Exclusion of a Transmembrane-Type Peptide from Ordered-Lipid Domains (Rafts) Detected by Fluorescence Quenching: Extension of Quenching Analysis to Account for the Effects of Domain Size and Domain Boundaries†

Palmitoylation modifies transmembrane adaptor protein PAG for ordered lipid environment: A molecular dynamics simulation study

We employed all-atom MD simulations to investigate the impact of palmitoylation on the PAG transmembrane peptide within various lipid environments, including the less explored boundary region separating lipid-ordered (Lo) and lipid-disordered (Ld) membrane phases. We found that palmitoylation of the peptide reduces its impact on membrane thickness, particularly within the Lo and boundary environments. Despite their hydrophobic nature, the palmitoyl chains on the peptide did not significantly affect the hydration of the surrounding membrane. Interestingly, the boundary membrane environment was found to be especially compatible with the palmitoylated peptide, suggesting its potential for accumulation in phase boundaries. Our findings highlight the importance of understanding how palmitoylation-modified peptides behave within membranes, with crucial implications for cell signaling and membrane organization. This knowledge may also inform the optimization of lipid membrane-based drug delivery systems, by improving our understanding of how drugs and excipients can be most effectively arranged within these carriers.

Egg White Peptides Increased the Membrane Liquid-Ordered Phase of Giant Unilamellar Vesicles: Visualization, Localization, and Phase Regulation Mechanism

Cell membranes are heterogeneous and consist of liquid-ordered (Lo) and liquid-disordered (Ld) phases due to phase separation. Membrane regulation of egg white peptides (LCAY and QVPLW) was confirmed in our previous study. However, the underlying mechanism of phase regulation by the peptides has not been elucidated. This study aimed to explore the effect of LCAY and QVPLW on the membrane phase separation and illustrate their mechanism by giant unilamellar vesicles (GUVs). Based on phase separation visualization, LCAY and QVPLW were found to increase the Lo phase by rearranging lipids and ordering the Ld phase. LCAY and QVPLW can bind to the GUVs and localize in the amphiphilic region of the membrane. By hydrogen bonds and hydrophobic interactions, LCAY and QVPLW may play a cholesterol-like role in regulating phase separation.

Photogenerated Radical Pair between Flavin and a Tryptophan-Containing Transmembrane-Type Peptide in a Large Unilamellar Vesicle

Electron-transfer (ET) reactions in biological systems, such as those with magnetic sensors based on flavoproteins and electron transport at biomembrane interfaces, are interesting and important issues that require understanding. As a model system of flavoproteins in biomimetic environments, we report the dynamics of the radical pair generated by photoinduced ET between riboflavin tetrabutylate (RFTB) and tryptophan (Trp) residues in a transmembrane-type polypeptide, both of which are distributed in a large unilamellar vesicle of 1,2-dimyristoyl-sn-glycero-3-phosphocholine. The Trp residues locate near the hydrophilic membrane interface, as confirmed by a dual-fluorescence quenching assay. The fluorescence and transient absorption upon photoexcitation of RFTB indicate that ET from both the singlet and triplet excited states occurs at the hydrophilic interface, whereas the RFTB in the hydrophobic region does not contribute to ET. The ET efficiency and the magnetic field effect (MFE) on the RFTB anion increase significantly above the gel-to-liquid crystal phase transition temperature due to a decrease in microviscosity. The MFE analysis indicates that the radical pair generated from the triplet ET channel exhibits a long lifetime as those in micellar systems due to the strong cage effect of the vesicle.

Defining raft domains in the plasma membrane

Many plasma membrane (PM) functions depend on the cholesterol concentration in the PM in strikingly nonlinear, cooperative ways: fully functional in the presence of physiological cholesterol levels (35~45 mol%), and nonfunctional below 25 mol% cholesterol; namely, still in the presence of high concentrations of cholesterol. This suggests the involvement of cholesterol-based complexes/domains formed cooperatively. In this review, by examining the results obtained by using fluorescent lipid analogs and avoiding the trap of circular logic, often found in the raft literature, we point out the fundamental similarities of liquid-ordered (Lo)-phase domains in giant unilamellar vesicles, Lo-phase-like domains formed at lower temperatures in giant PM vesicles, and detergent-resistant membranes: these domains are formed by cooperative interactions of cholesterol, saturated acyl chains, and unsaturated acyl chains, in the presence of >25 mol% cholesterol. The literature contains evidence, indicating that the domains formed by the same basic cooperative molecular interactions exist and play essential roles in signal transduction in the PM. Therefore, as a working definition, we propose that raft domains in the PM are liquid-like molecular complexes/domains formed by cooperative interactions of cholesterol with saturated acyl chains as well as unsaturated acyl chains, due to saturated acyl chains' weak multiple accommodating interactions with cholesterol and cholesterol's low miscibility with unsaturated acyl chains and TM proteins. Molecules move within raft domains and exchange with those in the bulk PM. We provide a logically established collection of fluorescent lipid probes that preferentially partition into raft and non-raft domains, as defined here, in the PM.

Effect of sterol structure on ordered membrane domain (raft) stability in symmetric and asymmetric vesicles

Sterol structure influences liquid ordered domains in membranes, and the dependence of biological functions on sterol structure can help identify processes dependent on ordered domains. In this study we compared the effect of sterol structure on ordered domain formation in symmetric vesicles composed of mixtures of sphingomyelin, 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and cholesterol, and in asymmetric vesicles in which sphingomyelin was introduced into the outer leaflet of vesicles composed of DOPC and cholesterol. In most cases, sterol behavior was similar in symmetric and asymmetric vesicles, with ordered domains most strongly stabilized by 7-dehydrocholesterol (7DHC) and cholesterol, stabilized to a moderate degree by lanosterol, epicholesterol and desmosterol, and very little if at all by 4-cholesten-3-one. However, in asymmetric vesicles desmosterol stabilized ordered domain almost as well as cholesterol, and to a much greater degree than epicholesterol, so that the ability to support ordered domains decreased in the order 7-DHC > cholesterol > desmosterol > lanosterol > epicholesterol > 4-cholesten-3-one. This contrasts with values for intermediate stabilizing sterols in symmetric vesicles in which the ranking was cholesterol > lanosterol ~ desmosterol ~ epicholesterol or prior studies in which the ranking was cholesterol ~ epicholesterol > lanosterol ~ desmosterol. The reasons for these differences are discussed. Based on these results, we re-evaluated our prior studies in cells and conclude that endocytosis levels and bacterial uptake are even more closely correlated with the ability of sterols to form ordered domains than previously thought, and do not necessarily require that a sterol have a 3β-OH group.

