Glucose-6-P control of glycogen synthase phosphorylation in yeast

Cellular Control of Viscosity Counters Changes in Temperature and Energy Availability

Cellular functioning requires the orchestration of thousands of molecular interactions in time and space. Yet most molecules in a cell move by diffusion, which is sensitive to external factors like temperature. How cells sustain complex, diffusion-based systems across wide temperature ranges is unknown. Here, we uncover a mechanism by which budding yeast modulate viscosity in response to temperature and energy availability. This "viscoadaptation" uses regulated synthesis of glycogen and trehalose to vary the viscosity of the cytosol. Viscoadaptation functions as a stress response and a homeostatic mechanism, allowing cells to maintain invariant diffusion across a 20°C temperature range. Perturbations to viscoadaptation affect solubility and phase separation, suggesting that viscoadaptation may have implications for multiple biophysical processes in the cell. Conditions that lower ATP trigger viscoadaptation, linking energy availability to rate regulation of diffusion-controlled processes. Viscoadaptation reveals viscosity to be a tunable property for regulating diffusion-controlled processes in a changing environment.

The AMP-Activated Protein Kinase Homolog Snf1 Concerts Carbon Utilization, Conidia Production and the Biosynthesis of Secondary Metabolites in the Taxol-Producer Pestalotiopsis microspora

Highly conserved, the Snf1/AMPK is a central regulator of carbon metabolism and energy production in the eukaryotes. However, its function in filamentous fungi has not been well established. In this study, we reported functional characterization of Snf1/AMPK in the growth, development and secondary metabolism in the filamentous fungus Pestalotiopsis microspora. By deletion of the yeast SNF1 homolog, we found that it regulated the utilization of carbon sources, e.g., sucrose, demonstrating a conserved function of this kinase in filamentous fungus. Importantly, several novel functions of SNF1 were unraveled. For instance, the deletion strain displayed remarkable retardation in vegetative growth and pigmentation and produced a diminished number of conidia, even in the presence of the primary carbon source glucose. Deletion of the gene caused damages in the cell wall as shown by its hypersensitivities to Calcofluor white and Congo red, suggesting a critical role of Snf1 in maintaining cell wall integrity. Furthermore, the mutant strain Δsnf1 was hypersensitive to stress, e.g., osmotic pressure (1 M sorbitol), drug G418 and heat shock, though the mechanism remains to be illustrated. Significantly, disruption of the gene altered the production of secondary metabolites. By high-performance liquid chromatography (HPLC) profiling, we found that Δsnf1 barely produced secondary metabolites, e.g., the known product pestalotiollide B. This study suggests that Snf1 is a key regulator in filamentous fungus Pestalotiopsis microspora concerting carbon metabolism and the filamentous growth, conidiation, cell wall integrity, stress tolerance and the biosynthesis of secondary metabolites.

Regulation of glycogen synthase from mammalian skeletal muscle--a unifying view of allosteric and covalent regulation

It is widely accepted that insufficient insulin-stimulated activation of muscle glycogen synthesis is one of the major components of non-insulin-dependent (type 2) diabetes mellitus. Glycogen synthase, a key enzyme in muscle glycogen synthesis, is extensively regulated, both allosterically (by glucose-6-phosphate, ATP, and others) and covalently (by phosphorylation). Although glycogen synthase has been a topic of intense study for more than 50 years, its kinetic characterization has been confounded by its large number of phosphorylation states. Questions remain regarding the function of glycogen synthase regulation and the relative importance of allosteric and covalent modification in fulfilling this function. In this review, we consider both earlier kinetic studies and more recent site-directed mutagenesis and crystal structure studies in a detailed qualitative discussion of the effects of regulation on the kinetics of glycogen synthase. We propose that both allosteric and covalent modification of glycogen synthase may be described by a Monod-Wyman-Changeux model in terms of apparent changes to L, the equilibrium constant for transition between the T and R conformers. As, with the exception of L, all parameters of this model are independent of the glycogen synthase phosphorylation state, the need to determine kinetic parameters for all possible states is eliminated; only the relationship between a particular state and L must be established. We conclude by suggesting that renewed efforts to characterize the relationship between phosphorylation and the kinetics of glycogen synthase are essential in order to obtain a better quantitative understanding of the function of glycogen synthesis regulation. The model we propose may prove useful in this regard.

Regulation of glycogen metabolism in yeast and bacteria

Microorganisms have the capacity to utilize a variety of nutrients and adapt to continuously changing environmental conditions. Many microorganisms, including yeast and bacteria, accumulate carbon and energy reserves to cope with the starvation conditions temporarily present in the environment. Glycogen biosynthesis is a main strategy for such metabolic storage, and a variety of sensing and signaling mechanisms have evolved in evolutionarily distant species to ensure the production of this homopolysaccharide. At the most fundamental level, the processes of glycogen synthesis and degradation in yeast and bacteria share certain broad similarities. However, the regulation of these processes is sometimes quite distinct, indicating that they have evolved separately to respond optimally to the habitat conditions of each species. This review aims to highlight the mechanisms, both at the transcriptional and at the post-transcriptional level, that regulate glycogen metabolism in yeast and bacteria, focusing on selected areas where the greatest increase in knowledge has occurred during the last few years. In the yeast system, we focus particularly on the various signaling pathways that control the activity of the enzymes of glycogen storage. We also discuss our recent understanding of the important role played by the vacuole in glycogen metabolism. In the case of bacterial glycogen, special emphasis is placed on aspects related to the genetic regulation of glycogen metabolism and its connection with other biological processes.

