Alexey Pronin

SELECTIVE REGULATION OF GALPHA Q/11 BY AN RGS DOMAIN IN THE G PROTEIN-COUPLED RECEPTOR KINASE, GRK2

G protein-coupled receptor kinases (GRKs) are well characterized regulators of G protein-coupled receptors, whereas regulators of G protein signaling (RGS) proteins directly control the activity of G protein α subunits. Interestingly, a recent report (Siderovski, D. P., Hessel, A., Chung, S., Mak, T. W., and Tyers, M. (1996) Curr. Biol. 6, 211–212) identified a region within the N terminus of GRKs that contained homology to RGS domains. Given that RGS domains demonstrate AlF4 −-dependent binding to G protein α subunits, we tested the ability of G proteins from a crude bovine brain extract to bind to GRK affinity columns in the absence or presence of AlF4 −. This revealed the specific ability of bovine brain Gαq/11 to bind to both GRK2 and GRK3 in an AlF4 −-dependent manner. In contrast, Gαs, Gαi, and Gα12/13 did not bind to GRK2 or GRK3 despite their presence in the extract. Additional studies revealed that bovine brain Gαq/11 could also bind to an N-terminal construct of GRK2, while no binding of Gαq/11, Gαs, Gαi, or Gα12/13 to comparable constructs of GRK5 or GRK6 was observed. Experiments using purified Gαq revealed significant binding of both GαqGDP/AlF4 − and Gαq(GTPγS), but not Gαq(GDP), to GRK2. Activation-dependent binding was also observed in both COS-1 and HEK293 cells as GRK2 significantly co-immunoprecipitated constitutively active Gαq(R183C) but not wild type Gαq. In vitro analysis revealed that GRK2 possesses weak GAP activity toward Gαq that is dependent on the presence of a G protein-coupled receptor. However, GRK2 effectively inhibited Gαq-mediated activation of phospholipase C-β both in vitro and in cells, possibly through sequestration of activated Gαq. 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.

Regulation of G Protein-Coupled Receptor Kinases

G protein-coupled receptor kinases (GRKs) specifically interact with the agonist-activated form of G protein-coupled receptors (GPCRs) to effect receptor phosphorylation and desensitization. Recent studies demonstrate that GRK function is a highly regulated process, and it is perhaps in this manner that a handful of GRKs (7 have been identified to date) are able to regulate the responsiveness of numerous GPCRs in a given cell type in a coordinated manner. The mechanisms by which GRK activity is regulated can be divided into 3 categories: 1) subcellular localization; 2) alterations in intrinsic kinase activity; and 3) alterations in GRK expression levels. This review will summarize our current understanding of each of these regulatory processes, and offer explanations as to how such mechanisms influence GPCR regulation under various physiologic conditions.

Molecular mechanism for the umami taste synergism

Umami is one of the 5 basic taste qualities. The umami taste of L-glutamate can be drastically enhanced by 5′ ribonucleotides and the synergy is a hallmark of this taste quality. The umami taste receptor is a heteromeric complex of 2 class C G-protein-coupled receptors, T1R1 and T1R3. Here we elucidate the molecular mechanism of the synergy using chimeric T1R receptors, site-directed mutagenesis, and molecular modeling. We propose a cooperative ligand-binding model involving the Venus flytrap domain of T1R1, where L-glutamate binds close to the hinge region, and 5′ ribonucleotides bind to an adjacent site close to the opening of the flytrap to further stabilize the closed conformation. This unique mechanism may apply to other class C G-protein-coupled receptors.

