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A family of synaptic vesicle-associated proteins involved in the short-term regulation of neurotransmitter release. Synapsin I (Protein I), the predominant member of this family, links synaptic vesicles to actin filaments in the presynaptic nerve terminal. These interactions are modulated by the reversible phosphorylation of synapsin I through various signal transduction pathways. The protein is also a substrate for cAMP- and calcium-calmodulin-dependent protein kinases. It is believed that these functional properties are also shared by synapsin II (Protein III).