Lateral segregation of proteins induced by cholesterol in bacteriorhodopsin-phospholipid vesicles

Mutation of host delta9 fatty acid desaturase inhibits brome mosaic virus RNA replication between template recognition and RNA synthesis

All positive-strand RNA viruses assemble their RNA replication complexes on intracellular membranes. Brome mosaic virus (BMV) replicates its RNA in endoplasmic reticulum (ER)-associated complexes in plant cells and the yeast Saccharomyces cerevisiae. BMV encodes RNA replication factors 1a, with domains implicated in RNA capping and helicase functions, and 2a, with a central polymerase-like domain. Factor 1a interacts independently with the ER membrane, viral RNA templates, and factor 2a to form RNA replication complexes on the perinuclear ER. We show that BMV RNA replication is severely inhibited by a mutation in OLE1, an essential yeast chromosomal gene encoding delta9 fatty acid desaturase, an integral ER membrane protein and the first enzyme in unsaturated fatty acid synthesis. OLE1 deletion and medium supplementation show that BMV RNA replication requires unsaturated fatty acids, not the Ole1 protein, and that viral RNA replication is much more sensitive than yeast growth to reduced unsaturated fatty acid levels. In ole1 mutant yeast, 1a still becomes membrane associated, recruits 2a to the membrane, and recognizes and stabilizes viral RNA templates normally. However, RNA replication is blocked prior to initiation of negative-strand RNA synthesis. The results show that viral RNA synthesis is highly sensitive to lipid composition and suggest that proper membrane fluidity or plasticity is essential for an early step in RNA replication. The strong unsaturated fatty acid dependence also demonstrates that modulating fatty acid balance can be an effective antiviral strategy.

Lateral diffusion of small compounds in human stratum corneum and model lipid bilayer systems

An image-based technique of fluorescence recovery after photobleaching (video-FRAP) was used to measure the lateral diffusion coefficients of a series of nine fluorescent probes in two model lipid bilayer systems, dimyristoylphosphatidylcholine (DMPC) and DMPC/cholesterol (40 mol%), as well as in human stratum corneum-extracted lipids. The probes were all lipophilic, varied in molecular weight from 223 to 854 Da, and were chosen to characterize the lateral diffusion of small compounds in these bilayer systems. A clear molecular weight dependence of the lateral diffusion coefficients in DMPC bilayers was observed. Values ranged from 6.72 x 10(-8) to 16.2 x 10(-8) cm2/s, with the smaller probes diffusing faster than the larger ones. Measurements in DMPC/cholesterol bilayers, which represent the most thorough characterization of small-solute diffusion in this system, exhibited a similar molecular weight dependence, although the diffusion coefficients were lower, ranging from 1.62 x 10(-8) to 5.60 x 10(-8) cm2/s. Lateral diffusion measurements in stratum corneum-extracted lipids, which represent a novel examination of diffusion in this unique lipid system, also exhibited a molecular weight dependence, with values ranging from 0.306 x 10(-8) to 2.34 x 10(-8) cm2/s. Literature data showed that these strong molecular weight dependencies extend to even smaller compounds than those examined in this study. A two-parameter empirical expression is presented that describes the lateral diffusion coefficient in terms of the solute's molecular weight and captures the size dependence over the range examined. This study illustrates the degree to which small-molecule lateral diffusion in stratum corneum-extracted lipids can be represented by diffusion in DMPC and DMPC/cholesterol bilayer systems, and may lead to a better understanding of small-solute transport across human stratum corneum.

Fluorescently labeled pulmonary surfactant protein C in spread phospholipid monolayers

Pulmonary surfactant, a lipid-protein complex, secreted into the fluid lining of lungs prevents alveolar collapse at low lung volumes. Pulmonary surfactant protein C (SP-C), an acylated, hydrophobic, alpha-helical peptide, enhances the surface activity of pulmonary surfactant lipids. Fluorescein-labeled SP-C (F-SP-C) (3, 6, 12 wt%) in dipalmitoylphosphatidylcholine (DPPC), and DPPC:dipalmitoylphosphatidylglycerol (DPPG) [DPPC:DPPG 7:3 mol/mol] in spread monolayers was studied by epifluorescence microscopy. Mass spectometry of F-SP-C indicated that the protein is partially deacylated and labeled with 1 mol fluorescein/1 mol protein. The protein partitioned into the fluid, or liquid expanded, phase. Increasing amounts of F-SP-C in DPPC or DPPC:DPPG monolayers decreased the size and total amounts of the condensed phase at all surface pressures. Calcium (1.6 mM) increased the amount of the condensed phase in monolayers of DPPC:DPPG but not of DPPC alone, and such monolayers were also perturbed by F-SP-C. The study indicates that SP-C perturbs the packing of neutral and anionic phospholipid monolayers even when the latter systems are condensed by calcium, indicating that interactions between SP-C and the lipids are predominantly hydrophobic in nature.

