Mapping of a cholinergic binding site by means of synthetic peptides, monoclonal antibodies, and alpha-bungarotoxin

Structural basis for α-bungarotoxin insensitivity of neuronal nicotinic acetylcholine receptors

The ten types of nicotinic acetylcholine receptor α-subunits show substantial sequence homology, yet some types confer high affinity for α-bungarotoxin, whereas others confer negligible affinity. Combining sequence alignments with structural data reveals three residues unique to α-toxin-refractory α-subunits that coalesce within the 3D structure of the α4β2 receptor and are predicted to fit between loops I and II of α-bungarotoxin. Mutating any one of these residues, Lys189, Ile196 or Lys153, to the α-toxin-permissive counterpart fails to confer α-bungarotoxin binding. However, mutating both Lys189 and Ile196 affords α-bungarotoxin binding with an apparent dissociation constant of 104 nM, while combining mutation of Lys153 reduces the dissociation constant to 22 nM. Analogous residue substitutions also confer high affinity α-bungarotoxin binding upon α-toxin-refractory α2 and α3 subunits. α4β2 receptors engineered to bind α-bungarotoxin exhibit slow rates of α-toxin association and dissociation, and competition by cholinergic ligands typical of muscle nicotinic receptors. Receptors engineered to bind α-bungarotoxin co-sediment with muscle nicotinic receptors on sucrose gradients, and mirror single channel signatures of their α-toxin-refractory counterparts. Thus the inability of α-bungarotoxin to bind to neuronal nicotinic receptors arises from three unique and interdependent residues that coalesce within the receptor's 3D structure.

Extracellular Domain Nicotinic Acetylcholine Receptors Formed by α4 and β2 Subunits

Models of the extracellular ligand-binding domain of nicotinic acetylcholine receptors (nAChRs), which are pentameric integral membrane proteins, are attractive for structural studies because they potentially are water-soluble and better candidates for x-ray crystallography and because their smaller size is more amenable for NMR spectroscopy. The complete N-terminal extracellular domain is a promising foundation for such models, based on previous studies of alpha7 and muscle-type subunits. Specific design requirements leading to high structural fidelity between extracellular domain nAChRs and full-length nAChRs, however, are not well understood. To study these requirements in heteromeric nAChRs, the extracellular domains of alpha4 and beta2 subunits with or without the first transmembrane domain (M1) were expressed in Xenopus oocytes and compared with alpha4beta2 nAChRs based on ligand binding and subunit assembly properties. Ligand affinities of detergent-solubilized, extracellular domain alpha4beta2 nAChRs formed from subunits with M1 were nearly identical to affinities of alpha4beta2 nAChRs when measured with [3H]epibatidine, cytisine, nicotine, and acetylcholine. Velocity sedimentation suggested that these extracellular domain nAChRs predominantly formed pentamers. The yield of these extracellular domain nAChRs was about half the yield of alpha4beta2 nAChRs. In contrast, [3H]epibatidine binding was not detected from the extracellular domain alpha4 and beta2 subunits without M1, implying no detectable expression of extracellular domain nAChRs from these subunits. These results suggest that M1 domains on both alpha4 and beta2 play an important role for efficient expression of extracellular domain alpha4beta2 nAChRs that are high fidelity structural models of full-length alpha4beta2 nAChRs.

Two-dimensional measurement of proton T1rho relaxation in unlabeled proteins: mobility changes in alpha-bungarotoxin upon binding of an acetylcholine receptor peptide

A method for the measurement of proton T(1)(rho) relaxation times in unlabeled proteins is described using a variable spin-lock pulse after the initial nonselective 90 degrees excitation in a HOHAHA pulse sequence. The experiment is applied to alpha-bungarotoxin (alpha-BTX) and its complex with a 25-residue peptide derived from the acetylcholine receptor (AChR) alpha-subunit. A good correlation between high T(1)(rho) values and increased local motion is revealed. In the free form, toxin residues associated with receptor binding according to the NMR structure of the alpha-BTX complex with an AChR peptide and the model for alpha-BTX with the AChR [Samson, A. O., et al. (2002) Neuron 35, 319-332] display high mobility. When the AChR peptide binds, a decrease in the relaxation times and the level of motion of residues involved in binding of the receptor alpha-subunit is exhibited, while residues implicated in binding gamma- and delta-subunits retain their mobility. In addition, the quantitative T(1)(rho) measurements enable us to corroborate the mapping of boundaries of the AChR determinant strongly interacting with the toxin [Samson, A. O., et al. (2001) Biochemistry 40, 5464-5473] and can similarly be applied to other protein complexes in which peptides represent one of the two interacting proteins. The presented method is advantageous because of its simplicity, generality, and time efficiency and paves the way for future investigation of proton relaxation rates in small unlabeled proteins.

Identification of regions involved in the binding of alpha-bungarotoxin to the human alpha7 neuronal nicotinic acetylcholine receptor using synthetic peptides

The neuronal alpha7 nicotinic acetylcholine receptor (AChR) binds the neurotoxin alpha-bungarotoxin (alpha-Bgt). Fine mapping of the alpha-Bgt-binding site on the human alpha7 AChR was performed using synthetic peptides covering the entire extracellular domain of the human alpha7 subunit (residues 1-206). Screening of these peptides for (125)I-alpha-Bgt binding resulted in the identification of at least two toxin-binding sites, one at residues 186-197, which exhibited the best (125)I-alpha-Bgt binding, and one at residues 159-165, with weak toxin-binding capacity; these correspond, respectively, to loops C and IV of the agonist-binding site. Toxin binding to the alpha7(186-197) peptide was almost completely inhibited by unlabelled alpha-Bgt or d -tubocurarine. Alanine substitutions within the sequence 186-198 revealed a predominant contribution of aromatic and negatively charged residues to the binding site. This sequence is homologous to the alpha-Bgt binding site of the alpha1 subunit (residues 188-200 in Torpedo AChR). In competition experiments, the soluble peptides alpha7(186-197) and Torpedo alpha1(184-200) inhibited the binding of (125)I-alpha-Bgt to the immobilized alpha7(186-197) peptide, to native Torpedo AChR, and to the extracellular domain of the human alpha1 subunit. These results suggest that the toxin-binding sites of the neuronal alpha7 and muscle-type AChRs bind to identical or overlapping sites on the alpha-Bgt molecule. In support of this, when synthetic alpha-Bgt peptides were tested for binding to the recombinant extracellular domains of the human alpha7 and alpha1 subunits, and to native Torpedo and alpha7 AChR, the results indicated that alpha-Bgt interacts with both neuronal and muscle-type AChRs through its central loop II and C-terminal tail.

