Endoglycosidase f cleaves the oligosaccharides from the glucose transporter of the human erythrocyte

Rapid GLUT-1 mediated glucose transport in erythrocytes from the grey-headed fruit bat (Pteropus poliocephalus)

D-Glucose entry into erythrocytes from adult grey-headed flying fox fruit bats (Pteropus poliocephalus) was rapid and showed saturation at high substrate concentrations. Kinetic parameters were estimated from the concentration dependence of initial rates of zero-trans D-glucose entry at 5.5 degrees C as Michaelis constant (K(m)) 1. 64+/-0.56 mM, and maximal velocity (V(max)) 1162+/-152 micromol.l. cell water(-1).min(-1). D-Glucose entry was inhibited by cytochalasin B; mass law analysis of D-glucose-displaceable cytochalasin B binding gave values of K(d) 37.1+/-5.0 nM and B(max) 361.2+/-9.1 pmol/mg membrane protein. Entry of 2-deoxy-D-glucose, and 3-O-methyl-D-glucose, into P. poliocephalus red cells was rapid, entry of D-fructose was very slow. Glucose transporter polypeptides were identified on immunoblots as a band M(r) 47000-54000 and their identity confirmed by D-glucose-sensitive photolabeling of membranes with [3H]-cytochalasin B. Peptide-N-glycanase F digestion of both human and bat erythrocyte membranes generated GLUT-1-derived bands M(r) 37000. Trypsin digestion of human and fruit bat erythrocyte membranes generated fragmentation patterns consistent with similar GLUT-1 polypeptide structures in both species. Erythrocytes from adult Australian ghost bats (Macroderma gigas), a carnivorous microchiropteran bat, also expressed high levels of GLUT-1.

N-glycosylation of glucose transporter-1 (Glut-1) is associated with increased transporter affinity for glucose in human leukemic cells

To elucidate the role of N-glycosylation in the functional activity of the universal glucose transporter, Glut-1, we investigated effects of the N-glycosylation inhibitor, tunicamycin, on glucose transport by human leukemic cell lines K562, U937 and HL60. Treatment with tunicamycin produced a 40-50% inhibition of 2-deoxyglucose uptake and this was associated with a 2-2.5-fold decrease in transporter affinity for glucose (Km) without a change in Vmax. Leukemic K562, U937 and HL60 cells expressed Glut-1 transporter protein. With K562 cells Glut-1 appeared as a broad band of 50-60 kDa, whereas with U937 and HL60 cells a diffuse band was observed at approximately 55 kDa. Treatment of K562 cells with tunicamycin for 18 h, resulted in extensive loss of the 50-60 kDa glycoprotein, appearance of a 30-40 kDa band and increased staining of a 45 kDa band. With U937 cells, tunicamycin treatment resulted in the appearance of a 30-40 kDa band and increased staining of a 45 kDa band. With HL60 cells loss of the 55 kDa Glut-1 band was observed and a band of 45 kDa appeared. Tunicamycin-treatment resulted in 75-90% inhibition in [3H]mannose incorporation but only 20-25% inhibition in [3H]thymidine and [3H]leucine incorporation. In contrast, tunicamycin had little effect on the viability and MTT responses of the cells used. These results suggest that in leukemic cells N-glycosylation of Glut-1 plays an important role in maintaining its structure and functional integration.

Monomeric human red cell glucose transporter (Glut1) in non-ionic detergent solution and a semi-elliptical torus model for detergent binding to membrane proteins

The self-association state of the human red cell glucose transporter (Glut1) in octaethylene glycol n-dodecyl ether (C12E8) and n-octyl beta-D-glucopyranoside (OG) solution was analyzed in the presence of reductant by gel filtration with light-scattering, refractivity and absorbance detection, and by ultracentrifugation. The C12E8-Glut1 complex was essentially monomeric, whereas OG-Glut1 also formed dimers and larger oligomers. C12E8-Glut1 retained substantial glucose transport activity even after depletion of endogenous lipids by gel filtration, as shown by reconstitution and transport measurements. Removal of endogenous lipids from OG-Glut1 abolished the activity unless phosphatidylcholine was included in the eluent. The binding of C12E8 and OG to Glut1 was determined by gel filtration with refractivity and absorbance detection or with radioactive tracer to be 1.86 +/- 0.07 and 1.84 +/- 0.09 g/g polypeptide, respectively. A structural model was proposed in which non-ionic detergent forms a semi-elliptical torus (SET) surrounding the transmembrane protein. The torus thickness was assumed to be equal to the radius (short half-axis) of a spherical (oblate ellipsoidal) free detergent micelle and the polar head groups of the detergent molecules were predicted to be situated just outside the hydrophobic surface of the protein. The experimental detergent binding values and those obtained from the SET model together confirmed that Glut1 was monomeric in C12E8 solution and provided constraints on the shape and size of the hydrophobic transmembrane region of Glut1 in alpha-helical and beta-barrel topology models.

Capillary and rotating-tube isoelectric focusing of a transmembrane protein, the human red cell glucose transporter

The human red cell glucose transporter (Glut1) is a transmembrane protein. Monomeric Glut1 was purified by ion-exchange chromatography in the presence of the non-ionic detergent n-dodecyl octaoxyethylene (C12E8). For focusing, the ionic strength of the solution of C12E8-Glut1 complexes with co-purified lipids was lowered by dialysis, the detergent concentration was increased and carrier ampholytes were added. Focusing was done for 5 min at 3000 V in a methyl cellulose-coated glass capillary (50 microns I.D.). The anolyte H3PO4 was then replaced by NaOH for mobilization towards the anode. Absorbance monitoring at 280 nm showed two groups of zones at pH 6 and 8. Similarly, isoelectric focusing in a rotating quartz tube (3 mm I.D.) gave Glut1 zones at pH 5.5 and 8.0. Phosphorus analysis revealed that the Glut1 zone at pH 8 contained more phospholipids than did the other one. The above results together with previously determined and calculated isoelectric points (pI) of Glut1 indicate that the Glut1 at pH 8 is monomeric and that the zone at pH 5.5-6 represents oligomeric materials. The pI 8.0 at 22 degrees C applies for monomeric Glut1 in the absence of urea. The results exemplify that capillary isoelectric focusing of hydrophobic membrane proteins is possible.

