Dual system for potassium transport in Saccharomyces cerevisiae

Assembly of plant Shaker-like K(out) channels requires two distinct sites of the channel alpha-subunit

SKOR and GORK are outward-rectifying plant potassium channels from Arabidopsis thaliana. They belong to the Shaker superfamily of voltage-dependent K(+) channels. Channels of this class are composed of four alpha-subunits and subunit assembly is a prerequisite for channel function. In this study the assembly mechanism of SKOR was investigated using the yeast two-hybrid system and functional assays in Xenopus oocytes and in yeast. We demonstrate that SKOR and GORK physically interact and assemble into heteromeric K(out) channels. Deletion mutants and chimeric proteins generated from SKOR and the K(in) channel alpha-subunit KAT1 revealed that the cytoplasmic C-terminus of SKOR determines channel assembly. Two domains that are crucial for channel assembly were identified: i), a proximal interacting region comprising a putative cyclic nucleotide-binding domain together with 33 amino acids just upstream of this domain, and ii), a distal interacting region showing some resemblance to the K(T) domain of KAT1. Both regions contributed differently to channel assembly. Whereas the proximal interacting region was found to be active on its own, the distal interacting region required an intact proximal interacting region to be active. K(out) alpha-subunits did not assemble with K(in) alpha-subunits because of the absence of interaction between their assembly sites.

Potassium transport at the plasma membrane of the food spoilage yeast Zygosaccharomyces bailii

Zygosaccharomyces bailii is a commercially important spoilage yeast capable of growth at low pH in the presence of weak organic acid preservatives, such as benzoic acid. A patch-clamp electrophysiological analysis of plasma membrane K+ transport revealed a high conductance pathway for low-affinity K+ uptake. In contrast to the equivalent K+ transporter in Saccharomyces cerevisiae, this system remained operative at low extracellular pH and may therefore facilitate K+ uptake in K(+)-rich and acidic beverages. Benzoate inhibited growth, increased intracellular K+ content, yet decreased the magnitude of the K+ uptake conductance; specifically, the hyperpolarization-activated inwardly-rectifying component was reduced. It is proposed that this adaptation helps maintain a hyperpolarized membrane voltage to effect continued ATPase-mediated H+ extrusion and so combat preservative-induced cytosolic acidosis. Again in contrast to S. cerevisiae, the K+ conductance was relatively insensitive to increased extracellular Ca2+. Paradoxically (and unlike S. cerevisiae) increasing extracellular Ca2+ inhibited growth, suggesting a simple expedient to limit spoilage by Z. bailii.

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.

New phenotypes of functional expression of the mKir2.1 channel in potassium efflux-deficientSaccharomyces cerevisiae strains

The functional expression of the mouse Kir2.1 potassium channel in yeast cells lacking transport systems for potassium and sodium efflux (ena1-4delta nha1delta) resulted in increased cell sensitivity to high external concentrations of potassium. The phenotype depended on the level of Kir2.1 expression and on the external pH. The activity of Kir2.1p in the yeast cells was almost negligible at pH 3.0 and the highest at pH 7.0. Kir2.1p was permeable for both potassium and rubidium cations, but neither sodium nor lithium were transported via the channel. Measurements of the cation contents in cells confirmed the higher concentration of potassium in cells with Kir2.1p. Specific inhibition of the mKir2.1 channel activity by Ba2+ cations was observed. The use of a mutant strain lacking both potassium efflux and uptake transporters (ena1-4delta nha1delta trk1delta trk2delta) enabled the monitoring of channel activity on two levels--the provision of the necessary amount of intracellular K+ in media with low potassium concentrations, and simultaneously, the channel's contribution to cell potassium sensitivity in the presence of high external K+. This combination of mutations proved to be a new, sensitive and practical tool for characterizing the properties of heterologously expressed transporters mediating both the efflux and influx of alkali-metal-cations.

Novel p-type ATPases mediate high-affinity potassium or sodium uptake in fungi

Fungi have an absolute requirement for K+, but K+ may be partially replaced by Na+. Na+ uptake in Ustilago maydis and Pichia sorbitophila was found to exhibit a fast rate, low Km, and apparent independence of the membrane potential. Searches of sequences with similarity to P-type ATPases in databases allowed us to identify three genes in these species, Umacu1, Umacu2, and PsACU1, that could encode P-type ATPases of a novel type. Deletion of the acu1 and acu2 genes proved that they encoded the transporters that mediated the high-affinity Na+ uptake of U. maydis. Heterologous expressions of the Umacu2 gene in K+ transport mutants of Saccharomyces cerevisiae and transport studies in the single and double Deltaacu1 and Deltaacu2 mutants of U. maydis revealed that the acu1 and acu2 genes encode transporters that mediated high-affinity K+ uptake in addition to Na+ uptake. Other fungi also have genes or pseudogenes whose translated sequences show high similarity to the ACU proteins of U. maydis and P. sorbitophila. In the phylogenetic tree of P-type ATPases all the identified ACU ATPases define a new cluster, which shows the lowest divergence with type IIC, animal Na+,K(+)-ATPases. The fungal high-affinity Na+ uptake mediated by ACU ATPases is functionally identical to the uptake that is mediated by some plant HKT transporters.

