Coated Vesicles, GTPases, and SNAREs in Cellular Transport

What are the three main types of coated vesicles?

Clathrin, COPI, and COPII.

What is the main role of COPII-coated vesicles?

To transport proteins from the ER to the Golgi apparatus (anterograde transport).

What is the main role of COPI-coated vesicles?

To retrieve proteins from the Golgi back to the ER (retrograde transport).

What is the GTPase associated with COPII vesicle formation?

Sar1.

What activates Sar1?

The ER membrane protein Sec12, a Sar1-GEF, which loads GTP onto Sar1.

What proteins form the inner COPII coat?

Sec23 and Sec24.

What proteins form the outer COPII coat?

Sec13 and Sec31.

What types of cargo do COPII vesicles carry?

Bulk proteins, Golgi enzymes, docking and fusion proteins, and cargo receptors.

What GTPase regulates COPI-coated vesicle formation?

ARF1.

How is ARF1 activated?

By Golgi-localised GEF proteins that exchange GDP for GTP.

What is the structure of the COPI coat?

A heptameric complex called coatomer composed of seven subunits.

What signal sequence ensures ER proteins are retrieved from the Golgi?

The KDEL sequence (Lys-Asp-Glu-Leu) at the C-terminus.

What receptor recognises the KDEL sequence?

The KDEL receptor in the cis-Golgi.

What happens when GTP hydrolysis is blocked during vesicle formation?

Coated vesicles accumulate because coat disassembly requires GTP hydrolysis.

Which small GTPase family regulates vesicle transport?

The Rab family of small GTPases.

What is the main function of Rab GTPases?

To regulate vesicle formation, transport, docking, and fusion through interactions with effector proteins.

How are Rab proteins localised to specific membranes?

By post-translational lipid modification (prenylation) and interactions with effectors.

What keeps Rab proteins inactive in the cytosol?

GDI (GDP Dissociation Inhibitor), which binds GDP-bound Rab and prevents membrane association.

What is the function of GDF (GDI displacement factor)?

To release GDI from Rab, allowing membrane insertion.

What converts Rab from its inactive to active form?

GEFs (Guanine nucleotide Exchange Factors) that exchange GDP for GTP.

What is the role of Rab effectors?

To mediate tethering, signalling, and vesicle fusion events.

What is a Rab cascade?

A process where one Rab activates the next Rab in a transport pathway, promoting vesicle maturation and identity changes.

What proteins mediate vesicle docking and fusion?

SNARE proteins (v-SNAREs and t-SNAREs).

Where are v-SNAREs located?

On vesicle membranes.

Where are t-SNAREs located?

On target membranes.

What is the function of SNARE proteins?

They pair to bring vesicle and target membranes into close proximity for fusion.

How does Rab facilitate SNARE-mediated docking?

Active Rab binds effector proteins that tether vesicle and target membranes together.

What is the trans-SNARE complex?

A coiled-coil structure formed by v-SNARE and t-SNARE helices that drives membrane fusion.

What disassembles SNARE complexes after fusion?

NSF (N-ethylmaleimide Sensitive Factor) and α-SNAP, using ATP hydrolysis.

What triggers synaptic vesicle fusion?

A rapid increase in calcium concentration.

What is complexin's role in synaptic fusion?

It stabilises the trans-SNARE complex in a primed state until calcium release triggers fusion.

What type of transport is regulated by SNAREs and Rab GTPases?

Targeted vesicle trafficking and membrane fusion specificity.