There are 3 variants of Complement component 4 binding protein (C4bp), which is composed of alpha and beta chains. The predominant form is a 570 kDa complex of 7 alpha chains and 1 beta chain. The other complexes formed are a 530 kDa homoheptamer of alpha chains or a 500 kDa complex of 6 alpha chains and 1 beta chain. C4bp regulates the activation of the classical complement pathway. It is a cofactor to the C3b/C4b inactivator, crucial to the proteolytic cleavage of C4b and important in cleavage of C3b. C4bp also regulates the C4bC2a complex (C3 convertase), accelerating its degradation by dissociating the complement fragment C2a. Both the C4bp alpha and beta chains interact with anticoagulant protein S and C4bp alpha binds protein S. C4bp is expressed in the blood serum.
C4b-binding protein alpha chain; C4bp; Proline-rich protein; PRP; RP11-164O23.4