The carbohydrate moiety of band 3 glycoprotein of human erythrocyte membranes. Structures of lower molecular weight oligosaccharides

N-Glycomics of Human Erythrocytes

Glycosylation is a complex post-translational modification that conveys functional diversity to glycoconjugates. Cell surface glycosylation mediates several biological activities such as induction of the intracellular signaling pathway and pathogen recognition. Red blood cell (RBC) membrane N-glycans determine blood type and influence cell lifespan. Although several proteomic studies have been carried out, the glycosylation of RBC membrane proteins has not been systematically investigated. This work aims at exploring the human RBC N-glycome by high-sensitivity MALDI-MS techniques to outline a fingerprint of RBC N-glycans. To this purpose, the MALDI-TOF spectra of healthy subjects harboring different blood groups were acquired. Results showed the predominant occurrence of neutral and sialylated complex N-glycans with bisected N-acetylglucosamine and core- and/or antennary fucosylation. In the higher mass region, these species presented with multiple N-acetyllactosamine repeating units. Amongst the detected glycoforms, the presence of glycans bearing ABO(H) antigens allowed us to define a distinctive spectrum for each blood group. For the first time, advanced glycomic techniques have been applied to a comprehensive exploration of human RBC N-glycosylation, providing a new tool for the early detection of distinct glycome changes associated with disease conditions as well as for understanding the molecular recognition of pathogens.

A Comprehensive Review of Our Current Understanding of Red Blood Cell (RBC) Glycoproteins

Human red blood cells (RBC), which are the cells most commonly used in the study of biological membranes, have some glycoproteins in their cell membrane. These membrane proteins are band 3 and glycophorins A-D, and some substoichiometric glycoproteins (e.g., CD44, CD47, Lu, Kell, Duffy). The oligosaccharide that band 3 contains has one N-linked oligosaccharide, and glycophorins possess mostly O-linked oligosaccharides. The end of the O-linked oligosaccharide is linked to sialic acid. In humans, this sialic acid is N-acetylneuraminic acid (NeuAc). Another sialic acid, N-glycolylneuraminic acid (NeuGc) is present in red blood cells of non-human origin. While the biological function of band 3 is well known as an anion exchanger, it has been suggested that the oligosaccharide of band 3 does not affect the anion transport function. Although band 3 has been studied in detail, the physiological functions of glycophorins remain unclear. This review mainly describes the sialo-oligosaccharide structures of band 3 and glycophorins, followed by a discussion of the physiological functions that have been reported in the literature to date. Moreover, other glycoproteins in red blood cell membranes of non-human origin are described, and the physiological function of glycophorin in carp red blood cell membranes is discussed with respect to its bacteriostatic activity.

Glycosylation of solute carriers: mechanisms and functional consequences

Solute carriers (SLCs) are one of the largest groups of multi-spanning membrane proteins in mammals and include ubiquitously expressed proteins as well as proteins with highly restricted tissue expression. A vast number of studies have addressed the function and organization of SLCs as well as their posttranslational regulation, but only relatively little is known about the role of SLC glycosylation. Glycosylation is one of the most abundant posttranslational modifications of animal proteins and through recent advances in our understanding of protein-glycan interactions, the functional roles of SLC glycosylation are slowly emerging. The purpose of this review is to provide a concise overview of the aspects of glycobiology most relevant to SLCs, to discuss the roles of glycosylation in the regulation and function of SLCs, and to outline the major open questions in this field, which can now be addressed given major technical advances in this and related fields of study in recent years.

Membrane damages under high pressure of human erythrocytes agglutinated by concanavalin A

Human erythrocytes are agglutinated by lectins such as concanavalin A (Con A). The behaviors of agglutinated erythrocytes under pressure are less well understood. Here, we report the effects of erythrocyte agglutination on pressure-induced membrane damages. Small clumps of intact erythrocytes by Con A were dissociated by a pressure of 200 MPa. Further, the observation by scanning electron microscopy demonstrated the generation of vesicles, fragmented particles, and membrane hole. On the other hand, large clumps of trypsin-digested erythrocytes by Con A seemed to be stable against 200 MPa. However, the erythrocytes dissociated from such pressure-treated clumps by methyl α-mannopyranoside also showed the existence of vesicles and fragmented particles except for the membrane hole. Pressure-induced hemolysis was greatly suppressed in such large clumps. Similar suppressive effects were observed in erythrocytes packed by centrifugation. However, the hemolysis occurred when the erythrocytes dissociated from 200 MPa-treated large clumps by methyl α-mannopyranoside were incubated at 0°C and atmospheric pressure. Pyrene excimer fluorescence due to spectrin denaturation was observed in Con A-agglutinated ghosts that were exposed to a pressure of 200 MPa. These results suggest that upon pressure treatment of tightly agglutinated erythrocytes, the hemolysis is greatly suppressed, but membrane damages occur such as spectrin denaturation and vesiculation.

