Glycophorin-enriched vesicles obtained by a selective extraction of human erythrocyte membranes with a non-ionic detergent

Anion exchanger and the resistance against thermal haemolysis

4,4'-Diiso-thiocyanato stilbene-2,2'-disulphonic acid (DIDS) is a membrane-impermeable, highly specific covalent inhibitor and powerful thermal stabiliser of the anion exchanger (AE1), the major integral protein of erythrocyte membrane (EM). Suspensions of control and DIDS-treated (15 µM, pH 8.2) human erythrocytes were heated from 20° to 70°C using various but constant heating rates (1-8°C/min). The cellular electrolyte leakage exhibited a sigmoidal response to temperature as detected by conductometry. The critical midpoint temperature of leakage, T(mo), extrapolated to low heating rate (0.5°C/min) was used as a measure for EM thermostability. T(mo) was greater for DIDS-treated erythrocytes, 63.2° ± 0.3°C, than for intact erythrocytes, 60.7° ± 0.2°C. The time, t(1/2), for 50% haemolysis of erythrocytes, exposed to 53°C was used as a measure for the resistance of erythrocytes against thermal haemolysis. The t(1/2) was also greater for DIDS-treated erythrocytes, 63 ± 3 min, than for intact erythrocytes, 38 ± 2 min. The fluorescent label N-(3-pyrenyl)maleimide and EPR spin label 3-maleimido-proxyl, covalently bound to sulphydryl groups of major EM proteins, were used to monitor the changes in molecular motions during transient heating. Both labels reported an intensification of the motional dynamics at the denaturation temperatures of spectrin (50°C) and AE1 (67°C), and, surprisingly, immobilisation of a major EM protein, presumably the AE1, at T(mo). The above results are interpreted in favour of the possible involvement of a predenaturational rearrangement of AE1 copies in the EM thermostability and the resistance against thermal haemolysis.

Neutral polymers elicit, and antibodies to spectrin, band 4.1 protein and cytoplasmic domain of band 3 protein inhibit the concanavalin A-mediated agglutination of human erythrocytes

Concanavalin A (Con A) is known to agglutinate human erythrocytes if the cells are pre-treated with a proteinase or neuraminidase. We report that untreated cells can also be made to agglutinate with the lectin if the lectin-bound cells are treated with anti-Con A antibodies, or if a neutral polymer such as serum albumin, polyvinylpyrrolidone or Ficoll is added. Thus, Con A falls in the category of 'incomplete' lectins. The polymer induces Con A-agglutinability without altering the receptor number, or deformability of the cells. If the polymer is sequestered within erythrocyte ghosts, Con A is unable to agglutinate them; but the presence of the polymer only on the outer surface (as in intact cells) or on both the surfaces permits agglutinability. Thus, the site of the polymer effect resides on the outer surface of the membrane. The polymer, however, is unable to induce agglutinability in erythrocyte vesicles, whose membrane lacks skeletal proteins. The result suggests a positive role for the membrane skeleton in the process of agglutination brought about by the polymer, as is true also for the agglutination of proteinase-treated cells. In order to obtain detailed information on the proteins participating in agglutination, monospecific antibodies to spectrins, band 4.1 protein, ankyrin and the cytoplasmic domain of band 3 protein were internalized in erythrocytes. It is found that anti-spectrin and anti-band 3 cytoplasmic domain, but not their Fab's, inhibit the Con A-mediated agglutinability partially, and anti-4.1 antibodies, as well as the Fab's, inhibit the agglutinability substantially. Anti-ankyrin, however, was without any effect. The results confirm a positive role for the membrane skeleton in the Con A-mediated agglutination of normal erythrocytes in the presence of a neutral polymer, or in proteinase treated cells. We also provide evidence for requirement of Mg-ATP in the agglutination process.