Overexpression of GLUT3 placental glucose transporter in diabetic rats

The localization of the two major placental glucose transporter isoforms, GLUT1 and GLUT3 was studied in 20-d pregnant rats. Immunocytochemical studies revealed that GLUT1 protein is expressed ubiquitously in the junctional zone (maternal side) and the labyrinthine zone (fetal side) of the placenta. In contrast, expression of GLUT3 protein is restricted to the labyrinthine zone, specialized in nutrient transfer. After 19-d maternal insulinopenic diabetes (streptozotocin), placental GLUT3 mRNA and protein levels were increased four-to-fivefold compared to nondiabetic rats, whereas GLUT1 mRNA and protein levels remained unmodified. Placental 2-deoxyglucose uptake and glycogen concentration were also increased fivefold in diabetic rats. These data suggest that GLUT3 plays a major role in placental glucose uptake and metabolism. The role of hyperglycemia in the regulation of GLUT3 expression was assessed by lowering the glycemia of diabetic pregnant rats. After a 5-d phlorizin infusion to pregnant diabetic rats, placental GLUT3 mRNA and protein levels returned to levels similar to those observed in nondiabetic rats. Furthermore, a short-term hyperglycemia (12 h), achieved by performing hyperglycemic clamps induced a fourfold increase in placental GLUT3 mRNA and protein with no concomitant change in GLUT1 expression. This study provides the first evidence that placental GLUT3 mRNA and protein expression can be stimulated in vivo under hyperglycemic conditions. Thus, GLUT3 transporter isoform appears to be highly sensitive to ambient glucose levels and may play a pivotal role in the severe alterations of placental function observed in diabetic pregnancies.

Isoelectric focusing in immobilized pH gradients of the glucose transporter from human red cell membranes

Isoelectric focusing of the human red cell glucose transporter (a transmembrane protein) was performed in immobilized pH gradients. Isoelectric focusing of integral membrane proteins presents problems that are related to the amphiphilic nature of these proteins. Solubilizing additives must be used to counteract hydrophobic effects. In our case, urea and the nonionic detergent, Triton X-100, were used. Focusing was done at 15 degrees C. The isoelectric point (pI) of the glucose transporter (freshly purified by anion-exchange chromatography in the presence of octyl glucoside) was determined at 8.4 +/- 0.05 (n = 9), in good agreement with our earlier determinations by two-dimensional electrophoresis with isoelectric focusing in the presence of carrier ampholytes in the first dimension. The width of the focused zone was approximately 0.1 pH unit, more narrow than after focusing with carrier ampholytes. In an immobilized pH gradient from pH 7 to 10, the transporter region at pH 8.4 comprised one major and one or two minor zones. The pH interval 4-10 was also used and showed a single transporter zone. The glucose transporter tends to self-associate in detergent solution. Octyl glucoside-purified glucose transporter formed oligomers during incubation at 37 degrees C. Upon focusing, these oligomers appeared in a wide pH interval far below pH 8.4.

The role of N-glycosylation in the targeting and stability of GLUT1 glucose transporter

The cDNAs encoding the GLUT1 glucose transporter protein were altered by site-directed mutagenesis at consensus sites for the addition of N-linked glycosylation. These cDNAs were transfected into CHO cells with an expression vector and the subcellular distribution and stability of the expressed glycosylation-defective GLUT1 protein were analyzed. Immunohistochemical analysis with a specific antibody demonstrated that a significant portion of glycosylation-defective GLUT1 protein remained in the intracellular compartment. By contrast, most of the wild-type GLUT1 protein expressed with the same procedures resided in the plasma membranes. Metabolic labeling studies revealed that the half-life of the glycosylation-defective GLUT1 protein was significantly shorter than that of wild-type GLUT1 protein. These results indicate that N-glycosylation of the glucose transporter affects its intracellular targeting and protein stability.

Characterization of functional human erythrocyte-type glucose transporter (GLUT1) expressed in insect cells using a recombinant baculovirus

The human erythrocyte-type glucose transporter (GLUT1) has been abundantly expressed in insect cells by using a recombinant baculovirus. At 4 days after infection with the virus, the insect cell-surface and intracellular membranes were found to contain greater than 200 pmol of D-glucose-sensitive binding sites for the transport inhibitor cytochalasin B per mg of protein. The characteristics of binding were identical with those of the erythrocyte transporter, although the two proteins differed substantially in apparent Mr, probably as a result of glycosylation differences.

