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

Distinct Roles of Ena ATP Family Proteins in Sodium Accumulation, Invasive Growth, and Full Virulence in Colletotrichum gloeosporioides

Colletotrichum gloeosporioides, a significant fungal pathogen of crops and trees, causes large economic losses worldwide. However, its pathogenic mechanism remains totally unclear. In this study, four Ena ATPases (Exitus natru-type adenosine triphosphatases), homology of yeast Ena proteins, were identified in C. gloeosporioides. Gene deletion mutants of ΔCgena1, ΔCgena2, ΔCgena3, and ΔCgena4 were obtained through the method of gene replacement. First, a subcellular localization pattern indicated that CgEna1 and CgEna4 were localized in the plasma membrane, while the CgEna2 and CgEna3 were distributed in the endoparasitic reticulum. Next, it was found that CgEna1 and CgEna4 were required for sodium accumulation in C. gloeosporioides. CgEna3 was required for extracellular ion stress of sodium and potassium. CgEna1 and CgEna3 were involved in conidial germination, appressorium formation, invasive hyphal development, and full virulence. The mutant of ΔCgena4 was more sensitive to the conditions of high concentrations of ion and the alkaline. Together, these results indicated that CgEna ATPase proteins have distinct roles in sodium accumulation, stress resistance, and full virulence in C. gloeosporioides.

The Toxic Effects of Ppz1 Overexpression Involve Nha1-Mediated Deregulation of K+ and H+ Homeostasis

The alteration of the fine-tuned balance of phospho/dephosphorylation reactions in the cell often results in functional disturbance. In the yeast Saccharomyces cerevisiae, the overexpression of Ser/Thr phosphatase Ppz1 drastically blocks cell proliferation, with a profound change in the transcriptomic and phosphoproteomic profiles. While the deleterious effect on growth likely derives from the alteration of multiple targets, the precise mechanisms are still obscure. Ppz1 is a negative effector of potassium influx. However, we show that the toxic effect of Ppz1 overexpression is unrelated to the Trk1/2 high-affinity potassium importers. Cells overexpressing Ppz1 exhibit decreased K⁺ content, increased cytosolic acidification, and fail to properly acidify the medium. These effects, as well as the growth defect, are counteracted by the deletion of NHA1 gene, which encodes a plasma membrane Na⁺, K⁺/H⁺ antiporter. The beneficial effect of a lack of Nha1 on the growth vanishes as the pH of the medium approaches neutrality, is not eliminated by the expression of two non-functional Nha1 variants (D145N or D177N), and is exacerbated by a hyperactive Nha1 version (S481A). All our results show that high levels of Ppz1 overactivate Nha1, leading to an excessive entry of H⁺ and efflux of K⁺, which is detrimental for growth.

P-type Na+/K+ ATPases essential and nonessential for cellular homeostasis and insect pathogenicity of Beauveria bassiana

ENA1 and ENA2 are P-type IID/ENA Na+/K+-ATPases required for cellular homeostasis in yeasts but remain poorly understood in filamentous fungal insect pathogens. Here, we characterized seven genes encoding five ENA1/2 homologues (ENA1a-c and ENA2a/b) and two P-type IIC/NK Na+/K+-ATPases (NK1/2) in Beauveria bassiana, an insect-pathogenic fungus serving as a main source of fungal insecticides worldwide. Most of these genes were highly responsive to alkaline pH and Na+/K+ cues at transcription level. Cellular Na+, K+ and H+ homeostasis was disturbed only in the absence of ena1a or ena2b. The disturbed homeostasis featured acceleration of vacuolar acidification, elevation of cytosolic Na+/K+ level at pH 5.0 to 9.0, and stabilization of extracellular H+ level to initial pH 7.5 during a 5-day period of submerged incubation. Despite little defect in hyphal growth and asexual development, the Δena1a and Δena2b mutants were less tolerant to metal cations (Na+, K+, Li+, Zn2+, Mn2+ and Fe3+), cell wall perturbation, oxidation, non-cation hyperosmolarity and UVB irradiation, severely compromised in insect pathogenicity via normal cuticle infection, and attenuated in virulence via hemocoel injection. The deletion mutants of five other ENA and NK genes showed little change in vacuolar pH and all examined phenotypes. Therefore, only ENA1a and ENA2b evidently involved in both transmembrane and vacuolar activities are essential for cellular cation homeostasis, insect pathogenicity and multiple stress tolerance in B. bassiana. These findings provide a novel insight into ENA1a- and ENA2b-dependent vacuolar pH stability, cation-homeostatic process and fungal fitness to host insect and environment.