The Ppz protein phosphatases regulate Trk-independent potassium influx in yeast

The Ppz protein phosphatases have been recently shown to negatively regulate the major potassium transport system in the yeast Saccharomyces cerevisiae, encoded by the TRK1 and TRK2 genes. We have found that, in the absence of the Trk system, Ppz mutants require abnormally high concentrations of potassium to proliferate. This can be explained by the observation that trk1 trk2 ppz1 or trk1 trk2 ppz1 ppz2 strains display a very poor rubidium uptake, with markedly increased Km values. These cells are very sensitive to the presence of several toxic cations in the medium, such as hygromicyn B or spermine, but not to lithium or sodium cations. At limiting potassium concentrations, addition of EGTA to the medium improves growth of these mutants. Therefore, our results indicate that, in addition to their role in regulating Trk potassium transporters, Ppz phosphatases (essentially Ppz1), positively affect the residual low affinity potassium transport mechanisms in yeast. These findings may provide a new way to elucidate the molecular nature of the low affinity potassium uptake system in yeast as well as a useful model to analyze the function of plant or mammalian potassium channels through heterologous expression in yeast.

Cloning and functional characterization of the high-affinity K+ transporter HAK1 of pepper

High-affinity K+ uptake in plants plays a crucial role in K+ nutrition and different systems have been postulated to contribute to the high-affinity K+ uptake. The results presented here with pepper (Capsicum annum) demonstrate that a HAK1-type transporter greatly contributes to the high-affinity K+ uptake observed in roots. Pepper plants starved of K+ for 3 d showed high-affinity K+ uptake (Km of 6 microM K+) that was very sensitive to NH and their roots expressed a high-affinity K+ transporter, CaHAK1, which clusters in group I of the KT/HAK/KUP family of transporters. When expressed in yeast ( Saccharomyces cerevisiae ), CaHAK1 mediated high-affinity K+ and Rb+ uptake with Km values of 3.3 and 1.9 microM, respectively. Rb+ uptake was competitively inhibited by micromolar concentrations of NH and Cs+, and by millimolar concentrations of Na+.

K+ fluxes in Schizosaccharomyces pombe

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

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

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

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.

Antimicrobial action of palmarosa oil (Cymbopogon martinii) on Saccharomyces cerevisiae

The essential oil extracted from palmarosa (Cymbopogon martinii) has proven anti-microbial properties against cells of Saccharomyces cerevisiae. Low concentrations of the oil (0.1%) inhibited the growth of S. cerevisiae cells completely. The composition of the sample of palmarosa oil was determined as 65% geraniol and 20% geranyl acetate as confirmed by GC-FTIR. The effect of palmarosa oil in causing K(+) leakage from yeast cells was attributed mainly to geraniol. Some leakage of magnesium ions was also observed. Blocking potassium membrane channels with caesium ions before addition of palmarosa oil did not change the extent of K(+) ion leakage, which was equal to the total sequestered K(+) in the cells. Palmarosa oil led to changes in the composition of the yeast cell membrane, with more saturated and less unsaturated fatty acids in the membrane after exposure of S. cerevisiae cells to the oil. Some of the palmarosa oil was lost by volatilization during incubation of the oil with the yeast cells. The actual concentration of the oil components affecting the yeast cells could not therefore be accurately determined.

Functional analysis of the M2(D) helix of the TRK1 potassium transporter of Saccharomyces cerevisiae

Eukaryotic KcsA-related K+ transporters mediate physiologically relevant K+ and Na+ fluxes in fungi and plants. ScTRK1 is a characteristic member of the group, and here we report a mutational analysis of the unique M2(D) helix of this transporter. Our results support the theoretical models placing this helix in a relevant position in the pore and interacting with P segments. Most single mutations eliminating positively charged or introducing negatively charged residues reduced the V(max) of Rb+ influx to a half, several together showed an additive effect, and four practically suppressed transport. In contrast, the introduction of only one positively charged residue practically abolished the function of the transporter. Almost all mutations in the M2(D) helix affected the two Rb+ binding sites of the transporter, mimicking mutations in the selectivity filter.

A novel intracellular K+/H+ antiporter related to Na+/H+ antiporters is important for K+ ion homeostasis in plants

In this study we have identified the first plant K+/H+ exchanger, LeNHX2 from tomato (Lycopersicon esculentum Mill. cv. Moneymaker), which is a member of the intracellular NHX exchanger protein family. The LeNHX2 protein, belonging to a subfamily of plant NHX proteins closely related to the yeast NHX1 protein, is abundant in roots and stems and is induced in leaves by short term salt or abscisic acid treatment. LeNHX2 complements the salt- and hygromycin-sensitive phenotype caused by NHX1 gene disruption in yeast, but affects accumulation of K+ and not Na+ in intracellular compartments. The LeNHX2 protein co-localizes with Prevacuolar and Golgi markers in a linear sucrose gradient in both yeast and plants. A histidine-tagged version of this protein could be purified and was shown to catalyze K+/H+ exchange but only minor Na+/H+ exchange in vitro. These data indicate that proper functioning of the endomembrane system relies on the regulation of K+ and H+ homeostasis by K+/H+ exchangers.

