Monosaccharide transport system of the human erythrocyte. Identification of the cytochalasin B binding component

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.

Differential labeling of the erythrocyte hexose carrier by N-ethylmaleimide: correlation of transport inhibition with reactive carrier sulfhydryl groups

Inhibition of hexose transport by N-ethylmaleimide was studied with regard to alkylation of different types of sulfhydryl group on the hexose carrier of the human erythrocyte. Uptake of 3-O-methylglucose was progressively and irreversibly inhibited by N-ethylmaleimide, with a half-maximal effect at 10-13 mM. A sulfhydryl group known to exist on the exofacial carrier was not involved in transport inhibition by N-ethylmaleimide, since reversible protection of this group by the impermeant sulfhydryl reagent 5,5'-dithiobis(2-nitrobenzoic acid) had no effect on the ability of N-ethylmaleimide to inhibit transport, or on its ability to decrease the affinity of the exofacial carrier for maltose. Nevertheless, the exofacial sulfhydryl was quite reactive with N-ethylmaleimide, since it was possible using a differential labeling technique to specifically label this group in protein-depleted ghosts with a half-maximal effect at 0.3 mM N-[3H]ethylmaleimide, and to localize it to the Mr 19,000 tryptic carrier fragment. Transport inhibition by N-ethylmaleimide correlated best with labeling of a single cytochalasin B-sensitive internal sulfhydryl group on the glycosylated Mr 23,000-40,000 tryptic fragment of the carrier, which was half-maximally labeled at about 4 mM reagent. Whereas N-ethylmaleimide readily alkylates the exofacial carrier sulfhydryl, it inhibits transport by reacting with at least one internal carrier sulfhydryl located on the glycosylated tryptic carrier fragment.

Interaction of a permeant maleimide derivative of cysteine with the erythrocyte glucose carrier. Differential labelling of an exofacial carrier thiol group and its role in the transport mechanism

S-(Bismaleimidomethyl ether)cysteine (Cys-Mal) was synthesized as a probe for reactive thiol groups on the erythrocyte glucose carrier. Although Cys-Mal entered cells, its reaction with intracellular GSH prevented alkylation of endofacial membrane proteins, limiting its effect to the cell surface at concentrations below 5 mM. Cys-Mal irreversibly inhibited hexose transport half-maximally at 1.5 mM by decreasing the maximal rate of transport, with no effect on the affinity of substrate for the carrier. Reaction occurred with the outward-facing form of the carrier, but did not affect the ability of the carrier to change orientation. In intact cells, several exofacial proteins were labelled by [35S]Cys-Mal, including the band-4.5 glucose carrier, the labelling of which occurred on a single site sensitive to transport inhibitors. The reactive exofacial group was a thiol group, since both transport inhibition and band-4.5 labelling by Cys-Mal were abolished by the thiol-specific and impermeant compound 5,5'-dithiobis(2-nitrobenzoic acid). Selectivity for carrier labelling in cells was increased by a double differential procedure, which in turn allowed localization of the exofacial thiol group to the Mr 18,000-20,000 membrane-bound tryptic carrier fragment. In protein-depleted ghosts the exofacial thiol group was preferentially labelled at low concentrations of [35S]Cys-Mal, whereas with the reagent at 10 mM the Mr 26,000-45,000 tryptic carrier fragment was also labelled. Cys-Mal should be useful in the study of carrier thiol-group location and function.

Selective labeling of the erythrocyte hexose carrier with a maleimide derivative of glucosamine: relationship of an exofacial sulfhydryl to carrier conformation and structure

Sulfhydryl-reactive derivatives of glucosamine were synthesized as potentially transportable affinity labels of the human erythrocyte hexose carrier. N-Maleoylglycyl derivatives of either 6- or 2-amino-2-deoxy-D-glucopyranose were the most potent inhibitors of 3-O-methylglucose uptake, with concentrations of half-maximal irreversible inhibition of about 1 mM. Surprisingly, these derivatives were very poorly transported into erythrocytes. They reacted rather with an exofacial sulfhydryl on the carrier following a reversible binding step, the latter possibly to the exofacial substrate binding site. However, their reactivity was determined primarily by access to the exofacial sulfhydryl, which, as predicted by the one-site model of transport, required a carrier conformation with the exofacial substrate binding site exposed. Once reacted, the carrier was "locked" in a conformation unable to reorient inwardly and bind cytochalasin B. In intact erythrocytes the N-maleoylglycyl derivative of 2-[3H]glucosamine labeled predominantly an Mr 45,000-66,000 protein on gel electrophoresis in a quantitative and cytochalasin B inhibitable fashion. By use of changes in carrier conformation induced by competitive transport inhibitors in a "double" differential labeling method, virtually complete selectivity of labeling of the carrier protein was achieved, the latter permitting localization of the reactive exofacial sulfhydryl to an Mr 18,000-20,000 tryptic fragment of the carrier.

