Specificity of lipid-protein interactions as determined by spectroscopic techniques

Interaction of staphylococcal delta-toxin and synthetic analogues with erythrocytes and phospholipid vesicles. Biological and physical properties of the amphipathic peptides

Staphylococcal delta-toxin, a 26-residue amphiphilic peptide is lytic for cells and phospholipid vesicles and is assumed to insert as an amphipathic helix and oligomerize in membranes. For the first time, the relationship between these properties and toxin structure is investigated by means of eight synthetic peptides, one identical in sequence to the natural toxin, five 26-residue analogues and two shorter peptides corresponding to residues 1-11 and 11-26. These peptides were designed by the Edmundson wheel axial projection in order to maintain: (a) the hydrophilic/hydrophobic balance while rationalizing the sequence, (b) the alpha-helical configuration and (c) the common epitopic structure. The fluorescence of the single Trp residue was used to monitor the behaviour of the natural toxin and analogues. All 26-residue analogues were hemolytically active although to a lesser extent than natural toxin. The peptide of residues 11-26 bound lipids weakly and was hemolytic at high concentration. The peptide of residues 1-11 did not bind lipids and was hemolytically inactive. All peptides except the latter cross-reacted in immunoprecipitation tests with the natural toxin. The study of a 26-residue analogue by circular dichroism revealed an alpha-helical configuration in both the free and lipid-bound state. Changes in the fluorescence of the peptides in the presence of lipid micelles and bilayers varied according to the position of the reporter group. When bound to lipids, Trp5, Trp16 and the Fmoc-1 positions of the analogues became buried while Trp15 of the natural toxin and its synthetic replicate remained more exposed. All changes are rationalized by the proposal of an amphipathic helix whose hydrophobic face is embedded within the apolar core of bilayers while the hydrophilic and charged face remains more exposed to solvent.

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.

Influence of proteins on the reorganization of phospholipid bilayers into large domains

Using large (5-10 microns) vesicles formed in the presence of phospholipids fluorescently labeled on the acyl chain and visualized using a fluorescence microscope, charge-coupled-device camera, and digital image processor, we examined the effects of membrane proteins on phospholipid domain formation. In vesicles composed of phosphatidic acid and phosphatidylcholine, incubation with cytochrome c induced the reorganization of phospholipids into large phosphatidic acid-enriched domains with the exclusion of phosphatidylcholine. Cytochrome c binding was demonstrated to be highest in the phosphatidic acid-enriched domain of the vesicle using the absorbance of the heme moiety for visualization. Both binding of cytochrome c and phospholipid reorganization were blocked by pretreatment of the vesicles with 0.1 M NaCl. The pore forming peptide gramicidin was examined for the effects of an integral protein on domain formation. Initially, gramicidin distributed randomly within the vesicle and showed no phospholipid specificity. Phosphatidic acid domain formation in the presence of 2.0 mM CaCl2 or 100 microM cytochrome c was not affected by the presence of 5 mol % gramicidin within the vesicles. In both cases, gramicidin was preferentially excluded from the phosphatidic acid-enriched domain and became associated with phosphatidylcholine-enriched areas of the vesicle. Thus, cytochrome c caused a major reorganization of both the phospholipids and the proteins in the bilayer.

Fatty acid composition of lipids which copurify with band 3

In a previous study (L. R. Maneri and P. S. Low (1988) J. Biol. Chem. 263, 16170-16178) we determined that the anion transport protein, band 3, was significantly stabilized by lipids containing saturated and/or long chain fatty acids. To determine whether this thermodynamic preference is reflected in the composition of lipids tightly associating with the anion transporter in vivo, we have analyzed the fatty acid content of phospholipids co-isolating with the purified integral domain of band 3. Our data demonstrate that although stearic acid comprises only 14% of the bulk lipid fatty acids of the red cell membrane, it constitutes -68% of the fatty acids of lipids co-isolating with band 3. Certain other long chain fatty acids were also enriched in the adherent lipids. These results suggest that the fatty acids which most effectively stabilize band 3 also have the highest affinity for the transport protein.

Perturbation of binary phospholipid mixtures by melittin: a fluorescence and raman spectroscopy study

The effect of melittin on different binary mixtures of phospholipids has been studied by polarization of DPH fluorescence in order to determine if melittin can induce phase separation. Since the interaction between lipids and melittin is sensitive to both electrostatic and hydrophobic forces, we have studied the effect of the acyl chain length and of the polar head group of the lipids. In spite of the difference of the chain length between dipalmitoylphosphatidylcholine (DPPC) and distearoylphosphatidylcholine (DSPC), no phase separation occurs in an equimolar mixture of these lipids in presence of melittin. However, when the charged lipid dipalmitoylphosphatidylglycerol (DPPG) is mixed with either DPPC or DSPC, the addition of melittin leads to phase separation. The DSPC/DPPG/melittin system, which shows a very complex thermotropism, has also been studied by Raman spectroscopy using DPPG with deuteriated chains in order to monitor each lipid independently. The results suggest that the higher affinity of melittin for DPPG leads to a partial phase separation. We propose the formation of DPPG-rich domains perturbed by melittin and peptide-free regions enriched in DSPC triggered by the head group charge and chain-length differences.

Weak interaction of spectrin with phosphatidylcholine-phosphatidylserine multilayers: a 2H and 31P NMR study

Spectrin from human erythrocytes binds to bilayer dispersions of both DMPC and DMPS:DMPC (1:1, w/w). However, no effect of bound spectrin on the conformation of the lipid head groups, as measured from the deuterium quadrupolar splittings of DMPC or DMPS specifically deuterated in the polar head groups, was detected in 1:1 mixtures of the two lipids containing either deuterated DMPC or DMPS. Neither the phase transition of the DMPS:DMPC mixtures, nor the spin-lattice relaxation time (T1) of the deuterated DMPS head group, was affected by spectrin. These results argue against any strong interaction of spectrin with phosphatidylserine and rule out the possibility that spectrin is responsible for the maintenance of PS in the inner monolayer of the erythrocyte membrane during the whole life-span of this cell.

