Nucleoside triphosphates are required to open the CFTR chloride channel

The CFTR Cl− channel contains two predicted nucleotide-binding domains (NBD1 and NBD2); therefore, we examined the effect of ATP on channel activity. Once phosphorylated by cAMP-dependent protein kinase (PKA), channels required cytosolic ATP to open. Activation occurred by a PKA-independent mechanism. ATPγS substituted for ATP in PKA phosphorylation, but it did not open the channel. Several hydrolyzable nucleotides (ATP > GTP > ITP ≈ UTP > CTP) reversibly activated phosphorylated channels, but nonhydrolyzable analogs and Mg2+-free ATP did not. Studies of CFTR mutants indicated that ATP controls channel activity independent of the R domain and suggested that hydrolysis of ATP by NBD1 may be sufficient for channel opening. The finding that nucleoside triphosphates regulate CFTR begins to explain why CF-associated mutations in the NBDs block Cl− channel function.