Membrane topology of the melibiose carrier of Escherichia coli

The substitution of Arg149 with Cys fixes the melibiose transporter in an inward-open conformation

The melibiose transporter from Escherichia coli (MelB) can use the electrochemical energy of either H(+), Na(+) or Li(+) to transport the disaccharide melibiose to the cell interior. By using spectroscopic and biochemical methods, we have analyzed the role of Arg149 by mutagenesis. According to Fourier transform infrared difference and fluorescence spectroscopy studies, R149C, R149Q and R149K all bind substrates in proteoliposomes, where the protein is disposed inside-out. Analysis of right-side-out (RSO) and inside-out (ISO) membrane vesicles showed that the functionally active R149Q and R149K mutants could bind externally added fluorescent sugar analog in both types of vesicles. In contrast, the non-transporting R149C mutant does bind the fluorescent sugar analog as well as melibiose and Na(+) in ISO, but not in RSO vesicles. Therefore, the mutation of Arg149 into cysteine restrains the orientation of transporter to an inward-open conformation, with the inherent consequences of a) reducing the frequency of access of outer substrates to the binding sites, and b) impairing active transport. It is concluded that Arg149, most likely located in the inner (cytoplasmic) half of transmembrane helix 5, is critically involved in the reorientation mechanism of the substrate-binding site accessibility in MelB.

G117C MelB, a mutant melibiose permease with a changed conformational equilibrium

Replacement of the glycine at position 117 by a cysteine in the melibiose permease creates an interesting phenotype: while the mutant transporter shows still transport activity comparable to the wild type its pre steady-state kinetic properties are drastically altered. The transient charge displacements after substrate concentration jumps are strongly reduced and the fluorescence changes disappear. Together with its maintained transport activity this indicates that substrate translocation in G117C melibiose permease is not impaired but that the initial conformation of the mutant transporter differs from that of the wild type permease. A kinetic model for the G117C melibiose permease based on a rapid dynamic equilibrium of the substrate free transporter is proposed. Implications of the kinetic model for the transport mechanism of the wild type permease are discussed.

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

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

Structural insights into the activation mechanism of melibiose permease by sodium binding

The melibiose carrier from Escherichia coli (MelB) couples the accumulation of the disaccharide melibiose to the downhill entry of H(+), Na(+), or Li(+). In this work, substrate-induced FTIR difference spectroscopy was used in combination with fluorescence spectroscopy to quantitatively compare the conformational properties of MelB mutants, implicated previously in sodium binding, with those of a fully functional Cys-less MelB permease. The results first suggest that Asp55 and Asp59 are essential ligands for Na(+) binding. Secondly, though Asp124 is not essential for Na(+) binding, this acidic residue may play a critical role, possibly by its interaction with the bound cation, in the full Na(+)-induced conformational changes required for efficient coupling between the ion- and sugar-binding sites; this residue may also be a sugar ligand. Thirdly, Asp19 does not participate in Na(+) binding but it is a melibiose ligand. The location of these residues in two independent threading models of MelB is consistent with their proposed role.

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

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

Mfsd2a encodes a novel major facilitator superfamily domain-containing protein highly induced in brown adipose tissue during fasting and adaptive thermogenesis

This study describes the identification of Mfsd2a (major facilitator superfamily domain-containing protein 2a), a novel mammalian major facilitator superfamily domain-containing protein, and an additional closely related protein, Mfsd2b. Most intron/exon junctions are conserved between the two genes, suggesting that they are derived from a common ancestor. Mfsd2a and Mfsd2b share a 12 transmembrane alpha-helical domain structure that bears greatest similarity to that of the bacterial Na(+)/melibiose symporters. Confocal microscopy demonstrated that Mfsd2a localizes to the endoplasmic reticulum. Mfsd2a is expressed in many tissues and is highly induced in liver and BAT (brown adipose tissue) during fasting. Mfsd2a displays an oscillatory expression profile in BAT and liver, consistent with a circadian rhythm. Although the basal level of Mfsd2a expression is relatively low in mouse BAT, it is greatly induced during cold-induced thermogenesis and after treatment with betaAR (beta-adrenergic receptor) agonists. This induction is totally abolished in beta-less (betaAR-deficient) mice. These findings indicate that Mfsd2a is greatly up-regulated in BAT during thermogenesis and that its induction is controlled by the betaAR signalling pathway. The observed induction of Mfsd2a expression in cultured BAT cells by dibutyryl-cAMP is in agreement with this conclusion. The present study suggests that Mfsd2a plays a role in adaptive thermogenesis.

FTIR spectroscopy of secondary-structure reorientation of melibiose permease modulated by substrate binding

Analysis of infrared polarized absorbance spectra and linear dichroism spectra of reconstituted melibiose permease from Escherichia coli shows that the oriented structures correspond mainly to tilted transmembrane alpha-helices, forming an average angle of approximately 26 degrees with the membrane normal in substrate-free medium. Examination of the deconvoluted linear dichroism spectra in H(2)O and D(2)O makes apparent two populations of alpha-helices differing by their tilt angle (helix types I and II). Moreover, the average helical tilt angle significantly varies upon substrate binding: it is increased upon Na(+) binding, whereas it decreases upon subsequent melibiose binding in the presence of Na(+). In contrast, melibiose binding in the presence of H(+) causes virtually no change in the average tilt angle. The data also suggest that the two helix populations change their tilting and H/D exchange level in different ways depending on the bound substrate(s). Notably, cation binding essentially influences type I helices, whereas melibiose binding modifies the tilting of both helix populations.

How phosphotransferase system-related protein phosphorylation regulates carbohydrate metabolism in bacteria

The phosphoenolpyruvate(PEP):carbohydrate phosphotransferase system (PTS) is found only in bacteria, where it catalyzes the transport and phosphorylation of numerous monosaccharides, disaccharides, amino sugars, polyols, and other sugar derivatives. To carry out its catalytic function in sugar transport and phosphorylation, the PTS uses PEP as an energy source and phosphoryl donor. The phosphoryl group of PEP is usually transferred via four distinct proteins (domains) to the transported sugar bound to the respective membrane component(s) (EIIC and EIID) of the PTS. The organization of the PTS as a four-step phosphoryl transfer system, in which all P derivatives exhibit similar energy (phosphorylation occurs at histidyl or cysteyl residues), is surprising, as a single protein (or domain) coupling energy transfer and sugar phosphorylation would be sufficient for PTS function. A possible explanation for the complexity of the PTS was provided by the discovery that the PTS also carries out numerous regulatory functions. Depending on their phosphorylation state, the four proteins (domains) forming the PTS phosphorylation cascade (EI, HPr, EIIA, and EIIB) can phosphorylate or interact with numerous non-PTS proteins and thereby regulate their activity. In addition, in certain bacteria, one of the PTS components (HPr) is phosphorylated by ATP at a seryl residue, which increases the complexity of PTS-mediated regulation. In this review, we try to summarize the known protein phosphorylation-related regulatory functions of the PTS. As we shall see, the PTS regulation network not only controls carbohydrate uptake and metabolism but also interferes with the utilization of nitrogen and phosphorus and the virulence of certain pathogens.

