Distinctive structure and function of human apolipoprotein variant ApoA-IV-2.
We have investigated the molecular structure, phospholipid binding, and lecithin-cholesterol acyltransferase catalytic activity of pure apoA-IV-2, a basic variant isoform of apoA-IV which is inherited as a classical Mendelian allele with a gene frequency of 0.09. Circular dichroism spectroscopy established that the alpha-helical content of apoA-IV-2 was 75% in the native state (versus 56% for apoA-IV-1), and increased to 88% in the presence of phospholipid. Fluorescence titration established that apoA-IV-2 bound to egg phospholipid vesicles with a Ka of 3.3 x 10(6) liter/mol, 2.4-fold greater than the affinity of apoA-IV-1. Fluorescence quenching studies revealed that, unlike apoA-IV-1, binding of apoA-IV-2 to phospholipid vesicles induced strong shielding of the amino-terminal tryptophan against iodide quenching. Enzyme kinetic studies using both saturated and unsaturated phospholipid substrates demonstrated that apoA-IV-2 was 36-71% more efficient in activating lecithin-cholesterol acyltransferase than apoA-IV-1. We conclude that apoA-IV-2 has more alpha-helical structure, is more stable in solution, and is more hydrophobic than apoA-IV-1, and that these distinctive structural features are associated with a higher affinity for phospholipid surfaces and an increased catalytic efficiency of lecithin:cholesterol acyltransferase activation. The biophysical basis for this latter characteristic may be the ability of apoA-IV-2 to penetrate phospholipid surfaces to a greater depth than apoA-IV-1. These molecular properties may be responsible for the increased levels of high density lipoproteins which have been observed in apoA-IV-2 heterozygotes.
Department of Medicine, University of Texas Health Science Center, Houston 77225.
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RB
Weinberg
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MK
Jordan
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Steinmetz
D000483:Alleles; D001054:Apolipoproteins A / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D002942:Circular Dichroism; D004789:Enzyme Activation; D006801:Humans; D066298:In Vitro Techniques; D015394:Molecular Structure; D007862:Phosphatidylcholine-Sterol O-Acyltransferase / Q000378:metabolism; D010743:Phospholipids / Q000378:metabolism; D011487:Protein Conformation; D011489:Protein Denaturation; D013050:Spectrometry, Fluorescence
D001054:Apolipoproteins A; D010743:Phospholipids; C022205:apolipoprotein A-IV; D007862:Phosphatidylcholine-Sterol O-Acyltransferase