H+/K+ exchange in reconstituted yeast plasma membrane vesicles

Effects of medium pH on the yeast plasma membrane potential

The effects of different pH incubation values and K+ on yeast plasma membrane potential (PMP) were studied both by the fluorescence changes and the accumulation of thioflavin T (ThT), a method that has been shown most adequate for both procedures. By the changes in fluorescence of ThT, the qualitative observation of PMP at the 3 evaluated pHs indicated that cells at pH 4.0 maintain a PMP lower, but close to the observed at pH 6.0 and 7.0. By measuring the accumulation of ThT and applying the Nernst equation on the different concentrations in and out, the values of PMP could also be estimated at the different pHs, resulting in values in mV, in agreement with our observations by following the fluorescence. Yeast cells at their native niches, or during fermentations must cope with low pHs, so the importance to maintain a robust PMP to survive. The contribution of bicarbonate, derived from the fermentation to the establishment of the PMP is also described. The experiments showed once more the efficacy of the methods used with this dye.

Monitoring of the proton electrochemical gradient in reconstituted vesicles: quantitative measurements of both transmembrane potential and intravesicular pH by ratiometric fluorescent probes

Proteoliposomes carrying reconstituted yeast plasma membrane H(+)-ATPase in their lipid membrane or plasma membrane vesicles are model systems convenient for studying basic electrochemical processes involved in formation of the proton electrochemical gradient (Deltamicro(H) (+)) across the microbial or plant cell membrane. Deltapsi- and pH-sensitive fluorescent probes were used to monitor the gradients formed between inner and outer volume of the reconstituted vesicles. The Deltapsi-sensitive fluorescent ratiometric probe oxonol VI is suitable for quantitative measurements of inside-positive Deltapsi generated by the reconstituted H(+)-ATPase. Its Deltapsi response can be calibrated by the K(+)/valinomycin method and ratiometric mode of fluorescence measurements reduces undesirable artefacts. In situ pH-sensitive fluorescent probe pyranine was used for quantitative measurements of pH inside the proteoliposomes. Calibration of pH-sensitive fluorescence response of pyranine entrapped inside proteoliposomes was performed with several ionophores combined in order to deplete the gradients passively formed across the membrane. Presented model system offers a suitable tool for simultaneous monitoring of both components of the proton electrochemical gradient, Deltapsi and DeltapH. This approach should help in further understanding how their formation is interconnected on biomembranes and even how transport of other ions is combined to it.

NHE8 mediates amiloride-sensitive Na+/H+ exchange across mosquito Malpighian tubules and catalyzes Na+ and K+ transport in reconstituted proteoliposomes

Following a blood meal, the mosquito Aedes aegypti will have acquired an enormous sodium load that must be rapidly excreted to restore ion homeostasis. It is a process that demands robust sodium and fluid transport capabilities. Even though the identities of the components involved in this ion transport across the mosquito Malpighian tubule epithelia have not been completely determined, electrophysiological studies suggest the contribution of a Na(+)/H(+) exchanger extruding cations into the lumen driven secondarily by the proton gradient created by the V-type H(+)-ATPase in the tubules' apical membrane. We have identified the putative exchanger and designated it AeNHE8. Immunolocalization studies demonstrated that AeNHE8 is expressed in the apical membranes of Malpighian tubules, gastric caecae, and rectum. When heterologously expressed in salt-sensitive yeast cells lacking Na(+) extrusion and Na(+)/H(+) exchange proteins, AeNHE8 rescues the salt-sensitive phenotype and restores the cells' ability to grow in high NaCl media. Furthermore, heterologous expression of AeNHE8 in NHE-deficient fibroblast cells results in an amiloride-sensitive (22)Na(+) uptake. To determine the exchanger's kinetic properties, we reconstituted membranes from yeast cells expressing the protein into lipid proteoliposomes and assayed for cation-dependent H(+) exchange by fluorimetric methods. Our results indicate that AeNHE8 mediates saturable exchange of Na(+) and K(+) for H(+). We propose that AeNHE8 may be coupled to the inward H(+) gradient across the Malpighian tubules and plays a role in the extrusion of excess sodium and potassium while maintaining steady intracellular pH in the principal cells.

