Chemical and functional characterization of a fragment of C1-s containing the epidermal growth factor homology region.

C1-s, one of the three subcomponents of C1-, the first component of complement, is a serine protease comprising two disulfide-linked chains, the B chain, containing the catalytic site, and the A chain, involved in Ca2+ binding and Ca2(+)-dependent interaction(s) with the other C1- subcomponents. In an attempt to identify the regions responsible for the latter functions, C1-s was submitted to limited proteolysis with plasmin, a treatment that split the A chain into three major fragments, alpha 1, alpha 2, and gamma. Fragment alpha 2, which comprised the epidermal growth factor-like (EGF-like) region of C1-s, was heterogeneous, starting at serine 97 or phenylalanine 105 and ending at lysine 195. This fragment was reduced and alkylated and then digested with elastase, and three peptides covering positions 131-135, 131-139, and 131-140 were characterized by amino acid analysis, Edman degradation, and mass spectrometry, showing that position 134 of C1-s is occupied partly by an asparagine (47%) and partly by an erythro-beta-hydroxyasparagine, in contrast with the homologous position (150) of C1-r which only contains erythro-beta-hydroxyasparagine. As measured by equilibrium dialysis, native alpha 2, like the other plasmin-cleavage fragments, did not retain the ability of intact C1-s to bind Ca2+. In the same way, plasmin cleavage abolished the ability of C1-s to dimerize or to associate with C1-r in the presence of Ca2+. In contrast, both alpha 2 and the N-terminal alpha 1 fragment, starting at serine 24 of the A chain, were able to compete significantly with intact C1s for the formation of the Ca2(+)-dependent C1-s-C1r-C1-r-C1-s tetramer.(ABSTRACT TRUNCATED AT 250 WORDS)