On the regulation of Na+/H+ and K+/H+ antiport in yeast mitochondria: evidence for the absence of an Na(+)-selective Na+/H+ antiporter

LETM1-Mediated K+ and Na+ Homeostasis Regulates Mitochondrial Ca2+ Efflux

Ca²⁺ transport across the inner membrane of mitochondria (IMM) is of major importance for their functions in bioenergetics, cell death and signaling. It is therefore tightly regulated. It has been recently proposed that LETM1—an IMM protein with a crucial role in mitochondrial K⁺/H⁺ exchange and volume homeostasis—also acts as a Ca²⁺/H⁺ exchanger. Here we show for the first time that lowering LETM1 gene expression by shRNA hampers mitochondrial K⁺/H⁺ and Na⁺/H⁺ exchange. Decreased exchange activity resulted in matrix K⁺ accumulation in these mitochondria. Furthermore, LETM1 depletion selectively decreased Na⁺/Ca²⁺ exchange mediated by NCLX, as observed in the presence of ruthenium red, a blocker of the Mitochondrial Ca²⁺ Uniporter (MCU). These data confirm a key role of LETM1 in monovalent cation homeostasis, and suggest that the effects of its modulation on mitochondrial transmembrane Ca²⁺ fluxes may reflect those on Na⁺/H⁺ exchange activity.

Mdm38 protein depletion causes loss of mitochondrial K+/H+ exchange activity, osmotic swelling and mitophagy

Loss of the MDM38 gene product in yeast mitochondria results in a variety of phenotypic effects including reduced content of respiratory chain complexes, altered mitochondrial morphology and loss of mitochondrial K(+)/H(+) exchange activity resulting in osmotic swelling. By use of doxycycline-regulated shut-off of MDM38 gene expression, we show here that loss of K(+)/H(+) exchange activity and mitochondrial swelling are early events, associated with a reduction in membrane potential and fragmentation of the mitochondrial reticulum. Changes in the pattern of mitochondrially encoded proteins are likely to be secondary to the loss of K(+)/H(+) exchange activity. The use of a novel fluorescent biosensor directed to the mitochondrial matrix revealed that the loss of K(+)/H(+) exchange activity was immediately followed by morphological changes of mitochondria and vacuoles, the close association of these organelles and finally uptake of mitochondrial material by vacuoles. Nigericin, a K(+)/H(+) ionophore, fully prevented these effects of Mdm38p depletion. We conclude that osmotic swelling of mitochondria triggers selective mitochondrial autophagy or mitophagy.

Release of Ca2+ and Mg2+ from yeast mitochondria is stimulated by increased ionic strength

Background

Divalent cations are required for many essential functions of mitochondrial metabolism. Yet the transporters that mediate the flux of these molecules into and out of the mitochondrion remain largely unknown. Previous studies in yeast have led to the molecular identification of a component of the major mitochondrial electrophoretic Mg²⁺ uptake system in this organism as well as a functional mammalian homolog. Other yeast mitochondrial studies have led to the characterization of an equilibrative fatty acid-stimulated Ca²⁺ transport activity. To gain a deeper understanding of the regulation of mitochondrial divalent cation levels we further characterized the efflux of Ca²⁺ and Mg²⁺ from yeast mitochondria.

Results

When isolated mitochondria from the yeast Saccharomyces cerevisiae were suspended in a salt-based suspension medium, Ca²⁺ and Mg²⁺ were released from the matrix space. Release did not spontaneously occur in a non-ionic mannitol media. When energized mitochondria were suspended in a mannitol medium in the presence of Ca²⁺ they were able to accumulate Ca²⁺ by the addition of the electrogenic Ca²⁺ ionophore ETH-129. However, in a KCl or choline Cl medium under the same conditions, they were unable to retain the Ca²⁺ that was taken up due to the activation of the Ca²⁺ efflux pathway, although a substantial membrane potential driving Ca²⁺ uptake was maintained. This Ca²⁺ efflux was independent of fatty acids, which have previously been shown to activate Ca²⁺ transport. Endogenous mitochondrial Mg²⁺ was also released when mitochondria were suspended in an ionic medium, but was retained in mitochondria upon fatty acid addition. When suspended in a mannitol medium, metal chelators released mitochondrial Mg²⁺, supporting the existence of an external divalent cation-binding site regulating release. Matrix space Mg²⁺ was also slowly released from mitochondria by the addition of Ca²⁺, respiratory substrates, increasing pH, or the nucleotides ATP, ADP, GTP, and ATP-gamma-S.

