Ran stimulates spindle assembly by altering microtubule dynamics and the balance of motor activities

The guanosine tri-phosphatase Ran stimulates assembly of microtubule spindl es. However, it is not known what aspects of the microtubule cytoskeleton a re subject to regulation by Ran in mitosis, Here we show that Ran-GTP stimu lates microtubule assembly by increasing the rescue frequency of microtubul es three- to eightfold. In addition to changing microtubule dynamics, Ran-G TP also alters the balance of motor activities, partly as a result of an in crease in the amount of motile Eg5, a plus end-directed microtubule motor t hat is essential for spindle formation. Thus, Ran regulates multiple proces ses that are involved in spindle assembly.