X-ray diffraction on biomembranes with emphasis on lipid moiety

Sodium selective ion channel formation in living cell membranes by polyamidoamine dendrimer

Polyamidoamine (PAMAM) dendrimers are highly charged hyperbranched protein-like polymers that are known to interact with cell membranes. In order to disclose the mechanisms of dendrimer-membrane interaction, we monitored the effect of PAMAM generation five (G5) dendrimer on the membrane permeability of living neuronal cells followed by exploring the underlying structural changes with infrared-visible sum frequency vibrational spectroscopy (SVFS), small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). G5 dendrimers were demonstrated to irreversibly increase the membrane permeability of neurons that could be blocked in low-[Na(+)], but not in low-[Ca(2+)] media suggesting the formation of specific Na(+) permeable channels. SFVS measurements on silica supported DPPG-DPPC bilayers suggested G5-specific trans-polarization of the membrane. SAXS data and freeze-fracture TEM imaging of self-organized DPPC vesicle systems demonstrated disruption of DPPC vesicle layers by G5 through polar interactions between G5 terminal amino groups and the anionic head groups of DPPC. We propose a nanoscale mechanism by which G5 incorporates into the membrane through multiple polar interactions that disrupt proximate membrane bilayer and shape a unique hydrophilic Na(+) ion permeable channel around the dendrimer. In addition, we tested whether these artificial Na(+) channels can be exploited as antibiotic tools. We showed that G5 quickly arrest the growth of resistant bacterial strains below 10μg/ml concentration, while they show no detrimental effect on red blood cell viability, offering the chance for the development of new generation anti-resistant antibiotics.

Effect of selenium on characteristics of rape chloroplasts modified by cadmium

Selenium appears to be an important protective agent that decreases cadmium-induced toxic effects in animals and plants. The aim of these studies was to investigate the changes of properties of chloroplast membranes obtained from Cd-treated rape seedlings caused by Se additions. Chloroplasts were isolated from leaves of 3-week-old rape plants cultured on Murashige-Skoog media supplied with 2 microM Na(2)SeO(4) and/or 400 microM CdCl(2) under in vitro conditions. The following physicochemical characteristics of chloroplasts were chosen as indicators of Se-effects: average size, zeta potential, ultrastructure, lipid and fatty acid composition and fluidity of envelope membrane. The results suggest that Se can partly counterbalance the destructive effects of Cd. This protective action led to an increase of chloroplast size reduced by Cd treatment and rebuilt, to some extent, the chloroplast ultrastructure. Lipid and fatty acid composition of chloroplast envelopes modified by Cd showed a decrease in digalactosyl-diacylglycerol content and an increase of content of monogalactosyl-diacylglycerol and phospholipid fractions, as well as an increase of fatty acid saturation of all lipids studied. The change in fatty acid saturation correlated well with a decrease of membrane fluidity and with a diminishing of absolute values of zeta potential. The presence of selenium in cultured media caused a partial reversal of the detected changes, which was especially visible in properties related to the hydrophobic part of an envelope, i.e. fatty acid saturation and fluidity.

Biophysical characterization of the fusogenic region of HCV envelope glycoprotein E1

We have studied the binding and interaction of the peptide E1(FP) with various model membranes. E1(FP) is derived from the amino acid segment 274-291 of the hepatitis C virus envelope glycoprotein E1, which was previously proposed to host the peptide responsible for fusion to target membranes. In the present study we addressed the changes which take place upon E1(FP) binding in both the peptide and the phospholipid bilayer, respectively, through a series of complementary experiments. We show that peptide E1(FP) binds to and interacts with phospholipid model membranes, modulates the polymorphic phase behavior of membrane phospholipids, is localized in a shallow position in the membrane and interacts preferentially with cholesterol. The capability of modifying the biophysical properties of model membranes supports its role in HCV-mediated membrane fusion and suggests that the mechanism of membrane fusion elicited by class I and II fusion proteins might be similar.

Screening a peptide library by DSC and SAXD: comparison with the biological function of the parent proteins

We have recently identified the membranotropic regions of the hepatitis C virus proteins E1, E2, core and p7 proteins by observing the effect of protein-derived peptide libraries on model membrane integrity. We have studied in this work the ability of selected sequences of these proteins to modulate the L_β-L_α and L_α-H_II phospholipid phase transitions as well as check the viability of using both DSC and SAXD to screen a protein-derived peptide library. We demonstrate that it is feasible to screen a library of peptides corresponding to one or several proteins by both SAXD and DSC. This methodological combination should allow the identification of essential regions of membrane-interacting proteins which might be implicated in the molecular mechanism of membrane fusion and/or budding.

