Fluid mosaic model lipid rafts-Fluid mosaic model: cell membranes article (article) | Khan Academy

The plasma membranes of cells contain combinations of glycosphingolipids , cholesterol and protein receptors organised in glycolipoprotein lipid microdomains termed lipid rafts. It has been proposed that they are specialised membrane microdomains which compartmentalise cellular processes by serving as organising centers for the assembly of signaling molecules , allowing a closer interaction of protein receptors and their effectors to promote kinetically favorable interactions necessary for the signal transduction. One key difference between lipid rafts and the plasma membranes from which they are derived is lipid composition. Research has shown that lipid rafts contain 3 to 5-fold the amount of cholesterol found in the surrounding bilayer. To offset the elevated sphingolipid levels, phosphatidylcholine levels are decreased which results in similar choline -containing lipid levels between the rafts and the surrounding plasma membrane.

Baird, and F. Some of these proteins have homology to eukaryotic raft associated proteins and many are involved in Amateur fetish model signaling and transport processes. According to this Fluid mosaic model lipid rafts modelthere is a lipid bilayer two molecules thick layer in which protein molecules are embedded. An interesting possibility is that the differential targeting of G proteins to caveolae or lipid rafts could lead to a parallel agonist-induced segregation of the receptors that interact with those G proteins in the same Fluif. Bulk plasma membrane gray rafhs less cholesterol, sphingomyelin, and gangliosides, and more phospholipids with unsaturated acyl chains.

Mouse model alzheimers. BRIEF HISTORICAL OVERVIEW

Amsterdam: Elsevier; 1—63 C, Almeida P. The transverse structure is a noticeable feature of a lipid bilayer, and is far from that of an isotropic fluid slab of hydrocarbons. Lipid rafts ilpid been suggested to play a rzfts role in this process. The extraction would take advantage of lipid raft resistance to non-ionic detergentssuch as Triton X or Brij at low temperatures e. Stock for the critical reading of this manuscript. Trends in Cell Biology. These studies allow for the observations of effects on neurotransmitter signaling upon reduction of cholesterol levels. When Fluid mosaic model lipid rafts a detergent is added to cells, Fluid mosaic model lipid rafts fluid membrane will dissolve while the lipid rafts may remain intact and could be extracted. LAT activation is the source of signal amplification. Although not all of the phospholipids within the raft are fully saturated, the hydrophobic chains of the Sara nude fake contained in the rafts are more saturated and tightly packed than the surrounding bilayer. To offset the elevated sphingolipid levels, phosphatidylcholine levels are decreased which results in similar choline -containing lipid levels between the rafts and the surrounding plasma membrane. J Lipid Res. Lipid asymmetry in membranes. Sequestration using filipin, nystatin or amphotericindepletion and removal using methyl-B-cyclodextrin and inhibition of cholesterol synthesis using HMG-CoA reductase inhibitors are ways cholesterol are manipulated in lipid raft studies.

Lipid rafts are subdomains of the plasma membrane that contain high concentrations of cholesterol and glycosphingolipids.

  • The plasma membranes of cells contain combinations of glycosphingolipids , cholesterol and protein receptors organised in glycolipoprotein lipid microdomains termed lipid rafts.
  • The structure, dynamics, and stability of lipid bilayers are controlled by thermodynamic forces, leading to overall tensionless membranes with a distinct lateral organization and a conspicuous lateral pressure profile.
  • The fluid mosaic model explains various observations regarding the structure of functional cell membranes.
  • .

Lipid rafts are subdomains of the plasma membrane that contain high concentrations of cholesterol and glycosphingolipids. They exist as distinct liquid-ordered regions of the membrane that are resistant to extraction with nonionic detergents. Rafts appear to be small in size, but may constitute a relatively large fraction of the plasma membrane.

While rafts have a distinctive protein and lipid composition, all rafts do not appear to be identical in terms of either the proteins or the lipids that they contain. A variety of proteins, especially those involved in cell signaling, have been shown to partition into lipid rafts. As a result, lipid rafts are thought to be involved in the regulation of signal transduction. Experimental evidence suggests that there are probably several different mechanisms through which rafts control cell signaling.

For example, rafts may contain incomplete signaling pathways that are activated when a receptor or other required molecule is recruited into the raft. Rafts may also be important in limiting signaling, either by physical sequestration of signaling components to block nonspecific interactions, or by suppressing the intrinsic activity of signaling proteins present within rafts. This review provides an overview of the physical characteristics of lipid rafts and summarizes studies that have helped to elucidate the role of lipid rafts in signaling via receptor tyrosine kinases and G protein-coupled receptors.

