Extent of raft composition in a model plasma membrane.

"Rafts" are nanometer-size inhomogeneities in the plasma membrane that, in the outer leaflet, are enriched in sphingomyelin and cholesterol. They are thought to provide a platform for proteins to carry out biological processes. Here, we employ a model asymmetric plasma membrane to address the question of the range of sphingomyelin and cholesterol compositions in which one would expect the formation of rafts. We define a weight for the likelihood of raft formation and evaluate it as a function of the sphingomyelin mole fraction in the outer leaflet for three bilayers with total cholesterol mole fractions of 0.30, 0.40, and 0.50. Not surprisingly, the weight decreases when there is little sphingomyelin. Less expected, we find that the weight also decreases when there is a large mole fraction of sphingomyelin. The weight is largest in the bilayer with a total cholesterol mole fraction of 0.30 and decreases rapidly with increasing total cholesterol. We explicate the reasons for these behaviors. In the 0.30 cholesterol bilayer, the largest weight occurs at a sphingomyelin mole fraction in the outer leaflet of approximately 0.23. The weight falls to one half its maximum value at sphingomyelin mole fractions of 0.15 and 0.33. In terms of the sphingomyelin mole fraction of the asymmetric bilayer, the maximum weight occurs at 0.12 and falls to half maximum at 0.08 and 0.17.