The influence of proteins and peptides on the phase properties of lipids

Proposed Mechanism for H(II) Phase Induction by Gramicidin in Model Membranes and Its Relation to Channel Formation

A model is proposed for the molecular mechanism of H(II) phase induction by gramicidin in model membranes. The model describes the sequence of events that occurs upon hydration of a mixed lipid/gramicidin film, relating them to gramicidin channel formation and to relevant literature on gramicidin and lipid structure.

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.

Integrative feedback and robustness in a lipid biosynthetic network

The homeostatic control of membrane lipid composition appears to be of central importance for cell functioning and survival. However, while lipid biosynthetic reaction networks have been mapped in detail, the underlying control architecture which underpins these networks remains elusive. A key problem in determining the control architectures of lipid biosynthetic pathways, and the mechanisms through which control is achieved, is that the compositional complexity of lipid membranes makes it difficult to determine which membrane parameter is under homeostatic control. Recently, we reported that membrane stored elastic energy provides a physical feedback signal which modulates the activity in vitro of CTP:phosphocholine cytidylyltransferase (CCT), an extrinsic membrane enzyme which catalyses a key step in the synthesis of phosphatidylcholine lipids in the Kennedy pathway (Kennedy 1953 J. Am. Chem. Soc. 75, 249-250). We postulate that stored elastic energy may be the main property of membranes that is under homeostatic control. Here we report the results of simulations based on this postulate, which reveal a possible control architecture for lipid biosynthesis networks in vivo.

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.

Use of reverse micelles in membrane protein structural biology

Membrane protein structural biology is a rapidly developing field with fundamental importance for elucidating key biological and biophysical processes including signal transduction, intercellular communication, and cellular transport. In addition to the intrinsic interest in this area of research, structural studies of membrane proteins have direct significance on the development of therapeutics that impact human health in diverse and important ways. In this article we demonstrate the potential of investigating the structure of membrane proteins using the reverse micelle forming surfactant dioctyl sulfosuccinate (AOT) in application to the prototypical model ion channel gramicidin A. Reverse micelles are surfactant based nanoparticles which have been employed to investigate fundamental physical properties of biomolecules. The results of this solution NMR based study indicate that the AOT reverse micelle system is capable of refolding and stabilizing relatively high concentrations of the native conformation of gramicidin A. Importantly, pulsed-field-gradient NMR diffusion and NOESY experiments reveal stable gramicidin A homodimer interactions that bridge reverse micelle particles. The spectroscopic benefit of reverse micelle-membrane protein solubilization is also explored, and significant enhancement over commonly used micelle based mimetic systems is demonstrated. These results establish the effectiveness of reverse micelle based studies of membrane proteins, and illustrate that membrane proteins solubilized by reverse micelles are compatible with high resolution solution NMR techniques.

Conformation study of the membrane models by the Maxwell displacement current technique and oxidative stress

The role of biological membranes as a target in biological radiation damage is still unclear. Recently much attention has been paid to the dynamic behaviour of the cell membrane. Maxwell displacement current technique (MDC) provides new possibility of conformation study of the membrane models. Oxidative stress can impair macromolecules in the cell on a molecular level. MDC technique enables to study the changes in molecular orientations and/or conformations of cell membranes. The combination of different methods in structural biology can clarify membrane chemical and physical properties.

The effects of gramicidin on the structure of phospholipid assemblies

Gramicidin is an antibiotic peptide that can be incorporated into the monolayers of cell membranes. Dimerization through hydrogen bonding between gramicidin monomers in opposing leaflets of the membrane results in the formation of an iontophoretic channel. Surrounding phospholipids influence the gating properties of this channel. Conversely, gramicidin incorporation has been shown to affect the structure of spontaneously formed lipid assemblies. Using small-angle x-ray diffraction and model systems composed of phospholipids and gramicidin, the effects produced by gramicidin on lipid layers were measured. These measurements explore how peptides are able to modulate the spontaneous curvature properties of phospholipid assemblies. The reverse hexagonal, H(II), phase formed by dioleoylphosphatidylethanolamine (DOPE) monolayers decreased in lattice dimension with increasing incorporation of gramicidin. This indicated that gramicidin itself was adding negative curvature to the lipid layers. In this system, gramicidin was measured to have an apparent intrinsic radius of curvature, R0pgram, of -7.1 A. The addition of up to 4 mol% gramicidin in DOPE did not result in the monolayers becoming stiffer, as measured by the monolayer bending moduli. Dioleoylphosphatidylcholine (DOPC) alone forms the lamellar (L(alpha)) phase when hydrated, but undergoes a transition into the reverse hexagonal (H(II)) phase when mixed with gramicidin. The lattice dimension decreases systematically with increased gramicidin content. Again, this indicated that gramicidin was adding negative curvature to the lipid monolayers but the mixture behaved structurally much less consistently than DOPE/gramicidin. Only at 12 mol% gramicidin in dioleoylphosphatidylcholine could an apparent radius of intrinsic curvature of gramicidin (R0pgram) be estimated as -7.4 A. This mixture formed monolayers that were very resistant to bending, with a measured bending modulus of 115 kT.

