Polylysine-induced 2H NMR-observable domains in phosphatidylserine/phosphatidylcholine lipid bilayers

Peripheral adsorption of polylysine on one leaflet of a lipid bilayer reduces the lipid diffusion of both leaflets

Understanding polycation-lipid interaction is essential not only in molecular biology but also in the biomedical industry and pharmacology. However, the effect of the polycation-lipid interaction on the molecular properties of lipids in biomembranes remains elusive. Here, two fluorescence correlation spectroscopies (FCSs), pulse-interleaved excitation (PIE) FCS and lifetime-based FCS, were performed to elucidate the change in the lipid diffusion of a model biomembrane induced by polylysine (PLL) adsorption. The results of PIE-FCS showed that the diffusions of both anionic and zwitterionic lipids become slower in the presence of PLL but the mobility of the anionic lipids is much reduced, suggesting the preferential interaction between the PLL and the anionic lipids due to the electrostatic attraction. Furthermore, leaflet-specific lipid diffusion analysis by lifetime-based FCS clearly showed that PLL adsorption on one leaflet of the membrane reduces the lipid diffusion of both leaflets in the same manner. This clearly indicates that the interleaflet coupling is strong in the presence of PLL.

A simple method for poly-D-lysine coating to enhance adhesion and maturation of primary cortical neuron cultures in vitro

Introduction

Glass coverslips are used as a substrate since Harrison's initial nerve cell culture experiments in 1910. In 1974, the first study of brain cells seeded onto polylysine (PL) coated substrate was published. Usually, neurons adhere quickly to PL coating. However, maintaining cortical neurons in culture on PL coating for a prolonged time is challenging.

Methods

A collaborative study between chemical engineers and neurobiologists was conducted to find a simple method to enhance neuronal maturation on poly-D-lysine (PDL). In this work, a simple protocol to coat PDL efficiently on coverslips is presented, characterized, and compared to a conventional adsorption method. We studied the adhesion and maturation of primary cortical neurons with various morphological and functional approaches, including phase contrast microscopy, immunocytochemistry, scanning electron microscopy, patch clamp recordings, and calcium imaging.

Results

We observed that several parameters of neuronal maturation are influenced by the substrate: neurons develop more dense and extended networks and synaptic activity is enhanced, when seeded on covalently bound PDL compared to adsorbed PDL.

Discussion

Hence, we established reproducible and optimal conditions enhancing maturation of primary cortical neurons in vitro. Our method allows higher reliability and yield of results and could also be profitable for laboratories using PL with other cell types.

Phospholipid lateral diffusion in the presence of cationic peptides as measured via 31P CODEX NMR

The effects of two cationic peptides on phospholipid lateral diffusion in binary mixtures of POPC with various anionic phospholipids were measured via ³¹P CODEX NMR. Large unilamellar vesicles composed of POPC/POPG (70/30 mol/mol), or POPC/DOPS (70/30 mol/mol), or POPC/TOCL (85/15 mol/mol), or POPC/DOPA (50/50 mol/mol) were exposed to either polylysine (pLYS, N = 134 monomers) or KL-14 (KKLL KKAKK LLKKL), a model amphipathic helical peptide, in an amount corresponding to 80% neutralization of the anionic phospholipid charge by the cationic lysine residues. In the absence of added peptide, phospholipid lateral diffusion coefficients (all measured at 10 °C) increased with increasing reduced temperature (T-T_m). The POPC/DOPA mixture was an exception to this generalization, in that lateral diffusion for both components was far slower than any other mixture investigated, an effect attributed to intermolecular hydrogen bonding. The addition of pLYS or KL-14 decreased lateral diffusion in the POPC/DOPS LUV, but had minimal effects in the POPC/POPG LUV, indicating that ease of access of the cationic peptide residues to the anionic phospholipid groups was important. Both cationic peptides produced the opposite effect in the POPC/DOPA case, in that lateral diffusion increased significantly in their presence, with KL-14 being most effective. This latter observation was interpreted in terms of the electrostatic / H-bond model proposed by Kooijman et al. [Journal of Biological Chemistry, 282:11356-11,364, 2007] to describe the mechanism of interaction between the phosphomonoester head group of PA and the tertiary amine of lysine.

Polycation-Anionic Lipid Membrane Interactions

Natural or synthetic polycations are used as biocides or as drug/gene carriers. Understanding the interactions between these macromolecules and cell membranes at the molecular level is therefore of great importance for the design of effective polymer biocides or biocompatible polycation-based delivery systems. Until now, details of the processes at the interface between polycations and biological systems have not been fully recognized. In this study, we consider the effect of strong polycations with quaternary ammonium groups on the properties of anionic lipid membranes that we use as a model system for protein-free cell membranes. For this purpose, we employed experimental measurements and atomic-scale molecular dynamics (MD) simulations. MD simulations reveal that the polycations are strongly hydrated in the aqueous phase and do not lose the water shell after adsorption at the bilayer surface. As a result of strong hydration, the polymer chains reside at the phospholipid headgroup and do not penetrate to the acyl chain region. The polycation adsorption involves the formation of anionic lipid-rich domains, and the density of anionic lipids in these domains depends on the length of the polycation chain. We observed the accumulation of anionic lipids only in the leaflet interacting with the polymer, which leads to the formation of compositionally asymmetric domains. Asymmetric adsorption of the polycation on only one leaflet of the anionic membrane strongly affects the membrane properties in the polycation-membrane contact areas: (i) anionic lipid accumulates in the region near the adsorbed polymer, (ii) acyl chain ordering and lipid packing are reduced, which results in a decrease in the thickness of the bilayer, and (iii) polycation-anionic membrane interactions are strongly influenced by the presence and concentration of salt. Our results provide an atomic-scale description of the interactions of polycations with anionic lipid bilayers and are fully supported by the experimental data. The outcomes are important for understanding the correlation of the structure of polycations with their activity on biomembranes.

