High constitutive activity of the human formyl peptide receptor

Co-expression of the beta2-adrenoceptor and dopamine D1-receptor with Gsalpha proteins in Sf9 insect cells: limitations in comparison with fusion proteins

The G-protein G(salpha) exists in three isoforms, the G(salpha) splice variants G(salphashort) (G(salphaS)) and G(salphalong) (G(salphaL)), and the G-protein G(alphaolf) that is not only involved in olfactory signaling but also in extrapyramidal motor regulation. Studies with beta(2)-adrenoceptor (beta(2)AR)-G(salpha) fusion proteins showed that G(salpha) proteins activate adenylyl cyclase (AC) in the order of efficacy G(salphaS)>G(salphaL) approximately G(alphaolf) and that G(salpha) proteins confer the hallmarks of constitutive activity to the beta(2)AR in the order of efficacy G(salphaL)>G(alphaolf)>G(salphaS). However, it is unclear whether such differences between G(salpha) proteins also exist in the nonfused state. In the present study, we co-expressed the beta(2)AR and dopamine D(1)-receptor (D(1)R) with G(salpha) proteins at different ratios in Sf9 insect cells. In agreement with the fusion protein studies, nonfused G(alphaolf) was less efficient than nonfused G(salphaS) and G(salphaL) at activating AC, but otherwise, we did not observe differences between the three G(salpha) isoforms. Thus, it is much easier to dissect differences between G(salpha) isoforms using beta(2)AR-G(salpha) fusion proteins than nonfused G(salpha) isoforms.

Functional differences between human formyl peptide receptor isoforms 26, 98, and G6

The formyl peptide receptor (FPR) is expressed in neutrophils, couples to G(i)-proteins and activates phospholipase C, chemotaxis and cytotoxic cell functions. FPR isoforms 26, 98, and G6 differ from each other in amino acids 101, 192 and 346 (FPR-26: V101, N192, E346; FPR-98: L101, N192, A346; FPR-G6: V101, K192, A346), but the functional significance of those structural differences is unknown. In order to address this question, we analyzed FPR-26, FPR-98 and FPR-G6 by co-expressing recombinant FLAG epitope-tagged FPRs with the G-protein G(i)alpha(2)beta(1)gamma(2) in Sf9 insect cells and measured high-affinity agonist binding and guanosine 5'- O-(3-thiotriphosphate) (GTPgammaS) binding. The B(max) values of high-affinity agonist binding with FPR-98 and FPR-G6 were much lower than with FPR-26. FPR-98 and FPR-G6 activated considerably fewer G(i)-proteins, and were much less constitutively active, than FPR-26. Whereas FPR-26 migrated as a monomer in SDS polyacrylamide electrophoresis, FPR-98 and FPR-G6 migrated as dimers and tetramers. In terms of immunoreactivity, FRP-98 and FPR-G6 were expressed at higher levels than FPR-26. Single amino acid exchanges at positions 101 (V-->L), 192 (N-->K) and 346 (E-->A) in FPR-26 revealed that E346 accounts for FPR-26 migrating as a monomer and the high constitutive activity of FPR-26. The V101L, N192K and E346A exchanges all reduced high-affinity agonist binding and the number of G(i)-proteins activated by FPR-26. We conclude that (i) FPR isoforms 98 and G6 exhibit a partial G(i)-protein coupling defect relative to FPR-26 and that (ii) E346 critically determines constitutive activity, G(i)-protein coupling and physical state of FPR-26.

2'(3')-O-(N-methylanthraniloyl)-substituted GTP analogs: a novel class of potent competitive adenylyl cyclase inhibitors

2'(3')-O-(N-Methylanthraniloyl)-(MANT)-substituted nucleotides are fluorescent and widely used for the kinetic analysis of enzymes and signaling proteins. We studied the effects of MANT-guanosine 5'-[gamma-thio]triphosphate (MANT-GTP gamma S) and MANT-guanosine 5'-[beta,gamma-imido]triphosphate (MANT-GppNHp) on G alpha(s)- and G alpha(i)-protein-mediated signaling. MANT-GTP gamma S/MANT-GppNHp had lower affinities for G alpha(s) and G alpha(i) than GTP gamma S/GppNHp as assessed by inhibition of GTP hydrolysis of receptor-G alpha fusion proteins. MANT-GTP gamma S was much less effective than GTP gamma S at disrupting the ternary complex between the formyl peptide receptor and G alpha(i2). MANT-GTP gamma S/MANT-GppNHp non-competitively inhibited GTP gamma S/GppNHp-, AlF(4)(-)-, beta(2)-adrenoceptor plus GTP-, cholera toxin plus GTP-, and forskolin-stimulated adenylyl cyclase (AC) in G alpha(s)-expressing Sf9 insect cell membranes and S49 wild-type lymphoma cell membranes. AC inhibition by MANT-GTP gamma S/MANT-GppNHp was not due to G alpha(s) inhibition because it was also observed in G alpha(s)-deficient S49 cyc(-) lymphoma cell membranes. Mn(2+) blocked AC inhibition by GTP gamma S/GppNHp in S49 cyc(-) membranes but enhanced the potency of MANT-GTP gamma S/MANT-GppNHp at inhibiting AC by approximately 4-8-fold. MANT-GTP gamma S and MANT-GppNHp competitively inhibited forskolin/Mn(2+)-stimulated AC in S49 cyc(-) membranes with K(i) values of 53 and 160 nm, respectively. The K(i) value for MANT-GppNHp at insect cell AC was 155 nm. Collectively, MANT-GTP gamma S/MANT-GppNHp bind to G alpha(s)- and G alpha(i)-proteins with low affinity and are ineffective at activating G alpha. Instead, MANT-GTP gamma S/MANT-GppNHp constitute a novel class of potent competitive AC inhibitors.

