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)].

In the nicotinic acetylcholine receptors (AChRs), the sequence segment surrounding two invariant vicinal cysteinyl residues at positions 192 and 193 of the alpha subunit contains important structural component(s) of the binding site for acetylcholine and high molecular weight cholinergic antagonists, like snake alpha-neurotoxins. At least a second sequence region contributes to the formation of the cholinergic site. Studying the binding of alpha-bungarotoxin and three different monoclonal antibodies, able to compete with alpha-neurotoxins and cholinergic ligands, to a panel of synthetic peptides as representative structural elements of the AChR from Torpedo, we recently identified the sequence segments alpha 181-200 and alpha 55-74 as contributing to form the cholinergic site (Conti-Tronconi et al., 1990). As a first attempt to elucidate the structural requirements for ligand binding to the subsite formed by the sequence alpha 181-200, we have now studied the binding of alpha-bungarotoxin and of antibody WF6 to the synthetic peptide alpha 181-200, and to a panel of peptide analogues differing from the parental sequence alpha 181-200 by substitution of a single amino acid residue. CD spectral analysis of the synthetic peptide analogues indicated that they all have comparable structures in solution, and they can therefore be used to analyze the influence of single amino acid residues on ligand binding. Distinct clusters of amino acid residues, discontinuously positioned along the sequence 181-200, seem to serve as attachment points for the two ligands studied, and the residues necessary for binding of alpha-bungarotoxin are different from those crucial for binding of antibody WF6. In particular, residues at positions 188-190 (VYY) and 192-194 (CCP) were necessary for binding of alpha-bungarotoxin, while residues W187, T191, and Y198 and the three residues at positions 193-195 (CPD) were necessary for binding of WF6. Comparison of the CD spectra of the toxin/peptide complexes, and those obtained for the same peptides and alpha-bungarotoxin in solution, indicates that structural changes of the ligand(s) occur upon binding, with a net increase of the beta-structure component. The cholinergic binding site is therefore a complex surface area, formed by discontinuous clusters of amino acid residues from different sequence regions. Such complex structural arrangement is similar to the "discontinuous epitopes" observed by X-ray diffraction studies of antibody/antigen complexes [reviewed in Davies et al. (1988)]. Within this relatively large structure, cholinergic ligands bind with multiple points of attachment, and ligand-specific patterns of the attachment points exist.(ABSTRACT TRUNCATED AT 400 WORDS)

The sequence segment 181-200 of the Torpedo nicotinic acetylcholine receptor (nAChR) alpha subunit forms a binding site for alpha-bungarotoxin (alpha-BTX) [e.g., see Conti-Tronconi, B. M., Tang, F., Diethelm, B. M., Spencer, S. R., Reinhardt-Maelicke, S., & Maelicke, A. (1990) Biochemistry 29, 6221-6230]. Synthetic peptides corresponding to the homologous sequences of human, calf, mouse, chicken, frog, and cobra muscle nAChR alpha 1 subunits were tested for their ability to bind 125I-alpha-BTX, and differences in alpha-BTX affinity were determined by using solution (IC50S) and solid-phase (KdS) assays. Panels of overlapping peptides corresponding to the complete alpha 1 subunit of mouse and human were also tested for alpha-BTX binding, but other sequence segments forming the alpha-BTX site were not consistently detectable. The Torpedo alpha 1(181-200) and the homologous frog and chicken peptides bound alpha-BTX with higher affinity (KdS approximately 1-2 microM, IC50s approximately 1-2 microM) than the human and calf peptides (Kds approximately 3-5 microM, IC50s approximately 15 microM). The mouse peptide bound alpha-BTX weakly when attached to a solid support (Kd approximately 8 microM) but was effective in competing for 125I-alpha-BTX in solution (IC50 approximately 1 microM). The cobra nAChR alpha 1-subunit peptide did not detectably bind alpha-BTX in either assay. Amino acid substitutions were correlated with alpha-BTX binding activity peptides from different species. The role of a putative vicinal disulfide bound between Cys-192 and -193, relative to the Torpedo sequence, was determined by modifying the peptides with sulfhydryl reagents. Reduction and alkylation of the peptides decreased alpha-BTX binding, whereas oxidation of the peptides had little effect. Modifications of the cysteine/cystine residues of the cobra peptide failed to induce alpha-BTX binding activity. These results indicate that while the adjacent cysteines are likely to be involved in forming the toxin/alpha 1-subunit interface a vicinal disulfide bound was not required for alpha-BTX binding.

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

Employing a panel of synthetic peptides as representative structural elements of the nicotinic acetylcholine receptor from Torpedo electric organ, we recently identified three sequence regions of the receptor (alpha 55-74, alpha 134-153 and alpha 181-200) serving as subsites for the binding of high molecular weight antagonists of acetylcholine (Conti-Tronconi et al. 1990). The relative binding affinities to these subsites of alpha-bungarotoxin and three competitive antibodies varied in a ligand-specific fashion. Employing a set of homologous synthetic peptides differing from alpha 181-200 by the exchange of single amino acid residues along the sequence, we now find that ligand binding crucially depends on the presence of particular amino acids within the subsite while others influence binding only marginally if at all. The existence of ligand-specific attachment points may account for the wide range in binding and kinetic parameters, pharmacological specificity and distinct mean open times of the receptor-integral cation channel observed for cholinergic ligands.