Interaction of melittin with negatively charged phospholipids: consequences for lipid organization

A characterization of the structural alterations induced by melittin in model-membranes of dioleoylphosphatidic acid and egg phosphatidylglycerol is presented, based on the use of 31P-NMR, freeze-fracture electron microscopy and small angle X-ray scattering. In accordance with earlier findings on the cardiolipin-melittin system, melittin is found to have an inverted phase inducing effect on these negatively charged lipids, in contrast to the influence on zwitterionic phospholipids. In phosphatidic acid this is expressed in the formation of an HII phase; in phosphatidylglycerol a less ordered, non-lamellar structure with low water content is adopted.     

Cytochrome c specifically induces non-bilayer structures in cardiolipin-containing model membranes

(1) The effect of cytochrome c addition on the phospholipid structure of liposomes composed of cardiolipin, phosphatidylserine, phosphatidylglycerol, phosphatidylcholine or phosphatidylethanolamine in a pure form or in mixtures was investigated by 31P-NMR and freeze-fracture techniques. (2) Cytochrome c specifically induces the hexagonal Hii phase and possibly an inverted micellar structure of part of the phospholipids in cardiolipin-containing model membranes. (3) These results are compared with the effect of Ca2+ on cardiolipin and are discussed in relation to the structure and function of the inner mitochondrial membrane.     

Penetration of a cardiotoxin into cardiolipin model membranes and its implications on lipid organization

The interaction of cardiotoxin II of Naja mossambica mossambica with cardiolipin model membranes was investigated by binding, fluorescence, resonance energy transfer, fluorescence quenching, 31P NMR, freeze-fracture, and small-angle X-ray experiments. An initially electrostatic binding appeared to be accompanied by a deep penetration, most likely into the acyl chain region of the phospholipids, indicating a hydrophobic contribution to the strong interaction (KD congruent to 5 X 10(-8) M). This binding results in a fusion of unilamellar vesicles as indicated by a fluorescence-based fusion assay, freeze-fracture, and X-ray diffraction. In these fused structures freeze-fracture electron microscopy reveals the appearance of particles, which is accompanied by the induction of an isotropic component in 31P NMR. The well-defined particles are interpreted as inverted micelles, and the localization of the cardiotoxin molecule in these structures is discussed.     

Molecular aspects of the bilayer stabilization induced by poly(L-lysines) of varying size in cardiolipin liposomes

The interaction between poly(L-lysines) of varying size with cardiolipin was investigated via binding assays, X-ray diffraction, freeze-fracture electron microscopy, and 31P- and 13C-NMR. Binding of polylysines to the lipid only occurred when three or more lysine residues were present per molecule. The strength of the binding was highly dependent on the polymerization degree, suggesting a cooperative interaction of the lysines within the polymer. Upon binding, a structural reorganization of the lipids takes place, resulting in a closely packed multilamellar system in which the polylysines are sandwiched in between subsequent bilayers. Acyl chain motion is reduced in these liquid-crystalline peptide-lipid complexes. From competition experiments with Ca2+ it could be concluded that when the affinity of the polylysine for cardiolipin was much larger than that of Ca2+, a lamellar polylysine-lipid complex was formed, irrespective of whether an excess of Ca2+ was added prior to or after the polypeptide. When the affinity of the polylysine for cardiolipin was less or of the same order as that of Ca2+, the lipid was organized in the hexagonal HII phase in the presence of Ca2+. These results are discussed in the light of the peptide specificity of bilayer (de)stabilization in cardiolipin model membranes.     

Phase behavior of the major lipids of Tetrahymena ciliary membranes

The major lipids of Tetrahymena membranes have been purified by thin-layer and high pressure liquid chromatography and the phosphatidylethanolamine and aminoethylphosphonate lipids were examined in detail. ³¹P-NMR, X-ray diffraction and freeze-fracture electron microscopy were employed to describe the phase behavior of these lipids. The phosphatidylethanolamine was found to form a hexagonal phase above 10°C. The aminoethylphosphonate formed a lamellar phase up to 20°C, but converted to a hexagonal phase structure at 40°C. Small amounts of phosphatidylcholine stabilized the lamellar phase for the aminoethylphosphonate. ³¹P-NMR spectra of the intact ciliary membranes were consistent with a phospholipid bilayer at 30°C, suggesting that phosphatidylcholine in the membrane stabilized the lamellar form, even though most of the lipid of that membrane prefers a hexagonal phase in pure form at 30°C. ³¹P-NMR spectra also showed a distinctive difference in the chemical shift tensor of the aminoethylphosphonolipid, when compared to that of phosphatidylethanolamine, due to the difference in chemical structure of the polar headgroups of the two lipids.     

