A galpha(s) carboxyl-terminal peptide prevents G(s) activation by the A(2A) adenosine receptor

The molecular mechanisms of interaction between G(s) and the A(2A) adenosine receptor were investigated using synthetic peptides corresponding to various segments of the Galpha(s) carboxyl terminus. Synthetic peptides were tested for their ability to modulate binding of a selective radiolabeled agonist, [(3)H]2-[4-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxam idoade nosine ([(3)H]CGS21680), to A(2A) adenosine receptors in rat striatal membranes. The Galpha(s) peptides stimulated specific binding both in the presence and absence of 100 microM guanosine-5'-O-(3-thiotriphosphate) (GTPgammaS). Three peptides, Galpha(s)(378-394)C(379)A, Galpha(s)(376-394)C(379)A, and Galpha(s)(374-394)C(379)A, were the most effective. In the presence of GTPgammaS, peptide Galpha(s)(374-394)C(379)A increased specific binding in a dose-dependent fashion. However, the peptide did not stabilize the high-affinity state of the A(2A) adenosine receptor for [(3)H]CGS21680. Binding assays with a radiolabeled selective antagonist, [(3)H]5-amino-7-(2-phenylethyl)-2-(2-furyl)pyrazolo[4, 3-e]-1,2,4-triazolo[1,5-c]pyrimidine ([(3)H]SCH58261), showed that the addition of the Galpha(s) peptide modified the slope of the 5'-N-ethylcarboxamidoadenosine (NECA) competition curve, suggesting modulation of receptor affinity states. In the presence of GTPgammaS, the displacement curve was right-shifted, whereas the addition of Galpha(s)(374-394)C(379)A caused a partial left-shift. Both curves were fitted by one-site models. This same Galpha(s) peptide was also able to disrupt G(s)-coupled signal transduction as indicated by inhibition of the A(2A) receptor-stimulated adenylyl cyclase activity without affecting either basal or forskolin-stimulated enzymatic activity in the same membrane preparations. Shorter peptides from Galpha(s) and Galpha(i1/2) carboxyl termini were not effective. NMR spectroscopy showed the strong propensity of peptide Galpha(s)(374-394)C(379)A to assume a compact carboxyl-terminal alpha-helical conformation in solution. Overall, our results point out the conformation requirement of Galpha(s) carboxyl-terminal peptides to modulate agonist binding to rat A(2A) adenosine receptors and disrupt signal transduction.

A structure-activity study of a C-terminal endothelin analogue

We report a structure-activity study of an endothelin (ET) analogue, obtained by introduction of a non-aminoacidic portion on the C-terminal ET pentapeptide. The peptidic moiety was modified with systematic replacement of each residue by alanine (Ala scan); further modifications were performed at the C-terminus. The biological activity was analyzed at both ET(A) and ET(B) receptor subtypes, showing that the two C-terminal residues (Ile-Trp) are very important for the activity. On the contrary, the aminoacidic central portion of the molecule appears to be much more tolerant toward modifications.

Lack of stereoselectivity of 8-hydroxy-2(di-N-propylamino)tetralin-mediated inhibition of forskolin-stimulated adenylyl cyclase activity in human pre- and post-synaptic brain regions

The stereoselectivity of the serotonin1A (5-HT1A) receptor compound 8-hydroxy-2(di-N-propylamino)tetralin (8-OH-DPAT) on forskolin-stimulated adenylyl cyclase activity was investigated in membranes from human 5-HT pre-synaptic (raphe nuclei) and post-synaptic (hippocampus and prefrontal cortex) regions of autopsy brains. After sample incubation with agonists and antagonists, results showed that both the racemic mixture of 8-OH-DPAT or its (+) and (-) enantiomers behaved as full agonists in the tested brain regions. Enantiomer potency (EC50, nM) and efficacy (percentage of maximal inhibition, %) values were similar in all regions under investigation. However, some inter and intra-region variations in racemic 8-OH-DPAT potency and efficacy have been observed. In particular, the potency of racemic 8-OH-DPAT was higher in the prefrontal cortex and raphe nuclei than in the hippocampus, where it was in fact lower than either single enantiomers. Agonist effects were competitively reversed by 5-HT1A antagonists, although once again a different profile was revealed in the hippocampus. The data underscores the lack of stereospecificity of 8-OH-DPAT-mediated inhibition of adenylyl cyclase activity in either pre- or post-synaptic human brain regions. Moreover, such results have significant implication, as they support the notion that human 5-HT1A receptors might vary from one brain region to the other.

