Single secretory granules of live cells recruit syntaxin-1 and synaptosomal associated protein 25 (SNAP-25) in large copy numbers

Before secretory vesicles undergo exocytosis, they must recruit the proteins syntaxin-1 and synaptosomal associated protein 25 (SNAP-25) in the plasma membrane. GFP-labeled versions of both proteins cluster at sites where secretory granules have docked. Single-particle tracking shows that minority populations of both molecules are strongly hindered in their mobility, consistent with their confinement in nanodomains. We measured the fluorescence of granule-associated clusters, the fluorescence of single molecules, and the numbers of unlabeled syntaxin-1 and SNAP-25 molecules per cell. There was a more than 10-fold excess of SNAP-25 over syntaxin-1. Fifty to seventy copies each of syntaxin-1 and SNAP-25 molecules were associated with a single docked granule, many more than have been reported to be required for fusion.