Immunological evidence that band 3 is the major glucose transporter of the human erythrocyte membrane

We have previously reported that human erythrocyte band 3 contains 90-95% of the reconstitutable glucose transport activity of the erythrocyte membrane (Shelton, R.L. and Langdon, R.G. (1983) Biochim. Biophys. Acta 733, 25-33). We have now found that monoclonal and polyclonal antibodies to epitopes on band 3 specifically removed band 3 and more than 90% of the reconstitutable glucose transport activity from unfractionated octylglucoside extracts of erythrocyte membranes; nonimmune serum removed neither. Western blots of whole membrane extracts revealed that the polyclonal antibody to band 4.5 used to isolate cDNA clones presumed to code for the transporter (Mueckler, M., Caruso, C., Baldwin, C.A., Pancio, M., Blench, J., Morris, H.B., Allard, W.J., Lienhard, G.E. and Lodish, H.F. (1985) Science 229, 941-945) reacts strongly with six discrete bands in the 4.5 region. A monoclonal antibody to band 3 also reacts with a Mr 55,000 component of band 4.5. We conclude that band 3 contains the major glucose transporter of human erythrocytes, and that the transport activity in band 4.5 might be attributable to a band 3 fragment. Band 3 is probably a multifunctional transport protein responsible for transport of glucose, anions, and water.

Inhibition of hexose transport and labelling of the hexose carrier in human erythrocytes by an impermeant maleimide derivative of maltose

Maltose-maleimide was synthesized as a potential affinity label for the facilitative hexose carrier with selectivity for exofacial sulphydryl groups. This reagent, although probably a mixture of isomers, did not significantly penetrate the plasma membrane of human erythrocytes at concentrations below 5 mM at 37 degrees C. When allowed to react to completion, it irreversibly inhibited the uptake of 3-O-methylglucose, with a half-maximal response at about 1.5-2.0 mM-reagent. The rate of transport inactivation was a saturable function of the maltose-maleimide concentration. Studies of reaction kinetics and effects of known transport inhibitors demonstrated that irreversible reaction occurred on the exofacial outward-facing carrier, although not at a site involved in substrate binding. Reaction of intact erythrocytes with [14C]maltose-maleimide resulted in labelling of a broad band 4.5 protein of Mr (average) 45,000-66,000 in electrophoretic gels. This protein was very likely the hexose carrier, since its labelling was inhibited by cytochalasin B. Exofacial band 4.5 labelling was stoichiometric with respect to transport inhibition, yielding an estimated 300,000 carriers/cell. These results suggest that the exofacial sulphydryl which reacts with maltose-maleimide is distinct from the substrate binding site on the hexose carrier, but that it confers substantial labelling selectivity to impermeant maleimides. Additionally, the high efficiency of carrier labelling obtained with maltose-maleimide is useful in quantifying numbers of carriers in whole cells.

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

Single half-turnovers of the glucose carrier of the human erythrocyte have been measured by recruiting carriers to the outward-facing conformation by (a) pre-exposing cells to extracellular maltose, or (b) pre-warming cells to 38 degrees C, before addition of D-[14C]glucose at 0 degrees C. Based on these experiments estimates of the number of glucose carriers per red cell range from 124,000 to 190,000.

Sulfhydryl substituents of the human erythrocyte hexose transport mechanism

Sulfhydryl substituents of the hexose transport mechanism of human erythrocyte membranes were studied with membrane-impermeant and -permeant maleimide derivatives. Three sulfhydryl classes have been identified on the basis of their reactivity toward the reagents and their effects on the transport mechanism. Type I sulfhydryl is located at the outer (exofacial) surface of the membrane and bound covalently on treatment of intact cells with the membrane-impermeant glutathione-maleimide. This sulfhydryl is required for the transport, and it is protected from alkylation, i.e., its reactivity toward maleimides is decreased by the presence of D-glucose or cytochalasin B. Type II sulfhydryl is also required for the transport, but it differs from type I in that D-glucose (but not cytochalasin B) increases the reactivity toward maleimides. Further, it is located at the endofacial surface of the membrane, since reaction with glutathione-maleimide occurs only in leaky ghosts and not in intact cells. Alkylation by glutathione-maleimide of type I and type II sulfhydryls increases the half-saturation for the binding of D-glucose to erythrocyte membranes. In contrast, inactivation of type III sulfhydryls by N-ethylmaleimide or dipyridyl disulfide decreases the half-saturation concentration for the binding of D-glucose and other transported hexoses to the membranes; nontransported sugars are not affected similarly. Type III sulfhydryl is not inactivated by the polar reagent glutathione-maleimide and is probably located in a nonpolar domain of the transport mechanism. Inactivation of either type I or II sulfhydryls decreases or eliminates the flux asymmetry of the hexose transport mechanism.