Highly conserved serine in the third transmembrane helix of the luteinizing hormone/human chorionic gonadotropin receptor regulates receptor activation

The elucidation of the role of highly conserved polar amino acids in the transmembrane helices of G-protein-coupled receptors (GPCRs) is important in understanding the mechanism of receptor activation. To this end, the significance of a highly conserved serine residue in the third transmembrane alpha-helix (TM3) of the luteinizing hormone/human chorionic gonadotropin receptor (LH/hCGR) in regulating receptor activation was examined. Results showed that mutation of serine 431 to alanine (S431A) decreased the ability of the receptor to mediate cAMP production in response to hCG, suggesting that S431 stabilizes the active state of the receptor. Homology with other GPCRs suggests that S431 may participate in the coordination of a Na(+) ion. Since Na(+) has been found to stabilize the active state of the receptor in the presence of hCG, the possibility that S431 promotes receptor activation by mediating the effects of Na(+) was explored. Results showed that the regulation of hormone-induced receptor activation by S431 was independent of Na(+). A rhodopsin-based homology model of the TM region of the LH/hCGR was developed to identify other amino acids that might mediate the effects of Na(+) on receptor function. Results indicate that substitution of an Asp at position 556 with Tyr alters the ability of Na(+) to regulate receptor activation. The homology model is used to explain this result as well as to identify a mechanism through which S431 may regulate receptor signaling. Taken together, these studies provide novel insights into the mechanism of LH/hCG receptor activation.

Formylpeptides trigger selective molecular pathways that are required in the physiological functions of human neutrophils

For-Met-Delta(z)Leu-Phe-OMe ([Delta(z)Leu(2)]) is a conformationally restricted for-Met-Leu-Phe-OMe (fMLP-OMe) analogue able to discriminate between different responses of human neutrophils. In contrast, [Delta(z)Leu(2)] significantly activates the transduction pathways-involving Ca(2+), inositol phosphate, and cyclic AMP (cAMP) enhancement, as is the case with the full agonist fMLP-OMe. Here, we have studied the specific involvement of protein kinase C (PKC) isoforms and mitogen activated protein kinases (MAPKs) in the presence or absence of extracellular Ca(2+), being the cation clearly involved in the activation of neutrophils by fMLP. A strong correlation has been found between PKC isoforms, MAPKs and the selective physiological functions by [Delta(z)Leu(2)]-activated neutrophils. In a calcium-free condition, our data suggest that the failure of PKC beta1 translocation and of p38 MAPK phosphorylation by the analogue refers to its inability to induce chemotaxis, and that the failure by both fMLP-OMe and [Delta(z)Leu(2)] to evoke extracellular response kinase 1 and 2 (ERK1/2) phosphorylation would suggest a reduction in superoxide anion production.

Functional coupling of the human dopamine D2 receptor with G alpha i1, G alpha i2, G alpha i3 and G alpha o G proteins: evidence for agonist regulation of G protein selectivity

(1) The human dopamine D(2long) (D(2L)) receptor was expressed with four different G proteins in Sf9 cells using the baculovirus expression system. When co-expressed with G(i)/G(o) G proteins (G(i1)alpha, G(i2)alpha, G(i3)alpha, or G(o)alpha, plus Gbeta(1) and Ggamma(2)), the receptor displayed a high-affinity binding site for the agonists (dopamine and NPA), which was sensitive to GTP (100 micro M), demonstrating interaction between the receptor and the different G proteins. (2) The receptor to G protein ratio (R : G ratio) was evaluated using [(3)H]-spiperone saturation binding (R) and [(35)S]-GTPgammaS saturation binding (G). R : G ratios of 1 : 12, 1 : 3, 1 : 14 and 1 : 5 were found for G(i1), G(i2), G(i3), and G(o) preparations, respectively. However, when R : G ratios of 1 : 2 and 1 : 12 were compared for G(i2) and G(o), no difference was found for the stimulation of [(35)S]-GTPgammaS binding. (3) Several agonists were tested for their ability to stimulate [(35)S]-GTPgammaS binding to membranes co-expressing the receptor and various G proteins. All the compounds tested showed agonist activity in preparations expressing G(i3) and G(o). However, for G(i2) and G(i1) preparations, compounds such as S-(-)-3-PPP and p-tyramine were unable to stimulate [(35)S]-GTPgammaS binding. (4) Most of the compounds showed higher relative efficacies (compared to dopamine) and higher potencies in the preparation expressing G(o). Comparison of the effects of different agonists in the different preparations showed that each agonist differentially activates the four G proteins. (5) We conclude that the degree of selectivity of G protein activation by the D(2L) receptor can depend on the conformation of the receptor stabilised by an agonist.

h5-HT(1B) receptor-mediated constitutive Galphai3-protein activation in stably transfected Chinese hamster ovary cells: an antibody capture assay reveals protean efficacy of 5-HT

1. Serotonin 5-HT(1B) receptors couple to G-proteins of the Gi/o family. However, their activation of specific G-protein subtypes is poorly characterised. Using an innovative antibody capture/guanosine-5'-0-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding strategy, we characterised Galpha(i3) subunit activation by h5-HT(1B) receptors stably expressed in Chinese hamster ovary (CHO) cells. 2. The agonists, 5-HT, alniditan and BMS181,101, stimulated Galpha(i3), whereas methiothepin and SB224,289 behaved as inverse agonists. The selective 5-HT(1B) receptor ligand, S18127, modestly stimulated Galpha(i3) and reversed the actions of both 5-HT and methiothepin. S18127 (1 micro M) also produced parallel, dextral shifts of the 5-HT and methiothepin isotherms. 3. Isotopic dilution experiments ([(35)S]GTPgammaS versus GTPgammaS) revealed high-affinity [(35)S]GTPgammaS binding to Galpha(i3) subunits in the absence of receptor ligands indicating constitutive activity. High-affinity [(35)S]GTPgammaS binding was increased 2.8-fold by 5-HT with an increase in the affinity of GTPgammaS for Galpha(i3) subunits. In contrast, methiothepin halved the number of high-affinity binding sites and decreased their affinity. 4. h5-HT(1B) receptor-mediated Galpha(i3) subunit activation was dependent on the concentration of NaCl. At 300 mM, 5-HT stimulated [(35)S]GTPgammaS binding, basal Galpha(i3) activation was low and methiothepin was inactive. In contrast, at 10 mM NaCl, basal activity was enhanced and the inverse agonist activity of methiothepin was accentuated. Under these conditions, 5-HT decreased Galpha(i3) activation. 5. In conclusion, at h5-HT(1B) receptors expressed in CHO cells: (i) inverse agonist induced inhibition of Galpha(i3), and its reversal by S18127, reveals constitutive activation of this Galpha subunit; (ii) constitutive Galpha(i3) activation can be quantified by isotopic dilution [(35)S]GTPgammaS binding and (iii) decreasing NaCl concentrations enhances Galpha(i3) activation and leads to protean agonist properties of 5-HT: that is a switch to inhibition of Galpha(i3).

