CFTR chloride channels and ABC active transporters undergo similar structural rearrangements as part of their operation
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Defects in the cystic fibrosis transmembrane conductance regulator (CFTR) underlie the lethal genetic disease cystic fibrosis. Although CFTR belongs to the ATP-binding cassette (ABC) transporter superfamily, it functions as a phosphorylation- and ATP-regulated chloride channel. The molecular architecture of CFTR encompasses two cytosolic nucleotide-binding domains (NBD1 and NBD2), two transmembrane domains (TMD1 and TMD2) and a regulatory (R) domain. Here, I adopted the cysteine scanning method to study two transmembrane segments (TM6 and TM12) of CFTR. Major findings are that (i) like other ABC proteins, both TM6 and TM12 of CFTR are [alpha]-helical and contribute to the lining of the permeation pathway, (ii) gating conformational changes in CFTR and structural rearrangements associated with the transport cycle of ABC exporters share analogous features in that both involve flip-flop motion in the TMDs and helical rotation in the TMs, and (iii) there is no physical gate at the cytoplasmic entrance of the CFTR channel. Collectively, these findings agree satisfactorily with the long-held hypothesis that the CFTR channel lacks the cytoplasmic-side gate of a primordial ABC exporter.
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