Modulation of beta-adrenergic agonist binding by guanylnucleotides in avian erythrocytes

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.

Gi affects the agonist-binding properties of beta-adrenoceptors in the presence of Gs

Pertussis-toxin-catalyzed ADP-ribosylation of Gi in S49 membranes, but not in S49AC- membranes, which lack Gs, induces a threefold reduction of isoproterenol affinity to the beta-adrenoceptors. A similar treatment of turkey erythrocyte membranes, which are devoid of functional Gi, has no effect on beta-agonist affinity to their beta-adrenoceptors. Non-hydrolyzable analogs such as GTP[S] induce a larger decrease in beta-adrenoceptor affinity in S49 cells towards the agonist isoproterenol as compared to pertussis-toxin-catalyzed ADP-ribosylation of Gi. These results suggest that Gi affects beta-adrenoceptor affinity to its agonist and that this interaction requires the presence of Gs. It seems, therefore, that Gi physically interacts with Gs to exert its effects on the receptor and probably on adenylate cyclase as well. Our ability to detect (a) the effect of pertussis-toxin-catalyzed ADP-ribosylation in S49 cells on beta-agonist affinity and (b) the quantitative difference between the effect of pertussis toxin (approx. threefold) and GTP[S] (fivefold to sevenfold) depends on the use of a simple but rigorous method to study in detail the affinity of beta-agonists to their receptors. This method seems to be superior to the analysis of displacement curves as a means to examine receptor-ligand interactions.

Differential sensitivity of basal and opioid-stimulated low Km GTPase to guanine nucleotide analogs

In membranes derived from NG108-15 cells, the opioid peptide [D-Ala2,D-Leu5]enkephalin (DADLE) stimulates a low Km GTPase. The nucleotide analogs guanosine 5'-O-(2-thio)diphosphate (GDP beta S), guanosine 5'-(beta,gamma-imido)triphosphate [Gpp(NH)p] and guanosine 5'-O-(3-thio)-triphosphate (GTP gamma S) inhibit the basal enzymatic activity with the order of potency GTP gamma S greater than Gpp (NH)p greater than GDP beta S. In the presence of DADLE, the inhibition isotherms of GDP beta S and Gpp(NH)p are shifted to the right five- and fourfold, respectively, compared to the inhibition observed in the absence of DADLE. In contrast, the IC50 of GTP gamma S for inhibiting the enzyme is reduced by 55% in the presence of the opioid. Both Gpp(NH)p and GTP gamma S produce a concentration-dependent increase in the Km(app) of GTPase, without affecting its Vmax, indicating a competitive inhibition. However, the replots of Km(app) versus inhibitor concentration are hyperbolic, suggesting a partial type of inhibition. Both Gpp(NH)p and GTP gamma S, but not GTP, induce an increase in the EC50 of DADLE for stimulating GTPase. These findings indicate that the basal and the opioid-stimulated low Km GTPase differ in their respective sensitivities to inhibition by guanine nucleotide analogs.

Message transmission: receptor controlled adenylate cyclase system

The adenylate cyclase system is composed of an activating hormone or neurotransmitter (H), its receptor (R), the guanosine triphosphate (GTP) binding protein (Gs), and the catalytic unit (C). The activation of the receptor R involves a transient change in conformation, from a loose binding of the neurotransmitter H to an extremely tight interaction, termed locking. The system is regulated in the activation steps and also by three deactivation processes. A guanosine triphosphatase activity is built into the Gs protein so that the active GsGTP has only a limited lifetime during which it is able to activate C. In addition, the continued occupation of R by H causes desensitization of R. Finally, there are inhibitory receptors, such as alpha-adrenergic and opiate receptors, which inhibit the adenylate cyclase by way of a specific GTP binding protein (Gi). Yet to be determined are the conformational transformations of pure R on binding of an agonist or a partial agonist; the genes that code for the many different receptors that activate the adenylate cyclase, and the possibility that the G components interact with systems in the cell other than the adenylate cyclase.

Trapping of the beta-adrenergic receptor in the hormone-induced state

Isoproterenol and other agonists readily dissociate from the beta-adrenergic receptor in turkey erythrocyte membranes. However, when a low concentration of deoxycholate is added, the receptor locks the prebound agonist; i.e., the rate of dissociation of the prebound agonist decreases drastically. The dissociation of prebound antagonists is slightly increased by deoxycholate. Locking, which is thus agonist specific, occurs in the cold, is reversed when detergent is removed from the membranes, and appears not to require the guanyl nucleotide binding protein of the adenylate cyclase system. It is suggested that this induced fit of a receptor to an agonist represents the specific conformational response that normally propagates in the receptor molecule in its interaction with the next component along the pathway of signal transmission.

Agonist binding to mammalian beta 1 and beta 2 adrenoceptors: modulation by guanine nucleotides and magnesium

Agonist interaction with beta 1 and beta 2 adrenoceptors in rat and rabbit lung has been examined using ligand binding techniques and the results are discussed in relation to current models of beta-adrenoceptor-adenylate cyclase coupling. Agonist binding has been assessed by examining the ability of isoprenaline or salbutamol to displace the labelled antagonist 3H-dihydroalprenolol (3H-DHA) from specific receptor sites. Beta 1 and beta 2 adrenoceptors, even within the same tissue, exhibited different ion and temperature requirements for guanine nucleotide modulation of agonist binding. Thus, in contrast to the situation at beta 2 sites, agonist binding to beta 1 adrenoceptors was only sensitive to GTP if incubations were performed at physiological temperatures in the presence of Mg++ ions. These findings suggest that there may be different receptor-effector coupling relationships between the two beta-adrenoceptor subtypes.

