Glutamate scanning reveals a pattern that alternates between cation and nonspecific permeability. (A–H) For each indicated glutamate mutant, reversal potentials were measured in solutions with different concentrations of sodium (solid) or chloride (open). Lines are GHK fits to the data. Reversal potentials were nearly unaffected by chloride concentration in mutants that are cation preferring (A, C, and E), while for nonselective mutants the reversal potentials were shifted by changes in sodium or chloride concentration (B, D, G, and H). Systematic substitution of glutamate for each amino acid from −2′ to 5′ exhibited a nearly alternating pattern of cation-preferring receptors and nonselective receptors.