Processing of mutant cystic fibrosis transmembrane conductance regulator is temperature-sensitive

Abstract

CYSTIC fibrosis transmembrane conductance regulator (CFTR) is a plasma membrane Cl⁻ channel regulated by cyclic AMP-dependent phosphorylation and by intracellular ATP^1–7. Mutations in CFTR cause cystic fibrosis^8–10 partly through loss of cAMP-regulated Cl⁻ permeability from the plasma membrane of affected epithelia^11,12. The most common mutation in cystic fibrosis is deletion of phenylalanine at residue 508 (CFTRΔF508) (ref. 10). Studies on the biosynthesis^13,14 and localization¹⁵ of CFTRΔF508 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-stimnlated Cl⁻ channels in cells expressing CFTRΔF508 (refs 16,17). Chloride channel activity was detected, however, when CFTRΔF508 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 CFTRΔF508. Here we show that the processing of CFTRΔF508 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.