Substrate-induced conformational change of human erythrocyte glucose transporter: inactivation by alkylating reagents

A three-dimensional model of the human facilitative glucose transporter Glut1

The human facilitative transporter Glut1 is the major glucose transporter present in all human cells, has a central role in metabolism, and is an archetype of the superfamily of major protein facilitators. Here we describe a three-dimensional structure of Glut1 based on helical packing schemes proposed for lactose permease and Glut1 and predictions of secondary structure, and refined using energy minimization, molecular dynamics simulations, and quality and environmental scores. The Ramachandran scores and the stereochemical quality of the structure obtained were as good as those for the known structures of the KcsA K(+) channel and aquaporin 1. We found two channels in Glut1. One of them traverses the structure completely, and is lined by many residues known to be solvent-accessible. Since it is delimited by the QLS motif and by several well conserved residues, it may serve as the substrate transport pathway. To validate our structure, we determined the distance between these channels and all the residues for which mutations are known. From the locations of sugar transporter signatures, motifs, and residues important to the transport function, we find that this Glut1 structure is consistent with mutagenesis and biochemical studies. It also accounts for functional deficits in seven pathogenic mutants.

Cysteine residues in the D-galactose-H+ symport protein of Escherichia coli: effects of mutagenesis on transport, reaction with N-ethylmaleimide and antibiotic binding

The galactose-H(+) membrane-transport protein, GalP, of Escherichia coli is similar in substrate specificity and susceptibility to cytochalasin B and forskolin, to the human GLUT1 sugar-transport protein; furthermore, they are about 30% identical in amino acid sequence. Transport activities of both GalP and GLUT1 are inhibited by the thiol-group-specific reagent, N-ethylmaleimide. GalP contains only three cysteine residues at positions 19, 374 and 389, each of which we have mutated, singly and in combination, to serine. Each single change of Cys-->Ser has only a minor effect on transport activity, whereas alteration of all three simultaneously profoundly diminishes V(max) for transport. The high level of expression of the GalP protein facilitates measurements of the reactivity of each mutant with N-ethylmaleimide or eosin 5-maleimide, which conclusively demonstrate that Cys(374) is the site of covalent modification by the reagents. By comparing the reactivity of Cys(374) in right-side-out and inside-out vesicles it appears that Cys(374) is located on the cytoplasmic face of the GalP protein. Although impaired in transport activity, the 'Cys-free' mutant, with all three cysteine residues mutated into serine, binds cytochalasin B and forskolin with wild-type affinities. All these results are interpreted in terms of a 12-helix model of the folding of the protein, in which the relative orientations of helix 10, containing the reactive Cys(374) residue, and helix 11, containing the unreactive Cys(389) residue, can now be defined.

Molecular characterization of the staphylococcal multidrug resistance export protein QacC

The QacC polypeptide is a member of a family of small membrane proteins which confer resistance to toxic compounds. The staphylococcal qacC gene confers resistance to toxic organic cations via proton-dependent export. The membrane topology of the QacC polypeptide was investigated by constructing and analyzing a series of qacC-phoA and qacC-lacZ fusions. From these analyses, most of the predicted features of the QacC protein were verified, although data regarding the possible orientation of the COOH region were not conclusive. The role of the sole cysteine residue, Cys-42, in QacC was studied by using the sulfhydryl reagent N-ethylmaleimide and site-directed mutagenesis. N-Ethylmaleimide was shown to inhibit qacC-mediated ethidium export. Multiple amino acid substitutions were made for Cys-42, and mutations at this location had various effects on resistance specificity. This suggests that the Cys-42 residue may be located near a region of QacC that is involved in substrate recognition. Mutagenesis of conserved residues in QacC indicated that Tyr-59 and Trp-62 also play an essential structural or functional role in QacC.