For 30 years, the fluid mosaic model of Singer and Nicolson 1 has provided the foundation for our understanding of the structure of cellular membranes. In this model, membrane proteins are viewed as icebergs floating in a sea of lipids.

However, work over the last decade has provided evidence that the plasma membrane is not a random ocean of lipids. Rather, there is structure within this sea of lipids that in turn imposes organization on the distribution of proteins in the bilayer.

Lipid rafts are localized regions of elevated cholesterol and glycosphingolipid content within cell membranes see Fig. The fatty-acid side chains of the phospholipids present in lipid rafts tend to be more highly saturated than those in the surrounding membrane. This allows close packing with the saturated acyl chains of sphingolipids, and probably leads to phase separation. Due to the presence of cholesterol, a liquid-ordered domain is formed that exhibits less fluidity than the surrounding plasma membrane.

This tight packing of lipids and phase separation is probably responsible for the signature property of lipid rafts: their insolubility in nonionic detergents 2. Structure of lipid rafts. Lipid rafts blue bilayer are specialized membrane domains containing high concentrations of cholesterol, sphingomyelin, and gangliosides. They are also enriched in phospholipids that contain saturated fatty acyl chains straight lines in lipid tails. This composition results in lateral phase separation and the generation of a liquid-ordered domain.

Bulk plasma membrane gray contains less cholesterol, sphingomyelin, and gangliosides, and more phospholipids with unsaturated acyl chains. As a result, it is more fluid than lipid rafts. A variety of proteins partition into lipid rafts: glycosylphosphatidylinositol-anchored proteins; transmembrane proteins TM ; dually acylated proteins Acyl. As shown in the diagram, not all lipid rafts have the identical protein or lipid composition Raft 1 vs. Raft 2. Not shown are invaginated caveolae, a subclass of lipid rafts that contains caveolin.

Caveolae are small plasma-membrane invaginations that can be viewed as a subset of lipid rafts. Like lipid rafts, caveolae have a high content of cholesterol and glycosphingolipids; however, caveolae are distinguished from lipid rafts by the presence of the cholesterol-binding protein caveolin-1 3 that appears to be responsible for stabilizing the invaginated structure of caveolae 4 , 5.

The presence within lipid rafts and caveolae of a variety of membrane proteins involved in cell signaling 6 , 7 has led to the consensus that these lipid domains play an important role in the process of signal transduction. This review will focus on the lipid rafts found in plasma membranes and their role in signal transduction. Except in specific cases, a distinction between flat rafts and invaginated caveolae will not be made, since most studies do not unequivocally distinguish between these raft subtypes.

Reviews focusing on the structure and function of invaginated caveolae have been published recently 7 — 9. This procedure yields a fairly consistent product that is enriched in cholesterol and raft marker proteins such as flotillin and glycosylphosphatidylinositol GPI -linked proteins. Differences can arise, however, if the extent of physical manipulation of the detergent lysates is varied.

For example, epidermal growth factor EGF receptors are retained in Triton Xresistant lipid rafts if the lysate is placed in a tube and simply inverted several times prior to centrifugation and flotation of rafts. By contrast, EGF receptors are lost from DRMs if the original detergent lysate is homogenized prior to centrifugation unpublished observations. Thus, care must be taken to be consistent in every aspect of the isolation procedure to obtain preparations that are comparable across experiments.

Recently, a wide variety of detergents other than Triton X have been used to isolate low-density detergent-insoluble membrane fractions. While there is substantial overlap in the protein and lipid content of lipid rafts prepared by these various methods, significant differences also exist among them 11 — This suggests that the various methods for lipid raft preparation do not yield identical membrane fractions.

Thus, to a large extent, detergent-insoluble lipid rafts are truly the unique product of the method by which they have been made. Several detergent-free preparations of lipid rafts have also been reported. One preparation involves the lysis of whole cells in a sodium carbonate buffer pH This buffer is used because the elevated pH helps in the removal of peripheral membrane proteins.

In addition, problems can arise because the material that is ultimately sonicated and separated by density-gradient centrifugation contains all of the intracellular membranes. Because rafts are present on intracellular membranes as well as the plasma membrane, there is no guarantee that a protein found in the light fraction at the end of this procedure was actually present in the plasma membrane at the beginning of the preparation.

