A single half-turnover of the glucose carrier of the human erythrocyte

Will the original glucose transporter isoform please stand up!

Monosaccharides enter cells by slow translipid bilayer diffusion by rapid, protein-mediated, cation-dependent cotransport and by rapid, protein-mediated equilibrative transport. This review addresses protein-mediated, equilibrative glucose transport catalyzed by GLUT1, the first equilibrative glucose transporter to be identified, purified, and cloned. GLUT1 is a polytopic, membrane-spanning protein that is one of 13 members of the human equilibrative glucose transport protein family. We review GLUT1 catalytic and ligand-binding properties and interpret these behaviors in the context of several putative mechanisms for protein-mediated transport. We conclude that no single model satisfactorily explains GLUT1 behavior. We then review GLUT1 topology, subunit architecture, and oligomeric structure and examine a new model for sugar transport that combines structural and kinetic analyses to satisfactorily reproduce GLUT1 behavior in human erythrocytes. We next review GLUT1 cell biology and the transcriptional and posttranscriptional regulation of GLUT1 expression in the context of development and in response to glucose perturbations and hypoxia in blood-tissue barriers. Emphasis is placed on transgenic GLUT1 overexpression and null mutant model systems, the latter serving as surrogates for the human GLUT1 deficiency syndrome. Finally, we review the role of GLUT1 in the absence or deficiency of a related isoform, GLUT3, toward establishing the physiological significance of coordination between these two isoforms.

Conventional transport assays underestimate sugar transport rates in human red cells

The time course of protein-mediated 3-O-methylglucose uptake by human red cells and by red cell ghosts containing or lacking 4 mM MgATP was measured at ice temperature and sub-saturating sugar levels by conventional sampling procedures and at 20 degrees C by use of a quench-flow apparatus. The temporal resolution of the quench-flow apparatus (as fast as 5-ms sample times) was confirmed by analysis of alkaline hydrolysis of dinitrophenolacetate. Red cell sugar uptake at 4 degrees C is consistent with two processes [fast (tau = 120 s) and slow (tau = 1100 s)] that occur in series. Intracellular ATP increases the size and the rate of equilibration of the fast compartment and slows the rate of filling of the slow compartment. Red cell ghost volume and protein content are unaffected by lysis/resealing in the presence of ATP. Uptake at 20 degrees C is also consistent with two processes [fast (tau = 10 ms) and slow (tau = 15 s)] that occur in series. ATP increases the size of both compartments and the rate of filling of the small compartment at 20 degrees C. Preliminary estimates indicate that the sugar uptake capacity of human red cells at 20 degrees C is underestimated by as much as 8-fold by measuring sugar uptake over 2 s vs. 26 ms. We discuss the implications of multiphasic sugar uptake in the context of models for protein-mediated sugar transport.

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.

Glucose transporter function is controlled by transporter oligomeric structure. A single, intramolecular disulfide promotes GLUT1 tetramerization