The topology of the major band 4.5 protein component of the human erythrocyte membrane: characterization of reactive cysteine residues

A preparation of band 4.5 protein of the red cell membrane, containing largely the sugar transporter, was labelled with the sulfhydryl reagent N-ethyl [14C]maleimide. In preparations denatured with sodium dodecyl sulfate (SDS), all five sulfhydryl groups present in the peptide, Mr 45 000 to 60 000, react with the alkylating agent within 20 min at 37 degrees C. If the peptide is reconstituted in lipid vesicles and cleaved with trypsin before extraction and denaturation with SDS, three sulfhydryl groups are found in a 30 kDa fragment and two in a 19 kDa fragment. In 'native' reconstituted protein only three groups react, even after two hours of exposure, two in the 30 kDa fragment and one in the 19 kDa fragment. Thus, one sulfhydryl group is cryptic, inaccessible to N-ethylmaleimide in each fragment. In intact cells, the single reactive group of the 19 kDa fragment can be protected against reaction with N-ethylmaleimide by the impermeant sulfhydryl reagent, p-chloromercuribenzene sulfonate (PCMBS). It is, therefore, considered to be exposed on the outer face of the membrane. The two reactive groups of the 30 kDa fragment are not protected by PCMBS and are, therefore, not considered to be exposed to the outside medium. Cytochalasin B, a competitive inhibitor of sugar transport affords temporary protection of the exofacial group of the 19 kDa against reaction with N-ethylmaleimide, and affords longer term protection of one of the reactive groups of the 30 kDa fragment. These findings allow conclusions about the topology of the sugar transport protein in the bilayer. Both proteolytic fragments must cross the bilayer. One of three reactive sulfhydryl groups is exofacial and two may be cytoplasmic. The two cryptic groups may be located within the bilayer.

Reconstitution of the glucose transport activity of rat adipocytes

Rat epididymal fat cell membrane proteins were extracted from adipocyte ghosts with octylglucoside and incorporated by detergent dialysis into unilamellar phosphatidylcholine vesicles approx. 200 nm in diameter. The rate of glucose transport into the vesicles under zero-trans conditions was substrate dependent, saturable and inhibited by phloretin and cytochalasin B. Their maximum specific transport activity was 35.6 mumol/min per mg protein, and their half saturation constant for glucose was 15 mM. Glucose transport into the reconstituted vesicles was inhibited by only those sugars which competitively inhibited glucose transport into intact adipocytes. A major protein component of the vesicles was a 100 kDa protein which we had previously found to react with the affinity label maltosyl isothiocyanate (Malchoff, D.M., Olansky, L., Pohl, S. and Langdon, R.G. (1981) Fed. Proc. 40, 1893). Removal of adipocyte ghost membrane extrinsic proteins with dimethylmaleic anhydride followed by extraction of the resulting membrane pellet with octylglucoside yielded a solution which contained two major proteins, of Mr 100 000 and 85 000, with very small quantities of lower Mr proteins. Vesicles into which these proteins were incorporated had average specific transport activities of 624 mumol/min per mg protein and half saturation constants of 22 mM. Our results strongly indicate that the native glucose transporter of the rat adipocyte, like that of the human erythrocyte (Shelton, R.L. and Langdon, R.G. (1983) Biochim. Biophys. Acta 733, 25-33), is a 100 kDa protein.

Characterization of a photosensitive glucose derivative. A photoaffinity reagent for the erythrocyte hexose transporter

The photosensitive reagent 6-N-(4-azido-2-hydroxy-3,5-diiodobenzoyl)-D-glucosamine has been assessed as a potential photoaffinity label for the hexose transporter. Under zero-trans conditions, transport experiments performed in the dark reveal that the reagent inhibits the uptake of D-glucose in resealed human erythrocyte ghosts. Increasing the concentration of glucose in the transport medium has a protective effect, reducing the inhibition. Kinetic analysis indicates that the probe acts as a competitive inhibitor with high affinity for the erythrocyte hexose transporter (Ki between 0.07 and 0.2 microM). Exposure to a 280 nm filtered high intensity mercury-vapor lamp results in a rapid and efficient photolysis. At low concentrations of the probe, specific labeling of membrane preparations was observed. Autoradiograms of 10% SDS gels revealed the specific labeling of bands 4.51 and 6. This labeling was concentration-dependent and protected by D-glucose (not the L-isomer) and phloretin in the medium. When subjected to multiple exposures of low concentration of the photoaffinity reagent, apparent saturation was achieved.

