Reconstitution of glucose transport using human erythrocyte band 3

A rapid method of reconstituting human erythrocyte sugar transport proteins

A rapid reconstitution procedure for human erythrocyte hexose transfer activity is described. The procedure (reverse-phase evaporation) avoids exposure of the isolated proteins to detergent, organic solvent, sonication, or freeze-thaw steps during insertion into synthetic membranes and may be effected within 15 min. The so-formed vesicles are unilamellar structures with a large encapsulated volume, narrow size range, and low passive permeabilities. Contamination by carry-through of endogenous (red cell) lipids is less than 1%. Reconstituted hexose transfer activity was examined by using unfractionated proteins (bands 3, 4.5, and 6) and purified proteins (bands 4.5 and 3). With unfractionated proteins, hexose transport activity is low [0.34 mumol X (mg of protein)-1 X min-1], is inhibited by cytochalasin B, and increases monotonically with protein concentration. Kinetic analysis indicates that Vmax values for both influx and efflux of D-glucose are identical. Reconstitution of the cytochalasin B binding protein (band 4.5) results in hexose transport with high specific activity [5 mumol X (mg of protein)-1 X min-1] and symmetry in transfer kinetics. Band 3 proteins also appear to mediate cytochalasin B sensitive D-glucose transport activity.

Reduction of erythrocyte membrane permeability and protein binding of low-molecular-weight drugs following glycoside derivitization

The permeability of glycosides (salicin, arbutin, glycyrritin, p-nitrophenyl-beta-D-glucopyranoside, p-nitrophenyl-beta-D-galactopyranoside, p-nitrophenyl-beta-D-lactopyranoside, and p-nitrophenyl-beta-D-maltopyranoside) and their aglycons through human erythrocyte membrane was investigated. The transport rate of the glycosides through human erythrocyte membrane was slower than that of their aglycons. Glycosides with a disaccharide did not permeate the erythrocyte membrane; this observation suggests that the introduction of disaccharide to drugs gives rise to a significant decrease in the leakage of drugs through the erythrocyte membrane. The derivatives of glycosides encapsulated in erythrocytes were not released from these erythrocytes into the outer medium. The transport capacity of the glycosides was not influenced by the kind of suspending medium, but that of the aglycons was influenced by the medium. The glycosides bound to human serum albumin more weakly than their aglycons. Particularly, the glycosides were more difficult to displace from 7-anilinocoumarin-4-acetic acid (site III drug) than their aglycons, except for glycyrritin.

The human red cell glucose transporter in octyl glucoside. High specific activity of monomers in the presence of membrane lipids

Human red cell membranes were stripped of peripheral proteins and partially solubilized with 50-260 mM octyl glucoside at 2-14 mg protein/ml, to find conditions that afford a high concentration of active glucose transporter after purification on DEAE-cellulose. Transporter-egg yolk phospholipid vesicles were prepared by gel filtration. The specific D-glucose equilibrium exchange activities increased with increasing dilution of the glucose transporter. At 260 mM octyl glucoside the glucose transporter became partially denaturated. At 225 mM detergent the DEAE-cellulose chromatography showed one main and one minor fraction of active glucose transporter. Nucleoside transport activity was enriched in the minor fraction. Solubilization with 75 mM octyl glucoside at 8 mg protein/ml gave a maximal concentration of purified transporter, 0.8 mg/ml, probably corresponding to complete solubilization. The phospholipids were partially retarded on the DEAE-cellulose. The specific D-glucose equilibrium exchange was high, up to 200 nmol glucose/micrograms transporter in two min at 50 mM glucose. High performance gel filtration in octyl glucoside indicated that the transporter formed dimers during the fractionation. These eluted at Mr 125,000, partially separated from the phospholipids, which appeared at Mr 55,000 (cf. Mascher, E. and Lundahl, P. (1987) J. Chromatogr. 397, 175-186). The D-glucose transport activity was low in the main fraction and high in the transporter-phospholipid fraction. Mixing of these fractions did not increase the activity. The glucose transporter is probably dependent on one or more specific membrane lipid(s). Presumably the transporter dimerizes and loses activity upon removal of these lipids.

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.

