Evidence for interactions between the energy-dependent transport of sugars and the membrane potential in the yeastRhodotorula gracilis (Rhodosporidium toruloides)

Determining mating type and ploidy in Rhodotorula toruloides and its effect on growth on sugars from lignocellulosic biomass

Rhodotorula toruloides is being developed for the use in industrial biotechnology processes because of its favorable physiology. This includes its ability to produce and store large amounts of lipids in the form of intracellular lipid bodies. Nineteen strains were characterized for mating type, ploidy, robustness for growth, and accumulation of lipids on inhibitory switchgrass hydrolysate (SGH). Mating type was determined using a novel polymerase chain reaction (PCR)-based assay, which was validated using the classical microscopic test. Three of the strains were heterozygous for mating type (A1/A2). Ploidy analysis revealed a complex pattern. Two strains were triploid, eight haploid, and eight either diploid or aneuploid. Two of the A1/A2 strains were compared to their parents for growth on 75%v/v concentrated SGH. The A1/A2 strains were much more robust than the parental strains, which either did not grow or had extended lag times. The entire set was evaluated in 60%v/v SGH batch cultures for growth kinetics and biomass and lipid production. Lipid titers were 2.33-9.40 g/L with a median of 6.12 g/L, excluding the two strains that did not grow. Lipid yields were 0.032-0.131 (g/g) and lipid contents were 13.5-53.7% (g/g). Four strains had significantly higher lipid yields and contents. One of these strains, which had among the highest lipid yield in this study (0.131 ± 0.007 g/g), has not been previously described in the literature.

SUMMARY

The yeast Rhodotorula toruloides was used to produce oil using sugars extracted from a bioenergy grass.

Stimulation of zero-trans rates of lactose and maltose uptake into yeasts by preincubation with hexose to increase the adenylate energy charge

Initial rates of sugar uptake (zero-trans rates) are often measured by incubating yeast cells with radiolabeled sugars for 5 to 30 s and determining the radioactivity entering the cells. The yeast cells used are usually harvested from growth medium, washed, suspended in nutrient-free buffer, and stored on ice before they are assayed. With this method, the specific rates of zero-trans lactose uptake by Kluyveromyces lactis or recombinant Saccharomyces cerevisiae strains harvested from lactose fermentations were three- to eightfold lower than the specific rates of lactose consumption during fermentation. No significant extracellular beta-galactosidase activity was detected. The ATP content and adenylate energy charge (EC) of the yeasts were relatively low before the [(14)C]lactose uptake reactions were started. A short (1- to 7-min) preincubation of the yeasts with 10 to 30 mM glucose caused 1.5- to 5-fold increases in the specific rates of lactose uptake. These increases correlated with increases in EC (from 0.6 to 0.9) and ATP (from 4 to 8 micromol x g dry yeast(-1)). Stimulation by glucose affected the transport V(max) values, with smaller increases in K(m) values. Similar observations were made for maltose transport, using a brewer's yeast. These findings suggest that the electrochemical proton potential that drives transport through sugar/H(+) symports is significantly lower in the starved yeast suspensions used for zero-trans assays than in actively metabolizing cells. Zero-trans assays with such starved yeast preparations can produce results that seriously underestimate the capacity of sugar/H(+) symports. A short exposure to glucose allows a closer approach to the sugar/H(+) symport capacity of actively metabolizing cells.

Transbilayer phosphatidylethanolamine movements in the yeast plasma membrane. Evidence for a protein-mediated, energy-dependent mechanism

Aminophospholipid movements in the plasma membrane of higher eukaryotic cells seem to be regulated by an ATP-dependent, protein-mediated process. To examine whether similar mechanisms exist in yeast cells, we have analysed phosphatidylethanolamine (PtdEtn) distributions in Saccharomyces cerevisiae (A184D) cells under a variety of conditions, with trinitrobenzenesulfonic acid and fluorescamine as the external membrane probes. The levels of external PtdEtn in the intact cells were reduced to about 50% by pretreatment of the cells with inhibitors of mitochondrial ATP synthesis, ATPase inhibitors or protein-sulfhydryl-group-modifying reagents, or by depletion of the cells of ATP by metabolic starvation. The levels of external PtdEtn could be restored to normal by repletion of the energy-depleted cells with ATP. Furthermore, treatment of the energy-depleted cells with sulfhydryl-modifying reagents did not cause further reduction in the external PtdEtn levels but decreased the accessibility of PtdEtn to fluorescamine after restoration of the cellular ATP levels to normal in these cells. These results demonstrate an involvement of an ATP-dependent, protein-mediated process(es) in the regulation of the PtdEtn distribution across the plasma-membrane bilayer of yeast cells. The results are discussed with regard to possible models that can generate and maintain the transbilayer phospholipid asymmetry in the yeast plasma membrane.

