The serine proteinase chain of human complement component C̅1s. Cyanogen bromide cleavage and N-terminal sequences of the fragments

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Abstract
Human complement component C1s was purified from fresh blood by conventional methods of precipitation and chromatography. The single-chain zymogen form was activated by treatment with .**GRAPHIC**. [activated C1r]. Reduction and carboxymethylation then allowed the L chain and H chain to be separated on DEAE-Sepharose CL-6B in 8M-urea. Liquid-phase sequencing of the L chain determined 50 residues from the N-terminus. CNBr [cyanogen bromide]-cleavage fragments of the L chain were separated by high-pressure liquid chromatography on gel-permeation and reverse-phase columns. N-terminal sequencing of these fragments determined the order of a further 138 residues, giving a total of 188 residues or about 75% of the L chain. Seven of these 8 sequences could be readily aligned with the amino acid sequences of other serine proteinases. The typical serine proteinase active-site residues are clearly conserved in .**GRAPHIC**. and the specificity-related side chain of the substrate-binding pocket is aspartic acid, as in trypsin, consistent with the proteolytic action of .**GRAPHIC**. on C4 at an arginine residue. When the .**GRAPHIC**. sequence is compared with that of complement subcomponent .**GRAPHIC**. the percentage difference (59%) is approximately the same as that found between the other mamalian serine proteinases (56-71%).