Adsorption of Synthetic Cationic Polymers on Model Phospholipid Membranes: Insight from Atomic-Scale Molecular Dynamics Simulations

Although synthetic cationic polymers represent a promising class of effective antibacterial agents, the molecular mechanisms behind their antimicrobial activity remain poorly understood. To this end, we employ atomic-scale molecular dynamics simulations to explore adsorption of several linear cationic polymers of different chemical structure and protonation (polyallylamine (PAA), polyethylenimine (PEI), polyvinylamine (PVA), and poly-l-lysine (PLL)) on model bacterial membranes (4:1 mixture of zwitterionic phosphatidylethanolamine (PE) and anionic phosphatidylglycerol (PG) lipids). Overall, our findings show that binding of polycations to the anionic membrane surface effectively neutralizes its charge, leading to the reorientation of water molecules close to the lipid/water interface and to the partial release of counterions to the water phase. In certain cases, one has even an overcharging of the membrane, which was shown to be a cooperative effect of polymer charges and lipid counterions. Protonated amine groups of polycations are found to interact preferably with head groups of anionic lipids, giving rise to formation of hydrogen bonds and to a noticeable lateral immobilization of the lipids. While all the above findings are mostly defined by the overall charge of a polymer, we found that the polymer architecture also matters. In particular, PVA and PEI are able to accumulate anionic PG lipids on the membrane surface, leading to lipid segregation. In turn, PLL whose charge twice exceeds charges of PVA/PEI does not induce such lipid segregation due to its considerably less compact architecture and relatively long side chains. We also show that partitioning of a polycation into the lipid/water interface is an interplay between its protonation level (the overall charge) and hydrophobicity of the backbone. Therefore, a possible strategy in creating highly efficient antimicrobial polymeric agents could be in tuning these polycation's properties through proper combination of protonated and hydrophobic blocks.

Synthesis of Perdeuterated 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine ([D82 ]POPC) and Characterisation of Its Lipid Bilayer Membrane Structure by Neutron Reflectometry

1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), an unsaturated acyl chain containing lipid, is often the predominant lipid in eukaryotic cell membranes in which it is crucial for the fluidity of membranes under physiological conditions. Commercially available, partially deuterated [D₃₁ ]1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine ([D₃₁ ]POPC) does not provide sufficient isotopic contrast for detailed structural studies of multicomponent membranes through neutron techniques. Herein, a relatively straightforward and generic chemical deuteration method is discussed for the asymmetric synthesis of perdeuterated [D₃₁ ]1-palmitoyl-[D₃₃ ]2-oleoyl-sn-[D₅ ]glycero-[D₁₃ ]3-phosphocholine ([D₈₂ ]POPC) that also allows selective deuteration of any of its constituent groups. Neutron reflectivity of a [D₈₂ ]POPC-supported bilayer was used to experimentally determine the neutron scattering length density profile of the lipid. The acyl chains of [D₈₂ ]POPC are closely contrast-matched to heavy water, whereas the very high scattering length density of the deuterated glycerophosphocholine head groups provides good contrast to membrane-binding agents in both deuterated and non-deuterated solvent environments.

β2-Microglobulin amyloid fibril-induced membrane disruption is enhanced by endosomal lipids and acidic pH

Although the molecular mechanisms underlying the pathology of amyloidoses are not well understood, the interaction between amyloid proteins and cell membranes is thought to play a role in several amyloid diseases. Amyloid fibrils of β2-microglobulin (β2m), associated with dialysis-related amyloidosis (DRA), have been shown to cause disruption of anionic lipid bilayers in vitro. However, the effect of lipid composition and the chemical environment in which β2m-lipid interactions occur have not been investigated previously. Here we examine membrane damage resulting from the interaction of β2m monomers and fibrils with lipid bilayers. Using dye release, tryptophan fluorescence quenching and fluorescence confocal microscopy assays we investigate the effect of anionic lipid composition and pH on the susceptibility of liposomes to fibril-induced membrane damage. We show that β2m fibril-induced membrane disruption is modulated by anionic lipid composition and is enhanced by acidic pH. Most strikingly, the greatest degree of membrane disruption is observed for liposomes containing bis(monoacylglycero)phosphate (BMP) at acidic pH, conditions likely to reflect those encountered in the endocytic pathway. The results suggest that the interaction between β2m fibrils and membranes of endosomal origin may play a role in the molecular mechanism of β2m amyloid-associated osteoarticular tissue destruction in DRA.

Phosphatidylinositol-4,5-bisphosphate enhances anionic lipid demixing by the C2 domain of PKCα

The C2 domain of PKCα (C2α) induces fluorescence self-quenching of NBD-PS in the presence of Ca²⁺, which is interpreted as the demixing of phosphatidylserine from a mixture of this phospholipid with phosphatidylcholine. Self-quenching of NBD-PS was considerably increased when phosphatidylinositol-4,5-bisphosphate (PIP₂) was present in the membrane. When PIP₂ was the labeled phospholipid, in the form of TopFluor-PIP₂, fluorescence self-quenching induced by the C2 domain was also observed, but this was dependent on the presence of phosphatidylserine. An independent indication of the phospholipid demixing effect given by the C2α domain was obtained by using ²H-NMR, since a shift of the transition temperature of deuterated phosphatidylcholine was observed as a consequence of the addition of the C2α domain, but only in the presence of PIP₂. The demixing induced by the C2α domain may have a physiological significance since it means that the binding of PKCα to membranes is accompanied by the formation of domains enriched in activating lipids, like phosphatidylserine and PIP₂. The formation of these domains may enhance the activation of the enzyme when it binds to membranes containing phosphatidylserine and PIP₂.

Tuning surface mechanical properties by amplified polyelectrolyte self-assembly: where "grafting-from" meets "grafting-to"

We report the interaction of surface-tethered weak polyacid brushes, poly(methacrylic acid), with a weak polybase poly(L-lysine)-graft-poly(ethylene glycol), in solution. The grafted polyacid brushes, grown directly from the silicon substrate by UVLED surface-initiated polymerization, act as a nanotemplate for the solution-phase polybase, which penetrates into the brushes, forming a polyelectrolyte complex (PEC), whose mechanical and nanotribological properties are markedly influenced by the electrostatic assembly conditions. The mechanical effects are amplified due to the architecture of the specific polybase used, which contributes approximately 2k Da per unit charge to the overall system, resulting in an efficient filling of the polyacid brushes, which thus acts as a scaffold. The distribution of the adsorbed copolymers in the PEC films has been investigated by means of confocal microscopy. The unique structure of the PEC films provides a system whose mechanical and nanotribological properties can be tuned over a wide range.

