This study shows that aggregation of U937 cell high affinity IgG Fc receptor (FcRI) results in the transient tyrosine phosphorylation of FcRI -chain but not the phosphorylation of -chains associated with nonaggregated IgA Fc receptors (FcR) on the same cells. Thus, normally, tyrosine phosphorylation of -chains is limited to FcR in aggregates. In contrast, aggregation of FcRI in the presence of vanadate induced the sustained tyrosine phosphorylation of FcRI -chains and the rapid and extensive phosphorylation of nonaggregated FcR -chains and low affinity IgG Fc receptors (FcRII). This global phosphorylation of motifs on nonaggregated FcR was also detected upon aggregation of FcR or FcRII, which induced the phosphorylation of nonaggregated FcRI -chains. Vanadate prevented dephosphorylation of proteins and increased kinase activity in stimulated cells. Evidence failed to support alternative explanations such as acquisition of phospho- through subunit exchange or a coalescence of nonaggregated with aggregated FcR. It is likely, therefore, that activated kinases interacted with nonaggregated FcR in stimulated cells. Pervanadate induced the tyrosine phosphorylation of -chains in the absence of FcR cross-linking, indicating that the kinases could be activated by phosphatase inhibition and could react with nonaggregated substrates. We conclude that under normal conditions there is a vanadate-sensitive mechanism that prevents tyrosine phosphorylation of nonaggregated FcR -chain motifs in activated cells, restricting their phosphorylation to aggregates.

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