Critical role of N-terminal N-glycosylation for proper folding of the human formyl peptide receptor

The human formyl peptide receptor (FPR) is N-glycosylated and activates phagocytes via G(i)-proteins. The FPR expressed with G(i)alpha(2)beta(1)gamma(2) in Sf9 insect cells exhibits high constitutive activity as assessed by strong inhibitory effects of an inverse agonist and Na(+) on basal guanosine 5(')-O-(3-thiotriphosphate) (GTPgammaS) binding. The aim of our study was to analyze the role of N-glycosylation in FPR function. Site-directed mutagenesis of extracellular Asn residues prevented FPR glycosylation but not FPR expression in Sf9 membranes. However, in terms of high-affinity agonist binding, kinetics of GTPgammaS binding, number of G(i)-proteins activated, and constitutive activity, non-glycosylated FPR was much less active than native FPR. FPR-Asn4Gln/Asn10Gln/Asn179Gln and FPR-Asn4Gln/Asn10/Gln exhibited similar defects. Our data indicate that N-glycosylation of N-terminal Asn4 and Asn10 but not of Asn179 in the second extracellular loop is essential for proper folding and, hence, function of FPR. FPR deglycosylation by bacterial glycosidases could be a mechanism by which bacteria compromise host defense.

Efficient adenylyl cyclase activation by a beta2-adrenoceptor-G(i)alpha2 fusion protein

The G-protein G(i)alpha can activate adenylyl cyclase (AC), but the relevance of this AC activation is unknown. We used receptor-G protein co-expression and receptor-G protein fusion proteins to investigate G(i)alpha(2) regulation of AC in Sf9 cells. G(i)alpha(2) was fused to the beta(2)-adrenoceptor (beta(2)AR), a preferentially G(s)-coupled receptor, or the formyl peptide receptor (FPR), a G(i)-coupled receptor. The FPR co-expressed with, or fused to, G(i)alpha(2), reduced AC activity. In contrast, the beta(2)AR fused to G(i)alpha(2) was a highly efficient AC activator, while the beta(2)AR co-expressed with G(i)alpha(2) was not. Agonist efficiently stimulated incorporation of [alpha-32P]GTP azidoanilide into beta(2)AR-G(i)alpha(2). We explain AC activation by beta(2)AR-G(i)alpha(2) by a model in which there is interaction of the beta(2)AR and AC, preventing tethered G(i)alpha(2) from interacting with the inhibitory G(i)alpha site of AC. The postulated beta(2)AR/AC interaction brings G(i)alpha(2) into close proximity of the G(s)alpha site of AC, enabling G(i)alpha(2) to activate AC.

HIV-1 envelope gp41 peptides promote migration of human Fc epsilon RI+ cells and inhibit IL-13 synthesis through interaction with formyl peptide receptors

We evaluated the effects of synthetic peptides (2017, 2019, 2020, 2021, 2023, 2027, 2029, 2030, 2031, and 2035) encompassing the structure of HIV-1(MN) envelope gp41 on both chemotaxis of human basophils and the release of preformed mediators (histamine) and of cytokines (IL-13). Peptides 2019 and 2021 were potent basophil chemoattractants, whereas the other peptides examined were ineffective. Preincubation of basophils with FMLP or gp41 2019 resulted in complete desensitization to a subsequent challenge with homologous stimulus. Incubation of basophils with low concentration (5 x 10(-7) M) of FMLP, which binds with high affinity to N-formyl peptide receptor (FPR), but not to FPR-like 1, did not affect the chemotactic response to a heterologous stimulus (gp41 2019). In contrast, a high concentration (10(-4) M) of FMLP, which binds also to FPR-like 1, significantly reduced the chemotactic response to gp41 2019. The FPR antagonist cyclosporin H inhibited chemotaxis induced by FMLP, but not by gp41 2019. None of these peptides singly induced the release of histamine or cytokines (IL-4 and IL-13) from basophils. However, low concentrations of peptides 2019 and 2021 (10(-8)-10(-6) M) inhibited histamine release from basophils challenged with FMLP but not the secretion caused by anti-IgE and gp120. Preincubation of basophils with peptides 2019 and 2021 inhibited the expression of both IL-13 mRNA, and the FMLP-induced release of IL-13 from basophils. These data highlight the complexity of the interactions between viral and bacterial peptides with FPR subtypes on human basophils.

Cyclosporins: structure-activity relationships for the inhibition of the human FPR1 formylpeptide receptor

The human formylpeptide receptor (FPR) is a seven-transmembranous G-protein-coupled receptor (7TM-GPCR) for chemotactic peptides of bacterial origins, possibly involved in the recruitment and activation of neutrophils in various inflammatory diseases of mucosal epithelia. Mutational analyses suggest that interactions of formylated peptides with FPR occur on the outer exoplasmic leaflet/domains of the plasma membrane. The immunosuppressive and antifungal antibiotic cyclic undecapeptide cyclosporin A (CsA; cyclo-[MeBmt(1)-Abu(2)-MeGly(3)-MeLeu(4)-Val(5)-MeLeu(6)-Ala(7)-D-Ala(8)-MeLeu(9)-MeLeu(10)-MeVal(11)]) and some tested analogues such as [Ala(2)]-CsA, [Thr(2)]-CsA, [Val(2)]-CsA, and [Nva(2)]-CsA were able of inhibiting the binding of formylpeptides to the FPR, with [D-MeVal(11)]-CsA (CsH) being much more active than the other analogues. CsH is devoid of immunosuppressive and antifungal activities, and its large potency for human FPR inhibition is of inverse agonism origin. Formylpeptide binding to FPR-expressing cells does not only induce chemotaxis; it also causes a rapid release of granule enzymes in the extracellular medium, allowing the easy monitoring of any inhibition of FPR function "in vivo" (with intact live cells). With such an assay, CsH was confirmed to be the most potent FPR inhibitory cyclosporin, although a far related immunosuppressive cyclosporin analogue, FR901459 ([Thr(2), Leu(5), Leu(10)]-CsA), was found to display a high FPR inhibitory activity (FPR-InhA). To establish structure-activity relationships (SAR) for FPR function inhibition, 59 cyclosporins were now studied by this standardized assay (with differentiated human leukemic cell line HL-60 as FPR-expressing cells and with N-acetyl-beta-D-glucosaminidase release as read-out). These SAR confirmed the low FPR-InhA of classical cyclosporins, where such activity was only seldom found: the most active ones ([Thr(2), Ile(5)]-CsA, [aMeIle(11)]-CsA, and [MeAla(11)]-CsA) remained 3-10-fold less potent than CsH. In contrast, the SAR disclosed that N(10)-desmethylated cyclosporins were particularly prone to display a large FPR-InhA: their most potent one was a [Thr(2), Gly(3), Leu(5), D-Hiv(8), Leu(10)]-CsA, found to be only 2-4-fold less active than [D-MeVal(11)]-CsA (CsH), with which it shows six differences out of 11 residues. Because the free conformations of both CsH and N(10)-desmethylated cyclosporins differ from those of "classical" (N(10)-methylated, [L-MeVal(11)]-using) cyclosporins, these potent FPR inhibitory cyclosporins probably bind to FPR pharmacophores for which classical cyclosporins show little affinity. Moreover, because the conformations of the N(10)-desmethylated cyclosporins widely differ from the CsH one, they probably bind to different pharmacophores on the FPR molecules.

