On the Atypical Cation-Conduction and Gating Properties of ELIC

Abstract

The superfamily of pentameric ligand-gated ion channels is unique among ionotropic receptors in that the same overall structure has evolved to generate multiple members displaying different combinations of agonist specificities and permeant-ion charge selectivities. However, aside from these differences, pLGICs have been typically regarded as a group that exhibits several invariant functional properties. These include pore blockade by extracellular quaternary-ammonium cations in the micromolar-to-millimolar concentration range, and a gain-of-function phenotype as a result of mutations that reduce the hydrophobicity of the transmembrane-pore lining. Here, we tested this notion on three distantly related cation-selective members of the superfamily: the muscle nAChR, and the bacterial GLIC and ELIC channels. Remarkably, we found that, whereas low millimolar concentrations of TMA (diameter: ∼5.7 A) and TEA (diameter: ∼6.9 A) block the nAChR and GLIC, neither of these two quaternary-ammonium cations blocks ELIC at such concentrations, and instead, both carry measurable inward currents when present as the only cations on the extracellular side. Thus, the narrowest constriction of ELIC in the open-channel conformation must be wider than that of the nAChR or GLIC. Also, we found that, whereas lidocaine binding speeds up the current-decay time courses of the nAChR and GLIC in the presence of saturating concentrations of agonists, the binding of lidocaine to ELIC slows this time course down. Furthermore, whereas mutations that reduce the hydrophobicity of the side chains at position 9’ of the M2 alpha-helices greatly slow down the deactivation time course of the nAChR and GLIC, we found that these mutations have little effect or even speed up deactivation when engineered in ELIC. Clearly, our data hint at the possibility that ELIC is a representative of a novel branch of the superfamily with markedly divergent pore properties despite a well-conserved three-dimensional architecture.