Degradation and biosynthesis of the glucose transporter protein in chicken embryo fibroblasts transformed by the src oncogene

The rate of glucose transport in cultured fibroblasts is regulated to a number of physiological variables, including malignant transformation by src, glucose starvation, and stimulation with mitogens. Much of this transport regulation can be accounted for by variations in the amount of transporter protein in the cells. To determine the mechanisms by which levels of the transporter are regulated, we measured the rates of synthesis and degradation of the transporter by pulse-chase experiments and immunoprecipitation of the transporter. We found that transformation by the src oncogene results in a large decrease in the rate at which the transporter protein is degraded but that it does not appreciably increase the rate of transporter biosynthesis. On the other hand, glucose starvation and mitogen stimulation increase the rate of transporter biosynthesis, although a role for control of degradation is possible in these circumstances also. Variations in the rate of glucose transport or the amount of the transporter are not associated with phosphorylation of the transporter protein.

Degradation and biosynthesis of the glucose transporter protein in chicken embryo fibroblasts transformed by the src oncogene

The rate of glucose transport in cultured fibroblasts is regulated to a number of physiological variables, including malignant transformation by src, glucose starvation, and stimulation with mitogens. Much of this transport regulation can be accounted for by variations in the amount of transporter protein in the cells. To determine the mechanisms by which levels of the transporter are regulated, we measured the rates of synthesis and degradation of the transporter by pulse-chase experiments and immunoprecipitation of the transporter. We found that transformation by the src oncogene results in a large decrease in the rate at which the transporter protein is degraded but that it does not appreciably increase the rate of transporter biosynthesis. On the other hand, glucose starvation and mitogen stimulation increase the rate of transporter biosynthesis, although a role for control of degradation is possible in these circumstances also. Variations in the rate of glucose transport or the amount of the transporter are not associated with phosphorylation of the transporter protein.

Asymmetric reconstitution of the glucose transporter from Ehrlich ascites cell plasma membrane: role of alkali cations

Gel chromatography of solubilized Ehrlich cell plasma membranes and preformed asolectin vesicles coupled to a freeze-thaw cycle results in the reconstitution of 3-O-methyl-D-glucose transport. The transport activity of the liposomes formed is critically dependent on the cation present during reconstitution. Liposomes formed in K+ show high levels of carrier-mediated 3-O-methyl-D-glucose uptake (495 pmol/min/mg protein) while those formed in Na+ do not (33 pmol/min/mg protein). The inactivity in Na+ is not due to a diminished incorporation of glucose transporter nor is it due to carrier molecules reconstituted with a different orientation from those in K+ liposomes. Instead, the low glucose transport level in Na+ liposomes is related to the small size of vesicles formed with Na+. A second freeze-thaw cycle in K+ causes a two- to threefold increase in the available intravesicular volume of Na+ liposomes and results in an eightfold increase in carrier-mediated 3-O-methyl-D-glucose uptake. K+ liposomes, treated in an identical manner, show only a twofold increase in uptake. The glucose transporter was identified as a protein with a molecular mass range of 44.7 to 66.8 kDa, by the D-glucose-inhibitable photoincorporation of [3H]cytochalasin B. The carrier protein is inserted in reconstituted vesicles in a nonrandom manner with at least 80% of the molecules oriented with the cytoplasmic domain accessible to the external medium. In contrast, the neutral Na+-dependent amino acid transport system appears to be randomly reconstituted.

The interaction of liposomes containing intrinsic erythrocyte membrane proteins with lipid monolayers at air/water and oil/water interfaces

The main intrinsic membrane proteins of the human erythrocyte membrane, glycophorin and the anion transporter, were isolated by extraction with Triton X-100 and ion-exchange chromatography. After removal of detergent the extract consisted of proteolipid vesicles with a lipid:protein molar ratio in the range 50-60 and a diameter of the order of 200 nm. The interaction between these vesicles and dipalmitoylphosphatidylcholine (DPPC), cholesterol and cholesterol:DPPC (2:1 molar ratio) monolayers at air/water and n-decane/water interfaces has been studied. The vesicles interact with the monolayers, rapidly causing large increases in surface pressure. Limiting values of surface pressure, 39.4-43 mN . m-1 at air/water and 31.5-33.4 mN . m-1 at the n-decane/water interface, were reached at protein levels above 1 microgram . ml-1. At the air/water interface, and probably at the n-decane/water, surface pressure increases were limited by monolayer collapse. Compression isotherms and surface potential measurements indicated that material from the proteolipid vesicles entered the monolayer phase. In contrast to proteolipid vesicles, injection of protein-free liposomes beneath the monolayer resulted in smaller, slower increases in surface pressure. Thus, the presence of intrinsic membrane proteins in vesicles greatly facilitated the transfer of material into the lipid monolayer.