Interaction of the most membranotropic region of the HCV E2 envelope glycoprotein with membranes. Biophysical characterization

The previously identified membrane-active regions of the hepatitis C virus (HCV) E1 and E2 envelope glycoproteins led us to identify different segments that might be implicated in viral membrane fusion, membrane interaction, and/or protein-protein binding. HCV E2 glycoprotein contains one of the most membranotropic segments, segment 603-634, which has been implicated in CD81 binding, E1/E2 and E2/E2 dimerization, and membrane interaction. Through a series of complementary experiments, we have carried out a study of the binding and interaction with the lipid bilayer of a peptide corresponding to segment 603-634, peptide E2(FP), as well as the structural changes induced by membrane binding that take place in both the peptide and the phospholipid molecules. Here, we demonstrate that peptide E2(FP) binds to and interacts with phospholipid model membranes, modulates the polymorphic phase behavior of membrane phospholipids, is localized in a shallow position in the membrane, and is probably oligomerized in the presence of membranes. These data support the role of E2(FP) in HCV-mediated membrane fusion, and sustain the notion that this segment of the E2 envelope glycoprotein, together with other segments of E2 and E1 glycoproteins, provides the driving force for the merging of the viral and target cell membranes.

Identification of the membrane-active regions of hepatitis C virus p7 protein: biophysical characterization of the loop region

We have identified the membrane-active regions of the hepatitis C virus p7 protein by performing an exhaustive study of membrane rupture, hemifusion, and fusion induced by a p7-derived peptide library on model membranes having different phospholipid compositions. We report the identification in p7 of a highly membranotropic region located at the loop domain of the protein. Here, we have investigated the interaction of a peptide patterned after the p7 loop (peptide p7(L)), studying its binding and interaction with the lipid bilayer, and evaluated the binding-induced structural changes of the peptide and the phospholipids. We show that positively rich p7(L) strongly binds to negatively charged phospholipids and it is localized in a shallow position in the bilayer. Furthermore, peptide p7(L) exhibits a high tendency to oligomerize in the presence of phospholipids, which could be the driving force for the formation of the active ion channel. Therefore, our findings suggest that the p7 loop could be an attractive candidate for antiviral drug development, because it could be a target for antiviral compounds that may lead to new vaccine strategies.

Electric and structural studies of hormone interaction with chloroplast envelope membranes isolated from vegetative and generative rape

The electric and structural properties of envelope membranes of chloroplasts obtained from vegetative and generative plants of rape and the effect of hormone (IAA, GA(3) and zearalenone) treatment were determined by zeta potential and small-angle X-ray scattering (SAXS) methods. Chloroplasts were isolated from leaves cut off from the vegetative (before cooling) and generative apical parts of plants. The lipid composition of chloroplast envelope membranes were analyzed by chromatographic techniques. Envelopes from generative plants contained higher levels of digalactosyldiacylglycerol (DGDG) and smaller amounts of phospholipids (PLs) in comparison to those obtained from vegetative ones. Moreover, envelopes of generative plants were characterized by higher fractions of unsaturated fatty acids. The zeta potential changes caused by hormone treatment were higher for chloroplasts isolated from vegetative plants in comparison to chloroplasts isolated from generative ones. An especially strong effect was observed for chloroplasts treated with IAA. The thickness of bilayers of untreated chloroplasts from vegetative plants were larger by 0.4 nm when comparing to the thickness of layers obtained from generative ones. The effect of hormones (GA(3) and zearalenone) was detected only for vegetative chloroplasts. Both applied methods indicated differences in the properties of untreated and hormone-treated chloroplasts obtained from vegetative and generative plants.

Effect of indole-3-acetic acid on surface properties of the wheat plastid lipids

Surface parameters of polar lipids extracted from winter wheat plastids were investigated by the Langmuir and X-ray differentiation scattering methods. Highly purified plastids were isolated from non-embryogenic (NE) and embryogenic (E) calli initiated from inflorescences. NE plastids contained more monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) and less phospholipids (PL) fraction than E plastids. Moreover, in E calli, unsaturated fatty acids were detected in a higher proportion than in NE for both MGDG and DGDG. No significant differences in fatty acids saturation of PL between NE and E objects were detected. Aqueous surface monolayers were prepared from separate lipids and from mixtures of glycolipids and PL. In the case of MGDG, isotherms showed specific shoulders, contrary to continuous isotherms obtained for other investigated lipids. On the base of pi-A isotherms, the surface parameters: limiting area (A(lim)) and collapse pressure (pi(coll)) were calculated. Indole-3-acetic acid (IAA) increased the A(lim) of all separated lipids about 4-10 angstrom2/mol. However, for NE lipid mixture, the effect of IAA was much smaller (about 2 angstroms2/mol) than for other objects (usually about 5 angstroms2/mol). X-ray experiments for liposomes, obtained from mixtures of glycolipids and PL of NE and E plastids, showed continuous scattering curves with maxima characteristic for lipid bilayer membranes. Calculations of distance distribution functions indicated that bilayer thickness was 41 and 38 angstroms for NE and E, respectively. IAA influence on membrane structures was detected especially in E liposomes and increased the distance between head groups by about 2 angstroms. It is suggested that changes occur during embryogenesis in specific structure of plastid membranes determined also the formation of domains, similar to that suggested for plasmalemma (Plant Sci. 165 (2003) 265). IAA treatment influenced the membrane structure, especially E plastids increasing distances between polar groups.

