Satoshi Tsukada

The cytoplasmic tyrosine kinase, Bruton's tyrosine kinase (Btk, formerly bpk or atk), is crucial for B cell development. Loss of kinase activity results in the human immunodeficiency, X-linked agammaglobulinemia, characterized by a failure to produce B cells. In the murine X-linked immunodeficiency (XID), B cells are present but respond abnormally to activating signals. The Btk gene, btk, was mapped to the xid region of the mouse X chromosome by interspecific backcross analysis. A single conserved residue within the amino terminal unique region of Btk was mutated in XID mice. This change in xid probably interferes with normal B cell signaling mediated by Btk protein interactions.

We evaluated in WEHI 231 B cells the time-dependent responses of Lyn, Blk, Btk, Syk, and three members of the Jak family of protein tyrosine kinases following antibody-mediated surface engagement of the B-cell antigen receptor. Our results show that the enzyme activities of Lyn and Blk were stimulated within seconds of antigen receptor engagement and correlated with the initial tyrosine phosphorylation of the Ig alpha and Ig beta subunits of the B-cell antigen receptor. Btk enzyme activity was also transiently stimulated and was maximal at approximately 5 min after B-cell receptor surface binding. Syk activity gradually increased to a maximum at 10-30 min following receptor ligation and was found to parallel the association of Syk with the tyrosine phosphorylated Ig alpha and Ig beta subunits of the receptor. While the specific activities of the Jak1, Jak2, and Tyk2 protein tyrosine kinases were unaltered following B-cell receptor ligation, the abundance of Jak1 and Jak2 were increased 3- to 4-fold within 10 min of receptor engagement. These results demonstrate that multiple families of non-transmembrane protein tyrosine kinases are temporally regulated during the process of B-cell antigen receptor-initiated intracellular signal transduction.

Bruton tyrosine kinase (Btk) has been implicated as the defective gene in both human and murine B-cell deficiencies. The identification of molecules that interact with Btk may shed light on critical processes in lymphocyte development. The N-terminal unique region of Btk contains a pleckstrin homology domain. This domain is found in a broad array of signaling molecules and implicated to function in protein-protein interactions. By using an in vitro binding assay and an in vivo competition assay, the pleckstrin homology domain of Btk was shown to interact with the beta gamma dimer of heterotrimeric guanine nucleotide-binding proteins (G proteins). A highly conserved tryptophan residue in subdomain 6 of the pleckstrin homology domain was shown to play a critical role in the binding. The interaction of Btk with beta gamma suggests the existence of a unique connection between cytoplasmic tyrosine kinases and G proteins in cellular signal transduction.