CHO1 (green) colocalizes with actin fibers (red).
Although CHO1 associates with microtubules, Kuriyama et al. show that it is not required for chromosome separation. The unique CHO1 region imparted an unexpected actin-binding activity both in vitro and in vivo. Inhibition of this actin binding did not impair early steps in cytokinesis, but did prevent the final stage. Although the daughter cells initially appeared to separate completely, within hours the two had merged.
The success of the initial stage of cytokinesis suggests that CHO1 is not essential for contractility. It may, however, help to anchor the midbody matrix to the cell membrane or bring in vesicles to complete membrane separation. Inhibition of the small GTPase Arf causes a similar cytokinesis-deficient phenotype in worms, and Arf is known to bind CHO1, rearrange actin, and be involved in vesicle secretion. The precise linkage between these components will have to await further experiments. ▪