Lecture 12 - Principles of vesicular transport Flashcards
What is a good model for transport vesicle formation?
COPII coated vesicles
How accurate are vesicles to getting to their destination?
Vesicles are targeted with high fidelity to their destination
What is transport mediated by?
Vesicles and tubules
What 3 things can coat transport vesicles?
- clathrin
- COPI
- COPII
Where do clathrin coated vesicles transport from and to?
from the plasma membrane to the endosome (a type of intracellular vesicle), and also in transport between different compartments of the Golgi apparatus.
Where do COPI coated vesicles transport from and to?
from the Golgi apparatus back to the endoplasmic reticulum (ER), or between Golgi compartments
Where do COPII coated vesicles transport from and to?
from the ER to the Golgi apparatus
What are the 3 essential components for all transport vesicles formation?
- GTPase
- adaptor proteins
- coat
What are small GTPases?
molecular switches
Where is GTP usually found?
GTP form is membrane associated
Where is GDP usually found?
usually in the cytosol
Where is Ras found?
in many forms of cancer
How is COPII coated vesicle a paradigm for transport vesicle formation?
COPII components:
- GTPase - Sar1 (member of the Arf family)
- Adaptor: Sec23/24
- Coat: Sec13/31
What ensures proper protein folding?
Chaperones
Describe the formation of a COPII vesicle
Soluble cargo selected through signalling
- high surface area:volume ratio
What recognises the proteins?
The adaptor proteins
What do coat proteins provide?
A structural scaffold
What type of experiment allows us to study COPII?
Reconstitution experiments allow us to understand what makes a COPII vesicle.
- centrifugation separates the different parts
What does the ER membrane contain?
Ribophorin
What do COPII vesicles contain?
p58 (cargo protein)
What are the COPII components?
- Sar1 GEF is Sec12
- Sec23/24
- Sec13/31
- ATP
- GTP
Where are GTPases recruited from?
the cytosol
What acts as a GAP for Sar1?
Sec23. GAP activity enhanced following recruitment of the coat Sec13/31
Describe features of GTPases
- mutant GTPases can be generated
- GDP mutant: sequester GEFs
- GTP mutant: cannot hydrolyse GTP
- Cycles of GTPase activity are important
- Mutant GTPases often have a dominant negative effect
- Expression of Sar1GDP inhibits COPII formation
What is transport through the secretory pathway mediated by?
Coated vesicles and tubules
What is COPII used to transport?
Newly synthesised proteins
What is COPI used to transport?
retrieved & newly synthesised proteins
What is Clathrin (TGN) used to transport?
Lysosomal proteins, regulated secretory proteins
What is Clathrin (PM) used to transport?
Endocytosed material
What do adaptor proteins do?
- recognise and select cargo ensuring specificity
- link the coat to the membrane
- adaptors recognise motifs (tags) in the cytoplasmic domains of the membrane protein
adaptor complexes also show a precise subcellular localization
What is a tyrosine-based sorting signal in endocytic proteins?
TfR (Transferrin Receptor)
What is a ubiquitin-based sorting signal in endocytic proteins?
EGFR (epidermal growth factor receptor)
What does AP2 do?
AP2 recognises a variety of peptide motifs e.g. u & o subunits recognise sorting signals
Explain the role of AP2 in endocytosis
AP2 is a major clathrin adaptor. Other adaptors allow cells to select what they internalize.
