Fang et al. describe two distinct pathways that target the tail of yeast myosin II to the bud neck to direct cell division.
Myosin II motors drive cytokinesis by assembling a contractile actomyosin ring at the site of cell division. The budding yeast myosin II, Myo1, is targeted to the bud neck by its C-terminal tail, and this domain alone is sufficient to separate mother and daughter cells, even though it lacks the actin-binding and motor activity of Myo1’s head domain.
By constructing a series of deletion mutants, Fang et al. discovered two separate mechanisms that localize the tail of Myo1 at different points in the cell cycle. Before cytokinesis, Myo1 localized to the bud neck by binding Bni5, a protein that in turn binds to septin GTPases. During cytokinesis, however, Myo1 targeting depended on the actin-binding protein IQGAP instead.
The Myo1 tail was sufficient to assemble an actomyosin ring at the bud neck, perhaps because IQGAP can compensate for the loss of Myo1’s actin-binding head. But how does the Myo1 tail promote cytokinesis in the absence of any motor activity? Fang et al. found evidence to suggest that the “headless” actomyosin ring directs the deposition of vesicles around the bud neck to form the primary septum—a specialized cell wall region that pushes the cytokinetic furrow inward. Senior author Erfei Bi says that the actomyosin ring therefore coordinates membrane deposition and contractile force during cytokinesis. He now wants to investigate how Myo1 and IQGAP promote actomyosin ring assembly and how the ring is coordinated with septum formation.