Bacterial transporters

Functional Roles of the Conserved Amino Acid Sequence Motif C, the Antiporter Motif, in Membrane Transporters of the Major Facilitator Superfamily

The biological membrane surrounding all living cells forms a hydrophobic barrier to the passage of biologically important molecules. Integral membrane proteins called transporters circumvent the cellular barrier and transport molecules across the cell membrane. These molecular transporters enable the uptake and exit of molecules for cell growth and homeostasis. One important collection of related transporters is the major facilitator superfamily (MFS). This large group of proteins harbors passive and secondary active transporters. The transporters of the MFS consist of uniporters, symporters, and antiporters, which share similarities in structures, predicted mechanism of transport, and highly conserved amino acid sequence motifs. In particular, the antiporter motif, called motif C, is found primarily in antiporters of the MFS. The antiporter motif's molecular elements mediate conformational changes and other molecular physiological roles during substrate transport across the membrane. This review article traces the history of the antiporter motif. It summarizes the physiological evidence reported that supports these biological roles.

Role of bacterial efflux pump proteins in antibiotic resistance across microbial species

Efflux proteins are transporter molecules that actively pump out a variety of substrates, including antibiotics, from cells to the environment. They are found in both Gram-positive and Gram-negative bacteria and eukaryotic cells. Based on their protein sequence homology, energy source, and overall structure, efflux proteins can be divided into seven groups. Multidrug efflux pumps are transmembrane proteins produced by microbes to enhance their survival in harsh environments and contribute to antibiotic resistance. These pumps are present in all bacterial genomes studied, indicating their ancestral origins. Many bacterial genes encoding efflux pumps are involved in transport, a significant contributor to antibiotic resistance in microbes. Efflux pumps are widely implicated in the extrusion of clinically relevant antibiotics from cells to the extracellular environment and, as such, represent a significant challenge to antimicrobial therapy. This review aims to provide an overview of the structures and mechanisms of action, substrate profiles, regulation, and possible inhibition of clinically relevant efflux pumps. Additionally, recent advances in research and the pharmacological exploitation of efflux pump inhibitors as a promising intervention for combating drug resistance will be discussed.

Inhibition of Multidrug Efflux Pumps Belonging to the Major Facilitator Superfamily in Bacterial Pathogens

Bacterial pathogens resistant to multiple structurally distinct antimicrobial agents are causative agents of infectious disease, and they thus constitute a serious concern for public health. Of the various bacterial mechanisms for antimicrobial resistance, active efflux is a well-known system that extrudes clinically relevant antimicrobial agents, rendering specific pathogens recalcitrant to the growth-inhibitory effects of multiple drugs. In particular, multidrug efflux pump members of the major facilitator superfamily constitute central resistance systems in bacterial pathogens. This review article addresses the recent efforts to modulate these antimicrobial efflux transporters from a molecular perspective. Such investigations can potentially restore the clinical efficacy of infectious disease chemotherapy.

The Major Facilitator Superfamily and Antimicrobial Resistance Efflux Pumps of the ESKAPEE Pathogen Staphylococcus aureus

The ESKAPEE bacterial pathogen Staphylococcus aureus has posed a serious public health concern for centuries. Throughout its evolutionary course, S. aureus has developed strains with resistance to antimicrobial agents. The bacterial pathogen has acquired multidrug resistance, causing, in many cases, untreatable infectious diseases and raising serious public safety and healthcare concerns. Amongst the various mechanisms for antimicrobial resistance, integral membrane proteins that serve as secondary active transporters from the major facilitator superfamily constitute a chief system of multidrug resistance. These MFS transporters actively export structurally different antimicrobial agents from the cells of S. aureus. This review article discusses the S. aureus-specific MFS multidrug efflux pump systems from a molecular mechanistic perspective, paying particular attention to structure-function relationships, modulation of antimicrobial resistance mediated by MFS drug efflux pumps, and direction for future investigation.

Functional Annotation and Curation of Hypothetical Proteins Present in A Newly Emerged Serotype 1c of Shigella flexneri: Emphasis on Selecting Targets for Virulence and Vaccine Design Studies

Shigella flexneri is the principal cause of bacillary dysentery, contributing significantly to the global burden of diarrheal disease. The appearance and increase in the multi-drug resistance among Shigella strains, necessitates further genetic studies and development of improved/new drugs against the pathogen. The presence of an abundance of hypothetical proteins in the genome and how little is known about them, make them interesting genetic targets. The present study aims to carry out characterization of the hypothetical proteins present in the genome of a newly emerged serotype of S. flexneri (strain Y394), toward their novel regulatory functions using various bioinformatics databases/tools. Analysis of the genome sequence rendered 4170 proteins, out of which 721 proteins were annotated as hypothetical proteins (HPs) with no known function. The amino acid sequences of these HPs were evaluated using a combination of latest bioinformatics tools based on homology search against functionally identified proteins. Functional domains were considered as the basis to infer the biological functions of HPs in this case and the annotation helped in assigning various classes to the proteins such as signal transducers, lipoproteins, enzymes, membrane proteins, transporters, virulence, and binding proteins. This study contributes to a better understanding of growth, survival, and disease mechanism at molecular level and provides potential new targets for designing drugs against Shigella infection.

Sensory quality and flavour of alginate coated and repetitive pulsed light treated fresh-cut cantaloupes (Cucumis melo L. Var. Reticulatus Cv. Glamour) during storage

Fresh-cut fruits are popular due to the convenience provided. However, fresh-cut processes damage fruit tissues and reduce the shelf life of products. Pulsed light (PL) treatment is a decontamination method of foods. PL treatment given repetitively at a certain interval during storage could further extend the shelf life of fresh-cut fruits. Edible coating preserves fresh-cut fruits by providing mechanical strength and reducing respiration and water loss. This study was to evaluate the effects of alginate coating combined with repetitive pulsed light (RPL) on sensory quality and flavour of fresh-cut cantaloupes during storage. Cantaloupes were treated with alginate (1.86%, w/v) and RPL (0.9 J/cm² at every 48 h up to 26 days) alone or in combination. Flavour analysis of fresh-cut cantaloupes was carried out every 12 days during storage at 4 ± 1 °C while sensory analysis was performed on day 32. Alginate coating and/or RPL retained sugar contents (17.92-20.01 g/kg FW for fructose, 18.77-19.98 g/kg FW for glucose and 23.02-29.41 g/kg FW for sucrose) in fresh-cut cantaloupes during storage. Combination of alginate with RPL reduced accumulation of lactic acid although alginate coating was more effective to minimise changes of other organic acids in fresh-cut cantaloupes. The combined treatment was also more effective than individual treatment in retaining total aroma compound concentration of fresh-cut cantaloupes during storage with the highest relative concentration, i.e. 3.174 on day 36. Overall, the combined alginate coating and RPL was effective to maintain the fresh-like sensory quality of fresh-cut cantaloupes with insignificant overall acceptability compared to the control.

