Turnover of the cystic fibrosis transmembrane conductance regulator (CFTR): Slow degradation of wild-type and ΔF508 CFTR in surface membrane preparations of immortalized airway epithelial cells

Abstract

The protein product of the cystic fibrosis (CF) gene, termed the cystic fibrosis transmembrane conductance regulator (CFTR), is known to function as an apical chloride channel at the surface of airway epithelial cells. It has been proposed that CFTR has additional intracellular functions and that there is altered processing of mutant forms. In examining these functions we found a stable form of CFTR with slow turnover in surface membrane preparations from CF and non-CF immortalized airway epithelial cell lines. The methods used to study the turnover of CFTR were pulse/chase experiments utilizing saturation labeling of [35S]Met with chase periods of 5-24 h in the presence of 8 mM Met and cell fractionation techniques. Preparations of morphologically identifiable surface membranes were compared to total cell membrane preparations containing intracellular membranes. Surface membrane CFTR had lower turnover defined by pulse/chase ratios than that of the total cell membrane preparations. Moreover, mutant CFTR was stable in the surface membrane fraction with little degradation even after a 24 h chase, whereas wild-type CFTR had a higher pulse/chase ratio at 24 h. In the presence of 50 μM castanospermine, which is an inhibitor of processing α-glucosidases, a more rapid turnover of mutant CFTR was found in the total cell membrane preparation, whereas wild-type CFTR had a lower response. The results are compatible with a pool of CFTR in or near the surface membranes which has an altered turnover in CF and a glycosylation-dependent alteration in the processing of mutant CFTR.