Clinical practice recommendations for infectious disease management of diabetic foot infection (DFI) - 2023 SPILF

In march 2020, the International Working Group on the Diabetic Foot (IWGDF) published an update of the 2015 guidelines on the diagnosis and management of diabetic foot infection (DFI). While we (the French ID society, SPILF) endorsed some of these recommendations, we wanted to update our own 2006 guidelines and specifically provide informative elements on modalities of microbiological diagnosis and antibiotic treatment (especially first- and second-line regiments, oral switch and duration). The recommendations put forward in the present guidelines are addressed to healthcare professionals managing patients with DFI and more specifically focused on infectious disease management of this type of infection, which clearly needs a multidisciplinary approach. Staging of the severity of the infection is mandatory using the classification drawn up by the IWGDF. Microbiological samples should be taken only in the event of clinical signs suggesting infection in accordance with a strict preliminarily established protocol. Empirical antibiotic therapy should be chosen according to the IWGDF grade of infection and duration of the wound, but must always cover methicillin-sensitive Staphylococcus aureus. Early reevaluation of the patient is a fundamental step, and duration of antibiotic therapy can be shortened in many situations. When osteomyelitis is suspected, standard foot radiograph is the first-line imagery examination and a bone biopsy should be performed for microbiological documentation. Histological analysis of the bone sample is no longer recommended. High dosages of antibiotics are recommended in cases of confirmed osteomyelitis.

Link between nasal carriage of Staphylococcus aureus and infected diabetic foot ulcers

AIMS

Nasal carriage of Staphylococcus aureus in diabetic patients may be a risk factor for diabetic foot lesion infections. The aims of this study were to compare the genotypic profiles of S. aureus strains isolated from nares and diabetic foot ulcers (DFUs) using microarray technology.

METHODS

Patients were included if they were admitted for diabetic foot infection (DFI) at any of three diabetology departments of Montpellier and Nîmes University Hospitals between 1 September 2010 to 30 June 2012. All S. aureus isolates were analyzed using oligonucleotides arrays; S. aureus resistance and virulence genes were determined and each isolate was affiliated to a clonal complex.

RESULTS

The prevalence of S. aureus nasal carriage among the 276 included patients was 39.5% (n=109), while 36.6% (n=101) had S. aureus at both sites (nares and foot wounds) and, of these patients, 65.3% of patients harboured the same strain at both sites. In addition, the spread of the methicillin-resistant S. aureus (MRSA) ST398 clone in DFI and its tropism for bone were also further confirmed.

CONCLUSION

These findings appear to provide new arguments in favour of the systematic detection of nasal S. aureus carriage to anticipate the management of DFI.

Active transport of biogenic amines in chromaffin granule membrane vesicles

Chromaffin granule membrane vesicles accumulate large amounts of catecholamines against their concentration gradients. This process is ATP-dependent, reserpine, FCCP and nigericin sensitive. Carrier-mediated, reserpine-sensitive accumulation has also been demonstrated in the absence of ATP when a pH gradient (delta pH) is artificially generated across the membrane. Crude preparation of 5-hydroxytryptamine storage vesicles from rat brain or from pig platelets showed similar requirement of a transmembrane pH gradient for accumulation of the amine. The catecholamine transporter from chromaffin granules has been solubilized by the use of detergents in the presence of phospholipids. Removal of the detergent either by Sephadex filtration or by dialysis results in the formation of proteoliposomes which catalyze delta pH-dependent, reserpine-sensitive catecholamine accumulation. Under proper conditions, the solubilized H+-translocating ATPase has been incorporated into the same proteoliposomes with the catecholamine transporter, and ATP-dependent transport has been measured. The reconstituted protein shows specificity and affinity towards catecholamines similar to the native one.

Treatment of acute Charcot foot with bisphosphonates: a systematic review of the literature

AIM/HYPOTHESIS

We undertook a systematic review of the literature concerning the efficacy and safety of bisphosphonates in acute Charcot neuropathic osteoarthropathy.

METHODS

MEDLINE, PubMed, the Cochrane Database of Systematic Reviews, and abstracts presented during the meetings of the American Diabetes Association and the European Association of Diabetes were searched for relevant publications from the period January 1990 to September 2011.

RESULTS

A total of ten studies on the treatment of acute Charcot osteoarthropathy with bisphosphonates were identified and included in the analysis. Only four clinical trials were published, three of which were randomised. Bisphosphonates appeared to induce significant reductions in skin temperature and bone turnover markers compared with placebo, without serious adverse events. Nevertheless, bisphosphonates did not shorten the immobilisation time. Moreover, no data were available regarding their long-term effects.

CONCLUSIONS/INTERPRETATIONS

Bisphosphonates have been shown to be effective for reducing bone turnover markers and skin temperature in some studies. Nevertheless, the long-term efficacy, specifically that regarding the occurrence of deformities and ulcerations, remains to be demonstrated as no follow-up studies have been published. Moreover, some studies have suggested that bisphosphonates may lengthen the resolution phase of the disease. In our opinion, the data are too weak to support the use of bisphosphonates as a routine treatment for acute Charcot neuroarthropathy.

Beneficial effects of implementing guidelines on microbiology and costs of infected diabetic foot ulcers

AIMS/HYPOTHESIS

In 2003, guidelines for management of diabetic foot infection (DFI) were written by the authors' team according to the guidelines of the International Working Group on the Diabetic Foot. The effects of implementing these guidelines on the microbiology and costs of infected diabetic foot ulcers were assessed.

METHODS

From 2003 to 2007, potential beneficial effects of implementing these guidelines were assessed by comparison over time of bacteriological data (number of bacterial samples, number of microorganisms isolated in cultures, prevalence of multidrug-resistant organisms [MDRO] and colonising flora), and costs related to use of antimicrobial agents and microbiology laboratory workload.

RESULTS

The study included 405 consecutive diabetic patients referred to the Diabetic Foot Unit for a suspected DFI. From 2003 to 2007, a significant decrease was observed in the median number of bacteria species per sample (from 4.1 to 1.6), prevalence of MDRO (35.2% vs 16.3%) and methicillin-resistant Staphylococcus aureus (52.2% vs 18.9%) (p < 0.001). Moreover, prevalence of pathogens considered as colonisers dramatically fell from 23.1% to 5.8% of all isolates (p < 0.001). In parallel, implementation of guidelines was associated with a saving of euro14,914 (US$20,046) related to a reduced microbiology laboratory workload and euro109,305 (US$147,536) due to reduced prescription of extended-spectrum antibiotic agents.

