Synapsins are a family of synaptic vesicle associated phosphoproteins consisting of three members: SYN1, SYN2, and SYN3, with several alternative splicing leading to 10 different isoforms. They are involved in synaptic transmission and plasticity, several stages of neurodevelopment, such as axon outgrowth and synapse formation. Synapsins are localized in presynaptic nerve terminals of central neurons¹.

Synapsin I is a peripheral membrane protein of synaptic vesicles that is phosphorylated at least at three different sites by various protein kinases. All synapsins have a single phosphorylation site in the short N-terminal domain A (site 1) that regulates neurite elongation and synaptic vesicle mobilization².

Synapsin I exhibits an asymmetric structure and exists in two forms, designated synapsin Ia and synapsin Ib. The primary structure of synapsin I is highly conserved, 95% of the protein’s residues are invariant and contain two major structural domains. The protein is divided into a hydrophobic, collagenase resistant head region and a proline rich, collagenase sensitive tail^3,4. 

Synapsin I has been suggested to play an important role in the regulation of neurotransmitter release and the organization of cytoskeletal organization including actin filaments, microtubules and spectrin in the presynaptic terminal⁵.

In vitro studies show that the dephospho-form of synapsin I binds synaptic vesicles, binds and bundles F-actin, and promotes G actin polymerization⁵.