Action of nicotine and analogs on acetylcholine receptors having mutations of transmitter-binding site residue αG153

A primary target for nicotine is the acetylcholine receptor channel (AChR). Some of the ability of nicotine to activate differentially AChR subtypes has been traced to a transmitter-binding site amino acid that is glycine in lower affinity and lysine in higher affinity AChRs. We studied the effects of mutations of this residue (αG153) in neuro-muscular AChRs activated by nicotine andeight other agonists including nornicotine and anabasine. All of the mutations increased the unligandedgating equilibrium constant. The affinity of the resting receptor (K_d) and the net binding energy from the agonist for gating (δG_B) were estimated by cross-concentration fitting of single-channel currents. In all but one of the agonist/mutant combinations there was a moderate decrease in K_d and essentially no change in δG_B. The exceptional case was nicotine plus lysine, which showed alarge, >8,000-fold decrease in K_d but no change in δG_B. The extraordinary specificity ofthis combination leads us to speculate that AChRs with a lysine at position αG153 may be exposedto a nicotine-like compound in vivo.