Kinetics and thermodynamics of the binding of forskolin to the galactose-H+ transport protein, GalP, of Escherichia coli

Amphipols outperform dodecylmaltoside micelles in stabilizing membrane protein structure in the gas phase

Noncovalent mass spectrometry (MS) is emerging as an invaluable technique to probe the structure, interactions, and dynamics of membrane proteins (MPs). However, maintaining native-like MP conformations in the gas phase using detergent solubilized proteins is often challenging and may limit structural analysis. Amphipols, such as the well characterized A8-35, are alternative reagents able to maintain the solubility of MPs in detergent-free solution. In this work, the ability of A8-35 to retain the structural integrity of MPs for interrogation by electrospray ionization-ion mobility spectrometry-mass spectrometry (ESI-IMS-MS) is compared systematically with the commonly used detergent dodecylmaltoside. MPs from the two major structural classes were selected for analysis, including two β-barrel outer MPs, PagP and OmpT (20.2 and 33.5 kDa, respectively), and two α-helical proteins, Mhp1 and GalP (54.6 and 51.7 kDa, respectively). Evaluation of the rotationally averaged collision cross sections of the observed ions revealed that the native structures of detergent solubilized MPs were not always retained in the gas phase, with both collapsed and unfolded species being detected. In contrast, ESI-IMS-MS analysis of the amphipol solubilized MPs studied resulted in charge state distributions consistent with less gas phase induced unfolding, and the presence of lowly charged ions which exhibit collision cross sections comparable with those calculated from high resolution structural data. The data demonstrate that A8-35 can be more effective than dodecylmaltoside at maintaining native MP structure and interactions in the gas phase, permitting noncovalent ESI-IMS-MS analysis of MPs from the two major structural classes, while gas phase dissociation from dodecylmaltoside micelles leads to significant gas phase unfolding, especially for the α-helical MPs studied.

Determinants of ligand binding affinity and cooperativity at the GLUT1 endofacial site

Cytochalasin B (CB) and forskolin (FSK) inhibit GLUT1-mediated sugar transport in red cells by binding at or close to the GLUT1 endofacial sugar binding site. Paradoxically, very low concentrations of each of these inhibitors produce a modest stimulation of sugar transport [ Cloherty, E. K., Levine, K. B., and Carruthers, A. ((2001)) The red blood cell glucose transporter presents multiple, nucleotide-sensitive sugar exit sites. Biochemistry 40 ((51)) 15549-15561]. This result is consistent with the hypothesis that the glucose transporter contains multiple, interacting, endofacial binding sites for CB and FSK. The present study tests this hypothesis directly and, by screening a library of cytochalasin and forskolin analogues, asks what structural features of endofacial site ligands determine binding site affinity and cooperativity. Like CB, FSK competitively inhibits exchange 3-O-methylglucose transport (sugar uptake in cells containing intracellular sugar) but noncompetitively inhibits sugar uptake into cells lacking sugar at 4 °C. This refutes the hypothesis that FSK binds at GLUT1 endofacial and exofacial sugar binding sites. Some forskolin derivatives and cytochalasins inhibit equilibrium [(3)H]-CB binding to red cell membranes depleted of peripheral proteins at 4 °C. Others produce a moderate stimulation of [(3)H]-CB binding when introduced at low concentrations but inhibit binding as their concentration is increased. Yet other analogues modestly stimulate [(3)H]-CB binding at all inhibitor concentrations applied. These findings are explained by a carrier that presents at least two interacting endofacial binding sites for CB or FSK. We discuss this result within the context of models for GLUT1-mediated sugar transport and GLUT1 quaternary structure, and we evaluate the major determinants of ligand binding affinity and cooperativity.

