In mast cells, antigen-mediated aggregation of the high-affinity receptor for immunoglobulin E, FcRI, stimulates tyrosine phosphorylation and activation of multiple signaling pathways leading to the release of several classes of mediators of the allergic response. Early events induced upon cross-linking of FcRI include tyrosine phosphorylation of FcRI subunits and activation of the tyrosine kinase p72 syk (Syk), which binds to tyrosine-phosphorylated FcRI. Clustering of Syk, as a result of its interaction with aggregated FcRI, may play a role in activating one or more of the signaling pathways leading to mediator release. To test this possibility, Syk was introduced into a model mast cell line (rat basophilic leukemia cells) as part of a chimeric transmembrane protein containing the extracellular and transmembrane domains of CD16 and CD7, respectively. Clustering of the Syk chimera, using antibodies against CD16, was found to be sufficient to stimulate early and late events normally induced by clustering of FcRI. Specifically, aggregation of Syk induced degranulation, leukotriene synthesis, and expression of cytokine genes. Induction of mediator release was dependent on the kinase activity of Syk. Consistent with this finding, clustering of Syk also induced the tyrosine phosphorylation of a profile of proteins, including phospholipase C-␥1 and mitogen-activated protein kinase, similar to that induced upon clustering of FcRI. These results strongly suggest that Syk is an early and critical mediator of multiple signaling pathways that emanate from the FcRI receptor and give rise to the allergic response.Antigen-induced aggregation of the high-affinity receptor for immunoglobulin E (IgE), FcεRI, induces mast cells to release several classes of mediators that activate both early and late phases of the allergic response (37, 49). Mediators released upon mast cell activation fall into three classes which are represented by (i) histamines, which are preformed and secreted from granules within minutes of activation; (ii) leukotrienes and prostaglandins, which are newly generated from lipid precursors and are also rapidly released upon activation; and (iii) cytokines, which are newly expressed and released within the first few hours after activation.Activation of the multiple signaling pathways that lead to the synthesis and release of these diverse mediators is preceded by aggregation-induced tyrosine phosphorylation of two subunits of FcεRI (33,40). Subsequent events include (i) the increased tyrosine phosphorylation and activation of a number of signaling molecules, including Syk (7, 23), phospholipase C-␥1 (PLC-␥1) (2,19,41), Vav (36), and mitogen-activated protein (MAP) kinase (46); (ii) activation of protein kinase C (55, 56) and phospholipase A 2 (20); and (iii) elevation of intracellular calcium levels (6). Tyrosine phosphorylation appears to play a critical role in propagating the signal leading to these events since mediator release is blocked by tyrosine kinase inhibitors (27,57).FcεRI is a multisubunit recept...