Peptide-specific antibody for the melibiose carrier of Escherichia coli localizes the carboxyl terminus to the cytoplasmic face of the membrane

Alteration of sugar-induced conformational changes of the melibiose permease by mutating Arg141 in loop 4-5

The melibiose permease (MelB) from Escherichia coli couples the uptake of melibiose to that of Na+, Li+, or H+. In this work, we applied attenuated total reflection Fourier transform infrared (ATR-FTIR) difference spectroscopy to obtain information about the structural changes involved in substrate interaction with the R141C mutant and with the wild-type MelB reacted with N-ethylmaleimide (NEM). These modified permeases have the ability to bind the substrates but fail to transport them. It is shown that the sugar-induced ATR-FTIR difference spectra of the R141C mutant are different from those corresponding to the Cys-less permease from which it is derived. There are alterations of peaks assigned to turns and beta-structures located most likely in loop 4-5. In addition, and quite notably, a peak at 1659 cm(-1), assigned to changes at the level of one alpha-helix subpopulation, disappears in the melibiose-induced difference spectrum in the presence of Na+, suggesting a reduction of the conformational change capacity of the mutated MelB. These helices may involve structural components that couple the cation- and sugar-binding sites. On the other hand, MelB-NEM difference spectra are proportionally less disrupted than the R141C ones. Hence, the transport cycle of these two permeases, modified at two different loops, is most likely impaired at a different stage. It is proposed that the R141C mutant leads to the generation of a partially defective ternary complex that is unable to catalyze the subsequent conformational change necessary for substrate translocation.

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.

Changes in secondary structures and acidic side chains of melibiose permease upon cosubstrates binding

Infrared difference spectroscopy analysis of the purified melibiose permease of Escherichia coli reconstituted into liposomes was carried out as a function of the presence of the two symporter substrates (Na(+), melibiose) in either H(2)O or in D(2)O media. Essentially, the data first show that addition of Na(+) induces appearance of peaks assigned to changes in the environment and/or orientation of alpha-helical domains of purified melibiose permease. Likewise, melibiose addition in the presence of Na(+) produces peaks corresponding to additional changes of alpha-helix environment or tilt. In addition to these changes, a pair of peaks (1599 (+) cm(-1)/1576 (-) cm(-1)) appearing in the Na(+)-induced difference spectrum is assigned to the antisymmetric stretching of COO(-) groups, since they show practically no shift upon H/D exchange. It is proposed that these acidic groups participate in Na(+) co-ordination. A corresponding pair of peaks, again fairly insensitive to H/D substitution (1591 (-) cm(-1)/1567 (+) cm(-1)), appear in the melibiose-induced difference spectra, and may again be assigned to COO(-) groups. The latter carboxyl groups may correspond to part or all of the acidic residues interacting with Lys or Arg in the resting state that become free upon melibiose binding.

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.

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.

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.

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.

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.

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.

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.

Mutational analysis of amiloride sensitivity of the NhaA Na+/H+ antiporter from Vibrio parahaemolyticus

The activity of the NhaA Na+/H+ antiporter of Vibrio parahaemolyticus is inhibited by amiloride. We found an amino acid sequence in the NhaA that was identical to a putative amiloride binding domain of the Na+/H+ exchanger in mammalian cells. We constructed mutant NhaAs that had amino acid substitutions in the putative amiloride binding domain by site-directed mutagenesis. These include V62L (Val62 replaced by Leu), F63Y, F64Y, and L65F. Most mutant NhaAs showed decreased sensitivity for amiloride. Among these, the F64Y mutant NhaA showed the least amiloride sensitivity, with a Ki value 7 to 10 times greater than that in the wild type. Thus, the sequence between residues V62 and L65 in NhaA, especially F64, is very important for the inhibitory effect of amiloride on the antiporter.

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.

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.

