Three possible disulfides in the acetylcholine receptor alpha-subunit

Interactions of Nereistoxin and Its Analogs with Vertebrate Nicotinic Acetylcholine Receptors and Molluscan ACh Binding Proteins

Nereistoxin (NTX) is a marine toxin isolated from an annelid worm that lives along the coasts of Japan. Its insecticidal properties were discovered decades ago and this stimulated the development of a variety of insecticides such as Cartap that are readily transformed into NTX. One unusual feature of NTX is that it is a small cyclic molecule that contains a disulfide bond. In spite of its size, it acts as an antagonist at insect and mammalian nicotinic acetylcholine receptors (nAChRs). The functional importance of the disulfide bond was assessed by determining the effects of inserting a methylene group between the two sulfur atoms, creating dimethylaminodithiane (DMA-DT). We also assessed the effect of methylating the NTX and DMA-DT dimethylamino groups on binding to three vertebrate nAChRs. Radioligand receptor binding experiments were carried out using washed membranes from rat brain and fish (Torpedo) electric organ; [³H]-cytisine displacement was used to assess binding to the predominantly high affinity alpha4beta2 nAChRs and [¹²⁵I]-alpha-bungarotoxin displacement was used to measure binding of NTX and analogs to the alpha7 and skeletal muscle type nAChRs. While the two quaternary nitrogen analogs, relative to their respective tertiary amines, displayed lower α4β2 nAChR binding affinities, both displayed much higher affinities for the Torpedo muscle nAChR and rat alpha7 brain receptors than their respective tertiary amine forms. The binding affinities of DMA-DT for the three nAChRs were lower than those of NTX and MeNTX. An AChBP mutant lacking the C loop disulfide bond that would potentially react with the NTX disulfide bond displayed an NTX affinity very similar to the parent AChBP. Inhibition of [³H]-epibatidine binding to the AChBPs was not affected by exposure to NTX or MeNTX for up to 24 hr prior to addition of the radioligand. Thus, the disulfide bond of NTX is not required to react with the vicinal disulfide in the AChBP C loop for inhibition of [³H]-epibatidine binding. However, a reversible disulfide interchange reaction of NTX with nAChRs might still occur, especially under reducing conditions. Labeled MeNTX, because it can be readily prepared with high specific radioactivity and possesses relatively high affinity for the nAChR-rich Torpedo nAChR, would be a useful probe to detect and identify any nereistoxin adducts.

Biochemical characterization of functional domains of the chaperone Cosmc

Cosmc is an endoplasmic reticulum chaperone necessary for normal protein O-GalNAc glycosylation through regulation of T-synthase, its single client. Loss-of-function of Cosmc results in expression of the Tn antigen, which is associated with multiple human diseases including cancer. Despite intense interest in dysregulated expression of the Tn antigen, little is known about the structure and function of Cosmc, including domain organization, secondary structure, oligomerization, and co-factors. Limited proteolysis experiments show that Cosmc contains a structured N-terminal domain (CosmcΔ256), and biochemical characterization of CosmcΔ256 reveals wild type chaperone activity. Interestingly, CosmcE152K, which shows loss of function in vivo, exhibits wild type-like activity in vitro. Cosmc and CosmcE152K heterogeneously oligomerize and form monomeric, dimeric, trimeric, and tetrameric species, while CosmcΔ256 is predominantly monomeric as characterized by chemical crosslinking and blue native page electrophoresis. Additionally, Cosmc selectively binds divalent cations in thermal shift assays and metal binding is abrogated by the CosmcΔ256 truncation, and perturbed by the E152K mutation. Therefore, the N-terminal domain of Cosmc mediates T-synthase binding and chaperone function, whereas the C-terminal domain is necessary for oligomerization and metal binding. Our results provide new structure-function insight to Cosmc, indicate that Cosmc behaves as a modular protein and suggests points of modulation or regulation of in vivo chaperone function.

