Differential capacities of the RGS1, RGS16 and RGS-GAIP regulators of G protein signaling to enhance alpha2A-adrenoreceptor agonist-stimulated GTPase activity of G(o1)alpha

Recombinant RGS1, RGS16 and RGS-GAIP, but not RGS2, were able to substantially further stimulate the maximal GTPase activity of G(o1)alpha promoted by agonists at the alpha2A-adrenoreceptor in a concentration-dependent manner. Kinetic analysis of the regulation of an alpha2A-adrenoreceptor-G(o1)alpha fusion protein by all three RGS proteins revealed that they had similar affinities for the receptor-G protein fusion. However, their maximal effects on GTP hydrolysis varied over threefold with RGS16 > RGS1 > RGS-GAIP. Both RGS1 and RGS16 reduced the potency of the alpha2A-adrenoreceptor agonist adrenaline by some 10-fold. A lower potency shift was observed for the partial agonist UK14304 and the effect was absent for the weak partial agonist oxymetazoline. Each of these RGS proteins altered the intrinsic activity of both UK14304 and oxymetazoline relative to adrenaline. Such results require the RGS interaction with G(o1)alpha to alter the conformation of the alpha2A-adrenoreceptor and are thus consistent with models invoking direct interactions between RGS proteins and receptors. These studies demonstrate that RGS1, RGS16 and RGS-GAIP show a high degree of selectivity to regulate alpha2A-adrenoreceptor-activated G(o1)alpha rather than G(i1)alpha, G(i2)alpha or G(i3)alpha and different capacities to inactivate this G protein.