How lipid headgroups sense the membrane environment: an application of ¹⁴N NMR

The orientation of lipid headgroups may serve as a powerful sensor of electrostatic interactions in membranes. As shown previously by (2)H NMR measurements, the headgroup of phosphatidylcholine (PC) behaves like an electrometer and varies its orientation according to the membrane surface charge. Here, we explored the use of solid-state (14)N NMR as a relatively simple and label-free method to study the orientation of the PC headgroup in model membrane systems of varying composition. We found that (14)N NMR is sufficiently sensitive to detect small changes in headgroup orientation upon introduction of positively and negatively charged lipids and we developed an approach to directly convert the (14)N quadrupolar splittings into an average orientation of the PC polar headgroup. Our results show that inclusion of cholesterol or mixing of lipids with different length acyl chains does not significantly affect the orientation of the PC headgroup. In contrast, measurements with cationic (KALP), neutral (Ac-KALP), and pH-sensitive (HALP) transmembrane peptides show very systematic changes in headgroup orientation, depending on the amount of charge in the peptide side chains and on their precise localization at the interface, as modulated by varying the extent of hydrophobic peptide/lipid mismatch. Finally, our measurements suggest an unexpectedly strong preferential enrichment of the anionic lipid phosphatidylglycerol around the cationic KALP peptide in ternary mixtures with PC. We believe that these results are important for understanding protein/lipid interactions and that they may help parametrization of membrane properties in computational studies.

Electrostatic field effects on membrane domain segregation and on lateral diffusion

Natural membranes are organized structures of neutral and charged molecules bearing dipole moments which generate local non-homogeneous electric fields. When subjected to such fields, the molecules experience net forces that can modify the lipid and protein organization, thus modulating cell activities and influencing (or even dominating) the biological functions. The energetics of electrostatic interactions in membranes is a long-range effect which can vary over distance within r^-1 to r^-3. In the case of a dipole interacting with a plane of dipoles, e.g. a protein interacting with a lipid domain, the interaction is stronger than two punctual dipoles and depends on the size of the domain. In this article, we review several contributions on how electrostatic interactions in the membrane plane can modulate the phase behavior, surface topography and mechanical properties in monolayers and bilayers.

Thermodynamics of lipid interactions with cell-penetrating peptides

Cationic peptides are efficiently taken up by biological cells through different pathways, which can be exploited for delivery of intracellular drugs. For example, their endocytosis is known since 1967, and this typically produces entrapment of the peptides in endocytotic vesicles. The resulting peptide (and cargo) degradation in lysosomes is of little therapeutic interest. Beside endocytosis (and various subtypes thereof), cationic cell-penetrating peptides (CPPs) may also gain access to cytosol and nucleus of livings cells. This process is known since 1988, but it is poorly understood whether the cytosolic CPP appearance requires an active cellular machinery with membrane proteins and signaling molecules, or whether this translocation occurs by passive diffusion and thus can be mimicked with model membranes devoid of proteins or glycans. In the present chapter, protocols are presented that allow for testing the membrane binding and disturbance of CPPs on model membranes with special focus on particular CPP properties. Protocols include vesicle preparation, lipid quantification, and analysis of membrane leakage, lipid polymorphism ((31)P NMR), and membrane binding (isothermal titration calorimetry). Using these protocols, a major difference among CPPs is observed: At low micromolar concentration, nonamphipathic CPPs, such as nona-arginine (WR(9)) and penetratin, have only a poor affinity for model membranes with a lipid composition typical of eukaryotic membranes. No membrane leakage is induced by these compounds at low micromolar concentration. In contrast, their amphipathic derivatives, such as acylated WR(9) (C(14), C(16), C(18)) or amphipathic penetratin mutant p2AL (Drin et al., Biochemistry 40:1824-1834, 2001), bind and disturb lipid model membranes already at low micromolar peptide concentration. This suggests that the mechanism for cytosolic CPP delivery (and potential toxicity) differs among CPPs despite their common name.

Antimicrobial polymers: mechanism of action, factors of activity, and applications

Complex epidemiological situation, nosocomial infections, microbial contamination, and infection risks in hospital and dental equipment have led to an ever-growing need for prevention of microbial infection in these various areas. Macromolecular systems, due to their properties, allow one to efficiently use them in various fields, including the creation of polymers with the antimicrobial activity. In the past decade, the intensive development of a large class of antimicrobial macromolecular systems, polymers, and copolymers, either quaternized or functionalized with bioactive groups, has been continued, and they have been successfully used as biocides. Various permanent microbicidal surfaces with non-leaching polymer antimicrobial coatings have been designed. Along with these trends, new moderately hydrophobic polymer structures have been synthesized and studied, which contain protonated primary or secondary/tertiary amine groups that exhibited rather high antimicrobial activity, often unlike their quaternary analogues. This mini-review briefly highlights and summarizes the results of studies during the past decade and especially in recent years, which concern the mechanism of action of different antimicrobial polymers and non-leaching microbicidal surfaces, and factors influencing their activity and toxicity, as well as major applications of antimicrobial polymers.

Morphological changes of supported lipid bilayers induced by lysozyme: planar domain formation vs. multilayer stacking

Total internal reflection fluorescence microscopy (TIRFM) has been utilized to explore the effect of cationic protein lysozyme (Lz) on the morphology of solid-supported lipid bilayers (SLBs) comprised of zwitterionic lipid phosphatidylcholine (PC) and its mixture with anionic lipid cardiolipin (CL). Kinetic TIRFM imaging of different systems revealed subtle interplay between lipid lateral segregation accompanied by exchange of neutral and acidic lipids in the protein-lipid interaction zone, and the formation of lipid multilayer stacks. The switch between these states was shown to be controlled by CL content. In weakly charged SLBs containing 5 mol% CL, assembling of CL molecules into planar domains upon Lz adsorption has been observed while at higher content of anionic lipid (25 mol%) in-plane domains tend to transform into multilayer stacks, thereby ensuring the most thermodynamically-favorable membrane conformation.

