Structural Basis for the Clamping and Ca²⁺Activation of SNARE-mediated Fusion by Synaptotagmin

Abstract

Synapotagmin-1 (Syt1) interacts with both SNARE proteins and lipid membranes to synchronize neurotransmitter release to Ca 2+ -influx. To understand the underlying molecular mechanism, we determined the structure of the Syt1-SNARE complex on lipid membranes using cryo-electron microscopy. Under resting conditions, the Syt1 C2 domains adopt a novel membrane orientation with a Mg 2+ -mediated partial insertion of the aliphatic loops, alongside weak interactions with the anionic lipid headgroups. The C2B domain concurrently binds the SNARE bundle via the 'primary' interface and is positioned between the SNAREpins and the membrane. In this configuration, Syt1 is projected to sterically delay the complete assembly of the associated SNAREpins and thus, contribute to clamping fusion. This Syt1-SNARE organization is disrupted upon Ca 2+ -influx as Syt1 reorients into the membrane, allowing the attached SNAREpins to complete zippering and drive fusion. Overall, we find cation (Mg 2+ /Ca 2+ ) dependent membrane interaction is a key determinant of the dual clamp/activator function of Syt1.The rapid Ca 2+ -triggered release of neurotransmitters at the synapse is a highly orchestrated process 1-3 . The proteins involved are known and are well-characterized 1-4 . This includes the SNARE (Soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins (VAMP2, Syntaxin, and SNAP25) that catalyze synaptic vesicle (SV) fusion as well as chaperones Munc13, Munc18 and regulators Complexin and Synaptotagmin 1-4 . Synaptotagmin-1 (Syt1) is a key component involved in all stages of the process, including SV docking and priming 5-9 , preventing un-initiated SV fusion 10,11 and triggering fusion upon Ca 2+ influx [12][13][14][15] . Remarkably, such broad specialization has not resulted in structural complexity of the protein.Syt-1 is anchored in the SV by a transmembrane domain (TMD), which is connected to tandem cytosolic C2 domains (C2A & C2B) via a 60-residue flexible linker. Both C2 domains are eight stranded anti-parallel beta sandwiches with one calcium sensing region consisting of two aliphatic loops containing Aspartic acid (Asp) residues that bind Ca 2+16-18 . These conserved Asp residues coordinate Ca 2+ with the acidic lipids, like phosphatidylserine (PS) and phosphatidylinositol 4, 5bisphosphate (PIP2), to facilitate the membrane insertion of the flanking aliphatic loops 5,18-21 . In addition, the C2B domain has a conserved poly-lysine motif that binds to PS/PIP2 headgroups to mediate Ca 2+ -independent membrane interaction 5,7,8 . Both the Ca 2+ -independent and dependent membrane interactions of the Syt1 C2B domain are functionally critical. The interaction of the poly-lysine motif with PIP2 clusters facilitates the initial docking of the SV at the active zone 6,7,22,23 and Ca 2+ -dependent membrane insertion of the aliphatic loops is physiologically required for triggering synaptic transmission 13,21,24,25 . The C2A domain does not have a pronounced polybasic patch like C2B and its Ca 2+ -dependent membran...

Independent Yet Synergistic Roles of Synaptotagmin-1 and Complexin in Calcium Regulated Neuronal Exocytosis

“…We posit that Syt1 C2B domain binds PIP2 (via the poly-lysine motif) on the plasma membrane and assembles into ring-like oligomeric structures (Bello et al 2018;Wang et al 2014;Zanetti et al 2016). The Syt1 oligomers concurrently binds the t-SNAREs via the 'primary' interface (Zhou et al 2015;Zhou et al 2017;Grushin et al 2019). The Syt1-t-SNARE interaction, which likely precedes the engagement of the v-and the t-SNAREs, positions the Syt1 such that it sterically blocks the full assembly of the associated SNAREpins (Grushin et al 2019).…”

“…The Syt1 oligomers concurrently binds the t-SNAREs via the 'primary' interface (Zhou et al 2015;Zhou et al 2017;Grushin et al 2019). The Syt1-t-SNARE interaction, which likely precedes the engagement of the v-and the t-SNAREs, positions the Syt1 such that it sterically blocks the full assembly of the associated SNAREpins (Grushin et al 2019). The Syt1 oligomers in addition to creating a stable steric impediment, could also radially restrain the assembling SNAREpins (Grushin et al 2019;Rothman et al 2017).…”

“…It involves Syt1 C2B domain binding both Ca 2+ and the SNARE complex (Figure 4). We have recently demonstrated that Ca 2+ -binding to Syt1 C2 domains induce a large-scale conformational rearrangement of the Syt1-SNARE complex on the lipid membrane surface, disrupting the pre-fusion clamped architecture (Grushin et al 2019). This, in effect, reverses the Syt1 clamp, allowing the associated SNAREs to complete zippering and drive fusion.…”

“…Syt1 is also involved in initial stage of docking of SVs to the plasma membrane (PM), in part mediated by its interaction with the anionic lipids, phosphatidylserine (PS) and phosphatidylinositol 4, 5bisphosphate (PIP2) on the plasma membrane (PM) (Honigmann et al 2013;Parisotto et al 2012;Park et al 2012). Besides the membrane interaction, the Syt1 also binds the neuronal t-SNAREs on the PM both independently (primary binding site) and in conjunction with Complexin (tripartite binding site) (Zhou et al 2015;Zhou et al 2017;Grushin et al 2019).…”

SNARE Complex Alters the Interactions of the Ca2+sensor Synaptotagmin 1 with Lipid Bilayers

“…Syt1 can interact with t-SNARE proteins, and multiple studies suggest an important role for Syt1-SNARE interactions during fusion [18][19][20][21][22][23]. However, other studies have argued against this possibility [24,25], and it is still debated how the Syt1-SNARE complex is formed in vivo and what is the role of the Syt1-SNARE interactions in the fusion process [26][27][28][29][30][31][32].…”