Chimeric mutations in the M2 segment of the 5-hydroxytryptamine-gated chloride channel MOD-1 define a minimal determinant of anion/cation permeability

The ionic selectivity of ligand-gated ion channels (LGICs) determines whether receptor activation produces an excitatory or inhibitory response. The determinants of anion/cation selectivity were investigated for a new member of the LGIC superfamily, MOD-1, a serotonin-gated chloride channel cloned from the nematode Caenorhabditis elegans. In common with other anionic LGICs (glycine receptors and GABA_A receptors), the selectivity triple mutant in the pore-forming M2 segment (proline insertion, Ala → Glu substitution at the central ring, and Thr → Val at the hydrophobic ring) converted the selectivity of MOD-1 from anionic to cationic. Unlike other LGICs, however, this mutant in MOD-1 was highly selective for K⁺ over other cations. Subsets of this selectivity triple mutant were studied to define the minimal change required for conversion from anion-permeable to cation-permeable. The double mutant at the central ring of charge (ΔPro-269/A270E) produced a non-selective cation channel. Charge reversal at the central ring alone (A270E) was sufficient to convert MOD-1 to cation-permeable. These results refine the determinants of ion-charge selectivity in LGICs and demonstrate the critical role of the central ring of charge formed by the M2 segments.