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

Electron microscopic evidence for nucleation and growth of 3D acetylcholine receptor microcrystals in structured lipid-detergent matrices

Nicotinic acetylcholine receptors (AChRs) belong to a superfamily of oligomeric proteins that transduce electric signals across the cell membrane on binding of neurotransmitters. These receptors harbor a large extracellular ligand-binding domain directly linked to an ion-conducting channel-forming domain that spans the cell membrane 20 times and considerably extends into the cytoplasm. Thus far, none of these receptor channels has been crystallized in three dimensions. The crystallization of the AChR from Torpedo marmorata electric organs is challenged here in lipidic-detergent matrices. Detergent-soluble AChR complexed with alpha-bungarotoxin (alphaBTx), a polypeptidic competitive antagonist, was purified. The AChR-alphaBTx complex was reconstituted in a lipidic matrix composed of monoolein bilayers that are structured in three dimensions. The alphaBTx was conjugated to a photo-stable fluorophore, enabling us to monitor the physical behavior of the receptor-toxin complex in the lipidic matrix under light stereomicroscope, and to freeze fracture regions containing the receptor-toxin complex for visualization under a transmission electron microscope. Conditions were established for forming 2D receptor-toxin lattices that are stacked in the third dimension. 3D AChR nanocrystals were thereby grown inside the highly viscous lipidic 3D matrix. Slow emulsification of the lipidic matrix converted these nanocrystals into 3D elongated thin crystal plates of micrometer size. The latter are stable in detergent-containing aqueous solutions and can currently be used for seeding and epitaxial growth, en route to crystals of appropriate dimensions for x-ray diffraction studies.

Muscarinic toxin-like proteins from Taiwan banded krait (Bungarus multicinctus) venom: purification, characterization and gene organization

Two novel proteins, BM8 and BM14, were isolated from Bungarus multicinctus (Taiwan banded krait) venom using the combination of chromatography on a SP-Sephadex C-25 column and a reverse-phase HPLC column. Both proteins contained 82 amino acid residues including 10 cysteine residues, but there were two amino acid substitutions at positions 37 and 38 (Glu37-Ala38 in BM8; Lys37-Lys38 in BM14). CD spectra and acrylamide quenching studies revealed that the gross conformation of BM8 and BM14 differed. In contrast to BM8, BM14 inhibited the binding of [3H]quinuclidinyl benzilate to the M2 muscarinic acetylcholine (mAchR) receptor subtype. Trinitrophenylation of Lys residues abolished the mAchR-binding activity of BM14, indicating that the Lys substitutions at positions 37 and 38 played a crucial role in the activity of BM14. The genomic DNA encoding the precursor of BM14 was amplified by PCR. The gene shared virtually identical structural organization with alpha-neurotoxin and cardiotoxin genes. The intron sequences of these genes shared a sequence identity up to 84%, but the protein-coding regions were highly variable. These results suggest that BM8, BM14, neurotoxins and cardiotoxins may have originated from a common ancestor, and the evolution of snake venom proteins shows a tendency to diversify their functions.

Characterization and gene organization of Taiwan banded krait (Bungarus multicinctus) gamma-bungarotoxin

gamma-Bungarotoxin was isolated from Bungarus multicinctus (Taiwan banded krait) venom using a combination of chromatography on a SP-Sephadex C-25 column and a reverse-phase high-performance liquid chromatography column. Circular dichroism (CD) measurement revealed that its secondary structure was dominant with beta-sheet structure as is that of snake venom alpha-neurotoxins and cardiotoxins. gamma-Bungarotoxin exhibits activity on inhibiting the binding of [3H]quinuclidinyl benzilate to the M2 muscarinic acetylcholine receptor subtype, and competes weakly with radioiodinated alpha-bungarotoxin for binding to the Torpedo nicotinic acetylcholine receptor. Moreover, the toxin inhibits collagen-induced platelet aggregation, with an IC50 of approximately 200 nM. The genomic DNA encoding the gamma-bungarotoxin precursor is amplified by polymerase chain reaction (PCR). The gene is organized with three exons separated by two introns, and shares virtually identical overall organization with those reported for alpha-neurotoxin and cardiotoxin genes, including similar intron insertions. The intron sequences of these genes share sequence identity up to 85%, but the exon sequences are highly variable. These observations suggest that gamma-bungarotoxin, alpha-neurotoxins, and cardiotoxins originate from a common ancestor, and the evolution of these genes shows a tendency to diversify the functions of snake venom proteins.

