Neuronal phosphoprotein that coats synaptic vesicles, binds to the cytoskeleton, and is believed to function in the regulation of neurotransmitter release. May play a role in noradrenaline secretion by sympathetic neurons. Interacts with CAPON. Expressed exclusively in neuronal cells. Isoform IIb is enriched in sympathetic cervical ganglion. The A region binds phospholipids with a preference for negatively charged species. Phosphorylation at Ser-10 dissociates synapsins from synaptic vesicles (By similarity). Phosphorylation at Ser-426 by MAPK1/ERK2 and/or MAPK3/ERK1 may play a role in noradrenaline secretion by sympathetic neurons. Belongs to the synapsin family.