Discovery of Pyruvate Kinase as a Novel Target of New Fungicide Candidate 3-(4-Methyl-1,2,3-thiadiazolyl)-6-trichloromethyl-[1,2,4]-triazolo-[3,4- b][1,3,4]-thiadizole

Target identification is an essential basis for novel-pesticide development in new molecular design and lead optimization. 3-(4-Methyl-1,2,3-thiadiazolyl)-6-trichloromethyl[1,2,4]triazolo[3,4- b][1,3,4]thiadizole (YZK-C22) is a novel fungicide candidate with specific antifungal activity. We investigated its mode of action, and our studies indicated that YZK-C22 showed no cross resistance against Saccharomyces cerevisiae mutants with classic fungicide targets. Mec1 and Rad53 are two kinases that respond to DNA-replication damage, and the efficacy test showed that YZK-C22 could not perform its fungicidal activity by inhibiting DNA repair. Target screening by drug-affinity-responsive target stability (DARTS) showed that pyruvate kinase (PK), a key enzyme in the glycolytic pathway, was the potent new fungicidal target of YZK-C22. Fifty-eight differentially expressed proteins (DEPs) primarily involved in the metabolic process were identified by isobaric tags for relative and absolute quantification analysis (iTRAQ) in S. cerevisiae, and protein expression in the citrate cycle decreased with treatment of 5 mg/L YZK-C22, which was consistent with the results of DARTS. Molecular-docking analysis further validated that YZK-C22 could dock into the active center of PK instead of phosphoenolpyruvate. The enzyme activity of PK from S. cerevisiae was competitively inhibited with a K_i of 3.33 ± 0.28 μmol/L, and the cell-growth inhibition of S. cerevisiae was released by supplementation with pyruvic acid, whereas the growth of S. cerevisiae was not recovered by adding PK's substrate (phosphoenolpyruvate) or allosteric regulator (fructose-1,6-bisphosphate). The present studies uncovered and validated the primary target of the new, potent fungicidal candidate YZK-C22; our results provide a successful, valuable, and applicable case of target discovery and identification for novel-fungicide development.

Bifunctional Enzyme SpoT Is Involved in Biofilm Formation of Helicobacter pylori with Multidrug Resistance by Upregulating Efflux Pump Hp1174 (gluP)

The drug resistance of Helicobacter pylori is gradually becoming a serious problem. Biofilm formation is an important factor that leads to multidrug resistance (MDR) in bacteria.

The drug resistance of Helicobacter pylori is gradually becoming a serious problem. Biofilm formation is an important factor that leads to multidrug resistance (MDR) in bacteria. The ability of H. pylori to form biofilms on the gastric mucosa is known. However, there are few studies on the regulatory mechanisms of H. pylori biofilm formation and multidrug resistance. Guanosine 3′-diphosphate 5′-triphosphate and guanosine 3′,5′-bispyrophosphate [(p)ppGpp] are global regulatory factors and are synthesized in H. pylori by the bifunctional enzyme SpoT. It has been reported that (p)ppGpp is involved in the biofilm formation and multidrug resistance of various bacteria. In this study, we found that SpoT also plays an important role in H. pylori biofilm formation and multidrug resistance. Therefore, it was necessary to carry out some further studies regarding its regulatory mechanism. Considering that efflux pumps are of great importance in the biofilm formation and multidrug resistance of bacteria, we tried to determine whether efflux pumps controlled by SpoT participate in these activities. We found that Hp1174 (glucose/galactose transporter [gluP]), an efflux pump of the major facilitator superfamily (MFS), is highly expressed in biofilm-forming and multidrug-resistant (MDR) H. pylori strains and is upregulated by SpoT. Through further research, we determined that gluP is involved in H. pylori biofilm formation and multidrug resistance. Furthermore, the average expression level of gluP in the clinical MDR strains (C-MDR) was considerably higher than that in the clinical drug-sensitive strains (C-DSS). Taken together, our results revealed a novel molecular mechanism of H. pylori resistance to multidrug exposure.

Hybrid rotors in F1Fo ATP synthases: subunit composition, distribution, and physiological significance

The c ring of the Na+ F1Fo ATP synthase from the anaerobic acetogenic bacterium Acetobacterium woodii is encoded by three different genes: atpE1, atpE2 and atpE3. Subunit c1 is similar to typical V-type c subunits and has four transmembrane helices with one ion binding site. Subunit c2 and c3 are identical at the amino acid level and are typical F-type c subunits with one ion binding site in two transmembrane helices. All three constitute a hybrid FoVo c ring, the first found in nature. To analyze whether other species may have similar hybrid rotors, we searched every genome sequence publicly available as of 23 February 2015 for F1Fo ATPase operons that have more than one gene encoding the c subunit. This revealed no other species that has three different c subunit encoding genes but twelve species that encode one Fo- and one Vo-type c subunit in one operon. Their c subunits have the conserved binding motif for Na+. The organisms are all anaerobic. The advantage of hybrid c rings for the organisms in their environments is discussed.

Closure of the cytoplasmic gate formed by TM5 and TM11 during transport in the oxalate/formate exchanger from Oxalobacter formigenes

OxlT, the oxalate/formate exchanger of Oxalobacter formigenes, is a member of the major facilitator superfamily of transporters. In the present work, substrate (oxalate) was found to enhance the reactivity of the cysteine mutant S336C on the cytoplasmic end of helix 11 to methanethiosulfonate ethyl carboxylate. In addition, S336C is found to spontaneously cross-link to S143C in TM5 in either native or reconstituted membranes under conditions that support transport. Continuous wave EPR measurements are consistent with this result and indicate that positions 143 and 336 are in close proximity in the presence of substrate. These two residues are localized within helix interacting GxxxG-like motifs (G₁₄₀LASG₁₄₄ and S₃₃₆DIFG₃₄₀) at the cytoplasmic poles of TM5 and TM11. Pulse EPR measurements were used to determine distances and distance distributions across the cytoplasmic or periplasmic ends of OxlT and were compared with the predictions of an inside-open homology model. The data indicate that a significant population of transporter is in an outside-open configuration in the presence of substrate; however, each end of the transporter exhibits significant conformational heterogeneity, where both inside-open and outside-open configurations are present. These data indicate that TM5 and TM11, which form part of the transport pathway, transiently close during transport and that there is a conformational equilibrium between inside-open and outside-open states of OxlT in the presence of substrate.

Modulation of Bacterial Multidrug Resistance Efflux Pumps of the Major Facilitator Superfamily

Bacterial infections pose a serious public health concern, especially when an infectious disease has a multidrug resistant causative agent. Such multidrug resistant bacteria can compromise the clinical utility of major chemotherapeutic antimicrobial agents. Drug and multidrug resistant bacteria harbor several distinct molecular mechanisms for resistance. Bacterial antimicrobial agent efflux pumps represent a major mechanism of clinical resistance. The major facilitator superfamily (MFS) is one of the largest groups of solute transporters to date and includes a significant number of bacterial drug and multidrug efflux pumps. We review recent work on the modulation of multidrug efflux pumps, paying special attention to those transporters belonging primarily to the MFS.

Oligomeric state of the oxalate transporter, OxlT

OxlT, the oxalate transporter of Oxalobacter formigenes, was studied to determine its oligomeric state in solution and in the membrane. Three independent approaches were used. First, we used triple-detector (SEC-LS) size exclusion chromatography to analyze purified OxlT in detergent/lipid micelles. These measurements evaluate protein mass in a manner independent of contributions from detergent and lipid; such work shows an average OxlT mass near 47 kDa for detergent-solubilized material, consistent with that expected for monomeric OxlT (46 kDa). A disulfide-linked OxlT mutant was used to verify that it was possible detect dimers under these conditions. A second approach used amino-reactive cross-linkers of varying spacer lengths to study OxlT in detergent/lipid micelles and in natural or artificial membranes, followed by analysis via sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These tests, performed under conditions where the presence of dimers can be documented for either of two known dimeric transporters (AdiC or TetL), indicate that OxlT exists as a monomer in the membrane and retains this status upon detergent solubilization. In a final test, we showed that reconstitution of OxlT into lipid vesicles at variable protein/lipid ratios has no effect on the specific activity of subsequent oxalate transport, as the OxlT content varies between 0.027 and 5.4 OxlT monomers/proteoliposome. We conclude that OxlT is a functional monomer in the membrane and in detergent/lipid micelles.

