Lipid RaftsVeronique Germain, CNRS-Universite de Bordeaux, France

Artemis Perraki, CNRS-Universite de Bordeaux, France

Sebastien Mongrand, CNRS-Universite de Bordeaux, France

The knowledge about the structure of the biological

membrane changed during the last 70 years. In the 70s,

Singer and Nicholson proposed the fluid mosaic model, a

major conceptual breakthrough in which amphiphilic

proteins reside within the lipid bilayer. In this dynamic

structure, components can move laterally. Further works

led to major modifications of this model. Indeed, func-

tional aspects of trafficking and signal transduction sug-

gestedthat lipids andproteins donot distribute randomly

but can be sequestered in small domains, thus enhancing

protein–protein interactions and speeding up signal

transduction and enzyme activity. The ‘raft hypothesis’

was born. Rafts are small and transient microdomains

enriched in sphingolipids and sterols, together with spe-

cific proteins with important functions. This hypothesis

explains the heterogeneity of the distribution of mem-

brane proteins by a spontaneous demixing of lipids to

form domains involved in signal transduction, cell traf-

ficking and host–pathogen relationship.

Introduction

The raft concept emerged from different laboratoriesto give tentative explanations of different biologicalobservations and, as often in experimental science, relied

on the development of new technological tools. Bio-physicists, cell biologists and lipid biochemists gathered tofurther strengthen this hypothesis.

The Rafts Hypothesis Born from CellBiology and Biophysics

The trafficking of proteins to the plasmamembrane

Most of the early evidences supporting the raft biologicalrelevancewere provided using an in vitro epithelial cellmodelcalled Madin-Darby canine kidney cells, MDCK. Thismodel displayed a strong differentiation between apical andbasolateral surfaces (Figure 1a). The apical membrane incontact with the external environment is enriched in pro-teins to ensure vectorial transport, transmittance of signalsand protection of the cell. Apical membrane proteins aremostly anchored by glycophosphatidyl-inositol (GPI). Interms of lipids, apical membranes are rich in glyco-sphingolipids like glucosylceramide and gangliosides(van’t Hof et al., 1992). These lipids located on the outerleaflet of the apical membrane cannot cross the tightjunctions that delineate into basolateral and apical regions(van Meer et al., 1986). Phosphoglycerolipids located inthe apical membrane are richer in saturated fattyacids (Brasitus and Schachter, 1984) in agreement with thefact that this membrane is relatively rigid, whereas thebasolateral membrane is more fluid (Le Grimellec et al.,1988). By contrast, basolateral membrane concentratesproteins involved in contacts with the basal blade. Thebasolateralmembrane is enriched in phosphoglycerolipids,and displays a lower cholesterol:phosphoglycerolipidsratio (Molitoris and Simon, 1986). The difference in

Advanced article

Article Contents

. Introduction

. The Rafts Hypothesis Born from Cell Biology and

Biophysics

. Phase Behaviour of Lipid-mixtures in Protein-free

Artificial Liposomes

. Methods for Isolation of Rafts from Biological

Membranes: Detergent-insoluble Membranes;

Pharmacological Tools Destabilising Membrane Rafts

. Lipids of Detergent-insoluble Membranes

. Proteins of Detergent-insoluble Membranes

. How to Visualise Rafts in Membranes?

. Controversy: Correspondence between DIM and Rafts in

Biological Membranes, the Use of Cold Detergent

. Physiological Roles of Rafts

. Conclusions, Perspectives

. Acknowledgements

Online posting date: 17th September 2012

eLS subject area: Plant Science

How to cite:Germain, Veronique; Perraki, Artemis; and Mongrand, Sebastien

(September 2012) Lipid Rafts. In: eLS. John Wiley & Sons, Ltd:Chichester.

DOI: 10.1002/9780470015902.a0023727

eLS & 2012, John Wiley & Sons, Ltd. www.els.net 1

composition of the basolateral and apical membranes alsoemerged through the work of van Meer and collaboratorswho found a difference in the viral envelope lipid com-position between viruses like the fowl plague virus that budat the apical membrane, and the vesicular stomatitis viruswhich bud at the basolateral membrane (van Meer andSimons, 1982). See also: Intracellular TransportPlant cells, by contrast with polarised animal cells, have

cell walls and lack tight junctions. Several plasma mem-brane (PM) domains have been documented in plantscalled basal, lateral, apical and nonpolar (Murphy et al.,2005). Although many plant proteins have been identifiedin these domains, like the plant hormone auxin transpor-ters (Figure 1b), the lipid composition of these cell domainsis still unknown. See also: Plant Cell: Overview

How could such segregation occur?

