Molecular genetics of yeast ion transport

Yeast Tok1p channel is a major contributor to membrane potential maintenance under chemical stress

Tok1p is a highly specific yeast plasma membrane potassium channel with strong outward directionality. Its opening is induced by membrane depolarization. Although the biophysical properties of Tok1p are well-described, its potentially important physiological role is currently largely unexplored. To address this issue, we examined the Tok1p activity following chemically-induced depolarization by measuring changes of plasma membrane potential (ΔΨ) using the diS-C₃(3) fluorescence assay in a Tok1p-expressing and a Tok1p-deficient strain. We report that Tok1p channel activity in response to chemical stress does not depend solely on the extent of depolarization, as might have been expected, but may also be negatively influenced by accompanying effects of the used compound. The stressors may interact with the plasma membrane or the channel itself, or cause cytosolic acidification. All of these effects may negatively influence the Tok1p channel opening. While ODDC-induced depolarization exhibits the cleanest Tok1p activation, restoring an astonishing 75% of lost ΔΨ, higher BAC concentrations reduce Tok1p activity, probably because of direct interactions with the channel and/or its lipid microenvironment. This is not only the first study of the physiological role of Tok1p in ΔΨ maintenance under chemical stress, but also the first estimate of the extent of depolarization the channel is able to counterbalance.

Cation-Stress-Responsive Transcription Factors SltA and CrzA Regulate Morphogenetic Processes and Pathogenicity of Colletotrichum gloeosporioides

Growth of Colletotrichum gloeosporioides in the presence of cation salts NaCl and KCl inhibited fungal growth and anthracnose symptom of colonization. Previous reports indicate that adaptation of Aspergillus nidulans to salt- and osmotic-stress conditions revealed the role of zinc-finger transcription factors SltA and CrzA in cation homeostasis. Homologs of A. nidulans SltA and CrzA were identified in C. gloeosporioides. The C. gloeosporioides CrzA homolog is a 682-amino acid protein, which contains a C₂H₂ zinc finger DNA-binding domain that is highly conserved among CrzA proteins from yeast and filamentous fungi. The C. gloeosporioides SltA homolog encodes a 775-amino acid protein with strong similarity to A. nidulans SltA and Trichoderma reesei ACE1, and highest conservation in the three zinc-finger regions with almost no changes compared to ACE1 sequences. Knockout of C. gloeosporioides crzA (ΔcrzA) resulted in a phenotype with inhibited growth, sporulation, germination and appressorium formation, indicating the importance of this calciu006D-activated transcription factor in regulating these morphogenetic processes. In contrast, knockout of C. gloeosporioides sltA (ΔsltA) mainly inhibited appressorium formation. Both mutants had reduced pathogenicity on mango and avocado fruit. Inhibition of the different morphogenetic stages in the ΔcrzA mutant was accompanied by drastic inhibition of chitin synthase A and B and glucan synthase, which was partially restored with Ca²⁺ supplementation. Inhibition of appressorium formation in ΔsltA mutants was accompanied by downregulation of the MAP kinase pmk1 and carnitine acetyl transferase (cat1), genes involved in appressorium formation and colonization, which was restored by Ca²⁺ supplementation. Furthermore, exposure of C. gloeosporioides ΔcrzA or ΔsltA mutants to cations such as Na⁺, K⁺ and Li⁺ at concentrations that the wild type C. gloeosporioides is not affected had further adverse morphogenetic effects on C. gloeosporioides which were partially or fully restored by Ca²⁺. Overall results suggest that both genes modulating alkali cation homeostasis have significant morphogenetic effects that reduce C. gloeosporioides colonization.

Identification of yeast genes involved in k homeostasis: loss of membrane traffic genes affects k uptake

Using the homozygous diploid Saccharomyces deletion collection, we searched for strains with defects in K(+) homeostasis. We identified 156 (of 4653 total) strains unable to grow in the presence of hygromycin B, a phenotype previously shown to be indicative of ion defects. The most abundant group was that with deletions of genes known to encode membrane traffic regulators. Nearly 80% of these membrane traffic defective strains showed defects in uptake of the K(+) homolog, (86)Rb(+). Since Trk1, a plasma membrane protein localized to lipid microdomains, is the major K(+) influx transporter, we examined the subcellular localization and Triton-X 100 insolubility of Trk1 in 29 of the traffic mutants. However, few of these showed defects in the steady state levels of Trk1, the localization of Trk1 to the plasma membrane, or the localization of Trk1 to lipid microdomains, and most defects were mild compared to wild-type. Three inositol kinase mutants were also identified, and in contrast, loss of these genes negatively affected Trk1 protein levels. In summary, this work reveals a nexus between K(+) homeostasis and membrane traffic, which does not involve traffic of the major influx transporter, Trk1.

Protective effect of ions against cell death induced by acid stress in Saccharomyces

Saccharomyces boulardii is a probiotic used to prevent or treat antibiotic-induced gastrointestinal disorders and acute enteritis. For probiotics to be effective they must first be able to survive the harsh gastrointestinal environment. In this work, we show that S. boulardii displayed the greatest tolerance to simulated gastric environments compared with several Saccharomyces cerevisiae strains tested. Under these conditions, a pH 2.0 was the main factor responsible for decreased cell viability. Importantly, the addition of low concentrations of sodium chloride (NaCl) protected cells in acidic conditions more effectively than other salts. In the absence of S. boulardii mutants, the protective effects of Na(+) in yeast viability in acidic conditions was tested using S. cerevisiae Na(+)-ATPases (ena1-4), Na(+)/H(+) antiporter (nha1Delta) and Na(+)/H(+) antiporter prevacuolar (nhx1Delta) null mutants, respectively. Moreover, we provide evidence suggesting that this protection is determined by the plasma membrane potential, once altered by low pH and low NaCl concentrations. Additionally, the absence or low expression/activity of Ena proteins seems to be closely related to the basal membrane potential of the cells.