The N-formylpeptide receptor (FPR) and a second G(i)-coupled receptor mediate fMet-Leu-Phe-stimulated activation of NADPH oxidase in murine neutrophils

N-Formylypeptides such as fMet-Leu-Phe (fMLF) potently induce superoxide production through NADPH oxidase activation. The receptors that mediate this response have not been defined. Here, we provide definitive proof using a mouse model that formyl peptide receptor (FPR) is a receptor, but not the only receptor, that mediates fMLF-induced oxidase activation. In wild-type (FPR(+/+)) mouse neutrophils, superoxide production is dependent on the concentration of fMLF with an EC(50) of approximately 5 microM and a peak at approximately 50 microM. In contrast, FPR-deficient (FPR(-/-)) mouse neutrophils produced markedly less superoxide with an EC(50) of approximately 50 microM and a peak at approximately 200 microM. Yet, FPR(+/+) and FPR(-/-) neutrophils showed similar oxidase activation kinetics and G(i) protein-dependent pharmacological sensitivities. These results suggested that a second receptor, likely FPR2, mediates superoxide production at high concentrations of fMLF. This less sensitive second pathway may permit continued oxidant generation in response to formyl peptides when FPR is desensitized in high concentrations of the chemotactic gradient.

Dexras1/AGS-1 inhibits signal transduction from the Gi-coupled formyl peptide receptor to Erk-1/2 MAP kinases

Dexras1 is a novel GTP-binding protein (G protein) that was recently discovered on the basis of rapid mRNA up-regulation by glucocorticoids in murine AtT-20 corticotroph cells and in several primary tissues. The human homologue of Dexras1, termed activator of G protein signaling-1 (AGS-1), has been reported to stimulate signaling by G(i) heterotrimeric G proteins independently of receptor activation. The effects of Dexras1/AGS-1 on receptor-initiated signaling by G(i) have not been examined. Here we report that Dexras1 inhibits ligand-dependent signaling by the G(i)-coupled N-formyl peptide receptor (FPR). Dexras1 and FPR were transiently co-expressed in both COS-7 and HEK-293 cells. Activation of FPR by ligand (N-formyl-methionine-leucine-phenylalanine (f-MLF)) caused phosphorylation of endogenous Erk-1/2 that was reduced by co-expression of Dexras1. Direct effects of Dexras1 on the activity of co-expressed, epitope-tagged Erk-2 (hemagglutinin (HA)-Erk-2) were measured by immune complex in vitro kinase assay. Expression of Dexras1 alone resulted in a 1.9- to 4.9-fold increase in HA-Erk-2 activity; expression of the unliganded FPR alone resulted in a 6.2- to 8.1-fold increase in HA-Erk-2 activity. Stimulation of FPR by f-MLF produced a further 8- to 10-fold increase in HA-Erk-2 activity over the basal (non-ligand-stimulated) state, and this ligand-dependent activity was attenuated at the time points of maximal activity by co-expression of Dexras1 (reduced 31 +/- 6.8% in COS-7 at 10 min and 86 +/- 9.2% in HEK-293 at 5 min, p < 0.01 for each). Expression of Dexras1 did not influence protein expression of FPR or Erk, suggesting that the inhibitory effects of Dexras1 reflect a functional alteration in the signaling cascade from FPR to Erk. Expression of Dexras1 had no effect on expression of G(i)alpha species, but significantly impaired pertussis toxin-catalyzed ADP-ribosylation of membrane-associated G(i)alpha. Expression of Dexras1 also significantly decreased in vitro binding of GTPgammaS in f-MLF-stimulated membranes of cells co-transfected with FPR. These data suggest that Dexras1 inhibits signal transduction from FPR to Erk-1/2 through an effect that is very proximal to receptor-G(i) coupling. While Dexras1 weakly activates Erk in the resting state, more potent effects are evident in the modulation of ligand-stimulated receptor signal transduction, where Dexras1 functions as an inhibitor rather than activator of the Erk mitogen-activated protein kinase signaling cascade.

C- and N-terminal residue effect on peptide derivatives' antagonism toward the formyl-peptide receptor

The biological action of several X-Phe-D-Leu-Phe-D-Leu-Z (X=3',5'-dimethylphenyl-ureido; Z=Phe, Lys, Glu, Tyr) analogues was analysed on human neutrophils to evaluate their ability to antagonize formyl-peptide receptors. X-Phe-D-Leu-Phe-D-Leu-Phe analogues obtained as C-terminal olo or amido derivatives and T-Phe-D-Leu-Phe-D-Leu-Phe analogues (T=thiazolyl-ureido) were also analysed. The activities of pentapeptide derivatives were compared with those of X-Phe-D-Leu-Phe-D-Leu-Phe chosen as reference antagonist. Our results demonstrate that X-Phe-D-Leu-Phe-D-Leu-Phe-olo, X-Phe-D-Leu-Phe-D-Leu-Glu and X-Phe-D-Leu-Phe-D-Leu-Tyr are more active antagonists than X-Phe-D-Leu-Phe-D-Leu-Phe. The presence of Lys (X-Phe-D-Leu-Phe-D-Leu-Lys) seems, instead, to inhibit the formyl-peptide receptor antagonist properties. The presence of the N-terminal thiazolyl-ureido group seems to considerably contribute to the receptor antagonist properties of T-Phe-D-Leu-Phe-D-Leu-Phe-OH. The introduction of the C-terminal methyl ester (T-Phe-D-Leu-Phe-D-Leu-Phe-OMe) or amido group (X-Phe-D-Leu-Phe-D-Leu-Phe-NH2) appears detrimental for the affinity and formyl-peptide receptor antagonist properties of the Phe-D-Leu-Phe-D-Leu-Phe derivatives. The examined peptides inhibit superoxide anion production and lysozyme release more efficaciously than neutrophil chemotaxis.

