Structural and functional studies of C4b-binding protein (C4BP)

The subject of this thesis is complement inhibitor C4b-binding protein (C4BP), on which we have performed both structural and functional studies. Complement is part of the innate immune defence and eliminates microbes, solubilises immune complexes and is involved in the clearance of apoptotic cells. The actions of the complement system must be carefully controlled by proteins such as C4BP to avoid self-damage of the host. C4BP is a polymeric protein composed of seven identical alpha-chains and one beta-chain. In human plasma, C4BP circulates in complex with protein S (PS), a cofactor in the anticoagulant system. The alpha-chains of C4BP bind C4b and thereby inhibit the actions of this activated complement factor in the classical pathway of complement. Our structural studies include determination of binding sites for C4b, C3b and heparin, structural requirements for the polymerisation of C4BP, and the structural stability of C4BP in adverse conditions. Based on the understanding of the polymerisation of C4BP we propose how to use the core of C4BP in the production of multimeric biotechnological tools. There is a need for therapeutic complement inhibitors, and the knowledge of the mechanisms of inhibition and the structural stability of natural complement inhibitors will be helpful in development of such drugs. The functional studies we have performed demonstrate the ability of C4BP to bind C3b and present it for degradation, thereby inhibiting the alternative pathway of complement, enhancement of C4BP activity in the presence of zinc and the role of C4BP in the phagocytosis of apoptotic cells. We found that the C4BP-PS complex inhibits phagocytosis of apoptotic cells by macrophages, suggesting an important physiological role for the complex