Tolerance to alkaline ambient pH in Aspergillus nidulans depends on the activity of ENA proteins

Tolerance of microorganisms to abiotic stress is enabled by regulatory mechanisms that coordinate the expression and activity of resistance genes. Alkalinity and high salt concentrations are major environmental physicochemical stresses. Here, we analyzed the roles of sodium-extrusion family (ENA) transporters EnaA, EnaB and EnaC in the response to these stress conditions in the filamentous fungus Aspergillus nidulans. While EnaC has a minor role, EnaB is a key element for tolerance to Na⁺ and Li⁺ toxicity. Adaptation to alkaline pH requires the concerted action of EnaB with EnaA. Accordingly, expression of enaA and enaB was induced by Na⁺, Li⁺ and pH 8. These expression patterns are altered in a sltAΔ background and completely inhibited in a mutant expressing non-functional PacC protein (palH72). However, a constitutively active PacC form was not sufficient to restore maximum enaA expression. In agreement with their predicted role as membrane ATPases, EnaA localized to the plasma membrane while EnaB accumulated at structures resembling the endoplasmic reticulum. Overall, results suggest different PacC- and SltA-dependent roles for EnaB in pH and salt homeostasis, acting in coordination with EnaA at pH 8 but independently under salt stress.

Monovalent cation transporters at the plasma membrane in yeasts

Maintenance of proper intracellular concentrations of monovalent cations, mainly sodium and potassium, is a requirement for survival of any cell. In the budding yeast Saccharomyces cerevisiae, monovalent cation homeostasis is determined by the active extrusion of protons through the Pma1 H⁺ -ATPase (reviewed in another chapter of this issue), the influx and efflux of these cations through the plasma membrane transporters (reviewed in this chapter), and the sequestration of toxic cations into the vacuoles. Here, we will describe the structure, function, and regulation of the plasma membrane transporters Trk1, Trk2, Tok1, Nha1, and Ena1, which play a key role in maintaining physiological intracellular concentrations of Na⁺ , K⁺ , and H⁺ , both under normal growth conditions and in response to stress.

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.

Survival defects of Cryptococcus neoformans mutants exposed to human cerebrospinal fluid result in attenuated virulence in an experimental model of meningitis

Cryptococcus neoformans is a fungal pathogen that encounters various microenvironments during growth in the mammalian host, including intracellular vacuoles, blood, and cerebrospinal fluid (CSF). Because the CSF is isolated by the blood-brain barrier, we hypothesize that CSF presents unique stresses that C. neoformans must overcome to establish an infection. We assayed 1,201 mutants for survival defects in growth media, saline, and human CSF. We assessed CSF-specific mutants for (i) mutant survival in both human bronchoalveolar lavage (BAL) fluid and fetal bovine serum (FBS), (ii) survival in macrophages, and (iii) virulence using both Caenorhabditis elegans and rabbit models of cryptococcosis. Thirteen mutants exhibited significant survival defects unique to CSF. The mutations of three of these mutants were recreated in the clinical serotype A strain H99: deletions of the genes for a cation ATPase transporter (ena1Δ), a putative NEDD8 ubiquitin-like protein (rub1Δ), and a phosphatidylinositol 4-kinase (pik1Δ). Mutant survival rates in yeast media, saline, and BAL fluid were similar to those of the wild type; however, survival in FBS was reduced but not to the levels in CSF. These mutant strains also exhibited decreased intracellular survival in macrophages, various degrees of virulence in nematodes, and severe attenuation of survival in a rabbit meningitis model. We analyzed the CSF by mass spectrometry for candidate compounds responsible for the survival defect. Our findings indicate that the genes required for C. neoformans survival in CSF ex vivo are necessary for survival and infection in this unique host environment.

Alkali metal cation transport and homeostasis in yeasts

The maintenance of appropriate intracellular concentrations of alkali metal cations, principally K(+) and Na(+), is of utmost importance for living cells, since they determine cell volume, intracellular pH, and potential across the plasma membrane, among other important cellular parameters. Yeasts have developed a number of strategies to adapt to large variations in the concentrations of these cations in the environment, basically by controlling transport processes. Plasma membrane high-affinity K(+) transporters allow intracellular accumulation of this cation even when it is scarce in the environment. Exposure to high concentrations of Na(+) can be tolerated due to the existence of an Na(+), K(+)-ATPase and an Na(+), K(+)/H(+)-antiporter, which contribute to the potassium balance as well. Cations can also be sequestered through various antiporters into intracellular organelles, such as the vacuole. Although some uncertainties still persist, the nature of the major structural components responsible for alkali metal cation fluxes across yeast membranes has been defined within the last 20 years. In contrast, the regulatory components and their interactions are, in many cases, still unclear. Conserved signaling pathways (e.g., calcineurin and HOG) are known to participate in the regulation of influx and efflux processes at the plasma membrane level, even though the molecular details are obscure. Similarly, very little is known about the regulation of organellar transport and homeostasis of alkali metal cations. The aim of this review is to provide a comprehensive and up-to-date vision of the mechanisms responsible for alkali metal cation transport and their regulation in the model yeast Saccharomyces cerevisiae and to establish, when possible, comparisons with other yeasts and higher plants.

