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Brisbois CA, Lee JC. Apolipoprotein C-III Nanodiscs Studied by Site-Specific Tryptophan Fluorescence. Biochemistry 2016; 55:4939-48. [PMID: 27529357 DOI: 10.1021/acs.biochem.6b00599] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Apolipoprotein C-III (ApoC-III) is found on high-density lipoproteins (HDLs) and remodels 1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicles into HDL-like particles known as nanodiscs. Using single-Trp-containing ApoC-III mutants, we have studied local side chain environments and interactions in nanodiscs at positions W42, W54, and W65. Using transmission electron microscopy and circular dichroism spectroscopy, nanodiscs were characterized at the ultrastructural and secondary conformational levels, respectively. Nearly identical particles (15 ± 2 nm) were produced from all proteins containing approximately 25 ± 4 proteins per particle with an average helicity of 45-51% per protein. Distinct residue-specific fluorescence properties were observed with W54 residing in the most hydrophobic environment followed by W42 and W65. Interestingly, time-resolved anisotropy measurements revealed that Trp side chain mobility is uncorrelated to the polarity of its surroundings. W54 is the most mobile compared to W65 and W42, which are more immobile in a nanodisc-bound state. On the basis of Trp spectral comparisons of ApoC-III in micellar and vesicle environments, ApoC-III binding within nanodiscs more closely resembles a bilayer-bound state. Despite the nanodiscs being structurally similar, we found marked differences during nanodisc formation by the Trp variants as a function of temperature, with W42 behaving the most like the wild-type protein. Our data suggest that despite the modest mutations of Trp to Phe at two of the three native sites, the interfacial location of W42 is important for lipid binding and nanodisc assembly, which may be biologically meaningful as of the three Trp residues, only W42 is invariant among mammals.
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Affiliation(s)
- Chase A Brisbois
- Laboratory of Protein Conformation and Dynamics, Biochemistry and Biophysics Center, National Heart, Lung, and Blood Institute, National Institutes of Health , Bethesda, Maryland 20892, United States
| | - Jennifer C Lee
- Laboratory of Protein Conformation and Dynamics, Biochemistry and Biophysics Center, National Heart, Lung, and Blood Institute, National Institutes of Health , Bethesda, Maryland 20892, United States
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Scheidelaar S, Koorengevel MC, Pardo JD, Meeldijk JD, Breukink E, Killian JA. Molecular model for the solubilization of membranes into nanodisks by styrene maleic Acid copolymers. Biophys J 2015; 108:279-90. [PMID: 25606677 PMCID: PMC4302193 DOI: 10.1016/j.bpj.2014.11.3464] [Citation(s) in RCA: 131] [Impact Index Per Article: 14.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2014] [Revised: 11/20/2014] [Accepted: 11/24/2014] [Indexed: 01/11/2023] Open
Abstract
A recent discovery in membrane research is the ability of styrene-maleic acid (SMA) copolymers to solubilize membranes in the form of nanodisks allowing extraction and purification of membrane proteins from their native environment in a single detergent-free step. This has important implications for membrane research because it allows isolation as well as characterization of proteins and lipids in a near-native environment. Here, we aimed to unravel the molecular mode of action of SMA copolymers by performing systematic studies using model membranes of varying compositions and employing complementary biophysical approaches. We found that the SMA copolymer is a highly efficient membrane-solubilizing agent and that lipid bilayer properties such as fluidity, thickness, lateral pressure profile, and charge density all play distinct roles in the kinetics of solubilization. More specifically, relatively thin membranes, decreased lateral chain pressure, low charge density at the membrane surface, and increased salt concentration promote the speed and yield of vesicle solubilization. Experiments using a native membrane lipid extract showed that the SMA copolymer does not discriminate between different lipids and thus retains the native lipid composition in the solubilized particles. A model is proposed for the mode of action of SMA copolymers in which membrane solubilization is mainly driven by the hydrophobic effect and is further favored by physical properties of the polymer such as its relatively small cross-sectional area and rigid pendant groups. These results may be helpful for development of novel applications for this new type of solubilizing agent, and for optimization of the SMA technology for solubilization of the wide variety of cell membranes found in nature.