Small lipid-soluble cations are not membrane voltage probes for Neurospora or Saccharomyces

Small lipid-soluble cations, such as tetraphenylphosphonium (TPP+) and tetraphenylarsonium (TPA+) are frequently used as probes of membrane voltage (delta psi, or Vm) for small animal cells, organelles, and vesicles. Because much controversy has accompanied corresponding measurements on 'walled' eukaryotic cells (plants, fungi), we studied their transport and relation to Vm in the large-celled fungus Neurospora crassa-where Vm can readily be determined with microelectrodes-as well as in the most commonly used model eukaryotic cell, the yeast Saccharomyces cerevisiae. We found no reasonable conditions under which the distribution of TPP+ or TPA+, between the cytoplasm (i) and extracellular solution (o), can serve to estimate Vm, even roughly, in either of these organisms. When applied at probe concentrations (i.e., < or = 100 microM, which did not depolarize the cells nor deplete ATP), TPP+ stabilized at ratios (i/o) below 30 in both organisms. That would imply apparent Vm values positive to -90 mV, in the face of directly measured Vm values (in Neurospora) negative to -180 mV. When applied at moderate or high concentrations (1-30 mM), TPP+ and TPA+ induced several phases of depolarization and changes of membrane resistance (Rm), as well as depletion of cytoplasmic energy stores. Only the first phase depolarization, occurring within the perfusion-turnover time and accompanied by a nearly proportionate decline of Rm, could have resulted from TPP+ or TPA+ currents per se. And the implied currents were small. Repeated testing, furthermore, greatly reduced the depolarizing effects of these lipid-soluble ions, implicating an active cellular response to decrease membrane permeability.

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 plasma membrane electrical gradient (membrane potential) in Leishmania donovani promastigotes and amastigotes

The equilibrium distribution of tetraphenylphosphonium bromide was used to measure the membrane potential in Leishmania donovani amastigotes and promastigotes and to investigate mechanisms underlying the maintenance of membrane potential. At pH 7.0, membrane potential ranges between -90 and -113 mV. Increasing the external concentrations of hydrogen or potassium ions decreased membrane potential as did treatments with carbonylcyanide chlorophenylhydrazone or valinomycin. These observations are consistent with a membrane potential set by hydrogen and potassium ion diffusion gradients. Anaerobiosis lowered membrane potential, suggesting the involvement of ATPase(s) in maintaining membrane potential. Membrane potential was insensitive to treatment with ouabain, demonstrating the absence of a Na+/K(+)-ATPase. Treatment with dicyclohexylcarbodiimide caused a temporary hyperpolarization of the membrane suggesting the participation of a proton ATPase in the maintenance of membrane potential. Determination of the membrane potential makes it possible to quantitate the total proton motive force which is the force for active transport across the parasite membrane.

Transport of L-tryptophan in Saccharomyces cerevisiae

In addition to the general amino acid transport system (GAP) of S. cerevisiae L-tryptophan is transported by another system with approximately 25% capacity of GAP, with a KT of 0.41 +/- 0.08 mmol/L and with a similar specificity as GAP (lower inhibition by Met, Pro, Ser, Thr and 2-aminoisobutyric acid; greater inhibition by Glu and His). The pH optimum of this system is at 5.0-5.5, activation energy above the transition point (20 degrees C) was 20 kJ/mol, below the transition point 55 kJ/mol. The transport by this system was virtually unidirectional, efflux amounting to at most 10% into a tryptophan-free medium. The transport itself was blocked by 2,4-dinitrophenol, antimycin A and uranyl nitrate. The system was synthesized de novo during preincubation with glucose = fructose greater than trehalose greater than ethanol within 30 min, and was degraded with a half-time of 15 min in the absence of further synthesis. The accumulation ratios of L-tryptophan in gap1 mutants were concentration-dependent (200:1 at 1 mumol L-Trp/L, 4:1 at 2.5 mmol L-Trp/L) and decreased with increasing suspension density from 200:1 to 5:1 (for 10 mumol L-Trp/L). The involvement of hydrogen ions in the uptake was clearly demonstrated by the effect of D2O even if it could not be established by either shifts of pHout or membrane depolarization.