Photoactivatable alpha-conotoxins reveal contacts with all subunits as well as antagonist-induced rearrangements in the Torpedo californica acetylcholine receptor

Azidobenzoyl (AzBz) and benzoylbenzoyl (BzBz) derivatives of alpha-conotoxin MI and L-benzoylphenylalanine (Bpa) analogs of alpha-conotoxin GI were synthesized. All these compounds, similarly to native alpha-conotoxins, completely displaced the radioiodinated MI or GI from the membrane-bound nicotinic acetylcholine receptor (AChR) of Torpedo californica. However, the GI(Bpa11) analog was considerably less potent than GI in competing with radioiodinated alpha-bungarotoxin (alphaBgt). Irradiation of iodinated AzBz derivatives bound to AChR resulted in labeling of all AChR subunits. The BzBz and Bpa derivatives gave lower levels of specific cross-linking but considerable labeling at additional sites that was enhanced, rather than suppressed, by an excess of native alpha-conotoxins or alphaBgt. Both equilibrium binding of benzophenone-derivatized alpha-conotoxins and their cross-linking could be totally abolished by physostigmine. The results obtained demonstrate that (a) specific binding sites for alpha-conotoxins and alphaBgt are overlapping but not identical, (b) each of the AChR subunits can be labeled with photoactivatable alpha-conotoxins and (c) enhancement of benzophenone-derivatized alpha-conotoxins cross-linking at additional (physostigmine-related) sites by alphaBgt or GI indicates that these antagonists induce structural alterations in the AChR outside their binding sites.

How do acetylcholine receptor ligands reach their binding sites?

The access pathway to the binding sites for large competitive antagonists of the nicotinic acetylcholine receptor from Torpedo californica electric tissue was analyzed by binding and photolabeling experiments with alpha-neurotoxins. Binding assays with [125I]alpha-bungarotoxin showed an increase in the number of accessible binding sites upon stepwise solubilization of the receptor-rich membranes. Similarily, ligand binding is facilitated upon fluidization of the membrane by increasing the temperature. The access to the binding sites seems to be sterically 'hindered' in the densely packed membrane state. Using a novel series of large biotinylated photoactivatable derivatives of neurotoxin II, we observed that the accessibility to the alpha/gamma- but not to the alpha/delta-binding site was considerably decreased for some derivatives under native conditions. This effect was less apparent at higher temperatures and could be abolished by complete solubilization. These observations support the nonequivalence of the receptor's binding sites. Together, our data suggest (a) that alpha-neurotoxins approach their binding sites from the membrane-facing periphery of the receptor's extramembrane domain rather than through the channel mouth and (b) that different entrance pathways to each binding site exist which vary in their sensitivity to the physical state of the plasma membrane.

Disulfide isomers of alpha-neurotoxins from King cobra (Ophiophagus hannah) venom

Two novel alpha-neurotoxins, Oh-6A and Oh-6B, isolated from the king cobra (Ophiophagus hannah) venom, consist of 70 amino acid residues with 10 cysteine residues and share the same amino acid sequences as determined by Edman degradation on the peptide fragments generated from the proteolytic hydrolysates. Their sequences share 46-53% homology with Oh-4, Oh-5, Toxin a, and Toxin b from the same venom. The finding that Oh-6A and Oh-6B had different retention times in the reversed-phase column suggested that the two toxin molecules should not have the same conformation. Selective reduction on the disulfide bond, Cys26--Cys30, at the tip of their loop II structures resulted in the production of the partially reduced derivatives eluted at the same position. Under redox conditions, the partially reduced Oh-6A and 6B exclusively converted into native Oh-6A as evidenced by HPLC analyses. This suggests that Oh-6A and Oh-6B are disulfide isomers which probably arise from cis-trans isomerization of the Cys26--Cys30 disulfide bond. Alternatively, the two toxins exhibited binding activity toward nAChR and lethal toxicity equally. It reflects that the diversity around the extra loop at the loop II structure does not exert a significant effect on the manifestation of the neurotoxicity of Oh-6A and Oh-6B.

