The presumed potassium carrier Trk2p in Saccharomyces cerevisiae determines an H+-dependent, K+-independent current

Electrophysiology of fluoride channels in the yeasts Saccharomyces cerevisiae and Candida albicans

Tight regulation of molecules moving through the cell membrane is particularly important for free-living microorganisms because of their small cell volumes and frequent changes in the chemical composition of the extracellular environment. This is true for nutrients, but even more so for toxic molecules. Traditionally, the transport of these diverse molecules in microorganisms has been studied on cell populations rather than on single cells, mainly because of technical difficulties. The goal of this chapter is to make available a detailed method to prepare yeast spheroplasts to study the movement of fluoride ions across the plasma membrane of single cells by the patch-clamp technique. In this procedure, three steps are critical to achieve high resistance (GΩ) seals between the membrane and the glass electrode: (1) appropriate removal of the cell wall by enzymatic treatment; (2) balance between the osmotic strength of sealing solutions and cell membrane turgor; and (3) meticulous morphological inspection of spheroplasts suitable for gigaseal formation. We show now that this method, originally developed for Saccharomyces cerevisiae, can also be applied to Candida albicans, an opportunistic human pathogen.

Yeast Fex1p Is a Constitutively Expressed Fluoride Channel with Functional Asymmetry of Its Two Homologous Domains

Fluoride is a ubiquitous environmental toxin with which all biological species must cope. A recently discovered family of fluoride export (FEX) proteins protects organisms from fluoride toxicity by removing it from the cell. We show here that FEX proteins in Saccharomyces cerevisiae function as ion channels that are selective for fluoride over chloride and that these proteins are constitutively expressed at the yeast plasma membrane. Continuous expression is in contrast to many other toxin exporters in yeast, and this, along with the fact that two nearly duplicate proteins are encoded in the yeast genome, suggests that the threat posed by fluoride ions is frequent and detrimental. Structurally, eukaryotic FEX proteins consist of two homologous four-transmembrane helix domains folded into an antiparallel dimer, where the orientation of the two domains is fixed by a single transmembrane linker helix. Using phylogenetic sequence conservation as a guide, we have identified several functionally important residues. There is substantial functional asymmetry in the effect of mutation at corresponding sites in the two domains. Specifically, mutations to residues in the C-terminal domain proved significantly more detrimental to function than did similar mutations in the N-terminal domain. Our data suggest particular residues that may be important to anion specificity, most notably the necessity of a positive charge near the end of TMH1 in the C-terminal domain. It is possible that a cationic charge at this location may create an electrostatic well for fluoride ions entering the channel from the cytoplasm.

Quantitative description of ion transport via plasma membrane of yeast and small cells

Modeling of ion transport via plasma membrane needs identification and quantitative understanding of the involved processes. Brief characterization of main ion transport systems of a yeast cell (Pma1, Ena1, TOK1, Nha1, Trk1, Trk2, non-selective cation conductance) and determining the exact number of molecules of each transporter per a typical cell allow us to predict the corresponding ion flows. In this review a comparison of ion transport in small yeast cell and several animal cell types is provided. The importance of cell volume to surface ratio is emphasized. The role of cell wall and lipid rafts is discussed in respect to required increase in spatial and temporary resolution of measurements. Conclusions are formulated to describe specific features of ion transport in a yeast cell. Potential directions of future research are outlined based on the assumptions.

Anion currents in yeast K+ transporters (TRK) characterize a structural homologue of ligand-gated ion channels

