Membrane binding controls ordered self-assembly of animal septins

Area of Science:

• Cell biology
• Biophysics
• Cytoskeletal dynamics

Background:

• Septins are crucial cytoskeletal proteins involved in cell cortex mechanics.
• Their interaction mechanisms with the plasma membrane are not well understood.

Purpose of the Study:

• To investigate the mechanisms of septin-lipid membrane interactions.
• To elucidate how septins self-assemble on membranes and form higher-order structures.

Main Methods:

• Cell-free reconstitution assays.
• Transmission electron microscopy (TEM).
• Atomic force microscopy (AFM) and quartz crystal microbalance (QCM).

Main Results:

• Septin binding to lipid membranes requires electrostatic interactions with anionic lipids.
• Membrane binding induces ordered self-assembly of fly septins into filamentous meshworks.
• Fly and mammalian septins form single and paired filament arrays; fly septin filaments form rigid double layers (12-18 nm thick), while truncated mutants form monolayers (4 nm thick).

Conclusions:

• Membrane binding is essential for the formation of ordered septin filament arrays.
• Septin filament stacking depends on C-terminal coiled coils of specific subunits.
• This study provides novel insights into septin regulation of cell surface mechanics.