Phospholipase C-β1 directly accelerates GTP hydrolysis by Gαq and acceleration is inhibited by Gβγ subunits

Phospholipase C-beta, the principal effector protein regulated by G alpha(q), has been shown to increase the agonist-stimulated, steady-state GTPase activity of G(q) in proteoliposomes that contain both heterotrimeric G(q) and ml muscarinic receptor. We now use a moderately stable complex of R183C G alpha(q) bound to GTP to show that PLC-beta 1 acts directly as a GTPase-activating protein (GAP) for isolated G alpha(q) in a membrane-free system. PLC-beta 1 accelerated the hydrolysis of G alpha(qR183C).GTP up to 20-fold. The K-m was 1.5 nM, which is similar both to the EC50 with which R183C and wild type G alpha(q) activate PLC-beta 1 and to the EC50 with which PLC-beta 1 acts as a G(q) GAP in the vesicle-based assay. The G alpha(q) GAP activity of RGS4 can also be quantitated by this assay; it accelerated hydrolysis of bound GTP about 100-fold. The G(q) GAP activities of both PLC-beta 1 and RGS4 are blocked by G beta gamma subunits, probably by a competitive mechanism. These data suggest either that the G beta gamma subunits are not continuously required for receptor-catalyzed GDP/GTP exchange during steady-state GTP hydrolysis or that GAPs, either PLC-beta or RGS proteins, can substitute for G beta gamma in this set of reactions.