Ceramide increases free volume voids in DPPC membranes

We have measured by positron annihilation lifetime spectroscopy (PALS) that ceramide increases the size of the free volume holes in DPPC lipid membranes.

Cholesterol modulation of sphingomyelinase activity at physiological temperatures

Bacillus cereus sphingomyelinase activity was assayed on large unilamellar vesicles composed of sphingomyelin (SM)/cholesterol (Ch) mixtures at varying proportions. Natural (egg) SM was used with a gel-fluid transition temperature at ca. 40 degrees C. When the enzyme was assayed at 37 degrees C, the activity on pure SM was exceedingly low, but a small increase was observed as soon as some Ch was added, and a large enhancement of activity occurred with Ch proportions above 25 mol%. The data were interpreted in terms of sphingomyelinase activity being higher in the cholesterol-induced liquid-ordered phase than in the gel phase. The abrupt increase in activity above 25 mol% Ch would occur as a result of a change in domain connectivity, when the Ch-rich liquid-ordered domains coalesced. In equimolar SM/Ch mixtures, that were in the liquid-ordered state in a wide range of temperatures, sphingomyelinase activity was virtually constant in the 30-70 degrees C range. The results demonstrate that at the mammalian and bird physiological temperatures Ch modulates sphingomyelinase activity, and that this can occur precisely because most SM have a gel-fluid transition temperature above the physiological temperature range. In addition, Ch activation of sphingomyelinase and the strong affinity of Ch for SM allow the rapid, localised and self-contained production of the metabolic signal ceramide in specific microdomains (rafts).

Purification and characterization of insulin-mimetic inositol phosphoglycan-like molecules from grass pea (Lathyrus sativus) seeds

BACKGROUND

Signal transduction through the hydrolysis of glycosyl-phosphatidylinositol (GPI) leading to the release of the water-soluble inositol phosphoglycan (IPG) molecules has been demonstrated to be important for mediating some of the actions of insulin and insulin-like growth factor-I (IGF-I).

MATERIALS AND METHODS

In the present study, GPI from grass pea (Lathyrus sativus) seeds has been purified and partially characterized on the basis of its chromatographic properties and its compositional analysis.

RESULTS

The results indicate that it shows similarities to GPI previously isolated from other sources such as rat liver. IPG was generated from L. sativus seed GPI by hydrolysis with a GPI-specific phospholipase D (GPI-PLD). This IPG inhibited protein kinase A (PKA) in an in vitro assay, caused cell proliferation in explanted cochleovestibular ganglia (CVG), and decreased 8-Br-cAMP-induced phosphoenolpyruvate carboxykinase (PEPCK) mRNA expression in cultured hepatoma cells.

CONCLUSIONS

Our data indicate that L. sativus seed IPG possess insulin-mimetic activities. This may explain why L. sativus seeds have been used in some traditional medicines to ameliorate diabetic symptoms.

Early stages of LDL oxidation: apolipoprotein B structural changes monitored by infrared spectroscopy

Changes in the conformation of apoliprotein B-100 in the early stages of copper-mediated low density lipoprotein oxidation have been monitored by infrared spectroscopy. During the lag phase no variation in structure is observed, indicating that copper binding to the protein does not significantly affect its structure. In the propagation phase, while hydroperoxides are formed but the protein is not modified, no changes in secondary structure are observed, but the thermal profile of the band corresponding to alpha-helix is displaced in frequency, indicating changes in tertiary structure associated with this conformation but not with beta-sheet components. When aldehyde formation starts, a decrease of approximately 3% in the area of bands corresponding to alpha-helix and beta-sheet is produced, concomitantly with an increase in beta-turns and unordered structure. The two bands corresponding to beta-turns vary as well under these conditions, indicating changes in these structures. Also at this stage the thermal profile shows variations in frequency for the bands corresponding to both alpha-helix and beta-sheet. The results are consistent with the hypothesis that as soon as the polyunsaturated fatty acids from the particle core are modified, this change is reflected at the surface, in the alpha-helical components contacting the monolayer.

Glycophorin as a receptor for Escherichia coli alpha-hemolysin in erythrocytes

Escherichia coli alpha-hemolysin (HlyA) can lyse both red blood cells (RBC) and liposomes. However, the cells are lysed at HlyA concentrations 1-2 orders of magnitude lower than liposomes (large unilamellar vesicles). Treatment of RBC with trypsin, but not with chymotrypsin, reduces the sensitivity of RBC toward HlyA to the level of the liposomes. Since glycophorin, one of the main proteins in the RBC surface, can be hydrolyzed by trypsin much more readily than by chymotrypsin, the possibility was tested of a specific binding of HlyA to glycophorin. With this purpose, a number of experiments were performed. (a) HlyA was preincubated with purified glycophorin, after which it was found to be inactive against both RBC and liposomes. (b) Treatment of RBC with an anti-glycophorin antibody protected the cells against HlyA lysis. (c) Immobilized HlyA was able to bind glycophorin present in a detergent lysate of RBC ghosts. (d) Incorporation of glycophorin into pure phosphatidylcholine liposomes increased notoriously the sensitivity of the vesicles toward HlyA. (e) Treatment of the glycophorin-containing liposomes with trypsin reverted the vesicles to their original low sensitivity. The above results are interpreted in terms of glycophorin acting as a receptor for HlyA in RBC. The binding constant of HlyA for glycophorin was estimated, in RBC at sublytic HlyA concentrations, to be 1.5 x 10(-9) m.

Diacylglycerol effects on phosphatidylinositol-specific phospholipase C activity and vesicle fusion

Diacylglycerol increased the hydrolytic activity of phosphatidylinositol-specific phospholipase C on large unilamellar vesicles containing 5-40% phosphatidylinositol. Moreover, diacylglycerol increased the rate and extent of vesicle fusion (contents mixing) induced by the enzyme. Kinetic studies of intervesicular lipid mixing revealed that fusion was limited by the frequency of contacts involving two diacylglycerol-rich domains.

Interaction of cholesterol with sphingomyelin in mixed membranes containing phosphatidylcholine, studied by spin-label ESR and IR spectroscopies. A possible stabilization of gel-phase sphingolipid domains by cholesterol

The ESR spectra from different positional isomers of sphingomyelin and phosphatidylcholine spin-labeled in their acyl chain have been studied in sphingomyelin(cerebroside)-phosphatidylcholine mixed membranes that contain cholesterol. The aim was to investigate mechanisms by which cholesterol could stabilize possible domain formation in sphingolipid-glycerolipid membranes. The outer hyperfine splittings in the ESR spectra of sphingomyelin and phosphatidylcholine spin-labeled on the 5 C atom of the acyl chain were consistent with mixing of the components, but the perturbations on adding cholesterol were greater in the membranes containing sphingomyelin than in those containing phosphatidylcholine. Infrared spectra of the amide I band of egg sphingomyelin were shifted and broadened in the presence of cholesterol to a greater extent than the carbonyl band of phosphatidylcholine, which was affected very little by cholesterol. Two-component ESR spectra were observed from lipids spin-labeled on the 14 C atom of the acyl chain in cholesterol-containing membranes composed of sphingolipids, with or without glycerolipids (sphingomyelin/cerebroside and sphingomyelin/cerebroside/phosphatidylcholine mixtures). These results indicate the existence of gel-phase domains in otherwise liquid-ordered membranes that contain cholesterol. In the gel phase of egg sphingomyelin, the outer hyperfine splittings of sphingomyelin spin-labeled on the 14-C atom of the acyl chain are smaller than those for the corresponding spin-labeled phosphatidylcholine. In the presence of cholesterol, this situation is reversed; the outer splitting of 14-C spin-labeled sphingomyelin is then greater than that of 14-C spin-labeled phosphatidylcholine. This result provides some support for the suggestion that transbilayer interdigitation induced by cholesterol stabilizes the coexistence of gel-phase and "liquid-ordered" domains in membranes containing sphingolipids.

