Revised subunit order of mammalian septin complexes explains their in vitro polymerization properties |
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| Authors: | Forooz Soroor Moshe S Kim Oliva Palander Yadu Balachandran Richard F Collins Samir Benlekbir John L Rubinstein William S Trimble |
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| Institution: | National Institutes of Health, NINDS;aCell Biology Program, Hospital for Sick Children, Toronto, ON M5G 1X8, Canada;cMolecular Medicine Program, Hospital for Sick Children, Toronto, ON M5G 1X8, Canada;bDepartment of Biochemistry, University of Toronto, Toronto, ON M5G 1A8, Canada |
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| Abstract: | Septins are conserved GTP-binding cytoskeletal proteins that polymerize into filaments by end-to-end joining of hetero-oligomeric complexes. In human cells, both hexamers and octamers exist, and crystallography studies predicted the order of the hexamers to be SEPT7-SEPT6-SEPT2-SEPT2-SEPT6-SEPT7, while octamers are thought to have the same core, but with SEPT9 at the ends. However, based on this septin organization, octamers and hexamers would not be expected to copolymerize due to incompatible ends. Here we isolated hexamers and octamers of specific composition from human cells and show that hexamers and octamers polymerize individually and, surprisingly, with each other. Binding of the Borg homology domain 3 (BD3) domain of Borg3 results in distinctive clustering of each filament type. Moreover, we show that the organization of hexameric and octameric complexes is inverted compared with its original prediction. This revised septin organization is congruent with the organization and behavior of yeast septins suggesting that their properties are more conserved than was previously thought. |
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