Roughness of Transmembrane Helices Reduces Lipid Membrane Dynamics

The dynamics of cellular membranes is primarily determined by lipid species forming a bilayer. Proteins are considered mainly as effector molecules of diverse cellular processes. In addition to large assemblies of proteins, which were found to influence properties of fluid membranes, biological membranes are densely populated by small, highly mobile proteins. However, little is known about the effect of such proteins on the dynamics of membranes. Using synthetic peptides, we demonstrate that transmembrane helices interfere with the mobility of membrane components by trapping lipid acyl chains on their rough surfaces. The effect is more pronounced in the presence of cholesterol, which segregates from the rough surface of helical peptides. This may contribute to the formation or stabilization of membrane heterogeneities. Since roughness is a general property of helical transmembrane segments, our results suggest that, independent of their size or cytoskeleton linkage, integral membrane proteins affect local membrane dynamics and organization.

Study of Förster Resonance Energy Transfer to Lipid Domain Markers Ascertains Partitioning of Semisynthetic Lipidated N-Ras in Lipid Raft Nanodomains

Cellular membranes are heterogeneous planar lipid bilayers displaying lateral phase separation with the nanometer-scale liquid-ordered phase (also known as "lipid rafts") surrounded by the liquid-disordered phase. Many membrane-associated proteins were found to permanently integrate into the lipid rafts, which is critical for their biological function. Isoforms H and N of Ras GTPase possess a unique ability to switch their lipid domain preference depending on the type of bound guanine nucleotide (GDP or GTP). This behavior, however, has never been demonstrated in vitro in model bilayers with recombinant proteins and therefore has been attributed to the action of binding of Ras to other proteins at the membrane surface. In this paper, we report the observation of the nucleotide-dependent switch of lipid domain preferences of the semisynthetic lipidated N-Ras in lipid raft vesicles in the absence of additional proteins. To detect segregation of Ras molecules in raft and disordered lipid domains, we measured Förster resonance energy transfer between the donor fluorophore, mant, attached to the protein-bound guanine nucleotides, and the acceptor, rhodamine-conjugated lipid, localized into the liquid-disordered domains. Herein, we established that N-Ras preferentially populated raft domains when bound to mant-GDP, while losing its preference for rafts when it was associated with a GTP mimic, mant-GppNHp. At the same time, the isolated lipidated C-terminal peptide of N-Ras was found to be localized outside of the liquid-ordered rafts, most likely in the bulk-disordered lipid. Substitution of the N-terminal G domain of N-Ras with a homologous G domain of H-Ras disrupted the nucleotide-dependent lipid domain switch.

Detection of Sphingomyelin Clusters by Raman Spectroscopy

Sphingomyelin (SM) is a major sphingolipid in mammalian cells that forms specific lipid domains in combination with cholesterol (Chol). Using molecular-dynamics simulation and density functional theory calculation, we identified a characteristic Raman band of SM at ∼1643 cm(-1) as amide I of the SM cluster. Experimental results indicate that this band is sensitive to the hydration of SM and the presence of Chol. We showed that this amide I Raman band can be utilized to examine the membrane distribution of SM. Similarly to SM, ceramide phosphoethanolamine (CerPE) exhibited an amide I Raman band in almost the same region, although CerPE lacks three methyl groups in the phosphocholine moiety of SM. In contrast to SM, the amide I band of CerPE was not affected by Chol, suggesting the importance of the methyl groups of SM in the SM-Chol interaction.

Influenza M2 Transmembrane Domain Senses Membrane Heterogeneity and Enhances Membrane Curvature

Targeting host cell membranes by M2 of influenza A virus is important for virus invasion and replication. We study the transmembrane domain of M2 (M2TM) interacting with mica-supported planar bilayers and free-standing giant unilamellar vesicles (GUVs). Using solution atomic force microscopy (AFM), we show that the size of M2TM oligomers is dependent on lipid composition. The addition of M2TM to lipid bilayers containing liquid-ordered (Lo) and liquid-disordered (Ld) phases reveals that M2TM preferentially partitions into the Ld phase; phase-dependent partitioning results in a larger rigidity of the Ld phase. We next use fluorescence microscopy to study the effects of M2TM on phase-coexisting GUVs. In particular, M2TM is found to increase GUVs' miscibility transition temperature Tmix. The augmented thermodynamic stability can be accounted for by considering an enhanced energy barrier of lipid mixing between coexisting phases. Our GUV study also shows that M2TM can elicit an array of vesicle shapes mimicking virus budding. M2TM enhanced membrane curvature is consistent with our AFM data, which show altered membrane rigidity and consequently line tension at domain edges. Together, our results highlight that in addition to conducting protons, M2TM can actively regulate membrane heterogeneity and augment membrane curvature.

Effect of lipid composition and amino acid sequence upon transmembrane peptide-accelerated lipid transleaflet diffusion (flip-flop)

We examined how hydrophobic peptide-accelerated transleaflet lipid movement (flip-flop) was affected by peptide sequence and vesicle composition and properties. A peptide with a completely hydrophobic sequence had little if any effect upon flip-flop. While peptides with a somewhat less hydrophobic sequence accelerated flip-flop, the half-time remained slow (hours) with substantial (0.5mol%) peptide in the membranes. It appears that peptide-accelerated lipid flip-flop involves a rare event that may reflect a rare state of the peptide or lipid bilayer. There was no simple relationship between peptide overall hydrophobicity and flip-flop. In addition, flip-flop was not closely linked to whether the peptides were in a transmembrane or non-transmembrane (interfacial) inserted state. Flip-flop was also not associated with peptide-induced pore formation. We found that peptide-accelerated flip-flop is initially faster in small (highly curved) unilamellar vesicles relative to that in large unilamellar vesicles. Peptide-accelerated flip-flop was also affected by lipid composition, being slowed in vesicles with thick bilayers or those containing 30% cholesterol. Interestingly, these factors also slow spontaneous lipid flip-flop in the absence of peptide. Combined with previous studies, the results are most consistent with acceleration of lipid flip-flop by peptide-induced thinning of bilayer width.

Lateral pressure-mediated protein partitioning into liquid-ordered/liquid-disordered domains

We have studied the contributions of stored elastic energies in liquid-ordered (Lo) and liquid-disordered (Ld) domains to transmembrane proteins using the lateral pressure concept. In particular we applied previously reported experimental data for the membrane thickness, intrinsic curvature and bending elasticities of coexisting Lo/Ld domains to calculate whether proteins of simple geometric shapes would preferentially diffuse into Lo or Ld domains and form oligomers of a certain size. For the studied lipid mixture we generally found that proteins with convex shapes prefer sorting to Ld phases and the formation of large clusters. Lo domains in turn would be enriched in monomers of concave shaped proteins. We further observed that proteins which are symmetric with respect to the bilayer center prefer symmetric Lo or Ld domains, while asymmetric proteins favor a location in domains with Lo/Ld asymmetry. In the latter case we additionally retrieved a strong dependence on protein directionality, thus providing a mechanism for transmembrane protein orientation.

