؊ . This revealed the specific ability of bovine brain G␣ q/11 to bind to both GRK2 and GRK3 in an AlF 4 ؊ -dependent manner. In contrast, G␣ s , G␣ i , and G␣ 12/13 did not bind to GRK2 or GRK3 despite their presence in the extract. Additional studies revealed that bovine brain G␣ q/11 could also bind to an N-terminal construct of GRK2, while no binding of G␣ q/11 , G␣ s , G␣ i , or G␣ 12/13 to comparable constructs of GRK5 or GRK6 was observed. Experiments using purified G␣ q revealed significant binding of both G␣ q GDP/AlF 4 ؊ and G␣ q (GTP␥S), but not G␣ q (GDP), to GRK2. Activation-dependent binding was also observed in both COS-1 and HEK293 cells as GRK2 significantly co-immunoprecipitated constitutively active G␣ q (R183C) but not wild type G␣ q . In vitro analysis revealed that GRK2 possesses weak GAP activity toward G␣ q that is dependent on the presence of a G proteincoupled receptor. However, GRK2 effectively inhibited G␣ q -mediated activation of phospholipase C-␤ both in vitro and in cells, possibly through sequestration of activated G␣ q . These data suggest that a subfamily of the GRKs may be bifunctional regulators of G protein-coupled receptor signaling operating directly on both receptors and G proteins.

β1- and β2-adrenergic receptors (βARs) are highly homologous, yet they play clearly distinct roles in cardiac physiology and pathology. Myocyte contraction, for instance, is readily stimulated by β1AR but not β2AR signaling, and chronic stimulation of the two receptors has opposing effects on myocyte apoptosis and cell survival. Differences in the assembly of macromolecular signaling complexes may explain the distinct biological outcomes. Here, we demonstrate that β1AR forms a signaling complex with a cAMP-specific phosphodiesterase (PDE) in a manner inherently different from a β2AR/β-arrestin/PDE complex reported previously. The β1AR binds a PDE variant, PDE4D8, in a direct manner, and occupancy of the receptor by an agonist causes dissociation of this complex. Conversely, agonist binding to the β2AR is a prerequisite for the recruitment of a complex consisting of β-arrestin and the PDE4D variant, PDE4D5, to the receptor. We propose that the distinct modes of interaction with PDEs result in divergent cAMP signals in the vicinity of the two receptors, thus, providing an additional layer of complexity to enforce the specificity of β1- and β2-adrenoceptor signaling.

G protein-coupled receptors (GPCRs) constitute the largest family of signaling proteins in mammals, mediating responses to hormones, neurotransmitters, and senses of sight, smell and taste. Mechanistic insight into GPCR signal transduction is limited by a paucity of high-resolution structural information. We describe the generation of a monoclonal antibody that recognizes the third intracellular loop (IL3) of the native human beta(2) adrenergic (beta(2)AR) receptor; this antibody was critical for acquiring diffraction-quality crystals.

G protein-coupled receptors (GPCRs) transduce cellular signals from hormones, neurotransmitters, light, and odorants by activating heterotrimeric guanine nucleotide-binding (G) proteins. For many GPCRs, short term regulation is initiated by agonist-dependent phosphorylation by GPCR kinases (GRKs), such as GRK2, resulting in G protein/receptor uncoupling. GRK2 also regulates signaling by binding G␣ q/ll and inhibiting G␣ q stimulation of the effector phospholipase C␤. The binding site for G␣ q/ll resides within the amino-terminal domain of GRK2, which is homologous to the regulator of G protein signaling (RGS) family of proteins. To map the G␣ q/ll binding site on GRK2, we carried out site-directed mutagenesis of the RGS homology (RH) domain and identified eight residues, which when mutated, alter binding to G␣ q/ll . These mutations do not alter the ability of full-length GRK2 to phosphorylate rhodopsin, an activity that also requires the amino-terminal domain. Mutations causing G␣ q/ll binding defects impair recruitment to the plasma membrane by activated G␣ q and regulation of G␣ q -stimulated phospholipase C␤ activity when introduced into full-length GRK2. Two different protein interaction sites have previously been identified on RH domains. The G␣ binding sites on RGS4 and RGS9, called the "A" site, is localized to the loops between helices ␣3 and ␣4, ␣5 and ␣6, and ␣7 and ␣8. The adenomatous polyposis coli (APC) binding site of axin involves residues on ␣ helices 3, 4, and 5 (the "B" site) of its RH domain. We demonstrate that the G␣ q/ll binding site on the GRK2 RH domain is distinct from the "A" and "B" sites and maps primarily to the COOH terminus of its ␣5 helix. We suggest that this novel protein interaction site on an RH domain be designated the "C" site.