ALTERED IONIC SELECTIVITY OF THE SODIUM-CHANNEL REVEALED BY CYSTEINE MUTATIONS WITHIN THE PORE

TO explore the role of pore-lining amino acids in Na+ channel ion-sele ctivity, pore residues were replaced serially with cysteine in cloned rat skeletal muscle Naf channels. Ionic selectivity was determined by measuring permeability and ionic current ratios of whole-cell currents in Xenopus oocytes. The rSkM1 channels displayed) ed an ionic selecti vity sequence Na+>Li+>NH4+>>K+>>K+>>Cs+ and were impermeable to divale nt cations. Replacement of residues in domain IV showed significantly enhanced current and permeability ratios of NH4+ and K+, and negative shifts in the reversal potentials recorded in the presence of external Na+ solutions when compared to cysteine mutants in domains I, II, and III (except KI237C). Mutants in domain IV showed altered selectivity sequences: W1531C (NH4+>K+>Na(+)greater than or equal to Li(+)approxim ate to CS+), D1532C, and G1533C (Na+>Li(+)greater than or equal to NH4 +>K+>Cs+). Conservative replacement of the aromatic residue in domain TV (W1531) with phenylalanine or tyrosine retained Nai selectivity of the channel while the alanine mutant (W1531C) reduced ion selectivity. A single mutation within the third pore forming region (K1237C) drama tically altered the selectivity sequence of the rSkM1 channel (NH4+>K>Na(+)greater than or equal to Li(+)approximate to Cs+) and was permea ble to divalent cations hating the selectivity sequence Ca(2+)greater than or equal to Sr2+>Mg2+>Ba2+. Sulfhydryl modification of K1237C, W1 531C or D1532C with methanethiosulfonate derivatives that introduce a positively charged ammonium group, large trimethylammonium moiety, or a negatively charged sulfonate group within the pore was ineffective i n restoring Na+ selectivity to these channels. Selectivity of D1532C m utants could be largely restored by increasing extracellular pH sugges ting altering the ionized state at this position influences selectivit y. These data suggest that K1237 in domain III and W1531, D1532, and G 1533 in domain IV play a critical role in determining the ionic select ivity of the Na+ channel.