In the alpha subunit of the Torpedo nicotinic cholinergic receptor (AChR), a sequence region surrounding a pair of adjacent cysteinyl residues at positions 192 and 193 contributes to a binding site for cholinergic ligands, including the snake alpha-neurotoxins. Synthetic and biosynthetic peptides corresponding to this region bind alpha-bungarotoxin (alpha-BTX) in the absence of other structural components of the AChR and, therefore, represent a "prototope" for alpha-BTX. Using synthetic peptides corresponding to the complete AChR alpha subunits of Torpedo electroplax and mammalian muscle, we previously defined a sequence segment corresponding to a universal prototope for alpha-BTX binding between amino acid residues 181 and 200 [Conti-Tronconi, B. M., Tang, F., Diethelm, B. M., Spencer, S. R. Reinhardt-Maelicke, S., & Maelicke, A. (1990) Biochemistry 29, 6221-6230; McLane, K. E., Wu, X., & Conti-Tronconi, B. M. (1990) J. Biol. Chem. 265, 1537-1544]. To elucidate the structural requirements for alpha-BTX binding, we initially used nonconservative single amino acid substitution analogues of the parental alpha(181-200) sequence, and we found that residues at positions 188-190 (VYY), and 192-194 (CCP) and several flanking residues seemed to be involved in alpha-BTX binding [Conti-Tronconi, B. M., Diethelm, B. M., Wu, X., Tang, F., Bertazzon, A., & Maelicke, A. (1991) Biochemistry 30, 2575-2584]. In the present study, amino acid residues previously found to affect alpha-BTX binding were replaced by different conservative single amino acid substitutions, in order to determine the nature of the amino acid side-chain interactions with alpha-BTX. Whereas V188 could be replaced by Ile or Thr with minor effects on alpha-BTX binding, substitution of Phe, His, or Thr for Y189 and Y190 resulted in large to moderate decreases in alpha-BTX binding. Similarly, alpha-BTX binding activity was intolerant to substitutions of C192 or C193 with Ser, His, or Val. Structural changes of the peptide alpha(181-200) induced by substitution of P194 or P197 with two adjacent Gly residues, and insertion of a Gly between C192 and C193, were also incompatible with alpha-BTX binding. Conservative substitutions of other aliphatic and aromatic residues resulted in only minor effects on alpha-BTX binding, as did replacements of K185 and D195 that changed or maintained the charge distribution of peptide alpha (181-200). The recognition site for alpha-BTX formed by the prototope alpha(181-200), therefore, involves important interactions with Y189, Y190, C192, and C193 that are highly specific to the amino acid residue at that position.(ABSTRACT TRUNCATED AT 400 WORDS)