Immunolocalization of betagranin: a chromogranin A-related protein of the pancreatic B-cell.

The tissue and subcellular distribution of betagranin, a chromogranin A-related, cosecreted protein produced in rat insulinoma tissue, has been investigated using a combination of density gradient centrifugation, immunoblotting, immunofluorescence, and immunoelectron microscopic techniques. Antibodies raised to insulinoma betagranin recognized antigens of the same molecular size (approximately 20,000 daltons) in insulinoma tissue and normal islets. Antigenicity was confined principally to secretory granules, and in insulinoma tissue was colocalized with insulin. Within the islet, all endocrine cells were immunoreactive, although subpopulations of beta- and alpha-cells displayed a more intense immunofluorescence. Adrenal tissue and anterior and posterior pituitaries were also highly immunoreactive, the antigen again being confined principally to the secretory granule. Higher molecular size species of 65,000, 85,000, and 100,000 daltons, which predominated in adrenal, were also present in pituitary along with equivalent amounts of the 20,000-dalton proteins. Isolated cells in the gastric antrum, small intestine, and colon were strongly immunofluorescent, but again, the molecular form differed from those of other tissues. Parallel experiments performed with antichromogranin A antisera suggested that betagranin in pancreatic B-cells is formed from chromogranin A by limited proteolysis within the secretory granule. It would appear that although chromogranin A is confined to tissues of the diffuse neuroendocrine system it can be processed differentially in tissues in this series. Potentially, the biological activity of chromogranin A resides in such derived peptides rather than in the parent molecule.