Alternate glutamate mutants at the intracellular end of TM2 are cation preferring. (A) Relative potassium permeability plotted against amino acid number shows the alternating pattern of cationic glutamate mutants (solid circles). Substitutions by positive lysine residues (gray circles) resulted in only small increases in potassium permeability. Substitution by neutral alanines (open circles) had no significant effect on ionic permeabilities. (B) Same as A, showing relative sodium permeability of mutant receptors.