31P NMR first spectral moment study of the partial magnetic orientation of phospholipid membranes

Structural data can be obtained on proteins inserted in magnetically oriented phospholipid membranes such as bicelles, which are most often made of a mixture of long and short chain phosphatidylcholine. Possible shapes for these magnetically oriented membranes have been postulated in the literature, such as discoidal structures with a thickness of one bilayer and with the short acyl chain phosphatidylcholine on the edges. In the present paper, a geometrical study of these oriented structures is done to determine the validity of this model. The method used is based on the determination of the first spectral moment of solid-state (31)P nuclear magnetic resonance spectra. From this first moment, an order parameter is defined that allows a quantitative analysis of partially oriented spectra. The validity of this method is demonstrated in the present study for oriented samples made of DMPC, DMPC:DHPC, DMPC:DHPC:gramicidin A and adriamycin:cardiolipin.

Control of cholesterol access to cytochrome P450scc in rat adrenal cells mediated by regulation of the steroidogenic acute regulatory protein

Cholesterol conversion to pregnenolone by cytochrome P450scc in steroidogenic cells, including those of the adrenal cortex, is determined by hormonal control of cholesterol availability. Intramitochondrial cholesterol movement to P450scc, which retains hormonal activation in isolated mitochondria, is apparently dependent on peripheral benzodiazepine receptor and the recently cloned steroidogenic acute regulatory (StAR) protein. In rat adrenal cells, StAR is formed as a 37-kDa precursor that is transferred to the mitochondrial inner membrane following phosphorylation by hormonally activated protein kinase A, and processed to multiple forms, some of which turn over very rapidly. In bovine cells, StAR undergoes three modifications forming a set of eight proteins seen in both glomerulosa and fasciculata cells. In the former, cyclic AMP and angiotensin II each decrease two forms and elevate six forms. Significantly, the major change seen after activation may not involve phosphorylation of StAR. Cholesterol transfer across mitochondrial membranes is also activated in isolated mitochondria by GTP and low concentrations of Ca2+, apparently prior to activation by StAR. Depletion of StAR by cycloheximide inhibits cholesterol transfer but is overcome by uptake of Ca2+ into the matrix. This activation of cellular cholesterol transport is sustained in adrenal cells permeabilized by Streptolysin O. In rat adrenal cells cAMP elevates 3.5- and 1.6-kb mRNA, hybridized by a 1.0-kb StAR cDNA. A 3.5-kb rat adrenal cDNA that encodes all except the 5' end of the longest StAR mRNA has been characterized. The corresponding gene sequence is distributed across seven exons. The shorter mRNA may arise from polyadenylation signals early in exon 7. However, the 3.5-kb mRNA comprises 80-90% of untreated rat adrenal StAR mRNA and may therefore provide the prime source for in vivo translation of StAR protein.

Phospholipid antibodies in alcoholic liver disease

Alcoholic liver injury has been reported to be directed preferentially against the proteins of the cell membrane, sparing the phospholipids. However, antiphospholipid antibodies against certain cell membrane phospholipids are known to be associated with a variety of diseases. We undertook this investigation to determine whether antiphospholipid antibodies were present in the serum of patients with alcoholic liver disease. We investigated seventy long-term alcoholic patients (> 80 gm ethanol/day for > 1 yr) and 8 normal nonalcoholic controls by means of enzyme-linked immunosorbent assay to determine whether serum antibodies were generated against the following membrane phospholipids: phosphatidylserine, phosphatidylinositol, phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol (cardiolipin) and phosphatidic acid. Group 1 comprised alcoholic patients with normal liver function (n = 13), group 2 comprised alcoholic patients with abnormal liver function (n = 16), group 3 comprised patients with alcoholic hepatitis or cirrhosis (n = 41) and group 4 comprised nonalcoholic controls (n = 8). The antibody prevalence was 15% in group 1, 31% in group 2, 81% in group 3 and 0% in group 4. In group 3, 20 of 41 patients had antibodies against several cell membrane phospholipids (i.e., phosphatidylethanolamine, phosphatidylserine, phosphatidylglycerol, phosphatidic acid, cardiolipin and phosphatidylinositol). The antiphosphatidylethanolamine isotype was IgA or IgM in 25 of 41 of these patients. Both IgA (p < 0.01) and IgM (p < 0.008) antiphosphatidylethanolamine correlated significantly with disease severity. Antiphospholipid antibodies in alcoholic patients seem to reflect disease progression and correlate significantly with disease severity.