Novel immunotoxin: a fusion protein consisting of gelonin and an acetylcholine receptor fragment as a potential immunotherapeutic agent for the treatment of Myasthenia gravis

In continuation of our attempts for antigen-specific suppression of the immune system [I.L. Urbatsch, R.K.M. Sterz, K. Peper, W.E. Trommer, Eur. J. Immunol. 23(1993) 776-779] a novel fusion protein composed of amino acids 4-181 of the extracellular domain of the alpha-subunit of the human muscle acetylcholine receptor and the plant toxin gelonin was expressed in Escherichia coli. The fusion protein formed inclusion bodies but could be solubilized in the presence of guanidinium hydrochloride. After a simple two step purification and refolding procedure, it exhibited a native structure at least in the main immunogenic region as shown by antibodies recognizing a conformational epitope. Half maximal inhibition of translation was achieved at 46 ng/ml as compared to 4.6 ng/ml for native and 2.4 for recombinant gelonin. Its use as therapeutic agent for the treatment of Myasthenia gravis was investigated in an animal model. Female Lewis rats were immunized with complete acetylcholine receptor from the electric ray Torpedo californica and developed thereafter experimental autoimmune M. gravis. Quantitative assessment of the disease was achieved by repetitive stimulation of the Nervus tibialis. Rats showed no symptoms of M. gravis, neither visually nor electrophysiologically after treatment with the fusion protein as determined one and seven weeks after the second application. This approach may also be useful for the therapy of further autoimmune diseases by substituting other autoantigens for the AchR fragment in the fusion protein.

Estimating the electrostatic potential at the acetylcholine receptor agonist site using power saturation EPR

Continuous wave EPR power saturation was used to measure electrostatic potentials at spin-labeled sites. Membrane surface potentials were estimated by power saturating the EPR spectrum of a membrane bound 14N spin-labeled amphiphile in the presence of a neutral or positively charged 15N labeled aqueous spin probe. The potentials that are measured are in good agreement with other probe measurements and with the predictions of the Gouy-Chapman-Stern theory, indicating that this is a valid approach to determine electrostatic potentials. A spin-labeled affinity probe based on maleimidobenzyltrimethylammonium was synthesized and could be derivatized to a sulfhydryl near either agonist site on the nicotinic acetylcholine receptor. The amplitudes of motion of the spin-probe on the ns time scale are significantly different when the two labeled sites are compared, and the probe is more restricted in its motion when attached to the more easily labeled site. When attached to this agonist site, power saturation EPR yields an electrostatic potential of -15 mV. Two other sulfhydryl-specific probes were used to label this site in reconstituted receptor containing membranes. These probes show less contact with the receptor and reduced electrostatic potentials, indicating that there is a strong spatial dependence to the potential at the agonist site. This work demonstrates that power saturation EPR provides a general method that can be used to estimate electrostatic potentials at any specifically spin-labeled macromolecular site.

Humoral immune response to immunocomplexed HIV envelope glycoprotein 120

To further our understanding of the nature of HIV-1 immunogenicity, we injected mice with the virus envelope protein gp120 in different configurations: free, complexed with its receptor CD4, and as an immunocomplex with a monoclonal antibody directed against the V3 loop of the protein. Analyses of the polyclonal sera, as well as of monoclonal antibodies produced in each case, allowed us to conclude that the quality of the humoral immune response depended on the complexation state of the antigen. For the free gp120 and gp120-CD4 complex the responses were directed mainly toward conformational epitopes. However, gp120 immunocomplexed with anti-V3 loop Mab produced, in addition, numerous MAbs directed toward linear epitopes. Epitopes were mapped using immunoblots of gp120 cleaved with S. aureus V8 protease and a combinatorial epitope phage-display library. It was found that some of the linear epitopes had been previously identified as T cell epitopes. These results suggest that the immunocomplexed gp120 may be particularly well taken up by antigen-presenting cells, leading to the processing of the gp120 and the efficient presentation of T cell epitopes. Thus immunocomplexation should afford a means for enhancing the immunogenicity of gp120 and improving its presentation.

