M&R 6.2 - Receptor Mediated Endocytosis

Question 1

What is receptor mediated endocytosis?

Answer 1

The selective internalisation of molecules due to activation of specific receptors

Question 2

What is the mechanism for endocytosis?

Answer 2

• Cell surface membrane of the vesicle fuses with the cell surface membrane of the cell
• Both membranes merge therefore releasing the molecule

Question 3

Describe the structure of a low density lipoprotein

Answer 3

• Hydrophobic core made from: Triacylglycerols and cholesterol esters
• Surface coat made from: phospholipids, cholesterol and Apoprotein B

Question 4

What kind of receptor do cells that require cholesterol synthesis? Where are these receptors located?

Answer 4

• LDL receptors that specifically recognise apoprotein B

- Located directly over Clathrin Coated Pits (cover ~2% of the cell surface)

Question 5

What happens when LDL binds to the receptors?

Answer 5

• Pit invaginates which encases the receptor and the LDL
• Then uncoats using energy from ATP
• Fuses with large, smooth vesicles (endosomes)

Question 6

What is an endosome?

Answer 6

A compartment for the uncoupling of the receptor and the ligand (CURL)

Question 7

What is the significance of the endosome having a lower pH than the cytoplasm? How is this maintained

Answer 7

• Decreases the affinity of the LDL receptor for the LDL which causes them to dissociate
• Maintained by the ATP dependent H+ pump

Question 8

What happens to the receptor once it has dissociated from the LDL?

Answer 8

• Moves into a separate area of the endosome
• Buds off
• Recycled to the plasma membrane

Question 9

What happens to the LDL once it has dissociated from the receptor?

Answer 9

• Endosome containing the LDL fuses with a lysosome
• Lysosome hydrolyses LDL
• Releases free cholesterol to the cell (along with esters)

Question 10

Describe 3 mutations that can cause hypercholesterolaemia

Answer 10

• LDL receptors aren’t above clathrin coated pits (cover entire surface rather than 2%) due to a deletion at the c-terminal - causes no interaction
• Mutation to receptor binding site = no LDL uptake
• Receptor deficiency due to mutation = less LDL uptake

Question 11

How is transferrin formed in circulation?

Answer 11

• 2x Fe3+ bind to Apotransferrin = Transferrin

Question 12

How is transferrin brought into the cell?

Answer 12

• Binds to a high affinity transferrin receptor on CSM
• Take in similarly to LDL (coated pit, then uncoated and fuses with endosome)
• pH competes for binding with transferrin which causes Fe3+ to be released (used with Hb)
• Apoferrin has a high affinity for the receptor at a lower pH so stays bound

Question 13

What happens to the transferrin receptor after dissociation?

Answer 13

• Recycled to the plasma membrane

- Apoferrin is released when pH returns to 7

Question 14

How does RME differ with insulin receptors? Why?

Answer 14

• The receptors ONLY congregate over the Clathrin coated ONCE THE AGONIST HAS BOUND
• Binding causes a conformational change that allows receptor to be recognised by the pit

Question 15

What happens to the insulin-receptor complex when in the endosome?

Answer 15

• Remains bound

- Targeted by lysosomes for degradation (receptor and insulin are degraded so receptor isn’t recycled)

Question 16

What is the significance of uptake of occupied insulin receptors?

Answer 16

• Reduces number of insulin receptors on CSM

- Desensitised to the constant high concentration of insulin in circulation

Question 17

Describe the development of type 2 diabetes

Answer 17

• Increased blood glucose levels means more insulin is secreted by beta cells
• Causes a down regulation of receptors (= resistance) which decreases the response to insulin
• Decreases glucose uptake so more is produced by the liver = HYPERGLYCAEMIA
• Less glucose transport, more insulin resistance
• Coupled with impaired beta cell function
• Resistance + deficiency = Type 2 diabetes

Question 18

What is transcytosis?

Answer 18

Transport of ligand + receptor ACROSS THE CELL

Question 19

Give 2 examples of transcytosis

Answer 19

• Maternal immunoglobulin to the foetus via the placenta

- Immunoglobulin A (IgA) from the circulation to bile in the liver

Question 20

What is the affect of pH on IgA

Answer 20

• Has no effect

- Budding off of vesicle contains both receptor & ligand

Question 21

What happens during the transport of IgA to the bile?

Answer 21

• Happens in a transfer vesicle (still bound)

- Proteolytic cleavage of the receptor leaves a small amount of the receptor bound to the ligand (secretory component)

Question 22

Which method is used by all mechanisms of RME?

Answer 22

• CURL

- Compartment for the uncoupling of the receptor and the ligand (endosome)

Question 23

How can RME be exploited by viruses/toxins?

Answer 23

• Binds to receptor and is internalised in the same way as a metabolite
• Lower pH of the endosome causes a conformational change of enveloping proteins therefore exposing hydrophobic domains
• Viral membrane can fuse with the endosomal membrane

Question 24

What happens after the fusion of the viral membrane with the endosomal membrane?

Answer 24

• Virus releases its own RNA into the cell

- Cell replicates itself and viral RNA to form a new virus

Question 25

Give an example of TWO toxins that can exploit RME

Answer 25

• Cholera toxin
• Diptheria toxin

(Both bind to GM1 ganglioside)