Mechanisms of CFTR Folding at the Endoplasmic Reticulum

Abstract

In the past decade much has been learned about how Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) folds and misfolds as the etiologic cause of cystic fibrosis (CF). CFTR folding is complex and hierarchical, takes place in multiple cellular compartments and physical environments, and involves several large networks of folding machineries. Insertion of transmembrane (TM) segments into the endoplasmic reticulum (ER) membrane and tertiary folding of cytosolic domains begin cotranslationally as the nascent polypeptide emerges from the ribosome, whereas posttranslational folding establishes critical domain–domain contacts needed to form a physiologically stable structure. Within the membrane, N- and C-terminal TM helices are sorted into bundles that project from the cytosol to form docking sites for nucleotide binding domains, NBD1 and NBD2, which in turn form a sandwich dimer for ATP binding. While tertiary folding is required for domain assembly, proper domain assembly also reciprocally affects folding of individual domains analogous to a jig-saw puzzle wherein the structure of each interlocking piece influences its neighbors. Superimposed on this process is an elaborate proteostatic network of cellular chaperones and folding machineries that facilitate the timing and coordination of specific folding steps in and across the ER membrane. While the details of this process require further refinement, we finally have a useful framework to understand key folding defect(s) caused by ΔF508 that provides a molecular target(s) for the next generation of CFTR small molecule correctors aimed at the specific defect present in the majority of CF patients.

Targeting ion channels in cystic fibrosis

“…Deletion of phenylalanine 508 causes multiple defects to CFTR protein stability, folding, trafficking, and gating [21,42,43]. Treatment of cells with low temperature or with high concentrations of chemical chaperones (glycerol, DMSO) was found to improve the trafficking of F508del-CFTR to the plasma membrane with a significant increase in chloride transport [44,45].…”

“…Class II mutations, in particular F508del that is the most frequent among all CF patients, instead impair the stability and folding of the protein [21]. The consequence is the degradation of mutant Fig.…”

CFTR: A new horizon in the pathomechanism and treatment of pancreatitis

“…Despite interactions with several cytosolic and endoplasmic reticulum (ER) chaperones (heat shock proteins Hsp70 and Hsp90, co-chaperones; Hdj2 [DNAJ1], HsBp1, Hop and p23, small Hsps, calnexin and calreticulin), only 30-60% of the newly synthesized nascent CFTR attains folded conformation, presumably due to the metastable nature of NBD1 and NBD2, the slow kinetics of domain assembly, and the highly efficient ER quality control [5]. The conformational maturation of CFTR is promoted by cytosolic ATP and the conjugation of Nglycan chains at the ER [3].…”

“…There are two adjacent polymorphic regions (poly-T and poly-TG tracts) in intron-8 which can alter inclusion of exon-9 (legacy numbering) during splicing [28][29][30]. A short poly-T tract (c.1210-12T [5], legacy name T5) in combination with a longer poly-TG tract (c.1210-34TG [12], legacy name TG12) results in more transcripts lacking exon-9 that give rise to inactive CFTR protein. A T5 allele combined with a severe CFTR mutation may cause CF and increases the penetrance of the mild p.R117H mutation.…”

“…Assembly of MSD1, NBD1, R domain and MSD2 is necessary and sufficient to form the minimal folding unit of CFTR [4]. These and other observations support the cooperative domain folding model and ensure the dynamic conformational coupling between the cytosolic NBDs and the pore-forming MSDs in the native molecule and provide a structural explanation for the cooperative domain unfolding, caused by cystic fibrosis (CF) mutations [4].Despite interactions with several cytosolic and endoplasmic reticulum (ER) chaperones (heat shock proteins Hsp70 and Hsp90, co-chaperones; Hdj2 [DNAJ1], HsBp1, Hop and p23, small Hsps, calnexin and calreticulin), only 30-60% of the newly synthesized nascent CFTR attains folded conformation, presumably due to the metastable nature of NBD1 and NBD2, the slow kinetics of domain assembly, and the highly efficient ER quality control [5]. The conformational maturation of CFTR is promoted by cytosolic ATP and the conjugation of Nglycan chains at the ER [3].…”