Visualizing association of lipidated signaling proteins in heterogeneous membranes−Partitioning into subdomains, lipid sorting, interfacial adsorption, and protein association

In a combined chemical biological and biophysical approach, we studied the partitioning of differently fluorescent-labeled palmitoyl and/or farnesyl lipidated peptides, which represent membrane recognition model systems, as well as the full lipidated N-Ras protein into various model membrane systems including canonical model raft mixtures. To this end, two-photon fluorescence microscopy on giant unilamellar vesicles, complemented by tapping-mode atomic force microscopy (AFM) measurements, was carried out. The measurements were performed over a wide temperature range, ranging from 30 to 80 °C to cover different lipid phase states (solid-ordered (gel), fluid/gel, liquid-ordered/liquid-disordered, all-fluid). The results provide direct evidence that partitioning of the lipidated peptides and N-Ras occurs preferentially into liquid-disordered lipid domains, which is also reflected in a faster kinetics of incorporation. The phase sequence of preferential binding of N-Ras to mixed-domain lipid vesicles is liquid-disordered > liquid-ordered ? solid-ordered. Intriguingly, we detect - using the better spatial resolution of AFM - also a large proportion of the lipidated protein located at the liquid-disordered/liquid-ordered phase boundary, thus leading to a favorable decrease in line tension that is associated with the rim of neighboring domains. In an all-liquid-ordered, cholesterol-rich phase, phase separation can be induced by an effective lipid sorting mechanism owing to the high affinity of the lipidated peptides and proteins to a fluid-like lipid environment. At low temperatures, where the overall acyl chain order parameter of the lipid bilayer has markedly increased, such an efficient lipid sorting mechanism is energetically too costly and self-association of the peptide into small clusters takes place. These data reveal the interesting ability of the lipidated peptides and proteins to induce formation of fluid microdomains at physiologically relevant high cholesterol concentrations. Furthermore, our results reveal self-association of the N-Ras protein at the domain boundaries which may serve as an important vehicle for association processes and nanoclustering, which has also been observed in in vivo studies.     

Fluorescence-based evaluation of the partitioning of lipids and lipidated peptides into liquid-ordered lipid microdomains: a model for molecular partitioning into "lipid rafts"

A fluorescence-quenching assay is described that can directly monitor the relative extents of partitioning of different but structurally homologous fluorescent molecules into liquid-ordered (l(o)) domains in lipid vesicles exhibiting liquid-ordered/liquid-disordered (l(o)/l(d)) phase coexistence. Applying this assay to a series of bimane-labeled diacyl phospholipid probes in cholesterol-containing ternary lipid mixtures exhibiting l(o)/l(d) phase separation, we demonstrate that partitioning into l(o)-phase domains is negligible for diunsaturated species and greatest for long-chain disaturated species. These conclusions agree well with those derived from previous studies of the association of lipids and lipid-anchored molecules with l(o)-phase domains, using methods based on the isolation of a detergent-insoluble fraction from model or biological membranes at low temperatures. However, we also find that monounsaturated and shorter-chain saturated species partition into l(o) phases with significant, albeit modest affinities, and that the level of partitioning of these latter species into l(o)-phase domains is significantly underestimated (relative to that of their long-chain saturated counterparts) by the criterion of low-temperature detergent insolubility. Finally, applying the fluorescence-quenching method to a family of lipid-modified peptides, we demonstrate that the S-palmitoyl/S-isoprenyl dual-lipidation motif found in proteins such as H- and N-ras and yeast Ste18p does not promote significant association with l(o) domains in l(o)/l(d)-phase-separated bilayers.     

