Cloning of cDNA coding for the beta chain of human complement component C4b-binding protein: sequence homology with the alpha chain.
- 1 February 1990
- journal article
- research article
- Published by Proceedings of the National Academy of Sciences in Proceedings of the National Academy of Sciences
- Vol. 87 (3), 1183-1187
- https://doi.org/10.1073/pnas.87.3.1183
Abstract
The major form of complement component C4b-binding protein, a regulator of the complement system, is composed of seven identical 70-kDa .alpha. chains, each containing a binding site for the complement protein C4b. We recently showed that C4b-binding protein also contains a unique 45-kDa .beta. chain. It is disulfide-linked to the central core and contains a binding site for the vitamin K-dependent protein S. We have now isolated and characterized full-length cDNA clones for the .beta. chain. In addition, 57% of the structure was determined by protein sequencing of tryptic and chymotryptic peptides. Two clones, A8 and C1, isolated from different libraries were sequenced. Except for a deleted triplet encoding Ala-3 in clone A8, the two clones were identical and coded for a leader sequence of 17 amino acids and a mature protein of 235 amino acids (including Ala-3). By N-terminal amino acid sequencing, the Ala-3 heterogeneity was confirmed and a third .beta.-chain species starting at Glu-4 was identified. The .beta. chains contains five potential N-linked glycosylation sites, and endoglycosidase digestion suggested that the .beta. chain contained multiple complex carbohydrate side chains. Northern blot analysis of human liver mRNA, using the .beta.-chain cDNA as the probe, demonstrated a major mRNA species of approximately 1.0 kilobase. From the N terminus, the .beta. chain contains three tandem repeat units (60 amino acids long) that are homologous to those present in the .alpha. chain. The C-terminal region, which was unrelated to the tandem repeats, demonstrated sequence similarity with the corresponding region of the .alpha. chain. In both .alpha. and .beta. chains these regions contain two cysteine residues that probably form the interchain disulfide bridges.This publication has 31 references indexed in Scilit:
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