The human erythrocyte glucose transporter is an allosteric complex of four GLUT1 proteins whose structure and substrate binding properties are stabilized by reductant-sensitive, noncovalent subunit interactions [Hebert, D. N., & Carruthers, A. (1992) J. Biol. Chem. 267, 23829-23838]. In the present study, we use biochemical and molecular approaches to isolate specific determinants of transporter oligomeric structure and transport function. When unfolded in denaturant, each subunit (GLUT1 protein) of the transporter complex exposes two sulfhydryl groups. Four additional thiol groups are accessible following subunit exposure to reductant. Assays of subunit disulfide bridge content suggest that two inaccessible sulfhydryl groups form an internal disulfide bridge. Differential alkylation/peptide mapping/N-terminal sequence analyses show that a GLUT1 carboxyl-terminal peptide (residues 232-492) contains three inaccessible sulfhydryl groups and that an N-terminal GLUT1 peptide (residues 147-261/299) contains two accessible thiols. The carboxyl-terminal peptide most likely contains the intramolecular disulfide bridge since neither its yield nor its electrophoretic mobility is altered by addition of reductant. Each GLUT1 cysteine was changed to serine by oligonucleotide-directed, in vitro mutagenesis. The resulting transport proteins were expressed in CHO cells and screened by immunofluorescence microscopy for their ability to expose tetrameric GLUT1-specific epitopes. Serine substitution at cysteine residues 133, 201, 207, and 429 does not inhibit exposure of tetrameric GLUT1-specific epitopes. Serine substitution at cysteines 347 or 421 prevents exposure of tetrameric GLUT1-specific epitopes. Hydrodynamic analysis of GLUT1/GLUT4 chimeras expressed in and subsequently solubilized from CHO cells indicates that GLUT1 residues 1-199 promote chimera dimerization and permit GLUT1/chimera heterotetramerization. This GLUT1 N-terminal domain is insufficient for chimera tetramerization which additionally requires GLUT1 residues 200-463. Extracellular reductants (dithiothreitol, beta-mercaptoethanol, or glutathione) reduce erythrocyte 3-O-methylglucose uptake by up to 15-fold. This noncompetitive inhibition of sugar uptake is reversed by the cell-impermeant, oxidized glutathione. Reductant is without effect on sugar exit from erythrocytes. Dithiothreitol doubles the cytochalasin B binding capacity of erythrocyte-resident glucose transporter, abolishes allosteric interactions between substrate binding sites on adjacent subunits, and occludes tetrameric GLUT1-specific GLUT1 epitopes in situ. CHO cell-resident GLUT1 structure and transport function are similarly affected by extracellular reductant. We conclude that each subunit of the glucose transporter contains an extracellular disulfide bridge (Cys347 and Cys421) that stabilizes transporter oligomeric structure and thereby accelerates transport function.

Re-examination of hexose exchanges using rat erythrocytes: evidence inconsistent with a one-site sequential exchange model, but consistent with a two-site simultaneous exchange model

(1). The kinetic parameters of zero-trans net uptake and infinite-trans uptake of 3-O-methyl-D-glucoside, 2-deoxy-D-glucose and D-mannose into rat red cells at 24 degrees C were measured after taking account of the linear diffusion components of flux. (2). Zero-trans exists of 3-O-methyl-D-glucoside and D-mannose from rat cells were also measured. (3). After correction for linear flux via non-specific routes, the Vmax of zero-trans uptake of 3-O-methyl-D-glucoside was significantly higher, (1.25 +/- 0.06 mumol (10 min)-1 (ml cell water)-1) than the corresponding parameters of mannose or 2-deoxy-D-glucose, (0.33 +/- 0.01 and 0.39 +/- 0.01 mumol(10 min)-1 (ml cell water)-1, respectively; P < 0.001). (4). After correction for linear flux via non-specific uptake routes, the Vmax of zero-trans exit of 3-O-methyl-D-glucoside is significantly higher (1.70 +/- 0.1 mumol (10 min)-1 (ml cell water)-1) than the corresponding value for mannose exit flux, (1.10 +/- 0.1 mumol (10 min)-1 (ml cell water)-1; P < 0.001). (5). The acceleration ratio, i.e., the ratio of infinite-trans influx Vmax/zero-trans influx Vmax of mannose by mannose (9.12 +/- 0.03) is significantly higher than that of 3-O-methyl-D-glucose by 3-O-methyl-D-glucose (2.77 +/- 0.14)(P < 0.001). (6). The one-site simple carrier model of glucose transport in which sugar exchange is viewed as a sequential process, predicts that the acceleration ratio of the more rapidly moving sugar 3-O-methyl-D-glucose by 3-O-methyl-D-glucose should be greater than that of the slower sugar, mannose by mannose. Hence, the observed findings are inconsistent with the one-site model, but confirm the earlier disputed studies of Miller, D.M. (1968; Biophys. J. 8, 1329-1338). (7). A two-site model, in which sugar exchange is considered as a simultaneous process, predicts that the acceleration ratio of mannose influx by mannose should be higher than for 3-O-methyl-D-glucose by 3-O-methyl-D-glucose. The data are, therefore, consistent with a two-site model.

