Sodium-dependent methyl 1-thio-beta-D-galactopyranoside transport in membrane vesicles isolated from Salmonella typhimurium

It's Better To Be Lucky Than Smart

Bacterial cytoplasmic membrane vesicles provide a unique experimental system for studying active transport. Vesicles are prepared by lysis of osmotically sensitized cells (i.e., protoplasts or spheroplasts) and comprise osmotically intact, unit-membrane-bound sacs that are approximately 0.5-1.0 μm in diameter and devoid of internal structure. Their metabolic activities are restricted to those provided by the enzymes of the membrane itself, and each vesicle is functional. The energy source for accumulation of a particular substrate can be determined by studying which compounds or experimental conditions drive solute accumulation, and metabolic conversion of the transported substrate or the energy source is minimal. These properties of the vesicle system constitute a considerable advantage over intact cells, as the system provides clear definition of the reactions involved in the transport process. This discussion is not intended as a general review but is concerned with respiration-dependent active transport in membrane vesicles from Escherichia coli. Emphasis is placed on experimental observations demonstrating that respiratory energy is converted primarily into work in the form of a solute concentration gradient that is driven by a proton electrochemical gradient, as postulated by the chemiosmotic theory of Peter Mitchell.

Thermodynamic mechanism for inhibition of lactose permease by the phosphotransferase protein IIAGlc

In a variety of bacteria, the phosphotransferase protein IIA(Glc) plays a key regulatory role in catabolite repression in addition to its role in the vectorial phosphorylation of glucose catalyzed by the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). The lactose permease (LacY) of Escherichia coli catalyzes stoichiometric symport of a galactoside with an H(+), using a mechanism in which sugar- and H(+)-binding sites become alternatively accessible to either side of the membrane. Both the expression (via regulation of cAMP levels) and the activity of LacY are subject to regulation by IIA(Glc) (inducer exclusion). Here we report the thermodynamic features of the IIA(Glc)-LacY interaction as measured by isothermal titration calorimetry (ITC). The studies show that IIA(Glc) binds to LacY with a Kd of about 5 μM and a stoichiometry of unity and that binding is driven by solvation entropy and opposed by enthalpy. Upon IIA(Glc) binding, the conformational entropy of LacY is restrained, which leads to a significant decrease in sugar affinity. By suppressing conformational dynamics, IIA(Glc) blocks inducer entry into cells and favors constitutive glucose uptake and utilization. Furthermore, the studies support the notion that sugar binding involves an induced-fit mechanism that is inhibited by IIA(Glc) binding. The precise mechanism of the inhibition of LacY by IIA(Glc) elucidated by ITC differs from the inhibition of melibiose permease (MelB), supporting the idea that permeases can differ in their thermodynamic response to binding IIA(Glc).

Insights into the inhibitory mechanisms of the regulatory protein IIA(Glc) on melibiose permease activity

The phosphotransfer protein IIA(Glc) of the bacterial phosphoenolpyruvate:carbohydrate phosphotransferase system plays a key role in the regulation of carbohydrate metabolism. Melibiose permease (MelB) is one among several permeases subject to IIA(Glc) regulation. The regulatory mechanisms are poorly understood; in addition, thermodynamic features of IIA(Glc) binding to other proteins are also unknown. Applying isothermal titration calorimetry and amine-specific cross-linking, we show that IIA(Glc) directly binds to MelB of Salmonella typhimurium (MelB(St)) and Escherichia coli MelB (MelB(Ec)) at a stoichiometry of unity in the absence or presence of melibiose. The dissociation constant values are 3-10 μM for MelB(St) and 25 μM for MelB(Ec). All of the binding is solely driven by favorable enthalpy forces. IIA(Glc) binding to MelB(St) in the absence or presence of melibiose yields a large negative heat capacity change; in addition, the conformational entropy is constrained upon the binding. We further found that the IIA(Glc)-bound MelB(St) exhibits a decreased binding affinity for melibiose or nitrophenyl-α-galactoside. It is believed that sugar binding to the permease is involved in an induced fit mechanism, and the transport process requires conformational cycling between different states. Thus, the thermodynamic data are consistent with the interpretation that IIA(Glc) inhibits the induced fit process and restricts the conformational dynamics of MelB(St).