Membrane-mediated regulation of the intrinsically disordered CD3ϵ cytoplasmic tail of the TCR

The regulation of T-cell-mediated immune responses depends on the phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) on T-cell receptors. Although many details of the signaling cascades are well understood, the initial mechanism and regulation of ITAM phosphorylation remains unknown. We used molecular dynamics simulations to study the influence of different compositions of lipid bilayers on the membrane association of the CD3ϵ cytoplasmic tails of the T-cell receptors. Our results show that binding of CD3ϵ to membranes is modulated by both the presence of negatively charged lipids and the lipid order of the membrane. Free-energy calculations reveal that the protein-membrane interaction is favored by the presence of nearby basic residues and the ITAM tyrosines. Phosphorylation minimizes membrane association, rendering the ITAM motif more accessible to binding partners. In systems mimicking biological membranes, the CD3ϵ chain localization is modulated by different facilitator lipids (e.g., gangliosides or phosphoinositols), revealing a plausible regulatory effect on activation through the regulation of lipid composition in cell membranes.

Raft-like membrane domains in pathogenic microorganisms

The lipid bilayer of the plasma membrane is thought to be compartmentalized by the presence of lipid-protein microdomains. In eukaryotic cells, microdomains composed of sterols and sphingolipids, commonly known as lipid rafts, are believed to exist, and reports on the presence of sterol- or protein-mediated microdomains in bacterial cell membranes are also appearing. Despite increasing attention, little is known about microdomains in the plasma membrane of pathogenic microorganisms. This review attempts to provide an overview of the current state of knowledge of lipid rafts in pathogenic fungi and bacteria. The current literature on characterization of microdomains in pathogens is reviewed, and their potential role in growth, pathogenesis, and drug resistance is discussed. Better insight into the structure and function of membrane microdomains in pathogenic microorganisms might lead to a better understanding of their pathogenesis and development of raft-mediated approaches for therapy.

The role of palmitoylation and transmembrane domain in sorting of transmembrane adaptor proteins

Plasma membrane proteins synthesised at the endoplasmic reticulum are delivered to cell surface via sorting pathways. Hydrophobic mismatch theory based on the length of transmembrane domain (TMD) dominates discussion about determinants required for protein sorting to the plasma membrane. Transmembrane adaptor proteins (TRAP) are involved in signalling events taking place at the plasma membrane. Members of this protein family have TMD of varying length. We were interested whether palmitoylation or other motifs contribute to the effective sorting of TRAP proteins. We found that palmitoylation is essential for some but not all TRAP proteins independent of their TMD length. We also provide evidence that palmitoylation and proximal sequences can modulate sorting of artificial proteins with TMD of suboptimal length. Our observations point to a unique character of each TMD defined by its primary amino acid sequence and its impact on membrane protein localisation. We conclude that, in addition to the TMD length, secondary sorting determinants such as palmitoylation or flanking sequences have evolved for the localisation of membrane proteins.

Transmembrane protein (perfringolysin o) association with ordered membrane domains (rafts) depends upon the raft-associating properties of protein-bound sterol

Because transmembrane (TM) protein localization, or nonlocalization, in ordered membrane domains (rafts) is a key to understanding membrane domain function, it is important to define the origin of protein-raft interaction. One hypothesis is that a tight noncovalent attachment of TM proteins to lipids that have a strong affinity for ordered domains can be sufficient to induce raft-protein interaction. The sterol-binding protein perfringolysin O (PFO) was used to test this hypothesis. PFO binds both to sterols that tend to localize in ordered domains (e.g., cholesterol), and to those that do not (e.g., coprostanol), but it does not bind to epicholesterol, a raft-promoting 3α-OH sterol. Using a fluorescence resonance energy transfer assay in model membrane vesicles containing coexisting ordered and disordered lipid domains, both TM and non-TM forms of PFO were found to concentrate in ordered domains in vesicles containing high and low-Tm lipids plus cholesterol or 1:1 (mol/mol) cholesterol/epicholesterol, whereas they concentrate in disordered domains in vesicles containing high-Tm and low-Tm lipids plus 1:1 (mol/mol) coprostanol/epicholesterol. Combined with previous studies this behavior indicates that TM protein association with ordered domains is dependent upon both the association of the protein-bound sterol with ordered domains and hydrophobic match between TM segments and rafts.

Bilayer asymmetry influences integrin sequestering in raft-mimicking lipid mixtures

There is growing recognition that lipid heterogeneities in cellular membranes play an important role in the distribution and functionality of membrane proteins. However, the detection and characterization of such heterogeneities at the cellular level remains challenging. Here we report on the poorly understood relationship between lipid bilayer asymmetry and membrane protein sequestering in raft-mimicking model membrane mixtures using a powerful experimental platform comprised of confocal spectroscopy XY-scan and photon-counting histogram analyses. This experimental approach is utilized to probe the domain-specific sequestering and oligomerization state of αvβ3 and α5β1 integrins in bilayers, which contain coexisting liquid-disordered/liquid-ordered (ld/lo) phase regions exclusively in the top leaflet of the bilayer (bottom leaflet contains ld phase). Comparison with previously reported integrin sequestering data in bilayer-spanning lo-ld phase separations demonstrates that bilayer asymmetry has a profound influence on αvβ3 and α5β1 sequestering behavior. For example, both integrins sequester preferentially to the lo phase in asymmetric bilayers, but to the ld phase in their symmetric counterparts. Furthermore, our data show that bilayer asymmetry significantly influences the role of native ligands in integrin sequestering.

