title: Molecular simulation of rapid translocation of cholesterol, diacylglycerol, and ceramide in model raft and nonraft membranes.
abstract: The translocation of lipids across membranes (flip-flop) is an important biological process. Slow exchange on a physiological timescale allows the creation of asymmetric distributions of lipids across cellular membranes. The location of lipids and their rate of exchange have important biological consequences, especially for lipids involved in cellular signaling. We investigated the translocation of cholesterol, ceramide, and diacylglycerol in two model bilayers using molecular dynamics simulations. We estimate half times for flip-flop for cholesterol, diacylglycerol, and ceramide of 20 μs, 30 μs, and 10 ms in a POPC bilayer, compared with approximately 30 min, 30 ms, and 30 s in a model raft bilayer (1:1:1 PSM, POPC, and cholesterol). Cholesterol has a large (54 kJ/mol) free energy of exchange between the POPC and raft bilayer, and therefore, it strongly prefers the more ordered and rigid raft bilayer over the more liquid POPC bilayer. Ceramide and diacylglycerol have relatively small free energies of exchange, suggesting nearly equal preference for both bilayers. This unexpected result may have implications for ceramide and diacylglycerol signaling and membrane localization.
authors.affiliation: Department of Biological Sciences and Institute for Biocomplexity and Informatics, University of Calgary, Calgary, Alberta T2N 1N4, Canada.
authors.forename: W F Drew
authors.initials: WFD
authors.lastname: Bennett
authors.affiliation: Department of Biological Sciences and Institute for Biocomplexity and Informatics, University of Calgary, Calgary, Alberta T2N 1N4, Canada. Electronic address: tieleman@ucalgary.ca.
authors.forename: D Peter
authors.initials: DP
authors.lastname: Tieleman
mesh_terms: D001692:Biological Transport; D002518:Ceramides / Q000737:chemistry / Q000378:metabolism*; D002784:Cholesterol / Q000737:chemistry / Q000378:metabolism*; D004075:Diglycerides / Q000737:chemistry / Q000378:metabolism*; D008051:Lipid Bilayers / Q000737:chemistry*; D056004:Molecular Dynamics Simulation
chemical_list: D002518:Ceramides; D004075:Diglycerides; D008051:Lipid Bilayers; D002784:Cholesterol