Human septins organize as octamer-based filaments and mediate actin-membrane anchoring in cells
Carla Silva Martins, Cyntia Taveneau, Gerard Castro-Linares, Mikhail Baibakov, Nicolas Buzhinsky, Mar Eroles, Violeta Milanović, Shizue Omi, J.D. Pédelacq, François Iv, Léa Bouillard, Alex Llewellyn, Maxime Gomes, Mayssa Belhabib, Mira Kuzmić, Pascal Verdier‐Pinard, Stacey Lee, Ali Badache, Sanjay Kumar, C. Chandré, Sophie Brasselet, Félix Rico, Olivier Rossier, Gijsje H. Koenderink, Jérôme Wenger, Stéphanie Cabantous, Manos Mavrakis
Abstract
Septins are cytoskeletal proteins conserved from algae and protists to mammals. A unique feature of septins is their presence as heteromeric complexes that polymerize into filaments in solution and on lipid membranes. Although animal septins associate extensively with actin-based structures in cells, whether septins organize as filaments in cells and if septin organization impacts septin function is not known. Customizing a tripartite split-GFP complementation assay, we show that all septins decorating actin stress fibers are octamer-containing filaments. Depleting octamers or preventing septins from polymerizing leads to a loss of stress fibers and reduced cell stiffness. Super-resolution microscopy revealed septin fibers with widths compatible with their organization as paired septin filaments. Nanometer-resolved distance measurements and single-protein tracking further showed that septin filaments are membrane bound and largely immobilized. Finally, reconstitution assays showed that septin filaments mediate actin-membrane anchoring. We propose that septin organization as octamer-based filaments is essential for septin function in anchoring and stabilizing actin filaments at the plasma membrane.