Evidence for trehalose-6-phosphate-dependent and -independent mechanisms in the control of sugar influx into yeast glycolysis

Short-term metabolome dynamics and carbon, electron, and ATP balances in chemostat-grown Saccharomyces cerevisiae CEN.PK 113-7D following a glucose pulse

The in vivo kinetics in Saccharomyces cerevisiae CEN.PK 113-7D was evaluated during a 300-second transient period after applying a glucose pulse to an aerobic, carbon-limited chemostat culture. We quantified the responses of extracellular metabolites, intracellular intermediates in primary metabolism, intracellular free amino acids, and in vivo rates of O(2) uptake and CO(2) evolution. With these measurements, dynamic carbon, electron, and ATP balances were set up to identify major carbon, electron, and energy sinks during the postpulse period. There were three distinct metabolic phases during this time. In phase I (0 to 50 seconds after the pulse), the carbon/electron balances closed up to 85%. The accumulation of glycolytic and storage compounds accounted for 60% of the consumed glucose, caused an energy depletion, and may have led to a temporary decrease in the anabolic flux. In phase II (50 to 150 seconds), the fermentative metabolism gradually became the most important carbon/electron sink. In phase III (150 to 300 seconds), 29% of the carbon uptake was not identified in the measurements, and the ATP balance had a large surplus. These results indicate an increase in the anabolic flux, which is consistent with macroscopic balances of extracellular fluxes and the observed increase in CO(2) evolution associated with nonfermentative metabolism. The identified metabolic processes involving major carbon, electron, and energy sinks must be taken into account in in vivo kinetic models based on short-term dynamic metabolome responses.

In vivo kinetics with rapid perturbation experiments in Saccharomyces cerevisiae using a second-generation BioScope

We present a robust second-generation BioScope: a system for continuous perturbation experiments. Firstly, the BioScope design parameters (i.e., pressure drop, overall oxygen (O2) and carbon dioxide (CO2) mass transfer, mean residence time distribution and plug flow characteristics) were evaluated. The average overall mass transfer coefficients were estimated to be 1.8E-5 m s(-1) for O2 and 0.34E-5 m s(-1) for CO2. It was determined that the O2/CO2 permeable membrane accounted for 75% and 95% of the overall resistance for O2 and CO2, respectively. The Peclet number (Pe) of the system was found to be >500 for liquid flow rates between 1 and 4 ml min(-1), ensuring plug flow characteristics. Secondly, steady-state intracellular metabolite concentrations obtained using direct rapid sampling from the fermentor were compared with those obtained by rapid sampling via the pre-perturbation sample port of the BioScope. With both methods the same metabolite levels were obtained. Thirdly, glucose perturbation experiments were carried out directly in the fermentor as well as in the BioScope, whereby steady-state Saccharomyces cerevisiae cells from a glucose/ethanol limited chemostat were perturbed by increasing the extracellular glucose concentration from 0.11 to 2.8 mM. Intracellular and extracellular metabolite levels were measured within a time window of 180 s. It was observed that the dynamic metabolite concentration profiles obtained from both perturbations were nearly the same, with the exception of the C4 metabolites of the TCA cycle, which might be due to differences in culture age.

The importance of a functional trehalose biosynthetic pathway for the life of yeasts and fungi

The view of the role of trehalose in yeast has changed in the last few years. For a long time considered a reserve carbohydrate, it gained new importance when its function in the acquisition of thermotolerance was demonstrated. More recently the cellular processes in which the trehalose biosynthetic pathway has been implicated range from the control of glycolysis to sporulation and infectivity by certain fungal pathogens. There is now enough experimental evidence to conclude that trehalose 6-phosphate, an intermediate of trehalose biosynthesis, is an important metabolic regulator in such different organisms as yeasts or plants. Its inhibition of hexokinase plays a key role in the control of the glycolytic flux in Saccharomyces cerevisiae but other, likely important, sites of action are still unknown. We present examples of the phenotypes produced by mutations in the two steps of the trehalose biosynthetic pathway in different yeasts and fungi, and whenever possible examine the molecular explanations advanced to interpret them.

