Low numbers of tSNAREs are within a functional distance of secretory vesicles.

<p>(<b>A</b>) A schematic representation of a secretory vesicle (<i>left</i>). A ribbon representation of the complex formed between syntaxin and SNAP-25 (<i>center</i>, based on PDB:3IPD <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049514#pone.0049514-Stein1" target="_blank">[31]</a>) and NMR structures of synaptobrevin (<i>right</i>, based on PDB:2KOG <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049514#pone.0049514-Ellena1" target="_blank">[30]</a>) both in synthetic lipid bilayers. The combined maximum reach of these proteins when in opposing bilayers is 17.8 nm. Assuming a distance of 0 nm between opposing bilayers, a chord 17.8 nm from the plasma membrane would have a half-length of 82.5 nm (<i>left</i>). This radial distance was used to calculate the number of plasma membrane SNAREs residing under each secretory vesicle. (<b>B</b>) A region of plasma membrane showing an overlay of rendered PALM data of SNAP-25 (<i>red</i>) and immunostained secretory vesicles (<i>green</i>). The center of mass of each secretory vesicle was calculated and used in a nearest neighbor analysis (<i>center</i>). The number of SNAP-25 molecules within 82.5 nm of each vesicle was calculated and is shown as a frequency histogram of mean ±SEM (n = 5 cells). (<b>C</b>) As in (B) but for PALM localized syntaxin (n = 5 cells).