A more selective procedure for the purification of nondetergent lipid rafts was reported by Smart et al. In this method, cells are lysed in isotonic sucrose buffer, and a postnuclear supernatant is isolated. A purified plasma membrane fraction is prepared by sedimentation of the postnuclear supernatant in a self-forming Percoll gradient. Plasma membranes are readily separated from Golgi, endoplasmic reticulum, and mitochondria by this method. The banding pattern of these various membrane fractions can be modified by altering the pH and ionic composition of the Percoll gradient to obtain optimal separation The purified plasma membranes are sonicated to release lipid rafts and caveolae , which are isolated by flotation in a continuous gradient of Opti-Prep in isotonic solution.

This method yields a highly purified lipid raft preparation that probably closely reflects the composition of these domains in intact cells. All of the above preparations isolate all forms of lipid rafts present in the cell, i. The caveolin-containing lipid rafts can be separated from noninvaginated rafts by anticaveolin immunoaffinity purification If caveolae are to be isolated from vascular endothelial cells, a procedure is available that physically separates caveolae from lipid rafts Rat lung vasculature is perfused with cationic colloidal silica particles to coat the extracellular side of the plasma membrane.

Lipid rafts, being flat, are surface-coated in this procedure. However, because the necks of caveolae are so narrow, the interior of these invaginations is not coated. Endothelial cells are isolated from these perfused vessels and, subsequently, plasma membranes are prepared. The plasma membranes are homogenized to release the caveolae, which float at a lower density in sucrose density gradients than do the silica-coated plasma membranes containing the flat lipid rafts. The plasma membranes, stripped of caveolae, can be washed with a high-salt solution to remove the silica.

Extraction of these membranes with Triton X and flotation in a density gradient results in the isolation of a low-density membrane fraction that is devoid of caveolin but enriched in other lipid raft markers, and probably corresponds to purified flat lipid rafts.

This preparation, though cumbersome and not generally applicable to all types of cells, generates what are probably the most highly purified preparations of caveolae and lipid rafts. Because caveolae represent a morphologically identifiable domain, the size of this subclass of lipid rafts can be readily determined via electron microscopy. However, caveolae are often found in grape-like clusters that have a much larger overall size.

In addition, caveolin-3, the muscle-specific form of the caveolar structural protein, caveolin-1, is involved in the development of T-tubules that can be microns in length The size of flattened lipid rafts cannot be measured directly, because the domains cannot be distinguished from the surrounding membrane.

Therefore, relatively indirect methods have been employed to determine the size of these domains. These studies have generated highly variable estimates of raft size. GPI-linked proteins are known to partition into lipid rafts and are often used as markers for these domains 10 , Analysis of the rate of lateral diffusion of GPI-linked proteins as well as gangliosides, a raft lipid marker, suggested that the domains are nm to nm in diameter 21 — Somewhat higher values 0.

By contrast, several studies using fluorescence resonance energy transfer have failed to find evidence for stable lipid domains of this size, and suggest that such domains may exist only transiently in some membranes 27 , Every technique applied to study lipid raft size has its own unique strengths and weaknesses. Based on the summation of currently available data, a conservative interpretation is that lipid rafts are probably structures with an average diameter in the range of nm to nm, well below the resolution of the light microscope.

Another important consideration with respect to raft size is the fraction of the plasma membrane that is actually covered by lipid rafts. The surface area encompassed by lipid rafts almost certainly varies among cell types, and this could account for the variability in published estimates of raft coverage. Firmer values for both the size and membrane fraction of lipid rafts await the development of better physical methods.

Nonetheless, the currently available information does provide an explanation for the observation that many proteins that can be localized to rafts biochemically often appear to be diffusely distributed on the cell membrane rather than present in a punctate pattern. A raft diameter below the limit of resolution of the light microscope coupled with the rather extensive coverage of the plasma membrane surface by these domains would result in an apparently even distribution of raft-localized proteins, as visualized by immunofluorescence methods.

Several different lipid raft preparations have been analyzed using various methodologies to determine their lipid composition. In general, these studies have shown that DRMs are enriched in cholesterol and glycosphingolipids, but are often poor in glycerophospholipids.

The DRMs were also enriched about 5-fold in glycolipids, such as gangliosides and sulfatides, as compared with intact cells. Similar results were reported by Prinetti et al. A tandem high resolution mass spectrometry analysis of 0. Thus, DRMs showed a moderate increase in saturated fatty acids as compared with plasma membranes. Analysis of lipid rafts prepared from KB cells by a detergent-free protocol demonstrated many similarities but also some differences from the above analyses of DRMs Interestingly, the nondetergent rafts were enriched in ethanolamine plasmalogens, particularly those containing arachidonic acid.

