title: Three-dimensional ultrastructure of the septin filament network in Saccharomyces cerevisiae.
abstract: Septins are conserved GTP-binding proteins involved in membrane compartmentalization and remodeling. In budding yeast, five mitotic septins localize at the bud neck, where the plasma membrane is enriched in phosphatidylinositol-4,5-bisphosphate (PtdIns4,5P(2)). We previously established the subunit organization within purified yeast septin complexes and how these hetero-octamers polymerize into filaments in solution and on PtdIns4,5P(2)-containing lipid monolayers. How septin ultrastructure in vitro relates to the septin-containing filaments observed at the neck in fixed cells by thin-section electron microscopy was unclear. A morphological description of these filaments in the crowded space of the cell is challenging, given their small cross section. To examine septin organization in situ, sections of dividing yeast cells were analyzed by electron tomography of freeze-substituted cells, as well as by cryo-electron tomography. We found networks of filaments both perpendicular and parallel to the mother-bud axis that resemble septin arrays on lipid monolayers, displaying a repeat pattern that mirrors the molecular dimensions of the corresponding septin preparations in vitro. Thus these in situ structures most likely represent septin filaments. In viable mutants lacking a single septin, in situ filaments are still present, although more disordered, consistent with other evidence that the in vivo function of septins requires filament formation.
authors.affiliation: Division of Biochemistry and Molecular Biology, Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720, USA.
authors.forename: Aurélie
authors.initials: A
authors.lastname: Bertin
authors.forename: Michael A
authors.initials: MA
authors.lastname: McMurray
authors.forename: Jason
authors.initials: J
authors.lastname: Pierson
authors.forename: Luong
authors.initials: L
authors.lastname: Thai
authors.forename: Kent L
authors.initials: KL
authors.lastname: McDonald
authors.forename: Elena A
authors.initials: EA
authors.lastname: Zehr
authors.forename: Galo
authors.initials: G
authors.lastname: García
authors.forename: Peter
authors.initials: P
authors.lastname: Peters
authors.forename: Jeremy
authors.initials: J
authors.lastname: Thorner
authors.forename: Eva
authors.initials: E
authors.lastname: Nogales
mesh_terms: D003599:Cytoskeleton / Q000737:chemistry / Q000648:ultrastructure; D021621:Imaging, Three-Dimensional; D009154:Mutation; D012441:Saccharomyces cerevisiae / Q000737:chemistry* / Q000648:ultrastructure; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000648:ultrastructure; D058112:Septins / Q000737:chemistry* / Q000648:ultrastructure
chemical_list: D029701:Saccharomyces cerevisiae Proteins; D058112:Septins