Sodium transport and HKT transporters: the rice model

Na+ uptake in the roots of K+-starved seedlings of barley, rice, and wheat was found to exhibit fast rate, low Km, and high sensitivity to K+. Sunflower plants responded in a similar manner but the uptake was not K+ sensitive. Ba2+ inhibited Na+ uptake, but not K+ uptake in rice roots. This demonstrated that Na+ and K+ uptake are mediated by different transporters, and that K+ blocked but was not transported by the Na+ transporter. The genome of rice cv. Nipponbare contains seven HKT genes, which may encode Na+ transporters, plus two HKT pseudogenes. Yeast expressions of OsHKT1 and OsHKT4 proved that they are Na+ transporters of high and low affinity, respectively, which are sensitive to K+ and Ba2+. Parallel experiments of K+ and Na+ uptake in yeast expressing the wheat or rice HKT1 transporters proved that they were very different; TaHKT1 transported K+ and Na+, and OsHKT1 only Na+. Transcript expressions in shoots of the OsHKT genes were fairly constant and insensitive to changes in the K+ and Na+ concentrations of the nutrient solution. In roots, the expressions were much lower than in shoots, except for OsHKT4 and OsHKT1 in K+-starved plants. We propose that OsHKT transporters are involved in Na+ movements in rice, and that OsHKT1 specifically mediates Na+ uptake in rice roots when the plants are K+ deficient. The incidence of HKT ESTs in several plant species suggests that the rice model with many HKT genes applies to other plants.

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.

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.

Molecular cloning and functional expression in bacteria of the potassium transporters CnHAK1 and CnHAK2 of the seagrass Cymodocea nodosa

The cDNAs CnHAK1 and CnHAK2, encoding K+ transporters, were amplified from the leaves of the seagrass Cymodocea nodosa. None of these transporters suppressed the K+ deficiency of a Saccharomyces cerevisiae mutant, but both suppressed the equivalent defect of an Escherichia coli mutant. Overexpression of the transporter CnHAKI, but not CnHAK2, mediated very rapid K+ or Rb+ influxes in the E. coli mutant. The concentration dependence of these influxes demonstrated that CnHAK1 is a low-affinity K+ transporter, which does not discriminate between K+ and Rb+. CnHAK1 expressed in E. coli worked in reverse when the external K+ concentrations were low, and we established the condition of a simple functional test of K+ loss for transporters of the Kup-HAK family. In comparison with its homologue barley transporter HvHAK2, CnHAKI was insensitive to Na+.

Cation/H+ antiporters mediate potassium and sodium fluxes in Pichia sorbitophila. Cloning of the PsNHA1 and PsNHA2 genes and expression in Saccharomyces cerevisiae

Pichia sorbitophila grows rapidly in the presence of very high NaCl concentrations. Under these conditions, even when the K(+) concentration is low, P. sorbitophila cells can maintain low Na(+) and high K(+) contents. This remarkable capacity of P. sorbitophila fails when the external pH is not acidic. This indicates that Na(+) efflux is mediated by an electroneutral Na(+)/H(+) antiporter. We have cloned and sequenced two genes designated as PsNHA1 and PsNHA2, which probably encode two antiporters of this type. The genes present high similarity with the corresponding genes from other yeasts. The heterologous expression of PsNHA1 or PsNHA2 in a Saccharomyces cerevisiae mutant lacking the Na(+) efflux systems and sensitive to high concentrations of Na(+) and K(+) rescued the tolerance and the ability to extrude both cations. The Accession Nos of the sequenced DNA fragments are: PsNHA1, AJ496431; PsNHA2, AJ496432. (TC 2.A.36)

Identification and characterization of a NaCl-inducible vacuolar Na+/H+ antiporter in Beta vulgaris

We have cloned, by RT-PCR and the use of degenerate oligonucleotide primers, a Na+/H+ antiporter from Beta vulgaris that is homologous to NHX1 of Arabidopsis thaliana and is a member of the family of recently cloned plant NHX-genes. This antiporter, BvNHX1, partially complements the salt-sensitive phenotype of a Deltaena1-4Deltanhx1 yeast strain. Antibodies were raised against a central portion of the BvNHX1 open reading frame that was predicted from the cloned cDNA. This antiporter was found to be highly enriched in tonoplast membranes isolated from plant tissues. BvNHX1 transcript abundance increased after salt treatments, in both suspension-cell cultures and whole plants. BvNHX1 protein abundance in the tonoplast-enriched membranes was also elevated after salt treatments. The vacuolar Na+/H+ antiporter activity increased up to 3-fold when the cell were exposed to 100 mM NaCl. The increase in protein abundance in response to the salt treatment, together with the salt-induced vacuolar Na+/H+ antiporter activity in B. vulgaris suggests that BvNHX1 plays an important role in salinity tolerance.