Measurements of plasma membrane potential changes inSaccharomyces cerevisiaecells reveal the importance of the Tok1 channel in membrane potential maintenance

K+ is one of the cations (besides protons) whose transport across the plasma membrane is believed to contribute to the maintenance of membrane potential. To ensure K+ transport, Saccharomyces cerevisiae cells possess several types of active and passive transporters mediating the K+ influx and efflux, respectively. A diS-C3(3) assay was used to compare the contributions of various potassium transporters to the membrane potential changes of S. cerevisiae cells in the exponential growth phase. Altogether, the contributions of six K+ transporters to the maintenance of a stable membrane potential were tested. As confirmed by the observed hyperpolarization of trk1 trk2 deletion strains, the diS-C3(3) assay is a suitable method for comparative studies of the membrane potential of yeast strains differing in the presence/absence of one or more cation transporters. We have shown that the presence of the Tok1 channel strongly influences membrane potential: deletion of the TOK1 gene results in significant plasma membrane depolarization, whereas strains overexpressing the TOK1 gene are hyperpolarized. We have also proved that plasma membrane potential is not the only parameter determining the hygromycin B sensitivity of yeast cells, and that the role of intracellular transporters in protecting against its toxic effects must also be considered.

Ability of Sit4p to promote K+ efflux via Nha1p is modulated by Sap155p and Sap185p

We demonstrate here that SAP155 encodes a negative modulator of K+ efflux in the yeast Saccharomyces cerevisiae. Overexpression of SAP155 decreases efflux, whereas deletion increases efflux. In contrast, a homolog of SAP155, called SAP185, encodes a positive modulator of K+ efflux: overexpression of SAP185 increases efflux, whereas deletion decreases efflux. Two other homologs, SAP4 and SAP190, are without effect on K+ homeostasis. Both SAP155 and SAP185 require the presence of SIT4 for function, which encodes a PP2A-like phosphatase important for the G1-S transition through the cell cycle. Overexpression of either the outwardly rectifying K+ channel, Tok1p, or the putative plasma membrane K+/H+ antiporter, Kha1p, increases efflux in both wild-type and sit4Delta strains. However, overexpression of the Na+-K+/H+ antiporter, Nha1p, is without effect in a sit4Delta strain, suggesting that Sit4p signals to Nha1p. In summary, the combined activities of Sap155p and Sap185p appear to control the function of Nha1p in K+ homeostasis via Sit4p.

Broad sensitivity of Saccharomyces cerevisiae lacking ribosome-associated chaperone ssb or zuo1 to cations, including aminoglycosides

The Hsp70 Ssb and J protein Zuo1 of Saccharomyces cerevisiae are ribosome-associated molecular chaperones, proposed to be involved in the folding of newly synthesized polypeptide chains. Cells lacking Ssb and/or Zuo1 have been reported to be hypersensitive to cationic aminoglycoside protein synthesis inhibitors that affect translational fidelity and to NaCl. Since we found that Deltassb1 Deltassb2 (Deltassb1,2), Deltazuo1, and wild-type cells have very similar levels of translational misreading in the absence of aminoglycosides, we asked whether the sensitivities to aminoglycosides and NaCl represent a general increase in sensitivity to cations. We found that Deltassb1,2 and Deltazuo1 cells are hypersensitive to a wide range of cations. This broad sensitivity is similar to that of cells having lowered activity of major plasma membrane transporters, such as the major K+ transporters Trk1 and Trk2 or their regulators Hal4 and Hal5. Like Deltahal4,5 cells, Deltassb1,2 and Deltazuo1 cells have increased intracellular levels of Na+ and Li+ upon challenge with higher-than-normal levels of these cations, due to an increased rate of influx. In the presence of aminoglycosides, Deltassb1,2, Deltazuo1, and Deltahal 4,5 cells have similarly increased levels of translational misreading. We conclude that, in vivo, the major cause of the aminoglycoside sensitivity of cells lacking ribosome-associated molecular chaperones is a general increase in cation influx, perhaps due to altered maturation of membrane proteins.

Yeast ARL1 encodes a regulator of K+ influx

A molecular genetic approach was undertaken in Saccharomyces cerevisiae to examine the functions of ARL1, encoding a G protein of the Ras superfamily. We show here that ARL1 is an important component of the control of intracellular K(+). The arl1 mutant was sensitive to toxic cations, including hygromycin B and other aminoglycoside antibiotics, tetramethylammonium ions, methylammonium ions and protons. The hygromycin-B-sensitive phenotype was suppressed by the inclusion of K(+) and complemented by wild-type ARL1 and an allele of ARL1 predicted to be unbound to nucleotide in vivo. The arl1 mutant strain internalized approximately 25% more [(14)C]-methylammonium ion than did the wild type, consistent with hyperpolarization of the plasma membrane. The arl1 strain took up 30-40% less (86)Rb(+) than did the wild type, showing an inability to regulate K(+) import properly, contributing to membrane hyperpolarity. By contrast, K(+) and H(+) efflux were undisturbed. The loss of ARL1 had no effect on the steady-state level or the localization of a tagged version of Trk1p. High copy suppressors of the hygromycin-B phenotype included SAP155, encoding a protein that interacts with the cell cycle regulator Sit4p, and HAL4 and HAL5, encoding Ser/Thr kinases that regulate the K(+)-influx mediators Trk1p and Trk2p. These results are consistent with a model in which ARL1, via regulation of HAL4/HAL5, governs K(+) homeostasis in cells.

K+ fluxes in Schizosaccharomyces pombe

All living cells accumulate high concentrations of K+ in order to keep themselves alive. To this end they have developed a great diversity of transporters. The internal level of K+ is the result of the net balance between the activities of the K+ influx and the K+ efflux transporters. Potassium fluxes have been extensively studied and characterized in Saccharomyces cerevisiae. However, this is not the case in the fission yeast and, in addition, the information available indicates that both yeasts present substantial and interesting differences. In this paper we have reviewed and summarized the information on K+ fluxes in Schizosaccharomyces pombe. We have included some unpublished results recently obtained in our laboratory and, in particular, we have highlighted the significant differences found between the well-known yeast S. cerevisiae and the fission yeast Sch. pombe.

