Lysosomes and Endosomes

Question 1

What is the name for the ‘digestive’ organelle in eukaryotes?

Answer 1

Lysosomes

Question 2

True or False: lysosomes can only degrade small proteins and lipids.

Answer 2

False. They degrade all sorts of macro molecules and organelles.

Question 3

What happens to the leftover products of degradation from the lysosome?

Answer 3

They are transported to the cytosol and reused.

Question 4

What role do ATPase proton pumps play in lysosome degradation?

Answer 4

It maintains the low pH needed for lysosomes to function by pumping H+.

Question 5

True or False: lysosomal proteins are O-glycosylated.

Answer 5

False. They are N-glycosylated, and thus start the process from the ER.

Question 6

What is the purpose of a mannose-6-phosphate (M6P) on a protein?

Answer 6

It acts as a lysosomal targeting signal when it is transported from the trans Golgi network

Question 7

True or False: M6P-bearing lysosomal proteins are transported in clathrin-coated vesicles.

Answer 7

True.

Question 8

Vesicles need what type of protein in order to be coated in clathrin?

Answer 8

GGA adaptor coat proteins.

Question 9

Name the protein that mediates recruitment of GGA adaptors to the TGN.

Answer 9

Arf1-GTP. It acts similarly to Sar1-GTP.

Question 10

What is the structure of clathrin?

Answer 10

A molecule consisting of 3 light chain polypeptides and 3 heavy chain polypeptides. They combine to form a 3 legged structure (triskelions).

Question 11

Clathrin transitioning from a hexagon to a pentagon serves what purpose in vesicle formation?

Answer 11

It acts as the driving force for the membrane to curve when forming the vesicle.

Question 12

True or False: dynamin pinches off clathrin-coated vesicles.

Answer 12

True. It’s a GTP-binding protein that surrounds the stalk. GTP hydrolysis pinches off the vesicle.

Question 13

What happens to the clathrin coat after pinching off of a vesicle?

Answer 13

It disassembles into the cytosol and returns to the TGN for another vesicle.

Question 14

True or False: a clathrin-coated vesicle must first fuse with an endosome before getting ‘eaten’ by lysosomes.

Answer 14

True. The material from the late endosome will be consumed by the lysosome along with the cargo.

Question 15

True or False: M6P receptors are ‘eaten’ by lysosomes along with the endosome.

Answer 15

False. They are recycled in order to be used for another round of trafficking.

Question 16

Name the two processes for materials to enter a cell.

Answer 16

Endocytosis (selective internalization) and Phagocytosis (cellular eating)

Question 17

What is another name for pinocytosis?

Answer 17

Bulk-phase endocytosis or cellular drinking.

Question 18

Which type of adaptor protein is required for clathrin coating on vesicles undergoing endocytosis?

Answer 18

AP2 adaptors. They work similarly to GGA adaptors.

Question 19

What is the role of an early endosome?

Answer 19

To sort and recycle extra-cellular materials via endocytosis.

Question 20

What happens to receptors that early endosomes recycle?

Answer 20

They are carried by vesicles back to the plasma membrane.

Question 21

What happens to any non-recyclable material in the early endosome?

Answer 21

A compartment detaches and matures into a late endosome. Fragments (MVBs) are then eaten by the lysosomes.