SRC FAMILY PROTEIN-TYROSINE KINASES INDUCE AUTOACTIVATION OF BRUTONS TYROSINE KINASE

Bruton's tyrosine kinase (Btk) is tyrosine phosphorylated and enzymati cally activated following ligation of the B-cell antigen receptor. The se events are temporally regulated, and Btk activation follows that of various members of the Src family of protein tyrosine kinases, thus r aising the possibility that Src kinases participate in the Btk activat ion process. We have evaluated the mechanism underlying Btk enzyme act ivation and have explored the potential regulatory relationship betwee n Btk and Src protein kinases. We demonstrate in COS transient-express ion assays that Btk can be activated through intramolecular autophosph orylation at tyrosine 551 and that Btk autophosphorylation is required for Btk catalytic functions. Coexpression of Btk with members of the Src family of protein tyrosine kinases, but not Syk, led to Btk tyrosi ne phosphorylation and activation. Using a series of point mutations i n Btk (a representative Src protein kinase) and Btk, we show that Src kinases activate Btk through an indirect mechanism that requires membr ane association of the Src enzymes as well as functional Btk SH3 and S H2 domains. Our results are compatible with the idea that Src protein tyrosine kinases contribute to Btk activation by indirectly stimulatin g Btk intramolecular autophosphorylation.