Anion selectivity and counter‐ion cation permeation in glycine receptor‐channels

Abstract

The polarity of the permeant ion (ion charge selectivity) is a major determinant of the physiological function of cys‐loop receptor‐channels at central synapses. Our studies aim to determine the molecular and biophysical basis of ion permeation in α1 glycine receptors (GlyRs) expressed in HEK‐293 cells, using patch clamp techniques. We previously reported a mutation to a critical M2 transmembrane domain residue (A251E) converts the channel from anion to cation selective, with the mean Cl− to Na+ permeability ratio (PCl/PNa) changing from 25.1 to 0.31. Mutations to neutralize or invert the charge of basic residues within an intracellular (“portals”) or extracellular (K104) component of the extended permeation pathway did not affect PCl/PNa, supporting the notion of a discrete ion charge selectivity filter at the internal end of the M2 delineated pore. With more precise consideration of liquid junction potentials (calculated using activities), the PCl/PNa in WT GlyRs is 11.8±0.4 (n=32) in the absence of divalent cations, and 16.2±0 (n=6) in 4 mM external Ca2+. PCl/PCs is even lower (5.0±0.5, n=6). A mutant anionic GlyR with a larger pore diameter (P250Δ) shows greater relative Na+ permeation. Hence counterion cations permeate through GlyRs, the relative extent of which depends on the pore size, the hydrated cation size and the presence of divalent cations. Funded by UNSW Faculty Research Grants.