Inventory and functional characterization of the HAK potassium transporters of rice

Plants take up large amounts of K(+) from the soil solution and distribute it to the cells of all organs, where it fulfills important physiological functions. Transport of K(+) from the soil solution to its final destination is mediated by channels and transporters. To better understand K(+) movements in plants, we intended to characterize the function of the large KT-HAK-KUP family of transporters in rice (Oryza sativa cv Nipponbare). By searching in databases and cDNA cloning, we have identified 17 genes (OsHAK1-17) encoding transporters of this family and obtained evidence of the existence of other two genes. Phylogenetic analysis of the encoded transporters reveals a great diversity among them, and three distant transporters, OsHAK1, OsHAK7, and OsHAK10, were expressed in yeast (Saccharomyces cerevisiae) and bacterial mutants to determine their functions. The three transporters mediate K(+) influxes or effluxes, depending on the conditions of the experiment. A comparative kinetic analysis of HAK-mediated K(+) influx in yeast and in roots of K(+)-starved rice seedlings demonstrated the involvement of HAK transporters in root K(+) uptake. We discuss that all HAK transporters may mediate K(+) transport, but probably not only in the plasma membrane. Transient expression of the OsHAK10-green fluorescent protein fusion protein in living onion epidermal cells targeted this protein to the tonoplast.

Molecular analysis of the mechanism of potassium uptake through the TRK1 transporter of Saccharomyces cerevisiae

The TRK-HKT family of K(+) transporters mediates K(+) and Na(+) uptake in fungi and plants. In this study, we have investigated the molecular mechanism involved in the movement of alkali cations through the TRK1 transporter of Saccharomyces cerevisiae. The model that best explains the activity of ScTRK1 is a cotransport of two K(+) or Rb(+), both of which bind the two binding sites of ScTRK1 with very high affinities in K(+)-starved cells. Na(+) can be transported in the same way but it exhibits a much lower affinity for the second binding site. Therefore, only at critical concentration ratios between K(+) and Na(+), or Rb(+) and Na(+), the transporter takes up Na(+) together with K(+) or Rb(+). Mutation analyses suggest that the two binding sites are located in the P fragment of the first MPM motif of the transporter, and that Gln(90) is involved in these binding sites. ScTRK1 can be in two states, medium or high affinity, and we have found that Leu(949) is involved in the oscillation of the transporter between these two states. ScTRK1 mediates active K(+) uptake. This is not Na(+)-coupled and direct coupling of ScTRK1 to a source of chemical energy seems more probable than K(+)-H(+) cotransport.

Characterization of a HKT-type transporter in rice as a general alkali cation transporter

We report the characterization of rice OsHKT1 (Oryza sativa ssp. indica) homologous to the wheat K+/Na+-symporter HKT1. Expression of OsHKT1 in the yeast strain CY162 defective in K+-uptake restored growth at mM and micro M concentrations of K+ and mediated hypersensitivity to Na+. When expressed in Xenopus oocytes, rice OsHKT1 showed uptake characteristics of a Na+-transporter but mediated transport of other alkali cations as well. OsHKT1 expression was analysed in salt-tolerant rice Pokkali and salt-sensitive IR29 in response to external cation concentrations. OsHKT1 is expressed in roots and leaves. Exposure to Na+, Rb+, Li+, and Cs+ reduced OsHKT1 transcript amounts in both varieties and, in some cases, incompletely spliced transcripts were observed. By in situ hybridizations the expression of OsHKT1 was localized to the root epidermis and the vascular tissue inside the endodermis. In leaves, OsHKT1 showed strongest signals in cells surrounding the vasculature. The repression of OsHKT1 in the two rice varieties during salt stress was different in various cell types with main differences in the root vascular tissue. The data suggest control over HKT expression as a factor that may distinguish salt stress-sensitive and stress-tolerant lines. Differences in transcript expression in space and time in different lines of the same species appear to be a component of ion homeostasis correlated with salt sensitivity and tolerance.

Reconstitution in yeast of the Arabidopsis SOS signaling pathway for Na+ homeostasis

The Arabidopsis thaliana SOS1 protein is a putative Na+/H+ antiporter that functions in Na+ extrusion and is essential for the NaCl tolerance of plants. sos1 mutant plants share phenotypic similarities with mutants lacking the protein kinase SOS2 and the Ca2+ sensor SOS3. To investigate whether the three SOS proteins function in the same response pathway, we have reconstituted the SOS system in yeast cells. Expression of SOS1 improved the Na+ tolerance of yeast mutants lacking endogenous Na+ transporters. Coexpression of SOS2 and SOS3 dramatically increased SOS1-dependent Na+ tolerance, whereas SOS2 or SOS3 individually had no effect. The SOS2/SOS3 kinase complex promoted the phosphorylation of SOS1. A constitutively active form of SOS2 phosphorylated SOS1 in vitro independently of SOS3, but could not fully substitute for the SOS2/SOS3 kinase complex for activation of SOS1 in vivo. Further, we show that SOS3 recruits SOS2 to the plasma membrane. Although sos1 mutant plants display defective K+ uptake at low external concentrations, neither the unmodified nor the SOS2/SOS3-activated SOS1 protein showed K+ transport capacity in vivo, suggesting that the role of SOS1 on K+ uptake is indirect. Our results provide an example of functional reconstitution of a plant response pathway in a heterologous system and demonstrate that the SOS1 ion transporter, the SOS2 protein kinase, and its associated Ca2+ sensor SOS3 constitute a functional module. We propose a model in which SOS3 activates and directs SOS2 to the plasma membrane for the stimulatory phosphorylation of the Na+ transporter SOS1.

