Cooperative lipid-protein interaction. Effect of pH and ionic strength on polymyxin binding to phosphatidic acid membranes

Meet the IUPAB Councilor-Hans-Joachim Galla

As one of the twelve Councilors, it is my pleasure to provide a short biographical sketch for the readers of Biophys. Rev. and for the members of the Biophysical Societies. I have been a member of the council in the former election period. Moreover, I served since decades in the German Biophysical Society (DGfB) as board member, secretary, vice president, and president. I hold a diploma degree in chemistry as well as PhD from the University of Göttingen. The experimental work for both qualifications has been performed at the Max Planck Institute for Biophysical Chemistry in Göttingen under the guidance of Erich Sackmann and the late Herman Träuble. When E. Sackmann moved to the University of Ulm, I joined his group as a research assistant performing my independent research on structure and dynamics of biological and artificial membranes and qualified for the “habilitation” thesis in Biophysical Chemistry. I have spent a research year at Stanford University supported by the Deutsche Forschungsgemeinschaft (DFG) and after coming back to Germany, I was appointed as a Heisenberg Fellow by the DFG and became Professor in Biophysical Chemistry in the Chemistry Department of the University of Darmstadt. Since 1990, I spent my career at the Institute for Biochemistry of the University of Muenster as full Professor and Director of the institute. I have trained numerous undergraduate, 150 graduate, and postdoctoral students from chemistry, physics, and also pharmacy as well as biology resulting in more than 350 published papers including reviews and book articles in excellent collaboration with colleagues from different academic disciplines in our university and also internationally, e.g., as a guest professor at the Chemistry Department of the Chinese Academy of Science in Beijing.

Highly synergistic antimicrobial activity of magainin 2 and PGLa peptides is rooted in the formation of supramolecular complexes with lipids

Magainin 2 and PGLa are cationic, amphipathic antimicrobial peptides which when added as equimolar mixture exhibit a pronounced synergism in both their antibacterial and pore-forming activities. Here we show for the first time that the peptides assemble into defined supramolecular structures along the membrane interface. The resulting mesophases are quantitatively described by state-of-the art fluorescence self-quenching and correlation spectroscopies. Notably, the synergistic behavior of magainin 2 and PGLa correlates with the formation of hetero-domains and an order-of-magnitude increased membrane affinity of both peptides. Enhanced membrane association of the peptide mixture is only observed in the presence of phophatidylethanolamines but not of phosphatidylcholines, lipids that dominate bacterial and eukaryotic membranes, respectively. Thereby the increased membrane-affinity of the peptide mixtures not only explains their synergistic antimicrobial activity, but at the same time provides a new concept to increase the therapeutic window of combinatorial drugs.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Thermodynamics and Structural Studies of the Interaction of Polymyxin B with Deep Rough Mutant Lipopolysaccharides

Deep rough mutant lipopolysaccharide (ReLPS) dissolved in aqueous solution spontaneously forms supramolecular structures which mainly consist of vesicles. Addition of Polymyxin B (PmB) to these ReLPS vesicles influence the shape of these structures as demonstrated here by electronmicroscopy and dynamic light scattering techniques. The main phase transition of the ReLPS is found at 21.3 +/- 0.1 degrees C for ReLPS from Escherichia coli and at 24.0 +/- 0.5 degrees C for ReLPS from Salmonella minnesota by differential scanning calorimetry (DSC). Using isothermal differential titration calorimetry (ITC), the thermodynamic behavior of the interaction of PmB with ReLPS vesicles has been studied. The stoichiometric ratio for the binding of PmB to ReLPS was found to lie between 0.6 and 1, as determined from ITC and monolayer experiments. No phase transition was observed for ReLPS monolayers saturated with PmB. The results indicate specific interaction of PmB with ReLPS. We propose a two-step mechanism for this interaction, which involves electrostatic attraction between charged parts of the molecules and, in the second step, hydrophobic interactions between the nonpolar parts of both compounds. Copyright 1999 Academic Press.

