Subunit Structure and Properties of Human C8

Studies of C8 subunit structure and conformation may provide important clues concerning the role of C8 in the formation of a transmembrane channel by the C5b-9 complex. Human C8, when examined by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis under reducing conditions, is composed of three polypeptide chains with the following molecular weights: α, 77,000; β, 63,000; and γ, 13,700. When using SDS and urea in the absence of reducing agents, two subunits are visualized: α-γ, 99,000 daltons, and β, 75,000 daltons. The α-chain appears to be largely concealed in the interior of the C8 molecule. This conclusion was deduced from the differential distribution of 125 I after labeling of C8 by the chloramine-T method, either in its native form or under conditions which cause unfolding and dissociation into subunits. Whereas the β- and γ-chains take up 125 I in proportion to their mass, the α-chain remains totally unlabeled in the native molecule.