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.

Uncoupling of the glucose growth defect and the deregulation of glycolysis in Saccharomyces cerevisiae Tps1 mutants expressing trehalose-6-phosphate-insensitive hexokinase from Schizosaccharomyces pombe

In the yeast Saccharomyces cerevisiae inactivation of trehalose-6-phosphate (Tre6P) synthase (Tps1) encoded by the TPS1 gene causes a specific growth defect in the presence of glucose in the medium. The growth inhibition is associated with deregulation of the initial part of glycolysis. Sugar phosphates, especially fructose-1,6-bisphosphate (Fru1,6bisP), hyperaccumulate while the levels of ATP, Pi and downstream metabolites are rapidly depleted. This was suggested to be due to the absence of Tre6P inhibition on hexokinase. Here we show that overexpression of Tre6P (as well as glucose-6-phosphate (Glu6P))-insensitive hexokinase from Schizosaccharomyces pombe in a wild-type strain does not affect growth on glucose but still transiently enhances initial sugar phosphate accumulation. We have in addition replaced the three endogenous glucose kinases of S. cerevisiae by the Tre6P-insensitive hexokinase from S. pombe. High hexokinase activity was measured in cell extracts and growth on glucose was somewhat reduced compared to an S. cerevisiae wild-type strain but expression of the Tre6P-insensitive S. pombe hexokinase never caused the typical tps1Delta phenotype. Moreover, deletion of TPS1 in this strain expressing only the Tre6P-insensitive S. pombe hexokinase still resulted in a severe drop in growth capacity on glucose as well as sensitivity to millimolar glucose levels in the presence of excess galactose. In this case, poor growth on glucose was associated with reduced rather than enhanced glucose influx into glycolysis. Initial glucose transport was not affected. Apparently, deletion of TPS1 causes reduced activity of the S. pombe hexokinase in vivo. Our results show that Tre6P inhibition of hexokinase is not the major mechanism by which Tps1 controls the influx of glucose into glycolysis or the capacity to grow on glucose. In addition, they show that a Tre6P-insensitive hexokinase can still be controlled by Tps1 in vivo.

Trehalose 6-phosphate is indispensable for carbohydrate utilization and growth in Arabidopsis thaliana

Genes for trehalose metabolism are widespread in higher plants. Insight into the physiological role of the trehalose pathway outside of resurrection plant species is lacking. To address this lack of insight, we express Escherichia coli genes for trehalose metabolism in Arabidopsis thaliana, which manipulates trehalose 6-phosphate (T6P) contents in the transgenic plants. Plants expressing otsA [encoding trehalose phosphate synthase (TPS)] accumulate T6P whereas those expressing either otsB [encoding trehalose phosphate phosphatase (TPP)] or treC [encoding trehalose phosphate hydrolase (TPH)] contain low levels of T6P. Expression of treF (encoding trehalase) yields plants with unaltered T6P content and a phenotype not distinguishable from wild type when grown on soil. The marked phenotype obtained of plants accumulating T6P is opposite to that of plants with low T6P levels obtained by expressing either TPP or TPH and consistent with a critical role for T6P in growth and development. Supplied sugar strongly inhibits growth of plants with reduced T6P content and leads to accumulation of respiratory intermediates. Remarkably, sugar improves growth of TPS expressors over wild type, a feat not previously accomplished by manipulation of metabolism. The data indicate that the T6P intermediate of the trehalose pathway controls carbohydrate utilization and thence growth via control of glycolysis in a manner analogous to that in yeast. Furthermore, embryolethal A. thaliana tps1 mutants are rescued by expression of E. coli TPS, but not by supply of trehalose, suggesting that T6P control over primary metabolism is indispensable for development.