On the role of Trk1 and Trk2 in Schizosaccharomyces pombe under different ion stress conditions

Trk1 and Trk2 are the major K(+) transport systems in Schizosaccharomyces pombe. Both transporters individually seem to be able to cope with K(+) requirements of the cells under normal conditions, since only the double mutant shows defective K(+) transport and defective growth at limiting K(+) concentrations. We have studied in detail the role of SpTrk1 and SpTrk2 under different ion stress conditions. Results show that the strain with only Trk1 (trk1(+)) is less sensitive to Li(+) and to hygromycin B, it grows better at low K(+) and it survives longer in a medium without K(+) than the strain expressing only Trk2 (trk2(+)). We conclude that Trk1 contributes more efficiently than Trk2 to the performance of the fission yeast under ion stress conditions. In the wild type both trk1(+) and trk2(+) genes are expressed and probably collaborate for the performance of the cells.

ARL1 participates with ATC1/LIC4 to regulate responses of yeast cells to ions

ATC1/LIC4, previously identified as a suppressor of the Li(+)-sensitive phenotype of calcineurin mutants, was also identified as a suppressor of the hygromycin B-sensitive phenotype of strains lacking the G protein gene, ARL1. Although loss of ARL1 confers several phenotypes, including sensitivity to hygromycin B and Li(+), reduced influx of K(+), and increased secretion of carboxypeptidase Y (CPY), loss of ATC1 was without effect by these and other measures. However, loss of ATC1 in an arl1 background exacerbated ion sensitivities, although not the CPY phenotype. Moreover, overexpression of ATC1 in an arl1 background partially suppressed ion sensitivities, but not the CPY phenotype. Additionally, expression of ENA1, the Na(+)/Li(+) efflux ATPase, and activated calcineurin, but not normal calcineurin, suppressed the Li(+)-sensitive phenotype of the arl1 atc1 double mutant. These results show ARL1 and ATC1 interact to control intracellular ion levels, but ATC1 has little influence on other functions of ARL1.

Characterization of potassium transport in wild-type and isogenic yeast strains carrying all combinations of trk1, trk2 and tok1 null mutations

Saccharomyces cerevisiae cells express three defined potassium-specific transport systems en-coded by TRK1, TRK2 and TOK1. To gain a more complete understanding of the physiological function of these transport proteins, we have constructed a set of isogenic yeast strains carrying all combinations of trk1delta, trk2delta and tok1delta null mutations. The in vivo K+ transport characteristics of each strain have been documented using growth-based assays, and the in vitro biochemical and electrophysiological properties associated with K+ transport have been determined. As has been reported previously, Trk1p and Trk2p facilitate high-affinity potassium uptake and appear to be functionally redundant under a wide range of environmental conditions. In the absence of TRK1 and TRK2, strains lack the ability specifically to take up K+, and trk1deltatrk2delta double mutant cells depend upon poorly understood non-specific cation uptake mechanisms for growth. Under conditions that impair the activity of the non-specific uptake system, termed NSC1, we have found that the presence of functional Tok1p renders cells sensitive to Cs+. Based on this finding, we have established a growth-based assay that monitors the in vivo activity of Tok1p.

Role of the Nha1 antiporter in regulating K(+) influx in Saccharomyces cerevisiae

NHA1 encodes a K(+) (Na(+))/H(+) antiporter in the plasma membrane of Saccharomyces cerevisiae. We report that cells expressing the NHA1 gene contained less K(+) than the mutant lacking the gene when grown without K(+) limitation. They also grew better at low K(+) and showed higher affinity of transport than the nha1 strain. In agreement with the function of an electroneutral cation/H(+) antiporter, the effect was only observed at acidic pH. The improved growth and transport depended on the presence of Trk1p (the main K(+) influx system) and did not require the product of TRK2. We propose that Nha1p regulates the potassium content of the cell and, as a consequence, can affect the activity of the main K(+) influx system (Trk1p).

Regulation of yeast H(+)-ATPase by protein kinases belonging to a family dedicated to activation of plasma membrane transporters

The regulation of electrical membrane potential is a fundamental property of living cells. This biophysical parameter determines nutrient uptake, intracellular potassium and turgor, uptake of toxic cations, and stress responses. In fungi and plants, an important determinant of membrane potential is the electrogenic proton-pumping ATPase, but the systems that modulate its activity remain largely unknown. We have characterized two genes from Saccharomyces cerevisiae, PTK2 and HRK1 (YOR267c), that encode protein kinases implicated in activation of the yeast plasma membrane H(+)-ATPase (Pma1) in response to glucose metabolism. These kinases mediate, directly or indirectly, an increase in affinity of Pma1 for ATP, which probably involves Ser-899 phosphorylation. Ptk2 has the strongest effect on Pma1, and ptk2 mutants exhibit a pleiotropic phenotype of tolerance to toxic cations, including sodium, lithium, manganese, tetramethylammonium, hygromycin B, and norspermidine. A plausible interpretation is that ptk2 mutants have a decreased membrane potential and that diverse cation transporters are voltage dependent. Accordingly, ptk2 mutants exhibited reduced uptake of lithium and methylammonium. Ptk2 and Hrk1 belong to a subgroup of yeast protein kinases dedicated to the regulation of plasma membrane transporters, which include Npr1 (regulator of Gap1 and Tat2 amino acid transporters) and Hal4 and Hal5 (regulators of Trk1 and Trk2 potassium transporters).

Individual functions of the HAK and TRK potassium transporters of Schwanniomyces occidentalis

We have cloned the gene encoding the TRK transporter of the soil yeast Schwanniomyces occidentalis and obtained the HAK1 trk1 delta and the hak1 delta TRK1 mutant strains. Analyses of the transport capacities of these mutants have shown that (i) the HAK1 and the TRK1 potassium transporters are the only transporters operating at low and medium K+ concentrations (< 1 mM); (ii) the HAK1 transporter is functional at low pH but fails at high pH; and (iii) the TRK1 transporter functions at neutral and high pH and fails at low pH. At neutral pH, both transporters are functional, but HAK1 is not expressed, except at very low K+ concentrations (< 50 microM) where HAK1 is very effective. TRK1 is also involved in the control of the membrane potential.

