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

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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 .alpha. chain, the receptor contains a single .beta. chain and two, disulfide-linked, .gamma. chains. From a cDNA library of a rat mucosal mast cell tumor, from which we recently cloned cDNAs coding for the .alpha. chain, we have now isolated cDNAs coding for the .beta. subunit. In vitro transcription-translation of the cDNA directed the synthesis of a polypeptide reactive with two distinctive anti-.beta. monoclonal antibodies and whose molecular weight was identical to that of authentic .beta. chains. Polyclonal antibodies to .beta. peptides expressed in Escherichia coli reacted with intact receptors and isolated .beta. 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 .beta. subunit are cytoplasmic. RNA transfer blots at high stringency failed to reveal mRNA for .beta. chains in a variety of cells (in particular, monocytes) that do not contain the high-affinity receptor for IgE.