Mutations that Disrupt Formation of Functional CFTR

Abstract

Cystic fibrosis (CF) is caused by a loss-of-function of the cystic fibrosis transmembrane conductance regulator (CFTR) channel. Many mutations that cause CF, including the most common disease allele ΔF508, interfere with CFTR function because the mutant protein does not efficiently fold into the native channel structure. Significantly, when these mutant proteins are induced to fold, in experimental systems, some CFTR function is recovered, suggesting an avenue for therapeutic development. Exploiting this opportunity requires detailed knowledge of the basic processes of CFTR folding and the steps altered by the disease-causing mutations. Models of CFTR place the critical F508 residue on the surface of one of the two nucleotide binding domains (NBD1) at a predicted interface with the intracellular loop (ICL4) in the second of two transmembrane domains (TMD2). A variety of biophysical, biochemical, and cell biological studies demonstrate that CFTR folds in a hierarchical manner, with folding of the domains occurring first, during translation and, later, the partially folded domains associating to form the final, functional CFTR structure. Consistent with the location of F508 in the structural models, its deletion interferes with both the folding of NBD1 and with subsequent steps of domain-domain association. The detailed energetics and kinetics of these processes provide insight into the fundamental mechanisms by which integral membrane proteins achieve their native structures and reveal obstacles to and suggest strategies for improving the folding efficiency of the mutant CFTR protein. Supported by NIH-NIDDK, NIH-NIDCR, Reata Pharmaceuticals, CFF.