A restatement of melittin-induced effects on the thermotropism of zwitterionic phospholipids

Melittin-Induced Lipid Extraction Modulated by the Methylation Level of Phosphatidylcholine Headgroups

Protein- and peptide-induced lipid extraction from membranes is a critical process for many biological events, including reverse cholesterol transport and sperm capacitation. In this work, we examine whether such processes could display specificity for some lipid species. Melittin, the main component of dry bee venom, was used as a model amphipathic α-helical peptide. We specifically determined the modulation of melittin-induced lipid extraction from membranes by the change of the methylation level of phospholipid headgroups. Phosphatidylcholine (PC) bilayers were demethylated either by substitution with phosphatidylethanolamine (PE) or chemically by using mono- and dimethylated PE. It is shown that demethylation reduces the association of melittin with membranes, likely because of the resulting tighter chain packing of the phospholipids, which reduces the capacity of the membranes to accommodate inserted melittin. This reduced binding of the peptide is accompanied by an inhibition of the lipid extraction caused by melittin. We demonstrate that melittin selectively extracts PC from PC/PE membranes. This selectivity is proposed to be a consequence of a PE depletion in the surroundings of bound melittin to minimize disruption of the interphospholipid interactions. The resulting PC-enriched vicinity of melittin would be responsible for the observed formation of PC-enriched lipid/peptide particles resulting from the lipid efflux. These findings reveal that modulating the methylation level of phospholipid headgroups is a simple way to control the specificity of lipid extraction from membranes by peptides/proteins and thereby modulate the lipid composition of the membranes.

Dynamical and Phase Behavior of a Phospholipid Membrane Altered by an Antimicrobial Peptide at Low Concentration

The mechanism of action of antimicrobial peptides is traditionally attributed to the formation of pores in the lipid cell membranes of pathogens, which requires a substantial peptide to lipid ratio. However, using incoherent neutron scattering, we show that even at a concentration too low for pore formation, an archetypal antimicrobial peptide, melittin, disrupts the regular phase behavior of the microscopic dynamics in a phospholipid membrane, dimyristoylphosphatidylcholine (DMPC). At the same time, another antimicrobial peptide, alamethicin, does not exert a similar effect on the DMPC microscopic dynamics. The melittin-altered lateral motion of DMPC at physiological temperature no longer resembles the fluid-phase behavior characteristic of functional membranes of the living cells. The disruptive effect demonstrated by melittin even at low concentrations reveals a new mechanism of antimicrobial action relevant in more realistic scenarios, when peptide concentration is not as high as would be required for pore formation, which may facilitate treatment with antimicrobial peptides.

Role of the Cationic C-Terminal Segment of Melittin on Membrane Fragmentation

The widespread distribution of cationic antimicrobial peptides capable of membrane fragmentation in nature underlines their importance to living organisms. In the present work, we determined the impact of the electrostatic interactions associated with the cationic C-terminal segment of melittin, a 26-amino acid peptide from bee venom (net charge +6), on its binding to model membranes and on the resulting fragmentation. In order to detail the role played by the C-terminal charges, we prepared a melittin analogue for which the four cationic amino acids in positions 21-24 were substituted with the polar residue citrulline, providing a peptide with the same length and amphiphilicity but with a lower net charge (+2). We compared the peptide bilayer affinity and the membrane fragmentation for bilayers prepared from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/1,2-dipalmitoyl-sn-glycero-3-phospho-l-serine (DPPS) mixtures. It is shown that neutralization of the C-terminal considerably increased melittin affinity for zwitterionic membranes. The unfavorable contribution associated with transferring the cationic C-terminal in a less polar environment was reduced, leaving the hydrophobic interactions, which drive the peptide insertion in bilayers, with limited counterbalancing interactions. The presence of negatively charged lipids (DPPS) in bilayers increased melittin binding by introducing attractive electrostatic interactions, the augmentation being, as expected, greater for native melittin than for its citrullinated analogue. The membrane fragmentation power of the peptide was shown to be controlled by electrostatic interactions and could be modulated by the charge carried by both the membrane and the lytic peptide. The analysis of the lipid composition of the extracted fragments from DPPC/DPPS bilayers revealed no lipid specificity. It is proposed that extended phase separations are more susceptible to lead to the extraction of a lipid species in a specific manner than a specific lipid-peptide affinity. The present work on the lipid extraction by melittin and citrullinated melittin with model membranes emphasizes the complex relation between the affinity, the lipid extraction/membrane fragmentation, and the lipid specificity.

Effect of antimicrobial peptide on the dynamics of phosphocholine membrane: role of cholesterol and physical state of bilayer

Antimicrobial peptides are universal in all forms of life and are well known for their strong interaction with the cell membrane. This makes them a popular target for investigation of peptide-lipid interactions. Here we report the effect of melittin, an important antimicrobial peptide, on the dynamics of membranes based on 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid in both the solid gel and fluid phases. To probe the phase transition, elastic neutron intensity temperature scans have been carried out on DMPC-based unilamellar vesicles (ULV) with and without melittin. We have found that addition of a small amount (0.2 mol%) melittin eliminates the steep fall in the elastic intensity at 296 K associated with the solid gel to fluid phase transition, which is observed for pure DMPC vesicles. Quasielastic neutron scattering (QENS) experiments have been carried out on DMPC ULV in the solid gel and fluid phases with and without 0.2 mol% melittin. The data analysis invariably shows the presence of lateral and internal motions of the DMPC molecule. We found that melittin does have a profound effect on the dynamics of lipid molecules, especially on the lateral motion, and affects it in a different way, depending on the phase of the bilayers. In the solid gel phase, it acts as a plasticizer, enhancing the lateral motion of DMPC. However, in the fluid phase it acts as a stiffening agent, restricting the lateral motion of the lipid molecules. These observations are consistent with the mean squared displacements extracted from the elastic intensity temperature scans. Their importance lies in the fact that many membrane processes, including signaling and energy transduction pathways, are controlled to a great extent by the lateral diffusion of lipids in the membrane. To investigate the effect of melittin on vesicles supplemented with cholesterol, QENS experiments have also been carried out on DMPC ULV with cholesterol in the presence and absence of 0.2 mol% melittin. Remarkably, the effects of melittin on the membrane dynamics disappear in the presence of 20 mol% cholesterol. Our measurements indicate that the destabilizing effect of the peptide melittin on membranes can be mitigated by the presence of cholesterol. This study might provide new insights into the mechanism of action of antimicrobial peptides and their selective toxicity towards foreign microorganisms.

Melittin: a membrane-active peptide with diverse functions

Melittin is the principal toxic component in the venom of the European honey bee Apis mellifera and is a cationic, hemolytic peptide. It is a small linear peptide composed of 26 amino acid residues in which the amino-terminal region is predominantly hydrophobic whereas the carboxy-terminal region is hydrophilic due to the presence of a stretch of positively charged amino acids. This amphiphilic property of melittin has resulted in melittin being used as a suitable model peptide for monitoring lipid-protein interactions in membranes. In this review, the solution and membrane properties of melittin are highlighted, with an emphasis on melittin-membrane interaction using biophysical approaches. The recent applications of melittin in various cellular processes are discussed.

