The Unique Domain as the Site on Lyn Kinase for Its Constitutive Association with the High Affinity Receptor for IgE

Aggregation of the high affinity receptor for IgE (FcεRI) leads to the phosphorylation of tyrosines on the β and γ chains of the receptor by the Src family kinase Lyn. We have studied the interaction between Lyn and the FcεRI in vivo using a transfection-based approach. FcεRI were stably transfected into Chinese hamster ovary cells. The small amount of endogenous Src family kinase was sufficient to phosphorylate receptor tyrosines upon extensive aggregation of FcεRI but not after addition of dimers of IgE. Upon stable co-transfection of Lyn kinase into the cells, dimers were now able to stimulate receptor phosphorylation and the response to more extensive aggregation was enhanced. In contrast, co-transfection with catalytically inactive Lyn inhibited the aggregation-induced phosphorylation by the endogenous kinase, and a quantitatively similar inhibition was observed in cells transfected with the SH4-containing unique domain of Lyn. Consistent with the results of others using alternative approaches, our additional studies using a yeast two-hybrid system detected a direct interaction between intact Lyn or its unique domain and the C-terminal cytoplasmic domain of the β chain but not with the receptor's other cytoplasmic domains.