One-step immunopurification and lectinochemical characterization of the Duffy atypical chemokine receptor from human erythrocytes

Duffy antigen/receptor for chemokines (DARC) is a glycosylated seven-transmembrane protein acting as a blood group antigen, a chemokine binding protein and a receptor for Plasmodium vivax malaria parasite. It is present on erythrocytes and endothelial cells of postcapillary venules. The N-terminal extracellular domain of the Duffy glycoprotein carries Fy(a)/Fy(b) blood group antigens and Fy6 linear epitope recognized by monoclonal antibodies. Previously, we have shown that recombinant Duffy protein expressed in K562 cells has three N-linked oligosaccharide chains, which are mainly of complex-type. Here we report a one-step purification method of Duffy protein from human erythrocytes. DARC was extracted from erythrocyte membranes in the presence of 1% n-dodecyl-β-D-maltoside (DDM) and 0.05% cholesteryl hemisuccinate (CHS) and purified by affinity chromatography using immobilized anti-Fy6 2C3 mouse monoclonal antibody. Duffy glycoprotein was eluted from the column with synthetic DFEDVWN peptide containing epitope for 2C3 monoclonal antibody. In this single-step immunoaffinity purification method we obtained highly purified DARC, which migrates in SDS-polyacrylamide gel as a major diffuse band corresponding to a molecular mass of 40-47 kDa. In ELISA purified Duffy glycoprotein binds anti-Duffy antibodies recognizing epitopes located on distinct regions of the molecule. Results of circular dichroism measurement indicate that purified DARC has a high content of α-helical secondary structure typical for chemokine receptors. Analysis of DARC glycans performed by means of lectin blotting and glycosidase digestion suggests that native Duffy N-glycans are mostly triantennary complex-type, terminated with α2-3- and α2-6-linked sialic acid residues with bisecting GlcNAc and α1-6-linked fucose at the core.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

Force profiles of protein pulling with or without cytoskeletal links studied by AFM

To test the capability of the atomic force microscope for distinguishing membrane proteins with/without cytoskeletal associations, we studied the pull-out mechanics of lipid tethers from the red blood cell (RBC). When wheat germ agglutinin, a glycophorin A (GLA) specific lectin, was used to pull out tethers from RBC, characteristic force curves for tether elongation having a long plateau force were observed but without force peaks which are usually attributed to the forced unbinding of membrane components from the cytoskeleton. The result was in agreement with the reports that GLA is substantially free of cytoskeletal interactions. On the contrary, when the Band 3 specific lectin, concanavalin A, was used, the force peaks were indeed observed together with a plateau supporting its reported cytoskeletal association. Based on these observations, we postulate that the state of cytoskeletal association of particular membrane proteins can be identified from the force profiles of their pull-out mechanics.

Affinity monolithic capillary columns for glycomics/proteomics: 1. Polymethacrylate monoliths with immobilized lectins for glycoprotein separation by affinity capillary electrochromatography and affinity nano-liquid chromatography in either a single column or columns coupled in series

In this report, microcolumn separation schemes involving monolithic capillary columns with immobilized lectins, and relevant to nanoglycomics/nanoproteomics were introduced. Positive and neutral monoliths based on poly(glycidyl methacrylate-co-ethylene dimethacrylate) were designed for achieving lectin affinity chromatography (LAC) by nano-LC and CEC. The positive monoliths (i.e., monoliths with cationic sites) afforded relatively high permeability in nano-LC but lack predictable EOF magnitude and direction, while neutral monoliths provided a good compromise between reasonable permeability in nano-LC and predictable EOF in CEC. Lectin affinity nano-LC permitted the enrichment of classes of different glycoproteins having similar N-glycans recognized by the immobilized lectin, whereas lectin affinity CEC provided the simultaneous capturing and separation of different glycoproteins due to differences in charge-to-mass ratio. Also, this investigation demonstrated for the first time the coupling of lectin capillary columns in series (i.e., tandem columns) for enhanced separation of glycoproteins by LAC using the CEC modality. Furthermore, in the coupled columns format, glycoforms of a given glycoprotein were readily separated.