Membrane fusion induced by phospholipase C and sphingomyelinases

In the past decade lipid vesicle fusion induced by either bacterial PC-preferring phospholipase C, phosphatidylinositol-specific phospholipase C, sphingomyelinase, or a combination of phospholipase C and sphingomyelinase has been demonstrated. In the present paper, the experimental evidence is reviewed, and discussed in terms of the underlying molecular mechanisms of fusion, and of the possible physiological relevance of these findings.

Spectroscopic techniques in the study of membrane solubilization, reconstitution and permeabilization by detergents

This review focuses on the use of spectroscopic techniques for the study of membrane solubilization, reconstitution, and permeabilization by detergents. Turbidity and light scattering, visible and infrared spectroscopic methods, fluorescence, nuclear magnetic resonance, electron spin resonance and X-ray diffraction are examined from the point of view of their applicability to the above detergent-mediated phenomena. A short introduction is provided about each of the techniques, and references are given for further study.

Lipids favoring inverted phase enhance the ability of aerolysin to permeabilize liposome bilayers

Channel formation by the bacterial toxin aerolysin follows oligomerization of the protein to produce heptamers that are capable of inserting into lipid bilayers. How insertion occurs is not understood, not only for aerolysin but also for other proteins that can penetrate membranes. We have studied aerolysin channel formation by measuring dye leakage from large unilamellar egg phosphatidylcholine vesicles containing varying amounts of other lipids. The rate of leakage was enhanced in a dose-dependent manner by the presence of phosphatidylethanolamine, diacylglycerol, cholesterol, or hexadecane, all of which are known to favor a lamellar-to-inverted hexagonal (L-H) phase transition. Phosphatidylethanolamine molecular species with low L-H transition temperatures had the largest effects on aerolysin activity. In contrast, the presence in the egg phosphatidylcholine liposomes of lipids that are known to stabilize the lamellar phase, such as sphingomyelin and saturated phosphatidylcholines, reduced the rate of channel formation, as did the presence of lysophosphatidylcholine, which favors positive membrane curvature. When two different lipids that favor hexagonal phase were present with egg PC in the liposomes, their stimulatory effects were additive. Phosphatidylethanolamine and lysophosphatidylcholine canceled each other's effect on channel formation.

Leaky vesicle fusion induced by phosphatidylinositol-specific phospholipase C: observation of mixing of vesicular inner monolayers

Large unilamellar vesicles containing phosphatidylinositol (PI), neutral phospholipids, and cholesterol are induced to fuse by the catalytic activity of phosphatidylinositol-specific phospholipase C (PI-PLC). PI cleavage by PI-PLC is followed by vesicle aggregation, intervesicular lipid mixing, and mixing of vesicular aqueous contents. An average of 2-3 vesicles merge into a large one in the fusion process. Vesicle fusion is accompanied by leakage of vesicular contents. A novel method has been developed to monitor mixing of lipids located in the inner monolayers of the vesicles involved in fusion. Using this method, the mixing of inner monolayer lipids and that of vesicular aqueous contents are seen to occur simultaneously, thus giving rise to the fusion pore. Kinetic studies show, for fusing vesicles, second-order dependence of lipid mixing on diacylglycerol concentration in the bilayer. Varying proportions of PI in the liposomal formulation lead to different physical effects of PI-PLC. Specifically, 30-40 mol % PI lead to vesicle fusion, while with 5-10 mol % PI only hemifusion is detected, i.e., mixing of outer monolayer lipids without mixing of aqueous contents. However, when diacylglycerol is included in the bilayers containing 5 mol % PI, PI-PLC activity leads to complete fusion.

Membrane interface-interacting sequences within the ectodomain of the human immunodeficiency virus type 1 envelope glycoprotein: putative role during viral fusion

We have identified a region within the ectodomain of the fusogenic human immunodeficiency virus type 1 (HIV-1) gp41, different from the fusion peptide, that interacts strongly with membranes. This conserved sequence, which immediately precedes the transmembrane anchor, is not highly hydrophobic according to the Kyte-Doolittle hydropathy prediction algorithm, yet it shows a high tendency to partition into the membrane interface, as revealed by the Wimley-White interfacial hydrophobicity scale. We have investigated here the membrane effects induced by NH(2)-DKWASLWNWFNITNWLWYIK-CONH(2) (HIV(c)), the membrane interface-partitioning region at the C terminus of the gp41 ectodomain, in comparison to those caused by NH(2)-AVGIGALFLGFLGAAGSTMGARS-CONH(2) (HIV(n)), the fusion peptide at the N terminus of the subunit. Both HIV(c) and HIV(n) were seen to induce membrane fusion and permeabilization, although lower doses of HIV(c) were required for comparable effects to be detected. Experiments in which equimolar mixtures of HIV(c) and HIV(n) were used indicated that both peptides may act in a cooperative way. Peptide-membrane and peptide-peptide interactions underlying those effects were further confirmed by analyzing the changes in fluorescence of peptide Trp residues. Replacement of the first three Trp residues by Ala, known to render a defective gp41 phenotype unable to mediate both cell-cell fusion and virus entry, also abrogated the HIV(c) ability to induce membrane fusion or form complexes with HIV(n) but not its ability to associate with vesicles. Hydropathy analysis indicated that the presence of two membrane-partitioning stretches separated by a collapsible intervening sequence is a common structural motif among other viral envelope proteins. Moreover, sequences with membrane surface-residing residues preceding the transmembrane anchor appeared to be a common feature in viral fusion proteins of several virus families. According to our experimental results, such a feature might be related to their fusogenic function.

Mixed membranes of sphingolipids and glycerolipids as studied by spin-label ESR spectroscopy. A search for domain formation

The temperature dependences of the ESR spectra from different positional isomers of sphingomyelin and of phosphatidylcholine spin-labeled in their acyl chain have been compared in mixed membranes composed of sphingolipids and glycerolipids. The purpose of the study was to identify the possible formation of sphingolipid-rich in-plane membrane domains. The principal mixtures that were studied contained sphingomyelin and the corresponding glycerolipid phosphatidylcholine, both from egg yolk. Other sphingolipids that were investigated were brain cerebrosides and brain gangliosides, in addition to sphingomyelins from brain and milk. The outer hyperfine splittings in the ESR spectra of sphingomyelin and of phosphatidylcholine spin-labeled on C-5 of the acyl chain were consistent with mixing of the sphingolipid and glycerolipid components, in fluid-phase membranes. In the gel phase of egg sphingomyelin and its mixtures with phosphatidylcholine, the outer hyperfine splittings of sphingomyelin spin-labeled at C-14 of the acyl chain of sphingomyelin are smaller than those of the corresponding sn-2 chain spin-labeled phosphatidylcholine. This is in contrast to the situation with sphingomyelin and phosphatidylcholine spin-labeled at C-5, for which the outer hyperfine splitting is always greater for the spin-labeled sphingomyelin. The behavior of the C-14 spin-labels is attributed to a different geometry of the acyl chain attachments of the sphingolipids and glycerolipids that is consistent with their respective crystal structures. The two-component ESR spectra of sphingomyelin and phosphatidylcholine spin-labeled at C-14 of the acyl chain directly demonstrate a broad two-phase region with coexisting gel and fluid domains in sphingolipid mixtures with phosphatidylcholine. Domain formation in membranes composed of sphingolipids and glycerolipids alone is related primarily to the higher chain-melting transition temperature of the sphingolipid component.