In situ study of K+ transport into the vacuole of Saccharomyces cerevisiae

Permeable spheroplasts were prepared from two strains of Saccharomyces cerevisiae by incubating with zymolyase without a permeabilizing agent. The loss of the plasma membrane barrier was confirmed by the nucleotide release, the activity of glucose 6-phosphate dehydrogenase with external substrates and by the effects on respiration of mitochondrial substrates and ADP. Mitochondrial integrity was maintained, as shown by respiration with lactate, pyruvate, glucose and ethanol, and its acceleration by ADP showed a coupled respiration. Potassium uptake into the vacuole was measured with a selective electrode and found to be taken up effectively by spheroplasts only in the presence of Mg-ATP; it was reverted by CCCP and PCP and inhibited by bafilomycin A1, but not by sodium vanadate or sodium azide. Potassium ions did not alter DeltaPsi of the vacuole, followed with oxonol V, but caused vacuolar alkalinization, as followed with pyranine. The increase of vacuolar pH was non-selective and observed at 50-200 mM of several monovalent cations. Isolated vacuoles with pyranine inside showed similar changes of the internal pH in the presence of KCl. Results indicate that some strains do not require a permeabilizing agent to directly access the vacuole in spheroplasts prepared with zymolyase. The hypothesis about the existence of a K+/H+ antiporter in the vacuolar membrane of S. cerevisiae is discussed.

Low-affinity potassium uptake by Saccharomyces cerevisiae is mediated by NSC1, a calcium-blocked non-specific cation channel

Previous descriptions by whole-cell patch clamping of the calcium-inhibited non-selective cation channel (NSC1) in the plasma membrane of Saccharomyces cerevisiae (H. Bihler, C.L. Slayman, A. Bertl, FEBS Lett. 432 (1998); S.K. Roberts, M. Fischer, G.K. Dixon, D.Sanders, J. Bacteriol. 181 (1999)) suggested that this inwardly rectifying pathway could relieve the growth inhibition normally imposed on yeast by disruption of its potassium transporters, Trk1p and Trk2p. Now, demonstration of multiple parallel effects produced by various agonists and antagonists on both NSC1 currents and growth (of trk1 Delta trk2 Delta strains), has identified this non-selective cation pathway as the primary low-affinity uptake route for potassium ions in yeast. Factors which suppress NSC1-mediated inward currents and inhibit growth of trk1 Delta trk2 Delta cells include (i) elevating extracellular calcium over the range of 10 microM-10 mM, (ii) lowering extracellular pH over the range 7.5-4, (iii) blockade of NSC1 by hygromycin B, and (iv) to a lesser extent by TEA(+). Growth of trk1 Delta trk2 Delta cells is also inhibited by lithium and ammonium; however, these ions do not inhibit NSC1, but instead enter yeast cells via NSC1. Growth inhibition by lithium ions is probably a toxic effect, whereas growth inhibition by ammonium ions probably results from competitive inhibition, i.e. displacement of intracellular potassium by entering ammonium.

Functional study of the Saccharomyces cerevisiae Nha1p C-terminus

Saccharomyces cerevisiae cells possess an alkali metal cation antiporter encoded by the NHA1 gene. Nha1p is unique in the family of yeast Na+/H+ antiporters on account of its broad substrate specificity (Na+, Li+, K+) and its long C-terminus (56% of the whole protein). In order to study the role of the C-terminus in Nha1p function, we constructed a series of 13 truncated NHA1 versions ranging from the complete one (2958 nucleotides, 985 amino acids) down to the shortest version (1416 nucleotides, 472 amino acids), with only 41 amino acid residues after the last putative transmembrane domain. Truncated NHA1 versions were expressed in an S. cerevisiae alkali metal cation-sensitive strain (B31; ena1-4Delta nha1Delta). We found that the entire Nha1p C-terminus domain is not necessary for either the proper localization of the antiporter in the plasma membrane or the transport of all four substrates (we identified rubidium as the fourth Nha1p substrate). Partial truncation of the C-terminus of about 70 terminal amino acids improves the tolerance of cells to Na+, Li+ and Rb+ compared with cells expressing the complete Nha1p. The presence of the neighbouring part of the C-terminus (amino acids 883-928), rich in aspartate and glutamate residues, is necessary for the maintenance of maximum Nha1p activity towards sodium and lithium. In the case of potassium, the participation of the long C-terminus in the regulation of intracellular potassium content is demonstrated. We also present evidence that the Nha1p C-terminus is involved in the cell response to sudden changes in environmental osmolarity.