Structure and dynamics of the myristoyl lipid modification of SRC peptides determined by 2H solid-state NMR spectroscopy

Lipid modifications of proteins are widespread in nature and play an important role in numerous biological processes. The nonreceptor tyrosine kinase Src is equipped with an N-terminal myristoyl chain and a cluster of basic amino acids for the stable membrane association of the protein. We used (2)H NMR spectroscopy to investigate the structure and dynamics of the myristoyl chain of myr-Src(2-19), and compare them with the hydrocarbon chains of the surrounding phospholipids in bilayers of varying surface potentials and chain lengths. The myristoyl chain of Src was well inserted in all bilayers investigated. In zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine membranes, the myristoyl chain of Src was significantly longer and appears "stiffer" than the phospholipid chains. This can be explained by an equilibrium between the attraction attributable to the insertion of the myristoyl chain and the Born repulsion. In a 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-[phospho-L-serine] membrane, where attractive electrostatic interactions come into play, the differences between the peptide and the phospholipid chain lengths were attenuated, and the molecular dynamics of all lipid chains were similar. In a much thicker 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1,2-dipalmitoyl-sn-glycero-3-[phospho-L-serine]/cholesterol membrane, the length of the myristoyl chain of Src was elongated nearly to its maximum, and the order parameters of the Src chain were comparable to those of the surrounding membrane.

Biomembrane sensitivity to structural changes in bound polymers

Anionic liposomes containing a 4:1 molar ratio of neutral to anionic phospholipids were treated with an excess of five zwitterionic polymers differing only in the spacer length separating their cationic and anionic moieties. Although the polymers do not disrupt the structural integrity of the liposomes, they can induce spacer-dependent molecular rearrangements within the liposomes. Thus, the following were observed: spacer length = 1, no binding to the liposomes; spacer length = 2, adsorption to the liposomes, but no molecular rearrangement; spacer length = 3, lateral lipid segregation but little or no flip-flop; spacer length = 4 or 5, lateral lipid segregation and flip-flop. These diverse behaviors are relevant to the use of biomedical formulations where polyelectrolytes play a role.

Vesicle diffusion close to a membrane: intermembrane interactions measured with fluorescence correlation spectroscopy

The protein machinery controlling membrane fusion (or fission) has been well studied; however, the role of vesicle diffusion near membranes in these critical processes remains unclear. We experimentally and theoretically investigated the dynamics of small vesicles (approximately 50 nm in diameter) that are diffusing near supported planar bilayers acting as "target" membranes. Using total internal reflection-fluorescence correlation spectroscopy, we examined the validity of theoretical analyses of vesicle-membrane interactions. Vesicles were hindered by hydrodynamic drag as a function of their proximity to the planar bilayer. The population distributions and diffusion kinetics of the vesicles were further affected by changing the ionic strength and pH of the buffer, as well as the lipid composition of the planar membrane. Effective surface charges on neutral bilayers were also analyzed by comparing experimental and theoretical data, and we show the possibility that vesicle dynamics can be modified by surface charge redistribution of the planar bilayer. Based on these results, we hypothesize that the dynamics of small vesicles, diffusing close to biomembranes, may be spatially restricted by altering local physiological conditions (e.g., salt concentration, lipid composition, and pH), which may represent an additional mechanism for controlling fusion (or fission) dynamics.

Polyelectrolyte-coated liposomes: stabilization of the interfacial complexes

Anionic liposomes, composed of egg lecithin (EL) or dipalmitoylphosphatidylcholine (DPPC) with 20 mol% of cardiolipin (CL(2-)), were mixed with cationic polymers, poly(4-vinylpyridine) fully quaternized with ethyl bromide (P2) or poly-L-lysine (PL). Polymer/liposome binding studies were carried out using electrophoretic mobility (EPM), fluorescence, and conductometry as the main analytical tools. Binding was also examined in the presence of added salt and polyacrylic acid (PAA). The following generalizations arose from the experiments: (a) Binding of P2 and PL to small EL/CL(2-) liposomes (60-80 nm in diameter) is electrostatic in nature and completely reversed by addition of salt or PAA. (b) Binding can be enhanced by hydrophobization of the polymer with cetyl groups. (c) Binding can also be enhanced by changing the phase state of the lipid bilayer from liquid to solid (i.e. going from EL to DPPC) or by increasing the size of the liposomes (i.e. going from 60-80 to 300 nm). By far the most promising systems, from the point of view of constructing polyelectrolyte multilayers on liposome cores without disruption of liposome integrity, involve small, liquid, anionic liposomes coated initially with polycations carrying pendant alkyl groups.

Membrane interactions of peptides representing the polybasic regions of three Rho GTPases are sensitive to the distribution of arginine and lysine residues

Rho GTPases are a multifunctional family of proteins that are localized at cellular membranes via an isoprenyl group covalently linked to a C-terminal cysteine. Close to this primary site of membrane anchoring there is often found an additional polybasic region (PBR), which plays a secondary role in membrane binding and targeting of the complex. Here, peptides derived from the PBRs of the Rho family proteins Rac1 (K(183)KRKRK), TCL (K(198)KKKKR) and Cdc42 (P(182)KKSRR) were prepared with hexalysine (K(6)) and hexaarginine (R(6)) to study their interactions with multilamellar vesicles of phosphatidylglycerol (DOPG) and headgroup-deuterated dimyristoylphosphatidylcholine (DMPC-d(4)) using (2)H and (31)P NMR. The membranes retained their lamellar architecture after peptide binding, but the (2)H NMR line shapes for DMPC-d(4) indicated that the bound peptides altered the orientation of the choline headgroups, consistent with a change in membrane surface charge. Rac1 and TCL peptides appeared to affect the headgroup orientation similarly to K(6), although the perturbations were weaker and unlike those induced by the Cdc42 peptide and R(6). Magic-angle spinning (31)P NMR spectra of the membranes showed significant and selective broadening of the peak for DMPC after addition of the peptides, with R(6) and the Cdc42 peptide having the greatest effect. The selective broadening may be a consequence of the lipids separating into short-lived domains enriched in peptide-bound DOPG and peptide-free DMPC. These results illustrate that a complex relationship exists between the sequence of PBRs and their behaviour at membrane surfaces, which may have implications for the cellular functions and localization of Rho GTPases.

Thermodynamic studies and binding mechanisms of cell-penetrating peptides with lipids and glycosaminoglycans

Cell-penetrating peptides (CPPs) traverse the membrane of biological cells at low micromolar concentrations and are able to take various cargo molecules along with. Despite large differences in their chemical structure, CPPs share the structural similarity of a high cationic charge density. This property confers to them the ability to bind electrostatically membrane constituents such as anionic lipids and glycosaminoglycans (GAGs). Controversies exist, however, about the biological response after the interaction of CPPs with such membrane constituents. Present review compares thermodynamic binding studies with conditions of the biological CPP uptake. It becomes evident that CPPs enter biological cells by different and probably competing mechanisms. For example, some amphipathic CPPs traverse pure lipid model membranes at low micromolar concentrations--at least in the absence of cargos. In contrast, no direct translocation at these conditions is observed for non-amphipathic CPPs. Finally, CPPs bind GAGs at low micromolar concentrations with potential consequences for endocytotic pathways.