Isolation and characterization of gastric microsomal glycoproteins. Evidence for a glycosylated beta-subunit of the H+/K(+)-ATPase

Detergent-solubilization of hog gastric microsomal membrane proteins followed by affinity chromatography using wheat germ agglutinin or Ricinus communis I agglutinin resulted in the isolation of five glycoproteins with the apparent molecular masses on sodium dodecyl sulfate polyacrylamide gels of (in kDa): 60-80 (two glycoproteins sharing this molecular mass); 125-150; and 190-210. In the nonionic detergent Nonidet P-40 (NP-40), the 94 kDa H+/K(+)-ATPase was recovered exclusively in the lectin-binding fraction; however, in the cationic detergent dodecyltrimethylammonium bromide, most of the ATPase was recovered in the nonbinding fraction. Detection of glycoproteins either by periodic acid-dansyl hydrazine staining of carbohydrate in polyacrylamide gels or by Western blots probed with lectins indicated that the majority of the ATPase molecules are not glycosylated. In addition, in the absence of microsomal glycoproteins, the NP-40-solubilized ATPase does not bind to a lectin column. Taken together, these results suggest that the recovery of NP-40-solubilized ATPase in the lectin-binding fraction is due to its noncovalent interaction with a gastric microsomal glycoprotein. Immunoprecipitation of the ATPase from NP-40-solubilized microsomal membrane proteins resulted in the co-precipitation of a single 60-80 kDa glycoprotein. Characterization of the 60-80 kDa glycoprotein associated with the ATPase revealed that: it is a transmembrane protein; it has an apparent core molecular mass of 32 kDa; and, it has five asparagine-linked oligosaccharide chains. Given its similarity to the glycosylated beta-subunit of the Na+/K(+)-ATPase, this 60-80 kDa gastric microsomal glycoprotein is suggested to be a beta-subunit of the H+/K(+)-ATPase.

Fatty acid acylation of membrane skeletal proteins in human erythrocytes

Fatty acid acylation of membrane proteins was studied on human erythrocytes by measuring incorporation of [3H]palmitate at different specific radioactivities. A 55 kDa polypeptide within the band 4.5 region was the main acceptor protein for acylation by fatty acids (palmitate, stearate, oleate), while other polypeptides (80, 65, 48, 30 kDa) incorporated [3H]palmitate slowly, in substoichiometric amounts. Integral membrane proteins were preferentially fatty acid acylated. Skeletal membrane proteins were, however, poorly labeled. Neither purified ankyrin nor band 4.1 protein were fatty acid acylated in human erythrocytes. On the other hand, label associated with high molecular weight skeletal proteins resisted low and high ionic strength extractions, and was extracted selectively by urea [corrected] along with a small subpopulation of spectrin which was also tightly associated with the membrane.

Glycophorin A interferes in the agglutination of human erythrocytes by concanavalin A. Explanation of the requirement for enzymic predigestion

Human erythrocytes become agglutinable with concanavalin A (Con A) after treatment with various proteinases or neuraminidase. The extent of agglutinability achieved with different enzymes is, however, different: Pronase, papain, trypsin, neuraminidase and chymotrypsin enhance the agglutinability in decreasing order, the last being barely effective. The actions of the enzymes on band 3, the Con A receptor, do not correlate with their abilities to increase the agglutinability: Pronase, papain and chymotrypsin cleave the protein, but not trypsin or neuraminidase. No significant differences are found in the number of Con A-binding sites or the affinities for the lectin between the normal and trypsin- or Pronase-treated cells. Thus the receptor does not seem to play a role in determining the Con A-agglutinability of erythrocytes. On the other hand, the cleavage of glycophorins, especially glycophorin A, and the release of sialic acid (in the peptide-bound form) are well-correlated with the enhancement in agglutination after the action of proteinases. The release of sialic acid by graded neuraminidase digestion and the increase in Con A-agglutinability show a correlation coefficient of 0.88. The major inhibitory role of glycophorin A in the process is indicated by the agglutination of En(a) heterozygous erythrocytes; the cells, known to bear about 50% glycophorin A molecules in their membrane, are agglutinated approximately half as well without proteolysis as are the trypsin-treated cells. Possible mechanisms by which glycophorin A could affect Con A-mediated agglutination are discussed.

