The guanosine tri-phosphatase Ran stimulates assembly of microtubule spindles. However, it is not known what aspects of the microtubule cytoskeleton are subject to regulation by Ran in mitosis. Here we show that Ran-GTP stimulates microtubule assembly by increasing the rescue frequency of microtubules three- to eightfold. In addition to changing microtubule dynamics, Ran-GTP also alters the balance of motor activities, partly as a result of an increase in the amount of motile Eg5, a plus-end-directed microtubule motor that is essential for spindle formation. Thus, Ran regulates multiple processes that are involved in spindle assembly.

Abstract. We used high-resolution video microscopy to visualize microtubule dynamic instability in extracts of interphase sea urchin eggs and to analyze the changes that occur upon addition of 0.8-2.5 #M okadaic acid, an inhibitor of phosphatase 1 and 2A (PP1, PP2a) (Bialojan, D., and A. Takai. 1988. Biochem. J. 256:283-290). Microtubule plus-ends in these extracts oscillated between the elongation and shortening phases of dynamic instability at frequencies typical for interphase cells. Switching from elongation to shortening (catastrophe) was frequent, but microtubules persisted and grew long because of frequent switching back to elongation (rescue). Addition of okadaic acid to the extract induced rapid (<5 min) conversion to short, dynamic microtubules typical of mitosis. The frequency of catastrophe doubled and the velocities of elongation and shortening increased slightly; however, the major change was an elimination of rescue. Thus, modulation of the rescue frequency by phosphorylationdependent mechanisms may be a major regulatory pathway for selectively controlling microtubule dynamics without dramatically changing velocities of microtubule elongation and shortening.

Microtubules (MTs) in newt mitotic spindles grow faster than MTs in the interphase cytoplasmic microtubule complex (CMTC), yet spindle MTs do not have the long lengths or lifetimes of the CMTC microtubules. Because MTs undergo dynamic instability, it is likely that changes in the durations of growth or shortening are responsible for this anomaly. We have used a Monte Carlo computer simulation to examine how changes in the number of MTs and changes in the catastrophe and rescue frequencies of dynamic instability may be responsible for the cell cycle dependent changes in MT characteristics. We used the computer simulations to model interphase-like or mitotic-like MT populations on the basis of the dynamic instability parameters available from newt lung epithelial cells in vivo. We started with parameters that produced MT populations similar to the interphase newt lung cell CMTC. In the simulation, increasing the number of MTs and either increasing the frequency of catastrophe or decreasing the frequency of rescue reproduced the changes in MT dynamics measured in vivo between interphase and mitosis.

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