Interaction of nucleotides with membrane-associated cystic fibrosis transmembrane conductance regulator

Cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl- channel that is regulated by cytosolic nucleotides and by cAMP-dependent phosphorylation. In excised membrane patches, CFTR Cl- channel activity requires hydrolyzable nucleotides and Mg2+, and is inhibited by ADP. We examined the interactions between CFTR and nucleotides using 8-azidoadenosine 5'-triphosphate (8-N3-ATP), a photoactivatable ATP analog. Because CFTR functions as a membrane ion channel, we studied CFTR in membranes of Sf9 insect cells. We found that [alpha-32P]8-N3ATP specifically photolabeled CFTR, with half-maximal labeling at 10 microM 8-N3ATP in the presence of Mg2+ and 100 microM in the absence of Mg2+. The 8-N3ATP also substituted for ATP in activating CFTR Cl- channels, indicating that it interacts with the active site(s). Both ATP and GTP prevented photolabeling with half-maximal inhibition at 1 mM. ADP and adenyl-5'-yl imidodiphosphate (AMP-PNP) prevented photolabeling but at much higher concentrations, whereas AMP did not inhibit photolabeling at concentrations of up to 100 mM. Phosphorylation of CFTR was not a prerequisite for nucleotide binding. These results demonstrate that CFTR interacts directly with nucleotides at concentrations that regulate CFTR Cl- channel activity.