Architecture and function of Bud3 and Bud4-induced septin structures

At the time of cytokinesis, a double septin ring is assembled at the division site in many eukaryotic cells. In budding yeast the double ring is made by two arrays of circumferential septin filaments at the two sides of the bud neck, which are thought to compartmentalize the membrane around the cleavage site. Integrity of the double septin ring requires the anillin Bud4 and its presumed partner Bud3, which associate with septins in mitosis and have separate, yet unknown, roles in stabilizing septin circumferential filaments. Through in vitro reconstitution assays using purified proteins, we show that Bud3 and Bud4 organise septin filaments in distinct ways, while together they cooperate to assemble higher-order septin networks. In agreement with their separate roles in septin organization, Bud3 and Bud4 bind to different septins and require septins to associate with each other, indicating that they modulate septin architecture independently but synergistically. We also provide evidence that Bud3 and Bud4 bind membranes in vivo and in vitro, consistent with the presence of lipid-binding domains in their primary sequence. Using bud3 bud4 double mutants that lack the septin double ring, we show that in vivo proteins marking cytokinetic remnants require the double ring to efficiently concentrate at the division site, while other proteins landing at the bud neck at cytokinesis are unaffected. Thus, we propose that a septin double ring may imprint a selective spatial memory for cytokinesis that is transmitted throughout subsequent cell divisions.