Down-regulation of the alpha-Gal epitope expression in N-glycans of swine endothelial cells by transfection with the N-acetylglucosaminyltransferase III gene. Modulation of the biosynthesis of terminal structures by a bisecting GlcNAc

The down-regulation of the alpha-Gal epitope (Galalpha1,3Galbeta-R) in swine tissues would be highly desirable, in terms of preventing hyperacute rejection in pig-to-human xenotransplantation. In an earlier study, we reported that the introduction of the beta1,4-N-acetylglucosaminyltransferase (GnT) III gene into swine endothelial cells resulted in a substantial reduction in the expression of the alpha-Gal epitope. In this study, we report on the mechanism for this down-regulation of the alpha-Gal epitope by means of structural and kinetic analyses. The structural analyses revealed that the amount of N-linked oligosaccharides bearing the alpha-Gal epitopes in the GnT-III-transfected cells was less than 10% that in parental cells, due to the alteration of the terminal structures as well as a decrease in branch formation. In addition, it appeared that the addition of a bisecting GlcNAc, which is catalyzed by GnT-III, leads to a more efficient sialylation rather than alpha-galactosylation. In vitro kinetic analyses showed that the bisecting GlcNAc has an inhibitory effect on alpha-galactosylation, but does not significantly affect the sialylation. These results suggest that the bisecting GlcNAc in the core is capable of modifying the biosynthesis of the terminal structures via its differential effects on the capping glycosyltransferase reactions. The findings may contribute to the development of a novel strategy to eliminate carbohydrate xenoantigens.

Two glycoprotein populations of band 3 dimers are present in human erythrocytes

The human erythrocyte Band 3 anion exchanger contains a single site of N-glycosylation that contains either a short complex oligosaccharide or an extended polylactosaminyl oligosaccharide. Approximately equal amounts of the different glycosylated forms of Band 3 are found in human red cells. As Band 3 exists predominantly as dimers, they may be uniform and consist of a subunit containing an extended oligosaccharide paired with a subunit containing a short oligosaccharide chain. Alternatively, Band 3 dimers may be comprised of subunits that either contain polylactosaminyl or short oligosaccharide chains. To distinguish between these two extremes, the ability of Band 3 membrane domain dimers to bind to immobilized tomato lectin, which specifically binds polylactosaminyl oligosaccharide, was tested. The dimeric membrane domain of Band 3 could be resolved into two fractions by tomato lectin chromatography. This shows that Band 3 dimers are not homogeneous and that two pools exist in red cells, some with a long polylactosaminyl oligosaccharide and the other with a short complex type. The amount of short chain form recovered in the unbound fraction was higher than expected for a random distribution of oligosaccharide chains on Band 3 dimers. Detergent extraction experiments showed that Band 3 glycoforms did not display a differential interaction with the cytoskeleton. The ability to separate Band 3 dimers into two glycoform populations suggests that subunit exchange between dimers does not occur in the membrane or in detergent solution. Furthermore, the results show that while one population of Band 3 dimers is processed to contain polylactosaminyl oligosaccharide, the other largely escapes this processing step.

Proteolytic cleavage sites of native AE2 anion exchanger in gastric mucosal membranes

The AE2 anion exchanger in pig and rabbit gastric mucosal membranes was subjected to limited proteolysis with trypsin, chymotrypsin, and papain, and to enzymatic N-deglycosylation. A monoclonal antibody to the AE2 C-terminal peptide was raised, characterized, and used to purify pig AE2 and its C-terminal cleavage products. Five distinct proteolytic cleavage sites within the AE2 transmembrane domain were defined by amino acid sequencing. The amino acid sequence of pig AE2 in the region encompassing the N-glycosylated Z-loop was also determined by RT-PCR. Tryptic cleavage of pig AE2 in the Z-loop produced C-terminal glycopeptides and was unaffected by deglycosylation, whereas the smaller rabbit AE2 C-terminal tryptic peptide lacked oligosaccharide, consistent with the respective amino acid sequences. The third consensus N-glycosylation site in pig Z-loop was heterogeneously glycosylated. Rapid papain cleavage in the Z-loop and slower cleavage in loop 7-8 produced C-terminal peptide products which were not N-glycosylated. Chymotryptic cleavage of the rabbit AE2 Z-loop required prior deglycosylation. Chymotryptic cleavage in the pig AE2 Z-loop produced C-terminal glycopeptides. Prior deglycosylation of pig AE2 unmasked novel, ionic strength-sensitive chymotryptic cleavage sites in the adjacent exofacial loop 7-8. These results provide experimental confirmation for some aspects of AE2 topography previously predicted from primary structure alone.

