Synergistic Activation of Phospholipase C-β3 by Gαq and Gβγ Describes a Simple Two-State Coincidence Detector

Abstract

Summary Background Receptors that couple to Gi and Gq often interact synergistically in cells to elicit cytosolic Ca2+ transients that are several-fold higher than the sum of those driven by each receptor alone. Such synergism is commonly assumed to be complex, requiring regulatory interaction between components, multiple pathways, or multiple states of the target protein. Results We show that cellular Gi-Gq synergism derives from direct supra-additive stimulation of phospholipase C-β3 (PLC-β3) by G protein subunits Gβγ and Gαq, the relevant components of the Gi and Gq signaling pathways. No additional pathway or proteins are required. Synergism is quantitatively explained by the classical and simple two-state (inactive↔active) allosteric mechanism. We show generally that synergistic activation of a two-state enzyme reflects enhanced conversion to the active state when both ligands are bound, not merely the enhancement of ligand affinity predicted by positive cooperativity. The two-state mechanism also explains why synergism is unique to PLC-β3 among the four PLC-β isoforms and, in general, why one enzyme may respond synergistically to two activators while another does not. Expression of synergism demands that an enzyme display low basal activity in the absence of ligand and becomes significant only when basal activity is ≤ 0.1% of maximal. Conclusions Synergism can be explained by a simple and general mechanism, and such a mechanism sets parameters for its occurrence. Any two-state enzyme is predicted to respond synergistically to multiple activating ligands if, but only if, its basal activity is strongly suppressed.

Membrane-induced Allosteric Control of Phospholipase C-β Isozymes

“…For PLC-␤2 and especially PLC-␤3, this activation can be synergistic and requires low basal phospholipase activity (52). We show here that low basal phospholipase activity requires both the XY-linker and HTH.…”

Structural Insights into Phospholipase C-β Function

“…For example, in both the crystal structure and single particle cryoelectron microscopy three-dimensional reconstruction of full-length PLCb3, the distal CTD interacts with the hydrophobic ridge of the catalytic core, sequestering the basic surface of the distal CTD and preventing the hydrophobic ridge from accessing the membrane (Lyon et al, 2013). These observations may help partially explain the cytosolic localization of PLCb3 and its lower basal activity compared with that of the other isoforms (Smrcka and Sternweis, 1993;Philip et al, 2010;Adjobo-Hermans et al, 2013). The PLCb2 C-terminal extension has also been shown to influence the equilibrium between membrane-bound and cytosolic populations of this enzyme (Illenberger et al, 2003b).…”

“…On the basis of biochemical and structural data, it is clear that Ga q has a distinct binding site and activation mechanism from Gbg and the Rho GTPases. Indeed, for some isoforms, regulation by these molecules has been shown to be synergistic (Roach et al, 2008;Philip et al, 2010;Rebres et al, 2011). Below we discuss the current state of knowledge regarding the molecular basis of PLCb activation by four key regulators: the phospholipid bilayer, Ga q , the Gbg heterodimer, and Rho GTPases.…”

Phosphoinositides: Tiny Lipids With Giant Impact on Cell Regulation

“…This broadly expressed isoform can be activated by G␣ q/11 as well as G␤␥ subunits (1215). The recent structure of PLC␤3 in complex with activated G␣ q defined the interface between the two proteins (1657).…”