30 Apr 2004 18:28 AR AR214-BB33-13.tex AR214-BB33-13.sgm LaTeX2e(2002/01/18) P1: FHD10.1146/annurev.biophys.32.110601.141803Annu. Rev. Biophys. Biomol. Struct. 2004. 33:269–95doi: 10.1146/annurev.biophys.32.110601.141803Copyrightc° 2004 by Annual Reviews. All rights reservedFirst published online as a Review in Advance on February 2, 2004MODEL SYSTEMS,LIPID RAFTS, ANDCELL MEMBRANES1Kai SimonsMax-Planck-Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108,01307 Dresden, Germany; email: [email protected] L.C. VazDepartamento de Qu´ımica, Universidade de Coimbra, 3004-535 Coimbra, Portugal;email: [email protected] Words sphingolipids, cholesterol, phase immiscibility, detergent resistance,membrane proteins■ Abstract Views of how cell membranes are organized are presently changing.The lipid bilayer that constitutes these membranes is no longer understood to be ahomogeneous fluid. Instead, lipid assemblies, termed rafts, have been introduced toprovide fluid platforms that segregate membrane components and dynamically com-partmentalize membranes. These assemblies are thought to be composed mainly ofsphingolipids and cholesterol in the outer leaflet, somehow connected to domains ofunknown composition in the inner leaflet. Specific classes of proteins are associatedwith the rafts. This review critically analyzes what is known of phase behavior andliquid-liquid immiscibility in model systems and compares these data with what isknown of domain formation in cell membranes.CONTENTSINTRODUCTION .....................................................270WHAT DO WE LEARN FROM MODEL SYSTEMS? ........................271HOW DOES CHOLESTEROL INTERACT WITH NEIGHBORINGPHOSPHOLIPIDS? ...................................................2721Abbreviations: AFM, atomic force microscopy; DOPC, 1,2-dioleoylphosphatidylcholine;DPPC, 1,2-dipalmitoylphosphatidylcholine; DPPE, 1,2-dipalmitoyl phosphatidylethano-lamine; ESR, electron spin resonance; F-DOPE, N-fluoresceincarboxamido-1,2-dioleoyl-phosphatidylcholine; F-DPPE, N-fluoresceincarboxamido-1,2-dipalmitoylphosphatidyl-choline; FRET, fluorescence resonance energy transfer; NMR, nuclear magneticresonance; PC, phosphatidylcholine; SpM, sphingomyelin; POPC, 1-palmitoyl-2-oleoyl-phosphatidylcholine.1056-8700/04/0609-0269$14.00269Annu. Rev. Biophys. Biomol. Struct. 2004.33:269-295. Downloaded from arjournals.annualreviews.orgby UNIVERSITY OF FLORIDA on 01/05/06. For personal use only.30 Apr 2004 18:28 AR AR214-BB33-13.tex AR214-BB33-13.sgm LaTeX2e(2002/01/18) P1: FHD270 SIMONS¥VA ZHOW DO THE MODEL MEMBRANE STUDIES RELATETO CELL MEMBRANES? .............................................278PROTEIN INTERACTIONS WITH LIPID RAFTS ..........................283WHAT DOES DETERGENT RESISTANCE TELL USABOUT LIPID DOMAINS? ............................................285PERSPECTIVES ......................................................287POSTSCRIPT CONCERNING TERMINOLOGY ...........................289INTRODUCTIONThe lipid bilayer that forms cell membranes is a two-dimensional liquid, the orga-nization of which has been the subject of intensive investigations for decades bybiochemists and biophysicists. Although the bulk of the bilayer has been consid-ered a homogeneous fluid, there have been repeated attempts to introduce lateralheterogeneities, lipid microdomains, into our modelforthestructureanddynamicsof the bilayer liquid (22, 37, 38, 89). The identification of boundary lipids aroundproteins created excitement in the 1970s but disappeared into the ESR realm ofresearch (48) when it was shown that on the timescale of NMR experiments theboundary lipids could not be observed.The realization that epithelial cells polarize their cell surfaces into apical andbasolateral domains with different protein and LIPID compositions in each ofthese domains initiated a new development that led to the “lipid raft” concept,which has stirred up commotion and a lively controversy in the field (78, 80). Theconceptofassembliesof sphingolipidsand cholesterolfunctioning asplatforms formembrane proteins was promoted by the observation that these assemblies seemedto survive detergent extraction (8). This operational breakthrough caused a floodof papers in which raft association was equated with resistance to Triton-X100extraction at 4◦C. The addition of another criterion, depletion of cholesterol usingmethyl-β-cyclodextrin (33, 72), leading to loss of detergent resistance, added evenmore papers to the field. But at the same time these new developments addedfurther controversies. Do lipid rafts exist in membranes or are they just artifacts ofdetergent extraction? Despite, or maybe because of all this controversy, the fieldhas matured and many new methods have added substance to the definition of lipiddomains in cell membranes and have significantly improved our understanding ofheterogeneity in membrane systems. There is now increasing support for a role oflipid assemblies in regulating numerous cellular processes including cell polarity,protein trafficking, and signal transduction. As Edidin (16) aptly put it, “Despitegreatreservationsabouttheinterpretationofclassicaloperationaldefinitionoflipidraft components and function, we are left with...the stubborn insistence by cellsthat raft lipids can be organized and segregated into membrane domains.” In thisreview we try to integratewhat we havelearned about phase immiscibility in modelsystems with what we know of lipid domain organizationin cell membranes today.Annu. Rev. Biophys. Biomol. Struct. 2004.33:269-295. Downloaded from arjournals.annualreviews.orgby UNIVERSITY OF FLORIDA on 01/05/06. For personal use only.30 Apr 2004 18:28 AR AR214-BB33-13.tex AR214-BB33-13.sgm LaTeX2e(2002/01/18) P1: FHDLIPID RAFTS 271WHAT DO WE LEARN FROM MODEL SYSTEMS?Cell membranes are two-dimensional liquids. Thus, lateral heterogeneity impliesliquid-liquidimmiscibility in the membrane plane.Hydrated lipid bilayers undergophase transitions as a function of temperature. These transitions, which occur atdefined temperatures for each lipid species, always involve some change in theorder of the system. The most important of these transitions is the so-called mainor chain-melting transition, in which the bilayer is transformed from a highlyorderedquasi-two-dimensionalcrystalline solid toa quasi-two-dimensional liquid.Itinvolvesadrasticchange in the orderofthe systems, in particularthetranslational(positional) order in the bilayer