The CFTR mutations database
The CFTR protein
The cystic fibrosis transmembrane conductance regulator (CFTR) is a 1,480-residue multidomain integral membrane glycoprotein localized at the apical membrane of epithelial cells (NP_000483). CFTR is an atypical member of the ATP binding cassette (ABC) transporter family, conserved in metazoan salt-transporting tissues and required to control ion and fluid homeostasis on epithelial surfaces.
Structure
As a member of the ABC superfamily, the CFTR protein (ABCC7) is organized in two homologous motifs, each comprising a transmembrane domain (TMD1 and TMD2) composed of six spanning regions, and a cytoplasmic nucleotide-binding domain (NBD1 and NBD2). In CFTR, these two motifs are connected by a regulatory (R) domain, unique among ABC transporters. Each of the two NBDs possesses the characteristic Walker A and Walker B sequences which bind ATP (adenosine 5'-triphosphate), and the signature sequence (LSGGQ/E) is conserved in all ABC transporters; TMDs are divergent among the members of the family. The R domain contains several consensus sites for phosphorylation by cAMP-dependant protein kinases A and C.
The membrane-spanning domains contribute to the formation of the channel pore whereas the nucleotide-binding domains bind and hydrolyze ATP to regulate channel gating, and phosphorylation of the R domain controls channel activity.
High-resolution 3D structure of human full-length CFTR has not been achieved yet, partly due to insufficient quantities of purified and reconstituted protein available for large-scale crystallization trials.
Biosynthetic processing and intracellular trafficking
A few minutes are required for a complete core-glycosylation of the nascent CFTR polypeptide through N-glycosylation of two sites in EC4 (extracellular loop 4); complete glycosylation with high-mannose oligosaccharide chains and maturation of the CFTR takes about two hours. Approximately one-third of the nascent chains are transformed in higher-molecular-weight mature products able to be exported from the endoplasmic reticulum (ER) and reach the Golgi and post-Golgi compartments, the remainder products being ubiquitylated and degraded by the proteasome. The large cytoplasmic domains of the CFTR must be folded and assembled correctly, and multiple chaperones and co-factors are implicated in these processes. Among ABC proteins, the CFTR seems to be a major target of complex quality control systems of the cell. The actin-based cytoskeleton plays a major role in the endocytic trafficking and recycling of the CFTR molecules.
Function
ABC transporters usually bind and hydrolyze ATP to perform active transport of substrate (against a concentration gradient). CFTR is the only known member of the large ABC protein family that functions as an ion channel. The gate that regulates chloride-ion flow through the membrane-spanning CFTR pore is opened by the binding of ATP to the two NBDs which interact closely together; the R domain must be phosphorylated by cAMP-dependent protein kinase A (PKA) before ATP is able to support channel opening. Hydrolysis of the ATP bound at the NBD2 catalytic site disrupts the interaction between the two NBDs and closes the channel. The control of CFTR gating by R domain phosphorylation seems more complex than a on/off switch, as the probability of channel opening is dependent on the balance between protein kinases (A and C) and phosphatases activities on multiple phosphorylation sites. Although it has for long been a subject of controversy, it is now generally admitted that ATP hydrolysis is not involved in channel opening. Nucleotide-induced changes in the NBDs coupled with conformational movements within and between the two NBDs and among the TMDs (transmembrane domains) contribute to the channel gating. Although Cl- is the major permeant anion, CFTR has a low selectivity and conducts a spectrum of anions. The CFTR also plays a role in HCO3- secretion and stimulates Cl-/HCO3- exchangers. Pancreatic failure in CF is partly due to loss of HCO3- fluid secretory capacity, and HCO3- conductance should be considered a major determinant in CF severity. In addition to its channel function, the CFTR plays other roles that impact other cellular proteins and functions.
The absence, dysfunction, or quantitative reduction of this regulated anion channel activity caused by mutations in the CFTR gene result in the failure of ionic and water homeostatis on exocrine epithelial surfaces. This causes accumulation of macromolecular secretions, which are dehydrated and altered. This occurs in most exocrine tissues but with the most serious consequences in the airways of the lung and in the pancreas, where failure of bicarbonate-rich fluid and enzyme secretion impair intestinal digestion and absorption. Viscous mucus accumulations and colonization by microorganisms cause inflammatory responses and tissue damaging.
References
* Aleksandrov AA, Aleksandrov LA, Riordan JR. 2007. CFTR (ABCC7) is a hydrolysable ligand-gated channel. Pflugers Arch 453(5):693-702. PMID: 17021796.
* Cheung JC, Deber CM. 2008. Misfolding of the cystic fibrosis transmembrane conductance regulator and disease. Biochemistry 47(6):1465-73. PMID: 18193900.
* Devidas S, Guggino WB. 1997. CFTR: domains, structure, and function. J Bioenerg Biomembr 29(5):443-51. PMID: 9511929.
* Ostedgaard LS, Baldursson O, Welsh MJ. 2001. Regulation of the cystic fibrosis transmembrane conductance regulator Cl- channel by its R domain. J Biol Chem 276(11):7689-92. PMID: 11244086.
* Quinton PM. 2007. Cystic fibrosis: lessons from the sweat gland. Physiology (Bethesda) 22:212-25. PMID: 17557942.
* Riordan JR. 2005. Assembly of functional CFTR chloride channels. Annu Rev Physiol 67:701-18. PMID: 15709975.
* Riordan JR, Rommens JM, Kerem B, Alon N, Rozmahel R, Grzelczak Z, Zielenski J, Lok S, Plavsic N, Chou JL and others. 1989. Identification of the cystic fibrosis gene: cloning and characterization of complementary DNA. Science 245(4922):1066-73. PMID: 2475911.