Prosomatostatin, the precursor of the hormone somatostatin, harbors an N-glycosylation site in its prodomain that has never been shown to be modified by the N-oligosaccharyl transferase (OST) of the endoplasmic reticulum (ER). The addition of Brefeldin A (BFA) to prosomatostatin transfected AtT20 cells leads to a quantitative glycosylation of the prohormone. Upon removal of the BFA the glycosylated hormone precursor is not deglycosylated, and is secreted after maturation of its oligosaccharide chain in the late secretory pathway. In addition, a significant proportion of the glycosylated hormone precursor remains in the cell. Since BFA is known to induce an effective collapse of the Golgi complex into the ER, the hypothesis that a prolonged exposure to the ER glycosylation machinery is responsible for the glycosylation was tested. No N-glycosylation was detected using a coupled in vitro transcription-translation system in the presence of canine pancreatic microsomes, indicating that rapid transit through the ER does not explain the lack of glycosylation observed in vivo in the absence of BFA. These observations show that co-translational glycosylation by OST becomes possible due to a still unidentified modification in the luminal environment brought about by the coalescence of the Golgi into the ER caused by BFA. (C) 2002 Elsevier Science (USA). All rights reserved.