Conclusion

In isolated yeast mitochondria Ca²⁺ and Mg²⁺ release was activated by increased ionic strength. Free nucleotides, metal ion chelators, and increased pH also stimulated release. In yeast cells this release is likely an important mechanism in the regulation of mitochondrial matrix space divalent cation concentrations.

A screen for nigericin-resistant yeast mutants revealed genes controlling mitochondrial volume and mitochondrial cation homeostasis

Little is known about the regulation of ion transport across the inner mitochondrial membrane in Saccharomyces cerevisiae. To approach this problem, we devised a screening procedure for facilitating the identification of proteins involved in mitochondrial ion homeostasis. Taking advantage of the growth inhibition of yeast cells by electroneutral K(+)/H(+) ionophore nigericin, we screened for genetic mutations that would render cells tolerant to this drug when grown on a nonfermentable carbon source and identified several candidate genes including MDM31, MDM32, NDI1, YMR088C (VBA1), CSR2, RSA1, YLR024C, and YNL136W (EAF7). Direct examination of intact cells by electron microscopy indicated that mutants lacking MDM31 and/or MDM32 genes contain dramatically enlarged, spherical mitochondria and that these morphological abnormalities can be alleviated by nigericin. Mitochondria isolated from the Deltamdm31 and Deltamdm32 mutants exhibited limited swelling in an isotonic solution of potassium acetate even in the presence of an exogenous K(+)/H(+) antiport. In addition, growth of the mutants was inhibited on ethanol-containing media in the presence of high concentrations of salts (KCl, NaCl, or MgSO(4)) and their mitochondria exhibited two- (Deltamdm31 and Deltamdm32) to threefold (Deltamdm31Deltamdm32) elevation in magnesium content. Taken together, these data indicate that Mdm31p and Mdm32p control mitochondrial morphology through regulation of mitochondrial cation homeostasis and the maintenance of proper matrix osmolarity.

Electroneutral K+/H+ exchange in mitochondrial membrane vesicles involves Yol027/Letm1 proteins

YOL027c in yeast and LETM1 in humans encode integral proteins of the inner mitochondrial membrane. They have been implicated in mitochondrial K+ homeostasis and volume control. To further characterize their role, we made use of submitochondrial particles (SMPs) with entrapped K+- and H+-sensitive fluorescent dyes PBFI and BCECF, respectively, to study the kinetics of K+ and H+ transport across the yeast inner mitochondrial membrane. Wild-type SMPs exhibited rapid, reciprocal translocations of K+ and H+ driven by concentration gradients of either of them. K+ and H+ translocations have stoichiometries similar to those mediated by the exogenous K+/H+ exchanger nigericin, and they are shown to be essentially electroneutral and obligatorily coupled. Moreover, [K+] gradients move H+ against its concentration gradient, and vice-versa. These features, as well as the sensitivity of K+ and H+ fluxes to quinine and Mg2+, qualify these activities as K+/H+ exchange reactions. Both activities are abolished when the yeast Yol027p protein is absent (yol027Delta mutant SMPs), indicating that it has an essential role in this reaction. The replacement of the yeast Yol027p by the human Letm1 protein restores K+/H+ exchange activity confirming functional homology of the yeast and human proteins. Considering their newly identified function, we propose to refer to the yeast YOL027c gene and the human LETM1 gene as yMKH1 and hMKH1, respectively.

The LETM1/YOL027 gene family encodes a factor of the mitochondrial K+ homeostasis with a potential role in the Wolf-Hirschhorn syndrome

The yeast open reading frames YOL027 and YPR125 and their orthologs in various eukaryotes encode proteins with a single predicted trans-membrane domain ranging in molecular mass from 45 to 85 kDa. Hemizygous deletion of their human homolog LETM1 is likely to contribute to the Wolf-Hirschhorn syndrome phenotype. We show here that in yeast and human cells, these genes encode integral proteins of the inner mitochondrial membrane. Deletion of the yeast YOL027 gene (yol027Delta mutation) results in mitochondrial dysfunction. This mutant phenotype is complemented by the expression of the human LETM1 gene in yeast, indicating a functional conservation of LetM1/Yol027 proteins from yeast to man. Mutant yol027Delta mitochondria have increased cation contents, particularly K+ and low-membrane-potential Deltapsi. They are massively swollen in situ and refractory to potassium acetate-induced swelling in vitro, which is indicative of a defect in K+/H+ exchange activity. Thus, YOL027/LETM1 are the first genes shown to encode factors involved in both K+ homeostasis and organelle volume control.

