Dynamics of the subcellular localization of RalBP1/RLIP through the cell cycle: the role of targeting signals and of protein-protein interactions.

The small G protein Ras regulates many cell processes, such as gene expression, proliferation, apoptosis, and cell differentiation. Its mutations are associated with one-third of all cancers. Ras functions are mediated, at least in part, by Ral proteins and their downstream effector the Ral-binding protein 1 (RalBP1). RalBP1 is involved in endocytosis and in regulating the dynamics of the actin cytoskeleton. It also regulates early development since it is required for the completion of gastrulation in Xenopus laevis. RalBP1 has also been reported to be the main transporter of glutathione electrophiles, and it is involved in multidrug resistance. Such a variety of functions could be explained by a differential regulation of RalBP1 localization. In this study, we have detected endogenous RalBP1 in the nucleus of interphasic cells. This nuclear targeting is mediated by nuclear localization sequences that map to the N-terminal third of the protein. Moreover, in X. laevis embryos, a C-terminal coiled-coil sequence mediates RalBP1 retention in the nucleus. We have also observed RalBP1 at the level of the actin cytoskeleton, a localization that depends on interaction of the protein with active Ral. During mitosis RalBP1 also associates with the mitotic spindle and the centrosome, a localization that could be negatively regulated by active Ral. Finally, we demonstrate the presence of post-transcriptional and post-translational isoforms of RalBP1 lacking the Ral-binding domain, which opens new possibilities for the existence of Ral-independent functions.