Cell division concludes with cytokinesis, a process driven by a contractile ring of actin and myosin that lies underneath the plasma membrane at the cell's equator. Although myosin is essential for cytokinesis in various animal models, whether and to what extent this reflects its motor activity or its ability to crosslink actin has been a matter of debate. Now Daniel Osorio, Ana Carvalho and colleagues tackle this problem with the help of CRISPR/Cas9 gene editing and C. elegans embryos. Mutations introduced into the ATPase domain of non-muscle myosin-2 (NMY-2, the sole myosin required for early cytokinesis) bind to actin but fail to translocate it in vitro. In the animal, these ‘motor-dead’ mutations lead to adult sterility and embryonic inviability, and fail to support cytokinesis. When motor activity is partially impaired, cytokinesis is prolonged and more sensitive to reductions in overall NMY-2 levels. Finally, although actin levels in the contractile ring are not affected by either NMY-2 motor activity or overall levels, motor activity is required for the alignment and compaction of actin at the ring and subsequent equatorial deformation. Thus, myosin's motor activity is essential for cytokinesis, and its ability to crosslink actin filaments is not sufficient.