SANS study on the effect of lanthanide ions and charged lipids on the morphology of phospholipid mixtures. Small-angle neutron scattering

The structural phase behavior of phospholipid mixtures consisting of short-chain (dihexanoyl phosphatidylcholine) and long-chain lipids (dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylglycerol), with and without lanthanide ions was investigated by small-angle neutron scattering (SANS). SANS profiles were obtained from 10 degrees C to 55 degrees C using lipid concentrations ranging from 0.0025 g/ml to 0.25 g/ml. The results reveal a wealth of distinct morphologies, including lamellae, multi-lamellar vesicles, unilamellar vesicles, and bicellar disks.

Studies of the structure and organization of cationic lipid bilayer membranes: calorimetric, spectroscopic, and x-ray diffraction studies of linear saturated P-O-ethyl phosphatidylcholines

Differential scanning calorimetry, x-ray diffraction, and infrared and (31)P-nuclear magnetic resonance ((31)P-NMR) spectroscopy were used to examine the thermotropic phase behavior and organization of cationic model membranes composed of the P-O-ethyl esters of a homologous series of n-saturated 1,2-diacyl phosphatidylcholines (Et-PCs). Differential scanning calorimetry studies indicate that on heating, these lipids exhibit single highly energetic and cooperative endothermic transitions whose temperatures and enthalpies are higher than those of the corresponding phosphatidylcholines (PCs). Upon cooling, these Et-PCs exhibit two exothermic transitions at temperatures slightly below the single endotherm observed upon heating. These cooling exotherms have both been assigned to transitions between the liquid-crystalline and gel phases of these lipids by x-ray diffraction. The x-ray diffraction data also show that unlike the parent PCs, the chain-melting phase transition of these Et-PCs involves a direct transformation of a chain-interdigitated gel phase to the lamellar liquid-crystalline phase for the homologous series of n > or = 14. Our (31)P-NMR spectroscopic studies indicate that the rates of phosphate headgroup reorientation in both gel and liquid-crystalline phases of these lipids are comparable to those of the corresponding PC bilayers. However, the shape of the (31)P-NMR spectra observed in the interdigitated gel phase indicates that phosphate headgroup reorientation is subject to constraints that are not encountered in the non-interdigitated gel phases of parent PCs. The infrared spectroscopic data indicate that the Et-PCs adopt a very compact form of hydrocarbon chain packing in the interdigitated gel phase and that the polar/apolar interfacial regions of these bilayers are less hydrated than those of corresponding PC bilayers in both the gel and liquid-crystalline phases. Our results indicate that esterification of PC phosphate headgroups results in many alterations of bilayer physical properties aside from the endowment of a positively charged surface. This fact should be considered in assessing the interactions of these compounds with naturally occurring lipids and with other biological materials.

Structural aspects of the interaction of peptidyl-glycylleucine-carboxyamide, a highly potent antimicrobial peptide from frog skin, with lipids

The interaction of PGLa (peptidyl-glycylleucine-carboxyamide), a 21-amino-acid residue cationic peptide, isolated from the skin of the South African clawed frog, Xenopus laevis, with model membrane systems was investigated. Our studies focussed on the importance of the difference in the phospholipid composition of bacterial and erythrocyte membranes. This is of particular interest to gain information on the specificity of membranolysis exhibited by this peptide against bacteria but not against erythrocytes. In phosphate buffer at physiological pH, as well as in the presence of the zwitterionic phosphatidylcholine and sphingomyelin. the peptide had a random structure but it adopted an alpha-helical conformation in the presence of negatively charged lipids. Furthermore, calorimetric experiments showed that PGLa had no effects on the thermotropic phase behavior of liposomes composed of the choline phosphatides, while separation of a distinct peptide-rich domain was observed for phosphatidylglycerol liposomes. In addition to the main transition of pure 1,2-dipalmitoylglycerophosphoglycerol at 40 degrees C a second transition owing to the peptide-perturbed lipid domains was found at 41 degrees C. This conclusion is supported by X-ray diffraction experiments which indicated that PGLa penetrates into the hydrophobic core of the bilayer inducing an untilting of the hydrocarbon chains as observed in the gel phase of the pure lipid. These results demonstrate that this antibacterial peptide specifically interacts with negatively charged lipid membranes, which are characteristic of bacterial membranes. This can be explained based on the structural features of PGLa.