Neuronal phosphoprotein that coats synaptic vesicles, binds to the cytoskeleton, and is believed to function in the regulation of neurotransmitter release. Regulation of neurotransmitter release. The complex formed with NOS1 and CAPON proteins is necessary for specific nitric-oxide functions at a presynaptic level. Homodimer. Interacts with CAPON. Forms a ternary complex with NOS1 (By similarity). Isoform Ib interacts with PRNP. The A region binds phospholipids with a preference for negatively charged species. Substrate of at least four different protein kinases. It is probable that phosphorylation plays a role in the regulation of synapsin-1 in the nerve terminal (By similarity). Phosphorylation at Ser-9 dissociates synapsins from synaptic vesicles. Belongs to the synapsin family.