C4b-binding protein

The main form of C4BP in human blood is composed of 7 identical alpha-chains and one unique beta-chain, which in turn binds anticoagulant, vitamin K-dependent protein S. C4BP is a large glycoprotein (500 kDa) with an estimated plasma concentration of 200 micrograms/mL synthesized mainly in the liver.

The genes coding for C4BP α-chain (C4BPA) and β-chain (C4BPB) are located in the regulators of complement activation (RCA) gene cluster on the long arm of chromosome 1 in the vicinity of other complement inhibitors.

C4BP binds apoptotic and necrotic cells as well as DNA, to clean up after injury.

The interaction with apoptotic and necrotic cells is mediated by the Gla domain of protein S and does not affect the ability of C4BP to inhibit complement.

A number of bacterial and fungal pathogens capture human C4BP and use it to prevent binding of C4b, which allows them to establish infection.