FIGURE 9:

Model for pineapple assembly. Randomly oriented, Taxol-stabilized microtubules (left) recruit the spindle assembly factors dynein, NUMA, and kinesin-5. Dynein and NUMA cluster minus ends, whereas kinesin-5 slides antiparallel microtubules apart (middle, blue arrow). Together with parallel bundling factors, these activities cause the microtubules to sort into two parallel arrays with minus ends clustered on the outside (middle). Only the lower array is shown in the right for clarity. Plus ends recruit midzone assembly factors, including the CPC and Kif4. Plus ends tend to grow, but Kif4 restrains this growth. Lateral aggregation of midzone factors (purple zigzags) causes them to self-organize into a single plane (right). All these processes probably occur simultaneously and are only separated for clarity.