Fractionation and reconstitution of the sarcoplasmic reticulum Ca2+ pump solubilized and stabilized by CHAPS/lipid micelles

A procedure for solubilization, fractionation, and reconstitution of sarcoplasmic reticulum (SR) protein is presented. The SR protein is solubilized with the zwitterionic detergent CHAPS in the presence of added 5-mM phosphatidylcholine and 20% glycerol, which stabilize the reconstitutable Ca2+ transport activity. For reconstitution the solubilized SR protein is incorporated into preformed French-pressed unilamellar vesicles that had been treated with 10-mM sodium cholate. By passing the proteoliposomes through a centrifuged Sephadex G-50 column that had been equilibrated with potassium oxalate, the detergent is removed, and the proteoliposomes become sealed with potassium oxalate trapped inside. This procedure requires less than 2 h and results in Ca2+ uptake active of over 1 mumol/min/mg of protein. The solubilized SR protein was fractionated on a DEAE-Biogel A column. A fraction containing the Ca2+-ATPase but not the Mr 55,000 glycoprotein had reconstitutable Ca2+ uptake activity of 2.2 mumol/min/mg of protein. Inclusion of the Mr 55,000 glycoprotein during the reconstitution procedure did not increase the Ca2+ uptake activity of the reconstituted fraction containing the Ca2+-ATPase. This result demonstrates that the glycoprotein is not required for Ca2+ uptake.

High frequency antigens of human erythrocyte membrane sialoglycoproteins, III. Studies on the EnaFR, Wrb and Wra antigens

The nature of the common erythrocyte antigens EnaFR and Wrb, that are both absent from En(a-) cells, and the rare Wra receptor, apparently encoded by an allele of Wrb, was investigated. Various modification, fractionation or cleavage products of erythrocyte membranes were used in hemagglutination inhibition assays. The EnaFR and Wrb antigens were shown to represent labile structures within the residues approx. 62-72 of the major (MN) sialoglycoprotein that require lipids, at least for complete expression of antigenic activity. During the course of these experiments, the arrangement of the MN glycoprotein's peptide chain with respect to the lipid bi-layer was also studied, using various proteinases. Furthermore, the MN glycoprotein was found to aggregate with the major membrane protein (band 3) in the presence of Triton X-100. The Wra antigen was shown to exhibit properties that differ considerably from those of the Wrb receptor. Analyses on the MN glycoprotein, isolated from the red cells of the only known Wra homozygote and two WraWrb individuals, did not reveal any amino-acid exchange within the residues 40-96 of the molecule. Therefore, the Wr locus that determines the presence or absence of the Wrb antigen on the MN glycoprotein might influence the post-translational modification of amino-acid residues, the structure of tightly bound lipids or the aggregation of the MN glycoprotein with a different protein such as band 3.

Optimization of erythrocyte membrane glycoprotein fluorescent labeling with dansylhydrazine after polyacrylamide gel electrophoresis

An improved procedure for the labeling of glycoproteins with dansylhydrazine subsequent to electrophoresis in polyacrylamide gels is reported. This procedure is derived from the work of Eckhardt et al. (1976, Anal. Biochem. 73, 192-197) and Weber and Hof (1975, Biochem. Biophys. Res. Commun. 65, 1298-1302) who showed that dansylhydrazine may be condensed with the aldehyde groups of oxidized glycoprotein carbohydrates and the resulting hydrazones reduced with dimethylamine borane and/or sodium borohydride. Using the known distribution of erythrocyte membrane glycoproteins as a benchmark the effect of variation of a number of process parameters was investigated and an optimal procedure identified. The procedure is shown to be relatively insensitive to moderate variations in reagent composition, pH, and time of incubation with dansylhydrazine solution or reducing agents. It is also shown that labeling patterns may be preserved in dried gels if dimethylsulfoxide is replaced or omitted from all of the process solutions and destaining is effected with 1 M sodium acetate, pH 5.6. While specifically developed for the labeling of erythrocyte membrane proteins, the procedure is demonstrated to be applicable to other glycoprotein containing preparations.

