Septin filaments build structures such as rings, lattices and gauzes that serve as platforms for localizing signaling and organizing cell membranes. How cells control the geometry of septin assemblies in poorly understood.  We show here that septins are isodesmic polymers, in contrast to cooperative polymerization exhibited by F-actin and microtubules.  We constructed a physical model to analyze and interpret how septin assemblies change in the presence of regulators in yeast extracts.  Notably filaments differ in length and curvature in yeast extract compared to pure protein indicating cellular regulators modulate intrinsic biophysical features.  Combining analysis of extracts from regulatory mutants with simulations, we found increased filament flexibility and reduced filament fragmentation promote assembly of septin rings, whereas reduced flexibility in crowded environments promotes local filament alignment. This work demonstrates how tuning of intrinsic features of septin filament assembly by regulatory proteins yields a diverse array of structures observed in cells.

Images were acquired via TIRF microscopy after incubating septins/extracts on planar supported lipid bilayers. Images were prepared using FIJI/ImageJ.