokinase. The presence of fructose-phosphorylating hexokinase in the cell was specifically needed for fructose repression of alcohol oxidase, catalase and maltase. Hence, glucose or fructose has to be phosphorylated in order to cause repression of the synthesis of these enzymes in H. polymorpha suggesting that sugar repression in this yeast therefore relies on the catalytic activity of hexose kinases.