Coupling of Ca2+ transport to ATP hydrolysis by the Ca2+-ATPase of sarcoplasmic reticulum: potential role of the 53-kilodalton glycoprotein

An essential feature of the function of the Ca2+-ATPase of sarcoplasmic reticulum (SR) is the close coupling between the hydrolysis of ATP and the active transport of Ca2+. The purpose of this study is to investigate the role of other components of the SR membrane in regulating the coupling of Ca2+-ATPase in SR isolated from rabbit skeletal muscle, reconstituted SR, and purified Ca2+-ATPase/phospholipid complexes. Our results suggest that (1) it is possible to systematically alter the degree of coupling obtained in reconstituted SR preparations by varying the [KC1] present during cholate solubilization, (2) the variation in coupling is not due to differences in the permeability of the reconstituted SR vesicles to Ca2+, and (3) vesicles reconstituted with purified Ca2+-ATPase are extensively uncoupled under our experimental conditions regardless of the lipid/protein ratio or phospholipid composition. In reconstituted SR preparations prepared by varying the [KC1] present during cholate treatment, we find a direct correlation between the relative degree of coupling between ATP hydrolysis and Ca2+ transport and the level of the 53-kilodalton (53-kDa) glycoprotein of the SR membrane. These results suggest that the 53-kDa glycoprotein may be involved in regulating the coupling between ATP hydrolysis and Ca2+ transport in the SR.

Spin-label studies of the oligomeric structure of band 3 protein in erythrocyte membranes and in reconstituted systems

A spin-labeled fatty acid (16-doxylstearic acid), linked by an ester bond to a maleimide or a nitrene residue, was covalently attached to band 3 of erythrocyte membranes. The electron spin resonance spectrum of the spin-labeled protein was examined at different temperatures in: (a) whole erythrocyte ghosts; (b) ghosts depleted of spectrin and actin; (c) alkaline-treated ghosts; (d) vesicles made with purified band 3 reassociated with dimyristoylphosphatidylcholine. Most spectra are composite with a major component corresponding to a large overall splitting. The determination of the percentage of the immobilized component was carried out by pairwise subtraction. At low temperatures (1-7 degrees C), the highest fraction of immobilized component was found in dimyristoylphosphatidylcholine vesicles (approx. 100%); alkaline-treated membranes had approx. 75% of the immobilized component at the same temperature; whole erythrocyte, spectrin/actin-depleted and spectrin/actin/ankyrin-depleted ghosts gave identical results (approx. 60% of immobilized component). The immobilized fraction decreased in all samples with increasing temperature or addition of a nonsolubilizing concentration of dodecyl octaethylene glycol monoether. In dimyristoylphosphatidylcholine vesicles, however, the modification in the ration of the two components was obtained only above the lipid transition temperature (23 degrees C). The strong immobilization of the spin-labeled lipid chain at all temperatures suggested trapping of the lipid chain between proteins. At low temperature, in dimyristoylphosphatidylcholine vesicles or in alkaline-treated ghosts, lipid-protein segregation is likely to take place. In whole erythrocyte ghosts, on the other hand, the large contribution of the motionally restricted component at physiological temperature indicates the oligomeric nature of band 3. Partial dissociation of the oligomers occurs as the temperature is increased, but the presence or absence of cytoskeletal proteins has no influence on the state of oligomerization of band 3.

