Comparative study of the fluid-phase proteolytic cleavage of human complement subcomponents C4 and C2 by C1s and...

The C3 convertase of the classical pathway of complement is composed of fragments C4b and C2a resulting from cleavage of C4 and C2 by activated C1. The limited proteolysis of these two different substrates by the same protease, C1s, has been studied in the fluid phase using purified proteins. The turnover numbers of C2 and C4 cleavage by C1s were affected to different extents, depending on whether C1s was alone or associated with C1r or with monoclonal antibodies to C1s. The binding of C2 to C4 favours the proteolysis of C2 by C1s, as revealed by the use of I2-treated C2.