Effects of phytohaemagglutinin, wheat-germ agglutinin, and concanavalin-A on the physical state of sialic acid and membrane proteins in human erythrocyte ghosts: a spin label study

Selective labeling of membrane protein sulfhydryl groups with methanethiosulfonate spin label

Electron paramagnetic resonance was used to characterize the first use of a thio-specific spin label in membranes. Procedures for use of the spin-label, 1-oxyl-2,2,5,5-tetramethyl-delta 3-pyrroline-3-methyl (methanethiosulfonate MTS) covalently attached to membrane proteins in human erythrocyte membranes are reported. The major findings are: (1) MTS was found to be thiol-specific in membranes as it is for soluble proteins; (2) MTS labels ghost proteins in as few as 30 min at room temperature, providing a distinct advantage when sensitive or fragile membranes are to be used; (3) the distribution of the spin label suggests that the major cytoskeletal protein, spectrin, and the major transmembrane protein (Band 3) incorporate the highest percentage of spin label. This procedure expands the tools with which the researcher can investigate the physical state of membrane proteins and its alteration upon interaction of membrane perturbants or in pathological conditions.

Alteration of the erythrocyte membrane via enzymatic degradation of ankyrin (band 2.1): subcellular surgery characterized by EPR spectroscopy

A fraction of band 3 protein, the major transmembrane protein of erythrocyte membranes, is held to the cytoskeletal protein spectrin via noncovalent interactions with the protein ankyrin (band 2.1). In this study, trypsin was used under defined conditions to selectively proteolyze ankyrin and thereby destroy the band 3-ankyrin linkage on the cytoplasmic side of erythrocyte ghost membranes. Electron paramagnetic resonance (EPR) spectroscopy, in conjunction with selective spin labeling methods, was used to monitor conformational changes occurring in cytoskeletal proteins or cell-surface carbohydrates as a result of this treatment. Treatment of RBC ghosts with TPCK-trypsin for 5 s at 0 degrees C caused an approx. 56% increase in the relevant EPR parameter of a maleimide spin label bound to spectrin (P < 0.004), indicative of increased segmental motion of the spin label and decreased protein-protein interactions. Analysis of the apparent rotational correlation time parameter tau of a spin label covalently and selectively bound to terminal sialic acid residues of glycophorin showed no significant effect from trypsin treatment. However, tau of spin label covalently and specifically bound to terminal galactose residues of cell-surface glycoconjugates of band 3 and other transmembrane glycoproteins significantly decreased with tryptic uncoupling of the ankyrin linkage (P < 0.005). These results suggest a marked conformational alteration in both cytoskeletal and transmembrane proteins as a result of uncoupling from ankyrin. Spermine (N,N'-bis(3-aminopropyl)tetramethylenediamine), a naturally occurring polyamine known to strengthen cytoskeletal protein-protein interactions (Wyse and Butterfield (1988) Biochim. Biophys. Acta 941, 141-149), was used to partially reverse the trypsin-induced cytoskeletal alterations. Addition of 2 mM spermine to ghosts previously treated with trypsin increased cytoskeletal protein-protein interactions as indicated by EPR (P < 0.002). SDS-PAGE was used to confirm the integrity of spectrin, band 3, and band 4.1 in all experiments. The results are discussed with reference to transmembrane signaling mechanisms and membrane-associated pathologies.

Spin-labeling studies of the conformation of the Ca(2+)-regulatory protein calmodulin in solution and bound to the membrane skeleton in erythrocyte ghosts: implications to transmembrane signaling

Electron paramagnetic resonance (EPR) studies of the Ca(2+)-regulatory protein calmodulin (CaM) have been performed. The conformation of CaM in solution changes upon binding of Ca2+ allowing the protein to bind to target proteins existing in the red blood cell membrane. In this study a maleimide spin label, covalently attached to the single cysteine residue of CaM located in the first Ca(2+)-binding domain, was used to monitor allosteric conformational changes induced by interaction of CaM with Ca2+ and subsequently with the red blood cell membrane. The results show, relative to apo-CaM, a significant increase in the apparent rotational correlation time, tau, of the spin label when Ca2+ was present in solution (P less than 0.001). When apo-CaM exposed to red blood cell membrane ghosts in the absence of Ca2+, no significant difference in spin label motion was seen relative to solution, consistent with the idea that Ca2+ is required for CaM to bind to skeletal proteins. When Ca2+ was added to CaM which was then exposed to ghosts, a highly significant increase in tau (decrease in motion) (P less than 0.000001) relative to apo-CaM exposed to ghosts was found. This latter increase in tau is significantly greater than that produced by the addition of Ca2+ to CaM in solution (P less than 0.001). The major interaction sites of CaM were found by photoaffinity labeling and autoradiography on SDS-PAGE to be on the principal skeletal protein, spectrin. EPR was also used to investigate the biophysical correlates of transmembrane signaling. Spin-labeled CaM was bound to the membrane skeleton in the presence of Ca2+. On the opposite side of the erythrocyte membrane a lectin was bound to the external glycoconjugate of Band 3, the major transmembrane protein of the erythrocyte. A highly significant increase in T of the maleimide spin probe was found relative to the control system in which the lectin was absent. (P < 0.00001). These results suggest that electron paramagnetic resonance spectra of spin-labeled CaM can provide useful information about protein structure and function when in solution and when bound to membranes.