Interactions of the HIV-1 fusion peptide with large unilamellar vesicles and monolayers. A cryo-TEM and spectroscopic study

We have examined the interaction of the human immunodeficiency virustype 1 fusion peptide (23 amino acid residues) and of a Trp-containing analog with vesicles composed of dioleoylphosphatidylcholine, dioleoylphosphatidylethanolamine and cholesterol (molar ratio, 1:1:1). Both the native and the Trp-substituted peptides bound the vesicles to the same extent and induced intervesicular lipid mixing with comparable efficiency. Infrared reflection-absorption spectroscopy data are compatible with the adoption by the peptide of a main beta-sheet structure in a cospread lipid/peptide monolayer. Cryo-transmission electron microscopy observations of peptide-treated vesicles reveal the existence of a peculiar morphology consisting of membrane tubular elongations protruding from single vesicles. Tryptophan fluorescence quenching by brominated phospholipids and by water-soluble acrylamide further indicated that the peptide penetrated into the acyl chain region closer to the interface rather than into the bilayer core. We conclude that the differential partition and shallow penetration of the fusion peptide into the outer monolayer of a surface-constrained bilayer may account for the detected morphological effects. Such single monolayer-restricted interaction and its structural consequences are compatible with specific predictions of current theories on viral fusion.

The pre-transmembrane region of the human immunodeficiency virus type-1 glycoprotein: a novel fusogenic sequence

We have investigated membrane interactions and perturbations induced by NH(2)-DKWASLWNWFNITNWLWYIK-COOH (HIV(c)), representing the membrane interface-partitioning region that precedes the transmembrane anchor of the human immunodeficiency virus type-1 gp41 fusion protein. The HIV(c) peptide bound with high affinity to electrically neutral vesicles composed of dioleoylphosphatidylcholine, dioleoylphosphatidylethanolamine and cholesterol (molar ratio, 1:1:1), and induced vesicle leakage and lipid mixing. Infrared spectra suggest that these effects were promoted by membrane-associated peptides adopting an alpha-helical conformation. A sequence representing a defective gp41 phenotype unable to mediate both cell-cell fusion and virus entry, was equally unable to induce vesicle fusion, and adopted a non-helical conformation in the membrane. We conclude that membrane perturbation and adoption of the alpha-helical conformation by this gp41 region might be functionally meaningful.

Interfacial enzyme activation, non-lamellar phase formation and membrane fusion. Is there a conducting thread?

Previous studies from this laboratory have shown that the enzymic generation of diacylglycerol in bilayers by phospholipase C may lead to membrane fusion through the formation of transient non-lamellar lipidic intermediates. The present paper intends to explore the correlations existing among the three main processes involved, namely (a) the induction (or inhibition) of lamellar-to-non-lamellar phase transitions in lipid mixtures through the addition of small (< 5 mol%) proportions of other lipids, (b) the promotion, by the latter lipids, of fusion in otherwise stable phospholipid vesicles (large unilamellar liposomes) under conditions leading to inverted hexagonal/inverted cubic phase formation in bulk lipid systems, and (c) the modulation, by the same small proportions of lipids, of phospholipase C hydrolysis of phosphatidylcholine in liposome bilayers. It is concluded that phospholipase C may give rise to non-lamellar lipidic structures that in turn permit liposomal fusion to occur, but neither enzyme activity is directly modulated by non-lamellar phase formation, nor will whatever kind of enzyme-induced non-lamellar structure give rise to fusion. Moreover, only under certain kinetic conditions will the enzyme give rise to the organization of non-lamellar structures that are conducive to the fusion event.

Structure and dynamics of membrane proteins as studied by infrared spectroscopy

Infrared (IR) spectroscopy is a useful technique in the study of protein conformation and dynamics. The possibilities of the technique become apparent specially when applied to large proteins in turbid suspensions, as is often the case with membrane proteins. The present review describes the applications of IR spectroscopy to the study of membrane proteins, with an emphasis on recent work and on spectra recorded in the transmission mode, rather than using reflectance techniques. Data treatment procedures are discussed, including band analysis and difference spectroscopy methods. A technique for the analysis of protein secondary and tertiary structures that combines band analysis by curve-fitting of original spectra with protein thermal denaturation is described in detail. The assignment of IR protein bands in H2O and in D2O, one of the more difficult points in protein IR spectroscopy, is also reviewed, including some cases of unclear assignments such as loops, beta-hairpins, or 3(10)-helices. The review includes monographic studies of some membrane proteins whose structure and function have been analysed in detail by IR spectroscopy. Special emphasis has been made on the role of subunit III in cytochrome c oxidase structure, and the proton pathways across this molecule, on the topology and functional cycle of sarcoplasmic reticulum Ca(2+)-ATPase, and on the role of lipids in determining the structure of the nicotinic acetylcholine receptor. In addition, shorter descriptions of retinal proteins and references to other membrane proteins that have been studied less extensively are also included.

Characterization of ATP and DNA binding activities of TrwB, the coupling protein essential in plasmid R388 conjugation

TrwB is the conjugative coupling protein of plasmid R388. TrwBDeltaN70 contains the soluble domain of TrwB. It was constructed by deletion of trwB sequences containing TrwB N-proximal transmembrane segments. Purified TrwBDeltaN70 protein bound tightly the fluorescent ATP analogue TNP-ATP (K(s) = 8.7 microM) but did not show measurable ATPase or GTPase activity. A single ATP binding site was found per TrwB monomer. An intact ATP-binding site was essential for R388 conjugation, since a TrwB mutant with a single amino acid alteration in the ATP-binding signature (K136T) was transfer-deficient. TrwBDeltaN70 also bound DNA nonspecifically. DNA binding enhanced TrwC nic cleavage, providing the first evidence that directly links TrwB with conjugative DNA processing. Since DNA bound by TrwBDeltaN70 also showed increased negative superhelicity (as shown by increased sensitivity to topoisomerase I), nic cleavage enhancement was assumed to be a consequence of the increased single-stranded nature of DNA around nic. The mutant protein TrwB(K136T)DeltaN70 was indistinguishable from TrwBDeltaN70 with respect to the above properties, indicating that TrwB ATP binding activity is not required for them. The reported properties of TrwB suggest potential functions for conjugative coupling proteins, both as triggers of conjugative DNA processing and as motors in the transport process.

Interbilayer lipid mixing induced by the human immunodeficiency virus type-1 fusion peptide on large unilamellar vesicles: the nature of the nonlamellar intermediates

A peptide corresponding to the 23 N-terminal amino acid residues of the human immunodeficiency virus type-1 (HIV-1) gp41 has the capacity to induce intervesicular lipid mixing in large unilamellar liposomes composed of dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylethanolamine (DOPE) and cholesterol (CHOL) (molar ratio, 1:1:1). Cryo-transmission electron microscopy (cryo-TEM) of diluted vesicles to which peptides has been externally added reveals a morphology that is compatible with the formation of nonlamellar lipidic aggregates during the time-course of lipid mixing. 31P-nuclear magnetic resonance and 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (TMADPH) steady-state anisotropy data at equilibrium indicate that the peptide is able to modulate the lipid polymorphism in pelletted membranes by: (i) promoting the thermotropic formation of inverted phases; and (ii) driving the lamellar-to-nonlamellar transition towards the formation of isotropic phases. Therefore, our combined morphological and spectroscopic data reveal the existence of a direct correlation between the ability of the externally added peptide to induce lipid-mixing in dilute liposome samples and its capacity to modulate lipid polymorphism in stacked bilayers.

Effect of Sublytic Concentrations of Sodium Cholate on Phospholipase C Hydrolysis of Phospholipid Bilayers

Phospholipase C activity has been assayed with phosphatidylcholine as substrate in the presence of sodium cholate at concentrations well below those producing lipid solubilization. With short-chain phosphatidylcholine, which exists in monomeric form in aqueous solution, cholate has little or no effect. However, when the substrate is egg phosphatidylcholine in the form of bilayers, small cholate concentrations (below 1 mM, corresponding to an effective surfactant:lipid ratio below 0.05) increase the maximum enzyme rates by about threefold, while decreasing drastically the latency periods of enzyme activity. Previous studies from this laboratory have associated the phospholipase enhancing activity of a variety of amphiphiles to their ability to facilitate the formation of inverted hexagonal phospholipid structures, yet sodium cholate has the opposite effect, stabilizing the lamellar versus the inverted hexagonal phase. This suggests that cholate is activating phospholipase C through a hitherto undescribed mechanism. Sodium cholate concentrations above 1 mM decrease further the enzyme lag time, but they are less effective in enhancing enzyme rates. These observations may be pertinent in the analysis of biochemical data with purified lipases, as well as in physiological studies of biliary function. Copyright 1999 Academic Press.

