Lecture 14 - The secretory pathway and exocytosis

Question 1

How do proteins enter the secretory pathway

Answer 1

Proteins enter the secretory pathway at the rough ER

Question 2

rER

Answer 2

Rough due to the many bound ribosomes

Found packing the cytoplasm in secretory cells like the pancreas

Question 3

Translocon

Answer 3

Protein complex membrane channel that allows protein movement (within the ER?)

Sec61

Question 4

N-linked glycosylation

Answer 4

Occurs at the ER

Post-translational modification

Asn-X-Ser/Thr

Oligosaccharyl transferase

Question 5

Proteins

Answer 5

Exit the ER in COPII vesicles

Vesicles are formed after specific signals

Those without ‘exit’ signals take longer to leave the ER

Misfolded - retained in ER by chaperones

Question 6

COPII vesicles

Answer 6

May directly go to their destination (?)
May fuse with each other - homotypic fusion, generating vesicular tubular clusters

Question 7

VTCs

Answer 7

Vesicular tubule clusters

Use motor proteins (Dynein) to travel along microtubules to enter the cis side of the Golgi apparatus

Question 8

COPI

Answer 8

Resets back to ‘1’ - retrograde transport

Bind to ‘retrieval’ signals

Going backwards in the secretory pathway into the ER

• Proteins that have escaped the ER
• Machinery (snare proteins etc)

Question 9

Golgi

Answer 9

Further modify proteins
Sort proteins

N-linked glycan - trimmed then formed into a complex oligosaccharide by sugar addition

O-linked glycans - Formed by the addition of sugars to OH residues of Ser/Thr, results in highly O-glycosylated proteoglycans (mainly EM proteins - skin/cartilage/bone)

Question 10

Glycosylation: whats the point?

Answer 10

• Assists protein folding (in ER lumen)
• Can be modified to act as a sorting signal
• Act as a ligand for cell-cell recognition events at the PM
• Protective function - restrict access for proteolytic enzymes

Question 11

Glycosylation and cancer cells

Answer 11

Altered glycosylation may result in altered cell-cell interactions (adhesion), metastases, signals affecting proliferation, differentiation, and survival

Can be targeted for therapy

Question 12

Golgi protein transport

Answer 12

Vesicular transport model - travels through cisternae by vesicles

Cisternal maturation model - move through the cisternae and mature as they move

Question 13

Secretion

Answer 13

Proteins with no tag naturally delivered to the plasma membrane

Constitutive secretion - Constant secretion

Regulated secretory - Wait until a signal then secreted

Question 14

Constitutive exocytosis

Answer 14

Supplies PM with expansion before cell division
Role in protein secretion

Question 15

Packing

Answer 15

May be dense - allows large release of material

Question 16

Further processing may happen in secretory vesicles: why?

Answer 16

Some proteins are too small to be tagged by the ER - need to join onto a bigger protein to be tagged correctly and can then be preoteolysed once it is in the correct vesicle

Allows for proteins to only be active once they have exited the cell (digestive enzymes)

Question 17

Release stimulation

Answer 17

Chemical messengers

Causing intracellular signal (Ca2+)

Causes Ca2+ influx, vesicle fusion

Question 18

Synaptic vesicle fusion

Answer 18

Docking

Priming I - synaptobrevin, syntaxin, and snap25 associate to form a partially assembled SNARE bundle

Priming II - complexin binds to prevent a fully assembled SNARE bundle

Fusion pore opening - Ca2+ binds to synaptotagmin which causes release of complexin and fusion with the PM

Question 19

Synaptic vesicles

Answer 19

May directly form vesicles from the PM - more efficient than travelling long axon distances

Question 20

Reciprocal endocytsosis

Answer 20

Prevents massive increase in PM (esp in secretory cells)

Balances with secretion

Question 21

Situations where PM growth is wanted

Answer 21

• Cytokinesis
• Phagocytosis
• PM repair
• Cellularisation

Question 22

Polarised cells

Answer 22

Sort cargoes to work with apical/basolateral surfaces that require different cargoes to each