PPi captures a short-lived, post-hydrolytic state. (A) A cartoon depicting the mechanism based in Fig. 1 A by which PPi locks open CFTR. O, open state with a dimerized NBD; C2, closed state with a partially separated NBD; LO, locked-open state with PPi occupying the second ABP, ABP2. (B, D, F, and G) Macroscopic current of WT-CFTR channels was activated by ATP to a steady state before carrying out different ligand-switch protocols: (B) washout for 10 s before applying PPi; (D) direct switch from ATP to PPi; (F) direct switched from ATP to a 1-s PPi pulse; and (G) washout for 3 s before applying a 1-s PPi pulse. (C) Computer simulation of macroscopic currents based on scenario 1. (E) A cartoon depicting a revised CFTR gating model, wherein state X can respond to PPi rapidly. Insets in B and D show the current rising phase upon PPi application; τ represents the relaxation time constant obtained by fitting the current rise with a single-exponential function (mean ± SEM was specified in Results). Bars above each trace mark the perfused ligand denoted on the very left (applied to every figure).