First, van Meer used a newly developed fluorescent cer-amide analogue, C6-(7-nitro-2,1,3-benzoxadiazol-4-yl)

aminocaproyl sphingosine, C6-(NBD)-ceramide, to studythe intracellular transport of de novo synthesised sphingo-lipids in MDCK cells (van Meer et al., 1987). The fluor-escent probe first accumulated in the Golgi, and wasquickly converted into NBD–sphingomyelin and NBD–glucosylceramide. Raising the temperature for 1 h resultedin loss of fluorescence from the Golgi and an intensestaining of PM. Quantification of the amount of NBD–lipids delivered to the apical and the basolateral PMshowed that the NBD–glucosylceramide was 2–4 foldenriched on the apical compared with basolateral domain,whereas NBD–sphingomyelin was about equally distrib-uted. The authors hypothesised that lipid sorting shouldinvolve the lateral segregation into areas of the membranethat would bud to form transport vesicles destined foreither the apical or the basolateral ones. At that time thefactors involved in this segregation process were unknown.Secondly, Michael Lisanti, Enrique Rodriguez-Boulan

and colleagues showed that six GPI-anchored proteinsfollowed the same apical distribution pattern as the NBD–glycolipid (Lisanti et al., 1988).

Nonpolar (symmetric) localization(PM-ATPase (plants))Polar (asymmetric) localization - Basolateral(carbonic anhydrase I (animals))

Polar localization - Basal (PIN1(plants))Polar localization - Lateral (COBRA (plants))Polar localization - Apical(carbonic anhydrase IV (animals), AUX1(plants))Basolateral redistribution(�(1)-integrin and Na/KATPase (animals))

Vectorial redistribution (PIN3 (plants))

Endoplasmicreticulam

Trans-golgiNetwork/golgi

EndosomesNucleus

Vacuole/lysosome

MitochondriumPlastid

Cell wall

Tightjunction

Circulatory system

Basement membrane

Gut

Animal Plant

(a) (b)

Figure 1 Animal and plant polarised cells. (a) Polarised animal intestinal epithelial cell exhibit targeting of PM proteins to the basolateral or apical

membranes with tight junctions that delineate the two regions by preventing lateral diffusion. Localisation of carbonic anhydrase XII, apical localisation of

carbonic anhydrase IV, and basolasteral redistribution of b(1)-integrin or Na+/K+ ATPase after a pathogen-induced change in cellular polarity. Dashed lines

denote redistributed proteins. (b) Plant cells have cell walls and lack tight junctions to define the polarity of the cell. Four types of PM domains have been

documented in plants. Nonpolar localisation of the ATPase, basal localisation of plant hormone auxin transporter PIN1, lateral localisation of GPI-anchored

protein COBRA, apical localisation of auxin transporter AUX1, and vectorial redistribution of PIN3 after a gravitropic stimulus. Reproduced from Murphy et al.

(2005) with permission of Annual Reviews, Inc.

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Finally, Deborah Brown and Jack Rose discovered in acornerstone paper that nonionic detergent extraction at4 8C allowed the isolation of low buoyant detergent-insol-uble membranes enriched in glycosphingolipids, sterolsand GPI-anchored proteins, but depleted in phosphogly-cerolipids (Brown and Rose, 1992), a content strikinglysimilar to the composition of the apical PM. Under theseconditions, the proteins found usually localised to theapical membrane were associated with detergent-insolublemembranes but not with proteins localised to the baso-lateral membrane. This work supports themodel proposedby Simons and van Meer for sorting of some protein–sphingolipid microdomains formed in the Golgi to theapical surface. These detergent-resistant or -Insolublemembranes (DRMorDIM) literally floated like rafts to thetop of the sucrose gradient, and this simple technique‘opened the field up to experimental analysis’, said vanMeer. The concept of lipid raft was coined. Brown andcolleagues proposed that the structure of the DIMwas dueto a separate, less fluid membrane phase regulated by tightpacking of cholesterol and aliphatic chains of sphingoli-pids. These observations raised the idea that lipids could beresponsible for such segregation. See also: Lipids; Mem-brane Lipid BiosynthesisFew thousand papers were published since the 90s

showing that rafts were also important for cell signallingand relationship with pathogens (Anderson and Jacobson,2002). In plant and yeast, DIM have been only recentlydescribed in 2001 and 2004, respectively (Bagnat et al.,2001; Borner et al., 2005; Mongrand et al., 2004), and only200 papers have been published yet. As discussed in section‘Proteins of Detergent-insoluble Membranes’, plant raftsare also enriched in GPI-anchored proteins, sphingolipidsand phytosterols and may act as signalling platforms par-ticularly in response to pathogens.

