REDISTRIBUTION OF A RAB3-LIKE GTP-BINDING PROTEIN FROM SECRETORY GRANULES TO THE GOLGI-COMPLEX IN PANCREATIC ACINAR-CELLS DURING REGULATED EXOCYTOSIS

Regulated secretion from pancreatic acinar cells occurs by exocytosis of zymogen granules (ZG) at the apical plasmalemma. ZGs originate from the TGN and undergo prolonged maturation and condensation. After exoc ytosis, the zymogen granule membrane (ZGM) is retrieved from the plasm a membrane and ultimately reaches the TGN. In this study, we analyzed the fate of a low M(r) GTP-binding protein during induced exocytosis a nd membrane retrieval using immunoblots as well as light and electron microscopic immunocytochemistry. This 27-kD protein, identified by a m onoclonal antibody that recognizes rab3A and B, may be a novel rab3 is oform. In resting acinar cells, the rab3-like protein was detected pri marily on the cytoplasmic face of ZGs, with little labeling of the Gol gi complex and no significant labeling of the apical plasmalemma or an y other intracellular membranes. Stimulation of pancreatic lobules in vitro by carbamylcholine for 15 min, resulted in massive exocytosis th at led to a near doubling of the area of the apical plasma membrane. H owever, no relocation of the rab3-like protein to the apical plasmalem ma was seen. After 3 h of induced exocytosis, during which time approx imately 90% of the ZGs is released, the rab3-like protein appeared to translocate to small vesicles and newly forming secretory granules in the TGN. No significant increase of the rab3-like protein was found in the cytosolic fraction at any time during stimulation. Since the prot ein is not detected on the apical plasmalemma after stimulation, we co nclude that recycling may involve a membrane dissociation-association cycle that accompanies regulated exocytosis.