Effects of W401 mutations on ligand exchange. (A) The response of a single-ATP–gated W401G channel to a sudden exposure of PATP (red trace). The prolongation of the open time occurred instantly upon ligand switch. A similar observation was seen in five other patches containing a single W401G-CFTR channel. (B) Single-channel kinetics was extracted every 12 s from A. (C) Macroscopic currents recorded from hundreds of W401G channels. The current rising phase elicited by PATP was fitted with a double-exponential function (inset), yielding two time constants. (D) The fast time constants and (E) the slow time constants for WT-CFTR and different W401 mutations. *, P < 0.01 when compared with WT-CFTR.