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PubMed2000

Multiple domains interacting with Gs in the porcine calcitonin receptor.

The molecular basis for Gs activation by the calcitonin (CT) receptor was investigated. Based upon the analysis of conserved regions in G protein-coupled receptors, two nonoverlapping regions in the heptahelical porcine CT receptor (CTR) were selected as candidate Gs-interacting domains: the third intracellular loop residues 327-344 (KLKESQEAESHMYLKAVR, P3 region) and the C-tail residues 404-418 (KRQWNQYQAQRWAGR, P4 region). To assess their Gs-interacting function, we expressed these sequences in hybrid insulin-like growth factor II receptors in which the receptor native Gi-interacting domain was converted to CTR sequences. In COS cells transfected with either P3- or P4-substituted hybrid receptor, membrane adenylyl cyclase activity significantly increased. The up-regulated activity of cAMP was confirmed by measuring the transcriptional activity of the cAMP response element in cells expressing either hybrid receptor. A mutant CTR lacking the P4 region maintained positive cAMP response but with an attenuated maximal capacity to produce cAMP. In contrast, we could not assess the function of the P3 region using a conventional deletion method, as CT bound poorly to cells transfected with either of the two P3-deficient CTRs (one lacking the P3 region and the other lacking P3 but having the P3 sequence in reverse orientation). These data suggest that the third intracellular loop and the C-tail in CTR have domain-specific roles in Gs activation and that the hybrid receptor approach used here, combined with a conventional mutagenesis approach, is useful for intact cell analysis and functional dissection of G protein-coupled receptors.

Adenylyl Cyclases↗

PubMed2005

Production of a monoclonal antibody specific for high molecular weight glutenin subunits (HMW-GS) in wheat and its antigenic determinant.

Wheat high molecular weight glutenin subunits (HMW-GS) 1Bx14 and 1By15 isolated by preparative SDS-PAGE are used as antigen to immunize BALB/c mice. Subcutaneous inoculation of the antigen is performed. The intra-peritoneal injection is completed 3 days before fusion with myeloma cell (SP2/0) via PEG-1500. The fusion cells are selected by indirect enzyme-linked immuno-sorbent assay (ELISA). Positive hybrid cells are further verified three times by limit dilution of the culture cells. A hybridoma cell line is successfully obtained. The monoclonal antibody belongs to IgG1 subclass. In immunoblotting, the antibody binds to all HMW-GS of T. aestivum cultivars, but does not bind to other storage proteins in seeds of wheat. This result is consisting with the high homology in amino acid sequences among the HMW glutenin subunits in wheat. The antibody also binds to HMW-GS storage proteins in Aegilops squarrosa and T. durum (durum wheat). Furthermore, it also binds to HMW storage proteins in Secale cereale (rye), Hordeum vulgare (barley). However, it never binds seed storage proteins in other cereals such as maize, oat, rice, foxtail millet, sorghum etc. The antigen determinant recognized by the antibody has been located within hexapeptide [PGQGQQ] or / and nonapeptide [GYYPTSPQQ] in the central repetitive region of HMW-GS.

Alleles↗

PubMed1994

Cholera toxin can ADP-ribosylate Gs as well as Gi in ACTH-unresponsive human adrenocortical cancer.

It is well known that cholera toxin (CT) stimulates ADP-ribosylation of Gs and also pertussis toxin (PT) does Gi. Each GTP-binding protein has its own action in the regulation of adenylate cyclase. A human non-functioning adrenocortical cancer tissue showed an unresponsiveness in adenylate cyclase to ACTH although ACTH and CT activated adenylate cyclase in a non-functioning adrenal adenoma tissue. CT ADP-ribosylated 43 kDa protein of the plasma membrane of the cancer tissue while CT and PT could ADP-ribosylate 43 kDa and 38 kDa protein in the adenoma tissue, respectively. Immunoblotting analysis of the cancer tissue demonstrated that 40 kDa protein was detected by anti-Gs antibody as well as by anti-Gi antibody. The present experiments demonstrated that CT could ADP-ribosylate Gs which has stimulatory action on adenylate cyclase and also Gi which inhibits adenylate cyclase. Thus it is suggested that CT can activate the ADP-ribosylation of Gs and also Gi in a human adrenocortical cancer tissue, partly resulting in abnormal regulation of adenylate cyclase which may be crossly related to ACTH-unresponsiveness.

