EVIDENCE FOR REGULAR DISTRIBUTION OF STEROLS IN LIQUID-CRYSTALLINE PHOSPHATIDYLCHOLINE BILAYERS

To investigate the lateral organization of sterols in membranes, the f luorescence intensity of dehydroergosterol at different mole fractions in liquid crystalline dimyristoyl phosphatidylcholine bilayers was ex amined. A number of intensity drops were observed at specific mole fra ctions, as predicted from a hexagonal super-lattice model. The fluores cence dips provide compelling evidence that a naturally occurring ster ol is regularly distributed at fixed compositional fractions, consiste nt with the presence of hexagonal superlattices in the fluid membranes . Regularly distributed regions, however, coexist with irregularly dis tributed regions. The extent of regular distribution varies periodical ly with sterol mole fraction and, consequently, similar variations tak e place in the membrane volume and lipid packing. This level of modula tion in local membrane structure by minute changes in sterol concentra tion should have profound implications for the functional role of chol esterol content in cell membranes.