Structure of the noncompetitive antagonist-binding site of the Torpedo nicotinic acetylcholine receptor. [3H]meproadifen mustard reacts selectively with alpha-subunit Glu-262.

[3H]Meproadifen mustard, an affinity label for the noncompetitive antagonist site of the nicotinic acetylcholine receptor (AChR), specifically alkylates the AChR alpha-subunit when the acetylcholine-binding sites are occupied by agonist (Dreyer, E. B., Hasan, F., Cohen, S. G., and Cohen, J. B. (1986) J. Biol. Chem. 261, 13727-13734). In this report, we identify the site of alkylation within the alpha-subunit as Glu-262. AChR-rich membranes from Torpedo californica electric organ were reacted with [3H]meproadifen mustard in the presence of carbamylcholine and in the absence or presence of nonradioactive meproadifen to define specific alkylation of the noncompetitive antagonist site. Alkylated alpha-subunits were isolated and subjected to chemical or enzymatic cleavage. When digests with CNBr in 70% trifluoroacetic acid or 70% formic acid were fractionated by gel filtration high performance liquid chromatography (HPLC), specifically labeled material was recovered in the void volume fractions. Based upon NH2-terminal sequence analysis, for both digests, the void volume fractions contained a fragment beginning at Gln-208 before the M1 hydrophobic sequence, whereas the sample from the digest in trifluoroacetic acid also contained as a primary sequence a fragment beginning at Thr-244 and extending through the M2 hydrophobic sequence. Sequence analysis revealed no release of 3H for the sample from digestion in formic acid, whereas for the trifluoroacetic acid digest, there was specific release of 3H in cycle 19, which would correspond to Glu-262. This site of alkylation was confirmed by isolation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and reversed-phase HPLC of a specifically labeled fragment from an endoproteinase Lys-C digest of the alkylated alpha-subunit. NH2-terminal amino acid sequencing revealed release of 3H at cycle 20 from a fragment beginning at Met-243 and extending into the M3 hydrophobic sequence. Because [3H]meproadifen mustard contains, as its reactive group, a positively charged quaternary aziridinium ion, Glu-262 of the alpha-subunit is identified as a contributor to the cation-binding domain of the noncompetitive antagonist-binding site and thus of the ion channel.     

Reaction of [3H]meproadifen mustard with membrane-bound Torpedo acetylcholine receptor

The Torpedo nicotinic acetylcholine receptor (AChR) contains a binding site for aromatic amine noncompetitive antagonists that is distinct from the binding site for agonists and competitive antagonists. To characterize the location and function of this allosteric antagonist site, an alkylating analog of meproadifen has been synthesized, 2-(chloroethylmethylamino)-ethyl-2, 2-diphenylpentanoate HCl (meproadifen mustard). Reaction of [3H]meproadifen mustard with AChR-rich membrane suspensions resulted in specific incorporation of label predominantly into the AChR alpha-subunit with minor incorporation into the beta-subunit. Specific labeling required the presence of high concentration of agonist and was inhibited by reversible noncompetitive antagonists including proadifen, meproadifen, perhydrohistrionicotoxin (HTX), and tetracaine when present at concentrations consistent with the binding affinity of these compounds for the allosteric antagonist site. No specific alkylation of the AChR alpha-subunit was detected in the absence of agonist, or in the presence of the partial agonist phenyltrimethylammonium or the competitive antagonists, d-tubocurarine, gallamine triethiodide, or decamethonium. Reaction with 35 microM meproadifen mustard for 70 min in the presence of carbamylcholine produced no alteration in the concentration of [3H]ACh-binding sites, but decreased by 38 +/- 4% the number of allosteric antagonist sites as measured by [3H]HTX binding. This decrease was not observed when the alkylation reaction was blocked by the presence of HTX. These results lead us to conclude that meproadifen mustard alkylates the allosteric antagonist site in the Torpedo AChR and that part of that site is associated with the AChR alpha-subunit.     