CONCLUSIONS/INTERPRETATION

Implementation of guidelines for obtaining specimens for culture from patients with DFI is cost-saving and provides interesting quality indicators in the global management of DFI.

[Epidemiology of diabetic foot problems]

Since diabetes mellitus is growing at epidemic proportions worldwide, the prevalence of diabetes-related complications is bound to increase. Diabetic foot disorders, a major source of disability and morbidity, are a significant burden for the community and a true public health problem. Many epidemiological data have been published on the diabetic foot but they are difficult to interpret because of variability in the methodology and in the definitions used in these studies. Moreover, there is a lack of consistency in population characteristics (ethnicity, social level, accessibility to care) and how results are expressed. In westernized countries, two of 100 diabetic patients are estimated to suffer from a foot ulcer every year. Amputation rates vary considerably: incidence ranges from 1 per thousand in the Madrid area and in Japan to up to 20 per thousand in some Indian tribes in North America. In metropolitan France, the incidence of lower-limb amputation is approximately 2 per thousand but with marked regional differences, and in French overseas territories, the incidence rate is much higher. Nevertheless, the risk for ulceration and amputation is much higher in diabetics compared to the nondiabetic population: the lifetime risk of a diabetic individual developing an ulcer is as high as 25% and it is estimated that every 30s an amputation is performed for a diabetic somewhere in the world. As reviewed in this paper, peripheral neuropathy, arterial disease, and foot deformities are the main factors accounting for this increased risk. Age and sex as well as social and cultural status are contributing factors. Knowing these factors is essential to classify every diabetic using a risk grading system and to take preventive measures accordingly.

The Internet and the diabetic foot: quality of online information in French language

The Internet has become a major source of health information for consumers. Nevertheless the quality of medical information varies widely and is generally poor.

AIM

This study aimed to evaluate the quality of information delivered on French-speaking Internet about the diabetic foot.

METHODS

Websites were selected using three popular search engines and introducing "foot+diabetes" and "foot+diabetic" as keywords. Two diabetologists independently evaluated the quality of information using a specially created scoring grid (range 0-52) based on acknowledged and published criteria with items relevant to general characteristics of the site and to information content. One hundred and twenty websites were selected but only 27 were included for analysis.

RESULTS

Agreement between the two raters was close for global score and site content but lower for site characteristics. Averaged global score ranged from 8 to 44. Only five sites were assessed as very advisable with a score higher than 39; in contrast 18 sites were judged as not advisable at all (score lower than 26).

CONCLUSION

This study, the first to be devoted to information about the diabetic foot on the Internet, clearly shows the variability and the general poor quality of information delivered by the great majority of French-speaking websites. Regulation organisms are urgently needed for checking and labelling public oriented health information in order to make the Internet a performing tool for patient information.

In vitro monomer swapping in EmrE, a multidrug transporter from Escherichia coli, reveals that the oligomer is the functional unit

EmrE is a small multidrug transporter, 110 amino acids long that extrudes various drugs in exchange with protons, thereby rendering Escherichia coli cells resistant to these compounds. Negative dominance studies and radiolabeled substrate-binding studies suggested that EmrE functions as an oligomer. Projection structure of two-dimensional crystals of the protein revealed an asymmetric dimer. To identify the functional unit of EmrE, a novel approach was developed. In this method, quantitative monomer swapping is induced in detergent-solubilized EmrE by exposure to 80 degrees C, a treatment that does not impair transport activity. Oligomer formation is highly specific as judged by several criteria, among them the fact that (35)S-EmrE can be "pulled out" from a mixture prepared from generally labeled cells. Using this technique, we show that inactive mutant subunits are functionally complemented when mixed with wild type subunits. The hetero-oligomers thus formed display a decreased affinity to substrates. In addition, sulfhydryl reagents inhibit the above hetero-oligomer even though Cys residues are present only in the inactive monomer. It is concluded that, in EmrE, the oligomer is the functional unit.

Functional analysis of novel multidrug transporters from human pathogens

Proteins of the Smr family are the smallest multidrug transporters, about 110 amino acids long, that extrude various drugs in exchange with protons, thereby rendering bacteria resistant to these compounds. One of these proteins, EmrE, is an Escherichia coli protein, which has been cloned based on its ability to confer resistance to ethidium and methyl viologen and which has been extensively characterized. More than 60 genes coding for Smr proteins have been identified in several bacteria based on amino acid sequence similarity to the emrE gene. In this work we have analyzed the sequence similarity among these homologues and identified some distinct signature sequence elements and several fully conserved residues. Five of these homologues, from human pathogens Mycobacterium tuberculosis, Bordetella pertussis, and Pseudomonas aeruginosa and from Escherichia coli, were cloned into an E. coli expression system. The proteins were further characterized and show varying degrees of methyl viologen uptake into proteoliposomes and [(3)H]TPP binding in solubilized membranes. The homologues can also form mixed oligomers with EmrE that exhibit intermediate binding characteristics. A comparative study of various homologous proteins provides a tool for deciphering structure-function relationship and monomer-monomer interaction in multidrug transporters and in membrane proteins in general.

Small is mighty: EmrE, a multidrug transporter as an experimental paradigm

EmrE is a multidrug transporter from Escherichia coli that functions as a homooligomer and is unique in its small size. In each monomer there are four tightly packed transmembrane segments and one membrane-embedded charged residue. This residue provides the basis for the coupling mechanism as part of a binding site "time shared" by substrates and protons.

A single carboxyl mutant of the multidrug transporter EmrE is fully functional

EmrE, a multidrug transporter from Escherichia coli removes toxic compounds from the cell in exchange with protons. Glu-14 is the only charged residue in the putative membrane domains and is fully conserved in more than 50 homologues of the protein. This residue was shown to be an essential part of the binding site, common to protons and substrate. EmrE bearing a single carboxylic residue, Glu-14, shows uptake and binding properties similar to those of the wild type. This suggests that a small protein bearing only 110 amino acids with a single carboxyl in position 14 is the most basic structure that shows ion-coupled transport activity. The role of Glu-14 in substrate binding was examined by using dicyclohexylcarbodiimide, a hydrophobic carbodiimide that is known to react with carboxyls. Tetraphenylphosphonium binding to both wild type and the single carboxyl mutant is inhibited by dicyclohexylcarbodiimide in a dose-dependent manner. Ethidium and other substrates of EmrE prevent this inhibition with an order of potency in accord with their apparent affinities. This suggests that dicyclohexylcarbodiimide binding is sterically prevented by the substrate, supporting the contention that Glu-14, the reactive residue, is part of the substrate-binding site.

