Domain Interdependence in the Biosynthetic Assembly of CFTR

Assembly and Misassembly of Cystic Fibrosis Transmembrane Conductance Regulator: Folding Defects Caused by Deletion of F508 Occur Before and After the Calnexin-dependent Association of Membrane Spanning Domain (MSD) 1 and MSD2

“…CFTR 1-1172 lacks NBD2 but folds to a conformation that passes ERQC and functions at the cell surface as a Cl Ϫ channel (Cui et al, 2007). Treatment of cells with CAS inhibits calnexin-dependent protein folding reactions (Hammond et al, 1994;Ellgaard and Helenius, 2003) and blocks calnexin binding to CFTR ( Figure 3D).…”

“…F508 of CFTR is located on the solvent-exposed surface of NBD1 (Lewis et al, 2004;Thibodeau et al, 2005), and the crystal structures of bacterial ABC transporters suggest that it makes contacts with cytosolic surface loops on MSD2 that are critical for stabilization of CFTR structure (Dawson and Locher, 2006;Serohijos et al, 2008). This supposition is supported by the observation that the F508 mutation disrupts assembly of CFTR 1-1172 (Cui et al, 2007) and makes similar CFTR fragments susceptible to cotranslational recognition by the ER-associated ubiquitin ligase RMA1 (Younger et al, 2006). NBD2 is the last domain on CFTR that is synthesized, and its folding and assembly into a complex with aminoterminal regions of CFTR seems to be slow and thus occur posttranslationally (Du et al, 2005).…”

“…The assembly of CFTR into an ion channel is complicated because it requires the coordinated folding and assembly of its membrane and cytoplasmic domains into a functional unit (Du et al, 2005;Riordan, 2005;Cui et al, 2007). CFTR is a modular protein, and its domains can collapse to a protease-resistant conformation independently (Zhang et al, 1998).…”

“…The folding pathway of this complex polytopic membrane protein has been a topic of great interest, because misfolding results in premature recognition of CFTR by the ER quality control system (ERQC) and degradation by the ubiquitin proteasome system (Skach, 2000). In fact, the most common disease-causing mutation of CFTR, ⌬F508CFTR, results in almost complete degradation of the protein by the ERQC system, which gives rise to a loss of function phenotype and lung disease (Ward and Kopito, 1994).The assembly of CFTR into an ion channel is complicated because it requires the coordinated folding and assembly of its membrane and cytoplasmic domains into a functional unit (Du et al, 2005;Riordan, 2005;Cui et al, 2007). CFTR is a modular protein, and its domains can collapse to a protease-resistant conformation independently (Zhang et al, 1998).…”

“…NBD2 is the last domain on CFTR that is synthesized, and its folding and assembly into a complex with aminoterminal regions of CFTR seems to be slow and thus occur posttranslationally (Du et al, 2005). Deletion of F508 hinders the folding of NBD2, but whether this occurs as a result of improper interaction of F508 NBD1 with NBD2 or global defects in CFTR assembly is not clear (Du et al, 2005;Cui et al, 2007).The folding and assembly of CFTR is not a spontaneous process, and some of the same chaperones that are involved in the selection of misfolded CFTR for degradation are also required for CFTR folding. The 40-kDa heat shock protein (Hsp40) Hdj2 is farnesylated and localized to the cytoplasmic face of the ER in which it binds CFTR translation intermediates to facilitate aspects of its cotranslational folding (Meacham et al, 1999).…”