Cholesterol modulation of molecular activity of reconstituted shark Na+,K(+)-ATPase

The cholesterol content of liposome bilayers has been varied between 0-40 mol% to study the effects on reconstituted Na+,K(+)-ATPase. The maximum hydrolytic activity of reconstituted Na+,K(+)-ATPase was increased by cholesterol at concentrations above 10 mol% for both the physiological Na+/K(+)-exchange reactions, as well as for the partial reactions Na+/Na(+)-exchange and uncoupled Na+ efflux. Omission of cholesterol from the liposome bilayer modified the activation by cytoplasmic Na+, indicating effects on both Vmax and on the Na(+)-affinity. Several other kinetic parameters were found to be strongly influenced as well, most notable the steady-state phosphorylation level, and the characteristics of the phosphorylation/dephosphorylation reactions. These results indicate that cholesterol interacts directly with the Na+,K(+)-ATPase as an essential effector perhaps by affecting its conformational mobility or monomer interaction.

The influence of TNF on the membrane fluidity of tumor cells

Possible TNF (tumor necrosis factor) effects on the membrane fluidity of tumor cells were investigated. Viable tumor cells, TNF sensitive, were obtained from the ascitic form of the SA-1 tumor bearing mice. The influence of in vitro and in vivo treatment of cells with the TNF analog was investigated by EPR (electron paramagnetic resonance). SA-1 cells were spin labeled with the methylester of 5-doxylpalmitate, which primarily dissolves in the membranes. The maximal hyperfine splitting was determined and the empirical correlation time calculated. The results show that TNF significantly decreases the correlation time, i.e. it increases the fluidity of SA-1 cell membranes. Such alteration could contribute to the cytotoxicity of TNF.

Interaction between glycophorin and a spin-labeled cholesterol analogue in reconstituted dimyristoylphosphatidylcholine bilayer vesicles

The interaction between glycophorin and a spin-labeled cholesterol analogue has been investigated by EPR spectroscopy. In vesicles which were reconstituted by the freeze and thaw technique, direct evidence was obtained for a reorganisation of the membrane at low protein content (protein/lipid ratio less than 1:300). From the spin exchange interaction we were able to show a protein-induced clustering of the steroid in fluid and in gel state membranes. Tryptic cleavage of the complete N-terminus of glycophorin vanishes the effect. Whereas the removal of the sialic acid residues by neuraminidase digest had no influence on the EPR spectra. The interaction seems to be cholestane spin label specific since it was not observed with an androstane spin-label.

Changes in receptor response by the effect of disease on membrane fluidity

Currently, changes that occur in receptor function under (patho-) physiological conditions are being investigated. The molecular mechanisms causing the observed variations are largely unknown. It is suggested that the pivotal role of the fluidity of the membrane has been neglected in the literature. It is hypothesized that aberrations in receptor function in diseases that are associated with a concomitant mild oxidative stress can be explained by membrane wavering.

Theory of protein-induced lateral phase separation in lipid membranes

An account is given of the current status of theoretical modeling of the phase equilibria in lipid membranes with intrinsic proteins. Special attention is paid to the description of lateral phase separation, which is important for membrane function since it may lead to biologically differentiated regions. We discuss in particular the mattress-model approach by Mouritsen and Bloom, who take matching between protein and lipid hydrophobic thicknesses as a determining factor for the phase behavior. The model has been developed in the framework of phenomenological thermodynamic solution theory. The predictions of the theory are compared to a variety of experimental measurements, including those of membrane recombinants of the protein content of the reaction center and antenna protein of the bacterial photosynthetic apparatus as well as the erythrocyte band 3 protein. The physical effects of lipid-protein interactions are contrasted to those of lipid-cholesterol interactions. The concept of hydrophobic matching is then used as a basis for discussing a possible relationship between membrane thickness and physiological function.

