Transphosphorylation as the mechanism by which the high-affinity receptor for IgE is phosphorylated upon aggregation

When aggregated, the high-affinity receptors for IgE on mast cells (Fc epsilon RI) launch a series of phosphorylations, particularly of protein tyrosines. We have analyzed how aggregation initiates this cascade. We examined Fc epsilon RI from unstimulated cells and from cells exposed to a polyvalent hapten conjugate that aggregates the Fc epsilon RI via the receptor-bound anti-hapten IgE. We also examined the latter receptors after they had been disaggregated in vitro with monovalent hapten. By an in vitro kinase assay: (i) Unaggregated and disaggregated receptors are associated with a kinase that phosphorylates an exogenous (peptide) substrate but minimally, or not at all, the subunits of Fc epsilon RI or associated proteins (endogenous substrates). After aggregation, phosphorylation of the exogenous substrate is linear with time, but the modification of the endogenous substrates reaches a plateau, presumably because only those endogenous substrates that are adjacent to the kinase are phosphorylated. (ii) Aggregated receptors and disaggregated receptors have enhanced kinase activity toward exogenous substrate. The state of phosphorylation of the receptor correlates strongly with the yield of enhanced kinase activity. We propose that upon aggregation of Fc epsilon RI, a constitutively associated kinase phosphorylates endogenous substrates by transphosphorylation. As a result, additional kinase activity becomes manifest and this promotes further transphosphorylation. In view of the homology between Fc epsilon RI and other receptors central to the immune response, the latter receptors likely utilize a similar transphosphorylation mechanism.