Kinetics of sulfate uptake by yeast

The Effect of pH on the Uptake of 35S(-II) by Wine Yeasts

The rates of uptake of 35S from S(-II) solutions by wine yeasts, Saccharomyces cerevisiae strains R92 and R104 and Saccharomyces chevalieri strain R93, were measured at a variety of solution pH values between pH 3.1 and pH 7.8. A pH effect was observed, the rates of uptake being higher at the lower pH values, but this effect was not related entirely to changes in the H2S or HS- concentration. The transport process of S(-II) appeared to be due to simple diffusion of H2S(aq) and carrier mediated transport of HS-(aq). The kinetic constants Km and V max were calculated for the carrier component of the mechanism at pH 7.2 and the permeability coefficient P was calculated for the diffusion of H2S(aq) at pH 3.1 and 7.2. By using these parameters, it was possible to calculate a theoretical ini tial rate of uptake over a range of extracellular S(-II) concentrations (0 to 50 mmoll-1) at pH 3.1 and pH 7.2. The experimentally determined initial rates were found to agree, within the experimental error, with the theoretical values. The initial rate of uptake of S(-II) and the values of Km for yeast strain R104 (a low sulfide producer) were found to be less than those for both strain R92 (a normal sulfide producer) and for strain R93 (a high sulfide producer).

Inorganic Phosphate and Sulfate Transport in S. cerevisiae

Inorganic ions such as phosphate and sulfate are essential macronutrients required for a broad spectrum of cellular functions and their regulation. In a constantly fluctuating environment microorganisms have for their survival developed specific nutrient sensing and transport systems ensuring that the cellular nutrient needs are met. This chapter focuses on the S. cerevisiae plasma membrane localized transporters, of which some are strongly induced under conditions of nutrient scarcity and facilitate the active uptake of inorganic phosphate and sulfate. Recent advances in studying the properties of the high-affinity phosphate and sulfate transporters by means of site-directed mutagenesis have provided further insight into the molecular mechanisms contributing to substrate selectivity and transporter functionality of this important class of membrane transporters.

Sul1 and Sul2 sulfate transceptors signal to protein kinase A upon exit of sulfur starvation

Sulfate is an essential nutrient with pronounced regulatory effects on cellular metabolism and proliferation. Little is known, however, about how sulfate is sensed by cells. Sul1 and Sul2 are sulfate transporters in the yeast Saccharomyces cerevisiae, strongly induced upon sulfur starvation and endocytosed upon the addition of sulfate. We reveal Sul1,2-dependent activation of PKA targets upon sulfate-induced exit from growth arrest after sulfur starvation. We provide two major arguments in favor of Sul1 and Sul2 acting as transceptors for signaling to PKA. First, the sulfate analogue, d-glucosamine 2-sulfate, acted as a non-transported agonist of signaling by Sul1 and Sul2. Second, mutagenesis to Gln of putative H⁺-binding residues, Glu-427 in Sul1 or Glu-443 in Sul2, abolished transport without affecting signaling. Hence, Sul1,2 can function as pure sulfate sensors. Sul1^E427Q and Sul2^E443Q are also deficient in sulfate-induced endocytosis, which can therefore be uncoupled from signaling. Overall, our data suggest that transceptors can undergo independent conformational changes, each responsible for triggering different downstream processes. The Sul1 and Sul2 transceptors are the first identified plasma membrane sensors for extracellular sulfate. High affinity transporters induced upon starvation for their substrate may generally act as transceptors during exit from starvation.