Differential surface accessibility of alpha(187-199) in the Torpedo acetylcholine receptor alpha subunits

We have probed the surface accessibility of residues alpha187 to alpha199 of the Torpedo acetylcholine receptor with monoclonal antibody 383C, which binds uniquely to these residues. However, 383C binds to only one of the two alpha subunits in the membrane-bound receptor, neither of the two subunits in carbamylcholine-desensitized receptor, and to both alpha subunits in Triton X-100 solubilized receptor. The kinetics of association and dissoci-ation of 383C with the peptide alpha(183-199) compared to those with the membrane-bound receptor suggest that all but a single hydrogen bond of affinity derives from contacts between this peptide and the monoclonal antibody paratope. Inhibition of 383C binding by alpha-bungarotoxin selectively directed to the alpha subunit correlated with the high-affinity d-tubocurarine binding site, along with a lack of inhibition by alpha-bungarotoxin directed to the alpha subunit correlated with the low-affinity d-tubocurarine binding site, suggests that the 383C epitope on the membrane-bound receptor resides on the alpha subunit associated with the high-affinity d-tubocurarine binding site. The results presented here suggest a structural basis for the differences between the two receptor acetylcholine binding sites.

Unfolding/folding studies on cobrotoxin from Taiwan cobra venom: pH and GSH/GSSG govern disulfide isomerization at the C-terminus

Refolding of cobrotoxin was assessed by the exposure degree of its single Trp determined by an acrylamide quenching study. The change in the accessibility of Trp for acrylamide quantitatively reflected the formation of folded cobrotoxin, and the data were confirmed by HPLC and gel electrophoresis analyses. However, the site-specific information provided by quenching Trp fluorescence revealed that the ordered structure in the neighborhood of Trp was attained prior to the complete formation of the tertiary structure of cobrotoxin. HPLC analyses showed that, in addition to refolded cobrotoxin, two novel species (cobrotoxin II and cobrotoxin III) with isomerization of disulfide bonds at the C-terminus of the toxin molecule were produced along the folding reaction. The disulfide pairings in cobrotoxin II and cobrotoxin III were Cys43-Cys55 and Cys54-Cys60 and Cys43-Cys60 and Cys54-Cys55, respectively. Among the three possible two-disulfide species at the C-terminus, the disulfide linkages Cys43-Cys60 and Cys54-Cys55 of cobrotoxin III caused a marked decrease in lethality and resulted in a conformation which was notably different from that observed with the native toxin molecule as evidenced by CD spectra. The refolding reaction was accelerated by the addition of GSH/GSSG, and the resulting products were mostly folded cobrotoxin. However, if GSH/GSSG was not added into the initial folding materials, the yields of cobrotoxin II and cobrotoxin III greatly increased. The conversion of cobrotoxin to its isomers was to be irreversible and pH-dependent: the higher the pH, the faster the rate of conversion. However, this conversion could be partly inhibited by GSH/GSSG. Cobrotoxin II and cobrotoxin III were purified from Taiwan cobra venom as well, and their yields in comparison to that of cobrotoxin in venom were similar to that noted with the folded products in the presence of GSH/GSSG. Moreover, the rate of disulfide isomerization was expected to be slow in venom fluid in which the pH was approximately pH 6.2. Thus, the finding that cobrotoxin represents the predominant neurotoxin species in Taiwan cobra venom is probably associated with the synergistic effects of GSH/GSSG and pH.

The alpha-bungarotoxin-binding nicotinic acetylcholine receptor from rat brain contains only the alpha7 subunit

When expressed in Xenopus oocytes, the rat alpha7 subunit forms homo-oligomeric nicotinic acetylcholine receptors, which are blocked by alpha-bungarotoxin. Since the pharmacological and physiological properties of the alpha7 receptor expressed in oocytes are similar to those of the alpha-bungarotoxin-sensitive nicotinic currents recorded from neuronal preparations and the distribution patterns of alpha7 mRNA and alpha-bungarotoxin-binding sites in the rat brain are very similar, alpha7 is thought to be the main component of the alpha-bungarotoxin-binding nicotinic receptor in the mammalian brain. However, while alpha7 is found in purified alpha-bungarotoxin-binding complexes from rat brain or PC12 cells, other proteins copurify with it. Therefore, the question whether alpha7 forms a homo-oligomeric alpha-bungarotoxin-binding nicotinic receptor in the mammalian brain remains. We have developed and characterized affinity-purified polyclonal antibodies and used these antibodies in Western blot analyses of alpha-bungarotoxin-binding proteins purified from rat brains. We report here that our experimental data support the current working hypothesis that the alpha-bungarotoxin-binding nicotinic receptor is a homo-oligomer of alpha7 subunits in the rat brain.