Key histidine residues in the nicotinic acetylcholine receptor

Reactivity of histidine residues of the Discopyge tschudii nicotinic acetylcholine receptor was studied by reaction with DEP and the influence of their modification on functional properties of the receptor was evaluated. Determination of two kinetically distinguishable classes was achieved. The fast-reacting class is composed of 7 histidine residues with an apparent velocity constant k1 = 0.0248 +/- 0.0031 min-1. The second includes--at least--21 histidine residues with a velocity constant k2 = 0.0016 +/- 0.0009 min-1. The circular dichroism spectra of the native receptor and the most DEP-derivative indicate no significant modifications in the alpha-helix content, and fourth derivative spectroscopy analyses show that the environment around the aromatic amino acids remains unchanged. DEP treatment of the receptor results in a time- and reagent concentration-dependent loss of its alpha-bungarotoxin binding ability; these results agree with those obtained with the membrane-bound receptor. The decrease in the neurotoxin binding capacity was correlated with the DEP-reaction extent of the slow groups. Incorporation of 1.93 +/- 0.23 mol of DEP accounted for the maximal binding capacity drop, thus indicating the involvement of two histidine residues per alpha-bungarotoxin binding site. Neither amino groups nor tyrosine residues were modified during the reaction with DEP, indicating that the derivatization of histidine residues is responsible for the observed effect. Faster-reacting residues appear to be involved in agonist-induced ion flux through the nAChR channel. These results strongly support the connection between histidine residues and the receptor functional activity and lead us to infer that the changes observed in alpha-bungarotoxin binding and ionic channel capacity are the consequence of independent events induced by reaction with DEP.

Distribution of six transplasma membrane NADH-dehydrogenases in rat brain tissue

Transplasma membrane redox plays a significant role in cellular activation and growth. Six isoenzymes could be prepared from purified rat brain synaptic plasma membrane. Polyclonal antibodies have been prepared against six transplasma membrane oxydoreductases (PMO-I to PMO-VI) and the tissue distribution of the various iso-enzymes have been investigated in adult rat brains by means of immunohistochemistry. PMO-I is densely observed in layers I, IV and V of the parietal cortex, in CA1 of the hippocampus (except for the molecular layer), in the caudate putamen, in the dorsal, granular and ventral parts of the auditory nuclei, in some loci of the vestibular nuclei as well as in the deep cerebellar nucleus and in the granular layer of the cerebellar cortex. PMO-II is mainly located in the polymorphic layer of the dentate gyrus and in the deep cerebellar nucleus and in the granular layer of the cerebellar cortex. PMO-III is abundant in the piriform cortex, in the pyramidal layers of both CA1 and CA2, in the diagonal band of the basal ganglia, in the supraoptic nucleus and in various loci of the magnetocellular paraventricular nucleus of the hippothalamus as well as in the vestibular nuclei from the brain stem. In addition PMO-III is also densely present in motor nuclei (oculomotor, facial, hypoglossal and ambiguus nuclei), in the reticular formation and in the deep cerebellar nucleus as well as in the Purkinje layer of the cerebellar cortex. PMO-IV has a similar location but is less abundant in the vestibular nuclei of the sensory brain stem and in the motor nucleus. PMO-V in contrast is poorly present in most brain areas compared to the other iso-enzymes, apart of the Purkinje layer of the cerebellar cortex. Finally PMO-VI is mainly present in the oriens layer and in the stratum radiatum of the hippocampus formation, in the supraoptic and lateral magnocellular nucleus of the hypothalamus, in the mesencephalic trigeminal nucleus, in the ventral auditory nucleus and in the facial nucleus of the brain stem as well as in red nucleus of the reticular formation and in the Purkinje layer of the cerebellar cortex. These data show that the iso-enzymes are located in specific brain nuclei. The significance of the results in respect to the yet very poorly defined function of PMO's is discussed.

Lipid-protein interactions and protein dynamics in vesicles containing the nicotinic acetylcholine receptor: a study with ethanol

