title: Molecular characterization of the catalytic domains of human complement serine protease C1r.
abstract: Limited cleavages of human C1r by extrinsic proteases of various specificity (plasmin, elastase, chymotrypsin, thermolysin) yield dimeric associations of two globular domains, each comprised of the intact B chain disulfide linked to gamma, the C-terminal fragment of the A chain. These (gamma-B)2 domains, which are homologous to those obtained from C1r by autolytic cleavage [Villiers, C. L., Arlaud, G. J., & Colomb, M. G. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 4477-4481], represent the core of the C1r molecule and are associated with the catalytic properties of the serine active site. V8 protease also yields (gamma-B)2 associations, although additional cleavages occur in the B chain. Sequence analysis shows that all cleavages generating the gamma fragments occur within a 13-residue sequence extending from positions 274 to 286 of the C1r A chain. Chemical cross-linking with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide of the (gamma-B)2 catalytic domains obtained from C1r autolytic cleavage indicates that each gamma-B domain interacts with its neighbor in a "head to tail" configuration, the gamma region of one domain interacting with the B chain of the other domain, and conversely. No evidence is found of gamma-gamma or B-B interactions. Such a head to tail configuration, placed in the context of the model proposed for the C1s-C1r-C1r-C1s catalytic subunit of C1 [Colomb, M. G., Arlaud, G. J., & Villiers, C. L. (1984) Philos. Trans. R. Soc. London, B 306, 283-292], is compatible with autolytic activation of C1r through an intramolecular cross-mechanism and with subsequent activation of C1s by activated C1r.
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D003166:Complement Activating Enzymes / Q000378:metabolism*; D015923:Complement C1r; D004220:Disulfides / Q000032:analysis; D006801:Humans; D046911:Macromolecular Substances; D008958:Models, Molecular; D010446:Peptide Fragments / Q000032:analysis; D010447:Peptide Hydrolases; D011487:Protein Conformation
chemical_list: D004220:Disulfides; D046911:Macromolecular Substances; D010446:Peptide Fragments; D003166:Complement Activating Enzymes; D010447:Peptide Hydrolases; D015923:Complement C1r