MECHANISTIC STUDIES OF STEROL CARRIER PROTEIN-2 EFFECTS ON L-CELL FIBROBLAST PLASMA-MEMBRANE STEROL DOMAINS

The factors which regulate intermembrane sterol domains and exchange i n biomembranes are not well understood. A new fluorescent sterol excha nge assay allowed correlation of changes in polarization to sterol tra nsfer. Analysis of spontaneous sterol exchange between L-cell plasma m embranes indicated two exchangeable and one very slowly or nonexchange able sterol domain. The exchangeable domains exhibited halftimes of 23 and 140 min with fractional contributions of 5 and 30%, respectively. Sterol carrier protein-2 (SCP-2) enhanced sterol exchange between L-c ell plasma membranes and altered sterol domain size in a concentration dependent manner. Previous model membrane studies indicate that SCP-2 alters sterol domains and exchange through interaction with anionic p hospholipids. In contrast to these observations, the ionic shielding a gents KCl, low pH, or neomycin were either totally or partially ineffe ctive inhibitors of SCP-2 action in L-cell plasma membrane exchanges. Thus the mechanism of SCP-2 in sterol transfer appears to be less char ge dependent in L-cell plasma membranes than in model membranes. The c holesterol lowering drug probucol was also capable of altering the ste rol exchange kinetics.