Transport protein synthesis in non-growing yeast cells

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.

Effects of the Fenton reagent on transport in yeast

In the facultatively anaerobic yeast Saccharomyces cerevisiae the uptake rate and the accumulation ratio of 2-aminoisobutyric acid was decreased by some 30% by Fenton's reagent (FR), a powerful source of OH. radicals. Likewise, the uptake of glutamic acid, leucine and arginine was diminished. The mediated diffusion of 6-deoxy-D-glucose was not affected. The H+ symport of maltose and trehalose was inhibited by some 40% both in the initial rate and in the accumulation ratio. FR had a dramatic inhibitory effect when present during preincubation with 50 mmol/L glucose. In the obligately aerobic Lodderomyces elongisporus the uptake of all amino acids tested was decreased by 15-30%, that of 6-deoxy-D-glucose by about 10%. The initial rates of uptake of maltose and trehalose were depressed by FR by 40% and the acceleration of uptake observed after 8 min of incubation, was abolished by FR completely. Acidification rate of the external medium by S. cerevisiae in the presence of glucose or galactose was enhanced three-fold, that after subsequently added K+ was substantially decreased. FR appears to have a dual effect on sugar and amino acid transport processes in yeast: (1) it blocks carrier protein synthesis; (2) it inhibits the source of energy for transport. It does not appreciably affect the carrier proteins themselves.

Effect of potassium on amino acid transport in yeast

Starved yeast cells accumulated potassium when the cation plus glucose was present in the incubation medium. Under these conditions, an increased amino acid transport capacity was developed within 30 to 60 min in comparison with cells incubated only with glucose. There seems to be a correlation between K+ accumulation and an increase of the amino acid transport activity. Preincubation in the presence of potassium also produced an increased general protein synthesis. Transport systems are strongly inactivated by ammonium and this effect is partially reverted by potassium preincubation. Results also showed that potassium could 'protect' the amino acid transport systems from the inactivation produced by ammonium. It appears that potassium preincubation may have some effect on the rate of synthesis of the amino acid carriers, but effects of potassium appear to exist also on the degradation or inactivation of the carriers.

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.

Effects of the physiological state of five yeast species on H(+)-ATPase-related processes

Effects of starvation and glucose preincubation on membrane potential, ATPase-mediated acidification and glutamic acid transport were studied in yeast species Saccharomyces cerevisiae, Schizosaccharomyces pombe, Dipodascus magnusii, Lodderomyces elongisporus and Rhodotorula gracilis. The membrane potential was highest after preincubation with glucose in all species but L. elongisporus and R. gracilis. In all cases the membranes were depolarized in the presence of 20 mmol/L KCl and hyperpolarized with 50 mumol/L diethylstilbestrol (DES). The extracellular acidification caused by addition of glucose was highest after preincubation with glucose in all cases except in R. gracilis where there was none. In all cases except in R. gracilis addition of KCl caused a marked increase in the acidification rate. Addition of DES with glucose caused a large decrease in rate in S. cerevisiae but had much less effect on the other species. Transport of glutamic acid was clearly increased after pretreatment with glucose in S. cerevisiae, S. pombe and D. magnusii (mainly due to enhanced synthesis of the carrier) but actually decreased in R. gracilis and L. elongisporus. Addition of DES had an inhibitory effect in all species but much more pronounced in S. cerevisiae and S. pombe than in others. In general, both the acidification and the transport of glutamate were enhanced after preincubation with glucose but much more so in the semianaerobic species, such as S. cerevisiae, than in the strict aerobes (R. gracilis) where the effect was occasionally negative. There was no relationship between the ATPase-mediated acidification and the membrane potential.