Nicotinic receptor signaling in nonexcitable cells

The finding that neuronal nicotinic acetylcholine receptors (nAChRs) are present in non-neuronal cells both within and outside the nervous system raises some interesting issues. The mechanisms underlying receptor signaling and its downstream consequences in these cells remain to be elucidated. Factors controlling the release of acetylcholine and the extent of its diffusion are likely to be different for these cells than for traditional neuronal synapses. Recent advances on the physiologic functions of some of these cell types have provided a better insight into possible functional roles for nAChRs in nonexcitable cells. The presence of nAChRs on these cells also implies a broader scope for the actions of nicotine that needs to be considered from a clinical viewpoint. Revealing the potential physiologic roles for nAChRs on nonexcitable cells is likely to provide a more complete understanding of cholinergic signaling.

Expression of soluble ligand- and antibody-binding extracellular domain of human muscle acetylcholine receptor alpha subunit in yeast Pichia pastoris. Role of glycosylation in alpha-bungarotoxin binding

The N-terminal extracellular domain (amino acids 1-210; halpha-(1-210)) of the alpha subunit of the human muscle nicotinic acetylcholine receptor (AChR), bearing the binding sites for cholinergic ligands and the main immunogenic region, the major target for anti-AChR antibodies in patients with myasthenia gravis, was expressed in the yeast, Pichia pastoris. The recombinant protein was water-soluble and glycosylated, and fast protein liquid chromatography analysis showed it to be a monomer. halpha-(1-210) bound (125)I-alpha-bungarotoxin with a high affinity (K(d) = 5.1 +/- 2.4 nm), and this binding was blocked by unlabeled d-tubocurarine and gallamine (K(i) approximately 7.5 mm). Interestingly, (125)I-alpha-bungarotoxin binding was markedly impaired by in vitro deglycosylation of halpha-(1-210). Several monoclonal antibodies that show partial or strict conformation-dependent binding to the AChR were able to bind to halpha-(1-210), as did antibodies from a large proportion of myasthenic patients. These results suggest that the extracellular domain of the human AChR alpha subunit expressed in P. pastoris has an apparently near native conformation. The correct folding of the recombinant protein, together with its relatively high expression yield, makes it suitable for structural studies on the nicotinic acetylcholine receptor and for use as an autoantigen in myasthenia gravis studies.

Structure of epitopes recognized by the antibodies to alpha(181-192) peptides of neuronal nicotinic acetylcholine receptors: extrapolation to the structure of acetylcholine-binding domain

Using the alpha(181-192) peptides of neuronal nicotinic acetylcholine receptor (nAChR) and Ala-substituted peptide analogues, amino acid residues critical for specific monoclonal antibody (mAb) binding were identified. By means of 2D nuclear magnetic resonance (2D-NMR) analysis followed by molecular modeling, it was found that mAb binding resulted in stabilization of the free alpha3(181-192) peptide flexible conformation yielding an extended structure with residues 6-11 of the peptide being in direct contact with the Ab. Since the Ab binds the native AChR as well, it is suggested that the corresponding fragment of AChR alpha3 subunit is exposed to solution and also appears in extended conformation.

The binding site of acetylcholine receptor as visualized in the X-Ray structure of a complex between alpha-bungarotoxin and a mimotope peptide

We have determined the crystal structure at 1.8 A resolution of a complex of alpha-bungarotoxin with a high affinity 13-residue peptide that is homologous to the binding region of the alpha subunit of acetylcholine receptor. The peptide fits snugly to the toxin and adopts a beta hairpin conformation. The structures of the bound peptide and the homologous loop of acetylcholine binding protein, a soluble analog of the extracellular domain of acetylcholine receptor, are remarkably similar. Their superposition indicates that the toxin wraps around the receptor binding site loop, and in addition, binds tightly at the interface of two of the receptor subunits where it inserts a finger into the ligand binding site, thus blocking access to the acetylcholine binding site and explaining its strong antagonistic activity.

Localization of agonist and competitive antagonist binding sites on nicotinic acetylcholine receptors

Identification of all residues involved in the recognition and binding of cholinergic ligands (e.g. agonists, competitive antagonists, and noncompetitive agonists) is a primary objective to understand which structural components are related to the physiological function of the nicotinic acetylcholine receptor (AChR). The picture for the localization of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are located mainly on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are identical, the observed high and low affinity for different ligands on the receptor is conditioned by the interaction of the alpha subunit with other non-alpha subunits. This molecular interaction takes place at the interface formed by the different subunits. For example, the high-affinity acetylcholine (ACh) binding site of the muscle-type AChR is located on the alphadelta subunit interface, whereas the low-affinity ACh binding site is located on the alphagamma subunit interface. Regarding homomeric AChRs (e.g. alpha7, alpha8, and alpha9), up to five binding sites may be located on the alphaalpha subunit interfaces. From the point of view of subunit arrangement, the gamma subunit is in between both alpha subunits and the delta subunit follows the alpha aligned in a clockwise manner from the gamma. Although some competitive antagonists such as lophotoxin and alpha-bungarotoxin bind to the same high- and low-affinity sites as ACh, other cholinergic drugs may bind with opposite specificity. For instance, the location of the high- and the low-affinity binding site for curare-related drugs as well as for agonists such as the alkaloid nicotine and the potent analgesic epibatidine (only when the AChR is in the desensitized state) is determined by the alphagamma and the alphadelta subunit interface, respectively. The case of alpha-conotoxins (alpha-CoTxs) is unique since each alpha-CoTx from different species is recognized by a specific AChR type. In addition, the specificity of alpha-CoTxs for each subunit interface is species-dependent. In general terms we may state that both alpha subunits carry the principal component for the agonist/competitive antagonist binding sites, whereas the non-alpha subunits bear the complementary component. Concerning homomeric AChRs, both the principal and the complementary component exist on the alpha subunit. The principal component on the muscle-type AChR involves three loops-forming binding domains (loops A-C). Loop A (from mouse sequence) is mainly formed by residue Y(93), loop B is molded by amino acids W(149), Y(152), and probably G(153), while loop C is shaped by residues Y(190), C(192), C(193), and Y(198). The complementary component corresponding to each non-alpha subunit probably contributes with at least four loops. More specifically, the loops at the gamma subunit are: loop D which is formed by residue K(34), loop E that is designed by W(55) and E(57), loop F which is built by a stretch of amino acids comprising L(109), S(111), C(115), I(116), and Y(117), and finally loop G that is shaped by F(172) and by the negatively-charged amino acids D(174) and E(183). The complementary component on the delta subunit, which corresponds to the high-affinity ACh binding site, is formed by homologous loops. Regarding alpha-neurotoxins, several snake and alpha-CoTxs bear specific residues that are energetically coupled with their corresponding pairs on the AChR binding site. The principal component for snake alpha-neurotoxins is located on the residue sequence alpha1W(184)-D(200), which includes loop C. In addition, amino acid sequence 55-74 from the alpha1 subunit (which includes loop E), and residues gammaL(119) (close to loop F) and gammaE(176) (close to loop G) at the low-affinity binding site, or deltaL(121) (close to the homologous region of loop G) at the high-affinity binding site, are i