Transbilayer effects of ethanol on fluidity of brain membrane leaflets

Previous work on membrane effects of ethanol focused on fluidization of the bulk membrane lipid bilayer. That work was extended in the present study to an examination of ethanol's effect on lipid domains. Two independent methods were developed to examine the effects of ethanol on the inner and outer leaflets of synaptic plasma membranes (SPM). First, differential polarized phase and modulation fluorometry and selective quenching of diphenyl-1,3,5-hexatriene (DPH) were used to examine individual leaflets. Both limiting anisotropy and rotational relaxation time of DPH in SPM indicated that the outer leaflet was more fluid than the inner leaflet. Second, plasma membrane sidedness selective fluorescent DPH derivatives, cationic 1-[4-(trimethylammonio)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH) and anionic 3-[p-6-phenyl)-1,3,5-hexatrienyl]phenylpropionic acid (PRO-DPH), confirmed this transmembrane fluidity difference. TMA-DPH and PRO-DPH preferentially localized in the inner and outer leaflets of SPM, respectively. Ethanol in vitro had a greater fluidizing effect in the outer leaflet as compared to the inner leaflet. Thus, ethanol exhibits a specific rather than nonspecific fluidizing action within transbilayer SPM domains. This preferential fluidization of the SPM outer leaflet may have a role in ethanol affecting transmembrane signaling in the nervous system.

Sulfogalactolipid binding protein SLIP 1: a conserved function for a conserved protein

We have studied the species and tissue expression of the 68kD sulfogalactolipid binding protein SLIP 1, originally detected in the male germ cells of the rat (Lingwood: Can. J. Biochem. Cell Biol., 63:1077-1085, 1985). Our results show that SLIP 1 has been highly conserved during evolution and is found in the testes of all vertebrates tested. In studies in the rat, we have found that SLIP 1 is, however, tissue restricted, being found only in the brain (also a major site of sulfogalactolipid biosynthesis) in addition to the testis. SLIP 1 was also detected in mammalian oocytes. The SLIP 1 species detected in brain and oocytes retain the sulfogalactolipid-binding characteristics of rat testicular SLIP 1, indicating that, in addition to immunological features, the glycolipid-binding function of SLIP 1 is conserved in these tissues.

Syntheses and properties of circular dichroism active phospholipids

A group of circular dichroism (CD) active phospholipids has been synthesised, in which one or both acyl chains has been replaced with a cinnamoyl or azobenzene chromophore-containing acid. Studies on the structure, CD activity and thermodynamic property of liposome membranes composed of CD active phospholipids were carried out. CD active liposomes were found to be stable, normal liposomes of approximately 550 A diameter based on the electron micrograph and dynamic light scattering, and to have thermodynamic property similar to the conventional phospholipid membranes without serious perturbation by aromatic bulk groups based on DSC. Liposomes composed of phospholipid having two trans-azobenzene chromophores showed an extremely large CD enhancement even well above Tc. This CD enhancement was drastically changed by the presence of cis-azobenzene chromophore and cis-cis isomer content after irradiation was higher than the theoretical value, suggesting the importance of interchromophore interaction in the liposome membranes.

Reconstitution of the nicotinic acetylcholine receptor using a lipid substitution technique

The nicotinic acetylcholine receptor was purified by affinity chromatography in the presence of dioleoylphosphatidylcholine (DOPC). A method for replacing the DOPC with other lipids was developed by using detergent solubilization with a large excess of the new lipid followed by sucrose density gradient centrifugation in detergent-free buffers to separate receptor-lipid complexes from excess lipid and detergent. Homogenous complexes of defined lipid composition could be easily prepared and the efficiency of substitution was independent of lipid type. However, the functional properties of the resulting lipid complexes depended on the lipid composition.

Solid-state 15N-NMR evidence that gramicidin A can adopt two different backbone conformations in dimyristoylphosphatidylcholine model membrane preparations

Using [15N-Val7]gramicidin A it is shown by solid state 15N-NMR that in dimyristoylphosphatidylcholine model membrane preparations evidence is obtained for two different backbone conformations of gramicidin. One of these conformations is the familiar channel state while a second conformation possesses very different dynamic and structural characteristics. The relative amounts of the conformations depend upon the solvent used to initially codissolve peptide and lipid. Furthermore, by incubation of the samples at modestly elevated temperatures a conversion can be induced from the non-channel to the channel state in a lipid environment.

Effect of acceptor membrane phosphatidylcholine on the catalytic activity of bovine liver phosphatidylcholine transfer protein

Protein-mediated transfer of phosphatidylcholine (PC) by bovine liver phosphatidylcholine transfer protein (PC-TP) was examined using a vesicle-vesicle assay system. Donor and acceptor membranes were prepared from Escherichia coli phospholipids and limiting amounts of egg yolk PC. PC transfer between vesicles of E. coli lipid/egg PC was markedly higher than transfer of PC from vesicles of E. coli lipid/egg PC to vesicles of E. coli lipid. Kinetic parameters of the interaction between PC-TP and E. coli lipid vesicles with or without PC was investigated. The apparent dissociation constants of the complex formed between PC-TP and these vesicles were determined kinetically and from double-reciprocal plots of intrinsic PC-TP fluorescence intensity increase versus vesicle concentration. The magnitude of the dissociation constant decreased as the PC content of the vesicles increased from 0 to 5 mol%. In addition, kinetic analysis revealed that the presence of PC in acceptor vesicles increased both the association and dissociation of PC-TP from vesicles. The effect of membrane PC molecules on transfer rates was examined using bis-phosphatidylcholine, a dimeric PC molecule which is not transferred by PC-TP. Rates of PC transfer to acceptor vesicles comprised of E. coli lipid/bis-PC were virtually identical to rates observed with acceptors vesicles prepared from E. coli lipid. The results suggest that transfer of PC by PC-TP is enhanced only when insertion of protein-bound PC occurs concurrently with the extraction of a molecule of membrane PC, i.e., a concerted, one-step catalytic mechanism for phospholipid exchange.

