# Complement component 1q > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Complement_component_1q > Markdown URL: https://mediated.wiki/source/Complement_component_1q.md > Source: https://en.wikipedia.org/wiki/Complement_component_1q > Source revision: 1333965645 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{cs1 config|name-list-style=vanc}} {{short description|Protein complex}} {{Infobox protein family | Symbol = C1q | Name = C1q domain | image = Complement c1q 1PK6.png | width = | caption = crystal structure of a collagen viii nc1 domain trimer | Pfam = PF00386 | Pfam_clan = CL0100 | InterPro = IPR001073 | SMART = | PROSITE = PDOC00857 | MEROPS = | SCOP = 1c28 | TCDB = | OPM family = | OPM protein = | CAZy = | CDD = }} {{protein | Name = complement component 1, q subcomponent, A chain | caption = | image = Complement c1q subcomponent subunit A 1PK6.png | width = | HGNCid = 1241 | Symbol = [C1QA](/source/C1QA) | AltSymbols = | EntrezGene = 712 | OMIM = 120550 | RefSeq = NM_015991 | UniProt = P02745 | PDB = | ECnumber = | Chromosome = 1 | Arm = p | Band = 36.3 | LocusSupplementaryData = -34.1 }} {{protein | Name = complement component 1, q subcomponent, B chain | caption = | image = Complement c1q subcomponent subunit B 1PK6.png | width = | HGNCid = 1242 | Symbol = C1QB | AltSymbols = | EntrezGene = 713 | OMIM = 120570 | RefSeq = NM_000491 | UniProt = P02746 | PDB = | ECnumber = | Chromosome = 1 | Arm = p | Band = 36.3 | LocusSupplementaryData = -34.1 }} {{protein | Name = complement component 1, q subcomponent, C chain | caption = | image = Complement c1q subcomponent subunit C 1PK6.png | width = | HGNCid = 1245 | Symbol = C1QC | AltSymbols = C1QG | EntrezGene = 714 | OMIM = 120575 | RefSeq = NM_172369 | UniProt = P02747 | PDB = | ECnumber = | Chromosome = 1 | Arm = p | Band = 36.11 | LocusSupplementaryData = }} The '''complement component 1q''' (or simply '''C1q''') is a [protein complex](/source/protein_complex) involved in the [complement system](/source/complement_system), which is part of the [innate immune system](/source/innate_immune_system). C1q together with [C1r](/source/C1r) and [C1s](/source/C1s) form the [C1 complex](/source/C1_complex). [Antibodies](/source/Antibody) of the [adaptive immune system](/source/adaptive_immune_system) can bind [antigen](/source/antigen), forming an [antigen-antibody complex](/source/antigen-antibody_complex). When C1q binds antigen-antibody complexes, the [C1 complex](/source/C1_complex) becomes activated. Activation of the C1 complex initiates the [classical complement pathway](/source/classical_complement_pathway) of the complement system. The antibodies [IgM](/source/IgM) and all [IgG](/source/Immunoglobulin_G) subclasses except [IgG4](/source/Immunoglobulin_G) are able to initiate the complement system. ==Structure== C1q is a 460 kDa protein formed from 18 peptide chains in 3 subunits of 6. Each 6 peptide subunit consists of a Y-shaped pair of triple peptide helices joined at the stem and ending in a globular non-helical head. The 80-amino acid helical component of each triple peptide contain many Gly-X-Y sequences, where X and Y are [proline](/source/proline), [isoleucine](/source/isoleucine), or [hydroxylysine](/source/hydroxylysine); they, therefore, strongly resemble [collagen](/source/collagen) [fibrils](/source/fibrils). ===C1q chains A, B and C=== C1q is composed of 18 polypeptide chains: six A-chains, six B-chains, and six C-chains. Each chain contains a collagen-like region located near the N terminus and a C-terminal globular region. The A-, B-, and C-chains are arranged in the order A-C-B on chromosome 1.{{cite web | title = Entrez Gene: C1QA complement component 1, q subcomponent, A chain| url = https://www.ncbi.nlm.nih.gov/gene?Db=gene&Cmd=ShowDetailView&TermToSearch=712}} == Domain == The C1q domain is a conserved [protein domain](/source/protein_domain). [C1q](/source/C1Q_complex) is a subunit of the C1 [enzyme](/source/enzyme) [complex](/source/Protein_complex) that activates the [serum](/source/blood_serum) [complement system](/source/complement_system). C1q comprises 6 A, 6 B and 6 C [chain](/source/polymer)s. These share the same topology, each possessing a small, globular [C-terminal](/source/C-terminal) domain, a collagen-like Gly/Pro-rich central region, and a [conserved](/source/conserved_sequence) C-terminal region, the C1q domain.