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

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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 MW of 73,000. Within 60 min after synthesis it is almost completely converted to the mature form (MW = 78,000-80,000). Only the precursor form of GP-2 is sensitive to digestion with the glycosidase endo-.beta.-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 MW decreasing from 73,000 to 61,000. GP-2 apparently contains 5 or 6 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.