CFTR: Molecular mechanisms of ion conduction and gating

Purpose: CFTR is a Cl channel found in a variety of epithelial cells that is activated by increases in cytosolic cAMP. The primary structure of CFTR can be envisioned as comprising two membrane-spanning domains (MSD1, MSD2) that are presumed to form a Cl-selective pore and three cytosolic domains, the R domain and two nucleotide binding folds (NBF's), that govern the opening and closing of the channels. Methods: The object of the studies presented here was to use mutant CFTR's expressed in Xenopus oocytes to investigate molecular mechanisms for ion conduction and gating. Results: Kinetic analysis of acivation of CFTR constucts bearing mutations in the nucleotide binding folds was consistent with a model in which the activation of CFTR requires ATP hydrolysis at NBF1 and deactivation requires ATP hydroysis at NBF2. Elimination of single serines in the R domain altered CFTR activation in a manner consistent with the existence of both stimulatory and inhibitory phosphorylation sites that regulate channel activity. Ion selectivity determinations indicated that the conduction path contains an anion binding site that exhibits a high affinity for thiocyanate (SCN). A patient mutation (G314E) that introduces a negative charge into the 5th membrane-spanning segment (TM5) of MSD1 markedly reduced SCN binding in the channel suggesting that TM5 may form a portion of the lining of the pore and contribute to the anion binding site. Conclusions: These results provide insights into the relationship between the structure of CFTR and its Cl channel function in health and disease.