TURNOVER OF THE CYSTIC-FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR (CFTR) - SLOW DEGRADATION OF WILD-TYPE AND DELTA-F508 CFTR IN SURFACE-MEMBRANE PREPARATIONS OF IMMORTALIZED AIRWAY EPITHELIAL-CELLS

The protein product of the cystic fibrosis (CF) gene, termed the cysti c fibrosis transmembrane conductance regulator (CFTR), is known to fun ction as an apical chloride channel at the surface of airway epithelia l cells. It has been proposed that CFTR has additional intracellular f unctions and that there is altered precessing of mutant forms. In exam ining these functions we found a stable form of CFTR with slow turnove r in surface membrane preparations from CF and non-CF immortalized air way epithelial cell lines. The methods used to study the turnover of C FTR were pulse/chase experiments utilizing saturation labeling of [S-3 5]Met with chase periods of 5-24 h in the presence of 8 mM Met and cel l fractionation techniques. Preparations of morphologically identifiab le surface membranes were compared to total cell membrane preparations containing intracellular membranes. Surface membrane CFTR had lower t urnover defined by pulse/chase ratios than that of the total cell memb rane preparations. Moreover, mutant CFTR was stable in the surface mem brane fraction with little degradation even after a 24 h chase, wherea s wild-type CFTR had a higher pulse/chase ratio at 24 h. In the presen ce of 50 mu M castanospermine, which is an inhibitor of processing alp ha-glucosidases, a more rapid turnover of mutant CFTR was found in the total cell membrane preparation, whereas wild-type CFTR had a lower r esponse. The results are compatible with a pool of CFTR in or near the surface membranes which has an altered turnover in CF and a glycosyla tion-dependent alteration in the processing of mutant CFTR. (C) 1996 W iley-Liss, Inc.