Deuterons cannot replace protons in active transport processes in yeast

Replacement of ordinary water with heavy water causes a sharp reduction of the rates of both primary hydrogen ion transport (at the plasma membrane ATPase) and secondary symports (H(+)-associated transports of sugars and amino acids) in several species of yeast. At the same time, the hydrolytic activity of the ATPase is affected only very little. Likewise, the membrane potential, the delta pH and, correspondingly, the accumulation ratios of the various symported solutes are altered much less. This serves as evidence that H+ or H3O+ ions are direct participants in the various active transports of nutrients in yeast.

Physiology of mutants with reduced expression of plasma membrane H+-ATPase

Two mutations containing insertions and deletions in the promoter in the plasma membrane H+-ATPase gene (PMA1) of Saccharomyces cerevisiae have been introduced into the genome by homologous recombination, replacing the wild-type gene. The resulting strains have 15 and 23% of the wild-type ATPase content. Decreased levels of ATPase correlate with decreased rates of proton efflux and decreased uptake rates of amino acids, methylamine, hygromycin B and tetraphenylphosphonium. This supports a central role of the enzyme in yeast bioenergetics. However, the final accumulation gradient of tetraphenylphosphonium is not affected by the mutations and that of methylamine and 2-aminoisobutyric acid is only decreased in the most extreme mutant. Apparently, kinetic constraints seem to prevent the equilibration of yeast active transports with the electrochemical proton gradient. As expected from their transport defects, the ATPase-deficient mutants are more resistant to hygromycin B and more sensitive to acidification than wild-type yeast. Mutant cells are very elongated, suggesting a structural role of the ATPase in the yeast surface.

Analysis of the H+/sugar symport in yeast under conditions of depolarized plasma membrane

H+/sugar symport in the obligatory aerobic yeast Rhodotorula glutinis was analyzed under conditions where the plasma membrane was selectively depolarized by the lipophilic cation tetraphenylphosphonium (TPP+). Control experiments showed that this treatment did not impair the transmembrane delta pH, the cell energy charge, and the function of plasma membrane H+-ATPase. The kinetic data were fitted to elementary functions derived from a model constructed on the basis of some simplifying premises for ordered (either C + H+ + S or C + S + H+) and random reaction mechanisms. In addition, the comparison of the kinetic parameters in fully energized and depolarized cells provided information about the free carrier charge. It was concluded that the binding sequence of formation of the ternary carrier/H+/substrate complex follows a random mechanism and that the carrier bears a negative charge.

Tetraphenylphosphonium is an indicator of negative membrane potential in Candida albicans

The characteristics of the uptake of lipophilic cations tetraphenylphosphonium (TPP+) into Candida albicans have been investigated to establish whether TPP+ can be used as a membrane potential probe for this yeast. A membrane potential (delta psi, negative inside) across the plasma membrane of C. albicans was indicated by the intracellular accumulation of TPP+. The steady-state distribution of TPP+ was reached within 60 min and varied according to the expected changes of delta psi. Agents known to depolarize membrane potential caused a rapid and complete efflux of accumulated TPP+. The initial influx of TPP+ was linear over a wide range of TPP+ concentrations (2.5-600 microM), indicating a non mediated uptake. Thus, TPP+ is a suitable delta psi probe for this yeast.

Phosphoinositides provide a regulatory mechanism of surface charge and active transport

Yeast cells, when grown in the presence of arsenate, are capable of accumulating phosphoinositides (PI) at the expense of inhibiting their degradation more than their synthesis. PI levels return to normal when the cells are cultured or exposed to media without arsenate. These reversible changes are employed as a tool to test the effect of inositide accumulation and dynamics on several membrane properties. In the PI-rich cells, phosphate and arsenate transport from low external concentrations (high affinity systems), as well as the transport of glycine, which enter the cells accompanied by protons, were increased. The proton ejection energized by glucose is also enhanced in the PI-rich cells that show a more efficient potassium inflow at pH 4.0-4.5. The membrane surface potential of the PI-rich cells was found to be 2-times higher than that of the normal cells, in agreement with the 2-fold increment in the PI. All the above mentioned alterations in membrane properties are reverted when the PI content of the PI-rich cells is reduced to the level of normal cells. The results show the participation of the phosphoinositides in the formation, maintenance and regulation of the membrane surface potential and their possible influence upon transport mechanisms.