The kinetics and thermodynamics of the binding of cytochalasin B to sugar transporters

The kinetics of the binding of cytochalasin B to the proton-linked L-arabinose (AraE) and D-galactose (GalP) symporters from Escherichia coli and to the human erythrocyte glucose transporter (GLUT1) have been investigated by exploiting the changes in protein fluorescence that occur upon binding the ligand. Steady-state measurements yielded Kd values of 1.1, 1.9 and 0.14 microM for the AraE, GalP and GLUT1 proteins, respectively. The association and dissociation rate constants for the binding of cytochalasin B have been determined by stopped-flow spectroscopy. In each case, the apparent Kd was calculated from the corresponding rate constants, yielding values of 1.5, 0.4 and 1.6 microM for AraE, GalP and GLUT1, respectively. The differences between these apparent Kd values and those measured by fluorescence titration is interpreted in terms of the following three step mechanism where CB represents cytochalasin B: [formula: see text] The transporter is proposed to alternate between two different conformational forms (T1 and T2), with cytochalasin B binding only to the T2 conformation, to induce a further conformational transition of the transporter to the T3 form. The values for the overall dissociation constants show that the T1 conformation is favoured by AraE and GalP in the absence of ligands, but the T2 conformation is favoured by GLUT1. Thus, the binding of cytochalasin B to GLUT1 alters the equilibrium towards the T3(CB) conformational state, producing the observed tight binding, in contrast to the changes in the equilibrium observed with the binding of cytochalasin B to AraE and GalP. A thermodynamic analysis of these conformational transitions has been performed. The T1 and T2 conformations may represent transporter states in which the binding site is facing outwards and inwards, respectively.

Tryptic digestion of the human erythrocyte glucose transporter: effects on ligand binding and tryptophan fluorescence

The conformation of the human erythrocyte glucose transport protein has been shown to determine its susceptibility to enzymatic cleavage on a large cytoplasmic loop. We took the converse approach and investigated the effects of tryptic digestion on the conformational structure of this protein. Exhaustive tryptic digestion of protein-depleted erythrocyte ghosts decreased the affinity of the residual transporter for cytochalasin B by 3-fold but did not affect the total number of binding sites. Tryptic digestion also increased the affinity of the residual transporter for D-glucose and inward-binding sugar phenyl beta-D-glucopyranoside but decreased that for the outward-binding 4,6-O-ethylidene glucose. These results suggest that tryptic cleavage stabilized the remaining transporter in an inward-facing conformation, but one with decreased affinity for cytochalasin B. The steady-state fluorescence emission scan of the purified reconstituted glucose transport protein was unaffected by tryptic digestion. Addition of increasing concentrations of potassium iodide resulted in linear Stern-Volmer plots, which were also unaffected by prior tryptic digestion. The tryptophan oxidant N-bromosuccinimide was investigated to provide a more sensitive measure of tryptophan environment. This agent irreversibly inhibited 3-O-methylglucose transport in intact erythrocytes and cytochalasin B binding in protein-depleted ghosts, with a half-maximal effect observed for each activity at about 0.3-0.4 nM. Treatment of purified glucose transport protein with N-bromosuccinimide resulted in a time-dependent quench of tryptophan fluorescence, which was resolved into two components by nonlinear regression using global analysis. Tryptic digestion retarded the rate of oxidation of the more slowly reacting class of tryptophans. (ABSTRACT TRUNCATED AT 250 WORDS)

Monitoring conformational change in the human erythrocyte glucose carrier: use of a fluorescent probe attached to an exofacial carrier sulfhydryl

