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 which probably form ten β strands interspersed by β-turns and/or loops [1]. Such a domain 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.

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 [5,6]. For collagen type X it has been suggested that the domain is important for initiation and maintenance of the correct assembly of the protein [7].

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.