Kojouharova M S, Panchev I D, Tchorbadjieva M I, Reid K B, Hoppe H J
Department of Biochemistry, Sofia University St. Climent Ohridsky, Bulgaria.
J Immunol. 1998 Oct 15;161(8):4325-31.
Two individual globular head regions (ghA and ghB) of the heterotrimeric C1q molecule (containing A, B, and C chains) were expressed in a bacterial expression system using a coproduction with the bacterial chaperone GroESL. The purified proteins were soluble and monomeric, as shown by gel-filtration analysis. No association into homotrimers was seen, which indicates that the ability to form heterotrimers is coupled with the discrimination against homotrimeric self-association. The individual globular heads retained their binding activities toward two ligands bound by the whole C1q molecule, i.e., IgG and the peptide P(601-613) derived from the HIV envelope glycoprotein gp41. The differential binding activities displayed for these ligands indicated a degree of structural independence of the binding sites from the regions responsible for heterotrimerization. It was found, using single chain recombinant anti-C1q Abs, that the binding sites on C1q for IgG and gp41 do not overlap, and this observation is also consistent with the view that specialization between the C1q polypeptide chains takes place within the C1q molecule regarding their ligand-binding activities.
异源三聚体C1q分子（包含A、B和C链）的两个独立球状头部区域（ghA和ghB），在细菌表达系统中与细菌伴侣蛋白GroESL共同表达。凝胶过滤分析表明，纯化后的蛋白质是可溶的单体。未观察到形成同三聚体，这表明形成异三聚体的能力与对同三聚体自我缔合的区分有关。单个球状头部保留了它们对整个C1q分子所结合的两种配体的结合活性，即IgG和源自HIV包膜糖蛋白gp41的肽P(601 - 613)。对这些配体表现出的不同结合活性表明，结合位点在一定程度上独立于负责异三聚化的区域的结构。使用单链重组抗C1q抗体发现，C1q上针对IgG和gp41的结合位点不重叠，这一观察结果也与以下观点一致：C1q多肽链之间的特化在C1q分子内就其配体结合活性而言是发生的。
