Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
Review
. 2018 Aug;28(8):631-645.
doi: 10.1016/j.tcb.2018.03.004. Epub 2018 Apr 26.

Ca2+-Triggered Synaptic Vesicle Fusion Initiated by Release of Inhibition

Axel T Brunger  1 , Jeremy Leitz  2 , Qiangjun Zhou  2 , Ucheor B Choi  2 , Ying Lai  2
Affiliations
Review

Ca2+-Triggered Synaptic Vesicle Fusion Initiated by Release of Inhibition

Axel T Brunger et al. Trends Cell Biol. 2018 Aug.

Abstract

Recent structural and functional studies of the synaptic vesicle fusion machinery suggest an inhibited tripartite complex consisting of neuronal soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs), synaptotagmin, and complexin prior to Ca2+-triggered synaptic vesicle fusion. We speculate that Ca2+-triggered fusion commences with the release of inhibition by Ca2+ binding to synaptotagmin C2 domains. Subsequently, fusion is assisted by SNARE complex zippering and by active membrane remodeling properties of synaptotagmin. This additional, inhibitory role of synaptotagmin may be a general principle since other recent studies suggest that Ca2+ binding to extended synaptotagmin C2 domains enables lipid transport by releasing an inhibited state of the system, and that Munc13 may nominally be in an inhibited state, which is released upon Ca2+ binding to one of its C2 domains.

Keywords: Ca(2+) triggering; action potential; fusion protein; synaptic vesicle fusion; synaptic vesicle priming.

