Metabolic and regulatory changes associated with growth of Saccharomyces cerevisiae in 1.4 M NaCl. Evidence for osmotic induction of glycerol dissimilation via the dihydroxyacetone pathway

Gene expression analysis of cold and freeze stress in Baker's yeast

We used mRNA differential display to assess yeast gene expression under cold or freeze shock stress conditions. We found both up- and down-regulation of genes, although repression was more common. We identified and sequenced several cold-induced genes exhibiting the largest differences. We confirmed, by Northern blotting, the specificity of the response for TPI1, which encodes triose-phosphate isomerase; ERG10, the gene for acetoacetyl coenzyme A thiolase; and IMH1, which encodes a protein implicated in protein transport. These genes also were induced under other stress conditions, suggesting that this cold response is mediated by a general stress mechanism. We determined the physiological significance of the cold-induced expression change of these genes in two baker's yeast strains with different sensitivities to freeze stress. The mRNA level of TPI1 and ERG10 genes was higher in freeze-stressed than in control samples of the tolerant strain. In contrast, both genes were repressed in frozen cells of the sensitive strain. Next, we examined the effects of ERG10 overexpression on cold and freeze-thaw tolerance. Growth of wild-type cells at 10 degrees C was not affected by high ERG10 expression. However, YEpERG10 transformant cells exhibited increased freezing tolerance. Consistent with this, cells of an erg10 mutant strain showed a clear phenotype of cold and freeze sensitivity. These results give support to the idea that a cause-and-effect relationship between differentially expressed genes and cryoresistance exists in Saccharomyces cerevisiae and open up the possibility of design strategies to improve the freeze tolerance of baker's yeast.

Osmotic stress signaling and osmoadaptation in yeasts

The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

Protein expression during lag phase and growth initiation in Saccharomyces cerevisiae

In order to obtain a better understanding of the biochemical events taking place in Saccharomyces cerevisiae during the lag phase, the proteins expressed during the first hours after inoculation were investigated by two-dimensional (2-D) gel electrophoresis and compared to those expressed in late respiratory growth phase. The studies were performed on a haploid strain (S288C) grown in defined minimal medium. Some of the abundant proteins, whose expression relative to total protein expression was induced during the lag phase, were identified by MALDI MS, and the expression of the corresponding genes was assessed by Northern blotting. The rate of protein synthesis was found to increase strongly during the lag phase and the number of spots detected on 2-D gels increased from 502 spots just after inoculation to 1533 spots at the end of the lag phase. During the first 20 min, the number of detectable spots was considerably reduced compared to the number of spots detected from the yeast in respiratory growth just prior to harvest and inoculation (747 spots), indicating an immediate pausing or shutdown in synthesis of many proteins just after inoculation. In this period, the cells got rid of most of their buds. The MALDI MS-identified, lag phase-induced proteins were adenosine kinase (Ado1p), whose cellular role is presently uncertain, cytosolic acetaldehyde dehydrogenase (Ald6p) and (DL)-glycerol-3-phosphatase 1, both involved in carbohydrate metabolism, a ribosomal protein (Asc1p), a fragment of the 70-kDa heat shock protein Ssb1, and translationally controlled tumour protein homologue (Yk1056cp), all involved in translation, and S-adenosylmethionine synthetase I involved in biosynthesis reactions. The level of mRNA of the corresponding genes was found to increase strongly after inoculation. By pattern matching using previously published 2-D maps of yeast proteins, several other lag phase-induced proteins were identified. These were also proteins involved in carbohydrate metabolism, translation, and biosynthesis reactions. The identified proteins together with other, yet unidentified, lag phase-induced proteins are expected to be important for yeast growth initiation and could be valuable biological markers for yeast performance. Such markers would be highly beneficial in the control and optimisation of industrial fermentations.

A tomato enzyme catalyzing the phosphorylation of 3,4-dihydroxy-2-butanone

A riboflavin biosynthesis ribB mutant of Escherichia coli deficient of 3,4-dihydroxy-2-butanone 4-phosphate synthase was complemented with a cDNA library from Lycopersicon esculentum. The complementing gene was isolated and expressed in E. coli. The resulting protein was shown to specify a 62 kDa protein which phosphorylates dihydroxyacetone, both enantiomers of 3,4-dihydroxy-2-butanone, and several other aldoses and ketoses. Sequence analysis revealed homology to dihydroacetone kinases (dak) genes from plants, animals, fungi and some eubacteria. Genes with similarity to the 5' part of the dak gene from tomato were found in many other eubacteria. The physiological role of the dak gene is still incompletely known.

Transient inhibition of translation initiation by osmotic stress

Cells respond and adapt to changes in the environment. In this study, we examined the effect of environmental stresses on protein synthesis in the yeast Saccharomyces cerevisiae. We found that osmotic stress causes irreversible inhibition of methionine uptake, transient inhibition of uracil uptake, transient stimulation of glucose uptake, transient repression of ribosomal protein (RP) genes such as CYH2 and RPS27, and the transient inhibition of translation initiation. Rapid inhibition of translation initiation by osmotic stress requires a novel pathway, different from the amino acid-sensing pathway, the glucose-sensing pathway, and the TOR pathway. The Hog1 MAP kinase pathway is not involved in the inhibition of either methionine uptake or translation initiation but is required for the adaptation of translation initiation after inhibition and the repression of RP genes by osmotic stress. These results suggest that the transient inhibition of translation initiation occurs as a result of a combination of both acute inhibition of translation and the long-term activation of translation by the Hog1 pathway.

Sulfur sparing in the yeast proteome in response to sulfur demand

Genome-wide studies have recently revealed the unexpected complexity of the genetic response to apparently simple physiological changes. Here, we show that when yeast cells are exposed to Cd(2+), most of the sulfur assimilated by the cells is converted into glutathione, a thiol-metabolite essential for detoxification. Cells adapt to this vital metabolite requirement by modifying globally their proteome to reduce the production of abundant sulfur-rich proteins. In particular, some abundant glycolytic enzymes are replaced by sulfur-depleted isozymes. This global change in protein expression allows an overall sulfur amino acid saving of 30%. This proteomic adaptation is essentially regulated at the mRNA level. The main transcriptional activator of the sulfate assimilation pathway, Met4p, plays an essential role in this sulfur-sparing response.

