| Abstract: | The intracellular transport and destination of the major glycoprotein associated with zymogen granule membranes in the pancreas (GP-2) was established. In suspensions of isolated acinar cells from rat pancreas, pulse-chase experiments were performed. The incorporation of the first newly synthesized GP-2 molecules into zymogen granule membranes occurred at about 60 min after beginning of the pulse. We demonstrated by using two different methods that newly made GP-2 reaches the cell surface within the same time span. After 6-8 h chase considerable more newly synthesized GP-2 has reached the cell surface than would be expected on account of secreted newly synthesized zymogens. These observations strongly suggest that at least part of the GP-2 molecules bypass the mature zymogen granule compartment on their way to the plasma membrane. GP-2 is the only protein that appears in discernable quantity in the plasma membrane during 1-4 h after a pulse label. Nevertheless GP-2 comprises only a small percentage of externally 125I-iodinated plasma membrane proteins. We conclude that GP-2 has a high turnover rate at the plasma membrane level. Treatment of the acinar cells with the N-glycosylation inhibitor tunicamycin does not block the intracellular transport of GP-2. |
