Protein Trafficking from the Golgi Apparatus

Question 1

What is Constitutive secretion ?

Answer 1

Vesicles move directly from the Golgi to the plasma membrane

Question 2

What is Regulated secretion?

Answer 2

Fusion of vesicles with the plasma membrane depends on a signal

Question 3

What is an example of Constitutive secretion ?

Answer 3

Exocytosis (process of moving materials from within a cell to the exterior of the cell)

Question 4

What is an example of Regulated secretion and explain ?

Answer 4

The release of insulin.

• Insulin secretion in beta cells of pancreas
• Triggered by rise in blood glucose
• Uptake of glucose by GLUT2 transporter
• Glycolysis leads to rise in ATP:ADP ratio
• This inactivates the potassium channel that depolarizes the membrane
• …causing the calcium channel to open
• Rise in Ca2+ causes release of insulin from vesicles
• SNAREs controlled by Ca2+ sensor
• neurons: ‘similar’

Question 5

What is Lysosome and it’s function ?

Answer 5

• membrane-bound sacs of hydrolytic enzymes
• digestion of extracellular material (endocytosis and phagocytosis)
• and ‘worn out’ cellular organelles (autophagy)

Question 6

Is the pH low or high for lysosome and why ?

Answer 6

• Low pH from an ATP-driven pump of H+ into the lumen

- Collection of hydrolytic enzymes active in acidic conditions

Question 7

What does the lysosome membrane contain ?

Answer 7

• Membrane contains transporters:
  transfer sugars, amino acids, nucleotides etc. to the cytoplasm
• All proteins delivered via ER and Golgi

Question 8

Why are Lysosomes dangerous ?

Answer 8

• Made as an inactive precursor with ‘extra’ peptide
• ‘Extra’ peptide cleaved off in Lysosome
• … and a sorting problem

Question 9

What are sorting (or signal) patches ?

Answer 9

They are a specific 3D arrangement on the surface of a folded protein similar to active sites in enzymes

Question 10

Cell surface to lysosomes; Endocytosis ?

Answer 10

Two examples:

• Phagocytosis (=cell eating): ingestion of large particles such as microorganisms
• Receptor-mediated endocytosis: specific cargo taken up by a transmembrane receptor

Question 11

What are macrophages ?

Answer 11

• macrophages (white blood cell type) defend against infection by ingesting invading microorganisms
• particle ‘swallowed’
• digested by lysosomal enzymes

Question 12

Explain the ‘Pinching off’: Dynamin and GTP ?

Answer 12

• vesicle formed: Dynamin polymerizes around neck
• hydrolysis of GTP drives conformational change
• vesicle separates from membrane

Question 13

Explain the first steps of receptor-mediated endocytosis ?

Answer 13

• Specific molecules taken up from extracellular fluid- – Bind to cell-surface receptors
• Receptors incorporated into clathrin-coated vesicles

Question 14

Receptor Mediated Endocytosis: Adaptins

Answer 14

• Adaptin complexes
• bind to ‘signal’ in cytoplasmic tail of receptor
• ……and to clathrin
• hence selective sorting into clathrin-coated vesicle

Question 15

What happens in the endosome ?

Answer 15

• first destination for endocytic vesicles
• sorting station for the endocytic pathway
• some molecules sent to lysosome, some recycled
• sorting by pH…….

Question 16

Where next from endosome depends on ?

Answer 16

Whether the ligand and receptor within the membrane of the vesicle endocytosed stay associated

Question 17

If receptor bound for recycling to plasma membrane it ?

Answer 17

It will go there

Question 18

If receptor bound to transfer to other edge of cell then they will ?

Answer 18

Transcytose

Question 19

If ligand and receptor do not remain bound (due to acidic nature of endosome – then?

Answer 19

It will go to lysosome for degradation

Question 20

Examples of Receptor-mediated endocytosis ?

Answer 20

1. LDL receptor
2. Transferrin receptor
3. EGF receptor

Question 21

Briefly explain Cholesterol uptake ?

Answer 21

• Low Density Lipoprotein ( LDL): ‘bad cholesterol’

- Lipoprotein particles : ‘packages’ of lipids and cholesterol bound by Apoprotein B-100

Question 22

Explain the LDL receptor?

Answer 22

• Transmembrane protein
• Binds to LDL outside cell and adaptins inside cell
• Adaptins bind Clathrin,
• LDL is internalized in coated vesicles
• Delivered to lysosomes

Question 23

Explain the Transferrin cycle?

Answer 23

Aim: get iron into the cell
3 components:
- Fe3+ ions, 
- transferrin 
- transferrin receptor 

- Iron-dependent binding:
Transferrin to receptor
- pH-dependent binding:
Fe3+ to transferrin,
tranferrin to receptor
- Endosomes are acidic
- Transferrin and receptor recycle 
to surface
- Nothing goes to lysosome

Question 24

Explain the EGF receptor?

Answer 24

• ‘Epidermal’ growth factor
• EGF binds
• Receptor becomes dimer
• Tyrosine kinase phosphorylates itself
• …and other things
• Cells proliferate

Question 25

What is the ‘on’ switch and the ‘off’ switch in the EGF receptor ?

Answer 25

• Growth factor = ‘on’ switch for cell proliferation
• Must have an ‘off’ switch
• ‘Off’ switch = endocytosis of EGF and receptor to lysosome

Question 26

Sorting in Endocytosis ?

Answer 26

• The ‘multi-vesicular body’
• Membranes within membranes
• Budding the ‘wrong’ way:
  cytoplasm to interior
• ‘Reverse’ coat in cytoplasm
• Escrt complex
• Internal vesicles degraded in
• lysosomes
• Hence can degrade membrane
  proteins
• i.e. breaks the topology rule