Gramicidin A/short-chain phospholipid dispersions: chain length dependence of gramicidin conformation and lipid organization

Gramicidin-lipid interactions were investigated using diacylphosphatidylcholines that contained two identical acyl chains of varying length, between 6 and 14 carbons. The gramicidin A (gA) conformation was monitored by circular dichroism (CD) spectroscopy and high-performance size-exclusion chromatography, and the lipid organization was investigated using 31P and 1H NMR spectroscopy and negative-stain electron microscopy. Diacylphosphatidylcholine (PC) lipids with chain lengths between 4 and 8 carbons have been previously shown to have a micellar organization in aqueous solution [Lin, T.-L., et al. (1986) J. Am. Chem. Soc. 108, 3499-3507]. CD spectra of aqueous gA/lipid dispersions, at a ratio of 1:28, demonstrated that the channel conformation of gA can be readily obtained when the acyl chain length is > or = 10, but not when the chain length is < or = 7. Size-exclusion chromatography revealed that the fraction of gA that could easily be dissociated into monomers in the dispersions increased with increasing acyl chain length, in agreement with the CD results. For a chain length of 8, the results were intermediate. The formation of the channel structure was found to depend on the "solvent-history", the temperature, the gA and lipid concentrations, the gA:lipid ratio, and consequently on the method of sample preparation. 1H and 31P NMR results suggest that codispersed gA increases the size of dioctanoyl-PC aggregates, but not of dihexanoyl-PC micelles. Negative-stain electron microscopy directly supports these findings. Dihexanoyl-PC (28 mM) was able to solubilize 1 mM gA in H2O, but the gA was not in the "channel" conformation.(ABSTRACT TRUNCATED AT 250 WORDS)

Solvent history dependence of gramicidin-lipid interactions: a Raman and infrared spectroscopic study

We have investigated the interactions between gramicidin and a model membrane composed of one phospholipid, dimyristoylphosphatidylcholine, as a function of the cosolubilization solvent and incubation time used in the sample preparation. Three organic solvents have been used; trifluoroethanol, a mixture of methanol/chloroform (1:1 v/v), and ethanol. Using Fourier transform infrared spectroscopy, we have demonstrated that the conformation adopted by gramicidin in the membrane is dependent upon the cosolubilization solvent used, and, only with trifluoroethanol, it is possible to incorporate gramicidin entirely as a beta 6.3-helix. Moreover, Raman spectroscopy results indicate that the orientation of the tryptophan side chains in gramicidin and their interaction with the hydrocarbon chains and the carbonyl groups of the lipids are also dependent on the cosolubilization solvent. On the other hand, the effect of the incorporation of gramicidin on the thermotropism of the lipid bilayer was found to be dependent upon the conformation of gramicidin in the lipid bilayers.

Regulation of cholesterol movement to mitochondrial cytochrome P450scc in steroid hormone synthesis

Transfer of cholesterol to cytochrome P450scc is generally the rate-limiting step in steroid synthesis. Depending on the steroidogenic cell, cholesterol is supplied from low or high density lipoproteins (LDL or HDL) or de novo synthesis. ACTH and gonadotropins stimulate this cholesterol transfer prior to activation of gene transcription, both through increasing the availability of cytosolic free cholesterol and through enhanced cholesterol transfer between the outer and inner mitochondrial membranes. Cytosolic free cholesterol from LDL or HDL is primarily increased through enhanced cholesterol ester hydrolysis and suppressed esterification, but increased de novo synthesis can be significant. Elements of the cytoskeleton, probably in conjunction with sterol carrier protein(2) (SCP(2)), mediate cholesterol transfer to the mitochondrial outer membranes. Several factors contribute to the transfer of cholesterol between mitochondrial membranes; steroidogenesis activator peptide acts synergistically with GTP and is supplemented by SCP(2). 5-Hydroperoxyeicosatrienoic acid, endozepine (at peripheral benzodiazepine receptors), and rapid changes in outer membrane phospholipid content may also contribute stimulatory effects at this step. It is suggested that hormonal activation, through these factors, alters membrane structure around mitochondrial intermembrane contact sites, which also function to transfer ADP, phospholipids, and proteins to the inner mitochondria. Cholesterol transfer may occur following a labile fusion of inner and outer membranes, stimulated through involvement of cardiolipin and phosphatidylethanolamine in hexagonal phase membrane domains. Ligand binding to benzodiazepine receptors and the mitochondrial uptake of 37 kDa phosphoproteins that uniquely characterize steroidogenic mitochondria could possibly facilitate these changes. ACTH activation of rat adrenals increases the susceptibility of mitochondrial outer membranes to digitonin solubilization, suggesting increased cholesterol availability. Proteins associated with contact sites were not solubilized, indicating that this part of the outer membrane is resistant to this treatment. Two pools of reactive cholesterol within adrenal mitochondria have been distinguished by different isocitrate- and succinate-supported metabolism. These pools appear to be differentially affected in vitro by the above stimulatory factors.

