ATP/PATP exchange for WT-CFTR channels locked open by ATP/PATP plus PPi. (A) Cartoon illustrating the configurations of two NBDs of CFTR for the effect of PPi. The lock-open state of CFTR induced by ATP and PPi is in a configuration where ATP occupies NBD1 but PPi binds in NBD2. (B) Cartoon showing a possible ligand exchange for WT-CFTR in the lock-open state. If ATP/PATP exchange in NBD1 can occur when CFTR is locked open, the channel will enter into a more stable lock-open state where NBD1 binds a PATP molecule. (C) Current traces showing WT-CFTR channels locked into open states by ATP plus PPi, PATP plus PPi, or ATP plus PPi, followed by PATP plus PPi. The currents decayed slowly after washout. (D) A comparison for current decay traces shown in C. (E) After the application of ATP plus PPi, the longer the subsequent exposure of WT-CFTR channels to PATP plus PPi, the slower the current relaxation upon washout. (F) The current decay time constants measured under different exposure times of PATP plus PPi were converted to the proportion of channels whose ATP in NBD1 has been replaced by PATP (see Materials and methods). Data points were fitted with a single-exponential function (red curve). It is noted that because significant closed durations are expected over an experimental time span of hundreds of seconds, the resulting time constant (522 s) likely underestimates the ATP dwell time in NBD1 when CFTR is in the lock-open state. Thus, the rate of ATP/PATP exchange, ∼0.002 s⁻¹ (or 1/522 s), can only be considered as an upper limit for the rate of exchange for the open state.