Deletion of phenylalanine 508 (delta F508) in the cystic fibrosis transmembrane-conductance
regulator (CFTR) prevents the otherwise functional protein from reaching the plasma
membrane and is the leading cause of cystic fibrosis. Indirect evidence suggests that
the mutant protein, delta F508 CFTR, is misfolded. We address this issue directly,
using comparative limited proteolysis of CFTR at steady steady state and during biosynthesis
in the native microsomal environment. Distinct protease susceptibilities suggest that
cytosolic domain conformations of wild type and delta F508 CFTR differ, not only near
F508, but globally. Moreover, delta F508 CFTR proteolytic cleavage patterns were indistinguishable
from those of the early folding intermediate of wild type CFTR. The results suggest
that the delta F508 mutation causes the accumulation of a form of the protein that
resembles an intermediate in the biogenesis of the wild type CFTR, rather than induces
the production of non-native variant.
