Title : 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