DiScipio R G, Hugli T E
J Biol Chem. 1985 Nov 25;260(27):14802-9.
The architecture of human complement component C9 and poly(C9) was investigated by transmission electron microscopy. Monomeric native C9 (Mr = 66,000) exhibits an ellipsoid appearance (70 X 50 A) with a crevice visible on one face. C9 polymerizes spontaneously to form hollow tubular structures consisting of 12-16 monomeric subunits. Poly(C9) is a cylinder (150 A-outer diameters and 90 A-inner diameter) rimmed by a torus (46-A thick) on one end. Electron micrographs of poly(C9) indicate that the torus is formed by radial strands of polypeptide. Each subunit of poly(C9) is apparently tilted relative to the central axis of the cylindrical structure. C9 can be cleaved by alpha-thrombin into two single-chain polypeptide fragments: C9a (Mr = 28,000) and C9b (Mr = 38,000), which are the amino- and carboxyl-terminal segments of the protein, respectively. The cleaved form of the protein, C9a,b, can be induced to polymerize under suitable conditions to form sodium dodecyl sulfate-resistant poly(C9), indicating that the resistance of poly(C9) to denaturation is a collective feature of both C9a and C9b. The C9a and C9b polypeptide regions have been mapped on poly(C9) by immunoelectron microscopy. Determinants for the C9a region were observed about the torus, base, and on the midsection of the poly(C9) cylinder. C9b epitopes are concentrated predominantly about the torus and base, but were rarely observed on the midsection of poly(C9). Thus, the C9a and C9b segments of the C9 polypeptide are not clearly segregated in poly(C9). The locations of oligosaccharide units on poly(C9) were visualized by electron microscopy after labeling of the complex with concanavalin A bound to colloidal gold. The oligosaccharide positions were found on the periphery of the torus and base. In summary, C9 appears to be a single-domain protein. Polymerization involves a major rearrangement. To form a subunit of poly(C9) the polypeptide chain must form at least one major fold parallel to the central axis of the tubule.
通过透射电子显微镜研究了人类补体成分C9和多聚C9（poly(C9)）的结构。单体天然C9（分子量=66,000）呈椭圆形外观（70×50埃），在一个面上可见一条裂缝。C9自发聚合形成由12 - 16个单体亚基组成的中空管状结构。poly(C9)是一个圆柱体（外径150埃，内径90埃），一端有一个圆环（厚46埃）环绕。poly(C9)的电子显微照片表明，圆环由多肽的径向链形成。poly(C9)的每个亚基显然相对于圆柱形结构的中心轴倾斜。C9可被α-凝血酶切割成两个单链多肽片段：C9a（分子量=28,000）和C9b（分子量=38,000），它们分别是该蛋白质的氨基末端和羧基末端片段。蛋白质的切割形式C9a,b在合适条件下可被诱导聚合形成耐十二烷基硫酸钠的poly(C9)，这表明poly(C9)对变性的抗性是C9a和C9b两者的共同特征。通过免疫电子显微镜已将C9a和C9b多肽区域定位在poly(C9)上。在圆环、基部以及poly(C9)圆柱体的中部观察到了C9a区域的决定簇。C9b表位主要集中在圆环和基部周围，但在poly(C9)的中部很少观察到。因此，C9多肽的C9a和C9b片段在poly(C9)中没有明显分开。在用与胶体金结合的伴刀豆球蛋白A标记复合物后，通过电子显微镜观察到了poly(C9)上寡糖单元的位置。寡糖位置位于圆环和基部的周边。总之，C9似乎是一种单结构域蛋白。聚合涉及主要的重排。为了形成poly(C9)的一个亚基，多肽链必须形成至少一个与小管中心轴平行的主要折叠。