Interaction of ferricytochrome c with zwitterionic phospholipid bilayers: a Raman spectroscopic study

The vibrational Raman spectra of both pure L-alpha-dipalmitoylphosphatidylcholine (DPPC) liposomes and DPPC multilayers reconstituted with ferricytochrome c under varying conditions of pH and ionic strength are reported as a function of temperature. Total integrated band intensities and relative peak height intensity ratios, two spectral scattering parameters used to determine bilayer disorder, are invariant to changes in pH and ionic strength but exhibit a sensitivity to the bilayer concentration of the ferricytochrome c. Protein concentrations were estimated by comparing the 1636 cm-1 resonance Raman line of known ferricytochrome c solutions to intensity values for the reconstituted multilayer samples. Temperature-dependent profiles of the 3100-2800 cm-1 C-H stretching, 1150-1000 cm-1 C-C stretching, 1440 cm-1 CH2 deformation, and 1295 cm-1 CH2 twisting mode regions characteristic of acyl chain vibrations reflect bilayer perturbations due to the weak interactions of ferricytochrome c. The DPPC multilamellar gel to liquid-crystalline phase transition temperature, TM, defined by either the C-H stretching mode I2935/I2880 or the C-C stretching mode I1061/I1090 peak height intensity ratios, is decreased by approximately 4 degrees C for the approximately 10(-4) M ferricytochrome c reconstituted DPPC liposomes. Other spectral features, such as the increase in the 2935 cm-1 C-H stretching mode region and the enhancement of higher frequency CH2 twisting modes, which arise in bilayers containing approximately 10(-4) M protein, are interpreted in terms of protein penetration into the hydrophobic region of the bilayer.

Application of percolation theory principles to the analysis of interaction of adenylate cyclase complex proteins in cell membranes

Lateral protein movement in cell membranes takes place in a medium with 'obstacles'. These obstacles are: (a) aggregates of major integral proteins immobilized by submembranous structures and cytoskeleton, and (b) membrane lipids in the gel phase. Hormonal activation of the adenylate cyclase complex is associated with lateral mobility of the constiutent proteins. Modification of the interaction of these proteins due to variation of the 'fluid' lipid fraction in reticulocyte membranes has been studied. A decrease in the percentage of 'fluid' lipids in membranes resulted in the inhibition (up to the full cessation) of the interaction of beta-adrenoreceptors with regulatory NS-proteins. The interaction of NS-proteins with catalytic proteins stopped as well. On the other hand, an increase in the 'fluid' lipid fraction led to a more intensive interaction. These facts do not arise from the functional damage of interacting proteins. Consequently, hormonal activation of the adenylate cyclase complex depends on the fraction of 'fluid' lipids in the membrane. The data obtained are in conformity with the percolation theory which makes it possible to characterize long-distance protein movement in a medium ('fluid' lipids) containing obstacles. Thus, interacting proteins prove to diffuse within distances greatly exceeding protein sizes. As a consequence, the intrinsic activity of a beta-agonist, isoproterenol, varies from 1 to 0 depending on the 'fluid' lipid fraction. Our findings also suggest that in vitro there are no beta-receptors precoupled with NS-proteins in rat reticulocyte membranes in the absence of guanine nucleotides.

Solution of the nitroxide spin-label spectral overlap problem using pulse electron spin resonance

Short-pulse saturation-recovery (SR) electron spin resonance (ESR) methods have been used to measure the lateral diffusion of a nitroxide-labeled cholesterol analogue (3-spiro-[2'-(N-oxyl-4',4'-dimethyloxazoladine)]-cholestane, CSL) in multilamellar liposomal dispersions. SR experiments were performed on samples containing 14NCSL:15NCSL pairs, and recovery signals were analyzed for initial conditions and multiexponential time constants by computer simulation. Rate equations describing the system were written and solved. The time constants contain combinations of electron spin lattice relaxation times Tle for both isotopes and the Heisenberg exchange rate constant Kx. We have investigated the complication that occurs from overlap of ESR spectral fragments from 14N and 15N moieties. The time constants of the multiexponential signals are independent of ESR line shape and position. From Kx, lateral diffusion constants of CSL in dimyristoylphosphatidylcholine (DMPC) were calculated (D = 1.7 x 10(-8) at 27 degrees C and 2.7 x 10(-8) cm2/s at 37 degrees C). It is shown that short-pulse saturation-recovery methods are able to overcome the ESR spectral overlap problem that is encountered in conventional ESR and continuous wave electron-electron double resonance (CW ELDOR) studies of spin-spin interactions. The present method can be extended to more complex situations involving spin labels in different environments with physical and chemical exchange.

