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 10(6) l/mole. These complexes dissociated on dilution of the protein solution to below 2 microM or decreasing the pH below 6. The binding site of these proteins was located in the CH2 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.