Oxalate efflux transporter from the brown rot fungus Fomitopsis palustris

An oxalate-fermenting brown rot fungus, Fomitopsis palustris, secretes large amounts of oxalic acid during wood decay. Secretion of oxalic acid is indispensable for the degradation of wood cell walls, but almost nothing is known about the transport mechanism by which oxalic acid is secreted from F. palustris hyphal cells. We characterized the mechanism for oxalate transport using membrane vesicles of F. palustris. Oxalate transport in F. palustris was ATP dependent and was strongly inhibited by several inhibitors, such as valinomycin and NH(4)(+), suggesting the presence of a secondary oxalate transporter in this fungus. We then isolated a cDNA, FpOAR (Fomitopsis palustris oxalic acid resistance), from F. palustris by functional screening of yeast transformants with cDNAs grown on oxalic acid-containing plates. FpOAR is predicted to be a membrane protein that possesses six transmembrane domains but shows no similarity with known oxalate transporters. The yeast transformant possessing FpOAR (FpOAR-transformant) acquired resistance to oxalic acid and contained less oxalate than the control transformant. Biochemical analyses using membrane vesicles of the FpOAR-transformant showed that the oxalate transport property of FpOAR was consistent with that observed in membrane vesicles of F. palustris. The quantity of FpOAR transcripts was correlated with increasing oxalic acid accumulation in the culture medium and was induced when exogenous oxalate was added to the medium. These results strongly suggest that FpOAR plays an important role in wood decay by acting as a secondary transporter responsible for secretion of oxalate by F. palustris.

Evidence for the transport of maltose by the sucrose permease, CscB, of Escherichia coli

The purpose of this study was to examine the sugar recognition and transport properties of the sucrose permease (CscB), a secondary active transporter from Escherichia coli. We tested the hypothesis that maltose transport is conferred by the wild-type CscB transporter. Cells of E. coli HS4006 harboring pSP72/cscB were red on maltose MacConkey agar indicator plates. We were able to measure "downhill" maltose transport and establish definitive kinetic behavior for maltose entry in such cells. Maltose was an effective competitor of sucrose transport in cells with CscB, suggesting that the respective maltose and sucrose binding sites and translocation pathways through the CscB channel overlap. Accumulation ("uphill" transport) of maltose by cells with CscB was profound, demonstrating active transport of maltose by CscB. Sequencing of cscB encoded on plasmid pSP72/cscB used in cells for transport studies indicate an unaltered primary CscB structure, ruling out the possibility that mutation conferred maltose transport by CscB. We conclude that maltose is a bona fide substrate for the sucrose permease of E. coli. Thus, future studies of sugar binding, transport, and permease structure should consider maltose, as well as sucrose.

The YvrI alternative sigma factor is essential for acid stress induction of oxalate decarboxylase in Bacillus subtilis

YvrI is a recently identified alternative sigma factor in Bacillus subtilis that requires the coactivator YvrHa to activate transcription. Previously, a strain engineered to overproduce YvrI was found to overproduce oxalate decarboxylase (OxdC), and further analysis identified three YvrI-activated promoters preceding the yvrI-yvrHa, yvrJ, and oxdC-yvrL operons. Independently, proteome analyses identified OxdC as a highly abundant, cell wall-associated protein that accumulated under acidic growth conditions. We show here that the accumulation of OxdC in the cell wall proteome under acidic growth conditions is absolutely dependent on YvrI and is correlated with enhanced transcription of both the yvrI-yvrHa and the oxdC-yvrL operons. Conversely, OxdC accumulates to a high level even under nonacidic growth conditions in cells lacking YvrL, a negative regulator of YvrI/YvrHa-dependent transcription. These results indicate that YvrI and its associated coregulators YvrHa and YvrL are required for the regulation of OxdC expression by acid stress. The high-level accumulation of OxdC depends, in part, on a strong oxdC promoter. A regulatory sequence with similarity to an upstream promoter element (UP) was identified upstream of the oxdC promoter and is required for high-level promoter activity. Conservation of the YvrI/YvrHa/YvrL regulatory system among related species allowed us to deduce an expanded consensus sequence for the compositionally unusual promoters recognized by this new sigma factor.

Cysteine scanning mutagenesis of TM5 reveals conformational changes in OxlT, the oxalate transporter of Oxalobacter formigenes

We constructed a single-cysteine panel encompassing TM5 of the oxalate transporter, OxlT. The 25 positions encompassed by TM5 were largely tolerant of mutagenesis, and functional product was recovered for 21 of the derived variants. For these derivatives, thiol-directed MTS-linked agents (MTSEA, MTSCE, and MTSES) were used as probes of transporter function, yielding 11 mutants that responded to probe treatment, as indicated by effects on oxalate transport. Further study identified three biochemical phenotypes among these responders. Group 1 included seven mutants, exemplified by G151C, displaying substrate protection against probe inhibition. Group 2 was comprised of a single mutant, P156C, which had unexpected behavior. In this case, we observed increased activity if weak acid/base or neutral probes were used, while exposure to probes introducing a fixed charge led to decreased function. In both instances, the presence of substrate prevented the observed response. Group 3 contained three mutants (e.g., S143C) in which probe sensitivity was increased by the presence of substrate. The finding of substrate-protectable probe modification in groups 1 and 2 suggests that TM5 lies on the permeation pathway, as do its structural counterparts, TM2, TM8, and TM11. In addition, we speculate that substrate binding facilitates TM5 conformational changes that allow new regions to become accessible to MTS-linked probes (group 3). These biochemical data are consistent with the recently developed OxlT homology model.

Ins and outs of major facilitator superfamily antiporters

The major facilitator superfamily (MFS) represents the largest group of secondary active membrane transporters, and its members transport a diverse range of substrates. Recent work shows that MFS antiporters, and perhaps all members of the MFS, share the same three-dimensional structure, consisting of two domains that surround a substrate translocation pore. The advent of crystal structures of three MFS antiporters sheds light on their fundamental mechanism; they operate via a single binding site, alternating-access mechanism that involves a rocker-switch type movement of the two halves of the protein. In the sn-glycerol-3-phosphate transporter (GlpT) from Escherichia coli, the substrate-binding site is formed by several charged residues and a histidine that can be protonated. Salt-bridge formation and breakage are involved in the conformational changes of the protein during transport. In this review, we attempt to give an account of a set of mechanistic principles that characterize all MFS antiporters.