The opsin family of proteins

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Promotion of tumor antigenicity in EL-4 leukemia cells by hydrostatic pressure

Subjection of EL-4-leukemia cells to hydrostatic pressure of 1200-1500 atm for 15 min increased their weak basal immunogenicity to a potent practical level. Injection of such pressure-treated and irradiated EL-4 cells into syngeneic naive C57Bl/6 mice significantly delayed tumor development and increased survival after subsequent challenge with untreated EL-4 cells. Application of pressure of 1500 atm for a longer period of time (e.g., 120 min) resulted in cell death and a smaller increase in tumor immunogenicity which could be partially accounted for by passive shedding of membrane material. Unlike previously studied tumor cells, incorporation of cholesteryl hemisuccinate (CHS) into the plasma membrane of EL-4 cells increased their apparent tumor immunogenicity only slightly. In addition, isolated EL-4 plasma membranes, untreated, CHS-treated or pressure-treated, as well as the material shed thereof by hydrostatic pressure, were all of weak immunogenicity. Modulation in the projection of surface antigens upon pressure treatment could account for the observed increase in tumor immunogenicity and was monitored via the Thy 1.2 antigen. Fluorescence cell sorting analysis indicated that upon application of 1500 atm for different periods of time the projection of Thy 1.2 progressively and irreversibly increased to a maximal level of about 140% at 15 min. At longer pressurization the availability of Thy 1.2 to antibody binding decreased sharply to levels below that of the untreated cells. It is suggested that pressure promotion of tumor immunogenicity is induced by changes in projection and surface distribution of the relevant antigens.

Protein rotational diffusion measurements on the interaction of bee venom melittin with bacteriorhodopsin in lipid vesicles

The rotational diffusion of bacteriorhodopsin reconstituted into dimyristoylphosphatidylcholine vesicles was measured by the technique of flash-induced transient dichroism. In the presence of melittin, a cell lysing peptide from honey bee (Apis mellifera) venom, dose-dependent loss of rotational mobility was observed. Chemically modified melittin derivatives, in which free amine groups were either acetylated or succinylated, were impaired in their ability to induce immobilisation of bacteriorhodopsin. Bacteriorhodopsin reconstitutions of differing lipid/protein ratio were tested and it was found that the bacteriorhodopsin immobilisation phenomena depended on the melittin/protein ratio, not the melittin/lipid ratio. This suggests that melittin produces its effect via direct interaction with bacteriorhodopsin. A mechanism is proposed in which the aggregation of bacteriorhodopsin is induced by electrostatic attraction between its anionic surface moieties and the highly cationic C-terminal segment of melittin.

Freeze-fracture study of filipin binding in photoreceptor outer segments and pigment epithelium of dystrophic and normal retinas

We have studied sterol distribution in the retinal pigment epithelial (RPE) microvillous and outer segment disc membranes of rats with inherited retinal degeneration (RCS; RCS-p/+) and of normal genetic controls (RCS-rdy+, RCS-rdy+-p/+) by using the polyene antibiotic filipin, which binds specifically to 3-B-hydroxy-sterols, and freeze-fracture techniques. Retinas were perfusion-fixed, incubated with filipin in the same fixative, and prepared routinely for freeze-fracture electron microscopy. In the normal retina, the distribution of filipin binding sites on both RPE microvillous and outer segment disc membranes changes during development. Prior to outer segment elongation and the onset of phagocytosis (10 days postnatal), filipin sterol complexes are homogeneously distributed in both microvillous and outer segment membranes. With the onset of phagocytosis (2 weeks postnatal and later) filipin binding in both tissues forms a proximal-to-distal gradient, and binding sites decrease as distance from the cell body increases. In the normal RPE microvillous membranes, binding sites are numerous proximally and sparse on the distal tips. In the normal outer segment disc membranes, binding sites are often present on the basal discs, but are sparse on the intact apical discs prior to shedding. As the discs are cast off and engulfed by the RPE, however, filipin binding increases on both disc and phagosome membranes. In the dystrophic retina, the distribution of filipin binding sites differs from the normal. First, in the microvillous membranes, the proximal-to-distal gradient in filipin binding is rarely present at 2 weeks postnatal and becomes prominent only after the buildup of membranous debris has begun (3-5 weeks postnatal). Second, as the photoreceptors degenerate and the membrane debris disappears (4 months postnatal), filipin binding on the microvillous membranes becomes relatively sparse and homogeneous. Third, filipin binding on the intact disc membranes does not change with outer segment elongation, and numerous filipin binding sites are present on both apical and basal outer segment disc membranes. Fourth, large aggregates of filipin binding sites occupy the vast expanses of particle-free areas of debris membranes which accumulate between the photoreceptors and the RPE. These changes in the amount and distribution of filipin binding sites in the dystrophic retina add to the evidence that the disease process involves outer segment as well as RPE membranes and suggest that alterations in cholesterol distribution could contribute to the phagocytic defect.