The isoelectric point of the human red cell glucose transporter

The isoelectric point (pI) of the human red cell glucose transporter (Glut 1) was determined. Inconsistent values of 6.0, 6.4-6.5 and 8 have been reported earlier. Integral membrane proteins from human red cells were analyzed by two-dimensional electrophoresis with isoelectric focusing and sodium dodecyl sulfate gel electrophoresis (2D-PAGE). A zone of monomeric Glut 1 was found at pH 8.7, but most of the Glut 1 focused at pH 6-7 together with the anion transporter and other components. Purified Glut 1 focused only at pH 8.5 +/- 0.2 (S.D., n = 12) and deglycosylated purified Glut 1 only at pH 8.4 +/- 0.1 (n = 5), as shown by 2D-PAGE. The absence of Glut 1 below pH 8 in the latter cases was confirmed by immunoblotting with a monoclonal antibody. Furthermore, Glut 1 was photoaffinity-labelled with [3H]cytochalasin B and subjected to isoelectric focusing in one dimension. The pI of the labelled Glut 1 was 8.6 +/- 0.3 (n = 11). A pI of 9.1 was calculated for the Glut 1 polypeptide on the basis of amino acid composition and pKa values for amino acid side groups. The sialic acid content of the glycosylated transporter from fresh red cells was determined at approximately 2.1 sialic acid residues per transporter, which corresponds to a calculated pI of 8.8. The pI values of other human glucose transporter polypeptides of the facilitative diffusion type (Glut 2, 3, 4 and 5) were calculated at 8.4, 7.4, 7.1 and 6.2, respectively.

Molecular anatomy of the blood-brain barrier as defined by immunocytochemistry

This review outlines the recent developments and improvements of our knowledge concerning the molecular composition of the BBB as revealed by immunocytochemistry. Data have been accumulated which show that the BBB exhibits a specific collection of structural and metabolic properties which are also found in tight transporting epithelia. This conclusion is substantiated by (i) the implementation of antibodies which recognize proteins of non-BBB origin, to show that these biochemical markers and the functions that they represent are localized in the BBB endothelium; and (ii) the characterization of target molecules to which polyclonal or monoclonal antibodies which have been generated to epitopes of the BBB endothelium or brain homogenates. According to these data the protein assemblies comprising the phenotypical appearance of the BBB can therefore be defined by the particular selection as well as topological expression of common epithelial antigens, rather than the expression of BBB-unique molecular species. In this respect the immunocytochemical data corroborate the physiological assumption that the BBB possesses the character of a specific polarized epithelium. Attention is also given to the description of developmental expression of BBB-related immunomarkers. By collecting the data from different sources we introduce a classification of the BBB marker proteins according to their developmental appearance. Three groups of proteins are classified with respect to their sequential expression around the time of BBB closure: Phase E (early) markers which appear before BBB closure, phase I (intermediate) markers which are expressed at the time of BBB tightening, and phase L (late) markers which are detectable after the closure of the BBB. Such a scheme may to be useful in better defining the maturation process of BBB, which apparently is not a momentary event in brain development, but rather consists of a temporally sequenced process of hierarchically structured gene expression which finally define the molecular properties of the BBB. This process continues even after parturition, especially with regard to the achievement of immunological properties of the mature BBB. By examining the developmental spatio-temporal expression of different BBB markers we conclude that the mechanisms governing the pattern of BBB maturation are not limited to the interactions occurring between glial and endothelial cells. We therefore suggest a heuristic model in a triangular interrelationship that includes differentiation effects of neurons on glia and of glia cells on the BBB endothelium.(ABSTRACT TRUNCATED AT 400 WORDS)

Intestinal absorption of dipeptides and beta-lactam antibiotics. II. Purification of the binding protein for dipeptides and beta-lactam antibiotics from rabbit small intestinal brush border membranes

By photoaffinity labeling of brush border membrane vesicles from rabbit small intestine with photoreactive derivatives of beta-lactam antibiotics and dipeptides, a binding protein for dipeptides and beta-lactam antibiotics with an apparent molecular weight of 127,000 was labeled. The labeled 127 kDa polypeptide could be solubilized with the non-ionic detergents Triton X-100, n-octyl glucoside or CHAPS. If the vesicles were solubilized prior to photoaffinity labeling, no clear incorporation of radioactivity into the 127 kDa polypeptide occurred indicating a loss of binding ability upon solubilization. By affinity chromatography of solubilized brush border membrane proteins on an agarose wheat germ lectin column, the binding protein for dipeptides and beta-lactam antibiotics of Mr 127,000 was retained on the column. With N-acetyl-D-glucosamine the photolabeled binding protein for beta-lactam antibiotics and dipeptides was eluted together with the brush border membrane-bound enzyme aminopeptidase N. Separation from aminopeptidase N and final purification was achieved by anion-exchange chromatography on DEAE-sephacel. Polyclonal antibodies against the purified binding protein were raised in guinea pigs. The photolabeled 127 kDa protein could be precipitated from solubilized brush border membranes with these antibodies. Incubation of brush border membrane vesicles with antiserum prior to photoaffinity labeling significantly reduced the extent of labeling of the 127 kDa protein. Treatment of brush border membrane vesicles with antiserum significantly inhibited the efflux of the alpha-aminocephalosporin cephalexin from the brush border membrane vesicles compared to vesicles treated with preimmune serum. These studies indicate that the binding protein for dipeptides and beta-lactam antibiotics of apparent molecular weight 127,000 in the brush border membrane of rabbit small intestinal enterocytes is directly involved in the uptake process of small peptides and orally active beta-lactam antibiotics across the enterocyte brush border membrane.

