MECHANISM OF DYSFUNCTION OF 2 NUCLEOTIDE-BINDING DOMAIN MUTATIONS IN CYSTIC-FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR THAT ARE ASSOCIATED WITH PANCREATIC SUFFICIENCY

Variability in the severity of cystic fibrosis (CF) is in part due to specific mutations in the CF transmembrane conductance regulator (CFTR ) gene. To understand better how mutations in CFTR disrupt Cl- channel function and to learn about the relationship between genotype and phe notype, we studied two CF mutants, A455E and P574H, that are associate d with pancreatic sufficiency. A455E and P574H are located close to co nserved ATP binding motifs in CFTR. Both mutants generated cAMP-stimul ated apical membrane Cl- currents in heterologous epithelial cells, bu t current magnitudes were reduced compared with wild-type. Patch-clamp analysis revealed that both mutants had normal conductive properties and regulation by phosphorylation and nucleotides. These mutants had n ormal or increased Cl- channel activity: A455E had an open-state proba bility (P-o) similar to wild-type, and P574H had an increased P-o beca use bursts of activity were prolonged. However, both mutants produced less mature glycosylated protein, although levels were greater than ob served with the Delta F508 mutant. These changes in channel activity a nd processing provide a quantitative explanation for the reduced apica l Cl- current. These data also dissociate structural requirements for channel function from features that determine processing. Finally, the results suggest that the residual function associated with these two mutants is sufficient to confer a milder clinical phenotype and infer approaches to developing treatments.