The effect of changes in gramicidin conformation on bilayer lipid properties

The effects of two different gramicidin conformations on lipid phase behaviour and dynamics are compared. Samples of chain-perdeuterated dimyristoylphosphatidylcholine containing gramicidin were first prepared with gramicidin in a state having a circular dichroism spectrum generally identified as corresponding to the non-channel conformation. The effects, on bilayer lipid properties, of gramicidin in this conformation were then determined using deuterium nuclear magnetic resonance measurements of acyl chain orientational order and transverse relaxation times as a function of temperature. These samples were then incubated at 65 degrees C to convert the gramicidin to a state with a circular dichroism spectrum of the type generally identified with the channel conformation. The nuclear magnetic resonance measurements were then repeated. In the gel phase, it was found that transverse relaxation time and chain orientational order of the lipid were insensitive to gramicidin conformation. In the liquid crystalline phase, gramicidin in the channel conformation was found to have a slightly larger effect on transverse relaxation and orientational order than gramicidin in the non-channel conformation. The perturbation of the phase behavior by gramicidin was found to be relatively insensitive to gramicidin conformation.

Synthesis of acylated gramicidins and the influence of acylation on the interfacial properties and conformational behavior of gramicidin A

Five gramicidin A analogs were synthesized in which various acyl chains, differing in length and unsaturation, were covalently coupled to the C-terminal ethanolamine group. The analogs were characterized by various spectroscopic techniques and their molecular properties were investigated using monolayer techniques and circular dichroism. It is demonstrated that neither the interfacial properties nor the conformational behavior of gramicidin A at the air/water interface are seriously affected upon acylation. It is proposed that at the limiting area the gramicidin molecule is oriented with its C-terminus towards the subphase with the covalently coupled acylchain located parallel to the helical axis in between the protruding tryptophans. Circular dichroism experiments, in which gramicidin-containing vesicles were prepared from different organic solvents, indicate that the presence of a covalently coupled fatty acylchain tends to stabilize the beta 6.3 helical conformation. It is demonstrated that, like for gramicidin A, also for the acylgramicidins the single-stranded beta 6.3 helical conformation, or channel conformation, is the preferred conformation upon incorporation in bilayers.

Radiation inactivation of ion channels formed by gramicidin A. Protection by lipid double bonds and by alpha-tocopherol

The conductance induced by the channel-forming peptide gramicidin A in lipid membranes is reduced by many orders of magnitude on exposure of the membrane and its aqueous environment to ionizing radiation. This results from an interaction of free radicals of water radiolysis with the tryptophan residues of gramicidin A. The sensitivity of the ion channels towards irradiation is strongly reduced in the presence of either vitamin E or of highly unsaturated lipids. An increase of the D37 dose up to a factor of 50 was found. The phenomena are interpreted via a reduction of the effective concentration of free radicals (such as OH.) in the membrane by reaction with unsaturated fatty acid residues or with vitamin E.

Polymorphic phospholipid phase transitions as tools to understand peptide-lipid interactions

The effect of peptides on bilayer----non-bilayer phase transitions can be used as a tool to investigate the molecular aspects of peptide-lipid interactions. In this contribution the action on membranes of the peptide antibiotic gramicidin A and the bee venom component melittin are compared. Although the known structures and locations of these peptides upon membrane binding are very different, their actions on membranes show striking parallels. A general model is proposed that explains the seemingly complex peptide-lipid interactions by making use of simple concepts.

Characterization of lipid domains in erythrocyte membranes

Fluorescence digital imaging microscopy was used to study the lateral distribution of the lipid components in erythrocyte membranes. Intact erythrocytes labeled with phospholipids containing a fluorophore attached to one fatty acid chain showed an uneven distribution of the phospholipids in the membrane thereby demonstrating the presence of membrane domains. The enrichment of the lipotropic compound chlor-promazine in domains in intact erythrocytes also suggested that the domains are lipid-enriched regions. Similar membrane domains were present in erythrocyte ghosts. The phospholipid enrichment was increased in the domains by inducing membrane protein aggregation. Double-labeling experiments were done to determine the relative distributions of different phospholipids in the membrane. Vesicles made from extracted lipids did not show the presence of domains consistent with the conclusion that membrane proteins were responsible for creating the domains. Overall, it was found that large domains exist in the red blood cell membrane with unequal enrichment of the different phospholipid species.

Hydrophobic mismatch in gramicidin A'/lecithin systems

Gramicidin A' (GA') has been added to three lipid systems of varying hydrophobic thicknesses: dimyristoyllecithin (DML), dipalmitoyllecithin (DPL), and distearoyllecithin (DSL). The similarity in length between the hydrophobic portion of GA' and the hydrocarbon chains of the lipid bilayers has been studied by using 31P and 2H NMR. Hydrophobic mismatch has been found to be most severe in the DML bilayer system and minimal in the case of DSL. In addition, the effects of hydrophobic mismatch on the cooperative properties of the bilayer have been obtained from 2H NMR relaxation measurements. The results indicate that incorporation of the peptide into the bilayer disrupts the cooperative director fluctuations characteristic of pure multilamellar lipid dispersions. Finally, the GA'/lecithin ratio at which the well-known transformation from bilayer to reverse hexagonal (HII) phase occurs (Van Echteld et al., 1982; Chupin et al., 1987) is shown to depend on the acyl chain length of the phospholipid. A rationale is proposed for this chain length dependence.

