Effects of the neuronal phosphoprotein synapsin I on actin polymerization. I. Evidence for a phosphorylation-dependent nucleating effect.

Synapsin I is a synaptic vesicle-specific phosphoprotein which is able to bind and bundle actin filaments in a phosphorylation-dependent fashion. In the present paper we have analyzed the effects of synapsin I on the kinetics of actin polymerization and their modulation by site-specific phosphorylation of synapsin I. We found that dephosphorylated synapsin I accelerates the initial rate of actin polymerization and decreases the rate of filament elongation. The effect was observed at both low and high ionic strength, was specific for synapsin I, and was still present when polymerization was triggered by F-actin seeds. Dephosphorylated synapsin I was also able to induce actin polymerization and bundle formation in the absence of KCl and MgCl2. The effects of synapsin I were strongly decreased after its phosphorylation by Ca2+/calmodulin-dependent protein kinase II. These observations suggest that synapsin I has a phosphorylation-dependent nucleating effect on actin polymerization. The data are compatible with the view that changes in the phosphorylation state of synapsin I play a functional role in regulating the interactions between the nerve terminal cytoskeleton and synaptic vesicles in various stages of the exoendocytotic cycle.