Glycerol export and glycerol-3-phosphate dehydrogenase, but not glycerol phosphatase, are rate limiting for glycerol production in Saccharomyces cerevisiae

Glycerol, one of the most important by-products of alcoholic fermentation, has positive effects on the sensory properties of fermented beverages. It was recently shown that the most direct approach for increasing glycerol formation is to overexpress GPD1, which encodes the glycerol-3-phosphate dehydrogenase (GPDH) isoform Gpd1p. We aimed to identify other steps in glycerol synthesis or transport that limit glycerol flux during glucose fermentation. We showed that the overexpression of GPD2, encoding the other isoform of glycerol-3-phosphate dehydrogenase (Gpd2p), is equally as effective as the overexpression of GPD1 in increasing glycerol production (3.3-fold increase compared to the wild-type strain) and has similar effects on yeast metabolism. In contrast, overexpression of GPP1, encoding glycerol 3-phosphatase (Gpp1p), did not enhance glycerol production. Strains that simultaneously overexpress GPD1 and GPP1 did not produce higher amounts of glycerol than a GPD1-overexpressing strain. These results demonstrate that GPDH, but not the glycerol 3-phosphatase, is rate-limiting for glycerol production. The channel protein Fps1p mediates glycerol export. It has recently been shown that mutants lacking a region in the N-terminal domain of Fps1p constitutively release glycerol. We showed that cells producing truncated Fps1p constructs during glucose fermentation compensate for glycerol loss by increasing glycerol production. Interestingly, the strain with a deregulated Fps1 glycerol channel had a different phenotype to the strain overexpressing GPD genes and showed poor growth during fermentation. Overexpression of GPD1 in this strain increased the amount of glycerol produced but led to a pronounced growth defect.

A PBS2 homologue from Debaryomyces hansenii shows a differential effect on calcofluor and polymyxin B sensitivity in Saccharomyces cerevisiae

The PBS2 gene encodes a MAP kinase kinase that plays a pivotal role in osmosensing signal-transduction pathway in the yeast Saccharomyces cerevisiae. Mutation in the PBS2 gene has a pleotropic effect. Besides being osmosensitive, pbs2 mutants show altered sensitivity to polymyxin B and calcofluor. Recent studies revealed that Pbs2p plays a different role in osmoadaptation and calcofluor sensitivity. We have isolated a gene homologous to PBS2 from the highly salt-tolerant yeast Debaryomyces hansenii by phenotypic complementation. DNA sequencing of the clone revealed that the gene encoded a protein of 683 amino acid residues. Like Pbs2p, this protein also has a proline-rich motif. Further characterization revealed that this gene could complement polymyxin B sensitivity but did not affect calcofluor sensitivity. Thus, it appeared that Pbs2p also has an independent role in these two physiological processes. The GenBank Accession No. of this sequence is AF371315.

The Saccharomyces cerevisiae Sko1p transcription factor mediates HOG pathway-dependent osmotic regulation of a set of genes encoding enzymes implicated in protection from oxidative damage

A major part of the transcriptional response of yeast cells to osmotic shock is controlled by the HOG pathway and several downstream transcription factors. Sko1p is a repressor that mediates HOG pathway-dependent regulation by binding to CRE sites in target promoters. Here, we report five target genes of Hog1p-Sko1p: GRE2, AHP1, SFA1, GLR1 and YML131w. The two CREs in the GRE2 promoter function as activating sequences and, hence, bind (an) activator protein(s). However, the two other yeast CRE-binding proteins, Aca1p and Aca2p, are not involved in regulation of the GRE2 promoter under osmotic stress. In the absence of the co-repressor complex Tup1p-Ssn6p/Cyc8p, which is recruited by Sko1p, stimulation by osmotic stress is still observed. These data indicate that Sko1p is not only required for repression, but also involved in induction upon osmotic shock. All five Sko1p targets encode oxidoreductases with demonstrated or predicted roles in repair of oxidative damage. Altered basal expression levels of these genes in hog1Delta and sko1Delta mutants may explain the oxidative stress phenotypes of these mutants. All five Sko1p target genes are induced by oxidative stress, and induction involves Yap1p. Although Sko1p and Yap1p appear to mediate osmotic and oxidative stress responses independently, Sko1p may affect Yap1p promoter access or activity. The five Sko1p target genes described here are suitable models for studying the interplay between osmotic and oxidative responses at the molecular and physiological levels.

Cloning of glycerol-3-phosphate dehydrogenase genes (ZrGPD1 and ZrGPD2) and glycerol dehydrogenase genes (ZrGCY1 and ZrGCY2) from the salt-tolerant yeast Zygosaccharomyces rouxii

The ZrGPD1 and ZrGPD2 genes encoding putative glycerol-3-phosphate dehydrogenases were isolated from the salt-tolerant yeast, Zygosaccharomyces rouxii. Both genes are homologous to GPD1 of Saccharomyces cerevisiae and are constitutively expressed in Z. rouxii cells. Putative glycerol dehydrogenase genes, ZrGCY1 and ZrGCY2, which are highly homologous to GCY1 of S. cerevisiae, were also isolated. Since the level of transcripts of ZrGCY1 and ZrGCY2 increased in Z. rouxii cells subjected to salt stress, it is suggested that the pathway of the signal transduction of salt stress controls the expression of these genes. The Accession Nos of these sequences in GenBank are as follows: ZrGPD1, AB047394; ZrGPD2, AB047395; ZrGCY1, AB047396; ZrGCY2, AB047397.

Transcript expression in Saccharomyces cerevisiae at high salinity

Transcript expression of Saccharomyces cerevisiae at high salinity was determined by microarray analysis of 6144 open reading frames (ORFs). From cells grown in 1 m NaCl for 10, 30, and 90 min, changes in transcript abundance >2-fold were classified. Salinity-induced ORFs increased over time: 107 (10 min), 243 (30 min), and 354 (90 min). Up-regulated, functionally unknown ORFs increased from 17 to 149 over this period. Expression patterns were similar early, with 67% of up-regulated transcripts after 10 min identical to those at 30 min. The expression profile after 90 min revealed different up-regulated transcripts (identities of 13% and 22%, respectively). Nucleotide and amino acid metabolism exemplified the earliest responses to salinity, followed by ORFs related to intracellular transport, protein synthesis, and destination. Transcripts related to energy production were up-regulated throughout the time course with respiration-associated transcripts strongly induced at 30 min. Highly expressed at 90 min were known salinity stress-induced genes, detoxification-related responses, transporters of the major facilitator superfamily, metabolism of energy reserves, nitrogen and sulfur compounds, and lipid, fatty acid/isoprenoid biosynthesis. We chose severe stress conditions to monitor responses in essential biochemical mechanisms. In the mutant, Deltagpd1/gpd2, lacking glycerol biosynthesis, the stress response was magnified with a partially different set of up-regulated ORFs.

