Roles of non-catalytic subunits in G beta gamma-induced activation of class I phosphoinositide 3-kinase isoforms beta and gamma

By using purified preparations we show that nanomolar concentrations of G b eta gamma significantly stimulated lipid kinase activity of phosphatidylino sitol 3-kinase (PI3K) beta and PI3K gamma in the presence as well as in the absence of non-catalytic subunits such as p85 alpha or p101. Concomitantly , G beta gamma stimulated autophosphorylation of the catalytic subunit of P I3K gamma (EC50, 30 nm; stoichiometry greater than or equal to 0.6 mol of P -i/mol of p110 gamma), which also occurred in the absence of p101. Surprisi ngly, we found that p101 affected the lipid substrate preference of PI3R ga mma in its G beta gamma-stimulated state. With phosphatidylinositol as subs trate, p110 gamma but not p101/p110 gamma was significantly stimulated by G beta gamma to form PI-3-phosphate (EC50, 20 nM). The opposite situation wa s found when PI-4,5-bisphosphate served as substrate. G beta gamma efficien tly and potently (EC50, 5 nM) activated the p101/p110 gamma heterodimer but negligibly stimulated the p110 gamma monomer to form PI-3,4,5-trisphosphat e. However, this weak stimulatory effect on p110 gamma was overcome by exce ss concentrations of G beta gamma (EC50, 100 nM). This finding is in accord ance with the in vivo situation, where activated PI3K catalyzes the formati on 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 PI3K gamma by sensitizing p110 gamma toward G beta gamma in the presence of PI-4,5 P-2 .