Altered substrate selectivity in a mutant of an intrahelical salt bridge in UhpT, the sugar phosphate carrier of Escherichia coli

Utilization of organophosphate:phosphate antiporter for isotope-labeling experiments in E. coli

The transport of organophosphates across the cytoplasma membrane is mediated by organophosphate:phosphate antiporter proteins. In this work, we present the application of a recombinant phosphoenolpyruvate:phosphate antiporter for isotopic labeling experiments in E. coli strains. The antiporters UhpT, UhpT-D388C, and PgtP were investigated regarding transport activity and growth on phosphoenolpyruvate as sole carbon source. The expression of the protein variant UhpT-D388C in a shikimic acid producing E. coli strain was used to show the successful isotopic labeling of shikimic acid from extracellular phosphoenolpyruvate. The results demonstrate the possibility of a direct incorporation of exogenously applicated glycolysis intermediates into E. coli cells for ¹³C-labeling experiments.

The SLC37 family of sugar-phosphate/phosphate exchangers

The SLC37 family members are endoplasmic reticulum (ER)-associated sugar-phosphate/phosphate (P(i)) exchangers. Three of the four members, SLC37A1, SLC37A2, and SLC37A4, function as Pi-linked glucose-6-phosphate (G6P) antiporters catalyzing G6P:P(i) and P(i):P(i) exchanges. The activity of SLC37A3 is unknown. SLC37A4, better known as the G6P transporter (G6PT), has been extensively characterized, functionally and structurally, and is the best characterized family member. G6PT contains 10 transmembrane helices with both N and C termini facing the cytoplasm. The primary in vivo function of the G6PT protein is to translocate G6P from the cytoplasm into the ER lumen where it couples with either the liver/kidney/intestine-restricted glucose-6-phosphatase-α (G6Pase-α or G6PC) or the ubiquitously expressed G6Pase-β (or G6PC3) to hydrolyze G6P to glucose and P(i). The G6PT/G6Pase-α complex maintains interprandial glucose homeostasis, and the G6PT/G6Pase-β complex maintains neutrophil energy homeostasis and functionality. G6PT is highly selective for G6P and is competitively inhibited by cholorogenic acid and its derivatives. Neither SLC37A1 nor SLC37A2 can couple functionally with G6Pase-α or G6Pase-β, and the antiporter activities of SLC37A1 or SLC37A2 are not inhibited by cholorogenic acid. Deficiencies in G6PT cause glycogen storage disease type Ib (GSD-Ib), a metabolic and immune disorder. To date, 91 separate SLC37A4 mutations, including 39 missense mutations, have been identified in GSD-Ib patients. Characterization of missense mutations has yielded valuable information on functionally important residues in the G6PT protein. The biological roles of the other SLC37 proteins remain to be determined and deficiencies have not yet been correlated to diseases.

Structural basis of substrate selectivity in the glycerol-3-phosphate: phosphate antiporter GlpT

Major facilitators represent the largest superfamily of secondary active transporter proteins and catalyze the transport of an enormous variety of small solute molecules across biological membranes. However, individual superfamily members, although they may be architecturally similar, exhibit strict specificity toward the substrates they transport. The structural basis of this specificity is poorly understood. A member of the major facilitator superfamily is the glycerol-3-phosphate (G3P) transporter (GlpT) from the Escherichia coli inner membrane. GlpT is an antiporter that transports G3P into the cell in exchange for inorganic phosphate (P(i)). By combining large-scale molecular-dynamics simulations, mutagenesis, substrate-binding affinity, and transport activity assays on GlpT, we were able to identify key amino acid residues that confer substrate specificity upon this protein. Our studies suggest that only a few amino acid residues that line the transporter lumen act as specificity determinants. Whereas R45, K80, H165, and, to a lesser extent Y38, Y42, and Y76 contribute to recognition of both free P(i) and the phosphate moiety of G3P, the residues N162, Y266, and Y393 function in recognition of only the glycerol moiety of G3P. It is the latter interactions that give the transporter a higher affinity to G3P over P(i).

