Synapsin
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The synapsins are a family of proteins that have long been implicated in the regulation of neurotransmitter release at synapses. Specifically, they are thought to be involved in regulating the number of synaptic vesicles available for release via exocytosis at any one time.
Synapsins are encoded by three different genes, synapsin I, II and III, and different neuron terminals will encode different amounts of these; synapsin will make up 1% of total brain protein at any one time.
Current studies suggest the following hypothesis for the role of synapsin: synapsins bind synaptic vesicles to components of the cytoskeleton which prevents them from migrating to the presynaptic membrane and releasing transmitter. During an action potential, synapsins are phosphorylated by Ca2+/calmodulin-dependent protein kinase, releasing the synaptic vesicles and allowing them to move to the membrane and release their neurotransmitter.
Gene knockout studies in mice (where the mouse is unable to produce synapsin) have had some surprising results, showing only mild effects for the mice. This has cast some doubt on the current hypothesis which places synapsin in a central role in synaptic transmission.
