Pro-opiomelanocortin Peptides in Mouse Pituitary Cells*

Pro-opiomelanocortin (POMC) is glycosylated and proteolytically cleaved to produce a number of smaller peptide hormones including adrenocorticotropic hormone (ACTH) and endorphin in mammalian pituitary and the mouse pituitary cell Line AtT-20/DI6,. When glycosylation of POMC is inhibited in AtT-20 cells with the drug tunicamycin, a 26,000-dalton protein appears in place of the glycosylated 29,000and 32,000-dalton forms of POMC. The 26,000-dalton form found in tunicamycin-treated cells has the same [3SS]methionine tryptic peptides as 29,000and 32,000-ddton POMC, indicating that the decrease in apparent m a s s is most likely due to loss of carbohydrate and not to changes in the peptide backbone. The 4,500-dalton form of a(139)ACTH and the 3,000and 11,000-dalton forms of endorphin are all present in tunicamycin-treated cells. The glycosylated form of a(l-39)ACTH, however, is missing and the glycosylated ACTH intermediates are replaced by unglycosylated ACTH intermediates. Pulsechase studies demonstrate that the 26,000-dalton unglycosylated POMC is the precursor of the smaller ACTH and endorphin molecules in tunicamycin-treated cells. Furthermore, all of the forms of ACTH and endorphin found in tunicamycin-treated cells are secreted. Thus, it appears that glycosylation is not an essential step fo r correct cleavage or secretion of POMC or its products.