annurev.biophys.32.110601

annurev.biophys.32.110601 - 30 Apr 2004 18:28 AR...

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Annu. Rev. Biophys. Biomol. Struct. 2004. 33:269–95 doi: 10.1146/annurev.biophys.32.110601.141803 Copyright c 2004 by Annual Reviews. All rights reserved First published online as a Review in Advance on February 2, 2004 M ODEL S YSTEMS , L IPID R AFTS , AND C ELL M EMBRANES 1 Kai Simons Max-Planck-Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, 01307 Dresden, Germany; email: [email protected] Winchil L.C. Vaz Departamento de Qu´ ımica, Universidade de Coimbra, 3004-535 Coimbra, Portugal; email: [email protected] Key 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 a homogeneous fluid. Instead, lipid assemblies, termed rafts, have been introduced to provide fluid platforms that segregate membrane components and dynamically com- partmentalize membranes. These assemblies are thought to be composed mainly of sphingolipids and cholesterol in the outer leaflet, somehow connected to domains of unknown composition in the inner leaflet. Specific classes of proteins are associated with the rafts. This review critically analyzes what is known of phase behavior and liquid-liquid immiscibility in model systems and compares these data with what is known of domain formation in cell membranes. CONTENTS INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 WHAT DO WE LEARN FROM MODEL SYSTEMS? . . . . . . . . . . . . . . . . . . . . . . . . 271 HOW DOES CHOLESTEROL INTERACT WITH NEIGHBORING PHOSPHOLIPIDS? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 1 Abbreviations: 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 magnetic resonance; PC, phosphatidylcholine; SpM, sphingomyelin; POPC, 1-palmitoyl-2-oleoyl- phosphatidylcholine. 1056-8700/04/0609-0269$14.00 269 Annu. Rev. Biophys. Biomol. Struct. 2004.33:269-295. Downloaded from arjournals.annualreviews.org by UNIVERSITY OF FLORIDA on 01/05/06. For personal use only.
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270 SIMONS ¥ VAZ HOW DO THE MODEL MEMBRANE STUDIES RELATE TO CELL MEMBRANES? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 PROTEIN INTERACTIONS WITH LIPID RAFTS . . . . . . . . . . . . . . . . . . . . . . . . . . 283 WHAT DOES DETERGENT RESISTANCE TELL US ABOUT LIPID DOMAINS? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 PERSPECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 POSTSCRIPT CONCERNING TERMINOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 INTRODUCTION The 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 by biochemists and biophysicists. Although the bulk of the bilayer has been consid- ered a homogeneous fluid, there have been repeated attempts to introduce lateral heterogeneities, lipid microdomains, into our model for the structure and dynamics of the bilayer liquid (22, 37, 38, 89). The identification of boundary lipids around proteins created excitement in the 1970s but disappeared into the ESR realm of research (48) when it was shown that on the timescale of NMR experiments the boundary lipids could not be observed.
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