ApoA-I structure on discs and spheres. Variable helix registry and conformational states.
Apolipoprotein A-I (apoA-I) readily forms discoidal high density lipoprotein (HDL) particles with phospholipids serving as an ideal transporter of plasma cholesterol. In the lipid-bound conformation, apoA-I activates the enzyme lecithin:cholesterol acyltransferase stimulating the formation of cholesterol esters from free cholesterol. As esterification proceeds cholesterol esters accumulate within the hydrophobic core of the discoidal phospholipid bilayer transforming it into a spherical HDL particle. To investigate the change in apoA-I conformation as it adapts to a spherical surface, fluorescence resonance energy transfer studies were performed. Discoidal rHDL particles containing two lipid-bound apoA-I molecules were prepared with acceptor and donor fluorescent probes attached to cysteine residues located at specific positions. Fluorescence quenching was measured for probe combinations located within repeats 5 and 5 (residue 132), repeats 5 and 6 (residues 132 and 154), and repeats 6 and 6 (residue 154). Results from these experiments indicated that each of the 2 molecules of discoidal bound apoA-I exists in multiple conformations and support the concept of a "variable registry" rather than a "fixed helix-helix registry." Additionally, discoidal rHDL were transformed in vitro to core-containing particles by incubation with lecithin:cholesterol acyltransferase. Compositional analysis showed that core-containing particles contained 11% less phospholipid and 633% more cholesterol ester and a total of 3 apoA-I molecules per particle. Spherical particles showed a lowering of acceptor to donor probe quenching when compared with starting rHDL. Therefore, we conclude that as lipid-bound apoA-I adjusts from a discoidal to a spherical surface its intermolecular interactions are significantly reduced presumably to cover the increased surface area of the particle.
Li
Hui-Hua
HH
Department of Pathology, The Wake Forest University Baptist Medical Center, Winston-Salem, North Carolina 27157, USA.
Lyles
Douglas S
DS
Pan
Wei
W
Alexander
Eric
E
Thomas
Michael J
MJ
Sorci-Thomas
Mary G
MG
D016632:Apolipoprotein A-I / Q000737:chemistry* / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism; D002788:Cholesterol Esters / Q000378:metabolism; D003432:Cross-Linking Reagents / Q000494:pharmacology; D003545:Cysteine / Q000737:chemistry; D004591:Electrophoresis, Polyacrylamide Gel; D031541:Fluorescence Resonance Energy Transfer; D006801:Humans; D008051:Lipid Bilayers; D050356:Lipid Metabolism; D008075:Lipoproteins, HDL / Q000737:chemistry; D008077:Lipoproteins, LDL / Q000378:metabolism; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D013696:Temperature; D013997:Time Factors
D016632:Apolipoprotein A-I; D002788:Cholesterol Esters; D003432:Cross-Linking Reagents; D008051:Lipid Bilayers; D008075:Lipoproteins, HDL; D008077:Lipoproteins, LDL; D002784:Cholesterol; D003545:Cysteine