The Open Question of How GPCRs Interact with GPCR Kinases (GRKs).

G protein-coupled receptors (GPCRs), which regulate a vast number of eukaryotic processes, are desensitized by various mechanisms but, most importantly, by the GPCR kinases (GRKs). Ever since GRKs were first identified, investigators have sought to determine which structural features of GRKs are used to select for the agonist-bound states of GPCRs and how this binding event in turn enhances GRK catalytic activity. Despite a wealth of molecular information from high-resolution crystal structures of GRKs, the mechanisms driving activation have remained elusive, in part because the GRK N-terminus and active site tether region, previously proposed to serve as a receptor docking site and to be key to kinase domain closure, are often disordered or adopt inconsistent conformations. However, two recent studies have implicated other regions of GRKs as being involved in direct interactions with active GPCRs. Atomic resolution structures of GPCR-GRK complexes would help refine these models but are, so far, lacking. Here, we assess three distinct models for how GRKs recognize activated GPCRs, discuss limitations in the approaches used to generate them, and then experimentally test a hypothetical GPCR interaction site in GRK2 suggested by the two newest models.

Mapping the putative G protein-coupled receptor (GPCR) docking site on GPCR kinase 2: insights from intact cell phosphorylation and recruitment assays.

G protein-coupled receptor kinases (GRKs) phosphorylate agonist-occupied receptors initiating the processes of desensitization and ß-arrestin-dependent signaling. Interaction of GRKs with activated receptors serves to stimulate their kinase activity. The extreme N-terminal helix (αN), the kinase small lobe, and the active site tether (AST) of the AGC kinase domain have previously been implicated in mediating the allosteric activation. Expanded mutagenesis of the αN and AST allowed us to further assess the role of these two regions in kinase activation and receptor phosphorylation in vitro and in intact cells. We also developed a bioluminescence resonance energy transfer-based assay to monitor the recruitment of GRK2 to activated α(2A)-adrenergic receptors (α(2A)ARs) in living cells. The bioluminescence resonance energy transfer signal exhibited a biphasic response to norepinephrine concentration, suggesting that GRK2 is recruited to Gßγ and α(2A)AR with EC50 values of 15 nM and 8 µM, respectively. We show that mutations in αN (L4A, V7E, L8E, V11A, S12A, Y13A, and M17A) and AST (G475I, V477D, and I485A) regions impair or potentiate receptor phosphorylation and/or recruitment. We suggest that a surface of GRK2, including Leu(4), Val(7), Leu(8), Val(11), and Ser(12), directly interacts with receptors, whereas residues such as Asp(10), Tyr(13), Ala(16), Met(17), Gly(475), Val(477), and Ile(485) are more important for kinase domain closure and activation. Taken together with data on GRK1 and GRK6, our data suggest that all three GRK subfamilies make conserved interactions with G protein-coupled receptors, but there may be unique interactions that influence selectivity.

Structural and functional analysis of the regulator of G protein signaling 2-gαq complex.

The heterotrimeric G protein Gαq is a key regulator of blood pressure, and excess Gαq signaling leads to hypertension. A specific inhibitor of Gαq is the GTPase activating protein (GAP) known as regulator of G protein signaling 2 (RGS2). The molecular basis for how Gαq/11 subunits serve as substrates for RGS proteins and how RGS2 mandates its selectivity for Gαq is poorly understood. In crystal structures of the RGS2-Gαq complex, RGS2 docks to Gαq in a different orientation from that observed in RGS-Gαi/o complexes. Despite its unique pose, RGS2 maintains canonical interactions with the switch regions of Gαq in part because its α6 helix adopts a distinct conformation. We show that RGS2 forms extensive interactions with the α-helical domain of Gαq that contribute to binding affinity and GAP potency. RGS subfamilies that do not serve as GAPs for Gαq are unlikely to form analogous stabilizing interactions.

Expression, purification, and analysis of G-protein-coupled receptor kinases.

G-protein-coupled receptor (GPCR) kinases (GRKs) were first identified based on their ability to specifically phosphorylate activated GPCRs. Although many soluble substrates have since been identified, the chief physiological role of GRKs still remains the uncoupling of GPCRs from heterotrimeric G-proteins by promoting ß-arrestin binding through the phosphorylation of the receptor. It is expected that GRKs recognize activated GPCRs through a docking site that not only recognizes the active conformation of the transmembrane domain of the receptor but also stabilizes a more catalytically competent state of the kinase domain. Many of the recent gains in understanding GRK-receptor interactions have been gleaned through biochemical and structural analysis of recombinantly expressed GRKs. Described herein are current techniques and procedures being used to express, purify, and assay GRKs in both in vitro and living cells.