Analysis of asparagine-linked oligosaccharides by sequential lectin affinity chromatography

Lectins are proteins that specifically bind to a particular carbohydrate structure. Affinity chromatography with immobilized lectins is a quite effective technique not only for the fractionation of glycoproteins or oligosaccharides but also their structural assessment. In this article, we focus on the separation of glycopeptides and oligosaccharides derived from glycoproteins by affinity chromatography on immobilized lectin columns.

ABO blood grouping of bloodstains by sandwich ELISA using monoclonal antibody specific for human red cell band 3

ABO blood grouping of human bloodstains was performed by a sandwich ELISA using a species-specific monoclonal antibody to the amino-terminal cytoplasmic domain of human red cell membrane band 3. In a blind trial, all A, B and O bloodstains (a 1 cm long thread) and AB bloodstains (a 1.5 cm long thread) were accurately typed by this method. Even when bloodstains were contaminated by other body fluids (e.g., semen and saliva), only the ABO blood group epitopes on band 3 of the red cell membrane were detected. Thus, identification of human blood and ABO blood grouping of bloodstains which were contaminated by other body fluids could be simultaneously performed by this method.

Elevation of the activities of glycosyl transferases involved in polylactosaminoglycan biosynthesis in autoimmune MRL lpr/lpr mouse T cells

Lymph node (LN) T cells from autoimmune MRL/MpJ-lpr/lpr (lpr) mice and control MRL/MpJ-(+)/+ (+/+) mice were compared as to glycosyl transferase activities involved in the biosynthesis of polylactosaminoglycans. The N-acetylglucosaminyl transferase (GlcNAc transferase) activity responsible for the extension of polylactosaminoglycans was assayed. The reaction products with this GlcNAc transferase were characterized by sequential glycosidase treatment and methylation analysis, and the enzyme was found to be classifiable as an UDP-GlcNAc:N-acetyllactosamine beta 1-3 GlcNAc transferase (polylactosamine extension enzyme). The activity of this GlcNAc transferase in T cells from enlarged LN of lpr mice was 3-6 times higher than that in T cells from +/+ mice. On the other hand, activated T cells from +/+ mice only showed about a 2-fold increase in the activity of the transferase, compared with that in resting T cells. B cells from +/+ mice also showed a significantly higher activity of the transferase than +/+ T cells, the enzyme activity being comparable to or slightly lower than that in lpr T cells. Furthermore, when the reaction mixture contained both UDP-GlcNAc and UDP-Gal as donors, extension of the Gal-GlcNAc residue was observed. These results indicated the biosynthetic basis for the abundance of polylactosaminoglycans in lpr T cells and normal B cells. We also found that lpr T cells exhibited significant UDP-GlcNAc:asialo-bovine submaxillary mucin GlcNAc transferase activity. Only weak activity of this enzyme was detected in +/+ resting and activated T cells, and B cells. This enzyme activity suggested the potential for polylactosaminoglycan formation on the mucin-type sugar chains on the surface of lpr T cells.

Carbohydrate binding specificities of several poly-N-acetyllactosamine-binding lectins

The structural requirements for the interaction of the Asn-linked poly-N-acetyllactosamine-type oligosaccharide moieties of glycoproteins with various N-acetylglucosamine-binding lectins were investigated by means of affinity chromatography on immobilized lectin-Sepharose columns. High molecular weight glycopeptides containing poly-N-acetyllactosamine-type oligosaccharides obtained by Pronase digestion of human erythrocyte ghosts were treated with 0.1 M trifluoroacetic acid at 100 degrees C for 40 min and then several oligosaccharide fragments were purified with an amino-bonded silica column. Among these oligosaccharide fragments, trisaccharide Gal beta 1-4GlcNAc beta 1-6Galol bound to the wheat germ agglutinin (WGA)- and pokeweed mitogen (PWM)-Sepharose columns, and also showed affinity to the Datura stramonium agglutinin (DSA)-, Lycopersicon esculentum (tomato) agglutinin- and Solanum tuberosum (potato) agglutinin-Sepharose columns. Pentasaccharide Gal beta 1-4GlcNAc beta 1-3(Gal beta 1-4GlcNAc beta 1-6)Galol showed weaker affinity to the WGA- and PWM-Sepharose columns, compared to the trisaccharide. Trisaccharide GlcNAc beta 1-3(GlcNAc beta 1-6)Galol showed weak affinity to the WGA-Sepharose column and did not show any affinity to the other lectin-Sepharose columns. Hexasaccharide Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAcol bound only to the DSA-Sepharose column, indicating that only DSA does not require a GlcNAc beta(1-6)- linkage for interaction.

