Gα_q binds to p110α/p85α phosphoinositide 3-kinase and displaces Ras

Several studies have reported that activation of G_q-coupled receptors inhibits PDK (phosphoinositide 3-kinase) signalling. In the present study, we used purified proteins to demonstrate that Gα_q directly inhibits p110α/p85α PI3EK in a GTP-dependent manner. Activated Gα_q binds to the p110α/p85α PI3K with an apparent affinity that is seven times stronger than that for Gα_q · GDP as measured by fluorescence spectroscopy. In contrast, Gα_q did not bind to the p110γ PI3K. Fluorescence spectroscopy experiments also showed that Gα_q, competes with Ras, a PI3K activator, for binding to p110α/p85α. Interestingly, co-precipitation studies using deletion mutants showed that Gα_q binds to the p85-binding domain of p110α and not to the Ras-binding domain. Expression of constitutively active Gα _qQ209L in cells inhibited Ras activation of the PI3K/Akt pathway but had no effect on Ras/Raf/MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase] signalling. These results suggest that activation of G_q-coupled receptors leads to increased binding of Gα_q · GTP to some isoforms of PI3K, which might explain why these receptors inhibit this signalling pathway in certain cell types.