Neurotransmitter release is triggered by a calcium-induced rearrangement in the Synaptotagmin-1/SNARE complex primary interface

Abstract

AbstractThe Ca2+sensor synaptotagmin-1 triggers neurotransmitter release together with the neuronal SNARE complex formed by syntaxin-1, SNAP25 and synaptobrevin. Moreover, synaptotagmin-1 increases synaptic vesicle priming and impairs spontaneous vesicle release. The synaptotagmin-1 C2B domain binds to the SNARE complex through a primary interface via two regions (I and II), but how exactly this interface mediates distinct functions of synaptotagmin-1, and the mechanism underlying Ca2+-triggering of release is unknown. Using mutagenesis and electrophysiological experiments, we show that region II is functionally and spatially subdivided: binding of C2B domain arginines to SNAP-25 acidic residues at one face of region II is crucial for Ca2+-evoked release but not for vesicle priming or clamping of spontaneous release, whereas other SNAP-25 and syntaxin-1 acidic residues at the other face mediate priming and clamping of spontaneous release but not evoked release. Mutations that disrupt region I impair the priming and clamping functions of synaptotagmin-1 while, strikingly, mutations that enhance binding through this region increase vesicle priming and clamping of spontaneous release, but strongly inhibit evoked release and vesicle fusogenicity. These results support previous findings that the primary interface mediates the functions of synaptotagmin-1 in vesicle priming and clamping of spontaneous release, and, importantly, show that Ca2+-triggering of release requires a rearrangement of the primary interface involving dissociation of region I, while region II remains bound. Together with modeling and biophysical studies presented in the accompanying paper, our data suggest a model whereby this rearrangement pulls the SNARE complex to facilitate fast synaptic vesicle fusion.Significance statementThe synaptic SNARE complex and synaptotagmin-1 are required for fast neurotransmitter release. The functions of synaptotagmin-1 in preparing synaptic vesicles for fusion and executing the triggering step have been proposed to be regulated through interactions with the SNARE complex via the so-called primary interface. Using site-directed mutagenesis and functional analysis in neurons, we now show that synaptotagmin-1 mediates its release preparatory functions via two contact sites with the SNARE complex at this interface. During Ca2+triggering, synaptotagmin-1 continues to contact the SNAREs at one site but disconnects the other site. We propose that this switch generates a pulling force on the SNARE complex that in turn triggers release. Biochemical and modeling studies described in the accompanying paper support this hypothesis.