Cloning of cDNA coding for the β chain of human complement component C4b-binding protein: Sequence homology with the α chain

Abstract

The major form of complement component C4b-binding protein, a regulator of the complement system, is composed of seven identical 70-kDa α chains, each containing a binding site for the complement protein C4b. We recently showed that C4b-binding protein also contains a unique 45-kDa β 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 β 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 β-chain species starting at Glu-4 was identified. The β chain contains five potential N-linked glycosylation sites, and endoglycosidase digestion suggested that the β chain contained multiple complex carbohydrate side chains. Northern blot analysis of human liver mRNA, using the β-chain cDNA as the probe, demonstrated a major mRNA species of approximately 1.0 kilobase. From the N terminus, the β chain contains three tandem repeat units (60 amino acids long) that are homologous to those present in the α chain. The C-terminal region, which was unrelated to the tandem repeats, demonstrated sequence similarity with the corresponding region of the α chain. In both a and β chains these regions contain two cysteine residues that probably form the interchain disulfide bridges.