Binding of a fluorescent dansylcadaverine-substance P analogue to negatively charged phospholipid membranes

We have investigated the binding of a new dansylcadaverine derivative of substance P (DNC-SP) with negatively charged small unilamellar vesicles composed of a mixture of phosphatidylcholine (PC) and either phosphatidylglycerol (PG) or phosphatidylserine (PS) using fluorescence spectroscopic techniques. The changes in fluorescence properties were used to obtain association isotherms at variable membrane negative charges and at different ionic strengths. The experimental association isotherms were analyzed using two binding approaches: (i) the Langmuir adsorption isotherm and the partition equilibrium model, that neglect the activity coefficients; and (ii) the partition equilibrium model combined with the Gouy-Chapman formalism that considers electrostatic effects. A consistent quantitative analysis of each DNC-SP binding curve at different lipid composition was achieved by means of the Gouy-Chapman approach using a peptide effective interfacial charge (v) value of (0.95 +/- 0.02), which is lower than the physical charge of the peptide. For PC/PG membranes, the partition equilibrium constant were 7.8 x 10(3) M(-1) (9/1, mol/mol) and 6.9 x 10(3) M(-1) (7/3, mol/mol), whereas for PC/PS membranes an average value of 6.8 x 10(3) M(-1) was estimated. These partition equilibrium constants were similar to those obtained for the interaction of DNC-SP with neutral PC membranes (4.9 x 10(3) M(-1)), as theoretically expected. We demonstrate that the v parameter is a determinant factor to obtain a unique value of the binding constant independently of the surface charge density of the vesicles. Also, the potential of fluorescent dansylated SP analogue in studies involving interactions with cell membranes is discussed.     

Presence of anionic phospholipids rules the membrane localization of phenothiazine type multidrug resistance modulator

Substances able to modulate multidrug resistance (MDR), including antipsychotic phenothiazine derivatives, are mainly cationic amphiphiles. The molecular mechanism of their action can involve interactions with transporter proteins as well as with membrane lipids. The interactions between anionic phospholipids and MDR modulators can be crucial for their action. In present work we study interactions of 2-trifluoromethyl-10-(4-[methanesulfonylamid]buthyl)-phenothiazine (FPhMS) with neutral (PC) and anionic lipids (PG and PS). Using microcalorimetry, steady-state and time-resolved fluorescence spectroscopy we show that FPhMS interacts with all lipids studied and drug location in membrane depends on lipid type. The electrostatic attraction between drug and lipid headgroups presumably keeps phenothiazine derivative molecules closer to surface of negatively charged membranes with respect to neutral ones. FPhMS effects on bilayer properties are not proportional to phosphatidylserine content in lipid mixtures. Behavior of equimolar PC:PS mixtures is similar to pure PS bilayers, while 2:1 or 1:2 (mole:mole) PC:PS mixtures resemble pure PC ones.     

Human opioid peptide Met-enkephalin binds to anionic phosphatidylserine in high preference to zwitterionic phosphatidylcholine: natural-abundance 13C NMR study on the binding state in large unilamellar vesicles

A human opioid neuropeptide, Met-enkephalin (M-Enk: Tyr1-Gly2-Gly3-Phe4-Met5), having no net charge binds to anionic phosphatidylserine (PS) in high preference to zwitterionic phosphatidylcholine (PC). The binding mechanism in the PS and PC bilayers was studied on the basis of the inter- and intramolecular interaction data obtained by natural-abundance 13C nuclear magnetic resonance (NMR) of the peptide. Prominent upfield changes of the 13C resonance were observed in the C-terminal residue upon binding to PS, whereas no such marked change was observed upon binding to PC. The upfield chemical shift changes with their characteristic carbon site dependence are ascribed to the electrostatic binding between the peptide C-terminal CO2- and the PS headgroup NH3+. Despite the net negative charge of the PS bilayer surface, M-Enk thus anchors the negatively charged C-terminus. In the N-terminal residue, on the other hand, marked downfield chemical shift changes are observed upon binding to both the PS and PC bilayers, the magnitude of the changes being much larger in the PS system. The downfield changes with their characteristic carbon site dependence are ascribed to the electrostatic binding between the peptide N-terminal NH3+ and the lipid headgroup negative charge(s) (CO2- or PO4- in PS, PO4- in PC). Perturbation on the signal half-widths due to membrane binding also indicates the preferential and deeper binding of M-Enk on the PS membrane surface than on the PC membrane surface. Local charge cancellation takes place efficiently between M-Enk termini and the PS headgroups and compensates for the strong electrostatic hydration of the ionic groups. Distribution of the charged (positive and negative) and uncharged sites in the headgroups along the bilayer normal is responsible for the marked difference between PS and PC headgroups in controlling the binding state of the zwitterionic M-Enk.     