Lipid lateral segregation driven by diacyl cyclodextrin interactions at the membrane surface

Cyclodextrins are hydrophilic molecular cages with a hydrophobic interior allowing the inclusion of water-insoluble drugs. Amphiphilic cyclodextrins obtained by appending a hydrophobic anchor were designed to improve the cell targeting of the drug-containing cavities through their liposome transportation in the organism. After insertion in model membranes, they were found to induce a lateral phase separation into a pure lipid phase and a fluid cyclodextrin-rich phase (L(CD)) with reduced acyl chain order parameters, as observed with a derivative containing a cholesterol anchor (M. Roux, R. Auzely-Velty, F. Djedaïni-Pilard, and B. Perly. 2002. Biophysical Journal, 8:813-822). We present another class of amphiphilic cyclodextrins obtained by grafting aspartic acid esterified by two lauryl chains on the oligosaccharide core via a succinyl spacer. The obtained dilauryl-beta-cyclodextrin (betaDLC) was inserted in chain perdeuterated dimyristoylphosphatidylcholine (DMPC-d54) membranes and studied by deuterium NMR ((2)H-NMR). A laterally segregated mixed phase was found to sequester three times more lipids than the cholesteryl derivative (approximately 4-5 lipids per monomer of betaDLC), and a quasipure L(CD) phase could be obtained with a 20% molar concentration of betaDLC. When cooled below the main fluid-to-gel transition of DMPC-d54 the betaDLC-rich phase stays fluid, coexisting with pure lipid in the gel state, and exhibits a sharp transition to a gel phase with frozen DMPC acyl chains at 12.5 degrees C. No lateral phase separation was observed with partially or fully methylated betaDLC, confirming that the stability of the segregated L(CD) phase was governed through hydrogen-bond-mediated intermolecular interactions between cyclodextrin headgroups at the membrane surface. As opposed to native betaDLC, the methylated derivatives were found to strongly increase the orientational order of DMPC acyl chains as the temperature reaches the membrane fluid-to-gel transition. The results are discussed in relation to the "anomalous swelling" of saturated phosphatidylcholine multilamellar membranes known to occur in the vicinity of the main fluid-to-gel transition.

Phosphatidylserine induces functional and structural alterations of the membrane-associated pleckstrin homology domain of phospholipase C-δ1

The membrane binding affinity of the pleckstrin homology (PH) domain of phospholipase C (PLC)-delta1 was investigated using a vesicle coprecipitation assay and the structure of the membrane-associated PH domain was probed using solid-state (13)C NMR spectroscopy. Twenty per cent phosphatidylserine (PS) in the membrane caused a moderate but significant reduction of the membrane binding affinity of the PH domain despite the predicted electrostatic attraction between the PH domain and the head groups of PS. Solid-state NMR spectra of the PH domain bound to the phosphatidylcholine (PC)/PS/phosphatidylinositol 4,5-bisphosphate (PIP(2)) (75 : 20 : 5) vesicle indicated loss of the interaction between the amphipathic alpha2-helix of the PH domain and the interface region of the membrane which was previously reported for the PH domain bound to PC/PIP(2) (95 : 5) vesicles. Characteristic local conformations in the vicinity of Ala88 and Ala112 induced by the hydrophobic interaction between the alpha2-helix and the membrane interface were lost in the structure of the PH domain at the surface of the PC/PS/PIP(2) vesicle, and consequently the structure becomes identical to the solution structure of the PH domain bound to d-myo-inositol 1,4,5-trisphosphate. These local structural changes reduce the membrane binding affinity of the PH domain. The effects of PS on the PH domain were reversed by NaCl and MgCl(2), suggesting that the effects are caused by electrostatic interaction between the protein and PS. These results generally suggest that the structure and function relationships among PLCs and other peripheral membrane proteins that have similar PH domains would be affected by the local lipid composition of membranes.

Grafting of polylysine with polyethylenoxide prevents demixing of O-pyromellitylgramicidin in lipid membranes

Both natural and synthetic polycations can induce demixing of negatively charged components in artificial and possibly in natural membranes. This process can result in formation of clusters (binding of several components to a polycation chain) and/or domains (aggregation of clusters and formation of a separate phase enriched in some particular component). In order to distinguish between these two phenomena, a model lipid membrane system containing ion channels, formed by a negatively charged peptide, O-pyromellitylgramicidin, and polycations of different structures was used. Microelectrophoresis of liposomes, changes in boundary potential of planar bilayers, the shape of compression curves and potentials of lipid and lipid/peptide monolayers were used to monitor the electrostatic factors in polymer adsorption to the membrane and peptide-polymer interactions. The synthesized PEO-grafted polylysine, PLL-PEO20000, did not induce peptide demixing monitored by stabilization of the gramicidin channels, in contrast to parent polylysine (PLL). Both polymers were shown to bind effectively to negatively charged liposomes and lipid monolayers, suggesting that the ineffectiveness of PLL-PEO20000 was not due to reduction of its binding. It was hypothesized that PLL-PEO20000 could not induce domain formation due to steric hindrance of long PEO chains preventing lateral fusion of clusters. Another copolymer, PLL-PEO4000, having four PEO chains of 4000 Da, exhibited intermediate effect between PLL and PLL-PEO20000, which shows the importance of the copolymer architecture for the effect on the lateral distribution of OPg channels. The model system can be relevant to regulation of lateral organization of ion channels and other components in natural membrane systems.

Interaction of poly(l-lysines) with negatively charged membranes: an FT-IR and DSC study

The influence of the binding of poly(L-lysine) (PLL) to negatively charged membranes containing phosphatidylglycerols (PG) was studied by DSC and FT-IR spectroscopy. We found a general increase in the main transition temperature as well as increase in hydrophobic order of the membrane upon PLL binding. Furthermore we observed stronger binding of hydration water to the lipid head groups after PLL binding. The secondary structure of the PLL after binding was studied by FT-IR spectroscopy. We found that PLL binds in an alpha-helical conformation to negatively charged DPPG membranes or membranes with DPPG-rich domains. Moreover we proved that PLL binding induces domain formation in the gel state of mixed DPPC/DPPG or DMPC/DPPG membranes as well as lipid remixing in the liquid-crystalline state. We studied these effects as a function of PLL chain length and found a significant dependence of the secondary structure, phase transition temperature and domain formation capacity on PLL chain length and also a correlation between the peptide secondary structure and the phase transition temperature of the membrane. We present a system in which the membrane phase transition triggers a highly cooperative secondary structure transition of the membrane-bound peptide from alpha-helix to random coil.

