C4b-binding protein

C4b-binding protein (C4BP) is a protein involved in the complement system where it acts as inhibitor. C4BP has a octopus-like structure with a central stalk and seven branching alpha-chains. 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 mg/L synthesized mainly in the liver.

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

Functions
It inhibits the action of C4, more specifically the classical and the lectin pathways. It also has ability to bind C3b. C4BP accelerates decay of C3 convertase and is a cofactor for serine protease factor I which cleaves C4b and C3b.

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, which allows them to establish infection.

No full deficiency of C4BP has been found yet.

C4BP interacts also with heparin, C-reactive protein (CRP), serum amyloid P component (SAP), fibromodulin, osteoadherin, chondroadherin, proline arginine-rich end leucine-rich repeat protein (PRELP), streptococcal M-proteins, gonococcal porins, Outer membrane protein A from E. coli, Ubiquitous surface protein 1 and 2 from Moraxella.