Role of ENA ATPase in Na(+) efflux at high pH in bryophytes

Potassium or Na(+) efflux ATPases, ENA ATPases, are present in all fungi and play a central role in Na(+) efflux and Na(+) tolerance. Flowering plants lack ENA ATPases but two ENA ATPases have been identified in the moss Physcomitrella patens, PpENA1 and PpENA2. PpENA1 mediates Na(+) efflux in Saccharomyces cerevisiae. To propose a general function of ENA ATPases in bryophytes it was necessary to demonstrate that these ATPases mediate Na(+) efflux in planta and that they exist in more bryophytes than P. patens. For these demonstrations (1) we cloned a third ATPase from P. patens, PpENA3, and studied the expression pattern of the three PpENA genes; (2) we constructed and studied the single and double Deltappena1 and Deltappena2 mutants; and (3) we cloned two ENA ATPases from the liverwort Marchantia polymorpha, MpENA1 and MpENA2, and expressed them in S. cerevisiae. The results from the first two approaches revealed that the expression of ENA ATPases was greatly enhanced at high pH and that Na(+) efflux at high pH depended on PpENA1. The ENA1 ATPase of M. polymorpha suppressed the defective growth of a S. cerevisiae mutant at high K(+) or Na(+) concentrations, especially at high K(+).

Growth at high pH and sodium and potassium tolerance in media above the cytoplasmic pH depend on ENA ATPases in Ustilago maydis

Potassium and Na(+) effluxes across the plasma membrane are crucial processes for the ionic homeostasis of cells. In fungal cells, these effluxes are mediated by cation/H(+) antiporters and ENA ATPases. We have cloned and studied the functions of the two ENA ATPases of Ustilago maydis, U. maydis Ena1 (UmEna1) and UmEna2. UmEna1 is a typical K(+) or Na(+) efflux ATPase whose function is indispensable for growth at pH 9.0 and for even modest Na(+) or K(+) tolerances above pH 8.0. UmEna1 locates to the plasma membrane and has the characteristics of the low-Na(+)/K(+)-discrimination ENA ATPases. However, it still protects U. maydis cells in high-Na(+) media because Na(+) showed a low cytoplasmic toxicity. The UmEna2 ATPase is phylogenetically distant from UmEna1 and is located mainly at the endoplasmic reticulum. The function of UmEna2 is not clear, but we found that it shares several similarities with Neurospora crassa ENA2, which suggests that endomembrane ENA ATPases may exist in many fungi. The expression of ena1 and ena2 transcripts in U. maydis was enhanced at high pH and at high K(+) and Na(+) concentrations. We discuss that there are two modes of Na(+) tolerance in fungi: the high-Na(+)-content mode, involving ENA ATPases with low Na(+)/K(+) discrimination, as described here for U. maydis, and the low-Na(+)-content mode, involving Na(+)-specific ENA ATPases, as in Neurospora crassa.

Identification of ENA1 as a virulence gene of the human pathogenic fungus Cryptococcus neoformans through signature-tagged insertional mutagenesis

A library of more than 4,500 signature-tagged insertion mutants of the human pathogenic fungus Cryptococcus neoformans was generated, and a subset was screened in a murine inhalation model to identify genes required for virulence. New genes that regulate aspects of C. neoformans virulence were also identified by screening the entire library for in vitro phenotypes related to the ability to cause disease, including melanin production, growth at high temperature, and growth under conditions of nutrient limitation. A screen of 10% of the strain collection in mice identified an avirulent mutant strain with an insertion in the ENA1 gene, which is predicted to encode a fungus-specific sodium or potassium P-type ATPase. The results of the deletion of the gene and complementation experiments confirmed its key role in mammalian virulence. ena1 mutant strains exhibited no change in sensitivity to high salt concentrations but were sensitive to alkaline pH conditions, providing evidence that the fungus may have to survive at elevated pH during infection of the mammalian host. The mutation of the well-characterized virulence factor calcineurin (CNA1) also rendered C. neoformans strains sensitive to elevated pH. ENA1 transcripts in wild-type and cna1 mutant strains were upregulated in response to high pH, and cna1 ena1 double mutant strains exhibited increased sensitivity to elevated pH, indicating that at least two pathways in the fungus mediate survival under alkaline conditions. Signature-tagged mutagenesis is an effective strategy for the discovery of new virulence genes in fungal pathogens of animals.

Identification and Characterization of Vnx1p, a Novel Type of Vacuolar Monovalent Cation/H+ Antiporter of Saccharomyces cerevisiae

We identified and characterized Vnx1p, a novel vacuolar monovalent cation/H+ antiporter encoded by the open reading frame YNL321w from Saccharomyces cerevisiae. Despite the homology of Vnx1p with other members of the CAX (calcium exchanger) family of transporters, Vnx1p is unable to mediate Ca2+ transport but is a low affinity Na+/H+ and K+/H+ anti-porter with a Km of 22.4 and 82.2 mm for Na+ and K+, respectively. Sequence analyses of Vnx1p revealed the absence of key amino acids shown to be essential for Ca2+/H+ exchange. vnx1Delta cells displayed growth inhibition when grown in the presence of hygromycin B or NaCl. Vnx1p activity was found in the vacuoles and shown to be dependent on the electrochemical potential gradient of H+ generated by the action of the V-type H+-ATPase. The presence of Vnx1p at the vacuolar membrane was further confirmed with cells expressing a VNX1::GFP chimeric gene. Similar to Nhx1p, the prevacuolar compartment-bound Na+/H+ antiporter, the vacuole-bound Vnx1p appears to play roles in the regulation of ion homeostasis and cellular pH.

