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: 10.37). 04/2011; 20(4):540-9. DOI: 10.1016/j.devcel.2011.02.004
Source: PubMed

ABSTRACT 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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    ABSTRACT: Septins belong to a family of polymerizing GTP binding proteins that are important for cytokinesis and other processes that involve spatial organization of the cell cortex. We have reconstituted a recombinant Drosophila septin complex and compared activities of the wild-type and several mutant septin complex variants both in vitro and in vivo. We show that Drosophila septin complex functions depend on the intact GTP binding and/or hydrolysis domains of Pnut, Sep1 and Sep2. The presence of the functional C-terminal domain of septins is required for the integrity of the complex. Drosophila Orc6 protein, the smallest subunit of the Origin Recognition Complex (ORC), directly binds to septin complex and facilitates septin filament formation. Orc6 forms dimers through the interactions of its N-terminal, TFIIB-like domains. This ability of the protein suggests a direct bridging role for Orc6 in stimulating septin polymerization in Drosophila. Studies reported here provide a functional dissection of a Drosophila septin complex and highlight the basic conserved and divergent features among metazoan septin complexes.
    Molecular Biology of the Cell 10/2014; 26(1). DOI:10.1091/mbc.E14-02-0734 · 4.55 Impact Factor
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    ABSTRACT: The septins are a family of GTP-binding proteins that form cytoskeletal filaments. Septins are highly conserved and evolutionarily ancient, but absent in land plants. The synthetic plant cytokinin forchlorfenuron (FCF) was previously shown to inhibit budding yeast cell division and induce ectopic septin structures. Subsequent studies in a wide range of eukaryotes have concluded that FCF exclusively inhibits septin function, yet the mechanism of FCF action in non-plant cells remains poorly understood. Here we report that the cellular effects of FCF are far more complex than previously described. The reported growth arrest of budding yeast cells treated with 1 mM FCF partly reflects sensitization caused by a bud4 mutation present in the W303 strain background. In wild-type (BUD4(+)) budding yeast, growth was inhibited at FCF concentrations that had no detectable effect on septin structure or function. Moreover, FCF severely inhibited the proliferation of fission yeast cells, in which septin function is non-essential. FCF induced fragmentation of budding yeast mitochondrial reticula and loss of mitochondrial membrane potential. Mitochondria also fragmented in cultured mammalian cells treated with concentrations of FCF that were previously assumed to solely target septins. Finally, FCF potently inhibited ciliation and motility and induced mitochondrial disorganization in Tetrahymena thermophila, without apparent alterations in septin structure. None of these effects was consistent with inhibition of septin function. Our findings point to non-septin targets as major concerns when using FCF.
    Eukaryotic Cell 09/2014; 13(11). DOI:10.1128/EC.00191-14 · 3.18 Impact Factor
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    ABSTRACT: Cytokinesis is the final step of cell division, and is a process that requires a precisely coordinated molecular machinery to fully separate the cytoplasm of the parent cell and to establish the intact outer cell barrier of the daughter cells. Among various cytoskeletal proteins involved, septins are known to be essential mediators of cytokinesis. In this Commentary, we present recent observations that specific cell divisions can proceed in the absence of the core mammalian septin SEPT7 and its Drosophila homolog Peanut (Pnut) and that thus challenge the view that septins have an essential role in cytokinesis. In the pnut mutant neuroepithelium, orthogonal cell divisions are successfully completed. Similarly, in the mouse, Sept7-null mutant early embryonic cells and, more importantly, planktonically growing adult hematopoietic cells undergo productive proliferation. Hence, as discussed here, mechanisms must exist that compensate for the lack of SEPT7 and the other core septins in a cell-type-specific manner. Despite there being crucial non-canonical immune-relevant functions of septins, septin depletion is well tolerated by the hematopoietic system. Thus differential targeting of cytokinesis could form the basis for more specific anti-proliferative therapies to combat malignancies arising from cell types that require septins for cytokinesis, such as carcinomas and sarcomas, without impairing hematopoiesis that is less dependent on septin.
    Journal of Cell Science 02/2015; DOI:10.1242/jcs.164830 · 5.33 Impact Factor

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