Amino acids that confer transport of raffinose and maltose sugars in the raffinose permease (RafB) of Escherichia coli as implicated by spontaneous mutations at Val-35, Ser-138, Ser-139, Gly-389 and Ile-391

In order to identify amino acid residues in the Escherichia coli raffinose-H(+) permease (RafB) that play a role in sugar selection and transport, we first incubated E. coli HS4006 containing plasmid pRU600 (expresses inducible raffinose permease and alpha-galactosidase) on maltose MacConkey indicator plates overnight. Initially, all colonies were white, indicating no fermentation of maltose. Upon further incubation, 100 mutants appeared red. pRU600 DNA was prepared from 55 mutants. Five mutants transferred the phenotype for fermentation of maltose (red). Plasmid DNA from five maltose-positive phenotype transformants was prepared and sequenced, revealing three distinct types of mutations. Two mutants exhibited Val-35-->Ala (MT1); one mutant had Ile-391-->Ser (MT2); and two mutants had Ser-138-->Asp, Ser-139-->Leu and Gly-389-->Ala (MT3). Transport studies of [(3)H]-maltose showed that cells harboring MT1, MT2 and MT3 had greater uptake (P <or= 0.05) than cells harboring wild-type RafB. However, [(14)C]-raffinose uptake was reduced in all mutant cells (P <or= 0.05) with MT1, MT2 and MT3 mutants compared to cells harboring wild-type RafB. Kinetic analysis showed enhanced apparent K (m) values for maltose and reduced V (max)/ K (m) ratios for raffinose compared to wild-type values. The apparent K (i) value of maltose for RafB indicates a competitive relationship between maltose and raffinose. Maltose "uphill" accumulation was greater for mutants (P <or= 0.05) than for cells with wild-type RafB. Thus, we implicate residues in RafB that are responsible for raffinose transport and suggest that the substituted residues in RafB dictate structures that enhance transport of maltose.

The Phosphorus Source Phytate Changes the Composition of the Cell Wall Proteome in Bacillus subtilis

Phytate is the most abundant phosphorus source in plants. Since Bacillus subtilis is a soil-dwelling bacterium, the focus of this study was to investigate whether it can use phytate as a phosphorus source. The extracellular proteome analysis revealed that phytate is an alternative phosphorus source to overcome the phosphate starvation response in B. subtilis. However, the phytase was not induced neither under phosphate starvation conditions nor by phytate addition. Surprisingly, the proteome analyses demonstrated a re-distribution of the major cell wall protease WprA from the cell wall to the extracellular medium in phytate-supplemented medium. In contrast, several cell wall proteins such as autolysins and autolysin modifier proteins (e.g., LytB, -C, -D, -E, -F) are increased in the cell wall proteome in response to phytate which is not accompanied by increased transcription of the corresponding genes. These effects of phytate on the composition of the B. subtilis cell wall proteome do not depend on the acidic conditions, the increased sodium ion concentration, and the increased cell lysis. In addition, the previously predicted as cytoplasmic protein oxalate decarboxylase OxdC was identified as the most abundant cell wall protein which was induced at the transcriptional level due to the acidic conditions caused by phytate.

A bacterial arginine-agmatine exchange transporter involved in extreme acid resistance

The arginine-dependent extreme acid resistance response of Escherichia coli operates by decarboxylating arginine. AdiC, a membrane antiporter, catalyzes arginine influx coupled to efflux of the decarboxylation product agmatine, effectively exporting a proton in each turnover. Using the adiC coding sequence under control of a tetracycline promoter in an E. coli vector, we expressed and purified the transport-protein with a yield of approximately 10 mg/liter bacterial culture. Glutaraldehyde cross-linking experiments indicate that the protein is a homodimer in detergent micelles and lipid membranes. Purified AdiC reconstituted into liposomes exchanges arginine and agmatine in a strictly coupled, electrogenic fashion. Kinetic analysis yields K(m) approximately 80 microm for Arg, in the same range as its dissociation constant determined by isothermal titration calorimetry.

A suppressor analysis of residues involved in cation transport in the lactose permease: identification of a coupling sensor

Four amino acids critical for lactose permease function were altered using site-directed mutagenesis. The resulting Quad mutant (E269Q/R302L/H322Q/E325Q) was expressed at 60% of wild-type levels but found to have negligible transport activity. The Quad mutant was used as a parental strain to isolate suppressors that regained the ability to ferment the alpha-galactoside melibiose. Six different suppressors were identified involving five discrete amino acid changes and one amino acid deletion (Q60L, V229G, Y236D, S306L, K319N and DeltaI298). All of the suppressors transported alpha-galactosides at substantial rates. In addition, the Q60L, DeltaI298 and K319N suppressors regained a small but detectable amount of lactose transport. Assays of sugar-driven cation transport showed that both the Q60L and K319N suppressors couple the influx of melibiose with cations (H(+) or H(3)O(+)). Taken together, the data show that the cation-binding domain in the lactose permease is not a fixed structure as proposed in previous models. Rather, the data are consistent with a model in which several ionizable residues form a dynamic coupling sensor that also may interact directly with the cation and lactose.

Separate ion pathways in a Cl-/H+ exchanger

CLC-ec1 is a prokaryotic CLC-type Cl⁻/H⁺ exchange transporter. Little is known about the mechanism of H⁺ coupling to Cl⁻. A critical glutamate residue, E148, was previously shown to be required for Cl⁻/H⁺ exchange by mediating proton transfer between the protein and the extracellular solution. To test whether an analogous H⁺ acceptor exists near the intracellular side of the protein, we performed a mutagenesis scan of inward-facing carboxyl-bearing residues and identified E203 as the unique residue whose neutralization abolishes H⁺ coupling to Cl⁻ transport. Glutamate at this position is strictly conserved in all known CLCs of the transporter subclass, while valine is always found here in CLC channels. The x-ray crystal structure of the E203Q mutant is similar to that of the wild-type protein. Cl⁻ transport rate in E203Q is inhibited at neutral pH, and the double mutant, E148A/E203Q, shows maximal Cl⁻ transport, independent of pH, as does the single mutant E148A. The results argue that substrate exchange by CLC-ec1 involves two separate but partially overlapping permeation pathways, one for Cl⁻ and one for H⁺. These pathways are congruent from the protein's extracellular surface to E148, and they diverge beyond this point toward the intracellular side. This picture demands a transport mechanism fundamentally different from familiar alternating-access schemes.

Experimental tests of a homology model for OxlT, the oxalate transporter of Oxalobacter formigenes

Using the x-ray structure of the glycerol 3-phosphate transporter (GlpT), we devised a model for the distantly related oxalate transporter, OxlT. The model accommodates all earlier biochemical information on OxlT, including the idea that Lys-355 lies on the permeation pathway, and predicts that Lys-355 and a second positive center, Arg-272, comprise the binding site for divalent oxalate. Study of R272K, R272A, and R272Q derivatives verifies that Arg-272 is essential, and comparisons with GlpT show that both anion transporters bind substrates within equivalent domains. In 22 single-cysteine variants in TM7 and TM8, topology as marked by accessibility to Oregon green maleimide is predicted by the model, with similar concordance for 52 positions probed earlier. The model also reconciles cross-linking of a cysteine pair placed near the periplasmic ends of TM2 and TM7, and retrospective study of TM2 and TM11 confirms that positions supporting disulfide trapping lie at a helical interface. Our work describes a pathway to the modeling of OxlT and other transporters in the major facilitator superfamily and outlines simple experimental tests to evaluate such proposals.

Membrane topology of the electrogenic aspartate-alanine antiporter AspT of Tetragenococcus halophilus

AspT is an electrogenic aspartate:alanine exchange protein that represents the vectorial component of a proton-motive metabolic cycle found in some strains of Tetragenococcus halophilus. AspT is the sole member of a new family, the Aspartate: Alanine Exchanger (AAE) family, in secondary transporters, according to the computational classification proposed by Saier et al. (http://www.biology.ucsd.edu/~msaier/transport/). We analyzed the topology of AspT biochemically, by using fusion methods in combination with alkaline phosphatase or beta-lactamase. These results suggested that AspT has a unique topology; 8 TMS, a large cytoplasmic loop (183 amino acids) between TMS5 and TMS6, and N- and C-termini that both face the periplasm. These results demonstrated a unique 2D-structure of AspT as the novel AAE family.