Membrane hyperpolarization and salt sensitivity induced by deletion of PMP3, a highly conserved small protein of yeast plasma membrane

Yeast plasma membranes contain a small 55 amino acid hydrophobic polypeptide, Pmp3p, which has high sequence similarity to a novel family of plant polypeptides that are overexpressed under high salt concentration or low temperature treatment. The PMP3 gene is not essential under normal growth conditions. However, its deletion increases the plasma membrane potential and confers sensitivity to cytotoxic cations, such as Na(+) and hygromycin B. Interestingly, the disruption of PMP3 exacerbates the NaCl sensitivity phenotype of a mutant strain lacking the Pmr2p/Enap Na(+)-ATPases and the Nha1p Na(+)/H(+) antiporter, and suppresses the potassium dependency of a strain lacking the K(+) transporters, Trk1p and Trk2p. All these phenotypes could be reversed by the addition of high Ca(2+) concentration to the medium. These genetic interactions indicate that the major effect of the PMP3 deletion is a hyperpolarization of the plasma membrane potential that probably promotes a non-specific influx of monovalent cations. Expression of plant RCI2A in yeast could substitute for the loss of Pmp3p, indicating a common role for Pmp3p and the plant homologue.

An NH2-terminal deleted plasma membrane H+-ATPase is a dominant negative mutant and is sequestered in endoplasmic reticulum derived structures

The NH2-terminus of the plasma membrane H+-ATPase is one of the least conserved segments of this protein among fungi. We constructed and expressed a mutant H+-ATPase from Saccharomyces cerevisiae deleted at an internal peptide within the cytoplasmic NH2-terminus (D44-F116). When the enzyme was subjected to limited trypsinolysis it was digested more rapidly than wild type H+-ATPase. Membrane fractionation experiments and immunofluorescence microscopy, using antibodies against H+-ATPase showed that the mutant ATPase is retained in the endoplasmic reticulum. The pattern observed in the immunofluorescence microscopy resembled structures similar to Russell bodies (modifications of the endoplasmic reticulum membranes) recently described in yeast. When the wild type H+-ATPase was co-expressed with the mutant, wild type H+-ATPase was also retained in the endoplasmic reticulum. Co-expression of both ATPases in a wild type yeast strain was lethal, demonstrating that this is a dominant negative mutant.

Cloning of two genes encoding potassium transporters in Neurospora crassa and expression of the corresponding cDNAs in Saccharomyces cerevisiae

Two Neurospora crassa genes, trk-1 and hak-1, encode K+ transporters that show sequence similarities to the TRK transporters described in Saccharomyces cerevisiae and Schizosaccharomyces pombe, and to the HAK transporters described in Schwanniomyces occidentalis and barley. The N. crassa TRK1 and HAK1 transporters expressed by the corresponding cDNAs in a trk1 delta trk2 delta mutant of S. cerevisiae exhibited a high affinity for Rb+ and K+. Northern blot analysis and comparison of the kinetic characteristics of the two transporters in the trk1 delta trk2 delta mutant with the kinetic characteristics of K+ uptake in N. crassa cells allowed TRK1 to be identified as the dominant K+ transporter and HAK1 as a transporter that is only expressed when the cells are K+ starved. The HAK1 transporter showed a high concentrative capacity and is identified as the K(+)-H+ symporter described in N. crassa, whereas TRK1 might be a K+ uniporter. Although the co-existence of K+ transporters of the TRK and HAK types in the same species had not been reported formerly, we discuss whether this co-existence may be the normal situation in soil fungi.

Single point mutations distributed in 10 soluble and membrane regions of the Nicotiana plumbaginifolia plasma membrane PMA2 H+-ATPase activate the enzyme and modify the structure of the C-terminal region

The Nicotiana plumbaginifolia pma2 (plasma membrane H+-ATPase) gene is capable of functionally replacing the H+-ATPase genes of the yeast Saccharomyces cerevisiae, provided that the external pH is kept above 5.0. Single point mutations within the pma2 gene were previously identified that improved H+-ATPase activity and allowed yeast growth at pH 4.0. The aim of the present study was to identify most of the PMA2 positions, the mutation of which would lead to improved growth and to determine whether all these mutations result in similar enzymatic and structural modifications. We selected additional mutants in total 42 distinct point mutations localized in 30 codons. They were distributed in 10 soluble and membrane regions of the enzyme. Most mutant PMA2 H+-ATPases were characterized by a higher specific activity, lower inhibition by ADP, and lower stimulation by lysophosphatidylcholine than wild-type PMA2. The mutants thus seem to be constitutively activated. Partial tryptic digestion and immunodetection showed that the PMA2 mutants had a conformational change making the C-terminal region more accessible. These data therefore support the hypothesis that point mutations in various H+-ATPase parts displace the inhibitory C-terminal region, resulting in enzyme activation. The high density of mutations within the first half of the C-terminal region suggests that this part is involved in the interaction between the inhibitory C-terminal region and the rest of the enzyme.

