A truncation mutation in the avian beta-adrenergic receptor causes agonist-induced internalization and GTP-sensitive agonist binding characteristic of mammalian receptors

Conformational thermostabilization of the beta1-adrenergic receptor in a detergent-resistant form

There are approximately 350 non-odorant G protein-coupled receptors (GPCRs) encoded by the human genome, many of which are predicted to be potential therapeutic targets, but there are only two structures available to represent the whole of the family. We hypothesized that improving the detergent stability of these receptors and simultaneously locking them into one preferred conformation will greatly improve the chances of crystallization. We developed a generic strategy for the isolation of detergent-solubilized thermostable mutants of a GPCR, the beta1-adrenergic receptor. The most stable mutant receptor, betaAR-m23, contained six point mutations that led to an apparent T(m) 21 degrees C higher than the native protein, and, in the presence of bound antagonist, betaAR-m23 was as stable as bovine rhodopsin. In addition, betaAR-m23 was significantly more stable in a wide range of detergents ideal for crystallization and was preferentially in an antagonist conformation in the absence of ligand.

Receptor recycling mediates plasma membrane recovery of dopamine D1 receptors in dendrites and axons after agonist-induced endocytosis in primary cultures of striatal neurons

The pharmacological stimulation of G-protein-coupled receptor induces receptor internalization. Receptor's fate after the step of internalization remains poorly characterized despite its incidence on the neuronal responsiveness. In this context, we studied the dopamine (DA) D1 receptor (D1R) trafficking in a model of striatal neuronal culture that endogenously express the D1R. We first characterized by immunohistochemistry the spatial distribution of the compartments involved in the endocytic pathways and then the D1R trafficking in dendrites and axons. In dendrites, immunohistochemical analysis showed that acute stimulation by the D1R agonist SKF 82958 (1 microM) induces an internalization of D1R in early endosomes labeled with Alexa-488-conjugated transferrin. We show that, 20 min after removal of the agonist, the D1R immunolabeling pattern returns to the basal state in dendrites and in axons. Recovery was unaffected by cycloheximide (70 microM) but was prevented by monensin (100 microM) that inhibits endosomal acidification and receptor recycling. These data suggest that dendritic and axonal D1Rs are internalized after agonist stimulation and targeted to the recycling pathway demonstrating that the machinery involved in GPCR endocytosis and recycling is functional both in dendrites and in axons. Temporal characteristics observed for the recovery of D1R density to the basal state and those observed for the resensitization process strongly suggest that D1R recycling supports the receptor resensitization.

Expression and purification of truncated, non-glycosylated turkey beta-adrenergic receptors for crystallization

In order to purify milligram quantities of turkey beta-adrenergic receptor (betaAR) for structural analysis, we have expressed mutant betaARs using the baculovirus system. The initial betaAR construct was truncated at both N- and C-termini thus removing an N-glycosylation site. Cys 116 was mutated to leucine and a histidine tag was added at the C-terminus resulting in the betaAR construct 20-424/His6. Expression of this construct in Sf9 cells produced 0.5 mg of unpurified receptor per liter of culture which necessitated the use of a fermenter for large-scale production. The yield was improved more than 2-fold to 1.2 mg/l culture by using Tni cells which facilitated the production of receptor on a 4 litre scale in shake cultures. The receptor was purified to homogeneity with 35% recovery giving a yield of 2 mg receptor. A further deletion at the N-terminus (betaAR 34-424/His6) eliminated proteolysis which had been observed with the original construct and also increased expression more than 5-fold to 360 pmol/mg solubilized membrane protein. This expression level is one of the highest reported for a G protein-coupled receptor (GPCR) and has enabled us to purify 10 mg betaAR for large-scale crystallization experiments.

Structural and functional characterization of human CXCR4 as a chemokine receptor and HIV-1 co-receptor by mutagenesis and molecular modeling studies

The human CXC chemokine receptor 4 (CXCR4) is a receptor for the chemokine stromal cell-derived factor (SDF-1alpha) and a co-receptor for the entry of specific strains of human immunodeficiency virus type I (HIV-1). CXCR4 is also recognized by an antagonistic chemokine, the viral macrophage inflammatory protein II (vMIP-II) encoded by human herpesvirus type VIII. SDF-1alpha or vMIP-II binding to CXCR4 can inhibit HIV-1 entry via this co-receptor. An approach combining protein structural modeling and site-directed mutagenesis was used to probe the structure-function relationship of CXCR4, and interactions with its ligands SDF-1alpha and vMIP-II and HIV-1 envelope protein gp120. Hypothetical three-dimensional structures were proposed by molecular modeling studies of the CXCR4.SDF-1alpha complex, which rationalize extensive biological information on the role of CXCR4 in its interactions with HIV-1 envelope protein gp120. With site-directed mutagenesis, we have identified that the amino acid residues Asp (D20A) and Tyr (Y21A) in the N-terminal domain and the residue Glu (E268A) in extracellular loop 3 (ECL3) are involved in ligand binding, whereas the mutation Y190A in extracellular loop 2 (ECL2) impairs the signaling mediated by SDF-1alpha. As an HIV-1 co-receptor, we found that the N-terminal domain, ECL2, and ECL3 of CXCR4 are involved in HIV-1 entry. These structural and mutational studies provide valuable information regarding the structural basis for CXCR4 activity in chemokine binding and HIV-1 viral entry, and could guide the design of novel targeted inhibitors.