Functional interactions between potassium and phosphate homeostasis in Saccharomyces cerevisiae

Maintenance of ion homeostatic mechanisms is essential for living cells, including the budding yeast Saccharomyces cerevisiae. Whereas the impact of changes in phosphate metabolism on metal ion homeostasis has been recently examined, the inverse effect is still largely unexplored. We show here that depletion of potassium from the medium or alteration of diverse regulatory pathways controlling potassium uptake, such as the Trk potassium transporters or the Pma1 H(+) -ATPase, triggers a response that mimics that of phosphate (Pi) deprivation, exemplified by accumulation of the high-affinity Pi transporter Pho84. This response is mediated by and requires the integrity of the PHO signaling pathway. Removal of potassium from the medium does not alter the amount of total or free intracellular Pi, but is accompanied by decreased ATP and ADP levels and rapid depletion of cellular polyphosphates. Therefore, our data do not support the notion of Pi being the major signaling molecule triggering phosphate-starvation responses. We also observe that cells with compromised potassium uptake cannot grow under limiting Pi conditions. The link between potassium and phosphate homeostasis reported here could explain the invasive phenotype, characteristic of nutrient deprivation, observed in potassium-deficient yeast cells.

Evolutionary relationships and functional diversity of plant sulfate transporters

Sulfate is an essential nutrient cycled in nature. Ion transporters that specifically facilitate the transport of sulfate across the membranes are found ubiquitously in living organisms. The phylogenetic analysis of known sulfate transporters and their homologous proteins from eukaryotic organisms indicate two evolutionarily distinct groups of sulfate transport systems. One major group named Tribe 1 represents yeast and fungal SUL, plant SULTR, and animal SLC26 families. The evolutionary origin of SULTR family members in land plants and green algae is suggested to be common with yeast and fungal SUL and animal anion exchangers (SLC26). The lineage of plant SULTR family is expanded into four subfamilies (SULTR1-SULTR4) in land plant species. By contrast, the putative SULTR homologs from Chlorophyte green algae are in two separate lineages; one with the subfamily of plant tonoplast-localized sulfate transporters (SULTR4), and the other diverged before the appearance of lineages for SUL, SULTR, and SLC26. There also was a group of yet undefined members of putative sulfate transporters in yeast and fungi divergent from these major lineages in Tribe 1. The other distinct group is Tribe 2, primarily composed of animal sodium-dependent sulfate/carboxylate transporters (SLC13) and plant tonoplast-localized dicarboxylate transporters (TDT). The putative sulfur-sensing protein (SAC1) and SAC1-like transporters (SLT) of Chlorophyte green algae, bryophyte, and lycophyte show low degrees of sequence similarities with SLC13 and TDT. However, the phylogenetic relationship between SAC1/SLT and the other two families, SLC13 and TDT in Tribe 2, is not clearly supported. In addition, the SAC1/SLT family is absent in the angiosperm species analyzed. The present study suggests distinct evolutionary trajectories of sulfate transport systems for land plants and green algae.

Chloride homeostasis in Saccharomyces cerevisiae: high affinity influx, V-ATPase-dependent sequestration, and identification of a candidate Cl- sensor

Chloride homeostasis in Saccharomyces cerevisiae has been characterized with the goal of identifying new Cl⁻ transport and regulatory pathways. Steady-state cellular Cl⁻ contents (∼0.2 mEq/liter cell water) differ by less than threefold in yeast grown in media containing 0.003–5 mM Cl⁻. Therefore, yeast have a potent mechanism for maintaining constant cellular Cl⁻ over a wide range of extracellular Cl⁻. The cell water:medium [Cl⁻] ratio is >20 in media containing 0.01 mM Cl⁻ and results in part from sequestration of Cl⁻ in organelles, as shown by the effect of deleting genes involved in vacuolar acidification. Organellar sequestration cannot account entirely for the Cl⁻ accumulation, however, because the cell water:medium [Cl⁻] ratio in low Cl⁻ medium is ∼10 at extracellular pH 4.0 even in vma1 yeast, which lack the vacuolar H⁺-ATPase. Cellular Cl⁻ accumulation is ATP dependent in both wild type and vma1 strains. The initial ³⁶Cl⁻ influx is a saturable function of extracellular [³⁶Cl⁻] with K_1/2 of 0.02 mM at pH 4.0 and >0.2 mM at pH 7, indicating the presence of a high affinity Cl⁻ transporter in the plasma membrane. The transporter can exchange ³⁶Cl⁻ for either Cl⁻ or Br⁻ far more rapidly than SO₄⁼, phosphate, formate, HCO₃⁻, or NO₃⁻. High affinity Cl⁻ influx is not affected by deletion of any of several genes for possible Cl⁻ transporters. The high affinity Cl⁻ transporter is activated over a period of ∼45 min after shifting cells from high-Cl⁻ to low-Cl⁻ media. Deletion of ORF YHL008c (formate-nitrite transporter family) strongly reduces the rate of activation of the flux. Therefore, Yhl008cp may be part of a Cl⁻-sensing mechanism that activates the high affinity transporter in a low Cl⁻ medium. This is the first example of a biological system that can regulate cellular Cl⁻ at concentrations far below 1 mM.