Protein-induced fusion of phospholipid vesicles of heterogeneous sizes

We have investigated the fusion of phospholipid vesicles induced by lysozyme and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Vesicles were composed of dimyristoylphosphatidylcholine/dioleoylphosphatidylethanolamine/ cholesterol (DMPC:DOPE:Chol, 2:1:1). Small unilamellar vesicles (SUV, diameter ca. 30 nm) obtained by extensive sonication or large unilamellar vesicles (LUV, diameters ranged from 100 to 400 nm) obtained by extrusion methods were used. Fusion of LUV induced by lysozyme and GAPDH was drastically decreased when the diameter of the vesicles increased over a value of 100 nm. Lysozyme effect was stopped at the aggregation step while GAPDH effect was stopped at the fusion (lipid mixing) step. Fusion of heterogeneous vesicle populations (SUV with LUV) was observed only with GAPDH and this happened only when the lipids were in the liquid-crystalline state.

Towards the in vitro reconstitution of caveolae. Asymmetric incorporation of glycosylphosphatidylinositol (GPI) and gangliosides into liposomal membranes

Large unilamellar vesicles consisting of phospholipids with or without cholesterol have been prepared containing GPI and/or gangliosides asymmetrically located in the outer leaflet of the bilayer. Such asymmetric distribution of GPI and gangliosides is found in 'rafts' and caveolae. Using these vesicles, GPI can be readily hydrolysed by phospholipases. Both cholesterol and ganglioside are seen to inhibit, in an additive way, the hydrolytic activity of GPI-specific phospholipase D.

Insertion of Escherichia coli alpha-haemolysin in lipid bilayers as a non-transmembrane integral protein: prediction and experiment

alpha-Haemolysin is an extracellular protein toxin (approximately 107 kDa) secreted by Escherichia coli that acts at the level of the plasma membranes of target eukaryotic cells. The nature of the toxin interaction with the membrane is not known at present, although it has been established that receptor-mediated binding is not essential. In this work, we have studied the perturbation produced by purified alpha-haemolysin on pure phosphatidylcholine bilayers in the form of large unilamellar vesicles, under conditions in which the toxin has been shown to induce vesicle leakage. The bilayer systems containing bound protein have been examined by differential scanning calorimetry, fluorescence spectroscopy, differential solubilization by Triton X-114, and freeze-fracture electron microscopy. All the data concur in indicating that alpha-haemolysin, under conditions leading to cell lysis, becomes inserted in the target membrane in the way of intrinsic or integral proteins. In addition, the experimental results support the idea that inserted alpha-haemolysin occupies only one of the membrane phospholipid monolayers, i.e. it is not a transmembrane protein. The experimental data are complemented by structure prediction studies according to which as many as ten amphipathic alpha-helices, appropriate for protein-lipid interaction, but no hydrophobic transmembrane helices are predicted in alpha-haemolysin. These observations and predictions have important consequences for the mechanism of cell lysis by alpha-haemolysin; in particular, a non-transmembrane arrangement of the toxin in the target membrane is not compatible with the concept of alpha-haemolysin as a pore-forming toxin.

Ceramides in phospholipid membranes: effects on bilayer stability and transition to nonlamellar phases

The effects of ceramides of natural origin on the gel-fluid and lamellar-inverted hexagonal phase transitions of phospholipids (mainly dielaidoylphosphatidylethanolamine) have been studied by differential scanning calorimetry, with additional support from infrared and 31P nuclear magnetic resonance (NMR) spectroscopy. In the lamellar phase, ceramides do not mix ideally with phospholipids, giving rise to the coexistence of domains that undergo the gel-fluid transition at different temperatures. The combination of differential scanning calorimetry and infrared spectroscopy, together with the use of deuterated lipids, allows the demonstration of independent melting temperatures for phospholipid and ceramide in the mixtures. In the lamellar-hexagonal phase transitions, ceramides (up to 15 mol %) decrease the transition temperature, without significantly modifying the transition enthalpy, thus facilitating the inverted hexagonal phase formation. 31P-NMR indicates the coexistence, within a certain range of temperatures, of lamellar and hexagonal phases, or hexagonal phase precursors. Ceramides from egg or from bovine brain are very similar in their effects on the lamellar-hexagonal transition. They are also comparable to diacylglycerides in this respect, although ceramides are less potent. These results are relevant in the interpretation of certain forms of interfacial enzyme activation and in the regulation and dynamics of the bilayer structure of cell membranes.

Structure and functional properties of diacylglycerols in membranes

1. 1,2-Diacyl-sn-glycerols (DAG) are minor components of cell membranes (about 1 mole% of the lipids) and yet they are potent regulators of both the physical properties of the lipid bilayer and the catalytic behaviour of several membrane-related enzymes. 2. In the pure state DAG's present a considerable polymorphism, with several crystalline phases in addition to the neat fluid phase. The most stable crystalline phase is the so-called beta' phase, a monoclinic crystalline form with orthorhombic perpendicular subcell chain packing, in which both acyl chains lie parallel to each other in a hairpinlike configuration about the sn-1 and sn-2 glycerol carbon atoms. The molecules are organized in a bilayer, with the glycerol backbone roughly parallel to the plane of the bilayer, and the acyl chains tilted at approximately 60 degrees with respect to that plane. Acyl chain unsaturation, and particularly a single cis unsaturation, impairs chain packing in mixed-chain DAG's, and this results in an increased number of metastable crystalline phases. 3. DAG's mix with phospholipids in fluid bilayers when their melting temperature is below or close enough to the melting temperature of the bilayer system. When incorporated in phospholipid bilayers, the conformation of DAG is such that the glycerol backbone is nearly perpendicular to the bilayer, with the sn-1 chain extending from the glycerol Cl carbon into the hydrophobic matrix of the bilayer and the sn-2 chain first extending parallel to the bilayer surface, then making a 90 degrees bend at the position of the sn-1 carbonyl to become parallel to the sn-1 chain. DAG's are located in phospholipid bilayers about two CH2 units deeper than the adjacent phospholipids. DAG's mix nonideally with phospholipids, giving rise to in-plane separations of DAG-rich and -poor domains, even in the fluid state. DAG molecules also increase the separation between phospholipid headgroups, and decrease the hydration of the bilayer surface. Also, because the transversal section of the DAG headgroup is small when compared to that of the acyl chains, DAG favours the (negative) curvature of the lipid monolayers, and DAG-phospholipid mixtures tend to convert into inverted nonlamellar hexagonal or cubic phases. 4. A number of membrane enzyme activities are modulated (activated) by DAG, most notably protein kinase C, phospholipases and other enzymes of lipid metabolism. Protein kinase C activation (and perhaps that of other enzymes as well) occurs as the combined result of a number of DAG-induced modifications of lipid bilayers that include: changes in lipid headgroup conformation, interspacing and hydration, changes in the bilayer propensity to form inverted nonlamellar phases, and lateral phase separations of DAG-rich and -poor domains. Among the DAG-activated enzymes, phospholipases C show the peculiarity of yielding the activator DAG as their reaction product, and this allows the self-induced transition from a low- to a high-activity status. 5. DAG's induce or enhance membrane fusion in a number of ways, mainly through partial dehydration of the bilayer surface, increase in lipid monolayer curvature and perhaps lateral phase separation. DAG-increased fusion rates have been demonstrated in several instances of cation-induced fusion of model membranes, as well as in Ca(2+)-induced fusion of chromaffin granules with plasma membrane vesicles. Also phospholipase C has been shown to induce vesicle aggregation and fusion through the catalytic generation of DAG in the bilayers. A rather general property of DAG is that it promotes vesicular or interparticle aggregation. 6. In the living cell, DAG is often generated through phospholipid degradation in response to an extracellular agonist binding a specific receptor in the cell surface. DAG is said to act as an intracellular second messenger. (ABSTRACT TRUNCATED)