Characterization and quantitation of fatty acids covalently bound to erythrocyte membrane proteins: anion transporter contains 1 mol of fatty acid thiol ester

Human erythrocyte membranes which had been thoroughly extracted with organic solvents contained 20 nmol of fatty acids/mg dry wt. The major fatty acids were palmitic and stearic with their monoethenoic derivatives as minor constituents. No other fatty acids were detected. When solvent-extracted membranes were digested with Pronase about 90% of the original content of fatty acids was retained in the insoluble residue. Fatty acids were linked to membrane proteins through alkali-labile bonds of which 30% were of a thiol ester and the remainder of an O-ester type. This conclusion is based on differential liberation of fatty acids by hydroxylamine at pH 7.0 and pH 11.0. Two extracts of membranes enriched in peripheral proteins (bands 1, 2, 5 and 2.1, 4.1, 4.2, 6) were prepared and extracted with organic solvents but each contained about six times less fatty acids than the parent solvent-extracted membranes. Glycophorin A contains little if any covalently bound fatty acids. Anion transporter (band 3) contains about 1 mol of thiol ester of fatty acid. This accounts for about half of the thiol ester-linked fatty acids in the parent solvent-extracted membranes. Most of the O-ester-linked fatty acids are linked to an undisclosed membrane protein.

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.

Poly-N-acetyllactosamine alterations of Thy-1 glycoprotein in lymphocyte differentiation

Developmental changes in murine lymphocyte Thy-1 include both quantitative and qualitative alterations involving N-linked oligosaccharides. Comparison of immature with mature T-cells has shown that the oligosaccharides of Thy-1 are characterized by an increase in the number of sialic acid residues responsible for the acidic pI of peripheral T-cell Thy-1, and a decrease in those oligosaccharides responsible for Mr heterogeneity of thymocyte Thy-1. The research reported here suggests that the basis of the large Mr heterogeneity in Thy-1 of immature T-cells is the presence of repeating N-acetyllactosamine (R'Gal beta 1,4GlcNAc beta 1,3R") units in the oligosaccharide portion of the molecule. Lymphocytes were surface iodinated and 125I-thy-1 was purified by immunoprecipitation and preparative nonequilibrium pH gradient electrophoresis. The minimal Mr of unglycosylated Thy-1 after endoglycosidase F digestion was 15,000-16,000. Digestion of Thy-1 with endo-beta-galactosidase suggested that the complex type N-linked glycans in thymocytes, but not in lymph node T-cells, contained increased levels of polylactosamine. The presence of polylactosamine was confirmed by binding to a Datura stramonium lectin column which retarded and bound approx. 50% of thymocyte Thy-1 and only about 18% of lymph node T-cell Thy-1. Affinity chromatography using anti-i antibody immobilized on agarose beads indicated that the polylactosamine is probably present in a predominantly linear form. Since alterations of polylactosamine structures have been implicated in development and transformation in several systems, the present results suggest an important role for these glycans in immune-cell differentiation.

Interaction of human erythrocyte Band 3 with Ricinus communis agglutinin and other lectins

Agglutination and competition studies suggest that human erythrocyte Band 3 can interact with both mannose/glucose- and galactose-specific lectins. Purified Band 3 reconstituted into lipid vesicles binds concanavalin A, but the nonspecific binding component, measured in the presence of alpha-methylmannoside, is very high. This glycoprotein also carries binding sites for the galactose-specific lectin Ricinus communis agglutinin. Binding was inhibited poorly by lactose, but much more effectively by desialylated fetuin glycopeptides, suggesting that the lectin recognizes a complex oligosaccharide sequence on Band 3. The glycoprotein bears two separate classes of binding sites for R. communis agglutinin. High-affinity binding sites exist which show strong positive cooperativity and correspond in number to the outward-facing Band 3 molecules. A low-affinity binding mode is abolished by 40% ethyleneglycol, suggesting the involvement of hydrophobic lectin-glycoprotein interactions. Studies on binding of R. communis agglutinin to human erythrocytes indicate positively cooperative binding to 7 X 10(5) very-high-affinity sites per cell, and lectin binding is completely inhibitable by lactose. Based on its binding characteristics in vesicles, it seems likely that Band 3 forms the major receptor for this lectin in human erythrocytes. Properties such as positive cooperativity thus appear to be a common feature of the interaction of Band 3 with a variety of lectins of different specificity, both in erythrocytes and lipid bilayers.

