| Fondo | Complement C1q subcomponent subunit A is a ~26 kDa protein. Core component of the complement C1 complex, a multiprotein complex that initiates the classical pathway of the complement system, a cascade of proteins that leads to phagocytosis and breakdown of pathogens and signaling that strengthens the adaptive immune system. The classical complement pathway is initiated by the C1Q subcomplex of the C1 complex, which specifically binds IgG or IgM immunoglobulins complexed with antigens, forming antigen-antibody complexes on the surface of pathogens: C1QA, together with C1QB and C1QC, specifically recognizes and binds the Fc regions of IgG or IgM via its C1q domain. Immunoglobulin-binding activates the proenzyme C1R, which cleaves C1S, initiating the proteolytic cascade of the complement system. The C1Q subcomplex is activated by a hexamer of IgG complexed with antigens, while it is activated by a pentameric IgM. The C1Q subcomplex also recognizes and binds phosphatidylserine exposed on the surface of cells undergoing programmed cell death, possibly promoting activation of the complement system. 1. Kishore, U. et al. (2003) Journal of immunology (Baltimore, Md. : 1950) 171, 812-20. PMID: 128472492. Gadjeva, MG. et al. (2008) Biochemistry 47, 13093-102. PMID: 190063213. Diebolder, CA. et al. (2014) Science (New York, N.Y.) 343, 1260-3. PMID: 246269304. Ugurlar, D. et al. (2018) Science (New York, N.Y.) 359, 794-797. PMID: 294494926. Zwarthoff, SA. et al. (2021) Proceedings of the National Academy of Sciences of the United States of America 118. PMID: 341551157. Lin, TY. et al. (1980) The Journal of biological chemistry 255, 7756-62. PMID: 6249812 |
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