PROCESSING OF MUTANT CYSTIC-FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR IS TEMPERATURE-SENSITIVE

CYSTIC fibrosis transmembrane conductance regulator (CFTR) is a plasma membrane Cl- channel regulated by cyclic AMP-dependent phosphorylation and by intracellular ATP1-7. Mutations in CFTR cause cystic fibrosis8-10 partly through loss of cAMP-regulated Cl- permeability from the plasma membrane of affected epithelia11,12. The most common mutation in cystic fibrosis is deletion of phenylalanine at residue 508 (CFTR-DELTA-F508) (ref. 10). Studies on the biosynthesis13,14 and localization15 of CFTR-DELTA-F508 indicate that the mutant protein is not processed correctly and, as a result, is not delivered to the plasma membrane. These conclusions are consistent with earlier functional studies which failed to detect cAMP-stimulated Cl- channels in cells expressing CFTR-DELTA-F508 (refs 16, 17). Chloride channel activity was detected, however, when CFTR-DELTA-F508 was expressed in Xenopus oocytes18, Vero cells19 and Sf9 insect cells20. Because oocytes and Sf9 cells are typically maintained at lower temperatures than mammalian cells, and because processing of nascent proteins can be sensitive to temperature21, we tested the effect of temperature on the processing of CFTR-DELTA-F508. Here we show that the processing of CFTR-DELTA-F508 reverts towards that of wild-type as the incubation temperature is reduced. When the processing defect is corrected, cAMP-regulated Cl- channels appear in the plasma membrane. These results reconcile previous contradictory observations and suggest that the mutant most commonly associated with cystic fibrosis is temperature-sensitive.