Novel GTP-binding proteins in plasma membranes and zymogen granule membranes from rat pancreas and in pancreatic AR 4-2J cell membranes.

Photoaffinity labelling with [alpha-32P]GTP allowed to detect a 54 kDa GTP-binding protein in rat pancreatic plasma membranes and in pancreatic AR 4-2J cell membranes. Like the 42 and 48 kDa Gs alpha subunits and the 41 kDa Gi alpha subunit, this protein was absent from zymogen granule membranes. Contrastingly, a new 28 kDa GTP-binding protein (detected by [alpha-32P]GTP binding on immobilized proteins) and a 25 kDa protein (ADP-ribosylated by botulinum toxin D) were found in all three membrane preparations. This is to our knowledge the first report on GTP-binding proteins in zymogen granule membranes.