Peter Zahler

Isolation of phospholipase A2 from sheep erythrocyte membranes in the presence of detergents

Isolation of phospholipase A2 (EC 3.1.1.4) from sheep erythrocyte membranes was carried out by a combination of (1) extraction of membranes at low ionic strength, (2) solubilization of extracted membranes with sodium dodecyl sulfate, (3) replacement of dodecyl sulfate with cholate by means of gel exclusion chromatography and (4) affinity chromatography on dialkyl-phosphatidylcholine-Sepharose in the presence of cholate. The phospholipase was prepared with good yield and purified to near homogeneity, as judged by sodium dodecyl sulfate gel electrophoresis. The protein is a minor component of the sheep erythrocyte membrane and has an apparent molecular weight of 18 500.

Lipid-protein interactions with native and modified myelin basic protein☆

The basic protein of central nervous system myelin has been shown to form complexes with acidic lipids in vitro. We measured the interaction of myelin basic protein with several charged and neutral lipids in a biphasic chloroform/methanol/water system and investigated the effect of decreasing the electrical charge of the basic amino groups of the myelin basic protein by acetylation. The modified myelin basic protein, which has an average of eight acetyl residues incorporated, was characterised by gel electrophoresis and circular dichroism. Complexes formed between the acetylated myelin basic protein and acidic lipids exhibited a reduction in the amount of lipids bound, a value that could be correlated with the number of modified amino groups. The significance of these experiments with reference to protein-lipid interaction in the myelin membrane is discussed.

Interaction of phenylisothiocyanate with human erythrocyte band 3 protein. II. Topology of phenylisothiocyanate binding sites and influence of p-sulfophenylisothiocyanate on phenylisothiocyanate modification

The two structurally related probes, the apolar phenylisothiocyanate and the polar, water-soluble p-sulfophenylisothiocyanate, were analysed for their topological interaction with human erythrocyte band 3 protein. Upon thermolytic and peptic digestion of labeled erythrocyte ghosts, the membrane-integrated segments of band 3 protein, the 17,000 and 10,000 dalton peptides, were isolated. At 2 mM initial label concentration, 90% of the hydrophobic probe phenylisothiocyanate was recovered in the 10,000 dalton peptide, the remaining amount of label being associated with the 17,000 dalton fragment. Pretreatment of the membranes with 5 mM p-sulfophenylisothiocyanate followed by labeling with 2 mM phenylisothiocyanate results in a consistent reduction in binding of phenylisothiocyanate by 1 mol/mol isolated band 3 protein. p-Sulfophenylisothiocyanate reportedly binds to the 17,000 dalton fragment (Drickamer, K. (1977), J. Biol. Chem. 252, 6909-6917). The interaction of the polar probe with the membrane protein affects binding of phenylisothiocyanate to the 10,000 dalton peptide by the equivalent of 1 mol/mol isolated peptide. The topological interrelation of the membrane-integrated segments is concluded.

Characterization of phenylisothiocyanate as a hydrophobic membrane label

Various labeling techniques have been widely employed to classify those portions ofmembrane proteins exposed to the aqueous environment [l-3]. With the recent introduction of hydrophobic azides, the lipophilic membrane-integrated domains became specifically accessible for chemical modification [4,5]. However, a marked disadvantage of the rapid, photoinduced reactivity of azides is that it is not exclusively directed towards selected functional groups of proteins [6]. Lipid components of the membrane system are quite highly labeled [7]. For specific modification of the hydrophobic moieties of membrane constituents group specific reagents are necessary. Successful modification in the apolar phase requires that the reagent be hydrophobic and partitions in favor of the apolar phase. Furthermore, the functional groups of the membrane protein must be in a reactive state. No less important, the reaction and reaction products formed are required to be nonionogenic and noncharged, respectively. Water should not be involved in the reaction, neither as reactant nor as product. PITC is able to fulfill these requirements. It reacts with ammo groups solely in the unprotonated form [8]. The reactivity of the protein functional groups can therefore be controlled by the pH in the aqueous phase. At neutral pH, the exposed protein amino groups are expected to be protonated. The buried, bulk pHindependent amino functions, however, can be in the reactive state. This implies selectivity of the reaction for non-aqueous systems.

Heterobifunctional crosslinking of bacteriorhodopsin by azidophenylisothiocyanate

Phenyliso~iocyanate has been described for group-specific modification of membrane proteins in the apolar membrane phase [I]. The method has been applied in various membrane systems [l-4]. Upon modification of purple membranes a single unique nucleophilic group of bateriorhodopsin (lysine 2 15) is labeled by phenylisothiocyanate [5]. The phenylisothiocyanate binding site is not accessible for the aqueously soluble analog p-sulfophenylisothiocyanate. Therefore, the phenylisothiocy~ate modification is concluded to occur in the hydrophobic membrane domain. The newly described probe p-azidophenylisothiocyanate combines the group specificity of phenylisothiocyanate and the advantages of photoinduced reactivity of arylazides. Since both reagents react with proteins from within the lipid bilayer, the new probe possesses the requirements necessary for hydrophobic, heterobifunctional crosslinking. The reagent interacts in a first step with deprotonated proteinaceous nucleophiles. The erosslink is then formed by light activation of the hetero-function (arylazide). In this study the investigation of molecular characteristics of p-azidophenylisothiocyanate, the modification of bateriorhodopsin by the bifunctional probe, and the flash-induced crosslink-formation are described. The versatility of the group-specific, heterobifunctional probe is discussed.