Mapping of the acetylcholine binding site of the nicotinic acetylcholine receptor: [3H]nicotine as an agonist photoaffinity label

The agonist [3H]nicotine was used as a photoaffinity label for the acetylcholine binding sites on the Torpedo nicotinic acetylcholine receptor (AChR). [3H]nicotine binds at equilibrium with Keq = 0.6 microM to the agonist binding sites. Irradiation with 254-nm light of AChR-rich membranes equilibrated with [3H]nicotine resulted in covalent incorporation into the alpha- and gamma-subunits, which was inhibited by agonists and competitive antagonists but not by noncompetitive antagonists. Inhibition of labeling by d-tubocurarine demonstrated that the alpha-subunit was labeled via both agonist sites but the gamma-subunit was labeled only via the site that binds d-tubocurarine with high affinity. Within the alpha-subunit, 93% of the labeling was contained within a 20-kDa Staphylococcus aureus V8 proteolytic fragment beginning at Ser-173. Sequence analysis of this peptide indicated that approximately 80% of the incorporation was into Tyr-198, approximately 13% was into Cys-192, and approximately 7% was into Tyr-190. Chymotryptic digestion of the alpha-subunit confirmed that Tyr-198 was the principal amino acid labeled by [3H]nicotine. This confirmation required a novel radio-sequencing strategy employing omicron-phthalaldehyde, since the efficiency of photolabeling was low (approximately 1.0%) and the labeled chymotryptic peptide was not isolated in sufficient quantity to be identified by mass. [3H]Nicotine, which is the first photoaffinity agonist used, labels primarily Tyr-198 in contrast to competitive antagonist affinity labels, which label primarily Tyr-190 and Cys-192/Cys-193.     

Location of ligand-binding sites on the nicotinic acetylcholine receptor alpha-subunit

The portions of the Torpedo californica nicotinic acetylcholine receptor (AChR) alpha-subunit that contribute to the allosteric antagonist-binding site and to the agonist-binding site have been localized by affinity labeling and proteolytic mapping. [3H]Meproadifen mustard was employed as an affinity label for the allosteric antagonist-binding site and [3H]tubocurare as a photoaffinity label for the agonist-binding site. Both labels were found in a 20-kDa proteolytic fragment generated from the AChR alpha-subunit by Staphylococcus aureus V8 protease. This 20-kDa peptide also contains the 3H-labeled 4-(N-maleimido)-alpha-benzyltrimethylammonium iodide-reactive site and binds 125I-alpha-bungarotoxin. N-terminal sequencing established that the 20-kDa fragment began at Ser-173 of the alpha-subunit. Fluorescein isothiocyanate-conjugated concanavalin A could be bound to the second of the two major V8 cleavage products, an 18-kDa peptide. This peptide was also sensitive to treatment with endo-beta-N-acetyl-glucosaminidase H, consistent with the presence of N-linked carbohydrate on this fragment. The N terminus of this peptide was found to be Val-46 of the alpha-subunit sequence. Experiments designed to map disulfide bonds within the AChR alpha-subunit indicate that no bonds exist between the 18-kDa fragment (containing Cys-128 and Cys-142) and the 20-kDa fragment (containing Cys-192, Cys-193, and Cys-222). These results establish that the 20-kDa fragment contributes to both the acetylcholine and the allosteric antagonist-binding sites, whereas there is no evidence that the 18-kDa fragment is part of either site.     

Structure of the agonist-binding sites of the Torpedo nicotinic acetylcholine receptor: affinity-labeling and mutational analyses identify gamma Tyr-111/delta Arg-113 as antagonist affinity determinants.