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.

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.

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).

Comparative functional features of plant potassium HvHAK1 and HvHAK2 transporters

Plant K(+) transporters of the HAK family belong to four rather divergent phylogenetic clusters, although most of the transporters belong to clusters I or II. A simple phylogenetic analysis of fungal and plant HAK transporters suggests that an original HAK gene duplicated even before fungi and plants diverged, generating transporters that at present fulfill different functions in the plant. The HvHAK1 transporter belongs to cluster I and mediates high-affinity K(+) uptake in barley roots, but no function is known for the cluster II transporter, HvHAK2, which is not functional in yeast. The function of HvHAK2 was investigated by constructing HvHAK1-HAK2 chimeric transporters, which were not functional even when they included only short fragments of HvHAK2. Then, amino acids characteristic of cluster II in the N terminus and in the first transmembrane domain were introduced into HvHAK1. All of these changes increased the Rb(+) K(m), introducing minimal changes in the Na(+) K(m), which suggested that HvHAK2 is a low-affinity, Na(+)-sensitive K(+) transporter. Using a K(+)-defective Escherichia coli mutant, we functionally expressed HvHAK2 and found that the predicted characteristics were correct, as well as discovering that the bacterial expression of HvHAK2 is functional at pH 5.5 but not at 7.5. We discuss whether HvHAK2 may be a tonoplast transporter effective for vacuolar K(+) depletion in K(+) starved plants.

Functional study of the Saccharomyces cerevisiae Nha1p C-terminus

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

The wheat cDNA LCT1 generates hypersensitivity to sodium in a salt-sensitive yeast strain

Salinity affects large areas of agricultural land, and all major crop species are intolerant to high levels of sodium ions. The principal route for Na(+) uptake into plant cells remains to be identified. Non-selective ion channels and high-affinity potassium transporters have emerged as potential pathways for Na(+) entry. A third candidate for Na(+) transport into plant cells is a low-affinity cation transporter represented by the wheat protein LCT1, which is known to be permeable for a wide range of cations when expressed in yeast (Saccharomyces cerevisiae). To investigate the role of LCT1 in salt tolerance we have used the yeast strain G19, which is disrupted in the genes encoding Na(+) export pumps and as a result displays salt sensitivity comparable with wheat. After transformation with LCT1, G19 cells became hypersensitive to NaCl. We show that LCT1 expression results in a strong decrease of intracellular K(+)/Na(+) ratio in G19 cells due to the combined effect of enhanced Na(+) accumulation and loss of intracellular K(+). Na(+) uptake through LCT1 was inhibited by K(+) and Ca(2+) at high concentrations and the addition of these ions rescued growth of LCT1-transformed G19 on saline medium. LCT1 was also shown to mediate the uptake of Li(+) and Cs(+). Expression of two mutant LCT1 cDNAs with N-terminal truncations resulted in decreased Ca(2+) uptake and increased Na(+) tolerance compared with expression of the full-length LCT1. Our findings strongly suggest that LCT1 represents a molecular link between Ca(2+) and Na(+) uptake into plant cells.

Two types of HKT transporters with different properties of Na+ and K+ transport in Oryza sativa

It is thought that Na+ and K+ homeostasis is crucial for salt-tolerance in plants. To better understand the Na+ and K+ homeostasis in important crop rice (Oryza sativa L.), a cDNA homologous to the wheat HKT1 encoding K+-Na+ symporter was isolated from japonica rice, cv Nipponbare (Ni-OsHKT1). We also isolated two cDNAs homologous to Ni-OsHKT1 from salt-tolerant indica rice, cv Pokkali (Po-OsHKT1, Po-OsHKT2). The predicted amino acid sequence of Ni-OsHKT1 shares 100% identity with Po-OsHKT1 and 91% identity with Po-OsHKT2, and they are 66-67% identical to wheat HKT1. Low-K+ conditions (less than 3 mM) induced the expression of all three OsHKT genes in roots, but mRNA accumulation was inhibited by the presence of 30 mM Na+. We further characterized the ion-transport properties of OsHKT1 and OsHKT2 using an expression system in the heterologous cells, yeast and Xenopus oocytes. OsHKT2 was capable of completely rescuing a K+-uptake deficiency mutation in yeast, whereas OsHKT1 was not under K+-limiting conditions. When OsHKTs were expressed in Na+-sensitive yeast, OsHKT1 rendered the cells more Na+-sensitive than did OsHKT2 in high NaCl conditions. The electrophysiological experiments for OsHKT1 expressed in Xenopus oocytes revealed that external Na+, but not K+, shifted the reversal potential toward depolarization. In contrast, for OsHKT2 either Na+ or K+ in the external solution shifted the reversal potential toward depolarization under the mixed Na+ and K+ containing solutions. These results suggest that two isoforms of HKT transporters, a Na+ transporter (OsHKT1) and a Na+- and K+-coupled transporter (OsHKT2), may act harmoniously in the salt tolerant indica rice.

