Complement Component 4 Binding Protein， Beta （C4BPB） belongs to a superfamily of proteins composed primarily of tandemly arrayed short consensus repeats of approximately 60 amino acids. C4BPB has a regulatory role in the coagulation system also， mediated via the beta-chain binding of protein S， a vitamin K-dependent protein which functions as a cofactor of activated protein C. A single， unique beta-chain of the C4BPB assembles with 7 identical alpha-chains into the principal isoform of C4BPB， which is a multimeric protein that controls activation of the complement cascade via the classical pathway. C4BPB binds as a cofactor to C3b/C4b inactivator （C3bINA）， which subsequently hydrolyzes the complement fragment C4b. In addition， C4BPB accelerates the degradation of the C4bC2a complex （C3 convertase） by detaching the complement fragment C2a. Furthermore， C4BPB interacts with anticoagulant protein S and with serum amyloid P component.
