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

Picosecond dynamics of a peptide from the acetylcholine receptor interacting with a neurotoxin probed by tailored tryptophan fluorescence

A tryptophan analog, dehydro-N-acetyl-L-tryptophanamide (delta-NATA), which is produced enzymatically via L-tryptophan 2',3'-oxidase from Chromobacterium violaceum, is newly used for time-resolved fluorescence. The absorption and emission maxima of delta-NATA at 332 and 417 nm, respectively, in 20% dimethylformamide-water are significantly shifted to the red with respect to those of tryptophan in water, permitting us to measure its fluorescence in the presence of tryptophan residues. We demonstrate that the steady-state spectra and the fluorescence decay of delta-NATA are very sensitive to environment, changing dramatically with solvent as the chromophore is localized within a protein and when this tagged protein binds to a peptide. The tryptophan oxidase was also used to modify the single Trp of a neurotoxin from snake (Naja nigricollis) venom. Modification of the toxin alpha (dehydrotryptophan-toxin alpha) permitted its investigation in complex with a synthetic 15-amino acid peptide corresponding to a loop of the agonist-binding site of acetylcholine receptor (AchR) from Torpedo marmorata species. The peptide alpha-185 possesses a single Trp at the third position (Trp187 of AchR) and a disulfide bridge between Cys192 and Cys193. A single-exponential rotational diffusion time with a constant of 1.65 ns is measured for the isolated 15-amino acid peptide. This suggests that Trp motion in the peptide in solution is strongly correlated with the residues downstream the peptide sequence, which may in part be attributed to long-range order imposed by the disulfide bond. The dynamics of the bound peptide are very different: the presence of two correlation times indicates that the Trp187 of the peptide has a fast motion (taur1 = 140 ps and r(0)1 = 0.14) relative to the overall rotation of the complex (taur2 = 3.4 ns and r(0)2 = 0.04). The correlation of the Trp residue with its neighboring amino acid residues and with the overall motion of the peptide is lost, giving rise to its rapid restricted motion. Thus, the internal dynamics of interacting peptides change on binding.

Identification of pairwise interactions in the alpha-neurotoxin-nicotinic acetylcholine receptor complex through double mutant cycles

alpha-Neurotoxins are potent inhibitors of the nicotinic acetylcholine receptor (nAChR), binding with high affinity to the two agonist sites located on the extracellular domain. Previous site-directed mutagenesis had identified three residues on the alpha-neurotoxin from Naja mossambica mossambica (Lys27, Arg33, and Lys47) and four residues on the mouse muscle nAChR alpha-subunit (Val188, Tyr190, Pro197, and Asp200) as contributing to binding. In this study, thermodynamic mutant cycle analysis was applied to these sets of residues to identify specific pairwise interactions. Amino acid variants of alpha-neurotoxin from N. mossambica mossambica at position 33 and of the nAChR at position 188 showed strong energetic couplings of 2-3 kcal/mol at both binding sites. Consistently smaller yet significant linkages of 1.6-2.1 kcal/mol were also observed between variants at position 27 on the toxin and position 188 on the receptor. Additionally, toxin residue 27 coupled to the receptor residues 190, 197, and 200 at the alphadelta binding site with observed coupling energies of 1.5-1.9 kcal/mol. No linkages were found between toxin residue Lys47 and the receptor residues studied here. These results provide direct evidence that the two conserved cationic residues Arg33 and Lys27, located on loop II of the toxin structure, are binding in close proximity to the alpha-subunit region between residues 188-200. The toxin residue Arg33 is closer to Val188, where it is likely stabilized by adjacent negative or aromatic residues on the receptor structure. Lys27 is positioned closer to Tyr190, Pro197, and Asp200, where it is likely stabilized through electrostatic interaction with Asp200 and/or cation/pi interactions with Tyr190.

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.

Amino acid sequence of two neurotoxins from the venom of the Egyptian black snake (Walterinnesia aegyptia)

The venom of the Egyptian black snake Walterinnesia aegyptia contains at least three toxins, which act postsynaptically to block the neuromuscular transmission of isolated rat phrenic nerve-diaphragm and chicken biventer cervicis muscle. The complete amino acid sequence of the two toxins, W-III and W-IV, consisting of 62 amino acid residues, was elucidated by Edman degradation of fragments obtained after Staphylococcus aureus protease and prolylpeptidase digestion. Although the toxins exhibit close structural homology to other short-chain postsynaptic neurotoxins from Elapidae venoms, toxin IV is unique by having a free SH-group (cysteine) at position 16. In position 35 of W-III, which is located at the tip of the central loop, threonine is replaced by lysine, which may alter the interaction of the toxin with the acetylcholine receptor, since the toxin is seven times less lethal than toxin W-IV.

