Proteolysis of the C5b-7 complex: Cleavage of the C5b and C6 subunits and its effect on the interaction of the complex with phospholipid bilayers

The present study was conducted to gain insight into the process of assembly of the C5b-7 complex on the phospholipid bilayer. The C5b-6 complex, C7, and 2 different forms of the C5b-7 complex (the C5b-7 complex either bound to artificial phospholipid bilayers or in fluid phase) were digested with trypsin, and the resulting products were analyzed by SDS-polyacrylamide gel electrophoresis. The α'-chains of C5b and C6 in the C5b-6 complex and C7 were susceptible to trypsin. The formation of the C5b-7 complex from the C5b-6 complex and C7 was accompanied by alteration in susceptibility to trypsin of C7: the C7 subunit of the fluid phase complex as well as of the complex bound to phospholipid vesicles is not susceptible to trypsin. On the other hand, the α'-chains of C5b and C6 in the C5b-7 complex bound to phospholipid vesicles as well as in fluid phase remained accessible and susceptible to trypsin, and cleaved as in the C5b-6 complex. Upon ultracentrifugation in a sucrose density gradient, the proteolyzed C5b-7 complex in fluid phase dissociated into subunits. However, none of the subunits in the proteolyzed C5b-7 complex bound to phospholipid vesicles dissociated from vesicles upon sucrose density gradient ultracentrifugation. Thus, the molecule of the C5b-7 complex became stable and resistant to proteolytic dissociation upon binding to the phospholipid bilayer. It is proposed that each subunit of the C5b-7 complex participates in the stable interaction of the complex with the phospholipid bilayer. In addition, the possible significance of the alteration in susceptibility to trypsin of C7 associated with the formation of the C5b-7 complex is discussed.