Proteolytic cleavages of cytochalasin B binding components of Band 4.5 proteins of the human red blood cell membrane

The putative hexose transport component of Band 4.5 protein of the human erythrocyte membrane was covalently photolabelled with [3H]cytochalasin B. Its transmembrane topology was investigated by electrophoretically monitoring the effect of proteinases applied to intact erythrocytes, unsealed ghosts, and a reconstituted system. Band 4.5 was resistant to proteolytic digestion at the extracellular face of the membrane in intact cells at both high and low ionic strengths. Proteolysis at the cytoplasmic face of the membrane in ghosts or reconstituted vesicles resulted in cleavage of the transporter into two membrane-bound fragments, a peptide of about 30 kDa that contained its carbohydrate moiety, and a 20 000 kDa nonglycosylated peptide that bore the cytochalasin B label. Because it is produced by a cleavage at the cytoplasmic face and because the carbohydrate moiety is known to be exposed to the outside, the larger fragment must cross the bilayer. It has been reported that the Band 4.5 sugar transporter may be derived from Band 3 peptides by endogenous proteolysis, but the cleavage pattern found in the present study differs markedly from that previously reported for Band 3. Minimization of endogenous proteolysis by use of fresh cells, proteinase inhibitors, immediate use of ghosts and omission of the alkaline wash resulted in no change in the incorporation of [3H]cytochalasin B into Band 4.5, and no labelling of Band 3 polypeptides. These results suggest that the cytochalasin B binding component of Band 4.5 is not the product of proteolytic degradation of a Band 3 component.

D-Glucose-sensitive and -insensitive cytochalasin B binding proteins from microvillous plasma membranes of human placenta. Identification of the D-glucose transporter

Cytochalasin B was found to bind to at least two distinct sites in human placental microvillous plasma membrane vesicles, one of which is likely to be intimately associated with the glucose transporter. These sites were distinguished by the specificity of agents able to displace bound cytochalasin B. [3H]Cytochalasin B was displaceable at one site by D-glucose but not by dihydrocytochalasin B; it was displaceable from the other by dihydrocytochalasin B but not by D-glucose. Some binding which could not be displaced by D-glucose + cytochalasin B binding site. Cytochalasin B can be photoincorporated into specific binding proteins by ultraviolet irradiation. D-Glucose specifically prevented such photoaffinity labeling of a microvillous protein component(s) of Mr = 60,000 +/- 2000 as determined by urea-sodium dodecyl sulfate acrylamide gel electrophoresis. This D-glucose-sensitive cytochalasin B binding site of the placenta is likely to be either the glucose transporter or be intimately associated with it. The molecular weight of the placental glucose transporter agrees well with the most widely accepted molecular weight for the human erythrocyte glucose transporter. Dihydrocytochalasin B prevented the photoincorporation of [3H]cytochalasin B into a polypeptide(s) of Mr = 53,000 +/- 2000. This component is probably not associated with placental glucose transport. This report presents the first identification of a sodium-independent glucose transporter from a normal human tissue other than the erythrocyte. It also presents the first molecular weight identification of a human glucose-insensitive high-affinity cytochalasin B binding protein.

The glucose transport system of muscle plasma membranes: characterization by means of [3H]cytochalasin B binding

A membrane-rich preparation was isolated from adult rat skeletal muscle in low salt media and further fractionated in sucrose gradients. Fraction F2, with a relative density of 1.092-1.119, consisted of sealed membrane vesicles which were enriched in plasma membrane markers. These vesicles were capable of stereospecific D-glucose uptake which was sensitive to cytochalasin B (CB). The membranes were also enriched in high affinity [3H]CB binding activity (Kd of 0.28 microM). [3H]CB binding to the glucose carrier of these plasma membranes, estimated as the fraction of binding protectable by D-glucose, ranged between 2.5 and 7.4 pmol/mg protein in several membrane preparations. The amount of [3H]CB binding to muscle membranes from newborn and adult rats was not markedly different. Trypsin, at low concentrations, altered the molecular weight of several membrane components, without affecting [3H]CB binding. Higher concentrations of trypsin abolished [3H]CB binding. Both 2,4-dinitrofluorobenzene (0.1 mM) and N-ethylmaleimide (15 mM) inhibited [3H]CB binding; inhibition by these reagents was prevented by inclusion of micromolar concentrations of CB in the reaction mixture. Several procedures that extracted specific proteins enriched the D-glucose-sensitive [3H]CB binding to the protein-depleted membranes. Antibody raised against the glucose carrier of human red cell membranes cross-reacted with a polypeptide of Mr about 45K of muscle membranes which might represent the glucose carrier.