Effect of phospholipid composition on an amphipathic peptide-mediated pore formation in bilayer vesicles

To better understand the influence of phospholipid acyl-chain composition on the formation of pores by cytotoxic amphipathic helices in biological membranes, the leakage of aqueous contents induced by the synthetic peptide GALA (WEAALAEALAE ALAEHLAEALAEALEALAA) from large unilamellar phospholipid vesicles of various compositions has been studied. Peptide-mediated leakage was examined at pH 5.0 from vesicles made of phosphatidylcholine (PC) and phosphatidylglycerol (PG) with the following acyl-chain compositions: 1-palmitoyl-2-oleoyl (PO), 1,2-dioleoyl (DO), 1, 2-dielaidoyl (DE), and 1,2-dipetroselinoyl (DPe). A mathematical model predicts and simulates the final extents of GALA-mediated leakage of 1-aminonaphthalene-3,6,8-trisulfonic acid (ANTS) and p-xylene-bis-pyridinium bromide (DPX) from 1-palmitoyl-2-oleoyl-phosphatidylcholine/1-palmitoyl-2-oleoyl-phospha tidylglycerol (POPC/POPG) and 1, 2-dielaidoyl-sn-glycero-3-phosphocholine/1, 2-dielaidoyl-phosphatidylglycerol (DEPC/DEPG) liposomes at pH 5.0 as a function of peptide concentration in the bilayer, by considering that GALA pores responsible for this leakage have a minimum size of 10 +/- 2 monomers and are formed by quasiirreversible aggregation of the peptide. With the phospholipid acyl-chain compositions tested, GALA-induced ANTS/DPX leakage follows the rank order POPC/POPG approximately DEPC/DEPG > DPePC/DPePG > DOPC/DOPG. Results from binding experiments reveal that this reduced leakage from DOPC/DOPG vesicles cannot be explained by a reduced binding affinity of the peptide to these membranes. As shown by monitoring the leakage of a fluorescent dextran, an increase in the minimum pore size also does not explain the reduction in ANTS/DPX leakage. The data suggest that surface-associated GALA monomers or aggregates are stabilized in bilayers composed of phospholipids containing a cis unsaturation per acyl chain (DO and DPe), while transbilayer peptide insertion is reduced. GALA-induced ANTS/DPX leakage is also decreased when the vesicles contain phosphatidylethanolamine (PE). This lends further support to the suggestion that factors stabilizing the surface state of the peptide reduce its insertion and subsequent pore formation in the bilayer.

Measurement of the lateral diffusion of dipalmitoylphosphatidylcholine adsorbed on silica beads in the absence and presence of melittin: a 31P two-dimensional exchange solid-state NMR study

31P two-dimensional exchange solid-state NMR spectroscopy was used to measure the lateral diffusion, D(L), in the fluid phase of dipalmitoylphosphatidylcholine (DPPC) in the presence and absence of melittin. The use of a spherical solid support with a radius of 320 +/- 20 nm, on which lipids and peptides are adsorbed together, and a novel way of analyzing the two-dimensional exchange patterns afforded a narrow distribution of D(L) centered at a value of (8.8 +/- 0.5) x 10(-8) cm2/s for the pure lipid system and a large distribution of D(L) spanning 1 x 10(-8) to 10 x 10(-8) cm2/s for the lipids in the presence of melittin. In addition, the determination of D(L) for nonsupported DPPC multilamellar vesicles (MLVs) suggests that the support does not slow down the lipid diffusion and that the radii of the bilayers vary from 300 to 800 nm. Finally, the DPPC-melittin complex is stabilized at the surface of the silica beads in the gel phase, opening the way to further study of the interaction between melittin and DPPC.

Stopped-flow fluorometric study of the interaction of melittin with phospholipid bilayers: importance of the physical state of the bilayer and the acyl chain length

Stopped-flow fluorometry has been employed to study the effects of melittin, the major protein component of bee venom, on dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) small unilamellar vesicles (SUVs) on the millisecond time scale, before melittin-induced vesicle fusion takes place. Use is made of 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH), which is an oriented fluorescent probe that anchors itself to the bilayer-water interface and is aligned parallel to the normal to the bilayer surface; its fluorescence anisotropy reports on the "fluidity" of the bilayer. For DMPC bilayers, melittin is found to decrease their fluidity only at their melting transition temperature. This perturbation appears to be exerted almost instantaneously on the millisecond time scale of the measurements, as deduced from the fact that its rate is comparable to that obtained by following the change in the fluorescence of the single tryptophan residue of melittin upon inserting itself into the bilayer. The perturbation is felt in the bilayer over a distance of at least 50 A, with measurements of transfer of electronic energy indicating that the protein is not sequestered in the neighborhood of TMA-DPH. The length of the acyl chains is found to be an important physical parameter in the melittin-membrane interaction: unlike the case of DMPC SUVs, melittin does not alter the fluidity of DPPC SUVs and has a considerably greater affinity for them. These results are discussed in terms of the concept of elastic distortion of the lipids, which results from a mismatch between the protein and the acyl chains that are attempting to accommodate it. Melittin is also found to cause a small (approximately 10%) enhancement in the total fluorescence intensity of TMA-DPH, which is interpreted as indicating a reduction in the degree of hydration of the bilayer.

Phases and phase transitions of the sphingolipids

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Herein, we present a review of the LIPIDAT data subset referring to sphingolipids together with an analysis of these data. It includes data collected over a 40-year period and consists of 867 records obtained from 112 articles in 25 different journals. An analysis of these data has allowed us to identify trends in hydrated sphingolipid phase behavior reflecting differences in fatty acyl chain length, saturation and hydroxylation, head group type, and sphingoid base identity. Information on the mesomorphism of biologically-derived and dry sphingolipids is also presented. This review includes 161 references.

Acyl chain length dependence in the stability of melittin-phosphatidylcholine complexes. A light scattering and 31P-NMR study

Light scattering and 31P-NMR have been used to monitor the effect of the bee-toxin, melittin, on phosphatidylcholine (PC) bilayers of variable acyl chain length (from C16:0 to C20:0). Melittin interacts with all lipids provided the interaction is initiated in the lipid fluid phase. For low-to-moderate amounts of toxin (lipid-peptide molar ratios, Ri > or = 15), the system takes the form of large spheroidal vesicles, in the fluid phase, whose radius increases from 750 A with dipalmitoyl-PC (DPPC) to 1500 A with diarachinoyl-PC (DAPC). These vesicles fragment into small discoids of 100-150 A radius when the system is cooled down below Tc (the gel-to-fluid phase transition temperature). Little chain length dependence is observed for the small objects. Small structures are also detected independently of the physical state of lipids (gel or fluid) when Ri < or = 5 and provided the interaction has been made above Tc. Small discs clearly characterized for DPPC and distearoyl-PC (DSPC) lipids are much less stable with DAPC. However in the long term, all these small structures fuse into large lipid lamellae. Discs are thermodynamically unstable and kinetics of disappearance of the small lipid-toxin complexes increases as the chain length increases in the sense: DAPC >> DSPC > DPPC. Kinetics of fusion of the small discs into extended bilayers is described by a pseudo-first-order law involving a lag time after which fusion starts. Increasing the chain length decreases the lag time and increases the rate of fusion. Formation of both the large vesicles in the fluid phase and the small discs in the gel phase as well as their stability is discussed in terms of relative shapes and dynamics of both lipids and toxin.