This finding suggests that these arachidonic acid-containing plasmalogens may be important for the function of lipid rafts. In this regard, a recent report that ethanolamine plasmalogens are required for the transport of cholesterol from the plasma membrane to the endoplasmic reticulum is particularly intriguing Most of the typical raft lipids e. By contrast, ethanolamine-containing glycerophospholipids are preferentially localized to the cytofacial leaflet of the plasma membrane.

The finding that rafts contain a distinct subset of these cytofacial lipids 33 suggests that the composition of both the exofacial and cytofacial leaflets of rafts are specific to these domains, and implies that rafts are probably bilayer structures. The DRMs are particularly low in inner leaflet lipids such as anionic phospholipids and phosphatidylethanolamine, and are not enriched in ethanolamine plasmalogens as are the nondetergent rafts.

These differences between the lipid composition of DRMs and nondetergent rafts suggest that detergent treatment of membranes may selectively extract the exofacial leaflet of rafts and leave behind the lipids from the inner leaflet.

Are there examples from naturally occurring membranes displaying micrometer-sized domains as observed in model membrane systems? Refinement of the fluid-mosaic model of membrane structure. Cold Spring Harb. Soft Matter Phys. Patchwork organization of the yeast plasma membrane into numerous coexisting domains. Confocal fluorescence images of natural membranes showing micrometer-sized domains.

Fluid mosaic model lipid rafts. BRIEF HISTORICAL OVERVIEW

Using detergent-extraction techniques is influenced by the way protein chemists work, isolating specific membrane proteins from biological material. However, membranes are self-assembled macromolecular structures in which a range of different molecular species organizes due to weak physical and thermally renormalized forces.

Seen from this point of view, adding detergents to membranes is the last thing you would do to study lateral organization. Even though it has been shown that detergents impinges a completely different structural and dynamical features to membranes Heerklotz, ; Sot et al. At this stage, however, the fact that detergents do not isolate preexisting membrane domains is more widely recognized Lingwood and Simons, Last but not least, conclusive experimental evidence about the existence of rafts in the plasma membrane remains elusive.

The structure, dynamics, and stability of lipid bilayers are controlled by thermodynamic forces, leading to overall tensionless membranes with a distinct lateral organization and a conspicuous lateral pressure profile reviewed in Bagatolli et al.

The transverse structure is a noticeable feature of a lipid bilayer, and is far from that of an isotropic fluid slab of hydrocarbons. The physics behind this profile is based on simple mechanics. In mechanical equilibrium in the tensionless state, the integral of the difference between the normal pressure and the lateral pressure, p N z —p L z , has to become zero. These variations can easily amount to the equivalent of hundreds of atmospheres pressure.

It is this very stressful environment integral membrane proteins have to come to terms with. The lateral pressure profile has recently been computed in 3D in contract to the initial 1D and used to determine the effect of the 3D transmembrane pressure distribution on membrane protein activation Samuli Ollila et al.

Schematic illustrations of: A the lateral pressure profile, p z , of a lipid bilayer, revealing regions of expansive positive pressures and regions of large tensile negative pressures; B lamellar and non-lamellar lipid aggregates formed by self-assembly processes in water. The different structures have different senses of curvature and are arranged in accordance with the value of the phenomenological molecular packing parameter P ; C Lipid monolayers with positive, zero, and negative from top to bottom curvature determined by the shape of the lipid molecules.

Stable lipid bilayer center formed by two opposing lipid monolayers. If the monolayers were not constrained by being in the bilayer, they may curve as shown at the top and the bottom illustrations.

In the latest cases, the stable bilayer would suffer from a built-in curvature stress. Adapted from Mouritsen a with permission. Bilayers are also subject to built-in curvature-stress instabilities that can be locally or globally released in terms of morphological changes Mouritsen, a , b , A crucial regulator of the bilayer propensity for forming curved structures is the lipid average molecular shape.

Of course a lipid molecule in a dynamic lipid aggregate cannot be assigned a shape as such, and the geometric parameters v , a , and l should therefore be considered as average molecular properties. Still, the value of P turns out to be surprisingly useful in predicting the structure of a lipid aggregate.

For instance, if the lipid composition in the two leaflets of a thermodynamically stable bilayer changes e. Via the curvature stress, molecular shape mediates also a coupling to membrane-protein function and provides a set of physical mechanisms for formation of lipid domains and laterally differentiated regions in the plane of the membrane Mouritsen, It has been suggested that the fluid mosaic model of membranes has been successful because it does not bias the researcher too strongly, allowing for broad interpretations of new experimental data and novel theoretical concepts Mouritsen and Andersen, ; Bagatolli et al.

