Endocytosis and Protein Degradation

Question 1

Why does clathrin need to bind to adapter proteins in order for the correct vesicles to be formed?

Answer 1

Adapter proteins are involved in selecting the correct cargo and membrane proteins for each vesicle.

Question 2

How does LDL receptor binding change with pH and how does that affect cholesterol absorption in the cell?

Answer 2

At physiological pH, the receptor can bind LDL proteins. The bound complex is endocytozed and the pH of the vesicle drops until the receptor undergoes conformational shift that causes the LDL to be released. LDL receptor is returned to the cell surface while free LDL is dissolved in the lysosome.

Question 3

What sort of membrane transporters must a lysosome contain?

Answer 3

Each lysosome must contain transporters for all sorts of “building block” materials it may contain. Amino acids, fatty acids, cholesterols, monosaccharides, nucleic acids

Question 4

What other process showcases the recycling of receptors?

Answer 4

The cycling of transferrin and transferrin receptor. Transferrin binds iron in blood and is then bound by a transferrin receptor. The complex is endocytosed by a clathrin coated vesicle which becomes increasingly acidic, causing the release of iron from transferrin. The transferrin and transferrin receptor are then recycled to the cell surface.

Question 5

What is unique about caveolae endocytosis/pinocytosis pathway?

Answer 5

It does not use vesicle coat proteins at all. Three genes make proteins called caveolins. Items taken in via caveolae include Cholera toxin, folic acid, albumin, and tetanus toxin.

Question 6

Four conceptual reasons why we need protein degradation

Answer 6

1) Misfolded proteins will create havoc if they are not destroyed
2) Proteins have finite lifetimes
3) Organelles get damaged, need to be removed and replaced
4) Both good and bad endocytosed materials need to be degraded

Question 7

What three structure-related processes occur in the ER during protein synthesis?

Answer 7

Co-tranlsational folding
Early post-translational modification
Quality control

Question 8

How does temperature shock affect CFTR expression in cystic fibrosis patients?

Answer 8

Major mutation in CFTR is F508, missing one amino acid, which prevents the CFTR protein from folding properly at human physiologic temperatures. Misfolding prevents it from making it to the cell surface

Question 9

What are two heat shock proteins and how do they function?

Answer 9

hsp70 - binds to exposed hydrophobic regions of misfolded proteins. Swells protein structure to encourage refolding.
hsp60 - forms large barrel around misfolded protein to prevent aggregation. Hydrophobic interior and binding sites that utilize ATP cause protein to expand and collapse into shape.

Question 10

What percentage of proteins will actually fold properly?

Answer 10

Roughly 30% will never fold properly and will be degraded.
30% will need the assistance of hsp70 and 1/3 of those (10%) will need hsp60 as well.
30-40% will naturally fold correctly.

Question 11

Process of protein folding quality control within the ER that involves glycosylation.

Answer 11

Lectin protein within the ER (CNX) binds the sugar on a target protein. Associated thio-oxireductase (ERp57) forms di-sulfide bonds. The protein is then released and if it is improperly formed it can be re-glycosylated (GT) and re-enters the cycle until it forms properly.

Question 12

What is ubiquitin and how is it utilized in protein degradation?

Answer 12

Ubiquitin is a 76 amino acid protein that is utilized in a variety of cellular processes and is highly conserved across species (74 of 76 aa match plants-humans, 76 of 76 aa match flies-humans) Proteins with a minimum of 4 ubiquitin molecules attached are degraded by proteases.

Question 13

Process of protein degradation by proteases

Answer 13

Marked proteins are placed into the ante-chambers and then fed in straight chains into the central chamber. All degradation occurs in the central chamber. Specific units within central chamber cleave after acidic, basic, or hydrophobic AAs. Expels 7-9 amino acid chunks.

Question 14

Summary of process of degradation for misfiled proteins

Answer 14

Protein folding fails in the ER
Protein exits ER through translocator protein
Sugars are removed in cytosol
Protein is poly-ubiquitinated
Protein is picked up by proteasome and degraded

Question 15

Unique properties of lysosomal degradation

Answer 15

Targeting occurs via the endocytic pathway
Monoubiquitinated transmembrane proteins may be degraded
Regulated at the multi vesicular body

Question 16

What effect might a lysosomal storage disorder have on Ebola?

Answer 16

Cells of patients with Neiman-Pick disease cannot be infected with Ebola because Ebola binds to the cholesterol transporters inside lysosomes to exit the lysosome. Neiman-Pick is a lysosomal storage disease that results from mutations in the cholesterol transporters. These same mutations prohibit Ebola from entering the cell.