Coupling of secondary active transport with a deltamu-H+

Most nutrients and ions in bacteria, yeasts, algae, and plants are transported uphill at the expense of a gradient of the electrochemical potential of protons deltamu-H+ (a type of secondary active transport). Diagnosis of such transports rests on the determination of the transmembrane electrical potential difference deltapsi and the difference of pH at the two membrane sides. The behavior of kinetic parameters K(T) (the half-saturation constant) and J(max), (the maximum rate of transport) upon changing driving ion concentrations and electrical potentials may be used to determine the molecular details of the transport reaction. Equilibrium accumulation ratios of driven solutes are expected to be in agreement with the deltapsi and deltapH measured independently, as well as with the Haldane-type expression involving K(T) and J(max). Different stoichiometries of H+/solute, as well as intramembrane effects of pH and deltapsi, may account for some of the observed inconsistencies.

Chloroplast sulfate transport in green algae--genes, proteins and effects

This review summarizes evidence at the molecular genetic, protein and regulatory levels concerning the existence and function of a putative ABC-type chloroplast envelope-localized sulfate transporter in the model unicellular green alga Chlamydomonas reinhardtii. From the four nuclear genes encoding this sulfate permease holocomplex, two are coding for chloroplast envelope-targeted transmembrane proteins (SulP and SulP2), a chloroplast stroma-targeted ATP-binding protein (Sabc) and a substrate (sulfate)-binding protein (Sbp) that is localized on the cytosolic side of the chloroplast envelope. The sulfate permease holocomplex is postulated to consist of a SulP-SulP2 chloroplast envelope transmembrane heterodimer, flanked by the Sabc and the Sbp proteins on the stroma side and the cytosolic side of the inner envelope, respectively. The mature SulP and SulP2 proteins contain seven transmembrane domains and one or two large hydrophilic loops, which are oriented toward the cytosol. The corresponding prokaryotic-origin genes (SulP and SulP2) probably migrated from the chloroplast to the nuclear genome during the evolution of Chlamydomonas reinhardtii. These genes, or any of its homologues, have not been retained in vascular plants, e.g. Arabidopsis thaliana, although they are encountered in the chloroplast genome of a liverwort (Marchantia polymorpha). The function of the SulP protein was probed in antisense transformants of C. reinhardtii having lower expression levels of the SulP gene. Results showed that cellular sulfate uptake capacity was lowered as a consequence of attenuated SulP gene expression in the cell, directly affecting rates of de novo protein biosynthesis in the chloroplast. The antisense transformants exhibited phenotypes of sulfate-deprived cells, displaying slow rates of light-saturated oxygen evolution, low levels of Rubisco in the chloroplast and low steady-state levels of the Photosystem II D1 reaction center protein. The role of the chloroplast sulfate transport in the uptake and assimilation of sulfate in Chlamydomonas reinhardtii is discussed along with its impact on the repair of Photosystem II from a frequently occurring photo-oxidative damage and H2-evolution related metabolism in this green alga.