Receptors for chemotactic formyl peptides as pharmacological targets

Leukocytes accumulate at sites of inflammation and immunological reaction in response to locally existing chemotactic mediators. N-formyl peptides, such as fMet-Leu-Phe (fMLF), are some of the first identified and most potent chemoattractants for phagocytic leukocytes. In addition to the bacterial peptide fMLF and the putative endogenously produced formylated peptides, a number of novel peptide agonists have recently been identified that selectively activate the high-affinity fMLF receptor FPR and/or its low-affinity variant FPRL1, both of which belong to the seven-transmembrane (STM), G protein-coupled receptor (GPCR) superfamily. These agonists include peptide domains derived from the envelope proteins of human immunodeficiency virus type 1 (HIV-1) and at least three amyloidogenic polypeptides, the human acute phase protein serum amyloid A, the 42 amino acid form of beta amyloid peptide and a 21 amino acid fragment of human prion. Furthermore, a cleavage fragment of neutrophil granule-derived bactericidal cathelicidin, LL-37, is also a chemotactic agonist for FPRL1. Activation of formyl peptide receptors results in increased cell migration, phagocytosis, release of proinflammatory mediators, and the signaling cascade culminates in heterologous desensitization of other STM receptors including chemokine receptors CCR5 and CXCR4, two coreceptors for HIV-1. Thus, by interacting with a variety of exogenous and host-derived agonists, formyl peptide receptors may play important roles in proinflammatory and immunological diseases and constitute a novel group of pharmacological targets.

Defective Gi protein coupling in two formyl peptide receptor mutants associated with localized juvenile periodontitis

The formyl peptide receptor (FPR) is a prototypical chemoattractant receptor expressed in neutrophils. It is well known that the FPR couples to G(i) proteins to activate phospholipase C, chemotaxis, and cytotoxic cell functions, but the in vivo role of the FPR in man has remained elusive. Recently, F110S and C126W mutations of the FPR have been associated with localized juvenile periodontitis. We studied FPR-F110S and FPR-C126W in comparison with wild-type FPR (FPR-WT) by coexpressing epitope-tagged versions of these receptors with the G protein Galpha(i2)beta(1)gamma(2) in Sf9 insect cells. FPRs were efficiently expressed in Sf9 membranes as assessed by immunoblotting using the beta(2)-adrenoreceptor as a standard. FPR-C126W differed from FPR-WT and FPR-F110S in migration on SDS-polyacrylamide gels and tunicamycin-sensitive glycosylation. FPR-WT efficiently reconstituted high-affinity agonist binding and agonist- and inverse agonist-regulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding to Galpha(i2)beta(1)gamma(2). In contrast, FPR-F110S only weakly reconstituted agonist-stimulated GTPgammaS binding, and FPR-C126W was completely inefficient. Collectively, our data show almost complete and complete loss of G(i) protein coupling in FPR-F110S and FPR-C126W, respectively. The severe functional defects in FPR-F110S and FPR-C126W contrast with the discrete clinical symptoms associated with these mutations, indicating that loss of FPR function in host defense is, for the most part, readily compensated.

Agonist regulation of D(2) dopamine receptor/G protein interaction. Evidence for agonist selection of G protein subtype

The D(2) dopamine receptor has been expressed in Sf21 insect cells together with the G proteins G(o) and G(i2), using the baculovirus system. Expression levels of receptor and G protein (alpha, beta, and gamma subunits) in the two preparations were similar as shown by binding of [(3)H]spiperone and quantitative Western blot, respectively. For several agonists, binding data were fitted best by a two-binding site model in either preparation, showing interaction of expressed receptor and G protein. For some agonists, binding to the higher affinity site was of higher affinity in D(2)/G(o) than in the D(2)/G(i2) preparation. Some agonists exhibited binding data that were best fitted by a two-binding site model in D(2)/G(o) and a one-binding site model in D(2)/G(i2). Therefore, receptor/G protein interaction seemed to be stronger in the D(2)/G(o) preparation. Agonist stimulation of [(35)S]GTP gamma S (guanosine 5'-3-O-(thio)triphosphate) binding in the two preparations also gave evidence for higher affinity D(2)/G(o) interaction. In the D(2)/G(o) preparation, agonist stimulation of [(35)S]GTP gamma S binding occurred at higher potency for several agonists, and a higher stimulation (relative to dopamine) was achieved in D(2)/G(o) compared with D(2)/G(i2). Some agonists were able to stimulate [(35)S]GTP gamma S binding in the D(2)/G(o) preparation but not in D(2)/G(i2). The extent of D(2) receptor selectivity for G(o) over G(i2) is therefore dependent on the agonist used, and thus agonists may stabilize different conformations of the receptor with different abilities to couple to and activate G proteins.

The synthetic peptide Trp-Lys-Tyr-Met-Val-Met-NH2 specifically activates neutrophils through FPRL1/lipoxin A4 receptors and is an agonist for the orphan monocyte-expressed chemoattractant receptor FPRL2