Transport properties of Asp-51-->Glu and Asp-120-->Glu mutants of the melibiose carrier of Escherichia coli

Asp-51-->Glu and Asp-120-->Glu mutants of the melibiose carrier of Escherichia coli were investigated for their cation/sugar cotransport properties. The carrier containing Glu-51 showed proton/melibiose cotransport but was extremely defective in Na+ or Li+ stimulation of sugar accumulation. On the other hand, the carrier containing Glu-120 had lost the ability to couple protons with melibiose uptake while retaining considerable Na+ or Li+ cotransport with melibiose (40-fold accumulation versus 90-fold for the wild type in the presence of Na+). It is concluded that both Asp-51 and Asp-120 are important for cation recognition.

Bacterial solute transport proteins in their lipid environment

The cytoplasmic membrane of bacteria is a selective barrier that restricts entry and exit of solutes. Transport of solutes across this membrane is catalyzed by specific membrane proteins. Integral membrane proteins usually require specific lipids for optimal activity and are inhibited by other lipid species. Their activities are also sensitive to the lipid bilayer dynamics and physico-chemical state. Bacteria can adapt to changes in the environments (respective temperature, hydrostatic pressure, and pH) by altering the lipid composition of the membrane. Homeoviscous adaptation results in the maintenance of the liquid-crystalline phase through alterations in the degree of acyl chain saturation and branching, acyl chain length and the sterol content of the membrane. Homeophasic adaptation prevents the formation of non-bilayer phases, which would disrupt membrane organization and increase permeability. A balance is maintained between the lamellar phase, preferring lipids, and those that adopt a non-bilayer organization. As a result, the membrane proteins are optimally active under physiological conditions. The molecular basis of lipid-protein interactions is still obscure. Annular lipids stabilize integral membrane proteins. Stabilization occurs through electrostatic and possibly other interactions between the lipid headgroups and the charged amino acid residues close to the phospholipid-water interface, and hydrophobic interactions between the fatty acyl chains and the membrane-spanning segments. Reconstitution techniques allow manipulation of the lipid composition of the membrane in a way that is difficult to achieve in vivo. The physical characteristics of membrane lipids that affect protein-mediated transport functions have been studied in liposomal systems that separate an inner and outer compartment. The activity of most transport proteins is modulated by the bulk physical characteristics of the lipid bilayer, while specific lipid requirements appear rare.

Identification, characterization, and molecular cloning of a novel transporter-like protein localized to the central nervous system

During the course of large scale purification of the D1 dopamine receptor from rat brain, a protein of approximately 87,000 daltons (p87) was observed to copurify with the D1 receptor through four chromatographic steps. To characterize the nature of this protein, bovine and rat cDNA clones were isolated and sequenced. The bovine and rat clones were highly conserved (98.5% identity). Each clone possessed an open reading frame of 2226 base pairs encoding a protein of 742 amino acids (calculated MW of 82,500), containing three stretches of peptide sequence obtained from p87 sequence analysis. Comparison of the deduced peptide sequence of this protein with those found in available databanks revealed that it was a novel protein related to the family of nutrient transport proteins from eukaryotes and bacteria, including, the mammalian facilitated glucose transporters, the yeast transporters for maltose, lactose, and glucose, and the proton-driven bacterial transporters for arabinose, xylose, and citrate. In addition p87 also shares with these transporters a similar hydropathicity profile that suggests the presence of 12 transmembrane segments. The mRNA for p87 appears to be localized primarily, if not exclusively, to the central nervous system. Northern blot analysis reveals a message of approximately 4.8 kb in cortex, hippocampus, brain stem, and cerebellum, but no detectable signal in peripheral tissues such as spleen, liver, kidney, lung, heart, or skeletal muscle. Evidence form Western blot analysis and immunohistochemistry suggests that this protein may be expressed in intracellular organelles or the membrane of synaptosomes rather than plasma membrane. Based on its structure and properties, p87 appears to define a new class of transporter-like proteins.

