18.15:
Fusion of Secretory Vesicles with the Plasma Membrane
Secretory vesicles first dock, then are primed for fusion, and finally fuse with the plasma membrane to release their contents. Vesicle fusion and exocytosis are aided by SNARE complexes and triggered by increased cytosolic calcium concentration.
In neuronal cells, synaptic vesicles, specialized secretory vesicles, fuse with the membrane. As the vesicles arrive at the membrane, the SNARE complex enables docking.
The SNARE complex consists of two sets of tethering proteins—vesicular or v-SNAREs and target or t-SNAREs. v-SNAREs interact and connect with t-SNAREs, bringing the vesicle closer to the plasma membrane.
In the priming step, complexins clamp the SNAREs together, locking them in a partially connected state. Complexins are in turn regulated by calcium and synaptotagmins, calcium-binding vesicular proteins.
When an action potential arrives at a neuron, voltage-gated calcium channels open, causing the influx of calcium ions.
Calcium binds to synaptotagmin, which causes the protein to bind to the membrane, displacing the complexin clamp.
The clamp release enables the SNAREs to fully connect, opening a pore to release the neurotransmitter.
18.15:
Fusion of Secretory Vesicles with the Plasma Membrane
Proteins and neurotransmitters in secretory vesicles can be released from a cell upon vesicle docking, priming, and fusion with the plasma membrane. Vesicles are docked and primed in preparation for the quick exocytosis of their contents in response to a stimulus. The fusion process is mainly carried out by a SNAP Receptor or SNARE complex, consisting of synaptobrevin, syntaxin-1, and SNAP-25.
In 1993, Jim Rothman proposed that the antiparallel pairing of vesicular and transmembrane SNAREs, or v- and t-SNAREs, was essential for vesicle docking. However, more recently, it has been shown that vesicle docking can also occur without SNAREs. Additionally, two soluble proteins, N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAP), bind to the SNARE complex to facilitate fusion.
The v-and t-SNAREs partially fuse in the priming process, forming a fusion-ready state. The protein complexin (Cpx) clamps the SNAREs and holds the vesicles in this partially fused state to prevent premature exocytosis. Vesicular membrane fusion begins when a stimulus, such as an action potential at the axonal terminal of a neuron, opens up a calcium channel, and calcium enters the cell. A total of five calcium ions bind to each synaptotagmin (Syt) – a vesicular membrane protein on either side of the vesicle. Calcium-bound Syt releases the Cpx clamp from the SNARE complexes and opens up the fusion pore to release the neurotransmitter. After fusion, NSF and SNAP proteins disassemble SNARE complexes for recycling.
Bacterial neurotoxins, such as botulinum from Clostridium botulinum or tetanus from Clostridium tetani, can inhibit secretory vesicle fusion by damaging the SNARE proteins, which prevents the fusion of secretory vesicles with the neuronal plasma membrane. As neurotransmitters are not released, action potentials are not generated, causing paralysis of muscles, and in some cases, death.
Suggested Reading
- Südhof, T. C. (2013). Neurotransmitter release: the last millisecond in the life of a synaptic vesicle. Neuron, 80(3), 675-690.
- Gagescu, R. (2000). SNARE hypothesis 2000. Nature Reviews Molecular Cell Biology, 1(1), 5-5.
- Neher, E. (2010). Complexin: does it deserve its name? Neuron 68(5), 803-806.