Patch clamp studies of the potassium-transport proteins TRK1,2 in Saccharomyces cerevisiae have revealed large chloride efflux currents: at clamp voltages negative to -100 mV, and intracellular chloride concentrations >10 mM (J. Membr. Biol. 198:177, 2004). Stationary-state current-voltage analysis led to an in-series two-barrier model for chloride activation: the lower barrier (α) being 10-13 kcal/mol located ~30% into the membrane from the cytoplasmic surface; and the higher one (β) being 12-16 kcal/mol located at the outer surface. Measurements carried out with lyotrophic anions and osmoprotective solutes have now demonstrated the following new properties: (1) selectivity for highly permeant anions changes with extracellular pH; at pH(o)= 5.5: I(-)≈ Br(-) >Cl(-) >SCN(-) >NO (3)(-) , and at pH(o) 7.5: I(-)≈ Br(-) > SCN(-) > NO(3)(-) >Cl(-). (2) NO(2)(-) acts like "superchoride", possibly enhancing the channel's intrinsic permeability to Cl(-). (3) SCN(-) and NO(3)(-) block chloride permeability. (4) The order of selectivity for several slightly permeant anions (at pH(o)= 5.5 only) is formate>gluconate>acetate>>phosphate(-1). (5) All anion conductances are modulated (choked) by osmoprotective solutes. (6) The data and descriptive two-barrier model evoke a hypothetical structure (Biophys. J. 77:789, 1999) consisting of an intramembrane homotetramer of fungal TRK molecules, arrayed radially around a central cluster of four single helices (TM7) from each monomer. (7) That tetrameric cluster would resemble the hydrophobic core of (pentameric) ligand-gated ion channels, and would suggest voltage-modulated hydrophobic gating to underlie anion permeation.

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.

Diversity in expression patterns and functional properties in the rice HKT transporter family

Plant growth under low K(+) availability or salt stress requires tight control of K(+) and Na(+) uptake, long-distance transport, and accumulation. The family of membrane transporters named HKT (for High-Affinity K(+) Transporters), permeable either to K(+) and Na(+) or to Na(+) only, is thought to play major roles in these functions. Whereas Arabidopsis (Arabidopsis thaliana) possesses a single HKT transporter, involved in Na(+) transport in vascular tissues, a larger number of HKT transporters are present in rice (Oryza sativa) as well as in other monocots. Here, we report on the expression patterns and functional properties of three rice HKT transporters, OsHKT1;1, OsHKT1;3, and OsHKT2;1. In situ hybridization experiments revealed overlapping but distinctive and complex expression patterns, wider than expected for such a transporter type, including vascular tissues and root periphery but also new locations, such as osmocontractile leaf bulliform cells (involved in leaf folding). Functional analyses in Xenopus laevis oocytes revealed striking diversity. OsHKT1;1 and OsHKT1;3, shown to be permeable to Na(+) only, are strongly different in terms of affinity for this cation and direction of transport (inward only or reversible). OsHKT2;1 displays diverse permeation modes, Na(+)-K(+) symport, Na(+) uniport, or inhibited states, depending on external Na(+) and K(+) concentrations within the physiological concentration range. The whole set of data indicates that HKT transporters fulfill distinctive roles at the whole plant level in rice, each system playing diverse roles in different cell types. Such a large diversity within the HKT transporter family might be central to the regulation of K(+) and Na(+) accumulation in monocots.

Chloride homeostasis in Saccharomyces cerevisiae: high affinity influx, V-ATPase-dependent sequestration, and identification of a candidate Cl- sensor

Chloride homeostasis in Saccharomyces cerevisiae has been characterized with the goal of identifying new Cl⁻ transport and regulatory pathways. Steady-state cellular Cl⁻ contents (∼0.2 mEq/liter cell water) differ by less than threefold in yeast grown in media containing 0.003–5 mM Cl⁻. Therefore, yeast have a potent mechanism for maintaining constant cellular Cl⁻ over a wide range of extracellular Cl⁻. The cell water:medium [Cl⁻] ratio is >20 in media containing 0.01 mM Cl⁻ and results in part from sequestration of Cl⁻ in organelles, as shown by the effect of deleting genes involved in vacuolar acidification. Organellar sequestration cannot account entirely for the Cl⁻ accumulation, however, because the cell water:medium [Cl⁻] ratio in low Cl⁻ medium is ∼10 at extracellular pH 4.0 even in vma1 yeast, which lack the vacuolar H⁺-ATPase. Cellular Cl⁻ accumulation is ATP dependent in both wild type and vma1 strains. The initial ³⁶Cl⁻ influx is a saturable function of extracellular [³⁶Cl⁻] with K_1/2 of 0.02 mM at pH 4.0 and >0.2 mM at pH 7, indicating the presence of a high affinity Cl⁻ transporter in the plasma membrane. The transporter can exchange ³⁶Cl⁻ for either Cl⁻ or Br⁻ far more rapidly than SO₄⁼, phosphate, formate, HCO₃⁻, or NO₃⁻. High affinity Cl⁻ influx is not affected by deletion of any of several genes for possible Cl⁻ transporters. The high affinity Cl⁻ transporter is activated over a period of ∼45 min after shifting cells from high-Cl⁻ to low-Cl⁻ media. Deletion of ORF YHL008c (formate-nitrite transporter family) strongly reduces the rate of activation of the flux. Therefore, Yhl008cp may be part of a Cl⁻-sensing mechanism that activates the high affinity transporter in a low Cl⁻ medium. This is the first example of a biological system that can regulate cellular Cl⁻ at concentrations far below 1 mM.