Poly-L-lysine-induced morphology changes in mixed anionic/zwitterionic and neat zwitterionic-supported phospholipid bilayers

Poly-L-lysine-induced morphological changes in liquid phase supported bilayers consisting of mixed anionic/zwitterionic and neat zwitterionic headgroup phospholipids were studied with atomic force microscopy and epifluorescence microscopy. Results obtained from these studies indicate that poly-L-lysine can induce domains, defects, and aggregate structures on both mixed bilayers and strictly zwitterionic bilayers. The structures formed on liquid phase supported bilayers were observed to be immobile from a timescale of 50 ms to several minutes. We propose that poly-L-lysine of sufficient length interacts with the mica substrate and phospholipids to create the stationary structures noted.

A study of the regional effects of alpha-synuclein on the organization and stability of phospholipid bilayers

Associations between the protein alpha-synuclein (alpha-syn) and presynaptic vesicles have been implicated in synaptic plasticity and neurotransmitter release and may also affect how the protein aggregates into fibrils found in Lewy bodies, the cellular inclusions associated with neurodegenerative diseases. This work investigated how alpha-syn interacts with model phospholipid membranes and examined what effect protein binding has upon the physical properties of lipid bilayers. Wide line 2H and 31P NMR spectra of phospholipid vesicles revealed that alpha-syn associates with membranes containing lipids with anionic headgroups and can disrupt the integrity of the lipid bilayer, but the protein has little effect on membranes of zwitterionic phosphatidylcholine. A peptide, alpha-syn(10-48), which corresponds to the lysine-rich N-terminal region of alpha-syn, was found to associate with lipid headgroups with a preference for a negative membrane surface charge. Another peptide, alpha-syn(120-140), which corresponds to the glutamate-rich C-terminal region, also associates weakly with lipid headgroups but with a slightly higher affinity for membranes with no net surface charge than for negatively charged membrane surfaces. Binding of alpha-syn(10-48) and alpha-syn(120-140) to the lipid vesicles did not disrupt the lamellar structure of the membranes, but both peptides appeared to induce the lateral segregation of the lipids into clusters of acidic lipid-enriched and acidic lipid-deficient domains. From these findings, it is speculated that the N-terminal and C-terminal domains of full-length alpha-syn might act in concert to organize the membrane components during normal protein function and perhaps play a role in presynaptic vesicle synthesis, maintenance, and fusion.

The cytoplasmic domains of phospholamban and phospholemman associate with phospholipid membrane surfaces

Phospholamban (PLB) and phospholemman (PLM, also called FXYD1) are small transmembrane proteins that interact with P-type ATPases and regulate ion transport in cardiac cells and other tissues. This work has investigated the hypothesis that the cytoplasmic domains of PLB and PLM, when not interacting with their regulatory targets, are stabilized through associations with the surface of the phospholipid membrane. Peptides representing the 35 C-terminal cytoplasmic residues of PLM (PLM(37-72)), the 23 N-terminal cytoplasmic residues of PLB (PLB(1-23)), and the same sequence phosphorylated at Ser-16 (P-PLB(1-23)) were synthesized to examine their interactions with model membranes composed of zwitterionic phosphatidylcholine (PC) lipids alone or in admixture with anionic phosphatidylglycerol (PG) lipids. Wide-line 2H NMR spectra of PC/PG membranes, with PC deuterated in the choline moiety, indicated that all three peptides interacted with the membrane surface and perturbed the orientation of the choline headgroups. Fluorescence and 31P magic-angle spinning (MAS) NMR measurements indicated that PLB(1-23) and P-PLB(1-23) had a higher affinity for PC/PG membranes, which carry an overall negative surface charge, than for PC membranes, which have no net surface charge. The 31P MAS NMR spectra of the PC/PG membranes in the presence of PLM(37-72), PLB(1-23), and P-PLB(1-23) indicated that all three peptides induced clustering of the lipids into PC-enriched and PG-enriched regions. These findings support the theory that the cytoplasmic domains of PLB and PLM are stabilized by interacting with lipid headgroups at the membrane surface, and it is speculated that such interactions may modulate the functional properties of biological membranes.

Structural Effects of a Basic Peptide on the Organization of Dipalmitoylphosphatidylcholine/Dipalmitoylphosphatidylserine Membranes: A Fluorescent Resonance Energy Transfer Study

We studied the effect of a model basic peptide, hexalysiltryptophan, on the organization of dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylserine unilamellar vesicles by means of fluorescent resonance energy transfer (FRET) between fluorescently labeled phospholipids. Several FRET theoretical models assuming different bilayer geometries and probe distributions were fitted to the time-resolved data. The experiments were carried out at two temperatures in different regions of the lipid mixture phase diagram. At 45 degrees C, the expected gel/fluid phase separation was verified by model fitting in peptide-free vesicles, which from the FRET approach means that domains are larger than approximately 200 A. No noticeable alteration of membrane organization was detected upon increasing the peptide concentration. At variance, for the single fluid phase at 60 degrees C, there was a large increase in FRET efficiency upon peptide addition to the lipid vesicles, mainly caused by peptide-induced vesicle aggregation. The system gradually changed from unilamellar lipid vesicles to a multibilayer geometry, and a limit lamellar repeat distance of approximately 57 A was recovered. Furthermore, no evidence for lateral domain formation on the FRET length scale was found at this temperature, the cationic peptide being only able to induce local lipid demixing, causing a short-range sequestration of 2-3 acidic lipids around each surface-adsorbed peptide.