Modulation of polymyxin B effects on mammalian urinary bladder

This report demonstrates that Ca2+, Mg2+, and protons alter the ability of polymyxin B (PX, a cationic antibiotic used clinically as a bactericidal agent) to increase the apical membrane conductance of the rabbit urinary bladder. Using electrophysiological methods, we determine that these alterations occur by two mechanisms. First, they blocked the PX-induced conductance in a rapid and reversible manner; second, they competed with PX for a membrane binding site. In addition, Ca2+ (but not Mg2+ or protons) altered the rate at which the induced conductance could be reversed. When solution pH was greater than 8.8, PX was not able to induce a conductance. This ability of high pH to inhibit the action of PX was due to a decrease in the number of positive charges on PX. Further studies demonstrated that for maximal activity, PX required its fatty acid tail. These data were used to develop a model describing the mechanism by which PX can induce a conductance in the apical membrane of the rabbit urinary bladder.

Structural characterization of free and membrane-bound nisin by infrared spectroscopy

This study reports two new trends about nisin affinity for lipid membranes. First, there is a very strong dependence of nisin binding on the membrane surface charge. As illustrated in this work, the binding of nisin is much greater for phosphatidylglycerol (PG) than for phosphatidylcholine (PC) membranes. This can be rationalized by electrostatic attraction between the positively charged peptide and the negatively charged PG. Second, the affinity of nisin shows a very weak dependence on the lipid phase, the binding to fluid or gel phase membranes being nearly equivalent. Therefore, our results suggest that nisin behaves as an extrinsic peptide. This work also presents the first piece of information relative to the structure of membrane-bound nisin. The Amide I band of the peptide is different for free nisin in water and for membrane-bound nisin. By analyzing this region using self-deconvolution and band fitting, and by comparing with results obtained from nisin dissolved in various H2O/trifluoroethanol mixtures, it can be inferred that the binding of nisin to phospholipid membranes leads to an increased proportion of beta-turns.

Submitochondrial localization and membrane topography of Ehrlich ascitic tumour cell glutaminase

The intramitocondrial localization of the phosphate-activated glutaminase from Ehrlich cells has been examined by a combination of techniques, including: mitochondria subfractionation studies, chemical modification with sulfhydryl group reagents of different permeability, enzymatic digestion in both sides of the inner mitochondrial membrane, and immunological studies. Using alkaline extraction at high ionic strength, hypoosmotic shock and freezing-thawing cycle techniques, the enzyme was found in the particulate fraction. On the contrary, glutaminase activity was labile when subfractionation was carried out by digitonin/lubrol method; Western blot analysis localized the inactive enzyme in the matrix fraction. In addition, glutaminase was fully inactivated when mitoplasts were incubated with phospholipase A2 and phospholipase C. The enzyme also showed a non-linear Arrhenius plot with a break at 24 degrees C. The membrane-impermeant thiol reagents mersalyl and p-chloromercuriphenylsulfonic acid do not inhibit glutaminase activity in freeze-thawed mitochondria and mitoplasts, but N-ethylmaleimide, which is membrane permeant, strongly inhibited the enzyme. However, mersalyl and p-chloromercuriphenylsulfonic acid were effective inhibitors when the alkylation was performed on the matrix side of mitoplasts or using detergent-solubilized enzyme. Furthermore, trypsin digestion of mitoplasts was only effective inactivating glutaminase when the proteolysis was carried out on the matrix side of the vesicles. Enzyme-linked immunosorbent assay of the soluble and membrane fractions obtained in the preparation of submitochondrial particles, revealed that most of the enzyme was solubilized, but in the inactive form. Phase separation with Triton X-114 rendered most of the protein in the aqueous phase. These results taken together discard a transmembrane localization for the protein, whereas they are consistent with anchorage of glutaminase on the matrix side of the inner mitochondrial membrane, the matrix portion of the enzyme being relevant for its function.

Membrane structure, toxins and phospholipase A2 activity

The phospholipid-hydrolyzing enzyme phospholipase A2 (PLA2) (EC 3.1.1.4) exists in several forms which can be located in the cytosol or on cellular membranes. We review briefly cellular regulatory mechanisms involving covalent modification by protein kinase C and the action of Ca2+, cytokines, G proteins and other cellular proteins. The major focus is the role of phospholipid structure on PLA2 activity, including (1) the mechanism of PLA2 action on synthetic phospholipid bilayers, (2) perturbation of synthetic and cellular membranes with lipophilic agents and membrane-interactive peptides and (3) the ability of these agents to activate endogenous PLA2 activity, with emphasis on the venom and plant toxins melittin, cardiotoxin and Pyrularia thionein.

