CHO1 (green) colocalizes with actin fibers (red).

The actin cytoskeleton is alone on the cytokinesis stage no longer. The final step of cytokinesis cannot be completed without interactions between actin and a microtubule-interacting protein, CHO1, according to a new study by Kuriyama et al. on page 783.Organization of microtubules into the central spindle is achieved in part by the MKLP1 microtubule-binding motor protein. Kuriyama has determined that alternative splicing of MKLP1 generates CHO1, which has an additional 100 amino acids in the COOH-terminal tail. The group shows that MKLP1 and CHO1 are coexpressed within mammalian cells, both tightly associated with microtubules.

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. ▪