Complement component 1q: Difference between revisions

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{{cs1 config|name-list-style=vanc}}

{{refimprove|date=October 2015}}

{{short description|Protein complex}}

{{context|date=December 2015}}

{{Infobox protein family

| Symbol = C1q

| Name = C1q ___domain

| image = PDB 1o91 EBI.jpg

| 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

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| width =

| HGNCid = 1241

| Symbol = [[C1QA]]

| AltSymbols =

| EntrezGene = 712

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| LocusSupplementaryData =

}}

The '''C1qcomplement complexcomponent 1q''' (or simply '''C1q''') is a [[protein complex]]. Itinvolved in the [[complement system]], which is part of the [[complementinnate immune system]]. C1q together with [[C1r]] and [[C1s]] form the [[C1 complex]].

[[Antibody|Antibodies]] of the [[adaptive immune system]] can bind [[antigen]], forming an [[antigen-antibody complex]]. When C1q binds antigen-antibody complexes, the [[C1 complex]] becomes activated. Activation of the C1 complex initiates the [[classical complement pathway]] of the complement system. The antibodies [[IgM]] and all [[Immunoglobulin G|IgG]] subclasses except [[Immunoglobulin G#Subclasses|IgG4]] are able to initiate the complement system.

It is potentially [[Valence (chemistry)|multivalent]] for attachment to the complement fixation sites of [[immunoglobulin]].

The sites are on the CH2 ___domain of [[IgG]] and, it is thought, on the CH4 ___domain of [[IgM]].

The appropriate [[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]].

==Structure==

C1q is a 400460 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]], [[isoleucine]], or [[hydroxylysine]]; they, therefore, strongly resemble [[collagen]] [[fibrils]].

===C1QA,C1q C1QBchains A, C1QCB 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. This gene encodes the A-chain polypeptide of human complement subcomponent C1q.<ref name="entrez ciqa">{{cite web | title = Entrez Gene: C1QA complement component 1, q subcomponent, A chain| url = httphttps://www.ncbi.nlm.nih.gov/sites/entrezgene?Db=gene&Cmd=ShowDetailView&TermToSearch=712| accessdate = }}</ref>

== Domain ==

The C1q ___domain is a conserved [[protein ___domain]]. [[C1Q complex|C1q]] is a subunit of the C1 [[enzyme]] [[Protein complex|complex]] that activates the [[blood serum|serum]] [[complement system]]. C1q comprises 6 A, 6 B and 6 C [[polymer|chain]]s. These share the same topology, each possessing a small, globular [[N-terminal]] ___domain, a collagen-like Gly/Pro-rich central region, and a [[conserved sequence|conserved]] C-terminal region, the C1q ___domain.<ref name="pmid1706597">{{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}}</ref> The C1q protein is produced in collagen-producing [[cell (biology)|cell]]s and shows [[sequence (biology)|sequence]] and structural similarity to [[collagen]]s VIII and X.<ref name="pmid2591537">{{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 = }}</ref><ref name="pmid2019595">{{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 }}</ref>

==Function==

[[ImageFile:Complement pathway.svg|thumb|270px|left|The classical and alternative complement pathways. C1q is the orange part of the [[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 suffering fromwith 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 suffer fromhave [[Systemic lupus erythematosus|SLE]]. C1q may also play a central role in the aging of cells.<ref name="Secret of ageing found: Japanese scientists pave way to everlasting life">{{cite web |title=SecretC1q ofdeficiency ageing- found:About Japanesethe scientistsDisease pave- wayGenetic and Rare Diseases toInformation everlastingCenter life|url=httphttps://wwwrarediseases.rtinfo.comnih.gov/newsdiseases/japan12958/c1q-scientists-stop-ageing-461/|work=Russiadeficiency Today|publisherwebsite=Russiararediseases.info.nih.gov Today|accessdatelanguage=10 June 2012en}}</ref>

C1q associates with C1r and C1s in order to yield the [[C1- complex]] (''C1qr²s²''), the first component of the serum [[complement system]]. Deficiency of C1q has been associated with [[lupus erythematosus]] and [[glomerulonephritis]].<ref name="entrez ciqa"/>

It is potentially [[Valence (chemistry)|multivalent]] for attachment to the complement fixation sites of [[immunoglobulin]].

The sites are on the CH2 ___domain of [[Immunoglobulin G|IgG]] and, it is thought, on the CH4 ___domain of [[IgM]]. IgG4 cannot bind C1q, but the other three IgG subclasses can.

The appropriate [[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]].

{{clear}}

==References==

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==Further reading==

* [httphttps://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==

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{{Complement system}}

{{gene-1-stub}}

[[Category:Complement system]]