Purification of the nicotinic acetylcholine receptor protein by affinity chromatography using a regioselectively modified and reversibly immobilized alpha-toxin from Naja nigricollis

A new method of affinity chromatography purification of the detergent-solubilized nicotinic acetylcholine receptor protein (nAChR) is presented, based on the reversible coupling of a chemically monomodified alpha-toxin from Naja nigricollis to a resin. The alpha-toxin was monothiolated on the epsilon-amino group of its lysine-15 by reaction with N-succinimidly-3-(2-pyridyldithio)propionate and was covalently linked in a reversible manner to a thiopropyl-activated agarose resin by thiol-disulfide exchange. We found that 50% of the immobilized toxin molecules were effective for purifying nAChR, indicating a high accessibility of resin-bound toxins to their binding sites on the receptor protein. Purified alpha-toxin/nAChR complexes were eluted with nearly 100% recovery by reduction of disulfide bridges with dithiothreitol. nAChR solutions of high purity were obtained, as shown by polyacrylamide gel electrophoresis. A comparison was made with two other procedures of affinity chromatography using: (1) alpha-bungarotoxin from Bungarus multicinctus polymodified on several amines and covalently linked to a resin in a reversible manner, and (2) a commercial agarose resin bearing irreversibly immobilized alpha-cobrotoxin from Naja naja kaouthia. We conclude that: (1) the use of a selected regioselective linking of a peptidic ligand to a chromatography resin results in an increased efficiency of protein binding, and (2) a high yield of protein recovery is obtained via reversible covalent linking.

A conserved disulfide loop facilitates conformational maturation in the subunits of the acetylcholine receptor

To examine the structural determinants for the assembly of ligand-gated receptors, we constructed mutant alpha, beta, gamma and delta subunits of the Torpedo acetylcholine receptor (AChR), lacking one of the conserved cysteine residues which forms a 13-amino acid disulfide loop in the amino terminal domain of each subunit. Mutant subunits were co-expressed with complementary wild-type subunits in Xenopus oocytes. Using subunit-specific antisera and monoclonal antibodies that recognize the two distinct alpha-bungarotoxin (alpha-BuTX) sites on the AChR, we were able to distinguish immature subunit associations from conformationally mature AChR complexes. Removal of the disulfide loop on the alpha subunit completely destroyed the formation of the two toxin-binding sites, while removal of the structure on the beta subunit had little effect. While mutant gamma and delta subunits were capable of forming associations (immature assembly) with other subunits, the formation of alpha-BTX sites between alpha and mutant gamma or mutant delta subunits was diminished. Interestingly, assembly of alpha beta gamma subunits remained efficient in the presence of mutant delta subunits, whereas assembly of alpha beta delta subunits was inefficient in the presence of mutant gamma subunits. Thus, these results indicate that the formation of the disulfide loop facilitates the conformational maturation of alpha gamma and alpha delta complexes, which may be conditional for correct subunit coupling in assembling receptors. Furthermore, it seems likely that the correct coupling between the alpha and gamma subunits is the most important step in subunit assembly.

Fluorescence correlation spectrometry of the interaction kinetics of tetramethylrhodamin α-bungarotoxin with Torpedo californica acetylcholine receptor