Defining topological features of membrane proteins by nanoelectrospray ionisation mass spectrometry

The D-galactose-H(+) symport protein, GalP, of Escherichia coli is the bacterial homologue of the human glucose transport protein, GLUT1. Here we demonstrate that mass spectrometry can be used to map modification by covalently bound reagents, and also to detect structural changes in the GalP protein that occur upon substrate binding. The small thiol-group-specific reagent N-ethylmaleimide (NEM) was used to modify the cysteine residues in GalP(His)(6) both alone and in the presence of D-glucose, a known substrate. Employing a mixture of proteolysis and thermal degradation methods, the three cysteine residues were found to undergo sequential reactions with NEM, with Cys374 being modified first, followed by Cys389 and finally Cys19, thus indicating their different accessibilities within the three-dimensional structure of the protein. Prior binding of the substrate D-glucose to the protein protected Cys19 and Cys374 against NEM modification, but not Cys389. Cys374 had been expected to be shielded by D-glucose binding while Cys389 had been expected to be unaffected, consistent with their proposed respective locations in the vicinity of, and distant from, the sugar binding site. However, the inaccessibility of Cys19 was unexpected and suggests a structural change in the protein promoted by D-glucose binding which changes the proximity of Cys19 with respect to the D-glucose-binding site.

Investigation of the structure and function of a Shewanella oneidensis arsenical-resistance family transporter

The toxic metalloid arsenic is an abundant element and most organisms possess transport systems involved in its detoxification. One such family of arsenite transporters, the ACR3 family, is widespread in fungi and bacteria. To gain a better understanding of the molecular mechanism of arsenic transport, we report here the expression and characterization of a family member, So_ACR3, from the bacterium Shewanella oneidensis MR-1. Surprisingly, expression of this transporter in the arsenic-hypersensitive Escherichia coli strain AW3110 conferred resistance to arsenate, but not to arsenite. Purification of a C-terminally His-tagged form of the protein allowed the binding of putative permeants to be directly tested: arsenate but not arsenite quenched its intrinsic fluorescence in a concentration-dependent fashion. Fourier transform infrared spectroscopy showed that the purified protein was predominantly alpha-helical. A mutant bearing a single cysteine residue at position 3 retained the ability to confer arsenate resistance, and was accessible to membrane impermeant thiol reagents in intact cells. In conjunction with successful C-terminal tagging with oligohistidine, this finding is consistent with the experimentally-determined topology of the homologous human apical sodium-dependent bile acid transporter, namely 7 transmembrane helices and a periplasmic N-terminus, although the presence of additional transmembrane segments cannot be excluded. Mutation to alanine of the conserved residue proline 190, in the fourth putative transmembrane region, abrogated the ability of the transporter to confer arsenic resistance, but did not prevent arsenate binding. An apparently increased thermal stability is consistent with the mutant being unable to undergo the conformational transitions required for permeant translocation.

Activity assay of membrane transport proteins

Membrane transport proteins are integral membrane proteins and considered as potential drug targets. Activity assay of transport proteins is essential for developing drugs to target these proteins. Major issues related to activity assessment of transport proteins include availability of transporters, transport activity of transporters, and interactions between ligands and transporters. Researchers need to consider the physiological status of proteins (bound in lipid membranes or purified), availability and specificity of substrates, and the purpose of the activity assay (screening, identifying, or comparing substrates and inhibitors) before choosing appropriate assay strategies and techniques. Transport proteins bound in vesicular membranes can be assayed for transporting substrate across membranes by means of uptake assay or entrance counterflow assay. Alternatively, transport proteins can be assayed for interactions with ligands by using techniques such as isothermal titration calorimetry, nuclear magnetic resonance spectroscopy, or surface plasmon resonance. Other methods and techniques such as fluorometry, scintillation proximity assay, electrophysiological assay, or stopped-flow assay could also be used for activity assay of transport proteins. In this paper the major strategies and techniques for activity assessment of membrane transport proteins are reviewed.

