Drosophila septins bundle and curve actin filaments. Manos Mavrakis¹, Yannick Azou¹, Feng-Ching Tsai², José Alvarado², Aurélie Bertin³, Francois Iv¹, Gijsje Koenderink², Thomas Lecuit¹. 1) Institut de Biologie du Développement de Marseille Luminy, CNRS UMR7288, Aix-Marseille University, Marseille, France; 2) FOM Institute AMOLF, Amsterdam, The Netherlands; 3) Institut de Biochimie et de Biophysique Moléculaire et Cellulaire, CNRS UMR8619, Université Paris-Sud, Orsay, France.

   During cell division, actin filaments and myosin motor proteins form a membrane-anchored contractile ring that constricts the cell midzone to give rise to two daughter cells. However, how actomyosin assembles in a cytokinesis-competent network remains an outstanding question. Here we focus on septins, one of the conserved core components of cytokinesis implicated in cancer and neurodegeneration. Although septins bind independently membranes, Anillin (an actin crosslinker) and nonmuscle Myosin-II in different model systems, the role of septins in the organization of actomyosin is not understood, and their precise biochemical function remains unknown. We explore the role of septins during Drosophila embryo cellularization, when actomyosin assembles into a cytokinesis-like ring at the tips of invaginating membranes. Septin depleted embryos fail to accumulate and to stabilize actomyosin, whereas actomyosin fails to assemble into ring-like structures and instead forms a loose coat beneath the cell membrane, suggesting that septins could be involved in compacting or/and curving actomyosin. To test this hypothesis we reconstituted actin filaments in the presence of purified septin complexes in vitro. Biochemical and microscopy assays showed that Drosophila septins constitute a bona fide actin crosslinker and bundler, and that septins further bend actin filament bundles into rings and highly curved geometries. Our data suggest that the functional organization of actomyosin during membrane constriction relies on the bundling and curving activity of septins on actin filaments.