In Saccharomyces cerevisiae, cations control the fate of the energy derived from oxidative metabolism through the opening and closing of the yeast mitochondrial unselective channel

The yeast mitochondrial unspecific channel (YMUC) sensitivity to inorganic (Ca2+ or Mg2+) or organic (hexyl or octyl-guanidine) cations was measured. The rate of oxygen consumption in State 3 and State 4, the transmembrane potential (deltapsi), mitochondrial swelling, and the polyethylene-glycol mediated recontraction were used to follow opening of the YMUC. Addition of 0.4 mM PO4 did not close the YMUC, although it did enhance the sensitivity to Ca2+ (I50 decreased from 50 to 0.3 mM) and Mg2+ (I50 decreased from 5 to 0.83 mM Mg2+). The Ca2+ concentration needed to close the YMUC was higher than the concentrations usually observed in the cell. Nonetheless, Mg2+, Ca2+, and PO4 exhibited additive effects. These cations did not inhibit contraction of preswollen mitochondria, suggesting that the YMUC/cation interaction was labile. Octyl-guanidine (OG-I50 7.5 microM) was the only cation which inhibited mitochondrial recontraction, probably as a result of membrane binding stabilization through its hydrophobic tail. The PO4-dependent, Ca(2+)/Mg(2+)-mediated closure of the YMUC may be a means to control the proportion of oxidative energy producing ATP or being lost as heat.

Mitochondrial potassium transport: the K(+) cycle

Potassium transport plays three distinct roles in mitochondria. Volume homeostasis to prevent excess matrix swelling is a housekeeping function that is essential for maintaining the structural integrity of the organelle. This function is mediated by the K(+)/H(+) antiporter and was first proposed by Peter Mitchell. Volume homeostasis to prevent excess matrix contraction is a recently discovered function that maintains a fully expanded matrix when diffusive K(+) influx declines due to membrane depolarization caused by high rates of electron transport. Maintaining matrix volume under these conditions is important because matrix contraction inhibits electron transport and also perturbs the structure-function of the intermembrane space (IMS). This volume regulation is mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). Cell signaling functions to protect the cell from ischemia-reperfusion injury and also to trigger transcription of genes required for cell growth. This function depends on the ability of mitoK(ATP) opening to trigger increased mitochondrial production of reactive oxygen species (ROS). This review discusses the properties of the mitochondrial K(+) cycle that help to understand the basis of these diverse effects.

Closure of the yeast mitochondria unspecific channel (YMUC) unmasks a Mg2+ and quinine sensitive K+ uptake pathway in Saccharomyces cerevisiae

The K+ uptake pathways in yeast mitochondria are still undefined. Nonetheless, the K+-mediated mitochondrial swelling observed in the absence of phosphate (PO4) and in the presence of a respiratory substrate has led to propose that large K+ movements occur in yeast mitochondria. Thus, the uptake of K+ by isolated yeast mitochondria was evaluated. Two parallel experiments were conducted to evaluate K+ transport; these were mitochondrial swelling and the uptake of the radioactive K+ analog 86Rb+. The opening of the yeast mitochondrial unspecific channel (YMUC) was regulated by different PO4 concentrations. The high protein concentrations used to measure 86Rb+ uptake resulted in a slight stabilization of the transmembrane potential at 0.4 mM PO4 but not at 0 or 4 mM PO4. At 4 mM PO4 swelling was inhibited while, in contrast, 86Rb+ uptake was still observed. The results suggest that an energy-dependent K+ uptake mechanism was unmasked when the YMUC was closed. To further analyze the properties of this K+ uptake system, the Mg2+ and quinine sensitivity of both swelling and 86Rb+ uptake were evaluated. Under the conditions where the unspecific pore was closed, K+ transport sensitivity to Mg2+ and quinine increased. In addition, when Zn2+ was added as an antiport inhibitor, uptake of 86Rb+ increased. It is suggested that in yeast mitochondria, the K+ concentration is highly regulated by the equilibrium of uptake and exit of this cation through two specific transporters.

