REGULATION OF MEMBRANE CHOLESTEROL DOMAINS BY STEROL CARRIER PROTEIN-2

Sterols are not randomly distributed in membranes but appear to be loc alized in multiple kinetic domains. Factors that regulate these sterol domains are not well-understood. A recently developed fluorescence po larization assay that measures molecular sterol transfer [Butko, P., H apala,I., Nemecz, G., of Schroeder, F. (1992) J. Biochem. Biophys. Met hods 24, 15-37] was used to examine the mechanism whereby anionic phos pholipids and liver sterol carrier protein-2 (SCP2) enhance sterol tra nsfer. Two exchangeable and one very slowly or nonexchangeable sterol domain were resolved in phosphatidylcholine (POPC)/sterol small unilam ellar vesicles (SUV). Inclusion of 10 mol % anionic phospholipids enha nced sterol exchange primarily by redistribution of sterol domain size s rather than by alteration of half-times of exchange. This effect was dependent primarily on the percent content rather than the net charge per anionic phospholipid. In contrast, SCP2 simultaneously altered bo th the distribution of sterol molecules between kinetic domains and th e exchange half-times of exchangeable sterol domains. The effects of S CP2 were much more pronounced when 10% acidic phospholipid was incorpo rated in the SUV. Compared to spontaneous sterol exchange, in the pres ence of 1.5 mu M SCP2, the rapidly exchanging pool was increased by 36 to 330%, depending on the SUV phospholipid composition. Concomitantly , exchange half-times for rapidly and slowly exchangeable sterol were reduced by 60 to 98% for (1)t(1/2) and 14 to 85% for (2)t(1/2), respec tively. The stimulatory effect of SCP2 was saturable and dependent bot h on protein concentration and on content of acidic phospholipids in m embranes. The effect of acidic phospholipids on SCP2 action. The effec t of SCP2 on sterol-exchange kinetics cannot be explained solely by it s ability to bind sterols as other sterol-binding protein (liver fatty acid binding protein) did not stimulate sterol exchange. The results indicate that the mechanism of SCP2 action involves both changes in me mbrane domain structure (relative sizes of sterol kinetic pools) and c hanges in the membrane-water interface (sterol desorption rate) induce d upon interaction of protein with the membrane surface.