C4b-binding protein

C4b-binding protein

C4b-binding protein (C4BP) is a crucial component of the complement system, which is part of the immune system that enhances the ability of antibodies and phagocytic cells to clear pathogens from an organism. C4BP is primarily involved in the regulation of the classical pathway and the lectin pathway of complement activation.

Structure

C4BP is a large, multimeric glycoprotein composed of several identical subunits. The most common form of C4BP is composed of seven α-chains and one β-chain, although other isoforms exist. Each α-chain contains multiple complement control protein (CCP) domains, which are also known as short consensus repeats (SCRs). These domains are crucial for the protein's ability to bind to its targets and exert its regulatory functions.

Function

C4BP primarily functions as a regulatory protein that inhibits the complement cascade. It achieves this by binding to the activated form of complement component 4, known as C4b. By binding to C4b, C4BP prevents the formation of the C3 convertase enzyme complex, which is essential for the propagation of the complement cascade. This action helps to prevent excessive inflammation and tissue damage that can result from uncontrolled complement activation.

Additionally, C4BP can bind to protein S, a vitamin K-dependent plasma protein that plays a role in the regulation of blood coagulation. This interaction is important for the anticoagulant properties of protein S.

Clinical Significance

Dysregulation of C4BP can lead to various pathological conditions. For instance, deficiencies or functional abnormalities in C4BP can result in increased susceptibility to infections due to inadequate regulation of the complement system. Conversely, overactivity of C4BP can contribute to autoimmune diseases by preventing the clearance of immune complexes and apoptotic cells.

C4BP levels can also be altered in certain diseases. For example, elevated levels of C4BP have been observed in patients with systemic lupus erythematosus (SLE), a chronic autoimmune disease.

Research and Therapeutic Potential

Research into C4BP is ongoing, with studies focusing on its role in disease and potential as a therapeutic target. Modulating C4BP activity could offer new avenues for treating diseases characterized by complement dysregulation, such as age-related macular degeneration and certain types of glomerulonephritis.

See Also

• Complement system
• Classical pathway
• Lectin pathway
• Protein S

External Links

• C4BP Gene - NCBI
• C4BP - UniProt