Optimal growth and ethanol production from xylose by recombinant Saccharomyces cerevisiae require moderate D-xylulokinase activity

D-Xylulokinase (XK) is essential for the metabolism of D-xylose in yeasts. However, overexpression of genes for XK, such as the Pichia stipitis XYL3 gene and the Saccharomyces cerevisiae XKS gene, can inhibit growth of S. cerevisiae on xylose. We varied the copy number and promoter strength of XYL3 or XKS1 to see how XK activity can affect xylose metabolism in S. cerevisiae. The S. cerevisiae genetic background included single integrated copies of P. stipitis XYL1 and XYL2 driven by the S. cerevisiae TDH1 promoter. Multicopy and single-copy constructs with either XYL3 or XKS1, likewise under control of the TDH1 promoter, or with the native P. stipitis promoter were introduced into the recombinant S. cerevisiae. In vitro enzymatic activity of XK increased with copy number and promoter strength. Overexpression of XYL3 and XKS1 inhibited growth on xylose but did not affect growth on glucose even though XK activities were three times higher in glucose-grown cells. Growth inhibition increased and ethanol yields from xylose decreased with increasing XK activity. Uncontrolled XK expression in recombinant S. cerevisiae is inhibitory in a manner analogous to the substrate-accelerated cell death observed with an S. cerevisiae tps1 mutant during glucose metabolism. To bypass this effect, we transformed cells with a tunable expression vector containing XYL3 under the control of its native promoter into the FPL-YS1020 strain and screened the transformants for growth on, and ethanol production from, xylose. The selected transformant had approximately four copies of XYL3 per haploid genome and had moderate XK activity. It converted xylose into ethanol efficiently.

Carbon metabolite feedback regulation of leaf photosynthesis and development

Photosynthesis is regulated as a two-way process. Light regulates the expression of genes for photosynthesis and the activity of the gene products (feedforward control). Rate of end-product use down-stream of the Calvin cycle, determined largely by nutrition and temperature, also affects photosynthetic activity and photosynthetic gene expression (feedback control). Whereas feedforward control ensures efficient light use, feedback mechanisms ensure that carbon flow is balanced through the pathways that produce and consume carbon, so that inorganic phosphate is recycled and nitrogen is distributed optimally to different processes to ensure growth and survival. Actual mechanisms are sketchy and complex, but carbon to nitrogen balance rather than carbon status per se is central to understanding carbon metabolite feedback control of photosynthesis. In addition to determining the activity of the metabolic machinery, carbon metabolite feedback mechanisms also regulate photosynthesis at the leaf level through the regulation of leaf development. This review summarizes the current sketchy, but growing, knowledge of the mechanisms through which carbon metabolite feedback mechanisms regulate leaf photosynthesis.

Disruption of the Candida albicans TPS2 gene encoding trehalose-6-phosphate phosphatase decreases infectivity without affecting hypha formation

Deletion of trehalose-6-phosphate phosphatase, encoded by TPS2, in Saccharomyces cerevisiae results in accumulation of trehalose-6-phosphate (Tre6P) instead of trehalose under stress conditions. Since trehalose is an important stress protectant and Tre6P accumulation is toxic, we have investigated whether Tre6P phosphatase could be a useful target for antifungals in Candida albicans. We have cloned the C. albicans TPS2 (CaTPS2) gene and constructed heterozygous and homozygous deletion strains. As in S. cerevisiae, complete inactivation of Tre6P phosphatase in C. albicans results in 50-fold hyperaccumulation of Tre6P, thermosensitivity, and rapid death of the cells after a few hours at 44 degrees C. As opposed to inactivation of Tre6P synthase by deletion of CaTPS1, deletion of CaTPS2 does not affect hypha formation on a solid glucose-containing medium. In spite of this, virulence of the homozygous deletion mutant is strongly reduced in a mouse model of systemic infection. The pathogenicity of the heterozygous deletion mutant is similar to that of the wild-type strain. CaTPS2 is a new example of a gene not required for growth under standard conditions but required for pathogenicity in a host. Our results suggest that Tre6P phosphatase may serve as a potential target for antifungal drugs. Neither Tre6P phosphatase nor its substrate is present in mammals, and assay of the enzymes is simple and easily automated for high-throughput screening.

