Molecular determinants of anion selectivity in the cystic fibrosis transmembrane conductance regulator chloride channel pore

Ionic selectivity in many cation channels is achieved over a short region o f the pore known as the selectivity filter, the molecular determinants of w hich have been identified in Ca2+, Na+, and K+ channels. However, a filter controlling selectivity among different anions has not previously been iden tified in any Cl- channel. In fact, because Cl- channels are only weakly se lective among small anions, and because their selectivity has proved so res istant to site-directed mutagenesis, the very existence of a discrete anion selectivity filter has been called into question. Here we show that mutati on of a putative pore-lining phenylalanine residue, F337, in the sixth memb rane-spanning region of the cystic fibrosis transmembrane conductance regul ator (CFTR) Cl- channel, dramatically alters the relative permeabilities of different anions in the channel. Specifically, mutations that reduce the s ize of the amino acid side chain present at this position virtually abolish the relationship between anion permeability and hydration energy, a relati onship that characterizes the anion selectivity not only of wild-type CFTR, but of most classes of Cl- channels. These results suggest that the pore o f CFTR may indeed contain a specialized region, analogous to the selectivit y filter of cation channels, at which discrimination between different perm eant anions takes place. Because F337 is adjacent to another amino acid res idue, T338, which also affects anion selectivity in CFTR, we suggest that s electivity is predominantly determined over a physically discrete region of the pore located near these important residues.