Ectopic potassium uptake in trk1 trk2 mutants of Saccharomyces cerevisiae correlates with a highly hyperpolarized membrane potential

Null trk1 trk2 mutants of Saccharomyces cerevisiae exhibit a low-affinity uptake of K+ and Rb+. We show that this low-affinity Rb+ uptake is mediated by several independent transporters, and that trk1Delta cells and especially trk1Delta trk2Delta cells are highly hyperpolarized. Differences in the membrane potentials were assessed for sensitivity to hygromycin B and by flow cytometric analyses of cellular DiOC6(3) fluorescence. On the basis of the latter analyses, it is proposed that Trk1p and Trk2p are involved in the control of the membrane potential, preventing excessive hyperpolarizations. K+ starvation and nitrogen starvation hyperpolarize both TRK1 TRK2 and trk1Delta trk2Delta cells, thus suggesting that other proteins, in addition to Trk1p and Trk2p, participate in the control of the membrane potential. The HAK1 K+ transporter from Schwanniomyces occidentalis suppresses the K+-defective transport of trk1Delta trk2Delta cells but not the high hyperpolarization, and the HKT1 K+ transporter from wheat suppresses both defects, in the presence of Na+. We discuss the mechanism involved in the control of the membrane potential by Trk1p and Trk2p and the causal relationship between the high membrane potential (negative inside) of trk1Delta trk2Delta cells and its ectopic transport of alkali cations.

Structure of yeast plasma membrane H(+)-ATPase: comparison of activated and basal-level enzyme forms

Plasma membrane H(+)-ATPase of the yeast Saccharomyces cerevisiae was isolated and purified in its two forms, the activated A-ATPase from glucose-metabolising cells, and the basal-level B-ATPase from cells with endogenous metabolism only. Structure of the two enzyme forms and the effects of beta, gamma-imidoadenosine 5'-triphosphate (AMP-PNP) and of diethylstilbestrol (DES) thereon were analysed by FT-IR spectroscopy. IR spectra revealed the presence of two populations of alpha-helices with different exposure to the solvent in both the A-ATPase and B-ATPase. AMP-PNP did not affect the secondary structure of A-ATPase while DES affected the ratio of the two alpha-helix populations. Thermal denaturation experiments suggested a more stable structure in the B-form than in the A-form. AMP-PNP stabilised the A-ATPase structure while DES destabilised both enzyme forms. IR spectra showed that 60% of the amide hydrogens were exchanged for deuterium in both forms at 20 degrees C. The remaining 40% were exchanged at higher temperatures. The maximum amount of H/D exchange was observed at 50-55 degrees C for both enzyme forms, while in the presence of DES it was observed at lower temperatures. The data do not contradict the possibility that the activation of H(+)-ATPase is due to the C-terminus of the enzyme dissociating from the ATP-binding site which is covered by it in the less active form.

Yeast gene YOR137c is involved in the activation of the yeast plasma membrane H+-ATPase by glucose

Glucose triggers transcriptional and post-transcriptional mechanisms that increase the level and activity of Saccharomyces cerevisiae plasma membrane H+-ATPase. We have studied the post-transcriptional activation of the enzyme by glucose and have found that the YOR137c gene product is implicated in this activation. Deletion of YOR137c does not affect the level of Pma1 at the plasma membrane, but disturbs the glucose-triggered Vmax increase of the enzyme. We propose that at least two independent mechanisms are involved in glucose activation of the H+-ATPase.

A homolog of mammalian, voltage-gated calcium channels mediates yeast pheromone-stimulated Ca2+ uptake and exacerbates the cdc1(Ts) growth defect

Previous studies attributed the yeast (Saccharomyces cerevisiae) cdc1(Ts) growth defect to loss of an Mn2+-dependent function. In this report we show that cdc1(Ts) temperature-sensitive growth is also associated with an increase in cytosolic Ca2+. We identified two recessive suppressors of the cdc1(Ts) temperature-sensitive growth which block Ca2+ uptake and accumulation, suggesting that cytosolic Ca2+ exacerbates or is responsible for the cdc1(Ts) growth defect. One of the cdc1(Ts) suppressors is identical to a gene, MID1, recently implicated in mating pheromone-stimulated Ca2+ uptake. The gene (CCH1) corresponding to the second suppressor encodes a protein that bears significant sequence similarity to the pore-forming subunit (alpha1) of plasma membrane, voltage-gated Ca2+ channels from higher eukaryotes. Strains lacking Mid1 or Cch1 protein exhibit a defect in pheromone-induced Ca2+ uptake and consequently lose viability upon mating arrest. The mid1delta and cch1delta mutants also display reduced tolerance to monovalent cations such as Li+, suggesting a role for Ca2+ uptake in the calcineurin-dependent ion stress response. Finally, mid1delta cch1delta double mutants are, by both physiological and genetic criteria, identical to single mutants. These and other results suggest Mid1 and Cch1 are components of a yeast Ca2+ channel that may mediate Ca2+ uptake in response to mating pheromone, salt stress, and Mn2+ depletion.

Physiological basis for the high salt tolerance of Debaryomyces hansenii

The effects of KCl, NaCl, and LiCl on the growth of Debaryomyces hansenii, usually considered a halotolerant yeast, and Saccharomyces cerevisiae were compared. KCl and NaCl had similar effects on D. hansenii, indicating that NaCl created only osmotic stress, while LiCl had a specific inhibitory effect, although relatively weaker than in S. cerevisiae. In media with low K+, Na+ was able to substitute for K+, restoring the specific growth rate and the final biomass of the culture. The intracellular concentration of Na+ reached values up to 800 mM, suggesting that metabolism is not affected by rather high concentrations of salt. The ability of D. hansenii to extrude Na+ and Li+ was similar to that described for S. cerevisiae, suggesting that this mechanism is not responsible for the increased halotolerance. Also, the kinetic parameters of Rb+ uptake in D. hansenii (Vmax, 4.2 nmol mg [dry weight]-1 min-1; K(m), 7.4 mM) indicate that the transport system was not more efficient than in S. cerevisiae. Sodium (50 mM) activated the transport of Rb+ by increasing the affinity for the substrate in D. hansenii, while the effect was opposite in S. cerevisiae. Lithium inhibited Rb+ uptake in D. hansenii. We propose that the metabolism of D. hansenii is less sensitive to intracellular Na+ than is that of S. cerevisiae, that Na+ substitutes for K+ when K+ is scarce, and that the transport of K+ is favored by the presence of Na+. In low K+ environments, D. hansenii behaved as a halophilic yeast.