A proteome analysis of the cadmium response in Saccharomyces cerevisiae

Cadmium is very toxic at low concentrations, but the basis for its toxicity is not clearly understood. We analyzed the proteomic response of yeast cells to acute cadmium stress and identified 54 induced and 43 repressed proteins. A striking result is the strong induction of 9 enzymes of the sulfur amino acid biosynthetic pathway. Accordingly, we observed that glutathione synthesis is strongly increased in response to cadmium treatment. Several proteins with antioxidant properties were also induced. The induction of nine proteins is dependent upon the transactivator Yap1p, consistent with the cadmium hypersensitive phenotype of the YAP1-disrupted strain. Most of these proteins are also overexpressed in a strain overexpressing Yap1p, a result that correlates with the cadmium hyper-resistant phenotype of this strain. Two of these Yap1p-dependent proteins, thioredoxin and thioredoxin reductase, play an important role in cadmium tolerance because strains lacking the corresponding genes are hypersensitive to this metal. Altogether, our data indicate that the two cellular thiol redox systems, glutathione and thioredoxin, are essential for cellular defense against cadmium.

The yeast glycerol 3-phosphatases Gpp1p and Gpp2p are required for glycerol biosynthesis and differentially involved in the cellular responses to osmotic, anaerobic, and oxidative stress

We have characterized the strongly homologous GPP1/RHR2 and GPP2/HOR2 genes, encoding isoforms of glycerol 3-phosphatase. Mutants lacking both GPP1 and GPP2 are devoid of glycerol 3-phosphatase activity and produce only a small amount of glycerol, confirming the essential role for this enzyme in glycerol biosynthesis. Overproduction of Gpp1p and Gpp2p did not significantly enhance glycerol production, indicating that glycerol phosphatase is not rate-limiting for glycerol production. Previous studies have shown that expression of both GPP1 and GPP2 is induced under hyperosmotic stress and that induction partially depends on the HOG (high osmolarity glycerol) pathway. We here show that expression of GPP1 is strongly decreased in strains having low protein kinase A activity, although it is still responsive to osmotic stress. The gpp1Delta/gpp2Delta double mutant is hypersensitive to high osmolarity, whereas the single mutants remain unaffected, indicating GPP1 and GPP2 substitute well for each other. Transfer to anaerobic conditions does not affect expression of GPP2, whereas GPP1 is transiently induced, and mutants lacking GPP1 show poor anaerobic growth. All gpp mutants show increased levels of glycerol 3-phosphate, which is especially pronounced when gpp1Delta and gpp1Delta/gpp2Delta mutants are transferred to anaerobic conditions. The addition of acetaldehyde, a strong oxidizer of NADH, leads to decreased glycerol 3-phosphate levels and restored anaerobic growth of the gpp1Delta/gpp2Delta mutant, indicating that the anaerobic accumulation of NADH causes glycerol 3-phosphate to reach growth-inhibiting levels. We also found the gpp1Delta/gpp2Delta mutant is hypersensitive to the superoxide anion generator, paraquat. Consistent with a role for glycerol 3-phosphatase in protection against oxidative stress, expression of GPP2 is induced in the presence of paraquat. This induction was only marginally affected by the general stress-response transcriptional factors Msn2p/4p or protein kinase A activity. We conclude that glycerol metabolism plays multiple roles in yeast adaptation to altered growth conditions, explaining the complex regulation of glycerol biosynthesis genes.

Three aldo-keto reductases of the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae is an industrially important yeast, which is also used extensively as a model eukaryote. The S. cerevisiae genome has been sequenced in its entirety and therefore represents an ideal organism in which to carry out functional analysis of genes. We have identified several open reading frames in the S. cerevisiae genome which show significant similarity to members of the aldo-keto reductase superfamily. The physiological roles of these gene products have not been previously determined, but their similarity to other enzymes suggests they may perform roles in carbohydrate metabolism and detoxification pathways. Cloning and expression of three of these enzymes has allowed their substrate specificities to be determined. Expression profiling and gene disruption analysis will allow potential roles for these enzymes within the cell to be examined.

Molecular and physiological characterization of the NAD-dependent glycerol 3-phosphate dehydrogenase in the filamentous fungus Aspergillus nidulans

In filamentous fungi, glycerol biosynthesis has been proposed to play an important role during conidiospore germination and in response to a hyperosmotic shock, but little is known about the genes involved. Here, we report on the characterization of the major Aspergillus nidulans glycerol 3-phosphate dehydrogenase (G3PDH)-encoding gene, gfdA. G3PDH is responsible for the conversion of dihydroxyacetone phosphate (DHAP) into glycerol 3-phosphate (G3P), which is subsequently converted into glycerol by an as yet uncharacterized phosphatase. Inactivation of gfdA does not abolish glycerol biosynthesis, showing that the other pathway from DHAP, via dihydroxyacetone (DHA), to glycerol is also functional in A. nidulans. The gfdA null mutant displays reduced G3P levels and an osmoremediable growth defect on various carbon sources except glycerol. This growth defect is associated with an abnormal hyphal morphology that is reminiscent of a cell wall defect. Furthermore, the growth defect at low osmolarity is enhanced in the presence of the chitin-interacting agent calcofluor and the membrane-destabilizing agent sodium dodecyl sulphate (SDS). As inactivation of gfdA has no impact on phospholipid biosynthesis or glycolytic intermediates levels, as might be expected from reduced G3P levels, a previously unsuspected link between G3P and cell wall integrity is proposed to occur in filamentous fungi.

