| Summary: | The contribution of ABCA1-mediated efflux of cellular phospholipid (PL) and cholesterol to human apolipoprotein A-I (apoA-I) to the formation of preβ1-HDL (or lipid-poor apoA-I) is not well defined. To explore this issue, we characterized the nascent HDL particles formed when lipid-free apoA-I was incubated with fibroblasts in which expression of the ABCA1 was upregulated. After a 2 h incubation, the extracellular medium contained small apoA-I/PL particles (preβ1-HDL; diameter = 7.5 ± 0.4 nm). The preβ1-HDL (or lipid-poor apoA-I) particles contained a single apoA-I molecule and three to four PL molecules and one to two cholesterol molecules. An apoA-I variant lacking the C-terminal α-helix did not form such particles when incubated with the cell, indicating that this helix is critical for the formation of lipid-poor apoA-I particles. These preβ1-HDL particles were as effective as lipid-free apoA-I molecules in mediating both the efflux of cellular lipids via ABCA1 and the formation of larger, discoidal HDL particles. In conclusion, preβ1-HDL is both a product and a substrate in the ABCA1-mediated reaction to efflux cellular PL and cholesterol to apoA-I. A monomeric apoA-I molecule associated with three to four PL molecules (i.e., lipid-poor apoA-I) has similar properties to the lipid-free apoA-I molecule.
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