This suggestion can somehow be extended to the raft hypothesis. Moreover, the assertion of a liquid-ordered structure is seldom verified directly but only indirectly by pointing to the high local concentration of cholesterol. This study suggested the existence of cholesterol concentration dependent domains of sizes around 20nm, where plasma membrane proteins dwell for periods of 10—20ms.

One way or another, it is clear that the raft hypothesis extends the mosaic nature of the membrane proposed by Singer and Nicolson to include now functionally important distinct fluid domains, selective in terms of both protein and lipid components. Notice that the generic view of the fluid mosaic model prevails again and no reference is made to relevant membrane physical features such as the transbilayer structure and the associated lateral pressure profile; Cantor, , curvature stress Miao et al.

Thus incorporation of other, more realistic, models, or modifications of the most popular ones are urgently required to interpret membrane related phenomena. Are there examples from naturally occurring membranes displaying micrometer-sized domains as observed in model membrane systems?

Yes, in very specialized membranes such as lung surfactant and skin stratum corneum, where lipids are the principal components, membrane-cytoskeleton anchorage is lacking, and local equilibrium conditions are likely attainable Bernardino de la Serna et al. Other examples have been reported, such as platelets upon activation Gousset et al. The message here is that generalizations can be perilous, and it is probably a good idea to pay attention to the compositional diversity of different membranes, including the way that processes evolve local equilibrium vs.

Confocal fluorescence images of natural membranes showing micrometer-sized domains. Left: skin stratum corneum lipids membranes from human. Right: pulmonary surfactant membranes from pig. This specialized membrane is mainly composed of phospholipids and small amounts of specifically associated proteins SP-B and SP-C. Among the phospholipids, significant amounts of dipalmitoylphosphatidylcholine DPPC and phosphatidylglycerol are present, both of which are unusual species in most animal membranes.

Mono-unsaturated phosphatidylcholines PC , phosphatidylinositol, and neutral lipids including cholesterol are also present in varying proportions Bernardino de la Serna et al. Since conclusive experimental evidence about the existence of domains in live cell plasma membranes remains elusive, fluctuations observed at compositions near the critical point, reported from phase diagrams of ternary mixtures containing cholesterol Veatch et al. This equilibrium phenomenon is claimed to be relevant to membrane function Veatch et al.

As mentioned previously Bagatolli et al. For example, minuscule mistuning near a critical point may lead dramatic changes in membrane structure and dynamics. It is more likely that a related phenomenon associated with non-equilibrium critical behavior, or self-organized critical behavior, which is robust and needs no tuning, may play a role in biology Jensen, Understanding these kinds of processes will prove very challenging, particularly considering that the biophysics of membrane organization under non-equilibrium conditions is in its infancy Sabra and Mouritsen, ; Girard et al.

In order to understand how membrane heterogeneity becomes controlled by the non-equilibrium state of the lipid matrix, it is vital to explore new experimental models and theory-based approaches Bagatolli et al.

For example, active membrane systems subject to transport, signaling, and enzymatic processes should be experimentally designed and studied Bouvrais et al. Last, but not least, it is worth mentioning that the behavior of biological systems including membrane related processes is generally viewed in terms of mass-action kinetics. However, natural systems exist far beyond the dilute concentration limit; consist of molecularly crowded environments with variable water activity and a collection of small sizes.

The impact of these conditions on membrane structure and dynamics is still obscure and waiting to be elucidated. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Stock for the critical reading of this manuscript. This work was supported in part by a grant from the Danish Research Council Notice however that other contribution in this special issue deals with this topic.

National Center for Biotechnology Information , U. Journal List Front Plant Sci v. Front Plant Sci. Published online Nov Luis A. Mouritsen 1, 3. Ole G. Author information Article notes Copyright and License information Disclaimer. Received Sep 6; Accepted Oct The use, distribution or reproduction in other forums is permitted, provided the original author s or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice.

No use, distribution or reproduction is permitted which does not comply with these terms. This article has been cited by other articles in PMC. Abstract The structure, dynamics, and stability of lipid bilayers are controlled by thermodynamic forces, leading to overall tensionless membranes with a distinct lateral organization and a conspicuous lateral pressure profile.

Keywords: raft hypothesis, fluid mosaic model, membrane lateral pressure profile, membrane compositional fluctuations, membrane curvature, membrane domains, membrane lateral organization. Open in a separate window. Conflict of Interest Statement The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Plasma membrane topography and interpretation of single-particle tracks. Methods 7 — Edward H. An outlook on organization of lipids in membranes: searching for a realistic connection with the organization of biological membranes. Lipid Res. Cholesterol rules: direct observation of the coexistence of two fluid phases in native pulmonary surfactant membranes at physiological temperatures. Modulation of rhodopsin function by properties of the membrane bilayer.