Identification and characterization of ENA ATPases HwENA1 and HwENA2 from the halophilic black yeast Hortaea werneckii

Two genes, HwENA1 and HwENA2, which encode ENA-like ATPases in the extremely halotolerant black yeast Hortaea werneckii, were cloned and sequenced. Although the expression of both genes is responsive to salt, the transcription of the HwENA1 gene was induced at a higher level when the cells were exposed to salt stress, and the expression of HwENA2 gene was higher in the adapted cells, suggesting their different roles in maintaining alkali cation homeostasis. According to the phylogenetic tree based on the amino acid sequences, they represent a new group of fungal P-type ATPases. The comparison of both amino acid sequences with other fungal ENA ATPases, together with salt- and pH-responsive gene expression, suggests that newly identified ENA genes could be involved in maintaining low Na(+)/K(+) content in H. werneckii.

Plant biotechnology--genetic engineering to enhance plant salt tolerance

Plants not only provide food to humans and animals, but also provide a large number of non-food products of industrial and chemical importance. Moreover, they have the ability to purify the air, soil and water on the earth. Various trials to genetically improve the potential of plants are actively in progress. Salt-tolerance would be an especially important ability to bestow upon plants for agricultural and industrial purposes, because high salinity conditions are ubiquitous on earth and represent major barriers to growth. Enhancement of resistance against both hyper-osmotic stress and Na+ toxity is necessary for successful molecular breeding of salt tolerant plants. Introduction of genes for osmolyte bio-synthesis is useful to increase hyperosmotic tolerance of plant cells. It is introduced in this review that genetically engineered ectoine synthesis results in increased hyperosmotic tolerance of tobacco cells. High concentrations of Na+ reduce cellular activity by interfering with vital Na+-sensitive enzymes and by affecting K+ transport. Understanding the regulation of K+ and Na+ homeostasis is thus indispensable for enhancement of plant Na+ tolerance. My research group is investigating the Na+ efflux activity of the yeast Na+-ATPase (Ena1) when installed in the plasma membrane of plant cells, and the rice K+-Na+ co-transporters (HKT) that contribute to the regulation of K+ and Na+ uptake in root cells.

Different anti-Candida activities of two human lactoferrin-derived peptides, Lfpep and kaliocin-1

The synthetic peptides Lfpep and kaliocin-1 include the sequences from positions 18 to 40 and 153 to 183 of human lactoferrin, respectively. Lfpep is a cationic peptide with bactericidal and giardicidal effects, whereas kaliocin-1 is a novel bactericidal peptide that corresponds to a highly homologous sequence present in the transferrin family of proteins. Both peptides presented fungicidal activity against Candida spp., including fluconazole- and amphotericin B-resistant clinical isolates. Lfpep exhibited higher antifungal activity (8- to 30-fold) and salt resistance than kaliocin-1. The killing activity of Lfpep was mediated by its permeabilizing activity on Candida albicans cells, whereas kaliocin-1 was unable to disrupt the cytoplasmic membrane, as indicated by its inability to allow permeation of propidium iodide and the small amount of K+ released. The amino acid sequence of kaliocin-1 includes the "multidimensional antimicrobial signature" conserved in disulfide-containing antimicrobial peptides and a striking similarity to brevinin-1Sa, an antimicrobial peptide from frog skin secretions, exhibiting a "Rana box"-like sequence. These features may be of interest in the design of new antifungals.

Mechanisms underlying the halotolerant way of

The yeast Debaryomyces hansenii is usually found in salty environments such as the sea and salted food. It is capable of accumulating sodium without being intoxicated even when potassium is present at low concentration in the environment. In addition, sodium improves growth and protects D. hansenii in the presence of additional stress factors such as high temperature and extreme pH. An array of advantageous factors, as compared with Saccharomyces cerevisiae, is putatively involved in the increased halotolerance of D. hansenii: glycerol, the main compatible solute, is kept inside the cell by an active glycerol-Na+ symporter; potassium uptake is not inhibited by sodium; sodium protein targets in D. hansenii seem to be more resistant. The whole genome of D. hansenii has been sequenced and is now available at http://cbi.labri.fr/Genolevures/ and, so far, no genes specifically responsible for the halotolerant behaviour of D. hansenii have been found.