Suppression of conformation-compromised mutants of Salmonella enterica serovar Typhimurium MelB

The crystal structure of the Na(+)-coupled melibiose permease of Salmonella enterica serovar Typhimurium (MelBSt) demonstrates that MelB is a member of the major facilitator superfamily of transporters. Arg residues at positions 295, 141, and 363 are involved in interdomain interactions at the cytoplasmic side by governing three clusters of electrostatic/polar interactions. Insertion of (one at a time) Glu, Leu, Gln, or Cys at positions R295, R141, and R363, or Lys at position R295, inhibits active transport of melibiose to a level of 2 to 20% of the value for wild-type (WT) MelBSt, with little effect on binding affinities for both sugar and Na(+). Interestingly, a spontaneous suppressor, D35E (periplasmic end of helix I), was isolated from the R363Q MelBSt mutant. Introduction of the D35E mutation in each of the mutants at R295, R141 (except R141E), or R363 rescues melibiose transport to up to 91% of the WT value. Single-site mutations for the pair of D35 and R175 (periplasmic end of helix VI) were constructed by replacing Asp with Glu, Gln, or Cys and R175 with Gln, Asn, or Cys. All mutants with mutations at R175 are active, indicating that a positive charge at R175 is not necessary. Mutant D35E shows reduced transport; D35Q and D35C are nearly inactivated. Surprisingly, the D35Q mutation partially rescues both R141C and R295Q mutations. The data support the idea that Arg at position 295 and a positive charge at positions 141 and 363 are required for melibiose transport catalyzed by MelBSt, and their mutation inhibits conformational cycling, which is suppressed by a minor modification at the opposite side of the membrane.

Reduced Na+ affinity increases turnover of Salmonella enterica serovar Typhimurium MelB

The melibiose permease of Salmonella enterica serovar Typhimurium (MelB(St)) catalyzes symport of melibiose with Na(+), Li(+), or H(+). Bioinformatics and mutational analyses indicate that a conserved Gly117 (helix IV) is a component of the Na(+)-binding site. In this study, Gly117 was mutated to Ser, Asn, or Cys. All three mutations increase the maximum rate (V(max)) for melibiose transport in Escherichia coli DW2 and greatly decrease Na(+) affinity, indicating that intracellular release of Na(+) is facilitated. Rapid melibiose transport, particularly by the G117N mutant, triggers osmotic lysis in the lag phase of growth. The findings support the previous conclusion that Gly117 plays an important role in cation binding and translocation. Furthermore, a spontaneous second-site mutation (P148L between loop(4-5) and helix V) in the G117C mutant prevents cell lysis. This mutation significantly decreases V(max) with little effect on cosubstrate binding in G117C, G117S, and G117N mutants. Thus, the P148L mutation specifically inhibits transport velocity and thereby blocks the lethal effect of elevated melibiose transport in the Gly117 mutants.

Role of Gly117 in the cation/melibiose symport of MelB of Salmonella typhimurium

The melibiose permease of Salmonella typhimurium (MelB(St)) catalyzes symport of melibiose with Na(+), Li(+), or H(+), and bioinformatics analysis indicates that a conserved Gly117 (helix IV) is part of the Na(+)-binding site. We mutated Gly117 to Ala, Pro, Trp, or Arg; the effects on melibiose transport and binding of cosubstrates depended on the physical-chemical properties of the side chain. Compared with WT MelB(St), the Gly117 → Ala mutant exhibited little difference in either cosubstrate binding or stimulation of melibiose transport by Na(+) or Li(+), but all other mutations reduced melibiose active transport and efflux, and decreased the apparent affinity for Na(+). The bulky Trp at position 117 caused the greatest inhibition of melibiose binding, and Gly117 → Arg yielded less than a 4-fold decrease in the apparent affinity for melibiose at saturating Na(+) or Li(+) concentration. Remarkably, the mutant Gly117 → Arg catalyzed melibiose exchange in the presence of Na(+) or Li(+), but did not catalyze melibiose translocation involving net flux of the coupling cation, indicating that sugar is released prior to release of the coupling cation. Taken together, the findings are consistent with the notion that Gly117 plays an important role in cation binding and translocation.

