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

Allosteric links between the hydrophilic N-terminus and transmembrane core of human Na+ /H+ antiporter NHA2

The human Na+ /H+ antiporter NHA2 (SLC9B2) transports Na+ or Li+ across the plasma membrane in exchange for protons, and is implicated in various pathologies. It is a 537 amino acids protein with an 82 residues long hydrophilic cytoplasmic N-terminus followed by a transmembrane part comprising 14 transmembrane helices. We optimized the functional expression of HsNHA2 in the plasma membrane of a salt-sensitive Saccharomyces cerevisiae strain and characterized in vivo a set of mutated or truncated versions of HsNHA2 in terms of their substrate specificity, transport activity, localization, and protein stability. We identified a highly conserved proline 246, located in the core of the protein, as being crucial for ion selectivity. The replacement of P246 with serine or threonine resulted in antiporters with altered substrate specificity that were not only highly active at acidic pH 4.0 (like the native antiporter), but also at neutral pH. P246T/S versions also exhibited increased resistance to the HsNHA2-specific inhibitor phloretin. We experimentally proved that a putative salt bridge between E215 and R432 is important for antiporter function, but also structural integrity. Truncations of the first 50-70 residues of the N-terminus doubled the transport activity of HsNHA2, while changes in the charge at positions E47, E56, K57, or K58 decreased the antiporter's transport activity. Thus, the hydrophilic N-terminal part of the protein appears to allosterically auto-inhibit cation transport of HsNHA2. Our data also show this in vivo approach to be useful for a rapid screening of SNP's effect on HsNHA2 activity.

Dynamic modeling reveals a three-step response of Saccharomyces cerevisiae to high CO2 levels accompanied by increasing ATP demands

Saccharomyces cerevisiae is often applied in large-scale bioreactors where gradients of dissolved CO2 exist. Under high CO2 pressure, the dissolved gas enters the microbe, causing multifold intracellular responses such as decrease of pH, increase of HCO3- and changes of ion balance. Effects of varying CO2 concentrations are multifold, hard to scale and hardly investigated. Hence, the multi-level response to CO2 shifts was summarized in a predicting ODE model with mass action kinetics, balancing electrochemical charges in steady-state growth conditions. Compared to experimental observations, the simulated dynamics of ion concentrations were found to be consistent. During CO2 shifts, the model predicts the initial depolarization of the membrane potential, the temporal pH drop and the activation of countermeasures such as Pma1-mediated H+ export and Trk1,2-mediated K+ import. In conclusion, extracellular cation concentrations and the cellular pH regulation are critical factors that determine physiology and cellular energy management. Consequently, pressure-induced CO2 gradients cause peaks of ATP demand which may occur in cells circulating in large-scale industrial bioreactors.

Physiological and transcriptomic analysis of a salt-resistant Saccharomyces cerevisiae mutant obtained by evolutionary engineering

Salt-resistant yeast strains are highly demanded by industry due to the exposure of yeast cells to high concentrations of salt, in various industrial bioprocesses. The aim of this study was to perform a physiological and transcriptomic analysis of a salt-resistant Saccharomyces cerevisiae (S. cerevisiae) mutant generated by evolutionary engineering. NaCl-resistant S. cerevisiae strains were obtained by ethyl methanesulfonate (EMS) mutagenesis followed by successive batch cultivations in the presence of gradually increasing NaCl concentrations, up to 8.5% w/v of NaCl (1.45 M). The most probable number (MPN) method, high-performance liquid chromatography (HPLC), and glucose oxidase/peroxidase method were used for physiological analysis, while Agilent yeast DNA microarray systems were used for transcriptome analysis. NaCl-resistant mutant strain T8 was highly cross-resistant to LiCl and highly sensitive to AlCl3. In the absence of NaCl stress, T8 strain had significantly higher trehalose and glycogen levels compared to the reference strain. Global transcriptome analysis by means of DNA microarrays showed that the genes related to stress response, carbohydrate transport, glycogen and trehalose biosynthesis, as well as biofilm formation, were upregulated. According to gene set enrichment analysis, 548 genes were upregulated and 22 downregulated in T8 strain, compared to the reference strain. Among the 548 upregulated genes, the highest upregulation was observed for the FLO11 (MUC1) gene (92-fold that of the reference strain). Overall, evolutionary engineering by chemical mutagenesis and increasing NaCl concentrations is a promising approach in developing industrial strains for biotechnological applications.