Partitioning of pyrene-labeled phospho- and sphingolipids between ordered and disordered bilayer domains

Here we have studied how the length of the pyrene-labeled acyl chain (n) of a phosphatidylcholine, sphingomyelin, or galactosylceramide affects the partitioning of these lipids between 1), gel and fluid domains coexisting in bovine brain sphingomyelin (BB-SM) or BB-SM/spin-labeled phosphatidylcholine (PC) bilayers or 2), between liquid-disordered and liquid-ordered domains in BB-SM/spin-labeled PC/cholesterol bilayers. The partitioning behavior was deduced either from modeling of pyrene excimer/monomer ratio versus temperature plots, or from quenching of the pyrene monomer fluorescence by spin-labeled PC. New methods were developed to model excimer formation and pyrene lipid quenching in segregated bilayers. The main result is that partition to either gel or liquid-ordered domains increased significantly with increasing length of the labeled acyl chain, probably because the pyrene moiety attached to a long chain perturbs these ordered domains less. Differences in partitioning were also observed between phosphatidylcholine, sphingomyelin, and galactosylceramide, thus indicating that the lipid backbone and headgroup-specific properties are not severely masked by the pyrene moiety. We conclude that pyrene-labeled lipids could be valuable tools when monitoring domain formation in model and biological membranes as well as when assessing the role of membrane domains in lipid trafficking and sorting.     

Permeabilization of raft-containing lipid vesicles by delta-lysin: a mechanism for cell sensitivity to cytotoxic peptides

Delta-lysin is a linear, 26-residue peptide that adopts an alpha-helical, amphipathic structure upon binding to membranes. Delta-lysin preferentially binds to mammalian cell membranes, the outer leaflets of which are enriched in sphingomyelin, cholesterol, and unsaturated phosphatidylcholine. Mixtures including these lipids have been shown to exhibit separation between liquid-disordered (l(d)) and liquid-ordered (l(o)) domains. When rich in sphingomyelin and cholesterol, these ordered domains have been called lipid "rafts". We found that delta-lysin binds poorly to the l(o) (raft) domains; therefore, in mixed-phase lipid vesicles, delta-lysin preferentially binds to the l(d) domains. This leads to the concentration of delta-lysin in l(d) domains, enhancing peptide aggregation and, consequently, the rate of peptide-induced dye efflux from lipid vesicles. The efficient lysis of eukaryotic cells by delta-lysin can thus be attributed not to specific delta-lysin-cholesterol or delta-lysin-sphingomyelin interactions but, rather, to the exclusion of delta-lysin from ordered rafts. The degree to which the kinetics of dye efflux are enhanced in mixed-phase vesicles over those observed in pure, unsaturated phosphatidylcholine vesicles directly reflects the amount of l(d) phase present in mixed-phase systems. This effect of lipid domains has broader consequences, beyond the hemolytic efficiency of delta-lysin. We discuss the hypothesis that bacterial sensitivity to antimicrobial peptides may be determined by a similar mechanism.     

Cholesterol does not induce segregation of liquid-ordered domains in bilayers modeling the inner leaflet of the plasma membrane

A fluorescence-quenching method has been used to assess the potential formation of segregated liquid-ordered domains in lipid bilayers combining cholesterol with mixtures of amino and choline phospholipids like those found in the cytoplasmic leaflet of the mammalian cell plasma membrane. When present in proportions >20-30 mol %, different saturated phospholipids show a strong proclivity to form segregated domains when combined with unsaturated phospholipids and cholesterol, in a manner that is only weakly affected by the nature of the phospholipid headgroups. By contrast, mixtures containing purely unsaturated phospholipids and cholesterol do not exhibit detectable segregation of domains, even in systems whose components differ in headgroup structure, mono- versus polyunsaturation and/or acyl chain heterogeneity. These results indicate that mixtures of phospholipids resembling those found in the inner leaflet of the plasma membrane do not spontaneously form segregated liquid-ordered domains. Instead, our findings suggest that factors extrinsic to the inner-monolayer lipids themselves (e.g., transbilayer penetration of long sphingolipid acyl chains) would be essential to confer a distinctive, more highly ordered organization to the cytoplasmic leaflet of "lipid raft" structures in animal cell membranes.     

Role of cholesterol in lipid raft formation: lessons from lipid model systems

Biochemical and cell-biological experiments have identified cholesterol as an important component of lipid 'rafts' and related structures (e.g., caveolae) in mammalian cell membranes, and membrane cholesterol levels as a key factor in determining raft stability and organization. Studies using cholesterol-containing bilayers as model systems have provided important insights into the roles that cholesterol plays in determining lipid raft behavior. This review will discuss recent progress in understanding two aspects of lipid-cholesterol interactions that are particularly relevant to understanding the formation and properties of lipid rafts. First, we will consider evidence that cholesterol interacts differentially with different membrane lipids, associating particularly strongly with saturated, high-melting phospho- and sphingolipids and particularly weakly with highly unsaturated lipid species. Second, we will review recent progress in reconstituting and directly observing segregated raft-like (liquid-ordered) domains in model membranes that mimic the lipid compositions of natural membranes incorporating raft domains.     

