Voltage-sensitive gating induced by a mutation in the fifth transmembrane domain of CFTR

A mutation in the fifth transmembrane domain of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channel (V317E) resulted in whole cell currents that exhibited marked outward rectification on expression in Xenopus oocytes. However, the single-channel unitary current (i)-voltage (V) relationship failed to account for the rectification of whole cell currents. In excised patches containing one to a few channels, the time-averaged single-channel current (I)-V relationship (I = N × P_o × i, where N is the number of active channels and P_o is open probability) of V317E CFTR displayed outward rectification, whereas that of wild-type CFTR was linear, indicating that the P_o of V317E CFTR is voltage dependent. The decrease in P_o at negative potentials was due to both a decreased burst duration and a decreased opening rate that could not be ameliorated by a 10-fold increase in ATP concentration. This behavior appears to reflect a true voltage dependence of the gating process because the P_o-V relationship did not depend on the direction of Cl^- movement. The results are consistent with the introduction, by a point mutation, of a novel voltage-dependent gating mode that may provide a useful tool for probing the portions of the protein that move in response to ATP-dependent gating.