Correcting mutant CFTR with a stabilizing nanobody reveals a novel active conformation of the channel

Defects in protein trafficking underlie many genetic diseases, including cystic fibrosis (CF), where the predominant F508del mutation destabilizes the cystic fibrosis transmembrane conductance regulator (CFTR) channel, leading to its degradation. To provide a protein-specific chaperone, we used lipid nanoparticles to deliver mRNA encoding T2a, a nanobody that thermally stabilizes CFTR via high-affinity binding to nucleotide-binding domain 1 (NBD1). When combined with clinically-approved correctors, T2a considerably improved F508del-CFTR maturation, plasma membrane expression, and channel activity. Single-channel recordings revealed that nanobody binding sustained channel activity by promoting both full open and sub-conductance gating states and protecting F508del-CFTR against thermal deactivation. Because T2a binding to NBD1 prevents the ATP-dependent NBD1-NBD2 association that drives canonical channel opening, the observation of channel activity in the presence of the nanobody suggested adoption of a novel open-channel CFTR structure. This inference was confirmed by cryo-EM analyses of CFTR in the presence of T2a, which revealed a novel open-channel conformation in which NBD1 binds to an alternative site on CFTR incompatible with NBD dimerization. Our findings establish a new paradigm to correct protein trafficking by stabilizing misfolded domains with targeted nanobodies and demonstrate a broadly applicable framework to treat CF and related protein misfolding diseases.