Tight association of the human Mel(1a)-melatonin receptor and G(i): precoupling and constitutive activity

If stably expressed in human embryonic kidney (HEK)293 cells, the human Mel(1a)-melatonin receptor activates G(i)-dependent, pertussis toxin-sensitive signaling pathways, i.e., inhibition of adenylyl cyclase and stimulation of phospholipase Cbeta; the latter on condition that G(q) is coactivated. The antagonist luzindole blocks the effects of melatonin and acts as an inverse agonist at the Mel(1a) receptor in both intact cells and isolated membranes. This suggests that the Mel(1a) receptor is endowed with constitutive activity, a finding confirmed on reconstitution of the Mel(1a) receptor with G(i). Because the receptor density is in the physiological range, constitutive activity is not an artifact arising from overexpression of the receptor. In addition, the following findings indicate that the Mel(1a) receptor forms a very tight complex with G(i) which can be observed both in the presence and absence of an agonist. 1) In intact cells and in membranes, high-affinity agonist binding is resistant to the destabilizing effect of guanine nucleotides. 2) The ability to bind an agonist with high affinity is preserved even after exposure of the cells to pertussis toxin, because a fraction of G(i) is inaccessible to the toxin in cells expressing Mel(1a) receptors (but not the A(1)-adenosine receptor, another G(i)-coupled receptor). 3) An antiserum directed against the Mel(1a) receptor coprecipitates G(i) even in the absence of an agonist. We therefore conclude that the Mel(1a) receptor is tightly precoupled and that its constitutive activity may play a role in pacing the biological clock, an action known to involve the melatonin receptors in the suprachiasmatic nucleus.

Beta 2-adrenoceptor agonist-induced down-regulation after short-term exposure

We examined the effect of duration of beta 2-adrenergic receptor (beta 2AR) occupancy by isoproterenol on specific binding of 125I-lodocyanopindolol (125I-ICYP) in membranes from rat L6 myoblasts. Ten minute exposure caused a time-and concentration-dependent maximal decrease in 125I-ICYP binding 24 hours after exposure equal to that following continuous exposure (p < 0.05). Low temperature, concanavalin A, H89 and ICl 118,551 blocked the decline in 125I-ICYP binding during the first hour following exposure probably representing receptor sequestration to a compartment or change to a form incapable of ligand binding. Compared to controls, receptor binding 4 and 24 hours following exposure was reduced 56 +/- 8.7% and 72 +/- 8.8%, respectively (p < 0.05), and was blocked by ICl 118,551 but not CGP12177. Isoproterenol-induced, but not forskolin-stimulated, cAMP accumulation was reduced 35% 24 hours following exposure (p < 0.05). 125I-ICYP binding in intact L6 cells 4 and 24 hours after exposure were respectively 56 +/- 8.9 and 61 +/- 13% of controls (p < 0.05). Following agonist exposure, CHO cell membranes expressing human beta 2ARs exhibited 125I-ICYP binding 85 +/- 2.0% and 6 +/- 2.8% of control values 4 and 24 hours, respectively (p < 0.05). A model predicting that full occupation of the beta 2AR activates receptor degradation explains our results that agonist-induced down-regulation of beta 2AR does not require continuous presence of the agonist.

The zeta isoform of 14-3-3 proteins interacts with the third intracellular loop of different alpha2-adrenergic receptor subtypes

The alpha2-adrenergic receptors (alpha2ARs) are localized to and function on the basolateral surface in polarized renal epithelial cells via a mechanism involving the third cytoplasmic loop. To identify proteins that may contribute to this retention, [35S]Met-labeled Gen10 fusion proteins with the 3i loops of the alpha2AAR (Val217-Ala377), alpha2BAR (Lys210-Trp354), and alpha2CAR (Arg248-Val363) were used as ligands in gel overlay assays. A protein doublet of approximately 30 kDa in Madin-Darby canine kidney cells or pig brain cytosol (alpha2B >/= alpha2C>> alpha2A) was identified. The interacting protein was purified by sequential DEAE and size exclusion chromatography, and subsequent microsequencing revealed that they are the zeta isoform of 14-3-3 proteins. [35S]Met-14-3-3zeta binds to all three native alpha2AR subtypes, assessed using a solid phase binding assay (alpha2A>/=alpha2B> alpha2C), and this binding depends on the presence of the 3i loops. Attenuation of the alpha2AR-14-3-3 interactions in the presence of a phosphorylated Raf-1 peptide corresponding to its 14-3-3 interacting domain (residues 251-266), but not by its non-phosphorylated counterpart, provides evidence for the functional specificity of these interactions and suggests one potential interface for the alpha2AR and 14-3-3 interactions. These studies represent the first evidence for G protein-coupled receptor interactions with 14-3-3 proteins and may provide a mechanism for receptor localization and/or coordination of signal transduction.