The subcellular localization of yeast glycogen synthase is dependent upon glycogen content

The budding yeast, Saccharomyces cerevisiae, accumulates the storage polysaccharide glycogen in response to nutrient limitation. Glycogen synthase, the major form of which is encoded by the GSY2 gene, catalyzes the key regulated step in glycogen storage. Here, we utilized Gsy2p fusions to green fluorescent protein (GFP) to determine where glycogen synthase was located within cells. We demonstrated that the localization pattern of Gsy2-GFP depended upon the glycogen content of the cell. When glycogen was abundant, Gsy2-GFP was found uniformly throughout the cytoplasm, but under low glycogen conditions, Gsy2-GFP localized to discrete spots within cells. Gsy2p is known to bind to glycogen, and we propose that the subcellular distribution of Gsy2-GFP reflects the distribution of glycogen particles. In the absence of glycogen, Gsy2p translocates into the nucleus. We hypothesize that Gsy2p is normally retained in the cytoplasm through its interaction with glycogen particles. When glycogen levels are reduced, Gsy2p loses this anchor and can traffic into the nucleus.

Identity of the growth-limiting nutrient strongly affects storage carbohydrate accumulation in anaerobic chemostat cultures of Saccharomyces cerevisiae

Accumulation of glycogen and trehalose in nutrient-limited cultures of Saccharomyces cerevisiae is negatively correlated with the specific growth rate. Additionally, glucose-excess conditions (i.e., growth limitation by nutrients other than glucose) are often implicated in high-level accumulation of these storage carbohydrates. The present study investigates how the identity of the growth-limiting nutrient affects accumulation of storage carbohydrates in cultures grown at a fixed specific growth rate. In anaerobic chemostat cultures (dilution rate, 0.10 h(-1)) of S. cerevisiae, the identity of the growth-limiting nutrient (glucose, ammonia, sulfate, phosphate, or zinc) strongly affected storage carbohydrate accumulation. The glycogen contents of the biomass from glucose- and ammonia-limited cultures were 10- to 14-fold higher than those of the biomass from cultures grown under the other three glucose-excess regimens. Trehalose levels were specifically higher under nitrogen-limited conditions. These results demonstrate that storage carbohydrate accumulation in nutrient-limited cultures of S. cerevisiae is not a generic response to excess glucose but instead is strongly dependent on the identity of the growth-limiting nutrient. While transcriptome analysis of wild-type and msn2Delta msn4Delta strains confirmed that transcriptional upregulation of glycogen and trehalose biosynthesis genes is mediated by Msn2p/Msn4p, transcriptional regulation could not quantitatively account for the drastic changes in storage carbohydrate accumulation. The results of assays of glycogen synthase and glycogen phosphorylase activities supported involvement of posttranscriptional regulation. Consistent with the high glycogen levels in ammonia-limited cultures, the ratio of glycogen synthase to glycogen phosphorylase in these cultures was up to eightfold higher than the ratio in the other glucose-excess cultures.

Trehalose-6-phosphate synthase 1 from Metarhizium anisopliae: clone, expression and properties of the recombinant

Trehalose, an important component in fungal spores, is a disaccharide which protects against several environmental stresses, such as heat, desiccation, salt. Trehalose-6-phosphate synthase 1 (TPS1) is a subunit of trehalose synthase complex in fungi; it plays a key role in the biosynthesis of trehalose. In this study, a full-length cDNA from Metarhizium anisopliae encoding TPS1 (designated as MaTPS1) was isolated. The MaTPS1 cDNA is composed of 1836 nucleotides encoding a protein of 517 amino acids with a molecular mass of 58 kDa. The amino acid sequence has a relatively high homology with the TPS1s in several other filamentous fungi. Southern blot analysis showed that MaTPS1 gene occurs as a single copy in the M. anisopliae genome. And MaTPS1 was cloned into Pichia pastoris KM71 and secretively expressed with a histamine tag to facilitate a rapid purification of recombinant MaTPS1 (designated reTPS1). The properties of reTPS1 were examined. The K(m) value of reTPS1 for UDP-glucose and glucose-6-phosphate was 9.6 mM and 3.9 mM, respectively, and the optimal pH and temperature were about 6.5 and 40 degrees C. The enzyme was highly specific to glucose-6-phosphate for glucosyl acceptor, and its activity decreased rapidly as the concentrations of phosphate increased. The expression system will provide sufficient amounts of reTPS1 for future structural characterization of the protein and use for further investigation of MaTPS1's function.

