Cation exchanges of yeast in the absence of magnesium

The H+-Translocating Inorganic Pyrophosphatase From Arabidopsis thaliana Is More Sensitive to Sodium Than Its Na+-Translocating Counterpart From Methanosarcina mazei

Overexpression of membrane-bound K+-dependent H+-translocating inorganic pyrophosphatases (H+-PPases) from higher plants has been widely used to alleviate the sensitivity toward NaCl in these organisms, a strategy that had been previously tested in Saccharomyces cerevisiae. On the other hand, H+-PPases have been reported to functionally complement the yeast cytosolic soluble pyrophosphatase (IPP1). Here, the efficiency of the K+-dependent Na+-PPase from the archaeon Methanosarcina mazei (MVP) to functionally complement IPP1 has been compared to that of its H+-pumping counterpart from Arabidopsis thaliana (AVP1). Both membrane-bound integral PPases (mPPases) supported yeast growth equally well under normal conditions, however, cells expressing MVP grew significantly better than those expressing AVP1 under salt stress. The subcellular distribution of the heterologously-expressed mPPases was crucial in order to observe the phenotypes associated with the complementation. In vitro studies showed that the PPase activity of MVP was less sensitive to Na+ than that of AVP1. Consistently, when yeast cells expressing MVP were grown in the presence of NaCl only a marginal increase in their internal PPi levels was observed with respect to control cells. By contrast, yeast cells that expressed AVP1 had significantly higher levels of this metabolite under the same conditions. The H+-pumping activity of AVP1 was also markedly inhibited by Na+. Our results suggest that mPPases primarily act by hydrolysing the PPi generated in the cytosol when expressed in yeast, and that AVP1 is more susceptible to Na+ inhibition than MVP both in vivo and in vitro. Based on this experimental evidence, we propose Na+-PPases as biotechnological tools to generate salt-tolerant plants.

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.

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.

The yeast plasma membrane protein Alr1 controls Mg2+ homeostasis and is subject to Mg2+-dependent control of its synthesis and degradation

The Saccharomyces cerevisiae ALR1 (YOL130w) gene product Alr1p is the first known candidate for a Mg(2+) transport system in eukaryotic cells and is distantly related to the bacterial CorA Mg(2+) transporter family. Here we provide the first experimental evidence for the location of Alr1p in the yeast plasma membrane and for the tight control of its expression and turnover by Mg(2+). Using well characterized npi1 and end3 mutants deficient in the endocytic pathway, we demonstrate that Alr1 protein turnover is dependent on ubiquitination and endocytosis. Furthermore, cells lacking the vacuolar protease Pep4p accumulated Alr1p in the vacuole. Mutants lacking Alr1p (Deltaalr1) showed a 60% reduction of total intracellular Mg(2+) compared with the wild type and failed to grow in standard media. When starved of Mg(2+), mutant and wild-type cells had similar low levels of intracellular Mg(2+); but upon addition of Mg(2+), wild-type cells replenished the intracellular Mg(2+) pool within a few hours, whereas Deltaalr1 mutant cells did not. Expression of the bacterial Mg(2+) transporter CorA in the yeast Deltaalr1 mutant partially restored growth in standard media. The results are discussed in terms of Alr1p being a plasma membrane transporter with high selectivity for Mg(2+).

Characterization of a glycerol/H+ symport in the halotolerant yeast Pichia sorbitophila

Pichia sorbitophila is a halotolerant yeast capable of surviving to extracellular NaCl concentrations up to 4 M in mineral medium when glucose or glycerol are the only carbon and energy sources. Evidence is presented here that glycerol, the main compatible solute this yeast accumulates so as to maintain osmotic balance, is actively co-transported with protons. This transport system was shown to be constitutive, not needing induction by either glycerol or salt, and was not repressible by glucose. In glucose- or glycerol-grown cells, a simple diffusion was detectable, and iterative calculations were performed to calculate kinetic parameters, in the presence and in the absence of NaCl. At 25 degrees C, pH 5.0, in glucose-grown cells these were: Km = 0.81 +/- 0.11 mM and Vmax = 634.2 +/- 164.8 mumol h-1 per g (glycerol); Km = 1.28 +/- 0.60 mM and Vmax = 558.6 +/- 100.6 mumol h-1 per g (protons). Correspondent stoichiometry was approximately 1, either for these conditions or in the presence of 1 M-NaCl. An increase in accumulation capacity was evident when different concentrations of NaCl were present. This capacity was shown to be dependent on delta pH and membrane potential, consistently with an electrogenic character. We suggest that the main role of this system is in osmoregulation, by keeping glycerol accumulated inside the cells, compensating for leakage, due to its liposoluble character.