Colocalization of neurotransmitter receptors on astrocytes in explant cultures of rat CNS

In recent years evidence has accumulated that astrocytes express functional receptors for a variety of neurotransmitters/neuromodulators. By means of electrophysiological and combined autoradiographic and immunohistochemical methods we have demonstrated the colocalization of cholinergic, adrenergic and peptidergic receptors on astrocytes in explant cultures from various regions of rat central nervous system. A great number of biochemical and electrophysiological studies from other laboratories have shown that most of the neurotransmitters exert their effects on second messenger systems and on Ca2+-activated K+-channels. Furthermore, certain neurotransmitters are involved in the regulation of energy metabolism by stimulating enzymatic breakdown of glycogen in astrocytes. It was suggested that there is a cross-talk between the various neurotransmitter receptors on the glial membrane and that these receptors act in a synergistic or antagonistic way. The coexistence of cholinergic and peptidergic receptors on astrocytes is of great interest since both neurotransmitter systems are involved in cognitive functions and are impaired in patients with Alzheimer's dementia. The question is therefore raised whether not only neurones but also astrocytes might be involved in neurodegenerative disorders such as Alzheimer's disease.

Alpha subunit composition of nicotinic acetylcholine receptors in the rat autonomic ganglia neurons as determined with subunit-specific anti-alpha(181-192) peptide antibodies

The subunit composition of nicotinic acetylcholine receptors of rat autonomic ganglia neurons was studied by means of antibodies, which differentiated between different alpha subunits and specifically blocked acetylcholine-induced membrane currents. Polyclonal rabbit antibodies and mouse monoclonal antibodies were raised against synthetic peptides matching in sequence the alpha(181-192) region of alpha3, alpha4, alpha5, and alpha7 subunits of rat neuronal nicotinic acetylcholine receptors. The antibodies discriminated among alpha3, alpha4, alpha5, and alpha7 peptides in enzyme-linked immunosorbent assay and bound to native acetylcholine receptors expressed in PC-12 cells. By means of immunoperoxidase staining of cultured rat autonomic neurons followed by transmission, dark-field and phase-contrast microscopy, it was found that all cells of the superior cervical ganglia expressed the alpha3, alpha5, and alpha7 nicotinic acetylcholine receptors, whereas approximately half of the cells were clearly alpha4-positive. In contrast, only about one-third of the intracardiac neurons were alpha3-positive, about 50% were alpha4-positive, one-seventh were alpha5-positive, and one-fifth were alpha7-positive. All antibodies tested blocked acetylcholine-induced currents in the neurons of the superior cervical ganglia as was demonstrated by whole-cell patch-clamp studies. Although each antibody could block up to 80% of the current, the degree of inhibition varied considerably from cell to cell. It is concluded that alpha3, alpha5, and alpha7 subunits are expressed in all neurons of the superior cervical ganglion and in some intracardiac neurons, whereas alpha4 subunits are expressed in some but not all neurons of both tissues. The neurons of the superior cervical ganglion express heterogeneous acetylcholine receptors and differ in relative amounts of acetylcholine receptor subtypes expressed.

Cellular localization of estrogen receptors on neurones in various regions of cultured rat CNS: coexistence with cholinergic and galanin receptors

Autoradiographic studies have shown that many neurones in explant cultures of rat neocortex, hippocampus, preoptic area and spinal cord express binding sites for [3H]-estradiol which are distributed over the cell bodies and primary processes. By means of immunohistochemistry, it was observed that neurones were labelled by monoclonal antibodies against estrogen alpha-receptors and a polyclonal antibody against estrogen beta-receptors. Immunoreactivity was distributed over the soma and primary processes of the cells, the nuclei being more intensely stained. Double-immunostaining revealed a colocalization of estrogen alpha- and beta-receptors on approximately half of the neurones in cultures from neocortex and hippocampus whereas in cultures from preoptic area and spinal cord only few cells were double-stained. On many neurones, a coexistence of estrogen receptors and cholinergic muscarinic or nicotinic sites was found. Furthermore, combined autoradiographic and immunohistochemical studies have shown a colocalization of receptors for estrogen and the neuropeptide [125I]-galanin. The coexistence of estrogen and cholinergic sites as well as of estrogen and galanin receptors on the same neurones are discussed with respect to neurodegenerative events such as Alzheimer's disease.

Neuronal nicotinic receptors in the locust Locusta migratoria. Cloning and expression

We have identified five cDNA clones that encode nicotinic acetylcholine receptor (nAChR) subunits expressed in the nervous system of the locust Locusta migratoria. Four of the subunits are ligand-binding alpha subunits, and the other is a structural beta subunit. The existence of at least one more nAChR gene, probably encoding a beta subunit, is indicated. Based on Northern analysis and in situ hybridization, the five subunit genes are expressed. localpha1, localpha3, and locbeta1 are the most abundant subunits and are expressed in similar areas of the head ganglia and retina of the adult locust. Because Loc<alpha3 binds alpha-bungarotoxin with high affinity, it may form a homomeric nAChR subtype such as the mammalian alpha7 nAChR. Localpha1 and Locbeta1 may then form the predominant heteromeric nAChR in the locust brain. localpha4 is mainly expressed in optic lobe ganglionic cells and localpha2 in peripherally located somata of mushroom body neurons. localpha3 mRNA was additionally detected in cells interspersed in the somatogastric epithelium of the locust embryo, suggesting that this isoform may also be involved in functions other than neuronal excitability. Transcription of all nAChR subunit genes begins approximately 3 days before hatching and continues throughout adult life. Electrophysiological recordings from head ganglionic neurons also indicate the existence of more than one functionally distinct nAChR subtype. Our results suggest the existence of several nAChR subtypes, at least some of them heteromeric, in this insect species.

Anatomy of the antigenic structure of a large membrane autoantigen, the muscle-type nicotinic acetylcholine receptor

The neuromuscular junction nicotinic acetylcholine receptor (AChR), a pentameric membrane glycoprotein, is the autoantigen involved in the autoimmune disease myasthenia gravis (MG). In animals immunized with intact AChR and in human MG, the anti-AChR antibody response is polyclonal. However, a small extracellular region of the AChR alpha-subunit, the main immunogenic region (MIR), seems to be a major target for anti-AChR antibodies. A major loop containing overlapping epitopes for several anti-MIR monoclonal antibodies (mAbs) lies within residues alpha 67-76 at the extreme synaptic end of each alpha-subunit: however, anti-MIR mAbs are functionally and structurally quite heterogeneous. Anti-MIR mAbs do not affect channel gating, but are very effective in the passive transfer of MG to animals; in contrast, their Fab or Fv fragments protect the AChR from the pathogenic effects of the intact antibodies. Antibodies against the cytoplasmic region of the AChR can be elicited by immunization with denatured AChR and the precise epitopes of many such mAbs have been identified; however, it is unlikely that such antibodies are present in significant amounts in human MG. Antibodies to other extracellular epitopes on all AChR subunits are present in both experimental and human MG; these include antibodies to the acetylcholine-binding site which affect AChR function in various ways and also induce acute experimental MG. Finally, anti-AChR antibodies cross-reactive with non-AChR antigens exist, suggesting that MG may result from molecular mimicry. Despite extensive studies, many gaps remain in our understanding of the antigenic structure of the AChR; especially in relation to human MG. A thorough understanding of the antigenic structure of the AChR is required for an in-depth understanding, and for possible specific immunotherapy, of MG.