Precious things come in little packages

The 110-amino acid multidrug transporter from E. coli, EmrE, is a member of the family of MiniTexan or Smr drug transporters. EmrE can transport acriflavine, ethidium bromide, tetraphenylphosphonium (TPP+), benzalkonium and several other drugs with relatively high affinities. EmrE is an H+/drug antiporter, utilizing the proton electrochemical gradient generated across the bacterial cytoplasmic membrane by exchanging two protons with one substrate molecule. The EmrE multidrug transporter is unique in its small size and hydrophobic nature. Hydropathic analysis of the EmrE sequence predicts four alpha-helical transmembrane segments. This model is experimentally supported by FTIR studies that confirm the high alpha-helicity of the protein and by high-resolution heteronuclear NMR analysis of the protein structure. The TMS of EmrE are tightly packed in the membrane without any continuous aqueous domain, as was shown by Cysteine scanning experiments. These results suggest the existence of a hydrophobic pathway through which the substrates are translocated. EmrE is functional as a homo-oligomer as suggested by several lines of evidence, including co-reconstitution experiments of wild-type protein with inactive mutants in which negative dominance has been observed. EmrE has only one membrane embedded charged residue, Glu-14, that is conserved in more than fifty homologous proteins and it is a simple model system to study the role of carboxylic residues in ion-coupled transporters. We have used mutagenesis and chemical modification to show that Glu-14 is part of the substrate-binding site. Its role in proton binding and translocation was shown by a study of the effect of pH on ligand binding, uptake, efflux and exchange reactions. We conclude that Glu-14 is an essential part of a binding site, common to substrates and protons. The occupancy of this site is mutually exclusive and provides the basis of the simplest coupling of two fluxes. Because of some of its properties and its size, EmrE provides a unique system to understand mechanisms of substrate recognition and translocation.

Vesicular monoamine transporters heterologously expressed in the yeast Saccharomyces cerevisiae display high-affinity tetrabenazine binding

A mammalian vesicular neurotransmitter transporter has been expressed in the yeast Saccharomyces cerevisiae. The gene encoding the rat vesicular monoamine transporter (rVMAT(1)) was cloned in several expression plasmids. The transporter was expressed at detectable levels only when short sequences using codons favored by S. cerevisiae were fused preceding the start of translation of rVMAT(1). The scarce expression of the wild-type protein was, most likely, due to the fact that part of the N-terminus of the protein is encoded by codons not preferred in S. cerevisiae. Furthermore, low growth temperatures increased rVMAT(1) expression and altered its processing. Whereas at 30 degrees C the protein is not glycosylated, at lower temperatures ( approximately 16 degrees C) half of the expressed transporters undergo core glycosylation. In addition, under these conditions the levels of protein expression significantly increase. Using a functional chimeric protein composed by VMAT and the green fluorescent protein (GFP), it is shown that the punctate pattern of intracellular distribution remains invariable at the different temperatures. Using a similar fusion sequence, the bovine VMAT isoform 2 (bVMAT(2)) was also expressed in yeast. The yeast-expressed bVMAT(2) binds [(3)H]dihydrotetrabenazine ([(3)H]TBZOH) with the same characteristics found in the native protein from bovine chromaffin granules. Dodecyl maltoside-solubilized bVMAT(2) retains the conformation required for [(3)H]TBZOH binding. We exploited the robust binding to follow the transporter during purification assays on a Ni(2+)-chelating column. In this report we describe for the first time the heterologous expression of a neurotransmitter transporter in the yeast S. cerevisiae.

A model for coupling of H(+) and substrate fluxes based on "time-sharing" of a common binding site

Both prokaryotic and eukaryotic cells contain an array of membrane transport systems maintaining the cellular homeostasis. Some of them (primary pumps) derive energy from redox reactions, ATP hydrolysis, or light absorption, whereas others (ion-coupled transporters) utilize ion electrochemical gradients for active transport. Remarkable progress has been made in understanding the molecular mechanism of coupling in some of these systems. In many cases carboxylic residues are essential for either binding or coupling. Here we suggest a model for the molecular mechanism of coupling in EmrE, an Escherichia coli 12-kDa multidrug transporter. EmrE confers resistance to a variety of toxic cations by removing them from the cell interior in exchange for two protons. EmrE has only one membrane-embedded charged residue, Glu-14, which is conserved in more than 50 homologous proteins. We have used mutagenesis and chemical modification to show that Glu-14 is part of the substrate-binding site. Its role in proton binding and translocation was shown by a study of the effect of pH on ligand binding, uptake, efflux, and exchange reactions. The studies suggest that Glu-14 is an essential part of a binding site, which is common to substrates and protons. The occupancy of this site by H(+) and substrate is mutually exclusive and provides the basis of the simplest coupling for two fluxes.

31P-CP-MAS NMR studies on TPP+ bound to the ion-coupled multidrug transport protein EmrE

The binding of tetraphenylphosphonium (TPP+) to EmrE, a membrane-bound, 110 residue Escherichia coli multidrug transport protein, has been observed by 31P cross-polarisation-magic-angle spinning nuclear magnetic resonance spectroscopy (CP-MAS NMR). EmrE has been reconstituted into dimyristoyl phosphatidylcholine bilayers. CP-MAS could selectively distinguish binding of TPP+ to EmrE in the fluid membrane. A population of bound ligand appears shifted 4 ppm to lower frequency compared to free ligand in solution, which suggests a rather direct and specific type of interaction of the ligand with the protein. This is also supported by the observed restricted motion of the bound ligand. The observation of another weakly bound substrate population arises from ligand binding to negatively charged residues in the protein loop regions.

A common binding site for substrates and protons in EmrE, an ion-coupled multidrug transporter

EmrE is an Escherichia coli 12-kDa multidrug transporter, which confers resistance to a variety of toxic cations by removing them from the cell interior in exchange with two protons. EmrE has only one membrane-embedded charged residue, Glu-14, that is conserved in more than 50 homologous proteins and it is a simple model system to study the role of carboxylic residues in ion-coupled transporters. We have used mutagenesis and chemical modification to show that Glu-14 is part of the substrate binding site. Its role in proton binding and translocation was shown by a study of the effect of pH on ligand binding, uptake, efflux and exchange reactions. We conclude that Glu-14 is an essential part of a binding site, common to substrates and protons. The occupancy of this site is mutually exclusive and provides the basis of the simplest coupling of two fluxes.