Control of glycoprotein synthesis. Bovine milk UDPgalactose:N-acetylglucosamine beta-4-galactosyltransferase catalyzes the preferential transfer of galactose to the GlcNAc beta 1,2Man alpha 1,3- branch of both bisected and nonbisected complex biantennary asparagine-linked oligosaccharides

Bovine milk UDPgalactose:N-acetylglucosamine beta-4-galactosyltransferase has been used to investigate the effect of a bisecting GlcNAc residue (linked beta 1,4 to the beta-linked mannose of the trimannosyl core of asparagine-linked complex oligosaccharides) on galactosylation of biantennary complex oligosaccharides. Columns of immobilized lectins (concanavalin A, erythroagglutinating phytohemagglutinin, and Ricinus communis agglutinin 120) were used to separate the various products of the reactions. Preferential galactosylation of the GlcNAc beta 1,2Man alpha 1,3 arm occurred both in the absence and in the presence of a bisecting GlcNAc residue; the ratio of the rates of galactosylation of the Man alpha 1,3 arm to the Man alpha 1,6 arm was 6.5 in the absence of a bisecting GlcNAc and 2.8 in its presence. The bisecting GlcNAc residue reduced galactosylation of the Man alpha 1,3 arm by about 78% probably due to steric hindrance of the GlcNAc beta 1,2Man alpha 1,3 beta 1,4 region of the substrate by the bisecting GlcNAc. This steric hindrance prevents the action of four other enzymes involved in assembly of complex asparagine-linked oligosaccharides and indicates the importance of the bisecting GlcNAc residue in the control of glycoprotein biosynthesis. The Man alpha 1,3 arm of biantennary oligosaccharides is believed to be freely accessible to enzyme action whereas the Man alpha 1,6 arm is believed to be folded back toward the core. This may explain the preferential action of Gal-transferase on the Man alpha 1,3 arm of both bisected and nonbisected oligosaccharides.

Arterial dermatan sulfate proteoglycan structure in pigeons susceptible to atherosclerosis

Arterial dermatan sulfate proteoglycans (DS-PG) were isolated from randomly bred White Carneau (RBWC) pigeons and a line of White Carneau pigeons (WC-2) genetically selected for increased atherosclerosis susceptibility. To characterize the basic proteoglycan (PG) structure and to identify structural differences that may relate to a specific arterial phenotype, embryos were labeled with radioactive sulfate in ovo and PG were isolated from aortas with 4.0 M GdnHCl. DS-PG were separated from chondroitin sulfate PG by gel chromatography and further purified by CsCl buoyant density ultracentrifugation and ion-exchange of chromatography. The DS-PG monomers from the two genetic lines of pigeons eluted at different positions on two high pressure liquid chromatography (HPLC) gel permeation systems, which suggested a structural difference between the two monomers. Analysis of the monomer components showed a similar protein core molecular weight of 50,000 for each and a similar amino acid composition. Glycosaminoglycans (GAG) molecular weights were estimated to be 15,000 for WC-2 and 18,000 for RBWC. The findings suggest a basic difference in post-translational processing of PG in the two pigeon types which may reflect distinct functional properties associated with resistance or susceptibility to atherosclerosis.

Plasmodium falciparum: carbohydrates as receptor sites of invasion

Monosaccharides, disaccharides, and trisaccharides were tested as inhibitors of the in vitro growth of Plasmodium falciparum (strain FCB). While certain monosaccharides (N-acetyl-D-glucosamine, D-mannose, and 3-O-methyl-D-glucose) proved to exhibit a toxic or reversibly retarding effect on the intraerythrocytic development of the parasite, the corresponding alpha- or beta-methylglycosides did not. Several methylglycosides, synthetic di- and tri-saccharides, and artificial blood group antigens were further tested for inhibitory effects on invasion of host red blood cells in vitro. The synthetic disaccharides beta DGlcNAc(1----4) alpha DManOMe and beta DGlcNAc(1----4) DGlcNAc (chitobiose) were good inhibitors of invasion at 10 mM concentration, whereas beta DGal(1----4)beta DGlcNAcOMe was negligibly inhibitory. The inhibition rate of N-acetyl-D-glucosamine, beta-glycosidically linked to bovine serum albumin (BSA) by an alipathic spacer, -(CH2)8CO-, was not enhanced, compared to the corresponding hapten, beta DGlcNAcO(CH2)8COOCH3. The inhibition rates of blood group A- and B-trisaccharide haptens, which were inhibitors of invasion, were also not significantly enhanced when coupled to BSA by way of the corresponding amide spacer, -(CH2)2NHCO(CH2)7CO-. A remarkable enhancement of the inhibition rate was, however, observed when beta DGal(1----3) alpha DGalNAcO(CH2)2NHCO(CH2)7COOCH3 (T-hapten) was coupled to BSA. A clear-cut decrease in the inhibition rates of different beta-glycosides of N-acetyl-D-glucosamine, beta DGlcNAcOR, was observed, depending on the nature of the aglycon R(p-nitrophenyl greater than -(CH2)8COOCH3 greater than -(CH2)2NHCO(CH2)2COOCH3 greater than -CH3). Also, p-nitrophenyl-alpha-D-glucopyranoside was a much better inhibitor of invasion than the corresponding methyl glycoside, alpha DGlcOMe, which was not inhibitory. The properties of the aglycon spacer, used for the covalent attachment of the carbohydrate to the carrier protein, may thus be crucial for the outcome of the inhibition rate.

