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Abstract

Fast synchronous neurotransmitter release is triggered by calcium that activates ​synaptotagmin-1 (​syt-1), resulting in fusion of synaptic vesicles with the presynaptic membrane. ​Syt-1 possesses two Ca2+-binding C2 domains that tether membranes via interactions with anionic phospholipids. It is capable of crosslinking membranes and has recently been speculated to trigger fusion by decreasing the gap between them. As quantitative information on membrane gaps is key to understanding general cellular mechanisms, including the role of ​syt-1, we developed a fluorescence-lifetime based inter-membrane distance ruler using membrane-anchored DNAs of various lengths as calibration standards. Wild-type and mutant data provide evidence that full-length ​syt-1 indeed regulates membrane gaps: without Ca2+, ​syt-1 maintains membranes at distances of ~7–8 nm. Activation with 100 μM Ca2+ decreases the distance to ~5 nm by binding the C2 domains to opposing membranes, respectively. These values reveal that activated ​syt-1 adjusts membrane distances to the level that promotes SNARE complex assembly.