Genomic expression programs in the response of yeast cells to environmental changes

We explored genomic expression patterns in the yeast Saccharomyces cerevisiae responding to diverse environmental transitions. DNA microarrays were used to measure changes in transcript levels over time for almost every yeast gene, as cells responded to temperature shocks, hydrogen peroxide, the superoxide-generating drug menadione, the sulfhydryl-oxidizing agent diamide, the disulfide-reducing agent dithiothreitol, hyper- and hypo-osmotic shock, amino acid starvation, nitrogen source depletion, and progression into stationary phase. A large set of genes (approximately 900) showed a similar drastic response to almost all of these environmental changes. Additional features of the genomic responses were specialized for specific conditions. Promoter analysis and subsequent characterization of the responses of mutant strains implicated the transcription factors Yap1p, as well as Msn2p and Msn4p, in mediating specific features of the transcriptional response, while the identification of novel sequence elements provided clues to novel regulators. Physiological themes in the genomic responses to specific environmental stresses provided insights into the effects of those stresses on the cell.

Microaerobic glycerol formation in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae produces large amounts of glycerol as an osmoregulator during hyperosmotic stress and as a redox sink at low oxygen availability. NAD(+)-dependent glycerol-3-phosphate dehydrogenase in S. cerevisiae is present in two isoforms, coded for by two different genes, GPD1 and GPD2. Mutants for either one or both of these genes were investigated under carefully controlled static and dynamic conditions in continuous cultures at low oxygen transfer rates. Our results show that S. cerevisiae controls the production of glycerol in response to hypoxic conditions by regulating the expression of several genes. At high demand for NADH reoxidation, a strong induction was seen not only of the GPD2 gene, but also of GPP1, encoding one of the molecular forms of glycerol-3-phosphatase. Induction of the GPP1 gene appears to play a decisive role at elevated growth rates. At low demand for NADH reoxidation via glycerol formation, the GPD1, GPD2, GPP1, and GPP2 genes were all expressed at basal levels. The dynamics of the gene induction and the glycerol formation at low demand for NADH reoxidation point to an important role of the Gpd1p; deletion of the GPD1 gene strongly altered the expression patterns of the GPD2 and GPP1 genes under such conditions. Furthermore, our results indicate that GCY1 and DAK1, tentatively encoding glycerol dehydrogenase and dihydroxyacetone kinase, respectively, may be involved in the redox regulation of S. cerevisiae.

Carbohydrate and energy-yielding metabolism in non-conventional yeasts

Sugars are excellent carbon sources for all yeasts. Since a vast amount of information is available on the components of the pathways of sugar utilization in Saccharomyces cerevisiae it has been tacitly assumed that other yeasts use sugars in the same way. However, although the pathways of sugar utilization follow the same theme in all yeasts, important biochemical and genetic variations on it exist. Basically, in most non-conventional yeasts, in contrast to S. cerevisiae, respiration in the presence of oxygen is prominent for the use of sugars. This review provides comparative information on the different steps of the fundamental pathways of sugar utilization in non-conventional yeasts: glycolysis, fermentation, tricarboxylic acid cycle, pentose phosphate pathway and respiration. We consider also gluconeogenesis and, briefly, catabolite repression. We have centered our attention in the genera Kluyveromyces, Candida, Pichia, Yarrowia and Schizosaccharomyces, although occasional reference to other genera is made. The review shows that basic knowledge is missing on many components of these pathways and also that studies on regulation of critical steps are scarce. Information on these points would be important to generate genetically engineered yeast strains for certain industrial uses.

Rap1p-binding sites in the saccharomyces cerevisiae GPD1 promoter are involved in its response to NaCl

Mechanisms involved in transcriptional regulation of the osmotically controlled GPD1 gene in Saccharomyces cerevisiae were investigated by promoter analysis. The GPD1 gene encodes NAD(+)-dependent glycerol-3-phosphate dehydrogenase, a key enzyme in the production of the compatible solute glycerol. By analysis of promoter deletions, we identified a region at nucleotides -478 to -324, in relation to start of translation, to be of great importance for both basal activity and osmotic induction of GPD1. Electrophoretic mobility shift and DNase I footprint analyses demonstrated protein binding to parts of this region that contain three consensus sequences for Rap1p (repressor activator protein 1)-binding sites. Actual binding of Rap1p to this region was confirmed by demonstrating enhanced electrophoretic mobility of the protein-DNA complex with extracts containing an N-terminally truncated version of Rap1p. The detected Rap1p-DNA interactions were not affected by changes in the osmolarity of the growth medium. Specific inactivation of the Rap1p-binding sites by a C-to-A point mutation in the core of the consensus showed that this factor is a major determinant of GPD1 expression since mutations in all three putative binding sites for Rap1p strongly hampered osmotic induction and drastically lowered basal activity. We also show that the Rap1p-binding sites appear functionally distinct; the most distal site (core of the consensus at position -386) exhibited the highest affinity for Rap1p and was strictly required for low salt induction (< or =0.6 m NaCl), but not for the response at higher salinities (> or =0.8 m NaCl). This indicates tha different molecular mechanisms might be operational for low and high salt responses of the GPD1 promoter.

Engineering of the pyruvate dehydrogenase bypass in Saccharomyces cerevisiae: role of the cytosolic Mg(2+) and mitochondrial K(+) acetaldehyde dehydrogenases Ald6p and Ald4p in acetate formation during alcoholic fermentation

Acetic acid plays a crucial role in the organoleptic balance of many fermented products. We have investigated the factors controlling the production of acetate by Saccharomyces cerevisiae during alcoholic fermentation by metabolic engineering of the enzymatic steps involved in its formation and its utilization. The impact of reduced pyruvate decarboxylase (PDC), limited acetaldehyde dehydrogenase (ACDH), or increased acetoacetyl coenzyme A synthetase (ACS) levels in a strain derived from a wine yeast strain was studied during alcoholic fermentation. In the strain with the PDC1 gene deleted exhibiting 25% of the PDC activity of the wild type, no significant differences were observed in the acetate yield or in the amounts of secondary metabolites formed. A strain overexpressing ACS2 and displaying a four- to sevenfold increase in ACS activity did not produce reduced acetate levels. In contrast, strains with one or two disrupted copies of ALD6, encoding the cytosolic Mg(2+)-activated NADP-dependent ACDH and exhibiting 60 and 30% of wild-type ACDH activity, showed a substantial decrease in acetate yield (the acetate production was 75 and 40% of wild-type production, respectively). This decrease was associated with a rerouting of carbon flux towards the formation of glycerol, succinate, and butanediol. The deletion of ALD4, encoding the mitochondrial K(+)-activated NAD(P)-linked ACDH, had no effect on the amount of acetate formed. In contrast, a strain lacking both Ald6p and Ald4p exhibited a long delay in growth and acetate production, suggesting that Ald4p can partially replace the Ald6p isoform. Moreover, the ald6 ald4 double mutant was still able to ferment large amounts of sugar and to produce acetate, suggesting the contribution of another member(s) of the ALD family.