Metabolism of sulfur amino acids in Saccharomyces cerevisiae

Sulfur amino acid biosynthesis in Saccharomyces cerevisiae involves a large number of enzymes required for the de novo biosynthesis of methionine and cysteine and the recycling of organic sulfur metabolites. This review summarizes the details of these processes and analyzes the molecular data which have been acquired in this metabolic area. Sulfur biochemistry appears not to be unique through terrestrial life, and S. cerevisiae is one of the species of sulfate-assimilatory organisms possessing a larger set of enzymes for sulfur metabolism. The review also deals with several enzyme deficiencies that lead to a nutritional requirement for organic sulfur, although they do not correspond to defects within the biosynthetic pathway. In S. cerevisiae, the sulfur amino acid biosynthetic pathway is tightly controlled: in response to an increase in the amount of intracellular S-adenosylmethionine (AdoMet), transcription of the coregulated genes is turned off. The second part of the review is devoted to the molecular mechanisms underlying this regulation. The coordinated response to AdoMet requires two cis-acting promoter elements. One centers on the sequence TCACGTG, which also constitutes a component of all S. cerevisiae centromeres. Situated upstream of the sulfur genes, this element is the binding site of a transcription activation complex consisting of a basic helix-loop-helix factor, Cbf1p, and two basic leucine zipper factors, Met4p and Met28p. Molecular studies have unraveled the specific functions for each subunit of the Cbf1p-Met4p-Met28p complex as well as the modalities of its assembly on the DNA. The Cbf1p-Met4p-Met28p complex contains only one transcription activation module, the Met4p subunit. Detailed mutational analysis of Met4p has elucidated its functional organization. In addition to its activation and bZIP domains, Met4p contains two regulatory domains, called the inhibitory region and the auxiliary domain. When the level of intracellular AdoMet increases, the transcription activation function of Met4 is prevented by Met30p, which binds to the Met4 inhibitory region. In addition to the Cbf1p-Met4p-Met28p complex, transcriptional regulation involves two zinc finger-containing proteins, Met31p and Met32p. The AdoMet-mediated control of the sulfur amino acid pathway illustrates the molecular strategies used by eucaryotic cells to couple gene expression to metabolic changes.

Flux distributions in anaerobic, glucose-limited continuous cultures of Saccharomyces cerevisiae

A stoichiometric model describing the anaerobic metabolism of Saccharomyces cerevisiae during growth on a defined medium was derived. The model was used to calculate intracellular fluxes based on measurements of the uptake of substrates from the medium, the secretion of products from the cells, and of the rate of biomass formation. Furthermore, measurements of the biomass composition and of the activity of key enzymes were used in the calculations. The stoichiometric network consists of 37 pathway reactions involving 43 compounds of which 13 were measured (acetate, CO2, ethanol, glucose, glycerol, NH4+, pyruvate, succinate, carbohydrates, DNA, lipids, proteins and RNA). The model was used to calculate the production rates of malate and fumarate and the ethanol measurement was used to validate the model. All rate measurements were performed on glucose-limited continuous cultures in a high-performance bioreactor. Carbon balances closed within 98%. The calculations comprised flux distributions at specific growth rates of 0.10 and 0.30 h-1. The fluxes through reactions located around important branch points of the metabolism were compared, i.e. the split between the pentose phosphate and the Embden-Meyerhoff-Parnas pathways. Also the model was used to show the probable existence of a redox shunt across the inner mitochondrial membrane consisting of the reactions catalysed by the mitochondrial and the cytosolic alcohol dehydrogenase. Finally it was concluded that cytosolic isocitrate dehydrogenase is probably not present during growth on glucose. The importance of basing the flux analysis on accurate measurements was demonstrated through a sensitivity analysis. It was found that the accuracy of the measurements of CO2, ethanol, glucose, glycerol and protein was critical for the correct calculation of the flux distribution.

Enhancement of synthesis and activity of yeast transport proteins by metabolic substrates

The transport rates of amino acids, ranging from L-Glu to L-Lys, uracil, adenine and sulfate and phosphate anions by Saccharomyces cerevisiae are greatly increased by preincubation with D-glucose in a nongrowth medium when a de novo synthesis of proteins takes place. In addition, some substrates, especially the inorganic anions, require the presence of glucose during their transport. This requirement has to do both with ongoing protein synthesis and degradation, as well as with providing energy and/or activating the plasma membrane H(+)-ATPase which supplies the protons to the H+ symports studied here.

Are proton symports in yeast directly linked to H(+)-ATPase acidification?