The complete inventory of the yeast Saccharomyces cerevisiae P-type transport ATPases

A total of sixteen open reading frames encoding for P-type ATPases have been identified in the complete genome sequence of Saccharomyces cerevisiae. Phylogenetic analysis distinguishes 6 distinct families. Topology predictions, identification of aminoacid sequence motifs and phenotype analysis of the available mutants suggest that these families correspond to ATPases transporting either H+ (2 members), Ca2+ (2 members), Na+ (3 members), heavy metals (2 members), possibly aminophospholipids (5 members including 4 new ones) or unknown substrates (2 new members). It is proposed that the latter family which has homologs in Tetrahymena thermophila, Plasmodium falciparum and Caenorhabditis elegans constitutes a new group called P4-ATPases with characteristic topology and aminoacid signatures.

Comparative physiology of salt tolerance in Candida tropicalis and Saccharomyces cerevisiae

The salt tolerance of the respiratory yeast Candida tropicalis and the fermentative yeast Saccharomyces cerevisiae have been compared in glucose media. C. tropicalis showed a better adaptation to Na+ and Li+ and maintained higher intracellular K+:Na+ and K+:Li+ ratios than S. cerevisiae. However, C. tropicalis showed a poorer adaptation to osmotic stress (produced by KCl and sorbitol) and exhibited reduced glycerol production as compared to S. cerevisiae. In media with the non-repressing sugar galactose as carbon source, S. cerevisiae exhibited reduced glycerol production and increased sensitivity to osmotic stress. Under these conditions, S. cerevisiae, but not C. tropicalis, utilized trehalose as a more important osmolyte than glycerol. These results suggest that the relative tolerance of yeast to the osmotic and cation toxicities of NaCl, and the underlying relative capabilities for osmolyte synthesis and cation transport, are modulated by the general catabolite control exerted by glucose.

Glucose-independent inhibition of yeast plasma-membrane H+-ATPase by calmodulin antagonists

Glucose metabolism causes activation of the yeast plasma-membrane H+-ATPase. The molecular mechanism of this regulation is not known, but it is probably mediated by phosphorylation of the enzyme. The involvement in this process of several kinases has been suggested but their actual role has not been proved. The physiological role of a calmodulin-dependent protein kinase in glucose-induced activation was investigated by studying the effect of specific calmodulin antagonists on the glucose-induced ATPase kinetic changes in wild-type and two mutant strains affected in the glucose regulation of the enzyme. Preincubation of the cells with calmidazolium or compound 48/80 impeded the increase in ATPase activity by reducing the Vmax of the enzyme without modifying the apparent affinity for ATP in the three strains. In one mutant, pma1-T912A, the putative calmodulin-dependent protein kinase-phosphorylatable Thr-912 was eliminated, and in the other, pma1-P536L, H+-ATPase was constitutively activated, suggesting that the antagonistic effect was not mediated by a calmodulin-dependent protein kinase and not related to glucose regulation. This was corroborated when the in vitro effect of the calmodulin antagonists on H+-ATPase activity was tested. Purified plasma membranes from glucose-starved or glucose-fermenting cells from both pma1-P890X, another constitutively activated ATPase mutant, and wild-type strains were preincubated with calmidazolium or melittin. In all cases, ATP hydrolysis was inhibited with an IC50 of approximately 1 microM. This inhibition was reversed by calmodulin. Analysis of the calmodulin-binding protein pattern in the plasma-membrane fraction eliminates ATPase as the calmodulin target protein. We conclude that H+-ATPase inhibition by calmodulin antagonists is mediated by an as yet unidentified calmodulin-dependent membrane protein.

Use of microphysiometry for analysis of heterologous ion channels expressed in yeast

Measurement of extracellular acidification rates by microphysiometry provides a means to analyze the function of ion channels expressed in yeast cells. These measurements depend on the proton pumping action of the H(+)-ATPase, a central component of the yeast plasma membrane. We used microphysiometry to analyze the activity of two ion channels expressed in yeast. In one example, an inwardly rectifying K+ channel, gpIRK1, provides a potassium uptake function when expressed in a potassium transporter-defective yeast strain. Rates of acidification in gpIRK1-expressing cells directly reflect channel function. Addition of cesium, an inhibitor of gpIRK1 activity, results in an immediate reduction in acidification rates. In a second example, expression of a nonselective cation channel, the influenza virus M2 protein, is believed to interfere with the maintenance of the electrochemical proton gradient by the H(+)-ATPase. In cells expressing the M2 channel, addition of inhibitors increases the rate of proton extrusion. Moreover, functional differences between two M2 inhibitors, amantadine and BL-1743, are distinguished by the microphysiometer. This application demonstrates the utility of the microphysiometer for functional studies of ion channels; it is adaptable to a screening process for compounds that modulate ion channel activity.

MEMBRANE TRANSPORT CARRIERS

▪ Abstract  Plant and fungal membrane proteins catalyzing the transmembrane translocation of small molecules without directly using ATP or acting as channels are discussed in this review. Facilitators, ion-cotransporters, and exchange translocators mainly for sugars, amino acids, and ions that have been cloned and characterized from Saccharomyces cerevisiae and from various plant sources have been tabulated. The membrane topology and structure of the most extensively studied carriers (lac permease of Escherichia coli, Glut1 of man, HUP1 of Chlorella) are discussed in detail as well as the kinetic analysis of specific Na+ and H+ cotransporters. Finally, the knowledge concerning regulatory phenomena of carriers—mainly of S. cerevisiae—is summarized.