Structural changes of carbohydrate chains of human thyroglobulin accompanying malignant transformations of thyroid glands

Carbohydrate moieties were released from human thyroglobulins prepared from normal and transformed, thyroid tissues by hydrazinolysis. The oligosaccharides thus prepared were fractionated by DEAE-cellulose, Bio-Gel P-4, concanavalin-A--Sepharose and Ricinus-communis-agglutinin--Sepharose columns and were characterized by exo-glycosidase and endo-glycosidase digestions, methylation analysis and 400-MHz 1H-NMR spectroscopy. These studies showed that the alterations accompanying malignancy occurred in complex-type carbohydrate chains and exhibited the following structural features: (a) a complex-type carbohydrate chain containing 1 phosphate in a diester linkage was present in all the malignant thyroglobulins. (b) Asialo-carbohydrate chains with biosynthetically incomplete structures were found in all of the malignant thyroglobulins. (c) Carbohydrate chains of higher molecular mass, which have repeating Gal-GlcNAc disaccharides and peripheral alpha-fucosyl residues were present in metastatic papillary carcinoma.

Incomplete glycosylation of erythrocyte membrane proteins in congenital dyserythropoietic anaemia type II (CDA II)

The alterations in the erythrocyte membrane proteins of individuals with congenital dyserythropoietic anaemia (CDA II) were studied. Alterations were observed in both the erythrocyte sialoglycoproteins and erythrocyte anion transport protein (Band 3). There was a decrease in the apparent molecular weight of the major sialoglycoprotein alpha (glycophorin A) as well as a general reduction in the intensity of staining of all the sialoglycoproteins by the PAS stain. Sialoglycoprotein alpha isolated from CDA II erythrocytes contained 30% less sialic acid than normal alpha. The anion transport protein of CDA II erythrocytes migrated as a band with a lower molecular weight than the normal protein on SDS-gel electrophoresis. The CDA II anion transport protein had a substantially reduced content of N-acetylglucosamine and galactose, which probably reflects a reduction in the number of N-acetyl-lactosamine units carried by the protein. Our results suggest that there is a general defect in glycosylation of the major membrane glycoproteins of CDA II erythrocytes. We suggest that this glycosylation defect is a consequence of bone marrow stress.

Interaction of pokeweed mitogen with poly(N-acetyllactosamine)-type carbohydrate chains

The carbohydrate-binding specificities of pokeweed mitogen (PWM) were studied by interaction between 125I-PWM and purified glycoproteins. Human erythrocyte-membrane, band-3 glycoprotein bound 125I-PWM, but other glycoproteins containing triple-branched (tri-antennary)-complex-, double-branched (biantennary)-complex-, hybrid-, high-mannose, or small-mucin-type carbohydrate chains failed to bind 125I-PWM. Pretreatment of human erythrocytes with endo-beta-D-galactosidase prevented 125I-PWM binding to the erythrocytes, as well as to band-3 glycoprotein from these cells. Poly(N-acetyllactosamine) glycopeptides and complex-type glycopeptides purified from band-3 glycoprotein and porcine thyroglobulin, respectively, were labeled at their nonreducing, terminal D-galactosyl or 2-acetamido-2-deoxy-D-galactosyl groups by D-galactose oxidase-sodium borotritide treatment and examined for interaction with agarose-immobilized PWM isolectins. From the binding behavior of these labeled glycopeptides on columns of immobilized-PWM isolectins, and from the interaction of various glycoproteins bearing known carbohydrate chains with 125I-PWM, it was concluded that all three major PWM isolectins (Pa-1, Pa-2, and Pa-4) bind specifically to poly(N-acetyllactosamine) structures.