Effects of SM-20550, a selective Na+-H+ exchange inhibitor, on the ion transport of myocardial mitochondria

The effect of a novel Na+-H+ exchange inhibitor, SM-20550 [N-(aminoiminomethyl)-1,4-dimethyl-1H-indole-2-carboxamide methanesulfonate] (SM) on the ion transport of myocardial mitochondria was studied using ion fluorometry and superfusion techniques. Isolated mitochondria from the guinea-pig heart were pre-loaded with fluoroprobes of either BCECF AM for H+, SBFI AM for Na+ or fura-2 AM for Ca2+. Initially, the treated mitochondria were superfused with a normal medium (MOPS-buffer, pH 7.4, 24 degrees C), subsequently fluorometric experiments on the Na+, H+, Ca2+ mobilization across the mitochondrial membrane were performed. The intramitochondrial pH (pHm) was increased by the superfusion of Na+ at physiological cytosolic concentrations of 10 mM, indicating the existence of a Na+-H+ exchange in mitochondrial membranes. The Na+ induced elevation of pH was dose-dependently inhibited by SM 1 microM (delta pHm; 45% as drug-free 100%), and 10 microM (delta pHm; 70%), as observed in our experiments with the myocardial sarcolemmal membrane. The selective Na+-H+ exchange inhibitor SM reduced such pHm elevations more markedly than that of EIPA [5-(N-ethyl-N-isopropyl) amiloride]. The Na+-H+ exchange inhibitors, SM and EIPA suppressed the intramitochondrial Ca2+ elevation ([Ca2+]m) brought on by external Ca2+ concentration changes: The pretreatment with SM 1 microM, 10 microM and EIPA 10 microM reduced the [Ca2+]m influx by 28.3, 56.5 and 63%, respectively. Additionally, the [Ca2+]m elevation induced by acidification of the perfusate was reduced by the prior infusion of SM and EIPA. Pretreatment of mitochondria with SM or EIPA which had beneficial effects on the left ventricular developed pressure (LVDP) in the ischemia-reperfusion injury of Langendorff hearts, reduced the intramitochondrial Na+ and pHm levels, indicating interplay of the inhibitory mechanism of Ca2+-uptake into mitochondria coupled with Na+-H+ exchange. These findings suggested that protective effects of Na+-H+ exchange inhibitors on reperfused myocardium are due in part to the Ca2+-paradox at the mitochondria level.

Expression of a mammalian Na+/H+ antiporter in Saccharomyces cerevisiae

The basolateral Na+/H+ antiporter (NHE) from LLC-PK1 cells was expressed in Saccharomyces cerevisiae. Two different strategies were tested for expression. In the first, we used a yeast strain that contains a temperature-sensitive mutation in the SEC-6 gene, whose product is required for the fusion of secretory vesicles with the plasma membrane. This strain was transformed with a vector containing the coding region of the NHE1 isoform under control of a heat shock (HS) promoter (pYNHE1-HS). In the second strategy, we replaced the heat shock promoter from pYNHE1-HS with a galactose (GAL) promoter (pYNHEI-GAL) and transformed wild-type yeast. In both cases, Northern blots demonstrated a transcript that hybridized against a probe containing the membrane region of the exchanger. When an antibody against the last 40 amino acids of the carboxy-terminus of NHE1 was used for immunoblots, a protein with a Mr of 73000 was seen in total membranes from both yeast transformants. Subcellular fractionation revealed that NHE1 was expressed in the endoplasmic reticulum. In the case of the pYNHEI-GAL transformant, the 100000 x g membrane pellet was reconstituted in phosphatidylcholine liposomes, and ethylisopropylamiloride-sensitive Na+/H+ exchange was observed. These results have paved the way for expression of the Na+/H+ exchanger in a genetically well-known microorganism.

Characterization of the yeast mitochondria unselective channel: a counterpart to the mammalian permeability transition pore?

Large and unselective permeabilities through the inner membrane of yeast mitochondria have been observed for more than 20 years, but the characterization of these permeabilities, leading to hypothesize the existence of a large-conductance unselective channel in yeast inner mitochondrial membrane, was done only recently by several groups. This channel has been tentatively identified as a yeast counterpart to the mammalian permeability transition pore, the crucial role of which is now well-documented in physiopathological phenomena, such as Ca2+ homeostasis, ischemic damages, or programmed cell death. The aim of this review is to make a point on the known characteristics of this yeast mitochondrial unselective channel (YMUC) and to analyze whether or not it can be considered as a "yeast permeability transition pore."

