A novel lipid raft-associated glycoprotein, TEC-21, activates rat basophilic leukemia cells independently of the type 1 Fcε receptor

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Abstract
Recent data suggest that initiation of signal transduction via type 1 Fcε receptor (FcεRI) and other immunoreceptors is spatially constrained to lipid rafts. In order to better understand the complexity and function of these structures, we prepared mAb against lipid rafts from the rat basophilic leukemia cell line, RBL-2H3, which is extensively used for analysis of FcεRI-mediated activation. One of the antibodies was found to recognize a novel glycosylphosphatidylinositol-anchored plasma membrane glycoprotein of 250 amino acids, designated TEC-21, containing a cysteine-rich domain homologous to those found in the urokinase plasminogen activator receptor/Ly-6/snake neurotoxin family. TEC-21 is abundant on the surface of RBL-2H3 cells (>10 6 molecules/cell), but is absent in numerous rat tissues except for testes. Aggregation of TEC-21 on RBL-2H3 cells induced a rapid increase in tyrosine phosphorylation of several substrates including Syk kinase and LAT adaptor, calcium flux, and release of secretory components. Similar but more profound activation events were observed in cells activated via FcεRI. However, aggregation of TEC-21 did not induce changes in density of IgE–FcεRI complexes, tyrosine phoshorylation of FcεRI β and γ subunits, and co-aggregation of Lyn kinase. TEC-21-induced activation events were also observed in FcεRI− mutants of RBL-2H3 cells. Thus, TEC-21 is a novel lipid raft component of RBL-2H3 cells whose aggregation induces activation independently of FcεRI.

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