The Inner Interhelix Loop 4–5 of the Melibiose Permease from Escherichia coli Takes Part in Conformational Changes after Sugar Binding

Cytoplasmic loop 4-5 of the melibiose permease from Escherichia coli is essential for the process of Na+-sugar translocation (Abdel-Dayem, M., Basquin, C., Pourcher, T., Cordat, E., and Leblanc, G. (2003) J. Biol. Chem. 278, 1518-1524). In the present report, we analyze functional consequences of mutating each of the three acidic amino acids in this loop into cysteines. Among the mutants, only the E142C substitution impairs selectively Na+-sugar translocation. Because R141C has a similar defect, we investigated these two mutants in more detail. Liposomes containing purified mutated melibiose permease were adsorbed onto a solid supported lipid membrane, and transient electrical currents resulting from different substrate concentration jumps were recorded. The currents evoked by a melibiose concentration jump in the presence of Na+, previously assigned to an electrogenic conformational transition (Meyer-Lipp, K., Ganea, C., Pourcher, T., Leblanc, G., and Fendler, K. (2004) Biochemistry 43, 12606-12613), were much smaller for the two mutants than the corresponding signals in cysteineless MelB. Furthermore, in R141C the stimulating effect of melibiose on Na+ affinity was lost. Finally, whereas tryptophan fluorescence spectroscopy revealed impaired conformational changes upon melibiose binding in the mutants, fluorescence resonance energy transfer measurements indicated that the mutants still show cooperative modification of their sugar binding sites by Na+. These data suggest that: 1) loop 4-5 contributes to the coordinated interactions between the ion and sugar binding sites; 2) it participates in an electrogenic conformational transition after melibiose binding that is essential for the subsequent obligatory coupled translocation of substrates. A two-step mechanism for substrate translocation in the melibiose permease is suggested.

Three-dimensional structure of the sugar symporter melibiose permease from cryo-electron microscopy

Melibiose permease (MelB) of Escherichia coli is a secondary transporter that couples the uptake of melibiose and various other galactosides to symport of cations that can be Na+, Li+ or H+. MelB belongs to the glycoside-pentoside-hexuronide: cation symporter family of porters and is suggested to have 12 transmembrane helices. We have determined the three-dimensional structure of MelB at 10A resolution in the membrane plane with cryo-electron microscopy from two-dimensional crystals. The three-dimensional map shows a heart-shaped molecule composed of two domains with a large central cavity between them. The structure is constricted at one side of the membrane while it is open to the other. The overall molecular shape resembles those of lactose permease and glycerol-3-phosphate transporter. However, organization of helices in MelB seems less symmetrical than in these two members of the major facilitator superfamily.

Hexose/Pentose and Hexitol/Pentitol Metabolism

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

Sugar binding induced charge translocation in the melibiose permease from Escherichia coli

Electrogenic events associated with the activity of the melibiose permease (MelB), a transporter from Escherichia coli, were investigated. Proteoliposomes containing purified MelB were adsorbed to a solid supported lipid membrane, activated by a substrate concentration jump, and transient currents were measured. When the transporter was preincubated with Na(+) at saturating concentrations, a charge translocation in the protein upon melibiose binding could still be observed. This result demonstrates that binding of the uncharged substrate melibiose triggers a charge displacement in the protein. Further analysis showed that the charge displacement is neither related to extra Na(+) binding to the transporter, nor to the displacement of already bound Na(+) within the transporter. The electrogenic melibiose binding process is explained by a conformational change with concomitant displacement of charged amino acid side chains and/or a reorientation of helix dipoles. A kinetic model is suggested, in which Na(+) and melibiose binding are distinct electrogenic processes associated with approximately the same charge displacement. These binding reactions are fast in the presence of the respective cosubstrate (k > 50 s(-1)).

Loop X/XI, the largest cytoplasmic loop in the membrane-bound melibiose carrier of Escherichia coli, is a functional re-entrant loop

The melibiose carrier of Escherichia coli is a membrane-bound sugar-cation cotransporter consisting of 12 transmembrane helices connected by cytoplasmic and periplasmic loops, with both N- and C-terminus on the cytoplasmic side. Using a functional cysteine-less carrier, cysteine was substituted individually for residues 347-378 that comprise the largest cytoplasmic loop X/XI. The majority of the cysteine mutants have good protein expression levels. The cysteine mutants were studied for their transport activities, and the inhibitory effects of two sulfhydryl reagents, PCMBS (7-A long) and BM (29-A long). Cysteine substitution resulted in substantial loss of transport in 12 mutants. While PCMBS caused significant inhibition in only two mutants, T373C and V376C, from the periplasmic side (in a substrate-protective manner), more extensive inhibition pattern was observed from the cytoplasmic side, in seven mutants: V353C, Y358C, V371C, Q372C, T373C, V376C and G378C, suggesting that these residues are along the sugar pathway in the aqueous channel, close to the cytoplasmic side. Furthermore, the inhibitory effect of BM on the inside-out vesicles of the above mutants was clearly less than that of PCMBS, suggesting channel space limitation to large molecules, consistent with those residues being inside the channel. Three second-site revertants (A350C/F268L, A350C/I22S, and A350C/I22N) were selected. They may suggest proximities between loop X/XI and helices I and VIII, in agreement with a re-entrant loop structure. Self thiol cross-linkings of the cysteine mutants on loop X/XI failed to form dimers, suggesting that most of the loop is not surface-exposed from cytoplasmic side. Together, these results strongly indicated a functional re-entrant loop mechanistically important in Na+-coupled transporters.

An investigation of cysteine mutants on the cytoplasmic loop X/XI in the melibiose transporter of Escherichia coli by using thiol reagents: implication of structural conservation of charged residues

The melibiose transporter (Mel B) of Escherichia coli is a cation-coupled (H(+), Li(+), and Na(+)) membrane protein (MW 50 kDa) consisting of 12 transmembrane helices that are connected by periplasmic and cytoplasmic loops, with both the C- and N-ends located on the cytoplasmic side of the membrane. Previous investigations on the largest cytoplasmic loop X/XI indicated that it is a functional re-entrant loop. In this communication, the cysteine mutants on loop X/XI were studied with charged thiol reagents MTSES, MTSET, and IAA for both the inhibition patterns and charge replacement/function rescue of inactive mutants in which the original charged residues were replaced by neutral cysteines. Strong inhibitions were observed in T373C and V376C by both MTSES and MTSET, consistent with previous results of PCMBS inhibition. The thiol reagents failed to recover the activities of inactive mutants D351C, D354C, and R363C and to inhibit active mutants E357C, K359C, and E365C to any significant extent, suggesting a structural conservation at D351, D354, and R363 and tolerance of structural variations at E357, K359, and E365. The results are consistent with previous observation of structural conservation of functionally charged residues in the transmembrane domains and extend to a loop the contention that in the melibiose transporter functionally important charged residues are structurally conserved.