The structural basis of substrate translocation by the Escherichia coli glycerol-3-phosphate transporter: a member of the major facilitator superfamily

The major facilitator superfamily represents the largest group of secondary active membrane transporters in the cell. The 3.3A resolution structure of a member of this protein superfamily, the glycerol-3-phosphate transporter from the Escherichia coli inner membrane, reveals two domains connected by a long central loop. These N- and C-terminal domains, each containing a six-helix bundle, are related by pseudo-twofold symmetry. A substrate translocation pore is located between the two domains and is open to the cytoplasm. Two arginines at the closed end of the pore comprise the substrate-binding site. Biochemical experiments show that, upon substrate binding, the protein adopts a more compact conformation. The crystal structure suggests that the transporter operates through a single binding site, alternating access mechanism via a rocker-switch type of movement of the N- and C-terminal domains. The structure and mechanism of the glycerol-3-phosphate transporter form a paradigm for other members of the major facilitator superfamily.

A Closed Conformation of Bacillus subtilis Oxalate Decarboxylase OxdC Provides Evidence for the True Identity of the Active Site

Oxalate decarboxylase (EC 4.1.1.2) catalyzes the conversion of oxalate to formate and carbon dioxide and utilizes dioxygen as a cofactor. By contrast, the evolutionarily related oxalate oxidase (EC 1.2.3.4) converts oxalate and dioxygen to carbon dioxide and hydrogen peroxide. Divergent free radical catalytic mechanisms have been proposed for these enzymes that involve the requirement of an active site proton donor in the decarboxylase but not the oxidase reaction. The oxidase possesses only one domain and manganese binding site per subunit, while the decarboxylase has two domains and two manganese sites per subunit. A structure of the decarboxylase together with a limited mutagenesis study has recently been interpreted as evidence that the C-terminal domain manganese binding site (site 2) is the catalytic site and that Glu-333 is the crucial proton donor (Anand, R., Dorrestein, P. C., Kinsland, C., Begley, T. P., and Ealick, S. E. (2002) Biochemistry 41, 7659-7669). The N-terminal binding site (site 1) of this structure is solvent-exposed (open) and lacks a suitable proton donor for the decarboxylase reaction. We report a new structure of the decarboxylase that shows a loop containing a 3(10) helix near site 1 in an alternative conformation. This loop adopts a "closed" conformation forming a lid covering the entrance to site 1. This conformational change brings Glu-162 close to the manganese ion, making it a new candidate for the crucial proton donor. Site-directed mutagenesis of equivalent residues in each domain provides evidence that Glu-162 performs this vital role and that the N-terminal domain is either the sole or the dominant catalytically active domain.

Active site geometry of oxalate decarboxylase from Flammulina velutipes: Role of histidine-coordinated manganese in substrate recognition

Oxalate decarboxylase (OXDC) from the wood-rotting fungus Flammulina velutipes, which catalyzes the conversion of oxalate to formic acid and CO(2) in a single-step reaction, is a duplicated double-domain germin family enzyme. It has agricultural as well as therapeutic importance. We reported earlier the purification and molecular cloning of OXDC. Knowledge-based modeling of the enzyme reveals a beta-barrel core in each of the two domains organized in the hexameric state. A cluster of three histidines suitably juxtaposed to coordinate a divalent metal ion exists in both the domains. Involvement of the two histidine clusters in the catalytic mechanism of the enzyme, possibly through coordination of a metal cofactor, has been hypothesized because all histidine knockout mutants showed total loss of decarboxylase activity. The atomic absorption spectroscopy analysis showed that OXDC contains Mn(2+) at up to 2.5 atoms per subunit. Docking of the oxalate in the active site indicates a similar electrostatic environment around the substrate-binding site in the two domains. We suggest that the histidine coordinated manganese is critical for substrate recognition and is directly involved in the catalysis of the enzyme.

Pyridoxal 5-phosphate inhibition of substrate selectivity mutants of UhpT, the sugar 6-phosphate carrier of Escherichia coli

In the sugar phosphate transporter UhpT, gain-of-function derivatives that prefer phosphoenolpyruvate (PEP) as substrate have an uncompensated lysine residue on transmembrane segment 11. We show here that these variants are also highly susceptible to substrate-protectable inhibition by covalent modification of lysine with pyridoxal 5-phosphate. The chemical requirements of this interaction provide evidence that the gain-of-function phenotype results from the pairing of the uncompensated lysines in these mutants with the anionic carboxyl group of PEP.

Structure/function relationships in OxlT, the oxalate/formate antiporter of Oxalobacter formigenes: assignment of transmembrane helix 2 to the translocation pathway

We constructed a single cysteine panel encompassing transmembrane helix two (TM2) of OxlT, the oxalate/formate antiporter of Oxalobacter formigenes. Among the 21 positions targeted, cysteine substitution identified one (phenylalanine 59) as essential to OxlT expression and three (glutamine 56, glutamine 66, and serine 69) as potentially critical to OxlT function. By probing membranes with a bulky hydrophilic probe (Oregon Green maleimide) we also located a central inaccessible core of at least eight residues in length, extending from leucine 61 to glycine 68. Functional assays based on reconstitution of crude detergent extracts showed that of single cysteine mutants within the TM2 core only the Q63C variant was substantially (> or =95%) inhibited by thiol-specific agents (carboxyethyl methanethiosulfonate and ethylsulfonate methanethiosulfonate). Subsequent analytical work using the purified Q63C protein showed that inhibition by ethylsulfonate methanethiosulfonate was blocked by substrate and that the concentration dependence of such substrate protection occurred with a binding constant of 0.16 mm oxalate, comparable with the Michaelis constant observed for oxalate transport (0.23 mm). These findings lead us to conclude that position 63 lies on the OxlT translocation pathway. Our conclusion is strengthened by the finding that position 63, along with most other positions relevant to TM2 function, is found on a helical face that can be cross-linked to the pathway-facing surface of TM11 (Fu, D., Sarker, R. I., Bolton, E., and Maloney, P. C. (2001) J. Biol. Chem. 276, 8753-8760).

Plasmid-encoded asp operon confers a proton motive metabolic cycle catalyzed by an aspartate-alanine exchange reaction

Tetragenococcus halophila D10 catalyzes the decarboxylation of L-aspartate with nearly stoichiometric release of L-alanine and CO(2). This trait is encoded on a 25-kb plasmid, pD1. We found in this plasmid a putative asp operon consisting of two genes, which we designated aspD and aspT, encoding an L-aspartate-beta-decarboxylase (AspD) and an aspartate-alanine antiporter (AspT), respectively, and determined the nucleotide sequences. The sequence analysis revealed that the genes of the asp operon in pD1 were in the following order: promoter --> aspD --> aspT. The deduced amino acid sequence of AspD showed similarity to the sequences of two known L-aspartate-beta-decarboxylases from Pseudomonas dacunhae and Alcaligenes faecalis. Hydropathy analyses suggested that the aspT gene product encodes a hydrophobic protein with multiple membrane-spanning regions. The operon was subcloned into the Escherichia coli expression vector pTrc99A, and the two genes were cotranscribed in the resulting plasmid, pTrcAsp. Expression of the asp operon in E. coli coincided with appearance of the capacity to catalyze the decarboxylation of aspartate to alanine. Histidine-tagged AspD (AspDHis) was also expressed in E. coli and purified from cell extracts. The purified AspDHis clearly exhibited activity of L-aspartate-beta-decarboxylase. Recombinant AspT was solubilized from E. coli membranes and reconstituted in proteoliposomes. The reconstituted AspT catalyzed self-exchange of aspartate and electrogenic heterologous exchange of aspartate with alanine. Thus, the asp operon confers a proton motive metabolic cycle consisting of the electrogenic aspartate-alanine antiporter and the aspartate decarboxylase, which keeps intracellular levels of alanine, the countersubstrate for aspartate, high.

