Cholesterol-induced fluid membrane domains: A compendium of lipid-raft ternary phase diagrams

The biophysical underpinning of the lipid-raft concept in cellular membranes is the liquid-ordered phase that is induced by moderately high concentrations of cholesterol. Although the crucial feature is the coexistence of phase-separated fluid domains, direct evidence for this in mixtures of cholesterol with a single lipid is extremely sparse. More extensive evidence comes from ternary mixtures of a high chain-melting lipid and a low chain-melting lipid with cholesterol, including those containing sphingomyelin that are taken to be a raft paradigm. There is, however, not complete agreement between the various phase diagrams and their interpretation. In this review, the different ternary phase diagrams of cholesterol-containing systems are presented in a uniform way, using simple xy-coordinates to increase accessibility for the non-specialist. It is then possible to appreciate the common features and examine critically the discrepancies and hence what direct biophysical evidence there is that supports the raft concept. (C) 2009 Elsevier B.V. All rights reserved.