Polarization fluorescence, spin label and ultracentrifugal studies of specific interaction of low molecular weight proteins with the Fc fragment of human immunoglobulin G

Low mol. wt (about 2000) proteins which were found in normal human serum formed complexes with the Fc fragment of IgG. The interaction constant was not less than 10⁶ 1/mole. These complexes dissociated on dilution of the protein solution to below 2 μM or decreasing the pH below 6. The binding site of these proteins was located in the C_H2 domains in close proximity to the carbohydrate moieties. The dissociation of IgG complexes with these proteins was accompanied by a conformational change in the Fc fragment.