Targeting of the zymogen-granule protein syncollin in AR42J and AtT-20 cells

Syncollin is a 13-kDa protein associated with the membranes of pancreatic z ymogen granules. Here we determine the in situ localization of syncollin in pancreatic acinar cells from adult and neonatal rats, and study the target ing of green fluorescent protein(GFP-) and His(6)-tagged syncollin chimaera s in model exocrine and endocrine secretory cells. Immunocytochemical analy sis of the distribution of syncollin in fully differentiated and neonatal a cinar cells revealed a granular pattern that corresponded with that of the zymogen-granule markers synaptobrevin 2 and amylase. In fully differentiate d acinar cells syncollin-positive vesicles were detected in the apical regi on of the cells, whereas in neonatal acinar cells they were found clustered near the cell nucleus. Both GFP- and His(6)-tagged syncollin entered the s ecretory pathway when transiently expressed in AR42J or AtT-20 cells. Synco llin-GFP was found predominantly in amylase- positive granules in AR42J cel ls and in adrenocorticotrophic hormone- (ACTH-) positive granules in AtT-20 cells. Syncollin-GFP was also present in the Golgi complex in AR42J cells. Syncollin-His(6) became localized in ACTH-containing granules in the neuri tic processes of AtT-20 cells. In AR42J cells syncollin-His(6) did not co-l ocalize with amylase, but was detected in acidic vesicles. These results sh ow that the exocrine protein syncollin contains intrinsic cell-type-indepen dent targeting information that is retained in both exocrine and endocrine cells after fusion to the GFP tag. In contrast, His(6)-tagged syncollin is efficiently targeted to secretory granules only in AtT-20 cells and not in AR42J cells.