Interaction of 2-deoxy-D-glucose and adenine with phosphate anion uptake in yeast

The transport of inorganic phosphate anions into yeast cells (after preincubation with glucose, fructose or another metabolizable sugar, and in the presence of glucose) shows two kinetic components with half-saturation constants of 40 mumol/L and 2.4 mmol/L. The uptake was strikingly stimulated by 2-deoxy-D-glucose (2-dGlc) at lower concentrations but inhibited above 100 mmol/L. A similar stimulation was caused by adenine (0.01-1 mmol/L) and a very small one by uracil and inorganic sulfate. It is suggested that either a phosphorylation reaction accompanies the transport (2-dGlc) or that some compounds stimulate the H(+)-ATPase more than inorganic phosphate itself and thus increase its rate of transport.

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.

Thialysine-resistant mutants and uptake of lysine in Schizosaccharomyces pombe

Mutants defective in lysine transport were isolated and characterized. After UV-mutagenesis colonies resistant to thialysine, a toxic analogue of lysine, were isolated and L-lysine uptake into the mutant strains was analyzed. Among the thialysine-resistant strains a group of mutants was found, where the half-saturation constant, KT, of the high-affinity transport system for lysine was higher than in the wild-type, the high-affinity transport system for basic amino acids being specifically affected. This was confirmed by a complementation test in which all the thialysine-resistant strains with a higher KT for lysine uptake belonged to one complementation group. Kinetic and genetic analysis showed that our mutants were identical with can1-1 mutants, showing that a single high-affinity system for the transport of basic amino acids exists in S. pombe.

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.

Peculiarities of amino acid transport in Schizosaccharomyces pombe: effects of growth medium

Transport systems for amino acids in the wild-type strain of Schizosaccharomyces pombe are not constitutive. During growth on different media no transport of acidic, neutral and basic amino acids is detectable. To acquire the ability to transport amino acids, cells must be preincubated with a metabolic source of energy, such as glucose. The appearance of transport activity is associated with protein synthesis (suppression by cycloheximide) at all phases of culture growth. After such preincubation the initial rate of amino acid uptake depends on the phase of growth of the culture and on the amount of glucose in the growth medium but not on the nitrogen source used. L-Proline and 2-aminoisobutyric acid are practically not transported under any of the conditions tested.

Incorporation of ionic channels from yeast plasma membranes into black lipid membranes

Recently, patch-clamping of yeast protoplasts has revealed the presence of plasma membrane K+ channels (Gustin, M. C., B. Martinac, Y. Saimi, M. R. Culberston, and C. Kung. 1986. Science (Wash. DC). 233:1195-1197). In this work we show that fusion of purified plasma membranes into planar bilayers allows the study of the yeast channels. The main cationic conductances detected were of 64 and 116 pS, however, larger and smaller conductances have been observed. The two main conductances were sensitive to the K+ channels blockers tetraethylammonium (TEA+) and Ba2+. Bionic experiments indicated that both conductances were K+ selective.

Transport of L-lysine in the fission yeast Schizosaccharomyces pombe

Systems of L-lysine transport in Schizosaccharomyces pombe are not constitutive, as at no phase of growth in a rich medium is lysine taken up. Transport activity appears only after preincubation of harvested cells with glucose or another suitable source of energy. If cycloheximide is added during this preincubation no transport systems are synthesized. After removal of glucose, the activity of the transport system decays with a half-time of 13 min. The transport of L-lysine into S. pombe cells from the stationary phase of growth preincubated for 60 min with 1% D-glucose is mediated by at least two systems, the high-affinity one with a Kt of 26 mumol/l and Jmax of 4.95 nmol/min per mg dry wt., the low-affinity one with a KT of 1.1 mmol/l and Jmax of 11.8 nmol/min per mg dry wt. The transport of lysine mediated by these two systems proceeds uphill. The high-affinity system has a pH optimum at 4.0-4.2, the accumulation ratio is highest at a cell density 2-5 mg dry wt. per ml and decreases with increasing lysine concentrations. Lysine accumulated by this system does not exit from cells. The only potent competitive inhibitors are L-arginine, L-histidine and D-lysine. The other amino acids tested do not behave as competitive inhibitors. Of the various metabolic inhibitors tested, the most potent were proton conductors and antimycin A.