Role of cholesterol in the formation and nature of lipid rafts in planar and spherical model membranes

Sterols play a crucial regulatory and structural role in the lateral organization of eukaryotic cell membranes. Cholesterol has been connected to the possible formation of ordered lipid domains (rafts) in mammalian cell membranes. Lipid rafts are composed of lipids in the liquid-ordered (l(o)) phase and are surrounded with lipids in the liquid-disordered (l(d)) phase. Cholesterol and sphingomyelin are thought to be the principal components of lipid rafts in cell and model membranes. We have used fluorescence microscopy and fluorescence recovery after photobleaching in planar supported lipid bilayers composed of porcine brain phosphatidylcholine (bPC), porcine brain sphingomyelin (bSM), and cholesterol to map the composition-dependence of l(d)/l(o) phase coexistence. Cholesterol decreases the fluidity of bPC bilayers, but disrupts the highly ordered gel phase of bSM, leading to a more fluid membrane. When mixed with bPC/bSM (1:1) or bPC/bSM (2:1), cholesterol induces the formation of l(o) phase domains. The fraction of the membrane in the l(o) phase was found to be directly proportional to the cholesterol concentration in both phospholipid mixtures, which implies that a significant fraction of bPC cosegregates into l(o) phase domains. Images reveal a percolation threshold, i.e., the point where rafts become connected and fluid domains disconnected, when 45-50% of the total membrane is converted to the l(o) phase. This happens between 20 and 25 mol % cholesterol in 1:1 bPC/bSM bilayers and between 25 and 30 mol % cholesterol in 2:1 bPC/bSM bilayers at room temperature, and at approximately 35 mol % cholesterol in 1:1 bPC/bSM bilayers at 37 degrees C. Area fractions of l(o) phase lipids obtained in multilamellar liposomes by a fluorescence resonance energy transfer method confirm and support the results obtained in planar lipid bilayers.     

Sphingolipid partitioning into ordered domains in cholesterol-free and cholesterol-containing lipid bilayers

We have used fluorescence-quenching measurements to characterize the partitioning of a variety of indolyl-labeled phospho- and sphingolipids between gel or liquid-ordered and liquid-disordered lipid domains in several types of lipid bilayers where such domains coexist. In both cholesterol-free and cholesterol-containing lipid mixtures, sphingolipids with diverse polar headgroups (ranging from sphingomyelin and monoglycosylceramides to ganglioside GM1) show a net preference for partitioning into ordered domains, which varies modestly in magnitude with varying headgroup structure. The affinities of different sphingolipids for ordered lipid domains do not vary in a consistent manner with the size or other simple structural properties of the polar headgroup, such that for example ganglioside GM1 partitions between ordered and disordered lipid domains in a manner very similar to sphingomyelin. Ceramide exhibits a dramatically higher affinity for ordered lipid domains in both cholesterol-free and cholesterol-containing bilayers than do other sphingolipids. Our findings suggest that sphingolipids with a variety of headgroup structures will be enriched by substantial factors in liquid-ordered versus liquid-disordered regions of membranes, in a manner that is only modestly dependent on the nature of the polar headgroup. Ceramide is predicted to show a very strong enrichment in such domains, supporting previous suggestions that ceramide-mediated signaling may be compartmentalized to liquid-ordered (raft and raft-related) domains in the plasma membrane.     

