Septin Filament Compaction Into Rings Requires the Anillin Mid2 and Contractile Ring Constriction

Septin filaments assemble into high-order molecular structures that associate with membranes, acting as diffusion barriers and scaffold proteins crucial for many cellular processes. However, how septin filaments organize in such structures is still not well understood. In this study, we used fission yeast to explore septin filament organization during cell division and decipher key factors responsible for their regulation. Live-imaging and polarization microscopy analysis uncovered that septin filaments are initially recruited as a diffuse meshwork surrounding the acto-myosin contractile ring (CR) in anaphase, which undergoes compaction into two rings when CR constriction is initiated. We found evidence that the anillin-like protein Mid2 is necessary to promote this novel compaction step, possibly acting as a bundler for septin filaments. We also found that Mid2-driven septin compaction requires inputs from the Septation Initiation Network (SIN) as well as CR constriction or the β-1-3 glucan synthase Bgs1. This work highlights the complex regulations that allow the coordination between septin ring assembly and cell cycle progression.