GRK2 activation by receptors: role of the kinase large lobe and carboxyl-terminal tail.

G protein-coupled receptor (GPCR) kinases (GRKs) were discovered by virtue of their ability to phosphorylate activated GPCRs. They constitute a branch of the AGC kinase superfamily, but their mechanism of activation is largely unknown. To initiate a study of GRK2 activation, we sought to identify sites on GRK2 remote from the active site that are involved in interactions with their substrate receptors. Using the atomic structure of GRK2 in complex with Gbetagamma as a guide, we predicted that residues on the surface of the kinase domain that face the cell membrane would interact with the intracellular loops and carboxyl-terminal tail of the GPCR. Our study focused on two regions: the kinase large lobe and an extension of the kinase domain known as the C-tail. Residues in the GRK2 large lobe whose side chains are solvent exposed and facing the membrane were targeted for mutagenesis. Residues in the C-tail of GRK2, although not ordered in the crystal structure, were also targeted because this region has been implicated in receptor binding and in the regulation of AGC kinase activity. Four substitutions out of 20, all within or adjacent to the C-tail, resulted in significant deficiencies in the ability of the enzyme to phosphorylate two different GPCRS: rhodopsin, and the beta(2)-adrenergic receptor. The mutant exhibiting the most dramatic impairment, V477D, also showed significant defects in phosphorylation of nonreceptor substrates. Interestingly, Michaelis-Menten kinetics suggested that V477D had a 12-fold lower k(cat), but no changes in K(M), suggesting a defect in acquisition or stabilization of the closed state of the kinase domain. V477D was also resistant to activation by agonist-treated beta(2)AR. Therefore, Val477 and other residues in the C-tail are expected to play a role in the activation of GRK2 by GPCRs.

The role of G beta gamma and domain interfaces in the activation of G protein-coupled receptor kinase 2.

In response to extracellular signals, G protein-coupled receptors (GPCRs) catalyze guanine nucleotide exchange on Galpha subunits, enabling both activated Galpha and Gbetagamma subunits to target downstream effector enzymes. One target of Gbetagamma is G protein-coupled receptor kinase 2 (GRK2), an enzyme that initiates homologous desensitization by phosphorylating activated GPCRs. GRK2 consists of three distinct domains: an RGS homology (RH) domain, a protein kinase domain, and a pleckstrin homology (PH) domain, through which it binds Gbetagamma. The crystal structure of the GRK2-Gbetagamma complex revealed that the domains of GRK2 are intimately associated and left open the possibility for allosteric regulation by Gbetagamma. In this paper, we report the 4.5 A structure of GRK2, which shows that the binding of Gbetagamma does not induce large domain rearrangements in GRK2, although small rotations of the RH and PH domains relative to the kinase domain are evident. Mutation of residues within the larger domain interfaces of GRK2 generally leads to diminished expression and activity, suggesting that these interfaces are important for stability and remain intact upon activation of GRK2. Geranylgeranylated Gbetagamma, but not a soluble mutant of Gbetagamma, protects GRK2 from clostripain digestion at a site within its kinase domain that is 80 A away from the Gbetagamma binding site. Equilibrium ultracentrifugation experiments indicate that neither abnormally large detergent micelles nor protein oligomerization can account for the observed protection. The Gbetagamma-mediated binding of GRK2 to CHAPS micelles or lipid bilayers therefore appears to rigidify the kinase domain, perhaps by encouraging stable contacts between the RH and kinase domains.

Phosphorylation-independent regulation of metabotropic glutamate receptor 1 signaling requires g protein-coupled receptor kinase 2 binding to the second intracellular loop.

Metabotropic glutamate receptors (mGluRs) are members of a unique class of G protein-coupled receptors (class III) that include the calcium-sensing and gamma-aminobutyric acid type B receptors. The activity of mGluRs is regulated by second messenger-dependent protein kinases and G protein-coupled receptor kinases (GRKs). The attenuation of both mGluR1a and mGluR1b signaling by GRK2 is phosphorylation- and beta-arrestin-independent and requires the concomitant association of GRK2 with both the receptor and Galpha(q/11). G protein interactions are mediated, in part, by the mGluR1 intracellular second loop, but the domains required for GRK2 binding are unknown. In the present study, we showed that GRK2 binds to the second intracellular loop of mGluR1a and mGluR1b and also to the mGluR1a carboxyl-terminal tail. Alanine scanning mutagenesis revealed a discrete domain within loop 2 that contributes to GRK2 binding, and the mutation of either lysine 691 or 692 to an alanine within this domain resulted in a loss of GRK2 binding to both mGluR1a and mGluR1b. Mutation of either Lys(691) or Lys(692) prevented GRK2-mediated attenuation of mGluR1b signaling, whereas the mutation of only Lys(692) prevented GRK2-mediated inhibition of mGluR1a signaling. Thus, the mGluR1a carboxyl-terminal tail may also be involved in regulating the signaling of the mGluR1a splice variant. Taken together, our findings indicated that kinase binding to an mGluR1 domain involved in G protein-coupling is essential for the phosphorylation-independent attenuation of signaling by GRK2.

