A Topological Switch in the Cystic Fibrosis Transmembrane Conductance Regulator Modulates Channel Activity and Sensitivity to Disease-Causing Mutation

The cystic fibrosis transmembrane conductance regulator (CFTR) anion channel is essential to maintain fluid homeostasis in key organs such as the lungs or the digestive systems. Functional impairment of CFTR due to mutation in the cftr gene lead to Cystic Fibrosis (CF) the most common lethal genetic disorder. Here we observe that the first nucleotide-binding domain (NBD1) of CFTR can spontaneously adopt an alternative conformation that departs from the canonical NBD fold previously observed for CFTR and other ATP-binding cassette (ABC) transporter proteins. Crystallography studies reveal that this conformation involves a topological reorganization of the β-subdomain of NBD1. This alternative state is adopted by wild-type CFTR in cells, where it leads to enhanced channel activity. Single-molecule fluorescence resonance energy transfer microscopy shows that the equilibrium between the conformations is regulated by ATP binding. However, under destabilizing conditions, such as a the prominent disease-causing mutation F508el , this conformational flexibility enables unfolding of the β-subdomain. Our data indicate that in wild-type CFTR switching to this topologically-swapped conformation of NBD1 regulates channel function, but, in the presence of the F508del mutation, it allows domain misfolding and subsequent protein degradation. Our work provides a framework to design conformation-specific therapeutics to prevent noxious transitions.