Cystic fibrosis transmembrane conductance regulator gating requires cytosolic electrolytes

Cystic fibrosis transmembrane conductance regulator (CFTR), which causes cy stic fibrosis when nonfunctional, is an anion channel and a member of the A TP binding cassette superfamily. After phosphorylation, CFTR gates by bindi ng and hydrolyzing ATP. We show that CFTR open probability (P-o) also depen ds on the electrolyte concentration of the cytosol. Inside-out patches from Calu-3 cells were transiently exposed to solutions of 160 mM salt or solut ions in which up to 90% of the salt was replaced by nonionic osmolytes such as sucrose. In lowered salt solutions, CFTR P-o declined within 1 s to a s table lower value that depended on the electrolyte concentration, (K-1/2 ap proximate to 80 mM NaCl). P-o was rapidly restored in normal salt concentra tions without regard to the electrolyte species. Reducing external electrol ytes did not affect CFTR P-o. The same results were obtained when CFTR was stably phosphorylated with adenosine 5'-O-(thiotriphosphate). The decrease in P-o resulted entirely from an increase in mean closed time. Increasing A TP levels up to 20-fold did not counteract the effect of low electrolytes. The same effect was observed for CFTR expressed in C127 cells but not for a different species of anion channel. Cytosolic electrolytes are an unsuspec ted, essential cofactor for CFTR gating.