Effect of ethanol on the specific transport system for L-lysine in Saccharomyces cerevisiae

Preincubation of resting cells of Saccharomyces cerevisiae double mutant can1 gap1 (with a single transport system for L-lysine) with metabolic substrates stimulated subsequent uptake of lysine. While in the wild type the stimulation is connected primarily with carrier protein synthesis (delayed, cycloheximide-inhibitable effect) in the mutant an immediate tapping of an energy source (antimycin-inhibited) is practically solely involved.

Effect of chloroquine on membrane permeability in yeast--release of cellular coproporphyrin

1. The influx and efflux of labelled substances with and without chloroquine was studied in yeast cells. 2. The uptake of delta-aminolevulinic acid by Saccharomyces cerevisiae is characterized by a KT of 3-4 mM and Jmax of 1.0-1.2 mumol min-1 g dry weight-1. 3. A method for loading yeast with labelled coproporphyrin is suggested. 4. The uptake of sorbitol and coproporphyrin was slightly stimulated, while the uptake of 6-deoxyglucose was slightly, that of 2-aminoisobutyric acid and leucine strongly inhibited by chloroquine. 5. The efflux of coproporphyrin, 2-aminoisobutyric acid and sorbitol was stimulated while that of leucine was not influenced by chloroquine. 6. The result showed that chloroquine influenced directly but nonspecifically the membrane permeability, apparently mainly that of the vacuolar membrane.

Possible role of histidine in the L-proline transport system of Saccharomyces cerevisiae

The L-proline transport system of Saccharomyces cerevisiae is shown to be specifically inactivated upon incubation of intact yeast cells with the histidine modifier diethylpyrocarbonate. The extent of inactivation is half-maximum at 0.5 mM diethylpyrocarbonate for an incubation of 2 min at 30 degrees C and pH 6.0. Under the same conditions, the time dependence of inactivation is monophasic with the second-order rate constant of 5.5 M-1 X s-1 and the maximum rate Jmax of L-proline transport is lowered by about 50%, while the KT value remains unchanged. Moreover, L-proline afforded significant protection against diethylpyrocarbonate inactivation. The complete reactivation of a partially inactivated L-proline transport system by neutral hydroxylamine and the elimination of the possibility that the modification of other amino acid residues are responsible for the inactivation, suggested that the transport protein inactivation occurs solely by a modification of histidine residues.

Absence of glucose-stimulated transport in yeast protoplasts

Protoplasts of Saccharomyces cerevisiae prepared by snail-gut juice treatment were compared in their transport properties with intact cells. 1. Constitutive monosaccharide transport (D-xylose, 6-deoxy-D-glucose), as well as inducible transport of D-galactose, were unaltered. 2. Phosphorylation-associated transport of 2-deoxy-D-glucose was enhanced in protoplasts, possibly as a consequence of removal of the unstirred layer of the cell wall. 3. Proton-driven transports of trehalose, L-leucine, L-proline and monophosphate could not be activated by preincubation with D-glucose, apparently owing to lack of proton-solute coupling in transport. Utilization of glucose was not depressed but respiration was reduced by about 50% while acidification of the external medium after glucose addition was inhibited by more than 90%. This may be related to the inability of protoplast plasma membrane H-ATPase to be activated by glucose and hence to impaired proton-translocating capacity. Uranyl ions inhibited generally much less in protoplasts than in intact cells although their binding to protoplasts was greater (maximum 0.68 fmol per cell but 3.2 fmol per protoplast).

