PARTICIPATION OF THE ENDOPLASMIC-RETICULUM CHAPERONE CALNEXIN (P88, IP90) IN THE BIOGENESIS OF THE CYSTIC-FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR

Deletion of phenylalanine at position 508 (Delta F508) in the first nu cleotide binding fold of the cystic fibrosis transmembrane conductance regulator (CFTR) is the most common mutation in patients with cystic fibrosis. Although retaining functional Cl- channel activity, this mut ant is recognized as abnormal by the cellular ''quality control'' mach inery and is retained within the endoplasmic reticulum (ER). We have u sed human epithelial cells and recombinant Chinese hamster ovary cells to identify molecular interactions that may contribute to this intrac ellular retention. Based upon coimmunoprecipitation and cosedimentatio n through glycerol density gradients, newly synthesized wild-type and Delta F508 mutant CFTRs associated specifically with calnexin, the cal cium-binding transmembrane chaperone of the ER. This association was r estricted to the immature (or ER-associated) forms of the CFTR protein s. Although the bulk of wild-type and Delta F508 CFTRs were present in itially in complexes containing calnexin, only wild-type CFTR was able to escape from this association and exit the ER. Calnexin retains mis folded or incompletely assembled proteins in the ER and thus is likely to contribute to the mislocalization of mutant CFTR.