Complement component 3

Complement component 3
250px PDB rendering based on 1c3d.
Available structures PDB Ortholog search: PDBe, RCSB List of PDB id codes 1C3D, 1GHQ, 1W2S, 2A73, 2A74, 2GOX, 2HR0, 2I07, 2ICE, 2ICF, 2NOJ, 2QKI, 2WII, 2WIN, 2WY7, 2WY8, 2XQW, 2XWB, 2XWJ, 3D5R, 3D5S, 3G6J, 3L3O, 3L5N, 3NMS, 3OED, 3OHX, 3OXU, 3RJ3, 3T4A, 4HW5, 4HWJ, 4I6O, 4M76, 4ONT
Identifiers
Symbols	C3 ; AHUS5; ARMD9; ASP; C3a; C3b; CPAMD1; HEL-S-62p
External IDs	OMIM: 120700 MGI: 88227 HomoloGene: 68031 ChEMBL: 4917 GeneCards: C3 Gene
EC number	3.4.21.43
Gene ontology Molecular function • endopeptidase inhibitor activity • receptor binding • protein binding • lipid binding • C5L2 anaphylatoxin chemotactic receptor binding • cofactor binding Cellular component • extracellular region • extracellular space • plasma membrane • extracellular exosome • blood microparticle Biological process • positive regulation of type IIa hypersensitivity • positive regulation of protein phosphorylation • positive regulation of activation of membrane attack complex • tolerance induction • fatty acid metabolic process • inflammatory response • immune response • complement activation • complement activation, alternative pathway • complement activation, classical pathway • signal transduction • G-protein coupled receptor signaling pathway • blood coagulation • positive regulation of vascular endothelial growth factor production • positive regulation of glucose transport • regulation of triglyceride biosynthetic process • positive regulation of lipid storage • negative regulation of endopeptidase activity • regulation of complement activation • response to magnesium ion • response to estradiol • response to progesterone • innate immune response • positive regulation of G-protein coupled receptor protein signaling pathway • positive regulation of angiogenesis • positive regulation of developmental growth • regulation of immune response • response to glucocorticoid • positive regulation of ERK1 and ERK2 cascade • positive regulation of apoptotic cell clearance Sources: Amigo / QuickGO
Orthologs
Species	Human	Mouse
Entrez	718	12266
Ensembl	ENSG00000125730	ENSMUSG00000024164
UniProt	P01024	P01027
RefSeq (mRNA)	NM_000064	NM_009778
RefSeq (protein)	NP_000055	NP_033908
Location (UCSC)	Chr 19: 6.68 – 6.73 Mb	Chr 17: 57.2 – 57.23 Mb
PubMed search	[1]	[2]

Complement component 3, often simply called C3, is a protein of the immune system. It plays a central role in the complement system and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3.

Function

C3 plays a central role in the activation of complement system. Its activation is required for both classical and alternative complement activation pathways. People with C3 deficiency are susceptible to bacterial infection.

One form of C3-convertase, also known as C4b2a, is formed by a heterodimer of activated forms of C4 and C2. It catalyzes the proteolytic cleavage of C3 into C3a and C3b, generated during activation through the classical pathway as well as the lectin pathway. C3a is an anaphylotoxin and the precursor of some cytokines such as ASP, and C3b serves as an opsonizing agent. Factor I can cleave C3b into C3c and C3d, the latter of which plays a role in enhancing B cell responses. In the alternative complement pathway, C3 is cleaved by C3bBb, another form of C3-convertase composed of activated forms of C3 (C3b) and factor B (Bb). Once C3 is activated to C3b, it exposes a reactive thioester that allows the peptide to covalently attach to any surface that can provide a nucleophile such as a primary amine or a hydroxyl group. Activated C3 can then interact with factor B. Factor B is then activated by factor D, to form Bb. The resultant complex, C3bBb, is called the alternative pathway (AP) C3 convertase.

C3bBb is deactivated in steps. First, the proteolytic component of the convertase, Bb, is removed by complement regulatory proteins having decay-accelerating factor (DAF) activity. Next, C3b is broken down progressively to first iC3b, then C3c + C3dg, and then finally C3d. Factor I is the protease that performs these cuts but it requires the help of another protein (Factor H, CR1, MCP or C4BP) to supply cofactor activity.

Structure

Several crystallographic structures of C3 have been determined and reveal that this protein contains 13 domains.

Clinical use

Complement tests

C4 (C)	FB (A)	C3	CH50	Conditions
·	↓	↓	↓	PSG, C3 NeF AA
↓	·	↓	·	HA, C4D
·	·	·	↓	TCPD
↓	· /↓	↓	↓	SLE
↑	↑	↑	↑	inflammation

Levels of C3 in the blood may be measured to support or refute a particular medical diagnosis. For example, low C3 levels are associated with some types of kidney disease such as post-infectious glomerulonephritis, membranoproliferative glomerulonephritis, and shunt nephritis.

Interactions

Complement component 3 has been shown to interact with Factor H.

External links

• GeneReviews/NCBI/NIH/UW entry on Atypical Hemolytic-Uremic Syndrome
• OMIM entries on Atypical Hemolytic-Uremic Syndrome
• GeneReviews/NCBI/NIH/UW entry on Dense Deposit Disease/Membranoproliferative Glomerulonephritis Type II
• Complement C3 at the US National Library of Medicine Medical Subject Headings (MeSH)

This article incorporates text from the United States National Library of Medicine, which is in the public domain.