Gender and age-related variation in adenylyl cyclase activity in the human prefrontal cortex, hippocampus and dorsal raphe nuclei

The influence of gender and age on adenylyl cyclase activity was investigated, through a Dowex-alumina double step chromatographic procedure, in the prefrontal cortex, hippocampus and dorsal raphe nuclei obtained from autopsy cadavers. Results showed that forskolin-stimulated enzyme activity in raphe nuclei was greater in men than in women; a region-dependent rank order of basal, forskolin-induced adenylyl cyclase activity and percentage forskolin-stimulation was observed in women only. Lastly, basal values correlated positively with forskolin-stimulated adenylyl cyclase activity in all areas except the prefrontal cortex of the male subjects. Positive significant correlations were also found between both forskolin-stimulated enzyme activity and percentage forskolin stimulation and aging in the prefrontal cortex. Overall, the findings suggest that sex and/or age-related differences in brain adenylyl cyclase vary from one cerebral region to the other.

Suc-[Glu9,Ala11,15]-endothelin-1 (8-21), IRL 1620, identifies two populations of ET(B) receptors in guinea-pig bronchus

The pharmacological properties of endothelin receptors (ETR) were investigated in guinea-pig bronchus by comparing binding and functional results. In binding assays, both the ET(B) agonists, endothelin-3 (ET-3) and N-suc-[Glu9,Ala11,15]ET-1(8-21) (IRL 1620), and the antagonist, N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D- 1-methoxycarbonyltryptophanyl-D-norleucine (BQ 788), showed biphasic inhibition curves of [125I]-endothelin-1 (ET-1) binding to bronchus membranes prepared from intact or epithelium-deprived tissue. IRL 1620 did not completely displace specifically [125I]-ET-1 bound to these tissue preparations. In the presence of the ET(A)-selective antagonist, cyclo(-D-Trp-D-Asp-L-Pro-D-Val-L-Leu) (BQ 123, 1 microM), IRL 1620 displacement curves were shallow but a complete inhibition was reached at a concentration of 1 microM. Both curves were better represented by two-site models. In addition, BQ 788 competition curves became monophasic when binding experiments were performed in the presence of 1 microM BQ 123. The non-selective agonist, ET-1, and BQ 123 inhibited [125I]-ET binding to bronchus membranes in dose-dependent fashions with monophasic curves. The contracting activity of IRL 1620 (0.55 nM- 1.6 microM) was tested on multiple-ring bronchial preparations pretreated with peptidase and cyclo-oxygenase inhibitors. BQ 788 shifted IRL1620 concentration-response curves to the right while BQ 123 did not influence bronchial responsiveness. In addition, a potentiation of the maximal response to the agonist was observed in BQ 788 treated bronchial rings. This effect was abolished by tissue pretreatment with Nomega-nitro-L-argininemethylester (L-NAME) or epithelium removal but not by pretreatment with atropine or iberiotoxin. Our results demonstrate that guinea-pig bronchus contains two populations of ET(B) receptors with different affinities for the ET(B)-selective agonist, IRL 1620. One ET(B) receptor population appears to activate bronchial muscle contraction while another on epithelial cells causes muscle relaxation through the release of nitric oxide (NO).

Synthesis and structure-activity relationship studies of new endothelin pseudopeptide analogues containing alkyl spacers

We replaced the Asp18-Ile19 dipeptide of the C-terminal ET analogue Ph-Ph-CH2-O-N=CH-CO-Phe-Asp-Ile-Ile-Trp-OH by alkyl spacers of various lengths to investigate the role of the aminoacidic central portion of the molecule and to define the N-terminal and C-terminal pharmacophoric regions of this analogue. The side-chains of the central dipeptide have been shown to be irrelevant for the binding of the molecule to the receptor, but the distance between the two postulated sites of interaction of the ligand with the ETB receptor appears to be fundamental.

