Complex alternative RNA splicing of epsilon-immunoglobulin transcripts produces mRNAs encoding four potential secreted protein isoforms.

In RNA from human IgE-producing lymphocytes, we previously discovered two alternatively spliced epsilon-immunoglobulin mRNA isoforms that encode a novel secreted form of IgE and a membrane-bound species. Further analysis using epsilon-specific reverse transcriptase polymerase chain reactions has elucidated several additional alternatively spliced species of epsilon mRNA. One RNA isoform is generated by splicing the CH4 exon to a novel distal splice acceptor site, forming an epsilon RNA species (designated CH4-M2") that encodes a secreted epsilon protein 6 amino acids larger than the classic secreted epsilon protein. The other three novel epsilon RNAs are generated by splicing from within CH4 to a new exon structure (designated CH5) that is located between CH4 and the membrane exons. Since the three new mRNAs using CH5 share the same stop codon in CH5, they all encode the same novel protein, which is 10 amino acids shorter than the classic secreted epsilon heavy chain. The new alternatively spliced epsilon mRNAs reported here, in addition to the previously reported forms encoding membrane and larger secreted IgE, appear to reflect the normal splicing pattern in humans, as we have detected all these epsilon RNAs in all the human IgE-secreting cells and cell lines tested.