Evaluation of predicted network modules in yeast metabolism using NMR-based metabolite profiling

Genome-scale metabolic models promise important insights into cell function. However, the definition of pathways and functional network modules within these models, and in the biochemical literature in general, is often based on intuitive reasoning. Although mathematical methods have been proposed to identify modules, which are defined as groups of reactions with correlated fluxes, there is a need for experimental verification. We show here that multivariate statistical analysis of the NMR-derived intra- and extracellular metabolite profiles of single-gene deletion mutants in specific metabolic pathways in the yeast Saccharomyces cerevisiae identified outliers whose profiles were markedly different from those of the other mutants in their respective pathways. Application of flux coupling analysis to a metabolic model of this yeast showed that the deleted gene in an outlying mutant encoded an enzyme that was not part of the same functional network module as the other enzymes in the pathway. We suggest that metabolomic methods such as this, which do not require any knowledge of how a gene deletion might perturb the metabolic network, provide an empirical method for validating and ultimately refining the predicted network structure.

Regulation of the nucleocytoplasmic distribution of Snf1-Gal83 protein kinase

Snf1 protein kinase containing the beta subunit Gal83 is localized in the cytoplasm during growth of Saccharomyces cerevisiae cells in abundant glucose and accumulates in the nucleus in response to glucose limitation. Nuclear localization of Snf1-Gal83 requires activation of the Snf1 catalytic subunit and depends on Gal83, but in the snf1Delta mutant, Gal83 exhibits glucose-regulated nuclear accumulation. We show here that the N terminus of Gal83, which is divergent from those of the other beta subunits, is necessary and sufficient for Snf1-independent, glucose-regulated localization. We identify a leucine-rich nuclear export signal in the N terminus and show that export depends on the Crm1 export receptor. We present evidence that catalytically inactive Snf1 promotes the cytoplasmic retention of Gal83 in glucose-grown cells through its interaction with the C terminus of Gal83; cytoplasmic localization of inactive Snf1-Gal83 maintains accessibility to the Snf1-activating kinases. Finally, we characterize the effects of glucose phosphorylation on localization. These studies define roles for Snf1 and Gal83 in determining the nucleocytoplasmic distribution of Snf1-Gal83 protein kinase.

Deviation of carbohydrate metabolism by the SIT4 phosphatase in Saccharomyces cerevisiae

A prominent phenotype of the yeast sit4 mutant, which lacks the Ser-Thr phosphatase Sit4, is hyper-accumulation of glycogen and the failure to grow on respiratory substrates. We investigated whether these two phenotypes are linked by studying the metabolic response to SIT4 deletion. Although the sit4 mutant failed to grow on respiratory substrates, in the exponential growth, phase respiration was de-repressed; active respiration was confirmed by measuring oxygen consumption and NADH generation. However, the fermentation rate and the internal glucose 6-phosphate and pyruvate levels were reduced, while glycogen content was high. Respiro-fermentative and respiratory substrates such as galactose, glycerol and ethanol were directed toward glycogen synthesis, which indicates that sit4 mutant deviates metabolism to glycogenesis by activating a glycogen futile cycle and depleting cells of Krebs cycle intermediates. An important feature of the sit4 mutant was the lack of growth under anaerobic conditions, suggesting that respiration is necessary to meet the energy requirements of the cell. Addition of aspartic acid, which can restore Krebs cycle intermediates, partially restored growth on ethanol. Our findings suggest that inhibition of Sit4 activity may be essential for redirecting carbohydrate flux to gluconeogenesis and glycogen storage.

Protein kinases associated with the yeast phosphoproteome

Background

Protein phosphorylation is an extremely important mechanism of cellular regulation. A large-scale study of phosphoproteins in a whole-cell lysate of Saccharomyces cerevisiae has previously identified 383 phosphorylation sites in 216 peptide sequences. However, the protein kinases responsible for the phosphorylation of the identified proteins have not previously been assigned.

Results

We used Predikin in combination with other bioinformatic tools, to predict which of 116 unique protein kinases in yeast phosphorylates each experimentally determined site in the phosphoproteome. The prediction was based on the match between the phosphorylated 7-residue sequence and the predicted substrate specificity of each kinase, with the highest weight applied to the residues or positions that contribute most to the substrate specificity. We estimated the reliability of the predictions by performing a parallel prediction on phosphopeptides for which the kinase has been experimentally determined.

Conclusion

The results reveal that the functions of the protein kinases and their predicted phosphoprotein substrates are often correlated, for example in endocytosis, cytokinesis, transcription, replication, carbohydrate metabolism and stress response. The predictions link phosphoproteins of unknown function with protein kinases with known functions and vice versa, suggesting functions for the uncharacterized proteins. The study indicates that the phosphoproteins and the associated protein kinases represented in our dataset have housekeeping cellular roles; certain kinases are not represented because they may only be activated during specific cellular responses. Our results demonstrate the utility of our previously reported protein kinase substrate prediction approach (Predikin) as a tool for establishing links between kinases and phosphoproteins that can subsequently be tested experimentally.

