Quantitative biophysical analysis of human septin hexamer and octamer self-assembly on model membranes

1Septins are GTP-binding cytoskeletal proteins that shape and compartmentalize the plasma membrane. Their complex interactome has made it difficult to understand the molecular factors that govern their assembly. Moreover, it is unclear whether human septin hexamers and octamers form distinct higher-order assemblies, especially at the plasma membrane. Here, we address this question by using label-free methods to probe binding and self-assembly of recombinant human septins on supported lipid bilayers. Quartz crystal microbalance with dissipation (QCM-D) monitoring revealed that septin-membrane binding is diffusion-limited and concentration-dependent. Hexamers and octamers showed distinct viscoelastic properties, suggestive of structural differences. Imaging by atomic force microscopy (AFM) revealed that septin hexamers formed aligned nematic filamentous networks, whereas septin octamers formed aligned curved structures including spirals. QCM-D and AFM measurements both showed that septins form double-layered filament networks. However, upon C-terminal truncation of the SEPT6 and SEPT7 subunits, hexamers no longer bound the membrane while octamers formed a single-layered network of filament spirals. Our findings reveal that human septin hexamers and octamers interact differently with membranes, providing a baseline to understand their functions in the cell. 2 Significance statementO_LISeptins are cytoskeletal proteins that control cell membrane shape and stiffness. It is poorly understood how septin oligomers, the basic building blocks of septin filaments, bind and assemble on membranes. C_LIO_LIWe used label-free biophysical assays to quantitatively compare the binding kinetics and self-assembly behavior of recombinant human septin hexamers and octamers on supported lipid bilayer membranes. C_LIO_LIOur findings reveal that human septin hexamers and octamers both form organized filamentous networks on membranes, but with different structural properties that may potentially translate into different biological functions. C_LI