The role of cysteine residues in glucose-transporter-GLUT1-mediated transport and transport inhibition

The role of cysteine residues in transport function of the glucose transporter GLUT1 was investigated by a mutagenesis-expression strategy. Each of the six cysteine residues was individually replaced by site-directed mutagenesis. Expression of the heterologous wild-type or mutant glucose transporters and transport measurements at two hexose concentrations (50 microM and 5 mM) were undertaken in Xenopus oocytes. The catalytic activity of GLUT1 was retained, despite substitution of each single cysteine residue, which indicated that no individual residue is essential for hexose transport. This finding questions the involvement of oligomerization or intramolecular stabilization by a single disulphide bond as a prerequisite for transporter activation under basal conditions. Application of the impermeant mercurial thiol-group-reactive reagent p-chloromercuribenzenesulphonate (pCMBS) to the external or internal surface of plasma membrane demonstrated that cysteine-429, within the sixth external loop, and cysteine-207, at the beginning of the large intracellular loop which connects transmembrane segments 6 and 7, are the residues which are involved in transport inhibition by impermeant thiol-group-reactive reagents from either side of the cell. These data support the predicted membrane topology of the transport protein by transport measurements. If residues other than the cysteines at positions 429 or 207 are exposed to either side of the plasma membrane by conformational changes, they do not contribute to the transport inhibition by pCMBS. Application of pCMBS to one side of the plasma membrane also inhibited transport from the opposite direction, most likely due to the hindrance of sugar-induced interconversion of transporter conformation.

Structural basis of human erythrocyte glucose transporter function: pH effects on intrinsic fluorescence

The effects of pH on the intrinsic fluorescence of purified human erythrocyte glucose transporter (HEGT) were studied to deduce the structure and the ligand-induced dynamics of this protein. D-Glucose increases tryptophan fluorescence of HEGT at a 320-nm peak with a concomitant reduction in a 350-nm peak, suggesting that glucose shifts a tryptophan residue from a polar to a nonpolar environment. Cytochalasin B or forskolin, on the other hand, only produces a reduction at the 350-nm peak. The pH titration of the intrinsic fluorescence of HEGT revealed that at least two tryptophan residues are quenched, one with a pKa of 5.5, the other with a pKa of 8.2, indicating involvement of histidine and cysteine protonation, respectively. D-Glucose abolishes both of these quenchings. Cytochalasin B or forskolin, on the other hand, abolishes the histidine quenching but not the cysteine quenching and induces a new pH quenching with a pKa of about 4, implicating involvement of a carboxyl group. These results, together with the known primary structure and the transmembrane disposition of this protein, predict the dynamic interactions between Trp388 and His337, Trp412 and Cys347, and Trp412 and Glu380, depending on liganded state of HEGT, and suggest the importance of the transmembrane helices 9, 10, and 11 in transport function.

Proton-linked sugar transport systems in bacteria

The cell membranes of various bacteria contain proton-linked transport systems for D-xylose, L-arabinose, D-galactose, D-glucose, L-rhamnose, L-fucose, lactose, and melibiose. The melibiose transporter of E. coli is linked to both Na+ and H+ translocation. The substrate and inhibitor specificities of the monosaccharide transporters are described. By locating, cloning, and sequencing the genes encoding the sugar/H+ transporters in E. coli, the primary sequences of the transport proteins have been deduced. Those for xylose/H+, arabinose/H+, and galactose/H+ transport are homologous to each other. Furthermore, they are just as similar to the primary sequences of the following: glucose transport proteins found in a Cyanobacterium, yeast, alga, rat, mouse, and man; proteins for transport of galactose, lactose, or maltose in species of yeast; and to a developmentally regulated protein of Leishmania for which a function is not yet established. Some of these proteins catalyze facilitated diffusion of the sugar without cation transport. From the alignments of the homologous amino acid sequences, predictions of common structural features can be made: there are likely to be twelve membrane-spanning alpha-helices, possibly in two groups of six; there is a central hydrophilic region, probably comprised largely of alpha-helix; the highly conserved amino acid residues (40-50 out of 472-522 total) form discrete patterns or motifs throughout the proteins that are presumably critical for substrate recognition and the molecular mechanism of transport. Some of these features are found also in other transport proteins for citrate, tetracycline, lactose, or melibiose, the primary sequences of which are not similar to each other or to the homologous series of transporters. The glucose/Na+ transporter of rabbit and man is different in primary sequence to all the other sugar transporters characterized, but it is homologous to the proline/Na+ transporter of E. coli, and there is evidence for its structural similarity to glucose/H+ transporters in Plants. In vivo and in vitro mutagenesis of the lactose/H+ and melibiose/Na+ (H+) transporters of E. coli has identified individual amino acid residues alterations of which affect sugar and/or cation recognition and parameters of transport. Most of the bacterial transport proteins have been identified and the lactose/H+ transporter has been purified. The directions of future investigations are discussed.