Mechanism of melibiose/cation symport of the melibiose permease of Salmonella typhimurium

The MelB permease of Salmonella typhimurium (MelB-ST) catalyzes the coupled symport of melibiose and Na(+), Li(+), or H(+). In right-side-out membrane vesicles, melibiose efflux is inhibited by an inwardly directed gradient of Na(+) or Li(+) and stimulated by equimolar concentrations of internal and external Na(+) or Li(+). Melibiose exchange is faster than efflux in the presence of H(+) or Na(+) and stimulated by an inwardly directed Na(+) gradient. Thus, sugar is released from MelB-ST externally prior to the release of cation in agreement with current models proposed for MelB of Escherichia coli (MelB-EC) and LacY. Although Li(+) stimulates efflux, and an outwardly directed Li(+) gradient increases exchange, it is striking that internal and external Li(+) with no gradient inhibits exchange. Furthermore, Trp → dansyl FRET measurements with a fluorescent sugar (2'-(N-dansyl)aminoalkyl-1-thio-β-D-galactopyranoside) demonstrate that MelB-ST, in the presence of Na(+) or Li(+), exhibits (app)K(d) values of ∼1 mM for melibiose. Na(+) and Li(+) compete for a common binding pocket with activation constants for FRET of ∼1 mM, whereas Rb(+) or Cs(+) exhibits little or no effect. Taken together, the findings indicate that MelB-ST utilizes H(+) in addition to Na(+) and Li(+). FRET studies also show symmetrical emission maximum at ∼500 nm with MelB-ST in the presence of 2'-(N-dansyl)aminoalkyl-1-thio-β-D-galactopyranoside and Na(+), Li(+), or H(+), which implies a relatively homogeneous distribution of conformers of MelB-ST ternary complexes in the membrane.

A 3D structure model of the melibiose permease of Escherichia coli represents a distinctive fold for Na+ symporters

The melibiose permease of Escherichia coli (MelB) catalyzes the coupled stoichiometric symport of a galactoside with a cation (either Na(+), Li(+), or H(+)), using free energy from the downhill translocation of one cosubstrate to catalyze the accumulation of the other. Here, we present a 3D structure model of MelB threaded through a crystal structure of the lactose permease of E. coli (LacY), manually adjusted, and energetically minimized. The model contains 442 consecutive residues ( approximately 94% of the polypeptide), including all 12 transmembrane helices and connecting loops, with no steric clashes and superimposes well with the template structure. The electrostatic surface potential calculated from the model is typical for a membrane protein and exhibits a characteristic ring of positive charges around the periphery of the cytoplasmic side. The 3D model indicates that MelB consists of two pseudosymmetrical 6-helix bundles lining an internal hydrophilic cavity, which faces the cytoplasmic side of the membrane. Both sugar and cation binding sites are proposed to lie within the internal cavity. The model is consistent with numerous previous mutational, biochemical/biophysical characterizations as well as low-resolution structural data. Thus, an alternating access mechanism with sequential binding is discussed. The proposed overall fold of MelB is different from the available crystal structures of other Na(+)-coupled transporters, suggesting a distinctive fold for Na(+) symporters.

Hexose/Pentose and Hexitol/Pentitol Metabolism

Escherichia coli and Salmonella enterica serovar Typhimurium exhibit a remarkable versatility in the usage of different sugars as the sole source of carbon and energy, reflecting their ability to make use of the digested meals of mammalia and of the ample offerings in the wild. Degradation of sugars starts with their energy-dependent uptake through the cytoplasmic membrane and is carried on further by specific enzymes in the cytoplasm, destined finally for degradation in central metabolic pathways. As variant as the different sugars are, the biochemical strategies to act on them are few. They include phosphorylation, keto-enol isomerization, oxido/reductions, and aldol cleavage. The catabolic repertoire for using carbohydrate sources is largely the same in E. coli and in serovar Typhimurium. Nonetheless, significant differences are found, even among the strains and substrains of each species. We have grouped the sugars to be discussed according to their first step in metabolism, which is their active transport, and follow their path to glycolysis, catalyzed by the sugar-specific enzymes. We will first discuss the phosphotransferase system (PTS) sugars, then the sugars transported by ATP-binding cassette (ABC) transporters, followed by those that are taken up via proton motive force (PMF)-dependent transporters. We have focused on the catabolism and pathway regulation of hexose and pentose monosaccharides as well as the corresponding sugar alcohols but have also included disaccharides and simple glycosides while excluding polysaccharide catabolism, except for maltodextrins.

