title: ABCA1 dimer-monomer interconversion during HDL generation revealed by single-molecule imaging.
abstract: The generation of high-density lipoprotein (HDL), one of the most critical events for preventing atherosclerosis, is mediated by ATP-binding cassette protein A1 (ABCA1). ABCA1 is known to transfer cellular cholesterol and phospholipids to apolipoprotein A-I (apoA-I) for generating discoidal HDL (dHDL) particles, composed of 100-200 lipid molecules surrounded by two apoA-I molecules; however, the regulatory mechanisms are still poorly understood. Here we observed ABCA1-GFP and apoA-I at the level of single molecules on the plasma membrane via a total internal reflection fluorescence microscope. We found that about 70% of total ABCA1-GFP spots are immobilized on the plasma membrane and estimated that about 89% of immobile ABCA1 molecules are in dimers. Furthermore, an ATPase-deficient ABCA1 mutant failed to be immobilized or form a dimer. We found that the lipid acceptor apoA-I interacts with the ABCA1 dimer to generate dHDL and is followed by ABCA1 dimer-monomer interconversion. This indicates that the formation of the ABCA1 dimer is the key for apoA-I binding and nascent HDL generation. Our findings suggest the physiological significance of conversion of the ABCA1 monomer to a dimer: The dimer serves as a receptor for two apoA-I molecules for dHDL particle generation.
authors.affiliation: Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Kyoto 606-8502, Japan.
authors.forename: Koh O
authors.initials: KO
authors.lastname: Nagata
authors.forename: Chieko
authors.initials: C
authors.lastname: Nakada
authors.forename: Rinshi S
authors.initials: RS
authors.lastname: Kasai
authors.forename: Akihiro
authors.initials: A
authors.lastname: Kusumi
authors.forename: Kazumitsu
authors.initials: K
authors.lastname: Ueda
mesh_terms: D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism*; D016632:Apolipoprotein A-I / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D008075:Lipoproteins, HDL / Q000235:genetics / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D009154:Mutation; D055503:Protein Multimerization / Q000502:physiology*
chemical_list: C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D016632:Apolipoprotein A-I; D008075:Lipoproteins, HDL