Action of melittin on the DPPC-cholesterol liquid-ordered phase: a solid state 2H-and 31P-NMR study

Solid-state deuterium and phosphorus-31 nuclear magnetic resonance studies of deuterium-labeled beta--[2,2',3,4,4',6-2H6]-cholesterol and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine have been undertaken to monitor the action of melittin on model membranes containing 30 mol% cholesterol, both at the molecular and macroscopic level. Cholesterol totally inhibits the toxin-triggered formation of large unilamellar vesicles and strongly restricts the appearance of small discs. The latter remain stable over a wide temperature range (20-60 degrees C) because of an increase in their cholesterol content as the temperature increases. This process is related to a constant disc hydrophobic thickness of approximately 29 A. The system, when not in the form of discs, appears to be composed of very large vesicles on which melittin promotes magnetically induced ellipsoidal deformation. This deformation is the greatest when the maximum of discs is observed. A model to describe both the disc formation and stability is proposed.

Effect of cholesterol on the polymorphism of dipalmitoylphosphatidylcholine/melittin complexes: an NMR study

In order to get insights into the effects of cholesterol on protein activity, the lytic power of melittin on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/cholesterol mixtures was studied using solid-state deuterium and phosphorus-31 nuclear magnetic resonance spectroscopy (2H and 31P-NMR). After incubation, melittin disrupts pure DPPC vesicles, leading to the formation of small lipid/peptide complexes below the phase transition temperature (Tm), whereas large bilayer assemblies are reformed above Tm; the transition between these two species is thermally reversible. This study reveals that cholesterol modifies this thermal behavior and that this modulation of the lytic power of melittin is indirect, since it is essentially related to the original effect of the sterol on the thermotropism of pure lipid bilayers. It is known that melittin does not lyse gel phase DPPC bilayers spontaneously. Our study shows that the addition of large amounts of sterol (30 mol%) does not promote the spontaneous lysis at 26 degrees C, despite the increased fluidity of the lipid system. The lysis takes place around 32 degrees C, regardless of the cholesterol concentration. This study also shows that high concentrations of cholesterol (> or = 30%) in DPPC bilayer inhibit the lysis. It is proposed that the tight lipid packing due to high cholesterol concentrations prevents the penetration of melittin into the bilayer. When melittin interacts with cholesterol-rich bilayers (30 mol%), the lysis is only partial, and leads to the formation of small cholesterol-depleted particles. Finally, DPPC which bears deuteriated acyl chains was used to determine the influence of melittin on the orientational order of the lipid chains in the large assemblies. The quadrupolar splittings obtained in the presence of melittin are not considerably different than those obtained in the absence of melittin.

High pressure FTIR study of interaction of melittin with dimyristoylphosphatidyl glycerol bilayers

Infrared spectra of hydrated dimyristoylphosphatidyl glycerol (DMPG) and of aqueous dispersions of melittin and DMPG at peptide:lipid molar ratios of 1:10 and 1:4 were recorded as a function of pressure from atmospheric to 22 kbar. Spectral features corresponding to vibrations of the amide linkages in melittin and to various functional groups in DMPG (carbonyl, methlylene, phosphate) were monitored in order to investigate the structure and dynamics of melittin:DMPG dispersions. Melittin was found to cause conformational and orientational disordering of the acyl chains in DMPG bilayers. The magnitude of these disorders was higher for higher concentration of melittin in DMPG. The higher concentration of melittin was also found to disrupt the DMPG bilayers through interactions with the lipid head groups. Such disruption may be related to some of the biological properties of melittin.

Calorimetry of tetraether lipids from Thermoplasma acidophilum: incorporation of alamethicin, melittin, valinomycin, and nonactin

The development and application of model membrane systems on the basis of tetraether lipids from Thermoplasma acidophilum has been proposed. In this respect incorporation of membrane proteins and ionophores is indispensable and is demonstrated in the case of alamethicin, melittin, nonactin, and valinomycin by calorimetry. Dipalmitoylphosphatidylcholine (DPPC) and dihexadecylmaltosylglycerol (DHMG) were chosen for comparison. Melittin and alamethicin prove to broaden the lipid phase transition and to reduce the melting temperature Tm and enthalpy change (delta H) of the main phospholipid from T. acidophilum (MPL) and DPPC. The decrease in Tm, however, is more pronounced in DPPC than in MPL. Valinomycin shows only a marginal effect on the temperature and width of the transition; delta H is reduced in MPL and remains constant in DPPC and DHMG. With nonactin the phase transition of DPPC is quenched, and delta H and the half-height width are increased. DHMG is affected to a lesser extent and MPL only marginally. The four ionophores exhibit different modulation of the phase transition behavior of the various lipids as expected from their varying molecular structures. Thus, the integral membrane protein alamethicin, the peripheral protein melittin, valinomycin, and nonactin interact primarily with lipid head groups and are readily incorporated into the tetraether lipid structures.

Anthrylvinyl-labeled phospholipids as fluorescent membrane probes. The action of melittin on multilipid systems

The interaction of melittin with multicomponent lipid mixtures composed of phosphatidylcholine, sphingomyelin and phosphatidylserine or phosphatidylglycerol was investigated by measuring the intrinsic fluorescence of the peptide, steady state fluorescence anisotropy of, and Trp-fluorescence energy transfer to fluorescent analogs of the same phospholipids bearing the anthrylvinyl fluorophore in one of the aliphatic chains at various distances from the polar head group. Based on the finding that at high lipid/peptide ratio the peptide induces unequal changes in the fluorescence parameters of phospholipid probes differing structurally only in their polar head groups, it is concluded that melittin induces lipid demixing in its nearest environment. Comparison of the fluorescence energy transfer from Trp to different lipid probes indicates that the depth of penetration of melittin into the bilayer depends on the polar head group composition of the phospholipid matrix and that certain segments of the melittin chain display a specific affinity for a given lipid head group.

