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.     

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.     

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 G_M1 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.     

Domain formation in model membranes studied by pulsed-field gradient-NMR: the role of lipid polyunsaturation

The effects of increased unsaturation in the sn-2 fatty acyl chain of phosphatidylcholines (PCs) on the lipid lateral diffusion have been investigated by pulsed-field gradient NMR. Macroscopically oriented bilayers containing a monosaturated PC, egg sphingomyelin, and cholesterol (CHOL) have been studied at temperatures between 0 degrees C and 60 degrees C, and the number of double bonds in the PC was one, two, four, or six. For PC bilayers, with and without the incorporation of egg sphingomyelin and CHOL, the lateral diffusion increased with increasing number of double bonds, as a consequence of the increased headgroup area caused by the unsaturation. Addition of CHOL caused a decrease in lipid diffusion due to the condensing effect of CHOL on the headgroup area. Phase separation into large domains of liquid-disordered and liquid-ordered phases were observed in the ternary systems with PCs containing four and six double bonds, as evidenced by the occurrence of two lipid diffusion coefficients. PC bilayers with one or two double bonds appear homogeneous on the length scales probed by the experiment, but the temperature dependence of the diffusion suggests that small domains may be present also in these ternary systems.     

The size of lipid rafts: an atomic force microscopy study of ganglioside GM1 domains in sphingomyelin/DOPC/cholesterol membranes

Atomic force microscopy has been used to study the distribution of ganglioside GM1 in model membranes composed of ternary lipid mixtures that mimic the composition of lipid rafts. The results demonstrate that addition of 1% GM1 to 1:1:1 sphingomyelin/dioleoylphosphatidylcholine/cholesterol monolayers leads to the formation of small ganglioside-rich microdomains (40-100 nm in size) that are localized preferentially in the more ordered sphingomyelin/cholesterol-rich phase. With 5% GM1 some GM1 microdomains are also detected in the dioleoylphosphatidylcholine-rich phase. A similar preferential localization of GM1 in the ordered phase is observed for bilayers with the same ternary lipid mixture in the upper leaflet. The small GM1-rich domains observed in these experiments are similar to the sizes for lipid rafts in natural membranes but considerably smaller than the ordered bilayer domains that have been shown to be enriched in GM1 in recent fluorescence microscopy studies of lipid bilayers. The combined data from a number of studies of model membranes indicate that lateral organization occurs on a variety of length scales and mimics many of the properties of natural membranes.     

Membrane microdomains: Role of ceramides in the maintenance of their structure and functions

Free-standing giant unilamellar vesicles were used to visualize the complex lateral heterogeneity, induced by ceramide in the membrane bilayer at micron scale using C₁₂-NBD-PC probe partitioning under the fluorescence microscope. Ceramide gel domains exist as leaf-like structures in glycerophospholipid/ceramide mixtures. Cholesterol readily increases ceramide miscibility with glycerophospholipids but cholesterol-ceramide interactions are not involved in the organization of the liquid-ordered phase as exemplified by sphingomyelin/cholesterol mixtures. Sphingomyelin stabilizes the gel phase and thus decreases ceramide miscibility in the presence of cholesterol. Gel/liquid-ordered/liquid-disordered phase coexistence was visualized in quaternary phosphatidylcholine/sphingomyelin/ceramide/cholesterol mixtures as occurrence of dark leaf-like and circular domains within a bright liquid phase. Sphingomyelin initiates specific ceramide-sphingomyelin interactions to form a highly ordered gel phase appearing at temperatures higher than pure ceramide gel phase in phosphatidylcholine/ceramide mixtures. Less sphingomyelin is engaged in formation of liquid-ordered phase leading to a shift in its formation to lower temperatures. Sphingomyelinase activity on substrate vesicles destroys micron L_o domains but induces the formation of a gel-like phase. The activation of phospholipase A₂ by ceramide on heterogeneous membranes was visualized. Changes in the phase state of the membrane bilayer initiates such morphological processes as membrane fragmentation, budding in and budding out was demonstrated.     