Properties of N-maleoylmethionine sulphone, a novel impermeant maleimide, and its use in the selective labelling of the erythrocyte glucose-transport system

1. The synthesis of N-maleoylmethionine sulphone (MMS), a membrane-impermeant protein-labelling reagent, is described. Radioactively labelled MMS can be readily prepared at high specific radioactivity from [35S]methionine. 2. The permeability of the erythrocyte membrane to the reagent was assessed by determining the extent of inactivation of glyceraldehyde 3-phosphate dehydrogenase after treatment of erythrocytes with MMS. Some inactivation of this enzyme was found when high concentrations (20mM) of the compound were used, but this could be prevented by pretreatment of the erythrocytes with 4,4'-di-isothiocyanatostilbene-2,2'-disulphonic acid, suggesting that MMS slowly enters the cells via the anion-transport system. 3. Treatment of erythrocytes with [35S]MMS resulted in the labelling of six major components. Labelling of erythrocyte membranes resulted in the intense labelling of many additional components. 4. MMS inhibited erythrocyte glucose transport. Cytochalasin b protected glucose transport against inactivation by MMS. Labelling experiments in erythrocytes in the presence and in the absence of cytochalasin b showed that the cytochalasin b-protected material was a broad band in the band-4.5 region.

Phloretinyl-3'-benzylazide: a high affinity probe for the sugar transporter in human erythrocytes. II. Irreversible transport inhibition is induced by photolysis

In the dark, phloretinyl-3'-benzylazide (PBAz), at a nominal concentration of 10 microM, will inhibit the transport of D-glucose in human erythrocytes by more than 90%. This inhibition can be completely reversed by percolating the cell suspension through a small column of Sephadex G-10; cells recovered after this treatment, and then loaded with 100 mM D-glucose, possess a transport capacity (glucose efflux) equal to untreated cells. The Sephadex matrix completely removes non-covalently bound inhibitor even though, under these conditions (subdued light, 0.2% hematocrit, 0 degrees C, pH 6.2 or 7.8), from 70 to 80% of the PBAz added is bound to the cells (mostly non-specifically to hemoglobin). However, when erythrocytes exposed to 10 microM inhibitor are irradiated with long wavelength ultraviolet light, the glucose transporter is irreversibly inhibited; after 1 min irradiation, about 50% of transporter activity cannot be restored by Sephadex treatment. Under identical conditions, control cells (no PBAz, but irradiated and treated with Sephadex) retain over 90% of carrier activity. The photolytic conversion of the inhibition to an irreversible form is directly dependent on PBAz concentration. The results reaffirm our earlier conclusions that PBAz is a potentially useful photoaffinity labeling agent for the glucose transporter in erythrocyte membranes.

Extraction and partial purification of the nucleoside-transport system from human erythrocytes based on the assay of nitrobenzylthioinosine-binding activity

Nitrobenzylthioinosine, a potent nucleoside-transport inhibitor, binds to high-affinity sites on the human erythrocyte membrane. This binding is a specific interaction with functional nucleoside-transport sites. The protein(s) responsible for high-affinity nitrobenzylthioinosine binding was purified 13-fold by treatment of haemoglobin-free 'ghosts' with EDTA (pH 11.2) to remove extrinsic proteins, extraction of the protein-depleted membranes with Triton X-100 and passage of the soluble extract through a DEAE-cellulose column equilibrated with Triton X-100. Void-volume fractions were collected and treated with Bio-Beads SM-2 to remove detergent. These fractions contained 31% of the starting nitrobenzylthioinosine-binding activity. They also contained D-glucose-sensitive cytochalasin B-binding activity. Nitrobenzylthioinosine binding to the partially purified preparation was saturable (apparent Kd 1.6 nM) and inhibited by nitrobenzylthioguanosine, dipyridamole and uridine. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis of pooled void-volume fractions revealed the presence of only two detectable protein bands, the broad zone 4.5 (containing glucose-transport protein) and a small amount of band 7.

