Role of beta gamma subunits of G proteins in targeting the beta-adrenergic receptor kinase to membrane-bound receptors

A constitutively active mutant beta 2-adrenergic receptor is constitutively desensitized and phosphorylated

The beta 2-adrenergic receptor (beta 2AR) can be constitutively activated by mutations in the third intracellular loop. Whereas the wild-type receptor exists predominantly in an inactive conformation (R) in the absence of agonist, the mutant receptor appears to spontaneously adopt an active conformation (R*). We now demonstrate that not only is the mutant beta 2AR constitutively active, it is also constitutively desensitized and down-regulated. To assess whether the mutant receptor can constitutively engage a known element of the cellular desensitization machinery, the receptor was purified and reconstituted into phospholipid vesicles. These preparations retained the essential properties of the constitutively active mutant receptor: agonist-independent activity [to stimulate guanine nucleotide-binding protein (Gs)-GTPase] and agonist-specific increase in binding affinity. Moreover, the purified mutant receptor, in the absence of agonist, was phosphorylated by recombinant beta AR-specific kinase (beta ARK) in a fashion comparable to the agonist-occupied wild-type receptor. Thus, the conformation of the mutated receptor is equivalent to the active conformation (R*), which stimulates Gs protein and is identical to the beta ARK substrate.

Rapid desensitization of neonatal rat liver beta-adrenergic receptors. A role for beta-adrenergic receptor kinase

Exposure of beta-adrenergic receptors (BAR) to agonists often leads to a rapid loss of receptor responsiveness. The proposed mechanisms of such rapid receptor desensitization include receptor phosphorylation by either cAMP-dependent protein kinase or the specific beta-adrenergic receptor kinase (BARK), leading to functional uncoupling from adenylyl cyclase and sequestration of the receptors away from the cell surface. To evaluate the physiological role of such mechanisms, we have investigated whether rapid regulation of BAR occurs in the neonatal rat liver immediately after birth, a physiological situation characterized by a dramatic but transient increase in plasma catecholamines. We have detected a rapid, transient uncoupling of liver plasma membrane BARs from adenylyl cyclase (corresponding to a desensitization of approximately 45%) within the first minutes of extrauterine life, followed by a transient sequestration of approximately 40% of the BARs away from the plasma membrane. In agreement with such pattern of desensitization, we have detected (by enzymatic and immunological assays) rapid changes in BARK specific activity in different neonatal rat liver subcellular fractions that take place within the same time frame of BAR uncoupling and sequestration. Our results provide new evidence on the potential role of BAR desensitization mechanisms in vivo and suggest that they are involved in modulating catecholamines actions at the moment of birth. Furthermore, our data indicate that in addition to its suggested role as a rapid modulator of adrenergic receptor function at synapse, rapid BARK-mediated receptor regulation may have functional relevance in other tissues in response to high circulating or local levels of agonists.

Adaptation of sweeteners in water and in tannic acid solutions

Repeated exposure to a tastant often leads to a decrease in magnitude of the perceived intensity; this phenomenon is termed adaptation. The purpose of this study was to determine the degree of adaptation of the sweet response for a variety of sweeteners in water and in the presence of two levels of tannic acid. Sweetness intensity ratings were given by a trained panel for 14 sweeteners: three sugars (fructose, glucose, sucrose), two polyhydric alcohols (mannitol, sorbitol), two terpenoid glycosides (rebaudioside-A, stevioside), two dipeptide derivatives (alitame, aspartame), one sulfamate (sodium cyclamate), one protein (thaumatin), two N-sulfonyl amides (acesulfame-K, sodium saccharin), and one dihydrochalcone (neohesperidin dihydrochalcone). Panelists were given four isointense concentrations of each sweetener by itself and in the presence of two concentrations of tannic acid. Each sweetener concentration was tasted and rated four consecutive times with a 30 s interval between each taste and a 2 min interval between each concentration. Within a taste session, a series of concentrations of a given sweetener was presented in ascending order of magnitude. Adaptation was calculated as the decrease in intensity from the first to the fourth sample. The greatest adaptation in water solutions was found for acesulfame-K, Na saccharin, rebaudioside-A, and stevioside. This was followed by the dipeptide sweeteners, alitame and aspartame. The least adaptation occurred with the sugars, polyhydric alcohols, and neohesperidin dihydrochalcone. Adaptation was greater in tannic acid solutions than in water for six sweeteners. Adaptation of sweet taste may result from the desensitization of sweetener receptors analogous to the homologous desensitization found in the beta adrenergic system.

