A negatively charged substitution at P290 (−2′) results in cation-preferring receptors. (A) Amplitudes of GABA-induced currents from P290E were measured at a continuously varying membrane potential using the ramp protocol described in the Materials and methods. Ramps obtained in low sodium OR2 are the dashed lines and those in low-chloride OR2 are the solid lines. The normal OR2 ramp is omitted for clarity, but the reversal potential is indicated with an arrow. For the graph on the right, reversal potentials were measured at different concentrations of sodium (solid) or chloride (open) and fit with the GHK equation (Eq. 2) to determine the relative permeability coefficients. Substitution of negatively charged glutamate for the native proline at position 290 (−2′) resulted in cation-preferring receptors, but receptors were chloride selective when this site was mutated to neutral alanine (B) or positively charged lysine (C).