Lecture 18 - Intracellular Vesicular Transport II

Question 1

What is a lysosome?

Answer 1

• compartment containing hydrolytic enzymes
• intracellular digestion of macromolecules
• acidic internal environment pH ~5
• vacuolar ATPase maintains pH
• derived from late endosome

Question 2

What pathways lead to the lysosome?

Answer 2

• phagocytosis
• endocytosis
• autophagy

Question 3

What is autophagy?

Answer 3

• damaged organelles are surrounded by a membrane and are isolated creating an autophagosome
• autophagosome fuses with lysosome and damaged organelle is degraded

Question 4

How are lysosomal proteins formed and transported to the lysosome?

Answer 4

• co-translationally transported into RER
• transported to TGN via Golgi, while in Golgi mannose-6-phosphate (M6P) is attached to the protein as a sorting signal by GlcNAc phosphotransferase
• M6P receptors bind M6P and facilitate formation of clathrin coated vesicles
• vesicle merges with endosome
• endosome fuses with lysosome

Question 5

What is GlcNAc phosphotransferase?

Answer 5

-protein responsible for phosphorylating mannose to make the signaling sequence for lysosomal proteins

Question 6

Hurlers disease and inclusion cell disease

Answer 6

• Hurlers Disease-Mutation in the enzyme required to break down GAG chains.
• Inclusion cell disease-ALL of the lysomal hydrolases are missing in many cell types. Undigested substrates accumulate as inclusions.

Question 7

What is endocytosis?

Answer 7

• uptake of macromolecules from exteriors

- invagination of cell membrane to form a vesicle

Question 8

What are the different types of endocytosis?

Answer 8

Pinocytosis:

• small particles
• pinocytic vesicles

Phagocytosis:

• large particles
• phagosomes

Question 9

What is receptor-mediated endocytosis?

Answer 9

• import of select molecules though endocytosis
• achieved using receptors for specific substance in clathrin coated pits

Used in cholesterol uptake

Question 10

What are the possible outcomes for a endocytosis receptor protein?

Answer 10

Degradation

Recycling
-return to original membrane region by recycling transport vesicle

Transcytosis
-insertion in opposition site of cell membrane

Question 11

What is the mechanism through which phagocytosis occurs?

Answer 11

• antibodies or other substances target a body to be phagocytized
• receptors on macrophage/neutrophil trigger formation of pseudopodium by actin fiber rearrangement when bound
• pseudopods envelop target body and form a phagosome
• phagosome is directed to lysosome by phosphoinositide signaling

Question 12

What are the two processes by which pinocytosis occurs?

Answer 12

Clathrin coated pits:
-membrane invaginates and pinches off

Caveolae:

• caveolae; flask shaped invaginations caused by high presence of cholesterol and glycosphingolipids
• structural protein is caveolin
• pinch of with dynamin
• from caveosome which does not fuse with lysosome

Question 13

What is exocytosis?

Answer 13

• movement of transport vesicles from TGN to cell membrane
• membrane proteins in vesicle incorporate into cell membrane
• soluble proteins are secreted

Question 14

What are the two types of exocytosis?

Answer 14

Constitutive:
-occurs continuously

Regulated:
-signal causes fusion of transport vesicle and is required for exocytosis