Transmembrane segment 11 of UhpT, the sugar phosphate carrier of Escherichia coli, is an alpha-helix that carries determinants of substrate selectivity

In Escherichia coli, transport of hexose 6-phosphates is mediated by the P(i)-linked antiport carrier, UhpT, a member of the major facilitator superfamily. We showed earlier that Lys(391), a member of an intrahelical salt bridge (Asp(388)/Lys(391)) in the eleventh transmembrane segment (TM11) of this transporter, can function as a determinant of substrate selectivity (Hall, J. A., Fann, M.-C., and Maloney, P. C. (1999) J. Biol. Chem. 274, 6148-6153). Here, we examine in detail the role of TM11 in setting substrate preference. Derivatives having an uncompensated cationic charge at either position 388 or 391 (the D388C, D388V, or D388K/K391C variants) are gain-of-function mutants in which phosphoenolpyruvate, not sugar 6-phosphate, is the preferred organic substrate. By contrast, when an uncompensated anionic charge is placed at position 388 (K391C), we observed behavior consistent with an increased preference for monovalent rather than divalent sugar 6-phosphate. Because positions 388 and 391 lie deep within the UhpT hydrophobic sector, these findings suggested that an extended length of TM11 may be accessible to external substrates and probes. To explore this issue, we used a panel of TM11 single cysteine variants to examine the transport of glucose 6-phosphate in the presence and absence of the membrane-impermeant, thiol-reactive agent p-chloromercuribenzosulfonate (PCMBS). Accessibility to PCMBS, together with the pattern of substrate protection against PCMBS inhibition, leads us to conclude that TM11 spans the membrane as an alpha-helix, with approximately two-thirds of its surface lining a substrate translocation pathway. We suggest that this feature is a general property of carrier proteins in the major facilitator superfamily and that for this reason residues in TM11 will serve to carry determinants of substrate selectivity.

Topology of OxlT, the oxalate transporter of Oxalobacter formigenes, determined by site-directed fluorescence labeling

The topology of OxlT, the oxalate:formate exchange protein of Oxalobacter formigenes, was established by site-directed fluorescence labeling, a simple strategy that generates topological information in the context of the intact protein. Accessibility of cysteine to the fluorescent thiol-directed probe Oregon green maleimide (OGM) was examined for a panel of 34 single-cysteine variants, each generated in a His(9)-tagged cysteine-less host. The reaction with OGM was readily scored by examining the fluorescence profile after sodium dodecyl sulfate-polyacrylamide gel electrophoresis of material purified by Ni2+ linked affinity chromatography. A position was assigned an external location if its single-cysteine derivative reacted with OGM added to intact cells; a position was designated internal if OGM labeling required cell lysis. We also showed that labeling of external, but not internal, positions was blocked by prior exposure of cells to the impermeable and nonfluorescent thiol-specific agent ethyltrimethylammonium methanethiosulfonate. Of the 34 positions examined in this way, 29 were assigned unambiguously to either an internal or external location; 5 positions could not be assigned, since the target cysteine failed to react with OGM. There was no evidence of false-positive assignment. Our findings document a simple and rapid method for establishing the topology of a membrane protein and show that OxlT has 12 transmembrane segments, confirming inferences from hydropathy analysis.

Structure/function relationships in OxlT, the oxalate-formate transporter of oxalobacter formigenes. Assignment of transmembrane helix 11 to the translocation pathway

OxlT, the oxalate:formate antiporter of Oxalobacter formigenes, has a lone charged residue, lysine 355 (Lys-355), at the center of transmembrane helix 11 (TM11). Because Lys-355 is the only charged residue in the hydrophobic sector, we tested the hypothesis that lysine 355 contributes to the binding site for the anionic substrate, oxalate. This idea was supported by mutational analysis, which showed that of five variants studied (Lys-355 --> Cys, Gly, Gln, Arg, or Thr), residual function was found for only the K355R derivative, in which catalytic efficiency had fallen 2,600-fold. Further insight came from a study of TM11 single-cysteine mutants, using the impermeant, thiol-specific reagents, carboxyethyl methanethiosulfonate and ethyltrimethylammonium methanethiosulfonate. Of the five reactive positions identified in TM11, four were at the cytoplasmic or periplasmic ends of TM11 (S344C and A345C, and G366C and A370C, respectively), whereas the fifth was at the center of the helix (S359C). Added study with carboxyethyl methanethiosulfonate and ethylsulfonate methylthiosulfonate showed that the attack on S359C could be blocked by the presence of the substrate, oxalate, and that protection could be predicted quantitatively by a kinetic model in which S359C is accessible only in the unliganded form of OxlT. Parallel study showed that the proteoliposomes used in such work contained OxlT of right side-out and inside-out orientations in about equal amounts. Accordingly, full inhibition of S359C by the impermeable methanethiosulfonate-linked probes must reflect an approach from both the cytosolic and periplasmic surfaces of the protein. This, coupled with the finding of substrate protection, leads us to conclude that S359C lies on the translocation pathway through OxlT. Since position 359 and 355 lie on the same helical face, we suggest that Lys-355 also lies on the translocation pathway, consistent with the idea that the essential nature of Lys-355 reflects its role in binding the anionic substrate, oxalate.

Sugar recognition mutants of the melibiose carrier of Escherichia coli: possible structural information concerning the arrangement of membrane-bound helices and sugar/cation recognition site

Melibiose carrier mutants, isolated by growing cells on melibiose plus the non-metabolizable competitive inhibitor thiomethyl-beta-galactoside (TMG), were studied to determine sugar and cation recognition abnormalities. Most of the mutants show good transport of melibiose but have lost the recognition of TMG. In addition, most mutants show little or no transport of lactose. Cation recognition is also affected as all of these mutants have lost the ability to transport protons with melibiose. The amino acids causing these mutations were determined by sequencing the melB gene on the plasmid. The mutations were located on helices I, IV, VII, X and XI. We propose that these five helices are in proximity with each other and that they line the sugar/cation transport channel.

Bacillus subtilis YvrK is an acid-induced oxalate decarboxylase

Bacillus subtilis has been shown to express a cytosolic oxalate decarboxylase (EC 4.1.1.2). The enzyme was induced in acidic growth media, particularly at pH 5.0, but not by oxalate. The enzyme was purified, and N-terminal sequencing identified the protein to be encoded by yvrK. The role of the first oxalate decarboxylase to be identified in a prokaryote is discussed.