Investigation of the interaction between melittin and dipalmitoylphosphatidylglycerol bilayers by vibrational spectroscopy

Melittin is shown to affect the structure of the charged phospholipid dipalmitoylphosphatidylglycerol (DPPG). In the gel phase, the presence of melittin leads to (i) an increased lipid interchain vibrational coupling, (ii) a shift of the rectangular to hexagonal lipid packing transition toward low temperatures, (iii) a very small conformational disordering effect, (iv) a decrease of the polarity or hydrogen bonding capability of the lipid ester group surrounding, (v) an important decrease of the water content in the complexes where the remaining water has a more disordered structure than bulk water, and (vi) an interlamellar repeat distance of 79 A. All these observations are rationalized by the following model: adjacent bilayers of DPPG are bridged by tetramers of melittin through electrostatic interactions inducing surface charge neutralization and partial dehydration of the complexes. Melittin also affects the thermotropic behavior of DPPG. When a small amount of the toxin is present, its affinity for charged lipids is such that a phase separation occurs, the domains being stable enough to have their own gel to liquid-crystalline phase transition. In the fluid state, a deeper penetration into the lipid matrix is proposed based on the downshift of the phase transition and the low vibrational interchain coupling. This study brings out general features of cationic species/anionic lipid complexes. The charge neutralization leads to stronger interchain coupling, and electrostatic bridging of adjacent bilayers seems to be common. The hydrophobicity of the peptide is a key factor in the modulation of the gel to liquid-crystalline phase transition and in its insertion in the fluid lipid matrix.

Reversible disc-micellization of dimyristoylphosphatidylcholine bilayers induced by melittin and [Ala-14]melittin

The properties of melittin and a synthetic analogue, [Ala-14]melittin (P14A), in inducing reversible transitions between vesicles and micelles at the liquid-crystalline to gel phase transition temperature (Tm) in complexes with saturated phosphatidylcholines has been studied by deuterium NMR and freeze-fracture electron microscopy (EM). At concentrations between 3 and 5 mol% relative to lipid, each peptide causes reversible micellization of dimyristoylphosphatidylcholine (DMPC) bilayers when the temperature is lowered below Tm. At concentrations of 5 mol% relative to lipid, the peptides induce macroscopic magnetic orientation of DMPC bilayers at temperatures around the centre of the lipid phase transition; at temperatures a few degrees above Tm, magnetic orientation is lost. These effects suggest a progressive phase separation of peptide and lipid on cooling the complexes through the phase transition, resulting in increased vesicle deformability. The rates of gel phase micellization, and of bilayer reformation from micelles at temperatures above Tm, are decreased by 100-fold in P14A:DMPC complexes compared with melittin: DMPC complexes. Freeze-fracture EM indicates that P14A suppresses the formation of the gel phase in DMPC bilayers at temperatures below Tm. EM observations of the time-dependence of the reformation of bilayers from micelles after incubating P14A:DMPC micellar complexes at temperatures above Tm indicate that micelles fuse to form growing bilayer sheets from which multilamellar vesicles eventually form. The presence of intramembranous particles (IP) on the fracture faces of both melittin: DMPC complexes and P14A:DMPC complexes in the fluid phase indicates that under the conditions of the study (50 mM Tris-HCl (pH 7.5), 5 mM EDTA) the peptides are organized as discrete aggregates that penetrate deeply into the bilayer.

Interaction of wasp venom mastoparan with biomembranes

Mastoparan-induced changes in the K+ permeability of rat peritoneal mast cells, human erythrocytes, Staphylococcus aureus and Escherichia coli were examined. Mastoparan did not efficiently increase the K+ permeability of cells except for S. aureus. The release of membrane phospholipids was also observed from S. aureus cells in the concentration range of the permeability enhancement. Mastoparan stimulated histamine release from mast cells, independently of a small efflux of K+. Mastoparan became markedly effective to E. coli cells whose outer membrane structure was chemically disrupted beforehand, showing that the peptide can enhance the permeability of the cytoplasmic membranes of both Gram-positive and -negative bacteria. In experiments using liposomes, mastoparan increased the permeability of the liposomes composed of egg phosphatidylethanolamine and egg phosphatidylglycerol, which are the lipid constituents of the cytoplasmic membrane of E. coli cells, while it showed a weak activity to the liposomes composed of egg phosphatidylcholine and cholesterol. The latter result related closely to the fact that this peptide acted weakly on erythrocytes and mast cells in which acidic lipids constitute a minor portion. Mastoparan decreased the phase transition temperature of dipalmitoylphosphatidylglycerol liposomes, but it did not affect that of dipalmitoylphosphatidylcholine liposomes. These results indicate that mastoparan penetrated into membranes mainly containing acidic phospholipids and disrupted the membrane structure to increase the permeability. The action of the wasp venom mastoparan was compared with that of a bee venom melittin.

Organic pesticides modify lipid-lipid and lipid-protein domains in model membranes. A laser Raman study

The effect of hexachlorocyclohexane (all isomers) on the thermal transition properties of phospholipid liposomes was determined by Raman spectroscopy. Raman spectra of liposomes with and without the presence of hexachlorocyclohexanes were recorded in the C-H stretching region which shows three major bands around 2850, 2880 and 2930 cm-1. Thermal transition properties were estimated from plots of I2880/I2850 and or I2930/I2850 vs. temperature, where I represents the intensity of the respective band. Our data on phospholipid liposomes reveal that delta- and gamma-hexachlorocyclohexanes drastically reduce and broaden the main thermal transitions of phospholipids at toxic level concentrations. These effects are more pronounced in liposomes containing 18 or more carbon atom long acyl chains. Alpha- and beta-isomers at similar concentrations show a minimum effect on the thermal transition properties of phospholipids. Raman analysis of phospholipid liposomes containing melittin, interestingly, reveal that the delta-isomer unlike the gamma-isomer strongly alters the transition properties of boundary lipids. These data suggest that the effect of hexachlorocyclohexanes on the thermal transition properties of membranes is stereo specific and that the delta-isomer preferably disrupts the lipid-protein domains. Results are explained on the basis of the dynamic flexibility owing to the equatorial and axial chlorine atoms of various hexachlorocyclohexane isomers.

Electrostatics of a peptide at a membrane/water interface. The pH dependence of melittin association with lipid vesicles

The association of the peptide melittin with small unilamellar DMPC vesicles was studied as a function of pH. The results are discussed quantitatively assuming a water-membrane partition equilibrium. Electrostatic surface charging is taken into account as more and more of the strongly basic peptide accumulates at the bilayer/water interface. The data could be well described in terms of a Gouy-Chapman approach involving an effective interfacial charge well below the actual physical charge carried by the individual peptide molecules. The partition coefficient turned out to be pH invariant, so that one can exclude deprotonation reactions upon insertion of the peptide into the bilayer. The effective interfacial charge per associated melittin molecule decreased over a broad range of pH (pH 7 to pH above 10). Contributions of the free amino terminus and of the arginine residues could be determined by comparing with results obtained using modified melittin (N-terminally formylated and fully acetylated). The data suggest approximately equal fractional contributions of the amino terminus and the three lysines to the effective interfacial charge. The two arginines contribute less. Thus, they may be located farther away from the interface or be closely associated with counter-ions. The analysis is extended to the effect of different ionic strengths.

