Abstract
Synaptotagmin-1 (Syt1) binds Ca(2+) through its tandem C2 domains (C2A and C2B) and triggers Ca(2+)-dependent neurotransmitter release. Here, we show that snt-1, the homolog of mammalian Syt1, functions as the Ca(2+) sensor for both tonic and evoked neurotransmitter release at the Caenorhabditis elegans neuromuscular junction. Mutations that disrupt Ca(2+) binding in double C2 domains of SNT-1 significantly impaired tonic release, whereas disrupting Ca(2+) binding in a single C2 domain had no effect, indicating that the Ca(2+) binding of the two C2 domains is functionally redundant for tonic release. Stimulus-evoked release was significantly reduced in snt-1 mutants, with prolonged release latency as well as faster rise and decay kinetics. Unlike tonic release, evoked release was triggered by Ca(2+) 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(2+) entry.SIGNIFICANCE STATEMENT We showed that SNT-1 in Caenorhabditis elegans regulates evoked neurotransmitter release through Ca(2+) 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 snt-1 mutants argues SNT-1 has a clamping function. Indeed, Ca(2+)-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(2+) 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.