Lipid lateral diffusion and membrane heterogeneity

The pulsed field gradient (pfg)-NMR method for measurements of translational diffusion of molecules in macroscopically aligned lipid bilayers is described. This technique is proposed to have an appreciable potential for investigations in the field of lipid and membrane biology. Transport of molecules in the plane of the bilayer can be successfully studied, as well as lateral phase separation of lipids and their dynamics within the bilayer organizations. Lateral diffusion coefficients depend on lipid packing and acyl chain ordering and investigations of order parameters of perdeuterated acyl chains, using (2)H NMR quadrupole splittings, are useful complements. In this review we summarize some of our recent achievements obtained on lipid membranes. In particular, bilayers exhibiting two-phase coexistence of liquid disordered (l(d)) and liquid ordered (l(o)) phases are considered in detail. Methods for obtaining good oriented lipid bilayers, necessary for the pfg-NMR method to be efficiently used, are also briefly described. Among our major results, besides determinations of l(d) and l(o) phases, belongs the finding that the lateral diffusion is the same for all components, independent of the molecular structure (including cholesterol (CHOL)), if they reside in the same domain or phase in the membrane. Furthermore, quite unexpectedly CHOL seems to partition into the l(d)and l(o) phases to roughly the same extent, indicating that CHOL has no strong preference for any of these phases, i.e. CHOL seems to have similar interactions with all of the lipids. We propose that the lateral phase separation in bilayers containing one high-T(m) and one low-T(m) lipid together with CHOL is driven by the increasing difficulty of incorporating an unsaturated or prenyl lipid into the highly ordered bilayer formed by a saturated lipid and CHOL, i.e. the phase transition is entropy driven to keep the disorder of the hydrocarbon chains of the unsaturated lipid.     

Role of cholesterol in lipid raft formation: lessons from lipid model systems

Biochemical and cell-biological experiments have identified cholesterol as an important component of lipid ‘rafts’ and related structures (e.g., caveolae) in mammalian cell membranes, and membrane cholesterol levels as a key factor in determining raft stability and organization. Studies using cholesterol-containing bilayers as model systems have provided important insights into the roles that cholesterol plays in determining lipid raft behavior. This review will discuss recent progress in understanding two aspects of lipid-cholesterol interactions that are particularly relevant to understanding the formation and properties of lipid rafts. First, we will consider evidence that cholesterol interacts differentially with different membrane lipids, associating particularly strongly with saturated, high-melting phospho- and sphingolipids and particularly weakly with highly unsaturated lipid species. Second, we will review recent progress in reconstituting and directly observing segregated raft-like (liquid-ordered) domains in model membranes that mimic the lipid compositions of natural membranes incorporating raft domains.     

The fluorescent cholesterol analog dehydroergosterol induces liquid-ordered domains in model membranes

The fluorescent sterol dehydroergosterol (DHE) is often used as a marker for cholesterol in cellular studies. We show by vesicle fluctuation analysis that DHE has a lower ability than cholesterol to stiffen lipid bilayers suggesting less efficient packing with phospholipid acyl chains. Despite this difference, we found by fluorescence and atomic force microscopy, that DHE induces liquid-ordered/-disordered coexistent domains in giant unilamellar vesicles (GUVs) and supported bilayers made of dipalmitoylphosphatidylcholine (DPPC), dioleylphosphatidylcholine (DOPC) and DHE or cholesterol. DHE-induced phases have a height difference of 0.9-1 nm similar as known for cholesterol-containing domains. DHE not only promotes formation of liquid-liquid immiscibility but also shows strong partition preference for the liquid-ordered phase further supporting its suitability as cholesterol probe.     

Use of cyclodextrin for AFM monitoring of model raft formation

The lipid rafts membrane microdomains, enriched in sphingolipids and cholesterol, are implicated in numerous functions of biological membranes. Using atomic force microscopy, we have examined the effects of cholesterol-loaded methyl-beta-cyclodextrin (MbetaCD-Chl) addition to liquid disordered (l(d))-gel phase separated dioleoylphosphatidylcholine (DOPC)/sphingomyelin (SM) and 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC)/SM supported bilayers. We observed that incubation with MbetaCD-Chl led to the disappearance of domains with the formation of a homogeneously flat bilayer, most likely in the liquid-ordered (l(o)) state. However, intermediate stages differed with the passage through the coexistence of l(o)-l(d) phases for DOPC/SM samples and of l(o)-gel phases for POPC/SM bilayers. Thus, gel phase SM domains surrounded by a l(o) matrix rich in cholesterol and POPC could be observed just before reaching the uniform l(o) state. This suggests that raft formation in biological membranes could occur not only via liquid-liquid but also via gel-liquid immiscibility. The data also demonstrate that MbetaCD-Chl as well as the unloaded cyclodextrin MbetaCD make holes and preferentially extract SM in supported bilayers. This strongly suggests that interpretation of MbetaCD and MbetaCD-Chl effects on cell membranes only in terms of cholesterol movements have to be treated with caution.     