Photoaffinity labeling of the Torpedo nicotinic acetylcholine receptor (nAChR) with [3H]d-tubocurarine (dTC) has identified a residue within the gamma-subunit which, along with the analogous residue in delta-subunit, confers selectivity in binding affinities between the two agonist sites for dTC and alpha-conotoxin (alpha Ctx) MI. nAChR gamma-subunit, isolated from nAChR-rich membranes photolabeled with [3H]dTC, was digested with Staphylococcus aureus V8 protease, and a 3H-labeled fragment was purified by reversed-phase high-performance liquid chromatography. Amino-terminal sequence analysis of this fragment identified 3H incorporation in gamma Tyr-111 and gamma Tyr-117 at about 5% and 1% of the efficiency of [3H]dTC photoincorporation at gamma Trp-55, the primary site of [3H]dTC photoincorporation within gamma-subunit [Chiara, D. C., and Cohen, J. B. (1997) J. Biol. Chem 272, 32940-32950]. The Torpedo nAChR delta-subunit residue corresponding to gamma Tyr-111 (delta Arg-113) contains a positive charge which could confer the lower binding affinity seen for some competitive antagonists at the alpha-delta agonist site. To test this hypothesis, we examined by voltage-clamp analysis and/or by [125I]alpha-bungarotoxin competition binding assays the interactions of acetylcholine (ACh), dTC, and alpha Ctx MI with nAChRs containing gamma Y111R or delta R113Y mutant subunits expressed in Xenopus oocytes. While these mutations affected neither ACh equilibrium binding affinity nor the concentration dependence of channel activation, the gamma Y111R mutation decreased by 10-fold dTC affinity and inhibition potency. Additionally, each mutation conferred a 1000-fold change in the equilibrium binding of alpha Ctx MI, with delta R113Y enhancing and gamma Y111R weakening affinity. Comparison of these results with previous results for mouse nAChR reveals that, while the same regions of gamma- (or delta-) subunit primary structure contribute to the agonist-binding sites, the particular amino acids that serve as antagonist affinity determinants are species-dependent.     

Evidence for Structural Dissimilarity in the Neurotransmitter Binding Region of Purified Acetylcholine Receptors from Human Muscle and Torpedo Electric Organ

Acetylcholine receptor (AChR) purified from human skeletal muscle affinity-alkylated with bromoacetyl[methyl-3H]choline bromide ([3H]BAC) in mildly reducing conditions to yield a specifically radiolabeled polypeptide, Mr 44,000, the alpha-subunit. The binding of [125I]alpha-bungarotoxin to AChR was completely inhibited by affinity-alkylation, indicating that the human AChR's binding site for alpha-bungarotoxin is closely associated with the alpha-subunit's acetylcholine binding site. Structures in the vicinity of the alpha-bungarotoxin binding sites of AChRs from human muscle and Torpedo electric organ were compared by varying the conditions of alkylation. Under optimal conditions of reduction and alkylation, both human and Torpedo AChR incorporated BAC in equivalence to the number of alpha-bungarotoxin binding sites. However, with limited conditions of reduction but sufficient BAC to alkylate 100% of the alpha-bungarotoxin binding sites of human AChR, only 71% of the Torpedo AChR's binding sites were alkylated. In optimal conditions of reduction but with the minimal concentration of BAC that permitted 100% alkylation of the human AChR's alpha-bungarotoxin sites, only 74% of the Torpedo AChR's binding sites were alkylated. These data suggest that the neurotransmitter binding region of human muscle AChR is structurally dissimilar from that of Torpedo electric organ, having a higher binding affinity for BAC and an adjacent disulfide bond that is more readily accessible to reducing agents.     

Ligand-induced variations in the reactivity of thio groups of the alpha-subunit of the acetylcholine receptor from Torpedo californica

We have studied alkylation of the acetylcholine receptor by N-[3H]ethylmaleimide ([3H]NEM) under various conditions. The radiolabeled preparations were submitted to sodium dodecyl sulfate-polyacrylamide gel electrophoresis to separate the receptor complex into subunits, and the incorporation of 3H into each type of chain was determined. We found the following: (i) When cysteines of native receptor in intact membranes were reacted with [3H]NEM, only the beta-subunit was labeled; the extent of alkylation did not change significantly if cholinergic effectors were present during this reaction. (ii) When the disulfide bonds of the receptor were reduced with dithiothreitol (DTT), the alpha- and beta-chains were labeled with [3H]NEM. The presence of receptor agonists and competitive antagonists during alkylation significantly altered the labeling patterns. Gallamine and hexamethonium markedly enhanced, while carbamylcholine and decamethonium markedly lessened, labeling of the alpha-subunit. Choline, d-tubocurarine, and alpha-neurotoxin induced small, but significant decreases in alkylation of the alpha-subunit, while procaine had no effect. (iii) When the same ligands were present during the reduction step, subsequent labeling with [3H]NEM produced patterns similar to those described in (ii). We also investigated the effects of gallamine and hexamethonium on reduction of the disulfide bond located near the acetylcholine binding site by using the affinity alkylating reagent (bromoacetyl)choline (BAC). Gallamine (0.1 mM) was able to increase the rate of reduction of this particular disulfide bond 3-fold in comparison to the control. In these experiments, alkylation by BAC blocked 50% of the toxin binding sites. Hexamethonium (1 mM) had a similar effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mapping the agonist binding site of the nicotinic acetylcholine receptor. Orientation requirements for activation by covalent agonist

