Reactions of fluorescent probes with normal and chemically modified myelin.

The fluorescent probes 8-anilino-1-naphthalenesulfonate (ANS) and 2-p-toluidinylnaphthalene-6-sulfonate (TNS) bind to highly purified myelin membranes obtained from bovine brain white matter. Binding of the dyes was markedly increased by environmental conditions which reduce the negative surface potential of the membrane, i.e., cations (La-3+ is greater than Ca-2+ is greater than Na-+,K-+), H-+, local anesthetics, and the antibiotic polymyxin B. Chemical alteration of accessible membrane charged groups affected dye binding in a manner consistent with the hypothesis that such binding is primarily dependent upon the membrane surface potential. Thus, binding was increased by blocking of carboxyl groups via carbodiimide activation and subsequent coupling with neutral amino acid esters, and even more so with a basic amino acid ester (e.g., arginine methyl ester). Dye binding was reduced by succinylation of amino groups, and by hydrolysis of choline and ethanolamine head groups of phospho- and sphingolipids by phospholipase C. Phospholipase C treatment of myelin, or sphingomyelin vesicles, reduced or abolished the augmentation of ANS and TNS binding due to cations, local anesthetics, or polymyxin B. Energy transfer from myelin tryptophan residues to bound ANS occurs, but with low efficiency. Oxidation of membrane tryptophan residues with N-bromosuccinimide, or alkylation with 2-hydroxy (or methoxy)-5-nitrobenzyl bromide, markedly reduced intrinsic membrane fluorescence and energy transfer to bound ANS, but did not significantly affect dye binding or the quantum yield of ANS fluorescence when excitation was at 380nm. Proteolytic digestion removed 6-30% of myelin protein, depending upon the enzyme used, but had no effect on fluorescent dye binding. It is concluded that the binding of the anionic fluorescent probes ANS and TNS to myelin is primarily a function of the membrane surface charge density and net surface potential, as is the case with other biological membranes. Conclusions about the degree of dye binding to membrane lipids or membrane proteins cannot be drawn unless additional studies are carried out on isolated water soluble membrane proteins.     

Intrinsic fluorescence of a hydrophobic myelin protein and some complexes with phospholipids

The fluorescence characteristics of lipophilin, a proteolipid apoprotein from human myelin, were determined in aqueous and lipid environments. In all cases the tryptophan residues were located in buried hydrophobic sites of uniform, but limited, accessibility to the permeant quenching agent acrylamide; only in the helicogenic solvent 2-chloroethanol were the protein fluorophores exposed to the medium. Quantum yields were dependent on the state of aggregation of the protein in aqueous solution and increased considerably on treatment with lysolecithin micelles, or when the protein was combined with phosphatidylcholine by codialysis from 2-chloroethanol into water. Fluorescence titrations indicated that lipophilin bound to lysolecithin with an association constant greater than 10(6) L/mol. Radiationless singlet excitation energy transfer from tyrosine to tryptophan residues was found to decrease markedly when the protein was combined with lipids. When the protein was introduced into dimyristoylphosphatidylcholine vesicles, the tryptophan fluorescence did not detect any solid-liquid phase change. These results were consistent with strong hydrophobic interactions between lipophilin and phospholipids, which lead to conformational adjustments in the protein, and to establishment of an immobilized layer of boundary lipid in bilayer systems.     

A study of carbachol-atropine interaction on intestinal smooth muscle vesicles, using a fluorescent probe.

Plasma membrane vesicles were prepared from guinea pig ileum longitudinal muscle. The vesicles were characterized by electron microscopy and analysis of lipid and protein content. They were shown to be free of gross contamination from actomyosin, sarcoplasmic reticulum, and mitochondria. 8-Anilino-1-naphthalene sulphonic acid (ANS) binding characteristics were similar to those found in other membranes. Both carbachol and atropine increased the fluorescence of ANS bound to this membrane, the maximum increase for atropine being greater than that for carbachol. Since neither drug effected the apparent affinity constant for the ANS-membrane interaction. It may be assumed that the increased fluorescence was due to an increase in the number of ANS binding sites. The carbachol-dependent increase in ANS fluorescence was blocked noncompetitively by atropine but not by tubocurarine or diphenhydramine. These latter two antagonists also increased ANS fluorescence but at much higher concentrations than either carbachol or atropine. Neither atropine nor carbachol increased ANS fluorescence on either erythrocyte ghosts or liposomes (prepared from a lipid extract of the muscle membrane).     