Specific interaction of the intermediate filament protein vimentin and its isolated N-terminus with negatively charged phospholipids as determined by vesicle aggregation, fusion, and leakage measurements

The interaction of the intermediate filament protein vimentin and its non-alpha-helical N-terminus with phosphatidylserine and phosphatidylinositol small unilamellar vesicles was investigated by measuring vesicle aggregation, fusion, and leakage. While the N-terminus suppressed Ca2(+)-induced fusion of phosphatidylserine vesicles, it caused their rapid aggregation in the absence of Ca2+; at a molar ratio of lipid to polypeptide of 25:3, the polypeptide/lipid complexes precipitated from the reaction mixture. This aggregation was efficiently diminished by NaCl. The phosphatidylinositol vesicles, on the other hand, became leaky when interacting with the N-terminus of vimentin, even at a molar ratio of lipid to polypeptide of 500:1. The leakage of phosphatidylinositol vesicles was suppressed by the addition of Ca2+ or NaCl to the reaction mixture. Intact vimentin also caused leakage of phosphatidylinositol vesicles, at low and high salt concentration. The results indicate specific and differential interactions of the N-terminus of vimentin with various negatively charged lipid species, although there is an electrostatic component common to these interactions.

Interaction of polymyxin B1 and polymyxin B1 nonapeptide with phosphatidic acid monolayer and bilayer membranes

The interactions of the antibiotic polymyxin B1 and its enzymatic cleavage product polymyxin B1 nonapeptide with phosphatidic acid monolayers and with bilayer membranes were investigated. Temperature-dependent pressure-area analysis of the monolayer reveals a linear increase of the lipid mean molecular area in the liquid condensed state for polymyxin concentrations between 10(-8) and 4 x 10(-7) M. Depending on the surface pressure, the area increase amounts to 30-70 A2. A linear dependence was also observed in the liquid expanded state but saturation is reached already at 10(-7) M polymyxin. The adsorption of polymyxin to phosphatidic acid bilayers is also linear and of a Langmuir type. Saturation is reached at a 1:4 polymyxin/lipid molar ratio. Polymyxin induces a phase separation in phosphatidic acid monolayers which was concluded from the thermotropic phase transition curves. In agreement with earlier bilayer experiments a second lowered phase transition appears in the presence of polymyxin. These fluidized domains again exhibit a linear polymyxin uptake comparable to the one of the liquid expanded monolayer at a temperature, where the undisturbed lipid is still in the condensed state. Polymyxin nonapeptide also causes an expansion of phosphatidic acid monolayers but only by maximally 10 A2. The thermotropic phase transition of the monolayer is reduced and considerably broadened by the nonapeptide. In phosphatidic acid bilayers we observed a decrease of the lipid phase transition temperature by 24 degrees C. The lateral chain packing is considerably disturbed by the peptide part of polymyxin.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

A simple assay to study protein-mediated lipid exchange by fluorescence polarization

We investigated the protein-mediated phospholipid transfer between small vesicles by fluorescence polarization measurements with diphenylhexatriene as optical probe. Thermotropic-phase-transition curves were taken after mixing two vesicle preparations of lipids exhibiting different gel-to-liquid phase transitions. From the heights of each phase-transition step we were able to follow the lipid transfer process without separating the two vesicle preparations.

Lipid domain structures in biological membranes

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

Calorimetric investigation of polymyxin binding to phosphatidic acid bilayers

The cooperative binding process between the antibiotic peptide polymyxin-B and negatively-charged phosphatidic acid bilayers was investigated by differential thermal analysis and completed by fluorescence polarization measurements. The sigmoidal binding curves were analyzed in terms of the interaction energy within a domain formed by polymyxin and phosphatidic acid molecules. The formation of such a heterogeneous domain structure was favoured by high concentration of external monovalent ions. The cooperativity of the binding increased while a charge-induced decrease in the phase transition temperature of the pure lipid phase was observed with increasing ion concentration at a given pH. The reduced lateral coupling within the lipid bilayer in the presence of salt ions, as demonstrated by an increase in the lipid phase transition enthalpy, was considered to facilitate the cooperative domain formation. Moreover, an increase in the cooperativity of the polymyxin binding could be observed if phosphatidic acids of smaller chain length and thus of a lowered phase transition temperature were used. By the use of chemically-modified polymyxin we were able to demonstrate the effect of electrostatic and hydrophobic interaction. Acetylated polymyxin with a reduced positive charge was used to demonstrate the pure hydrophobic effect of polymyxin binding leading to a decrease in the phosphatidic acid phase transition temperature by about 20 degrees C. The cooperativity of the binding was strongly reduced. Cleavage of the hydrophobic polymyxin tail yielded a colistinnonapeptide which caused an electrostatically-induced increase in the phosphatidic acid phase transition temperature. With unmodified polymyxin we observed the combined effects of electrostatic as well as hydrophobic interaction making this model system interesting for the understanding of lipid-protein interactions. Evidence is presented that the formation of the polymyxin-phosphatidic acid complex is a lateral phase separation phenomenon.