Cytoplasmic loop connecting helices IV and V of the melibiose permease from Escherichia coli is involved in the process of Na+-coupled sugar translocation

Previous photolabeling and limited proteolysis studies suggested that one of the four basic residues (Arg-141) of the N-terminal cytoplasmic loop connecting helices IV and V (loop 4-5) of the melibiose permease (MelB) from Escherichia coli has a potential role in its symport function (Ambroise, Y., Leblanc, G., and Rousseau, B. (2000) Biochemistry 39, 1338-1345). A mutagenesis study of Arg-141 and of the other three basic residues of loop 4-5 was undertaken to further examine this hypothesis. Cys replacement analysis indicated that Arg-141 and Arg-149, but not Lys-138 and Arg-139, are essential for MelB transport activity. Replacement of Arg-141 by neutral residues (Cys or Gln) inactivated transport and energy-independent carrier-mediated flows of substrates (counterflow, efflux), whereas it had a limited effect on co-substrate binding. R141C sugar transport was partially rescued on reintroducing a positive charge with a charged and permeant thiol reagent. Whereas R149C was completely inactive, R149K and R149Q remained functional. Strikingly, introduction of an additional mutation in the C-terminal helix X (Gly for Val-343) of R149C restored sugar transport. Impermeant thiol reagents inhibited R149C/V343G transport activity in right-side-out membrane vesicles and prevented sugar binding in a sugar-protected manner. All these data suggest that MelB loop 4-5 is close to the sugar binding site and that the charged residue Arg-141 is involved in the reaction of co-substrate translocation or substrate release in the inner compartment.

Study of amide-proton exchange of Escherichia coli melibiose permease by attenuated total reflection-Fourier transform infrared spectroscopy: evidence of structure modulation by substrate binding

The accessibility of Escherichia coli melibiose permease to aqueous solvent was studied following hydrogen-deuterium exchange kinetics monitored by attenuated total reflection-Fourier transform infrared spectroscopy under four distinct conditions where MelB forms different complexes with its substrates (H(+), Na(+), melibiose). Analysis of the amide II band upon (2)H(2)O exposure discloses a significant sugar protection of the protein against aqueous solvent, resulting in an 8% less exchange of the corresponding H(+)*melibiose*MelB complex compared with the protein in the absence of sugar. Investigation of the amide I exchange reveals clear substrate effects on beta-sheet accessibility, with the complex H(+)*melibiose*MelB being the most protected state against exchange, followed by Na(+)*melibiose*MelB. Although of smaller magnitude, similar changes in alpha-helices plus non-ordered structures are detected. Finally, no differences are observed when analyzing reverse turn structures. The results suggest that sugar binding induces a remarkable compactness of the carrier's structure, affecting mainly beta-sheet domains of the transporter, which, according to secondary structure predictions, may include cytoplasmic loops 4-5 and 10-11. A possible catalytic role of these two loops in the functioning of MelB is hypothesized.

Evidence for intraprotein charge transfer during the transport activity of the melibiose permease from Escherichia coli

Electrogenic activity associated with the activity of the melibiose permease (MelB) of Escherichia coli was investigated by using proteoliposomes containing purified MelB adsorbed onto a solid-supported membrane. Transient currents were selectively recorded by applying concentration jumps of Na+ ions (or Li+) and/or of different sugar substrates of MelB (melibiose, thio-methyl galactoside, raffinose) using a fast-flow solution exchange system. Characteristically, the transient current response was fast, including a single decay exponential component (tau approximately 15 ms) on applying a Na+ (or Li+) concentration jump in the absence of sugar. On imposing a Na+ (or Li+) jump on proteoliposomes preincubated with the sugar, a sugar jump in a preparation preincubated with the cation, or a simultaneous jump of the cation and sugar substrates, the electrical transients were biphasic and comprised both the fast and an additional slow (tau approximately 350 ms) decay components. Finally, selective inactivation of the cosubstrate translocation step by acylation of MelB cysteins with N-ethyl maleimide suppressed the slow response components and had no effect on the fast transient one. We suggest that the fast transient response reflects charge transfer within MelB during cosubstrate binding while the slow component is associated with charge transfer across the proteoliposome membrane. From the time course of the transient currents, we estimate a rate constant for Na+ binding in the absence and presence of melibiose of k > 50 s(-1) and one for melibiose binding in the absence of Na+ of k approximately 10 s(-1).

The proximity between helix I and helix XI in the melibiose carrier of Escherichia coli as determined by cross-linking

The melibiose carrier of Escherichia coli is a transmembrane protein that comprises 12 transmembrane helices connected by periplasmic and cytoplasmic loops, with both the N- and C-termini located on the cytoplasmic side. Our previous studies of second-site revertants suggested proximity between several helices, including helices XI and I. In this study, we constructed six double cysteine mutants, each having one cysteine in helix I and the other in helix XI: three mutants, K18C/S380C, D19C/S380C, and F20C/S380C, have their cysteine pairs near the cytoplasmic side of the carrier, and the other three, T34C/G395C, D35C/G395C, and V36C/G395C, have their cysteine pairs near the periplasmic side. In the absence of substrate, disulfide formations catalyzed by iodine and copper-(1,10-phenanthroline)(3) indicate that helix I and helix XI are in immediate proximity to each other on the periplasmic side but not on the cytoplasmic side, as shown by protease cleavage analyses. We infer that the two helices are tilted with respect to each other, with the periplasmic sides in close proximity.

The effect of modifications of the charged residues in the transmembrane helices on the transport activity of the melibiose carrier of Escherichia coli

The melibiose transport carrier of Escherichia coli (coded by melB gene) is a cotransport system which couples the transport of a-galactosides to protons, sodium, or lithium ions. The charged amino acid residues in membrane-spanning helices are of considerable interest because many of them have important function in substrate recognition. In most cases changing these charged residue to an uncharged residue (cysteine) results in total loss of activity. In this communication we describe experiments in which the cysteine substitution for a charged residue was chemically changed by sulfhydryl reagents (MTSEA and MTSET to restore a positive charge and MTSES a negative charge) or by iodoacetic acid or through oxidation by hydrogen peroxide so as to regain the original negative charge. In two cases (D55C and D124C) the reconstructed negative charges via the oxidation of the thiol to the sulfinic and/or sulfonic acid resulted in partial recovery of transport: D55C up to 27% of the normal and D124C up to 4% of the normal in melibiose accumulation; D55C up to 36% of the normal and D124 up to 4.5% of the normal in downhill transport. Sulfhydryl reagents and iodoacetic acid failed to recover transport in all cases. We infer that the configurations of the charges as well as the structure of the side chains that carry them are critical in the maintenance of the transport.