Interaction of wheat alpha-thionin with large unilamellar vesicles

The interaction of the wheat antibacterial peptide alpha-thionin with large unilamellar vesicles has been investigated by means of fluorescence spectroscopy. Binding of the peptide to the vesicles is followed by the release of vesicle contents, vesicle aggregation, and lipid mixing. Vesicle fusion, i.e., mixing of the aqueous contents, was not observed. Peptide binding is governed by electrostatic interactions and shows no cooperativity. The amphipatic nature of wheat alpha-thionin seems to destabilize the membrane bilayer and trigger the aggregation of the vesicles and lipid mixing. The presence of distearoylphosphatidylethanolamine-poly(ethylene glycol 2000) (PEG-PE) within the membrane provides a steric barrier that inhibits vesicle aggregation and lipid mixing but does not prevent leakage. Vesicle leakage through discrete membrane channels is unlikely, because the release of encapsulated large fluorescent dextrans is very similar to that of 8-aminonaphthalene-1,3,6,trisulfonic acid (ANTS). A minimum number of 700 peptide molecules must bind to each vesicle to produce complete leakage, which suggests a mechanism in which the overall destabilization of the membrane is due to the formation of transient pores rather than discrete channels.

Infrared studies of protein-induced perturbation of lipids in lipoproteins and membranes

The paper reviews the main recent publications concerning infrared (IR) spectroscopy as applied to the study of lipid-protein interactions in model and cell membranes, lipoproteins, and related systems (e.g. lung surfactant). The review focuses mainly on transmission IR. Based on the available data, a number of general conclusions are presented on the perturbations caused by proteins on either the hydrocarbon chains, the polar headgroups or the interface region. Lipid-protein interactions in native cell membranes do not reveal significant differences from what is observed in semisynthetic model systems.

Vesicle membrane fusion induced by the concerted activities of sphingomyelinase and phospholipase C

When vesicles composed of an equimolar mixture of sphingomyelin, phosphatidylcholine, phosphatidylethanolamine, and cholesterol are treated with phospholipase C, phospholipid hydrolysis occurs without major changes in vesicle architecture. In the same way, addition of sphingomyelinase leads only to sphingomyelin cleavage. However, when both enzymes are added together, their joint hydrolytic activities give rise to leakage-free vesicle aggregation, lipid mixing, and aqueous contents mixing, i.e. vesicle fusion. The contribution of both enzymes is unequal, the main role of sphingomyelinase being the production of relatively large amounts of ceramide that will facilitate the lamellar-to-nonlamellar transition in the formation of the fusion pore, whereas phospholipase C provides mainly a localized, asymmetric, high concentration of diacylglycerol that constitutes the trigger for the fusion process. The lipidic end-products of both enzymes cooperate in destabilizing and fusing the membranes in a way that is never achieved through the action of any of the enzymes individually, nor by the products themselves when premixed with the other lipids during liposome preparation. Thus the enzymes appear to be coupled through their reaction products. This is the first observation of membrane fusion induced by the concerted activities of two enzymes. Besides, considering that both diacylglycerol and ceramide are important metabolites involved in cell signaling, it may also provide new ideas in the exploration of "cross-talk" phenomena between different signal transduction pathways.

Phospholipase C hydrolysis of phospholipids in bilayers of mixed lipid compositions

Phosphatidylcholine phospholipase C (EC 3.1.4.3) from Bacillus cereus has been assayed with substrates in the form of large unilamellar vesicles. Phosphatidylcholine, phosphatidylethanolamine (also a substrate for the enzyme), sphingomyelin, and cholesterol have been mixed in various proportions, in binary, ternary, and quaternary mixtures. A lag period, followed by a burst of enzyme activity, has been found in all cases. The activity burst was always accompanied by an increase in turbidity of the vesicle suspension. Varying lipid compositions while keeping constant all the other parameters leads to a range of lag times extending over 2 orders of magnitude (from 0.13 to 38.0 min), and a similar variability is found in maximal enzyme rates (from 0.40 to 55.9 min-1). Meanwhile, the proportion of substrate that is hydrolyzed during the lag period remains relatively constant at 0.10% moles of total lipid, in agreement with the idea that enzyme activation is linked to vesicle aggregation through diacylglycerol-rich patches. Phosphatidylethanolamine and cholesterol enhance the enzyme activity in a dose-dependent way: they reduce the lag times and increase the maximal rates. The opposite is true of sphingomyelin. These lipids exert each its own peculiar effect, positive or negative, either alone or in combination, so that the susceptibility of a given mixture to the enzyme activity can be to some extent predicted from its composition. Phospholipase C activity is not directly influenced by the formation of nonlamellar structures. However, the presence of lipids with a tendency to form nonlamellar phases, such as phosphatidylethanolamine or cholesterol, stimulates the enzyme even under conditions at which purely lamellar phases exist. Conversely sphingomyelin, a well-known stabilizer of the lamellar phase, inhibits the enzyme. Thus phospholipase C appears to be regulated by the overall geometry and composition of the bilayer.

Detergent solubilisation of phospholipid bilayers in the gel state: the role of polar and hydrophobic forces

Testing the solubilisation of phosphatidylcholine (PC) bilayers by Triton X-100 reveals that in the gel state, but not in the fluid state, the amount of detergent required to solubilise the phospholipid is highly dependent on the chain length. Saturated C16 and C18 PC are virtually insoluble at 4 degreesC. However, addition of water-soluble reagents that perturb hydrogen bonding, e.g. urea, or of small proportions of non-bilayer lipids, make the bilayers amenable to detergent solubilisation, even at low temperatures. These results are relevant in the explanation of the origin of detergent-resistant membrane fragments as found, e.g. in caveolae or 'rafts'.

Phospholipase cleavage of glycosylphosphatidylinositol reconstituted in liposomal membranes

Glycosylphosphatidylinositol (GPI) purified from rat liver lipids was incorporated into lipid bilayers of defined compositions, in the form of large unilamellar vesicles. The GPI concentration in the bilayers was kept constant at 25 mole%, whereas the remaining lipids being phosphatidylcholine, phosphastidylethanolamine, sphingomyelin and/or cholesterol were varied. The resulting liposomes consisted of spherical vesicles, approximately 100 nm in diameter, that could keep their aqueous contents separated from the extravesicular medium. When these liposomes were treated with either Bacillus cereus phosphatidylinositol-phospholipase C, Trypanosoma brucei GPI-phospholipase C, or bovine serum GPI-phospholipase D, GPI was hydrolyzed at different rates, depending on the enzyme and the bilayer lipid composition. These observations open the way to biophysical and biochemical studies of enzymic GPI cleavage under defined conditions. Extensive GPI hydrolysis was observed in certain cases that could allow the use of these systems for the preparation of inositol phosphoglycans, proposed second messengers of a wide variety of hormones, cytokines and growth factors.

E. coli alpha-hemolysin: a membrane-active protein toxin

alpha-Hemolysin is synthesized as a 1024-amino acid polypeptide, then intracellularly activated by specific fatty acylation. A second activation step takes place in the extracellular medium through binding of Ca2+ ions. Even in the absence of fatty acids and Ca2+ HlyA is an amphipathic protein, with a tendency to self-aggregation. However, Ca(2+)-binding appears to expose hydrophobic patches on the protein surface, facilitating both self-aggregation and irreversible insertion into membranes. The protein may somehow bind membranes in the absence of divalent cations, but only when Ca2+ (or Sr2+, or Ba2+) is bound to the toxin in aqueous suspensions, i.e., prior to its interaction with bilayers, can alpha-hemolysin bind irreversibly model or cell membranes in such a way that the integrity of the membrane barrier is lost, and cell or vesicle leakage ensues. Leakage is not due to the formation of proteinaceous pores, but rather to the transient disruption of the bilayer, due to the protein insertion into the outer membrane monolayer, and subsequent perturbations in the bilayer lateral tension. Protein or glycoprotein receptors for alpha-hemolysin may exist on the cell surface, but the toxin is also active on pure lipid bilayers.