Phase Behaviour of Lipid-mixtures inProtein-free Artificial Liposomes

Liposomes turned out to be very useful to studymembranedomain segregation. They are lipid bilayers organised asvesicles, composed of defined mixtures of synthetic ornatural lipids. They are considered as highly simplifiedmodels mimicking biological membranes.

Formation of phases in the lipid mixtures

A lipid bilayer exists in different states called phases,depending on the medium and the temperature. Thecohesion of lipids depends on the low-energy bondsbetween hydrophobic chains (van der Waals interactions)and between polar heads (hydrogen and electrostaticbonds). The transition temperature or Tm (melting tem-perature) is defined as the temperature at which a mem-brane composedof one type of lipid changes phase.TheTmdepends on the length and the degree of unsaturation of the

fatty acid chains but also on the nature of the polar group(Silvius et al., 1979). At low temperatures, themembrane isin a gel phase called solid-ordered (So), in which lipids aretightly packed and lateral motions are very slow. The gelphase does not exist in biological membranes. At highertemperatures, the membrane is in the form of a fluid phasecalled liquid disordered (Ld or La), in which lipids aremuch less condensed, acyl chains are mobile and looselypacked, and lateral diffusion coefficients are high. Liquidordered (Lo) phase is an intermediate phase between the geland theLdphase, inwhich a high degree of acyl chain orderis observed, but the lipid lateral diffusion coefficients arecomparable to those of the Ld phases.Themixture of lipidsmimickingPMof eukaryotic cells is

a ternary mixture, which consists of sphingolipids, sterolsand phosphoglycerolipids (Figure 2a) with different Tm,giving a high potential to generate lateral phase separationsat a given temperature. Cholesterol increases the rigidity ofthe membrane while increasing the lateral mobility of theother compounds. Depending on the cholesterol ratio,several phases can be formed. Based on these results, phasediagrams can be established (Figure 2b). It is clear from invitro studies that a coexistenceofLoandLddomains exists.The polar head of phospho/sphingolipids acts as a coverfor cholesterol molecules intercalated below in interactionswith saturated aliphatic chains rather than unsaturatedones, called the ‘Cholesterol’s Umbrella Effect’ (de Meyerand Smit, 2009; Figure 2c). Using fluorescent probes, itwas possible to observe the immiscibility of a mixture oflipids and formation of lipid domains in mixture 1:1:1of phosphoglycerolipids:sphingolipids:cholesterol. Lo wasenriched in sphingolipids and sterols similar with raftdomains, Ld enriched in phosphoglycerolipids. See also:Lipids; Lipid BilayersAlthough mammalian and fungal cells contain only one

major sterol (cholesterol and ergosterol, respectively),plant cells display a large variety of sterols, with sitosterol,stigmasterol and 24-methylcholesterol as the most repre-sented compounds. Plant sterols mainly accumulate in thePM, where they play a major role in regulating the mem-brane fluidity and permeability. Phytosterols are alsoenriched in DIM, and display similar properties to chol-esterol. Thepresence of amix of phytosterols appears likelyto be the evolution response for plant adaptation to largetemperature variations (Dufourc, 2008).