Fluorescence spectral resolution of myelin basic protein conformers in complexes with lysophosphatidylcholine

The structure of (Deibler) myelin basic protein in solution and in a lysolecithin lipid complex has been studied by using the emission properties of the single tryptophan residue of the protein (Trp-115). The studies have been carried out using both static and time-resolved fluorescence techniques. Relative to the free protein, the lipid bound myelin basic protein showed a, twofold increase in fluorescence intensity and a marked blue-shift in the emission maximum wavelength. The multiexponential fluorescence decays and the decay associated spectra indicated that the protein exists in at least three different conformations both in buffer and in lipids. Fluorescence polarization and acrylamide quenching experiments showed that the tryptophan containing region of the protein is embedded in the lipid matrix. The binding of the protein to the lipid appears to be comparable with that predicted for the interaction of amphipathic helices with nonpolar lipids.     

Binding of naphthalene dyes to the N and A conformers of bovine α-lactalbumin

Contrary to earlier findings, monomeric native α-lactalbumin does bind naphthalene dyes such as ANS and TNS with marked enhancement of their fluorescence. Nanosecond decay measurements indicate there to be two dye binding sites per protein molecule with lifetimes of ca. 2 and 15 ns for ANS and 5 and 11 ns for TNS. The fluorescence titrations curves of α-lactalbumin with ANS and TNS reflect this site multiplicity, i.e., it was not possible to analyze such curves with a single K_diss. The apparent dissociation constants for binding of ANS and TNS to native bovine α-lactalbumin, as determined by an ultracentrifugal technique, ca. 950 and 900 μm, respectively, indicate that such binding is considerably weaker than previously supposed. The A conformer (metal ion-free form) of α-lactalbumin binds ANS and TNS more tightly than the N (native) form of the protein with marked fluorescence enhancement. The A conformer has two dye binding sites with lifetimes for ANS and TNS comparable with those seen with native protein.     

Polymyxin binding to charged lipid membranes. An example of cooperative lipid-protein interaction.

The binding of polymyxin-B to lipid bilayer vesicles of synthetic phosphatidic acid was studied using fluorescence, ESR spectroscopy and electron microscopy. 1,6-Diphenylhexatriene (which exhibits polarized fluorescence) and pyrene decanoic acid (which forms excimers) were used as fluorescence probes to study the lipid phase transition. The polymyxin binds strongly to negatively charged lipid layers. As a result of lipid/polymyxin chain-chain interactions, the transition temperature of the lipid. This can be explained in terms of a slight expansion of the crystalline lipid lattice (Lindeman's rule). Upon addition of polymyxin to phosphatidic acid vesicles two rather sharp phase transitions (width deltaT = 5 degrees C) are observed. The upper transition (at Tu) is that of the pure lipid and the lower transition (at T1) concerns the lipid bound to the peptide. The sharpness of these transitions strongly indicates that the bilayer is characterized by a heterogeneous lateral distribution of free and bound lipid regions, one in the crystalline and the other in the fluid state. Such a domain structure was directly observed by electron microscopy (freeze etching technique). In (1 : 1) mixtures of dipalmitoyl phosphatidic acid and egg lecithin, polymyxin induces the formation of domains of charged lipid within the fluid regions of egg lecithin. With both fluorescence methods the fraction of lipid bound to polymyxin-B as a function of the peptide concentration was determined. S-shaped binding curves were obtained. The same type of binding curve is obtained for the interaction of Ca2+ with phosphatidic acid lamellae, while the binding of polylysine to such membranes is characterized by a linear or Langmuir type binding curve. The S-shaped binding curve can be explained in terms of a cooperative lipid-ligand (Ca2+, polymyxin) interaction. A model is proposed which explains the association of polymyxin within the membrane plane in terms of elastic forces caused by the elastic distortion of the (liquid crystalline) lipid layer by this highly asymmetric peptide.     

