The mechanism ofGαqregulation ofPLCβ3-catalyzedPIP2hydrolysis

Abstract

AbstractPLCβenzymes cleavePIP2producing IP3 and DAG.PIP2modulates the function of many ion channels, while IP3 and DAG regulate intracellular Ca2+levels and protein phosphorylation by protein kinase C, respectively.PLCβenzymes are under the control of GPCR signaling through direct interactions with G proteinsGβγandGαqand have been shown to be coincidence detectors for dual stimulation ofGαqand Gαicoupled receptors.PLCβsare aqueous-soluble cytoplasmic enzymes, but partition onto the membrane surface to access their lipid substrate, complicating their functional and structural characterization. Using newly developed methods, we recently showed thatGβγactivatesPLCβ3by recruiting it to the membrane. Using these same methods, here we show thatGαqincreases the catalytic rate constant,kcat, ofPLCβ3. Since stimulation ofPLCβ3byGαqdepends on an autoinhibitory element (the X-Y linker), we propose thatGαqproduces partial relief of the X-Y linker autoinhibition through an allosteric mechanism. We also determined membrane-bound structures of thePLCβ3-Gαq,andPLCβ3-Gβγ(2)-Gαqcomplexes, which show that these G proteins can bind simultaneously and independently of each other to regulatePLCβ3activity. The structures rationalize a finding in the enzyme assay, that co-stimulation by both G proteins follows a product rule of each independent stimulus. We conclude that baseline activity ofPLCβ3is strongly suppressed, but the effect of G proteins, especially acting together, provides a robust stimulus upon G protein stimulation.Significance StatementFor certain cellular signaling processes, the background activity of signaling enzymes must be minimal and stimulus-dependent activation robust. Nowhere is this truer than in signaling byPLCβ3, whose activity regulates intracellular Ca2+, phosphorylation by Protein Kinase C, and the activity of numerous ion channels and membrane receptors. In this study we show howPLCβ3enzymes are regulated by two kinds of G proteins,GβγandGαq. Enzyme activity studies and structures on membranes show how these G proteins act by separate, independent mechanisms, leading to a product rule of co-stimulation when they act together. The findings explain how cells achieve robust stimulation ofPLCβ3in the setting of very low background activity, properties essential to cell health and survival.