TRIS+/NA+ PERMEABILITY RATIOS OF NICOTINIC ACETYLCHOLINE-RECEPTORS ARE REDUCED BY MUTATIONS NEAR THE INTRACELLULAR END OF THE M2 REGION

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' (alpha-Thr244, beta-Gly255, gamma-Thr253, delta-Ser258) 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 delta had a greater effect on relative Tris+ permeability than did comparable mutations in gamma; omission of the mouse delta-subunit (delta(0) receptor) or replacement of mouse delta with Xenopus delta-dramatically reduced relative Tris+ permeability. The WT mouse muscle receptor (alpha-beta-gamma-delta) 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.