The high-affinity immunoglobulin-E receptor (FcεRI) is endocytosed by an AP-2/clathrin-independent, dynamin-dependent mechanism

Aggregation of the high-affinity immunoglobulin E (IgE) receptor (Fc epsilon RI), expressed on mast cells and basophils, initiates the immediate hypersensitivity reaction. Aggregated Fc epsilon RI has been reported to rapidly migrate to lipid rafts in RBL-2H3 cells. We confirmed that aggregated Fc epsilon RI is found in the lipid raft fractions of cellular lysates. Furthermore, we show that the cross-linked Fc epsilon RI remains associated with detergent-resistant structures upon internalization. Previous morphological studies have reported that aggregated Fc epsilon RI is endocytosed via clathrin-coated pits, which in general are not lipid raft associated. To address this apparent discrepancy, we employed siRNA to suppress expression of components of the clathrin-mediated internalization machinery, namely, clathrin heavy chain, and the AP-2 (alpha-adaptin or mu 2-subunit). Transferrin receptor (TfR) is endocytosed by a clathrin-mediated process and, as expected, each transfected siRNA caused a two to threefold elevation of TfR surface expression and almost completely inhibited its endocytosis. In contrast, there was no effect on surface expression levels of Fc epsilon RI nor on the endocytosis of the dinitrophenyl-human serum albumin (DNP-HSA)/IgE/Fc epsilon RI complex. On the contrary, internalization of DNP-HSA/IgE/Fc epsilon RI was inhibited by overexpression of a dominant-negative dynamin mutant. We conclude that internalization of cross-linked Fc epsilon RI does not require the AP-2/clathrin complex but is dynamin-dependent and may be lipid raft mediated.