Apolipoprotein A-I conformation in discoidal particles: evidence for alternate structures.
To define the conformation of apolipoprotein A-I in discoidal particles, the immunoreactivity of a series of epitopes distributed along the apoA-I sequence has been evaluated in lipid-free apoA-I and in lipid-bound form. To this end, reconstituted discoidal lipoproteins, here called LpA-I, and defined by number of apoA-I per particle (e.g., Lp2A-I), have been prepared with palmitoyloleoylphosphatidylcholine, cholesterol, and apoA-I. Four LpA-I have been obtained and studied: two in the Lp2A-I class, 7.8 and 9.6 nm in diameter, and two in the Lp3A-I class, 10.8 and 13.4 nm. The immunoreactivity of all the epitopes tested was significantly different in LpA-I particles compared to lipid-free apoA-I, demonstrating that binding to lipids produces a drastic change in apoA-I conformation. Specific domains in the primary sequence become highly exposed while others are masked. Although the variation in immunoreactivity of the epitopes between various LpA-I was not drastic, significant differences in the calculated ED50 values were observed for a number of antibodies in small versus large particles within each class (Lp2A-I or Lp3A-I), indicating that particle size can modulate apoA-I conformation. In addition, when the competition between pairs of mAbs was analyzed in order to understand the relative position of epitopes, highly significant differences were observed as a function of particle size within each class. In particular, the competition between mAbs recognizing epitopes in the central region of apoA-I was greater in the larger particles than in their small counterparts.(ABSTRACT TRUNCATED AT 250 WORDS)
Calabresi
L
L
Lipoproteins and Atherosclerosis Group, University of Ottawa Heart Institute, Ontario, Canada.
Meng
Q H
QH
Castro
G R
GR
Marcel
Y L
YL
D000911:Antibodies, Monoclonal / Q000378:metabolism; D016632:Apolipoprotein A-I / Q000737:chemistry* / Q000302:isolation & purification; D001666:Binding Sites, Antibody; D001667:Binding, Competitive; D002784:Cholesterol; D000939:Epitopes / Q000032:analysis; D006801:Humans; D008077:Lipoproteins, LDL / Q000097:blood / Q000302:isolation & purification; D008081:Liposomes; D008961:Models, Structural; D010713:Phosphatidylcholines; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011863:Radioimmunoassay; D012016:Reference Values
D000911:Antibodies, Monoclonal; D016632:Apolipoprotein A-I; D000939:Epitopes; D008077:Lipoproteins, LDL; D008081:Liposomes; D010713:Phosphatidylcholines; D002784:Cholesterol; C028694:1-palmitoyl-2-oleoylphosphatidylcholine