Cloning of an intracellular receptor for protein kinase C: a homolog of the beta subunit of G proteins

Protein kinase C (PKC) translocates from the soluble to the cell particulate fraction on activation. Intracellular receptors that bind activated PKC in the particulate fraction have been implicated by a number of studies. Previous work identified 30- to 36-kDa proteins in the particulate fraction of heart and brain that bound activated PKC in a specific and saturable manner. These proteins were termed receptors for activated C-kinase, or RACKs. In the following study, we describe the cloning of a cDNA encoding a 36-kDa protein (RACK1) that fulfills the criteria for RACKs. (i) RACK1 bound PKC in the presence of PKC activators, but not in their absence. (ii) PKC binding to the recombinant RACK1 was not inhibited by a pseudosubstrate peptide or by a substrate peptide derived from the pseudosubstrate sequence, indicating that the binding did not reflect simply PKC association with its substrate. (iii) Binding of PKC to RACK1 was saturable and specific; two other protein kinases did not bind to RACK1. (iv) RACK1 contains two short sequences homologous to a PKC binding sequence previously identified in annexin I and in the brain PKC inhibitor KCIP. Peptides derived from these sequences inhibited PKC binding to RACK1. Finally, RACK1 is a homolog of the beta subunit of G proteins, which were recently implicated in membrane anchorage of the beta-adrenergic receptor kinase [Pitcher, J., Inglese, L., Higgins, J. B., Arriza, J. A., Casey, P. J., Kim, C., Benovic, J. L., Kwatra, M. M., Caron, M. G. & Lefkowitz, R. J. (1992) Science 257, 1264-1267]. Our in vitro data suggest a role for RACK1 in PKC-mediated signaling.

Isolation and expression of a novel chick G-protein cDNA coding for a G alpha i3 protein with a G alpha 0 N-terminus

We have cloned cDNAs coding for G-protein alpha subunits from a chick brain cDNA library. Based on sequence similarity to G-protein alpha subunits from other eukaryotes, one clone was designated G alpha i3. A second clone, G alpha i3-o, was identical to the G alpha i3 clone over 932 bases on the 3' end. The 5' end of G alpha i3-o, however, contained an alternative sequence in which the first 45 amino acids coded for are 100% identical to the conserved N-terminus of G alpha o from species such as rat, mouse, human, bovine and hamster. Both clones were found to be expressed in all tissues studied. The unusual alpha o-alpha i3-like G-protein chimera, G alpha i3-o, was found to be expressed at significantly lower levels than G alpha i3. In vitro transcription and translation of the G alpha i3-o cDNA clone gave a protein of approx. 41 kDa which stably bound guanosine 5'-[gamma-thio]triphosphate. G alpha i3-o appears to be the first G-protein alpha subunit cloned which contains ends that are homologous to two different alpha subunit isoforms, G alpha o and G alpha i3.

Signal-transducing G proteins: basic and clinical implications

The pivotal role that G proteins play in transmembrane signal transduction is highlighted by the rapidly expanding list of receptors and effector molecules that are coupled through G proteins. G proteins are poised to allow discrimination and diversification of cellular signals into the cytosolic milieu. The utilization of an evolutionarily conserved "GTPase clock" by G proteins, offers insight into the fundamental role these proteins play in biology. Knowledge of the implication of altered expression or function of G proteins in human disease is now emerging. It is not surprising that deficiency or expression of altered forms of these important proteins can lead to global or restricted metabolic disturbances, depending upon the distribution and role of the G protein. Human disorders, including heart failure, alcoholism, endocrine abnormalities, and neoplasia, are now recognized as due in part to altered expression or function of G proteins.

