5-HT3 receptor ion size selectivity is a property of the transmembrane channel, not the cytoplasmic vestibule portals

摘要: 5-HT3A receptors select among permeant ions based on size and charge. The membrane-associated (MA) helix lines the portals into channel’s cytoplasmic vestibule in 4-A resolution structure of homologous acetylcholine receptor. MA residues are important determinants single-channel conductance. It is unknown whether also determine selectivity ions. We sought to form filter. Recently, we showed that channels functioned when entire M3–M4 loop was replaced by heptapeptide sequence from GLIC, a bacterial Cys-loop neurotransmitter gated ion channel homologue Gloebacter violaceus. used homomeric with either wild-type (WT) or chimeric (5-HT3A–glvM3M4) loop, i.e., without portals. In Na+-containing buffer, WT receptor current–voltage relationship inwardly rectifying. contrast, 5-HT3A–glvM3M4 construct had negative slope conductance region at voltages less than −80 mV. Glutamine substitution for arginine eliminated region. measured relative permeabilities conductances series inorganic organic cations ranging 0.9 4.5 A radius (Li+, Na+, ammonium, methylammonium, ethanolammonium, 2-methylethanolammonium, dimethylammonium, diethanolammonium, tetramethylammonium, choline, tris [hydroxymethyl] aminomethane, N-methyl-d-glucamine). Both constructs measurable Li+, methylammonium (size range 0.9–1.8-A radius). Many >2.4 acted as competitive antagonists complicating measurement ratios. Analysis permeability ratios excluded volume theory indicates minimal pore 5-HT3–glvM3M4 similar, ∼5 A. infer filter located transmembrane not vestibule. Thus, physically distinct regions protein.