Transport of amino acids in Saccharomyces cerevisiae is an H(+)-driven secondary active transport. Inhibitors of the plasma membrane H(+)-ATPase, particularly heavy water, diethylstilbestrol and suloctidil, were shown to affect the H(+)-extruding ATPase activity as well as the ATP-hydrolyzing activity, to a similar degree as they inhibited the transport of amino acids. The inhibitors had virtually no effect on the membrane electric potential or on the delta pH which constitute the thermodynamically relevant source of energy for these transports. Transport of acidic amino acids was affected much more than that of the neutral and especially of the basic ones. The effects were greater with higher amino acid concentrations. All this is taken as evidence that the amino acid carriers respond kinetically to the presence of protons directly at the membrane site where they are extruded by the H(+)-ATPase, rather than to the overall protonmotive force.

The stimulation by salts of hexose phosphate uptake by Escherichia coli

Hexose phosphate uptake in Escherichia coli is stimulated by salts. KCl and MgCl2 stimulate to about the same extent, but Mg2+ is effective at a tenth the concentration of K+. At higher concentrations, both salts inhibit. The stimulation by a series of salts correlates strongly with the hydrated radius of the cation, with small ions more effective than large. There are effects by the anion, but they do not correlate with any simple property. Cells accumulate glucose 6-phosphate to a higher concentration in the presence of KCl than in its absence. The maximum velocity of glucose 6-phosphate uptake is stimulated by KCl, as is the ratio V/Km.

Evidence for two distinct intracellular pools of inorganic sulfate in Penicillium notatum

A strain of Penicillium notatum unable to metabolize inorganic sulfate can accumulate sulfate internally to an apparent equilibrium concentration 10(5) greater than that remaining in the medium. The apparent Keq is near constant at all initial external sulfate concentrations below that which would eventually exceed the internal capacity of the cells. Under equilibrium conditions of zero net flux, external 35SO42- exchanges with internal, unlabeled SO42- at a rate consistent with the kinetic constants with the sulfate transport system. Efflux experiments demonstrated that sulfate occupies two distinct intracellular pools. Pool 1 is characterized by the rapid release of 35SO42- when the suspension of preloaded cells is adjusted to 10 mM azide at pH 8.4 (t 1/2, 0.38 min). 35SO42- in pool 1 also rapidly exchanges with unlabeled medium sulfate. Pool 2 is characterized by the slow release of 35SO42- induced by azide at pH 8.4 or unlabeled sulfate (t 1/2, 32 to 49 min). Early in the 35SO42- accumulation process, up to 78% of the total transported substrate is found in pool 1. At equilibrium, pool 1 accounts for only about 2% of the total accumulated 35SO42-. The kinetics of 35SO42- accumulation is consistent with the following sequential process: medium----pool 1----pool 2. Monensin (33 microns) accelerates the transfer of 35SO42- from pool 1 to pool 2. Valinomycin (0.2 microM) and tetraphenylboron- (1 mM) retard the transfer of 35SO42- from pool 1 to pool 2. At the concentrations used, neither of the ionophores nor tetraphenylboron- affect total 35SO42- uptake. Pool 2 may reside in a vacuole or other intracellular organelle. A model for the transfer of sulfate from pool 1 to pool 2 is presented.

Evidence for proton/sulfate cotransport and its kinetics inLemna gibba G1

Sulfate uptake into duckweed (Lemna gibba G1) was studied by means of [(35)S]sulfate influx and measurements of electrical membrane potential. Uptake was strongly regulated by the intracellular content of soluble sulfate. At the onset of sulfate uptake the membrane potential was transiently depolarized. Fusicoccin stimulated uptake up to 165% of the control even at pH 8. It is suggested that sulfate uptake is energized in the whole pH range by a 3H(+)/sulfate cotransport mechanism. Kinetics of sulfate uptake and sulfate-induced membrane depolarization in the concentration range of 5 μM to 1 mM sulfate at pH 5.7 was best described by two Michaelis-Menten terms without any linear component. The second system had a lower affinity for sulfate and was fully active only at sufficiently high proton concentrations.