Adsorption of ruthenium red to phospholipid membranes.

We have measured the distribution of the hexavalent ruthenium red cation (RuR) between water and phospholipid membranes, have shown the critical importance of membrane negative surface charge for RuR binding, and determined the association constant of RuR for different phospholipid bilayers. The studies were performed with liposomes made of mixtures of zwitterionic L-alpha-phosphatidylcholine (PC), and one of the negatively charged phospholipids: L-alpha-phosphatidylserine (PS), L-alpha-phosphatidylinositol (PI), or L-alpha-phosphatidylglycerol (PG). Lipid composition of PC:PX membranes was 1:0, 19:1, 9:1, and 4:1. Liposomes were processed using freeze-and-thaw treatment, and their size distribution was characterized by light scattering and electron microscopy. Experimental distribution isotherms of RuR obtained by ultracentrifugation and spectrophotometry can be reproduced with the Langmuir-Stern-Grahame model, assuming that RuR behaves in the diffuse double layer as an ion with effective valency < 6. In terms of this model, PC-PS, PC-PI, and PC-PG membranes were found to be electrostatically equivalent and the intrinsic association constants of RuR were obtained. RuR has highest affinity to PS-containing membranes; its association constant for PC-PI and PC-PG membranes is about 5 times smaller than that for PC-PS membranes. From the comparison of RuR binding to mixed negatively charged phospholipid membranes and RuR binding to sarcoplasmic reticulum (SR), we conclude that the low-affinity RuR binding sites may indeed be associated with the lipid bilayer of SR.     

Membrane surface-charge titration probed by gramicidin A channel conductance.

We manipulate lipid bilayer surface charge and gauge its influence on gramicidin A channel conductance by two strategies: titration of the lipid charge through bulk solution pH and dilution of a charged lipid by neutral. Using diphytanoyl phosphatidylserine (PS) bilayers with CsCl aqueous solutions, we show that the effects of lipid charge titration on channel conductance are masked 1) by conductance saturation with Cs+ ions in the neutral pH range and 2) by increased proton concentration when the bathing solution pH is less than 3. A smeared charge model permits us to separate different contributions to the channel conductance and to introduce a new method for "bilayer pKa" determination. We use the Gouy-Chapman expression for the charged surface potential to obtain equilibria of protons and cations with lipid charges. To calculate cation concentration at the channel mouth, we compare different models for the ion distribution, exact and linearized forms of the planar Poisson-Boltzmann equation, as well as the construction of a "Gibbs dividing surface" between salt bath and charged membrane. All approximations yield the intrinsic pKain of PS lipid in 0.1 M CsCl to be in the range 2.5-3.0. By diluting PS surface charge at a fixed pH with admixed neutral diphytanoyl phosphatidylcholine (PC), we obtain a conductance decrease in magnitude greater than expected from the electrostatic model. This observation is in accord with the different conductance saturation values for PS and PC lipids reported earlier (, Biochim. Biophys. Acta. 552:369-378) and verified in the present work for solvent-free membranes. In addition to electrostatic effects of surface charge, gramicidin A channel conductance is also influenced by lipid-dependent structural factors.     