Phospholipid in the hexagonal II phase is immunogenic: evidence for immunorecognition of nonbilayer lipid phases in vivo

Immunization of mice with phosphatidylethanolamine in the hexagonal II phase but not the bilayer phase resulted in the induction of anti-phospholipid antibodies. These antibodies, which were strongly reactive with phosphatidylethanolamine and crossreactive with cardiolipin, had functional lupus anticoagulant activity and were characteristic of autoantibodies common in patients with autoimmune disease. Recognition of the hexagonal II phase by the afferent limb of the immune system suggests that nonbilayer phospholipids can arise in the course of membrane remodeling and induce the autoantibodies of disease.

Optimizing and characterizing alignment of oriented lipid bilayers containing gramicidin D

31P NMR spectroscopy and optical microscopy have been used to characterize samples of gramicidin D in oriented lipid bilayers. Correlations have been made between the defect structures observed under crossed polarizers by optical microscopy and characteristic features of 31P NMR spectra. The sample preparation protocol has been improved using these techniques to achieve minimal dispersion of the bilayer normal and minimal amounts of unoriented sample. The molar ratio of gramicidin to dimyristoyl-phosphatidylcholine, the extent of hydration, and the cosolubilizing solvent system were used as the protocol variables. While hydration level and solvent system had profound effects on the sample orientation the molar ratio did not. However, the 31P chemical shift anisotropy is very sensitive to the molar ratio and can be used as an in situ method for determining the molar ratio.

Effects of cholesterol or gramicidin on slow and fast motions of phospholipids in oriented bilayers

Nuclear spin-lattice relaxation both in the rotating frame and in the laboratory frame is used to investigate the slow and fast molecular motions of phospholipids in oriented bilayers in the liquid crystalline phase. The bilayers are prepared from a perdeuterated phospholipid labeled with a pair of 19F atoms at the 7 position of the 2-sn acyl chain. Phospholipid-cholesterol or phospholipid-gramicidin interactions are characterized by measuring the relaxation rates as a function of the bilayer orientation, the locking field, and the temperature. Our studies show that cholesterol or gramicidin can specifically enhance the relaxation due to slow motions in phospholipid bilayers with correlation times tau s longer than 10(-8) sec. The perturbations of the geometry of the slow motions induced by cholesterol are qualitatively different from those induced by gramicidin. In contrast, the presence of cholesterol or gramicidin slightly suppresses the fast motions with correlation times tau f = 10(-9) to 10(-10) sec without significantly affecting their geometry. Weak locking-field and temperature dependences are observed for both pure lipid bilayers and bilayers containing either cholesterol or gramicidin, suggesting that the motions of phospholipid acyl chains may have dispersed correlation times.

Increased cellular internalization of amphiphiles in a multidrug-resistant CHO cell line

The uptake of labeled palmitoyl carnitine and palmitoyl lysophosphatidylcholine by CHO cells was studied by measuring the extractability of these amphiphiles by bovine serum albumin. A multidrug-resistant cell line, CHRC5, showed a more rapid uptake, compared with the parental line, of these amphiphiles into a pool that was no longer susceptible to extraction with bovine serum albumin. The more rapid uptake by the drug-resistant cell line was reversed back to the rates observed with the parental cell line in the presence of verapamil, quinacrine or cyclosporin A. These latter three drugs also reverse the multidrug-resistant phenotype. These results demonstrate a relationship between the rate of amphiphile uptake and multidrug resistance.

Conformational transmission in condensed lipid model compounds. Chain packing modification by head-group-induced conformational changes in the glyceryl backbone

On changing the phosphorus coordination from four to five via a trigonal bipyramidal geometry in phospholipid model systems, a conformational change is initiated around the C2-C3 linkage caused by enhanced electrostatic repulsion between the atoms O3 and O2, situated in the axis of the trigonal bipyramid. This is supported by the absence of conformational change upon substitution of O2 by methylene. With solid-state cross-polarization and magic-angle spinning 13C-NMR it is shown that such a conformational transmission leads to an increased packing density of the lipid chains. It seems to us that changes in lipid packing caused by conformational transmission may also occur when pentacoordination is present transiently, as under biological conditions, thereby inducing an activation of the transport proteins that are embedded in the membrane.