Modifications of anionic-lipid domains preceding membrane fusion in guinea pig sperm

The relationship between anionic-lipid concentration and the functional properties of plasma-membrane domains was explored using the guinea-pig sperm membrane as a model, with polymyxin B (PXB) as a probe. Areas of plasmalemma specialized for fusion during the acrosome reaction had a higher affinity for the probe than adjacent nonfusigenic regions. In addition, capacitation--a process preceding acrosome:plasma-membrane fusion--markedly enlarged the area susceptible to PXB binding over the acrosomal cap. Protease treatment mimicked capacitation by increasing the acrosome-reaction incidence as well as PXB binding, at enzyme concentrations not affecting the surface coat nor altering filipin/sterol localization. Both proteolytic digestion and capacitation failed to augment PXB- or filipin-affinity in nonfusigenic zones, such as the post-acrosomal segment, including its particle-free maculae. Incubation of sperm in capacitating medium supplemented with 32P-labeled phosphate, followed by lipid extraction, thin-layer chromatography, and autoradiography, revealed a radioactive band comigrating with cardiolipin and phosphatidic acid. Vermiform protrusions elicited by PXB in the outer lamellae of cardiolipin-phosphatidylcholine liposomes resembled those seen in fusional regions of sperm membrane. We conclude that (a) differing concentrations of anionic lipids are found in adjacent domains of the sperm plasma membrane; (b) these domains mirror the functional regions of the membrane, with higher anionic-lipid concentrations localized over fusional zones; (c) the surface coat does not participate in the maintenance of such domains; (d) anionic-lipid synthesis may contribute to their formation; and (e) anionic-lipid concentrations increase as the membrane becomes fusionally competent, indicating that cellular modulation of lipid domains accompanies regulation of membrane function.

Lipid alterations in cell envelopes of polymyxin-resistant Pseudomonas aeruginosa isolates

The lipid composition of cells of Pseudomonas aeruginosa strains resistant to polymyxin was compared with the lipid composition of cells of polymyxin-sensitive strains as to their content of readily extractable lipids (RELs), acid-extractable lipids, the fatty acid composition of RELs, and the contents of various phospholipids in the REL fraction. The polymyxin-resistant strains had an increased content of RELs, but a decreased phospholipid content. The REL fraction from the polymyxin-resistant strains had an increased content of unsaturated fatty acids accompanied by a decreased content of cyclopropane fatty acids as compared with the fatty acid composition of RELs from polymyxin-sensitive strains. The phosphatidylethanolamine content was greatly reduced in the polymyxin-resistant strains, whereas the content of an unidentified lipid, thought to be a neutral lipid lacking either a phosphate, free amino, or choline moiety, was greatly increased. Cell envelopes of the polymyxin-resistant strains contained reduced concentrations of Mg2+ and Ca2+ as compared with the cell envelopes of polymyxin-sensitive strains. It appears that polymyxin resistance in these strains is associated with a significant alteration in the lipid composition and divalent cation content of the cell envelope.

Polymyxin interaction with negatively charged lipid bilayer membranes and the competitive effect of Ca2+

The binding of cationic polymyxin-B to negatively charged phosphatidic acid and phosphatidylglycerol membranes has been investigated by fluorescence polarization study. Competition experiments with Ca2+ were performed. 1. Binding of polymyxin-B to mixed dipalmitoylphosphatidic acid/distearoylphosphatidylcholine membranes leads to a phase separation. Domains of polymyxin-bound phosphatidic acid are formed. 2. Ca2+ is found to be a strong competitor in displacing polymyxin from the complex in the mixed membrane system. Complete displacement is obtained at pH 9.0. With decreasing pH value, Ca2+ becomes a less strong competitor and is ineffective at pH 5.0. 3. Binding of polymyxin to dipalmitoylphosphatidylglycerol membranes is observed. Incorporation of polymyxin lowers the lipid phase transition by 10 degrees C. One polymyxin is found to bind five phosphatidylglycerol molecules. The binding curve is determined and in contrast to phosphatidic acid membranes, a noncooperative binding could be established. 4. Addition of Ca2+ decreases the amount of phosphatidylglycerol bound to polymyxin by about 20%. No complete displacement is achieved even at 10-fold excess of Ca2+ with respect to phosphatidylglycerol.

