Neuronal phosphoprotein that coats synaptic vesicles. Plays a role in the establishment of functional synaptic connections and in the regulation of their activity-dependent short-term plasticity. Localized in the contact area between the pre- and postsynaptic cells and in varicose structures along presynaptic neurites projecting onto the postsynaptic cell. Ser-9 is likely to be phosphorylated by CaMK1/4 and/or PKA. It is probable that the phosphorylation plays a role in short-term synaptic enhancement (STE). Ser-36 and/or Ser-42 are likely to be phosphorylated by MAPK; the phosphorylation may play a dual role in regulating both functional synapse formation and their activity-dependent short-term plasticity. Belongs to the synapsin family.