Abstract

[(3)H]Tetracaine is a noncompetitive antagonist of the Torpedo nicotinic acetylcholine receptor (nAChR) that binds with high affinity in the absence of cholinergic agonist (K(eq) = 0.5 microM) and weakly (K(eq) = 30 microM) in the presence of agonist (i.e., to nAChR in the desensitized state). In the absence of agonist, irradiation at 302 nm of nAChR-rich membranes equilibrated with [(3)H]tetracaine results in specific photoincorporation of [(3)H]tetracaine into each nAChR subunit. In this report, we identify the amino acids of each nAChR subunit specifically photolabeled by [(3)H]tetracaine that contribute to the high-affinity binding site. Subunits isolated from nAChR-rich membranes photolabeled with [(3)H]tetracaine were subjected to enzymatic digestion, and peptides containing (3)H were purified by SDS-polyacrylamide gel electrophoresis followed by reversed phase HPLC. N-terminal sequence analysis of the isolated peptides demonstrated that [(3)H]tetracaine specifically labeled two sets of homologous hydrophobic residues (alphaLeu(251), betaLeu(257), gammaLeu(260), and deltaLeu(265); alphaVal(255) and deltaVal(269)) as well as alphaIle(247) and deltaAla(268) within the M2 hydrophobic segments of each subunit. The labeling of these residues establishes that the high-affinity [(3)H]tetracaine-binding site is located within the lumen of the closed ion channel and provides a definition of the surface of the M2 helices facing the channel lumen.