Analysis of the specific association of the eighth and ninth components of human complement: identification of a direct role for the .alpha. subunit of C8
The basic for the physical association between C8 and C9 in solution was examined by isolating the noncovalently associated .alpha.-.gamma. and .beta. subunits of C8 and determining their respective affinities for C9. Results indicate that only .alpha.-.gamma. associates with C9 and this association, though reversible, is complete at near equimolar ratios of each component. Further experiments using purified .alpha. or .gamma. revealed that only .alpha. was capable of forming a stable complex with C9. Although the strength of this interaction was dependent on salt concentration, association was observed in buffer of physiological ionic strength and in human serum. These results establish that the domain on C8 responsible for interaction with C9 is located entirely within .alpha.. In related experiments, addition of .beta. to preformed dimers of either (.alpha.-.gamma. + C9) of (.alpha. + C9) resulted in complete association of this subunit. These particular results indicate that there are two physically distinct sites on .alpha. that separately mediate association of .alpha. with .beta. and with C9. Furthermore, occupation of one site does not impair interaction at the other.