Glycation of the human erythrocyte glucose transporter in vitro and its functional consequences

Glycation of human erythrocyte membrane proteins was induced by incubation in vitro with high concentrations (80 mM or 200 mM) of D-glucose for 3 or 6 days. The extent of glycation was quantified from the covalent incorporation of 3H by reduction of the glucose glycation products with NaB3H4. For membranes incubated for 3 days with 80 mM-D-glucose, glycation in vitro of Band 4.5 (containing the glucose transporter) was equivalent to 0.11 mol of glucose/mol of glucose transporter, compared with 3H labelling in 3-day-incubated control membranes of 0.055 mol of glucose/mol of glucose transporter. In membranes incubated for 6 days with 200 mM-D-glucose, glycation increased to 0.21 mol of glucose/mol of glucose transporter, whereas the controls without glucose had 0.11 mol of glucose/mol of glucose transporter. Glycation in vitro was accompanied by a fall in the Bmax of binding of [3H]cytochalasin B (a competitive inhibitor of glucose transport), without any change in the binding affinity. The data suggest that glycated glucose transporters have decreased ability to bind cytochalasin B. It is proposed that glycation can alter glucose transporter activity.

A radioimmunoassay to screen for antibodies to native conformational antigens and analyse ligand-induced structural states of antigenic proteins

A radioimmunoassay is described in which antigenic protein was immobilized by incubating nitrocellulose filters of defined diameter with antigen-containing solutions. The amount of adsorbed antigen increased in a linear fashion over a wide range of antigen concentrations. The antigen-antibody reactions and the indicator reactions were performed by incubating the filters with appropriate solutions. During the test any contact of the antigen with air was avoided. Antigenic sites which are sensitive to protein denaturation by drying could be detected with the assay. The assay was also used to screen hybridoma supernatants for antibodies directed against Na+ cotransport proteins from renal brush-border membranes. Monoclonal antibodies were selected which showed different binding characteristics depending on whether or not substrates of Na+ cotransporters were present. Since binding of several antibodies was altered by two different substrates and not by non-transported control substances, these monoclonal antibodies were believed to interact with more than one transport system. One of the antibodies, which showed different antibody binding after addition of D-glucose or L-lactate, bound to a polypeptide component of the renal Na+-D-glucose cotransporter and was able to inhibit Na+ gradient-dependent D-glucose uptake in brush-border membrane vesicles (Koepsell, Korn, Raszeja-Specht, Bernotat-Danielowski, Ollig, 1988, J. Biol. Chem., in press). To investigate the effects of D-glucose and L-lactate on the binding of this antibody concentration dependence was measured. High and low affinity binding sites for D-glucose and L-lactate were characterized thereby demonstrating that the radioimmunoassay permits investigations of the properties of high and low affinity substrate binding sites.

Proteolytic dissection as a probe of conformational changes in the human erythrocyte glucose transport protein

Tryptic digestion has been used to investigate the conformational changes associated with substrate translocation by the human erythrocyte glucose transporter. The effects of substrates and inhibitors of transport on the rates of tryptic cleavage at the cytoplasmic surface of the membrane have confirmed previous observations that this protein can adopt at least two conformations. In the presence of phloretin or 4,6-O-ethylidene-D-glucose, the rate of cleavage is slowed. Because these inhibitors bind preferentially at the extracellular surface of the transporter, their effects must result from a conformational change rather than from steric hindrance. A conformational change must also be responsible for the effect of the physiological substrate D-glucose, which is to increase the rate of cleavage. The regions of the protein involved in the conformational changes include both of the large cytoplasmic regions that are cleaved by trypsin: these are the central hydrophilic region of the sequence (residues 213-269) and the hydrophilic C-terminal region (residues 457-492).