The low-affinity component of the glucose transport system in Saccharomyces cerevisiae is not due to passive diffusion

Quantifying intracellular glucose levels when yeast is grown in glucose media

In Saccharomyces cerevisiae, intracellular glucose levels impact glucose transport and regulate carbon metabolism via various glucose sensors. To investigate mechanisms of glucose sensing, it is essential to know the intracellular glucose concentrations. Measuring intracellular glucose concentrations, however, is challenging when cells are grown on glucose, as glucose in the water phase around cells or stuck to the cell surface can be carried over during cell sampling and in the following attributed to intracellular glucose, resulting in an overestimation of intracellular glucose concentrations. Using lactose as a carryover marker in the growth medium, we found that glucose carryover originates from both the water phase and from sticking to the cell surface. Using a hexokinase null strain to estimate the glucose carryover from the cell surface, we found that glucose stuck on the cell surface only contributes a minor fraction of the carryover. To correct the glucose carryover, we revisited L-glucose as a carryover marker. Here, we found that L-glucose slowly enters cells. Thus, we added L-glucose to yeast cultures growing on uniformly 13C-labeled D-glucose only shortly before sampling. Using GC-MS to distinguish between the two differently labeled sugars and subtracting the carryover effect, we determined the intracellular glucose concentrations among two yeast strains with distinct kinetics of glucose transport to be at 0.89 mM in the wild-type strain and around 0.24 mM in a mutant with compromised glucose uptake. Together, our study provides insight into the origin of the glucose carryover effect and suggests that L-glucose added to the culture shortly before sampling is a possible method that yet has limitations with regard to measurement accuracy.

In and out of the plant storage vacuole

The plant storage vacuole is involved in a wide variety of metabolic functions a great many of which necessitate the transport of substances across the tonoplast. Some solutes, depending on the origin, have to cross the plasma membrane as well. The cell is equipped with a complex web of transport systems, cellular routes, and unique intracellular environments that support their transport and accumulation against a concentration gradient. These are capable of processing a diverse nature of substances of distinct sizes, chemical properties, and origins. In this review we describe the various mechanism involved in solute transport into the vacuole of storage cells with special emphasis placed on solutes arriving through the apoplast. Transport of solutes from the cytosol to the vacuole is carried out by tonoplast-bound ABC transporters, solute/H(+) antiporters, and ion channels whereas transport from the apoplast requires additional plasma membrane-bound solute/H(+) symporters and fluid-phase endocytosis. In addition, and based on new evidence accumulated within the last decade, we re-evaluate the current notion of extracellular solute uptake as partially based on facilitated diffusion, and offer an alternative interpretation that involves membrane bound transporters and fluid-phase endocytosis. Finally, we make several assertions in regards to solute export from the vacuole as predicted by the limited available data suggesting that both membrane-bound carriers and vesicle mediated exocytosis are involved during solute mobilization.

Characteristics of glucose uptake by glucose- and NH4-limited grown Penicillium ochrochloron at low, medium and high glucose concentration

Glucose uptake by Penicillium ochrochloron (formerly Penicillium simplicissimum) was studied from 0.01 to 400 mM glucose using chemostat culture and bioreactor batch culture. The characteristics of glucose uptake varied considerably with the conditions of growth, harvest and uptake assay. Glucose-limited grown mycelium showed one saturable transport system [K(S) below 0.01 mM; v(max) 1.1-1.2 mmol (g dry weight)(-1)h(-1)] plus a first order process (permeability P=1.2x10(-7)cm s(-1)). Ammonium-limited grown mycelium showed only one saturable transport system [K(S) 0.3-0.7 mM; v(max) 0.5-0.8 mmol (g dry weight)(-1)h(-1)]. During exponential growth at high glucose concentration (300-400 mM) a first order process was found with a P value of 5.6-9.3x10(-7)cm s(-1). After ammonium exhaustion a second first order phase showed a lower P value (6.1-9.3x10(-8)cm s(-1)). A similar change in permeability was also found after a re-evaluation of published data for Gibberella fujikuroi, Aspergillus niger, Aspergillus awamori and Saccharomycopsis lipolytica. For the first order processes simple diffusion was ruled out as a mechanism for glucose uptake. Glucose uptake by P. ochrochloron was controlled more strongly by metabolism than by transport and was not rate limiting for overflow metabolism.