Electron paramagnetic resonance (EPR) spectroscopy was used to study the action of ethanol on the protein side chain motions of the nicotinic acetylcholine receptor (nAcChoR) in alkaline extracted membranes from Torpedo nobiliana. EPR spectra of the nAcChoR derivatized with maleimide spin label contain both strongly and weakly immobilized components. The rotational correlation time of the strongly immobilized component decreases by a factor of 2-3-fold with the addition of 1.6 M ethanol, while that of the weakly immobilized component is not significantly altered. EPR spectroscopy was also used to probe the lipid environment immediately surrounding the nAcChoR with stearic acid and phosphatidylcholine spin labeled at the fourteenth acyl carbons (14-SASL and 14-PCSL, respectively), and the steroid spin label androstanol (ASL). EPR spectra of these probes reveal a component corresponding to lipids that are motionally restricted by the receptor (annular lipids) in addition to a more fluid component arising from bulk lipid. Using spectral subtraction, the order of selectivity of these spin labels for the nAcChoR was determined to be ASL > or = 14-SASL > 14-PCSL. The estimated rotational correlation times of the high affinity 14-SASL and ASL probes ranged from approx. 20 to 35 ns. The correlation times of the lower affinity 14-PCSL were generally shorter than those for 14-SASL and ASL and ranged from about 10 to 25 ns. The addition of up to 0.9 M ethanol altered neither the affinity nor the mobility of the motionally restricted EPR component. This suggests that ethanol's actions on the nAcChoR are not mediated via changes at the lipid/protein interface near the center of the bilayer.

The active site of methanol dehydrogenase contains a disulphide bridge between adjacent cysteine residues

Adjacent cysteine residues can only form disulphide bridges in a distorted structure containing a cis-peptide link. Such bridges are extremely uncommon, identified so far in the acetyl choline receptor alone where the structure of the bridge is undetermined. Here we present the first molecular description of a disulphide bridge of this type in the quinoprotein methanol dehydrogenase from Methylobacterium extorquens. We show that this structure occurs in close proximity to the pyrrolo-quinoline quinone prosthetic group and a calcium ion in the active site of the enzyme. This unusual disulphide bridge appears to play a role in the electron transfer reaction mediated by methanol dehydrogenase.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Protein-lipid interactions and Torpedo californica nicotinic acetylcholine receptor function. 1. Spatial disposition of cysteine residues in the gamma subunit analyzed by fluorescence-quenching and energy-transfer measurements

The nicotinic acetylcholine receptor from Torpedo californica was labeled with a fluorescent, lipophilic probe, N-(1-pyrenyl)maleimide, specific for sulfhydryls in a hydrophobic environment, and was found to alkylate Cys 416, Cys 420 and Cys 451 in the gamma subunit [Li, L., Schuchard, M., Palma, A., Pradier, L., & McNamee, M.G. (1990) Biochemistry 29, 5428-5436]. The spatial disposition of the acetylcholine receptor-bound pyrene with respect to the membrane bilayer was assessed by a combination of fluorescence-quenching and resonance energy transfer measurements, under conditions of selective labeling of the gamma subunit. Quenching of pyrene fluorescence by spin-labeled fatty acids with the doxyl group at positions C-5 and C-12 revealed that the former was more effective, with a Stern-Volmer quenching constant of 0.187 compared to 0.072 for the latter, suggesting that the fluorophore(s) are located closer to the membrane-water interface rather than the hydrophobic interior. Energy transfer was found to occur from tryptophan in the acetylcholine receptor to cysteine-bound pyrene with a distance of separation of approximately 18 A. However, there was no energy transfer when pyrene-labeled AChR was reconstituted into membranes containing brominated phospholipids and cholesterol, suggesting that the fluorophore(s) responsible for energy transfer are located in the membrane domain. Thus, the N-(1-pyrenyl)maleimide can be used to monitor lipid-protein interactions of the AChR.

A Raman spectroscopic study of acetylcholine receptor-rich membranes from Torpedo marmorata. Interaction of the receptor with carbamylcholine and (+)-tubocurarine

Raman spectroscopy is used to determine structural features of alkali-treated subsynaptic membrane fragments from Torpedo marmorata electric organ, rich in native functional AcChR. Distinct vibrations attributable to the membrane proteins and lipids were identified and studied before and after addition of the agonist carbamylcholine and the competitive antagonist (+)-tubocurarine. The protein secondary structure determined by using amide-I polypeptide vibrational analysis, indicates 47% alpha-helices, 25% beta-sheets, 18% turns and 11% undefined structure. The secondary structure of the AcChR molecule was not subject to large modifications upon addition of carbamylcholine. But, the presence of the (+)-tubocurarine leads to detectable changes in the amide-I region which might be interpreted as reflecting different contributions of alpha-helices and turns in the secondary structure. In addition, Raman spectra provide information about the environment of aromatic amino acids (tyrosine and tryptophan), the (C-C) bonds, the CH2 and CH3 groups of aliphatic side chains, as well as the disulfide (S-S) and cystein (C-S) bonds. The tyrosines seem 'exposed' to the aqueous medium. The Raman spectra of the AcChR-carbamylcholine complex suggest 'exposed' tryptophans, while those of the unliganded membrane-bound AcChR or of the receptor with (+)-tubocurarine are shown 'buried'. The disulfide bridges in the AcChR subunits show identical conformation in the absence and presence of carbamylcholine. On the contrary, considerable changes are found in the AcChR-(+)-tubocurarine complex. Carbamylcholine and especially (+)-tubocurarine decrease lipid fluidity.

