IDENTIFICATION OF A VESICLE-ASSOCIATED MEMBRANE-PROTEIN (VAMP)-LIKE MEMBRANE-PROTEIN IN ZYMOGEN GRANULES OF THE RAT EXOCRINE PANCREAS

Zymogen granules of the exocrine pancreas are the secretory organelles responsible for the regulated secretion of digestive enzymes. Several proteins are associated with or are integral components of the lipid bilayer that forms the zymogen granule membrane. These proteins likely represent important components in the regulated secretion of digestiv e enzymes. VAMPs (vesicle-associated membrane proteins)/synaptobrevins are a family of 18-kDa integral membrane proteins originally characte rized in synaptic vesicles. Polyclonal antisera raised against either a VAMP/glutathione S-transferase (GST) fusion protein or rat brain syn aptic vesicles, defected an 18-kDa immunoreactive protein in zymogen g ranule membranes that co-migrates electrophoretically with rat brain s ynaptic vesicle VAMP. Rat brain synaptic vesicle VAMP was detected by both antisera. Botulinum-B toxin treatment of zymogen granule membrane s did not result in cleavage of zymogen granule membrane VAMP, indicat ing that exocrine pancreatic VAMP is either VAMP1 or a novel VAMP-isof orm Immunofluorescent studies demonstrated that exocrine pancreatic VA MP localized with GP2, a zymogen granule membrane protein, to the apic al region of pancreatic acinar cells. No significant labeling was obse rved in basolateral regions of pancreatic acinar cells. These results establish the presence of a VAMP protein in the zymogen granule of the rat pancreas and suggest that VAMPs have a role in exocrine secretion .