Formation of Two Intramolecular Disulfide Bonds Is Necessary for ApoA-I-dependent Cholesterol Efflux Mediated by ABCA1

ABCA1 plays a major role in cholesterol homeostasis and high density lipoprotein (HDL) metabolism. ABCA1 contains disulfide bond(s) between its N- and C-terminal halves, but it remains unclear whether disulfide bond formation is important for the functions of ABCA1 and which cysteines are involved in disulfide bond formation. To answer these questions, we constructed >30 ABCA1 mutants in which 16 extracellular domain (ECD) cysteines were replaced with serines and examined disulfide bond formation, apoA-I binding, and HDL formation in these mutants. From the single cysteine replacements, two cysteines (Cys(75) and Cys(309)) in ECD1 were found to be essential for apoA-I binding. In contrast, in ECD2, only Cys(1477) was found to be essential for HDL formation, and no single cysteine replacement impaired apoA-I binding. The concurrent replacement of two cysteines, Cys(1463) and Cys(1465), impaired apoA-I binding and HDL formation, suggesting that four of five extracellular cysteines (Cys(75), Cys(309), Cys(1463), Cys(1465), and Cys(1477)) are involved in these functions of ABCA1. Trypsin digestion experiments suggested that one disulfide bond is not sufficient and that two intramolecular disulfide bonds (between Cys(75) and Cys(309) in ECD1 and either Cys(1463) or Cys(1465) and Cys(1477) in ECD2) are required for ABCA1 to be fully functional.