The macromolecular structure of the first component of complement.

The binding of C1 to IgG and the interactions between C1 subcomponents have been studied by affinity chromatography of serum C1 and purified C1 proteins on Sepharose-IgG. Affinity chromatography of serum on Sepharose-IgG resulted in the binding of C1; subsequent washing with EDTA removed only C1s and C1t. Affinity chromatography of serum-EDTA on Sepharose-IgG resulted in binding of only C1q and C1r. Affinity chromatography of serum on Sepharose-tryptophan-modified-IgG resulted in the binding of only C1r and C1s. By the use of purified C1 proteins and Sepharose-IgG in binding studies it was confirmed that both C1q and C1r bind independently to sites on IgG and hold C1s and C1t by Ca++-dependent bonds. Measurements of the hemolytic activity of various combinations of C1 subcomponents confirmed the data obtained by the affinity binding studies. Both C1t and C1r independently enhanced the C1 activity of C1q-C1s mixtures; maximal activity required all four subcomponents. Sucrose gradient ultracentrifugation of mixtures of C1 proteins showed formation of the following complexes: C1qs (12S), C1qrs (15S), C1qst (18-23S), and C1qrst (19S). The evidence suggests that the spatial sequence of the components of the Sepharose-IgG-Serum C1 complex is: Sepharose-IgG: C1q: C1t: C1s: C1r: IgG-Sepharose. The probable physiologic significance of this model is discussed.