A Mass Spectrometric Determination of the Conformation of Dimeric Apolipoprotein A-I in Discoidal High Density Lipoproteins

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• 作者：R. A. Gangani D. Silva ; George M. Hilliard ; Ling Li ; Jere P. Segrest ; and W. Sean Davidson
• 刊名：Biochemistry
• 出版年：2005
• 出版时间：June 21, 2005
• 年：2005
• 卷：44
• 期：24
• 页码：8600 - 8607
• 全文大小：286K
• 年卷期：v.44,no.24(June 21, 2005)
• ISSN：1520-4995

文摘

Discoidal forms of high density lipoproteins (HDL) are critical intermediates between lipid-poor apolipoprotein A-I (apo A-I), the major protein constituent of HDL, and the mature spherical formsthat comprise the bulk of circulating particles. Thus, many studies have focused on understanding apoA-Istructure in discs reconstituted in vitro. Recent theoretical and experimental work supports a "belt" modelfor apoA-I in which repeating amphipathic helical domains run parallel to the plane of the lipid disc.However, disc-associated apoA-I can adopt several tertiary arrangements that are consistent with a beltorientation. To distinguish among these, we cross-linked near-neighbor Lys groups in homogeneous 96Å discs containing exactly two molecules of apoA-I. After delipidation and tryptic digestion, massspectrometry was used to identify 9 intermolecular and 11 intramolecular cross-links. The cross-linkingpattern strongly suggests a "double-belt" molecular arrangement for apoA-I in which two apoA-I moleculeswrap around the lipid bilayer disc forming two stacked rings in an antiparallel orientation with helix 5 ofeach apoA-I in juxtaposition (LL5/5 orientation). The data also suggests the presence of an additionaldouble-belt orientation with a shifted helical registry (LL5/2 orientation). Furthermore, a 78 Å particlewith two molecules of apoA-I fit a similar double-belt motif with evidence for conformational changesin the N-terminus and the region near helix 5. A comparison of this work to a previous study is suggestivethat a third molecule of apoA-I can form a hairpin in larger particles containing three molecules ofapoA-I.