Intracellular localisation of SNARE proteins in rat parotid acinar cells: SNARE complexes on the apical plasma membrane.

Intracellular localisation of soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein receptors (SNAREs) is an important factor in clarifying whether SNAREs regulate exocytosis in salivary glands. We investigated intracellular localisation of syntaxins 2, 3 and 4 and SNAP-23, which are thought to be target membrane (t)-SNAREs, in rat parotid gland by Western blotting and immunocytochemistry. Syntaxins 2 and 3 were localised in the apical plasma membrane (APM), and syntaxin 4 was localised in the plasma membrane. SNAP-23 was localised in the APM and intracellular membrane (ICM). In a yeast two-hybrid assay, syntaxins 2, 3 and 4 interacted with SNAP-23 and VAMP-3. Using immunoprecipitation methods, syntaxins 3 and 4 were seen to interact with VAMP-8 and SNAP-23 at the APM, respectively. SNAP-23 interacted with syntaxin 3, syntaxin 4, VAMP-2, VAMP-3 and VAMP-8. Many SNARE complexes were detected under non-stimulated/basic conditions in the parotid APM. Some of these complexes may have a role in exocytosis from parotid acinar cells.