Effect of lipid composition upon fusion of liposomes with Sendai virus membranes

How the lipid composition of liposomes determines their ability to fuse with Sendai virus membranes was tested. Liposomes were made of compositions designed to test postulated mechanisms of membrane fusion that require specific lipids. Fusion does not require the presence of lipids that can form micelles such as gangliosides or lipids that can undergo lamellar to hexagonal phase transitions such as phosphatidylethanolamine (PE), nor is a phosphatidylinositol (PI) to phosphatidic acid (PA) conversion required, since fusion occurs with liposomes containing phosphatidylcholine (PC) and any one of many different negatively charged lipids such as gangliosides, phosphatidylserine (PS), phosphatidylglycerol, dicetyl phosphate, PI, or PA. A negatively charged lipid is required since fusion does not occur with neutral liposomes containing PC and a neutral lipid such as globoside, sphingomyelin, or PE. Fusion of Sendai virus membranes with liposomes that contain PC and PS does not require Ca2+, so an anhydrous complex with Ca2+ or a Ca2+-induced lateral phase separation is not required although the possibility remains that viral binding causes a lateral phase separation. Sendai virus membranes can fuse with liposomes containing only PS, so a packing defect between domains of two different lipids is not required. The concentration of PS required for fusion to occur is approximately 10-fold higher than that required for ganglioside GD1a, which has been shown to act as a Sendai virus receptor. When cholesterol is added as a third lipid to liposomes containing PC and GD1a, the amount of fusion decreases if the GD1a concentration is low.(ABSTRACT TRUNCATED AT 250 WORDS)     

Quantitative fluorescence analysis of the adsorption of lysozyme to phospholipid vesicles

Experiments directed to measure the interaction of lysozyme with liposomes consisting of phosphatidylcholine (PC) and phosphatidylserine (PS) have been conducted by monitoring both protein and lipid fluorescence and fluorescence anisotropy of the protein. The binding of lysozyme to the unilamellar vesicles was quantified using a novel method of analysis in which the fractional contribution at moderate binding conditions is determined from either total fluorescence decay or anisotropy decay curves of tryptophan at limiting binding conditions. In the energy transfer experiments PC and PS lipids labelled with two pyrene acyl chains served as energy acceptors of the excited tryptophan residues in lysozyme. The binding was strongly dependent on the molar fraction of negatively charged PS in neutral PC membranes and on the ionic strength. Changes in the tryptophan fluorescence decay characteristics were found to be connected with long correlation times, indicating conformational rearrangements induced by binding of the protein to these lipid membranes. The dynamics of membrane bound protein appeared to be dependent on the physical state of the membrane. Independent of protein fluorescence studies, formation of a protein-membrane complex can also be observed from the lipid properties of the system. The interaction of lysozyme with di-pyrenyl-labelled phosphatidylserine in anionic PS/PC membranes resulted in a substantial decrease of the intramolecular excimer formation, while the excimer formation of dipyrenyl-labelled phosphatidylcholine in neutral PC membranes barely changed in the presence of lysozyme.Abbreviations dipyr₄ sn-1,2-(pyrenylbutyl) - dipyr₁₀ sn-1,2-(pyrenyldecanoyl). - DMPC dimyristoyl-phosphatidylcholine - DOPC dioleoyl-phosphatidylcholine - DPPC dipalmitoyl-phosphatidylcholine - DPPC dipalmitoylphosphatidylcholine - PC phosphatidylcholine - PS phosphatidylserine Correspondence to: A. J. W. G. Visser     

Intrinsic selectivity in binding of matrix metalloproteinase-7 to differently charged lipid membranes

We provide evidence that matrix metalloproteinase-7 (MMP-7) interacts with anionic, cationic and neutral lipid membranes, although it interacts strongest with anionic membranes. While the catalytic activity of the enzyme remains unaffected upon binding to neutral and negatively charged membranes, it is drastically impaired upon binding to the positively charged membranes. The structural data reveal that the origin of these features lies in the "bipolar" distribution of the electrostatic surface potentials on the crystallographic structure of MMP-7.     

