EFFECT OF TUNICAMYCIN ON THE SYNTHESIS, PROCESSING, AND SECRETION OF PRO-OPIOMELANOCORTIN PEPTIDES IN MOUSE PITUITARY-CELLS

Pro-opiomelanocortin (POMC) is glycosylated and proteolytically cleaved to produce a number of smaller peptide hormones including ACTH and endorphin in mammalian pituitary and the mouse pituitary cell line AtT-20/D16v. 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 29,000- and 32,000-dalton forms of POMC. The 26,000-dalton form found in tunicamycin-treated cells has the same [35S]methionine tryptic peptides as 29,000- and 32,000-dalton POMC, indicating that the decreease in apparent mass is most likely due to loss of carbohydrate and not to changes in the peptide backbone. The 4,500-dalton form of .alpha.(1-39)ACTH and the 3,000- and 11,000-dalton forms of endorphin are all present in tunicamycin-treated cells. The glycosylated form of .alpha.(1-39)ATCH, however, is missing and the glycosylated ACTH intermediates are replaced by unglycosylated ACTH intermediates. Pulse chase 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. It appears that glycosylation is not an essential step for correct cleavage or secretion of POMC or its products.