Lipids 73 — The lateral pressure profile in membranes: a physical mechanism of general anesthesia. Biochemistry 36 — The influence of membrane lateral pressures on simple geometric models of protein conformational equilibria. Lipids 45—56 Lipid composition and the lateral pressure profile in bilayers. Micrometric segregation of fluorescent membrane lipids: relevance for endogenous lipids and biogenesis in erythrocytes.

Direct observation of the nanoscale dynamics of membrane lipids in a living cell. Nature — Influence of nonequilibrium lipid transport, membrane compartmentalization, and membrane proteins on the lateral organization of the plasma membrane. After transported into late endosomes, low-pH-dependent conformation change of HA induces fusion, and viral ribonucleoprotein complexes RNP are released by proton influx of viral ion channel M2 protein that requires binding with cholesterol.

Semliki Forest virus SFV and Sindbis virus SIN require cholesterol and sphingolipids in target membrane lipid rafts for envelope glycoprotein-mediated membrane fusion and entry. An alternative receptor for HIV-1 envelope glycoprotein on epithelial cells is glycosphingolipid galactosyl-ceramide GalCer , which enriches at lipid raft.

One of the primary reasons for the controversy over lipid rafts has stemmed from the challenges of studying lipid rafts in living cells, which are not in thermodynamic equilibrium. Currently synthetic membranes are studied; however, there are many drawbacks to using these membranes.

First, synthetic membranes have a lower concentration of proteins compared to biomembranes. Also, it is difficult to model membrane-cytoskeletal interactions which are present in biomembranes. Other pitfalls include lack of natural asymmetry and inability to study the membranes in non-equilibrium conditions. For example, fluorophores conjugated to cholera-toxin B-subunit, which binds to the raft constituent ganglioside GM1 is used extensively. Also used are lipophilic membrane dyes which either partition between rafts and the bulk membrane, or change their fluorescent properties in response to membrane phase.

Laurdan is one of the prime examples of such a dye. Rafts may also be labeled by genetic expression of fluorescent fusion proteins such as Lck-GFP. Manipulation of cholesterol is one of the most widely used techniques for studying lipid rafts.

Sequestration using filipin, nystatin or amphotericin , depletion and removal using methyl-B-cyclodextrin and inhibition of cholesterol synthesis using HMG-CoA reductase inhibitors are ways cholesterol are manipulated in lipid raft studies.

These studies allow for the observations of effects on neurotransmitter signaling upon reduction of cholesterol levels. This allows information of the diffusivity of particles in the membrane to be extracted as well as revealing membrane corrals, barriers and sites of confinement.

Not only optical techniques, but also scanning probe techniques like atomic force microscopy AFM or Scanning Ion Conductance Microscopy SICM can be used to detect the topological and mechanical properties of synthetic lipids [56] or native cell membranes [57] isolated by cell unroofing. Also used are dual polarisation interferometry , Nuclear Magnetic Resonance NMR although fluorescence microscopy remains the dominant technique. In the future it is hoped that super-resolution microscopy such as Stimulated Emission Depletion STED [58] or various forms of structured illumination microscopy may overcome the problems imposed by the diffraction limit.

The role of rafts in cellular signaling, trafficking, and structure has yet to be determined despite many experiments involving several different methods, and their very existence is controversial despite all the above.

A first rebuttal to this point suggests that the Lo phase of the rafts is more tightly packed due to the intermolecular hydrogen bonding exhibited between sphingolipids and cholesterol that is not seen elsewhere. A second argument questions the effectiveness of the experimental design when disrupting lipid rafts. Pike and Miller discuss potential pitfalls of using cholesterol depletion to determine lipid raft function.

Finally, while lipid rafts are believed to be connected in some way to proteins, Edidin argues that proteins attract the lipids in the raft by interactions of proteins with the acyl chains on the lipids, and not the other way around. From Wikipedia, the free encyclopedia. Molecular Immunology. Archivum Immunologiae et Therapiae Experimentalis. Current Medicinal Chemistry.

Retrieved The Journal of Lipid Research. Journal of Clinical Investigation. Frontiers in Physiology. Expert Reviews in Molecular Medicine. Biochemical Journal. Gisou Trends in Cell Biology. Biophysical Journal. Bibcode : BpJ Bibcode : Sci The Journal of Cell Biology. Quarterly Reviews of Biophysics.