Metabolic engineering of Saccharomyces cerevisiae for xylose utilization

Metabolic engineering of Saccharomyces cerevisiae for ethanolic fermentation of xylose is summarized with emphasis on progress made during the last decade. Advances in xylose transport, initial xylose metabolism, selection of host strains, transformation and classical breeding techniques applied to industrial polyploid strains as well as modeling of xylose metabolism are discussed. The production and composition of the substrates--lignocellulosic hydrolysates--is briefly summarized. In a future outlook iterative strategies involving the techniques of classical breeding, quantitative physiology, proteomics, DNA micro arrays, and genetic engineering are proposed for the development of efficient xylose-fermenting recombinant strains of S. cerevisiae.

An unexpected plethora of trehalose biosynthesis genes in Arabidopsis thaliana

Trehalose accumulation has been documented in many organisms, such as bacteria and fungi, where it serves a storage and stress-protection role. Although conspicuously absent in most plants, trehalose biosynthesis genes were discovered recently in higher plants. We have uncovered a family of 11 TPS genes in Arabidopsis thaliana, one of which encodes a trehalose-6-phosphate (Tre6P) synthase, and a subfamily of which might encode the still elusive Tre6P phosphatases. A regulatory role in carbon metabolism is likely but might not be restricted to the TPS control of hexokinase activity as documented for yeast. Incompatibility between high trehalose levels and chaperone-assisted protein folding might be a reason why plants have evolved to accumulate some alternative stress-protection compounds to trehalose.

Trehalose is required for the acquisition of tolerance to a variety of stresses in the filamentous fungus Aspergillus nidulans

Trehalose is a non-reducing disaccharide found at high concentrations in Aspergillus nidulans conidia and rapidly degraded upon induction of conidial germination. Furthermore, trehalose is accumulated in response to a heat shock or to an oxidative shock. The authors have characterized the A. nidulans tpsA gene encoding trehalose-6-phosphate synthase, which catalyses the first step in trehalose biosynthesis. Expression of tpsA in a Saccharomyces cerevisiae tps1 mutant revealed that the tpsA gene product is a functional equivalent of the yeast Tps1 trehalose-6-phosphate synthase. The A. nidulans tpsA-null mutant does not produce trehalose during conidiation or in response to various stress conditions. While germlings of the tpsA mutant show an increased sensitivity to moderate stress conditions (growth at 45 degrees C or in the presence of 2 mM H(2)O(2)), they display a response to severe stress (60 min at 50 degrees C or in the presence of 100 mM H(2)O(2)) similar to that of wild-type germlings. Furthermore, conidia of the tpsA mutant show a rapid loss of viability upon storage. These results are consistent with a role of trehalose in the acquisition of stress tolerance. Inactivation of the tpsA gene also results in increased steady-state levels of sugar phosphates but does not prevent growth on rapidly metabolizable carbon sources (glucose, fructose) as seen in Saccharomyces cerevisiae. This suggests that trehalose 6-phosphate is a physiological inhibitor of hexokinase but that this control is not essential for proper glycolytic flux in A. nidulans. Interestingly, tpsA transcription is not induced in response to heat shock or during conidiation, indicating that trehalose accumulation is probably due to a post-translational activation process of the trehalose 6-phosphate synthase.

Reserve carbohydrates metabolism in the yeast Saccharomyces cerevisiae

Glycogen and trehalose are the two glucose stores of yeast cells. The large variations in the cell content of these two compounds in response to different environmental changes indicate that their metabolism is controlled by complex regulatory systems. In this review we present information on the regulation of the activity of the enzymes implicated in the pathways of synthesis and degradation of glycogen and trehalose as well as on the transcriptional control of the genes encoding them. cAMP and the protein kinases Snf1 and Pho85 appear as major actors in this regulation. From a metabolic point of view, glucose-6-phosphate seems the major effector in the net synthesis of glycogen and trehalose. We discuss also the implication of the recently elucidated TOR-dependent nutrient signalling pathway in the control of the yeast glucose stores and its integration in growth and cell division. The unexpected roles of glycogen and trehalose found in the control of glycolytic flux, stress responses and energy stores for the budding process, demonstrate that their presence confers survival and reproductive advantages to the cell. The findings discussed provide for the first time a teleonomic value for the presence of two different glucose stores in the yeast cell.

