REGULATION OF BTK FUNCTION BY A MAJOR AUTOPHOSPHORYLATION SITE WITHINTHE SH3 DOMAIN

Bruton's tyrosine kinase (Btk) plays a crucial role in B cell developm ent. Overexpression of Btk with a Src family kinase increases tyrosine phosphorylation and catalytic activity of Btk. This occurs by transph osphorylation at Y551 in the Btk catalytic domain and the enhancement of Btk autophosphorylation at a second site. A gain-of-function mutant called Btk containing E41 to K change within the pleckstrin homology domain induces fibroblast transformation. Btk enhances the transphos phorylation of Y551 by endogenous Src family tyrosine kinases and auto phosphorylation at the second site. We mapped the major Btk autophosph orylation site to Y223 within the SH3 domain. Mutation of Y223 to F bl ocks Btk autophosphorylation and dramatically potentiates the transfor ming activity of Btk in fibroblasts. The location of Y223 in a potent ial ligand-binding pocket suggests that autophosphorylation regulates SH3-mediated signaling by Btk.