Tn917-lac mutagenesis of Streptococcus mutans to identify environmentally regulated genes

Previously, we demonstrated successful Tn917 mutagenesis of the oral pathogen Streptococcus mutans using pTV1-OK (Km(r), repATs), a temperature conditional replicative delivery vector carrying a lactococcal pWVO1Ts backbone. In this report we describe the construction and utilization of pTV32-OK, a plasmid harboring Tn917-lac (em(r), beta-gal(+)) that was employed to isolate transcriptional fusions of the Escherichia coli lacZ reporter gene with streptococcal promoters in S. mutans strain NG8. Tn917-lac transposition occurred at a frequency of ca. 10(-6) with 20% of the resultant em(r) clones displaying varying levels of lacZ expression. Tn917-lac mutants that expressed beta-galactosidase activity under growth conditions of glucose limitation, acidic pH, 35 mM NaCl, and elevated (42 degrees C) temperature were isolated. Further characterization of one of the mutants with increased beta-gal activity under glucose limitation, strain AS42, revealed maximal activity in batch culture in stationary phase after glucose depletion. The beta-gal activity of AS42 also was found to be repressed 3-fold in medium containing 2% glucose relative to measured activity from cells suspended in the same medium containing no glucose. Further phenotypic analysis revealed that AS42 had a 30% lower growth yield than the parent strain NG8 when grown in pH 5 medium. Sequence analysis of the region harboring the transposon revealed that the lacZ fusion occurred near the 3'-end of a gene encoding a homolog of an ATP binding protein from a family of Gram-positive ABC transporters. These findings demonstrate that Tn917-lac mutagenesis can be used to identify environmentally regulated genes in S. mutans and possibly in other medically relevant streptococcal species.

TRAP transporters: an ancient family of extracytoplasmic solute-receptor-dependent secondary active transporters

Tripartite ATP-independent periplasmic transporters (TRAP-T) represent a novel type of secondary active transporter that functions in conjunction with an extracytoplasmic solute-binding receptor. The best characterized TRAP-T family member is from Rhodobacter capsulatus and is specific for C4-dicarboxylates [Forward, J. A., Behrendt, M. C., Wyborn, N. R., Cross, R. & Kelly, D. J. (1997). J Bacteriol 179, 5482-5493]. It consists of three essential proteins, DctP, a periplasmic C4-dicarboxylate-binding receptor, and two integral membrane proteins, DctM and DctQ, which probably span the membrane 12 and 4 times, respectively. Homologues of DctM, DctP and DctQ were identified in all major bacterial subdivisions as well as in archaea. An orphan DctP homologue in the Gram-positive bacterium Bacillus subtilis may serve as a receptor for a two-component transcriptional regulatory system rather than as a constituent of a TRAP-T system. Phylogenetic data suggest that all present day TRAP-T systems probably evolved from a single ancestral transporter with minimal shuffling of constituents between systems. Homologous TRAP-T constituents exhibit decreasing degrees of sequence identity in the order DctM > DctP > DctQ. DctM appears to belong to a large superfamily of transporters, the ion transporter (IT) superfamily, one member of which can function by either protonmotive force- or ATP-dependent energization. It is proposed that IT superfamily members exhibit the unusual capacity to function in conjunction with auxiliary proteins that modify the transport process by providing (i) high-affinity solute reception, (ii) altered energy coupling and (iii) additional yet to be defined functions.

Arg-52 in the melibiose carrier of Escherichia coli is important for cation-coupled sugar transport and participates in an intrahelical salt bridge

Arg-52 of the Escherichia coli melibiose carrier was replaced by Ser (R52S), Gln (R52Q), or Val (R52V). While the level of carrier in the membrane for each mutant remained similar to that for the wild type, analysis of melibiose transport showed an uncoupling of proton cotransport and a drastic reduction in Na(+)-coupled transport. Second-site revertants were selected on MacConkey plates containing melibiose, and substitutions were found at nine distinct locations in the carrier. Eight revertant substitutions were isolated from the R52S strain: Asp-19-->Gly, Asp-55-->Asn, Pro-60-->Gln, Trp-116-->Arg, Asn-244-->Ser, Ser-247-->Arg, Asn-248-->Lys, and Ile-352-->Val. Two revertants were also isolated from the R52V strain: Trp-116-->Arg and Thr-338-->Arg revertants. The R52Q strain yielded an Asp-55-->Asn substitution and a first-site revertant, Lys-52 (R52K). The R52K strain had transport properties similar to those of the wild type. Analysis of melibiose accumulation showed that proton-driven accumulation was still defective in the second-site revertant strains, and only the Trp-116-->Arg, Ser-247-->Arg, and Asn-248-->Lys revertants regained significant Na(+)-coupled accumulation. In general, downhill melibiose transport in the presence of Na(+) was better in the revertant strains than in the parental mutants. Three revertant strains, Asp-19-->Gly, Asp-55-->Asn, and Thr-338-->Arg strains, required a high Na(+) concentration (100 mM) for maximal activity. Kinetic measurements showed that the N248K and W116R revertants lowered the K(m) for melibiose, while other revertants restored transport velocity. We suggest that the insertion of positive charges on membrane helices is compensating for the loss of Arg-52 and that helix II is close to helix IV and VII. We also suggest that Arg-52 is salt bridged to Asp-55 (helix II) and Asp-19 (helix I).

Structure-function relationships in OxlT, the oxalate/formate transporter of Oxalobacter formigenes. Topological features of transmembrane helix 11 as visualized by site-directed fluorescent labeling

Analysis of hydropathy suggests that in OxlT, the oxalate/formate antiporter of Oxalobacter formigenes, lysine 355 is within transmembrane helix no. 11. To test this idea, we used single-cysteine, histidine-tagged OxlT variants to study the organization of a 30-residue segment (residues 344-373) containing this region. Topology was examined by probing the A345C and A370C proteins with Oregon Green maleimide carboxylic acid, an impermeant and fluorescent thiol-reactive agent. Examination of purified protein showed that only A370C was fluorescent after treating intact cells with the probe, while both proteins were modified in tests with isolated membrane ghosts. In addition, labeling of A370C, but not A345C, was blocked when external cysteines were protected with the impermeant and nonfluorescent agent, methanethiosulfonate ethyltrimethylammonium. These findings confirm that A345 faces the cytoplasm, while A370C faces the periplasm. A similar study focused on 13 single-cysteine variants positioned throughout the target segment. That work revealed a striking discontinuity in reactivity toward Oregon Green maleimide; cysteines within a 10-residue central core (residues 351-360) were not labeled when membranes were probed, but were readily modified after protein denaturation. We suggest this core resides within the lipid bilayer, unavailable to an impermeant reporter. Since this region includes position 355, we also suggest that lysine 355 lies within the OxlT hydrophobic sector, where it may facilitate the binding and translocation of the anionic substrates, oxalate and formate.

Identification of an Na+-dependent malonate transporter of Malonomonas rubra and its dependence on two separate genes

Two membrane proteins encoded by the malonate fermentation gene cluster of Malonomonas rubra, MadL and MadM, have been synthesized in Escherichia coli. MadL and MadM were shown to function together as a malonate transport system, whereas each protein alone was unable to catalyze malonate transport. Malonate transport by MadLM is Na+ dependent, and imposition of a DeltapNa+ markedly enhanced the rate of malonate uptake. The kinetics of malonate uptake into E. coli BL21(DE3) cells synthesizing MadLM at different pH values indicated that Hmalonate- is the transported malonate species. The stimulation of malonate uptake by Na+ ions showed Michaelis-Menten kinetics, and a Km for Na+ of 1.2 mM was determined. These results suggest that MadLM is an electroneutral Na+/Hmalonate- symporter and that it is dependent on two separate genes.