Cysteine mutagenesis of the amino acid residues of transmembrane helix I in the melibiose carrier of Escherichia coli

The melibiose carrier of Escherichia coli is a sugar-cation cotransport system that utilizes Na(+), Li(+), or H(+). This membrane transport protein consists of 12 transmembrane helices. Starting with the cysteine-less melibiose carrier, cysteine has been substituted individually for amino acids 17-37, which includes all of the residues in membrane helix I. The carriers with cysteine substitutions were studied for their transport activity and the effect of the water soluble sulfhydryl reagent p-chloro- mercuribenzenesulfonic acid (PCMBS). Cysteine substitution caused loss of transport activity in six of the mutants (G17C, K18C, D19C, Y32C, T34C, and D35C). PCMBS caused greater than 50% inhibition in eleven mutants (F20C, A21C, I22C, G23C, I24C, V25C, Y26C, M27C, Y28C, M30C, and Y31C). We suggest that the residues whose cysteine derivatives were inhibited by PCMBS face the aqueous channel and that helix I is completely surrounded by aqueous environment. Second site revertants were isolated from K18C and Y31C. The revertants were found to have mutations in helices I, IV, and VII.

Sodium-substrate cotransport in bacteria

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

Genes involved in control of galactose uptake in Lactobacillus brevis and reconstitution of the regulatory system in Bacillus subtilis

The heterofermentative lactic acid bacterium Lactobacillus brevis transports galactose and the nonmetabolizable galactose analogue thiomethyl-beta-galactoside (TMG) by a permease-catalyzed sugar:H(+) symport mechanism. Addition of glucose to L. brevis cells loaded with [(14)C]TMG promotes efflux and prevents accumulation of the galactoside, probably by converting the proton symporter into a uniporter. Such a process manifests itself physiologically in phenomena termed inducer expulsion and exclusion. Previous evidence suggested a direct allosteric mechanism whereby the phosphocarrier protein, HPr, phosphorylated at serine-46 [HPr(Ser-P)], binds to the galactose:H(+) symporter to uncouple sugar transport from proton symport. To elucidate the molecular mechanism of inducer control in L. brevis, we have cloned the genes encoding the HPr(Ser) kinase, HPr, enzyme I, and the galactose:H(+) symporter. The sequences of these genes were determined, and the relevant phylogenetic trees are presented. Mutant HPr derivatives in which the regulatory serine was changed to either alanine or aspartate were constructed. The cloned galP gene was integrated into the chromosome of Bacillus subtilis, and synthesis of the mutant HPr proteins in this organism was shown to promote regulation of GalP, as expected for a direct allosteric mechanism. We have thus reconstituted inducer control in an organism that does not otherwise exhibit this phenomenon. These results are consistent with the conclusion that inducer exclusion and expulsion in L. brevis operates via a multicomponent signal transduction mechanism wherein the presence of glycolytic intermediates such as fructose 1,6-bisphosphate (the intracellular effector), derived from exogenous glucose (the extracellular effector), activates HPr(Ser) kinase (the sensor) to phosphorylate HPr on Ser-46 (the messenger), which binds to the galactose:H(+) symporter (the target), resulting in uncoupling of sugar transport from proton symport (the response). This cascade allows bacteria to quickly respond to changes in external sugar concentrations. Understanding the molecular mechanism of inducer control advances our knowledge of the link between metabolic and transport processes in bacteria.

Physiological evidence for an interaction between helices II and XI in the melibiose carrier of Escherichia coli

The melibiose carrier from Escherichia coli is a cation-substrate cotransporter that catalyzes the accumulation of galactosides at the expense of H(+), Na(+), or Li(+) electrochemical gradients. Charged residues on transmembrane domains in the amino-terminal portion of this carrier play an important role in the recognition of cations, while the carboxyl portion of the protein seems to be important for sugar recognition. In the present study, we substituted Lys-377 on helix XI with Val. This mutant carrier, K377V, had reduced melibiose transport activity. We subsequently used this mutant for the isolation of functional second-site revertants. Revertant strains showed the additional substitutions of Val or Asn for Asp-59 (helix II), or Leu for Phe-20 (helix I). Isolation of revertant strains where both Lys-377 and Asp-59 are substituted with neutral residues suggested the possibility that a salt bridge exists between helix II and helix XI. To further test this idea, we constructed three additional site-directed mutants: Asp-59-->Lys (D59K), Lys-377-->Asp (K377D), and a double mutant, Asp-59-->Lys/Lys-377-->Asp (D59K/K377D), in which the position of these charges was exchanged. K377D accumulated melibiose only marginally while D59K could not accumulate. However, the D59K/K377D double mutant accumulated melibiose to a modest level although this activity was no longer stimulated by Na(+). We suggest that Asp-59 and Lys-377 interact via a salt bridge that brings helix II and helix XI close to one another in the three-dimensional structure of the carrier.

Topological organization of the hyaluronan synthase from Streptococcus pyogenes

Since we first reported (DeAngelis, P. L., Papaconstantinou, J., and Weigel, P. H. (1993) J. Biol. Chem. 268, 19181-19184) the cloning of the hyaluronan (HA) synthase from Streptococcus pyogenes (spHAS), numerous membrane-bound HA synthases have been discovered in both prokaryotes and eukaryotes. The HASs are unique among enzymes studied to date because they mediate 6-7 discrete functions in order to assemble a polysaccharide containing hetero-disaccharide units and simultaneously effect translocation of the growing HA chain through the plasma membrane. To understand how the relatively small spHAS performs these various functions, we investigated the topological organization of the protein utilizing fusion analysis with two reporter enzymes, alkaline phosphatase and beta-galactosidase, as well as several other approaches. From these studies, we conclude that the NH2 terminus and the COOH terminus, as well as the major portion of a large central domain are localized intracellularly. The first two predicted membrane domains were confirmed to be transmembrane domains and give rise to a very small extracellular loop that is inaccessible to proteases. Several regions of the large internal central domain appear to be associated with, but do not traverse, the membrane. Following the central domain, there are two additional transmembrane domains connected by a second small extracellular loop that also is inaccessible to proteases. The COOH-terminal approximately 25% of spHAS also contains a membrane domain that does not traverse the membrane and may contain extensive re-entrant loops or amphipathic helices. Numerous membrane associations of this latter COOH-terminal region and the central domain may be required to create a pore-like structure through which a growing HA chain can be extruded to the cell exterior. Based on the high degree of similarity among Class I HAS family members, these enzymes may have a similar topological organization for their spHAS-related domains.

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.