HIV-1 antibodies and vaccine antigen selectively interact with lipid domains

The rare, broadly neutralizing antibodies, 4E10 and 2F5, that target the HIV-1 membrane proximal external region also associate with HIV-1 membrane lipids as part of a required first-step in HIV-1 neutralization. HIV-1 virions have high concentration of cholesterol and sphingomyelin, which are able to organize into liquid-ordered domains (i.e., lipid rafts), and could influence the interaction of neutralizing antibodies with epitopes proximal to the membrane. The objective of this research is to understand how these lipid domains contribute to 2F5/4E10 membrane interactions and to antigen presentation in liposomal form of HIV-1 vaccines. To this end we have engineered biomimetic supported lipid bilayers and are able to use atomic force microscopy to visualize membrane domains, antigen clustering, and antibody–membrane interactions. Our results demonstrate that 2F5/4E10 do not interact with highly ordered gel and liquid-ordered domains and exclusively bind to a liquid-disordered lipid phase. This suggests that vaccine liposomes that contain key viral membrane components, such as high cholesterol content, may not be advantageous for 2F5/4E10 vaccine strategies. Rather, vaccine liposomes that primarily contain a liquid-disordered phase may be more likely to elicit production of lipid reactive, 2F5- and 4E10-like antibodies.     

NBD-cholesterol probes to track cholesterol distribution in model membranes

A series of cholesterol (Chol) probes with NBD and Dansyl fluorophores attached to the 3-hydroxyl position via carbamate linkers has been designed and synthesized and their ability to mimic the behavior of natural cholesterol in bilayer membranes has been examined. Fluorescence spectroscopy data indicate that the NBD-labeled lipids are located in the polar headgroup region of the bilayer with their position varying with the method of fluorophore attachment and the linker length. The partitioning of the Chol probes between liquid-ordered (L_o) and liquid-disordered (L_o) phases in supported bilayers prepared from ternary lipid mixtures of DOPC, Chol and either egg sphingomyelin or DPPC was examined by fluorescence microscopy. The carbamate-linked NBD-Chols show a stronger preference for partitioning into L_o domains than does a structurally similar probe with an ester linkage, indicating the importance of careful optimization of probe and linker to provide the best Chol mimic. Comparison of the partitioning of NBD probes to literature data for native Chol indicates that the probes reproduce well the modest enrichment of Chol in L_o domains as well as the ceramide-induced displacement of Chol. One NBD probe was used to follow the dynamic redistribution of Chol in phase separated membranes in response to in situ ceramide generation. This provides the first direct optical visualization of Chol redistribution during enzymatic ceramide generation and allows the assignment of new bilayer regions that exclude dye and have high lateral adhesion to ceramide-rich regions.     

Partitioning of lipidated peptide sequences into liquid-ordered lipid domains in model and biological membranes.

We have used a fluorescence assay and detergent fractionation to examine the partitioning of different fluorescent lipidated peptides, with sequences and lipid substituents matching those found in various classes of lipidated cellular proteins, into liquid-ordered (raft-like) domains in lipid bilayers. Peptides incorporating isoprenyl groups, or multiple unsaturated acyl chains, show negligible affinity for liquid-ordered domains in mixed-phase liquid-ordered/liquid-disordered (l(o)/l(d)) bilayers composed of dipalmitoylphosphatidylcholine, a spin-labeled unsaturated phosphatidylcholine, and cholesterol. By contrast, peptides incorporating multiple S- and/or N-acyl chains, or a cholesterol residue plus an N-terminal palmitoyl chain, show significant partitioning into liquid-ordered domains under the same conditions. Interestingly, the affinity of a lipidated peptide for l(o) domains can be strongly influenced, not only by the structures of the lipid substituents but also by the nature and the positions of their attachment to the peptide chain. These results are well correlated with those obtained from parallel assays based on low-temperature detergent fractionation. Using the latter approach, we further demonstrate that a truly minimal l(o) domain partitioning motif [myristoylGlyCys(palmitoyl)-] can mediate efficient incorporation into the "raft" fraction of COS-7 cell membranes.     