A proton NMR study of the mechanism of the erythrocyte glucose transporter

A generalizable 1H NMR technique is developed and used to monitor beta-D-glucose binding to glucose transport sites on erythrocyte membranes. This technique provides resolution of beta-D-glucose binding sites on opposite sides of the membrane, thereby enabling study of recruitment of transport sites from one side of the membrane to the other. Cytochalasin B, which competitively and specifically inhibits glucose binding to the inward-facing glucose transport site, recruits all glucose transport sites on both sides of the membrane to the inward-facing conformation. This result strongly supports a one-site model in which a single transport site alternates between distinct inward- and outward-facing conformations. The rate-limiting step in the transport process is translocation of the transport site between the two conformations, since the beta-D-glucose binding and dissociation events at both the inward- and outward-facing transport sites are shown to be fast compared to the known turnover rate of the glucose transport cycle. A model is presented for the transport machinery in which the glucose molecule binds in a cleft between channel-forming transmembrane helices, and during the transport event a sliding barrier moves past the transport site, thereby exposing the site to the opposite solution compartment.

Human erythrocyte glucose transporter: normal asymmetric orientation and function in liposomes

The transport function and orientation of the reconstituted human erythrocyte glucose transporter was studied with liposomes made with bovine brain lipid or Escherichia coli lipid. Reconstitution was achieved by a simple octyl glucoside dilution method. The reconstituted transporters with either lipid showed identical counterflow transport activity, the same response to various inhibitors, and characteristic cytochalasin B (CB) labeling. Functional location and purification of the glucose transporter was performed by using gel-permeation high-performance liquid chromatography with octyl glucoside-containing buffer. The reconstituted transport activity was associated only with band 4.5 protein (preactin) and not with band 3 protein. Both CB binding and transport function of the reconstituted transporters were resistant to trypsin but susceptible to chymotrypsin digestion. However, both trypsin and chymotrypsin treatment of unsealed ghosts completely eliminated the CB labeling and transport function of the glucose transporter. In our reconstitution system the glucose transporters maintained a normal asymmetrical (right-side-out) orientation and good transport function. A specific monoclonal antibody against the glucose transporter inhibited CB labeling of the transporters on unsealed ghosts. This was not found with the reconstituted system; however, after freeze-thawing there was a significant inhibition of CB binding by the antibody. These findings suggest that the CB-binding site of the reconstituted transporter is on the inner side of the proteoliposomes.

Improved preparation of the integral membrane proteins of human red cells, with special reference to the glucose transporter

Human red cell membranes were isolated and partially stripped of peripheral proteins by gel filtration of hemolysates on a Sepharose CL-4B column at pH 8 connected in tandem to a Sepharose CL-6B column at pH 10.5. The eluted material was washed by centrifugations, once at pH 10.5 and twice at pH 12. In this way, water-soluble proteins and peripheral membrane proteins were thoroughly removed, and 0.2 g of integral membrane proteins could be prepared within 10 h from 0.2 litre of red cells. The exposure to high pH did not lower the D-glucose transport activity, and electrophoretically pure glucose transport protein could be isolated from this preparation. Gel filtration in sodium dodecyl sulfate separated the integral membrane components into four fractions, one of them containing 4.5-material; gel electrophoresis showed about 14 zones and two-dimensional electrophoresis resolved up to 100 mostly minor components, among which the glucose transporter focused around pH 7. However, purified glucose transporter focused around pH 8. Glucose and nucleoside transport proteins were co-purified in active form on DEAE-cellulose and a fraction isolated by adsorption to Mono Q was used for immunization of mice and production of monoclonal antibodies. One hybridoma produced antibodies that reacted with material in the 4.5-region, possibly the glucose transport protein, and not with band 3-material. Upon two-dimensional electrophoresis of integral membrane components that had been solubilized with octyl glucoside the immunoreactive and the silver-stained 4.5-material focused in a broad range from pH 6 to pH 9. A possible explanation for this heterogeneity might be interaction between the glucose and nucleoside transport proteins and negatively charged lipids.