Formation of raft-like assemblies within clusters of influenza hemagglutinin observed by MD simulations

The association of hemagglutinin (HA) with lipid rafts in the plasma membrane is an important feature of the assembly process of influenza virus A. Lipid rafts are thought to be small, fluctuating patches of membrane enriched in saturated phospholipids, sphingolipids, cholesterol and certain types of protein. However, raft-associating transmembrane (TM) proteins generally partition into Ld domains in model membranes, which are enriched in unsaturated lipids and depleted in saturated lipids and cholesterol. The reason for this apparent disparity in behavior is unclear, but model membranes differ from the plasma membrane in a number of ways. In particular, the higher protein concentration in the plasma membrane may influence the partitioning of membrane proteins for rafts. To investigate the effect of high local protein concentration, we have conducted coarse-grained molecular dynamics (CG MD) simulations of HA clusters in domain-forming bilayers. During the simulations, we observed a continuous increase in the proportion of raft-type lipids (saturated phospholipids and cholesterol) within the area of membrane spanned by the protein cluster. Lateral diffusion of unsaturated lipids was significantly attenuated within the cluster, while saturated lipids were relatively unaffected. On this basis, we suggest a possible explanation for the change in lipid distribution, namely that steric crowding by the slow-diffusing proteins increases the chemical potential for unsaturated lipids within the cluster region. We therefore suggest that a local aggregation of HA can be sufficient to drive association of the protein with raft-type lipids. This may also represent a general mechanism for the targeting of TM proteins to rafts in the plasma membrane, which is of functional importance in a wide range of cellular processes.

The cell membrane is composed of a wide variety of lipids and proteins. Until recently, these were thought to be mixed evenly, but we now have evidence of the existence of “lipid rafts” — small, slow-moving areas of membrane in which certain types of lipid and protein accumulate. Rafts have many important biological functions in healthy cells, but also play a role in the assembly of influenza virus. For example, after the viral protein hemagglutinin is made inside the host cell, it accumulates in rafts. Exiting virus particles then take these portions of cell membrane with them as they leave the host cell. However, the mechanism by which proteins associate with lipid rafts is unclear. Here, we have used computers to simulate lipid membranes containing hemagglutinin. The simulations allow us to look in detail at the motions and interactions of individual proteins and lipids. We found that clusters of proteins altered the properties of nearby lipids, leading to accumulation of raft-type lipids. It therefore appears that aggregation of hemagglutinin may be enough to drive its association with rafts. This helps us to better understand both the influenza assembly process and the properties of lipid rafts.

Sphingolipids and membrane domains: recent advances

There is growing evidence that cell membranes can contain domains with different lipid and protein compositions and with different physical properties. Furthermore, it is increasingly appreciated that sphingolipids play a crucial role in the formation and properties of ordered lipid domains (rafts) in cell membranes. This review describes recent advances in our understanding of ordered membrane domains in both cells and model membranes. In addition, how the structure of sphingolipids influences their ability to participate in the formation of ordered domains, as well as how sphingolipid structure alters ordered domain properties, is described. The diversity of sphingolipid structure is likely to play an important role in modulating the biologically relevant properties of "rafts" in cell membranes.

Altering hydrophobic sequence lengths shows that hydrophobic mismatch controls affinity for ordered lipid domains (rafts) in the multitransmembrane strand protein perfringolysin O

The hypothesis that mismatch between transmembrane (TM) length and bilayer width controls TM protein affinity for ordered lipid domains (rafts) was tested using perfringolysin O (PFO), a pore-forming cholesterol-dependent cytolysin. PFO forms a multimeric barrel with many TM segments. The properties of PFO mutants with lengthened or shortened TM segments were compared with that of PFO with wild type TM sequences. Both mutant and wild type length PFO exhibited cholesterol-dependent membrane insertion. Maximal PFO-induced pore formation occurred in vesicles with wider bilayers for lengthened TM segments and in thinner bilayers for shortened TM segments. In diC(18:0) phosphatidylcholine (PC)/diC(14:1) PC/cholesterol vesicles, which form ordered domains with a relatively thick bilayer and disordered domains with a relatively thin bilayer, affinity for ordered domains was greatest with lengthened TM segments and least with shortened TM segments as judged by FRET. Similar results were observed by microscopy in giant vesicles containing sphingomyelin in place of diC(18:0) PC. In contrast, in diC(16:0) PC/diC(14:0) PC/diC(20:1) PC/cholesterol vesicles, which should form ordered domains with a relatively thin bilayer and disordered domains with a relatively thick bilayer, relative affinity for ordered domains was greatest with shortened TM segments and least with lengthened TM segments. The inability of multi-TM segment proteins (unlike single TM segment proteins) to adapt to mismatch by tilting may explain the sensitivity of raft affinity to mismatch. The difference in width sensitivity for single and multi-TM helix proteins may link raft affinity to multimeric state and thus control the assembly of multimeric TM complexes in rafts.

Measurement of lipid nanodomain (raft) formation and size in sphingomyelin/POPC/cholesterol vesicles shows TX-100 and transmembrane helices increase domain size by coalescing preexisting nanodomains but do not induce domain formation

Mixtures of unsaturated lipids, sphingolipids, and cholesterol form coexisting liquid-disordered and sphingolipid and cholesterol-rich liquid-ordered (Lo) phases in water. The detergent Triton X-100 does not readily solubilize Lo domains, but does solubilize liquid-disordered domains, and is commonly used to prepare detergent-resistant membranes from cells and model membranes. However, it has been proposed that in membranes with mixtures of sphingomyelin (SM), 1-palmitoyl 2-oleoyl phosphatidylcholine (POPC), and cholesterol Triton X-100 may induce Lo domain formation, and therefore detergent-resistant membranes may not reflect the presence of preexisting domains. To examine this hypothesis, the effect of Triton on Lo domain formation was measured in SM/POPC/cholesterol vesicles. Nitroxide quenching methods that can detect ordered nanodomains with radii >12 Å showed that in the absence of Triton X-100 this mixture formed ordered state domains that melt with a midpoint (= T(mid)) at ∼45°C. However, T(mid) was lower when detected using various fluorescence resonance energy transfer (FRET) pairs. Furthermore, the T(mid) value was Ro dependent, and decreased as Ro increased. Because FRET can only readily detect domains with radii >Ro, this result can be explained by domain radii that are close to Ro and decrease as temperature increases. An analysis of FRET and quenching data suggests that nanodomain radius gradually decreases from ≥150 Å to <40 Å as temperature increases from 10 to 45°C. Interestingly, the presence of Triton X-100 or a transmembrane-type peptide did not stabilize ordered state formation when detected by nitroxide quenching, i.e., did not increase T(mid). However, FRET-detected T(mid) did increase in the presence of Triton X-100 or a transmembrane peptide, indicating that both increased domain size. Controls showed that the results could not be accounted for by probe-induced perturbations. Thus, SM/POPC/cholesterol, a mixture similar to that in the outer leaflet of plasma membranes, forms nanodomains at physiological temperatures, and TX-100 does not induce domain formation or increase the fraction of the bilayer in the ordered state, although it does increase domain size by coalescing preexisting domains.

