D110 forms a salt bridge with K892 when CFTR is in the closed state. (A) Representative single-channel current traces of K892E-, D110R/K892E-, and D110C/K892C-CFTR recorded with the same experimental conditions as Fig. 2 (left), their all-points amplitude histograms (middle), and their mean burst durations (right). *, P < 0.05 indicates a significant difference between D110R- and D110R/K892E-CFTR; #, P < 0.001 for D110C/K892C-CFTR compared with D110R alone. (B) Exposure to 1 mM DTT backfilled into the pipette had no effect on WT-CFTR. Two traces recorded from the same inside-out patch from a WT-CFTR–expressing oocyte at V_M = −100 mV, with MgATP and PKA in the intracellular solution. −DTT, DTT in the pipette solution but early in the recording; +DTT, late in the recoding after DTT perfused to the tip of pipette. Window current every 1 min was measured with Clampfit 10.2 and normalized to the maximum window current (I_max). I/I_max is plotted with its real time scale shown in the right panel. The data for WT-CFTR were fit with a single-exponential function with τ = 5.33 min (red line). (C) Exposure to 1 mM DTT, backfilled into the pipette, increased the number of active channels in D110C/K892C-CFTR. Two traces recorded from the same inside-out patch at V_M = −100 mV, with MgATP and PKA in the intracellular solution. The third trace is a small part of the second trace expanded for viewing. −DTT, pipette solution alone; +DTT, DTT perfused to the tip of pipette. Window current data were fit using nonlinear regression, and no time constant was determined (green line). Mean ± SEM is shown.