Several fluorescent sulfhydryl reagents were tested as probes for assessing substrate-induced conformational change of the human erythrocyte glucose carrier. Of these, 2-(4'-maleimidylanilino)-naphthalene-6-sulfonic acid (Mal-ANS) inhibited 3-O-methylglucose transport most strongly and specifically labeled a previously characterized exofacial sulfhydryl on the glucose carrier. Analysis of equilibrium cytochalasin B binding in cells treated with Mal-ANS suggested that the inhibition of transport was due to a partial channel-blocking effect, and not to competition for the substrate binding site or to hindrance of carrier conformational change. In purified glucose carrier prepared from cells labeled on the exofacial sulfhydryl with Mal-ANS, a blue shift in the peak of fluorescence indicated that the fluorophore was in a relatively hydrophobic environment. Mal-ANS fluorescence in such preparations was quenched by ligands with affinity for the outward-facing carrier (ethylidene glucose, D-glucose, and maltose), but not by inhibitors considered to bind to the inward-facing carrier conformation (cytochalasin B or phenyl beta-D-glucoside). The effect of ethylidene glucose appeared to be related to an interaction with the glucose carrier, since the concentration dependence of ethylidene glucose-induced quench correlated well with the ability of the sugar analog to inhibit cytochalasin B binding to intact cells. The hydrophilic quenchers iodide and acrylamide decreased carrier-bound Mal-ANS fluorescence, resulting in downward-curving Stern-Volmer plots. Whereas ethylidene glucose enhanced iodide-induced quench, it had no effect on that of acrylamide.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel method for the observation of membrane transporter dynamics

A new method is proposed for measuring the dynamic properties of a membrane transporter by means of steady-state fluxes. Any voltage-sensitive transporter will give a flow of substrate in the presence of a steady-state periodic membrane potential. The periodic steady-state flow, averaged over one period, is a flux that can be measured by traditional steady-state techniques, such as the radioactive tracer method. The average flux, solely due to the periodic field, is described by a set of Lorentzian functions that depend on the applied periodic field amplitude and frequency. The normal mode amplitudes and frequencies of these Lorentzians are model-independent parameters of the transport mechanism. Measurement of the average flux as a function of the applied periodic frequency permits determination of system relaxation times as the reciprocals of the midpoints of the Lorentzian curves, which in turn can be used to estimate individual rate constants of specific models. It was found by simulation of a six-state model of the electrogenic Na+/glucose cotransporter, using published estimates of the model rate constants, that the periodic field effects can be large and rich with measurable details that can be used to study the mechanism thoroughly. The new method serves in this case to complement and expand on the information obtainable by means of the voltage clamp method. It was also found by means of simulations of a nonelectrogenic six-state cotransporter model that experimentally measurable effects are expected and that results can be used to distinguished among alternative kinetic models as well as to estimate individual rate constants. The range of dynamic information available with this method is not accessible by voltage clamp or other pre-steady-state methods presently in use.

Effects of pressure on glucose transport in human erythrocytes

The operation of the human red cell glucose transporter has been studied at normal and high hydrostatic pressure to identify the step(s) which involve a volume change. Pressure inhibited zero-trans and equilibrium exchange influx to similar extents, by decreasing the Vmax but not significantly changing the Km. The Bmax and Kd of specific [3H]cytochalasin B binding were unaffected by pressure indicating no change to the number or affinity of functional transporters at pressure. Passive glucose transport was inhibited by pressure in a manner consistent with permeation across the lipid bilayer. These data indicate that there is a major change in volume during the translocation step of the glucose transporter which is rate-limiting for transport.

The one-site model of human erythrocyte glucose transport: testing its predictions using network thermodynamic computer simulations

Network thermodynamic computer simulations were carried out using parameters experimentally derived by Lowe and Walmsley ((1987) Biochim. Biophys. Acta 903, 547-550) for two tests of the one-site model of human erythrocyte glucose transport. In the temperature-jump experiment, the simulations predicted the amplitude and relaxation time of accelerated uptake, but underestimated the net uptake due to an unexpectedly low measured basal rate. In the maltose-acceleration experiment, the dissociation constant of maltose was assessed at 0 degrees C by measuring the inhibitory effects of maltose on both cytochalasin B binding and on 3-O-methylglucose uptake, and using this value (52 mM) to calculate the dissociation constant (2.9 mM). The simulated experiment then did show a transient acceleration in uptake comparable in magnitude to that observed experimentally, except that the relaxation time was more than 10-fold longer in the simulations. Measurements of the temperature dependence of the inhibition of cytochalasin B binding by maltose and 3-O-methylglucose indicated that apparent sugar affinity is sensitive to carrier orientation at low temperatures, whereas at more physiologic temperatures the intrinsic dissociation constant predominated.

