C4b-binding protein alpha chain (C4BPA)

C4b-binding protein alpha chain (C4BPA) is a major soluble regulator of the classical and lectin pathways of the complement system, primarily synthesized in the liver (UniProt P04003). It typically exists as a large hetero-oligomeric complex composed of seven alpha chains and one beta chain, though homopolymeric forms also occur. C4BPA functions by binding to C4b, where it acts as a cofactor for Factor I-mediated cleavage and accelerates the decay of the C3 convertase (C4b2a), effectively protecting host tissues from complement-mediated damage (Ermert et al., 2015). Beyond its immune functions, C4BPA interacts with Protein S to link the complement and coagulation systems and plays a role in the non-inflammatory clearance of apoptotic cells. In clinical pathology, C4BPA is often hijacked by pathogens such as Neisseria gonorrhoeae and Streptococcus pyogenes to evade immune detection, and its deficiency is associated with autoimmune diseases like systemic lupus erythematosus (Blom et al., 2004). Therapeutic development focuses on utilizing recombinant C4BPA or C4BPA-fusion proteins to treat complement-driven inflammatory conditions or developing strategies to block pathogen recruitment of the protein.

Other names

C4BPComplement component 4-binding protein alphaProline-rich proteinC4BP alpha

C4BPA acts as a critical cofactor for the serine protease Factor I, facilitating the proteolytic cleavage of C4b into C4c and C4d, which prevents the formation of the C3 convertase (UniProt P04003). Additionally, it exhibits decay-accelerating activity by actively dissociating the C2a catalytic subunit from the C4b2a complex, thereby halting the classical and lectin complement cascades (Blom et al., 2004). It also binds to C3b, albeit with lower affinity, to assist in its degradation.