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

Abstract

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

Under the spotlight: The organic–inorganic hybrid halide perovskite for optoelectronic applications

“…Methylammonium (MA). The most widely used A cation for hybrid perovskites, the methylammonium ion (MA; CH 3 NH 3 + ; R MA = 0.18 nm) [43], has achieved efficiencies of 15% and above for MAPbI 3 devices. At room temperature a tetragonal symmetry is formed for MAPbI 3 , rather than cubic, due to the small size of the MA ion.…”

“…Ionic radii of commonly employed hybrid perovskite components.Source: Adapted with permission from Refs [15,35,43,45]…”

Mutagenic Analysis of the Intracellular Portals of the Human 5-HT3A Receptor

“…The same region in the 5-HT 3 A receptor additionally impacts upon divalent versus monovalent cation permeability, channel gating, and the kinetics of desensitization (34–36). However, ion size selectivity for monovalent cations appears to depend upon the M2 domain (37), and the M3-M4 loop is not essential for receptor function (38). …”

Progress and Prospect on Stability of Perovskite Photovoltaics

“…Tin based perovskite PV contains Sn which oxidizes from Sn 2+ to Sn 4+ which is metal like because of the band gap [42,49,51]. Despite this problem in Sn based perovskite PV, [50] and reported a working Sn-based PV.…”