G proteins: critical control points for transmembrane signals

Heterotrimeric GTP-binding proteins (G proteins) that are made up of alpha and beta gamma subunits couple many kinds of cell-surface receptors to intracellular effector enzymes or ion channels. Every cell contains several types of receptors, G proteins, and effectors. The specificity with which G protein subunits interact with receptors and effectors defines the range of responses a cell is able to make to an external signal. Thus, the G proteins act as a critical control point that determines whether a signal spreads through several pathways or is focused to a single pathway. In this review, I will summarize some features of the structure and function of mammalian G protein subunits, discuss the role of both alpha and beta gamma subunits in regulation of effectors, the role of the beta gamma subunit in macromolecular assembly, and the mechanisms that might make some responses extremely specific and others rather diffuse.

Specific enhancement of beta-adrenergic receptor kinase activity by defined G-protein beta and gamma subunits

The beta and gamma subunits of heterotrimeric guanine nucleotide-binding proteins (G proteins) have recently been shown to play an active role in signal transduction. Among other effects they enable translocation of the beta-adrenergic receptor kinase (beta ARK) from the cytosol to the plasma membrane and thus permit phosphorylation and ultimately desensitization of beta-adrenergic receptors and other G-protein-coupled receptors. To investigate the specificity of this effect, we have purified various combinations of recombinant beta and gamma subunits expressed in Sf9 cells and measured their effects on beta ARK-catalyzed phosphorylation of beta 2-adrenergic receptors and of rhodopsin. The combinations tested were beta 1 gamma 2, beta 1 gamma 3, beta 2 gamma 2, beta 2 gamma 3, and transducin beta gamma (beta 1 gamma 1). There were clear differences in enhancement of rhodopsin phosphorylation, with an order of efficacy beta 2 gamma 2 > beta 1 gamma 2 >> beta 2 gamma 3 approximately beta 1 gamma 3 approximately beta 1 gamma 1. The first two combinations had larger effects than a mixed beta gamma preparation from bovine brain. In enhancing phosphorylation of beta 2-adrenergic receptors, beta 1 gamma 2 was more efficient and potent than all other combinations. These data suggest a twofold specificity of beta gamma complexes in enhancing beta ARK-catalyzed receptor phosphorylation: the gamma subunits may be important in interacting with beta ARK, with gamma 2 being more potent than gamma 3, whereas the beta subunits may determine coupling to the receptors, with beta 2 being more effective than beta 1 for rhodopsin and beta 1 being more effective than beta 2 for beta 2-adrenergic receptors.

Identification of additional members of human G-protein-coupled receptor kinase multigene family

Human neutrophils express several distinct guanine nucleotide binding (G)-protein-coupled receptors that mediate their responsiveness to chemoattractants. Phosphorylation by receptor-specific and second messenger-activated protein kinases is a common mechanism for regulation of G-protein-coupled receptors. To explore the possibility that chemoattractant receptors are regulated by unique receptor kinases, we utilized PCR to identify receptor kinases in human neutrophils. Here, we report the isolation of three G-protein-coupled-receptor-kinase (GPRK)-like sequences termed GPRK5, GPRK6, and GPRK7 in addition to the beta-adrenergic receptor kinase (beta ARK) 1 and 2 isoforms (beta ARK1 and beta ARK2). Two, GPRK5 and GPRK6, showed high homology at the amino acid level to the recently identified receptor-kinase-like sequence localized close to the Huntington disease locus. GPRK7 is of interest in that it contains a DLG (Asp-Leu-Gly) amino acid motif of receptor kinases preceded by a DFD (Asp-Phe-Asp) motif. We isolated cDNAs corresponding to GPRK6; the complete sequence shows > 66% identity and 81% similarity at the amino acid level to the GPRK from the Huntington disease locus. The GPRK6 cDNA probe hybridizes to two mRNAs of 2.9 and 2.1 kb that were expressed in all the tested human tissues including HL-60 cells and neutrophils. Genomic Southern blot analysis and chromosome mapping showed that GPRK6 hybridizes to two closely related genes located on chromosomes 5 and 13 and are, therefore, distinct from the GPRK located near the Huntington disease locus on chromosome 4. The identification herein of three putative receptor kinases indicates that in addition to beta ARK and rhodopsin kinase subfamilies, there are other receptor-kinase subfamilies that regulate the broad spectrum of G-protein-coupled receptors.

