Conformational change of Syntaxin-3b in regulating SNARE complex assembly in the ribbon synapses

Neurotransmitter release of synaptic vesicles relies on the assembly of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, consisting of syntaxin and SNAP-25 on the plasma membrane and synaptobrevin on the synaptic vesicle. The formation of the SNARE complex progressively zippers towards the membranes, which drives membrane fusion between the plasma membrane and the synaptic vesicle. However, the underlying molecular mechanism of SNARE complex regulation is unclear. In this study, we investigated the syntaxin-3b isoform found in the retinal ribbon synapses using single-molecule fluorescence resonance energy transfer (smFRET) to monitor the conformational changes of syntaxin-3b that modulate the SNARE complex formation. We found that syntaxin-3b is predominantly in a self-inhibiting closed conformation, inefficiently forming the ternary SNARE complex. Conversely, a phosphomimetic mutation (T14E) at the N-terminal region of syntaxin-3b promoted the open conformation, similar to the constitutively open form of syntaxin LE mutant. When syntaxin-3b is bound to Munc18-1, SNARE complex formation is almost completely blocked. Surprisingly, the T14E mutation of syntaxin-3b partially abolishes Munc18-1 regulation, acting as a conformational switch to trigger SNARE complex assembly. Thus, we suggest a model where the conformational change of syntaxin-3b induced by phosphorylation initiates the release of neurotransmitters in the ribbon synapses.