SRC-INDUCED ACTIVATION OF INDUCIBLE T-CELL KINASE (ITK) REQUIRES PHOSPHATIDYLINOSITOL 3-KINASE ACTIVITY AND THE PLECKSTRIN HOMOLOGY DOMAIN OF INDUCIBLE T-CELL KINASE

The Tec family of tyrosine kinases are involved in signals emanating f rom cytokine receptors, antigen receptors, and other lymphoid cell sur face receptors. One family member, ITK (inducible T cell kinase), is i nvolved in T cell activation and can be activated by the T cell recept or and the CD28 cell surface receptor. This stimulation of tyrosine ph osphorylation and activation of ITK can be mimicked by the Src family kinase Lck We have explored the mechanism of this requirement for Src family kinases in the activation of ITK, We found that coexpression of ITK and Src results in increased membrane association, tyrosine phosp horylation and activation of ITK, which could be blocked by inhibitors of the lipid kinase phosphatidylinositol 3-kinase (PI 3-kinase) as we ll as overexpression of the p85 subunit of PI 3-kinase. Removal of the Pleckstrin homology domain (PH) of ITK resulted in a kinase that coul d no longer be induced to localize to the membrane or be activated by Src. The PH of ITK was also able to bind inositol phosphates phosphory lated at the D3 position. Membrane targeting of ITK without the PH rec overed its ability to be activated by Src, These results suggest that ITK can be activated by a combination of Src and PI 3-kinase.