Characterization of Schizosaccharomyces pombe malate permease by expression in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, L-malic acid transport is not carrier mediated and is limited to slow, simple diffusion of the undissociated acid. Expression in S. cerevisiae of the MAE1 gene, encoding Schizosaccharomyces pombe malate permease, markedly increased L-malic acid uptake in this yeast. In this strain, at pH 3.5 (encountered in industrial processes), L-malic acid uptake involves Mae1p-mediated transport of the monoanionic form of the acid (apparent kinetic parameters: Vmax = 8.7 nmol/mg/min; Km = 1.6 mM) and some simple diffusion of the undissociated L-malic acid (Kd = 0.057 min(-1)). As total L-malic acid transport involved only low levels of diffusion, the Mae1p permease was further characterized in the recombinant strain. L-Malic acid transport was reversible and accumulative and depended on both the transmembrane gradient of the monoanionic acid form and the DeltapH component of the proton motive force. Dicarboxylic acids with stearic occupation closely related to L-malic acid, such as maleic, oxaloacetic, malonic, succinic and fumaric acids, inhibited L-malic acid uptake, suggesting that these compounds use the same carrier. We found that increasing external pH directly inhibited malate uptake, resulting in a lower initial rate of uptake and a lower level of substrate accumulation. In S. pombe, proton movements, as shown by internal acidification, accompanied malate uptake, consistent with the proton/dicarboxylate mechanism previously proposed. Surprisingly, no proton fluxes were observed during Mae1p-mediated L-malic acid import in S. cerevisiae, and intracellular pH remained constant. This suggests that, in S. cerevisiae, either there is a proton counterflow or the Mae1p permease functions differently from a proton/dicarboxylate symport.

Regulation of monovalent ion homeostasis and pH by the Ser-Thr protein phosphatase SIT4 in Saccharomyces cerevisiae

A gene, SIT4, was identified as corresponding to a serine/threonine protein phosphatase and when overexpressed confers lithium tolerance in galactose medium to the budding yeast Saccharomyces cerevisiae. This gene has been previously identified as a regulator of the cell cycle and involved in nitrogen sensing. It is shown that the transcription levels of SIT4 are induced by low concentrations of Li(+) in a time-dependent manner. Na(+) and K(+) at high concentrations, but not sorbitol, also induce transcription. As a response to Na(+) or Li(+) stress, yeast cells lower the intracellular K(+) content. This effect is enhanced in cells overexpressing SIT4, which also increase (86)Rb efflux after the addition of Na(+) or Li(+) to the extracellular medium. Another feature of SIT4-overexpressing cells is that they maintain a more alkaline pH of 6.64 compared with 6.17 in the wild type cells. It has been proposed that the main pathway of salt tolerance in yeast is mediated by a P-type ATPase, encoded by PMR2A/ENA1. However, our results show that in a sit4 strain, expression of ENA1 is still induced by monovalent cations, and overexpression of SIT4 does not alter the amount of ENA1 transcript. These results show that SIT4 acts in a parallel pathway not involving induction of transcription of ENA1 and suggest a novel function for SIT4 in response to salt stress.

Expression of a mammalian Na+/H+ antiporter in Saccharomyces cerevisiae

The basolateral Na+/H+ antiporter (NHE) from LLC-PK1 cells was expressed in Saccharomyces cerevisiae. Two different strategies were tested for expression. In the first, we used a yeast strain that contains a temperature-sensitive mutation in the SEC-6 gene, whose product is required for the fusion of secretory vesicles with the plasma membrane. This strain was transformed with a vector containing the coding region of the NHE1 isoform under control of a heat shock (HS) promoter (pYNHE1-HS). In the second strategy, we replaced the heat shock promoter from pYNHE1-HS with a galactose (GAL) promoter (pYNHEI-GAL) and transformed wild-type yeast. In both cases, Northern blots demonstrated a transcript that hybridized against a probe containing the membrane region of the exchanger. When an antibody against the last 40 amino acids of the carboxy-terminus of NHE1 was used for immunoblots, a protein with a Mr of 73000 was seen in total membranes from both yeast transformants. Subcellular fractionation revealed that NHE1 was expressed in the endoplasmic reticulum. In the case of the pYNHEI-GAL transformant, the 100000 x g membrane pellet was reconstituted in phosphatidylcholine liposomes, and ethylisopropylamiloride-sensitive Na+/H+ exchange was observed. These results have paved the way for expression of the Na+/H+ exchanger in a genetically well-known microorganism.

Potassium ion efflux induced by cationic compounds in yeast

Potassium efflux in yeast induced by several cationic compounds showed different characteristics. All of the observed efflux required glucose as substrate at the concentrations used. For most of them, the phenomenon required binding of the cationic compound to the cell surface and increased with the negative cell surface charge, and for all the compounds tested, it depended on a metabolizable substrate. Efflux induced with terbium chloride appeared more likely due to the function of a K+/H+ antiporter. With DEAE-dextran and dihydrostreptomycin, potassium efflux was dependent on the cell potassium content and was also sensitive to osmotic changes of the medium. DEAE-dextran-provoked efflux was not due to cell disruption. Dihydrostreptomycin seemed to activate a potassium efflux system which could not be studied in isolation, but its inhibition of potassium uptake may also be involved. Except for cells treated with ethidium bromide, no appreciable cell disruption was observed. The potassium efflux observed appears to be a membrane phenomenon reversible after washing with magnesium chloride.