Peak temperature influences on heating rate effect in hyperthermic cytotoxicity

Exponentially growing Chinese hamster V79 cells were heated from 37 degrees C up to 42 degrees C or 43 degrees C at four different heating rates: immediate exposure, 1/2 hour, 1 hour, and 2 hours. The D0's of the heat survival curves for both 42 degrees C and 43 degrees C were determined for the different heating rates. The results show that the decrease in toxicity to cells at 43 degrees C resulting from a slower heating rate appears to be less pronounced than that at 42 degrees C. However, the thermotolerance ratio is found to depend on the specific heating time (the reciprocal of the heating rate), in a linear fashion and it does not seem to depend on the peak temperature. We propose that the difference in the heat exchange on cell membranes at different rates of heating may contribute to the causes for the difference in cell sensitivity to hyperthermia.

Strong interactions between a spin-labeled cholesterol analog and erythrocyte proteins in the human erythrocyte membrane

We have used a spin label analog of cholesterol bearing a nitroxide on the alkyl chain (26-nor-25-doxylcholestanol) to study cholesterol-protein interactions in the human erythrocyte membrane. As judged from the ESR spectrum, the spin label is readily incorporated into the membrane when added from a concentrated ethanolic solution to a cell or ghost suspension. With intact erythrocytes or white ghosts in isotonic buffer, the ESR spectrum is a superposition of a mobile component and a strongly immobilized component (outer hyperfine splitting 61-63 G). The latter corresponds to approx. 45% of the signal, a percentage which is barely affected by varying the temperature between 5 and 37 degrees C. Removal of the cytoskeletal proteins spectrin and actin by low ionic strength treatment or of all extrinsic proteins by alkali treatment of ghosts reduces the immobilized fraction to approx. 25%. The effect of controlled proteolysis of intrinsic proteins was also tested. Pre-treatment of cells with chymotrypsin or pre-treatment of unsealed ghosts with trypsin has no effect on the ESR spectrum obtained with alkali-treated membranes. On the other hand, after chymotrypsin treatment of unsealed ghost, which reduces the band 3 protein to a 17.5 kDa membrane fragment, the strongly immobilized component is no longer observable. These data show that the cholesterol analog 26-nor-25-doxylcholestanol interacts strongly with one or several proteins of the erythrocyte membrane. That the intrinsic protein band 3 is involved is suggested by the disappearance of the immobilized fraction occurring upon chymotrypsin digestion of this protein. Our results are thus consistent with the proposal of a selective cholesterol-band 3 interaction in the erythrocyte membrane (Schubert, D. and Boss, K. (1982) FEBS Lett. 150, 4-8). Our data also suggest that this interaction is influenced by cytoskeletal proteins, an effect which can be explained considering the known linking of band 3 to the erythrocyte cytoskeleton via ankyrin. Experiments have also been carried out with 3-doxylandrostanol, a more commonly used cholesterol spin-label analog. With this spin label, at all temperatures investigated, we found it impossible to demonstrate unambiguously the existence of two spectral components. It is suggested that 26-nor-25-doxylcholestanol is a better reporter of cholesterol behavior in membranes.