GUP1 and its close homologue GUP2, encoding multimembrane-spanning proteins involved in active glycerol uptake in Saccharomyces cerevisiae

Many yeast species can utilize glycerol, both as a sole carbon source and as an osmolyte. In Saccharomyces cerevisiae, physiological studies have previously shown the presence of an active uptake system driven by electrogenic proton symport. We have used transposon mutagenesis to isolate mutants affected in the transport of glycerol into the cell. Here we present the identification of YGL084c, encoding a multimembrane-spanning protein, as being essential for proton symport of glycerol into S. cerevisiae. The gene is named GUP1 (glycerol uptake) and, for growth on glycerol, is important as a carbon and energy source. In addition, in strains deficient in glycerol production it also provides osmotic protection by the addition of glycerol. Another open reading frame (ORF), YPL189w, presenting a high degree of homology to YGL084c, similarly appears to be involved in active glycerol uptake in salt-containing glucose-based media in strains deficient in glycerol production. Analogously, this gene is named GUP2. To our knowledge, this is the first report on a gene product involved in active transport of glycerol in yeasts. Mutations with the same phenotypes occurred in two other ORFs of previously unknown function, YDL074c and YPL180w.

Response of Saccharomyces cerevisiae to severe osmotic stress: evidence for a novel activation mechanism of the HOG MAP kinase pathway

The HOG/p38 MAP kinase route is an important stress-activated signal transduction pathway that is well conserved among eukaryotes. Here, we describe a novel mechanism of activation of the HOG pathway in budding yeast. This mechanism operates upon severe osmostress conditions (1.4 M NaCl) and is independent of the Sln1p and Sho1p osmosensors. The alternative input feeds into the HOG pathway MAPKK Pbs2p and requires activation of Pbs2p by phosphorylation. We show that, upon severe osmotic shock, Hog1p nuclear accumulation and phosphorylation is delayed compared with mild stress. Moreover, both events lost their transient pattern, presumably because of the absence of negative feedback mediated by Ptp2p tyrosine phosphatase, which we found to be localized in the nucleus. Under severe osmotic stress conditions, the delayed nuclear accumulation correlates with a delay in stress-responsive gene expression. Severe osmoshock leads to a situation in which active and nuclear-localized Hog1p is transiently unable to induce transcription of osmotic stress-responsive genes. It also appeared from our studies that the Sho1p osmosensor is less active under severe osmotic stress conditions, whereas the Sln1p/Ypd1p/Ssk1p sensor and signal transducer functions normally under these circumstances.

The control of intracellular glycerol in Saccharomyces cerevisiae influences osmotic stress response and resistance to increased temperature

Glycerol has been demonstrated to serve as the major osmolyte of Saccharomyces cerevisiae. Consistently, mutant strains gpd1gpd2 and gpp1gpp2, which are devoid of the main glycerol biosynthesis pathway, have been shown to be osmosensitive. In addition, the primary hyperosmotic stress response is affected in these strains. Hog1p phosphorylation turned out to be prolonged and osmostress-induced gene expression is delayed compared with the kinetics observed in wild-type cells. A hog1 deletion strain was previously found to contain lower internal glycerol and therefore displays an osmosensitive phenotype. Here, we show that the osmosensitivity of hog1 is suppressed by growth at 37 degrees C. We reasoned that this temperature-remedial osmoresistance might be caused by a higher intracellular glycerol level at the elevated temperature. This hypothesis was confirmed by measurement of the glycerol concentration, which was shown to be similar for wild type and hog1 cells only at elevated growth temperatures. In agreement with this finding, hog1 cells containing an fps1 allele, encoding a constitutively open glycerol channel, have lost their temperature-remedial osmoresistance. Furthermore, gpd1gpd2 and gpp1gpp2 strains were found to be temperature sensitive. The growth defect of these strains could be suppressed by adding external glycerol. In conclusion, the ability to control glycerol levels influences proper osmostress-induced signalling and the cellular potential to grow at elevated temperatures. These data point to an important, as yet unidentified, role of glycerol in cellular functioning.

Aca1 and Aca2, ATF/CREB activators in Saccharomyces cerevisiae, are important for carbon source utilization but not the response to stress

In Saccharomyces cerevisiae, the family of ATF/CREB transcriptional regulators consists of a repressor, Acr1 (Sko1), and two activators, Aca1 and Aca2. The AP-1 factor Gen4 does not activate transcription through ATF/CREB sites in vivo even though it binds these sites in vitro. Unlike ATF/CREB activators in other species, Aca1- and Aca2-dependent transcription is not affected by protein kinase A or by stress, and Aca1 and Aca2 are not required for Hog1-dependent salt induction of transcription through an optimal ATF/CREB site. Aca2 is important for a variety of biological functions including growth on nonoptimal carbon sources, and Aca2-dependent activation is modestly regulated by carbon source. Strains lacking Aca1 are phenotypically normal, but overexpression of Aca1 suppresses some defects associated with the loss of Aca2, indicating a functional overlap between Aca1 and Aca2. Acr1 represses transcription both by recruiting the Cyc8-Tup1 corepressor and by directly competing with Aca1 and Aca2 for target sites. Acr1 does not fully account for osmotic regulation through ATF/CREB sites, and a novel Hog1-dependent activator(s) that is not a bZIP protein is required for ATF/CREB site activation in response to high salt. In addition, Acr1 does not affect a number of phenotypes that arise from loss of Aca2. Thus, members of the S. cerevisiae ATF/CREB family have overlapping, but distinct, biological functions and target genes.

