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Morphological complexity and plasticity are hallmarks of polyextremotolerant fungi. Septins are conserved cytoskeletal proteins and key contributors to cell polarity and morphogenesis. They sense membrane curvature, coordinate cell division, and influence diffusion at the plasma membrane. Four septin homologues are conserved from yeasts to humans, the systems in which septins have been most studied. But there is also a fifth family of opisthokont septins that remain biochemically mysterious. Members of this family, Group 5 septins, appear in the genomes of filamentous fungi, but are understudied due to their absence from ascomycete yeasts. Knufia petricola is an emerging model polyextremotolerant black fungus that can also serve as a model system for Group 5 septins. We have recombinantly expressed and biochemically characterized KpAspE, a Group 5 septin from K. petricola. This septin––by itself in vitro––recapitulates many functions of canonical septin hetero-octamers. KpAspE is an active GTPase that forms diverse homo-oligomers, binds shallow membrane curvatures, and interacts with the terminal subunit of canonical septin hetero-octamers. These findings raise the possibility that Group 5 septins govern the higher-order structures formed by canonical septins, which in K. petricola cells form extended filaments, and provide insight into how septin hetero-oligomers evolved from ancient homomers. 

Abstract Morphological complexity and plasticity are hallmarks of polyextremotolerant fungi. Septins are conserved cytoskeletal proteins and key contributors to cell polarity and morphogenesis. They sense membrane curvature, coordinate cell division, and influence diffusion at the plasma membrane. Four septins homologs are conserved from yeasts to humans, the two systems in which septins have been studied most extensively. But there is also a fifth family of septin proteins that remain biochemically mysterious. Members of this family, known as Group 5 septins, appear in the genomes of filamentous fungi, and thus have been understudied due to their absence from ascomycete yeasts.Knufia petricolais an emerging model polyextremotolerant black fungus that can serve as a model system for understudied Group 5 septins. We have recombinantly expressed and biochemically characterizedKpAspE, a Group 5 septin fromK. petricola, demonstrating that this septin––by itselfin vitro–– recapitulates many of the functions of canonical septin hetero-octamers.KpAspE is an active GTPase that forms diverse homo-oligomers, senses membrane curvature, and interacts with the terminal subunit of canonical septin hetero-octamers. These findings raise the possibility that Group 5 septins govern the higher order structures formed by canonical septins, which inK. petricolacells form extended filaments. These findings provide insight into how septin hetero-oligomers evolved from ancient homomers and raise the possibility that Group 5 septins govern the higher order structures formed by canonical septins. Significance StatementSeptins are understudied cytoskeletal proteins. Here, we biochemically characterizedKpAspE, a novel Group 5 septin from a polyextremotolerant black fungus.KpAspE in isolation recapitulates many properties of canonical septin hetero-octamersin vitroand interacts with the Cdc11, the terminal subunit of those octamers.These findings provide insight into how ancient septins may have evolved and diversified from homopolymers and suggest that many of the septin functions were present in the ancestral protein.