Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)

What is CFTR?

The CFTR protein is a cell-surface anion channel that permeates the chloride and bicarbonate ions called cystic fibrosis transmembrane conductance regulator (CFTR). 5 It is part of the ATP-binding cassette (ABC) transporter family, but functions as an ATP-gated anion channel. CFTR is an ATP-gated anion channel because CFTR requires ATP binding to induce conformational changes in the protein that would result in opening the protein for the anions to pass inside the epithelial cell to outside of the cell. Chloride and bicarbonate ions are unable to pass through the membrane bilayer without protein channels because ionic molecules are hydrophilic and are repelled by the hydrophobic tails in the interior of the bilayer; thus an integrated channel protein is needed to pass through the membrane bilayer.  3  

CFTR is an integral membrane protein that spans the entire membrane lipid bilayer and is composed of different domains in different compartments. There are five functional domains (tertiary structures): two transmembrane domains (also referred to as membrane-spanning domains (MSD)), TMD1 and TMD2, two cytoplasmic nucleotide-binding domains, NBD1 and NBD2, and a regulatory, R, domain. 7 Together these domains form the quaternary structure of the CFTR protein. The transmembrane domains are each composed of six alpha helices that form the channel pore for chloride ions. The helices are connected by cytosolic loops and extracellular loops, that couple the nucleotide-binding domains to the transmembrane domains. 9, 18, 20 The nucleotide-binding domains consist of alpha helices and antiparallel beta sheets that form dimers creating two sites that bind and hydrolyze adenosine triphosphate (ATP) and regulate channel gating. As can be seen in Figure 2, the transmembrane domains (labeled MSD) are hydrophobic while the nuclear binding domains are hydrophilic. 13 The transmembrane domains are embedded in the membrane with the lipid tails and the nuclear binding domains are outside of the membrane interacting with the lipid head groups. The chamber formed by the two TMDs where negatively charged ions traverse is hydrophilic. 11 The R domain, which is unique to the CFTR protein, is an unstructured helical segment wedged between the two nucleotide-binding domains and connects the carboxy terminus of NBD1 to the amine terminus of TMD2. 6, 20 It is encoded by exon 13, which contains eight serine amino acids in consensus motifs. These make up the nineteen phosphorylation sites that control channel activity in the CFTR protein. 14, 20

Where can CFTR be found?

CFTR can be found in the epithelial cells of multiple organs including the lungs, liver, pancreas, digestive tract, and both males and  females reproductive organs. 18 CFTR are strongly expressed in the specialized cells called the pulmonary ionocytes of the lungs, as well as in the sebaceous and eccrine sweat glands. 19 

CFTR Mechanism

In biochemical processes, phosphorylation is the addition of a phosphate to an organic compound to make it usable. In this case two enzymes, protein kinases and phosphatases, are used to catalyze the transfer of a phosphate group. “A protein kinase catalyzes the transfer of γ-phosphate from ATP (or GTP) to its protein substrates while a protein phosphatase catalyzes the transfer of the phosphate from a phosphoprotein to a water molecule.” 4  Both enzymes catalyze opposing reactions to regulate the structure and functions of proteins. 4

In the CFTR protein, phosphorylation is a mechanism for the rapid and reversible control of channel activity. When a phosphate is added to amino acids a negative charge is added to the side chains of the amino acids. 15 The R domain in the CFTR protein is mainly phosphorylated by the enzyme, cAMP-dependent protein kinase A (PKA) and less often by the enzyme protein kinase C. 8, 9, 14 When the serine residues that make up the R domain are phosphorylated, the R domain is stabilized away from the NBD interfaces to permit conformational changes necessary for channel opening. 10, 20 ATP binds to both NBD1 and NBD2 to induce dimerization of the nucleotide-binding domains to form a closed dimer. 12, 20 ATP must bind to both NBD1 and NBD2 for the channel to open. 9 When both nucleotide-binding domains are in closed dimer formation, the pore remains connected to the cytosol through a gap between TMD helices four and six. On the extracellular side, a small opening can be seen between TMD helices one and six. 20 ATP hydrolysis and dephosphorylation of the R domain by protein phosphatases mark the closing of the CFTR protein channel. 18, 20 In this dephosphorylated state, “the R domain is wedged between TMD helices 9, 10, and 12 and extends into the cytosol between the two NBDs” in an inward-facing conformation where it acts as a steric block. 10, 20 This inhibits the nucleotide-binding domains from forming a closed dimer. 10