Neuronal phosphoprotein that coats synaptic vesicles, binds to the cytoskeleton, and is believed to function in the regulation of neurotransmitter release. The complex formed with NOS1 and CAPON proteins is necessary for specific nitric-oxid functions at a presynaptic level (By similarity). Homodimer. Interacts with CAPON. Forms a ternary complex with NOS1. Interacts with PRNP (By similarity). Belongs to the synapsin family.