DEPHOSPHORYLATED SYNAPSIN-I ANCHORS SYNAPTIC VESICLES TO ACTIN CYTOSKELETON - AN ANALYSIS BY VIDEOMICROSCOPY

Synapsin I is a synaptic vesicle-associated protein which inhibits neu rotransmitter release, an effect which is abolished upon its phosphory lation by Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). Based on indirect evidence, it was suggested that this effect on neur otransmitter release may be achieved by the reversible anchoring of sy naptic vesicles to the actin cytoskeleton of the nerve terminal. Using video-enhanced microscopy, we have now obtained experimental evidence in support of this model: the presence of dephosphorylated synapsin I is necessary for synaptic vesicles to bind actin; synapsin I is able to promote actin polymerization and bundling of actin filaments in the presence of synaptic vesicles; the ability to cross-link synaptic ves icles and actin is specific for synapsin I and is not shared by other basic proteins; the cross-linking between synaptic vesicles and actin is specific for the membrane of synaptic vesicles and does not reflect either a non-specific binding of membranes to the highly surface acti ve synapsin I molecule or trapping of vesicles within the thick bundle s of actin filaments; the formation of the ternary complex is virtuall y abolished when synapsin I is phosphorylated by CaM kinase II. The da ta indicate that synapsin I markedly affects synaptic vesicle traffic and cytoskeleton assembly in the nerve terminal and provide a molecula r basis for the ability of synapsin I to regulate the availability of synaptic vesicles for exocytosis and thereby the efficiency of neurotr ansmitter release.