Structure-function study of the glucose-6-phosphate transporter, an eukaryotic antiporter deficient in glycogen storage disease type Ib

Glycogen storage disease type Ib is caused by deficiencies in the glucose-6-phosphate transporter (G6PT), a phosphate (P(i))-linked antiporter capable of homologous (P(i):P(i)) and heterologous (G6P:P(i)) exchanges similar to the bacterial hexose-6-phosphate transporter, UhpT. Protease protection and glycosylation scanning assays have suggested that G6PT is anchored to the endoplasmic reticulum by 10 transmembrane domains. However, recent homology modeling proposed that G6PT may contain 12 helices and that amino acids essential for the functions of UhpT also play important roles in G6PT. Site-directed mutagenesis and in vitro expression assays demonstrated that only one of the four residues critical for UhpT activity is essential in G6PT. Furthermore, glycosylation scanning and protease sensitivity assays showed that the 10-domain model of G6PT is more probable than the 12-domain UhpT-like model.

Functional analysis of mutations in the glucose-6-phosphate transporter that cause glycogen storage disease type Ib

The glucose-6-phosphate transporter (G6PT) deficient in glycogen storage disease type Ib is a phosphate (P(i))-linked antiporter capable of G6P: P(i) and P(i):P(i) exchanges. We previously characterized G6PT mutations by measuring G6P uptake activities in microsomes co-expressing G6PT and glucose-6-phosphatase-alpha. Here we report a new assay, based on reconstituted proteoliposomes carrying only G6PT, and characterize G6P and P(i) uptake activities of 23 G6PT mutations. We show that co-expression and G6PT-only assays are equivalent in measuring G6PT activity. However, the p.Q133P mutation exhibits differential G6P and P(i) transport activities, suggesting that characterizing G6P and P(i) transport activities of G6PT mutations may yield insights to this genetic disorder.

Salt-bridge dynamics control substrate-induced conformational change in the membrane transporter GlpT

Active transport of substrates across cytoplasmic membranes is of great physiological, medical and pharmaceutical importance. The glycerol-3-phosphate (G3P) transporter (GlpT) of the E. coli inner membrane is a secondary active antiporter from the ubiquitous major facilitator superfamily that couples the import of G3P to the efflux of inorganic phosphate (P(i)) down its concentration gradient. Integrating information from a novel combination of structural, molecular dynamics simulations and biochemical studies, we identify the residues involved directly in binding of substrate to the inward-facing conformation of GlpT, thus defining the structural basis for the substrate-specificity of this transporter. The substrate binding mechanism involves protonation of a histidine residue at the binding site. Furthermore, our data suggest that the formation and breaking of inter- and intradomain salt bridges control the conformational change of the transporter that accompanies substrate translocation across the membrane. The mechanism we propose may be a paradigm for organophosphate:phosphate antiporters.

Ins and outs of major facilitator superfamily antiporters

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

Glycerol-3-phosphate transporter of Escherichia coli: structure, function and regulation

Glycerol-3-phosphate (G3P) plays a major role in glycolysis and phospholipid biosynthesis in the cell. Escherichia coli uses a secondary membrane transporter protein, GlpT, to uptake G3P into the cytoplasm. The crystal structure of the protein was recently determined to 3.3 A resolution. The protein consists of an N- and a C-terminal domain, each formed by a compact bundle of six transmembrane alpha-helices. The substrate-translocation pore is found at the domain interface and faces the cytoplasm. At the closed end of the pore is the substrate binding site, which is formed by two arginine residues. In combination with biochemical data, the crystal structure suggests a single binding site, alternating access mechanism for substrate translocation, namely, the substrate bound at the N- and C-terminal domain interface is transported across the membrane via a rocker-switch type of movement of the domains. Furthermore, GlpT may serve as a structural and mechanistic paradigm for other secondary active membrane transporters.

Altered oxyanion selectivity in mutants of UhpT, the Pi-linked sugar phosphate carrier of Escherichia coli

In Escherichia coli, the UhpT transporter catalyzes the electroneutral accumulation of sugar 6-phosphate by exchange with internal inorganic phosphate (Pi). The substrate specificity of UhpT is regulated at least in part by constituents of an Asp388-Lys391 intrahelical salt bridge, and mutations that remove one but not both of these residues alter UhpT preference for organophosphate substrates. Using site-directed mutagenesis, we examined the role played by these two positions in the selection of the oxyanion countersubstrate. We show that derivatives having aliphatic or polar residues at positions 388 and 391 are gain-of-function mutants capable of transporting SO4 as well as Pi. These oxyanions share similar structures but differ significantly in the presence of a proton(s) on Pi. Our findings therefore lead us to suggest that the Asp388-Lys391 ion pair acts normally as a filter that prevents substrates lacking a proton that can be donated from occupying the UhpT active site.