Membrane binding properties of IRSp53-missing in metastasis domain (IMD) protein

The 53-kDa insulin receptor substrate protein (IRSp53) organizes the actin cytoskeleton in response to stimulation of small GTPases, promoting the formation of cell protrusions such as filopodia and lamellipodia. IMD is the N-terminal 250 amino acid domain (IRSp53/MIM Homology Domain) of IRSp53 (also called I-BAR), which can bind to negatively charged lipid molecules. Overexpression of IMD induces filopodia formation in cells and purified IMD assembles finger-like protrusions in reconstituted lipid membranes. IMD was shown by several groups to bundle actin filaments, but other groups showed that it also binds to membranes. IMD binds to negatively charged lipid molecules with preference to clusters of PI(4,5)P₂. Here, we performed a range of different in vitro fluorescence experiments to determine the binding properties of the IMD to phospholipids. We used different constructs of large unilamellar vesicles (LUVETs), containing neutral or negatively charged phospholipids. We found that IMD has a stronger binding interaction with negatively charged PI(4,5)P₂ or PS lipids than PS/PC or neutral PC lipids. The equilibrium dissociation constant for the IMD–lipid interaction falls into the 78–170 μM range for all the lipids tested. The solvent accessibility of the fluorescence labels on the IMD during its binding to lipids is also reduced as the lipids become more negatively charged. Actin affects the IMD–lipid interaction, depending on its polymerization state. Monomeric actin partially disrupts the binding, while filamentous actin can further stabilize the IMD–lipid interaction.     

Melittin binding to mixed phosphatidylglycerol/phosphatidylcholine membranes

The binding of bee venom melittin to negatively charged unilamellar vesicles and planar lipid bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) was studied with circular dichroism and deuterium NMR spectroscopy. The melittin binding isotherm was measured for small unilamellar vesicles containing 10 or 20 mol % POPG. Due to electrostatic attraction, binding of the positively charged melittin was much enhanced as compared to the binding to neutral lipid vesicles. However, after correction for electrostatic effects by means of the Gouy-Chapman theory, all melittin binding isotherms could be described by a partition Kp = (4.5 +/- 0.6) x 10(4) M-1. It was estimated that about 50% of the total melittin surface was embedded in a hydrophobic environment. The melittin partition constant for small unilamellar vesicles was by a factor of 20 larger than that of planar bilayers and attests to the tighter lipid packing in the nonsonicated bilayers. Deuterium NMR studies were performed with coarse lipid dispersions. Binding of melittin to POPC/POPG (80/20 mol/mol) membranes caused systematic changes in the conformation of the phosphocholine and phosphoglycerol head groups which were ascribed to the influence of electrostatic charge on the choline dipole. While the negative charge of phosphatidylglycerol moved the N+ end of the choline -P-N+ dipole toward the bilayer interior, the binding of melittin reversed this effect and rotated the N+ end toward the aqueous phase. No specific melittin-POPG complexes could be detected. The phosphoglycerol head group was less affected by melittin binding than its choline counterpart.     

Membrane binding and pore formation of the antibacterial peptide PGLa: thermodynamic and mechanistic aspects