PubMed Disclaimer

Figures

Figure 1
Figure 1. More complete reconstitution improves fusion probability in vitro
(A) Domain diagrams of SNAREs, synaptotagmin-1, and complexin-1. (B) Histogram of vesicle fusion time upon entry of 500 µM Ca2+ (at time = 0 sec), as measured in a single-vesicle fusion assay [50,59]. The inset shows the corresponding cumulative probability histogram. Synthetic proteoliposomes that mimic the synaptic vesicle (SV) are reconstituted with synaptobrevin-2 and synaptotagmin-1, while plasma membrane-mimic proteoliposomes (PM) are reconstituted with syntaxin-1 and SNAP-25. The histograms were normalized by the number of associated (docked) vesicle pairs. Alone, this simple reconstitution (blue) produces only ~0.5 % fusing vesicle pairs immediately upon Ca2+ entry. Addition of complexin-1 to this fusion system (green) increases both the number of fusion-competent vesicle pairs as well as the fraction of vesicles that fuse immediately upon Ca2+ entry. In the more complete reconstitution system (red) NSF, α-SNAP and Munc18 are incubated with PM vesicles in the presence of ATP and Mg2+ to disassemble pre-formed binary (syntaxin-1/SNAP-25) SNARE complexes and allowing Munc18 to trap free syntaxin-1 (we refer to these syntaxin-Munc18 vesicles as SM vesicles). Subsequent addition of SNAP-25, Munc13, and complexin-1 with the SV vesicles greatly enhances the total number of fusion competent vesicle pairs as well as the number of vesicles that fuse immediately upon calcium addition. Data are taken from [50,59]. SV: synaptic vesicle mimicking proteoliposome; PM: plasma membrane mimicking proteoliposome; TM: transmembrane domain.
Figure 2
Figure 2. Atomic-resolution structures of SNARE/complexin-1, SNARE/synaptotagmin-1, and SNARE/complexin-1/synaptotagmin-1 complexes
(A) Crystal structure of the pairwise complex between complexin-1 (yellow) and the SNARE complex (synaptobrevin-2, blue; SNAP-25, green, syntaxin-1A, red) [63] (PBD ID 1KIL). (B) Superposition of the Ca2+ and Mg2+-bound crystal structures of the pairwise complex between the SNARE complex (synaptobrevin-2, blue; SNAP-25, green, syntaxin-1A, red), and synaptotagmin-1 C2B (gray, green, purple, blue, and gold) [3] (PDB IDs 5CCG and 5CCI). For clarity, only the primary C2B-SNARE interface is shown. (C) Crystal structure of the Ca2+-free tripartite complex between the half-zippered SNARE complex (synaptobrevin-2, blue; SNAP-25, green, syntaxin-1A, red), complexin-1 (yellow), and synaptotagmin-1 C2B (gray, green, purple, blue, and gold) [16] (PDB ID 5W5C). (D) Functional regions and interfaces of the synaptotagmin-1 C2B domain. The colors indicate the loops involved in Ca2+-binding (gold), the primary SNARE-synaptotagmin-1 interface (green), the tripartite SNARE/complexin-1/synaptotagmin-1 interface (purple), and the polybasic region (blue).
Figure 3
Figure 3. Models of quaternary arrangements of the SNARE/complexin-1/synaptotagmin-1 pre-fusion complex
(A) Schema of the trans SNARE complex interacting with two synaptotagmin-1 C2B domains and the central α-helix of complexin-1 in the core quaternary arrangement. The trans SNARE complex consists of synaptobrevin-2 (blue), syntaxin-1 (red) and SNAP-25 (green). The trans SNARE complex forms two interfaces (referred to as primary and tripartite interfaces) with two synaptotagmin-1 C2B domains (represented as multicolored ellipsoids), one of which also involves the central α-helix of complexin-1 (yellow) [16]. The colors of the C2B ellipsoid indicate the loops involved in Ca2+-binding (gold), the primary SNARE/synaptotagmin-1 interface (green), the tripartite SNARE/complexin-1/synaptotagmin-1 interface (purple), and the polybasic region (blue). For clarity, the rest of synaptotagmin-1 including the C2A domain and the transmembrane domain, has been omitted. For the trans SNARE complex, the primary interface mainly involves SNAP-25 while the tripartite interface involves synaptobrevin-2, syntaxin-1 and complexin-1 (yellow). (B) and (C) Two orthogonal views of the core arrangement consisting of one trans SNARE complex that interacts with one central α-helix of complexin-1, and two synaptotagmin-1 C2B domains. See also Supplementary Video 1. (C) The same core arrangement viewed from above. (D) and (E) Two orthogonal views of the bridge quaternary arrangement consisting of two trans SNARE/complexin-1 complexes that interact with a single C2B domain via primary and tripartite interfaces, respectively. To exemplify the potentially iterative nature of this interaction, additional synaptotagmin-1 C2B domains are shown in white. See also Supplementary Video 2. (E) A top-down view of (D). In panels (B)–(E), the primary and tripartite molecular interface areas are indicated by green and purple colored surfaces, respectively, and the known crystal structure of the SNARE/complexin-1/synaptotagmin-1 complex (PDB ID 5W5C) is shown in ribbon representation.
See this image and copyright information in PMC

Similar articles

See all similar articles

Cited by

See all "Cited by" articles

References

    1. Südhof TC. Neurotransmitter Release: The Last Millisecond in the Life of a Synaptic Vesicle. Neuron. 2013;80:675–690. - PMC - PubMed
    1. Rothman JE. The Principle of Membrane Fusion in the Cell (Nobel Lecture) Angew. Chemie Int. Ed. 2014 doi: 10.1002/anie.201402380. - DOI - PubMed
    1. Zhou Q, et al. Architecture of the synaptotagmin-SNARE machinery for neuronal exocytosis. Nature. 2015;525:62–67. - PMC - PubMed
    1. Hui E, et al. Synaptotagmin-mediated bending of the target membrane is a critical step in Ca(2+)-regulated fusion. Cell. 2009;138:709–21. - PMC - PubMed
    1. Martens S, et al. How Synaptotagmin Promotes Membrane Fusion. Science (80-.) 2007;316:1205–1208. - PubMed

Publication types

MeSH terms

LinkOut - more resources

-