Methods for Isolation of Rafts fromBiological Membranes: Detergent-insoluble Membranes; PharmacologicalTools Destabilising Membrane Rafts

Detergents are invaluable tools for the isolation and puri-fication of membrane proteins. They are amphiphilicmolecules, consisting of a polar head and a hydrophobicchain, able to incorporate in the membranes and solubilise

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proteins by replacing their lipid environment. The deter-gent insolubility implies that the solubilisation neverbecomes full, and that a fraction of the membrane ismaintained as large bilayer fragments. The DIM are

separated after centrifugation in a gradient in low-densityfractions, whereas the detergent-soluble membranesremain at the bottom of the gradient, trapped by highconcentration of sucrose. DIMmay vary depending on the

Phospholipid Sphingolipid

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Figure 2 Spatial organisation of membrane rafts; Ternary lipid phase diagram for a lipid mixture imitating plasma membrane. (a) Structure of main lipids

found in biological membranes. Reproduced from Munro (2003) with permission from Elsevier. (b) Spatial organisation of sterols, phosphoglycerolipids and

sphingolipids in raft domains. Reproduced from Quinn (2010) with permission from Elsevier. (c) Phase diagrams at 37 8C of three-component bilayer mixture

at different concentration (egg-phosphatidylcholine:egg-sphingomyelin:cholesterol, that is, a high- and low-melting point lipids together with sterol)

showing the co-existence of Lo and Ld phases. Around the diagram is depicted structures of lipid bilayers with G:gel phase, solid ordered; Ld: liquid

disordered phase; Lo=liquid ordered phase. Reproduced from Simons K and Ikonen E (1997) Functional Rafts in Cell Membranes. Nature 387(6633):

569–572.

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type of detergent used, the ratio detergent/lipid andthe cell type studied. There are several other names fordetergent insoluble membrane (DIM) in the literature:detergent-insoluble glycolipid-enriched complexes, glyco-sphingolipid-enriched membranes, Triton-insoluble float-ing fractions. The detergent most commonly used is theTriton X-100, however, Lubrol, Brij and Tween have beenreported (Schuck et al., 2003). Methods based on thesonication of PMwere also proposed (Brown andLondon,1997; Macdonald and Pike, 2005; Peskan et al., 2000).The idea to in vivomodify the integrity of rafts appears a

good way to prove their existence, assess their biologicalrole and validate the association of specific proteins. Theprime target is obviously the sterol concentration, reviewedin Zidovetzki and Levitan (2007): (1) Sterols can besequestered by antibiotics, such as filipin. (2) Sterols can beextracted fromPMby cyclodextrins. Thismethod has beencriticised and must be used with caution because it leads tolarge perturbations of the PM. (3) The cholesterol synthesiscan be inhibited by drugs such as lovastatin, often used intandem with cyclodextrin.

Lipids of Detergent-insolubleMembranes

Pioneer work by Brown and Rose (Brown and Rose, 1992)showed enrichment in glycosphingolipids and depletion ofglycerophospholipids in DIM of MDCK. The enrichmentin sphingolipids and cholesterol was also found in DIMisolated from mast cells, Jurkat cells and CHO ChineseHamster Ovary (CHO) cells (e.g. Fridriksson et al., 1999;Pike et al., 2005; Schuck et al., 2003).

In plants, DIM are enriched in a mix of sterols (Borneret al., 2005; Mongrand et al., 2004), and also in phytos-terols conjugated to sugars, called steryl glucosides andacyl steryl glucosides (Lefebvre et al., 2007; Figure3). Like inanimal cells, glycerophospholipids were depleted in plantDIM (Mongrand et al., 2004). In term of sphingolipids,glucosylceramides are conserved across kingdoms, butsphingomyelin, the main sphingolipid of animal mem-branes, as well as gangliosides and globosides are absent inplants. Conversely, glycosyl-inositol-phospho-ceramides(GIPC) are only encountered in plant and fungus organ-isms (Sperling et al., 2005). Indirect evidence suggests thatGIPC are likely the main sphingolipids of plant DIM(Borner et al., 2005; Lefebvre et al., 2007), but this remainsto be demonstrated. See also: Lipid Bilayers

Proteins of Detergent-insolubleMembranes

DIM contain relatively low protein content compared tolipids. Several teams carried out proteomic analysis ofDIM fractions of plant and animal cells. These analysesshowed the presence of many families of proteins, mostly

signalling, cytoskeleton and scaffolding proteins (e.g.Kierszniowska et al., 2009; Morel et al., 2006; Nebl et al.,2002). Proteins specifically found in DIM are GPI-anchored proteins, they are localised exclusively in theouter leaflet of the PM (Borner et al., 2005; Garner et al.,2007), see Figure 4a. Another class of proteins found inDIMis of acylated proteins containing post-translationalmodifications of N-myristoylation and palmitoylationtype (Melkonian et al., 1999). Few proteins with trans-membrane domains associate with DIM.Caveolin are localised in caveolae (see Glossary). These

proteins are distributed exclusively in the DIM fraction,making it a goodmarker forDIM, just as flotillin (Morrowand Parton, 2005). Recently it was shown that flotillinshave properties very similar to caveolins and induce theformation of membrane microdomains which structure issimilar to those of caveolae (Anderson and Jacobson,2002).Caveolin is absent in plants, but flotillin homologueshave been identified (Haney et al., 2011). See also:Membrane Rafts and Caveolae; Protein Association withMembrane Rafts

How to Visualise Rafts in Membranes?

