Simulations of the dynamic encounters of secretory vesicles and tSNARE molecules.

<p>(<b>A</b>) Initial positions of secretory vesicles and SNAP-25 were derived from PALM experiments. Over 5 s molecules and secretory vesicles were allowed to diffuse, bound by the restrictions imposed by the model. A combined image (<i>left panel</i>) of all tracks (<i>grey</i>) and secretory vesicles (<i>green, disc signifies sampling window and dashed line circumference of the vesicle</i>) is shown. A single molecular path is shown (center) with trajectory sampled at 10 µs (<i>grey</i>), sampled every 50 ms as in the sptPALM (<i>blue</i>) and secretory vesicles (<i>green</i>) The number of SNAP-25 molecules within the sampling window is shown over time for a single secretory vesicle over 5 s (<i>upper right</i>) and enlarged over 500 ms (<i>lower right</i>). (<b>B</b>) As in (A) but using initial positions for secretory vesicles and syntaxin derived from PALM experiments (<b>C</b>) The probability of each vesicle having a tSNARE within the sampling window at any particular time was calculated using the traces shown in panels a and b. The number of tSNAREs is plotted against frequency for each secretory vesicle in the simulation (<i>vertical bars, dark grey to light grey</i>). The mean for all secretory vesicles in each bin is shown (<i>blue line</i>). This shows that at any given point in time the majority of vesicles experience three or fewer SNARE molecules.