Antigenicity, storage, and aging: physiologic autoantibodies to cell membrane and serum proteins and the senescent cell antigen

Normal human serum contains autoantibodies to a wide range of cellular and serum proteins. IgG autoantibodies to cell membrane proteins spectrin, syndein (Band 2.1), Band 3 degradation products, and the senescent cell antigen are among them. Physiologic autoantibodies to the senescent cell antigen, a approximately 62 000 dalton glycopeptide derived from Band 3, initiate removal of senescent, damaged, and stored cells in vivo. The senescent cell antigen is one of the two Band 3 degradation products (Mr approximately 66 000 and 62 000) observed in freshly prepared ghosts. Since the senescent cell antigen is observed on red cells aged in situ, data suggest that Band 3 undergoes proteolysis in situ. IgG eluted from blood stored for transfusion binds to the senescent cell antigen. The amount of IgG on red cells increases during storage suggesting accumulation of the senescent cell antigen. Autoantibodies to other cell and serum proteins are discussed as possible regulators of homeostasis. The effect of age on physiologic autoantibodies is reviewed.

Molecular properties of calcium-pumping ATPase from human erythrocytes

The Ca2+-pumping ATPase from human erythrocyte membranes, purified by the method previously reported [Niggli, V., Penniston, J. T., & Carafoli, E. (1979) J. Biol. Chem. 254, 9955-9958], was freed of minor impurities by extensive washing while bound to the calmodulin-Sepharose column. The pure enzyme showed a single band of Mr 138000, which contained no stainable carbohydrate. The enzyme retained calmodulin-stimulable ATPase activity; with appropriate assay conditions, an activity of 21.2 mumol/(mg x min) was obtained. Amino acid analysis showed that the ATPase had a larger proportion of polar amino acids than do other integral membrane proteins. Despite this, the ATPase showed a tendency to form dimers and higher aggregates even in the presence of sodium dodecyl sulfate and urea. The enzyme required Mg2+ but showed little activity unless a second ion was added. With regard to this second ion, the enzyme responded to alkaline earth metal ions in the order Ca2+ greater than Sr2+ much greater than Ba2+. It was highly specific for ATP and was stimulated by Na+ or K+; in all of these properties it resembled the enzyme in unfractionated membranes. Limited proteolysis using trypsin yielded, at short times, many fragments of various molecular weights; continued proteolysis resulted in two trypsin-resistant fragments of Mr 81000 and 33500. Analysis of the time course of proteolysis indicated that the ATPase existed in two or more conformations that had differing susceptibilities to proteolysis. It is suggested that these correspond to active and inactive conformers of the enzyme.

Decrease of carbohydrate in membrane glycoproteins during human erythrocyte ageing in vivo

Membrane glycoproteins from young human erythrocytes and erythrocytes aged in vivo were fractionated by gel filtration. Three major groups of glycoproteins were obtained. The neutral hexoses and sialic acid contents of each group of glycoproteins from the old cells were found to be significantly reduced by comparison to the values found in the glycoproteins of the young cells. Thus, the previously observed decrease in neutral hexoses and in sialic acid contents of the full erythrocyte membrane during in vivo ageing does not affect only one particular group of glycoproteins but each of the groups of glycoproteins tested, including the major glycoproteins of the erythrocyte membrane, that is to say glycophorins. It is shown in addition, that the previously observed decrease per cell of the surface galactose and N-acetylgalactosamine residues of the ageing erythrocyte affect several groups of membrane glycoproteins including band 3, PAS-1, PAS-4 and PAS-3. The physiological significance of these experimental data is discussed.

Anti-Rho(D) IgG binds to band 3 glycoprotein of the human erythrocyte membrane

Alkali-extracted erythrocyte ghost membranes from Rho(D)-positive and Rho(D)-negative donors were incubated with human immune anti-Rho(D) IgG and nonimmune IgG. After sensitization with IgG, the integral membrane proteins were solubilized in Brij 36T nonionic detergent and chromatographed by gel filtration. There was a distinct resolution of IgG into free and membrane-complexed forms. The IgG-complexed membrane proteins were isolated by the use of a staphylococcal protein A affinity support. The protein A-bound complexes were examined for polypeptide composition by gel electrophoresis after elution. Only Rho(D)-positive membrane proteins incubated with immune anti-Rho(D) IgG revealed intact band 3. Control Rh-negative membrane proteins that had reacted with immune anti-Rho(D) IgG and the Rh-positive membranes that had reacted with nonimmune IgG showed only low molecular weight fragments of band 3 that bound nonspecifically to IgG. Arguments are presented supporting a band 3 localization for the Rh antigen.

