ABC-transporter CFTR folds with high fidelity through a modular, stepwise pathway
The question how proteins fold is especially pointed for large multidomain, multispanning membrane proteins with complex topologies. We have uncovered the sequence of events that encompass proper folding of the ABC transporter CFTR in live cells, by combining kinetic radiolabeling with protease-susceptibility assays. We found that CFTR folds in two clearly distinct stages. The first, co-translational, stage involves folding of the 2 transmembrane domains TMD1 and TMD2, plus one nucleotide-binding domain, NBD1. The second stage is a simultaneous, post-translational increase in protease resistance for both TMDs and NBD2 caused by assembly of these domains onto NBD1. Our technology probes every 2-3 residues (on average) in CFTR. This in-depth analysis at amino-acid level allows detailed analysis of domain folding and importantly also the next level: the assembly of the domains to native, folded CFTR. Defects and changes brought about by medicines, chaperones or mutations also are amenable to analysis. We here show that the DXD motif in NBD1 that was identified to be required for export of CFTR from the ER turned out to be required for proper domain folding and assembly instead, upstream of transport. CFTR mutated in this motif phenocopies the misfolding and degradation of the well-known disease-causing mutant F508del that established cystic fibrosis as protein-folding disease. The highly modular process of domain folding and stepwise domain assembly explains the relatively high fidelity of folding and the importance of a step-wise folding process for such complex proteins.