The actions of melittin on membranes

The molecular mechanisms underlying the various effects of melittin on membranes have not been completely defined and much of the evidence described indicates that different molecular mechanisms may underlie different actions of the peptide. Ideas about the formation of transbilayer aggregates of melittin under the influence of a transbilayer potential, and for bilayer structural perturbation arising from the location of the peptide helix within the head group region of the membrane have been made based on the crystal structure of the peptide, the kinetics and concentration dependence of melittins membrane actions, together with simple ideas about the conformational properties of amphipathic helical peptides and their interactions with membranes. Physical studies of the interaction of melittin with model membranes have been useful in determining the potential of the peptide to adopt different locations, orientations and association states within membranes under different conditions, but the relationship of the results obtained to the actions of melittin in cell membranes or under the influence of a membrane potential are unclear. Experimental definition of the interaction of melittin with more complex membranes, including the erythrocyte membrane or in bilayers under the influence of a transmembrane potential, will require direct study in these membranes. Experiments employing labeled melittins for ESR, NMR or fluorescence experiments are promising both for their sensitivity (ESR and fluorescence) and the ability to focus on the peptide within the background of endogenous proteins within cell membranes. The study of melittin in model membranes has been useful for the development of methodology for determination of membrane protein structures. Despite the structural complexity of integral membrane proteins, it is interesting that in some respects their study be more straightforward, lacking as they do the elusive properties of melittin (and other structurally labile membrane peptides) which limit the possibility of defining their interaction with membranes in terms of a single conformation, location, orientation and association state within the membrane.

Interaction of the pore-forming domain of colicin A with phospholipid vesicles

The interaction of the 20-kDa pore-forming domain of colicin A with phospholipid vesicles was investigated by gel permeation chromatography, analytical centrifugation, and electron microscopy. Under the experimental conditions of this study, this peptide was found to interact only with vesicles containing negatively charged phospholipids. It forms a well-defined disklike complex with phosphatidylglycerols with a preference for those containing 12-14 atoms of carbon in their fatty acid chain. This complex has a diameter of 120 A and is about one bilayer thick. It contains nine molecules of peptide and is formed both at acidic pH (pH 5.0) and at neutral pH (pH 7.2).

Characterization of plasma-stabilized liposomes composed of dioleoylphosphatidylethanolamine and oleic acid [published errtum appears in Biochem Biophys Res Commun 1989 Sep 29;163(3):1539]

We have previously reported that small unilamellar liposomes (d less than or equal to 200 nm) composed of dioleoylphosphatidylethanolamine and oleic acid can be stabilized by incubating with normal human plasma (Liu and Huang, Biochemistry 1989, in press). The stabilized liposomes were very stable even under relatively harsh conditions such as extreme pH, high salt and trypsin treatment. Fluorescence depolarization of diphenylhexatriene showed that the stabilized liposome had a high microviscosity in the lipid core, which did not decrease even after the majority of proteins were removed by trypsin. These data suggest that plasma proteins inserted into the lipid bilayer are probably responsible for the stabilization activity. After i.v. injection into mouse, stabilized liposomes showed a relatively low affinity to liver and spleen as compared to a conventional liposome composition.

A high-sensitivity differential scanning calorimetric study of the interaction of melittin with dipalmitoylphosphatidylcholine fused unilamellar vesicles

High-sensitivity differential scanning calorimetry has been used to examine the interaction of bee venom melittin with dipalmitoylphosphatidylcholine fused unilamellar vesicles. Experiments were performed under conditions for which melittin in solution is either monomeric (in low salt) or tetrameric (in high salt). It was found that under both sets of conditions melittin abolishes the pretransition at a relatively high lipid-to-protein molar incubation ratio, Ri (about 200) and that at intermediate values of Ri it broadens the main transition profile and reduces the transition enthalpy. This provides evidence which suggests that melittin is at least partially inserted into the apolar region of the bilayer. Evident at low values of Ri are two peaks in the lipid thermal transition profiles, which may arise from a heterogeneous population of lipid vesicles formed through fusion induced by melittin, or by lipid phase separation. For those profiles which exhibited only one peak, transition enthalpies, normalized to those of the lipid in the absence of the protein, are plotted vs. the bound protein-to-lipid molar ratios for the experiments performed under the conditions which give monomeric and tetrameric melittin in solution. These plots yield straight lines, the slopes of which give the number of lipid molecules each protein molecule excludes from participating in the phase transition. These were found to be 9.9 +/- 0.7 and 4.1 +/- 0.5 for monomeric and tetrameric melittin, respectively. The results are discussed in terms of possible models for the binding of melittin to phospholipid vesicles. For simple hexagonal packing of lipid molecules, incorporation as an aggregate is favored when melittin is tetrameric in solution, whereas incorporation as a monomer is favored when melittin is monomeric in solution. For low-salt solutions, evidence is obtained for the contribution of free melittin to lipid fusion, perhaps by the formation of protein bridges between apposed vesicles.

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.

Membrane structure and dynamics by 2H- and 31P-NMR. Effects of amphipatic peptidic toxins on phospholipid and biological membranes

The actions of bee venom melittin and delta-lysin from Staphylococcus aureus on membranes have been monitored by solid-state deuterium and phosphorus NMR and shown to differ depending on temperature and on the lipid-to-peptide molar ratio Ri. In the gel phase of phosphatidylcholine model membranes, for lipid-to-peptide ratios Ri greater than 15, melittin induces isotropic lines interpreted as reflecting the presence of small discoidal structures, whereas delta-lysin does not. These small objects are metastable, that is, within a time-scale of hours they return to large lipid bilayers. The kinetics of this process depend on the lecithin chain length. In the fluid phases, at temperatures greater than that of the gel-to-fluid transition Tc, analysis of the quadruplar splittings in terms of chain ordering indicates that both melittin and delta-lysin similarly disorder the membrane. At temperatures above but close to Tc, melittin preferentially orders the center of the bilayer, while delta-lysin promotes ordering throughout the entire bilayer thickness. These effects are interpreted as reflecting different locations of the peptides with respect to the membrane surface. The addition of greater amounts of toxins, Ri = 4, on phosphatidylcholine model membranes induces very small structures irrespective of the temperature in the case of melittin, but only above Tc for delta-lysin. NMR spectral features similar to those characterizing the small fast-tumbling objects with phosphatidylcholine are also observed with egg phosphatidylethanolamine and erythrocyte membranes. The formation of small structures is thus inferred as a general process which reflects membrane supramolecular reorganization.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of alpha-lactalbumin on the thermotropic phase behaviour of phosphatidylcholine bilayers, studied by fluorescence polarization, differential scanning calorimetry and Raman spectroscopy

The effects of bovine alpha-lactalbumin on the thermotropic properties of dimyristoylphosphatidylcholine liposomes are studied by Raman spectroscopy, fluorescence polarization and differential scanning calorimetry. The Raman spectrum reveals the drastic effects of the protein on the phospholipid structure. The transition temperature shifts downwards and the inter- and intrachain order in the lipid matrix progressively diminish with increasing protein concentration. Up to a lipid to protein molar ratio R = 25, the bilayer structure however is maintained. From fluorescence polarization data we conclude that the protein restricts the mobility of the DPH probe. In view of the Raman results, the lower probe mobility obviously cannot be associated with a more rigid lipid matrix. Nevertheless the transition temperatures of the alpha-lactalbumin-phospholipid complex increases. DSC measurements give no decisive way out for this discrepancy. These results confirm that different types of lipid order are involved in lipid-protein interactions. Compared to the free protein, the alpha-helicity of the protein has increased in the complex.

