Computational modeling reveals how interplay between components of a GTPase-cycle module regulates signal transduction

Abstract

Heterotrimeric G protein signaling is regulated by signaling modules composed of heterotrimeric G proteins, active G proteincoupled receptors (Rs), which activate G proteins, and GTPaseactivating proteins (GAPs), which deactivate G proteins. We term these modules GTPase-cycle modules. The local concentrations of these proteins are spatially regulated between plasma membrane microdomains and between the plasma membrane and cytosol, but no data or models are available that quantitatively explain the effect of such regulation on signaling. We present a computational model of the GTPase-cycle module that predicts that the interplay of local G protein, R, and GAP concentrations gives rise to 16 distinct signaling regimes and numerous intermediate signaling phenomena. The regimes suggest alternative modes of the GTPasecycle module that occur based on defined local concentrations of the component proteins. In one mode, signaling occurs while G protein and receptor are unclustered and GAP eliminates signaling; in another, G protein and receptor are clustered and GAP can rapidly modulate signaling but does not eliminate it. Experimental data from multiple GTPase-cycle modules is interpreted in light of these predictions. The latter mode explains previously paradoxical data in which GAP does not alter maximal current amplitude of G protein-activated ion channels, but hastens signaling. The predictions indicate how variations in local concentrations of the component proteins create GTPase-cycle modules with distinctive phenotypes. They provide a quantitative framework for investigating how regulation of local concentrations of components of the GTPase-cycle module affects signaling.T he timing and amplitude of signaling in cellular signaling networks depends on the local concentrations and kinetics of component proteins. The complexity of these networks has precluded comprehensive quantitative analysis of signaling. However, the networks can be divided into discrete signaling modules that can be quantitatively modeled and analyzed independently (1-5). Later, such models of modules can be experimentally verified and then reconnected to build larger signaling networks.G protein-mediated signaling networks (6) are ideal for such quantitative analysis. A key signaling module in these networks is the GTPase-cycle module, which controls signaling by regulating G protein activity. This module consists of a heterotrimeric G protein, a guanine nucleotide exchange factor (GEF) such as agonist-bound and active G protein-coupled receptor, a GTPase-activating protein (GAP) such as regulator of G protein signaling (RGS) protein or phospholipase C-␤, GTP, GDP, and P i . The extent of G protein activation reflects the balance between the rates of GDP͞GTP exchange, which activates the G protein, and GTP hydrolysis, which deactivates it (7-9). GEFs facilitate GDP͞GTP exchange (9), and GAPs accelerate GTP hydrolysis (10, 11). Additional roles of GEFs (12) and GAPs (10,11,13,14) are not included in this module. At present, we lack quantitativ...

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