Aurora B associates with F-actin in the gap region and is required to stabilize microtubules for polarization. (A) TRITC-phalloidin staining revealed abundant F-actin cables between the furrow-like cortex and chromosomes (C). These cables run parallel to the polarized microtubules and are present in all optical sections, as shown in YZ and XZ views with the reference mark (x). (B) Aurora B associated with actin filaments in the gap region. In the pear stage, Aurora B appeared as puncta that decorate TRITC-phalloidin stained actin cables between microtubule tips and the cortex. Boxed regions are magnified below. (C) Monopolar HeLa cells treated with purvalanol A and 200 nM VX680. Examples of both nonpolarized cells (78%) and polarized cells are shown. In the nonpolarized cells the microtubules do not extend to the cortex. Aurora B itself is delocalized from microtubule tips and tends to associate with chromosomes (second-to-bottom row, yellow arrow). It is also less evident on actin cables in the cells that do manage to polarize (bottom row, white arrow). PRC1, MKLP1, and mDia are all delocalized as well. Bars, 5 μm.