To characterize the structural requirements for ligand orientation compatible with activation of the Torpedo nicotinic acetylcholine receptor (nAChR), we used Cys mutagenesis in conjunction with sulfhydryl-reactive reagents to tether primary or quaternary amines at defined positions within the agonist binding site of nAChRs containing mutant alpha- or gamma-subunits expressed in Xenopus oocytes. 4-(N-Maleimido)benzyltrimethylammonium and 2-aminoethylmethanethiosulfonate acted as irreversible antagonists when tethered at alphaY93C, alphaY198C, or gammaE57C, as well as at alphaN94C (2-aminoethylmethanethiosulfonate only). [2-(Trimethylammonium)-ethyl]-methanethiosulfonate (MTSET), which attaches thiocholine to binding site Cys, also acted as an irreversible antagonist when tethered at alphaY93C, alphaN94C, or gammaE57C. However, MTSET modification of alphaY198C resulted in prolonged activation of the nAChR not reversible by washing but inhibitable by subsequent exposure to non-competitive antagonists. Modification of alphaY198C (or any of the other positions tested) by [(trimethylammonium)methyl]methanethiosulfonate resulted only in irreversible inhibition, while modification of alphaY198C by [3-(trimethylammonium)propyl]methanethiosulfonate resulted in irreversible activation of nAChR, but at lower efficacy than by MTSET. Thus changing the length of the tethering arm by less than 1 A in either direction markedly effects the ability of the covalent trimethylammonium to activate the nAChR, and agonist activation depends on a very selective orientation of the quaternary ammonium within the agonist binding site.     

The hydrophobic photoreagent 3-(trifluoromethyl)-3-m-([125I] iodophenyl) diazirine is a novel noncompetitive antagonist of the nicotinic acetylcholine receptor

We have shown previously that the lipophilic photoreagent 3-(trifluoromethyl)3-m-([125I]iodophenyl)-diazirine ([125I]TID) photolabels all four subunits of the Torpedo nicotinic acetylcholine receptor (AChR) and that greater than 70% of this photoincorporation is inhibited by cholinergic agonists and some noncompetitive antagonists, including histrionicotoxin (HTX), but not phencyclidine (PCP; White, B.H., and Cohen, J.B. (1988) Biochemistry 27, 8741-8751). We have now examined the effects of nonradioactive TID on (a) AChR photoincorporation of [125I]TID, (b) AChR-mediated ion transport, and (c) AChR binding of several cholinergic ligands. We find that TID inhibits [125I]TID photoincorporation into the AChR to the same extent as carbamylcholine. The saturable component of [125I]TID photolabeling is half-maximal at 4 microM [125I]TID with 0.5 mol specifically incorporated per mol of AChR after 30 min photolysis with 60 microM [125I]TID. Repeated labeling of membranes at a fixed [125I]TID concentration gave results consistent with a maximal incorporation of one [125I]TID molecule per AChR. Nonradioactive TID also noncompetitively inhibits agonist-stimulated 22Na+ efflux from Torpedo vesicles with an IC50 of 1 microM. Furthermore, TID inhibits allosterically the binding of [3H]HTX, decreasing its affinity for the AChR 5-fold both in the presence and absence of agonist. In contrast, TID has little effect on [3H]PCP binding in the absence of agonist but completely inhibits it in the presence of agonist. TID enhances the cooperativity of [3H]nicotine binding. [125I]TID is thus a photoaffinity label for a novel noncompetitive antagonist binding site on the AChR that is linked allosterically to the binding sites of both agonists and other noncompetitive antagonists. The [125I]TID site is presumably located within the central pore of the AChR.     