An essential glutamyl residue in EmrE, a multidrug antiporter from Escherichia coli

EmrE is an Escherichia coli 12-kDa protein that confers resistance to toxic compounds, by actively removing them in exchange with protons. The protein includes eight charged residues. Seven of these residues are located in the hydrophilic loops and can be replaced with either Cys or another amino acid bearing the same charge, without impairing transport activity. Glu-14 is the only charged residue in the membrane domain and is conserved in all the proteins of the family. We show here that this residue is the site of action of dicyclohexylcarbodiimide, a carbodiimide known to act in hydrophobic environments. When Glu-14 was replaced with either Cys or Asp, resistance was abolished. Whereas the E14C mutant displays no transport activity, the E14D protein shows efflux and exchange at rates about 30-50% that of the wild type. The maximal DeltapH-driven uptake rate of E14D is only 10% that of the wild type. The mutant shows a different pH profile in all the transport modes. Our results support the notion that Glu-14 is an essential part of a binding domain shared by substrates and protons but mutually exclusive in time. This notion provides the molecular basis for the obligatory exchange catalyzed by EmrE.

A membrane-embedded glutamate is required for ligand binding to the multidrug transporter EmrE

EmrE is an Escherichia coli multidrug transporter that confers resistance to a variety of toxins by removing them in exchange for hydrogen ions. The detergent-solubilized protein binds tetraphenylphosphonium (TPP(+)) with a K(D) of 10 nM. One mole of ligand is bound per approximately 3 mol of EmrE, suggesting that there is one binding site per trimer. The steep pH dependence of binding suggests that one or more residues, with an apparent pK of approximately 7.5, release protons prior to ligand binding. A conservative Asp replacement (E14D) at position 14 of the only membrane-embedded charged residue shows little transport activity, but binds TPP(+) at levels similar to those of the wild-type protein. The apparent pK of the Asp shifts to <5.0. The data are consistent with a mechanism requiring Glu14 for both substrate and proton recognition. We propose a model in which two of the three Glu14s in the postulated trimeric EmrE homooligomer deprotonate upon ligand binding. The ligand is released on the other face of the membrane after binding of protons to Glu14.

Scanning cysteine accessibility of EmrE, an H+-coupled multidrug transporter from Escherichia coli, reveals a hydrophobic pathway for solutes

EmrE is a 12-kDa Escherichia coli multidrug transporter that confers resistance to a wide variety of toxic reagents by actively removing them in exchange for hydrogen ions. The three native Cys residues in EmrE are inaccessible to N-ethylmaleimide (NEM) and a series of other sulfhydryls. In addition, each of the three residues can be replaced with Ser without significant loss of activity. A protein without all the three Cys residues (Cys-less) has been generated and shown to be functional. Using this Cys-less protein, we have now generated a series of 48 single Cys replacements throughout the protein. The majority of them (43) show transport activity as judged from the ability of the mutant proteins to confer resistance against toxic compounds and from in vitro analysis of their activity in proteoliposomes. Here we describe the use of these mutants to study the accessibility to NEM, a membrane permeant sulfhydryl reagent. The study has been done systematically so that in one transmembrane segment (TMS2) each single residue was replaced. In each of the other three transmembrane segments, at least four residues covering one turn of the helix were replaced. The results show that although the residues in putative hydrophilic loops readily react with NEM, none of the residues in putative transmembrane domains are accessible to the reagent. The results imply very tight packing of the protein without any continuous aqueous domain. Based on the findings described in this work, we conclude that in EmrE the substrates are translocated through a hydrophobic pathway.

Projection structure of NhaA, a secondary transporter from Escherichia coli, at 4.0 A resolution

Electron cryomicroscopy of frozen-hydrated two-dimensional crystals of NhaA, a Na+/H+ antiporter from Escherichia coli predicted to have 12 transmembrane alpha-helices, has facilitated the calculation of a projection map of NhaA at 4.0 A resolution. NhaA was homologously expressed in E.coli with a His6 tag, solubilized in dodecyl maltoside and purified in a single step using Ni2+ affinity chromatography. Two-dimensional crystals were obtained after reconstitution of purified protein with E.coli lipids. The projection map reveals that this secondary transporter has a highly asymmetric structure in projection. NhaA exhibits overall dimensions of approximately 38x48 A with a ring-shaped density feature probably corresponding to a bundle of tilted helices, adjacent to an elongated region of density containing several peaks indicative of transmembrane helices. Two crystal forms with p22121 symmetry show tightly packed dimers of NhaA which differ in the interactions between adjacent dimers. This work provides the first direct glimpse into the structure of a secondary transporter.

EmrE, a small Escherichia coli multidrug transporter, protects Saccharomyces cerevisiae from toxins by sequestration in the vacuole

In this report we describe the functional expression of EmrE, a 110-amino-acid multidrug transporter from Escherichia coli, in the yeast Saccharomyces cerevisiae. To allow for phenotypic complementation, a mutant strain sensitive to a series of cationic lipophilic drugs was first identified. A hemagglutinin epitope-tagged version of EmrE (HA-EmrE) conferring resistance to a wide variety of drugs, including acriflavine, ethidium, methyl viologen, and the neurotoxin 1-methyl-4-phenylpyridinium (MPP+), was functionally expressed in this strain. HA-EmrE is expressed in yeast at relatively high levels (0.5 mg/liter), is soluble in a mixture of organic solvents, and can be functionally reconstituted in proteoliposomes. In bacterial cells, EmrE removes toxic compounds by active transport through the plasma membrane, lowering their cytosolic concentration. However, yeast cells expressing HA-EmrE take up 14C-methyl viologen as well as control cells do. Thus, we investigated the basis of the enhanced resistance to the above compounds. Using Cu2+ ions or methylamine, we could selectively permeabilize the plasma membrane or deplete the proton electrochemical gradients across the vacuolar membrane, respectively. Incubation of yeast cells with copper ions caused an increase in 14C-methyl viologen uptake. In contrast, treatment with methylamine markedly diminished the extent of uptake. Conversely, the effect of Cu2+ and methylamine on a plasma membrane uptake system, proline, was essentially the opposite: while inhibited by the addition of Cu2+, it remained unaffected when cells were treated with methylamine. To examine the intracellular distribution of HA-EmrE, a functional chimera between HA-EmrE and the green fluorescent protein (HA-EmrE-GFP) was prepared. The pattern of HA-EmrE-GFP fluorescence distribution was virtually identical to that of the vacuolar marker FM 4-64, indicating that the transporter is found mainly in this organelle. Therefore, HA-EmrE protects yeast cells by lowering the cytoplasmic concentrations through removal of the toxin to the vacuole. This novel way of detoxification has been previously suggested to function in organisms in which a large vacuolar compartment exists. This report represents the first molecular description of such a mechanism.

