Week 7: Friday

Question 1

Cause of familial hypercholesterolemia

Answer 1

Mutations in the LDL receptor

Question 2

What does the cytoplasmic tail of the LDL receptor interact with?

Answer 2

The cytoplasmic tail of the LDL receptor contains an endocytosis signal.
It binds to an adaptin in clathrin-coated pits.

Question 3

ARH

Answer 3

Autosomal recessive hypercholesterolemia. This is the adaptin that binds the receptor tail.

Question 4

Receptor mediated endocytosis

Answer 4

The process by which ARH and AP-2 work together ot endocytose LDL into CCVs.

Question 5

Three types of LDL mutants

Answer 5

1. LDL receptor mutants that did not fold properly and were retained in the ER.
2. LDL receptor mutants that were transported normally to the plasma membrane, but could not bind LDL (binding site mutants).
3. The third class was represented by a single patient “JD”, whose LDL receptor was transported normally to the plasma membrane, bound LDL normally, but failed to get endocytosed.

Question 6

How is LDL internalized?

Answer 6

LDL is internalized by receptor mediated endocytosis.

Question 7

Where is the LDL receptor with bound cargo transported to?

Answer 7

An early endosome.

Question 8

Effect of acidic pH on the LDL receptor

Answer 8

The acidic pH of the early endosome causes receptor dissociation and LDL is dropped off in the lumen.

Question 9

What happens to the protein component of LDL?

Answer 9

The protein component of LDL is degraded, cholesterol is transported out of the lysosome and sent to the ER.

Question 10

Herceptin

Answer 10

Herceptin is a monoclonal antibody that recognizes the human epidermal growth factor receptor 2 (HER2) and is used to treat women with HER2-positive breast cancer.

Question 11

What type of receptor is EGFR?

Answer 11

A tyrosine kinase

Question 12

What happens when EGFR binds to EGF?

Answer 12

When EGFR binds EGF the receptor dimerizes, becomes activated and phosphorylates itself.

Question 13

What happens after EGFR is phosphorylated?

Answer 13

The phosphorylated receptor recruits a RasGEF (Grb2-SOS) to the membrane that activates Ras (RasGDPRasGTP), which activates a cascade of protein kinases. EGFR also activates PI3K (both directly and because RasGTP stimulates PI3K activity). The downstream kinases drive gene expression changes that promote cell division.

Question 14

Modification made to activated EGR receptor and effect

Answer 14

Activated EGR receptor is ubiquitinated and this modification serves as an endocytosis signal (recognized by a co-adaptor with AP-2

Question 15

Fate of ubiquitinated plasma membrane proteins (such as the EGF receptor)

Answer 15

Ubiquitinated plasma membrane proteins (such as the EGF receptor) are endocytosed and sorted into the multivesicular body.

Question 16

Two roles of ubiquitin

Answer 16

Ubiquitin can serve as both an endocytosis signal and MVB sorting signal

Question 17

Effect of a failure to sort EGFR into the lumenal vesicles

Answer 17

A failure to sort EGFR into the lumenal vesicles allows recycling of the active receptor back to the plasma membrane (a pro-oncogenic situtation).

Question 18

MVB pathway

Answer 18

Vesicles bud into the lumen of the organelle instead of budding into the cytosol.

Question 19

How does HIV interact with the ESCRT complex?

Answer 19

HIV borrows the MVB machinery (ESCRT complex) in order to bud viral particles from the plasma membrane

Question 20

Structure of ESCRT III

Answer 20

ESCRT III forms spiral structures that somehow forces the membrane to bud outwards.

Question 21

How do many viruses enter the cell?

Answer 21

Many viruses, like influenza H1N1, use receptor-mediated endocytosis to enter cells

Question 22

How is the flu virus internalized into the cell?

Answer 22

• The flu virus binds to cell surface glycoproteins bearing sialic acid on their N-linked oligos.
• The virus is internalized by endocytosis.
• The viral coats will then fuse with the endosome membrane to deposit the viral genome into the cytosol.

Question 23

What type of process makes synaptic vesicles?

Answer 23

Synaptic vesicles are made through a clathrin-mediated endocytosis

Question 24

How are synaptic vesicles regenerated?

Answer 24

Vesicles are regenerated by clathrin and refilled by the combined action of the V-ATPase and neurotransmitter/H+ antiporter

Question 25

Kiss and run hypothesis

Answer 25

The synaptic vesicle may not completely fuse to the plasma membrane – In this hypothesis, the fusion pore opens transiently to allow neurotransmitter release and then close back up and the vesicle is release back to the cytosol.

Question 26

What type of protein plays a role in pinching off synaptic vesicles?

Answer 26

Dynamin