Ion Selectivity in Acid-Sensing Ion Channels and Epithelial Sodium Channels

Abstract

Members of the epithelial Na+ channel/degenerin (ENaC/DEG) superfamily are Na+-selective channels with a common overall structure. Each of the three subunits within these trimeric channels has two transmembrane domains (M1 and M2), of which M2 forms the pore. Included in this family are the proton-gated acid-sensing ion channels (ASICs), which are formed by identical or homologous subunits. ASICs mediate excitatory Na+ currents (relative Na+/K+ permeability of ∼ 10/1) and play key roles in many physiological processes, including nociception and cell death following ischemic stroke. ENaCs are obligate heterotrimers comprised of the principal α subunits, as well as β and γ subunits and display significantly higher Na+ selectivity than ASICs (relative Na+/K+ permeability of ∼ 100/1). Previous work had indicated that the selectivity filter in both ENaCs and ASICs is formed by a conserved G-X-S motif in the pore. However, we have recently shown the mASIC1a selectivity filter to be composed of two carboxylate pairs in the lower part of the pore, namely M2 E18’ and D21’. As both the G-X-S motif and the carboxylates in 18’ and 21’ are conserved between ASICs and ENaCs, there are likely other factors contributing to the stark differences in Na+ selectivity. We have thus probed the contribution of residues in M1 to ion selectivity in ASICs and ENaCs. By introducing point-mutations, bulky side chains of the ENaC M1 were replaced by less bulky side chains, and the converse was performed in ASIC1a. We show that the bulky residues in M1 are important for Na+ selectivity, conceivably by influencing the size of the pore.