Figure 8.

Model: pathway of spindle pole positioning determined by dynein, KLP61F, and Ncd activity during mitosis. Schematic illustrations depict the proposed timing and mechanism of action for dynein, KLP61F, and Ncd during the assembly and function of mitotic spindles in Drosophila early embryos. (A) Prophase. Dynein, anchored to the cortex by dynactin, pulls on astral MTs to separate spindle poles, whereas Ncd cross-links and slides together antiparallel MTs between the spindle poles to act as a brake. KLP61F is sequestered in the nucleus and does not participate in these movements. (B) Prometaphase/Metaphase. After NEB, KLP61F can cross-link and slide apart antiparallel MTs balancing the “inward” force generated by Ncd on these same MTs and offsetting spindle collapse. Cortical dynein pulls on astral MTs and in concert with KLP61F drives prometaphase spindle elongation. (C) Anaphase B. The inactivation of Ncd at anaphase allows KLP61F and cortical dynein to drive the poles farther apart. KLP61F appears to act before dynein in this process.