Lipid tails modulate antimicrobial peptide membrane incorporation and activity

Membrane disrupting antimicrobial peptides (AMPs) are often amphipathic peptides that interact directly with lipid bilayers. AMPs are generally thought to interact mostly with lipid head groups, but it is less clear how the lipid alkyl chain length and saturation modulate interactions with membranes. Here, we used native mass spectrometry to measure the stoichiometry of three different AMPs—LL-37, indolicidin, and magainin-2—in lipid nanodiscs. We also measured the activity of these AMPs in unilamellar vesicle leakage assays. We found that LL-37 formed specific hexamer complexes but with different intermediates and affinities that depended on the bilayer thickness. LL-37 was also most active in lipid bilayers containing longer, unsaturated lipids. In contrast, indolicidin incorporated to a higher degree into more fluid lipid bilayers but was more active with bilayers with thinner, less fluid lipids. Finally, magainin-2 incorporated to a higher degree into bilayers with longer, unsaturated alkyl chains and showed more activity in these same conditions. Together, these data show that higher amounts of peptide incorporation generally led to higher activity and that AMPs tend to incorporate more into longer unsaturated lipid bilayers. However, the activity of AMPs was not always directly related to amount of peptide incorporated.     

Bilayer polarity and its thermal dependency in the l(o) and l(d) phases of binary phosphatidylcholine/cholesterol mixtures

Diverse variations in membrane properties are observed in binary phosphatidylcholine/cholesterol mixtures. These mixtures are nonideal, displaying single or phase coexistence, depending on chemical composition and other thermodynamic parameters. When compared with pure phospholipid bilayers, there are changes in water permeability, bilayer thickness and thermomechanical properties, molecular packing and conformational freedom of phospholipid acyl chains, in internal dipolar potential and in lipid lateral diffusion. Based on the phase diagrams for DMPC/cholesterol and DPPC/cholesterol, we compare the equivalent polarity of pure bilayers with specific compositions of these mixtures, by using the Py empirical scale of polarity. Besides the contrast between pure and mixed lipid bilayers, we find that liquid-ordered (l(o)) and liquid-disordered (l(d)) phases display significantly different polarities. Moreover, in the l(o) phase, the polarities of bilayers and their thermal dependences vary with the chemical composition, showing noteworthy differences for cholesterol proportions at 35, 40, and 45 mol%. At 20 degrees C, for DMPC/cholesterol at 35 and 45 mol%, the equivalent dielectric constants are 21.8 and 23.8, respectively. Additionally, we illustrate potential implications of polarity in various membrane-based processes and reactions, proposing that for cholesterol containing bilayers, it may also go along with the occurrence of lateral heterogeneity in biological membranes.     

The effects of temperature, pressure and peptide incorporation on ternary model raft mixtures--a Laurdan fluorescence spectroscopy study

Recently, an increasing evidence accumulated for the existence of lipid microdomains, called lipid rafts, in cell membranes, which may play an important role in many important membrane-associated biological processes. Suitable model systems for studying biophysical properties of lipid rafts are lipid vesicles composed of three-component lipid mixtures, such as POPC/SM/cholesterol, which exhibit a rich phase diagram, including raft-like liquid-ordered/liquid-disordered phase coexistence regions. We explored the temperature, pressure and concentration-dependent phase behavior of such canonical model raft mixtures using the Laurdan fluorescence spectroscopic technique. Hydrostatic pressure has not only been used as a physical parameter for studying the stability and energetics of these systems, but also because high pressure is an important feature of certain natural membrane environments. We show that the liquid-disordered/liquid-ordered phase coexistence regions of POPC/SM/cholesterol model raft mixtures extends over a very wide temperature range of about 50 degrees C. Upon pressurization, an overall ordered membrane state is reached at pressures of approximately 1,000 bar at 20 degrees C, and of approximately 2,000 bar at 40 degrees C. Incorporation of 5 mol% gramicidin as a model ion channel slightly increases the overall order parameter profile in the l(o)+l(d) two-phase coexistence region, probably by selectively partitioning into l(d) domains, does not change the overall phase behavior, however. This behavior is in contrast to the effect of the peptide incorporation into simple, one-component phospholipid bilayer systems.     

