SNT-1 Functions as the Ca Sensor for Tonic and Evoked Neurotransmitter Release in .

Synaptotagmin-1 (Syt1) binds Ca through its tandem C2 domains (C2A and C2B) and triggers Ca-dependent neurotransmitter release. Here, we show that , the homolog of mammalian Syt1, functions as the Ca sensor for both tonic and evoked neurotransmitter release at the neuromuscular junction. Mutations that disrupt Ca binding in double C2 domains of SNT-1 significantly impaired tonic release, whereas disrupting Ca binding in a single C2 domain had no effect, indicating that the Ca binding of the two C2 domains is functionally redundant for tonic release. Stimulus-evoked release was significantly reduced in mutants, with prolonged release latency as well as faster rise and decay kinetics. Unlike tonic release, evoked release was triggered by Ca binding solely to the C2B domain. Moreover, we showed that SNT-1 plays an essential role in the priming process in different subpopulations of synaptic vesicles with tight or loose coupling to Ca entry. We showed that SNT-1 in regulates evoked neurotransmitter release through Ca binding to its C2B domain in a similar way to Syt1 in the mouse CNS and the fly neuromuscular junction. However, the largely decreased tonic release in mutants argues SNT-1 has a clamping function. Indeed, Ca-binding mutations in the C2 domains in SNT-1 significantly reduced the frequency of the miniature EPSC, indicating that SNT-1 also acts as a Ca sensor for tonic release. Therefore, revealing the differential mechanisms between invertebrates and vertebrates will provide significant insights into our understanding how synaptic vesicle fusion is regulated.