Mimicry between receptors and antibodies. Identification of snake toxin determinants recognized by the acetylcholine receptor and an acetylcholine receptor-mimicking monoclonal antibody

In several instances, a monoclonal antibody raised against a receptor ligand has been claimed to mimic the ligand receptor. Thus, a specific monoclonal antibody (Malpha2-3) raised against a short-chain toxin from snake was proposed to mimic the nicotinic acetylcholine receptor (AChR) (). Further confirming this mimicry, we show that (i) like AChR, Malpha2-3 elicits anti-AChR antibodies, which in turn elicit anti-toxin antibodies; and (ii) the region 106-122 of the alpha-chain of AChR shares 66% primary structure identity with complementarity-determining regions of Malpha2-3. Also, a mutational analysis of erabutoxin a reveals that the epitope recognized by Malpha2-3 consists of 10 residues, distributed within the three toxin loops. Eight of these residues also belong to the 10-residue epitope recognized by AChR, a result that offers an explanation as to the functional similarities between the receptor and the antibody. Strikingly, however, most of the residues common to the two epitopes contribute differentially to the energetic formation of the antibody-toxin and the receptor-toxin complexes. Together, the data suggest that the mimicry between AChR and Malpha2-3 is partial only.

The emerging three-dimensional structure of a receptor. The nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is the neurotransmitter receptor with the most-characterized protein structure. The amino acid sequences of its five subunits have been elucidated by cDNA cloning and sequencing. Its shape and dimensions (approximately 12.5 nm x 8 nm) were deduced from electron-microscopy studies. Its subunits are arranged around a five-fold axis of pseudosymmetry in the order (clockwise) alpha H gamma alpha L delta beta. Its two agonist/competitive-antagonist-binding sites have been localized by photolabelling studies to a deep gorge between the subunits near the membrane surface. Its ion channel is formed by five membrane-spanning (M2) helices that are contributed by the five subunits. This finding has been generalized as the Helix M2 model for the superfamily of ligand-gated ion channels. The binding site for regulatory non-competitive antagonists has been localized by photolabelling and site-directed-mutagenesis studies within this ion channel. Therefore a three-dimensional image of the nicotinic acetylcholine receptor is emerging, the most prominent feature of which is an active site that combines the agonist/ competitive-antagonist-binding sites, the regulatory site and the ion channel within a relatively narrow space close to and within the bilayer membrane.

Key histidine residues in the nicotinic acetylcholine receptor

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

Nuclear magnetic resonance (NMR) analysis of ligand receptor interactions: the cholinergic system--a model

Elucidation of the molecular mechanisms that govern ligand-receptor recognition is essential to the rational design of specific pharmacological reagents. Whereas often the receptor and its binding site are the target of investigation, study of the ligand in its free and bound state can also reveal important information regarding this recognition process. Nuclear magnetic resonance (NMR) spectroscopy can be extremely useful for such studies. In this review, we discuss the attributes of NMR in the study of ligand receptor interactions. The cholinergic receptor and its binding to the neurotransmitter, acetylcholine, and cholinergic antagonists serve as a model system, illustrating the power of ligand analysis by NMR. The results discussed prove that the region of residues alpha 180-205 of the nicotinic acetylcholine receptor are an essential component of the cholinergic binding site and that ligand binding involves a positively charged hydrophobic motif.

Genetic engineering of snake toxins. The functional site of Erabutoxin a, as delineated by site-directed mutagenesis, includes variant residues

Using site-directed mutagenesis, we previously identified some residues that probably belong to the site by which Erabutoxin a (Ea), a sea snake toxin, recognizes the nicotinic acetylcholine receptor (AcChoR) (Pillet, L., Trémeau, O., Ducancel, F. Drevet, P., Zinn-Justin, S., Pinkasfeld, S., Boulain, J.-C., and Ménez, A. (1993) J. Biol. Chem. 268, 909-916). We have now studied the effect of mutating 26 new positions on the affinity of Ea for AcChoR. The mutations are F4A, N5V, H6A, Q7L, S9G, Q10A, P11N, Q12A, T13V, T14A, K15A, T16A, delta S18, E21A, Y25F, Q28A, S30A, T35A, I36R, P44V, T45A, V46A, K47A, P48Q, I50Q, and S53A. Binding affinity decreases upon mutation at Gln-7, Gln-10 and to a lesser extent at His-6, Ser-9 and Tyr-25 whereas it increases upon mutation at Ile-36. Other mutations have no effect on Ea affinity. In addition, new mutations of the previously explored Ser-8, Asp-31, Arg-33, and Glu-38 better explain the functional role of these residues in Ea. The previous and present mutational analysis suggest that the "functional" site of Ea covers a homogeneous surface of at least 680 A2, encompassing the three toxin loops, and includes both conserved and variant residues. The variable residues might contribute to the selectivity of Ea for some AcChoRs, including those from fish, the prey of sea snakes.