Identification of synapsin I peptides that insert into lipid membranes

The synapsins constitute a family of synaptic vesicle-associated phosphopro teins essential for regulating neurotransmitter release and synaptogenesis, The molecular mechanisms underlying the selective targeting of synapsin I to synaptic vesicles are thought to involve specific protein-protein intera ctions, while the high-affinity binding to the synaptic vesicle membrane ma y involve both protein-protein and protein-lipid interactions. The highly h ydrophobic N-terminal region of the protein has been shown to bind with hig h affinity to the acidic phospholipids phosphatidylserine and phosphatidyli nositol and to penetrate the hydrophobic core of the lipid bilayer, To prec isely identify the domains of synapsin I which mediate the interaction with lipids, synapsin I was bound to liposomes containing the membrane-directed carbene-generating reagent 3-(trifluoromethyl)-3-(m-[I-125]iodophenyl)diaz irine and subjected to photolysis. Isolation and N-terminal amino acid sequ encing of I-125-labelled synapsin I peptides derived from CNBr cleavage ind icated that three distinct regions in the highly conserved domain C of syna psin I insert into the hydrophobic core of the phospholipid bilayer, The bo undaries of the regions encompass residues 166-192, 233-258 and 275-327 of bovine synapsin I. These regions are surface-exposed in the crystal structu re of domain C of bovine synapsin I and are evolutionarily conserved among isoforms across species. The present data offer a molecular explanation for the high-affinity binding of synapsin I to phospholipid bilayers and synap tic vesicles.