Exofacial photoaffinity labelling of the human erythrocyte sugar transporter

The 4-azidosalicylate derivative of 1,3-bis(D-mannos-4'-yloxy)-2-[2-3H]propylamine (ASA-[2-3H]BMPA) has been tested as a photoaffinity label for the sugar transporter in human erythrocytes. When photolysed in the presence of intact erythrocytes, ASA-[2-3H]BMPA covalently binds to the exofacial surface of the transporter. This labelled protein appears as a broad band in the 4.5 region in sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis. The peak of radiolabel incorporation gives an apparent Mr of approx. 50 000 on 5-20% acrylamide gels. The binding is 80% inhibitable by 320 mM 4,6-O-ethylidene-D-glucose, by 320 mM D-glucose and by 50 microM cytochalasin B. Photoirradiation of a saturating concentration of ASA-BMPA in the presence of erythrocytes results in a 25-30% loss of D-galactose transport activity. From transport inactivation data and estimations of the amount of ASA-[2-3H]BMPA binding to the transporter it is calculated that there are approx. 220 000 exofacial hexose-transport binding sites per erythrocyte. The labelling of the transporter has been carried out using freshly drawn blood and 4-weeks-old transfusion blood. No change in the binding profile on SDS-polyacrylamide gel electrophoresis was observed. Proteolytic digestion of the ASA-[2-3H]BMPA-labelled transporter with either trypsin or alpha-chymotrypsin results in the appearance of a labelled 19 kDa fragment on SDS-polyacrylamide gel electrophoresis.

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.

Rhodopsin-egg phosphatidylcholine reconstitution by an octyl glucoside dilution procedure

The transmembrane protein bovine rhodopsin was reconstituted with egg phosphatidylcholine (PC) by using a modified detergent dilution technique employing the nonionic detergent octyl-beta-D-glucoside (octyl glucoside). Using this technique, reconstituted membranes having molar phospholipid/protein ratios between 60:1 and 255:1 were prepared. This is in contrast to the results obtained when an octyl glucoside dialysis technique was employed (Jackson, M.L. and Litman, B.J. (1982) Biochemistry 21, 5601-5608). In the latter case, the highest molar phospholipid/protein ratio that could be obtained when reconstituting rhodopsin with egg PC was approximately 50:1. Reconstituted vesicles prepared by the octyl glucoside dilution technique were examined by negative stain and freeze-fracture electron microscopy, and it was found that the vesicles were unilamellar providing the molar PC/protein ratio was below about 200:1, whereas in preparations having ratios higher than this, a significant number of the vesicles were multilamellar. The mean vesicle diameter showed no trend based on the molar PC/protein ratio within the range of 82:1 to 186:1. The mean diameters of the preparations were between 520 and 850 A. Approximately equal numbers of protein particles were observed on the concave and convex fracture faces of the freeze-fracture micrographs of the reconstituted membranes which is indicative of a symmetric distribution of the protein across the bilayer.

Fractionation of human red cell membrane proteins by ion-exchange chromatography in detergent on Mono Q, with special reference to the glucose transporter

The anion exchanger Mono Q has been used for rapid and efficient fractionation of human red cell membrane proteins in the easily removable detergents n-octyl-beta-D-glucopyranoside or nonanoyl-N-methylglucamide. In practice the chromatographic resolution of membrane proteins was lower than for water-soluble proteins, perhaps due to protein-protein interactions and microheterogeneity, but several components, or groups of components, separated well upon salt gradient elution. The glucose transporter (or transportase) was eluted early, glycophorins later, and the anion transporter still later. The detergents Berol 185 and the zwitter-ionic derivatives of cholate, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulphonat e and 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propan esulphonate, gave similar chromatographic results but differed in solubilization selectivity. A relatively pure material was also fractionated; viz., a glucose transportase which had been prepared by DEAE-cellulose chromatography. Mono Q, in the presence of octyl glucoside, afforded additional purification, which made automatic sequence determination possible for eighteen amino acid residues. The results indicate that two polypeptides were present in about equimolar amounts.

The influence of lipid composition on the barrier properties of band 3-containing lipid vesicles

Band 3 protein has been incorporated into lipid vesicles consisting of 94:6 (molar ratio) egg phosphatidylcholine-bovine heart phosphatidylserine or total erythrocyte lipids by means of a Triton X-100 Bio-Beads method, with an additional sonication step prior to the removal of the detergent. This methods results, for both types of band 3 lipid vesicles, in rather homogeneous vesicles with comparable protein content and vesicle trap. Freeze-fracture electron microscopy revealed that band 3-egg phosphatidylcholine-bovine heart phosphatidylserine vesicles have considerably more intramembrane particles as compared to the band 3-erythrocyte lipid vesicles. The dimensions of the nonspecific permeation pathways present in the band 3-lipid vesicles were measured using an influx assay procedure for nonelectrolytes of different size, in which the vesicles were sampled and subsequently freed from nonenclosed labeled permeant by means of gel-filtration. The band 3-egg phosphatidylcholine-bovine heart phosphatidylserine vesicles have nonspecific permeation pathways (pores), with diameters of up to 60 A. In contrast, the band 3-total erythrocyte lipid vesicles are more homogeneous and show much smaller nonspecific permeation pathways, having a diameter of about 12 A. These results suggest that the nonspecific permeability of the band 3-lipid vesicles is strongly lipid-dependent. Increase in specific anion permeability expected as a consequence of the presence of band 3 in the erythrocyte lipid vesicles was found to be very limited. However, stereospecific, phloretin-inhibitable D-glucose permeability could clearly be demonstrated in these vesicles. The difference of the nonspecific permeability of the band 3-egg phosphatidylcholine-bovine heart phosphatidylserine vesicles and band 3-erythrocyte lipid vesicles, is discussed in the light of the presence of defects at the lipid/protein interface and protein aggregation, which may induce formation of pores.