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Affiliation(s)
- Stefan Scheidelaar
- Membrane Biochemistry & Biophysics, Bijvoet Center for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Padualaan, Utrecht, The Netherlands.
| | - Martijn C Koorengevel
- Membrane Biochemistry & Biophysics, Bijvoet Center for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Padualaan, Utrecht, The Netherlands
| | - Juan Dominguez Pardo
- Membrane Biochemistry & Biophysics, Bijvoet Center for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Padualaan, Utrecht, The Netherlands
| | - Johannes D Meeldijk
- Electron Microscopy Utrecht, Debye Institute of Nanomaterials Science, Faculty of Science, Utrecht University, Padualaan, Utrecht, The Netherlands
| | - Eefjan Breukink
- Membrane Biochemistry & Biophysics, Bijvoet Center for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Padualaan, Utrecht, The Netherlands
| | - J Antoinette Killian
- Membrane Biochemistry & Biophysics, Bijvoet Center for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Padualaan, Utrecht, The Netherlands
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Wan CPL, Chiu MH, Wu X, Lee SK, Prenner EJ, Weers PMM. Apolipoprotein-induced conversion of phosphatidylcholine bilayer vesicles into nanodisks. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2010; 1808:606-13. [PMID: 21111706 DOI: 10.1016/j.bbamem.2010.11.020] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/03/2010] [Revised: 11/03/2010] [Accepted: 11/15/2010] [Indexed: 11/27/2022]
Abstract
Apolipoprotein mediated formation of nanodisks was studied in detail using apolipophorin III (apoLp-III), thereby providing insight in apolipoprotein-lipid binding interactions. The spontaneous solubilization of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles occured only in a very narrow temperature range at the gel-liquid-crystalline phase transition temperature, exhibiting a net exothermic interaction based on isothermal titration calorimetry analysis. The resulting nanodisks were protected from proteolysis by trypsin, endoproteinase Glu-C, chymotrypsin and elastase. DMPC solubilization and the simultaneous formation of nanodisks were promoted by increasing the vesicle diameter, protein to lipid ratio and concentration. Inclusion of cholesterol in DMPC dramatically enhanced the rate of nanodisk formation, presumably by stabilization of lattice defects which form the main insertion sites for apolipoprotein α-helices. The presence of fully saturated acyl chains with a length of 13 or 14 carbons in phosphatidylcholine allowed the spontaneous vesicle solubilization upon apolipoprotein addition. Nanodisks with C13:0-phosphatidylcholine were significantly smaller with a diameter of 11.7 ± 3.1nm compared to 18.5 ± 5.6 nm for DMPC nanodisks determined by transmission electron microscopy. Nanodisk formation was not observed when the phosphatidylcholine vesicles contained acyl chains of 15 or 16 carbons. However, using very high concentrations of lipid and protein (>10mg/ml), 1,2,-dipalmitoyl-sn-glycero-3-phosphocholine nanodisks could be produced spontaneously although the efficiency remained low.
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Affiliation(s)
- Chung-Ping Leon Wan
- Department of Chemistry and Biochemistry, California State University Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840, USA
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Koepsell H. Methodological aspects of purification and reconstitution of transport proteins from mammalian plasma membranes. Rev Physiol Biochem Pharmacol 2006; 104:65-137. [PMID: 2940665 DOI: 10.1007/bfb0031013] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
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5
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Pownall HJ, Knapp RD, Gotto AM, Massey JB. Helical amphipathic moment: application to plasma lipoproteins. FEBS Lett 2001. [DOI: 10.1016/0014-5793(83)80408-6] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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Bussell SJ, Koch DL, Hammer DA. Effect of hydrodynamic interactions on the diffusion of integral membrane proteins: diffusion in plasma membranes. Biophys J 1995; 68:1836-49. [PMID: 7612825 PMCID: PMC1282086 DOI: 10.1016/s0006-3495(95)80360-7] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
Tracer diffusion coefficients of integral membrane proteins (IMPs) in intact plasma membranes are often much lower than those found in blebbed, organelle, and reconstituted membranes. We calculate the contribution of hydrodynamic interactions to the tracer, gradient, and rotational diffusion of IMPs in plasma membranes. Because of the presence of immobile IMPs, Brinkman's equation governs the hydrodynamics in plasma membranes. Solutions of Brinkman's equation enable the calculation of short-time diffusion coefficients of IMPs. There is a large reduction in particle mobilities when a fraction of them is immobile, and as the fraction increases, the mobilities of the mobile particles continue to decrease. Combination of the hydrodynamic mobilities with Monte Carlo simulation results, which incorporate excluded area effects, enable the calculation of long-time diffusion coefficients. We use our calculations to analyze results for tracer diffusivities in several different systems. In erythrocytes, we find that the hydrodynamic theory, when combined with excluded area effects, closes the gap between existing theory and experiment for the mobility of band 3, with the remaining discrepancy likely due to direct obstruction of band 3 lateral mobility by the spectrin network. In lymphocytes, the combined hydrodynamic-excluded area theory provides a plausible explanation for the reduced mobility of sIg molecules induced by binding concanavalin A-coated platelets. However, the theory does not explain all reported cases of "anchorage modulation" in all cell types in which receptor mobilities are reduced after binding by concanavalin A-coated platelets. The hydrodynamic theory provides an explanation of why protein lateral mobilities are restricted in plasma membranes and why, in many systems, deletion of the cytoplasmic tail of a receptor has little effect on diffusion rates. However, much more data are needed to test the theory definitively. We also predict that gradient and tracer diffusivities are the same to leading order. Finally, we have calculated rotational diffusion coefficients in plasma membranes. They decrease less rapidly than translational diffusion coefficients with increasing protein immobilization, and the results agree qualitatively with the limited experimental data available.
