A defective flexible loop contributes to the processing and gating defects of the predominant cystic fibrosis-causing mutation

People with the genetic disease cystic fibrosis (CF) often carry a deletion mutation ΔF508 on the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) Cl‒ channel. This mutation greatly reduces the CFTR maturation process and slows the channel opening rate. Here, we investigate whether residues near F508 contribute to these defects in ΔF508-CFTR. Most deletion mutations, but not alanine substitutions, of individual residues from position 503 to 513 impaired CFTR maturation. Interestingly, only protein processing of ΔY512-CFTR, like that of ΔF508-CFTR, was greatly improved by low-temperature culture at 27 °C or small molecule corrector C18. The two mutant Cl‒ channels were equally slow to open, suggesting that they may share common structural flaws. Studies on the H3-H4 loop that links residues F508 and Y512 demonstrate that G509A/V510G mutations, moving G509 one position backward in the loop, markedly enhanced ΔF508-CFTR maturation and opening rate, while promoting protein stability and persistence of the H3 helix in ΔF508 NBD1.
Moreover, V510A/S511A mutations noticeably increased ΔY512-CFTR maturation at 27 °C and its opening rate. Thus, loop abnormalities may contribute to ΔF508- and ΔY512-CFTR defects. Importantly, correcting
defects from G509 displacement in ΔF508-CFTR may offer a new avenue for drug discovery and CF treatments.