Practical aspects of overexpressing bacterial secondary membrane transporters for structural studies

Membrane transporter proteins play critical physiological roles in the cell and constitute 5-10% of prokaryotic and eukaryotic genomes. High-resolution structural information is essential for understanding the functional mechanism of these proteins. A prerequisite for structural study is to overexpress such proteins in large quantities. In the last few years, over 20 bacterial membrane transporters were overexpressed at a level of 1 mg/l of culture or higher, most often in Escherichia coli. In this review, we analyzed those factors that affect the quantity and quality of the protein produced, and summarized recent progress in overexpression of membrane transporters from bacterial inner membrane. Rapid progress in genome sequencing provides opportunities for expressing several homologues and orthologues of the target protein simultaneously, while the availability of various expression vectors allows flexible experimental design. Careful optimization of cell culture conditions can drastically improve the expression level and homogeneity of the target protein. New sample preparation techniques for mass spectrometry of membrane proteins have enabled one to identity the rigid protein core, which can be subsequently overexpressed. Size-exclusion chromatography on HPLC has proven to be an efficient method in screening detergent, pH an other conditions required for maintaining the stability and monodispersity of the protein. Such high-quality preparations of membrane transporter proteins will probably lead to successful crystallization and structure determination of these proteins in the next few years.

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.

Structure and function of facilitative sugar transporters

Sugar transporters from one group of the major facilitator superfamily of membrane transporters. A conserved common central pore structure lies at the heart of these transporters and diverse functionality is brought about by alterations to this pore or regions associated with it. Recent mutagenesis studies of sugar transporters within the framework of tenable models for the distantly related lactose permease argue that this model is a good paradigm for other members of the major facilitator superfamily.

On the analogy between forskolin and D-glucose

2-O-Acetyl-D-glucose was synthesized in order to evaluate the influence of an acyl group on the binding with the glucose carrier protein (GluT); as its affinity neighbours that of glucose itself, the glucose-forskolin analogy appears to be coincidental and several explanations are proposed.

Sugar transporters from bacteria, parasites and mammals: structure-activity relationships

Sugar transport across the plasma membrane is one of the most important transport processes. The cloning and expression of cDNAs from a superfamily of related sugar transporters that all adopt a 12-membrane-spanning-domain structure has opened new avenues of investigation, including presteady-state kinetic analysis. Structure-function analyses of mammalian and bacterial sugar transporters, and comparisons of these transporters with those of parasitic trypanosomatids, indicate that different environmental pressures have tailored the evolution of the various members of the sugar-transporter superfamily. Subtle distinctions in the function of these proteins can be related to particular amino acid residue substitutions.

Asparagine 394 in putative helix 11 of the galactose-H+ symport protein (GalP) from Escherichia coli is associated with the internal binding site for cytochalasin B and sugar

The galactose-H+ symport protein (GalP) of Escherichia coli is very similar to the human glucose transport protein, GLUT1, and both contain a highly conserved Asn residue in predicted helix 11 that is different in a cytochalasin B-resistant member of this sugar transport family (XylE). The role of the Asn394 residue (which is predicted to be in putative trans-membrane alpha-helix 11) in the structure/activity relationship of the D-galactose-H+ symporter (GalP) was therefore assessed by measuring the interaction of sugar substrates and the inhibitory antibiotics, cytochalasin B, and forskolin with the wild-type and Asn394 --> Gln mutant proteins. Steady-state fluorescence quenching experiments show that the mutant protein binds cytochalasin B with a Kd 37-53-fold higher than the wild type. This low affinity binding was not detected with equilibrium binding or photolabeling experiments. In contrast, the mutant protein binds forskolin with a Kd similar to that of the wild type and is photolabeled by 3-125I-4-azido-phenethylamido-7-O-succinyl-desacetyl-forskolin. The mutant protein displays an increased amount of steady-state fluorescence quenching with the binding of forskolin, suggesting that the substitution of the Asn residue has altered the environment of a tryptophan, probably Trp395, in a conformationally active region of the protein. Time-resolved fluorescence measurements on the mutant protein provided association and dissociation rate constants (k2 and k-2), describing the initial interaction of cytochalasin B to the inward-facing binding site (Ti), that are decreased (9-fold) and increased (4.9-fold) compared with the wild type. This yielded a dissociation constant (K2) for cytochalasin B to the inward-facing binding site 44-fold higher than that of the wild type. The binding of forskolin gave values for k2 and k-2 3.9- and 3.6-fold lower, respectively, yielding a K2 value for Ti similar to that of the wild type. The low overall affinity (high Kd) of the mutant protein for cytochalasin B is due mainly to a disruption in binding to the Ti conformation. It is proposed that Asn394 forms either a direct binding interaction with cytochalasin B or is part of the immediate environment of the binding site and that Asn394 is in the immediate environment, but not part, of the forskolin binding site. The ability of the mutant protein to catalyze energized transport is only mildly impaired with 4.8- and 2.1-fold reduction in Vmax/Km values for D-galactose and D-glucose, respectively. In stark contrast, the overall Kd describing binding of D-galactose and D-glucose to the inward-facing conformation of the mutant and their subsequent translocation across the membrane is substantially increased (64-fold for D-galactose and 163.3-fold for D-glucose). These data indicate that Asn394 is associated with both the cytochalasin B and internal sugar binding sites. This conclusion is also supported by data showing that the sugar specificity of the mutant protein has been altered for D-xylose. This work powerfully illustrates how comparisons of the aligned amino acid sequences of homologous membrane proteins of unknown structure and characterization of their phenotypes can be used to map substrate and ligand binding sites.

