Identification of the sites of incorporation of [H-3]ethidium diazide within the torpedo nicotinic acetylcholine receptor ion channel

The binding sites of ethidium, a noncompetitive antagonist of the nicotinic acetylcholine receptor (nAChR), have been localized in the Torpedo nAChR i n the desensitized state by use of a photoactivatible derivative, [H-3]ethi dium diazide. At 10 mu M [H-3]ethidium diazide, incorporation into the alph a-, beta-, and delta-subunits was inhibited by the presence of phencyclidin e (PCP). Within the alpha-subunit, the incorporation was mapped to a 20-kDa fragment beginning at alpha Ser-173 and containing the first three transme mbrane segments, alpha M1, alpha M2, and alpha M3. Further digestion of thi s fragment generated two fragments with PCP-inhibitable incorporation, one containing alpha M1 and one containing both alpha M2 and alpha M3. Within a lpha M2, specific incorporation was present in alpha Leu-251 and alpha Ser- 252, residues that have been previously shown to line the lumen of the ion channel. Digestion of the g-subunit with S. aureus V8 protease generated a 14-kDa and a 20-kDa fragment, both of which began at Ile-192 and contained PCP-inhibitable labeling. The 14-kDa fragment, containing delta M1 and delt a M2, was further digested to generate a 3-kDa fragment, containing delta M 2 alone, with PCP-inhibitable incorporation. Digestion of the 20-kDa fragme nt, which contained delta M1, dM2, and delta M3, generated two fragments wi th incorporation, one containing the delta M1 segment and the other contain ing dM2 and delta M3. These results establish that in the desensitized stat e of the nAChR, the high-affinity binding site of ethidium is within the lu men of the ion channel and that the bound drug is in contact with amino aci ds from both the M1 and M2 hydrophobic segments.