An X-ray diffraction study of model membrane raft structures

Protein sorting and assembly in membrane biogenesis and function involves the creation of ordered domains of lipids known as membrane rafts. The rafts are comprised of all the major classes of lipids, including glycerophospholipids, sphingolipids and sterol. Cholesterol is known to interact with sphingomyelin to form a liquid-ordered bilayer phase. Domains formed by sphingomyelin and cholesterol, however, represent relatively small proportions of the lipids found in membrane rafts and the properties of other raft lipids are not well characterized. We examined the structure of lipid bilayers comprised of aqueous dispersions of ternary mixtures of phosphatidylcholines and sphingomyelins from tissue extracts and cholesterol using synchrotron X-ray powder diffraction methods. Analysis of the Bragg reflections using peak-fitting methods enables the distinction of three coexisting bilayer structures: (a) a quasicrystalline structure comprised of equimolar proportions of phosphatidylcholine and sphingomyelin, (b) a liquid-ordered bilayer of phospholipid and cholesterol, and (c) fluid phospholipid bilayers. The structures have been assigned on the basis of lamellar repeat spacings, relative scattering intensities and bilayer thickness of binary and ternary lipid mixtures of varying composition subjected to thermal scans between 20 and 50 °C. The results suggest that the order created by the quasicrystalline phase may provide an appropriate scaffold for the organization and assembly of raft proteins on both sides of the membrane. Co-existing liquid-ordered structures comprised of phospholipid and cholesterol provides an additional membrane environment for assembly of different raft proteins.     

Kinetics and thermodynamics of association of a phospholipid derivative with lipid bilayers in liquid-disordered and liquid-ordered phases

We have measured the rates of insertion into, desorption from, and spontaneous interlayer translocation (flip-flop) in liquid-disordered and liquid-ordered phase lipid bilayer membranes, of the fluorescent phospholipid derivative NBD-dimyristoylphosphatidyl ethanolamine. This study made use of a recently described method that exploits a detailed knowledge of the binding kinetics of an amphiphile to bovine serum albumin, to recover the insertion and desorption rate constants when the albumin-bound amphiphile is transferred through the aqueous phase to the membrane and vice versa. The lipid bilayers, studied as large unilamellar vesicles, were prepared from pure 1-palmitoyl-2-oleoylphosphatidylcholine in the liquid-disordered phase; and from two cholesterol-containing binary lipid mixtures, 1-palmitoyl-2-oleoylphosphatidylcholine and cholesterol (molar ratio of 1:1), and egg sphingomyelin and cholesterol (molar ratio of 6:4), both in the liquid-ordered phase. Insertion, desorption, and translocation rate constants and equilibrium constants for association of the amphiphile monomer with the lipid bilayers were directly measured between 15 degrees and 35 degrees C, and the standard free energies, enthalpies, and entropies, as well as the activation energies for these processes, were derived from this data. The equilibrium partition coefficients for partitioning of the amphiphile between the aqueous phase and the different membrane phases were also derived, and permitted the estimation of hypothetical partition coefficients and the respective energetic parameters for partitioning between the different lipid phases if these were to coexist in the same membrane.     

Association of ethanol with lipid membranes containing cholesterol, sphingomyelin and ganglioside: a titration calorimetry study.

The association of ethanol at physiologically relevant concentrations with lipid bilayers of different lipid composition has been investigated by use of isothermal titration calorimetry (ITC). The liposomes examined were composed of combinations of lipids commonly found in neural cell membranes: dimyristoyl phosphatidylcholine (DMPC), ganglioside (GM(1)), sphingomyelin and cholesterol. The calorimetric results show that the interaction of ethanol with fluid lipid bilayers is endothermic and strongly dependent on the lipid composition of the liposomes. The data have been used to estimate partitioning coefficients for ethanol into the fluid lipid bilayer phase and the results are discussed in terms of the thermodynamics of partitioning. The presence of 10 mol% sphingomyelin or ganglioside in DMPC liposomes enhances the partitioning coefficient by a factor of 3. Correspondingly, cholesterol (30 mol%) reduces the partitioning coefficient by a factor of 3. This connection between lipid composition and partitioning coefficient correlates with in vivo observations. Comparison of the data with the molecular structure of the lipid molecules suggests that ethanol partitioning is highly sensitive to changes in the lipid backbone (glycerol or ceramide) while it appears much less sensitive to the nature of the head group.     