Regulation of the G Protein-coupled Receptor Kinase GRK5 by Protein Kinase C

G protein-coupled receptor kinases (GRKs) specifically recognize and phosphorylate the hormone-occupied form of numerous G protein-coupled receptors, ultimately resulting in termination of receptor signaling. While little is presently known about the regulation of GRK function, recent studies suggest a role for protein kinase C (PKC) phosphorylation of the β-adrenergic receptor kinase in membrane association and activation of the kinase. To assess a potential general role for PKC in regulating GRK function, we characterized the ability of PKC to phosphorylate GRK5, a recently identified member of the GRK family. We demonstrate that GRK5 can be rapidly and stoichiometrically phosphorylated by PKC in vitro Intact cell studies reveal that GRK5 is also phosphorylated when transiently expressed in COS-1 cells following treatment with the PKC activator, phorbol 12-myristate 13-acetate. In vitro analysis reveals two major sites of PKC phosphorylation within the C-terminal 26 amino acids of GRK5. GRK5 phosphorylation by PKC dramatically reduces its ability to phosphorylate both receptor (light-activated rhodopsin) and non-receptor (casein and phosvitin) substrates. Kinetic analysis reveals an ∼5-fold increased Km and ∼3-fold decreased Vmax for rhodopsin, with no change in the Km for ATP. The reduced affinity of PKC-phosphorylated GRK5 for rhodopsin was also evident in a decreased ability to bind to rhodopsin-containing membranes, while direct binding of GRK5 to phospholipids appeared unaltered. These results suggest that PKC might play an important role in modulating the ability of GRK5 to regulate receptor signaling and that GRK phosphorylation by PKC may serve as a disparate mechanism for regulating GRK activity.

Involvement of G protein-coupled receptor kinase-6 in desensitization of CGRP receptors

This investigation was undertaken to study the mechanisms of calcitonin gene-related peptide (CGRP)-mediated desensitization using recombinant porcine CGRP receptors stably expressed in human embryonic kidney (HEK-293) cells. Pretreatment of these cells with human alphaCGRP resulted in an approximately 60% decrease in CGRP-stimulated adenylyl cyclase activity and an approximately 10-fold rightward shift in the dose-response curve of CGRP. This effect was rapid (t(1/2) approximately 5 min) and was accompanied by a significant decrease in [125I]CGRP binding to membrane preparations from CGRP-pretreated cells. In contrast, CGRP pretreatment had no effect on isoproterenol- or forskolin-stimulated adenylyl cyclase activity in these cells. The potential involvement of protein kinase A or protein kinase C in CGRP-mediated desensitization was studied using selective inhibitors or activators of these kinases. Pretreatment of the cells with forskolin (adenylyl cyclase activator) or phorbol dibutyrate (protein kinase C activator) had no effect on CGRP-mediated adenylyl cyclase activity and did not influence CGRP-mediated desensitization. However, pretreatment of the cells with 2-(8-[(dimethylamino)methyl]-6,7,8, 9-tetrahydropyrido[1,2-a]indol-3-yl]-3-(1-methylindol-3-yl)m aleimide hydrochloride (Ro 32-0432) (a potent inhibitor of protein kinase C) resulted in significant attenuation of CGRP-mediated desensitization with an IC(50) approximately 3 microM. To establish whether this effect might be due to inhibition of other protein kinases by Ro 32-0432, its effect was tested against several G protein-coupled receptor kinases (GRKs). Ro 32-0432 was found to inhibit GRK2, GRK5, and GRK6 with IC(50) values of 29, 3.6, and 16 microM, respectively, suggesting that its effect on CGRP-mediated desensitization might be a result of GRK inhibition. To further test this hypothesis, as well as the potential GRK specificity, the cells were treated with antisense oligonucleotides to GRK2, GRK5, and GRK6. While GRK2 and GRK5 antisense nucleotides had no effect on CGRP-mediated desensitization, the GRK6 antisense nucleotide treatment significantly reversed CGRP-mediated desensitization. These results suggest the involvement of GRK6 in CGRP-mediated desensitization in HEK-293 cells.