Close approximation of putative alpha -helices II, IV, VII, X, and XI in the translocation pathway of the lactose transport protein of Streptococcus thermophilus

The lactose transport protein (LacS) of Streptococcus thermophilus belongs to a family of transporters in which putative alpha-helices II and IV have been implicated in cation binding and the coupled transport of the substrate and the cation. Here, the analysis of site-directed mutants shows that a positive and negative charge at positions 64 and 71 in helix II are essential for transport, but not for lactose binding. The conservation of charge/side-chain properties is less critical for Glu-67 and Ile-70 in helix II, and Asp-133 and Lys-139 in helix IV, but these residues are important for the coupled transport of lactose together with a proton. The analysis of second-site suppressor mutants indicates an ion pair exists between helices II and IV, and thus a close approximation of these helices can be made. The second-site suppressor analysis also suggests ion pairing between helix II and the intracellular loops 6-7 and 10-11. Because the C-terminal region of the transmembrane domain, especially helix XI and loop 10-11, is important for substrate binding in this family of proteins, we propose that sugar and proton binding and translocation are performed by the joint action of these regions in the protein. Indeed, substrate protection of maleimide labeling of single cysteine mutants confirms that alpha-helices II and IV are directly interacting or at least conformationally involved in sugar binding and/or translocation. On the basis of new and published data, we reason that the helices II, IV, VII, X, and XI and the intracellular loops 6-7 and 10-11 are in close proximity and form the binding sites and/or the translocation pathway in the transporters of the galactosides-pentosides-hexuronides family.

Secondary structure components and properties of the melibiose permease from Escherichia coli: a fourier transform infrared spectroscopy analysis

The structure of the melibiose permease from Escherichia coli has been investigated by Fourier transform infrared spectroscopy, using the purified transporter either in the solubilized state or reconstituted in E. coli lipids. In both instances, the spectra suggest that the permease secondary structure is dominated by alpha-helical components (up to 50%) and contains beta-structure (20%) and additional components assigned to turns, 3(10) helix, and nonordered structures (30%). Two distinct and strong absorption bands are recorded at 1660 and 1653 cm(-1), i.e., in the usual range of absorption of helices of membrane proteins. Moreover, conditions that preserve the transporter functionality (reconstitution in liposomes or solubilization with dodecyl maltoside) make possible the detection of two separate alpha-helical bands of comparable intensity. In contrast, a single intense band, centered at approximately 1656 cm(-1), is recorded from the inactive permease in Triton X-100, or a merged and broader signal is recorded after the solubilized protein is heated in dodecyl maltoside. It is suggested that in the functional permease, distinct signals at 1660 and 1653 cm(-1) arise from two different populations of alpha-helical domains. Furthermore, the sodium- and/or melibiose-induced changes in amide I line shape, and in particular, in the relative amplitudes of the 1660 and 1653 cm(-1) bands, indicate that the secondary structure is modified during the early step of sugar transport. Finally, the observation that approximately 80% of the backbone amide protons can be exchanged suggests high conformational flexibility and/or a large accessibility of the membrane domains to the aqueous solvent.

Syntheses and properties of photoactivatable sugar derivatives designed to probe the sugar-binding site of melibiose permease

Three new photoreactive sugar analogues bearing an azido, a diazonium salt or a diazirine group as the photophore as well as a tritium atom were developed. Two of these new photoactivatable compounds gave excellent preliminary results, with a high affinity for the melibiose permease of Escherichia coli.

Consequences of hydrophobic mismatch between lipids and melibiose permease on melibiose transport

The structural and functional consequences of a mismatch between the hydrophobic thickness d(P) of a transmembrane protein and that d(L) of the supporting lipid bilayer were investigated using melibiose permease (MelB) from Escherichia coli reconstituted in a set of bis saturated and monounsaturated phosphatidylcholine species differing in acyl-chain length. Influence of MelB on the midpoint gel-to-liquid-phase transition temperature, T(m), of the saturated lipids was investigated through fluorescence polarization experiments, with 1,6-diphenyl-1,3,5-hexatriene as the probe, for varying protein/lipid molar ratio. Diagrams in temperature versus MelB concentration showed positive or negative shifts in T(m) with the short-chain lipids DiC12:0-PC and DiC14:0-PC or the long-chain lipids DiC16:0-PC and DiC18:0-PC, respectively. Theoretical analysis of the data yielded a d(L) value of 3.0 +/- 0.1 nm for the protein, similar to the 3.02 nm estimated from hydropathy profiles. Influence of the acyl chain length on the carrier activity of MelB was investigated in the liquid phase, using the monounsaturated PCs. Binding of the sugar to the transporter showed no dependence on the acyl chain length. In contrast, counterflow and Deltapsi-driven experiments revealed strong dependence of melibiose transport on the lipid acyl chain length. Similar bell-shaped transport versus acyl chain length profiles were obtained, optimal activity being supported by diC16:1-PC. On account of a d(P) value of 2.65 nm for the lipid and of various local constraints which would all tend to elongate the acyl chains in contact with the protein, one can conclude that maximal activity was obtained when the hydrophobic thickness of the bilayer matched that of the protein.

Evidence for a role of helix IV in connecting cation- and sugar-binding sites of Escherichia coli melibiose permease

To improve the structural organization model of melibiose permease, we assessed the individual contributions of the N-terminal tryptophans to the transporter fluorescence variations induced by the binding of cations and beta-configured sugars, by replacement of the six N-terminal tryptophans by phenylalanines and the study of the signal changes. Only two mutations, W116F located in helix IV and W128F located in the cytoplasmic loop 4-5, impair permease activity. The intrinsic fluorescence spectroscopy analysis of the other mutants suggests that W54, located in helix II, W116, and W128 are mostly responsible for the cation-induced fluorescence variations. These tryptophans, W116 and W128, would also be responsible for the beta-galactoside-induced fluorescence changes observed in the N-terminal domain of the transporter. The implication of W116 and W128 in both the cation- and beta-galactoside-induced fluorescence variations led us to investigate in detail the effects of their mutations on the functional properties of the permease. The results obtained suggest that the domains harboring the two tryptophans, or the residues themselves, play a critical role in the mechanism of Na(+)/sugar symport. Taken together, the results presented in this paper and previous results are consistent with a fundamental role of helix IV in connecting cation- and sugar-binding sites of the melibiose permease.

Physiological evidence for an interaction between helix XI and helices I, II, and V in the melibiose carrier of Escherichia coli

In a previous study 23 residues in helix XI of the cysteine-less melibiose carrier were changed individually to cysteine. Several of these cysteine mutants (K377C, A383C, F385C, L391C, G395C) had low transport activity and they were white on melibiose MacConkey fermentation plates. After several days of incubation of these white clones on melibiose MacConkey plates a rare red mutant appeared. The plasmid DNA was then isolated and sequenced. The two second site revertants from K377C were I22S and D59A. This change of aspartic acid to a neutral residue suggests that physiologically there is an interaction between K377 and D59 (possibly a salt bridge). The revertants from A383C were in positions 20 (F20L) and 22 (I22S and I22N). Revertants of F385C were intrahelical changes (I387M and A388G) and a change in C-terminal loop (R441C). Revertants of L391C were in helix I (I22N, I22T and D19E) and helix V (A152S). Revertants of G395C were in helix I (D19E and I22N). We suggest that there is an interaction between helix XI and helices I, II, and V and proximity between these helices.

