Conformational and Temperature-sensitive Stability Defects of the ΔF508 Cystic Fibrosis Transmembrane Conductance Regulator in Post-endoplasmic Reticulum Compartments

Abstract

Deletion of phenylalanine at position 508 (ΑF508) is the most common cystic fibrosis (CF)-associated mutation in the CF transmembrane conductance regulator (CFTR), a cAMP-regulated chloride channel. The consensus notion is that ΔF508 imposes a temperature-sensitive folding defect and targets newly synthesized CFTR for degradation at endoplasmic reticulum (ER). A limited amount of CFTR activity, however, appears at the cell surface in the epithelia of homozygous ΔF508 CFTR mice and patients, suggesting that the ER retention is not absolute in native tissues. To further eluci. date the reasons behind the inability of ΔF508 CFTR to accumulate at the plasma membrane, its stability was determined subsequent to escape from the ER, induced by reduced temperature and glycerol. Biochemical and functional measurements show that rescued ΔF508 CFTR has a temperature-sensitive stability defect in post-ER compartments, including the cell surface. The more than 4-20-fold accelerated degradation rate between 37 and 40 °C is, most likely, due to decreased conformational stability of the rescued ΔF508 CFTR, demonstrated by in situ protease susceptibility and SDS-resistant thermoaggregation assays. We propose that the decreased stability of the spontaneously or pharmacologically rescued mutant may contribute to its inability to accumulate at the cell surface. Thus, therapeutic efforts to correct the folding defect should be combined with stabilization of the native ΔF508 CFTR.