The RGS14 GoLoco domain discriminates among Gα_i isoforms

Regulators of G protein signaling (RGS) modulate G protein activity by functioning as GTPase-activating proteins (GAPs) for α-subunits of heterotrimeric G proteins. RGS14 regulates G protein nucleotide exchange and hydrolysis by acting as a GAP through its RGS domain and as a guanine nucleotide dissociation inhibitor (GDI) through its GoLoco motif. RGS14 exerts GDI activity on Gα_i1, but not Gα_o. Selective interactions are mediated by contacts between the αA and αB helices of the Gα_i1 helical domain and the GoLoco C terminus (Kimple, R. J., Kimple, M. E., Betts, L., Sondek, J., and Siderovski, D. P. (2002) Nature 416, 878-881). Three isoforms of Gα_i exist in mammalian cells. In this study, we tested whether all three isoforms were subject to RGS14 GDI activity. We found that RGS14 inhibits guanine nucleotide exchange on Gα_i1 and Gα_i3, but not Gα_i2. Gα_i2 could be rendered sensitive to RGS14 GDI activity by replacement of residues within the α-helical domain. In addition to the contact residues in the αA and αB helices previously identified, we found that the αA/αB and αB/αC loops are important determinants of Gα_i selectivity. The striking selectivity observed for RGS14 GDI activity in vitro points to Gα_i1 and Gα_i3 as the likely targets of RGS14-GoLoco regulation in vivo.