The Syp1/FCHo2 protein induces septin filament bundling through its intrinsically disordered domain

The septin collar of budding yeast is an ordered array of septin filaments that serves a scaffolding function for the cytokinetic machinery at the bud neck and compartmentalizes the membrane between mother and daughter cell. How septin architecture is aided by septin-binding proteins is largely unknown. Syp1 is an endocytic protein that was implicated in the timely recruitment of septins to the newly forming collar through an unknown mechanism. Using advanced microscopy and in vitro reconstitution assays, we show that Syp1 is able to align laterally and tightly pack septin filaments, thereby forming flat bundles or sheets. This property is shared by the Syp1 mammalian counterpart FCHo2, thus emphasizing conserved protein functions. Interestingly, the septin-bundling activity of Syp1 resides mainly in its intrinsically disordered region. Our data uncover the mechanism through which Syp1 promotes septin collar assembly and offer another example of functional diversity of unstructured protein domains. [Display omitted] •The budding yeast F-BAR protein Syp1 promotes septin recycling•Syp1 induces septin filament bundling in vitro•The septin-bundling activity of Syp1 is shared by its mammalian counterpart FCHo2•Septin bundling relies mainly on the intrinsically disordered domain of Syp1 A septin collar at the division site is essential for budding yeast cytokinesis. Yet, how it gets organized is not understood. Using in vitro reconstitution assays, Ibanes et al. show that the F-BAR protein Syp1 aligns and bundles septin filaments into ordered arrays, thereby contributing to septin collar assembly.