Mapping of a binding site for ATP within the extracellular region of the Torpedo nicotinic acetylcholine receptor beta-subunit

Using 2,8,5'-[3H]ATP as a direct photoaffinity label for membrane-bound nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata, we have identified a binding site for ATP in the extracellular region of the beta-subunit of the receptor. Photolabeling was completely inhibited in the presence of saturating concentrations of nonradioactive ATP, whereas neither the purinoreceptor antagonists suramin, theophyllin, and caffeine nor the nAChR antagonists alpha-bungarotoxin and d-tubocurarine affected the labeling reaction. Competitive and noncompetitive nicotinic agonists and Ca2+ increased the yield of the photoreaction by up to 50%, suggesting that the respective binding sites are allosterically linked with the ATP site. The dissociation constant KD of binding of ATP to the identified site on the nAChR was of the order of 10(-4) M. Sites of labeling were found in the sequence regions Leu11-Pro17 and Asp152-His163 of the nAChR beta-subunit. These regions may represent parts of a single binding site for ATP, which is discontinuously distributed within the primary structure of the N-terminal extracellular domain. The existence of an extracellular binding site for ATP confirms, on the molecular level, that this nucleotide can directly act on nicotinic receptors, as has been suggested from previous electrophysiological and biochemical studies.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Involvement of histidine 134 in the binding of alpha-bungarotoxin to the nicotinic acetylcholine receptor

Peptides corresponding to the sequence alpha 124-147 of the Torpedo californica and Homo sapiens nicotinic cholinergic receptors were synthesized. The His residue at position 134 was ethoxyformylated or substituted by Ala. Effects of such modifications were studied by: (a) a toxin blot assay and (b) a competition assay between each peptide and the Discopyge Ischudii receptor for 125I alpha-bungarotoxin, in solution. Apparent Kd values were 0.1 and 0.8 microM for Torpedo californica and Homo sapiens native peptides, respectively, and no binding was observed when the His residue was modified or substituted by Ala. ic50 values for the Torpedo californica and Homo sapiens fragments were 1.0 and 0.8 microM, respectively, and no significant displacement occurred when His 134 was ethoxyformylated or substituted by Ala. Hydroxylamine treatment restored 80-100% of their binding ability. Results strongly support the involvement of His 134 in the binding of alpha-bungarotoxin either to the Torpedo californica or the Homo sapiens receptor.

Evidence for the existence of galanin receptors on cultured astrocytes of rat CNS: colocalization with cholinergic receptors

The cellular localization of binding sites for [125I]galanin was studied in explant cultures of rat neocortex, cerebellum, locus coeruleus and spinal cord by means by of autoradiography. Binding sites for the peptide were observed on a great number of astrocytes in all CNS regions studied. In addition to astrocytes, many neurones were intensely labelled by [125I]galanin. Binding of [125I]galanin (10(-8) M) to both astrocytes and neurones was markedly reduced or inhibited by the unlabelled peptide at high concentration (10(-6) M), suggesting 'specific' binding of the radioligand. Evidence for the colocalization of galanin and cholinergic receptors on astrocytes was provided by combined autoradiographic and immunohistochemical studies. Many astrocytes were labelled by [125I]galanin and immunostained with antibodies to either muscarinic or nicotinic receptors. Electrophysiological studies revealed that addition of galanin (10(-9) to 10(-7) M) to the bathing fluid caused a dose-dependent hyperpolarization of the majority of astrocytes studied. When galanin (10(-8) M) and the cholinergic agonists muscarine and nicotine (10(-6) M) were tested on the same astrocyte, all three compounds induced a hyperpolarization, suggesting a colocalization of functional galanin and cholinergic receptors on the glial membrane.

Localization of histidine residues relevant for the binding of alpha-bungarotoxin to the acetylcholine receptor alpha-subunit in V8-proteolytic fragments

Histidine residues have been shown to be critical for alpha-BgTx binding to the acetylcholine receptor (Lacorazza et al., 1992; Bouzat et al., 1993; Lacorazza et al., 1995). Receptor subunits from Discopyge tschudii were modified with diethylpyrocarbonate (DEP). DEP treatment produces a concentration-dependent decrease of [125I] alpha-BgTx binding to the alpha-subunit. The neurotoxin binding capacity was fully restored by adding the nucleophile hydroxylamine. By proteolytic mapping of the alpha-subunit with V8-protease, we determined that the binding capacity to the fragment alpha V8-19 decreased 80% by DEP treatment. In addition, the [125I] alpha-BgTx binding to the same fragment decreased by 70% when the subunits were reduced and affinity-alkylated. We report the N-terminal sequence of both subunits and V8-fragments (alpha V8-10, alpha V8-13, and alpha V8-18), which constitute a first contribution to the knowledge of the primary structure of the Discopyge tschudii receptor. We propose that the fragment alpha V8-19 contains one or more of the histidine residues involved in the alpha-BgTx binding and probably includes the Cys alpha 192-193 disulfide bond. Only two histidine residues are present in the extracellular sequence of Torpedo californica for such fragments: His alpha 186 and alpha 204.

Activation and blockade of mouse muscle nicotinic channels by antibodies directed against the binding site of the acetylcholine receptor

1. Using the patch-clamp technique, we have found that mouse muscle nicotinic acetylcholine receptor (nAChR) channels can be activated by low concentrations of a monoclonal antibody (MoAb), referred to as WF6, which is directed against the acetylcholine (ACh) binding site. Similar effects were seen using IgG or F(ab)2 fragments from the sera of patients with myasthenia gravis (MG), which contain polyclonal anti-nAChR antibodies. 2. The mean open times of MoAb and the slope conductance of single WF6-activated single channels were similar to those of ACh-activated channels under the same experimental conditions. 3. On outside-out patches, single channel activity was elicited by MoAb WF6 and MG F(ab)2 fragments, and was blocked by (+)-tubocurarine. We therefore concluded that MoAb WF6 and the MG F(ab)2 fragments activate the nAChR. 4. MoAb WF6 and MG F(ab)2 fragments blocked the current activated by pulsed application of 10(-4) M ACh to a significant extent. The block was partly reversible. The rate constants for the binding and dissociation of MoAb WF6 from the receptor were determined quantitatively.

