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

Abstract

Photoaffinity labelling with [α‐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 G_sα subunits and the 41 kDa G_iα subunit, this protein was absent from zymogen granule membranes. Contrastingly, a new 28 kDa GTP‐binding protein (detected by [α‐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.