C1Q complex

complement component 1, q subcomponent, A chain
Identifiers
Symbol	C1QA
Entrez	712
HUGO	1241
OMIM	120550
RefSeq	NM_015991
UniProt	P02745
Other data
Locus	Chr. 1 p36.3-34.1

complement component 1, q subcomponent, B chain
Identifiers
Symbol	C1QB
Entrez	713
HUGO	1242
OMIM	120570
RefSeq	NM_000491
UniProt	P02746
Other data
Locus	Chr. 1 p36.3-34.1

complement component 1, q subcomponent, C chain
Identifiers
Symbol	C1QC
Alt. symbols	C1QG
Entrez	714
HUGO	1245
OMIM	120575
RefSeq	NM_172369
UniProt	P02747
Other data
Locus	Chr. 1 p36.11

The C1q complex is potentially 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 400 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.

Function

The classical and alternative complement pathways.

It is assumed that the globular ends are the sites for multivalent attachment to the complement fixing sites in immune complexed immunoglobulin. Patients suffering from 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 from SLE. C1q may also play a central role in the aging of cells.

External links

• Complement C1q at the US National Library of Medicine Medical Subject Headings (MeSH)