All four putative selectivity filter glycine residues in KtrB are essential for high affinity and selective K+ uptake by the KtrAB system from Vibrio alginolyticus

The subunit KtrB of bacterial Na+-dependent K+-translocating KtrAB systems belongs to a superfamily of K+ transporters. These proteins contain four repeated domains, each composed of two transmembrane helices connected by a putative pore loop (p-loop). The four p-loops harbor a conserved glycine residue at a position equivalent to a glycine selectivity filter residue in K+ channels. We investigated whether these glycines also form a selectivity filter in KtrB. The single residues Gly70, Gly185, Gly290, and Gly402 from p-loops P(A) to P(D) of Vibrio alginolyticus KtrB were replaced with alanine, serine, or aspartate. The three alanine variants KtrB(A70), KtrB(A185), and KtrB(A290) maintained a substantial activity in KtrAB-mediated K+ uptake in Escherichia coli. This activity was associated with a decrease in the affinity for K+ by 2 orders of magnitude, with little effect on Vmax. Minor activities were also observed for three other variants: KtrB(A402), KtrB(S70), and KtrB(D185). With all of these variants, the property of Na+ dependence of K+ transport was preserved. Only the four serine variants mediated Na+ uptake, and these variants differed considerably in their K+/Na+ selectivity. Experiments on cloned ktrB in the pBAD18 vector showed that V. alginolyticus KtrB alone was still active in E. coli. It mediated Na+-independent, slow, high affinity, and mutation-specific K+ uptake as well as K+-independent Na+ uptake. These data demonstrate that KtrB contains a selectivity filter for K+ ions and that all four conserved p-loop glycine residues are part of this filter. They also indicate that the role of KtrA lies in conferring velocity and ion coupling to the Ktr complex.

In the yeast potassium channel, Tok1p, the external ring of aspartate residues modulates both gating and conductance

The yeast plasma-membrane potassium channel, Tok1p, is a voltage-dependent outward rectifier, the gating and steady-state conductance of which are conspicuously modulated by extracellular [K(+)] ([K(+)](o)). Activation is slow at high [K(+)](o), showing time constants (tau(a)) of approximately 90 ms when [K(+)](o) is 150 mM (depolarizing step to +100 mV), and inactivation is weak (<30%) during sustained depolarization. Lowering [K(+)](o) accelerates activation, increases peak current, and enhances inactivation, so that at 15 mM [K(+)](o) tau(a) is less than 50 ms and inactivation suppresses approximately 60% of peak current. Two negative residues, Asp292 and Asp426, near the mouth of the assembled channel, modulate both kinetics and conductance of the channel. Charge neutralization in the mutant Asp292Asn allows fast activation (tau(a) approximately 20 ms) at high [K(+)](o), peak currents diminishing with decreasing [K(+)](o), and fast, nearly complete, inactivation. The voltage dependence of tau(a) persists in the mutant, but the [K(+)](o) dependence almost disappears. Similar but smaller changes are seen in the Asp426Asn mutant, implying that pore geometry in the functional channel has twofold, not fourfold, symmetry.