Adenoma↗

PubMed1998

Toxic thyroid adenoma: absence of DNA mutations of the TSH receptor and Gs alpha.

DNA point mutations of the TSH receptor and of the alpha subunit of the stimulatory GTP-binding protein (Gs alpha) have been suggested as major causes of hyperfunctioning thyroid adenomas. However, significant differences in the prevalence of these mutations (from 0.3 to 84%) have been found in different populations. The present study was designed to evaluate further the presence of mutations in discrete fragments of cDNA encoding critical regions of the TSH receptor and of the Gs alpha involved in signal transduction and cAMP production. Genomic DNA extracted from 15 thyroid adenomas and surrounding quiescent thyroid tissues was used as a template to amplify four DNA fragments of TSH receptor and one DNA fragment of Gs alpha. TSH receptor and Gs alpha DNAs were analyzed by a number of techniques. We did not detect any mutations (new or previously described) in our patients. These results confirm that the causes of solitary toxic adenomas are protean, and only some of them may be somatic DNA point mutations. Since the clinical features of solitary toxic adenoma are homogeneous, it could be important to establish the specific molecular defect underlying each case, in order to follow up the patients and to assess their clinical evolution.

Adenoma↗

PubMed1993

Functional domains of the Gs alpha subunit: role of the C-terminus in the receptor-dependent and receptor-independent activation.

We have developed a rapid and simple model for studying functional domains of Gs alpha subunit, the GTP binding protein involved in adenylyl cyclase activation. Cyc- membranes prepared from a variant S49 cell line which does not express the alpha subunit of Gs are reconstituted by the in vitro translated Gs alpha subunit. Since the messenger RNA used for in vitro translation is generated from in vitro transcription of the cDNA encoding Gs alpha subunit, it is possible to introduce genetic modifications at the nucleotide level and analyze their consequences at the amino-acid level on the functional properties of the protein. We have constructed mutated alpha chains which correspond to various deletions of the carboxy-terminal region. Removal of the 9 carboxy-terminal residues uncoupled the alpha subunit from the receptor whereas deletion of the 26 carboxy-terminal residues blocked any activation induced either in a receptor-dependent or in a receptor-independent manner.

Adenosine Diphosphate Ribose↗

PubMed1992

The guanine nucleotide-binding protein Gs activates a novel calcium transporter in Xenopus oocytes.

Calcium influx is an important aspect of receptor-mediated signal transduction, yet limited information is available regarding the pathways of calcium influx into nonexcitable cells. We show that treatment of oocytes from Xenopus laevis with cholera toxin, a potent activator of the guanine nucleotide-binding protein Gs, specifically stimulates a sustained inward whole cell flux of calcium through a novel membrane transporter. The calcium is distributed into a mobilizable pool. The flux is voltage-independent and is completely and specifically blocked by microinjection of oocytes with an antiserum directed against Gs alpha. The flux is not activated by treatment of the cells with forskolin or 8-bromo-cyclic adenosine monophosphate indicating that the effect of Gs alpha on the transporter occurs independently of adenylylcyclase activation. Transporter activity is insensitive to benzyl amiloride, does not require a sodium gradient, and is not stimulated by external calcium, indicating that it is not a sodium-calcium exchanger. The Gs-activated flux is dramatically potentiated by lanthanum ion and other trivalent cations but not by any of six divalent cations that were tested; all other known calcium channels and exchangers are, in contrast, potently blocked by lanthanum. The divalent cation cadmium inhibited transporter activity in a concentration-dependent manner. This novel calcium transporter may be important for receptor-mediated calcium influx in the oocyte and perhaps other cell types.

