Conversion of the ion selectivity of the 5-HT3A receptor from cationic to anionic reveals a conserved feature of the ligand-gated ion channel superfamily.

The 5-hydroxytryptamines3 (5-HT3) receptor is a member of a superfamily of ligand-gated ion channels, which includes nicotinic acetylcholine, γ-aminobutyric acid, and glycine receptors. The receptors are either cation or anion selective, leading to their distinctive involvement in either excitatory or inhibitory neurotransmission. Using a combination of site-directed mutagenesis and electrophysiological characterization of homomeric 5-HT3A receptors expressed in HEK293 cells, we have identified a set of mutations that convert the ion selectivity of the 5-HT 3A receptor from cationic to anionic; these were substitution of V13′7 in M2 together with neutralization of glutamate residues (E-1′A) and the adjacent insertion of a proline residue (P-1′) in the M1-M2 loop. Mutant receptors showed significant chloride permeability (Pcl/PNa = 12.3, PNa/PCl = 0.08), whereas WT receptors are predominantly permeable to sodium (P Na/PCl > 20, PCl/PNa < 0.05). Since the equivalent mutations have previously been shown to convert α7 nicotinic acetylcholine receptors from cationic to anionic (Galzi J.-L., Devillers-Thiery, A, Hussy, N., Bertrand, S. Changeux, J. P., and Bertrand, D. (1992) Nature 359, 500-505) and, recently, the converse mutations have allowed the construction of a cation selective glycine receptor (Keramidas, A., Moorhouse, A. J., French, C. R., Schofield, P. R., and Barry, P. H. (2000) Biophys. J. 78, 247-259), it appears that the determinants of ion selectivity represent a conserved feature of the ligand-gated ion channel superfamily.