Phospholipid-induced changes of gamma-aminobutyric acid transport in cortex grey matter in culture

The function of membrane phospholipids (PL) in the regulation of gamma-aminobutyric acid (GABA) transport and GABA carrier binding has been investigated in organized cultures of rat cerebral cortex. The cellular lipid composition has been changed by growing the cells in a delipidated nutrient solution or by short-term exposure of the cells to PL emulsions. Introduction of PL into the cellular matrix was monitored by analysis of biologically active fluorescently labeled phosphatidylcholine (PC) or phosphatidylethanolamine (PE). Parinaroyl and dansyl derivatives were used. Conditions of maintenance as well as exogenously given PL affected the transport of GABA. Two transport systems were observed, one first-order system and one cooperative system. Saturated species of PC or PE reduced first-order GABA uptake with increase in chain length of the fatty acid residues. The effects of unsaturated PL were dependent upon the polar head. Unsaturated PC enhanced the capacity of the first-order transport of the amino acid. In comparison to cultures grown in lipid-free medium, introduction of diarachinoyl-PC into the cells increased the density of the first-order active transport sites by a factor of 8 and the affinity constant by a factor of 17. Diarachinoyl-PE reduced both kinetic parameters. GABA uptake via the cooperative system was enhanced by the unsaturated PE, not by PC. The role of endogenous PL and their asymmetric distribution was studied by application of phospholipase A2, C, and D. Stimulation of carrier activity was induced by hydrolysis of PL on the external leaflet. Inhibition occurred upon enzymatic degradation of external and cytoplasmic PL. Lipolysis also affected GABA receptor binding, suggesting that the effects observed represent the activity of both classes of binding sites, the carrier and the receptor. However the latter accounted for a small fraction of the binding. Transport of the amino acid was temperature sensitive. The temperature curve was shifted within two discontinuities, appearing in the Arrhenius plot as a function of membrane lipids. The results suggest a partitioning of the proteins between fluid and ordered lipid domains. Displacement of the protein may govern the rate constants and/or the effective protein concentration.

Statistical mechanics of lipid membranes. Protein correlation functions and lipid ordering

An expression is derived for the lipid-mediated intermolecular interaction between protein molecules embedded in a lipid bilayer. It is assumed that protein particles are accommodated by the bilayer, but they distort the lipids in some manner from their equilibrium protein-free configuration. We treat this situation by expanding the free energy density in the plane of the membrane as a Taylor series in some arbitrary parameter and its gradient. Minimization of the total membrane energy for a given particle configuration yields the interparticle interaction energy for that configuration. A test of the model is provided by measurement of the protein-protein pair distribution function from freeze-fracture micrographs of partially aggregated membranes. The measured functions can be simulated by adjustment of two parameters (a) a lipid correlation length that characterizes the distance over which a distortion of the bilayers is transmitted laterally through the bilayer, and (b) a term quantifying the energy of the protein-lipid interaction at the protein-lipid boundary. Correlation lengths obtained by fitting the calculated particle distribution functions to the data are found to be several nanometers. Protein-lipid interaction energies are of the order of a few kT.

Thermotropic lipid phase separations in human erythrocyte ghosts and cholesterol-enriched rat liver plasma membranes

Electron spin resonance (ESR) studies of human erythrocyte ghosts labeled with 5-nitroxide stearate, I(12,3), indicate that a temperature-dependent lipid phase separation occurs with a high onset at 38 degrees C. Cooling below 38 degrees C induces I(12,3) clustering. Similar phase separations were previously identified in human platelet and cholesterol-loaded [cholesterol/phospholipid molar ratio (C/P) = 0.85] rat liver plasma membranes [L.M. Gordon et al., 1983; J. Membrane Biol. 76; 139-149]; these were attributed to redistribution of endogenous lipid components such that I(12,3) is excluded from cholesterol-rich domains and tends to reside in cholesterol-poor domains. Further enrichment of rat liver plasma membranes to C/P ratios of 0.94-0.98 creates an "artificial" system equivalent to human erythrocyte ghosts (C/P = 0.90), using such criteria as probe flexibility, temperature dependent I(12,3) clustering; and polarity of the probe environment. Consequently, cholesterol-rich and -poor domains probably exist in both erythrocyte ghosts and high cholesterol liver membranes at physiologic temperatures. The temperature dependence of cold-induced hypertonic lysis of intact human erythrocytes was examined by incubating cells in 0.9 M sucrose for 10 min at 1 degree C intervals between 9 and 46 degrees C (Stage 1), and then subjecting them to 0 degrees C for 10 min (Stage 2). Plots of released hemoglobin are approx. sigmoidal, with no lysis below 18 degrees C and maximal lysis above 40 degrees C. The protective effect of low temperatures during Stage 1 may be due to the formation of cholesterol-rich domains that alter the bilayer distribution and/or conformation of critical membrane-associated proteins.