Polymorphic phase behaviour of cardiolipin as detected by 31P NMR and freeze-fracture techniques. Effects of calcium, dibucaine and chlorpromazine.

1. The influence of Ca2+ on the polymorphic phase behaviour of cardiolipin has been investigated employing 31P NMR and freeze-fracture techniques. The close correlation between the results obtained here and previous X-ray studies (Rand, R.P. and Sengupta, S. (1972) Biochim. Biophys. Acta 255, 484--492) confirms 31P NMR as a useful analytical procedure for investigating the polymorphic phase behaviour of hydrated phospholipids. 2. Ca2+ induces formation of the hexagonal (H11) phase via an intermediary phase which is observed at Ca2+/cardiolipin ratios of less than 1 (mol/mol). This intermediary appears to consist of "inverted' structure which lies adjacent to regions of bilayer structure. 3. The local anaesthetics dibucaine and chlorpromazine produce similar phase changes for cardiolipin as does Ca2+. It is suggested that the anaesthetics interact with the membrane in their charged form and induce their effects by charge neutralization.     

Effective charge of melittin upon interaction with POPC vesicles

The binding of bee venom melittin to small unilamellar vesicles and large nonsonicated multilamellar bilayer membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was studied by means of circular dichroism, 31P-NMR and electrophoretic mobility. The melittin binding isotherm for small unilamellar vesicles (SUV) could be described by a partition equilibrium with Kp = (6 +/- 1).10(4) M-1. Electrostatic effects were taken into account by means of the Gouy-Chapman theory. Combining the partition equilibrium with the Gouy-Chapman analysis suggested an effective charge for melittin of Zp = 1.9, which is lower than the true electric charge of 5-6. The variation of the 31P-NMR signal of SUV showed the change in potential at the phosphodiester moiety of the lipid upon addition of melittin. This potential change was lower than that for an ion with an electrical charge of 5-6 and corresponded to a charge of 1.5. Electrophoretic mobility measurements with multilamellar vesicles confirmed the charge reduction effect. These experimental results show that the use of the simple Gouy-Chapman theory requires an effective electrical charge of the melittin which is lower than the formal charge.     

Molecular aspects of the bilayer stabilization induced by poly(l-lysines) of varying size in cardiolipin liposomes

The interaction between poly(l-lysines) of varying size with cardiolipin was investigated via binding assays, X-ray diffraction, freeze-fracture electron microscopy, and ³¹P- and ¹³C-NMR. Binding of polylysines to the lipid only occurred when three or more lysine residues were present per molecule. The strength of the binding was highly dependent on the polymerization degree, suggesting a cooperative interaction of the lysines within the polymer. Upon binding, a structural reorganization of the lipids takes place, resulting in a closely packed multilamellar system in which the polylysines are sandwiched in between subsequent bilayers. Acyl chain motion is reduced in these liquid-crystalline peptide-lipid complexes. From competition experiments with Ca²⁺ it could be concluded that when the affinity of the polylysine for cardiolipin was much larger than that of Ca²⁺, a lamellar polylysine-lipid complex was formed, irrespective of whether an excess of Ca²⁺ was added prior to or after the polypeptide. When the affinity of the polylysine for cardiolipin was less or of the same order as that of Ca²⁺, the lipid was organized in the hexagonal H_II phase in the presence of Ca²⁺. These results are discussed in the light of the peptide specificity of bilayer (de)stabilization in cardiolipin model membranes.     

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.     