CHX14 is a plasma membrane K-efflux transporter that regulates K(+) redistribution in Arabidopsis thaliana

Potassium (K(+) ) is essential for plant growth and development, yet the molecular identity of many K(+) transporters remains elusive. Here we characterized cation/H(+) exchanger (CHX) 14 as a plasma membrane K(+) transporter. CHX14 expression was induced by elevated K(+) and histochemical analysis of CHX14 promoter::GUS transgenic plants indicated that CHX14 was expressed in xylem parenchyma of root and shoot vascular tissues of seedlings. CHX14 knockout (chx14) and CHX14 overexpression seedlings displayed different growth phenotypes during K(+) stress as compared with wild-type seedlings. Roots of mutant seedlings displayed higher K(+) uptake rates than wild-type roots. CHX14 expression in yeast cells deficient in K(+) uptake renders the mutant cells more sensitive to deficiencies of K(+) in the medium. CHX14 mediates K(+) efflux in yeast cells loaded with high K(+) . Uptake experiments using (86) Rb(+) as a tracer for K(+) with both yeast and plant mutants demonstrated that CHX14 expression in yeast and in planta mediated low-affinity K(+) efflux. Functional green fluorescent protein (GFP)-tagged versions of CHX14 were localized to both the yeast and plant plasma membranes. Taken together, we suggest that CHX14 is a plasma membrane K(+) efflux transporter involved in K(+) homeostasis and K(+) recirculation.

Potassium starvation in yeast: mechanisms of homeostasis revealed by mathematical modeling

The intrinsic ability of cells to adapt to a wide range of environmental conditions is a fundamental process required for survival. Potassium is the most abundant cation in living cells and is required for essential cellular processes, including the regulation of cell volume, pH and protein synthesis. Yeast cells can grow from low micromolar to molar potassium concentrations and utilize sophisticated control mechanisms to keep the internal potassium concentration in a viable range. We developed a mathematical model for Saccharomyces cerevisiae to explore the complex interplay between biophysical forces and molecular regulation facilitating potassium homeostasis. By using a novel inference method (“the reverse tracking algorithm”) we predicted and then verified experimentally that the main regulators under conditions of potassium starvation are proton fluxes responding to changes of potassium concentrations. In contrast to the prevailing view, we show that regulation of the main potassium transport systems (Trk1,2 and Nha1) in the plasma membrane is not sufficient to achieve homeostasis.

Without potassium, all living cells will die; it has to be present in sufficient amounts for the proper function of most cell types. Disturbances in potassium levels in animal cells result in potentially fatal conditions and it is also an essential nutrient for plants and fungi. Cells have developed effective mechanisms for surviving under adverse environmental conditions of low external potassium. The question is how. Using the eukaryotic model organism, baker's yeast (Saccharomyces cerevisiae), we modeled how potassium homeostasis takes place. This is because, through mathematical modeling and experimentation, we found that the electro-chemical forces regulating potassium concentrations are coupled to proton fluxes, which respond to external conditions in order to maintain a viable potassium level within the cells. Our results challenge the current understanding of potassium homeostasis in baker's yeast, and could potentially be extended to other microorganisms, including non-conventional yeasts such as the pathogenic Candida albicans, and plant cells. In the future, the fundamental bases for this descriptive and predictive model might contribute to the development of new treatments for fungal infections, or developments in crop sciences.

