EXOCRINE GRANULE SPECIFIC PACKAGING SIGNALS ARE PRESENT IN THE POLYPEPTIDE MOIETY OF THE PANCREATIC GRANULE MEMBRANE-PROTEIN GP2 AND IN AMYLASE - IMPLICATIONS FOR PROTEIN TARGETING TO SECRETORY GRANULES

The mechanisms for segregation of secretory and membrane proteins incorporated into storage granules from those transported constitutively have been thought to be conserved in diverse cell types, including exocrine and endocrine cells. However, GP2, the major protein of pancreatic zymogen granule membranes, in its native glycosyl phosphatidylinositol (GPI)-linked form, is incorporated into secretory granules when expressed in exocrine pancreatic AR42J cells, but not in the endocrine cells such as pituitary AtT20. To determine whether the protein moiety of GP2 contains the cell-type specific information for packaging into granules, a secretory form of GP2 (GP2-GPI(-)), with the GPI attachment site deleted, was generated and introduced into AR42J and AtT20 cells. Like native GP2, GP2-GPI(-) localized to the zymogen-like granules of AR42J cells and underwent regulated secretion. In AtT20 cells expressing GP2-GPI(-), however, the protein was secreted by the constitutive pathway. Thus, a granule packaging signal is present in the luminal portion of GP2 that is functional only in the exocrine cells. However, this cell-type dependent sorting process is not limited to GP2 or membrane proteins. Amylase, a major content protein of pancreatic acinar and serous salivary gland granules, was also secreted exclusively by the constitutive pathway when expressed in AtT20 cells. The cell-type specific targeting of GP2 to granules correlated with its behavior in an in vitro aggregation assay where it co-aggregated more effectively with content proteins from pancreatic zymogen granules than with those from pituitary granules. The results indicate that membrane and content proteins can interact during granule formation and that the packaging of exocrine proteins like GP2 and amylase is likely to be dependent on heterotypic binding to exocrine specific proteins.