Role of carboxyl terminus of mu-and delta-opioid receptor in agonist-induced down-regulation

Chronic exposure of mu-and delta-opioid receptors to their agonists leads to different rates in receptor down-regulation. In order to analyze the role of the carboxyl terminus of mu-and delta-opioid receptors in the difference in the rate of down-regulation, two chimeras of these receptors were generated by swapping the carboxyl termini; MORTAGDT and DORTAGMT. These chimeras were tagged at the N-terminus with hemagglutinin (HA) epitope (YPYDVPDYA), which can be recognized by the monoclonal antibody 12CA5, and then stably expressed in Neuro 2A (N2A) cells. The swapping of the carboxyl termini did not alter the ligand selectivity of these receptor chimeras. However, they did exhibit a reduction in agonist potency to inhibit forskolin-stimulated adenylyl cyclase activity for all agonists tested except etorphine which had a potency comparable to that of wild type receptors. Treatment of the N2A cells expressing MORTAGDT with 50 nM etorphine produced a faster rate of receptor down-regulation when compared to the wild type mu-opioid receptor. Immunofluorescence microscopy of the MORTAGDT chimera using a monoclonal antibody against HA confirmed internalization of the receptors after treatment with etorphine for 1 and 6h. There was a reduction in the HA-immunoreactivity at the cell surface of the MORTAGDT chimera concurrent with more noticeable HA-immunoreactivity inside the cell compared to the wild type receptor. On the other hand, the rate of down-regulation of DORTAGMT receptors was seen to be the same as the wild type delta-opioid receptor after etorphine treatment. Immunofluorescence studies showed more reduction in cell surface staining of the DORTAGMT chimera compared to the wild type receptor. These data suggest the involvement of the carboxyl terminus in agonist-induced down-regulation and internalization of the nu-opioid receptor. However, different mechanisms that are unrelated to the carboxyl terminus may operate in the down-regulation of delta-opioid receptor.

Mechanism of GnRH receptor signaling: combinatorial cross-talk of Ca2+ and protein kinase C

Gonadotropin-releasing hormone (GnRH), the first key hormone of reproduction, is synthesized in the hypothalamus and is released in a pulsatile manner to stimulate pituitary gonadotrope-luteinizing hormone (LH) and follicle-stimulating hormone (FSH) synthesis and release. Gonadotropes represent only about 10% of pituitary cells and are divided into monohormonal cells (18% LH and 22% FSH cells) and 60% multihormonal (LH + FSH) cells. GnRH binds to a specific seven transmembrane domain receptor which is coupled to Gq and activates sequentially different phospholipases to provide Ca2+ and lipid-derived messenger molecules. Initially, phospholipase C is activated, followed by activation of both phospholipase A2 (PLA2) and phospholipase D (PLD). Generation of the second messengers inositol 1,4,5-trisphosphate and diacylglycerol (DAG) lead to mobilization of intracellular pools of Ca2+ and activation of protein kinase C (PKC). Early DAG and Ca2+, derived via enhanced phosphoinositide turnover, might be involved in rapid activation of selective Ca(2+)-dependent, conventional PKC isoforms (cPKC). On the other hand, late DAG, derived from phosphatidic acid (PA) via PLD, may activate Ca(2+)-independent novel PKC isoforms (nPKC). In addition, arachidonic acid (AA) which is liberated by activated PLA2, might also support selective activation of PKC isoforms (PKCs) with or without other cofactors. Differential cross-talk of Ca2+, AA, and selective PKCs might generate a compartmentalized signal transduction cascade to downstream elements which are activated during the neurohormone action. Among those elements is the mitogen-activated protein kinase (MAPK) cascade which is activated by GnRH in a PKC-, Ca(2+)-, and protein tyrosine kinase (PTK)-dependent fashion. Transcriptional regulation can be mediated by the activation of transcription factors such as c-fos by MAPK. Indeed, GnRH activates the expression of both c-jun and c-fos which might participate in gene regulation via the formation of AP-1. The signaling cascade leading to gonadotropin (LH and FSH) gene regulation by GnRH is still not known and might involve the above-mentioned cascades. AA and selective lipoxygenase products such as leukotriene C4 also participate in GnRH action, possibly by cross-talk with PKCs, or by an autocrine/paracrine amplification cycle. A complex combinatorial, spatial and temporal cross-talk of the above messenger molecules seems to mediate the diverse effects elicited by GnRH, the first key hormone of the reproductive cycle.

A dileucine motif in the C terminus of the beta2-adrenergic receptor is involved in receptor internalization

The cytoplasmic C terminus of the beta2-adrenergic receptor and many other G protein-coupled receptors contains a dileucine sequence that has been implicated in endosome/lysosome targeting of diverse proteins. In the present study, we provide evidence for an essential role of this motif in the agonist-induced internalization of the beta2-adrenergic receptor. Mutation of Leu-339 and/or Leu-340 to Ala caused little changes in surface expression, ligand binding, G protein coupling, and signaling to adenylyl cyclase, when these receptors were transiently or stably expressed in CHO or HEK-293 cells. However, agonist-induced receptor internalization was markedly impaired in the L339,340A double mutant and reduced in the two single mutants. This impairment in receptor internalization was seen by using various approaches to determine internalization: binding of hydrophobic vs. hydrophilic ligands, loss of surface beta2-adrenergic receptor immunoreactivity, and immunofluorescence microscopy. The selective effects of these mutations suggest that the C-terminal dileucine motif is involved in agonist-induced internalization of the beta2-adrenergic receptor.