Localization of the forskolin photolabelling site within the monosaccharide transporter of human erythrocytes

Chemical and proteolytic digestion of intact erythrocyte glucose transporter as well as purified transporter protein has been used to localize the derivatization site for the photoaffinity agent 3-[125I]iodo-4-azido-phenethylamino-7-O-succinyldeacetylforskol in [( 125I]IAPS-forskolin). Comparison of the partial amino acid sequence of the labelled 18 kDa tryptic fragment with the known amino acid sequence for the HepG2 glucose transporter confirmed that the binding site for IAPS-forskolin is between the amino acid residues Glu254 and Tyr456. Digestion of intact glucose transporter with Pronase suggests that this site is within the membrane bilayer. Digestion of labelled transporter with CNBr generated a major radiolabelled fragment of Mr approximately 5800 putatively identified as residues 365-420. Isoelectric focusing of Staphylococcus aureus V8 proteinase-treated purified labelled tryptic fragment identified two peptides which likely correspond to amino acid residues 360-380 and 381-393. The common region for these radiolabelled peptides is the tenth putative transmembrane helix of the erythrocyte glucose transporter, comprising amino acid residues 369-389. Additional support for this conclusion comes from studies in which [125I]APS-forskolin was photoincorporated into the L-arabinose/H(+)-transport protein of Escherichia coli. Labelling of this transport protein was protected by both cytochalasin B and D-glucose. The region of the erythrocyte glucose transporter thought to be derivatized with IAPS-forskolin contains a tryptophan residue (Trp388) that is conserved in the sequence of the E. coli arabinose-transport protein.

Identification and characterization of glucose transport proteins in plasma membrane- and Golgi vesicle-enriched fractions prepared from lactating rat mammary gland

Plasma membrane- and Golgi vesicle-enriched membrane fractions were prepared from day-10 lactating rat mammary glands. Each fraction was found to contain a single set of D-glucose-inhibitable cytochalasin B-binding sites: plasma membranes and Golgi vesicles bound 20 +/- 2 and 53 +/- 4 pmol of cytochalasin/mg of membrane protein (means +/- S.E.M.), with dissociation constants of 259 +/- 47 and 520 +/- 47 nM respectively. Anti-peptide antibodies against the C-terminal region (residues 477-492) of the rat brain/human erythrocyte glucose transporter labelled a sharp band of apparent Mr 50,000 on Western blots of both fractions. Treatment with endoglycosidase F before blotting decreased the apparent Mr of this band to 38,000, indicating that it corresponded to a glycoprotein. Confirmation that this immunologically cross-reactive band was a glucose transporter was provided by the demonstration that it could be photoaffinity-labelled, in a D-glucose-sensitive fashion, with cytochalasin B. Quantitative Western blotting studies yielded values of 28 +/- 5 and 23 +/- 3 pmol of immunologically cross-reactive glucose transporters/mg of membrane protein in the plasma membrane and Golgi vesicle fractions respectively. From comparison with the concentration of cytochalasin B-binding sites, it is concluded that a protein homologous to the rat brain glucose transporter constitutes the major glucose transport species in the plasma membranes of mammary gland epithelial cells. Glucose transporters are also found in the Golgi membranes of these cells, at least half of them being similar, if not identical, to the transporters of the plasma membrane. However, their function in this location remains unclear.

Biotin-conjugated reagents as site-specific probes of membrane protein structure: application to the study of the human erythrocyte hexose transporter

A novel labeling procedure using biotin-conjugated protein-modifying reagents has been employed to study the structure and function of the human erythrocyte hexose transporter. The carbohydrate moiety of the isolated, reconstituted transporter was labeled by using galactose oxidase/biotin hydrazide. Cysteine residues, which are essential for transporter function, were tagged with a biotin-conjugated maleimide. Labeling with this reagent inhibited the binding of cytochalasin B to the transporter. Following sodium dodecyl sulfate-gel electrophoresis, labeling of the transporter and its proteolytic fragments was detected by Western blotting and probing with alkaline phosphatase-conjugated avidin. After tryptic cleavage of the transporter into two membrane domains, preparations reacted with galactose oxidase/biotin hydrazide were labeled on the 25-kDa glycosylated fragment, but not on the carbohydrate-free 19-kDa peptide. Biotin-maleimide-labeled cysteine residues on both peptides. Transporter polypeptide was fragmented more extensively using Staphylococcus aureus V8 protease. Limited digestion produced a broad band of 30-50 kDa and sharper bands of 23 and 21 kDa. More extensive digestion resulted in the disappearance of the 23-kDa peptide and the appearance of sharp bands of 20, 19, 17, 13, 11, 8, and 7 kDa. Biotin label introduced with galactose oxidase/biotin hydrazide was found on the broad 30-kDa band, confirming its identity as a glycopeptide. All of the peptides weighing more than 11 kDa contained cysteine residues labeled with biotin maleimide, while the 8- and 7-kDa peptides were unlabeled. These results demonstrate the potential usefulness of biotin-conjugated reagents as site-specific probes of membrane protein structure.

Chromatographic characterization of nitrobenzylthioinosine binding proteins in band 4.5 of human erythrocytes: purification of a 40 kDa truncated nucleoside transporter