Active-site-directed photolabeling of the melibiose permease of Escherichia coli

Covalent photolabeling of the melibiose permease (MelB) of Escherichia coli has been undertaken with the sugar analogue [(3)H]-p-azidophenyl alpha-D-galactopyranoside ([(3)H]-alpha-PAPG) with the purpose of identifying the domains forming the MelB sugar-binding site. We show that alpha-PAPG is a high-affinity substrate of MelB (K(d) = 1 x 10(-)(6) M). Its binding to or transport by MelB is Na-dependent and is competitively prevented by melibiose or by the high-affinity ligand p-nitrophenyl alpha-D-galactopyranoside (alpha-NPG). Membrane vesicles containing overexpressed histidine-tagged recombinant MelB were photolabeled in the presence of [(3)H]-alpha-PAPG by irradiation with UV light (lambda = 250 nm). Eighty-five percent of the radioactivity covalently associated with the vesicles was incorporated in a polypeptide corresponding to MelB monomer. MelB labeling was completely prevented by an excess of melibiose or alpha-NPG during the assay. Radioactivity analysis of CNBr cleavage or limited proteolysis products of the purified [(3)H]-alpha-PAPG-labeled transporter suggests that several domains of MelB are targets for labeling. One of the labeled CNBr cleavage products is a peptide with an apparent molecular mass of 5.5 kDa. It is shown that (i) its amino acid sequence is that of the Asp124-Met181 domain of MelB (7.5 kDa), which includes the cytoplasmic loop 4-5 connecting helices IV and V, the hydrophobic helix V, and the outer loop connecting helices V-VI, and (ii) that Arg141 in loop 4-5 is the only labeled amino acid of this peptide. Labeling of loop 4-5 provides independent evidence that this specific domain plays a significant role in MelB transport. Comparison with the well-characterized equivalent domain of LacY suggests that sugar transporters with similar structure and substrate specificity may have conserved domains involved in sugar recognition.

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).

Melibiose carrier of Escherichia coli: use of cysteine mutagenesis to identify the amino acids on the hydrophilic face of transmembrane helix 2

The melibiose carrier from Escherichia coli is a galactoside-cation symporter. Based on both experimental evidence and hydropathy analysis, 12 transmembrane helices have been assigned to this integral membrane protein. Transmembrane helix 2 contains several charged and polar amino acids that have been shown to be essential for the cation-coupled transport of melibiose. Starting with the cysteine-less melibiose carrier, we have individually substituted cysteine for amino acids 39-66, which includes the proposed transmembrane helix 2. In the resulting derivative carriers, we measured the transport of melibiose, determined the effect of the hydrophilic sulfhydryl reagent, p-chloromercuribenzenesulfonic acid (PCMBS), on transport in intact cells and inside out vesicles, and examined the ability of melibiose to protect the carrier from inactivation by the sulfhydryl reagent. We found a set of seven positions in which the reaction with the sulfhydryl reagent caused partial or complete loss of carrier function measured in intact cells or inside-out vesicles. The presence of melibiose protected five of these positions from reaction with PCMBS. The reaction of two additional positions with PCMBS resulted in the partial loss of transport function only in inside-out vesicles. Melibiose protected these two positions from reaction with the reagent. Together, the PCMBS-sensitive sites and charged residues assigned to helix 2 form a cluster of amino acids that map in three rows with each row comprised of every fourth residue. This is the pattern expected of residues that are part of an alpha-helical structure and thus the rows are tilted at an angle of 25 degrees to the helical axis. We suggest that these residues line the path of melibiose and its associated cation through the carrier.

Structural information on a membrane transport protein from nuclear magnetic resonance spectroscopy using sequence-selective nitroxide labeling

The lactose transport protein (LacS) from Streptococcus thermophilus bearing a single cysteine mutation, K373C, within the putative interhelix loop 10-11 has been overexpressed in native membranes. Cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy (NMR) could selectively distinguish binding of (13)C-labeled substrate to just 50-60 nmol of LacS(K373C) in the native fluid membranes. Nitroxide electron spin-label at the K373C location was essentially immobile on the time scale of both conventional electron spin resonance spectroscopy (ESR) (<10(-8)s) and saturation-transfer ESR (<10(-3)s), under the same conditions as used in the NMR studies. The presence of the nitroxide spin-label effectively obscured the high-resolution NMR signal from bound substrate, even though (13)C-labeled substrate was shown to be within the binding center of the protein. The interhelix loop 10-11 is concluded to be in reasonably close proximity to the substrate binding site(s) of LacS (<15 A), and the loop region is expected to penetrate between the transmembrane segments of the protein that are involved in the translocation process.

Structural analysis of a plant sucrose carrier using monoclonal antibodies and bacteriophage lambda surface display

Monoclonal antibodies were raised and selected against recombinant Plantago major PmSUC2 sucrose carrier protein. Epitopes of two monoclonal antibodies (PS2-1A2 and PS2-4D4) were mapped using N-terminally truncated PmSUC2 proteins and a lambda library displaying random PmSUC2 peptides. PS2-1A2 recognizes an octapeptide close to the N-terminus of PmSUC2, PS2-4D4 binds to a decapeptide at the very C-terminus. Analyses of antibody binding to yeast protoplasts with functionally active, tagged PmSUC2 protein revealed that both epitopes are located in cytoplasmic domains of PmSUC2. These results support a model for plant sucrose transporters containing 12 transmembrane helices with the N-terminus and the C-terminus on the cytoplasmic side of the plasma membrane.

Structural studies of the melibiose permease of Escherichia coli by fluorescence resonance energy transfer. I. Evidence for ion-induced conformational change

Further insight into the cosubstrate-induced structural change of the melibiose permease (MelB) of Escherichia coli has been sought by investigating the binding and spectroscopic properties of the fluorescent sugar 2'-(N-5-dimethylaminonaphthalene-1-sulfonyl)aminoethyl 1-thio-beta-D-galactopyranoside (Dns2-S-Gal) and related analogs (Dns3-S-Gal or Dns6-S-Gal with a propyl or hexyl instead of an ethyl linker, respectively) interacting with MelB in membrane vesicles or in proteoliposomes. The three analogs efficiently inhibit melibiose transport and bind to MelB in a sodium-dependent fashion. Their dissociation constants (Kd) are in the micromolar range in the presence of NaCl and an order of magnitude higher in its absence. In the presence of NaCl and Dns2-S-Gal, sample excitation at 335 or 297 nm gives rise to a fluorescent signal at around 465 nm, whereas Dns3-S-Gal or Dns6-S-Gal emits a fluorescence light at 490 or 506 nm, respectively. Detailed study of the Dns2-S-Gal signal elicited by a 297 nm illumination indicates that a tryptophan-mediated fluorescence resonance energy transfer phenomenon is involved in the response. All fluorescence signals below 500 nm are prevented by addition of melibiose in excess, and the kinetic constants describing their dependence on the probe or NaCl concentrations closely correlate with the probe binding constants. Finally, the Dns2-S-Gal signal recorded in sodium-free medium is red shifted by up to 25 nm from that recorded in the presence of NaCl. Taken together, these results suggest (i) that the fluorescence signals below 500 nm arise from Dns-S-Gal molecules bound to MelB, (ii) the presence of a highly hydrophobic environment close to or at the sugar-binding site, the polarity of which increases on moving away from the sugar-binding site, and (iii) that the interaction of sodium ions with MelB enhances the hydrophobicity of this environment. These results are consistent with the induction of a cooperative change of the structure of the sugar-binding site or of its immediate vicinity by the ions.

