A receptor in which the Asn or Asp had
been substituted for another amino acid failed to respond, or responded only very poorly,
to acetylcholine (when tested electrophysiologically).A receptor in which the Asn or Asp had
been substituted for another amino acid failed to respond, or responded only very poorly,
to acetylcholine (when tested electrophysiologically).
It could be shown that such receptors bound radioactive acetylcholine only very poorly.
Likewise, mutations that disrupted the disulphide bridges (e.g. substitution of Cys for another amino acid) also had detrimental effects on ligand binding and receptor function.
Conclusion: results of site-directed mutagenesis experiments support the conclusion that the beta-structural loop is the ligand binding domain.