Growing up to be a mature and stable ion channel appears to be especially difficult for the cystic fibrosis transmembrane conductance regulator (CFTR)1.Riordan J.R. Cystic fibrosis as a disease of misprocessing of the cystic fibrosis transmembrane conductance regulator glycoprotein.Am J Hum Genet. 1999; 64: 1499-1504Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar. Whereas other adenine nucleotide binding cassette (ABC) transporters including members of the same subfamily (ABCC) mature efficiently, only about 30% of wild-type nascent CFTR polypeptides mature and proceed beyond the endoplasmic reticulum (ER) in mammalian cells. The remaining 70% are polyubiquitinated and degraded by the proteasome2.Jensen T.J. Loo M.A. Pind S. et al.Multiple proteolytic systems, including the proteasome, contribute to CFTR processing.Cell. 1995; 83: 129-135Abstract Full Text PDF PubMed Scopus (742) Google Scholar. The δF508 present in 90% of cystic fibrosis (CF) patients and many different missense mutations completely preclude maturation. Although detectable at steady state, the immature core-glycosylated form of neither the wild-type nor δF508 CFTR accumulate in the ER. There is some evidence from both mammalian cells and yeast that nascent CFTR may be “retrotranslocated” from the ER membrane enabling proteolysis by ER-associated proteasomes. Crucial questions that need to be answered to understand the biosynthetic lability of CFTR include (1) what are the features of the molecule that prevent it from maturing efficiently, and (2) how are molecules that can mature distinguished from those that cannot? CFTR most obviously differs from other ABCC family members by the presence of charged amino acids in membrane-spanning sequences, the large central R-domain rich in phosphorylation sites and an extended C-terminal domain beyond the second nucleotide-binding domain. Phosphorylation sites of the R-domain apparently play no crucial role in maturation3.Seibert F.S. Chang X.-B. Aleksandrov A.A. et al.Influence of phosphorylation by protein kinase A on CFTR at the cell surface and endoplasmic reticulum.Biochim Biophys Acta. 1999; 1461: 275-283Crossref PubMed Scopus (62) Google Scholar. C-terminal sequences were shown to impact maturation; however, a truncated CFTR molecule without C-terminal domain shows still a substantial degree of maturation4.Gentzsch M. Riordan J.R. Localization of sequences within the C-terminal domain of CFTR which impact maturation and stability.J Biol Chem. 2001; 276: 1291-1298Crossref PubMed Scopus (32) Google Scholar. Thus, neither the R-domain phosphorylation nor the C-terminal extension appears to be exclusively responsible for the failure to mature. Mutations identified in patients with cystic fibrosis are distributed all over the CFTR gene. Examination of more than 40 missense mutations indicated that a large proportion (60%) of those in cytoplasmic loops caused misprocessing, while none in the extracytoplasmic loops had this effect5.Seibert F.S. Loo T.W. Clarke D.M. Riordan J.R. Cystic fibrosis: Channel, catalytic, and folding properties of the CFTR protein.J Bioenerg Biomembr. 1997; 29: 429-442Crossref PubMed Scopus (37) Google Scholar. Hence, detection appears to occur on the cytoplasmic rather than the luminal side of the ER membrane. Consistent with this interpretation, although nascent CFTR normally binds calnexin, the conformation-sensing mechanism that this chaperone provides together with UDP-glucosyl glucose transferase, does not seem to be involved since prevention of glycosylation neither interferes with transport of wild-type CFTR to the cell surface nor enables δF508 to do so. The cytoplasmic chaperones Hsp70 and Hsp90 bind nascent CFTR and inhibition of the latter interaction with ansamycin drugs further disrupts maturation and accelerates proteolysis6.Loo M.A. Jensen T.J. Cui L. et al.Perturbation of Hsp90 interaction with nascent CFTR prevents its maturation and accelerates its degradation by the proteasome.EMBO J. 1998; 17: 6879-6887Crossref PubMed Scopus (290) Google Scholar. As a Hsp90 substrate, nascent CFTR is typical of a small set of proteins that mature with difficulty. Because missense mutations at nearly all locations along the polypeptide may prevent maturation, it seems likely that achievement of a native overall tertiary structure is the criterion that must be met to enable export from the ER. Sequence changes elsewhere in the molecule can suppress or compensate for the null export phenotype of δF508. An example is the combined substitutions R29K and R555K that rescue at least one third of δF508 molecules7.Chang X. Cui L. Hou Y. et al.Removal of multiple arginine-framed trafficking signals overcomes misprocessing of delta F508 CFTR present in most patients with cystic fibrosis.Mol Cell. 1999; 4: 137-142Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar.