Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that cause loss of function of the CFTR channel on the apical surface of epithelial cells. The major CF-causing mutation, F508del-CFTR, is misfolded, retained in the endoplasmic reticulum, and degraded. Small molecule corrector compounds have been identified using high throughput screens, which partially rescue the trafficking defect of F508del-CFTR, allowing a fraction of the mutant protein to escape endoplasmic reticulum retention and traffic to the plasma membrane, where it exhibits partial function as a cAMP-regulated chloride channel. A subset of such corrector compounds binds directly to the mutant protein, prompting the hypothesis that they rescue the biosynthetic defect by inducing improved protein conformation. We tested this hypothesis directly by evaluating the consequences of a corrector compound on the conformation of each nucleotide binding domain (NBD) in the context of the full-length mutant protein in limited proteolytic digest studies. Interestingly, we found that VRT-325 was capable of partially restoring compactness in NBD1. However, VRT-325 had no detectable effect on the conformation of the second half of the molecule. In comparison, ablation of the di-arginine sequence, R(553)XR(555) (F508del-KXK-CFTR), modified protease susceptibility of NBD1, NBD2, and the full-length protein. Singly, each intervention led to a partial correction of the processing defect. Together, these interventions restored processing of F508del-CFTR to near wild type. Importantly, however, a defect in NBD1 conformation persisted, as did a defect in channel activation after the combined interventions. Importantly, this defect in channel activation can be fully corrected by the addition of the potentiator, VX-770.