Conformational changes at the carboxyl terminus of Galpha occur during G protein activation

To understand the dynamics of conformational changes during G protein activation, surface exposed cysteine residues on Galpha were fluorescently labeled. Limited trypsinolysis and mutational analysis of recombinant Galphat/Galphai1 determined that two cysteines are the major fluorescent labeling sites, Cys210, located in the switch II region, and Cys347 at the C terminus. Mutants with serines replacing Cys210 (Chi6a) and Cys347 (Chi6b) were single fluorescently labeled with lucifer yellow (LY), while a double mutant (Chi6ab) was no longer labeled. When Chi6b was labeled with LY on Cys210, AlF4- caused a 220% increase in LY fluorescence, indicating that the fluorescent group at Cys210 is a reporter of conformational change in the switch II region. Chi6a labeled at Cys347 also showed an AlF4--dependent increase in LY fluorescence (91%), indicating that Galpha activation leads to a conformational change at the COOH terminus. Preactivation of the protein with AlF4- before labeling led to a decreased incorporation of LY into Cys347 suggesting that Galpha activation buries Cys347. This COOH-terminal conformational change may provide the structural basis for communication between the GDP-binding site on Galpha and activated receptors, and may contribute to dissociation of activated Galpha subunit from activated receptor.

Effects of postmortem delay on serotonin and (+)8-OH-DPAT-mediated inhibition of adenylyl cyclase activity in rat and human brain tissues

The reproducibility of serotonin (5-HT) and (+)8-OH-DPAT-mediated inhibition of adenylyl cyclase activity was assessed in membranes, stimulated by forskolin, of rat frontal cortex postmortem as well as of human fronto-cortical, hippocampal and dorsal raphe tissues obtained from autopsy brains. The results revealed that differences between basal and forskolin-stimulated enzyme activities were still significant after 48 h postmortem in rat cortex and in all human brain regions up to 46 h after death. However, a decrease of about 17 and 26% in forskolin-stimulated adenylyl cyclase activity was observed at 24 and 48 h, respectively, in rat cortex. 5-HT and the 5-HT1A receptor agonist, (+)8-hydroxy-2(di-N-propylamino)tetraline (8-OH-DPAT), were able to inhibit forskolin-stimulated adenylyl cyclase activity in a dose-dependent manner for 48 h after death in rat and human brain. In rat cortex, both 5-HT and (+)8-OH-DPAT potencies (EC50, nM) and efficacies (percent of maximum inhibition capacity, %) varied significantly with postmortem delay. Conversely, in human tissues, postmortem delay and subject age did not modify agonist potencies and efficacies. Furthermore, a regionality of 5-HT potency and efficacy was revealed in the human brain. 5-HT was equally potent in cortex and raphe nuclei, while being more potent but less effective in hippocampus. (+)8-OH-DPAT was more active in hippocampus and raphe nuclei than in cortex. (+)8-OH-DPAT behaved as an agonist in all areas, as its efficacy was similar or greater than those obtained with 5-HT. The (+)8-OH-DPAT dose-response curve was completely reversed by 5-HT1A receptor antagonists in rat cortex and all human brain areas. In conclusion, we suggest here that differences between rat and human brain might exist at the level of postmortem degradation of 5-HT-sensitive adenylyl cyclase activity. In human brain, 5-HT1A receptor-mediated inhibition of adenylyl cyclase seems to be reproducible, suggesting that reliable experiments can be carried out on postmortem specimens from patients with neuropsychiatric disorders.

Toward the rational development of peptidomimetic analogs of the C-terminal endothelin hexapeptide: development of a theoretical model