Functional consequences of A1 adenosine-receptor phosphorylation by the beta-adrenergic receptor kinase

Treatment of smooth-muscle cells with R-phenylisopropyladenosine (R-PIA) leads to a loss of A1 adenosine receptor (A1AR)-mediated inhibition of adenylate cyclase, a decrease in receptor number and an increase in receptor phosphorylation. In this study, the role of the beta-adrenergic receptor kinase (beta ARK) in the phosphorylation and inactivation of the A1AR was examined. A1ARs were purified from bovine brain and reconstituted into phospholipid vesicles, with or without a 10-fold excess of Gi/Go (a 50:50 mixture). The reconstituted receptor preparations were phosphorylated with beta ARK in the absence (control) or presence (treated) of R-PIA. R-PIA stimulated A1AR phosphorylation by 2-3-fold over control. Phosphorylation of the A1AR was blocked by XAC, and A1AR antagonist, underscoring its agonist dependence. The stoichiometry of phosphorylation obtained was approx. 1.3 mol of phosphate per mol of A1AR. Phosphorylation of the A1AR by beta ARK was enhanced by an additional 42% when G beta gamma (30 nM) was included in the phosphorylation mixture. In order to test the role of phosphorylation on receptor function, the purified A1AR was reconstituted with a mixture of Gi/Go, phosphorylated with beta ARK and used to determine high-affinity [125I]APNEA (A1AR agonist) binding. Agonist binding was reduced by about 50% in the treated preparations compared to control. In contrast, antagonist ([3H]XAC) binding was increased by about 50%. These data are consistent with an uncoupling of the A1AR from G proteins following receptor phosphorylation. In control preparations, R-PIA stimulated GTPase activity from 0.08 to 0.164 pmol Pi released/pmol Gi/Go per min. Phosphorylation of receptor by beta ARK reduced R-PIA-stimulated GTPase activity by 35%. In addition, phosphorylation of the A1AR by beta ARK decreased R-PIA-stimulated GTP gamma S binding by 62%. These data provide evidence that A1AR phosphorylation by beta ARK results in a diminished receptor-G-protein interaction.

New roles for G-protein beta gamma-dimers in transmembrane signalling

When a membrane-bound receptor acts on a G protein, the GTP-binding or G alpha subunit dissociates from the G beta gamma dimer. Until recently, the G alpha subunit alone was thought to act on the enzymes and ion channels controlled by these proteins. Newer evidence indicates that the G beta gamma dimer also plays a major part in signal transmission, enhancing the complexity of the possible interactions between the G proteins and their targets.

Modification of eukaryotic signaling proteins by C-terminal methylation reactions

Eukaryotic polypeptides that are initially synthesized with the C-terminal sequence -Cys-Xaa-Xaa-Xaa, including a variety of signal-transducing proteins, such as small G-proteins, large G-proteins and cGMP phosphodiesterases, can be targeted for a series of sequential post-translational modifications. This processing pathway includes the isoprenylation of the cysteine residue with a farnesyl or geranylgeranyl moiety, followed by proteolysis of the three terminal residues and alpha-carboxyl methyl esterification of the cysteine residue. The potential reversibility of the last step suggests that it may be involved in modulating the function of these proteins. Firstly, methylation may play a role in the activation of cellular peptides or proteins. Secondly, this modification may aid in the membrane attachment of cytosolic precursor proteins. Thirdly, methylation may protect the polypeptide from C-terminal proteolytic degradation once the three terminal amino acid residues are removed. Finally, reversible methylation may directly regulate the function of its target proteins. Therapeutically, inhibitors of C-terminal isoprenylcysteine methylation or demethylation reactions may prove to be useful pharmacological tools as anti-cancer and anti-inflammatory agents.

