The membrane attack complex of complement: relation of C7 to the metastable membrane binding site of the intermediate complex C5b-7.

Isolated C7 (m.w. 120,000) in 1% deoxycholate (DOC) forms dimers with an apparent m.w. of 230,000 and a DOC-binding capacity of 82 mol per mol of dimer. Dimerization of C7 also occurs in the presence of DOC-phospholipid mixed micelles and eventuates in the insertion of C7 dimers into the lipid bilayer upon the removal of the detergent. C5b-7 complex formation in the fluid phase or on lipid vesicles likewise involves polymerization. C5b-7 sedimented with 17-40S, which suggests a dimeric to hexameric composition. In avidin-biotin binding experiments in which two differentially labeled forms of C5b,6 (biotinyl 125I-C5b,6, and 131I-C5b,6) were used in equimolar amounts to assemble C5b-7, more than 50% of the biotinyl 125I-C5b,6-containing complexes also contained 131I label; again suggesting that C5b-7 consisted of oligomers rather than monomers. The conformation of C7 in C5b-7 and in dimeric C7 appeared similar by the following criteria. On formation of C5b-7 from C5b,6 and C7, a 20% increase in beta-pleated sheet structure was observed by circular dichroism spectroscopy, and a similar change occurred on dimerization of isolated C7. Tryptic and thermolytic digests of C5b-7 and C7 dimers containing 125I-C7 were analyzed by autoradiography after SDS-polyacrylamide gel electrophoresis and were found to contain similar peptides that were distinct from those in the digests of monomeric C7. Direct evidence showing that the metastable membrane binding site of the C5b-7 complex resides in the C7 subunit was obtained by using the conjugates of C5b,6 and colloidal gold. Viewed in the electron microscope, these conjugates were aggregated upon the addition of isolated C7. In contrast, when conjugates of C7 and colloidal gold were treated with soluble C5b,6, no such aggregates occurred, but instead, individual C5b-7 complexes were observed arranged around single gold particles, resulting in star-like structures. The results strongly suggest that structures of C7 are responsible for the expression of the membrane binding site of metastable C5b-7.