Bibcode : PNAS Bibcode : Natur. Nature Reviews Neuroscience. Seminars in Immunology. Sci STKE. Bibcode : PNAS.. J Lipid Res. Sci Rep. Bibcode : NatSR J Biol Chem. Journal of Investigative Dermatology. Journal of Cell Science.

Journal of Cellular Physiology. Current Opinion in Chemical Biology. Biophysical Chemistry. International Immunology. Current Opinion in Immunology. Biochemical and Biophysical Research Communications. Microbiology and Molecular Biology Reviews. Annals of Medicine. Molecular Membrane Biology. M; Crowe, S. M; Mak, J Journal of Clinical Virology. Nature Cell Biology. Journal of Immunological Methods.

The Journal of Immunology. In Quinn, Peter J. Membrane Dynamics and Domains. Sub-Cellular Biochemistry. Subcellular Biochemistry.

The traditional fluid mosaic model of biological membranes stated that lipids are homogeneously mixed. However, biological membranes are actually composed of a complex mixture of lipid domains that exist in different degrees of order, ranging from the high order gel phase to low order liquid phase.

These rafts have been estimated to be nm in size. Small rafts can merge to form large rafts through protein-protein and protein-lipid interactions. The rafts are important because certain membrane proteins preferentially localize to these domains and biological processes such as signal transduction occur there as a result. Proteins are often targeted to rafts by palmitoylation and myristoylation, which are covalent attachments of fatty acids.

Glycosylphosphatidylinositol GPI -anchors also localize proteins to rafts. Rafts exist as planar domains or invaginated structures known as caveolae 1. Caveole type rafts contain the cholesterol binding protein cavin, which are located in the inner membrane leaflet and are needed for membrane invagination. Cavins are peripheral membrane proteins that bind to caveolar phosphatidylserines.

Caveolin is another caveolar protein that is involved in signaling processes. Lipid rafts are also found in the endomembrane system in addition to the plasma membrane 1.

Sphingolipids and cholesterol are synthesized in the ER. They are then trafficked to the Golgi, where they associate to form rafts. Raft containing vesicles are then sent to the plasma membrane through the trans Golgi network, which is involved in raft recycling as well. Rafts also participate in sorting and membrane targeting of lipids in the trans Golgi network. Rafts are composed of sphingolipids such as sphingomyelin, cholesterol, and phospholipids.

Rafts are often therefore referred to as detergent resistant membranes DRMs. Phase separation caused by higher order rafts and lower order bulk lipids is the driving force for raft formation. Raft associated lipids are more saturated than bulk lipids and therefore are more tightly packed. Cholesterol preferentially localizes to rafts because it has a rigid structure that prefers the higher order state of the raft.

The hydroxyl group in cholesterol interacts with the sphingosine amide, which contributes to the higher ordering. In fact, the DRM rafts disappear if cholesterol is extracted from the membrane.

Another driving force for raft formation is phase separation caused by differential hydrophobic acyl chain length in raft associated and non-associated lipids. The acyl chains of shpingomyelin are longer than that of typical phospholipids found in the bulk lipid of biological membranes. A homogenous mixure of long and short chain lipids would result in higher exposure of the hydrophobic region of the long chain lipids to the surrounding water than if phase separation were to occur.

Therefore, phase separation of long and short chain lipids decreases the free energy of the membrane. Hydrophobic mismatch, or the difference between protein hydrophobic transmembrane domain lengths and hydrophobic membrane widths, can cause proteins to associate or not associate with rafts. Hydrophobic mismatch can also effect protein conformation and therefore protein activity. The Singer and Nicolson fluid mosaic model of biological membranes was proposed in By the early s it became clear that membranes lipids preferentially segregated into different phases under physiological conditions and therefore existed as a heterogeneous mixture in membranes 3.

However, there was still a question of whether the DRM was an artifact of the cold treatment. To answer this question, resonance energy transfer RET was measured in cells expressing two types of membrane associated fluorescent folate analogs, GPI anchored folate receptors and transmembrane anchored folate receptors 5. Also, when cholesterol was extracted from the membrane, the GPI anchored probes were found to be randomly distributed.

This data suggests that the DRM rafts are in fact real. Another key piece of evidence for the existence of rafts came from membrane protein cross-linking experiments 6. Membrane proteins were cross-linked by aggregating a certain protein with an antibody and cross-linking nearby proteins with aldehyde fixatives.

Membrane proteins associated with the DRM rafts were colocalized, while proteins not associated with the DRM rafts did not colocalize with raft proteins. The close proximity of raft-associated proteins suggests that the DRM rafts are real membrane microdomains. Many proteins involved in signal transduction have been found to colocalize with the raft microdomains. Rafts can be important for signal transduction because they provide a microenvironment where specific signal responses can occur upon ligand binding to the receptor.