Molecular convergence of bacterial and eukaryotic surface order

The conservation of fluidity is a theme common to all cell membranes. In this study, an analysis of lipid packing was conducted via C-laurdan spectroscopy of cell surface membranes prepared from representative species of Bacteria and Eukarya. We found that despite their radical differences in composition (namely the presence and absence of membrane-rigidifying sterol) the membrane order of all taxa converges on a remarkably similar level. To understand how this similarity is constructed, we reconstituted membranes with either bacterial or eukaryotic components. We found that transmembrane segments of proteins have an important role in buffering lipid-mediated packing. This buffering ensures that sterol-free and sterol-containing membranes exhibit similar barrier properties.

Lateral sorting in model membranes by cholesterol-mediated hydrophobic matching

Theoretical studies predict hydrophobic matching between transmembrane domains of proteins and bilayer lipids to be a physical mechanism by which membranes laterally self-organize. We now experimentally study the direct consequences of mismatching of transmembrane peptides of different length with bilayers of different thicknesses at the molecular level. In both model membranes and simulations we show that cholesterol critically constrains structural adaptations at the peptide-lipid interface under mismatch. These constraints translate into a sorting potential and lead to selective lateral segregation of peptides and lipids according to their hydrophobic length.

Transmembrane helices can induce domain formation in crowded model membranes

We studied compositionally heterogeneous multi-component model membranes comprised of saturated lipids, unsaturated lipids, cholesterol, and α-helical TM protein models using coarse-grained molecular dynamics simulations. Reducing the mismatch between the length of the saturated and unsaturated lipid tails reduced the driving force for segregation into liquid-ordered (l(o)) and liquid-disordered (l(d)) lipid domains. Cholesterol depletion had a similar effect, and binary lipid mixtures without cholesterol did not undergo large-scale phase separation under the simulation conditions. The phase-separating ternary dipalmitoyl-phosphatidylcholine (DPPC)/dilinoleoyl-PC (DLiPC)/cholesterol bilayer was found to segregate into l(o) and l(d) domains also in the presence of a high concentration of ΤΜ helices. The l(d) domain was highly crowded with TM helices (protein-to-lipid ratio ~1:5), slowing down lateral diffusion by a factor of 5-10 as compared to the dilute case, with anomalous (sub)-diffusion on the μs time scale. The membrane with the less strongly unsaturated palmitoyl-linoleoyl-PC instead of DLiPC, which in the absence of TM α-helices less strongly deviated from ideal mixing, could be brought closer to a miscibility critical point by introducing a high concentration of TM helices. Finally, the 7-TM protein bacteriorhodopsin was found to partition into the l(d) domains irrespective of hydrophobic matching. These results show that it is possible to directly study the lateral reorganization of lipids and proteins in compositionally heterogeneous and crowded model biomembranes with coarse-grained molecular dynamics simulations, a step toward simulations of realistic, compositionally complex cellular membranes. This article is part of a Special Issue entitled: Protein Folding in Membranes.

Perfringolysin O association with ordered lipid domains: implications for transmembrane protein raft affinity

Upon interaction with cholesterol, perfringolysin O (PFO) inserts into membranes and forms a rigid transmembrane (TM) β-barrel. PFO is believed to interact with liquid ordered lipid domains (lipid rafts). Because the origin of TM protein affinity for rafts is poorly understood, we investigated PFO raft affinity in vesicles having coexisting ordered and disordered lipid domains. Fluorescence resonance energy transfer (FRET) from PFO Trp to domain-localized acceptors indicated that PFO generally has a raft affinity between that of LW peptide (low raft affinity) and cholera toxin B (high raft affinity) in vesicles containing ordered domains rich in brain sphingomyelin or distearoylphosphatidylcholine. FRET also showed that ceramide, which increases exposure of cholesterol to water and thus displaces it from rafts, does not displace PFO from ordered domains. This can be explained by shielding of PFO-bound cholesterol from water. Finally, FRET showed that PFO affinity for ordered domains was higher in its non-TM (prepore) form than in its TM form, demonstrating that the TM portion of PFO interacts unfavorably with rafts. Microscopy studies in giant unilamellar vesicles confirmed that PFO exhibits intermediate raft affinity, and showed that TM PFO (but not non-TM PFO) concentrated at the edges of liquid ordered domains. These studies suggest that a combination of binding to raft-associating molecules and having a rigid TM structure that is unable to pack well in a highly ordered lipid environment can control TM protein domain localization. To accommodate these constraints, raft-associated TM proteins in cells may tend to locate within liquid disordered shells encapsulated within ordered domains.

Comparison of three ternary lipid bilayer mixtures: FRET and ESR reveal nanodomains

Phase diagrams of ternary lipid mixtures containing cholesterol have provided valuable insight into cell membrane behaviors, especially by describing regions of coexisting liquid-disordered (Ld) and liquid-ordered (Lo) phases. Fluorescence microscopy imaging of giant unilamellar vesicles has greatly assisted the determination of phase behavior in these systems. However, the requirement for optically resolved Ld + Lo domains can lead to the incorrect inference that in lipid-only mixtures, Ld + Lo domain coexistence generally shows macroscopic domains. Here we show this inference is incorrect for the low melting temperature phosphatidylcholines abundant in mammalian plasma membranes. By use of high compositional resolution Förster resonance energy transfer measurements, together with electron spin resonance data and spectral simulation, we find that ternary mixtures of DSPC and cholesterol together with either POPC or SOPC, do indeed have regions of Ld + Lo coexistence. However, phase domains are much smaller than the optical resolution limit, likely on the order of the Förster distance for energy transfer (R(0), ∼2-8 nm).

Lipid packing drives the segregation of transmembrane helices into disordered lipid domains in model membranes

Cell membranes are comprised of multicomponent lipid and protein mixtures that exhibit a complex partitioning behavior. Regions of structural and compositional heterogeneity play a major role in the sorting and self-assembly of proteins, and their clustering into higher-order oligomers. Here, we use computer simulations and optical microscopy to study the sorting of transmembrane helices into the liquid-disordered domains of phase-separated model membranes, irrespective of peptide-lipid hydrophobic mismatch. Free energy calculations show that the enthalpic contribution due to the packing of the lipids drives the lateral sorting of the helices. Hydrophobic mismatch regulates the clustering into either small dynamic or large static aggregates. These results reveal important molecular driving forces for the lateral organization and self-assembly of transmembrane helices in heterogeneous model membranes, with implications for the formation of functional protein complexes in real cells.