Rice Na+/H+-antiporter Nhx1 partially complements the alkali-metal-cation sensitivity of yeast strains lacking three sodium transporters

A triple mutant strain of Saccharomyces cerevisiae lacking its own Na+-ATPases and Na+/H+ antiporters (enal-4delta nha1delta nhx1delta) was used for the expression of the Oryza sativa NHX1 gene encoding a putative vacuolar Na+/H+ exchanger. Upon expression in yeast cells, the OsNhx 1p is not a transport system specific only for sodium cations but it has a broad substrate specificity for at least four alkali metal cations (Na+, Li+, K+ and Rb+) and is able to substitute for the endogenous yeast ScNhx1 antiporter. Its activity contributes to sequestration of alkali metal cations in intracellular vesicles.

pH response transcription factor PacC controls salt stress tolerance and expression of the P-Type Na+ -ATPase Ena1 in Fusarium oxysporum

Fungi possess efficient mechanisms of pH and ion homeostasis, allowing them to grow over a wide range of environmental conditions. In this study, we addressed the role of the pH response transcription factor PacC in salt tolerance of the vascular wilt pathogen Fusarium oxysporum. Loss-of-function pacC(+/-) mutants showed increased sensitivity to Li(+) and Na(+) and accumulated higher levels of these cations than the wild type. In contrast, strains expressing a dominant activating pacC(c) allele were more salt tolerant and had lower intracellular Li(+) and Na(+) concentrations. Although the kinetics of Li(+) influx were not altered by mutations in pacC, we found that Li(+) efflux at an alkaline, but not at an acidic, ambient pH was significantly reduced in pacC(+/-) loss-of-function mutants. To explore the presence of a PacC-dependent efflux mechanism in F. oxysporum, we cloned ena1 encoding an orthologue of the yeast P-type Na(+)-ATPase ENA1. Northern analysis revealed that efficient transcriptional activation of ena1 in F. oxysporum required the presence of high Na(+) concentrations and alkaline ambient pH and was dependent on PacC function. We propose a model in which PacC controls ion homeostasis in F. oxysporum at a high pH by activating expression of ena1 coordinately with a second Na(+)-responsive signaling pathway.

Modulation of in vitro fungicidal activity of human lactoferrin against Candida albicans by extracellular cation concentration and target cell metabolic activity

The anti-Candida activity of the innate defense protein human lactoferrin was investigated. Lactoferrin displayed a clear fungicidal effect against Candida albicans only under low-strength conditions. This candidacidal activity was inversely correlated with the extracellular concentration of the monovalent cations and was prevented by Na(+) and K(+) (> or 30 mM) and by divalent cations (Ca(2+) and Mg(2+) at > or 4 mM). A slight cellular release of K(+), cytosolic acidification, and a change in the membrane potential were observed in C. albicans cells treated with lactoferrin, suggesting that this protein directly or indirectly interacts with the cytoplasmic membrane. Mitochondrial inhibitors (carbonyl cyanide m-chlorophenylhydrazone, 2,4-dinitrophenol, azide, and antimycin) as well as anaerobic conditions significantly reduced the killing effect of lactoferrin. These results suggest that low-strength conditions and the cellular metabolic state may modulate the candidacidal activity of human lactoferrin.

Yeast plasma membrane Ena1p ATPase alters alkali-cation homeostasis and confers increased salt tolerance in tobacco cultured cells

In plants, the plasma membrane Na(+)/H(+) antiporter is the only key enzyme that extrudes cytosolic Na(+) and contributes to salt tolerance. But in fungi, the plasma membrane Na(+)/H(+) antiporter and Na(+)-ATPase are known to be key enzymes for salt tolerance. Saccharomyces cerevisiae Ena1p ATPase encoded by the ENA1/PMR2A gene is primarily responsible for Na(+) and Li(+) efflux across the plasma membrane during salt stress and for K(+) efflux at high pH and high K(+). To test if the yeast ATPase would improve salt tolerance in plants, we expressed a triple hemagglutinin (HA)-tagged Ena1p (Ena1p-3HA) in cultured tobacco (Nicotiana tabacum L.) cv Bright Yellow 2 (BY2) cells. The Ena1p-3HA proteins were correctly localized to the plasma membrane of transgenic BY2 cells and conferred increased NaCl and LiCl tolerance to the cells. Under moderate salt stress conditions, the Ena1p-3HA-expressing BY2 clones accumulated lower levels of Na(+) and Li(+) than nonexpressing BY2 clones. Moreover, the Ena1p-3HA expressing BY2 clones accumulated lower levels of K(+) than nonexpressing cells under no-stress conditions. These results suggest that the yeast Ena1p can also function as an alkali-cation (Na(+), Li(+), and K(+)) ATPase and alter alkali-cation homeostasis in plant cells. We conclude that, even with K(+)-ATPase activity, Na(+)-ATPase activity of the yeast Ena1p confers increased salt tolerance to plant cells during salt stress.

Molecular cloning and characterization of a sodium-pump ATPase of the moss Physcomitrella patens

Physcomitrella patens grew slowly at 600 mm Na+, pH 6.0, affected by the low water potential but without signs of suffering Na+ toxicity. At pH 8.0, tolerance seemed to be lower but it grew at 200 mm Na+, again without signs of Na+ toxicity. The resistance of Physcomitrella cells to the toxic effects of Na+ can be accounted for by their capacity to keep high K+:Na+ ratios and to extrude Na+ by a system that is not dependent on DeltapH. Physcomitrella expresses two P-type ATPases similar in sequence to fungal ENA-type Na+-ATPases. A functional study in yeast demonstrated that one of these ATPases, PpENA1, is an Na+-pump. We also found that P. patens has a plant-type SOS1 Na+/H+ antiporter. We discuss that Na+-ATPases existed in early land plants but that they were lost during the evolution of bryophytes to flowering plants.