DEAE-column-purified band 4.5 polypeptides of human erythrocyte membranes are mostly glucose transporters with nucleoside transporters as a minor component. The purpose of the present work was to differentially identify and isolate the nucleoside transporters in band 4.5 free from glucose transporters. Equilibrium binding studies demonstrated that the band 4.5 preparation binds nibrobenzylthioinosine (NBTI), a potent nucleoside transport inhibitor, at two distinct sites, one with a high affinity (dissociation constant, KD of 1 nM) with a small capacity, BT (0.4 nmol/mg protein), and the other with a low affinity (KD of 15 microM) with a large BT (14-16 nmol/mg protein). The BT of the low-affinity site was equal to that of the cytochalasin B binding site in the preparation. A gel-filtration chromatography of band 4.5 photolabeled with [3H]NBTI and [3H]cytochalasin B identified three polypeptides of apparent Mr 55,000, 50,000 and 40,000. Of these, the 55 kDa polypeptide was specifically labeled by cytochalasin B (p55GT), indicating that it is a glucose transporter. Both the 50 and 40 kDa polypeptides were labeled with NBTI at low ligand concentrations (less than 0.1 microM), which was abolished by an excess (20 microM) of nitrobenzylthioguanosine, indicating that they are two forms (p50NT and p40NT, respectively) of the high affinity NBTI binding protein or nucleoside transporter. At higher (not less than 10 microM) NBTI concentrations, however, p55GT was also labeled with NBTI, indicating that the low-affinity NBTI binding is due to a glucose transporter. Treatment of band 4.5 with trypsin reduced the p50NT labeling with a concomitant and stoichiometric increase in the p40NT NBTI labeling without affecting the high-affinity NBTI binding of the preparation. These findings indicate that the nucleoside transporter is slightly smaller in mass than the glucose transporter and that trypsin digestion produces a truncated nucleoside transporter of apparent Mr 40,000 which retains the high-affinity NBTI binding activity of intact nucleoside transporter. Both p55GT and p50 NT were coeluted in a major protein fraction, P1 in the chromatography, while p40NT was eluted separately as a minor protein fraction, P1a. All three polypeptides formed mixed dimers, which were eluted in a fraction PO. We have purified and partially characterized the truncated nucleoside transporter, p40NT. The purified p40NT may be useful for biochemical characterization of the nucleoside transporter.

Binding of sodium dodecyl sulphate to an integral membrane protein and to a water-soluble enzyme. Determination by molecular-sieve chromatography with flow scintillation detection

We have determined the binding of sodium dodecyl sulphate (SDS) to the human red cell glucose transporter (polypeptide, Mr 54,117) and to a water-soluble enzyme, N-5'-phosphoribosylanthranilate isomerase-indole-3-glycerol-phosphate synthase (PRAI-IGPS) from Escherichia coli (Mr 49,484). [35S]SDS was equilibrated with each protein on molecular-sieve chromatography at a series of SDS concentrations. The binding ratios of SDS to protein were determined by flow scintillation detection and automated amino acid analyses. Unexpectedly the glucose transporter, which is a transmembrane protein, bound about the same amount of SDS per gram of protein as did the enzyme. At 1.6 mM SDS, slightly below the critical micelle concentration (CMC) (1.8 mM) in the eluent, the binding ratio was 1.6 g SDS/g protein for both the glucose transporter and PRAI-IGPS. At 2.0 mM SDS (above the CMC) the glucose transporter showed a binding ratio of 1.7 g SDS/g protein. The corresponding value for the enzyme was about 1.5 g/g. The SDS-glucose transporter complex seems to be more compact than the SDS-enzyme complex as judged by molecular-sieve chromatography and by SDS-polyacrylamide gel electrophoresis. Recent neutron scattering results have shown a protein-decorated triple-micelle structure for the SDS-PRAI-IGPS complex. Hypothetically, the more compact SDS-glucose transporter complex may therefore consist of a dual-micelle structure. The molecular-sieve gel beads bound considerable amounts of SDS. The SDS binding to the gel matrix and to the proteins increased with increasing SDS concentration up to at least 1.6-2.0 mM SDS. In the case of the water-soluble enzyme a shoulder was observed in the binding curve at 1 mM SDS, probably reflecting a change in the conformation of the complex.

Mapping the binding site of the nucleoside transporter protein: a 3D-OSAR study

The nucleoside transporter is an intrinsic membrane protein that mediates salvage of nucleosides from the extracellular medium. In this report, its binding sites have been characterized by a 3D-QSAR (three-dimensional structure-directed quantitative structure-activity relationships) receptor mapping technique. REMOTEDISC. The algorithm is applied to a set of 19 nucleoside analogues, each of which binds to the transporter. The methodology includes: (i) conformational analysis of each ligand; (ii) estimation of physicochemical properties of each ligand at the atomic level; (iii) structural comparison of the low energy conformation of each ligand in the series with a reference structure on the basis of physicochemical property matching; (iv) construction of a predicted binding site cavity from the alignments of step (iii); and (v) multiple regression analysis of the binding data with respect to the 3-dimensional physicochemical descriptors in different 'site-pockets' of the binding cavity. The pharmacophore model that emerges consists of the geometry of the binding site cavity and the relative weights of various properties in different pockets for each of the ligands considered. The study suggests that binding free energy is sensitive to the composition, size and hydrophobicity of the heterocyclic base in the ligand. Though both syn and anti conformations are tried as active forms, the anti conformation gives a better solution and is chosen for modeling the binding site cavity. The best model obtained divides the binding site into six pockets and uses nine independent variables, fitting the observed data with a correlation coefficient of 0.94, a standard deviation of 0.22 and an explained variance of 0.80. Results of our model are consistent with a hypothesis that the 5'-OH group hydrogen bonds with the receptor. This model provides tentative design criteria for development of new nucleoside drugs and transport inhibitors. The model will undoubtedly continue to evolve (i) as the 3D-QSAR algorithm is further refined, and (ii) as data on additional nucleoside analogues become available.

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.

