Synapsin I is a highly surface-active molecule

Synapsin I is a neuron-specific phosphoprotein localized on the surface of small synaptic vesicles to which it binds with high affinity (Kd = 10 nM). Synapsin I exhibits a tendency to self-associate, suggesting that it might have amphiphilic properties. We have now found that synapsin I forms a stable monolayer at an air-water interface which can be compressed under a lateral force of up to 60 dynes/cm, indicating the presence of amphiphilic characteristics in its structure. This interpretation was also supported by circular dichroism spectra of synapsin I, which showed induction of secondary structure in the presence of trifluoroethanol. The various phosphorylated forms of synapsin I did not show any noticeable differences in the force-area isotherms. The monolayer properties of synapsin I fragments derived by cysteine-specific cleavage indicated the presence of amphiphilic characteristics throughout the entire sequence, although the C-terminal region showed less of such surfactant properties. Compositional studies of these fragments revealed that there is little interaction between the N-terminal and middle fragment regions, but that there may be some interaction between the C-terminal and middle fragment regions which affects the surface area occupied by these fragments. Based on this information, we propose a molecular topology for synapsin I consisting of amphiphilic regions and a hydrophilic region.