Imaging the Lipid-Phase-Dependent Pore Formation of Equinatoxin II in Droplet Interface Bilayers

Using phase-separated droplet interface bilayers, we observe membrane binding and pore formation of a eukaryotic cytolysin, Equinatoxin II (EqtII). EqtII activity is known to depend on the presence of sphingomyelin in the target membrane and is enhanced by lipid phase separation. By imaging the ionic flux through individual pores in vitro, we observe that EqtII pores form predominantly within the liquid-disordered phase. We observe preferential binding of labeled EqtII at liquid-ordered/liquid-disordered domain boundaries before it accumulates in the liquid-disordered phase.     

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.     

Sorting of streptavidin protein coats on phase-separating model membranes

Heterogeneities in cell membranes due to the ordering of lipids and proteins are thought to play an important role in enabling protein and lipid trafficking throughout the secretory pathway and in maintaining cell polarization. Protein-coated vesicles provide a major mechanism for intracellular transport of select cargo, which may be sorted into lipid microdomains; however, the mechanisms and physical constraints for lipid sorting by protein coats are relatively unexplored. We studied the influence of membrane-tethered protein coats on the sorting, morphology, and phase behavior of liquid-ordered lipid domains in a model system of giant unilamellar vesicles composed of dioleoylphosphatidylcholine, sphingomyelin, and cholesterol. We created protein-coated membranes by forming giant unilamellar vesicles containing a small amount of biotinylated lipid, thereby creating binding sites for streptavidin and avidin proteins in solution. We found that individual tethered proteins colocalize with the liquid-disordered phase, whereas ordered protein domains on the membrane surface colocalize with the liquid-ordered phase. These observations may be explained by considering the thermodynamics of this coupled system, which maximizes its entropy by cosegregating ordered protein and lipid domains. In addition, protein ordering inhibits lipid domain rearrangement and modifies the morphology and miscibility transition temperature of the membrane, most dramatically near the critical point in the membrane phase diagram. This observation suggests that liquid-ordered domains are stabilized by contact with ordered protein domains; it also hints at an approach to the stabilization of lipid microdomains by cross-linked protein clusters or ordered protein coats.     

Coupling of cholesterol-rich lipid phases in asymmetric bilayers

We showed previously that cholesterol-rich liquid-ordered domains with lipid compositions typically found in the outer leaflet of plasma membranes induce liquid-ordered domains in adjacent regions of asymmetric lipid bilayers with apposed leaflets composed of typical inner leaflet lipid mixtures [Kiessling, V., Crane, J. M., and Tamm, L. K. (2006) Biophys. J. 91, 3313-26]. To further examine the nature of transbilayer couplings in asymmetric cholesterol-rich lipid bilayers, the effects on the lipid phase behavior in asymmetric bilayers of different lipid compositions were investigated. We established systems containing several combinations of natural extracted and synthetic lipids that exhibited coexisting liquid-ordered (lo) and liquid-disordered (ld) domains in a supported bilayer format. We find that lo phase domains are induced in all quaternary inner leaflet combinations composed of PCs, PEs, PSs, and cholesterol. Ternary mixtures of PCs/PEs/Chol, PCs/PSs/Chol also exhibit lo phases adjacent to outer leaflet lo phases. However, with the exception of brain PC extracts, binary PC/Chol mixtures are not induced to form lo phases by adjacent outer leaflet lo phases. Higher melting lipid ad-mixtures of PEs and PSs are needed for lo phase induction in the inner leaflet. It appears that the phase behavior of the inner leaflet mixtures is dominated by the intrinsic chain melting temperatures of the lipid components, rather than by their specific headgroup classes. In addition, similar studies with synthetic, completely saturated lipids and cholesterol show that lipid oxidation is not a factor in the observed phase behavior.     