The N-terminal coiled-coil domain of beta is essential for gamma association: a model for G-protein beta gamma subunit interaction

We have identified the N terminus of the beta subunit as an essential domain for G-protein beta gamma assembly. A C-terminal fragment, beta 1-(130-340), fails to bind gamma unless coexpressed with the complementary N-terminal fragment, beta 1-(1-129). Deletion of the N-terminal 33 residues of beta 1, a region identified by computer algorithm to favor coiled-coil formation, abolishes gamma 2 association. On the basis of these findings, we propose a coiled-coil model of beta gamma interaction and refine this by computer-assisted molecular modeling. The model is tested by further mutagenesis: reversing the charge of residues in beta 1 that are hypothesized to be involved in interhelical salt bridges precludes gamma association. Insertions in the coiled-coil region, which disrupt the proposed hydrophobic interface, prevent gamma association. This structural basis for beta gamma dimerization provides a starting point for the design of beta and gamma mutants that can be used to map regions in beta gamma critical for interactions with the alpha subunit, receptors, and effectors.

The importance of G-protein βλ subunits

As the properties of more and more isoforms of the molecules involved in G-protein-mediated signal transduction pathways are unravelled, surprising diversity and versatility are being revealed. The path from receptor to effector is not dictated exclusively by the alpha subunits of heterotrimetric G proteins. The nature of the beta lambda subunit complex probably controls interactions of G(alpha) with receptors. In addition, dissociation of G(alpha)-GTP from G(beta lambda)provides two signalling complexes, and these proteins regulate effectors independently or synergistically. Synergistic or conditional regulation of effectors by G(alpha) and G(beta lambda)can provide a molecular signal that records the association of independent events.

Cloning and expression of GRK5: a member of the G protein-coupled receptor kinase family

Guanine nucleotide binding protein (G-protein)-coupled receptor kinases (GRKs) specifically phosphorylate the agonist-occupied form of G-protein-coupled receptors such as the beta 2-adrenergic receptor and rhodopsin. The best characterized members of this family include the beta-adrenergic receptor kinase (beta ARK) and rhodopsin kinase. To identify additional members of the GRK family, the polymerase chain reaction was used to amplify human heart cDNA using degenerate oligonucleotide primers from highly conserved regions unique to the GRK family. Here we report the isolation of a cDNA that encodes a 590-amino acid protein kinase, termed GRK5, which has 34.8% and 47.2% amino acid identities with beta ARK and rhodopsin kinase, respectively. Interestingly, GRK5 has an even higher homology with Drosophila GPRK-2 (71.0% identity) and the recently identified human IT11 (69.1% identity). Northern blot analysis of GRK5 with selected human tissues reveals a message of approximately 3 kilobases with highest levels in heart, placenta, lung > skeletal muscle > brain, liver, pancreas > kidney. GRK5, overexpressed in Sf9 insect cells using the baculovirus system, was able to phosphorylate rhodopsin in a light-dependent manner. In addition, GRK5 neither contains a consensus sequence for isoprenylation like rhodopsin kinase nor is activated by G-protein beta gamma subunits like beta ARK1. Thus, GRK5 represents a member of the GRK family that likely has a unique physiological role.

Purification and functional characterization of beta-adrenergic receptor kinase expressed in insect cells

The beta-adrenergic receptor kinase mediates agonist-dependent phosphorylation of beta-adrenergic receptors, which is thought to represent the first step of homologous desensitization. We have expressed bovine and human beta ARK1 in Sf9 cells and purified them to apparent homogeneity in milligram quantities. The Km-values of the enzyme were 3.8 microM for rhodopsin and 22 microM for ATP; the Vmax-value was 9.9 mol phosphate/mol beta ARK/min. These data indicate that the two recombinant kinases were at least as active as preparations previously obtained from bovine brain. There were no differences in the functional activity of human and bovine beta ARK.