Guanine nucleotide mediated desensitization of adenylate cyclase in cell free preparations from a Leydig cell tumour

Cell free desensitization of a tumour Leydig cell plasma membrane adenylate cyclase has been demonstrated in the presence of guanine nucleotides. In experiments in which the membranes were pre-incubated with various nucleotides and LH, it was shown that this decreased adenylate cyclase activity was dependent on the presence of GTP and occurred both in the presence and absence of ATP. While pre-treatment with LH alone appeared to enhance subsequent adenylate cyclase activity, this hormone was able to potentiate the desensitizing effect of GTP. The desensitizing effect of GTP was not inhibited by sodium fluoride. In contrast, the GTP analogue p(NH)ppG (guanosine 5'beta, gamma-imido triphosphate) caused a persistent activation of the adenylate cyclase. GMP and guanosine also initially inhibited the adenylate cyclase activity, but this was entirely reversed by p(NH)ppG plus LH. GDP in addition to GTP caused desensitization but this was only partially reversed by p(NH)ppG plus LH. It is proposed that in similarity with the ovary (Bockaert et al. 1976; Ezra & Salomon 1982a) desensitization of Leydig tumour cell plasma membrane adenylate cyclase may involve a GTP-mediated phosphorylation step.

Activation of adenylate cyclase in bovine corpus-luteum membranes by human choriogonadotropin, guanine nucleotides and NaF

1. Preincubation of luteal membranes with human choriogonadotropin results in the formation of an activated state of adenylate cyclase which is not reversed by washing and which is limited only by the absence of guanine nucleotides, whereas preincubation with GTP yields only a partially activated adenylate cyclase which requires the presence of both GTP and human choriogonadotropin during assay to demonstrate maximal activity. 2. Preincubation of luteal membranes with GTP and human choriogonadotropin does not lead to a synergistic increase in wash-resistant activity. 3. Luteal membranes that had been preincubated with GTP and hormone exhibited a decreasing rate of cyclic AMP synthesis during the adenylate cyclase assay incubation; addition of GTP during the assay incubation reversed the decrease. 4. Membranes that had been preincubated in the absence of guanine nucleotide and hormone showed a ;burst' phase of cyclic AMP synthesis when GTP was present in the assay incubation and a ;lag' phase with p[NH]ppG (guanosine 5'-[beta,gamma-imido]triphosphate) present in the assay. The presence of human choriogonadotropin with either nucleotide in the assay incubation eliminated the curvatures in plots observed with guanine nucleotides alone. 5. Luteal adenylate cyclase was persistently activated by preincubation with p[NH]ppG alone or in combination with human choriogonadotropin; the activation caused by p[NH]ppG alone was still increasing after 70min of preincubation, whereas that caused by p[NH]ppG in the presence of hormone was essentially complete within 10min of preincubation. 6. Luteal adenylate cyclase that had been partially preactivated by preincubation with p[NH]ppG was slightly increased in activity by the inclusion of further p[NH]ppG in the adenylate cyclase assay incubation, but more so with p[NH]ppG and hormone. Human choriogonadotropin alone caused no further increase in the activity of the partially stimulated preparation unless p[NH]ppG was also added to the assay incubation. 7. GTP decreased the activity of adenylate cyclase in membranes that had been partially preactivated in the presence of p[NH]ppG; the decrease in activity was greater when GTP and hormone were present simultaneously in the assay. 8. The results indicate that stable activation states of adenylate cyclase can be induced by preincubation of luteal membranes in vitro with human choriogonadotropin or p[NH]ppG, and that in the presence of p[NH]ppG the hormone may accelerate events subsequent to guanine nucleotide binding. Stable activation of luteal adenylate cyclase by prior exposure to GTP is not achieved. The involvement of GTPase activity and of hormone-promoted guanine nucleotide exchange in the modulation of luteal adenylate cyclase activity is discussed.

Functional desensitisation of beta-adrenergic receptors of avian erythrocytes by catecholamines and adenosine 3',5'-phosphate

Prolonged exposure to beta-adrenergic agonists of pigeon erythrocytes causes a reversible loss (70%) of catecholamine-stimulated adenylate cyclase activity without reduction in the number of beta-adrenergic receptors. In addition a less pronounced decrease in non-stimulated and NaF-stimulated adenylate cyclase activity (15-22%) is observed, appearing at different agonist concentrations and at a different rate. Dibutyryladenosine 3',5'-phosphate and the phosphodiesterase inhibitor methylisobutylxanthine partially mimick the action of the beta-adrenergic agonist, thus pointing to a possible role of adenosine 3',5'-phosphate in establishing desensitization. When adenylate cyclase from desensitized cells is stimulated with 5'-guanylyl-imidodiphosphate in the presence or absence of catecholamines the lag period preceding the attainment of maximal activity is extended. Likewise the rate of reversal by GTP or GTP of persistent activation of adenylate cyclase is slowed down. This is therefore interpreted to mean that the loss in hormonal stimulation on treatment of pigeon red blood cells with beta-adrenergic agonists is due to a delayed exchange of GDP against GTP on the regulatory GTP-binding protein. Furthermore, we conclude that events causing the refractory state in avian erythrocytes should occur at a site distal to the beta-adrenergic receptor.