Reconstitution of ethanolic fermentation in permeabilized spheroplasts of wild-type and trehalose-6-phosphate synthase mutants of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, TPS1-encoded trehalose-6-phosphate synthase (TPS) exerts an essential control on the influx of glucose into glycolysis, presumably by restricting hexokinase activity. Deletion of TPS1 results in severe hyperaccumulation of sugar phosphates and near absence of ethanol formation. To investigate whether trehalose 6-phosphate (Tre6P) is the sole mediator of hexokinase inhibition, we have reconstituted ethanolic fermentation from glucose in permeabilized spheroplasts of the wild-type, tps1Delta and tps2Delta (Tre6P phosphatase) strains. For the tps1Delta strain, ethanol production was significantly lower and was associated with hyperaccumulation of Glu6P and Fru6P. A tps2Delta strain shows reduced accumulation of Glu6P and Fru6P both in intact cells and in permeabilized spheroplasts. These results are not consistent with Tre6P being the sole mediator of hexokinase inhibition. Reconstitution of ethanolic fermentation in permeabilized spheroplasts with glycolytic intermediates indicates additional target site(s) for the Tps1 control. Addition of Tre6P partially shifts the ethanol production rate and the metabolite pattern in permeabilized tps1Delta spheroplasts to those of the wild-type strain, but only with glucose as substrate. This is observed at a very high ratio of glucose to Tre6P. Inhibition of hexokinase activity by Tre6P is less efficiently counteracted by glucose in permeabilized spheroplasts compared to cell extracts, and this effect is largely abolished by deletion of TPS2 but not TPS1. In permeabilized spheroplasts, hexokinase activity is significantly lower in a tps2Delta strain compared to a wild-type strain and this difference is strongly reduced by additional deletion of TPS1. These results indicate that Tps1-mediated protein-protein interactions are important for control of glucose influx into yeast glycolysis, that Tre6P inhibition of hexokinase might not be competitive with respect to glucose in vivo and that also Tps2 appears to play a role in the control of hexokinase activity.

Improved protocols for quantitative determination of metabolites from biological samples using high performance ionic-exchange chromatography with conductimetric and pulsed amperometric detection

Simple and reliable protocols are described for an extensive analysis of metabolites in extracts from different biological sources. The separation was performed by high performance ionic-exchange chromatography (HPIC) at alkaline pH using two types of chromatography columns and two detection methods. Organic acids and inorganic anions were separated on an ionPac AS11 column using a 0.5 to 35 mM Na0H gradient. Detection limits in the range of milligrams per liter were achieved by use of a conductivity detector equipped with an anion self-regenerating suppressor. Twelve phosphorylated compounds belonging to the glycolytic and the pentose phosphate pathways could be resolved on a CarboPac PA1 column using a Na0H/Na-acetate gradient. Quantification was achieved by pulsed amperometry with detection limits in the micromolar range. Cell extracts obtained by extraction in boiling buffered ethanol described previously could be directly injected onto HPIC columns for the separation of metabolites because the extraction procedure affected neither the retention time nor the stability of most of the metabolites, and yielded very clean chromatograms. These improved protocols were applied for a dynamic analysis of intracellular metabolites in Saccharomyces cerevisiae in response to a glucose pulse.