Animals↗

PubMed1992

Beta 1- and beta 2-adrenergic receptors display subtype-selective coupling to Gs.

beta-Adrenergic receptor (beta AR) subtypes differ in their affinities for some agonists and antagonists and thus may potentially impart different cellular effects based on this ligand-binding specificity. However, the possibility that there may be subtype-specific events subsequent to ligand binding has not been evaluated extensively. In particular, although beta ARs stimulate adenylyl cyclase by coupling to the guanine nucleotide-binding protein Gs, no studies have directly assessed the coupling efficiencies among isolated beta AR subtypes. We, therefore, permanently transfected the mammalian fibroblast cell line CHW-1102 with beta 1- or beta 2AR cDNAs and studied the coupling characteristics of these two receptor subtypes, each expressed at approximately 335 fmol/mg of protein. Both receptors mediated equivalent maximal increases in adenylyl cyclase activities (6.63 +/- 1.85-fold for beta 1AR versus 6.10 +/- 0.53-fold for beta 2AR; p = not significant). However, the isoproterenol dose-response curves for the beta 2AR were shifted to the left, compared with those for the beta 1AR (EC50 of 52.3 +/- 2.87 nM and 191 +/- 10.5 nM, respectively; p less than 0.05), resulting in an approximately 4-fold greater potency for the beta 2AR versus the beta 1AR. Thus, at the submaximal isoproterenol concentration of 30 nM, the beta 2AR stimulated adenylyl cyclase approximately 50% more than did the beta 1AR. This finding was not due to a difference in the affinities of isoproterenol for these receptors, which were found to be the same, as determined by competition binding studies with 125I-cyanopindolol in the presence of GTP. The ability of beta 1- and beta 2ARs to form the high affinity ternary complex was assessed in agonist competition studies without guanine nucleotide. We found that, whereas the proportion of receptors in the high affinity state was equivalent between the two receptor subtypes, the affinity of this state for isoproterenol was approximately 5-fold greater for the beta 2AR, compared with the beta 1AR (KH for beta 2AR, 11.8 +/- 3.1 nM; KH for beta 1AR, 61.7 +/- 18.3 nM; p less than 0.05). In addition, we examined physical and functional coupling of beta 1- and beta 2ARs to Gs using the agonist epinephrine, which also has equal binding affinity for both receptor subtypes. As with isoproterenol, epinephrine was more potent in stimulating adenylyl cyclase and promoted a higher affinity ternary complex for the beta 2AR. Thus, a greater degree of both physical and functional agonist-promoted coupling occurs between Gs and beta 2AR, compared with beta 1AR. We conclude that coupling to Gs by beta 1- and beta 2ARs is subtype selective and is a potentially important distinguishing feature among these members of the beta AR family.

Adrenergic beta-Agonists↗

PubMed1991

Expression and analysis of Gs alpha mutants with decreased ability to activate adenylylcyclase.

We have constructed mutants of the alpha subunit of Gs in an attempt to identify sites in the protein that are important for its interaction with adenylylcyclase. Some residues specific for those G proteins that activate adenylylcyclase were replaced with residues characteristic of Gi alpha. Mutant proteins were expressed in Escherichia coli, and two of these were purified to homogeneity and characterized in detail. Mutation of trp263, leu268, or arg269 caused a significant loss of the capacity of Gs alpha to stimulate adenylylcyclase, and the triple mutant had less than 1% of the ability of wild type Gs alpha to activate the enzyme. Guanine nucleotide binding and GTP hydrolysis by the mutant proteins were unaltered, as was guanosine 5'-3-O-(thio)triphosphate-induced enhancement of intrinsic tryptophan fluorescence. Mutant proteins also appeared to have a reduced affinity for the G protein beta gamma subunit complex. Secondary structure analysis and comparison with the structure of p21ras suggests that the region of Gs alpha that we have identified is part of a loop that may be involved in interaction of the protein with adenylylcyclase. Although these residues are essential for activation of adenylylcyclase, they are not sufficient to do this when placed in the context of another G protein alpha subunit.

Adenylyl Cyclases↗

PubMed1991

Coupling of a mutated form of the human beta 2-adrenergic receptor to Gi and Gs. Requirement for multiple cytoplasmic domains in the coupling process.