Sodium-substrate cotransport in bacteria

A variety of sodium-substrate cotransport systems are known in bacteria. Sodium enters the cell down an electrochemical concentration gradient. There is obligatory coupling between the entry of the ion and the entry of substrate with a stoichiometry (in the cases studied) of 1:1. Thus, the downhill movement of sodium ion into the cell leads to the accumulation of substrate within the cell. The melibiose carrier of Escherichia coli is perhaps the most carefully studied of the sodium cotransport systems in bacteria. This carrier is of special interest because it can also use protons or lithium ions for cotransport. Other sodium cotransport carriers that have been studied recently are for proline, glutamate, serine-threonine, citrate and branched chain amino acids.

Lysine excretion by Corynebacterium glutamicum. 2. Energetics and mechanism of the transport system

Lysine excretion in Corynebacterium glutamicum was characterized as secondary transport process. It is modulated by three forces: the membrane potential, the chemical potential of lysine, and the proton gradient. The ATP content of the cells did not correlate with the export activity. Lysine is excreted in symport with presumably two OH- ions which is not distinguishable experimentally from an antiport mechanism against two protons. The substrate-loaded carrier is uncharged. When the external substrate concentration is low and no proton gradient present, reorientation of the positively charged, unloaded carrier is rate-limiting. Export then depends on the membrane potential. When the external substrate is high, translocation of the loaded, uncharged carrier is rate-limiting, and export is not modulated by the membrane potential. The lysine secretion system in C. glutamicum is shown to be well adapted to the requirements of metabolite export.

Respiratory Na+ pump and Na+-dependent energetics in Vibrio alginolyticus

The marine bacterium Vibrio alginolyticus was found to possess the respiratory Na+ pump that generates an electrochemical potential of Na+, which plays a central role in bioenergetics of V. alginolyticus, as a direct result of respiration. Mutants defective in the Na+ pump revealed that one of the two kinds of NADH: quinone oxidoreductase requires Na+ for activity and functions as the Na+ pump. The Na+ pump composed of three subunits was purified and reconstituted into liposomes. Generation of membrane potential by the reconstituted proteoliposomes required Na+. The respiratory Na+ pump coupled to the NADH: quinone oxidoreductase was found in wide varieties of Gram-negative marine bacteria belonging to the genera Alcaligenes, Alteromonas, and Vibrio, and showed a striking similarity in the mode of electron transfer and enzymic properties. Na+ extrusion seemed to be coupled to a dismutation reaction, which leads to the formation of quinol and quinone from semiquinone radical.

Na+(Li+)/branched-chain amino acid cotransport in Pseudomonas aeruginosa

A transport system for branched-chain amino acids (designated as LIV-II system) in Pseudomonas aeruginosa requires Na+ for its operation. Coupling cation for this system was identified by measuring cation movement during substrate entry using cation-selective electrodes. Uptakes of Na+ and Li+ were induced by the imposition of an inwardly-directed concentration gradient of leucine, isoleucine, or valine. No uptake of H+ was found, however, under the same conditions. In addition, effects of Na+ and Li+ on the kinetic property of the system were examined. At chloride salt concentration of 2.5 mM, values of apparent Km and Vmax for leucine uptake were larger in the presence of Na+ than Li+. These results indicate that the LIV-II transport system is a Na+(Li+)/substrate cotransport system, although effects of Na+ and Li+ on kinetics of the system are different.

