Identification and partial characterization of a domain in CFTR that may bind cyclic nucleotides directly

Background: The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that is activated by cAMP-dependent phosphorylation. CFTR channel activity is also stimulated by cGMP-dependent protein kinase and protein kinase C. Results Here, we show that CFTR channel activation by cGMP may also occur directly. In oocytes from one-third of Xenopus donors, the activation of CFTR by cGMP averaged 87 % of the level achieved by cAMP. The currents activated by either cyclic nucleotide displayed similar current–voltage relationships, kinetics, pharmacology and halide selectivity. Sequential stimulation by cAMP and cGMP was not additive, suggesting that both cyclic nucleotides activate the same channel; cGMP was one order of magnitude more potent than cAMP, and its action was insensitive to protein kinase inhibitors. Analysis of the amino-acid sequence of CFTR revealed a domain in the amino-terminal portion of the third cytoplasmic loop that resembles a class of cyclic-nucleotide-binding domains related to that of the catabolite-gene activator protein, CAP. Two CFTR residues in this domain — Val397 and Lys420 — were identified which, when changed to alanine, altered the response to cGMP independently of the response to cAMP. Conclusion We conclude that direct cyclic nucleotide binding may play a role in channel gating of CFTR. The cGMP-binding domain may provide a useful target for pharmacologic intervention in cystic fibrosis.