Regulation of phospholipase C-β₁GTPase-activating protein (GAP) function and relationship to G_qefficacy

Abstract

How cells regulate G q efficacy (initiation and termination of G q signaling) to effect response remains a central question in pharmacology and drug discovery. Phospholipase C-b 1 (PLCb 1 ) is an effector and a GTPase activating protein (GAP) specific to Ga q . The physiological function of PLC-b 1 GAP remains unclear and controversial. GAPs are generally thought to function in deactivation of G q signaling. However, PLC-b 1 GAP has also been shown to increase signaling efficiency through kinetic coupling with the ligand-activated GPCR. GPCRs function as guanine nucleotide exchange factors (GEF) on the G protein activation cycle. This article sets forth a new hypothesis that could unify these conflicting paradigms as it pertains to physiological signaling and native levels of protein. It is proposed that the physiological function of PLC-b 1 GAP is context-dependent and regulated by phosphatidic acid (PA). PA stimulates PLC-b 1 GAP activity. In the absence of ligand, PLC-b 1 GAP does indeed deactivate G q signaling, limiting leaky activation to set the threshold for stimulation to sharpen signal kinetics. However in the presence of activating ligand, the increase in levels of PA would stimulate PLC-b 1 GAP to kinetically couple with GPCR GEF to increase signaling efficiency. We found that PA-increased G q efficiency is dependent on signaling via the unique PLC-b 1 PA binding domain. V C 2013 IUBMB Life, 65(11):936-940, 2013

Dissociation of the G protein βγ from the Gq–PLCβ complex partially attenuates PIP2 hydrolysis

“…It would be surprising to see any activity of Gαq GTP –PLCβ, if PLCβ could induce 1000-fold higher hydrolysis of GTP on Gαq in cells, as seen in vitro ( 2 , 10 , 24 , 25 , 58 , 59 , 60 ). In cells, the activated GPCRs are proposed to suppress the GAP activity of PLCβ mentioned above ( 24 ). Also, it was suggested that the enhanced GEF activity of GPCR (to activate the heterotrimer) in cells outcompete PLCβ–GAP activity ( 23 ).…”

“…Also, it was suggested that the enhanced GEF activity of GPCR (to activate the heterotrimer) in cells outcompete PLCβ–GAP activity ( 23 ). It was also predicted that PLCβ GAP activity does not downregulate Gαq GTP -induced PLC activation but enhances it ( 24 ). These theories collectively suggest that the GAP activity of PLCβ behaves differently in living cells compared with its demonstrated efficacy in vitro .…”

“…In addition, G protein coupled receptors (GPCRs) and GAPs have been proposed to equally control the rates of signal initiation and termination, allowing for signaling to be nearly instantaneously turned on and off ( 23 ). Although it seems counterintuitive, the GAP activity of PLCβ1 has also been shown to increase the signaling efficiency of Gαq GTP ( 23 , 24 ). It has been suggested that the ability of this GAP activity to stabilize the activated M1R and the Gαq and PLCβ1 complex could be responsible for this proposed enhanced signaling efficacy ( 21 , 25 ).…”