Characterization of GRK2 RH domain-dependent regulation of GPCR coupling to heterotrimeric G proteins.

Heterotrimeric guanine nucleotide (G)-coupled receptors (GPCRs) form the largest family of integral membrane proteins. GPCR activation by an agonist promotes the exchange of GDP for GTP on the Galpha subunit of the heterotrimeric G protein. The dissociated Galpha and Gbetagamma subunits subsequently modulate the activity of a diverse assortment of effector systems. GPCR signaling via heterotrimeric G proteins is attenuated rapidly by the engagement of protein kinases. The canonical model for GPCR desensitization involves G protein-coupled receptor kinase (GRK)-dependent receptor phosphorylation to promote the binding of arrestin proteins that function to sterically block receptor:G-protein interactions. GRK2 and GRK3 have been shown to interact with Galphaq via the regulator of G-protein signaling (RGS) homology (RH) domain localized within their amino-terminal domains. It now appears that the G-protein uncoupling of many GPCRs linked to Galphaq, in particularly metabotropic glutamate receptors, may be mediated by the GRK2 RH domain via a phosphorylation-independent mechanism. This article reviews much of the background and methodology required for the characterization of the GRK2 phosphorylation-independent attenuation of GPCR signaling.

Characterization of the GRK2 binding site of Galphaq.

Heterotrimeric guanine nucleotide-binding proteins (G proteins) transmit signals from membrane bound G protein-coupled receptors (GPCRs) to intracellular effector proteins. The G(q) subfamily of Galpha subunits couples GPCR activation to the enzymatic activity of phospholipase C-beta (PLC-beta). Regulators of G protein signaling (RGS) proteins bind to activated Galpha subunits, including Galpha(q), and regulate Galpha signaling by acting as GTPase activating proteins (GAPs), increasing the rate of the intrinsic GTPase activity, or by acting as effector antagonists for Galpha subunits. GPCR kinases (GRKs) phosphorylate agonist-bound receptors in the first step of receptor desensitization. The amino termini of all GRKs contain an RGS homology (RH) domain, and binding of the GRK2 RH domain to Galpha(q) attenuates PLC-beta activity. The RH domain of GRK2 interacts with Galpha(q/11) through a novel Galpha binding surface termed the "C" site. Here, molecular modeling of the Galpha(q).GRK2 complex and site-directed mutagenesis of Galpha(q) were used to identify residues in Galpha(q) that interact with GRK2. The model identifies Pro(185) in Switch I of Galpha(q) as being at the crux of the interface, and mutation of this residue to lysine disrupts Galpha(q) binding to the GRK2-RH domain. Switch III also appears to play a role in GRK2 binding because the mutations Galpha(q)-V240A, Galpha(q)-D243A, both residues within Switch III, and Galpha(q)-Q152A, a residue that structurally supports Switch III, are defective in binding GRK2. Furthermore, GRK2-mediated inhibition of Galpha(q)-Q152A-R183C-stimulated inositol phosphate release is reduced in comparison to Galpha(q)-R183C. Interestingly, the model also predicts that residues in the helical domain of Galpha(q) interact with GRK2. In fact, the mutants Galpha(q)-K77A, Galpha(q)-L78D, Galpha(q)-Q81A, and Galpha(q)-R92A have reduced binding to the GRK2-RH domain. Finally, although the mutant Galpha(q)-T187K has greatly reduced binding to RGS2 and RGS4, it has little to no effect on binding to GRK2. Thus the RH domain A and C sites for Galpha(q) interaction rely on contacts with distinct regions and different Switch I residues in Galpha(q).

G Protein-coupled receptor kinase 2 regulator of G protein signaling homology domain binds to both metabotropic glutamate receptor 1a and Galphaq 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 gamma-aminobutyric acid B receptors. Here we show that GRK2 mutants impaired in Galphaq/11 binding (R106A, D110A, and M114A), bind effectively to mGluR1a, but do not mediate mGluR1a adaptation. Galphaq/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/Galphaq/11 complex. However, these mGluR1a/Galphaq/11 interactions are not antagonized by the overexpression of either GRK2 mutants defective in Galphaq/11 binding or RGS4. We have also identified a GRK2-D527A mutant that binds Galphaq/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 Galphaq/11 and mGluR1a. This serves to coordinate GRK2 interactions with Galphaq/11 and to disrupt receptor/Galphaq/11 complexes. Our findings indicate that GRK2 regulates receptor/G protein interactions, in addition to its traditional role as a receptor kinase.