In an early report on the structure-activity relationship of endothelin (ET) peptides, it was reported that the C-terminal hexapeptide ET(16-21), His-Leu-Asp-Ile-Ile-Trp, is the minimum ET fragment which maintains biological activity in some, but not all the tissues responding to ETs. Subsequently, other authors described a series of analogs of this peptide, in which the His 16 residue was replaced by non-natural amino acids, characterized by bulky aromatic side chains. Among them, two well-characterized non-selective ETA/ETB antagonists were PD 142893 and PD 145065; interest in these potent ET antagonists was, however, reduced by their peptidic structure which was likely to lead to undesirable properties such as poor bioavailability and short duration of action. On the basis of these premises, our previous studies led to the development of a peptidomimetic ligand of ET receptors (compound 3), based on the replacement of the His 16 residue of ET(16-21) with an (E)-N-(benzyloxy)iminoacyl moiety; compound 3 proved to possess a certain affinity for ET receptors, albeit lower than that shown by PD 142893 and PD 145065. We report here on ETA/ETB binding affinity of compounds 4-12, designed as a new series of ET(16-21) analogs. Compounds 4 and 5 were practically devoid of any affinity; derivatives 6-12 exhibited appreciable affinity indices for ETB receptors higher than that shown by 3, even if still lower than that obtained for PD 145065. This paper also describes the development of a pharmacophoric model able to explain the ET receptor binding properties of our hexapeptide analogs compared with those of PD 142893 and PD 145065 and IRL2500, recently reported as a potent ETB selective endothelin antagonist.

Antagonists of the receptor-G protein interface block Gi-coupled signal transduction

The carboxyl terminus of heterotrimeric G protein alpha subunits plays an important role in receptor interaction. We demonstrate that peptides corresponding to the last 11 residues of Galphai1/2 or Galphao1 impair agonist binding to A1 adenosine receptors, whereas Galphas or Galphat peptides have no effect. Previously, by using a combinatorial library we identified a series of Galphat peptide analogs that bind rhodopsin with high affinity (Martin, E. L., Rens-Domiano, S., Schatz, P. J., and Hamm, H. E. (1996) J. Biol. Chem. 271, 361-366). Native Galphai1/2 peptide as well as several analogs were tested for their ability to modulate agonist binding or antagonist-agonist competition using cells overexpressing human A1 adenosine receptors. Three peptide analogs decreased the Ki, suggesting that they disrupt the high affinity receptor-G protein interaction and stabilize an intermediate affinity state. To study the ability of the peptides to compete with endogenous Galphai proteins and block signal transduction in a native setting, we measured activation of G protein-coupled K+ channels through A1 adenosine or gamma-aminobutyric acid, type B, receptors in hippocampal CA1 pyramidal neurons. Native Galphai1/2, peptide, and certain analog peptides inhibited receptor-mediated K+ channel gating, dependent on which receptor was activated. This differential perturbation of receptor-G protein interaction suggests that receptors that act on the same G protein can be selectively disrupted.

Structure-activity analysis of C-terminal endothelin analogues

Several synthetic endothelin (ET) analogues of the C-terminal ET hexapeptide (ET16-21) were analyzed by radio-receptor competition binding assays and biologic activity using both ETA and ETB receptor subtypes. In addition, we produced a hybridoma monoclonal antibody, anti-ET15-21, that appeared to crossreact with the entire ET molecule and was able to neutralize its biologic activity. Antibody binding was measured with competition enzyme-linked immunosorbent assays and a surface plasmon resonance-based biosensor (BIA technology). The ET16-21 moiety was modified with systematic replacement of each residue by alanine (Ala-scan). Whereas the C-terminal residues (Asp18, Ile20, and particularly Trp21) were very important for both receptor binding and immunologic activity, Ala substitution in positions 16, 17, and 19 hardly affected such activities. Analysis of another series of synthetic ET16-21 analogues with the His16 residue replaced by a non-amino-acidic block confirmed that the last two C-terminal residues are essential for receptor and antibody binding, whereas the central region of this hexapeptide is much more tolerant to modification. However, a critical steric conformation of the active hexapeptide is necessary.

2'-C-Methyl analogues of selective adenosine receptor agonists: synthesis and binding studies

2'-C-Methyl analogues of selective adenosine receptor agonists such as (R)-PIA, CPA, CCPA, NECA, and IB-MECA were synthesized in order to further investigate the subdomain that binds the ribose moiety. Binding affinities of these new compounds at A1 and A2A receptors in bovine brain membranes and at A3 in rat testis membranes were determined and compared. It was found that the 2'-C-methyl modification resulted in a decrease of the affinity, particularly at A2A and A3 receptors. When such modification was combined with N6-substitutions with groups which induce high potency and selectivity at A1 receptors, the high affinity was retained and the selectivity was increased. Thus, 2-chloro-2'-C-methyl-N6-cyclopentyladenosine (2'-Me-CCPA), which displayed a Ki value of 1.8 nM at A1 receptors, was selective for A1 vs A2A and A3 receptors by 2166- and 2777-fold, respectively, resulting in one of the most potent and A1-selective agonists so far known. In functional assay, this compound inhibited forskolin-stimulated adenylyl cyclase activity with an IC50 value of 13.1 nM, acting as a full agonist.

