CFTR Cl- channel and CFTR-associated ATP channel: distinct pores regulated by common gates

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Abstract
The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that is regulated by phosphorylation of the R domain and ATP hydrolysis at two nucleotide‐binding domains (NBDs). It is controversial whether CFTR conducts ATP or whether CFTR might be closely associated with a separate ATP conductance. To characterize ATP channels associated with CFTR, we analyzed Cl− and ATP single channel‐currents in excised inside‐out membrane patches from MDCK epithelial cells transiently expressing CFTR. With 100 mM ATP in the pipette and 140 mM Cl− in the bath, ATP channels were associated with CFTR Cl− channels in two‐thirds of patches that included CFTR. CFTR Cl− channels and CFTR‐associated ATP channels had slope conductances of 7.4 pS and 5.2 pS, respectively, and had distinct reversal potentials and sensitivities to channel blockers. CFTR‐associated ATP channels exhibited slow gating kinetics that depended on the presence of protein kinase A and cytoplasmic ATP, similar to CFTR Cl− channels. Gating kinetics of the ATP channels as well as the CFTR Cl− channels were similarly affected by non‐hydrolyzable ATP analogues and mutations in the CFTR R domain and NBDs. Our results indicate that phosphorylation‐ and nucleotide‐hydrolysis‐dependent gating of CFTR is directly involved in gating of an associated ATP channel. However, the permeation pathways for Cl− and ATP are distinct and the ATP conduction pathway is not obligatorily associated with the expression of CFTR.