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Membrane binding controls ordered self-assembly of animal septins
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23 Septins are conserved cytoskeletal proteins that regulate cell cortex mechanics. The mechanisms of 24 their interactions with the plasma membrane remain poorly understood. Here we show by cell-free 25 reconstitution that membrane binding requires electrostatic interactions of septins with anionic lipids 26 and promotes the ordered self-assembly of fly septins into filamentous meshworks. Transmission 27 electron microscopy reveals that both fly and mammalian septins form arrays of single and paired 28 filaments. Atomic force microscopy and quartz crystal microbalance demonstrate that the fly filaments 29 form mechanically rigid, 12 to 18 nm thick, double layers of septins. By contrast, C-terminally truncated 30 septin mutants form 4 nm thin monolayers, indicating that stacking requires the C-terminal coiled coils 31 on DSep2 and Pnut subunits. Our work shows that membrane binding is required for fly septins to form 32 ordered arrays of single and paired filaments and provides new insights into the mechanisms by which 33 septins may regulate cell surface mechanics.
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