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 Baculovirus-insect cell expression system. We found that human ABCA1 in isolated membranes showed a specific, Mg(2+)-dependent ATP binding but had no measurable ATPase activity. Nevertheless, conformational changes in ABCA1 could be demonstrated by nucleotide occlusion, even without arresting the catalytic cycle by phosphate-mimicking anions. Addition of potential lipid substrates or lipid acceptors (apolipoprotein A-I) did not modify the ATPase activity or nucleotide occlusion by ABCA1. Our data indicate that ATP hydrolysis by ABCA1 occurs at a very low rate, suggesting that ABCA1 may not function as an effective active transporter as previously assumed. In the light of the observed conformational changes we propose a regulatory function for human ABCA1.
Szakács
G
G
National Institute of Haematology and Immunology, Membrane Research Group of the Hungarian Academy of Sciences, Daróczi út 24, H-113 Budapest, Hungary.
Langmann
T
T
Ozvegy
C
C
Orsó
E
E
Schmitz
G
G
Váradi
A
A
Sarkadi
B
B
D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000502:physiology*; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D016632:Apolipoprotein A-I / Q000494:pharmacology; D016367:Baculoviridae / Q000235:genetics; D001693:Biological Transport, Active; D005822:Genetic Vectors; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism; D050356:Lipid Metabolism; D018411:Spodoptera / Q000235:genetics; D014162:Transfection
C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D016632:Apolipoprotein A-I; D000251:Adenosine Triphosphatases