Evidence from the anti-idiotypic network that the acetylcholine receptor is a rabies virus receptor

We have developed idiotype-anti-idiotype monoclonal antibodies that provide evidence for rabies virus binding to the acetylcholine receptor (AChR). Hybridoma cell lines 7.12 and 7.25 resulted after fusion of NS-1 myeloma cells with spleen cells from a BALB/c mouse immunized with rabies virus strain CVS. Antibody 7.12 reacted with viral glycoprotein and neutralized virus infectivity in vivo. It also neutralized infectivity in vitro when PC12 cells, which express neuronal AChR, but not CER cells or neuroblastoma cells (clone N18), which have no AChR, were used. Antibody 7.25 reacted with nucleocapsid protein. Anti-idiotypic monoclonal antibody B9 was produced from fusion of NS-1 cells with spleen cells from a mouse immunized with 7.12 Fab. In an enzyme-linked immunosorbent assay and immunoprecipitation, B9 reacted with 7.12, polyclonal rabies virus immune dog serum, and purified AChR. The binding of B9 to 7.12 and immune dog serum was inhibited by AChR. B9 also inhibited the binding of 7.12 to rabies virus both in vitro and in vivo. Indirect immunofluorescence revealed that B9 reacted at neuromuscular junctions of mouse tissue. B9 also reacted in indirect immunofluorescence with distinct neurons in mouse and monkey brain tissue as well as with PC12 cells. B9 staining of neuronal elements in brain tissue of rabies virus-infected mice was greatly reduced. Rabies virus inhibited the binding of B9 to PC12 cells. Mice immunized with B9 developed low-titer rabies virus-neutralizing antibody. These mice were protected from lethal intramuscular rabies virus challenge. In contrast, anti-idiotypic antibody raised against nucleocapsid antibody 7.25 did not react with AChR.

The functional architecture of the acetylcholine nicotinic receptor explored by affinity labelling and site-directed mutagenesis

The scientific community will remember Peter Läuger as an exceptional man combining a generous personality and a sharp and skilful mind. He was able to attract by his views the interest of a large spectrum of biologists concerned by the mechanism of ion translocation through membranes. Yet, he was not a man with a single technique or theory. Using an authentically multidisciplinary approach, his ambition was to ‘understand transmembrane transport at the microscopic level, to capture its dynamics in the course of defined physiological processes’ (1987). According to him, ‘new concepts in the molecular physics of proteins’ had to be imagined, and ‘the traditional static picture of proteins has been replaced by the notions that proteins represent dynamic structures, subjected to conformational fluctuations covering a very wide time-range’ (1987).

Brain nicotinic receptors isolated by a monospecific antibody against a synthetic alpha-3 subunit receptor peptide compared to a monoclonal anti-idiotypic (to nicotine) antibody

Nicotinic cholinergic receptor proteins purified from rat brain by immunoaffinity chromatography were characterized using the anti-S3 polyclonal antibody vs. the anti-idiotypic monoclonal antibody 422F11 (generated against an antibody to nicotine). Anti-S3 IgG was purified to homogeneity; anti-S3-Sepharose 4B and 422F11-Sepharose 4B each depleted 3H-nicotine binding sites from brain. Nicotinic receptors isolated from both immunoaffinity columns showed major bands (silver-stained) at 55K and 70K. Using anti-S3 serum as probe, Western blots of nicotinic receptors isolated by the two immunoaffinity gels also showed major bands at 55 and 70K. However, Western blots of fresh brain extracts revealed a major band at 80K and minor bands at 55K and 70K. These results show similar nicotinic cholinergic receptor proteins isolated by the anti-S3 and 422F11 anti-idiotypic antibodies; 80K was dominant only when fresh brain extract was subjected to Western blotting without prior immunoaffinity purification.