Trehalase activation in yeasts is mediated by an internal acidification

It has been reported that the addition of glucose, uncouplers and nystatin to yeast cells grown in a sugarfree medium causes trehalase activation; it has been postulated that this activation might be mediated by the depolarization of the plasma membrane. In this article the values of membrane potential and pH gradient across the plasma membrane of Saccharomyces cerevisiae have been determined under the same conditions as those in which trehalase is activated. Membrane potential was evaluated from the distribution of triphenylmethylphosphonium, the pH gradient from the distribution of benzoic acid across the plasma membrane. When the effect of several agents on the two components of the electrochemical proton gradient across the plasma membrane of ethanol-grown yeast cells were studied, under trehalase activation conditions, the following observations were made. (a) The addition of glucose activated trehalase and caused internal acidification of the cells, but had practically no effect on the membrane potential. (b) The addition of 200 mM KCl depolarized the cell membrane but did not affect the internal pH, nor trehalase activity. (c) Although carbonyl cyanide m-chlorophenylhydrazone depolarized the cells at external pH 6.0 and 7.0, it only activated trehalase at an external pH 6.0, leading to the acidification of the internal medium at this pH. (d) Nystatin caused an increase in the triphenylmethylphosphonium accumulation at external pH 6.0 and 7.0, but only activated trehalase at external pH 6.0, causing acidification of the cell interior at this pH. (e) Activation of trehalase was also observed when the internal acidification was caused by addition of a weak acid such as acetate. It is concluded that trehalase activation is mediated by an intracellular acidification and is independent of the membrane potential.

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.

The electrochemical H+ gradient in the yeast Rhodotorula glutinis

The electrochemical gradient of protons, delta mu H+, was estimated in the obligatory aerobic yeast Rhodotorula glutinis in the pH0 range from 3 to 8.5. The membrane potential, delta psi, was measured by steady-state distribution of the hydrophobic ions, tetraphenylphosphonium (TPP+) for negative delta psi above pH0 4.5, and thiocyanate (SCN-) for positive delta psi below pH0 4.5. The chemical gradient of H+ was determined by measuring the chemical shift of intracellular Pi by 31P-NMR at given pH0 values. The values of pHi increased almost linearly from 7.3 at pH0 3 to 7.8 at pH0 8.5. In the physiological pH0 range from 3.5 to 6, delta mu H+ was fairly constant at values between 17-18 KJ mol-1, gradually decreasing at pH0 above 6. In deenergized cells, the intracellular pHi decreased to values as low as 6, regardless of whether the cell suspension was buffered at pH0 4.5 or 7.5. There was no membrane potential detectable in deenergized cells.

Possible energization of K+ accumulation into metabolizing yeast by the protonmotive force. Binding correction to be applied in the calculation of the yeast membrane potential from tetraphenylphosphonium distribution

Membrane potentials of yeast cells, Saccharomyces cerevisiae, calculated from the equilibrium distribution of tetraphenylphosphonium (TPP) between cell-water and medium should be corrected for a contribution due to binding of TPP to intracellular constituents. The magnitude of this correction depends upon the way in which it is determined. In cells permeabilized by boiling, cell-binding is much higher than in cells permeabilized by repeated freezing and thawing. The binding corrections are 75 +/- 1 mV and 49 +/- 7 mV, respectively. The binding correction obtained from TPP distribution between deenergized cells and medium is much lower and amounts to 19 +/- 9 mV. The latter value is probably more reliable. It is supposed that permeabilization of the cells by boiling or repeated freezing and thawing unmasks potential TPP binding groups in the cell. The K+ accumulation into anaerobically metabolizing yeast cells can be accounted for almost quantitatively by a cotransport of protons and K+ ions if the lower binding correction is applied. This means that K+ accumulation into the yeast cell may be driven by the sum of the protonmotive force and the membrane potential.

