FIGURE 9.

Mutations I1575A and I1575C increase selectivity for Na⁺ ions in K1237E. The permeability of the channels for the large organic cation choline was assessed from the shift in reversal potential produced by replacing 90 mm Na⁺ of the bath solution with 90 mm choline. Current-voltage relationships were fit with the function Equation 2 to derive the reversal potential. Permeability ratios were then calculated from the shift in reversal potential with Equation 3. In K1237E/I1575A and K1237E/I1575C permeability to choline was significantly smaller than in K1237E alone or in the double mutant K1237E/F1579A, indicating that the mutations at site 1575 reduce the diameter of the selectivity filter (n = 6 for each construct).