Glycosylation of a vesicular monoamine transporter: a mutation in a conserved proline residue affects the activity, glycosylation, and localization of the transporter

The role of N-glycosylation in the expression, ligand recognition, activity, and intracellular localization of a rat vesicular monoamine transporter (rVMAT1) was investigated. The glycosylation inhibitor tunicamycin induced a dose-dependent decrease in the rVMAT1-mediated uptake of [3H]serotonin. Part of this effect was due to a general toxic effect of the drug. Therefore, to assess the contribution of each of the glycosylation sites to the transporter activity, the three putative N-glycosylation sites were mutated individually, in combination, and in toto ("triple" mutant). Mutation of each glycosylation site caused a minor and additive decrease in activity, up to the triple mutant, which retained at least 50% of the wild-type activity. No significant differences were found either in the time dependence of uptake or the apparent affinity for ligands of the triple mutant compared with the wild-type protein. It is interesting that in contrast to plasma-membrane neurotransmitter transporters, the unglycosylated form of rVMAT1 distributed in the cell as the wild-type protein. Pro43 is a highly conserved residue located at the beginning of the large loop in which all the potential glycosylation sites are found. A Pro43Leu mutant transporter was inactive. It is remarkable that despite the presence of glycosylation sites, the mutant transporter was not glycosylated. Moreover, the distribution pattern of the Pro43Leu mutant clearly differed from that of the wild type. In contrast, a Pro43Gly mutant displayed an activity practically identical to the wild-type protein. As this replacement generated a protein with wild-type characteristics, we suggest that the conformation conferred by the amino acid at this position is essential for activity.

NMR investigation of the multidrug transporter EmrE, an integral membrane protein

EmrE is an Escherichia coli multidrug transport protein that confers resistance to a wide range of toxicants by active transport across the bacterial cell membrane. The highly hydrophobic polytopic integral membrane protein has been purified and studied in its full-length form by high-resolution NMR spectroscopy in a mixture of chloroform/methanol/water (6:6:1, by vol.). Full activity is maintained after reconstitution of the protein into proteoliposomes from this solvent mixture. A series of heteronuclear (1H-15N) two- and three-dimensional experiments, as well as triple resonance experiments, were applied to the 110-residue protein and led to the assignment of the 1H, 15N and a large part of the 13C backbone resonances as well as many of the sidechain resonances. A preliminary analysis of the secondary structure, based on sequential NOE connectivities, deviation of chemical shifts from random coil values and 3J(NH-H alpha) coupling constants supports a model where the protein forms four alpha-helices between residues 4-26 (TM1), 32-53 (TM2), 58-76 (TM3) and 85-106 (TM4). For the residues of helices TM2 and TM3 a significant line broadening occurs due to slow conformational processes.

Probing the conformation of NhaA, a Na+/H+ antiporter from Escherichia coli, with trypsin

One of the most striking features of NhaA, an Escherichia coli Na+/H+ antiporter, is its extreme sensitivity to pH. The activity of NhaA increases 2000-fold between pH 6.5 and 8.5. In this work, we investigated whether the activation of NhaA by pH is accompanied by conformational changes which can be detected using trypsin as a probe. We have found that NhaA is susceptible to proteolytic digestion at the pH range where it is activated, suggesting that these two events may be related; at alkaline pH, the protein becomes active and adopts an "open" conformational state in which more domains are exposed to the enzyme. This idea was further supported by results from two mutants of NhaA in which His-225, a residue involved in pH sensing, has been replaced by either Arg or Asp. The mutant H225R is activated at more acidic pH values, while H225D at more alkaline pH. In accordance with the results described for the wild-type protein, H225R was susceptible to digestion by trypsin at the pH at which it undergoes main activation. NhaA has many potential tryptic cleavage sites. However, analysis of the tryptic digestion fragments suggests that at alkaline pH, the protein is exposed to cleavage mainly at hydrophilic loops 6, 7, and 8. Thus, upon activation, NhaA appears to undergo a change in conformation that is reflected in specific regions of the protein.

Mutations affecting substrate specificity of the Bacillus subtilis multidrug transporter Bmr

The Bacillus subtilis multidrug transporter Bmr, a member of the major facilitator superfamily of transporters, causes the efflux of a number of structurally unrelated toxic compounds from cells. We have shown previously that the activity of Bmr can be inhibited by the plant alkaloid reserpine. Here we demonstrate that various substitutions of residues Phe143 and Phe306 of Bmr not only reduce its sensitivity to reserpine inhibition but also significantly change its substrate specificity. Cross-resistance profiles of bacteria expressing mutant forms of the transporter differ from each other and from the cross-resistance profile of cells expressing wild-type Bmr. This result strongly suggests that Bmr interacts with its transported drugs directly, with residues Phe143 and Phe306 likely to be involved in substrate recognition.

EmrE, the smallest ion-coupled transporter, provides a unique paradigm for structure-function studies

EmrE is an Escherichia coli multidrug transporter which confers resistance to a wide variety of toxicants by actively removing them in exchange for hydrogen ions. EmrE is a highly hydrophobic 12 kDa protein which has been purified by taking advantage of its unique solubility in organic solvents. After solubilization and purification, the protein retains its ability to transport as judged from the fact that it can be reconstituted in a functional form. Hydrophobicity analysis of the sequence yielded four putative transmembrane domains of similar sizes. Results from transmission Fourier transform infrared measurements agree remarkably well with this hypothesis and yielded alpha-helical estimates of 78% and 80% for EmrE in CHCl3:MeOH and 1,2-dimyristoyl phosphocholine, respectively. Furthermore, the fact that most of the amide groups in the protein do not undergo amide-proton H/D exchange implies that most (approximately 80%) of the residues are embedded in the bilayer. These observations are only consistent with four transmembrane helices. A domain lined by Cys41 and Cys95 accessible only to substrates such as the organic mercurial 4-(chloromercuri)benzoic acid has been identified. Both residues are asymmetric in their location with respect to the plane of the membrane, Cys95 being closer than Cys41 to the outside face of the membrane. In co-reconstitution experiments of wild-type protein with three different inactive mutants, negative dominance has been observed. This phenomenon suggests that EmrE is functional as a homo-oligomer.