The antibacterial peptide PGLa exerts its activity by permeabilizing bacterial membranes whereas eukaryotic membranes are not affected. To provide insight into the selectivity and the permeabilization mechanism, the binding of PGLa to neutral and negatively charged model membranes was studied with high-sensitivity isothermal titration calorimetry (ITC), circular dichroism (CD), and solid-state deuterium nuclear magnetic resonance ((2)H NMR). The binding of PGLa to negatively charged phosphatidylcholine (PC)/phosphatidylglycerol (PG) (3:1) vesicles was by a factor of approximately 50 larger than that to neutral PC vesicles. The negatively charged membrane accumulates the cationic peptide at the lipid-water interface, thus facilitating the binding to the membrane. However, if bulk concentrations are replaced by surface concentrations, very similar binding constants are obtained for neutral and charged membranes (K approximately 800-1500 M(-)(1)). Membrane selectivity is thus caused almost exclusively by electrostatic attraction to the membrane surface and not by hydrophobic insertion. Membrane insertion is driven by an exothermic enthalpy (DeltaH approximately -11 to -15 kcal/mol) but opposed by entropy. An important contribution to the binding process is the membrane-induced random coil --> alpha-helix transition of PGLa. The peptide is random coil in solution but adopts an approximately 80% alpha-helical conformation when bound to the membrane. Helix formation is an exothermic process, contributing approximately 70% to the binding enthalpy and approximately 30% to the free energy of binding. The (2)H NMR measurements with selectively deuterated lipids revealed small structural changes in the lipid headgroups and in the hydrocarbon interior upon peptide binding which were continuous over the whole concentration range. In contrast, isothermal titration calorimetry of PGLa solutions with PC/PG(3:1) vesicles gave rise to two processes: (i) an exothermic binding of PGLa to the membrane followed by (ii) a slower endothermic process. The latter is only detected at peptide-to-lipid ratios >17 mmol/mol and is paralleled by the induction of membrane leakiness. Dye efflux measurements are consistent with the critical limit derived from ITC measurements. The endothermic process is assigned to peptide pore formation and/or lipid perturbation. The enthalpy of pore formation is 9.7 kcal/mol of peptide. If the same excess enthalpy is assigned to the lipid phase, the lipid perturbation enthalpy is 180 cal/mol of lipid. The functional synergism between PGLa and magainin 2 amide could also be followed by ITC and dye release experiments and is traced back to an enhanced pore formation activity of a peptide mixture.     

Nonideal mixing of phosphatidylserine and phosphatidylcholine in the fluid lamellar phase.

The mixing of phosphatidylserine (PS) and phosphatidylcholine (PC) in fluid bilayer model membranes was studied by measuring binding of aqueous Ca2+ ions. The measured [Ca2+]aq was used to derive the activity coefficient for PS, gamma PS, in the lipid mixture. For (16:0, 18:1) PS in binary mixtures with either (16:0, 18:1)PC, (14:1, 14:1)PC, or (18:1, 18:1)PC, gamma PS > 1; i.e., mixing is nonideal, with PS and PC clustered rather than randomly distributed, despite the electrostatic repulsion between PS headgroups. To understand better this mixing behavior, Monte Carlo simulations of the PS/PC distributions were performed, using Kawasaki relaxation. The excess energy was divided into an electrostatic term Uel and one adjustable term including all other nonideal energy contributions, delta Em. Uel was calculated using a discrete charge theory. Kirkwood's coupling parameter method was used to calculate the excess free energy of mixing, delta GEmix, hence In gamma PS,calc. The values of In gamma PS,calc were equalized by adjusting delta Em in order to find the simulated PS/PC distribution that corresponded to the experimental results. We were thus able to compare the smeared charge calculation of [Ca2+]surf with a calculation ("masked evaluation method") that recognized clustering of the negatively charged PS: clustering was found to have a modest effect on [Ca2+]surf, relative to the smeared charge model. Even though both PS and PC tend to cluster, the long-range nature of the electrostatic repulsion reduces the extent of PS clustering at low PS mole fraction compared to PC clustering at an equivalent low PC mole fraction.     

Anion binding to neutral and positively charged lipid membranes

Aqueous anion binding to bilayer membranes consisting of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was investigated by using deuterium and phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy. Only those anions that exhibit chaotropic properties showed significant binding to POPC membranes. A detailed investigation of thiocyanate binding to neutral POPC and to positively charged mixed POPC/dihexadecyldimethylammonium bromide (DHDMAB) (8:2 mol/mol) membranes revealed changes in the 2H NMR quadrupole splittings from POPC specifically deuteriated at either the alpha-segment or the beta-segment of the choline head group which were consistent with a progressive accumulation of excess negative charge at the membrane surface with increasing SCN- concentration. Both the 2H and 31P NMR spectra indicated the presence of fluid lipids in a bilayer configuration up to at least 1.0 M NaSCN with no indication of any phase separation of lipid domains. Calibration of the relationship between the change in the 2H NMR quadrupole splitting and the amount of SCN- binding provided thiocyanate binding isotherms. At a given SCN- concentration the positively charged membranes bound levels of SCN- 3 times that of the neutral membranes. The binding isotherms were analyzed by considering both the electrostatic and the chemical equilibrium contributions to SCN- binding. Electrostatic considerations were accounted for by using the Gouy-Chapman theory. For 100% POPC membranes as well as for mixed POPC/DHDMAB (8:2 mol/mol) membranes the thiocyanate binding up to concentrations of 100 mM was characterized by a partition equilibrium with an association constant of K approximately 1.4 +/- 0.3 M-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of phospholipids on conformational structure of bovine pancreatic trypsin inhibitor (BPTI) and its thermolabile mutants

