Specific docking of apolipoprotein A-I at the cell surface requires a functional ABCA1 transporter.

The identification of defects in ABCA1 as the molecular basis of Tangier disease has highlighted its crucial role in the loading with phospholipids and cholesterol of nascent apolipoprotein particles. Indeed the expression of ABCA1 affects apolipoprotein A-I (apoA-I)-mediated removal of lipids from cell membranes, and the possible role of ABCA1 as an apoA-I surface receptor has been recently suggested. In the present study, we have investigated the role of the ABCA1 transporter as an apoA-I receptor with the analysis of a panel of transfectants expressing functional or mutant forms of ABCA1. We provide experimental evidence that the forced expression of a functional ABCA1 transporter confers surface competence for apoA-I binding. This, however, appears to be dependent on ABCA1 function. Structurally intact but ATPase-deficient forms of the transporter fail to elicit a specific cell association of the ligand. In addition the diffusion parameters of membrane-associated apoA-I indicate an interaction with membrane lipids rather than proteins. These results do not support a direct molecular interaction between ABCA1 and apoA-I, but rather suggest that the ABCA1-induced modification of the lipid distribution in the membrane, evidenced by the phosphatidylserine exofacial flopping, generates a biophysical microenvironment required for the docking of apoA-I at the cell surface.

ABC1 promotes engulfment of apoptotic cells and transbilayer redistribution of phosphatidylserine.

ATP-binding-cassette transporter 1 (ABC1) has been implicated in processes related to membrane-lipid turnover. Here, using in vivo loss-of-function and in vitro gain-of-function models, we show that ABC1 promotes Ca2+-induced exposure of phosphatidylserine at the membrane, as determined by a prothrombinase assay, membrane microvesiculation and measurement of transbilayer redistribution of spin-labelled phospholipids. That ABC1 promotes engulfment of dead cells is shown by the impaired ability of ABC1-deficient macrophages to engulf apoptotic preys and by the acquisition of phagocytic behaviour by ABC1 transfectants. Release of membrane phospholipids and cholesterol to apo-AI, the protein core of the cholesterol-shuttling high-density lipoprotein (HDL) particle, is also ABC1-dependent. We propose that both the efficiency of apoptotic-cell engulfment and the efflux of cellular lipids depend on ABC1-induced perturbation of membrane phosphatidylserine turnover. Transient local exposure of anionic phospholipids in the outer membrane leaflet may be sufficient to alter the general properties of the membrane and thus influence discrete physiological functions.

Transport of lipids from golgi to plasma membrane is defective in tangier disease patients and Abc1-deficient mice.

Mutations in the gene encoding ATP-binding cassette transporter 1 ( ABC1) have been reported in Tangier disease (TD), an autosomal recessive disorder that is characterized by almost complete absence of plasma high-density lipoprotein (HDL), deposition of cholesteryl esters in the reticulo-endothelial system (RES) and aberrant cellular lipid trafficking. We demonstrate here that mice with a targeted inactivation of Abc1 display morphologic abnormalities and perturbations in their lipoprotein metabolism concordant with TD. ABC1 is expressed on the plasma membrane and the Golgi complex, mediates apo-AI associated export of cholesterol and phospholipids from the cell, and is regulated by cholesterol flux. Structural and functional abnormalities in caveolar processing and the trans-Golgi secretory pathway of cells lacking functional ABC1 indicate that lipid export processes involving vesicular budding between the Golgi and the plasma membrane are severely disturbed.