Phosphoinositide 3-kinase-dependent phosphorylation of the dual adaptor for phosphotyrosine and 3-phosphoinositides by the Src family of tyrosine kinase

We recently identified a novel adaptor protein, termed dual adaptor for pho sphotyrosine and 3-phosphoinositides (DAPP1), that possesses a Src homology (SH2) domain and a pleckstrin homology (PH) domain. DAPP1 exhibits a high- affinity interaction with PtdIns(3,4,5)P-3 and PtdIns(3,4)P-2, which bind t o the PW domain. In the present study we show that when DAPP1 is expressed in HEK-293 cells, the agonists insulin, insulin-like growth factor-1 and ep idermal growth factor induce the phosphorylation of DAPP1 at Tyr(139). Trea tment of cells with phosphoinositide 3-kinase (PI 3-kinase) inhibitors or e xpression of a dominant-negative PI 3-kinase prevent phosphorylation of DAP P1 at Tyr(130), and a PH-domain mutant of DAPP1, which does not interact wi th PtdIns(3,4,5)P-3 or PtdIns(3,4)P-2 is not phosphorylated at Tyr(139) fol lowing agonist stimulation of cells. Overexpression of a constitutively act ive form of PI 3-kinase induced the phosphorylation of DAPP1 in unstimulate d cells. We demonstrated that Tyr(139) of DAPP1 is likely to be phosphoryla ted in vivo by a Src-family tyrosine kinase, since the specific Src-family inhibitor, PP2, but not an inactive variant of this drug, PP3, prevented th e agonist-induced tyrosine phosphorylation of DAPP1. Src, Lyn and Lck tyros ine kinases phosphorylate DAPP1 at Tyr(139) in vitro at similar rates in th e presence or absence of PtdIns(3,4,5)P-3, and overexpression of these kina ses in HEK-293 cells induces the phosphorylation of Tyr(139). These finding s indicate that, following activation of PI 3-kinases, PtdIns(3,4,5)P-3 or PtdIns(3,4)P-2 bind to DAPP1, recruiting it to the plasma membrane where it becomes phosphorylated at Tyr(139) by a Src-family tyrosine kinase.