Influence of proteins on the reorganization of phospholipid bilayers into large domains

Using large (5-10 microns) vesicles formed in the presence of phospholipids fluorescently labeled on the acyl chain and visualized using a fluorescence microscope, charge-coupled-device camera, and digital image processor, we examined the effects of membrane proteins on phospholipid domain formation. In vesicles composed of phosphatidic acid and phosphatidylcholine, incubation with cytochrome c induced the reorganization of phospholipids into large phosphatidic acid-enriched domains with the exclusion of phosphatidylcholine. Cytochrome c binding was demonstrated to be highest in the phosphatidic acid-enriched domain of the vesicle using the absorbance of the heme moiety for visualization. Both binding of cytochrome c and phospholipid reorganization were blocked by pretreatment of the vesicles with 0.1 M NaCl. The pore forming peptide gramicidin was examined for the effects of an integral protein on domain formation. Initially, gramicidin distributed randomly within the vesicle and showed no phospholipid specificity. Phosphatidic acid domain formation in the presence of 2.0 mM CaCl2 or 100 microM cytochrome c was not affected by the presence of 5 mol % gramicidin within the vesicles. In both cases, gramicidin was preferentially excluded from the phosphatidic acid-enriched domain and became associated with phosphatidylcholine-enriched areas of the vesicle. Thus, cytochrome c caused a major reorganization of both the phospholipids and the proteins in the bilayer.

Interfacial properties of gramicidin and gramicidin-lipid mixtures measured with static and dynamic monolayer techniques

Gramicidin films at the air/water interface are shown to exhibit a phase transition at 225 A2/molecule which might be caused by either cluster formation, reorientation of molecules, conformational changes or multilayer formation. It is further shown that coupling of a charged group on either NH2- or COOH-terminus or elongation of the peptide by two amino acids, only slightly affects the surface area characteristics whereas modification of the tryptophans or even replacement of a single tryptophan by phenylalanine leads to drastic alterations in the surface-area characteristics and a (partial) loss of the phase transition demonstrating that the tryptophans play an important role in the interfacial behavior of gramicidin. The lack of a solvent history effect on the interfacial behavior indicates a rapid conformational interconversion of the peptide at the air/water interface. Gramicidin in mixtures with dioleoylphosphatidylcholine and lysopalmitoylphosphatidylcholine shows a condensing effect whereas gramicidin shows ideal mixing with dioleoylphosphatidylethanolamine. The condensing effect most likely is related to the aggregational state of the peptides which is different in phosphatidylcholines and phosphatidylethanolamines.

A mismatch between the length of gramicidin and the lipid acyl chains is a prerequisite for HII phase formation in phosphatidylcholine model membranes

Previously it was shown that gramicidin can induce HII phase formation in diacylphosphatidylcholine model membranes only when the lipid acyl chain length exceeds 16 carbon atoms (Van Echteld, C.J.A., De Kruijff, B., Verkleij, A.J., Leunissen-Bijvelt, J. and De Gier, J. (1982) Biochim. Biophys. Acta 692, 126-138). Using 31P-NMR and small angle X-ray diffraction we now demonstrate that upon increasing the length of gramicidin, the peptide loses its ability to induce HII phase formation in di-C18:1c-PC but not in the longer chained di-C22:1c-PC. It is concluded that a mismatch in length between gramicidin and the lipid acyl chains, when the latter would provide excess bilayer thickness, is a prerequisite for HII phase formation in phosphatidylcholine model membranes.

Dielectric spectroscopy of L-alpha-lysolecithin-packaged gramicidin A

The complex permittivities of L-alpha-lysolecithin in the absence and presence of the gramicidin A ion channel were measured over the temperature range 0-60 degrees C and over the frequency range 1-1000 MHz. One dielectric relaxation/loss has been observed. It is located at 103.3 MHz (1.54 ns) for a micellar 0.4 M L-alpha-lysolecithin solution at 20 degrees C, whereas it is shifted to 71.7 MHz (2.22 ns) for a lamellar L-alpha-lysolecithin-gramicidin A aqueous solution (0.4 M L-alpha-lysolecithin, 0.0308 M gramicidin A) at 20 degrees C. The dielectric relaxation decreases and the relaxation time increases when gramicidin A is incorporated into L-alpha-lysolecithin. These dielectric changes are related, in part, to the micellar-to-lamellar lipid phase transition induced by the incorporation of gramicidin A into lysolecithin. We suggest that the diffuse rotational motion of the polar head group of L-alpha-lysolecithin contributes to the dielectric relaxation/loss at around 100 MHz.

Conformation of gramicidin in relation to its ability to form bilayers with lysophosphatidylcholine

The ability of gramicidin to induce bilayer formation in lysophosphatidylcholine (LPC) systems was investigated as a function of the conformation of the peptide. The conformation was varied by using different solvents to cosolubilize gramicidin and lipid. Using circular dichroism (CD), it was found that when codissolved in trifluoroethanol (TFE), after drying and subsequent hydration, gramicidin is mainly present in the single-stranded beta 6.3-helical configuration, whereas when using chloroform/methanol or ethanol as the solvent, it is proposed that the dominant conformation of gramicidin in the membrane is that of the double-stranded antiparallel dimer. Employing 31P NMR, the stoichiometry for bilayer formation was found to be 6 to 7 lipid molecules per gramicidin monomer, when samples were prepared from TFE, whereas a stoichiometry of 4 was found when chloroform/methanol or ethanol was the solvent. Upon heating the latter samples, a conversion was observed in the CD pattern toward that indicative of the beta 6.3-helical configuration. This change was accompanied by an increase in the extent of bilayer formation. Next, it was investigated whether the conformation of gramicidin and its ability to induce bilayer formation were dependent on the lipid acyl chain length. CD measurements of samples prepared from TFE indicated that gramicidin, independent of acyl chain length, was present in the beta 6.3-helical configuration but the intensity of the ellipticities at 218 nm increased with the length of the acyl chain. The extent of bilayer formation in these samples was found to be largely chain length independent.(ABSTRACT TRUNCATED AT 250 WORDS)