Effect of cholesterol on molecular order and dynamics in highly polyunsaturated phospholipid bilayers.

The effect of cholesterol on phospholipid acyl chain packing in bilayers consisting of highly unsaturated acyl chains in the liquid crystalline phase was examined for a series of symmetrically and asymmetrically substituted phosphatidylcholines (PCs). The time-resolved fluorescence emission and decay of fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to characterize equilibrium and dynamic structural properties of bilayers containing 30 mol % cholesterol. The bilayers were composed of symmetrically substituted PCs with acyl chains of 14:0, 18:1n9, 20:4n6, or 22:6n3, containing 0, 1, 4, or 6 double bonds, respectively, and mixed-chain PCs with a saturated 16:0 sn-1 chain and 1, 4, or 6 double bonds in the sn-2 chain. DPH excited-state lifetime was fit to a Lorentzian lifetime distribution, the center of which was increased 1-2 ns by 30 mol % cholesterol relative to the cholesterol-free bilayers. Lifetime distributions were dramatically narrowed by the addition of cholesterol in all bilayers except the two consisting of dipolyunsaturated PCs. DPH anisotropy decay was interpreted in terms of the Brownian rotational diffusion model. The effect of cholesterol on both the perpendicular diffusion coefficient D perpendicular and the orientational distribution function f(theta) varied with acyl chain unsaturation. In all bilayers, except the two dipolyunsaturated PCs, 30 mol % cholesterol dramatically slowed DPH rotational motion and restricted DPH orientational freedom. The effect of cholesterol was especially diminished in di-22:6n3 PC, suggesting that this phospholipid may be particularly effective at promoting lateral domains, which are cholesterol-rich and unsaturation-rich, respectively. The results are discussed in terms of a model for lipid packing in membranes containing cholesterol and PCs with highly unsaturated acyl chains.     

How the molecular features of glycosphingolipids affect domain formation in fluid membranes

Glycosphingolipids, sphingomyelin and cholesterol are often all found in the detergent resistant fraction of biological membranes and are therefore recognized as raft components, but they do not necessarily co-localize in the same lateral domains. From cell biological studies it is evident that different sphingolipid species can be found in different lateral regions within the same cellular membrane. Biophysical studies have shown that their tendency to co-localize with each other and with other membrane components is largely governed by structural features of all lipids present. Glycosphingolipids form gel-phase like domains in fluid lipid bilayers. Sphingomyelin readily associates with cholesterol, forming liquid-ordered phase domains, but glycosphingolipids do not readily form cholesterol-enriched domains by themselves. However, mixed sphingomyelin- and glycosphingolipid-rich domains appear to incorporate cholesterol. Recent studies indicate that the ceramide backbone structure as well as the number of sugar units and presence of charge in the glycosphingolipid head group will influence the partitioning of these lipids between lateral membrane domains. The properties of the domains will be largely influenced by the presence of glycosphingolipids, which have very high melting temperatures. The lateral partitioning of glycosphingolipid molecular species has only recently been studied more intensively, and a lot remains to be done in this field of research.     

Transmembrane Peptides Influence the Affinity of Sterols for Phospholipid Bilayers