Evidence of the coevolution of a snake toxin and its endogenous antitoxin cloning, sequence and expression of a serum albumin cDNA of the Chinese cobra

A full-length cDNA of the serum albumin (CSA) of the cobra (Naja naja kaouthia) was cloned from a lambda gt 11 library. It encodes a mature protein of 614 amino-acid residues homologous to the precursor of mammalian serum albumins. The 1 degree and 2 degrees structures of the CSA resemble those of the human variety. The putative toxin binding sites are mainly located in the subdomains IIA and IIIA. The relation between structural homology and function of the serum albumins (SA) is discussed. An analysis of their evolutionary tree revealed that anti-toxicity arose by < 90 amino-acid exchanges. The rate of substitution is much higher in the SA than in cytochrome C, which probably reflects the difference in evolutionary driving forces. The evolutionary period of the SA (6.7 +/- 0.1 M.Y.) significantly exceeds that of hemoglobin (5.8 M.Y.). Eight tripeptides in the nicotinic acetylcholine receptor (ACR), all flanking the putative toxin binding site, are also found in the CSA where they join to form 1 octa-, 1 penta- and 4 tripeptides, thus indicating the concerted evolution of two functionally linked proteins: toxin and antitoxin (CSA).

Nicotinic acetylcholine receptors on the surface of the blood fluke Schistosoma

Blood dwelling stages of schistosomes have acetylcholinesterase (AChE) on their teguments. As an initial step towards understanding the function of tegumental AChE, we have used specific ligand-binding assays to identify nicotinic acetylcholine receptors (nAChR) on the schistosome surface. AChR could not be detected on migratory stages using fluoroscein isothiocyanate-alpha-bungarotoxin binding but the amount of specific labelling increased on sexual pairing and as the parasites matured into egg-producing adults. Both AChE and nAChR were concentrated on the dorsal surface of the adult male. These results indicate a role for AChE and AChR associated with the transporting function of this membrane.

Clustering of B and T epitopes within short sequence regions of the nicotinic acetylcholine receptor

The epitope repertoire of B cells, due to their selective ability to process their specific antigen and the potential bias imposed on the resulting peptides by the surface immunoglobulins bound to the antigen, may influence the T-helper repertoire. Immunization of C57B1/6 mice with Torpedo acetylcholine receptor (TAChR) causes experimental autoimmune myasthenia gravis (EAMG). Anti-TAChR CD4+ cells recognize epitopes within three sequence regions of the TAChR alpha subunit ('dominant epitopes'). Immunization of mice with denatured or synthetic TAChR antigens sensitizes CD4+ cells to other TAChR sequence regions ('cryptic epitopes'). We investigated here whether clustering of B and T epitopes within the same short sequence segments occurs during the anti-TAChR response, as previously described for the response to hexogenous antigens unrelated to homologous self proteins. Twelve 19-20 residue synthetic sequences of the TAChR alpha, gamma and delta subunits, containing dominant or cryptic CD4+ epitopes for C57B1/6 mice, were tested for ability to induce anti-peptide antibody production. C57B1/6 mice were immunized with the individual peptides. Ten peptides stimulated antibody production. Therefore > 80% of these short TAChR sequences also contain B epitopes. Therefore also in the anti-TAChR response leading to EAMG T and B cell epitopes frequently reside within the same short sequence segment.

Expression of nicotinic acetylcholine receptors in the rat superior cervical ganglion on mRNA and protein level

The expression of nicotinic acetylcholine receptors (nAChR) in the rat superior cervical ganglion was investigated by Western blotting, immunohistochemistry and non-radioactive in situ hybridization applying probes for the alpha 4-1 and beta 2 subunit mRNA. Immunoblot analysis of homogenized ganglia using the anti-nAChRs antibody WF6 revealed a labeled protein band of apparent molecular weight of 40 kDa which is typical for the alpha subunit of nAChRs. Applying double-labeling immunofluorescence with antibodies against tyrosine hydroxylase, nAChR-like molecules were identified in most postganglionic neurons and in a subpopulation of small intensely fluorescent (SIF) cells. alpha 4-1 and beta 2 subunit mRNAs were detected in all perikarya of postganglionic sympathetic neurons but not in SIF cells. These results suggest that antibodies raised against purified Torpedo AChR bind to nAChR in sympathetic ganglia and indicate that alpha 4-1 and beta 2 subunits are constituents of nAChRs in sympathetic postganglionic neurons but not of SIF cells.

Colocalization of binding sites for somatostatin, muscarine and nicotine on cultured neurones of rat neocortex, cerebellum, brain stem and spinal cord: combined autoradiographic and immunohistochemical studies

The cellular localization of binding sites for [125I]1-tyramine somatostatin ([125I]SS) was studied in explant cultures of rat CNS by autoradiography. In cultures from cortex, brain stem and spinal cord many neurones revealed binding sites for the peptide whereas in cerebellar cultures only little binding of [125I]SS was observed. In addition to neurones, astrocytes were also labelled by the peptide. By combined immunohistochemical and autoradiographic techniques, it was demonstrated that the majority of neurones which expressed binding sites for [125I]SS were also immunostained by the monoclonal cholinergic muscarinic or nicotinic receptor antibodies (M 35 and W 6, respectively), providing evidence for a colocalization of cholinergic and somatostatin receptors on the neuronal membrane.

Interaction of protein ligands with receptor fragments. On the residues of curaremimetic toxins that recognize fragments 128-142 and 185-199 of the alpha-subunit of the nicotinic acetylcholine receptor

Using a solid-phase assay, we found that 3H-labeled alpha Cobtx from Naja naja siamensis, a long-chain curaremimetic toxin, and 3H-labelled toxin alpha from Naja nigricollis, a short-chain toxin both bind specifically but with substantially different affinities (Kd = 4 x 10(-7) M and 50 x 10(-6) M) to fragment 185-199 (T alpha 185-199) of the alpha-subunit of the acetylcholine receptor (AcChoR) from Torpedo marmorata. Then we show that monoderivatizations of residues common to both long-chain and short-chain toxins (Tyr-25, Lys-27, Trp-29, and Lys-53) or to long-chain toxins only (Cys-30 and Cys-34) do not affect the binding of the toxins to T alpha 185-199, suggesting that none of these invariant residues in implicated in the recognition of this AcChoR region. alpha Cobtx and toxin alpha bind to the fragment 128-142 (T alpha 128-142) with more similar affinities (Kd = 3 x 10(-7) M and 1.4 x 10(-6) M) and their binding is dramatically affected by the single abolition of the positive charge of Lys-53, an invariant residue that contributes to AcChoR recognition. Therefore, the data indicate that Lys-53 more specifically recognizes the 128-142 region of AcChoR. Other monoderivatizations have no effect on toxin binding. The approach described in this paper may be of great help to identify toxin residues that establish direct contact with receptor fragments.

