CFTR folding and maturation (norm and CF)
The cystic fibrosis transmembrane conductance regulator ( CFTR ) is a member of the ATP-binding cassette transporter superfamily. It acts in the apical part of the epithelial cells as a plasma-membrane cyclic AMP-activated chloride anion, bicarbonate anion and glutathione channel , , . The most common CFTR mutation is loss of a Phe residue at position 508 ( deltaF508-CFTR ) .
Folding and maturation of wt- CFTR and deltaF508- CFTR are somewhat different. In both cases, folding starts when Oligosaccharyltransferase complex ( OST complex ) binds to the newly synthesized wt- CFTR or deltaF508- CFTR via Asn-X-Ser/Thr consensus sequences that are co-translationally inserted into the endoplasmatic reticulum (ER) by a branched 14-unit oligosaccharide (two N -acetylglucosamines, nine mannoses, and three glucoses) . This process is known as N-glycosylation or core-glycosylation (see in detail about N-glycosylation  and OST complex ,  ).
Then, most of deltaF508- CFTR is degraded via Heat shock 70kDa protein 8 (Hsc70)-dependent ubiquitination. Unlike deltaF508- CFTR, wt- CFTR proceeds in the folding pathway through an interaction of its N-glycosyl residues. Glycan moiety is recognized by any Glucosidases I and II, which trim two glucose residues sequentially. The exact Glucosidases which participate in this process are unknown. Monoglucosylated oligosaccharide structure is then recognized by the lectin Calnexin , .
Chaperones that are currently thought to significantly affect CFTR ER-associated folding pathways, include cytosolic chaperones DnaJ homolog subfamily B members 1 and 6 ( Hsp40 and Hdj-1 ), Heat shock proteins 70 and 90kDa ( Hsp70 and Hsp90 ) ,  and others . ATPase activity of Hsp70 may be regulated by Hsp70-interacting protein ( HspBP1 ). HspBP1 can bind Hsp70, changes the conformation of the ATPase domain, and inhibits Hsp70 -associated protein folding . The exact role of most chaperones is unknown .
Dissociation from Calnexin coincides with trimming of the third glucose residue by Glucosidases II. If the CFTR is folded at this stage, it proceeds to the secretory pathway to Golgi. However, misfolded CFTR is specifically recognized as such by UDP-glycoprotein glucosyltransferase (e.g., UGCGL1 ), which reglucosylates them. Reglucosylated CFTR again may interact with Calnexin , .
In addition, prolonged presence in the Calnexin cycle may cause misfolded CFTR to become a substrate of Mannosidase alpha class 1B member 1 ( MA1B1 ). MA1B1 removes mannose from the middle branch CFTR, forming a Man8B isomer which in turn is recognized by another lectin ER degradation enhancer, mannosidase alpha-like 1 ( EDEM ) that targets it to proteasomal Glycoprotein endoplasmic reticulum-associated degradation (GERAD) , . GERAD is a major degradative pathway for misfolded CFTR.