Figure 4.

Cytoplasmic microtubules interact dynamically with the cell cortex and result in associated movements of the spindle during mitosis. (A) The cytoplasmic microtubule sweeps back and forth along the bud cortex over the course of the time sequence. A short, faint cytoplasmic microtubule can also be seen in the mother cell, as well as the two spindle pole bodies and long spindle that spans the bud neck. In this cell, a bright spot of GFP staining, not corresponding to any microtubule structure, is seen in the mother cell. (B) Sweeping of a cytoplasmic microtubule along the cortex of the mother cell results in the spindle being pulled further into the mother cell. Note how the end of the spindle and the spindle pole body in the bud cell are pulled up into the neck by the third frame. (C) Shrinking of a microtubule at the cortex results in spindle movement toward the direction of the cortical attachment. A cytoplasmic microtubule appears in focus by t = 290 s and remains attached at the mother cortex while shrinking. This shrinking is coupled to a movement of the spindle toward the cortex. By the last frame the left spindle pole body is close to the cortex, and only a short cytoplasmic microtubule remains between them. Bars, 2 μm.