Differential scanning calorimetry and 2H NMR studies of the phase behavior of gramicidin-phosphatidylcholine mixtures

The extents of two-phase coexistence in the phase diagrams of mixtures of gramicidin with 1,2-bis(perdeuteriopalmitoyl)-sn-glycero-3-phosphocholine (DPPC-d62) and with 1,2-bis(perdeuteriomyristoyl)-sn-glycero-3-phosphocholine (DMPC-d54) mixtures have been explored with differential scanning calorimetry (DSC) and deuterium nuclear magnetic resonance (2H NMR). For both systems, increased gramicidin content causes a decrease in transition enthalpy and a broadening of the peak in excess heat capacity at the transition. In DMPC-d54-based mixtures, the broadening is roughly symmetric about the pure lipid transition temperature. Addition of gramicidin to DPPC-d62 extends the excess heat capacity peak on the low-temperature side, resulting in a slightly asymmetric scan. Deuterium NMR spectra showing a superposition of gel and liquid-crystalline components, observed for both mixtures, indicate the presence of two-phase coexistence. For the DPPC-d62-based mixtures, two-phase coexistence is restricted to an approximately 2 degrees C temperature range below the pure transition temperature. For DMPC-d54-based mixtures, the region of two-phase coexistence is even narrower. For both mixtures, beyond a gramicidin mole fraction of 2%, distinct gel and liquid-crystal contributions to the spectra cannot be distinguished. Along with the broad featureless nature of the DSC scan in this region, this is taken to indicate that the transition has been replaced by a continuous phase change. These results are consistent with the existence of a closed two-phase region having a critical concentration of gramicidin below 2 mol%.

Refolding of bacteriorhodopsin in lipid bilayers. A thermodynamically controlled two-stage process

Possible steps in the folding of bacteriorhodopsin are revealed by studying the refolding and interaction of two fragments of the molecule reconstituted in lipid vesicles. (1) Two denatured bacteriorhodopsin fragments have been purified starting from chymotryptically cleaved bacteriorhodopsin. Cleaved bacteriorhodopsin has been renatured from a mixture of the fragments in Halobacterium lipids/retinal/dodecyl sulfate solution following removal of dodecyl sulfate by precipitation with potassium. The renatured molecules have the same absorption spectrum and extinction coefficient as native cleaved bacteriorhodopsin. They are integrated into small lipid vesicles as a mixture of monomers and aggregates. Extended lattices form during the partial dehydration process used to orient samples for X-ray and neutron crystallography. (2) Correct refolding of cleaved bacterioopsin occurs upon renaturation in the absence of retinal. Regeneration of the chromophore and reformation of the purple membrane lattice are observed following subsequent addition of all-trans retinal. (3) The two chymotryptic fragments have been reinserted separately into lipid vesicles and refolded in the absence of retinal. Circular dichroism spectra of the polypeptide backbone transitions indicate that they have regained a highly alpha-helical structure. The kinetics of chromophore regeneration following reassociation have been studied by absorption spectroscopy. Upon vesicle fusion, the refolded fragments first reassociate, then bind retinal and finally regenerate cleaved bacteriorhodopsin. The complex formed in the absence of retinal is kinetically indistinguishable from cleaved bacterioopsin. The refolded fragments in lipid vesicles are stable for months, both as separate entities and after reassociation. These observations provide further evidence that the native folded structure of bacteriorhodopsin lies at a free energy minimum. They are interpreted in terms of a two-stage folding mechanism for membrane proteins in which stable transmembrane helices are first formed. They subsequently pack without major rearrangement to produce the tertiary structure.

Nuclear magnetic resonance methods to characterize lipid-protein interactions at membrane surfaces

Specific molecular interactions that determine many of the functions of a biomembrane have a high probability of occurring at the surface of that membrane. However, unlike their hydrophobic core, the polar-apolar interface of biomembranes has been somewhat neglected experimentally. Reasons for this are that the chemical heterogeneity encountered makes a simple description difficult and that probing the membrane surface often involves a perturbation of those very interactions being studied. Classical methods for obtaining structural information about biomolecules, including X-ray diffraction, electron microscopy, and more recently high-resolution 2D nuclear magnetic resonance techniques are inappropriate for all but the simplest of membrane problems. In an effort to throw light on how membrane surfaces are organized, both architecturally and dynamically, protons in lipids and proteins have been selectively replaced by deuterons and the resultant deuterium NMR spectrum analyzed to give structural and dynamic information about the molecular associations between a range of membrane components. In principle, lipids, proteins, and oligosaccharides can be studied by this method and the information gained related to biochemical integrity and function. With one or two notable exceptions, the majority of the studies reported so far have been on model systems. A comprehensive review of the literature will not be presented here. However, protein-lipid molecular specificity in membranes, peptide-induced lateral separation, and the ionization behavior of deuterated phospholipids and peripheral proteins will all be demonstrated predominantly using deuterium NMR methods. Some suggestions for future work are also presented.

Interactions of lipids and proteins: some general principles

Methods to describe the binding of phospholipids to membrane proteins are described. It is shown that it is difficult to obtain estimates of the number of phospholipids bound to the surface of a membrane protein from ESR experiments in which plots of free to bound spin label (y) vs. molar ratio of lipid to protein are extrapolated to y = 0. The relative advantages and disadvantages of ESR and fluorescence methods for measuring relative binding constants of phospholipids to membrane proteins are discussed. The particular problems associated with comparing binding constants of molecules of very different sizes (e.g., fatty acids and cardiolipin) are described and equations are presented to account for these problems. The possible effects of membrane viscosity and thickness on activity of membrane proteins are discussed, but it is concluded that effects of phospholipid structure on activity can only be understood in terms of a reasonably complete kinetic model for the protein.

Selectivity of lipid-protein interactions

The spin label ESR and intrinsic fluorescence quenching methods of determining the selectivity of interactions of lipids with integral membrane proteins are summarized. The selectivity patterns of phospholipids, fatty acids, and steroids are reviewed for a variety of integral proteins. Where appropriate, correlations are established with biochemical assays of the effects of specific lipids on enzymatic activity and transport function.

