The RGS14 GoLoco domain discriminates among Gαi isoforms

Regulators of G protein signaling (RGS) modulate G protein activity by functioning as GTPase-activating proteins (GAPs) for alpha-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 Galpha(i1), but not Galpha(0). Selective interactions are mediated by contacts between the alphaA and alphaB helices of the Galpha(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 Galpha(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 Galpha(i1) and Galpha(i3), but not Galpha(i2). Galpha(i2) could be rendered sensitive to RGS14 GDI activity by replacement of residues within the alpha-helical domain. In addition to the contact residues in the alphaA and alphaB helices previously identified, we found that the alphaA/alphaB and alphaB/alphaC loops are important determinants of Galpha(i) selectivity. The striking selectivity observed for RGS14 GDI activity in vitro points to Galpha(i1) and Galpha(i3) as the likely targets of RGS14-GoLoco regulation in vivo.