Isolation of the phagocytosis-inducing IgG-binding antigen on senescent somatic cells

To remove senescent red blood cells (RBCs) from the circulation, macrophages must distinguish them from mature RBCs. That is achieved by a specific recognition system. An antigen that develops on the surface of a senescing RBC is recognized and bound by the Fab region of an IgG autoantibody in the serum. Subsequently the Fc region of the autoantibody is recognized and bound by a macrophage, which proceeds to phagocytose the RBC. The antigenic molecule can be extracted from senescent but not young RBCs with Triton X-100 (ref. 4), although 10--30% as much antigen can be extracted from middle-aged as from senescent RBCs. I have now used IgG autoantibodies eluted from senescent RBCs to isolate and purify the IgG-binding antigen on senescent RBCs, and to detect the antigen on other somatic cells. The antigen is a congruent to 62,000-Mr protein which is present on stored platelets, lymphocytes and neutrophils, and on cultured human adult liver and embryonic kidney cells, as well as senescent RBCs.

An age-specific cell antigen is present on senescent human red blood cell membranes. A brief note

Immunoglobulin G autoantibodies selectively bind to senescent human red blood cells (RBC) in situ and initiate their removal by phagocytosis. In this paper, we characterize the IgG binding receptor appearing on senescent RBC using glycophorin-enriched vesicles prepared by Triton X-100 extraction of young, middle-aged, and old RBC populations. These vesicles contain all known sialoglycoproteins and trace contaminants of other proteins. IgG binds predominantly to vesicles from old cells, as determined by both 125I-labeled protein A binding to IgG molecules and an erythrophagocytosis-inhibition assay. Addition of lipids does not alter IgG binding. Liposomes prepared from lipids of young and old cell fractions do not bind significant amounts of IgG. IgG binding is reduced following trypsin treatment of vesicles. The data suggest that the age-specific cell antigen is a protein which co-purifies with sialoglycoproteins, but is not identical with glycophorin. Since it is extracted predominantly from senescent cells, a chemical modification within the membrane may either form the age-specific cell antigen during aging or render it accessible during senescence.

Reconstitution of human erythrocyte membrane acetylcholinesterase in phospholipid vesicles. Analysis of the molecular forms by cross-linking studies

Unilamellar lipid vesicles were formed upon removal of Triton X-100 with Amberlite XAD-2 from a mixture of egg phosphatidylcholine and Triton-solubilized pure human erythrocyte membrane acetylcholinesterase. A majority of large (230 nm diameter) vesicles together with a minor population of smaller (30 nm diameter) strictures were observed in freeze-fracture electron micrographs. Reconstitution experiments performed with [phenyl-3H(n)]-Triton X-100 showed that only one detergent molecule per 600 lipid molecules was present in the vesicles. Density gradient centrifugation showed co-sedimentation of acetylcholinesterase with the lipid vesicles. About 60% of the incorporated enzyme was directed towards the vesicle exterior and could be partially degraded by papain. Mainly dimeric acetylcholinesterase was found when the reconstituted or, alternatively, the lipid-free but Triton-solubilized enzyme were cross-linked with glutaraldehyde. Aggregates were observed when the detergent-depleted oligomeric forms of the enzyme were cross-linked. The results thus indicate that mainly the dimeric enzyme form is present in a phospholipid environment.