A theory of carcinogenesis based on an analysis of the effects of carcinogens

Carcinogenic stimuli appear to act on target cells (and their daughters) by one or more of three mechanisms. The first is by oxidation of membrane component molecules on the extracellular surfaces of their plasma membranes. The second is by chronic and continuous impingement of electrons on the extracellular surfaces of their plasma membranes and the third is by relocation of predominantly basic molecules to the cytoplasmic surfaces of their plasma membranes. This latter effect in turn causes electrostatic attraction of image charged acidic molecules to the extracellular surfaces to balance the transmembrane charge of the target cells. Each of the above mechanisms results in a condition of increased electronegativity of the extracellular surfaces of plasma membranes of the target cells and their daughters. A theory of transformation is advanced based on the above related modes of action and it is used to explain some previously unexplainable properties of tumors.

Influence of cholesterol on equilibrium and dynamic bilayer structure of unsaturated acyl chain phosphatidylcholine vesicles as determined from higher order analysis of fluorescence anisotropy decay

The influence of cholesterol on equilibrium and dynamic bilayer structure in minimally to highly unsaturated phosphatidylcholine (PC) vesicles has been examined by characterization of the dynamic fluorescence properties of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH). Large, unilamellar egg PC, palmitoyloleoyl-PC (POPC), dioleoyl-PC (DOPC), palmitoylarachidonoyl-PC (PAPC), and palmitoyldocosahexaenoyl-PC (P-22:6-PC) vesicles containing no cholesterol or approximately 15 or 30 mol % cholesterol have been examined. Equilibrium and dynamic DPH orientational properties were analyzed according to an orthogonal, bimodal orientational distribution function [Straume, M., & Litman, B.J. (1987) Biochemistry (preceding paper in this issue)]. The same mathematical formalism was applied to TMA-DPH except that probe orientational probability was permitted only in the distribution peak aligned parallel to the bilayer normal. TMA-DPH fluorescence lifetimes were consistently increased by incorporation of cholesterol into these vesicles. Greater acyl chain unsaturation and increasing temperature each promoted reduction of lifetimes in the presence or absence of cholesterol. DPH lifetimes were much less sensitive than those of TMA-DPH to changes in composition or temperature. This behavior is consistent with reduced water penetrability into liquid-crystalline bilayers as cholesterol content is increased and as acyl chain unsaturation and temperature are reduced. Cholesterol also induces substantial equilibrium ordering of the bilayer both at the hydrophobic core and at the bilayer-water interface. DPH orientational distributions were shifted in favor of alignment parallel to the acyl side chains. The distributions of both probes were narrowed in response to incorporation of cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Equilibrium and dynamic structure of large, unilamellar, unsaturated acyl chain phosphatidylcholine vesicles. Higher order analysis of 1,6-diphenyl-1,3,5-hexatriene and 1-[4-(trimethylammonio)phenyl]- 6-phenyl-1,3,5-hexatriene anisotropy decay

Equilibrium and dynamic structural properties of minimally to highly unsaturated acyl chain, large, unilamellar phosphatidylcholine (PC) vesicles have been characterized by the dynamic fluorescence properties of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH). Fluorescence lifetimes and equilibrium and dynamic rotational properties of these probes were analyzed by limited-frequency phase-modulation fluorometry in egg PC, palmitoyloleoyl-PC (POPC), dioleoyl-PC (DOPC), palmitoylarachidonoyl-PC (PAPC), and palmitoyldocosahexaenoyl-PC (P-22:6-PC) vesicles over a temperature range from 5 to 37 degrees C. DPH equilibrium orientational distributions were derived according to a model permitting bimodal orientational distributions in which the parallel probability maximum was aligned parallel to the bilayer normal and the orthogonal probability maximum was oriented parallel to the plane of the bilayer. TMA-DPH orientational distributions were derived according to the same model except that all probability was constrained to the parallel orientation. TMA-DPH fluorescence lifetimes were much more sensitive than those of DPH to variations in acyl chain composition and temperature although the same qualitative behavior was generally observed with both probes. Greater acyl chain unsaturation and higher sample temperatures each gave rise to shorter lifetimes consistent with increased water penetrability into the bilayers. Equilibrium order of the hydrocarbon core (as probed by DPH) and of the interfacial and head group regions of the bilayers (as probed by TMA-DPH) was reduced by increasing levels of unsaturation and by higher sample temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)

Micrometer-scale domains in fibroblast plasma membranes

We have used the technique of fluorescence photobleaching recovery to measure the lateral diffusion coefficients and the mobile fractions of a fluorescent lipid probe, 1-acyl-2-(12-[(7-nitro-2-1, 3-benzoxadiazol-4-yl)aminododecanoyl]) phosphatidylcholine (NBD-PC), and of labeled membrane proteins of human fibroblasts. Values for mobile fractions decrease monotonically with increasing size of the laser spot used for the measurements, over a range of 0.35-5.0 microns. Values for NBD-PC diffusion coefficients increase in part of this range to reach a plateau at larger laser spots. This variation is not an artifact of the measuring system, since the effects are not seen if diffusion of the probe is measured in liposomes. We also find that the distribution of diffusion coefficients measured with small laser spots is heterogeneous indicating that these small spots can sample different regions of the membrane. These regions appear to differ in protein concentration. Our data strongly indicate that fibroblast surface membranes consist of protein-rich domains approximately 1 micron in diameter, embedded in a relatively protein-poor lipid continuum. These features appear in photographs of labeled cell surfaces illuminated by the expanded laser beam.