Fluorescence spectral resolution of myelin basic protein conformers in complexes with lysophosphatidylcholine

The structure of (Deibler) myelin basic protein in solution and in a lysolecithin++ lipid complex has been studied by using the emission properties of the single tryptophan residue of the protein (Trp-115). The studies have been carried out using both static and time-resolved fluorescence techniques. Relative to the free protein, the lipid bound myelin basic protein showed a twofold increase in fluorescence intensity and a marked blue-shift in the emission maximum wavelength. The multiexponential fluorescence decays and the decay associated spectra indicated that the protein exists in at least three different conformations both in buffer and in lipids. Fluorescence polarization and acrylamide quenching experiments showed that the tryptophan containing region of the protein is embedded in the lipid matrix. The binding of the protein to the lipid appears to be comparable with that predicted for the interaction of amphipathic helices with nonpolar lipids.     

Interactions between lipid-anchored and transmembrane proteins. Spin-label ESR studies on avidin-biotinyl phosphatidylethanolamine in membrane recombinants with myelin proteolipid proteins

Interactions between lipid-anchored and transmembrane proteins are relevant to the intracellular membrane sorting of glycosyl phosphatidylinositol-linked proteins. We have studied the interaction of a spin-labeled biotinyl diacyl phospholipid, with and without specifically bound avidin, with the myelin proteolipid protein (or the DM-20 isoform) reconstituted in dimyristoylphosphatidylcholine. Tetrameric avidin bound to the N-biotinyl lipid headgroup is a surface-anchored protein, and the myelin proteolipid is an integral protein containing four transmembrane helices. The electron spin resonance (ESR) spectrum of N-biotinyl phosphatidylethanolamine spin-labeled at the C-14 position of the sn-2 chain consists of two components in fluid-phase membranes of dimyristoylphosphatidylcholine containing the proteolipid. In the absence of avidin, this is characteristic of lipid-protein interactions with integral transmembrane proteins. The more motionally restricted component represents the lipid population in direct contact with the intramembranous surface of the integral protein, and the more mobile component corresponds to the bulk fluid lipid environment of the bilayer. In the presence of avidin, the biotin-lipid chains have reduced mobility because of the binding to avidin, even in the absence of the proteolipid [Swamy, M. J., and Marsh, D. (1997) Biochemistry 36, 7403-7407]. In the presence of the proteolipid, the major fraction of the avidin-anchored chains is further restricted in its mobility by interaction with the transmembrane protein. At a biotin-lipid concentration of 1 mol %, approximately 80% of the avidin-linked chains are restricted in membranes with a phosphatidylcholine:proteolipid molar ratio of 37:1. This relatively high stoichiometry of interaction can be explained when allowance is made for the closest interaction distance between the lipid-anchored avidin tetramer and the transmembrane proteolipid hexamer, without any specific interaction between the two types of membrane-associated proteins. The interaction is essentially one of steric exclusion, but the lipid chains are rendered more sensitive to interaction with the integral protein by being linked to avidin, even though they are removed from the immediate intramembrane protein-lipid interface. This could have implications for the tendency of lipid-anchored chains to associate with membrane domains with reduced lipid mobility.     

A permeability change of myelin membrane vesicles towards cations is induced by MgATP but not by phosphorylation of myelin basic proteins

The existence of an endogenous protein kinase activity and protein phosphatase activity in myelin membrane from mammalian brain has now been well established. We found that under all conditions tested the myelin basic protein is almost the only substrate of the endogenous protein kinase in myelin of bovine brain. The protein kinase activity is stimulated by Ca2+ in the micromolar range. Optimal activity is reached at a free Ca2+ concentration of about 2 microM. Myelin membrane vesicles were prepared and then shown to be sealed by a light-scattering technique. After preloading with 45Ca2+, 86Rb+, or 22Na+, the self-diffusion (passive outflux) of these ions from myelin membrane vesicles was measured. Ionophores induced a rapid, concentration-dependent outflux of 80--90% of the cations, indicating that only a small fraction of the trapped ions was membrane bound. There was no difference in the diffusion rates of the three cations whether phosphorylated (about 1 mol phosphate per myelin basic protein) or non-phosphorylated vesicles were tested. In contrast, a small but significant decrease in permeability for Rb+ and Na+ was measured, when the vesicles were pretreated with ATP and Mg2+.     