Hemagglutinin of influenza virus partitions into the nonraft domain of model membranes

The HA of influenza virus is a paradigm for a transmembrane protein thought to be associated with membrane-rafts, liquid-ordered like nanodomains of the plasma membrane enriched in cholesterol, glycosphingolipids, and saturated phospholipids. Due to their submicron size in cells, rafts can not be visualized directly and raft-association of HA was hitherto analyzed by indirect methods. In this study, we have used GUVs and GPMVs, showing liquid disordered and liquid ordered domains, to directly visualize partition of HA by fluorescence microscopy. We show that HA is exclusively (GUVs) or predominantly (GPMVs) present in the liquid disordered domain, regardless of whether authentic HA or domains containing its raft targeting signals were reconstituted into model membranes. The preferential partition of HA into ld domains and the difference between lo partition in GUV and GPMV are discussed with respect to differences in packaging of lipids in membranes of model systems and living cells suggesting that physical properties of lipid domains in biological membranes are tightly regulated by protein-lipid interactions.

Orientation and dynamics of transmembrane peptides: the power of simple models

In this review we discuss recent insights obtained from well-characterized model systems into the factors that determine the orientation and tilt angles of transmembrane peptides in lipid bilayers. We will compare tilt angles of synthetic peptides with those of natural peptides and proteins, and we will discuss how tilt can be modulated by hydrophobic mismatch between the thickness of the bilayer and the length of the membrane spanning part of the peptide or protein. In particular, we will focus on results obtained on tryptophan-flanked model peptides (WALP peptides) as a case study to illustrate possible consequences of hydrophobic mismatch in molecular detail and to highlight the importance of peptide dynamics for the experimental determination of tilt angles. We will conclude with discussing some future prospects and challenges concerning the use of simple peptide/lipid model systems as a tool to understand membrane structure and function.

Order of lipid phases in model and plasma membranes

Lipid rafts are nanoscopic assemblies of sphingolipids, cholesterol, and specific membrane proteins that contribute to lateral heterogeneity in eukaryotic membranes. Separation of artificial membranes into liquid-ordered (Lo) and liquid-disordered phases is regarded as a common model for this compartmentalization. However, tight lipid packing in Lo phases seems to conflict with efficient partitioning of raft-associated transmembrane (TM) proteins. To assess membrane order as a component of raft organization, we performed fluorescence spectroscopy and microscopy with the membrane probes Laurdan and C-laurdan. First, we assessed lipid packing in model membranes of various compositions and found cholesterol and acyl chain dependence of membrane order. Then we probed cell membranes by using two novel systems that exhibit inducible phase separation: giant plasma membrane vesicles [Baumgart et al. (2007) Proc Natl Acad Sci USA 104:3165-3170] and plasma membrane spheres. Notably, only the latter support selective inclusion of raft TM proteins with the ganglioside GM1 into one phase. We measured comparable small differences in order between the separated phases of both biomembranes. Lateral packing in the ordered phase of giant plasma membrane vesicles resembled the Lo domain of model membranes, whereas the GM1 phase in plasma membrane spheres exhibited considerably lower order, consistent with different partitioning of lipid and TM protein markers. Thus, lipid-mediated coalescence of the GM1 raft domain seems to be distinct from the formation of a Lo phase, suggesting additional interactions between proteins and lipids to be effective.

Preparation and properties of asymmetric vesicles that mimic cell membranes: effect upon lipid raft formation and transmembrane helix orientation

A methyl-beta-cyclodextrin-induced lipid exchange technique was devised to prepare small unilamellar vesicles with stable asymmetric lipid compositions. Asymmetric vesicles that mimic biological membranes were prepared with sphingomyelin (SM) or SM mixed with 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) as the predominant lipids in the outer leaflet and dioleoylphosphatidylcholine (DOPC), POPC, 1-palmitoyl-2-oleoyl-phosphatidyl-L-serine (POPS), or POPS mixed with 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) in the inner leaflet. Fluorescence-based assays were developed to confirm lipid asymmetry. Cholesterol was introduced into these vesicles using a second methyl-beta-cyclodextrin exchange step. In asymmetric vesicles composed of SM outside, DOPC inside (SMo/DOPCi) or SM outside, 2:1 mol:mol POPE:POPS inside (SMo/2:1 POPE:POPSi) the outer leaflet SM formed an ordered state with a thermal stability similar to that in pure SM vesicles and significantly greater than that in symmetric vesicles with the same overall lipid composition. Analogous behavior was observed in vesicles containing cholesterol. This shows that an asymmetric lipid distribution like that in eukaryotic plasma membranes can be conducive to ordered domain (raft) formation. Furthermore asymmetric vesicles containing approximately 25 mol % cholesterol formed ordered domains more thermally stable than those in asymmetric vesicles lacking cholesterol, showing that the crucial ability of cholesterol to stabilize ordered domain formation is likely to contribute to ordered domain formation in cell membranes. Additional studies demonstrated that hydrophobic helix orientation is affected by lipid asymmetry with asymmetry favoring formation of the transmembrane configuration. The ability to form asymmetric vesicles represents an important improvement in model membrane studies and should find many applications in the future.

A high-cholesterol diet increases the association between caveolae and insulin receptors in rat liver

Caveolin-1, a component of caveolae, regulates signaling pathway compartmentalization by interacting with tyrosine (Tyr) kinase receptors and their substrates. Perturbations in caveolae lipid composition have been shown in vitro to displace proteins from lipid microdomains, thereby altering their functionality and subsequent downstream signaling. The role of caveolin-1 in insulin receptor (IR) signaling has been widely investigated in vitro mainly in 3T3-L1 adipocyte cells. However, in vivo experiments investigating this connection in liver tissue have not been carried out. The objective of the present study was to investigate the effects of a high-cholesterol diet on caveolin-1 expression and IR localization and activity in the rat liver. Compared with a standard diet, rats fed with diet rich in cholesterol significantly altered liver caveolae by increasing both caveolin-1 (66%, P < 0.05) and caveolin-2 (55%, P < 0.05) expression while caveolin-1 mRNA levels were reduced. Concomitantly, a 25% increase in localization of the caveolae-resident signaling protein IR was observed. The distribution of caveolar and noncaveolar phosphorylated IR was unaffected but insulin-induced IR activation was significantly enhanced following consumption of the high-cholesterol diet (120%, P < 0.001). However, the downstream molecules IRS-1 and Akt have shown impaired activity in cholesterol-fed rats suggesting insulin resistance condition. Insulin stimulation failed to induce Tyr phosphorylation of caveolin-1 in cholesterol-fed rats. These findings suggest a mechanism by which a high-cholesterol diet altered caveolin-1 expression in vivo accompanied by altered IR localization and activity.

