Characterization of Protein-tyrosine Phosphatases That Dephosphorylate the High Affinity IgE Receptor

Abstract

An early event that follows aggregation of the high affinity receptor for IgE (FcepsilonRI) is the phosphorylation of protein tyrosines, especially those on the beta- and gamma-subunits of the receptor. Disaggregation of the receptors leads to their rapid dephosphorylation, but even stably aggregated receptors undergo continual rounds of phosphorylation and dephosphorylation. We developed assays to study dephosphorylation of the receptors and other cellular proteins. Whole cell extracts dephosphorylated both subunits of the receptors rapidly and were as active against aggregated as against disaggregated FcepsilonRI. Upon disaggregation, the in vivo dephosphorylation of the FcepsilonRI and several other proteins followed first-order kinetics with closely similar rate constants despite substantial differences in the extent of phosphorylation. These results suggest that the level of phosphorylation of FcepsilonRI is largely controlled by the aggregation-induced action of kinase(s) and not from changes in susceptibility to or activity of the phosphatases. Much of the total phosphatase is lost when the cells are permeabilized, but the rate of dephosphorylation of disaggregated FcepsilonRI was comparable in intact and permeabilized cells. Thus, much of the activity utilized by the cell to dephosphorylate the FcepsilonRI is likely to be associated with the plasma membrane.