Septin Filament Formation Is Essential in Budding Yeast

Division of Biochemistry and Molecular Biology, Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
Developmental Cell (Impact Factor: 9.71). 04/2011; 20(4):540-9. DOI: 10.1016/j.devcel.2011.02.004
Source: PubMed


Septins are GTP-binding proteins that form ordered, rod-like multimeric complexes and polymerize into filaments, but how such supramolecular structure is related to septin function was unclear. In Saccharomyces cerevisiae, four septins form an apolar hetero-octamer (Cdc11-Cdc12-Cdc3-Cdc10-Cdc10-Cdc3-Cdc12-Cdc11) that associates end-to-end to form filaments. We show that septin filament assembly displays previously unanticipated plasticity. Cells lacking Cdc10 or Cdc11 are able to divide because the now-exposed subunits (Cdc3 or Cdc12, respectively) retain an ability to homodimerize via their so-called G interface, thereby allowing for filament assembly. In such cdc10Δ and cdc11Δ cells, the remaining septins, like wild-type complexes, localize to the cortex at the bud neck and compartmentalize nonseptin factors, consistent with a diffusion barrier composed of continuous filaments in intimate contact with the plasma membrane. Conversely, Cdc10 or Cdc11 mutants that cannot self-associate, but "cap" Cdc3 or Cdc12, respectively, prevent filament formation, block cortical localization, and kill cells.

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Available from: Michael Mcmurray, Jul 25, 2014
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    • "Previous studies have demonstrated the presence of septin heterooctamers in fungi and heterohexamers in mammals [13], [14], [16], [17], [30]. Our results show that similar heteropolymer units are formed in A. nidulans by core septins AspACdc11, AspBCdc3, AspCCdc12 and AspDCdc10 throughout early development and that AspE is not a member of these heteropolymers. "
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    ABSTRACT: Septins are important components of the cytoskeleton that are highly conserved in eukaryotes and play major roles in cytokinesis, patterning, and many developmental processes. Septins form heteropolymers which assemble into higher-order structures including rings, filaments, and gauzes. In contrast to actin filaments and microtubules, the molecular mechanism by which septins assemble is not well-understood. Here, we report that in the filamentous fungus Aspergillus nidulans, four core septins form heteropolymeric complexes. AspE, a fifth septin lacking in unicellular yeasts, interacts with only one of the core septins, and only during multicellular growth. AspE is required for proper localization of three of the core septins, and requires this same subset of core septins for its own unique cortical localization. The ΔaspE mutant lacks developmentally-specific septin higher-order structures and shows reduced spore production and slow growth with low temperatures and osmotic stress. Our results show that at least two distinct septin heteropolymer populations co-exist in A. nidulans, and that while AspE is not a subunit of either heteropolymer, it is required for assembly of septin higher-order structures found in multicellular development.
    PLoS ONE 03/2014; 9(3):e92819. DOI:10.1371/journal.pone.0092819 · 3.23 Impact Factor
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    • "In S. cerevisiae, four septins oligomerize to form an octameric septin complex (Bertin et al. 2008). In the absence of one septin, the remaining three septins can still form a less optimal hexameric complex that is sufficient for cytokinesis (McMurray et al. 2011). Many human septins have complex tissue-specific expression patterns, such that many tissues contain multiple septin genes from the same subgroup (Cao et al. 2007). "
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    ABSTRACT: Septins are cytoskeletal proteins that form hetero-oligomeric complexes and function in many biological processes, including cytokinesis. Drosophila melanogaster has five septin genes. Sep5, which is the most recently evolved septin gene in Drosophila, is a retrogene copy of Sep2. Sep5 mutants appear wild type, whereas Sep2 mutant females are semisterile. Their ovaries have egg chambers containing abnormal numbers of nurse cells. The egg chamber phenotype is rescued to wild type by expressing a Sep2 cDNA, but it is only partially rescued by expressing a Sep5 cDNA, showing that these paralogs have diverged in function at the protein level. Sep2 Sep5 double mutants have an early pupal lethal phenotype and lack imaginal discs, suggesting that these genes have redundant functions during imaginal cell proliferation.
    Genome 12/2013; 56(12):753-8. DOI:10.1139/gen-2013-0210 · 1.42 Impact Factor
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    • "A case in point is that septin disk enlargement by SEPT9 isoforms lacking the N-terminal extension of SEPT9(a) is not seen using the cognate AcGFP-tagged isoforms, which were nonlocalized in most cells (compare Figures 5 and 9D). Deletion of the α0 helix at the N-terminal end of the budding yeast septin CDC11 weakens homotypic CDC11–CDC11 interactions at the NC-interface of octamers, which prevents end-to-end heteromer polymerization (Bertin et al., 2008; McMurray et al., 2011). Moreover, expression of SEPT9 lacking the entire N-terminus, including the α0 helix, has been shown to disrupt septin filament formation, which supports the notion that octamers polymerize through a SEPT9–SEPT9 NC-interface (Kim et al., 2011). "
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    ABSTRACT: Septin family proteins assemble into rod-shaped heteromeric complexes that form higher-order arrangements at the cell cortex where they serve apparently conserved functions as diffusion barriers and molecular scaffolds. There are 13 confirmed septin paralogs in mammals that may be ubiquitous or tissue-specific. Septin heterooligomerization appears homology subgroup-directed, which in turn determines the subunit arrangement of six- to eight-subunit core heteromers. Here we addressed functional properties of human SEPT9 that, due to variable mRNA-splicing, exist as multiple isoforms that differ between tissues. Myeloid K562 cells express three SEPT9 isoforms, all of which have an equal propensity to heterooligomerize with SEPT7-containing hexamers to generate octameric heteromers. However, due to limiting amounts of SEPT9, K562 cells contain both hexameric and octameric heteromers. To generate cell lines with controllable hexamer to octamer ratios and that express single SEPT9 isoforms, a gene product replacement-strategy was developed. By this means we identified SEPT9 isoform-specific properties that either facilitate septin heteromer polymerization along microtubules or modulate the size-range of submembranous septin disks - a prevalent septin structure in non-adhered cells. Our findings show that the SEPT9 expression level directs the hexamer to octamer ratio, while the isoform-composition and expression level together determine higher-order arrangements of septin filaments.
    Molecular biology of the cell 09/2012; 23(21). DOI:10.1091/mbc.E12-06-0486 · 4.47 Impact Factor
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