Interactions of La2+ with phosphatidylserine vesicles: binding, phase transition, leakage, 31P-NMR and fusion

The interaction of La2+ with phosphatidylserine vesicles is studied by differential scanning calorimetry, 140La binding, 31P-NMR chemical shifts and relaxation rates, carboxyfluorescein and [14C]sucrose release, X-ray diffraction and freeze-fracture electron microscopy. In the presence of La3+ concentrations above 1 mM and an incubation temperature of 38 degrees C, i.e., at the phase transition temperature of the complex La/phosphatidylserine, the binding ratio of La/lipid exceeds a 1/3 ratio, reaching saturation at a 1/2 ratio. Analysis, employing a modified Gouy-Chapman equation, indicates a significant increase in the intrinsic binding constant of La/phosphatidylserine when the La3+ concentrations exceeds the threshold concentration for leakage. The analysis illustrates that at the molecular level the binding of La3+ can be comparable to or even weaker than that of Ca2+, but that even when present at smaller concentrations La3+ competes with and partially displaces Ca2+ from membranes or other negatively charged surfaces. The results suggest that the sequence La3+ greater than Ca2+ greater than Mg2+ reflects both the binding strength of these cations to phosphatidylserine as well as their ability to induce leakage, enhancement of 31P spin-lattice relaxation rates, fusion and other structural changes. The leakage, fusion, and other structural changes are more pronounced at the phase transition temperature of the La/lipid complex.     

Melittin-induced changes of the macroscopic structure of phosphatidylethanolamines

The binding of melittin to phosphatidylethanolamine model systems and its influence on the supramolecular organization of the lipid were investigated with binding assays, differential scanning calorimetry, 31P NMR, freeze-fracture electron microscopy, and small-angle X-ray scattering. The results are compared with binding to an analogous phosphatidylcholine and structural consequences thereof. Melittin binds with similar affinity to both lipid types in the liquid-crystalline state; at gel-phase temperatures, in contrast, interaction with phosphatidylethanolamine is much weaker and does not lead to the bilayer fragmentation observed for phosphatidylcholines. With regard to phosphatidylethanolamine polymorphism, it is shown that melittin acts as an inhibitor of HII-phase formation and as a stabilizer of the bilayer organization. It is demonstrated that the remarkable variety of effects of melittin on the polymorphism of different membrane phospholipids can be understood in a relatively simple concept, taking into account the relative position and the shape of the interacting components.     

Adriamycin inhibits the formation of non-bilayer lipid structures in cardiolipin-containing model membranes

The effect of adriamycin on cardiolipin-containing model membrane systems have been studied by ³¹P-NMR, freeze-fracture electron microscopy and binding experiments. Adriamycin effectively inhibits the formation of non-bilayer lipid structures induced by Ca²⁺ and cytochrome c in cardiolipin-containing liposomes. This drug also strongly inhibits the uptake of Ca²⁺ by cardiolipin into an organic phase. These results are discussed in relation to the cardiotoxic effect of adriamycin and the possible importance of non-bilayer lipid structures for the functioning of the mitochondrion.     

Polymorphic phase behaviour of dilinoleoylphosphatidylethanolamine and palmitoyloleoylphosphatidylcholine mixtures. Structural changes between hexagonal, cubic and bilayer phases

The polymorphic phase behaviour of dilinoleoylphosphatidyethanolamine (DLPE) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) is investigated by freeze-fracture electron microscopy, X-ray diffraction and 31P-NMR. The structures at 5% or less POPC are predominantly inverted hexagonal (HII), whereas at 15% or more POPC, the structure is mostly bilayer (L), interrupted by defects (lipidic particles). A cubic phase structure is observed in the transition range between H and L phases; the cubic arrangement deteriorates at higher temperatures into an amorphous aggregate of spherical units. Both cubic and amorphous structures contribute to the isotropic 31P resonance, with no preference for PC or PE partitioning in the isotropic motion as observed by high resolution NMR. The existence of the cubic phase seems to depend cirtically on the homogeneity and the degree unsaturation of the phospholipids.     