Phenotypes of sphingolipid-dependent strains of Saccharomyces cerevisiae

To study sphingolipid function(s) in Saccharomyces cerevisiae, we have investigated the effects of environmental stress on mutant (SLC) strains (R. C. Dickson, G. B. Wells, A. Schmidt, and R. L. Lester, Mol. Cell. Biol. 10:2176-2181, 1990) that either contain or lack sphingolipids, depending on whether they are cultured with a sphingolipid long-chain base. Strains lacking sphingolipid were unable to grow at low pH, at 37 degrees C, or with high salt concentrations in the medium; these environmental stresses are known to inhibit the growth of some S. cerevisiae strains with a defective plasma membrane H(+)-ATPase. We found that sphingolipids were essential for proton extrusion at low pH and furthermore found that cells lacking sphingolipid no longer exhibited net proton extrusion at normal pH after a 1-min exposure to pH 3. Cells lacking sphingolipid appeared to rapidly become almost completely permeable to protons at low pH. The deleterious effects of low pH could be partially prevented by 1 M sorbitol in the suspension of cells lacking sphingolipid. Proton extrusion at normal pH (pH 6) was significantly inhibited at 39 degrees C only in cells lacking sphingolipid. Thus, the product of an SLC suppressor gene permits life without sphingolipids only in a limited range of environments. Outside this range, sphingolipids appear to be essential for maintaining proton permeability barriers and/or for proton extrusion.

Dual system for potassium transport in Saccharomyces cerevisiae

In a newly formulated growth medium lacking Na+ and NH4+, Saccharomyces cerevisiae grew maximally at 5 microM K+. Cells grown under these conditions transported K+ with an apparent Km of 24 microM, whereas cells grown in customary high-K+ medium had a significantly higher Km (2 mM K+). The two types of transport also differed in carbonyl cyanide-m-chlorophenyl hydrazone sensitivity, response to ATP depletion, and temperature dependence. The results can be accounted for either by two transport systems or by one system operating in two different ways.

Na+/H+ antiporters

Na+/H+ antiports or exchange reactions have been found widely, if not ubiquitously, in prokaryotic and eukaryotic membranes. In any given experimental system, the multiplicity of ion conductance pathways and the absence of specific inhibitors complicate efforts to establish that the antiport observed actually results from the activity of a specific secondary porter which catalyzes coupled exchanged of the two ions. Nevertheless, a large body of evidence suggests that at least some prokaryotes possess a delta psi-dependent, mutable Na+/H+ antiporter which catalyzes Na+ extrusion in exchange for H+; in other bacterial species, the antiporter my function electroneutrally, at least at some external pH values. The bacterial Na+/H+ antiporter constitutes a critical limb of Na+ circulation, functioning to maintain a delta mu Na+ for use by Na+-coupled bioenergetic processes. The prokaryotic antiporter is also involved in pH homeostasis in the alkaline pH range. Studies of mutant strains that are deficient in Na+/H+ antiporter activity also indicate the existence of a relationship, e.g., a common subunit or regulatory factor, between the Na+/H+ antiporter and Na+/solute symporters in several bacterial species. In eukaryotes, an electroneutral, amiloride-sensitive Na+/H+ antiport has been found in a wide variety of cell and tissue types. Generally, the normal direction of the antiport appears to be that of Na+ uptake and H+ extrusion. The activity is thus implicated as part of a complex system for Na+ circulation, e.g., in transepithelial transport, and might have some role in acidification in the renal proximal tubule. In many experimental systems, the Na+/H+ antiport appears to influence intracellular pH. In addition to a role in general pH homeostasis, such Na+-dependent changes in intracellular pH could be part of the early events in a variety of differentiating and proliferative systems. Reconstitution and structural studies, as well as detailed analysis of gene loci and products which affect the antiport activity, are in their very early stages. These studies will be important in further clarification of the precise structural nature and role(s) of the Na+/H+ antiporters. In neither prokaryotes nor eukaryotes systems is there yet incontrovertible evidence that a specific protein carrier, that catalyzes Na+/H+ antiport, is actually responsible for any of the multitude of effects attributed to such antiporters. The Na+-H+ exchange might turn out to be side reactions of other porters or the additive effects of several conductance pathways; or, as appears most likely in at least some bacteria and in renal tissue, the antiporter may be a discrete, complex carr

