Engineering differential charge selectivity from a single structural template

Abstract

For any ion channel, both the identity and fundamental physiological roles are largely defined by ion selectivity. It is the regulated passage of some ions, but not others, across a cell membrane that determines specific membrane potential (Vm) changes that occur with channel activation. For example, opening of potassium (K+)-selective channels typically drives Vm to negative values, generally reducing excitability, whereas activation of voltage-dependent sodium (Na+)- or calcium (Ca2+)-selective channels contributes to strong depolarization. For the superfamily of tetrameric, strongly cation-selective channels, the determinants of selectivity arise in part from favored occupancy by permeant ions at one or more specific sites within the permeation pathway, sites readily identified in available structures (1, 2). Specific coordination of permeant cations by carbonyl oxygens or carboxylic acid side chains favors occupancy by the permeating species over other cations of slightly different radii (3, 4). However, other ion channels are not so explicitly selective and have evolved alternative filtering strategies. A remarkable case is that of members of the pentameric ligand-gated ion channel (pLGIC) superfamily, in which each member shares similar structural features, but some are cation-selective whereas others are anion-selective. How such functional distinctions arise among channels with highly conserved features has been a long-standing question. Previous work has focused on some specific charged loci in one or a few pLGIC members to identify potential influences on charge selectivity (5⇓⇓–8). However, as noted in a paper by Cymes and Grosman in PNAS (9), a detailed analysis of the literature reveals that a large number of discrepancies exist and that no consensus mechanism has yet been attained. For example, the elementary question as to whether charge selectivity is governed by interactions of the permeating ions with backbone atoms or side-chain atoms had not …