title: Calcium-linked self-association of human complement C1s.
abstract: The weight-average molecular weight of C1s, an activated serine protease subcomponent of human complement C1, has been measured by means of sedimentation equilibrium over a wide range of both protein and calcium ion concentrations. The combined data may be accounted for quantitatively by a simple model for Ca(2+)-dependent self-association of C1s to a dimer. According to this model, the monomer contains a single Ca2+ binding site with K approximately equal to 3 x 10(5) M-1, and the dimer contains three independent Ca binding sites, two having a Ca2+ affinity lower than that of the monomer (K approximately equal to 3 x 10(4) M-1). The third binding site in the dimer, which presumably lies at the interface between the two amino-terminal alpha domains, has a higher Ca2+ affinity (K approximately equal to 1 x 10(8) M-1) and provides the driving force for C1s dimerization in the presence of calcium.
authors.affiliation: Laboratory of Biochemical Pharmacology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: G
authors.initials: G
authors.lastname: Rivas
authors.forename: K C
authors.initials: KC
authors.lastname: Ingham
authors.forename: A P
authors.initials: AP
authors.lastname: Minton
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002498:Centrifugation; D003173:Complement C1s / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D046911:Macromolecular Substances; D008970:Molecular Weight
chemical_list: D046911:Macromolecular Substances; D003173:Complement C1s; D002118:Calcium