Tyrosine phosphorylation of p85 relieves its inhibitory activity on phosphatidylinositol 3-kinase

Under resting conditions, the p85 regulatory subunit of phosphatidylinosito l S-kinase (PI3K) serves to both stabilize and inactivate the p110 catalyti c subunit. The inhibitory activity of p85 is relieved by occupancy of the N H2-terminal SH2 domain of p85 by phosphorylated tyrosine. Src family kinase s phosphorylate tyrosine 688 in p85, a process that we have shown to be rev ersed by the activity of the p85-associated SH2 domain-containing phosphata se SHP1. We demonstrate that phosphorylation of the downstream PI3K target Akt is increased in cells lacking SHP1, implicating phosphorylation of p85 in the regulation of PI3K activity. Furthermore, the in vitro specific acti vity of PI3K associated with tyrosine-phosphorylated p85 is higher than tha t associated with nonphosphorylated p85, Expression of wild-type p85 inhibi ts PI3K enzyme activity as indicated by PI3K-dependent Akt phosphorylation. The inhibitory activity of p85 is accentuated by mutation of tyrosine 688 to alanine and reversed by mutation of tyrosine 688 to aspartic acid, chang es that block and mimic tyrosine phosphorylation, respectively. Strikingly, mutation of tyrosine 688 to aspartic acid completely reverses the inhibito ry activity of p85 on cell viability and activation of the downstream targe ts Akt and NF kappaB, indicative of the physiological relevance of p85 phos phorylation, Tyrosine phosphorylation of Tyr(688) or mutation of tyrosine 6 88 to aspartic acid is sufficient to allow binding to the NH2-terminal SH2 domain of p85. Thus an intramolecular interaction between phosphorylated Ty r(688) and the NH2-terminal SH2 domain of p85 can relieve the inhibitory ac tivity of p85 on p110. Taken together, the data indicate that phosphorylati on of Tyr(688) in p85 leads to a novel mechanism of PI3K regulation.