SNAP-23 and syntaxin-2 localize to the extracellular surface of the platelet plasma membrane

SNARE proteins direct membrane fusion events required for platelet granule secretion. These proteins are oriented in cell membranes such that most of the protein resides in a cytosolic compartment. Evaluation of SNARE protein localization in activated platelets using immunonanogold staining and electron microscopy, however, demonstrated expression of SNAP-23 and syntaxin-2 on the extracellular surface of the platelet plasma membrane. Flow cytometry of intact platelets confirmed trypsin-sensitive SNAP-23 and syntaxin-2 localization to the extracellular surface of the plasma membrane. Acyl-protein thioesterase 1 and botulinum toxin C light chain released SNAP-23 and syntaxin-2, respectively, from the surface of intact platelets. When resting platelets were incubated with both acyl-protein thioesterase 1 and botulinum toxin C light chain, a complex that included both SNAP-23 and syntaxin-2 was detected in supernatants, indicating that extracellular SNARE proteins retain their ability to bind one another. These observations represent the first description of SNARE proteins on the extracellular surface of a cell.