Cooperativity in septin polymerization is tunable by ionic strength and membrane adsorption.

Area of Science:

• Cell biology
• Biophysics
• Polymer science

Background:

• Cells utilize cytoskeletal polymers for essential functions like movement and division.
• Septins, a fourth cytoskeletal component, play roles in membrane dynamics and cell shape but are poorly understood.
• Understanding septin polymerization is crucial due to their link to human diseases.

Purpose of the Study:

• To investigate the polymerization mechanisms of septins under varying conditions.
• To determine how membranes influence septin assembly and cooperativity.
• To elucidate the versatility of septin assembly modes.

Main Methods:

• Reactive Brownian dynamics simulations to model membrane-induced cooperativity.
• Fluorescence correlation spectroscopy (FCS) to analyze septin filament formation in solution.
• Quantitative microscopy to study septin polymerization on lipid bilayers of varying geometries.

Main Results:

• Septins exhibit salt-dependent cooperative assembly in solution.
• Membrane binding limits the cooperativity of septin polymerization.
• Septin assembly demonstrates characteristics of both isodesmic and cooperative polymerization.

Conclusions:

• Septin polymerization is significantly influenced by extrinsic factors and substrate properties.
• The adaptable assembly of septins contributes to their diverse functions and locations.
• This study provides insights into the fundamental mechanisms governing septin cytoskeleton dynamics.