Endocytosis and Protein Degradation

Question 1

Routes for small volume endocytosis (2)

Answer 1

Phagocytosis: engulfing of material followed by delivery to lysosome.
Pinocytosis: small volume endocytosis mediated by ligand/receptor uptake via clathrin or caveolae.

Question 2

Clathrin

Answer 2

Receptor binds cargo, R recognized by cytoplasmic adaptor, clathrin associates, dynamin pinches off vesicle.

Question 3

Quality control in ER

Answer 3

30% of protein gets misfolded and degraded because they will otherwise aggregate
ER provides oxidation environment for folding and oligomeric assembly

Question 4

Hsp 60 and Hsp 70

Answer 4

Hsp60: ATP-dependent, places protein in isolation chamber, “pulls” protein to have it “spring back” into correct conformation.
Hsp70 (BiP): Binds to hydrophobic regions of protein during folding

Question 5

ERp57

Answer 5

thiol oxidoreductase, assists with disulphide bond formation. Associates with Calnexin and Calreticulin

Question 6

Calnexin and Calreticulin

Answer 6

Calnexin (trasmembrane) and Calreticulin: ER proteins that bind glucose residues added to proteins in ER and promote interactions with folding proteins. Glucose cleaved with dissociation.

Question 7

Glycosyl Transferase

Answer 7

Adds glucose to UGGT sequence of protein (marker for misfolding). Causes association with Calnexin/Calreticulin to promote folding assistance.

Question 8

Proteasome targeting

Answer 8

Requires 4+ ubiquitin chain on lysine residue

Question 9

Addition of Ubiquitin process

Answer 9

3 steps:

1. E1 (activation) adds Ubq to E2 protein
2. E2 (conjugation) carries Ubq and associates with E3
3. E3 (ligation) recognizes protein and transfers Ubq from E2 to lysine residue or other Ubq

Question 10

Protein at Proteasome

Answer 10

ATP utilized to unfold protein. Protein is fed into antechamber of proteasome composed of alpha subunits.
Protein enters central chamber of beta subunits which cleave protein into 7-9 AA pieces.
No ATP is required for cleavage.

Question 11

B1 subunits

Answer 11

caspase-like activity for acidic AAs

Question 12

B2 subunits

Answer 12

trypsin-like activity for basic AAs

Question 13

B3 subuints

Answer 13

chymotrypsin-like activity for hydrophobic AAs

Question 14

Lysosome function

Answer 14

Degrades endocytosed material and some intracellular components via enzymes functioning at low pH within vesicle. Low pH achieved through proton pump.