Ammonia accumulation in acetate-growing yeast

During growth on acetate, the pH of yeast cultures rises from 5.8 to around 7-8 in the stationary phase. This was found to result from acetic acid uptake and accompanying H+ loss. In addition, acetate-growing yeast were found to accumulate ammonia. The influence of pH on ammonia transport and accumulation was studied with the analogue [14C]methylamine with the following results. (a) Methylamine uptake kinetics from 0.1-50 mM were consistent with a single-component uptake system (NH+4 permease) at pH values more acidic than 6.5, and with a two-component system (NH+4 permease and NH3 diffusion) above pH 7.5. (b) Equilibrium accumulation of methylamine was found to increase with increasing pH. (c) Methylamine efflux from methylamine-loaded cells increased as the external pH decreased. It was concluded from measurements of the internal pH under various culture conditions that the accumulation of ammonia in acetate-growing alkaline cultures resulted from the sum of two processes: (1) an energy-driven NH+4 transport; and (2) NH3 diffusion dependent on the delta pH.

39K, 23Na, and 31P NMR studies of ion transport in Saccharomyces cerevisiae

The relationship between efflux and influx of K+, Na+, and intracellular pH (pHin) in yeast cells upon energizing by oxygenation was studied by using the noninvasive technique of 39K, 23Na, and 31P NMR spectroscopy. By introducing an anionic paramagnetic shift reagent, Dy3+(P3O5(-10))2, into the medium, NMR signals of intra- and extracellular K+ and Na+ could be resolved, enabling us to study ion transport processes by NMR. Measurements showed that 40% of the intracellular K+ and Na+ in yeast cells contributed to the NMR intensities. By applying this correction factor, the intracellular ion concentrations were determined to be 130-170 mM K+ and 2.5 mM Na+ for fresh yeast cells. With the aid of a home-built solenoidal coil probe for 39K and a double-tuned probe for 23Na and 31P, we could follow time courses of K+ and Na+ transport and of pHin with a time resolution of 1 min. It was shown that H+ extrusion is correlated with K+ uptake and not with Na+ uptake upon energizing yeast cells by oxygenation. When the cells were deenergized after the aerobic period, K+ efflux, H+ influx, and Na+ influx were calculated to be 1.6, 1.5, and 0.15 mumol/min per ml of cell water, respectively. Therefore, under the present conditions, K+ efflux is balanced by exchange for H+ with an approximate stoichiometry of 1:1.

Acidification power: indicator of metabolic activity and autolytic changes in Saccharomyces cerevisiae

Acidification power, defined as the sum of the spontaneous pH change determined after suspending yeast cells in water and the substrate-induced pH change after addition of glucose to the resulting suspension, reflects the level of cellular energy sources. Its use as an indicator of metabolic state of the cells was tested during a 120-h aerobic starvation. Its changes coincided with changes in cell viability, initial rate of endogenous oxygen consumption rate, cell ATP, extra- and intracellular buffering capacity, and the ability of cell-free extract to produce acidity by glucose fermentation. It was used as a sensitive marker of metabolic changes occurring during starvation, on treatment with glycolytic and respiratory inhibitors, and at elevated temperature.

The effect of monovalent cations on calcium efflux in yeasts

The properties of the calcium efflux system in the yeast Saccharomyces cerevisiae were investigated. After growing the cells overnight in medium containing 45Ca, the cells were transferred to medium containing glucose. Herpes buffer (pH 5.2) and monovalent cations. The presence of potassium or sodium in the medium induced efflux of calcium from the cells. The magnitude of the efflux was dependent on the concentration of these cations in the medium. The time course of calcium efflux was analyzed, and two types of exchangeable calcium pools, which turned over at different rates, were detected: 'Fast turnover' and 'slow turnover'. Increase in the concentration of monovalent cations in the medium caused an increase in the fraction of cellular calcium which turned over at a fast rate, and activation of calcium efflux from the 'slow turnover' calcium pool. The specific changes in the parameters of calcium efflux induced by monovalent cations were different from those reported previously to be induced by divalent cations. Both processes, i.e. activation of calcium efflux by monovalent and by divalent cations, were found to be additive, indicating that they operate via different mechanisms. Experiments using the respiratory inhibitor Antimycin A, showed that stimulation of calcium efflux by monovalent cations is energy dependent. Lanthanum ions which are known to inhibit calcium influx into yeast cells, inhibited the activation of calcium efflux by both divalent and monovalent cations. Determination of the cationic composition of the cells indicated that the stimulation of calcium efflux was accompanied by influx of potassium or sodium into the cells.

Energy source for lithium efflux in yeast

The efflux of Li+ in yeast was found to depend on the protonmotive force. The ATP content of the cell regulated the efflux that was also sensitive to the decrease in the cell pH. We propose an electrogenic H+/Li+ antiport as the mechanism for the efflux of Li+.