Bni5 regulates and coordinates septin architecture and myosin-II functions at the cell division site.

The spatiotemporal coordination of septins and myosin-II in processes like cytokinesis is not well understood. In Saccharomyces cerevisiae, Bni5 links the myosin-II heavy chain Myo1 to the septin hourglass at the bud neck prior to cytokinesis, but the underlying mechanisms and functions remain unclear. Here, we show that Bni5 binds septin filaments, the septin-associated kinase Elm1, and Myo1 via distinct domains. Bni5 regulates the architecture and stability of the septin hourglass until it dissociates from the bud neck at the onset of cytokinesis. This dissociation, facilitated through phosphorylation of Bni5 by Gin4, an Elm1-interacting kinase, enables timely remodeling of the septin hourglass into a double ring. Bni5 also mediates the role of Myo1 in retrograde actin cable flow during polarized growth and ensures maximal accumulation of Myo1 at the bud neck before cytokinesis, reinforcing the actomyosin ring and buffering it against perturbations. These findings establish Bni5 as a key regulator and coordinator of septins and myosin-II at the division site.