There are two models for raft mediated signal transduction 2. One model is that ligand bound receptors migrate to rafts where the downstream signal transduction occurs. The other model is that ligand binding causes several rafts with different signaling components to merge and produce the downstream signal response.

A combination of the two models where receptor migration to a raft causes different rafts to merge is also possible. The first discovered example of raft associated signal transduction was the tyrosine kinase signal transduction of immunoglobulin E IgE signaling in the allergic immune response 2.

The crosslinked receptor complexes are recruited to rafts where Lyn, a tyrosine kinase, phosphorylates the receptor complex, which starts the phosphorylation signaling cascade. Rafts have also been shown to be involved in other types of signal transduction such as G-protein coupled signal transduction 2. A new and exciting aspect of the lipid raft mediated signal transduction is the interaction of rafts with the cytoskeleton.

Actin binds to caveolins in caveoles and tetraspanins in planar rafts. Through this interaction with the cytoskeleton, rafts have been shown to be involved in cellular polarity, cell migration, neuronal signaling, neuronal membrane repair, and T-cell activation 1. Another major role for lipid rafts is in entry and shedding of certain viruses 7. Both envelope lacking and enveloped viruses can depend on lipid rafts for cell entry.

The mechanism of attachment and entry can depend on lipid-lipid, lipid-protein, and protein-protein interactions. Many viral proteins have been shown to interact with cholesterol and sphingolipids in the rafts. Caveoles can also be involved in endocytosis of certain viruses. When certain viruses are released from the host cell, they bud off and their membrane is formed from the host cell plasma membrane.

The similarity of certain viral and raft lipid composition indicates that these raft-dependent viruses bud off at raft sites. Since a single raft is not large enough to form a full viral membrane, multiple lipid rafts are likely recruited before virus budding. HIV is an example of a virus that has raft like lipid composition 7. HIV Gag proteins have positively charged residues and a myristate group that targets it to phophatidylinositol PI 4,5 P2 in the plasma membrane inner leaflet.

Gag aggregation in the plasma membrane creates a saturated lipid environment that recruits raft microdomains. HIV budding then proceeds at the raft microdomain. A study on the flu virus has shown that removing cholesterol from the host membrane increases viral budding, but produces viruses with very low infectivity 8.

This suggests that rafts are more important for cell entry than for viral shedding. Rafts have historically been studied in animal membranes, but plant membrane rafts have more recently been studied as well. Plants do not contain cholesterol, but other sterols contribute to plant lipid rafts. As with animal rafts, plant rafts also mediate signal transduction events.

Another recent study has revealed an association of lipid rafts to plasmadesmata, which are direct cytosolic connections between plant cells The full implications of this finding are yet to be elucidated. Rafts have also been described in other eukaryotes such as fungi Rafts may not be exclusive to eukaryotes. Recent work in Bacillus subtilus has shown differential protein localization in DRMs Some of these proteins have homology to eukaryotic raft associated proteins and many are involved in bacterial signaling and transport processes.

Other microdomains that are not cholesterol-associated rafts also exist in biological membranes. These non-raft domains are likely quite diverse in composition and function, but have not been well characterized. Many non-raft microdomains are made up of poly-unsaturated lipids, which exclude cholesterol. Some membrane proteins are thought to have differential activity when localized to raft or non-raft microdomains.

Structure and Formation Rafts exist as planar domains or invaginated structures known as caveolae 1. Lipid Composition and Biophysical Properties Rafts are composed of sphingolipids such as sphingomyelin, cholesterol, and phospholipids. Discovery and Evidence The Singer and Nicolson fluid mosaic model of biological membranes was proposed in Signal Transduction Many proteins involved in signal transduction have been found to colocalize with the raft microdomains.

Viral Interactions Another major role for lipid rafts is in entry and shedding of certain viruses 7. Prevalence of Lipid Rafts Rafts have historically been studied in animal membranes, but plant membrane rafts have more recently been studied as well. Non-Raft Microdomains Other microdomains that are not cholesterol-associated rafts also exist in biological membranes.

References Head, B. Biochimica et biophysica acta , , doi Simons, K. Lipid rafts and signal transduction. Nature reviews. Molecular cell biology 1 , , doi Karnovsky, M. The concept of lipid domains in membranes. The Journal of cell biology 94 , Brown, D. Functions of lipid rafts in biological membranes. Annual review of cell and developmental biology 14 , , doi Varma, R.

GPI-anchored proteins are organized in submicron domains at the cell surface.