Rotational diffusion of band 3 proteins in membranes from En(a-) and neuraminidase-treated normal human erythrocytes

Recent experiments have demonstrated an association between band 3 and glycophorin A in the human eythrocyte membrane (Nigg, E.A., Bron, C., Girardet, M. and Cherry, R.J. (1980) Biochemistry 19, 1887-1893). Here, the influence of sialoglycoproteins on the rotational diffusion of band 3 in the human erythrocyte membrane was investigated by studying membranes from En(a-) and neuraminidase-treated erythrocytres. Rotational diffusion was measured by observing flash-induced transient dichroism of eosin-labeled band 3. Although erythrocytes of the rare phenotype En(a-) lack the major sialoglycoprotein, glycophorin A, no significant difference in band 3 rotation at pH 7.4 and 37 degrees C could be detected between En(a-) and normal erythrocyte membranes. Band 3 rotation at pH 7.4 was also insensitive to the enzymatic removal of sialic acid from the surface of normal erythrocytes. Moreover, the existence of an essentially similar temperature-dependent equilibrium between band 3 proteins with different mobilities was observed in normal, En(a-) and neuraminidase-treated erythrocytes. From these results it is concluded that glycophorin A contributes less than 15% to the cross-sectional diameter of the band 3-glycophorin A complex in the plane of the normal membrane. The rotation of the complex at pH 7.4 is not significantly influenced by either steric or electrostatic interactions involving the oligosaccharide moiety of glycophorin A.

Lipid-protein interactions in human erythrocyte-membrane acetylcholinesterase. Modulation of enzyme activity by lipids

Purified human erythrocyte membrane acetylcholinesterase was incorporated into vesicles of various lipid compositions. The activities of the free and the lipid-associated enzyme were assayed at temperatures between 4 degrees C and 40 degrees C and the results were visualized as plots of log v versus 1/T (Arrhenius plots). For the purified, detergent-depleted enzyme a linear relation was obtained. If Triton X-100 was added to the assay medium a curved plot resulted. For acetylcholinesterase incorporated into dimyristoylphosphatidylcholine vesicles a clear break in the plot was observed at the phase transition temperature of the lipid. With lipids not experiencing a phase transition within the temperature range investigated, again a linear relation was obtained. These results show that the activity of human erythrocyte membrane acetylcholinesterase is strongly modulated by its hydrophobic environment.

Band 3-glycophorin A association in erythrocyte membrane demonstrated by combining protein diffusion measurements with antibody-induced cross-linking

A new approach to the study of molecular protein interactions in biological membranes is presented. The technique is based on measuring the rotation of a membrane protein in the presence and absence of specific antibodies directed toward a purported complex partner. As a first illustration of the method, the putative association of band 3 with glycophorin A in the human erythrocyte membrane was investigated. The rotational diffusion of band 3 was strongly reduced following cross-linking of glycophorin A with divalent antibodies. However, little or no effect on band 3 rotation was produced by monovalent antiglycophorin A Fab fragments, antispectrinor nonspecific antibodies, ruling out major effects on band 3 mobility due to steric hindrance, unspecific antibody adsorption, or transmembrane interactions involving spectrin. It is concluded that immobilization of band 3 by antiglycophorin A antibodies is directly caused by cross-linking of a preexisting band 3-glycophorin A complex in the human erythrocyte membrane.

Lipid phase states influence glycophorin reconstitution

Reconstitution of glycophorin into dimyristoyl phosphatidylcholine and sphingomyelin vesicles was sub-maximal below the phase transition temperatures of these lipids. Reconstitution of glycophorin into diisostearoyl phosphatidylcholine and dioleoyl phosphatidylcholine liposomes was maximal within a range of temperatures below the phase transition temperatures of dimyristoyl phosphatidylcholine and sphingomyelin but above the phase transition temperatures of diisostearoyl phosphatidylcholine and dioleoyl phosphatidylcholine. These findings indicate a greater tendency for reconstitution of glycophorin into fluid as opposed to solid lipid phases.