{PDOC00857}
{PS50871; C1Q}
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* C1q domain profile *
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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
[1-4]. 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  beta-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.

-Sequences known to belong to this class detected by the profile: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.

-Expert(s) to contact by email:
           Reid K.B.M.;
kbmreid@biochemistry.oxford.ac.uk

-Last update: March 2013 / Profile and text revised.

[ 1] Smith K.F., Haris P.I., Chapman D., Reid K.B.M., Perkins S.J.
     "Beta-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."
     Biochem. J. 301:249-256(1994).
     PubMed=8037678
[ 2] Brass A., Kadler K.E., Thomas J.T., Grant M.E., Boot-Handford R.P.
     "The fibrillar collagens, collagen VIII, collagen X and the C1q
     complement proteins share a similar domain in their C-terminal
     non-collagenous regions."
     FEBS Lett. 303:126-128(1992).
     PubMed=1607009
[ 3] Petry F., Reid K.B.M., Loos M.
     "Isolation, 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."
     Eur. J. Biochem. 209:129-134(1992).
     PubMed=1396691
[ 4] Bork P.
     Unpublished observations (1995).
[ 5] Seldin M.M., Peterson J.M., Byerly M.S., Wei Z., Wong G.W.
     "Myonectin (CTRP15), a novel myokine that links skeletal muscle to
     systemic lipid  homeostasis."
     J. Biol. Chem. 287:11968-11980(2012).
     PubMed=22351773; DOI=10.1074/jbc.M111.336834
[ 6] Rosenbloom J., Endo R., Harsch M.
     J. Biol. Chem. 251:2070-2076(1976).
[ 7] Engel J., Prockop D.J.
     "The zipper-like folding of collagen triple helices and the effects of
     mutations that disrupt the zipper."
     Annu. Rev. Biophys. Biophys. Chem. 20:137-152(1991).
     PubMed=1867713
[ 8] Kwan A.P.L., Cummings C.E., Chapman J.A., Grant M.E.
     "Macromolecular organization of chicken type X collagen in vitro."
     J. Cell Biol. 114:597-604(1991).
     PubMed=1860888
[ 9] Tu X., Palczewski K.
     "Crystal structure of the globular domain of C1QTNF5: Implications for
     late-onset  retinal macular degeneration."
     J. Struct. Biol. 180:439-446(2012).
     PubMed=22892318; DOI=10.1016/j.jsb.2012.07.011

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