The transmembrane electrochemical gradient of Na+ as driving force for methanol oxidation in Methanosarcina barkeri

A sodium ion gradient (inside low) across the cytoplasmic membrane of Methanosarcina barkeri was required for methanogenesis from methanol. This could be concluded from the following results. (a) Inhibition of the Na+/H+ antiporter by K+ or amiloride led to an inhibition of methanogenesis from methanol. (b) Upon addition of the sodium ionophore monensin the Na+ gradient was abolished and at the same time methanogenesis from methanol was inhibited. (c) Methanogenesis was impaired when the Na+ gradient had the opposite orientation (inside high). All these inhibitory effects were not observed when H2 was present in addition to methanol indicating that the oxidation of methanol to CO2 was driven by a sodium-motive force. In accordance with this, a methanol-dependent influx of Na+ and a corresponding decrease of the membrane potential could be observed, when the Na+/H+ antiporter was inhibited by amiloride. This influx was indicative of the presence of a Na+ transport system which was functional when the oxidation of methanol had to be driven, but was not functional when H2 was present for reduction of methanol to methane.

Cation specificity for sugar substrates of the melibiose carrier in Escherichia coli

A study has been made of the sugar substrate specificities and the cation specificities of the melibiose transport system of Escherichia coli. The following beta-galactosides were found to be transported: lactose, L-arabinose-beta-D-galactoside, D-fructose-beta-D-galactoside, o- and p-nitrophenyl-beta-D-galactosides. These beta-galactosides were cotransported with Na+ but not with H+. The alpha-galactosides raffinose, melibiose and p-nitrophenyl-alpha-galactoside were transported with either H+ or Na+. Of the monosaccharides tested D-galactose could use either Na+ or H+ for cotransport whereas D-fucose, L-arabinose and D-galactosamine could use only Na+. The sugar specificity requirements for H+ cotransport are therefore more exacting than those for Na+ cotransport.

Sodium ion transport decarboxylases and other aspects of sodium ion cycling in bacteria

Images

Biochemical and morphological properties of membranes of unsaturated fatty acid auxotrophs of Salmonella typhimurium: effects of fluorinated myristic acids

In order to investigate the utility of the fluorine-19 nucleus as a spectroscopic probe, a fluorinated analog of myristic acid has been incorporated into the membrane lipids of an unsaturated fatty acid auxotroph of Salmonella typhimurium. It is capable of supporting limited growth at temperatures above 37 degrees C. Freeze-fracture electron microscopic examinations of the membrane ultrastructure show a temperature and fatty acid supplement-dependent segregation of intramembranous protein particles into distinct patches in the auxotrophic membrane leaving intramembranous protein-denuded areas. The occurrence of these patches seems to be related to the phase separation of membrane lipids. Corresponding changes in the transport and accumulation of methyl thio-beta-D-galactopyranoside and tetracycline are observed. However, transport of histidine does not appear to be dependent on the physical state of the membrane lipids. The auxotroph shows differences in growth and morphological characteristics from those of the wild type. Functions of both inner and outer membranes are shown to be affected as a response to the fatty acid chain composition of the lipids.

Proline uptake through the major transport system of Salmonella typhimurium is coupled to sodium ions

Strains of Salmonella typhimurium deficient in one or more of the proline transport systems have been constructed and used to study the mechanism of energy coupling to transport. Proline uptake through the major proline permease (PP-I, putP) is shown to be absolutely coupled to Na+ ions and not to H+ ions as has previously been assumed. Transport through the minor proline permease (PP-II, proP), however, is unaffected by the presence or absence of Na+. The effect of Na+ on the kinetics of proline uptake shows that external Na+ increases the Vmax for transport. It seems probable that proline transport through PP-I is also coupled to Na+ ions in Escherichia coli.

Amiloride, a diuretic with in vitro antimicrobial activity

The effect of amiloride, an inhibitor of passive sodium influx in animal cells, was investigated on the in vitro bacterial growth. Amiloride blocked the growth of different bacterial strains at concentrations ranging from 25 to 1,300 micrograms/ml. While generally the block was bacteriostatic and bacteria, on amiloride removal, recovered their ability to growth, the drug showed a killing activity on hemolytic streptococci. Gram-positive bacteria revealed a greater susceptibility to amiloride than gram-negative ones. Although an hitherto unknown effect of amiloride cannot be excluded, from the known mechanism of action of amiloride on animal cells it might be suggested that sodium permeability plays a critical role on bacterial multiplication.