Interaction of phenylisothiocyanate with human erythrocyte band 3 protein I. Covalent modification and inhibition of phosphate transport

The hydrophobic probe phenylisothiocyanate is utilized for chemical modification of human erythrocyte band 3 protein. The binding of phenylisothiocyanate to this protein is characterized in whole erythrocytes, erythrocyte ghost membranes and in isolated band 3 protein. The label, reactive with nucleophiles in their deprotonated form is found in all three preparations to be covalently bound to band 3 protein. Under saturation conditions, 4--5 mol phenylisothiocyanate are covalently bound per mol protein (molecular weight 95 000). The described modification effects inhibition of phosphate entry into erythrocytes. 50% inhibition of phosphate transport is obtained following a preincubation of erythrocytes with 0.45 mM phenylisothiocyanate. Both phenylisothiocyanate binding and transport inhibition are saturating processes. The relationship of the two parameters is non-linear.

Interaction of phosphatidylcholine liposomes and plasma lipoproteins with sheep erythrocyte membranes: Preferential transfer of phosphatidylcholine containing unsaturated fatty acids

The interaction of sheep erythrocyte membranes with phosphatidylcholine vesicles (liposomes) or human plasma lipoproteins is described. Isolated sheep red cell membranes were incubated with liposomes containing [14C]phosphatidylcholine or [3H]phosphatidylcholine in the presence of EDTA. A time-dependent uptake of phosphatidylcholine into the membranes could be observed. The content of this phospholipid was increased from 2 to 5%. The rate of transfer was dependent on temperature, the amount of phosphatidylcholine present in the incubation mixture and on the fatty acid composition of the liposomal phosphatidylcholine. A possible adsorption of lipid vesicles to the membranes could be monitored by adding cholesteryl [14C]oleate to the liposomal preparation. As cholesterylesters are not transferred between membranes [1], it was possible to differentiate between transfer of phosphatidylcholine molecules from the liposomes into the membranes and adsorption of liposomes to the membranes. The phosphatidylcholine incorporated in the membranes was isolated, and its fatty acids were analysed by gas chromatography. It could be shown that there was a preferential transfer of phosphatidylcholine molecules containing two unsaturated fatty acids.

Preparation of membrane vesicles from isolated myelin. Studies on functional and structural properties

Myelin membranes purified from bovine brain are shown to form membrane vesicles when incubated in hypotonic buffer. Following restoration of isotonicity a resealing of the membrane occurs as judged by a significant decrease in 22Na+ permeability. Electron spin resonance measurements using stearic acid spin label I indicate a small decrease in membrane fluidity with increasing ionic strength between 50 and 80 mM NaCl. Iodination of myelin membrane vesicles by lactoperoxidase shows a four-fold increase in the amount of iodine incorporation into the myeline basic protein from 0--150 mM NaCl, while the iodination of the proteolipid protein remains essentially unaffected by the change in ionic strength. This dependence of the iodination of the myelin basic protein on the ionic strength can be explained by the electrostatic interactions of this protein with membrane lipids. In view of striking analogies with studies on model membranes correlating protein binding with membrane permeability changes, we suggest a similar structure-function relationship for the myelin basic protein.

Effects of pH during recombination of human erythrocyte membrane apoprotein and lipid

The recombinates from human red cell membrane proteins and lipids resulting from dialysis of the components in 2-chloroethanol against aqueous buffers from pH2-12 have been studied by density gradient centrifugation, polyacrylamide gel electrophoresis and freeze-fracture electron microscopy. Between pH 4 and 10 most of the proteins were found in the recombinates whereas below pH 4 and above pH 10 only part of them were recovered in the lipoprotein band after density gradient centrifugation. At low pH, increasing incorporation of the major glycoprotein into the recombinates was detected by gel electrophoresis and in parallel increasing amounts of particles were found in the freeze-fracture membrane faces. The necessity of working at low pH values from pH 2-4, however, and a critical evaluation of all the data presently available leads to the conclusion that the 2-choloroethanol technique is not adequate for recombination studies tending to membrane reconsitution.

Hereditary Leaky Red Cell Syndrome in a Swiss Family

A Swiss family was found to have a hereditary hemolytic disorder associated with excessively leaky red cell membranes. Hemolysis was mild and fully compensated. Membrane lipid analysis revealed an increased phosphatidylcholine:phosphatidylethanolamine ratio. Membrane leaks included an increased permeability to sodium, potassium, calcium and, possibly, creatine. It is suggested that in hemolytic states, the combined finding of reticulocytosis and normal red cell creatine might be an easily obtainable clue to the presence of a leaky red cell syndrome.