Topological analysis of NhaA, a Na+/H+ antiporter from Escherichia coli

Analysis of the hydropathic profile of the amino acid sequence of NhaA, a Na+/H+ antiporter from Escherichia coli has previously suggested the existence of 11 putative transmembrane segments (Taglicht, D., Padan, E., and Schuldiner, S. (1991) J. Biol. Chem. 266, 11289-11294). In the present work to test the location of the C terminus, right-side-out and inside-out membrane vesicles were digested with carboxypeptidase B and probed with an antibody raised against a synthetic peptide whose sequence was based on the C terminus sequence. The results demonstrate that the C terminus is facing the cell interior because it is available for digestion only from the inside. Previous evidence from an NhaA-beta-galactosidase fusion to loop 5 of NhaA indicated that this loop is also facing the cytoplasm (Karpel, R., Alon, T., Glaser, G., Schuldiner, S., and Padan, E. (1991) J. Biol. Chem. 266, 21753-21759) and therefore was not consistent with the position of the C terminus in an 11-transmembrane segment model. Therefore, the model was re-evaluated. For this purpose, 10 nhaA'-'phoA gene fusions were constructed and assayed for alkaline phosphatase activity. The results support a 12-transmembrane segment model with the N and C termini located in the cytoplasm. The evidence indicates that two very short segments, 14 and 16 amino acids long, must cross the membrane in an unknown conformation.

Negative dominance studies demonstrate the oligomeric structure of EmrE, a multidrug antiporter from Escherichia coli

EmrE, the smallest known ion-coupled transporter, is an Escherichia coli 12-kDa protein 80% helical and soluble in organic solvents. EmrE is a polyspecific antiporter that exchanges hydrogen ions with aromatic toxic cations such as methyl viologen. Since it is many times smaller than the classical consensus 12 transmembrane segments transporters, it was particularly interesting to determine its oligomeric state. For this purpose, a series of nonfunctional mutants has been generated and characterized to test their effect on the activity of the wild-type protein upon mixing. As opposed to the wild type, these mutants do not confer resistance to methyl viologen, ethidium bromide, or a series of other toxicants. Co-expression of each of the nonfunctional mutants with the wild-type protein results in a reduction in the ability of the functional transporter to confer resistance to several toxicants. To perform mixing experiments in vitro, all the mutants have been purified by extraction with organic solvents, reconstituted in proteoliposomes, and found to be inactive. When co-reconstituted with wild-type protein, they inhibit the activity of the latter in a dose-dependent form up to full inhibition. We assume that this inhibition is due to the formation of mixed oligomers in which the presence of one nonfunctional subunit causes full inactivation. A binomial analysis of the results based on the latter assumptions do not provide statistically significant answers but suggests that the oligomer is composed of three subunits. The results described provide the first in vitro demonstration of the functional oligomeric structure of an ion-coupled transporter.

Identification of residues in the translocation pathway of EmrE, a multidrug antiporter from Escherichia coli

EmrE is a small, 12-kDa, highly polyspecific antiporter, which exchanges hydrogen ions with aromatic cations such as methyl viologen. EmrE-mediated transport is inhibited by the sulfhydryl-reactive reagent 4-(chloromercuri)benzoic acid (PCMB) but not by a variety of other sulfhydryl reagents. This differential effect is due to the fact that the organic mercurial is a substrate of the transporter and can reach domains otherwise inaccessible to the different reagents. To find out which of the three cysteine residues in EmrE is reacting with PCMB, each was replaced with serine and it was shown that none of them is essential for transport activity. A protein completely devoid of Cys residues (CL) is also capable of substrate accumulation albeit at a slower rate. Mutated proteins in which only one of the native cysteines was left whereas the other changed to serine were also constructed. The use of these proteins demonstrated that two of the three Cys in EmrE, Cys-41 and Cys-95, but not Cys-39, react with PCMB. A related mercurial, 4-(chloromercuri)benzenesulfonic acid (PCMBS), is only a very poor inhibitor, probably because of the negative charge it bears. PCMBS reacts with EmrE in an asymmetric and unique way. It reacts with the mutant bearing a single Cys residue in position 95 (CL-C95) only when the reagent is present in the outside face of the membrane and with the mutant CL-C41 only when allowed to permeate to the cell interior; as expected, it does not react with the mutant protein bearing a single Cys at position 39 (CL-C39). It is concluded that PCMB permeates through the substrate pathway of EmrE and covalently reacts with the two exposed residues, Cys-95 and Cys-41, but not with Cys-39, located on the opposite face of the helix relative to residue 41. In addition, because of the asymmetric reactivity to PCMBS, an inhibitor that does not permeate through the protein, it is concluded that Cys-41 is closer to the cytoplasmic face than Cys-95. The results demonstrate the existence of a domain accessible only to substrates and provide a unique tool for studying the substrate permeation pathway of an ion-coupled transporter.

Determining the secondary structure and orientation of EmrE, a multi-drug transporter, indicates a transmembrane four-helix bundle

EmrE is a member of a newly emerging family of MiniTEXANS, a family of multi-drug antiporters from bacteria characterized by their small size of roughly 100 amino acids. In this report we have obtained transmission FTIR spectra of EmrE in CHCl3:MeOH, DMPC vesicles, and Escherichia coli lipid vesicles. Secondary structure analysis has shown that both in DMPC vesicles and in CHCl3: MeOH the protein adopts a highly helical secondary structure that correlates remarkably well with that predicted by hydropathy analysis. The protein was shown to be resistant to amide proton H/D exchange, providing evidence that most of the protein is embedded in the lipid bilayer. Polarized ATR-FTIR spectra of the protein in DMPC vesicles have shown that the helices are oriented with an average tilt angle of 27 degrees from the bilayer normal. The protein was found to be less oriented in E. coli lipid vesicles, most likely as a result of the poor orientation of the bilayer lipids themselves. Thus, the protein is identified as a transmembrane four-helix bundle providing valuable structural data for this family of multi-drug transporters. The results set the stage for further studies aimed at deriving a detailed model for this protein.

