Transmembrane signaling by the high-affinity IgE receptor on membrane preparations

Aggregating the receptor with high affinity for IgE (Fc epsilon RI) stimulates a variety of phenomena in mast cells. Previous efforts to reproduce some of these events in broken-cell preparations such as isolated membranes have had limited success, possibly because the phenomena being monitored were too distal from the initial events. One of the earliest responses is now known to be the phosphorylation of tyrosine residues on several proteins, including the beta and gamma subunits of Fc epsilon RI. We show that in cell sonicates or on partially purified membranes derived from tumor mast cells, aggregating Fc epsilon RI stimulates phosphorylation of receptor tyrosine residues. As in the intact cells, receptor-mediated phosphorylation occurs only on receptors that are themselves aggregated. Because even in the unfractionated sonicates the phosphorylation of other cellular components was not detectably enhanced, and because the evidence is against the receptor itself being a kinase, our results suggest that phosphorylation of Fc epsilon RI is one of the earliest events stimulated by the receptor--an event that can now be investigated on simpler biological preparations than previously available.