Species- and subtype-specific recognition by antibody WF6 of a sequence segment forming an alpha-bungarotoxin binding site on the nicotinic acetylcholine receptor alpha subunit

The monoclonal antibody WF6 competes with acetylcholine and alpha-bungarotoxin (alpha-BGT) for binding to the Torpedo nicotinic acetylcholine receptor (nAChR) alpha 1 subunit. Using synthetic peptides corresponding to the complete Torpedo nAChR alpha 1 subunit, we previously mapped a continuous epitope recognized by WF6, and the prototope for alpha-BGT, to the sequence segment alpha 1(181-200). Single amino acid substitution analogs have been used as an initial approach to determine the critical amino acids for WF6 and alpha-BGT binding. In the present study, we continue our analysis of the structural features of the WF6 epitope by comparing its cross-reactivity with synthetic peptides corresponding to the alpha 1 subunits from the muscle nAChRs of different species, the rat brain alpha 2, alpha 3, alpha 4 and alpha 5 nAChR subtypes, and the chick brain alpha-BGT binding protein subunits, alpha BGTBP alpha 1 and alpha BGTBP alpha 2. Our results indicate that WF6 is able to cross-react with the muscle alpha 1 subunits of different species by virtue of conservation of several critical amino acid residues between positions 190-198 of the alpha 1 subunit. These studies further define the essential structural features of the sequence segment alpha 1(181-200) required to form the epitope for WF6.

Structural determinants within residues 180-199 of the rodent alpha 5 nicotinic acetylcholine receptor subunit involved in alpha-bungarotoxin binding

Synthetic peptides corresponding to sequence segments of the nicotinic acetylcholine receptor (nAChR) alpha subunits have been used to identify regions that contribute to formation of the binding sites for cholinergic ligands. We have previously defined alpha-bungarotoxin (alpha-BTX) binding sequences between residues 180 and 199 of a putative rat neuronal nAChR alpha subunit, designated alpha 5 [McLane, K. E., Wu, X., & Conti-Tronconi, B. M. (1990) J. Biol. Chem. 265, 9816-9824], and between residues 181 and 200 of the chick neuronal alpha 7 and alpha 8 subunits [McLane, K. E., Wu, X., Schoepfer, R., Lindstrom, J., & Conti-Tronconi, B. M. (1991) J. Biol. Chem. (in press)]. These sequences are relatively divergent compared with the Torpedo and muscle nAChR alpha 1 alpha-BTX binding sites, which indicates a serious limitation of predicting functional domains of proteins based on homology in general. Given the highly divergent nature of the alpha 5 sequence, we were interested in determining the critical amino acid residues for alpha-BTX binding. In the present study, the effects of single amino acid substitutions of Gly or Ala for each residue of the rat alpha 5(180-199) sequence were tested, using a competition assay, in which peptides compete for 125I-alpha-BTX binding with native Torpedo nAChR.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of a key cysteine residue in the gating of the acetylcholine receptor

We have examined changes in single-channel behavior that result from conservative amino acid substitutions at the Cys230 residue in the putative first transmembrane region (M1) of the murine nicotinic acetylcholine receptor. Mutations made in the gamma subunit altered the energy barrier for a single closing rate constant in proportion to the size of the substituted side chain. One of these substitutions, when made in the alpha subunits, had no effect on gating. No mutations altered permeation. We conclude that the region surrounding the M1 Cys is involved in the gating of the nicotinic acetylcholine receptor and that the gamma subunit contributes significantly to the control of channel closure.

The inhibitory glycine receptor: a ligand-gated chloride channel of the central nervous system

The postsynaptic glycine receptor (GlyR) is a major inhibitory chloride channel protein in the central nervous system. The affinity-purified receptor contains polypeptides of 48 kDa, 58 kDa, and 93 kDa. The 48-kDa (alpha) and 58 kDa (beta) subunits span the postsynaptic membrane in a pentameric arrangement to form the anion channel of the receptor. The 93-kDa polypeptide is cytoplasmically localized and may have an anchoring function. Molecular cloning revealed that different structural characteristics are shared by the membrane-spanning subunits of the GlyR and those of other ligand-gated ion channel proteins. Developmental regulation of the GlyR is characterized by alterations in antagonist binding, heterogeneity of alpha subunits, and increased levels of the 93-kDa polypeptide. Glycine receptor function can be reconstituted by expression of cloned alpha subunits in heterologous cell systems. Positive charges found at the presumed mouths of the GlyR channel appear to be important determinants of ion selectivity. These data establish the anion-conducting GlyR as a homolog of other ligand-gated ion channel proteins and suggest that the diversity of these channels originates from divergent evolution of a primordial channel protein early in phylogeny.