Salt tolerance in plants and microorganisms: toxicity targets and defense responses

Salt tolerance of crops could be improved by genetic engineering if basic questions on mechanisms of salt toxicity and defense responses could be solved at the molecular level. Mutant plants accumulating proline and transgenic plants engineered to accumulate mannitol or fructans exhibit improved salt tolerance. A target of salt toxicity has been identified in Saccharomyces cerevisiae: it is a sodium-sensitive nucleotidase involved in sulfate activation and encoded by the HAL2 gene. The major sodium-extrusion system of S. cerevisiae is a P-ATPase encoded by the ENA1 gene. The regulatory system of ENA1 expression includes the protein phosphatase calcineurin and the product of the HAL3 gene. In Escherichia coli, the Na(+)-H+ antiporter encoded by the nhaA gene is essential for salt tolerance. No sodium transport system has been identified at the molecular level in plants. Ion transport at the vacuole is of crucial importance for salt accumulation in this compartment, a conspicuous feature of halophytic plants. The primary sensors of osmotic stress have been identified only in E. coli. In S. cerevisiae, a protein kinase cascade (the HOG pathway) mediates the osmotic induction of many, but not all, stress-responsive genes. In plants, the hormone abscisic acid mediates many stress responses and both a protein phosphatase and a transcription factor (encoded by the ABI1 and ABI3 genes, respectively) participate in its action.

An overview of membrane transport proteins in Saccharomyces cerevisiae

All eukaryotic cells contain a wide variety of proteins embedded in the plasma and internal membranes, which ensure transmembrane solute transport. It is now established that a large proportion of these transport proteins can be grouped into families apparently conserved throughout organisms. This article presents the data of an in silicio analysis aimed at establishing a preliminary classification of membrane transport proteins in Saccharomyces cerevisiae. This analysis was conducted at a time when about 65% of all yeast genes were available in public databases. In addition to approximately 60 transport proteins whose function was at least partially known, approximately 100 deduced protein sequences of unknown function display significant sequence similarity to membrane transport proteins characterized in yeast and/or other organisms. While some protein families have been well characterized by classical genetic experimental approaches, others have largely if not totally escaped characterization. The proteins revealed by this in silicio analysis also include a putative K+ channel, proteins similar to aquaporins of plant and animal origin, proteins similar to Na+-solute symporters, a protein very similar to electroneural cation-chloride cotransporters, and a putative Na+-H+ antiporter. A new research area is anticipated: the functional analysis of many transport proteins whose existence was revealed by genome sequencing.

Functional expression of a vertebrate inwardly rectifying K+ channel in yeast

We describe the expression of gpIRK1, an inwardly rectifying K+ channel obtained from guinea pig cardiac cDNA. gpIRK1 is a homologue of the mouse IRK1 channel identified in macrophage cells. Expression of gpIRK1 in Xenopus oocytes produces inwardly rectifying K+ current, similar to the cardiac inward rectifier current IK1. This current is blocked by external Ba2+ and Cs+. Plasmids containing the gpIRK1 coding region under the transcriptional control of constitutive (PGK) or inducible (GAL) promoters were constructed for expression in Saccharomyces cerevisiae. Several observations suggest that gpIRK1 forms functional ion channels when expressed in yeast. gpIRK1 complements a trk1 delta trk2 delta strain, which is defective in potassium uptake. Expression of gpIRK1 in this mutant restores growth on low potassium media. Growth dependent on gpIRK1 is inhibited by external Cs+. The strain expressing gpIRK1 provides a versatile genetic system for studying the assembly and composition of inwardly rectifying K+ channels.

Isolation of a cDNA from Saccharomyces cerevisiae that encodes a high affinity sulphate transporter at the plasma membrane

Resistance to selenate and chromate, toxic analogues of sulphate, was used to isolate a mutant of Saccharomyces cerevisiae deficient in the capacity to transport sulphate into the cells. A clone which complements this mutation was isolated from a cDNA library prepared from S. cerevisiae poly(A)+ RNA. This clone contains an insert which is 2775 bp in length and has a single open reading frame that encodes a 859 amino acid polypeptide with a molecular mass of 96 kDa. Sequence motifs within the deduced amino acid sequence of this cDNA (SUL1) show homology with conserved areas of sulphate transport proteins from other organisms. Sequence analysis predicts the position of 12 putative membrane spanning domains in SUL1. When the cDNA for SUL1 was expressed in S. cerevisiae, a high affinity sulphate uptake activity (Km = 7.5 +/- 0.6 microM for SO2-4) was observed. A genomic mutant of S. cerevisiae in which 1096 bp were deleted from the SUL1 coding region was constructed. This mutant was unable to grow on media containing less than 5 mM sulphate unless complemented with a plasmid containing the SUL1 cDNA. We conclude that the SUL1 cDNA encodes a S. cerevisiae high affinity sulphate transporter that is responsible for the transfer of sulphate across the plasma membrane from the external medium.

The plasma membrane of Saccharomyces cerevisiae: structure, function, and biogenesis

The composition of phospholipids, sphingolipids, and sterols in the plasma membrane has a strong influence on the activity of the proteins associated or embedded in the lipid bilayer. Since most lipid-synthesizing enzymes in Saccharomyces cerevisiae are located in intracellular organelles, an extensive flux of lipids from these organelles to the plasma membrane is required. Although the pathway of protein traffic to the plasma membrane is similar to that of most of the lipids, the bulk flow of lipids is separate from vesicle-mediated protein transport. Recent advances in the analysis of membrane budding and membrane fusion indicate that the mechanisms of protein transport from the endoplasmic reticulum to the Golgi and from the Golgi to plasma membrane are similar. The majority of plasma membrane proteins transport solutes across the membrane. A number of ATP-dependent export systems have been detected that couple the hydrolysis of ATP to transport of molecules out of the cell. The hydrolysis of ATP by the plasma membrane H(+)-ATPase generates a proton motive force which is used to drive secondary transport processes. In S. cerevisiae, many substrates are transported by more than one system. Transport of monosaccharide is catalyzed by uniport systems, while transport of disaccharides, amino acids, and nucleosides is mediated by proton symport systems. Transport activity can be regulated at the level of transcription, e.g., induction and (catabolite) repression, but transport proteins can also be affected posttranslationally by a process termed catabolite inactivation. Catabolite inactivation is triggered by the addition of fermentable sugars, intracellular acidification, stress conditions, and/or nitrogen starvation. Phosphorylation and/or ubiquitination of the transport proteins has been proposed as an initial step in the controlled inactivation and degradation of the target enzyme. The use of artificial membranes, like secretory vesicles and plasma membranes fused with proteoliposomes, as model systems for studies on the mechanism and regulation of transport is evaluated.

