Role of the beta subunit in interaction of the eighth component of human complement with the membrane-bound cytolytic complex.

This study is concerned with the subunits of the eighth component of human complement (C8) and their role in facilitating specific incorporation of C8 into the membrane-bound cytolytic complex of complement. The noncovalently associated alpha-gamma and beta subunits of C8 were purified and examined for their ability to interact independently with the C8 binding site on C5b-7, the membrane-bound precursor of the cytolytic complex. Using erythrocyte-bound C5b-7 (EAC1-7), it was observed that native C8 and its isolated beta subunit have similar high affinities for this complex. Binding of beta to EAC1-7 was specific for the C8 binding site as evidenced by the fact that 1) nearly identical molar amounts of either C8 or beta were required to saturate C8 binding sites on EAC1-7; 2) pretreatment of EAC1-7 with saturating amounts of beta prevented binding of C8; and 3) pretreatment of EAC1-7 with saturating amounts of C8 prevented binding of beta. In related experiments, alpha-gamma alone had no affinity for EAC1-7, however, binding comparable to that exhibited by native C8 occurred if alpha-gamma was first incubated with an equimolar amount of beta. Other experiments showed that if EAC1-7 was first saturated with beta to produce EAC1-7(beta), alpha-gamma also bound to yield functional C8 on the cell surface. These results provide conclusive evidence that structural features of C8 which are essential for specific recognition by membrane-bound C5b-7 are contained in the beta subunit.