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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.

ABSTRACT: 1. The sequence of the N-terminal 42 amino acid residues and the identity of residue 45 of the C chain of subcomponent C1q were established by the use of the automatic protein sequencer. 2. A comparison of the amino acid sequences of the A and C chains of subcomponent C1q shows that they are identical at 18 positions out of the first 45. However, 12 of the amino acid residues in the positions of identity are glycine residues occurring in the repeating triplet sequence Gly-X-Y. 3. Position 36 in the C chain was found to be 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. 4. The sequence information presented here therefore indicates that there should be a break, or distortion, located approximately half-way along each of the six collagen-like triple-helical regions proposed to be present in subcomponent C1q [Reid & Porter (1976) Biochem. J. 155, 19-23] and this is consistent with what is seen in electron micrographs of intact and pepsin-digested subcomponent C1q.

SUBMITTER: Reid KB

PROVIDER: S-EPMC1164502 | biostudies-other | 1977 Feb

REPOSITORIES: biostudies-other

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Project description:1. Unreduced human subcomponent C1q was shown by electrophoresis on polyacrylamide gels run in the presence of sodium dodecyl sulphate to be composed of two types of non-covalently linked subunits of apparent mol.wts. 69 000 and 54 000. The ratio of the two subunits was markedly affected by the ionic strength of the applied sample. At a low ionic strength of applied sample, which gave the optimum value for the 54 000-apparent mol.wt. subunit, a ratio of 1.99:1.00 was obtained for the ratio of the 69 000-apparent mol.wt. subunit to the 5400-apparent-mol.wt. subunit. The amount of the 54 000-apparent-mol.wt. subunit detected in the expected position on the gel was found to be inversely proportional to increases in the ionic strength of the applled sample. 2. Human subcomponent C1q on reduction and alkylation, or oxidation, yields equimolar amounts of three chains designated A, B and C [Reid et al. (1972) Biochem. J. 130, 749-763]. The results obtained by Yonemasu & Stroud [(1972) Immunochemistry 9, 545-554], which showed that the 69 000-apparent-mol.wt. subunit was a disulphide-linked dimer of the A and B chains and that the 54 000-apparent-mol.wt. subunit was a disulphide-linked dimer of the C chain, were confirmed. 3. Gel filtration on Sephadex G-200 in 6.0M-guanidinium chloride showed that both types of unreduced subunit were eluted together as a single symmetrical peak of apparent mol.wt. 49 000-50 000 when globular proteins were used as markers. The molecular weights of the oxidized or reduced A, B and C chains have been shown previously to be very similar all being in the range 23 000-24 000 [Reid et al. (1972) Biochem. J. 130, 749-763; Reid (1974) Biochem. J. 141, 189-203]. 4. It is proposed that subcomponent C1q (mol.wt. 410000) is composed of nine non-covalently linked subunits, i.e. six A-B dimers and three C-C dimers. 5. A structure for subcomponent C1q is proposed and is based on the assumption that the collagen-like regions of 78 residues in each of the A, B and C chains are combined to form a triple-helical structure of the same type as is found in collagens.

Dataset Information

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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