Membrane entrapped Saccharomyces cerevisiae in a biosensor-like device as a generic rapid method to study cellular metabolism

We describe a new, faster and convenient method to study some metabolic characteristics - by the successful application of immobilized yeast cells (S. cerevisiae) in a microbial biosensor-like device. Microbial biosensors consist of microorganisms immobilized on the surface of a membrane or in a gel, in close contact with a transducer. Almost all works published to date have used biosensors for analyses in which a concentration-related property of the external medium is measured. A different approach is presented here; we have successfully used S. cerevisiae and a carbon dioxide electrode as the main components of a biosensor-like device, used as a proof of concept, for a system useful to characterize metabolic parameters of the microbial cells immobilized on a carbon dioxide electrode. The biosensor-like device we are presenting allows us to calculate Michaelis-Menten parameters related to the kinetics of transport and degradation of several carbohydrates (i.e., glucose, fructose, galactose, sucrose and xylose, with K(m(app)) of 6.0, 5.8, 0.9, 2.0, and 147 mM, respectively), and the study of the kinetics of expression of non-constitutive proteins related to the transport and degradation of galactose.

Utilization and transport of glucose in Olea Europaea cell suspensions

Cell suspensions of Olea europaea var. Galega Vulgar grown in batch culture with 0.5% (w/v) glucose were able to transport D-[(14)C]glucose according to Michaelis-Menten kinetics associated with a first-order kinetics. The monosaccharide carrier exhibited high affinity (K(m) approximately 50 micro M) and was able to transport D-glucose, D-fructose, D-galactose, D-xylose, 2-deoxy-D-glucose and 3-O-methyl-D-glucose, but not D-arabinose, D-mannitol or L-glucose. D-[(14)C]glucose uptake was associated with proton uptake, which also followed Michaelis-Menten kinetics. The transport of 3-O-methyl-D-glucose was accumulative (40-fold, at pH 5.0) and the protonophore carbonyl cyanide m-chlorophenylhydrazone strongly inhibited sugar accumulation. The results were consistent with the involvement of a monosaccharide: proton symporter with a stoichiometry of 1 : 1. When cells were grown with 3% (w/v) glucose, the uptake of D-[(14)C]glucose followed first-order kinetics and monosaccharide:proton symporter activity was not detected. The value obtained for the permeability coefficient of hexoses in O. europaea cells supported the hypothesis that the first-order kinetics observed in 0.5% and 3% sugar-grown cells was produced exclusively by passive diffusion of the sugar. The results indicate that in O. europaea cells sugar levels have a regulatory effect on sugar transport, because the activity for monosaccharide transport was repressed by high sugar concentrations.

Kinetics of active alpha-glucoside transport in Saccharomyces cerevisiae

alpha-Glucosides are the most abundant fermentable sugars in the industrial applications of Saccharomyces cerevisiae, and the active transport across the plasma membrane is the rate-limiting step for their metabolism. In this report we performed a detailed kinetic analysis of the active alpha-glucoside transport system(s) present in a wild-type strain, and in strains with defined alpha-glucoside permeases. Our results indicate that the wild-type strain harbors active transporters with high and low affinity for maltose and trehalose, and low-affinity transport systems for maltotriose and alpha-methylglucoside. The maltose permease encoded by the MAL21 gene showed a high affinity (K(m) approximately 5 mM) for maltose, and a low affinity (K(m) approximately 90 mM) for trehalose. On the other hand, the alpha-glucoside permease encoded by the AGT1 gene had a high affinity (K(m) approximately 7 mM) for trehalose, a low affinity (K(m) approximately 18 mM) for maltose and maltotriose, and a very low affinity (K(m) approximately 35 mM) for alpha-methylglucoside.

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

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

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

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

The molecular genetics of hexose transport in yeasts

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

A novel signal transduction pathway in Saccharomyces cerevisiae defined by Snf3-regulated expression of HXT6

We show that cells deleted for SNF3, HXT1, HXT2, HXT3, HXT4, HXT6, and HXT7 do not take up glucose and cannot grow on media containing glucose as a sole carbon source. The expression of Hxt1, Hxt2, Hxt3, Hxt6, or Gal2 in these cells resulted in glucose transport and allowed growth on glucose media. In contrast, the expression of Snf3 failed to confer glucose uptake or growth on glucose. HXT6 is highly expressed on raffinose, low glucose, or nonfermentable carbon sources but is repressed in the presence of high concentrations of glucose. The maintenance of HXT6 glucose repression is strictly dependent on Snf3 and not on intracellular glucose. In snf3 delta cells expression of HXT6 is constitutive even when the entire repertoire of HXT genes is present and glucose uptake is abundant. In addition, glucose repression of HXT6 does not require glucose uptake by HXT1, HXT2, HXT3 or HXT4. We show that a signal transduction pathway defined by the Snf3-dependent hexose regulation of HXT6 is distinct from but also overlaps with general glucose regulation pathways in Saccharomyces cerevisiae. Finally, glucose repression of ADH2 and SUC2 is intact in snf3 delta hxt1 delta hxt2 delta hxt3 delta hxt4 delta hxt6 delta hxt7 delta gal2 cells, suggesting that the sensing and signaling mechanism for general glucose repression is independent from glucose uptake.