Interaction of free and synaptic vesicle-bound synapsin I with F-actin.

Synapsin I is a neuron-specific phosphoprotein that binds to small synaptic vesicles and F-actin in a phosphorylation-dependent fashion. We have found that dephosphorylated synapsin I induces a dose-dependent increase in the number of actin filaments, which at high ionic strength is abolished by synapsin I phosphorylation. The increase in filament number appears to be due to a nucleating effect of synapsin I and not to a barbed-end capping/severing activity. Synaptic vesicle-bound synapsin I was as effective as free synapsin I in increasing the number of filaments. These data support the view that synapsin I is involved in the regulation of the dynamics of the actin-based network during the exo-endocytotic cycle.