Melibiose permease of Escherichia coli: mutation of histidine-94 alters expression and stability rather than catalytic activity

Previous studies utilizing site-directed mutagenesis [Pourcher et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 468-472] indicate that out of seven histidinyl residues in the melibiose (mel) permease of Escherichia coli, only His94 is important. The role of His94 has now been investigated by replacing the residue with Asn, Gln, or Arg. Cells expressing mel permease with Asn94 or Gln94 retain 30% or 20% of wild-type activity, respectively, and surprisingly, immunological assays demonstrate that diminished transport activity is due to a proportional reduction in the amount of permease in the membrane. Moreover, kinetic analyses of transport and ligand binding studies with right-side-out membrane vesicles indicate that both substrate recognition and turnover (kcat) are comparable in the mutant permeases and the wild-type. Mel permease with Arg in place of His94 also binds ligand and catalyzes sugar accumulation, but only when the cells are grown at 30 degrees C, and evidence is presented that Arg94 permease is inactivated at 37 degrees C. Finally, labeling studies demonstrate that expression and/or insertion of the permease, but not degradation, is strongly dependent on the amino acid present at position 94 and temperature. The findings indicate that an imidazole group at position 94 is required for proper insertion and stability of mel permease, but not for transport activity per se. Since replacement of the other six histidinyl residues in mel permease with Arg has little or no effect on transport activity, it is concluded that histidinyl residues do not play a direct role in the mechanism of this secondary transport protein.

Asp-51 and Asp-120 are important for the transport function of the Escherichia coli melibiose carrier

Asp-51 and Asp-120 of the Escherichia coli melibiose carrier on plasmid pKKMB were separately replaced by amber codons and transformed into eight amber suppressor strains, producing eight amino acid substitutions for each site. Glu-51 and Glu-120 were the only replacements in the carrier that allowed the cells to ferment melibiose and that showed transport of melibiose against a concentration gradient. Revertants to Glu-51 and Glu-120 show less activity than the wild type. The Asp-51 position is more crucial for Na(+)-stimulated melibiose accumulation than is the Asp-120 site.

Melibiose permease of Escherichia coli: mutation of aspartic acid 55 in putative helix II abolishes activation of sugar binding by Na+ ions

An aspartic residue (Asp55) located in the putative transmembrane alpha-helix II of the melibiose(mel) permease of Escherichia coli was replaced by Cys using oligonucleotide-directed, site-specific mutagenesis. Although D55C permease is expressed at 0.7 times the level of wild type permease, the mutated mel permease loses the ability to catalyse Na+ or H+ coupled melibiose transport against a concentration gradient. (3H) p-nitrophenyl-alpha-D-galactoside (NPG) binding studies demonstrated that D55C permease binds the sugar co-substrate but Na+ (or Li+) ions do no longer enhance the affinity of D55C permease for the co-transported sugar. In addition sugar binding on D55C permease but not on wild type permease is inactivated by sulfhydryl reagents and the inhibition protected by an excess of melibiose. These observations suggest 1) that the negatively-charged Asp55 residue, expected to be within the membrane embedded domain near the NH2 extremity of mel permease, is in or near the Na(+)-binding site and 2) that the cation and sugar binding sites may be overlapping.

Identification and initial topological analysis of the Rickettsia prowazekii ATP/ADP translocase

The Rickettsia prowazekii ATP/ADP translocase was identified by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) and immunoblot analysis using antibodies raised against a synthetic peptide corresponding in sequence to the carboxyl-terminal 17 amino acids of the carrier. Both the translocase of R. prowazekii and that expressed by Escherichia coli transformants containing the rickettsial gene had an apparent molecular mass of 36,500 Da by SDS-PAGE analysis, a mass considerably less than that deduced from the sequence of the gene. The SDS-solubilized translocase aggregated upon heating at 100 degrees C in the presence of disulfide bond-reducing agents. Similar concentrations of disulfide bond-reducing agents inhibited the exchange transport of adenine nucleotides by both R. prowazekii and translocase-expressing E. coli. These data suggested that an intramolecular disulfide bond in the translocase was essential for transport activity. The antipeptide antibodies used for identification of the translocase bound preferentially to inside-out membrane vesicles of translocase-expressing E. coli relative to right-side-out spheroplasts, thus indicating that the carboxyl terminus of the carrier is located on the cytoplasmic side of the bacterial inner membrane. Protease studies were unable to localize the carboxyl terminus because of the resistance of this region of the native translocase to proteolytic cleavage. These data in conjunction with hydrophobicity analysis were used to construct an initial topological model of the translocase within the cell membrane.