Erythrocyte membrane structure and function

The structure and function of the proteins of the human erythrocyte membrane are discussed. The major integral proteins comprise the anion transport protein (band 3), the glucose transporter and four sialic acid-rich polypeptides. The anion transport protein equilibrates Cl- and HCO-3 between the plasma and red cell and also provides an anchorage site for peripheral proteins including those in the red cell cytoskeleton. The sialic acid-rich proteins are predominantly exposed at the surface of the cell. The bulk of the peripheral proteins are organized in a complex fashion to form a skeletal meshwork at the cytoplasmic surface of the membrane which maintains the shape and deformability of the red cell. The changes in membrane components during differentiation of the red cell are discussed.

High-speed gel-permeation chromatography of glycosaminoglycans: its application to the analysis of heparan sulfate of embryonic carcinoma and its degradation products by tumor cell-derived heparanase

A high-speed gel-permeation chromatographic system for analyzing glycosaminoglycans which uses two 0.7 X 75-cm stainless-steel columns containing Fractogel (Toyopearl) TSK HW-55(S), was developed. Glycosaminoglycans were applied and eluted with a 0.2 M sodium chloride solution and monitored by ultraviolet absorption at 210 nm or radioactivity. The best resolution of glycans was obtained at 55 degrees C at a flow rate of 1.0 ml/min. Acidic and neutral glycans in the molecular weight (Mr) range 600-60,000 eluted within 45 min. A linear relationship was found between retention time and molecular weight using standard glycosaminoglycans, chitin oligosaccharides, and a porcine thyroglobulin glycoprotide. This system was used to analyze the heparan sulfate synthesized by PYS-2 embryonic carcinoma cells and the degradation products produced by incubating it with extracted glycosidases from metastatic B16 melanoma cells. The results indicated that B16 melanoma cells contain at least two different heparan sulfate degradative activities, one of which appears to be an endoglycosidase.

The concanavalin A receptor from human erythrocytes in lipid bilayer membranes. Interaction with concanavalin A and succinyl-concanavalin A

The concanavalin A receptor from human erythrocyte membranes has been isolated by affinity chromatography using the mild, readily-dialyzable detergent dodecyltrimethylammonium bromide. The purified protein has been reincorporated into large unilamellar phospholipid vesicles using a detergent dialysis technique. The mean diameter of these vesicles increases as the lipid: protein ratio decreases. Binding of succinyl-concanavalin A to these vesicles was quantitated using 125I-labelled lectin in a filtration assay. The concanavalin A receptor in lipid bilayer vesicles provides specific high affinity binding sites for succinyl-concanavalin A with an association constant of 2.13 . 10(6) M-1. Scatchard plots indicate positive cooperativity of binding at very low lectin concentrations, a characteristic also seen in concanavalin A binding to intact human erythrocytes. The presence of bovine serum albumin has little effect on lectin binding and is not required for expression of cooperativity. Concanavalin A effectively competes with succinyl-concanavalin A for binding to the vesicles with an association constant of 4.83 . 10(6) M-1. Receptor-bearing vesicles are readily agglutinated by concanavalin A but not by its succinylated derivative. The kinetics of vesicle agglutination are biphasic, with an initial rapid phase followed by a pseudo-first order process. We suggest that studies on reassembled receptor proteins in lipid bilayers can provide valuable insight into receptor involvement in transmembrane signalling events and the factors involved in cell membrane behaviour and cell agglutination.

Separation of acidic oligosaccharides by liquid chromatography: application to analysis of sugar chains of glycoproteins

An ion-exchange, chromatographic system was developed to separate acidic oligosaccharides. Application of this system for the analysis of sialylated carbohydrate chains of glycoproteins was examined, and it was found that this system affords high resolution of acidic oligosaccharide isomers. (1) 3'-O-Neuraminyllactose, 6'-O-neuraminyllactose, and N-acetyl-6'-O-neuraminyllactosamine could be separated from bovine colostrum. (2) Subjection, to this system, of the oligosaccharide fraction released from bovine submaxillary mucin by treatment with alkaline borohydride resulted in good separation of the trisaccharide-alditol GlcNAc-(1 to 3)-[NeuAc-(2 to 6)]-GalNAcol and the disaccharide-alditol NeuAc- or NeuGc-(2 to 6)-GalNAcol. (3) When the serine- or threonine-linked oligosaccharide fraction obtained from human glycophorin A was subjected to this system, two monosialosylated, trisaccharide-alditol isomers, namely, NeuAc-(2 to 3)-Gal(1 to 3)-GalNAcol and Gal-(2 to 3)-[NeuAc-(2 to 6)]-GalNAcol, could be separated from each other. (4) The separation of so-called triantennary and biantennary, complex type of sugar chains of porcine thyroglobulin was achieved with this system.