Attenuated total reflectance Fourier transform infrared studies of the interaction of melittin, two fragments of melittin, and delta-hemolysin with phosphatidylcholines

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR) has been used to monitor alterations in phospholipid organization in thin layers of 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), induced by the membrane lytic peptide melittin, its fragments 1-15 (hydrophobic fragment) and 16-26 (hydrophilic fragment), and delta-hemolysin. In addition, the secondary structures of the peptides and the orientation of helical fragments were determined with respect to the bilayer. The insertion of melittin into POPC caused large perturbations in the order and increased rates of motion of the acyl chains, as monitored by the frequency and half-width of the symmetric CH2 stretching vibration near 2850 cm-1, as well as by the ATR dichroic ratio for this mode. Changes in DPPC organization were less and were consistent with peptide-induced static disordering (gauche rotamer formation) in the acyl chains. Melittin adopted primarily an alpha-helical secondary structure, although varying small proportions of beta and/or aggregated forms were noted. The helical segments were preferentially oriented perpendicular to the bilayer plane. Several modes of melittin/lipid interaction were considered in an attempt to semiquantitatively understand the observed dichroic ratios. By considering the peptide as a bent rigid rod, a plausible model for its lytic properties has been developed. The hydrophilic fragment in DPPC showed a secondary structure with little alpha-helix present. As judged by its effect on phospholipid acyl chain organizational parameters, the fragment did not penetrate the bilayer substantially. The hydrophobic fragment in DPPC gave amide I spectral patterns consistent with a mixture of predominantly beta-antiparallel pleated sheet with a smaller fraction of alpha-helix.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of melittin-induced fusion of dipalmitoylphosphatidylcholine small unilamellar vesicles

We have studied the kinetics of fusion of dipalmitoylphosphatidylcholine small unilamellar vesicles at 51 degrees C which is induced by bee venom melittin at a protein-to-lipid molar ratio of 1/60. This was done by following with a stopped-flow fluorometer the reduction in the ratio of the excimer to monomer fluorescence intensities of 1-palmitoyl-2-(10-pyrenyldecanoyl)-sn-glycero-3-phosphorylcholine that accompanies fusion. At a low melittin concentration and low ionic strength, for which case the protein is monomeric, the value of the rate constant for fusion is 0.006 s-1. This is much smaller than that of 0.06 s-1 obtained for a high melittin concentration at low ionic strength, i.e. for the protein in the tetrameric form which is not induced by a high salt concentration. The value of the rate constant for fusion for a low melittin concentration in the presence of 2 M NaCl, i.e. for the protein in the tetrameric form which is induced by a high salt concentration, is 0.12 s-1. This is twice as large as that for fusion induced by the tetramer in a low ionic strength solution. These findings show that the state of aggregation of the protein in solution and, to a lesser extent, electrostatic interactions play an important role in the kinetics of melittin-induced fusion of vesicles.

Melittin induces HII phase formation in cardiolipin model membranes

The interaction of melittin with bovine heart cardiolipin model membranes was investigated via binding assays, 31P-NMR, freeze-fracture electron microscopy, small angle X-ray diffraction and fluorescence based fusion assays. A strong binding (Kd less than 10(-7) M) appeared to be accompanied by the formation of large structures, resulting from a fusion process of extremely fast initial rate. As the melittin content is increased, bilayer structure is gradually lost and from a cardiolipin to melittin ratio of about 6 the lipid starts to organize itself in an hexagonal HII phase. At lower temperatures (T less than 40 degrees C) the coexistence of another structure is observed, characterized by a broad isotropic 31P-NMR signal and giving rise to sharp X-ray reflections, most probably a cubic phase, as suggested also be freeze-fracture images, showing orderly stacked particles. The results are discussed in relation to contrasting observations on the structural changes induced by melittin in the zwitterionic phospholipid system of dipalmitoylphosphatidylcholine (Dufourcq. J. et al. (1986) Biochim. Biophys. Acta 859, 33-48). The biological relevance of the observations with respect to the process of protein insertion into membranes is indicated.

A study of the structure of polymyxin B-dipalmitoylphosphatidylglycerol complexes by vibrational spectroscopy

The effect of the antibiotic polymyxin B on dipalmitoylphosphatidylglycerol (DPPG) bilayers has been studied by Raman and infrared spectroscopies and small-angle X-ray diffraction. Each polymyxin B molecule binds five DPPG molecules at physiological pH and induces a macroscopic phase separation of the complex rather than a lateral phase separation. Below the phase transition of DPPG/polymyxin B bilayers, the results obtained show that the intermolecular vibrational coupling is high and suggest that the acyl chains of the bound lipid are interdigitated and that the hydrophobic tail of the antibiotic does not penetrate this tight assembly. On the other hand, the phase transition of DPPG is shifted down from 41 degrees C to 37 degrees C in the complexes and remains highly cooperative. Above the phase transition of the complexes, the conformation of the acyl chains of DPPG is slightly more disordered as a result of the penetration of the polymyxin chain, but the structure of the glycerol backbone of the lipid does not seem to be affected. However, the rotational rate of the lipid appears to be restricted by the peptide.

Lipid specific penetration of melittin into phospholipid model membranes

The relative depth of penetration of melittin into egg phosphatidylcholine and bovine heart cardiolipin model membranes was investigated using fluorescence spectroscopy techniques. The tryptophan intrinsic fluorescence shift suggests a more hydrophobic surrounding of this residue in cardiolipin, while the accessibility for charged and uncharged aqueous quenchers is decreased in the cardiolipin system when compared with the phosphatidylcholine-bound situation. A lipid incorporated hydrophobic, collisional quencher and a resonance energy transfer acceptor on the other hand are more effective in quenching the tryptophan fluorescence of cardiolipin bound melittin. The combination of these results is interpreted as prove of a deeper positioning of the tryptophan containing part of the peptide molecule in the cardiolipin system in comparison with the situation in phosphatidylcholine. Models that take this difference into account are presented, which try to explain the opposite effect of melittin binding to the two lipid systems with respect to supramolecular structure, as reported in the preceding article (Batenburg, A.M., Hibbeln, J.C.L., Verkleij, A.J. and De Kruijff, B. (1987) Biochim. Biophys. Acta 903, 142-154).