A genomewide screen for tolerance to cationic drugs reveals genes important for potassium homeostasis in Saccharomyces cerevisiae

Potassium homeostasis is crucial for living cells. In the yeast Saccharomyces cerevisiae, the uptake of potassium is driven by the electrochemical gradient generated by the Pma1 H(+)-ATPase, and this process represents a major consumer of the gradient. We considered that any mutation resulting in an alteration of the electrochemical gradient could give rise to anomalous sensitivity to any cationic drug independently of its toxicity mechanism. Here, we describe a genomewide screen for mutants that present altered tolerance to hygromycin B, spermine, and tetramethylammonium. Two hundred twenty-six mutant strains displayed altered tolerance to all three drugs (202 hypersensitive and 24 hypertolerant), and more than 50% presented a strong or moderate growth defect at a limiting potassium concentration (1 mM). Functional groups such as protein kinases and phosphatases, intracellular trafficking, transcription, or cell cycle and DNA processing were enriched. Essentially, our screen has identified a substantial number of genes that were not previously described to play a direct or indirect role in potassium homeostasis. A subset of 27 representative mutants were selected and subjected to diverse biochemical tests that, in some cases, allowed us to postulate the basis for the observed phenotypes.

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.

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.

Cytosolic calcium and pH signaling in plants under salinity stress

Calcium is one of the essential nutrients for growth and development of plants. It is an important component of various structures in cell wall and membranes. Besides some fundamental roles under normal condition, calcium functions as a major secondary-messenger molecule in plants under different developmental cues and various stress conditions including salinity stress. Also changes in cytosolic pH, pH(cyt), either individually, or in coordination with changes in cytosolic Ca(2+) concentration, [Ca(2+)](cyt), evoke a wide range of cellular functions in plants including signal transduction in plant-defense responses against stresses. It is believed that salinity stress, like other stresses, is perceived at cell membrane, either extra cellular or intracellular, which then triggers an intracellular-signaling cascade including the generation of secondary messenger molecules like Ca(2+) and protons. The variety and complexity of Ca(2+) and pH signaling result from the nature of the stresses as well as the tolerance level of the plant species against that specific stress. The nature of changes in [Ca(2+)](cyt) concentration, in terms of amplitude, frequency and duration, is likely very important for decoding the specific downstream responses for salinity stress tolerance in planta. It has been observed that the signatures of [Ca(2+)](cyt) and pH differ in various studies reported so far depending on the techniques used to measure them, and also depending on the plant organs where they are measured, such as root, shoot tissues or cells. This review describes the recent advances about the changes in [Ca(2+)](cyt) and pH(cyt) at both cellular and whole-plant levels under salinity stress condition, and in various salinity-tolerant and -sensitive plant species.

Identification of two partners from the bacterial Kef exchanger family for the apical plasma membrane V-ATPase of Metazoa

The vital task of vectorial solute transport is often energised by a plasma membrane, proton-motive V-ATPase. However, its proposed partner, an apical alkali-metal/proton exchanger, has remained elusive. Here, both FlyAtlas microarray data and in situ analyses demonstrate that the bacterial kefB and kefC (members of the CPA2 family) homologues in Drosophila, CG10806 and CG31052, respectively, are both co-expressed with V-ATPase genes in transporting epithelia. Immunocytochemistry localises endogenous CG10806 and CG31052 to the apical plasma membrane of the Malpighian (renal) tubule. YFP-tagged CG10806 and CG31052 both localise to the plasma membrane of Drosophila S2 cells, and when driven in principal cells of the Malpighian tubule, they localise specifically to the apical plasma membrane. V-ATPase-energised fluid secretion is affected by overexpression of CG10806, but not CG31052; in the former case, overexpression causes higher basal rates, but lower stimulated rates, of fluid secretion compared with parental controls. Overexpression also impacts levels of secreted Na+ and K+. Both genes rescue exchanger-deficient (nha1 nhx1) yeast, but act differently; CG10806 is driven predominantly to the plasma membrane and confers protection against excess K+, whereas CG31052 is expressed predominantly on the vacuolar membrane and protects against excess Na+. Thus, both CG10806 and CG31052 are functionally members of the CPA2 gene family, colocalise to the same apical membrane as the plasma membrane V-ATPase and show distinct ion specificities, as expected for the Wieczorek exchanger.