Freeze-fracture studies of photoreceptor membranes: new observations bearing upon the distribution of cholesterol

We performed electron microscopy of replicas from freeze-fractured retinas exposed during or after fixation to the cholesterol-binding antibiotic, filipin. We observed characteristic filipin-induced perturbations throughout the disk and plasma membranes of retinal rod outer segments of various species. It is evident that a prolonged exposure to filipin in fixative enhances rather than reduces presumptive cholesterol detection in the vertebrate photoreceptor cell. In agreement with the pattern seen in our previous study (Andrews, L.D., and A. I. Cohen, 1979, J. Cell Biol., 81:215-228), filipin-binding in membranes exhibiting particle-free patches seemed largely confined to these patches. Favorably fractured photoreceptors exhibited marked filipin-binding in apical inner segment plasma membrane topologically confluent with and proximate to the outer segment plasma membrane, which was comparatively free of filipin binding. A possible boundary between these differing membrane domains was suggested in a number of replicas exhibiting lower filipin binding to the apical plasma membrane of the inner segment in the area surrounding the cilium. This area contains a structure (Andrews, L. D., 1982, Freeze-fracture studies of vertebrate photoreceptors, In Structure of the Eye, J. G. Hollyfield and E. Acosta Vidrio, editors, Elsevier/North-Holland, New York, 11-23) that resembles the active zones of the nerve terminals for the frog neuromuscular junction. These observations lead us to hypothesize that these structures may function to direct vesicle fusion to occur near them, in a domain of membrane more closely resembling outer than inner segment plasma membrane. The above evidence supports the views that (a) all disk membranes contain cholesterol, but the particle-free patches present in some disks trap cholesterol from contiguous particulate membrane regions; (b) contiguous inner and outer segment membranes may greatly differ in cholesterol content; and (c) the suggested higher cholesterol in the inner segment than in the outer segment plasma membrane may help direct newly inserted photopigment molecules to the outer segment.

Elevated membrane cholesterol concentrations inhibit glucagon-stimulated adenylate cyclase

A method was devised which increases the cholesterol concentration of rat liver plasma membranes by exchange from cholesterol-rich liposomes at low temperature (4 degrees C). When the cholesterol concentration of liver plasma membranes is increased, there is an increase in lipid order as detected by a decrease in mobility of an incorporated fatty acid spin probe. This is accompanied by an inhibition of adenylate cyclase activity. The various ligand-stimulated adenylate cyclase activities exhibit different sensitivities to inhibition by cholesterol, with inhibition of glucagon-stimulated greater than fluoride-stimulated greater than basal activity. The bilayer-fluidizing agent benzyl alcohol is able to reverse the inhibitory effect of cholesterol on adenylate cyclase activity in full. The thermostability of fluoride-stimulated cyclase is increased in the cholesterol-rich membranes. Elevated cholesterol concentrations abolish the lipid-phase separation occurring at 28 degrees C in native membranes as detected by an incorporated fatty acid spin probe. This causes Arrhenius plots of glucagon-stimulated adenylate cyclase activity to become linear, rather than exhibiting a break at 28 degrees C. It is suggested that the cholesterol contents of both halves of the bilayer are increased by the method used and that inhibition of adenylate cyclase ensues, owing to the increase in lipid order and promotion of protein-protein and specific cholesterol-phospholipid interactions.

Lipid domain structures in biological membranes

Phase separation represents a possibility for segregation of lipidic membrane components into structurally distinct domains. Freeze-fracture electronmicroscopy is a useful method for detection of lipid domains. Indications of a possible domain-nature of structures are a regular pattern within a separated area, a regular outline of such an area and a local modulation of curvature (evagination or invagination). Candidates for domain structures in biological membranes are smooth particle-free areas and arrays of regularly arranged particles. The interpretation of the particle-free areas is more reliable than that of the arrays with regularly arranged particles. Phase separation in biological membranes can be induced experimentally by lowering the temperature, but physiologically the isothermically induced domains are more important. Factors in control of isothermic domain formation are divalent cations, proteins, cholesterol etc. Suggestions on the biological relevance of domain formation concern mainly their role in the mechanism of membrane fusion, but domains in form of transient or stable membrane structures seem to occur also otherwise and disturbances in domain formation or artificially induced domains can be suitable for pathological alterations.