Specific features of changes in levels of endogenous respiration substrates in Saccharomyces cerevisiae cells at low temperature

The rate of endogenous respiration of Saccharomyces cerevisiae cells incubated at 0 degrees C under aerobic conditions in the absence of exogenous substrates decreased exponentially with a half-period of about 5 h when measured at 30 degrees C. This was associated with an indirectly shown decrease in the level of oxaloacetate in the mitochondria in situ. The initial concentration of oxaloacetate significantly decreased the activity of succinate dehydrogenase. The rate of cell respiration in the presence of acetate and other exogenous substrates producing acetyl-CoA in mitochondria also decreased, whereas the respiration rate on succinate increased. These changes were accompanied by an at least threefold increase in the L-malate concentration in the cells within 24 h. It is suggested that the increase in the L-malate level in the cells and the concurrent decrease in the oxaloacetate level in the mitochondria should be associated with a deceleration at 0 degrees C of the transport of endogenous respiration substrates from the cytosol into the mitochondria. This deceleration is likely to be caused by a high Arrhenius activation energy specific for transporters. The physiological significance of L-malate in regulation of the S. cerevisiae cell respiration is discussed.

Glycogen synthesis in the absence of glycogenin in the yeast Saccharomyces cerevisiae

In eukaryotic cells, glycogenin is a self-glucosylating protein that primes glycogen synthesis. In yeast, the loss of function of GLG1 and GLG2, which encode glycogenin, normally leads to the inability of cells to synthesize glycogen. In this report, we show that a small fraction of colonies from glg1glg2 mutants can switch on glycogen synthesis to levels comparable to wild-type strain. The occurrence of glycogen positive glg1glg2 colonies is strongly enhanced by the presence of a hyperactive glycogen synthase and increased even more upon deletion of TPS1. In all cases, this phenotype is reversible, indicating the stochastic nature of this synthesis, which is furthermore illustrated by colour-sectoring of colonies upon iodine-staining. Altogether, these data suggest that glycogen synthesis in the absence of glycogenin relies on a combination of several factors, including an activated glycogen synthase and as yet unknown alternative primers whose synthesis and/or distribution may be controlled by TPS1 or under epigenetic silencing.

Role of reserve carbohydrates in the growth dynamics of Saccharomyces cerevisiae

The purpose of this study was to explore the role of glycogen and trehalose in the ability of Saccharomyces cerevisiae to respond to a sudden rise of the carbon flux. To this end, aerobic glucose-limited continuous cultures were challenged with a sudden increase of the dilution rate from 0.05 to 0.15 h(-1). Under this condition, a rapid mobilization of glycogen and trehalose was observed which coincided with a transient burst of budding and a decrease of cell biomass. Experiments carried out with mutants defective in storage carbohydrates indicated a predominant role of glycogen in the adaptation to this perturbation. However, the real importance of trehalose in this response was veiled by the unexpected phenotypes harboured by the tps1 mutant, chosen for its inability to synthesize trehalose. First, the biomass yield of this mutant was 25% lower than that of the isogenic wild-type strain at dilution rate of 0.05 h(-1), and this difference was annulled when cultures were run at a higher dilution rate of 0.15 h(-1). Second, the tps1 mutant was more effective to sustain the dilution rate shift-up, apparently because it had a faster glycolytic rate and an apparent higher capacity to consume glucose with oxidative phosphorylation than the wild type. Consequently, a tps1gsy1gsy2 mutant was able to adapt to the dilution rate shift-up after a long delay, likely because the detrimental effects from the absence of glycogen was compensated for by the tps1 mutation. Third, a glg1Deltaglg2Delta strain, defective in glycogen synthesis because of the lack of the glycogen initiation protein, recovered glycogen accumulation upon further deletion of TPS1. This recovery, however, required glycogen synthase. Finally, we demonstrated that the rapid breakdown of reserve carbohydrates triggered by the shift-up is merely due to changes in the concentrations of hexose-6-phosphate and UDPglucose, which are the main metabolic effectors of the rate-limiting enzymes of glycogen and trehalose pathways.

Effect of nitrogen limitation and surplus upon trehalose metabolism in wine yeast

Trehalose metabolism in yeast has been related to stress and could be used as a stress indicator. Winemaking conditions are stressful for yeast and understanding trehalose metabolism under these conditions could be useful for controlling alcoholic fermentation. In this study, we analysed trehalose metabolism of a commercial wine yeast strain during alcoholic fermentation by varying the nitrogen levels from low (below adequate) to high (excess). We determined trehalose, nitrogen, sugar consumption and expression of NTH1, NTH2 and TPS1. Our results show that trehalose metabolism is slightly affected by nitrogen availability and that the main consumption of nitrogen occurs in the first 24 h. After this period, nitrogen is hardly taken up by the yeast cells. Although nitrogen and sugar are still available, no further growth is observed in high concentrations of nitrogen. Increased expression of genes involved in trehalose metabolism occurs mainly at the end of the growth period. This could be related to an adaptive mechanism for fine tuning of glycolysis during alcoholic tumultuous fermentation, as both anabolic and catabolic pathways are affected by such expression.

