Studies of the IgE binding sites to rat mast cell receptor with proteolytic fragments and with a monoclonal antibody directed against epsilon heavy chain: Evidence that the combining sites are located in the C ϵ3 domain

Abstract

The binding sites of rat IgE to mast cell receptor were investigated by the use of proteolytic fragments and a monoclonal antibody to epsilon chain (MARE-1). Three main fragments were characterized by short-time papain digestion of IgE: F(ab') 2-ϵ, a fragment related to the C ϵ4 domain, and an asymmetric fragment corresponding probably to an IgE molecule with one proteolyzed C ϵ 3 domain. Neither F(ab') 2-ϵ nor C ϵ4 could interfere with the binding of IgE to rat mast cells. These two fragments did not show significant polymerization upon heating at 56°C, while large amounts of polymers were produced from whole IgE. MARE-1 monoclonal antibody was found to react neither with F (ab') 2 nor with C ϵ4, thereby suggesting its interaction with the C,3 domain. MARE-1 was found to inhibit partially (about 55%) the binding of IgE to its receptor. Taken together the results indicate that the binding sites of IgE to rat mast cell receptor are located within the C ϵ3 domain. In addition, isolation of the C ϵ4 domain will be useful to evaluate its participation in the affinity of IgE to receptors of other cells such as lymphocytes or macrophages.