Synaptotagmin-1 may be a distance regulator acting upstream of SNARE nucleation

Synaptotagmin-1 triggers Ca2+-sensitive, rapid neurotransmitter release by promoting interactions between SNARE proteins on synaptic vesicles and the plasma membrane. How synaptotagmin-1 promotes this interaction is unclear, and the massive increase in membrane fusion efficiency of Ca2+-bound synaptotagmin-1 has not been reproduced in vitro. However, previous experiments have been performed at relatively high salt concentrations, screening potentially important electrostatic interactions. Using functional reconstitution in liposomes, we show here that at low ionic strength SNARE-mediated membrane fusion becomes strictly dependent on both Ca2+ and synaptotagmin-1. Under these conditions, synaptotagmin-1 functions as a distance regulator that tethers the liposomes too far from the plasma membrane for SNARE nucleation in the absence of Ca2+, but while bringing the liposomes close enough for membrane fusion in the presence of Ca2+. These results may explain how the relatively weak electrostatic interactions between synaptotagmin-1 and membranes substantially accelerate fusion.