Structural Determinants of G-protein α Subunit Selectivity by Regulator of G-protein Signaling 2 (RGS2)

Regulator of G-protein signaling (RGS) proteins facilitate the termination of G protein-coupled receptor (GPCR) signaling via their ability to increase the intrinsic GTP hydrolysis rate of G alpha subunits (known as GTPase-accelerating protein or GAP activity). RGS2 is unique in its in vitro potency and selectivity as a GAP for G alpha(q) subunits. As many vasoconstrictive hormones signal via G(q) heterotrimer-coupled receptors, it is perhaps not surprising that RGS2-deficient mice exhibit constitutive hypertension. However, to date the particular structural features within RGS2 determining its selectivity for G alpha(q) over G alpha(i/o) substrates have not been completely characterized. Here, we examine a trio of point mutations to RGS2 that elicits G alpha(i)-directed binding and GAP activities without perturbing its association with G alpha(q). Using x-ray crystallography, we determined a model of the triple mutant RGS2 in complex with a transition state mimetic form of G alpha(i) at 2.8-angstrom resolution. Structural comparison with unliganded, wild type RGS2 and of other RGS domain/G alpha complexes highlighted the roles of these residues in wild type RGS2 that weaken G alpha(i) subunit association. Moreover, these three amino acids are seen to be evolutionarily conserved among organisms with modern cardiovascular systems, suggesting that RGS2 arose from the R4-subfamily of RGS proteins to have specialized activity as a potent and selective G alpha(q) GAP that modulates cardiovascular function.