Molecular Analysis of the /5-Chain of Human C4B-Binding Protein

Abstract

C4b-binding protein (C4BP) is a multimeric glycoprotein in plasma with important regulatory functions in the complement system. It occurs in two forms, as free protein and in a non-covalent bimolecular complex with the vitamin K-dependent protein S. Protein S is an important anticoagulant and enhances the rate of inactivation by activated protein C of blood coagulation factors, Va and VIIIa. Protein S bound to C4BP is inactive as an anticoagulant, indicating C4BP to have a regulatory function in the blood coagulation process. Approximately 50% of C4BP in plasma circulates in complex with protein S, but little has been known about as to how these proteins interact. This report describes the structure of C4BP and its relation to protein S binding. A novel C4BP subunit, designated the beta-chain, which in all likelihood contains the protein S binding site, has been identified, isolated and characterized. The major form of C4BP is composed of seven alpha-chains and one beta-chain, and the subunits are covalently linked by their carboxy-terminal regions giving the molecule a spider-like quaternary structure. A subpopulation of C4BP, which does not bind protein S, was found to lack the beta-chain. This provides support for the concept that the single protein S binding site is located on the beta-chain. The beta-chain is structurally related to the alpha-chain of C4BP, and both subunits belong to the superfamily of C3b/C4b-binding proteins. The genes coding for the alpha- and beta-chains of C4BP were found to be closely linked within a cluster of genes, coding for structurally related proteins, on the long arm of human chromosome 1.