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Affiliation(s)
- S J Bussell
- School of Chemical Engineering, Cornell University, Ithaca, New York 14853, USA
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Segrest JP, Garber DW, Brouillette CG, Harvey SC, Anantharamaiah GM. The amphipathic alpha helix: a multifunctional structural motif in plasma apolipoproteins. ADVANCES IN PROTEIN CHEMISTRY 1994; 45:303-69. [PMID: 8154372 DOI: 10.1016/s0065-3233(08)60643-9] [Citation(s) in RCA: 226] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- J P Segrest
- Department of Medicine, University of Alabama 35294
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Segrest JP, Jones MK, De Loof H, Brouillette CG, Venkatachalapathi YV, Anantharamaiah GM. The amphipathic helix in the exchangeable apolipoproteins: a review of secondary structure and function. J Lipid Res 1992. [DOI: 10.1016/s0022-2275(20)41536-6] [Citation(s) in RCA: 702] [Impact Index Per Article: 21.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
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McLean LR, Hagaman KA, Owen TJ, Payne MH, Davidson WS, Krstenansky JL. Examination of the peptide sequence requirements for lipid-binding. Alternative pathways for promoting the interaction of amphipathic alpha-helical peptides with phosphatidylcholine. BIOCHIMICA ET BIOPHYSICA ACTA 1991; 1086:106-14. [PMID: 1954237 DOI: 10.1016/0005-2760(91)90161-a] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
To examine the relationship between peptide sequence and the interaction of amphipathic alpha-helical peptides with phosphatidylcholines, various methods of mixing the peptide and lipid were explored. A series of amphipathic alpha-helical peptides containing from 10 to 18 residues were synthesized by solid-phase techniques. An 18-residue peptide and two relatively hydrophobic 10-residue peptides did not disrupt dimyristoylphosphatidylcholine liposomes when added to the lipid in buffer. However, when the peptides were premixed with lipid in a suitable organic solvent and then reconstituted with aqueous buffer, clear micelles were formed, indicating association of the amphipathic alpha-helical peptide with lipid. In general, the best solvent for this purpose was trifluoroethanol. The circular dichroic and fluorescence spectra of peptides which readily formed clear mixtures when mixed in buffer with dimyristoylphosphatidylcholine liposomes were similar when prepared either by the alternative pathway technique using trifluoroethanol or by a cholate removal technique. For the peptides which did not clear liposomes in buffer, first mixing with dimyristoylphosphatidylcholine in trifluoroethanol resulted in an increase in the alpha-helicity of the peptides as judged by circular dichroic spectra and a blue-shift in the fluorescence emission maxima of the single tryptophan residue in each peptide. These data are consistent with formation of an amphipathic alpha-helix in lipid by peptides which based on mixing experiments with dimyristoylphosphatidylcholine liposomes in buffer at the phase transition temperature of the lipid would be considered ineffective in lipid binding. Thus, simple mixing of peptides with liposomes may give misleading results concerning the intrinsic affinity of a particular peptide sequence for lipid. In addition, the data demonstrate that relatively hydrophobic amphipathic alpha-helical peptides which do not form small micelles with dimyristoylphosphatidylcholine spontaneously in aqueous solution may interact with lipid as typical amphipathic alpha-helices when mixed by an alternative pathway.
