SYNAPTIC VESICLE-ASSOCIATED CA2+/CALMODULIN-DEPENDENT PROTEIN KINASE-II IS A BINDING-PROTEIN FOR SYNAPSIN-I

SYNAPSIN I is a synaptic vesicle-associated phosphoprotein that is involved in the modulation of neurotransmitter release1. Ca2+ calmodulin-dependent protein kinase II, which phosphorylates two sites in the carboxy-terminal region of synapsin I, causes synapsin I to dissociate from synaptic vesicles2 and increases nerotransmitter release3,4. Conversely, the dephosphorylated form of synapsin I, but not the form phosphorylated by Ca2+/calmodulin-dependent protein kinase II, inhibits neurotransmitter release4-6. The amino-terminal region of synapsin I interacts with membrane phospholipids, whereas the C-terminal region binds to a protein component of synaptic vesicles7,8. Here we demonstrate that the binding of the C-terminal region of synapsin I involves the regulatory domain of a synaptic vesicle-associated form of Ca2+/calmodulin-dependent protein kinase II. Our results indicate that this form of the kinase functions both as a binding protein for synapsin I, and as an enzyme that phosphorylates synapsin I and promotes its dissociation from the vesicles.