Interaction of lipid-bound myelin basic protein with actin filaments and calmodulin

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocytes (OLs) and is believed to be responsible for adhesion of these surfaces in the multilayered myelin sheath. MBP in solution has been shown by others to bind to both G- and F-actin, to bundle F-actin filaments, and to induce polymerization of G-actin. Here we show that MBP bound to acidic lipids can also bind to both G- and F-actin and cause their sedimentation together with MBP-lipid vesicles. Thus it can simultaneously utilize some of its basic residues to bind to the lipid bilayer and some to bind to actin. The amount of actin bound to the MBP-lipid vesicles decreased with increasing net negative surface charge of the lipid vesicles. It was also less for vesicles containing the lipid composition predicted for the cytosolic surface of myelin than for PC vesicles containing a similar amount of an acidic lipid. Calmodulin caused dissociation of actin from MBP and of the MBP-actin complex from the vesicles. However, it did not cause dissociation of bundles of actin filaments once these had formed as long as some MBP was still present. These results suggest that MBP could be a membrane actin-binding protein in OLs/myelin and its actin binding can be regulated by calmodulin and by the lipid composition of the membrane. Actin binding to MBP decreased the labeling of MBP by the hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[(125)I]iodophenyl)diazirine (TID), indicating that it decreased the hydrophobic interactions of MBP with the bilayer. This change in interaction of MBP with the bilayer could then create a cytosol to membrane signal caused by changes in interaction of the cytoskeleton with the membrane.     

Lipid/myelin basic protein multilayers. A model for the cytoplasmic space in central nervous system myelin

A multilayered complex forms when a solution of myelin basic protein is added to single-bilayer vesicles formed by sonicating myelin lipids. Vesicles and multilayers have been studied by electron microscopy, biochemical analysis, and X-ray diffraction. Freeze-fracture electron microscopy shows well-separated vesicles before myelin basic protein is added, but afterward there are aggregated, possibly multilayered, vesicles and extensive planar multilayers. The vesicles aggregate and fuse within seconds after the protein is added, and the multilayers form within minutes. No intra-bilayer particles are seen, with or without the protein. Some myelin basic protein, but no lipid, remains in the supernatant after the protein is added and the complex sedimented for X-ray diffraction. A rather variable proportion of the protein is bound. X-ray diffraction patterns show that the vesicles are stable in the absence of myelin basic protein, even under high g-forces. After the protein is added, however, lipid/myelin basic protein multilayers predominate over single-bilayer vesicles. The protein is in every space between lipid bilayers. Thus the vesicles are torn open by strong interaction with myelin basic protein. The inter-bilayer spaces in the multilayers are comparable to the cytoplasmic spaces in central nervous system myelins . The diffraction indicates the same lipid bilayer thickness in vesicles and multilayers, to within 1 A. By comparing electron-density profiles of vesicles and multilayers, most of the myelin basic protein is located in the inter-bilayer space while up to one-third may be inserted between lipid headgroups. When cytochrome c is added in place of myelin basic protein, multilayers also form. In this case the protein is located entirely outside the unchanged bilayer. Comparison of the various profiles emphasizes the close and extensive apposition of myelin basic protein to the lipid bilayer. Numerous bonds may form between myelin basic protein and lipids. Cholesterol may enhance binding by opening gaps between diacyl-lipid headgroups.     

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 myelin 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.     

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.     

The intrinsic fluorescence of isolated central-nervous-system myelin-sheath preparations.

The intrinsic fluorescence characteristics of tyrosine and tryptophan residues in the proteins of isolated central-nervous-system myelin were investigated to gain information concerning the location of these residues within the intact membrane system. Tryptophan fluorescence from isolated myelin has an emission maximum at 325 nm that appears to arise from at least two different populations of tryptophan residues. Further evidence for heterogeneity of tryptophan location in the membrane is obtained from quenching studies with chloroform and acrylamide. It is speculated that one tryptophan population is hydrophobically situated and may be derived from the proteolipid protein of myelin, whereas the other tryptophan population is located at the membrane surface and may arise from the extrinsic basic protein. A significant tyrosine fluorescence is detected from isolated myelin, indicating that some of these residues are not quenched by structural interactions within the lipid--protein membrane system. Studies with freeze-dried resuspended myelin suggest that the structural arrangement of protein components in the dried rehydrated membrane system differs significantly from that of the freshly isolated myelin membrane.     

