Characterization of the ATPase cycle of human ABCA1: Implications for its function as a regulator rather than an active transporter

ABCA1 plays a key role in cellular cholesterol and phospholipid traffic. To explore the biochemical properties of this membrane protein we applied a B aculovirus-insect cell expression system. We found that human ABCA1 in isol ated membranes showed a specific, Mg2+-dependent ATP binding but had no mea surable ATPase activity. Nevertheless, conformational changes in ABCA1 coul d be demonstrated by nucleotide occlusion, even without arresting the catal ytic cycle by phosphate-mimicking anions. Addition of potential lipid subst rates or lipid acceptors (apolipoprotein A-I) did not modify the ATPase act ivity or nucleotide occlusion by ABCA1. Our data indicate that ATP hydrolys is by ABCA1 occurs at a very low rate, suggesting that ABCA1 may not functi on as an effective active transporter as previously assumed. In the light o f the observed conformational changes we propose a regulatory function for human ABCA1. (C) 2001 Academic Press.