Novel localization of a Na+/H+ exchanger in a late endosomal compartment of yeast. Implications for vacuole biogenesis

Na+/H+ exchangers catalyze the electrically silent countertransport of Na+ and H+, controlling the transmembrane movement of salt, water, and acid-base equivalents, and are therefore critical for Na+ tolerance, cell volume control, and pH regulation. In contrast to numerous well studied plasma membrane isoforms (NHE1-4), much less is known about intracellular Na+/H+ exchangers, and thus far no vertebrate isoform has been shown to have an exclusively endosomal distribution. In this context, we show that the yeast NHE homologue, Nhx1 (Nass, R., Cunningham, K. W., and Rao, R. (1997) J. Biol. Chem. 272, 26145-26152), localizes uniquely to prevacuolar compartments, equivalent to late endosomes of animal cells. In living yeast, we show that these compartments closely abut the vacuolar membrane in a striking bipolar distribution, suggesting that vacuole biogenesis occurs at distinct sites. Nhx1 is the founding member of a newly emergent cluster of exchanger homologues, from yeasts, worms, and humans that may share a common intracellular localization. By compartmentalizing Na+, intracellular exchangers play an important role in halotolerance; furthermore, we hypothesize that salt and water movement into vesicles may regulate vesicle volume and pH and thus contribute to vacuole biogenesis.

Identification of a mitochondrial Na+/H+ exchanger

The electroneutral exchange of protons for Na+ and K+ across the mitochondrial inner membrane contributes to organellar volume and Ca2+ homeostasis. The molecular nature of these transporters remains unknown. In this report, we characterize a novel gene (YDR456w; renamed NHA2) in Saccharomyces cerevisiae whose deduced protein sequence is homologous to members of the mammalian Na+/H+ exchanger gene family. Fluorescence microscopy showed that a Nha2-green fluorescent protein chimera colocalizes with 4',6-diamidino-2-phenylindole staining of mitochondrial DNA. To assess the function of Nha2, we deleted the NHA2 gene by homologous disruption and found that benzamil-inhibitable, acid-activated 22Na+ uptake into mitochondria was abolished in the mutant strain. It also showed retarded growth on nonfermentable carbon sources and severely reduced survival during the stationary phase of the cell cycle compared with the parental strain, consistent with a defect in aerobic metabolism. Sequence comparisons revealed that Nha2 has highest identity to a putative Na+/H+ exchanger homologue (KIAA0267; renamed NHE6) in humans. Northern blot analysis demonstrated that NHE6 is ubiquitously expressed but is most abundant in mitochondrion-rich tissues such as brain, skeletal muscle, and heart. Fluorescence microscopy showed that a NHE6-green fluorescent protein chimera also accumulates in mitochondria of transfected HeLa cells. These data indicate that NHA2 and NHE6 encode homologous Na+/H+ exchangers and suggest they may be important for mitochondrial function in lower and higher eukaryotes, respectively.

Potassium collapses the deltaP in yeast mitochondria while the rate of ATP synthesis is inhibited only partially: modulation by phosphate

Addition of increasing concentrations of K+ to yeast mitochondria in the presence of 0 to 400 microM phosphate and 200 microM Mg2+ led to uncoupled respiration and decreased protonmotive force (deltaP):at 0 K+ deltaP = 213 mV, negative inside, where deltapsi = 180 mV and deltapH = 33 mV, while at 20 mM K+ deltaP = 28 mV, where deltapsi = 16 mV and deltapH = 12 mV. In contrast, the synthesis of ATP resulted in smaller values for the Km and the Vmax in 400 microM Pi and increasing ADP: in 0 K+, Km = 18.6 microM and Vmax = 75.4 nmol (min x mg protein)-1, while in 20 mM K+, Km = 5.2 microM and Vmax = 46.0 nmol (min x mg protein)-1, i.e., when K+ depleted most of the deltaP, and at ADP concentrations below the Km, the rate of ATP synthesis was essentially the same as in the absence of K+. At saturating ADP, the rate of ATP synthesis in the presence of K+ was about 60% of the rate observed without K+. The synthesis of ATP by yeast mitochondria was inhibited by oligomycin or uncouplers. K+ had no effects on rat liver mitochondria. Adenylate kinase activity was much smaller in yeast mitochondria than in rat liver mitochondria and thus did not account for the synthesis of ATP observed in the presence of K+. The effects of K+ on the deltaP of yeast mitochondria were prevented by increasing concentrations of phosphate (1 to 4 mM). At 4 mM phosphate, the deltaP was always above 200 mV and the kinetics of ATP synthesis were as follows: 0 K+ Km = 10.0 microM and Vmax = 88.3 nmol (min x mg protein)-1. At 20 mM K+, Km = 7.4 microM and Vmax = 133 nmol (min x mg protein)-1.