Structure-Function Analysis of G Protein-coupled Receptor Kinase-5 ROLE OF THE CARBOXYL TERMINUS IN KINASE REGULATION

Many G protein-coupled receptors are phosphorylated and regulated by a distinct family of G protein-coupled receptor kinases (GRKs) that specifically target the activated form of the receptor. Recent studies have revealed that the GRKs are also subject to post-translational regulation. For example, GRK5 activity is strongly inhibited by protein kinase C phosphorylation and by Ca2+-calmodulin binding. Ca2+-calmodulin binding also promotes GRK5 autophosphorylation, which further contributes to kinase inhibition. In this study we identify two important structural domains in GRK5, a phospholipid binding domain (residues 552–562) and an autoinhibitory domain (residues 563–590), that significantly contribute to GRK5 localization and function. We demonstrate that the C-terminal region of GRK5 (residues 563–590) contains residues autophosphorylated in the presence of calmodulin as well as the residues phosphorylated by protein kinase C. Deletion of this domain increases the apparent affinity of GRK5 for receptor substrates 3–4-fold but has no effect on nonreceptor substrates. These findings define residues 563–590 of GRK5 as an autoinhibitory domain with efficacy that is regulated by phosphorylation. Another C-terminal domain in GRK5 that appears to be functionally important is found between residues 552 and 562. Deletion of this region significantly inhibits kinase phosphorylation of membrane-bound receptor substrates but has no effect on soluble substrates. Additional studies reveal that this domain is critical for GRK5 interaction with phospholipids and for the intracellular localization of the kinase. Interestingly, similar regions in GRK4 and GRK6 appear to be palmitoylated (and involved in membrane interaction), suggesting evolutionary conservation of the function of this domain.

Proper processing of a G protein γ subunit depends on complex formation with a β subunit

G protein β and γ subunits function as a tightly associated complex. We show that complex formation with the β subunit is a critical step for post‐translational processing of a γ subunit. When expressed alone in a cell line, the γ3 subunit type is isoprenylated but degraded; co‐expression with theβ1 subunit type stabilizes the γ3 protein. Furthermore, our experiments with partial cell fractionation indicate that the γ3 protein is localized differently in the cell depending on whether or not it is bound to the β subunit. Binding of the γ subunit to the β subunit is thus one of the prerequisites for the appropriate intracellular localization of the βγ complex and potentially, for normal G‐protein function.

[38] Characterization of antibodies for various G-protein β and γ subunits

Publisher Summary Antibodies to the β and γ subunits of G proteins have been invaluable in the investigations that have begun to dissect the structure and function of these subunits. The cDNAs for four β subunit types and for several γ subunits have now been characterized. The most useful antisera are those raised against peptides specific to the amino acid sequence encoded by cDNAs for different β and γ subunit types. Depending on the region of the protein chosen for synthesizing a corresponding peptide, these antibodies are capable of reacting with a particular subunit type specifically or with several different subunits. Of an immunoblot of a mixture of different subunit types, specific peptide antisera can distinguish among different members of the β and γ subunit families of proteins. Although the antisera are raised against a peptide corresponding to a relatively small portion of the protein, several of the antisera against the G-protein β and γ subunits are capable of immunoprecipitating the βγ complex. They can also be used for immunocytochemical localization of the β or γ subunits present in transient transfectants.

Characterization of G protein-coupled receptor kinases

Publisher Summary A basic feature of most cells is the ability to dynamically regulate their responsiveness to extracellular stimuli. Numerous stimuli transmit their signals via interaction with cell surface G protein-coupled receptors (GPCRs). GPCRs are subject to three principal modes of regulation: (1) desensitization, the process by which a receptor becomes refractory to continued stimuli, (2) internalization, whereby receptors are physically removed from the cell surface by endocytosis, and (3) downregulation, where total cellular receptor levels are decreased. This chapter describes some of the current methodologies for analyzing endogenous and expressed GPCR kinases (GRKs) in mammalian cells and strategies for in vitro analysis of GRK phosphorylation of GPCRs. GPCR desensitization is primarily mediated by second messenger responsive kinases—such as protein kinase A (PKA) and protein kinase C (PKC)—and by GRKs. GRKs specifically phosphorylate agonist-occupied GPCRs and initiate the recruitment of additional proteins, termed “arrestins,” which further receptor desensitization and internalization. The seven mammalian GRKs identified can be divided into three subfamilies based on their overall structural organization and homology: (1) GRKl (rhodopsin kinase) and GRK7, (2) GRK2 and GRK3, and (3) GRK4, GRKS, and GRKG.