Molecular cloning of the plasma membrane H(+)-ATPase from Kluyveromyces lactis: a single nucleotide substitution in the gene confers ethidium bromide resistance and deficiency in K+ uptake

A Kluyveromyces lactis strain resistant to ethidium bromide and deficient in potassium uptake was isolated. Studies on the proton-pumping activity of the mutant strain showed that a decreased H(+)-ATPase specific activity was responsible for the observed phenotypes. The putative K. lactis PMA1 gene encoding the plasma membrane H(+)-ATPase was cloned by its ability to relieve the potassium transport defect of this mutant and by reversing its resistance to ethidium bromide. Its deduced amino acid sequence predicts a protein 899 residues long that is structurally colinear in its full length to H(+)-ATPases cloned from different yeasts, except for the presence of a variable N-terminal domain. By PCR-mediated amplification, we identified a transition from G to A that rendered the substitution of the fully conserved methionine at position 699 by isoleucine. We attribute to this amino acid change the low capacity of the mutant H(+)-ATPase to pump out protons.

Genetic analysis of the fluorescein isothiocyanate binding site of the yeast plasma membrane H(+)-ATPase

The highly conserved motif of Saccharomyces cerevisiae H(+)-ATPase 474KGAP has been proposed to participate in the formation of the phosphorylated intermediate during the catalytic cycle (Portillo, F., and Serrano, R. (1988) EMBO J. 7, 1793-1798). In addition, Lys-474 is the FITC binding site of the yeast enzyme (Portillo, F. and Serrano, R. (1989) Eur. J. Biochem. 186, 501-507). We have performed an intragenic suppressor analysis of the K474R mutation to identify the interacting regions involved in these functions. Random in vitro mutagenesis of the K474R allele resulted in seven suppressor (second-site) mutations. One mutation (V396I), located 18 residues away from the Asp-378 residue, which is phosphorylated during catalysis, is allele-specific. This provides genetic evidence of a direct interaction between the KGAP motif and the phosphorylation domain during the catalytic cycle. Three mutations (V484I, V484I/E485K, and E485K/E486K) are located near Lys-474 and may compense the structural alteration introduced by the K474R mutation. Two substitutions at the end of the predicted transmembrane stretch 2 (A165V and V169I/D170N) and another in the predicted ATP binding domain (P536L) may act as allele-nonspecific suppressors, as they are also able to suppress a mutation at the enzyme's carboxyl terminus.

Targeting of the yeast plasma membrane [H+]ATPase: a novel gene AST1 prevents mislocalization of mutant ATPase to the vacuole

We have characterized a class of mutations in PMA1, (encoding plasma membrane ATPase) that is ideal for the analysis of membrane targeting in Saccharomyces cerevisiae. This class of pma1 mutants undergoes growth arrest at the restrictive temperature because newly synthesized ATPase fails to be targeted to the cell surface. Instead, mutant ATPase is delivered to the vacuole, where it is degraded. Delivery to the vacuole occurs without previous arrival at the plasma membrane because degradation of mutant ATPase is not prevented when internalization from the cell surface is blocked. Disruption of PEP4, encoding vacuolar proteinase A, blocks ATPase degradation, but fails to restore growth because the ATPase is still improperly targeted. One of these pma1 mutants was used to select multicopy suppressors that would permit growth at the nonpermissive temperature. A novel gene, AST1, identified by this selection, suppresses several pma1 alleles defective for targeting. The basis for suppression is that multicopy AST1 causes rerouting of mutant ATPase from the vacuole to the cell surface. pma1 mutants deleted for AST1 have a synthetic growth defect at the permissive temperature, providing genetic evidence for interaction between AST1 and PMA1. Ast1 is a cytoplasmic protein that associates with membranes, and is localized to multiple compartments, including the plasma membrane. The identification of AST1 homologues suggests that Ast1 belongs to a novel family of proteins that participates in membrane traffic.

TRK2 is not a low-affinity potassium transporter in Saccharomyces cerevisiae

TRK1 and TRK2 encode proteins involved in K+ uptake in Saccharomyces cerevisiae. A kinetic study of Rb+ influx in trk1 TRK2, trk1 TRK2D, and trk1 trk2 mutants reveals that TRK2 shows moderate affinity for Rb+. K(+)-starved trk1 delta TRK2 cells show a low-affinity component accounting for almost the total Vmax of the influx and a moderate-affinity component exhibiting a very low Vmax. Overexpression of TRK2 in trk1 delta TRK2D cells increases the Vmax of the moderate-affinity component, and this component disappears in trk1 delta trk2 delta cells. In contrast, the low-affinity component of Rb+ influx in trk1 delta TRK2 cells is not affected by mutations in TRK2. Consistent with the different levels of activity of the moderate-affinity Rb+ influx, trk1 delta TRK2 cells grow slowly in micromolar K+, trk1 delta TRK2D cells grow rapidly, and trk1 delta trk2 delta cells fail to grow. The existence of a unique K+ uptake system composed of several proteins is also discussed.

Development of improved cell-based assays and screens in Saccharomyces through the combination of molecular and classical genetics

Traditionally, the discovery of pharmaceutical and agrochemical products has largely depended on mass screening. Over the years, screen design and screening programs have evolved in terms of the sensitivity with which active material can be identified, the number of samples that can be tested, and the types of molecular targets and cellular functions that can be conveniently assayed. More recently, screens with desirable properties have been developed for a great variety of molecular targets through the exploitation of Saccharomyces molecular biology and genetics. Recent advances have enabled researchers to develop yeast-based screens for agents acting on a number of new therapeutic targets: G-protein linked receptors, cytoplasmic receptors, ion (potassium) channels, novel fungal cell wall enzymes, fungal sterol biosynthesis enzymes, antiviral targets, immunosuppressive targets, cyclic nucleotide phosphodiesterase, oncogenes and the multiple drug resistance (MDR) protein.