Pressure-induced changes in the molecular organization of a lipid-peptide complex: polymyxin binding to phosphatidic acid membranes

The effect of 100 atm pressure on the organization of the lipid-peptide complex formed between polymyxin and dipalmitoyl phosphatidic acid has been investigated. Phase transition curves were obtained by electron paramagnetic resonance by measuring the partition coefficient of the spin label, 2, 2, 5, 5-tetramethylpiperidine-N-oxyl. The three-step phase transition curve previously obtained with fluorescence polarization measurements was confirmed, demonstrating three distinct phosphatidic acid domains in the bilayer. Pressure increases binding of polymyxin to phosphatidic acid bilayers and alters the proportions of the two domains that differ in the mode of binding between phosphatidic acid and polymyxin. The binding curves of polymyxin to phosphatidic acid bilayers wre determined and it was shown that application of pressure reduces the cooperativity of the binding curve.

Anionic lipid domains: correlation with functional topography in a mammalian cell membrane

Polymyxin B was used to explore distribution of anionic phospholipids in sperm plasma membranes by electron microscopy of freeze-fracture replicas. After exposure to Hepes/Tris-buffered polymyxin at 4 mM, phosphatidylcholine liposomes showed no perturbations nor did they fluoresce with dansylated incubation. When phosphatidylethanolamine was included in the liposomes, they became perturbed and fluoresced. Plasma membranes of Drosophila larval cells, containing or lacking cholesterol, were also disrupted by polymyxin. The cell membranes of guinea pig sperm were likewise disrupted but in specific functional areas. Fusional membrane domains showed protrusions; the stable membrane of the flagellum revealed diffuse bubbling. Regions of well-defined particle arrays and the postacrosomal segment maintained smooth contours. By fluorescence microscopy, we detected the same heterogeneous binding of the polymyxin dansyl derivative.

Asymmetric antagonistic effects of an inhalation anesthetic and high pressure on the phase transition temperature of dipalmitoyl phosphatidic acid bilayers

The phase transition temperature (Tt) of dipalmitoyl phosphatidic acid multilamellar liposomes is depressed 10 degrees C by the inhalation anesthetic methoxyflurane at a concentration of 100 mmol/mol lipid. Application of 100 atm of helium pressure to pure phosphatidic acid liposomes increased Tt only 1.5 degrees C. However, application of 100 atm helium pressure to dipalmitoyl phosphatidic acid lipsomes containing 100 mmol methoxyflurane/mol lipid almost completely antagonized the effect of the anesthetic. A non-linear pressure effect is observed. In a previous study, a concentration of 60 mmol methoxyflurane/mol dipalmitoyl phosphatidylcholine depressed Tt only 1.5 degrees C, exhibiting a linear pressure effect. The completely different behavior in the charged membrane is best explained by extrusion of the anesthetic from the lipid phase.

Effects of carbon sources on chemical composition of cell envelopes of Pseudomonas aeruginosa in association with polymyxin resistance

Cells of Pseudomonas aeruginosa 015 were grown in basal medium with isobutyrate, DL-2-methylbutyrate, isovalerate, L-valine, L-isoleucine, L-leucine, D-glucose, or L-glutamate as the carbon source. Their resultant susceptibility to polymyxin B varied from a minimal inhibitory concentration of 2 U of polymyxin per ml for isobutyrate-grown cells to 975 U/ml for L-glutamate-grown cells. Cell envelopes from cells grown with each carbon source were compared with cell envelopes from cells grown in Mueller-Hinton broth as to their content of total protein, carbohydrate, and 2-keto-3-deoxyoctonate and as to their protein composition as determined by slab polyacrylamide gel electrophoresis. No pattern of cell envelope content of total protein, carbohydrate, 2-keto-3-deoxyoctonate, or outer membrane protein concentrations could be correlated with the degree of resistance to polymyxin. In these cells increased resistance to polymyxin was not associated with the loss of outer membrane proteins and lipopolysaccharide by the cell envelope.