Structure of the Regulator of G Protein Signaling 8 (RGS8)-Gαq Complex MOLECULAR BASIS FOR Gα SELECTIVITY

Regulator of G protein signaling (RGS) proteins interact with activated G subunits via their RGS domains and accelerate the hydrolysis of GTP. Although the R4 subfamily of RGS proteins generally accepts both G(i/o) and G(q/11) subunits as substrates, the R7 and R12 subfamilies select against G(q/11). In contrast, only one RGS protein, RGS2, is known to be selective for G(q/11). The molecular basis for this selectivity is not clear. Previously, the crystal structure of RGS2 in complex with G(q) revealed a non-canonical interaction that could be due to interfacial differences imposed by RGS2, the G subunit, or both. To resolve this ambiguity, the 2.6 crystal structure of RGS8, an R4 subfamily member, was determined in complex with G(q). RGS8 adopts the same pose on G(q) as it does when bound to G(i3), indicating that the non-canonical interaction of RGS2 with G(q) is due to unique features of RGS2. Based on the RGS8-G(q) structure, residues in RGS8 that contact a unique -helical domain loop of G(q) were converted to those typically found in R12 subfamily members, and the reverse substitutions were introduced into RGS10, an R12 subfamily member. Although these substitutions perturbed their ability to stimulate GTP hydrolysis, they did not reverse selectivity. Instead, selectivity for G(q) seems more likely determined by whether strong contacts can be maintained between 6 of the RGS domain and Switch III of G(q), regions of high sequence and conformational diversity in both protein families.