Cholesterol is distributed unevenly between different cellular membrane compartments, and the cholesterol content increases from the inner bilayers toward the plasma membrane. It has been suggested that this cholesterol gradient is important in the sorting of transmembrane proteins. Cholesterol has also been to shown play an important role in lateral organization of eukaryotic cell membranes. In this study the aim was to determine how transmembrane proteins influence the lateral distribution of cholesterol in phospholipid bilayers. Insight into this can be obtained by studying how cholesterol interacts with bilayer membranes of different composition in the presence of designed peptides that mimic the transmembrane helices of proteins. For this purpose we developed an assay in which the partitioning of the fluorescent cholesterol analog CTL between LUVs and mβCD can be measured. Comparison of how cholesterol and CTL partitioning between mβCD and phospholipid bilayers with different composition suggests that CTL sensed changes in bilayer composition similarly as cholesterol. Therefore, the results obtained with CTL can be used to understand cholesterol distribution in lipid bilayers. The effect of WALP23 on CTL partitioning between DMPC bilayers and mβCD was measured. From the results it was clear that WALP23 increased both the order in the bilayers (as seen from CTL and DPH anisotropy) and the affinity of the sterol for the bilayer in a concentration dependent way. Although WALP23 also increased the order in DLPC and POPC bilayers the effects on CTL partitioning was much smaller with these lipids. This indicates that proteins have the largest effect on sterol interactions with phospholipids that have longer and saturated acyl chains. KALP23 did not significantly affect the acyl chain order in the phospholipid bilayers, and inclusion of KALP23 into DMPC bilayers slightly decreased CTL partitioning into the bilayer. This shows that transmembrane proteins can both decrease and increase the affinity of sterols for the lipid bilayers surrounding proteins. This is likely to affect the sterol distribution within the bilayer and thereby the lateral organization in biomembranes.     

Probing the lipid-protein interface using model transmembrane peptides with a covalently linked acyl chain

The aim of this study was to gain insight into how interactions between proteins and lipids in membranes are sensed at the protein-lipid interface. As a probe to analyze this interface, we used deuterium-labeled acyl chains that were covalently linked to a model transmembrane peptide. First, a perdeuterated palmitoyl chain was coupled to the Trp-flanked peptide WALP23 (Ac-CGWW(LA)₈LWWA-NH₂), and the deuterium NMR spectrum was analyzed in di-C18:1-phosphatidylcholine (PC) bilayers. We found that the chain order of this peptide-linked chain is rather similar to that of a noncovalently coupled perdeuterated palmitoyl chain, except that it exhibits a slightly lower order. Similar results were obtained when site-specific deuterium labels were used and when the palmitoyl chain was attached to the more-hydrophobic model peptide WLP23 (Ac-CGWWL₁₇WWA-NH₂) or to the Lys-flanked peptide KALP23 (Ac-CGKK(LA)₈LKKA-NH₂). The experiments showed that the order of both the peptide-linked chains and the noncovalently coupled palmitoyl chains in the phospholipid bilayer increases in the order KALP23 < WALP23 < WLP23. Furthermore, changes in the bulk lipid bilayer thickness caused by varying the lipid composition from di-C14:1-PC to di-C18:1-PC or by including cholesterol were sensed rather similarly by the covalently coupled chain and the noncovalently coupled palmitoyl chains. The results indicate that the properties of lipids adjacent to transmembrane peptides mostly reflect the properties of the surrounding lipid bilayer, and hence that (at least for the single-span model peptides used in this study) annular lipids do not play a highly specific role in protein-lipid interactions.     

Role of ceramide in membrane protein organization investigated by combined AFM and FCS

Ceramide-induced alterations in the lateral organization of membrane proteins can be involved in several biological contexts, ranging from apoptosis to viral infections. In order to investigate such alterations in a simple model, we used a combined approach of atomic force microscopy, scanning fluorescence correlation spectroscopy and confocal fluorescence imaging to study the partitioning of different membrane components in sphingomyelin/dioleoyl-phosphatidylcholine/cholesterol/ceramide supported bilayers. Such model membranes exhibit coexistence of liquid-disordered, liquid-ordered (raft-like) and ceramide-rich lipid phases. Our results show that components with poor affinity toward the liquid-ordered phase, such as several fluorescent lipid analogues or the synaptic protein Synaptobrevin 2, are excluded from ceramide-rich domains. Conversely, we show for the first time that the raft-associated protein placental alkaline phosphatase (GPI-PLAP) and the ganglioside GM1 are enriched in such domains, while exhibiting a strong decrease in lateral diffusion. Analogue modulation of the local concentration and dynamics of membrane proteins/receptors by ceramide can be of crucial importance for the biological functions of cell membranes.