The Role of the Carbohydrate in the Stabilization, Processing, and Packaging of the Glycosylated Adrenocorticotropin-Endorphin Common Precursor in Toad Pituitaries
The neurointermediate lobes of dark adapted toads, Xenopus laeuis, were incubated for 30 min in [3H]arginine, [3H]arginine plus [14C]glucosamine, or [3H]glucosamine and then chased for various time periods ranging from 1–3 h. The labeled polypeptides synthesized and secreted by the lobes were analyzed by acid-urea polyacrylamide gel electrophoresis. A glycosylated ACTH-endorphin precursor (32,000 mol wt) was synthesized during the pulse and identified by immunoprecipitation by ACTH-(11-24) antiserum. During the chase, this precursor was processed to various glycopeptides and peptides, including ACTH, β-lipotropin, and α-MSH, which were subsequently secreted into the medium. An immunoprecipitable ACTH-related glycoprotein (approximately 150,000 mol wt) and other nonimmunoprecipitable glycoproteins (approximately 80,000–100,000 mol wt) were also synthesized and secreted by the neurointermediate lobe. The secretion of these glycoproteins and peptides was inhibited by dopamine. The significance of glycosylation of the precursor for the biosynthesis, processing, and secretion of the ACTH, β-lipotropin-, and MSH-related peptides was examined by using a specific inhibitor of glycosylation, tunicamycin. Tunicamycin treatment did not affect the synthesis of the 32,000 mol wt ACTH-endorphin precursor but did prevent its glycosylation. The absence of carbohydrate on the precursor resulted in its rapid intracellular degradation. Precursors that escaped degradation were processed incompletely, leading to the formation and secretion of an unglycosylated intermediate and various other abnormal peptides. The data indicate that glycosylation of the ACTH-endorphin precursor may not be involved in the processes of intracellular transport, packaging, and secretion per se but, rather, may provide specific conformational stability to the precursor as a signal for directed limited proteolysis.