RabSD localizes to zymogen granules in rat pancreatic acini and other exocrine glands

Rab3 proteins are members of the family of Ras-like monomeric GTP-binding proteins that have been implicated in secretion in neuronal cells. Although an isoform of RabS has been assumed to exist in pancreatic acini, its identity has not yet been established. We now report that RabSD is present in rat pancreatic acini and is localized to the zymogen granule membrane. Reverse transcription-polymerase chain reaction (PCR) was used with primers based on mouse RabSD to amplify RabSD from rat pancreas. The PCR product without primer sites consisted of 580 base pairs and was 94% identical to the mouse RabSD cDNA sequence previously cloned from adipocytes. Western blotting with a polyclonal antiserum raised against RabSD-specific carboxyterminal amino acids identified RabSD in rat pancreatic acini and revealed its concentration on zymogen granule membranes. Immunocytochemistry of pancreatic lobules showed that RabSD localized to the apical region in a pattern similar to amylase. Confocal fluorescence microscopy of lobules double immunolabeled with antibodies to RabSD and the granule membrane marker protein glycoprotein-2 (GP-2) revealed a similar localization of these proteins to zymogen granules. Immunocytochemistry also revealed the presence of RabSD in chief and enterochromaffm-like cells in the stomach, acinar cells in lacrimal and parotid gland, and Paneth cells in the intestine. These results show that RabSD is expressed in rat pancreatic acini and other exocrine secretory cells. Its location implies it may be involved in regulated exocytosis.