Bh. Qu et al., LOCALIZATION AND SUPPRESSION OF A KINETIC DEFECT IN CYSTIC-FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR FOLDING, The Journal of biological chemistry, 272(25), 1997, pp. 15739-15744
A growing body of evidence indicates that the most common cystic fibro
sis-causing mutation, Delta F508, alters the ability of the cystic fib
rosis transmembrane conductance regulator (CFTR) protein to fold and t
ransit to the plasma membrane, Here we present evidence that the Delta
F508 mutation affects a step on the folding pathway prior to formatio
n of the ATP binding site in the nucleotide binding domain (NBD), Nota
bly, stabilization of the native state with 4 mM ATP does not alter th
e temperature-dependent folding yield of the mutant Delta F508 NBD1 in
vitro. In contrast, glycerol, which promotes Delta F508-CFTR maturati
on in vivo, increases the folding yield of NBD1 Delta F and reduces th
e off pathway rate in vitro, although it does not significantly alter
the free energy of stability, Likewise a second site mutation, R553M,
which corrects the maturation defect in vivo, is a superfolder which c
ounters the effects of Delta F508 on the temperature-dependent folding
yield in vitro, but does not significantly alter the free energy of s
tability, A disease-causing mutation, G551D, which does not alter the
maturation of CFTR in vivo but rather its function as a chloride chann
el, and the S549R maturation mutation have no discernible effect on th
e folding of the domain, These results demonstrate that Delta F508 is
a kinetic folding mutation that affects a step early in the process, a
nd that there is a significant energy barrier between the native state
and the step affected by the mutation precluding the use of native st
ate ligands to promote folding, The implications for protein folding i
n general are that the primary sequence may not necessarily simply def
ine the most stable native structure, but rather a stable structure th
at is kinetically accessible.