High-resolution structure of RGS17 suggests a role for Ca(2+) in promoting the GTPase-activating protein activity by RZ subfamily members.

Regulator of G protein signaling (RGS) proteins are negative regulators of G protein-coupled receptor (GPCR) signaling through their ability to act as GTPase-activating proteins (GAPs) for activated Gα subunits. Members of the RZ subfamily of RGS proteins bind to activated Gα(o), Gα(z), and Gα(i1-3) proteins in the nervous system and thereby inhibit downstream pathways, including those involved in Ca(2+)-dependent signaling. In contrast to other RGS proteins, little is known about RZ subfamily structure and regulation. Herein, we present the 1.5-à crystal structure of RGS17, the most complete and highest-resolution structure of an RZ subfamily member to date. RGS17 cocrystallized with Ca(2+) bound to conserved positions on the predicted Gα-binding surface of the protein. Using NMR chemical shift perturbations, we confirmed that Ca(2+) binds in solution to the same site. Furthermore, RGS17 had greater than 55-fold higher affinity for Ca(2+) than for Mg(2+) Finally, we found that Ca(2+) promotes interactions between RGS17 and activated Gα and decreases the K(m) for GTP hydrolysis, potentially by altering the binding mechanism between these proteins. Taken together, these findings suggest that Ca(2+) positively regulates RGS17, which may represent a general mechanism by which increased Ca(2+) concentration promotes the GAP activity of the RZ subfamily, leading to RZ-mediated inhibition of Ca(2+) signaling.