Primary structure of the A chain of human complement-classical-pathway enzyme C1̄r. N-Terminal sequences and alignment of autolytic fragments and CNBr-cleavages peptides

Abstract

Activated human complement-classical-pathway enzyme C1̄r has previously been shown to undergo autolytic cleavages occurring in the A chain. Chemical analysis of the autolytic products confirms that the A chain undergoes two major cleavages, generating three fragments, which have now been isolated and characterized. The N-terminal α fragment (approx. 210 residues long) has a blocked N-terminus, as does the whole A chain, whereas N-terminal sequences of fragments β adn γ (approx. 66 and 176 residues long respectively) do not, and their N-terminal sequences were determined. Fragments α, β and γ, which are not interconnected by disulphide bridges, are located in this order within C1̄r A chain. Fragment γ is disulphide-linked to the B chain of C1̄r, which is C-terminal in the single polypeptide chain of precursor C1r. CNBr cleavage of C1̄r A chain yields seven major peptides, CN1b, CN4a, CN2a, CN1a, CN3, CN4b and CN2b, which were positioned in that order, on the basis of N-terminal sequences of the methionine-containing peptides generated from tryptic cleavage of the succinylated (3-carboxypropionylated) C1̄r A chain. About 60% of the sequence of C1̄r A chain (440-460 residues long) was determined, including the complete sequence of the C-terminal 95 residues. This region shows homology with the corresponding parts of plasminogen and chymotrypsinogen and, more surprisingly, with the α1 chain of human haptoglobin 1-1, a serine proteinase homologue.