Correlative AFM and fluorescence imaging demonstrate nanoscale membrane remodeling and ring-like and tubular structure formation by septins

Septins are ubiquitous cytoskeletal filaments that interact with the inner plasma membrane and are essential for cell division in eukaryotes. In cellular contexts, septins are often localized at micrometric Gaussian curvatures, where they assemble onto ring-like structures. The behavior of budding yeast septins depends on their specific interaction with inositol phospholipids, enriched at the inner leaflet of the plasma membrane. Septin filaments are built from the non-polar self-assembly of short rods into filaments. However, the molecular mechanisms regulating the interplay with the inner plasma membrane and the resulting interaction with specific curvatures are not fully understood. In this report, we have imaged dynamical molecular assemblies of budding yeast septins on PIP2-containing supported lipid bilayers using a combination of high-speed AFM and correlative AFM-fluorescence microscopy. Our results clearly demonstrate that septins are able to bind to flat supported lipid bilayers and thereafter induce the remodeling of membranes. Short septin rods (octamers subunits) can indeed destabilize supported lipid bilayers and reshape the membrane to form 3D structures such as rings and tubes, demonstrating that long filaments are not necessary for septin-induced membrane buckling.