An inventory of the P-type ATPases in the fission yeast Schizosaccharomyces pombe

The analysis of the Schizosaccharomyces pombe genome revealed the presence of 14 putative P-type ATPases. The clustering of ATPases resembles that of Saccharomyces cerevisiae, indicating that the main classes of pumps were already present before the split of the Archiascomycetes from the other Ascomycota. The overall amino acid identity between fission and budding yeast P-type ATPases is generally low (30-50%). This is similar to the fungus-plant and fungus-animal comparisons, suggesting that fungal ATPases underwent an extensive process of diversification. Unlike Sac. cerevisiae, fission yeast lacks Na(+)-ATPases, has a single heavy-metal ATPase and three ATPases of unknown specificity. The observed divergence within these fungi might reflect physiological differences, including adaptation to environmental stresses.

Development of a reporter system for the yeast Schwanniomyces occidentalis: influence of DNA composition and codon usage

In this paper we report on searching for suitable reporters to monitor gene expression and protein secretion in the amylolytic yeast Schwanniomyces occidentalis. Several potential reporter and marker genes, formerly shown to be functional in other yeasts, were cloned downstream from the homologous invertase gene (INV) promoter and their activity was followed in conditions of repression and derepression of the INV promoter. However, neither beta-glucuronidase nor beta-lactamase nor phleomycin resistance-conferring gene, all originating from E. coli, were expressed in S. occidentalis cells to such a level to allow for monitoring of their activity. All the reporter genes tested have a higher percentage of GC (47-62%) in their DNA compared to the DNA composition of S. occidentalis genes that are more AT-rich (36% GC). The codon usage of all the reporter genes also varies from that of 16 so far sequenced S. occidentalis genes. This suggests that an appropriate composition of DNA and a codon usage similar to S. occidentalis genes might be very important parameters for an efficient expression of a heterologous gene in Schwanniomyces occidentalis. Indeed, two genes originating from Staphylococcus aureus, with an AT-content in their DNA similar to that of S. occidentalis, were functionally expressed in S. occidentalis cells. Both a phleomycin resistance-conferring gene and a chloramphenicol acetyltransferase-encoding gene thus represent suitable reporters of gene expression and protein secretion in S. occidentalis. Additionally, we show in this work that the transcription-regulating region and the signal peptide sequence of the S. occidentalis invertase gene were efficient to direct gene expression and subsequent protein secretion in Saccharomyces cerevisiae.

Physiological studies on long-term adaptation to salt stress in the extremely halotolerant yeast Candida versatilis CBS 4019 (syn. C. halophila)

Candida halophila CBS 4019 (syn. C. versatilis) is an extremely salt-tolerant yeast. It was chosen to study the physiology of long-term resistance to salt stress in cells cultivated at increasing NaCl concentrations up to 4 or 5 M. Growth under stress was slow, severely affected not by salt, but rather by initial external pH. Growing on glucose, glycerol and mannitol were produced. Glycerol is the osmolyte and is transported by H(+)/symport. Transport-driven accumulation was though not affected by salt. The role of mannitol is unknown. Internal pH and intracellular volume were constant during growth at all initial pH/salt combinations. H(+)-ATPase activity was not affected by salt.

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.

Difference in substrate specificity divides the yeast alkali-metal-cation/H(+) antiporters into two subfamilies

Yeast plasma membrane Na(+)/H(+) antiporters (TC 2.A.36) share a high degree of similarity at the protein level. Expression of four antiporters (Saccharomyces cerevisiae Nha1p, Candida albicans Cnh1p, Zygosaccharomyces rouxii ZrSod2-22p and Schizosaccharomyces pombe sod2p) in a SACCH: cerevisiae mutant strain lacking both Na(+)-ATPase and Na(+)/H(+) antiporter genes made it possible to study the transport properties and contribution to cell salt tolerance of all antiporters under the same conditions. The ZrSod2-22p of the osmotolerant yeast Z. rouxii has the highest transport capacity for lithium and sodium but, like the SCHIZ: pombe sod2p, it does not recognize K(+) and Rb(+) as substrates. The SACCH: cerevisiae Nha1p and C. albicans Cnh1p have a broad substrate specificity for at least four alkali metal cations (Na(+), Li(+), K(+), Rb(+)), but their contribution to overall cell tolerance to high external concentration of toxic Na(+) and Li(+) cations seems to be lower compared to the antiporters of SCHIZ: pombe and especially Z. rouxii.

