Structure of the high-affinity mast cell receptor for IgE

Mast cells and related rat tumor basophils have a surface receptor that binds monomeric IgE with high avidity. The receptor in situ is unclustered, mobile, and univalent, and its aggregation into dimers and higher oligomers triggers degranulation. It has two subunits, alpha and beta. The alpha chain has a molecular weight of about 50,000, of which 30% is carbohydrate. Heterogeneity in the latter accounts at least in part and perhaps fully for the heterogeneity of the alpha chain. Studies with proteases have delineated two domains: alpha 1 appears slightly larger on sizing gels, and incorporation studies suggest it has more carbohydrate than alpha 2. The alpha 2 domain and a 24,000-dalton subfragment of it contain the principal site whose surface labeling is blocked by the presence of IgE. The alpha subunit is firmly but noncovalently bound to beta in a 1:1 complex in situ and in detergent extracts. Nevertheless, during thorough washing of IgE-receptor complexes with detergent, beta dissociates. A portion of beta, beta 1, is integrated in the membrane; it is this domain that is labeled when the receptor is isolated from cells reacted with the intramembranous probe iodonapthylnitrene and that interacts with the alpha chain. These results and others have been used to draw a provisional model of the receptor.