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
Human precerebellin, which is located within postsynaptic structures of
Purkinje cells, probably membrane-bound. Cerebellin is involved in synaptic
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
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 .
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 . 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>) .
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.
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.
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