A Molecular-Level View of the Sorting of Lipidated Peptides and the Origin of Cholesterol's Condensing Effect in Fluid Bilayers

The lateral organization of lipids in cell membranes is one of the most vexing problems facing chemists, biologists, and biophysicists today. Among the techniques used to probe membrane organization, the Nearest Neighbor Recognition (NNR) method developed in our laboratory is unique in its ability to provide quantitative, molecular level information about the interactions between lipids in a bilayer. The NNR method was applied to two problems of biological relevance: "lipid sorting," i.e. the partitioning of membrane components into discrete domains, and the origin of cholesterol's condensing effect.;Lipid sorting was probed by measuring the free energy of interaction between a mimic of the known raft associating peptide motif [(myristoyl)GlyCys(palmitoyl)], as well as an analogous peptide bearing an unsaturated acyl chain, and mimics of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol. The NNR results, which represent the first direct measurement of lipidated peptide-lipid interaction energies, were then used in Monte Carlo simulations to produce a physical picture of the partitioning of the peptide between liquid ordered (lo) and liquid disordered ( ld) domains in a model membrane. The peptide motif [(myristoyl)GlyCys(palmitoyl)] mixed ideally across both domains, while the analogous peptide bearing an unsaturated acyl chain was found to have a slight preference for ld domains. The lack of a clear preference for the lo phase suggests that hydrophobic interactions between lipidated proteins and membrane lipids may be less important than previously hypothesized for lipid sorting, with other factors such as hydrogen bonding potentially playing a greater role.;The origin of cholesterol's condensing effect was revealed via the use of NNR measurements to compare the relative condensing power of three sterols: cholesterol, coprostanol, and dihydrocholesterol. Contrary to what is predicted by the widely accepted "umbrella model," coprostanol was found to be a weaker condensing agent than the other sterols, despite its larger cross-sectional area. To explain this observation, we propose a "template model" for cholesterol's condensing effect, wherein the rigid planar core acts as a template upon which the acyl chains of phospholipids can extend and condense, maximizing hydrophobic contact.