Compartmentation protects trypanosomes from the dangerous design of glycolysis

Unlike in other organisms, in trypanosomes and other Kinetoplastida the larger part of glycolysis takes place in a specialized organelle, called the glycosome. At present it is impossible to remove the glycosome without changing much of the rest of the cell. It would seem impossible, therefore, to assess the metabolic consequences of this compartmentation. Therefore, we here develop a computer experimentation approach, which we call computational cell biology. A validated molecular kinetic computer replica was built of glycolysis in the parasite Trypanosoma brucei. Removing the glycosome membrane in that replica had little effect on the steady-state flux, which argues against the prevalent speculation that glycosomes serve to increase flux by concentrating the enzymes. Removal of the membrane did cause (i) the sugar phosphates to rise to unphysiologically high levels, which must have pathological effects, and (ii) a failure to recover from glucose deprivation. We explain these effects on the basis of the biochemical organization of the glycosome. We conclude (i) that the glycosome protects trypanosomes from the negative side effects of the "turbo" structure of glycolysis and (ii) that computer experimentation based on solid molecular data is a powerful tool to address questions that are not, or not yet, accessible to experimentation.

Trehalose hydrolysis is not required for human serum-induced dimorphic transition in Candida albicans: evidence from a tps1/tps1 mutant deficient in trehalose synthesis

Exponential yeast-like cells of a Candida albicans wild-type strain exhibited strong capacity for germ tube formation in a glucose-containing medium (YPD) after induction with human serum at 37 degrees C, whereas the isogenic double disruptant tps1/tps1 mutant, which is deficient in trehalose synthesis, failed to produce germ tubes. In a medium without glucose (YP), the morphological transition fraction was roughly equivalent in both strains. Substitution of glucose by galactose or glycerol increased the number of wild-type proliferating cells able to enter the dimorphic program with no noticeable change in their trehalose content, while stationary cells, which accumulate a large amount of trehalose, did not form germ tubes. When fresh medium was added, a high proportion of these resting cells recovered their ability to carry out dimorphic transition. The tps1/tps1 mutant followed the same pattern of hyphae formation, despite the fact that it was unable to accumulate trehalose either during dimorphism induction or after several stress challenges. Furthermore, trehalose-6-phosphate synthase activity was barely detectable in the mutant. These results strongly suggest that serum-induced dimorphic transition does not require trehalose mobilization; they also support the idea that TPS1 is the only activity involved in trehalose biosynthesis in C. albicans.

A Selaginella lepidophylla trehalose-6-phosphate synthase complements growth and stress-tolerance defects in a yeast tps1 mutant

The accumulation of the disaccharide trehalose in anhydrobiotic organisms allows them to survive severe environmental stress. A plant cDNA, SlTPS1, encoding a 109-kD protein, was isolated from the resurrection plant Selaginella lepidophylla, which accumulates high levels of trehalose. Protein-sequence comparison showed that SlTPS1 shares high similarity to trehalose-6-phosphate synthase genes from prokaryotes and eukaryotes. SlTPS1 mRNA was constitutively expressed in S. lepidophylla. DNA gel-blot analysis indicated that SlTPS1 is present as a single-copy gene. Transformation of a Saccharomyces cerevisiae tps1Delta mutant disrupted in the ScTPS1 gene with S. lepidophylla SlTPS1 restored growth on fermentable sugars and the synthesis of trehalose at high levels. Moreover, the SlTPS1 gene introduced into the tps1Delta mutant was able to complement both deficiencies: sensitivity to sublethal heat treatment at 39 degrees C and induced thermotolerance at 50 degrees C. The osmosensitive phenotype of the yeast tps1Delta mutant grown in NaCl and sorbitol was also restored by the SlTPS1 gene. Thus, SlTPS1 protein is a functional plant homolog capable of sustaining trehalose biosynthesis and could play a major role in stress tolerance in S. lepidophylla.