Neutrophils express the G protein-coupled N-formyl peptide receptor (FPR) and its homologue FPRL1, whereas monocytes express FPR, FPRL1, and FPRL2, an orphan receptor sharing 83% amino acid identity with FPRL1. FPRL1 is a promiscuous receptor activated by serum amyloid A and by different synthetic peptides, including the hexapeptide Trp-Lys-Tyr-Met-Val-d-Met-NH(2) (WKYMVm). By measuring calcium flux in HL-60 cells transfected with FPR, FPRL1, or FPRL2, we show that WKYMVm activated all three receptors, whereas the l-conformer WKYMVM activated exclusively FPRL1 and FPRL2. The functionality of FPRL2 was further assessed by the ability of HL-60-FPRL2 cells to migrate toward nanomolar concentrations of hexapeptides. The half-maximal effective concentrations of WKYMVM for calcium mobilization in HL-60-FPRL1 and HL-60-FPRL2 cells were 2 and 80 nm, respectively. Those of WKYMVm were 75 pm and 3 nm. The tritiated peptide WK[3,5-(3)H(2)]YMVM bound to FPRL1 (K(D) approximately 160 nm), but not to FPR. The two conformers similarly inhibited binding of (125)I-labeled WKYMVm to FPRL2-expressing cells (IC(50) approximately 2.5-3 micrometer). Metabolic labeling with orthophosphoric acid revealed that FPRL1 was differentially phosphorylated upon addition of the l- or d-conformer, indicating that it induced different conformational changes. In contrast to FPRL1, FPRL2 was already phosphorylated in the absence of agonist and not evenly distributed in the plasma membrane of unstimulated cells. However, both receptors were internalized upon addition of either of the two conformers. Taken together, the results indicate that neutrophils are activated by WKYMVM through FPRL1 and that FPRL2 is a chemotactic receptor transducing signals in myeloid cells.

Two active molecular phenotypes of the tachykinin NK1 receptor revealed by G-protein fusions and mutagenesis

The NK1 neurokinin receptor presents two non-ideal binding phenomena, two-component binding curves for all agonists and significant differences between agonist affinity determined by homologous versus heterologous competition binding. Whole cell binding with fusion proteins constructed between either Galpha(s) or Galpha(q) and the NK1 receptor with a truncated tail, which secured non-promiscuous G-protein interaction, demonstrated monocomponent agonist binding closely corresponding to either of the two affinity states found in the wild-type receptor. High affinity binding of both substance P and neurokinin A was observed in the tail-truncated Galpha(s) fusion construct, whereas the lower affinity component was displayed by the tail-truncated Galpha(q) fusion. The elusive difference between the affinity determined in heterologous versus homologous binding assays for substance P and especially for neurokinin A was eliminated in the G-protein fusions. An NK1 receptor mutant with a single substitution at the extracellular end of TM-III-(F111S), which totally uncoupled the receptor from Galpha(s) signaling, showed binding properties that were monocomponent and otherwise very similar to those observed in the tail-truncated Galpha(q) fusion construct. Thus, the heterogenous pharmacological phenotype displayed by the NK1 receptor is a reflection of the occurrence of two active conformations or molecular phenotypes representing complexes with the Galpha(s) and Galpha(q) species, respectively. We propose that these molecular forms do not interchange readily, conceivably because of the occurrence of microdomains or "signal-transductosomes" within the cell membrane.

Characterization of the binding site on the formyl peptide receptor using three receptor mutants and analogs of Met-Leu-Phe and Met-Met-Trp-Leu-Leu

The formyl peptide receptor (FPR) is a chemotactic G protein-coupled receptor found on the surface of phagocytes. We have previously shown that the formyl peptide binding site maps to the membrane-spanning region (Miettinen, H. M., Mills, J. S., Gripentrog, J. M., Dratz, E. A., Granger, B. L., and Jesaitis, A. J. (1997) J. Immunol. 159, 4045-4054). Recent reports have indicated that non-formylated peptides, such as MMWLL can also activate this receptor (Chen, J., Bernstein, H. S., Chen, M., Wang, L., Ishi, M., Turck, C. W., and Coughlin, S. R. (1995) J. Biol. Chem. 270, 23398-23401.) Here we show that the selectivity for the binding of different NH(2)-terminal analogs of MMWLL or MLF can be markedly altered by mutating Asp-106 to asparagine or Arg-201 to alanine. Both D106N and R201A produced a similar change in ligand specificity, including an enhanced ability to bind the HIV-1 peptide DP178. In contrast, the mutation R205A exhibited altered specificity at the COOH terminus of fMLF, with R205A binding fMLF-O-butyl > fMLF-O-methyl > fMLF, whereas wt FPR bound fMLF > fMLF-O-methyl approximately fMLF-O-butyl. These data, taken together with our previous finding that the leucine side chain of fMLF is probably bound to FPR near FPR (93)VRK(95) (Mills, J. S., Miettinen, H. M., Barnidge, D., Vlases, M. J., Wimer-Mackin, S., Dratz, E. A., and Jesaitis, A. J. (1998) J. Biol. Chem. 273, 10428-10435.), indicate that the most likely positioning of fMLF in the binding pocket of FPR is approximately parallel to the fifth transmembrane helix with the formamide group of fMLF hydrogen-bonded to both Asp-106 and Arg-201, the leucine side chain pointing toward the second transmembrane region, and the COOH-terminal carboxyl group of fMLF ion-paired with Arg-205.

Molecular analysis of beta(2)-adrenoceptor coupling to G(s)-, G(i)-, and G(q)-proteins

The beta(2)-adrenoceptor (beta(2)AR) couples to the G-protein G(s) to activate adenylyl cyclase. Intriguingly, several studies have demonstrated that the beta(2)AR can also interact with G-proteins of the G(i)- and G(q)-family. To assess the efficiency of beta(2)AR interaction with various G-protein alpha-subunits (G(xalpha)), we expressed fusion proteins of the beta(2)AR with the long (G(salphaL)) and short (G(salphaS)) splice variants of G(salpha), the G(i)-proteins G(ialpha2) and G(ialpha3), and the G(q)-proteins G(qalpha) and G(16alpha) in Sf9 cells. Fusion proteins provide a rigorous approach for comparing the coupling of a given receptor to G(xalpha) because of the defined 1:1 stoichiometry of receptor and G-protein and the efficient coupling. Here, we show that the beta(2)AR couples to G(s)-, G(i)-, and G(q)-proteins as assessed by ternary complex formation and ligand-regulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding. The combined analysis of ternary complex formation, GTPgammaS binding, agonist efficacies, and agonist potencies revealed substantial differences in the interaction of the beta(2)AR with the various classes of G-proteins. Comparison of the coupling of the beta(2)AR and formyl peptide receptor to G(ialpha2) revealed receptor-specific differences in the kinetics of GTPgammaS binding. We also detected highly efficient stimulation of GTPgammaS dissociation from G(salphaL), but not from G(qalpha) and G(16alpha), by a beta(2)AR agonist. Moreover, we show that the 1:1 stoichiometry of receptor to G-protein in fusion proteins reflects the in vivo stoichiometry of receptor/G-protein coupling more closely than was previously assumed. Collectively, our data show 1) that the beta(2)AR couples differentially to G(s)-, G(i)-, and G(q)-proteins, 2) that there is ligand-specific coupling of the beta(2)AR to G-proteins, 3) that receptor-specific G-protein conformational states may exist, and 4) that nucleotide dissociation is an important mechanism for G-protein deactivation.

