Decoding F508del misfolding in cystic fibrosis

Abstract

The functional deficiency of the cystic fibrosis transmembrane conductance regulator (CFTR), a plasma membrane chloride channel, leads to the development of cystic fibrosis. The deletion of a phenylalanine at residue 508 (F508del) is the most common cause of CFTR misfolding leading to the disease. The F508del misfolding originates in the first nucleotide-binding domain (NBD1), which induces a global conformational change in CFTR through NBD1's interactions with other domains. Such global misfolding produces a mutant chloride channel that is impaired in exocytic trafficking, peripheral stability, and channel gating. The nature and atomic details of F508del misfolding have been subject to extensive research during the past decade. Current data support a central role for NBD1 in F508del misfolding and rescue. Many cis-acting NBD1 second-site mutations rescue F508del misfolding in the context of full-length CFTR. While some of these mutations appear to specifically counteract the F508del-induced misfolding, others release certain inherent conformational constraints of the human wild-type CFTR. Several small-molecule correctors were recently found to act on key interdomain interfaces of F508del CFTR. Potential rational approaches have been proposed in an attempt to develop highly effective small molecule modulators that improve the cell surface functional expression of F508del CFTR.

Figure 1

Misfolding and rescue of F508del cystic fibrosis transmembrane conductance regulator (CFTR). Shown are domain conformation of human full-length wild-type (wt) CFTR, its paralogue p-glycoprotein (p-gp), and other CFTR homologues and mutants, as indicated. In each panel, a full-length ABC protein is anchored to the membrane bilayer (in yellow) through two six-pass membrane-spanning domains (white). The extracellular and cytoplasmic loops are in blue, and the major cytoplasmic domains such as the two nucleotide-binding domains (NBD1 and NBD2) and the regulatory domain (R) are in grey. Glycosylation sites on the extracellular loops are labeled in orange (p-gp has three and CFTR has two). The different shapes and orientations of the major cytoplasmic domains represent their different conformations. Key sequence variations in NBD1 are labeled. I539T/4P refers to the combined I539T and four proline substitutions occurring in chicken CFTR. “F508del” refers to the chicken version of the human F508del mutation. RI refers to the presence of the regulatory insertion. Key interdomain contacts for CFTR are labeled by pink circles. The proteins are arranged in the order of conformational stability. Green arrows represent potential F508del rescue strategies.