Effects of domain-specific erythrocyte membrane modulators on acetylcholinesterase and NADH:cytochrome b5 reductase activities

Previously, we showed using electron paramagnetic resonance that the physical state of one side of erythrocyte membranes could be modulated by agents which interact with the opposite side (reviewed in Butterfield, 1989, Biological and Synthetic Membranes, A. R. Liss, Inc., New York). The present study was undertaken to determine whether membrane-bound enzymes would exhibit a similar transmembrane modulation effect. The effects of known, domain-specific modulators of the physical state of erythrocyte membranes on the activity of two membrane-bound enzymes were investigated. Acetylcholinesterase, an enzyme having its active site situated on the extracellular side of the membrane, seemed to be unaffected by most of the modulators employed in this study, with the exception of reversible inhibition by benzyl alcohol. Conversely, the activity of NADH:cytochrome b5 reductase, an enzyme whose active site is located on the cytoplasmic side of the erythrocyte membrane, was increased by those agents that interact primarily with skeletal proteins to increase skeletal protein-protein interactions; however, those agents which interact primarily with the skeleton to decrease protein-protein interactions decreased the activity of NADH:cytochrome b5 reductase. This enzyme's activity was also significantly altered by lectins which bind specifically to the external face of glycophorin A on the opposite side of the membrane, but it's activity was unaffected by concanavalin A, a lectin which binds to the external face of band 3. The results of these biochemical studies suggested that NADH:cytochrome b5 reductase can interact with and its activity can be modulated by skeletal or transmembrane proteins. In addition, these results support the hypothesis that in transmembrane signaling processes, biophysical and biochemical changes are correlated.

The influence of poly(ethylene glycol) on the molecular dynamics within the glycocalyx

Interaction of polymers with cell surfaces is a question of general interest for cell aggregation and fusion. The molecular dynamics within the surface coat of human erythrocytes as well as alterations of membrane protein arrangement (IMPs) in the presence of poly(ethylene glycol) (PEG) were investigated by EPR spin labeling techniques and freeze-fracture electron microscopy, respectively. AT PEG concentrations which induce aggregation of erythrocytes the surface coat and the protein arrangement is not disturbed by the polymer. This implicate an exclusion of the polymer from the cell surface.

Interaction of hemin with erythrocyte membranes: alterations in the physical state of the major sialoglycoprotein

Hemin has been shown to disrupt erythrocyte membrane skeletal protein-protein interactions, initially those involving band 4.1 (Shaklai et. al. (1986) Biochem. Int. 13, 467-477). We have used electron spin resonance (ESR) spin labels specific for cell-surface carbohydrates, skeletal membrane proteins, or bilayer lipids to find: (1) simultaneous reaction of the protein-specific spin label, MAL-6, which binds to skeletal protein SH residues, and 10 microM hemin suggested that hemin decreased skeletal protein-protein interactions; (2) 10 microM hemin markedly decreased (greater than 60%, P less than 0.001) the rotational motion of spin-labeled erythrocyte membrane cell-surface sialic acid residues, 70% of which are located on the major transmembrane sialoglycoprotein, glycophorin A; and (3) 10 microM hemin caused a small, but significant (P less than 0.02), decrease in the motion of a lipid bilayer specific spin label (5-NS) in the erythrocyte membrane. Since glycophorin A is reportedly linked to the erythrocyte membrane skeletal protein network by band 4.1, it is conceivable that hemin-induced disruption of skeletal protein interactions, particularly those of band 4.1, could subsequently lead to the alterations in the motion of cell-surface sialic acid presented in this report.

Electron spin resonance (ESR) studies of skeletal protein interactions in human erythrocyte membranes exposed to polyanions and in membranes prepared from inositol hexaphosphate (IHP)-incorporated low-affinity erythrocytes

Previous biophysical investigations, including those from our laboratories, have reported that polyphosphates weaken RBC membrane skeletal protein-protein interactions and decrease hemoglobin affinity for oxygen. We have additionally demonstrated that low-affinity intact RBC's may be produced by inositol hexaphosphate (IHP) incorporation via an osmotic pulse method. In the present electron spin resonance (ESR) study, IHP was shown to cause a concentration-dependent increase in the segmental motion of ghost membrane skeletal proteins, but no alterations in spin-labeled terminal sialic acid. Pyrophosphate and inositol hexasulfate were significantly less effective in altering the physical state of skeletal proteins than was IHP. Additional ESR studies of both the interaction of IHP with membrane skeletal proteins in the presence of hemoglobin and of membranes obtained from osmotic pulse-treated intact cells were performed. The results of all these studies are discussed in terms of previous biophysical investigations of the effects of polyphosphates on membranes and of possible molecular events that occur during the osmotic pulse procedure.

