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

Several studies have reported that activation of G(q)-coupled receptors inhibits PI3K (phosphoinositide 3-kinase) signalling. In the present study, we used purified proteins to demonstrate that G alpha(q) directly inhibits p110 alpha/p85 alpha PI3K in a GTP-dependent manner. Activated Gaq binds to the p110 alpha/p85 alpha PI3K with an apparent affinity that is seven times stronger than that for G alpha(q) (.) GDP as measured by fluorescence spectroscopy. In contrast, Gaq did not bind to the p110 gamma PI3K. Fluorescence spectroscopy experiments also showed that G alpha(q) competes with Ras, a PI3K activator, for binding to p110 alpha/p85 alpha. Interestingly, co-precipitation studies using deletion mutants showed that G alpha(q) binds to the p85-binding domain of p110 alpha and not to the Ras-binding domain. Expression of constitutively active G alpha(q)Q209L in cells inhibited Ras activation of the PI3K/Akt pathway but had no effect on Ras/Raf/MEK [MAPK (mitogen-activated protein kinase)/ERK (extra-cell ular-signal-regulated kinase) kinase] signalling. These results suggest that activation of G(q)-coupled receptors leads to increased binding of G alpha(q) (.) GTP to some isoforms of PI3K, which might explain why these receptors inhibit this signalling pathway in certain cell types.