Biosynthesis of the Major Glycoprotein Associated with Zymogen‐Granule Membranes in the Pancreas

Abstract

The biosynthesis of GP‐2, the major glycoprotein associated with zymogen‐granule membranes in the pancreas, was studied in acinar cell suspensions from rat pancreas. Pulse‐chase experiments, using [35S]methionine, were performed and the processing of GP‐2 was analyzed by immunoprecipitation and sodium dodecyl sulfate/polyacrylamide gel electrophoresis. GP‐2 is synthesized as a precursor glycoprotein with apparent molecular weight Mr= 73000. Within 60 min after synthesis it is almost completely converted to the mature form (Mr= 78000–80000). Only the precursor form of GP‐2 is sensitive to digestion with the glycosidase endo‐β‐N‐acetylglucosaminidase H, indicating that the observed conversion reflects the processing of ‘high‐mannose’ oligosaccharides into complex type oligosaccharides. Acinar cells cultured in the presence of increasing concentrations of the N‐glycosylation inhibitor tunicamycin synthesize 5–6 distinct precursor GP‐2 species with apparent molecular weights decreasing from 73000–61000. We conclude that GP‐2 contains five or six N‐linked carbohydrate chains. From cell fractionation studies it was established that the precursor GP‐2 is present in a microsomal fraction with high density (> 1.169 g/ml) presumably derived from the rough endoplasmic reticulum; mature GP‐2 is localized in low density microsomes (< 1.130 g/ml) probably Golgi vesicles. The GP‐2 in zymogen granule membranes is also in the mature form. Copyright © 1983, Wiley Blackwell. All rights reserved