Solid-state 15N-NMR evidence that gramicidin A can adopt two different backbone conformations in dimyristoylphosphatidylcholine model membrane preparations

Using [15N-Val7]gramicidin A it is shown by solid state 15N-NMR that in dimyristoylphosphatidylcholine model membrane preparations evidence is obtained for two different backbone conformations of gramicidin. One of these conformations is the familiar channel state while a second conformation possesses very different dynamic and structural characteristics. The relative amounts of the conformations depend upon the solvent used to initially codissolve peptide and lipid. Furthermore, by incubation of the samples at modestly elevated temperatures a conversion can be induced from the non-channel to the channel state in a lipid environment.

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.

Dielectric relaxation spectroscopy on dimyristoylphosphatidylcholine-packaged gramicidin A

The complex permittivities of aqueous suspensions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and of DMPC packaged gramicidin A' (DMPC-GA) have been determined over the frequency range of 1 MHz to 1 GHz and the temperature range of 0-60 degrees C. A dielectric relaxation/loss has been observed at about 66 MHz for the DMPC suspension (30 degrees C) and at about 57 MHz for the DMPC-GA suspension (30 degrees C). This dielectric relaxation/loss has been attributed to the rotational mobility of the zwitterionic group of DMPC. The temperature dependence (from 60 degrees C to 0 degrees C) of this dispersion/absorption process of the DMPC suspension indicates a sharp reduction of the dielectric relaxation at about 20 degrees C. This dielectric change is related to the conversions of shape and structure of bilayer aggregates. This sharp reduction of the dielectric relaxation disappears or broadens when GA is incorporated into the DMPC aqueous suspension. The interpretation of these results is that the GA addition into the DMPC aqueous suspension induces a small decrease of the rotational mobility of the zwitterionic group above the lipid phase transition, and a small increase of the rotational mobility of the zwitterionic group below the lipid phase transition.

Modulation of membrane surface curvature by peptide-lipid interactions

Recent reports on the interaction of cardiotoxin and melittin with phospholipid model membranes are reviewed and analyzed. These types of peptide toxins are able to modulate lipid surface curvature and polymorphism in a highly lipid-specific way. It is demonstrated that the remarkable variety of effects of melittin on the organization of different membrane phospholipids can be understood in a relatively simple model, based on the shape-structure concept of lipid polymorphism and taking into account the position of the peptide molecule with respect to the lipids. Based on the strong preference of the peptides for negatively charged lipids and the structural consequences thereof, and on preliminary studies of signal peptide-lipid interaction, a role of inverted or concave lipid structures in the process of protein translocation across membranes is suggested.

Phospholipids and bile acids as diffusional carriers of Na+ across nonpolar media

Phospholipids and bile acids, by virtue of their amphiphilic properties, can interact in nonpolar media forming "inverted" structures (micelles) which presumably have an hydrophilic core and might act as diffusional carriers (ionophores) of electrolytes across low dielectric constant media or lipid membranes. The Na+ ionophoretic capability of various purified phospholipids and the modulating effects of bile acids and phosphatidylcholine was examined by: (a) measurement of 22Na+ partition into the organic phase (chloroform) of a two-phase system and (b) direct measurement of the translocation of 22Na+ across a bulk chloroform phase separating two aqueous phases in a Pressman cell. All phospholipids tested, except for phosphatidylcholine, showed ionophoretic capability for Na+ at micromolar concentrations. Cardiolipin and phosphatidylserine were the most efficient Na+ carriers, comparable with monensin, an established Na+ ionophore. In contrast, cholic acid as well as other bile acids demonstrated only marginal or no Na+ ionophoretic capability. However, hydroxylated bile acids (particularly cholic acid), sodium dodecyl sulfate and Triton X-100, which can induce and stabilize inverted structures in lipid membranes, were able to increase 5- to 8-fold the phospholipid-mediated Na+ transport. Interaction of cardiolipin with Na+ in the chloroform phase followed a rectangular hyperbolic function with an apparent Kd within the physiological Na+ concentration range (16.9 +/- 5.1 mM). Addition of cholic acid to the cardiolipin-containing organic phase resulted in a 10-fold increase of maximal Na+ uptake and no change in apparent Kd. The effect of cholic acid on both cardiolipin-mediated Na+ partition and Na+ translocation across the chloroform phase showed a marked dependence on pH, being greater at pH 7.4.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of ferricytochrome c with zwitterionic phospholipid bilayers: a Raman spectroscopic study