Characterisation of glucose transport in the hyperthermophilic Archaeon Sulfolobus solfataricus

Sulfolobus solfiataricus is a hyperthermophilic Archaeon growing at 80 degrees C, pH 3. The glucose transport system of this organism has been characterised kinetically at this temperature and pH using 2-deoxy-D-glucose: the sugar analogue is transported into the cells with a Km = 1.8 +/- 0.3 microM and a Vmax = 3.6 +/- 0.1 nmol min(-1) (mg protein)(-1), with an intracellular accumulation of up to 200-fold over the extracellular concentration. Transport was significantly reduced at pH 5. Inhibition of 2-deoxy-D-glucose transport was investigated using a variety of sugars and sugar analogues; D-glucose, D-galactose and D-mannose showed the highest affinity for the transporter, with D-glucose possessing a Ki = 120 +/- 20 nM.

The role of tryptophans 371 and 395 in the binding of antibiotics and the transport of sugars by the D-galactose-H+ symport protein (GalP) from Escherichia coli

The interactions between the D-galactose-H+ symporter (GalP) from Escherichia coli and the inhibitory antibiotics, cytochalasin B and forskolin, and the substrates, D-galactose and H+, have been investigated for the wild-type protein and the mutants Trp-371-->Phe and Trp-395-->Phe, so that the roles of these residues in the structure-activity relationship could be assessed. Neither mutation prevented photolabeling by either [4-3H]cytochalasin B or by 3-[125I]iodo-4-azidophenethyl-amido-7-O-succinyldesacetylforskolin ([125I]APS-forskolin). However, measurements of protein fluorescence show that both residues are in structural domains, the conformations of which are perturbed by the binding of cytochalasin B or forskolin. Moreover, both mutations cause a substantial decrease in the affinity of the inward-facing site of the GalP protein for cytochalasin B, 10- and 43-fold, respectively, but have little effect upon the affinity of this site for forskolin, 0.8- and 2.6-fold reductions, respectively. Both these mutations change the equilibrium between the putative outward- (T1) and inward-facing (T2) conformations, so that the inward-facing form is more favored. They also stabilize a different conformational state, "T3-antibiotic," in which the initial interactions between the protein and antibiotics are tightened. Overall, this has the effect of compensating for the reduction in affinity for cytochalasin B, so that the respective overall Kd values are 0.74- and 3.5-fold that of the wild type, while causing a slight increase, 1.5- and 3.2-fold, respectively, in affinity of the mutants for forskolin. The Trp-371-->Phe mutation causes a 15-fold reduction in the affinity of the inward-facing site for D-galactose, suggesting that this residue forms part of the sugar binding site. In contrast, the Trp-395-->Phe mutation has no effect upon the affinity of the inward-facing site for D-galactose. These effects may be related to the reduction in galactose-H+ symport activity only in the Trp-371-->Phe mutant, although it still effects active transport to the same extent as the Trp395-->Phe mutant. However, there is a 10-20-fold increase in the Km values for energized transport of D-galactose for both mutants.