Complement C3 (C3) Native Protein

Function: 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. Ref.5 Ref.11 Ref.14 Ref.15 Ref.16 Ref.22 Ref.25 Ref.29Derived from proteolytic degradation of complement C3, C3a anaphylatoxin is a mediator of local inflammatory process. It induces the contraction of smooth muscle, increases vascular permeability and causes histamine release from mast cells and basophilic leukocytes. Ref.5 Ref.11 Ref.14 Ref.15 Ref.16 Ref.22 Ref.25 Ref.29Acylation stimulating protein (ASP): adipogenic hormone that stimulates triglyceride (TG) synthesis and glucose transport in adipocytes, regulating fat storage and playing a role in postprandial TG clearance. Appears to stimulate TG synthesis via activation of the PLC, MAPK and AKT signaling pathways. Ligand for C5AR2. Promotes the phosphorylation, ARRB2-mediated internalization and recycling of C5AR2. Ref.5 Ref.11 Ref.14 Ref.15 Ref.16 Ref.22 Ref.25 Ref.29

Subunit structure: C3 precursor is first processed by the removal of 4 Arg residues, forming two chains, beta and alpha, linked by a disulfide bond. C3 convertase activates C3 by cleaving the alpha chain, releasing C3a anaphylatoxin and generating C3b (beta chain + alpha' chain). C3dg interacts with CR2 (via the N-terminal Sushi domains 1 and 2). During pregnancy, C3dg exists as a complex (probably a 2:2:2 heterohexamer) with AGT and the proform of PRG2. Interacts with VSIG4. C3b interacts with herpes simplex virus 1 (HHV-1) and herpes simplex virus 2 (HHV-2) envelope glycoprotein C; this interaction inhibits the activation of the complement system. Interacts with S.aureus immunoglobulin-binding protein sbi, this prevents interaction between C3dg and CR2. Interacts with S.aureus fib. Interacts (both C3a and ASP) with C5AR2; the interaction occurs with higher affinity for ASP, enhancing the phosphorylation and activation of C5AR2, recruitment of ARRB2 to the cell surface and endocytosis of GRP77. Ref.7 Ref.10 Ref.17 Ref.18 Ref.30 Ref.31 Ref.43 Ref.44

Subcellular location: Secreted.

Tissue specificity: Plasma. The acylation stimulating protein (ASP) is expressed in adiopocytes and released into the plasma during both the fasting and postprandial periods. Ref.15 Ref.22

Post-translational modification: C3b is rapidly split in two positions by factor I and a cofactor to form iC3b (inactivated C3b) and C3f which is released. Then iC3b is slowly cleaved (possibly by factor I) to form C3c (beta chain + alpha' chain fragment 1 + alpha' chain fragment 2), C3dg and C3f. Other proteases produce other fragments such as C3d or C3g. C3a is further processed by carboxypeptidases to release the C-terminal arginine residue generating the acylation stimulating protein (ASP). Levels of ASP are increased in adipocytes in the postprandial period and by insulin and dietary chylomicrons.Phosphorylation sites are present in the extracellular medium.

Polymorphism: There are two alleles: C3S (C3 slow), the most common allele in all races and C3F (C3 fast), relatively frequent in Caucasians, less common in Black Americans, extremely rare in Orientals.

Involvement in disease: Complement component 3 deficiency (C3D) [MIM:613779]: 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.Note: The disease is caused by mutations affecting the gene represented in this entry. Ref.32 Ref.35 Ref.36 Ref.37 Ref.48 Ref.49Age-related macular degeneration 9 (ARMD9) [MIM:611378]: A form of age-related macular degeneration, 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.Note: Disease susceptibility is associated with variations affecting the gene represented in this entry. Ref.32 Ref.50Hemolytic uremic syndrome atypical 5 (AHUS5) [MIM:612925]: 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.Note: Disease susceptibility is associated with variations affecting the gene represented in this entry. Other genes may play a role in modifying the phenotype. Ref.32 Ref.51 Ref.52Increased 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. Ref.32

Sequence similarities: Contains 1 anaphylatoxin-like domain.Contains 1 NTR domain.

Caution: According to Ref.44, the interaction surface between C3 and CR2 reported in Ref.36 is artifactual and can be ascribed to the presence of zinc acetate in the buffer.