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.
Ribbes, C. Liu, and P. 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 mpdel al. Nature — External link. Membrane microdomains, rafts, and detergent-resistant membranes in plants and fungi. Acylation targets endothelial nitric-oxide synthase to plasmalemmal caveolae. Fluorescence depolarization studies of phase transitions and fluidity in phospholipid bilayers.
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Right: pulmonary surfactant membranes from pig. According to this biological modelthere is a lipid bilayer two molecules thick layer in which protein molecules are embedded. Budding transitions of fluid-bilayer vesicles: the effect of area-difference mosaix. Current views on structural and dynamical aspects of biological membranes have been profoundly influenced and to some extent biased rats the fluid mosaic model, proposed by Singer and Nicolson Lipid asymmetry in membranes. Functional rafts in cell membranes. Furthermore, membrane regions induced by lipid-protein interactions were proposed as a physical basis for membrane-mediated Fluid mosaic model lipid rafts Marcelja, ; Mouritsen and Bloom, ; Sackmann, Gisou At around that time, Israelachvili proposed another Youtube bikini girl to account for ,ipid need of membrane proteins and lipids to adjust to each other Israelachvili, Molecular Membrane Biology. Laurdan generalized polarization fluctuations measures membrane packing micro-heterogeneity in vivo.
Lipid rafts are subdomains of the plasma membrane that contain high concentrations of cholesterol and glycosphingolipids.
- 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 plasma membranes of cells contain combinations of glycosphingolipids , cholesterol and protein receptors organised in glycolipoprotein lipid microdomains termed lipid rafts.
- The fluid mosaic model explains various observations regarding the structure of functional cell membranes.
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. Bilayers are also subject to built-in curvature-stress instabilities that may be released locally or globally in terms of morphological changes leading to the formation of non-lamellar and curved structures.
Via the curvature stress, molecular shape mediates 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. Unfortunately, these relevant physical features of membranes are often ignored in the most popular models for biological membranes. Results from a number of experimental and theoretical studies emphasize the significance of these fundamental physical properties and call for a refinement of the fluid mosaic model and the accompanying raft hypothesis.
Current views on structural and dynamical aspects of biological membranes have been profoundly influenced and to some extent biased by the fluid mosaic model, proposed by Singer and Nicolson This model supports the idea of lipids forming a more or less randomly organized fluid, flat, bi-dimensional matrix in which proteins perform their distinct functions. Although lipid-mediated lateral heterogeneity in membranes was concurrently described during the s, this feature was not considered in the nascent Singer and Nicolson model.
Along with these observations, it was proposed that lipid compositional heterogeneity may play a role in the modulation of relevant physical properties of natural membranes. Lipid lateral segregation, which might arise under particular environmental plausibly found in physiological states, would be one of these Gebhardt et al. Furthermore, membrane regions induced by lipid-protein interactions were proposed as a physical basis for membrane-mediated processes Marcelja, ; Mouritsen and Bloom, ; Sackmann, To account for lipid-mediated lateral heterogeneity alternative models of biological membranes have been proposed.
At around that time, Israelachvili proposed another model to account for the need of membrane proteins and lipids to adjust to each other Israelachvili, This type of phenomenon in turns gives rise to interfacial tension between lipid and proteins, resulting in clustering of specific lipid molecules around a protein or lipid-mediated protein—protein interactions due to capillary forces.
In addition, a model accounting for the importance of the cytoskeleton and the glycocalyx on membrane organization was developed by Sackmann 1. Regrettably, many of the important physical mechanism highlighted by these models are generally ignored when membrane-related phenomena are addressed e.
These authors envisaged the formation of lipid domains as an early event in the sorting process in the plasma membrane of epithelial cells. These domains were surmised to be functionally associated with specific proteins involved in intracellular lipid traffic and cell signaling Simons and Ikonen, The idea that these rafts, by being enriched in cholesterol, should have special physical properties arose from original observations in model membranes reported by Ipsen et al.
The liquid-ordered phase combines free rotational and translational diffusion of lipids as found in the L d phase with a low proportion of gauche rotamers in the hydrocarbon chains i. Since , the raft hypothesis has become very popular among researchers in the biosciences, spawning thousands of projects and publications in multiple areas of cell biology, biochemistry, and biophysics. However, accurate definitions of the physical phenomena that would underlie the raft hypothesis are still lacking, a fact that has resulted in numerous reformulations over the last few years.
It remains to be established whether membranes are best described as being near local equilibrium at some time scale thus allowing phase separation , or whether they can be more appropriately perceived as metastable regions caused by fluctuations originating from non-equilibrium conditions. Perhaps one of the more questionable aspects of the raft hypothesis was its original operational definition, which was based on detergent extraction methods. 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.
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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.
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Direct observation of the nanoscale dynamics of membrane lipids in a living cell. SV40 utilizes two different receptors to bind onto cell surface: ganglioside GM1 located in lipid rafts and major histocompatibility MHC class I molecule. Vereb; et al. To account for lipid-mediated lateral heterogeneity alternative models of biological membranes have been proposed. These models were not well supported by microscopy and thermodynamic data, and did not accommodate evidence for dynamic membrane properties. Pike and Miller discuss potential pitfalls of using cholesterol depletion to determine lipid raft function. Nicolson in , describes the cell membrane as a two-dimensional liquid that restricts the lateral diffusion of membrane components.
Fluid mosaic model lipid rafts. BRIEF HISTORICAL OVERVIEW
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.