Role of cysteine residues of rat A2a adenosine receptors in agonist binding

In the present study, we investigated the role of disulfide bridges and sulfhydryl groups in A2a adenosine receptor binding of the agonist 2-p-(2-carboxyethyl)phenylethylamino)-5'-N-ethylcarboxamidoadenosi ne (CGS 21680). To evaluate the presence of essential disulfide bridges, rat striatal membranes were incubated with [3H]CGS 21680 in the presence of dithiothreitol and binding of the agonist to membranes was measured. The amount of [3H]CGS 21680 which specifically bound, decreased progressively upon pretreatment of membranes with increasing concentrations of dithiothreitol. Pretreatment of rat striatal membranes with 12.5 mM dithiothreitol for 15 min at 25 degrees C resulted in a 2-fold decrease of A2a adenosine receptor affinity for [3H]CGS 21680, and a reduction in the maximal number of binding sites. The presence of agonist or antagonist ligands protected the A2a adenosine receptor sites from the effect of dithiothreitol. We also examined the susceptibility of A2a adenosine receptors to inactivation by the sulfhydryl alkylating reagent, N-ethylmaleimide. When rat striatal membranes were pretreated with N-ethylmaleimide for 30 minutes at 37 degrees C, a decrease in specific [3H]CGS 21680 binding was observed. Pretreatment of membranes with 1 mM N-ethylmaleimide also resulted in a 2-fold reduction of A2a adenosine receptor affinity for [3H]CGS 21680, as well as a slight decrease in the maximal number of binding sites. Neither agonist nor antagonist ligands were effective in protecting the receptor sites from inactivation by N-ethylmaleimide. In contrast, addition of 100 microM guanosine-5'-O-(3-thiotriphosphate) or 5'-guanylylimidodiphosphate were both effective in protecting the receptor sites from inactivation by N-ethylmaleimide. This protective effect was significant but not complete. Our data suggest that disulfide bridges play a role in the structural integrity of the A2a adenosine receptor, furthermore, reduced sulfhydryl groups appear to be important but we do not yet know if they are on the receptor or on the Gs alpha subunit.

Interaction of transducin with light-activated rhodopsin protects It from proteolytic digestion by trypsin

The tryptic cleavage pattern of transducin (Gt) in solution was compared with that in the presence of phospholipid vesicles, rod outer segment (ROS) membranes kept in the dark, or ROS membranes containing light-activated rhodopsin, metarhodopsin II (Rh*). When Gt was in the high affinity complex with Rh*, the alphat subunit was almost completely protected from proteolysis. The protection of alphat at Arg310 was complete, while Arg204 was substantially protected. The cleavage of alphat at Lys18 was protected in the presence of phospholipid vesicles, ROS membranes kept in the dark, or ROS membranes containing Rh*. The cleavage of betat was slower in the presence of ROS membranes or phospholipid vesicles. When the Rh*. Gt complex was incubated with guanyl-5'-yl thiophosphate, a guanine nucleotide analog known to release the high affinity interaction between Gt and Rh*, the protection at Arg310 and Arg204 was diminished. From our results, we propose that Rh* either physically blocks access of trypsin to Arg204 and Arg310 or maintains the heterotrimer in such a conformation that these cleavage sites are not available. Since Arg204 is involved in the switch interface with betagammat (Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319), it may be that betagammat is implicated in protecting this cleavage site in the receptor-bound, stabilized heterotrimer. Arg310 is not near the betagammat subunit, thus we believe that the high affinity binding of Gt to Rh* physically or sterically blocks access of trypsin to this site. Thus, Arg310, only a few angstroms away from the carboxyl terminus of alphat, which is known to directly bind to Rh*, is likely to also be a part of the Rh* binding site. This is in agreement with other studies and has implications for the mechanism by which receptors catalyze GDP release from G proteins. The protection of Lys18 in the presence of phospholipid vesicles suggests that the amino-terminal region is in contact with the membrane, consistent with the crystal structure of the heterotrimer (Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319).