Polymorphic phase behaviour of dilinoleoylphosphatidylethanolamine and palmitoyloleoylphosphatidylcholine mixtures: Structural changes between hexagonal,cubic and bilayer phases

The polymorphic phase behaviour of dilinoleoylphosphatidyethanolamine (DLPE) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) is investigated by freeze-fracture electron microscopy, X-ray diffraction and ³¹P-NMR. The structures at 5% or less POPC are predominantly inverted hexagonal (H_II), whereas at 15% or more POPC, the structure is mostly bilayer (L), interrupted by defects (lipidic particles). A cubic phase structure is observed in the transition range between H and L phases; the cubic arrangement deteriorates at higher temperatures into an amorphous aggregate of spherical units. Both cubic and amorphous structures contribute to the isotropic ³¹P resonance, with no preference for PC or PE partitioning in the isotropic motion as observed by high resolution NMR. The existence of the cubic phase seems to depend critially on the homogeneity and the degree unsaturation of the phospholipids.     

Naja naja oxiana Cobra Venom Cytotoxins CTI and CTII Disrupt Mitochondrial Membrane Integrity: Implications for Basic Three-Fingered Cytotoxins

Cobra venom cytotoxins are basic three-fingered, amphipathic, non-enzymatic proteins that constitute a major fraction of cobra venom. While cytotoxins cause mitochondrial dysfunction in different cell types, the mechanisms by which cytotoxins bind to mitochondria remain unknown. We analyzed the abilities of CTI and CTII, S-type and P-type cytotoxins from Naja naja oxiana respectively, to associate with isolated mitochondrial fractions or with model membranes that simulate the mitochondrial lipid environment by using a myriad of biophysical techniques. Phosphorus-31 nuclear magnetic resonance (³¹P-NMR) spectroscopy data suggest that both cytotoxins bind to isolated mitochondrial fractions and promote the formation of aberrant non-bilayer structures. We then hypothesized that CTI and CTII bind to cardiolipin (CL) to disrupt mitochondrial membranes. Collectively, ³¹P-NMR, electron paramagnetic resonance (EPR), proton NMR (¹H-NMR), deuterium NMR (²H-NMR) spectroscopy, differential scanning calorimetry, and erythrosine phosphorescence assays suggest that CTI and CTII bind to CL to generate non-bilayer structures and promote the permeabilization, dehydration and fusion of large unilamellar phosphatidylcholine (PC) liposomes enriched with CL. On the other hand, CTII but not CTI caused biophysical alterations of large unilamellar PC liposomes enriched with phosphatidylserine (PS). Mechanistically, single molecule docking simulations identified putative CL, PS and PC binding sites in CTI and CTII. While the predicted binding sites for PS and PC share a high number of interactive amino acid residues in CTI and CTII, the CL biding sites in CTII and CTI are more divergent as it contains additional interactive amino acid residues. Overall, our data suggest that cytotoxins physically associate with mitochondrial membranes by binding to CL to disrupt mitochondrial structural integrity.     

Effect of acyl chain composition on salt-induced lamellar to inverted hexagonal phase transitions in cardiolipin

Salt-induced fluid lamellar (L alpha) to inverted hexagonal (HII) phase transitions have been studied in diphosphatidylglycerols (cardiolipins) with different acyl chain compositions, using 31P nuclear magnetic resonance (NMR) spectroscopy. Cardiolipins with four myristoyl chains, tetramyristoyl cardiolipin (TMCL), and with four oleoyl chains, tetraoleoyl cardiolipin (TOCL), were synthesized chemically. TMCL was found to undergo a thermotropic lamellar gel to lamellar liquid-crystalline phase transition at 33-35 degrees C. This lipid exhibited an axially symmetric 31P-NMR spectrum corresponding to a lamellar phase at all NaCl concentrations between 0 and 6 M. In the case of TOCL, formation of an HII phase was induced by salt concentrations of 3.5 M NaCl or greater. These observations, taken together with earlier findings that bovine heart cardiolipin aqueous dispersions adopt an HII phase at salt concentrations of 1.5 M NaCl or greater, indicate that increasing unsaturation and length of the acyl chains favour formation of the HII phase in diphosphatidylglycerols.     