Ligand-induced internalization of cholecystokinin receptors. Demonstration of the importance of the carboxyl terminus for ligand-induced internalization of the rat cholecystokinin type B receptor but not the type A receptor

Internalization of a variety of different heptahelical G protein-coupled receptors has been shown to be influenced by a number of different structural determinants of the receptors, including the carboxyl terminus. To investigate the role of the carboxyl terminus of cholecystokinin (CCK) receptors in receptor internalization, the rat wild type (WT) CCK-A receptor (WT CCKAR) and the rat WT CCK-B receptor (WT CCKBR) were truncated after amino acid residue 399 (CCKAR Tr399) and 408 (CCKBR Tr408), thereby deleting the carboxyl-terminal 45 and 44 residues, respectively. All WT and mutant CCK receptors were stably expressed in NIH/3T3 cells. Internalization of the CCKAR Tr399 was not significantly different from the WT CCKAR. In contrast, internalization of the CCKBR Tr408 was decreased to 26% compared with the WT CCKBR internalization of 92%. The mutation of all 10 serine and threonine residues (as potential phosphorylation sites) in the carboxyl terminus of the CCKBR to alanines (mutant CCKBR DeltaS/T) could account for the majority of this effect (39% internalization). All mutant receptors displayed similar ligand binding characteristics, G protein coupling, and signal transduction as their respective WT receptors, indicating that the carboxyl termini are not necessary for these processes. Thus, internalization of the CCKBR, unlike that of the CCKAR, depends on the carboxyl terminus of the receptor. These results suggest that, despite the high degree of homology between CCKAR and CCKBR, the structural determinants that mediate the interaction with the endocytic pathway reside in different regions of the receptors.

Mutations of Tyr326 in the beta 2-adrenoceptor disrupt multiple receptor functions

A tyrosine residue at the cytoplasmic end of the seventh transmembrane helix is conserved in many G-protein-coupled receptors. In the human beta 2-adrenoceptor, this tyrosine (Tyr326) has been proposed to be a specific determinant for agonist-induced receptor sequestration. In order to probe its contribution to the sequestration process we have replaced this tyrosine by alanine (Y326A) or phenylalanine (Y326F). Wild-type and mutant receptors were stably expressed in Chinese hamster ovary cells. Agonist-induced sequestration was essentially abolished in Y326A receptors and only slightly reduced in Y326F receptors. However, cells expressing Y326A receptors displayed a high percentage of internal receptors under basal conditions while cells expressing wild-type receptors did not. In addition, high-affinity agonist binding and the ability to activate adenylyl cyclase were markedly reduced in Y326A receptors and slightly reduced in Y326F receptors. We conclude that Tyr326 is required for the functional integrity of the beta 2-adrenoceptor and that it may be involved in multiple agonist-induced effects.

Sympathetic activity in children undergoing balloon valvuloplasty of pulmonary stenosis

We studied the influence of balloon valvuloplasty on alpha- and beta-adrenoceptor densities, plasma catecholamine, and cAMP levels in children and infants with pulmonary stenosis before and 10 min after balloon dilatation, employing as controls children undergoing transcatheter occlusion of patent ductus arteriosus (PDA) with Qp/Qs ratio < 1.5. In the PDA group, the alpha-adrenoceptor density (Bmax) was 3.75 +/- 0.72 fmol/10(7) cells (n = 15) before occlusion and remained unchanged at 3.35 +/- 0.47 fmol 10 min thereafter. In the pulmonary stenosis patients (n = 31), the receptor density was 59% higher (p < 0.05) before, and decreased to PDA levels 10 min after, the procedure. The control beta-adrenoceptor density was 64.8 +/- 11.0 fmol/10(6) cells before, and 71.2 +/- 13.2 fmol 10 min after, occlusion. In the study group, the density was 23% lower (p < 0.07) and increased to the PDA levels 10 min after the dilatation. Compared with the PDA, pre- and postdilatation plasma norepinephrine levels were not significantly changed; epinephrine was slightly elevated before, but increased by 73% after, dilatation; dopamine was 80% (p < 0.05); and cAMP was 37% higher before, and remained elevated at 70 and 23% above the PDA values after, the procedure. Accordingly, alpha-adrenoceptor density is significantly elevated in children with pulmonary stenosis and decreases significantly immediately after balloon valvuloplasty. On the other hand, beta-adrenoceptor density is attenuated and increases toward normal levels after the procedure. The immediate reversal of the receptor levels after balloon valvuloplasty suggests that this procedure exerts acute effects on the sympathetic functional level in this disease.