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.

Effects of mutations of Torpedo acetylcholine receptor alpha 1 subunit residues 184-200 on alpha-bungarotoxin binding in a recombinant fusion protein

Residues between positions 184 and 200 of the Torpedo acetylcholine receptor alpha 1 subunit were changed by oligonucleotide-directed mutagenesis in a recombinant fusion protein containing residues 166-211. Amino acids were substituted with residues present in the snake alpha subunit, with an alanine, or with a functionally dissimilar residue. The competitive antagonist alpha-bungarotoxin bound to the fusion protein with high apparent affinity (IC50 = 3.2 x 10(-8) M), and binding was competed by agonists and antagonists. Mutation of His-186, Tyr-189, Tyr-190, Cys-192, Cys-193, Pro-194, and Asp-195 greatly reduced or abolished alpha-bungarotoxin binding, while mutation of Tyr-198 reduced binding, indicating these residues play an important role in binding either through functional interaction with neurotoxin residues or by stabilizing the conformation of the binding site. Molecular modeling of acetylcholine receptor residues 184-200 and knowledge of both neurotoxin and receptor residues essential for binding allow analysis of possible structure-function relationships of the interaction of alpha-bungarotoxin with this region of the receptor.

Photoaffinity labeling of Torpedo acetylcholine receptor by physostigmine

The plant alkaloid physostigmine, an established anti-cholinesterase agent of the carbamate type, has recently been shown to bind to the nicotinic acetylcholine receptor from Torpedo marmorata electrocytes [Okonjo, K. O., Kuhlmann, J. & Maelicke, A. (1991) Eur. J. Biochem. 200, 671-677]. Pharmacological studies of physostigmine-induced ion flux into nicotinic-acetylcholine-receptor-rich membrane vesicles, indicated distinct binding sites for physostigmine and acetylcholine. As shown in this study by photoaffinity labeling with [phenyl-(n)-3H](-)physostigmine, the physostigmine-binding site is located within the same subunit (alpha polypeptide) of the receptor as the acetylcholine-binding site. Using a variety of proteolytic cleavage conditions for the purified alpha polypeptide, several [3H]physostigmine-labeled peptides were isolated and sequenced. From the radioactivity released in the course of the Edman degradations of the labeled peptides, it was found that the label was associated in all cases with Lys125. These results identify a novel ligand-binding site for the Torpedo nicotinic acetylcholine receptor that is different in location from binding sites identified previously for acetylcholine, its established agonists and antagonists, and direct channel blockers.

A novel cholinergic-specific antigen (Chol-2) in mammalian brain

Three new antisera have been raised in sheep against cholinergic electromotor presynaptic plasma membranes prepared from the electric organs of the electric ray, Torpedo marmorata. They all recognized one or more cholinergic-specific antigens in the mammalian nervous system by the following criteria: they sensitized the cholinergic subpopulation of rat-brain synaptosomes--and only this subpopulation--to lysis by the complement system and, in an immunocytochemical study, selectively stained choline acetyltransferase-positive cholinergic neurons in the rat spinal cord. However, two of the three antisera failed to recognize Chol-1 alpha and -beta, two closely related minor gangliosides already identified as the cholinergic-specific antigens recognized by previous anti-Torpedo presynaptic plasma membrane antisera or indeed any other ganglioside and the third recognized only Chol-1 alpha. A further investigation of the antigen(s) recognized by the most antigenic of the new antisera indicated that it is proteinaceous in nature, but has epitopes in common with electric organ gangliosides.

Sodium dodecyl sulfate- and carbamylcholine-induced changes in circular dichroism spectra of acetylcholine receptor synthetic peptides

The effect of sodium dodecyl sulfate (SDS) on the conformation of acetylcholine receptor alpha-subunit synthetic peptides was investigated by circular dichroism. In the presence of SDS (0.01-0.02%), the affinity of a 173-204 32 residue peptide and a 172-227 56 residue peptide for the competitive antagonist alpha-bungarotoxin increases about 10-fold to the nanomolar range. Circular dichroism spectroscopy of these peptides revealed significant changes in the secondary structure of the peptides in the presence of SDS at concentrations below the critical micelle concentration. It is concluded that SDS induces a conformation of the peptides that is conductive to high affinity binding. Carbamylcholine, an acetylcholine analog, produced small but significant changes in the spectrum of the 173-204 peptide. This change could be the result of agonist-induced conformational changes in this region of the acetylcholine receptor alpha-subunit or to changes in the asymmetric environments of aromatic chromophores in the binding site. These studies demonstrate that synthetic peptides alone are capable of retaining significant functional activity and contain significant secondary structure.

Acetylcholine receptor binding characteristics of snake and cone snail venom postsynaptic neurotoxins: further studies with a non-radioactive assay

The binding of postsynaptic neurotoxins from snake and marine cone snail (Conus sp.) venoms to nicotinic acetylcholine receptor (AchR) was investigated with an ELISA-based, non-radioactive assay. Three snake postsynaptic toxins from the long-chain group (Naja naja kaouthia cobratoxin, Naja oxiana neurotoxin I, Bungarus multicinctus alpha-bungarotoxin) and short-chain group (Naja naja atra cobrotoxin, Naja oxiana neurotoxin II, and Laticauda semifasciata erabutoxin b) were studied. Both types of snake postsynaptic toxins showed a dose-response with constant AchR (50 micrograms/ml) and varying toxin concentrations (50-0.035 micrograms/ml). The minimum detection limits of the assay for snake toxins ranged from 310 to 1240 ng/ml (40-160 pmole/ml), depending on the toxin. Unlike any of the short-chain toxins, long-chain toxins consistently bound less receptor and reached maximum absorbance levels with toxin concentrations of 10-50 micrograms/ml. Competition for AchR binding between cone snail postsynaptic neurotoxins (conotoxins GI, MI, SI) and alpha-bungarotoxin or cobrotoxin resulted in a dose-response. The postsynaptic conotoxins were uniformly better competitors for AchR binding with alpha-bungarotoxin than with cobrotoxin. Heat stability studies with neurotoxin I, erabutoxin b, or cobrotoxin revealed a loss in AchR binding activity with increasing temperature. alpha-Bungarotoxin heated at 90 degrees C had increased AchR binding activity by 105%, relative to 25 degrees C samples, but lost the majority of its binding activity after 100 degrees C. The enhanced binding of heated alpha-bungarotoxin to AchR was specific, as evidenced by a competitive dose-response with unheated alpha-bungarotoxin, but heated toxin lacked any biological activity in the mouse lethal assay. When conotoxins GI or MI were heated at 100 degrees C, there was no detectable loss in AchR binding activity, and only a slight decrease in mouse lethality.

