Nine Residues Influence the Binding of α‐Bungarotoxin in α‐Subunit Region 185–200 of Human Muscle Acetylcholine Receptor

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Abstract
Identification of residues in the skeletal muscle nicotinic acetylcholine receptor (AChR) that bind snake venom a‐neurotoxin antagonists of acetylcholine [e.g., α‐bungarotoxin (α‐BTx)] provides structural information about the neurotransmitter binding region of the receptor. Using synthetic peptides of the human AChR α‐subunit region 177–208, we previously localized a pharmacologically specific binding site for α‐BTx in segment 185–199. To define in more detail the residues that influence the binding of α‐BTx to this region, we prepared 16 peptide analogues of the α‐subunit segment 185–200, with the amino acid Lalanine 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 α‐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 α 185–200 in solution‐phase competition with native human AChR for binding of 125I‐labeled α‐BTx. The binding of α‐BTx by analogue peptides with alanine substituted for Tyr190, Cys192, or Cys193 was greatly diminished. Binding of α‐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 α‐BTx binding (p < 0.003). The data imply that these nine amino acids influence the binding of the antagonist, α‐BTx, to the nicotinic acetylcholine receptor of human skeletal muscle, and confirm previous reports for certain contact residues for α‐BTX that were found in region α181‐200 of the Torpedo AChR.

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