SNAP25 (red) moves into the bulge (outlined) where sequential exocytosis is taking place (left to right).
Vesicles heading to the plasma membrane sometimes selectively bind to another that has already fused there. This process, called sequential exocytosis, is efficient at large-scale secretion because vesicles in the cytosol can be mobilized without being transported all the way to the plasma membrane. Using 2-photon imaging, the authors show that sequential exocytosis is directed by a plasma membrane SNARE called SNAP25.
Sequential exocytosis was examined in insulin-secreting pancreatic β cells, in which SNAP25 diffused into the bulge where the leading exiting vesicle had fused with the plasma membrane. SNAP25 rarely diffused into spots where only one vesicle exited, but was seen...