Quantitative modeling of chloride conductance in yeast TRK potassium transporters

So-called TRK proteins are responsible for active accumulation of potassium in plants, fungi, and bacteria. A pair of these proteins in the plasma membrane of Saccharomyces cerevisiae, ScTrk1p and ScTrk2p, also admit large, adventitious, chloride currents during patch-recording (Cl- efflux). Resulting steady-state current-voltage curves can be described by two simple kinetic models, most interestingly, voltage-driven channeling of ions through a pair of activation-energy barriers that lie within the membrane dielectric, near the inner (alpha) and outer (beta) surfaces. Two barrier heights (E(alpha) and E(beta)) and two relative distances (a1 and b2) from the surfaces specify the model. Measured current amplitude parallels intracellular chloride concentration and is strongly enhanced by acidic extracellular pH. The former implies an exponential variation of a1, between approximately 0.2 and approximately 0.4 of the membrane thickness, whereas the latter implies a linear variation of E(beta), by 0.69 Kcal mol(-1)/pH. The model requires membrane slope conductance to rise exponentially with increasingly large negative membrane voltage, as verified by data from a few yeast spheroplasts that tolerated voltage clamping at -200 to -300 mV. The behaviors of E(beta) and a1 accord qualitatively with a hypothetical structural model for fungal TRK proteins, suggesting that chloride ions flow through a central pore formed by symmetric aggregation of four TRK monomers.

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.

Chloride channel function in the yeast TRK-potassium transporters

The TRK proteins-Trk1p and Trk2p- are the main agents responsible for "active" accumulation of potassium by the yeast Saccharomyces cerevisiae. In previous studies, inward currents measured through those proteins by whole-cell patch-clamping proved very unresponsive to changes of extracellular potassium concentration, although they did increase with extracellular proton concentration-qualitatively as expected for H(+) coupling to K(+) uptake. These puzzling observations have now been explored in greater detail, with the following major findings: a) the large inward TRK currents are not carried by influx of either K(+) or H(+), but rather by an efflux of chloride ions; b) with normal expression levels for Trk1p and Trk2p in potassium-replete cells, the inward TRK currents are contributed approximately half by Trk1p and half by Trk2p; but c) strain background strongly influences the absolute magnitude of these currents, which are nearly twice as large in W303-derived spheroplasts as in S288c-derived cells (same cell-size and identical recording conditions); d) incorporation of mutations that increase cell size (deletion of the Golgi calcium pump, Pmr1p) or that upregulate the TRK2 promoter, can further substantially increase the TRK currents; e) removal of intracellular chloride (e.g., replacement by sulfate or gluconate) reveals small inward currents that are K(+)-dependent and can be enhanced by K(+) starvation; and f) finally, the latter currents display two saturating kinetic components, with preliminary estimates of K(0.5) at 46 micro M [K(+)](out) and 6.8 m M [K(+)](out), and saturating fluxes of approximately 5 m M/min and approximately 10 m M/min (referred to intracellular water). These numbers are compatible with the normal K(+)-transport properties of Trk1p and Trk2p, respectively.

The TRK1 potassium transporter is the critical effector for killing of Candida albicans by the cationic protein, Histatin 5

The principal feature of killing of Candida albicans and other pathogenic fungi by the catonic protein Histatin 5 (Hst 5) is loss of cytoplasmic small molecules and ions, including ATP and K(+), which can be blocked by the anion channel inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. We constructed C. albicans strains expressing one, two, or three copies of the TRK1 gene in order to investigate possible roles of Trk1p (the organism's principal K(+) transporter) in the actions of Hst 5. All measured parameters (Hst 5 killing, Hst 5-stimulated ATP efflux, normal Trk1p-mediated K(+) ((86)Rb(+)) influx, and Trk1p-mediated chloride conductance) were similarly reduced (5-7-fold) by removal of a single copy of the TRK1 gene from this diploid organism and were fully restored by complementation of the missing allele. A TRK1 overexpression strain of C. albicans, constructed by integrating an additional TRK1 gene into wild-type cells, demonstrated cytoplasmic sequestration of Trk1 protein, along with somewhat diminished toxicity of Hst 5. These results could be produced either by depletion of intracellular free Hst 5 due to sequestered binding, or to cooperativity in Hst 5-protein interactions at the plasma membrane. Furthermore, Trk1p-mediated chloride conductance was blocked by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid in all of the tested strains, strongly suggesting that the TRK1 protein provides the essential pathway for ATP loss and is the critical effector for Hst 5 toxicity in C. albicans.

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.

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.

