Figure 2.

MOPS⁻ competes with NPPB for binding in the pore but not at the gating site. (A) Macroscopic locked-open E1371S CFTR current at −120 mV after removal of ATP from the bath; exposures to various NPPB concentrations (brown bars) in the continued presence of 80 mM MOPS⁻ (green bar; magnified in yellow inset). Dotted line marks zero-current level. (B) Dose–response curves for NPPB block at −120 mV in the absence (open symbols; replotted from Csanády and Töröcsik [2014]) and presence (green-filled symbols) of 80 mM MOPS⁻. Leftmost symbols in these and all other dose–response plots represent zero drug concentration. Fits to the Michaelis-Menten equation (dotted and solid line) reveal an approximate ninefold increase in the apparent K_I of NPPB in the presence of 80 mM MOPS⁻ (K_I = 8.3 mM for MOPS⁻; compare with Csanády and Töröcsik [2014]). (C) Macroscopic WT CFTR currents at −120 mV elicited by brief exposures to 2 mM ATP in the absence or presence of blockers. Current relaxations after ATP removal were fitted by single exponentials (colored lines), and colored numbers are time constants (in milliseconds). (D) Macroscopic closing rates of WT CFTR in the absence of blocker (gray) and in the presence of 100 µM NPPB (brown), 80 mM MOPS⁻ (green), or 100 µM NPPB + 80 mM MOPS⁻ (striped). (E) Macroscopic K1250A CFTR currents at −40 mV elicited by brief exposures to 10 mM ATP in the absence or presence of blockers. Current relaxations after ATP removal were fitted by single exponentials with time constants indicated. (F) Macroscopic K1250A closing rates in the absence of blocker (gray) and in the presence of 100 µM NPPB (brown), 80 mM MOPS⁻ (green), or 100 µM NPPB + 80 mM MOPS⁻ (striped). Mean ± SEM is shown.