A collagen-like amino acid sequence in a polypeptide chain of human C1q (a subcomponent of the first component of complement).

1. A partial amino acid sequence of 95 residues of the 191 residues in the oxidized A chain of human subcomponent C1q was determined. The partial nature of the sequence is because one overlapping peptide is missing in the proposed sequence, also the proof of some of the overlapping peptides depends partly on their amino acid composition and not on their complete sequence. 2. This region of the A chain contained a repeating sequence of glycine-X-Y (where X is often proline and Y is often hydroxyproline) for 78 residues. 3. The five hydroxylysine residues and the five hydroxyproline residues present in the oxidized A chain were all in these 78 residues and only in the Y position of the repeating sequence. 4. Prolonged collagenase digestion of the oxidized A chain yielded a large, apparently C-terminal, peptide which contained most of the non-collagenous sequences present in the chain. 5. It is concluded that there is a collagen-like region in the A chain of subcomponent C1q which constitutes most of the N-terminal half of the chain and that similar collagen-like regions will be found in the B and C chains.