Cystic Fibrosis Transmembrane Conductance Regulator: Physical Basis for Lyotropic Anion Selectivity Patterns

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl channel exhibits lyotropic anion selectivity. Anions that are more readily dehydrated than Cl exhibit permeability ratios ( P S / P Cl ) greater than unity and also bind more tightly in the channel. We compared the selectivity of CFTR to that of a synthetic anionselective membrane [poly(vinyl chloride)–tridodecylmethylammonium chloride; PVC-TDMAC] for which the nature of the physical process that governs the anion-selective response is more readily apparent. The permeability and binding selectivity patterns of CFTR differed only by a multiplicative constant from that of the PVC-TDMAC membrane; and a continuum electrostatic model suggested that both patterns could be understood in terms of the differences in the relative stabilization of anions by water and the polarizable interior of the channel or synthetic membrane. The calculated energies of anion–channel interaction, derived from measurements of either permeability or binding, varied as a linear function of inverse ionic radius (1/ r ), as expected from a Born-type model of ion charging in a medium characterized by an effective dielectric constant of 19. The model predicts that large anions, like SCN, although they experience weaker interactions (relative to Cl) with water and also with the channel, are more permeant than Cl because anion–water energy is a steeper function of 1/ r than is the anion–channel energy. These large anions also bind more tightly for the same reason: the reduced energy of hydration allows the net transfer energy (the well depth) to be more negative. This simple selectivity mechanism that governs permeability and binding acts to optimize the function of CFTR as a Cl filter. Anions that are smaller (more difficult to dehydrate) than Cl are energetically retarded from entering the channel, while the larger (more readily dehydrated) anions are retarded in their passage by “sticking” within the channel. key words: hydration energy • anion binding • pseudohalides • ion-selective electrodes • anion channels I N T R O D U C T I O N The cystic fibrosis transmembrane conductance regulator (CFTR) 1 channel functions as a PKA-activated ion channel that is predominantly expressed in epithelial cells. The channel selects for anions over cations ( P Na / P Cl < 0.03) (Tabcharani et al., 1997) and exhibits modest discrimination among anions as judged either from reversal potential measurements or the block of Cl conduction by other permeant ions (Anderson et al., 1991; Sheppard et al., 1993; Linsdell et al., 1997b; Tabcharani et al., 1997; Mansoura et al., 1998). Within the framework of a two-barrier, one-site permeation scheme, permeability selectivity may be thought of as a reflection of the relative ease with which anions enter the channel (barrier height) while blockade of conduction measures the tightness of anion binding (well depth) within the channel (Bezanilla and Armstrong, 1972; Hille, 1975b; Dawson, and Smith, 1997; Mansoura, and Dawson, 1998; Dawson et al., 1999). The selectivity patterns for both anion permeability and anion binding fall in the so called “lyotropic” sequence. Anions that are more readily dehydrated than Cl experience a reduced barrier to entry into the pore and also stick more tightly inside the pore than Cl (i.e., they see a deeper well). These parallel changes in peak height and well depth are evident for an anion like SCN, which is three times more permeant than Cl ( P SCN / P Cl , 3.4), but also blocks Cl flow (Tabcharani et al., 1993; Mansoura et al., 1998). Anion binding differs from anion entry, however, in that it is highly sensitive to changes in pore structure, whereas permeability ratios are much less so (Mansoura et al., 1998). To gain insight into the nature of the physical interactions that are reflected in the peak and well energies that characterize anion permeation, we compared the Dr. Smith’s and Dr. Dawson’s present address is Department of Physiology & Pharmacology, Oregon Health Sciences University, Port-