Question

1a..Write a list of bullets telling the life story of the CFTR protein starting from the CFTR gene and finishing with the CFTR protein working in the membrane. Make sure to comment on the machinery needed to execute each step, any relevant signals and means of moving through and between cellular structures. Be concise and precise.

1b. Pick one class of CFTR mutations and brainstorm 2 examples of specific mutations that could fall in the class you have chosen. Many versions of correct answers are possible for each class mutations. Use your understanding of how the cell works and propose 2 mutations. Explain how they will work. For example, class I mutations is associated with no CFTR protein being made. Think how could that happen.

Answer 1

Explanations:

Cystic fibrosis is a genetic disease that predominantly affects lungs as well as digestive system. It affects cells that produce mucus, digestive juices and sweat.

1a. Story of CFTR gene conversion to protein functioning in the membrane

· CFTR (Cystic Fibrosis Trans membrane Regulator) gene present in the DNA of cell nucleus instructs to produce CFTR transmembrane proteins.

· CFTR gene is 189 kb in length that forms a CFTR glycoprotein with 1480 amino acids. The protein consists of five domains which include: six transmembrane helices (TM), two transmembrane domains (TMD) which are connected to a nucleotide binding domain (NBD) in the cytoplasm and a regulatory domain (R).

· Through the process of transcription and translation of the CFTR protein is formed. The chain form is folded into a specific folded 3-D shape with the help of chaperone molecules which is protease resistant and is transported to the Golgi. The mature folded CFTR protein moves to the cell surface, a process called as trafficking.

· Two conformations occur:

o ATP bound form where the ATP hydrolysis occurs for substrates across membrane.

o ATP free where ATP binds to each nucleotide binding domain resulting in NBD dimerization and reorganization of the transmembrane helices.

· ATP driven conformational changes in CFTR protein opens or closes the ion channel gate that allows anion (Cl−) flow through the membrane.

· A CFTR protein acts as a channel protein that transports negatively charged chloride ions (Cl−) out of a cell and positively charged sodium ions (Na+) across cell membranes that helps to maintain the precise balance of fluids in organs such as lung, liver, digestive tract and reproductive organs.

1b. Types of CFTR mutations with examples

Cystic fibrosis mutations are a recessive disease that occurs when a CFTR protein is formed abnormally. The mutation result in formation of thick, sticky mucus rather than thin, watery mucus.

There are six types of cystic fibrosis mutations,

o Class I – Protein synthesis mutation

o Class II – Protein maturation mutation.

o Class III – Gated channel regulation mutation.

o Class IV – Chloride conductance mutation.

o Class V – Affected functioning CFTR at the apical membrane

o Class VI – Mutations affecting the regulation of other channels.

Class I mutation is a nonsense mutation where CFTR protein formation stops abruptly. The cell stops finishing formation of protein and no functional protein is formed.

Class IV mutation is chloride conductance is prevented where the 3D shape of protein does not function properly and so movement of chloride ions through the channel is prevented. This is known as conductance mutation.