Calcitonin-derived peptide carriers: mechanisms and application

Among the family of the so-called cell-penetrating peptides (CPP) sequences derived from the native peptide hormone human calcitonin (hCT) have recently proven to translocate different bioactive molecules across cellular membranes. Herein, we give an extensive summary of the development of hCT-derived carrier peptides, beginning with the therapeutic nasal administration of full-length hCT. Hence, N-terminally truncated hCT fragments were investigated and subsequently optimised to extend their field of application. The latest generation of hCT-derived carrier peptides are highly effective, branched peptides. The current state of the art is reviewed concerning the structural requirements, mechanistic assumptions and metabolic features of these peptides as well as experiments proving their excellent carrier potential.

The phenyltetraene lysophospholipid analog PTE-ET-18-OMe as a fluorescent anisotropy probe of liquid ordered membrane domains (lipid rafts) and ceramide-rich membrane domains

The conjugated phenyltetraene PTE-ET-18-OMe (all-(E)-1-O-(15'-phenylpentadeca-8',10',12',14'-tetraenyl)-2-O-methyl-rac-glycero-3-phosphocholine) is a recently developed fluorescent lysophospholipid analog of edelfosine, (Quesada et al. (2004) J. Med. Chem. 47, 5333-5335). We investigated the use of this analog as a probe of membrane structure. PTE-ET-18-OMe was found to have several properties that are favorable for fluorescence anisotropy (polarization) experiments in membranes, including low fluorescence in water and moderately strong association with lipid bilayers. PTE-ET-18-OMe has absorbance and fluorescence properties similar to those of diphenylhexatriene (DPH) probes, with about as large a difference between its fluorescence anisotropy in liquid disordered (Ld) and ordered states (gel and Lo) as observed for DPH. Also like DPH, PTE-ET-18-OMe has a moderate affinity for both gel state ordered domains and Lo state ordered domains (rafts). However, unlike fluorescent sterols or DPH (Megha and London (2004) J. Biol. Chem. 279, 9997-10004), PTE-ET-18-OMe is not displaced from ordered domains by ceramide. Also unlike DPH, PTE-ET-18-OMe shows only slow exchange between the inner and outer leaflets of membrane bilayers, and can thus be used to examine anisotropy of an individual leaflet of a lipid bilayer. Since PTE-ET-18-OMe is a zwitterionic molecule, it should not be as influenced by electrostatic interactions as are other probes that do not cross the lipid bilayer but have a net charge. We conclude that PTE-ET-18-OMe has some unique properties that should make it a useful fluorescence probe of membrane structure.

Effect of the structure of lipids favoring disordered domain formation on the stability of cholesterol-containing ordered domains (lipid rafts): identification of multiple raft-stabilization mechanisms

Despite the importance of lipid rafts, commonly defined as liquid-ordered domains rich in cholesterol and in lipids with high gel-to-fluid melting temperatures (T(m)), the rules for raft formation in membranes are not completely understood. Here, a fluorescence-quenching strategy was used to define how lipids with low T(m), which tend to form disordered fluid domains at physiological temperatures, can stabilize ordered domain formation by cholesterol and high-T(m) lipids (either sphingomyelin or dipalmitoylphosphatidylcholine). In bilayers containing mixtures of low-T(m) phosphatidylcholines, cholesterol, and high-T(m) lipid, the thermal stability of ordered domains decreased with the acyl-chain structure of low-T(m) lipids in the following order: diarachadonyl > diphytanoyl > 1-palmitoyl 2-docosahexenoyl = 1,2 dioleoyl = dimyristoleoyl = 1-palmitoyl, 2-oleoyl (PO). This shows that low-T(m) lipids with two acyl chains having very poor tight-packing propensities can stabilize ordered domain formation by high-T(m) lipids and cholesterol. The effect of headgroup structure was also studied. We found that even in the absence of high-T(m) lipids, mixtures of cholesterol with PO phosphatidylethanolamine (POPE) and PO phosphatidylserine (POPS) or with brain PE and brain PS showed a (borderline) tendency to form ordered domains. Because these lipids are abundant in the inner (cytofacial) leaflet of mammalian membranes, this raises the possibility that PE and PS could participate in inner-leaflet raft formation or stabilization. In bilayers containing ternary mixtures of PO lipids, cholesterol, and high-T(m) lipids, the thermal stability of ordered domains decreased with the polar headgroup structure of PO lipids in the order PE > PS > phosphatidylcholine (PC). Analogous experiments using diphytanoyl acyl chain lipids in place of PO acyl chain lipids showed that the stabilization of ordered lipid domains by acyl chain and headgroup structure was not additive. This implies that it is likely that there are two largely mutually exclusive mechanisms by which low-T(m) lipids can stabilize ordered domain formation by high-T(m) lipids and cholesterol: 1), by having structures resulting in immiscibility of low-T(m) and high-T(m) lipids, and 2), by having structures allowing them to pack tightly within ordered domains to a significant degree.

Localization of the human breast cancer resistance protein (BCRP/ABCG2) in lipid rafts/caveolae and modulation of its activity by cholesterol in vitro

Breast cancer resistance protein (BCRP/ABCG2) is an active efflux pump that belongs to the ATP-binding cassette (ABC) transporter family. It is located in various tissues involved in drug absorption, distribution, and elimination and plays an important role in multidrug resistance. For P-glycoprotein, another member of the ABC transporter family, it is well established that it is at least partly located in cholesterol and sphingolipid-enriched domains of the plasma membrane called "lipid rafts" and that the composition of the membrane lipids may modulate its efflux activity. This study addressed the compartmentalization of BCRP in the plasma membrane and the influence of membrane cholesterol on the efflux activity of BCRP. As a cell model, we used the canine kidney epithelial cell line MDCKII-BCRP transfected with the cDNA encoding human BCRP and the corresponding parental cell line MDCKII. Cholesterol depletion with methyl-beta-cyclodextrin (MbetaCD) provoked a 40% decrease in BCRP activity (p < 0.01) assessed with flow cytometry (pheophorbide A efflux assay). Cholesterol repletion with MbetaCD/cholesterol-inclusion complexes restored BCRP function, and cholesterol saturation of native cells did not further enhance BCRP activity. Coimmunoprecipitation experiments indicated a physical interaction between BCRP and caveolin-1, and Western blot analysis after density gradient ultracentrifugation demonstrated that BCRP is located in detergent-resistant membranes that also contain caveolin-1. In conclusion, our results demonstrate for the first time that BCRP is located in membrane rafts and that cholesterol has impact on its efflux activity.