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

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

Functional analysis of an inosine-guanosine transporter from Leishmania donovani. The role of conserved residues, aspartate 389 and arginine 393

Equilibrative nucleoside transporters encompass two conserved, charged residues that occur within predicted transmembrane domain 8. To assess the role of these "signature" residues in transporter function, the Asp389 and Arg393 residues within the LdNT2 nucleoside transporter from Leishmania donovani were mutated and the resultant phenotypes evaluated after transfection into Delta ldnt2 parasites. Whereas an R393K mutant retained transporter activity similar to that of wild type LdNT2, the R393L, D389E, and D389N mutations resulted in dramatic losses of transport capability. Tagging the wild type and mutant ldnt2 proteins with green fluorescent protein demonstrated that the D389N and D389E mutants targeted properly to the parasite cell surface and flagellum, whereas the expression of R393L at the cell surface was profoundly compromised. To test whether Asp389 and Arg393 interact, a series of mutants was generated, D389R/R393R, D389D/R393D, and D389R/R393D, within the green fluorescent protein-tagged LdNT2 construct. Although all of these ldnt2 mutants were transport-deficient, D389R/R393D localized properly to the plasma membrane, while neither D389R/R393R nor D389D/R393D could be detected. Moreover, a transport-incompetent D389N/R393N double ldnt2 mutant also localized to the parasite membrane, whereas a D389L/R393L ldnt2 mutant did not, suggesting that an interaction between residues 389 and 393 may be involved in LdNT2 membrane targeting. These studies establish genetically that Asp389 is critical for optimal transporter function and that a positively charged or polar residue at Arg393 is essential for proper expression of LdNT2 at the plasma membrane.

Structure and mechanism of the glycerol-3-phosphate transporter from Escherichia coli

The major facilitator superfamily represents the largest group of secondary membrane transporters in the cell. Here we report the 3.3 angstrom resolution structure of a member of this superfamily, GlpT, which transports glycerol-3-phosphate into the cytoplasm and inorganic phosphate into the periplasm. The amino- and carboxyl-terminal halves of the protein exhibit a pseudo two-fold symmetry. Closed off to the periplasm, a centrally located substrate-translocation pore contains two arginines at its closed end, which comprise the substrate-binding site. Upon substrate binding, the protein adopts a more compact conformation. We propose that GlpT operates by a single-binding site, alternating-access mechanism through a rocker-switch type of movement.

Functional characterization of cysteine residues in GlpT, the glycerol 3-phosphate transporter of Escherichia coli

In Escherichia coli, the GlpT transporter, a member of the major facilitator superfamily, moves external glycerol 3-phosphate (G3P) into the cytoplasm in exchange for cytoplasmic phosphate. Study of intact cells showed that both GlpT and HisGlpT, a variant with an N-terminal six-histidine tag, are inhibited (50% inhibitory concentration approximately 35 microM) by the hydrophilic thiol-specific agent p-mercurichlorobenzosulfonate (PCMBS) in a substrate-protectable fashion; by contrast, two other thiol-directed probes, N-maleimidylpropionylbiocytin (MPB) and [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET), have no effect. Use of variants in which the HisGlpT native cysteines are replaced individually by serine or glycine implicates Cys-176, on transmembrane helix 5 (TM5), as the major target for PCMBS. The inhibitor sensitivity of purified and reconstituted HisGlpT or its cysteine substitution derivatives was found to be consistent with the findings with intact cells, except that a partial response to PCMBS was found for the C176G mutant, suggesting the presence of a mixed population of both right-side-out (RSO) (resistant) and inside-out (ISO) (sensitive) orientations after reconstitution. To clarify this issue, we studied a derivative (P290C) in which the RSO molecules can be blocked independently due to an MPB-responsive cysteine in an extracellular loop. In this derivative, comparisons of variants with (P290C) and without (P290C/C176G) Cys-176 indicated that this residue shows substrate-protectable inhibition by PCMBS in the ISO orientation in proteoliposomes. Since PCMBS gains access to Cys-176 from both periplasmic and cytoplasmic surfaces of the protein (in intact cells and in a reconstituted ISO orientation, respectively) and since access is unavailable when the substrate is present, we propose that Cys-176 is located on the transport pathway and that TM5 has a role in lining this pathway.