Characterization of cytochalasin B binding to adult rat liver parenchymal cells in primary culture

The characterization of cytochalasin B binding and the resulting effect on hexose transport in rat liver parenchymal cells in primary culture were studied. The cells were isolated from adult rats by perfusing the liver in situ with collagenase and separating the hepatocytes from the other cell types by differential centrifugation. The cells were established in primary culture on collagen-coated dishes. The binding of [4-3H]cytochalasin B and transport of 3-O-methyl-D-[14C]glucose into cells were investigated in monolayer culture followed by digestion of cells and scintillation counting of radioactivity. The binding of cytochalasin B to cells was rapid and reversible with association and dissociation being essentially complete within 2 min. Analysis of the kinetics of cytochalasin B binding by Scatchard plots revealed that binding was biphasic, with the parenchymal cell being extremely rich in high-affinity binding sites. The high-affinity site, thought to be the glucose-transport carrier, exhibited a KD of 2.86 x 10(-7) M, while the low-affinity site had a KD of 1.13 x 10(-5) M. Sugar transport was monitored by 3-O-methyl-D-glucose uptake and it was found that cytochalasin B (10(-5) M) drastically inhibited transport. However, D-glucose (10(-5) M) did not displace cytochalasin B, and cytochalasin E, which does not inhibit transport, was competitive for cytochalasin B at only the low-affinity site, demonstrating that the cytochalasin B inhibition of sugar transport occurs at the high-affinity site but that the inhibition is non-competitive in nature. Therefore, the liver parenchymal cells may represent an unusually rich source of glucose-transport system which may be useful in the isolation of this important membrane carrier.

Immunological identification of the human erythrocyte glucose transporter

A rabbit antibody against the human erythrocyte glucose transporter was purified by affinity chromatography and used to determine the distribution of transporter on polyacrylamide gels after electrophoresis in sodium dodecyl sulfate. Fresh erythrocyte ghosts showed transporter only at the broad 55,000 Mr band, as did the isolated transporter. HeLa cell plasma membranes showed a similar band of crossreacting material at Mr 55,000. The amount of crossreacting material in human erythrocyte ghosts and in plasma membranes from human HeLa cells and mouse L-1210 cells was determined in an enzyme-linked immunosorbent assay which gave results consistent with the extent of glucose-reversible binding of cytochalasin B.

The glucose transport activity of human erythrocyte membranes. Reconstitution in phospholipid liposomes and fractionation by molecular sieve and ion exchange chromatography

Human erythrocyte membranes, at a protein concentration of 1-2 g/l, were solubilized with 0.12 M cholate in the presence of 0.06 M phospholipid (egg yolk phospholipids or phosphatidylcholine). More than 40% of the protein was solubilized. Cholate was removed by molecular sieve chromatography, whereby liposomes formed. These liposomes exchanged D-glucose faster than L-glucose. The recovery of glucose transport activity in the reconstituted system was estimated to be higher than 16%. The liposomes were heterogenous in size, as shown by molecular sieve chromatography on Sepharose 4B, and small liposomes predominated. In liposomes formed with phosphatidylcholine, the distribution of glucose transport activity did not parallel the distribution of protein or phospholipid, and the activity was found mainly in the smallest liposomes. The proteins were incorporated mainly in the liposomes that eluted at the lowest ionic strength upon ion exchange chromatography. The glucose transport activity separated into three main peaks upon ion exchange chromatography of egg yolk phospholipid liposomes. The activity eluted at low ionic strength. The liposomes contained proteins mainly from the 3- and 4.5-regions (nomenclature according to Steck, T.L. (1974) J. Cell Biol. 62, 1-19). The activity peaks were highest in the first part of the chromatogram. The protein distribution did not coincide with the variation in activity over each peak. Therefore, it cannot be excluded that a minor component not seen in the electrophoretic analyses might be responsible for the glucose transport activity.

Evidence for allosteric inhibition sites in the glucose carrier of erythrocytes

2,4-Fluorodinitrobenzene and 2,3-butanedione, which irreversibly inactivate the glucose transfer system of erythrocytes, have been used as probes to determine whether the substrate site and inner and outer sites for reversible inhibitors are located in the same or different regions of the carrier. Inhibitors bound at an inhibition site exposed in the inward-facing but not the outward-facing form of the carrier (cytochalasin B, androstendione and androstandione) protect the transport system against inactivation by 2,4-fluorodinitrobenzene. Inhibitors bound at an external inhibition site (phloretin) and substrates bound at the transfer site do not protect. In contrast inactivation by 2,3-butanedione is slightly accelerated by internally bound inhibitors, while substrates and substrate analogs bound at the transfer site protect the system. It is shown that fluorodinitrobenzene reacts in the inner inhibition site and butanedione in the substrate site; and further that these sites may be separate binding areas in the carrier linked by allosteric interaction. The consequence of this linkage is that binding of a ligand at the substrate site precludes binding of another ligand at the internal or external inhibition site.