Heterogeneity of the glucose transporter in malignant and suppressed hybrid cells

Previous work has demonstrated unequivocally that the kinetics of glucose transport of tumorigenic and suppressed hybrid cells show a consistent difference. This lies in an increased affinity for glucose in the tumorigenic cell lines (i.e., a reduced Km). Evidence has also been presented that the degree of glycosylation of the transporter may affect the Km. When a suitable antiserum to the transporter present in these cells became available, it was of interest to examine the patterns of binding to immunoblots of extracts of the hybrid cell pairs. It became apparent that there was a clear difference between the two parental cell lines and that this difference was reflected in the hybrid cells. Both the tumorigenic parent and the tumorigenic hybrid cell presented a much more heterogeneous distribution of apparent molecular weights than the nontumorigenic parent or suppressed cell line. That this was probably due to differences in glycosylation was indicated by the effect of tunicamycin on the cells; this gave rise to a more homogeneous band of lower molecular weight. The significance of these differences is discussed in relation to results obtained with other tumorigenic cell lines and to the published structure of the human erythrocyte glucose transporter.

Sodium dodecyl sulphate-protein complexes. Changes in size or shape below the critical micelle concentration, as monitored by high-performance agarose gel chromatography

We have determined the sodium dodecyl sulphate (SDS) concentration needed to complete the formation of SDS-protein complexes. A Superose-6 column was equilibrated with SDS for 7 h. A sample of a native protein or an SDS-protein complex was applied, and the elution volume, Ve, was determined. Then the SDS concentration, CSDS, was changed, etc., i.e., Ve was determined as a function of CSDS. The critical micelle concentration of SDS (cmcSDS) was 1.8 mM in the eluent (ionic strength 0.10 M). Native bovine carbonic anhydrase (BCA) formed an SDS complex above 0.2 mM SDS. As CSDS was increased, Ve decreased gradually in two main transitions, (TI) at 0.2-1.0 mM and (TII) at 1.2-2.0 mM SDS. These concentrations are corrected for a lag in the column equilibration with SDS. SDS-BCA, pre-equilibrated at 1.6 mM SDS, showed transitions similar to those observed with native BCA, except that transition TII included a minor transition at 2.0-2.2 mM SDS. The SDS complexes of reduced and carboxamidomethylated bovine serum albumin, of N-5'-phosphoribosylanthranilate isomerase-indole-3-glycerol-phosphate synthase from Escherichia coli (PRAI-IGPS) and of two tryptic fragments of this enzyme behaved similarly. For SDS-PRAI-IGPS the major part of transition TII was completed at 1.6-1.7 mM SDS, as shown by analyses after 20-h column equilibrations with increasing as well as decreasing CSDS. The SDS complex of an integral membrane protein, the glucose transporter from human red cells, was smaller or less elongated than the SDS complexes of water-soluble proteins of the same polypeptide length. The formation of all five SDS-protein complexes investigated was practically completed at cmcSDS.

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

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

Structure, biosynthesis, and function of the hexose transporter in Chinese hamster ovary cells deficient in N-acetylglucosaminyltransferase 1 activity

We have used a Chinese hamster ovary cell line deficient in N-acetylglucosaminyltransferase 1 activity (Lec1) to study the effects of altered asparagine-linked oligosaccharides on the structure, biosynthesis, and function of glucose transporter protein. Immunoblots of membranes of Lec1 cells show a glucose transporter protein of Mr 40,000, whereas membranes of wild-type (WT) cells contain a broadly migrating Mr 55,000 form similar to that observed in several other mammalian tissues. The total content of immunoreactive glucose transporters in Lec1 cells is 3.5-fold greater than that of WT cells. Digestion with endoglycosidases, treatment with inhibitors of glycosylation, and interactions with agarose-bound lectins demonstrate that glucose transporters of both cell lines derive from a similar Mr 38,000 core polypeptide and that both contain asparagine-linked oligosaccharide. Transporters in Lec1 cells contain primarily "undecorated" but "trimmed" mannose-type asparagine-linked oligosaccharides, while the protein in WT cells contains a mixture of "decorated" and "trimmed" asparagine-linked oligosaccharides. Biosynthetic and turnover studies demonstrate that Lec1 cells, in contrast to WT cells, are unable fully to process the core asparagine-linked oligosaccharides of maturing glucose transporters. When radiolabeled in methionine-deficient medium both Lec1 and WT cells show similar rates of synthesis and turnover of glucose transporter proteins. It should be noted, however, that starvation for a critical amino acid may alter the ability of the cell to synthesize or degrade proteins. The abilities of Lec1 and WT cells to transport hexoses and to interact with the inhibitor cytochalasin B are very similar. The results indicate that, although altered asparagine-linked glycosylation can affect the content and biogenesis of glucose transporters, these changes do not greatly modify cellular hexose uptake. The possibility that alterations in asparagine-linked glycosylation may change the cell surface localization or acquisition of a "functional conformation" of the glucose transporter is also suggested.

Effect of endoglycosidase F-peptidyl N-glycosidase F preparations on the surface components of the human erythrocyte

Endo-N-acetyl-beta-D-glucosaminidase F-Peptidyl N-glycosidase F preparations (abbreviated Endo F) and endo-beta-D-galactosidase were used to study the major human erythrocyte membrane glycoproteins and the components carrying the blood group A, B, Rhesus (D), and Duffy (Fya) antigens. The results are consistent with the known presence of an N-glycosyl-linked oligosaccharide on sialoglycoprotein alpha and the absence of such an oligosaccharide from sialoglycoprotein delta. Under the conditions used, only a portion of the N-glycosyl-linked oligosaccharides on band 3 molecules were cleaved by Endo F alone or by Endo F in combination with endo-beta-D-galactosidase. Immunoblotting experiments showed that treatment of red cells with Endo F alone had little effect on the components carrying blood group A and B antigen activity. However, Endo F used in combination with endo-beta-D-galactosidase caused a substantial reduction in the binding of monoclonal anti-A and anti-B antibodies. The results clearly show that sialoglycoproteins alpha and delta carry little or no blood group A or B activity. Endo F alone, or in combination with endo-beta-D-galactosidase, had no effect on the electrophoretic mobility of the Rh(D) polypeptide, supporting previous suggestions that this membrane polypeptide is unusual in not being glycosylated. Endo F had a dramatic effect on the electrophoretic mobility of the component(s) carrying blood group Fya activity. The diffuse Fya component of Mr 38,500-90,000 was sharpened to a band of Mr 26,000. Either endo-beta-D-galactosidase or neuraminidase treatment reduced the Mr of the Fya component(s) but did not significantly sharpen the bands, suggesting that the Fya component contains between 40-50% by mass of N-glycosyl-linked oligosaccharides.

