The C1q complex
is potentially multivalent
for attachment to the complement fixation sites of immunoglobulin
The sites are on the CH2 domain of IgG and probably on the CH4 domain of IgM.
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.
C1q is a 400 KDa(1 KDa=1000 Da) 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.
It is assumed that the globular ends are the sites for multivalent attachment to the complement fixing sites in immune complexed immunoglobulin.