We constructed five genes encoding mutant human beta 2-adrenergic receptor sequence (beta 2AR) which contained 12-22 amino acid substitutions with corresponding sequence from the human alpha 2AAR in order to assess the receptor domains involved in Gs versus Gi recognition and coupling. Mutant beta 2AR with substitutions in the N (S1)- and C-terminal (S2) portions of the third intracellular loop, the proximal cytoplasmic tail (S3), and two combinations thereof (S2,3 and S1,2,3), were stably expressed in Chinese hamster fibrobasts (CHW-1102), as were the human beta 2AR and alpha 2AAR at comparable receptor levels. All mutant receptors with S2 substitutions (i.e. S2, S2,3, S1,2,3) were significantly (approximately 85%) uncoupled from Gs. Upon exposure to pertussis toxin, which uncouples receptors from Gi, S1,2,3 exhibited a 526 +/- 99% increase in agonist-stimulated adenylylcyclase activity compared with a 59 +/- 13% increase with the wild type receptor. This enhanced ability of S1,2,3 to interact with Gs following pertussis toxin treatment indicates that, in the absence of toxin exposure, substantial coupling occurs between the mutant receptor and Gi. Mutant beta 2AR bearing only one or two alpha 2AAR-substituted sequences showed no such enhancement. Forskolin-stimulated enzyme activities were increased by pertussis toxin treatment to similar degrees in all clones examined, indicating that the observed effects are confined to the receptor-mediated pathway. In the absence of GTP, competition binding experiments with S1,2,3, beta 2AR and alpha 2AAR revealed that approximately 40-50% of the receptors formed a high affinity binding state for agonist. Pertussis toxin treatment markedly reduced this to approximately 19% with S1,2,3, while having no effect on beta 2AR and completely eliminating high affinity agonist binding to alpha 2AAR. These results suggest that S1,2,3 interacts with Gi as well as Gs, and that receptor:G protein coupling requires the concerted participation of multiple cytoplasmic receptor domains.

Adenosine Diphosphate Ribose↗

PubMed1989

Coupling of the stimulatory GTP-binding protein Gs to rat synaptic membrane adenylate cyclase is enhanced subsequent to chronic antidepressant treatment.

In an attempt to resolve a unified postreceptor mechanism of action for antidepressant therapy, rats were treated with amitriptyline, desipramine or iprindole. Chronic treatment with these antidepressant drugs increased guanylylimidodiphosphate-[Gpp(NH)p-], NaF-, or forskolin-activated adenylate cyclase in synaptic membranes prepared from cerebral cortexes of treated rats. Gpp(NH)p-dependent inhibition of adenylate cyclase was unaffected. Maximal binding of the photoaffinity GTP analog azidoanilido-GTP (AAGTP) to the adenylate cyclase stimulatory (Gs alpha) and inhibitory (Gi alpha) G proteins was unaffected by antidepressant treatment. The chemical elimination of Gs (low pH treatment) eliminated all differences between control and antidepressant-treated groups. Further, nonneural tissues from rats receiving chronic antidepressants showed no changes in adenylate cyclase activity or AAGTP binding. The results of these studies suggest that chronic antidepressant administration promoted increased coupling between Gs and catalytic unit of adenylate cyclase. Thus, the molecular locus of antidepressant action may reside at the stimulatory GTP-binding protein, Gs.

Adenylyl Cyclase Inhibitors↗

PubMed1987

Mutations that uncouple the beta-adrenergic receptor from Gs and increase agonist affinity.

The deletion of residues 239-272 from the hamster beta-adrenergic receptor resulted in a loss of the ability of the receptor, expressed in mouse L cells, to stimulate adenylate cyclase (Dixon, R. A. F., Sigal, I. S., Rands, E., Register, R. B., Candelore, M. R., Blake, A. D., and Strader, C. D. (1987) Nature 326, 73-77). This mutant receptor (D(239-272)beta AR) bound the agonist isoproterenol with a single class of binding sites, in contrast to the wild-type beta-adrenergic receptor, which exhibited two classes of agonist affinity sites. We now report that the affinity of D(239-272)beta AR for isoproterenol is relatively insensitive to detergent solubilization or to treatment with either GTP or NaF, indicating the absence of a receptor-Gs interaction. Whereas deletions within the region of amino acids 229-258 did not reduce the ability of the receptor to couple to Gs or to stimulate adenylate cyclase, the deletion of either of the regions 222-229 or 258-270 resulted in receptors which were unable to couple to Gs. The affinities of D(222-229)beta AR, D(239-272)beta AR, and D(258-270)beta AR toward isoproterenol were greater than that observed for the low affinity, uncoupled form of the wild-type receptor. These results suggest a role for the regions of the beta-adrenergic receptor encompassing amino acids 222-229 and 258-270, which are predicted to form amphiphilic helices, in the agonist-promoted activation of Gs.

Adenylyl Cyclases↗

PubMed1987

The hydrophobic tryptic core of the beta-adrenergic receptor retains Gs regulatory activity in response to agonists and thiols.