Increased glycogen storage in yeast results in less branched glycogen

Glycogen is a branched polymer of glucose, synthesized as a reserve of both energy and carbon. The branched nature of glycogen is important for its function and polyglucosan bodies, particles that contain a glycogen-like polymer with reduced branching, are a feature of several disease states. The degree of glycogen branching is thought to be governed by the balance between glycogen synthesis and branching activities. However, there have been reports that the intrinsic properties of individual branching enzymes govern the degree of branching. To address the relationship between synthesis and branching more fully, we made use of the yeast Saccharomyces cerevisiae. The glycogen content of yeast cells was manipulated by using different growth conditions or by the introduction of specific mutations. Whenever glycogen storage was elevated, the polysaccharide formed was found to be less branched but normal branching could be restored by overexpression of branching enzyme.

Glycogen synthase sensitivity to glucose-6-P is important for controlling glycogen accumulation in Saccharomyces cerevisiae

Glycogen is a storage form of glucose utilized as an energy reserve by many organisms. Glycogen synthase, which is essential for synthesizing this glucose polymer, is regulated by both covalent phosphorylation and the concentration of glucose-6-P. With the yeast glycogen synthase Gsy2p, we recently identified two mutants, R579A/R580A/R582A [corrected] and R586A/R588A/R591A, in which multiple arginine residues were mutated to alanine that were completely insensitive to activation by glucose-6-P in vitro (Pederson, B. A., Cheng, C., Wilson, W. A., and Roach, P. J. (2000) J. Biol. Chem. 275, 27753-27761). We report here the expression of these mutants in Saccharomyces cerevisiae and, as expected from our findings in vitro, they were not activated by glucose-6-P. The R579A/R580A/R582A [corrected] mutant, which is also resistant to inhibition by phosphorylation, caused hyperaccumulation of glycogen. In contrast, the mutant R586A/R588A/R591A, which retains the ability to be inactivated by phosphorylation, resulted in lower glycogen accumulation when compared with wild-type cells. When intracellular glucose-6-P levels were increased by mutating the PFK2 gene, glycogen storage due to the wild-type enzyme was increased, whereas that associated with R579A/R580A/R582A [corrected] was not greatly changed. This is the first direct demonstration that activation of glycogen synthase by glucose-6-P in vivo is necessary for normal glycogen accumulation.

Frog oocyte glycogen synthase: enzyme regulation under in vitro and in vivo conditions

Frog oocyte glycogen synthase properties differ significantly under in vitro or in vivo conditions. The K(mapp) for UDP-glucose in vivo was 1.4mM (in the presence or absence of glucose-6-P). The in vitro value was 6mM and was reduced by glucose-6-P to 0.8mM. Under both conditions (in vitro and in vivo) V(max) was 0.2 m Units per oocyte in the absence of glucose-6-P. V(max) in vivo was stimulated 2-fold by glucose-6-P, whereas, in vitro, a 10-fold increase was obtained. Glucose-6-P required for 50% activation in vivo was 15 microM and, depending on substrate concentrations, 50-100 microM in vitro. The prevailing enzyme obtained in vitro was the glucose-6-P-dependent form, which may be converted to the independent species by dephosphorylation. This transformation could not be observed in vivo. We suggest that enzyme activation by glucose-6-P in vivo is due to allosteric effects rather than to dephosphorylation of the enzyme. Regulatory mechanisms other than allosteric activation and covalent phosphorylation are discussed.

Trehalose and glycogen accumulation is related to the duration of the G1 phase of Saccharomyces cerevisiae

Several factors may control trehalose and glycogen synthesis, like the glucose flux, the growth rate, the intracellular glucose-6-phosphate level and the glucose concentration in the medium. Here, the possible relation of these putative inducers to reserve carbohydrate accumulation was studied under well-defined growth conditions in nitrogen-limited continuous cultures. We showed that the amounts of accumulated trehalose and glycogen were regulated by the growth rate imposed on the culture, whereas other implicated inducers did not exhibit a correlation with reserve carbohydrate accumulation. Trehalose accumulation was induced at a dilution rate (D)</=0.10 h(-1), whereas glycogen accumulation gradually increased at decreasing growth rates. The growth rate dependency of trehalose accumulation was supported by studies in cells overexpressing the G(1)-cyclin CLN3. The trehalose level appeared to be dependent on the duration of the G(1) phase, as trehalose was only accumulated at a G(1) phase duration of more than 5 h in both wild-type and CLN3-overexpressing cells. On the other hand, the glycogen level was reduced by CLN3 overexpression in a cell cycle-independent manner. A possible regulatory mechanism that links trehalose and glycogen accumulation to the growth rate is discussed.