Lactose carrier mutants of Escherichia coli with changes in sugar recognition (lactose versus melibiose)

The purpose of this research was to identify amino acid residues that mediate substrate recognition in the lactose carrier of Escherichia coli. The lactose carrier transports the alpha-galactoside sugar melibiose as well as the beta-galactoside sugar lactose. Mutants from cells containing the lac genes on an F factor were selected by the ability to grow on succinate in the presence of the toxic galactoside beta-thio-o-nitrophenylgalactoside. Mutants that grew on melibiose minimal plates but failed to grow on lactose minimal plates were picked. In sugar transport assays, mutant cells showed the striking result of having low levels of lactose downhill transport but high levels of melibiose downhill transport. Accumulation (uphill) of melibiose was completely defective in all of the mutants. Kinetic analysis of melibiose transport in the mutants showed either no change or a greater than normal apparent affinity for melibiose. PCR was used to amplify the lacY DNA of each mutant, which was then sequenced by the Sanger method. The following six mutations were found in the lacY structural genes of individual mutants: Tyr-26-->Asp, Phe-27-->Tyr, Phe-29-->Leu, Asp-240-->Val, Leu-321-->Gln, and His-322-->Tyr. We conclude from these experiments that Tyr-26, Phe-27, Phe-29 (helix 1), Asp-240 (helix 7), Leu-321, and His-322 (helix 10) either directly or indirectly mediate sugar recognition in the lactose carrier of E. coli.

AP1-mediated multidrug resistance in Saccharomyces cerevisiae requires FLR1 encoding a transporter of the major facilitator superfamily

We have isolated a Candida albicans gene that confers resistance to the azole derivative fluconazole (FCZ) when overexpressed in Saccharomyces cerevisiae. This gene encodes a protein highly homologous to S. cerevisiae yAP-1, a bZip transcription factor known to mediate cellular resistance to toxicants such as cycloheximide (CYH), 4-nitroquinoline N-oxide (4-NQO), cadmium, and hydrogen peroxide. The gene was named CAP1, for C. albicans AP-1. Cap1 and yAP-1 are functional homologues, since CAP1 expression in a yap1 mutant strain partially restores the ability of the cells to grow on toxic concentrations of cadmium or hydrogen peroxide. We have found that the expression of YBR008c, an open reading frame identified in the yeast genome sequencing project and predicted to code for a multidrug transporter of the major facilitator superfamily, is dramatically induced in S. cerevisiae cells overexpressing CAP1. Overexpression of either CAP1 or YAP1 in a wild-type strain results in resistance to FCZ, CYH, and 4-NQO, whereas such resistance is completely abrogated (FCZ and CYH) or strongly reduced (4-NQO) in a ybr008c deletion mutant, demonstrating that YBR008c is involved in YAP1- and CAP1-mediated multidrug resistance. YBR008c has been renamed FLR1, for fluconazole resistance 1. The expression of an FLR1-lacZ reporter construct is strongly induced by the overexpression of either CAP1 or YAP1, indicating that the FLR1 gene is transcriptionally regulated by the Cap1 and yAP-1 proteins. Taken collectively, our results demonstrate that FLR1 represents a new YAP1-controlled multidrug resistance molecular determinant in S. cerevisiae. A similar detoxification pathway is also likely to operate in C. albicans.

Purification of UhpT, the sugar phosphate transporter of Escherichia coli

To purify UhpT, the sugar phosphate carrier of Escherichia coli, we constructed a variant (HisUhpT) in which 10 tandem histidine residues were placed at the UhpT N terminus and then used Ni(2+)-agarose affinity chromatography of detergent-solubilized proteins. Membrane vesicles from a strain overexpressing His-UhpT were extracted at pH 7.4 with either 1.5% n-octyl-beta-D-glucopyranoside (octylglucoside) or 1.5% n-dodecyl-beta-D-maltoside (dodecylmaltoside) in 200 mM sodium chloride, 100 mM potassium phosphate, 50 mM glucose 6-phosphate, 10-20% glycerol, 0.2% E. coli phospholipid, and 5 mM beta-mercaptoethanol. After the detergent extract was applied to a Ni(2+)-agarose column, nonspecifically bound material was removed by washing at pH 7 with the same buffer also containing 50 mM imidazole. Purified HisUhpT was released subsequently, when sodium chloride was replaced with 300 mM imidazole or 100 mM EDTA, giving an overall yield of about 25 micrograms HisUhpT/mg vesicle protein. Whether eluted by imidazole or EDTA in either octylglucoside or dodecylmaltoside, purified HisUhpT showed a specific activity of 2.5-3 mumol/min per milligram of protein as monitored by [14C]glucose 6-phosphate transport by proteoliposomes loaded with 100 mM potassium phosphate. This corresponded to a calculated turnover number near 20 s-1 for the heterologous exchange of external sugar phosphate with internal phosphate. At low temperature (4 degrees C) HisUhpT retained full activity in either octylglucoside or dodecylmaltoside; however, at elevated temperature (> or = 23 degrees C), the protein displayed a marked lability in octylglucoside (t1/2 = 11 min), but not in dodecylmaltoside (t1/2 > or = 200-300 min).

Exchange of glutamate and gamma-aminobutyrate in a Lactobacillus strain

Lactobacillus sp. strain E1 catalyzed the decarboxylation of glutamate (Glu), resulting in a nearly stoichiometric release of the products gamma-aminobutyrate (GABA) and CO2. This decarboxylation was associated with the net synthesis of ATP. ATP synthesis was inhibited almost completely by nigericin and about 70% by N,N'-dicyclohexylcarbodiimide (DCCD), without inhibition of the decarboxylation. These findings are consistent with the possibility that a proton motive force arises from the cytoplasmic proton consumption that accompanies glutamate decarboxylation and the electrogenic Glu/GABA antiporter and the possibility that this proton motive force is coupled with ATP synthesis by DCCD-sensitive ATPase.

Multiple ionic conductances of the human dopamine transporter: the actions of dopamine and psychostimulants

Electrophysiological and pharmacological studies of a cloned human dopamine transporter (hDAT) were undertaken to investigate the mechanisms of transporter function and the actions of drugs at this target. Using two-electrode voltage-clamp techniques with hDAT-expressing Xenopus laevis oocytes, we show that hDAT can be considered electrogenic by two criteria. (1) Uptake of hDAT substrates gives rise to a pharmacologically appropriate "transport-associated" current. (2) The velocity of DA uptake measured in oocytes clamped at various membrane potentials was voltage-dependent, increasing with hyperpolarization. Concurrent measurement of transport-associated current and substrate flux in individual oocytes revealed that charge movement during substrate translocation was greater than would be expected for a transport mechanism with fixed stoichiometry of 2 Na+ and 1 Cl- per DA+ molecule. In addition to the transport-associated current, hDAT also mediates a constitutive leak current, the voltage and ionic dependencies of which differ markedly from those of the transport-associated current. Ion substitution experiments suggest that alkali cations and protons are carried by the hDAT leak conductance. In contrast to the transport-associated functions, the leak does not require Na+ or Cl-, and DAT ligands readily interact with the transporter even in the absence of these ions. The currents that hDAT mediates provide a functional assay that readily distinguishes the modes of action of amphetamine-like "DA-releasing" drugs from cocaine-like translocation blockers. In addition, the voltage dependence of DA uptake suggests a mechanism through which presynaptic DA autoreceptor activation may accelerate the termination of dopaminergic neurotransmission in vivo.