Table III. Amino acid substitutions at key residues of FaNaC TM1.

Residue	Substitution	pNa+/pK+
Wild type	none	2.84 ± 0.49
Val89	arginine	inactive
	lysine	2.14 ± 0.41
	tyrosine	2.36 ± 0.54
Thr79	phenylalanine	2.67 ± 0.24
	aspartic acid	inactive
Leu72	glycine	2.48 ± 0.24
	phenylalanine	4.96 ± 0.46
	tyrosine	inactive
	aspartic acid	inactive

Various substitutions were introduced by site-directed mutagenesis, and the corresponding mutants were tested for their functional expression, and selectivity between sodium and potassium (pNa⁺/pK⁺). Note the presence of several inactive mutants for each tested position, and the change of relative permeability observed for the Leu72Phe mutant.