Temperature effects on 1,8-anilinonaphthalene sulfonic acid fluorescence with sarcolemma vesicles.

Increased temperature produces a red shift and decreased fluorescence intensity of the emission peak of 1,8-anilinonaphthalene sulfonic acid (ANS) in suspensions of biomembrane vesicles. These changes have been attributed to a conjectured increase in polarity of the microenvironment of ANS. If the conjecture is correct, fluorescence lifetimes must be decreased with warming. We showed than ANS binds to both protein and lipid protein of sarcolemma, that there are two kinds of sarcolemma-lipid ANS-binding sites, and that there are three fluorescence lifetimes of excited sarcolemma-bound ANS. The three fluroescence lifetimes were unchanged on warming, or decreased too little to account for the observations. Fluorescence lifetime data were consistent with the notion that the effect of increasing temperature is to decrease the amount of ANS bound to sarcolemma. From studies of liposomes prepared from lipid extracts of sarcolemma, and of proteins from sarcolemma it was deduced that warming reducted the amount of ANS bound to both of these sarcolemma components, probably mainly by reducing binding capacity. There might also be a shift of affinities such that the ratio, KA sarcolemma lipid/KA sarcolemma protein, is larger at higher temperature. Except at very small concentration ratios of ANS/sarcolemma, more than twice as much ANS was bound to sarcolemma lipids as to proteins.     

Circular dichroic analysis of the secondary structure of myelin basic protein and derived peptides bound to detergents and to lipid vesicles.

In aqueous solution bovine myelin basic protein exhibits no significant alpha-helical or beta-pleated sheet structure. However, in vivo this protein is associated largely with the myelin membrane: experiments have therefore been performed to determine the structure of the protein when bound to lipid bilayers. Circular dichroism spectra show that this protein undergoes a major conformational change on binding to lipid bilayer vesicles formed from diacylphosphatidylserine or diacylphosphatidic acid, and on binding to micelles of several detergents. Association with diacylphosphatidylcholine failed to induce a structural change: this observation is interpreted in terms of an earlier report that lysophosphatidylcholine does increase the alpha-helical content of basic protein. These circular dichroism measurements and studies of the binding to the bilayer-forming lipids appear to provide support for significant hydrophobic lipid-protein interactions. Similar studies using two peptides produced by cleavf basic protein indicate that a major structure-forming region in the middle of the protein has been disrupted by this scission.     

Interaction of mammalian Hsp22 with lipid membranes

Hsp22/HspB8 is a member of the small heat-shock protein family, whose function is not yet completely understood. Our immunolocalization studies in a human neuroblastoma cell line, SK-N-SH, using confocal microscopy show that a significant fraction of Hsp22 is localized to the plasma membrane. We therefore investigated its interactions with lipid vesicles in vitro. Intrinsic tryptophan fluorescence is quenched in the presence of lipid vesicles derived from either bovine brain lipid extract or purified lipids. Time-resolved fluorescence studies show a decrease in the lifetimes of the tryptophan residues. Both of these results indicate burial of some tryptophan residues of Hsp22 upon interaction with lipid vesicles. Membrane interactions also lead to increase in fluorescence polarization of Hsp22. Gel-filtration chromatography shows that Hsp22 binds stably with lipid vesicles; the extent of binding depends on the nature of the lipid. Hsp22 binds more strongly to vesicles made of lipids containing a phosphatidic acid, phosphatidylinositol or phosphatidylserine headgroup (known to be present in the inner leaflet of plasma membrane) compared with lipid vesicles made of a phosphatidylcholine head-group alone. Far-UV CD spectra reveal conformational changes upon binding to the lipid vesicles or in membrane-mimetic solvent, trifluoroethanol. Thus our fluorescence, CD and gel-filtration studies show that Hsp22 interacts with membrane and this interaction leads to stable binding and conformational changes. The present study therefore clearly demonstrates that Hsp22 exhibits potential membrane interaction that may play an important role in its cellular functions.     

