Model Plasma Membrane exhibits a Microemulsion in both Leaves providing a Foundation for “Rafts”

Abstract

AbstractWe consider a model plasma membrane, one that describes the outer leaf as consisting of sphingomyelin, phosphatidylcholine, and cholesterol, and the inner leaf of phosphatidylethanolamine, phosphatidylserine, phosphatidyl-choline, and cholesterol. Their relative compositions are taken from experiment, and the cholesterol freely interchanges between leaves. Fluctuations in composition are coupled to fluctuations in the membrane height as in the Leibler-Andelman mechanism. Provided that the membrane is of relatively constant thickness, this coupling of fluctuations also provides a coupling between the composition fluctuations of the two leaves. Structure functions display, for components in both leaves, a peak at non-zero wavevector. This indicates that the disordered fluid membrane is characterized by structure on a scale given by membrane properties. From measurements on the plasma membrane, this scale is on the order of 100 nm. The theory provides a tenable basis for the origin of “rafts”.Statement of SignificanceThe hypothesis that the plasma membrane is not homogeneous, but rather is heterogeneous, with rafts” of one composition floating in a sea of another, has overturned conventional views of this membrane and how it functions. Proteins prefer either the raft or the sea, and so are not uniformly distributed. Hence they perform more efficiently. From experiment, rafts are thought to be about 100 nm. However there is no realistic model that provides: a length scale for the rafts; a raft in both leaves of the membrane; the composition of the raft. We provide such a model. In contrast to other theories, the raft and sea are distinguished not only by composition, but also by a difference in curvature.