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Affiliation(s)
- L R McLean
- Marion Merrell Dow Research Institute, Cincinnati, OH 45215
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Krupka RM. Role of substrate binding forces in exchange-only transport systems: II. Implications for the mechanism of the anion exchanger of red cells. J Membr Biol 1989; 109:159-71. [PMID: 2671377 DOI: 10.1007/bf01870855] [Citation(s) in RCA: 29] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
The transition-state theory of exchange-only membrane transport is applied to experimental results in the literature on the anion exchanger of red cells. Two central features of the system are in accord with the theory: (i) forming the transition state in translocation involves a carrier conformational change; (ii) substrate specificity is expressed in transport rates rather than affinities. The expression of specificity is consistent with other evidence for a conformational intermediate (not the transition state) formed in the translocation of all substrates. The theory, in conjunction with concepts derived from the chemistry of macrocyclic ion inclusion complexes, prescribes certain essential properties in the transport site. Separate subsites are required for the preferred substrates, Cl- and HCO3-, to account for tight binding in the transition state (Kdiss congruent to 1 microM). Further, the following mechanism is suggested. A substrate anion initially forms a loose surface complex at one subsite, but in the transition state the subsites converge to form an inclusion complex in which the binding forces are greatly increased through a chelation effect. The conformational change at the substrate site, which is driven by the mounting forces of binding, sets in train a wider conformational change that converts the carrier from an immobile to a mobile form. Though simple, this composite-site mechanism explains many unusual features of the system. It accounts for substrate inhibition, partially noncompetitive inhibition of one substrate by another, and "tunneling," which is net transport under conditions where exchange should prevail, according to other models. All three types of behavior result from the formation of a ternary complex in which substrate anions are bound at both subsites. The mechanism also accounts for the enormous range of substrate structures accepted by the system, for the complex inhibition by the organic sulfate NAP-taurine, and for the involvement of several cationic side chains and two different protein domains in the transport site.
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Affiliation(s)
- R M Krupka
- Agriculture Canada, Research Centre, London, Ontario
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11
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McLean LR, Hagaman KA. Probucol reduces the rate of association of apolipoprotein C-III with dimyristoylphosphatidylcholine. BIOCHIMICA ET BIOPHYSICA ACTA 1988; 959:201-5. [PMID: 3355846 DOI: 10.1016/0005-2760(88)90191-9] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
The effect of low concentrations of probucol and cholesterol on the association of dimyristoylphosphatidylcholine with human plasma apolipoprotein C-III was studied. Liposomes of dimyristoylphosphatidylcholine with or without probucol or cholesterol were prepared by swelling the lipids in buffer at 37 degrees C. The association of apolipoprotein C-III with the liposomes was determined at 24 degrees C by measuring the rate of clearing of turbidity at 400 nm following addition of protein. At a weight ratio of probucol/dimyristoylphosphatidylcholine of 1:25 (5 mol% probucol), the rate of clearing of liposomes was decreased by 60%; 5 mol% cholesterol had no effect on the clearing rate. Liposomes were then added to the preformed apolipoprotein C-III/lipid micelles. In the absence of probucol, the added liposomes cleared rapidly regardless of the presence or absence of cholesterol. With 5 mol% probucol, almost no decrease in absorbance was noted on addition of liposomes to the micelles. These data show that probucol reduces the rate of association of an apolipoprotein with lipid and suggests that the interaction of probucol with lipid may modify the assembly and/or metabolism of lipoproteins.
