Detergent-Resistant Membrane Domains Are Required for Mast Cell Activation but Dispensable for Tyrosine Phosphorylation upon Aggregation of the High Affinity Receptor for IgE

Aggregation of the high affinity receptor for IgE (FceRI) on mast cells results in the rapid phosphorylation of tyrosines on the p and 7 chains of the receptor by the Src family kLnase Lyn, which initiates the signaling cascades leading to secretion of inflmnnmtory mediators. The detergent-resistant membranes (DRMs) have been implicated in FceRI signaling because aggregated receptors emigrate to DRMs that are enriched in certain signaling components. We evaluated the role of DRMs in FceRI signaling by disruption of DRMs using a cholesterol-binding agent, methyl-P-cyclodextrin (MBCD). While treatment of rat basopbilic leukemia cells with MBCD inhibits degranulation and Ca2+ mobilization upon aggregation of FceRI, MBCD hardly affects the aggregation-induced tyrosine phosphorylation of FceRI as well as other signaling molecules such as phospholipase C-7I (PLC-7I). MBCD delocalizes phosphatidylinositol 4,5-bisphosphate from DRMs, which may prevent MBCD-treated cells from producing inositol 1,4,5-trisphosphate by means of activated PLC-7I. These data suggest an indispensable role for DRMs in the Ca2+ response rather than tyrosine phosphorylation, and support a model of receptor phosphorylation in which aggregated FceRI is tyrosine phosphorylated outside DRMs by constitutively associated Src family kinase Lyn via a transphosphorylation mechanism.