Complement Component 4 Binding Protein
C4-binding protein (C4BP) is the principal fluid-phase regulator of the classical and lectin complement pathways. Structured like an octopus, it assembles from multiple alpha chains, optionally with a beta chain, each arm built from short consensus repeat (SCR) modules. The alpha chains act as cofactors for factor I-mediated cleavage of C4b and accelerate the decay of the C4b2a convertase, limiting opsonin generation. C4BP also binds vitamin K-dependent protein S and localizes to apoptotic cells and enveloped viruses, coupling complement control with clearance of dying cells. Recombinant C4BP supports convertase-decay assays and cofactor-activity studies.
