Determinants of competitive antagonist sensitivity on neuronal nicotinic receptor beta subunits.

We constructed a series of chimeric and mutant neuronal nicotinic acetylcholine receptor beta subunits to map amino acid residues that determine sensitivity to competitive antagonists. The beta 2 and beta 4 subunits form pharmacologically distinct receptors when expressed in combination with the alpha 3 subunit in Xenopus oocytes. At equipotent acetylcholine concentrations, alpha 3 beta 2 is 56-fold more sensitive to blockage by dihydro-beta-erythroidine than is alpha 3 beta 4. The alpha 3 beta 2 combination is also sensitive to long-term blockade by neuronal bungarotoxin, whereas alpha 3 beta 4 is not. Pharmacological analysis of receptors formed by chimeric beta subunits reveals that amino acid residues that determine both dihydro-beta-erythroidine and neuronal bungarotoxin sensitivity are located within several sequence segments. The major determinant of sensitivity to both competitive antagonists is located between residues 54 and 63. A minor determinant of sensitivity to both antagonists lies between residues 1 and 54, whereas a minor determinant of NBT sensitivity lies between residues 74 and 80. Within region 54-63 of beta 2, mutant beta 2 subunits were used to identify threonine 59 as a residue critical in determining competitive antagonist sensitivity. Changing threonine 59 to lysine, as occurs in beta 4, causes a 9-fold decrease in dihydro-beta-erythroidine sensitivity and a 71-fold decrease in neuronal bungarotoxin sensitivity. Changing polar threonine 59 to negatively charged aspartate causes a 2.5-fold increase in neuronal bungarotoxin sensitivity and has no effect on dihydro-beta-erythroidine sensitivity.