Adenosine transporters in chromaffin cells: subcellular distribution and characterization

Adenosine transporters in freshly isolated and cultured chromaffin cells were quantified by the [3H]dipyridamole binding technique, showing a maximal bound capacity of 0.4 +/- 0.05 pmol/10(6) cells (240,000 +/- 20,000 transporters by cell). Scatchard analysis showed a similar affinity for [3H]dipyridamole in isolated cells and subcellular fractions (Kd = 5 +/- 0.6 nM). For enriched plasma membrane preparations and chromaffin granule membranes, the maximal binding capacities were also very similar, 2.3 +/- 0.3 and 1.8 +/- 0.4 pmol/mg protein, respectively. When [3H]nitrobenzylthioinosine was employed as a radioligand, the maximal bound capacity in cultured chromaffin cells was 0.053 +/- 0.004 pmol/10(6) cells (32,000 +/- 3000 transporters per cell) with a high affinity constant (Kd = 0.25 +/- 0.03 nM); similar values were obtained in all subcellular fractions (Kd = 0.1 +/- 0.01). Also, plasma and chromaffin granule membranes showed similar maximal binding values (0.4 +/- 0.06 pmol/mg protein). Photoincorporation studies with [3H]nitrobenzylthioinosine into plasma membrane polypeptides showed the presence of three molecular species of 115 +/- 10; 58 +/- 6 and 42 +/- 5 kDa. Chromaffin granule membranes showed only the 105 +/- 9 and 51 +/- 4 molecular species.

The blood-nerve barrier is rich in glucose transporter

The glucose transporter of the facilitated diffusion type has been localized in sections of innervated rat diaphragm muscle and sciatic nerve by immunofluorescence, using affinity-purified antibodies against both the entire transporter and the carboxy-terminal peptide. In both tissues the transporter was very abundant in the perineurial sheath of cells surrounding the nerve fibres. The transporter also appeared to be abundant in the endoneurial blood vessels of the sciatic nerve. The identity of the antigen as the glucose transporter was established by extracting sciatic nerve with sodium dodecylsulphate and immunoblotting the extract. A single reactive polypeptide with the expected molecular weight of 55,000 was found. The high concentration of glucose transporter in the cells of the blood-nerve barrier presumably ensures an adequate supply of glucose to the nerve fibres.

Investigation of the structure and function of the human erythrocyte glucose transporter by proteolytic dissection

Tryptic and papain digestion have been employed to investigate the structure and function of the human erythrocyte glucose transporter. Trypsin cleaves the native protein into two large, membrane-embedded fragments and a number of small peptides that are released from the membrane. These fragments have been isolated and located within the transporter sequence by fast atom bombardment mass spectrometry and amino acid analysis. The results indicate that the segments of the sequence comprising residues 213-269 and 457-492 are cleaved from the cytoplasmic surface of the membrane by trypsin treatment. These findings are compatible with a model previously proposed for the arrangement of the polypeptide in the membrane (Mueckler, M., et al. (1985) Science 229, 941-945). Despite the loss of these 93 residues, the portion of the protein remaining embedded in the membrane is still able to bind cytochalasin B. This binding is inhibited by D-glucose, indicating that the membrane-embedded fragments retain the substrate-binding site. Fourier transform infrared spectroscopic analysis of the protein before and after proteolytic digestion shows that the intramembranous part of the protein is largely alpha-helical, although some beta-sheet structure appears also to be present. The spectroscopic findings also indicate that the extramembranous, cytoplasmic domain of the transporter, which is removed by trypsin, contains alpha-helical structure.

Identification and characterization of the glucose-transport protein of the bovine blood/brain barrier

The glucose-transport protein from bovine cerebral-cortex microvessels has been identified and characterized by virtue of its ability to bind the ligand [4-3H]cytochalasin B. Microvessel membranes were found to contain a single set of glucose-inhibitable high-affinity cytochalasin B-binding sites [113 +/- 16 (S.E.M.) pmol/mg of membrane protein], with an association constant of 6.8 +/- 1.8 (S.E.M.) micron-1. D-Glucose inhibited the binding to these sites with a Ki of 31 mM. The transport protein was identified by photoaffinity labelling with [4-3H]cytochalasin B and was found to migrate as a broad band of apparent Mr 55,000 on SDS/polyacrylamide gels. Labelling was inhibited by D-glucose, but not by L-glucose. Treatment with endoglycosidase F yielded a sharper band of apparent Mr 46,000, indicating that the transport protein is glycosylated. However, in contrast with the human erythrocyte glucose transporter, digestion with endo-beta-galactosidase had little effect on the electrophoretic mobility of the microvessel protein. Tryptic digestion of the photolabelled protein yielded a radioactive fragment of apparent Mr 18,000, similar to that of the fragment produced by digestion of the labelled human erythrocyte glucose transporter. In addition, a protein of Mr identical with that of the photolabelled transporter was labelled on Western blots of microvessel membranes by antisera raised against the intact erythrocyte transporter and against synthetic peptides corresponding to its N- and C-terminal regions. It is concluded that the glucose-transport protein of bovine cerebral-cortex microvessel endothelial cells shows structural homology with the human erythrocyte glucose transporter.