Reversible denaturation, self-aggregation, and membrane activity of Escherichia coli alpha-hemolysin, a protein stable in 6 M urea

Escherichia coli alpha-hemolysin (HlyA) is an extracellular protein toxin (107 kDa) whose cell lytic activity may be preserved for months at -20 degreesC in the presence of 6 M urea, although it decays rapidly in urea-free buffers. This paper describes experiments addressed to unravel the role of urea in HlyA stabilization. Urea up to 8 M inhibits the Ca2+-binding and hemolytic activities of the protein, alters its secondary and tertiary structures, and reduces its tendency to self-aggregation. All these changes are largely reversed upon urea removal by dilution or dialysis, suggesting that they are interrelated. Furthermore, the extent of recovery of the native activities and structural features of alpha-hemolysin that follows urea removal increases with the concentration of urea during the previous phase. Thus, it seems that urea elicits the reversible transition of HlyA to a less active but more stable state whose structure differs significantly from that of the native protein. Moreover dialysis equilibration of the protein with buffers containing 3 M urea induces the formation of a molecular form of HlyA 5-10 times more active than the native protein in the absence of urea. This hyperactive intermediate appears to keep the native secondary structure of HlyA, but with a less compact tertiary structure, that increases the number of exchangeable Ca2+ ions under these conditions. Changes in the intrinsic fluorescence of HlyA also support the notion of a conformational change in the high-activity intermediate. The intermediate is only detected when assayed in the presence of Ca2+ and 3 M urea and can bind a large number of calcium ions (approximately 12 vs approximately 3 for the native protein); it shows a large tendency to self-aggregation and presumably, in the presence of membranes, a similar tendency to irreversible insertion, which may be the reason for its high lytic activity.

Topology of sarcoplasmic reticulum Ca2+-ATPase: an infrared study of thermal denaturation and limited proteolysis

Sarcoplasmic reticulum Ca2+-ATPase structure and organization in the membrane has been studied by infrared spectroscopy by decomposition of the amide I band. Besides the component bands assignable to secondary structure elements such as alpha-helix, beta-sheet, etc...., two unusual bands, one at 1,645 cm(-1) in H2O buffer and the other at 1,625 cm(-1) in D2O buffer are present. By perturbing the protein using temperature and limited proteolysis, the band at 1,645 cm(-1) is tentatively assigned to alpha-helical segments located in the cytoplasmic domain and coupled to beta-sheet structure, whereas the band at 1,625 cm(-1) arises probably from monomer-monomer contacts in the native oligomeric protein. The secondary structure obtained is 33% alpha-helical segments in the transmembrane plus stalk domain; 20% alpha-helix and 22% beta-sheet in the cytoplasmic domain plus 19% turns and 6% unordered structure. Thermal unfolding of Ca2+-ATPase is a complex process that cannot be described as a two-state denaturation. The results obtained are compatible with the idea that the protein is an oligomer at room temperature. The loss of the 1,625 cm(-1) band upon heating would be consistent with a disruption of the oligomers in a process that later gives rise to aggregates (appearance of the 1,618 cm(-1) band). This picture would also be compatible with early results suggesting that processes governing Ca2+ accumulation and ATPase activity are uncoupled at temperatures above 37 degrees C, so that while ATPase activity proceeds at high rates, Ca2+ accumulation is inhibited.

Effect of single chain lipids on phospholipase C-promoted vesicle fusion. A test for the stalk hypothesis of membrane fusion

The effect of low proportions (up to 5 mol %) of single-chain lipids on phospholipase C-promoted fusion of large unilamellar vesicles has been investigated with the aim of testing the so-called stalk model of membrane fusion. This model is known in two main versions, the one originally published by Kozlov and Markin [Kozlov, M. M. and Markin, V. S. (1983) Biofizika 28, 255-261] and what is known as the "modified stalk model" [Siegel, D. P. (1993) Biophys. J. 65, 2124-2140], that differ in a number of predictions. In the view of the latter author, hydrocarbons or other nonpolar lipids should help fusion by decreasing the interstitial energy of the stalk connecting the two apposed bilayers. We show that small amounts of hexadecane or squalene increase significantly the fusion rates in our system. Changes in monolayer curvature are the object of different predictions by the original and modified stalk theories. According to the original form, fusion would be promoted by lipids inducing a negative curvature in the closest (cis) monolayers of the fusing membranes and inhibited by the same lipids in the trans monolayers; the opposite would happen with lipids inducing a positive curvature. The modified stalk model predicts that fusion is helped by increasing the negative curvature of both monolayers. In our system, symmetrically distributed arachidonic acid, which increases the negative curvature, enhances lipid and content mixing, and the opposite is found with symmetrically distributed lysophosphatidylcholine or palmitoylcarnitine, which facilitate a positive monolayer curvature. In addition, fluorescence polarization and 31P NMR studies of the lamellar-to-isotropic (Q224 cubic) thermotropic transition of a lipid mixture corresponding to our liposomal composition reveal that all lipids that facilitate fusion decrease the transition temperature, while fusion inhibitors increase the transition temperature. Moreover, fusion (content mixing) rates show a maximum at the lamellar-to-isotropic transition temperature. These observations support the involvement of inverted lipid structures, as occurring in the inverted cubic phases, in membrane fusion. All these data are in full agreement with the stalk model of membrane fusion, particularly in its modified version.

Phosphatidylinositol-dependent membrane fusion induced by a putative fusogenic sequence of Ebola virus

The membrane-interacting abilities of three sequences representing the putative fusogenic subdomain of the Ebola virus transmembrane protein have been investigated. In the presence of calcium, the sequence EBO(GE) (GAAIGLAWIPYFGPAAE) efficiently fused unilamellar vesicles composed of phosphatidylcholine, phosphatidylethanolamine, cholesterol, and phosphatidylinositol (molar ratio, 2:1:1:0.5), a mixture that roughly resembles the lipid composition of the hepatocyte plasma membrane. Analysis of the lipid dependence of the process demonstrated that the fusion activity of EBO(GE) was promoted by phosphatidylinositol but not by other acidic phospholipids. In comparison, EBO(EA) (EGAAIGLAWIPYFGPAA) and EBO(EE) (EGAAIGLAWIPYFGPAAE) sequences, which are similar to EBO(GE) except that they bear the negatively charged glutamate residue at the N terminus and at both the N and C termini, respectively, induced fusion to a lesser extent. As revealed by binding experiments, the glutamate residue at the N terminus severely impaired peptide-vesicle interaction. In addition, the fusion-competent EBO(GE) sequence did not associate significantly with vesicles lacking phosphatidylinositol. Tryptophan fluorescence quenching by vesicles containing brominated phospholipids indicated that the EBO(GE) peptide penetrated to the acyl chain level only when the membranes contained phosphatidylinositol. We conclude that binding and further penetration of the Ebola virus putative fusion peptide into membranes might be governed by the nature of the N-terminal residue and by the presence of phosphatidylinositol in the target membrane. Moreover, since insertion of such a peptide leads to membrane destabilization and fusion, the present data would be compatible with the involvement of this sequence in Ebola virus fusion.