Influence of cholesterol on the rotation and self-association of band 3 in the human erythrocyte membrane

The cholesterol/phospholipid mole ratio (C/P) in the human erythrocyte membrane was varied by incubating cells with liposomes. The rotational mobility of band 3 proteins was measured in these membranes by observing flash-induced transient dichroism of the triplet probe eosin maleimide. Measurements were performed with membranes in which associations of band 3 with cytoskeletal proteins were removed by mild proteolysis with trypsin. It was found that decreasing C/P resulted in a more rapid decay of the flash-induced anisotropy. The anisotropy decay curves were analyzed by curve-fitting procedures, which indicated the existence of different sized small aggregates of band 3. The changes in the decay curves with varying C/P can be explained by an effect of cholesterol on the size distribution of these aggregates. The experiments suggest a possible role of cholesterol in regulating associations between integral membrane proteins.

Relationship between the density distribution of intramembrane particles and electron transfer in the mitochondrial inner membrane as revealed by cholesterol incorporation

A low pH method of liposome-membrane fusion (Schneider et al., 1980, Proc. Natl. Acad. Sci. U. S. A. 77:442) was used to enrich the mitochondrial inner membrane lipid bilayer 30-700% with exogenous phospholipid and cholesterol. By varying the phospholipid-to-cholesterol ratio of the liposomes it was possible to incorporate specific amounts of cholesterol (up to 44 mol %) into the inner membrane bilayer in a controlled fashion. The membrane surface area increased proportionally to the increase in total membrane bilayer lipid. Inner membrane enriched with phospholipid only, or with phospholipid plus cholesterol up to 20 mol %, showed randomly distributed intramembrane particles (integral proteins) in the membrane plane, and the average distance between intramembrane particles increased proportionally to the amount of newly incorporated lipid. Membranes containing between 20 and 27 mol % cholesterol exhibited small clusters of intramembrane particles while cholesterol contents above 27 mol % resulted in larger aggregations of intramembrane particles. In phospholipid-enriched membranes with randomly dispersed intramembrane particles, electron transfer activities from NADH- and succinate-dehydrogenase to cytochrome c decreased proportionally to the increase in distance between the particles. In contrast, these electron-transfer activities increased with decreasing distances between intramembrane particles brought about by cholesterol incorporation. These results indicate that (a) catalytically interacting redox components in the mitochondrial inner membrane such as the dehydrogenase complexes, ubiquinone, and heme proteins are independent, laterally diffusible components; (b) the average distance between these redox components is effected by the available surface area of the membrane lipid bilayer; and (c) the distance over which redox components diffuse before collision and electron transfer mediates the rate of such transfer.

Alamethicin-induced changes in lipid bilayer morphology

We have found that alamethicin, in the absence of an electric field, modifies both the hydrophilic surface and hydrophobic core of lipid bilayers. As shown by freeze-fracture and X-ray diffraction experiments with multiwalled vesicles, alamethicin increases the fluid space between bilayers by as much as 50 nm, and at the same time perturbs the hydrocarbon regions of the bilayers. For suspensions of gel-state lipid treated with alamethicin, uniformly spaced rows of particles cover the fracture faces and corresponding linear arrays of stain-collecting depressions cover the hydrophilic surfaces. In the liquid-crystalline state, alamethicin induces an irregular granular texture on the fracture faces.

Freeze-fracture study of photoreceptor outer segments and pigment epithelium in dystrophic and normal retinas

The intramembrane organization of outer segment (OS) membranes and pigment epithelial (PE) microvilli has been studied in rats (10-17 postnatal days) with inherited retinal degeneration (RCS) and in normal retinas from genetically controlled rats (RCS-rdy+). The OS plasma membranes of both dystrophic and normal retinas are characterized by large particles surrounding circular, particle-free zones on the P-faces (cytoplasmic leaflets) and a sparse distribution of particles on the E-faces (external leaflets). No regional differences in particle distribution are observed in either basal or distal plasma membrane regions. Outer segment disc membranes are characterized by large, densely packed P-face particles and ridged E-faces with very few particles. Small, particle-free patches of membrane are present in the basal disc P-faces of both normal and dystrophic retinas, which Andrews and Cohen ("79) have described as characteristic of newly added disc membrane. In dystrophic retinas, larger, particle-free domains are observed in the distal disc membranes (P-faces) and accumulating membranous debris. In older retinas, which have accumulated more debris, the particle-free domains occupy vast areas of the membrane faces and it is not possible to identify these membranes as belonging to either discs of plasma membranes. No comparable areas of particle-free membrane are observed in the distal discs and OS plasma membranes of normal retinas. Pigment epithelial microvillus membranes are characterized by an intermixture of large and medium-sized particles surrounding irregular particle-free areas, but no differences between normal and dystrophic PE membranes are observed. The changes in particle distribution observed in the dystrophic retinas suggests that the intramembrane molecular composition of older disc membranes has become altered or rearranged as the OS membranes accumulate as debris.