The effects of negatively charged phosphatidylserine-prepared membranes (PS) and neutral phosphatidylcholine-prepared membranes (PC) on the structure of wild-type and mutant bovine pancreatic trypsin inhibitor (BPTI) at neutral pH were investigated. The presence of PC did not have any effect on the protein structure while PS induced a non-native structure in three mutant BPTI proteins. However, the negatively charged membrane did not have any effect on wild-type BPTI. The findings revealed that (i) elimination of some disulphide bonds results in dramatic change in protein structure, and, (ii) that this biochemical interaction is surface-driven and electrostatic interactions may play a very strong role in influencing the fore-stated changes in protein structure. Of further interest were the results obtained from investigating the possible role of PS fluidity and concentration in altering mutant. When the value of Gibbs free-energy change of unfolding (DeltaG(U)) was positive, various non-native structures were formed in a concentration-dependent manner. However, when the value of DeltaG(U) was negative, only two types of non-native structures were formed: one with high beta structure content at low PS fluidity state, and the other with a high alpha-helical content at high PS fluidity state. Our study reveals how particular combinations of phospholipid:protein interactions can induce a protein conformation transition from a native to a non-native one at neutral pH, especially when the native structure is predestabilized by amino acid substitutions. This revelation may open up opportunities to explore alternative ways in which phospholipids may play a role in protein mis-folding and the related pathologies.     

Interaction of verapamil with lipid membranes and P-glycoprotein: connecting thermodynamics and membrane structure with functional activity

Verapamil and amlodipine are calcium ion influx inhibitors of wide clinical use. They are partially charged at neutral pH and exhibit amphiphilic properties. The noncharged species can easily cross the lipid membrane. We have measured with solid-state NMR the structural changes induced by verapamil upon incorporation into phospholipid bilayers and have compared them with earlier data on amlodipine and nimodipine. Verapamil and amlodipine produce a rotation of the phosphocholine headgroup away from the membrane surface and a disordering of the fatty acid chains. We have determined the thermodynamics of verapamil partitioning into neutral and negatively charged membranes with isothermal titration calorimetry. Verapamil undergoes a pK-shift of DeltapK(a) = 1.2 units in neutral lipid membranes and the percentage of the noncharged species increases from 5% to 45%. Verapamil partitioning is increased for negatively charged membranes and the binding isotherms are strongly affected by the salt concentration. The electrostatic screening can be explained with the Gouy-Chapman theory. Using a functional phosphate assay we have measured the affinity of verapamil, amlodipine, and nimodipine for P-glycoprotein, and have calculated the free energy of drug binding from the aqueous phase to the active center of P-glycoprotein in the lipid phase. By combining the latter results with the lipid partitioning data it was possible, for the first time, to determine the true affinity of the three drugs for the P-glycoprotein active center if the reaction takes place exclusively in the lipid matrix.     

Phosphatidic acid and phosphatidylserine have distinct structural and functional interactions with the nicotinic acetylcholine receptor

