Complement component 1r

Complement component 1r (C1r) is a protein involved in the complement system, which is an integral part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear pathogens from an organism. C1r is a serine protease that is part of the C1 complex, which also includes C1q and C1s. The C1 complex is the first component of the classical pathway of the complement system.

Structure

C1r is a modular enzyme composed of several regions: two CUB domains (C1r/C1s, Uegf, Bmp1), two EGF-like domains, a CCP (complement control protein) domain, and the serine protease domain. This structure is essential for its interaction with C1q and C1s, and for its enzymatic activity.

Function

Upon activation by C1q binding to antibodies that are themselves bound to antigens, C1r is auto-activated through a cleavage that occurs in its serine protease domain. Activated C1r then cleaves and activates C1s, another serine protease, which subsequently leads to the activation of the complement cascade. This cascade results in several immune responses, including opsonization, cell lysis, and inflammation.

Regulation

The activity of C1r is tightly regulated to prevent spontaneous activation of the complement system, which could lead to tissue damage. C1 inhibitor (C1-INH) is a key regulator that binds to activated C1r (and C1s), inhibiting its protease activity and preventing further activation of the complement system.

Clinical Significance

Deficiencies or dysfunctions in C1r can lead to immune system-related diseases. For example, a deficiency in C1-INH can result in hereditary angioedema, characterized by sudden and severe swelling in various parts of the body. Additionally, excessive activation of C1r has been implicated in autoimmune diseases, where the immune system attacks the body's own tissues.

Research

Research into C1r focuses on understanding its structure-function relationships, its role in disease, and its potential as a therapeutic target. Inhibitors of C1r are being explored as treatments for conditions involving excessive complement activation, such as certain autoimmune diseases and ischemia-reperfusion injury.

See Also

• Complement system
• Immune system
• Serine protease
• Hereditary angioedema
• Autoimmune diseases

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