Evidence for a close association between helix IV and helix XI in the melibiose carrier of Escherichia coli

The melibiose carrier of Escherichia coli is a cation-sugar cotransport protein. Asp124 in membrane-spanning helix IV of the carrier protein was replaced with Ser, Ile or Phe by site-directed mutagenesis of a plasmid containing the melB gene. Each of these mutants failed to show membrane transport of melibiose and melibiose-positive revertants could be isolated on melibiose MacConkey indicator plates. D124F showed only one type of revertant (D124C) and D124I showed only revertants to the normal (D124). Second site revertants were not found with either of these mutants. S124, however, showed two types of second site revertants: D124S/V375A and D124S/V375G. The revertant D124S/V375A had lost melibiose/proton cotransport but showed 25% of normal melibiose (20 mM) uptake in the presence of 10 mM sodium ion at 37 degrees C. The value for the parental strain D124S was 2%. The second revertant D124S/V375G showed greater activity than S124 in the presence of 100 mM NaCl at both 20 degrees C and 37 degrees C. It was concluded that in the normal carrier protein Asp124 in helix IV is probably close to Val375 in helix XI. Since Lys377 is close to Val375, it is possible that Asp124 may interact with Lys377 to form a salt bridge.

The construction of a cysteine-less melibiose carrier from E. coli

The melibiose carrier of E. coli is a cation-sugar cotransport system. This membrane protein contains four cysteine residues and the transport function is inhibited by sulfhydryl reagents. In order to investigate the importance of the cysteines, we have constructed a set of four melibiose transporters each of which has one cysteine replaced with serine or valine. The sensitivity of this set of carriers to N-ethylmaleimide was tested and Cys364 was identified as the target of the reagent. In addition, we constructed a melibiose transporter in which all 4 cysteines were replaced with either serine (Cys110, Cys310, and Cys364) or valine (Cys235) and we found that, as expected, the resulting cysteine-less transporter was resistant to the action of N-ethylmaleimide. The cysteine-less melibiose carrier had no significant decrease in ability to accumulate melibiose with cotransported sodium ions or protons. Thus, none of the 4 cysteines are necessary for the function of the melibiose carrier.

Sequence of a melibiose transporter gene of Enterobacter cloacae

We cloned a fragment of the chromosomal DNA of Enterobacter cloacae, which enabled a melibiose-negative Escherichia coli mutant lacking melB to grow on melibiose as the sole source of carbon. Transformed cells harboring the hybrid plasmid carrying the cloned DNA showed melibiose transport activity. The nucleotide sequence of the DNA region was determined. One complete open reading frame (ORF) and a part of another ORF were found in the region, and the amino acid sequences were deduced. The complete ORF was found to encode a melibiose transporter which consisted of 425 amino acid residues. Hydropathy analysis revealed that there are about 12 hydrophobic domains in this transporter. The incomplete ORF which exists in the upstream region of the transporter gene seemed to encode an alpha-galactosidase.

Proteolytic mapping and substrate protection of the Escherichia coli melibiose permease

The topology and substrate-induced conformational change(s) of the Na+ (Li+ or H+)-melibiose cotransporter (MelB) of Escherichia coli were investigated by limited protease digestion. To facilitate these analyses, MelB was epitope-tagged both at its carboxyl-terminus and at its amino-terminus. Limited digestion with different proteases indicates that the cytoplasmic loops connecting transmembrane domains 4-5, 6-7, and 10-11 together with the carboxyl-terminus of MelB are exposed in the cytoplasm. In contrast, periplasmic loops are highly resistant to all the proteases examined, including nonspecific proteases such as proteinase K and thermolysin. The effect of Na+ or Li+ and/or melibiose on the rate of protease digestion of the cytoplasmic loops was also analyzed. The rate of protease digestion of loop 4-5 is specifically reduced, by approximately 3-fold, by the presence of Na+ or Li+. These results suggest that loop 4-5 is near or part of the cation binding site. Moreover, the presence of both melibiose and either Na+ or Li+ further reduced the rate of protease digestion of this loop 4-5 by up to 9-fold, although no protection from protease digestion was observed when melibiose was added alone. The increase in resistance to proteases observed in the presence of the cation alone or the cation plus melibiose suggests that the interaction of the two cosubstrate with MelB results in change(s) of MelB conformation.

A melibiose transporter and an operon containing its gene in Enterobacter cloacae

We detected inducible melibiose transport activity in cells of Enterobacter cloacae IID977. H+, but not Na+, was found to be the coupling cation for this transporter. We cloned and sequenced the gene encoding the melibiose transporter. A homology search of a protein sequence database revealed that this melibiose transporter has high sequence similarity with the lactose transporter (LacY) and the raffinose transporter (RafB) and has some similarity with the melibiose transporter (MelB) of Escherichia coli.

Characteristics of the melibiose transporter and its primary structure in Enterobacter aerogenes

Cells of Enterobacter aerogenes can grow on melibiose as a sole source of carbon. This suggests the presence of melibiose operon in this organism. We found that E. aerogenes cells possess both alpha-galactosidase activity and melibiose transport activity, which were induced by melibiose. Neither Na+ nor Li+ stimulated the melibiose transport. However, transport of methyl-beta-thiogalactoside (TMG) was stimulated by Li+ but not by Na+. These findings suggest that the major coupling cation for the melibiose transporter in E. aerogenes is H+. In fact, we observed H+ entry into cells caused by an influx of melibiose and some of its analogs. We cloned the melB gene which encodes the melibiose transporter, and sequenced it. Deduced amino acid sequence of the transporter revealed that the melibiose transporter consists of 471 amino acid residues and the molecular weight was calculated to be 52214 Da. The sequence showed high homology with the sequences of the melibiose transporters of Escherichia coli, Salmonella typhimurium and Klebsiella pneumoniae. Higher homology was found with the melibiose transporter of K. pneumoniae than with that of E. coli and S. typhimurium.

