Question

Describe the life cycle of a healthy CFTR protein molecule, including:

• translation at ribosomes
• folding and membrane insertion in the endoplasmic reticulum
• trafficking to the Golgi apparatus via vesicles, and subsequent transport to the cell membrane by secretory vesicles
• internalisation through the endosomal system
• degradation in the lysosome

Answer 1

Cystic fibrosis transmembrane conductance regulator (CFTR)

CFTR gene, encodes for CFTR protein which is a chloride selective anion channel expressed by cells producing saliva, tears, mucus, digestive enzymes and sweat etc. CFTR protein regulates the in and out movement of chloride ions thereby maintaining the osmotic balance and controlling the movement of water in cells and tissues and thus, helps in maintaining free flowing mucus without any lumps and coagulation. The mucus lubricates the lining of alimentary canal, nasal airway, and many other organs. In this way CFTR protein works. CFTR also helps in movement of other positive ions such as potassium ion and sodium ion through transporters.

Lifecycle

Ribosomes- CFTR gene encodes for a 1480 amino acid protein having a transmembrane domain with 6 helices and an ATP binding domain (NBD) attached through a hydrophilic regulatory domain. During co-translation, transporter translocon Sec 61 integrates the CFTR polypeptide into the endoplasmic reticulum.

In the ER, there is N-linked glycosylation of extracellular loop of CFTR. After addition of two core glycosylated groups to CFTR, chaperon proteins Hsp70 and calnexin folds the polypeptide correctly. The protein becomes protease resistant and is transported to Golgi apparatus.

In Golgi apparatus, in trans-Golgi network the glycosylated groups added on the CFTR in ER are further modified into complex carbohydrates and the protein forms a mature CFTR protein of 170 kDa. The mature CFTR is now transported to cell membrane expressing CFTR protein.

In Plasma membrane, the CFTR undergoes rounds and rounds of endocytosis with the help of clathrin coated vesicles. There is an internalization signal (Tyrosine based) at the C-terminal end of CFTR, which helps in endocytosis internalization. The recycling of CFTR is controlled by the concentration of cAMP. Increase in cAMP concentration increases the amount of CFTR expressed on the cell membrane.

Lysosomal degradation – the half life of a mature CFTR protein is 16 hours, thereafter signal is generated for degradation of CFTR proteins. The CFTR is then targeted to the lysosomes for degradation. There is an interaction between the AP-2 adaptor of clathrin coated vesicles and YDSI motif of CFTR. The endocytosis of CFTR requires motor protein myosin-VI and Rab GTPase. Rab4 mediates the exit of CFTR from endosomes and returning back to plasma membrane. While, Rab7 mediates CFTR degradation by increasing trafficking between late endosome and lysosomes

Misfolded CFTR proteins degradation- During folding of CFTR protein in the ER, many times only 25% protein folds in a way to gain the protease resistance while remaining CFTR protein cannot fold in that form and is subjected to ubiquitin dependent degradation by cytosolic 26S proteasome.