Synthesis and Intracellular Transport of Common Antigens of Secretory Granule Membrane.

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Monoclonal antibodies (mAbs) were generated to the secretory granule membrane of rat and rabbit parotid gland, respectively. The mAbs recognized integral membrane glycoproteins localized on the luminal side of the secretory granules with N-linked carbohydrates : molecular weight ; 98-65 kD in the rat, 55-65 kD in the rabbit.Immunohistochemical studies indicated that both the secretory granule membrane antigens were found in the secretory granules of both exocrine and endocrine cells, and in the lysosomes of various cells in the rat and rabbit. These results indicated that common antigens are present in the secretory granule membranes. Immunoelectron microscopy with immunogold revealed that antigen was present on the membrane of the secretory granules, lysosomes, the Golgi vesicles and condensing vacuoles in pancreatic and parotid acinar cells and AR-42J rat pancreatic tumor cells ; the Golgi stacks exhibited no immunoreaction. In the rabbit, the antigen was localized exclusively along the membranes of secretory granules in the parotid acinar cells, exocrine pancreatic cells and adenohypophyseal cells.Employing HRP labeled mAb, we followed the retrieval of the antigen after exocytosis in AR42J cells. The mAb was internalized specifically with antigen-mediated endocytosis. It was transported to endosomes, subsequently to the trans-Golgi network, and then packaged into secretory granules. However, the Golgi stacks revealed no uptake of the labeled antibody. These results indicated that the common antigen of secretory granule memrane is reused after exocytosis, and suggested that the pool of the antigen might present in the trans-Golgi network.