Melibiose permease of Escherichia coli: structural organization of cosubstrate binding sites as deduced from tryptophan fluorescence analyses

Binding of the coupling ion (Na+ or Li+) and sugars to the purified melibiose permease of Escherichia coli, reconstituted in proteoliposomes, produces selective and cooperative changes of the transporter tryptophan fluorescence. To assess the individual contribution of N- or C-terminal domains of the permease to these substrate-induced fluorescence variations, we replaced the two tryptophans located in its C-terminal half (W299 and W342) by a phenylalanine and compared the signal change in mutants and wild-type permease. None of the mutations significantly impairs transport activity. Persistence of the ion-induced signal quenching in a permease carrying only the six other tryptophans of the N-terminal domain is consistent with a previous suggestion that this domain accommodates the ion-binding site. On the other hand, the sugar-induced fluorescence increase varies from mutant to mutant in a sugar-specific fashion. While alpha-galactosides increase essentially the fluorescence of W299 and W342, beta-galactosides enhance the signal of W299 and of one (or more) of the N-terminal tryptophans but quench that of W342. Moreover, addition of sugars producers a 10 nm blue shift of both W299 and W342 emission spectra, suggesting reduced accessibility of these residues to solvent following substrate binding. These data suggest that W299 and W342 are at or close to the sugar binding site and that this latter is lined by the C-terminal helices IX and X. Moreover, as sugars with the beta-configuration also enhance the fluorescence of the N-terminal tryptophans, it is suggested that one (or more) helix of the N-terminal half may be also at or near the sugar binding site. This implies close proximity and/or tight functional linkage between some N-terminal helices and helices IX and X of the C-terminal domain of the transporter.

Alteration of Na(+)-coupled transport in site-directed mutants of the melibiose carrier of Escherichia coli

Asn-58 of the Escherichia coli melibiose carrier was replaced by Ala, Leu, Ser, and Gln. Trp-54 was replaced by Leu and a double mutant Leu-54/Ala-58 was constructed using site-directed mutagenesis. Cation/sugar cotransport and sugar-induced cation uptake were studied for each mutant. The change of Asn-58 to Ala results in a nearly complete loss of Na(+)-stimulated galactoside transport as well as sugar-stimulated Na+ uptake. Substitutions of Leu, Gln, and Ser for Asn-58 were also defective in Na(+)-stimulated sugar transport. The Trp-54 to Leu mutant shows moderate sugar accumulation with cation selectivity similar to wild-type. The double mutant Leu-54/Ala-58 shows elevated H(+)-melibiose cotransport as well as reduced Na(+)-stimulated melibiose cotransport. These results suggest that Asn-58 is important for Na+ recognition.

Membrane topology of the melibiose permease of Escherichia coli studied by melB-phoA fusion analysis

In order to study the secondary structure of the melibiose permease of Escherichia coli, 57 melB-phoA gene fusions were constructed and assayed for alkaline phosphatase activity. In general agreement with a previously suggested secondary structure model of melibiose permease [Botfield, M. C., Naguchi, K., Tsuchiya, T., & Wilson, T.H. (1992) J. Biol. Chem. 267, 1818], clusters of fusions exhibiting low and high phosphatase activity fusions alternate along the primary sequence. Fusions with high activity generally cluster at residues predicted to be in the periplasmic half of transmembrane domains or in periplasmic loops, while fusions with low activity cluster at residues predicted to be in the cytoplasmic half of transmembrane domains or in cytoplasmic loops. Taken together, the findings strongly support the contention that melibiose permease contains 12 transmembrane domains that traverse the membrane in zigzag fashion connected by hydrophilic loops that are exposed alternatively on the periplasmic or cytoplasmic surfaces of the membrane with the N and C termini on the cytoplasmic face of the membrane. Moreover, on the basis of the finding that the cytoplasmic half of an out-going segment is sufficient for alkaline phosphatase export to the periplasm while the periplasmic half of an in-going segment prevents it [Calamia, T., & Manoil, C. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 4937], the activity profile of the melibiose permease-alkaline phosphatase fusions is consistent with the predicted topology of seven of 12 transmembrane segments. However, five transmembrane domains require adjustment, and as a consequence, the size of the central cytoplasmic loop is reduced and a significant number of charged residues are shifted from a hydrophilic to a hydrophobic domain in this region of the transporter.

Ligand recognition properties of the Escherichia coli 4-aminobutyrate transporter encoded by gabP. Specificity of Gab permease for heterocyclic inhibitors

4-aminobutyrate metabolism in Escherichia coli begins with transport across the cytoplasmic membrane via the GabP, which is encoded by gabP. Although GabP is specific and exhibits poor affinity for many cellular constituents such as the alpha-amino acids, the range of compounds recognized with high affinity has yet to be investigated. In order to address this gap in knowledge, we developed a gabP-negative host strain, which permits evaluation of test compounds for inhibitory effects on cloned GabP (expression inducible by isopropyl-1-thio-beta-D-galactopyranoside). Using this inducible expression system, three structurally distinct categories of high affinity transport inhibitor were identified. The structural dissimilarity of these inhibitors significantly alters our view of ligand recognition by GabP. Any complete model must now account for the observation that inhibition of 4-aminobutyrate transport can be mediated either (i) by open chain analogs of 4-aminobutyrate, (ii) by cyclic amino acid analogs, or (iii) by planar heterocyclic compounds lacking a carboxyl group. Such results do not support a previously sustainable view of GabP that features a restrictive ligand recognition domain, unable to accommodate structures that differ very much from the native substrate, 4-aminobutyrate.

Melibiose permease of Escherichia coli: substrate-induced conformational changes monitored by tryptophan fluorescence spectroscopy

Tryptophan fluorescence spectroscopy has been used to investigate the effects of sugars and coupling cations (H+, Na+, or Li+) on the conformational properties of purified melibiose permease after reconstitution in liposomes. Melibiose permease emission fluorescence is selectively enhanced by sugars, which serve as substrates for the symport reaction, alpha-galactosides producing larger variations (13-17%) than beta-galactosides (7%). Moreover, the sugar-dependent fluorescence increase is specifically potentiated by NaCl and LiCl (5-7 times), which are well-established activators of sugar binding and transport by the permease. The potentiation effect is greater in the presence of LiCl than NaCl. On their own, sodium and lithium ions produce quenching of the fluorescence signal (2%). Evidence suggesting that sugars and cations compete for their respective binding sites is also given. Both the sugar-induced fluorescence variation and the NaCl(or LiCl)-dependent potentiation effect exhibit saturation kinetics. In each ionic condition, the half-maximal fluorescence change is found at a sugar concentration corresponding to the sugar-binding constant. Also, half-maximal potentiation of the fluorescence change by sodium or lithium occurs at a concentration comparable to the activation constant of sugar binding by each ion. The sugar- and ion-dependent fluorescence variations still take place after selective inactivation of the permease substrate translocation capacity by N-ethylmaleimide. Taken together, the data suggest that the changes in permease fluorescence reflect conformational changes occurring upon the formation of ternary sugar/cation/permease complexes.