Small GTP-binding protein, Rab6, is associated with secretory granules in atrial myocytes

Rab proteins, a subfamily of small GTP-binding proteins, have been shown to play key roles in regulation of vesicular traffic in eukaryotic cells. In this study, we have intended to identify the atrial granule-associated Rab proteins that seem to be required for formation or intracellular transport of the granules. Atrial granules contained at least four small GTP-binding proteins, and we have demonstrated by biochemical analysis that one of the small GTP-binding proteins associated with the atrial granules is a Rab6 protein (Rab6p). Rab6p was also detected in highly purified zymogen granules of pancreatic exocrine gland. Immunogold electron microscopy performed on ultrathin cryosections of rat auricle revealed that Rab6p was associated with the atrial granule membranes. Association of Rab6p with the atrial granule membranes was also confirmed by immunodiffusion electron microscopy in agarose-embedded atrial granules. These data indicate that Rab6p is associated with the atrial granules and that it might function in the intracellular traffic of the secretory granules in the atrial myocytes.