Activation of trehalase by membrane-depolarizing agents in yeast vegetative cells and ascospores

The membrane-depolarizing agents 2,4-dinitrophenol, carbonylcyanide m-chlorophenylhydrazone, and nystatin are known to cause a rapid increase in the cyclic AMP level in fungal cells. Addition of these proton ionophores to yeast stationary-phase cells or ascospores causes an immediate 10-fold increase in trehalase activity. This observation is in agreement with a role for cyclic AMP-induced phosphorylation in the activation process of trehalase. It also provides an explanation for previous results on the induction of trehalose breakdown by 2,4-dinitrophenol in resting yeast cells.

Membrane potential and cation permeability. A study with a nystatin-resistant mutant of Rhodotorula gracilis (Rhodosporidium toruloides)

Cells of a nystatin-resistant mutant of the obligatory aerobic yeast Rhodotorula gracilis displayed an electrical potential difference, delta psi, across the plasma membrane which was, in contrast to the wild-strain cells, virtually independent of the pH of cell suspensions down to 4.5. In addition, the delta psi in mutant cells was insensitive to extracellular K+ concentrations. The mutant cells failed to cotransport measurable amounts of H+ by the onset of monosaccharide transport and to take up K+ in exchange for H+. Taking into account the lower passive permeability of the mutant plasma membrane for cations, it has been concluded that the pH dependency of delta psi in wild-strain cells is correlated with the electrogenic leak of H+ back into the cells in course of increasing delta pH across the plasma membrane.

Passive fluxes of K+ and H+ in wild strain and nystatin-resistant mutant of Rhodotorula gracilis (ATCC 26194)

The passive fluxes of protons and potassium ions have been studied in the obligatory aerobic yeast Rhodotorula gracilis. The cellular energy metabolism was suspended by introducing anaerobic conditions. The H+-permeability of the plasma membrane was modified by adding an uncoupler under both aerobic and anaerobic conditions. Unfortunately, the plasma membrane of R. gracilis was insensitive to K+-ionophores. The passive flows of H+ and K+ under anaerobic and/or uncoupled conditions were electrically coupled and exhibited a constant stoicheiometry of 1:1. The H+ permeability of the plasmalemma was shown to determine the velocity of the passive K+-H+ exchange. The nystatin-resistant mutant M 67 displayed distinctly lower permeability for both H+ and K+, which can explain the observed differences in some transport characteristics of the two strains. In order to account for the properties of passive K+ flows, a membrane-potential-gated channel for K+ has been proposed. Evidence is presented that the inhibitor of the plasmalemma-bound H+-ATPase, N,N'-dicyclohexylcarbodiimide (DCCD), reduced at first the permeability for both K+ and H+ and only upon prolonged incubation the ATPase itself. Since DCCD effected an immediate hyperpolarization of the membrane potential, it has been concluded that the H+ does not slip through the H+-ATPase under deenergized conditions.

The energetics of D-fucose transport in Saccharomyces fragilis. The influence of the protonmotive force on sugar accumulation

The protonmotive force in Saccharomyces fragilis has been estimated under various experimental conditions. The transmembrane potential has been monitored with tetraphenylphosphonium and 3,3'-dipropylthiadicarbocyanine. The distribution ratio of these cations between intracellular and extracellular water appeared to be governed by the electrical potential difference across the membrane of this yeast strain. The transmembrane pH difference was deduced from dimethyloxazolidinedione uptake experiments and from direct measurements of intracellular pH after freezing and boiling of the cells. Both methods yielded similar results. D-Fucose is transported by S. fragilis via H+ symport, with a H+/fucose stoichiometry of approximately 1. Accumulation of this sugar appeared to be closely correlated with the protonmotive force.

A nystatin-resistant mutant of Rhodotorula gracilis. Transport properties and sterol content

A nystatin-resistant mutant of Rhodotorula gracilis was obtained by treatment of the wild strain cells with N-methyl-N-nitro-N-nitrosoguanidine and selected on agar plates containing 150 micrograms nystatin/ml. Three important transport functions of the plasma membrane of mutant cells: the accumulation of monosaccharides, the generation and maintenance of the pH-gradient and of the membrane potential, as well as the cell respiration were insensitive to at least 10(-5) M nystatin. This concentration of nystatin inhibited completely all these processes in wild strain cells. Analysis of cellular sterols revealed a defect of ergosterol biosynthesis in the mutant, which was localized at the last oxidative step between 5,6-dihydroergosterol and ergosterol.

