Nim1-related kinases regulate septin organization and cytokinesis by modulating Hof1 at the cell division site

The septin scaffold recruits and organizes the actomyosin ring (AMR) components, thus ensuring faithful cytokinesis. The septin-associated kinases - Elm1, Gin4, Hsl1, and Kcc4 are believed to stabilize the septins at the bud neck, but the underlying mechanisms are largely unknown. Here, we present a comprehensive, quantitative analysis of these four septin regulatory kinases and reveal major roles for Elm1 and Gin4 in septin stability. We find that Elm1 and Gin4 play an overlooked role in actomyosin ring organization and constriction. We report that Gin4 kinase directly interacts with F-BAR protein Hof1 via its C-terminal membrane-binding domain and may be involved in proper organization and anchoring of AMR component Hof1 at the bud neck, representing an unappreciated mode of regulation of cytokinesis by the septin kinase network. We also show that Gin4 controls septin organisation and AMR constriction in a kinase-independent manner similar to Elm1. We have also performed an extensive GFP-GBP-based tethering screen in {Delta}elm1 and {Delta}gin4 cells and found an important role for Hsl1 in maintaining septin organisation and cell shape in coordination with Elm1, Gin4, and Kcc4. Furthermore, our data indicate that Hsl1 acts downstream of Elm1, with its membrane-binding KA1 domain being critical for its function. Together, these findings reveal new insights into modes of cytokinesis regulation by kinases Gin4 and Elm1 and highlight a redundant role for Hsl1 in controlling septin organization and cytokinesis, revealing the in-built adaptability of the septin kinase network in S. cerevisiae.