Alternations in lipid membrane fluidity and the physical state of cell-surface sialic acid in zinc-deficient rat erythrocyte ghosts

Erythrocyte ghosts, prepared from the blood of rats fed zinc-deficient diets, were evaluated for membrane fluidity and surface sialic acid properties using spin-labeled probes and electron spin resonance (ESR) spectroscopy. These physical parameters of the erythrocyte ghosts from the zinc-deficient group were compared to those for erythrocyte ghosts obtained from ad libitum and pair fed controls consuming zinc-adequate diets. As the animals became progressively zinc deficient, the erythrocyte ghost membranes became more fluid than those from the control groups. In addition, the apparent rotational correlation time of Tempamine spin probes on surface sialic acid residues was smaller for the zinc deficient group, indicative of an increased rotational mobility of the spin label. These results suggest that zinc deficiency can have pronounced effects on the physical state of membrane bilayer lipids and cell surface carbohydrates and supports the view that many of the pathological signs of zinc deficiency are due to a general membrane defect.

Association of gangliosides with the lymphocyte plasma membrane studied using radiolabels and spin labels

Gangliosides are known to act as potent suppressors of lectin-stimulated lymphocyte activation when added to the culture medium. Since this effect may be mediated via ganglioside association with (or insertion into) the plasma membrane, we have used 3H- and spin-labelled derivatives of mixed gangliosides to probe the nature of this interaction. Gangliosides bind rapidly to the lymphocyte membrane and show no preference for association with either inside-out or right-side-out membrane vesicles. Around 20% of the bound gangliosides can be removed by repetitive washing, and a further 22-28% by treatment with pronase for 1 h, suggesting that this fraction is tightly bound to membrane proteins at the cell surface. The ESR spectrum of membrane-bound gangliosides did not resemble the spin-exchanged spectrum of micellar spin-labelled gangliosides in aqueous solution, but was similar to that seen for 5 mol% ganglioside spin label in liposomes of egg phosphatidylcholine. This suggests that the bulk of the membrane-bound gangliosides are inserted and molecular dispersed in the lymphocyte membrane. Binding of wheat-germ agglutinin to lymphocyte-associated gangliosides results in specific immobilization of the carbohydrate headgroup, while concanavalin A and other lectins have little or no effect on oligosaccharide mobility. Membrane-inserted gangliosides show a response to lectin binding which is qualitatively different from that seen for gangliosides in bilayers of phosphatidylcholine.

ESR studies of the erythrocyte membrane skeletal protein network: influence of the state of aggregation of spectrin on the physical state of membrane proteins, bilayer lipids, and cell surface carbohydrates

The stability of the human erythrocyte membrane skeletal network is reported to be dependent on the state of aggregation of spectrin and decreased or increased by polyphosphate anions or the polyamine, spermine, respectively. We have employed polyacrylamide gel electrophoresis and electron spin resonance (ESR) utilizing spin labels specific for membrane proteins, bilayer lipids, or cell-surface sialic acid in order to gain insight into these observations and into the reliability of the ESR spectra of the protein-specific spin label used to correctly report the interactions of the skeletal protein network. The major findings are: (1) We confirm previous reports that the preferred state of spectrin aggregation in the skeletal network is tetrameric and that spectrin can be reversibly transformed to dimeric spectrin and back to tetrameric spectrin on the membrane. (2) The ESR spectra of the protein specific maleimide spin label employed accurately reflect the state of aggregation of spectrin. (3) As dimeric spectrin is increased on the membrane or when 2,3-bisphosphoglycerate was added to spin-labeled membranes, increased segmental motion of protein spin label binding sites reflecting decreased protein-protein interactions in the skeletal network is observed (P less than 0.002 and P less than 0.005, respectively). (4) Conversely, as protein-protein interactions between skeletal proteins or between skeletal proteins and the bilayer are increased by spermine (reflected in the total inability to extract spectrin from the membrane in contrast to control membranes), highly decreased segmental motion of the protein specific spin label binding site is observed (P less than 0.005). (5) The dimeric-tetrameric state of spectrin aggregation on the membrane does not have influence on the order or motion of bilayer lipids nor on the rotational rate of spin-labeled, cell-surface sialic acid, a result also observed when protein-protein interactions were decreased by 2,3-bisphosphoglycerate. In contrast, increased protein-protein interactions by addition of spermine produced a small, but significant, increase in order and decrease in motion of bilayer lipids near the membrane surface as well as a nearly 40% decrease in the apparent rotational correlation time of spin labeled, cell surface sialic acid (P less than 0.002). These latter observations are discussed with reference to possible associations of phospholipids and the major, transmembrane sialoglycoprotein with the skeletal protein network.

