Multifarious hormones, neurotransmitters, growth factors and other extracellular stimuli produce their physiological effects by activating G protein–coupled receptors (GPCRs) and their associated heterotrimeric G proteins1
. RGS proteins attenuate heterotrimeric G-protein signaling by enhancing the intrinsic GTPase activity of Gα subunits and are vital for proper signal transduction kinetics2–4
. The R7 (class C) subfamily of RGS proteins encompasses the four mammalian proteins RGS6, RGS7, RGS9 and RGS11 (ref. 4
), which are all highly expressed in the central nervous system5,6
. RGS9 is the best-characterized R7-RGS protein and is expressed in two isoforms: RGS9-1 mediates the rate-limiting step during response recovery of rod phototransduction7
, whereas RGS9-2 is required for proper signal transduction downstream of certain opioid and dopamine receptors8–10
. Humans lacking functional RGS9-1 are temporarily visually impaired by sudden changes in light levels11
. Mice without RGS9-2 develop exacerbated dependence and withdrawal to morphine and exhibit dyskinesias induced by the stimulation of D2
-dopamine receptors that are similar to the side effects observed in patients treated for psychoses and Parkinson’s disease10
Members of the R7 subfamily of RGS proteins contain functional domains in addition to the characteristic RGS domain, which provides catalytic GTPase activating function. A Dishevelled/Egl-10/Pleckstrin homology (DEP) domain participates in proper subcellular localization of these RGS proteins through interaction with the recently discovered membrane-targeting proteins RGS9 anchor protein (R9AP) and R7 binding protein (R7BP)12–14
. A G protein γ (Gγ)-like (GGL) domain, which shares sequence homology with the Gγ subunits of heterotrimeric G proteins, mediates obligate heterodimerization with the most divergent G protein β (Gβ) subunit, Gβ5 (ref. 15
). Although it has been suggested that Gβ5–R7-RGS complexes might support nucleotide exchange on G protein α (Gα) subunits in conjunction with agonist-stimulated GPCRs16
, the function of Gβ5 in these complexes remains unknown.
To better understand the functional implication of Gβ5–R7-RGS complexes in mediating the activation and deactivation of G protein–mediated signaling cascades, we present here the high-resolution crystal structure of Gβ5–RGS9. In this structure, Gβ5 scaffolds the various domains of RGS9 and contains a highly conserved surface that is consistent with its ability to bind Gα subunits but that is occluded by RGS9. The structure highlights features that dictate the exclusive pairing of Gβ5 with R7-RGS proteins and suggests a mechanism for filtering the interactions between activated Gα proteins and the RGS domain that uses other portions of Gβ5–RGS9.