Functional role of the cysteine 451 thiol group in the M4 helix of the gamma subunit of Torpedo californica acetylcholine receptor

Previous chemical modification studies of the acetylcholine receptor [Yee, A.S., Corey, D.E., & McNamee, M.G. (1986) Biochemistry 25, 2110-2119] were extended by using fluorescent N-pyrenylmaleimide to alkylate purified Torpedo californica nicotinic acetylcholine receptor (AChR). Peptide sequencing of the tryptic fragments of the labeled AChR gamma subunit identified cysteines 416, 420, and 451 as the modified residues. The functional role of Cys-451 in the M4 transmembrane domain of the AChR gamma subunit was further investigated by studying the functional consequences of the site-specific mutation of this cysteine to either serine or tryptophan by using AChR mRNAs injected into Xenopus laevis oocytes. Both mutants displayed about 50% reduction in the normalized channel activity of the receptor measured as the ACh-induced conductance per femtomole of surface alpha-bungarotoxin binding sites. However, the mutations did not change other AChR functional properties such as agonist binding ability, the slow phase of desensitization, and blockade by competitive and noncompetitive antagonists. The significant reduction in AChR ion channel activity associated with the above point mutations, especially the simple change of the -SH group on Cys-451 to the -OH group, suggests that this thiol group in the M4 helix of gamma subunit may play an important role in AChR ion channel function. Previous site-directed mutations of the Cys-416 and -420 residues showed a decreased response when both of these residues were changed to phenylalanine, but not when they were changed to serine [Pradier, L., Yee, A.S., & McNamee, M.G. (1989) Biochemistry 28, 6562-6571].(ABSTRACT TRUNCATED AT 250 WORDS)

Altered patterns of N-linked glycosylation of the Torpedo acetylcholine receptor expressed in Xenopus oocytes

The nicotinic acetylcholine receptor (AChR) from Torpedo electroplax is an oligomeric transmembrane glycoprotein made up of four highly homologous subunits in a stoichiometry of alpha 2 beta gamma delta. The role of N-linked glycosylation of the AChR has been studied in several cell lines and these studies have suggested that the addition of carbohydrate may be important for receptor expression. While Xenopus oocytes have proven to be an invaluable tool for studying the AChR, little is known about N-linked glycosylation of the oocyte-expressed receptor. The present report demonstrates that the oocyte-expressed AChR is glycosylated and contains the same number of oligosaccharide residues per subunit as the native receptor. However, unlike the native Torpedo receptor which contains both high mannose and complex oligosaccharides, the oocyte-expressed AChR contains only high mannose oligosaccharide modifications. However, as has been well documented, the Torpedo AChR expressed in oocytes is fully functional, demonstrating that the precise nature of the oligosaccharide modification is not critical for receptor function. The role of the oligosaccharide component of the AChR in receptor function was examined using tunicamycin (TM) to inhibit N-linked protein glycosylation. TM treatment resulted in a 70-80% inhibition of AChR expression in oocytes. Functional, unglycosylated receptors were not expressed; receptors expressed in TM-treated oocytes were functional wild-type, glycosylated AChR, formed only during the initial 12 hr of TM exposure. These data suggest that while glycosylation of the oocyte-expressed Torpedo AChR is required for assembly of subunits into a functional receptor, as has been demonstrated in other cells, oocyte modification of normal Torpedo glycosylation patterns does not affect receptor function or assembly.

