Synaptic vesicle exocytosis

Question 1

What are the different types of synaptotagmin (Syt) and how are they recognised?

Answer 1

• Syt1 functions as a calcium sensor, binding to SNARE complexes in a calcium-dependent manner
• Syt2 exhibits the faster rise and decay kinetics of calcium release and is primarily expressed in synapses.
• Syt9 exhibit slow calcium release and are involved in emotions
• Syt12 is not able to bind to calcium

Question 2

What is the role of complexins in neurotransmitter release, and how do they interact with the SNARE complex?

Answer 2

Complexins are small proteins that bind to the partially assembled SNARE complex. They act as both activators and clamps of neurotransmitter release. Complexin’s central alpha helix and Syt1 bind to SNARE complexes at overlapping sites. Calcium binding to Syt1 triggers the displacement of complexin from the SNARE complex, promoting synaptic vesicle fusion.

Question 3

What are chaperones and how do they support SNARE protein function?

Answer 3

Chaperones are proteins that assist in the conformational folding and unfolding of other proteins. In the context of SNARE proteins, chaperones like CPSα and α/β/γ-synucleins form classical and nonclassical chaperone complexes, respectively. These complexes support the functional competence of SNARE proteins in engaging in SNARE complexes.

Question 4

Explain the process of clathrin-mediated endocytosis and name molecules that are included in the process

Answer 4

It starts with the interaction of clathrin adaptors like AP-2 with the lipid bilayer and with membrane proteins. They will bind to PiP2, which is present on the presynaptic membrane. This binding will mediate the recruitment of clathrin. Clathrin then interacts with actin to stabilise the vesicle. After stabilisation, N-BAR proteins such as endophiin and amphilysin interact which generates and maintains the curvature of the vesicle. The GTPase dynamin is then required to part the neck from the bud to free the vesicle (budding off/fission). Disassembly or the uncoating of the clathrin coat is ATP-dependent and requires ATPase sc70 and its cofactor auxilin. The shedding of adaptors is dependent on the hydrolysis of PIP2 by synaptojanin.

Question 5

What is the classical chaperone complex?

Answer 5

It consists of;
* CSPα
* Hsc70
* SGT
This complex binds to SNAP-25 and engages in the SNARE complex.

Question 6

What is the non-classical chaperon complex?

Answer 6

This consists of;
* α/β/γ-synucleins  bound to phospholidpids
* synaptobrevin/vAMP  on vesicles
They bind to the assembling SNARE complexes to support their folding.