Calcium-dependent conformation of E. coli alpha-haemolysin. Implications for the mechanism of membrane insertion and lysis

Previous studies from this laboratory had shown that calcium ions were essential for the membrane lytic activity of E. coli alpha-haemolysin (HlyA), while zinc ions did not sustain such a lytic activity. The present data indicate that calcium-binding does not lead to major changes in the secondary structure, judging from circular dichroism spectra. However binding to Ca2+ exposes new hydrophobic residues at the protein surface, as indicated by the increased binding of the fluorescent probe aniline naphtholsulphonate (ANS), and by the increased tendency of the Ca2+-bound protein to self-aggregate. In addition zinc ions are seen to decrease the thermal stability of HlyA which, according to intrinsic fluorescence and differential scanning calorimetry data, is stable below 95 degrees C when bound to calcium, while it undergoes irreversible denaturation above 60 degrees C in the zinc-bound form. Binding to phosphatidylcholine bilayers is quantitatively similar in the presence of both cations, but about one-third of the zinc-bound HlyA is released in the presence of 2 M NaCl. Differential scanning calorimetry of dimyristoylglycerophosphocholine large unilamellar vesicles reveals that Zn2+-HlyA interaction with the lipid bilayer has a strong polar component, while Ca2+-HlyA appears to interact mainly through hydrophobic forces. Experiments in which HIyA transfer is measured from phospholipid vesicles to red blood cells demonstrate that Ca2+ ions promote the irreversible binding of the toxin to bilayers. All these data can be interpreted in terms of a specific Ca2+ effect that increases the surface hydrophobicity of the protein, thus facilitating its irreversible bilayer insertion in the fashion of intrinsic membrane proteins.

TrwD, a protein encoded by the IncW plasmid R388, displays an ATP hydrolase activity essential for bacterial conjugation

A 1.7-kilobase pair segment from the conjugative transfer region of plasmid R388 DNA was cloned and sequenced. It contained trwD, a gene essential for plasmid R388 conjugation, for expression of the conjugative W-pilus and for sensitivity to phage PRD1. The deduced amino acid sequence of TrwD showed homology to the PulE/VirB11 superfamily of potential ATPases involved in various types of transport processes. A fusion of trwD with the glutathione S-transferase (GST) was constructed, and the resulting fusion protein was purified from overproducing bacteria. Factor Xa hydrolysis of GST-TrwD and further purification rendered TrwD protein with more than 95% purity. Antibodies raised against TrwD localized it both in the soluble fraction and in the outer membrane of Escherichia coli. TrwD is probably a peripheral outer membrane protein because it could be solubilized by increasing salt concentration to 0.5 M NaCl in the lysis buffer. Both purified GST-TrwD and TrwD could hydrolize ATP. ATPase activity increased 2-fold in the presence of detergent-phospholipid mixed micelles. To study the importance of the nucleotide-binding site, Walker box A (GXXGXGK(T/S)), present in TrwD, the conserved lysine residue was replaced by glutamine. The mutant protein, expressed and purified under the same conditions as the wild type, did not exhibit ATPase activity. TrwD(K203Q) was not able to complement the mutation in trwD of the R388 mutant plasmid, suggesting the essentiality of the ATPase activity of the protein in the conjugative process. Furthermore, the dominant character of this mutation suggested that GST-TrwD(K432Q) was still able to interact either with itself or with other component(s) of the conjugative machinery.

Permeabilization and fusion of uncharged lipid vesicles induced by the HIV-1 fusion peptide adopting an extended conformation: dose and sequence effects

The peptide HIV(arg), corresponding to a sequence of 23 amino acid residues at the N-terminus of HIV-1 gp41 (LAV1a strain), has the capacity to destabilize negatively charged large unilamellar vesicles. As revealed by infrared spectroscopy, the peptide associated with those vesicles showed conformational polymorphism: in the absence of cations the main structure was a pore-forming alpha-helix, whereas in the presence of Ca2+ the conformation switched to a fusogenic, predominantly extended beta-type structure. Here we show that an extended structure can also be involved in electrically neutral vesicle destabilization induced by the HIV-1 fusion peptide when it binds the vesicle from the aqueous phase. In the absence of cations, neutral liposomes composed of phosphatidylcholine, phosphatidylethanolamine, and cholesterol (molar ratio 1:1:1) selected for an extended structure that became fusogenic in a dose-dependent fashion. At subfusogenic doses this structure caused the release of trapped 8-aminonaphtalene-1,3,6-trisulfonic acid sodium salt/p-xylenebis(pyridinium)bromide from liposomes, indicating the existence of a peptide-mediated membrane destabilizing process before and independent of the development of fusion. When compared to HIV(arg), the fusion activity of HIV(ala) (bearing the R22 --> A substitution) was reduced by 70%. Fusogenicity was completely abolished when a second substitution (V2 --> E) was included to generate HIV(ala-E2), a sequence representing the N-terminus of an inactive gp41. However, the three sequences associated with vesicles to the same extent, and the three adopted a similar extended structure in the membrane. Whereas 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene emission anisotropy was unaffected by the three peptides, DPH emission anisotropy in membranes was increased only by the fusogenic sequences. Taken together, our observations strongly argue that it is not an alpha-helical but an extended structure adopted by the HIV-1 fusion peptide what actively destabilizes cholesterol-containing, electrically neutral membranes. Moreover, membrane destabilization is modulated by the amino acid sequence in the extended structure. The effect displayed by the aforementioned V2 --> E substitution suggests that the fusion process described here could be reflecting a physiologically relevant phenomenon.

Membrane fusion induced by the HIV type 1 fusion peptide: modulation by factors affecting glycoprotein 41 activity and potential anti-HIV compounds

Peptides representing a sequence of 23 amino acid residues at the N terminus of human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp41 bind and subsequently induce fusion of large unilamellar vesicles (LUV), an activity presumably related to gp41 function in viral infection. These in vitro effects can be modulated by several factors that are known to affect HIV-1 infectivity and gp41-mediated virus-cell fusion. Peptide-induced membrane fusion but not peptide binding can be inhibited by two factors known to block gp41 activity: a polar amino acid substitution V --> E in position 2 and the presence of the N-terminal hexapeptide of gp41 in addition to the parent sequence. Whereas inclusion of the alternative gp120 receptor galactosylceramide in membranes has virtually no effect, membrane cholesterol stimulates fusion activity. In view of its putative physiological relevance, we have used the fusion activity of the peptides as a tool to evaluate the inhibitory effect of antivirals that might target this sequence. We describe three dissimilar effects: Amphotericin B inhibits in a cholesterol-independent way peptide-induced fusion but not binding, human serum albumin inhibits binding and consequently fusion, and dextran sulfate (M(r) 5000) does not affect either binding or fusion.

Balance of electrostatic and hydrophobic interactions in the lysis of model membranes by E. coli alpha-haemolysin

The relative weight of electrostatic interactions and hydrophobic forces in the process of membrane disruption caused by E. coli alpha-haemolysin (HlyA) has been studied with a purified protein preparation and a model system consisting of large unilamellar vesicles loaded with water-soluble fluorescent probes. Vesicles were prepared in buffers of different ionic strengths, or pHs, and the net surface charge of the bilayers was also modified by addition of negatively (e.g., phosphatidylinositol) or positively (e.g., stearylamine) charged lipids. The results can be interpreted in terms of a multiple equilibrium in which alpha-haemolysin may exist: aggregated HlyA <==> monomeric HlyA <==> membrane-bound HlyA. In these equilibria both electrostatic and hydrophobic forces are significant. Electrostatic forces become substantial under certain circumstances, e.g., membrane binding when bilayer and protein have opposite electric charges. Protein adsorption to the bilayer is more sensitive to electrostatic forces than membrane disruption itself. In the latter case, the irreversible nature of protein insertion may overcome electrostatic repulsions. Also of interest is the complex effect of pH on the degree of aggregation of an amphipathic toxin like alpha-haemolysin, since pH changes are not only influencing the net protein charge but may also be inducing protein conformational transitions shown by changes in the protein intrinsic fluorescence and in its susceptibility to protease digestion, that appear to regulate the presence of hydrophobic patches at the surface of the molecule, thus modifying the ability of the toxin to either aggregate or become inserted in membranes.