In vivo glucose activation of the yeast plasma membrane ATPase

The addition of glucose to yeast cells activates proton efflux mediated by the plasma membrane ATPase. Accordingly, the ATPase activity of purified plasma membranes is increased up to 10-fold. The activated ATPase has a more alkaline pH optimum, better affinity for ATP and greater sensitivity to vanadate than the non-activated enzyme. All these changes are reversed by washing the cells free of glucose. This suggests two states of the ATPase which are interconverted by a covalent modification. As glucose does not affect the phosphorylation of plasma membrane polypeptides, other type of covalent modification may be involved.

Transport of nutrients in yeast protoplasts

Protoplasts were prepared with snail-gut juice from Saccharomyces cerevisiae and Candida utilis. Transport of D-xylose (A), trehalose (B), 2-deoxy-D-glucose (C), L-leucine (D), L-proline (E), inorganic phosphate (F) and H+ ions (G) was studied with special emphasis on the first species. Transport of A which is not sensitive to glucose stimulation in its synthesis was not affected by protoplast formation. Similarly unaffected were transports of B, D, E and F in their "residual" form in starved cells. However, transport of these substances after glucose-stimulated synthesis was practically fully suppressed by protoplast formation. This may be connected with the virtually complete inhibition of the proton pump (G) in protoplasts with the implication that B, D, E and F are transported in conjunction with protons but only those that are immediately produced by the proton pump (glucose consumption, oxygen utilization and membrane potential were not substantially altered in protoplasts). Transport of C was stimulated nearly two-fold in protoplasts. Uranyl ions (0.1 mM) had a pronouncedly lower inhibitory effect on all the transports studied in protoplasts.

L-Proline transport in Saccharomyces cerevisiae

Transport of L-proline into Saccharomyces cerevisiae K is mediated by two systems, one with a KT of 31 microM and Jmax of 40 nmol . s-1 . (g dry wt.)-1, the other with KT greater than 2.5 mM and Jmax of 150-165 nmol . s-1 . (g dry wt.)-1. The kinetic properties of the high-affinity system were studied in detail. It proved to be highly specific, the only potent competitive inhibitors being (i) L-proline and its analogs L-azetidine-2-carboxylic acid, sarcosine, D-proline and 3,4-dehydro-DL-proline, and (ii) L-alanine. The other amino acids tested behaved as noncompetitive inhibitors. The high-affinity system is active, has a sharp pH optimum at 5.8-5.9 and, in an Arrhenius plot, exhibits two inflection points at 15 degrees C and 20-21 degrees C. It is trans-inhibited by most amino acids (but probably only the natural substrates act in a trans-noncompetitive manner) and its activity depends to a considerable extent on growth conditions. In cells grown in a rich medium with yeast extract maximum activity is attained during the stationary phase, on a poor medium it is maximal during the early exponential phase. Some 50-60% of accumulated L-proline can leave cells in 90 min (and more if washing is done repeatedly), the efflux being insensitive to 0.5 mM 2,4-dinitrophenol and uranyl ions, the pH between 3 and 7.3, as well as to the presence of 10-100 mM unlabeled L-proline in the outside medium. Its rate and extent are increased by 1% D-glucose and by 10 micrograms nystatin per ml.

Maltotriose transport and utilization in baker's and brewer's yeast

Maltotriose is metabolized by baker's and brewer's yeast only oxidatively, with a respiratory quotient of 1.0, the QCO2Ar being, depending on the strain used, 0-11, as compared with QCO2air of 6-42 microL CO2 per h per mg dry substance. The transport appeared to proceed by facilitated diffusion (no effects of NaF, iodoacetamide and 3-chlorophenylhydrazonomalononitrile) with a KT of more than 50 mM and was inhibited by maltose greater than maltotriose greater than methyl-alpha-D-glucoside greater than maltotetraose greater than D-fructose greater than D-glucose. The transport was present constitutively in both Saccharomyces cerevisiae (baker's yeast) and in S. uvarum (brewer's yeast) and it was not significantly stimulated by preincubation with glucose or maltose. The pH optimum was 4.5-5.5, the temperature dependence yielded an activation energy of 26 kJ/mol.