Beyond Nuclear Transport: Ran-Gtp as a Determinant of Spindle Assembly

It's not easy being a nuclear transport factor. During the G1, S, and G2 phases of the cell cycle, the nucleocytoplasmic transport machinery must tirelessly zigzag between the nucleus and cytoplasm while directing other proteins to the correct subcellular compartment. When M phase finally arrives, and the nucleus breaks down, do these proteins finally get to take a break? Apparently not. In the case of the Ran-GTPase and its associated nuclear transport factors, there is no rest for the weary. Instead, Ran and its associated proteins shift their focus from nuclear transport to the regulation of microtubule dynamics. Recent findings by several groups have shown that the GTP-bound form of Ran is necessary for the nucleation and organization of microtubule structures during M phase. Moreover, the implication that the active form of Ran is concentrated in the vicinity...