Critical Roles of a RhoGEF-Anillin Module in Septin Architectural Remodeling During Cytokinesis

How septin architecture is remodeled from an hourglass to a double ring during cytokinesis in fungal and animal cells remains unknown.  Here, we show that during the hourglass-to-double ring transition in budding yeast, septins acquire a striking “zonal architecture” in which paired septin filaments that are organized along the mother-bud axis associate with circumferential single septin filaments, the RhoGEF Bud3, and the anillin-like protein Bud4 exclusively at the outer zones and with myosin-II filaments the middle zone.  Deletion of Bud3 or its Bud4-interacting domain, but not its RhoGEF domain, leads to a complete loss of single filaments whereas deletion of Bud4 or its Bud3-interacting domain destabilizes the transitional hourglass, especially at the mother side, with partial loss of both filament types.  This and further analysis indicate that Bud3 stabilizes the single filaments whereas Bud4 fastens the junction between the paired and single filaments.  Deletion of BUD3 and BUD4 together abolishes transitional hourglass and prevents double ring formation.  The absence of the septin structures in the double mutant presents a unique opportunity to examine the significance of the septin double ring in cytokinesis.  We demonstrate that the septin double ring is dispensable for cytokinesis unless the actomyosin ring is also compromised.  Thus, the two cytokinetic structures, which are pre-patterned by and simultaneously generated from the transitional hourglass, normally act in concert to ensure efficient cytokinesis.  This study establishes a mechanism and function for the role of a RhoGEF-anillin module in septin architectural remodeling at the cell division site.