Studies on the mechanism of activation of a protease in the complement system, C3 convertase.

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Complement system is a humoral host defense system composed of multiple protease reactions and C3 convertase is a key enzyme in the complement system. It is assembled from two components, C4b and C2a, which are the activation products of C4 and C2. In this project, we have focused on the functional domains of C4b which are implicated in the molecular assembly of C3 convertase and obtained following results.1) A model of inter-chain disulfide bonds of C4 which is composed of three polypeptide chains ( <alpha> , <beta> , <gamma> ). There is a single S-S bond between <alpha> and <beta> chains, while two S-S bonds are linking between the <alpha> and <gamma> chains. 2) By mild reduction of C4b, two different binding sites for C2a (C-terminal side of C2) and C2b (N-terminal side of C2) were found. The site for C2b is located on the <alpha> - <beta> chains, whereas that for C2a is lost upon the mild reduction.3) Three functionally different monoclonal antibodies (MAb) against C4b were obtained. Using these MAb, it was found that a novel functional site is present on the <beta> chain which labilizes the binding of C2a to C4b.4) Binding of C2b to C3 convertase occurs and results in the dissociation of C3 convertase, suggesting that C2b may play as a regulator for C3 convertase.