The organisation of intracellular traffic - secretory pathway
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What is the function of the COPII coat?
It mediates vesicle formation at the ER for anterograde (forward) transport to the Golgi.
What does “anterograde transport” mean?
Movement of cargo from the ER toward the Golgi apparatus.
What does “retrograde transport” refer to?
Movement from the Golgi back to the ER, typically COPI-dependent.
Where does COPII-mediated budding occur?
At specialized ER subdomains called ER exit sites (ERES).
What are the two main coat protein complexes involved in the secretory pathway?
COPII (ER → Golgi) and COPI (Golgi → ER and intra-Golgi).
How does COPII coat formation begin?
Activation of the small GTPase Sar1 by its GEF, Sec12, on the ER membrane.
What triggers Sar1 activation?
Exchange of GDP for GTP catalyzed by Sec12.
What happens when Sar1 binds GTP?
A hydrophobic N-terminal amphipathic helix inserts into the ER membrane, initiating curvature and recruiting other COPII components.
What are the main COPII coat components?
Sar1, Sec23, Sec24, Sec13, Sec31, and Sec16.
Which COPII subunits form the inner coat?
Sec23 and Sec24.
Which subunits form the outer coat?
Sec13 and Sec31.
What is the function of Sec23?
Serves as a GTPase-activating protein (GAP) for Sar1 and scaffolds the inner coat.
What is the function of Sec24?
Cargo selection via recognition of ER export motifs on cargo or adaptors.
What does Sec13/31 do?
Forms a polyhedral cage that shapes the vesicle.
What role does Sec16 play?
Acts as a scaffold at ER exit sites and helps organize coat assembly.
What drives membrane deformation during vesicle budding?
Insertion of Sar1’s amphipathic helix and coat polymerization.
What event triggers coat disassembly?
GTP hydrolysis on Sar1, promoted by Sec23, leading to coat release after budding.
What is the approximate size of a typical COPII vesicle?
~60–90 nm in diameter.
How do COPII vesicles maintain directionality?
Through Rab GTPases and tethering factors that target them to the cis-Golgi or ERGIC.
How does Sec24 recognize cargo?
It binds short ER export motifs on cytosolic cargo tails or adaptors.
What is the function of cargo receptors?
They capture soluble luminal cargo and link it to Sec24 for export.
Name an example of a cargo receptor for soluble proteins.
ERGIC-53 or p24 family proteins.
What ensures only correctly folded proteins exit the ER?
ER quality control retains misfolded or unassembled proteins via chaperones and retrieval signals.
What happens to misfolded cargo that escapes the ER?
It can be retrieved by COPI vesicles using ER retrieval motifs like KDEL.