Intracellular glucose 1-phosphate and glucose 6-phosphate levels modulate Ca2+ homeostasis in Saccharomyces cerevisiae

The enzyme phosphoglucomutase plays a key role in cellular metabolism by virtue of its ability to interconvert Glc-1-P and Glc-6-P. It was recently shown that a yeast strain lacking the major isoform of phosphoglucomutase (pgm2Delta) accumulates a high level of Glc-1-P and exhibits several phenotypes related to altered Ca(2+) homeostasis when d-galactose is utilized as the carbon source (Fu, L., Miseta, A., Hunton, D., Marchase, R. B., and Bedwell, D. M. (2000) J. Biol. Chem. 275, 5431-5440). These phenotypes include increased Ca(2+) uptake and accumulation and sensitivity to high environmental Ca(2+) levels. In the present study, we overproduced the enzyme UDP-Glc pyrophosphorylase to test whether the overproduction of a downstream metabolite produced from Glc-1-P can also mediate changes in Ca(2+) homeostasis. We found that overproduction of UDP-Glc did not cause any alterations in Ca(2+) uptake or accumulation. We also examined whether Glc-6-P can influence cellular Ca(2+) homeostasis. A yeast strain lacking the beta-subunit of phosphofructokinase (pfk2Delta) accumulates a high level of Glc-6-P (Huang, D., Wilson, W. A., and Roach, P. J. (1997) J. Biol. Chem. 272, 22495-22501). We found that this increase in Glc-6-P led to a 1.5-2-fold increase in total cellular Ca(2+). We also found that the pgm2Delta/pfk2Delta strain, which accumulated high levels of both Glc-6-P and Glc-1-P, no longer exhibited the Ca(2+)-related phenotypes associated with high Glc-1-P levels in the pgm2Delta mutant. These results provide strong evidence that cellular Ca(2+) homeostasis is coupled to the relative levels of Glc-6-P and Glc-1-P in yeast.

Coordinate regulation of sugar flux and translation by PAS kinase

PAS kinase is a serine/threonine kinase regulated in cis by a PAS domain. A genetic study of the two PAS kinase genes in budding yeast gave evidence of the involvement of these enzymes in the control of sugar metabolism and translation. Using a biochemical screen for PAS kinase substrates, three translation factors were identified as direct phosphorylation targets. PAS kinase was also found to phosphorylate UDP-glucose pyrophosphorylase and glycogen synthase, the enzymes catalyzing the two final steps in the glycogen biosynthetic pathway. Genetic, biochemical, and physiological data provide evidence that both of these enzymes are inhibited by PAS kinase-dependent phosphorylation, thereby downregulating carbohydrate storage. These studies provide evidence of a cell-autonomous signaling system that both controls and connects the balance of fuel consumption/storage to protein synthesis.

Systematic identification of the genes affecting glycogen storage in the yeast Saccharomyces cerevisiae: implication of the vacuole as a determinant of glycogen level

At the onset of nutrient limitation, the yeast Saccharomyces cerevisiae synthesizes glycogen to serve as a carbon and energy reserve. We undertook a systematic survey for the genes that affect glycogen accumulation by taking advantage of the strain deletion set generated by the Saccharomyces Genome Deletion Project. The strain collection analyzed contained some 4600 diploid homozygous null deletants, representing approximately 88% of all viable haploid disruptants. We identified 324 strains with low and 242 with elevated glycogen stores, accounting for 12.4% of the genes analyzed. The screen was validated by the identification of many of the genes known already to influence glycogen accumulation. Many of the mutants could be placed into coherent families. For example, 195 or 60% of the hypoaccumulators carry mutations linked to respiratory function, a class of mutants well known to be defective in glycogen storage. The second largest group consists of approximately 60 genes involved in vesicular trafficking and vacuolar function, including genes encoding 13 of 17 proteins involved in the structure or assembly of the vacuolar ATPase. These data are consistent with our recent findings that the process of autophagy has a significant impact on glycogen storage (Wang, Z., Wilson, W. A., Fujino, M. A., and Roach, P. J. (2001) Antagonistic controls of autophagy and glycogen accumulation by Snf1p, the yeast homolog of AMP-activated protein kinase, and the cyclin-dependent kinase Pho85p. Mol. Cell. Biol. 21, 5742-5752). Autophagy delivers glycogen to the vacuole, and we propose that the impaired vacuolar function associated with ATPase mutants (vma10 or vma22) results in reduced degradation and subsequent hyperaccumulation of glycogen.

Effect of AICAR treatment on glycogen metabolism in skeletal muscle

AMP-activated protein kinase (AMPK) is proposed to stimulate fat and carbohydrate catabolism to maintain cellular energy status. Recent studies demonstrate that pharmacologic activation of AMPK and mutations in the enzyme are associated with elevated muscle glycogen content in vivo. Our purpose was to determine the mechanism for increased muscle glycogen associated with AMPK activity in vivo. AMPK activity and glycogen metabolism were studied in red and white gastrocnemius muscles from rats treated with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) in vivo, and also in muscles incubated with AICAR in vitro. In vivo AICAR treatment reduced blood glucose and increased blood lactate compared with basal values. AICAR increased muscle alpha2 AMPK activity, glycogen, and glucose-6-phosphate concentrations. Glycogen synthase activity was increased in the red gastrocnemius but was decreased in the white gastrocnemius. Glycogen phosphorylase activity increased in both muscles, with an inhibition initially observed in the red gastrocnemius. In vitro incubation with AICAR activated alpha2 AMPK but had no effect on either glycogen synthase or glycogen phosphorylase. These results suggest that AICAR treatment does not promote glycogen accumulation in skeletal muscle in vivo by altering glycogen synthase and glycogen phosphorylase. Rather, the increased glycogen is due to the well-known effects of AICAR to increase glucose uptake.