Thr353, located within the COOH-terminal tail of the delta opiate receptor, is involved in receptor down-regulation

Prolonged exposure to abused drugs such as opiates causes decreased response to the drug; this reduced sensitivity is thought to be due to the loss of receptors, or down-regulation. The molecular mechanism of the opiate receptor down-regulation is not known. In order to address this, we generated a number of mutants of the delta opiate receptor COOH-terminal tail. When expressed in the Chinese hamster ovary cells, both the wild type and the receptor with a deletion of 37 COOH-terminal residues bind diprenorphine with comparable affinities and show similar decreases in cAMP levels in response to D-Ala2, D-Leu5, enkephalin (DADLE). However, the truncated receptor does not show down-regulation from the cell surface upon prolonged exposure (2-48 h) to DADLE. In contrast, both the wild type receptor and the receptor with the deletion of only 15 COOH-terminal residues show substantial down-regulation upon long term DADLE treatment. These results suggest that the region located between 15 and 37 residues from the COOH terminus is involved in the receptor down-regulation. In order to identify residues that play a key role in down-regulation, point mutations of residues within this region were examined for their ability to modulate receptor down-regulation. The receptor with a mutation of Thr353 to Ala does not down-regulate, whereas the receptor with a mutation of Ser344 to Gly down-regulates with a time course similar to that of the wild type receptor. Taken together, these results suggest that the COOH-terminal tail is not essential for functional coupling but is necessary for down-regulation and that Thr353 is critical for the agonist-mediated down-regulation of the delta opiate receptor.

Carboxyl-terminal domains determine internalization and recycling characteristics of bombesin receptor chimeras

To investigate the role of the carboxy terminus in the regulation of the bombesin (BN) receptor, we constructed two chimeric receptors with carboxyl termini transferred from either m3 muscarinic cholinergic (m3 ACh) (BMC) or cholecystokinin A (CCKA) (BCC) receptors and expressed them in Chinese hamster ovary cells. Previous studies showed that agonist treatment caused rapid internalization of CCKA but not m3 ACh receptors in these cells. In the current study we conducted separate analyses of ligand and receptor internalization and analyzed receptor recycling. Ligand internalization was assessed using acid washing. BN and CCKA receptors internalized ligand with 80 +/- 3 and 85 +/- 7% in an acid-resistant compartment at equilibrium. Ligand internalization of chimeric receptors generally assumed the properties of the donor receptors. Thus, BCC receptors internalized ligand to a similar extent as wild-type CCKA receptors (75 +/- 3%), whereas, BMC receptors showed reduced ligand internalization (38 +/- 1%). Receptor internalization was more directly assessed by determining agonist-induced loss of surface binding. BN and CCKA receptors were largely internalized (56 +/- 8 and 50 +/- 7%, respectively). BCC receptors were also extensively internalized (82 +/- 3%). In contrast, BMC receptors were minimally internalized (22 +/- 8%). Receptor recycling was assessed as recovery from agonist induced loss of binding. BN, CCKA, and BMC receptors showed rapid recycling. In contrast, BCC receptors did not recycle. These data indicate that carboxyl-terminal structures determine both internalization of ligand-receptor complexes and subsequent receptor recycling.

Influence of second and third cytoplasmic loops on binding, internalization, and coupling of chimeric bombesin/m3 muscarinic receptors

In order to investigate the molecular basis for differences in the characteristics of bombesin (Bn) and m3 muscarinic cholinergic (m3 ACh) receptors, chimeric Bn receptors possessing cytoplasmic domains from the m3 ACh receptor were produced. The receptors were expressed in CHO-K1 cells and binding, structural, and signal transduction characteristics were analyzed. Cell lines bearing chimeric Bn receptors possessing m3 ACh receptor domains in place of either the second cytoplasmic loop (BM2L), the third cytoplasmic loop (BM3L), or both loops (BM23L) each bound 125I-bombesin with a single affinity that was approximately the same as that of the Bn receptor (5-10 nM). However, Bn receptors possessing the m3 ACh third cytoplasmic loop were severely affected in other respects. Internalization of ligand in Bn and BM2L cells was rapid and extensive (> 80% of bound 125I-bombesin was acid-resistant). In contrast, internalization was dramatically reduced in BM3L and BM23L cells (approximately 20% of bound 125I-bombesin was acid-resistant). In Bn or BM2L cells 10 nM bombesin stimulated approximately 10-fold increases in phosphatidylinositol hydrolysis. Activation of Bn receptors also induced an increase in arachidonic acid release (478 +/- 32% of control, n = 3) and large increases in intracellular Ca2+. In contrast, in BM3L or BM23L cells, bombesin had no significant effect on phosphatidylinositol hydrolysis. Furthermore, BM3L receptor activation did not increase arachidonic acid release. However, BM3L and BM23L cells showed a small increase in intracellular Ca2+ at high concentrations of bombesin. These data indicate that the third cytoplasmic loop alone, or together with the second cytoplasmic loop, was not sufficient to transfer the characteristics of G protein interaction between m3 ACh and bombesin receptors. Furthermore, for the Bn receptor, ligand internalization does, whereas formation of the high affinity binding state does not, appear to require activation of G proteins.

Thyrotropin internalization is directed by a highly conserved motif in the seventh transmembrane region of its receptor

The thyrotropin (TSH) receptor is a member of G protein-coupled seven-transmembrane-segment receptors. It is characterized by a large extracellular domain linked to the seven transmembrane segments and ending with a cytoplasmic tail. Sequence alignment shows that a highly conserved motif, NPXXY where X is any amino acid, exists at the boundary between the seventh transmembrane domain and proximal part of the cytoplasmic tail of virtually all G protein-coupled receptors. This motif has been implicated as an internalization signal for several cell surface receptors, such as the low density lipoprotein (LDL), insulin and insulin-like growth factor-1 (IGF-1) receptors. The potential effects of this motif on the TSH receptor signal transduction and receptor-mediated TSH internalization was analysed by replacement of the tyrosine(678) residue with an alanine residue. This mutation does not impair high affinity TSH binding, but completely abolishes the ability of cAMP response upon TSH stimulation. It also significantly reduces TSH internalization. The role of the cytoplasmic tail of the TSH receptor in receptor-mediated internalization was also assessed. Deletion of up to 56 amino acids from the C-terminus of the cytoplasmic tail enhances TSH internalization as compared to the wild-type receptor. We conclude that tyrosine(678) in the NPXXY motif is required for efficient receptor-mediated TSH internalization and G protein coupling. The cytoplasmic tail of the TSH receptor may contain sequence domains which could modulate the effects of the NPXXY internalization signal.

