Sterol domains in phospholipid membranes: dehydroergosterol polarization measures molecular sterol transfer

Abstract

The domain structure of cholesterol in membranes and factors affecting it are not well understood. A method, based on kinetics of Δ5,7,9,(11),22-ergostatetraen-3β-ol (dehydroergosterol) fluorescence polarization change and not requiring separation of donor and acceptor membranes, was used to examine sterol domains in three-component cholesterol: dehydroergosterol: phospholipid small unilamellar vesicles (SUV). A new mathematical data treatment was developed to provide a direct correlation between molecular sterol exchnage and steady-state dehydroergosterol fluorescence polarization measurements. The method identified multiple kinetic pools of sterol in SUV: a small but rapidly exchanging pool, a predominant slowly exchanging pool, and a very slowly exchangeable (nonexchangeable) pool. The relative sizes of the pools and half-times of exchange were highly dependent on the presence of acidic phospholipids and on cytosolic proteins involved in sterol transfer. Thus, the method provides a direct measure of molecular sterol transfer between membranes without separating donor and acceptor membranes.