Characterization of a low affinity binding site for N6-substituted adenosine derivatives in rat testis membranes

The binding characteristics of radiolabeled N6-(cyclohexyl)adenosine ([3H]CHA), N6-(R-phenylisopropyl)adenosine ([3H]R-PIA), 5'-N-ethyl-carboxamidoadenosine ([3H]NECA), and 2-[4-(2-carboxyethyl)phenyl]ethyl-amino-5'-N-ethylcarboxamidoadenosine ([3H]CGS 21680), to rat testis membranes were investigated. Specific binding of [3H]CGS 21680, a selective agonist for the A2a adenosine receptor, was very modest whilst the nonselective agonist [3H]NECA bound to rat testis membranes showing high binding capacity. At least two types of binding sites for [3H]NECA could be identified in rat testis membranes: high affinity sites and high capacity sites. Selective agonists for the A1 adenosine receptor, [3H]CHA and [3H]R-PIA bound with high affinity to a single class of binding sites. This high affinity binding site showed the typical pharmacological specificity of the A1 adenosine receptor with a potency order for agonists of CHA > or = R-PIA > NECA > N6-(S-phenylisopropyl)adenosine (S-PIA). In order to detect the presence of the A3 adenosine receptor in these membranes we selectively blocked the A1 receptor with a large molar excess of a xanthine antagonist, either 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) or xanthine amine congener (XAC). In the presence of an antagonist a low affinity binding site for [3H]CHA and [3H]R-PIA was detected. This low affinity binding site showed a different pharmacological specificity than the high affinity binding site. In fact the potency order for agonists was CHA > or = NECA = R-PIA > S-PIA. This finding suggests that the low affinity binding site represents the A3 adenosine receptor.

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

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

Tryptophan207 is involved in the GTP-dependent conformational switch in the alpha subunit of the G protein transducin: chymotryptic digestion patterns of the GTP gamma S and GDP-bound forms

The limited proteolytic pattern of transducin, Gt, and its purified subunits with chymotrypsin were analyzed and the cleavage sites on the alpha t subunit were identified. The alpha t subunit in the GTP gamma S bound form was cleaved into a major 38 kD fragment, whereas alpha t-GDP was progressively digested into 38, 23, 21, and 15 kD fragments. The beta gamma t subunit was not very sensitive to proteolytic digestion with chymotrypsin. The gamma t subunit was not cleaved and only a small portion of beta t was digested into several fragments. In order to determine which proteolytic fragment of alpha t still contained the carboxyl terminal region, chymotrypsinization was carried out using Gt previously 32P-labeled at Cys347 by pertussis toxin-catalyzed ADP-ribosylation. The 32P-label was mainly associated with the alpha t subunit and a 15 kD fragment. The 23 and 21 kD fragments were not 32P-labeled. Analysis of amino terminal sequences of 38, 21, and 15 kD proteolytic bands allowed the identification of the major cleavage sites. Chymotrypsin had two cleavage sites in the amino terminal region of alpha t, at Leu15 and Leu19. Chymotrypsin removed 15-19 amino acid residues from the amino terminus of alpha t, generating two peptides (38 kD) which comigrates in gel electrophoresis. Chymotrypsin also cleaved at Trp207 in a conformation-dependent manner. Trp207 of alpha t-GTP gamma S was resistant to proteolysis but alpha t-GDP and the 38 kD fragments of alpha t-GDP produced the 23 and 21 kD fragments, respectively, and a 15 kD fragment containing the carboxyl terminus.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural analysis of rod GTP-binding protein, Gt. Limited proteolytic digestion pattern of Gt with four proteases defines monoclonal antibody epitope

