Symmetry of binding sites of a mouse IgA myeloma protein (MOPC 315).

We have investigated the mechanism of monovalency of the 7S subunit of a mouse IgA myeloma protein (MOPC 315) against a large antigen. This subunit, although it clearly can bind two molecules of a small hapten, fails to precipitate or hemagglutinate the relevant multivalent antigen. In an equilibrium Farr assay, we have shown that the subunit has only one valence for a univalent 40,000 molecular weight antigen (dinitrophenyl-dextran). We have investigated how various levels of affinity labeling quantitatively affect (a) the valence observed in the equilibrium Farr assay against a large antigen, and (b) the binding of the MOPC 315 to an insoluble antigenic matrix. Our results indicate that the Fab regions of the 7S subunit are arranged symmetrically and that the inactivity of one of them toward a large antigen is probably due to steric hindrance caused by the antigen bound to the adjacent site.