Modification of the pH profile and tetrabenazine sensitivity of rat VMAT1 by replacement of aspartate 404 with glutamate

Vesicular monoamine transporters (VMAT) catalyze transport of serotonin, dopamine, epinephrine, and norepinephrine into subcellular storage organelles in a variety of cells. Accumulation of the neurotransmitter depends on the proton electrochemical gradient (Delta micro H+) across the organelle membrane and involves VMAT-mediated exchange of two lumenal protons with one cytoplasmic amine. Mutagenic analysis of the role of two conserved Asp residues located in transmembrane segments X and XI of rat VMAT type I reveals an important role of these two residues in catalysis. Replacement of Asp 431 with either Glu or Ser inhibits VMAT-mediated [3H]serotonin transport. The mutated proteins are unimpaired in ligand recognition as measured with the high affinity ligand [3H]reserpine or coupling to the proton electrochemical gradient as judged by its ability to accelerate [3H]reserpine binding. Therefore, the Asp residue is needed as such in this position and even a conservative replacement with Glu generates a protein that can catalyze only partial reactions but cannot complete the transport cycle. Replacement of Asp 404 with either Ser or Cys inhibits all VMAT-mediated reactions measured. However, replacement with Glu generated a protein that catalyzed [3H]serotonin transport with modified properties. Whereas the mutated protein binds [3H]reserpine to normal levels and the pH optimum of this reaction is only slightly affected, the optimum pH for transport activity shifted to the acid side and became very sharp; in addition the sensitivity to the inhibitor tetrabenazine increased significantly in this mutated protein. The results point to the need of a carboxyl moiety in position 404. A slight change in its relative location or in the environment around it has a significant effect on the pK of group(s) involved in steps after ligand recognition and coupling to the first H+.

The pharmacological profile of the vesicular monoamine transporter resembles that of multidrug transporters

Vesicular neurotransmitter transporters function in synaptic vesicles and other subcellular organelles and they were thought to be involved only in neurotransmitter storage. Several findings have led us to test novel aspects of their function. Cells expressing a c-DNA coding for one of the rat monoamine transporters (VMAT1) become resistant to the neurotoxin N-methyl-4-phenylpyridinium (MPP+) [Liu et al. (1992) Cell, 70, 539-551]. The basis of the resistance is the VMAT1-mediated transport and sequestration of the toxin into subcellular compartments. In addition, the deduced sequence of VMAT1 predicts a protein that shows a distinct homology to a class of bacterial drug resistance transporters (TEXANs) that share some substrates with mammalian multidrug resistance transporters (MDR) such as the P-glycoprotein. These findings induced us to test whether compounds that are typically transported by MDR interact also with vesicular transporters. The use of [3H]reserpine binding to determine drug interactions with VMAT allowed assessment of the ability of various drugs to bind to the substrate site of the transporter. Cytotoxic compounds such as ethidium, isometamidium, tetraphenylphosphonium, rhodamine, tacrine and doxorubicin, interact specifically with vesicular monoamine transporters. Verapamil, a calcium channel blocker, is also a competitive inhibitor of transport. In the case of rhodamine, fluorescence measurements in digitonin-permeabilized cells demonstrated ATP-dependent VMAT-mediated transport. The results imply that even though the bacterial and vesicular transporters are structurally different from the P-glycoprotein, they share a similar substrate range. These findings suggest a novel possible way of protection from the effects of toxic compounds by removal to subcellular compartments.

Replacements of histidine 226 of NhaA-Na+/H+ antiporter of Escherichia coli. Cysteine (H226C) or serine (H226S) retain both normal activity and pH sensitivity, aspartate (H226D) shifts the pH profile toward basic pH, and alanine (H226A) inactivates the carrier at all pH values

We have previously shown that replacement of His-226 in the NhaA Na+/H+ antiporter of Escherichia coli to Arg (H226R) shifts the pH profile of the antiporter toward acidic pH and as a result of delta nhaA delta nhaB strain bearing this mutation is Na+ sensitive at alkaline pH (Gerchman, Y., Olami, Y., Rimon, A., Taglicht, D., Schuldiner, S. and Padan, E. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 1212-1216). In the present work the role of His-226 in the response of NhaA to pH has been studied in detail. The Na+ sensitivity of the delta nhaA delta nhaB mutant bearing the H226R-NhaA plasmid at alkaline pH provided a very powerful tool to isolate revertants and suppressants of H226R growing on high Na+ at alkaline pH. With this approach cysteine (H226C) and serine (H226S) replacements were found to efficiently replace His-226 and yield an antiporter, which like the wild-type protein, is activated by pH between pH 7 and 8. These results imply that polarity and/or hydrogen bonding, the common properties shared by these amino acid residues, are essential at position 226 for pH regulation of NhaA. This suggestion was substantiated by site-directed mutagenesis of His-226 either to alanine (H226A) or aspartate (H226D). Whereas H226A-NhaA shows very low activity which is not activated by pH, H226D-NhaA is active and regulated by pH. The pH profile of H226D is shifted by half a pH unit toward alkaline pH, as opposed to the previously isolated mutant H226R which has a pH profile shift, to the same extent, but toward acidic pH. It is suggested that charge modifies the pH profile but is not essential for the pH regulation of NhaA.

Amiloride and harmaline are potent inhibitors of NhaB, a Na+/H+ antiporter from Escherichia coli

The diuretic drug amiloride is a specific inhibitor of sodium transporting proteins in several cell types. Attempts to inhibit this activity in membrane vesicles derived from various bacteria, did not yield clear results. Therefore, we tested the effect of amiloride and its derivatives on the purified Na+/H+ antiporters of E. coli reconstituted in functional form in proteoliposomes. Whereas NhaA is not inhibited by amiloride, both amiloride and harmaline are potent inhibitors of NhaB with K0.5 of 6 and 15 microM, respectively. The pattern of inhibition by amiloride derivatives is different from that reported for mammalian antiporters but similar to that reported for the Na+/H+ antiporter of D. salina [Katz, A., Kleyman, T.R. and Pick, U. (1994) Biochemistry 33, 2389-2393]. Clonidine is a poor inhibitor (K0.5 = 200 microM) while cimetidine had no effect on the antiporter up to concentration of 1 mM. These new potent inhibitors provide us with important tools for the study of the mechanism of action of NhaB.