The epitope of monoclonal antibody (mAb 4A), which recognizes the alpha subunit of the rod G protein, Gt, has been suggested to be both at the carboxyl terminus (Deretic, D., and Hamm, H.E. (1987) J. Biol. Chem. 262, 10839-10847) and the amino terminus (Navon, S.E., and Fung, B.K.-K. (1988) J. Biol. Chem. 263, 489-496) of the molecule. To characterize further the mAb 4A binding site on alpha t and to resolve the discrepancy between these results limited proteolytic digestion of Gt or alpha t using four proteases with different substrate specificities has been performed. Endoproteinase Arg-C, which cleaves the peptide bond at the carboxylic side of arginine residues, cleaved the majority of alpha t into two fragments of 34 and 5 kDa. The alpha t 34-kDa fragment in the holoprotein, but not alpha t-guanosine 5'-O-(3-thiotriphosphate), was converted further to a 23-kDa fragment. A small fraction of alpha t-GDP was cleaved into 23- and 15-kDa fragments. Endoproteinase Lys-C, which selectively cleaves at lysine residues, progressively removed 17 and then 8 residues from the amino terminus, forming 38- and 36-kDa fragments. Staphylococcus aureus V8 protease is known to remove 21 amino acid residues from the amino-terminal region of alpha t, with the formation of a 38-kDa fragment. L-1-Tosylamido-2-phenylethyl chloromethyl ketone-treated trypsin cleaved alpha t progressively into fragments of known amino acid sequences (38, then 32 and 5, then 21 and 12 kDa) and a transient 34 kDa fragment. The binding of mAb 4A to proteolytic fragments was analyzed by Western blot and immunoprecipitation. The major fragments recognized by mAb 4A on Western blots were the 34- and 23-kDa fragments obtained by endoproteinase Arg-C and tryptic digestion. Under conditions that allowed sequencing of the 15- and 5-kDa fragments neither the 34- nor the 23-kDa fragments could be sequenced by Edman degradation, indicating that they contained a blocked amino terminus. The smallest fragment that retained mAb 4A binding was the 23-kDa fragment containing Met1 to Arg204. Thus the main portion of the mAb 4A antigenic site was located within this fragment, indicating that the carboxyl-terminal residues from Lys205 to Phe350 were not required for recognition by the antibody. Additionally, the antibody did not bind the 38- and 36-kDa or other fragments containing the carboxyl terminus, showing that the amino-terminal residues from Met1 to Lys17 were essential for antibody binding to alpha t.

Superoxide dismutase-like activity of 1,2,3-triazole derivatives

This paper reports the synthesis of some sulfurated 1,2,3-triazole derivatives and the biological evaluation, for the superoxide dismutase-like activity, of these new compounds together with several 1,2,3-triazoles previously described by us. All the compounds possess a SOD-like activity and some of them showed an activity equivalent to or higher than those of penicillamine or thiopronin, which are well known therapeutic drugs in the treatment of rheumatoid arthritis.

Effect of monoclonal antibody binding on alpha-beta gamma subunit interactions in the rod outer segment G protein, Gt

The guanyl nucleotide binding regulatory protein of retinal rod outer segments, called Gt, that couples the photon receptor rhodopsin with the light-activated cGMP phosphodiesterase, can be resolved into two functional components, alpha t and beta gamma t. The effect of monoclonal antibody binding to the alpha t subunit of Gt on subunit association has been investigated in the present study. It was previously shown that this monoclonal antibody, mAb 4A, blocks interactions with rhodopsin and its epitope was located within the region Arg310-Phe350 at the COOH terminus of the alpha t subunit. In this paper, we show that mAb 4A disrupts the Gt complex. Gt migrates in 5-20% linear sucrose density gradients as a monomer, with a sedimentation coefficient of 4.1 +/- 0.07 S, while in the presence of mAb 4A, the alpha t and beta gamma t subunits show sedimentation coefficients of 7.7 +/- 0.2 and 3.7 +/- 0.1 S, respectively. The beta gamma t subunit migrates with the same sedimentation rate as pure beta gamma t. Nonimmune rabbit IgG does not modify the sedimentation behavior of Gt. The Fab fragment of mAb 4A also dissociates the Gt complex, as suggested by the change of the sedimentation rate of alpha t. This effect of mAb 4A on Gt subunit association was also confirmed by immunoprecipitation studies in the presence of detergent. In the presence of detergent, subunit association is not affected, but the formation of Gt oligomers and, therefore, the nonspecific precipitation of beta gamma t subunit are reduced.(ABSTRACT TRUNCATED AT 250 WORDS)