A Saccharomyces cerevisiae mutant lacking a K+/H+ exchanger

The KHA1 gene corresponding to the open reading frame YJL094c (2.62 kb) encoding a putative K+/H+ antiporter (873 amino acids) in Saccharomyces cerevisiae was disrupted by homologous recombination. The core protein is similar to the putative Na+/H+ antiporters from Enterococcus hirae (NAPA gene) and Lactococcus lactis (LLUPP gene) and the putative K+/H+ exchanger from Escherichia coli (KEFC gene). Disruption of the KHA1 gene resulted in an increased K+ accumulation and net influx without a significant difference in efflux, as well as an increased growth rate, smaller cells, and twice the cell yield per glucose used. Flow cytometry analysis showed an increase of the DNA duplication rate in the mutant. Kinetic studies of 86Rb+ uptake showed the same saturable system for wild-type and disruptant strains. Mutant cells also produced a greater acidification of the medium coincident with an internal pH alkalinization and showed a higher oxygen consumption velocity. We speculate that higher K+ accumulation and increased osmotic pressure accelerate the cell cycle and metabolic activity.

The Nha1 antiporter of Saccharomyces cerevisiae mediates sodium and potassium efflux

The NHA1 gene of Saccharomyces cerevisiae, transcribed into a 3.5 kb mRNA, encodes a protein mediating Na+ and K+ efflux through the plasma membrane that is required for alkali cation tolerance at acidic pH. Deletion of the gene in a wild-type strain resulted in higher sensitivity to both K+ and Na+ at acidic pH. Measurements of cation loss in strains carrying deleted or overexpressed alleles of NHA1 demonstrated its role in K+ and Na+ efflux. In addition, high K+ and Na+ efflux observed upon alkalinization of the cytoplasm implies a role of Nha1p in the regulation of intracellular pH. Moreover, the overexpression of ENA1 and NHA1 genes in an ena1-4 delta-nha1 delta strain showed that the Nha1 alkalication antiporter is responsible for growth on high concentrations of KCl and NaCl at acidic pH, and Ena alkali-cation ATPases are necessary at higher pH values. Both systems have a complementary action to maintain the intracellular steady-state concentration of K+ and Na+.

Influence of monovalent cations on yeast cytoplasmic and vacuolar pH

The effects of monovalent cations on the internal pH of yeast were studied. Our former procedure was modified, inducing maximal alkalinization of the cells with 100 mM-NH4OH instead of Tris base. The pH values were lower than reported before (Peña et al., J. Baceteriol. 1995 177, 1017-1022). With glucose as substrate, the internal cytoplasmic pH reached higher values when incubating at an external pH of 6.0, as compared to pH 4.0. Monovalent cations added approximately 5 min after glucose produced a further increase in the internal pH, which was higher at a previous incubation pH of 4.0 than that observed at pH 6.0. The selectivity of the changes followed a similar order to that of the transport system for monovalent cations. When incubating cells with glucose for more than 30 min, the initial changes of the internal pH appeared to be regulated by the cell. However, under the fluorescence microscope, it was observed that pyranine, which was confined to the cytoplasm during the first 15 min, was progressively concentrated in the vacuole. By studying the fluorescence changes of cells electroporated and then incubated with glucose or glucose plus potassium, we could follow the internal pH of this organelle, obtaining values within the range reported by other authors. Also, in cells preincubated with glucose for 60 min, and electroporated afterwards, the fluorescence of pyranine, which only entered the cytoplasm, allowed us to measure the pH of this compartment, showing that it was more alkaline than the vacuole. Moreover, the cytoplasmic pH increased upon addition of glucose or potassium. The vacuolar pH, on the other hand, increased upon addition of potassium after glucose, but decreased upon addition of glucose. In addition, incubation of the cells with glucose with or without pyranine produced vesiculation of the vacuole.

P-type ATPases mediate sodium and potassium effluxes in Schwanniomyces occidentalis

Two genes isolated from Schwanniomyces occidentalis, ENA1 and ENA2, encode P-type ATPases highly homologous to the Na-ATPases of Saccharomyces cerevisiae and complement the Na+ sensitivity of an S. cerevisiae mutant strain lacking its own Na-ATPases. The expression of both ENA1 and ENA2 was highly dependent on a high external pH, but whereas a high pH was sufficient for the expression of ENA2, the expression of ENA1 required a high pH and the presence of Na+. Disruption of ENA1 rendered the cells less tolerant to Na+ than the wild-type strain and decreased their capacity for Na+ extrusion. Disruption of ENA2 did not affect Na+ tolerance, but decreased both the growth at high pH and K+ efflux. We discuss these results and propose that fungal Na-ATPases should be considered alkali cation ATPases. By sequence comparison, we found that fungal Na-ATPases form a homogeneous group that can be distinguished from other cation-pumping P-type ATPases, except from the cta3 Ca-ATPase of Schizosaccharomyces pombe.