A deuterium and phosphorus-31 nuclear magnetic resonance study of the interaction of melittin with dimyristoylphosphatidylcholine bilayers and the effects of contaminating phospholipase A2

The interaction of bee venom melittin with dimyristolphosphatidylcholine (DMPC) selectively deuteriated in the choline head group has been studied by deuterium and phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy. The action of residual phospholipase A2 in melittin samples resulted in mixtures of DMPC and its hydrolytic products that underwent reversible transitions at temperatures between 30 and 35 degrees C from extended bilayers to micellar particles which gave narrow single-line deuterium and phosphorus-31 NMR spectra. Similar transitions were observed in DMPC-myristoyllysophosphatidylcholine (lysoPC)-myristic acid mixtures containing melittin but not in melittin-free mixtures, indicating that melittin is able to stabilize extended bilayers containing DMPC and its hydrolytic products in the liquid-crystalline phase. Melittin, free of phospholipase A2 activity, and at 3-5 mol% relative to DMPC, induced reversible transitions between extended bilayers and micellar particles on passing through the liquid-crystalline to gel phase transition temperature of the lipid, effects similar to those observed in melittin-acyl chain deuterated dipalmitoylphosphatidylcholine (DPPC) mixtures [Dufourc, E. J., Smith, I. C. P., & Dufourcq, J. (1986) Biochemistry 25, 6448-6455]. LysoPC at concentrations of 20 mol% or greater relative to DMPC induced transitions between extended bilayers and micellar particles with characteristics similar to those induced by melittin. It is proposed that these melittin- and lysoPC-induced transitions share similar mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Incorporation of highly purified melittin into phosphatidylcholine bilayer vesicles

Melittin free of phospholipase A2 was prepared. In the absence of salt this highly pure protein starts to aggregate in solution at a protein concentration of Cp greater than 10(-3) M. In high salt solution (2 M) aggregation starts at Cp greater than 10(-6) M. This was determined from the blue shift of the intrinsic fluorescence of the protein. Reinvestigation of the quenching behaviour clearly shows that self-aggregation cannot be deduced from quenching experiments using nitrate or 2,2,6,6-tetramethylpiperidine-1-oxyl as quencher. The incorporation of melittin into phosphatidylcholine bilayer vesicles was studied by fluorescence quenching and by energy-transfer experiments using 2- and 6-anthroyloxypalmitic acid as acceptor and peptide tryptophan as donor. Incorporation of melittin into small unilamellar vesicles was found to be reduced below the lipid phase transition temperature, Tt, whereas it incorporates and distributes more randomly above Tt. Cooling the temperature below Tt after incubation at T greater than Tt leads to a deeper incorporation of the peptide into the lipid bilayer due to electrostatic interaction between the lipid phosphate groups and the positively charged amino acids. This stabilizing effect is lost above Tt and melittin is extruded to the polar phase. Quenching experiments support this finding. EPR measurements clearly demonstrate that even in the presence of high amounts of melittin up to 10 mol% with respect to the lipid broadening of the phase transition curves was only observed with fatty acid spin labels, where the doxyl group is localized near the bilayer surface. The order degree of the inner part of the bilayer remains almost unchanged even in the presence of high melittin content.

Effects of bee venom melittin on the order and dynamics of dimyristoylphosphatidylcholine unilamellar and multilamellar vesicles

The effects of bee venom melittin on the order and dynamics of dimyristoylphosphatidylcholine unilamellar and multilamellar vesicles at a protein-to-lipid molar ratio of 1:60 have been investigated by employing the techniques of nanosecond emission anisotropy with 1,6-diphenyl-1,3,5-hexatriene as the fluorescent probe, enhancement by polar groups of the weakly allowed 0-0 vibronic transition in the fluorescence spectrum of pyrene, and Raman spectroscopy. The emission anisotropy results, which are found to be consistent with the wobble-in-cone model, show that the protein induces an increase in the order parameter, S, of the acyl chains of unilamellar vesicles below, at, and above their phase transition temperature, Tt, and it decreases strongly the diffusion rate, Dw, only below Tt. On the other hand, for multilamellar vesicles, the protein induces a decrease in S only at Tt and does not affect Dw. These effects are consistent with the observed changes in the degree of enhancement of the 0-0 vibronic transition of pyrene. Moreover, the protein broadens the thermal transition profile of multilamellar vesicles but sharpens dramatically that of unilamellar vesicles and fuses them without changing significantly the Tt in either case. On the other hand, the Raman data detect a decrease in the inter- and intramolecular order of the acyl chains of multilamellar vesicles below Tt and a decrease of only the former Tt. This disparity between the Raman and the nanosecond emission anisotropy data is discussed in terms of differences in the time scales of the two techniques and in the state of aggregation of the lipid-bound melittin. The data for the enhancement of the 0-0 vibronic transition of pyrene suggest that, for a melittin-to-lipid ratio of 1:60, the size or structure of channels formed in the bilayer by melittin does not allow the penetration of a neutral molecule the size of pyrene deeply into the bilayer.

Anisotropy decay of diphenylhexatriene in melittin-phospholipid complexes by multifrequency phase-modulation fluorometry

Frequency-domain fluorometry was used to investigate the intensity and anisotropy decays of diphenylhexatriene (DPH) in melittin-lipid complexes. Simulated and experimental data indicate that correlation times ranging from 0.3 to 500 ns can be determined using data from 1 to 200 MHz. For the melittin-lipid complexes the hindered rotator model was not adequate to account for the anisotropy decays, especially at temperatures above the transition temperatures. At high protein-to-lipid ratios the data revealed the formation of small particles (100 A) of melittin and dipalmitoylphosphatidylcholine and the disruption of membrane order in bilayers of dipalmitoylphosphatidic acid.

Molecular details of melittin-induced lysis of phospholipid membranes as revealed by deuterium and phosphorus NMR

Solid-state deuterium and phosphorus-31 nuclear magnetic resonance (2H and 31P NMR) studies of deuterium-enriched phosphatidylcholine [( 3',3'-2H2]DPPC, [sn-2-2H31]DPPC) and ditetradecylphosphatidylglycerol (DMPG-diether), as water dispersions, were undertaken to investigate the action of melittin on zwitterionic and negatively charged membrane phospholipids. When the lipid-to-protein ratio (Ri) is greater than or equal to 20, the 2H and 31P NMR spectral features indicate that the system is constituted by large bilayer structures of several thousand angstrom curvature radius, at T greater than Tc (Tc, temperature of "gel-to-liquid crystal" phase transition of pure lipid dispersions). At T approximately Tc, a detailed analysis of the lipid chain ordering shows that melittin induces a slight disordering of the "plateau" positions concomitantly with a substantial ordering of positions near the bilayer center. At T much greater than Tc, an apparent general chain disordering is observed. These findings suggest that melittin is in contact with the acyl chain segments and that its position within the bilayer may depend on the temperature. On a cooling down below Tc, for Ri greater than 20, two-phase spectra are observed, i.e., narrow single resonances superimposed on gel-type phosphorus and deuterium powder patterns. These narrow resonances are characteristic of small structures (vesicles, micelles, ... of a few hundred angstrom curvature radius) undergoing fast isotropic reorientation, which averages to zero both the quadrupolar and chemical shift anisotropy interactions. On an increase of the temperature above Tc, the NMR spectra indicate that the system returns reversibly to large bilayer structures.(ABSTRACT TRUNCATED AT 250 WORDS)

