Identification of Rab 5 but not Rab 3A in rat pancreatic zymogen granule membranes.

Low molecular weight GTP-binding (LMWG) proteins are known to be present on secretory organelles and a role for Rab 3A in exocytosis of synaptic vesicles has been postulated. Pancreatic zymogen granules possess multiple LMWG proteins which were shown to be on the outside of the granules by pronase susceptibility. Rab 3A, however, could not be identified in rat pancreatic acinar cells or on zymogen granules by immunoblots or by northern blots or PCR amplification although all three techniques demonstrated its presence in rat brain, RINm5F and AR42J cells. Immunoblots revealed expression of Rab 5, Rab 7, Rab 11 and ARF in acini. Of these, Rab 5 was identified in purified zymogen granule membranes where it may be involved in the endocytotic retrieval of granule membrane following exocytosis.