Key points Malfunction of the cystic fibrosis transmembrane conductance regulator (CFTR), a gated pathway for chloride movement, causes the common life‐shortening genetic disease cystic fibrosis (CF). Gene changes (called second‐site mutations or revertants) that restore function to F508del, the most common CF mutation, also alter the behaviour of the CF mutant G551D. Revertants have direct impact on the structure of CFTR, but they exert their effects in a mutation‐specific way. Information about the action of revertants assists the development of new therapies that target the root cause of CF. Abstract Cystic fibrosis (CF) is caused by dysfunction of the epithelial anion channel cystic fibrosis transmembrane conductance regulator (CFTR). One strategy to restore function to CF mutants is to suppress defects in CFTR processing and function using revertant mutations. Here, we investigate the effects of the revertant mutations G550E and 4RK (the simultaneous disruption of four arginine‐framed tripeptides (AFTs): R29K, R516K, R555K and R766K) on the CF mutant G551D, which impairs severely channel gating without altering protein processing and which affects a residue in the same α‐helix as G550 and R555. Both G550E and 4RK augmented strongly CFTR‐mediated iodide efflux from BHK cells expressing G551D‐CFTR. To learn how revertant mutations influence G551D‐CFTR function, we studied protein processing and single‐channel behaviour. Neither G550E nor 4RK altered the expression and maturation of G551D‐CFTR protein. By contrast, both revertants had marked effects on G551D‐CFTR channel gating, increasing strongly opening frequency, while 4RK also diminished noticeably the duration of channel openings. Because G551D‐CFTR channel gating is ATP independent, we investigated whether revertant mutations restore ATP dependence to G551D‐CFTR. Like wild‐type CFTR, the activity of 4RK‐G551D‐CFTR varied with ATP concentration, suggesting that 4RK confers some ATP dependence on the G551D‐CFTR channel. Thus, the revertant mutations G550E and 4RK alter the gating pattern and ATP dependence of G551D‐CFTR without restoring single‐channel activity to wild‐type levels. Based on their impact on the CF mutants F508del and G551D, we conclude that G550E and 4RK have direct effects on CFTR structure, but that their action on CFTR processing and channel function is CF mutation specific.