Complement component C1 is a large inactive protein complex in the plasma, which is composed of one C1q, two C1r, and two C1s. The C1q subunit can bind to the Fc regions of two antigen-specific antibodies that have bound in close proximity to antigen fixed on the surface of a pathogen. Binding of the C1q subunit activates the C1r subunits and then C1s subunits are activated. Following, the activated C1s subunits can cleave serum C4 into C4a and C4b. Then after a series of reactions, the C3 convertase and C5 convertase formed and classical pathway is activated.
Fig.1 Schematic illustration of C1 complex responses to nucleoli in live, apoptotic, and necrotic cells. (Cai, 2017)
C1q is made up of six heterotrimers each containing the C1qA, C1qB, and C1qC chains. Each chain of C1q has a central region forming a collagen stem together with the equivalent regions of the other two chains. The structures of C1r and C1s are similar, and both of them have the domain architecture CUB1-EGF-CUB2-CCP1-CCP2-SP. Recently, several researchers have found broad C1 functions beyond the complement system. C1q binds to the Frizzled receptors to activate C1s, which cleaves lipoprotein receptor-related protein 6 to trigger aging-associated Wnt receptor signaling. Beside, C1q binds to apoptotic cells and the activated C1 proteases cleave nuclear antigens.
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Reference
Cai, Yitian., et al. Broad susceptibility of nucleolar proteins and autoantigens to complement C1 protease degradation. The Journal of Immunology. 2017, 199.12: 3981-3990.
Fig.1 Schematic illustration of C1 complex responses to nucleoli in live, apoptotic, and necrotic cells. (Cai, 2017)