A lever hypothesis for Synaptotagmin-1 action in neurotransmitter release

Neurotransmitter release is triggered in microseconds by Ca2+- binding to the dles between synaptic vesicles and plasma membranes, but the coupling mechanism between Ca2+- sensing and membrane fusion is unknown. Release requires extension of SNARE helices into juxtamembrane linkers that precede transmembrane regions (linker through a "primary interface" comprising two regions (I and II). The Synaptotagmin- 1 curvature, perturbing lipid bilayers, or helping bridge the membranes, but SNARE from the fusion site, hindering these putative activities. To clarify this paradox, we have used NMR and fluorescence spectroscopy. NMR experiments reveal that binding of C2B domain arginines to SNARE acidic residues at region II remains after disruption of region I, and that a mutation that impairs spontaneous and Ca2+- triggered neurotransSNARE complex but causes reorientation of the C2B domain. Based on these results domain reorients on the membrane and dissociates from the SNAREs at region I but linker zippering or other SNARE structural changes required for fast membrane fusion.