On the mechanism of membrane damage by C: exposure of hydrophobic sites on activated C proteins.

In previous papers we have presented evidence that peptides from C proteins C5b, C7, C8, and C9 become inserted in the lipid bilayer membranes and form a transmembrane channel. Presumably, this insertion follows exposure of hydrophobic domains by C activation. In the present experiments liposomes were made with 14C-phosphatidyl choline (PC) and Forssman antigen in the bilayer, and with 86Rb+ in the aqueous compartments. When such liposomes were incubated with anti-Forssman antibody (A) and guinea pig serum (GPS) as a source of C, substantially more 14C-PC and 86Rb+ were released than from liposomes treated with A and C4-deficient GPS, or with A and heated C, or with C alone, or with A alone. The specific release of PC was dependent on the dose of C. Prior treatment of GPS with cobra venom factor abolished its capacity to release PC. The release of PC by A and C7-deficient human serum (C7D-HS) was the same as that of GPS alone, i.e., there was no specific release. A and C8D-HS produced much less specific release than A and GPS; addition of purified guinea pig C7 or C8 to C7D-HS or C8D-HS, respectively, restored the PC release to its full extent. Hence, part of the PC removal is mediated by C5b,6,7; the remainder is attributable to C8 and/or C9.