title: Ca(2+)-linked association of human complement C1s and C1r.
abstract: The weight-average molecular weight of Clr, an activated serine protease subcomponent of complement Cl, was measured in the presence of widely varying concentrations of Ca2+ and the other serine protease subcomponent, Cls, by utilizing the technique of tracer sedimentation equilibrium. A quantitative model for heteroassociation between the two subcomponents, which takes into account the previously observed Ca(2+)-dependent self-association of Cls, was fit to the combined data at each Ca2+ concentration. The results indicate that Clr, which exists as a dimer under all of the conditions explored in this work, can bind up to two molecules of Cls at both low and high Ca2+ concentrations, but the association constant for binding a single molecule of Cls to dimeric Clr is estimated to increase on the order of a 1000-fold as [Ca2+] increases from 1 nM to 1.0 mM. Heteroassociation of Clr and Cls is favored over self-association of Cls at all conditions. The results clearly indicate the necessity of taking into account a multiplicity of states of association when attempting to understand the equilibrium average properties of a mixture of the two subcomponents and their binding to Clq in solution.
authors.affiliation: Laboratory of Biochemical Pharmacology, National Institute of Diabetes and Digestive 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*; D015923:Complement C1r / Q000378:metabolism*; D003173:Complement C1s / Q000378:metabolism*; D006801:Humans; D008956:Models, Chemical
chemical_list: D015923:Complement C1r; D003173:Complement C1s; D002118:Calcium