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ABSTRACT Cells employ cytoskeletal polymers to move, divide, and pass information inside and outside of the cell. Previous work on eukaryotic cytoskeletal elements such as actin, microtubules, and intermediate filaments investigating the mechanisms of polymerization have been critical to understand how cells control the assembly of the cytoskeleton. Most biophysical analyses have considered cooperative versus isodesmic modes of polymerization; this framework is useful for specifying functions of regulatory proteins that control nucleation and understanding how cells regulate elongation in time and space. The septins are considered a fourth component of the eukaryotic cytoskeleton, but they are poorly understood in many ways despite their conserved roles in membrane dynamics, cytokinesis, and cell shape, and in their links to a myriad of human diseases. Because septin function is intimately linked to their assembled state, we set out to investigate the mechanisms by which septin polymers elongate under different conditions. We used simulations,in vitroreconstitution of purified septin complexes, and quantitative microscopy to directly interrogate septin polymerization behaviors in solution and on synthetic lipid bilayers of different geometries. We first used reactive Brownian dynamics simulations to determine if the presence of a membrane induces cooperativity to septin polymerization. We then used fluorescence correlation spectroscopy (FCS) to assess septins’ ability to form filaments in solution at different salt conditions. Finally, we investigated septin membrane adsorption and polymerization on planar and curved supported lipid bilayers. Septins clearly show signs of salt-dependent cooperative assembly in solution, but cooperativity is limited by binding a membrane. Thus, septin assembly is dramatically influenced by extrinsic conditions and substrate properties and can show properties of both isodesmic and cooperative polymers. This versatility in assembly modes may explain the extensive array of assembly types, functions, and subcellular locations in which septins act. SIGNIFICANCEThe septin cytoskeleton plays conserved and essential roles in cell division, membrane remodeling, and intracellular signaling with links to varied human diseases. Unlike actin and microtubules, whose polymerization dynamics have been extensively characterized, the molecular details of septin polymerization remain poorly understood. Here, we investigate the mode of septin polymerization through the lens of isodesmic and cooperative polymer assembly models in solution, on planar and curved supported membranes, and under different ionic conditions. Our findings show that the mechanisms of septin assembly are highly sensitive to ionic conditions, membrane geometry, and protein concentrations. Notably, assembly can show either cooperative or isodesmic properties depending on context, thereby revealing unexpected plasticity.