A sulfate, sulfite and thiosulfate incorporating system in Candida utilis

Sulfate, sulfite and thiosulfate incorporation in the yeast Candida utilis is inhibited by extracellular sulfate, sulfite and thiosulfate and by sulfate analogues selenate, chromate and molybdate. The three processes are blocked if sulfate, sulfite, thiosulfate, cysteine and homocysteine are allowed to accumulate endogenously. Incorporation of the three inorganic sulfur oxy anions is inactivated by heat at the same rate. Mutants previously shown to be defective in sulfate incorporation are also affected in sulfite and thiosulfate uptake. Revertants of these mutants selected by plating in ethionine-supplemented minimal medium recovered the capacity to incorporate sulfate, sulfite and thiosulfate. These results taken together with previous evidence demonstrate the existence of a common sulfate, sulfite and thiosulfate incorporating system in this yeast.

Kinetic analysis of H+/methyl beta-D-thiogalactoside symport in Saccharomyces fragilis

A theoretical description of initial uptake kinetics of H+/sugar symport is given, with emphasis on the differences between carrier and non-carrier systems. Transport of methyl beta-D-thiogalactoside in Saccharomyces fragilis is shown to proceed via the inducible lactose transporter. Uptake of this sugar stimulates electrogenic H+ influx. Together with the correlation between methyl beta-D-thiogalactoside accumulation and the proton-motive force this shows that transport proceeds via H+ symport. Kinetic analysis of initial influx revealed that transport proceeds via a single transport system, sensitive to changes in membrane potential. The pH dependence of the kinetic parameters showed that Kapp is almost pH insensitive, whereas Vapp decreases strongly at increasing extracellular pH. It is shown that transport proceeds, most likely, via a non-carrier system, with random binding of H+ and sugar, in a system where binding of the first ligand does not influence binding of the second.

Transmembrane ferricyanide reduction by cells of the yeast Saccharomyces cerevisiae

Both respiratory-competent and respiratory-deficient yeast cells reduce external ferricyanide. The reduction is stimulated by ethanol and inhibited by the alcohol dehydrogenase inhibitor, pyrazole. The reduction of ferricyanide is not inhibited by inhibitors of mitochondrial or microsomal ferricyanide reduction. Cells in exponential-phase growth show a much higher rate of ferricyanide reduction. The reduction of ferricyanide is accompanied by increased release of protons by the yeast cells. We propose that the ferricyanide reduction is carried out by a transmembrane NADH dehydrogenase.

Proton-sulfate co-transport: mechanism of H+ and sulfate addition to the chloride transporter of human red blood cells

Proton and sulfate inhibition of the obligatory chloride-chloride exchange of human erythrocytes was measured at 0 degrees C to determine their mechanism of reaction with the anion transporter. The proton and sulfate that are co-transported by this mechanism at higher temperatures behaved as nontransported inhibitors at 0 degrees C. We analyzed the data in terms of four molecular mechanisms: (1) HSO4- addition to the transporter; (2) ordered addition with the proton first; (3) ordered addition with the sulfate first; (4) random addition to the transporter. The Dixon plots of 1/MCl vs. [SO4] at different proton concentrations were not parallel. Thus protons and sulfate ions were not mutually exclusive inhibitors. The slope of these Dixon plots was independent of pH above 7.0, which indicates that sulfate could bind to the unprotonated carrier and excludes the first two mechanisms. Protons were inhibitors of chloride flux in the absence of sulfate, which indicates that protons could bind to the unloaded carrier and excludes mechanism 3. The KI for sulfate was 4.35 +/0 0.36 mM. The pK for the protonatable group was 5.03 +/- 0.02. The binding of either a proton or sulfate to the carrier decreased the KI of the other by ninefold. The only simple mechanism consistent with the data is a random-ordered mechanism with more transporters loaded with a sulfate than loaded with a proton at the pH and sulfate concentrations of plasma.

Energy source for lithium efflux in yeast

The efflux of Li+ in yeast was found to depend on the protonmotive force. The ATP content of the cell regulated the efflux that was also sensitive to the decrease in the cell pH. We propose an electrogenic H+/Li+ antiport as the mechanism for the efflux of Li+.