The hypothesis of rafts has long been controversial becausein the absence of stimulation, rafts could barely beobserved by conventional light microscopy with theexception of caveolae. To explain the lack of visualisationof rafts in cell membranes, it has been proposed that raftsare very small and dynamic. To visualise very small areasbelow the lightmicroscopy resolution limit and understandtheir dynamic properties, newmicroscopy techniques weredeveloped, reviewed in (Jacobson et al., 2007; Marguetet al., 2006): (1) Fluorescence Lifetime Imaging Micro-scopy, which determines the lifetime of a fluorophore indifferent spatial positions; (2) Fluorescence probes (likeLaurdan and Di-4-aANEPPDHQ) that has spectralproperty dependant on its lipid environment and thus thetype of phase; (3) Fluorescence Recovery After Photo-bleaching determines the lateral mobility of fluorescentmolecules in the membrane and thus, establishes whether aprotein has a tendency to settle in a lipid environment.When the photobleaching is combined with Forster Res-onance Energy Transfer, this method provides details ofthe close proximity of membrane components (less than8 nm); (4) Fluorescence Correlation Spectroscopy quanti-fies the diffusion parameters with high-time resolution, andcan distinguish diffusion rates between raft and nonraftphases; (5) single particle tracking observes with a high-resolution microscope the translational mobility of pro-teins or lipids labelled with specific markers (gold particleor fluorescent label). This technique provides a significantspatial motion accuracy of the particle, and can separatespecific movements fromBrownian movement; (6) AtomicForce Microscopy is the most interesting technique tostudy the microstructure of the membrane at the order of 1to 10 nm. AFM is achieved by measuring the deflection

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CH

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Figure 3 Chemical structures of some animal and plant raft lipids. Reproduced from Bodin S, Tronchere H and Payrastre B (2003) Lipid Rafts Are Critical Membrane Domains in Blood Platelet Activation

Processes. Biochimica et Biophysica Acta 1610: 247–257, with permission from Elsevier.

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forces exerted on a thin needle in a very close proximityof the membrane to be mapped. See also: Atomic ForceMicroscopy; Fluorescence Microscopy; Single-MoleculeLight MicroscopyThe rafts are expected to exist in membranes containing

sphingolipids and cholesterol in sufficient quantity.Because cells display an increasing gradient of cholesterolfrom the endoplasmic reticulum (ER) to the PM, andbecause the sphingolipid biosynthesis occurs in the golgi, itseems that rafts are formed in this latter compartment inplants and animals (Laloi et al., 2007; van Meer et al.,1986). Depending on cell type, the distribution of rafts inthe PM is not homogeneous. In polarised epithelial cells,the rafts are almost found exclusively at the apical pole ofthe cells (see Introduction). In neuronal cells, the raftswould also be preferentially localised at the axonal PM(Kamiguchi, 2006). However for quiescent and non-polarised cells such as fibroblasts, the distribution of theseareas most often appears homogeneous to the membranesurface.

Controversy: Correspondencebetween DIM and Rafts in BiologicalMembranes, the Use of ColdDetergent

The question of the existence of rafts in biological mem-branes is still in debate, mainly because the main evidenceof their existence is based on extraction using detergents(Munro, 2003). It was proposed that rafts correspond tothe Lo phase, the remaining membrane lipids beingorganised into phase Ld, and that the high ordering of fattyacid chains is responsible for the insolubility in detergents.However, extrapolation from model membrane to a realbiologicalmembrane is very difficult because: (1)Biologicalmembranes are composed of hundreds of different lipids,which by their arrangements that can influence the mem-brane; (2) Biologicalmembranes are rich in proteins, whichby their structure or enzymatic activity alter the properties

out

in

(b)

(a)

Without stimuli

After stimuli

out

in

out

in

PM

PM

Figure 4 Proteins found enriched in membrane raft of the plasma membrane according to their anchorage and putative coalescence of raft after stimulus.