The function of structural domains of the beta-adrenergic receptor were probed by studying the ability of tryptic fragments of the receptor to catalyze the binding of guanosine-5'-O-(3-thiotriphosphate (GTP gamma S) to the GTP-binding regulatory protein, Gs. beta-Adrenergic receptor purified from turkey erythrocytes was treated with trypsin under nondenaturing conditions. Such treatment decreased beta-adrenergic ligand binding activity by only 15-25%. Active components of the limit digest were repurified by affinity chromatography on alprenolol-agarose and then reconstituted with purified Gs into unilamellar phospholipid vesicles. After reconstitution, the proteolyzed receptor was able to catalyze agonist-stimulated binding of GTP gamma S to Gs at a rate and extent equivalent to that of the nonproteolyzed receptor. The proteolyzed receptor was also partially activated upon reduction by dithiothreitol, as previously reported for the intact receptor (Pedersen, S.E., and Ross, E.M. (1985) J. Biol. Chem. 260, 14150-14157). The repurified, active tryptic digest contained two detectable peptides. One, of approximately 2 X 10(4) Da, contained either four or five of the amino-terminal membrane-spanning domains plus the intervening hydrophilic loops but not the amino-terminal extracellular, glycosylated peptide. The second, of 9,000-10,000 Da, was composed essentially of the two carboxyl-terminal membrane-spanning domains and the intervening extracellular, hydrophilic loop. These data indicate that most of the large intracellular hydrophilic loop and the hydrophilic, carboxyl-terminal region of the receptor are not necessary for the agonist-stimulated regulation of Gs.

Adrenergic beta-Agonists↗

PubMed1985

[Characteristics of the anti-arrhythmic action of 3-carbethoxyamino-5-dimethylaminoacetyl-imino-dibenzyl hydrochloride (Bonnecor, AWD 19-166, GS 015)].

The action of GS 015, a substance out of the series of the novel 5-(dialkyl-amino-acyl)-3-carbalkoxyamino-10,11-dihydro-5H-dibenz[b ,f]azepines, on the alteration of the bioelectric activity in the sympathetic nerves of the heart in case of disturbed cardiac rhythm and ventricular fibrillation was tested on anaesthetized cats. The arrhythmias were induced by two different methods: Artificial electric induction by high-frequency electrical stimulation of the ventricles, occlusion and reperfusion of the anterior descending branch of the arteria coronaria sinistra. GS 015 decreases the tonic bioelectric activity in the sympathetic nerves of the heart and prevents their activation in case of disturbed rhythm which is provoked or by electric stimulation or by occlusion of the coronary artery. With an electric stimulation of the ventricles GS 015, given at doses of 0.5, 1.0, and 2.0 mg/kg, decreases the maximally reproducible frequency in dependence on the dose, which corresponds to an increase of the effective refractory period. At the same time the fibrillation threshold of the ventricles is enhanced in a very intense and long-lasting manner excelling considerably the action of the reference preparations Ethmozin and lidocaine. It may be concluded from the present results that the antiarrhythmic and antifibrillatory effects of GS 015 are basing not only on its immediate action on the myocardial cell but also on a decrease of the activity in the sympathetic nerves of the heart.

Animals↗

PubMed1995

Hamster alpha 1B-adrenergic receptor directly activates Gs in the transfected Chinese hamster ovary cells.

