Specific binding of ApoA-I, enhanced cholesterol efflux, and altered plasma membrane morphology in cells expressing ABC1

Mutations of the ABC1 transporter have been identified as the defect in Tan gier disease, characterized by low HDL and cholesterol ester accumulation i n macrophages. A full-length mouse ABC1 cDNA was used to investigate the me chanisms of lipid efflux to apoA-I or HDL in transfected 293 cells, ABC1 ex pression markedly increased cellular cholesterol and phospholipid efflux to apoA-I but had only minor effects on lipid efflux to HDL. The increased li pid efflux appears to involve a direct interaction between apoA-I and ABC1, because ABC1 expression substantially increased apoA-I binding at the cell , surface, and chemical cross-linking and immunoprecipitation analysis show ed that apoA-I binds directly to ABC1. In contrast to scavenger receptor BI (SR-BI), another cell surface molecule capable of facilitating cholesterol efflux, ABC1 preferentially bound lipid-free apoA-I but not HDL, Immunoflu orescence confocal microscopy showed that ABC1 is primarily localized on th e cell surface. In the absence of apoA-I, cells overexpressing ABC1 display ed a distinctive morphology, characterized by plasma membrane protrusions a nd resembling echinocytes that form when there are excess lipids in the out er membrane hemileaflet, The studies provide evidence for a direct interact ion between ABC1 and apoA-I, but not HDL, indicating that free apoA-I is th e metabolic substrate for ABC1, Plasma membrane ABC1 may act as a phospholi pid/cholesterol flippase, providing lipid to bound apoA-I, or to the outer membrane hemileaflet.