In vivo and in vitro treatment with the synthetic cannabinoid CP55, 940 decreases the in vitro migration of macrophages in the rat: involvement of both CB1 and CB2 receptors

Cannabinoids have been shown to affect immune responses, acting on different populations of immune cells. In the present paper we analyze the ability of in vivo and in vitro treatment with the potent synthetic cannabinoid CP55,940 to interfere with an important function of rat peritoneal macrophages, i.e. spontaneous migration and formyl-metionyl-leucine-phenylalanine (fMLP)-induced chemotaxis, that were assessed by the use of a Boyden-modified microchemotaxis chamber. When added in vitro, CP55,940 induced a significant and dose-dependent inhibition of both spontaneous migration and fMLP-induced chemotaxis. Both the Cannabinoid Receptor 1 (CB1) and the Cannabinoid Receptor 2 (CB2) antagonists were able to block the CP55,940-induced inhibition of spontaneous migration, although the CB2 antagonist was more potent and only the CB2 antagonist was able to reverse the effect of CP55,940 on fMLP-induced chemotaxis. Similarly, in the in vivo experiments, 1 h after the acute subcutaneous administration of 0.4 mg/kg of CP55,940, both spontaneous motility and chemotaxis were reduced. The pretreatment with the CB2 antagonist, but not with the CB1 antagonist, was able to prevent this effect. Our data confirm that cannabinoids can affect some macrophage functions, mainly throughout CB2 receptors, and suggest that the development of specific CB2 ligands may lead to an interesting new class of anti-inflammatory drugs.

Effects of sodium on agonist efficacy for G-protein activation in mu-opioid receptor-transfected CHO cells and rat thalamus

1. Sodium ions inhibit spontaneous G(i)/G(o)-coupled receptor activity and promote agonist-induced responses in vitro. The effects of sodium on the relative efficacy of opioid agonists for G-protein activation was measured by guanosine-5'-O-(gamma-(35)S)-triphosphate ([(35)S]-GTPgammaS) binding in membranes from two mu-opioid receptor-containing systems: CHO cells stably transfected with mouse mureceptors (mMOR-CHO cells) and rat thalamus. 2. NaCl inhibited basal [(35)S]-GTPgammaS binding in both systems, and this effect was partially mimicked by KCl. In mMOR-CHO membranes, net [(35)S]-GTPgammaS binding stimulated by partial but not full agonists was inhibited by NaCl with a potency that was inversely proportional to agonist efficacy. Monovalent cations were required for agonist-stimulated [(35)S]-GTPgammaS binding in this system, and increasing NaCl concentrations magnified relative efficacy differences among agonists. 3. In thalamic membranes, which contain a lower receptor:G-protein ratio than mMOR-CHO cells, similar monovalent cation effects were observed, with two exceptions: (1) [(35)S]-GTPgammaS binding stimulated by both full and partial agonists was inhibited by NaCl; and (2) monovalent cations were not required to observe agonist-stimulated [(35)S]-GTPgammaS binding. 4. Basal [(35)S]-GTPgammaS binding stimulated by the absence of monovalent cations resembled that of agonist-stimulated binding and was blocked by pretreatment of mMOR-CHO cells with pertussis toxin. 5. These results indicate that sodium inhibits spontaneous and agonist-occupied mu receptor-mediated G-protein activation in a manner inversely proportional to the efficacy of the agonist, and that spontaneous mu receptor activity and the relative efficacy of partial agonists acting at these receptors are both increased by increases in the stoichiometric ratio of receptors:G-proteins.

The synthetic chemoattractant Trp-Lys-Tyr-Met-Val-DMet activates neutrophils preferentially through the lipoxin A4 receptor

A D-methionine–containing peptide, Trp-Lys-Tyr-Met-Val-D-Met-NH2 (WKYMVm), featuring a unique receptor specificity was investigated with respect to its ability to activate neutrophil effector functions. The peptide was found to be more potent than the N-formylated peptide N-formyl-Met-Leu-Phe (fMLF) at inducing neutrophil chemotaxis, mobilization of neutrophil complement receptor 3 (CR3), and activation of the neutrophil NADPH-oxidase. The fact that binding of fML[3H]F was inhibited by both fMLF and WKYMVm suggests that N-formyl peptide receptor (FPR) is shared by these peptides. However, the neutrophil response induced by the WKYMVm peptide was insensitive to the fMLF antagonists, cyclosporin H, and Boc-FLFLF that specifically block the function of the FPR. These results suggest that even though WKYMVm may bind FPR the cells are activated preferentially through a receptor distinct from the FPR. Using transfected HL-60 cells expressing either the FPR or its neutrophil homologue FPRL1, also referred to as LXA4R because it has been shown to bind lipoxin A4, we show that WKYMVm is about 300-fold more active at mobilizing intracellular calcium through FPRL1 than through FPR. The WKYMVm activates FPRL1-expressing cells in a cyclosporin H-independent manner with an EC50 of around 75 pmol/L, whereas it activates FPR-expressing cells with an EC50 of around 25 nmol/L. The observation that exudated cells are primed in their response to WKYMVm suggests that FPRL1/LXA4R like FPR is stored in mobilizable organelles.

Quantitative analysis of formyl peptide receptor coupling to g(i)alpha(1), g(i)alpha(2), and g(i)alpha(3)

The human formyl peptide receptor (FPR) is a prototypical G(i) protein-coupled receptor, but little is known about quantitative aspects of FPR-G(i) protein coupling. To address this issue, we fused the FPR to G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) and expressed the fusion proteins in Sf9 insect cells. Fusion of a receptor to Galpha ensures a defined 1:1 stoichiometry of the signaling partners. By analyzing high affinity agonist binding, the kinetics of agonist- and inverse agonist-regulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding and GTP hydrolysis and photolabeling of Galpha, we demonstrate highly efficient coupling of the FPR to fused G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) without cross-talk of the receptor to insect cell G proteins. The FPR displayed high constitutive activity when coupled to all three G(i)alpha isoforms. The K(d) values of high affinity agonist binding were approximately 100-fold lower than the EC(50) (concentration that gives half-maximal stimulation) values of agonist for GTPase activation. Based on the B(max) values of agonist saturation binding and ligand-regulated GTPgammaS binding, it was previously proposed that the FPR activates G proteins catalytically, i.e. one FPR activates several G(i) proteins. Analysis of agonist saturation binding, ligand-regulated GTPgammaS saturation binding and quantitative immunoblotting with membranes expressing FPR-G(i)alpha fusion proteins and nonfused FPR now reveals that FPR agonist binding greatly underestimates the actual FPR expression level. Our data show the following: (i) the FPR couples to G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) with similar efficiency; (ii) the FPR can exist in a state of low agonist affinity that couples efficiently to G proteins; and (iii) in contrast to the previously held view, the FPR appears to activate G(i) proteins linearly and not catalytically.