A prototypic Ca(2+)-mobilizing hormone receptor, alpha 1-adrenergic receptor (alpha 1AR), stimulates cAMP accumulation. The mechanism underlying this phenomenon was previously suggested to be secondary to phosphatidylinositol hydrolysis-protein kinase C activation in some cells. We transfected Chinese hamster ovary (CHO)-K1 cells with hamster alpha(1B)AR cDNA and isolated cells stably expressing alpha(1B)AR (CHO alpha 1B cells). We investigated the molecular mechanism underlying the alpha 1AR-mediated cAMP production in the CHO alpha 1B cells. Norepinephrine (NE) stimulated intracellular calcium mobilization and cAMP production through alpha(1B)AR. Pretreatment with a phospholipase C inhibitor, U-73,122 (10 microM), abolished the NE-induced intracellular calcium response, whereas it did not affect the NE-stimulated cAMP production. Treatment with various agents (protein kinase C inhibitors, calcium ionophore, cyclo-oxygenase inhibitor, or pertussis toxin) had little effect on the NE-induced cAMP production. The parent CHO and CHO alpha 1B cells contained similar amounts of Gs alpha (42 and 45 kDa, respectively), as detected with immunoblot analysis, and exhibited similar extents of cAMP synthesis with cholera toxin and forskolin. Adenylyl cyclase activity in the CHO alpha 1B cell membranes was also enhanced by NE. Furthermore, incubation of CHO alpha 1B cell membranes with antiserum directed against the carboxyl-terminal portion of Gs alpha inhibited the NE-stimulated adenylyl cyclase activity. Taken together, the results indicate that the alpha(1B)AR-mediated cAMP synthesis in CHO alpha 1B cells reflects direct stimulation of Gs-adenylyl cyclase. Therefore, the alpha 1AR-stimulated cAMP production observed in some native tissues may involve the multiple mechanisms of the direct activation of Gs-adenylyl cyclase and a secondary effect through activation of phosphatidylinositol hydrolysis.

Adenylate Cyclase Toxin↗

PubMed1994

Synergistic activation of the type I adenylyl cyclase by Ca2+ and Gs-coupled receptors in vivo.

The type I adenylyl cyclase is directly stimulated by Ca2+ and calmodulin in vivo (Choi, E. J., Wong, S. T., Hinds, T. R. and Storm, D. R. (1992) J. Biol. Chem. 267, 12440-12442; Wu, Z., Wong, S. T., and Storm, D. R. (1993) J. Biol. Chem. 268, 23766-23768). In this study, we examined the sensitivity of the type I adenylyl cyclase expressed in HEK-293 cells to beta-adrenergic agonists or glucagon when intracellular Ca2+ was elevated by Ca2+ ionophore or carbachol. Although previous studies have shown that this enzyme can be directly stimulated by activated Gs in vitro, we demonstrate that it is not stimulated by Gs-coupled receptors in vivo. However, the enzyme was stimulated by Gs-coupled receptors in vivo when it was activated by intracellular Ca2+. For example, the Ca2+ ionophore A23187 stimulated the enzyme 3 +/- 0.5-fold (n = 9) and isoproterenol alone did not stimulate the enzyme, but the combination of the two stimulated type I adenylyl cyclase 13 +/- 2-fold (n = 9) in vivo. Similarly, 500 nM glucagon alone did not stimulate the enzyme but the combination of A23187 and glucagon activated the enzyme 90 +/- 8-fold (n = 4). Synergistic stimulation of type I adenylyl cyclase activity was also obtained with combinations of carbachol and isoproterenol or glucagon. This phenomenon was not observed with a mutant enzyme that is insensitive to Ca2+ and calmodulin, suggesting that conformational changes caused by binding of calmodulin to the type I adenylyl cyclase enhance binding or coupling to activated Gs. These data illustrate that this adenylyl cyclase can couple Ca2+ and neurotransmitter signals to generate optimal cAMP levels, a property of the enzyme that may be important for its role in learning and memory in mammals.

Adenylyl Cyclases↗

PubMed1994

GS-X pump is functionally overexpressed in cis-diamminedichloroplatinum (II)-resistant human leukemia HL-60 cells and down-regulated by cell differentiation.

