Computer modeling of synapsin I binding to synaptic vesicles and F-actin: implications for regulation of neurotransmitter release.

Synapsin I is a neuron-specific phosphoprotein that binds to small synaptic vesicles and actin filaments in a phosphorylation-dependent fashion. It has been hypothesized that dephosphorylated synapsin I inhibits neurotransmitter release either by forming a cage around synaptic vesicles (cage model) or by anchoring them to the F-actin cytoskeleton of the nerve terminal (crosslinking model). Computer modeling was performed with the aim of testing the impact of phosphorylation on the molecular interactions of synapsin I within the nerve terminal. The results of the simulation experiments demonstrate that in the crosslinking model the phosphorylation of synapsin I causes a severalfold increase in the number of vesicles released from the cytoskeleton and that in the cage model the phosphorylation induces a 2-fold increase in the number of vesicles bearing one or more unsaturated synapsin I binding sites. These data are compatible with the view that the function of synapsin I in the short-term regulation of neurotransmitter release is to induce a phosphorylation-dependent transition of synaptic vesicles from a "reserve pool" to a readily "releasable pool" of vesicles.