Sequential Quality-Control Checkpoints Triage Misfolded Cystic Fibrosis Transmembrane Conductance Regulator

SummaryCystic fibrosis arises from the misfolding and premature degradation of CFTRΔF508, a Cl− ion channel with a single amino acid deletion. Yet, the quality-control machinery that selects CFTRΔF508 for degradation and the mechanism for its misfolding are not well defined. We identified an ER membrane-associated ubiquitin ligase complex containing the E3 RMA1, the E2 Ubc6e, and Derlin-1 that cooperates with the cytosolic Hsc70/CHIP E3 complex to triage CFTR and CFTRΔF508. Derlin-1 serves to retain CFTR in the ER membrane and interacts with RMA1 and Ubc6e to promote CFTR's proteasomal degradation. RMA1 is capable of recognizing folding defects in CFTRΔF508 coincident with translation, whereas the CHIP E3 appears to act posttranslationally. A folding defect in CFTRΔF508 detected by RMA1 involves the inability of CFTR's second membrane-spanning domain to productively interact with amino-terminal domains. Thus, the RMA1 and CHIP E3 ubiquitin ligases act sequentially in ER membrane and cytosol to monitor the folding status of CFTR and CFTRΔF508.