Voltage-dependent proton fluxes in liposomes

Liposomes containing buffered KCl were prepared from bacterial lipids, were diluted into K+-free media and were treated with valinomycin to induce the formation of a diffusion potential (delta psi). Upon formation of such a potential, substantial proton influx was observed, as assayed by the quenching of 9-aminoacridine fluorescence. Complete reversal of fluorescence quenching occurred when the potential was collapsed by addition of KCl or when methylamine was added. Studies of proton influx as a function of the theoretical magnitude of the delta psi indicated that the phenomenon occurred only above a delta psi of about -60 mV. Establishment of a Na+ diffusion potential also resulted in proton influx. Treatment of K+-loaded liposomes with N,N'-dicyclohexylcarbodiimide did not reduce the delta psi-dependent proton influx. Moreover, proton influx could be demonstrated upon imposition of a diffusion potential in liposomes prepared from a synthetic lipid. The proton fluxes associated with generation of a diffusion potential in liposomes may complicate studies of reconstituted systems in which proton translocation should occur, and may affect the magnitude of the electrochemical proton gradient that is operant under some conditions.

Na+ and K+ transport in Nitrosomonas europaea and Nitrobacter agilis

Potassium-depleted cells of Nitrosomonas europaea and Nitrobacter agilis were prepared by diethanolamine treatment and contained less than 5 mM intracellular K+. The addition of K+ to K+-depleted cells of N. europaea and N. agilis resulted in a depolarization of membrane potential (delta psi) by about 5 and 10 mV, respectively. This depolarization was, however, compensated by an equivalent increase in transmembrane pH gradient (delta pH), so that the total proton-motive force (delta p) remained constant, indicating that K+ transport was electrogenic in both bacteria. Using 22Na+-loaded cells, it is shown that both bacteria lack a respiration-dependent Na+ pump; however, antiporters for Na+/H+, K+/Na+ and K+/H+ were detected. Of these, at least the K+/Na+ antiporter required an electrochemical gradient for its operation. It is also shown that the unprotonated form of NH+4 is transported into these bacteria by a simple diffusion mechanism.

Energetics of sodium efflux from Escherichia coli

When energy-starved cells of Escherichia coli were passively loaded with 22Na+, efflux of sodium could be initiated by addition of a source of metabolic energy. Conditions were established where the source of energy was phosphate bond energy, an electrochemical proton gradient, or both. Only an electrochemical proton gradient was required for efflux from intact cells. These results are consistent with secondary exchange of Na+ for H+ catalyzed by a sodium/proton antiporter.

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

Characterization of the Na+/H+ antiporter of alkalophilic bacilli in vivo: delta psi-dependent 22Na+ efflux from whole cells

The Na+/H+ antiporter of Bacillus alcalophilus was studied by measuring 22Na+ efflux from starved, cyanide-inhibited cells which were energized by means of a valinomycin-induced potassium diffusion potential, positive out (delta psi). In the absence of a delta psi, 22Na+ efflux at pH 9.0 was slow and appreciably inhibited by N-ethylmaleimide. Upon imposition of a delta psi, a very rapid rate of 22Na+ efflux occurred. This rapid rate of 22Na+ efflux was competitively inhibited by Li+ and varied directly with the magnitude of the delta psi. Kinetic experiments with B. alcalophilus and alkalophilic Bacillus firmus RAB indicated that the delta psi caused a pronounced increase in the Vmax for 22Na+ efflux. The Km values for Na+ were unaffected by the delta psi. Upon imposition of a delta psi at pH 7.0, a retardation of the slow 22Na+ efflux rate at pH 7.0 was caused by the delta psi. This showed that inactivity of the Na+/H+ antiporter at pH 7.0 was not secondary to a low delta psi generated by respiration at this pH. Indeed, 22Na+ efflux activity appeared to be inhibited by a relatively high internal proton concentration. By contrast, at a constant internal pH, there was little variation in the activity at external pH values from 7.0 to 9.0; at an external pH of 10.0, the rate of 22Na+ efflux declined. This decline at typical pH values for growth may be due to an insufficiency of protons when a diffusion potential rather than respiration is the driving force. Non-alkalophilic mutant strains of B. alcalophilus and B. firmus RAB exhibited a slow rate of 22Na+ efflux which was not enhanced by a delta psi at either pH 7.0 or 9.0.