Photolabeling of membrane-bound Torpedo nicotinic acetylcholine receptor with the hydrophobic probe 3-trifluoromethyl-3-(m-[125I]iodophenyl)diazirine

The hydrophobic, photoactivatable probe 3-trifluoromethyl-3-(m-[125I]iodophenyl)diazirine ([125I]TID) was used to label acetylcholine receptor rich membranes purified from Torpedo californica electric organ. All four subunits of the acetylcholine receptor (AChR) were found to incorporate label, with the gamma-subunit incorporating approximately 4 times as much as each of the other subunits. Carbamylcholine, an agonist, and histrionicotoxin, a noncompetitive antagonist, both strongly inhibited labeling of all AChR subunits in a specific and dose-dependent manner. In contrast, the competitive antagonist alpha-bungarotoxin and the noncompetitive antagonist phencyclidine had only modest effects on [125I]TID labeling of the AChR. The regions of the AChR alpha-subunit that incorporate [125I]TID were mapped by Staphylococcus aureus V8 protease digestion. The carbamylcholine-sensitive site of labeling was localized to a 20-kDa V8 cleavage fragment that begins at Ser-173 and is of sufficient length to contain the three hydrophobic regions M1, M2, and M3. A 10-kDa fragment beginning at Asn-339 and containing the hydrophobic region M4 also incorporated [125I]TID but in a carbamylcholine-insensitive manner. Two further cleavage fragments, which together span about one-third of the alpha-subunit amino terminus, incorporated no detectable [125I]TID. The mapping results place constraints on suggested models of AChR subunit topology.     

Mapping the alpha-subunit site photolabeled by the noncompetitive inhibitor [3H]quinacrine azide in the active state of the nicotinic acetylcholine receptor

We have characterized the time-resolved labeling of a site on the Torpedo californica electrocyte acetylcholine receptor (ACHR) by the photoreactive noncompetitive inhibitor derivative quinacrine azide (QA). The dependence of [3H]QA labeling on acetylcholine (ACH) concentration and on time is consistent with the preferential labeling by [3H]QA of ACHR in the open state. The ACH-dependent [3H]QA labeling, which was associated predominantly with the alpha-subunit, was blocked by other noncompetitive inhibitors including quinacrine, chlorpromazine, proadifen, histrionicotoxin, and bupivacaine. alpha-Subunit from ACHR labeled with [3H]QA 20 ms after the addition of ACH was cleaved with CNBr, and the fragments were separated by high pressure liquid chromatography. A peptide containing a major site of specific labeling was purified on two different reverse-phase columns. By N-terminal sequencing, amino acid composition, binding to mercurial-agarose, and apparent molecular weight, this [3H]QA-labeled peptide was identified as alpha-208-243, a CNBr fragment containing the putative membrane-spanning helix M1.     

Cembranoid and long-chain alkanol sites on the nicotinic acetylcholine receptor and their allosteric interaction

Long-chain alkanols are general anesthetics which can also act as uncharged noncompetitive inhibitors of the peripheral nicotinic acetylcholine receptor (AChR) by binding to one or more specific sites on the AChR. Cembranoids are naturally occurring, uncharged noncompetitive inhibitors of peripheral and neuronal AChRs, which have no demonstrable general anesthetic activity in vivo. In this study, [3H]tenocyclidine ([3H]TCP), an analogue of the cationic noncompetitive inhibitor phencyclidine (PCP), was used to characterize the cembranoid and long-chain alkanol sites on the desensitized Torpedo californica AChR and to investigate if these sites interact. These studies confirm that there is a single cembranoid site which sterically overlaps the [3H]TCP channel site. This cembranoid site probably also overlaps the sites for the cationic noncompetitive inhibitors, procaine and quinacrine. Evidence is also presented for one or more allosteric cembranoid sites which negatively modulate cembranoid affinity for the inhibitory site. In contrast, long-chain alkanols inhibit [3H]TCP binding through an allosteric mechanism involving two or more alkanol sites which display positive cooperativity toward each other. Double inhibitor studies show that the cembranoid inhibitory site and the alkanol sites are not independent of each other but interfere allosterically with each other's inhibition of [3H]TCP binding. The simplest models consistent with the observed data are presented and discussed.     

