Differential acidic pH sensitivity of delta F508 CFTR Cl- channel activity in lipid bilayers

Cystic fibrosis transmembrane conductance regulator (CFTR) is present in acidic intracellular vesicles. Human normal and delta F508 CFTR Cl- channel characteristics at pH 7.4 and pH 4.5 were determined by fusing Xenopus laevis oocyte plasma membranes containing the expressed channels to planar lipid bilayers. At pH 7.4, both channels exhibited linear current-voltage curves, a 10 +/- 0.3-pS conductance using 800 mM CsCl, and a 9:1 Cl-/Cs+ discrimination ratio obtained from a 32 +/- 2 mV reversal potential with a fivefold gradient. At -80 mV, the open probability (Po) of mutant CFTR was 53% that of normal CFTR. Reduction of the trans-pH from 7.4 to 4.5 had no effect on the above characteristics except for Po, where it caused a 47% reduction in normal CFTR Po (due to a 75% decrease in mean open time) and a 75% reduction in delta F508 CFTR Po (due to a 6-fold increase in mean closed time). Normal CFTR can thus function in the environment of acidic intracellular organelles, whereas activity of mutant CFTR would be greatly reduced. These results may be of significance to understanding the cystic fibrosis defect.