Ultrastructure of cobra venom factor-dependent C3/C5 convertase and its zymogen, factor B of human complement

The molecular architecture of the cobra venom factor (CVF)-dependent C3/C5 convertase (EC 3.4.21.47) of human complement was deduced from electron microscopy of the purified bimolecular complex (CVF,Bb) and its isolated subunits, CVF and Bb. Negatively stained CVF imaged as an irregularly shaped cylindrical structure, with approximate dimensions of 137 A x 82 A (length x diameter). The zymogen Factor B appeared globular with a diameter of about 80 A and its image suggested a bipartite structure of two compact, closely associated regions, one about twice as large as the other. Bb, the larger, catalytic site-bearing cleavage fragment of Factor B, revealed two globular domains, each 40 A in diameter, connected by a short linker region about 10 A long and thick. CVF and Bb could be distinguished in micrographs of the CVF,Bb complex: Bb attached to one end of the CVF molecule through only one of its two domains, and in an orientation making the long axes of both molecules approximately orthogonal. It is suggested the naturally occurring C3 convertases (C3b,Bb and C4b,2a) have a similar morphology.