Effect of a phase transition on the binding of 1-anilino-8-naphthalenesulfonate to phospholipid membranes.

The binding of 1-anilino-8-naphthalenesulfonate (ANS) to dipalmitoyl-phosphatidycholine (DPPC)-sonicated vesicles was measured by a fluorimetric method in the vicinity of the gel-to-liquid crystalline phase transition temperature (Tm). A similar measurement was performed on large multimellar DPPC vesicles through equilibrium dialysis. Both measurements demonstrated anomalous dye binding in the temperature region of the Tm and slightly above (prefreezing region). The amount of ANS bound at this temperature region was in excess of what would be expected based on extrapolation of the high temperature binding data; just below the Tm, the amount of bound dye decreased abruptly. The fluorimetric studies on vesicles also indicated that inner monolayer binding of ANS was markedly inhibited below the Tm. The possibility that the increase in bound dye in the prefreezing region was caused by enhanced lateral compressibility, density fluctuations, or additional binding sites at the boundary of transient gel-like clusters is discussed and the general topic of anomalous increases in various membrane processes in the vicinity of a phase transition is briefly reviewed.     

Immune lysis of reconstituted myelin basic protein--lipid vesicles and myelin vesicles

Complement-mediated lysis of reconstituted lipid-myelin basic protein (BP) vesicles and myelin vesicles due to antibody raised against BP and isolated myelin is measured by determination of the amount of a water-soluble spin label, tempocholine chloride, released from the vesicles. The response is shown to be antigen-specific, antibody-dependent, and complement mediated. The relative response to different anti-BP antibody samples is similar to that determined by radioimmunoassay procedures. In contrast to immunoassays with BP in aqueous solution, this method measures immune recognition of the protein in either a synthetic or a natural membranous environment. This is important because this protein has been shown to have a different conformation when bound to lipid bilayers than in aqueous solution and its conformation depends on lipid composition. It is also a more rapid method because no separation of spin label still trapped in the vesicles and that released due to immune lysis is required. In synthetic membranes consisting of sphingomyelin, cholesterol, and an acidic lipid, either phosphatidylglycerol, phosphatidic acid, or phosphatidylserine, the response was greatest when the acidic lipid was phosphatidic acid. The response did not depend significantly on the antigen concentration expressed as molar ratio of BP to sphingomyelin, over the range 0.15:600 to 2:600, although it decreased at molar ratios less than 0.15:600. The antigen density required for immune lysis of vesicles containing this protein antigen is similar to that reported elsewhere for lipid antigens, although the time required for maximal lysis was greater. Both anti-BP and anti-myelin antibodies caused a greater specific complement-mediated response with synthetic vesicles than with myelin vesicles, which may be due to the different lipid and/or protein composition of myelin. Response was also obtained with the myelin vesicles, however, indicating that some determinants of BP can be recognized on the surface of the bilayer in isolated myelin by anti-BP.     

Non-covalent cross-linking of lipid bilayers by myelin basic protein: a possible role in myelin formation.

Myelin basic protein associates with bilayer vesicles of pure egg phosphatidylcholine, L-alpha-dimyristoyl phosphatidylcholine and DL-alpha-dipalmitoyl phosphatidylcholine. Under optimum conditions the vesicles contain 15-18% of protein by weight. The binding to dipalmitoyl phosphatidylcholine is facilitated above its gel-to-liquid crystalline transition temperature. At low ionic strength the protein provokes a large increase in vesicle size and aggregation of these enlarged vesicles. Above a sodium chloride concentration of 0.07 M vesicle fusion is far less marked but aggregation persists. The pH- and ionic strength-dependence of this aggregation follows that of the protein alone; in both cases it occurs despite appreciable electrostatic repulsion between the associated species. A similar interaction was observed with diacyl phosphatidylserine vesicles. These observations, which contrast with earlier reports in the literature of a lack of binding of basic protein to phosphatidylcholine-containing lipids, demonstrate the ability of this protein to interact non-ionically with lipid bilayers. The strong cross-linking of lipid bilayers suggests a role for basic protein in myelin, raising the possibility that the protein is instrumental in collapsing the oligodendrocyte cell membrane and thus initiating myelin formation.