Chemical and Biological Folding Contribute to Temperature-Sensitive ΔF508 CFTR Trafficking

Proteostasis (Balch WE, Morimoto RI, Dillin A, Kelly JW. Adapting proteostasis for disease intervention. Science 2008;319:916-919) refers to the biology that maintains the proteome in health and disease. Proteostasis is challenged by the most common mutant in cystic fibrosis, Delta F508, a chloride channel [the cystic fibrosis transmembrane conductance regulator (CFTR)] that exhibits a temperature-sensitive phenotype for coupling to the coatomer complex II (COPII) transport machine for exit from the endoplasmic reticulum. Whether rescue of export of Delta F508 CFTR at reduced temperature simply reflects energetic stabilization of the chemical fold defined by its primary sequence or requires a unique proteostasis environment is unknown. We now show that reduced temperature (30 degrees C) export of Delta F508 does not occur in some cell types, despite efficient export of wild-type CFTR. We find that Delta F508 export requires a local biological folding environment that is sensitive to heat/stress-inducible factors found in some cell types, suggesting that the energetic stabilization by reduced temperature is necessary, but not sufficient, for export of Delta F508. Thus, the cell may require a proteostasis environment that is in part distinct from the wild-type pathway to restore Delta F508 coupling to COPII. These results are discussed in the context of the energetics of the protein fold and the potential application of small molecules to achieve a proteostasis environment favoring export of a functional form of Delta F508.