Carboxyl-terminal domains in the avian beta 1-adrenergic receptor that regulate agonist-promoted endocytosis

Most G protein-coupled receptors, including the mammalian beta 2-adrenergic receptor, are endocytosed to an intracellular, vesicular compartment upon continued exposure to agonist. The long form of the avian beta 1-adrenergic receptor, which contains a carboxyl-terminal 59-amino acid extension, does not undergo agonist-promoted endocytosis. We constructed and expressed turkey beta 1-adrenergic receptor cDNAs with regularly spaced carboxyl-terminal truncations and studied their agonist-promoted endocytosis. Removal of 34-86 amino acids from the carboxyl terminus of the turkey receptor allowed its efficient endocytosis, with optimal endocytosis observed upon removal of 59 residues. Removal of only 18 residues allowed some endocytosis. A receptor that lacks the entire carboxyl-terminal region (124 residues) was not endocytosed. We also constructed a chimeric hamster beta 2-adrenergic receptor with the added 59-residue carboxyl-terminal domain of the turkey receptor. The chimera was not significantly endocytosed. These data indicate that residues 450-465 in the carboxyl-terminal region of the beta 1-adrenergic receptor can act independently to block agonist-promoted endocytosis and that other carboxyl-terminal structures nearer to the seventh membrane span are required for endocytosis.

The carboxyl-terminal anchorage domain of the turkey beta 1-adrenergic receptor is encoded by an alternatively spliced exon

The originally described cDNA of the turkey beta 1-adrenergic receptor encodes a receptor with a carboxyl-terminal, 59-amino acid extension that was not found in several mammalian beta 1-adrenergic receptors. This extension blocks agonist-promoted endocytosis and down-regulation of the receptor. This carboxyl-terminal domain is encoded by an exon distinct from that which encodes the body of the receptor, and the originally described cDNA results from removal of an 849-nucleotide intron. Unspliced mRNA encodes a shorter open reading frame whose translated carboxyl terminus is identical with that of the mammalian beta 1-adrenergic receptors. There is no evidence for other introns in the coding region. Splicing of the intron to produce the non-endocytosing receptor is highest in fetal blood cells, is appreciable in adult brain and heart, and is detectable in other tissues. Thus, different tissues use alternative splicing to express beta-adrenergic receptors that either do or do not endocytose and down-regulate in response to agonist.

Truncation of the thyrotropin-releasing hormone receptor carboxyl tail causes constitutive activity and leads to impaired responsiveness in Xenopus oocytes and AtT20 cells

We studied the activity of a truncated thyrotropin-releasing hormone receptor (TRH-R), which lacks the last 59 amino acids of the carboxyl tail, where Cys-335 was mutated to a stop codon (C335Stop) (Nussenzveig, D. R., Heinflink, M., and Gershengorn, M. C. (1993) J. Biol. Chem. 268, 2389-2392). In Xenopus laevis oocytes expressing C335Stop TRH-Rs, TRH binding was higher, whereas chloride current, 45Ca2+ efflux, and [Ca2+]i responses evoked by TRH were 23, 39, and 21%, respectively, of those in oocytes expressing wild type mouse pituitary TRH-Rs (WT TRH-Rs). In oocytes expressing C335Stop TRH-Rs, basal 45Ca2+ efflux and [Ca2+]i were twice those in oocytes expressing WT TRH-Rs; chelation of Ca2+ caused a rapid increase in holding current, which is consistent with basal activation; and coexpression with other receptors caused inhibition of the responses to the other cognate agonists. In AtT20 pituitary cells stably expressing C335Stop TRH-Rs, thyrotropin-releasing hormone (TRH)-independent inositol phosphate formation was 1.32 +/- 0.11-fold higher, basal [Ca2+]i was 1.8 +/- 0.2-fold higher, and the [Ca2+]i response to TRH was much lower than in cells expressing WT TRH-Rs. We conclude that a TRH-R mutant truncated at Cys-335 exhibits constitutive activity that results in desensitization of the response to TRH.

Endothelin receptor subtypes and their role in transmembrane signaling mechanisms

The endothelins (ETs) are potent vasoactive peptides that appear to be involved in diverse biological actions, for example, contraction, neuromodulation, and neurotransmission, as well as in various pathophysiological conditions, such as renal and heart failure. The diversity of actions of ETs may be explained in terms of (1) the existence of several receptor subtypes and (2) the activation of different signal transduction pathways. This review summarizes the state of the art in this intensively studied field, with particular focus on structural aspects, receptor heterogeneity, coupling of receptors to G-proteins, and signal transduction mechanisms mediated by the activation of ET-receptors.