Molecular mechanisms and regulation of K+ transport in higher plants

Potassium (K+) plays a number of important roles in plant growth and development. Over the past few years, molecular approaches associated with electrophysiological analyses have greatly advanced our understanding of K+ transport in plants. A large number of genes encoding K+ transport systems have been identified, revealing a high level of complexity. Characterization of some transport systems is providing exciting information at the molecular level on functions such as root K+ uptake and secretion into the xylem sap, K+ transport in guard cells, or K+ influx into growing pollen tubes. In this review, we take stock of this recent molecular information. The main families of plant K+ transport systems (Shaker and KCO channels, KUP/HAK/KT and HKT transporters) are described, along with molecular data on how these systems are regulated. Finally, we discuss a few physiological questions on which molecular studies have shed new light.

Large Mg(2+)-dependent currents are associated with the increased expression of ALR1 in Saccharomyces cerevisiae

Two genes in Saccharomyces cerevisiae, ALR1 and ALR2, encode proteins putatively involved in Mg(2+) uptake. The present study supports this role for ALR1 and provides the first electrophysiological characterisation of this protein. The patch-clamp technique was used to measure whole-cell ion currents in protoplasts prepared from the wild-type strain, the alr1 alr2 double mutant (CM66), and the double mutant over-expressing the ALR1 gene (CM66+ALR1). With 50 mM Mg(2+) in the bathing solution, the inward current in protoplasts of CM66+ALR1 averaged -264+/-48 pA at -150 mV. Inward currents measured in the wild-type and CM66 protoplasts were more than five-fold smaller. When Mg(2+) was the major cation in the pipette solution, time-dependent outward currents were also detected in CM66+ALR1 protoplasts suggesting ALR1 can facilitate Mg(2+) efflux as well as uptake. We conclude that the ALR1 gene encodes a transport protein. The large magnitude of the Mg(2+)-dependent currents suggests that ALR1 could function as a cation channel.

Protein phosphatase type 1 regulates ion homeostasis in Saccharomyces cerevisiae

Protein phosphatase type 1 (PP1) is encoded by the essential gene GLC7 in Saccharomyces cerevisiae. glc7-109 (K259A, R260A) has a dominant, hyperglycogen defect and a recessive, ion and drug sensitivity. Surprisingly, the hyperglycogen phenotype is partially retained in null mutants of GAC1, GIP2, and PIG1, which encode potential glycogen-targeting subunits of Glc7. The R260A substitution in GLC7 is responsible for the dominant and recessive traits of glc7-109. Another mutation at this residue, glc7-R260P, confers only salt sensitivity, indicating that the glycogen and salt traits of glc7-109 are due to defects in distinct physiological pathways. The glc7-109 mutant is sensitive to cations, aminoglycosides, and alkaline pH and exhibits increased rates of l-leucine and 3,3'-dihexyloxacarbocyanine iodide uptake, but it is resistant to molar concentrations of sorbitol or KCl, indicating that it has normal osmoregulation. KCl suppresses the ion and drug sensitivities of the glc7-109 mutant. The CsCl sensitivity of this mutant is suppressed by recessive mutations in PMA1, which encodes the essential plasma membrane H(+)ATPase. Together, these results indicate that Glc7 regulates ion homeostasis by controlling ion transport and/or plasma membrane potential, a new role for Glc7 in budding yeast.

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).

Characterization of two HKT1 homologues from Eucalyptus camaldulensis that display intrinsic osmosensing capability

Plants have multiple potassium (K(+)) uptake and efflux mechanisms that are expressed throughout plant tissues to fulfill different physiological functions. Several different classes of K(+) channels and carriers have been identified at the molecular level in plants. K(+) transporters of the HKT1 superfamily have been cloned from wheat (Triticum aestivum), Arabidopsis, and Eucalyptus camaldulensis. The functional characteristics as well as the primary structure of these transporters are diverse with orthologues found in bacterial and fungal genomes. In this report, we provide a detailed characterization of the functional characteristics, as expressed in Xenopus laevis oocytes, of two cDNAs isolated from E. camaldulensis that encode proteins belonging to the HKT1 superfamily of K(+)/Na(+) transporters. The transport of K(+) in EcHKT-expressing oocytes is enhanced by Na(+), but K(+) was also transported in the absence of Na(+). Na(+) is transported in the absence of K(+) as has been demonstrated for HKT1 and AtHKT1. Overall, the E. camaldulensis transporters show some similarities and differences in ionic selectivity to HKT1 and AtHKT1. One striking difference between HKT1 and EcHKT is the sensitivity to changes in the external osmolarity of the solution. Hypotonic solutions increased EcHKT induced currents in oocytes by 100% as compared with no increased current in HKT1 expressing or uninjected oocytes. These osmotically sensitive currents were not enhanced by voltage and may mediate water flux. The physiological function of these osmotically induced increases in currents may be related to the ecological niches that E. camaldulensis inhabits, which are periodically flooded. Therefore, the osmosensing function of EcHKT may provide this species with a competitive advantage in maintaining K(+) homeostasis under certain conditions.

