Endocytosis and protein degradation

Question 1

What carries out phagocytosis?

Answer 1

specialized cells (e.g. macrophages and neutrophils)

Question 2

What is pinocytosis?

Answer 2

• involves small volumes, usually is associated with specific uptake of ligands and receptors

Question 3

Vesicles are typically formed by what two mechanisms?

Answer 3

clathrin coat proteins or caveolae

Question 4

What is the clathrin coat pathway?

Answer 4

• cargo molecules bind to a transmembrane receptor (has a short motif on the cytoplasmic domain that is recognized and binds to an adaptor protein, enable a clathrin coat to assemble on the budding vesicle)
• vesicle remains attached to the membrane until dyamin pinches it off
• the adaptor complex and clathrin rapidly dissociate from the endocytose vesicle

Question 5

What is the general scheme of the LDL receptor pathway?

Answer 5

• cycles between the cell surface and the lysosome, bringing LDL particles to the lysosome where they are degraded

Question 6

How does LDLR bring LDL to the lysosome for degradation?

Answer 6

• LDLRs are clustered in membrane pits where adaptor protein complex AP2 binds to the receptor and clathrin
• upon binding to LDL, clathrin assembles over the surface of the nascent vesicle, dyamin pinches off the neck of the membrane, creating a clathrin-coated vesicle
• shortly after the clathrin coat dissembles, this vesicle (early endosome) moves to the next compartment (late endosome) with low pH
• LDLR is recycle and LDL is broken down into cholesterol, fatty acids, and amino acids

Question 7

What are caveolae?

Answer 7

small endocytic vesicles that form without coat proteins

- require the structural protein cave-in (scaffolding protein)

Question 8

What are the three major protein degradation pathways?

Answer 8

• ubiquitin-proteasome system (UPS)
• lysosome (endocytosis pathway)
• autophagy

Question 9

What is ubiquitin-proteasome system (UPS) responsible for?

Answer 9

rapid degradation of proteins when fast adaption is needed

Question 10

What is autophagy mainly involved in?

Answer 10

degradation of long-lived proteins and entire organelles

Question 11

How does Hsp70 work?

Answer 11

helps fold a protein by binding to exposed hydrophobic patches in incompletely folded proteins and prevents aggregation

Question 12

How does Hsp60 work?

Answer 12

helps fold a protein by forming an elaborate, large, barrel-shaped structure that acts as an isolated chamber (prevents aggregation)

Question 13

How does the ER establish an environment conducive for protein folding?

Answer 13

• contains folding enzymes like ERp57 (allows formation of disulfide bonds)
• contains molecular chaperones
• there are folding sensors that monitor unfolded proteins and hold them in the ER until they fold properly or are shuttled to a degradation pathway

Question 14

What is BiP?

Answer 14

an Hsp70-like protein that uses ATP to help proteins fold

Question 15

What do calnexin and calreticulin do?

Answer 15

• they are lectins (bind to sugars) that monitor unfolded proteins and hold them in the ER until they fold properly or are shuttled to a degradation pathway
• bind to the oligosaccharide chain if there is a glucose
• when the glucose is removed by glucosidase, the protein is released by calnexin and calreticulin and leaves the ER
• if not correctly folded, it is recognized and bound by a glucosyltransferase that puts the glucose back on the sugar chain

Question 16

What is the ubiquitin-proteasome degradation system?

Answer 16

proteins retrotranslocated out of the ER and have multiple ubiquitin molecules are targeted to the proteasome, a huge complex of proteins that unwinds the misfiled protein and feeds it into a compartment that cuts the protein into short peptides

Question 17

Where does cleavage occur in the proteasome? Does it require energy?

Answer 17

the central cylinder; yes, it uses ATP to unfold the protein

Question 18

What does the attachment of ubiquitin to a protein require?

Answer 18

Three ligase enzymes: E1, E2, and E3

• E1 binds and activates ubiquitin
• transfered to E2
• E3 attaches a string of additional ubiquitins to the first one

Question 19

What does mono- and multiubiquitins do?

Answer 19

used as regulatory signals

Question 20

What does the lysosome do?

Answer 20

primarily degrade extracellular materials taken up by endocytosis and some intracellular components (lumen is acidic, pH 5)