A Residue outside the Binding Site Determines the Gα Binding Specificity of GoLoco Motifs

Abstract

GoLoco motif-containing proteins regulate the nucleotide-binding state of Gα proteins in various signaling pathways. As guanine nucleotide dissociation inhibitors (GDIs), they bind Gα·GDP and inhibit GDP to GTP exchange. GoLoco proteins show binding selectivity toward different members of the Gα family. Although the Gαi1·GDP/RGS14 crystal structure explains the specific binding selectivity of the RGS14 GoLoco domain well, the mechanism of selective binding has not been understood for the more general features of short GoLoco domains found in tandem arrays in proteins like GPSM2/LGN/ dPins and GPSM1/AGS3. We explored the mechanism of differential interactions of GoLoco protein LGN with hGαi3 and hGαo. By combining mutagenesis experiments and molecular dynamics simulations, we identified a residue (Asp229 in hGαi3) away from the binding interface that remarkably affects the interaction between LGN and hGαi/o. A negatively charged residue at this position is required for high binding affinity. This affinity regulation mechanism was further verified by the cases of hGαi2 and dGαo, suggesting that this pathway is conserved among members of the Gα family.