Potassium- or sodium-efflux ATPase, a key enzyme in the evolution of fungi

Potassium is the most abundant cation in cells. Therefore, plant-associated fungi and intracellular parasites are permanently or circumstantially exposed to high K(+) and must avoid excessive K(+) accumulation activating K(+) efflux systems. Because high K(+) and high pH are compatible in natural environments, free-living organisms cannot keep a permanent transmembrane DeltapH and cannot rely only on K(+)/H(+) antiporters, as do mitochondria. This study shows that the Schizosaccharomyces pombe CTA3 is a K(+)-efflux ATPase, and that other fungi are furnished with Na(+)-efflux ATPases, which also pump Na(+). All these fungal ATPases, including those pumping only Na(+), form a phylogenetic group, IID or ENA, among P-type ATPases. By searching in databases and partial cloning of ENA genes in species of Zygomycetes and Basidiomycetes, the authors conclude that probably all fungi have these genes. This study indicates that fungal K(+)- or Na(+)-ATPases evolved from an ancestral K(+)-ATPase, through processes of gene duplication. In yeast hemiascomycetes these duplications have occurred recently and produced bifunctional ATPases, whereas in Neurospora, and probably in other euascomycetes, they occurred earlier in evolution and produced specialized ATPases. In Schizosaccharomyces, adaptation to Na(+) did not involve the duplication of the K(+)-ATPase and thus it retains an enzyme which is probably close to the original one. The parasites Leishmania and Trypanosoma have ATPases phylogenetically related to fungal K(+)-ATPases, which are probably functional homologues of the fungal enzymes.

Role of the glutamic and aspartic residues in Na+-ATPase function in the ZrENA1 gene of Zygosaccharomyces rouxii

The effect of replacement of negatively charged amino acid residues on the function of Na+ transport proteins of the salt-tolerant yeast Zygosaccharomyces rouxii was examined by heterologous expression of mutant proteins in a strain of Saccharomyces cerevisiae, RH16.6, that lacks native Na+-ATPase activity due to null mutations of ENA1, ENA2, ENA3, and ENA4. Mutants of Na+/H+ antiporter gene (ZrSOD2) and Na+-ATPase gene (ZrENA1) of Z. rouxii were generated by site-directed mutagenesis. The significance of two aspartic residues arranged in tandem (D265 and D266) was demonstrated in Z. rouxii Na+/H+ antiporter. Some Z. rouxii Na+-ATPase mutant genes, namely E778A, D852A, and E981A present in the transmembrane domains (TMDs) and D736A, D743A, D748A, D749A, D759A, and D760A present in the cytoplasmic space were constructed. A lower level of salt tolerance was bestowed by the mutant genes D852A and E981A present in TMDs and D748A and D749A present in cytoplasmic space, compared with the wild-type gene (ZrENA1).

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.

Cloning and expression of two genes coding for sodium pumps in the salt-tolerant yeast Debaryomyces hansenii

Two genes encoding Na(+)-ATPases from Debaryomyces hansenii were cloned and sequenced. The genes, designated ENA1 from D. hansenii (DhENA1) and DhENA2, exhibited high homology with the corresponding genes from Schwanniomyces occidentalis. DhENA1 was expressed in the presence of high Na(+) concentrations, while the expression of DhENA2 also required high pH. A mutant of Saccharomyces cerevisiae lacking the Na(+) efflux systems and sensitive to Na(+), when transformed with DhENA1 or DhENA2, recovered Na(+) tolerance and also the ability to extrude Na(+).

Effect of pH, carbon source and K+ on the Na+-inhibited germ tube formation of Candida albicans

The effect of pH, carbon source and K+ on the Na+ -inhibited germ tube formation of the pathogenic fungus Candida albicans was examined in the arginine-phosphate modified (APM) medium. All C. albicans cells formed germ tubes in APM medium at pH 5.0-9.0. Na+ inhibited germ tube formation in a concentration dependent manner ranging from 0.2 to 1.0 M, and was further influenced by the pH of the medium. The inhibitory effect of Na+ was lowest at pH 8.0, and germ tube formation ceased at 1.0 M Na+ for any pH (4.0-9.0). At pH > or = 6.0, non-germ tube-forming cells did not show yeast growth; whereas at pH < or = 5.0, Na+ inhibited only germ tube formation but did not inhibit yeast growth. The inhibitory effect of Na+ was stronger in glucose medium than in galactose medium as carbon source. K+, at 0-0.8 M, had almost no effect on germ tube formation. However, in the presence of Na+, a very low concentration of K+ (0.5 mM) was able to release the cells from Na+ arrest and produced an increase in the rate as well as the percentage of germ tube formation. Intracellular Na+/K+ ratios increased with the increase in extracellular Na+ concentration, whereas the ratios decreased and remained within nontoxic levels when the extracellular K+ concentration was increased.

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.

Molecular cloning of the calcium and sodium ATPases in Neurospora crassa

Using PCR, reverse transcription-PCR (RT-PCR) and colony hybridization in a genomic library, we isolated six genes which encode type II P-type ATPases in Neurospora crassa. The six full-length cDNAs were cloned in a yeast expression vector and transformed into Saccharomyces cerevisiae null Ca2+- or Na+-ATPase mutants. Three cDNAs suppressed the defect of the Ca2+ mutant and two of these protected from Mn2+ toxicity. One cDNA suppressed the defect of the Na+ mutant and two cDNAs were not functional in S. cerevisiae. The expression of the transcripts of the six genes in the presence of Ca2+, Na+, high pH or supporting an osmotic shock indicated that, with the exception of one of the Ca2+-ATPases, the main function of the cloned ATPases is the adaptation to stress conditions. The relationship between the cloned fungal Ca2+- and Na+-ATPases and plant type II P-ATPases is discussed.

