Regulators of G protein signaling (RGS) are GTPaseaccelerating proteins (GAPs), which can inhibit heterotrimeric G protein pathways. In this study, we provide experimental and theoretical evidence that high concentrations of receptors (as at a synapse) can lead to saturation of GDP-GTP exchange making GTP hydrolysis rate-limiting. This results in local depletion of inactive heterotrimeric G-GDP, which is reversed by RGS GAP activity. Thus, RGS enhances receptor-mediated G protein activation even as it deactivates the G protein. A critical question in cellular signaling is what determines the specificity of signal transduction processes. There is much recent evidence for the formation of complexes maintained by protein scaffolds to control signaling specificity. This contrasts with a classical model in the G protein signaling field, the collision-coupling model (1), which relies entirely on the structure of receptor-G protein and G protein-effector contact sites to determine signaling specificity. The collision coupling model also suggests that there would be significant spread of G protein signals in a cell upon receptor activation, since all components are freely diffusable. There have been numerous studies indicating that such free transfer of information over long distances may not occur for G i or G q mediated signals (2, 3). Thus, similar to the localized signaling by postsynaptic ionotropic receptors via protein complex assembly (4), mechanisms to limit the "spread" of G protein signaling appear necessary.G protein-coupled receptors (GPCR) 1 activate cellular signals by inducing nucleotide exchange on the G protein ␣ subunit, while inactivation occurs upon GTP hydrolysis by the intrinsic G␣ GTPase (5). Regulator of G protein signaling (RGS) proteins are a recently discovered family of proteins which act as GTPase-activating proteins (GAPs) for G␣ subunits (6 -9). The GAP activity of RGS proteins generally reduces steady state levels of GTP-bound G␣ subunits and inhibits the activity of G proteins (6, 10). However, some studies of receptor-stimulated signaling show that RGS proteins can speed the kinetics of responses without compromising steady state signaling strength (11-13). The mechanism and significance of this paradoxical result is not understood.The maintained signaling in the face of RGS-enhanced GTPase activity suggests that the RGS proteins somehow increase the efficiency of G protein activation. One possible mechanism for this could be "physical scaffolding" in which the RGS protein binds to both receptor and G protein and stabilizes a complex between them. This could involve the diverse aminoand carboxyl-terminal domains of the RGS proteins such as GGL, DEP, DH/PH, and PDZ domains (6,10,14,15). Indeed, RGS12 does bind to the carboxyl terminus of the IL8 receptor through a PDZ domain (16). Alternatively, Ross and co-workers (17) have suggested that the GAP activity of phospholipase C-1, which is both a G q GAP and its effector, serves to enhance muscarinic receptor-G q coupling (6,17). In that ...