Direct interaction of a small-molecule modulator with G551D-CFTR, a cystic fibrosis-causing mutation associated with severe disease

CF (cystic fibrosis) is caused by mutations in CFTR (CF transmembrane conductance regulator), which cause its mistrafficking and/or dysfunction as a regulated chloride channel on the apical surface of epithelia. CFTR is a member of the ABC (ATP-binding-cassette) superfamily of membrane proteins and a disease-causing missense mutation within the ABC signature sequence; G551D-CFTR exhibits defective phosphorylation and ATP-dependent channel gating. Studies of the purified and reconstituted G551D-CFTR protein revealed that faulty gating is associated with defective ATP binding and ATPase activity, reflecting the key role of G551 in these functions. Recently, high-throughput screens of chemical libraries led to identification of modulators that enhance channel activity of G551D-CFTR. However, the molecular target(s) for these modulators and their mechanism of action remain unclear. In the present study, we evaluated the mechanism of action of one small-molecule modulator, VRT-532, identified as a specific modulator of CF-causing mutants. First, we confirmed that VRT-532 causes a significant increase in channel activity of G551D-CFTR using a novel assay of CFTR function in inside-out membrane vesicles. Biochemical studies of purified and reconstituted G551D-CFTR revealed that potentiation of the ATPase activity of VRT-532 is mediated by enhancing the affinity of the mutant for ATP. Interestingly, VRT-532 did not affect the ATPase activity of the Wt (wild-type) CFTR, supporting the idea that this compound corrects the specific molecular defect in this mutant. To summarize, these studies provide direct evidence that this compound binds to G551D-CFTR to rescue its specific defect in ATP binding and hydrolysis.