Determinants of Nicotinic Receptor Gating in Natural and Unnatural Side Chain Structures at the M2 9′ Position

Abstract

A nonsense suppression method was employed to incorporate a total of four natural and six unnatural residues at the 9′ position of the M2 region in the β, γ, and δ subunits of muscle nicotinic receptors. In 33 pairwise comparisons of functional properties as influenced by structural features including side chain length, branching, and substitution of oxygen for methylene carbons, it is concluded that increased polarity in the side chains at the 9′ position consistently increases the sensitivity to acetylcholine. In addition, the stereochemistry of the side chain can have marked influences on the EC₅₀, primarily because of changes in the single-channel open time. For the case of isoleucine versus allo-isoleucine in the δ subunit, these changes are themselves modified by mutations at the 9′ position in other subunits. The data suggest an especially strong interaction between the β and δ subunits in the pore region, leading in turn to a suggested arrangement of subunits within the pentamer.