Roles of trehalose phosphate synthase in yeast glycogen metabolism and sporulation

Trehalose is a major storage carbohydrate in budding yeast, Saccharomyces cerevisiae. Alterations in trehalose synthesis affect carbon source-dependent growth, accumulation of glycogen and sporulation. Trehalose is synthesized by trehalose phosphate synthase (TPS), which is a complex of at least four proteins. In this work, we show that the Tps1p subunit protein catalyses trehalose phosphate synthesis in the absence of other TPS components. The tps1-H223Y allele (glc6-1) that causes a semidominant decrease in glycogen accumulation exhibits greater enzyme activity than wild-type TPS1 because, unlike the wild-type enzyme, TPS activity in tps1-H223Y cells is not inhibited by phosphate. Poor sporulation in tps1 null diploids is caused by reduced expression of meiotic inducers encoded by IME1, IME2 and MCK1. Furthermore, high-copy MCK1 or heterozygous hxk2 mutations can suppress the tps1 sporulation trait. These results suggest that the trehalose-6-phosphate inhibition of hexokinase activity is required for full induction of MCK1 in sporulating yeast cells.

Hyperactive glycogen synthase mutants of Saccharomyces cerevisiae suppress the glc7-1 protein phosphatase mutant

A yeast glc7-1 mutant expressing a variant of protein phosphatase type 1 fails to accumulate glycogen. This defect is associated with hyperphosphorylated and inactive glycogen synthase, consistent with Glc7p acting directly to dephosphorylate and activate glycogen synthase. To characterize the glycogen synthesis defect of this mutant in more detail, we isolated 26 pseudorevertants of the glc7-1 mutant. All pseudoreversion events were due to missense mutations in GSY2, the gene encoding the major isoform of glycogen synthase. A majority of the mutations responsible for the suppression were in the 3' end of the gene, corresponding to the phosphorylated COOH terminus of Gsy2p. Phosphorylation of the mutant proteins was reduced, suggesting that they are poor substrates for glycogen synthase kinases. Suppressor mutations outside this domain did not decrease the phosphorylation of the resulting proteins, indicating that these proteins are immune to the regulatory effects of phosphorylation. Since no growth defect has been observed for strains with altered glycogen levels, the relative levels of fitness of GSY2 mutants that fail to accumulate glycogen and that hyperaccumulate glycogen were assayed by cocultivation experiments. A wild-type strain outcompeted both hypo- and hyperaccumulating strains, suggesting that glycogen levels contribute substantially to the fitness of yeast.

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.

Mitochondrial respiratory mutants of Saccharomyces cerevisiae accumulate glycogen and readily mobilize it in a glucose-depleted medium

Mutant strains of Saccharomyces cerevisiae defective in respiration have been reported to be unable to store glycogen, as revealed by the iodine-staining method. In this report, it is shown that in contrast to this claim, mitochondrial respiratory mutants accumulated even more glycogen than wild-type cells during the fermentative growth on glucose. However, as soon as glucose was exhausted in the medium, these mutants readily and completely mobilized their glycogen content, contrary to wild-type cells which only transiently degraded this polymer. The mobilization of glycogen was a specific trait resulting from a defect in mitochondrial function that could not be suppressed by mutations in the cAMP- and Pho85 protein kinase-dependent nutrient-sensing pathways, and by other mutations which favour glycogen synthesis. To account for this mobilization, it was found that respiration-defective cells not only contained a less active glycogen synthase, but also a more active glycogen phosphorylase. Since glucose 6-phosphate (Glc6P) is a potent inhibitor of the phosphorylation and an activator of the dephosphorylation processes of glycogen synthase and glycogen phosphorylase, it is suggested that the drop in Glc6P observed at the onset of glucose depletion in respiration-deficient cells triggers this rapid and sustained glycogen mobilization. It is also proposed that this degradation provides the energy for the viability of respiratory mutants in glucose-starved medium.

Dynamic responses of reserve carbohydrate metabolism under carbon and nitrogen limitations in Saccharomyces cerevisiae

The dynamic responses of reserve carbohydrates with respect to shortage of either carbon or nitrogen source was studied to obtain a sound basis for further investigations devoted to the characterization of mechanisms by which the yeast Saccharomyces cerevisiae can cope with nutrient limitation during growth. This study was carried out in well-controlled bioreactors which allow accurate monitoring of growth and frequent sampling without disturbing the culture. Under glucose limitation, genes involved in glycogen and trehalose biosynthesis (GLG1, GSY1, GSY2, GAC1, GLC3, TPS1), in their degradation (GPH1, NTHI), and the typical stress-responsive CTT1 gene were coordinately induced in parallel with glycogen, when the growth has left the pure exponential phase and while glucose was still plentiful in the medium. Trehalose accumulation was delayed until the diauxic shift, although TPS1 was induced much earlier, due to hydrolysis of trehalose by high trehalase activity. In contrast, under nitrogen limitation, both glycogen and trehalose began to accumulate at the precise time when the nitrogen source was exhausted from the medium, coincidentally with the transcriptional activation of genes involved in their metabolism. While this response to nitrogen starvation was likely mediated by the stress-responsive elements (STREs) in the promoter of these genes, we found that these elements were not responsible for the co-induction of genes involved in reserve carbohydrate metabolism during glucose limitation, since GLG1, which does not contain any STRE, was coordinately induced with GSY2 and TPS1.

