Sterols are not randomly distributed in membranes but appear to be localized in multiple kinetic domains. Factors that regulate these sterol domains are not well-understood. A recently developed fluorescence polarization assay that measures molecular sterol transfer [Butko, P., Hapala, I., Nemecz, G., of Schroeder, F. (1992) J. Biochem. Biophys. Methods 24, 15-37] was used to examine the mechanism whereby anionic phospholipids and liver sterol carrier protein-2 (SCP2) enhance sterol transfer. Two exchangeable and one very slowly or nonexchangeable sterol domain were resolved in phosphatidylcholine (POPC)/sterol small unilamellar vesicles (SUV). Inclusion of 10 mol % anionic phospholipids enhanced sterol exchange primarily by redistribution of sterol domain sizes 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 both the distribution of sterol molecules between kinetic domains and the exchange half-times of exchangeable sterol domains. The effects of SCP2 were much more pronounced when 10% acidic phospholipid was incorporated in the SUV. Compared to spontaneous sterol exchange, in the presence of 1.5 microM 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 1t1/2 and 14 to 85% for 2t1/2, respectively. The stimulatory effect of SCP2 was saturable and dependent both on protein concentration and on content of acidic phospholipids in membranes.(ABSTRACT TRUNCATED AT 250 WORDS)