The Binding of Antibodies to the Solubilized and Membrane-Integrated Mouse and Rat Receptor for IgE

Abstract Rabbit antiserum to rat basophilic leukemia cells (anti-RBL) inhibited the binding of 125I-IgE to and caused agglutination of RBL and mouse mastocytoma cells. Immunoglobulin G, Fab′, and Fab derived from this antiserum inhibited binding of 125I-IgE to whole rat and mouse tumor cells, normal rat peritoneal mast cells, and to detergent-solubilized receptors from either cell line. The inhibition was less efficient with receptors from the mouse mastocytoma and no inhibition of binding of 125I-IgE to normal mouse peritoneal mast cells was observed. The degree of inhibition of IgE binding to whole cells and solubilized rat and mouse receptors was inversely proportional to the time of incubation with and concentration of 125I-IgE. In a double immune precipitation system, anti-RBL reacted with detergent-solubilized receptors and IgE — receptor complexes derived from either mouse or rat cells. However, although nearly 100% of the free rat receptors could be precipitated no more than 70% of free mouse receptor were maximally precipitated. The efficiency of precipitation of detergent-solubilized free receptors, and the maximal precipitation of receptor — IgE complexes by anti-RBL were decreased when the detergent was above the critical micelle concentration. However, neither the precipitation of receptor — IgE complexes by anti-IgE, nor the binding of 125I-IgE to the free receptor was significantly affected under the same conditions. The results suggest that the anti-RBL contains antibodies to at least two discrete determinants on the receptor. One determinant (or group of determinants) is sufficiently close to the combining site for IgE to interfere with IgE binding and in turn is blocked from binding by the presence of IgE. The second is distal from the first, is exposed at low detergent concentration even in the presence of IgE, but is blocked when the receptor interacts with larger amounts of detergent. Taken together with other data these findings suggest that the receptor is substantially embedded in the surface membrane.