Ras is an indispensable coregulator of the class IB phosphoinositide 3-kinase p87/p110γ

Class I-B phosphoinositide 3-kinase gamma (PI3K gamma) elicits various immunologic and cardiovascular responses; however, the molecular basis for this signal heterogeneity is unclear. PI3K gamma consists of a catalytic p110 gamma and a regulatory p87(PIKAP) (p87, also p84) or p101 subunit. Hitherto p87 and p101 are generally assumed to exhibit redundant functions in receptor-induced and G protein beta gamma (G beta gamma)-mediated PI3K gamma regulation. Here we investigated the molecular mechanism for receptor-dependent p87/p110 gamma activation. By analyzing GFP-tagged proteins expressed in HEK293 cells, PI3K gamma-complemented bone marrow-derived mast cells (BMMCs) from p110 gamma(-/-) mice, and purified recombinant proteins reconstituted to lipid vesicles, we elucidated a novel pathway of p87-dependent, G protein-coupled receptor (GPCR)-induced PI3K gamma activation. Although p101 strongly interacted with G beta gamma, thereby mediating PI3K gamma membrane recruitment and stimulation, p87 exhibited only a weak interaction, resulting in modest kinase activation and lack of membrane recruitment. Surprisingly, Ras-GTP substituted the missing G beta gamma-dependent membrane recruitment of p87/p110 gamma by direct interaction with p110 gamma, suggesting the indispensability of Ras for activation of p87/p110 gamma. Consequently, interference with Ras signaling indeed selectively blocked p87/p110 gamma, but not p101/p110 gamma, kinase activity in HEK293 and BMMC cells, revealing an important crosstalk between monomeric and trimeric G proteins for p87/p110 gamma activation. Our data display distinct signaling requirements of p87 and p101, conferring signaling specificity to PI3K gamma that could open up new possibilities for therapeutic intervention.