Complement Component 3 (C3) Active Protein

This gene encodes complement protein C3 which plays a central role in the classical, alternative and lectin activation pathways of the complement system. The encoded preproprotein undergoes a multi-step processing to generate various functional peptides. Mice deficient in the encoded protein fail to clear bacteria from the blood stream upon infection, display diminished airway hyperresponsiveness and lung eosinophilia upon allergen-induced pulmonary allergy, and develop severe lung injury after deposition of IgG immune complexes. Deficiency of the homolog of the encoded protein in humans was found to be associated with increased susceptibility to infections, age-related macular degeneration, and atypical hemolytic uremic syndrome. [provided by RefSeq, Mar 2015]

C3: C3 plays a central role in the activation of the complement system. Its processing by C3 convertase is the central reaction in both classical and alternative complement pathways. After activation C3b can bind covalently, via its reactive thioester, to cell surface carbohydrates or immune aggregates. Defects in C3 are the cause of complement component 3 deficiency (C3D). A rare defect of the complement classical pathway. Patients develop recurrent, severe, pyogenic infections because of ineffective opsonization of pathogens. Some patients may also develop autoimmune disorders, such as arthralgia and vasculitic rashes, lupus-like syndrome and membranoproliferative glomerulonephritis. Genetic variation in C3 is associated with susceptibility to age-related macular degeneration type 9 (ARMD9). ARMD is a multifactorial eye disease and the most common cause of irreversible vision loss in the developed world. In most patients, the disease is manifest as ophthalmoscopically visible yellowish accumulations of protein and lipid that lie beneath the retinal pigment epithelium and within an elastin- containing structure known as Bruch membrane. Defects in C3 are a cause of susceptibility to hemolytic uremic syndrome atypical type 5 (AHUS5). An atypical form of hemolytic uremic syndrome. It is a complex genetic disease characterized by microangiopathic hemolytic anemia, thrombocytopenia, renal failure and absence of episodes of enterocolitis and diarrhea. In contrast to typical hemolytic uremic syndrome, atypical forms have a poorer prognosis, with higher death rates and frequent progression to end-stage renal disease. Susceptibility to the development of atypical hemolytic uremic syndrome can be conferred by mutations in various components of or regulatory factors in the complement cascade system. Other genes may play a role in modifying the phenotype. Increased levels of C3 and its cleavage product ASP, are associated with obesity, diabetes and coronary heart disease. Short-term endurance training reduces baseline ASP levels and subsequently fat storage.

Protein type: Secreted; Inhibitor; Secreted, signal peptide

Cellular Component: extracellular space; extracellular region

Molecular Function: protein binding; endopeptidase inhibitor activity; cofactor binding; lipid binding; C5L2 anaphylatoxin chemotactic receptor binding

Biological Process: positive regulation of developmental growth; immune system process; complement activation, alternative pathway; fatty acid metabolic process; complement activation; positive regulation of angiogenesis; positive regulation of activation of membrane attack complex; positive regulation of phagocytosis; positive regulation of G-protein coupled receptor protein signaling pathway; positive regulation of type IIa hypersensitivity; innate immune response; positive regulation of protein amino acid phosphorylation; lipid metabolic process; blood coagulation; inflammatory response; complement activation, classical pathway