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From the * International Center for Genetic Engineering and Biotechnology, Area Science Park,
Padriciano 99, 34012-Trieste, Italy; and The human C Ospedale di Pediatria Burlo Garofalo, Via Dell'Istria 65, 34147-Trieste, Italy
gene expresses two membrane IgE heavy chain mRNAs which differ in the sequence that encodes their extracellular membrane-proximal domain. In the long IgE isoform
(mLIgE), this domain contains a stretch of 52 amino acids which are absent in the short variant
(mSIgE). We have now generated B cell transfectoma cell lines that express these two isoforms
and show that both types of mIgE form functional B cell antigen receptors (BCR). Both receptors associate with the Ig-
/Ig-
heterodimer, as well as with protein kinases that are capable
of phosphorylating this complex. Upon their cross-linking, both receptors can activate protein
tyrosine kinases that phosphorylate the same substrate proteins. Both IgE receptors also associate with two novel proteins that do not bind to mIgM. Apart from these similarities, the two IgE-BCRs show several differences of which some are analogous to the differences between
the IgM- and IgD-BCRs. First, the mSIgE is transported to the cell surface at a higher rate than
the mLIgE. Second, the two IgE-BCRs associate with differently glycosylated Ig- proteins,
the mLIgE associates with the completely glycosylated form, whereas the mSIgE associates with
an Ig- glycoform that is partially sensitive to endoglycosidase H. Third, the kinetics of protein
tyrosine phosphorylation induced by receptor cross-linking is significantly different for the two
IgE-BCRs. Finally, cross-linking of the mSIgE-BCR leads to growth inhibition of the B cell
transfectoma, whereas signaling through the mLIgE-BCR does not affect the cellular proliferation. These data show that the two human membrane IgE isoforms assemble into functionally
distinct antigen receptors which can induce different cellular responses.
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