A Ras subfamily GTPase shows cell cycle-dependent nuclear localization

Previously characterized Ras subfamily proteins have been found to be predo minantly associated with the plasma membrane where they function in signal transduction pathways to convey extracellular signals to intracellular targ ets. Here, we provide evidence that the Dictyostelium Ras subfamily protein RasB has a novel subcellular localization and function. The protein is pre dominantly localized in the nucleus during most of the cell cycle. Furtherm ore, during mitosis and cytokinesis RasB assumes a diffuse cellular localiz ation despite the fact that the nuclear membrane stays intact. The linkage between the position of RasB in the cell and division suggests that it may have a role in nuclear division. Consistent with this idea, rasB(-) cells e xhibit severe growth defects and cells overexpressing an activated version of RasB are multinucleate.