Fig. 4: Computational model of synergistic activation of vesicular fusion by Syt1 and Syt7.

a Schematic illustration of the release of inhibition model. At rest, fusion of vesicles is inhibited (‘clamped’) by binding of Syt1 and Syt7 along with CPX to partially assembled SNAREpins. Upon Ca²⁺ binding, the C2 domains of Syt1 and Syt7 insert into the membrane, leading to the removal of the fusion clamp. This allows the complete zippering of the SNARE complexes, resulting in vesicular fusion. Inset shows that two clamp architectures are considered in the default model: dual Syt1/Syt1 or dual Syt1/Syt7 clamp (see Supplementary Fig. 6 for additional clamp architectures tested). b Kinetic reaction schemes describing Ca²⁺-triggered release of the fusion clamp. Each modeled C2 domain sequentially binds two Ca²⁺ ions which triggers the insertion of its aliphatic loop into the membrane. Scheme 1 assumes that membrane insertion results in the instantaneous removal of the Synaptotagmin fusion clamp, while Scheme 2 assumes a delay between membrane insertion and the removal of the clamp. S₀, S₁, S₂ refer to 0, 1 or 2 Ca²⁺ bound state of the C2 domains, while I₂ refers to membrane inserted state of the Ca²⁺-bound C2 domain. The prefixes ^c and ^u refer to the ‘clamped’ or ‘unclamped’ state respectively. c The time course of vesicular fusion (Model Output) simulated in response to the experimentally constrained Ca²⁺ signal (Supplementary Fig. 3) for models with different clamp architecture and kinetics of clamp reversal. Experimental data (mean ± standard deviation from Fig. 1c) for the Ca²⁺-triggered fusion of Syt1 containing vesicles in the absence (Syt1_EXP) or the presence of saturating levels of Syt7 (Syt7_EXP) are plotted for comparison. The model suggests that experimentally observed fusion kinetics can be explained by the mechanism with differential rates of fusion clamp removal for Syt1 (instantaneous) and Syt7 (delayed). For each modeled condition a minimum of 1000 stochastic simulations were performed to calculate the average response. The source data is provided as a ‘Source Data’ file. Figure 4a created in BioRender. Krishnakumar, S. (2023) BioRender.com/x49d271.