The vibrational Raman spectra of both pure L-alpha-dipalmitoylphosphatidylcholine (DPPC) liposomes and DPPC multilayers reconstituted with ferricytochrome c under varying conditions of pH and ionic strength are reported as a function of temperature. Total integrated band intensities and relative peak height intensity ratios, two spectral scattering parameters used to determine bilayer disorder, are invariant to changes in pH and ionic strength but exhibit a sensitivity to the bilayer concentration of the ferricytochrome c. Protein concentrations were estimated by comparing the 1636 cm-1 resonance Raman line of known ferricytochrome c solutions to intensity values for the reconstituted multilayer samples. Temperature-dependent profiles of the 3100-2800 cm-1 C-H stretching, 1150-1000 cm-1 C-C stretching, 1440 cm-1 CH2 deformation, and 1295 cm-1 CH2 twisting mode regions characteristic of acyl chain vibrations reflect bilayer perturbations due to the weak interactions of ferricytochrome c. The DPPC multilamellar gel to liquid-crystalline phase transition temperature, TM, defined by either the C-H stretching mode I2935/I2880 or the C-C stretching mode I1061/I1090 peak height intensity ratios, is decreased by approximately 4 degrees C for the approximately 10(-4) M ferricytochrome c reconstituted DPPC liposomes. Other spectral features, such as the increase in the 2935 cm-1 C-H stretching mode region and the enhancement of higher frequency CH2 twisting modes, which arise in bilayers containing approximately 10(-4) M protein, are interpreted in terms of protein penetration into the hydrophobic region of the bilayer.

Influence of cholesterol on gramicidin-induced HII phase formation in phosphatidylcholine model membranes

The influence of cholesterol incorporation on gramicidin-induced hexagonal HII phase formation in different phosphatidylcholine model systems was investigated by 31P- and 2H-NMR, small-angle X-ray diffraction and differential scanning calorimetry. In liquid-crystalline distearoylphosphatidylcholine systems cholesterol inhibits gramicidin-induced HII phase formation. In dioleoylphosphatidylcholine the opposite effect is observed. Cholesterol appears to preferentially interact with gramicidin under liquid-crystalline conditions in both systems. Two phenomena that had been reported for gramicidin-treated erythrocyte membranes and derived liposomes (Tournois, H., Leunissen-Bijvelt, J., Haest, C.W.M., De Gier, J. and De Kruijff, B. (1987) Biochemistry, 26, 6613-6621) could also be observed in more simple dioleoylphosphatidylcholine-gramicidin-cholesterol systems. These are (i) an increase in tube diameter in the gramicidin-induced HII phase with increasing temperature, which is ascribed to the presence of cholesterol in this phase, and (ii) the loss of the hexagonal HII phase related 31P-NMR line shape at lower temperatures despite the presence of this phase as demonstrated with X-ray diffraction. This latter phenomenon appears to be due to restrictions in the rate of lateral diffusion of the phospholipids around the HII tubes due to the presence of gramicidin.

Interrelationships between tyrocidine and gramicidin A' in their interaction with phospholipids in model membranes

(1) The interaction of tyrocidine with different lipids is studied in model membranes and the results are compared to the gramicinid-lipid interaction. (2) The tyrocidine-dielaidoylphosphatidylethanolamine interaction gives rise to a population of phospholipids with a lower gel to liquid-crystalline transition temperature and to an abolition of the bilayer to HII phase transition, resulting in a macroscopic organization with dynamic and structural properties different from those of the pure lipid. (3) Tyrocidine has a strong fluidizing effect on the acyl chains of phosphatidylcholines, manifested by a decrease in enthalpy of the main thermotropic transition. (4) No evidence of a gramicidin A'-like lipid-structure modulating activity was found. However, tyrocidine inhibits the formation by gramicidin of an HII phase in dioleoylphosphatidylcholine model membranes. Instead, a cubic type of lipid organization is observed. (5) Tyrocidine greatly perturbs the barrier properties of dioleoylphosphatidylcholine model membrane. (6) Gramicidin A' reverses the effect of tyrocidine on membrane permeability by forming a complex in the model membrane with an apparent 1:1 stoichiometry. (7) The results suggest that both peptide antibiotics, which are produced by Bacillus brevis ATC 8185 prior to sporulation, show antagonism in their effect on membrane structure similar to their effect on superhelical DNA (Bogh, A. and Ristow, H. (1986) Eur. J. Biochem. 160, 587-591. The possible underlying basic mechanism is indicated.

Gramicidin A--phospholipid model systems

Gramicidin A forms ion-conducting channels which can traverse the hydrocarbon core of lipid bilayer membranes. The structures formed by gramicidin A are among the best characterized of all membrane-bound polypeptides or proteins. In this review a brief summary is given of the occurrence, conformation, and synthesis of gramicidin A, and of its use as a model for ion transport and the interaction of proteins and lipids in biological membranes.