Collet X, Perret B, Simard G, Raffaï E, Marcel Y L
Laboratory of Lipoprotein Metabolism, Institut de Recherches Cliniques de Montreal, Québec, Canada.
J Biol Chem. 1991 May 15;266(14):9145-52.
Recently identified epitopes in apoA-I define a distinct N-terminal region with a complex tertiary structure, characterized by multiple discontinuous epitopes. Other epitopes are constituted of short domains centered either on beta-turns or random coils or on the 22-mer amphipathic alpha-helices (Marcel, Y. L., Provost, P. R., Koa, H., Raffaï, E., Vu Dac, N., Fruchart, J.-C., and Rassart, E. (1991) J. Biol. Chem. 266, 3644-3653). The compared immunoreactivity of seven epitopes studies here in response first to delipidation of high density lipoprotein (HDL) apoA-I by detergents, and second to modifications of HDL lipid composition by phospholipase A2 or by enrichment in surface lipids demonstrates that apoA-I has a flexible conformation which is readily responsive to the nature and concentration of bound lipids and that the structure of lipid-free apoA-I is significantly different from that of HDL-bound apoA-I, possibly representing a condensed molecule with several masked domains. In HDL apoA-I, these epitopes define five distinct domains which are characterized by particular responses to lipid modifications. However, two domains, each starting at the N-terminal beta-turn of an amphipathic alpha-helical repeat (residues 99-121 and 186-209, respectively) have almost identical immunoreactivity whether after detergent treatment or after changes in cholesterol and phospholipid levels, a property which probably reflects the known periodicity of apoA-I structural 22-mers. The immunoreactivity of a discontinuous epitope, representative of the N-terminal domain, is inversely related to the concentration of phospholipids, a unique characteristic among the epitopes tested here which indicates that the complex N-terminal region interacts with phospholipids, either directly or indirectly. These studies demonstrate that the conformation of multiple domains of HDL apoA-I is dependent on lipid phase composition and differentially affected by cholesterol and phospholipids.
载脂蛋白A-I中最近鉴定出的表位定义了一个具有复杂三级结构的独特N端区域，其特征是多个不连续表位。其他表位由以β-转角、无规卷曲或22聚体两亲性α-螺旋为中心的短结构域组成（Marcel, Y. L., Provost, P. R., Koa, H., Raffaï, E., Vu Dac, N., Fruchart, J.-C., and Rassart, E. (1991) J. Biol. Chem. 266, 3644 - 3653）。本文研究的七个表位的免疫反应性首先针对去污剂对高密度脂蛋白（HDL）载脂蛋白A-I的脱脂作用，其次针对磷脂酶A2对HDL脂质组成的修饰或表面脂质富集，结果表明载脂蛋白A-I具有灵活的构象，很容易对结合脂质的性质和浓度做出反应，并且无脂质载脂蛋白A-I的结构与HDL结合的载脂蛋白A-I的结构显著不同，可能代表一个具有多个被掩盖结构域的浓缩分子。在HDL载脂蛋白A-I中，这些表位定义了五个不同的结构域，其特征是对脂质修饰有特定反应。然而，两个结构域，分别从两亲性α-螺旋重复序列的N端β-转角开始（分别为残基99 - 121和186 - 209），无论是在去污剂处理后还是在胆固醇和磷脂水平变化后，都具有几乎相同的免疫反应性，这一特性可能反映了载脂蛋白A-I结构22聚体已知的周期性。一个代表N端结构域的不连续表位的免疫反应性与磷脂浓度呈负相关，这是本文测试的表位中独一无二的特征，表明复杂的N端区域直接或间接与磷脂相互作用。这些研究表明，HDL载脂蛋白A-I多个结构域的构象取决于脂质相组成，并受到胆固醇和磷脂的不同影响。
