The homozygous deletion of the phenylalanine at position 508 (ΔPhe508) in the first nucleotide-binding domain (NBD1) of the cystic fibrosis transmembrane conductance regulator (CFTR) is the most common CF-causing genetic defect. It has been proposed that the propensity of NBD1 to aggregate may lead to a lower display of the CFTR chloride channel to the cell membrane and to the disease, thus opening an avenue for the pharmacological development of CFTR folding correctors. Here, we show that a human monoclonal antibody fragment specific to the folded conformation of NBD1 inhibits the aggregation of NBD1 in vitro. However, in contrast to the previously published observations, we proved experimentally that NBD1 of wild-type and ΔPhe508 version of CFTR display comparable propensities to aggregate in vitro and that the corresponding full-length CFTR protein reaches the cell membrane with comparable efficiency in mammalian cell expression systems. On the basis of our results, the ‘folding defect' hypothesis seems unlikely to represent the causal mechanism for the pathogenesis of CF. A solid understanding of how the ΔPhe508 deletion leads to the disease represents an absolute requirement for the development of effective drugs against C