Fluorescence correlation spectroscopy (FCS) is suited to determine low concentrations (10(-8) M) of slowly interacting molecules with different translational diffusion coefficients on the level of single molecule counting. This new technique was applied to characterize the interaction dynamics of tetramethylrhodamin labelled alpha-bungarotoxin (B( *)) with the detergent solubilized nicotinic acetylcholine receptor (AChR) of Torpedo californica electric organ. At pseudo-first-order conditions for AChR, the complex formation with B( *) is monophasic. The association rate coefficient of the monoliganded species AChR . B is k(ass)' = 3.8 . 10(3) s(-1) at 293 K (20 degrees C). The dissociation of bound B( *) from the monomer species AChR . B( *) . B (and AChR . B(2)( *)), initiated by adding an excess of nonlabelled alpha-bungarotoxin (B), is biphasic suggesting a three state cascade for the B-sites: R(alpha) --> R(alpha)' --> R(alpha)'' with the exchange dissociation constants: (k(diss)')(B) = 5.5(+/-1) . 10(-5) s(-1) and (k(diss)'')(B) = 3(+/-1) . 10(-6) s(-1) at 293 K. The data are consistent with dissociative intermediate steps of ligand exchange on two different interconvertible conformations of one binding site. The dissociation of bound B( *) by excess of the neurotransmitter acetylcholine (ACh) is biphasic. At [ACh] = 0.1 M both B( *) are released from the AChR . B(2)( *) species. The mechanism involves associative ternary intermediates (AChR . B( *)A, AChR . B( *)A(2) and AChR . B(2)( *)A(2)). The equilibrium constants (K(A)) and dissociation rate constants (k(-A)) for ACh in the ternary complex state R(alpha)' and R(alpha)'', respectively, are K(A)' = 1.1 . 10(-2) M and k(-A)' = 3 . 10(5) s(-1) and K(A)'' = 7.5 . 10(-2) M and k(-A)'' = 2 . 10(6) s(-1). It is of physiological importance that the FCS data indicate that the AChR monomer species (M(r) = 290 000), which normally at [ACh] 1 mM only binds one ACh molecule, does bind two ACh molecules at [ACh] 0.1 M.

Chemical modification of cationic residues in toxin a from king cobra (Ophiophagus hannah) venom

The cationic groups of arginine and lysine residues in alpha-neurotoxin, Toxin a, isolated from king cobra (Ophiophagus hannah) venom were subjected to modification with trinitrobenzene sulfonate (TNBS) and p-hydroxyphenylglyoxal (HPG), respectively. The trinitrophenylated (TNP) derivatives of Toxin a at Lys-10, 56, or 71 showed approximately 25% residual lethality, and modifications on Lys-10 and 56 or Lys-10 and 50 resulted in a decrease of lethality by 84% and 86%, respectively. Modifications on Arg-34, 37, and 70 and Arg-34, 37, and 72 in Toxin a caused a decrease in lethality by 92% and 93%, respectively, and it almost completely lost its lethality and binding activity to nicotinic acetylcholine receptor (nAChR) when all four arginine residues were modified. These results indicate that in addition to the cationic residues on loop II (Arg-34, 37), loop III (Lys-50, 56), and the C-terminal tail (Arg-70, 72; Lys-71), Lys-10 on loop I is also related to the neurotoxicity of Toxin a.

Interaction of d-tubocurarine analogs with the Torpedo nicotinic acetylcholine receptor. Methylation and stereoisomerization affect site-selective competitive binding and binding to the noncompetitive site

Analogs of d-tubocurarine were used to determine the individual effects of methylation, stereoisomerization, and halogenation of d-tubocurarine on the affinity for each of the two acetylcholine (ACh) binding sites of the Torpedo nicotinic acetylcholine receptor (AChR) and for the noncompetitive antagonist site. Eight analogs were synthesized, including three new compounds: 7'-O-methyl-chondocurarine, 12'-O-methyl-chondocurarine, and 13'-bromo-d-tubocurarine. The two ACh sites differ in their affinities for d-tubocurarine by 400-fold, as shown by inhibition of [3H]ACh binding, whereas the affinity ratio for metocurine, the trimethylated derivative of d-tubocurarine, is reduced to 30 due to a decreased affinity for the high affinity site. Binding analysis of five d-tubocurarine analogs demonstrates that methylation of the phenols alone is responsible for the observed changes in affinity. Substitution with bromine or iodine at the 13'-position affected affinity at both sites with a net increase in site selectivity. Stereoisomers of d-tubocurare had decreased affinity for only the high affinity ACh site. Thus, the ring systems, including the 12'- and 13'-positions and the 1-position stereocenter, appear to be important in discriminating between the two ACh binding sites. Desensitization of the AChR was measured by increased affinity for [3H]phencyclidine. Binding to only the single, high affinity acetylcholine binding site, comprised by the alpha gamma-subunits, was required for partial desensitization of the AChR by d-tubocurarine and its analogs. Stronger desensitization, to the same extent observed in the presence of the agonist carbamylcholine, occurred upon binding by iodonated or brominated d-tubocurarine. Interaction of the analogs at the noncompetitive antagonist site of the AChR was also measured by [3H]phencyclidine binding. The bis-tertiary ammonium analogs of either the d- or l-stereoisomers bound to the noncompetitive antagonist binding site of the AChR with 100-fold higher affinity than the corresponding quaternary ammonium analogs.

