Neuronal phosphoprotein that coats synaptic vesicles, binds to the cytoskeleton, and is believed to function in the regulation of neurotransmitter release. Homodimer (By similarity). Interacts with CAPON. Forms a ternary complex with NOS1. Isoform Ib interacts with PRNP (By similarity). 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. Sequencing errors.