title: Ca2+ binding properties and Ca2(+)-dependent interactions of the isolated NH2-terminal alpha fragments of human complement proteases C1-r and C1-s.
abstract: The NH2-terminal alpha fragments of human complement proteases C1-r and C1-s were obtained by limited proteolysis of the native proteins with trypsin, and isolated. C1-r alpha extended from residues 1 to 208 of C1-r A chain, with at least two cleavage sites within disulfide loops, after lysine 134 and arginine 202. C1-s alpha comprised residues 1-192 of the C1-s A chain, with one cleavage site within a disulfide loop, after arginine 186. C1-r alpha was monomeric either in the presence or absence of Ca2+ but formed Ca2(+)-dependent dimers with native C1-s. C1-s alpha dimerized in the presence of Ca2+ and formed Ca2(+)-dependent tetramers (C1-s alpha-C1-r-C1-r-C1-s alpha) with native C1-r. C1-r alpha and C1-s alpha associated in the presence of Ca2+ to form C1-r alpha-C1-s alpha heterodimers. Equilibrium dialysis studies indicated that each alpha region binds Ca2+ with a dissociation constant ranging from 19 microM (native proteins) to 38 microM (fragments). C1-r alpha, C1-r alpha-C1-s alpha, and the native C1-s-C1-r-C1-r-C1-s tetramer bound 0.9, 1.9, and 4.0 Ca2+ atoms/mol, respectively, whereas dimers C1-s alpha-C1-s alpha and C1-s-C1-s incorporated 2.9 and 3.0 Ca2+ atoms/mol. It is concluded that each alpha region contains one high affinity Ca2+ binding site. This 1:1 stoichiometry is maintained upon heterologous (C1-r-C1-s) interaction, whereas the homologous (C1-s-C1-s) interaction provides one additional binding site.
authors.affiliation: Département de Recherches Fondamentales, Centre d'Etudes Nucléaires de Grenoble, France.
authors.forename: N M
authors.initials: NM
authors.lastname: Thielens
authors.forename: C A
authors.initials: CA
authors.lastname: Aude
authors.forename: M B
authors.initials: MB
authors.lastname: Lacroix
authors.forename: J
authors.initials: J
authors.lastname: Gagnon
authors.forename: G J
authors.initials: GJ
authors.lastname: Arlaud
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002852:Chromatography, Ion Exchange; D015923:Complement C1r / Q000378:metabolism*; D003173:Complement C1s / Q000378:metabolism*; D004220:Disulfides / Q000032:analysis; D006801:Humans; D007700:Kinetics; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000302:isolation & purification / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D011489:Protein Denaturation; D014357:Trypsin
chemical_list: D004220:Disulfides; D010446:Peptide Fragments; D014357:Trypsin; D015923:Complement C1r; D003173:Complement C1s; D002118:Calcium