Dolichyl phosphate induces non-bilayer structures, vesicle fusion and transbilayer movement of lipids: a model membrane study

The effect of dolichol and dolichyl phosphate on fusion between large unilamellar vesicles comprised of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) was studied using a fluorescence resonance energy transfer assay. The influence of dolichyl phosphate on the transbilayer movement of DOPC in multilamellar vesicles (MLV) and large unilamellar vesicles (LUV) composed of DOPC and DOPE (1:2) was investigated by using the phosphatidylcholine-specific transfer protein. 31P-NMR and freeze-fracture electron microscopy were employed to study the macroscopic organization of DOPC and DOPE containing model membranes in the absence or presence of dolichyl phosphate. The results indicate that both dolichol and dolichyl phosphate enhance vesicle fusion in a comparable and concentration-dependent way; the amount of exchangeable PC from MLVs is increased by dolichyl phosphate, probably as a result of fusion processes; dolichyl phosphate destabilizes the bilayer organization in MLVs comprised of DOPE and DOPC, resulting in the formation of hexagonal (HII) phase and 'lipidic' particles.     

N-succinyldioleoylphosphatidylethanolamine: structural preferences in pure and mixed model membranes

The structural preferences of the pH-sensitive phospholipid, N-succinyldioleoylphosphatidylethanolamine (N-succinyl-DOPE), have been examined alone and in mixtures with DOPE by 31P-NMR, fluorescence energy transfer, and freeze-fracture techniques. The basic polymorphic behavior of pure N-succinyl-DOPE and DOPE/N-succinyl-DOPE lipid systems and the influence of calcium and pH were investigated. It is shown that, similar to other negatively charged acidic phospholipids, N-succinyl-DOPE adopts the bilayer organization upon hydration. This structure is maintained at both pH 7.4 and 4.0 in the presence or absence of calcium. In the mixed lipid system, N-succinyl-DOPE can stabilize the non-bilayer lipid, DOPE, into a bilayer structure at both pH 7.4 and 4.0 at more than 10 mol% N-succinyl-DOPE, although a narrow 31P-NMR lineshape is observed at acidic pH values. This corresponds to the presence of smaller vesicles as shown by quasi-elastic light scattering measurements. Addition of equimolar calcium (with respect to N-succinyl-DOPE) to the DOPE/N-succinyl-DOPE systems induces the hexagonal HII phase at both pH values. In unilamellar systems with similar lipid composition the addition of Ca2+ results in membrane fusion as indicated by fluorescence energy-transfer experiments. These findings are discussed with regard to the molecular mechanism of the bilayer to hexagonal HII phase transition and membrane fusion and the utility of N-succinyl-DOPE containing pH-sensitive vesicles as drug-delivery vehicles.     

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).     

Comparable interaction of doxorubicin with various acidic phospholipids results in changes of lipid order and dynamics

We have characterized the interaction of the antitumor drug doxorubicin with model membranes of the anionic phospholipids dioleoylphosphatidic acid (DOPA), dioleoylphosphatidylserine (DOPS), cardiolipin and dioleoylphosphatidylglycerol (DOPG) as compared to the zwitterionic dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylethanolamine (DOPE). The saturating binding levels were: 2.4 (DOPA), 1.3 (cardiolipin), 1.5 (DOPS, DOPG) and 0.02 (DOPC) doxorubicin per lipid phosphorus (mol/mol). The half-saturating free drug concentrations were comparable for DOPA, cardiolipin, DOPS and DOPG: 20, 16, 35 and 18 microM, respectively. Doxorubicin fluorescence revealed the simultaneous existence of at least two populations of bound drug in the various anionic phospholipids: (1) fluorescent molecules with chromophores that reside between the lipid molecules and (2) above 0.01-0.02 doxorubicin bound per lipid phosphorus: non-fluorescent drug-stacks that are closer to the aqueous phase than the fluorescent molecules. Small-angle X-ray scattering indicated that doxorubicin can reorganize anionic phospholipid dispersions into closely-packed multilamellar structures. Addition of the drug caused leakage of entrapped 6-carboxyfluorescein. Neither 2H-NMR on [2-2H]serine-labelled DOPS nor 31P-NMR revealed any significant effect of doxorubicin on headgroup conformation, but 2H-NMR on di[11,11-2H2]oleoyl-labelled phospholipids showed that the drug had a strong acyl chain-disordering effect on anionic phospholipids. 2H-NMR relaxation measurements indicated that the drug immobilized the headgroups and acyl chains of anionic phospholipids. The implications of these observations for the cellular activity of the drug are indicated.