Function of subunits within the eighth component of human complement: selective removal of the .gamma. chain reveals it has no direct role in cytolysis

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Abstract
The 8th component of human complement (C8) consists of a disulfide-linked .alpha.-.gamma. dimer that is noncovalently associated with .beta.. Two of these subunits have distinct roles in the cytolytic function of C8. Binding of C8 to the precursive C5b-7 complex is mediated strictly through .beta., while interaction between C8 and the lipid bilayer of target membranes [sheep erythrocytes] occurs primarily through .alpha.. The importance of the .gamma. subunit in cytolysis was examined by characterizing functional properties of C8'', a derivative of C8 that lacks .gamma.. Preparation of this derivative was accomplished by limited cleavage of disulfide bonds in purified .alpha.-.gamma. and separation of .alpha. from .gamma.. When mixed, purified .alpha. and .beta. combined to form C8''. When tested for functional similarity to normal C8, the following results were obtained. Specific and saturable binding of C8'' to the C8 binding site on C5b-7 could be achieved. Significantly, the resulting C5b-8'' supported subsequent C9 binding and cell lysis, which was equivalent to that observed with C5b-8. Complement activation of (C8 + C9)-depleted serum containing C8'' resulted in formation of SC5b-8''. Inclusion of C9 in the serum resulted in further conversion to SC5b-(8'')9. These observations indicate that C8'' is functionally similar to C8. Unlike .alpha. and .beta., the .gamma. subunit has no direct role in facilitating C8 interaction with the nascent cytolytic complex or in mediating C9 binding and membrane lysis. On the basis of these and earlier results, a model is proposed that depicts the arrangement of C8 subunits within membrane-bound C5b-8.