Probing an Open CFTR Pore with Organic Anion Blockers

The cystic fibrosis transmembrane conductance regulator (CFTR) is an ion channel that conducts Cl⁻ current. We explored the CFTR pore by studying voltage-dependent blockade of the channel by two organic anions: glibenclamide and isethionate. To simplify the kinetic analysis, a CFTR mutant, K1250A-CFTR, was used because this mutant channel, once opened, can remain open for minutes. Dose–response relationships of both blockers follow a simple Michaelis-Menten function with K_d values that differ by three orders of magnitude. Glibenclamide blocks CFTR from the intracellular side of the membrane with slow kinetics. Both the on and off rates of glibenclamide block are voltage dependent. Removing external Cl⁻ increases affinity of glibenclamide due to a decrease of the off rate and an increase of the on rate, suggesting the presence of a Cl⁻ binding site external to the glibenclamide binding site. Isethionate blocks the channel from the cytoplasmic side with fast kinetics, but has no measurable effect when applied extracellularly. Increasing the internal Cl⁻ concentration reduces isethionate block without affecting its voltage dependence, suggesting that Cl⁻ and isethionate compete for a binding site in the pore. The voltage dependence and external Cl⁻ concentration dependence of isethionate block are nearly identical to those of glibenclamide block, suggesting that these two blockers may bind to a common binding site, an idea further supported by kinetic studies of blocking with glibenclamide/isethionate mixtures. By comparing the physical and chemical natures of these two blockers, we propose that CFTR channel has an asymmetric pore with a wide internal entrance and a deeply embedded blocker binding site where local charges as well as hydrophobic components determine the affinity of the blockers.