Nucleoside transporter of pig erythrocytes. Kinetic properties, isolation and reaction with nitrobenzylthioinosine and dipyridamole

Rapid kinetic techniques were used to measure the transport of uridine in pig erythrocytes in zero-trans entry and exit and equilibrium exchange protocols. The kinetic parameters were computed by fitting appropriate integrated rate equations to the time-courses of transmembrane equilibration of radiolabeled uridine. Transport of uridine conformed to the simple carrier model with directional symmetry, but differential mobility of substrate-loaded and empty carrier. At 5 degrees C, the carrier moved about 30-times faster when loaded than when empty. Uridine transport was inhibited in a concentration-dependent manner by nitrobenzylthioinosine and dipyridamole and the inhibition correlated with the binding of the inhibitors to high-affinity binding sites on the cells (Kd about 1 and 10 nM, respectively). Thus, in its kinetic properties, differential mobility when empty and loaded, and sensitivity to inhibition by nitrobenzylthioinosine and dipyridamole, the transporter of pig erythrocytes is very similar to that of human erythrocytes. Also, the total number of high-affinity binding sites for nitrobenzylthioinosine and dipyridamole/cell were similar for the two cell types and the [3H]nitrobenzylthioinosine-labeled carrier of pig erythrocytes, just as that of human red cells, was mainly recovered in the band 4.5 protein fraction of Triton X-100-solubilized membranes. However, sodium dodecylsulfate-polyacrylamide gel electrophoresis of photoaffinity-labeled band 4.5 membrane proteins indicated a slightly higher molecular weight for the transporter from pig than human erythrocytes. We have also confirmed the lack of functional sugar transport in erythrocytes from adult pigs by measuring the uptake of various radiolabeled sugars. But in spite of the lack of functional sugar transport we recovered as much band 4.5 protein from pig as from human erythrocyte membranes.

High-performance agarose gel chromatography in octyl glucoside of integral membrane proteins from human red cells, with special reference to the glucose transporter

Integral membrane proteins and lipids from human red cells were fractionated in the presence of octyl glucoside by high-performance gel chromatography on a 22-ml column of the small-bead cross-linked agarose gel Superose 6, at 5 degrees C, pH 7.6 and 30-50 mM detergent. To avoid aggregation a relatively high flow-rate, 9 ml/h, was chosen. At low ionic strength four main fractions were resolved, which contained anion transporter multimers(I), glycophorin oligomers(II), glucose transporter dimers(III) and phospholipids(IV). In 0.5 M sodium chloride the resolution was lower but the yield of the glucose transporter was markedly higher, and chromatography of partially purified glucose transporter gave a protein recovery of about 90%. The apparent Mr values for the octyl glucoside complexes of the main components were: anion transporter, 900,000; glycophorin A, 210,000-360,000, dependent on ionic strength; glucose transporter, 110,000-160,000; lipids, 70,000. Some components aggregated with time: at a flow-rate of 1 ml/h mainly glycophorins and the glucose transporter were eluted, but no anion transporter, and fractionation performed 20 h after solubilization showed extensive aggregation of proteins. Superose-6 chromatography of glucose transporter and membrane lipids that had been isolated on DEAE-cellulose partially resolved the transporter and the phospholipid fractions. In this case, the resolution was better with 50 than with 30 mM detergent. The maximum glucose transport activity was approximately one-tenth of that observed before fractionation and appeared in two main fractions, at the main transporter fraction as well as at the overlap between the transporter and the lipids. The activity level was the same in both fractions, although the protein concentration was much lower in the second one. Addition of 2 mM egg-yolk phospholipids to the eluent did not increase the activity. This strongly indicates that the glucose transporter needs some specific membrane lipids to retain high transport activity. At the concentration of ca. 0.3 mg/ml used, the glucose transporter was probably eluted as a dimer in the absence of phospholipids and as a monomer in their presence.

Characterization of monoclonal antibodies that recognize band 4.5 polypeptides associated with nucleoside transport in pig erythrocytes

Three monoclonal antibodies have been raised against partially purified band 4.5 polypeptides [Steck (1974) J. Cell Biol. 62, 1-19] from pig erythrocyte membranes. The antibodies were capable of binding to both intact pig erythrocytes and protein-depleted membrane preparations and recognized detergent-solubilized polypeptides from adult and neonatal pig erythrocytes that were photolabelled with [G-3H]nitrobenzylthioinosine (NBMPR), a potent specific inhibitor of nucleoside transport. The antibodies did not recognize polypeptides from neonatal pig erythrocytes that were photolabelled with the glucose-transport inhibitor [3H]cytochalasin B. Reactivity with polypeptides of apparent Mr 64,000 [10% (w/v) acrylamide gels] was demonstrated by Western-blot analysis. The antibodies recognized pig band 4.5 polypeptides after prolonged treatment with endoglycosidase F, a finding consistent with reactivity against polypeptide, rather than carbohydrate, determinants. Trypsin digestion of NBMPR-labelled protein-depleted pig erythrocyte membranes generated two labelled polypeptide fragments (Mr 43,000 and 26,000). Two of the antibodies recognized both fragments on Western blots, whereas the third bound to the larger, but not to the smaller, fragment. The antibodies had no significant effect on reversible binding of NBMPR to protein-depleted pig erythrocyte membranes and did not bind to NBMPR-labelled polypeptides in human, rabbit or mouse erythrocytes.