Distinct central amphipathic alpha-helices in apolipoprotein A-I contribute to the in vivo maturation of high density lipoprotein by either activating lecithin-cholesterol acyltransferase or binding lipids.
Recombinant adenoviruses with cDNAs for human apolipoprotein A-I (wild type (wt) apoA-I) and three mutants, referred to as Delta4-5A-I, Delta5-6A-I, and Delta6-7A-I, that have deletions removing regions coding for amino acids 100-143, 122-165, and 144-186, respectively, were created to study structure/function relationships of apoA-I in vivo. All mutants were expressed at lower concentrations than wt apoA-I in plasma of fasting apoA-I-deficient mice. The Delta5-6A-I mutant was found primarily in the lipid-poor high density lipoprotein (HDL) pool and at lower concentrations than Delta4-5A-I and Delta6-7A-I that formed more buoyant HDL(2/3) particles. At an elevated adenovirus dose and earlier blood sampling from fed mice, both Delta5-6A-I and Delta6-7A-I increased HDL-free cholesterol and phospholipid but not cholesteryl ester. In contrast, wt apoA-I and Delta4-5A-I produced significant increases in HDL cholesteryl ester. Further analysis showed that Delta6-7A-I and native apoA-I could bind similar amounts of phospholipid and cholesterol that were reduced slightly for Delta5-6A-I and greatly for Delta4-5A-I. We conclude from these findings that amino acids (aa) 100-143, specifically helix 4 (aa 100-121), contributes to the maturation of HDL through a role in lipid binding and that the downstream sequence (aa 144-186) centered around helix 6 (aa 144-165) is responsible for the activation of lecithin-cholesterol acyltransferase.
McManus
D C
DC
Lipoprotein and Atherosclerosis Research Group and the Department of Pathology and Laboratory Medicine, University of Ottawa Heart Institute, Ottawa, Ontario K1Y 4W7, Canada.
Scott
B R
BR
Frank
P G
PG
Franklin
V
V
Schultz
J R
JR
Marcel
Y L
YL
D000818:Animals; D016632:Apolipoprotein A-I / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018076:DNA, Complementary; D004789:Enzyme Activation; D006801:Humans; D050356:Lipid Metabolism; D008075:Lipoproteins, HDL / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D009154:Mutation; D007862:Phosphatidylcholine-Sterol O-Acyltransferase / Q000378:metabolism*; D011487:Protein Conformation; D011499:Protein Processing, Post-Translational; D017384:Sequence Deletion; D014461:Ultracentrifugation
D016632:Apolipoprotein A-I; D018076:DNA, Complementary; D008075:Lipoproteins, HDL; D007862:Phosphatidylcholine-Sterol O-Acyltransferase