Mechanistic insights into neurotransmitter release and presynaptic plasticity from the crystal structure of Munc13-1 C1C2BMUN

Abstract

Munc13–1 acts as a master regulator of neurotransmitter release, mediating docking-priming of synaptic vesicles and diverse presynaptic plasticity processes. It is unclear how the functions of the multiple domains of Munc13–1 are coordinated. The crystal structure of a Munc13–1 fragment including its C1, C2B and MUN domains (C1C2BMUN) reveals a 19.5 nm-long multi-helical structure with the C1 and C2B domains packed at one end. The similar orientations of the respective diacyglycerol- and Ca2+-binding sites of the C1 and C2B domains suggest that the two domains cooperate in plasma-membrane binding and that activation of Munc13–1 by Ca2+ and diacylglycerol during short-term presynaptic plasticity are closely interrelated. Electrophysiological experiments in mouse neurons support the functional importance of the domain interfaces observed in C1C2BMUN. The structure imposes key constraints for models of neurotransmitter release and suggests that Munc13–1 bridges the vesicle and plasma membranes from the periphery of the membrane-membrane interface.DOI: http://dx.doi.org/10.7554/eLife.22567.001

Hypothesis – buttressed rings assemble, clamp, and release SNAREpins for synaptic transmission

“…Finally, a very recent and important high‐resolution structure of the complete core functional domain of Munc13 (the MUN domain) from the laboratories of Rizo and Ma 27, 28 reveals a rigid, curved, planar shape (Fig 1D), building on their earlier, less complete structures 29.…”

Molecular Mechanisms of Synaptic Vesicle Priming by Munc13 and Munc18

“…We next investigated the effect of the larger C1C2BMUN and C1C2BMUNC2C fragments (Liu et al, 2016; Xu et al, 2017) in our fusion assay. We used the same protocol as in Figure 2A except that the MUN domain was replaced with one of these fragments at the specified concentrations (Figures 3A, S2B, S2G, S4 and Table S4).…”

Molecular Mechanisms of Synaptic Vesicle Priming by Munc13 and Munc18

“…Ca 2+ -binding to the C2B domain, the synergy between C1, C2B, and C2C domains , and the Ca 2+ -dependent interaction between calmodulin and Munc13 (Junge et al, 2004) may localize Munc13 to sites of docked synaptic vesicles and increase its ability to assist in proper SNARE complex assembly, as corroborated by the concentration dependence of the various Munc13 fragments in our fusion experiments (Figures 3C-E). Indeed, mutations of the Munc13-2 C2B domain affect neurotransmitter release upon a single action potential as well as short-term synaptic plasticity (Shin et al, 2010) and mutations in the interface regions between the C1, C2B and MUN domains have differential effects on evoked release and the readily releasable pool (Xu et al, 2017), suggesting that the two molecular functions of Munc13 may be differentially regulated in the neuron. It is of course possible that Munc13 has a more direct role in the fusion process although our fusion experiments suggest that Munc13 can be removed after proper assembly and prior to Ca 2+ -triggering without a significant effect on Ca 2+ -triggered fusion.…”

“…We next investigated the effect of the larger C1C2BMUN and C1C2BMUNC2C fragments Xu et al, 2017) in our fusion assay. We used the same protocol as in Figure 2A except that the MUN domain was replaced with one of these fragments at the specified concentrations ( Figures 3A, S2B, S2G, S4 and Table S4).…”