Cloning and characterization of two K+ transporters of Debaryomyces hansenii

Two genes from the halotolerant yeast Debaryomyces hansenii were cloned, DhTRK1 and DhHAK1. These genes encode K(+) transporters with sequence similarities to the TRK and HAK transporters from Debaryomyces occidentalis and Candida albicans. The DhHAK1p transporter was only expressed in K(+)-starved cells, as shown by Northern blot analysis. Both DhTRK1p and DhHAK1p were expressed in a trk1Delta trk2Delta mutant of Saccharomyces cerevisiae, unable to grow at low K(+). This expression resulted in partial recovery of growth and ability to retain K(+) at low concentrations. In liquid media, 0.5 M NaCl affected growth of these S. cerevisiae transformants as it does in D. hansenii, resulting in a much less deleterious effect than in wild-type S. cerevisiae. Kinetics of Rb(+) uptake in the transformants suggest that DhTRK1p and DhHAK1p code for moderate-affinity K(+) transporters exhibiting a sigmoid response against Rb(+) concentration and presenting a deviation from classic Michaelis-Menten kinetics at low substrate concentrations. Rb(+) uptake by the DhTRK1p transporter was stimulated by millimolar concentrations of Na(+) at pH 4.5. The good performance of DhTRK1p in the presence of NaCl may be a key feature in the halotolerance of D. hansenii.

Molecular characterization of PeSOS1: the putative Na(+)/H (+) antiporter of Populus euphratica

Populus euphratica is a salt-tolerant tree species growing in semi-arid saline areas. A Na(+)/H(+) antiporter gene was successfully isolated from this species through RACE cloning, and named PeSOS1. The isolated cDNA was 3665 bp long and contained a 3438 bp open reading frame that was predicted to encode a 127-kDa protein with 12 hypothetical transmembrane domains in the N-terminal part and a long hydrophilic cytoplasmic tail in the C-terminal part. The amino acid sequence of this PeSOS1 gene showed 64% identity with the previously isolated SOS1 gene from the glycophyte Arabidopsis thaliana. The level of protein expressed by PeSOS1 in the leaves of P. euphratica was significantly up-regulated in the presence of high (200 mM) concentrations of NaCl, while the mRNA level in the leaves remained relatively constant. Immunoanalysis suggested that the protein encoded by PeSOS1 is localized in the plasma membrane. Expression of PeSOS1 partially suppressed the salt sensitive phenotypes of the EP432 bacterial strain, which lacks the activity of the two Na(+)/H(+) antiporters EcNhaA and EcNhaB. These results suggest that PeSOS1 may play an essential role in the salt tolerance of P. euphratica and may be useful for improving salt tolerance in other tree species.

Key role for intracellular K+ and protein kinases Sat4/Hal4 and Hal5 in the plasma membrane stabilization of yeast nutrient transporters

K+ transport in living cells must be tightly controlled because it affects basic physiological parameters such as turgor, membrane potential, ionic strength, and pH. In yeast, the major high-affinity K+ transporter, Trk1, is inhibited by high intracellular K+ levels and positively regulated by two redundant "halotolerance" protein kinases, Sat4/Hal4 and Hal5. Here we show that these kinases are not required for Trk1 activity; rather, they stabilize the transporter at the plasma membrane under low K+ conditions, preventing its endocytosis and vacuolar degradation. High concentrations (0.2 M) of K+, but not Na+ or sorbitol, transported by undefined low-affinity systems, maintain Trk1 at the plasma membrane in the hal4 hal5 mutant. Other nutrient transporters, such as Can1 (arginine permease), Fur4 (uracil permease), and Hxt1 (low-affinity glucose permease), are also destabilized in the hal4 hal5 mutant under low K+ conditions and, in the case of Can1, are stabilized by high K+ concentrations. Other plasma membrane proteins such as Pma1 (H+ -pumping ATPase) and Sur7 (an eisosomal protein) are not regulated by halotolerance kinases or by high K+ levels. This novel regulatory mechanism of nutrient transporters may participate in the quiescence/growth transition and could result from effects of intracellular K+ and halotolerance kinases on membrane trafficking and/or on the transporters themselves.

