Protein misfolding

Question 1

How does a mutation in a1-antitrypsin cause disruption of the protein?

Answer 1

A (Glu342Lys) Glutamic acid –> lysine mutation at position 342 causes the formation of an unstable intermediate.
Mutation occurs at the head of the fifth strand of the beta-sheet A at the base of the mobile reactive loop and causes the opening of the beta-sheet and leads it unable to form a salt-bridge. The beta-sheet can now accept the loop of another molecule to become a dimer, and the process continues, leading to polymer formation and then the formation of aggregates in the ER of liver cells.

Question 2

What is the normal structure of a1-antitrypsin and what is its function?

Answer 2

Normal structure is composed of 3 beta sheets and 9 alpha helices with reactive loops in the centre of the molecule. These loops act as a pseudo substrate for neutrophil elastases, which cleave the loop and result in the incorporation of the NE, to form a forth strand in the beta-sheet A.

Question 3

What is the structure of the CFTR protein?

Answer 3

Each CFTR protein is composed of two membrane spanning domains (MSD1 and MSD2), two nucleotide binding domains (NBD1 and NBD2), which participate in ATP binding and hydrolysis and an regulatory domain (R), whose phosphorylation regulates channel gating.

Question 4

How is native CFTR folded?

Answer 4

In native CFTR, phenylalanine 508 is located on the N-terminal of NBD1 and interfaces with CL4 and CL1 in MSD2 and MSD1. NBD2 associates with CL2 and CL3 of MSD1 and MSD2.

Question 5

What happens when CFTR misfolds?

Answer 5

ΔF508 causes kinetic and thermodynamic changes to NBD1. Impaired assembly of the interface between NBD1 and MSD1/MSD2.
ΔF508 destabilises the conformations of MSD1, MSD2 and NBD2.
Non-native ΔF508 CFTR cannot leave the ER and if it does, it has limited metabolic stability at the plasma membrane.