Conditions allowing different states of ATP- and GDP-induced permeability in mitochondria from different strains of Saccharomyces cerevisiae

The effect of ATP and other nucleotides on the respiration of Saccharomyces cerevisiae mitochondria was investigated. It was observed that ATP induced a stimulation of the respiration rate only in the presence of a salt in mitochondria from the baker's yeast Yeast Foam, whereas an ATP-induced stimulation occurred even in the absence of salt in mitochondria from three different laboratory strains. In both cases, the stimulation was related to a collapse of the transmembrane potential, suggesting the opening of ion- and/or proton-conducting pathways. Not only ATP, but also GTP and CTP, induced these pathways. Moreover, a similar stimulation was obtained with GDP and its analog GDP-beta-S. The fact that, as opposed to NTPs, GDP did not induce any non-specific anion channel, allowed us to use it to demonstrate unambiguously that a proton-conducting pathway was opened through the inner mitochondrial membrane of laboratory strains but not of Yeast Foam. Three additional aspects of this nucleotide-induced permeability were investigated. (i) The proton-conducting pathway was insensitive to Mg2+, whereas the anion-conducting pathway was fully inhibited by 4 mM Mg2-. (ii) The proton-conducting pathway of mitochondria isolated from laboratory strains was opened by the action of nucleotides outside the mitochondrion, since it was fully insensitive to (carboxy)atractyloside, and fully active in mitochondria isolated from op1 and delta anc strains. On the other hand, the cation-conducting pathway of Yeast Foam mitochondria was partly sensitive to (carboxy)atractyloside and insensitive to bongkrekic acid, suggesting a role of the conformational state of ANC in this activity. (iii) Both the proton and cation-conducting pathways were inhibited by very low concentrations of vanadate, under conditions where this oxyanion was polymerized to decavanadate: a competitor to nucleotide-binding sites on some enzymes.

H+/K+ exchange in reconstituted yeast plasma membrane vesicles

The activity of a putative H+/K+ exchange system in the plasma membrane of yeast was studied following the alkalinization of the interior of vesicles prepared with lecithin and yeast plasma membrane containing pyranine entrapped inside. The fluorescence of pyranine was used as an indicator of the internal pH of the vesicles. The addition of monovalent cations produced an increase of the internal pH, probably due to the activity of an exchange system, allowing H+ to leave the vesicle in an exchange for the cation added. The system showed partial selectivity towards K+ against other monovalent cations, and it was inhibited by amiloride. The activity of this system required the presence of the yeast plasma membrane in the vesicles, and it did not produce important changes of the membrane potential of the vesicles. The exchange depended partially on the relative values of the internal and the external pH of the vesicles. The system shows low affinity for the cations, and appears to be different from the mitochondrial H+/K+ exchange system, which is non-selective toward the different monovalent cations. This system could be involved in the regulation of the internal pH of the cells when they accumulate high concentrations of K+.

Activation of the uncoupling protein by fatty acids is modulated by mutations in the C-terminal region of the protein

The transport properties of the uncoupling protein (UCP) from brown adipose tissue have been studied in mutants where Cys304 has been replaced by either Gly, Ala, Ser, Thr, Ile or Trp. This position is only two residues away from the C-terminus of the protein, a region that faces the cytosolic side of the mitochondrial inner membrane. Mutant proteins have been expressed in Saccharomyces cerevisiae and their activity determined in situ by comparing yeast growth rates in the presence and absence of 2-bromopalmitate. Their bioenergetic properties have been studied in isolated mitochondria by determining the effects of fatty acids and nucleotides on the proton permeability and NADH oxidation rate. It is revealed that substitution of Cys304 by non-charged residues alters the response of UCP to fatty acids. The most effective substitution is Cys for Gly since it greatly enhances the sensitivity to palmitate, decreasing threefold the concentration required for half-maximal stimulation of respiration. The opposite extreme is the substitution by Ala which increases twofold the half-maximal concentration. We conclude that the C-terminal region participates in the fatty acid regulation of UCP activity. The observed correlation between yeast growth rates in the presence of bromopalmitate and the calculated activation constants for respiration in isolated mitochondria validates growth analysis as a method to screen the in situ activity of UCP mutants.