The molecular biology of Schwanniomyces occidentalis klocker

This review describes the molecular studies of Schwanniomyces occidentalis (Debaryomyces occidentalis) concerning transformation, genome, gene cloning, gene structure, gene expression and its characteristics to application. Schw. occidentalis appears to have at least five or seven chromosomes and no native plasmid from the yeast has been reported. Four transformation systems based on complement of Schw. occidentalis auxotrophic mutants were established. Vectors with the replicon of 2-micron plasmid and autonomous replication sequences (ARS) of Saccharomyces cerevisiae and Schw. occidentalis ARS replicated extrachromosomally in Schw. occidentalis transformants, without modification of the transformed vector DNA. So far, at least 21 Schw. occidentalis genes encoding 14 different proteins have been cloned. Most of the Schw. occidentalis genes have shown homologies (45 to 91%) with the corresponding genes of other organisms, especially of S. cerevisiae. However, some Schw. occidentalis genes possess other unique structures for their operators, promoters, transcription initiation sites, and terminators. Some foreign genes were expressed in Schw. occidentalis, while Schw. occidentalis genes functioned in other yeasts and bacteria, Escherichia coli, and Streptomyces lividans. Due to a strong ability of secretion and low level of glycosylation, Schw. occidentalis might be a promising host to produce heterologous proteins.

The Schizosaccharomyces pombe Pzh1 protein phosphatase regulates Na+ ion influx in a Trk1-independent fashion

We have previously shown that fission yeast encodes a PPZ-like phosphatase, designated Pzhl, which is an important determinant of cation homeostasis. pzh1 delta mutants display increased tolerance to Na+ ions, but they are hypersensitive to KC1 [Balcells, L., Gómez, N., Casamayor, A., Clotet, J. & Ariño, J. (1997) Eur. J. Biochem. 250, 476-483]. We have immunodetected Pzh1 in yeast extracts and found that this phosphatase is largely associated with particulate fractions. Cells defective in Pzh1 do not show altered efflux of Na+ or Li+ ions, but they accumulate these cations more slowly than wild-type cells. K+ ion content of pzh1 delta cells is about twice that of wild-type cells, and this can be explained by decreased efflux of K+. Therefore, Pzh1 may regulate both Na+ influx and K+ efflux in fission yeast. To test the possible relationship between K+ uptake, Na+ tolerance and Pzh1 function, we deleted the trk1+ gene, which encodes a putative high-affinity transporter of K+ ions. trkl delta mutants grew well even at relatively low concentrations of KCl and did not show significantly altered content or influx of K+ ions. However, they showed a Na(+)-sensitive phenotype which was greatly intensified by deletion of the sod2+ gene (which encodes the major determinant for efflux of Na+ ions), and clearly ameliorated by deletion of the pzh1 phosphatase, as well as by moderate concentrations of KCl in the medium. These results suggest that Trk1 does not mediate the effect of Pzh1 on NaCl tolerance and that fission yeast contains efficient systems, other than Trk1, for uptake of K+ ions.

Characterization of the Na+-ATPase gene (ZENA1) from the salt-tolerant yeast Zygosaccharomyces rouxii

In order to clarify the relationship between the salt tolerance of Zygosaccharomyces rouxii and the function of Na+-ATPase, a gene which exhibited homology to the Na+-ATPase gene (ZENA1) of Saccharomyces cerevisiae was isolated from Z. rouxii. This newly isolated gene (ZENA1) encoded a product of 1048 amino acids. The predicted amino-acid sequence of Zena1p was highly homologous to that of S. cerevisiae Ena1p and Ena2p, and Schwanniomyces occidentalis Ena1p and Ena2p, but showed low homology to that of Zpma1p, which is the product of the Z. rouxii plasma membrane H+.ATPase gene (ZENA1). Zena1p shares the peptide motifs which have been suggested to participate in the function of ATPase. Expression of ZENA1 was observed, but was independent of NaCl shock. When ZENA1 was expressed in salt-sensitive S. cerevisiae under the regulation of a GAL1 promoter by using the expression vector pYES2, salt tolerance of the transformants was observed. The growth characteristics of Zena1Delta-disruptants of Z. rouxii and the pH profiles of their plasma membrane ATPase activity were almost the same as those of the wild-type strain, indicating that the function of Zena1p is of little relevance to the salt tolerance property of Z. rouxii. By considering the close relationship between the salt tolerance of Z. rouxii and the function of its Na+/H+-antiporter, we concluded that the extrusion of Na+ across the plasma membrane in Z. rouxii cells might be carried out mainly via the function of the Na+/H+-antiporter in a high salinity environment.

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+.