The transcriptional response of Saccharomyces cerevisiae to osmotic shock. Hot1p and Msn2p/Msn4p are required for the induction of subsets of high osmolarity glycerol pathway-dependent genes

We have analyzed the transcriptional response to osmotic shock in the yeast Saccharomyces cerevisiae. The mRNA level of 186 genes increased at least 3-fold after a shift to NaCl or sorbitol, whereas that of more than 100 genes was at least 1.5-fold diminished. Many induced genes encode proteins that presumably contribute to protection against different types of damage or encode enzymes in glycerol, trehalose, and glycogen metabolism. Several genes, which encode poorly expressed isoforms of enzymes in carbohydrate metabolism, were induced. The high osmolarity glycerol (HOG) pathway is required for full induction of many but not all genes. The recently characterized Hot1p transcription factor is required for normal expression of a subset of the HOG pathway-dependent responses. Stimulated expression of the genes that required the general stress-response transcription factors Msn2p and Msn4p was also reduced in a hog1 mutant, suggesting that Msn2p/Msn4p might be regulated by the HOG pathway. The expression of genes that are known to be controlled by the mating pheromone response pathway was stimulated by osmotic shock specifically in a hog1 mutant. Inappropriate activation of the mating response may contribute to the growth defect of a hog1 mutant in high osmolarity medium.

The level of cAMP-dependent protein kinase A activity strongly affects osmotolerance and osmo-instigated gene expression changes in Saccharomyces cerevisiae

The influence of cAMP-dependent protein kinase (PKA) on protein expression during exponential growth under osmotic stress was studied by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). The responses of isogenic strains (tpk2Deltatpk3Delta) with either constitutively low (tpk1(w1)), regulated (TPK1) or constitutively high (TPK1bcy1Delta) PKA activity were compared. The activity of cAMP-dependent protein kinase (PKA) was shown to be a major determinant of osmotic shock tolerance. Proteins with increased expression during growth under sodium chloride stress could be grouped into three classes with respect to PKA activity, with the glycerol metabolic proteins GPD1, GPP2 and DAK1 standing out as independent of PKA. The other osmotically induced proteins displayed a variable dependence on PKA activity; fully PKA-dependent genes were TPS1 and GCY1, partly PKA-dependent genes were ENO1, TDH1, ALD3 and CTT1. The proteins repressed by osmotic stress also fell into distinct classes of PKA-dependency. Ymr116c was PKA-independent, while Pgi1p, Sam1p, Gdh1p and Vma1p were fully PKA-dependent. Hxk2p, Pdc1p, Ssb1p, Met6p, Atp2p and Hsp60p displayed a partially PKA-dependent repression. The promotors of all induced PKA-dependent genes have STRE sites in their promotors suggestive of a mechanism acting via Msn2/4p. The mechanisms governing the expression of the other classes are unknown. From the protein expression data we conclude that a low PKA activity causes a protein expression resembling that of osmotically stressed cells, and furthermore makes cells tolerant to this type of stress.

Isolation and sequence of the HOG1 homologue from Debaryomyces hansenii by complementation of the hog1Delta strain of Saccharomyces cerevisiae

The HOG1 gene encodes a MAP kinase that plays an essential role in maintaining water homeostasis in the yeast Saccharomyces cerevisiae. A gene homologous to S. cerevisiae HOG1 has been isolated from a highly salt-tolerant yeast, Debaryomyces hansenii, by phenotypic complementation. DNA sequencing of the clone revealed the presence of an open reading frame encoding a protein 387 amino acids long. The deduced amino acid sequence showed very high similarity with homologous genes identified from S. cerevisiae, Candida albicans and Zygosaccharomyces rouxii. In addition, it has also TGY motif characteristics of hyperosmolarity-activated MAP kinases. The Genbank Accession No. of this sequence is AF185278.

Metabolic surprises in Saccharomyces cerevisiae during adaptation to saline conditions: questions, some answers and a model

This review describes the metabolic alterations and adaptations of yeast cells in response to osmotic stress. The basic theme of the cellular response is known to be exclusion of the extracellular stress agent salt and intracellular accumulation of the compatible solute glycerol. Molecular details of these basic processes are currently rather well known. However, analysis of expression changes during adaptation to salt has revealed a number of metabolic surprises. These include the induced expression of genes involved in glycerol dissimilation as well as trehalose turnover. The physiological rationale for these responses to osmotic stress is discussed. A model is presented in which it is hypothesised that the two pathways function as glycolytic safety valves during adaptation to stress.

Competition for enzymes in metabolic pathways: implications for optimal distributions of enzyme concentrations and for the distribution of flux control

The structures of biochemical pathways are assumed to be determined by evolutionary optimization processes. In the framework of mathematical models, these structures should be explained by the formulation of optimization principles. In the present work, the principle of minimal total enzyme concentration at fixed steady state fluxes is applied to metabolic networks. According to this principle there exists a competition of the reactions for the available amount of enzymes such that all biological functions are maintained. In states which fulfil these optimization criteria the enzyme concentrations are distributed in a non-uniform manner among the reactions. This result has consequences for the distribution of flux control. It is shown that the flux control matrix c, the elasticity matrix epsilon, and the vector e of enzyme concentrations fulfil in optimal states the relations c(T)e = e and epsilon(T)e = 0. Starting from a well-balanced distribution of enzymes the minimization of total enzyme concentration leads to a lowering of the SD of the flux control coefficients.

Hyperosmotic stress represses the transcription of HXT2 and HXT4 genes in Saccharomyces cerevisiae

Effects of hyperosmotic stress on the transcriptional regulation of the HXT2 and HXT4 genes of Saccharomyces cerevisiae were investigated under glucose-repressed and -depressed growth conditions. Hyperosmotic stress repressed the transcription of these HXT genes up to 81% depending on growth conditions. Preconditioning of yeast cells for the hyperosmotic stress resulted in a much stronger repression of both HXT genes. The negative effect of hyperosmotic stress was much higher for HXT4 than HXT2. These results also show that hyperosmotic stress interferes with the glucose-dependent transcriptional activation or derepression of HXT2 and HXT4 genes transcription in S. cerevisiae.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.

Cloning and characterization of two genes encoding dihydroxyacetone kinase from Schizosaccharomyces pombe IFO 0354

We report the cloning and characterization of two genes encoding dihydroxyacetone kinase (EC 2.7.1.29), SpDAK1 and SpDAK2, from Schizosaccharomyces pombe IFO 0354. The open reading frames of both genes encode 591 amino acids and have Mrs of 62158 and 62170, respectively. Both predicted amino acid sequences exhibited a high identity to each other (99.8%) and relatively high identities (30% to 76%) to other putative dihydroxyacetone kinase gene products. A Western blot analysis showed that these enzymes are induced by glycerol and repressed by glucose. A genomic Southern blot analysis indicated the presence of SpDAK1 and the absence of SpDAK2 in a standard laboratory strain, S. pombe 972h-.

