Acton S, Resnick D, Freeman M, Ekkel Y, Ashkenas J, Krieger M
Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.
J Biol Chem. 1993 Feb 15;268(5):3530-7.
Macrophage scavenger receptors have been implicated in the development of atherosclerosis and other macrophage-associated functions, including host defense. The mechanism by which these receptors bind a wide array of polyanions, such as acetylated low density lipoprotein (Ac-LDL), with high affinity has not yet been elucidated; however, it has been proposed that the positively charged extracellular collagenous domain of scavenger receptors plays a key role in ligand binding. To test this proposal, we generated truncation mutants of the bovine and murine scavenger receptors and studied their expression in transiently transfected COS cells. These mutants contain only 8 (bovine) or 5 (murine) of the 24 Gly-X-Y tripeptide repeats found in the collagenous domains of the full-length receptors. Immunochemical analyses established that the truncation of the bovine scavenger receptor did not interfere significantly with its synthesis, trimerization, post-translational processing, intracellular transport, surface expression, or stability. However, unlike their full-length counterparts, the truncated bovine and murine receptors were unable to bind Ac-LDL. Thus, the collagenous domain was necessary for normal ligand binding. In addition, cotransfection of the expression vector for the truncated bovine scavenger receptor with that for the full-length receptor resulted in dramatically reduced activity of the full-length construct (dominant negative effect). A ligand bead-binding assay was used to show that the isolated collagenous domain from a different protein, complement component C1q, could bind a wide variety of polyanions with a specificity which was similar, but not identical, to that of scavenger receptors. These results suggest that the collagenous domain of the scavenger receptor is both necessary and sufficient to determine the broad binding specificity that characterizes this unusual receptor. Scavenger receptors and C1q, along with the mannose-binding protein, conglutinin, and lung surfactant apoprotein A, help define a set of proteins which all contain short collagenous domains and which all appear to participate in host defense. Their short collagenous domains may contribute significantly to their host-defense functions.
巨噬细胞清道夫受体与动脉粥样硬化的发展以及其他巨噬细胞相关功能（包括宿主防御）有关。这些受体以高亲和力结合多种多阴离子（如乙酰化低密度脂蛋白（Ac-LDL））的机制尚未阐明；然而，有人提出清道夫受体带正电荷的细胞外胶原结构域在配体结合中起关键作用。为了验证这一观点，我们构建了牛和鼠清道夫受体的截短突变体，并研究了它们在瞬时转染的COS细胞中的表达。这些突变体在全长受体胶原结构域中发现的24个Gly-X-Y三肽重复序列中仅包含8个（牛）或5个（鼠）。免疫化学分析表明，牛清道夫受体的截短对其合成、三聚化、翻译后加工、细胞内运输、表面表达或稳定性没有显著干扰。然而，与全长对应物不同，截短的牛和鼠受体无法结合Ac-LDL。因此，胶原结构域对于正常的配体结合是必需的。此外，将截短的牛清道夫受体表达载体与全长受体表达载体共转染导致全长构建体的活性显著降低（显性负效应）。配体珠结合试验表明，从另一种蛋白质补体成分C1q分离的胶原结构域可以结合多种多阴离子，其特异性与清道夫受体相似但不完全相同。这些结果表明，清道夫受体的胶原结构域对于确定该特殊受体的广泛结合特异性既是必需的也是充分的。清道夫受体和C1q，以及甘露糖结合蛋白、凝聚素和肺表面活性物质载脂蛋白A，有助于定义一组都含有短胶原结构域且似乎都参与宿主防御的蛋白质。它们的短胶原结构域可能对其宿主防御功能有显著贡献。
