Roles of Non-catalytic Subunits in Gβγ-induced Activation of Class I Phosphoinositide 3-Kinase Isoforms β and Î

By using purified preparations we show that nanomolar concentrations of Gβγ significantly stimulated lipid kinase activity of phosphatidylinositol 3-kinase (PI3K) β and PI3Kγ in the presence as well as in the absence of non-catalytic subunits such as p85α or p101. Concomitantly, Gβγ stimulated autophosphorylation of the catalytic subunit of PI3Kγ (EC 50 , 30 n m ; stoichiometry ≥0.6 mol of P i /mol of p110γ), which also occurred in the absence of p101. Surprisingly, we found that p101 affected the lipid substrate preference of PI3Kγ in its Gβγ-stimulated state. With phosphatidylinositol as substrate, p110γ but not p101/p110γ was significantly stimulated by Gβγ to form PI-3-phosphate (EC 50 , 20 n m ). The opposite situation was found when PI-4,5-bisphosphate served as substrate. Gβγ efficiently and potently (EC 50 , 5 n m ) activated the p101/p110γ heterodimer but negligibly stimulated the p110γ monomer to form PI-3,4,5-trisphosphate. However, this weak stimulatory effect on p110γ was overcome by excess concentrations of Gβγ (EC 50 , 100 n m ). This finding is in accordance with the in vivo situation, where activated PI3K catalyzes the formation of PI-3,4,5-trisphosphate but not PI-3-phosphate. We conclude that p101 is responsible for PI-4,5-bisphosphate substrate selectivity of PI3Kγ by sensitizing p110γ toward Gβγ in the presence of PI-4,5-P 2 .