Rab4 associates with the actin terminal web in developing rat pancreatic acinar cells

Rab4 is a small GTP-binding protein that has been implicated in the regulation of membrane traffic and recycling of transferrin receptors and GLUT4 transporters along the endocytic pathway. Here we present data that suggest a novel and very different role for rab4 during development in the rat exocrine pancreas. On immunoblots of pancreatic homogenates, a dramatic increase in rab4 expression occurred over the first 40 h after birth, concomitant with the time of acquisition of stimulus-secretion coupling. Following high-speed centrifugation of postnuclear supernatants prepared from 1-day neonatal pancreatic homogenates, rab4 partitioned into a Triton X-100 insoluble particulate fraction and was partially solubilized upon extraction with 0.1 M Na2CO3, pH 11.5, or 1 M KCl, suggesting that rab4 was not an integral membrane protein. This was confirmed by Triton X-114 extractions of post-nuclear supernatants showing that rab4 partitioned into the aqueous phase of Triton X-114, which is indicative of a lack of isoprenylation. Confocal and electron microscopic immunocytochemistry revealed that rab4 colocalized with the actin terminal web and microvilli in the apical region of the exocrine acinar cells. In view of these findings, we propose that rab4 is involved in the maturation of the regulated secretory pathway in pancreatic acinar cells through an interaction with the apical actin cytoskeleton.