The dimerization of their two nucleotide binding domains (NBDs) in a so-called "nucleotide-sandwich"
is the hallmark of ATP cassette binding (ABC) proteins and the basis of their catalytic
activities. The major disease-causing mutation in the cystic fibrosis transmembrane
conductance regulator (CFTR or ABCC7), deletion of Phe508 in NBD1, does not grossly
alter the structure of that domain but prevents conformational maturation of the whole
CFTR protein, possibly by disrupting the native interaction between NBD1 and NBD2.
However, the role of inter-domain interactions in CFTR folding has been brought into
question by a recent report that all CFTR domains fold independently. Here we show
that in addition to domain folding, correct inter-domain assembly is essential to
form a stable unit that satisfies endoplasmic reticulum (ER) quality control. N-terminal
domains depend on their more C-terminal neighbors, most essentially the second membrane-spanning
domain (MSD2) but significantly, not NBD2. Wild-type C-terminal truncation constructs,
completely devoid of NBD2 are transported out of the ER and to the cell surface where
they form characteristic CFTR chloride channels with low open probability. The DeltaNBD2
wild-type protein matures and has similar stability as its full-length counterpart.
Therefore, the catalytically crucial inter-NBD associations are not required to satisfy
ER quality control mechanisms. The DeltaF508 mutation arrests the maturation of DeltaNBD2
just as it does full-length CFTR, indicating that DeltaF508 perturbs other portions
of the molecule in addition to NBD2. We find that the mutation prevents formation
of a compact MSD1, reflected in its susceptibility to protease digestion. This perturbation
of MSD1 may in turn prevent its normal integration with MSD2. The dispensability of
NBD2 in the folding of more N-terminal domains stands in contrast to the known hypersensitivity
to proteolysis of NBD2 in the DeltaF508 protein.
