Functional coordination of three mitotic motors in Drosophila embryos

It is well established that multiple microtubule-based motors contribute to the formation and function of the mitotic spindle, but how the activities of these motors interrelate remains unclear. Here we visualize spindle form ation in living Drosophila embryos to show that spindle pole movements are directed by a temporally coordinated balance of forces generated by three m itotic motors, cytoplasmic dynein, KLP61F, and Ncd. Specifically, our findi ngs suggest that dynein acts to move the poles apart throughout mitosis and that this activity is augmented by KLP61F after the fenestration of the nu clear envelope, a process analogous to nuclear envelope breakdown, which oc curs at the onset of prometaphase. Conversely, we find that Ncd generates f orces that pull the poles together between interphase and metaphase, antago nizing the activity of both dynein and KLP61F and serving as a brake for sp indle assembly. During anaphase, however, Ncd appears to have no effect on spindle pole movements, suggesting that its activity is downregulated at th is time, allowing dynein and KLP61F to drive spindle elongation during anap hase B.