Deletions around the F508 residue revealed the importance of regional integrity in protein processing of the cystic fibrosis transmembrane conductance regulator

Abstract

The ΔF508 mutation missing a phenylalanine at position 508 in the cystic fibrosis transmembrane conductance regulator (CFTR) is found most commonly in patients with cystic fibrosis, and it greatly reduces protein processing of CFTR. Absent side chain interactions and abnormal protein folding caused by the ΔF508 mutation are proposed currently to account for the processing defect of ΔF508-CFTR. However, it is unclear whether the residues around F508 also have important function in maintaining the structural integrity required for CFTR processing, but disrupted by deletion of the F508 residue. To test this hypothesis, we performed immunoblot experiments to measure protein processing of several CFTR deletion mutants, in which each mutant has a residue deleted at the position from 503 to 513. Our data demonstrate that fully glycosylated and mature CFTR were abundantly produced in the cells expressing ΔV510- and ΔS511-CFTR, but only some for ΔY512-CFTR and little for all other mutations. These data suggest that similar to the ΔF508 mutation, most deletions around the F508 residue significantly reduced protein processing. To investigate whether these deletion mutants and ΔF508-CFTR cause the processing defects by similar mechanisms, cells transiently expressing a deletion mutant were treated with low temperature culture at 27 or CFTR correctors C1 or C18. Our data indicate that low temperature culture only enhanced the production of mature ΔF508- and ΔY512-CFTR. By contrast, corrector C1 showed little enhancement on the amount of mature proteins among all mutants, whereas corrector C18 significantly increased the amount of mature ΔF508-, ΔG509- and ΔY512-CFTR. These data suggest that the ΔY512 and ΔF508 mutations may have defective protein processing by similar mechanisms. Interestingly, adding corrector C18 at low temperature further enhanced mature protein expression of both ΔF508- and ΔY512-CFTR among other mutants with a deletion from 506 to 513. These data suggest that low temperature and C18 may rescue CFTR processing by different mechanisms. In addition, our data demonstrate that the alanine replacement on the residue at position from 505 to 513 did not significantly affect CFTR processing, except that the amount of mature protein was moderately decreased in cells expressing D513A-CFTR and largely reduced for N505A-CFTR. The data suggest that the side chain of the N505 residue is important for structural integrity required for normal CFTR processing. Moreover, these data suggest that in the region around the F508 residue, protein processing of CFTR is more vulnerable to the deletion of the residue backbone than the absence of its side chain. This work was supported by RGC, NACF, University of Hong Kong and Cystic Fibrosis Foundation Therapeutics