EFFECTS OF MEMBRANE PROPERTIES ON THE LOCALIZATION AND ASSEMBLY OF THE SEPTIN CYTOSKELETON

The septin cytoskeleton comprises a well-conserved class of membrane binding proteins critical for organizing major changes to cell shape. Septins do this by assembling into filament arrays, often at sites of membrane curvature, to organize cell signaling factors. In vitro, septins display a narrow preference for 1 µm diameter membrane curvatures, similarly to their most common binding sites in cells. Like many other curvature sensing proteins, septins possess an amphipathic helix that, in the case of septins, is necessary and sufficient for localization to their preferred curvature both in vitro and in cells. However, septins can assemble on an array of membrane geometries, suggesting additional cues for their localization. This work describes the impact of membrane properties, specifically the property of lipid packing dictated by both membrane curvature and lipid composition, on the localization and assembly of the septin cytoskeleton. Here, we designed lipid mixtures varying in lipid packing by molecular dynamics simulations and incorporated these compositions onto supported lipid bilayers to measure septin adsorption in vitro. Septins strongly favor loosely packed, disordered lipid bilayers but additional geometry cues act in conjunction with this membrane property. Introducing tighter lipid packing in cells disrupted septin structures in a curvature dependent manner, specifically limiting septin assembly and retention along flat regions of the plasma membrane. This work demonstrates that packing defects and geometry jointly regulate septin localization and highlights how multiple membrane properties are integrated to control organization of the septin cytoskeleton.