Priming of the synaptic protein complex

Overview. The SNARE/complexin-1/synaptotagmin-1 complex (Zhou et al., 2017) is important for the primed state of synaptic vesicles (i.e.,the association of synaptic vesicles with presynaptic proteins that enables them to undergo fast Ca2+-triggered fusion). There are several factors involved in priming, including Munc18 and Munc13, but the molecular basis of the priming function of these molecules had been unknown until recently. We discovered that one of the functions of such priming factors is to ensure the proper assembly of SNARE complexes (Lai et al., 2017). This finding led us to propose a model where regulation of priming may be in part accomplished at the molecular level by ensuring the proper assembly of the synaptic complexes.

Why is it necessary to assist the proper assembly of the SNARE complex? When soluble fragments of SNARE proteins are simply mixed in solution, we previously observed that improper (e.g.,anti-parallel) configurations may occur (Choi et al., 2016), (Weninger et al., 2003). In search for factors that ensure proper (i.e., parallel) SNARE complex formation, we discovered that Munc13-1 promotes the proper parallel subconfiguration between syntaxin-1A and synaptobrevin-2 when assembling the ternary SNARE complex (Lai et al., 2017). Additionally, Munc13 also cooperates with Munc18 to promote the proper syntaxin-1A/SNAP-25A subconfiguration within the assembled ternary SNARE complex.

Model of concerted Munc18-1 / Munc13-1 action to properly assemble ternary SNARE complex. Model of the combined effect of Munc13-1 and Munc18-1 on Ca2+-triggered fusion. (A) Munc13-1 and Munc18-1 cooperate to produce proper trans ternary SNARE complex. (B) Without Munc13-1 and Munc18-1, trans ternary SNARE complex assembly is error prone resulting in much reduced Ca2+-triggered fusion probability.

Key insights. Taken together, Munc13 and Munc18 can be viewed as assembly factors for establishing all proper subconfigurations of the ternary SNARE complex assembly. The cooperation of Munc18 and Munc13 in promoting the proper SNARE complex assembly explains the severe effect of deletion of Munc18 and Munc13 in neurons. Moreover, consistent with their molecular functions, Munc18 and Munc13 substantially increased the efficiency of  Ca2+-triggered vesicle fusion in our reconstituted fusion assay  (Lai et al., 2017).