Deletion of phenylalanine-508 (Phe-508) from the N-terminal nucleotide-binding domain
(NBD1) of the cystic fibrosis transmembrane conductance regulator (CFTR), a member
of the ATP-binding cassette (ABC) transporter family, disrupts both its folding and
function and causes most cystic fibrosis. Most mutant nascent chains do not pass quality
control in the ER, and those that do remain thermally unstable, only partially functional,
and are rapidly endocytosed and degraded. Although the lack of the Phe-508 peptide
backbone diminishes the NBD1 folding yield, the absence of the aromatic side chain
is primarily responsible for defective CFTR assembly and channel gating. However,
the site of interdomain contact by the side chain is unknown as is the high-resolution
3D structure of the complete protein. Here we present a 3D structure of CFTR, constructed
by molecular modeling and supported biochemically, in which Phe-508 mediates a tertiary
interaction between the surface of NBD1 and a cytoplasmic loop (CL4) in the C-terminal
membrane-spanning domain (MSD2). This crucial cytoplasmic membrane interface, which
is dynamically involved in regulation of channel gating, explains the known sensitivity
of CFTR assembly to many disease-associated mutations in CL4 as well as NBD1 and provides
a sharply focused target for small molecules to treat CF. In addition to identifying
a key intramolecular site to be repaired therapeutically, our findings advance understanding
of CFTR structure and function and provide a platform for focused biochemical studies
of other features of this unique ABC ion channel.
