Lecture 14 - The secretory pathway and exocytosis Flashcards
How do proteins enter the secretory pathway
Proteins enter the secretory pathway at the rough ER
rER
Rough due to the many bound ribosomes
Found packing the cytoplasm in secretory cells like the pancreas
Translocon
Protein complex membrane channel that allows protein movement (within the ER?)
Sec61
N-linked glycosylation
Occurs at the ER
Post-translational modification
Asn-X-Ser/Thr
Oligosaccharyl transferase
Proteins
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
COPII vesicles
May directly go to their destination (?)
May fuse with each other - homotypic fusion, generating vesicular tubular clusters
VTCs
Vesicular tubule clusters
Use motor proteins (Dynein) to travel along microtubules to enter the cis side of the Golgi apparatus
COPI
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)
Golgi
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)
Glycosylation: whats the point?
- 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
Glycosylation and cancer cells
Altered glycosylation may result in altered cell-cell interactions (adhesion), metastases, signals affecting proliferation, differentiation, and survival
Can be targeted for therapy
Golgi protein transport
Vesicular transport model - travels through cisternae by vesicles
Cisternal maturation model - move through the cisternae and mature as they move
Secretion
Proteins with no tag naturally delivered to the plasma membrane
Constitutive secretion - Constant secretion
Regulated secretory - Wait until a signal then secreted
Constitutive exocytosis
Supplies PM with expansion before cell division
Role in protein secretion
Packing
May be dense - allows large release of material
Further processing may happen in secretory vesicles: why?
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)
Release stimulation
Chemical messengers
Causing intracellular signal (Ca2+)
Causes Ca2+ influx, vesicle fusion
Synaptic vesicle fusion
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
Synaptic vesicles
May directly form vesicles from the PM - more efficient than travelling long axon distances
Reciprocal endocytsosis
Prevents massive increase in PM (esp in secretory cells)
Balances with secretion
Situations where PM growth is wanted
- Cytokinesis
- Phagocytosis
- PM repair
- Cellularisation
Polarised cells
Sort cargoes to work with apical/basolateral surfaces that require different cargoes to each