Bilayers containing phosphatidylcholine (PC) and the anionic lipid phosphatidic acid (PA) are particularly effective at stabilizing the nicotinic acetylcholine receptor (nAChR) in a functional conformation that undergoes agonist-induced conformational change. The physical properties of PC membranes containing PA are also substantially altered upon incorporation of the nAChR. To test whether or not the negative charge of PA is responsible for this "bi-directional coupling," the nAChR was reconstituted into membranes composed of PC with varying levels of the net negatively charged lipid phosphatidylserine (PS). In contrast to PA, increasing levels of PS in PC membranes do not stabilize an increasing proportion of nAChRs in a functional resting conformation, nor do they slow nAChR peptide hydrogen exchange kinetics. Incorporation of the nAChR had little effect on the physical properties of the PC/PS membranes, as monitored by the gel-to-liquid crystal phase transition temperatures of the bilayers. These results show that a net negative charge alone is not sufficient to account for the unique interactions that occur between the nAChR and PC/PA membranes. Incorporation of the receptor into PC/PS membranes, however, did lead to an altered head group conformation of PS possibly by recruiting divalent cations to the membrane surface. The results show that the nAChR has complex and unique interactions with both PA and PS. The interactions between the nAChR and PS may be bridged by divalent cations, such as calcium.     

Lipid demixing and protein-protein interactions in the adsorption of charged proteins on mixed membranes

The adsorption free energy of charged proteins on mixed membranes, containing varying amounts of (oppositely) charged lipids, is calculated based on a mean-field free energy expression that accounts explicitly for the ability of the lipids to demix locally, and for lateral interactions between the adsorbed proteins. Minimization of this free energy functional yields the familiar nonlinear Poisson-Boltzmann equation and the boundary condition at the membrane surface that allows for lipid charge rearrangement. These two self-consistent equations are solved simultaneously. The proteins are modeled as uniformly charged spheres and the (bare) membrane as an ideal two-dimensional binary mixture of charged and neutral lipids. Substantial variations in the lipid charge density profiles are found when highly charged proteins adsorb on weakly charged membranes; the lipids, at a certain demixing entropy penalty, adjust their concentration in the vicinity of the adsorbed protein to achieve optimal charge matching. Lateral repulsive interactions between the adsorbed proteins affect the lipid modulation profile and, at high densities, result in substantial lowering of the binding energy. Adsorption isotherms demonstrating the importance of lipid mobility and protein-protein interactions are calculated using an adsorption equation with a coverage-dependent binding constant. Typically, at bulk-surface equilibrium (i.e., when the membrane surface is "saturated" by adsorbed proteins), the membrane charges are "overcompensated" by the protein charges, because only about half of the protein charges (those on the hemispheres facing the membrane) are involved in charge neutralization. Finally, it is argued that the formation of lipid-protein domains may be enhanced by electrostatic adsorption of proteins, but its origin (e.g., elastic deformations associated with lipid demixing) is not purely electrostatic.     

Fluorescent probes of electrostatic potential 1 nm from the membrane surface

We measured the electrostatic potential 1 nm from the surface of charged phospholipid bilayer membranes to test the predictions of the Gouy-Chapman theory. Fluorescent probes (anthraniloyl, 5-(dimethylamino)naphthalene-1-sulfonyl, Lucifer yellow) were attached covalently to the sialic acid residue of the ganglioside galactosyl-N-acetylgalactosaminyl(N-acetylneuraminyl)galactosylglucosylc eramide (GM1). These fluorescent gangliosides were incorporated into neutral [phosphatidylcholine (PC)] or charged [phosphatidylserine (PS)] phospholipid bilayers, and the fluorescence was quenched with the cations thallium and 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (tempamine). We calculated the electrostatic potential at the chromophore from the quenching ratio using the Boltzmann relation: the average potential was -30 mV for PS bilayers in 0.1 M NaNO3. We assume the chromophore is 1 nm from the surface because X-ray diffraction measurements demonstrate that the sialic acid residue of GM1 is 1 nm from the surface of a PC/GM1 bilayer [McDaniel, R. V., & McIntosh, T. J. (1986) Biophys. J. 49, 94-96]. We also used thallium and tempamine to quench the fluorescence of chromophores located at the surface of the PS membranes; in 0.1 M NaNO3 the average surface potential was -80 mV, which agrees with other measurements. The Gouy-Chapman theory predicts that the potential 1 nm from a membrane with a surface potential of -80 mV is -24 mV; this prediction agrees qualitatively with the experimental results obtained with fluorescent gangliosides.