Rab3B regulates ZO-1 targeting and actin organization in PC12 neuroendocrine cells

Rab3B is a monomeric GTPase that modulates norepinephrine secretion when ex pressed in PC12 neuroendocrine cells. In the present study we determined wh ether rab3B also regulates the organization of intercellular junctions, sin ce this GTPase localizes to regions of cell contact in multiple cell types. The stable expression of rab3B, but not the closely related rab3A, led to two morphological phenotypes in PC12 cells: (i) reorganization of F-actin i nto long filopodia and (ii) redistribution of the junction-associated prote in ZO-1, ZO-1 localization was not appreciably affected by the expression o f a GTP binding mutant of rab3B (N135I) that stimulates norepinephrine secr etion by PC12 cells. The apparent diversity of these rab3B phenotypes impli es that this GTPase is capable of influencing cell signaling pathways that in turn, modulate the cytoskeleton and junction organization. In support of this hypothesis we observed that rab3B expression also altered the profile of proteins that interact with the signaling molecule, phosphatidylinosito l 3-kinase (PI3-binase). The effect of rab3B on protein interactions with P I3-kinase was reversed by inhibitors of this kinase, Furthermore, PI3-kinas e inhibitors virtually abolished ZO-1 localization at the surfaces of cells that express rab3B, but not rab3A, whereas these inhibitors had no effect on rab3B-dependent norepinephrine secretion. Our results indicate that rab3 B can influence junctional protein targeting and secretion by distinct mech anisms. (C) 2000 Academic Press.