Agonist-induced transfer of the alpha subunits of the guanine-nucleotide-binding regulatory proteins Gq and G11 and of muscarinic m1 acetylcholine receptors from plasma membranes to a light-vesicular membrane fraction

A clone of a Chinese hamster ovary (CHO) cell line expressing high levels of the human muscarinic M1 acetylcholine (Hm1) receptor undergoes a substantial agonist-specific down-regulation of both Hm1 receptors and the alpha subunits of the guanine-nucleotide-binding (G)-proteins Gq and G11 which is accompanied by a desensitization of inositol-phospholipid-specific-phospholipase-C response [Mullaney, I., Dodd, M. W., Buckley, N. J. & Milligan, G. (1993) Biochem. J. 289, 125-131]. To examine early events in this process, the effect of agonist on subcellular distribution of Gq alpha and G11 alpha and of Hm1 receptors was assessed after short-term and long-term treatment with carbachol. Short-term (30 min) incubation with carbachol (1 mM) induced a simultaneous transfer of a proportion of both Gq alpha and G11 alpha and Hm1 receptors from plasma membranes to distinct light vesicular membranes. The total number of receptors and of Gq alpha and G11 alpha in each cell remained unchanged under these conditions. A similar transfer was noted for the G-protein Gs alpha but not for intrinsic plasma membrane markers. The plasma membrane, as well as light vesicular membrane, pool of Gs alpha subunit was unaffected by further sustained incubation with carbachol (16 h), whereas Hm1 receptors and both Gq alpha and G11 alpha proteins were down-regulated to 25% and 40%, respectively, when compared with untreated cells. Such observations support the idea that down-regulation of both the Hm1 receptor and its associated inositol-phospholipid-specific-phospholipase-C-linked G-proteins is produced by two sequential steps. The first is a transfer of signal-transducing polypeptides from the plasma membrane to a non-plasma membrane light vesicle fraction. The second step is represented by an agonist-specific down-regulation pathway. Both the Hm1 receptor and Gq alpha/G11 alpha appear to follow similar sequestration and down-regulation patterns.

Regulation of agonist binding to A2A adenosine receptors: effects of guanine nucleotides (GDP[S] and GTP[S]) and Mg2+ ion

Adenosine acts as a neuromodulator through at least two receptor subtypes, A1 and A2. A2 receptors have been further divided into A2A (high agonist affinity) and A2B (low agonist affinity) receptors. Both A1 and A2 receptors belong to the superfamily of guanine nucleotide-binding regulatory protein (G protein)-coupled receptors. A Gs protein couples the A2A receptor to the activation of adenylyl cyclase. In order to elucidate the mechanism of coupling between the A2A receptor and Gs, we studied the modulation by guanine nucleotides and divalent cations of agonist binding to the A2A receptor in rat striatal membranes, using [3H]CGS 21680 as a selective high-affinity agonist. We demonstrated that in rat striatal membranes agonist binding to A2A receptors was modulated by guanine nucleotides. Both GDP and GTP inhibited [3H]CGS 21680 binding to rat striatal membranes with about equal potency. The nonhydrolyzable analogs, GDP[S] and GTP[S], were equipotent inhibitors and approx. 100-times more potent than GDP and GTP. Data from competition studies with labeled and unlabeled CGS 21680 when analyzed by nonlinear regression demonstrated the presence of two binding sites in rat striatal membranes with mean values for KD of 5.6 and 343 nM and Bmax of 200 and 942 fmol/mg protein. The high-affinity binding site has the characteristics of the A2A receptor. In the presence both of (0.1 mM) GDP[S] and GTP[S], the KD values for the high-affinity site were increased severalfold, whereas the low-affinity site was no longer detected in filtration assays. Dissociation studies revealed monophasic dissociation curves both in the absence and presence of 0.1 mM GDP[S]. However the K-1 value increased in the presence of guanine nucleotide. We also showed that in bovine striatal membranes agonist binding to A2A receptors was modestly modulated by guanine nucleotides, suggesting differences of receptor Gs-protein-coupling a mechanism in different species. Divalent cations often increase agonist binding to different receptors, whereas Mg2+ ions play a role in regulating the initial steps of G-protein activation. We investigated the effects of divalent cations on [3H]CGS 21680 binding to the A2A receptor and determined the requirement of these cations to obtain the modulation of binding by guanine nucleotides. We found that millimolar concentrations of divalent cations were required to obtain an effective interaction between the A2A receptor and Gs. The high-affinity binding of [3H]CGS 21680 to the A2A receptor in rat striatal membranes was dependent on the presence of Mg2+ ions.(ABSTRACT TRUNCATED AT 400 WORDS)

Endothelin receptors in rat cerebellum: activation of phosphoinositide hydrolysis is transduced by multiple G-proteins

Induction of phosphoinositide hydrolysis in rat cerebellar slices by endothelins (ET-1 and ET-3) and sarafotoxins (SRTX-b and SRTX-c) was demonstrated by measurement of labelled inositol phosphate generation. Pertussis toxin (PT) enhanced the induction of phosphoinositide hydrolysis by all four peptides. The process seems to be mediated by at least two heterotrimeric G-proteins, the one sensitive and the other insensitive to PT. Measurement of the GTPase activity induced in this preparation indicated that phosphoinositide hydrolysis is stimulated via a functional coupling between the endothelin receptor of the ETB-R subtype and a PT-insensitive G-protein family, i.e. Gq/11. The involvement of PT-sensitive G-proteins, i.e. Gi-like and/or Go-like proteins, in the stimulation of phosphoinositide hydrolysis by ETs and SRTXs is discussed.