Formation and properties of tetramers of band 3 protein from human erythrocyte membranes in planar lipid bilayers

Lipid bilayer experiments were performed in the presence of solubilized band 3 protein from human red cell membranes. Band 3 protein increased the conductance of the lipid membranes by several orders of magnitude. Membrane conductance was found to be dependent on the fourth power of protein concentration. This shows that four band 3 subunits form an ion permeable pathway in the lipid bilayer membranes. It also shows that, in the membranes, the protein molecules undergo an association equilibrium which involves at least the monomer and the tetramer of the protein, relaxation towards equilibrium being rapid on the time scale of the experiment. The increase in bilayer conductance induced by the band 3 tetramer could be inhibited by pretreatment of the protein with several SH-reagents (pCMB, pCMBS, DTNB) which also inhibit water transport across the human red cell membrane. Other SH-reagents which do not influence water transport (iodoacetamide, N-ethylmaleimide) did not show any influence on the band 3 induced conductance increase. A band 3-mediated exchange of anions comparable to that in the erythrocyte membrane did not occur in the system studied by us. Our results suggest that, in the human erythrocyte membrane, a pore formed by the band 3 tetramer could be the pathway responsible for the protein-mediated part of water transport.

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.

Quantitation of the major proteins of the human erythrocyte membrane by amino acid analysis

The proteins of the human erythrocyte membrane have been separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the resulting gel cut into 2-mm sections, and the amino acid content and composition of each slice measured using a sensitive method of amino acid analysis. The distribution of proteins among bands coincides closely with that estimated using staining intensity. Composition data for the major bands agree well with those reported for the purified proteins in all cases except that of band 4.5. Using quantitative amino acid analysis and resistive particle counting the total protein content of purified membranes was found to be 3.75 X 10(-13) g/cell, which is substantially less than previous estimates based on indirect methods. These data are used to calculate the number of copies of each major protein in a single erythrocyte.

Proteolytic and chemical dissection of the human erythrocyte glucose transporter

Treatment of the purified, reconstituted, human erythrocyte glucose transporter with trypsin lowered its affinity for cytochalasin B more than 2-fold, and produced two large, membrane-bound fragments. The smaller fragment (apparent Mr 18000) ran as a sharp band on sodium dodecyl sulphate (SDS)/polyacrylamide-gel electrophoresis. When the transporter was photoaffinity labelled with [4-3H]cytochalasin B before tryptic digestion, this fragment became radiolabelled and so probably comprises a part of the cytochalasin B binding site, which is known to lie on the cytoplasmic face of the erythrocyte membrane. In contrast, the larger fragment was not radiolabelled, and ran as a diffuse band on electrophoresis (apparent Mr 23000-42000). It could be converted to a sharper band (apparent Mr 23000) by treatment with endo-beta-galactosidase from Bacteroides fragilis and so probably contains one or more sites at which an oligosaccharide of the poly(N-acetyl-lactosamine) type is attached. Since the transporter bears oligosaccharides only on its extracellular domain, whereas trypsin is known to cleave the protein only at the cytoplasmic surface, this fragment must span the membrane. Cleavage of the intact, endo-beta-galactosidase-treated, photoaffinity-labelled protein at its cysteine residues with 2-nitro-5-thiocyanobenzoic acid yielded a prominent, unlabelled fragment of apparent Mr 38000 and several smaller fragments which stained less intensely on SDS/polyacrylamide gels. Radioactivity was found predominantly in a fragment of apparent Mr 15500. Therefore it appears that the site(s) labelled by [4-3H]cytochalasin B lies within the N-terminal or C-terminal third of the intact polypeptide chain.