The H2O2 stimulon in Saccharomyces cerevisiae

The changes in gene expression underlying the yeast adaptive stress response to H2O2 were analyzed by comparative two-dimensional gel electrophoresis of total cell proteins. The synthesis of at least 115 proteins is stimulated by H2O2, whereas 52 other proteins are repressed by this treatment. We have identified 71 of the stimulated and 44 of the repressed targets. The kinetics and dose-response parameters of the H2O2 genomic response were also analyzed. Identification of these proteins and their mapping into specific cellular processes give a distinct picture of the way in which yeast cells adapt to oxidative stress. As expected, H2O2-responsive targets include an important number of heat shock proteins and proteins with reactive oxygen intermediate scavenging activities. Exposure to H2O2 also results in a slowdown of protein biosynthetic processes and a stimulation of protein degradation pathways. Finally, the most remarkable result inferred from this study is the resetting of carbohydrate metabolism minutes after the exposure to H2O2. Carbohydrate fluxes are redirected to the regeneration of NADPH at the expense of glycolysis. This study represents the first genome-wide characterization of a H2O2-inducible stimulon in a eukaryote.

The Pichia pastoris dihydroxyacetone kinase is a PTS1-containing, but cytosolic, protein that is essential for growth on methanol

Dihydroxyacetone kinase (DAK) is essential for methanol assimilation in methylotrophic yeasts. We have cloned the DAK gene from Pichia pastoris by functional complementation of a mutant that was unable to grow on methanol. An open reading frame of 1824 bp was identified that encodes a 65.3 kDa protein with high homology to DAK from Saccharomyces cerevisiae. Although DAK from P. pastoris contained a C-terminal tripeptide, TKL, which we showed can act as a peroxisomal targeting signal when fused to the green fluorescent protein, the enzyme was primarily cytosolic. The TKL tripeptide was not required for the biochemical function of DAK because a deletion construct lacking the DNA encoding this tripeptide was able to complement the P. pastoris dak delta mutant. Peroxisomes, which are essential for growth of P. pastoris on methanol, were present in the dak delta mutant and the import of peroxisomal proteins was not disturbed. The dak delta mutant grew at normal rates on glycerol and oleate media. However, unlike the wild-type cells, the dak delta mutant was unable to grow on methanol as the sole carbon source but was able to grow on dihydroxyacetone at a much slower rate. The metabolic pathway explaining the reduced growth rate of the dak delta mutant on dihydroxyacetone is discussed. The nucleotide sequence reported in this paper has been submitted to GenBank with Accession Number AF019198.

The Ssn6-Tup1 repressor complex of Saccharomyces cerevisiae is involved in the osmotic induction of HOG-dependent and -independent genes

The response of yeast to osmotic stress has been proposed to rely on the HOG-MAP kinase signalling pathway and on transcriptional activation mediated by STRE promoter elements. However, the osmotic induction of HAL1, an important determinant of salt tolerance, is HOG independent and occurs through the release of transcriptional repression. We have identified an upstream repressing sequence in HAL1 promoter (URSHAL1) located between -231 and -156. This promoter region was able to repress transcription from a heterologous promoter and to bind proteins in non-stressed cells, but not in salt-treated cells. The repression conferred by URSHAL1 is mediated through the Ssn6-Tup1 protein complex and is abolished in the presence of osmotic stress. The Ssn6-Tup1 co-repressor is also involved in the regulation of HOG-dependent genes such as GPD1, CTT1, ALD2, ENA1 and SIP18, and its deletion can suppress the osmotic sensitivity of hog1 mutants. We propose that the Ssn6-Tup1 repressor complex might be a general component in the regulation of osmostress responses at the transcriptional level of both HOG-dependent and -independent genes.

Msn2p and Msn4p control a large number of genes induced at the diauxic transition which are repressed by cyclic AMP in Saccharomyces cerevisiae

The multicopy suppressors of the snf1 defect, Msn2p and Msn4p transcription factors (Msn2/4p), activate genes through the stress-responsive cis element (CCCCT) in response to various stresses. This cis element is also the target for repression by the cyclic AMP (cAMP)-signaling pathway. We analyzed the two-dimensional gel electrophoresis pattern of protein synthesis of the msn2 msn4 double mutant and compared it with that of the wild-type strain during exponential growth phase and at the diauxic transition. Thirty-nine gene products (including those of ALD3, GDH3, GLK1, GPP2, HSP104, HXK1, PGM2, SOD2, SSA3, SSA4, TKL2, TPS1, and YBR149W) are dependent upon Msn2/4p for their induction at the diauxic transition. The expression of all these genes is repressed by cAMP. Thirty other genes identified during this study are still inducible in the mutant. A subset of these genes were found to be superinduced at the diauxic transition, and others were subject to cAMP repression (including ACH1, ADH2, ALD6, ATP2, GPD1, ICL1, and KGD2). We conclude from this analysis that Msn2/4p control a large number of genes induced at the diauxic transition but that other, as-yet-uncharacterized regulators, also contribute to this response. In addition, we show here that cAMP repression applies to both Msn2/4p-dependent and -independent control of gene expression at the diauxic shift. Furthermore, the fact that all the Msn2/4p gene targets are subject to cAMP repression suggests that these regulators could be targets for the cAMP-signaling pathway.

