Calcium ions play an essential role in many intracellular processes, including transmitter and hormone release. Which ONE of the following proteins is the Ca2+ sensor in the release apparatus of synaptic terminals in the brain?

Question

Calcium ions play an essential role in many intracellular processes, including transmitter and hormone release.  Which ONE of the following proteins is the Ca2+ sensor in the release apparatus of synaptic terminals in the brain?

BlackTom AI · Accepted Answer

Synaptotagmin-1 is the Ca2+ sensor that triggers fast, synchronous neurotransmitter release at central synapses.

1) Relevant knowledge points and concepts
- Synaptic vesicle fusion is driven by the SNARE complex (syntaxin, SNAP-25, synaptobrevin) which brings vesicle and plasma membranes together.
- A separate Ca2+ sensor is required to trigger rapid fusion in response to action-potential–evoked Ca2+ influx.
- Synaptotagmin-1 (Syt1) is an integral synaptic vesicle protein containing two C2 domains (C2A and C2B) that bind Ca2+ and phospholipids, coupling Ca2+ entry to membrane fusion.

2) Step-by-step reasoning process
- Action potentials open voltage-gated Ca2+ channels, producing a localized rise in intracellular Ca2+ near release sites.
- Synaptotagmin-1, positioned on vesicles adjacent to release sites, binds Ca2+ with its C2 domains.
- Ca2+-bound Syt1 undergoes conformational change, increasing affinity for anionic phospholipids (e.g., PIP2) and interacting with the SNARE complex.
- These interactions lower the energy barrier for membrane merger, rapidly triggering synchronous neurotransmitter release.

3) Intermediate steps / experimental evidence (no numerical calculation required)
- Genetic deletion or mutation of Syt1 abolishes fast synchronous release and leaves slower, asynchronous release intact, demonstrating its specific sensor role.
- Biochemical and structural studies show Ca2+-dependent phospholipid binding by the C2 domains and interaction with SNARE proteins, explaining the mechanistic coupling.

4) Why the correct answer is correct
- Syt1’s localization, Ca2+-binding C2 domains, and loss-of-function phenotypes directly demonstrate it is the principal fast Ca2+ sensor in brain synaptic terminals.

5) Why other candidate proteins are incorrect
- Calmodulin and protein kinases respond to Ca2+ but are too slow and mediate signaling cascades, not the millisecond triggering of vesicle fusion.
- Complexin regulates SNARE-mediated fusion but does not itself bind Ca2+ to trigger release.
- Other synaptotagmin isoforms (e.g., Syt2, Syt7) can act in different tissues or modes (faster/slower), but Syt1 is the principal sensor in many central synapses.

6) Conclusion
These mechanistic, genetic, and biochemical lines of evidence confirm Synaptotagmin-1 as the Ca2+ sensor in the release apparatus of synaptic terminals, matching the provided answer: Synaptotagmin-1.

Calcium ions play an essential role in many intracellular processes, including transmitter and hormone release. Which ONE of the following proteins is the Ca2+ sensor in the release apparatus of synaptic terminals in the brain?Single choice

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