11. Principles of Receptor-Mediated Endocytosis

Question 1

Define phagocytosis.

Answer 1

Internalisation of particulate matter.

Question 2

Define pinocytosis.

Answer 2

Invagination of the the plasma membrane to form a vesicle, permits uptake of extracellular solutes.

Question 3

Define endocytosis.

Answer 3

The selective internalisation of molecules into the cell by binding to specific cell surface receptors.

Question 4

How is cholesterol uptake not into a cell?

Answer 4

Using receptor mediated endocytosis.

Question 5

What is the structure of low density lipoproteins?

Answer 5

Core of esterified cholesterol esters that are covered by a phospholipid and cholesterol mono layer and contain apoprotein B.

Question 6

What are triskelions?

Answer 6

Coat structures made up of hexagons and pentagons. The hexagonal and pentagonal structures are formed from clathrin triskelions. Triskelions comprise 3 clathrin heavy chains and 3 light chains.

Question 7

How are clathrin coats formed?

Answer 7

Spontaneously.

Question 8

How are clathrin coats disassembled?

Answer 8

They’re uncalled by an ATP-dependent uncoating protein.

Question 9

What mutations affect the LDL receptor in hypercholestolaemia?

Answer 9

Non-functioning receptor so no binding of LDL but normal coated pits and internalisation.
Receptor binding normal but no internalisation even with LDL receptors distributed over whole cell surface but there is deletion of C-terminal cytoplasmic domain that prevents interaction with the clathrin clot.

Question 10

Give a brief overview of receptor mediated endocytosis.

Answer 10

LDL attaches to a receptor in a coated pit. The pit invaginates to become a coated vesicle. It is uncoated by ATP-dependent uncoating and becomes an uncoated vesicle. Uncoated vesicle fuses with the endosome, where the pH is low inside but there are no lysosomal enzymes (prelysosomal). The endosome is the compartment of uncoupling of receptor and ligand as it dissociates the receptor outside due to acidic conditions. The endosome buds off as lysosomes. The receptors bud off and find their way back to the membrane.

Question 11

What makes endosome prelysosomal?

Answer 11

They don’t have any lysosomal enzymes.

Question 12

What is the fate of the ligand and receptor in endocytosis of LDL?

Answer 12

Ligand is degraded, receptor is recycled.

Question 13

What happens to the intracellular [59Fe] as the surface concentration drops?

Answer 13

It rises.

Question 14

What happens to the protein transferrin concentration intracellularly over time?

Answer 14

It increase for a short time but then decreases as the culture medium rises. This means the protein that carries Fe ions is recycled.

Question 15

What do ferric ions bind to at pH7.0?

Answer 15

Apotranferrin to form ferrotransferrin.

Question 16

What happens to the uncoated vesicles of Fe3+?

Answer 16

They bind to endosome, much like RME of LDL.

Question 17

What is the fate of the ligand and receptor after endocytosis of transferrin?

Answer 17

Ligand is recycled and receptor is also recycled.

Question 18

What is the fate of the ligand and receptor in endocytosis of insulin?

Answer 18

Ligand is degraded and receptor is also degraded.

Question 19

What is the fate of ligands and receptors after endocytosis of immunoglobulin transcytosis?

Answer 19

Ligand is transported and receptor is also transported.

Question 20

What’s an example of receptor-mediated endocytosis Where the receptor is recycled and the ligand is degraded?

Answer 20

LDL.

Question 21

What’s an example of receptor-mediated endocytosis where the receptor is recycled and the ligand is recycled too?

Answer 21

Transferrin.

Question 22

What’s an example of receptor-mediated endocytosis where the receptor is degraded and ligand is degraded too?

Answer 22

Insulin.

Question 23

What’s an example of receptor-mediated endocytosis where the receptor is transported and the ligand is transported too?

Answer 23

Immunoglobulin A and G.

Question 24

How is RME affected in type II diabetes?

Answer 24

Raised glucose through diet, means more insulin is secreted. If insulin is raised for a prolonged period, the receptors are down regulated so there are fewer receptors. This is an insulin resistance. So there is a reduced response to insulin and lower glucose uptake. The lack of insulin triggers glucose production and further hyperglycaemia and further down regulation.

Question 25

How do membrane-enveloped viruses enter cells?

Answer 25

Using receptor-mediated endocytosis. They bind to cells by fortuitous association with cell receptors. Then enter cells via clathrin-coated pits and unfold hydrophobic domains in membrane fusion proteins in response to the acidic pH of the endosome. They use the pH to change the conformation of the protein to reveal finger like protrusions. This allows fusion into the endosome membrane and introduces the genetic material into the cytoplasm. The RNA genome is translated and reproduced and reforms on the membrane of the cell and buds off.

Question 26

Which toxins can enter by RME?

Answer 26

Cholera toxin, diphtheria toxin. Both bind to GM1 ganglioside.

Question 27

What is the journey of vesicles?

Answer 27

Bud from the donor organelle, trafficked to their destination, fuse with the recipient organelle.