Role of amino and carboxyl groups of cobrotoxin in the conformational stability and the interaction with acetylcholine receptor

To study the functional involvements of the common interaction of the Leu-1 alpha-amino group and Asp-58 in cobrotoxin, the lysine epsilon-amino groups of cobrotoxin were initially guanidinated with o-methylisourea. The alpha-amino group of Leu-1 was them modified with TNBS after the guanidination of cobrotoxin. Both modified derivatives displayed no significant changes in the secondary structure and antigenicity of cobrotoxin, whereas the binding affinity for nicotinic acetylcholine receptor (nAChR) was pronouncedly decreased when Leu-1 was modified. Six out of seven free carboxyl groups and the remaining buried Glu-21 carboxyl group of cobrotoxin were modified with glycine methyl ester in the absence and presence of guanidine HCl, respectively. Alternation in the beta-sheet secondary structure of cobrotoxin was observed with the carboxyl-group modified derivatives, which caused a decrease in the binding activity of the toxin molecule to the antibody and nAChR. Moreover, modification of the Glu-21 carboxyl group of cobrotoxin further reduced the nAChR binding activity, while the antigenicity remained unchange. Thus, our results conclude that the Glu-21 residue and the common interaction of the terminal Leu-1 alpha-amino group and the Asp-58 carboxyl group are related to the nAChR-binding activity of cobrotoxin, and the free carboxyl groups in cobrotoxin are conformation-essential.

Chemical modification of tryptophan residues in alpha-neurotoxins from Ophiophagus hannah (king cobra) venom

Two alpha-neurotoxins, Oh-4 and Oh-7, from the king cobra (Ophiophagus hannah) venom were subjected to Trp modification with 2-nitrophenylsulfenyl chloride (NPS-Cl). One major NPS derivative was isolated from the modified mixtures of Oh-4 and two from Oh-7 by HPLC. Amino acid analysis and sequence determination revealed that Trp-27 in Oh-4, and Trp-30 and Trp-26 and 30 in the two Oh-7 derivatives, were modified, respectively. Sulfenylation of Trp-27 in Oh-4 caused about 70% drop in lethal toxicity and nicotinic acetylcholine receptor-binding activity. Modification of Trp-30 in Oh-7 resulted in the decrease of lethal toxicity by 36% and binding activity by 61%. The activities were further lost when the conserved Trp-26 in Oh-7 was modified. Sulfenylation of the Trp residues did not significantly affect the secondary structure of the toxins as revealed by the CD spectra. These results indicate that the Trp residues in these two long alpha-neurotoxins may be involved in the receptor binding.

Relationship between the binding sites for an alpha-conotoxin and snake venom neurotoxins in the nicotinic acetylcholine receptor from Torpedo californica

Photoinduced cross-links between the iodinated Lys26-p-azidobenzoyl derivative of neurotoxin II from Naja naja oxiana cobra venom and nicotinic acetylcholine receptor from Torpedo californica (AChR) have been studied in the presence of alpha-conotoxin GI from the marine snail C. geographus. Preincubation of the AChR-enriched membranes with increasing concentrations of alpha-conotoxin GI protects first the gamma subunit from photolabelling and then the delta subunit, the IC50 values being 0.76 and 5.01 microM, respectively. The results obtained, in view of the relevant data in literature, demonstrate that the (alpha + gamma) site, which is the high affinity site for d-tubocurarine, has also a higher affinity for an alpha-conotoxin than the (alpha + delta) containing site. The latter has a somewhat higher affinity than the (alpha + gamma) site towards some naturally occurring snake venom alpha-neurotoxins or their derivatives.

Diterpenoids from Caribbean gorgonians act as noncompetitive inhibitors of the nicotinic acetylcholine receptor

1. Three cyclic diterpenoids isolated from gorgonians of the Eunicea genus and characterized as eupalmerin acetate (EUAC), 12,13-bisepieupalmerin (BEEP), and eunicin (EUNI) were found to be pharmacologically active on the nicotinic acetylcholine receptor (AChR). 2. The receptor from the BC3H-1 muscle cell line was expressed in Xenopus laevis oocytes and studied with a two-electrode voltage clamp apparatus. 3. All three compounds reversibly inhibited ACh-induced currents, with IC50's from 6 to 35 microM. ACh dose-response curves suggested that his inhibition was noncompetitive. The cembranoids also increased the rate of receptor desensitization. 4. Radioligand-binding studies using AChR-rich membranes from Torpedo electric organ indicated that all three cembranoids inhibited high-affinity [3H]phencyclidine binding, with IC50's of 0.8, 11.6, and 63.8 microM for EUNI, EUAC, and BEEP, respectively. The cembranoids at a 100 microM concentration did not inhibit [alpha-125I]bungarotoxin binding to either membrane-bound or solubilized AChR. 5. It is concluded that these compounds act as noncompetitive inhibitors of peripheral 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).

