ATP hydrolysis-dependent conformational changes in the extracellular domain of ABCA1 are associated with apoA-I binding.

ATP-binding cassette protein A1 (ABCA1) plays a major role in cholesterol homeostasis and high-density lipoprotein (HDL) metabolism. Although it is predicted that apolipoprotein A-I (apoA-I) directly binds to ABCA1, the physiological importance of this interaction is still controversial and the conformation required for apoA-I binding is unclear. In this study, the role of the two nucleotide-binding domains (NBD) of ABCA1 in apoA-I binding was determined by inserting a TEV protease recognition sequence in the linker region of ABCA1. Analyses of ATP binding and occlusion to wild-type ABCA1 and various NBD mutants revealed that ATP binds equally to both NBDs and is hydrolyzed at both NBDs. The interaction with apoA-I and the apoA-I-dependent cholesterol efflux required not only ATP binding but also hydrolysis in both NBDs. NBD mutations and cellular ATP depletion decreased the accessibility of antibodies to a hemagglutinin (HA) epitope that was inserted at position 443 in the extracellular domain (ECD), suggesting that the conformation of ECDs is altered by ATP hydrolysis at both NBDs. These results suggest that ATP hydrolysis at both NBDs induces conformational changes in the ECDs, which are associated with apoA-I binding and cholesterol efflux.

The COP9 signalosome controls ubiquitinylation of ABCA1.

ATP-binding cassette protein A1 (ABCA1) mediates the transfer of cellular free cholesterol and phospholipids to apolipoprotein A-I (apoA-I), an extracellular acceptor in plasma, to form high-density lipoprotein (HDL). ABCA1 has been suggested to be degraded by proteasome in cholesterol-loaded macrophages, however, the mechanism and regulation of proteasomal degradation of ABCA1 remain unclear. In this study, we analyzed the putative interaction between ABCA1 and COP9 signalosome (CSN), a key molecule in controlling protein ubiquitination and deubiquitination. CSN2 and CSN5, subunits of COP9 CSN complex, were coprecipitated with ABCA1 when coexpressed in HEK293 cells and proteasomal degradation was inhibited by MG132. Overexpression of CSN2 increased endogenous CSN7 and CSN8, and decreased ubiquitinylated forms of ABCA1. These results suggest that CSN is a key molecule which controls the ubiquitinylation and deubiquitinylation of ABCA1, and is thus an important target for developing potential drugs to prevent atherosclerosis.

Retroendocytosis pathway of ABCA1/apoA-I contributes to HDL formation.

ATP-binding cassette protein A1 (ABCA1) mediates transfer of cellular free cholesterol and phospholipids to apolipoprotein A-I (apoA-I), an extracellular acceptor in plasma, to form high-density lipoprotein (HDL). It is currently unknown to what extent ABCA1 endocytosis and recycling contribute to the HDL formation. To address this issue, we expressed human ABCA1 constructs with either an extracellular HA tag or an intracellular GFP tag in cells, and used this system to characterize endocytosis and recycling of ABCA1 and apoA-I. Under basal conditions, ABCA1 and apoA-I are endocytosed via a clathrin- and Rab5-mediated pathway and recycled rapidly back to the cell surface, at least in part via a Rab4-mediated route; approximately 30% of the endocytosed ABCA1 is recycled back to the cell surface. When receptor-mediated endocytosis is inhibited, the level of ABCA1 at the cell surface increases and apoA-I internalization is blocked. Under these conditions, apoA-I mediated cholesterol efflux from cells that have accumulated lipoprotein-derived cholesterol is decreased, whereas efflux from cells without excess cholesterol is increased. These results suggest that the retroendocytosis pathway of ABCA1/apoA-I contributes to HDL formation when excess lipoprotein-derived cholesterol has accumulated in cells.