How a single mutation in CFTR causes the systemic disease cystic fibrosis: interactions, PTMs, and structure

Protein conformation is dynamic as it is influenced by post-translational modifications (PTMs) and interactions with other proteins. In vivo characterization of protein structures and protein structural changes after perturbation is a major challenge. Therefore, experiments to characterize protein structures are typically performed in vitro and with highly purified proteins or protein complexes, revealing a static picture of the protein. To identify the true conformational space occupied by proteins in vivo, we developed a novel low-resolution method named Covalent Protein Painting (CPP) that allows the characterization of protein conformations in vivo. Here, we report how an ion channel, the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR), is conformationally changed during biogenesis and channel opening in the cell. Our study led to the identification of a novel opening mechanism for CFTR by revealing that the interaction of the intracellular loop 2 (ICL2) with the nucleotide binding domain 2 (NDB2) of CFTR is needed for channel gating, and this interaction occurs concomitantly with changes to the narrow part of the pore and the walker A lysine in NBD1 for wt CFTR. However, the ICL2:NBD2 interface, which forms a “ball-in-a-socket” motif, is uncoupled during biogenesis, likely to prevent inadvertent channel activation during transport. Mutation of K273 in the ICL2 loop severely impaired CFTR biogenesis and led to accumulation of CFTR in the Golgi and TGN. CPP further revealed that, even upon treatment with current approved drugs such as Trikafta or at permissive temperature, the uncoupled state of ICL2 is a prominent feature of the misfolded CFTR mutants ∆F508 and N1303K that cause Cystic Fibrosis. Although Trikafta treatment reduced the amount of uncoupled ICL2:NBD2 interfaces, more than 75% of F508 CFTR remained in the uncoupled state, suggesting that stabilization of this interface could produce a more efficient CF drug.