Effect of chemical modification of extracellular histidyl residues on the channel properties of the nicotinic acetylcholine receptor.

We have examined the effect of chemical modification with diethyl pyrocarbonate (DEP) on the properties of acetylcholine (ACh)-activated channels in the cloned muscle-cell line BC3H-1. After protein modification, patch-clamp recordings showed alterations in the kinetics of the nicotinic acetylcholine receptor (AChR) channel. The major effect was observed in the channel mean open time, which was reduced up to about 12-fold at 466 microM DEP. The specificity of the effect was first established through comparison with both untreated cells and cells treated with inactivated DEP. Consistent with an increase in the number of unprotonated histidine residues (pKa = 6.0), this effect increased concomitantly with the pH of the reaction medium, being faster at pH 8 than at pH 6. The changes were dependent on time and DEP concentration, with an apparent EC50 = 114 microM. Modified channels also showed an increase in the number of events per burst of openings together with a decrease in burst durations. The amplitude of the channel-closed time component of about 1 ms increased with respect to the longest-duration-closed component. The number of alpha-bungarotoxin sites was slightly reduced after the modification, without affecting ligand binding affinity. The results suggest that DEP affects extracellular histidine residues involved in the ion translocation function of the AChR, but not its toxin-recognition ability. DEP could, therefore, induce a dissociation between toxin and agonist binding, as is often observed in neuronal AChR.