Complex biomembrane mimetics on the sub-nanometer scale

Biomimetic lipid vesicles are indispensable tools for gaining insight into the biophysics of cell physiology on the molecular level. The level of complexity of these model systems has steadily increased, and now spans from domain-forming lipid mixtures to asymmetric lipid bilayers. Here, we review recent progress in the development and application of elastic neutron and X-ray scattering techniques for studying these systems in situ and under physiologically relevant conditions on the nanometer to sub-nanometer length scales. In particular, we focus on: (1) structural details of coexisting liquid-ordered and liquid-disordered domains, including their thickness and lipid packing mismatch as a function of a size transition from nanoscopic to macroscopic domains; (2) membrane-mediated protein partitioning into lipid domains; (3) the role of the aqueous medium in tuning interactions between membranes and domains; and (4) leaflet-specific structure in asymmetric bilayers and passive lipid flip-flop.     

Thermodynamics of membrane domains

The concept of lipid rafts and the intense work toward their characterization in biological membranes has spurred a renewed interest in the understanding of domain formation, particularly in the case of cholesterol-containing membranes. The thermodynamic principles underlying formation of domains, rafts, or cholesterol/phospholipid complexes are reviewed here, along with recent work in model and biological membranes. A major motivation for this review was to present those concepts in a way appropriate for the broad readership that has been drawn to the field. Evidence from a number of different techniques points to the conclusion that lipid-lipid interactions are generally weak; therefore, in most cases, massive phase separations are not to be expected in membranes. On the contrary, small, dynamic lipid domains, possibly stabilized by proteins are the most likely outcome. The results on mixed lipid bilayers are used to discuss recent experiments in biological membranes. The clear indication is that proteins partition preferentially into fluid, disordered lipid domains, which is contrary to their localization in ordered, cholesterol/sphingomyelin rafts inferred from detergent extraction experiments on cell membranes. Globally, the evidence appears most consistent with a membrane model in which the majority of the lipid is in a liquid-ordered phase, with dispersed, small, liquid-disordered domains, where most proteins reside. Co-clustering of proteins and their concentration in some membrane areas may occur because of similar preferences for a particular domain but also because of simultaneous exclusion from other lipid phases. Specialized structures, such as caveolae, which contain high concentrations of cholesterol and caveolin are not necessarily similar to bulk liquid-ordered phase.     

Imaging the Lipid-Phase-Dependent Pore Formation of Equinatoxin II in Droplet Interface Bilayers

Using phase-separated droplet interface bilayers, we observe membrane binding and pore formation of a eukaryotic cytolysin, Equinatoxin II (EqtII). EqtII activity is known to depend on the presence of sphingomyelin in the target membrane and is enhanced by lipid phase separation. By imaging the ionic flux through individual pores in vitro, we observe that EqtII pores form predominantly within the liquid-disordered phase. We observe preferential binding of labeled EqtII at liquid-ordered/liquid-disordered domain boundaries before it accumulates in the liquid-disordered phase.     

Probing lipid mobility of raft-exhibiting model membranes by fluorescence correlation spectroscopy

Confocal fluorescence microscopy and fluorescence correlation spectroscopy (FCS) have been employed to investigate the lipid spatial and dynamic organization in giant unilamellar vesicles (GUVs) prepared from ternary mixtures of dioleoyl-phosphatidylcholine/sphingomyelin/cholesterol. For a certain range of cholesterol concentration, formation of domains with raft-like properties was observed. Strikingly, the lipophilic probe 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI-C18) was excluded from sphingomyelin-enriched regions, where the raft marker ganglioside GM1 was localized. Cholesterol was shown to promote lipid segregation in dioleoyl-phosphatidylcholine-enriched, liquid-disordered, and sphingomyelin-enriched, liquid-ordered phases. Most importantly, the lipid mobility in sphingomyelin-enriched regions significantly increased by increasing the cholesterol concentration. These results pinpoint the key role, played by cholesterol in tuning lipid dynamics in membranes. At cholesterol concentrations >50 mol%, domains vanished and the lipid diffusion slowed down upon further addition of cholesterol. By taking the molecular diffusion coefficients as a fingerprint of membrane phase compositions, FCS is proven to evaluate domain lipid compositions. Moreover, FCS data from ternary and binary mixtures have been used to build a ternary phase diagram, which shows areas of phase coexistence, transition points, and, importantly, how lipid dynamics varies between and within phase regions.