Poly(ethylene glycol)-lipid conjugates inhibit phospholipase C-induced lipid hydrolysis, liposome aggregation and fusion through independent mechanisms

Poly(ethylene glycol)-phosphatidylethanolamine (PEG-PE) conjugates have been introduced in liposomal compositions. The resulting large unilamellar vesicles were subjected to the action of phospholipase C. Enzyme-promoted vesicle aggregation and fusion were assayed in liposomes containing various proportions of PEG-PE. At PEG-PE concentrations above 1 mol% the rate of phospholipid hydrolysis decreases, perhaps because the PEG moiety hinders the enzyme from reaching the membrane surface. At concentrations above 0.1 mol% vesicle aggregation occurs at a slower rate, presumably because of the repulsive barrier properties or surface-grafted PEG. Lipid mixing decreases in parallel with vesicle aggregation. Finally, liposomal fusion rates measured as mixing of vesicle aqueous contents are decreased at or even below 0.1 mol%. The latter inhibition is due, apart from the reduced rates of lipid hydrolysis, vesicle aggregation and lipid mixing, to a PEG-PE-based stabilization of the lipid bilayer structure. Thus the observed low rates of contents mixing arise from three combined and independent inhibitory effects of PEG-PE.

Differential effects of five types of antipathogenic plant peptides on model membranes

The effects of five antipathogenic plant peptides, wheat alpha-thionin, potato PTH1 defensin, barley LTP2 lipid transfer protein, and potato tuber DL1 and DL2 defensins, have been tested against phospholipid vesicles (liposomes). Wheat thionin very actively induces aggregation and leakage of negatively charged vesicles. LTP2 displays the same activities, although to a limited extent. Under certain conditions PTH1 and DL2 induce vesicle aggregation, but not leakage. Potato defensin DL1 failed to show any effect on liposomes. The same peptides have been assayed against a plant pathogenic bacterium, both the membrane-active and -inactive compounds having efficient antibacterial action.

Morphological changes induced by phospholipase C and by sphingomyelinase on large unilamellar vesicles: a cryo-transmission electron microscopy study of liposome fusion

Cryo-transmission electron microscopy has been applied to the study of the changes induced by phospholipase C on large unilamellar vesicles containing phosphatidylcholine, as well as to the action of sphingomyelinase on vesicles containing sphingomyelin. In both cases vesicle aggregation occurs as the earliest detectable phenomenon; later, each system behaves differently. Phospholipase C induces vesicle fusion through an intermediate consisting of aggregated and closely packed vesicles (the "honeycomb structure") that finally transforms into large spherical vesicles. The same honeycomb structure is also observed in the absence of enzyme when diacylglycerols are mixed with the other lipids in organic solution, before hydration. In this case the sample then evolves toward a cubic phase. The fact that the same honeycomb intermediate can lead to vesicle fusion (with enzyme-generated diacylglycerol) or to a cubic phase (when diacylglycerol is premixed with the lipids) is taken in support of the hypothesis according to which a highly curved lipid structure ("stalk") would act as a structural intermediate in membrane fusion. Sphingomyelinase produces complete leakage of vesicle aqueous contents and an increase in size of about one-third of the vesicles. A mechanism of vesicle opening and reassembling is proposed in this case.

Origin of the lag period in the phospholipase C cleavage of phospholipids in membranes. Concomitant vesicle aggregation and enzyme activation

When phospholipase C is added to a suspension of large unilamellar vesicles of egg phosphatidylcholine, maximal rates of hydrolysis occur only after a latency period. No lag period is seen when the substrate is in the form of small (sonicated) vesicles, or of short-chain phosphatidylcholine monomers. For a given vesicle concentration, the lag time may vary as a function of Ca2+, enzyme concentration, or temperature, but activation occurs at a fixed molar fraction of diacylglycerol produced. Lag times decrease gradually with vesicle size, and also with the amount of diacylglycerol present in the bilayers when it is mixed with phospholipid prior to enzyme addition. Parallel recordings of enzyme activity and suspension turbidity reveal that in all cases the latency period ends concomitantly with the start of a process of vesicle aggregation. Both the lag time and the amount of diacylglycerol formed before activation decrease with vesicle concentration, suggesting that enzyme activation is somehow related to vesicle aggregation. The latency period of phospholipase C may be explained in terms of a hypothesis according to which (a) full enzyme activity requires the presence of membrane surface irregularities or defects, (b) the diacylglycerol generated in the lag phase produces some kind of phase separation, with the formation of diacylglycerol-rich "patches" or domains, (c) vesicles aggregate through contacts between those patches, and (d) aggregation causes (and/or increases, and/or stabilizes) the surface inhomogeneities that allow fast enzyme activity. These data and suggestions may be relevant to the process of model membrane fusion promoted by phospholipase C.

Different effects of enzyme-generated ceramides and diacylglycerols in phospholipid membrane fusion and leakage

When large unilamellar vesicles consisting of sphingomyelin:phosphatidylethanolamine:cholesterol (2:1:1 molar ratio) are treated with sphingomyelinase, production of ceramides in the bilayer is accompanied by leakage of vesicle aqueous contents and by vesicle aggregation in the absence of lipid mixing or vesicle fusion. This is in contrast to the situation of phosphatidylcholine:phosphatidylethanolamine:cholesterol (2:1:1 molar ratio) liposomes when treated with phospholipase C. In that case, in situ generation of diacylglycerol leads to vesicle aggregation followed by vesicle fusion in the absence of leakage (Nieva, J. L., Goñi, F. M., and Alonso, A. (1989) Biochemistry 28, 7364-7367). Moreover, when ceramides (5-10 mol %) are included in the formulation of the phosphatidylcholine-containing vesicles, they reduce the lag time of phospholipase C-induced fusion, although they are less active than diacylglycerols in this respect. 31P NMR studies of aqueous lipid dispersions show that diacylglycerols as well as ceramides induce a thermotropic lamellar to non-lamellar phase transition in both phospholipid:cholesterol mixtures under study although sphingomyelin-containing bilayers are more stable than those containing phosphatidylcholine, and ceramide is less active than diacylglycerol in promoting non-lamellar phase formation. These observations are relevant to both the physiological role of ceramides and the current views on the mechanism of membrane fusion.

Reversible adsorption and nonreversible insertion of Escherichia coli alpha-hemolysin into lipid bilayers

Alpha-Hemolysin is an extracellular protein toxin (107 kDa) produced by some pathogenic strains of Escherichia coli. Although stable in aqueous medium, it can bind to lipid bilayers and produce membrane disruption in model and cell membranes. Previous studies had shown that toxin binding to the bilayer did not always lead to membrane lysis. In this paper, we find that alpha-hemolysin may bind the membranes in at least two ways, a reversible adsorption and an irreversible insertion. Reversibility is detected by the ability of liposome-bound toxin to induce hemolysis of added horse erythrocytes; insertion is accompanied by an increase in the protein intrinsic fluorescence. Toxin insertion does not necessarily lead to membrane lysis. Studies of alpha-hemolysin insertion into bilayers formed from a variety of single phospholipids, or binary mixtures of phospholipids, or of phospholipid and cholesterol, reveal that irreversible insertion is favored by fluid over gel states, by low over high cholesterol concentrations, by disordered liquid phases over gel or ordered liquid phases, and by gel over ordered liquid phases. These results are relevant to the mechanism of action of alpha-hemolysin and provide new insights into the membrane insertion of large proteins.

Palmitoylcarnitine, a surface-active metabolite

Palmitoylcarnitine is a well-known intermediate in mitochondrial fatty acid oxidation. Less known are its properties as a surfactant, with a capacity to solubilize biological membranes similar to that of many synthetic detergents used in the biochemical laboratory. Some of the physico-chemical properties of palmitoylcarnitine may help to explain the need for coenzyme A-carnitine-coenzyme A acyl exchange during mitochondrial fatty acid import. The amphiphilic character of palmitoylcarnitine may also explain its proposed involvement in the pathogenesis of myocardial ischemia.