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Affiliation(s)
- L R McLean
- Merrell Dow Research Institute, Cincinnati, OH 45215
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12
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Studies of synthetic peptide analogs of the amphipathic helix. Effect of charge distribution, hydrophobicity, and secondary structure on lipid association and lecithin:cholesterol acyltransferase activation. J Biol Chem 1987. [DOI: 10.1016/s0021-9258(18)48092-1] [Citation(s) in RCA: 67] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
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13
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Jain MK, Zakim D. The spontaneous incorporation of proteins into preformed bilayers. BIOCHIMICA ET BIOPHYSICA ACTA 1987; 906:33-68. [PMID: 3032257 DOI: 10.1016/0304-4157(87)90004-9] [Citation(s) in RCA: 79] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
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14
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Passow H. Molecular aspects of band 3 protein-mediated anion transport across the red blood cell membrane. Rev Physiol Biochem Pharmacol 1986; 103:61-203. [PMID: 2421388 DOI: 10.1007/3540153330_2] [Citation(s) in RCA: 59] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
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15
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Peters R. Fluorescence microphotolysis. Diffusion measurements in single cells. THE SCIENCE OF NATURE - NATURWISSENSCHAFTEN 1983; 70:294-302. [PMID: 6877390 DOI: 10.1007/bf00404836] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
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Rauenbuehler PB, Cordes KA, Salhany JM. Identification of the hemoglobin binding sites on the inner surface of the erythrocyte membrane. BIOCHIMICA ET BIOPHYSICA ACTA 1982; 692:361-70. [PMID: 7171600 DOI: 10.1016/0005-2736(82)90385-6] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Abstract
The hemoglobin binding sites on the inner surface of the erythrocyte membrane were identified by measuring the fraction of hemoglobin released following selective proteolytic or lipolytic enzyme digestion. In addition, binding stoichiometry to and fractional hemoglobin release from inside-out vesicle preparations of human and rabbit membranes were compared since rabbit membranes differ significantly from human membranes only in that they lack glycophorin. Our results show that rabbit inside-out vesicles bind about 65% less human or rabbit hemoglobin under conditions of optimal and stoichiometric binding, despite being otherwise similar in composition. We suggest that this difference is either directly or indirectly due to the absence of glycophorin in rabbit membranes. Further supportive evidence includes demonstrating (a) that neuraminidase treatment of human membranes did not affect hemoglobin binding and (b) that reconstitution of isolated glycophorin into phospholipid vesicles increased the hemoglobin binding capacity in a manner proportional to the fraction of glycophorin molecules oriented with their cytoplasmic sides exposed to the exterior of the vesicle. Proteolysis of human inside-out vesicles either before or after addition of hemoglobin reduced the binding capacity by about 25%. This is consistent with the known proportion of total hemoglobin binding sites involving band 3 protein and the selective lability of the cytoplasmic aspect of band 3 protein to proteolysis. Phospholipid involvement in hemoglobin binding was determined using various phospholipase C preparations which differ in their reactivity profiles. Approximately 38% of the bound hemoglobin was released upon cleavage of phospholipid headgroups. These results suggest that the predominant sites of binding for hemoglobin on the inner surface of the red cell membrane are the two major integral membrane glycoproteins.
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Kampmann L, Lepke S, Fasold H, Fritzsch G, Passow H. The kinetics of intramolecular cross-linking of the band 3 protein in the red blood cell membrane by 4,4'-diisothiocyano dihydrostilbene-2,2'-disulfonic acid (H2DIDS). J Membr Biol 1982; 70:199-216. [PMID: 7186941 DOI: 10.1007/bf01870563] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Abstract
The two isothiocyanate groups of the anion transport inhibitor 4,4'-diisothiocyano dihydrostilbene-2-2'-disulfonate (H2DIDS) may react covalently with two lysine residues called a and b that reside on the chymotryptic 60,000 Dalton and 35,000 Dalton segments, respectively, of the band 3 protein of the human erythrocyte membrane. Under suitable conditions, the reaction leads to the establishment of intramolecular cross-links between a and b (M.L. Jennings & H. Passow, 1979, Biochim. Biophys. Acta 554:498-519). In the present work, the time course of the reactions with a and b, and of the establishment of the cross-link were investigated experimentally and compared with simple mathematical models of the reaction sequence. The rates of reaction with a and b were found to increase with increasing pH. Regardless of pH, the rate of reaction with a exceeds that with b several-fold. Once the H2DIDS molecule has reacted with a, the rate of the subsequent reaction of the other isothiocyanate group with b is reduced by about 1/30. The reactions that follow the unilateral attachment to site b are not yet clear. A more detailed analysis of the time course of the cross-linking reaction suggests that a satisfactory description of the kinetics requires the assumption that the H2DIDS binding site may exist in two different states, and that the transition from one state to the other is associated with changes of the reactivities of either lys a alone or of both lys a and b. This led to the formulation of the two-states model of the H2DIDS binding site, which is supported by other pieces of independent evidence. The analysis of the pH dependence of the rate of thiocyanylation of b shows that the apparent pK value of that lysine residue is about 9.9 to 10.0 and hence slightly lower than the intrinsic pK of a lysine residue in an aqueous environment.
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