Regulation of G protein-signaling by RGS proteins in cardiovascular system

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RGS proteins (regulators of G protein signaling) serve as GTPase-activating proteins (GAPs) for Gα subunits and negatively regulate G protein-coupled receptor signaling. In this study, we characterized biochemical properties of RGS5 and its N terminal (1-33)-deleted mutant (ΔN-RGS5). RGS5 bound to Gαi1, Gαi2, Gαi3, Gαo and Gαq but not to Gαs and Gα13 in the presence of GDP/AlF, and accelerated the catalytic rate of GTP hydrolysis of Gαi3 subunit. When expressed in 293T cells stably expressing angiotensin (Ang) AT1a receptors (AT1a-293T cells), RGS5 suppressed Ang II-and endothelin (ET)-1-induced intracellular Ca transients. The effect of RGS5 was concentration-dependent, and the slope of the concentration-response relationship showed that a 10-fold increase in amounts of RGS5 induced about 20-25% reduction of the Ca signaling. Furthermore, a comparison study of three sets of 293T cells with different expression levels of AT1a receptors showed that RGS5 inhibited Ang II-induced responses more effectively in 293T cells with the lower density of AT1a receptors, suggesting that the degree of inhibition by RGS proteins reflects the ratio of amounts of RGS proteins to those of activated Gα subunits after receptor stimulation by agonists. When expressed in AT1a-293T cells, ΔN-RGS5 was localized almost exclusively in the cytosolic fraction, and exerted the inhibitory effects as potently as RGS5 which was present in both membrane and cytosolic fractions. Studies on relationship between subcellular localization and inhibitory effects of RGS5 and ΔN-RGS5 revealed that the N terminal (1-33) of RGS5 plays a role in targeting this protein to membranes, and that the N terminal region of RGS5 is not essential for exerting activities.