A study of protein dynamics from anisotropy decays obtained by variable frequency phase-modulation fluorometry: internal motions of N-methylanthraniloyl melittin

Internal motions of melittin and its lipid complexes were studied by anisotropy decays determined by frequency-domain fluorometry. A covalent anthraniloyl probe was attached, probably to lysine-21. The emission spectra indicate that the anthraniloyl moiety is exposed to solvent in both monomeric and tetrameric forms and is present at the lipid-water interfacial region in the lipid complexes. The fluorescence intensity decay of melittin in solution and its lipid complexes was characterized by three lifetimes. The lifetimes were near 1-2 ns, 6-7 ns and 10 ns. At increased temperatures there was an increase in the amplitude of the intermediate lifetime and a decrease in that of the longer lifetime. For all the melittin systems, at least three correlation times were required to fit the anisotropy data. Of the three correlation times, the shortest correlation time represents the local motions of the probe, while the longest represents global motions of the whole molecule. The intermediate correlation time probably represents the dynamics of domains/helices within the molecule. The melittin monomer is highly flexible, with greater than 90% of its anisotropy being lost by the local motions. Even though it is well organized (greater than 75% helical), the tetramer is still a highly flexible molecule, with 70% of its anisotropy being lost by the local motions. The internal motions of melittin decrease upon binding to lipids and are sensitive to the phase state of the lipid complexes.

Investigation of polylysine-dipalmitoylphosphatidylglycerol interactions in model membranes

The effect of poly(L-lysine) on dipalmitoylphosphatidylglycerol bilayers has been studied by Raman and infrared spectroscopies, small-angle X-ray diffraction, and carboxyfluorescein escape experiments. The polypeptide is shown to induce a stabilization of the bilayer detected by the increase of interchain vibrational coupling and a slight decrease of the overall disorder. In addition, long polylysine (Mr 150,000) induces a positive shift of the gel to fluid transition temperature and, at lipid to lysine molar ratios greater than 1, a lateral phase separation within the bilayer. Raman and infrared spectra indicate modifications at the head group level. In contrast, short polylysine (Mr 4,000) leads to a decrease of the lipid thermotropic transition temperature, and no modification of the polar head group and no phase separation could be observed. These differences between short and long polypeptides are correlated with the conformation the polypeptide adopts upon binding to the lipid, which favors the formation of alpha-helices in the case of long polypeptides (Mr greater than or equal to 14,000). The X-ray data suggest that the basic polypeptide acts as a bridge between neighboring bilayers, thus causing their aggregation and dehydration.

Morphological changes of phosphatidylcholine bilayers induced by melittin: vesicularization, fusion, discoidal particles

Morphological changes induced by the melittin tetramer on bilayers of egg phosphatidylcholine and dipalmitoylphosphatidylcholine have been studied by quasi-elastic light scattering, gel filtration and freeze-fracture electron microscopy. It is concluded that melittin similarly binds and changes the morphology of both single and multilamellar vesicles, provided that their hydrocarbon chains have a disordered conformation, i.e., at temperatures higher than that of the transition, Tm. When the hydrocarbon chains are ordered (gel phase), only small unilamellar vesicles are morphologically affected by melittin. However after incubation at T greater than Tm, major structural changes are detected in the gel phase, regardless of the initial morphology of the lipids. Results from all techniques agree on the following points. At low melittin content, phospholipid-to-peptide molar ratios, Ri greater than 30, heterogeneous systems are observed, the new structures coexisting with the original ones. For lipids in the fluid phase and Ri greater than 12, the complexes formed are large unilamellar vesicles of about 1300 +/- 300 A diameter and showing on freeze-fracture images rough fracture surfaces. For lipids in the gel phase, T less than Tm after passage above Tm, and for 5 less than Ri less than 50, disc-like complexes are observed and isolated. They have a diameter of 235 +/- 23 A and are about one bilayer thick; their composition corresponds to one melittin for about 20 +/- 2 lipid molecules. It is proposed that the discs are constituted by about 1500 lipid molecules arranged in a bilayer and surrounded by a belt of melittin in which the mellitin rods are perpendicular to the bilayer. For high amounts of melittin, Ri less than 2, much smaller and more spherical objects are observed. They are interpreted as corresponding to lipid-peptide co-micelles in which probably no more bilayer structure is left. It is concluded that melittin induces a reorganization of lipid assemblies which can involve different processes, depending on experimental conditions: vesicularization of multibilayers; fusion of small lipid vesicles; fragmentation into discs and micelles. Such processes are discussed in connexion with the mechanism of action of melittin: the lysis of biological membranes and the synergism between melittin and phospholipases.

Interaction of myelin basic protein with different ionization states of phosphatidic acid and phosphatidylserine

Myelin basic protein (BP) has a perturbing effect on some lipids, causing, among other effects, a decrease in the temperature and enthalpy of the phase transition. This is believed to be a result of penetration of some hydrophobic residues of the protein partway into the lipid bilayer. Variations in the perturbing effect of BP on different acidic lipids has been attributed to the ability of the lipids to participate in intermolecular hydrogen bonding which inhibits penetration of the protein. Participation in intermolecular hydrogen bonding depends on the ionization state of the lipid as well as the type of lipid. In order to further test the dependence of the degree of penetration of BP on the intermolecular hydrogen bonding properties of lipids, the effect of BP on the phase transition of lipids in different ionization states was studied using differential scanning calorimetry. Dipalmitoylphosphatidic acid (DPPA) and dimyristoylphosphatidylserine (DMPS) were studied at different pH-values from 4 to 9.5. The results were compared to data obtained earlier with phosphatidylglycerol (PG), which is in the same ionization state at pH-values above 4, in order to distinguish the effects of pH on the protein from effects on the lipids. The perturbing effect of BP on PG increases with increase in pH. This is probably a result of the increasing hydrophobicity of the protein as the histidines become deprotonated, which allows greater penetration of the protein into the bilayer. In contrast, the effect on DPPA was greatest at low pH, where the state of ionization of the lipid is less than 1 and protein binding utilizes all of the hydrogen bond accepting sites (P-O-) on the lipid. BP had no perturbing effect on DPPA at higher pH where the state of ionization is between 1 and 1.5, and hydrogen bond accepting and donating sites (P-OH) are still available even after binding of the protein. Thus hydrogen bonding occurs at high pH and penetration of hydrophobic residues of the protein into DPPA is inhibited. BP had a large perturbing effect on DMPS at all pH values above 4 suggesting that lipid intermolecular hydrogen bonding does not occur in the presence of the protein and its hydrophobic residues consequently can penetrate into the bilayer. The protein may inhibit hydrogen bonding by binding electrostatically to the anionic hydrogen bond accepting group of PS.