Cytochalasin B-binding proteins in rabbit erythrocyte membranes and their post-natal change in relation to the glucose carrier activity

Two distinct, carrier-mediated glucose uptake processes, a fast, cytochalasin B-sensitive and a slow, cytochalasin B-insensitive flux are identified in parallel in newborn rabbit erythrocytes. The fast, cytochalasin B-sensitive carrier function disappears as rabbits age, and only the slow cytochalasin B-insensitive carrier function is observed with adult rabbit erythrocytes. Three different cytochalasin B binding sites are distinguished in newborn rabbit erythrocytes; a glucose-sensitive site (site I), a cytochalasin E-sensitive site (site II), and a site insensitive to both glucose and cytochalasin E. With adult rabbit erythrocytes, only a cytochalasin E-sensitive site is detected. With glucose-sensitive site disappears as rabbits age, with a time course which is comparable to that of the disappearance of the cytochalasin B-sensitive glucose carrier function. The cytochalasin E-sensitive cytochalasin B binding site does not increase during this change, thus the disappearance of the glucose-sensitive site is not due to its conversion to a cytochalasin E-sensitive site. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of rabbit erythrocyte ghosts revealed a partial decrease in each of the membrane polypeptides of approximate molecular weights of 240 000, 160 000 and 50 000 as rabbits aged. It is concluded that the cytochalasin B-sensitive glucose carrier of fetal rabbit erythrocytes, like that of the human erythrocyte, is tightly associated with the site I cytochalasin B-binding protein, while the cytochalasin B-insensitive glucose carrier, operative in adult rabbit erythrocytes, is not.

Maltosyl isothiocyanate: an affinity label for the glucose transporter of the human erythrocyte membrane. 1. Inhibition of glucose transport

Maltosyl isothiocyanate (MITC) has been synthesized from maltose with an overall yield of 88%. It has been found to be a potent irreversible inhibitor of zero trans influx of glucose with human erythrocytes. Kinetic analysis of glucose transport after treatment of erythrocytes with MITC revealed that VT was diminished while KT was unchanged. Transportable sugars and competitive inhibitors of monosaccharide transport protected against MITC inhibition, while carbohydrates which do not interact with the transporter gave no protection. Covalent inhibitors of anion transport were without effect on glucose transport. MITC fulfilled the kinetic requirements for an affinity label of the glucose transporter of human erythrocytes [Groman, E. V., Schultz, R. M., & Engel, L. L. (1977) Methods Enzymol. 46, 54].

Maltosyl isothiocyanate: an affinity label for the glucose transporter of the human erythrocyte membrane. 2. Identification of the transporter

Maltosyl isothiocyanate (MITC), a potent irreversible inhibitor of glucose transport in human erythrocytes [Mullins, R. E., & Langdon, R. G. (1980) Biochemistry (preceding paper in this issue)], has been found to react almost exclusively with band 3 of the human erythrocyte membrane. The incorporation of [14C]MITC into band 3 was found to be antagonized by transportable sugars or competitive inhibitors of transport. On the basis of [14C]MITC incorporation into band 3 and MITC inhibition of transport, it is estimated that there are 3 x 10(5) glucose transporters present in the erythrocyte membrane. It was found that [14C]MITC-labeled band 3 could be converted into 14C-labeled band 4.5 during the Triton X-100 extraction procedure described by Kasahara & Hinkle [Kasahara, M., & Hinkel, P. C. (1977) J. Biol. Chem. 252, 7384]. On the basis of the evidence presented here and in the preceding paper, it is suggested that in the native erythrocyte membrane a component of band 3 is the glucose transport protein and that during purification with nonionic detergents the transport protein may be enzymatically degraded with some retention of activity.

An alternative synthesis of 4-deoxy-4-fluoro-D-glucose and its transport in the human erythrocyte

Treatment of methyl 4-O-mesy-alpha-D-galactopyranoside with benzyl bromide in N,N-dimethylformamide in the presence of silver oxide yielded methyl 2,3,6-tri-O-benzyl-4-O-mesyl-alpha-D-galactopyranoside which with tert-butylammonium fluoride at reflux underwent nucleophilic displacement to give methyl 2,3,6-tri-O-benzyl-4-deoxy-4-fluoro-alpha-D-glucopyranoside. This compound on hydrogenolysis provided crystalline methyl 4-deoxy-4-fluoro-alpha-D-glucopyranoside (9). The structure of 9 was established by its conversion to the 2,3,6-tri-O-acetyl derivative and by n.m.r. and m.s. analysis. Acid hydrolysis of 9 gave 4-deoxy-4-fluoro-D-glucose (1). A modification of an established synthesis of 4-deoxy-D-xylo-hexose (2) from methyl 2,3,6-tri-O-benzoyl-alpha-D-galactopyranoside is described. A systematic comparison was made of the transport parameters (Kx and Vmax) of D-glucose, 2, and 1 in human erythrocytes. The Kx values observed for the above sugars are: 4.0mM, 4.5mM, and 4.6mM, respectively. These results indicate that O-4 in beta-D-glucopyranose is not involved in hydrogen bonding to the carrier protein associated with the transport of D-glucose in the erythrocyte.

