An evaluation of the expression, subcellular localization, and function ofrab4 in the exocrine pancreas

The small GTP-binding protein, rab4, is involved in recycling of transferri n receptors and translocation of GLUT4. Recent studies suggest that rab4 co ntrols regulated exocytosis in the exocrine pancreas. We conducted the pres ent study to further investigate the role of rab4 in the exocrine pancreas. We found that the exocrine pancreas expresses two rab4 immunoanalogs, one of similar to 28 kDa identified previously in neonatal glands, and one of s imilar to 24 kDa which is similar to rab4 characterized in other systems. T he latter species was mostly membrane-anchored and localized to endosome-li ke structures in a supranuclear region that was immunopositive for the tran sferrin receptor. The similar to 24-kDa rab4 form also localized to the api cal plasmamembrane, and this immunofluorescence increased greatly in tissue challenged with a secretagogue. We propose that the similar to 24-kDa rab4 species is involved in compensatory membrane retrieval following regulated exocytosis, and that rab4-positive endocytic vesicles move through a supra nuclear recycling compartment. (C) 2000 Academic Press.