T-helper epitopes on human nicotinic acetylcholine receptor in myasthenia gravis

The synthesis of AChR antibodies requires intervention of AChR-specific Th cells. Because of the paucity of anti-AChR Th cells in the blood of myasthenia gravis (MG) patients, direct studies of these autoimmune cells in the blood are seldom possible. Propagation in vitro of anti-AChR T cells from MG patients by cycles of stimulation with AChR antigens selectively enriches and expands the autoimmune T-cell clones, allowing investigation of their function and epitope specificity. Torpedo electroplax AChR was initially used for propagation of anti-AChR T-cell lines. Those studies demonstrated the feasibility of in vitro propagation of AChR-specific T cells. These are bona fide CD4+ Th cells, which stimulate production in vitro of anti-AChR antibodies by B cells of myasthenic patients and recognize equally well denatured and native AChR, suggesting the usefulness of synthetic human AChR sequences as antigens for propagation of the autoimmune Th cells. We used pools of overlapping synthetic peptides, corresponding to the complete sequences of the human AChR alpha-, beta-, gamma-, and delta-subunits, to propagate AChR-specific Th cells from the blood of MG patients. The AChR sequence regions forming epitopes recognized by the autoimmune T cells were determined by challenging the lines with individual synthetic peptides, 20 residues long, screening the AChR subunit sequences. Although each line had an individual pattern of epitope recognition--as expected from their different HLA-DR haplotype--some peptides were recognized by most of all the CD4+ T-cell lines, irrespective of their DR haplotype. The existence of immunodominant regions of the AChR sequence was verified by investigating the response of unselected CD4+ cells from the blood of a relatively large number of MG patients to the individual peptides screening the human alpha-, gamma-, and delta-subunit sequences. Those studies confirmed that each patient has an individual pattern of peptide recognition. The studies also identified a large number of T epitopes of the human AChR and verified the existence of sequence regions immunodominant for T-helper sensitization, because a limited number of sequence regions, including all those immunodominant for the T-helper lines, were recognized by most patients. Anti-AChR CD4+ T lines could be propagated from some healthy controls only for a brief period of time. They recognized AChR sequences poorly, suggesting a low affinity of their T-cell receptors for the corresponding AChR epitopes.(ABSTRACT TRUNCATED AT 400 WORDS)

Physostigmine and neuromuscular transmission

Single channel studies carried out in cultured rat myoballs and cultured hippocampal neurons, and ion flux studies performed on Torpedo electrocyte membrane vesicles, showed that physostigmine (Phy), a well-established acetylcholinesterase inhibitor, interacts directly with nicotinic acetylcholine receptors (nAChR). Low concentrations (0.1 microM) of Phy activate the receptor integral channel, whereas higher concentrations blocked the channel in its opened state. In contrast to channel activation by acetylcholine (ACh) and classical cholinergic agonists, however, Phy was capable of activating the nAChR channel even when the ACh binding sites were blocked by competitive antagonists, such as alpha-neurotoxins and d-tubocurarine, or when the nAChR was desensitized by preincubation with high concentrations of ACh. The binding site at which Phy binds and activates the nAChR was mapped. It was located within the N-terminal extracellular region of the alpha-polypeptide, in close proximity to the binding site of the natural transmitter. These data identify a novel binding site at nAChRs from many species and tissues that may be involved in receptor regulatory processes.

Nine residues influence the binding of alpha-bungarotoxin in alpha-subunit region 185-200 of human muscle acetylcholine receptor

Identification of residues in the skeletal muscle nicotinic acetylcholine receptor (AChR) that bind snake venom alpha-neurotoxin antagonists of acetylcholine [e.g., alpha-bungarotoxin (alpha-BTx)] provides structural information about the neurotransmitter binding region of the receptor. Using synthetic peptides of the human AChR alpha-subunit region 177-208, we previously localized a pharmacologically specific binding site for alpha-BTx in segment 185-199. To define in more detail the residues that influence the binding of alpha-BTx to this region, we prepared 16 peptide analogues of the alpha-subunit segment 185-200, with the amino acid L-alanine sequentially replacing each native amino acid. Circular dichroism spectroscopy did not reveal changes in the secondary structure of the peptides except for the analogue in which Pro194 was substituted with alanine. This implies that any change in alpha-BTx binding could be attributed to replacement of the native residue's side chain by alanine's methyl group, rather than to a change in the structure of the peptide. The influence of each substitution with alanine was determined by comparing the analogue to the parental sequence alpha 185-200 in solution-phase competition with native human AChR for binding of 125I-labeled alpha-BTx. The binding of alpha-BTx by analogue peptides with alanine substituted for Tyr190, Cys192, or Cys193 was greatly diminished. Binding of alpha-BTx to peptides containing alanine replacements at Val188, Thr189, Pro194, Asp195, or Tyr198 was also reduced significantly (p < 0.003). An unanticipated finding was that substitution of alanine for Ser191 significantly increased alpha-BTx binding (p < 0.003).(ABSTRACT TRUNCATED AT 250 WORDS)

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.

Biochemical characterization of a novel channel-activating site on nicotinic acetylcholine receptors

We have studied the interaction of the reversible acetylcholine esterase inhibitor (-)physostigmine and several structurally related compounds with the nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata electric tissue by means of ligand-induced ion flux into nAChR-rich membrane vesicles, direct binding studies and photoaffinity labeling. (-)Physostigmine acts as a channel-activating ligand at low concentrations and as a direct channel blocker at elevated concentrations. Channel activation is not inhibited by desensitizing concentrations of ACh or ACh-competitive ligands (including alpha-bungarotoxin and D-tubocurarine) but is inhibited by antibody FK1 and several other compounds. From photoaffinity labeling using tritiated physostigmine and mapping of the epitope for the Phy-competitive antibody FK1, the binding site for physostigmine is located within the alpha-subunit of the Torpedo nAChR and is distinct from the acetylcholine binding site. Our data suggest a second pathway of nAChR channel activation that may function physiologically as an allosteric control of receptor activity.