{{cite journal |vauthors=Sellar GC, Blake DJ, Reid KB | title = Characterization and organization of the genes encoding the A-, B- and C-chains of human complement subcomponent C1q. The complete derived amino acid sequence of human C1q | journal = Biochem. J. | volume = 274 | issue = 2| pages = 481–90 |date=March 1991 | pmid = 1706597 | pmc = 1150164 | doi = 10.1042/bj2740481}} The C1q protein is produced in collagen-producing [cell](/source/cell_(biology))s and shows [sequence](/source/sequence_(biology)) and structural similarity to [collagen](/source/collagen)s VIII and X.{{cite journal |vauthors=Petry F, Reid KB, Loos M | title = Molecular cloning and characterization of the complementary DNA coding for the B-chain of murine Clq | journal = FEBS Lett. | volume = 258 | issue = 1 | pages = 89–93 |date=November 1989 | pmid = 2591537 | doi = 10.1016/0014-5793(89)81622-9| s2cid = 44986344 | doi-access = }}{{cite journal |vauthors=Muragaki Y, Jacenko O, Apte S, Mattei MG, Ninomiya Y, Olsen BR | title = The alpha 2(VIII) collagen gene. A novel member of the short chain collagen family located on the human chromosome 1 | journal = J. Biol. Chem. | volume = 266 | issue = 12 | pages = 7721–7 |date=April 1991 | doi = 10.1016/S0021-9258(20)89508-8 | pmid = 2019595 | doi-access = free }} ==Function== [[File:Complement pathway.svg|thumb|270px|left|The classical and alternative complement pathways. C1q is the orange part of the [C1 complex](/source/C1_complex) at the top of the image.]] It is assumed that the globular ends are the sites for multivalent attachment to the complement fixing sites in immune complexed immunoglobulin. Patients with Lupus erythematosus often have deficient expression of C1q. Genetic deficiency of C1q is extremely rare (approximately 75 known cases) although the majority (>90%) of those have [SLE](/source/Systemic_lupus_erythematosus).{{cite web |title=C1q deficiency - About the Disease - Genetic and Rare Diseases Information Center |url=https://rarediseases.info.nih.gov/diseases/12958/c1q-deficiency |website=rarediseases.info.nih.gov |language=en}} C1q associates with C1r and C1s in order to yield the [C1 complex](/source/C1_complex) (''C1qr2s2''), the first component of the serum [complement system](/source/complement_system). Deficiency of C1q has been associated with [lupus erythematosus](/source/lupus_erythematosus) and [glomerulonephritis](/source/glomerulonephritis). It is potentially [multivalent](/source/Valence_(chemistry)) for attachment to the complement fixation sites of [immunoglobulin](/source/immunoglobulin). The sites are on the CH2 domain of [IgG](/source/Immunoglobulin_G) and, it is thought, on the CH4 domain of [IgM](/source/IgM). IgG4 cannot bind C1q, but the other three IgG subclasses can. The appropriate [peptide sequence](/source/peptide_sequence) of the complement fixing site might become exposed following complexing of the immunoglobulin, or the sites might always be available, but might require multiple attachment by C1q with critical geometry in order to achieve the necessary [avidity](/source/avidity). {{clear}} ==References== {{Reflist}} ==Further reading== * [https://www.ncbi.nlm.nih.gov/pubmed/10904115 C1q: structure, function, and receptors. Kishore U1, Reid KB. Immunopharmacology. 2000 Aug;49(1-2):159-70.] * [http://www.jimmunol.org/content/187/8/4369.full Functional Complement C1q Abnormality Leads to Impaired Immune Complexes and Apoptotic Cell Clearance.] * [http://journal.frontiersin.org/article/10.3389/fimmu.2014.00565/full Deciphering the fine details of C1 assembly and activation mechanisms: "mission impossible"?] - detailed diagrams ==External links== * {{MeshName|Complement+C1q}} {{Complement system}} Category:Complement system --- Adapted from the Wikipedia article [Complement component 1q](https://en.wikipedia.org/wiki/Complement_component_1q) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Complement_component_1q?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.