Nitrate starvation induces homeoviscous regulation of lipids in the cell envelope of the blue-green alga, Anacystis nidulans

Replacement of the normal culture liquid to a nitrate-free medium resulted in an immediate drop in the ratio of protein to lipid in isolated cell envelopes of Anacystis nidulans cells. The relative fluidity of the envelope membranes or liposomes, made from the extracted lipids of the envelope, was estimated by measuring the steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene. A thermotrophic phase transition of lipids within the cytoplasmic membrane of intact cells was also revealed by detecting the temperature-dependent absorption changes in the proportion of zeaxanthin at 390 nm. It became evident that a decrease in the proportion of protein to lipid within the cell envelope was accompanied neither by changes in the microviscosity level, nor by shifting of characteristic temperatures of the liquid-crystalline-to-gel transition of lipids. In parallel with nitrate starvation, however, the proportion of saturated fatty acids of the envelope lipids increased markedly. Accumulation of saturated, longer-chain (C18) fatty acids at the cost of C16 counterparts upon nitrate deprivation occurred in all of the complex lipids. In accordance with these findings, a pronounced decrease in the fluidity was demonstrated for the liposomes prepared from the envelope polar lipids of nitrate-starved cells compared with the corresponding control, throughout the temperature range (45-5 degrees C) studied. We propose that the fluidizing effect due to a fall in the ratio of protein to lipid was compensated by a rapidly triggering regulatory process which enables the preservation of the fluidity characteristics at an optimal level within the cell envelope of A. nidulans.

Shape transformations induced by amphiphiles in erythrocytes

Shape alterations induced in human erythrocytes by cationic, anionic, zwitterionic and nonionic amphiphiles (C10-C16) at antihaemolytic concentrations (CAH50 and CAHmax) and at a slightly lytic concentration (2-10% haemolysis) were studied. Anionic (sodium alkyl sulphates) and zwitterionic amphiphiles (3-(alkyldimethylammonio)-1-propanesulfonates) proved to be potent echinocytogenic agents. Among the nonionic amphiphiles there were potent stomatocytogenicagents (octaethyleneglycol alkyl ethers, pentaethyleneglycol dodecyl ether), one potent echinocytogenic agent (dodecyl D-maltoside) and one weak echinocytogenic agent (decyl beta-D-glucopyranoside). Shape alterations induced by cationic amphiphiles (alkyltrimethylammonium bromides, cetylpyridinium chloride and dodecylamine hydrochloride) showed a strong time-dependence. These amphiphiles immediately induced strongly crenated erythrocytes which during incubation shifted to less crenated erythrocytes or to stomatocytes. All of the echinocytogenic amphiphiles induced echinocytes immediately, and there were only small alterations of the induced shape during incubation. Among the stomatocytogenic amphiphiles there were some that induced stomatocytes immediately or after a short lag time while others first passed the erythrocytes through echinocytic stages before stomatocytic shapes were attained. Erythrocytes treated with amphiphiles did not recover their normal discoid shape following repeated washing and reincubation for 1 h in amphiphile-free medium. Our study shows that shape alterations induced by amphiphiles in erythrocytes cannot be explained solely by assuming a selective intercalation of differently charged amphiphiles into the monolayers of the lipid bilayer as suggested in the bilayer couple hypothesis (Sheetz, M.P. and Singer, S.J. (1976) J. Cell Biol. 70, 247-251). We suggest that amphiphiles, when intercalated into the lipid bilayer, trigger a rapid formation of intrabilayer non-bilayer phases which protect the bilayer against a collapse and bring about a transbilayer redistribution of intercalated amphiphiles as well as of bilayer lipids.

Alterations in lateral lipid mobility in the plasma membrane of urodelean ectodermal cells during gastrulation

The mobility characteristics of lipids were studied in the plasmalemma of dissociated presumptive ectodermal cells from embryos of Pleurodeles Waltl at different stages of development, from early blastula to early neurula, using a Fluorescence Recovery After Photobleaching technique (FRAP), after incorporation of the lipophilic fluorescent probe 5N-(hexadecanoyl)-aminofluoresceine (HEDAF) into the cell plasma membrane. At all stages of development, fluorescence recovery was found to extrapolate to 100%, which suggested that the lipid phase in these plasma membranes can be regarded as dynamically homogeneous (no immobilized fraction). It appears as a continuum over a wide cell surface area, in which lipids are free to move laterally. The lateral diffusion coefficient of the probe, obtained from statistical analysis of the fluorescence recovery data, was found to decrease significantly from blastula to gastrula, slightly increasing at the neurula stage. These changes in the dynamic properties of the lipid probe HEDAF during gastrulation suggest that the lipid phase of the plasma membrane of these ectodermal cells undergo structural changes. The results lend support to the idea that the plasma membrane of these cells is actively involved in the morphogenetic movements which characterize the development of the embryo.

2H-nuclear magnetic resonance investigations on phospholipid acyl chain order and dynamics in the gramicidin-induced hexagonal HII phase

The following results are reported in this paper: The interaction of gramicidin with [11,11-2H2]dioleoylphosphatidylcholine (DOPC) and [11,11-2H2]dioleoylphosphatidylethanolamine (DOPE) at different stages of hydration was studied by 2H- and 31P-nuclear magnetic resonance. In the L alpha phase in excess water the acyl chains of phosphatidylethanolamine (PE) are more ordered than phosphatidylcholine (PC) most likely as the result of the lower headgroup hydration of the former lipid. In excess water gramicidin incorporation above 5 mol % in DOPC causes a bilayer----hexagonal HII phase change. In the HII phase acyl chain order is virtually unaffected by gramicidin but the peptide restricts the fast chain motions. At low water content gramicidin cannot induce the HII phase but it markedly decreases chain order in the DOPC bilayer. Increasing water content results in separation between a gramicidin-poor and a gramicidin-rich L alpha phase with decreased order of the entire lipid molecule. Further increase in hydration reverts at low gramicidin contents the phase separation and at high gramicidin contents results in a direct change of the disordered lamellar to the hexagonal HII phase. Gramicidin also promotes HII phase formation in the PE system but interacts much less strongly with PE than with PC. The results support our hypothesis that gramicidin, by a combination of strong intermolecular attraction forces and its pronounced cone shape, both involving the four tryptophans at the COOH-terminus, has a strong tendency to organize, with the appropriate lipid, in intramembranous cylindrical structures such as is found in the HII phase.