Importance of cholesterol-phospholipid interaction in determining dynamics of normal and abetalipoproteinemia red blood cell membrane

Acanthocytic red blood cells in patients with abetalipoproteinemia have a decrease membrane fluidity that is associated with increased sphingomyelin/phosphatidylcholine (SM/PC) ratios. Here we describe studies designed to gain better insight into (i) the interrelationship between the composition of lipoprotein and red blood cell membrane in abetalipoproteinemia patients and normal controls; and (ii) how the differences in lipid composition of the red blood cell membrane affect its fluidity. The increased SM/PC ratio found in abetalipoproteinemia plasma high density lipoproteins (HDL) (3 times greater than controls) was paralleled by an increase in this ratio in acanthocytic red cells, but to a lesser degree (almost twice greater than control red cells). Cholesterol/phospholipid mole ratios (C/P) were increased 3-fold in abetalipoproteinemia HDL, but only slightly increased in red cells compared to controls values. As in the controls, 80-85% of abetalipoproteinemia red cell sphingomyelin was found to be in the outer half of the erythrocyte membrane. Membrane fluidity was defined in terms of microviscosity (eta) between 5 and 42 degrees C by the fluorescent polarization of 1,6-diphenylhexatriene (DPH) present in erythrocyte ghost membranes. At all temperatures, membrane microviscosity was higher in abetalipoproteinemia ghosts than controls, but these differences decreased at higher temperatures (12.34 vs 9.79 poise, respectively at 10 degrees C; 4.63 vs 4.04 poise at 37 degrees C). These differences were eliminated after oxidation of all membrane cholesterol to cholest-4-en-3-one by incubation with cholesterol oxidase. Following cholesterol oxidation, the membrane microviscosity decreased in patient ghosts more than in normal red blood cells so that at all temperatures no significant differences were present relative to control ghosts, in which the apparent microviscosity was also diminished but to a lesser degree. Therefore, although increased SM/PC ratios in abetalipoproteinemia may be responsible for decreased erythrocyte membrane fluidity, these effects are dependent upon normal interactions of cholesterol with red cell phospholipid.

Membrane lipid heterogeneity associated with acetylcholine receptor particle aggregates in Xenopus embryonic muscle cells

Filipin, digitonin, and saponin react with membrane cholesterol to produce unique membrane alterations (sterol-specific complexes) that are easily discernible in freeze-fracture replicas. We have treated both noninnervated and innervated Xenopus embryonic muscle cells in culture with these agents. Freeze-fracture of these treated muscle cells showed that most areas of the muscle plasma membrane contain sterol-specific complexes (19- to 40-nm protuberances and dimples with filipin, a scalloped appearance with digitonin, or an irregular, rough appearance with saponin). However, these complexes were virtually absent from membrane areas of junctional and nonjunctional aggregates of acetylcholine receptor particles. This result suggests that the membrane matrix of these aggregates is low in cholesterol and that this membrane lipid heterogeneity may be linked to the mechanisms involved in their formation and stabilization on muscle cells in culture.

Absence of filipin-sterol complexes from the membranes of active zones and acetylcholine receptor aggregates at frog neuromuscular junctions

The polyene antibiotic filipin reacts specifically with membrane cholesterol and produces distinctive membrane lesions. We treated frog cutaneous and sartorius muscles with 0.04% filipin in a glutaraldehyde solution with or without prefixation with glutaraldehyde. Freeze-fracture of these muscles revealed numerous 19 to 38-nm protuberances and depressions (filipin-sterol complexes) in most areas of muscle, axon, and Schwann cell membranes. In the presynaptic membrane, however, these filipin-sterol complexes were absent from active zones consisting of ridges bordered with double rows of particles. In the postsynaptic membrane, filipin-sterol complexes were also virtually absent from the areas occupied by aggregates of large particles representing acetylcholine receptors. These results suggest that the membrane regions of active zones and acetylcholine receptor aggregates have a low cholesterol content.