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.

Characterization of the ion transport activity of the budding yeast Na+/H+ antiporter, Nha1p, using isolated secretory vesicles

The Saccharomyces cerevisiae Nha1p, a plasma membrane protein belonging to the monovalent cation/proton antiporter family, plays a key role in the salt tolerance and pH regulation of cells. We examined the molecular function of Nha1p by using secretory vesicles isolated from a temperature sensitive secretory mutant, sec4-2, in vitro. The isolated secretory vesicles contained newly synthesized Nha1p en route to the plasma membrane and showed antiporter activity exchanging H+ for monovalent alkali metal cations. An amino acid substitution in Nha1p (D266N, Asp-266 to Asn) almost completely abolished the Na+/H+ but not K+/H+ antiport activity, confirming the validity of this assay system as well as the functional importance of Asp-266, especially for selectivity of substrate cations. Nha1p catalyzes transport of Na+ and K+ with similar affinity (12.7 mM and 12.4 mM), and with lower affinity for Rb+ and Li+. Nha1p activity is associated with a net charge movement across the membrane, transporting more protons per single sodium ion (i.e., electrogenic). This feature is similar to the bacterial Na+/H+ antiporters, whereas other known eukaryotic Na+/H+ antiporters are electroneutral. The ion selectivity and the stoichiometry suggest a unique physiological role of Nha1p which is distinct from that of other known Na+/H+ antiporters.

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.

Evolutionary origins of eukaryotic sodium/proton exchangers

More than 200 genes annotated as Na+/H+ hydrogen exchangers (NHEs) currently reside in bioinformation databases such as GenBank and Pfam. We performed detailed phylogenetic analyses of these NHEs in an effort to better understand their specific functions and physiological roles. This analysis initially required examining the entire monovalent cation proton antiporter (CPA) superfamily that includes the CPA1, CPA2, and NaT-DC families of transporters, each of which has a unique set of bacterial ancestors. We have concluded that there are nine human NHE (or SLC9A) paralogs as well as two previously unknown human CPA2 genes, which we have named HsNHA1 and HsNHA2. The eukaryotic NHE family is composed of five phylogenetically distinct clades that differ in subcellular location, drug sensitivity, cation selectivity, and sequence length. The major subgroups are plasma membrane (recycling and resident) and intracellular (endosomal/TGN, NHE8-like, and plant vacuolar). HsNHE1, the first cloned eukaryotic NHE gene, belongs to the resident plasma membrane clade. The latter is the most recent to emerge, being found exclusively in vertebrates. In contrast, the intracellular clades are ubiquitously distributed and are likely precursors to the plasma membrane NHE. Yeast endosomal ScNHX1 was the first intracellular NHE to be described and is closely related to HsNHE6, HsNHE7, and HsNHE9 in humans. Our results link the appearance of NHE on the plasma membrane of animal cells to the use of the Na+/K(+)-ATPase to generate the membrane potential. These novel observations have allowed us to use comparative biology to predict physiological roles for the nine human NHE paralogs and to propose appropriate model organisms in which to study the unique properties of each NHE subclass.

The yeast endosomal Na+K+/H+ exchanger Nhx1 regulates cellular pH to control vesicle trafficking

The relationship between endosomal pH and function is well documented in viral entry, endosomal maturation, receptor recycling, and vesicle targeting within the endocytic pathway. However, specific molecular mechanisms that either sense or regulate luminal pH to mediate these processes have not been identified. Herein we describe the use of novel, compartment-specific pH indicators to demonstrate that yeast Nhx1, an endosomal member of the ubiquitous NHE family of Na+/H+ exchangers, regulates luminal and cytoplasmic pH to control vesicle trafficking out of the endosome. Loss of Nhx1 confers growth sensitivity to low pH stress, and concomitant acidification and trafficking defects, which can be alleviated by weak bases. Conversely, weak acids cause wild-type yeast to present nhx1Delta trafficking phenotypes. Finally, we report that Nhx1 transports K+ in addition to Na+, suggesting that a single mechanism may responsible for both pH and K+-dependent endosomal processes. This presents the newly defined family of eukaryotic endosomal NHE as novel targets for pharmacological inhibition to alleviate pathological states associated with organellar alkalinization.

