Lyn dissociation from phosphorylated Fc epsilon RI subunits: A new regulatory step in the Fc epsilon RI signaling cascade revealed by studies of Fc epsilon RI dimer signaling activity

Cross-linking the heterotrimeric (alpha beta gamma 2) IgE receptor, Fc epsi lon RI, of mast cells activates two tyrosine kinases: Lyn, which phosphoryl ates beta and gamma subunit immunoreceptor tyrosine-based activation motifs , and Syk, which binds gamma-phospho-immunoreceptor tyrosine-based activati on motifs and initiates cellular responses. We studied three Fc epsilon RI- dimerizing mAbs that maintain similar dispersed distributions over the surf ace of RBL-2H3 mast cells but elicit very different signaling responses. Sp ecifically, mAb H10 receptor dimers induce very little inositol 1,4,5-trisp hosphate synthesis, Ca2+ mobilization, secretion, spreading, ruffling, and actin plaque assembly, whereas dimers generated with the other anti-Fc epsi lon RI mAbs induce responses that are only modestly lower than that to mult ivalent Ag. H10 receptor dimers activate Lyn and support Fc epsilon RI beta and gamma subunit phosphorylation but are poor Syk activators compared wit h Ag and the other anti-Fc epsilon RI mAbs, H10 receptor diners have two ot her distinguishing features. First, they induce stable complexes between ac tivated Lyn and receptor subunits, Second, the predominant Lyn-binding phos pho-beta isoform found in mAb H10-treated cells is a less tyrosine phosphor ylated, more electrophoretically mobile species than the predominant isofor m in Ag-treated cells that does not coprecipitate with Lyn, These studies i mplicate Lyn dissociation from highly phosphorylated receptor subunits as a new regulatory step in the Fc epsilon RI signaling cascade required for Sy k activation and signal progression.