Photoaffinity labeling of Torpedo nicotinic receptor with the agonist [3H]DCTA: identification of amino acid residues which contribute to the binding of the ester moiety of acetylcholine

Torpedo marmorata acetylcholine binding sites were photolabeled using 360 nm light, at equilibrium in the desensitized state, with the agonist [3H]DCTA utilizing the CeIV/glutathione procedure described previously (Grutter, et al. (1999) Biochemistry 38, 7476-7484). Photoincorporation of [3H]DCTA was concentration-dependent with a maximum of 7.5% specific labeling on the alpha-subunit and 1.2% on the gamma-subunit. The apparent dissociation constants for labeling of the alpha- and gamma-subunits were 2.2 +/- 1.1 and 3.6 +/- 2.8 microM, respectively. The alpha-chains isolated from receptor-rich membranes photolabeled in the absence or in the presence of carbamylcholine were cleaved with CNBr using an efficient "in gel" procedure. The resulting peptide fragments were purified by HPLC and further submitted to trypsinolysis. The digest was analyzed by HPLC leading to a single radioactive peak which, by microsequencing, revealed two sequences extending from alpha Lys-179 and from alpha His-186, respectively. Radioactive signals could be unambiguously attributed to positions corresponding to residues alpha Tyr-190, alpha Cys-192, alpha Cys-193, and alpha Tyr-198. These four identified [3H]DCTA-labeled residues, which have been also labeled with other affinity and photoaffinity probes including the agonist [3H]nicotine, belong to loop C of the ACh binding site. The chemical structure of [3H]DCTA, together with its well-defined and powerful photochemical reactivity, provides convincing evidence that loop C-labeled residues are primarily involved in the interaction with the ester moiety of acetylcholine.     

The noncompetitive blocker [(3)H]chlorpromazine labels segment M2 but not segment M1 of the nicotinic acetylcholine receptor alpha-subunit

The membrane bound acetylcholine receptor from Torpedo marmorata was photolabeled by the noncompetitive channel blocker ]3H]chlorpromazine under equilibrium conditions in the presence of the agonist carbamoylcholine. The radioactivity incorporated into the AChR subunits was reduced by addition of phencyclidine, a specific ligand for the high-affinity side for noncompetitive blockers. The alpha-subunit was purified and digested with trypsin and/or CNBr and the resulting fragments fractionated by HPLC. Sequence analysis resulted in the identification of Ser-248 as a major residue labeled by [3H]chlorpromazine in a phencyclidine-sensitive manner. This residue is located in the hydrophobic and putative transmembrane segment M2 of the alpha-subunit, a region homologous to that containing the chlorpromazine-labeled Ser-262 in the delta-chain [1] and Ser-254 and Leu-257 in the beta-chain [2]. Extended sequence analysis of the hydrophobic segment M1 further showed that no labeling-occurred in this region.     

Decidium. A novel fluorescent probe of the agonist/antagonist and noncompetitive inhibitor sites on the nicotinic acetylcholine receptor

