Site of Resting State Inhibition of the Nicotinic Acetylcholine Receptor by a Hydrophobic Inhibitor

The lipophilic photoactivatable probe 3-(trifluoromethyl)-3-(m-iodophenyl) diazirine (TID) is a noncompetitive, resting-state inhibitor of the nicotinic acetylcholine receptor (nAChR) that requires tens of milliseconds of preincubation to inhibit agonist-induced cation efflux. At equilibrium, [(125)I]TID photoincorporates into both the ion channel and the lipid-protein interface of the Torpedo nAChR. To determine which of these regions is responsible for resting-state inhibition, we characterized the interactions between [(125)I]TID and nAChR-rich membranes milliseconds after mixing, by use of time-resolved photolabeling. Photolabeling was performed after preincubation times of 2 ms or 600 s (equilibrium), and the efficiencies of incorporation at specific residues were determined by amino-terminal sequence analysis of nAChR-subunit proteolytic fragments isolated by SDS-PAGE and/or reversed-phase HPLC. Equilibration of TID with lipid was complete within a millisecond as determined by both stopped-flow fluorescence quenching of diphenylhexatriene in lipid bilayers and photoincorporation into nAChR-rich membrane phospholipids. Equilibration with the lipid-protein interface (alphaM4) was slightly slower, reaching approximately 50% that at equilibrium after 2 ms preincubation. In contrast, equilibration with the channel region (alpha 2 and deltaM2) was much slower, reaching only 10% that at equilibrium after 2 ms preincubation. Within the ion channel, the ratio of [(125)I]TID incorporation between M2 residues 9', 13', and 16' was independent of preincubation time. We conclude that TID's access to the ion channel is more restricted than to the lipid-protein interface and that TID bound within the ion channel is responsible for flux inhibition upon activation of the nAChR.