C4b-binding protein

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Structure of C4b-binding protein

C4b-binding protein (C4BP) is a plasma protein that plays a crucial role in the complement system, which is part of the immune system. It is primarily involved in the regulation of the classical pathway of complement activation.

Structure

C4BP is a large, multimeric protein composed of several identical subunits. Each subunit contains multiple complement control protein (CCP) domains, which are also known as short consensus repeats (SCRs). These domains are responsible for the protein's ability to bind to its targets, such as C4b and heparin.

Function

C4BP functions as a regulatory protein in the complement system. Its main role is to bind to the activated form of C4, known as C4b, thereby preventing the formation of the C3 convertase enzyme complex. This action inhibits the further progression of the complement cascade, thus protecting host cells from damage.

C4BP also acts as a cofactor for the serine protease factor I, which cleaves and inactivates C4b. This further contributes to the regulation of the complement system and helps maintain immune homeostasis.

Clinical significance

Alterations in the levels or function of C4BP can lead to various immune disorders. For example, deficiencies in C4BP can result in increased susceptibility to infections due to uncontrolled complement activation. Conversely, elevated levels of C4BP have been associated with certain autoimmune diseases, where excessive complement inhibition may contribute to disease pathology.

Related pages

• Complement system
• Immune system
• C4b
• Factor I
• Autoimmune disease