Stimulation of oxidative phosphorylation by electrophoretic K+ entry associated to electroneutral K+/H+ exchange in yeast mitochondria

The effect of the addition of KCl, at constant osmolarity, was investigated on oxidative phosphorylation in isolated yeast mitochondria. KCl stimulated both respiration and ATP synthesis rates without changing the ATP/O ratio. KCl did not change the relationships between respiration rates and the protonmotive force. Since the K+/H+ exchange activity was active under these conditions, the stimulatory effect of respiration could be explained by the net proton entry caused by the electrophoretic K+ entry/electroneutral K+/H+ exchange cycle. On the other hand, K+ entry stimulated phosphate accumulation and transport under non-phosphorylating conditions and decreased the kinetic control by phosphate transport under phosphorylating conditions. Additionally, the stimulation of ATP synthesis strongly depended on the activity of phosphate transport. Taken together, these data showed that electrophoretic K(+)-entry and electroneutral K+/H+ exchange occurred in phosphorylating yeast mitochondria but did not promote any uncoupling between respiration and ATP synthesis.

Investigations of the inhibitory effect of propranolol, chlorpromazine, quinine, and dicyclohexylcarbodiimide on the swelling of yeast mitochondria in potassium acetate. Evidences for indirect effects mediated by the lipid phase

The mode of action of propranolol, chlorpromazine, and quinine, three cationic drugs inhibiting swelling of yeast mitochondria in potassium acetate, was investigated by looking at their effect on fluorescent probes of the polar heads and of the nonpolar moiety of the membranes, under inhibitory conditions of swelling. As expected, propranolol and chlorpromazine exhibited specificity for anionic phospholipids since they increased the binding of the anionic probe 1-anilino 8-naphthalenesulfonate (ANS). Although propranolol did not release 1,6-diphenyl-1,3,5-hexatriene (DPH) from the hydrophobic moiety of the membrane, it increased the excimer/monomer fluorescence ratio of 10-(1-pyrene)decanoate, suggesting that it induced a limitation in the movements of the aliphatic chains of phospholipids. Opposite to propranolol, chlorpromazine removed DPH from the membrane, suggesting that it bound essentially to the hydrophobic moiety. However, chloramphenicol, which was also able to remove DPH but did not increase the binding of ANS, did not inhibit swelling. Inhibition by chlorpromazine therefore appeared to be related to its binding to the hydrophobic moiety of anionic phospholipids. Quinine had no effect on membrane properties: at inhibitory concentrations of swelling in potassium acetate, it did not inhibit swelling in ammonium phosphate (mediated by the phosphate/H+ cotransporter), whereas propranolol and chlorpromazine did, suggesting a more specific effect of quinine on (a) protein(s) involved in the K+/H+ exchange. Dicyclohexylcarbodiimide (DCCD), which irreversibly inhibits the swelling in potassium acetate, bound to ethanolamine heads; despite this effect, DCCD had no major consequences on the binding of the probes. Consequently, propranolol and chlorpromazine are of no help for characterizing protein(s) catalyzing the K+/H+ exchange, although their effect on lipids seems to involve limited zones of the inner mitochondrial membrane. Quinine and DCCD, although they also bind to lipids, may inhibit the activity by acting on a limited number of proteins.

Cation transport systems in mitochondria: Na+ and K+ uniports and exchangers

It is now well established that mitochondria contain three antiporters that transport monovalent cations. A latent, allosterically regulated K+/H+ antiport appears to serve as a cation-extruding device that helps maintain mitochondrial volume homeostasis. An apparently unregulated Na+/H+ antiport keeps matrix [Na+] low and the Na(+)-gradient equal to the H(+)-gradient. A Na+/Ca2+ antiport provides a Ca(2+)-extruding mechanism that permits the mitochondrion to regulate matrix [Ca2+] by balancing Ca2+ efflux against influx on the Ca(2+)-uniport. All three antiports have well-defined physiological roles and their molecular properties and regulatory features are now being determined. Mitochondria also contain monovalent cation uniports, such as the recently described ATP- and glibenclamide-sensitive K+ channel and ruthenium red-sensitive uniports for Na+ and K+. A physiological role of such uniports has not been established and their properties are just beginning to be defined.