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The cytoskeleton of a cell controls all the aspects of cell shape changes and motility from its physiological functions for survival to reproduction to death. The structure and dynamics of the cytoskeletal components: actin, microtubules, intermediate filaments, and septins – recently regarded as the fourth member of the cytoskeleton family – are conserved during evolution. Such conserved and effective control over the mechanics of the cell makes the cytoskeletal components great candidates for in vitro reconstitution and bottom-up synthetic biology studies. Here, we review the recent efforts in reconstitution of the cytoskeleton in and on membrane-enclosed biomimetic systems and argue that co-reconstitution and synergistic interplay between cytoskeletal filaments might be indispensable for efficient mechanical functionality of active minimal cells. Further, mechanical equilibrium in adherent eukaryotic cells is achieved by the formation of integrin-based focal contacts with extracellular matrix (ECM) and the transmission of stresses generated by actomyosin contraction to ECM. Therefore, a minimal mimic of such balance of forces and quasi-static kinetics of the cell by bottom-up reconstitution requires a careful construction of contractile machineries and their link with adhesive contacts. In this review, in addition to cytoskeletal crosstalk, we provide a perspective on reconstruction of cell mechanical equilibrium by reconstitution of cortical actomyosin networks in lipid membrane vesicles adhered on compliant substrates and also discuss future perspectives of this active research area.
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This content will become publicly available on February 16, 2026
Cooperativity in septin polymerization is tunable by ionic strength and membrane adsorption
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.
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- Award ID(s):
- 2401042
- PAR ID:
- 10586672
- Publisher / Repository:
- bioRxiv
- Date Published:
- Format(s):
- Medium: X
- Institution:
- bioRxiv
- Sponsoring Org:
- National Science Foundation
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