Abstract
Class I(B) phosphoinositide 3-kinase gamma (PI3Kgamma) elicits various
immunologic and cardiovascular responses; however, the molecular
basis for this signal heterogeneity is unclear. PI3Kgamma consists
of a catalytic p110gamma 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 betagamma (Gbetagamma)-mediated
PI3Kgamma regulation. Here we investigated the molecular mechanism
for receptor-dependent p87/p110gamma activation. By analyzing GFP-tagged
proteins expressed in HEK293 cells, PI3Kgamma-complemented bone marrow-derived
mast cells (BMMCs) from p110gamma(-/-) mice, and purified recombinant
proteins reconstituted to lipid vesicles, we elucidated a novel pathway
of p87-dependent, G protein-coupled receptor (GPCR)-induced PI3Kgamma
activation. Although p101 strongly interacted with Gbetagamma, thereby
mediating PI3Kgamma 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 Gbetagamma-dependent membrane recruitment of p87/p110gamma
by direct interaction with p110gamma, suggesting the indispensability
of Ras for activation of p87/p110gamma. Consequently, interference
with Ras signaling indeed selectively blocked p87/p110gamma, but
not p101/p110gamma, kinase activity in HEK293 and BMMC cells, revealing
an important crosstalk between monomeric and trimeric G proteins
for p87/p110gamma activation. Our data display distinct signaling
requirements of p87 and p101, conferring signaling specificity to
PI3Kgamma that could open up new possibilities for therapeutic intervention.
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