Identification of putative sites of interaction between the human formyl peptide receptor and G protein

Wild-type and 35 mutant formyl peptide receptors (FPRs) were stably expressed in Chinese hamster ovary cells. All cell surface-expressed mutant receptors bound N-formyl peptide with similar affinities as wild-type FPR, suggesting that the mutations did not affect the ligand-binding site. G protein coupling was examined by quantitative analysis of N-formyl-methionyl-leucyl-phenylalanine-induced increase in binding of (35)S-labeled guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) to membranes. The most prominent uncoupled FPR mutants were located in the N-terminal part of the second transmembrane domain (S63W and D71A) and the C-terminal interface of the third transmembrane domain (R123A and C124S/C126S). In addition, less pronounced uncoupling was detected with deletion mutations in the third cytoplasmic loop and in the cytoplasmic tail. Further analysis of some of the mutants that were judged to be uncoupled based on the [(35)S]GTPgammaS membrane-binding assay were found to transduce a signal, as evidenced by intracellular calcium mobilization and activation of p42/44 MAPK. Thus, these single point mutations in FPR did not completely abolish the interaction with G protein, emphasizing that the coupling site is coordinated by several different regions of the receptor. Mutations located in the putative fifth and sixth transmembrane domains near the N- and C-terminal parts of the third cytoplasmic loop did not result in uncoupling. These regions have previously been shown to be critical for G protein coupling to many other G protein-coupled receptors. Thus, FPR appears to have a G protein-interacting site distinct from the adrenergic receptors, the muscarinic receptors, and the angiotensin receptors.

The luteinizing hormone receptor activates phospholipase C via preferential coupling to Gi2

Binding of lutropin/choriogonadotropin (LH/CG) to its cognate receptor results in the activation of adenylyl cyclase and phospholipase C. This divergent signaling of the LH receptor is based on the independent activation of distinct G protein subfamilies, i.e. , Gs, Gi, and potentially also Gq. To examine the selectivity of LH receptor coupling to phospholipase C beta-activating G proteins, we used an in vivo reconstitution system based on the coexpression of the LH receptor and different G proteins in baculovirus-infected insect cells. In this paper, we describe a refined expression strategy for the LH receptor in insect cells. The receptor protein was inserted into the cell membrane at an expression level of 0.8 pmol/mg of membrane protein. Sf9 cells expressing the LH receptor responded to hCG challenge with a concentration-dependent accumulation of intracellular cAMP (EC50 = 630 nM) but not of inositol phosphates, whereas stimulation of the histamine H1 receptor in Sf9 cells led to increased phospholipase C (PLC) activity. Immunoblotting experiments using G protein-specific antisera revealed the absence of quantitative amounts of alpha i in Sf9 cells, whereas alpha s and alpha q/11 were detected. We therefore attempted to restore the hCG-dependent PLC activation by infection of Sf9 cells with viruses encoding the LH receptor and different G protein alpha subunits. HCG stimulation of cells coexpressing the LH receptor and exogenous alpha i2 resulted in stimulation of PLC activity. In cells coinfected with an alpha i3-baculovirus, hCG challenge led to a minor activation of PLC, whereas no hCG-dependent PLC stimulation was observed in cells coexpressing alpha i1. Most notably, coinfection with baculoviruses encoding alpha q or alpha 11 did not reproduce the PLC activation by the LH receptor. Thus, the murine LH receptor activates adenylyl cyclase via Gs and PLC via selective coupling to Gi2.

GPCR-Galpha fusion proteins: molecular analysis of receptor-G-protein coupling

The efficiency of interactions between G-protein-coupled receptors (GPCRs) and heterotrimeric guanine nucleotide-binding proteins (G proteins) is greatly influenced by the absolute and relative densities of these proteins in the plasma membrane. The study of these interactions has been facilitated by the use of GPCR-Galpha fusion proteins, which are formed by the fusion of GPCR to Galpha. These fusion proteins ensure a defined 1:1 stoichiometry of GPCR to Galpha and force the physical proximity of the signalling partners. Thus, fusion of GPCR to Galpha enhances coupling efficiency can be used to study aspects of receptor-G-protein coupling that could not otherwise be examined by co-expressing GPCRs and G proteins as separate proteins. The results of studies that have made use of GPCR-Galpha fusion proteins will be discussed in this article, along with the strengths and limitations of this approach.

Effects of guanine, inosine, and xanthine nucleotides on beta(2)-adrenergic receptor/G(s) interactions: evidence for multiple receptor conformations

The aim of our study was to examine the effects of different purine nucleotides [GTP, ITP, and xanthosine 5'-triphosphate (XTP)] on receptor/G protein coupling. As a model system, we used a fusion protein of the beta(2)-adrenergic receptor and the alpha subunit of the G protein G(s). GTP was more potent and efficient than ITP and XTP at inhibiting ternary complex formation and supporting adenylyl cyclase (AC) activation. We also studied the effects of several beta(2)-adrenergic receptor ligands on nucleotide hydrolysis and on AC activity in the presence of GTP, ITP, and XTP. The efficacy of agonists at promoting GTP hydrolysis correlated well with the efficacy of agonists for stimulating AC in the presence of GTP. This was, however, not the case for ITP hydrolysis and AC activity in the presence of ITP. The efficacy of ligands at stimulating AC in the presence of XTP differed considerably from the efficacies of ligands in the presence of GTP and ITP, and there was no evidence for receptor-regulated XTP hydrolysis. Our findings support the concept of multiple ligand-specific receptor conformations and demonstrate the usefulness of purine nucleotides as tools to study conformational states of receptors.