In this study, we compared domain formation in raftlike mixtures of cholesterol and dioleoylphosphatidylcholine (DOPC) with either sphingomyelin (SM) or dipalmitoylphosphatidylcholine (DPPC). Using 2 H nuclear magnetic resonance, we studied the properties of the lipid enriched in the £uid phase, DOPC. We found that acyl chain 2 H-labeled DOPC is much less ordered in SM-containing mixtures than in those containing DPPC, suggesting that DOPC in the SM-containing mixture senses a lower concentration of cholesterol in its direct environment. Atomic force microscopy experiments demonstrated large di¡erences in the size and shape of domains in the di¡erent mixtures. We propose that these various di¡erences are a consequence of the preferential interaction of cholesterol for sphingolipids over glycerophospholipids.

The aim of this study was to gain insight into the mechanism through which transmembrane proteins are targeted to liquid ordered (L o ) phase domains or rafts. This was investigated by analyzing the Triton X-100 resistance of designed transmembrane peptides in model membranes of 1,2-dioleoylsn-glycero-3-phosphocholine, sphingomyelin and cholesterol (1/ 1/1, molar ratio), which contain both L o phase domains and £uid bilayers. By using peptides with one or two palmitate chains covalently linked to their N-terminus or with variable hydrophobic lengths, the roles of protein palmitoylation and of mismatch between the transmembrane segment of the protein and the bilayer thickness, respectively, were investigated. The results show that neither hydrophobic matching nor palmitoylation is su⁄cient for partitioning of peptides into L o phase domains. It is concluded that the L o phase itself, due to the tight packing of the lipids, constitutes an unfavorable environment for accommodation of protein transmembrane segments. ß

The aim of this study was to gain insight into the structural consequences of hydrophobic mismatch for membrane proteins in lipid bilayers that contain cholesterol. For this purpose, tryptophanflanked peptides, designed to mimic transmembrane segments of membrane proteins, were incorporated in model membranes of unsaturated phosphatidylcholine bilayers of varying thickness and containing varying amounts of cholesterol. Analysis of the lipid organization by 31 P NMR and cryo-TEM demonstrated the formation of an isotropic phase, most likely representing a cubic phase, which occurred exclusively in mixtures containing lipids with relatively long acyl chains. Formation of this phase was inhibited by incorporation of lysophosphatidylcholine. These results indicate that the isotropic phase is formed as a consequence of negative hydrophobic mismatch and that its formation is related to a negative membrane curvature. When either peptide or cholesterol was omitted from the mixture, isotropic-phase formation did not occur, not even when the concentrations of these compounds were significantly increased. This suggests that formation of the isotropic phase is the result of a synergistic effect between the peptides and cholesterol. Interestingly, isotropic-phase formation was not observed when the tryptophans in the peptide were replaced by either lysines or histidines. We propose a model for the mechanism of this synergistic effect, in which its dependence on the flanking residues is explained by preferential interactions between cholesterol and tryptophan residues.

No abstract

No abstract