Probing perturbation of bovine lung surfactant extracts by albumin using DSC and 2H-NMR

Lung surfactant (LS), a lipid-protein mixture, forms films at the lung air-water interface and prevents alveolar collapse at end expiration. In lung disease and injury, the surface activity of LS is inhibited by leakage of serum proteins such as albumin into the alveolar hypophase. Multilamellar vesicular dispersions of a clinically used replacement, bovine lipid extract surfactant (BLES), to which (2% by weight) chain-perdeuterated dipalmitoylphosphatidycholine (DPPG mixtures-d(62)) had been added, were studied using deuterium-NMR spectroscopy ((2)H-NMR) and differential scanning calorimetry (DSC). DSC scans of BLES showed a broad gel to liquid-crystalline phase transition between 10-35 degrees C, with a temperature of maximum heat flow (T(max)) around 27 degrees C. Incorporation of the DPPC-d(62) into BLES-reconstituted vesicles did not alter the T(max) or the transition range as observed by DSC or the hydrocarbon stretching modes of the lipids observed using infrared spectroscopy. Transition enthalpy change and (2)H-NMR order parameter profiles were not significantly altered by addition of calcium and cholesterol to BLES. (2)H-NMR spectra of the DPPC-d(62) probes in these samples were characteristic of a single average lipid environment at all temperatures. This suggested either continuous ordering of the bilayer through the transition during cooling or averaging of the DPPC-d(62) environment by rapid diffusion between small domains on a short timescale relative to that characteristic of the (2)H-NMR experiment. Addition of 10% by weight of soluble bovine serum albumin (1:0.1, BLES/albumin, dry wt/wt) broadened the transition slightly and resulted in the superposition of (2)H-NMR spectral features characteristic of coexisting fluid and ordered phases. This suggests the persistence of phase-separated domains throughout the transition regime (5-35 degrees C) of BLES with albumin. The study suggests albumin can cause segregation of protein bound-lipid domains in surfactant at NMR timescales (10(-5) s). Persistent phase separation at physiological temperature may provide for a basis for loss of surface activity of surfactant in dysfunction and disease.

Interaction of poly(L-lysine)-g-poly(ethylene glycol) with supported phospholipid bilayers

Interactions between the graft copolymer poly(L-lysine)-g-poly(ethylene glycol), PLL-g-PEG, and two kinds of surface-supported lipidic systems (supported phospholipid bilayers and supported vesicular layers) were investigated by a combination of microscopic and spectroscopic techniques. It was found that the application of the copolymer to zwitterionic or negatively charged supported bilayers in a buffer of low ionic strength led to their decomposition, with the resulting formation of free copolymer-lipid complexes. The same copolymer had no destructive effect on a supported vesicular layer made up of vesicles of identical composition. A comparison between poly(L-lysine), which did not induce decomposition of supported bilayers, and PLL-g-PEG copolymers with various amounts of PEG side chains per backbone lysine unit, suggested that steric repulsion between the PEG chains that developed upon adsorption of the polymer to the nearly planar surface of a supported phospholipid bilayer (SPB) was one of the factors responsible for the destruction of the SPBs by the copolymer. Other factors included the ionic strength of the buffer used and the quality of the bilayers, pointing toward the important role defects present in the SPBs play in the decomposition process.

Domain formation induced by the adsorption of charged proteins on mixed lipid membranes

Peripheral proteins can trigger the formation of domains in mixed fluid-like lipid membranes. We analyze the mechanism underlying this process for proteins that bind electrostatically onto a flat two-component membrane, composed of charged and neutral lipid species. Of particular interest are membranes in which the hydrocarbon lipid tails tend to segregate owing to nonideal chain mixing, but the (protein-free) lipid membrane is nevertheless stable due to the electrostatic repulsion between the charged lipid headgroups. The adsorption of charged, say basic, proteins onto a membrane containing anionic lipids induces local lipid demixing, whereby charged lipids migrate toward (or away from) the adsorption site, so as to minimize the electrostatic binding free energy. Apart from reducing lipid headgroup repulsion, this process creates a gradient in lipid composition around the adsorption zone, and hence a line energy whose magnitude depends on the protein's size and charge and the extent of lipid chain nonideality. Above a certain critical lipid nonideality, the line energy is large enough to induce domain formation, i.e., protein aggregation and, concomitantly, macroscopic lipid phase separation. We quantitatively analyze the thermodynamic stability of the dressed membrane based on nonlinear Poisson-Boltzmann theory, accounting for both the microscopic characteristics of the proteins and lipid composition modulations at and around the adsorption zone. Spinodal surfaces and critical points of the dressed membranes are calculated for several different model proteins of spherical and disk-like shapes. Among the models studied we find the most substantial protein-induced membrane destabilization for disk-like proteins whose charges are concentrated in the membrane-facing surface. If additional charges reside on the side faces of the proteins, direct protein-protein repulsion diminishes considerably the propensity for domain formation. Generally, a highly charged flat face of a macroion appears most efficient in inducing large compositional gradients, hence a large and unfavorable line energy and consequently lateral macroion aggregation and, concomitantly, macroscopic lipid phase separation.

Topographical pattern dynamics in passive adhesion of cell membranes

Strong adhesion of highly active cells often nucleates focal adhesions, synapses, and related structures. Red cells lack such complex adhesion systems and are also nonmotile, but they are shown here to dynamically evolve complex spatial patterns beyond an electrostatic threshold for strong adhesion. Spreading of the cell onto a dense, homogeneous poly-L-lysine surface appears complete in <1 s with occasional blisters that form and dissipate on a similar timescale; distinct rippled or stippled patterns in fluorescently labeled membrane components emerge later, however, on timescales more typical of long-range lipid diffusion (approximately minutes). Within the contact zone, the anionic fluorescent lipid fluorescein phosphoethanolamine is seen to rearrange, forming worm-like rippled or stippled domains of <500 nm that prove independent of whether the cell is intact and sustaining a tension or ruptured. Lipid patterns are accompanied by visible perturbations in Band 3 distribution and weaker perturbations in membrane skeleton actin. Pressing down on the membrane quenches the lipid patterns, revealing a clear topographical basis for pattern formation. Counterion screening and membrane fluctuations likely contribute, but the results primarily highlight the fact that even in adhesion of a passive red cell, regions of strong contact slowly evolve to become interspersed with regions where the membrane is more distant from the surface.