We have examined the interaction of the nicotinic acetylcholine receptor with decidium diiodide, a bisquaternary analogue of ethidium containing 10 methylene groups between the endocyclic and trimethylamino quaternary nitrogens. Decidium inhibits mono-[125I]iodo-alpha-toxin binding, inhibits agonist-elicited 22Na+ influx in intact cells, augments agonist competition with mono-[125I]iodo-alpha-toxin binding, and enhances [3H]phencyclidine (PCP) binding to a noncompetitive inhibitor site. These effects occur over similar concentration ranges (half-maximum effects between 0.1 and 0.4 microM). Thus, decidium binds to the agonist site and converts the receptor to a desensitized state exhibiting increased affinity for agonist and heterotropic inhibitors. These properties are similar to metaphilic antagonists characterized in classical pharmacology. At higher concentrations decidium associates directly with the noncompetitive inhibitor site identified by [3H]phencyclidine binding. Dissociation constants of decidium at this site in the resting and desensitized states are determined to be 29 and 1.2 microM, respectively. Analysis of fluorescence excitation and emission maxima reveal that binding to both the agonist and noncompetitive inhibitor sites is associated with approximately 2-fold enhancement of fluorescence. The excitation maximum for decidium bound at the agonist site appears at 490 nm while that for decidium bound at the noncompetitive inhibitor site appears at 530 compared to 480 nm in buffer. These results suggest that decidium experiences a more hydrophobic environment upon binding to the nicotinic acetylcholine receptor sites, particularly to the noncompetitive inhibitor site. Fluorescence energy transfer between N'-fluorescein isothiocyanate-lysine-23 alpha-toxin (FITC-toxin), and decidium is not detected when each is bound to one of the two agonist sites on the receptor. This allows a minimal distance to be estimated between fluorophores. In contrast, energy transfer is observed between decidium nonspecifically associated with the membrane or with nonspecific sites and the FITC-toxin at the agonist sites.     

Photoaffinity labeling of the acetylcholine transporter.

The acetylcholine (AcCh) binding site in the AcCh transporter-vesamicol receptor (AcChT-VR) present in synaptic vesicles isolated from the electric organ of Torpedo was characterized. A high-affinity analogue of AcCh containing an aryl azido group, namely, cyclohexylmethyl cis-N-(4-azidophenacyl)-N-methylisonipecotate bromide (AzidoAcCh), was synthesized in nonradioactive and highly tritiated forms. AzidoAcCh was shown to be a competitive inhibitor of [3H]AcCh active transport and binding of [3H]-vesamicol to the allosteric site. The [3H]AzidoAcCh saturation curve was determined. In all cases the AcChT.AzidoAcCh complex exhibited an inhibition or dissociation constant of about 0.3 microM. Binding of [3H]AzidoAcCh was inhibited by vesamicol and AcCh. AzidoAcCh irreversibly blocked greater than 90% of the [3H]vesamicol binding sites after multiple rounds of photolysis and reequilibration with fresh ligand. Autofluorographs of synaptic vesicles photoaffinity-labeled with [3H]AzidoAcCh showed specific labeling of material exhibiting a continuous distribution from 50 to 250 kDa after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The result demonstrates that the AcChT has an unexpected structure highly suggestive of the synaptic vesicle proteoglycan.     

Identification of a cytoplasmic region of the Torpedo nicotinic acetylcholine receptor alpha-subunit by epitope mapping.

Analysis of the binding of monoclonal antibodies (mAbs) by Torpedo nicotinic acetylcholine receptor (AChR) has demonstrated that a region of the alpha-subunit between alpha-156 and alpha-179 is exposed on the cytoplasmic surface of the nicotinic post-synaptic membrane. A panel of mAbs was produced that recognized sodium dodecyl sulfate-denatured subunits of the Torpedo AChR. Antibodies recognizing alpha-subunit were distinguished in terms of their ability to bind alpha-subunit fragments generated by Staphylococcus aureus V8 protease: an 18-kDa fragment beginning at Val-46, a 20-kDa fragment beginning at Ser-173/Ser-162, and a 10 kDa fragment beginning at Asn-339. Three mAbs, selected for binding to each of the V8-protease alpha-subunit fragments, respectively, were characterized in detail. The location of epitopes recognized by both anti-V8-18 and anti-V8-20 mAbs was determined to be within alpha-156 to alpha-179 by isolation of small immunoreactive peptides from proteolytic digests of the alpha-subunit, while the mAb reactive to V8-10 was bound to an epitope within alpha-339 to alpha-386. Quantitative evaluation of binding of the anti-V8-18 and anti-V8-20 mAbs to overlapping synthetic peptides corresponding to alpha-147 to alpha-179 localized the epitopes to distinct portions of this region. Further screening of the panel of mAbs using these synthetic peptides revealed three additional mAbs that bind in this region. The mAbs that bound the three distinct V8-protease alpha-subunit fragments were shown to bind to native AChR by indirect immunofluorescence on frozen sections of Torpedo electric organ. Binding to the native AChR was to the cytoplasmic surface of the AChR since the mAbs could bind to AChR in native vesicles, in which the AChR is oriented right-side-out, only after permeabilization of the vesicles by alkaline treatment or after scrambling of the orientation of the AChR by solubilization and reconstitution into liposomes. The location of the mAb-binding sites at the cytoplasmic surface of the AChR was visualized directly by freeze-etch immunoelectron microscopy. The identification of alpha-156 and alpha-179 as containing a cytoplasmic exposed sequence implies the existence of two non-hydrophobic transmembrane sequences between the site of N-glycosylation (Asn-141) and Cys-192, a site alkylated by the cholinergic affinity labels.     

