Carboxyl methylation of rab3D is developmentally regulated in the rat pancreas: correlation with exocrine function.

Several GTPases of the rab family, including rab3A, are methylesterifled on their carboxy-terminal prenylcysteine residue. The significance of this reversible posttranslational modification for the function of rab proteins is unknown, although it has been postulated that carboxyl methylation facilitates the membrane association of prenylated proteins through a hydrophobic mechanism. We here demonstrate, that pancreatic rab3D undergoes developmentally regulated carboxyl methylation concurrently with the maturation of the regulated secretory apparatus in pancreatic acinar cells: in fetal glands, which are refractive to hormone stimulation, the majority of the rab3D protein was methylated, whereas in neonatal and adult glands, which are secretory competent, only 50% was methylated. The methylated form of rab3D was also predominant in a transplantable acinar cell tumor which displays impaired secretory responsiveness and morphological characteristics reminiscent of the fetal pancreas. In addition, treatment of AR42J pancreatic acinar tumor cells with dexamethasone to induce a regulated secretory pathway, led to a significant increase in the size of the unmethylated pool of a rab3-like protein. Strikingly, membrane preparations from adult pancreata and parotid glands contained both methylated and unmethylated forms of rab3D indiscriminately. These results suggest that the acquisition of stimulus-secretion coupling by the exocrine pancreas correlates with the methylation state of rab3D, and that carboxyl methylation plays no significant role in enhancing the membrane association or determining the subcellular distribution of rab3D.