Cholesterol/phospholipid interactions in hybrid bilayer membranes

The interactions between cholesterol and saturated phospholipids in hybrid bilayer membranes (HBMs) were investigated using the interface-sensitive technique of vibrational sum frequency spectroscopy (VSFS). The unique sensitivity of VSFS to order/disorder transitions of the lipid acyl chains was used to determine the main gel to liquid crystal phase transition temperature, Tm, for HBMs of binary cholesterol/phospholipid mixtures on octadecanethiolate self-assembled monolayers. The phase transition temperature and the breadth of the transition were shown to increase with cholesterol content, and the phase boundaries observed in the cholesterol/phospholipid HBMs were comparable to the published phase diagrams of binary cholesterol/phospholipid vesicles. A thermodynamic assessment of the cooperative units of the HBM phase transitions revealed the presence of <10 nm diameter domains that were independent of the cholesterol composition.

Differential detergent resistance of the apical and basolateral NPPases: relationship with polarized targeting

Targeting of glycosylphosphatidylinositol-anchored proteins to the apical surface of epithelial cells involves clustering in Triton X-100-resistant membrane microdomains or rafts. The role of these microdomains in sorting transmembrane proteins is more questionable because, unlike glycosylphosphatidylinositol-anchored proteins, apical transmembrane proteins are rather soluble in Triton X-100. They are, however, resistant to milder detergents such as Lubrol WX or Tween 20. It has been proposed that specific membrane microdomains, defined by resistance to these detergents, would carry transmembrane proteins to the apical surface. We have used MDCK cells stably transfected with the apical and basolateral pyrophosphatases/phosphodiesterases, NPP3 and NPP1, to examine the relationship between detergent resistance and apical targeting. The apically expressed wild-type NPP3 was insoluble in Lubrol WX whereas wild-type NPP1, which is expressed basolaterally, was essentially soluble. By using tail mutants and chimeric constructs that combine the cytoplasmic, transmembrane and extracellular domains of NPP1 and NPP3, we show that there is not a strict correlation between detergent resistance and apical targeting. Lubrol resistance is an intrinsic property of NPP3, which is acquired early during the biosynthetic process irrespective of its final destination, and depends on positively charged residues in its cytoplasmic tail.

Detecting ordered domain formation (lipid rafts) in model membranes using Tempo

Short-range fluorescence quenching has proven to be an effective method to detect the presence of coexisting ordered and disordered state lipid domains in model membranes. In this approach a fluorescent group and fluorescence-quenching molecule are incorporated into the lipid bilayer of interest. In a typical experiment, the fluorophore chosen partitions into ordered domains to a significant degree, whereas the quencher partitions more favorably into disordered domains. Thus, in the presence of lipid mixtures forming coexisting ordered and disordered domains, fluorophore and quencher segregate so that fluorescence intensity is much stronger than in homogeneous lipid bilayers lacking separate domains. The small nitroxide-labeled molecule tempo (2,2,6,6 tetramethylpiperidine-1-oxyl) is a useful quencher for such experiments. Protocols for using tempo to detect ordered domains and ordered domain thermal stability are described. The advantages and disadvantages of use of tempo as opposed to nitroxide-labeled lipids are also described.

Lipid Rafts, Detergent-Resistant Membranes, and Raft Targeting Signals

Lipid rafts are liquid-ordered (lo) phase microdomains proposed to exist in biological membranes. Rafts have been widely studied by isolating lo-phase detergent-resistant membranes (DRMs) from cells. Recent findings have shown that DRMs are not the same as preexisting rafts, prompting a major revision of the raft model. Nevertheless, raft-targeting signals identified by DRM analysis are often required for protein function, implicating rafts in a variety of cell processes.

Peptides in lipid bilayers: the power of simple models

Interactions between proteins and lipids lie at the heart of virtually all membrane processes, but on a molecular level they are still poorly understood. Nowadays, simple model systems comprising designed transmembrane peptides in synthetic lipid bilayers are increasingly being recognized as powerful tools to uncover basic principles of protein-lipid interactions. Such model systems enable detailed analysis of how the properties of lipids influence the structure and dynamics of transmembrane helices, how these helices are anchored at the lipid-water interface, and how the length and composition of transmembrane segments influence the organization and dynamics of membrane lipids. In addition, well-characterized model systems have proven useful to refine computational approaches and to develop new techniques for studies of protein-lipid interactions.

Temperature and composition dependence of the interaction of delta-lysin with ternary mixtures of sphingomyelin/cholesterol/POPC

The kinetics of carboxyfluorescein efflux induced by the amphipathic peptide delta-lysin from vesicles of porcine brain sphingomyelin (BSM), 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC), and cholesterol (Chol) were investigated as a function of temperature and composition. Sphingomyelin (SM)/Chol mixtures form a liquid-ordered (L(o)) phase whereas POPC exists in the liquid-disordered (L(d)) phase at ambient temperature. delta-Lysin binds strongly to L(d) and poorly to L(o) phase. In BSM/Chol/POPC vesicles the rate of carboxyfluorescein efflux induced by delta-lysin increases as the POPC content decreases. This is explained by the increase of delta-lysin concentration in L(d) domains, which enhances membrane perturbation by the peptide. Phase separations in the micrometer scale have been observed by fluorescence microscopy in SM/Chol/POPC mixtures for some SM, though not for BSM. Thus, delta-lysin must detect heterogeneities (domains) in BSM/Chol/POPC on a much smaller scale. Advantage was taken of the inverse variation of the efflux rate with the L(d) content of BSM/Chol/POPC vesicles to estimate the L(d) fraction in those mixtures. These results were combined with differential scanning calorimetry to obtain the BSM/Chol/POPC phase diagram as a function of temperature.

Coexisting domains in the plasma membranes of live cells characterized by spin-label ESR spectroscopy

The importance of membrane-based compartmentalization in eukaryotic cell function has become broadly appreciated, and a number of studies indicate that these eukaryotic cell membranes contain coexisting liquid-ordered (L(o)) and liquid-disordered (L(d)) lipid domains. However, the current evidence for such phase separation is indirect, and so far there has been no direct demonstration of differences in the ordering and dynamics for the lipids in these two types of regions or their relative amounts in the plasma membranes of live cells. In this study, we provide direct evidence for the presence of two different types of lipid populations in the plasma membranes of live cells from four different cell lines by electron spin resonance. Analysis of the electron spin resonance spectra recorded over a range of temperatures, from 5 to 37 degrees C, shows that the spin-labeled phospholipids incorporated experience two types of environments, L(o) and L(d), with distinct order parameters and rotational diffusion coefficients but with some differences among the four cell lines. These results suggest that coexistence of lipid domains that differ significantly in their dynamic order in the plasma membrane is a general phenomenon. The L(o) region is found to be a major component in contrast to a model in which small liquid-ordered lipid rafts exist in a 'sea' of disordered lipids. The results on ordering and dynamics for the live cells are also compared with those from model membranes exhibiting coexisting L(o) and L(d) phases.