Synthesis of an antigenic site of native acetylcholine receptor peptide 159-169 of Torpedo acetylcholine receptor alpha-chain.

A region of the alpha-subunit of the nicotinic acetylcholine receptor (AChR) of the Torpedo electric organ, containing residues 161-166, has been proposed to be a major antigenic site in the native AChR protein. We report the synthesis of a peptide corresponding to residues 159-169, which contains the proposed antigenic region. In quantitative radiometric titrations, radiolabelled anti-(native AChR) antibodies from three different species, rabbit, rat and dog, exhibited considerable binding (approx. 15% relative to native AChR) to Sepharose-immobilized peptide 159-169, but did not bind significantly to Sepharose-immobilized unrelated proteins or peptides. Specificity was further confirmed by the finding that no rabbit anti-AChR antibodies bound to the peptide after absorption with native AChR. These data indicate that the region 159-169 contains an antigenic site that is readily accessible in solubilized native Torpedo AChR.     

Identification of a cDNA clone coding for the acetylcholine binding subunit of Torpedo marmorata acetylcholine receptor.

A recombinant DNA plasmid has been constructed that contains sequences of the gene coding for the acetylcholine binding subunit (alpha-subunit, 40 000 daltons) of Torpedo marmorata acetylcholine receptor protein (AChR). Polyadenylated RNA purified from Torpedo electric organ was used to construct a cDNA library. The AChR alpha-subunit cDNA clone was then identified by a two-step screening of 700 recombinant clones. As AChR is present in Torpedo electric organ but not in Torpedo liver or spleen, differential screening led to the selection of 12 clones specific for the electric organ. We then tested the ability of cDNA inserts to hybridize alpha-subunit mRNA specifically, as judged by cell-free translation and immunoprecipitation. The insert from one clone, p alpha-1, selectively hybridized with a mRNA species which elicited the synthesis of a 38 000 mol. wt. polypeptide. This polypeptide was precipitated by: (1) a rabbit serum raised against purified denatured alpha-subunit (the pure alpha-subunit displaced the complex); and (2) a rat monoclonal antibody specific for the denatured alpha-subunit. It was thus identified as a precursor of the alpha chain. Blot hybridization analysis of polyadenylated RNA from Torpedo electric organ with the p alpha-1 probe revealed a major species of 2.0 kb, which thus contains approximately 800 non-coding nucleotides.     

The ion channel of the nicotinic acetylcholine receptor is formed by the homologous helices M II of the receptor subunits

A binding site for the channel-blocking noncompetitive antagonist [3H]triphenylmethylphosphonium ([3H]TPMP+) was localized in the alpha-, beta- and delta-chains of the nicotinic acetylcholine receptor (AChR) from Torpedo marmorata electric tissue. The photolabel was found in homologous positions of the highly conserved sequence helix II, alpha 248, beta 254, and delta 262. The site of the photoreaction appears to not be affected by the functional state of the receptor. [3H]TPMP+ was found in position delta 262 independent of whether photolabeling was performed with the receptor in its resting, desensitized or antagonist state. A model of the AChR ion channel is proposed, according to which the channel is formed by the five helices II contributed by the five receptor subunits.