MCB Lecture 33 Endocytic Pathway

Question 0

What types of enzymes are found in lysosomes?

Where do they come from?

Answer 0

Hydrolysis enzymes

They are synthesised in the ER, processed in the Golgi, then transported to the lysosome

Question 1

What is the function of the early endosome?

Answer 1

It is the sorting station for materials taken into the cell by phagocytosis

Question 2

What is a lysosome?

What are the features?

Answer 2

A lysosome is an organelle in which materials are digested
Low pH
Hydrolytic enzymes

Question 3

How is pH maintained in a lysosome?

Answer 3

There is a proton pump

Question 4

What are the two ways that hydrolytic enzymes are prevented from randomly digesting the contents of a cell?

Answer 4

1. Sequestered in the lysosome

2. pH activated. If they do end up in the cytosol, the pH will be such that the enzymes are not active

Question 5

What other bodies go on to form a lysosome?

Answer 5

Phagosome
Autophagosome
Endosome

Question 6

Endosomes mature into …

Answer 6

Late endosome, Lysosomes

Question 7

What is an endolysosome?

Answer 7

This is when a lysosome fuses with an endosome

Question 8

Describe the synthesis and targeting of lysosomal proteins to the lysosome

Answer 8

1. Synthesised in ER
2. Transported via a COPII coated vesicle to the Golgi
3. Processed in the Golgi:
   a. N-acetyl glucosamine added by Glucosamine transferase
   b. N-acetyl glucosamine removed, leaving a phosphate on the mannose
   c. M6P
4. Protein with Mannose-6-Phosphate produced at end of Golgi
5. M6P binds to a M6P receptor on Golgi membrane
6. Clathrin coated out forms
7. Vesicle targeted to an early endosome
8. Cargo released and enzymes activated due to low pH
9. Vesicle with empty receptors is recycled back to the Golgi

Question 9

Which enzyme adds a phosphate to the mannose on a hydrolysis enzyme? What is the function of this?

Answer 9

Glucosamine transferase

The M6P targets it to a lysosome.

Question 10

What are two lysosomal storage diseases? Describe them

Answer 10

1. Hurler’s Disease: mutation in an enzyme that breaks down GAG
2. I-Cell disease: defective glucosamine transferase

The effect is that materials aren’t digested

Question 11

Briefly describe the endocytic pathway

Answer 11

Materials are taken in from outside the cell when the membrane extends around the material, pinching off to form a phagosome. This early endosome, phagosome or auto phagosome matures to become a lysosome

Question 12

Compare pino and phagocytosis

Answer 12

Phagocytosis: large molecules, bacteria endocytosed
Pinocytosis: smaller molecules or fluids endocytosed

Question 13

Describe how LDL is taken into cells

Answer 13

1. There are LDL Receptors on the outside of a cell to which LDL binds
2. A clathrin coated pit forms, with the loaded LDL receptors being concentrated in the vesicle

Question 14

Describe what happens when there is a mutation in the LDL receptors

Answer 14

When there is a mutation, the loaded LDL receptors are not concentrated in the clathrin coated pit, an so LDL is only randomly taken into the cell. This leads to high concentration of LDL in the blood, and formation of atherosclerosis

Question 15

What are fusion of endosomes, and how does it occur?

Answer 15

This is when endosomes fuse together due to Ran-GTP that brings about fusion.
This evolves larger and larger endosomes

Question 16

How is the surface area of early endosomes decreased?

Answer 16

Invagination a of the membrane of endosomes to form multi vesicular bodies

Question 17

Describe the differences between exocytosis and the transport pathway

Answer 17

Exocytosis: the protein product moves from Golgi out of the cell
Transport: COPII

Question 18

Describe cargo concentration, and why it occurs

Answer 18

This is when secretory vesicles fuse together, so that the protein product is concentrated into fewer vesicles

Cell membrane is then removed and recycled back to the Golgi or ER. This makes the secretory vesicles even more concentrated

Question 19

Describe how secretory vesicle fusion occurs in neurons

Answer 19

Kiss and Run

The secretory vesicle doesn’t completely fuse with the membrane.
In this way, the vesicle membrane is very rapidly recycled back to create more vesicles, and the membrane does not become too large too suddenly

Question 20

Which process causes an endosome to mature into a lysosome?

Answer 20

Fusion with vesicles from the Golgi containing hydrolytic enzymes

Question 21

How are hydrolytic enzymes recognised, so that they may have the phosphate added to the mannose?

Answer 21

They have a signal patch that is recognised

Question 22

Differentiate between constitutive and mediated exocytosis

Answer 22

Constitutive: the vesicles will fuse with the cell membrane under normal conditions

Mediated: there is a requirement of a signal before vesicles will fuse with the membrane. The vesicles just hang out inside the cell (think of synaptic vesicles)

Question 23

Which vesicle coat protein recycles membrane from concentrated secretory vesicles?

Answer 23

Clathrin