Erythrocyte membrane rigidity induced by glycophorin A-ligand interaction. Evidence for a ligand-induced association between glycophorin A and skeletal proteins

Erythrocyte skeletal proteins are known to play an important role in determining membrane deformability. In order to see whether transmembrane proteins also influence deformability and, if so, whether this influence is mediated by an interaction with the membrane skeleton, we examined the effect on deformability of ligands specific for transmembrane proteins. We found membrane deformability markedly reduced in erythrocytes that were pretreated with glycophorin A-specific ligands. In contrast, ligands specific for band 3 and A and B blood group antigens had no effect. The increase in membrane rigidity appeared to depend upon a transmembrane event and not upon a rigidity-inducing lattice on the outside surface of the cell in that a monovalent Fab of antiglycophorin IgG caused decreased deformability. We therefore looked for a ligand-induced association of glycophorin and the skeletal proteins and found, in Triton X-100-insoluble residues, a partitioning of glycophorin with the skeletal proteins only after preincubation with a ligand specific for glycophorin. We then studied cells and resealed membranes with skeletal protein abnormalities. In spectrin-deficient and protein 4.1-deficient erythrocytes and in 2,3-diphosphoglycerate-treated resealed membranes, the antiglycophorin IgG was only one-third as effective in decreasing deformability as it was in normal cells. Thus, normal skeletal proteins appear to be essential for liganded glycophorin to affect membrane deformability maximally. Taken together, these observations indicate that there is a ligand-induced interaction between glycophorin A and skeletal proteins and that this interaction can directly influence membrane deformability.

The alteration of membrane proteins in human erythrocyte membranes induced by quinolinic acid, an endogenous neurotoxin. Correlation of effect with structure

Quinolinic acid (2,3-pyridinedicarboxylic acid), an endogenous metabolite of L-tryptophan, reportedly via the kynurenine pathway, has been previously shown to possess neurotoxic properties when injected into rat striatum (Schwarcz, R., Whetsell, W.O., Jr. and Mangano, R.M. (1983) Science 219, 316-318) and to alter the physical state of human erythrocyte membrane proteins, as judged by ESR spectroscopy (Farmer, B.T., II and Butterfield, D.A. (1984) Life Sci. 35, 501-509). Both the morphologic and ESR studies employed nicotinic acid as one comparative control and found that the effect of quinolinic acid is significantly different from that of nicotinic acid. In the present study, we report that the effects of several structural analogues and positional isomers of quinolinic acid on the ESR parameter associated with the physical state of membrane proteins in human erythrocyte membranes suggest the following conclusions concerning the structure-effect relationship of quinolinic acid: The alteration in the conformation of membrane proteins: (1) requires the presence of two carboxylic acid groups; (2) is independent of their relationship to one another on the pyridine ring; (3) is slightly dependent on the presence of the pyridine nitrogen atom but is independent of the positional relationship of the two carboxylic acid moieties to the heteroatom; and (4) seems to depend upon the presence of restricted internal motion derived from the aromaticity in these compounds.

Selective spin labeling of terminal galactose and N-acetylgalactosamine residues on the membrane surface of erythrocytes

A method to selectively spin label galactose and N-acetylgalactosamine residues of erythrocyte membrane glycoconjugates is described. The method is based on the activation of the C-6 CH2OH group of these two sugars by galactose oxidase followed by reductive amination with 2,2,6,6-tetramethyl-4-aminopiperidine-1-oxyl in the presence of a mild reducing agent, NaBH3CN. The extent and distribution of the spin labeling suggest that the major sialoglycoprotein, PAS-1, incorporates the greatest amount of spin label while the glycolipids incorporate less than 10% of the spin label.

Quinolinic acid, an endogenous metabolite with neurotoxic properties, alters the physical state of membrane proteins in human erythrocytes

Quinolinic acid (2,3-pyridinedicarboxylic acid), an endogenous, tryptophan metabolite, is neurotoxic when injected into rat striatum (1). To begin to investigate the molecular interactions of quinolinic acid with membranes, electron spin resonance studies of the effects of this neurotoxin on the physical state of lipids, proteins, and cell-surface sialic acid in human erythrocyte ghosts have been performed. Quinolinic acid induced a highly significant alteration in the physical state of membrane proteins (P less than 0.01) while that of sialic acid and membrane lipids was unaffected. These results are similar to those induced by ibotenic acid, an exogenous neurotoxin, and are discussed with reference to possible molecular characteristics of the interaction of these neurotoxins with membrane proteins.