A stress-inducible plasma membrane protein 3 (AcPMP3) in a monocotyledonous halophyte, Aneurolepidium chinense, regulates cellular Na(+) and K(+) accumulation under salt stress

Regulation of ion homeostasis is fundamental to physiological activities in plants. Here, we report on the functional characterization of AcPMP3 [Aneurolepidium chinense (a monocotyledonous halophyte) plasma membrane protein 3] under salt stress. Expression of AcPMP3-1 and AcPMP3-2 genes was highly induced by various abiotic stresses, such as salt, cold and drought. Furthermore, abscisic acid, H(2)O(2) and salicylic acid also triggered expression of AcPMP3 genes. In the Deltanha1 Deltapmr2 Deltapmp3 yeast mutant, which lacks the major Na(+) efflux systems (Na(+)/H(+) antiporter and Na(+)-ATPase), its salt-sensitive phenotype was restored by expressing the AcPMP3-1 gene, and the transformants accumulated lesser amounts of Na(+) and K(+) than mutant cells under 50 mM NaCl and 500 mM KCl conditions, respectively. These results suggested that AcPMP3-1 plays a role as a regulator of both Na(+) and K(+) accumulation in the cells. In situ hybridization showed that the AcPMP3-1 transcript was localized in cells of the root cap and root epidermis, which strongly suggested that AcPMP3-1 is essential for regulating Na(+)/K(+) transportation between plant roots and the outer environment under salt stress.

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.

Mutagenesis analysis of the yeast Nha1 Na+/H+ antiporter carboxy-terminal tail reveals residues required for function in cell cycle

The yeast Nha1 Na(+),K(+)/H(+) antiporter may play an important role in regulation of cell cycle, as high-copy expression of the NHA1 gene is able to rescue the blockage at the G(1)/S transition of cells lacking Sit4 protein phosphatase and Hal3 activities. Interestingly, this function was independent of the role of the antiporter in improving tolerance to sodium cations, it required the integrity of a relatively large region (from residues 800 to 948) of its carboxy-terminal moiety, and was not performed by the fission yeast homolog antiporter Sod2, which lacks a carboxy-terminal tail. Here we show that a hybrid protein composed of the Sod2 antiporter fused to the carboxy-terminal half of Nha1 strongly increased sodium tolerance, but did not allow growth at high potassium nor did rescue growth of the sit4 hal3 conditional mutant strain. Deletion of Nha1 residues from 800 to 849, 900 to 925 or 926 to 954 abolished the function of Nha1 in cell cycle without affecting sodium tolerance. A screening for loss-of-function mutations at the 775-980 carboxy-terminal tail of Nha1 has revealed a number of residues required for function in cell cycle, most of them clustering in two regions, from residues 869 to 876 (cluster A) and 918 to 927 (cluster B). The later is rather conserved in other related antiporters, while the former is not.

Simultaneous determination of potassium and rubidium content in yeast

Rubidium is widely used as a potassium analogue in transport studies in yeast and other organisms. As rubidium (potassium) uptake is modulated by the internal potassium concentration, it is often necessary to determine both Rb(+) and K(+) concentrations in the same cell extract. Current methods based on atomic absorption/emission spectroscopy require separate analysis for each cation. Alternatively, unsafe radioactive isotopes can be used. Here we report a convenient, non-radioactive, HPLC/conductivity-based method that allows a complete analysis of both cations with a single injection from a cell extract. The increase in Rb(+) uptake during K(+) starvation in yeast is easily demonstrated with this method.