The yeast Na+/H+ exchanger Nhx1 is an N-linked glycoprotein. Topological implications

Nhx1, the endosomal Na(+)/H(+) exchanger of Saccharomyces cerevisiae represents the founding member of a newly emerging subfamily of intracellular Na(+)/H(+) exchangers. These proteins share significantly greater sequence homology to one another than to members of the mammalian Na(+)/H(+) exchanger (NHE) family encoding plasma membrane Na(+)/H(+) exchangers. Members of both subtypes are predicted to share a common organization, with an N-terminal transporter domain of transmembrane helices followed by a C-terminal hydrophilic tail. In the present study, we show that Nhx1 is an asparagine-linked glycoprotein and that the sites of glycosylation map to two residues within the C-terminal stretch of the polypeptide. This is the first evidence, to date, for glycosylation of the C-terminal region of any known NHE isoform. Importantly, the mapping of N-linked glycosylation to the C-terminal domain of Nhx1 is indicative of an unexpected membrane topology, particularly with regard to the orientation of the tail region. Although one recent study demonstrated that certain epitopes in the C-terminal domain of NHE3 were accessible from the exoplasmic side of the plasma membrane (Biemesderfer, D., DeGray, B., and Aronson, P. S. (1998) J. Biol. Chem. 273, 12391-12396), numerous other studies implicate a cytosolic disposition for the hydrophilic C-terminal tail of plasma membrane NHE isoforms. Our analysis of the glycosylation of Nhx1 is strongly indicative of residence of at least some portion of the hydrophilic tail domain within the endosomal lumen. These findings imply that the organization of the tail domain may be more complex than previously assumed.

The low-temperature- and salt-induced RCI2A gene of Arabidopsis complements the sodium sensitivity caused by a deletion of the homologous yeast gene SNA1

Two closely related, tandemly arranged, low-temperature- and salt-induced Arabidopsis genes, corresponding to the previously isolated cDNAs RCI2A and RCI2B, were isolated and characterized. The RCI2A transcript accumulated primarily in response to low temperature or high salinity, and to a lesser extent in response to ABA treatment or water deficit stress. The RCI2B transcript was present at much lower levels than RCI2A, and could only be detected by reverse transcription-PCR amplification. The predicted 6 kDa RCI2 proteins are highly hydrophobic and contain two putative membrane-spanning regions. The polypeptides exhibit extensive similarity to deduced low-temperature- and/or salt-induced proteins from barley, wheat grass and strawberry, and to predicted proteins from bacteria, fungi, nematodes and yeast. Interestingly, we found that a deletion of the RCI2 homologous gene, SNA1 (YRD276c), in yeast causes a salt-sensitive phenotype. This effect is specific for sodium, since no growth defect was observed for the sna1 mutant on 1.7 M sorbitol, 1 M KCl or 0.6 M LiCl. Finally, we found that the Arabidopsis RCI2A cDNA can complement the sna1 mutant when expressed in yeast, indicating that the plant and yeast proteins have similar functions during high salt stress.

TRH1 encodes a potassium transporter required for tip growth in Arabidopsis root hairs

Root hair initiation involves the formation of a bulge at the basal end of the trichoblast by localized diffuse growth. Tip growth occurs subsequently at this initiation site and is accompanied by the establishment of a polarized cytoplasmic organization. Arabidopsis plants homozygous for a complete loss-of-function tiny root hair 1 (trh1) mutation were generated by means of the T-DNA-tagging method. Trichoblasts of trh1 plants form initiation sites but fail to undergo tip growth. A predicted primary structure of TRH1 indicates that it belongs to the AtKT/AtKUP/HAK K(+) transporter family. The proposed function of TRH1 as a K(+) transporter was confirmed in (86)Rb uptake experiments, which demonstrated that trh1 plants are partially impaired in K(+) transport. In line with these results, TRH1 was able to complement the trk1 potassium transporter mutant of Saccharomyces, which is defective in high-affinity K(+) uptake. Surprisingly, the trh1 phenotype was not restored when mutant seedlings were grown at high external potassium concentrations. These data demonstrate that TRH1 mediates K(+) transport in Arabidopsis roots and is responsible for specific K(+) translocation, which is essential for root hair elongation.

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.

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.

