C4b-binding protein (C4BP) is a multimeric plasma glycoprotein that functions as a regulator of the classical and lectin pathways of the complement system. It is a member of the complement control protein (CCP) family and is encoded by the C4BPB and C4BPA genes in humans.
Structure
C4BP exists primarily as a hexamer composed of one α-chain (C4BPα) and up to five β-chains (C4BPβ), linked together by disulfide bonds. The α-chain contains eight complement control protein (CCP) domains, whereas the β-chain contains three CCP domains and a short peptide that serves as a binding site for protein S, a vitamin K–dependent anticoagulant cofactor. The protein is heavily glycosylated, contributing to its stability and circulatory half‑life.
Function
C4BP regulates complement activation by binding to the complement component C4b. This interaction accelerates the decay of the C3/C5 convertase (C4b2a) and acts as a cofactor for factor I–mediated proteolytic inactivation of C4b. In addition to its role in the classical and lectin pathways, C4BP can bind to other complement fragments (e.g., C3b) and to certain pathogens, thereby contributing to immune evasion mechanisms.
Genetics
In humans, the α‑chain is encoded by the C4BPA gene located on chromosome 1p31.3, while the β‑chain is encoded by the C4BPB gene on chromosome 1p31.3, adjacent to C4BPA. Alternative splicing yields isoforms with variable numbers of β‑chains, which influence the protein’s affinity for protein S.
Clinical significance
- Deficiency: Rare hereditary deficiencies of C4BP can lead to uncontrolled complement activation, predisposing individuals to inflammatory and autoimmune conditions.
- Autoantibodies: Autoantibodies against C4BP have been reported in systemic lupus erythematosus (SLE) and other autoimmune diseases, potentially impairing its regulatory function.
- Infection: Certain bacteria (e.g., Streptococcus pyogenes, Neisseria meningitidis) express surface proteins that bind C4BP, allowing them to evade complement‑mediated killing.
- Thrombosis: Through its interaction with protein S, C4BP may influence coagulation pathways, although the clinical relevance of this interaction remains under investigation.
History
C4BP was first identified in the 1970s as a plasma protein that binds to C4b and inhibits complement-mediated hemolysis. Subsequent biochemical and molecular studies elucidated its multimeric structure and regulatory mechanisms.
References
(References are omitted per instruction; information is drawn from established immunology and hematology literature.)