Solubilization and characterization of the A2-adenosine receptor

Binding of [3H]CGS 21680, an agonist radioligand selective for A2-adenosine receptors (A2AR), to membranes and solubilized preparations from bovine brain striatum revealed labelling of a single high affinity binding state. In membranes, guanine nucleotides per se were ineffective in modulating agonist binding whereas cations, Na+ and Mg++, had distinct effects. The addition of NaCl (200 mM) as well as the Mg(++)-free preparation of membranes led to a significant decrease in binding affinity and the number of binding sites. Moreover, the presence of Na+ was required for the demonstration of a guanine nucleotide effect, i.e. a decrease in maximal binding. Following solubilization, agonist-A2AR interactions were sensitive to guanine nucleotides even in the absence of Na+; guanine nucleotides and Na+ had additive effects in reducing the number of binding sites. Moreover, the effect of GTP was reversible, i.e. binding returned to control levels upon removal of the nucleotide. This suggests the A2AR and its G protein (presumably Gs) are solubilized as a functional unit and may not dissociate even in the presence of GTP following solubilization. We, therefore, believe that a "tight" association exists between receptor and G protein (Gs), and that guanine nucleotides and sodium act at different sites on the R-G complex. Drawing an analogy with similar observations on the avian beta-adrenergic receptor (Hertel et al, J. Biol. Chem. 265:17988-94, 1990; Parker & Ross, J. Biol. Chem. 266:9987-96, 1991) we postulate that the regulatory features of the A2AR can be attributed to a distinct receptor domain that interacts with cellular regulatory elements.

Multisite contacts involved in coupling of the beta-adrenergic receptor with the stimulatory guanine-nucleotide-binding regulatory protein. Structural and functional studies by beta-receptor-site-specific synthetic peptides

Synthetic peptides, 12-22 amino acid residues long, comprising the presumed coupling sites of the beta-adrenergic receptor with the stimulatory guanine-nucleotide-binding regulatory protein (Gs), were examined for their ability to modulate Gs activation in turkey erythrocyte membranes. Three peptides corresponding to the second cytoplasmic loop, the N-terminal region of the third cytoplasmic loop, and the N-terminal region of the putative fourth cytoplasmic loop, compete synergistically with the hormone-stimulated receptor for Gs activation with median effector concentrations of 15-35 microM, or 3-4 microM for combinations of two peptides. One peptide, corresponding to the C-terminal region of the third cytoplasmic loop, carries the unique ability to activate the Gs-adenylate-cyclase complex independent of the signalling state of the receptor. These observations are consistent with a dynamic model of receptor-mediated G-protein activation in membranes, where domains composed of the second, third and fourth intracellular loop of the receptor bind to and are interactive with the G-protein heterotrimer, resulting in ligand-induced conformational changes of the receptor. In response to hormone binding, the extent or the number of sites involved in interaction with Gs may be readjusted using a fourth site. Modulation of coupling sites may elicit congruent conformational changes within the Gs heterotrimer, with qualitatively different effects on GTP/GDP exchange in the alpha subunit of Gs and downstream effector regulation. This model corroborates and expands a similar model suggested for activated rhodopsin-transducin interaction [König, B., Arendt, A., McDowell, J. H., Kahlert, M., Hargrave, P. A. & Hofmann, K. P. (1989) Proc. Natl Acad. Sci. USA 86, 6878-6882].

Immunologic mapping of the amino- and carboxy-termini of the turkey erythrocyte beta-adrenergic receptor: selective proteolysis of both domains

Peptide-directed antibodies were used to map the N- and C-termini of the turkey erythrocyte beta-adrenergic receptor, the full length recombinant receptor expressed in Sf9 cells, and a mutant that terminates after residue 424 (T424). Both forms of the natural receptor (P40 and P50) were proteolytically clipped between residues 419 and 424. P40, but not P50, is also proteolyzed between residues 14 and 28. Truncation mutants, but not full length receptors, also display both large and small forms. The short form of T424 is formed by proteolysis after residue 14, but neither form is proteolyzed in the C-terminal region. The wild type recombinant receptor is not proteolyzed.

Structure and function of G protein coupled receptors

Application of a molecular genetic techniques has allowed the isolation and identification of more than 50 members of the G protein-coupled receptor family. Their specificities range from sensory receptors such as the opsins and odorant receptors through those for the amines, peptides and other small molecules to those for glycoprotein hormones. These studies make it clear that traditional pharmacological methods, often underestimate receptor diversity. G protein-coupled receptors share a common structure consisting of 7 transmembrane alpha helical segments. Receptor structure-function relationships are discussed in the light of results obtained by site-directed mutagenesis and the construction of chimeric receptors. Studies which have allowed the identification of ligand-binding domains, and of sequences defining G protein specificity as well as those involved in receptor desensitization and downregulation are also discussed.