Alzheimer's disease: paired helical filaments and cytomembranes

Observations were made with the electron microscope on biopsies from the frontal cerebral cortex of four patients showing the clinical symptoms of advanced Alzheimer's disease (AD). Sections from all four biopsies showed neuronal dendrites, and to a lesser extent, perikarya, packed with paired helical filaments (PHFs) and a correlated complete loss of the normal content of microtubules. It was usually impossible to visualize the beginning and end of the PHF since it passed out of the plane of section. However, not infrequently, PHFs could be seen apparently arising at or from the surfaces of cytomembranes. Such membranes (in perikarya or dendrite) often formed irregular stacks or lamellated bodies. These membranes were invariably agranular (i.e. not studded with ribosomes) and so might be considered as pathological forms of smooth endoplasmic reticulum (SER). A correlation is established between the known biochemical nature of PHFs and their postulated membrane origin.

Orientation and vertical fluctuations of spin-labeled analogues of cholesterol and androstanol in phospholipid bilayers

We have used ESR and NMR linewidth broadening by spin-labels to determine the overall orientation of spin-labeled analogues of cholesterol and androstanol in egg lecithin bilayers. While the cholesterol analogues were found to have a single orientation in each monolayer, with the acyl chain pointing towards the center of the bilayer, the androstanol analogue appeared, at least in sonicated vesicles, to experience two opposite orientations in the same monolayer, very likely with a rapid reorientation. The possibility of rapid vertical fluctuations of the sterol molecules within the phospholipid bilayer is also discussed.

Preferential lipid association and mode of penetration of apocytochrome c in mixed model membranes as monitored by tryptophanyl fluorescence quenching using brominated phospholipids

The fluorescence of the single tryptophan residue at position 59 in apocytochrome c, the biosynthetic precursor of the inner mitochondrial membrane protein cytochrome c, was studied in small unilamellar vesicles composed of phosphatidylserine (PS) and phosphatidylcholine (PC) with or without specifically Br-labelled acyl chains at the sn-2 position. The protein has a very high affinity for PS-containing vesicles (dissociation constant Kd less than 1 microM). From the relative quenching efficiency by the brominated phospholipids, it could be concluded that the protein specifically associates with the PS component in mixed vesicles and that maximal quenching occurred with phospholipids in which the bromine was present at the 6,7-position of the 2-acyl chain suggesting that (part of) the bound protein penetrates 7-8 A deep into the hydrophobic core of the bilayer.

Modulation of membrane protein function by bilayer lipids

In many instances, the composition of fatty acyl groups of membrane phospholipids can be modified to achieve a range of fatty acyl unsaturation without any detectable change in bulk membrane fluidity. At the same time, the function of membrane proteins may be considerably altered, raising questions concerning the property of the lipids that brings about this altered protein function. There is some evidence that the lipids may be laterally distributed in a heterogeneous manner throughout the membrane, and changes in this distribution could be responsible for the effects on proteins. There is also increasing evidence for specific interactions between individual molecular species and membrane proteins that may also modulate membrane protein function.

Association of ferri- and ferro-cytochrome c with lipid multilayers: a 31P solid-state NMR study

The 31P nuclear magnetic resonance anisotropies of dispersions of diacylphosphatidic acid and diacylphosphatidylserine were slightly increased in the presence of cytochrome c: no increase was observed with cardiolipin. However, the 31P spin-lattice relaxation times (T1) for all of these lipids were reduced markedly by the protein. As similar effects were observed with ferri-cytochrome c and with the reduced protein, which is diamagnetic, we suggest that the changes in T1 reflect a reduction in the spectral density of fast motions for the lipid headgroups attendant on binding of protein, rather than paramagnetic relaxation of the phosphorus nuclear spin.

Ionic strength-dependent alterations of membrane structure of red blood cells

Electron paramagnetic resonance (EPR) measurements using various fatty acid spin labels were performed on membranes of intact human erythrocytes at physiological, and at low ionic strength. In the case of spin probes bearing the nitroxide near the polar head group, a less restricted motion at low ionic strength was seen than with those labels with a nitroxide deeper within the hydrophobic tail of the membrane. Although these data clearly show an influence of ionic strength on membrane structure, and possibly a modified protein-lipid interaction, they cannot be simply discussed in terms of an altered membrane fluidity.

Monte Carlo calculations of order parameter profiles in models of lipid-protein interactions in bilayers

The Monte Carlo method has been utilized to calculate lipid chain order parameters in model monomolecular layers (half-bilayers) containing several different model polypeptides. The systems all consist of a periodic array of identical cells, each containing 35 hydrocarbon chains and 1 "perturbant" (a small model polypeptide or protein). The lipid chains are each 10 CH2 subunits long, have one end constrained to lie in the bilayer plane, and interact via van der Waals forces between all subunits. The chains also interact with the perturbant via van der Waals forces. With standard Monte Carlo procedures order parameter profiles are calculated for chains that are close to the perturbant and for the nonneighboring chains. In order to examine a wide range of possibilities, several different model polypeptides are considered: (i) a rigid smooth cylinder, (ii) a cylinder with identical side chains at alpha-helical positions, (iii) a cylinder with nonidentical side chains at alpha-helical positions, and (iv) a cylinder identical with (ii) but which only extends about halfway through the monolayer. Although results differ for the different systems studied, in all cases only slight conformational differences between the bulk chains and the chains that are nearest the perturbants are found, and it is not possible to characterize the boundary chains as "more ordered" or "less ordered" than the nonboundary chains.

