SR-BI mediates cholesterol efflux via its interactions with lipid-bound ApoE. Structural mutations in SR-BI diminish cholesterol efflux

Apolipoprotein E (apoE) and the lipoprotein receptor SR-BI play critical roles in lipid and lipoprotein metabolism. We have examined the cholesterol efflux from wild-type (WT) and mutant forms of SR-BI expressed in ldlA-7 cells using reconstituted discoidal particles consisting of apoE, 1-palmitoyl-2-oleoyl-l-phospatidylcholine (POPC), and cholesterol (C) as acceptors. POPC/C-apoE particles generated using apoE2, apoE3, apoE4, or carboxy-terminally truncated forms apoE4-165, apoE4-202, apoE4-229, and apoE4-259 caused similar (20-25%) cholesterol efflux from WT SR-BI. Cholesterol efflux mediated by POPC/C-apoE was not enhanced in the presence of lipid-free apoE. The rate of cholesterol efflux mediated by particles containing the WT or carboxy-terminally truncated forms of apoE was decreased to approximately 30% of the WT control with the Q402R/Q418R mutant SR-BI form that is unable to bind native HDL normally but binds LDL. The rate of cholesterol efflux was further decreased to approximately 7% of the WT control with another SR-BI mutant (M158R) that binds neither HDL nor LDL. The level of binding of POPC/C-apoE particles (150 microg/mL) to SR-BI mutant forms Q402R/Q418R and M158R was 70 and 8% of the WT control, respectively. SR-BI-dependent binding of lipid-free apoE to cells was undetectable, and cholesterol efflux was less than 0.5%. The findings establish that only lipid-bound apoE promotes SR-BI-mediated cholesterol efflux and that the amino-terminal region of residues 1-165 of apoE is sufficient for both receptor binding and cholesterol efflux. The SR-BI-apoE interactions may contribute to overall cholesterol homeostasis in cells and tissues that express SR-BI and apoE.