Expression of the glyoxalase I gene of Saccharomyces cerevisiae is regulated by high osmolarity glycerol mitogen-activated protein kinase pathway in osmotic stress response

Methylglyoxal is a cytotoxic metabolite derived from dihydroxyacetone phosphate, an intermediate of glycolysis. Detoxification of methylglyoxal is performed by glyoxalase I. Expression of the structural gene of glyoxalase I (GLO1) of Saccharomyces cerevisiae under several stress conditions was investigated using the GLO1-lacZ fusion gene, and expression of the GLO1 gene was found to be specifically induced by osmotic stress. The Hog1p is one of the mitogen-activated protein kinases (MAPKs) in S. cerevisiae, and both Msn2p and Msn4p are the transcriptional regulators that are thought to be under the control of Hog1p-MAPK. Expression of the GLO1 gene under osmotic stress was completely repressed in hog1Delta disruptant and was repressed approximately 80 and 50% in msn2Delta and msn4Delta disruptants, respectively. A double mutant of the MSN2 and MSN4 gene was unable to induce expression of the GLO1 gene under highly osmotic conditions. Glucose consumption increased approximately 30% during the adaptive period in osmotic stress in the wild type strain. On the contrary, it was reduced by 15% in the hog1Delta mutant. When the yeast cell is exposed to highly osmotic conditions, glycerol is synthesized as a compatible solute. Glycerol is synthesized from glucose, and a rate-limiting enzyme in glycerol biosynthesis is glycerol-3-phosphate dehydrogenase (GPD1 gene product), which catalyzes reduction of dihydroxyacetone phosphate to glycerol 3-phosphate. Expression of the GPD1 gene is also under the control of Hog1p-MAPK. Methylglyoxal is also synthesized from dihydroxyacetone phosphate; therefore, induction of the GLO1 gene expression by osmotic stress was thought to scavenge methylglyoxal, which increased during glycerol production for adaptation to osmotic stress.

Amino acid uptake is strongly affected during exponential growth of Saccharomyces cerevisiae in 0.7 M NaCl medium

Labelling of Saccharomyces cerevisiae grown in 0.7 M NaCl with 35S-methionine revealed a 5-6 fold lowering of the methionine incorporation into protein, which could not be attributed solely to the approximately 50% longer generation time of cells grown in 0.7 M NaCl. Subsequent studies of the high affinity methionine uptake system showed a strongly reduced uptake of methionine during growth in 0.7 M NaCl medium. This reduced uptake was shown to be strain-independent and caused mainly by an approximately 20-fold lowered maximum velocity (Vmax) of the transport system, while the substrate affinity (Km) displayed only a minor change. A salt-instigated reduction of uptake was furthermore demonstrated for the leucine and histidine high affinity uptake systems and also for a mixture of 15 different amino acids. We therefore suggest that the reduced amino acid uptake is a general phenomenon observed in salt-grown cells.

Two-dimensional electrophoretic separation of yeast proteins using a non-linear wide range (pH 3-10) immobilized pH gradient in the first dimension; reproducibility and evidence for isoelectric focusing of alkaline (pI > 7) proteins

The proteome of the yeast Saccharomyces cerevisiae was analysed by two-dimensional (2D) polyacrylamide gel electrophoresis utilizing a non-linear immobilized pH gradient (3-10) in the first-dimensional separation. Cells were labelled by [35S]methionine incorporation in the respiro-fermentative phase during exponential growth on glucose. Gels were run, visualized with phosphoimager technology and all resolved proteins automatically quantified. Proteins were well resolved over the whole pH interval, and evidence for isoelectric focusing on the basic side of the pattern was generated by sequencing of some spots, revealing the 2D positions of Tef1p, Pgk1p, Gpm1p, Tdh1p and Shm2p. Roughly 25% of the spots were resolved at the alkaline side of the pattern (pI > 7). The position reproducibility was high and in the range 1-2 mm in the x- and y-dimension, respectively. No quantitative variation was linked to a certain size or charge class of resolved proteins, and the average quantitative standard deviation was 17 +/- 11%. The obtained immobilized pH gradient based pattern could easily be compared to the old ampholine-based 2D pattern, and the previously reported identifications could thus be transferred. Our yeast pattern currently contains 43 known proteins, all identified by protein sequencing. Utilizing these identified proteins, relevant pI and Mr scales in the pattern were constructed. Normalization of the expression of identified spots by compensating for the number of methionine residues a protein contains allowed stoichiometric comparisons. The most dominant proteins under these growth conditions were Tdh3p, Fba1p, Eno2p and Tef1p/Tef2p, all being expressed at more than 500,000 copies per cell. The differential carbon source response during exponential growth on either glucose, galactose or ethanol was examined for the alkaline proteins identified by micro-sequencing in this study.

Osmoregulation and glycerol metabolism in the yeast Saccharomyces cerevisiae

Glycerol is the main compatible solute in Saccharomyces cerevisiae. It is accumulated intracellularly when cells are exposed to decreased extracellular water activity. In general, increased intracellular accumulation of a solute may be caused by enhanced production, restricted dissimilation, increased retention by the plasma membrane and increased uptake from the medium. In this review, we evaluate current knowledge concerning mechanisms leading to the accumulation of glycerol in osmotically stressed cells of S. cerevisiae at the molecular and metabolic levels. An overview of glycerol metabolism in S. cerevisiae is provided.

Osmoresponsive proteins and functional assessment strategies in Saccharomyces cerevisiae

Cells respond to increased external osmolarities by enhanced accumulation of compatible solutes. In yeast-cells, mainly exemplified by Saccharomyces cerevisiae, the premier compatible solute is the polyhydroxy-alcohol glycerol, the production of which is accompanied by overall metabolic changes. By applying two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) coupled to computerized image quantification, a large body of valuable physiological information relating to this stress-adaptation has been gathered. One of the presumed key-enzymes in the production of glycerol in the cell is glycerol 3-phosphate dehydrogenase encoded by the GPD1 gene. The amount of this protein is enhanced during saline stress, and from 2-D analysis linked to microsequencing it became apparent that the osmo-regulated from contained a putative presequence. Sequence analysis of another salt-induced spot in the 2-D pattern revealed identity to a gene, YER062c, with previously unknown function. Biochemical characterization of this protein, including standard purification via chromatography and subsequent activity/specificity measurements, identified this salt-regulated protein as the missing protein/gene in glycerol production, namely the glycerol 3-phosphatase. The sequence of another salt regulated protein resolved in the 2-D gel revealed identity to a bacterial dihydroxyacetone kinase, thus indicating salt induced glycerol dissimilation. Comparing Northern data to the 2-D generated expression pattern revealed a strong correlation, indicating mainly regulation at the transcriptional level. In addition, altered expression during saline growth of some of the glycolytic enzymes was also apparent. Signalling mutants, either in the cAMP-dependent protein kinase A pathway or in a protein kinase cascade, have been analyzed during osmotic stress via 2-D PAGE, grouping proteins/genes apparently regulated via similar mechanismus. Proteome analysis has proven invaluable in the unravelling of the molecular physiology of yeast cells during adaptation and growth under osmotic stress, identifying vital components not selected by purely genetic approaches.