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 a protein complex involved in the complement system, which is part of the innate immune system. C1q is part of the C1-complex.

C1q can bind to the antibodies IgM and IgG (all but IgG4) when the antibody is bound to an antigen. When C1q binds to IgM or IgG complexed with antigen, the C1-complex is activated. Activation of the C1-complex intitiates the classical complement pathway.

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.

C1QA, C1QB, C1QC

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.[1]

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.[2]

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.[1]

It 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. IgG4 cannot bind C1q, but the other three IgG types 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.

References

  1. 1 2 "Entrez Gene: C1QA complement component 1, q subcomponent, A chain".
  2. ↑ "Secret of ageing found: Japanese scientists pave way to everlasting life". Russia Today. Russia Today. Retrieved 10 June 2012.

Further reading

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