Reconstitution of Escherichia coli membrane vesicles with D-amino acid dehydrogenase

When purified D-amino acid dehydrogenase [Olsiewski, P. J., Kaczorowski, G. J., & Walsh, C. T. (1980) J. Biol. Chem. 255, 4487] is incubated with right-side-out membrane vesicles from Escherichia coli, the enzyme binds to the membrane in a time- and concentration-dependent manner. As a result, the vesicles acquire the ability to oxidize D-alanine and catalyze D-alanine-dependent active transport. Similarly, incubation of D-amino acid dehydrogenase with inside-out vesicles results in binding of enzyme and D-alanine oxidase activity. Antibody inhibition studies indicate that the enzyme is bound exclusively to the inner cytoplasmic surface of the membrane in native vesicles (i.e., membrane vesicles prepared from cells induced for D-amino acid dehydrogenase). In contrast, similar studies with reconstituted vesicles demonstrate that enzyme binds to the surface exposed to the medium regardless of the orientation of the membrane. Thus, enzyme bound to right-side-out vesicles is located on the opposite side of the membrane from where it is normally found. Remarkably, in the presence of D-alanine, reconstituted right-side-out and inside-out vesicles generate electrochemical proton gradients of similar magnitude but opposite polarity, indicating that enzyme bound to either surface of the membrane is physiologically functional. The results suggest that vectorial proton translocation via the respiratory chain occurs at a point distal to the site where electrons enter the respiratory chain from the primary dehydrogenase, a conclusion that is inconsistent with the notion that the dehydrogenase forms part of a proton-translocating loop.

Potassium ion is required for the generation of pH-dependent membrane potential and delta pH by the marine bacterium Vibrio alginolyticus

The electrochemical potential gradient of protons in the marine bacterium Vibrio alginolyticus was measured as a function of external pH. In K+-containing cells, the membrane potential (delta psi) and delta pH vary with external pH as reported in other bacteria. On the other hand, K+-depleted cells show little pH dependence in the magnitude of delta psi from pH 6.0 to 8.5. The cytoplasmic pH in these cells varies depending on external pH, resulting in the generation of a small delta pH at acidic pH. Addition of K+ to K+-depleted cells leads to partial dissipation of delta psi and concomitant generation of delta pH. Strikingly, this effect of K+ is dependent on external pH. Collapse of delta psi and generation of delta pH by the addition of K+ decrease with increasing external pH. Thus, the delta psi and delta pH obtained after addition of K+ are essentially the same as those determined in K+-containing cells, and cytoplasmic pH becomes less dependent on external pH. The results suggest that the variation of delta psi and delta pH with external pH is controlled by K+ transport.

ATP-driven active transport in right-side-out bacterial membrane vesicles

Membrane vesicles from Salmonella typhimurium induced for phosphoglycerate transport, were loaded with pyruvate kinase and ADP by lysing spheroplasts under appropriate conditions. Vesicles so prepared catalyze active transport of proline and serine in the presence of phosphoenolpyruvate; this activity is abolished by the protonophore carbonyl cyanide-m-chlorophenylhydrazone and by the H+-ATPase inhibitor N,N' dicyclohexylcarbodiimide but not by anoxia or cyanide. In contrast, D-lactate-driven active transport is abolished by the hydrazone and by anoxia or cyanide but not by the carbodiimide. Moreover, phosphoenolpyruvate does not drive transport effectively in vesicles that lack the phosphoglycerate transport system. The results are consistent with an overall mechanism in which phosphoenolpyruvate gains access to the interior of the vesicles by means of the phosphoglycerate transporter and is then acted on by pyruvate kinase to phosphorylate ADP. ATP formed inside of the vesicles is then hydrolyzed by the H+-ATPase, leading to the generation of a proton electrochemical gradient that drives H+/solute symport. By using pBR322 as vector and Escherichia coli as host, a fragment of S. typhimurium DNA coding for the phosphoglycerate transport system has been cloned. E. coli membrane vesicles containing the phosphoglycerate transport system also catalyze transport in the presence of phosphoenolpyruvate when they are loaded with pyruvate kinase and ADP.