Snare proteins essential for cyclic AMP-regulated exocytosis in salivary glands

Abstract

Rat parotid acinar cells secrete amylase through the stimulation of beta-adrenoceptors followed by accumulation of intracellular cAMP. However, it remains unclear at the molecular level how secretory granules fuse with the apical membranes. We have examined whether SNARE proteins are involved in exocytosis in the salivary glands, and have found that one of the SNARE proteins, VAMP-2, is localized at the secretory granule membrane of rat parotid acinar cells. Moreover, botulinum neurotoxin B, which has endoprotease activity that cleaves VAMP-2, inhibited cAMP-dependent amylase release but did not inhibit basal secretion in the absence of cAMP. These results suggest that VAMP-2 is essential for cAMP-regulated exocytosis in rat parotid acinar cells. In contrast, both neurotoxins A and C1 (endoproteases that cleave SNAP-25 and syntaxin 1 respectively) failed to inhibit cAMP-dependent amylase release. Therefore, neither SNAP-25 nor syntaxin 1 are involved in amylase secretion in the parotid glands. Clarification of the mechanism of secretion will require the identification of proteins that interact and function cooperatively with VAMP-2. This approach may also reveal details of the molecular mechanism by which the cAMP facilitates secretion in other systems, including neurotransmission.