BPS2026 – Fast reclamping controls synchrony and release probability during neuronal signaling

Synchronous neurotransmitter release with millisecond precision is essential for information processing and other nervous system functions. How the release machinery accomplishes synchrony at neuronal synapses is unknown since its architecture was not established, but recent advances present a picture in which a core calcium-sensitive ring of oligomerized synaptotagmin (Syt) C2B domains blocks fusion by binding (“clamping”) multiple SNARE fusogens and spacing membranes (Ramakrishnan et al., 2020). This has opened the door to computational simulation of the full machinery for the first time. Here, we used coarse-grained molecular dynamics to study the multicomponent release machinery. Following action-potential triggered calcium entry, the Syt ring disassembles and releases (“unclamps”) SNAREs. We find unclamping is rate-limited by Ca 2+ -C2B binding kinetics, so the electrophysiologically measured ∼ 0.1 ms delay time (Sabatini and Regehr, 1996) to first post-synaptic response (often interpreted as a characteristic fusion time) is unrelated to fusion, but rather the characteristic unclamping time. While unclamped, SNAREs exert entropic forces that drive fusion (Butu et al., 2025). We find synchrony requires fast reclamping when calcium subsides, requiring short Syt linker domains (LDs) to confine C2B domains near the fusion site. Consistently, Syt LDs have reduced effective lengths, due to intra-LD (Lai et al., 2013) or LD-membrane interactions (Lu et al., 2014). An outer ring of Munc13 MUN domains (Rothman et al., 2017, Grushin et al., 2022) may also promote fast reclamping. Fast reclamping also imposes low vesicle release probabilities P ves , a fundamental feature of neuronal signaling, since the unclamping window is much shorter than the mean fusion time due to entropic forces from unclamped SNAREs. We predict P ves depending on the number of active SNAREs, explaining higher release probabilities with more SNAREs (Acuna et al., 2014) or lowered fusion barriers (Basu et al., 2007).