Amino acid sequence of the N-terminal forty-two amino acid residues of the C chain of subcomponent C1q of the first component of human complement

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Abstract
The sequence of the N-terminal 42 amino acid residues and the identity of residue 45 of the C chain of C1q [q fragment of complement component 1] were established by the use of the automatic protein sequencer. A comparison of the amino acid sequences of the A and C chains of C1q shows that they are identical at 18 positions out of the first 45. Twelve of the amino acid residues in the positions of identity are glycine residues occurring in the repeating triplet sequence Gly-X-Y. Position 36 in the C chain was apparently alanine, which was unexpected since the residue in this position would have to be glycine if the repeating triplet sequence Gly-X-Y were to extend, uniformly, throughout the entire length of the C-chain collagen-like region. This break in the repeating triplet sequence would prevent residue 36 in the C chain from taking part in collagen-like triple-helix formation. The sequence information presented here indicates that there should be a break, or distortion, located approximately half-way along each of the 6 collagen-like triple-helical regions proposed to be present in C1q. This is consistent with what is seen in electron micrographs of intact and pepsin-digested C1q.