Methyl acceptors for protein methylase II from human-erythrocyte membrane

Membrane proteins from human erythrocytes were methylated with purified protein methylase II (S-adenosylmethionine:protein-carboxyl O-methyltransferase, EC.2.1.1.24). The methylated proteins were analyzed by dodecyl sulfate/polyacrylamide gel electrophoresis. Monomeric and dimeric glycophorin A (NaIO4/Schiff-2 and NaIO4/Schiff-1 positive bands) and 'band 4.5' were identified as two major classes of methyl-acceptor polypeptides for protein methylase II. In rabbit erythrocyte membrane where glycophorin A is absent, 'band 4.5' was the only major methyl-acceptor protein component. Extracted and purified glycophorin A from human erythrocytes was also found to be an excellent substrate for protein methylase II with a Km of 35.7 microM. The role of erythrocyte membrane protein methylation is discussed with regard to membrane function.

Purification of the cytochalasin B binding component of the human erythrocyte monosaccharide transport system

The cytochalasin B binding component of the human erythrocyte monosaccharide transport system has been purified. The preparation appears to contain one major protein with an apparent polypeptide chain molecular weight of 55,000 and about 0.4 binding sites per chain. Cytochalasin B binds to the reconstituted preparation with a dissociation constant of 1.3.10(-7) M, a value which is similar to that reported for the transport system in the intact erythrocyte.

The band 3 protein of the human red cell membrane: a review

Band 3 is the predominant polypeptide and the purported mediator of anion transport in the human erythrocyte membrane. Against a background of minor and apparently unrelated polypeptides of similar electrophoretic mobility, and despite apparent heterogeneity in its glycosylation, the bulk of band 3 exhibits uniform and characteristic behavior. This integral glycoprotein appears to exist as a noncovalent dimer of two approximately 93,000-dalton chains which span the membrane asymmetrically. The protein is hydrophobic in its composition and in its behavior in aqueous solution and is best solubilized and purified in detergent. It can be cleaved while membrane-bound into large, topographically defined segments. An integral, outer-surface, 38,000-dalton fragment bears most of the band 3 carbohydrate. A 17,000-dalton, hydrophobic glycopeptide fragment spans the membrane. A approximately 40,000-dalton hydrophilic segment represents the cytoplasmic domain. In vitro, glyceraldehyde 3-P dehydrogenase and aldolase bind reversibly, in a metabolie-sensitive fashion, to this cytoplasmic segment. The cytoplasmic domain also bears the amino terminus of this polypeptide, in contrast to other integral membrane proteins. Recent electron microscopic analysis suggests that the poles of the band 3 molecule can be seen by freeze-etching at the two original membrane surfaces, while freeze-fracture reveals the transmembrane disposition of band 3 dimer particles. There is strong evidence that band 3 mediates 1:1 anion exchange across the membrane through a conformational cycle while remaining fixed and asymmetrical. Its cytoplasmic pole can be variously perturbed and even excised without a significant alteration of transport function. However, digestion of the outer-surface region leads to inhibition of transport, so that both this segment and the membrane-spanning piece (which is selectively labeled by covalent inhibitors of transport) may be presumed to be involved in transport. Genetic polymorphism has been observed in the structure and immunogenicity of the band 3 polypeptide but this feature has not been related to variation in anion transport or other band 3 activities.

Characterization of the glucose transporter from human erythrocytes

The D-glucose transporter from human erythrocytes has been purified and reconstituted by Kasahara and Hinkle (J Biol Chem 252:7394--7390). Using a similar purification scheme, we have isolated the protein with 65% of the extracted phospholipid at a lipid-protein ratio of 14:1 by weight. The KD (0.14 micrometer) and extent (11 nmoles/mg protein) for binding of 3H-cytochalasin B was determined by equilibrium dialysis. Glucose was a linear competitive inhibitor of binding of cytochalasin B, with an inhibition constant of 30 mM. To further characterize the protein, samples were filtered in the presence of sodium dodecyl sulfate (SDS) through Sepharose 6B to remove 95% of the lipid followed by filtration of Sephadex G150 to remove the remaining lipid and a contaminating amount of a minor, lower-molecular-weight protein. This preparation contains only 24% acidic and basic amino acids. The protein also contains 5% neutral sugars (of which 3% is galactose), 7% glucosamine, and 5% sialic acid.