EmrE, an Escherichia coli 12-kDa multidrug transporter, exchanges toxic cations and H+ and is soluble in organic solvents

The smallest membrane protein shown to catalyze ion-coupled transport is documented in this report. A gene coding for a small 110-amino acid membrane protein (emrE or mvrC) has been previously identified and cloned and shown to render Escherichia coli cells resistant to methyl viologen and to ethidium. In this report, it is shown that the resistance is due to extrusion of the toxic compounds in a process that requires a proton electrochemical gradient rather than ATP. For this purpose, cells in which the unc gene was inactivated were used so that the interconversion between the proton gradient and ATP is not possible, and the effect of agents, which specifically affect either of them, was tested on transport of ethidium in the intact cell. In addition, EmrE has been overexpressed and metabolically labeled with [35S]methionine. Strikingly, the protein can be quantitatively extracted with a mixture of organic solvents such as chloroform:methanol and is practically pure after this extraction. Moreover, after addition of E. coli lipids to the chloroform:methanol extract, EmrE has been reconstituted in proteoliposomes loaded with ammonium chloride. Upon dilution of the proteoliposomes in ammonium-free medium, a pH gradient was formed that drove transport of ethidium and methyl viologen into the proteoliposome. Both substrates compete with each other and exchange with previously transported solute. EmrE is a multidrug transporter of a novel type, and, because of its size and its solubility properties, it provides a unique model to study structure-function aspects of transport reactions in ion-coupled processes.

MH1, a second-site revertant of an Escherichia coli mutant lacking Na+/H+ antiporters (delta nhaA delta nhaB), regains Na+ resistance and a capacity to excrete Na+ in a delta microH(+)-independent fashion

The Escherichia coli mutant delta nhaA delta nhaB (EP432), which lacks the two specific Na+/H+ antiporter genes, is incapable of efficiently excreting Na+. Accordingly at low K+ (6 mM) medium, its intracellular Na+ concentration is only slightly lower (1.5-2x) than the extracellular concentration (50 mM), explaining the high sensitivity to Na+ (> or = 30 mM) of the mutant. This Na+ sensitivity is shown to be a powerful selection for spontaneous second-site suppressor mutations that allow growth on high Na+ (< or = 0.6 M) with a rate similar to that of the wild type. One such mutation, MH1, maps at 25.7 min on the E. coli chromosome. It confers Na+ but not Li+ resistance upon delta nhaA delta nhaB cells and exposes a Na(+)-excreting capacity, maintaining a Na+ gradient of about 8-10 (at 50 mM extracellular Na+), which is similar to that of the wild type. Although lower, Na+ excretion capacity is also observed in the delta nhaA delta nhaB mutant when grown in medium containing higher K+ (70 mM). This capacity is accompanied with a shift in the sensitivity of the mutant to higher Na+ concentrations (> or = 300 mM). Whereas Na+ excretion by a wild type carrying delta unc is uncoupler sensitive, that of MH1 delta unc is dependent on respiration in an uncoupler-insensitive fashion. It is concluded that under some conditions (high K+ in the medium or in MH1-like mutants), a primary pump driven by respiration is responsible for Na+ extrusion when the Na+/H+ antiporters are not active.

Histidine-419 plays a role in energy coupling in the vesicular monoamine transporter from rat

Vesicular monoamine transporters (VMAT) catalyze transport of serotonin, dopamine, epinephrine and norepinephrine into subcellular storage organelles in a variety of cells. Accumulation of the neurotransmitter depends on the proton electrochemical gradient across the organelle membrane and involves VMAT-mediated exchange of two lumenal protons with one cytoplasmic amine. It has been suggested in the past that His residues play a role in H+ movement or in its coupling to active transport in H(+)-symporters and antiporters. Indeed VMAT-mediated transport is inhibited by reagents specific for His residues. We have identified one His residue in VMAT1 from rat which is conserved in other vesicular neurotransmitter transporters. Mutagenesis of this His (H419) to either Arg or Cys completely inhibits [3H]serotonin and [3H]dopamine accumulation. Mutagenesis also inhibits other H(+)-dependent partial reactions of VMAT such as the acceleration of binding of the high affinity ligand reserpine, but does not inhibit the [3H]reserpine binding which is not dependent on H+ translocation. It is concluded that His-419 plays a role in energy coupling in r-VMAT1.

Molecular physiology of the Na+/H+ antiporter in Escherichia coli

All living cells maintain an inwardly directed Na+ gradient and a constant intracellular pH. Na+/H+ antiporters have been assigned an essential role in these homeostatic mechanisms in all cells. In Escherichia coli, two Na+/H+ antiporter genes, nhaA and nhaB, have been cloned. Deletion of either one or both showed that NhaA is essential for adaptation to high salinity, for growth at alkaline pH in the presence of Na+ and for challenging Li+ toxicity. NhaB confers tolerance to low levels of Na+ and becomes essential when the activity of NhaA limits growth. The adaptive response to Na+ is mediated by the positive regulator nhaR, which transduces the signal (intracellular Na+) to expression of the nhaA gene. We have identified Glu-134 of NhaR as part of the 'Na+ sensor' of NhaA. In agreement with the role of NhaA in pH homeostasis, its Na(+)-dependent expression is enhanced at alkaline pH. Reconstitution of pure NhaA and NhaB in proteoliposomes demonstrates that, whereas both are electrogenic (the H+/Na+ stoichiometry of NhaA is 2), only NhaA is pH-dependent, increasing its activity 1000-fold between pH 7 and 8.5. Mutating all the histidines of NhaA shows that His-226 is part of the 'pH sensor' of NhaA.

Kinetic properties of NhaB, a Na+/H+ antiporter from Escherichia coli

NhaB, a Na+/H+ antiporter from Escherichia coli, was overproduced, purified, and reconstituted in a functional state, demonstrating that a single polypeptide, the product of the nhaB gene, can catalyze full activity. NhaB is a minor protein that accounts for less than 0.1% of the total membrane protein. The use of proteoliposomes made possible the determination of important kinetic and pharmacological properties in the absence of passive and mediated leaks. The activity of NhaB was found to have some pH dependence; the apparent Km for Na+ changes by 10-fold from 1.55 mM at pH 8.5 to 16.66 mM at pH 7.2, while the Vmax remains constant. It was demonstrated that NhaB is electrogenic and translocates more H+ than Na+ per cycle; the rate of sodium efflux from proteoliposomes was accelerated by a membrane potential, negative inside, and NhaB activity generated a membrane potential as monitored by two techniques. The stoichiometry of NhaB was estimated by a thermodynamic method in which the magnitude of delta psi generated by NhaB was measured at various Na+ gradients. A kinetic method, in which the electrophoretic movement of 86Rb+ (in the presence of valinomycin) was monitored in parallel with measurements of NhaB-mediated 22Na+ uptake, allowed us to determine a stoichiometry of 3H+/2Na+. The significance of the existence of two antiporters with different stoichiometries, NhaA and NhaB, active in the same cell, is discussed.