Interaction between permeation and gating in a putative pore domain mutantin the cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chlorid e channel with distinctive kinetics. At the whole-cell level, CFTR currents in response to voltage steps are time independent for wild type and for th e many mutants reported so far. Single channels open for periods lasting up to tens of seconds; the openings are interrupted by brief closures at hype rpolarized, but not depolarized, potentials. Here we report a serine-to-phe nylalanine mutation (S1118F) in the 11th transmembrane domain that confers voltage-dependent, single-exponential current relaxations and moderate inwa rd rectification of the macroscopic currents upon expression in Xenopus ooc ytes. At steady state, the S1118F-CFTR single-channel conductance rectifies , corresponding to the whole-cell rectification. in addition, the open-chan nel burst duration is decreased 10-fold compared with wild-type channels. S 1118F-CFTR currents are blocked in a voltage-dependent manner by diphenylam ine-2-carboxylate (DPC); the affinity of S1118F-CFTR for DPC is similar to that of the wild-type channel, but blockade exhibits moderately reduced vol tage dependence. Selectivity of the channel to a range of anions is also af fected by this mutation. furthermore, the permeation properties change duri ng the relaxations, which suggests that there is an interaction between gat ing and permeation in this mutant. The existence of a mutation that confers voltage dependence upon CFTR currents and that changes kinetics and permea tion properties of the channel suggests a functional role for the 11th tran smembrane domain in the pore in the wild-type channel.