Composition and functional analysis of the Saccharomyces cerevisiae trehalose synthase complex

In the yeast Saccharomyces cerevisiae, trehalose-6-phosphate synthase (TPS) and trehalose-6-phosphate phosphatase (TPP), which convert glucose 6-phosphate plus UDP-glucose to trehalose, are part of the trehalose synthase complex. In addition to the TPS1 (previously also called GGS1, CIF1, BYP1, FDP1, GLC6, and TSS1) and TPS2 (also described as HOG2 and PFK3) gene products, this complex also contains a regulatory subunit encoded by TSL1. We have constructed a set of isogenic strains carrying all possible combinations of deletions of these three genes and of TPS3, a homologue of TSL1 identified by systematic sequencing. Deletion of TPS1 totally abolished TPS activity and measurable trehalose, whereas deletion of any of the other genes in most cases reduced both. Similarly, deletion of TPS2 completely abolished TPP activity, and deletion of any of the other genes resulted in a reduction of this activity. Therefore, it appears that all subunits are required for optimal enzymatic activity. Since we observed measurable trehalose in strains lacking all but the TPS1 gene, some phosphatase activity in addition to Tps2 can hydrolyze trehalose 6-phosphate. Deletion of TPS3, in particular in a tsl1Delta background, reduced both TPS and TPP activities and trehalose content. Deletion of TPS2, TSL1, or TPS3 and, in particular, of TSL1 plus TPS3 destabilized the trehalose synthase complex. We conclude that Tps3 is a fourth subunit of the complex with functions partially redundant to those of Tsl1. Among the four genes studied, TPS1 is necessary and sufficient for growth on glucose and fructose. Even when overproduced, none of the other subunits could take over this function of Tps1 despite the homology shared by all four proteins. A portion of Tps1 appears to occur in a form not bound by the complex. Whereas TPS activity in the complex is inhibited by Pi, Pi stimulates the monomeric form of Tps1. We discuss the possible role of differentially regulated Tps1 in a complex-bound or monomeric form in light of the requirement of Tps1 for trehalose production and for growth on glucose and fructose.

Thermotolerance in Saccharomyces cerevisiae: the Yin and Yang of trehalose

Trehalose, a sugar produced by a wide variety of organisms, has long been known for its role in protecting certain organisms from desiccation. Recent work in yeast indicates that trehalose also promotes survival under conditions of extreme heat, by enabling proteins to retain their native conformation at elevated temperatures and suppressing the aggregation of denatured proteins. The latter property, however, seems to impair the recovery of cells from heat shock if they fail to degrade trehalose after the stress has passed. These multiple effects of trehalose on protein stability and folding suggest a host of promising applications.

Starting up yeast glycolysis

Successfully igniting the yeast glycolytic flux during the transition from gluconeogenic to fermentative growth seems to be a matter of balance and coordination between a multitude of events. The contours of the sugar sensing and signalling pathways that regulate this transition are only beginning to emerge.

The danger of metabolic pathways with turbo design

Many catabolic pathways begin with an ATP-requiring activation step, after which further metabolism yields a surplus of ATP. Such a 'turbo' principle is useful but also contains an inherent risk. This is illustrated by a detailed kinetic analysis of a paradoxical Saccharomyces cerevisiae mutant; the mutant fails to grow on glucose because of overactive initial enzymes of glycolysis, but is defective only in an enzyme (trehalose 6-phosphate synthase) that appears to have little relevance to glycolysis. The ubiquity of pathways that possess an initial activation step, suggests that there might be many more genes that, when deleted, cause rather paradoxical regulation phenotypes (i.e. growth defects caused by enhanced utilization of growth substrate).

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

We describe here that the HXK2 gene product, isoenzyme PII of hexokinase, is located in both the nucleus and the cytoplasm of Saccharomyces cerevisiae cells. This conclusion is supported by assays of hexokinase-specific activity in isolated nuclei from wild-type and hxk1lhxk2 double mutant strains, by immunoblot experiments using anti-Hxk2 antibodies and by observation of the fluorescence distribution of a Hxk2-GFP fusion protein in cells transformed with the HXK2::gfp gene.

Intracellular glucose concentration in derepressed yeast cells consuming glucose is high enough to reduce the glucose transport rate by 50%

In Saccharomyces cerevisiae cells exhibiting high-affinity glucose transport, the glucose consumption rate at extracellular concentrations above 10 mM was only half of the zero trans-influx rate. To determine if this regulation of glucose transport might be a consequence of intracellular free glucose we developed a new method to measure intracellular glucose concentrations in cells metabolizing glucose, which compares glucose stereoisomers to correct for adhering glucose. The intracellular glucose concentration was 1.5 mM, much higher than in most earlier reports. We show that for the simplest model of a glucose carrier, this concentration is sufficient to reduce the glucose influx by 50%. We conclude that intracellular glucose is the most likely candidate for the observed regulation of glucose import and hence glycolysis. We discuss the possibility that intracellular glucose functions as a primary signal molecule in these cells.

