G Protein-coupled Receptor Kinase 2 Regulator of G Protein Signaling Homology Domain Binds to Both Metabotropic Glutamate Receptor 1a and Gαq to Attenuate Signaling

Heterotrimeric guanine nucleotide-binding (G) protein-coupled receptor kinases (GRKs) are cytosolic proteins that contribute to the adaptation of G protein-coupled receptor signaling. The canonical model for GRK-dependent receptor desensitization involves GRK-mediated receptor phosphorylation to promote the binding of arrestin proteins that sterically block receptor coupling to G proteins. However, GRK-mediated desensitization, in the absence of phosphorylation and arrestin binding, has been reported for metabotropic glutamate receptor 1 (mGluR1) and γ-aminobutyric acid B receptors. Here we show that GRK2 mutants impaired in Gαq/11 binding (R106A, D110A, and M114A), bind effectively to mGluR1a, but do not mediate mGluR1a adaptation. Gαq/11 is immunoprecipitated as a complex with mGluR1a in the absence of agonist, and either agonist treatment or GRK2 overexpression promotes the dissociation of the receptor/Gαq/11 complex. However, these mGluR1a/Gαq/11 interactions are not antagonized by the overexpression of either GRK2 mutants defective in Gαq/11 binding or RGS4. We have also identified a GRK2-D527A mutant that binds Gαq/11 in an AlF4--dependent manner but is unable to either bind mGluR1a or attenuate mGluR1a signaling. We conclude that the mechanism underlying GRK2 phosphorylation-independent attenuation of mGluR1a signaling is RH domain-dependent, requiring the binding of GRK2 to both Gαq/11 and mGluR1a. This serves to coordinate GRK2 interactions with Gαq/11 and to disrupt receptor/Gαq/11 complexes. Our findings indicate that GRK2 regulates receptor/G protein interactions, in addition to its traditional role as a receptor kinase.