Mitochondrial respiratory mutants in yeast inhibit glycogen accumulation by blocking activation of glycogen synthase

Control of glycogen synthase activity by protein phosphorylation is important for regulating the synthesis of glycogen. In this report, we describe a regulatory linkage between the ability of yeast cells to respire and activation of glycogen synthase. Strains containing respiration-deficient mutations in genes such as COQ3, required for the synthesis of coenzyme Q, were reduced in their ability to accumulate glycogen in response to limiting glucose. This lowered glycogen accumulation results from inactivation of the rate-determining enzyme, glycogen synthase (Gsy2p). Reduced glycogen synthase activity is coincident with lowered glucose 6-phosphate and ATP levels in the respiration-deficient cells deprived of glucose. Alanine substitutions of three previously characterized phosphorylation sites in Gsy2p, Ser-650, Ser-654, or Thr-667, each suppressed the glycogen defect in cells unable to respire, suggesting that inactivation of this enzyme is mediated by phosphorylation of these residues. Inactivation of glycogen synthase requires the RAS signaling pathway that controls cAMP-dependent protein kinase and is independent of Pho85p previously identified as a Gsy2p kinase. These results suggest that yeast cells unable to shift from a fermentative to a respiratory metabolic regimen block accumulation of glycogen by inactivating Gsy2p through protein phosphorylation.

Cyclin partners determine Pho85 protein kinase substrate specificity in vitro and in vivo: control of glycogen biosynthesis by Pcl8 and Pcl10

In Saccharomyces cerevisiae, PHO85 encodes a cyclin-dependent protein kinase (Cdk) with multiple roles in cell cycle and metabolic controls. In association with the cyclin Pho80, Pho85 controls acid phosphatase gene expression through phosphorylation of the transcription factor Pho4. Pho85 has also been implicated as a kinase that phosphorylates and negatively regulates glycogen synthase (Gsy2), and deletion of PHO85 causes glycogen overaccumulation. We report that the Pcl8/Pcl10 subgroup of cyclins directs Pho85 to phosphorylate glycogen synthase both in vivo and in vitro. Disruption of PCL8 and PCL10 caused hyperaccumulation of glycogen, activation of glycogen synthase, and a reduction in glycogen synthase kinase activity in vivo. However, unlike pho85 mutants, pcl8 pcl10 cells had normal morphologies, grew on glycerol, and showed proper regulation of acid phosphatase gene expression. In vitro, Pho80-Pho85 complexes effectively phosphorylated Pho4 but had much lower activity toward Gsy2. In contrast, Pcl10-Pho85 complexes phosphorylated Gsy2 at Ser-654 and Thr-667, two physiologically relevant sites, but only poorly phosphorylated Pho4. Thus, both the in vitro and in vivo substrate specificity of Pho85 is determined by the cyclin partner. Mutation of PHO85 suppressed the glycogen storage deficiency of snf1 or glc7-1 mutants in which glycogen synthase is locked in an inactive state. Deletion of PCL8 and PCL10 corrected the deficit in glycogen synthase activity in both the snf1 and glc7-1 mutants, but glycogen synthesis was restored only in the glc7-1 mutant strain. This genetic result suggests an additional role for Pho85 in the negative regulation of glycogen accumulation that is independent of Pcl8 and Pcl10.

Control analysis of muscle glycogen metabolism

A minimal model of glycogen metabolism in muscle tissue is analyzed in accordance with metabolic control analysis. The model contains two branch points. Rather than contributing to complexity of the analysis, this branching allows expression of the control coefficients in a simplified form. Glucose 6-phosphate is the metabolite at the first branch point, and the analysis is simplified further by the fact that glucose 6-phosphate is the substrate for enzymes which catalyze near-equilibrium reactions. Control of the concentration of glucose 6-phosphate is of interest because of its pivotal location in the metabolic system, but also because it interacts with an allosteric site on glycogen synthase to stimulate glycogen synthase activity. It is shown that the control which the transporter and enzymes involved in glycogen synthesis exert on glycolytic flux is proportional to the control which these components exert on glucose 6-phosphate concentration. Thus, glycolysis plays a major role in control of glucose 6-phosphate concentration. It is concluded that control of glycogen synthesis is not a rigid parameter of any component of this metabolic system. Rather the distribution of control is flexible and shifts from one portion of the system to another in response to shifts in the physiological state. An important element in determining the distribution of control of glycogen synthesis is the change in the sensitivity of the allosteric site of glycogen synthase to glucose 6-phosphate which is brought about by conversion of glycogen synthase to the dephosphorylated, glucose 6-phosphate-independent, state.