Nonequivalence of alpha-bungarotoxin binding sites in the native nicotinic receptor molecule

In the native, membrane-bound form of the nicotinic acetylcholine receptor (M-AcChR) the two sites for the cholinergic antagonist alpha-bungarotoxin (alpha-BGT) have different binding properties. One site has high affinity, and the M-AcChR/alpha-BGT complexes thus formed dissociate very slowly, similar to the complexes formed with detergent-solubilized AcChR (S-AcChR). The second site has much lower affinity (KD approximately 59 +/- 35 nM) and forms quickly reversible complexes. The nondenaturing detergent Triton X-100 is known to solubilize the AcChR in a form unable, upon binding of cholinergic ligands, to open the ion channel and to become desensitized. Solubilization of the AcChR in Triton X-100 affects the binding properties of this second site and converts it to a high-affinity, slowly reversible site. Prolonged incubation of M-AcChR at 4 degrees C converts the low-affinity site to a high-affinity site similar to those observed in the presence of Triton X-100. Although the two sites have similar properties when the AcChR is solubilized in Triton X-100, their nonequivalence can be demonstrated by the effect on alpha-BGT binding of concanavalin A, which strongly reduces the association rate of one site only. The Bmax of alpha-BGT to either Triton-solubilized AcChR or M-AcChR is not affected by the presence of concanavalin A. Occupancy of the high-affinity, slowly reversible site in M-AcChR inhibits the Triton X-100 induced conversion to irreversibility of the second site. At difference with alpha-BGT, the long alpha-neurotoxin from Naja naja siamensis venom (alpha-NTX) binds with high affinity and in a very slowly reversible fashion to two sites in the M-AcChR (Conti-Tronconi & Raftery, 1986). We confirm here that Triton-solubilized AcChR or M-AcChR binds in a very slowly reversible fashion the same amount of alpha-NTX.(ABSTRACT TRUNCATED AT 250 WORDS)

Monospecific antibodies against a synthetic peptide predicted from the alpha-3 nicotinic receptor cDNA inhibit binding of [3H]nicotine to rat brain nicotinic cholinergic receptor

Polyclonal antibodies were raised against a synthetic decapeptide (designated S3) predicted from a segment of the alpha-3 subunit cDNA (amino acid residues 130-139) encoding the rat brain nicotinic cholinergic receptor. This segment was selected because it may be proximate to the nicotine/acetylcholine-binding site of the receptor (1). By radioligand binding assays and sucrose density gradient centrifugation, these monospecific antibodies were shown to inhibit the binding of [3H]nicotine to both the large molecular weight rat brain receptor (240 kDa) and to an SDS-disaggregated nicotine-binding subunit species (80 kDa), in a dose-dependent manner. The neutralizing effect of the anti-S3 antibodies supports the view that this region of the protein is closely related to the agonist binding site.

Consensus residues at the acetylcholine binding site of cholinergic proteins

The nicotinic (nAcChR) and muscarinic (mAcCh) acetylcholine receptors and acetylcholinesterase (AcChEase) are structurally unrelated but share a common functional property: interaction with acetylcholine (AcCh). Alignment of the probable AcCh binding site regions of the nAcChR and mAcChR protein sequences revealed the presence of ten nearly identically spaced consensus residues, six of which contain potentially ligand-interactive side chains. Important elements of the consensus residues also were found in one unique sequence region of the AcChEases. Alignments among the two receptors and AcChEase outside the apparent binding region were rare, and the consensus AcCh binding residues were largely substituted in the homologous proteins, which do not bind AcCh. The consensus residues include two possible anionic subsite Asp residues and a Ser that may hydrogen bond to the AcCh carbonyl in the receptors. These residues correspond to positions Asp-166, Ser-173, and Asp-200 in the neuromuscular nAcChR; Asp-71, Ser-78, and Asp-105 in the M1 mAcChR; and Asp-93 and Asp-128 in Torpedo AcChEase. No corresponding consensus Ser is found in the AcChEase sequence; this is expected because of a downstream esterase active-site Ser-200 (Torpedo). A receptor-conserved and disulfide-linked Cys corresponding to neuromuscular nAcChR residue 193 and M1 mAcChR residue 97 may be important in energy transduction associated with agonist-mediated events. The presence of additional binding-site aromatic residues that may form a hydrophobic environment near the anionic subsite are aligned within, but not between, the three cholinergic protein groups. These observations target specific regions and residues within these proteins for structure-function studies of the cholinergic binding domain.