Receptor binding

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a. Model of the 4-TM receptors based on data obtained from the nAChR. The model shows the ligand binding site, the membrane bilayer, and the position of the channel gate. b. Electron density map of the nAChR in profile with the membrane indicated by dots. The bend alpha-helical segment, believed to be TM2, is depicted as solid lines. The square-like density in the cytoplasmic region arises from an associated 43-kDa protein.