The ATP-dependent glutathione S-conjugate export pump, named GS-X pump, has been shown to eliminate a potentially cytotoxic glutathione-platinum (GS.Pt) complex from tumor cells, thereby modulating glutathione (GSH)-associated resistance to cis-diamminedichloroplatinum(II) (cisplatin) (Ishikawa, T., and Ali-Osman, F. (1993) J. Biol. Chem. 268, 20116-20125). The present study provides evidence that the GS-X pump is functionally overexpressed in cisplatin-resistant human promyelocytic leukemia HL-60 (HL-60/R-CP) cells, in which the cellular GSH level was substantially enhanced. Indeed, the rate of ATP-dependent transport of the GS.Pt complex, measured with plasma membrane vesicles, was about 4-fold greater in HL-60/R-CP cells than in HL-60 cells. Three membrane proteins with apparent molecular masses of 200, 110, and 70 kDa were overexpressed in HL-60/R-CP cells, whereas P-glycoprotein (MDR1) was not immunologically detected in the membrane preparations from resistant and sensitive cells. Unlike in HL-60 cells, increased numbers of intracellular vesicles were observed in the cytoplasm of HL-60/R-CP cells. Fluorescence microscopy with syn-(CICH2,CH3)-1,5-diazabicyclo-[3.3.0]-octa-3,6-dione-2,8-dione (monochlorobimane) revealed that the fluorescent glutathione S-conjugate accumulated in intracellular vesicles of the cisplatin-resistant cells in an energy-dependent manner. The GS-X pump is suggested to contribute to vesicle-mediated excretion of GSH-drug conjugates from cells. In addition, both HL-60 and HL-60/R-CP cells underwent cell differentiation in response to 12-O-tetradecanoylphorbol-13-acetate, retinoic acid, and dimethyl sulfoxide, resulting in proliferation arrest as well as a remarkable decrease in the c-myc mRNA levels. After cell differentiation, a significant decrease was observed in the activity of ATP-dependent transport of the GS.Pt complex in membrane vesicles prepared from both HL-60/R-CP and HL-60 cells. These results suggest that the expression of the GS-X pump in both cisplatin-resistant and -sensitive cells is related to cell proliferation.

Adenosine Triphosphate↗

PubMed1993

ADP-ribosylation of Gs by cholera toxin is potentiated by agonist activation of beta-adrenergic receptors in the absence of GTP.

Purified Gs is a substrate for ADP-ribosylation catalyzed by cholera toxin (CTx). In S49 cyc- membranes complemented with in vitro translated Gs alpha, the beta-adrenergic agonist isoproterenol enhanced the ADP-ribosylation rate. This effect was maximal if all guanyl nucleotides were suppressed but was blocked by the beta-adrenergic antagonist alprenolol. Enhancement was partially diminished if addition of GDP followed that of isoproterenol. When added in the absence of agonist, the GTP analogues guanosine 5'-O-(gamma-thiotriphosphate) and guanosine 5'-(beta, gamma-imido)triphosphate potentiated CTx-catalyzed ADP-ribosylation of Gs alpha consistent with their activating ADP-ribosylation factors. However, this effect was lessened when the same nucleotides were tested in the presence of agonist. Taken altogether, these results indicate that like Gt and Gi, Gs is an optimal substrate for CTx when coupled to an agonist-activated receptor and depleted of nucleotide. Therefore, coupling to the receptor and subsequent departure of the GDP turn out to be the common features underlying the sensitivity of all GTP-binding proteins to CTx-catalyzed ADP-ribosylation.

Adenosine Diphosphate Ribose↗

PubMed1994

Gs regulation of endosome fusion suggests a role for signal transduction pathways in endocytosis.

Work from several laboratories indicates that guanine nucleotide-binding proteins (GTP-binding proteins) are required for intracellular vesicular transport. In a previous report we presented evidence indicating that one or more heterotrimeric G proteins regulate fusion between endosomes (Colombo, M. I., Mayorga, L. S., Casey, P. J., and Stahl, P. D. (1992) Science 255, 1695-1697). We now report on experiments showing that Gs plays a role in endosome fusion. We have used several reagents known to modulate Gs function including (i) peptides corresponding to the cytoplasmic domains of G protein-coupled receptors and peptides that mimic interaction of receptors with G proteins, (ii) anti-G protein antibodies, and (iii) cholera toxin. Synthetic peptides corresponding to the third cytoplasmic loop of the beta 2-adrenergic receptor which putatively interact with G alpha s inhibited endosomal fusion. The inhibitory effect of these peptides was prevented by a short preincubation of endosomes with guanosine-5'-3-O-(thio)triphosphate or by phosphorylating the peptide with cAMP-dependent protein kinase. The involvement of Gs in endosome recognition and/or the fusion process was assessed by testing an antibody against the COOH terminus of G alpha s. Anti-G alpha s IgG completely abolished fusion between endosomes. Lastly, preincubation of endosomal vesicles with cholera toxin abrogated fusion in the presence of NAD, whereas no effect was observed in the absence of the cofactor. Taken together these findings indicate a role for Gs in either the mechanism or the regulation of fusion among endosomes. These results raise the possibility that signal transduction through cytoplasmic domains of receptors may participate in the regulation of endocytic trafficking.

Amino Acid Sequence↗