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. Author manuscript; available in PMC: 2019 Aug 1.
Published in final edited form as: Trends Cell Biol. 2018 Apr 26;28(8):631–645. doi: 10.1016/j.tcb.2018.03.004

Figure 3. Models of quaternary arrangements of the SNARE/complexin-1/synaptotagmin-1 pre-fusion complex.

Figure 3

(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.