Connection of transport and sensing by UhpC, the sensor for external glucose-6-phosphate in Escherichia coli

UhpC is a membrane-bound sensor protein in Escherichia coli required for recognizing external glucose-6-phosphate (Glc6P) and induction of the transport protein UhpT. Recently, it was shown that UhpC is also able to transport Glc6P. In this study we investigated whether these transport and sensing activities are obligatorily coupled in UhpC. We expressed a His-UhpC protein in a UhpC-deficient E. coli strain and verified that this construct does not alter the basic biochemical properties of the Glc6P sensor system. The effects of arginine replacements, mutations of the central loop, and introduction of a salt bridge in UhpC on transport and sensing were compared. The exchanges R46C, R266C and R149C moderately affected transport by UhpC but strongly decreased the sensing ability. This suggested that the affinity for Glc6P as a transported substrate is uncoupled in UhpC from its affinity for Glc6P as an inducer. Four of the 11 arginine mutants showed a constitutive phenotype but had near wild-type transport activity suggesting that Glc6P can be transported by a molecule locked in the inducing conformation. Introduction of an intrahelical salt bridge increased the transport activity of UhpC but abolished sensing. Three conserved residues from the central loop were mutated and although none of these showed transport, one exhibited increased affinity for sensing. Taken together, these data show that transport by UhpC is not required for sensing, that conserved arginine residues are important for sensing and not for transport, and that residues located in the central hydrophilic loop are critical for transport and for sensing.

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

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

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

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

Properties of the glucose-6-phosphate transporter from Chlamydia pneumoniae (HPTcp) and the glucose-6-phosphate sensor from Escherichia coli (UhpC)

The amino acid sequence of the proposed glucose-6-phosphate (Glc6P) transporter from Chlamydia pneumoniae (HPTcp; hexose phosphate transporter [Chlamydia pneumoniae]) exhibits a higher degree of similarity to the Escherichia coli Glc6P sensor (UhpC) than to the E. coli Glc6P transporter (UhpT). Overexpression of His-UhpC in a UhpT-deficient E. coli strain revealed that the sensor protein is also able to transport Glc6P and exhibits an apparent K(m) ((Glc6P)) of 25 microM, whereas His-HPTcp exhibits an apparent K(m)( (Glc6P)) of 98 microM. His-HPTcp showed a four-times-lower specific activity than His-UhpT but a 56-times-higher specific activity than His-UhpC. Like His-UhpT and His-UhpC, the carrier His-HPTcp performs a sugar-phosphate/inorganic-phosphate antiporter mode of transport. Surprisingly, while physiological concentrations of inorganic phosphate competitively inhibited transport mediated by the E. coli proteins His-UhpT and His-UhpC, transport mediated by His-HPTcp was not inhibited. Interestingly, C(3)-organophosphates stimulated His-HPTcp activity but not His-UhpT- or His-UhpC-catalyzed Glc6P transport. In contrast to His-UhpC, the His-HPTcp protein does not act as a Glc6P sensor in the uhp regulon.

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

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

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

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

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

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

Glucose-6-phosphate transporter and receptor functions of the glucose 6-phosphatase system analyzed from a consensus defined by multiple alignments

The cDNA encoding the protein (P46) that is mutated in glycogen storage disease type-1b (GSD-1b) has been previously cloned by homology with bacterial sequences of the uhp (upper hexose phosphate) system. Hydropathic profiles, transmembrane-prediction analysis, and a multiple alignment of 14 sequences related to P46 (with percentage of identity around 30%) allowed to identify two large domains in the proteins linked by a large variable loop. Highly conserved transmembrane (TM) segments, TM1 and TM4 in the first domain and TM5 in the second one, were identified almost in all the integral proteins related to P46. The multiple alignment allowed definition of a consensus involving the 14 sequences related to P46. The detailed comparison of the consensus with the UhpT (the bacterial G6P transporter) and with UhpC (the bacterial G6P receptor) sequences reveals that the P46 protein could carry both G6P receptor and transporter functions.

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

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