Amino acid residues within the sequence region alpha 55-74 of Torpedo nicotinic acetylcholine receptor interacting with antibodies to the main immunogenic region and with snake alpha-neurotoxins

The sequence region 55-74 of the alpha-subunit of the acetylcholine receptor (AChR) from Torpedo californica electroplax comprises the amino-terminal end of a sequence segment--residues alpha 67-76--forming the main immunogenic region (MIR), which is most frequently recognized by anti-AChR autoantibodies in myasthenia gravis. The synthetic sequence alpha 55-74 of Torpedo AChR binds alpha-bungarotoxin (alpha BTX), suggesting that amino acid residues within this sequence region may contribute to formation of an alpha BTX binding site. Using single-residue substituted synthetic analogues of the sequence alpha 55-74 of Torpedo AChR, in which each residue was sequentially substituted by either glycine or alanine, we sought identification of the amino acids involved in interaction with alpha-neurotoxins and with three different anti-MIR monoclonal antibodies (mAbs 6, 22, and 198). Substitution of Arg55, Arg57, Trp60, Arg64, Leu65, Arg66, Trp67, or Asn68 strongly inhibited alpha-toxin binding, whereas substitutions of Ile61, Val63, Pro69, Ala70, Asp71, or Tyr72 had marginal effects. Substitutions within the region alpha 68-72 significantly diminished binding of anti-MIR mAbs, although residue preferences differed among mAbs. Further, substituting Trp60 substantially reduced binding of mAb 198, and moderately affected binding of mAb 6, and substitution of Asp62 slightly but consistently affected binding of mAbs 6 and 22.

Nuclear magnetic resonance solution structure of the alpha-neurotoxin from the black mamba (Dendroaspis polylepis polylepis)

The three-dimensional structure in solution of the alpha-neurotoxin from the black mamba (Dendroaspis polylepis polylepis) has been determined by nuclear magnetic resonance spectroscopy. A high quality structure for this 60-residue protein was obtained from 656 NOE distance constraints and 143 dihedral angle constraints, using the distance geometry program DIANA for the structure calculation and AMBER for restrained energy minimization. For a group of 20 conformers used to represent the solution structure, the average root-mean-square deviation value calculated for the polypeptide backbone heavy atoms relative to the mean structure was 0.45 A. The protein consists of a core region from which three finger-like loops extend outwards. It includes a short, two-stranded antiparallel beta-sheet of residues 1-5 and 13-17, a three-stranded antiparallel beta-sheet involving residues 23-31, 34-42 and 51-55, and four disulfide bridges in the core region. There is also extensive non-regular hydrogen bonding between the carboxy-terminal tail of the polypeptide chain and the rest of the core region. Comparison with the crystal structure of erabutoxin-b indicates that the structure of alpha-neurotoxin is quite similar to other neurotoxin structures, but that local structural differences are seen in regions thought to be important for binding of neurotoxins to the acetylcholine receptor. For two regions of the alpha-neurotoxin structure there is evidence for an equilibrium between multiple conformations, which might be related to conformational rearrangements upon binding to the receptor. Overall, the alpha-neurotoxin presents itself as a protein with a stable core and flexible surface areas that interact with the acetylcholine receptor in such a way that high affinity binding is achieved by conformational rearrangements of the deformable regions of the neurotoxin structure.

Epitope mapping of polyclonal and monoclonal antibodies against two alpha-bungarotoxin-binding alpha subunits from neuronal nicotinic receptors

Recently, cDNAs for alpha subunits of two different neuronal alpha-bungarotoxin-binding proteins (alpha BgtBP) were isolated from chick brain, designated alpha BgtBP alpha 1 and alpha BgtBP alpha 2. These are now also referred to as subunits alpha 7 and alpha 8, respectively. Expression studies in Xenopus oocytes have indicated that alpha 7 subunits are able to form cation channels that are sensitive to nicotinic ligands, and therefore represent bona fide nicotinic acetylcholine receptor subunits. Polyclonal and monoclonal antibodies (mAbs) have been produced against: (i) affinity-purified chick brain alpha BgtBP; and (ii) fusion proteins containing the unique cytoplasmic sequences alpha 7(327-412) and alpha 8(293-435). Here, synthetic overlapping peptides corresponding to their deduced amino acid sequences are used to map the epitopes recognized by the different antibodies. The polyclonal response to affinity-purified alpha BgtBPs and the fusion proteins indicates that sequence segments 290-420 of both subunits contain several major and minor epitopes. mAbs selected for their ability to bind both native and denatured alpha BgtBPs isolated from chick brain also recognize subunit-specific sequential epitopes within the sequence segment 290-420. The epitopes recognized by the mAbs correspond to the minor epitopes defined using antisera. The mAbs characterized in these studies will provide useful probes for further studies of alpha BgtBP structure and histological localization.

Nicotinic cholinoceptors in the rat pineal gland as analyzed by western blot, light- and electron microscopy

The monoclonal antibody WF6, raised against purified Torpedo nicotinic acetylcholine receptor (nAChR) was used to study the distribution of cholinoceptors in the rat pineal gland by means of Western blot analysis, light- and electron microscopy. The immunoblot analysis using homogenized pineal gland revealed a labeled protein band of apparent molecular weight 40 kDa which was identified as alpha-subunit of a nAChR. In the light microscope, approximately one-fourth of the pinealocytes exhibited cytoplasmic immunoreactivity (IR) of varying density. In the electron microscope, IR was seen as patchy staining of cell membranes of pinealocyte somata and processes. Presynaptic IR material was not found. Distribution and intensity of the observed IR was not significantly different in pineal sections from ganglionectomized rats, nor were any alterations found that would relate to the animals' sex or to the time of killing (day vs night). Our results provide further evidence for the existence of cholinergic receptors in the mammalian pineal. They may be important for the understanding of the gland's regulation.

Substitution of Torpedo acetylcholine receptor alpha 1-subunit residues with snake alpha 1- and rat nerve alpha 3-subunit residues in recombinant fusion proteins: effect on alpha-bungarotoxin binding

A fusion protein consisting of the TrpE protein and residues 166-211 of the Torpedo acetylcholine receptor alpha 1 subunit was produced in Escherichia coli using a pATH10 expression vector. Residues in the Torpedo sequence were changed by means of oligonucleotide-directed mutagenesis to residues present in snake alpha 1 subunit and rat nerve alpha 3 subunit which do not bind alpha-bungarotoxin. The fusion protein of the Torpedo sequence bound 125I-alpha-bungarotoxin with high affinity (IC50 = 2.5 x 10(-8) M from competition with unlabeled toxin, KD = 2.3 x 10(-8) M from equilibrium saturation binding data). Mutation of three Torpedo residues to snake residues, W184F, K185W, and W187S, had no effect on binding. Conversion of two additional Torpedo residues to snake, T191S and P194L, reduced alpha-bungarotoxin binding to undetectable levels. The P194L mutation alone abolished toxin binding. Mutation of three Torpedo alpha 1 residues to neuronal alpha 3-subunit residues, W187E, Y189K, and T191N, also abolished detectable alpha-bungarotoxin binding. Conversion of Try-189 to Asn which is present in the snake sequence (Y189N) abolished toxin binding. It is concluded that in the sequence of the alpha subunit of Torpedo encompassing Cys-192 and Cys-193, Try-189 and Pro-194 are important determinants of alpha-bungarotoxin binding. Tyr-189 may interact directly with cationic groups or participate in aromatic-aromatic interactions while Pro-194 may be necessary to maintain a conformation conductive to neurotoxin binding.