Hillarp A, Dahlbäck B
Department of Clinical Chemistry, University of Lund, Malmö General Hospital, Sweden.
J Biol Chem. 1988 Sep 5;263(25):12759-64.
C4b-binding protein (C4BP) is a multimeric protein with regulatory functions in the complement system. It also interacts with vitamin K-dependent protein S, which is involved in the regulation of the coagulation system. It has been demonstrated that C4BP consists of seven disulfide-linked, identical 70-kDa subunits, which are arranged to give the molecule a spider-like structure. We now have evidence for the presence of a new subunit in C4BP. On sodium dodecyl sulfate-poly-acrylamide gel electrophoresis it appears as a weakly stainable band with a molecular weight of approximately 45,000. The subunit was isolated by gel filtration in 6 M guanidine hydrochloride of reduced and carboxymethylated C4BP. Its amino-terminal sequence is distinct from previously known protein sequences. The stoichiometry of 45- to 70-kDa subunits was estimated to be 1:9, indicating the presence of one 45-kDa subunit per C4BP molecule. The new subunit was demonstrated to be a disulfide-linked component of the central core of C4BP. It was sensitive to proteolysis by chymotrypsin, and when cleaved the protein S binding ability of C4BP was lost. With protein S bound to C4BP, the 45-kDa subunit was protected from degradation by chymotrypsin, and the protein S binding site remained intact. These data suggest that the new subunit is directly involved in protein S binding.
C4b结合蛋白（C4BP）是一种在补体系统中具有调节功能的多聚体蛋白。它还与维生素K依赖性蛋白S相互作用，后者参与凝血系统的调节。已经证明C4BP由七个通过二硫键连接的相同的70 kDa亚基组成，这些亚基排列后使分子呈蜘蛛状结构。我们现在有证据表明C4BP中存在一种新的亚基。在十二烷基硫酸钠-聚丙烯酰胺凝胶电泳上，它表现为一条染色较弱的带，分子量约为45,000。通过在6 M盐酸胍中对还原和羧甲基化的C4BP进行凝胶过滤分离出该亚基。其氨基末端序列与先前已知的蛋白质序列不同。45 kDa至70 kDa亚基的化学计量比估计为1:9，表明每个C4BP分子存在一个45 kDa亚基。新亚基被证明是C4BP中心核心的二硫键连接成分。它对胰凝乳蛋白酶的蛋白水解敏感，裂解后C4BP的蛋白S结合能力丧失。当蛋白S与C4BP结合时，45 kDa亚基受到胰凝乳蛋白酶降解的保护，且蛋白S结合位点保持完整。这些数据表明新亚基直接参与蛋白S的结合。
