cDNA heterogeneity suggests structural variants related to the high-affinity IgE receptor

The high-affinity IgE receptor present on mast cells and basophils is responsible for the IgE-mediated activation of these cells. The current model for this receptor depicts a four-subunit structure, αβγ₂. A cDNA for the α subunit was recently cloned and predicts a structure consisting of two homologous extracellular domains, a transmembrane segment, and a cytoplasmic tail. Using a synthetic oligonucleotide corresponding to the amino-terminal sequence of the α subunit, we identified a number of cDNA clones from a rat basophilic leukaemia cell cDNA library. Nucleotide sequencing estabished four different forms of cDNA: one is nearly identical to the published cDNA; the second differs from the first in the 5' untranslated sequence; the other two forms use either one or the other of the 5'-end sequences as above and lack 163 base pairs in the region coding for the second extracellular domain. RNase protection analysis with radioactive RNA probes established the heterogeneity of rat basophilic leukemia cell mRNA with regard to both the 5' end and the internal sequences. Our results suggest the existence of at least four different protein forms related to the α subunit of the high-affinity IgE receptor.