During the initiation of fermentation overexpression of hexokinase PII in yeast transiently causes a similar deregulation of glycolysis as deletion of Tps1

In the yeast Saccharomyces cerevisiae a novel control exerted by TPS1 (= GGS1 = FDP1 = BYP1 = CIF1 = GLC6 = TSS1)-encoded trehalose-6-phosphate synthase, is essential for restriction of glucose influx into glycolysis apparently by inhibiting hexokinase activity in vivo. We show that up to 50-fold overexpression of hexokinase does not noticeably affect growth on glucose or fructose in wild-type cells. However, it causes higher levels of glucose-6-phosphate, fructose-6-phosphate and also faster accumulation of fructose-1,6-bisphosphate during the initiation of fermentation. The levels of ATP and Pi correlated inversely with the higher sugar phosphate levels. In the first minutes after glucose addition, the metabolite pattern observed was intermediate between those of the tps1 delta mutant and the wild-type strain. Apparently, during the start-up of fermentation hexokinase is more rate-limiting in the first section of glycolysis than phosphofructokinase. We have developed a method to measure the free intracellular glucose level which is based on the simultaneous addition of D-glucose and an equal concentration of radiolabelled L-glucose. Since the latter is not transported, the free intracellular glucose level can be calculated as the difference between the total D-glucose measured (intracellular + periplasmic/extracellular) and the total L-glucose measured (periplasmic/extracellular). The intracellular glucose level rose in 5 min after addition of 100 mM-glucose to 0.5-2 mM in the wild-type strain, +/- 10 mM in a hxk1 delta hxk2 delta glk1 delta and 2-3 mM in a tps1 delta strain. In the strains overexpressing hexokinase PII the level of free intracellular glucose was not reduced. Overexpression of hexokinase PII never produced a strong effect on the rate of ethanol production and glucose consumption. Our results show that overexpression of hexokinase does not cause the same phenotype as deletion of Tps1. However, it mimics it transiently during the initiation of fermentation. Afterwards, the Tps1-dependent control system is apparently able to restrict properly up to 50-fold higher hexokinase activity.

Differential requirement of the yeast sugar kinases for sugar sensing in establishing the catabolite-repressed state

Addition of rapidly fermentable sugars to cells of the yeast Saccharomyces cerevisiae grown on nonfermentable carbon sources causes a variety of short-term and long-term regulatory effects, leading to an adaptation to fermentative metabolism. One important feature of this metabolic switch is the occurrence of extensive transcriptional repression of a large group of genes. We have investigated transcriptional regulation of the SUC2 gene encoding repressible invertase, and of HXK1, HXK2 and GLK1 encoding the three known yeast hexose kinases during transition from derepressed to repressed growth conditions. Comparing yeast strains that express various combinations of the hexose kinase genes, we have determined the importance of each of these kinases for establishing the catabolite-repressed state. We show that catabolite repression involves two distinct mechanisms. An initial rapid response is mediated through any kinase, including Glk1, which is able to phosphorylate the available sugar. In contrast, long-term repression specifically requires Hxk2 on glucose and either Hxk1 or Hxk2 on fructose. Both HXK1 and GLK1 are repressed upon addition of glucose or fructose. However, fructose repression of Hxk1 is only transient, which is in line with its preference for fructose as substrate and its requirement for long-term fructose repression. In addition, expression of HXK1 and GLK1 is regulated through cAMP-dependent protein kinase. These results indicate that sugar sensing and establishment of catabolite repression are controlled by an interregulatory network, involving all three yeast sugar kinases and the Ras-cAMP pathway.