Identification of SNAREs involved in regulated exocytosis in the pancreatic acinar cell

The molecular basis of exocytotic membrane fusion in the pancreatic acinar cell was investigated using an in vitro assay that measures both zymogen gr anule-plasma membrane fusion and granule-granule fusion. These two fusion e vents were differentially sensitive to Ca2+ suggesting that they are contro lled by different Ca2+-sensing mechanisms. Botulinum neurotoxin C (BoNT/C) treatment of the plasma membranes caused cleavage of syntaxin 2, the apical isoform of this Q-SNARE, but did not affect syntaxin 4, the basolateral is oform, BoNT/C also cleaved syntaxin 3, the zymogen granule isoform. BoNT/C treatment of plasma membranes abolished granule-plasma membrane fusion, whe reas toxin treatment of the granules reduced granule-plasma membrane fusion and abolished granule-granule fusion, Tetanus toxin cleaved granule-associ ated synaptobrevin 2 but caused only a small reduction in both granule-plas ma membrane fusion and granule-granule fusion. Our results indicate that sy ntaxin 2 is the isoform that mediates fusion between zymogen granules and t he apical plasma membrane of the acinar cell, Syntaxin 3 mediates granule-g ranule fusion, which might be involved in compound exocytosis. In contrast, the major R-SNARE on the zymogen granule remains to be identified.