Tyrosine phosphorylation of several cellular proteins is one of the earliest signaling events induced by cross-linking of the high-affinity receptor for immunoglobulin E (FcERI) on mast cells or basophils. Tyrosine kinases activated during this process include the Src family kinases, Lyn, c-Yes, and c-Src, and members of another subfamily, Syk and PTK72 (identical or highly related to Syk). Recently, some of us described two novel tyrosine kinases, Emb and Emt, whose expression was limited to subsets of hematopoietic cells, including mast cells. Emb turned out to be identical to Btk, a gene product defective in human X-linked agammaglobulinemia and in X-linked immunodeficient (xid) mice. Here we report that FcrRI cross-linking induced rapid phosphorylation on tyrosine, serine, and threonine residues and activation of Btk in mouse bone marrowderived mast cells. A small fraction of Btk translocated from the cytosol to the membrane compartment following receptor cross-linking. Tyrosine phosphorylation of Btk was not induced by either a Ca2+ ionophore (A23187), phorbol 12-myristate 13-acetate, or a combination of the two reagents. Coimmunoprecipitation between Btk and receptor subunit i or y was not detected. The data collectively suggest that Btk is not associated with FceRI but that its activation takes place prior to protein kinase C activation and plays a novel role in the FcERI signaling pathway.Cross-linking of the high-affinity immunoglobulin E (IgE) receptor (FccRI) on mast cells and basophils induces a variety of morphological and biochemical changes, culminating in the release of chemical mediators (17). Among early intracellular signaling events, tyrosine phosphorylation of several proteins is thought to be of primary importance (2,4,7,18,21,26,40). Thus, protein-tyrosine kinase (PTK) inhibitors prevent not only tyrosine phosphorylation of cellular proteins but also other early events, including the production of inositol 1,4,5-trisphosphate and histamine release (18). Therefore, PTK activation is thought to be proximal to the activation of phospholipase C (PLC) and protein kinase C (PKC), two enzymes considered essential for mast cell activation.Two subfamilies of PTKs are implicated in this process. The Src family PTKs p56'Y' in RBL-2H3 (a rat basophilic leukemia cell line) and p62cYes in PT-18 (a mouse mast cell line) were found to be associated with FccRI and activated upon receptor cross-linking (8). p60csrc in RBL-2H3 cells was also activated subsequent to receptor cross-linking, although its association with the receptor was not detected. Members of another PTK subfamily, p72"k and PTK72 (which is identical or highly related to p72SYk), were also shown to be activated by FcuRI cross-linking (3, 16). The association of p72syk with the receptor depends on tyrosine phosphorylation of the -y subunit of the receptor (3).In a recent report, some of us described two novel PTK genes expressed in murine mast cells (37). One, termed emb, turned out to be identical to the btk gene, whose mutations in humans a...