Fig. 8: Simple model quantifying the impact of Munc13 clustering and oligomerization on SNARE assembly.

A The distribution of Munc13 monomers under a 50 nm vesicle (gray) assuming a uniform random two-dimensional distribution of Munc13 on the plasma membrane with various densities as indicated. B Failure rate (defined as the probability of fewer than three SNAREpins forming under a vesicle) is plotted versus the probability of an individual SNAREpin successfully assembling on a Munc13 (p). The density corresponding to 50–100 copies of Munc13 per AZ is highlighted (magenta). C. Clustering of Munc13 at a fixed AZ density of 500 per µm² was modeled by associating a range of binding energies (ε_B in units of k_BT) with Munc13 monomers under a vesicle as indicated. These distributions were used to model Munc13 clustering observed at presynaptic sites^7,8,40. D Failure rate versus SNAREpin probability for the clustered Munc13 distributions. E Probability of being in the loosely docked state versus tightly docked state as a function of Munc13 monomer number under a vesicle. The baseline energy penalty of transiting to the tightly docked state was set to 35 kT and the docking energy contribution per Munc13 monomer (ε_Z) was varied between 1 and 10 kT as indicated. F Failure rate versus SNAREpin probability for clustered Munc13 distributions using a binding energy ε_B = 10 kT and varying the docking energy ε_Z as indicated. G An equilibrium reaction scheme for individual SNAREs (S, orange and blue) binding to an oligomer of Munc13 (M, yellow) to form assembled SNAREpins where M is composed of N Munc13 monomers. H Failure rate versus SNAREpin probability for oligomers of size N = 3 (red), N = 6 (black), and N = 8 (green) using several cooperativity factor values: γ = 1 (no cooperativity), 2, 4, and 6. Oligomers of N = 6 with modest cooperativity can essentially eliminate the chance of failure to form three SNAREpins over a broad range of individual SNAREpin probabilities. See Methods for additional details.