Labeling of the proteins at the surface of the pancreatic zymogen granule using diazotized [125I]iodosulfanilic acid

Purified pig and rat pancreatic zymogen granules have been covalently labeled with the membrane impermeant agent diazotized 125I]iodosulfanilic acid. Following alkaline lysis, the radioactivity was almost entirely (92%) recovered in a dense protein pellet designated as the 1 M sucrose pellet. The rest (8%) of the label was recovered in the membrane fraction. The specificity of this procedure in labeling the cytoplasmic aspect of the granule is demonstrated by the absence of label from granule content proteins and by the removal of iodinatable proteins following protease treatment of intact granules. No characteristic integral membrane proteins were labeled. In the pig, four major protein bands were labeled in both subfractions at Mr of 15,000, 33,000, 35,000 and 38,000. In the rat, a similar set of protein bands was labeled except for that of 15,000 Mr which was poorly labeled. Due to their location, it is suggested that these proteins may play an important role in the recognition between the granule membrane and the cell membrane and thereby the control of the exocytosis process.