Tris+/Na + permeability ratios of nicotinic acetylcholine r ceptors are reduced by mutations near the intracellular end of the M2 region.Journal of General Physiology

ABSTRACT Tris+/Na + permeability ratios were measured from shifts in the biionic reversal potentials of the macroscopic ACh-induced currents for 3 wild-type (WT), 1 hybrid, 2 subunit-deficient, and 25 mutant nicotinic receptors expressed in Xenopus oocytes. At two positions near the putative intracellular end of M2, 2' ({xThr244, 13Gly255,-yThr253, 8Ser258) and-1 ' , point mutations reduced the relative Tris + permeability of the mouse receptor as much as threefold. Comparable mutations at several other positions had no effects on relative Tris ÷ permeability. Mutations in ~ had a greater effect on relative Tris + permeability than did comparable mutations in ~/; omission of the mouse ~ subunit (~0 receptor) or replacement of mouse ~ with Xenopus ~ dramatically reduced relative Tris + perme-ability. The WT mouse muscle receptor (a15~8) had a higher relative permeability to Tris ÷ than the wild-type Torpedo receptor. Analyses of the data show that (a) changes in the Tris+/Na + permeability ratio produced by mutations correlate better with the hydrophobicity of the amino acid residues in M2 than with their volume; and (b) the mole-fraction dependence of the reversal potential in mixed Na+/Tris + solutions is approximately consistent with the Goldman-Hodgkin-Katz voltage equation. The results suggest that the main ion selectivity filter for large monovalent cations in the ACh receptor channel is the region delimited by positions-1 ' and 2 ' near the intracellular end of the M2 helix