(a) Caveolin are shown in green in a caveolae, a morphologically discrete flask-shaped structure with raft properties. Acylated proteins (orange) partition into

rafts (yellow) in the inner leaflet of the PM, and in close proximity to caveolin. Glycosphingolipid-enriched domains (pink) are shown in the outer leaflet of

the PM, enriched in GPI-anchored proteins (red). All these domains are too small to resolve by fluorescence microscopy, with the exception of caveolae. (b)

Stimuli-induced raft (in red) coalescence, which facilitates the clustering of signalling-related molecules allowing a rapid signal transduction inside the

cells.Reproduced from Kenworthy A (2002) Peering Inside Lipid Rafts and Caveolae. Trends in Biochemical Sciences 27(9): 435–437, with permission from

Elsevier.

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of the membrane; (3) Rafts are interacting with the cyto-skeleton, which forms a network below the membrane andcan influence the compartmentalisation and the stability ofthese domains; (4)Modelmembranes consist of two similarlayers symmetrically organised as opposed to asymmetricorganisation of the two sheets of biological membraneswhich influence membrane biophysical properties; (5)Several studies on model membranes have shown that theisolation ofDIMbyTritonmay lead to underestimation ofthe amount of lipids present in the areas Lo beforeextraction (Schroeder et al., 1998; Xu and London, 2000);(6) Heerklotz (2002) showed onmodel membranes that theTriton X-100 increases the size and severely perturbs pre-existing Lo-domains and also causes the redistribution ofsome lipids in the form of clusters.In conclusion,DIMcannot simply be assimilated to rafts

as well as rafts do not correspond to the phases Lo (Lich-tenberg et al., 2005). The DIM is the result of incompletesolubilisation of membranes by detergents, and must beused as a fast tool to study rafts. Imagery approaches mustbe further used to insure the clustering of membranecomponents in the plane of the bilayer.

Physiological Roles of Rafts

Analyses of the protein composition ofDIMhaveprovideda list of potential raft-associated molecules and it soonbecame apparent that the rafts could also be involved in awide variety of biological processes. Because of raft abilityto concentrate proteins or exclude others, rafts may pro-mote protein–protein and protein–lipid interactions.Therefore, rafts are considered to play a major role inintracellular trafficking, signal transduction, immuneresponse and the entry and exit of pathogens.

Sorting and intracellular trafficking

Rafts were originally proposed as a platform for the intra-cellular trafficking of proteins to the apical domain ofpolarised cells. The hypothesis of the role of rafts in theapical sorting was reformulated to reflect data showing thatmost transmembrane proteins do not have inherently a highaffinity for rafts. It is proposed that the oligomerisationstatus of proteins may increase – at least transiently theiraffinity in rafts or stabilise them (Schuck and Simons, 2004).

Signal transduction and activation of theimmune response

In signal transduction pathways, onemust consider rafts asplatforms for the recruitment of receptors and transducers.If receptor activation takes place in the rafts, the signallingcomplex is then protected from the nonrafts enzymes, likephosphatases that could affect signalling (Brown, 2006). Ingeneral, rafts recruit proteins in a microenvironment inwhich the phosphorylation state can be changed by localkinases and phosphatases, to modulate signalling. There

are several examples of signalling pathways involving raftsin animal cells like IgE or insulin signalling, and signallingvia T- or B-cells. Receptors recruited by oligomerisationinduce raft coalescence and induce signal transductionpathways, see Figure 4b. The functional architecture of thiscoupling is stabilised by the cytoskeleton adjacent to themembrane. See also: LymphocytesSimilarly, plants, which are constantly exposed to a wide

variety of microorganisms such as bacteria, fungi andoomycetes, lack a network of circulating cells. Therefore,they rely on the innate immunity of each cell and on sys-temic signals emanating from infection sites. Pathogen-associatedmolecular patterns (PAMP, also called elicitors)that trigger plant defence responses have been isolatedfrom a variety of phytopathogenic and nonpathogenicmicroorganisms. The dynamic association of specific pro-teins with DIM upon PAMP exposure has been reported,confirming a possible role for plant rafts as signal trans-duction platforms, particularly during biotic interactions(Simon-Plas et al., 2011). See also: Systemic Signalling inPlant Defence

