PROSITE documentation PDOC00857
C1q domain profile


C1q is a subunit of the C1 enzyme complex that activates the serum complement system. It is composed of 9 disulfide-linked dimers of the chains A, B and C, which share a common structure which consist of a N-terminal nonhelical region, a triple helical (collagenous) region and a C-terminal globular head which is called the C1q domain. That domain consists of about 136 amino acids and has been found in the C-terminus of vertebrate secreted or membrane-bound proteins which are mostly short-chain collagens and collagen-like molecules [1,2,3,4]. These proteins are listed below:

  • Complement C1q subcomponent chains A, B and C. Efficient activation of C1 takes place on interaction of the globular heads of C1q with the Fc regions of IgG or IgM antibody present in immune complexes.
  • Vertebrate short-chain collagen type VIII, the major component of the basement membrane of corneal endothelial cells. It is composed of a triple helical domain in between a short N-terminal and a larger C-terminal globule which contains the C1q domain.
  • Vertebrate collagen type X, which has the same structure than collagen type VIII. It is a product of hyperthrophic chondrotocytes.
  • Bluegill inner-ear specific structural protein. This short-chain collagen forms a microstructural matrix within the otolithic membrane.
  • Chipmunk hibernation-associated plasma proteins HP-20, HP-25 and HP-27. These proteins disappear from blood specifically during hibernation. They contain a collagen-like domain near the N-terminus and a C-terminal C1q domain.
  • Human precerebellin, which is located within postsynaptic structures of Purkinje cells, probably membrane-bound. Cerebellin is involved in synaptic activity.
  • Rat precerebellin-like glycoprotein, a probable membrane protein. The C1q domain is located at the C-terminal extracellular extremity.
  • Human endothelial cell multimerin (ECM), a carrier protein for platelet factor V/VA.
  • Vertebrate 30 Kd adipocyte complement-related protein (ACRP30), also known as ApM1 or AdipoQ.
  • Vertebrate myonectin (CTRP15), a nutrient responsive myokine secreted by skeletal muscle to regulate whole-body fatty acid metabolism [5].

The C-terminal globular domain of the C1q subcomponents and collagen types VIII and X is important both for the correct folding and alignment of the triple helix and for protein-protein recognition events [6,7]. For collagen type X it has been suggested that the domain is important for initiation and maintenance of the correct assembly of the protein [8]. The globular head is a trimer of C1q domains. Each individual C1q adopts a 10-strand Jelly-roll fold arranged in two antiparallel 5-stranded β-sheets (see <PDB:4F3J>) [9]. There are two well conserved regions within the C1q domain: an aromatic motif is located within the first half of the domain, the other conserved region is located near the C-terminal extremity.

Expert(s) to contact by email:

Reid K.B.M.;

Last update:

March 2013 / Profile and text revised.


Technical section

PROSITE method (with tools and information) covered by this documentation:

C1Q, PS50871; C1q domain profile  (MATRIX)


1AuthorsSmith K.F. Haris P.I. Chapman D. Reid K.B.M. Perkins S.J.
TitleBeta-sheet secondary structure of the trimeric globular domain of C1q of complement and collagen types VIII and X by Fourier-transform infrared spectroscopy and averaged structure predictions.
SourceBiochem. J. 301:249-256(1994).
PubMed ID8037678

2AuthorsBrass A. Kadler K.E. Thomas J.T. Grant M.E. Boot-Handford R.P.
TitleThe fibrillar collagens, collagen VIII, collagen X and the C1q complement proteins share a similar domain in their C-terminal non-collagenous regions.
SourceFEBS Lett. 303:126-128(1992).
PubMed ID1607009

3AuthorsPetry F. Reid K.B.M. Loos M.
TitleIsolation, sequence analysis and characterization of cDNA clones coding for the C chain of mouse C1q. Sequence similarity of complement subcomponent C1q, collagen type VIII and type X and precerebellin.
SourceEur. J. Biochem. 209:129-134(1992).
PubMed ID1396691

4AuthorsBork P.
SourceUnpublished observations (1995).

5AuthorsSeldin M.M. Peterson J.M. Byerly M.S. Wei Z. Wong G.W.
TitleMyonectin (CTRP15), a novel myokine that links skeletal muscle to systemic lipid homeostasis.
SourceJ. Biol. Chem. 287:11968-11980(2012).
PubMed ID22351773

6AuthorsRosenbloom J. Endo R. Harsch M.
SourceJ. Biol. Chem. 251:2070-2076(1976).

7AuthorsEngel J. Prockop D.J.
TitleThe zipper-like folding of collagen triple helices and the effects of mutations that disrupt the zipper.
SourceAnnu. Rev. Biophys. Biophys. Chem. 20:137-152(1991).
PubMed ID1867713

8AuthorsKwan A.P.L. Cummings C.E. Chapman J.A. Grant M.E.
TitleMacromolecular organization of chicken type X collagen in vitro.
SourceJ. Cell Biol. 114:597-604(1991).
PubMed ID1860888

9AuthorsTu X. Palczewski K.
TitleCrystal structure of the globular domain of C1QTNF5: Implications for late-onset retinal macular degeneration.
SourceJ. Struct. Biol. 180:439-446(2012).
PubMed ID22892318

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