Interaction of cytochrome c with mixed dimyristoylphosphatidylcholine-dimyristoylphosphatidylserine bilayers: a deuterium nuclear magnetic resonance study

Deuterium nuclear magnetic resonance (2H NMR) was used to study the interaction of cytochrome c (from horse heart) with bilayers of mixed dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS). Three types of labeled lipids were used: chain-perdeuterated phosphatidylcholine (DMPC-d54), chain-perdeuterated phosphatidylserine (DMPS-d54), and phosphatidylserine labeled at the alpha-position of the head group (DMPS-d2). Liposomes containing equimolar mixtures of DMPC and DMPS were found to bind cytochrome c with a maximum ratio of about 1 mg of cytochrome c per 1 mg of DMPS. The 2H NMR spectra of equimolar mixtures of DMPC-d54-DMPS and DMPC-DMPS-d54 were examined with and without cytochrome c. No change of the NMR spectra of either DMPC or DMPS could be detected after protein addition, for temperatures both above and below the phospholipid phase transition region. On the other hand, in the liquid-crystalline state, the transverse relaxation time, T2e, was reduced by 30-40% after protein addition. Measurements of the spin-lattice relaxation time, T1, showed, under all circumstances, multiple components. For simplicity, we have examined the shape of the relaxation curves at short and long times. Addition of protein increased by 2-fold the value of the slow T1 component of DMPS-d54 but not that of DMPC-d54. Partially relaxed spectroscopy allowed us to assign this slow component (at least in part) to the methyl group and C2H2 groups near the methyl end of the chains, i.e., far from the binding sites of the extrinsic protein.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of membrane proteins on lipid dynamics

The application of electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) to the study of phospholipid dynamics in membranes is discussed. Using these complementary spectroscopic techniques it is possible to investigate the dynamics of lipids in membranes over a time scale range of from 10(-10) to 1 s. A rather detailed, quantitative description of phospholipid dynamics in pure lipid/water bilayer dispersions has emerged. For example, the correlation time for phosphate group reorientation has been shown to be of the order of 10(-9) s. Chain dynamics can be modelled in terms of three basic types of motion: reorientation about the long axis, fluctuation of the long axis with respect to the bilayer normal, and gauche-trans isomerization about C-C bonds. In the fluid phase, all of these chain motions are in the fast limit on the NMR time scale, but only the gauche-trans isomerization is fast on the EPR time scale. In the gel phase, all of these motions are in the intermediate time scale regime for NMR. While a similarly detailed description of the influence of protein on lipid dynamics has not yet been obtained, these techniques have demonstrated their capability to perform that task. The limited data available suggest that the major effect of protein on lipid dynamics is to increase the relative importance of motions at lower frequency. This is most clearly evident as a slight increase in the correlation time for phosphate group reorientation. The strongest evidence for slower motion of the hydrocarbon chains is from NMR relaxation time and line width measurements. The interpretation of changes in lipid dynamics in terms of protein/lipid interactions will require further studies of protein/lipid phase equilibria as well as molecular dynamics.

Deuterium NMR study of head-group deuterated phosphatidylserine in pure and binary phospholipid bilayers. Interactions with monovalent cations Na+ and Li+

Head-group deuterated 1,2-dimyristoyl-sn-glycero-3-phosphorylserine (DMPS) was synthesized. 2H NMR spectra reflect the ionic strength-dependent polymorphism of DMPS aqueous dispersions. Results obtained with pure DMPS and mixed bilayers with phosphatidylcholine or phosphatidylethanolamine at various NaCl or LiCl concentrations indicate that interactions with Na+ and Li+ have very different effects upon the head-group quadrupole splittings.

Topology of the binding site of blood-clotting factors in model membranes. A fluorescence study

Factors II, X and IX are blood-clotting proteins which bind to phospholipid interfaces in the presence of Ca2+ to activate coagulation. The topology of their binding site on the membrane was investigated in two ways. First, the transition temperature changes of equimolar mixtures of dipalmitoylglycerophosphocholine/phosphatidylserine and dimyristoylglycerophosphocholine/dipalmitoylglycerophosphoserin e were examined by the fluorescence polarization of 1,6-diphenylhexatriene. Results show that Ca2+ triggers a shift of about 3-4 degrees C and that blood-clotting factors further increase this shift by about 1.5 degree C. This suggests that in the gel phase, Ca2+ induces some aggregation of the phosphatidylserine molecules which is reinforced by blood proteins. Second, isothermal energy transfer experiments were performed with natural lipids in their fluid phase. The tryptophan residues of the factors were the energy donors, and pyrene covalently bound to a fatty acid chain of either phosphatidylcholine or phosphatidic acid was the energy acceptor. These pyrene-phospholipids probe either the neutral or the acidic component of phospholipid mixtures. It is concluded that the binding sites of the factors are constituted by both types of lipids and that their composition depends on the membrane. Factor II exhibits some specificity for acidic phospholipids and seems to be surrounded by non-interacting zwitterionic lipids. Factor IX appears to be surrounded by statistically the same amount of charged and zwitterionic lipids. We also demonstrate that binding can also occur without Ca2+. This Ca2+-independent binding probably involves electrostatic and hydrophobic forces but its physiological significance remains to be elucidated.