Functional conservation between yeast and plant endosomal Na(+)/H(+) antiporters

Vacuolar compartmentation of Na(+) is an essential mechanism for salinity tolerance since it lowers cytosolic Na(+) levels while contributing to osmotic adjustment for cell turgor and expansion. The AtNHX1 protein of Arabidopsis thaliana substituted functionally for ScNHX1, the endosomal Na(+)/H(+) antiporter of yeast. Ion tolerance conferred by AtNHX1 and ScNHX1 correlated with ion uptake into an intracellular pool that was energetically dependent on the vacuolar (H(+))ATPase. AtNHX1 localized to vacuolar membrane fractions of yeast. Hence, both transporters share an evolutionarily conserved function in Na(+) compartmentation. AtNHX1 mRNA levels were upregulated by ABA and NaCl treatment in leaf but not in root tissue.

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.

Tpr1, a Schizosaccharomyces pombe protein involved in potassium transport

The Schizosaccharomyces pombe Tpr1 was isolated as suppressor of the Saccharomyces cerevisiae Delta trk1,2 potassium uptake deficient phenotype. Tpr1, for tetratrico peptide repeat, encodes a 1039 amino acid residues protein with several reiterated TPR units displaying significant homology to p150(TSP), a recently identified phosphoprotein of mouse, to S. cerevisiae CTR9 and to related sequences of human, Caenorhabditis elegans, Methanoccocus jannaschii and Arabidopsis thaliana. Expression of Tpr1 restored growth on 0.2 mM K(+) media, induced K(+) transport with a K(T) of 4.6 mM and resumed inward currents of -90 pA at -250 mV (pH 7.2) conducting K(+) and other alkali-metal ions. The tetratrico peptide repeat is a degenerate motif of 34 amino acids that is repeated several times within TPR-containing proteins and has been suggested to mediate protein-protein interactions. The sequence and putative binding properties of Tpr1 suggest the protein unlikely as transporter but involved in the enhancement of K(+) uptake via conventional carriers.

Tolerance to toxic metals by a gene family of phytochelatin synthases from plants and yeast

Phytochelatins play major roles in metal detoxification in plants and fungi. However, genes encoding phytochelatin synthases have not yet been identified. By screening for plant genes mediating metal tolerance we identified a wheat cDNA, TaPCS1, whose expression in Saccharomyces cerevisiae results in a dramatic increase in cadmium tolerance. TaPCS1 encodes a protein of approximately 55 kDa with no similarity to proteins of known function. We identified homologs of this new gene family from Arabidopsis thaliana, Schizosaccharomyces pombe, and interestingly also Caenorhabditis elegans. The Arabidopsis and S.pombe genes were also demonstrated to confer substantial increases in metal tolerance in yeast. PCS-expressing cells accumulate more Cd2+ than controls. PCS expression mediates Cd2+ tolerance even in yeast mutants that are either deficient in vacuolar acidification or impaired in vacuolar biogenesis. PCS-induced metal resistance is lost upon exposure to an inhibitor of glutathione biosynthesis, a process necessary for phytochelatin formation. Schizosaccharomyces pombe cells disrupted in the PCS gene exhibit hypersensitivity to Cd2+ and Cu2+ and are unable to synthesize phytochelatins upon Cd2+ exposure as determined by HPLC analysis. Saccharomyces cerevisiae cells expressing PCS produce phytochelatins. Moreover, the recombinant purified S.pombe PCS protein displays phytochelatin synthase activity. These data demonstrate that PCS genes encode phytochelatin synthases and mediate metal detoxification in eukaryotes.

Characterisation of a novel gene family of putative cyclic nucleotide- and calmodulin-regulated ion channels in Arabidopsis thaliana

In plants, cyclic GMP is involved in signal transduction in response to light and gibberellic acid. For cyclic AMP, a potential role during the plant cell cycle was recently reported. However, cellular targets for cyclic nucleotides in plants are largely unknown. Here we report on the identification and characterisation of a new gene family in Arabidopsis, which share features with cyclic nucleotide-gated channels from animals and inward-rectifying K+ channels from plants. The identified gene family comprises six members (Arabidopsis thaliana cyclic nucleotide-gated channels, AtCNGC1-6) with significant homology among the deduced proteins. Hydrophobicity analysis predicted six membrane-spanning domains flanked by hydrophilic amino and carboxy termini. A putative cyclic nucleotide binding domain (CNBD) which contains several residues that are invariant in other CNBDs was located in the carboxy terminus. This domain overlaps with a predicted calmodulin (CaM) binding site, suggesting interaction between cyclic nucleotide and CaM regulation. We demonstrated interaction of the carboxy termini of AtCNGC1 and AtCNGC2 with CaM in yeast, indicating that the CaM binding sites are functional. Furthermore, it was shown that both AtCNGC1 and AtCNGC2 can partly complement the K(+)-uptake-deficient yeast mutant CY162. Therefore, we propose that the identified genes constitute a family of plant cyclic nucleotide- and CaM-regulated ion channels.

