Synaptotagmin-1 binds to PIP(2)-containing membrane but not to SNAREs at physiological ionic strength

Ca(2+)-sensor synaptotagmin-1 is thought to trigger membrane fusion by binding to acidic membrane lipids and SNARE proteins. Previous work has shown that binding is mediated by electrostatic interactions that are sensitive to the ionic environment. However, the influence of divalent or polyvalent ions, at physiological concentrations, on synaptotagmin binding to membranes or SNAREs has not been explored. Here we show that binding of rat synaptotagmin-1 to membranes containing PIP(2) is regulated by charge shielding caused by the presence of divalent cations. Surprisingly, polyvalent ions such as ATP and Mg(2+) completely abrogate synaptotagmin-1 binding to SNAREs regardless of whether Ca(2+) is present or not. Altogether, our data suggest that at physiological ion concentrations Ca(2+)-dependent synaptotagmin-1 binding is confined to PIP(2)-containing membrane patches in the plasma membrane, suggesting that membrane interaction of synaptotagmin-1 rather than SNARE binding triggers exocytosis of vesicles.