The architecture of complement component C9 and poly(C9)

Abstract

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.