The expression pattern of rab3D in the developing rat exocrine pancreas coincides with the acquisition of regulated exocytosis

Although the secretory apparatus of rat pancreatic acinar cells already has a mature appearance in late stage fetuses, the regulated exocytotic pathway becomes functional only after birth. In this study we tested the hypothesis that the acquisition of stimulus-secretion coupling in the acinar cells depends on the developmental expression of rab3D, a small GTP-binding protein which we have previously shown to be associated with zymogen granule membranes in the adult pancreatic acinar cell. On immunoblots of pancreatic homogenates, rab3D became detectable on gestational day 18. This developmental pattern was consistent with that observed by confocal immunocytochemistry. In 20- and 21-day-old fetal pancreata, rab3D was mostly associated with the cytosolic fraction, whereas a redistribution to the membrane fraction occurred after birth, concomitant with a decrease in rab-GDI expression. Overlays of Western blots with P-32-alpha-GTP revealed that rab3D immunoprecipitated from fetal pancreatic homogenates possessed very little GTP-binding capacity as compared to adult homogenates. Triton X-114 extractions showed that the fetal rab3D partitioned into the detergent phase, suggesting that it was posttranslationally isoprenylated. Taken together, the present data indicate that the expression and localization of rab3D is developmentally regulated and strongly suggest that the maturation of the regulated exocytotic pathway in the exocrine pancreas depends on the membrane association of rab3D.