Differential interaction of GRK2 with members of the G alpha q family.

Regulators of G protein signaling (RGS) proteins bind to active G alpha subunits and accelerate the rate of GTP hydrolysis and/or block interaction with effector molecules, thereby decreasing signal duration and strength. RGS proteins are defined by the presence of a conserved 120-residue region termed the RGS domain. Recently, it was shown that the G protein-coupled receptor kinase 2 (GRK2) contains an RGS domain that binds to the active form of G alpha(q). Here, the ability of GRK2 to interact with other members of the G alpha(q) family, G alpha(11), G alpha(14), and G alpha(16), was tested. The signaling of all members of the G alpha(q) family, with the exception of G alpha(16), was inhibited by GRK2. Immunoprecipitation of full-length GRK2 or pull down of GST-GRK2-(45-178) resulted in the detection of G alpha(q), but not G alpha(16), in an activation-dependent manner. Moreover, activated G alpha(16) failed to promote plasma membrane (PM) recruitment of a GRK2-(45-178)-GFP fusion protein. Assays with chimeric G alpha(q)(-)(16) subunits indicated that the C-terminus of G alpha(q) mediates binding to GRK2. Despite showing no interaction with GRK2, G alpha(16) does interact with RGS2, in both inositol phosphate and PM recruitment assays. Thus, GRK2 is the first identified RGS protein that discriminates between members of the G alpha(q) family, while another RGS protein, RGS2, binds to both G alpha(q) and G alpha(16).

G protein-coupled receptor Kinase 2/G alpha q/11 interaction. A novel surface on a regulator of G protein signaling homology domain for binding G alpha subunits.

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 alpha(q/ll) and inhibiting G alpha(q) stimulation of the effector phospholipase C beta. The binding site for G alpha(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 Galpha(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 alpha(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 alpha(q/ll) binding defects impair recruitment to the plasma membrane by activated G alpha(q) and regulation of G alpha(q)-stimulated phospholipase C beta activity when introduced into full-length GRK2. Two different protein interaction sites have previously been identified on RH domains. The G alpha binding sites on RGS4 and RGS9, called the "A" site, is localized to the loops between helices alpha 3 and alpha 4, alpha 5 and alpha 6, and alpha 7 and alpha 8. The adenomatous polyposis coli (APC) binding site of axin involves residues on alpha helices 3, 4, and 5 (the "B" site) of its RH domain. We demonstrate that the G alpha(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 alpha 5 helix. We suggest that this novel protein interaction site on an RH domain be designated the "C" site.

Phosphorylation-independent regulation of metabotropic glutamate receptor signaling by G protein-coupled receptor kinase 2.

The accepted paradigm for G protein-coupled receptor kinase (GRK)-mediated desensitization of G protein-coupled receptors involves GRK-mediated receptor phosphorylation followed by the binding of arrestin proteins. Although GRKs contribute to metabotropic glutamate receptor 1 (mGluR1) inactivation, beta-arrestins do not appear to be required for mGluR1 G protein uncoupling. Therefore, we investigated whether the phosphorylation of serine and threonine residues localized within the C terminus of mGluR1a is sufficient to allow GRK2-mediated attenuation of mGluR1a signaling. We find that the truncation of the mGluR1a C-terminal tail prevents mGluR1a phosphorylation and that GRK2 does not contribute to the phosphorylation of an mGluR1 splice variant (mGluR1b). However, mGluR1a-866Delta- and mGluR1b-stimulated inositol phosphate formation is attenuated following GRK2 expression. The expression of the GRK2 C-terminal domain to block membrane translocation of endogenous GRK2 increases mGluR1a-866Delta- and mGluR1b-stimulated inositol phosphate formation, presumably by blocking membrane translocation of GRK2. In contrast, expression of the kinase-deficient GRK2-K220R mutant inhibits inositol phosphate formation by these unphosphorylated receptors. Expression of the GRK2 N-terminal domain (residues 45-185) also attenuates both constitutive and agonist-stimulated mGluR1a, mGluR1a-866Delta, and mGluR1b signaling, and the GRK2 N terminus co-precipitates with mGluR1a. Taken together, our observations indicate that attenuation of mGluR1 signaling by GRK2 is phosphorylation-independent and that the interaction of the N-terminal domain of GRK2 with mGluR1 contributes to the regulation of mGluR1 G protein coupling.