Phosphorylation of Synaptic Vesicle Protein 2 Modulates Binding to Synaptotagmin

Synaptic vesicle protein 2 (SV2) is a component of all synaptic vesicles that is required for normal neurotransmission. Here we report that in intact synaptic terminals SV2 is a phosphoprotein. Phosphopeptide mapping studies indicate that a major site of phosphorylation is located on the cytoplasmic amino terminus. SV2 is phosphorylated on serine and threonine but not on tyrosine residues, indicating that it is a substrate for serine/threonine kinases. Phosphopeptide mapping, in gel kinase assays, and surveys of kinase inhibitors suggest that casein kinase I is a primary SV2 kinase. The amino terminus of SV2 was previously shown to mediate its interaction with synaptotagmin, a calcium-binding protein also required for normal neurotransmission. Comparison of synaptotagmin binding with phosphorylated and unphosphorylated SV2 amino-terminal peptides reveals an increase in binding with phosphorylation. These results suggest that the affinity of SV2 for synaptotagmin is modulated by phosphorylation of SV2.