Isolation and characterization of cDNAs coding for the β subunit of the high-affinity receptor for immunoglobulin E

Abstract

Among receptors that bind the Fc region of immunoglobulins ('Fc receptors'), only the one with high affinity for immunoglobulin E (IgE) is known to consist of more than a single polypeptide. In addition to the IgE-binding α chain, the receptor contains a single β chain and two, disulfide-linked, γ chains. From a cDNA library of a rat mucosal mast cell tumor, from which we recently cloned cDNAs coding for the α chain, we have now isolated cDNAs coding for the β subunit. In vitro transcription-translation of the cDNA directed the synthesis of a polypeptide reactive with two distinctive anti-β monoclonal antibodies and whose molecular weight was identical to that of authentic β chains. Polyclonal antibodies to β peptides expressed in Escherichia coli reacted with intact receptors and isolated β chains. The gene encodes a protein of 243 residues with no leader sequence. A hydropathicity plot suggests that the polypeptide crosses the plasma membrane four times. The epitope recognized by one of the monoclonal antibodies was localized to the NH2 terminus; that by the other was localized to the COOH terminus. Since those antibodies react with membranes and not with intact cells, we suggest that both ends of the β subunit are cytoplasmic. RNA transfer blots at high stringency failed to reveal mRNA for β chains in a variety of cells (in particular, monocytes) that do not contain the high-affinity receptor for IgE.