Studies on the status of amino groups in alpha-bungarotoxin

The positive charges of amino groups in alpha-bungarotoxin (alpha-BuTX) were neutralized by reaction with trinitrobenzene sulfonate (TNBS) and were converted into negative charges with 4-chloro-3,5-dinitrobenzoate (CDNB). Three derivatives monotrinitrophenylated (TNP-) at Lys-38, 64, or 70; three di-TNP at Lys-38 and 64, Lys-38 and 70, and Lys-64 and 70; one tri-TNP at Lys-38, 64 and 70; and one penta-TNP at Lys-38, 51, 52, 64 and 70 as well as one mono-carboxydinitrophenylated (CDNP-) at Lys-38; di-CDNP at Lys-38 and 70, and tri-CDNP at Lys-38, 64 and 70 were isolated, respectively. The epsilon-amino groups at Lys-38, 64 and 70 are the most accessible to trinitrophenylation, Lys-51 and 52 are less reactive, while the N-terminus and Lys-26 are the least reactive. Each mono-TNP and CDNP derivative showed approximately 50% residual binding activity to nicotinic acetylcholine receptor (AChR)-rich membranes isolated from Torpedo californica and 50% of the lethality of alpha-BuTX. However, the activities were progressively lost as the accumulative modifications proceeded, and led finally to the formation of almost inactive penta-TNP derivative. The antigenicity of alpha-BuTX was still retained essentially intact after one or two amino groups of lysine residues were modified, whereas tri-TNP and CDNP-derivatives modified at Lys-38, 64 and 70 lost 46 and 70% of their antigenicity, respectively. Pronounced alteration in antigenicity was observed after five amino groups were trinitrophenylated. The present study indicates that the amino groups in alpha-BuTX may participate in the multipoint contact between the toxin and AChR, but none of the individual amino groups is definitely essential for the binding.

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

Previous studies by several laboratories have identified a narrow sequence region of the nicotinic acetylcholine receptor (AChR) alpha subunit, flanking the cysteinyl residues at positions 192 and 193, as containing major elements of, if not all, the binding site for cholinergic ligands. In the present study, we used a panel of synthetic peptides as representative structural elements of the AChR to investigate whether additional segments of the AChR sequences are able to bind alpha-bungarotoxin (alpha-BTX) and several alpha-BTX-competitive monoclonal antibodies (mAbs). The mAbs used (WF6, WF5, and W2) were raised against native Torpedo AChR, specifically recognize the alpha subunit, and bind to AChR is inhibited by all cholinergic ligands. WF6 competes with agonists, but not with low mol. wt. antagonists, for AChR binding. The synthetic peptides used in this study were approximately 20 residue long, overlapped each other by 4-6 residues, and corresponded to the complete sequence of Torpedo AChR alpha subunit. Also, overlapping peptides, corresponding to the sequence segments of each Torpedo AChR subunit homologous to alpha 166-203, were synthesized. alpha-BTX bound to a peptide containing the sequence alpha 181-200 and also, albeit to a lesser extent, to a peptide containing the sequence alpha 55-74. WF6 bound to alpha 181-200 and to a lesser extent to alpha 55-74 and alpha 134-153. The two other mAbs predominantly bound to alpha 55-74, and to a lesser extent to alpha 181-200. Peptides alpha 181-200 and alpha 55-74 both inhibited binding of 125I-alpha-BTX to native Torpedo AChR. None of the peptides corresponding to sequence segments from other subunits bound alpha-BTX or WF6, or interfered with their binding. Therefore, the cholinergic binding site is not a single narrow sequence region, but rather two or more discontinuous sequence segments within the N-terminal extracellular region of the AChR alpha subunit, folded together in the native structure of the receptor, contribute to form a cholinergic binding region. Such a structural arrangement is similar to the "discontinuous epitopes" observed by X-ray diffraction studies of antibody-antigen complexes [reviewed in Davies et al. (1988)].