Remodeling of yeast genome expression in response to environmental changes

We used genome-wide expression analysis to explore how gene expression in Saccharomyces cerevisiae is remodeled in response to various changes in extracellular environment, including changes in temperature, oxidation, nutrients, pH, and osmolarity. The results demonstrate that more than half of the genome is involved in various responses to environmental change and identify the global set of genes induced and repressed by each condition. These data implicate a substantial number of previously uncharacterized genes in these responses and reveal a signature common to environmental responses that involves approximately 10% of yeast genes. The results of expression analysis with MSN2/MSN4 mutants support the model that the Msn2/Msn4 activators induce the common response to environmental change. These results provide a global description of the transcriptional response to environmental change and extend our understanding of the role of activators in effecting this response.

Partial deletion of a loop region in the high affinity K+ transporter HKT1 changes ionic permeability leading to increased salt tolerance

HKT1 is a high affinity K(+) transporter protein that is a member of a large superfamily of transporters found in plants, bacteria, and fungi. These transporters are primarily involved in K(+) uptake and are energized by Na(+) or H(+). HKT1 is energized by Na(+) but also mediates low affinity Na(+) uptake and may therefore be a pathway for Na(+) uptake, which is toxic to plants. The aim of this study was to identify regions of HKT1 that are involved in K(+)/Na(+) selectivity and alter the amino acid composition in those regions to increase the ionic selectivity of the transporter. A highly charged loop was identified, and two deletions were created that resulted in the removal of charged and uncharged amino acids. The functional changes caused by the deletions were studied in yeast and Xenopus oocytes. The deletions improved the K(+)/Na(+) selectivity of the transporter and increased the salt tolerance of the yeast cells in which they were expressed. In light of recent structural models of members of this symporter superfamily, it was necessary to determine the orientation of this highly charged loop. Introduction of an epitope tag allowed us to demonstrate that this loop faces the outside of the membrane where it is likely to facilitate the interaction with cations such as K(+) and Na(+). This study has identified an important structural feature in HKT1 that in part determines its K(+)/Na(+) selectivity. Understanding the structural basis of the functional characteristics in transporters such as HKT1 may have important implications for increasing the salt tolerance of higher plants.

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.

Trk1 and Trk2 define the major K(+) transport system in fission yeast

The trk1(+) gene has been proposed as a component of the K(+) influx system in the fission yeast Schizosaccharomyces pombe. Previous work from our laboratories revealed that trk1 mutants do not show significantly altered content or influx of K(+), although they are more sensitive to Na(+). Genome database searches revealed that S. pombe encodes a putative gene (designated here trk2(+)) that shows significant identity to trk1(+). We have analyzed the characteristics of potassium influx in S. pombe by using trk1 trk2 mutants. Unlike budding yeast, fission yeast displays a biphasic transport kinetics. trk2 mutants do not show altered K(+) transport and exhibit only a slightly reduced Na(+) tolerance. However, trk1 trk2 double mutants fail to grow at low K(+) concentrations and show a dramatic decrease in Rb(+) influx, as a result of loss of the high-affinity transport component. Furthermore, trk1 trk2 cells are very sensitive to Na(+), as would be expected for a strain showing defective potassium transport. When trk1 trk2 cells are maintained in K(+)-free medium, the potassium content remains higher than that of the wild type or trk single mutants. In addition, the trk1 trk2 strain displays increased sensitivity to hygromycin B. These results are consistent with a hyperpolarized state of the plasma membrane. An additional phenotype of cells lacking both Trk components is a failure to grow at acidic pH. In conclusion, the Trk1 and Trk2 proteins define the major K(+) transport system in fission yeast, and in contrast to what is known for budding yeast, the presence of any of these two proteins is sufficient to allow growth at normal potassium levels.