Virus Transport

Membrane rafts are widely used by pathogens, whichinclude bacteria, fungi, virus and parasites to interact withtarget cells, and many receptors associated with rafts areinvolved in the entry of pathogens (Riethmuller et al., 2006)for review. For example, the receptors CD4 involved in theentry of human immunodeficiency virus (HIV) are raft-associated. The association with rafts also helps the bud-ding and the release of viral particles to coordinate theirexit. The viral envelope is frequently decorated withreceptors and membrane lipids of the host cells that canserve as a cover-up for the immune system but willalso increase the efficiency of infection of new target cells.See also: History of Virology; Virus Host Cell ReceptorsIn plants, Raffaele et al. (2009) showed that a group of

proteins specific to plants, called Remorins type 1, wereassociatedwith rafts and localised to discrete patches in thePM (70 nm in diameter) and plasmodesmata, microscopicchannels which traverse the cell walls of plant cells enablingcommunication between cells and transport of viruses. Theauthors also showed that Remorins interferes with cell-to-cell movement of Potato Virus X (PVX) in tomato plantsadding weight to the hypothesis that plant rafts play animportant role in virus trafficking. See also: Plasmo-desmata; Plant Virus Movement and the Impact of RNASilencing

Role of the cytoskeleton

Themechanisms that control the biogenesis, the size and thedynamic of rafts are still very poorly understood. In thisregard, the role of cytoskeletal elements is emerging. It seemsthat microtubules and actin microfilaments are privilegedpartners of rafts.TheworkofKusumi’s lab suggests that thediffusion of proteins and lipids in rafts was affected by thecytoskeleton below the membrane, the so-called fence and

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picket model. Membrane proteins could be temporarilyconfined in areas of about 30nm to few hundred nm withfastdiffusion (Murase et al., 2004).Transitionalpassages arecalled ‘hop or jump’, and occur between membrane areasdefined by the cytoskeleton called ‘corrals’ (Kusumi et al.,2005; http://www.nanobio.frontier.kyoto-u.ac.jp/research/slide-show/slide02/). Recently, the hypothesis of Kusumihas been questioned by different methodologicalapproaches (Wieser et al., 2007). The cytoskeleton plays arole in the stabilisation of microdomains, allowing therecruitment of protein partners in a complex functional forsignal transduction, for example, in immune response(Meiri, 2005), neuronal signalling (Allen et al., 2007).See also: Cytoskeleton

Conclusions, Perspectives

The Keystone Symposium on Rafts in 2006 favours theterm ‘membrane raft’ because both proteins and lipidscontribute to the genesis of these microdomains. ‘Mem-brane rafts are small (10–200 nm), heterogeneous, highlydynamic, sterol- and sphingolipid-enriched domains thatcompartmentalise cellular processes’ (Pike, 2006).Whatever the real picture of the PM is, it seems that the

old notion of cell surface lipids as a passive, equilibrated,two-dimensional solvent implied by the fluid-mosaicmodelwill have to be replaced by a radically different model. Theconcept of rafts, which assumes that lipids play a role in theformation of membrane microdomains has generatedmuch controversy but stimulated the development ofresearch on lipid–protein interactions. It is now acceptedthat there is heterogeneity in the lateral membranes andthat formation of domains is essential to many biologicalfunctions. According to (Maxfield andWustner, 2002), themembrane is a collection of contiguous rafts within fluidinclusions. Beside, themodel of ‘lipid shells’ (Anderson andJacobson, 2002) proposed that proteins with affinity forcholesterol and sphingolipids are embedded in lipid shells.These highly mobile small units would be able to cluster inmore stable areas to be functional. These differences illus-trate the difficulty to characterise structurally the cellmembrane. Rafts may then be considered as pre-existing,scale-dependent active structures, poised to be induced toform larger and more stable structures, which may beutilised for specific cellular purposes.A primary question is what are the common organising

principles governing the structural and functional archi-tecture of rafts and the dynamic nature of lipid assemblies.Imagery methods to dynamically visualise the cellularprocesses involving rafts must be developed in differentbiological contexts and tissues.

Acknowledgements

Membrane raft research acknowledge funding of theFrench Agence Nationale de la Recherche Programme

blanc ‘PANACEA’ NT09_517917 (‘ANR blanc’ contractsto SM).

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Further Reading

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Science 15: 656–663.

Lipid Rafts

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