Mechanisms for the facilitated diffusion of substrates across cell membranes

Two classes of theoretical mechanisms for protein-mediated, passive, transmembrane substrate transport (facilitated diffusion) are compared. The simple carrier describes a carrier protein that exposes substrate influx and efflux sites alternately but never both sites simultaneously. Two-site models for substrate transport describe carrier proteins containing influx and efflux sites simultaneously. Velocity equations describing transport by these mechanisms are derived. These equations take the same general form, being characterized by five experimental constants. Simple carrier-mediated transport is restricted to hyperbolic kinetics under all conditions. Two-site carrier-mediated transport may deviate from hyperbolic kinetics only under equilibrium exchange conditions. When both simple- and two-site carriers display hyperbolic kinetics under equilibrium exchange conditions, these models are indistinguishable by using steady-state transport data alone. Seven sugar transport systems are analyzed. Five of these systems are consistent with both models for sugar transport. Uridine, leucine, and cAMP transport by human red cells are consistent with both simple- and two-site models for transport. Human erythrocyte sugar transport can be modeled by simple- and two-site carrier mechanisms, allowing for compartmentalization of intracellular sugars. In this instance, resolution of the intrinsic properties of the human red cell sugar carrier at 20 degrees C requires the use of submillisecond transport measurements.

Analysis of protein-mediated 3-O-methylglucose transport in rat erythrocytes: rejection of the alternating conformation carrier model for sugar transport

3-O-Methylglucose (3OMG) transport in rat erythrocytes (RBCs) is mediated by a low-capacity, facilitated diffusion-type process. This study examines whether the characteristics of sugar transport in rat RBCs are consistent with the predictions of two diametric, theoretical mechanisms for sugar transport. The one-site carrier describes a transport mechanism in which sugar influx and efflux substrate binding sites are mutually exclusive. The two-site carrier describes a transport mechanism in which sugar influx and efflux substrate binding sites can exist simultaneously but may interact in a cooperative fashion when occupied by substrate. Michaelis and velocity parameters for saturable 3OMG transport in rat erythrocytes at 24 degrees C were obtained from initial rate measurements of 3OMG transport. The results are incompatible with the predictions of the one-site carrier but are consistent with the predictions of a symmetric two-site carrier, displaying negligible cooperativity between substrate binding sites. This allows reduction of the two-site carrier transport equations to a form containing fewer constants than the one-site carrier equations without limiting their predictive success. While the available evidence does not prove that rat erythrocyte sugar transport is mediated by a two-site mechanism, we conclude that adoption of the formally more complex one-site model for sugar transport in rat erythrocytes is unnecessary and unwarranted. Counterflow experiments have also been performed in which the time course of radiolabeled 3OMG uptake is measured in cells containing saturating levels of 3OMG. The results of these experiments are consistent with the hypothesis [Naftalin et al. (1985) Biochim. Biophys. Acta 820, 235-249] that exchange of sugar between intracellular compartments (cell water and hemoglobin) can be rate limiting for transport under certain conditions.

Pre-steady-state uptake of D-glucose by the human erythrocyte is inconsistent with a circulating carrier mechanism

Simulation shows that the four-state mobile carrier model for sugar transport in which the asymmetry arises from unequal rate constants of inward and outward translation of the free-carrier and carrier-sugar complex, does not fit with the observed data for pre-steady-state uptake recently obtained by A.G. Lowe and A.R. Walmsley [1987) Biochim. Biophys. Acta 903, 547-550). The main reason for this discrepancy is that pre-steady-state fluxes are determined mainly by the dissociation constants Ks of glucose and maltose for the external sites, rather than the Km (zero-transoi) of glucose and the Ki of maltose. The data are also inconsistent with other forms of asymmetric carrier but are fairly consistent with a symmetrical carrier with high-affinity sites for D-glucose or with a fixed site carrier model.