The influence of uncouplers on facilitated diffusion of sorbose in Saccharomyces cerevisiae

Sorbose uptake in Saccharomyces cerevisiae, strain Delft 1, proceeds via mediated passive transport. In the cell sorbose is distributed in at least two compartments. Efflux studies showed that sorbose uptake in one of these compartments is not readily reversible. Uncouplers of oxidative phosphorylation inhibit both transport velocity and steady-state uptake level. It could be shown that these two effects are caused by different modes of action of the uncouplers. None of these two effects could be ascribed to changes of the electrochemical H+ gradient or of the intracellular pH. It is suggested that the inhibition of uptake velocity is caused by binding of the uncoupler to the sorbose translocator, thus lowering the transport activity. The uncoupler binding site is probably located at the intracellular fragment of the carrier. The second effect, reduction of the steady-state uptake level, is probably due to blocking of sorbose influx into the compartment that exhibits poor reversibility.

Some characteristics of tetraphenylphosphonium uptake into Saccharomyces cerevisiae

The characteristics of the uptake of the lipophilic cation tetraphenylphosphonium (TPP+) into Saccharomyces cerevisiae have been investigated in order to establish whether this compound can be used to monitor the membrane potential of his organism. Unlike dibenzyldimethylammonium, TPP+ is not translocated via the thiamine transport system, nor via another inducible translocation mechanism. On changing the experimental conditions the equilibrium potential of TPP+ varies according to expected changes of the membrane potential. TPP+ accumulation is higher in metabolizing cells than in non-metabolizing cells. In addition, decreasing the medium pH, addition of the proton conductor 2,4-dinitrophenol and addition of K+ all cause an apparent depolarization, whereas Ca2+ apparently hyperpolarizes the cell membrane. It is concluded that TPP+, if applied at low concentrations, can be used to measure the membrane potential of S. cerevisiae.

Membrane potentials in respiring and respiration-deficient yeasts monitored by a fluorescent dye

Changes in fluorescence of 3,3'-dipropylthiodicarbocyanine iodide which had been equilibrated with suspensions of the wild-type yeast Saccharomyces cerevisiae and of respiration-deficient mutants were followed. The changes have been attributed to changes of yeast membrane potentials, since the fluorescence with wild-type yeast could be affected in a predictable manner by uncouplers and the pore-forming agent nystatin. As in other systems, a rise of steady-state fluorescence was ascribed to depolarization and a drop of the fluorescence to hyperpolarization. (1) A considerable rise in steady-state fluorescence was brought about by addition of antimycin A or some other mitochondrial inhibitors to respiring cells. A major part of the composite membrane potential monitored in intact yeast cells appeared to be represented by the membrane potential of mitochondria. (2) Addition of D-glucose and of other substrates of hexokinase, including non-metabolizable 2-deoxy-D-glucose, induced a two-phase response of fluorescence, indicating transient depolarization followed by repolarization. Such a response was not elicited by other sugars which had been reported to be transported into the cells by a glucose carrier or by D-galactose in galactose-adapted cells. The depolarization was explained by electrogenic ATP exit from mitochondria to replenish the ATP consumed in the Hexokinase reaction and the repolarization by subsequent activation of respiration. (3) In non-respiring cells only a drop in fluorescence was induced by glucose and this was ascribed to an ATP-dependent polarization of the plasma membrane. (4) Steady-state fluorescence in suspensions of respiration-deficient mutants, lacking cytochrome a, cytochrome b, or both, was high an remained unaffected by uncouplers and nystatin. This indicates that membranes of the mutants may have been entirely depolarized. A partial polarization, apparently restricted to the plasma membrane, could be achieved by glucose addition.

Membrane potentials in yeast cells measured by direct and indirect methods

The membrane potential, delta psi, of various yeasts estimated from the distribution of tetraphenylphosphonium cations ranged from -50 to -120 mV, depending on species, incubation conditions and technique of measurement. Values obtained directly with a microelectrode in Endomyces magnusii were consistently lower than those determined indirectly.