Screening Molecular Associations with Lipid Membranes Using Natural Abundance 13C Cross-Polarization Magic-Angle Spinning NMR and Principal Component Analysis

We describe an NMR approach for detecting the interactions between phospholipid membranes and proteins, peptides, or small molecules. First, 1H-13C dipolar coupling profiles are obtained from hydrated lipid samples at natural isotope abundance using cross-polarization magic-angle spinning NMR methods. Principal component analysis of dipolar coupling profiles for synthetic lipid membranes in the presence of a range of biologically active additives reveals clusters that relate to different modes of interaction of the additives with the lipid bilayer. Finally, by representing profiles from multiple samples in the form of contour plots, it is possible to reveal statistically significant changes in dipolar couplings, which reflect perturbations in the lipid molecules at the membrane surface or within the hydrophobic interior.

Protein transduction domains of HIV-1 and SIV TAT interact with charged lipid vesicles. Binding mechanism and thermodynamic analysis

Cell-penetrating peptides (CPPs) traverse cell membranes of cultured cells very efficiently by a mechanism not yet identified. Recent theories for the translocation suggest either the binding of the CPPs to extracellular glycosaminoglycans or the formation of inverted micelles with negatively charged lipids. In the present study, the binding of the protein transduction domains (PTD) of human (HIV-1) and simian immunodeficiency virus (SIV) TAT peptide (amino acid residues 47-57, electric charge z(p) = +8) to membranes containing various proportions of negatively charged lipid (POPG) is characterized. Monolayer expansion measurements demonstrate that TAT-PTD insertion between lipids requires loosely packed monolayer films. For densely packed monolayers (pi > 29 mN/m) and lipid bilayers, no insertion is possible, and binding occurs via electrostatic adsorption to the membrane surface. Light scattering experiments show an aggregation of anionic lipid vesicles when the electric surface charge is neutralized by TAT-PTD, the observed stoichiometry being close to the theoretical value of 1:8. Membrane binding was quantitated with isothermal titration calorimetry and three further methods. The reaction enthalpy is Delta H degrees approximately equal to -1.5 kcal/mol peptide and is almost temperature-independent with Delta C(p) degrees approximately 0 kcal/(mol K), indicating equal contributions of polar and hydrophobic interactions to the reaction heat capacity. The binding of TAT-PTD to the anionic membrane is described by an electrostatic attraction/chemical partition model. The electrostatic attraction energy, calculated with the Gouy-Chapman theory, accounts for approximately 80% of the binding energy. The overall binding constant, K(app), is approximately 10(3)-10(4) M(-1). The intrinsic binding constant (K(p)), corrected for electrostatic effects and describing the partitioning of the peptide between the lipid-water interface and the membrane, is small and is K(p) approximately 1-10 M(-1). Deuterium and phosphorus-31 nuclear magnetic resonance demonstrate that the lipid bilayer remains intact upon TAT-PTD binding. The NMR data provide no evidence for nonbilayer structures and also not for domain formation. This is further supported by the absence of dye efflux from single-walled lipid vesicles. The electrostatic interaction between TAT-PTD and anionic phosphatidylglycerol is strong enough to induce a change in the headgroup conformation of the anionic lipid, indicating a short-lived but distinct correlation between the TAT-PTD and the anionic lipids on the membrane outside. TAT-PTD has a much lower affinity for lipid membranes than for glycosaminoglycans, making the latter interaction a more probable pathway for CPP binding to biological membranes.

Migration of poly-L-lysine through a lipid bilayer

When a giant vesicle, composed of neutral and anionic lipid (90:10 mol %), comes into contact with various poly-l-lysines (MW 500-29 300), ropelike structures form within the vesicle interior. By using fluorescence lipids and epi-fluorescence microscopy, we have shown that both neutral and anionic lipids are constituents of the ropes. Evidence that the ropes are also comprised of poly-l-lysine comes from two experiments: (a) direct microinjection of poly(acrylic acid) into rope-containing vesicles causes the ropes to contract into small particles, an observation consistent with a polycation/polyanion interaction; and (b) direct microinjection of fluorescein isothiocyanate (a compound that covalently labels poly-l-lysine with a fluorescent moiety) into rope-containing vesicles leads to fluorescent ropes. The results may be explained by a model in which poly-l-lysine binds to the vesicle exterior, forms a domain, and enters the vesicle through defects or at the domain boundary. The model helps explain the ability of poly-l-lysine to mediate the permeation of a cancer drug, doxorubicine, into the vesicle interior.

Cholesterol dynamics in membranes of raft composition: a molecular point of view from 2H and 31P solid-state NMR

Lipidic membrane systems that have been reported to be composed of sphingomyelin (SM)-cholesterol (Chol) microdomains or "rafts" by Dietrich et al. [palmitoyloleoyl-phosphatidylcholine(POPC)/SM/Chol, 1/1/1; Dietrich, C., Bagatolli, L. A., Volovyk, Z. N., Thompson, N. L., Levi, M., Jacobson, K., and Gratton, E. (2001) Biophys. J. 80, 1417-1428] and by Schroeder et al. [SCRL: Liver-PC/Liver-phosphatidylethanolamine/SM/Cerebrosides/Chol, 1/1/1/1/2; Schroeder, R., London, E., and Brown, D. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 12130-12134] were investigated under the form of fully hydrated liposomes by the noninvasive solid-state (31)P and (2)H NMR method. Liposomes of binary lipid composition POPC/Chol and SM/Chol were also studied as boundary/control systems. All systems are found to be in the liquid-ordered phase (Lo) at physiological temperatures. Use of deuterium-labeled cholesterol afforded finding both the position of the sterol motional axis and its molecular order parameter. The axis of anisotropic rotation of cholesterol is such that the molecule is, on average, quasiperpendicular to the membrane plane, in all of the four systems investigated. Cholesterol order parameters greater than 0.8 are observed, indicating that the sterol is in a very motionally restricted environment in the temperature range 0-60 degrees C. The binary mixtures present "boundary" situations with the lowest values for POPC/Chol and the highest for SM/Chol. The SCRL raft mixture has the same ordering as the SM/Chol, i.e., the highest order parameter values over the temperature range. It demonstrates that in the SCRL mixture cholesterol dynamics is as in the binary system SM/Chol, therefore, suggesting that it might be depleted from the rest of the membrane to form complexes as if it were alone with SM. On the other hand, the mixture POPC/SM/Chol exhibits an intermediate ordering situation between those of SM/Chol and POPC/Chol. This strongly suggests that cholesterol could be in fast exchange, at the NMR time scale (milli- to microseconds), between two or more membrane regions of different dynamics and questions the statement of "rigid domains" made of SM and cholesterol in the model "raft" system POPC/SM/Chol.