Activation of phospholipase C beta 2 by the alpha and beta gamma subunits of trimeric GTP-binding protein

Cotransfection assays were used to show that the members of the GTP-binding protein Gq class of alpha subunits could activate phospholipase C (PLC) beta 2. Similar experiments also demonstrated that G beta 1 gamma 1, G beta 1 gamma 5, and G beta 2 gamma 5 could activate the beta 2 isoform of PLC but not the beta 1 isoform, while G beta 2 gamma 1 did not activate PLC beta 2. To determine which portions of PLC beta 2 are required for activation by G beta gamma or G alpha, a number of PLC beta 2 deletion mutants and chimeras composed of various portions of PLC beta 1 and PLC beta 2 were prepared. We identified the N-terminal segment of PLC beta 2 with amino acid sequence extending to the end of the Y box as the region required for activation by G beta gamma and the C-terminal region as the segment containing amino acid sequences required for activation by G alpha. Furthermore, we found that coexpression of G alpha 16 and G beta 1 gamma 1 but not G beta 1 gamma 5 in COS-7 cells was able to synergistically activate recombinant PLC beta 2. We suggest that G alpha 16 may act together with free G beta 1 gamma 1 to activate PLC beta 2, while G alpha 16 may form heterotrimeric complexes with G beta 1 gamma 5 and be stabilized in an inactive form. We conclude that the regions of PLC beta 2 required for activation by G beta gamma and G alpha are physically separate and that the nature of the G beta subunit may play a role in determining the relative specificity of the G beta gamma complex for effector activation while the nature of the G gamma subunit isoform may be important for determining the affinity of the G beta gamma complex for specific G alpha proteins.

Oocyte maturation in starfish is mediated by the beta gamma-subunit complex of a G-protein

The stimulation of meiotic maturation of starfish oocytes by the hormone 1-methyladenine is mimicked by injection of beta gamma subunits of G-proteins from either retina or brain. Conversely, the hormone response is inhibited by injection of the GDP-bound forms of alpha i1 or alpha t subunits, or by injection of phosducin; all of these proteins should bind free beta gamma. alpha-subunit forms with reduced affinity for beta gamma (alpha i1 or alpha t bound to hydrolysis-resistant GTP analogs, or alpha i1-GMPPCP treated with trypsin to remove the amino terminus of the protein) are less effective inhibitors of 1-methyladenine action. These results indicate that the beta gamma subunit of a G-protein mediates 1-methyladenine stimulation of oocyte maturation.

Farnesylcysteine analogues inhibit chemotactic peptide receptor-mediated G-protein activation in human HL-60 granulocyte membranes

Analogues of S-prenylated cysteine like N-acetyl-S-trans,trans-farnesyl-L-cysteine (AFC) have previously been shown to inhibit the carboxyl methylation of proteins carrying a C-terminal S-prenylated cysteine residue and to block the endotoxin-activated serum-elicited chemotactic response of mouse macrophages. Here, we show that AFC inhibits both basal and formyl peptide receptor-stimulated binding of guanosine 5'-O-(3-thiotriphosphate) (GTP[S]) to and hydrolysis of GTP by membranes of myeloid differentiated HL-60 granulocytes. Receptor-stimulated GTP[S] binding and GTP hydrolysis are more sensitive to AFC inhibition than basal G-protein functions. Inhibition of formyl peptide receptor-mediated G-protein activation is also observed for S-trans,trans-farnesyl-3-thiopropionic acid, but not for N-acetyl-S-trans-geranyl-L-cysteine, N-acetyl-L-cysteine, or the methyl ester of AFC, suggesting that the farnesyl moiety and the carboxyl group, but not the peptide bond of AFC are required for inhibition. The observations that exogeneous S-adenosyl-L-methionine is apparently not required for and S-adenosyl-L-homocysteine does not attenuate the inhibitory action of AFC raise the distinct possibility that AFC inhibits receptor-mediated G-protein interaction by a mechanism other than inhibition of protein carboxyl methylation.