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

Ionic currents related to the major potassium uptake systems in Saccharomyces cerevisiae were examined by whole cell patch-clamping, under K+ replete conditions. Those currents have the following properties. They (1) are inward under all conditions investigated, (2) arise instantaneously with appropriate voltage steps, (3) depend solely upon the moderate affinity transporter Trk2p, not upon the high affinity transporter Trk1p. They (4) appear to be independent of the extracellular K+ concentration, (5) are also independent of extracellular Ca2+, Mg2+ and Cl- but (6) are strongly dependent on extracellular pH, being large at low pH (up to several hundred pA at -200 mV and pH 4) and near zero at high pH (above 7.5). They (7) increase in proportion to log[H+]o, rather than directly in proportion to the proton concentration and (8) behave kinetically as if each transporter cycle moved one proton plus one (high pH) or two (low pH) other ions, as yet unidentified. In view of background knowledge on K+ transport related to Trk2p, the new results suggest that the K+ status of yeast cells modulates both the kinetics of Trk2p-mediated transport and the identity of ions involved. That modulation could act either on the Trk2 protein itself or on interactions of Trk2 with other proteins in a hypothetical transporter complex. Structural considerations suggest a strong analogy to the KtrAB system in Vibrio alginolyticus and/or the TrkH system in Escherichia coli.

Channel-mediated high-affinity K+ uptake into guard cells from Arabidopsis

Potassium uptake by higher plants is the result of high- or low-affinity transport accomplished by different sets of transporters. Although K+ channels were thought to mediate low-affinity uptake only, the molecular mechanism of the high-affinity, proton-dependent K+ uptake system is still scant. Taking advantage of the high-current resolution of the patch-clamp technique when applied to the small Arabidopsis thaliana guard cells densely packed with voltage-dependent K+ channels, we could directly record channels working in the concentration range of high-affinity K+ uptake systems. Here we show that the K+ channel KAT1 expressed in Arabidopsis guard cells and yeast is capable of mediating potassium uptake from media containing as little as 10 microM of external K+. Upon reduction of the external K+ content to the micromolar level the voltage dependence of the channel remained unaffected, indicating that this channel type represents a voltage sensor rather than a K+-sensing valve. This behavior results in K+ release through K+ uptake channels whenever the Nernst potential is negative to the activation threshold of the channel. In contrast to the H+-coupled K+ symport shown to account for high-affinity K+ uptake in roots, pH-dependent K+ uptake into guard cells is a result of a shift in the voltage dependence of the K+ channel. We conclude that plant K+ channels activated by acid pH may play an essential role in K+ uptake even from dilute solutions.

Genetic selection of mutations in the high affinity K+ transporter HKT1 that define functions of a loop site for reduced Na+ permeability and increased Na+ tolerance

Potassium is an important macronutrient required for plant growth, whereas sodium (Na+) can be toxic at high concentrations. The wheat K+ uptake transporter HKT1 has been shown to function in yeast and oocytes as a high affinity K+-Na+ cotransporter, and as a low affinity Na+ transporter at high external Na+. A previous study showed that point mutations in HKT1, which confer enhancement of Na+ tolerance to yeast, can be isolated by genetic selection. Here we report on the isolation of mutations in new domains of HKT1 showing further large increases in Na+ tolerance. By selection in a Na+ ATPase deletion mutant of yeast that shows a high Na+ sensitivity, new HKT1 mutants at positions Gln-270 and Asn-365 were isolated. Several independent mutations were isolated at the Asn-365 site. N365S dramatically increased Na+ tolerance in yeast compared with all other HKT1 mutants. Cation uptake experiments in yeast and biophysical characterization in Xenopus oocytes showed that the mechanisms underlying the Na+ tolerance conferred by the N365S mutant were: reduced inhibition of high affinity Rb+ (K+) uptake at high Na+ concentrations, reduced low affinity Na+ uptake, and reduced Na+ to K+ content ratios in yeast. In addition, the N365S mutant could be clearly distinguished from less Na+-tolerant HKT1 mutants by a markedly decreased relative permeability for Na+ at high Na+ concentrations. The new mutations contribute to the identification of new functional domains and an amino acid in a loop domain that is involved in cation specificity of a plant high affinity K+ transporter and will be valuable for molecular analyses of Na+ transport mechanisms and stress in plants.

The Arabidopsis thaliana proton transporters, AtNhx1 and Avp1, can function in cation detoxification in yeast

Overexpression of the Arabidopsis thaliana vacuolar H+-pyrophosphatase (AVP1) confers salt tolerance to the salt-sensitive ena1 mutant of Saccharomyces cerevisiae. Suppression of salt sensitivity requires two ion transporters, the Gef1 Cl- channel and the Nhx1 Na+/H+ exchanger. These two proteins colocalize to the prevacuolar compartment of yeast and are thought to be required for optimal acidification of this compartment. Overexpression of AtNHX1, the plant homologue of the yeast Na+/H+ exchanger, suppresses some of the mutant phenotypes of the yeast nhx1 mutant. Moreover, the level of AtNHX1 mRNA in Arabidopsis is increased in the presence of NaCl. The regulation of AtNHX1 by NaCl and the ability of the plant gene to suppress the yeast nhx1 mutant suggest that the mechanism by which cations are detoxified in yeast and plants may be similar.

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.