Transmembrane Protein Folding: Effects of Disease-Causing Mutations on CFTR Folding and Assembly



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The biosyntheses of multi-domain membrane proteins are complex processes which involve the translation, folding, and assembly of domains to reach the native state. The nascent chain of a membrane protein must interact with multiple solvent environments, ribosome and chaperone components, and processing and trafficking machinery, and each of these steps are at least partially determined by the protein sequence and structure. Alterations to protein sequences often perturb these processes by impacting any of a number of structural states of the protein, and while some mutations impact the native state structures of proteins directly, others impact the folding process and have little direct effect on the native state structure. A growing number of mutations have been shown to impact these folding processes in the cystic fibrosis transmembrane conductance regulator (CFTR), a multi-domain transmembrane protein associated with cystic fibrosis. Two such mutations are detailed in this work: F508del and P205S.
The most common CF-causing mutation, F508del, is the deletion of a single phenylalanine residue in a cytosolic domain of CFTR and results in a protein which fails to fold at physiological temperature, is retained in the ER and is degraded by the proteasome. The resulting loss of protein is the underlying basis for cystic fibrosis. The loss of the backbone at this position induces the misfolding of the domain, while changes in sidechain character impact subsequent domain-domain assembly. The rescue of full-length F508del CFTR by second-site suppressors correlates with the rescue of the folding of the soluble domain, further suggesting the direct role of Phe508 in domain folding. The mutation of equivalent residues in homologous proteins results in similar phenotypes, suggesting an evolutionary conservation of function for this position. The Pro205 residue, in the first transmembrane domain, has been shown to facilitate proper folding by disfavoring alternate, non-native protein conformations. A computational study of proline residues in transmembrane helices suggests that this mechanism is also conserved evolutionarily. With these data, a hierarchical model for CFTR folding is presented and mechanisms by which these mutations specifically impact the stepwise folding and assembly of CFTR are suggested.